Sealed glass tube reactors Reactant(s) A Gaseous transporting agent B Temperature gradient furnace T ~ 50 o C Equilibrium established A(s) + B(g)

• Sealed glass tube reactors

• Reactant(s) A

• Gaseous transporting agent B

• Temperature gradient furnace T ~ 50oC

• Equilibrium established

• A(s) + B(g) AB(g)

VAPOR PHASE TRANSPORT VPC MATERIALS SYNTHESIS, CRYSTAL GROWTH, PURIFICATION

T2 T1

B(g)A(s) AB(g)A(s)

Glass tube


B(g)A(s)

T2 T1

AB(g)A(s)

Glass tube• Equilibrium constant K

• A + B react at T2

• Gaseous transport by AB(g)

• Decomposes back to A(s) at T1

• Creates crystals of pure A


B(g)A(s)

T2 T1

AB(g)A(s)

Glass tube• Temperature dependent K

• Equilibrium concentration of AB(s) changes with T

• Different at T2 and T1

• Concentration gradient of AB(g) provides thermodynamic driving force for gaseous diffusion from T2 to T1

THERMODYNAMICS OF CVT

• A(s) + B(g) AB(g)

• Reversible equilibrium needed: Go = -RTlnKequ

• Consider case of “exothermic” reaction with - Go

• Thus Go = RTlnKequ

• Smaller T implies larger Kequ

• Forms at cooler end - decomposes at hotter end of reactor

• Consider case of “endothermic” reaction with +Go

• Thus Go = -RTlnKequ = RTln(1/Kequ)

• Larger T implies larger Kequ

• Forms at hotter end - decomposes at cooler end of reactor

USES OF VPT

• synthesis of new solid state materials

• growth of single crystals

• purification of solids

• Endothermic reaction

• PtO2 forms at hot end

• Diffuses to cool end

• Deposits well formed Pt crystals

• Observed in furnaces containing Pt heating elements

• CVT, T2 > T1, provides concentration gradient and free energy thermodynamic driving force for gaseous diffusion of vapor phase transport agent PtO2

PLATINUM HEATER ELEMENTS IN FURNACES

THEY MOVE!! Pt(s) + O2(g) PtO2(g)

VPT agent PtO2(g)

Atmosphere O2(g)

Pt(s) PtO2(g)

T2 T1

APPLICATIONS OF CVT METHODS

• Purification of Metals

• Van Arkel Method

• Cr(s) + I2(g) (T2) (T1) CrI2(g)

• Exothermic, CrI2(g) forms at T1, pure Cr(s) deposited at T2

• Useful for Ti, Hf, V, Nb, Cu, Ta, Fe, Th

• Removes metals from carbide, nitride, boride, silicide, oxide impurities!!!

DOUBLE TRANSPORT INVOLVING OPPOSING EXOTHERMIC-ENDOTHERMIC REACTIONS

• Endothermic

• WO2(s) + I2(g) (T1 800oC) (T2 1000oC) WO2I2(g)

• Exothermic

• W(s) + 2H2O(g) + 3I2(g) (T2 1000oC) (T1 800oC) WO2I2 (g) + 4HI(g)

• The antitheticalantithetical nature of these two reactions allows W/WO2 mixtures which often form together to be separated at different ends of the gradient reactor using H2O/I2 as the VPT reagents

VAPOR PHASE TRANSPORT FOR SYNTHESIS SYNTHESIS

• A(s) + B(g) (T1) (T2) AB(g)

• AB(g) + C(s) (T2) (T1) AC(s) + B(g)

• Concept: couple VPT with subsequent chemical reaction to give overall reaction and desired product :

• A(s) + C(s) + B(g) (T2) AC(s) + B(g) (T1)

REAL EXAMPLES VPT SYNTHESIS DIRECT REACTIONDIRECT REACTION

• SnO2(s) + 2CaO(s) Ca2SnO4(s)

• Sluggish reaction even at high T for a useful phosphor - luminescent cations like Mn2+, Cu+, Ag+ isomorphously replace Ca2+ sites in crystal lattice

• Greatly speeded up with CO as VPT agent

• SnO2(s) + CO(g) SnO(g) + CO2(g)

• SnO(g) + CO2(g) + 2CaO(s) Ca2SnO4(s) + CO(g)

REAL EXAMPLES VPT SYNTHESISDIRECT REACTIONDIRECT REACTION

• Cr2O3(s) + NiO(s) NiCr2O4(s)

• Greatly enhanced rate to magnetic Spinel with O2 VPT agent

• Cr2O3(s) + 3/2O2 2CrO3(g)

• 2CrO3(g) + NiO(s) NiCr2O4(s) + 3/2O2(g)

OVERCOMING PASSIVATIONPASSIVATION IN SOLID STATE SYNTHESIS THROUGH VPT

• 2Al(s) + 3S(s) Al2S3(s) passivating skin stops reaction

• In presence of cleansing VPT agent I2 the Al2S3 skin is removed at hot end to reveal fresh Al surface to react with S to form Al2S3 by VPT at cooler end according to:

• Endothermic: Al2S3(s) + 3I2(g) (T1 700oC) (T2 800oC) 2AlI3(g) + 3/2S2(g)

• Zn(s) + S(s) ZnS(s) passivation prevents reaction proceeding to completion and again I2 cleans surface of ZnS to reveal fresh Zn to react with S to form ZnS by VPT at the cooler end according to:

• Endothermic: ZnS(s) + I2(g) (T1 800oC) (T2 900oC) ZnI2(g) + 1/2S2(g)

• Endothermic reaction forms at hotter end and crystallizes at cooler end according to the VPT reaction

• Fe3O4(s) + 8HCl(g) 1020K 1FeCl2(g) + 2FeCl3(g) + 4H2O(g) 1270K

• Inverted Spinel ferromagnetic Magnetite crystals grow at cooler end - B(AB)O4 - Fe(III)(Td)[Fe(III)Fe(II)(Oh)]O4

VPT GROWTH OF FERROMAGNETIC MAGNETITE SINGLE CRYSTALS FROM POWDERED MAGNETITE

VPT agent FeCl2/FeCl3(g)

Atmosphere HCl(g)

Fe3O4(s)

1270K 1020K

FERROMAGNETIC INVERTED SPINEL MAGNETITE B(AB)O4

Fe(III)(Td)[Fe(III)Fe(II)(Oh)]O4

Multi Weiss domain paramagnet above Tc

Multi Weiss domain ferromagnet below Tc

Single domain superparamagnet

Field H

M

H

Ms

Mr

Hc

Magnetization Hysteresis M vs H Diagnostic of Ferromagnetism

Ms saturation magnetization

Mr remnant magnetization

Hc coercive field

• Endothermic reaction forms at hotter end and crystallizes at cooler end - also removes passivating TiS2 skin on Ti

• (T1) TiS2(s) + 2Br2(g) (T2) TiBr4(g) + S2(g)

• TiS2 plate morphology crystal grow at cooler end• Interesting for studying intercalation reactions - kinetics,

mechanism, structure• Historically relevant for use of TiS2 as a LSSB cathode

VPT SYNTHESIS AND CRYSTAL GROWTH OF TiS2 FROM POWDERED Ti/S

VPT agent TiBr4(g)

Atmosphere Br2(g)

Ti/S(s)

550-685oC (T2) 510-645oC (T1)

TiS2

LITHIUM SOLID STATE BATTERY MATERIAL

Li + TiS2 LixTiS2

• TiS2 hcp packing S(-II) 3p filled VB Oh Ti(IV) 3d t2g empty CB

• Li+ intercalates between hcp S2- layers in well defined LiS4 Td crystal sites

• Charge balancing electrons injected into t2g Ti(IV) CB

• TiS2 semiconductor LixTiS2 conductivity increases upon insertion of Li(+) and e(-)

• Hopping semiconductor localized mixed valence description xLi(I)xTi(III)(1-x)Ti(IV)SHopping semiconductor localized mixed valence description xLi(I)xTi(III)(1-x)Ti(IV)S22

Li insertionLi insertion

LITHIUM SOLID STATE BATTERY MATERIAL

Li + TiS2 LixTiS2

• Li intercalation varies from 1 x 0, 10% lattice expansion, TiS2 LiTiS2

• Microscopic intercalation manifest macroscopically – expansion of thickness of plate crystal

• Capacity ~ 250 A-h/kg, Voltage ~ 1.9 Volts - too low for SS cathode

• Energy density ~ 480 W-h/kg

Li insertionLi insertion

• WO3(s) + 2Cl2(g) (T2 1060oC) (T2 1060oC) WO2Cl2(g) + Cl2O(g)

• WO2Cl2(g) + Cl2O(g) + ZnO(s) (T2 1060oC) ZnWO4(s) + Cl2(g) (T1 980oC)

VPT SYNTHESIS OF ZnWO4

A REAL PHOSPHOR HOST CRYSTAL FOR LUMINESCENT Ag(I), Cu(I), Mn(II) Isomorphous Replacement of Non-Luminescent Zn(II) Cations by Luminescent Ones

Endothermic reaction

VPT agent WO2Cl2(g) + Cl2O(g) formed at hot end in an atmosphere Cl2(g)

WO3/ZnO(s)1060oC (T2) 980oC (T1)ZnWO4(s)

GaAs(s) + HCl(g) GaCl(g) + 1/2H2(g) + 1/4As4(g)

VPT GROWTH OF EPITAXIAL GaAs FILMS ON LATTICE MATCHING SUBSTRATE OR GROWTH OF SINGLE CRYSTALS

USING CONVENIENT STARTING MATERIALS

Endothermic VPT agent GaCl/As4/H2(g) formed at hot

end deposits GaAs at cold end in an atmosphere of HCl(g)

(T2) (T1)GaAs(s)GaAs(s)

MgB2 SAT ON THE SHELF DOING NOTHING FOR HALF A CENTURY

AND THEN THE BIGGEST SURPRISE

SINCE HIGH Tc CERAMIC

SUPERCONDUCTORS

SUPERCONDUCTIVITY IN MgB2 AT 39K

A SENSATIONAL AND CURIOUS DISCOVERY

Mg

B

Mg

Note basic repeat unit is 1Mg + 6/3B = MgB2Note basic repeat unit is 1Mg + 6/3B = MgB2

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

ORIGIN OF SUPERCONDUCTIVITY IN MgB2

Graphite like B22- Mg2+MgB2

ip

p

*op

3s

3p

3s-

3p

3p*

3s-*

E

N(E)

Responsible for metallic behaviour

BCS THEORY OF SUPERCONDUCTIVITY

Tc = 1.13hD/2kB{exp[-1/N(EF)V]}

Debye cut off frequency - highest phonon mode - temperature dependent, D m-1/2 - expected isotope effect on (Tc(m1)/Tc(m2) = (m2/m1)1/2

DOS of electrons at Fermi level - larger N(EF) - larger Tc

Matrix element characteristic of e-ph-e coupling of Cooper pairs - larger V - larger Tc - requires high frequency phonon modes



• Metallic MgB2 known since 1953• Direct synthesis from reacting Mg/B solids• Akimitsu Nature 2001, 410, 63 • Tc of 39K, surprising• Tc Nb3Ge 23K, LaxSr1-xCuO4 40K, YBa2Cu3O7 90K• Graphitic B2

2- sheets sandwiching hcc Mg2+ layers• Isoelectronic graphite NOT a superconductor – but

when doped C8K becomes one with Tc = 0.15KTc = 0.15K



• Strong 3p bonding between B6 rings and Mg• Band diagram 3p stabilized wrt 3s-* of graphitic-like

B22- sheets

• BCS Isotope effect of 1K on Tc for Mg10B2 higher than Mg11B2 implicates phonons

• Cooper pairs (e-p-e coupling) generated by excitation of 3p electrons into 3s-*

• MgxAl1-xB2 - smaller more highly charged Al3+ isomorphously substitutes for Mg2+ results in stronger Al3+ 3p, larger pto p* and hence 3p-* gaps

• Fewer Cooper pairs, lower Tc

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/VLS 1000°C

Sealed quartz tube

B NWs-MgO/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

AuSi dewetting on MgO on heating and nano cluster formation on MgO

VLS growth of B NWs on AuSi clusters

CONVERSION OF B NANOWIRES TO SUPERCONDUCTING MgB2 NANOWIRES

Mg 800-900°

B NWs on AuSi clusters MgB2 NWs on AuSi 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 AuSi nanoclusters supported on MgO substrate

• Vapor-solid 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 –magnetic field uncouples Cooper pairs zero field maintains them

• The existence of superconductivity within the sample is demonstrated by these measurements of perfect diamagnetism and the Meissner effect at ~ 33K of total exclusion of an external magnetic field (diamagnetic supercurrent and Lenz’s law)

• Potentially useful as building blocks in superconducting nanodevices and as low power dissipation superconducting interconnects in nanoscale electronics

• Recently Recently epitaxial epitaxial thin films made thin films made for superconducting electronics for superconducting electronics and nanohelices!and nanohelices!

Tc

ZFC

Meissner Effect

• The Meissner effect is the total exclusion of any magnetic flux from the interior of a superconductor

• It is often referred to as perfect diamagnetism

• In the effect, there is an exclusion of magnetic flux brought about by electrical screening currents that flow at the surface of the superconductor and which generate a magnetic field that exactly cancels (repels) the externally applied field inside the superconductor (Lenz’s law).

• The Meissner effect is one of the defining features of superconductivity, and its discovery served to establish that the onset of superconductivity is a phase transition between uncoupled and phonon coupled electrons

• Superconducting magnetic levitation is due to the Meissner effect which repels a permanent magnet Mag Lev high speed train

SUPERCONDUCTING MAGNESIUM DIBORIDE HELICES

Superconducting nanocoils may have practical applicationsas nanoactuators or in flexible superconducting cable.

Mg(s) + B2H6 (770-800°C VPTVPT flow of N2 and H2) MgB2

RT ULTRAVIOLET ZnO NANOWIRE NANOLASERS

VPT SYNTHESIS AND GROWTH

RT ULTRAVIOLET NANOWIRE NANOLASERS

VPT SYNTHESIS AND GROWTH

VPT carbo-thermal reduction

ZnO/C 905°C ===> ZnCO VPT ===> ZnO VLS NW 880°C

VPT AND VLS SYNTHESIS AND GROWTH OF ORIENTED ZnO NANOWIRES

VLS growth ZnO wires on 1-3.5 nm Aun nanoclusters 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 UV e-h excitonic lasing action in ZnO nanowire arrays demonstrated

• Self-organized Wurtzite <0001> oriented ZnO nanowires grown (epitaxially) on 1-3.5 nm thick Au coated sapphire substrate, dewetting makes Au nanoclusters – thickness of Au film controls diameter of Au nanocluster – ZnO nanowires grow from Au nanoclustrs - nanowire morphology related to fastest rate of growth of <0001> face

• VPT carbothermal reduction ZnO/C 905°C ZnCO ZnO VLS NW growth at 880°C - alumina crucible, Ar flow, condensation process

• Wide band-gap ZnO SC nanowires, faceted end and epitaxial sapphire end reflectors, high RI ZnO that is cladded by lower RI air and sapphire form natural TIR waveguiding laser cavities, nanowire diameters 20-150 nm with lengths up to 10 m

PXRD – SHOWS PREFERRED GROWTH OF NANOWIRES ALONG C-AXIS OF ZnO

RT ULTRAVIOLET NANOWIRE

NANOLASERS

• PXRD pattern of ZnO nanowires on a sapphire substrate

• Only (000l ) peaks observed owing to well-oriented <0001> growth

• (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

RT ULTRAVIOLET NANOWIRE

NANOLASERS

• (C) Schematic of a nanowire as a resonance cavity with two naturally faceted hexagonal end faces acting as reflecting mirrors

• Stimulated emission from the nanowires collected in the direction along the nanowire’s end-plane normal (the symmetric axis)

• The 266-nm pump beam focused on nanowire array at angle 10° to the end-plane normal, all experiments were carried out at RT


• QSEs cause substantial DOS at band edges and enhances e-h 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 and patterned arrays could have myriad applications, including optical computing, information storage and on chip microanalysis and chemical/biochemical sensing platforms

GaN NW LASER - TOPOGRAPHIC AND OPTICAL IMAGE OF UV LASING ACTION – DEFINES NW END EMISSION

VLS SYNTHESIS AND GROWTH OF ORIENTED GaN NANOWIRES

Ga or Me3Ga + NH3/900°C

Wurtzite type GaN <0001> growth

sapphire Nin

3MeH or 3/2H2

SINGLE GaN NANOWIRE LASERS

individual GaN NW UV lasing action

Lasing from ends

lasing

photoluminescence

COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS – BANDGAP ENGINEERING

ABS PL

COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS

FULL COLOR TUNING OF FULL COLOR TUNING OF PHOTOLUMINESCENCEPHOTOLUMINESCENCE

The reactor consists of three inner quartz tubes, which supply the reactive gases, InCl3, GaCl3 (N2 carrier) and NH3, and an outer quartz tube, which supplies inert gas (N2) and houses the reaction in a horizontal tube furnace.

Two independently controlled heating tapes were used to tune the vapour pressure of the InCl3 and GaCl3 precursors.

The positioning of the reactive gas outlets results in the observed InGaN compositional gradient.

Shown below the furnace is the temperature profile, indicating that the centre of the furnace is maintained at 700 C, whereas the substrates are at 550 C.

Inset: Photograph of an as-made sample on quartz (left) showing ABS and a colour image from PL of a section of substrate (right).


Vegard’s Law on Unit Cell Dimensions


Vegard’s Law on Unit Cell Dimensions

Wire morphology and XRD at varying InGaN composition.

a, SEM images of the nanowire morphology, with increasing In concentration from images 1 to 13. The wire morphology changes most noticeably in 10–11 from the smaller to larger wires at around 75–90% In.

b, 100, 002 and 101 Wurtzite XRD peaks from left to right of the nanowires, with increasing In concentration from images 1 to 13.

c, Lattice constants a and c derived from the 100 and 002 diffraction peaks respectively, plotted as a function of In concentration determined by EDS, and Vegard-law values for the respective a and c lattice constants as a function of indium concentration (red and blue lines).

COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESISVegard’s Law on Electronic Bandgap


Vegard’s Law on Electronic Bandgap

Optical characterization of the InGaN nanowires.

a, Colour CCD images,

b, visible PL emission (x =0–0.6),

c, corrected peak intensities and

d, optical absorption spectra (x =0–1.0) of the InxGa1−xN nanowire arrays taken at intervals across the substrates with varying concentration x.

e, Energy plotted as a function of In concentration x determined by EDS for PL, absorption and EELS and bowing equation fit to absorption spectra.

bowing equation: E(x)=(P1)(1−x)+(P2)x−(B)x(1−x)

E(x) is the energy gap as a function of composition x. P1 and P2 represent the bandgaps at x =0 and x =1 respectivelyB is the bowing parameter. The following values were obtained: GaN, P1 = 3.43 eV; InN, P2 = 1.12 eV; B = 1.01 eV.

Documents

Sealed glass tube reactors Reactant(s) A Gaseous transporting agent B Temperature gradient furnace T ~ 50 o C Equilibrium established A(s) + B(g)