Transcript SURFACE AREA OF PRECURSORS

IN SOLID STATE MATERIALS CHEMISTRY - SHAPE,
SIZE AND DEFECTS ARE EVERYTHING!
• Form, habit, morphology and physical size of product
controls synthesis method of choice, rate and extent of
reaction and reactivity (exposed contacted crystal faces)
• Single crystal, phase pure, defect free solids - do not exist
and if they did not likely of much interest – any thoughts ?!?
• Single crystal (SC) that has been defect modified with
dopants - intrinsic vs extrinsic or non-stoichiometry controls chemical and physical properties, function, utility
• SC preferred over microcrystalline powders for structure
and properties characterization and nanocrystals have
distinct size dependent properties
SHAPE IS EVERYTHING!
• Microcrystalline powder Used for characterization when
single crystal can not be easily obtained, preferred for industrial production
and certain applications, where large surface area useful like control of
reactivity, catalytic chemistry, separation materials, battery and fuel cell energy
materials, diffusion length control
• Polycrystalline shapes like pellet, tube, rod,
wire made of microcrystal forms Engineering superconducting ceramic wires, ceramic engines, aeronautical parts, magnets
• Single crystal or polycrystalline film Widespread
use in microelectronics, optical telecommunications, photonic devices, magnetic
data storage applications, coatings – protective, antireflection, self-cleaning
• Epitaxial film – single and multilayer
superlattice films - lattice matching with
substrate - tolerance factor - elastic strain,
defects important for fabrication of electronic, magnetic, optical
planarized devices – minimizing deleterious defects
SHAPE IS EVERYTHING!
• Non-crystalline – amorphous glassy - fibers, films, tubes, plates No
long range translational order – just short range local order
- control mechanical, optical-electrical-magnetic properties
like fiber optic cables, fiber lasers, optical components
• Nanocrystalline – below a certain
dimensions properties of materials
scale with size Quantum size effect materials –
electronic, optical, magnetic devices - discrete electronic
energy levels rather than continuous electronic bands – also
useful in nanomedicine like diagnostics, therapeutic drug
delivery, cancer therapy, imaging contrast agents MRI, CT,
PET, and fuel, battery, solar cell materials
MORE ASPECTS OF SOLID-SOLID REACTIONS
• Conventional solid state synthesis - heating mixtures of
two or more solids to form a solid phase product.
• Repeat - unlike gas phase and solution reactions
• Limiting factor in solid-solid reactions usually diffusion,
driven thermodynamically by a concentration gradient.
• Described by 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)
MORE 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 T 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 can still be 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
• How do we think about this???
Thinking About A, d, x Particle 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 mix)
•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
ELEMENT M + 2X
MODULATED
SUPERLATTICES DEPOSITED AND
THERMALLY POST
TREATED TO GIVE
LAYERED METAL
DICHALCOGENIDES MX2
COMPUTER
MODELLING OF
DIFFUSION
CONTROLLED SOLID
STATE REACTION OF
JOHNSON
SUPERLATTICE
Metal Dichalcogenides MX2
• M = Ti, V, Cr, Zr, Hf, Nb, Ta, Mo, W
• X = S, Se, Te
• Oh octahedral and D3h trigonal pyramidal
MX6 building blocks
• Edge sharing trigonal packed MX6/3 units
• Parallel stacked MX2 layers
• Strong M-X covalent forces in layers
• Weak VdW forces between layers
• VdW gap between adjacent layers
• Chemistry between the sheets
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
AT LOW T THE
SUPERLATTICE REAGENTS
YIELD SUPERLATTICE
ARTIFICIAL CRYSTAL
PRODUCT
Confirms correlation between
precursor heterostructure sequence and
superlattice ordering of final product
Note NbSe2 is a superconductor !!!
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
Johnson 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
ELEMENTAL MODULATED
SUPERLATTICES
• Several important synthetic parameters and in situ probes
• Reactants prepared using standard thin film deposition
techniques – more on this later - and consist of nm scale
thickness controlled 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
kinetics and mechanism as a function of inter-diffusion distance and
temperature
• Multi-layer repeat distances easily verified in prepared reactants and
products made under different conditions using low angle XRD
• Think about how to make a BaTiO3-SrTiO3 Perovskite superlattice or a
MgAl2O4-ZnAl2O4 Spinel superlattice and then a BaxSr1-xTiO3 and
MgxZn1-xAl2O4 solid solution ??? 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(III) precursor (acetate, acetylacetonate) - NaBH4 reductant
• with surfactants - oleic acid or oleylamine  ConLm
• arrested nucleation and growth of ligand capped cobalt nanoclusters
• surfactant functions as high temperature capping ligand and solvent
• then surfactant-sulfur injection - coating of sulfur shell on nanocluster
• cobalt sesquisulfide Co2S3 product shell layer formed at interface
Oleic Acid C17H33CO2H
Arrested nucleation and growth of nanocrystals – use of
surfactant, ligand, high temperature solvent properties
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
• size rather than charge effect determines diffusion
• generated vacancies agglomerate in core to form a void
• hollow core created which grows as the product shell thickens
• end result – a hollow nanosphere made of nc cobalt sesquisulfide Co2S3
• shell not perfectly sealed - has some porosity between nanocrystals –
• magnetic drug delivery and magnetohyperthermia cancer therapeutics
THINGS ARE NEVER
THAT SIMPLE!!!
Different diffusion
processes in the growth
of different architecture
hollow nanostructures
induced by the Kirkendall
effect
Air vacancies white,
cobalt orange, Co2S3
product blue
I don’t believe it !!!
Small Sept 2007
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
• Kirkendall effect - 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
TURNING NANOSTRUCTURES INSIDE-OUT
• When starting with perfect building blocks such as
monodisperse cobalt nanocrystals a reaction
meeting the Kirkendall criteria can lead to supersaturation and agglomeration of vacancies
exclusively in the center of the nanocrystal.
• General route to hollow nanocrystals of almost any
given material and shape – like nanocubes,
nanotriangles, nanorods and chains of nanoshells
• First proof-of-concept experiment - 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
Scheme of magnetic dipole-dipole coupling of superparamagnetic
nanocrystals into magnetic nanocrystal chains
Superparamagnetism – cooperative magnetic coupling of unpaired
electron spins in a single Weiss domain ferromagnetic nanocrystal
Magnetotactic bacteria – vesicle templated nucleation and
growth of superparamagnetic nanocrystal dipole chain
Communication and cooperative
behaviour between bacteria
communities – relevant to
evolutionary biology ???
I know my magnetic North !!!
Works for Hollow ZnAl2O4 Spinel Nanotubes!!!
Hollow ZnAl2O4 Spinel Nanotubes
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How does it work – VPT VLS growth (see later)
ZnO(s) +C(s)  ZnCO(g)
Aun(l) + ZnCO(g)  ZnO(NW) + C(s)
Coat ZnO NW with hydrolysable-polymerizable AlX3 (X = Cl,
OR) precursor in solution or vapor phase – solgel chemistry
AlX3 + 3H2O  Al(OH)3 + 3HCl
AlOH + HOAl  Al-O-Al + H2O
Thermally treat to make Al2O3 coated ZnO NWs
Heat further to induce interdiffusion of core and corona
Zn2+ more mobile than Al3+
Creates Zn2+ vacancies in the core ZnO nanowire
Vacancies agglomerate in core and create Kirkendall porosity
Final product a ZnAl2O4 hollow Spinel nanotube
And what would you use them for – nanofluidics, ionic
nanodiode or transistor, drug storage and delivery vehicle
???
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
Tutorial - Surface Chemistry of Silica
Polydimethylsiloxane PDMS
(H3C)3SiO[Si(CH3)2O]nSi(CH3)3
n Si(CH3)2Cl2 + n H2O → [Si(CH3)2O]n + 2n HCl
Condensation polymerization synthesis of PDMS - a
very famous polymer – elastomeric and hydrophobic
- let’s make a micromold and do chemistry
PDMS MASTER FOR SOFT LITHOGRAPHY
MICROCONTACT PRINTING mCP
Whitesides
Whitesides
PDMS MASTER
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Schematic illustration of the procedure
for casting PDMS replicas from a
master having relief structures on its
surface.
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The master is silanized and made
hydrophobic by exposure to
CF3(CF2)6(CH2)2SiCl3 vapor
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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
prevents adhesive tearing of mold
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Each master can be used to fabricate
more than 50 PDMS replicas.
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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) – ferromagnetic or
superparamagnetic depending on size
(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 – ferromagnetic or superparamagnetic .
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 atomically
mixed Mg(II)/Fe(III) multidentate 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.
BEYOND MICROCONTACT PRINTING
GOING EVEN SMALLER WITH DIP PEN NANOLITHOGRAPHY
Throw Away the Micron Scale PDMS Stamp – Use a nm Scale AFM Tip
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Direct-write "dip-pen"
nanolithography (DPN)
has been developed
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Delivers collections of
molecules in a positive
printing mode
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Proof-of concept
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Alkanethiols on gold
controls surface
wettability, chemical
reactivity at scale well
below a micron
Chad Mirkin, Science 283, 661, 1999
PATTERNING INORGANIC SOLID-STATE CHEMISTRY VIA DIPPEN NANOLITHOGRAPHY WITH SOL-GEL -BASED INKS
Pluronic PPO-PEO-PPO
triblock copolymer surfactant
solvent and carrier to enable
solgel chemistry
tin oxide
aluminum oxide
Nano gas
sensor
Nano catalyst
support
silicon oxide
Nano optical
waveguide
calcined
SO YOU THOUGHT YOU SAW EVERYTHING!!!
Massively parallel DPN with a passive 2D cantilever
array - 55,000 Tips – really pushing the envelope for
solid state nanomaterials synthesis !!!
Angew Chem Int Ed., Mirkin et al, 25th September 2005
55,000-pen array was used to
HOW
GOOD???
generate
approximately
88,000,000 million dot features
Each pen generated 1600 dots
in a 40 x 40 array, where the
dot-to-dot distance was 400 nm.
The dots had a diameter of
(100±20) nm, a height of 30 nm,
and were spaced by 20 mm in
the x direction and 90 mm in the
y direction corresponding to the
distances determined by the
array architecture.
DOING ‘REAL’ SOLID STATE SYNTHESIS 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 MgAl2O4
• Intimate mixing of precursor reagents
• Homogenization of solid 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 – know your
solid state chemistry
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 – tunable
magnetic 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 complex oxides like Spinels,
Perovskites
• Disadvantage: often difficult to
prepare high purity, accurate
stoichiometric phases
DOUBLE SALT PRECURSORS
• Precisely 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 – see later – synthesis in
high T high P aqueous environment
BASICS: FERROELECTRIC BARIUM TITANATE
Paraelectric a = 4.018Å
Displacive
Transition
Ferroelectric a = 3.997Å, c = 4.031Å
Ti moves off center
Above 120C (Tc) - Cubic perovskite
equivalent O-Ti-O bonds in BaTiO3
Below Tc Tetragonal perovskite long-short axial O-Ti—O
bonds induced aligned electric dipoles in BaTiO3
Note - small grains –
complications - tetragonal to
cubic surface gradients ferroelectricity is particle size
dependent and can be lost
Cubic dielectric above Tc – paraelectric
state - below Tc multi-domain state with
cooperative electric dipole interactions
within each domain – aligned in domain
but randomly oriented between domains
Multidomain ferroelectric
dipoles align in E field below Tc
Single domain superparaelectric
HYSTERESIS OF POLARIZATION OF
FERROELECTRIC BaTiO3 IN APPLIED FIELD E
Field E
Random domain dielectric
P
Ps saturation polarization
Ps
Pr remnant polarization
Ec coercive field
Aligned domain ferroelectric
Pr
Pc
Single domain superparaelectric
E
Polarization Hysteresis Behavior P vs E
Diagnostic of Ferroelectric
Synthesis of a Ferroelectric Random
Access Memory (FeRAM) 0.5 Tbit/in2
Polarization Switching by Changing Direction of Applied Electric Field
DPN Direct Reaction Solid State Chemistry
Synthesis of Ferroelectric PbTiO3 Array
Synthesis Precursor Sol
PbO + TiO2  PbTiO3
DPN Synthesis of PbTiO3 (PTO)
• Schematic drawings illustrating
the dip-pen nanolithography
(DPN) of ferroelectric PbTiO3
(PTO) nanodots.
• (a) Patterns of PTO nanodots
formed by DPN.
• (b) Formation of a nanopattern
using a PTO precursor sol on
the surface of epitaxially
mateched Nb-doped SrTiO3 by
DPN.
• (c) To obtain highly crystallized
• PTO nanodots, an annealing
process is carried out after the
lithography of the PTO
nanopattern is performed.
Conducting AFM tip and substrate enable PFM and
EFM ferroelectricity measurements on individual dots
DPN Size and Thickness Control of PbTiO3 Nanodots
DPN PbTiO3 Nanodots – How Small Can You Go?
DPN Synthesis of FeRAM – Characterization of Ferroelectricity By
Piezoelectric and Electric Field Force Microscopy (PFM, EFM)
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 – CHANGED THE WORLD!
• Indium sesquioxide In2O3 (wide Eg semiconductor) electrical conductivity
enhanced by n-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, OLEDs, 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
Doping Basics on TCOs – The Big Three
• ITO: Sn doped In2O3 - 1: 9 solid solution – electrons in CB
• n-doped with Sn(IV) isomorphously replacing In(III)
• ATO: Sb doped SnO2 – how would you make it?
• n-doped with Sb(V) isomorphously replacing Sn(IV)
• FTO: F doped SnO2 – how would you make it?
• n-doped with F(-I) isomorphously replacing O(-II)
• AZO: Al doped ZnO – how would you make it ?
• n-doped with Al(III) isomorphously replacing Zn(II)
TCO Materials are NOT that Simple
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Objective is to optimize optical transparency and electrical conductivity
ITO – Sn:In = 1: 9 solid solution – Linear Vegard Law
Si classical semiconductor doping normally ppm B and P dopants
Contrast higher ITO doping creates some [O] vacancies
To balance Sn(IV) vs In(III) charge differences
Reality – general formula of ITO
• In2-xSnxO3-2x
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Replacing xIn(III) with xSn(IIV) requires 2x[O(-II)]
Effect is to reduces number of electron n-dopants
Reduces conductivity
Also some unwanted Sn(II) formed in synthesis introduces holes
 s = se + sh
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Reduces conductivity
Optimizing electrical conductivity of ITO by materials chemistry is not so simple
Nanocrystalline Antimony
Doped Tin Dioxide - ncATO
A Nanomaterial that Could
Change the World
How, Why, When !!! ???
Welcome to Beautiful ncATO
TCO’s
• Optical transparency and electrical conductivity of TCO’s critical for
thin layer electrodes in a wide range of high technology devices
• Solar cells
• Flat panel displays
• Smart energy saving electrochromic windows
• Electronics
• Chemical sensors
• OLEDs
• Lasers
• Currently industry favourite is ITO on glass
• Made by vacuum thermal deposition or sputter deposition
• Works on thermally stable substrates but not on plastics
• ITO expensive as In rare – Canada is a major supplier !!!
• Dire need for low cost easy to make alternative
• Film formation at RT on plastic substrates would be great asset
• Example of how to get rich quickly through materials chemistry
A Little Nanochemistry Secret
Benzyl Alcohol High T
Solvent and Reactant
Source of Oxygen in
Non-Aqueous SolGel
Chemistry
Nanochemistry Synthesis of ncATO
• Non-aqueous sol-gel in C6H5CH2OH
• Benzyl Alcohol solvent and source of oxygen
• ROH + MCl  MOH + RCl
• Or
• ROH + MOEt  MOH + ROEt
• MOH + HOM  MOM
Benzolate Capping of ncATO
ncATO
Why ncATO?
• Wide bandgap optically transparent semiconductor Eg = 3.6eV and
n-doped
• Control over size, shape, surface charge allows colloidally and air
stable dispersions in common solvents like H2O, EtOH, THF
• Note add a little acid to water dispersion - colloidally stable
• Enable thin films and patterns to be made on any substrate under
ambient conditions
• Spin, dip, aerosol, IJP coating and printing strategies
• Synthesis non-aqueous solgel
• Arrested nucleation and growth of ncATO
• Solvent and reactant benzyl alcohol C6H5CH2OH
• Reagent precursors SbCl3, Sb(OEt)3, Sb(O2CCH3)3
• Anhydrous synthesis conditions 150oC, 2 hours
Meet ncATO
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Morphology, size and dispersibility of
ATO nanoparticles:
STEM-HAADF images of 10 % ATO
nanoparticles prepared using Sb(ac)3
at 100 oC (a) and Sb(ac)3 at 150 oC
(b).
The insets 6 x 6 nm in size show high
resolution STEM-HAADF images of a
single nanoparticle.
Size distribution of 10 % ATO
nanoparticles prepared using Sb(ac)3
at 150 oC determined from HRTEM
images of ca. 100 nanoparticles (gray
bars) and from DLS measurement (red
line) of a colloidal dispersion in EtOH
of the same nanoparticles
(c). The inset 6 x 6 nm in size shows
high resolution TEM image of a single
nanoparticle.
Images of as prepared differently
doped ATO nanoparticles synthesized
at 150 °C using Sb(ac)3 (d): dried
particles (top) and their colloidal
dispersions in THF (particle
concentration of 5 wt%) (bottom).
ncATO Diagnostics
• Key objectives in nanochemistry strategy
• Command over size, shape, surface charge
• Composition and solvent solubility/dispersibility
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PXRD – phase purity and particle size
HRTEM, DLS – particle size and particle size distribution
XPS – Sb(V) : Sb(III)
EDX – elemental compositition
• Conductivity – nc size and nc
dopant concentration and oxidation
state dependence
ELECTRON BEAM LITHOGRAPHY
Top Down NanoFabrication - High Spatial Resolution Patterning at
the Nanoscale Using Energetic Short Wavelength Electron Beams
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
How Good is EBL?