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

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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 ... inset) the threshold, pump power for these spectra are 20, 100, and 150 kW/cm2 , respectively. ... – PowerPoint PPT presentation

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


1
BONDING AND ELECTRONIC STRUCTURE IN MAGNESIUM
DIBORIDE - DOS - THINKING ABOUT ORIGIN OF
SUPERCONDUCTIVITY IN MgB2
Graphite like B22-
Mg2
MgB2
2
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

3
VPT AND VAPOR-LIQUID-SOLID (VLS) SYNTHESIS OF
BORON NANOWIRES AND THEIR CONVERSION TO
SUPERCONDUCTING MgB2 NANOWIRES
4
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
5
CONVERSION OF B NANOWIRES TO SUPERCONDUCTING MgB2
NANOWIRES
MgB2 NWs on Au clusters
B NWs on Au clusters
Mg 800-900
6
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
7
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
8
RT ULTRAVIOLET ZnO NANOWIRE NANOLASERSVPT
SYNTHESIS AND GROWTH
9
RT ULTRAVIOLET NANOWIRE NANOLASERSVPT SYNTHESIS
AND GROWTH VPT carbothermal reduction ZnO/C
905C gt ZnCO VPT gt ZnO NW 880C
10
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
11
VPT-VLS SYNTHESIS AND GROWTH OF ORIENTED ZnO
NANOWIRES
ZnO lt0001gt growth
ZnCO
C
sapphire
Aun
12
ZnO NW LASER
266 nm excitation
385 nm laser emission
13
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

14
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

15
GaN NW LASER - TOPOGRAPHIC AND OPTICAL IMAGE OF
UV LASING ACTION
16
SINGLE GaN NANOWIRE LASERS
17
VLS SYNTHESIS AND GROWTH OF ORIENTED GaN
NANOWIRES
Wurtzite type GaN lt0001gt growth
Ga or Me3Ga/NH3/900C
sapphire
Nin
18
individual GaN NW UV lasing action
Lasing from ends
lasing
photoluminescence
19
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

20
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

21
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

22
GRAPHITE
23
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
24
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
25
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

26
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

27
BUTTON CELLS LITHIUM-GRAPHITE FLUORIDE BATTERY
28
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

29
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

30
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

31
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)

32
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

33
STRUCTURE OF BORON GRAPHITE BC3
4Cx1/4 2Cx1/2 10Cx1 12C
6Bx1/2 1Bx1 4B
Probable layer atomic arrangement with
stoichiometry BC3
34
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)

35
4-PROBE CONDUCTIVITY MEASUREMENTS
36
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)

37
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

38
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

39
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

40
SEEING INTERCALATION - DIRECT VISUALIZATION
OPTICAL MICROSCOPY
Intercalating lithium - see the layers spread
apart
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