BONDING AND ELECTRONIC STRUCTURE IN MAGNESIUM DIBORIDE - DOS - THINKING ABOUT

ORIGIN OF SUPERCONDUCTIVITY IN MgB2

Graphite like gg

3s

3p

3s-

p

p

3s-*

E

N(E)

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- bonding interactions between B6 rings and Mg

• p- stabilized wrt s-* of graphitic-like B22- sheets

• Cooper pairs from excitation of 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

B/I2/Si/1100°C BI3/SiI4 VPT

MgO/5nm Au/B NWs/1000°C

Sealed quartz tube

B NWs/Mg/800-900°C MgB2 NWs

Tantalum tube

VPT AND VAPOR-LIQUID-SOLID (VLS) SYNTHESIS OF BORON NANOWIRES AND THEIR CONVERSION TO

SUPERCONDUCTING MgB2 NANOWIRES

Au film on MgO

Au dewetting on MgO on heating and cluster formation on MgO

VLS growth of B NWs on Au clusters

CONVERSION OF B NANOWIRES TO SUPERCONDUCTING MgB2 NANOWIRES

Mg 800-900°

B NWs on Au clusters MgB2 NWs on Au clusters

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

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

Tc

ZFC

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

VLS growth ZnO wires on 1-3.5 nm Aun on sapphire 880°C

Sealed quartz tube reactor - fate of carbon deposited on glass

Alumina boat

ZnO/C/905°C ZnCO VPT

VPT-VLS SYNTHESIS AND GROWTH OF ORIENTED ZnO NANOWIRES

ZnCO C

ZnO <0001> growth

sapphire Aun

ZnO NW LASER

266 nm excitation

385 nm laser emission

RT ULTRAVIOLET NANOWIRE NANOLASERS

• 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 m

• 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

• 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

Ga or Me3Ga/NH3/900°C

Wurtzite type GaN <0001> growth

sapphire Nin

individual GaN NW UV lasing action

Lasing from ends

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 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

GRAPHITE

A

A

B

out of plane p orbitals - * delocalized bands

VDW gap 3.35Å

sp2 in plane bonding

C-C 1.41Å, BO 1.33

ABAB stacked hexagonal graphite

Pristine graphite - filled band - empty * band - narrow gap - semimetal

GRAPHITE INTERCALATION COMPOUNDS

G (s) + K (melt or vapor) C8K (bronze) C8K (vacuum, heat) C24K C36K C48K C60KStaging, ordered guests, K to G charge transferAAAA sheet stacking sequence K nesting between parallel eclipsed hexagons, Typical of many graphite H-G inclusion compounds

4x1/4 K = 1

8x1 C = 8

C8Kstoichiometry

GRAPHITE INTERCALATION ELECTRON DONORS AND ACCEPTORS

SALCAOs of the -pi-type create the valence and * 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

N(E)

C C8Br electron depletion

from C2p VB - metallic

C8K electron transfer to

C2p CB - metallic

E(F)

E(F)Eg

CB

VB

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

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 *-CB filling or 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

SS contact

Li anode

Li+/PEO

CFx/C cathode

Al contact

e

BUTTON CELLSLITHIUM-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

• 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

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-1cm-1)

4-PROBE CONDUCTIVITY MEASUREMENTS4-PROBE CONDUCTIVITY MEASUREMENTS

I = V1/R1

Rsample = V2/I

Rsample = (V2R1)/V1

= Rsample (A/L)

= 1

LA

I

V2

V1

R1

Current Source

Ohmeter

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 + Na+Naphthalide-/THF (BC3)xNa (bronze, first stage, Ic ~ 4.3 Å)

• 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

• 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%

SEEING INTERCALATION - DIRECT VISUALIZATION OPTICAL MICROSCOPY

Intercalating lithium - see the layers spread apart