High quality VO2 thin films synthesized from V2O5 powder for sensitive near-infrared detection

Vapor transport method has been successfully used to synthesize high quality VO2 thin films on SiO2/Si substrate using V2O5 as a precursor in an inert-gas environment. The morphological and structural evolutions of the intermediate phases during the nucleation and growth processes were investigated by SEM and Raman spectroscopy, respectively. The results showed that the conversion of V2O5 powder to VO2 thin films was dominated by a melting-evaporation-nucleation-growth mechanism. Further characterization results demonstrated that the high quality crystals of monoclinic VO2 thin films exhibit a sharp resistance change up to 4 orders of magnitude. In addition, the VO2 thin films exhibited good near-infrared response, high stability, and reproducibility under ambient conditions, which should be promising for sensitive near-infrared detection. Our work not only provided a simple and direct approach to synthesize high quality VO2 thin films with distinct phase transition properties but also demonstrated the possible infrared sensing application in the future.

. However, little attention has been paid to the nucleation and growth processes of VO 2 thin films. In addition, as a narrow-band gap semiconductor (~ 0.65 eV), VO 2 should be a suitable material for infrared detectors. In order to realize its practical application, infrared photodetection of VO 2 thin films need to explore in detail.
In this paper, we present a simple and direct method to synthesize high quality VO 2 thin films by using V 2 O 5 powder as a precursor via vapor transport in an inert-gas environment. The conversion processes of V 2 O 5 powder to VO 2 thin films are identified as the melting and evaporation of V 2 O 5 precursor (forming droplets on the SiO 2 surface), the nucleation of VO 2 crystals in V 2 O 5 droplets, the growth of VO 2 thin films. The high and pure crystal quality of the as-synthesized VO 2 thin films is demonstrated by Raman spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) exhibiting an obvious resistance change up to 4 orders of magnitude and a very small hysteresis across the MIT. High quality VO 2 thin film has many potential applications, as exemplified by its eminent suitability for near-infrared detector with fast response speed and high stability in atmospheric condition at room temperature. Figure 1a shows a schematic setup of vapor transport system used in the VO 2 thin films synthesis. The substrate was upside down over the quartz boat, as shown in Fig. 1b. Figure 1c shows a typical SEM image of a complete VO 2 thin film, which reveals that the as-synthesized film consists of smooth with irregular shaped grains (on a large scale) called microplates, the connectivity between these irregularly-shaped microplates is quite good. The inset of Fig. 1c shows the cross-sectional morphology of the thin film with a thickness of about 1.2 μm. As shown in Fig. 1d, the mean lateral size of microplates is about 47.7 μm as clearly seen from the size distribution of microplates in a statistical histogram. Histogram has been plotted by measuring the largest dimension of about two hundred microplates in a given image. The dimensions of VO 2 thin films proved to be sensitively dependent on the synthetic parameters (e.g. precursor flux, not shown here). Additionally, it should be mentioned that because of multivalent of V atom, its oxides can exist in a wide range of stable and metastable stoichiometries that are mutually transformable at specific synthetic conditions as previously reported 45 . Accordingly, the reductive growth of the VO 2 micro-and nanostructures from the vanadia precursor is considered to be a multistep process, which is triggered by the interplay between the kinetics of vanadia reduction and the thermodynamic stability of different phases 45 .

Results and discussion
Considering this, first we perform ex-situ investigations on the surface morphological evolution of the VO 2 thin film during its nucleation and growth process using SEM. As noted above, the samples at different growth stages are achieved at different temperatures during the thermal ramping in argon flow. The corresponding surface morphologies during the conversion process are shown in Fig. 2a-e. It is expected that initially the fine powder of V 2 O 5 starts to melt down near its melting temperature (~ 690 °C), and subsequently evaporates from the quartz boat when the furnace is ramped up to 750 °C, forming small droplets (cooled to be microparticles) on the SiO 2 surface (stage I), as shown in Fig. 2a. Upon further precursor deposition and temperature rise (800 °C, www.nature.com/scientificreports/ stage II), the small microparticles aggregate melt consistently into large droplets (circled in dotted line), which may occur because of the surface diffusion at higher temperatures results in greater aggregation by Ostwald ripening 45 . Particularly, it is important to note that some parts of the droplets already contain crystals of the guest phase (microbeams, circled in red dotted line), which further undergo a coalescence process that enlarges the width and the length of the microbeams (shown by the yellow arrow). There are obvious traces of grooves left aside around the microbeams due to the dissolution of SiO 2 during the growth process. When heated up to 850 °C (stage III), one can observe that the microbeams are converted to quasi-2D microplates morphology and the density of these droplets reduces in the vicinity of the microplates, as shown in Fig. 2c. This can be attributed to the fact that high growth temperatures favor the fusing of microbeams along with feeding from the neighboring droplets and enhance their lateral growth. At even higher temperature (900 °C), the growth of single-crystal quasi-2D plates continues until they meet each other, namely, these plates stretch and merge to form a porous film (stage IV, Fig. 2d). After 5 h annealing at 900 °C, the porous film eventually crystallizes and transforms into a complete polycrystalline film due to mass transport by surface diffusion (stage V, Fig. 2e). It is demonstrated that Raman technique is a powerful method to identify phase structure due to its high sensibility of crystalline lattice variation of vanadium oxides 46 . To complement the morphological studies, the phase structural evolution of the corresponding sample is identified by recording room temperature micro-Raman spectra. Group theory predicts that there are 21 allowed Raman-active modes (14A g + 7B g ) for the α-V 2 O 5 structure and 18 Raman-active vibrations (9A g + 9B g ) for the VO 2 (M1) structure, but there is a difference in numbers of experimentally observed Raman-active modes of vanadium oxides due to the different synthetic strategies and polymorphs 46 . In our work, experimentally observed positions of the Raman-active modes are summarized for all stages of growth in Table 1 (extracted from Fig. 3a). Despite this, the Raman spectrum is typical of α-V 2 O 5 , with its main features at 147, 285, 305, 405, 702, and 995 cm −1 , whereas the monoclinic VO 2 phase has a set of strong well-resolved bands, located at 194, 224, 262, 310, 387, and 612 cm −146 . Among these, the mode at 147 cm −1 (ω o (2) ) is associated with the in-phase oscillation of vanadium (shear-like distortions), the band at 702 cm −1 (ω v-o(2)-v) ) is assigned to the asymmetric stretching of V-O (2) -V bridges, and the highest intensity feature at 995 cm −1 (ω v=o(1) ) corresponds to the stretching vibrations of V=O (1) vanadyl bonds in V 2 O 5 28,36,44,46 . Besides, the low-energy peaks at 194 and 224 cm −1 (ω v1 and ω v2 ) correspond to the motion of vanadium atoms along the c-axis of the VO 2 crystal (stretching motion of V-V dimers), while the high-energy one at 612 cm −1 (ω o ) is associated with the V-O vibrations 32,36,46 . We subsequently compare these data with our own experimental Raman spectra to examine the samples' phase structure of each stage, as shown in Fig. 3a. The Raman response of small droplets in stage I demonstrates the typical features of V 2 O 5 phase, with its main bands at 145, 285, 404,   Fig. 3b. In stages IV and V, the film structures are determined to be mostly VO 2 by their main Raman bands found at 195, 223, 309, and 617 cm −1 . Additionally, it is important to notice that the relative intensities of the lines at 145, 702, and 995 cm −1 are substantially decreased or even disappeared from the initial stage to the final stage, whereas the relative intensities of the lines at 195, 223, and 617 cm −1 are noticeably increased. This is a result of the significant increase in relative concentration of VO 2 crystals in the resultant film.
Combining the above results on morphological and structural changes, the conversion processes of VO 2 thin films from V 2 O 5 powder can be summarized as follows. Initially V 2 O 5 powder starts to melt down in the vicinity of its melting temperature, and subsequently evaporates from the quartz boat when the temperature rises, forming small V 2 O 5 droplets on the oxide substrate. Later, VO 2 nucleates inside the V 2 O 5 droplets and the nuclei grow into microbeams, and theses beams further stretch to form microplates at higher temperature by feeding from the neighboring V 2 O 5 droplets. As the temperature further rises, the irregular quasi-2D microplates join together to form a porous VO 2 thin film, and finally crystallize into a complete VO 2 thin film after a long duration. Also, the thermal decomposition of V 2 O 5 powder of the as above the discussed phase transformations can be described as Table 1. Summary of the experimentally observed positions of the Raman-active modes of the growth products at different growth stages.  www.nature.com/scientificreports/ In addition to Raman analysis, chemical composition data are desired to ensure that the final phase consisted mostly of VO 2 . Figure 4a shows high resolution XPS spectrum of V 2p and O 1 s collected from the final film structures. The V 2p spectrum shows the main peaks of 2p 3/2 centered at 516.1 eV and 2p 1/2 centered at 523.7 eV, which corresponds to the V 4+ oxidation state 11,17,47 . The small shoulder peak at 517.3 eV is attributable to V 5+ ions due to surface oxidation of the VO 2 thin films or the presence of minor component of V 2 O 5 11,17,47 The O 1 s spectrum can be deconvoluted into two peaks at 529.9 and 531.9 eV, which correspond to VO 2 and CO 2 , H 2 O, respectively. In our case, the XPS results are consistent with the Raman results. The analysis of the crystal structure is further studied by XRD, as shown in Fig. 4b. XRD patterns of the film confirms the presence of highly crystalline VO 2 structure, exhibiting two indexed peaks (JCPDS 82-0661), (011) and (022) respectively, located at ~ 27.9° and ~ 57.5° with narrow full width half maximum of ~ 0.15° and ~ 0.3°, consistent with the monoclinic VO 2 structure 17,26 . The strong diffraction peak at ~ 27.9° (011) of monoclinic VO 2 suggests that the film has a preferential orientation (011). It is reported that highly (011) orientated monoclinic VO 2 thin film in the insulator state is considered to result in an excellent phase transition performance 26 . To verify this, we measure the temperature dependent electrical resistance from a VO 2 thin film device. As shown in Fig. 4c, with increasing temperatures, the two-terminal resistance gradually decreases, exhibiting the classical activated semiconductor behavior and switches to metallic behavior for temperature above 339 K. Upon cooling, the film displays a reverse jump to the insulating phase at temperature of 337 K, thus a very narrow hysteresis is exhibited. There is a dramatic change in resistance ( �R = R 300K /R 350K ) by over four orders of magnitude over the MIT, which shows comparable or better phase transition properties than that of VO 2 thin films synthesized by molecular beam epitaxy 34 and magnetron sputtering 8,12,35 . The inset of Fig. 4c shows the temperature-dependent conductance of the device followed the Arrhenius behavior well with a thermal activation energy ( E a ) of 0.4 eV 48 . Figure 4d is the first derivative of the temperature curves ( d log(R) /dT ) in Fig. 4c for a clear view of the phase transition temperature of the thin film.
In case of VO 2 thin films, most studies in the literatures have been mainly focused on their synthesis and related applications in smart coatings 18 , optical switching devices 49 , laser protective materials 50 , thermal stealth materials 51 and sensors 21 . However, little attention has been paid to demonstrate the viability of VO 2 thin films in efficient IR photodetectors. In this work, we investigate the photoelectron characteristics of the as-synthesized VO 2 thin film under the illumination of an IR light (850 nm). Figure 5a shows the schematic diagram and a representative SEM image of our device. Figure 5b shows that the room-temperature IR response of the channel current under chopping light conditions at a bias voltage of 5.0 V, where a high-current state and a low-current state are observed when light is switched on and off, respectively. Under illumination, the photocurrent www.nature.com/scientificreports/ ( I ph = I light − I dark ) increases rapidly, and then decreases to its initial level in the dark. This is understandable that VO 2 structure exhibits semiconductor state at room temperature, which can absorb the IR photons and excite photoelectrons from valence band to conduction band, producing an efficient IR response. It should be reminded that the photoresponse can be influenced by the surface morphology of VO 2 thin films, such as grain boundary density and surface roughness because of their influences on the separation and collection of photogenerated carriers. It can be seen in Fig. 5c that the currents of "on" and "off " states remain almost unchanged for more than 150 cycles, demonstrating that our device exhibits high reversibility and stable characteristics. For the IR photodetectors, the stability and response time are two of the most critical parameters. Response times (rise and fall times) are defined as times required for the current to increase from 10 to 90% or decrease from 90 to 10% of the maximum photocurrent upon on/off cycling 30 . Accordingly, the rise and fall times are respectively measured as 60 and 85 ms from a single on/off cycle (shown in Fig. 5d), showing that the fabricated photodetector was superior to the previously reported IR photodetectors based on VO 2 nanostructures (e.g. nano-rods/ clusters 31,32,47 , and microwire 27 ), which exhibit rise and fall processes on a time scale of seconds. The fast response is attributed to the large single-crystal size and good connectivity of the microplates in VO 2 thin films, demonstrating that our VO 2 thin films are suitable as IR photodetectors. Furthermore, the dependence of the photocurrent as a function of light power at a bias of 5.0 V is shown in Fig. 6a. It can be seen that the photocurrent increases gradually with an increase in the light power, and simultaneously deviates linearly with the light power, which is due to the high trap state density (between the Fermi level and conduction band edge of VO 2 ) 31,32 . According to the formula R = I ph /P 52,53 , where R is responsivity, I ph is the photocurrent and P is the power intensity of incident light, a highest responsivity of our device is calculated to be around 16 mA/W. Although there is still room for improvement in responsivity compared to previously reported semiconductor IR photodetectors 54,55 , the values of photocurrent are comparable or higher than that of other previously reported VO 2 nanostructures-based photodetectors 27,31,47 . Figure 6b shows the current-voltage (I-V) characteristics of the photodetector under dark and light (850 nm IR radiation of different intensities) conditions, revealing that the observed I-V curves exhibit almost linear, thus indicating that the Au electrodes made Ohmic contact with the film and that the junction resistance was relatively smaller than the total resistance of the device. Figure 6c shows the dependence of the photocurrent with varying individual bias voltage under different light powers. The photocurrent significantly increases under illumination, particularly at high bias voltage. The above results imply that the photoresponse characteristics of our device can be effectively tuned by light intensities and bias voltages. Figure 6d shows dynamic photocurrent measured in vacuum and air at light power of 22 μW and bias voltage 5.0 V. The photocurrent is enhanced obviously in vacuum, suggesting that conductance could be increased by decreasing ambient pressure, which is related to a surface oxygen adsorption-desorption mechanism as previously reported in VO 2 nanowires-based devices 27,30-32 . Overall, the www.nature.com/scientificreports/ IR response of the VO 2 thin film device can be understood from the point view of a traditional photon-induced carries transport process (see the inset of Fig. 5a) 47 . Upon 850 nm light illumination, the photons will be absorbed by VO 2 because of its ~ 0.65 eV bandgap, and electrons will be directly excited from valence band into conduction band, leaving the holes in valence band. The electrons and holes will be further separated by applying bias voltage on the terminal of electrode, resulting in the enhanced channel current and causing efficient IR response.

Conclusions
In summary, we reported a simple and direct approach to synthesize high quality VO 2 thin films by the reduction of V 2 O 5 powder in argon gas flow, and the growth mechanism of the resultant films was systematically examined by interrupting the growth at different temperatures. SEM and Raman spectroscopy were employed to distinguish the morphological and structural changes of the thin films at different growth stages, respectively. Stated simply, the conversion processes of V 2 O 5 powder to VO 2 thin films were identified as the melting and evaporation of V 2 O 5 precursor (forming droplets on the SiO 2 surface), the nucleation of VO 2 crystals in V 2 O 5 droplets, the growth of VO 2 thin films. XPS and XRD analyses collectively demonstrated that the film structures have high and pure crystal quality of monoclinic VO 2 phase. Furthermore, a two terminal device structure was fabricated to study the MIT behavior and IR response. The results showed that the VO 2 film exhibited a dramatic change in resistance by 4 orders of magnitude and a very small hysteresis across the MIT. Compare to previously reported IR photodetectors based on VO 2 nanostructures (grown by more complicated techniques), the fabricated photodetector exhibited low cost and high-performing IR response with fast response speed (rise and fall times were 60 and 85 ms), and high stability (more than 150 cycles) in atmospheric condition at room temperature.

Methods
Synthesis of VO 2 thin films. Commercial V 2 O 5 powder (99.99%, Sigma-Aldrich) was used as the vanadium source for the growth of VO 2 films. Prior to VO 2 films growth, the 500 nm SiO 2 /Si substrate was ultrasonically rinsed in acetone, ethanol, and deionized water to remove contaminants. The fine powder was loaded on a quartz boat positioned at the center of temperature regions of the tube furnace, and the substrate was upside down over the quartz boat. The amount of precursor was 0.1 g, flow rate of argon was 5 sccm, the pressure was 10 Pa, and the distance between the SiO 2 surface and the bottom of boat was about 1 cm. The samples at different growth states were obtained when ramped up to 750 °C (stage I), 800 °C (stage II), 850 °C (stage III) and 900 °C (stage IV) at a rate of ~ 15 °C/min, and then cooled to room temperature at a rate of ~ 5 °C/min, respectively. For the growth of a complete VO 2 polycrystalline thin film (stage V), the temperature was kept constant at 900 °C for 5 h. www.nature.com/scientificreports/ Morphology and structure characterization of VO 2 thin films. The morphologies and structures of samples at different growth stages were obtained using a field-emission scanning electron microscope (Nova NanoSEM 450). Raman spectra were conducted using a Renishaw micro-Raman system 2000 spectrometers with a wavelength of 532 nm. To avoid unintentional heating during Raman analysis, the incident laser power is limited to 0.2 mW. XRD spectrum was collected using a Bruker D8 Advance scanning X-ray diffractometer equipped with a monochromatic source of Cu Kα radiation at 1.6 kW (40 kV, 40 mA). X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Scientific Escalab Xisystem using Al Kα radiation, and the XPS data was calibrated to the C1s binding energy of 285.0 eV.
Phase transition and photoresponse measurements of VO 2 thin films. The VO 2 thin film devices were fabricated using reactive ion etching, the metal contacts (15 nm Cr/200 nm Au as electrodes) were deposited by thermal evaporation. The thickness of as-grown VO 2 thin film was ~ 1.2 μm with lateral dimensions 30 × 45 μm, respectively. Phase transition measurements were performed using a cryogenic probe station (Lake Shore TTPX) under a vacuum chamber with pressure about 1 × 10 −4 Pa, the temperature was controlled by a Lake Shore 330 temperature controller with high temperature stability. Near infrared (NIR) response characteristics of the device were measured using a Keithley 2612 analyzer under dark and illuminated conditions. Light with a wavelength of 850 nm was switched on/off using a mechanical chopper (at a working frequency of 2 Hz).
Optical attenuator was introduced to change the input power. The light was focused on the film with a 50 × objective (NA = 0.5), and the spot size of light is ∼1 μm.

Data availability
The data that support the findings of this study are available within the article.