Nanostructured Vanadium Dioxide Materials for Optical Sensing Applications

Vanadium dioxide (VO2) is one of the strongly correlated materials exhibiting a reversible insulator–metal phase transition accompanied by a structural transition from a low-temperature monoclinic phase to high-temperature rutile phase near room temperature. Due to the dramatic change in electrical resistance and optical transmittance of VO2, it has attracted considerable attention towards the electronic and optical device applications, such as switching devices, memory devices, memristors, smart windows, sensors, actuators, etc. The present review provides an overview of several methods for the synthesis of nanostructured VO2, such as solution-based chemical approaches (sol-gel process and hydrothermal synthesis) and gas or vapor phase synthesis techniques (pulsed laser deposition, sputtering method, and chemical vapor deposition). This review also presents stoichiometry, strain, and doping engineering as modulation strategies of physical properties for nanostructured VO2. In particular, this review describes ultraviolet-visible-near infrared photodetectors, optical switches, and color modulators as optical sensing applications associated with nanostructured VO2 materials. Finally, current research trends and perspectives are also discussed.


Introduction
The complex interplay between charge, spin, orbital, and lattice degrees of freedom results in the novel electronic and magnetic phenomena in strongly correlated materials (SCMs), as an interesting class of materials in condensed-matter physics [1]. Among SCMs, vanadium dioxide (VO 2 ) has attracted considerable attention, due to the reversible and dramatic changes in conductance and transmittance during metal-insulator transition (MIT), which is a first-order phase transition accompanied by a crystal structure change from a low-temperature monoclinic phase to a high-temperature rutile phase at near-roomtemperature (Tc~340 K), as shown in Figure 1a [2,3]. VO 2 is a tetragonal rutile (R) structure with space group P4 2 /mnm and lattice constants a = b ≈ 4.55 Å and c ≈ 2.85 Å above Tc, whereas it is a monoclinic M1 structure with space group P2 1 /c and lattice constants a ≈ 5.75 Å, b ≈ 4.53 Å, c ≈ 5.38 Å, b = 122.6 • [4]. According to the band theory proposed by Goodenough, the vanadium (V) 3d orbitals are split into σ* (e g ) symmetry and π* (t 2g ) symmetry states, and the t 2g states are further split into two d π orbitals and one d orbital [5]. In the R structure, the Fermi level falls between the π* band and the d band, whereas in the monoclinic structure, the d band is split into two energy bands (d and d *), and a forbidden band with the bandwidth of approximately 0.7 eV between the d band and the π* band is formed [5].
The driving mechanisms behind the MIT in VO 2 have been a topic of controversy for decades whether the transition is driven by electron-electron correlations (Mott transition) or by a structure distortion (Peierls transition). Recently, a collaborative Mott-structural transition mechanism in the phase-transition process has also been proposed as an alternative to the two abovementioned mechanisms of the MIT, because both the structural and electron-correlation aspects are important for describing the MIT behavior in VO 2 [6,7]. Park and co-workers studied a series of epitaxial VO 2 films with different deposition temperatures to understand the cooperation effect between Peierls and Mott transitions in VO 2 [6]. They proposed the diagram of band structures, which provides insights into the role of the strain and multivalent V states on the phase transition of VO 2 , as shown in Figure 1b [6]. In addition, they inferred electronic band structures corresponding to insulating M1 + M2 coexisting phases and metallic M1 and R phases, on the basis of experimental results through hydrogen incorporation in VO 2 , as shown in Figure 1c [8].
The driving mechanisms behind the MIT in VO2 have been a topic of controversy for decades whether the transition is driven by electron-electron correlations (Mott transition) or by a structure distortion (Peierls transition). Recently, a collaborative Mott-structural transition mechanism in the phase-transition process has also been proposed as an alternative to the two abovementioned mechanisms of the MIT, because both the structural and electron-correlation aspects are important for describing the MIT behavior in VO2 [6,7]. Park and co-workers studied a series of epitaxial VO2 films with different deposition temperatures to understand the cooperation effect between Peierls and Mott transitions in VO2 [6]. They proposed the diagram of band structures, which provides insights into the role of the strain and multivalent V states on the phase transition of VO2, as shown in Figure 1b [6]. In addition, they inferred electronic band structures corresponding to insulating M1 + M2 coexisting phases and metallic M1 and R phases, on the basis of experimental results through hydrogen incorporation in VO2, as shown in Figure 1c [8].  [6], Copyright 2020, American Chemical Society. (c) Band structures corresponding to M1, M2, and R phases. Reproduced with permission from [8], Copyright 2020, American Chemical Society.
Although the MIT mechanism is still unclear, the modification of the phase transition and the manipulation of physical properties in VO2 are possible by a variety of external stimuli, such as light, temperature, stress, stoichiometry, doping, pressure, electric field, and magnetic field [9]. The distinctive properties of MIT triggered by these stimuli have enabled the demonstration of a wide range of applications shown in Figure 2 [9], such as sensors, switches, smart windows, actuators, memory devices, camouflage, and memristors, including electromagnetic absorption materials [10,11].
In the present review, we focus on emerging optical sensing applications based on nanostructured VO2 materials. Firstly, we introduced briefly several synthesis methods of VO2 nanostructures and modification techniques of its physical properties. In addition, we describe the potential applications of VO2 nanostructures for optical sensing-e.g., Figure 1. (a) Schematic of crystal and electronic band structures of VO 2 in the high-temperature metallic rutile (R) phase and the low-temperature insulating monoclinic (M) phase. In VO 2 (R), V 4+ ions occupied the corner and center positions and each V 4+ is surrounded by six O 2− , where the closest V-V distance is approximately 2.85 Å in chains along the c-axis. In VO 2 (M), the unit cell is a distorted rutile structure of VO 2 (R) and two different V-V distances of 3.19 and 2.60 Å between the nearest V atoms form the zigzag atom chains. Reproduced with permission from [2], Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Schematic diagram of band structures depicting Peierls, Mott, and collaborative Mott-Peierls transitions. Reproduced with permission from [6], Copyright 2020, American Chemical Society. (c) Band structures corresponding to M1, M2, and R phases. Reproduced with permission from [8], Copyright 2020, American Chemical Society.
Although the MIT mechanism is still unclear, the modification of the phase transition and the manipulation of physical properties in VO 2 are possible by a variety of external stimuli, such as light, temperature, stress, stoichiometry, doping, pressure, electric field, and magnetic field [9]. The distinctive properties of MIT triggered by these stimuli have enabled the demonstration of a wide range of applications shown in Figure 2 [9], such as sensors, switches, smart windows, actuators, memory devices, camouflage, and memristors, including electromagnetic absorption materials [10,11].

Synthesis Methods of Nanostructured VO2
The morphology of VO2 depends on synthesis methods, which are primarily categorized solution-and gas-phase-based synthesis methods. For example, sol-gel process and hydrothermal synthesis are representative solution-based chemical approaches, while pulsed laser deposition (PLD), sputtering method, and chemical vapor deposition (CVD) are gas-or vapor-phase synthesis techniques. In previous reports [3,[12][13][14][15], various techniques for the fabrication of nanostructured VO2 materials have been described in detail. The advantages and limitations for some of these synthesis methods are summarized in Table 1. Sol-gel or hydrothermal approaches have been used to synthesize nanostructured VO2, mainly for the application of thermochromic smart windows. Meanwhile, PLD, sputtering, and CVD have been used to fabricate high quality VO2 thin films or single-crystals for the application of MIT-related devices. The various nanostructures (e.g., nanowire, nanorod, nanobeam, nanosheet, nanoparticle, and nanoplate), as well as thin films, can be fabricated by using these synthesis methods. The optical sensing applications based on VO2 with different morphologies will be described in Section 3 and, in particular, nanostructured VO2-based photodetectors will be summarized in Table 2. In the present review, we focus on emerging optical sensing applications based on nanostructured VO 2 materials. Firstly, we introduced briefly several synthesis methods of VO 2 nanostructures and modification techniques of its physical properties. In addition, we describe the potential applications of VO 2 nanostructures for optical sensing-e.g., photodetectors, optical switches, and color modulators. Finally, the current research trends and prospective research areas of VO 2 in future applications are also briefly mentioned.

Synthesis Methods of Nanostructured VO 2
The morphology of VO 2 depends on synthesis methods, which are primarily categorized solution-and gas-phase-based synthesis methods. For example, sol-gel process and hydrothermal synthesis are representative solution-based chemical approaches, while pulsed laser deposition (PLD), sputtering method, and chemical vapor deposition (CVD) are gas-or vapor-phase synthesis techniques. In previous reports [3,[12][13][14][15], various techniques for the fabrication of nanostructured VO 2 materials have been described in detail. The advantages and limitations for some of these synthesis methods are summarized in Table 1. Sol-gel or hydrothermal approaches have been used to synthesize nanostructured VO 2 , mainly for the application of thermochromic smart windows. Meanwhile, PLD, sputtering, and CVD have been used to fabricate high quality VO 2 thin films or single-crystals for the application of MIT-related devices. The various nanostructures (e.g., nanowire, nanorod, nanobeam, nanosheet, nanoparticle, and nanoplate), as well as thin films, can be fabricated by using these synthesis methods. The optical sensing applications based on VO 2 with different morphologies will be described in Section 3 and, in particular, nanostructured VO 2 -based photodetectors will be summarized in Table 2. Table 1. Synthesis methods of nanostructured VO 2 [3,[12][13][14][15].

Synthesis Method Advantages Limitations
Sol-gel

Modulation of Physical Properties of Nanostructured VO 2
In recent years, considerable efforts have been devoted to manipulate physical properties (e.g., electrical and optical properties) of nanostructured VO 2 materials for a variety of applications, such as optical switches, smart window coating, Mott transistors, memristors, sensors, and thermal actuators [12,15]. Most recently, Shi et al. [16] demonstrated the effective phase management of the metallic R phase and insulating phases of monoclinic (M1, M2) and triclinic (T) structures in single-crystalline VO 2 microbeams through stoichiometry engineering, as shown in Figure 3 [16]. Figure 3a shows the synthesis process of VO 2 microbeams in the nucleation/growth stage, driven by the reduction of high-valence vanadium precursors (V 2 O 5 or V 6 O 13 ) at T < 850 • C and the stoichiometry-modulation stage for the oxidation or deoxidation of VO 2 under different partial pressures of oxygen (P O 2 ) at T = 850 • C. Using these stoichiometry modulations by adding an appropriate amount of WO 2 , the single-crystalline W-doped VO 2 actuator with a stoichiometry gradient and selective phase stability was proposed, as shown in Figure 3b. In Figure 3c, the VO 2 microbeam actuators showed a clear laterally asymmetric configuration and evolution of domains and deflection with increasing temperature. The formation of a radially asymmetric M2-T-M1 domain pattern led to the initial bending at the beginning of the heating stage and with a further increase in temperature, the oxygen-deficient side was gradually occupied by R domains (the oxygen-rich side is occupied by M2 domains). At 60 • C, the entire VO 2 beam was transformed into the pure R phase of the straight state. As mentioned in ref. [16], the stoichiometry engineering, which was used to selectively stabilize all the three insulating phases (M1, T, M2) in single-crystalline VO 2 microbeams, may open opportunities for designing and controlling phase inhomogeneity and domains of VO 2 . and high compressive stresses, the system was in pure M phase, while it was in pure I phase when η = 0 at low temperatures and high tensile stresses. The coexistence of M and I phases with the spatial arrangement and relative fraction was shown at intermediate temperatures and stresses. In Figure 4c, room-temperature I-V characteristics of a VO2 microbeam under different axial compressions show the significant reduction of threshold voltages and currents by the external compression upon MIT, implying the possibility of novel device applications using drastic reduction of the operation power through strain engineering of VO2. The epitaxial VO2 nanostructures grown on single-crystal substrates can be strongly affected by the lattice mismatch with substrate or crystal orientations, resulting in determining the relationship between the stress and strain [17,20]. Figure 4d shows resistivitytemperature curves and the surface morphology of VO2 films grown on TiO2 and Al2O3 single crystals with various crystallographic orientations [21]. The results show that substrate-dependent strains in the VO2 films result in different MIT temperatures. This suggests an enhanced ability to manipulate the MIT properties of VO2 by using lattice strain control through the implementation of a microstructured buffer layer in heteroepitaxial In addition to stoichiometry engineering, the ability to control domain structures and phase transitions of VO 2 by strain or stress may lead to a deeper understanding of the correlated electron materials exhibiting the MIT, superconductivity, and magnetoresistance [15,17,18]. Cao et al. [19] demonstrated the manipulation of ordered arrays of metal (M) and insulator (I) domains along single-crystal VO 2 microbeams by strain engineering, where uniaxial external stress was used to engineer M-I domains and to observe the Mott transition at room temperature, as shown in Figure 4a-c [19]. Figure 4a shows an array of triangle M-I domains which are nucleated and co-stabilized by tensile and compressive strain during heating in a mechanically bent VO 2 microbeam. In the stress-temperature phase diagram (Figure 4b), when the M phase fraction η = 1 at high temperatures and high compressive stresses, the system was in pure M phase, while it was in pure I phase when η = 0 at low temperatures and high tensile stresses. The coexistence of M and I phases with the spatial arrangement and relative fraction was shown at intermediate temperatures and stresses. In Figure 4c, room-temperature I-V characteristics of a VO 2 microbeam under different axial compressions show the significant reduction of threshold voltages and currents by the external compression upon MIT, implying the possibility of novel device applications using drastic reduction of the operation power through strain engineering of VO 2 .
The epitaxial VO 2 nanostructures grown on single-crystal substrates can be strongly affected by the lattice mismatch with substrate or crystal orientations, resulting in determining the relationship between the stress and strain [17,20]. Figure 4d shows resistivity-temperature curves and the surface morphology of VO 2 films grown on TiO 2 and Al 2 O 3 single crystals with various crystallographic orientations [21]. The results show that substrate-dependent strains in the VO 2 films result in different MIT temperatures. This suggests an enhanced ability to manipulate the MIT properties of VO 2 by using lattice strain control through the implementation of a microstructured buffer layer in heteroepitaxial oxide thin films. More recently, Shin et al. [22] demonstrated core-shell heterostructure-enabled stress engineering on MIT, providing accommodation of uniform axial stress and control of the phase-transition pathway and properties in VO 2 nanobeams. In this previous study [22], core-shell VO 2 -Al 2 O 3 (CS-VO 2 ) nanobeams exhibited a simple and direct M1-R phase-transition pathway at a lower temperature without the appearance of metastable intermediate phases (M2 or T), compared to pristine VO 2 nanobeams with an M1-M2-R transition pathway, as shown in Figure 4e. These results provide the unique insight that the formation of uniform stress states through core-shell architectures can be applied to the design of phase-transition paths and physical properties for VO 2 -based device applications using the MIT process.  [22] demonstrated core-shell heterostructureenabled stress engineering on MIT, providing accommodation of uniform axial stress and control of the phase-transition pathway and properties in VO2 nanobeams. In this previous study [22], core-shell VO2-Al2O3 (CS-VO2) nanobeams exhibited a simple and direct M1-R phase-transition pathway at a lower temperature without the appearance of metastable intermediate phases (M2 or T), compared to pristine VO2 nanobeams with an M1-M2-R transition pathway, as shown in Figure 4e. These results provide the unique insight that the formation of uniform stress states through core-shell architectures can be applied to the design of phase-transition paths and physical properties for VO2-based device applications using the MIT process. Meanwhile, doping in VO2 has attracted much attention as an effective way for its electrical and optical modulation for electronic and optical device applications [5,[23][24][25]. Shao et al. [5] reviewed previous works by Yoon et al. [26] and Zou et al. [27]: (1) a twostep insulator (M-VO2)-to-metal (HxVO2)-to-insulator (HVO2) modulation as the hydrogen concentration increases in nano-sized Pt-island-decorated VO2 layers during annealing the samples at 120 °C, under forming gas containing 5% hydrogen gas (Figure 5a, upper panel); (2) a facile approach to hydrogenate monoclinic VO2 in an acidic solution under ambient conditions, by placing a small piece of low-work function metal (Al, Cu, Stress-temperature phase diagram for CS-VO 2 nanobeams (purple-colored arrow) and pristine VO 2 nanobeams (red-colored arrow). The arrows show the phase-transition traces on the phase diagram during heating. Temperature-dependent resistance during the heating process for CS-VO 2 and pristine VO 2 nanobeams. Reproduced with permission from [22], Copyright 2021, Elsevier.
Meanwhile, doping in VO 2 has attracted much attention as an effective way for its electrical and optical modulation for electronic and optical device applications [5,[23][24][25]. Shao et al. [5] reviewed previous works by Yoon et al. [26] and Zou et al. [27]: (1) a two-step insulator (M-VO 2 )-to-metal (HxVO 2 )-to-insulator (HVO 2 ) modulation as the hydrogen concentration increases in nano-sized Pt-island-decorated VO 2 layers during annealing the samples at 120 • C, under forming gas containing 5% hydrogen gas (Figure 5a, upper panel); (2) a facile approach to hydrogenate monoclinic VO 2 in an acidic solution under ambient conditions, by placing a small piece of low-work function metal (Al, Cu, Ag, Zn, or Fe) on the VO 2 surface (Figure 5a, lower panel). Recently, Chet et al. [24] modulated the insertion/extraction of hydrogen into/from the VO 2 lattice at room temperature through a solid electrolyte-assisted gating control, resulting in tristate phase transitions that enable the control of light transmittance, as shown in Figure 5b. Strelcov et al. [25] proposed a new high-yield method of doping VO 2 nanostructures with aluminum, which could provide possible stabilization of the monoclinic M2 phase for realization of a purely electronic Mott transition field-effect transistor (Figure 5c). According to previous reports [28,29], uniaxial stress and doping can stabilize the M2 phase at ambient conditions. In the schematic diagram depicting phase transformations of VO 2 phases by metal-ion dopants (Figure 5c), dopants of higher oxidation states (M = W 6+ , Nb 5+ , and Mo 6+ ) lower the transition temperature, whereas dopants of lower oxidation states (M = Cr 3+ , Al 3+ , Fe 3+ , or Ga 3+ ) stabilize the M2 and T phases of VO 2 at room temperature [25,30,31]. This behavior shows the influences of reduction and oxidation of the V 4+ ions, respectively, in which the oxidation effect is similar to the effect of application of uniaxial stress along the [110] direction of the R phase [25]. Ag, Zn, or Fe) on the VO2 surface ( Figure 5a, lower panel). Recently, Chet et al. [24] modulated the insertion/extraction of hydrogen into/from the VO2 lattice at room temperature through a solid electrolyte-assisted gating control, resulting in tristate phase transitions that enable the control of light transmittance, as shown in Figure 5b. Strelcov et al. [25] proposed a new high-yield method of doping VO2 nanostructures with aluminum, which could provide possible stabilization of the monoclinic M2 phase for realization of a purely electronic Mott transition field-effect transistor (Figure 5c). According to previous reports [28,29], uniaxial stress and doping can stabilize the M2 phase at ambient conditions. In the schematic diagram depicting phase transformations of VO2 phases by metal-ion dopants (Figure 5c), dopants of higher oxidation states (M = W 6+ , Nb 5+ , and Mo 6+ ) lower the transition temperature, whereas dopants of lower oxidation states (M = Cr 3+ , Al 3+ , Fe 3+ , or Ga 3+ ) stabilize the M2 and T phases of VO2 at room temperature [25,30,31]. This behavior shows the influences of reduction and oxidation of the V 4+ ions, respectively, in which the oxidation effect is similar to the effect of application of uniaxial stress along the [110] direction of the R phase [25].  (100)

Optical Sensing Applications
In general, the detectable wavelength ranges for photodetectors are categorized as ultraviolet (UV, 10-400 nm), visible (400-760 nm), near-infrared (NIR, 760-1000 nm), shortwavelength infrared (SWIR, 1-3 µm), mid-wavelength infrared (MWIR, 3-5 µm), and long-wavelength infrared (LWIR, 8-12 µm) [32]. According to these detectable ranges, photodetectors can be used in a variety of potential applications, as shown in Figure 6. VO 2 has an optical bandgap (∼0.7 eV) and high-temperature coefficient of resistance (TCR), suggesting its potential for optical sensing applications over a wide wavelength range. In addition, the MIT of VO 2 allows optical switching in the IR wavelength range and color modulation. In this section, we describe the representative photodetection mechanism and review up-to-date studies for VO 2 -based photodetectors. Other optical applications of transmission and color modulation are briefly reviewed.

Optical Sensing Applications
In general, the detectable wavelength ranges for photodetectors are categorized as ultraviolet (UV, 10-400 nm), visible (400-760 nm), near-infrared (NIR, 760-1000 nm), short-wavelength infrared (SWIR, 1-3 µm), mid-wavelength infrared (MWIR, 3-5 µm), and long-wavelength infrared (LWIR, 8-12 µm) [32]. According to these detectable ranges, photodetectors can be used in a variety of potential applications, as shown in Figure 6. VO2 has an optical bandgap (−0.7 eV) and high-temperature coefficient of resistance (TCR), suggesting its potential for optical sensing applications over a wide wavelength range. In addition, the MIT of VO2 allows optical switching in the IR wavelength range and color modulation. In this section, we describe the representative photodetection mechanism and review up-to-date studies for VO2-based photodetectors. Other optical applications of transmission and color modulation are briefly reviewed.

Light-Induced Phase Transition
VO2 typically has a phase-transition characteristic from the insulating M1 phase to metallic R phase above critical temperature (TC) of approximately 67 °C. Instead of thermal sources, optical excitation allows the phase transition of VO2 to occur on a picosecond time scale. However, the metallic state is not permanent and is transformed back into the insulating state [33,34]. Unlike a temporary phase transition, exposure of UV light on a VO2 can induce a permanent phase transition from an insulating to a metallic state [35]. Li et al. [36,37] also reported that a longer UV exposure duration led to a greater reduction in the resistance of the VO2 film, suggesting the possibility of UV detection. It is believed that photo-induced oxygen vacancies can induce an electronic phase transition leading to electrical resistance changes in the VO2 film during UV exposure.

Photoconductive Effect
By the incident photons with the energies greater than the energy bandgap (Eg), the electrical conductivity of the conduction channel can be modulated owing to the genera-

Light-Induced Phase Transition
VO 2 typically has a phase-transition characteristic from the insulating M1 phase to metallic R phase above critical temperature (T C ) of approximately 67 • C. Instead of thermal sources, optical excitation allows the phase transition of VO 2 to occur on a picosecond time scale. However, the metallic state is not permanent and is transformed back into the insulating state [33,34]. Unlike a temporary phase transition, exposure of UV light on a VO 2 can induce a permanent phase transition from an insulating to a metallic state [35]. Li et al. [36,37] also reported that a longer UV exposure duration led to a greater reduction in the resistance of the VO 2 film, suggesting the possibility of UV detection. It is believed that photo-induced oxygen vacancies can induce an electronic phase transition leading to electrical resistance changes in the VO 2 film during UV exposure.

Photoconductive Effect
By the incident photons with the energies greater than the energy bandgap (E g ), the electrical conductivity of the conduction channel can be modulated owing to the generation of electron-hole pairs, which change the carrier concentration in the conduction channel. The electron-hole pairs are separated by the applied electric field, generating a photocurrent (I ph ). I ph depends on the applied potential, charge carrier mobility, and carrier lifetime. Because the reported E g value of monoclinic VO 2 was approximately 0.7 eV, the photoconductive effect can be a dominant detection mechanism for the photodetectors using a VO 2 as a conductive channel under UV to NIR illumination Because VO 2 nanostructures have a large surface-to-volume ratio, oxygen molecules adsorbed on the surface can significantly affect their electrical conductivity by acting as electron acceptors. Under UV illumination, the adsorbed oxygen molecules are desorbed from the surface by recombination with photo-generated holes. Such absorption and desorption of oxygen molecules on the surface of VO 2 nanostructures under illumination can modulate the carrier concentration. The incident light acts as a gate for carrier modulation (photogating effect).

Photovoltaic Effect
When the incident photon energy is higher than the E g of the semiconductor materials, electrons are excited from the valence band to the conduction band, generating electronhole pairs. Such photoexcited carriers are driven by a built-in electric field arising from semiconductor-semiconductor or semiconductor-metal junctions such as p-n junctions and Schottky junctions. Because a VO 2 (M1) with an insulating phase is considered an ntype semiconductor, the formation of contacts with other semiconductor materials enables photodetection via the photovoltaic effect. Recently, heterojunctions between 2D materials and VO 2 have been employed for photodetectors using the photovoltaic effect; the examples are explained in the following section.

Photobolometric Effect
The absorption of photon energy can induce resistance changes in materials by increasing temperature. The figure-of-merit of the photodetector based on the photobolometric effect is the TCR, where TCR = dR/R·dT. It is well known that VO 2 has high TCR (−4%·K −1 ) and such a property enables commercial IR photodetectors (microbolometer) [38]. In VO 2based photodetectors, the photobolometric effect is believed to be dominant in the MWIR and LWIR regions.

Ultraviolet (UV) Photodetection
The structural change in VO 2 from the M1 to the R phase at the phase-transition temperature can induce lattice expansion, leading to tensile stress in the interfaced film. Xin et al. [39] reported a UV photodetector based on a ZnO/VO 2 heterostructure with a high responsivity in the UV range (Figure 7a). In this device architecture, the photodetectors exhibited high photoresponsivity and a fast photoresponse at the phase-transition temperature of the VO 2 thin film. The structural phase transition of VO 2 can induce tensile stress in ZnO thin films owing to lattice expansion. In addition, it is well known that ZnO has piezoelectric properties and that polarization can occur in the crystal. Therefore, the phase transition of VO 2 could lead to an internal electric field in ZnO thin films. Such an electric field boosted the charge separation of the electron-hole pairs generated by UV illumination, resulting in a high photoresponsivity (R = 10.07 A·W −1 ) and a faster photoresponse (rise = 0.020 s, decay = 0.032 s), compared to the device below the phase-transition temperature (R = 0.32 A·W −1 , rise = 2.49 s, decay = 2.06 s).
Basyooni et al. [40] reported a UV photodetector with a vertically stacked device configuration consisting of a VO 2 /MoS 2 /Si thin film and asymmetric metal contacts for energy-band alignment (Figure 7b). Here, the insertion of the VO 2 thin film enabled higher conductivity and photocurrent owing to the high carrier mobility and enhanced photon absorption characteristics, compared to the MoS 2 /Si device. Consequently, an increase in the photoresponsivity and specific detectivity was observed in the VO 2 /MoS 2 /Si heterostructure. The maximum photoresponsivity of the device was 4.7 A·W −1 .
Employing nanostructures into the photodetectors is promising due to their large surfaceto-volume ratio, which allows for enhanced light absorption and response. Wu et al. [41] demonstrated a UV photodetector using a single VO 2 microwire with a one-dimensional (1D) structure (Figure 7c). It is well known that a 1D structure has a relatively large surface-to-volume ratio, which significantly affects the sensing characteristics of devices. In particular, oxygen molecules can be easily adsorbed on the surface of the VO 2 microwire, capturing conduction electrons. This can induce surface depletion of the VO 2 microwire. When illuminated with UV light, electron-hole pairs can be generated, and the holes can migrate to the surface and recombine with the adsorbed oxygen molecules, reducing surface depletion. As a result, the device showed a significant enhancement in detection performance owing to the photogating effect. The reported photoresponsivity of the device was approximately 7069 A·W −1 .
ture. The maximum photoresponsivity of the device was 4.7 A·W .
Employing nanostructures into the photodetectors is promising due to their large surface-to-volume ratio, which allows for enhanced light absorption and response. Wu et al. [41] demonstrated a UV photodetector using a single VO2 microwire with a one-dimensional (1D) structure (Figure 7c). It is well known that a 1D structure has a relatively large surface-to-volume ratio, which significantly affects the sensing characteristics of devices. In particular, oxygen molecules can be easily adsorbed on the surface of the VO2 microwire, capturing conduction electrons. This can induce surface depletion of the VO2 microwire. When illuminated with UV light, electron-hole pairs can be generated, and the holes can migrate to the surface and recombine with the adsorbed oxygen molecules, reducing surface depletion. As a result, the device showed a significant enhancement in detection performance owing to the photogating effect. The reported photoresponsivity of the device was approximately 7069 A·W −1 . UV exposure of VO2 can induce a phase transition as described previously. Li et al. [36,37] reported that UV illumination at an intensity of 64 mW·cm 2 caused the nonvolatile and gradual conductance change in a VO2 by the phase transition, while the illumination of visible light induced volatile conductance change in a VO2 due to rapid recombination UV exposure of VO 2 can induce a phase transition as described previously. Li et al. [36,37] reported that UV illumination at an intensity of 64 mW·cm 2 caused the nonvolatile and gradual conductance change in a VO 2 by the phase transition, while the illumination of visible light induced volatile conductance change in a VO 2 due to rapid recombination after turning off the light, as shown in Figure 6d. Based on these unique optoelectronic properties, the authors demonstrated the application of artificial synaptic devices.

Visible Photodetection
Regarding VO 2 -based photodetectors, interfacing with two-dimensional (2D) materials enables photodetection in the visible range because of the E g of the 2D materials. Oliva et al. [42] demonstrated a photodetector based on a MoS 2 /VO 2 heterojunction (Figure 8a). The heterojunction exhibited rectification behavior due to the energy band alignment, leading to a lower leakage current in the reverse bias region. In addition, it showed a relatively higher photoresponsivity in the visible range than multilayer MoS 2 devices reported in other studies. The maximum photoresponsivity of the device was approximately 1.25 A·W −1 . Luo et al. [43] introduced a WSe 2 /VO 2 heterojunction to form a p-n junction, as shown in Figure 8b. The fabricated photodetector showed dual-mode operation depending on the VO 2 phase transition temperature. At room temperature, VO 2 had an insulating phase, and the photodetector exhibited photovoltaic properties due to the built-in potential formed by the p-n junction. However, when VO 2 had a metallic phase at 90 • C, the photodetector was operated via photoconductive effect, forming a Schottky contact between WSe 2 and VO 2 . The WSe 2 /VO 2 photodetector indicated a relatively high photoresponsivity of 2.4 A·W −1 at room temperature and 6.6 A·W −1 at 90 • C.
after turning off the light, as shown in Figure 6d. Based on these unique optoelectronic properties, the authors demonstrated the application of artificial synaptic devices.

Visible Photodetection
Regarding VO2-based photodetectors, interfacing with two-dimensional (2D) materials enables photodetection in the visible range because of the Eg of the 2D materials. Oliva et al. [42] demonstrated a photodetector based on a MoS2/VO2 heterojunction (Figure 8a). The heterojunction exhibited rectification behavior due to the energy band alignment, leading to a lower leakage current in the reverse bias region. In addition, it showed a relatively higher photoresponsivity in the visible range than multilayer MoS2 devices reported in other studies. The maximum photoresponsivity of the device was approximately 1.25 A·W -1 . Luo et al. [43] introduced a WSe2/VO2 heterojunction to form a p-n junction, as shown in Figure 8b. The fabricated photodetector showed dual-mode operation depending on the VO2 phase transition temperature. At room temperature, VO2 had an insulating phase, and the photodetector exhibited photovoltaic properties due to the built-in potential formed by the p-n junction. However, when VO2 had a metallic phase at 90 °C, the photodetector was operated via photoconductive effect, forming a Schottky contact between WSe2 and VO2. The WSe2/VO2 photodetector indicated a relatively high photoresponsivity of 2.4 A·W −1 at room temperature and 6.6 A·W −1 at 90 °C.

Near-IR (NIR) Photodetection
Recently, a VO2-based NIR photodetector using the localized surface plasmon resonance (LSPR) effect was demonstrated to enhance the photodetector performance, as shown in Figure 9a [44]. The device showed significant enhancement in Iph under the illumination of light (λ = 808 nm, P = 8.59 W·cm 2 ) due to the MIT property and LSPR effect leading to the hot electron injection. The fabricated NIR photodetector showed the maximum photoresponsivity of 502.1 mA·W −1 .
Xie et al. [45] reported the highly ordered W-doped VO2 nanowire arrays for NIR detection (Figure 9b). One-dimensional nanowire arrays increased the effective area for photon absorption. In addition, it was reported that the Eg of W-doped VO2 was smaller than that of bare VO2 [46], and doped W could prevent the recombination of electron-hole

Near-IR (NIR) Photodetection
Recently, a VO 2 -based NIR photodetector using the localized surface plasmon resonance (LSPR) effect was demonstrated to enhance the photodetector performance, as shown in Figure 9a [44]. The device showed significant enhancement in I ph under the illumination of light (λ = 808 nm, P = 8.59 W·cm 2 ) due to the MIT property and LSPR effect leading to the hot electron injection. The fabricated NIR photodetector showed the maximum photoresponsivity of 502.1 mA·W −1 .
Xie et al. [45] reported the highly ordered W-doped VO 2 nanowire arrays for NIR detection (Figure 9b). One-dimensional nanowire arrays increased the effective area for photon absorption. In addition, it was reported that the E g of W-doped VO 2 was smaller than that of bare VO 2 [46], and doped W could prevent the recombination of electron-hole pairs, thereby extending the exciton lifetime [47]. As a result, the W-doped VO 2 nanowire arrays showed a much higher photocurrent and photoresponsivity than the bare VO 2 nanowire arrays. The photoresponsivity of the W-doped VO 2 nanowire array and bare VO 2 nanowire array were 21.4 mA·W −1 and 0.29 mA·W −1 , respectively.
A VO 2 /n-Si heterojuction for NIR photodetection, which showed low dark current and linear photoresponse characteristics, was also demonstrated as shown in Figure 9c [46]. In particular, enhanced photoresponsivity and a faster photoresponse were observed at relatively high electric fields and optical power densities. The enhanced optoelectronic performance could be resulted from the MIT of VO 2 by the applied electric field and NIR illumination (λ = 940 nm), leading to the efficient collection of the photoexcited electronhole pairs in n-Si, as shown in the energy band diagram in Figure 9c. The maximum photoresponsivity was 1.01 mA·W −1 .
OR PEER REVIEW 12 of 22 pairs, thereby extending the exciton lifetime [47]. As a result, the W-doped VO2 nanowire arrays showed a much higher photocurrent and photoresponsivity than the bare VO2 nanowire arrays. The photoresponsivity of the W-doped VO2 nanowire array and bare VO2 nanowire array were 21.4 mA·W 1 and 0.29 mA·W −1 , respectively. A VO2/n-Si heterojuction for NIR photodetection, which showed low dark current and linear photoresponse characteristics, was also demonstrated as shown in Figure 9c [46]. In particular, enhanced photoresponsivity and a faster photoresponse were observed at relatively high electric fields and optical power densities. The enhanced optoelectronic performance could be resulted from the MIT of VO2 by the applied electric field and NIR illumination (λ = 940 nm), leading to the efficient collection of the photoexcited electronhole pairs in n-Si, as shown in the energy band diagram in Figure 9c. The maximum photoresponsivity was 1.01 mA·W −1 . Guo et al. [49] demonstrated an NIR photodetector based on a VO2 film synthesized via CVD in which a VO2 film was used as the photoconductor (Figure 9d). When the NIR light (λ = 850 nm) was illuminated on the VO2 channel, a photocurrent was generated by the photoexcited electron-hole pairs and an applied electric field (photoconductive effect). The photoresponsivity of the device was approximately 16 mA·W −1 . Guo et al. [49] demonstrated an NIR photodetector based on a VO 2 film synthesized via CVD in which a VO 2 film was used as the photoconductor (Figure 9d). When the NIR light (λ = 850 nm) was illuminated on the VO 2 channel, a photocurrent was generated by the photoexcited electron-hole pairs and an applied electric field (photoconductive effect). The photoresponsivity of the device was approximately 16 mA·W −1 .

IR Photodetection
Infrared (IR) with wavelength longer than NIR is classified into SWIR (1-3 µm), MWIR (3-5 µm), and LWIR (8-12 µm). Because VO 2 has a small E g (∼0.7 eV) and a high TCR, photoconductive and photobolometric effects are the dominant photodetection mechanisms. Rajeswaran et al. [50] demonstrated a VO 2 -based SWIR photodetector (Figure 10a) and observed its electrical properties under illumination (λ = 1550 nm). Because the absorbed photon energy (∼0.8 eV) is higher than the E g of VO 2 , the photoexcited electron-hole pairs can be generated and contribute to the photocurrent of the device. At high optical power density and applied bias region, the device showed a high photoresponse and then the maximum photoresponsivity was 7.13 × 10 −2 mA·W −1 .

IR Photodetection
Infrared (IR) with wavelength longer than NIR is classified into SWIR (1-3 µm), MWIR (3-5 µm), and LWIR (8-12 µm). Because VO2 has a small Eg (−0.7 eV) and a high TCR, photoconductive and photobolometric effects are the dominant photodetection mechanisms. Rajeswaran et al. [50] demonstrated a VO2-based SWIR photodetector (Figure 10a) and observed its electrical properties under illumination (λ = 1550 nm). Because the absorbed photon energy (−0.8 eV) is higher than the Eg of VO2, the photoexcited electron-hole pairs can be generated and contribute to the photocurrent of the device. At high optical power density and applied bias region, the device showed a high photoresponse and then the maximum photoresponsivity was 7.13 × 10 −2 mA·W −1 . Fu et al. [51] fabricated the photodetector based on vertically stacked 1D VO2 nanowire/carbon nanotube (CNT) composite film (Figure 10b). The composite-film-based photodetector exhibited both enhanced photoresponsivity and a faster photoresponse than the VO2-nanowire-based photodetector. The CNT film played a role as a medium for heat absorption and transfer between the CNT and VO2 films, leading to improved IR response characteristics. The photoresponsivity of the device was 17.83 mA·W −1 . Ma et al. [52] used a VO2/silicon nitride (SN) composite film for IR photodetection. A flexible and freestanding thin film photodetector was fabricated using SN nanotubes. The device showed strong IR absorption and low heat capacity, leading to enhanced IR photodetection.

Broadband Photodetection
Recently, VO2-based broadband photodetectors have been intensively studied due to their potential applications. Kabir et al. [53] demonstrated a broadband photodetector based on a VO2 thin film synthesized by DC sputtering and annealing in ambient air (Figure 11a). According to the literature, the broadband photodetection of a device is attributed to the photo-excitation, electrical excitation, and thermal excitation, simultaneously. In particular, the enhanced photoresponse after the phase transition from the insulating to the metallic phases of VO2 is attributed to free carriers. The highest photoresponsivity of the device was approximately 2 A·W −1 at the metallic phase of VO2 and in the visible range. Umar et al. reported similar device configurations but used different metal Fu et al. [51] fabricated the photodetector based on vertically stacked 1D VO 2 nanowire/ carbon nanotube (CNT) composite film (Figure 10b). The composite-film-based photodetector exhibited both enhanced photoresponsivity and a faster photoresponse than the VO 2 -nanowire-based photodetector. The CNT film played a role as a medium for heat absorption and transfer between the CNT and VO 2 films, leading to improved IR response characteristics. The photoresponsivity of the device was 17.83 mA·W −1 . Ma et al. [52] used a VO 2 /silicon nitride (SN) composite film for IR photodetection. A flexible and freestanding thin film photodetector was fabricated using SN nanotubes. The device showed strong IR absorption and low heat capacity, leading to enhanced IR photodetection.

Broadband Photodetection
Recently, VO 2 -based broadband photodetectors have been intensively studied due to their potential applications. Kabir et al. [53] demonstrated a broadband photodetector based on a VO 2 thin film synthesized by DC sputtering and annealing in ambient air (Figure 11a). According to the literature, the broadband photodetection of a device is attributed to the photo-excitation, electrical excitation, and thermal excitation, simultaneously. In particular, the enhanced photoresponse after the phase transition from the insulating to the metallic phases of VO 2 is attributed to free carriers. The highest photoresponsivity of the device was approximately 2 A·W −1 at the metallic phase of VO 2 and in the visible range. Umar et al. reported similar device configurations but used different metal contacts (Ag). The authors suggested that the photocurrent was generated by a photon energy higher than the E g of VO 2 [54]. Hong et al. [55] introduced a glancing angle deposition method to form vertically aligned VO 2 nanorods, as shown in Figure 11b. The porous VO 2 nanorods provided a wide specific area for light absorption. In addition, Ag-nanoparticle-decorated VO 2 was employed to induce an LSPR effect. The electric field at the interface between the Ag nanoparticles and VO 2 nanorods enabled the broadband photodetector (visible to NIR) and enhanced the photoresponse of the device. Interestingly, the device showed high photoresponsivity from visible to NIR ranges and the maximum photoresponsivity was approximately 10 3 A·W −1 in the NIR range.
Hassan et al. [56] demonstrated a photodetector employing a vertical VO 2 /p-Si heterojunction (Figure 11c). The formed p-n junction enabled the self-powered operation of the photodetectors. The excited photocarriers could be separated using the built-in electric field of the device. The maximum photoresponsivity of the photodetector was approximately 0.02 mA·W −1 . Jiang et al. [57] introduced a VO 2 /MoTe 2 heterojunction into a photodetector to form a p-n junction (Figure 11d). The fabricated photodetector exhibited broadband photodetection properties in the visible-to-SWIR range. In addition, due to the high TCR of the VO 2 film, the MWIR and LWIR were detectable. The photoresponsivity observed in the NIR range (λ = 830 nm) was approximately 0.22 A·W −1 .
, 23, x FOR PEER REVIEW 14 of 22 enhanced the photoresponse of the device. Interestingly, the device showed high photoresponsivity from visible to NIR ranges and the maximum photoresponsivity was approximately 10 3 A·W-1 in the NIR range.
Hassan et al. [56] demonstrated a photodetector employing a vertical VO2/p-Si heterojunction (Figure 11c). The formed p-n junction enabled the self-powered operation of the photodetectors. The excited photocarriers could be separated using the built-in electric field of the device. The maximum photoresponsivity of the photodetector was approximately 0.02 mA·W −1 . Jiang et al. [57] introduced a VO2/MoTe2 heterojunction into a photodetector to form a p-n junction (Figure 11d). The fabricated photodetector exhibited broadband photodetection properties in the visible-to-SWIR range. In addition, due to the high TCR of the VO2 film, the MWIR and LWIR were detectable. The photoresponsivity observed in the NIR range (λ = 830 nm) was approximately 0.22 A·W −1 . Figure 11. VO2-based broadband photodetectors. (a) Broadband photodetector that is detectable from UV to NIR. Such a photodetection property contributes to the photo-excitation, electrical excitation, and thermal excitation, simultaneously. Reproduced with permission from [53], Copyright 2020, Elsevier. (b) Vertically aligned and Ag-decorated VO2 nanorod-based broadband photodetector allowing detection from visible to NIR using LSPR effect by Ag nanoparticles. Reproduced with permission from [55], Copyright 2019, American Chemical Society. (c) VO2/p-Si heterojunctionbased broadband photodetector (visible to NIR) using photovoltaic effect. The p-n junction forms the built-in potential leading to the efficient separation of photoexcited carriers. Reproduced with permission from [56], Copyright 2022, Elsevier. (d) VO2/MoTe2 heterojunction-based broadband photodetectors with dual-mode operation. The device operates by photovoltaic effect in visible-to-SWIR range, and by photobolometric effect in MWIR-to-LWIR range. Reproduced with permission from [57], Copyright 2020, Springer Nature. Table 2 shows a summary of the up-to-date VO2-based photodetectors. In this table, we summarize the materials, detectable wavelengths, deposition methods for VO2, photoresponsivity (R), and specific detectivity (D *) of the photodetectors. Figure 11. VO 2 -based broadband photodetectors. (a) Broadband photodetector that is detectable from UV to NIR. Such a photodetection property contributes to the photo-excitation, electrical excitation, and thermal excitation, simultaneously. Reproduced with permission from [53], Copyright 2020, Elsevier. (b) Vertically aligned and Ag-decorated VO 2 nanorod-based broadband photodetector allowing detection from visible to NIR using LSPR effect by Ag nanoparticles. Reproduced with permission from [55], Copyright 2019, American Chemical Society. (c) VO 2 /p-Si heterojunction-based broadband photodetector (visible to NIR) using photovoltaic effect. The p-n junction forms the builtin potential leading to the efficient separation of photoexcited carriers. Reproduced with permission from [56], Copyright 2022, Elsevier. (d) VO 2 /MoTe 2 heterojunction-based broadband photodetectors with dual-mode operation. The device operates by photovoltaic effect in visible-to-SWIR range, and by photobolometric effect in MWIR-to-LWIR range. Reproduced with permission from [57], Copyright 2020, Springer Nature. Table 2 shows a summary of the up-to-date VO 2 -based photodetectors. In this table, we summarize the materials, detectable wavelengths, deposition methods for VO 2 , photoresponsivity (R), and specific detectivity (D *) of the photodetectors.

Optical Switching and Color Modulator Applications
VO 2 has been extensively investigated as a tunable material for optical modulation systems due to the drastic change in the IR transmittance and the refractive index across the MIT [63][64][65]. Long and co-workers demonstrated static visible light tunability and dynamic NIR modulation of two-dimensional SiO 2 -VO 2 core-shell photonic crystal films, as shown in Figure 12a [63]. The SiO 2 -VO 2 core-shell photonic crystal-based thermochromic smart window can show the tunable functionality via selectively reflecting and blocking light (indicated by red, blue, orange, and green arrows) and simultaneously maintaining (attenuating) IR transmission at low temperature (at high temperature) (Figure 12(ai)). The finite difference time domain simulations for transmission spectra of the photonic crystal structure predict that the transmittance can be tuned across the visible spectrum, while maintaining good solar regulation efficiency (∆T sol = 11.0%) and high solar transmittance (T lum = 49.6) (Figure 12(aii,iii)).
Liang et al. [64] presented dual-band modulation of visible and NIR transmittance through voltage and temperature in a hybrid micro-nano composite film, which contains the microsized liquid crystals domains with a negative dielectric constant and tungsten-doped vanadium dioxide (W-VO 2 ) nanocrystals (Figure 12b). The light mod-ulation performance of the films with 2.5 and 5.0 wt.% W-VO 2 nanocrystals showed transparency in the visible region and a drastic change in NIR transmittance at different temperatures (Figure 12(bi)). This result indicates that NIR light transmittance of the hybrid composite film can be passively modulated according to the temperature variations. In addition, the visible light transmittance of the hybrid composite film can be independently and dynamically regulated by the external voltages (Figure 12(bii)). Specifically, Vis/NIR spectra of the films with 2.5 wt.% of W-VO 2 /PVP nanocrystals showed that the visible light transmittance of the film gradually decreased due to a spatial variation of IR between the micro-liquid crystal domains and the polymer during the increase in the applied voltages from 0 to 35 V, resulting from the parallel alignment to the direction of the electric field of liquid crystals (Figure 12(bii)).
Wan et al. [65] demonstrated a VO 2 -based limiting optical diode as a nonlinear device that features asymmetric transmission of light, which was bidirectionally transparent at low power but opaque during illumination of a sufficiently intense light incident from a particular direction. The proof-of-concept of a VO 2 -based limiting optical diode comprising a transparent sapphire substrate, a thin VO 2 layer, and a semitransparent gold film shows the asymmetric absorption of a VO 2 thin film to selectively trigger the MIT, enabling asymmetric transmission (Figure 12(ci)). For the case of forward incidence, a significant amount of power is reflected before it reaches the VO 2 , whereas there is substantially more absorption of the light by the VO 2 in the case of backward incidence. This could be supported by the temperature-dependent infrared refractive indices of the VO 2 film. Due to a large change in n and κ for a relatively small change in temperature, a VO 2 -based limiting optical diode can be designed to operate over a broad wavelength range (1-3.5 µm) (Figure 12(cii)) and, by using VO 2 films with narrower transitions, maximal asymmetry can be reached in the simple thin-film geometry (Figure 12(ciii)).
In addition to light transmission modulation of VO 2 , the device applications using dynamic color modulation based on the phase transition of VO 2 , in combination with nanostructured metals, were recently reported [66][67][68]. Shu et al. [66] demonstrated color generation for display and imaging applications through the integration of plasmonic nanostructures with periodic silver-nanodisk arrays on VO 2 film ( Figure 13a). As illustrated in Figure 13a, the reflection images of samples can readily be tuned by adjusting the geometric parameters (diameter and periodicity) of the nanodisks, and the color of the sample changes with the increase in nanodisk diameter at both 20 and 80 • C. In Figure 13a, the scanning electron microscopy (SEM) images for four different patterns of VO 2 and periodic silver-nanodisk arrays showed distinctively different reflection color images of the patterns at 20 and 80 • C, indicating the realization of abundant color variation due to the MIT of VO 2 and surface plasmon effect of metal nanostructures. Liu and co-workers demonstrated reconfigurable multistate optical systems enabled by phase transitions in VO 2 , which could be modulated by thermal tuning, hydrogen (H)-doping, and electron (e)-doping, as shown in Figure 13b [67]. Specifically, they presented a quadruple-state dynamic plasmonic display based on stacked structures of aluminum (Al)/Al 2 O 3 nanodisks on a VO 2 /Au mirror substrate with Pd dots, in response to a combination of temperature and H-doping (Figure 13b, bottom panels). Meanwhile, In et al. [68] proposed composites of self-organized gold network (SGN) and VO 2 as promising templates for photonic applications, combining advantages of both MIT hysteresis and strong light-matter interactions. They demonstrated thermoactive cyan-magenta-yellow color filters based on SGN-VO 2 hybrid films, which were fabricated on 2 in. sapphire wafers with various VO 2 thickness, as shown in Figure 13c.

Summary and Outlook
In this review, we first introduced several solution-based and gas-phase-based synthesis methods of nanostructured VO2 and modulation approaches of its properties, such as stoichiometry, strain (or stress), and doping. Among the potential applications of VO2 (c) Self-organized gold network (SGN)-VO 2 hybrid films for color filter application. Photographs of bare VO 2 (80 nm) film (1) and SGN-VO 2 hybrid films (2-4) for various Au thicknesses (VO 2 thickness fixed at 80 nm). Photographs of SGN-VO 2 hybrid films (5-8) for various VO 2 thicknesses (Au thickness fixed at 50 nm). Reproduced with permission from [68], Copyright 2020, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Summary and Outlook
In this review, we first introduced several solution-based and gas-phase-based synthesis methods of nanostructured VO 2 and modulation approaches of its properties, such as stoichiometry, strain (or stress), and doping. Among the potential applications of VO 2 nanostructures and optical sensing devices, including photodetectors, optical switches, and color modulators, were discussed. Specifically, we reviewed and summarized photodetection mechanisms and VO 2 -based photodetectors for UV, visible, NIR, and IR lights, including optical transmission and dynamic color modulations. Many researchers have devoted to modulate the MIT properties and have demonstrated the design of various device applications.
In addition to the applications mentioned in this review, very recently, VO 2 has been considered as one of the promising materials for energy-efficient neuromorphic computing applications, owing to the rise of artificial intelligence related to the fourth industrial revolution [23,[70][71][72][73][74]. Moreover, VO 2 has attracted much attention as a promising material for adaptive radiative cooling due to a thermochromic property, offering a potential way to reduce energy consumption in buildings [75][76][77]. However, there are still some challenges, such as scalable and reliable fabrication of nanostructured VO 2 materials and precise control of their phase-transition properties (transition temperature, resistivity ratio, hysteresis, transition pathway, phase coexistence, etc.). Thus, it is still necessary to achieve a comprehensive understanding of the MIT in VO 2 , substantial progress for practical uses, and enhanced regulation of the physical and chemical processes associated with future research fields, including the applications discussed above. Finally, further advancements in VO 2 will pave the way for new possibilities and opportunities, enabling the expansion of VO 2 -based devices into a wider range of innovative functional applications.