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DIRECT VISUALIZATION OF ATOM SCALE DEFECTS BY TRANSMISSION ELECTRON MICROSCOPY

DIRECT VISUALIZATION OF ATOM SCALE DEFECTS BY TRANSMISSION ELECTRON MICROSCOPY. VISUALIZING MISFIT DISLOCATIONS AT INTERFACE BETWEEN MBE DEPOSITED Si AND GaAs SUBSTRATE . MISFIT DISLOCATION. 111 FAULTS. O 2-. Mg 2+. Mg 2+. O 2-. O 2-. Mg 2+. Mg 2+. Mg 2+. O 2-. O 2-. O 2-.

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DIRECT VISUALIZATION OF ATOM SCALE DEFECTS BY TRANSMISSION ELECTRON MICROSCOPY

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  1. DIRECT VISUALIZATION OF ATOM SCALE DEFECTS BY TRANSMISSION ELECTRON MICROSCOPY

  2. VISUALIZING MISFIT DISLOCATIONS AT INTERFACE BETWEEN MBE DEPOSITED Si AND GaAs SUBSTRATE MISFIT DISLOCATION 111 FAULTS

  3. O2- Mg 2+ Mg 2+ O2- O2- Mg 2+ Mg 2+ Mg 2+ O2- O2- O2- Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ O2- O2- O2- O2- O2- O2- O2- O2- Mg 2+ O2- Mg 2+ O2- O2- O2- Mg 2+ {100} face {111} face {111} face Mg 2+ Mg 2+ SURFACE STRUCTURE AND REACTIVITY EFFECTS ON DIRECT REACTION OF SOLIDS 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

  4. Needle growth Platelet growth {111} vs {100} growth rates: cube, cubooctahedral or octahedral shape 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

  5. Needle growth Platelet growth {111} vs {100} growth rates: cube, cubooctahedral or octahedral shape 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 rolein reactivity, nucleation, crystal growth, materials properties (electronic, optical, magnetic, charge-transport, mechanical, thermal)

  6. CRYSTAL NUCLEATION AND GROWTH • DG°(crystal) = DG° (surface) - DG° (bulk) • Induction period - growth of viable crystal nuclei • Growth and dissolution of seed • Equilibrium growth condition whenDG°(crystal) = 0 • DG° (surface) = DG° (bulk) • Condition to creates critical size nuclei • Crystal growth favored when DG° (surface) < DG° (bulk) • Sigmoid-shaped nucleation-growth-depletion curve • Large crystals grow at expense of small ones • Crystal growth ceases when nutrients depleted

  7. ABALONE SHELL - BIOMINERALIZATION - VECTORIAL CONTROL OF CRYSTAL NUCLEATION, GROWTH AND FORM IN NATURE 95wt% inorganic - site specific calcite platelet oriented growth 5wt% b-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

  8. “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

  9. 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

  10. 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

  11. 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

  12. PRECURSOR SOLID STATE SYNTHESIS METHOD • Co-precipitation, high degree of homogenization, high reaction rate - applicable to nitrates, acetates, oxalates, alkoxides, b-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

  13. PROBLEMS WITH CO-PRECIPITATION METHOD • Co-precipitation applicable to nitrates, acetates, oxalates, alkoxides, b-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

  14. 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

  15. 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 DOUBLE SALT PRECURSORS • 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

  16. 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

  17. BASICS: FERROELECTRIC BARIUM TITANATE Cubic perovskite BaTiO3 Tetragonal perovskite BaTiO3 Small grains, tetragonal to cubic surface gradients, ferroelectricity particle size dependent Multidomain ferroelectric below Tc Multidomain paraelectric above Tc Cooperative electric dipole interactions within each domain Single domain superparaelectric

  18. 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 920C 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

  19. 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 + D + O2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

  20. 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 50-100C, heat at 350C in air to produce ITO

  21. 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 TiOxand crystalline TiO2 films. Choosing the right solid state precursor

  22. 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

  23. 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

  24. Nanometer scale tolerances

  25. MAGNETIC GARNETS, YxGd3-xFe5O12TUNABLE 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, enablesUC dimensions to be determined as a function of Y: Ga ratio over range 0 < x < 3

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

  27. aqueous basic medium, mineralizes, temperature gradient, transports, deposits reactants on seed crystal to grow product yttrium aluminum oxide crystal baffles T2 T1 T2 Y2O3 Al2O3 Seed crystal to grow Y3Al5O12 crystal HYDROTHERMAL SYNTHESIS AND CRYSTAL GROWTH OF YTTRIUM ALUMINUM GARNET

  28. 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)

  29. ONE OCTANT OF CUBIC UNIT CELL OF YAG Faces 3 dodecahedral Y(3+) sites Corners and center 2Oh AlO6 sites Faces 3Td AlO4 sites One octant of cubic unit cell of garnet

  30. 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

  31. 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

  32. 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 averageP of the end-member materials

  33. MAGNETIC GARNETS, YxGd3-xFe5O12TUNABLE MAGNETIC MATERIALS • Cubic unit cell parameter a versus x for YxGd3-xFe5O12 • CompositionLattice 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 1.2468 R(Gd(3+)) = 0.938Å > R(Y(3+)) = 0.900Å

  34. MAGNETIC GARNETS, YxGd3-xFe5O12TUNABLE 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

  35. 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

  36. 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

  37. VEGARD LAW AT THE NANOSCALE SYNTHESIS OF COMPOSITION TUNABLE MONODISPERSE CAPPED ZnxCd1-xSe ALLOY NANOCRYSTALS

  38. Spatial and quantum confinement and dimensionality

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