Current induced polycrystalline-to-crystalline transformation in vanadium dioxide nanowires

Vanadium dioxide (VO2) exhibits a reversible insulator-metal phase transition that is of significant interest in energy-efficient nanoelectronic and nanophotonic devices. In these applications, crystalline materials are usually preferred for their superior electrical transport characteristics as well as spatial homogeneity and low surface roughness over the device area for reduced scattering. Here, we show applied electrical currents can induce a permanent reconfiguration of polycrystalline VO2 nanowires into crystalline nanowires, resulting in a dramatically reduced hysteresis across the phase transition and reduced resistivity. Low currents below 3 mA were sufficient to cause the local temperature in the VO2 to reach about 1780 K to activate the irreversible polycrystalline-to-crystalline transformation. The crystallinity was confirmed by electron microscopy and diffraction analyses. This simple yet localized post-processing of insulator-metal phase transition materials may enable new methods of studying and fabricating nanoscale structures and devices formed from these materials.

o Table S1. Rcon for various VO2 wire widths before crystallization in the insulator phase.

Critical Current
Figure S1 (a) and (b) shows the abrupt irreversible voltage drop in the VO2 wire at their respective critical currents, IC, 2.3 mA and 0.7 mA. The VI plot show two different paths as the current is ramped up ("before" VI) and back down to 0 mA ("after" VI).

Determination of the Contact Resistance and VO2 Resistivity
The contact resistance (Rcon) of the 4 different scenarios (the insulator and metallic states before and after crystallization) are calculated using a modified transmission line method (TLM). Typical TLM involves a series of contact pads with varying lengths of separation on a single wire. In our case, we fabricated sets of wires with varying lengths and widths but kept the contact area between VO2 and Pd constant, as explained in the main article. For each wire width, W, we plot the total resistance (Rtotal = RVO2 + 2Rcon) vs. length as shown in Figure S2. The dimensions of W1 to W6 are listed in Table S1. 3 The total resistance was found by linearly fitting the first 5 data points from the ramp-up (0 mA to 0.3 mA) VI curve for the VO2 insulating phase and the last 5 data points for the VO2 metallic phase. In Figure S2, different wire widths lead to different slopes corresponding to RVO2, and Rcon should be roughly constant. To determine Rcon, we averaged the intercept resistance values in Figure S2 and took the standard deviation to be the uncertainty in Rcon. The results are summarized in Table S1.  Similarly, Rcon for the metallic phase before crystallization and Rcon after crystallization in both phases were calculated. The values of Rcon were used in the calculation of VO2 resistivity (e.g., Figure 2c and 2d) and in the thermal simulations. To account for the change in contact resistance as a function of temperature over the phase transition in Figure 2d, we interpolated Rcon(T) using a lineshape function, F(T), as follows: where α = 0.6 is a unit-less parameter obtained from fitting F(T) to the measured total resistance, and Rcon,i and Rcon,m are respectively the insulator and metallic phases contact resistance. An interpolation procedure for the contact resistance was not applied for the resistance vs. current data due to the abrupt changes in the voltage. Rcon,i was used for the range between 0 mA and the first step in the transition, and Rcon,m was used for currents greater than that at the first transition.
With Rcon known, the resistivity of the VO2 wire (e.g., Figure 2c,d) was determined from where W is the width, t is thickness, and L is the length of the VO2 wire. 5

Crystallization in Wide VO2 Wires
For wide (> 5 μm) VO2 wires, the crystallized region did not cover the entire width of the wire. In Figure S3a (W = 10 μm), only the top region was smooth, and in Figure S3b (W = 50 μm), the crystallized region was only about 3 μm wide along the mid-section of the wire. Figure S4c and S3d display the VI characteristics before and after this partial crystallization. In Figure S4d, the "before" VI curve of a wide VO2 wire shows only a single-step transition, 31 but the "after" VI curve exhibits the two-step transition characteristic of a narrow VO2 wire. This suggests the current was channelled into a narrow, low resistivity VO2 filament after the crystallization. μm and W = 10 μm and (b) L = 3.4 μm and W = 50 μm after Ic was applied. The VO2 was not uniformly crystallized. The respective "before" and "after" VI plots are shown in (c) and (d). 6

X-ray Diffraction and Electron Microscopy
The X-ray diffraction was taken of the as-deposited VO2 film using an X-ray diffractometer (Bruker D2 Phaser) with CuK radiation. The DIFFRAC.EVA V3.0 was used to calculate the weight percentage of VO2 and V2O5.
For the TEM and SAED, the specimens were prepared using focused ion beam (FIB, Zeiss NVision 40). The FIB sectioned slices of 10 × 10 m 2 samples with thicknesses between 50 nm to 75 nm of the as-deposited VO2 film and crystallized VO2 wire. TEM images and SAED patterns of the samples in the insulator and metallic phase were taken using a JEOL 2010F TEM/STEM with a temperature-controlled stage operated at 200 kV.
The electron diffraction patterns of the annealed VO2 wire for both in the insulating phase and metallic phase are shown in Figure S4a and S4c, respectively (the metallic phase diffraction pattern is also shown in the main article). The computed diffraction patterns are simulated using SingleCrystal and the lattice parameters in ref 33. The results for the insulating (monoclinic) and metallic phase (rutile) are shown as black spots in Figure S4b and S4d, respectively. Only diffraction points with computed relative intensities >3.5% have been included. The faint spots near the centre of the diffraction patterns in Figure S4a and S4c are attributed to V2O5. In Figure   S4b and S4d, we superimpose the computed diffraction pattern of V2O5 in red to find a good match