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Abstract
We demonstrate the in situ localized synthesis of vanadium oxides on glass and plastic substrates using localized laser illumination. The proposed technique is efficient and simple in terms of thermal budget and fabrication complexity. The physical properties of the laser-induced vanadium oxide channel region, which is mainly composed of VO2 and V2O5, are assessed by Raman analysis, while its electrical properties and phase transition characteristics with respect to the input optical powers and voltage biases are carefully examined by various resistance measurements. At a bias voltage of the order of 1 V, for example, optically triggered irreversible resistance switching through insulator-to-metal transition of vanadium oxide materials can be observed. The resistance switching ratio before and after the temperature-dependent phase transition exceeds two orders of magnitude. The obtained results confirm the applicability of the photothermally fabricated vanadium oxide devices to illumination-based resistance switching and temperature-dependent current flow management with large dynamic ranges. The proposed fabrication technique can also be applied to other transition metal oxide materials, which are currently grown at high temperatures or vacuum environments, for flexible electronics applications.
© 2017 Optical Society of America
1. Introduction
Vanadium oxides have attracted considerable attention among other transition-metal oxides because of their reversible temperature-dependent electrical and optical properties, which are associated with crystallographic transformation and phase transition at the critical temperature [1]. These interesting properties of vanadium oxides have been extensively exploited in a variety of applications such as thermal and optical sensors [2, 3], electronic devices [4], electrochromic and thermochromic films [5], solar cell windows [6], and marine anti-biofouling [7]. Well-known examples of vanadium oxides are vanadium sesquioxide (V2O3), vanadium dioxide (VO2), and vanadium pentoxide (V2O5). VO2 has been particularly noted for its phase transition between the insulating and metallic states near room temperature, while V2O5 is known to exhibit the most stable phase properties [8, 9].
Vanadium oxides can be prepared by various methods such as the sol-gel method [10], pulsed-laser deposition [11], magnetron sputtering [12], and chemical-vapor deposition [13]. However, most of these techniques typically require vacuum environments and high-temperature treatment during the thin-film growth and post-deposition annealing processes. Owing to such temperature requirements, the fabrication and integration of these transition-metal oxides are often limited to temperature-stable solid-state substrates, and it has been difficult to utilize flexible plastic substrates with relatively low thermal budgets.
In the present study, we demonstrated the localized synthesis of semiconducting vanadium oxide structures that are capable of illumination-dependent resistance switching by more than two orders of magnitude. Although theoretical and experimental works have been carried out on the photo-induced insulator-to-metal transition in VO2 [14, 15], laser-stimulated growth of vanadium oxides has rarely been reported [16, 17]. Very recently, photothermal oxidation of graphene is also presented for the first time [18]. By means of simple, localized photothermal oxidation with a focused near-infrared (NIR) laser beam, the illuminated region of a vanadium metal strip on a glass or plastic substrate was permanently converted into vanadium oxide under typical atmospheric pressure and at room temperature. The unexposed region of the vanadium metal remained unchanged, providing self-aligned ohmic contact for the oxidized region.
To explore the potential applications of the suggested fabrication technique for a light-controlled resistive switching element, we investigated the static and transient electrical responses of the photothermally generated vanadium oxide channel devices with respect to the NIR illumination power under various voltage biases. Photothermal heating with NIR illumination leads to the temperature-dependent phase transition of vanadium oxide, and the interplay between the illumination- and current-based heating processes allows active control of its hysteresis characteristics. For example, irreversible illumination-triggered resistance switching can be achieved with a voltage bias at ~1 V, while lower voltage biases resulted in optically gated current flow control through the phase transition of vanadium oxide.
Furthermore, the proposed localized oxidation technique enables the seamless integration of vanadium oxide-based phase transition materials on a flexible or organic substrate, which has been difficult to achieve with conventional deposition methods. Because the localized oxidation technique prevents the overall temperature increases for the entire substrate, it provides a simple and viable solution for the selective deposition of an inorganic metal-oxide material on organic substrates with low melting points [19–21]. To the best of our knowledge, we present the first attempt to directly fabricate vanadium oxide-based resistance switching devices on a PET substrate.
2. Experimental results and discussions
Figure 1(a) shows a schematic of the experimental setup used to demonstrate the laser-based localized photothermal oxidation. The basic idea is to locally oxidize the vanadium metal to semiconducting vanadium oxides by irradiating the high-intensity laser beam (on the order of 10 kW cm–2) and increase the temperature of the illuminated region through absorption for accelerated oxidation reactions. The inset in Fig. 1(a) is an optical microscope image of a typical sample. Figure 1(b) shows a scanning electron microscope (SEM) image of the synthesized vanadium oxides after high-intensity laser irradiation. Because of its distinct surface morphology, the vanadium oxide region is clearly distinguishable from the outer regions consisting of unoxidized vanadium. As shown in the magnified SEM image included in Fig. 1(b), the active vanadium oxide region comprises an array of randomly oriented nanorods, each of which typically measured more than 1 μm in length. The proposed localized oxidation technique provides the simple, swift, and open-chamber solution to selective growth avoiding an overall temperature increase of the whole substrate.
Fig. 1 (a) Schematic of the experimental setup for localized photothermal vanadium oxidation. The inset is an optical microscope image of the vanadium metal device (L = 50 μm, W = 10 μm) fabricated on a glass substrate. (b) SEM image of the vanadium oxide region after laser-induced photothermal oxidation; the magnified image in the inset shows the randomly oriented vanadium oxide rods. The scale bars in (b) and in the inset represent 5 and 2 μm, respectively. (c) Raman spectrum of the vanadium oxide region, showing spectral peaks of both VO2 (filled circles) and V2O5 (empty diamonds). (d) Calculated two-dimensional temperature distribution when the vanadium is subjected to 40-mW optical illumination; the scale bar represents 5 μm. The estimated transient response for up to 1 ms during photothermal oxidation on the onset of 40-mW laser illumination is also shown.
Raman spectra were obtained at room temperature to further investigate the material compositions of such nanorods in the photothermally oxidized region. Although our synthesis technique is capable of forming many different vanadium oxide phases under various processing conditions, the representative Raman spectrum in Fig. 1(c) reveals that the oxidized region was mainly a mix of VO2 and V2O5. By comparing the obtained spectrum with previous works [22], the peaks of the Raman spectra at 149, 200, 285, 306, 406, 482, 525, 700, and 1000 cm−1 (empty diamonds in Fig. 1(c)) were determined to originate from V2O5, while those at 222 and 613 cm−1 (black circles in Fig. 1(c)) correspond to VO2 [23]. To investigate the spatial uniformity and homogeneity of the synthesized film, we further obtained the position-resolved Raman intensity measurements at three representative Raman spectral bands, namely, 120–160, 580–620, and 680–720 cm−1, labeled as bands I, II, and III, respectively, in Fig. 1(c). The spatial distributions of the peak Raman wavenumbers within the spectral bands are presented in Fig. 2. Although there were some minor spatial variations, most of the peak wavenumbers within the synthesis area (~10 × 10 µm) were close to the major Raman peaks of V2O5 (149 and 700 cm−1) and VO2 (613 cm−1).
Fig. 2 Frequency peaks were mapped as a function of the space using ~1-μm-resolution position-resolved Raman spectrum measurements within three representative spectral bands, namely, (a) 120–160 cm−1 (I in Fig. 1(c)); (b) 580–620 cm−1 (II in Fig. 1(c)); and (c) 680–720 cm−1 (III in Fig. 1(c)), which contain major Raman peaks for the VO2 and V2O5. All spectral information was obtained from the active region (10 × 10 μm) on the oxidized sample.
We also calculated the heat generation and transfer in the original vanadium strip with respect to the incident optical power to estimate the temperature distribution produced by the incident laser beam. Figure 1(d) shows the two-dimensional spatial temperature distribution for an incident laser wavelength of 975 nm with a corresponding optical power of ~40 mW (peak intensity of ~50 kW cm–2 at the beam center). According to the numerical simulations based on the finite element method, the peak temperature of the vanadium strip was estimated to be 580 °C, which is sufficiently high for vanadium oxides growth [24, 25]. The absorption coefficient at the wavelength of 975 nm is 3.49 × 105 cm−1. Included in Fig. 1(d) is the simulated temporal evolution of the peak temperature during high-intensity laser illumination, signifying the speed of the photothermal heating process. Since the electromagnetic absorption is proportional to the input optical power, the peak temperature was observed to linearly increase with the laser irradiation power.
To investigate the phase-transition properties of the fabricated vanadium oxide channel devices through laser-based photothermal optical gating, we measured the resistance (or current) across the active region as a function of the optical illumination power and the applied voltage bias VS using the same setup as shown in Fig. 1(a). However, the laser illumination power in this case (<8 mW) was much smaller than the optical power used for oxidation (~40 mW). Under this situation, further oxidation and chemical conversion of the vanadium oxides do not actively occur becuase of relatively low temperature variation (<100 K), and the local temperature of the illuminated vanadium oxide channel region increases with the absorbed illumination power. Figures 3(a)–(c) show the electrical resistance versus input optical power curves for bias voltage VS values of 0.5, 0.8, and 1.1 V, respectively. To highlight the hysteresis characteristics, the resistance curves for increasing and decreasing optical power are separately shown in red and blue, respectively.
Fig. 3 Resistance variation of the vanadium oxides fabricated on a glass substrate with respect to the incident optical power for the bias voltage (VS) values of (a) 0.5 V, (b) 0.8 V, and (c) 1.1 V. (d) Optical power of the first (Pt1) and second (Pt2) forward transition with respect to the voltage bias. (e) Current–voltage contour map of the vanadium oxide material fabricated on the glass substrate with various incident optical powers. The colors represent the current through the vanadium oxide channel in the logarithmic scale. (f) The resistance variation (R(T)/R(30 °C)) of the vanadium oxides as a function of temperature.
As shown by the increasing-optical-power curve for VS = 0.5 V (red curve in Fig. 3(a)), the vanadium oxide resistance decreased linearly when the input optical power is between 0 and 3.64 mW (Region I). Within this low optical power regime, the layer of vanadium oxide nanorods behaved as an insulating phase (high resistance state) and exhibited better conductances at higher illumination intensities (which result in higher temperatures). Because the illumination photon energy at a laser wavelength of 975 nm was much lower than the typical bandgap of vanadium oxides in the ultraviolet range, the observed behavior was not due to the direct photoconduction across the bandgap, but rather due to the effect of the increased temperature or defect-assisted conduction.
The first abrupt and drastic transition of the vanadium oxide channel resistance was observed at an illumination power of Pt1 = 3.64 mW. After the first forward transition involving a sharp decrease in the resistance up to an illumination power of 3.94 mW, the resistance continued to decrease at a much slower rate (Region II). However, at the second transition optical power, Pt2, of 5.86 mW, a second abrupt change in resistance was observed. Subsequently, the resistance remained nearly constant up to the maximum illumination power of 8 mW (Region III, low resistance state). At higher bias voltage values of VS = 0.8 and 1.1 V, two similar abrupt transitions were observed in the forward traces as shown in Fig. 3(b) and (c).
In the decreasing-optical-power curves indicated by the blue lines in Figs. 3(a)–(c), however, the device resistance followed paths that were different from those for increasing optical power, and the overall hysteresis properties as well as the number of abrupt transitions in resistance heavily depended on the applied voltage bias across the active region. For VS = 0.5 V, two abrupt transitions in electrical resistance were still observed as in the forward curves, and the first and second reverse transitions occurred at illumination powers of 4.76 and 3.08 mW, respectively. The hysteresis widths in terms of the differences in the transition optical powers for the first and second transitions were 0.56 and 1.10 mW, respectively. However, for VS = 0.8 V, there was only one reverse transition point at 2.31 mW, and it was much lower than that for VS = 0.5 V. Such reduction was mainly due to the increased Joule heating and local temperature at the higher bias voltage. Furthermore, no reverse transition was observed for VS = 1.1 V, indicating that the vanadium oxides maintained their low resistance state even after the external illumination was turned off. At room temperature, the vanadium oxide resistances are very large (>100 kΩ), and the voltage biases on the order of 1 V cannot generate enough heat (<0.01 mW) to reach the phase-transition threshold. When the vanadium oxide channel is in its insulating phase (high resistance state), thermal effects from the voltage bias are therefore very little. However, once the local temperature increases and the phase transition occurs through external illumination, the electrical resistance of the vanadium oxide channel decreases by more than two orders of magnitude, resulting in significant increase in Joule heating in the active region. Although the V2O5 within the channel region do not exhibit the phase transition and remain at the high resistance state regardless of the illumination intensity, we expect that most vanadium oxide domains including VO2 are converted to the low-resistance metallic state when the local temperature exceeds the critical phase transition temperature due to the input light absorption. By applying the electric current through the conducting paths within the active channel region, the local domain temperature can be further increased by Joule heating. If the amount of such Joule heating is sufficient to sustain the phase transition temperature, the low-resistance state can be maintained even after the external illumination is turned off [26, 27]. The interplay between the photothermal and Joule heating can affect the overall phase transition behavior such as forward and reverse transition points, hysteresis width, and reversible (or irreversible) switching phenomena. At VS = 1.1 V, for example, the current through the vanadium oxide region generated excess Joule heat that is large enough to sustain the high conductance state above the phase-transition threshold temperature even without external illumination.
On the basis of our observations in Figs. 3(a)–(c), Fig. 3(d) summarizes the first and second forward transition optical powers, Pt1 and Pt2, with respect to the voltage bias to assess the illumination-dependent phase transition characteristics controlled by an external voltage bias. Although the forward transition powers (Pt1 and Pt2) only slightly decreased with the increasing voltage bias because of additional Joule heating, the reverse transition powers and therefore the overall hysteresis properties were significantly changed. To further illustrate the effects of the applied voltage bias and incident optical power, the current–voltage relationships at various levels of input optical power were plotted in Fig. 3(e). It signifying that the phase transition behavior can be controlled by an external voltage bias. In order to investigate the temperature-dependent resistance variation of the active region, the resistance measurements (heating and cooling process) are performed as shown in Fig. 3(f). The resistance variation, R(T)/R(30 °C) with a different slope is observed in the vicinity of the VO2 phase transition temperature (70~90 °C). The hysteresis width is approximately ~10 °C.
We believe that the first and second transitions (resistance drops) were mainly induced by the formation of conducting filaments (paths) or a conducting network across the array of randomly oriented vanadium oxide nanorods. We also performed heat simulations to predict the temperature changes in the VO2 and V2O5 films as functions of the incident optical power as shown in Fig. 4(a) and (b). The absorption coefficient of the insulating VO2, metallic VO2, and V2O5 at the wavelength of 975 nm are 4.33 × 104 cm−1, 1.87 × 105 cm−1 and 2.31 × 105 cm−1, respectively. Prior to the first transition, VO2 and V2O5 remained in the insulating phase (Region I in Fig. 3(a)). Based on the resistance-temperature measurements in Fig. 3(f) and the temperature profile simulation, we believe that VO2 was the main contributor to the first abrupt decrease in the resistance between Regions I and II. In Region II (after Pt1), most VO2 states were therefore in its metallic phase, while thermally excited V2O5 was still in the insulating phase. The second abrupt resistance variation between Regions II and III might be affected by the coexistence of the insulating vanadium oxides phases (mainly V2O5) and photothermally excited (semi)conducting ones. We postulate that the (semi)conducting networks across the array of randomly oriented vanadium oxide nanorods are abruptly changed to lower-resistance metallic domains at a higher critical temperature condition [28, 29].
Fig. 4 (a) Calculated maximum temperature TMax of VO2 fabricated on a glass substrate, as a function of the illumination power. The squares and circles represent TMax for the insulating and metallic phases, respectively, of VO2. In the numerical simulations, the optical constants, namely, the refractive indexes and extinction coefficients, of the two phases were considered to be 3.05 and i0.5, and 1.53 and i1.45, respectively. (b) Calculated maximum temperature of V2O5 fabricated on a glass substrate, as a function of the illumination power. The optical constants of the insulating V2O5 were assumed to be 2.47 and i1.794. The overall temperature linearly increases with the optical power.
Next, we investigated the NIR illumination-dependent (or optically gated) resistance switching response with respect to the bias voltage, as shown in Fig. 5. The time-domain responses were measured by turning the 975-nm laser on (7 mW) and off (0 mW) under different external bias voltages. The illumination power of 7 mW was chosen to be just above the second transition power, Pt2, which is associated with the formation of the mixed metallic phases of VO2 and V2O5. At VS = 0.8 V (Fig. 5(a)), the current through the active region increased to a stable value under laser illumination (shaded regions in Fig. 5) as the forward transition described in Fig. 5 was triggered optically. When the illumination is turned off, the electric current returns to the initial value near zero, which implies that the reversible resistance switching between a high and low resistance state can be achieved by external illumination. At VS = 1.1 V, however, the current level did not return to the initial value even after the laser beam is turned off, and the transition between a high and low resistance state became irreversible. As explained before, this is because of the increased Joule heating from the applied voltage bias after the phase transition of the vanadium oxide channel. The bias voltage effect is expected to be beneficially applied to optimize the volatile memory-like operation of the laser-induced switching. Moreover, the switching time of the vanadium oxide-device was estimated in the unit of ~ms in previous report, which is comparable to the switching speed of the optical switch [30].
Fig. 5 (a) Transient response (optical resistance switching) measured under alternate illumination conditions (7 and 0 mW for on and off periods, respectively) using external bias voltage VS values of (a) 0.8 V and (b) 1.1 V.
Considering that the proposed localized photothermal synthesis/oxidation can be performed at low ambient temperatures and in normal-pressure environments using only short illumination durations, it can be applied to the conversion of vanadium metal to vanadium oxides on a flexible and temperature-sensitive substrate such as polyethylene terephthalate (PET). Here, we report, to the best of our knowledge, the first attempt to directly and locally fabricate vanadium oxide-based resistance switching devices on a PET substrate. The inset in Fig. 6(a) shows the fabricated vanadium oxide devices on the PET substrate. The dimensions (length × width) of a single vanadium strip pattern were 50 × 5 μm. According to our simulations, the applied illumination power of ~5.85 mW generates an amount of heat that can increase the temperature of the active region to >300 °C. This high temperature caused partial melting and deformation of the PET substrate near the laser beam illumination spot, resulting in the unintended suspension of the vanadium oxide strip, as shown in Fig. 6(b). Included in Fig. 6(b) is a magnified image of the vanadium oxide grains. Because the photothermal oxidation temperature is estimated to be lower than the typical crystallization temperature for vanadium oxide growth, the produced vanadium oxide became amorphous, and its surface morphology was different from the nanorod shape for the glass substrate case shown in Fig. 1(b). The measured resistances under a fixed VS value of 0.1 V shown in Fig. 6(a) indicate that the vanadium metal was successfully oxidized and converted to vanadium oxides, but with a smaller hysteresis region. Compared to the case when a glass substrate was used (~40 mW), we found that the required optical power for the oxidation of vanadium on a PET substrate was very low (<6 mW). Owing to the low heat dissipation and thermal diffusion through the PET substrate, the absorbed electromagnetic energy was mainly confined within the device region, and this significantly reduced the required illumination intensity. Higher thermal insulation from the suspended structure further increased the overall efficiency of the photothermal heating process.
Fig. 6 (a) Evolution of the device resistance with respect to the incident optical power. The inset shows the vanadium oxide device array fabricated on a flexible PET substrate. (b) SEM image of the vanadium oxide region after the completion of the laser-induced photothermal oxidation. The deformation and shrinkage of the substrate was due to the low melting point of the PET, and it caused a reduction in the active oxidized area when compared to the glass substrate case. The included magnified image shows the amorphous vanadium oxide grains. The scale bars in (b) and in the inset represent 5 and 1 μm, respectively. (c) An example of current variation with increasing illumination power for a fixed voltage bias of VS = 0.1 V. Three regions can be identified: before the transition, during the transition, and after the transition. The numbers with arrows indicate the illumination power in mW. (d) Transient responses (reversible resistance switching by external NIR illumination) measured under alternate illumination conditions. The inset shows the optical power dependence of the device current under a voltage bias of 0.1 V, indicating a strong correlation between the current and the illumination power.
After the illuminated vanadium metal region was converted to the amorphous vanadium oxides, we monitored the time-dependent current changes with increasing illumination power under a fixed VS value of 0.1 V, as shown in Fig. 6(c). The same experimental setup shown in Fig. 1(a) was used. As in the previous case, the optical powers were limited to be much lower (<1.2 mW) than the oxidation power (~5.85 mW) to prevent damages to the vanadium oxide structure. We found that there were three regions between the high and low resistance states, namely, regions corresponding to the semiconducting (before the phase transition), intermediate (during the transition), and metallic (after the transition) phases. At an incident optical power of <0.1 mW, the current through the active vanadium oxide region remained almost unchanged, implying that the optical power was not sufficient to activate the photothermal phase transition toward the low resistance state. Meanwhile, drastic current variations were observed with higher NIR illumination intensities in the transition region where the high and low resistance phases coexisted. When the incident optical power exceeded 0.713 mW, the abrupt current steps disappeared, suggesting the domination of the low-resistance phases within the vanadium oxide region.
To demonstrate reversible resistance switching by NIR illumination on the PET substrate, we measured the variation of the current with respect to the optical power. As shown in Fig. 6(d), the current increased with increasing illumination power, which is consistent with the results in Fig. 6(c). The dependence of the current on the optical power was examined at a low bias voltage of 0.1 V to minimize electric heating. It can be seen that when the optical power was within a range of 0–0.1 mW, the current started increasing dramatically. This abrupt current increase strongly indicates that the phase transition occurred as a result of laser illumination. When the optical power exceeded 0.713 mW (corresponding to the peak illumination intensity of ~1.4 kW cm–2), the current began to saturate. Our results indicate that the resistance level of the vanadium oxide structure is strongly correlated with the NIR illumination power, and such variation can be reversibly obtained.
3. Conclusion
We demonstrated the in situ localized synthesis of vanadium oxides on glass and plastic substrates using localized NIR laser illumination. The proposed technique is efficient and simple in terms of thermal budget and fabrication complexity because it does not require post-deposition thermal processing as well as additional metal electrode patterning. The physical properties of the NIR laser-induced vanadium oxide channel region, which was mainly composed of VO2 and V2O5, was assessed by Raman analysis, while phase transition characteristics with respect to the input optical powers and voltage biases were carefully examined by various resistance measurements. At a bias voltage on the order of 1 V, for example, optically triggered irreversible resistance switching through insulator-to-metal phase transition of vanadium oxide materials could be observed. The proposed fabrication technique can also be applied to other transition metal oxide materials, which are currently grown at high temperatures or vacuum environments, for flexible electronics applications.
Funding
Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2016M3D1A1900035); Center for Advanced Meta-Materials funded by the Ministry of Science, ICT and Future Planning (CAMM-2014M3A6B3063709).
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