Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films.


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1. The temperature-dependent XRD θ-2θ scans of 60 nm (001)-VO 2 /TiO 2 Figure S1 shows the temperature-dependent XRD θ-2θ scans of 60 nm (001)-VO 2 /TiO 2 and the obvious peak shifts at low (~30 o C) and high temperature (~90 o C) are observed. Based on this result, the lattice constant c was calculated and shown in Figure 3 in the main text. The lattice constant c has a sharp jump across the MIT, indicating an existing structural phase transition in the thicker films.

Figure S1
The temperature-dependent XRD θ-2θ scans of 60 nm (001)-VO 2 /TiO 2 . Figure S2 shows the R-T curve and its differential curve of 60 nm (001)-VO 2 /TiO 2 . It can be seen from Figure S2 that the resistance jumps sharply about four orders at about 65 °C across the MIT, which indicates the high quality of the VO 2 film grown on the TiO 2 substrate by magnetron sputtering techniques. From the inset of differential curve, the MIT ranges from 50 °C to 65 °C, which was also labeled in Figure 3 in the manuscript.

Figure S2
The R-T curve and its differential curve of 60 nm (001)-VO 2 /TiO 2 . S4 3. (001)-VO 2 /TiO 2 epitaxial films both at heating and cooling process. Figure S3 shows the complete temperature-dependent Raman spectrum of 13-nm-thick (001)-VO 2 /TiO 2 epitaxial film both at heating and cooling process. There are no strong sharp peaks belong to the monoclinic VO 2 phase and the 358 cm -1 and 413 cm -1 Raman peaks belong to tetragonal VO 2 phase are existed both at low and high temperature. Furthermore, the positions and intensities of them are not changed anymore, which suggests that the structural phase transition should be absent across the MIT both at heating and cooling process.

Figure S3
The complete temperature-dependent Raman spectrum of (001)-VO 2 /TiO 2 epitaxial films in 13-nm thick both at (a) heating and (b) cooling process.

The temperature-dependent Raman spectrum of bare TiO 2 substrate.
For comparison, Figure S4 shows the temperature-dependent Raman spectrum of bare (001)-TiO 2 substrate and the characteristic peaks (140 cm -1 , 242 cm -1 , 446 cm -1 and 609 cm -1 ) are labeled. It can be seen that the Raman peaks are not changed no matter for the positions or intensities. This result is useful for eliminating the substrate effect on the Raman peaks for the (001)-VO 2 /TiO 2 epitaxial thin films.

Figure S4
The temperature-dependent Raman spectrum of bare TiO 2 substrate. S6 5. The normalized intensities of V 3d electron state at Fermi energy E F as a function of temperature. Figure S5 shows the temperature-dependent intensities of V 3d electron state at Fermi energy E F . The intensities were normalized to the integrated intensity from Binding energy from -0.2 to 0.5 eV as shown in Figure 5b in our manuscript. The intensities shows a change across MIT, which implies that the VO 2 should undergo an electronic phase transition across the MIT, which was well consistent with the previous studies (see references in the main text). This may further verifies that the electronic phase transition should lead to the occurrence of the MIT of the strained 13-nm (001)-VO 2 /TiO 2 epitaxial thin films. We adopted DFT+U method to study the DOS spectra in the 13-nm VO 2 film in the present work. The crystal structure was determined by our experiments and did not change for the cases of U=0.0 eV and U=4.5 eV. This assumption was intended to show the dominant role of electron-electron correlation. Here, we adopted the generalized gradient approximation (GGA) for the exchange correlation along with double-ζ-double polarized basis set for the electron wave function. The computed projected density of states (PDOS) is shown in Figure S6. It is seen that the DOS spectra with U=0.0 eV that the VO 2 was metallic due to the electron occupancy at Fermi energy E F in Figure S6 (a). On the other hand, the DOS spectra with U=4.5 eV in Figure S6 (b) demonstrated the VO 2 was insulating state because there was a band gap of 0.606 eV near Fermi energy E F . The band gap (~0.606 eV) with U=4.5 eV, agrees well with the orbital distributions theoretically calculated by Quackenbush et al. [ Nano Lett. 2013, 13, 4857. ] and Gabriel Kotliar et al. [Phys. Rev. B 2010, 81, 115117.] Moreover, the open band gap is in excellent agreement with the experimental results of 0.6 V. [Rev. B 2013, 87, 115121. Phys. Rev. B 2013.] Therefore, with the addition of electron-electron correlation interaction U term (~4.5 eV), the V d-orbitals split into a lower band and an upper band and eventually produce a band gap without changing the crystal structure. Consequently, the electron-electron correlation may induce the MIT, where the structural phase transition is not the mandatory requirement for the MIT. Figure S6 The calculated DOS spectra of with (a) U=0.0 eV and (b) U=4.5 eV.

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S9 7. The temperature-dependent XRD of the 24-nm-thick VO 2 /TiO 2 film. Figure S7(a) and (b) shows the temperature-dependent XRD θ-2θ scans of 24-nm (001)-VO 2 /TiO 2 and there is no obvious peak shifts during both heating and cooling process. Moreover, in Figure S7(c), the lattice constant c has no jumping behavior and has a linear relationship with respect to the temperature. These results indicate that there is also no structural phase transition in the 24-nm-thick (001)-VO 2 /TiO 2 film.
Comparing the XRD results of the 13-nm and 60-nm VO 2 films, we can conclude that the VO 2 films thinner than the critical thickness (~26.5 nm) may have no structural phase transition across the MIT. S10 Figure S7 The temperature-dependent XRD θ-2θ scans of (002) peaks of the 24-nm-thick VO 2 /TiO 2 film. The heating and cooling processes are shown in (a) and (b), respectively. (c) The lattice constant c is as function of the temperature. The dotted lines show that region of the MIT. S11 8. The temperature-dependent Raman spectrum of the 24-nm-thick VO 2 /TiO 2 film. Figure S8 shows the complete temperature-dependent Raman spectrum of the 24-nm-thick (001)-VO 2 /TiO 2 epitaxial film during MIT. There are no strong sharp Figure S8 The temperature-dependent Raman spectrum of the 24-nm-thick VO 2 /TiO 2 film. peaks belong to the monoclinic VO 2 phase and the 358 cm -1 and 413 cm -1 Raman peaks belong to tetragonal VO 2 phase are existed both at low and high temperature.
This suggests that the structural phase transition should be absent across the MIT.
Comparing the Raman spectrum of the 13-nm and 60-nm VO 2 films, we can conclude that the VO 2 films thinner than the critical thickness (~26.5 nm) maintain tetragonal phase even at room temperature by epitaxial strain.