ASPECTS OF SOLID-SOLID REACTIONS
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Transcript of ASPECTS OF SOLID-SOLID REACTIONS
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ASPECTS OF SOLID-SOLID REACTIONS
• Conventional solid state synthesis - heating mixtures of two or more solids to form a solid phase product.
• Unlike gas phase and solution reactions
• Limiting factor in solid-solid reactions usually diffusion, driven thermodynamically by a concentration gradient.
• Fick’s law : J = -D(dc/dx)
• J = Flux of diffusing species (#/cm2s)
• D = Diffusion coefficient (cm2/s)
• (dc/dx) = Concentration Gradient (#/cm4)
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ASPECTS OF SOLID-SOLID REACTIONS
• The average distance a diffusing species will travel <x>
• <x> (2Dt)1/2 where t is the time.
• To obtain good rates of reaction you typically need the diffusion coefficient D to be larger than ~ 10-12 cm2/s.
• D = Doexp(-Ea/RT) diffusion coefficient increases with temperature, rapidly as you approach the melting point.
• This concept leads to empirical Tamman’s Rule : Extensive reaction will not occur until temperature reaches at least 1/3 of the melting point of one or more of the reactants.
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RATES OF REACTIONS IN SOLID STATE SYNTHESIS ARE CONTROLLED BY THREE MAIN FACTORS
1. Contact area: surface area of reacting solids
2. Rates of diffusion: of ions through various phases, reactants and products
3. Rate of nucleation: of product phase
Let us examine each of the above in turnLet us examine each of the above in turn
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SURFACE AREA OF PRECURSORS
• Seems trivial - vital consideration in solid state synthesis
• Consider MgO, 1 cm3 cubes, density 3.5 gcm-3
• 1 cm cubes: SA 6x10-4 m2/g • 10-3 cm cubes: SA 6x10-1 m2/g (109x6x10-6/104)• 10-6 cm cubes: SA 6x102 m2/g (1018x6x10-12/104)
• The latter is equal to a 100 meter running track!!!
• Clearly reaction rate influenced by SA of precursors as contact area depends roughly on SA of the particles
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EXTRA CONSIDERATIONS IN SOLID STATE SYNTHESIS – GETTING PRECURSORS TOGETHER
• High pressure squeezing of reactive powders into pellets, for instance using 105 psi to reduce inter-grain porosity and enhance contact area between precursor grains
• 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
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EXTRA CONSIDERATIONS IN SOLID STATE SYNTHESIS
• x(thickness planar layer) 1/A(contact area)
• A(contact area) 1/d(particle size)
• Thus particle sizes and surface area connected
• Hence x d
• Therefore A and d affect interfacial thickness x!!!
• These relations suggest some strategies for rate enhancement in direct solid state reactions by controlling diffusion lengths!!! controlling diffusion lengths!!!
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MINIMIZING DIFFUSION LENGTHS <x> (2Dt)1/2 FOR RAPID AND COMPLETE DIRECT REACTION
BETWEEN SOLID STATE MATERIALS AT LOWEST T
Particle surface area A
Product interface thickness x
Particle size d
dx/dt = k/x = k’A =k"/d
Decreasing particle size to nanocrystalline range Hot pressing densification of particles Atomic mixing in composite precursor compounds Coated particle mixed component reagents, corona/core precursors
All aimed to increase A and decrease x and minimize diffusion length scale
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MINIMIZING DIFFUSION LENGTHS <x> (2Dt)1/2 FOR RAPID AND COMPLETE DIRECT REACTION
BETWEEN SOLID STATE MATERIALS AT LOWEST T
All aimed to increase A and decrease x and minimize diffusion length scale
Core-corona reactants in intimate contact, made by salt precipitation, sol-gel deposition, CVD
COATED PARTICLE SOLID STATE REAGENTS
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MINIMIZING DIFFUSION LENGTHS <x> (2Dt)1/2 FOR RAPID AND COMPLETE DIRECT REACTION
BETWEEN SOLID STATE MATERIALS AT LOWEST T
•Johnson superlattice precursor •Deposition of thin film reactants•Controlled thickness, composition•Metals, semiconductors, oxides•Binary, ternary compounds•Modulated structures•Solid solutions (statistical reagent mixing)•Diffusion length x control •Thickness control of reaction rate•Low T solid state reaction•Designer element precursor layers•Coherent directed product nucleation•Oriented product crystal growth•LT metastable hetero-structures•HT thermodynamic product
SUPERLATTICE SUPERLATTICE REAGENTSREAGENTS
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ELEMENTALLY MODULATED
SUPERLATTICES -DEPOSITED AND
THERMALLY POST TREATED TO GIVE LAYERED METAL
DICHALCOGENIDES MX2
COMPUTER MODELLING OF
SOLID STATE REACTION OF
JOHNSON SUPERLATTICE
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• Several important synthetic parameters and in situ probes.
• Reactants prepared using thin film deposition techniques and consist of nm scale layers of the elements to be reacted.
• One element easily substituted for another
• Allows rapid surveys over a class of related reactions and synthesis of iso-structural compounds.
ELEMENTALLY MODULATED ELEMENTALLY MODULATED SUPERLATTICESSUPERLATTICES
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ELEMENTALLY MODULATED ELEMENTALLY MODULATED SUPERLATTICESSUPERLATTICES
• Diffusion distance is determined by the multilayer repeat distance which can be continuously varied.
• An important advantage, allowing experimental demonstration of changes in reaction mechanism as a function of inter-diffusion distance and temperature.
• Multi-layer repeat distances can be easily verified in the prepared reactants and products made under different conditions using low angle X-ray diffraction.
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MINIMIZING DIFFUSION LENGTHS <x> (2Dt)1/2 FOR RAPID AND COMPLETE DIRECT REACTION
BETWEEN SOLID STATE MATERIALS AT LOWEST T
Johnson superlattice reagent design
{(Ti-2Se)6(Nb-2Se)6}n
Low T annealing reaction
{(TiSe2)6(NbSe2)6}n
Metastable ternary modulated layered metal dichalcogenide (hcp Se2- layers, Ti4+/Nb4+ Oh/D3h interlayer sites) superlattice well defined PXRD
Confirms correlation between precursor heterostructure sequence and superlattice ordering of final product
SUPERLATTICE REAGENTS YIELD SUPERLATTICE ARTIFICIAL CRYSTAL PRODUCT
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Superlattice precursor sequence 6(Ti-2Se)-6(Nb-2Se) yields ternary modulated superlattice composition {(TiSe 2)6(NbSe 2)6}n with 62 well defined PXRD reflections – good exercise – give it a try
Confirms correlation between precursor heterostructure sequence and superlattice ordering of final product
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MINIMIZING DIFFUSION LENGTHS <x> (2Dt)1/2 FOR RAPID AND COMPLETE DIRECT REACTION
BETWEEN SOLID STATE MATERIALS AT LOWEST T
John superlattice reagent design
{(Ti-2Se)6(Nb-2Se)6}n
High T annealing reaction
{(Ti0.5Nb0.5Se2)}n
Thermodynamic linear Vegard type solid solution ternary metal dichalcogenide product with identical layers
Properties of ternary product is the atomic fraction weighted average of binary end member components
P(TixNb(1-x)Se2) = xP(TiSe2) + (1-x)PNbSe2
SUPERLATTICE REAGENTS YIELD HOMOGENEOUS SOLID SOLUTION PRODUCT
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NANOCLUSTER PRECURSOR BASED KIRKENDALL SYNTHESIS OF HOLLOW NANOCLUSTERS
V[Co]
Co(2+)
S(2-)
e(-)
Co
S
Co2S3
• Synthesis of surfactant-capped cobalt nanoclusters: • Co(3+)/BH4(-) reduction in oleic acid, oleylamine ConLm in a high temperature surfactant solvent, • surfactant-sulfur injection, coating of sulfur shell on nanocluster• cobalt sesquisulfide product shell layer formed at interface• counter-diffusion of Co(2+)/2e(-) and S(2-) across thickening shell• faster diffusion of Co(2+) than S(2-) creates V[Co] in core• vacancies agglomerate in core• hollow core created which grows as the product shell thickens • end result – a hollow nanosphere made of cobalt sesquisulfide Co2S3
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TURNING NANOSTRUCTURES INSIDE-OUT
• Kirkendall effect a well-known phenomenon discovered in 1930’s.
• Occurs during reaction of two solid-state materials and involves the counter diffusion of reactant species, like ions, across product interface usually at different rates.
• Special case of movement of fast-diffusing component cannot be balanced by movement of slow component the net mass flow is accompanied by a net flow of atomic vacancies in the opposite direction.
• Leads to Kirkendall porosity, formed through super-saturation of vacancies into hollow pores
• When starting with perfect building blocks such as monodisperse cobalt nanocrystals a reaction meeting the Kirkendall criteria can lead to super-saturation of vacancies exclusively in the center of the nanocrystal.
• General route to hollow nanocrystals of almost any given material
• Proof-of-concept - synthesis of Co2S3 nanoshell starting from Co nanocluster.
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Time evolution of a hollow Co2S3 nanocrystal grown from a Co nanocrystal via the nanoscale Kirkendall effect
Science 2004, 304, 711
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NANOSCALE PATTERNING OF SHAKE-AND-BAKE SOLID-STATE CHEMISTRY
Younan Xia
MINIMIZING DIFFUSION LENGTHS <x> (2Dt)1/2 FOR RAPID AND COMPLETE
DIRECT REACTION BETWEEN SOLID STATE MATERIALS AT LOWEST T
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PDMS MASTER Whitesides
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PDMS MASTER
• Schematic illustration of the procedure for casting PDMS replicas from a master having relief structures on its surface.
• The master is silanized and made hydrophobic by exposure to CF3(CF2)6(CH2)2SiCl3 vapor
• SiCl bind to surface OH groups and anchor perfluoroalkylsilane to surface of silicon master CF3(CF2)6(CH2)2SiO3 for easy removal of PDMS mold
• Each master can be used to fabricate more than 50 PDMS replicas.
• Representative ranges of values for h, d, and l are 0.2 - 20, 0.5 - 200, and 0.5 - 200 mm respectively.
Whitesides
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NANOSCALE PATTERNING OF SHAKE-AND-BAKE SOLID-STATE CHEMISTRY
Younan Xia
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NANOSCALE PATTERNING OF SHAKE-AND-BAKE SOLID-STATE CHEMISTRY
(A) Optical micrograph (dark field) of an ordered 2-D array of nanoparticles of Co(NO3)2 that was fabricated on a Si/SiO2 substrate by selective de-wetting from a 0.01 M nitrate solution in 2-propanol. The surface was patterned with an array of hydrophilic Si-SiO2 grids of 5 x 5 m2 in area and separated by 5 m.
(B) An SEM image of the patterned array shown in (A), after the nitrate had been decomposed into Co3O4 by heating the sample in air at 600 °C for 3 h. These Co3O4 particles have a hemispherical shape (see the inset for an oblique view).
(C) An AFM image (tapping mode) of the 2-D array shown in (B), after it had been heated in a flow of hydrogen gas at 400 °C for 2 h. These Co particles were on average 460 nm in lateral dimensions and 230 nm in height.
Co(NO3)2
Co3O4
Co
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NANOSCALE PATTERNING OF SHAKE-AND-BAKE SOLID-STATE CHEMISTRY
AFM image of an ordered 2-D array of (A) MgFe2O4 and (B) NiFe2O4 that was fabricated on the surface of a Si/SiO2 substrate by selective de-wetting from the 2-propanol solution (0.02 M) that contained a mixture of two nitrates [e.g. 1:2 between Mg(NO3)2 and Fe(NO3)3].
The PDMS stamp contained an array of parallel lines that were 2 mm in width and separated by 2 mm. Twice stamped orthogonally. Citric acid HOC(CH2CO2H)3 forms mixed Mg(II)/Fe(III) complex - added to reduce the reaction temperature between these two nitrate solids in forming the ferrite.
Ferrite nanoparticles ~300 nm in lateral dimensions and ~100 nm in height.
MgFe2O4
NiFe2O4
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NUCLEATION OF PRODUCT PHASE
MgO ccp O(2-) with Mg(2+) diffusion
Al2O3 hcp O(2-) with Al(3+) diffusion
Nucleation at interface favored by structural similarity of reagents and products
Minimal structural reorganizationPromotes nucleation and growthBoth MgO and MgAl2O4 similar O(2-) ccpSpinel lattice nuclei matches MgO at interfaceEssentially continuous across reaction interface
STRUCTURAL SIMILARITY OF REACTANTS AND PRODUCTS PROMOTES NUCLEATION RATE AND RATE OF PRODUCT GROWTH
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SURFACE STRUCTURE AND REACTIVITY EFFECTS ON DIRECT REACTION OF SOLIDS
O2- O2-
O2- O2-
O2- O2-
O2- O2-
O2- O2-
O2-
O2-
Mg 2+ O2- Mg 2+ O2-
Mg 2+O2- Mg 2+ O2-
Mg 2+ O2- Mg 2+ O2-
Mg 2+ Mg 2+
Mg 2+ Mg 2+
Mg 2+ Mg 2+
Mg 2+ Mg 2+
Mg 2+ Mg 2+
Mg 2+
Mg 2+
{100} face {111} face {111} face
Nucleation depends on surface structure of reacting phases - crystal faces in contact - MgO rock salt - different Miller index faces exposed - ion arrangements in crystal face different - distinct crystal habits (octahedral, cubooctahedral, cubic) possible depending on growth conditions and additives - {100} alternating Mg(2+) and O(2-) at corners of square grid - {111} Mg(2+) or O(2-) in hexagonal arrangement - implies different surface structures, energies and reactivities
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DIRECT REACTION OF SOLIDS - “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 for well over the past 50 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 trial-and-error methods to the new chemistry and a rational and systematic approach to the synthesis of materials
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THINKING ABOUT REAGENTS
• Drying reagents MgO/Al2O3 200-800°C, maximum SA
• In situ decomposition of precursors at 600-800°C MgCO3/Al(OH)3 MgO/Al2O3
• Intimate mixing of precursor reagents
• Homogenization of reactants using organic solvents, grinding, ball milling, ultra-sonification
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THINKING ABOUT CONTAINER MATERIALS
• Chemically inert crucibles, boats
• Noble metals Nb, Ta, Au, Pt, Ni, Rh, Ir
• Refractories, alumina, zirconia, silica, boron nitride, graphite
• Reactivity with containers at high temperatures needs to be carefully evaluated for each system
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THINKING ABOUT SOLID STATE SYNTHESIS HEATING PROGRAM
• Furnaces, RF, microwave, lasers, ion and electron beams
• Prior reactions and frequent cooling, grinding and regrinding, boost SA of reacting grains
• Overcoming sintering, grain growth, brings up SA, fresh surfaces, enhanced contact area
• Pellet and hot press reagents – densification and porosity reduction, higher surface contact area, enhances rate, extent of reaction
• Care with unwanted preferential component volatilization if T too high, composition dependent
• Need INERT atmosphere for unstable oxidation states
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PRECURSOR SOLID STATE SYNTHESIS METHOD
• Co-precipitation, high degree of homogenization, high reaction rate - applicable to nitrates, acetates, citrates, carboxylates, oxalates, alkoxides, -diketonates, glycolates
• Concept: precursors to magnetic Spinels - recording media
• Zn(CO2)2/Fe2[(CO2)2]3/H2O 1 : 1 solution phase mixing
• H2O evaporation, salts co-precipitated – solid solution mixing on atomic/molecular scale, filter, calcine in air
• Zn(CO2)2 + Fe2[(CO2)2]3 ZnFe2O4 + 4CO + 4CO2
• High degree of homogenization, smaller diffusion lengths, fast rate at lower reaction temperature
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PROBLEMS WITH CO-PRECIPITATION METHOD
• Co-precipitation requires:
• Similar salt solubilities
• Similar precipitation rates
• Avoid super-saturation as poor control of co-precipitation
• Useful for synthesizing Spinels, Perovskites
• Disadvantage: often difficult to prepare high purity, accurate stoichiometric phases
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DOUBLE SALT PRECURSORS
• Known stoichiometry double salts having controlled stoichiometry
• Ni3Fe6(CH3CO2)17O3(OH).12Py
• Basic double acetate pyridinate
• Burn off organics at 200-300oC, then calcine at 1000oC in air for 2-3 days
• Product highly crystalline phase pure NiFe2O4 spinel
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Good way to make chromite Spinels, important tunable magnetic materials - juggling electronic-magnetic properties of the A Oh and B Td ions in the Spinel lattice
• Chromite Spinel Precursor compound 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
DOUBLE SALT PRECURSORS
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PEROVSKITE FERROELECTRICS BARIUM TITANATE
• Control of grain size determines ferroelectric properties, important for capacitors, microelectronics
• Direct heating of solid state precursors is of limited value in this respect – lack of size and morphology controllack of size and morphology control
• BaCO3(s) + TiO2(s) BaTiO3(s)
• Sol-gel reagents useful to create single source barium titanate precursor with correct stoichiometry
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SINGLE SOURCE PRECURSOR SYNTHESIS OF BARIUM TITANATE - FERROELECTRIC MATERIAL
• Ti(OBu)4(aq) + 4H2O Ti(OH)4(s) + 4BuOH(aq)
• Ti(OH)4(s) + C2O42-(aq) TiO(C2O4)(aq) + 2OH-(aq) + H2O
• Ba2+(aq) + C2O42-(aq) + TiO(C2O4)(aq) Ba[TiO(C2O4)2](s)
• Precipitate contains barium and titanium in correct ratio and at 920Precipitate contains barium and titanium in correct ratio and at 920 C C decomposes to barium titanate according to:decomposes to barium titanate according to:
• Ba[TiO(C2O4)2](s) BaTiO3(s) + 2CO(g) + 2CO2(g)
• Grain size important for control of ferroelectric properties
• Used to grow single crystals hydrothermally
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BASICS: FERROELECTRIC BARIUM TITANATE
CubicCubic perovskite equivalent O-Ti-O bonds in BaTiO3
TetragonalTetragonal perovskite long-short axial O-Ti—O bonds in BaTiO3
Multidomain paraelectric above Tc Cooperative electric dipole interactions within each domain – aligned in domain but random between
Multidomain ferroelectric dipoles align in E field and/or below Tc
Single domain superparaelectric
Small grains, tetragonal to cubic surface gradients, ferroelectricity particle size dependent
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SOL-GEL SINGLE SOURCE PRECURSORS TO LITHIUM NIOBATE - NLO MATERIAL
• LiOEt + EtOH + Nb(OEt)5 LiNb(OEt)6 LiNbO3
• LiNb(OEt)6 + H2O LiNb(OEt)n(OH)6-n gel
• LiNb(OEt)n(OH)6-n + + O2 LiNbO3
• Lithium niobate, ferroelectric Perovskite, nonlinear optical NLO material, used as electrooptical switch
• Bimetallic alkoxides - single source precursor
• Sol-gel chemistry - hydrolytic polycondensation gel
• MOH + M’OH MOM’ + H2O
• Yields glassy product
• Sintering product in air - induces crystallization
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INDIUM TIN OXIDE -ITO
• Indium sesquioxide In2O3 (wide Eg semiconductor) electrical conductivity enhanced by p-doping with (10%) Sn(4+)
• ITO is SnnIn2-nO3
• ITO is optically transparent, electrically conducting, thin films are vital as electrode material for solar cells, electrochromic windows/mirrors, LEDs, LC displays, electronic ink, photonic crystal ink and so forth
• Precursors - EtOH solution of (2-n)In(OBu)3/nSn(OBu)4
• Hydrolytic poly-condensation to form gel, spin coat gel onto glass substrate to make thin film
• Dry gel at 50-100C, heat at 350C in air to produce ITO
• Check electrical conductivity and optical transparency
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SUB -10 NM NANOSCALE DIRECT SOLID STATE REACTION – TiO2
Electron Beam Nanolithography of Spin-Coated Sol-Gel TiO2 Based Resists
Choosing the right solid state precursor to make resist
benzoyl acetone
tetrabutoxyorthotitanate
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SUB -10 NM NANOSCALE DIRECT SOLID STATE REACTION
Electron Beam Nanolithography Using Spin-Coated TiO2 Resists
• Utilization of spin-coated sol gel based TiO2 resists by chemically reacting titanium n-butoxide with benzoylacetone in methyl alcohol.
• They have an electron beam sensitivity of 35 mC cm-2 and are >107 times more sensitive to an electron beam than sputtered TiO2 and crystalline TiO2 films.
Choosing the right solid state precursor
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Sub-10 nm Electron Beam Nanolithography Using Spin-Coated TiO2 Resists
• Fourier transform infrared studies suggest that exposure to an electron beam results in the gradual removal of organic material from the resist.
• This makes the exposed resist insoluble in organic solvents such as acetone, thereby providing high-resolution negative patterns as small as 8 nm wide.
• Such negative patterns can be written with a pitch as close as 30 nm.
Choosing the right solid state precursor
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Nanometer scale precision structures
Nanoscale TiO2 structures offer new opportunities for developing next generation solar cells, optical wave-guides, gas sensors, electrochromic
displays, photocatalysts, photocatalytic mCP, battery materials
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Nanometer scale tolerances