BONDING AND ELECTRONIC STRUCTURE IN MAGNESIUM DIBORIDE - DOS - THINKING ABOUT ORIGIN OF SUPERCONDUCTIVITY IN MgB2

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BONDING AND ELECTRONIC STRUCTURE IN MAGNESIUM
DIBORIDE - DOS - THINKING ABOUT ORIGIN OF
SUPERCONDUCTIVITY IN MgB2
Graphite like B22-
Mg2
MgB2
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SUPERCONDUCTIVITY IN MgB2 AT 39K A SENSATIONAL
AND CURIOUS DISCOVERY

• Metallic MgB2 known 1953, direct synthesis from
  Mg/B
• Akimitsu Nature 2001, 410, 63
• Tc of 39K, surprising - Tc Nb3Ge 23K,
  LaxSr1-xCuO4 40K, YBa2Cu3O7 90K
• Graphitic B22- sheets sandwiching hcc Mg2 layers
• Isoelectronic graphite is not a superconductor -
  only when doped at 5K?
• Strong p-p bonding interactions between B6 rings
  and Mg
• p-p stabilized wrt s-? of graphitic-like B22-
  sheets
• Cooper pairs from excitation of p-p electrons
  into s-?
• MgxAl1-xB2 substitution extra electron fills s-?
  and reduces Tc
• BCS Isotope effect of 1K on Tc for Mg10B2 higher
  than Mg11B2

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VPT AND VAPOR-LIQUID-SOLID (VLS) SYNTHESIS OF
BORON NANOWIRES AND THEIR CONVERSION TO
SUPERCONDUCTING MgB2 NANOWIRES
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VPT AND VAPOR-LIQUID-SOLID (VLS) SYNTHESIS OF
BORON NANOWIRES AND THEIR CONVERSION TO
SUPERCONDUCTING MgB2 NANOWIRES
Au dewetting on MgO on heating and cluster
formation on MgO
VLS growth of B NWs on Au clusters
Au film on MgO
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CONVERSION OF B NANOWIRES TO SUPERCONDUCTING MgB2
NANOWIRES
MgB2 NWs on Au clusters
B NWs on Au clusters
Mg 800-900
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SYNTHESIS OF SUPERCONDUCTING MAGNESIUM BORIDE
NANOWIRES

• Planar hexagonal net of stacked B2- anionic
  layers with hexagonally ordered Mg2 cations
  between the layers
• VPT agent BI3/SiI4
• VLS growth of B NWs, diameter 50-400 nm, on
  controlled size Au/Si nanoclusters supported on
  MgO substrate
• Vapor phase transformation of amorphous boron
  nanowires to crystalline magnesium boride
  nanowires

B
MgB2
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SUPERCONDUCTIVITY OF MAGNESIUM BORIDE NANOWIRES

• Magnetization of MgB2 nanowires as a function of
  temperature under conditions of zero field
  cooling and field cooling at 100G
• The existence of superconductivity within the
  sample is demonstrated by these measurements and
  the Meissner effect at 33K
• Potentially useful as building blocks in
  superconducting nanodevices and as low power
  dissipation interconnects in nanoscale
  electronics
• Recently epitaxial thin films made for
  superconducting electronics

ZFC
Tc
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RT ULTRAVIOLET ZnO NANOWIRE NANOLASERSVPT
SYNTHESIS AND GROWTH
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RT ULTRAVIOLET NANOWIRE NANOLASERSVPT SYNTHESIS
AND GROWTH VPT carbothermal reduction ZnO/C
905C gt ZnCO VPT gt ZnO NW 880C
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VPT AND VLS SYNTHESIS AND GROWTH OF ORIENTED ZnO
NANOWIRES
Sealed quartz tube reactor - fate of carbon
deposited on glass
VLS growth ZnO wires on 1-3.5 nm Aun on sapphire
880C
Alumina boat
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VPT-VLS SYNTHESIS AND GROWTH OF ORIENTED ZnO
NANOWIRES
ZnO lt0001gt growth
ZnCO
C
sapphire
Aun
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ZnO NW LASER
266 nm excitation
385 nm laser emission
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RT ULTRAVIOLET NANOWIRE NANOLASERS

• RT UV excitonic lasing action in ZnO nanowire
  arrays demonstrated
• Self-organized lt0001gtoriented ZnO nanowires grown
  on 1-3.5 nm thick Au coated sapphire substrate,
  morphology related to fastest rate of growth of
  lt0001gt face
• VPT carbothermal reduction ZnO/C 905C ---gt ZnCO
  ---gt ZnO NW 880C alumina boat, Ar flow,
  condensation process
• Wide band-gap ZnO SC nanowires, faceted end and
  sapphire end reflectors, high RI ZnO cladded by
  lower RI air and sapphire, form natural laser
  cavities, diameters 20-150 nm, lengths up to 10
  mm
• QSEs yield substantial DOS at band edges and
  enhance radiative recombination due to carrier
  confinement
• Under 266 nm optical excitation, surface-emitting
  lasing action observed at 385 nm with emission
  line width lt 0.3 nm
• The chemical flexibility and the
  one-dimensionality of these quantum confined
  nanowires make them ideal miniaturized laser
  light sources
• UV nanolasers could have myriad applications,
  including optical computing, information storage,
  and microanalysis

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RT ULTRAVIOLET NANOWIRE NANOLASERS

• PXRD pattern of ZnO nanowires on a sapphire
  substrate
• Only (000l ) peaks observed, owing to
  well-oriented lt0001gt growth configuration
• (A) PL emission spectra from nanowire arrays
  below (line a) and lasing emission above (line b
  and inset) the threshold, pump power for these
  spectra are 20, 100, and 150 kW/cm2 ,
  respectively.
• (B) Integrated emission intensity from nanowires
  as a function of optical pumping energy intensity
• (C) Schematic illustration of a nanowire as a
  resonance cavity with two naturally faceted
  hexagonal end faces acting as reflecting mirrors
• Stimulated emission from the nanowires was
  collected in the direction along the nanowires
  end-plane normal (the symmetric axis)
• The 266-nm pump beam was focused to the nanowire
  array at an angle 10 to the end-plane normal,
  all experiments were carried out at RT

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GaN NW LASER - TOPOGRAPHIC AND OPTICAL IMAGE OF
UV LASING ACTION
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SINGLE GaN NANOWIRE LASERS
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VLS SYNTHESIS AND GROWTH OF ORIENTED GaN
NANOWIRES
Wurtzite type GaN lt0001gt growth
Ga or Me3Ga/NH3/900C
sapphire
Nin
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individual GaN NW UV lasing action
Lasing from ends
lasing
photoluminescence
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TOPOTACTIC SOLID-STATE SYNTHESIS METHODS
HOST-GUEST INCLUSION CHEMISTRY

• Ion-exchange, injection, intercalation type
  synthesis
• Ways of modifying existing solid state structures
  while maintaining the integrity of the overall
  structure
• Precursor structure
• Open framework
• Ready diffusion of guest atoms, ions, organic
  molecules, polymers, organometallics,
  coordination compounds into and out of the
  structure/crystals

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TOPOTACTIC SOLID-STATE SYNTHESIS METHODS
HOST-GUEST INCLUSION CHEMISTRY

• Penetration into interlamellar spaces 2-D
  intercalation
• Into 1-D channel voids 1-D injection
• Into cavity spaces 3-D injection
• Classic materials for this kind of topotactic
  chemistry
• Zeolites, TiO2, WO3 channels, cavities
• Graphite, TiS2, NbSe2, MoO3 interlayer spaces
• Beta alumina interlayer spaces, conduction
  planes
• Polyacetylene, NbSe3 inter chain channel spaces

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TOPOTACTIC SOLID-STATE SYNTHESIS METHODS
HOST-GUEST INCLUSION CHEMISTRY

• Ion exchange, ion-electron injection, atom,
  molecule intercalation, achievable by
  non-aqueous, aqueous, gas phase, melt techniques
• Chemical, electrochemical synthesis methods
• This type of solid state chemistry creates new
  materials with novel properties, useful functions
  and wide ranging technologies

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GRAPHITE
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GRAPHITE INTERCALATION COMPOUNDS
4x1/4 K 1
8x1 C 8
C8Kstoichiometry
G (s) K (melt or vapor) C8K (bronze) C8K
(vacuum, heat) C24K C36K C48K
C60K Staging, ordered guests, K to G charge
transfer AAAA sheet stacking sequence K nesting
between parallel eclipsed hexagons, Typical of
many graphite H-G inclusion compounds
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GRAPHITE INTERCALATION ELECTRON DONORS AND
ACCEPTORS
SALCAOs of the p-pi-type create the p valence and
p conduction bands of graphite, very small band
gap, essentially metallic conductivity properties
in-plane 104 times that of out-of plane
conductivity - thermal, electrical properties
tuned by degree of CB band filling or VB emptying
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TYPICAL INTERCALATION REACTIONS OF GRAPHITE

• G (HF/F2/25oC) ? C3.3F to C40F
• intercalation via HF2- not F- - less strongly
  interacting -more facile diffusion
• G (HF/F2/450oC) ? CF0.68 to CF (white)
• G (H2SO4 conc.) ? C24(HSO4).2H2SO4 H2
• G (FeCl3 vapor) ? CnFeCl3
• G (Br2 vapor) ? C8Br

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PROPERTIES OF INTERCALATED GRAPHITE

• Structural planarity of layers often unaffected
  by intercalation - bending of layers has been
  observed - intercalation often reversible
• Modification of thermal and electrical
  conductivity behavior by tuning the degree of
  p-CB filling or p-VB emptying
• Anisotropic properties of graphite intercalation
  systems usually observed - layer spacing varies
  with nature of the guest and the loading
• CF 6.6 Å, C4F 5.5 Å, C8F 5.4 Å

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BUTTON CELLS LITHIUM-GRAPHITE FLUORIDE BATTERY
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BUTTON CELLSLITHIUM-GRAPHITE FLUORIDE BATTERY

• Cell electrochemistry
• xLi CFx ? xLiF C
• xLi ? xLi e-
• CxxF- xLi xe- ? C xLiF Nominal cell
  voltage 2.7 V
• CFx safe storage of fluorine, intercalation of
  graphite by fluorine
• Millions of batteries sold yearly, first
  commercial Li battery, Panasonic
• Lightweight high energy density battery, just
  C/Li/F, cell requires SS anode/lithium anode/Li
  ion conductor/CFx-acetylene black/aluminum
  cathode

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SYNTHESIS OF BORON AND NITROGEN GRAPHITES -
INTRALAYER DOPING

• New ways of modifying the properties of graphite
• Instead of tuning the degree of CB/VB filling
  with electrons and holes using the traditional
  methods involve interlayer doping
• Put B or N into the graphite layers, deficient
  and rich in carriers, enables intralayer doping
  with holes and electrons respectively
• Also provides a new intercalation chemistry

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SYNTHESIS OF AND BC3THEN PROVING IT IS SINGLE
PHASE?

• Traditional heat and beat
• xB yC (2350oC) ? BCx
• Maximum 2.35 at B incorporation in C
• Poor quality not well-defined materials
• New approach, soft chemistry, low T, flow
  reaction quartz tube
• 2BCl3 C6H6 (800oC) ? 2BC3 (lustrous film on
  walls) 6HCl

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CHEMICAL AND PHYSICAL CHARACTERIZATION OF BC3

• BC3 15/2F2 ? BF3 3CF4
• Fluorine burn technique
• BF3 CF4 1 3
• Shows BC3 composition
• Electron and Powder X-Ray Diffraction Analysis
• Shows graphite like interlayer reflections (00l)

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CHEMICAL AND PHYSICAL CHARACTERIZATION OF BC3

• 2BC3 (polycryst) 3Cl2 (300oC) ? 6C (amorph)
  2BCl3
• C (cryst graphite) Cl2 (300oC) ? C (cryst
  graphite)
• This neat experiment proves B is truly a
  "chemical" constituent of the graphite sheet and
  not an amorphous component of a "physical"
  mixture with graphite
• Synthesis, analysis, structural findings all
  indicate a graphite like structure for BC3 with
  an ordered B, C arrangement in the layers

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STRUCTURE OF BORON GRAPHITE BC3
4Cx1/4 2Cx1/2 10Cx1 12C
6Bx1/2 1Bx1 4B
Probable layer atomic arrangement with
stoichiometry BC3
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CHEMICAL AND PHYSICAL CHARACTERIZATION OF BC3

• BC3 interlayer spacing similar to graphite
• Also similar to graphite like BN made from
  thermolysis of borazine B3N3H6
• Four probe basal plane resistivity on BC3 flakes
• s(BC3)AB 1.1 s(G)AB, (greater than 2 x 104
  ohm-1cm-1)

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4-PROBE CONDUCTIVITY MEASUREMENTS
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REPRESENTATIVE BC3 INTERCALATION CHEMISTRY

• BC3 S2O6F2 ? (BC3)2SO3F Oxidative
  Intercalation
• Note O2FS-O--OSO2F, peroxydisulphuryl fluoride,
  weak peroxy-linkage, easily reduced to 2SO3F-
• (BC3)2SO3F Ic 8.1 Å, (C7)SO3F Ic
  7.73 Å, (BN)3SO3F Ic 8.06 Å
• BC3 Ic 3-4 Å , C
  Ic 3.35 Å, BN Ic 3.33 Å
• More Juicy intercalation chemistry for BC3
• BC3 NaNaphthalide-/THF ? (BC3)xNa (bronze,
  first stage, Ic 4.3 Å)
• BC3 Br2(l) ? (BC3)15/4Br (deep blue)

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ATTEMPT TO INCORPORATE NITROGEN INTO THE GRAPHITE
SHEETS, EVIDENCE FOR C5N

• Pyridine Cl2 (800oC, flow, quartz tube) ?
  silvery deposit (PXRD Ic 3.42 Å)
• Fluorine burning of silver deposit ? CF4/NF3/N2
• No signs of HF, ClF1,3,5 in F2 burning reaction
• Superior conductivity wrt graphite
• Try to balance the fluorine burning reaction to
  give the nitrogen graphite stoichiometry of C5N -
  a challenge for your senses!!! 4C5N 43F2 ?
  20CF4 2NF3 N2

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INTERCALATION SYNTHESIS OF TRANSITION METAL
DICHALCOGENIDES

• Group IV, V, VI MS2 and MSe2 Compounds
• Layered structures
• Most studied is TiS2
• hcp S2-
• Ti4 in Oh sites
• Van der Waals gap

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INTERCALATION SYNTHESIS OF TRANSITION METAL
DICHALCOGENIDES

• Li intercalated between the layers
• Li resides in well-defined Td S4 interlayer
  sites
• Electrons injected into Ti4 t2g CB states
• LixTiS2 with tunable band filling and unfilling
• Localized xTi(III)-(1-x) Ti(IV) vs delocalized
  Ti(IV-x) electronic bonding models
• VDW gap prized apart by 10

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SEEING INTERCALATION - DIRECT VISUALIZATION
OPTICAL MICROSCOPY
Intercalating lithium - see the layers spread
apart