SOLID STATE MATERIALS CHEMISTRY 1999

INSTRUCTOR: Geoffrey A. Ozin, Professor of Materials Chemistry
 

This course is designed as a follow-up to CHM 325, Polymer and Materials Chemistry, which focused on structure-property-function relations of selected classes of polymeric and inorganic materials. In this course we will be concerned with a comprehensive investigation of a wide range of synthetic methods for preparing diverse classes of inorganic materials with properties that are intentionally tailored for a particular use. The lectures begin with a primer that covers key aspects of the background solid-state materials, electronic band description of solids, and connections between molecules and bonds in materials chemistry and solids and bands in solid-state physics. This is followed by a survey of archetypical solids that have had a dramatic influence on the materials world, new and exciting developments in materials chemistry and a look into the crystal ball at perceived future developments in materials research, development and technology. Strategies for synthesizing many different classes of materials with intentionally designed structures and compositions, textures and morphologies are then explored in detail emphasizing how to control the relations between structure and property of materials and ultimately function and utility. A number of contemporary issues in materials research are critically evaluated to introduce the student to recent highlights in the field of materials chemistry - an emerging sub-discipline of chemistry.   Solid-state materials – synthesis methods Combinatorial materials chemistry – robotic synthesis Contemporary issues in solid-state materials chemistry – case histories Recommended text: A. R. West, Solid State Chemistry and its Applications, Wiley, 1997. Reference texts: D. W. Bruce, D. O’Hare, Inorganic Materials, Second Edition, Wiley, 1997. L. V. Interrante, M. J. Hampden-Smith, Chemistry of Advanced Materials, Wiley-VCH, 1998. C. N. R. Rao, J. Gopalakrishnan, New Directions in Solid State Chemistry, Second Edition, Cambridge University Press, 1997. L. Smart and E. Moore, Solid State Chemistry, An Introduction, Chapman and Hall, London, Second Edition. P. Ball, Made to Measure, New Materials for the 21st Century, Princeton University Press, 1997.

Course evaluation      Mid-term test 90 min. (25%) Written term paper 3000 words (15%) Written/oral assignments (10%) Final examination 180 min. (50%)

• Assignment 1: 18th October 2001 - short answer paper
• Assignment 2: 25th October 2001, oral presentation - mini-symposium, 6-9 pm
• Assignment 3: 15th November 2001 - 90 minute mid-term test
• Assignment 4: 29th November 2001 - term paper
• Assignment 5: Final exam to be announced - 10-19th December 2001
  
   

Topics to be covered (more-or-less!?)
 

Solid state primer What's exciting in materials chemistry  Materials synthesis Low dimensional solids  Synthetic electrical conductors Open-framework solids  Self-assembling nanoscale wires, films, clusters, colloids and shapes Biomineralization and biomimetics Materials future

 

SOLID STATE CENTURY

    Materials Chemistry, Umbrella View

• Classes of solids, insulators, semiconductors, semimetals, metals, superconductors
• Organic, inorganic, organometallic, polymer, ceramic, composites
• Solids, surfaces, interfaces, epitaxy, topotaxy, stability and reactivity of solids
• Defects, dopants, non-stoichiometry, effects on properties
• Simple to complex and close-packed to open structures, building-blocks, dimensionality, polymorphs, polytypes
• Amorphous, crystalline, quasicrystalline, paracrystalline, nanocrystalline, liquid crystalline materials
• Molecules-solids, bonds-bands, quantum size effects, scaling laws
• Irrational to rational synthesis, combinatorial chemistry, self-assembly, 2,3-D patterning, templating, biomimetics
• Nanosynthesis, nanofabrication, nanoelectronics, nanophotonics, nanomachines, nanofuture
• Characterization of materials, diffraction, microscopy, spectroscopy, thermal, transport, optical, mechanical
• Synthesis, structure, morphology, property, function, application, user sector

• Electronic, diodes, transistors, photovoltaics, photodetectors, solar cells, photoelectrochemical devices,
• Optical, fiber communication, CDs, LEDs, lasers, NLO SHG/THG, transphasors, photon gap, phosphors, lighting, TV screens, FP displays
• Magnetic, actuators, switches, data storage discs, tapes, heads, NMR, MRI, SQUIDS
• Ionic batteries, fuel cells, sensors, displays, electrochromics, smart windows and mirrors, ion-selective electrodes, detergents
• Energy, catalysts, photocatalysts, chemicals, fuels, electrocataysts
• Mechanical, earth, space vehicles, construction, tools, polymers, ceramics, composites, metals, alloys
• Environmental, car, industrial pollution NO_x, SO_x, CO₂, water clean-up heavy metals, organics
• Monitoring, chemical sensors, natural gas, glucose, electronic nose (food, perfume, drugs, chemical and biological warfare)
• Separations, gases, liquids, ions, molecular sieves, membranes, chromatography
• Biomedical, bone, skin, organ replacement, augmentation, repair, drug delivery

Where does a materials chemist fit into this scheme of things? (Interrrante).
 

MATERIALS THROUGH CIVILIZATION
 
 

The evolution of the relative importance of different classes of materials from the findings of archaeologists, educators and prognosticators (Ashby).
 

BASICS, MATERIALS CHEMISTRY

    THINGS YOU NEED TO REVISE

1. Bonding in solids, ionic and covalent
2. Most solids are not purely ionic or covalent, polarization, dipolar, dispersion, van der Waals forces
3. Close packing concepts, hard spheres, coordination number, substitutional-interstitial sites
4. Primitive unit cell, standard crystal systems (seven), lattices (fourteen Bravais), translational and rotational symmetry (230 space groups)
5. Factors controlling structure, stoichiometry, stability (charge, size, space-filling concepts) of solids
6. Basic concepts in bonding and electronic properties of solids
7. Defects, dopants, non-stoichiometry, effects on chemical, physical and mechanical properties of solid state materials
8. Rudiments of electronic, optical, magnetic, charge-transport behavior of solids

 
BASICS,  ELECTRONIC PROPERTIES OF SOLIDS

    PRIMER, BLOCH-WILSON BAND DESCRIPTION OF SOLIDS
 

1. Free electron traveling wave exp(ikx)
2. Electron l, wave vector k = 2p/l, p = h/l = (h/2p)k quasi-momentum
3. Description of electrons in solids
4. Modulated electron waves in a periodic crystal potential U(x)
5. Bloch orbitals Y (x) = exp(ikx)U(x)
6. Electron wavelengths from ¥ to lattice spacing 2a
7. Scattering of e’s by nuclei, standing waves at Bragg condition nl = 2a
8. Gives rise to forbidden energy band gap, E_g, and VB and CB
9. First Brillouin zone runs from k = ± p/a
10. Band description in terms of density of states (DOS), n(E)
11. Density of occupied and unoccupied states, n(E) = f_FD(E)N(E)
12. Fermi Dirac distribution of electrons, probability of state E being occupied: f_FD(E) = 1/(1 + exp(E_F-E)/k_BT)
13. E_F chemical potential of metal essentially highest occupied level of VB
14. E_F chemical potential of electrons, pinned for intrinsic SCs at E_F = 1/2(E_v + E_c)
15. Electronic selection rules, optical transitions, momentum k, electric dipole m
16. Direct transitions, Dk = 0, k_v = k_c, conservation of momentum, dipole allowed or forbidden
17. Indirect transitions, for Dk = 0, k_v + k_ph = k_c, conservation of momentum, dipole allowed or forbidden
18. Doping, Hydrogen impurity model, n/p-doping, radius and energies of electrons/holes
19. Effective mass of electrons/holes in solids, m_e,h^* = (h/2p)²/(d²E/dk²)
20. Tight binding description of bands Y _k = S ₁ⁿexp(ikna)f _n, periodic SALCAO Bloch orbitals
21. Essentially EHMO approximation for solids, H_ii coulomb, H_ij resonance integrals, nearest neighbor orbital overlap, yields E(k) vs k dispersion plots
22. useful relations, orbital overlap, band width, delocalization, band gap, band curvature, m*, mobility, conductivity
23. Junctions between SCs, Guass’s theorem, contact potential, band bending
24. Semiconductor np-junction diodes
25. M-SC junctions, Schottky barriers/diodes, ohmic contacts
26. Photovoltaics, photodetectors
27. Semiconductor-liquid junctions
28. Solar cells and photoelectrochemical cells
29. Semiconductor pnp and npn-junction bipolar transistors, amplifiers, switches
30. Metal-oxide-semiconductor junction field effect transistor, MOS-FET
31. Semiconductor LEDs, lasers, detectors
32. Organic LEDs, FETs
33. Quantum confined semiconductors, sheets, wires, dots
34. Quantum superlattices
35. Quantum devices, electronic/optical switches, MQW lasers, SETs
36. Nanomaterials, nanoelectronics, nanophotonics, nanomachines, nanofuture

SOLID STATE MATERIALS CHEMISTRY MEETS CONDENSED MATTER PHYSICS

OVERCOMING THE JARGON BARRIER

MOLECULES TO SOLIDS, BONDS TO BANDS
 

SOLID STATE BAND	MOLECULAR ORBITAL
Valence band, VB, continuous energy states	HOMO, discrete energy states
Conduction band, CB, continuous energy states	LUMO, discrete energy states
Fermi energy, EF	(Electro)chemical potential
Bloch orbital, delocalized	Molecular orbital, localized/delocalized
Tight binding band calculation	EH molecular orbital calculation
n-doping	Reduction, pH scale base
p-doping	Oxidation, pH scale acid
Band gap, Eg	HOMO-LUMO gap
Direct band gap, momentum/dipole selection rules	Dipole allowed
Indirect band gap, momentum/phonon/dipole selection rules	Dipole forbidden, vibronically allowed
Phonon or lattice vibration/libration	Molecule vibration/rotation
Peierls distortion, charge density wave	Jahn-Teller distortion
Polarons, magnons, plasmons	No analogues in molecules

 
 

  

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MATERIALS: STUFF THAT DOES STUFF
 

• WHAT ARE THE PROBLEMS A MATERIALS CHEMIST CAN HELP SOLVE?
 
• MINIATURIZATION · Moore’s law, doubling number of transistors on a chip every 1.5 years
•  smaller does not mean faster, low (insulation, t(RC)) and high (gate, V_br) e materials
•  contact with the outside world, making nanowires and nanoscale interconnects
•  quantum size effects, feature size < l_F(e,h), discrete states, dispersity, quantum noise
•  SETs, coulomb blockade (capacitor charging energy) and coulomb cascade (tunneling)
•  soft lithography, room at the top and bottom, applications when smaller is not better
•  self-assembling chips, molecular electronics, challenges for the nanofuture
•  SPMs, imaging, resolution, fabricating nanostructures, ICs, massive parallel arrays
• DATA STORAGE · magnetic, magneto-optical optical, which is the way to go
• higher storage density discs, sensitivity of disc head, read-write-erase, archival life
• BATTERIES · electrodes, electrolytes, strengths and limitations, V_oc, I_sc, interfaces
• FUEL CELLS · failure, cyclability (primary vs secondary), safety, environmental
• PECs, PVs · electrocatalytic activity, SA, 3-phase boundary, charge transport (electronic and ionic)
• SOLAR CELLS · overpotential, photoefficiency, photocorrosion, surface traps, tandem PV-PEC cells
• PLASTIC FUTURE · LEDs, FETs, ELDs, LCDs, lasers, smart windows and mirrors, flat panel displays, carrier mobility
• challenge, do not go up against the silicon steamroller (silicon Samouri)
• SENSORS · large-small molecules, arrays, neural net, pattern recognition, electronic nose,
• molecule selectivity, interference, response speed, reversibility, poisoning,
• SYNTHESIS · ad hoc approach to materials no longer acceptable, the “new materials chemistry”
• direct synthesis (high T, P), interfaces, diffusion lengths, nanoscale reactants
• soft chemistry, intentional design, rational synthesis, metastable materials
• self-assembly, templating, crystal engineering
• combinatorial discovery of new materials, rapid evaluation of huge libraries

CHM 434F, MATERIALS CHEMISTRY

    ASSIGNMENT 1, September 1999

    INSTRUCTOR: Professor Geoffrey A. Ozin

    Why have the solids listed below had a dramatic influence on the materials world?

    This question is intentionally designed to get you to read the required and recommended texts, and around the subject, a one line answer is required!
 

ZrO2
Na1+xAl11O17+x/2
alpha-SiO2
Si
GaAs
a-SiH
Na56Al56Si136O384
(amine)xTaS2
BaPb0.8Bi0.2O3
YBa2Cu3O7-x
BaTiO3
LiNbO3
LaNi5
Nb3Ge
Ca10(PO4)6(OH)2
TiS2
ZnS
(Si,Al)3(O,N)4
h-BN
PbMo6Se8
Y3Al5O12
K2[Pt(CN)4]Br0.3
(CH)n
TTF(TCNQ)
c-C, h-C
C60
C60K3
SiOPc
(SN)x
HxWO3
WO3-x
TiO2
CrxAl3-xO3
AgBr
Cu2HgI4
gamma-AgI
VO2
CrO2
AlxGa1-xPyAs1-y
SmCo5
Fe3O4
PEO(LiClO4)

 
 

CHM 434F, MATERIALS CHEMISTRY

ASSIGNMENT 2, September 1999

INSTRUCTOR: Professor Geoffrey A. Ozin

CONTEMPORARY ISSUES IN MATERIALS CHEMISTRY

(Note that these questions will require considerable background reading, and may not be able to be addresed until well into the course)
 
 

1. why would the recently discovered MoS₂ faux fullerenes make ideal solid lubricants?
2. how would you use chemistry to make water flow uphill?
3. how would you synthesize hexagonal mesoporous silica from a lyotropic liquid crystal?
4. why does nanocrystalline TiO₂ enhance the RT Li⁺ ionic conductivity of the polymer electrolyte PEO-LiClO₄ in a solid state Li intercalation battery?
5. how and why would you solublize a single wall carbon nanotube?
6. how can an electroluminescent thin film device be made from monodispersed surfactant-capped CdSe clusters?
7. what are the advantages of using a single walled carbon nanotube as the tip in an atomic force microscope?
8. how might you synthesize a concrete spring?
9. how would you synthesize a zeolite-like material with a framework based upon a metal sulfide rather than an aluminosilicate?
10. why and how does the color and luminescence of monodispersed surfactant-capped CdSe clusters change with the size of the clusters?
11. how would you make an abacus from C₆₀?
12. how would you use a thermotropic liquid crystal and a polymer to electrically control the transmission of light through a glass window?
13. how and why does the magnetotactic bacteria synthesize a chain of ferromagnetic clusters?
14. how could you build a chemical sensor from monodispersed latex spheres?
15. how does the intermetallic LaNi₅H_x function as a cathode in an alkaline-nickel hydroxide battery?
16. how would you use a combinatorial materials chemistry approach to find a better solid phosphor?
17. how can information be stored in CoCuCo metal magnetic multilayers?
18. how would you synthesize a plastic light emitting diode?
19. how and why would you synthesize a diamond opal?
20. why is a membrane made out of Nafion, a perfluorosulphonic acid, the solid electrolyte-separator of choice in a hydrogen-oxygen fuel cell?
21. why does the T_c of BiSrCuO type ceramic superconductors not change on intercalating a (cetylpyridinium)₂HgI₄ 5 nm thickness bilayer between the BiO layer-planes?
22. how can a single electron transistor (SET) be made from a single 5 nm diameter CdSe cluster?
23. how can a diode be made from just one single walled carbon nanotube?
24. why does the jewelers chisel preferentially cleave diamond along {111}?
25. why does single crystal Si display chemical anisotropic etching in alkaline solutions, faster along {111} than {100}?
26. why does an ensemble of monodisperse 5 nm CdS nanoclusters, excited with UV light, display continuous bright green-blue luminescence, whereas a single nanocluster shows flashing green?
27. why does nitric acid preferentially open the end of a closed carbon nanotube?
28. why are Fe, Co, Ni the only ferromagnetic transition metals?
29. why does nanocrystalline TiO₂ greatly enhance the RT Li⁺ ionic conductivity of the polymer electrolyte PEO-LiClO₄ in a solid state Li intercalation battery?
30. why does dye-sensitized nanocrystalline nc-TiO₂ greatly enhance the light-to-electricity conversion efficiency of a photoregenerative solar cell with the following construction ITO|nc-TiO₂, Ru(bipy)₃²⁺|I^-,I₂,CH₃CN|Pt?
31. why is the fracture toughness of the calcite nacre shell of the mollusk 1000x that of calcite itself?
32. how can you tune the wavelength of a Bragg reflector built of a face centered cubic colloidal crystal array of silica spheres?
33. taking lessons from the synthesis of semiconductive, weakly electroluminescent, amorphous hydrogenated silicon a-HSi, devise a synthesis of the tritiated analogue a-TSi, and surmise whether it would exhibit any unusual property-function relations that portend potentially useful applications.
34. how does the anodic oxidation of a wafer of p-Si in aqueous HF, lead to self-limiting monodispersed pore formation and a novel material that is photo-, cathodo- and electroluminescent? with this knowledge how would you build an array of wavelength tunable, individually addressable LEDs on a Si wafer based on this chemistry, that could be used for an active matrix flat panel display?
35. how does anodic oxidation of an Al wafer in aqueous phosphoric or oxalic acid lead to an alumina membrane bearing a periodic array of monodispersed mesopores? devise a way to utilize this chemistry to synthesize a Pt membrane with an identical pore structure and use it to convert H₂ and O₂ into electricity?
36. how would you synthesize Ca₂C₆₀? assuming a fcc arrangement of C₆₀ molecules and Ca residing in octahedral interstices, explain why the material is semiconducting?
37. given just a glass slide, curved lens, polarizers and cholesteryl esters, how would you make a clinical thermometer with a precision of ± 0.1^oC?
38. which organic, inorganic and polymeric materials are in the global battle for control of the electroluminescent, electrochromic, photoluminescent and liquid crystal active matrix flat panel display market, and what properties of the material will make it a winner?
39. how would you mimic biomineralization of magnetotactic bacteria to synthesize better data storage materials?
40. how might you make a buckyball switch?
41. given Pt, how would you devise a resistless lithography for Si wafers?

 

CHM 434F, MATERIALS CHEMISTRY

INDEPENDENT WRITTEN PROJECT (TERM PAPER)

Suggested topics:
 

1. Evoking light emission from silicon.
2. Endohedral and exohedral fullerenes.
3. Inorganic polymers, materials for the next century?
4. Non-oxide open-framework materials, past, present and future.
5. Materials harder than diamond, can they be made?.
6. Supramolecular templating of mesostructured inorganics.
7. Plastic electronics for the next millenium.
8. Carbon nanotubes, better than Buckminsterfullerene C₆₀?
9. Capped semiconductor clusters and cluster superlattices.
10. Capped gold clusters and cluster superlattices.
11. Electrides, chemistry with the electron.
12. Magic of magnetic multilayers, giant magnetoresistance materials.
13. Molecular magnetism, a basis for new materials?
14. Photorefractive materials for manipulating light.
15. Nanoscale patterning and imaging with scanning probe microscopes.
16. High Tc superconductors, will they ever reach RT and be processable?
17. Kinetics of intercalation, getting between the sheets as fast as possible.
18. Layer-by-layer assembly of inorganic thin films, designer multilayers.
19. Alkane thiol self-assembled monolayers (SAMs), synthesis, characterization, what are they good for?
20. Biomimetic inorganic materials chemistry, why steal Nature’s best ideas?
21. Why grow inorganic crystals in space?
22. Information storage materials, how dense can you get?
23. Microelectrochemical transistors, diodes, sensors.
24. Photon band gap materials for a photonics revolution.
25. Dye sensitized nanocrystalline semiconductors for high efficiency solar cells.
26. Fuel cell materials and the future of the electric vehicle.
27. Smart window materials, energy conservation and privacy.
28. Forbidden symmetry, quasi-crystals for quasi-technologies?
29. Nanocrystalline materials, impact on science and technology.
30. Why do 3-D self-assembly of shaped mesoscale objects?
31. A layer of silicon must be at least 4-5 atoms thick to function as an insulator. This suggests that silicon-based microchips will reach the physcal limits of miniaturization early next century, year 2012!!! Is this the end of the road for silicon?
32. MEMS, microelectromechanical machines, tiny machines to do big things.

 
PRIMER: SOLID STATE SYNTHESIS

THINKING ABOUT MATERIALS

Chemical and physical properties of infinite non-molecular solids

Different techniques and concepts for synthesis and characterization of solid state materials from those conventionally applied to molecular solids, liquids, liquid crystals, solutions and gases

Various classes of solid state synthesis

Form or morphology of product controls synthesis method of choice

Single crystal, phase pure, defect free, interestingly not likely of much interest!

Single crystal, defect modified, dopants, intrinsic, extrinsic, non-stoichiometry, vacancies

Microcrystalline powder

Polycrystalline pellet, tube, rod

Single crystal or polycrystalline film, thin or thick, epitaxial, lattice matching, tolerance factor, elastic strain, defects

Non-crystalline, amorphous, glassy, fibers, films, tubes, plates

Nanocrystalline, paracrystalline, liquid crystalline, quasicrystalline
 

CLASSES OF SOLID STATE SYNTHETIC METHODS

Direct reaction

Precursor method

Crystallization techniques, solutions, melts, glasses, gels, hydrothermal, molten salt, high P/T

Vapour phase transport, synthesis, purification, crystal growth, doping

Ion-exchange methods, solution, melt

Injection, intercalation: chemical, electrochemical, pressure

Chimie Douce, soft-chemistry methods, synthesis of novel metastable materials, such as, open framework phases

Electrochemical synthesis, redox preparations, anodic oxidation, oxidative polymerization

Preparation of thin films and superlattices, chemical, electrochemical, physical, self-assembling mono- and multilayers, exfoliation-reassembly

Growth of single crystals, vapour, liquid, solid phase chemical, electrochemical

High pressure methods, hydrothermal, diamond anvils

Combinatorial materials chemistry, creation and rapid evaluation of of gigantic libraries of related materials.

WHAT’S EXCITING IN SOLID STATE SYNTHESIS: CASE HISTORIES OF CONTEMPORARY MATERIALS SYNTHESES

• Porous silicon, getting light out of a non-emissive material

• High Tc superconducting films, on the way to SC devices even with a refractory material

• Buckyballs and buckytubes, nanoscale device opportunities

• Diamond films, cheap and a myriad of opportunities

• Graphite fluoride, the unwettable

• Hydrogen fuel cells based on Pt and Nafion membranes, do not miss the bus

• A compositionally tunable superconducting organometallic intercalate

FACTORS INFLUENCING REACTIONS OF SOLIDS

Reaction conditions, temperature, pressure, atmosphere

Structural considerations

Reaction mechanism

Surface area of precursors

Defect concentration, defect type

Nucleation of one phase within another

Diffusion rates of atoms, ions, molecules in solids

Epitactic and topotactic reactions

Surface structure and reactivity of different crystal planes/faces

ARCHETYPE: DIRECT REACTION OF SOLIDS

Thermodynamic and kinetic factors need to be understood

Model reaction MgO + Al₂O₃ ® MgAl₂O₄ Spinel (ccp O^2-, Mg²⁺ 1/8 T_d, Al³⁺ 1/2 O_h)

Single crystals of precursors, interfaces between reactants, temperature T

On reaction, new reactant-product MgO/MgAl₂O₄ and Al₂O₃/MgAl₂O₄ interfaces

Free energy negative, favours reaction

Extremely slow at normal temperatures

Complete reaction can take several days at 1500^oC

Interfacial growth rates 3 : 1

Linear dependence of interface thickness x² versus t

Why is nucleation, mass transport so difficult?

MgO ccp O^2-, Mg²⁺ in O_h sites

Al₂O₃ hcp O^2-, Al³⁺ in 2/3 O_h sites

MgAl₂O₄ ccp O^2-, Mg²⁺ 1/8 T_d, Al³⁺ 1/2 O_h

Structural differences between reactants and products

Major structural reorganization in forming product spinel

Making and breaking strong bonds (mainly ionic)

Long range counter-diffusion of Mg²⁺ and Al³⁺ cations across interface, usually RDS

Requires ionic conductivity, substitutional or interstitial hopping of cations from site to site to effect mass transport

High temperature process as D(Mg²⁺) and D(Al³⁺) large for small highly charged cations

Nucleation of product spinel at interface, ions diffuse across thickening interface

Oxide ion reorganization at nucleation site

Decreasing rate as spinel product layer thickens

Parabolic rate law: dx/dt = k/x

x² = kt

Easily monitored with coloured product at interface, T and t

NiO + Al₂O₃ ® NiAl₂O₄

Linear x² vs t plots observed

lnk vs 1/T experiments provides Arrhenius activation energy Ea for the solid state reaction

Reaction mechanism requires charge balance to be maintained in the solid state interfacial reaction

3Mg²⁺ diffuse in opposite way to 2Al³⁺

MgO/MgAl₂O₄ Interface

2Al³⁺ -3Mg²⁺ + 4MgO ® MgAl₂O₄

MgAl₂O₄/Al₂O₃ Interface

3Mg²⁺ -2Al³⁺ + Al₂O₃ ® 3MgAl₂O₄

Overall Reaction

4MgO + 4Al₂O₃ ® 4MgAl₂O₄

RHS/LHS growth rate of interface = 3/1

This is called the Kirkendall Effect

MgO + Fe₂O₃ ® MgFe₂O₄

Different colour interfaces

Easily monitored rates

Other examples, give them a try to calculate the Kirkendall ratio:

SrO + TiO₂ ®

KF + NiF₂ ®

SiO₂ + Li₂O ®

KEY FACTORS IN SOLID STATE SYNTHESIS

Contact area: surface area of reacting solids, controls:

Rate of nucleation of product phase

Rates of diffusion of ions through various phases, reactants and products

Let us examine each of the above in turn

Surface Area of Precursors

Seems trivial but this is a vital consideration in solid state synthesis:

Consider MgO, 1cm³ cubes, density 3.5 gcm^-3

1 cm cubes: SA 6x10^-4 m²/g

10^-3 cm cubes: SA 6x10^-1 m²/g

10^-6 cm cubes: 6x10² m²/g, a 100 metre running track!!!

Clearly reaction rate is greatly influenced by the SA of precursors as contact area depends roughly on SA of the particles

Extra considerations

High pressure squeezing of reactive powders into pellets, for instance using 10⁵ psi

Pressed pellets still 20-40% porous

Hot pressing improves densification

Note: contact area not in planar layer lattice diffusion model for thickness change with time,
dx/dt = k/x

But x µ 1/A_contact

And A_{contact µ} 1/d_particle

Thus particle sizes and surface area inextricably connected and obviously x µ d and SA and particle size affect the interfacial thickness

These relations suggest some strategies for rate enhancement in direct reactions

Methods for increasing solid state reaction rates

Decreasing particle size

Hot pressing densification of particles

Atomic mixing in composite precursor compounds

Coated particle mixed component reagents, corona/core precursors

Nanocrystalline precursors

Thin layer superlattice reagents, Johnson superlattices

Aimed to increase interfacial reaction area A and decrease interface thickness x

dx/dt = k/x = k’A =k"/d

Minimizes diffusion length scales

NUCLEATION AND DIFFUSION CONCEPTS IN SOLID STATE REACTIONS

Nucleation, requires structural similarity of reactants and products, less reorganization energy, faster nucleation of product phase within reactants

MgO, Al₂O₃, MgAl₂O₄ example

MgO and MgAl₂O₄ rock salt and spinel, similar ccp O^2-

Distinct to hcp O^2- in Al₂O₃ phase

Spinel nuclei, matching of structure at MgO interface

Oxide arrangement essentially continuous across MgO/MgAl₂O₄ interface

Bottom line: structural similarity of reactants and products promotes nucleation and growth of one phase within another

Lattice of oxide anions, mobile Mg²⁺ and Al³⁺ cations

Factors influencing cation diffusion rates

Charge, mass and temperature

Interstitial versus substitutional diffusion

Depends on number and types of defects in reactant and product phases

Point, line, planar defects, grain boundaries

Enhanced ionic diffusion with defects and grain boundaries

What about orientation effects in the bulk and surface regions of solids?

Topotactic and epitactic reactions

Implies structural relationships between two phases

Topotaxy occurs in bulk, 1-, 2- or 3-D

Epitaxy occurs at interfaces, 2-D

Epitactic reactions requires 2-D structural similarity

Lattice matching within 15% to tolerate oriented nucleation

Otherwise mismatch over large contact area

Strained interface, missing atoms

Example: MgO/BaO

Both rock salt lattices, cannot be lattice matched over large contact area

Lattice matched crystalline growth

Best with less than 0.1% lattice mismatch

Causes elastic strain at interface

Slight atom displacement from equilibrium position

Strain energy reduced by misfit-dislocation

Creates dangling bonds, localized electronic states, carrier scattering by defects, luminescence quenching, killer traps, generally reduces efficacy of electronic and optical devices

Can be visualized by HR-TEM imaging

Topotactic reactions

More specific, require interfacial and bulk crystalline structural similarity, lattice matching

Topotaxy: involves lock-and-key ideas of self-assembly, molecule recognition, host-guest inclusion, clearly requires available space or creates space in the process of adsorption, injection, intercalation etc

SURFACE STRUCTURE AND REACTIVITY

Nucleation depends on actual surface structure of reacting phases

Example MgO rock salt

Different Miller index faces exposed, atom arrangements different

Different crystal habits possible, depends on rate of growth of different faces, octahedral, cubooctahedral, cubic possible and variants in between

{100} MgO alternating Mg²⁺, O^2- at corners of square grid

{111} MgO, Mg²⁺ or O^2- hexagonal arrangement

Different surface structures, implies distinct surface reactivities

CRYSTAL GROWTH

Most prominent surfaces, slower growth

Growth rate of specific surfaces controls morphology

Depends on area of a face, structure of exposed face, accessibility of a face, adsorption at surface sites, surface defects

All types of defects, intrisic or extrinsic, vacancies, interstitials, lines, planes, dislocations, grain boundaries enhance diffusion of ions

Play major role in reactivity, nucleation, crystal growth, materials properties (electronic, optical, magnetic, charge-transport, mechanical, thermal, acoustical etc)

DIRECT "SHAKE-AND-BAKE" SOLID STATE SYNTHESIS

Although this approach may seem to be ad hoc and a little irrational at times, the technique has served solid state chemistry well over the years

It has given birth to the majority of high technology devices and products that we take for granted every day of our lives

Thus it behooves us to look critically and carefully at the methods used if one is to move beyond to the new chemistry and a rational synthesis of materials

Reagents

Drying, maximum SA, in situ decomposition MgCO₃/Al(OH)₃

Intimate mixing, precursor reagents, homogenization, organic solvents, grinding, ball milling, ultrasonification

Container Materials

Chemically inert crucibles, boats, noble metals Au, Pt, Ni, Rh, Ir

Refractories, alumina, zirconia, silica, boron nitride, graphite

Reactivities with containers at high temperatures needs to be carefully evaluated for each system

Heating Programme

Furnaces, RF, microwave, lasers, ions, electrons

Prior decompositions

Frequent cooling, grinding, boost SA

Overcoming sintering, grain growth, brings up SA, fresh surfaces, enhanced contact area

Pelleting, hot pressing, enhances rate, extent of reaction

Preferential component volatilization if T too high, composition dependent

Controlled atmosphere for unstable oxidation states
 

Precursor techniques to improve reagent mixing
 

Coprecipitation, high degree of homogenization, high reaction rate

Concept: precursors to spinels

Zn(CO₂)₂/Fe₂[(CO₂)₂]₃/H₂O 1 : 1 mixing

H₂O evaporation, salts coprecipitated

Solid-solution mixing on atomic scale, filter, calcine in air

Zn(CO₂)₂ + Fe₂[(CO₂)₂]₃ ® ZnFe₂O₄ + 4CO + 4CO₂

High degree of homogenization, lower reaction temperature, faster rate

Magnetic garnets, tunable magnetic materials, aqueous precursor technique:

Y(NO₃)₃ + Gd(NO₃)₃ + FeCl₃ + NaOH ® Y_xGd₃-_xFe₅O₁₂

Firing 900^oC, 18-24 hrs., pellets, regrinding, repelletizing, repeated firings, removes RFeO₃ perovskite impurity

Isomorphous replacement of Y³⁺ for Gd³⁺ on dodecahedral sites, solid solution, similar rare earth ionic radii

complete family accessible, 0 < x < 3, 2Fe³⁺ Oh sites, 3Fe³⁺ Td sites, 3RE³⁺ dodecahedral sites

eight formula units in a unit cell, total of 160 atoms, cubic lattice unit cell parameter follows Vegard law behavior:
 

P(Y_xGd_3-xFe₅O₁₂) = Px/3(Y₃Fe₅O₁₂) + P(3-x)/3(Gd₃Fe₅O₁₂)
 

Any property P of a solid-solution member is the atom fraction weighted average of the end-members

Tunable magnetic properties by tuning the x value in the binary garnet  Y_xGd₃-_xFe₅O₁₂

3 Td Fe³⁺ sites, 5 UPEs

2 Oh Fe³⁺ sites, 5UPEs

Ferrimagnetically coupled material, oppositely aligned electron spins on the Td and Oh Fe³⁺ magnetic sublattices

Counting spins Y₃Fe₃O₁₂ ferrimagnetic at low T:  3 x 5 - 2 x 5 = 5UPEs

Counting spins Gd₃Fe₃O₁₂ ferrimagnetic at low T: 3 x 7 -3 x 5 + 2 x 5 = 16 UPEs
 
 
Y_xGd₃-_xFe₅O₁₂ creates a tunable magnetic garnet that is strongly temperature and composition dependent, applications in permanent magnets, magnetic recording media, magnetic bubble memories and so forth, similar concepts apply to magnetic spinels

 
Coprecipitation applicable to nitrates, acetates, oxalates, alkoxides, beta-diketonates and so forth

Requires:

similar salt solubilities

similar precipitation rates

no supersaturation, poor control

Useful for spinels and the like

Disadvantage: difficult to prepare high purity, accurate stoichiometric phases

Double Salt Precursors

Known stoichiometry double salts, controlled stoichiometry such as:

Ni₃Fe₆(CH₃CO₂)₁₇O₃(OH).12Py

Basic double acetate pyridinate

Burn off organics 200-300^oC, then 1000^oC in air for 2-3 days

Product highly crystalline phase pure NiFe₂O₄ spinel

Good way to make chromite spinels, important tunable magnetic materials

Juggling the electronic-magnetic properties of the A O_h and B T_d ions in the spinel lattice

 
 

Chromite spinel	Precursor	Ignition T, oC
MgCr2O4	(NH4)2Mg(CrO4)2.6H2O	1100-1200
NiCr2O4	(NH4)2Ni(CrO4)2.6H2O	1100
MnCr2O4	MnCr2O7.4C5H5N	1100
CoCr2O4	CoCr2O7.4C5H5N	1200
CuCr2O4	(NH4)2Cu(CrO4)2.2NH3	700-800
ZnCr2O4	(NH4)2Zn(CrO4)2. 2NH3	1400
FeCr2O4	(NH4)2fe(CrO4)2	1150

 

COMBOMATERIALS, ROBOTS CAN DO SOLID STATE SYNTHESIS!

Combinatorial materials chemistry, the wave of the future, beware traditional solid state chemists!

Methods (compositional spread and discrete deposition) combines thin film deposition and physical masking, direct reaction similar to those described above

Used for parallel synthesis of large spatially addressable libraries, rapid screening by parallel measurements, clever analytical techniques, massive amounts of data

Applied to high T_c superconductors, inorganic phosphors, giant magnetoresistant GMR mixed valency perovskites, high dielectric constant rutile type oxides, mixed metal catalysts and electrocatalysts, and even hydrothermal synthesis of zeolites (see later)

CRYSTALLIZING SOLUTIONS, MELTS, GLASSES AND GELS

Prerequisite, single phase, homogeneous (T, C), amorphous

Homogeneous nucleation, few crystal seeds

Slow transport of precursors to seed

Lowest possible crystallization temperature

Metastable phases accessible, often impossible to prepare by other methods

Crystallizing an inorganic glass, lithium disilicate

Li₂O + 2SiO₂ ® 1032^oC, Pt crucible, Li₂Si₂O₅

Li₂Si₂O₅ forms as a melt

Hold at 1100^oC for 2-3 hrs

Homogeneous, rapid cooling, fast viscosity increase, quenches transparent glass

Li₂Si₂O₅, glass 500-700^oC, T_g ~ 450^oC from DSC ® Li₂Si₂O₅, crystals in 2-3 hrs. principle of crystallizing a glass above its glass transition

Classic case hydrothermal crystallization of aluminosilicate gels to prepare zeolites, molecular sieves

Condensation-polymerization, pH, T, P, t control for a particular structure

Typical zeolite synthesis:

NaAl(OH)₄(aq) + Na₂SiO₃(aq) + NaOH(aq), 25^oC, condensation-polymerization,

Na(H₂O)_n⁺ template effect ®  Na_a(AlO₂)_b(SiO₂)_c.NaOH.H₂O(gel) ® 25-175^oC,

hydrothermal crystallization of amorphous gel Na_x(AlO₂)_x(SiO₂)_y.zH₂O(crystals)

Extraframework charge-balancing cations, ion-exchangeable

Framework Al(III)O₄ and Si(IV)O₄ tetrahedral primary building-blocks

Occluded water, removed by 25-500^oC vacuum thermal dehydration

Organic cationic templates, quaternary alkylammonium, structure-directing, space-filling, charge-balancing, discovery of new structures,

Zeolites by chance, go combinatorial zeolite synthesis to improve your chances of discovering a new zeolite

Building-block approach to zeolite synthesis, structures and properties

Primary tetrahedral units, AlO₂^-, SiO₂, PO₂⁺ and so forth, combined to give open-framework and framework charge, balanced by extraframework cations, for instance (AlO₂)(PO₂) = AlPO₄ aluminophosphate molecular sieves

Existence of primary and secondary units in a synthesis mixture, lots of characterization work reported

4R, 6R, 8R, D4R, D6R, 5-1, cubooctahedron

Most well known zeolite archetypes

SOD, LTA, FAU, MOR, MFI

Wide range of solid state characterization methods for zeolites, diffraction, microscopy, spectroscopy, thermal, adsorption and so forth

Zeolite post modification for controlling properties of zeolites

Tailoring channel, cage, window dimensions, adsorbents, gas separation, purification

Tuning Bronsted acidity, hydrocarbon cracking

Ion exchange capacity, Lewis acid-base character, water softening, detergents (believe it or not 25wt% zeolite)

Size-shape selective catalysis, separations, sensing, reactant, product, transition state selectivity

Host-guest inclusion, atoms, ions, molecules, radicals, organometallics, coordination compounds, clusters, polymers (conducting, insulating), nanoreaction chambers

Advanced zeolite devices, electronic, optical, magnetic applications

Entry to nanoscale materials, size tunable properties, QSEs

Changing face of Zeolite Science and Technology

Zeolites and the periodic table, compositions galore thanks to Edith Flanigan

Oxide and non-oxide frameworks, sulfides, selenides and more

Coordination frameworks, supramolecular zeolites

The quest for larger and larger pore sizes, where will it end?

Supramolecular templating, inorganic micelles, lyotropic liquid crystals, block-copolymers, breaking the 1 nm pore size barrier

Size and compositional tunable mesostructures, essentially inorganic replicas of the templating mesophase, one of the most exciting new fields of materials chemistry!!!

Biomimetic approach to inorganics with natural form, connection to nature’s biomineralization process, stealing nature's best ideas and using them to make new materials

VAPOR PHASE TRANSPORT SYNTHESIS

Sealed glass tube reactors

Reactant(s) A, gaseous transporting agent B

Temperature gradient furnace DT ~ 50^oC

Equilibrium established

A(s) + B(g) « AB(g)

Equilibrium constant K

A + B react at T₂

Gaseous transport by AB(g)

Decomposes back to A(s) at T₁

Creates crystals of pure A

Temperature dependent K

Equilibrium concentration of AB(s) changes with T

Different at T₂ and T₁

Concentration gradient of AB(g) provides driving force for gaseous diffusion

Example: Pt(s) + O₂(g) « PtO₂(g)

Endothermic reaction

PtO₂ forms at hot end

Diffuses to cool end

Deposits well formed Pt crystals

Observed in furnaces containing Pt heating elements

Chemical vapor transport, T₂ > T₁, provides concentration gradient and thermodynamic driving force for gaseous diffusion of vapor phase transport agent AB(g)

Uses of VPT

• synthesis of new solid state materials

• growth of single crystals

• purification of solids

Reversible equilibrium needed: DG^o = -RTlnK_equ

Consider case of - DG^o exothermic

Thus DG^o = RTlnK_equ

Smaller T implies larger K_equ

Forms at cooler end, decomposes at hotter end of reactor

Consider case of DG^o endothermic

Thus DG^o = -RTlnK_equ = RTln(1/K_equ)

Larger T implies larger K_equ

Forms at hotter end, decomposes at cooler end of reactor

Applications of CVT Methods

Purification of Metals

Van Arkel Method

Cr(s) + I₂(g) (T₂) « (T₁) CrI₂(g)

Exothermic, CrI₂(g) forms at T₁, pure Cr(s) deposited at T₂

Useful for Ti, Hf, V, Nb, Cu, Ta, Fe, Th

Removes metals from carbide, nitride, oxide impurities
 

Double Transport Involving Opposing Exothermic-Endothermic Reactions

Endothermic:

WO₂(s) + I₂(g) (T₁ 800^oC) « (T₂ 1000^oC) WO₂I₂(g)

Exothermic: W(s) + 2H₂O(g) + 3I₂(g) (T₂ 1000^oC) « (T₁ 800^oC) WO₂I₂(g) + 4HI(g)

The antithetical nature of these two reactions allows W/WO₂ mixtures to be separated at diferent ends of the gradient reactor using H₂O/I₂ as the transporting VP reagents

Vapor Phase Transport for Synthesis
 

A(s) + B(g) (T₁) « (T₂) AB(g)

AB(g) + C(g) (T₂) « (T₁) AC(s) + B(g)
 

Concept: couple VPT with subsequent reaction to give overall reaction:
 

A(s) + C(s) (T₂) « (T₁) AC(s)

Real Examples, Direct Reaction:
 

SnO₂(s) + 2CaO(s) ® Ca₂SnO₄(s)

Sluggish reaction even at high T, useful phosphor

Greatly speeded up with CO as VPT agent:
 

SnO₂(s) + CO(g) « SnO(g) + CO₂(g)

SnO(g) + CO₂(g) + 2CaO(s) « Ca₂SnO₄(s) + CO(g)

Direct Reaction:

Cr₂O3(s) + NiO(s) ® NiCr2O₄(s)

Greatly enhanced rate with O2 VPT agent:

Cr₂O₃(s) + 3/2O₂ « 2CrO₃(g)

2CrO₃(g) + NiO(s) « NiCr₂O₄(s) + 3/2O₂(g)

Overcoming Passivation Through VPT

Al(s) + 3S(s) ® Al₂S₃(s) passivating skin stops reaction

In presence of cleansing VPT agent I₂:

Endothermic: Al₂S₃(s) + 3I₂(g) (T₁ 700^oC) « (T₂ 800^oC) 2AlI₃(g) + 3/2S₂(g)

Zn(s) + S(s) ® ZnS(s) passivation prevents reaction to completion

Endothermic: ZnS(s) + I₂(g) (T₁ 800^oC) « (T₂ 900^oC) ZnI₂(g) + 1/2S₂(g)

VPT Synthesis of ZnWO₄: A Real Phosphor Host Crystal for Ag⁺, Cu⁺, Mn²⁺

WO₃(s) + 2Cl₂(g) (T₁ 980^oC) « (T₂ 1060^oC) WO₂Cl₂(g) + Cl₂O(g)

WO₂Cl₂(g) + Cl₂O(g) + ZnO(s) (T₂ 1060^oC) « ZnWO₄(s) + Cl₂(g)

Growing Epitaxial GaAs Films by VPT Using Convenient Starting Materials

GaAs(s) + HCl(g) « GaCl(g) + 1/2H₂(g) + 1/4As₄(g)

AsCl₃(g) + Ga(s) + 3/2H₂ « GaAs(s) + 3HCl(g)

Serves to establish initial equilibrium
 

ION-EXCHANGE, INJECTION, INTERCALATION TYPE SYNTHESIS

TOPOTACTIC SOLID-STATE METHODS

Ways of modifying existing solid state structures while maintaining the integrity of the overall structure

Precursor structure

Open framework

Ready diffusion of guest atoms, ions, organic molecules, polymers, organometallics, coordination compounds into and out of the structure/crystals

Penetration into interlamellar spaces

2-D intercalation

Into 1-D channel voids

1-D injection

Into cavity spaces

3-D injection

All examples of host-guest inclusion chemistry

Classic materials for this kind of topotactic chemistry

Zeolites: channels, cavities

Graphite: interlayer spaces

Beta alumina: interlayer spaces, conduction planes

Polyacetylene: inter chain channel spaces

Niobium triselenide: interchain channels

Ion exchange achievable by non-aqueous, aqueous, gas phase, melt techniques

Chemical, electrochemical methods

Creates new materials with novel proerties, useful functions and wide ranging technologies

GRAPHITE INTERCALATION (beyond inserting a day in the calendar!)

G (s) + K (melt or vapour) ® C₈K (bronze)

C₈K (vacuum, heat) ® C₂₄K ® C₃₆K ® C₄₈K ® C₆₀K

Graphite sp² sigma-bonding in-plane

p-pi-bonding out of plane

Two layer ABAB stacking sequence

Hexagonal graphite

SALCAOs of the p-pi-type create the valence and conduction bands of graphite, very small band gap, essentially metallic conductivity properties in-plane, 10⁴ times that of out-of plane conductivity

C₈K potassium graphite ordered structure

Ordered K guests between the sheets, K to G charge transfer

AAAA stacking sequence, reduction of graphite sheets, electrons enter CB

K found nesting between parallel eclipsed hexagonal planar carbon six-rings

Typical of many donor-acceptor complexes of graphite

Many graphite intercalation compounds known

Guests include atoms, cations, anions, molecules

Examples include alkali cations, halide anions, oxyanions, metal halides

Typical reactions of graphite

G (HF/F₂/25^oC) ® C₃.₃F to C₄₀F (intercalation via HF₂^- not F^-, through more facile diffusion)

G (HF/F₂/450^oC) ® CF₀.₆₈ to CF (white)

G (H₂SO₄ conc.) ® C₂₄(HSO₄).2H₂SO₄ + H₂

G (FeCl₃ vapour) ® C_nFeCl₃

G (Br₂ vapour) ® C₈Br

Structural planarity of layers often unaffected by intercalation

Bending of layers has been observed, elastic deformations due to intercalation, relevant to phenomenon of staging of graphite

Intercalation often reversible

C₈K DOS diagram shows electron transfer from K into C2p-pi CB, metallic

C₈Br DOS diagram shows depletion of charge from C2p-pi VB, metallic

Modification of thermal and electrical conductivity behavior by tuning the degree of band filling (CB) or un-filling (VB)

Anisotropic properties of graphite intercalation systems usually observed

Layer spacing varies with nature of the guest and the loading

CF: 6.6 Å

C₄F: 5.5 Å

C₈F: 5.4 Å

BUTTON CELLS: LITHIUM-GRAPHITE FLUORIDE BATTERY

Cell electrochemistry:

xLi + CF_x « xLiF + C

xLi ® xLi⁺ + e^-

C^x+xF^- + xLi⁺ + xe^- ® C + xLiF

Nominal cell voltage 2.7 V

CF_x safe storage of fluorine, intercalation of graphite by fluorine

Millions of batteries sold yearly

First commercial Li battery, matsushita Panasonic

Lightweight high energy density battery

Cell requires solid state anode/lithium anode/Li⁺ ion conductor/CF_x-acetylene black/aluminum cathode

SYNTHESIS OF BORON AND NITROGEN GRAPHITES

New ways of modifying the properties of graphite

Instead of tuning the degree of CB/VB filling with electrons and holes

Traditional methods involve interlayer doping

Put B or N into the graphite layers, deficient and rich in carriers, enables intralayer doping with holes and electrons respectively

Also provides a new intercalation chemistry

SYNTHESIS OF BC₃

Traditional heat and beat

xB + yC (2350^oC) ® BC_x

2.35 at % B in C

Poor quality not well-defined materials

New approach, softer chemistry, low T, flow reaction in quartz tube

2BCl₃ + C₆H₆ (800^oC) ® 2BC₃ (lustrous film on walls) + 6HCl

CHEMICAL AND PHYSICAL CHARACTERIZATION OF BC₃

BC₃ + 15/2F₂ ® BF₃ + 3CF₄

Fluorine burn technique

BF₃ : CF₄ = 1 : 3

Shows BC₃ composition

Electron and Powder X-Ray Diffraction Analysis

Shows graphite like interlayer reflections (00l)

2BC₃ (polycrystalline) + 3Cl₂ (300^oC) ® 6C (amorphous) + 2BCl₃

C (crystalline graphite) + Cl₂ (300^oC) ® C (crystalline graphite)

This neat experiment proves B is truly a "chemical" constituent of the graphite sheet and not an amorphous component of a "physical" mixture with graphite

Synthesis, analysis, structural findings all indicate a graphite like structure for BC₃ with an ordered B, C arrangement in the layers

BC₃ interlayer spacing similar to graphite

Also similar to graphite like BN made from thermolysis of borazine B3N3H6

Four probe basal plane resistivity on BC₃ flakes

Sigma(BC₃)_AB ~ 1.1sigma(Graphite)_AB, (greater than 2 x 10⁴ ohm^-1cm^-1)

REPRESENTATIVE BC₃ INTERCALATION CHEMISTRY

BC₃ + S₂O₆F₂ ® (BC₃)₂SO₃F

Note: O₂FS-O--OSO₂F, peroxydisulphuryl fluoride

Weak peroxy-linkage, easily reduced to 2SO₃F^-

(BC₃)₂SO₃F I_c = 8.1 Å

(C₇)SO₃F I_c = 7.73 Å

(BN)₃SO₃F I_c = 8.06 Å

BN I_c = 3.33 Å

C I_c = 3.35 Å

BC₃ I_c = 3-4 Å

More Juicy intercalation chemistry for BC3

BC₃ + Na⁺Naphthalide^- (THF) ® (BC₃)_xNa (bronze, first stage, I_c ~ 4.3 Å)

BC₃ + Br₂(l) ® (BC₃)_15/4Br (deep blue)

Attempt to Incorporate Nitrogen

Pyridine + Cl₂ (800^oC, flow, quartz tube) ® silvery deposit (I_c ~ 3.42 Å)

Silver deposit + F₂ ® CF₄/NF₃/N₂

No signs of HF, ClF₁,_3,5 in F₂ burning reaction

Superior conductivity to graphite

Try to balance the fluorine burning reaction to give the nitrogen graphite stoichiometry of C₅N, a challenge for your senses!!!

INTERCALATION SYNTHESIS OF TRANSITION METAL DICHALCOGENIDES

Group IV, V, VI MS₂ and MSe₂ Compounds

Layered structures

Most studied is TiS₂

hcp S^2-

Ti⁴⁺ in O_h sites

Van der Waals gap

Li⁺ intercalated between the layers

Li⁺ resides in well-defined T_d S₄ interlayer sites

Electrons injected into Ti⁴⁺ t_2g CB states

Li_xTiS₂ results with tunable band filling and unfilling properties

Van der Waals gap prized apart by about 10%

CHEMICAL SYNTHESIS OF Li_xTiS₂

xC₄H₉Li + TiS₂ (hexane, N₂/RT) ® Li_xTiS₂ (filter, hexane wash)+ x/2C₈H₁₈

ELECTROCHEMICAL SYNTHESIS OF Li_xTiS₂

TiS₂ (polymer bonded to metal cathode)/LiClO₄, dioxolane non-aqueous solvent/Li(anode)

TiS₂ (polymer bonded to metal cathode)/(PEO)₈LiSO₃CF₃ polymer electrolyte/Li(anode)

Overall cell reaction:

TiS₂ + Li⁺ + e^- ® Li_xTiS₂

Controlled potential coulometry

Votage controlled intercalation rate

Reversing potential culminates intercalation at selected value of x charge equivalents

Li/TiS₂ attractive energy source

Energy density of Li/TiS₂ > 4 x Energy density of Pb/H₂SO₄ battery of same weight

Technical problems need to be overcome

Cycling causes Li dendritic growth at anode

Mechanical deterioration of multiple intercalation-deintercalation cycles at the cathode

Cause lifetime, corrosion, reactivity, and safety hazards

OTHER INTERCALATION SYNTHESES WITH TITANIUM DISULFIDE

Cu⁺, Ag⁺, H⁺, NH₃, Cp₂Co

Cobaltacene especially interesting, (Cp₂Co)_x⁺Ti_x³⁺Ti_1-x⁴⁺S₂ chemical-electronic description consistent with structure spectroscopy, mixed valence, localized states, hopping conductivity, rather than delocalized partially filled band description

Solid state wide line NMR shows two forms of ring wizzing and molecule tumpling dynamics, Cp₂Co⁺ molecular axis orthogonal and parallel to layers

INTERCALATION ZOO

Channel, layer and framework materials

1-D, 2-D, 3-D structures

Classics:

MS₂, MSe₂, NiPS₃ [Ni₂(P₂S₆), ABAB CdI₂ packing, octahedral alternating layers of NiS₆ and P₂S₆ groupings with Van der Waals gap), FeOCl

V₂O₅.nH₂O (layered structure), MoO₃, TiO₂

MnO₂, CoO₂, WO₃, Mo₆S₈, Mo₆Se₈ (Chevrel phases)

Tungsten bronzes especially notorious

ReO3 structure type

O^2- coordination number 2, centers of edges of a cube

W⁶⁺ coordination number 6, corners of a cube

ELECTROCHEMICAL OR CHEMICAL SYNTHESIS OF M_xWO₃

xNa⁺ + xe^- + WO₃ ® Na_xW_x⁵⁺W_1-x⁶⁺O₃

xH⁺ + xe^- + WO₃ ® H_xW_x⁵⁺W_1-x⁶⁺O₃

Injection of alkali metal cations generates perovskite structure types

M⁺ coordination number 12, resides at center of cube

Many applications of this chemistry and materials

Electrochemical devices, chemical sensors, pH responsive microelectrochemical displays, smart windows, advanced batteries

Behave as low dopant semiconductors

High dopant metals

Electronic and color changes best understood by reference to simple band picture of Na_xW_x⁵⁺W_1-x⁶⁺O₃

SEMICONDUCTOR TO METAL TRANSITION WITH DOPING IN M_xWO₃

WO₃: Band gap excitation from O^2-(2pp) to W⁶⁺ (5d), essentially LMCT in UV region, wide band gap insulator

Na_xW_x⁵⁺W_1-x⁶⁺O₃: Low doping level, narrow band gap semiconductor, narrow localized W⁵⁺ (d¹) VB, visible absorption, essentially IVCT

Na_xW_x⁵⁺W_1-x⁶⁺O₃: High doping level, partially filled metallic valence band, narrow delocalized W⁵⁺ (d¹) VB, visible absorption, IVCT, reflectivity

ION EXCHANGE SYNTHESIS

Requirements: anionic open channel, layer or framework structure

Replacement of some or all of charge balancing cations

Classics zeolites, clays, beta-alumina's

Recall the high T synthesis of beta-alumina:

(1+x)/2Na₂O + 5.5Al₂O₃ ® Na1+xAl₁₁O_17+x/2

Structural reminders:

Na₂O: Antifluorite ccp Na⁺, O^2- in T_d sites

Al₂O₃: Corundum ccp O^2-, Al³⁺ in 2/3 Oh sites

Na_1+xAl₁₁O_17+x/2: Defect spinel, O^2- vacancies in conduction plane, controlled by x ~ 0.2, spinel blocks 9Å, bridging oxygen columns, mobile Na⁺ cations, 2-D fast-ion conductor

Melt ion-exchange of crystals

Equilibria between beta-alumina and MNO₃ and MCl melts, 300-350^oC

Extent of exchange depends on time and melt composition

Monovalents: Li⁺, K⁺, Rb⁺, Ag⁺, Cu⁺, Tl⁺, NH₄⁺, In⁺, Ga⁺, NO⁺, H₃O⁺

Higher valent cations: Ca²+, Eu³⁺, Pb²⁺

Higher T melts required, strong cation binding, slower cation diffusion, 600-800^oC typical

Charge-balance requirements:

2Na⁺ for 1Ca²⁺

3Na⁺ for 1La³⁺

Controlled partial exchange by control of melt composition:

e.g., xNaNO₃ : (1-x)AgNO₃

Electrochemical synthesis for ion-exchange of beta-alumina also possible

Kinetics of Ion-Exchange

Controlled by ionic mobility of the cation

Mass, charge, radius, temperature, solvent, solid state structural properties

Thermodynamics, Extent of Ion-Exchange

Ion-exchange equilibrium for cations

Binding activities between melt and crystal phases

Site preferences

Binding energetics

Charge : radius ratios

Think about the kinetic/thermodynamic factors for ion-exchange in the following

Na₂Si₂O₅ (sheet silicate) + AgNO₃ (melt) ® Ag₂Si₂O₅ +2NaNO₃

CHIMIE DOUCE: SOFT CHEMISTRY

Synthesis of new metastable phases

Not accessible by other methods

Precursor method

Often a close relation structurally between precursor phase and product

Classics:

Tournaux synthesis of new TiO₂

K₂O + 4TiO₂ (rutile, 1000^oC) ® K₂Ti₄O₉

KNO₃ (T^oC) ® K₂O (source)

K₂Ti₄O₉ + HNO₃ (RT) ® H₂Ti₄O₉.H₂O

H₂Ti₄O₉.H₂O (500^oC) ® 4TiO₂ (new slab structure) + 2H₂O

Figlarz synthesis of new WO₃

Na₂WO₄ + HCl (aq) ® gel

Gel (hydrothermal) ® 3WO₃.H₂O

3WO₃.H₂O (air, 420^oC) ® WO₃ (hexagonal tunnel structure)

ELECTROCHEMICAL REDUCTIVE SYNTHESIS, CRYSTAL GROWTH

Molten mixtures of precursors

Product crystallizes from melt

Examples of reduction of TM ions to lower oxidation states in solid state structures

Melt electrochemistry:

CaTi(IV)O₃ (perovskite)/CaCl₂ (850^oC) ® CaTi(III)₂O₄ (spinel)

Na₂Mo(VI)O₄/Mo(VI)O₃ (675^oC) ® Mo(IV)O₂ (large crystals)

Li₂B₄O₇/LiF/Ta(V)₂O₅ (950^oC) ® Ta(II)B₂

Na₂B₄O₇/NaF/V(V)₂O₅/Fe(III)₂O₃ (850^oC) ® Fe(II)V(III)₂O₄ (spinel)

Electrochemical reduction can be extended to a range of materials:

Phosphates ® phosphides

Carbonates ® carbides

Borates ® borides

Sulfates ® sulfides

Silicides ® silicides

Germanates ® germides

Na₂CrO₄/Na₂SiF₆ (T^oC) ® Cr₃Si

Na₂Ge₂O₅/NaF/NiO ® Ni₂Ge

SYNTHESIS OF THIN FILMS

Crystalline

Amorphous

Microcrystalline

Monolayer, multilayer, superlattice, junctions

Free-standing, supported

Epitaxial (commensurate), incommensurate

THIN FILMS ARE VITAL IN MODERN TECHNOLOGY:

Protective coatings

Optical coatings

Filters, mirrors, lenses

Microelectronic devices

Optoelectronic devices

Photonic devices

Electrode surfaces

Photoelectric devices, photovoltaics, solar cells

Xerography

Photography

Lithography

Catalyst surfaces

Information storage, magnetic, magneto-optical, optical memories

FILM PROPERTIES DEPEND ON NUMEROUS CONSIDERATIONS:

Thickness

Surface : volume ratio

Structure, surface versus bulk, surface reconstruction, surface roughness

Hydrophobicity, hydrophilicy

Composition

Texture, single crystal, microcrystalline, domains, orientation

Form, supported or unsupported, nature of substrate

MAIN METHODS OF SYNTHESIZING THIN FILMS

CHEMICAL, ELECTROCHEMICAL, PHYSICAL

Cathodic deposition, Anodic deposition, Electroless deposition

Thermal oxidation, nitridation

Chemical vapour deposition, Metal organic chemical vapour deposition

Molecular beam epitaxy, supersonic cluster beams, aerosol deposition

Liquid phase epitaxy

Self-assembly, surface anchoring

Discharge techniques, RF, microwave

Laser ablation

Cathode sputtering, vacuum evaporation

CATHODIC DEPOSITION

Two electrodes

Dipped into electrolyte solution

External potential applied

Metal deposition onto the cathode as thin film

Anode metal slowly dissolves

ELECTROLESS DEPOSITION

Spontaneous

No applied potential

Depends on electrochemical potential difference between electrode and solution redox active species to be deposited

Both methods limited to metallic films on conducting substrates

ANODIC DEPOSITION

Deposition of oxide films, such as alumina, titania

Deposition of conducting polymer films by oxidative polymerization of monomer, such as thiophene, pyrolle, aniline

Oxide films formed from metallic electrode in aqueous salts or acids

Example: anodic oxidation of aluminum in oxalic or phosphoric acid

Pt|H₃PO₄, H₂O|Al

Al ® Al³⁺ + 3e^- anode

PO₄^3- +2e^- ® PO₃^3- + O^2- cathode

2Al³⁺ + 3O^2- ® gamma-Al₂O₃ (annealing) ® alpha-Al₂O₃

OVERALL ELECTROCHEMISTRY

2Al + 3PO₄^3- ® Al₂O₃ + PO₃^3-

Interesting film forming process, as the applied potential controls the oxide thickness and the rate at which it forms, oxide anions from solution have to diffuse through an Al₂O₃ layer of growing thickness on the reacting Al substrate, to attain an equilibrium thickness of the alumina film

Fascinating self-organizing process observed, whereby a regular array of size tunable hcp pores form and permeate orthogonally through the alumina film

The origin of this effect relates to defect sites, defect strain, and curvature induced electric fields that are produced at the pore entrance which templates or propagates its further growth

Thus one has a self-limiting pore forming process that can be made even more regular by initially creating a hcp pattern of indentation reactive sites on the Al

Exceptionally useful process for creating controlled porosity membranes, photonic gap materials, template for synthesizing semiconductor nanostructures, host for synthesizing and organizing aligned carbon nanotubes for high intensity electron emission displays, and last but not least, fuel cell electrode materials (Chairman Martin Moskovits is a leader in this field)

THERMAL OXIDATION

Anodic layers, metal exposed to a glow discharge

Al + O₂ ® (RT) Al₂O₃, thickness 3-4 nm

Similar method applicable to other metals, Ti, V, W, Zr etc

Not restricted to oxides, nitrides too, exceptionally hard, high temperature protective coating

Ti + NH₃ ® TiN

Al + NH₃ ® AlN

CHEMICAL VAPOUR DEPOSITION

Pyrolysis, photolysis, chemical reaction, discharges, RF, microwave

Epitaxial films, correct matching to substrate lattice
 

EXAMPLES OF CVD

CH₄ + H₂ (RF, MW) ® C, diamond films

Et₄Si (thermal, air) ® SiO₂

SiCl₄ or SiH₄ (thermal, H₂) ® a-HSi

SiH₄ + PH₃ (RF) ® n-Si

Si₂H₆ + B₂H₆ (RF) ® p-Si

SiH₃SiH₂SiH₂PH₂ (RF) ® n-Si

Me₃Ga (laser photolysis, heating) ® Ga

Me₃Ga + AsH₃ + H₂ ® GaAs + CH₄

Si (laser evaporation, supersonic jet) Si_n⁺ (size selected cluster deposition) ® Si

METAL ORGANIC CHEMICAL VAPOR DEPOSITION, MOCVD

Invented by Mansevit in 1968

Recognized high volatility of metal organic compounds as sources for semiconductor thin film preparations

Enabling chemistry for electronic and optical quantum devices

Quantum wells and superlattices

Occurs for 5-500 angstrom layers

Known as artificial superlattices

Quantum confined electrons and holes when thickness of quantum well L is comparable to the wavelength of an electron or hole at the Fermi level of the material, band diagram shows confined particle states and quantization effects for electrical and optical properties

Discrete electronic energy states rather than continuous bands, given by solution to the simple particle in a box equation, assuming infinite barriers for the wells, m* is the effective mass of electrons and holes

E_n = n²p²h²/2m*L²

Tunable thickness, tailorable composition materials, do it yourself quantum mechanics materials for the semiconductor industry

Quantum well structure synthesized by depositing a controlled thickness superlattice of a narrow band gap GaAs layer sandwiched by two wide band gap Ga_xAl_1-xAs layers using MOCVD or MBE (see later)

Known as artificial superlattices, designer periodicity of layers, quantum confined lattices, thin layers, epitaxially grown

Example: Ga_xAl_1-xAs|GaAs|Ga_xAl_1-xAs

n- and p-doping also achievable by having excess As or Ga respectively in a GaAs layer

Composition and carrier concentration controls refractive index and electrical conductivity, therefore total internal reflection can be achieved in a semiconducting superlattice, enabling quantum and photon confinement for electronic and optoelectronic and optical devices, multiple quantum well lasers, quantum cascade lasers, distributed feedback lasers, high mobility ballistic transistors, laser diodes and so forth

BAND GAP ENGINEERING OF SEMICONDUCTORS

The MOCVD, LPE, CVD, CVT, MBE are all deposition techniques that provide angstrom precise control of film thickness

Together with composition control one has a beautiful synthetic method for fine tuning the electronic band gap and hence most of the important properties of a semiconductor quantized film

The key thing is to achieve epitaxial lattice matching of the film with the underlying substrate

This avoids things like lattice strain at the interface, elastic deformation, misfit dislocations, defects

All of this problems serve to increase carrier scattering and quenching of e-h recombination luminescence (killer traps), thereby reducing the efficacy of the material for advanced device applications

MOCVD PRECURSORS, SINGLE SOURCE MATERIALS

Me₃Ga, Me₃Al, Et₃In

NH₃, PH₃, AsH₃

H₂S, H₂Se

Me₂Te, Me₂Hg, Me₂Zn, Me₄Pb, Et₂Cd

Example for IR detectors: Me₂Cd + Me₂Hg + Me₂Te (H₂, 500^oC) ® Cd_xHg_1-xTe

All pretty toxic materials

Specially designed MOCVD reactors, controlled flow of precursors to single crystal heated substrate

Creates a problem for semiconductor manufacturers in terms of safe disposal of toxic waste

Most reactions occur in range 400-1300^oC

Complications of diffusion at interfaces, disruption of atomically flat epitaxial surfaces/interfaces occurs during deposition

Photolytic processes (photoepitaxy) help to bring the deposition temperatures to more reasonable temperatures

REQUIREMENTS OF MOCVD PRECURSORS

RT stable

No polymerization, decomposition

Easy handling

Simple storage

Not too reactive

Vaporization without decomposition

Modest < 100^oC temperatures

Low rate of homogeneous pyrolysis, gas phase, wrt heterogeneous decomposition

HOMO : HETERO rates ~ 1 : 1000

Heterogeneous reaction on substrate

Greater than on other hot surfaces in reactor

Not on supports, vessel etc

Ready chemisorption of precursor on substrate

Detailed surface and gas phase studies of structure of adsorbed species, reactive intermediates, kineticss, vital for quantifying film nucleation and growth processes

Electronic and optical films synthesized in this way

Semiconductors, metals, silicides, nitrides, oxides, mixed oxides (e.g., high Tc superconductors)

CRITICAL PARAMETERS IN MATERIALS PREPARATION FOR SYNTHESIS OF THIN FILMS

Composition control

Variety of materials to be deposited

Good film uniformity

Large areas to be covered, > 100 cm²

Precise reproducibility

Growth rate, thickness control

2-2000 nm layer thicknesses

Precise control of film thickness

Accurate control of deposition, film growth rate

Crystal quality, epitaxy

High degree of film perfection

Defects degrade device performance

Reduces useable wafer yields

Purity of precursors

Usually less than 10^-9 impurity levels

Stringent demands on starting material purity

Challenge for chemistry, purifying and analyzing at the ppb level

Demands exceptionally clean growth system otherwise defeats the object of controlled doping of films for device applications

Interface widths

Abrupt changes of composition, dopant concentration required

Vital for quantum confined structures

30-40 sequential layers often needed

Alternating composition and graded composition films

0.5-50 nm thicknesses required with atomic level precision

All of the above has been more-or-less perfected in the electronics and optics industries

SEVERAL TECHNIQUES USED TO GROW SEMICONDUCTOR FILMS AND MULTILAYERED FILMS

MOCVD

Liquid phase epitaxy

Chemical vapor transport

Molecular beam epitaxy

Laser ablation

All methods have been used for band gap engineering

Example: obtaining semiconductor materials with operating wavelengths for important optical in the near IR

PHYSICAL METHODS FOR PREPARING THIN FILMS

CATHODE SPUTTERING

Bell jar equipment

10^-1 to 10^-2 torr of Ar, Kr, Xe

Glow discharge created

Positively charged rare gas ions

Accelerated in a high voltage to cathode target

High energy ions collide with cathode

Sputter material from cathode

Deposits on substrate opposite cathode to form thin film

Multi-target sputtering also possible

Creates composite or multilayer films

THERMAL VACUUM EVAPORATION

High vacuum bell jar > 10^-6 torr

Heating e-beam, resistive, laser

Gaseous material deposited on substrate

Thin films nucleate and grow

Containers must be chemically inert, W, Ta, Nb, Pt, BN, Al₂O₃, ZrO₂, Graphite

Substrates include insulators, metals, glass, alkali halides, silicon

Sources include metals, alloys, semiconductors, insulators, inorganic salts

MOLECULAR BEAM EPITAXY

Million dollar thin film machine, ideal for preparing high quality artificial semiconductor quantum superlattices, ferroelectrics, superconductors

Ultrahigh vacuum system >10^-12 torr

What's in the chamber?

Elemental or compound sources in shutter controlled Knudsen effusion cells

Ar⁺ ion gun for cleaning substrate surface or depth profiling sample using auger analyzer

High energy electron diffraction for surface structure analysis

Mass spectrometer for control and detection of vapor species

e-gun for heating the substrate etc etc etc

PHOTOEPITAXY

Making atomically perfect thin films under milder and more controlled conditions

Mullin and Tunnicliffe 1984

Et₂Te + Hg (pool) + H₂ (hn , 200^oC) ® HgTe + 2C₂H₆

MOCVD preparation requires 500^oC using Me₂Te + Me2Hg

Advantages of photoepitaxy

Lower temperature operation

Multilayer formation

Less damage of layers

Lower interlayer diffusion

Easy to fabricate

Abrupt boundaries

Less defects, strain, irregularities at interfaces

Note that H₂ gas window in apparatus prevents deposition of HgTe on observation port

CdTe can be deposited onto GaAs at 200-250^oC even with a 14% lattice mismatch

GaAs is susceptible to damage under MOCVD conditions 650-750^oC

EXTENSIONS OF PHOTOLYTIC METHODS, LASER WRITING AND LASER ETCHING

Laser writing:

Substrate GaAs

Me₃Al or Me₂Zn adsorbed layer or gas phase

Focussed UV laser on film

Photodissociation of organometallic precursor:

Me₃Al ® Al + C₂H₆

Creates sub-micron lines of Al or Zn

Laser photoetching:

GaAs substrate

Gaseous or adsorbed layer of CH₃Br

Focussed UV laser

Creates reactive Br atoms

CH₃Br(g) (hn ) ® CH₃(g) + Br(g)

Br(g) + GaAs(s) ® GaAs…Br_n(ad)

GaAs…Br_n(ad) ® GaBr_n(g) + AsBr_n(g)

Adsorbed reactive surface Br atoms erode surface regions irradiated with laser

Vaporization of volatile gallium and arsenic bromides from surface

Creates sub-micron etched line

 GROWTH OF SINGLE CRYSTALS, MICRONS TO METERS

Vapor, liquid, solid phase crystallization techniques

Single crystals vital for meaningful property measurements of materials

Single crystals allow measurement of anisotropic phenomena (electrical, optical, magnetic, mechanical, thermal, and so forth) in crystals with symmetry lower than cubic (isotropic)

Single crystals important for fabrication of certain devices, like yttrium aluminum garnet (YAG) and beta-beryllium borate (BBO) for doubling and tripling the frequency of CW or pulsed laser light, quartz crystal oscillators for mass monitors, lithium niobate for photorefractive applications and so forth

LET'S GROW CRYSTALS

Key point to remember when leaning how to be a crystal grower (incidentally, an exceptionally rare profession, extraordinarily well paid I might add):

Many different crystal growing techniques exist, hence one must think very carefully as to which method is the most appropriate for the material under consideration, size of crystal desired, stability in air, morphology or crystal habit required and so forth, so let's proceed to look at some case histories.
 

CZOCHRALSKI METHOD

Interesting crystal pulling technique (but can you pronounce the name!)

Single crystal growth from the melt precursor(s)

Crystal seed of material to be grown placed in contact with surface of melt

Temperature of melt held just above melting point, highest viscosity, lowest vapor pressure

Seed gradually pulled out of the melt (not with your hands of course, special crystal pulling equipment is used)

Melt solidifies on surface of seed

Melt and seed usually rotated counterclockwise with respect to each other to maintain constant temperature and to facilitate uniformity of the melt during crystal growth, produces higher quality crystals, less defects

Inert atmosphere, often under pressure around growing crystal and melt to prevent any materials loss
 

GROWING BIMETALLIC CRYSTALS LIKE GaAs REQUIRES A MODIFICATION OF THE CZOCHRALSKI METHOD

Layer of molten inert oxide like B₂O₃ spread on to of the molten feed material to prevent preferential volatilization of the more volatile component of the bimetal, this is critical for maintaining precise stoichiometry, for example Ga_1+xAs and GaAs_1+x which are respectively rich in Ga and As, become p-doped and n-doped (try to explain this)!!!

The Czochralski crystal pulling technique has proven invaluable for growing many large single crystals in the form of a rod, which can subsequently be cut and polished for various applications, some important examples are listed below, what do you think these have been used for:

Si

Ge

GaAs

LiNbO₃

SrTiO₃

NdCa(NbO₃)₂
 

BRIDGMAN AND STOCKBARGER METHODS

Stockbarger method is based on a crystal growing from the melt, involves the relative displacement of melt and a temperature gradient furnace, fixed gradient and a moving melt/crystal

Bridgman method is again based on crystal growth from a melt, but now a temperature gradient furnace is gradually cooled and crystallization begins at the cooler end, fixed crystal and changing temperature gradient

Both methods are founded on the controlled solidification of a stoichiometric melt of the material to be crystallized

Enables oriented solidification

Melt passes through a temperature gradient

Crystallization occurs at the cooler end

Both methods benefit from seed crystals and controlled atmospheres
 

ZONE MELTING CRYSTAL GROWTH AND PURIFICATION OF SOLIDS

Method related to the Stockbarger technique

Thermal profile furnace employed

Material contained in a boat (must be inert to the melt)

Only a small region of the charge is melted at any one time

Initially part of the melt is in contact with the seed

Boat containing sample pulled at a controlled velocity through the thermal profile furnace

Zone of material melted, hence the name of the method

Oriented solidification of crystal occurs on the seed

Simultaneously more of the charge melts

Partitioning of impurities occurs between melt and the crystal

This is the basis of the zone refining methods for purifying solids

Impurities concentrate in liquid more than the solid phase, swept out of crystal by moving the liquid zone

Used for purifying materials like W, Si, Ge to ppb level of impurities, often required for device applications
 

VERNEUIL FUSION FLAME METHOD

1904 first recorded use of the method

Useful for growing crystals of extremely high melting metal oxides

Examples include:

Ruby from Cr³⁺/Al₂O₃ powder

Sapphire from Cr₂⁶⁺/Al₂O₃ powder

Luminescent host CaO powder

Starting material fine powder

Passed through O₂/H₂ flame or plasma torch (ouch they are hot!)

Melting of the powder occurs in the flame

Molten droplets fall onto the surface of a seed or growing crystal

Leads to controlled crystal growth

NOTE THAT ALL OF THE ABOVE CRYSTAL GROWING METHODS, COCHRALSKI, BRIDGMAN, STOCKBARGER, ZONE MELTING, VERNEUIL

All have the advantage of rapid growth rates of large crystals required for many advanced device applications

The crystal quality obtained by all of these techniques must be checked for inhomogeneities (composition, structure), impurities, defects, grain boundaries and so forth (think about how you might go about this if you were confronted with a 12"x12"x12" crystal)

HYDROTHERMAL SYNTHESIS OF CRYSTALS

Basic methodology

Water medium

High temperature growth, above normal boiling point

Water acts as a pressure transmitting agent

Water functions as solublizing phase

Often a mineralizing agent is added to assist with the transport of reactants and crystal growth

Speeds up chemical reactions between solids

Useful technique for the synthesis and crystal growth of phases that are unstable in a high temperature preparation in the absence of water

Crystal growth hydrothermally involves:

Temperature gradient reactor

Dissolution of reactants at one end

Transport with help of mineralizer to seed at the other end

Crystallization at the other end.

Note that because some materials have negative solubility coefficients, crystals can actually grow at the hotter end in a temperature gradient hyrdothermal reactor, counterintuitive but true, good example is alpha-AlPO₄ known as Berlinite, important for its high piezoelectric coefficient (yes larger than alpha-quartz with which it is isoelectronic) and use as a high frequency oscillator
 

Hydrothermal growth of quartz crystals

Water medium

Nutrients 400^oC

Seed 360^oC

Pressure 1.7 Kbar

Mineralizer 1M NaOH

Uses of single crystal quartz:

Radar, sonar, piezoelectric transducers, monochromators, XRD

Annual global production hundreds of tons of quartz crystals, amazing

Hydrothermal crystal growth is also suitable for growing single crystals of:

Ruby: Cr³⁺/Al₂O₃

Corundum: alpha-Al₂O₃

Sapphire: Cr₂⁶⁺/Al₂O₃

Emerald: Cr³⁺/Be₃Al₂Si₆O₁₈

Berlinite: alpha-AlPO₄

Metals: Au, Ag, Pt, Co, Ni, Tl, As

Role of the mineralizer

Consider the growth of quartz crystals

Control of crystal growth rate, through choice of mineralizer, temperature and pressure

Solubility of quartz in water is important

SiO₂ + 2H₂O « Si(OH)₄

Solubility about 0.3 wt% even at supercritical temperatures >374^oC

A mineralizer is a complexing agent (not too stable) for the reactants/precursors that need to be solublized (not too much) and transported to the growing crystal

Some mineralizing reactions:

NaOH mineralizer, dissolving reaction, 1.3-2.0 KBar

3SiO₂ + 6OH^- « Si₃O₉^6- + 3H₂O

Na₂CO₃ mineralizer, dissolving reaction, 0.7-1.3 KBar

SiO₂ + 2OH_- « SiO₃^2- + H₂O

CO₃^2- + H₂O « HCO₃^- + OH^-

NaOH creates growth rates about 2x greater than with Na₂CO₃ because of different concentrations of hydroxide mineralizer

Examples of hydrothermal crystal growth and mineralizers

Berlinite alpha-AlPO4

Powdered AlPO₄ cool end of reactor, negative solubility coefficient!!!

H₃PO₄/H₂O mineralizer

AlPO₄ seed crystal at hot end

Emeralds Cr³⁺Be₃Al₂Si₆O₁₈

SiO₂ powder at hot end 600^oC

NH₄Cl or HCl/H₂O mineralizer, 0.7-1.4 Kbar, cool central region for seed, 500^oC

Al₂O₃/BeO/Cr³⁺ dopant powder mixture at other hot end 600^oC

6SiO₂ + Al₂O₃ + 3BeO ® Be₃Al₂Si₆O₁₈

Beryl contains Si₆O₁₈^12- six rings

Metal crystals (amazing, what would you use these for?)

Metal powder at cool end 480^oC

Mineralizer 10M HI/I₂

Metal seed at cool end 500^oC

Dissolving reaction that also transports Au to the seed crystal:

Au + 3/2I₂ + I^- « AuI₄^-

Metal crystals grown this way include Au, Ag, Pt, Co, Ni, Tl, As at 480-500^oC

Hydrothermal synthesis necessitates knowledge of what is going on in an autoclave under different degrees of filling and temperature

Pressure, volume, temperature tables of dense fluids like water are well documented

Critical point of water 374^oC, 222 Bar, density of gas and liquid water the same 0.32 gcm^-3, at this point liquid cannot exist, so fluid in autoclave by definition is a gas!

Density of liquid water decreases with T

Density of water vapor increases with T

Liquid level in autoclave rises for > 32% volume filling

Autoclave filled at 250^oC for > 32% volume filling

For 32% volume filling liquid level remains unchanged and becomes fluid at critical temperature

Tables of pressure versus temperature for different initial volume filling of autoclave must be consulted to establish a particular set of reaction conditions for a hydrothermal synthesis or crystallization

Safety: if this is not done correctly, with proper protection equipment in place, you can have an autoclave explosion that can kill!!!

DRY HIGH PRESURE METHODS OF SOLID STATE SYNTHESIS

Pressures up to Gigabars accessible, at high temperatures, and with insitu observations by diffraction, spectroscopy to probe chemical reactions, structural transformations, crystallization, amorphization, phase transitions and so forth

Methods of obtaining high pressures

Anvils, diamond tetrahedral and octahedral pressure transmission

Shock waves

Explosions

Go to another planet, recall hydrogen is metallic at 100 Gbar (explain why this is so?)

Pressure techniques useful for synthesis of unusual structures, metastable yet stable when pressure released (why?)

Often high pressure phases have a higher density, higher coodination number

In fact ruby is used for calibrating a high pressure diamond anvil, so explain how this method works?
 

Examples of high pressure polymorphism for some simple solids:
 

What about diamonds, are they REALLY a "persons" best friend?

So why is it so difficult to transform commonal garden graphite into diamond?

Industrial diamonds made from graphite around 3000oC and 130 kbar

The problem is that the activation energy required for a sp² 3-coordinate to a sp³ 4-coordinate structural transformation is very high, so requires extreme conditions

Ways of getting round the difficulty

Squeezing and heating buckyball whose carbons are already intermediate between sp^2-3. In the case of C₆₀, diamond anvil, 20 GPa instantaneous transformation to bulk crystalline diamond, highly efficient process, fast kinetics

Using CH₄/H₂ microwave discharges to create reactive atomic carbon whose valencies are more-or-less free to form sp³ diamond, in this case with atomic hydrogen this is the route for making diamond films

Organic molecule theory of diamond cleavage

Why does the jeweler's chisel if placed correctly on a diamond, with a well oriented blow, always cause cleavage along {111} greater than 90% of the time, imagine the cost of a mistake with a large crystal

First note that the number of bonds broken per unit area (that is, surface energies) for different planes does not explain the observations of preferential {111} cleavage!!!

Diamond viewed in terms of layers of polycondensed cyclohexane rings with axial bonds between layers and equatorial bonds within layers

Theory is founded on unfavourable axial-axial C-C bond interactions at 2.51 Å versus equatorial-equatorial at 2.96 Å

Evidence for this is seen in model compounds like cis-decalin versus trans-decaline, comprised of two fused cyclohexane rings where trans-decalin is 11-12 KJmol^-1 more stable because cis-strain cannot be relieved by bond rotation as in cyclohexane itself, cis can onlycan only isomerize to trans by bond cleavage followed by recombination, hence origin of the high activation energy for the cis-to-trans isomerization of decalin

With all of this in mind the macromolecular organic view of diamond cleavage is in terms of a breaking molecule theory, where axial-axial unfavorable interactiopns cause the mechanical energy of the jeweler's chisel to be funneled into preferential breakage of an axial C-C bond

This then induces a kind of domino effect whereby the adjacent axial C-C bonds break and C-C bonds throughout the entire {111} plane are severed

Thus it is the poly-cyclohexane-like structure of diamond that can provide a natural explanation of the {111} preferred cleavage properties of diamond, chemistry rather than physics view of the phenomenon!!!

WITH ALL OF THIS NEW FOUND KNOWLEDGE WE CAN NOW LOOK AT SOME MORE ADVANCED CASE HISTORIES OF SOLID STATE SYNTHESIS AND HOW THEY HAVE IMPACTED THE MATERIALS WORLD

Note that I will cover these more specialized topics in the lectures but I will not be putting the notes up on the web, please ensure that you make excellent notes on these topics as some of them will likely appear in assignments, the mid-term test and the final, you will have to read around the subject to come to grips with this part of the course, more challenging stuff that does stuff.
 

Porous silicon, getting light out of non-emissive material, how do they do that?

High Tc superconducting films, on the way to SC devices even with a refractory material?

Buckyballs and buckytubes, nanoscale device opportunities for the next millenium

Diamond films, simple, cheap and a myriad of opportunities

Graphite fluoride, the unwettable

Hydrogen fuel cells based on Pt and Nafion membranes, do not miss the bus!

A compositionally tunable and dopable superconducting organometallic intercalate, what a mouthful!

  LUMINESCENT SILICON

Silicon wafer, photolytic or electrical excitation of electron and hole pair, extremely weakly emissive at band gap energy of 1.1 eV with some transverse and longitudinal phonon side-bands

Classic case of semiconductor with an indirect electronic band gap, top of VB does not lie at the same k value as bottom of CB

Light emission forbidden (weakly allowed actually) because of momentum selection rule, D k ¹ 0, requires participation of a phonon (lattice vibration), that is k_VB + k_ph = k_CB

Phonon coupling in solids is analogous to vibronic coupling in molecular systems when discussing selection rules for electronic transitions

The indirect character of bulk silicon is a major impediment to its incorporation into silicon technology for optoelectronic and photonic applications

Canham discovered about a decade ago that anodic oxidation of a p-doped Si wafer in aqueous HF created a material that was strongly photoluminescent (PL) and electroluminescent (PL) in the visible wavelength range.

The electrochemical oxidative etching turns out to be a fascinating self-limiting pore formation process

The luminescence maximum was found to scale with the porosity of the Si wafer, that is, higher porosity shifts the luminescence energy into the blue.

Microscopy studies show that the anodic oxidation process leads to a network of pores with walls comprised of interlaced quantum wires and dots

Similarly, laser ablation of silicon clusters from bulk silicon, pulsed laser pyrolysis of disilane precursor, ultrasonic dispersion of silicon in an organic solvent, are all methods that produce silicon quantum dots, that also display intense luminescence that scales with the size of the clusters.

The question then is the origin of the luminescence and this is still under debate one decade after Canham's breakthrough (recall a breakthrough is a discovery pregnant with promise and then the hard graft begins)

The central idea is that when bulk Si drops in size below the Fermi wavelength of an electron in silicon, which happens to be about 5 nm (that is really small, considering GaAs is about 30 nm, why the difference?) the continuous energy level structure of the bands in bulk silicon become discrete in the silicon clusters (quantum confinement, e-h in a box model) and quantum size effects are to be expected.

So has the band structure of bulk silicon changed from indirect to direct as a result of spatial and quantum confinement of charge carriers?

The silicon maintains the diamond lattice in porous silicon and nanocrystalline silicon and the emission is intense and size tunable (1/R²), so what is the problem with an argument based upon quantum confinement?

Lifetimes of the luminescence are the problem, they are too long for an allowed transition, millisecond instead of nanosecond

While the lifetimes do decrease as the cluster size decreases, the trend shows they would have to be < 1 nm to fall in the nanosecond time domain, so what do we think of next?

The accepted explanation these days is that the probability of radiative recombination of e-h pairs increases in the cluster compared to the bulk because of spatial confinement (note: e-h can easily dissociate in the delocalized bands of the bulk), so the luminescence intensity increases, but the diamond lattice and indirect selection rule remain and keep the lifetime long

Meanwhile one has to be sure that the luminescence is not from some adventious source such as surface impurities like a siloxane polymer or surface states associated with hydride, fluoride, oxide (we will have more to say about this point)

A relevant side issue that has important implications on the origin of luminescence in QSE silicon is amorphous hydrogenated silicon (aH-Si) and making it into a useful semiconductor that is now finding its way into photovoltaics for solar heating, watches, calculators and so forth, how does this all work?

A RF discharge of disilane is a good way to CVD films of a-Si, however the dangling surface bonds and unpaired electrons create mid-gap states between VB and CB states, act as traps for electrons, hence a-Si is a poor semiconductor, low charge transport as localized rather than delocalized states, hopping and tunneling mobility, high Ea process.

By performing the RF discharge in H₂ the dangling bonds become capped with hydride in aH-Si, this forms sigma(Si-H) and sigma*(Si-H) bonding and antibonding orbitals which shift out of the mid gap below and above the silicon VB and CB states

This removes trapping and scattering of charge carriers making amorphous hydrogenated silicon a goo semiconductor but not as good as crystalline silicon, good enough though to build a whole range of devices from CVD amorphous films which is much more economical than working with expensive crystalline silicon

This information suggests ways of evoking luminescence from silicon, make clusters and wires with at least one dimension small compared to the Fermi wavelength and remove dangling bond states by capping with hydride or oxide or fluoride

Hence competing non-radiative relaxation pathways for photo- or electro-generated e-h pairs via mid gap states from dangling bonds can be eliminated or reduced by capping and at the same time promoting radiative relaxation and luminescence

Based on these ideas numerous creative chemical syntheses have been devized to evoke luminescence out of silicon for applications in optoelectronics, sensing and so forth

One way involves CVD topotaxy of disilane inside the supercages of acid form of zeolite Y. Recall zeolite Y is built of supercages (alpha) and cubooctahedral (beta) cages in a fcc unit cell, there are 8 alpha and 8 beta cages in the unit cell of Na₅₆Al₅₆Si₁₃₆O₃₈₄, the 56 charge balancing sodium cations translate into 4 in the alpha and three in the beta cages, which can be chemically transformed to Brf nsted acid sites SiOH for anchoring of the disilane precursor to the silicon clusters. So this is the basis of the following CVD synthesis:

Na₅₆Y (NH₄OH, H₂O) ® (NH₄)₅₆Y (RT ® 500^oC, vacuum thermal deamination, dehydration) ® H₅₆Y (Si₂H₆, 25-100_oC, anchoring) ® (Si₂H₅)₃₂aH₂₄bY (400-800^oC, reductive-elimination, cluster self-assembly, supergage) ® (Si₈)₈aY (diamond lattice of luminescent Si₈ clusters in supergages of zeolite Y, capped with framework oxygen, crown ether zeolate analogy, removes dangling bond states, promotes luminescence, short lifetimes, change in structure from diamond lattice, alteration of selection rules, become allowed)

The spectroscopy of this system shows nuclearity and composition dependent shifts in the absorption edge and luminescence maximum consistent with quantum confinement.

The Si₈(OZ)₈ zeolate capping idea is consistent with the results obtained for octasilacubane cluster Si₈R8 materials that have been synthesized

Using this approach the optical properties of the luminescent silicon clusters have been tuned: n/2Si₂H₆ + (4-n/2)Ge₂H₆ + H₅₆Y ® Si_nGe_8-nY, the results show red shifting of the absorption edge and luminescence maximum with increase atomic fractions of Ge, poorer overlap smaller band gap, smaller band width ideas.

MOCVD ROUTE TO HIGH Tc SUPERCONDUCTING FILMS

The philosophy behind this work is the discovery of:

1. volatile organometallic precursors
2. sometimes single source containing more than one of the required elements
3. that are pure enough
4. and cleanly produce the required elements on a desired substrate
5. at as low a temperature as possible
6. often epitaxially to minimize interfacial defects, Perovskite substrates make a lot of sense like BaPbO₃ also by CVD
7. and multilayer with abrupt boundaries, superlattices, Josephson junctions

Consider the classic case of YBa₂Cu₃O_7-d

Recall that d controls whether this material is an insulator, semiconductor or metal/superconductor

In essence one tunes the number of oxygen vacancies in a defect Perovskite through the thermal vacuum treatment in the synthesis, this in turn controls the crystal structure of the material, from copper-oxide chains to sheets, the Cu²⁺/Cu³⁺ ratio, the number of unpaired electrons, and the density of states at the Fermi level. When d = 0 the system is optimized for superconductivity and the Cooper pairs move mainly in the copper-oxide layer planes, when d = 1/2 the system is a semiconductor and d = 1/2 it is an insulator

Question, how do we get volatile precursors that cleanly generate this high Tc ceramic superconductor as an epitaxial film and on which substrate???

MOCVD provides an answer to both of these questions

The concept is to encapsulate as well as possible the central metal atom of interest in a bulky organic ligand that promotes small intermolecular interactions in the solid state, low polarity, encourages poor packing in the unit cell, reduces lattice cohesive energies

Also, ligands that minimize oligomerization of the precursor and facilitate vapor phase transport, especially important for those complexes based on large central metals with a small charge to radius ratio, like barium, lead and so forth

A favorite ligand is the bulky 2,2',6,6'-tetramethyl-3,5-heptanedionate, basically a bulky acac ligand (TMHD)

Y(TMHD)₃ T_sub = 160^oC, Ba(TMHD)₂ T_sub = 70-190^oC, and Cu(TMHD)₂ T_sub = 125_oC are very useful MOCVD precursors used in the presence of H₂ for assisting loss of the ligand as TMHDH and NH₃/N₂ carrier gas for complexing ammonia to the barium center to saturate metal coordination sphere and minimize oligomerization and facilitate transport

As well as sterically encumbered non-polar ligands the low polarizability of the small fluorine ligand is also useful for enhancing volatility, like hfacac, so materials like Ba(hfacac)₂ are very appealing but they tend to suffer from fluoride contamination of the film in the form of say BaF₂

The idea that oligomerization can be reduced by Lewis bases like ammonia and ethers suggested the brilliant idea of building the Lewis base into the ligand. This involves ligand design for materials containing barium, that also do away with the fluorine contamination problem, such as, beta-ketoiminates (acac ligands but with one iminine, DIKI^-, TRIKI^-) with an appended oligoether which performs the Lewis base complexation internally:

BaH₂ + 2HL (Ar) ® BaL₂ + H₂

Fully encapsulated BaL₂ MOCVD precursors

BaL₂ sublimes 80-120^oC at 10^-5 Torr

Minor decomposition

Major products Ba²⁺ and HL

Crystal structures determined

Distorted trigonal dodecahedral Ba²⁺

Show coordinated polyether appendages

R(BaO) = 2.54-2.91 Å; R(BaN) = 2.824-2.832 Å

What about making a perovskite film that is useful for epitaxial matching with a defect perovskite superconductor

Ba(DIKI)₂ + Pb(PIVOLATE)₂ + H₂O/O₂ ({001} face of MgO single crystal substrate, T = 450^oC) ® BaPbO₃ (PXRD shows oriented film, trace baO contaminant)

Most classes of high Tc superconductors can be made as thin films using MOCVD precursors of the type described:

All of the following have been synthesized by MOCVD: YBa₂Cu3O₇-_d, Bi₂Sr₂Ca_mCu_nO_x, Tl_mBa₂Ca_n-1Cu_nO_m+2m+2, Nd_2-xCe_xCuO4_-y

Useful precursors for this work include: Sr(TMHD)₂, Ca(TMDH)₂, BiPh₃, Ca(hfacac)₂.TEG, Tl(TMHD), Nd(TMDH)₃

SINGLE SOURCE MOCVD PRECURSORS TO METAL, METAL OXIDE, NITRIDE AND SULFIDE FILMS

Best precursors for copper films used in microelectronics are Cu(hfacac)₂ (VP 0.25 Torr at 60^oC) at 250-350^oC, and Cu(hfacac)PR₃ (VP 0.1 Torr at 60^oC) at 120-350^oC

Rare earth doped semiconductor films make use of the sterically crowded encapsulated (C₅H₄Me)₃Nd and (C₅H₄CMe₃)₃Nd can sublime at 110^oC and 10^-3 Torr allowing them to be doped into III-V semiconductors, the idea is to excite the sharp 4f-4f intra-shell luminescence of the rare earth center optically and electrically via the host semiconductor crystals, which is of interest in fiber optical communication:

GaMe₃ + AsH₃ + (C₅H₄Me)₃Nd ® Nd:GaAs

Nitride films are important as they display unique properties including metallic behavior, extreme hardness, very high melting points, high chemical resistance

This has generated considerable interest in MOCVD precursors to nitride films

Homoleptic dialkyamides and ammonia react at temperatures as low as 200oC to afford excellent quality TiN films:

Ti(NMe₂)₄ + NH₃ (200-450^oC) ® TiN + organics

Similar approach used for VN, NbN, Ta₃N₅, Si₃N₄, Sn₃N₄, GaN, AlN

Dialkyamides have good volatility and low deposition teperatures

The precursor TiCl₄(NH₃)₂ is carbon free and can be sublimed at ~ 100^oC and 0.1 Torr and provides gold colored titanium nitride films with undetectable chlorine contamination by XPS

Sulfide films possess a wide range of fascinating solid state properties and have been the focus of much MOCVD research. Most prominent application is in the area of cathodes for thin film lithium batteries.

Promising materials are TiS₂ and MoS₂

TiCl₄(HSC₆H₁₁)₂ (VP 1-2 Torr, 25^oC, single source precursor, ~200^oC) ® TiS₂ + 2HCl + 2C₆H₁₁Cl

Resulting films show no detectable levels of carbon or chlorine by XPS, also crystallographic orientation of the films ideal for cathode in lithium batteries

The tetraakanethiolate Ti(S^tBu)₄ is also a good source of TiS₂ film by MOCVD

NEW FORMS OF CARBON, FROM DIAMOND FILMS TO BUCKYBALL TO CARBON NANOTUBES

Known forms (allotropes) of carbon must be expanded to now include diamond c-C, Lonsdaleite h-C in meteorites, graphite, single-walled and multi-walled fullerenes and single-walled and multi-walled carbon nanotubes

Bonding in diamond is sp³, in graphite is sp² and in buckyballs and buckytubes intermediate sp^2+x

Single crystal synthetic diamonds (high temperature, pressure synthesis) make excellent heat sinks for semiconductors in device applications

Example of high thermal conductivity of diamond laser diode heat drain, conductivity of diamond 4x greater than copper or silver at RT (10-20 watts/cm^oC)

Yellow diamond n-doped with N, blue diamond p-doped with B

CVD can be used to produce diamond at low pressure and around 1000oC

By 1996 it was estimated that semiconductor applications could take 60% of worldwide diamond thin film market, other contenders for use of diamond film made by CVD are coated tools (abrasion resistance), optical disk coatings (protective coatings), lens and window coatings, loudspeakers (sound distortion control), UV laser coatings (reduces laser heating)

Synthetic methods for making diamond films all employ low pressure deposition of 1%CH₄/H₂ onto 1000^oC substrate

Heated filament method uses a hot wire to decompose the methane, 2200^oC, produces atomic C/H, 50 Torr pressure silica bell jar, diamond film deposited on 1000^oC substrate

Direct current plasma jet arc discharge focuses coating on a small area of substrate and can be scanned across a substrate

Microwave plasma discharge is used for commercial production of diamond films

Films characterized and distinguished from amorphous carbon and graphite by a combination of XRD, Auger, XPS, UPS, EELS, FTR

Raman particularly valuable quick diagnostic as sharp 1332 cm^-1 band specifies diamond, while broad bands in 1345, 1540 cm^-1 region, signals graphite and amorphous carbon in the film.

ED and TEM show {111} twinning which is commonly found in diamond (the famous cleavage plane of the jeweler and the layer of fused cyclohexane rings, easy to see how twins would grow out of this plane!!!

FULLERENE ZOO: THE CELESTIAL SPHERE THAT FELL TO EARTH

Patriarch C₆₀, an aromatic English football and the architect of the geodesic dome Buckminsterfuller

From astrophysics and carbon clusters to a single peak in a mass spectrum at 720 amu (plus a little friend at 840 amu) to a red solution in toluene to seeing-is-believing with a tiny tip to meteorites to a race for the Nobel

Well done Sir Harry (my good friend from my home town Brighton, Sussex)

Icospiral nucleation and growth model, to-close-or-not-to-close, that is the entropy and enthalpy of the story, getting rid of those nasty dangling bonds helps

Magic number fullerenes, 20, 24, 28, 32, 36, 50, 60, 70, even Steven, pentagon separation rule-the strain is too much, avoid abutting

Graphene Origami, pentagonal and heptagonal defects

Nature did it billions of years ago, just talk to Aulonia, and see those buckytubes later

From corannulene to soot particles an old story, Onions, giant fullerenes, nested spheres, Iijima expanded icosahedra, yes I discovered the buckytubes too!!!

C₆₀ superatom ideas, my that is a polarizable beast

The LUMO story of C₆₀^q- (q = 0-6), reducible representation of 60 2pp AO's in I_h point group, a tough group theory cookie, electronic properties of C₆₀, filling the famous LUMOs t_1u, t_1g and emptying the HOMO h_u,

Order-disorder transitions in solid C₆₀ from rigidly fixed to freely rotating, solid state NMR sees all, anisotropy to isotropy from below to above 77K

Alkali and alkaline earth fullerides M_xC₆₀ a long and super-interesting story

Structure-property relations, the fcc M₃C₆₀ and bcc M₆C₆₀ tale of two structures and where are those sites and electrons

Superconducting buckyball materials, BCS, ¹³C isotope effect, alkali effect on T_c, 18-45K superconductors, can we go any further (compare with other organic superconductors, conducting polymers, charge-transfer salts)

Where art thou Cooper pairs (DOS at E_F, strength of phonon-electron coupling), superconducting energy gap

Electrochemistry with C₆₀, Li₁₂C₆₀, my, that is a lot of lithium, so build a bucky-battery, compare with graphite/Li cells

Iodine intercalation or charge-transfer that is the question, C₆₀(I₂)₂, can you get back the energy cost of C₆₀⁺ and I^-/I₃^- through the Madalung energy, you get what you pay for! Analogy with graphite-iodine

Charge-transfer complexes of bucky with strong electron doors make ferromagnetic buckysolids, C₆₀(TME)

Teflon balls, C₆₀F₆₀, slippery thoughts, fluorinating the living daylights out of buckyball, compare with graphite fluoride and fluorographite

Platinum balls C₆₀[Pt(PPh₃)₂]₆, Leonardo's Pawnbroker

The inside story, ³He(I = 1/2)-C₆₀, shielding and Packman, pentagon-hexagon thermal triplets and how my friend did you get inside to look at the outside, C₇₀ inner is more strongly shielding than C₆₀, is this shear numbers of 2pp or something related to ball-currents!!!

Endohedral and exohedral fullerenes, start of another zoo

Organics, organometallics

Polymer fullerenes, string-of-pearls, necklaces, ladders, sheets and frameworks

Vaporizing rare earth doped carbon, MC₈₂, M₃C₈₂ stories, endohedral controversy, making them pure and in large enough quantities, EPR shows where the electron resides most of the time, electronic description in terms of M²⁺(C₈₂^2-), M₃²⁺(C₈₂^2-), small metal hfs and ¹³C satellites gives it away

Perfect host, perfect fit, C₆₀-VPI5, putting bucky down the 1.25 nm channels of a 18-ring aluminophosphate molecular sieve, new photophysics and photochemistry, brighter luminescence

Doping bucky, C₅₉B using boron-graphite pellets and laser evaporation

Faux fullerenes, electron beam heating of MoS₂, BN, NiCl₂ layered materials, inorganics bend too, but what are the defects in this kind of Origami, new materials, new chemistry, new properties

Buckyballs to buckydiamonds at room temperature, curvature and moving away from sp² carbon helps

Bucky-Switch, putting the squeeze on bucky, single molecule conductivity of squashed bucky under the STM tip is different to un-squashed

Bucky-SAMs, C₆₀H-HN(CH₂)_nSH-Au self-assembled bucky monolayeres on gold

Cure for HIV so stuff C₆₀ into the active site

So where are the technologies, problem, $5/kg!!!

FULLERENES, CLOSED CARBON CAGES OF WHICH BUCKMINSTERFULLERENE IS THE MOST NOTORIOUS

Mass spectra of laser ablated carbon, showed magic number stability of particular even number naked carbon clusters, 20, 24, 28, 32, 36, 50, 60, 70

Under certain conditions C₆₀ dominated with minor amounts of C₇₀

Something was special about these species that led Kroto, Smalley, Curl and Heath to propose that in particular C₆₀ had unique stability and a novel structure based upon an icosahedron with 12 isolated pentagons surrounded by 20 hexagons, a European football shape, a geodesic structure like that of the architect Buckminsterfuller, especially noteworthy being the U.S. space pavillion at the Montreal Expo 67.

Amazingly, the geodesic domes require pentagons mixed in with the hexagons in the strutts to allow the structure to curve with minimal amount of strain, also the diatom Aulonia has pentagons in its spherical microskeleton built of silica based six membered rings!

Nature knew how to do this billions of years before materials researchers stumbled on the problem of curving and closing graphene sheets

In the C₆₀ structure the pentagons are as isolated as possible to avoid the known instability inherent in fused pentagon configurations

Following this requirement structure and stability of magic number fullerenes can be nicely rationalized

Symmetrically distributed curvature related ring strain minimized

12,500 resonance structures, large aromaticity and stabilization

Closed electronic shell LUMO's are t_1u⁰t_1g⁰ in C₆₀ and are responsible for a lot of the chemical and physical properties

C₆₀ unique, all carbons are equivalent, seen in ¹³C NMR, compared to C₇₀ (egg-shaped, band of 10 extra carbons compared to C₆₀) that has five distinct ¹³C resonances from five distinct carbons (structure again avoids unstable abutting carbons)

Mode of formation of C₆₀ still under debate (has not been synthesized by an organic chemist yet, what's taking them so long)

Icospiral nucleation theory considers process to start from a corannulene C₂₀ saucer-shaped molecule which accretes carbon atoms in the gas phase, grows the sheet and has a chance to close

Although entropically highly unfavored the act of closure gets rid of all the dangling unsatisfied valencies on the edge carbons and the gain in bond energy released on eliminating reactive edges drives the process of closure.

Ones that do not close go on to grow a multi-walled onion-like fullerene through a type of epitaxial growth process of one shell over the other, at graphite separations of 0.35 nm

Iijima originally noted that soot particles had a structure based on polyhedral concentric shells consistent with the graphite icospiral model

Incidently, Iijima discovered the single and multi-walled carbon nanotubes that look like they may turn out to be scientifically and technologically more relevant than the celestial sphere that fell to earth

Giant fullerenes, multi-walled have now been discovered in the electron microscope when various forms of carbon are heated, form graphene sheets and curl in the electron beam

Similar faux fullerenes based on layered materials like BN, MoS₂, NiCl₂ are now being discovered and portend all sorts of interesting materials research for the future

C₆₀ itself undergoes interesting order-disorder transitions in the solid state as a function of temperature

Has a fcc lattice in which the C₆₀ icosahedra are locked into place below liquid nitrogen 77K temperatures, above this temperature they become freely rotating, this is nicely seen in the ¹³C solid state NMR where the line shape changes from axial anisotropic to isotropic

The EA = 2.65 eV, IP = 7.58 eV so easier to reduce C₆₀ than oxidize it, accounts for large number of anionic species C₆₀^x- where x = 1-6, chemical and electrochemical reduction

E_g = 1.7 eV, band structure based on SALCAO of sp² and pp orbitals under the icosohedral point group I_h

Thus depending on the degree of filling of the LUMO pp orbitals t_1ut_1g the properties such as electrical conductivity alter in a rather dramatic fashion

Some of these materials involving electron transfer to C₆₀ (reduction by alkaline and alkaline earth metals, metal alkyl reagents, electrochemical) become superconducting at low temperature

The T_c (superconducting-metal transition temperature) can be tuned with the size of the alkaline earth M₃C₆₀ where K < Rb < Cs

Also ¹³C isotope substitution in ¹³C₆₀ shows that the T_c decreases proportionally to the reduced mass of ¹³C relative to ¹²C, this observation provides strong support for a BCS mechanism of charge transport involving Cooper electron superconducting pairs and a phonon-electron coupling model which will depend on the mass of the atoms involved

How can we begin to understand some of these observations?

Graphite (T^oC, p_He, laser ablation, graphite arc) ® Condensate (solvent like toluene extract, LC) ® C₆₀ (alkali or alkaline earth sublime, RM, M/R₃N, electrochemical) ® M_xC₆₀

In the case of alkali metals K, Rb, Cs

x = 3 (superconductor low T, metal RT)

x = 0, 6 (insulator)

Let us first look at the structure of the solid C₆₀

fcc buckyballs

ABCABC close packing of the buckyball layers

M located in all O_h and T_d sites, so let us start counting sites in the fcc cell

Unit cell description: M₃C₆₀ = (T_d)₈(O_h)₄(C₆₀)₄

Note M₆C₆₀ goes bcc and M lie in the Wigner-Seitz quasi-tetrahedral sites

With M₆C₆₀ unit cell description (T_d)₁₂(C₆₀)₂

Critical superconducting transition depends on the size, electronic properties and number of metal dopants in the unit cell

M₀C₆₀,p(t_1u⁰) ® M₃C₆₀,p(t_1u³) ® M₆C₆₀,p(t_1u⁶)

This is the isolated molecule description from which one can see that a mini-band description from electronically coupling these moieties in the unit cell can lead to empty, half-filled and filled band descriptions of an insulator, metal-superconductor, insulator

This is obviously a simple description and does not handled the details, but one of the key features of the BCS model of superconductivity is the strength of electron-phonon coupling and the DOS at the Fermi level

This is what is being tuned with variations of the size of the alkali metal cation in M₃C₆₀, charge-to-radius ratio, interaction strength with the C₆₀ and so forth

Electrochemical synthesis, Li|(PEO)₈LiClO₄|C₆₀ or G cell design, coin-type

Reversible injection of Li⁺ and e^- for q = 0.5, 2, 3, 4

Maximum injection observed Li₁₂C₆₀, similar to saturation value of LiC₆ for graphite intercalation

Lithium ion nested between the six-rings of C₆₀ compared to nested between the six-rings in the graphene sheets, interesting analogy

Intercalation of solid C₆₀ with I₂ also occurs (250^oC, evacuated pyrex tube synthesis) but energetic cost of C₆₀⁺/I^- or I₃^- not offset by the gain in Madalung energy, so intercalates as molecular C₆₀(I₂)₂ layered compound AAA rather than oxidative intercalation

Alkali and alkaline earth fullerides M_xC₆₀ (0 < x £ 6), sublimation of metal vapor, or MR or M/R₃M or electrochemical syntheses

C₆₀,p(t_1u⁰) ® K₃C₆₀,p(t_1u³) fcc ® K₄C₆₀,p(t_1u⁴) ® K₆C₆₀,p(t_1u⁶) bcc

Insulator Low T Superconductor Insulator Insulator

Room T Metal

C₆₀,p(t_1u⁰) ® Ca₂C₆₀,p(t_1u⁴) ® Ca₃C₆₀,p(t_1u⁶) ® Ca₅C₆₀,p(t_1u⁶t_1g⁴)

Insulator RT Metal Insulator Low T Superconductor, RT Metal

Key points concerning variation of electrical conductivity properties, are filled or partially filled t_1u and t_1g bands, M-NM transitions (Jahn-Teller and Peierls effects in partially filled degenerate t_1u), ¹³C isotope effect on Tc and BCS, strength of coupling band widths, DOS at E_F

Thinking about endohedral fullerenes, probing the inside surface using ³He (I = 1/2) NMR, 600^oC, 2500 atm., Packman mechanism, opening up a C-C bond between a pentagon-hexagon pair, thermally populated triplet, He pops inside,

NMR shielding increases ³He < ³HeC₆₀ < ³HeC₇₀, reason not clear, extra electron density from 70 vs 60 2pp surface orthogonal orbitals, buckyball shielding currents (analogous to deshielding in aromatic ring compounds)

First solution phase EPR spectrum of a metallofullerene La@C82, 8-isotropic lines, hfs ~ 1G, ¹³C satellite lines on every hf line, gaseous La²⁺ hfs cc = 196 G, observed g = 2.0010 close to that of C₆₀^-, La (I = 7/2), La (5d²6s¹), so interpret this data in terms of an electronic-bonding description of La@C82

GRAPHITE FLUORIDE, MAKING GRAPHITE INTO THE UNWETTABLE ONE

Direct fluorination of graphite, p(F₂) ~ 0.29 x 10⁵ Pa reaction pressure, produces a series of materials from 375^oC to 600oC reaction temperature, 120 to 0.33 hours reaction time, F : C = 0.58 to 1.0

These CF to C₂F materials are to be distinguished from the HF/F₂ reductive intercalation to form CF_x used earlier in Li solid state batteries

So what are these unwettable graphite fluoride materials, with the largest known contact angles wrto water?

CF has a structure based on fully fluorinated single sheets of fused cyclohexane rings (puckered chair conformation) eclipsed sheets, 0.59 nm width of a sheet

C₂F has a structure based upon half fluorinated double sheets of fused cyclohexane rings held together between layers by alternating C-C bonds, staggered sheets, 0.81 nm width of a double sheet

Dangling bonds at the edges of the CF and C₂F layers satisfied with C-F bonds, different structure for edges of single and double layers

The C-F bonds encapsulating the sheets are the origin of the hydrophobicity of the materials and there inability to be wetted by water, short strong C-F bonds, very small polarizability, weak interactions with water

SAMs, CONTROL OF WETTABILITY OF SURFACES, PATTERNING WETTABILITY FROM NM TO MICROMETER SCALES, SOFT LITHOGRAPHY, MICROCONTACT PRINTING, MICROMACHINING AND MORE

Si substrate, evaporated Ti adhesion layer for Au layer, chemisorption of alkane thiolates, self-assembly into well ordered monolayer on the Au, S-Au bonding, immense possibilities for tuning the chemical and physical properties of the SAM, length of alkane chain, end functionality, PDMS elastomeric stamp for patterning alkanethiolates on gold, chemical and physical patterns from nm to microns, 2-D and 3-D patterning, planar and curved surfaces, soft lithography

Consider a hydrophilic SAM with carboxyl end-group functionality, use scalpel to write a line in the SAMs, removes hydrophylic alkanethiolates, replace by dipping in hydrophobic alkanethiolates (usually in EtOH solvent, removed with nitrogen), creates patterned regions of hydrophylic and hydrophobic SAMs, causes patterning of wettability, condensation figures

The whole SAM story, which will win a Nobel in chemistry (my prediction) will be laid out in all of its glory in the new graduate course on Supramolecular Materials, this is just a taste of what is to come.

TUNING THE BAND PROPERTIES OF A SUPERCONDUCTING ORGANOMETALLIC INTERCALATE

This is quite a mouthful, what do I mean:

Sn{S_xSe_2-x}_1-y{P}_y{Cp₂Co}_0.31

Tuning the VB/CB electronic structure of a layered tin dichalcogenide using a solid-solution (Vegard law) of wide band semiconducting sulfide and narrow band metallic selenide 0 £ x £ 2

Doping with electron deficient 5-electron P isomorphously replacing the 6-electron chalcogenide in the hcp layer plane, p-doping the tin dichalcogenide

Redox active organometallic intercalate, electron donor forming Cp₂Co⁺ interlamellar guests with electron injected into the CB of SnS_xSe_2-x, oxidative intercalation

Can even perform lithium cointercalation reactions with BuLi and LiI

Synthesis of the host material from vapor phase transport of the elements in a quartz evacuated tube with Sn/S/Se/1%P/Br₂ at 550-685^oC with product formed at 510-645^oC

The Br₂ has the dual function of transporting agent and prevents passivation of the Sn with a SnS₂ skin

SnS₂ + 2Br₂ ® SnBr₄ + S₂ ® SnS₂ + 2Br₂

Crystals formed orange (SnS₂) ® red to dark red (SnS_xSe_2-x) ® black (SnSe₂)

So why are the colors changing in this way with selenium incorporation

Oxidative-intercalation synthesis:

Sn{S_xSe_2-x}_1-y{P}_y + Cp₂Co (CH₃CN, 65^oC, 5-21 days, CH₃CN wash, vacuum dry) ® Sn{S_xSe_2-x}_1-y{P}_y{Cp₂Co}_0.31

How come this saturation loading of 0.31 cobaltacenes/cobaltaciniums?

Relates to the most efficient packing of the metallocene with its long axis parallel to the sheets and matching trigonl symmetry within the sheet, this gives a natural limit to packing 3SnS₂ to 1Cp₂Co with molecular axis orthogonal to the sheets

The cobaltacene is almost a spherical molecule, the interlayer gap opens by 5.29 angstoms on intercalating Cp₂Co into the layer space, essentially the size of the cobaltacene

^{2D Wide Line Solid State NMR reveals some interesting dynamics going on between the sheets using single crystal materials and deuterium labelling of the cyclopentadienyl rings of the cobaltacene guest, great experiment!!!}

Low T , 80 K E_a = 0.13 kcal/mole for ring-whizzing process

High T > 245K, E_a = 8.17 kcal/mole for molecule rotation process about axis orthogonal to layers

This is all very nice but what about explaining the observation that the electrical conductivity of the material with increased selenium content transforms from a hopping semiconductor to a metal which goes superconducting at low T?

This requires thinking about the band picture for the materials

On the sulfur rich side of the picture the Cp₂Co guests have a HOMO level (recall it is the friendly 19-electron easily oxidized guest) that lies below the Sn⁴⁺ CB of the dichalcogenide and creates a localized state and a material that is best described as Sn⁴⁺-Sn²⁺-Co²⁺-Co³⁺ mixed valence with electrons hopping (Hall, Seebeck measurements show they are really electrons that move and not holes)

So this end is best described ads a mixed valence hopping semiconductor (high Ea gives this away)

Also characterized effectively from combination of XPS, UPS, Mossbauer data

Some good and difficult questions to address are exactly how do the electrons move in this material, do they hop exclusively in the tin chalcogenide layer planes, or exclusively in the cobaltacene planes or actually between these planes, tough question!!!

Possibly the anisotropy of the conductivity would help in this regard

So what happens at the selenium rich end. Here the cobaltacene HOMO overlaps with the Sn⁴⁺ CB to create a partially filled CB rather than localized states. This increasing overlap of the CB of the host with the HOMO of the cobltacene guest is the cause of the gradual change from a hopping semiconductor towards itinerant metallic behavior.
 

 
MOLECULAR METALS

A combination of noun and adjective that would have appeared quite paradoxical not so long ago. Metals are formed from extended close-packed lattices of atoms while molecular crystals do not usually conduct electricity.

The focus of this part of the course is to look more closely at the subject of synthetic electrical conductors, the main classes of materials, synthesis-structure-property relations and utility in a range of devices.

SYNTHETIC ELECTRICAL CONDUCTORS

Some of the most important materials to be covered are listed below:

K₂Pt(CN)₄Br_0.3.3H₂O

(SiPcO)_n

(SN)_n

Hg_3-xAsF₆

NbI₄

VO₂

H_x

Poly(acetylene), (CH)_n

(TTF)TCNQ

(TTF)Br_x

(TMTSF)₂ClO₄

Poly(pyrrole)

Poly(thiophene)

Poly(aniline)

Poly(phenylene)

Poly(phenylenevinylene)

Poly(phenylenesulfide)

This is quite a long list of synthetic electrical conductors. We will not have time to cover all aspects of them in detail and so key points will be highlighted with plenty of real world applications. The number of recent reviews and books on the subject is very large so you should have no difficulty in tracking down information on this exciting branch of materials chemistry in order to supplement these brief lecture notes and my lecture presentations.

The overall goal is to look at rational approaches to their synthesis, relations between structure and bonding, chemical and electrochemical doping, charge-transport behavior and optical properties, and how this knowledge can be utilized to construct smart windows, displays, batteries, diodes, transistors, and electroluminescent devices.

Some more specialized but very important topics of central interest in the field of molecular metals will be touched upon. These include electrical and optical anisotropy, Peierls instabilty, metal-nonmetal transitions, commensurate and incommensurate charge-density waves (CDWs), spinless electrical conductivity, solitons, polarons, bipolarons, superconductivity.

CHRONOLOGY OF MILESTONES IN THE EMERGING FIELD OF SYNTHETIC ELECTRICAL CONDUCTORS
 
 

1911. First suggestion that organic solids could be electrical conductors

1911. H.K. Onnes discovers that Hg superconducts

1954. Bromine salt of perylene shows electrical conductivity

1957. Bardeen, Cooper, Schrieffer BCS theory of superconductivity

1962. Synthesis of tetracyanoquinodimethane, TCNQ

1964. Little suggests for the first time RT superconductivity is possible

1968. Krogmann's salts re-investigated with electrical properties now as the focus

1970. Synthesis of tetrathiofulvalene, TTF

1973. TTF-TCNQ synthesized, first organic metal, M-NM transition at 54K

1975. (SN)_n found to be an electrical conductor and superconductor at 0.36K

1977. (SNBr_0.4)_n doped poly(thiazyl) superconducts at 0.36K

1977. Macrocyclic transition metal planar stacks found to electrically conduct

1979. TMTSF-DMTCNQ synthesizied, s = 10⁵ W ^-1 cm^-1 at 10 kbar and 1K

1979. Bechgaard salts (TMTSF)₂X, where X = monovalent anion, superconductors under pressure and 1K

1981. (TMTSF)₂ClO₄ first ambient pressure superconductor at 1.4K

1983. (BEDT-TTF)₂ReO₄ first sulfur based organic superconductor under pressure and 2K

1984-86. b-(BETD-TTF)₂X where X = a collection of complex anions, yields a diverse group of ambient pressure sulfur-based superconductors with T_c gradually creeping up to 20K!!!

This more-or-less describes the current trend and activity in the field of molecular metals. The discovery of the high T_c ceramic oxide perovskite-based superconductors in 1986-87 changed the emphasis of the field and then the discovery of Buckyball based semiconductors, metals and superconductors added yet another exciting dimension to the field of synthetic electrical conductors.

While all this was happening Shirakawa's discovery of poly(acetylene) and its doping to the metallic state by MacDiamid caused an explosion of activity in the area of conducting conjugated polymers and the commencement of the "all-plastics" electronics, optoelectronics and photonics revolution.

An interesting aspect of the charge-transport properties of materials is that electrical conductivity extends from around 10^-18 ohm^-1 cm^-1 for good insulators like sulfur and Teflon to around 10^4-6 ohm^-1 cm^-1 for excellent metals. The difference between the two extremes is around 10²⁴ and represents the largest range known for any property of a material!!!

Of interest is the fact that doped conjugated polymers, charge-transfer salts, organic radical-anion salts and doped macrocyclic transition metal complexes and polymeric columnar stacks have electrical conductivities that can span the semiconducting 10^-6-10² ohm^-1 cm^-1 and metallic 10⁰-10⁶ ohm^-1 cm^-1 ranges. This makes them amenable to a high degree of tailorability and is what makes the materials so exciting and relevant both scientifically and technologically.

Note that to really distinguish a semiconductor from a metal one needs temperature dependent measurements of the conductivity.

FOUR PROBE ELECTRICAL CONDUCTIVITY MEASUREMENTS

s = r ^-1 = (I/V)(L/A) W ^-1 cm^-1

Relationship between resistivity, conductivity, sample dimensions, and the voltage drop measured for a constant current across the sample

s (T) = s ₀exp(-E_g/2kT)

Semiconductor behavior, conductivity increases with T according to energy gap law

r (T) = r ₀ + bT^g

Metallic behavior, conductivity decreases with T because of resistivity, surface and impurity/defect scattering

Usually four-probe technique applied to single crystals

Nature of the contacts important, ohmic junctions rather than Schottky barrier

Constant current source applied across sample cross sectional area A

Voltage drop measured across length of sample L

Hall or Seebeck measurements provide the sign of the majority charge carrier, whether electrons or holes.

ELECTRICALLY CONDUCTIVE PLANAR CONJUGATED MOLECULAR STACKS

Key points to consider with this class of materials and other molecular metals

Stacking architecture of molecular building units

Close spatial proximity of units

Similar crystallographic environments, high degree of order

Energetically favorable extended pathway for charge transport

Extended band structure from orbital overlap of molecules comprising a stack

Hückel-like tight binding description of the electronic band structure

Molecular metallic state requires partial oxidation (or reduction) and unfilling (or filling) of the valence (or conduction) band.

Control of this "doping" process provides a means of finely adjusting the degree of filling or emptying of electronic bands and a way of tailoring the properties of the materials (i.e., magnetic, electrical conductivity, optical absorption, photo- or electroluminescence and so forth).

Partial oxidation or reduction of planar stacks of molecules creates a charge z⁺ on each component of the stack and this process must be charge-balanced by counter anions A^- that have to diffuse into the structure.

When considering band formation for this class of partially charged stacked planar molecules we can conceptualize the process by starting with a single molecular building block and it HOMO and examine how the orbital structure gradually transforms into bands with an increasing number of sub-units and HOMOs in the stack.

The most bonding energy levels are comprised of the in-phase HOMO combinations and lie at the bottom of the band. Conversely the out-of-phase combinations lie at the top of the band and are most antibonding.

The Fermi energy for a partially filled band is close to the highest filled level in the band taking into account the shape of the DOS and the so-called Maxwellian tail located at the band edge.

At the Fermi level there is an electronic degeneracy k = -k and this can give rise to a Peierls instability and the phenomenon of a charge density wave originating from electron-phonon coupling. Associated with this effect may be a concomitant distortion of the lattice and a metal-nonmetal transition.

The width of the band in the tight binding approximation is denoted 4t where t is the tight binding transfer integral, which is analogous to the Hückel resonance integral b.

The degree of band filling is controlled by the extent of oxidative or reductive doping (chemical or electrochemical).

All of this discussion relates to the "isolated" stack model with negligible interstack interactions.

To evaluate the various factors that contribute to the charge-transport properties of a partially charged columnar stack of planar transition metal complexes linked or not linked together into a polymer, planar stacks of charge transfer salts, or stacks of planar organic radical anion salts, there are certain key points that have to be taken into account.

These include:

Partial charge on the components in the stack

Nature of the counteranion, charge and size

Degree of order in the stack

Electron-electron repulsions in the partially filled band

Band width determined by orbital overlap

Interstack and intrastack separations

Interstack and intrastack interactions.

The subtle interplay of these factors decides whether the material behaves as an electrical insulator or a metal, whether and what kind of metal-nonmetal transition occurs in the system as a function of temperature and/or pressure (e.g., Mott, Hubbard, Peierls, Anderson).

FAVORITE STACKERS

Basic building blocks often used for synthesizing conducting molecular stacks of different kinds.

Tetrachalcogenafulvalenes

Tetracyanoquinodimethanes

Tetrachalcogenatetracenes

Tetracyanoplatinates

Glyoximates

Phthalocyanines

Porphyrins

Dibenzotetraazaannulenes
 

Basic metal phthalocyanine stacking unit.

When unconnected the architecture of the stack is not under control.

By making the planar phthalocyanines part of a 1-D polymer chain the architecture and state of oxidation is under better control.

This was the concept that drove Tobin Mark's work on structurally enforced electrically conductive metallomacrocyclic polymer arrays.

Synthetic strategy:

H₂Pc + SiCl₄ ® Si(Pc)Cl₂ + 2HCl

Si(Pc)Cl₂ + H₂O ® Si(Pc)(OH)₂ + 2HCl

Si(Pc)(OH)₂ ® [Si(Pc)O]_n + H₂O

The final condensation-polymerization provides a "phthalocyanine silicone" polymer comprised of parallel planar silicon phthalocyanine complexes linked together by linear Si-O-Si bonds.

Similar chemistry has been used to make the germanium and tin phthalocyanine polymer analogues.

Oxidative chemical doping of the polymer with halogens, nitrosonium salts, quinones as well as electrochemical oxidation yields an electrically conductive polymer.

Some examples of this doping process are given below:

[M(Pc)O]_n + 0.55nX₂ ® {[M(Pc)O]X_1.1}_n where X = Br, I

[M(Pc)O]_n + 0.35nNO⁺Y^-® {[M(Pc)O]Y_0.35}_n +0.35nNO(g) where Y = BF₄, PF₆

[M(Pc)O]_n + nyQ ® {[M(Pc)O]Q_y}_n where Q = TCNQ, DDQ

Single crystal XRD structures have been obtained for some of these polymers in the pristine and undoped states.

They often display "staggered" packing of the phthalocyanine ligands, in the case of {[Si(Pc)O]Cl_1.1}_n the space group is P 4/mcc with the chloride anions packed in a linear array down the four fold axis in the channel spaces between the polymer chains.