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• 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
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VAPOR PHASE TRANSPORT VPC MATERIALS SYNTHESIS, CRYSTAL GROWTH, PURIFICATION
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
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VAPOR PHASE TRANSPORT VPC MATERIALS SYNTHESIS, CRYSTAL GROWTH, PURIFICATION
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
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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
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USES OF VPT
• synthesis of new solid state materials
• growth of single crystals
• purification of solids
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• 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
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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!!!
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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
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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)
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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)
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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)
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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)
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• 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
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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
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• 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
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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
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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
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• 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)
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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)
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MgB2 SAT ON THE SHELF DOING NOTHING FOR HALF A CENTURY
AND THEN THE BIGGEST SURPRISE
SINCE HIGH Tc CERAMIC
SUPERCONDUCTORS
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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
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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
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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
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SUPERCONDUCTIVITY IN MgB2 AT 39K
A SENSATIONAL AND CURIOUS DISCOVERY
• 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
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SUPERCONDUCTIVITY IN MgB2 AT 39K
A SENSATIONAL AND CURIOUS DISCOVERY
• 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
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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
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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
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CONVERSION OF B NANOWIRES TO SUPERCONDUCTING MgB2 NANOWIRES
Mg 800-900°
B NWs on AuSi clusters MgB2 NWs on AuSi clusters
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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 AuSi nanoclusters supported on MgO substrate
• Vapor-solid 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 –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
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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
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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
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RT ULTRAVIOLET ZnO NANOWIRE NANOLASERS
VPT SYNTHESIS AND GROWTH
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RT ULTRAVIOLET NANOWIRE NANOLASERS
VPT SYNTHESIS AND GROWTH
VPT carbo-thermal reduction
ZnO/C 905°C ===> ZnCO VPT ===> ZnO VLS NW 880°C
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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
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VPT-VLS SYNTHESIS AND GROWTH OF ORIENTED ZnO NANOWIRES
ZnCO C
ZnO <0001> growth
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 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
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PXRD – SHOWS PREFERRED GROWTH OF NANOWIRES ALONG C-AXIS OF ZnO
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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 <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
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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
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RT ULTRAVIOLET NANOWIRE NANOLASERS
• 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
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GaN NW LASER - TOPOGRAPHIC AND OPTICAL IMAGE OF UV LASING ACTION – DEFINES NW END EMISSION
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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
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SINGLE GaN NANOWIRE LASERS
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individual GaN NW UV lasing action
Lasing from ends
lasing
photoluminescence
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COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS – BANDGAP ENGINEERING
ABS PL
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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).
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COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS
Vegard’s Law on Unit Cell Dimensions
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COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS
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).
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COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESISVegard’s Law on Electronic Bandgap
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COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS
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.
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