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Woong-Ki Hong, SeungNam Cha, Jung Inn Sohn, Jong Min Kim, "Metal-Insulator Phase Transition in Quasi-One-Dimensional VO2 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 VO2 Structures
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 (VO2) 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) VO2 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 VO2 structures. Furthermore, we will review recent studies on realizing novel functional devices based on quasi-1D VO2 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.
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 (VO2) 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, P21/c) to a high-temperature rutile phase (R, P42/mnm) [3, 4, 21]. Although there have been continued debates on whether the MIT in VO2 is usually driven by strong electron-electron correlations associated with the Mott transition or electron-phonon interactions associated with the Peierls transition, VO2 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) VO2 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 VO2 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 VO2 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 VO2 related to metallic and insulating phases and the representative growth method of quasi-1D VO2 structures based on the vapor phase transport process. Then, we will review recent research carried out on the quasi-1D VO2 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 VO2 structures for gas and strain sensing, electrical switching, and thermal and nonvolatile electrical memory technologies.
2. Crystal and Molecular Orbital Structures of VO2
VO2 exhibits at least four different phases: the monoclinic M1, triclinic T, monoclinic M2, and rutile R phases. The electronic metal-insulator phase transition in VO2 is coupled with a SPT from a high-temperature metallic rutile (R) phase (P42/mnm space group) to a low-temperature insulating monoclinic (M1) phase (P21/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 VO6 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 VO6 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 VO2 micro/nanocrystals were reported to exist in the M1 and the R phase through stabilization by tensile stresses resulting from VO2 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].
The SPT in VO2 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 VO6 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 VO2 Structures
In recent years, considerable efforts have been made to grow single-crystalline VO2 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 VO2 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.  reported that liquid droplets of V2O5 nucleate initially and then these droplets may become nucleation sites for the growth of VO2 nanowires. Strelcov et al.  conducted direct in situ optical and photoelectron emission microscopy observations of the nucleation and growth of VO2 nanostructures using thermal transport of V2O5 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  also studied controlled morphology, density, and site-specificity of VO2 nanobeams using a two-step vapor transport method. As seen in Figure 2(b), they observed three distinctive morphologies of VO2 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 VO2 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].
4. Stimuli Effects on MITs in Quasi-1D VO2 Structures
4.1. Influence of Doping on MIT
The ability to incorporate transition metal ions into quasi-1D VO2 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 VO2. It has been reported that the substitution of V4+ ions with metal-ion dopants of higher oxidation states, such as W6+, Nb5+, and Mo6+, lowers the transition temperature , which is identical to reduction of the V4+ ions. In contrast, metal-ion dopants of lower oxidation states, such as Cr3+, Al3+, Fe3+, and Ga3+, stabilize the M2 and T phases of VO2 at room temperature , which is identical to oxidation of the V4+ ions. A schematic diagram (Figure 3(a)) shows mutual transformations of VO2 phases as a function of reduction and oxidation induced by metal-ion dopants. Furthermore, Strelcov et al.  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 VO2 nanostructures during the growth in which the doping level was varied from zero to . In addition, Lee et al.  also demonstrated the axially graded-tungsten- (W-) doped VO2 nanowires and measured resistance (R)-temperature (T) curves of the graded-W-doped and undoped VO2 nanowires, as shown in Figure 3(c). The undoped VO2 nanowire shows an abrupt resistance change at 67°C, whereas resistance of the graded-W-doped VO2 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 VO2 nanowire.
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.  demonstrated that hydrogen doping into VO2 is completely reversible process and that the MIT in VO2 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 . Figures 4(a) and 4(b) show electrical resistivities and structural phases before and after the hydrogen doping of VO2 microcrystals. In Figure 4(a), the two-terminal device made from an as-grown VO2 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 VO2 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 VO2 microcrystal. This is also well supported by the optical microscopy images of a VO2 microcrystal before and after hydrogen doping.
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 VO2 micro/nanostructures [33–35, 50]. In particular, the surface stresses associated with the interaction between a nanobeam and a substrate for VO2 nanobeams with and without epitaxial interfaces significantly affect the MIT behavior in VO2 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), VO2 nanobeams lying on a SiO2 substrate (referred to as on-substrate VO2 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 . Figure 5(b) shows that the electrical resistance of devices made from the on-substrate VO2 nanobeams changes in many discrete steps over a much wider temperature range during the heating and cooling cycles .
Sohn et al.  demonstrated how the epitaxial interface stress affects the phase transition behavior in VO2 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 VO2 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 VO2 nanobeams . 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 VO2 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 VO2 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.
4.3. Influence of External Stress on MIT
The control of the domain structure and phase transitions through external stress in VO2 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 VO2 beams have been explored by introducing the external stress [3, 36, 37, 41, 42, 51]. For example, Cao et al.  demonstrated that periodic domains of metallic and insulating phases along single-crystal VO2 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 VO2 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 VO2 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 VO2 microbeam clamped onto a soft substrate. The uniaxial compression reversibly induces a phase transition between metal () and insulator () at room temperature in the clamped VO2 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 VO2 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 VO2 microbeam device.
To investigate the influence of external stress on crystallographic phase transition behavior in VO2 microcrystals, Atkin et al.  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 VO2 microbeam subjected to external uniaxial strain, as shown in Figures 8(a) and 8(b). Figure 8(a) shows Raman spectra of a VO2 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)).
4.4. Influence of Stoichiometry and/or Defects on MIT
The MIT properties of VO2 are significantly affected by stoichiometry and/or defects due to the fact that vanadium can exist in multiple valence states such as V3+, V4+, and V5+ [38, 39, 52–55]. Recently, Zhang and coworkers  investigated the influence of stoichiometry on the structural phase transition in suspended single-crystalline VO2 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 VO2 nanobeams (Figure 9(b)). Most recently, Hong et al.  demonstrated a morphotropic phase transformation, which is the phase transition due to the compositional variation, in single-crystalline VO2 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 . Figures 9(c) and 9(d) show that the work function and the electrical resistance of the reduced VO2 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 VO2 nanobeams are closely correlated with the compositional stoichiometry and/or defects in the nanobeam.
VO2 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 . Recently, several efforts have been devoted to the demonstration of potential devices utilizing MITs in VO2 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.  demonstrated a novel gas sensing concept based on suspended VO2 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.  fabricated a flexible strain sensor and a single domain electrical switch based on a VO2 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.  also showed that the self-heated VO2 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 VO2 nanobeam.
In addition to the sensor and switching device applications, Xie et al.  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.  also reported a two-terminal memory device based on single VO2 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 VO2 nanowire. The multiple retainable resistances of the single VO2 nanowire when two different voltage pulses are applied repeatedly show the possibility of nonvolatile memory device utilizing a MIT behavior.
In this review, we first present the basic crystal and molecular orbital structures of VO2 in metallic and insulating phases and then discuss the growth characteristics of single-crystalline quasi-1D VO2 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 VO2 structures. Lastly, we present snapshots of the research carried out emerging applications of quasi-1D VO2 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 VO2 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.
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).
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