VAPOR PHASE TRANSPORT VPC MATERIALS
SYNTHESIS, CRYSTAL GROWTH, PURIFICATION
A(s)
B(g)
• Sealed glass tube reactors
A(s)
T2
• Reactant(s) A
• Gaseous transporting agent B
• Temperature gradient furnace DT ~ 50oC
• Equilibrium established
• A(s) + B(g) AB(g)
AB(g)
T1
Glass tube
VAPOR PHASE TRANSPORT VPC MATERIALS
SYNTHESIS, CRYSTAL GROWTH, PURIFICATION
A(s)
B(g)
A(s)
AB(g)
T2
• 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
T1
Glass tube
VAPOR PHASE TRANSPORT VPC MATERIALS
SYNTHESIS, CRYSTAL GROWTH, PURIFICATION
A(s)
B(g)
A(s)
AB(g)
T2
• Temperature dependent K
T1
Glass tube
• 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: DGo = -RTlnKequ
• Consider case of “exothermic” reaction with - DGo
• Thus DGo = RTlnKequ
• Smaller T implies larger Kequ
• Forms at cooler end - decomposes at hotter end of reactor
• Consider case of “endothermic” reaction with +DGo
• Thus DGo = -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
PLATINUM HEATER ELEMENTS IN FURNACES
THEY MOVE!! Pt(s) + O2(g)  PtO2(g)
• Endothermic reaction
T2
• PtO2 forms at hot end
Pt(s)
T1
PtO2(g)
• Diffuses to cool end
VPT agent PtO2(g)
• Deposits well formed Pt crystals
Atmosphere O2(g)
• 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
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 antithetical 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
• 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 REACTION
•
SnO2(s) + 2CaO(s)  Ca2SnO4(s)
• Sluggish reaction even at high T for a useful phosphor
• 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 SYNTHESIS
DIRECT 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 PASSIVATION IN SOLID STATE
SYNTHESIS THROUGH VPT
• Al(s) + 3S(s) Al2S3(s)
passivating skin stops reaction
• In presence of cleansing VPT agent I2 skin removed to
reveal fresh Al surface to react with S 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 according to:
• Endothermic: ZnS(s) + I2(g) (T1 800oC)  (T2 900oC)
ZnI2(g) + 1/2S2(g)
VPT GROWTH OF FERROMAGNETIC MAGNETITE
CRYSTALS FROM POWDERED MAGNETITE
Fe3O4(s)
1270K
1020K
VPT agent FeCl2/FeCl3(g)
Atmosphere HCl(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
FERROMAGNETIC INVERTED SPINEL MAGNETITE B(AB)O4
Fe(III)(Td)[Fe(III)Fe(II)(Oh)]O4
Field H
Multidomain
paramagnet above Tc
Multidomain
ferromagnet below Tc
M
Ms
Mr
Hc
Single domain
superparamagnet
H
Magnetization Hysteresis M vs H
Diagnostic of Ferromagnetism
VPT SYNTHESIS AND CRYSTAL GROWTH OF TiS2
FROM POWDERED Ti/S
TiS2
Ti/S(s)
550-685oC (T2)
510-645oC (T1)
VPT agent TiBr4(g)
Atmosphere Br2(g)
• 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
• Relevant to use of TiS2 as a LSSB cathode
LITHIUM SOLID STATE BATTERY MATERIAL
Li + TiS2  LixTiS2
Li insertion
•
•
•
•
•
•
•
•
•
TiS2 structure hcp packing of S(-II) 3pp filled VB, octahedral Ti(IV) t2g 3d empty CB
Li+ intercalates between hcp S2- layers into well defined LiS4 Td crystallographic sites
Charge balancing electrons injected into t2g Ti(IV) CB
TiS2 semiconductor, LixTiS2 conductivity increases upon insertion of Li(+) and e(-)
Hopping semiconductor mixed valence description xLi(I)xTi(III)(1-x)Ti(IV)S2
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
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
1060oC (T2)
ZnWO4(s)
WO3/ZnO(s)
980oC (T1)
Endothermic reaction
VPT agent WO2Cl2(g) + Cl2O(g) formed at hot end in an atmosphere Cl2(g)
•
WO3(s) + 2Cl2(g) (T2 1060oC)  (T2 1060oC)
WO2Cl2(g) + Cl O(g)
2
• WO2Cl2(g) + Cl O(g) + ZnO(s) (T2 1060 C)  ZnWO (s) + Cl (g) (T1 980 C)
2
o
4
2
o
VPT GROWTH OF EPITAXIAL GaAs FILMS ON LATTICE MATCHING
SUBSTRATE OR GROWTH OF SINGLE CRYSTALS USING
CONVENIENT STARTING MATERIALS
(T2)
GaAs(s)
GaAs(s)
(T1)
Endothermic VPT agent GaCl/As4/H2(g) formed
at hot end in an atmosphere of HCl(g)
GaAs(s) + HCl(g)  GaCl(g) + 1/2H2(g) + 1/4As4(g)
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 = MgB2
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+
BCS THEORY OF SUPERCONDUCTIVITY
Tc = 1.13hwD/2pkB{exp[-1/N(EF)V]}
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
Debye cut off frequency - highest phonon mode temperature dependent, wD  m-1/2 - expected isotope
effect on (Tc(m1)/Tc(m2) = (m2/m1)1/2
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 B22- sheets sandwiching hcc Mg2+ layers
Isoelectronic graphite NOT a superconductor – but
when doped C8K Tc = 0.15K
SUPERCONDUCTIVITY IN MgB2 AT 39K
A SENSATIONAL AND CURIOUS DISCOVERY
• Strong 3p-p bonding interactions between B6 rings and Mg
• Band diagram 3p-p stabilized wrt 3s-* of graphitic-like B22sheets
• 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-p
electrons into 3s-*
• MgxAl1-xB2 stronger Al3+ 3p-p, larger 3p-p/3p-p* gap
• Fewer Cooper pairs, lower Tc
SURPRISE-SURPRISE
NATURE PHYSICS 2005, 1, 39
HIGH TC INTERCALATED SUPERCONDUCTING C6Y AND C6Ca
GRAPHITES - VPT OF Yb AND Ca INTO GRAPHITE
XRD shows every layer filled with Yb, stage 1, interlayer spacing 4.57 Å,
AA registered graphite layers and a-b offset triangular array Yb layers,
superconductivity mechanism under investigation
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-MgO/Mg/800-900°C
MgO/5nm Au/B NWs/VLS 1000°C
MgB2 NWs
VPT AND VAPOR-LIQUID-SOLID (VLS) SYNTHESIS OF
BORON NANOWIRES AND THEIR CONVERSION TO
SUPERCONDUCTING MgB2 NANOWIRES
AuSi dewetting on MgO
on heating and nano
cluster formation on MgO
Au film on MgO
VLS growth of B
NWs on AuSi clusters
CONVERSION OF B NANOWIRES TO
SUPERCONDUCTING MgB2 NANOWIRES
B NWs on AuSi clusters
MgB2 NWs on AuSi clusters
Mg 800-900°
SYNTHESIS OF SUPERCONDUCTING
MAGNESIUM BORIDE NANOWIRES
B
MgB2
•
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 phase transformation of
amorphous boron nanowires to
crystalline magnesium boride
nanowires
SUPERCONDUCTIVITY OF
MAGNESIUM BORIDE NANOWIRES
•
ZFC
Tc
•
•
•
Magnetization of MgB2 nanowires as a
function of temperature under conditions of
zero field cooling and field cooling at 100G –
latter uncouples Cooper pairs former
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 interconnects in nanoscale
electronics
Recently epitaxial thin films made for
superconducting electronics and nanohelices!
•
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
Meissner
Effect
SUPERCONDUCTING MAGNESIUM DIBORIDE HELICES
Superconducting nanocoils and solenoids (with iron nanorod cores) may have practical applications
as nanoactuators or in flexible superconducting cable.
Mg(s) + B2H6 (770-800 °C VPT 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
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 nanoclusters 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
•
RT UV e-h excitonic lasing action in ZnO nanowire arrays demonstrated
•
Self-organized <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 boat, 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 laser cavities, nanowire diameters
20-150 nm with lengths up to 10 mm
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
RT ULTRAVIOLET
NANOWIRE NANOLASERS
•
QSEs yield 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
GaN NW LASER - TOPOGRAPHIC AND OPTICAL IMAGE
OF UV LASING ACTION – NW END EMISSION SEEN
VLS SYNTHESIS AND GROWTH OF
ORIENTED GaN NANOWIRES
Wurtzite type GaN <0001> growth
Ga or Me3Ga/NH3/900°C
sapphire
Nin
SINGLE GaN NANOWIRE LASERS
Lasing from ends
individual GaN NW UV
lasing action
lasing
photoluminescence
COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS
FULL COLOR TUNING OF PHOTOLUMINESCENCE
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) and a colour image from PL of a section of substrate (right).
COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS
Vegard’s Law on Unit Cell Dimensions and Electronic Bandgap
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 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 SYNTHESIS
Vegard’s Law on Unit Cell Dimensions and Electronic Bandgap
The data were fitted by the following standard bowing equation: F(x)=P1(1−x)+P2x−Bx(1−x),where F(x) is the
energy gap as a function of composition x. P1 and P2 represent the bandgaps at x =0 and x =1 respectively, and B
is the bowing parameter. The following values were obtained: GaN, P1 = 3.43 eV; InN, P2 = 1.12 eV;B = 1.01 eV.
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.