Microscopic and Nanoscale Perspective of the Metal−Insulator Phase

PERSPECTIVE pubs.acs.org/JPCL

Microscopic and Nanoscale Perspective of the Metal
Insulator Phase Transitions of VO2: Some New Twists to an Old Tale Luisa Whittaker, Christopher J. Patridge, and Sarbajit Banerjee* Department of Chemistry, University at Buffalo, Buffalo, New York 14260-3000, United States ABSTRACT: The peculiarities in the electronic structure of the seemingly simple binary vanadium oxide VO2, as manifested in a pronounced metal
insulator phase transition in proximity to room temperature, have made it the subject of extensive theoretical and experimental investigations over the last several decades. We review some recent advances in theoretical treatments of strongly correlated systems along with ultrafast measurements of VO2 samples that provide unprecedented mechanistic insight into the nature of the phase transition. Scaling VO2 to nanoscale dimensions has recently been possible and has allowed well-defined VO2 nanostructures to serve as model systems for measurements of intrinsic properties without obscuration from grain boundary connectivities and domain dynamics. Geometric confinement, substrate interactions, and varying defect densities of VO2 nanostructures give rise to an electronic and structural phase diagram that is substantially altered from the bulk. We postulate that design principles deduced from fundamental understanding of phase transitions in nanostructures will allow the predictive and rational design of systems with tunable charge and spin ordering.

T

he metal
insulator transition in VO2 enjoys near-iconic status in physical chemistry and condensed matter physics as one of the original (and still most cherished!) examples of the consequences of coupled orbital and lattice instabilities mediated by strong electronic correlations.1
3 The macroscopic manifestations of electronic and lattice instabilities in VO2 are truly breathtaking—discontinuous jumps in electrical conductivity and optical transmittance that can span 5 orders of magnitude at temperatures in reasonable proximity to room temperature (∼67 °C in the bulk).4 Less-pronounced but equally discontinuous alterations are noted at the phase-transition temperature for other physical properties such as the magnetic susceptibility, specific heat, and the Seebeck coefficient.2 Since the first discovery of this phenomenon by Morin more than half of a century ago,5 the structural and electronic peculiarities of VO2 have attracted waves of attention from theorists and experimentalists alike, and a fierce debate has raged over several decades regarding the microscopic mechanism of the phase transition. There are several reasons for the abiding interest in the peculiarities of this seemingly simple binary oxide; structurally and compositionally more complex materials have now been identified, several of them also oxides such as cuprates and manganites, which exhibit analogous aspects of electroncorrelation-induced inhomogeneity of electrical and magnetic domains within compositionally identical material.6
8 Consequently, VO2 serves as somewhat of a model system to understand and deconvolute the distinctive roles of electron
phonon and electron
electron coupling manifested in exotic modulations of magnetic and transport properties such as high-Tc superconductivity, charge stripes, phase separations, quantized spin and charge fluctuations, and ferroelectricity.6,7 Another compelling argument is that VO2 is important in its own right and has potential for applications in a diverse array of r 2011 American Chemical Society

technological applications given the proximity of the phasetransition temperature to room temperature, the approximately 0.5
0.6 eV band gap of the insulating phase, and the tunability of the phase transition (and thus infrared optical transmittance and electrical resistivity) with doping, strain, and scaling to finite size, as will be discussed below in more detail.9 Proposed applications for VO2 include thermochromic window elements with switchable infrared reflectivity that can function as “smart windows”, thus increasing heating/air-conditioning efficiency by being infrared-transparent at low temperatures and infrared-reflective at higher temperatures (without compromising transparency in the visible region of the electro-

The structural and electronic peculiarities of VO2 have attracted waves of attention from theorists and experimentalists alike, and a fierce debate has raged over several decades regarding the microscopic mechanism of the phase transition. magnetic spectrum).10 Another tantalizing possibility is the Mott field effect transistor, which can be switched between insulating Received: December 6, 2010 Accepted: March 4, 2011 Published: March 11, 2011 745

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Figure 1. Schematic depictions of the crystal structures of the lowtemperature monoclinic M1, intermediate M2, and high-temperature tetragonal rutile R phases. V
V distances are highlighted.

and metallic phases (ideally with fast turnover) by the application of a gate voltage.11,12 Other practical applications envisaged for this material include thermally sensitive switching, infrared modulators such as those for missile testing, infrared polarizers and fiber optic waveguides, optical data storage media, and electro-optical sensing elements.9,11
15 In this Perspective, we will review some distinctive characteristics of the metal
insulator phase transition of VO2 unveiled upon scaling to finite size. In some cases, finite size and the concomitant surface energetics and geometric effects enable tuning of the phase-transition behavior, whereas in other examples, single-crystalline nanostructures serve as an enabling technology for observations of intrinsic single/few-domain behavior without obscuration from grain boundary connectivity and complex domain dynamics.8,16
24 Concurrent with explorations of finite size effects on the phase transition of VO2, the last 5 years have seen a flurry of activity aimed at unraveling the microscopic mechanisms for the phase transition. Recent advances in dynamical mean field theory,25 static lattice calculations,26 ultrafast optical spectroscopy and electron diffraction,27
31 nanoscale imaging techniques,7 and total scattering32 now provide an unprecedented level of detail with superb spatial and temporal resolution for piecing together the electronic and structural aspects of this puzzle. We will also review some of these findings, even though they pertain to measurements of bulk or polycrystalline thin films of VO2. In the interest of brevity and readability, we have selected a few illustrative examples of recent advances instead of attempting to be exhaustive in scope. Structural and Mechanistic Aspects of the Phase Transition: The Many Phases of VO2. It is incontrovertibly known that the metal
insulator phase transition in VO2 is associated with a structural phase transformation from a tetragonal (rutile-like) phase, which crystallizes in the P4/2mnm space group, to a monoclinic phase in the P21/c space group.33 In essence, a pronounced lowering of symmetry is observed upon cooling though the phase transition (Figure 1). The transition is firstorder in nature with a latent heat on the order of 1020 cal/mol, although the relative contributions of lattice enthalpy and entropy changes of conduction electrons remain to be deconvoluted.2 It also remains to be settled whether the structural phase transformation is a prerequisite for the electronic metal
insulator phase transition, although the balance of evidence now does seem to point toward delinking of the two phenomena should sufficient carrier density become available in the material.21,31,34
36 In other words, a metallic phase can perhaps be induced even without a structural phase transformation if optical/thermal/gate-voltage-induced excitation of

Figure 2. Molecular orbital diagram depicting the electronic structure of the monoclinic and tetragonal phases of VO2. The left MO diagram corresponds to the undistorted metallic phase of VO2, whereas the diagram on the right shows the altered MO diagram upon transition to the distorted insulating phase of VO2.

carriers reaches a threshold density. This represents a key piece of evidence in elucidating the nature of the phase transition. Nevertheless, we begin by first reviewing the structures of the rutile (commonly metallic) and monoclinic (commonly insulating) phases of VO2. The rutile phase is fairly symmetric with two formula units per unit cell, and although the VO6 octahedra show local Jahn
Teller distortions, chains of cornersharing VO6 octahedra are linearly aligned along the crystallographic c axis with unique V
V distances of ∼2.85 Å (Figure 1).33,37 Upon cooling through the phase transition, the unit cell is doubled in size due to pairing and tilting of VO6 octahedra along the rutile c axis (Figure 1). Two distinctive sets of short and long V
V bond distances are observed for the monoclinic M1 phase at 2.65 and 3.12 Å. The distinctive crystalline lattice distortion and formation of “molecular” V
V dimers observed upon cooling through the phase transition are suggestive of the importance of a Peierls-type phenomenon in opening a band gap for the monoclinic phase. According to this model, higher-order crystal field terms and alterations of the band structure at surfaces of the reduced Brillouin zone, both a consequence of the lowering of symmetry and increase in periodicity of the crystal lattice, cause energetic gaps to open up for the low-temperature monoclinic phase.2,3,33 In other words, the driving force for the metal
insulator transition is the reduction in electronic energy; the transition occurs only when lowering of electronic energy more than counteracts the strain energy induced by lattice distortions. Goodenough proposed a more rigorously molecular picture of this process based on relatively simple crystal field theory considerations, as illustrated in Figure 2.33,38 In tetragonal VO2, the vanadium atoms are aligned along the c axis, as depicted in Figure 1. The crystal field of the oxide ligands splits the d orbitals into t2g and eg sets; the latter dz2 and dx2
y2 orbitals point directly toward the ligands and give rise to strongly σ bonding and antibonding sets of Vd
Op molecular orbitals.2,25,33 The t2g set is further split because of the tetragonal distortion of the VO6 octahedra. The dxz and dyz orbitals are involved in a sideways π 746

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Heisenberg chains).41
43 Specifically, V0.985Al0.015O2 crystallizes in the monoclinic M2 phase at 323 K with half of the vanadium chains showing characteristics of molecular dimerization with alternating short (2.540 Å) and long (3.261 Å) V
V separations. In contrast, the other half of the vanadium chains are tilted but undimerized with equally spaced (2.935 Å) zigzag V chains along the rutile c axis (Figure 1).43 This phase is still insulating (and again, DFT-LDA fails to yield a true band gap), suggesting that the absence of a Peierls-type structural dimerization does not necessarily predicate induction of metallic transport. An additional discrepancy is the renormalization of electron occupancies of the frontier orbitals evidenced by photoemission spectroscopy when VO2 undergoes a phase transition.44 The insulating phase is characterized by a state
0.8 eV below the Fermi level, which is apparently at a higher energy than the state centered at ∼
1.2 eV for the metallic phase.44 The alternative Mott picture attributes the presence of a band gap in monoclinic VO2 to strong electronic correlations. When the kinetic energy gained by delocalizing electrons across a collective band structure is less than the increase in potential energy due to Coulomb repulsion between the electrons, the system exhibits a strong preference for an insulating ground state.3
5 However, if a certain critical carrier density can be reached in the material, the excitonic electron
hole attractions are screened by the free carriers until such a point that bound states can no longer be stabilized, and a discontinuous step change in the free-carrier density is observed with a transition to a correlated metallic phase. Hubbard was able to devise Hamiltonians that allow incorporation of electron correlation effects into band theory calculations, although obtaining viable solutions to this many-body problem for d orbitals has been a challenge that has kept theorists occupied for the better part of four decades.25,40 The influence of such correlation effects on the metal
insulator transitions of VO2 is underlined by several experimental observations. First, the metallic phase of VO2 exhibits relatively low mobility values (1
10 cm2/(V s)) and is characterized by a short electron mean free path on the order of the lattice constant. This characteristic “poor metal” behavior is suggestive of electron correlations in the metallic phase. Next, the photoexcitation of charge carriers for insulating VO2 has been observed to induce a nonthermal phase transition to the metallic state. Optical pump
probe experiments typically use a near-IR pulse to excite the bonding d state of the insulator, followed by varying THz domain or electron diffraction probe pulses to probe the dynamics of the phase transition. It is clear from these experiments that the transition to the metallic phase is induced only above a critical flux of the incident laser beam, which is consistent with the Mott
Hubbard picture of requiring sufficient density of free carriers to screen excitonic interactions due to strong electron correlations.29
31 Furthermore, the critical threshold flux required to trigger a phase transition is seen to decrease with increasing temperature, suggesting that the phase transition depends uniquely on the density of excited charge carriers regardless of their origin (thermal, optical, or polarizationinduced).30,45 Recent findings, as will be summarized in the subsequent sections, have allowed further refinements of our understanding of mechanistic aspects of this phase transition and suggest that it is both structurally and electronically driven, and perhaps, VO2 is best considered as a specific case of a chargeordered Mott insulator.7

overlap with the oxide ligands, whereas the dxy orbital is projected along the crystallographic c axis and is directed toward adjacent V

A metallic phase can perhaps be induced even without a structural phase transformation if optical/ thermal/gate-voltage-induced excitation of carriers reaches a threshold density. This represents a key piece of evidence in elucidating the nature of the phase transition.

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atoms that are at a distance of ∼2.85 Å on either side. Although, this d level is nominally nonbonding (Figure 2), it is appropriately positioned in terms of geometric and energetic considerations to mediate V
V interactions. Upon cooling through the phase transition and stabilization of the M1 phase, the dimerization and tilting of VO6 octahedra have two notable effects on the energy level diagram. The tilting of the VO6 octahedra facilitates improved π overlap between the V t2g and O 2p levels and thus raises the antibonding π* level due to a concurrent stabilization of the bonding states. More importantly, the d bands no longer remain nonbonding in the monoclinic phase but instead strongly interact within the molecular dimers and are split into a d bonding and antibonding combination.38 The single d electrons from each of the vanadium atoms in the dimer occupy the bonding d level, opening up a gap between this band and the lowest-unoccupied states, which are derived from the π* orbitals that are at slightly higher energies than those in the rutile structure, owing to the canting of the octahedra along the c axis.25,33 This simple but elegant model serves well to qualitatively explain the nature of the metal
insulator phase transition in VO2 but is plagued by several discrepancies upon comparison to experimental data. This situation has inspired the invocation of a Mott
Hubbard picture wherein electron correlation effects also play an important role in defining a band gap for the dielectric phase.25,35,39,40 From a band structure perspective, crystalline lattice distortion and concomitant symmetry lowering can induce a true band gap only when the bandwidths are on the order of the induced gap. In other words, only materials having exceedingly narrowed but partially filled bands will derive sufficient energetic gain from a lattice deformation.3 While the spatial extent of vanadium d orbitals in VO2 is far more constricted than that of Mo and W in their metallic binary oxides, there is no evidence for extremely narrow bands. Indeed, density functional theory (DFT) calculations in the local density approximation (LDA) do not yield a band gap for the insulating M1 phase without exaggeration of the lattice distortion to remove remnant overlap between the d and π* states.25 Other findings that suggest that the Peierls distortion model does not adequately describe this metal
insulator phase transition include the observation that a monoclinic M2 phase of VO2 stabilized under strain or by the inclusion of small amounts of Cr or Al substitutional dopants is insulating even though half of the chains along the rutile c axis are undimerized (spin 1/2 747

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Recent Measurements and Calculations of VO2: Surprises Galore. As noted above, DFT-LDA does not yield a band gap for the monoclinic M1 and M2 phases, suggesting an inadequate treatment of electron correlations, albeit a Peierls instability of the d band and its separation from eg-derived π* states is indeed qualitatively captured.40 In this context, the cluster dynamical mean field theory calculations by Biermann et al. represent a major advance wherein structural distortions and strong electronic Coulomb interactions are explicitly taken into consideration in determining the electron occupancies.25 These authors have found a quasiparticle coherence peak proximal to the Fermi level and distinctive Hubbard bands
1.5 eV below and 2.5 eV above the Fermi level for the metallic phase, consistent with a description of a strongly correlated metal. Perhaps more importantly, a 0.6 eV band gap is captured for the M1 phase with pronounced redistribution of electron density into the bonding d states (0.8 electrons per vanadium in d ) as compared to a more even distribution across d and t2g-derived π states observed for the rutile phase (0.36 electrons per vanadium site in d states). This description essentially treats the insulating state as being stabilized by Heitler
London “molecular” interactions with both electrons occupying a localized correlated singlet state on the dimer rather than being part of a more delocalized uncorrelated bonding set of molecular orbitals.31 According to this picture, the insulating phase is not a usual Mott insulator but rather a renormalized Peierls insulator exhibiting a quasiparticle coherence peak at
0.8 eV below the Fermi level and an upper Hubbard band centered 2.2. eV above the Fermi level.25 This description is consistent with the renormalization of spectral weight observed in photoemission spectra of VO2 across the phase transition, taking into account that the
0.8 eV band is a quasiparticle coherence peak and not a Hubbard band.44 Such a mixed Peierls
Mott character treatment of the insulating phase thus provides one of the most realistic descriptions of the structure, consistent with optical measurements of the band gap and photoemission measurements of shifts in spectral weights across the phase transition. Notably, the pronounced redistribution of electron occupancies across the phase transition is also exquisitely captured in near-edge X-ray absorption fine structure (NEXAFS) spectra acquired for VO2 at the O K-edge, which map transitions from O 1s core levels to unoccupied and partially filled O 2p states hybridized with V 3d orbitals.24,28,38,46 Transition to the metallic state results in diminution of spectral features attributed to states derived from d bands because these orbitals no longer form Heitler
London bonding/antibonding combinations. In some more recent band structure calculations, Gatti et al. have demonstrated that a parameter-free GW calculation can correctly describe the band structure of both insulating and metallic phases as long as the quasiparticle energies and wave functions are self-consistently calculated, suggesting that static correlation is adequate to describe the photoemission spectra in the screened quasiparticle picture.47 Their calculations suggest that a satellite peak observed at 1.3 eV in the photoemission spectrum of the metallic phase corresponds to a plasmon resonance and disappears upon cooling to the insulating phase. A characteristic signature of a correlation-induced Mott phase transition is a divergent quasiparticle mass due to pronounced alterations in screening effects.48 Direct evidence for such a phenomenon comes from scanning near-field infrared nanoimaging studies performed by Qazilbash et al.7 These measurements enable resolution of thin-film domains with )

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distinctive optical constants. Because insulating and metallic phases have very different values of optical conductivity and the real part of the dielectric function, the metal
insulator phase transition can be followed with unparalleled spatial resolution. In studies of a polycrystalline VO2 film, with increasing temperature, the authors observed the nucleation of high-scattering contrast “metallic puddles” against a backdrop of the uniformly low-scattering insulating phase. The metallic domains grow with increasing temperature and coalesce until the entire sample transforms to the metallic phase. These studies provided the first evidence for the percolative nature of the first-order metal
insulator phase transition and indicate a coexistence regime where both metallic and insulating domains are present. Interestingly, the optical conductivity of incipient metallic puddles extracted in close proximity to the phase-transition temperature shows a rather peculiar dependence on frequency that is very significantly different from the high-temperature metallic phase. Strong divergence is seen for the optical effective mass in proximity to the phase transition, suggesting the formation of rather heavy quasiparticles as the band gap of the insulating phase is gradually filled. Additionally, a pseudogap is also observed and can be attributed to electronic excitations across a slice of the Fermi surface, as has previously been observed for other Mott insulators.1,7 On the basis of the divergent quasiparticle optical effective mass and pseudogap, the authors postulate that insulating domains first transform into a intermediate strongly correlated metal phase that then evolve to form the regular poor metal phase, with the observations suggesting a strong role for Mott
Hubbard correlations in mediating the phase transition.1,7,48 In more recent work, the authors have identified discrete metallic and insulating domains within a single compositionally identical crystalline grain near the phase-transition temperature, further corroborating the idea of a coexistence regime with charge and orbital inhomogeneities.49 Using statistical analysis of their near-field scattering images, they have demonstrated that upon approaching the phase transition from below, metallic phases are nucleated within 10 nm of grain boundaries and crevices.49 Nevertheless, recent pair distribution function (PDF) analysis of X-ray scattering data of polycrystalline VO2 powder with relatively large grain sizes, while corroborating a coexistence regime, precludes the presence of intermediate structures other than the monoclinic M1 and rutile polymorphs at least for bulk samples. The experimental PDFs close to the phase-transition temperature can be fitted as a simple combination of the two end members without having to invoke an intermediate crystal structure.32 Optical pump
probe experiments have provided valuable insight into the nature of the metal
insulator phase transition of VO2.27
31 These experiments typically use photoexcitation from a near-infrared laser pulse to initiate the phase transition and then subsequently follow the dynamics of the phase transition using a probe pulse. The optical experiments demonstrate a strong sensitivity to the incident fluence, and the phase transition is induced only above a threshold excitation power. This pronounced nonlinearity is further indicative of a Mott-type transition, where excitonic states need to be screened by a sufficient density of free carriers before occurrence of the transition to the metallic phase. Hilton et al. have used THz probe pulses to monitor the dynamics of the phase transition and have determined that the critical fluence required to induce the phase transition decreases with increasing temperature, 748

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suggesting a softening of the insulating phase due to nucleation of metallic domains.29 Using effective medium theory analysis, these authors establish the presence of a coexistence regime wherein both metallic and insulating phases are present and further corroborate the equivalence of thermally and optically excited carriers in mediating the phase transition.29 In another notable ultrafast experiment, Kubler et al. have provided perhaps the most convincing evidence for delinking of the structural and electronic phase transitions.31 These authors use a broad-band THz probe pulse and are able to track the dynamics of the phase transition over an extended frequency range that includes well-separated contributions from lattice vibrations and the optical conductivity. This enables the THz contributions from structural phonon modes and the induced electronic conductivity to be simultaneously monitored. Remarkably, above the threshold fluence, photoexcitation launches a strong phase oscillation of the phonon features that persists over several cycles, whereas the electronic conductivity settles at a maximum, suggestive of establishment of a metallic phase, after only a single oscillation cycle coherent with the phonon fluctuations. The implication here is that metallic conductivity is established well before the lattice has had the chance to settle into an equilibrium configuration. The authors postulate that excitation of an electron residing in the Heitler
London bonding orbitals derived from d states causes occupation of an antibonding level, which weakens the V
V singlet dimer (Figure 3).25 At moderate fluences, electron correlation is rapidly re-established, and the electron returns to the ground state. However, above a critical threshold, electron correlation can no longer be rapidly established, and the result is a spectacular breakdown of the Born
Oppenheimer approximation with metallic conductivity being established first and structural transformation to the rutile phase only being reached later as the excited electrons in the antibonding levels locate to the potential energy well of the rutile phase (Figure 3).31 A very different set of experiments further confirms delinking of the structural and electronic phase transitions. Kim et al. induced the insulator
metal phase transition in VO2 thin films by the application of a dc electric field and were further able to use Raman microprobe and microdiffraction measurements to simultaneously monitor the structural phase transformation.34,36 These authors noted that electronic phase transition could be induced in advance of the structural monoclinic f rutile phase transformation, suggesting again the importance of Mott physics.34
36,45 We conclude this section by reviewing some recent atomistic details of the structural phase transition that have emerged from ultrafast electron diffraction experiments and static lattice calculations.26,30 Zewail and co-workers determined that the photoexcitation-induced phase transition in VO2 occurred through a stepwise nonconcerted mechanism with discrete intermediates and at least three distinctive time scales. Upon photoexcitation of the d -derived Heitler
London orbitals into an antibonding state, as expected from the schematic in Figure 3, the rapid dilation of the V
V bond occurs on a time scale of ∼307 fs, thus breaking the localized singlet dimers; a subsequent slower tranverse motion is also identified with a time scale of ∼9.2 ps as VO6 octahedra locally rearrange to adopt a more rutile-like geometry. The experiments also discern a far slower ∼100 ps motion that is attributed to shear movements propagating at the speed of sound, likely across hundreds of nanometers, as the lattice recrystallizes through dislocations and slips to adopt )

Figure 3. Schematic potential energy surface depicting adjacent V
V interactions in two different polymorphs of VO2 (modified diagram based on a schematic by Kubler et al., copyright 2007 by the American Physical Society).31 QM1 and QR represent the interactomic distances in the low-temperature M1 and the high-temperature R phases, respectively.

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the rutile geometry.30 By searching an extensive energy landscape using static lattice calculations, Netsianda and co-workers have identified a saddle point intermediate between the rutile and M1 polymorphs that, perhaps not surprisingly, is very similar in structure to the M2 polymorph noted above (and depicted in Figure 1).26,41,42 The identified transition-state intermediate has a Peierls distortion for half of the V
V chains along the rutile c axis. According to this picture, when approaching the phase transition from above, spontaneous Peierls distortion is initially initiated for a subset of the cation chains. Subsequently, the extent of distortion increases before analogous Peierls distortion spreads to orthogonal chains.26 Interestingly, although there is no evidence for isolation of a stable M2 intermediate phase for bulk and polycrystalline thin films of VO2 (as noted above), it does seem apparent that such an intermediate phase mediates the transition from the rutile to the M1 phase in nanostructures where it finds particular stabilization as a result of strong surfaceinduced stress.21,22 Nanoscale VO2: Many Roads to VO2. The complex domain dynamics and coexistence of discrete metallic and insulating domains even within single grains of compositionally homogeneous VO249 make it difficult to correlate specifics of atomistic geometric and electronic structure to macroscopic observations of variations in electrical conductivity and optical transmittance. It is well established that thermally cycling single crystals of VO2 or even large crystallites or thin films of the material across various heating/cooling cycles leads to cracking, progressive mechanical degradation, and diminution in grain size due to large strains induced by the substantial distortions required to transform the M1 to the rutile phase and vice versa given the substantial (∼1%) differences in the lattice constants of the two phases.2,38 Scaling VO2 to nanoscale dimensions offers the possibility of much simplified domain behavior and opportunities to study the intrinsic properties of the material at the single-domain limit. Furthermore, the inevitable lattice stresses introduced as a result of both intrinsic geometric effects of high surface-to-volume ratios of nanostructures as well as extrinsic 749

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strong substrate interactions portend modifications of the thermodynamic stabilities of the different phases. Such a situation is expected to lead to dramatic modifications of the phase diagram of VO2 from the bulk, thereby altering the phase-transition temperatures and hystereses and providing an additional knob other than substitutional doping to tune the location and hysteresis of the phase transition. Additionally, as per the mesoscopic nucleation, growth, and coalescence percolative description of the phase transition noted above,7,49 the metallic phase needs to be nucleated within a certain region of the material. The nucleation of one phase within another is likely strongly affected by the presence of intrinsic defects (oxygen vacancies, vanadium interstitials) and dopant atoms, and consequently, the degree of perfection of a nanocrystal can strongly alter the density of probable nucleation sites.50 More specifically, the facile migration of defects to the surface often results in “selfpurification” and stabilization of single-crystalline nanostructures with far lower defect density than that in the bulk.51 A number of different synthetic strategies have been developed for the preparation of VO2 nanostructures of varying dimensions and morphology. There is still considerable spread in the observed phase-transition temperatures, strengths of the metal
insulator phase transition, and extents of hystereses, which is not surprising given the variations in synthesis parameters and the influence of such parameters on the surface energetics, defect density, and crystallinity of the obtained materials.8,18
20,23,24,52,53 Some approaches reported for the fabrication of VO2 nanostructures include sol
gel synthesis,54

nanostructures and is amenable to doping with tungsten by introduction of an appropriate precursor to the solid source.18 Hydrothermal synthesis, which occurs under autogenous pressure and at temperatures above the boiling point of water, provides a facile green chemical alternative for fabrication of highly pure and single-crystalline VO2 nanostructures with desired composition, reproducible shape, and tunable morphology.23,24,61 A major challenge for hydrothermal synthesis is circumventing the formation of metastable VO2(A) and VO2(B) polymorphs that represent local minima on the potential energy surface and, thereby, shifting the equilibrium toward stabilization of the rutile phase, which yields the monoclinic M1 phase upon cooling.23,61
63 The VO2(A) and VO2(B) polymorphs crystallize in far more open (less dense) crystal structures with larger unit cells and VO6 octahedra aligned exclusively along a single crystallographic direction, in contrast to rutile VO2 where octahedra are aligned along two mutually perpendicular directions; neither of the metastable polymorphs show any evidence for a metal
insulator phase transition.62,63 Upon annealing, the VO2(B) polymorph is transformed to VO2(R) at a temperature of ∼450 °C. This phase transition can be viewed as a reconstructive order
disorder structural transformation wherein the disorder of vacancies and vanadium atoms is progressively increased until, above a certain threshold of vacancies, the highly disordered phase recrystallizes into the more densely packed rutile structure with ordered chains of VO6 octahedra aligned along two mutually perpendicular crystallographic axes.62 The requisite threshold of disorder needs to be achieved under hydrothermal conditions via control of temperature, autogenous pressure, and the addition of suitable structure-directing agents and seeds to induce stabilization of the rutile phase of VO2. Figure 4 shows examples of VO2 nanostructures with varying morphology prepared in our laboratories by hydrothermal methods. Variations on two different synthetic strategies allow us to grow VO2 nanowires with considerable control of dimensions, morphology, and substitutional doping. The first approach involves the hydrothermal reduction of V2O5 by small-molecule aliphatic alcohols and ketones, wherein under high pressures, the layered V2O5 structure is intercalated by the molecular reducing agents that then serve to exfoliate sheets of reduced V2O5. Depending on the added structure-directing agent, nanosheets of VO2 can be stabilized, or splitting of the said nanosheets along specific crystallographic directions to relax generated stresses can yield faceted nanowires and other morphologies.23 This method is especially versatile and enables facile and tunable incorporation of substitutional dopant species such as Mo and W by the inclusion of tungstic and molybdic acid precursors in the hydrothermal reaction vessel.61 A second synthetic strategy involves the use of bulk V2O4 as the precursor.24 Hydration of V2O4 yields a layered structure analogous to hydrated phases discovered by Whittingham and co-workers.64
66 Intercalation, exfoliation, and splitting of these layered hydrated vanadates by structure-directing small molecules such as aliphatic alcohols, bifunctional alcohols, and carboxylic acids enables stabilization of faceted nanowires. The hydrothermal reaction time and concentration of the structuredirecting agent can be optimized to yield control over the nanobelt dimensions, whereas the choice of added structuredirecting agent and additional seeds can enable some degree of control over the morphology of the obtained nanostructures.24 For example, in recent work, the addition of a small amount of

Scaling VO2 to nanoscale dimensions offers the possibility of much simplified domain behavior and opportunities to study the intrinsic properties of the material at the single-domain limit. controlled oxidation and sputtering,55 physical vapor transport,8,56,57 ion implantation,19,52,58 hydrothermal syntheses,23,24,53,59 and thermolysis.60 Vapor transport and hydrothermal methods yield the most substantial control of dimensions, morphology, and added substitutional dopants and, not surprisingly, have emerged as the most popular synthetic methodologies. The diversity of synthetic approaches explains some of the spread in the data, a process like ion implantation, is incredibly rapid and likely freezes VO2 clusters in a kinetically trapped metastable phase, whereas hydrothermal reactions typically proceed for scores of hours, providing greater opportunities for self-purification and surface reconstruction. Guiton et al. pioneered a self-catalyzed vapor-transport process that yields single-crystalline VO2 nanobeams based on the vaporization and transport of granular VO2 precursors onto a Si3N4 substrate placed at a set distance away from the source within a tube furnace.8,57 The nanobeams have regular faceted rectangular cross sections and grow along the M1 [100] direction with the sidewalls bound by (011) and (01
1) facets.8,22,57 The surface stresses in the nanobeams can be somewhat tuned by epitaxial matching to the underlying substrate.22 The vaportransport process yields high-quality single-crystalline 750

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Figure 4. Scanning electron micrographs showing different morphologies of VO2 upon varying the hydrothermal growth conditions.

VO with perfectly octahedral symmetry (rock salt structure) facilitates the growth of six-armed VO2 nanostars exhibiting sixfold symmetry and complete preservation of electronic structure (Figure 4, unpublished results). Son et al. have recently reported the single-step hydrothermal growth of VO2 nanostars by the hydrazine reduction of V2O5 and have templated the nanocrystals onto polystyrene sphere assemblies to obtain opalescent VO2 constructs.67 Finite Size Effects on the Metal
Insulator Phase Transitions of VO2: Why (Small) Size Can Make All the Difference. Lopez et al. used ion implantation to obtain quasi-spherical and slightly oblate VO2 nanoparticles embedded within an amorphous silica matrix.19,52,58 Using optical transmittance studies, these authors observed a pronounced size-dependent increase of the insulator f metal and an equally significant decrease of the metal f insulator phase-transition temperatures, thus indicating substantial hystereses on the order of tens of degrees Celsius upon scaling VO2 to nanoscale dimensions. The role of strain was discounted as a contributing factor in mediating such hysteretic switching, although the fact of the particles being embedded in the matrix does not readily permit direct evaluation of lattice parameters during thermal cycling. These authors suggested that the high degree of crystalline perfection achieved within the surface-passsivated nanostructures decreases the density of available sites that can serve to nucleate the other phase. Consequently, an additional driving force in the form of overheating of the insulating phase or supercooling of the metallic phase is required before the phase transition can be initiated. Wu and co-workers determined that for VO2 nanobeams grown by the vapor-transport process outlined in the previous section, the domain dynamics and metal
insulator switching are strongly dependent upon strain imposed on the nanobeam via substrate interactions.8,57 Suspended nanobeam devices where

the active element does not contact the substrate show crisp although hysteretic cycling between the metallic and insulating phases in four-terminal single-nanobeam resistance measurements.8,18 On the basis of differences in scattering of visible light by the metallic and insulating domains, it is clear from optical microscopy images that the suspended nanobeams exhibit essentially single-domain behavior, and indeed, the propagation of a single metal/insulator domain wall can be studied in real time within an individual nanobeam from one electrode to another during a current-driven phase-transition process.18 Not surprisingly for the switching of an isolated single domain, the nanobeams exhibit a characteristic four orders of magnitude transition in electrical resistivity within a temperature range of 4 orders of magnitude in resistance is seen. Inset: SEM image of the device structure and the high yield and purity of the hydrothermal synthesis process yielding δ-KxV2O5 nanowires.

electron correlations and its relationship to geometric structure in the model VO2 system will thus allow design of other metal oxides exhibiting tunable spin and charge fluctuations. Greater synthetic control, improved spatial and temporal resolution in geometric and electronic structure measurements, and advances in theory pave the way for a more comprehensive understanding of the peculiarities of VO2 that have defied comprehensive elucidation for the last 5 decades.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: sb244@buffalo.edu. Fax: 716-645-6963.

’ BIOGRAPHIES Luisa Whittaker is a Ph.D. student at the University at Buffalo, SUNY. She obtained her B.S. degree from the University of Panama and M.S. degrees from the Latin
American University of Science and Technology (Panama) and the University at Buffalo, where she was a Fulbright Fellow from 2007 to 2009. Christopher J. Patridge is a Ph.D. student at the University at Buffalo, SUNY, with an anticipated graduation date of Summer 2011. He received a B.A. degree from the University at Buffalo and a M.S. degree in Forensic Science from the John Jay College of Criminal Justice, CUNY. Sarbajit Banerjee is an Assistant Professor of Chemistry at the University at Buffalo, SUNY. His research involves the study of finite size effects in strongly correlated systems, fabrication of nanostructured thin films, and X-ray absorption methodologies for probing the geometric and electronic structure of nanomaterials. For more information, see http://www.chemistry.buffalo. edu/people/faculty/banerjee/. ’ ACKNOWLEDGMENT This work was primarily supported by the National Science Foundation under DMR 0847169. S.B. also acknowledges the Research Corporation for Science Advancement for support through a Cottrell Scholar Award. We warmly acknowledge Prof. 756

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Sambandamurthy Ganapathy (UB Physics) and Dr. Daniel Fischer (NIST) for many insightful discussions.

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