Transcript CHM 434F/1206F SOLID STATE MATERIALS CHEMISTRY

BONDING AND ELECTRONIC STRUCTURE IN
MAGNESIUM DIBORIDE - DOS - THINKING ABOUT
ORIGIN OF SUPERCONDUCTIVITY IN MgB2
E
p*
3p-p*
p
3p
3s-*
3s

3p-p
3s-
Graphite like B22-
N(E)
MgB2
Mg2+
SUPERCONDUCTIVITY IN MgB2 AT 39K
A SENSATIONAL AND CURIOUS DISCOVERY
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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
VPT AND VAPOR-LIQUID-SOLID (VLS) SYNTHESIS OF
BORON NANOWIRES AND THEIR CONVERSION TO
SUPERCONDUCTING MgB2 NANOWIRES
Sealed quartz tube
B/I2/Si/1100°C
BI3/SiI4 VPT
Tantalum tube
B NWs/Mg/800-900°C
MgO/5nm Au/B NWs/1000°C
MgB2 NWs
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
Au film on MgO
VLS growth of B
NWs on Au clusters
CONVERSION OF B NANOWIRES TO
SUPERCONDUCTING MgB2 NANOWIRES
B NWs on Au clusters
MgB2 NWs on Au
clusters
Mg 800-900°
SYNTHESIS OF SUPERCONDUCTING
MAGNESIUM BORIDE NANOWIRES
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B
MgB2
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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
SUPERCONDUCTIVITY OF
MAGNESIUM BORIDE NANOWIRES
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ZFC
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Tc
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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
RT ULTRAVIOLET ZnO NANOWIRE NANOLASERS
VPT SYNTHESIS AND GROWTH
RT ULTRAVIOLET NANOWIRE NANOLASERS
VPT SYNTHESIS AND GROWTH
VPT carbothermal reduction
ZnO/C 905°C ===> ZnCO VPT ===> ZnO NW 880°C
VPT AND VLS SYNTHESIS AND GROWTH OF
ORIENTED ZnO NANOWIRES
Sealed quartz tube reactor - fate of carbon
deposited on glass
ZnO/C/905°C
Alumina boat
ZnCO VPT
VLS growth ZnO wires on 1-3.5 nm
Aun on sapphire 880°C
VPT-VLS SYNTHESIS AND GROWTH OF
ORIENTED ZnO NANOWIRES
ZnO <0001> growth
ZnCO
sapphire
C
Aun
ZnO NW
LASER
266 nm
excitation
385 nm laser
emission
RT ULTRAVIOLET NANOWIRE
NANOLASERS
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RT UV excitonic lasing action in ZnO nanowire arrays demonstrated
Self-organized <0001>oriented ZnO nanowires grown on 1-3.5 nm thick Au
coated sapphire substrate, morphology related to fastest rate of growth of
<0001> face
VPT carbothermal reduction ZnO/C 905°C ---> ZnCO ---> ZnO NW 880°C
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 < 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
RT ULTRAVIOLET
NANOWIRE
NANOLASERS
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PXRD pattern of ZnO nanowires on a sapphire substrate
Only (000l ) peaks observed, owing to well-oriented <0001> 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 nanowire’s 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
GaN NW LASER - TOPOGRAPHIC AND OPTICAL IMAGE
OF UV LASING ACTION
SINGLE GaN NANOWIRE LASERS
VLS SYNTHESIS AND GROWTH OF
ORIENTED GaN NANOWIRES
Wurtzite type GaN <0001> growth
Ga or Me3Ga/NH3/900°C
sapphire
Nin
Lasing from ends
individual GaN NW UV
lasing action
lasing
photoluminescence
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
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
TOPOTACTIC SOLID-STATE SYNTHESIS METHODS:
HOST-GUEST INCLUSION CHEMISTRY
• Ion exchange, ion-electron injection, atom,
molecule intercalation, achievable by nonaqueous, 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
GRAPHITE
out of plane pp orbitals - p/p* delocalized bands
A
B
sp2 in plane  bonding
VDW gap 3.35Å
C-C 1.41Å, BO 1.33
A
ABAB stacked
hexagonal graphite
Pristine graphite - filled p band - empty
p* band - narrow gap - semimetal
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
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
E
C
p*
CB
C8K electron transfer to
C8Br electron depletion
C2pp CB - metallic
from C2pp VB - metallic
p*
Eg
p

VB
p

E(F)
p*
p
E(F)

N(E)
TYPICAL INTERCALATION REACTIONS OF GRAPHITE
• G (HF/F2/25oC)  C3.3F to C40F
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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
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
BUTTON CELLS
LITHIUM-GRAPHITE FLUORIDE BATTERY
e
Al contact
SS contact
Li anode
CFx/C cathode
Li+/PEO
BUTTON CELLS
LITHIUM-GRAPHITE FLUORIDE BATTERY
• Cell electrochemistry
• xLi + CFx  xLiF + C
• xLi  xLi+ + e• Cx+xF- + xLi+ + xe-  C + xLiF Nominal cell voltage 2.7 V
• CFx safe storage of fluorine, intercalation of graphite by fluorine
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Millions of batteries sold yearly, first commercial Li battery, Panasonic
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Lightweight high energy density battery, just C/Li/F, cell requires SS
anode/lithium anode/Li+ ion conductor/CFx-acetylene black/aluminum
cathode
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
SYNTHESIS OF AND BC3
THEN 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
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)
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
STRUCTURE OF BORON GRAPHITE BC3
4Cx1/4 + 2Cx1/2 + 10Cx1 = 12C
6Bx1/2 + 1Bx1 = 4B
Probable layer atomic arrangement with stoichiometry BC3
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
 (BC3)AB ~ 1.1 (G)AB, (greater than 2 x 104 ohm-1cm1)
4-PROBE CONDUCTIVITY MEASUREMENTS
L
A
I = V1/R1
I
Rsample = V2/I
Rsample = (V2R1)/V1
r = Rsample (A/L)
V2
Current
Source
R1
 = 1/r
V1
Ohmeter
REPRESENTATIVE BC3 INTERCALATION CHEMISTRY
• BC3 + S2O6F2  (BC3)2SO3F
Oxidative Intercalation
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Note: O2FS-O--OSO2F, peroxydisulphuryl fluoride, weak peroxylinkage, easily reduced to 2SO3F-
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(BC3)2SO3F
8.06 Å
Ic = 8.1 Å,
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BC3
3.33 Å
Ic = 3-4 Å ,
(C7)SO3F
C
Ic = 7.73 Å,
Ic = 3.35 Å,
(BN)3SO3F
BN
Ic =
Ic =
• More Juicy intercalation chemistry for BC3
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BC3 + Na+Naphthalide-/THF  (BC3)xNa (bronze, first stage, Ic ~ 4.3 Å)
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BC3 + Br2(l)  (BC3)15/4Br (deep blue)
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
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
INTERCALATION SYNTHESIS OF TRANSITION
METAL DICHALCOGENIDES
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Li+ intercalated between the layers
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Li+ resides in well-defined Td S4 interlayer
sites
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Electrons injected into Ti4+ t2g CB states
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LixTiS2 with tunable band filling and unfilling
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Localized xTi(III)-(1-x) Ti(IV) vs delocalized
Ti(IV-x) electronic bonding models
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VDW gap prized apart by 10%
SEEING INTERCALATION - DIRECT VISUALIZATION
OPTICAL MICROSCOPY
Intercalating lithium - see the layers spread apart