Transcript VAPOR PHASE TRANSPORT VPC MATERIALS SYNTHESIS, …

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 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 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
• 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)
VPT GROWTH OF FERROMAGNETIC MAGNETITE
SINGLE 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
Multi Weiss domain
paramagnet above Tc
M
Ms saturation magnetization
Ms
Mr remnant magnetization
Hc coercive field
Multi Weiss domain
ferromagnet below Tc
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
• Historically relevant for use of TiS2 as a LSSB cathode
LITHIUM SOLID STATE BATTERY MATERIAL
Li + TiS2  LixTiS2
Li insertion
•
TiS2 hcp packing S(-II) 3pp 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)S2
LITHIUM SOLID STATE BATTERY MATERIAL
Li + TiS2  LixTiS2
Li insertion
• 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 deposits GaAs at cold 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*op
3p-p*
pop
3p
3s-*
Responsible for metallic behaviour
3s
3p-p
ip
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 becomes one with Tc = 0.15K
SUPERCONDUCTIVITY IN MgB2 AT 39K
A SENSATIONAL AND CURIOUS DISCOVERY
• Strong 3p-p bonding between B6 rings and Mg
• Band diagram 3p-p 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-p electrons into 3s-*
• MgxAl1-xB2 - smaller more highly charged Al3+
isomorphously substitutes for Mg2+ results in stronger
Al3+ 3p-p, larger 3p-pto 3p-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
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-solid 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 –
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 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 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 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 mm
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
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
GaN NW LASER - TOPOGRAPHIC AND OPTICAL IMAGE OF
UV LASING ACTION – DEFINES NW END EMISSION
VLS SYNTHESIS AND GROWTH OF
ORIENTED GaN NANOWIRES
Wurtzite type GaN <0001> growth
Ga or Me3Ga +
NH3/900°C
sapphire
3MeH or
3/2H2
Nin
SINGLE GaN NANOWIRE LASERS
Lasing from ends
individual GaN NW UV
lasing action
lasing
photoluminescence
COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS – BANDGAP ENGINEERING
ABS
PL
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) showing ABS and a
colour image from PL of a section of substrate (right).
COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS
Vegard’s Law on Unit Cell Dimensions
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).
COMBINATORIAL Ga1-xInxN NANOWIRE SYNTHESIS
Vegard’s Law on Electronic Bandgap
COMBINATORIAL Ga1-xInxN
NANOWIRE SYNTHESIS
Vegard’s Law on Electronic Bandgap
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 respectively
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.