Transcript SURFACE AREA OF PRECURSORS

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)
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J = Flux of diffusing species (#/cm2s)
D = Diffusion coefficient (cm2/s)
(dc/dx) = Concentration Gradient (#/cm4)
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.
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 turn
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
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
A, d, x Relations
Small d
Large d
Large SA/V
Small SA/V
Small x
Large x
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!!!
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
All aimed to increase A and decrease x
and minimize diffusion length scale
Decreasing particle size to
nanocrystalline range
Hot pressing densification of particles
Atomic scale mixing in composite
precursor compounds
Coated particle mixed component
reagents, corona/core precursors
Johnson superlattice layered precursors
MINIMIZING DIFFUSION LENGTHS <x>  (2Dt)1/2 FOR
RAPID AND COMPLETE DIRECT REACTION
BETWEEN SOLID STATE MATERIALS AT LOWEST T
Core-corona reactants in intimate
contact, made by precursor
precipitation, sol-gel deposition, CVD
All aimed to increase A and decrease x
and minimize diffusion length scale
COATED PARTICLE
MIXED SOLID STATE
REAGENTS
SYNTHESIS OF COMPOSITION TUNABLE
MONODISPERSE ZnxCd1-xSe ALLOY NANOCRYSTALS –
ELECTRONIC BAND GAP ENGINEERING
x controlled by size of core and corona
more on this later
MINIMIZING DIFFUSION LENGTHS <x>  (2Dt)1/2 FOR
RAPID AND COMPLETE DIRECT REACTION
BETWEEN SOLID STATE MATERIALS AT LOWEST T
SUPERLATTICE
REAGENTS
•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
ELEMENTALLY
MODULATED
SUPERLATTICES DEPOSITED AND
THERMALLY POST
TREATED TO GIVE
LAYERED METAL
DICHALCOGENIDES MX2
COMPUTER
MODELLING OF
SOLID STATE
REACTION OF
JOHNSON
SUPERLATTICE
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
AT LOW T THE
SUPERLATTICE REAGENTS
YIELD SUPERLATTICE
ARTIFICIAL CRYSTAL
PRODUCT
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 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
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 “alloy”
product with identical layers
AT HIGH T THE
SUPERLATTICE REAGENTS
YIELD HOMOGENEOUS
SOLID SOLUTION PRODUCT
Properties of ternary product is the atomic
fraction weighted average of binary end
member components – Vegard Law
P(TixNb(1-x)Se2) = xP(TiSe2) + (1-x)PNbSe2
ELEMENTALLY MODULATED
SUPERLATTICES
• Several important synthetic parameters and in situ probes
• Reactants prepared using thin film deposition techniques –
more on this later - and consist of nm scale layers of the
elements to be reacted.
• Elements easily substituted for another
• Allows rapid surveys over a class of related reactions and
synthesis of iso-structural compounds.
ELEMENTALLY MODULATED
SUPERLATTICES
• Diffusion distance is determined by the multilayer repeat distance which
can be continuously varied
• An important advantage, allowing experimental probe of 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
• Think about how to make a BaTiO3-SrTiO3 Perovskite superlattice or a
MgAl2O4-ZnAl2O4 Spinel superlattice and why would you do this ???
CORE-CORONA NANOCLUSTER PRECURSOR BASED KIRKENDALL
SYNTHESIS OF HOLLOW NANOCLUSTERS
S
Co
V[Co]
e(-)
Co(3+)
S(2-)
Co2S3
• Synthesis
of surfactant-capped cobalt nanoclusters:
• Co(3+)/BH4(-) reduction in oleic acid, oleylamine  ConLm
• arrested nucleation and growth of ligand capped cobalt nanoclusters
• surfactant functions as high temperature capping ligand and solvent
• surfactant-sulfur injection, coating of sulfur shell on nanocluster
• cobalt sesquisulfide product shell layer formed at interface
CORE-CORONA NANOCLUSTER PRECURSOR BASED KIRKENDALL
SYNTHESIS OF HOLLOW NANOCLUSTERS
S
Co
V[Co]
e(-)
Co(3+)
S(2-)
Co2S3
• counter-diffusion of Co(3+)/2e(-) and S(2-) across thickening shell
• faster diffusion of Co(3+) than S(2-) creates vacancies 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
THINGS ARE NEVER
THAT SIMPLE!!!
Different diffusion
processes in the
growth of hollow
nanostructures
induced by the
Kirkendall effect
Small Sept 2007
asap web
Time evolution of a hollow Co2S3 nanocrystal grown from a
Co nanocrystal via the nanoscale Kirkendall effect
Science 2004, 304, 711
TURNING NANOSTRUCTURES INSIDE-OUT
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Kirkendall effect a well-known phenomenon discovered in 1930’s.
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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.
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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.
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Leads to Kirkendall porosity, formed through super-saturation of vacancies
into hollow pores
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When starting with perfect building blocks such as monodisperse cobalt
nanocrystals a reaction meeting the Kirkendall criteria can lead to supersaturation of vacancies exclusively in the center of the nanocrystal.
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General route to hollow nanocrystals of almost any given material and shape –
like nanorods – see next example
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Proof-of-concept - synthesis of Co2S3 nanoshell starting from Co nanocluster.
Time evolution of a hollow CoSe2 nanocrystal magnetic
dipole chain grown from a Co nanocrystal and selenium
in surfactant capping ligand and solvent via the
nanoscale Kirkendall effect – Small September 2007
Works for Hollow ZnAl2O4 Spinel Nanotubes!!!
Li2NH Hollow Nanospheres from the
Kirkendall Reaction of Li Nanospheres
and Ammonia - Hydrogen Storage
Materials with Superior AdsorptionDesorption Kinetics
Chemistry Materials December 9th 2007
SYNTHESIS OF Li2NH HOLLOW NANOSPHERES AND
THEIR REVERSIBLE REACTION WITH H2
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Vaporization of Li metal as spherical nanodroplets
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Reaction of nanodroplets with gaseous NH3
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Kirkendall effect of small fast diffusing Li reacting with NH3 forms
shell of Li2NH and core of vacancies which coalesce
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Li2NH hollow nanospheres with high surface area and thin shell
enables fast H2 adsorption kinetics (6wt%, 470K, Ea 106 kJ/mole) to
form LiNH2 and LiH and fast de-sorption kinetics 503K for reaction
of LiNH2 with LiH enabled by small diffusion lengths
•
Significantly improved kinetics compared to micron size particles
of Li2NH (610K, Ea 225kJ/mole ads, 618K des)
SYNTHESIS OF Li2NH HOLLOW NANOSPHERES AND THEIR
REVERSIBLE REACTION WITH H2 WITH SHAPE RETENTION
• (a) SEM image
• (b) TEM image (inset:
magnified TEM image) of the
as-prepared Li2NH hollow
nanospheres
• (c) TEM image of the Li2NH
hollow nanospheres
annealing at 573 K under
vacuum for 1 h
• (d) TEM image of the Li2NH
hollow nanospheres after
hydrogenated at 573 K under
35 bar of hydrogen for 1 h.
PXRD Characterization
• XRD patterns of (a)
Li2NH hollow
nanospheres
• (b) Li2NH hollow
nanospheres after
hydrogenated at 573
K under 35 bar of
hydrogen for 1 h.
DSC Characterization
• (a) DSC curves of
hydrogenation at a heating
rate of 10 K/min under 35
bar of H2 and
• (b) DSC curves of
desorption at a heating rate
of 10 K/min under flowing Ar
after hydrogenated under 35
bar of H2 at 573K for 1 h.
• N ) Li2NH hollow
nanospheres, and M ) Li2NH
micrometer particles.
Adsorption Characterization
• Hydrogenation
absorption curves of
the obtained samples at
different temperature
under an initial
hydrogen pressure
about 35 bar of H2 of
(N) Li2NH hollow
nanospheres and (M)
Li2NH micrometer
particles.
Pressure Hydrogen Content Temperature
Behaviour of Li2NH Hollow Nanospheres
NANOSCALE PATTERNING OF SHAKE-AND-BAKE
SOLID-STATE CHEMISTRY
MINIMIZING DIFFUSION LENGTHS
<x>  (2Dt)1/2 FOR RAPID AND COMPLETE
DIRECT REACTION BETWEEN SOLID STATE
MATERIALS AT LOWEST T
Younan Xia
PDMS MASTER FOR SOFT LITHOGRAPHY
MICROCONTACT PRINTING
Whitesides
Whitesides
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.
NANOSCALE PATTERNING OF SHAKE-AND-BAKE
SOLID-STATE CHEMISTRY
Younan Xia
NANOSCALE PATTERNING OF SHAKE-AND-BAKE
SOLID-STATE CHEMISTRY
Co(NO3)2
Co3O4
Co
(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 dewetting 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 mm2 in area and
separated by 5 mm.
(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.
NANOSCALE PATTERNING OF SHAKE-AND-BAKE
SOLID-STATE CHEMISTRY
MgFe2O4
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 dewetting 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.
NiFe2O4
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.
ACTUALLY DOING IT IN THE LAB
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 in the lab if one is to move beyond trial-anderror methods to the new solid state chemistry and a
rational and systematic approach to synthesis of materials
THINKING ABOUT MIXING SOLID 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
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
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
PRECURSOR SOLID STATE SYNTHESIS METHOD
• Co-precipitation, high degree of homogenization, high
reaction rate - applicable to nitrates, acetates, citrates,
carboxylates, oxalates, alkoxides, b-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
PROBLEMS WITH CO-PRECIPITATION METHOD
• Co-precipitation requirements:
• 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
DOUBLE SALT PRECURSORS
• Known stoichiometry double salts have controlled
element 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
DOUBLE SALT PRECURSORS
• Chromite Spinel Precursor compound
Ignition T, oC
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1100-1200
1100
1100
1200
700-800
1400
1150
MgCr2O4
NiCr2O4
MnCr2O4
CoCr2O4
CuCr2O4
ZnCr2O4
FeCr2O4
(NH4)2Mg(CrO4)2.6H2O
(NH4)2Ni(CrO4)2.6H2O
MnCr2O7.4C5H5N
CoCr2O7.4C5H5N
(NH4)2Cu(CrO4)2.2NH3
(NH4)2Zn(CrO4)2. 2NH3
(NH4)2Fe(CrO4)2
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
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 stoichiometry, size and morphology
control
• BaCO3(s) + TiO2(s)  BaTiO3(s)
• Sol-gel reagents useful to create single source
barium titanate precursor with correct stoichiometry
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 920C
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
BASICS: FERROELECTRIC BARIUM TITANATE
Cubic perovskite equivalent
O-Ti-O bonds in BaTiO3
Tetragonal perovskite long-short
axial O-Ti—O bonds in BaTiO3
Small grains, tetragonal
to cubic surface
gradients, ferroelectricity
particle size dependent
Multidomain paraelectric above Curie
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
SOL-GEL SINGLE SOURCE PRECURSORS TO LITHIUM
NIOBATE - NLO MATERIAL
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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 + D + O2  LiNbO3
Lithium niobate, ferroelectric Perovskite, nonlinear
optical NLO material, used as electrooptical switch –
voltage control of refractive index – random vs aligned
electric dipoles
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
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: InOH + HOSn  InOSn
• Dry gel at 50-100C, heat at 350C in air to produce ITO
• Check electrical conductivity and optical transparency
SUB -10 NM NANOSCALE
DIRECT SOLID STATE REACTION – TiO2
Electron Beam Nanolithography of Spin-Coated Sol-Gel TiO2 Based Resists
LOCALIZED HEATING AT THE NANOMETER SCALE
benzoyl acetone
tetrabutoxyorthotitanate
Choosing the right solid state precursor to make resist
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.
Choosing the right
solid state precursor
• 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.
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,
unexposed is soluble, thereby providing
high-resolution negative patterns as small
as 8 nm wide.
Choosing the right
solid state precursor
• Such negative patterns can be written with
a pitch as close as 30 nm.
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
Nanometer scale tolerances