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Solid State Synthetic Methods
A. P. WilkinsonCHEM 6182, March 2001
Overview� High temperature direct reaction
� Limited by slow diffusion in solids� May be limited by phase diagram
� Control of oxidation state� CO/CO2 buffers, getering etc.
� Precursor methods for reducing path lengths� Stoichiometric precursors, sol-gel methods, co-precipitation
� Insertion, intercalation, ion exchange� Vapor phase transport� Fluxes� Electrochemical crystal growth� High pressure techniques� Thin film growth methods
� CVD, sputtering evaporation, sol-gel etc.� Large single crystal growth methods
The standard ceramic route
� The easiest way to make many solid state materials is direct reaction of their components at high temperatures
� PbO + TiO2 --(~900oC)---> PbTiO3
� Grind/mix powdered reactants, press into a pellet and heat
Problems with ‘heat and beat’
� Requires high temperature because reaction is diffusion limited– can be expensive– may give incomplete reaction– may give compositionally inhomogeneous products– there may be some loss of the reactants– there is little chance of getting kinetic control– may not get desired microstructure
Formation of MgAl2O4
� MgO + Al2O3 → MgAl2O4– Reaction only occurs at contact points between grains of MgO
and Al2O3
� Get nucleation near contact point and then growth of product– Growth requires diffusion of Mg2+ / Al3+ through the product – Very slow
Traditional solid state synthesis� Reaction is diffusion limited
– high temperatures to get diffusion» use absolute temperature that is > 2/3
of the MP of lowest melting reactant– high temperatures usually lead to
thermodynamically stable products
NiO Al2O3NiAl2O4
Ni2+
Al3+
NiO(s) + Al2O3(s) NiAl2O4(s) x = product layer thickness
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Nucleation� Nucleation of desired phase is a key step� For MgAl2O4 reaction, the reactants and products all
have structures based on close packed oxide– As the lattice constants of reactants and products are not
dissimilar you can get nucleation on surface of reactants� This leads to epitaxial growth – product orientation is
defined by substrate it is growing off
Self Propagating High Temperature Synthesis (SHS)
� Extreme exothermiticity of a reaction can be used to provide high temperatures needed for diffusion– Thermite Fe2O3 + Al → Al2O3 +Fe
� Has been used to make a number of useful materials including refractory ceramic parts that can be pressed and machined to final size– AlN+TiB2
– Si3N4 + SiC + TiN– Can produce functionally graded
materials. Have a composition and hence property gradient
Solid Flame propagation� Combustion front sweeps through powder mixture
Overcoming the diffusion barrier� Need intimate mixture of reactants� Can be obtained in several ways
– very small particle size reactants– find molecular precursor that has the needed elements in the
correct ratio» eg. Ba[TiO(C2O4)2] for BaTiO3
– Make a solution of needed metals and dry the solution out without demixing the components
» co-precipitate reactants in a solid solution salt� e.g. carbonates for Brownmillerite (Ca2Fe2O5)
» crystallize from gels prepared using sol-gel chemistry
Ferrites from stoichiometric precursors 1
�Can use series of oxalates MFe2(C2O4)3.6H2O precipitated from solution, M = Mn2+, Co2+, Ni2+, Zn2+
– MFe2(C2O4)3.6H2O + 2O2 → MFe2O4 + 6H2O + 6CO2
– Forms spinel at low temperature but does not give good control of stoichiometry because solubilities of metals are not the same
Ferrites from stoichiometric precursors 2
�Can use pyridinates M3Fe6(AcO)17O3OH.12Py precipitated from solution, M = Mg2+, Mn2+, Co2+, Ni2+
�Give ferrites with excellent control of stoichiometry
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Chromites from stoichiometric precursors
� Chromites MCr2O4 require very high temperature for direct preparation from oxides ( 1400 – 1700 °C)
� Decompose precursors such as – (NH4)2Mg(CrO4)2.6H2O– MnCr2O7.4Py– (NH4)2Mn(CrO4)2.2NH3
– NH4Fe(CrO4)2
� Get good control over stoichiomtery
What is sol-gel chemistry?
� Polymerize a solution of precursor molecules to form a sol or gel
Si(OEt)4(soln) + 2H2O ---->SiO2(gel) + 4EtOH
Steps in sol formation� Precursor initially undergoes hydrolysis
– Si(OEt)4 + H2O → Si(OEt)3OH + EtOH� Then get condensation
– Si(OEt)3OH + Si(OEt)3OH → (EtO)3SiOSi(OEt)3 + H2O– Or Si(OEt)3OH + Si(OEt)4 → (EtO)3SiOSi(OEt)3 + EtOH
� Both steps are often accelerated using an acid or base catalyst– Base catalysis leads to highly branched polymers, acid
catalysis does not» This effects properties of sol/gel
A PZT (Pb[Zr1-xTix]O3) gel
Sol-Gel PZT (Pb[Zr1-xTix]O3)�PZT is one of the best known perovksite
ferroelectrics– used in SPMs, sonar, memory devices, thermal
imaging.......�The sol-gel processing of PZT has been
extensively explored– thin film applications
�Typically, hydrolyze solution of Pb(OAc)2, Ti(iOPr)4, Zr(OBu)4 in 2-methoxyethanol
Sol-gel PbTiO3
� Mix Pb(OAc)2 and Ti(OR)4 in dry alcohol
� Add water– solution starts to become viscous and eventually
becomes an elastic gel
� This gel is a polymer with a - O -Ti(X2) - O -backbone and the lead is either incorporated into the polymer or trapped in the polymer network
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PbTiO3 sol-gel continued
� Take the wet gel (alcogel) and dry it– drying in air causes a collapse of the gel to give a
xerogel– drying using supercritical solvents can give aerogels
� Heat dry gel in air to burn off organics– usually gives amorphous (glassy) PbTiO3
– anneal the glass to form a crystalline product– crystallizes at low temperatures (<500oC for PT)
Advantages sol-gel chemistry
� Cheap way to make thin films and fibres compared to CVD but can be expensive relative to heat and beat
� Molecular mixing of precursors prior to hydrolysis helps get good homogeneity
� Can make high purity materials� Low firing temperatures
– metastable phases– reduced loss of volatile components
Compositional homogeneity� May not get perfectly homogeneous products even with sol-gel
chemistry
Alkoxide A
Alkoxide B
A hydrolyzes faster than B
A hydrolyzes at the same rate as B
Different hydrolysis rates can lead to compositional segregationAlso some components in solution may not be incorporated into polymer backbone
Nonhydrolytic sol-gel syntheses�The reactions forming polymers by the addition
of water to an alkoxide are hydrolytic process.�Can form similar polymer but get bridging
oxygen from ether or alkoxide– As there is no water this type of process is referred
to as nonhydrolytic» Not widely used but supposedly capable of forming very
homogeneous gels
Low temperature synthesis of ZrW2O8
� Decomposition into ZrO2 and WO3 occurs above 1050K– need method that gives atomic scale mixing of
zirconium and tungsten» decomposition of soluble precursors?
� used by Sleight, still needs high temperatures» alkoxide sol-gel?» non-hydrolytic sol-gel?» other route?
ZrW2O8 by non-hydrolytic sol-gel methods
� Non-hydrolytic routes are “advertised” as providing good compositional homogeneity– is this true?
� ZrX4 + 2WX6 + 8R2O --> “ZrW2O8” + 16RX– R should not be a primary alkyl group
� Zr(OR’)4 + 2WX6 + 4R2O -> “ZrW2O8” + 8RX + 4R’X ?
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Synthetic conditions� Use CHCl3 or CH2Cl2 as a solvent� Use iPr2O as an oxygen donor� Solubility of metal halides and alkoxides
can be a problem– use THF to enhance solubility of zirconium
alkoxide – CHCl3 gives better solubility of WCl6– Heat at 110oC for several days
Precipitate formation� Use of WCl6 always gave
a blue precipitate rather than a gel
Gel-formation� WCl6 does not give a gel� WBr5 gives a gel
Crystallization of “ZrW2O8” gels
750oC
600oC500oC
WCl6, Zr(iOPr)4, iPr2O, THF, CHCl3
Cubic
“Hexagonal” ZrW2O8
Cheap variations on sol-gel
� Aqueous solutions of metal salts can sometimes be made into a gel by the use of a complexing agent (say citrate) and a solvent like ethyleneglycol− This “Puchini” process is very commonly used
− the citrate and ethylene glycol not only bind the metals they form a polyester polymer
− Can be used to form films like alkoxide sol-gel method
− Metal salts like nitrates are much cheaper than alkoxides
Soft chemistry (Chimie Douce)
� Many solid state synthesis techniques rely on high temperatures and produce thermodynamic products
� Synthesis that use low temperatures to producemetastable (kinetic) products are often referred to as examples of ‘soft chemistry’.� Dehydration, Ion exchange, insertion, deintercalation etc.
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The synthesis of TiO2(B)
� 2KNO3 + 4TiO2 -(900oC)---> K2Ti4O9
� K2Ti4O9 + HNO3(aq) -(RT)--> H2Ti4O9.H2O
– this is an ion exchange process
� H2Ti4O9.H2O ---(500oC)---> TiO2(B) + 2H2O
– a topotactic dehydration
Hexagonal WO3� WO3 crystallizes at high T with a ReO3 structure� WO3•0.5H2O can be prepared by hydrothermal treatment of
tungstic acid (WO3•2H2O ) – from acidification of Na2WO4 soln.� Dehyration of the hemihydrate leads to an orthorhombic WO3 that
then transforms to hexagonal WO3
Pyrochlore WO3
� Obtain [(NH4)2O]xW2O6, x = 0.5 by hydrothermal treatment of (NH4)10W12O41•5H2O in acidic ethylene glycol
� Produce W2O6•xH2O by ion exchange of ammonium in acidic solution
� Dehydrate at 100 °C to get pyrochlore WO3
Ion exchange
� Many solid state materials can be ion exchanged under moderate conditions
– zeolites and α zirconium phosphates can be ion exchanged in water
– β aluminas and NaZr2(PO4)3 type materials can be ion exchanged in molten salts
� NaAl11O17 + AgNO3 --(300oC) --> AgAl11O17 + NaNO3
Insertion reactions
� Many inorganic solids have cavities can that can be subsequently filled by other ions. This process is referred to as insertion.
� Take WO3, coat with a little H2PtCl6 solution and then heat the material. This gives WO3 with Pt metal particles on the surface
� WO3(Pt) + H2 -----> HxWO3
– this is an example of hydrogen spillover
Intercalation� It is possible to modify many layered materials by
introducing species into the interlayer space� C + K → C8K� TiS2 + nBuLi → LixTiS2� ZrS2 + CoCp2 –(120ºC/tol)� (CoCp2)xZrS2
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Intercalation into graphite�Graphite will react with alkali metal and
halogens to form quite well defined compounds by intercalation
C8KGraphite
The intercalation of Li into TiS2
Intercalation of alkali metals into TaS2
� TaS2 + xNa → NaxTaS2 x = 0.4 – 0.7
The structure of TaS2
The electronic structure of TaS2Staging in intercalation compounds
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Vapor Phase Chemical Transport� Can overcome diffusion limitations on reaction by getting
one or preferably both reactants into the gas phase– Set up equilibrium between solid products/reactants and gas
phase species
Transport agent B reactswith solid A to form a gasthat decomposes in a cooler
or hotter region
For exothermic reactions deposit A at hotter end
For endothermic reactions deposit A at cooler end
Examples of Vapor Phase Transport� 2-MnP2 two layer stacking varient of MnP2
– Mn + 2P -(1at% I2, sealed tube)� MnP2» Hot end of tube at ~ 800K cool end 80 K lower, MnP2 formed at cooler end
of tube» Both components are transportable
� CaSn2O4– 2CaO + SnO2 –(900 °C traces CO or H2)-> CaSn2O4
» Reaction accelerated because SnO is volatile
� NiCr2O4– NiO + C2O3 –(1100 °C traces O2)-> NiCr2O4
» Reaction accelerated by the formation of volatile CrO3
� Nb5Si3– 11Nb + 3SiO2 –(1000 °C, trace H2)�Nb5Si3 + 6NbO
» Reaction does not occur in absence of H2, volatile SiO is formed
Examples of Vapor Phase Transport� If the reaction product is transportable get faster
reaction than if only one reactant is transportable– Also can easily get single crystals
� Al + S ---( traces I2 sealed SiO2 tube)-> Al2S3– Very slow reaction in absence of I2 due to formation on
Al2S3 layer on surface of Al» With I2 get rapid reaction due to formation of volatile S2(g) and AlI3(g)
» Large crystals are formed
Crystallization from melts
� Several different approaches − For many conducting reduced phases it is possible to
electrolyze a melt and form crystals of the desired reduced product on the electrode
− MOLTEN SALT ELECTROLYSIS− dissolve the compound of interest or appropriate
reactants in a molten salt and try to recrystallize the compound from the molten salt (flux)
− FLUX GROWTH− melt the composition of interest and cool it so that it
crystallizes
Phase diagram can give problems� Crystal growth of an
incongruently melting phase can be very difficult– Can not just go straight
from a stoichiometric melt» May need to use flux or
other method
� A4B can be grown directly from stoichiometric melt AB3can not
Advantages of Molten Salt Electrolysis� Can produce single crystals� Electrochemical control of growth� Isothermal process� Solutions may be purified by coulometry� Reproducible� Small changes in temperature do not normally
influence growth rate� Relatively short reaction times� Apparatus simple and inexpensive
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Disadvantages of Molten Salt Electrolysis
� Red-Ox potential may be unfavorable� Low current efficiencies� Difficulty of finding a suitable solvent restricts
use� Container and electrode attack� Products often sensitive to melt composition;
changes with time during electrolysis� High nucleation rates lead to twinning, inclusions,
agglomerations etc.
Oxides made by MSE� Metallic oxides
- NaxWO3, x = 0.85 – 0.3
� Quasi-low dimensional materials- A0.9Mo6O17 ( A = Li, K, Rb, Tl)- K0.30MoO3- La2Mo2O7- La5Mo4O16 (ferromagnetic semiconductor)
� Superconducting materials- YBa2Cu3O7 ( Eutectic, stab. Zirconia anode)- (K,Ba)BiO3 (NaOH-KOH saturated with water vapor)- LiTi2O4
Mo oxides with M-M bonds by MSE� Isolated or Quasi-isolated clusters
- La2Mo2O7 Digonal- La5Mo4O16 Digonal- ZnMo3O8 Triangular- Y4Mo18O32 Rhomboidal Mo4- LaMo2O5 Mo6 octahedra and Mo3 cluster- LaMo8O14 Bi-faced capped octahedron- SrMo5O8 Edge-shared bi-octahedron
� Compounds with extended metal-metal bonds- KMo4O6- NaxMo2O4- SrMo5O8
1 cm La2Mo2O7 crystal
CMR materials by MSE� La0.94Mn0.98O3 (Tc = 240 K)� Na0.12La0.86MnO3 (Tc = 310 K)� Sr0.34La0.66MnO3 (Tc = 370 K)� Sr0.12La0.84Mn0.99O3 (Tc = 325 K)
La0.94Mn0.98O3
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Electrocrystallization of Ba1-xKxBiO3(BKBO)
� The high Tc superconductor Bi0.6K0.4BiO3 can be crystallized by electrolyzing a melt of composition KOH:Ba(OH)2.8H2O:Bi2O352.4:0.54:1.00 under a water saturated Aratmosphere
� Melt can be used at 180oC. Bi2O3 never fully dissolves. Platinum electrodes are used and crystals grow on the anode.
Electrocrystallization of tungsten bronzes (NaxWO3)
� Melts of Na2WO4 and WO3 (750oC) can be electrolyzed to give large crystals of NaxWO3
� x can take values between 0.32 and 0.93
� Compounds with high values of x have a red-orange coloration (look similar to metallic copper) and are good electrical conductors
Flux growth
� Oxides like Bi2O3 and PbO have low melting points and may be used as solvents– PbTiO3 can be crystallized from PbO/PbF2
mixtures– need to pick a flux that is compatible with the
desired product� Alkali and alkaline earth metal hydroxides
and halides are also frequently used as fluxes
The PbO/PbF2 phase diagram
The NaOH / KOH pseudobinaryphase diagram Growing from wet KOH/NaOH
� Many superconducting copper oxides have been grown by this method (Stacy et al.)
� Synthesis of EuBa2Cu3O7-δ– take stoichiometric amounts of CuO, Eu2O3 and
Ba(OH)2.8H2O and dissolve in molten KOH/NaOH at 450oC
– gives clear blue solution– solution held at 450oC under flowing dry air– as water is lost the product crystallizes out
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Reactive halide fluxes� Growth from halide fluxes in the presence of trace
amounts of water can lead to interesting products– BaCl2 + 6Fe2O3 + H2O → BFe12O19 + 2HCl
» Grow large single xtals
� From CaCl2 flux grew CaFeO4, Ca3Al10O18, CaCrO4, CaSiO4 by adding Fe2O3, Al2O3, Cr2O3 and SiO2 to flux
� From BaCl2 flux grew BaFe12O19, BaWO4, BaSi2O5, BaPbO3, BaTi3O7 by addition of metal oxide to flux
Molten metal fluxes� Molten metals can sometimes be used as solvents.
However, metal should not form stable compounds with reactants– Mn + 2Si –(Cu liquid/1200 °C)-> MnSi2
» Heat materials in sealed ampul. Using a Cu solvent avoids Mn loss due to heating at high temps
– Ru + 2P –(Sn liquid)-> RuP2» 1:2:100 ratio of Ru, P and Sn sealed evacuated quartz tube. Heated
to 1200 °C and then slow cooled. Crystals recovered from Sn by washing with HCl
� Kaner et al., Mater. Res. Bull. 12, 1143 (1977)
Large single crystals from a melt�The preparation of large high quality single
crystals is a crucial stage in the manufacture of many technologically important devices– Silicon is grown for the semiconductor industry– LiNbO3 is melt grown for telecommunications
applications– YAG (Y3Ga5O12) is grown for laser applications
Single crystal silicon
The Czochralski Method The crystal growth procedure�Seed is brought into
contact with melt and pulled out slowly while rotating
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The LiNbO3 phase diagram Commercial LiNbO3 crystals
Gadolinium Gallium Garnet�Used as substrate for growth of devices
Stockbarger method�Move the crucible containing a seed and the
melt through a temperature gradient so that the melt crystallizes onto the seed crystal
Bridgman method�Adjust furnace so that temperature gradient
varies with time and the melt grows of a crystal in the crucible
Zone melting�Sweep a molten zone through the crucible in
such a way that the melt crystallize onto a seed– Method used for purifying existing crystals as
impurities tend to stay with the liquid
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Floating zone growth of silicon�Move polycrystalline ingot into hot zone a
seed formation of single crystal
Verneuil method
�Used for growing large crystals of high melting point solids– for example, Ruby Cr2O3
doped Al2O3
High pressure methods� Three types of high pressure reaction
– Hydro or solvo thermal. Solids are heated in a liquid medium and the reaction takes place by virtue of reactants dissolving in solvent at temperature / pressure
– High pressure solid / gas reactions. May arrange things so that solid is exposed to very high O2 or F2 pressures
» Facilitates prep of high oxidation state metals
– Solid – solid reactions involving compression of reactants in flexible/compressible container
» Favors formation of dense high coordination number phases
Piston Cylinder Press� Can achieve 50 kbar and 1800K
– Sample is placed in container (Pt, Au ..) and the container is embedded in a pyropholite block
» Pyropholite acts as a pressure transmitting medium– Squeeze sample by forcing WC piston into WC cylinder
Multianvil Press� Can achieve ~200 kbar and over 2000K
– Sample volume may be low for some designs
Belt design� Can achieve 150 kbar, 2300K
– Relatively large sample volume– Sample in Au/Pr container or for chalocogenides BN or MgO
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Synthetic diamond� Prepared as a powder on a vast scale for cutting
tools and other applications– Requires high pressure to make bulk diamond
The production of diamonds
Large synthetic diamonds Why use high pressures?� High pressure allows the preparation of new
compositions, new structures, unusual oxidation states– PbSnO3 does not form as a perovskite at ambient pressure,
but will at high pressure– CaFeO3 can only be prepared at high P. At ambient P
Brownmillerite (CaFeO2.5) forms– Superconducting oxygen excess La2CuO4+δ can be prepared
at high oxygen P– La2Pd2O7 can be prepared at high oxygen P. Normally only
get Pd2+ oxides» Oxygen P can be generated in-situ by decomposition of say KClO3.
However, beware! KCl may be incorporated into the product
Hydrothermal growth of quartz crystals� Large quartz crystals are needed as oscillators for
timing applications– Are grown from basic aqueous solution at high P/T due to
improved solubility of SiO2
Hydrothermally grown quartz
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Hydrothermal reactor designs� Depending on design may be useable to 10 kbar
Applicability and value of hydrothermal synthesis� Hydrothermal techniques can be used to
synthesize a wide variety of materials– zeolites and aluminophosphates
– optical materials like KTP (KTiOPO4)
– BaTiO3 (widely used ferroelectric)
� Synthesis can be carried out at low temperature (relative to direct reaction of solids)
� High quality samples can be made
Templated growth from solution
� Many hydrothermal synthesis of materials such as zeolites or mesoporous materials use ‘templates’ as part of the synthesis
� A molecule or ion is added to the synthesis mix to direct the formation of the solid– the solid grows around the template
Self assembled templates� In zeolitic materials the template is a single
molecule or ion� Self assembled aggregates of molecules or ions
can also serve as templates– Surfactants aggregate into a variety of structures
depending on conditions
Self assembled surfactant structures� In solution surfactants can self assemble to form
micelles, rods, sheets and 3D structures– All of these can in principle be used as templates– Rod like surfactant aggregates have provided some of
the most interesting structures� A lot of work has been done exploring the
formation of silicate structures using self assembled templates– Other inorganic oxides have also been examined
Materials containing rod like assemblies
Material is highly ordered and gives a diffraction pattern but thesilicate walls are not crystalline, they are glass like
MCM-41 silicate structures
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A typical MCM-41 synthesis� MCM-41 contains rod like surfactant aggregates. � MCM-41 materials can be made in many ways.� Aluminosilicate materials have been prepared as
follows� C16H33NMe3OH, catapal alumina, TMA silicate and
amorphous silica stirred in water� Heated in autoclave for 48 hours at 150 º C� Recover by filtration and remove template by heating in
nitrogen to 540 ºC for 1 hour and then heating in air for 6 hours� Surfactant decomposes by Hoffman elimination reaction
Pore size distribution� The pore size distribution in MCM-41 is usually
quite narrow as well ordered materials can be made, but it is not as tightly defined as that for a zeolite as MCM-41 is not a crystalline product
Growth of thin films� Can be done in many different ways
– Electrochemical deposition– Coat substrate with sol and heat– Decompose organometallic precursor over substrate– Flame pyrolysis– Evaporate material onto substrate– Sputter material of target onto substrate– Laser ablation – use laser to blow a plume of material of a
target and allow plume to impinge on substrate
Sputtering
�Produce ions from gas and accelerate ions into target so that they knock material off the surface of the target
Evaporation�Heat material so that it evaporates onto
substrate
Thin film CVD diamond�Can deposit diamond film from hydrogen rich
flame or arc containing C2H2 or CH4.– Conditions are far from equilibrium but growth is
kinetically favored– Can monitor film quality by Raman spectroscopy
» Often contains some sp2 carbon and may not be nice cubic diamond, but diamond like carbon
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Diamond growth�Hydrogen atoms in flame or arc remove H from
growing surface and provide clean C sites for further reaction with carbon containing species
CVD GaAs�Can make materials like GaAs in many
different ways– Flow mix of GaMe3 and AsH3 over heated
substrate– Flow over heated substrate
– Using single source precursor can get around some problems such as control of stoichiometry and prereaction leading to snow formation in vapor phase
GaAsGaAstBu
tBu
tBu
tBuMe Me
MeMe
CVD amorphous Si�Doped thin film silicon can be very useful
but simple CVD process produces amorphous material that can not be doped– Dangling bonds act as traps for doped in
electrons or holes– Preparation in presence of hydrogen ties up all
the dangling bonds and allows good control of doping
Band structure of amorphous Si
�Hydrogenation ties up electrons in orbitals between the bands