Transcript CHM 434F/1206F SOLID STATE MATERIALS CHEMISTRY

EgC = EgB + (h2/8R2)(1/me* + 1/mh*) - 1.8e2/R
Quantum
localization term
Coulomb interaction
between e-h
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
ARRESTED GROWTH
OF MONODISPERSED
NANOCLUSTERS
CRYSTALS, FILMS
ANDLITHOGRAPHIC
PATTERNS
nMe2Cd + nnBu3PSe + mnOct3PO  (nOct3PO)m(CdSe)n + n/2C2H6 + nnBu3P
BASICS OF NANOCLUSTER NUCLEATION,
GROWTH, CRYSTALLIZATION AND CAPPING
STABILIZATION
Gb > Gs
supersaturation
nucleation
Addition
of reagent
aggregation
capping and
stabilization
nMe2Cd + nnBu3PSe + mnOct3PO  (nOct3PO)m(CdSe)n + n/2C2H6 + nnBu3P
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
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
SYNTHESIS OF COMPOSITION TUNABLE ZnxCd1-xSe
ALLOY NANOCRYSTALS
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Sequence of steps for synthesis of core-shell precursor nanoclusters
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Cd(stearate)2 + (octyl)3PO + solvent octadecylamine
Reaction temperature 310-330°C
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Se + (octyl)3P
Mixing temperature 270-300°C
Provides core nanocluster precursor (CdSe)n(TOPO)m
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Add ZnEt2 + (octyl)3P in controlled stoichiometry increments
Mixing temperature 290-320°C
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Monitor photoluminescence until constant wavelength emission
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Desired alloy nanocluster product (ZnxCd1-xSe)n(TOPO)m
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
ABSORPTION-EMISSION SPECTRA OF COMPOSITION
TUNABLE ZnxCd1-xSe ALLOY NANOCRYSTALS
EXPECTED BLUE SHIFT OF ABSORPTION AND EMISSION WITH
INCREASING AMOUNTS OF WIDE BAND GAP ZnSe COMPONENT IN
NARROW BAND GAP CdSe NANOCRYSTALS
PXRD PATTERNS OF COMPOSITION TUNABLE
ZnxCd1-xSe ALLOY NANOCRYSTALS
EXPECTED DECREASE IN UNIT CELL DIMENSIONS WITH
INCREASING AMOUNTS OF SMALLER UNIT CELL ZnSe
COMPONENT IN LARGER UNIT CELL CdSe NANOCRYSTALS
MODE OF FORMATION OF COMPOSITION TUNABLE
ZnxCd1-xSe ALLOY NANOCRYSTALS
2 MoCl5 + 5 Na2S  2 MoS2 + 10 NaCl + S
Richard Kaner: Rapid Solid State Synthesis of Materials
RAPID SS PRECURSOR SYNTHESIS OF MATERIALS:
LixQy + MClx  MQy + xLiCl
Q = 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 (500-3000°C) 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
RAPID SS PRECURSOR SYNTHESIS OF MATERIALS
LixQy + MClx  MQy + xLiCl
Q = 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 1000°C for
several days.
• New SSS gives pure, crystalline MoS2 from a selfinitiated reaction between the solids MoCl5 and Na2S
in seconds
• 2 MoCl5 + 5 Na2S --> 2 MoS2 + 10 NaCl + S
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NaCl byproduct is simply washed away.
Other layered transition MS2 can be produced in analogous rapid
solid-solid reactions: M = W, Nb, Ta, Rh
PARTICLE SIZE CONTROL:
USE AN INERT DILUENT LIKE NaCl TO AMELIORATE
THE HEAT OF REACTION
• MoCl5/NaCl
• 1:0
• 1:4
• 1:16
MoS2 Paricle Size nm
45
18
• NaCl washed away after reaction
8
RAPID SS PRECURSOR SYNTHESIS OF MATERIALS:
LixQy + MClx  MQy + xLiCl
Q = 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 Mo1xWxS2 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
SOLID SOLUTION PRECURSORS
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REACTANT A
Na2(S,Se)
Na3(P,As)
PRODUCT
Ga(P,As)
Mo(S,Se)2
W(S,Se)2
(Mo,W)S2
REACTANT B
GaCl3
MoCl5
WCl6
RAPID SS PRECURSOR SYNTHESIS OF MATERIALS:
LixQy + MClx  MQy + xLiCl
Q = 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.
~ 3000°C) can be produced in seconds from ZrCl4
and Li3N
• ZrCl4 + 4/3Li3N  ZrN + 4LiCl + 1/6N2
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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
RAPID SS PRECURSOR SYNTHESIS OF MATERIALS:
LixQy + MClx  MQy + xLiCl
Q = 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
SUMMARIZING KEY FEATURES OF RAPID SOLID
STATE SYNTHESIS OF MATERIALS
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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
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
VAPOR PHASE TRANSPORT VPC MATERIALS
SYNTHESIS, CRYSTAL GROWTH, PURIFICATION
B(g)
A(s)
• Sealed glass tube reactors
T2
A(s)
AB(g)
T1
Glass tube
• Reactant(s) A, gaseous transporting agent B
• Temperature gradient furnace T ~ 50oC
• Equilibrium established
• A(s) + B(g) AB(g)
VAPOR PHASE TRANSPORT VPC MATERIALS
SYNTHESIS, CRYSTAL GROWTH, PURIFICATION
A(s)
B(g)
A(s)
AB(g)
T2
• 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
T1
Glass tube
VAPOR PHASE TRANSPORT VPC MATERIALS
SYNTHESIS, CRYSTAL GROWTH, PURIFICATION
A(s)
B(g)
A(s)
AB(g)
T2
• Temperature dependent K
T1
Glass tube
• Equilibrium concentration of AB(s) changes with T
• Different at T2 and T1
• Concentration gradient of AB(g) provides driving
force for gaseous diffusion
• A(s) + B(g)  AB(g)
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THERMODYNAMICS
OF CVT
Reversible equilibrium needed: Go = -RTlnKequ
• Consider case of exothermic reaction with - Go
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Thus Go = RTlnKequ
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Smaller T implies larger Kequ
• Forms at cooler end, decomposes at hotter end of
reactor
• Consider case of endothermic reaction with +Go
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Thus Go = -RTlnKequ = RTln(1/Kequ)
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Larger T implies larger Kequ
• Forms at hotter end, decomposes at cooler end of
reactor
USES OF VPT
• synthesis of new solid state materials
• growth of single crystals
• purification of solids
PLATINUM HEATER ELEMENTS IN FURNACES
THEY MOVE!! Pt(s) + O2(g)  PtO2(g)
• Endothermic reaction
T2
• PtO2 forms at hot end
Pt(s)
• Diffuses to cool end
T1
PtO2(g)
VPT agent PtO2(g)
Atmosphere O2(g)
• Deposits well formed Pt crystals
• Observed in furnaces containing Pt heating
elements
• CVT, T2 > T1, provides concentration gradient
and thermodynamic driving force for gaseous
diffusion of vapor phase transport agent PtO2
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 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
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)
REAL EXAMPLES VPT DIRECT REACTION
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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)
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)
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)
VPT GROWTH OF MAGNETITE CRYSTALS FROM
POWDERED MAGNETITE
Fe3O4(s)
1270K
1020K
VPT agent FeCl2/FeCl3(g)
Atmosphere HCl(g)
• 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
FERROMAGNETIC INVERTED SPINEL MAGNETITE B(AB)O4
Fe(III)(Td)[Fe(III)Fe(II)(Oh)]O4
Field H
Multidomain
paramagnet above Tc
Multidomain
ferromagnet below Tc
M
Ms
Mr
Hc
H
Single domain
superparamagnet
VPT SYNTHESIS AND CRYSTAL GROWTH OF TiS2
FROM POWDERED Ti/S
TiS2
Ti/S(s)
550-685oC (T2)
510-645oC (T1)
VPT agent TiBr4(g)
Atmosphere Br2(g)
• 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,
LITHIUM SOLID STATE BATTERY MATERIAL
Li + TiS2  LixTiS2
Li insertion
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TiS2 structure hcp packing of S(-II), octahedral Ti(IV)
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Li+ intercalates between hcp S2- layers, electrons injected into t2g
Ti(IV) CB
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TiS2 is a semiconductor, conductivity increases upon insertion of Li
ions and electrons
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Li intercalation varies from 1  x  0, 10% lattice expansion, TiS2 
LiTiS2
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Capacity ~ 250 A-h/kg, voltage ~ 1.9 Volts (too low for SS cathode)
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Energy density ~ 480 W-h/kg
VPT SYNTHESIS OF ZnWO4: A REAL PHOSPHOR
HOST CRYSTAL FOR Ag(I), Cu(I), Mn(II)
1060oC (T2)
WO3/ZnO(s)
ZnWO4(s)
980oC (T1)
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)
VPT GROWTH OF EPITAXIAL GaAs FILMS/CRYSTALS
USING CONVENIENT STARTING MATERIALS
(T2)
GaAs(s)
GaAs(s)
(T1)
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)
MgB2 SAT ON THE
SHELF DOING NOTHING
FOR HALF A CENTURY
AND THEN THE
BIGGEST SURPRISE
SINCE HIGH Tc CERAMIC
SUPERCONDUCTORS
SUPERCONDUCTIVITY IN MgB2 AT 39K
A SENSATIONAL AND CURIOUS DISCOVERY
Mg
B
Mg