Phase coexistence and pressure-temperature phase evolution of VO2(A) nanorods near the semiconductor-semiconductor transition (original) (raw)
Related papers
Nano Letters, 2010
The ability to synthesize VO 2 in the form of single-crystalline nanobeams and nano-and microcrystals uncovered a number of previously unknown aspects of the metal-insulator transition (MIT) in this oxide. In particular, several reports demonstrated that the MIT can proceed through competition between two monoclinic (insulating) phases M1 and M2 and the tetragonal (metallic) R phase under influence of strain. The nature of such phase behavior has been not identified. Here we show that the competition between M1 and M2 phases is purely lattice-symmetry-driven. Within the framework of the Ginzburg-Landau formalism, both M phases correspond to different directions of the same four-component structural order parameter, and as a consequence, the M2 phase can appear under a small perturbation of the M1 structure such as doping or stress. We analyze the strain-controlled phase diagram of VO 2 in the vicinity of the R-M2-M1 triple point using the Ginzburg-Landau formalism and identify and experimentally verify the pathways for strain-control of the transition. These insights open the door toward more systematic approaches to synthesis of VO 2 nanostructures in desired phase states and to use of external fields in the control of the VO 2 phase states. Additionally, we report observation of the triclinic T phase at the heterophase domain boundaries in strained quasi-two-dimensional VO 2 nanoplatelets, and theoretically predict phases that have not been previously observed.
High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films
ACS Applied Electronic Materials
Vanadium dioxide (VO 2) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long and short range elastic distortions, as well as the symmetry change, and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue towards mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO 2 while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO 2-metal contact is observed 1
Evidence of pressure induced compressibility enhancement in pure and Cr-doped vanadium dioxide
2011
We present structural studies of V1−xCrxO2 (pure, 0.7% and 2.5% Cr doped) compounds at room temperature in a diamond anvil cell for pressures up to 20 GPa using synchrotron x-ray powder diffraction. All the samples studied show a persistence of the monoclinic M1 symmetry between 4 and 12 GPa. Above 12 GPa, the monoclinic M1 symmetry changes to isostructural Mx phase (space group P 21/c) with a significant anisotropy in lattice compression of the b-c plane of the M1 phase.
Evidence of a Pressure-Induced Metallization Process in Monoclinic VO2
Physical Review Letters, 2007
Raman and combined trasmission and reflectivity mid infrared measurements have been carried out on monoclinic VO2 at room temperature over the 0-19 GPa and 0-14 GPa pressure ranges, respectively. The pressure dependence obtained for both lattice dynamics and optical gap shows a remarkable stability of the system up to P*∼10 GPa. Evidence of subtle modifications of V ion arrangements within the monoclinic lattice together with the onset of a metallization process via band gap filling are observed for P>P*. Differently from ambient pressure, where the VO2 metal phase is found only in conjunction with the rutile structure above 340 K, a new room temperature metallic phase coupled to a monoclinic structure appears accessible in the high pressure regime, thus opening to new important queries on the physics of VO2.
Physical Review B, 2016
Recent experiments have revealed an intriguing pressure-induced isostructural transition of the low temperature monoclinic VO 2 and hinted to the existence of a new metallization mechanism in this system. The physics behind this isostructural phase transition and the metallization remains unresolved. In this work, we show that the isostructural transition is a result of pressure-induced instability of a phonon mode that relates to a CaCl 2-type of rotation of the oxygen octahedra which alleviates, but does not completely remove, the dimerization and zigzagging arrangement of V atoms in the M1 phase. This phonon mode shows an increasing softening with pressure, ultimately leading to an isostructural phase transition characterized by the degree of the rotation of the oxygen octahedra. We also find that this phase transition is accompanied by an anisotropic compression, in excellent agreement with experiments. More interestingly, in addition to the experimentally identified M1' phase, we find a closely related M1" phase which is nearly degenerate with the M1' phase. Unlike the M1' phase which has a nearly pressure-independent electronic band gap, the gap of the M1" drops quickly at high pressures and vanishes at a theoretical pressure of about 40 GPa.
Pressure-induced phase transitions and metallization in VO 2
Physical Review B, 2015
We report the results of pressure-induced phase transitions and metallization in VO 2 based on synchrotron x-ray diffraction, electrical resistivity, and Raman spectroscopy. Our isothermal compression experiments at room temperature and 383 K show that the room temperature monoclinic phase (M1, P 2 1 /c) and the high-temperature rutile phase (R, P 4 2 /mnm) of VO 2 undergo phase transitions to a distorted M1 monoclinic phase (M1 , P 2 1 /c) above 13.0 GPa and to an orthorhombic phase (CaCl 2 -like, P nnm) above 13.7 GPa, respectively. Upon further compression, both high-pressure phases transform into a new phase (phase X) above 34.3 and 38.3 GPa at room temperature and 383 K, respectively. The room temperature M1-M1 phase transition structurally resembles the R-CaCl 2 phase transition at 383 K, suggesting a second-order displacive type of transition. Contrary to previous studies, our electrical resistivity results, Raman measurements, as well as ab initio calculations indicate that the new phase X, rather than the M1 phase, is responsible for the metallization under pressure. The metallization mechanism is discussed based on the proposed crystal structure.
Extended Mapping and Exploration of the Vanadium Dioxide Stress-Temperature Phase Diagram
Nano Letters, 2010
Single-crystal micro-and nanomaterials often exhibit higher yield strength than their bulk counterparts. This enhancement is widely recognized in structural materials but is rarely exploited to probe fundamental physics of electronic materials. Vanadium dioxide exhibits coupled electronic and structural phase transitions that involve different structures existing at different strain states. Full understanding of the driving mechanism of these coupled transitions necessitates concurrent structural and electrical measurements over a wide phase space. Taking advantages of the superior mechanical property of micro/nanocrystals of VO 2 , we map and explore its stress-temperature phase diagram over a phase space that is more than an order of magnitude broader than previously attained. New structural and electronic aspects were observed crossing phase boundaries at high-strain states. Our work shows that the actively tuning strain in micro/nanoscale electronic materials provides an effective route to investigate their fundamental properties beyond what can be accessed in their bulk counterpart.
Physical Review B, 2012
In addition to its metal-insulator transition (MIT), VO 2 exhibits a rich phase behavior of insulating monoclinic (M1,M2) and triclinic (T) phases. By using micro-Raman spectroscopy and independent control of temperature and uniaxial strain in individual single-crystal microbeams, we map these insulating phases with their associated structural changes as represented by their respective phonon frequencies. The competition between these structural forms is dictated by the internal strain due to differing lattice constants, the experimentally applied external strain, and the temperature-dependent phase stability. We identify the nature of the triclinic phase as a continuously distorted variant of the M1 monoclinic phase, while a discontinuous transition into the M2 phase occurs from both the M1 and T phases. The results suggest that understanding the driving forces that determine the interplay between M1, M2, and T phases near the MIT could be critical for the identification of the underlying mechanism behind the MIT itself.