Metal-semiconductor domain configurations during switching of VO2single crystals

Sliding twin-domains in self-heated needle-like VO2 single crystals

The prototypical metal-insulator transition in VO₂ at 340 K is from a high-temperature rutile phase to a low-temperature monoclinic phase. The lower symmetry of the monoclinic structure removes the degeneracy of the two equivalent directions of the tetragonal structure, giving rise to twin domains. Since formation of domain walls require energy most needle-like monoclinic single crystal are single-domain. The mixed metal-insulator state in self-heated needle-like single crystals exhibits various domain patterns, the most remarkable being static insulating triangular domains embedded in the metal and narrow insulating domains sliding along the metallic background in the direction of the electric current. Reported here are results obtained for some rare needle-like twinned VO₂ single crystals. Such sample revealed a unique feature: joint static triangular twins emit sliding twin domains, first overlapping and later disjoining. Dark and bright twins and dim metallic background were seen for optimal orientation under a microscope, due to polarization by reflection.

Switching VO₂ Single Crystals and Related Phenomena: Sliding Domains and Crack Formation

VO₂ is the prototype material for insulator–metal transition (IMT). Its transition at T_IMT = 340 K is fast and consists of a large resistance jump (up to approximately five orders of magnitude), a large change in its optical properties in the visible range, and symmetry change from monoclinic to tetragonal (expansion by 1% along the tetragonal c-axis and 0.5% contraction in the perpendicular direction). It is a candidate for potential applications such as smart windows, fast optoelectronic switches, and field-effect transistors. The change in optical properties at the IMT allows distinguishing between the insulating and the metallic phases in the mixed state. Static or dynamic domain patterns in the mixed-state of self-heated single crystals during electric-field induced switching are in strong contrast with the percolative nature of the mixed state in switching VO₂ films. The most impressive effect—so far unique to VO₂—is the sliding of narrow semiconducting domains within a metallic background in the positive sense of the electric current. Here we show images from videos obtained using optical microscopy for sliding domains along VO₂ needles and confirm a relation suggested in the past for their velocity. We also show images for the disturbing damage induced by the structural changes in switching VO₂ crystals obtained for only a few current–voltage cycles.

Electromechanical actuation and current-induced metastable states in suspended single-crystalline VO₂ nanoplatelets

Current-induced electromechanical actuation enabled by the metal-insulator transition in VO(2) nanoplatelets is demonstrated. The Joule heating by a sufficient current flowing through suspended nanoplatelets results in formation of heterophase domain patterns and is accompanied by nanoplatelet deformation. The actuation action can be achieved in a wide temperature range below the bulk phase transition temperature (68 °C). The observed current-sustained heterophase domain structures should be interpreted as distinct metastable states in free-standing and end-clamped VO(2) samples. We analyze the main prerequisites for the realization of a current-controlled actuator based on the proposed concept.

Thermoelectric effect across the metal-insulator domain walls in VO2 microbeams

We report on measurements of Seebeck effect in single-crystal VO(2) microbeams across their metal-insulator phase transition. One-dimensionally aligned metal-insulator domain walls were reversibly created and eliminated along single VO(2) beams by varying temperature, which allows for accurate extraction of the net contribution to the Seebeck effect from these domain walls. We observed significantly lower Seebeck coefficient in the metal-insulator coexisting regime than predicted by a linear combination of contributions from the insulator and metal domains. This indicates that the net contribution of the domain walls has an opposite sign from that of the insulator and metal phases separately. Possible origins that may be responsible for this unexpected effect were discussed in the context of complications in this correlated electron material.

New aspects of the metal-insulator transition in single-domain vanadium dioxide nanobeams

Many strongly correlated electronic materials have a domain structure that greatly influences the bulk properties and obscures the fundamental properties of the homogeneous material. Nanoscale samples, on the other hand, can be smaller than the characteristic domain size, thus making it possible to explore these fundamental properties in detail. Here, we report new aspects of the metal-insulator transition, studied in single-domain vanadium dioxide nanobeams. We have observed supercooling of the metallic phase by 50 degrees C, an activation energy in the insulating phase that is consistent with the optical gap, and a connection between the metal-insulator transition and the equilibrium carrier density in the insulating phase. Our devices also provide a nanomechanical method for determining the transition temperature, enable measurements on individual metal-insulator interphase walls to be made, and allow general investigations of phase transitions in quasi-one-dimensional geometries.

Current-driven phase oscillation and domain-wall propagation in WxV1-xO2 nanobeams

We report the observation of a current-driven metal (M)-insulator (I) phase oscillation in two-terminal devices incorporating individual WxV1-xO2 nanobeams connected to parallel shunt capacitors. The frequency of the phase oscillation reaches above 5 MHz for approximately 1 mum long devices. The M-I phase oscillation, which coincides with the charging/discharging of the capacitor, occurs through the axial drift of a single M-I domain wall driven by Joule heating and the Peltier effect.

Strain-induced self organization of metal-insulator domains in single-crystalline VO2 nanobeams

We investigated the effect of substrate-induced strain on the metal-insulator transition (MIT) in single-crystalline VO(2) nanobeams. A simple nanobeam-substrate adhesion leads to uniaxial strain along the nanobeam length because of the nanobeam's unique morphology. The strain changes the relative stability of the metal (M) and insulator (I) phases and leads to spontaneous formation of periodic, alternating M-I domain patterns during the MIT. The spatial periodicity of the M-I domains can be modified by changing the nanobeam thickness and the Young's modulus of the substrate.