We report the size effects on the metal-insulator phase transition of vanadium dioxide (VO2) nanowires prepared by chemical vapor deposition. The phase transition temperature can be tuned from 67 °C in the bulk VO2 to as low as 29 °C by reducing the diameter of VO2 nanowires to nanoscale. Temperature-dependent Raman spectra display a clear dynamic picture on the metal-insulator phase transition process of the VO2 nanowires. Whilst, Raman study shows no remarkable strain effect on the phase transition behaviors of our samples. The increasing surface defect density with reducing nanowire size facilitates the decreasing phase transition temperature. In addition, the polarized-photocurrent effect was observed, resulting from the anisotropy of the photoresponse and also caused by the reduced dimensionality. Our results indicate that size of VO2 nanostructures can dominate their thermoelectric and photoelectrical properties.
© 2014 Optical Society of America
Metal-insulator phase transition exhibited by a number of transition-metal oxide compounds is an intensified research topic . Among these compounds, VO2 is one of the most attractive examples because its phase transition occurs close to room temperature (Tc ~67 °C) and it can exhibit a 5 orders of magnitude increase in electrical conductivity [2–4]. The first-order phase transition in VO2 is accompanied by a structural phase transformation: below the phase transition temperature, VO2 presents a monoclinic structure; beyond the critical temperature, VO2 converts to a tetragonal structure. In addition, the phase transition is also accompanied by a large change in the optical absorbance. VO2 is transparent over the infrared region in its insulator phase, while it is opaque at most frequencies in its metallic phase, which is very attractive for a wide variety of potential applications, such as Mott field-effect transistor, optical waveguide, sensing components and thermochromic devices [5–8]. For imperious demands of room-temperature thermoelectric devices, there is tremendous interest in being able to depress the metal-insulator phase transition temperature of VO2 to lower temperatures. Some recent reports by Lopez and associates [9–11] suggested intriguing possibility of shifting the phase transition temperature and hysteresis loops in VO2 by reducing the dimension to nanoscale. In their reports, the authors demonstrated a strong size effect on the phase transition temperature and hysteresis loop in VO2 nanoparticles prepared by top-down methods such as ion implantation or laser ablation in SiO2 matrices. These authors postulated that the defects in VO2 nanoparticles can act as nucleation sites of the phase transition and elimination of defects can result in the need for a stronger driving force. More recently, Whittaker et al.  observed a strong size effect on solution-grown VO2 nanostructures. The authors demonstrated the phase transition temperature can be tuned by scaling VO2 to nanoscale dimensions and it can be depressed to 33 °C in VO2 nanostructures. Nevertheless, these studies did not give clear pictures on the relationship between the phase transition behaviors and the dimension of the nanostructures. Baik and associates  synthesized the VO2 nanowires by physical vapor deposition and carried out current-induced metal-insulator phase transition and calculated the phase transition temperature by an indirectly method. They observed a downward trend in phase transition temperature with decreasing diameter of the nanowire. However, the VO2 nanowires used in their investigation were grown on the SiO2/Si substrate. Thus the strain effect, arising from nanowire-substrate interaction, is introduced in the system and the observed effect is not a pure size effect. Up to date, a systematic study of the variation of the metal-insulator phase transition temperature with reduced dimensionality is still absent. In this paper, we present a systematic study of the thermally induced and current-induced metal-insulator phase transition process in individual VO2 nanowires with various diameters prepared by chemical vapor deposition (CVD). The phase transition temperature shows clear dependence on the size of the nanowires. The temperature-dependent Raman study on different size VO2 nanowires shows that the phase transition temperature can be depressed to as low as 29 °C by reducing the size of the nanowire. By careful analysis of Raman spectra, the relationship between the phase transition temperature and the sample dimensionality is well displayed. Meanwhile, the dynamic process of the phase transition is also clearly revealed by the temperature dependent Raman spectra. In addition, size effect on anisotropy of the photoconductivity is also observed in these nanowires. Our results help to provide a greater insight into the effect of size on the metal-insulator phase transition and optical properties of VO2 nanostructures, as well as to promote the applications of VO2 nanostructures.
2. Methods and experiments
VO2 nanowires were synthesized through a vapor-solid (VS) approach with V2O5 power (Sigma Aldrich) as the precursor in a conventional CVD furnace . The synthesis was carried out in a horizontal tube furnace with Ar (99.9%) as the carrier gas. A ceramic boat loaded with V2O5 power (~0.25 g) was inserted into a quartz tube with diameter of ~1.5 cm. A Si(100) wafer was placed inside the quartz tube at 2 cm away from the source powder. The furnace was first pumped down to a base pressure of 9 × 10−3 mbar before flowing Ar gas at a constant flow rate of 300 sccm with the pressure regulated and maintained at ~1 mbar. The furnace temperature was ramped to 850 °C at a rate of 60 °C/min, and the system was kept at this temperature for 1 h before cooling down to room temperature naturally. After the synthesis, we chose a series of nanowires with the diameters ranging from ~100 nm to 3.5 μm as the specimen of our investigation.
The thermally induced metal-insulator phase transition in individual VO2 nanowires was investigated using micro-Raman spectroscopy (Renishaw inVia) with a 532-nm laser as the light source. The current induced insulator-metal transition was studied through individual nanowire devices. Heavily n-doped Si substrates covering with 200-nm thick insulating nitride layer were used as substrates to construct single nanowire devices. The VO2 nanowires were contact transferred from the growth substrates to the Si3N4/Si substrates. 200-nm thick Au layers were deposited onto the two ends of individual VO2 nanowires to form electrical contacts. The Au electrodes in two-terminal configuration were deposited using a conventional e-beam evaporation system (Edwards Auto306) and patterned by standard photolithography. The electrical characterization (I-V) was carried out using a Keithley 6430 sourcemeter. For the measurements at elevated temperatures, the nanowire device was heated at a rate of 5 °C/min by using a heating stage (Linkam Scientific Instruments, TMS 94). The temperature of the sample was measured using a thermocouple placed on the substrate. The Raman spectra were measured at a low laser power to avoid phase transition induced by the laser irradiation.
3. Results and discussions
Figure 1 shows representative temperature dependence of the resistance of individual VO2 wires with different diameters as temperature is changed across the phase transition region. The voltage applied on nanowires maintains a relative low value (0.5V) during measurement to avoid Joule heat inducing phase transition. According to the finite element method (FEM) simulation, the temperature raise due to Joule heating is less than 0.1 °C, as shown in Fig. 1(e) and 1(f). As expected, the room-temperature resistance increases with reducing wire diameter. The phase transition temperature and the width of the hysteresis loop exhibit a correlation with the size of VO2 wires. In heating process, a sharp decrease in resistance was observed upon phase transition. The phase transition temperature in heating process, Tinc, decreases with decreasing the diameter of VO2 wire. As shown in Fig. 1, the metal-insulator phase transition occurs at ~65 °C for the wire with the diameter of 3 μm, which is comparable with the phase transition temperature of bulk VO2 . The phase transition temperature Tinc gradually shifts to lower temperatures with reducing the diameter of the wire. In cooling process, the phase transition temperature of the wires, Tdec, shifts to lower temperature compared to the bulk value, and Tdec decreases with reducing the wire diameter. In addition, an increasing hysteresis width is observed when the diameter of VO2 wires is reduced. In previous studies [12, 15–17], several possible origins, including strain, doping with other transition metals (such as W or Mo), and reducing size to nanoscale, were proposed for the depression of phase transition temperature in VO2 nanowires. Cao et al. mapped and explored stress-temperature phase diagram and demonstrated the phase transition temperature can be tuned by applying tensile/compress strain on VO2 nanobeams . However, the nanowires used in our measurements were transferred to the substrates from their growth substrates. Therefore, the influence of the lattice mismatch and the difference of coefficients of thermal expansion between the wire and substrate are significantly reduced. The strain effect would not be the dominant reason for the reduction of the phase transition temperature. Wu and associates reported the phase transition temperature decreased with increasing W content in WxV1-xO2 nanowires . In our sample synthesis process, there are no other transition metals in the ingredient, ruling out doping effect as the underlying origin for the reduction of the phase transition temperature. The Raman spectra and XRD patterns (shown in our previous report)  of the as-grown nanowires present pure M1 phase without dopants, impurities or contaminants are observed. Recently, Lopez et al.  fabricated VO2 nanoparticles via an ion implantation method. They demonstrated that the decreasing of phase transition temperature arises from the nanoscale effects and pointed out defects in VO2 act as the nucleation sites for phase transition. The nature of the nucleation sites may include simple vacancies, substitutions wall dislocations etc . Oxygen vacancies are frequently reported as the common defects on metal oxide nanostructures surface [19–23]. Consequently, defects on surface (oxygen vacancies) are believed to play an important role in the nucleation of phase transformation in VO2 wires used in the present study. As the dimension of wires decreases, the surface-to-volume ratio increase, thus the effect of surface defects is more significant in smaller wires. Hence, nucleation defect density is higher in the smaller wires, enabling the decrease of the phase transition temperature in smaller wires.
Figure 2 shows representative I-V curves of VO2 wires with different diameters measured at 20°C, 40°C and 70°C, respectively. When the temperature is below the phase transition temperature (20°C and 40 °C), at lower applied voltages, the current increases linearly with the voltage with a slope corresponding to the conductivity of the wire, characterizing the VO2 wire in its insulating phase. Gradually, a threshold voltage, VM, is approached at which the current increases sharply. When the bias voltage increases beyond VM, the current continues to increase but with a bigger slope, indicating higher conductance of the wire in its metallic phase. As shown by the thermal simulation in Fig. 1 and previous reports [24, 25], despite the resistive heating is provided, it is insufficient to make the temperature reach the phase transition temperature and cause the metal-insulator phase transition over the whole wire. The obtained phase transition at the temperature (20°C and 40 °C) below the phase transition temperature is facilitated by the “many-body effect” induced by electrical field [26, 27]. When the measurement is performed at a temperature higher than the phase transition temperature, the I-V curve has a bigger slope, corresponding to the higher conductance of the VO2 wire in metal phase, as the metal-insulator phase transition has occurred. Comparing the I-V characteristics of the wider wire (~3 μm, Fig. 2(a)) with that of the narrower wire (~0.5 μm, Fig. 2(b)), the wider wire has a higher current with the same bias, smaller resistance and a higher phase transition voltage VM at the same substrate temperature than the narrower wire. This phenomenon is consistent with the observation from Fig. 1.
In order to clearly describe the dynamic process of metal-insulator phase transition and how the size of VO2 nanowires affect the metal-insulator phase transition, temperature-dependent Raman spectra of a series of VO2 nanowires with different diameters were recorded. Temperature-dependent Raman spectra of a single VO2 nanowire with a diameter of 1.2 μm are shown in Fig. 3. At temperatures below the phase transition temperature, the nanowire exhibits characteristic Raman peak of the insulator (M1) phase at 608 cm−1. Upon heating from 25 °C, the spectral signature of the M1 phase is retained without significant change in intensity up to 49 °C. After that, reduction of the M1 Raman band is observed with further heating from 49 °C to 51 °C, and the M1 band disappears at 52 °C. The decreased intensity of the M1 Raman band suggests the nucleation of the metallic (R) phase and the coexistence of the insulator and metal phases between 49 and 51 °C, indicating a phase evolution of M1→M1 + R→ R, as has been observed in the studies of VO2 thin films [28, 29] and doped VO2 nanowires . An analogous R→M1 + R→M1 transition is observed during cooling process with a hysteresis (Fig. 3(b)).
In Fig. 4(a), we track the phase transition of the nanowire with the diameter of 0.1 μm, of which the metal-insulator transition has been depressed to near room temperature. In heating process, from 32 to 33 °C, the nanowire abruptly converts to the R phase, and the Raman characteristic band of the M1 phase is no longer discernible. No significant reduction in intensity or shifts of the M1 Raman band are observed before phase transition, indicating nucleation of the R phase and an abrupt phase transition without any intermediate phase, e.g. T phase or M2 phase. The identical M1→R transition behavior was also observed for stoichiometric VO2 nanobeams [15, 31] and doped VO2 nanowires , in which the nanobeams/nanowires are relaxed or reside on compliant substrates. In our case, the nanowires used in Raman measurements were contact transferred onto the glass substrates through unidirectional sliding with light pressure, so tensile strain generated due to the differential thermal expansion coefficients of VO2 and the substrates should be negligible . In all Raman spectra we obtained from nanowires with various diameters, there is no obvious distortion or split in the band of M1 phase, indicating our nanowires were in a relaxed state on the glass substrate without observable strain [33, 34]. Thus, the decrease of phase transition temperature with the decrease of the nanowire diameter we observed is a size effect other than a strain effect.
As shown in Fig. 4(b), we plot the phase transition temperature as a function of the diameter of the nanowire in heating process. The phase transition temperatures exhibit a saturation effect, which can be well fitted by , where t is nanowire diameter. It is shown that the phase transition temperatures are approaching the bulk phase transition temperature when the diameter of nanowires increases. The VO2 phase transition temperature can correlate to the nanowire crystallinity quality. It has been reported that in VO2 nanoparticles the phase transition temperature is suppressed with reducing diameter . The authors suggested that the smaller nanoparticles possess higher density of heterogeneous nucleation centers, such as oxygen vacancies, which plays an essential role in the phase transformation. The influence of oxygen vacancy is also demonstrated in VO2 thin film, in which the metal-insulator phase transition temperature could be significantly suppressed by creation of oxygen vacancies in VO2 and the metallic state could stabilize even at 5 K . Hence, the phase transition temperature is closely related to the density of oxygen vacancies. The VO2 nanowires with smaller size are estimated with more oxygen vacancies on their surfaces, providing more nucleation sites for phase transition. Build on this, it is easier to implement the phase transition in smaller nanowires at reasonable lower temperature.
Figure 5 shows the polarization dependent photocurrent of the single VO2 nanowire with diameter of 3.4 µm, 2.3 µm and 0.5 µm. The nanowires are applied with a constant bias of 20 mV with periodic irradiation of 532-nm laser at the intensity of ~0.1 W/cm2. This value is much lower than the light-induced phase transition threshold (~20 kW/cm2) of VO2 . Evidently, the output current shows different degrees of angle-dependent. The photocurrent reaches maximum when the laser polarization is parallel to the nanowire length, whilst, the photocurrent becomes minimum when the laser polarization is perpendicular to the nanowire length. The photocurrent anisotropy ratio can be defined as, where and are the photocurrent when the laser polarization is parallel and perpendicular to the long axis of nanowire, respectively. The photocurrent anisotropy ratios for the nanowires of 3.4 µm, 2.3 µm and 0.5 µm are 0.014, 0.028, and 0.15, respectively. Remarkably, with reducing size, the anisotropy of the photoelectrical property is more significant, suggesting a strong size-dependent polarization effect in VO2 nanowires.
We have investigated the size dependence of the metal-insulator phase transition temperature and hysteresis loops in VO2 nanowires, and found that the phase transition temperature decreases with decreasing nanowire diameters. Temperature-dependent Raman spectroscopy study has shed light on the phase transition dynamics of VO2 nanowires with different sizes. The phase transition temperature suppression is facilitated by the increasing surface defect density with decreasing nanowire diameter. The controlling of phase transition temperature in VO2 nanowires via altering their sizes offers a new-concept thermoelectronic devices.
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