CAPPED MONODISPERSED SEMICONDUCTOR NANOCLUSTERS

nMe2Cd + nnBu3PSe + mnOct3PO (nOct3PO)m(CdSe)n + n/2C2H6 + nnBu3P

EgC = Eg

B + (h2/8R2)(1/me* + 1/mh*) - 1.8e2/R

Coulomb interaction between e-h

Quantum localization term

TUNING CHEMICAL AND PHYSICAL PROPERTIES OF MATERIALS WITH SIZE AS WELL AS COMPOSITION AND STRUCTURE

ARRESTED GROWTH OF MONODISPERSED

NANOCLUSTERS

CRYSTALS, FILMS ANDLITHOGRAPHIC

PATTERNS

nMe2Cd + nnBu3PSe + mnOct3PO (nOct3PO)m(CdSe)n + n/2C2H6 + nnBu3P

Additionof reagent

nucleation

aggregation

BASICS OF NANOCLUSTER NUCLEATION, GROWTH, CRYSTALLIZATION AND CAPPING STABILIZATION

capping and stabilization

nMe2Cd + nnBu3PSe + mnOct3PO (nOct3PO)m(CdSe)n + n/2C2H6 + nnBu3P

Gb > Gs

supersaturation

THINK SMALL DO BIG THINGS!!!

EgC = Eg

B + (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

• Sequence of steps for synthesis of core-shell precursor nanoclusters

• Cd(stearate)2 + (octyl)3PO + solvent octadecylamine • Reaction temperature 310-330°C

• Se + (octyl)3P• Mixing temperature 270-300°C • Provides core nanocluster precursor (CdSe)n(TOPO)m

• Add ZnEt2 + (octyl)3P in controlled stoichiometry increments • Mixing temperature 290-320°C

• Monitor photoluminescence until constant wavelength emission

• 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 reactions are thermodynamically downhill by 100-200 kcal/mol or more.

• 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 self-initiated reaction between the solids MoCl5 and Na2S in seconds

• 2 MoCl5 + 5 Na2S --> 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

RAPID SS PRECURSOR SYNTHESIS OF MATERIALS LixQy + MClx MQy + xLiCl

Q = N, P, As (PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si (CARBIDES, SILICIDES)

PARTICLE SIZE CONTROL: USE AN INERT DILUENT LIKE NaCl TO AMELIORATE

THE HEAT OF REACTION

• MoCl5/NaCl MoS2 Paricle Size nm

• 1:0 45• 1:4 18• 1:16 8

• NaCl washed away after reaction

• 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!!!

RAPID SS PRECURSOR SYNTHESIS OF MATERIALS: LixQy + MClx MQy + xLiCl

Q = N, P, As (PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si (CARBIDES, SILICIDES)

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

• 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

• 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

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

RAPID SS PRECURSOR SYNTHESIS OF MATERIALS: LixQy + MClx MQy + xLiCl

Q = N, P, As (PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si (CARBIDES, SILICIDES)

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

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 (C2

2-)!!!

• Inert, hard, refractory conducting ceramics

• Used for cutting tools, crucibles, catalysts, hard steel manufacture

• Sealed glass tube reactors

• 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

T2 T1

B(g)A(s) AB(g)A(s)

Glass tube

VAPOR PHASE TRANSPORT VPC MATERIALS SYNTHESIS, CRYSTAL GROWTH, PURIFICATION

B(g)A(s)

T2 T1

AB(g)A(s)

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

VAPOR PHASE TRANSPORT VPC MATERIALS SYNTHESIS, CRYSTAL GROWTH, PURIFICATION

B(g)A(s)

T2 T1

AB(g)A(s)

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

THERMODYNAMICS OF CVT

• A(s) + B(g) AB(g)

• Reversible equilibrium needed: Go = -RTlnKequ

• Consider case of exothermic reaction with - Go

• Thus Go = RTlnKequ

• Smaller T implies larger Kequ

• Forms at cooler end, decomposes at hotter end of reactor

• Consider case of endothermic reaction with +Go

• Thus Go = -RTlnKequ = RTln(1/Kequ)

• 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

• 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 > T1, provides concentration gradient and thermodynamic driving force for gaseous diffusion of vapor phase transport agent PtO2

PLATINUM HEATER ELEMENTS IN FURNACES

THEY MOVE!! Pt(s) + O2(g) PtO2(g)

VPT agent PtO2(g)

Atmosphere O2(g)

Pt(s) PtO2(g)

T2 T1

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

• 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)

• 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

VPT GROWTH OF MAGNETITE CRYSTALS FROM POWDERED MAGNETITE

VPT agent FeCl2/FeCl3(g)

Atmosphere HCl(g)

Fe3O4(s)

1270K 1020K

FERROMAGNETIC INVERTED SPINEL MAGNETITE B(AB)O4

Fe(III)(Td)[Fe(III)Fe(II)(Oh)]O4

Multidomain paramagnet above Tc

Multidomain ferromagnet below Tc

Single domain superparamagnet

Field H

M

H

Ms

Mr

Hc

• 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

VPT SYNTHESIS AND CRYSTAL GROWTH OF TiS2 FROM POWDERED Ti/S

VPT agent TiBr4(g)

Atmosphere Br2(g)

Ti/S(s)

550-685oC (T2) 510-645oC (T1)

TiS2

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

Li insertionLi insertion

• 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 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/ZnO(s)1060oC (T2) 980oC (T1)ZnWO4(s)

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

VPT GROWTH OF EPITAXIAL GaAs FILMS/CRYSTALS USING CONVENIENT STARTING MATERIALS

VPT agent GaCl/As4/H2(g) formed at hot end, atmosphere HCl(g)

(T2) (T1)GaAs(s)GaAs(s)

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