CHM 434F/1206F SOLID STATE MATERIALS CHEMISTRY

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EgC EgB (h2/8R2)(1/me 1/mh) - 1.8e2/?R
Coulomb interaction between e-h
Quantum localization term
CAPPED MONODISPERSED SEMICONDUCTOR NANOCLUSTERS
TUNING CHEMICAL AND PHYSICAL PROPERTIES OF
MATERIALS WITH SIZE AS WELL AS COMPOSITION AND
STRUCTURE
nMe2Cd nnBu3PSe mnOct3PO ? (nOct3PO)m(CdSe)n
n/2C2H6 nnBu3P
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ARRESTED GROWTH OF MONODISPERSED NANOCLUSTERS
CRYSTALS, FILMS ANDLITHOGRAPHIC PATTERNS
nMe2Cd nnBu3PSe mnOct3PO ? (nOct3PO)m(CdSe)n
n/2C2H6 nnBu3P
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BASICS OF NANOCLUSTER NUCLEATION, GROWTH,
CRYSTALLIZATION AND CAPPING STABILIZATION
?Gb gt ?Gs
supersaturation
nucleation
aggregation
capping and stabilization
nMe2Cd nnBu3PSe mnOct3PO ? (nOct3PO)m(CdSe)n
n/2C2H6 nnBu3P
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THINK SMALL DO BIG THINGS!!!
EgC EgB (h2/8R2)(1/me 1/mh) - 1.8e2/?R
tuning chemical and physical properties of
materials with size as well as composition and
structure
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SYNTHESIS OF COMPOSITION TUNABLE ZnxCd1-xSe ALLOY
NANOCRYSTALS

• High structural and optical quality ZnxCd1-xSe
  semiconductor alloy nanocrystals successfully
  prepared using core-corona precursor made by
  incorporating stoichiometric amounts of Zn and Se
  into pre-prepared CdSe nanocrystal seeds and
  thermally inducing alloy nanocluster formation by
  interdiffusion of element components within
  nanocluster - diffusion length control of
  reaction between two solid reagents
• With increasing Zn content, a composition-tunable
  photoemission across most of the visible spectrum
  has been demonstrated by a systematic blue-shift
  in emission wavelength (QSE) demonstrating alloy
  nanocluster formation and not phase separation
• A rapid alloying process is observed at the
  alloying point as the core and corona
  components mix to provide a homogeneous Vegard
  law type distribution of elements in the
  nanoclusters

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SYNTHESIS OF COMPOSITION TUNABLE ZnxCd1-xSe ALLOY
NANOCRYSTALS

• Sequence of steps for synthesis of core-shell
  precursor nanoclusters
• Cd(stearate)2 (octyl)3PO solvent
  octadecylamine
• Reaction temperature 310-330C
• Se (octyl)3P
• Mixing temperature 270-300C
• Provides core nanocluster precursor
  (CdSe)n(TOPO)m
• Add ZnEt2 (octyl)3P in controlled stoichiometry
  increments
• Mixing temperature 290-320C
• Monitor photoluminescence until constant
  wavelength emission
• Desired alloy nanocluster product
  (ZnxCd1-xSe)n(TOPO)m

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TEM OF COMPOSITION TUNABLE ZnxCd1-xSe ALLOY
NANOCRYSTALS SHOWS MONOTONIC INCREASE IN
DIAMETER OF NANOCRYSTALS WITH ADDITION OF ZnSe
CORONA TO CdSe CORE
SPATIALLY RESOLVED EDX SHOWS NANOCRYSTAL
COMPOSITIONAL HOMOGENIETY
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ABSORPTION-EMISSION SPECTRA OF COMPOSITION
TUNABLE ZnxCd1-xSe ALLOY NANOCRYSTALSEXPECTED
BLUE SHIFT OF ABSORPTION AND EMISSION WITH
INCREASING AMOUNTS OF WIDE BAND GAP ZnSe
COMPONENT IN NARROW BAND GAP CdSe NANOCRYSTALS
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PXRD PATTERNS OF COMPOSITION TUNABLE ZnxCd1-xSe
ALLOY NANOCRYSTALSEXPECTED DECREASE IN UNIT
CELL DIMENSIONS WITH INCREASING AMOUNTS OF
SMALLER UNIT CELL ZnSe COMPONENT IN LARGER UNIT
CELL CdSe NANOCRYSTALS
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MODE OF FORMATION OF COMPOSITION TUNABLE
ZnxCd1-xSe ALLOY NANOCRYSTALS
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2 MoCl5 5 Na2S ? 2 MoS2 10 NaCl S
Richard Kaner Rapid Solid State Synthesis of
Materials
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RAPID SS PRECURSOR SYNTHESIS OF MATERIALS
LixQy MClx ?? MQy xLiClQ N, P, As
(PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si
(CARBIDES, SILICIDES)

• Many useful materials, such as ceramics, are most
  often produced from high temperature reactions
  (500-3000C) which often take many days due to
  the slow nature of solid-solid diffusion.
• Rapid SS new method which enables high quality
  refractory materials to be synthesized in seconds
  from appropriate solid state precursors.
• Basic idea is to react stable high oxidation
  state metal halides with alkali or alkaline earth
  compounds to produce the desired product plus an
  alkali(ne) halide salt which can simply be washed
  away.
• Since alkali(ne) salt formation is very favorable
  many of these reactions are thermodynamically
  downhill by 100-200 kcal/mol or more.

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RAPID SS PRECURSOR SYNTHESIS OF MATERIALS LixQy
MClx ?? MQy xLiClQ N, P, As (PNICTIDES),
S, Se, Te (CHALCOGENIDES), C, Si (CARBIDES,
SILICIDES)

• MoS2, a material used as a lubricant in aerospace
  applications, as a cathode for rechargeable
  batteries and as a hydrodesulfurization catalyst,
  is normally prepared by heating the elements to
  1000C for several days.
• New SSS gives pure, crystalline MoS2 from a
  self-initiated reaction between the solids MoCl5
  and Na2S in seconds
• 2 MoCl5 5 Na2S --gt 2 MoS2 10 NaCl S
• NaCl byproduct is simply washed away.
• Other layered transition MS2 can be produced in
  analogous rapid solid-solid reactions M W, Nb,
  Ta, Rh
• Na2Se used for MSe2 syntheses

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PARTICLE SIZE CONTROL USE AN INERT DILUENT
LIKE NaCl TO AMELIORATE THE HEAT OF REACTION

• MoCl5/NaCl MoS2 Paricle Size nm
• 10 45
• 14 18
• 116 8
• NaCl washed away after reaction

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RAPID SS PRECURSOR SYNTHESIS OF MATERIALS
LixQy MClx ?? MQy xLiClQ N, P, As
(PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si
(CARBIDES, SILICIDES)

• High quality anion solid solutions such as
  MoS1-xSex can be made using the precursor
  Na2S1-xSex formed by co-precipitation of
  Na2S/Na2Se mixtures from liquid ammonia
• High quality cation solid solutions such as
  Mo1-xWxS2 can be made by melting together the
  metal halides MoCl5 and WCl6, followed by
  reaction with Na2S
• The solid-solution products can be analyzed by
  studying the MoW alloys formed after reduction in
  hydrogen - ASSUMING NO SEGREGATION!!!

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SOLID SOLUTION PRECURSORS

• REACTANT A REACTANT B
• Na2(S,Se) GaCl3
• Na3(P,As) MoCl5
• WCl6
• PRODUCT
• Ga(P,As)
• Mo(S,Se)2
• W(S,Se)2
• (Mo,W)S2

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RAPID SS PRECURSOR SYNTHESIS OF MATERIALS
LixQy MClx ?? MQy xLiClQ N, P, As
(PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si
(CARBIDES, SILICIDES)

• These SS metathesis reactions are becoming a
  general process for synthesizing important
  materials.
• For example, refractory ceramics such as ZrN
  (m.p. 3000C) can be produced in seconds from
  ZrCl4 and Li3N
• ZrCl4 4/3Li3N ? ZrN 4LiCl 1/6N2
• NOTE CHANGE IN OXIDATION STATE Zr(IV) REDUCED TO
  Zr(III) WITH OXIDATION OF N(-III) TO N(0)
• MoSi2, a material used in high temperature
  furnace elements, can be made from MoCl5 and Mg2Si

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RAPID SS PRECURSOR SYNTHESIS OF MATERIALS
LixQy MClx ?? MQy xLiClQ N, P, As
(PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si
(CARBIDES, SILICIDES)

• The III-V semiconductors GaP and GaAs can be made
  in seconds from the solid precursors GaCl3 and
  Na3P or Na3As
• Recently, high pressure methods have been
  employed to allow the use of metathesis to
  synthesize gallium nitride (GaN) using Li3N, very
  important blue laser diode material, a synthesis
  which was not possible using the methods for GaP
  or GaAs

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SUMMARIZING KEY FEATURES OF RAPID SOLID STATE
SYNTHESIS OF MATERIALS

• Metathesis - exchange pathway
• Access to large number of materials
• Extremely rapid about 1 s
• Initiated at or near RT
• Self propagating
• Thermodynamic driving force of alkali(ne) halides
• Control of particle size with inert alkali(ne)
  halide matrix
• Solid solution materials synthesis
• Most recent addition to metathesis zoo are
  carbides

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METAL CARBIDES - TRY TO BALANCE THESE EQUATIONS -
OXIDATION STATE CHALLENGE

• 3ZrCl4 Al4C3 ? 3ZrC 4AlCl3
• 2WCl4 4CaC2 ? 2WC 4CaCl2 6C
• 2TiCl3 3CaC2 ? 2TiC 3CaCl2 4C
• DO NOT CONFUSE CARBIDE C4- FROM ACETYLIDE
  (C22-)!!!
• Inert, hard, refractory conducting ceramics
• Used for cutting tools, crucibles, catalysts,
  hard steel manufacture

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VAPOR PHASE TRANSPORT VPC MATERIALS SYNTHESIS,
CRYSTAL GROWTH, PURIFICATION

• Sealed glass tube reactors
• Reactant(s) A, gaseous transporting agent B
• Temperature gradient furnace DT 50oC
• Equilibrium established
• A(s) B(g) AB(g)

T2
T1
Glass tube
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VAPOR PHASE TRANSPORT VPC MATERIALS SYNTHESIS,
CRYSTAL GROWTH, PURIFICATION
Glass tube

• 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

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VAPOR PHASE TRANSPORT VPC MATERIALS SYNTHESIS,
CRYSTAL GROWTH, PURIFICATION
Glass tube

• 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

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THERMODYNAMICS OF CVT

• A(s) B(g) ? AB(g)
• Reversible equilibrium needed DGo -RTlnKequ
• Consider case of exothermic reaction with - DGo
• Thus DGo RTlnKequ
• Smaller T implies larger Kequ
• Forms at cooler end, decomposes at hotter end of
  reactor
• Consider case of endothermic reaction with DGo
• Thus DGo -RTlnKequ RTln(1/Kequ)
• Larger T implies larger Kequ
• Forms at hotter end, decomposes at cooler end of
  reactor

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USES OF VPT

• synthesis of new solid state materials
• growth of single crystals
• purification of solids

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PLATINUM HEATER ELEMENTS IN FURNACES THEY
MOVE!! Pt(s) O2(g) PtO2(g)
T2
T1

• Endothermic reaction
• PtO2 forms at hot end
• Diffuses to cool end
• Deposits well formed Pt crystals
• Observed in furnaces containing Pt heating
  elements
• CVT, T2 gt T1, provides concentration gradient and
  thermodynamic driving force for gaseous diffusion
  of vapor phase transport agent PtO2

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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!!!

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DOUBLE TRANSPORT INVOLVING OPPOSING
EXOTHERMIC-ENDOTHERMIC REACTIONS

• Endothermic
• WO2(s) I2(g) (T1 800oC) (T2 1000oC) WO2I2(g)
• Exothermic
• W(s) 2H2O(g) 3I2(g) (T2 1000oC) (T1 800oC)
  WO2I2 (g) 4HI(g)
• The antithetical nature of these two reactions
  allows W/WO2 mixtures to be separated at
  different ends of the gradient reactor using
  H2O/I2 as the VPT reagents

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VAPOR PHASE TRANSPORT FOR SYNTHESIS

• A(s) B(g) (T1) (T2) AB(g)
• AB(g) C(s) (T2) (T1) AC(s) B(g)
• Concept couple VPT with subsequent reaction to
  give overall reaction
• A(s) C(s) B(g) (T2) AC(s) B(g) (T1)

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REAL EXAMPLES VPT 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)

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REAL EXAMPLES VPT 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)

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OVERCOMING PASSIVATION IN SOLID STATE SYNTHESIS
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 700oC) (T2
  800oC) 2AlI3(g) 3/2S2(g)
• Zn(s) S(s) ZnS(s) passivation prevents
  reaction to completion
• Endothermic ZnS(s) I2(g) (T1 800oC) (T2
  900oC) ZnI2(g) 1/2S2(g)

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VPT GROWTH OF MAGNETITE CRYSTALS FROM POWDERED
MAGNETITE

• Endothermic reaction forms at hotter end,
  crystallizes at cooler end
• Fe3O4(s) 8HCl(g) ? 1FeCl2(g) 2FeCl3(g)
  4H2O(g)
• Inverted Spinel Magnetite crystals grow at cooler
  end - B(AB)O4 - Fe(III)(Td)Fe(III)Fe(II)(Oh)O4

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FERROMAGNETIC INVERTED SPINEL MAGNETITE
B(AB)O4Fe(III)(Td)Fe(III)Fe(II)(Oh)O4
Field H
Multidomain paramagnet above Tc
Multidomain ferromagnet below Tc
M
Ms
Mr
Single domain superparamagnet
Hc
H
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VPT SYNTHESIS AND CRYSTAL GROWTH OF TiS2 FROM
POWDERED Ti/S

• Endothermic reaction forms at hotter end,
  crystallizes at cooler end - also removes
  passivating TiS2 skin on Ti
• (T1) TiS2(s) 2Br2(g) ? (T2) TiBr4(g) S2(g)
• TiS2 crystals grow at cooler end - interesting
  for studying intercalation reactions - kinetics,
  mechanism, structure

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LITHIUM SOLID STATE BATTERY MATERIAL Li TiS2 ?
LixTiS2

• TiS2 structure hcp packing of S(-II), octahedral
  Ti(IV)
• Li intercalates between hcp S2- layers,
  electrons injected into t2g Ti(IV) CB
• TiS2 is a semiconductor, conductivity increases
  upon insertion of Li ions and electrons
• Li intercalation varies from 1 ? x ? 0, 10
  lattice expansion, TiS2 ? LiTiS2
• Capacity 250 A-h/kg, voltage 1.9 Volts (too
  low for SS cathode)
• Energy density 480 W-h/kg

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VPT SYNTHESIS OF ZnWO4 A REAL PHOSPHOR HOST
CRYSTAL FOR Ag(I), Cu(I), Mn(II)
Endothermic reaction VPT agent WO2Cl2(g)
Cl2O(g) formed at hot end, atmosphere Cl2(g)

• WO3(s) 2Cl2(g) (T2 1060oC) (T2 1060oC)
  WO2Cl2(g) Cl2O(g)
• WO2Cl2(g) Cl2O(g) ZnO(s) (T2 1060oC)
  ZnWO4(s) Cl2(g) (T1 980oC)

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VPT GROWTH OF EPITAXIAL GaAs FILMS/CRYSTALS USING
CONVENIENT STARTING MATERIALS
VPT agent GaCl/As4/H2(g) formed at hot end,
atmosphere HCl(g)

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

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MgB2 SAT ON THE SHELF DOING NOTHING FOR HALF A
CENTURY AND THEN THE BIGGEST SURPRISE SINCE HIGH
Tc CERAMIC SUPERCONDUCTORS
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SUPERCONDUCTIVITY IN MgB2 AT 39K A SENSATIONAL
AND CURIOUS DISCOVERY
Mg
B
Mg