DIRECT VISUALIZATION OF ATOM SCALE DEFECTS BY TRANSMISSION ELECTRON MICROSCOPY

VISUALIZING MISFIT DISLOCATIONS AT INTERFACE BETWEEN MBE DEPOSITED Si AND GaAs

SUBSTRATE

111 FAULTS

MISFIT DISLOCATION

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 - also distinct crystal habits (octahedral, cubooctahedral, cubic) possible depending on growth conditions and additives - {100} alternating Mg(2+), O(2-) at corners of square grid - {111} Mg(2+) or O(2-) in hexagonal arrangement - implies different surface structures and reactivities

FACTORS WHICH CONTROL CRYSTAL GROWTH AND MORPHOLOGY

Most prominent surfaces exhibit slower growth Growth rate of specific surfaces controls morphology of crystal Depends on area of a face - structure of exposed face - accessibility of a face - surface energy - surface reconstruction - adsorption at surface sites - surface defects

{111} vs {100} growth rates: cube, cubooctahedral or octahedral shape

Needle growth

Platelet growth

FACTORS WHICH CONTROL CRYSTAL GROWTH AND MORPHOLOGY

All types of defects, intrisic or extrinsic, vacancies, interstitials, lines, planes, dislocations, grain boundaries, enhance diffusion of ions and crystal growth rates Defects play major role in reactivity, nucleation, crystal growth, materials properties (electronic, optical, magnetic, charge-transport, mechanical, thermal)

{111} vs {100} growth rates: cube, cubooctahedral or octahedral shape

Needle growth

Platelet growth

CRYSTAL NUCLEATION AND GROWTH

• G°(crystal) = G° (surface) - G° (bulk)

• Induction period - growth of viable crystal nuclei

• Growth and dissolution of seed

• Equilibrium growth condition when G°(crystal) = 0

• G° (surface) = G° (bulk)

• Condition to creates critical size nuclei

• Crystal growth favored when G° (surface) < G° (bulk)

• Sigmoid-shaped nucleation-growth-depletion curve

• Large crystals grow at expense of small ones

• Crystal growth ceases when nutrients depleted

ABALONE SHELL - BIOMINERALIZATION - VECTORIAL CONTROL OF CRYSTAL NUCLEATION, GROWTH AND

FORM IN NATURE

95wt% inorganic - site specific calcite platelet oriented growth

5wt% -sheet protein organic structure directing matrix

Organic-inorganic nanocomposite 1000x fracture toughness of bulk calcite – impact energy on shell dissipated in soft protein layers rather than in hard calcite preventing cracking - learning from Nature - biomimetic inorganic materials chemistry - technology transfer from biology

“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 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 to the new chemistry and a rational synthesis of materials

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, ultrasonification

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

SOLID STATE SYNTHESIS HEATING PROGRAM

• Furnaces, RF, microwave, lasers, ion and electron beams

• Prior decompositions and frequent cooling, grinding, boost SA of reacting grains

• Overcoming sintering, grain growth, brings up SA, fresh surfaces, enhanced contact area

• Pellet and hot press reagents - higher surface contact area, enhances rate, extent of reaction

• Care with unwanted preferential component volatilization if T too high, composition dependent

• Need controlled atmosphere for unstable oxidation states

PRECURSOR SOLID STATE SYNTHESIS METHOD

• Co-precipitation, high degree of homogenization, high reaction rate - applicable to nitrates, acetates, oxalates, alkoxides, -diketonates

• Concept: precursors to magnetic spinels - recording media

• Zn(CO2)2/Fe2[(CO2)2]3/H2O 1 : 1 mixing

• H2O evaporation, salts co-precipitated - solid-solution mixing on atomic scale, filter, calcine in air

• Zn(CO2)2 + Fe2[(CO2)2]3 ZnFe2O4 + 4CO + 4CO2

• High degree of homogenization, lower reaction temperature, faster rate

PROBLEMS WITH CO-PRECIPITATION METHOD

• Co-precipitation applicable to nitrates, acetates, oxalates, alkoxides, -diketonates and so forth requires:

• Similar salt solubilities

• Similar precipitation rates

• Avoid super-saturation as poor control of co-precipitation

• Useful for spinels

• Disadvantage: difficult to prepare high purity, accurate stoichiometric phases

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

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

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

• BaCO3(s) + TiO2(s) BaTiO3(s)

• Sol-gel reagents useful to create single source barium titanate precursor with correct stoichiometry

BASICS: FERROELECTRIC BARIUM TITANATE

Cubic perovskite BaTiO3 Tetragonal perovskite BaTiO3

Multidomain paraelectric above Tc Cooperative electric dipole interactions within each domain

Multidomain ferroelectric below Tc

Single domain superparaelectric

Small grains, tetragonal to cubic surface gradients, ferroelectricity particle size dependent

SINGLE SOURCE PRECURSOR SYNTHESIS OF BARIUM TITANATE - FERROELECTRIC MATERIAL

• Ti(OBu)4(aq) + 4H2O Ti(OH)4(s) + 4BuOH(aq)

• Ti(OH)4(s) + (COO)22-(aq) TiO(COO)(aq) + 2OH-(aq) + H2O

• Ba2+(aq) + (COO)22-(aq) + TiO(COO)(aq) Ba[TiO(COO)2](s)

• Precipitate contains barium and titanium in correct ratio and at 920C decomposes to barium titanate according to:

• Ba[TiO(COO)2](s) BaTiO3(s) + 2CO(g)

• Grain size important for control of ferroelectric properties

• Used to grow single crystals hydrothermally

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 + LiNbO3

• Lithium niobate, ferroelectric perovskite, nonlinear optical NLO material, used as electrooptical switch

• Bimetallic alkoxides - single source precursor• Sol-gel chemistry - hydrolytic polycondensation

• MOH + M’OH MOM’ + H2O

• Yields glassy product • Sintering product in air - induces crystallization

INDIUM TIN OXIDE -ITO

• Indium sesquioxide In2O3 (wide band gap 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, electronic ink

• 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

SUB -10 NM NANOSCALE DIRECT SOLID STATE REACTION

Electron Beam Nanolithography Using Spin-Coatable TiO2 Resists

• Utilization of spin-coatable 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 TiOx and crystalline TiO2 films.

Choosing the right solid state precursor

Sub-10 nm Electron Beam Nanolithography Using Spin-Coatable 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

Nanometer scale precision structures

Nanoscale TiO2 structures offer new opportunities for developing next generation solar cells, optical waveguides, gas sensors, electrochromic

displays, photocatalysts, photocatalytic mCP, battery materials

Nanometer scale tolerances

MAGNETIC GARNETS, YxGd3-xFe5O12

TUNABLE MAGNETIC MATERIALS

• Y(NO3)3 + Gd(NO3)3 + FeCl3 + NaOH YxGd3-xFe5O12

• Mixed metal hydroxide aqueous precursor synthesis method, reactants red brown, solid products olive green

• Firing pellets at 900oC, 18-24 hrs, regrinding, repelletizing, repeated firings, removes REFeO3 perovskite impurity

• PXRD used to identify garnet phase, detects any crystalline impurity phase like REFeO3, enables UC dimensions to be determined as a function of Y: Ga ratio over range 0 < x < 3

PXRD OF SOLID PRODUCTS OF Y(NO3)3 + Gd(NO3)3 + FeCl3 + NaOH REACTION

HYDROTHERMAL SYNTHESIS AND CRYSTAL GROWTH OF YTTRIUM ALUMINUM GARNET

Al2O3

Seed crystal to grow Y3Al5O12 crystal

Y2O3

baffles

aqueous basic medium, mineralizes, temperature gradient, transports, deposits reactants on seed crystal to grow product yttrium aluminum oxide crystal

T2 T1 T2

GARNETS DISPLAY INTERESTING COOPERATIVE MAGNETIC BEHAVIOR

• Tunable magnet by varying magnetic superlattice components without disrupting garnet structure

• Similar idea to magnetic spinel AB2O4 solid solution behavior - in which one has magnetically tunable Td (A) and Oh (B) metal sites

• Rare earth garnets R3Fe5O12

• General Formula C3A2D3O12 (8 formula units per cubic unit cell - total 160 atoms)

ONE OCTANT OF CUBIC UNIT CELL OF YAG

One octant of cubic unit cell of garnet

Faces 3 dodecahedral Y(3+) sites

Corners and center 2Oh AlO6 sites

Faces 3Td AlO4 sites

GARNETS DISPLAY INTERESTING COOPERATIVE MAGNETIC BEHAVIOR

• C3A2D3O12 isomorphous replacement of Y(3+) for Gd(3+) on dodecahedral C cation sites (works for all rare earths except La, Ce, Pr, Nd)

• Forms solid solution as similar ionic radii,

• R(Gd(3+) = 0.938Å > R(Y(3+) = 0.900Å

• Complete family accessible, YxGd3-xFe5O12, 0 x 3

• 2Fe(3+) Oh A-sites, 3Fe(3+) D-Td sites, 3RE(3+) C dodecahedral sites

MODELS FOR DETERMINING THE Y(3+)/Gd(3+) DISTRIBUTION IN YxGd3-xFe5O12

1. Solid solution - random distribution of two components - EDX mapping

2. Physical mixture of two end members - phase segregation - PXRD

3. Compositional gradients - STEM imaging - EDX mapping

4. Core-corona - cherry model - surface free energy driven - EDX mapping

5. Domains smaller than 10 nm - PXRD line broadening

6. Ordered superlattice - ED

MODELS FOR DETERMINING THE Y(3+)/Gd(3+) DISTRIBUTION IN YxGd3-xFe5O12

• Interesting problem in solid state materials characterization

• If any measured physical property P of the product follows Vegard law behavior this defines a solid solution for the Y(3+)/Gd(3+) distribution

• P(YxGd3-xFe5O12) = Px/3(Y3Fe5O12) + P(3-x)/3(Gd3Fe5O12)

• Measured P of product is the atomic/mole fraction weighted average P of the end-member materials

MAGNETIC GARNETS, YxGd3-xFe5O12

TUNABLE MAGNETIC MATERIALS

• Cubic unit cell parameter a versus x for YxGd3-xFe5O12

• Composition Lattice parameter, nm

• Y3Fe5O12 1.2370

• Y2.5Gd0.5Fe5O12 1.2382

• Y2Gd1Fe5O12 1.2402

• Y1.5Gd1.5Fe5O12 1.2423

• Y1Gd2Fe5O12 1.2437

• Y0.5Gd2.5Fe5O12 1.2450

• Gd3Fe5O12 1.2468R(Gd(3+)) = 0.938Å > R(Y(3+)) = 0.900Å

MAGNETIC GARNETS, YxGd3-xFe5O12

TUNABLE MAGNETIC MATERIALS

• Isomorphous random replacement of Y3+ for Gd3+on dodecahedral sites of cubic lattice

• Vegard law behavior

• P(YxGd3-xFe5O12) = Px/3(Y3Fe5O12) + P(3-x)/3(Gd3Fe5O12)

• Any property of a solid-solution member is the atom/mole fraction weighted average of the end-members - distinguishes statistical from other types of mixtures (core-corona, phase separation, domains, gradients, superlattices)

• Cubic lattice parameter a shows linear Vegard law behavior with x

TUNABLE MAGNETIC PROPERTIES BY VARYING x IN THE BINARY GARNET YxGd3-xFe5O12

• Counting electrons and unpaired electron spins

• x dodec Y(3+) sites 4d0, 4f0 0 UPEs

• (3-x) dodec Gd(3+) sites HS 4f7 7 UPEs

• 3 Td Fe(3+) sites HS 3d5 5 UPEs

• 2 Oh Fe(3+) sites HS 3d5 5UPEs

TUNABLE MAGNETIC PROPERTIES BY VARYING x IN THE BINARY GARNET YxGd3-xFe5O12

• Ferrimagnetically coupled material, oppositely aligned electron spins on Td and Oh Fe(3+) magnetic sub-lattices

• Counting spins Y3Fe5O12 ferrimagnetic at low T

• 3 x 5 - 2 x 5 = 5UPEs

• Counting spins Gd3Fe5O12 ferrimagnetic at low T

• 3 x 7 -3 x 5 + 2 x 5 = 16UPEs

• Tunable magnetic garnet from 16 to 5 UPEs

VEGARD LAW AT THE NANOSCALE

SYNTHESIS OF COMPOSITION TUNABLE MONODISPERSE CAPPED ZnxCd1-xSe ALLOY NANOCRYSTALS

Spatial and quantum confinement and dimensionality