We present the first comprehensive broadband optical spectroscopy data on two insulating phases of vanadium dioxide (VO 2 ): monoclinic M 2 and triclinic. The main result of our work is that the energy gap and the electronic structure are essentially unaltered by the first-order structural phase transition between the M 2 and triclinic phases. Moreover, the optical interband features in the M 2 and triclinic phases are remarkably similar to those observed in the well-studied monoclinic M 1 insulating phase of VO 2 . As the energy gap is insensitive to the different lattice structures of the three insulating phases, we rule out vanadium-vanadium pairing (the Peierls component) as the dominant contributor to the opening of the gap. Rather, the energy gap arises primarily from intra-atomic Coulomb correlations.

The characteristics of the voltage-induced metal insulator transition (MIT) of VO2 film devices are investigated as a function of ambient temperature and length. At the onset of voltage-induced MIT, an abrupt formation of a conduction channel is observed within the insulating phase. The carrier density of the device varies with ambient temperature (TA) and device length (L) across MIT. As the device length is reduced, a statistically random appearance of the conduction channel is observed. Our results suggest that the primary operation principles of the VO2 device can be chosen between Joule heating effect and the electric field effect.

We report the characteristics of a voltage-induced metal-insulator transition (MIT) in macro-sized VO2 crystals. The square of MIT onset voltage (VCMIT2) value shows a linear dependence with the ambient temperature, suggesting that the Joule heating effect is the likely cause to the voltage-induced MIT. The combination of optical microscope images and Laue microdiffraction patterns show the simultaneous presence of a metallic phase in the bulk of the crystal with partially insulating surface layers even after the MIT occurs. A large asymmetry in the heating power just before and after the MIT reflects the sudden exchange of Joule heat to its environment.

Vanadium dioxide undergoes a first order metalinsulator transition (MIT) from the high temperature metallic rutile (R) phase to an insulating monoclinic (M1) phase at a commercially accessible temperature. Two separate mechanisms have been proposed to explain the nature of the MIT: the Mott and Peierls mechanisms [1][2][3][4][5]. Electron-electron interactions drive the MIT according to the Mott mechanism [3, 5] while the Peierls mechanism explains the transition in terms of electron -lattice interactions [1]. Between these two competing models, understanding the intermediate insulating M2 phase has been the most critical issue. The M2 phase of VO 2 is an electronic insulator despite band structure calculations suggesting that undimerized V atoms in the M2 phase should lead to conducting states [3]. For this reason, the structural properties of the M2 phase prepared by applying stress or through Cr doping have been thoroughly investigated [6].In recent years, with advances in nanoscale crystal synthesis, single domain VO 2 nanocrystals (nanorods) have been prepared, leading to new insights into the intrinsic properties of VO 2 [7-9]. Previously, most VO 2 samples were prepared as thin films on foreign substrate. Nonuniform stress from substrate often resulted in multiple domain boundaries. It is often pointed out that the lack of extended single domains in thin film VO 2 prevents experimental determination of the intrinsic origin of the MIT [9]. On the other hand, VO 2 nanocrystals are free from these extrinsic factors and exhibit intrinsic VO 2 properties with single domains. Recently, Wu's group synthesized free-standing, single-crystal nanomaterials of VO 2 and presented the stress-temperature phase diagram of these nanocrystals [8]. In their report, the structural phase transition between monoclinic M1, M2 and rutile R phases is controlled with applied stress.In this Letter, we report the growth of pristine VO 2 single crystals using self-flux evaporation. A new structural phase transition (SPT) is observed in these macro-sized VO 2 single crystals. Compared to previously reported VO 2 VO 2 single crystals with unprecedented quality, exhibiting a first-order metal -insulator transition (MIT) at 67.8 °C and an insulatorinsulator transition (IIT) at ~49 °C, are grown using a self-flux evaporation method. Using synchrotronbased X-ray microdiffraction analysis, it is shown that the IIT is related to a structural phase transition (SPT) from the monoclinic M2 phase to the M1 phase upon heating while the MIT occurs together with a SPT of M1 to the rutile R phase. All previous reports have shown that VO 2 exists in the M1 phase at room temperature in contrast to the M2 phase observed in this work. We suggest that internal strain inside single crystal VO 2 may generate the previously unobserved IIT and the unusual room temperature structure.

We investigate the control of two important parameters in VO2 crystals -the temperature and speed of phase transition -by varying the crystal size. By decreasing the width of the square cylinder-shaped microcrystals from ~80 to ~1 μm, the phase transition temperature varies as much as 26.1 °C (19.7 °C) during heating (cooling) while the phase transition speed increases by a factor of 37 from 4.6 × 10 2 to 1.7 × 10 4 μm/s. The interplay between phase transition temperature and size of VO2 microcrystals can be explained through the statistical behavior of first-order phase transition and size dependent thermal dissipation effect.