the nanoscale insulator-metal transition in vo 2 : structure, size and dynamics

Egyptian Materials Research Society Slide 1

The Nanoscale Insulator-Metal Transition in VO2: Structure, Size and Dynamics

IMS 320 — Vanderbilt University — 8 October 2008

37

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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 VO2 nanoparticles

• Optical properties of VO2 nanoparticles

• Dynamics of the metal-insulator transition

• What have we learned, and where are we going?

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3

… strongly correlated electrons

Kotliar and Volllhardt, Physics Today,March 2004 for review of DMFT

Itinerant electrons (Fermi liquid)

Localized electrons (Mott insulator)

CORRELATED ELECTRONS

Tradeoff between hopping rate tij (kinetic energy) and Hubbard U

(on-site Coulomb potential)

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• Morin, PRL, 1959

• First-order phase transition

• Structural rearrangement

• Gives (conductivity)~104-105

• Large change in optical T, R

• Can be triggered by laser

• Entropy cost S~1.6kB/V ion

• Antiferromagnetic above TC

VO2 metal-insulator transition

Temperature dependence of resistivity in VO2 films

Hysteresis loop; typical first order transition feature.

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VOx focal-plane array bolometers

• 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

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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

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

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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First-order thermodynamics and hysteresis

Avalanche-mediated transformation path: athermal activation thermal fluctuations not operative very recently observed in VO2 nanojunctions

First-order phase transformation: discontinuous first derivatives of Gibbs free energy entropy change latent heat of transformation need for undercooling and overheating hysteresis around Tc

Generic bistable potential linear tilt controlled by driving field h (e.g., |T – Tc|)

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• Motivation: exploit the metal-insulator transition


o Ion implantation in bare SiO2 substrates

o Pulsed laser deposition of V in O2 atmosphere

o Fabricating nanoparticle arrays of VO2



• What have we learned, where are we going?

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Ion separationmagnet

Ion extraction

ElectrostaticDeflection(Rastering)

Ion acceleration

Ion source

Target

nc-VO2 by ion implantation

O @ 55 keV, 3.0 x 1017 ions/cm2

V @ 150 keV, 1.5 x 1017 ions/cm2

1000 ºC

Anneal

C-axis

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Fabrication of Au::VO2 nanostructures

• VO2 film by PLD

• Stoichiometry by RBS

• Switching by Topt(IR)

• Morphology by SEM

• Location by microscopy

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Long anneal times

• Hysteresis width and transition temperatures correlate with increasing nanocrystal size to Tanneal~450˚C

01020304050607080

0 20 40 60 80 100

Reflection (A.U.)

40 minutes

01020304050607080

0 20 40 60 80 100Temperature (C)

Reflections (A.U.)

80 minutes

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VO2 vs V2O5 growth

• 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

t=15 nm, T=450˚C, 250 mTorr O2, 40 min

t=15 nm, T=550˚C, 250 mTorr O2, 40 min

2.00 µm

1.00 µm2.00 µm

1.00 µm

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o Measuring the optical response of nanoparticles

o Making valid comparisons for varying NP sizes

o From characterization to modeling



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Transmission experiments

• Broadband white-light source

• CCD spectrometer (0.3-1.2 µm)

• Measure transmission vs temperature

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Optical response vs size

V, 1.5 x 1017 ions/cm2

O, 3.0 x 1017 ions/cm2

Anneal in Ar 1000 ºC

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

20 25 30 35 40 45 50 55 60 65 70 75 80 85

2 min

5 min

9 min

15 min

20 min

60 min

Temperature (°C)

Tra

ns

mis

sio

n

IncreasingVO2 size

=2.0 µm

Reff

37 nm

67 nm80 nm89 nm87 nm

b/a1.3

1.92.73.23.5

Nanoparticles by ion implantation Lopez et al., Phys. Rev. B (2002)

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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 V is V

o The probability of finding more than 1 defect in that V is negligible;

o The probability of finding that defect is independent of other V’s;

o Then Poisson statistics apply, and ...

Density of defects is

Defect probability is

€

=C Δgex T −Tc( )[ ]y

€

F =1− exp −ρ ⋅V[ ]

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Optical signature of nucleation

€

Th =e−Noσhz, Tc =e−Noσcz, T =e−( No−N )σhz+Nσcz[ ]

€

T =1− r 2

( )2e−Nσz

1− r 4( )e

−2Nσz

€

≈−N ⋅z σ c −σ h( )−No ⋅z σ c −σ h( )

=NNo

≡ F

€

T −Th

Tc −Th=

e−Nz σ c−σ h( ) −1[ ]

e−Noz σ c−σ h( ) −1[ ]

0

0.2

0.4

0.6

0.8

1

25 35 45 55 65 75

Temperature (ºC)

Sw

itch

ing T

Tc

Th

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Size dependence of MIT

Remember: Small is different! (“Small” depends on property.)

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Jun

e 2

00

7R

ice

Un

ive

rsity

EC

E S

em

ina

r

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nc VO2 arrays

• Remove VOx-coated PMMA by standard lift-off technique

• Anneal in 250 mTorr O2 at 450C for up to 30 min.

o RESULT: VO2 nanoarrayso Limited by PMMA thickness

nc-VO2, typical disk diameter 60 nm, height variable, spacing variable.

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… and it is size-dependent

• Measured scattered (white) light, dispersed in CCD spectrometer

• VO2 nanoparticles 120 nm diam

• Lattice constant 280 nm

• Resonance at 460 nm

• Double hysteresis loop

500 nm

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An order-disorder transition

• Note that the differing widths of the “bumps” parallels the differing gex dependence of heating and cooling transitions!

y = 2.0461x - 10.053

R2 = 0.989

y = 3.0456x - 13.164

R2 = 0.9388

-8

-7.5

-7

-6.5

-6

-5.5

-5

-4.5

-4

1 1.5 2 2.5 3

warming up

Cooling down

Log gex

(J/mole)

Log

(-ln

(1-F

) /

V)

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• Appearance of a metallic plasmon response

• THz probe of AC conductivity

• A model supporting recent theory


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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

4 MHz Ti:sapph amplifiertp = 12 fs; Ephot = 1.55 eVOpt. Lett. 28, 2118 (2003)

i-InP, d = 230 nmVO2, d = 100 nm

VD1

tD

GaSe

EOX

electro-optic analysisof both transmitted THzamplitude and phase

/4

WPbalanced

differential detector:ETHz(T), ETHz(T, tD)

VD2

T

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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 have we learned, where are we going?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

• SiO2 microspheres on glass by micropipette

• Monolayer polycrystalline colloidal film

• Microsphere diameter 1.54 µm in all cases

• Laser heating and laser probing during MIT

Gla

ssVO2 film

Probelaser

Heating laser

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Transmission measurements

• Samples heated by ns Nd:YAG laser (532 nm)

• Heating fluence ~ 10 mJ/cm2

• Transmission measured at 980 nm (cw diode)

• Transmission on µsphere array increases!

100 nm film

140 nm film

140 nm on SiO2

100 nm on SiO2

VO2 thin film samples VO2 film on SiO2 µspheres

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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!

Tc~72˚C

Tc~82˚C• Epi-VO2 on TiO2 shows that Tc shifts higher with increasing stress (thinner films?)

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Materials, geometries … nanophotonics

• Oriented nanostructures

• Better material(epi-VO2)

• Exploit optical near field

• Nonlinear optics (SHG, 3)

• Other correlated materials?

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What is to learn?

• New materials and nanoscale structureso 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 studieso 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 …

Thanks to the National Science Foundation and the United States Department of Energy for $$$!

2.31 µmPicasso

“Don Quixote”(in VO2)

“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

Eugene Donev

Matthew McMahon

the nanoscale insulator-metal transition in vo 2 : structure, size and dynamics

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