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Substrate Temperature-Dependent Structural, Optical, and Electrical Properties of Thermochromic VO2(M) Nanostructured Films Grown by a One-Step Pulsed Laser Deposition Process on Smooth Quartz Substrates

Substrate Temperature-Dependent Structural, Optical, and Electrical Properties of Thermochromic... Hindawi Advances in Condensed Matter Physics Volume 2021, Article ID 7700676, 8 pages https://doi.org/10.1155/2021/7700676 Research Article Substrate Temperature-Dependent Structural, Optical, and Electrical Properties of Thermochromic VO (M) Nanostructured Films Grown by a One-Step Pulsed Laser Deposition Process on Smooth Quartz Substrates Ali Hendaoui Physics Department, College of Science and General Studies, Alfaisal University, P.O. Box 50927, Riyadh 11533, Saudi Arabia Correspondence should be addressed to Ali Hendaoui; ali.hendaoui@gmail.com Received 11 June 2021; Accepted 27 August 2021; Published 6 September 2021 Academic Editor: Prasenjit Guptasarma Copyright © 2021 Ali Hendaoui. &is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. &ermochromic M-phase vanadium dioxide VO (M) films with different morphologies have been grown directly on smooth fused quartz substrates using low deposition rate pulsed laser deposition without posttreatment. When the substrate temperature ° ° was increased in the range 450 C–750 C, better (011) texturization of VO (M) films was observed along with an enhancement of their crystallinity. Morphology evolved from small-grained and densely packed VO (M) grains at 450 C to less packed micro/ nanowires at 750 C. Mechanisms behind the crystallinity/morphology evolution were discussed and correlated with the effect of 5+ the temperature on the diffusion of the adatoms as well as on the V valence states content in VO (M) films. Resistivity measurements as a function of temperature revealed that the insulator-to-metal transition features of VO (M) films (i.e., transition temperature (T ), resistivity variation (ΔR), hysteresis width (ΔH), and transition sharpness (ΔT)) are strongly IMT dependent on the processing temperature. In terms of optical properties, it was found that the open (i.e., porous) structure of the films achieved at high temperature induced an improvement of their luminous transmittance. Simultaneously, the enhancement of the films crystallinity with the temperature resulted in better IR modulation ability. &e present contribution provides a one- step process to control the morphology of VO (M) films grown on smooth quartz substrates for applications as switches, memory devices, and smart windows. windows applications, especially since the critical temper- 1. Introduction ature can be decreased to room temperature by donor-level &ermochromic M-phase Vanadium dioxide VO (M) un- doping [2]. Full exploitation of the IMT in VO (M) requires 2 2 dergoes an insulator-to-metal transition (IMT) that takes a thorough control of its IMT features, such as T , hys- IMT place around a temperature of T ≈ 340K. Below T , teresis width ΔH, and modulation capability of its electrical IMT IMT VO (M) has a monoclinic phase characterized by a high and/or optical properties depending on the targeted appli- cation. For example, a sensor would require a small hys- resistivity (insulator). Above T , VO (M) displays a te- IMT 2 tragonal phase with metallic characteristics. &e IMT is teresis, sharp transition, and large modulation, while a reversible and takes place at ultrafast timescales and is memristor requires a large hysteresis. It is worth mentioning characterized by a dramatic change in its resistivity as well as that the IMT characteristics in VO (M) films depend on their in its infrared optical properties from being highly trans- crystallinity and grain morphology, in addition to the im- missive to being highly reflective, while the optical properties purity/dopants content [4, 5]. in the visible range remain almost unchanged across T Several reports in the literature describe studies on IMT [1–3]. &is makes VO (M) very promising for ultrafast VO (M) with controlled IMT properties for targeted ap- 2 2 electronic switching devices, memristors, and smart plications [1–8]. Among them, pulsed laser deposition 2 Advances in Condensed Matter Physics (PLD) holds a privileged position for the productions of pure the films is obtained. In addition, we will show that the VO (M) thin films with controlled composition, crystal- improvement of VO (M) crystallinity with the temperature 2 2 linity, and morphology [2–5]. It has also the potential for results in an improvement of infrared (IR) transmittance large-scale production, especially if the thin films’ synthesis modulation ability toward smart windows applications. is performed at low deposition rates [9]. &e crystallinity and morphology of the VO (M) films can be controlled via 2. Materials and Methods several parameters, including the substrate’s temperature during the deposition process. By controlling the substrate PLD was performed using KrF excimer pulsed laser ° −2 temperature at 700 C, in a recent report, Lafane et al. re- (λ � 248 nm, fluence � 1.8 J.cm ) focused on Vanadium ported the synthesis of VO (M) polycrystalline nanoplatelets target (99.9% pure) under 5 mTorr of oxygen ambient. on glass substrate by PLD using Vanadium Pentoxide V O 2 5 Such a low pressure was chosen as it is expected to be target under oxygen ambient [10]. However, Lafane et al. did beneficial for producing elongated structures due to the not report information about the composition of the films. enhanced mobility of the adatoms on the substrate. &e In addition, the functional properties of the grown nano- total number of laser pulses on the target for each de- platelets are not reported therein [10]. In summary, despite position experiment was set at 18000 pulses. Smooth fused the importance of the substrate temperature for controlling quartz, used as the substrate, was kept at 7 cm away from the IMT characteristics of PLD-grown nanostructured the target and the substrate temperature was varied for the VO (M) films, the related studies remain relatively scarce. In different experiments. &e laser was pulsed at a frequency addition, the influence of the substrate temperature on the of 2 Hz. &e choice of this value is based on preliminary vanadium valence content of the PLD-grown nano- tests on the influence of the laser pulsing frequency on the structured VO (M) films on smooth quartz substrates re- morphology of the grown films toward the synthesis of mains, to the best of our knowledge, unexplored. VO (M) micro/nanowires. In fact, as shown in Figure S1 Another important topic of interest related to the present on the supplemental file, scanning electron microscope study is related to the synthesis of VO (M) micro/nanowires (SEM) images revealed that a pulsing frequency of 2 Hz is (MNWs). In fact, many approaches have been developed to suitable for achieving elongated, rods-like structures for achieve VO (M) MNWs. In most of the cases, the proposed VO (M) grains for films grown at the same substrate methods resulted in relatively low yield (i.e., surface density) temperature. for the micro/nanowires [11, 12]. As a remedial solution, X-ray photoelectron spectroscopy (XPS) measurements roughening the substrate surface, patterning it, or using were made using a VG Escalab 220I-XL system with Al Kα epitaxial growth were proposed [11–14]. However, these (h] � 1486.6 eV) radiation. Etching with Argon was per- approaches are either not suitable for optical applications formed for 900-second prior measurements to surface (roughness and patterning), or not applicable for large-scale contamination and/or overoxidation. More details about the production (epitaxy). For example, optical applications such deconvolution analysis of the binding energy of the V2p 3/2 as smart windows require large transmittance of the samples core level peak to determine the vanadium valence state in the visible range of the spectrum. In this sense, smooth content of the samples are given in the supplemental file (cf. surface substrates are needed because the presence of Figure S2 in the supplemental file). roughness or patterns on the substrates surface would &e crystalline structure of the samples was analyzed by negatively impact the optical transmittance. As for the ep- X-ray diffraction (PANalytical’s X’Pert, Cu Kα radiation). itaxy, it could be a limiting factor for large scale, that is, &eir morphology was studied using scanning electron commercial production of thin films, since it requires the use microscopy (JEOL JSM-6300F). &e resistivity of the films of costly single crystalline substrates with atomic-level ° ° was measured in the range 25 C–100 C using four-point smoothness and specific lattice characteristics, such as single probe. Optical transmittance was analyzed in the range of crystalline titanium dioxide or sapphire substrates for 250–2500 nm using a spectrophotometer (Agilent, Cary growing VO (M), in order to ensure lattice matching be- 5000) at normal incidence. tween the substrate and the films. &e integral luminous transmittance T (390–830 nm) lum In this paper, we will investigate the influence of the and IR transmittance T (830–2500 nm) were calculated IR temperature on the composition, structure, and electrical using the following equation: and optical properties of VO (M) films directly grown on smooth fused quartz substrates by a simple PLD approach at 􏽒 φ (λ)T(λ) dλ lum/IR a low deposition rate without posttreatment. Smooth quartz T � , (1) lum/IR 􏽒 φ (λ) dλ lum/IR substrates were chosen as they are convenient for resistivity measurements and suitable for optical applications. We will demonstrate that a control of the substrate where φ (λ) is the IR irradiance spectrum for air mass 1.5 IR temperature of the PLD-grown VO (M) films allows the for a 37 tilted surface [15] and φ (λ) is the CIE (2008) 2 lum control of their IMT features as revealed by resistivity physiologically relevant luminous efficiency function for measurement. On the other hand, we will demonstrate that, photopic vision [16]. as the morphology changes from densely packed small &e modulation ΔT is defined as ΔT � T IR IR IR,RT grains to less packed micro/nanowires with increasing the − T , where T and T are, respectively, the ° ° IR,90 C IR,RT IR,90 C temperature, an enhancement of luminous transmittance of integral IR transmittance at room temperature and at 90 C. Advances in Condensed Matter Physics 3 3. Results and Discussion 3.1. Composition Analysis. Figure 1 shows the evolution of 5+ 4+ 2+ the V , V , and V valence states content with the substrate temperature extracted from XPS measurements. 4+ As can be seen in Figure 1, V is the dominant valence in the 4+ sample, which corresponds to the state related to VO . V 5+ content decreases in favor of an increase in the V content with increasing the temperature. &erefore, higher oxidation of the films is obtained with increasing the temperature. On 2+ the other hand, the content in V remains relatively con- stant as a function of the temperature as it originates from the creation of oxygen vacancies during the Argon etching process rather than the films PLD synthesis process itself. 450 500 550 600 650 700 750 3.2. Microstructure and Morphology Analysis. Figure 2 shows Temperature (°C) the XRD patterns of the VO (M) films. All the peaks could be identified using Joint Committee on Powder Diffraction V4+ Standards (JCPDS) Card No. 44-0252 and were attributed to V2+ V5+ VO (M) monoclinic phase. (011) preferred orientation of the films was identified for the peak present at∼28 indicating Figure 1: Vanadium valence in the PLD-grown VO (M) films. texturization of VO (M) along the (011) plane as it is the energetically favored one [17, 18]. &e preferential crystal growth along the (011) plane is enhanced as the substrate 0.42 ° ° temperature increases from 450 C to 750 C as shown by the 0.40 increase in the (011) peak intensity. &e inset in Figure 1 shows that the full width at half maximum (FWHM) of peak 0.38 (011) decreases with increasing the substrate temperature, indicating an improvement of the crystallinity for the VO 0.36 (M). 500 600 700 Figure 3 presents the top-view SEM images of VO (M) Temperature (°C) films obtained at different substrate temperatures. At 450 C, 750°C the VO (M) film shows a small-grained, densely packed structure due to the relatively low diffusion of adatoms alongside the high nucleation rate that characterizes the PLD process. At 550 C, the structure displays the coexistence of 650°C grains and platelets. &e sample synthesized at 650 C shows the formation of micro/nanorods with well-defined facets and a low aspect ratio. &e evolution of the microstructure and morphology of 550°C VO (M) films with varying the processing temperature from (011) ° ° 450 C to 650 C can be explained by the increase of the diffusion due to a concurrent effect of the temperature and 5+ the V content. In fact, increasing the temperature not only (020) (200) improves the diffusion of the ad-atoms but also increases 450°C (210) 5+ 5+ V content in the films. Since V state suggests the ex- 20 25 30 35 40 45 50 istence of V O , bulk diffusion is favored due to the low 2 5 2θ (°) melting temperature of V O (∼680 C) in accordance with 2 5 Figure 2: XRD patterns of the PLD VO (M) films grown at dif- the structural zone model for film growth described by ferent temperatures. &e inset shows the full width at half maxi- Movchan-Demchishin [19]. More pronounced (011) tex- mum (FWHM) of the peak (011) versus growth temperature. turization and better crystallinity of the VO (M) films are obtained as the consequence of enhanced diffusion of the ratio. &is temperature is above the melting point of V O adatoms to grow the planes with the lowest energy [17, 18]. 2 5 At the same time, the improvement of the diffusion helps in (∼680 C), which can exist as an intermediate liquid phase during the PLD growth of VO (M) structures. &e liquid minimizing surface and interface energies by allowing the growth of large grains at the expense of smaller grains. V O enhances the formation of micro/nanowires through 2 5 the wetting assisted growth mechanism, as described by At 750 C, the structure of VO (M) changes significantly with the formation of micro/nanowires with a high aspect Strelcov et al. [18]. At the same time, the high nucleation rate Vanadium valence content (%) Intensity (a.u) FWHM (°) 4 Advances in Condensed Matter Physics 1 µm 1 µm 1 µm 1 µm ° ° ° ° Figure 3: SEM images of the PLD VO (M) films grown at different substrate temperatures: (a) 450 C, (b) 550 C, (c) 650 C, and (d) 750 C. for the PLD process is beneficial for increasing the surface transition) segment. Finally, the transition sharpness (ΔT) density (i.e., the yield) of the micro/nanowires on smooth corresponds to the FWHM of the Gaussian fit curves. &e fused quartz substrates, while the high mobility of the PLD corresponding results are summarized in Table 1. adatoms is expected to increase the aspect ratio of the micro/ T is observed to increase with increasing the substrate IMT nanowires for a temperature lower than those reported for temperature (cf. Table 1). &is can be explained by the ac- 5+ thermal evaporation-based techniques [20]. ceptor-level doping of the films due to the increase in the V valence content that tends to shift T to higher values. &e IMT largest ΔR was achieved for the sample deposited at 450 C 3.3. Electrical Characterization. &e resistivity measurement (3.18 orders). ΔR decreases with increasing the substrate ° ° as a function of the temperature of the VO samples de- temperature from 450 C to 650 C (cf. Table 1). &is result is 4+ 4+ ° ° ° posited at T � 450 C, 550 C, and 650 C is shown in Figure 4. correlated to the V content, so that large V content &e resistivity of the film deposited at 750 C could not be corresponds to a largerΔR. In parallel, the hysteresis loopΔH measured as the related values were beyond the upper limit increases for samples processed at higher temperature. &is of the four-point probe setup. &e increase of the resistivity can be attributed to the increase of grain size as explained by of the films can be explained by two main reasons: first, high Suh et al. [22]. Finally, the transition sharpness (ΔT) is known 5+ V content at high temperature is correlated to the exis- to depend on the type of defects and their concentration in the tence of excessive oxygen atoms that will induce holes (i.e., films as well as on the mechanical stress in the grains of acceptor) doping in the VO films [21]. Second, as the different sizes [4, 22–27]. At low substrate temperature, temperature increases, the films become less dense (cf. SEM VO (M) grains are of a relatively small size and display a low images in Figure 3), which will further contribute to the discrepancy in the size (cf. Figure 3(a)). In this case,ΔT is low increase of their overall resistivity. indicating a sharp transition as a result of a low density of bulk &e IMT features were obtained from the resistivity defects [4, 23]. In addition, a symmetric hysteresis loop is curves as follows: the resistivity variation, ΔR, is defined as observed. For VO (M) films processed at high temperatures, ΔR � log (R ° /R ° ), where R ° and R ° are the re- the grain size increases along with the exacerbation of the 10 25 C 100 C 25 C 100 C ° ° sistivity values at 25 C and 100 C, respectively. &e first discrepancy in the grain size (Figures 3(b) and 3(c)). As a derivative of the resistivity versus temperature was fitted result, ΔT increases and an asymmetric hysteresis loop is with a Gaussian function (cf. Figure 5). &e insulator-to- observed due to the more pronounced difference in the values metal transition temperature (T ) is obtained from the of ΔT for the heating and cooling segments of the resistivity IMT position of the minimum of the Gaussian fit of the first curves of the same sample. derivative of the curve resistivity � f(T) for the heating segment, while the hysteresis width (ΔH) is calculated as the difference between the minimum of the Gaussian fit of the 3.4. Optical Properties of the VO (M) Films toward Smart first derivative for the heating segment (insulator-to-metal Windows Application. Several approaches were reported to transition) and that for the cooling (metal-to-insulator improve the properties of VO (M) for smart windows 2 Advances in Condensed Matter Physics 5 0.1 0.01 1E-3 20 40 60 80 100 Temperature (°C) 650°C Heating 550°C Cooling 650°C Cooling 450°C Heating 550°C Heating 450°C Cooling Figure 4: Resistivity versus temperature of the PLD VO (M) films. 0.0 0.0 -0.1 -0.1 -0.2 -0.2 -0.3 -0.3 -0.4 -0.4 20 30 40 50 60 70 80 90 100 110 20 30 40 50 60 70 80 90 100 110 Temperature (C) Temperature (C) Heating Gauss fit heating Heating Gauss fit heating Cooling Gauss fit cooling Cooling Gauss fit cooling (a) (b) 0.0 -0.1 -0.2 -0.3 -0.4 20 30 40 50 60 70 80 90 100 110 Temperature (C) Heating Gauss fit heating Cooling Gauss fit cooling (c) Figure 5: Derivative of the resistivity versus temperature of the PLD VO (M) films and the related fit of the results using a Gaussian: ° ° ° (a) 450 C, (b) 550 C, and (c) 650 C. Derivative resistivity 450C Resistivity (Ω.cm) Derivative resistivity 650C Derivative resistivity 550C 6 Advances in Condensed Matter Physics Table 1: &e characteristics of the IMT of the PLD VO (M) films grown at different temperatures. ΔT ( C) ° ° ° Substrate temperature ( C) ΔR (orders of magnitude) T ( C) ΔH ( C) IMT Heating Cooling 450 3.184 72 8 5 7 550 2.718 75 11 6 10. 650 1.705 81 29 12 32 80 80 60 60 40 40 20 20 0 0 500 1000 1500 2000 2500 500 1000 1500 2000 2500 Wavelength (nm) Wavelength (nm) 750°C 550°C 750°C 550°C 650°C 450°C 650°C 450°C (a) (b) Figure 6: Spectral transmittance at room temperature (a) and at 90 C (b) of PLD VO (M) films grown at different temperatures. Table 2: Optical properties of the PLD VO (M) films grown at applications, such as doping, multilayer films, core-shell 2 different temperatures. nanostructures, and patterning [28]. &e main target is to improve the luminous transmittance along with achieving T (%) T (%) lum IR Substrate temperature ( C) ΔT (%) good IR transmittance modulation across T . Despite the IR IMT ° ° RT 90 C RT 90 C achieved promising results, the proposed approaches in- 450 38.4 37.2 55.7 43.0 12.7 volve complicated synthesis and/or fabrication procedures 550 40.1 38.9 59.1 45.1 14 that may severely limit practical application. 650 41.1 40.1 61.2 46.5 14.7 &e spectral transmittance measured at room temper- 750 44.6 43.0 68.3 49.4 18.9 ature presented in Figure 6(a) shows an enhancement when the substrate temperature of VO (M) films increases. VO (M) layer displays a more open structure when the windows applications since the objective is to have a stable substrate temperature increases (cf. Figure 3). &e medium visible luminosity while ensuring a good modulation of the made of VO (M) and pores will have a lower refractive transmittance in the IR. index, which results in an increase in the transmittance with &e IR transmittance modulation ΔT is observed to IR the porosity [29]. As a result, the integral luminous trans- ° increase from 12.7% for VO (M) film deposited at 450 C to mittance at room temperature T increases from 38.4% lum,RT 18.9% for the micro/nanowire VO (M) sample deposited at for VO (M) film deposited at 450 C to 44.6% for the micro/ ° 750 C (cf. Table 2). By correlating these results to the de- nanowire VO (M) sample deposited at 750 C (cf. Table 2). crease in the FWHM (cf. inset in Figure 2), the improvement &is represents a relative increase in T by 16.1%. &e in the modulation properties can be explained by the im- lum,RT spectral transmittance measured at 90 C is presented in provement in the crystallinity of VO (M) with increasing the Figure 6(b). Similarly to the trend observed at room tem- temperature [30]. perature, the integral luminous transmittance at 90 C T increases from 37.2% for VO (M) film deposited at lum,90° C 2 4. Conclusion 450 C to 43.0% for the micro/nanowire VO (M) sample deposited at 750 C (cf. Table 2). &is represents a relative VO (M) films with different morphologies were directly increase in T by 15.6%. It is important to highlight the grown on smooth fused quartz substrates by a simple PLD lum,90 C fact that T values are very similar at both room tem- approach at a low deposition rate without posttreatment. It lum perature (RT) and 90 C, which means that the visible lu- was found that the increase in the substrate’s temperature minosity remains very stable across the IMT critical not only results in an enhancement of the adatoms diffusion, 5+ temperature. &is factor might be very convenient for smart but also increases the V state content, resulting in a further Transmission (%) Transmission (%) Advances in Condensed Matter Physics 7 ° ° ° ° improvement of bulk diffusion due to the low melting point 450 C, (b) 550 C, (c) 650 C, and (d) 750 C. (Supplementary 5+ of vanadium oxides containing V valence state. As a result, Materials) XRD revealed better (011) texturization and improved crystallinity with increasing the temperature from 450 C to References 750 C. In addition, the morphology the VO (M) grains evolved from small-grained, closely packed structure at [1] F. J. 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Substrate Temperature-Dependent Structural, Optical, and Electrical Properties of Thermochromic VO2(M) Nanostructured Films Grown by a One-Step Pulsed Laser Deposition Process on Smooth Quartz Substrates

Advances in Condensed Matter Physics , Volume 2021 – Sep 6, 2021

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Copyright © 2021 Ali Hendaoui. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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10.1155/2021/7700676
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Hindawi Advances in Condensed Matter Physics Volume 2021, Article ID 7700676, 8 pages https://doi.org/10.1155/2021/7700676 Research Article Substrate Temperature-Dependent Structural, Optical, and Electrical Properties of Thermochromic VO (M) Nanostructured Films Grown by a One-Step Pulsed Laser Deposition Process on Smooth Quartz Substrates Ali Hendaoui Physics Department, College of Science and General Studies, Alfaisal University, P.O. Box 50927, Riyadh 11533, Saudi Arabia Correspondence should be addressed to Ali Hendaoui; ali.hendaoui@gmail.com Received 11 June 2021; Accepted 27 August 2021; Published 6 September 2021 Academic Editor: Prasenjit Guptasarma Copyright © 2021 Ali Hendaoui. &is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. &ermochromic M-phase vanadium dioxide VO (M) films with different morphologies have been grown directly on smooth fused quartz substrates using low deposition rate pulsed laser deposition without posttreatment. When the substrate temperature ° ° was increased in the range 450 C–750 C, better (011) texturization of VO (M) films was observed along with an enhancement of their crystallinity. Morphology evolved from small-grained and densely packed VO (M) grains at 450 C to less packed micro/ nanowires at 750 C. Mechanisms behind the crystallinity/morphology evolution were discussed and correlated with the effect of 5+ the temperature on the diffusion of the adatoms as well as on the V valence states content in VO (M) films. Resistivity measurements as a function of temperature revealed that the insulator-to-metal transition features of VO (M) films (i.e., transition temperature (T ), resistivity variation (ΔR), hysteresis width (ΔH), and transition sharpness (ΔT)) are strongly IMT dependent on the processing temperature. In terms of optical properties, it was found that the open (i.e., porous) structure of the films achieved at high temperature induced an improvement of their luminous transmittance. Simultaneously, the enhancement of the films crystallinity with the temperature resulted in better IR modulation ability. &e present contribution provides a one- step process to control the morphology of VO (M) films grown on smooth quartz substrates for applications as switches, memory devices, and smart windows. windows applications, especially since the critical temper- 1. Introduction ature can be decreased to room temperature by donor-level &ermochromic M-phase Vanadium dioxide VO (M) un- doping [2]. Full exploitation of the IMT in VO (M) requires 2 2 dergoes an insulator-to-metal transition (IMT) that takes a thorough control of its IMT features, such as T , hys- IMT place around a temperature of T ≈ 340K. Below T , teresis width ΔH, and modulation capability of its electrical IMT IMT VO (M) has a monoclinic phase characterized by a high and/or optical properties depending on the targeted appli- cation. For example, a sensor would require a small hys- resistivity (insulator). Above T , VO (M) displays a te- IMT 2 tragonal phase with metallic characteristics. &e IMT is teresis, sharp transition, and large modulation, while a reversible and takes place at ultrafast timescales and is memristor requires a large hysteresis. It is worth mentioning characterized by a dramatic change in its resistivity as well as that the IMT characteristics in VO (M) films depend on their in its infrared optical properties from being highly trans- crystallinity and grain morphology, in addition to the im- missive to being highly reflective, while the optical properties purity/dopants content [4, 5]. in the visible range remain almost unchanged across T Several reports in the literature describe studies on IMT [1–3]. &is makes VO (M) very promising for ultrafast VO (M) with controlled IMT properties for targeted ap- 2 2 electronic switching devices, memristors, and smart plications [1–8]. Among them, pulsed laser deposition 2 Advances in Condensed Matter Physics (PLD) holds a privileged position for the productions of pure the films is obtained. In addition, we will show that the VO (M) thin films with controlled composition, crystal- improvement of VO (M) crystallinity with the temperature 2 2 linity, and morphology [2–5]. It has also the potential for results in an improvement of infrared (IR) transmittance large-scale production, especially if the thin films’ synthesis modulation ability toward smart windows applications. is performed at low deposition rates [9]. &e crystallinity and morphology of the VO (M) films can be controlled via 2. Materials and Methods several parameters, including the substrate’s temperature during the deposition process. By controlling the substrate PLD was performed using KrF excimer pulsed laser ° −2 temperature at 700 C, in a recent report, Lafane et al. re- (λ � 248 nm, fluence � 1.8 J.cm ) focused on Vanadium ported the synthesis of VO (M) polycrystalline nanoplatelets target (99.9% pure) under 5 mTorr of oxygen ambient. on glass substrate by PLD using Vanadium Pentoxide V O 2 5 Such a low pressure was chosen as it is expected to be target under oxygen ambient [10]. However, Lafane et al. did beneficial for producing elongated structures due to the not report information about the composition of the films. enhanced mobility of the adatoms on the substrate. &e In addition, the functional properties of the grown nano- total number of laser pulses on the target for each de- platelets are not reported therein [10]. In summary, despite position experiment was set at 18000 pulses. Smooth fused the importance of the substrate temperature for controlling quartz, used as the substrate, was kept at 7 cm away from the IMT characteristics of PLD-grown nanostructured the target and the substrate temperature was varied for the VO (M) films, the related studies remain relatively scarce. In different experiments. &e laser was pulsed at a frequency addition, the influence of the substrate temperature on the of 2 Hz. &e choice of this value is based on preliminary vanadium valence content of the PLD-grown nano- tests on the influence of the laser pulsing frequency on the structured VO (M) films on smooth quartz substrates re- morphology of the grown films toward the synthesis of mains, to the best of our knowledge, unexplored. VO (M) micro/nanowires. In fact, as shown in Figure S1 Another important topic of interest related to the present on the supplemental file, scanning electron microscope study is related to the synthesis of VO (M) micro/nanowires (SEM) images revealed that a pulsing frequency of 2 Hz is (MNWs). In fact, many approaches have been developed to suitable for achieving elongated, rods-like structures for achieve VO (M) MNWs. In most of the cases, the proposed VO (M) grains for films grown at the same substrate methods resulted in relatively low yield (i.e., surface density) temperature. for the micro/nanowires [11, 12]. As a remedial solution, X-ray photoelectron spectroscopy (XPS) measurements roughening the substrate surface, patterning it, or using were made using a VG Escalab 220I-XL system with Al Kα epitaxial growth were proposed [11–14]. However, these (h] � 1486.6 eV) radiation. Etching with Argon was per- approaches are either not suitable for optical applications formed for 900-second prior measurements to surface (roughness and patterning), or not applicable for large-scale contamination and/or overoxidation. More details about the production (epitaxy). For example, optical applications such deconvolution analysis of the binding energy of the V2p 3/2 as smart windows require large transmittance of the samples core level peak to determine the vanadium valence state in the visible range of the spectrum. In this sense, smooth content of the samples are given in the supplemental file (cf. surface substrates are needed because the presence of Figure S2 in the supplemental file). roughness or patterns on the substrates surface would &e crystalline structure of the samples was analyzed by negatively impact the optical transmittance. As for the ep- X-ray diffraction (PANalytical’s X’Pert, Cu Kα radiation). itaxy, it could be a limiting factor for large scale, that is, &eir morphology was studied using scanning electron commercial production of thin films, since it requires the use microscopy (JEOL JSM-6300F). &e resistivity of the films of costly single crystalline substrates with atomic-level ° ° was measured in the range 25 C–100 C using four-point smoothness and specific lattice characteristics, such as single probe. Optical transmittance was analyzed in the range of crystalline titanium dioxide or sapphire substrates for 250–2500 nm using a spectrophotometer (Agilent, Cary growing VO (M), in order to ensure lattice matching be- 5000) at normal incidence. tween the substrate and the films. &e integral luminous transmittance T (390–830 nm) lum In this paper, we will investigate the influence of the and IR transmittance T (830–2500 nm) were calculated IR temperature on the composition, structure, and electrical using the following equation: and optical properties of VO (M) films directly grown on smooth fused quartz substrates by a simple PLD approach at 􏽒 φ (λ)T(λ) dλ lum/IR a low deposition rate without posttreatment. Smooth quartz T � , (1) lum/IR 􏽒 φ (λ) dλ lum/IR substrates were chosen as they are convenient for resistivity measurements and suitable for optical applications. We will demonstrate that a control of the substrate where φ (λ) is the IR irradiance spectrum for air mass 1.5 IR temperature of the PLD-grown VO (M) films allows the for a 37 tilted surface [15] and φ (λ) is the CIE (2008) 2 lum control of their IMT features as revealed by resistivity physiologically relevant luminous efficiency function for measurement. On the other hand, we will demonstrate that, photopic vision [16]. as the morphology changes from densely packed small &e modulation ΔT is defined as ΔT � T IR IR IR,RT grains to less packed micro/nanowires with increasing the − T , where T and T are, respectively, the ° ° IR,90 C IR,RT IR,90 C temperature, an enhancement of luminous transmittance of integral IR transmittance at room temperature and at 90 C. Advances in Condensed Matter Physics 3 3. Results and Discussion 3.1. Composition Analysis. Figure 1 shows the evolution of 5+ 4+ 2+ the V , V , and V valence states content with the substrate temperature extracted from XPS measurements. 4+ As can be seen in Figure 1, V is the dominant valence in the 4+ sample, which corresponds to the state related to VO . V 5+ content decreases in favor of an increase in the V content with increasing the temperature. &erefore, higher oxidation of the films is obtained with increasing the temperature. On 2+ the other hand, the content in V remains relatively con- stant as a function of the temperature as it originates from the creation of oxygen vacancies during the Argon etching process rather than the films PLD synthesis process itself. 450 500 550 600 650 700 750 3.2. Microstructure and Morphology Analysis. Figure 2 shows Temperature (°C) the XRD patterns of the VO (M) films. All the peaks could be identified using Joint Committee on Powder Diffraction V4+ Standards (JCPDS) Card No. 44-0252 and were attributed to V2+ V5+ VO (M) monoclinic phase. (011) preferred orientation of the films was identified for the peak present at∼28 indicating Figure 1: Vanadium valence in the PLD-grown VO (M) films. texturization of VO (M) along the (011) plane as it is the energetically favored one [17, 18]. &e preferential crystal growth along the (011) plane is enhanced as the substrate 0.42 ° ° temperature increases from 450 C to 750 C as shown by the 0.40 increase in the (011) peak intensity. &e inset in Figure 1 shows that the full width at half maximum (FWHM) of peak 0.38 (011) decreases with increasing the substrate temperature, indicating an improvement of the crystallinity for the VO 0.36 (M). 500 600 700 Figure 3 presents the top-view SEM images of VO (M) Temperature (°C) films obtained at different substrate temperatures. At 450 C, 750°C the VO (M) film shows a small-grained, densely packed structure due to the relatively low diffusion of adatoms alongside the high nucleation rate that characterizes the PLD process. At 550 C, the structure displays the coexistence of 650°C grains and platelets. &e sample synthesized at 650 C shows the formation of micro/nanorods with well-defined facets and a low aspect ratio. &e evolution of the microstructure and morphology of 550°C VO (M) films with varying the processing temperature from (011) ° ° 450 C to 650 C can be explained by the increase of the diffusion due to a concurrent effect of the temperature and 5+ the V content. In fact, increasing the temperature not only (020) (200) improves the diffusion of the ad-atoms but also increases 450°C (210) 5+ 5+ V content in the films. Since V state suggests the ex- 20 25 30 35 40 45 50 istence of V O , bulk diffusion is favored due to the low 2 5 2θ (°) melting temperature of V O (∼680 C) in accordance with 2 5 Figure 2: XRD patterns of the PLD VO (M) films grown at dif- the structural zone model for film growth described by ferent temperatures. &e inset shows the full width at half maxi- Movchan-Demchishin [19]. More pronounced (011) tex- mum (FWHM) of the peak (011) versus growth temperature. turization and better crystallinity of the VO (M) films are obtained as the consequence of enhanced diffusion of the ratio. &is temperature is above the melting point of V O adatoms to grow the planes with the lowest energy [17, 18]. 2 5 At the same time, the improvement of the diffusion helps in (∼680 C), which can exist as an intermediate liquid phase during the PLD growth of VO (M) structures. &e liquid minimizing surface and interface energies by allowing the growth of large grains at the expense of smaller grains. V O enhances the formation of micro/nanowires through 2 5 the wetting assisted growth mechanism, as described by At 750 C, the structure of VO (M) changes significantly with the formation of micro/nanowires with a high aspect Strelcov et al. [18]. At the same time, the high nucleation rate Vanadium valence content (%) Intensity (a.u) FWHM (°) 4 Advances in Condensed Matter Physics 1 µm 1 µm 1 µm 1 µm ° ° ° ° Figure 3: SEM images of the PLD VO (M) films grown at different substrate temperatures: (a) 450 C, (b) 550 C, (c) 650 C, and (d) 750 C. for the PLD process is beneficial for increasing the surface transition) segment. Finally, the transition sharpness (ΔT) density (i.e., the yield) of the micro/nanowires on smooth corresponds to the FWHM of the Gaussian fit curves. &e fused quartz substrates, while the high mobility of the PLD corresponding results are summarized in Table 1. adatoms is expected to increase the aspect ratio of the micro/ T is observed to increase with increasing the substrate IMT nanowires for a temperature lower than those reported for temperature (cf. Table 1). &is can be explained by the ac- 5+ thermal evaporation-based techniques [20]. ceptor-level doping of the films due to the increase in the V valence content that tends to shift T to higher values. &e IMT largest ΔR was achieved for the sample deposited at 450 C 3.3. Electrical Characterization. &e resistivity measurement (3.18 orders). ΔR decreases with increasing the substrate ° ° as a function of the temperature of the VO samples de- temperature from 450 C to 650 C (cf. Table 1). &is result is 4+ 4+ ° ° ° posited at T � 450 C, 550 C, and 650 C is shown in Figure 4. correlated to the V content, so that large V content &e resistivity of the film deposited at 750 C could not be corresponds to a largerΔR. In parallel, the hysteresis loopΔH measured as the related values were beyond the upper limit increases for samples processed at higher temperature. &is of the four-point probe setup. &e increase of the resistivity can be attributed to the increase of grain size as explained by of the films can be explained by two main reasons: first, high Suh et al. [22]. Finally, the transition sharpness (ΔT) is known 5+ V content at high temperature is correlated to the exis- to depend on the type of defects and their concentration in the tence of excessive oxygen atoms that will induce holes (i.e., films as well as on the mechanical stress in the grains of acceptor) doping in the VO films [21]. Second, as the different sizes [4, 22–27]. At low substrate temperature, temperature increases, the films become less dense (cf. SEM VO (M) grains are of a relatively small size and display a low images in Figure 3), which will further contribute to the discrepancy in the size (cf. Figure 3(a)). In this case,ΔT is low increase of their overall resistivity. indicating a sharp transition as a result of a low density of bulk &e IMT features were obtained from the resistivity defects [4, 23]. In addition, a symmetric hysteresis loop is curves as follows: the resistivity variation, ΔR, is defined as observed. For VO (M) films processed at high temperatures, ΔR � log (R ° /R ° ), where R ° and R ° are the re- the grain size increases along with the exacerbation of the 10 25 C 100 C 25 C 100 C ° ° sistivity values at 25 C and 100 C, respectively. &e first discrepancy in the grain size (Figures 3(b) and 3(c)). As a derivative of the resistivity versus temperature was fitted result, ΔT increases and an asymmetric hysteresis loop is with a Gaussian function (cf. Figure 5). &e insulator-to- observed due to the more pronounced difference in the values metal transition temperature (T ) is obtained from the of ΔT for the heating and cooling segments of the resistivity IMT position of the minimum of the Gaussian fit of the first curves of the same sample. derivative of the curve resistivity � f(T) for the heating segment, while the hysteresis width (ΔH) is calculated as the difference between the minimum of the Gaussian fit of the 3.4. Optical Properties of the VO (M) Films toward Smart first derivative for the heating segment (insulator-to-metal Windows Application. Several approaches were reported to transition) and that for the cooling (metal-to-insulator improve the properties of VO (M) for smart windows 2 Advances in Condensed Matter Physics 5 0.1 0.01 1E-3 20 40 60 80 100 Temperature (°C) 650°C Heating 550°C Cooling 650°C Cooling 450°C Heating 550°C Heating 450°C Cooling Figure 4: Resistivity versus temperature of the PLD VO (M) films. 0.0 0.0 -0.1 -0.1 -0.2 -0.2 -0.3 -0.3 -0.4 -0.4 20 30 40 50 60 70 80 90 100 110 20 30 40 50 60 70 80 90 100 110 Temperature (C) Temperature (C) Heating Gauss fit heating Heating Gauss fit heating Cooling Gauss fit cooling Cooling Gauss fit cooling (a) (b) 0.0 -0.1 -0.2 -0.3 -0.4 20 30 40 50 60 70 80 90 100 110 Temperature (C) Heating Gauss fit heating Cooling Gauss fit cooling (c) Figure 5: Derivative of the resistivity versus temperature of the PLD VO (M) films and the related fit of the results using a Gaussian: ° ° ° (a) 450 C, (b) 550 C, and (c) 650 C. Derivative resistivity 450C Resistivity (Ω.cm) Derivative resistivity 650C Derivative resistivity 550C 6 Advances in Condensed Matter Physics Table 1: &e characteristics of the IMT of the PLD VO (M) films grown at different temperatures. ΔT ( C) ° ° ° Substrate temperature ( C) ΔR (orders of magnitude) T ( C) ΔH ( C) IMT Heating Cooling 450 3.184 72 8 5 7 550 2.718 75 11 6 10. 650 1.705 81 29 12 32 80 80 60 60 40 40 20 20 0 0 500 1000 1500 2000 2500 500 1000 1500 2000 2500 Wavelength (nm) Wavelength (nm) 750°C 550°C 750°C 550°C 650°C 450°C 650°C 450°C (a) (b) Figure 6: Spectral transmittance at room temperature (a) and at 90 C (b) of PLD VO (M) films grown at different temperatures. Table 2: Optical properties of the PLD VO (M) films grown at applications, such as doping, multilayer films, core-shell 2 different temperatures. nanostructures, and patterning [28]. &e main target is to improve the luminous transmittance along with achieving T (%) T (%) lum IR Substrate temperature ( C) ΔT (%) good IR transmittance modulation across T . Despite the IR IMT ° ° RT 90 C RT 90 C achieved promising results, the proposed approaches in- 450 38.4 37.2 55.7 43.0 12.7 volve complicated synthesis and/or fabrication procedures 550 40.1 38.9 59.1 45.1 14 that may severely limit practical application. 650 41.1 40.1 61.2 46.5 14.7 &e spectral transmittance measured at room temper- 750 44.6 43.0 68.3 49.4 18.9 ature presented in Figure 6(a) shows an enhancement when the substrate temperature of VO (M) films increases. VO (M) layer displays a more open structure when the windows applications since the objective is to have a stable substrate temperature increases (cf. Figure 3). &e medium visible luminosity while ensuring a good modulation of the made of VO (M) and pores will have a lower refractive transmittance in the IR. index, which results in an increase in the transmittance with &e IR transmittance modulation ΔT is observed to IR the porosity [29]. As a result, the integral luminous trans- ° increase from 12.7% for VO (M) film deposited at 450 C to mittance at room temperature T increases from 38.4% lum,RT 18.9% for the micro/nanowire VO (M) sample deposited at for VO (M) film deposited at 450 C to 44.6% for the micro/ ° 750 C (cf. Table 2). By correlating these results to the de- nanowire VO (M) sample deposited at 750 C (cf. Table 2). crease in the FWHM (cf. inset in Figure 2), the improvement &is represents a relative increase in T by 16.1%. &e in the modulation properties can be explained by the im- lum,RT spectral transmittance measured at 90 C is presented in provement in the crystallinity of VO (M) with increasing the Figure 6(b). Similarly to the trend observed at room tem- temperature [30]. perature, the integral luminous transmittance at 90 C T increases from 37.2% for VO (M) film deposited at lum,90° C 2 4. Conclusion 450 C to 43.0% for the micro/nanowire VO (M) sample deposited at 750 C (cf. Table 2). &is represents a relative VO (M) films with different morphologies were directly increase in T by 15.6%. It is important to highlight the grown on smooth fused quartz substrates by a simple PLD lum,90 C fact that T values are very similar at both room tem- approach at a low deposition rate without posttreatment. It lum perature (RT) and 90 C, which means that the visible lu- was found that the increase in the substrate’s temperature minosity remains very stable across the IMT critical not only results in an enhancement of the adatoms diffusion, 5+ temperature. &is factor might be very convenient for smart but also increases the V state content, resulting in a further Transmission (%) Transmission (%) Advances in Condensed Matter Physics 7 ° ° ° ° improvement of bulk diffusion due to the low melting point 450 C, (b) 550 C, (c) 650 C, and (d) 750 C. (Supplementary 5+ of vanadium oxides containing V valence state. As a result, Materials) XRD revealed better (011) texturization and improved crystallinity with increasing the temperature from 450 C to References 750 C. In addition, the morphology the VO (M) grains evolved from small-grained, closely packed structure at [1] F. J. Morin, “Oxides which show a metal-to-insulator tran- ° ° 450 C, to less packed micro/nanowires structure at 750 C. sition at the neel temperature,” Physical Review Letters, vol. 3, no. 1, pp. 34–36, 1959. Resistivity variation as a function of temperature revealed [2] A. Hendaoui, N. Emond, S. Dorval, M. Chaker, and that VO (M) films obtained at low substrate temperature E. Haddad, “VO -based smart coatings with improved display low insulator-to-metal transition temperature emittance-switching properties for an energy-efficient near (T ), large resistivity variation (ΔR), narrow hysteresis IMT room-temperature thermal control of spacecrafts,” Solar width (ΔH), sharp transition, and symmetric hysteresis loop. Energy Materials and Solar Cells, vol. 117, pp. 494–498, 2013. Increasing substrate temperature resulted in VO films with [3] V. R. Morrison, R. P. Chatelain, K. L. 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Published: Sep 6, 2021

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