The Nanoscale Insulator-Metal Transition
in VO2: Structure, Size and Dynamics
IMS 320 — Vanderbilt University — 8 October 2008
Egyptian
Materials Research Society
Slide 1
What you are about to hear …
• Motivation:
exploiting the metal-insulator transition
o Highly correlated solids and the phase transition
o Smart or functional nanoparticles
o Nanoscale properties of metal oxides
• Fabrication of VO nanoparticles
• Optical properties of VO nanoparticles
• Dynamics of the metal-insulator transition
• What have we learned, and where are we going?
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… strongly correlated electrons
Itinerant electrons (Fermi liquid)
CORRELATED
ELECTRONS
Tradeoff between hopping rate tij
(kinetic energy) and Hubbard U
(on-site Coulomb potential)
Localized electrons (Mott insulator)
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Kotliar and Volllhardt, Physics Today,
March 2004 for review of DMFT
VO2 metal-insulator transition
• Morin, PRL, 1959
• First-order phase transition
• Structural rearrangement
• Gives D(conductivity)~10 -10
• Large change in optical T, R
• Can be triggered by laser
• Entropy cost DS~1.6k /V ion
• Antiferromagnetic above T
4
5
B
C
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Hysteresis loop;
typical first order
transition feature.
Temperature dependence
of resistivity in VO2 films
VOx focal-plane array bolometers
•
•
•
•
5
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Vox bolometers are being developed for
use in uncooled focal-plane IR detectors
(8-14 µm).
Small size is critical, since it sets the
spatial resolution of the focal-plane array
given camera parameters
Little is known about the effects of
granularity, stoichiometry and other
materials parameters on detector
performance (noise limits, etc.)
Photo credits: Raytheon Corporation
VO2: a martensitic phase transition
 Structural phase transition alongside SMT:
 T < Tc  monoclinic
 T > Tc  rutile (tetragonal)
 Monoclinic phase:
 pairing and tilting of V cations
 doubling of unit cell
 One valence electron per V cation  3d
compound  narrow bands  e-e
correlations
 Which comes first, lattice change or SMT?
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Mechanisms of metal-insulator transition
 Dimerization of atoms  unit-cell
doubling
 Opening of new band gap at Fermi level
 metal-insulator transition
 Peierls deformation  lowering of
electronic energy (mostly near kF) vs.
increase in elastic energy
 Quasi-1D metals (e.g., VO2) 
susceptible to Peierls instability
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 Hubbard U: Coulomb repulsion between on-site
electrons  energy “penalty” for electron transport
 Bandwidth W: determined by hopping between
sites  kinetic energy of electrons
 U ~ W  itinerant vs. localized behavior  Mott
metal-insulator transition
 Narrow-band systems (e.g., VO2)  strong
electron-electron correlations
.
First-order thermodynamics and hysteresis
 First-order phase transformation:
 discontinuous first derivatives of Gibbs free
 Avalanche-mediated transformation path:
energy
 athermal activation  thermal fluctuations
 entropy change  latent heat of transformation
not operative
 need for undercooling and overheating 
 very recently observed in VO2
hysteresis around Tc
nanojunctions
 Generic bistable potential  linear tilt
controlled by driving field h (e.g., |T –
Tc|)
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What you are about to hear …
• Motivation: exploit the metal-insulator transition
• Fabrication of VO nanoparticles
2
o Ion implantation in bare SiO2 substrates
o Pulsed laser deposition of V in O2 atmosphere
o Fabricating nanoparticle arrays of VO2
• Optical properties of VO nanoparticles
• Dynamics of the metal-insulator transition
• What have we learned, where are we going?
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nc-VO2 by ion implantation
Ion acceleration
Ion separation
magnet
Electrostatic
Deflection
(Rastering)
Target
Ion extraction
O @ 55 keV, 3.0 x 1017 ions/cm2
V @ 150 keV, 1.5 x 1017 ions/cm2
Ion source
Anneal
1000 ºC
C-axis
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Fabrication of Au::VO2 nanostructures
• VO film by PLD
• Stoichiometry by RBS
• Switching by T (IR)
• Morphology by SEM
• Location by microscopy
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opt
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Long anneal times
Reflection (A.U.)
80
70
60
50
40
30
20
10
0
40 minutes
0
Reflections (A.U.)
80
70
60
50
40
30
20
10
0
20
40
60
80
100
80
100
80 minutes
0
20
40
60
Temperature (C)
• Hysteresis width and transition temperatures correlate
with increasing nanocrystal size to Tanneal~450˚C
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VO2 vs V2O5 growth
t=15 nm, T=450˚C, 250 mTorr O2, 40 min
2.00 µm
1.00 µm
t=15 nm, T=550˚C, 250 mTorr O2, 40 min
2.00 µm
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•
•
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1.00 µm
VO2 NCs are hemispherical, implying no wetting of the Si substrate
X-ray data confirm that 550˚C anneal produces substantial V2O5
Shape of high-temperature anneal NCs shows surface wetting
What you are about to hear …
• Motivation: exploit the metal-insulator transition
• Fabrication of VO nanoparticles
• Optical properties of VO nanoparticles
2
2
o Measuring the optical response of nanoparticles
o Making valid comparisons for varying NP sizes
o From characterization to modeling
• Dynamics of the metal-insulator transition
• What have we learned, where are we going?
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Transmission experiments
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• Broadband white-light source
• CCD spectrometer (0.3-1.2 µm)
• Measure transmission vs temperature
Optical response vs size
1.05
l=2.0 µm
1
Transmission
0.95
0.9
2 min
Increasing
VO2 size
0.85
0.8
Reff
37 nm
b/a
1.3
67 nm
80 nm
89 nm
87 nm
1.9
2.7
3.2
3.5
5 min
9 min
15 min
20 min
0.75
60 min
0.7
0.65
0.6
20
25
30
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40
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50
55
60
65
70
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80
85
Temperature (°C)
V, 1.5 x 1017 ions/cm2
O, 3.0 x 1017 ions/cm2
Anneal in Ar 1000 ºC
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Nanoparticles by ion implantation Lopez et al., Phys. Rev. B (2002)
Nucleation and size-dependence of hysteresis
Lopez et al., Phys. Rev. B 65, 224113 (2002)
 Energy barrier too high for homogeneous
nucleation  VO2 transition nucleates at
heterogeneous “potent sites”
 Availability F of potent sites depends on:
 nanoparticle volume V
 thermal driving “force” |T – Tc|
Smaller NPs  larger driving force needed
to transform  wider hysteresis
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Heterogeneous nucleation
• Nucleation at special sites (structural or point defects?).
• Not all defects have the same potency to nucleate the transition.
• This potency must be thermally activated
IF :
o The probability of finding an
activated defect in a DV is  DV
o The probability of finding more than
1 defect in that DV is negligible;
o The probability of finding that defect

is independent of other DV’s;
o Then Poisson statistics apply, and ...
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Density of defects is
  C Dgex T  Tc 
y
Defect probability is
F  1 exp V 
Optical signature of nucleation
1 r  e

T
4 2 Nz
1
r
 e
2 2  Nz
1
Switching
0.8
T
0.6
Th
Tc
0.4
0.2
0
25
35
45
55
65
Temperature (ºC)
 No  h z
Th  e

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

 No  c z
, Tc  e


e Nz c  h  1

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 ( No  N ) h zN c z 
, T e
N  z c   h  N
T  Th


F
  N z  
N o  z c   h  N o
Tc  Th e o c h 1
Size dependence of MIT
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Remember: Small is different! (“Small” depends on property.)
29 June 2007 Rice University ECE Seminar
nc VO2 arrays
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• Remove VO -coated
x
PMMA
by standard lift-off
technique
Anneal in 250 mTorr O2
at 450C for up to 30 min.
•
o RESULT: VO2 nanoarrays
o Limited by PMMA thickness
nc-VO2, typical disk diameter 60 nm,
height variable, spacing variable.
… and it is size-dependent
• Measured scattered (white) light,
dispersed in CCD spectrometer
• VO nanoparticles 120 nm diam
• Lattice constant 280 nm
• Resonance at 460 nm
• Double hysteresis loop
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500 nm
An order-disorder transition
-4
warming up
Log(-ln(1-F) / V)
-4.5
Cooling down
-5
-5.5
y = 2.0461x - 10.053
R2 = 0.989
-6
-6.5
-7
y = 3.0456x - 13.164
R2 = 0.9388
-7.5
-8
1
1.5
2
2.5
3
Log Dgex (J/mole)
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•
Note that the differing widths of the “bumps” parallels the differing Dgex
dependence of heating and cooling transitions!
What you are about to hear …
• Motivation: exploit the metal-insulator transition
• Fabrication of VO nanoparticles
• Optical properties of VO nanoparticles
• Dynamics of the metal-insulator transition
• Appearance of a metallic plasmon response
• THz probe of AC conductivity
• A model supporting recent theory
• What have we learned, where are we going?
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2
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fs response of VO2 films and nc-VO2
• fs pump at 800 nm, fs IR
•
probe
LSPR response as in
adiabatic thermal phase
transition
Lopez et al., Applied Physics Letters (2004)
M. Rini, R. Lopez, A Cavalleri et al., Optics Letters (2005)
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Ultrabroadband THz study of VO2
Nd:YVO4, 18 W
VD2
VD1
T
tD
4 MHz Ti:sapph amplifier
tp = 12 fs; Ephot = 1.55 eV
Opt. Lett. 28, 2118 (2003)
EOX
GaS
e
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WP
l/4
balanced
differential detector:
ETHz(T), DETHz(T, tD)
electro-optic analysis
of both transmitted THz
amplitude and phase
i-InP, d = 230 nm
VO2, d = 100 nm
Integrated THz response
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Two-dimensional spectra
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Coherent phonon generation
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Model developed from THz experiments
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What you are about to hear …
• Motivation: exploit the metal-insulator transition
• Fabrication of VO nanoparticles
• Optical properties of VO nanoparticles
• Dynamics of the metal-insulator transition
• What have we learned, where are we going?
2
2
o Novel geometries, stress and strain
o Better materials and shorter pulses
o Modeling the electric field effects
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One more variation on geometry
Probe
laser
Heating laser
Glass
VO2 film
• SiO microspheres on glass by micropipette
• Monolayer polycrystalline colloidal film
• Microsphere diameter 1.54 µm in all cases
• Laser heating and laser probing during MIT
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Transmission measurements
VO2 thin film samples
VO2 film on SiO2 µspheres
100 nm on SiO2
100 nm film
140 nm on SiO2
140 nm film
• Samples heated by ns Nd:YAG laser (532 nm)
• Heating fluence ~ 10 mJ/cm
• Transmission measured at 980 nm (cw diode)
• Transmission on µsphere array increases!
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So what is happening … and why?
• The µsphere array is a diffraction grating …
• … with light in both zeroth and first orders.
• Measurement shows that MIT shifts intensity …
• … from first to zeroth order in µsphere array.
• It could be stress!
• Epi-VO
on TiO2
shows that Tc
shifts higher with
increasing stress
(thinner films?)
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Tc~82˚C
Tc~72˚C
Materials, geometries … nanophotonics
• Oriented nanostructures
• Better material(epi-VO )
• Exploit optical near field
• Nonlinear optics (SHG, 3)
• Other correlated materials?
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What is to learn?
• New materials and nanoscale structures
o Materials: V2O3, VxCr1-xO2, WO3, …
o Novel structures (e.g., arrays with curved surfaces, Konstanz)
o Embedding materials designed for particular effects (e.g., NLO)
• Ultrafast and angle-resolved studies of the effect:
o Switching nonlinear effects using ultrashort laser pulses
o Exploring the wavelength- and surface-dependence
o What about the effect of the VO2 SPR (~1.3 µm)?
• Ultrafast, THz and FIR studies
o THz radiation could look at properties of the excited electron gas
o FIR spectroscopy could help resolve controversial Raman results.
o Early fs THz studies hint at MIT-related IR modes (Konstanz)
• Nanoscale geometrical structure brings advantages of
optical coherence to nanoscale differences!
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The end …
Picasso
“Don Quixote”
(in VO2)
2.31 µm
“The legitimate purpose of research
can only be, to make two questions
grow where there was only one
before.” [Thorsten Veblen]
Jae Suh
René Lopez
Matthew
McMahon
Eugene Donev
Thanks to the National Science Foundation
and the United States Department of Energy for $$$!
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