## Functional Nanomaterials for Electronics, Optoelectronics, and Bioelectronics

View this Special IssueReview Article | Open Access

Woong-Ki Hong, SeungNam Cha, Jung Inn Sohn, Jong Min Kim, "Metal-Insulator Phase Transition in Quasi-One-Dimensional VO_{2} Structures", *Journal of Nanomaterials*, vol. 2015, Article ID 538954, 15 pages, 2015. https://doi.org/10.1155/2015/538954

# Metal-Insulator Phase Transition in Quasi-One-Dimensional VO_{2} Structures

**Academic Editor:**Chetna Dhand

#### Abstract

The metal-insulator transition (MIT) in strongly correlated oxides has attracted considerable attention from both theoretical and experimental researchers. Among the strongly correlated oxides, vanadium dioxide (VO_{2}) has been extensively studied in the last decade because of a sharp, reversible change in its optical, electrical, and magnetic properties at approximately 341 K, which would be possible and promising to develop functional devices with advanced technology by utilizing MITs. However, taking the step towards successful commercialization requires the comprehensive understanding of MIT mechanisms, enabling us to manipulate the nature of transitions. In this regard, recently, quasi-one-dimensional (quasi-1D) VO_{2} structures have been intensively investigated due to their attractive geometry and unique physical properties to observe new aspects of transitions compared with their bulk counterparts. Thus, in this review, we will address recent research progress in the development of various approaches for the modification of MITs in quasi-1D VO_{2} structures. Furthermore, we will review recent studies on realizing novel functional devices based on quasi-1D VO_{2} structures for a wide range of applications, such as a gas sensor, a flexible strain sensor, an electrical switch, a thermal memory, and a nonvolatile electrical memory with multiple resistance.

#### 1. Introduction

Strongly correlated oxide materials undergoing reversible transitions between metallic and insulating states have been gaining interest because of their unique physical properties coupled with various phase transitions as well as their potential for application in electronic devices, thermochromic devices, optical and holographic devices, sensors, actuators, and power meter or thermometer [1–22]. However, in spite of the attractive features of strongly correlated systems associated with metal-insulator transitions (MITs), it has been difficult to move forward towards commercially viable industrial applications. These problems have been mainly associated with the lack of not only comprehensive and fundamental understandings of underlying physics accounting for the precise transition mechanism but also appropriate materials and technology. Therefore, recent research has been focused on MITs in single-domain nanostructures due to their unique geometry and favorable domain size, providing a simple and homogeneous system to explore the intrinsic property of individual phases or single-domain phenomena, which are obscured in bulk samples.

Among the strongly correlated materials, vanadium dioxide (VO_{2}) is the most interesting because of its first-order MIT near easily accessible temperature (approximately 341 K) that is accompanied by a structural phase transition (SPT) from a low-temperature monoclinic phase (M1, P2_{1}/*c*) to a high-temperature rutile phase (R, P4_{2}/*mnm*) [3, 4, 21]. Although there have been continued debates on whether the MIT in VO_{2} is usually driven by strong electron-electron correlations associated with the Mott transition or electron-phonon interactions associated with the Peierls transition, VO_{2} has attracted significant attention as a potential candidate for electronic and photonic devices based on MITs because of its tunable electrical and optical switching features at ultrafast time scale [1–23]. In particular, quasi-one-dimensional (quasi-1D) VO_{2} structures can provide new opportunities to explore, understand, and ultimately engineer MIT properties for developing novel functional devices as they exhibit significantly different properties compared with their bulk counterparts due to surface effects and unique dimensionality [3, 4, 21]. In addition, phase transitions in quasi-1D VO_{2} structures can be significantly affected and tuned by doping, interfacial stress, external stress, and stoichiometry and/or defects. Accordingly, considerable and extensive efforts have been recently devoted to the understanding of MIT behavior and fundamental mechanisms in quasi-1D VO_{2} structures and their practical applications, such as sensors, switching, and memory devices [24–44]. In this review, therefore, we will first describe the basic crystal and electronic structures of VO_{2} related to metallic and insulating phases and the representative growth method of quasi-1D VO_{2} structures based on the vapor phase transport process. Then, we will review recent research carried out on the quasi-1D VO_{2} structures, particularly focusing on the development of various approaches for tunable MITs by doping, surface stress, external stress, and stoichiometry and/or defects. Lastly, we will discuss recent functional applications based on quasi-1D VO_{2} structures for gas and strain sensing, electrical switching, and thermal and nonvolatile electrical memory technologies.

#### 2. Crystal and Molecular Orbital Structures of VO_{2}

VO_{2} exhibits at least four different phases: the monoclinic M1, triclinic T, monoclinic M2, and rutile R phases. The electronic metal-insulator phase transition in VO_{2} is coupled with a SPT from a high-temperature metallic rutile (R) phase (P4_{2}/*mnm* space group) to a low-temperature insulating monoclinic (M1) phase (P2_{1}/*c* space group) at a temperature of approximately 341 K (Figure 1) [3, 4, 22, 23]. All V atoms are equally spaced along linear chains of VO_{6} octahedra parallel to the crystallographic -axis () with V–V distances of 2.86 Å in the R phase, as shown in Figure 1(a). During the MIT from a metallic R phase to an insulating M1 phase, two distinctive sets of V–V bond distances are observed at 2.65 and 3.12 Å for the monoclinic M1 phase due to the pairing and tilting of VO_{6} octahedra with respect to the rutile -axis, (Figure 1(a)) [22, 23]. Another monoclinic phase (M2, C2/m space group) has two types of V chains consisting of equal-spaced tilted V chains and paired V chains. Recently, M2 phases in VO_{2} micro/nanocrystals were reported to exist in the M1 and the R phase through stabilization by tensile stresses resulting from VO_{2} crystals bent or clamped to the substrate as well as stoichiometric defects due to the variation of lattice constants [38, 39, 45–47]. The insulating character of the metastable M2 phase has been described as a Mott insulator driven by electron-electron correlation [3, 4, 21]. The T phase is a transitional phase between the M1 and M2 phases [3, 21, 39].

**(a)**

**(b)**

The SPT in VO_{2} is accompanied by a change of the electronic structures in the metallic and insulating states which was described in terms of molecular orbital theory [22, 23, 48, 49]. In the high-temperature metallic state (as shown schematically in the left side of Figure 1(b)), the density of states at is formed from a mixture of the half-filled band oriented along the and antibonding band. Across the MIT, the dimerization of the V ions along the and the tilting of the VO_{6} octahedra splits the bands that mediate V–V bonds into a bonding combination and an antibonding ( and ) combination. This results in the orbital polarization with the bonding band being fully occupied and the and being empty.

#### 3. Growth of Quasi-1D VO_{2} Structures

In recent years, considerable efforts have been made to grow single-crystalline VO_{2} nanobeams or nanowires using a vapor phase transport method because of difficulties of growth associated with the presence of various competing vanadium oxide phases [24–29]. It has been reported that the growth characteristics, morphology and composition features, and density of VO_{2} nanostructures are significantly affected by growth parameters such as temperature, gas flow rate, oxygen partial pressure, precursor deposition rate, and crystallographic plane of growth substrates [24–29]. To explain this phenomenon, Kim et al. [24] reported that liquid droplets of V_{2}O_{5} nucleate initially and then these droplets may become nucleation sites for the growth of VO_{2} nanowires. Strelcov et al. [25] conducted direct in situ optical and photoelectron emission microscopy observations of the nucleation and growth of VO_{2} nanostructures using thermal transport of V_{2}O_{5} precursor in a vacuum or in an inert gas environment. They observed the coexistence and transformation of the intermediate oxide phases and morphologies during nanostructure reductive growth, as shown in Figure 2(a). In Figure 2(a), the temperature-composition phase diagram shows that vanadium oxides can have a variety of stoichiometries due to multiple oxidation states of vanadium in which the stoichiometries are mutually transformable at specific temperatures and oxygen partial pressures. Kim and Lauhon [26] also studied controlled morphology, density, and site-specificity of VO_{2} nanobeams using a two-step vapor transport method. As seen in Figure 2(b), they observed three distinctive morphologies of VO_{2} nanostructures, such as nanoparticles, nanowires, and nanosheets, depending on local source supersaturation and temperature. In addition, as shown in Figures 2(c)–2(f), some previous studies have also shown that VO_{2} nanowires can form on various substrate surfaces and display either in-plane or out-of-plane growth, depending on the crystallographic orientation and lattice mismatch of growth substrates as well as the temperature of the reactor [27–29].

**(a)**

**(b)**

**(c)**

**(d)**

**(e)**

**(f)**

#### 4. Stimuli Effects on MITs in Quasi-1D VO_{2} Structures

##### 4.1. Influence of Doping on MIT

The ability to incorporate transition metal ions into quasi-1D VO_{2} structures, which can play a key role in determining their MIT properties, is extremely important for a variety of applications such as optical switches, smart window coating, Mott transistors, memristors, sensors, and thermal actuators [30–32, 40–44]. Figures 3(a) and 3(b) show that the doping of metal ions has a profound influence on the phase transition behavior and transition temperatures of VO_{2}. It has been reported that the substitution of V^{4+} ions with metal-ion dopants of higher oxidation states, such as W^{6+}, Nb^{5+}, and Mo^{6+}, lowers the transition temperature , which is identical to reduction of the V^{4+} ions. In contrast, metal-ion dopants of lower oxidation states, such as Cr^{3+}, Al^{3+}, Fe^{3+}, and Ga^{3+}, stabilize the M2 and T phases of VO_{2} at room temperature [30], which is identical to oxidation of the V^{4+} ions. A schematic diagram (Figure 3(a)) shows mutual transformations of VO_{2} phases as a function of reduction and oxidation induced by metal-ion dopants. Furthermore, Strelcov et al. [30] have recently demonstrated a practical synthesis procedure for stabilization of the M2 phase at ambient conditions* via* doping metal ions, which can open a way for realization of a purely electronic Mott transition field-effect transistor without an accompanying structural transition. As shown in Figure 3(b), the authors also produced high-quality uniformly doped single-crystalline structure and demonstrated a temperature-doping level phase diagram in the temperature range close to the ambient conditions by doping aluminum (Al) into VO_{2} nanostructures during the growth in which the doping level was varied from zero to . In addition, Lee et al. [31] also demonstrated the axially graded-tungsten- (W-) doped VO_{2} nanowires and measured resistance (*R*)-temperature (*T*) curves of the graded-W-doped and undoped VO_{2} nanowires, as shown in Figure 3(c). The undoped VO_{2} nanowire shows an abrupt resistance change at 67°C, whereas resistance of the graded-W-doped VO_{2} nanowire decreases gradually from room temperature to 60°C without the abrupt resistance change. As shown in Figure 3(d), with the increase in temperature, the metallic phase grows out of the two ends of the W-doped nanowire, followed by a progressive invasion into the insulating phase toward the middle of the W-doped nanowire, and the W-doped nanowire entirely turns into a single metallic phase at 55–60°C compared with the undoped VO_{2} nanowire.

**(a)**

**(b)**

**(c)**

**(d)**

Although the doping of metal ions into vanadium oxides is usually regarded as the effective way to control the electron concentration, this process is not reversible. Recently, Wei et al. [32] demonstrated that hydrogen doping into VO_{2} is completely reversible process and that the MIT in VO_{2} nanostructures can be strongly modified by doping with atomic hydrogen using the catalytic spillover method, which results in the electronic phase transition (i.e., the Mott transition). The authors also demonstrated that the MIT accompanied by a structural phase transition could be reversibly modified by hydrogen doping using a catalytic spillover method [32]. Figures 4(a) and 4(b) show electrical resistivities and structural phases before and after the hydrogen doping of VO_{2} microcrystals. In Figure 4(a), the two-terminal device made from an as-grown VO_{2} microcrystal shows thermally activated conduction exhibiting an energy gap close to 0.6 eV (black curve). The two-terminal device that baked in flushing hydrogen gas at 150°C for 20 min shows an energy gap close to 0.2 eV (green curve). The energy gap of the device after further baking at 180°C for 20 min (red curve) is nearly zero. The device after annealing at 190°C for 20 min eventually stabilized in the metallic state with a characteristic negative slope (purple curve). The two-terminal device that was annealed in air at 250°C for 20 min recovered the original phase transition and temperature dependence (blue curve). As shown in the SEM images of Figure 4(b), the VO_{2} nanobeam becomes straight after hydrogen doping to the fully metallic state, indicating that the fully hydrogen-doped nanobeam has a shorter lattice constant than a monoclinic as-grown VO_{2} microcrystal. This is also well supported by the optical microscopy images of a VO_{2} microcrystal before and after hydrogen doping.

**(a)**

**(b)**

##### 4.2. Influence of Surface Stress on MIT

The surface stress, affecting the lattice structure and relative stability of competing phases, plays an important role in determining the phase state of VO_{2} micro/nanostructures [33–35, 50]. In particular, the surface stresses associated with the interaction between a nanobeam and a substrate for VO_{2} nanobeams with and without epitaxial interfaces significantly affect the MIT behavior in VO_{2} nanobeams, the spontaneous formation of metal-insulator domains, and the spatial phase transitions as well as the formation and stabilization of an M2 phase. For example, as shown in Figure 5(a), VO_{2} nanobeams lying on a SiO_{2} substrate (referred to as on-substrate VO_{2} nanobeams) without metal contacts exhibit the spontaneous formation of alternating metal-insulator domains along the nanobeam length, resulting from an adhesive interaction between the nanobeam and the substrate leading to a coherent uniaxial strain on the nanobeam [33]. Figure 5(b) shows that the electrical resistance of devices made from the on-substrate VO_{2} nanobeams changes in many discrete steps over a much wider temperature range during the heating and cooling cycles [33].

**(a)**

**(b)**

Sohn et al. [34] demonstrated how the epitaxial interface stress affects the phase transition behavior in VO_{2} nanobeams epitaxially grown on c-cut sapphire. Figure 6(a) shows the temperature-dependent evolution of X-ray diffraction (XRD) spectra related to the (011)M1 and (020)M1 planes. Contour plots exhibit coexisting characteristics within the temperature region of 54–64°C and 68–80°C for corresponding (011)M1 and (020)M1 planes (marked by yellow dotted lines), respectively. In particular, in Figure 6(a), the peak corresponding to the plane of M2 is broader than that expected at low temperature and its peak position shifts slightly upward compared to the value of an M2 phase in VO_{2} nanobeams without the epitaxial interface. A SPT in the (011) plane occurs from 54°C, whereas a peak of the (020) plane splits into two peaks of (200)R and (002)M2 planes corresponding to the (020) plane of an M1 phase from 68°C, indicating the coexistence of M2 and R phases. Figure 6(b) shows temperature-dependent Raman and XRD spectra for VO_{2} nanobeams [35]. The temperature-dependent Raman spectra, which are obtained from the straight part (marked by A in the upper inset) and bent part (marked by B in the lower inset) of a bent VO_{2} nanobeam on a c-cut sapphire substrate, demonstrate the stress-induced structural transitions and the coexistence of three distinct M1, M2, and R phases. The evolution of Raman spectra of the straight region of a nanobeam (A) exhibits direct structural changes from M1 to M2 phases, whereas those of the bent part of a nanobeam (B) display coexistence of both M1 and M2 phases with increasing temperature and peaks associated with only M1 and M2 phases are observed even at room temperature. The XRD spectra from ensembles of epitaxially grown VO_{2} nanobeams were obtained at the temperature range of 6–303 K during the cooling process. At 303 K, peaks of (011)M1 and corresponding M2 planes coexist and the coexisting region exists down to 50 K through the direct transformation of the remaining M2 phase to an M1 phase.

**(a)**

**(b)**

##### 4.3. Influence of External Stress on MIT

The control of the domain structure and phase transitions through external stress in VO_{2} could lead not only to deeply understanding the correlated electron materials but also to providing a novel way to control their electrical and optical properties for device applications. Recently, the phase transitions and domain dynamics between metallic and insulating phases in single-crystalline qausi-1D VO_{2} beams have been explored by introducing the external stress [3, 36, 37, 41, 42, 51]. For example, Cao et al. [36] demonstrated that periodic domains of metallic and insulating phases along single-crystal VO_{2} microbeams were nucleated and manipulated by tuning the strain over a wide range of values, as shown in Figure 7. Figure 7(a) shows the evolution of domains of triangular shape along a bent VO_{2} microbeam at different temperatures. The bent microbeam was in an insulating phase at room temperature and periodic triangular domains of the metallic phase started to nucleate at the inner edge of the bent region (compressive strain) at elevated temperatures. At a temperature near 341 K, the straight part of the microbeam transformed abruptly to the metallic phase, whereas the bent part of the microbeam showed a coexistence of domains of the metallic and insulating phases. The uniaxial stress (*σ*)-temperature (*T*) phase diagram in Figure 7(b) shows the fraction of the metallic and insulating phases as a function of temperature (-axis) and uniaxial stress (left -axis) or strain (right -axis). In the diagram, the VO_{2} phase is a pure metallic phase (metallic phase fraction ) at high temperatures and high compressive stresses and a pure insulating phase () at low temperatures and high tensile stresses. At intermediate temperatures and stresses, metallic and insulating phases coexisted. Figure 7(c) also shows how the metallic phase fraction (* η*) changed with external compressive stress along the length of a VO

_{2}microbeam clamped onto a soft substrate. The uniaxial compression reversibly induces a phase transition between metal () and insulator () at room temperature in the clamped VO

_{2}microbeams. The microbeam can be self-heated into the metallic phase when the applied bias voltage exceeded a threshold transition voltage and the operation power of the self-heated VO

_{2}microbeam can be drastically reduced under the uniaxial compression at room temperature. Figure 7(d) shows the experimental observation of a MIT behavior induced by Joule heating under the external compression in the VO

_{2}microbeam device.

**(a)**

**(b)**

**(c)**

**(d)**

To investigate the influence of external stress on crystallographic phase transition behavior in VO_{2} microcrystals, Atkin et al. [37] employed Raman spectroscopy, which is a facile, rapid, and nondestructive tool for studying the phase transition properties of individual nano/microstructures. The authors demonstrated that, with increasing tensile strain, an M1–T–M2 structural phase transition occurs at temperatures below approximately 305 K over a wide range of strain values in an individual, homogeneous VO_{2} microbeam subjected to external uniaxial strain, as shown in Figures 8(a) and 8(b). Figure 8(a) shows Raman spectra of a VO_{2} microcrystal showing the evolution in phonon modes with increasing tensile strain at room temperature. From these Raman spectra, a Raman frequency map is presented based on the spectral position of as a fingerprint for the three different phases (M1, T, and M2 phases) (Figure 8(b)).

**(a)**

**(b)**

##### 4.4. Influence of Stoichiometry and/or Defects on MIT

The MIT properties of VO_{2} are significantly affected by stoichiometry and/or defects due to the fact that vanadium can exist in multiple valence states such as V^{3+}, V^{4+}, and V^{5+} [38, 39, 52–55]. Recently, Zhang and coworkers [38] investigated the influence of stoichiometry on the structural phase transition in suspended single-crystalline VO_{2} nanobeams and established a pseudo-*T*-*δ* phase diagram with dimensions of temperature and stoichiometry, as shown in Figure 9(a). The authors also demonstrated that the annealing of nanobeams under vacuum conditions stabilized the rutile phase to temperatures as low as 103 K due to the fact that oxygen deficiency contributed to the enhancement of conductivity, providing direct evidence of substantial electron doping in VO_{2} nanobeams (Figure 9(b)). Most recently, Hong et al. [39] demonstrated a morphotropic phase transformation, which is the phase transition due to the compositional variation, in single-crystalline VO_{2} nanobeams caused by thermal reduction in a high-pressure hydrogen gas, leading to the stabilization of metallic phases. The authors showed that hydrogen significantly reduced oxygen in the nanobeams with characteristic nonlinear reduction kinetics which depend on the annealing time [39]. Figures 9(c) and 9(d) show that the work function and the electrical resistance of the reduced VO_{2} nanobeams follow a similar trend to the compositional variation due to the oxygen deficiency and related defects. These results imply that the structural properties and the electrical resistivity of VO_{2} nanobeams are closely correlated with the compositional stoichiometry and/or defects in the nanobeam.

**(a)**

**(b)**

**(c)**

**(d)**

#### 5. Applications

VO_{2} has attracted considerable attention because of a variety of potential applications based on abrupt reversible phase transitions at ultrafast timescales in which the phase transition can be triggered by external perturbations, such as thermal, electrical, or optical perturbations as well as strain [1]. Recently, several efforts have been devoted to the demonstration of potential devices utilizing MITs in VO_{2} nanostructures, such as gas sensors, strain sensors, electrical switches, a thermal memory, and electrical memory devices [40–44, 56, 57].

For example, Strelcov et al. [40] demonstrated a novel gas sensing concept based on suspended VO_{2} nanowires, as shown in Figure 10(a), in which the transition properties of nanowires strongly depend on the changes in molecular composition, pressure, and temperature of the ambient gas environment. Hu et al. [41] fabricated a flexible strain sensor and a single domain electrical switch based on a VO_{2} nanobeam. Figure 10(b) shows the change of the current (*I*)-voltage (*V*) behavior solely dependent on the loading strains (compressive and tensile strains) in which the different types of strain lead to the distinct response from the phase transition between M1 and M2 phases. Hu et al. [42] also showed that the self-heated VO_{2} nanobeam under the application of a bias voltage that exceeds a threshold transition voltage can be easily switched based on a single domain transition by stretching or compressing the substrate. Figure 10(c) shows a single domain switch based on phase transitions induced by the coupling of self-heating and external strain in a VO_{2} nanobeam.

**(a)**

**(b)**

**(c)**

In addition to the sensor and switching device applications, Xie et al. [43] demonstrated a solid-state thermal memory that can store and retain thermal information with high/low (HI/LO) temperature states, as shown in Figure 11(a). In Figure 11(a), HI/LO temperature states of repeated Write HI-Read-Write LO-Read cycles using heating and cooling pulses show the switching performance and repeatability of the thermal memory. Bae et al. [44] also reported a two-terminal memory device based on single VO_{2} nanowires that were synthesized by a hydrothermal method, followed by thermal annealing process to form a monoclinic phase (Figure 11(b)). As shown in Figure 11(b), the MIT induced by the Joule heating and the hysteresis behavior leads to the nonlinear* R-V* characteristic and eventually enables the switchable resistance to be maintained, resulting from the mixed states of metallic and insulating phases during the MIT of the single VO_{2} nanowire. The multiple retainable resistances of the single VO_{2} nanowire when two different voltage pulses are applied repeatedly show the possibility of nonvolatile memory device utilizing a MIT behavior.

**(a)**

**(b)**

#### 6. Summary

In this review, we first present the basic crystal and molecular orbital structures of VO_{2} in metallic and insulating phases and then discuss the growth characteristics of single-crystalline quasi-1D VO_{2} structures in terms of their morphology, composition, and density, which can be significantly affected by growth conditions such as temperature, gas flow rate, oxygen partial pressure, precursor deposition rate, and crystallographic plane of growth substrates. Next, we discuss the influence of doping, surface stress, external stress, and stoichiometry and/or defects on various aspects of phase transitions in the quasi-1D VO_{2} structures. Lastly, we present snapshots of the research carried out emerging applications of quasi-1D VO_{2} structures such as a gas sensor, a flexible strain sensor, an electrical switch, a thermal memory, and a nonvolatile electrical memory. We expect that this review will give insights not only into understanding the basic important aspects of mechanism and properties of quasi-1D VO_{2} materials but also into developing practical applications for successful commercialization based on MIT technology.

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

#### Acknowledgments

Woong-Ki Hong acknowledges the financial support from the National Research Foundation of Korea (NRF) grant funded by the Korean Government (NRF-2013-R1A1A2009884 and NRF-2014M2B2A4030807). SeungNam Cha, Jung Inn Sohn, and Jong Min Kim acknowledge the support from the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry and Energy, Republic of Korea (no. 20128510010080).

#### References

- Z. Yang, C. Ko, and S. Ramanathan, “Oxide electronics utilizing ultrafast metal-insulator transitions,”
*Annual Review of Materials Research*, vol. 41, pp. 337–367, 2011. View at: Publisher Site | Google Scholar - J. Jeong, N. Aetukuri, T. Graf, T. D. Schladt, M. G. Samant, and S. S. P. Parkin, “Suppression of metal-insulator transition in VO
_{2}by electric field-induced oxygen vacancy formation,”*Science*, vol. 339, no. 6126, pp. 1402–1405, 2013. View at: Publisher Site | Google Scholar - J. H. Park, J. M. Coy, T. S. Kasirga et al., “Measurement of a solid-state triple point at the metal-insulator transition in VO
_{2},”*Nature*, vol. 500, no. 7463, pp. 431–434, 2013. View at: Publisher Site | Google Scholar - T. S. Kasirga, D. Sun, J. H. Park et al., “Photoresponse of a strongly correlated material determined by scanning photocurrent microscopy,”
*Nature Nanotechnology*, vol. 7, no. 11, pp. 723–727, 2012. View at: Publisher Site | Google Scholar - M. M. Qazilbash, M. Brehm, B.-G. Chae et al., “Mott transition in VO
_{2}revealed by infrared spectroscopy and nano-imaging,”*Science*, vol. 318, no. 5857, pp. 1750–1753, 2007. View at: Publisher Site | Google Scholar - C. Kübler, H. Ehrke, R. Huber et al., “Coherent structural dynamics and electronic correlations during an ultrafast insulator-to-metal phase transition in VO
_{2},”*Physical Review Letters*, vol. 99, no. 11, Article ID 116401, 2007. View at: Publisher Site | Google Scholar - K. Appavoo and R. F. Haglund Jr., “Detecting nanoscale size dependence in VO
_{2}phase transition using a split-ring resonator metamaterial,”*Nano Letters*, vol. 11, no. 3, pp. 1025–1031, 2011. View at: Publisher Site | Google Scholar - R. Lopez, L. A. Boatner, T. E. Haynes, R. F. Haglund Jr., and L. C. Feldman, “Switchable reflectivity on silicon from a composite VO
_{2}-SiO_{2}protecting layer,”*Applied Physics Letters*, vol. 85, no. 8, pp. 1410–1412, 2004. View at: Publisher Site | Google Scholar - A. A. Bugayev and M. C. Gupta, “Femtosecond holographic interferometry for studies of semiconductor ablation using vanadium dioxide film,”
*Optics Letters*, vol. 28, no. 16, pp. 1463–1465, 2003. View at: Publisher Site | Google Scholar - A. Cavalleri, C. Tóth, C. W. Siders et al., “Femtosecond structural dynamics in VO
_{2}during an ultrafast solid-solid phase transition,”*Physical Review Letters*, vol. 87, Article ID 237401, 2001. View at: Google Scholar - D. Ruzmetov, G. Gopalakrishnan, J. Deng, V. Narayanamurti, and S. Ramanathan, “Electrical triggering of metal-insulator transition in nanoscale vanadium oxide junctions,”
*Journal of Applied Physics*, vol. 106, no. 8, Article ID 083702, 2009. View at: Publisher Site | Google Scholar - M. J. Dicken, K. Aydin, I. M. Pryce et al., “Frequency tunable near-infrared metamaterials based on VO
_{2}phase transition,”*Optics Express*, vol. 17, no. 20, pp. 18330–18339, 2009. View at: Publisher Site | Google Scholar - T. Driscoll, H.-T. Kim, B.-G. Chae, M. Di Ventra, and D. N. Basov, “Phase-transition driven memristive system,”
*Applied Physics Letters*, vol. 95, no. 4, Article ID 043503, 2009. View at: Publisher Site | Google Scholar - D. Ruzmetov, G. Gopalakrishnan, C. Ko, V. Narayanamurti, and S. Ramanathan, “Three-terminal field effect devices utilizing thin film vanadium oxide as the channel layer,”
*Journal of Applied Physics*, vol. 107, no. 11, Article ID 114516, 2010. View at: Publisher Site | Google Scholar - Q. Gu, A. Falk, J. Q. Wu, L. Ouyang, and H. Park, “Current-driven phase oscillation and domain-wall propagation in W
_{x}v_{1−x}O_{2}nanobeams,”*Nano Letters*, vol. 7, no. 2, pp. 363–366, 2007. View at: Publisher Site | Google Scholar - B.-J. Kim, Y. W. Lee, B.-G. Chae et al., “Temperature dependence of the first-order metal-insulator transition in VO
_{2}and programmable critical temperature sensor,”*Applied Physics Letters*, vol. 90, no. 2, Article ID 023515, 2007. View at: Publisher Site | Google Scholar - R. M. Briggs, I. M. Pryce, and H. A. Atwater, “Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition,”
*Optics Express*, vol. 18, no. 11, pp. 11192–11201, 2010. View at: Publisher Site | Google Scholar - J. Cao, W. Fan, Q. Zhou et al., “Colossal thermal-mechanical actuation via phase transition in single-crystal VO
_{2}microcantilevers,”*Journal of Applied Physics*, vol. 108, no. 8, Article ID 083538, 2010. View at: Publisher Site | Google Scholar - H. Guo, M. I. Khan, C. Cheng et al., “VO
_{2}nanowire-based microthermometer for quantitative evaluation of electron beam heating,”*Nature Communications*, vol. 5, pp. 1–5, 2014. View at: Google Scholar - C. Cheng, D. Fu, K. Liu et al., “Directly metering light absorption and heat transfer in single nanowires using metal-insulator transition in VO
_{2},”*Advanced Optical Materials*, vol. 3, no. 3, pp. 336–341, 2015. View at: Publisher Site | Google Scholar - S. J. Chang, J. B. Park, G. Lee et al., “In situ probing of doping- and stress-mediated phase transitions in a single-crystalline VO
_{2}nanobeam by spatially resolved Raman spectroscopy,”*Nanoscale*, vol. 6, no. 14, pp. 8068–8074, 2014. View at: Publisher Site | Google Scholar - L. Whittaker, C. J. Patridge, and S. Banerjee, “Microscopic and nanoscale perspective of the metal-insulator phase transitions of VO
_{2}: some new twists to an old tale,”*Journal of Physical Chemistry Letters*, vol. 2, no. 7, pp. 745–758, 2011. View at: Publisher Site | Google Scholar - Y. Wu, L. Fan, W. Huang et al., “Depressed transition temperature of W
_{x}V_{1−x}O_{2}: mechanistic insights from the X-ray absorption fine structure (XAFS) spectroscopy,”*Physical Chemistry Chemical Physics*, vol. 16, no. 33, pp. 17705–17714, 2014. View at: Publisher Site | Google Scholar - M. H. Kim, B. Lee, S. Lee et al., “Growth of metal oxide nanowires from supercooled liquid nanodroplets,”
*Nano Letters*, vol. 9, no. 12, pp. 4138–4146, 2009. View at: Publisher Site | Google Scholar - E. Strelcov, A. V. Davydov, U. Lanke, C. Watts, and A. Kolmakov, “
*In situ*monitoring of the growth, intermediate phase transformations and templating of single crystal VO_{2}nanowires and nanoplatelets,”*ACS Nano*, vol. 5, no. 4, pp. 3373–3384, 2011. View at: Publisher Site | Google Scholar - I. S. Kim and L. J. Lauhon, “Increased yield and uniformity of vanadium dioxide nanobeam growth via two-step physical vapor transport process,”
*Crystal Growth and Design*, vol. 12, no. 3, pp. 1383–1387, 2012. View at: Publisher Site | Google Scholar - B. S. Guiton, Q. Gu, A. L. Prieto, M. S. Gudiksen, and H. Park, “Single-crystalline vanadium dioxide nanowires with rectangular cross sections,”
*Journal of the American Chemical Society*, vol. 127, no. 2, pp. 498–499, 2005. View at: Publisher Site | Google Scholar - C. Cheng, H. Guo, A. Amini et al., “Self-assembly and horizontal orientation growth of VO
_{2}nanowires,”*Scientific Reports*, vol. 4, pp. 1–5, 2014. View at: Publisher Site | Google Scholar - J. I. Sohn, H. J. Joo, A. E. Porter et al., “Direct observation of the structural component of the metal-insulator phase transition and growth habits of epitaxially grown VO
_{2}nanowires,”*Nano Letters*, vol. 7, no. 6, pp. 1570–1574, 2007. View at: Publisher Site | Google Scholar - E. Strelcov, A. Tselev, I. Ivanov et al., “Doping-based stabilization of the M2 phase in free-standing VO
_{2}nanostructures at room temperature,”*Nano Letters*, vol. 12, no. 12, pp. 6198–6205, 2012. View at: Publisher Site | Google Scholar - S. Lee, C. Cheng, H. Guo et al., “Axially engineered metal-insulator phase transition by graded doping VO
_{2}nanowires,”*Journal of the American Chemical Society*, vol. 135, no. 12, pp. 4850–4855, 2013. View at: Publisher Site | Google Scholar - J. Wei, H. Ji, W. Guo, A. H. Nevidomskyy, and D. Natelson, “Hydrogen stabilization of metallic vanadium dioxide in single-crystal nanobeams,”
*Nature Nanotechnology*, vol. 7, no. 6, pp. 357–362, 2012. View at: Publisher Site | Google Scholar - J. Wu, Q. Gu, B. S. Guiton, N. P. de Leon, L. Ouyang, and H. Park, “Strain-induced self organization of metal-insulator domains in single-crystalline VO
_{2}nanobeams,”*Nano Letters*, vol. 6, no. 10, pp. 2313–2317, 2006. View at: Publisher Site | Google Scholar - J. I. Sohn, H. J. Joo, D. Ahn et al., “Surface-stress-induced Mott transition and nature of associated spatial phase transition in single crystalline VO
_{2}nanowires,”*Nano Letters*, vol. 9, no. 10, pp. 3392–3397, 2009. View at: Publisher Site | Google Scholar - J. I. Sohn, H. J. Joo, K. S. Kim et al., “Stress-induced domain dynamics and phase transitions in epitaxially grown VO
_{2}nanowires,”*Nanotechnology*, vol. 23, no. 20, Article ID 205707, 2012. View at: Publisher Site | Google Scholar - J. Cao, E. Ertekin, V. Srinivasan et al., “Strain engineering and one-dimensional organization of metal–insulator domains in single-crystal vanadium dioxide beams,”
*Nature Nanotechnology*, vol. 4, no. 11, pp. 732–737, 2009. View at: Google Scholar - J. M. Atkin, S. Berweger, E. K. Chavez et al., “Strain and temperature dependence of the insulating phases of VO
_{2}near the metal-insulator transition,”*Physical Review B—Condensed Matter and Materials Physics*, vol. 85, no. 2, Article ID 020101, 2012. View at: Publisher Site | Google Scholar - S. Zhang, I. S. Kim, and L. J. Lauhon, “Stoichiometry engineering of monoclinic to rutile phase transition in suspended single crystalline vanadium dioxide nanobeams,”
*Nano Letters*, vol. 11, no. 4, pp. 1443–1447, 2011. View at: Publisher Site | Google Scholar - W.-K. Hong, J. B. Park, J. Yoon et al., “Hydrogen-induced morphotropic phase transformation of single-crystalline vanadium dioxide nanobeams,”
*Nano Letters*, vol. 13, no. 4, pp. 1822–1828, 2013. View at: Publisher Site | Google Scholar - E. Strelcov, Y. Lilach, and A. Kolmakov, “Gas sensor based on metal-insulator transition in VO
_{2}nanowire thermistor,”*Nano Letters*, vol. 9, no. 6, pp. 2322–2326, 2009. View at: Publisher Site | Google Scholar - B. Hu, Y. Ding, W. Chen et al., “External-strain induced insulating phase transition in VO
_{2}nanobeam and its application as flexible strain sensor,”*Advanced Materials*, vol. 22, no. 45, pp. 5134–5139, 2010. View at: Publisher Site | Google Scholar - B. Hu, Y. Zhang, W. Chen, C. Xu, and Z. L. Wang, “Self-heating and external strain coupling induced phase transition of VO
_{2}nanobeam as single domain switch,”*Advanced Materials*, vol. 23, no. 31, pp. 3536–3541, 2011. View at: Publisher Site | Google Scholar - R. Xie, C. T. Bui, B. Varghese et al., “An electrically tuned solid-state thermal memory based on metal-insulator transition of single-crystalline VO
_{2}nanobeams,”*Advanced Functional Materials*, vol. 21, no. 9, pp. 1602–1607, 2011. View at: Publisher Site | Google Scholar - S.-H. Bae, S. Lee, H. Koo et al., “The memristive properties of a single VO
_{2}nanowire with switching controlled by self-heating,”*Advanced Materials*, vol. 25, no. 36, pp. 5098–5103, 2013. View at: Publisher Site | Google Scholar - J. Cao, Y. Gu, W. Fan et al., “Extended mapping and exploration of the vanadium dioxide stress-temperature phase diagram,”
*Nano Letters*, vol. 10, no. 7, pp. 2667–2673, 2010. View at: Publisher Site | Google Scholar - S. Zhang, J. Y. Chou, and L. J. Lauhon, “Direct correlation of structural domain formation with the metal insulator transition in a VO
_{2}nanobeam,”*Nano Letters*, vol. 9, no. 12, pp. 4527–4532, 2009. View at: Publisher Site | Google Scholar - A. C. Jones, S. Berweger, J. Wei, D. Cobden, and M. B. Raschke, “Nano-optical investigations of the metal-insulator phase behavior of individual VO
_{2}microcrystals,”*Nano Letters*, vol. 10, no. 5, pp. 1574–1581, 2010. View at: Publisher Site | Google Scholar - N. B. Aetukuri, A. X. Gray, M. Drouard et al., “Control of the metal-insulator transition in vanadium dioxide by modifying orbital occupancy,”
*Nature Physics*, vol. 9, no. 10, pp. 661–666, 2013. View at: Publisher Site | Google Scholar - C. Miller, M. Triplett, J. Lammatao et al., “Unusually long free carrier lifetime and metal-insulator band offset in vanadium dioxide,”
*Physical Review B*, vol. 85, no. 8, Article ID 085111, 2012. View at: Publisher Site | Google Scholar - Y. Cheng, T. Zhang, Y. Cai, K. M. Ho, K. K. Fung, and N. Wang, “Structure and metal-to-insulator transition of VO
_{2}nanowires grown on sapphire substrates,”*European Journal of Inorganic Chemistry*, vol. 2010, no. 27, pp. 4332–4338, 2010. View at: Publisher Site | Google Scholar - H. Guo, K. Wang, Y. Deng et al., “Nanomechanical actuation from phase transitions in individual VO
_{2}micro-beams,”*Applied Physics Letters*, vol. 102, no. 23, Article ID 231909, 2013. View at: Publisher Site | Google Scholar - C. H. Griffiths and H. K. Eastwood, “Influence of stoichiometry on the metal-semiconductor transition in vanadium dioxide,”
*Journal of Applied Physics*, vol. 45, no. 5, pp. 2201–2206, 1974. View at: Publisher Site | Google Scholar - C. Ko, Z. Yang, and S. Ramanathan, “Work function of vanadium dioxide thin films across the metal-insulator transition and the role of surface nonstoichiometry,”
*ACS Applied Materials and Interfaces*, vol. 3, no. 9, pp. 3396–3401, 2011. View at: Publisher Site | Google Scholar - E. Hryha, E. Rutqvist, and L. Nyborg, “Stoichiometric vanadium oxides studied by XPS,”
*Surface and Interface Analysis*, vol. 44, no. 8, pp. 1022–1025, 2012. View at: Publisher Site | Google Scholar - C. Clavero, J. L. Slack, and A. Anders, “Size and composition-controlled fabrication of thermochromic metal oxide nanocrystals,”
*Journal of Physics D: Applied Physics*, vol. 46, no. 36, Article ID 362001, 2013. View at: Publisher Site | Google Scholar - A. Simo, B. Mwakikunga, B. T. Sone, B. Julies, R. Madjoe, and M. Maaza, “VO
_{2}nanostructures based chemiresistors for low power energy consumption hydrogen sensing,”*International Journal of Hydrogen Energy*, vol. 39, no. 15, pp. 8147–8157, 2014. View at: Publisher Site | Google Scholar - K. J. Choi and H. W. Jang, “One-dimensional oxide nanostructures as gas-sensing materials: review and issues,”
*Sensors*, vol. 10, no. 4, pp. 4083–4099, 2010. View at: Publisher Site | Google Scholar

#### Copyright

Copyright © 2015 Woong-Ki Hong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.