Vanadium oxides (VxOy) are classic “smart functional materials” used in a wide array of thermochromic, electronic, and catalytic applications. Specifically, vanadium dioxide (VO2) class nanomaterials are of enormous interest due to their unique first order reversible metal-insulator phase transition (MIT) behavior accompanied by a structural phase transition, inducing dramatic changes in electrical and optical properties with large lattice deformation. To date, a plethora of reports exemplifying the MIT characteristics of VO2, synthetic methods of VO2, and modulating VO2 phase transition temperatures (Tc) have been published. In this Tutorial Review, we present an overview on the fundamentals of the VO2 band structure and principles of MIT and outline various reported synthetic approaches for VO2 thin films, including dimensionally oriented VO2 nanostructures. Discussion on recent trends in VO2 applications, challenges in VO2 synthesis, and future perspectives are also elaborated in detail.
A. Vanadium oxides and their properties
Vanadium oxides are ideal prototypical materials with strong electron–electron correlations discovered to demonstrate remarkable changes in their electrical properties from the insulating to the metallic state by applying external stimuli. A sudden change in the intrinsic electrical conductivity of vanadium oxides is normally related to a reversible semiconductor-to-metal transition (SMT), which is more widely known as a metal-to-insulator transition (MIT). Vanadium dioxide (VO2), for example, undergoes fully reversible MIT between insulating monoclinic VO2(M) to metallic rutile VO2(R) phases or vice versa triggered by externally applied stimuli such as heat or stress. The phase transition corresponds to a four orders of magnitude dramatic change in resistivity and optical properties, which can be reversible via natural cooling. Since the discovery of MIT in VO2(M) in the late 1950s,1 vanadium oxides have attracted extensive recognition as resistance switching material for highly sensitive smart device applications. This heightened the interest in the other phases of vanadium oxides, and a series of vanadium oxide systems were realized to demonstrate MIT behavior. Non-stoichiometric vanadium oxides exhibiting MIT are categorized into two families, namely, Magneli-type vanadium oxides with a general formula of VnO2n−1, and Wadsley phases with structural compositions of VnO2n + 1.2 The Wadsley phases are less oxidized compared with the Magneli phases and do not exhibit MIT, except V6O13.3 On the other hand, most Magneli phases, comprised of corundum and rutile-like layers with structural similarity to dioxide and sesquioxide, undergo reversible phase transition from metallic to insulating states at certain temperatures.4 Figure 1(a) shows typical examples of Magneli phase VnO2n−1 oxides and their corresponding MIT temperatures.
A Magneli-type VO2 is an ideal textbook example of a system that exhibits abrupt structural transition from the metallic to the insulator state with an electrical conductivity change up to five orders of magnitude. To date, many VO2 crystalline polymorphs have been reported, including monoclinic VO2(M1) (P21/c),5 tetragonal VO2(R) (P42/mnm),5 monoclinic VO2(M2) (C2/m),6 tetragonal VO2(A) (P42/nmc),7 triclinic VO2 [P*(2)],8 VO2(C),9 orthorhombic VO2⋅H2O (P222),10 tetragonal VO2⋅0.5H2O (I4/mmm),11 monoclinic V2O4 (P21/c), and V2O4.2H2O.12 In addition, the VO2(D) phase, which is isostructural to monoclinic NiWO4, with formation energies very close to VO2(R),13 and orthorhombic paramontroseite VO2(P),14 are also reported to exist. However, only rutile-type VO2 phases exhibit fully reversible MIT phenomena near the phase transition temperature (Tc) of 68 °C.1 In the rutile-type family, at 68 °C, the insulating M1 (monoclinic, P21/c) phase (low T) transforms into the tetragonal metallic (tetragonal, P42/mnm) R-phase (high T) under ambient pressure. Under applied hydrostatic pressure, the M1 phase can drive into another isostructural and more conductive M1′ phase and then toward a new metallic X (monoclinic) phase at room temperature.15 Additionally, the application of uniaxial stress induces the formation of a second metastable monoclinic (C2/m) M2 phase in VO2 between the M1 and R phases.16 Unlike in the M1 phase, only half of the V-atom pairs dimerize in the M2 phase, and the other half of V atoms do not undergo dimerization, which results in more insulating characteristics than in the M1 phase.17 Hence, the M1 phase can be interpreted as the overlap of two M2 phases. A metastable M2 phase is reported to form either by metal doping or by applying mechanical stress.18 Furthermore, a triclinic (T) phase is reported to exist as an intermediate between M1 and M2.19
Due to the variable oxidation states of vanadium, it is highly complicated to sketch the phase diagram of VOx compounds. In fact, a small deviation in the V–O stoichiometry results in the formation of different VOx systems with variable crystalline structures. However, a phase diagram of the VO–VO2.5 system constructed using thermodynamic models is shown in Fig. 1(b), suggesting that the correct stoichiometry for VO2(M1) phase formation is tuned by adjusting the annealing temperature and mole fraction ratios of V and O.20
Although VO2 generates immense interest as multifunctional inorganic materials for energy-saving and smart device applications, the experimental synthesis of the pure VO2(M1) phase is highly challenging owing to the presence of other stable oxide phases, and the formation of VO2(M1) occurs only at a very narrow range of oxygen partial pressure. The current state-of-art preparation methods are still complicated for the large-scale production of the impurity free VO2(M) phase. More often, VO2 nanostructures grown on a selective substrate have limited size and poor yield, indicating the demand for a simplified and efficient large-scale method/process to produce a single crystalline VO2(M) phase. It should be noted that there is still no MIT model that can clarify all of the experimental results, and the precise MIT mechanism in VO2 can be best described as a dispute between the electron-driven Mott transition and the structure-driven Peierls transition, and as a cooperation of these two mechanisms. This tutorial summarizes the fundamentals of MIT in VO2, current synthetic strategies, applications, and challenges are outlined for beginners working in the field of VO2 synthesis.
B. Metal-insulator transition (MIT) in vanadium dioxide
MIT in VO2 is a consequence of structural alteration from monoclinic to rutile states or vice versa. Nevertheless, the origin of MIT in VO2 is still a topic of debate due to the controversies in electronic band descriptions in single particle theories and correlated electronic states in Mott–Hubbard theory. As given in Fig. 2(a), VO2(R) has a tetragonal rutile crystal structure where V4+ ions occupy body center and corner positions, with each V-atom surrounded by six O2− atoms and forming an octahedral unit cell. The infinite vanadium ions in tetragonal VO2(R) are aligned in linear chains, where the adjacent V–V bond distance along the rutile c axis is ≈2.85 Å. Therefore, all metal V atoms in the V–V linear chain have equal shares of d-orbital electrons, resulting in metallic conductivity with a low resistivity of about 10−6 Ω m.21 In contrast, the monoclinic VO2(M) crystal structure is characterized by a distorted rutile crystal structure, where the unit cell volume of VO2(M) is double that of VO2(R). The phase transition from a high symmetry tetragonal structure to a low symmetry monoclinic deviates from the position of the V atoms. In this case, dimerization creates zigzag V-atom chains with two different V–V distances of 3.19 and 2.60 Å between the nearest adjacent V atoms [Fig. 2(b)]. Dimerization patterns of V chains localize d-orbital electrons within the dimer, which gives VO2(M) an insulating character with resistance in the order of 0.1 Ω m.22
According to the crystal field band structure model by Goodenough,23 VO2 adopts an unstrained stoichiometric monoclinic (M) crystal structure at a low temperature and a tetragonal rutile (R) phase at a high temperature. In the molecular orbital structure of VO2, the V 3d orbital undergoes octahedral splitting into eg and t2g symmetries. Owing to the orthorhombic component, the t2g state further splits into two dπ orbitals and one d|| orbital. At a high temperature (i.e., above Tc), the VO2(R) phase forms a π-bond and π* anti-bond between the V and O orbitals along with a d|| non-bond between adjacent V orbitals. Although the VO2(R) phase has an energy bandgap of about 2.5 eV, an overlap of partially filled π* and d|| bands, and the Fermi energy level near the overlap point of the π* and d|| bands, this material shows a metal-type conductivity [Fig. 2(c)]. Below Tc, the zigzag distortion changes the VO6 octahedral position in VO2(M), which increases the π overlap in V–O orbitals, thereby upwardly shifting the anti-bonding π* level. The significant structural band change further splits the d|| band into d|| bonding and d||* anti-bonding components. As a result, a narrow bandgap of 0.7 eV is generated between the π* and d|| bonds, which induces an insulating character to the VO2(M) phase. Although Goodenough clarified the nature of MIT in VO2, modern experimental and theoretical studies contradict the findings of this model.24
A single crystal VO2 nanobeam phase transition using a nanomechanical strain apparatus revealed that at zero strain, the MIT temperature of the insulating M1 and M2 phases is degenerate with the metallic R-phase [Fig. 3(a)].25 In unstrained VO2 or undoped VO2, the free energies of all three phases are degenerate at its triple point. Figure 3(b) is a pressure–temperature (P–T) phase diagram of VO2, including M1, M1′, R, O, and X phases constructed using optical reflectance, Raman, and data of electrical properties obtained under isothermal and/or isobaric conditions.15 The P–T phase diagram features a non-monotonic phase boundary between low temperature insulating phases (M1 and M1′) and high temperature metallic phases (R, O, and X). Along the pressure axis of the P–T phase diagram, the more insulating M1 progressively transforms into the less insulating M1′ phase and, finally, into the metallic X phases. For the transition from the insulating M1 to the metallic R-phase, the dT/dP phase boundary slope is determined to be 1.4 °C/GPa over an average pressure range of 0 to 16 GPa.
The extrinsic doping of VO2 with another metal element can adjust the phases and the MIT temperature.26 Figure 3(c) shows the substitution effect on the MIT temperature of VO2. The substitution of the V-atom with dopants higher than a +4 valence of vanadium tends to decrease the MIT temperature.27 The doping of VO2 with W reduces the Tc up to 27 K/at. % of doping, and at 7.5 at. % of W doping, the lowest Tc of −50 °C was demonstrated.28 On the other hand, dopants with valences lower than vanadium can increase the MIT temperature,29 with the Ga-atom being the most effective to enhance the Tc at 8 K/at. %, but 1% Ga doping fraction is the saturation limit.30 As was evident, Cr-doped Cr0.2V0.8O2 demonstrated the highest Tc of 100 °C.31 Metal doping of VO2 can work up to a maximum doping fraction, beyond which MIT might disappear or materials may have more complex phases than the M1 and R phases.
C. Applications of VO2 nanostructures
Despite six decades of MIT discovery, the synthesis of single phase VO2 remains a challenging task. In contrast, VO2 attracted great attention as a smart material for a number of electronic and thermochromic applications. The ultrafast resistive switching feature of VO2 is extensively explored in smart electronic applications such as transistors, bolometers, ultrafast optical switching, and so on (Fig. 4). Some important applications of VO2 are briefly summarized herewith.
D. Mott, electrolyte-gated transistors, and bolometer
A field-effect transistor (FET) is a three terminal device used to control current flow using an electric field. The three terminals of FET are source, gate, and drain. FET consists of a semiconductor channel to conduct electricity and a capacitor as a gate to amplify the carrier density of the channel by electrostatic charging effects. Despite groundbreaking and novel metal oxide semiconductor FETs (MOFET) in modern electronics, MOFET miniaturization is limited by the short channel effect and current leakage. Henceforth, new types of FETs, which work with a different operational mechanism, are of great interest. Strongly electron-correlated materials, such as VO2 (Mott insulator), can be utilized as the channel in an FET and can operate with different mechanisms, namely, “Mott-transistor” (MT).32 MT supersedes MOFETs in switching speed, sharp threshold swing, and minimal fluctuation effect. Therefore, VO2 has been explored as a channel material in MTs. However, the threshold carrier density of VO2 MIT (1018–1019/cm3) is too high to attain by solid gating. As result, liquid gating using an electrolyte or ionic liquid is typically adopted in the construction of VO2-based FETs [Fig. 5(a)].33 To prevent the electrochemical reaction of electrolyte ions with oxides, Zhou et al.34 applied monolayer graphene as an inert ion diffusion barrier at the oxide–electrolyte interface, which significantly suppressed the resistance modulation and enabled retention of the insulating phase [Fig. 5(b)]. Nakano et al.35 developed ionic liquid-gated, VO2-based electric double layer transistors, and the structural transition of VO2 was attributed to the electrostatic gating effect. In addition, electrochemically driven doping of hydrogen ions or oxygen vacancies were attributed to the gating mechanism.36
A bolometer detects and measures incident electromagnetic radiation. Ideal bolometers have an active material whose electrical resistance changes with the temperature induced by radiation absorption. Bolometer senses IR light intensity by screening the change in resistance on a detecting layer. For the high sensitivity of a bolometer, it is desired to have a large thermal coefficient of resistance (TCR) and low resistivity (ρ) component as the active material.37 Typical bolometers employ small TCR materials such as nickel, titanium, and platinum as active materials with low sensitivities. Owing to large TCRs at the MIT region, VO2 applied as an active material in microbolometers [Fig. 5(c)].38 The MIT phenomenon of VO2 suits its application as an IR sensing material in a bolometer. Nevertheless, the MIT of VO2 in a narrow range hinders continuous temperature sensing, hysteresis during heating and cooling processes, and high resistivity (1 Ω.cm) in insulating state limits with VO2 as a detecting material. Research efforts have been devoted to broadening the MIT temperature range, reducing the hysteresis by small temperature differences, and lowering the resistivity by doping. The undoped VO2 nanowires display a rapid resistance switching around 67 °C, while the resistance of graded W-doped VO2 nanowires progressively decreased from room temperature to 60 °C, without sudden resistance changes [Fig. 5(d)]. Particularly, graded W-doped VO2 nanowires were reported to have a high TCR of 10%/K and a low resistivity of 0.001 Ω cm, which should be reliable for highly sensitive IR materials in microbolometers [Fig. 5(e)].39
E. Thermochromics, smart windows, and actuators
Recently, the main purpose of developing VO2-based transparent thin films has shifted to energy conservation. The heat reflection and absorbing properties of VO2 are pertinent as smart window materials. The energy efficiency of VO2 thin films is estimated by building model houses with VO2-coated glass substrates mounted as smart windows or roof. Gao et al.40 fabricated a 2% tungsten-doped VO2-coated float glass (30 × 40 cm2) and fixed it to a model house as a front room window. For comparison, a blank float glass was used as a front window for another identical model house. Both model houses were equipped with thermocouples to monitor the temperature, and IR lamps as light sources were kept at an equal distance. The model houses were irradiated with IR light, and the relative temperature change noted in the VO2-coated model house was approximately 9 °C lower than the uncoated glass model house [Fig. 5(f)]. A composite of Cr2O3/VO2/SiO2 (CVS) coated glass substrates showed an ultrahigh solar modulation ability of 16.1% and excellent luminous transmittance of 54.0%.41 The model house equipped with CVS-coated glass exhibits rather slow temperature increments of 29.8%, whereas the blank uncoated house reflected rapid temperature increments by 84.8%. Polycrystalline VO2 shows several magnitudes of conductivity change across the MIT, which led to an increased use of VO2 for thermally conductive modulation applications as a thermal switch. The lattice thermal conductivity of single crystalline VO2 nanowires measured by a suspended microdevice system was found to be almost a constant (∼6 W/mK) across the MIT [Fig. 5(g)].42 The total thermal conductivity of the VO2 nanowires was typically lower than the value expected from the Wiedemann–Franz law. However, W-doped VO2 shows an abrupt increase in the total thermal conductivity across the MIT, where such thermal conductivity changes normally suggest possible modes of thermal conduction switching using WxV1−xO2 compounds. Additionally, the reversible phase transition of VO2 drives its use in actuator applications. A flexible actuator composed of VO2 and carbon nanotube biomorph produced high responses of ∼100 Hz and longer lifetime actuation cycles [Fig. 5(h)].43 The power consumption of such microscopic bending in actuators can be lowered by reducing MIT via VO2 doping.
II. SYNTHESIS OF VO2 THIN FILMS AND NANOPARTICLES
Typically, synthetic conditions have a significant influence on the morphology and defects of VO2 products, which directly affects their MIT behavior. A plethora of methods have been designed and developed to form the VO2(M) phase. Based on the reaction medium, these methods are primarily categorized as gas phase synthesis and solution-based methods. Gas phase deposition techniques frequently used for the synthesis of VO2(M) structures include chemical vapor deposition, physical vapor deposition, and sputtering methods. In contrast, solution-based approaches mainly consist of a sol–gel process, hydrothermal synthesis, and polymer assisted deposition.
A. Gas phase synthesis
Gas or vapor phase deposition techniques are commonly used for the fabrication of high-quality thin films, where one or more gaseous precursors are volatilized onto a target substrate to form a solid thin film. Thin film quality is highly dependent on the deposition rate, the distance between source and target, and the partial pressure of the system.
1. Chemical vapor deposition
The first report of the chemical vapor deposition (CVD) of VO2 films can be traced back to 1967.44 Since then, CVD has been extensively used in the fabrication of high-quality VO2 films. Depending on the synthetic configuration, CVD techniques used for the synthesis of VO2(M) nanostructures are atmospheric pressure CVD (APCVD), aerosol assisted CVD (AACVD), metal-organic CVD (MOCVD), plasma enhanced CVD (PECVD), and so forth. The principle composition and quality of CVD-deposited VO2 films are mainly determined by the vanadium precursor and the gas carrier type used. The APCVD system only utilizes volatile precursors, which is a limitation. Two types of vanadium precursors are used in the APCVD technique, namely, vanadium halides, such as VCl4 and VOCl3, and vanadium organometallics [VO(acac)2 and VO(OC3H7)3]. Maruyama and Ikuta used vanadium (III) acetylacetonate in the APCVD technique for the growth of high-quality polycrystalline VO2 films on fused quartz and sapphire substrates.45 Qureshi et al.46 inspected the formation of VO2 film on a glass substrate via APCVD reaction of VOCl3 and water at 650 °C. The incorporation of TiCl4 as a co-reactant affected the growth morphology of VO2 thin films without titanium contamination. The morphology of VO2 films deposited at 650 °C displayed an irregular ball shaped morphology, while decreasing the deposition temperature to 550 °C produced “warm-like” particles of VO2 and spherical ball-like TiO2 particles [Figs. 6(a) and 6(b)]. Interestingly, the introduction of TiCl4 in the VO2 films reduced the thermochromic switching temperature from 69 °C in bulk VO2 to 49 °C. Gaskell et al.47 used VCl4 and water to fabricate smooth monoclinic VO2 thin films with an optimized APCVD technique on an fluorine tin oxide (FTO) substrate. The deposited VO2 films exhibited a lower surface roughness and good transmittance properties. Parkin and Manning developed W-doped VO2 films on a glass substrate using an APCVD method with VOCl3, H2O, and WCl6 precursors [Fig. 6(c)].48 They found that tungsten incorporation into VO2 films reduced the transition temperature by 19 °C per W at. % doping. The significant difference in VCl4 and VOCl3 systems are the redox reactions of VOCl3⋅H2O and the hydrolysis reaction of the VCl4.H2O system. Therefore, VCl4 systems are easier to control than the VOCl3 system and are suggested for APCVD VO2 growth.
A pure monoclinic phase of VO2 was grown on CMOS-compatible Si substrates using an MOCVD technique at a deposition temperature range of 520 °C to 550 °C.49 The morphology of VO2 films significantly varied with the deposition temperature; however, films retained the M1 phase of VO2. Notably, the morphology of the thin film deposited at 535 °C showed sharp grain boundaries with an average grain size of 200 nm [Figs. 6(d)–6(f)]. The temperature dependent resistance behavior of a VO2 film grown at 535 °C was better than that of the films grown at other temperatures of 520 °C and 550 °C. MOCVD deposition temperature induces various fractions of vanadium surface defects in the form of V3+, V4+, and V5+ oxidation states. The fractions of these components vary with the deposition temperature, and the fraction of V-defects-content regulates the MIT behavior of VO2 thin films. Blackburn et al.50 designed a single molecular precursor dichloro(oxo) vanadium (IV) diethyl malonate [[VOCl2[CH2(COOEt)2]]4] for the synthesis of the VO2 monoclinic phase using AACVD [Fig. 6(g)]. The developed single molecular precursor has advantages over typical poly-molecular precursors including high solubility in AACVD-preferred solvents such as hexane, dichloromethane, and chloroform, whereas other precursor compounds, such as vanadium acetate, are sparingly soluble. Furthermore, single molecular vanadium precursors have high oxygen reactivity due to the presence of volatile chloride ligands. The morphology of VO2(M) films deposited by AACVD using single molecular precursors at 550 °C was highly disordered with large rod-like assemblies of 300–500 nm [Fig. 6(h)], while W doping greatly altered its morphology into densely packed round vertical protrusions. The thermochromic characteristics of AACVD-deposited VO2(M) films are competitive (a solar modulation of 15.9%), and small amounts of W doping into the molecular precursor reduced the MIT temperature from 70 °C in undoped VO2 to 50 °C in W-doped VO2.50
Compared with other chemical deposition techniques, PECVD offers a low temperature synthesis of VO2 thin films using either a strong electric field or microwaves to produce plasma that causes the rapid decomposition of vanadium precursors and produces thin films. Yet, there are very few reports on VO2 synthesis using PECVD due to its complexity of deposition. Warwick et al.51 applied a potential difference between the top plate and bottom substrate to generate an electrical field with a positive bias at the end substrate to produce uniform VO2(M) thin layers. The presence of the electrical field has led to substantial changes in the thin film microstructure, such as increased electrical field-reduced crystalline size and improved near-infrared transmittance property of VO2(M). The limitation of the PECVD technique is that there are several oxidant and reductant by-products that prevent the growth of pure VO2(M). Therefore, it is recommended that an appropriate single molecular vanadium precursor, such as VCl4 and VO(acac)2, be used for PECVD.
2. Physical vapor deposition
PVD is a well-known gaseous technique to fabricate vanadium oxide thin films, which usually involves evaporation, transportation, reaction, and deposition steps. In PVD, vanadium solid precursors are bombarded by high energy sources such as electron beam or ions under reduced pressure. PVD offers a low temperature synthesis of VO2 thin films and an eco-friendly method with ease of multilayer deposition. Pulse laser deposition and sputtering have been the most common PVD deposition processes to fabricate VO2 thin films. The optoelectronic characteristics of VO2 films deposited by the PVD process are strongly influenced by the nature of the substrate materials. For instance, the morphology, structural phase, and optical properties of VO2 thin films were investigated by changing substrate materials such as glass,52 silicon,53 quartz,54 and sapphire.55 Fuls et al.56 from Bell Labs first used reactive sputtering techniques to obtain VO2 nanostructures under an argon–oxygen atmosphere. Thereafter, various sputtering developments were designed to obtain high-quality VO2 films. To date, direct current (DC), radio frequency, and magnetron sputtering have been used for the fabrication of VO2 films. The advantages of sputtering processes are good homogeneity, scalability over large substrate areas, and high-quality thin films. To standardize the sputter deposition of VO2 thin films, deposition parameters such as temperature, oxygen partial pressure, and vanadium to oxygen ratio critically influence the structural, optical, and electrical properties of VO2 thin films. Some of the notable findings on standard sputter deposition parameters can be tracked to the contributions of Jin et al.57 and Razavi et al.58
Recent developments in sputter deposition have accompanied great enhancements in VO2 thin film thermochromic properties. Compared with conventional DC-sputtered VO2, reactive pulsed DC-sputtered VO2 films exhibited improved thermochromic behavior.59 Indeed, asymmetric-bipolar pulsed DC power supply enabled an increase in sputter yield by periodically reversing the voltage of the electrode, thus neutralizing target surface poisoning. In addition, pulsed DC sputtering reduces working gas pressure and increases ion current density and ion bombardment, which improves the film density/crystallinity. Moreover, optimal Nb-doping into reactive pulsed DC sputter deposited VO2 films decreased the phase transition temperature from 59 °C to 34 °C. However, the doping of Nb into VO2 films degraded the optical transmittance of thin films due to the introduction of large defects in the crystal lattice. Chen et al.60 developed VO2 thin films using reactive ion beam sputter deposition with a low phase transition temperature of 35 °C [Fig. 7(a)]. The substantial suppression in the MIT temperature of VO2 films for room temperature applications is ascribed to decreases in grain size and lower crystallite sizes of fabricated VO2 thin films. The morphology of VO2 thin films deposited on silicon substrates shows fine nanostructures with average crystallite sizes of 20 nm [Fig. 7(b)]. This study highlights that the phase transition temperature of VO2 can be lowered by reducing the particle size of VO2 rather than common doping methods. Later, Zhang et al.61 studied the relationship between grain size and hysteresis width in VO2 thin films [Fig. 7(c)]. DC magnetron-sputtered VO2 thin films with varied particle size were grown by controlling the annealing time. Extending the annealing time increased the VO2 particle size and, after 45 min of annealing time, the grain sizes of VO2 particles decreased. Although the authors did not mention the reasons for this phenomenon, the VO2 films fabricated under optimal annealing time represented improved optical transmittance. In recent years, high power impulse magnetron sputtering (HiPIMS) has emerged as a modern PVD technique for growing high-quality VO2 thin films. The HiPIMS technique offers extremely high peak power density and a high plasma density with very short plasma pulse durations. In HiPIMS, the high fraction of ionization and high plasma density and bias voltage are applied to control the bombardment momentum and improve the quality of VO2 thin films. Lin et al.62 studied the effects of pulse bias voltage on the thermochromic properties of VO2 films prepared by the HiPIMS technique. The crystalline orientation of VO2 thin films changed with varied magnitude of applied bias, and films deposited with a bias voltage of −250 V produced smaller crystalline sizes of ∼12 nm. As a result, the MIT temperature of the VO2 films was reduced from 54 °C to 31.5 °C when the applied bias voltage increased from −54 to −250 V.
Pulsed laser deposition (PLD) is a relatively new field of the PVD process for thin film deposition. PLD-assisted VO2 thin films were first reported in 1993,63 where a highly powered pulsed laser beam was focused on a metallic vanadium target that evaporated the source and deposited on the target substrate as a thin film inside a vacuum chamber with Ar/O2 process gas. The PLD technique is highly efficient in controlling the stoichiometry and orientation of VO2 thin films. The oxygen content of the VO2 film was optimized by controlling the partial pressure of oxygen, which is critical for growing the pure VO2 phase. Moreover, in PLD, oxygen flow rate, laser wavelength, and target substrate are vital parameters that influence the MIT characteristics of developed VO2 thin films. The epitaxial grown VO2 thin films on a sapphire substrate exhibit a lower MIT temperature than bulk or polycrystalline materials. Fu et al.64 comprehensively presented a study of MIT in PLD epitaxial VO2 thin films grown on a c-plane sapphire substrate. Their findings indicated that the growth direction of epitaxial VO2 thin films on sapphire is oriented along M1, instead of M1. The electron effective mass of the metallic and insulating phase was determined by Hall and Seebeck measurements. Across the MIT region of PLD-grown VO2, electrical conductivity changes were mainly accountable to the variation in electron density of the metallic and insulating states.64 Pauli et al.65 used synchronous PLD to grow VO2 films on Si (001) and Al2O3 (0001) substrates. The VO2 film grown on the Si (001) substrate at room temperature was amorphous and annealing these films produced crystalline VO2 nanoparticles of hemispherical islands [Fig. 7(d)]. In contrast, VO2 deposited on the Al2O3(0001) substrate produced preferentially oriented rod-like nanoparticles [Fig. 7(e)]. The optical measurements of VO2 thin films grown on Si (001) substrates evidenced sharp thermal hysteresis and good switching characteristics compared with VO2 deposited on the sapphire substrate [Figs. 7(f) and 7(g)].65 The variation in the thermal switching of VO2 films on different substrates was governed by the crystallinity of the VO2 phase and strain relaxation flattening of the nanoparticles on the sapphire substrate. Hajlaoui et al.66 used boron doping effects in reactive PLD-grown VO2 thin films. A small amount of boron doping stabilized the VO2(R) phase, and the (011) diffraction peak of VO2 shifted toward higher wavelengths and stabilized the VO2 R-phase. Boron doping drastically reduced the MIT behavior of PLD-grown VO2 films as large as 31.5 °C per atomic percentage of boron, where similar behavior was reported for tungsten-doped VO2.67 Kumi-Barimah et al.68 used femtosecond PLD for high-quality VO2(M1) thin films on sapphire substrates with V2O5 as an ablation target. Thin films were deposited at 400 °C and 600 °C, representing the crystalline VO2(M1) phase. The VO2(M1) films deposited at 600 °C exhibited sharp and abrupt MIT behavior at a temperature of 66 °C with a narrow hysteresis width of 3.9 °C. The noticeable electrical properties of VO2 films deposited at 600 °C were ascribed to the surface roughness of the films induced by the agglomeration of VO2 nanoparticles.
3. Vapor transportation method
The vapor transportation method (VTM) is widely used to synthesize VO2 nanowires. VO2 nanowires grown via VTM usually have rectangular cross sections with exposed  facets.69 The length, width, and density of VO2 nanowires grown using VTM can vary with the growth substrate, and this is mostly because of the difference in capillary interactions and crystallographic directions of the substrate. Phase selective growth of free-standing and substrate-bound VO2 micro/nanowires with various shapes can be grown using liquid droplet condensation over a preferentially oriented substrate. Previously, one-dimensional VO2 nanowires were thought to grow from a selective facet of VO2 nuclei by condensing precursors of evaporated VO2.70 It was later discovered that the evaporated and condensed precursor are traces of V2O5,71 and this showed that VO2 seeds are not required for the preferential growth of VO2 nanowires. In situ optical and photoelectron emission microscopy analysis revealed that the evaporation of V2O5 at low temperatures (< ∼ 700 °C) forms V2O5 droplets on the substrate, which later transform into VO2 nanowires at a high temperature (>800 °C) via reductive reactions in inert atmospheres (vacuum or Ar). Under these conditions, VO2 nanowires directly crystallize from molten V2O5 droplets at high heating rates (ca. 500 °C/min), and when the heating rate is very low, the product undergoes multiple intermediate phase transformations such as V2O5, V6O13, and VO2 (Fig. 8).71 More examples reporting the synthesis of VO2 nanowires/microbeams via VTM are described in Sec. III A.
B. Solution phase synthesis
Solution phase synthesis of VO2 nanostructures is relatively simpler than gas phase methods. The usual setup in solution phase methods is comparatively less complex and more cost-effective than gas phase methods. Sol–gel and hydrothermal synthesis are common solution phase routes to obtain VO2(M) nanostructures.
1. Sol–gel methods for films
Sol–gel is a wet-chemical process of producing solid nanoassemblies from a monomeric colloidal solution (sol), which acts as a precursor for a polymeric network (gel) of discrete particles. Typical precursors of the sol–gel process are metal alkoxides. Greenberg first deposited gel assisted VO2 thin films using a VO(OC3H7)3 precursor.72 Vapor deposition of metal-alkoxide precursors first produce metal hydroxide complexes, and subsequent annealing reduces the vanadium hydroxide complex into the VO2 phase, which is shown as VO(OC3H7)3 → VO(OH)3 → V2O5 → V2O3 → VO2. Although other phase formations over VO2 are competitive, controlled reduction of V2O5/V2O3 results in the VO2 phase. The doping of VO(OC3H7)3 gel precursors with tungsten, molybdenum, and niobium shifted the transition temperature of the as-obtained VO2 thin films below 68 °C. Similarly, Guo et al.73 used sol–gel-derived VO(OC3H7)3 precursors to fabricate VO2 thin films onto a sapphire (0001) substrate. Variations in the annealing time of thin films produced greater influence on grain size and transition temperature. Prolonging the annealing time from 1 h to 7 h transformed V2O3 to V3O5 and then to VO2, respectively. The most common precursors for sol–gel VO2 thin films are tetravalent vanadium alkoxides, which are usually expensive and harmful and require high annealing temperatures for thermal polymerization. Hence, efforts have been focused on finding alternate vanadium precursors that are low cost, safe to handle, and can form a metal oxide network at lower temperatures.
Huang and co-workers used the cetyltrimethylammonium vanadate (CTAV) precursor for nanoparticle assembled VO2 films.74 As obtained, these VO2 films exhibited a nano-hierarchical porous network induced by self-assembly of the long-chained CTAV. The visible light transmittance behavior of porous films was 25% higher than the continuous VO2 films. The porosity of VO2 films was largely tuned by adjusting the dip coating speed, which had great influence on the transmittance and switching efficiency of VO2 films. Cao et al.75 employed freeze drying methods to fabricate nanoporous VO2(M) films with excellent thermochromic characteristics. For this, vanadium powder precursors were dissolved into solution by the addition of optimal hydrogen peroxide (H2O2) and then to gelation on fused silica substrates either by freeze drying or by solvent evaporation [Fig. 9(a)]. The VO2 films fabricated by freeze drying with 7.5 ml of H2O2 produced a nanoporous film with an average mean pore size of ∼28 nm [Fig. 9(b)]. Importantly, the nanoporous nature of VO2 films rendered a highly luminous transmittance (Tlum = 50%) and a large solar modulation ability (14.7%).75
Induced doping in the sol–gel synthesis of VO2 thin films is known to alter optical and thermochemical characteristics. For instance, sol–gel-derived Ce-doped VO2 films on muscovite substrates exhibited an appreciable decrease in the phase transition temperature.76 In addition, Ce doping narrowed the interfacial surface energy between the substrate and the VO2 films, forming compact, (011) oriented films. The morphology of Ce-doped VO2 films represented relatively small grain sizes than the undoped films [Figs. 9(c) and 9(d)] because Ce partially substituted V atoms with Ce4+ and Ce3+ ions in the crystal lattice, and, hence, reduced the particle size by suppressing the grain growth. After Ce doping, the MIT Tc of the VO2 film decreased by 4.5 °C per 1 at. %, and 1.68 at. % Ce-doping Tc was reduced to 60 °C. However, the metal-insulator transition hysteresis width (Tw) of 0.43 at. % and 1.68 at. % Ce-doped VO2 films was 14 °C and 16 °C, which was higher than that of the undoped VO2 films (Tw: 12 °C). Likewise, sol–gel assisted Zr-doped VO2 films fabricated on a glass substrate signified a compact and continuous growth of films with a relative decrease in the phase transition temperature [Fig. 9(e)].77 A solution mixture of vanadium(IV)-oxy acetylacetonate, polyvinyl pyrrolidone, and Zr4+ ions were aged for 24 h to form a transparent uniform sol. The sol was coated on the treated glass substrate and subsequently annealed at 550 °C to obtain Zr-doped VO2 films. Zirconium doping into VO2 films induced lattice constant enlargements to the large ionic radii of Zr4+ ions. Zr-doping did not alter visible transmissivity; however, about a 2 wt. % increment in Zr-doping reduced the critical phase transition temperature to 50 °C.
2. Polymer assisted deposition
Polymer assisted deposition (PAD) is similar to sol–gel synthesis where vanadium salts are mixed with a potential polymer to produce aqueous transparent solutions, and then mixed precursor solutions are transformed as VO2 films by traditional spin or dip coating methods. Porous films with low optical constants and controlled thickness can have a strong influence on the optical properties of VO2 thin films.75,78 Kang et al.78 demonstrated polyvinylpyrrolidone (PVP)-assisted deposition of nanoporous VO2 films with low optical constants and enhanced luminous transmittance. PVP mixed aqueous solution of VOCl2 was spin coated on a fused silica substrate, dried, and annealed at 500 °C to produce an interconnected VO2 particle nanoporous network [Fig. 9(f)]. The VO2 films with different thicknesses represented porosity variations, and increased porosity reduced the optical constants. The significant change in optical constants modulated the infrared transmittance properties of VO2 films with a luminous transmittance of 43.3% and a solar modulation of 14.1%.78 Analogously, Cao and group used poly(tetrafluoroethylene) (PTFE) as a self-templated structural material for the preparation of nanoporous VO2 films.79 The PTFE-assisted VO2 films exhibited excellent optical performance with an ultrahigh luminous transmittance of 78.0% and a solar modulation ability of 14.1%. The developed films provide anti-reflection functionality with potential applications as smart window materials with high solar modulation ability. Laser sintering of VO2 films processed by the PAD method demonstrated two orders of magnitude of improvement in SMT.80 The main feature of this work is a replacement of a common PAD water solvent with propylene glycol (PPG). Replacing water with PPG offered control over the precursor solution viscosity, concentration, and environmental stability. The high viscosity-glycol based precursor solution enables highly reproducible thickness and properties that can be patterned using direct-write techniques, such as laser-based patterning and laser annealing. A precursor with 4:1 EDTA:NH4VO3 and polyethylene glycol dissolved in water solvents after water mixing was replaced with PPG by subsequent heating of solution. The resulting viscous precursor was found to be extremely stable, which was then spin coated and laser annealed. The room temperature optical transmittance of laser sintered VO2 films was measured as 86% and the high temperature transmittance was measured at 42%, corresponding to a total transmittance of 44% [Fig. 9(g)].80 These findings conclude that high viscosity precursors offer uniform thickness, and annealing of VO2 at 500–550 °C yields high-quality films, whereas lowering the sintering temperature results in poor crystallinity and degradation in thermochromic properties. Lee and Kim81 investigated the effects of organic polymer additives in precursor solutions (VO2+ ion) and the physicochemical properties of thermally deposited VO2 films on sapphire (0001) planes. Organic additive-(sucrose, polyvinyl alcohol [PVA], and polyvinylpyrrolidone [PVP]) added VO2 films displayed an MIT, but a control, additive free VO2 film revealed no abrupt resistance change [Fig. 9(h)]. The absence of MIT in the additive free VO2 films is ascribed to the presence of the oxidized V2O5 phase, which may scatter the conduction process and act as a potential barrier for the VO2 transition. In contrast, VO2 films without V2O5 impurity and an organic additive polymer show phase transition properties with order of reduction potential of the added polymer.
3. Hydrothermal synthesis
Hydrothermal synthesis is a common solution method in synthesizing VO2 nanostructures. However, the synthesis of a single VO2(M) phase is challenging as most hydrothermal products have a mixture of other polymeric forms such as VO2(B), VO2(P), VO2(D), and so on. Nevertheless, several polymeric VO2 forms were transformed into pure VO2(M) phase by hydrothermal heating. Hydrothermal heating of vanadium precursors commonly produces the VO2(B) phase, and subsequent postheat treatment transforms it into the VO2(M) phase. The paramount feature of hydrothermally transformed VO2(M) is the retention of the parent phase morphology.82 There are only a few reports that claim morphology changes after hydrothermal transformation from VO2(B) to VO2(M).83 The Pollet group84 described rapid synthesis of the VO2(B) phase using hydrothermal heating of V2O5 and citric acid precursors. The VO2(B) phase is stable until 400 °C, and further heating above 400 °C resulted in irreversible phase transition into the VO2(M) phase. In parallel, several studies report the indirect phase transition of other VO2 (A,13,85 D,86 P,87 C,9 and hydrate88 polymorphs into VO2(M) of various shapes by changing hydrothermal parameters such as annealing temperature and time.
Guo and co-workers89 developed a symmetrically confined growth methodology to fabricate a geometrical pattern of the VO2 nanonet. Single crystal-like VO2 nanonet superstructures are realized by a controlled one-step hydrothermal heating. VO2 films grown on rutile (001)-TiO2 substrates reflect a uniform orthogonal nanostructure network with an average nano-width wall of 56 nm [Fig. 10(a)]. The interfacial mismatch between the surface energy of (001)-TiO2 and VO2 directs the horizontal growth of vanadium particles as a nanonet. The unique VO2 nanonetwork displayed outstanding thermochromic properties.89 The solar modulation ability of the VO2 nanonet was up to 11.82% with visible light transmittance greater than 70%. Sun et al.87 used hydrothermal methods to prepare paramontroseite VO2(P) nanocrystals, and rapid heating of VO2(P) nanocrystals at 400 °C for 40 s in an air atmosphere resulted in VO2(M) phase nanocrystals. VO2(M) nanocrystals of size 10, 20, and 30 nm were obtained by the direct heating of VO2(P) phase nanocrystals [Fig. 10(b)]. The single-domain VO2(M) nanocrystals exhibited single size dependent MIT behavior, and increases in particle size gave an increased MIT temperature but cooling broadened the hysteresis width.87 Alternatively, W-doped VO2(M) nanorods prepared by in situ stirring hydrothermal synthesis reflected improved infrared switching behavior. Mao et al.90 proposed an in situ stirring approach for the hydrothermal synthesis of VO2(M), verified phase composition, morphology, and the MIT temperature of different amounts of W-doped VO2(M). The morphology of in situ stirred and hydrothermally heated undoped VO2(M) exhibited thin nanobelt shapes, and W-doped VO2(M) retained nanobelt or nanorod-like morphology [Fig. 10(c)].90 The MIT temperature of W-doped VO2(M) decreased with increasing the in situ stirring time. The reduction in MIT temperature with a longer stirring time is ascribed to the fine dispersion of W atoms into the VO2 crystal lattice, which improves the doping efficiency. The MIT temperature of 1.0 at. % W-doped VO2(M) nanorods was reduced to as low as 24.55 °C/at. %W doping in the range of 0.75–2.0 at. %. The W-doped VO2(M) films coated on PET substrates represented high light transmittance and flexibility, which are desirable for practical thermochromic applications.90
Zhou et al.91 synthesized boron-doped VO2 powders by hydrothermal heating followed by a subsequent annealing step. A relative doping level of 9.0 at. % was realized for boron-doped VO2. Boron doping induced reduction in the VO2 particle size and endorsed uniform distribution and a compact doping effect. The VO2 samples with a boron doping content of 6.0 at. % represented a fractured nanorod-like morphology of uneven size and length [Fig. 10(d)]. A DSC analysis of boron-doped VO2 with different boron contents revealed clear variation in the endo- and exothermic peaks on heating and cooling ramps, which is an indication of first order phase transition. Doping induced decreases in VO2 grain size, promoted reduction in the hysteresis loop, and with a boron doping content of 6.0 at. %, achieved an outstanding MIT temperature as low as 28.1 °C, which was very close to room temperature. The transmittance spectra of 6.0 at. % boron-doped VO2 presented large differences in thermochromic properties at two different temperatures. The visible light transmittance and the solar modulation ability of the optimal 6.0 at. % boron-doped VO2 were noted as 54.3% and 12.5%, respectively. Significant research efforts are focused on one-step hydrothermal syntheses of the pure VO2(M) phase.92 More detailed examples are described in the nanoparticle section. However, longer times and higher temperatures during the hydrothermal reaction are needed, which lead to larger VO2(M) particles than the products from the above-mentioned methods using phase transformation.
Directional growth of VO2 nanostructures shows lattice deformation with more abrupt and intrinsic MIT than bulk VO2. Since Morin's first report on the hydrothermal synthesis of 0.1 mm scale VO2 crystals, the hydrothermal method has made great progress in the synthesis of various VO2 nanostructures. The advantage of the hydrothermal method is that it provides control over the size, morphology, and phase structure of VO2. However, pure thermochromic VO2(M) nanostructures can hardly be produced by the hydrothermal method, due to the presence of multiple polymorphic phases, including VO2(B), VO2(A), VO2(D), and VO2(P). Moreover, the end product always has traces of VO2(B) and so on. Recent research has focused more on the hydrothermal synthesis of the pure VO2(M) phase for thermochromic applications.
In this regard, an indirect hydrothermal synthesis of pure VO2(M) has been realized by simultaneous VO2(B) formation and its rapid conversion into the VO2(M) phase.84 Banerjee and group83 presented a stepwise scalable hydrothermal synthesis of free-standing VO2(M) phase nanowires. The VO2(M) nanowires were prepared by the initial oxalic acid reduction of V2O2 powders into V3O7.H2O nanowires [Fig. 11(a)] and subsequent reduction using a 1:1 (v/v) mixture of water and 2-proponal, followed by postannealing [Fig. 11(b)]. These VO2(M) nanowires exhibited almost four orders of magnitude change in electrical conductivity across the MIT. Similarly, Hou et al.93 demonstrated a facile synthesis of VO2(M1) nanorods through a low temperature hydrothermal reaction, followed by annealing for the transformation of VO2(A) to the VO2(M) phase. At first, metastable VO2(A) nanorods [Fig. 11(c)] were synthesized by hydrothermal treatment of the VOSO4 precursor, and monoclinic VO2(M) nanorods were obtained by subsequent annealing of VO2(A) nanorods at 500 °C under an inert atmosphere for 1 h [Fig. 11(d)]. Shi et al. prepared linear gradient MIT behavior, hydrogen doped single-domain VO2(M) nanowires through hydrothermal synthesis, and hydrogen engineering via postannealing treatment. The intrinsic H-doping concentration in the VO2(M) nanowires was adjusted by filling ratio and reducing agent concentration. Furthermore, the annealing treatment was shown to eliminate and redistribute dopant/vacancies in the hydric VO2(M) nanowires.
C. Miscellaneous methods to obtain VO2 nanostructures
VO2 in the form of single crystal nanowires exhibit more abrupt and intrinsic MIT than polycrystalline based films/nanostructures. For various reasons (low cost and ease of processing), other nanostructures are also desired over thin films. A variety of high temperature synthetic approaches have been developed to grow doped VO2 micro/nanowires on embedded substrates. Axially grading the tungsten (W) doping level in VO2 nanowires from the tips to the center of the nanowire resulted in axial MIT behavior.39 At high temperatures, the graded WxV1−xO2 nanowires showed a gradual MIT by growth of the metallic phase from the tips into the center of the nanowire. Doping VO2 with atomic hydrogen was also reported to effect the MIT.94 Annealing VO2 nanobeams in contact with catalytic metals (Au, Pd, Cu, and Ni) under a hydrogen atmosphere showed rapid hydrogen diffusion along the CR axis.94 A coexistence of metallic and insulating domains was observed in free-standing oxygen deficient/rich VO2 nanobeams.95 The deficiency of oxygen stabilized the rutile phase even at a low temperature of 103 K and suppressed phase transition, while oxygen-rich growth conditions stabilized metastable monoclinic M2 and triclinic T phases. Cheng et al. reported ultra-long, highly dense, free-standing, single crystalline VO2 nanowires of about 5 mm grown using VTM.96 A low-melting point source V2O5 and rough quartz substrate facilitated the enhanced VO2 nucleation and growth rate. Unidirectionally long VO2 nanowires are typically fabricated by epitaxial growth. It is believed that epitaxial single crystalline VO2 nanowires on a c-cut sapphire substrate are known to grow laterally on the basal plane along the direction [Figs. 11(e) and 11(f)].69 In contrast, r-cut and a-cut sapphire substrate VO2 nanowires were grown out of r and a basal-planes [Fig. 11(g)].69 Furthermore, the use of Si(100) and rough surface quartz as substrates in VTM produced highly dense and ultra-long VO2 micro/nanowires [Figs. 11(h) and 11(i)].96,97 VO2 nanowires naturally grow to the  plane of a rutile structure, as it is a tetragonal system, and the surface energy of the (001) plane is larger than others.98 Nevertheless, epitaxial growth yields long unidirectional VO2 nanostructures usually clamped on the substrate, which makes postprocessing difficult and limits their further application. In a recent development, well-aligned vertical VO2 nanowire arrays were grown on Si (001) nanoimprinted substrates. The size of the nanowires was smaller than 1 μm; however, complicated Si substrate preparation impedes the practical use of this process in real time. Recently, Zhao et al.99 presented a simple and fast fabrication process for single crystal VO2 microtube arrays as a thermal oxidation route. The synthesis of VO2 microtube arrays includes a two-step procedure: first, high temperature heating of vanadium foil in air to approximately 1700 °C in 10 s with the aid of a direct current that produces V2O5 and then rapid successive cooling of as-synthesized V2O5 to below 250 °C enables the growth of single crystal VO2 microtube arrays. The single crystal rod shaped VO2 arrays grew vertically on the layered V2O5 substrate [Fig. 11(j)] and the interiors of the VO2 rods were hollow with uniform rectangular cross sections [Fig. 11(k)]. Optical microscopy observations demonstrated the MIT phenomenon in VO2 microtube arrays. Insulator-metal transition induced strain-free VO2 microtubes and an obvious shrinkage was detected in reverse transition due to the lattice difference in both phases [Figs. 11(a) and 11(b)].
In general, vapor deposition and hydrothermal approaches are used in the deposition of VO2 films, which are normally expensive and complicated. On the other hand, hydrothermal reactions are economical and offer control over morphology and particle size. Unfortunately, postheat treatment after a hydrothermal reaction at elevated temperature results in the agglomeration of VO2 NPs into large size particles. Therefore, a recent advancement to the conventional hydrothermal process is the one-step hydrothermal synthesis, where pure VO2(M) powders are obtained without an additional annealing step. Guo and co-workers100 reported an annealing-free hydrothermal approach to prepare ultrafine VO2(M) NPs. Hydrogen peroxide was used as the oxidizing and oxygen replenishing agent. A Teflon-lined autoclave was equipped with a quartz vessel and the gap between the quartz vessel and the Teflon-lined autoclave was filled with H2O2 solution [Fig. 12(a)]. During the hydrothermal process, H2O2 boils and slowly decomposes into water and oxygen, providing a strong oxidizing atmosphere favorable for the formation of VO2(M) NPs. As obtained, VO2(M) NPs show desirable dispersity with an average particle size of ∼30 nm [Fig. 12(b)]. These ultra-small VO2(M) nanoparticles uniformly coated on poly(ethylene terephthalate) (PET) substrates exhibited high flexibility and good visible transmittance, which is propitious for light scattering [Fig. 12(c)] and provides opportunities for thermochromic applications. Likewise, Chen et al.101 developed a one-step hydrothermal heating up process to obtain W-doped VO2(M) NPs ranging from 25 to 45 nm depending on the hydrothermal holding temperature [Fig. 12(d)]. Highly crystalline VO2 NPs were dispersed in polyurethane and uniformly coated on PET substrates for flexible films [Fig. 12(e)]. In contrast to pure VO2 NP based films, W doping into VO2 NPs largely influenced the light transmittance behavior at 20 and 100 °C. W-doped VO2 films represent a comparatively low reflectance (below 8%) than pure VO2 films (above 30%).
The sol–gel method is another common technique to prepare VO2 NPs. Ding and group74 suggested cetyltrimethylammonium vanadate (CTAV) as a precursor for VO2 NPs via the sol–gel route. Dip coating of the as-prepared CTAV precursors onto a fused silica substrate were followed by air annealing at 500 °C, and subsequent reduction under H2/Ar gas resulted in VO2 nanoparticle assembled (NPA) films [Fig. 12(f)]. The sol–gel assisted VO2 NPA film exhibited enhanced visible light transmittance, and dip coating speed was used to adjust the film optical properties such as visible light transmittance and near-infrared switching efficiency. However, the disadvantage of the dip coating technique is that the deposited VO2 films are rigid, which impedes their practical application [Fig. 12(g)].
As given in Table I, the main goal of developing VO2(M) nanostructures is to modulate VO2's bulk electronic properties and reduce MIT temperature for near-room-temperature switching applications. The band structure of VO2 varies with particle size and the underlying MIT behavior depends on the size of the VO2 particle. The fine control growth of VO2 particle size can be realized by preparation temperature, annealing time, and rate of VO2 deposition as thin films. For instance, R → M1 transition temperature or vice versa of single-domain VO2(M) nanocrystals with 10, 20, and 30 nm of particle size, increased with increasing particle size and a gradual broadening of hysteresis width.87 The crystallinity of VO2 particles was also controlled by adjusting the thickness of the precursor film and annealing time. A negligible hysteresis and a lower MIT temperature of 41 °C has been reported for a dispersed VO2 particle film with an average particle size of 1.16 μm.102 The MIT behavior of large VO2 particles is dominated by compressive stress, while tensile stress and defect density are prevalent in smaller sized particles. Furthermore, VO2 nanoscale heterostructures integrated with other nanomaterials impart synergistic structure-functional properties, which are different from bulk VO2 or low-dimension VO2. Nano-heterostructures with distinct size and particle–particle interface changes the MIT behavior of VO2. The VO2 nanorods with distinct VO2–TiO2–VO2 interfaces revealed a notable difference in the phase transition temperature.103 MIT variation of VO2–TiO2–VO2 interfaces is due to the difference in the size of nanostructure domain interfaces and orientation of VO2 seed growth. Therefore, the feasibility of incorporating VO2(M) into hybrid nanoparticles opens several classes of VO2-hybrid nanomaterials with characteristic switching properties. Hence, the nano-engineering of VO2 aids in control over crystallinity, phase purity, and the lowering of MIT for integration into small-scale device architectures operational at near room temperature.
|Category .||Preparation method .||Microstructure/morphology .||Transition temperature, Tc (°C) .||Reference .|
|Gas phase reaction||CVD||Warm-like particles||49.0||46|
|Thermal oxidation||Microtube arrays||67.2||99|
|Solution phase reaction||Hydrothermal||Nanorod/belts||41.5||90|
|Long snowflake shaped||68.0||105|
|Category .||Preparation method .||Microstructure/morphology .||Transition temperature, Tc (°C) .||Reference .|
|Gas phase reaction||CVD||Warm-like particles||49.0||46|
|Thermal oxidation||Microtube arrays||67.2||99|
|Solution phase reaction||Hydrothermal||Nanorod/belts||41.5||90|
|Long snowflake shaped||68.0||105|
Hydrothermal heating induces abrupt changes in the morphology of VO2 nanostructures and phase formation. Verma et al.105 analyzed hydrothermal growth parameters to obtain various shapes of VO2 nanostructures. In particular, increasing the hydrothermal growth time promoted the formation of the VO2(M) phase with varied morphologies. Adjusting the hydrothermal growth time for 20, 40, 48, and 60 h, and postannealing produced spherical aggregates, platelike microstructures, 3D nanocrystals, and flower shaped VO2(M) phase particles [Figs. 13(a)–13(d)].105 In another report, Wu and co-workers106 prepared VO2(M) nanospherical assemblies by hydrothermal and calcination of metastable VO2(B/D) powders at 300–500 °C for 2 h [Fig. 13(e)]. Rectangular plate-like VO2(M)-SiO2 structures were synthesized by the cumulative hydrothermal method and postannealing of VO2(B) phase powder [Figs. 13(f) and 13(g)].109 As obtained, nanoporous VO2(M)@SiO2 exhibited excellent optical performance for smart windows applications [Fig. 13(h)]. A report regarding the effects of precursor aging time on the shape and phase formation of VO2 nanostructures demonstrated a vanadium oxyacetylacetone precursor assisted synthesis of pure VO2(M) phase nanofacets with good thermochromic properties [Fig. 13(i)].110
Son et al.107 reported a direct one-step hydrothermal synthesis of VO2(M) micro- and nanocrystals. For the preparation of nanocrystals, a hydrous precipitate of VO2+ solution was realized by mixing optimized amounts of hydrazine and NaOH solution to the VO2+ precursor. The fresh amorphous VO2+, OH− precipitate was directly applied in the hydrothermal reaction. Under optimal hydrothermal reaction parameters, asterisk-shaped microcrystals, small nanocrystals, and hexagonal crystals were obtained with the pure VO2(M) phase [Figs. 13(j)–13(l)]. Wu et al.108 reported a controlled oxidation reaction route for the synthesis of single-domain monoclinic VO2(M) nanorods using solution-based synthesis. The developed method produced grain boundary free single crystalline VO2(M) nanorods [Fig. 14(a)] with rapid magnetization change upon passing the phase transition. The first order transition of VO2(M) to VO2(R) brings a magnetic susceptibility change upon passing the MIT from the disordered to the ordered magnetic moment. The magnetic susceptibility curves of single-domain VO2(M) nanorods obtained from solution synthesis exhibit sharper increases near the Tc than the VO2 samples prepared from other methods [Fig. 14(b)].108 Further, the differential magnetic susceptibility (∂x/∂T) values of VO2(M) nanorods were comparatively higher than the solid-state samples. The large magnetic entropy changes of VO2(M) nanorods near room temperature conjecture the magnetocaloric effects of VO2 and the potential for magnetic refrigeration applications.
III. CHALLENGES AND FUTURE OUTLOOK
Developing appropriate synthesis methods for VO2(M/R) is strongly demanded for studying the fundamental physics of the material and exploring commercial applications. A large-scale, low-cost universal method of obtaining the VO2(M) phase is highly desirable. Industrially compatible protocols to develop free-standing single-domain VO2(M) micro/nanowires and nanostructure growth remains a significant challenge. Even after decades of studies, gaseous and solution phase reactions mainly used to fabricate VO2 suffer from by-products or non-stoichiometric VO2 phases. In this context, the synthesis of desirable VO2(M/R) films or powders for each study is the first essential step. The synthesis of single phase VO2(M/R) with more uniform and smaller sizes, better dispersity, perfect stoichiometry, and higher crystallinity is still a task that remains to be achieved.
As discussed above, VO2 has been a breakthrough material with strong electron–electron correlation; yet the real time use of VO2 as a smart material requires the solving of several challenges.
First, following significant progress in VO2 MIT, the exact mechanism of MIT still needs a thorough explanation. Purely Mott or Peierls scenarios are insufficient to elucidate the origin of MIT. Investigations to bridge Mott and Peierls theory can provide more insights into the MIT origin. The stimulant response of VO2 to ultrafast optical fields, electrostatic fields, THz pulses, or stress can complement the current understanding of the MIT mechanism.
Second, the exploration of MIT in VO2 is limited to certain (transistors, actuators, smart switches, and sensors) applications. The use of thermochromic VO2 smart windows is gaining ground; yet, practical difficulties in uniform large area VO2 coatings need to be precisely addressed. A balance between Tc, transmittance, and solar modulation ability can be extended with structure engineering, controlled porosity, and atomic level doping. Even though VO2 serves as an energy-saving material, practical VO2-based devices are relatively lacking. Accelerated research into the application of VO2 to meet real-world demands can expand its scientific merit.
The modus operandi of exploring VO2 for more commercial and industrially useful applications remains in combined simulations and experimental investigations. The coupling of VO2 with other functional materials can alter its physicochemical properties and the developed hybrid VO2 composites have potentially superior performance.
Given these problems, we hope that this Tutorial instigates the demand for novel approaches and ideas to accelerate the current understanding of VO2 research and provide support to other inorganic materials.
This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (No. NRF- 2019R1A2C1084010).