Supporting Information for Effects of Thickness on the Metal-Insulator Transition in Free-Standing Vanadium Dioxide Nanocrystals

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1 Supporting Information for Effects of Thickness on the Metal-Insulator Transition in Free-Standing Vanadium Dioxide Nanocrystals Mustafa M. Fadlelmula 1,2, Engin C. Sürmeli 1,2, Mehdi Ramezani 1,2, T. Serkan Kasırga 1,2,3 * 1 National Nanotechnology Research Center, Bilkent University, Bilkent, Ankara Turkey. 2 Institute of Materials Science and Nanotechnology, Bilkent University, Bilkent, Ankara Turkey. 3 Department of Physics, Bilkent University, Bilkent, Ankara Turkey. Estimation of Thomas-Fermi Screening Length in VO 2 The Thomas-Fermi Screening length, is a measure of how far the electrostatic effects persist in a solid. The electric potential within a solid will decrease by e -1 in magnitude by every = /. Here, is the dielectric permittivity, is the Boltzmann s constant, is the temperature, and is the elementary charge. To estimate the in VO 2, we use the range of values in the literature for the critical carrier concentration, and relative dielectric constant, in the insulating phase of VO 2. 1,2,3 Crystal Etch Rate As mentioned in the main text, we determined the crystal etch rate based on two methods. Crystals masked with photoresist can only withstand ~10 mins of etching before being removed completely under the parameters used for etching. We performed 8 sets of measurements on different samples using 1 kev ion-beam energy with medium monatomic flux. The average etch rates and the methods used S1

2 for masking are given in the table below. Average of all samples give 3.3 nm/min with ±0.3 nm/min error. Table S1 Etch rates determined from different masking methods of VO 2 from Ar-ion bombardment. Sample Masking Method Average Etch Rate Sample 1 Photoresist 3.7 Sample 2 Photoresist 3.7 Sample 3 Indium Shadowed Crystal 3.0 Sample 4 Crystal Masked 3.2 Sample 5 Crystal Masked 3.2 Sample 6 Crystal Shadowed 3.3 Sample 7 Crystal Shadowed 3.4 Sample 8 Crystal Shadowed 3.2 Average of all samples 3.3 (nm/min) Some Examples for Formation of SiO 2 Terrace After Etching Figure S1 shows some examples of different crystals etched using XPS Ar-ion gun for various durations. In all cases, formation of the terrace due to Ar-ion gun being at an oblique angle is clear. S2

3 Figure S1 SEM micrograph of a. 28 minute b. 57 minute c. 60 minute etched crystals of similar thickness in pristine form. d. Shows the optical microscope image of the same region in c. Black arrow from c to d shows the same crystal in the optical microscope image as an aid. Indium-Contacted Devices for RT Measurements Resistance vs. temperature measurements are taken from devices with indium contacts. We use indium as contacts to VO2, as we can rapidly produce devices opposed to metal contacts patterned with optical lithography and metal evaporation. Moreover, since the indium pins are micron thick, even after elongated milling S3

4 durations, they remain intact, unlike several hundred nanometer thick gold/titanium contacts. The nanobeams are places on exfoliated h-bn flakes to remove any nonuniform stress due to surface adhesion. Microscope images of the device are shown after each fabrication step in Figure S2. Figure S2 Fabrication steps of two-terminal devices. First, a suitable crystal is placed on a multi-layer h-bn flake as shown in a. using a micromanipulator. b. Pt is deposited to selected regions using Focused Ion Beam (FIB) to act as fixtures on the h-bn flake as the crystal tends to move in the following steps. c. After a few second dip in buffered oxide etchant to remove the surface oxide on VO 2, indium pins are placed at an elevated temperature. d. SEM micrograph shows the completed device. Scale bars are 10µm. S4

5 Phase Transition in 4 nm Thick VO 2 In Figure S3 we present a series of microscope images taken while warming an etched crystal of total thickness of 10 nm (4 nm of VO 2 beneath ~6 nm of amorphous film). Indium contacts are for RT measurements and the crystal rests on h- BN/SiO 2 /Si surface. The phase transition is optically visible under the microscope. Figure S3 Series of optical microscope images of a thinned VO 2 crystal taken at a. 55 C b. 61 C c. 63 C d. 67 C e. 70 C f. 75 C. The length between the contacts is 25 µm. Figure S4 SEM micrographs of the device shown in Figure S3. a. A wide view of the crystal is shown. Buckled part of the crystal on the left hand side of the device is clearly visible. Scale bar is 10 µm. b. A close-up view of the buckled part is shown. Scale bar is 4 µm. HR-TEM Cross-Sections for 10 and 40 minute Etched Crystals In figure S5 cross-sectional views of two crystals, one etched for 10 min. the other etched for 40 min., taken by HR-TEM are shown. For the 10 min. etched crystal, the S5

6 amorphous surface film thickness is about 4 nm, while for the 40 min. etched crystal it is about 6 nm. This is in agreement with our findings about the amorphous surface film thickness given in Figure 3c. Figure S5 HR-TEM cross-sectional views of a. a 10 min. and b. a 40 min. etched VO 2 crystals. Amorphous surface film formation is indicated by yellow dashed lines and doubleheaded arrow. The scale bars are 5 nm. Ar 2p XPS Survey from Bare SiO 2 Compared to VO 2 Grown SiO 2 XPS surveys around argon 2p peak, both taken under same conditions (30 scans, 400 µm X-ray spot size, total dwell time of 5 minutes and 15 seconds) after 28 minutes of etching show that there is almost no difference between bare SiO 2 surface and SiO 2 with VO 2 nanocrystals on top, in terms of the entrapped argon content. This measurement is shown in Figure S6. S6

7 Figure S6 Argon 2p XPS surveys taken from samples one with VO 2 grown on SiO 2 and the other bare SiO 2, etched for the same duration. Peaks are similar in both cases showing that most of the Ar ions are trapped on SiO 2 surface. Simulation Analysis of Ar-Ion Beam Induced Damage on VO 2 Crystals The formation of this amorphous layer is due to the ion beam induced damage on the crystal. As the Ar-ions collide with the surface, some atoms get sputtered from the crystal and some atoms are displaced to a certain depth in the nanoplate. We used the software SRIM-2013 (Stopping and Range of Ions in Matter) by Ziegler et al., which is based on the method of binary collision approximation 4, to simulate the Ar-ion damage effect. For the simulation parameters, in order to replicate our milling process we used Ar-ions with an energy of 1 kev, targeted at 32 to the surface. Figure S7 demonstrates the ion penetration depth with respect to the number of ions that collide with the surface. It shows that only logarithmic increments in the number of ions result in similar increase in penetration depth, thus the beam induced damage on the crystal builds up at a decelerating rate. Two limiting factors can be stated about this event; first of all, the ion penetration is a strong function of the S7

8 energy of ions, therefore their relatively low energy in our experiments leaves a small amount of damaged surface film on the crystal. Secondly, even as the penetration depth grows, the sputtering of surface atoms from the amorphous film reduces its thickness, as stated in the main text as well. Figure S7 SRIM results showing the ion damage on the crystal with increasing number of ions. Additionally, the simulation results that show the evolution of the ion distribution in the crystal is given in Figure S8. The y-axis of the plots correspond to the ratio of target density to the ion dose (i.e. the fluence). Notice that even after a short time the Ar-ion bombardment begins, the majority of the ions get concentrated at 1.7 nm in the crystal, where most of the damage is done. It can be inferred that the oxygenpoor Magneli phases should be located here. As the ion damage is reduced S8

9 exponentially through the crystal, the amorphous film may show VO and VO 2 at these places. S9

10 Figure S8 Evolution of the Ar-ion distribution in the VO 2 crystal. Each image corresponds to an ion number in increasing order from Figure S7. References 1 A. Mansingh, R. Singh, Phys. Status Solidi A 1978, 49, Z. Yang, C. Ko, V. Balakrishnan, G. Gopalakrishnan, S. Ramanathan, Phys. Rev. B 2010, 82, H. Paik, J. A. Moyer, T. Spila, J. W. Tashman, J. A. Mundy, E. Freeman, N. Shukla, J. M. Lapano, R. Engel-Herbert, W. Zander, J. Schubert, D. A. Muller, S. Datta, P. Schiffer, D. G. Schlom, Appl. Phys. Lett. 2015, 107, Ziegler, J. F.; Ziegler, M.; Biersack, J. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 2010, 268 (11-12), S10