Changwoo Park

Role of joule heating effect and bulk-surface phases in voltage-driven metal-insulator transition in VO2 crystal

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.

Investigation of length-dependent characteristics of the voltage-induced metal insulator transition in VO2 film devices

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.

Controlling the Temperature and Speed of the Phase Transition of VO2 Microcrystals

We investigated the control of two important parameters of vanadium dioxide (VO2) microcrystals, the phase transition temperature and speed, by varying microcrystal width. By using the reflectivity change between insulating and metallic phases, phase transition temperature is measured by optical microscopy. As the width of square cylinder-shaped microcrystals decreases from ∼70 to ∼1 μm, the phase transition temperature (67 °C for bulk) varied as much as 26.1 °C (19.7 °C) during heating (cooling). In addition, the propagation speed of phase boundary in the microcrystal, i.e., phase transition speed, is monitored at the onset of phase transition by using the high-speed resistance measurement. The phase transition speed increases from 4.6 × 10(2) to 1.7 × 10(4) μm/s as the width decreases from ∼50 to ∼2 μm. While the statistical description for a heterogeneous nucleation process explains the size dependence on phase transition temperature of VO2, the increase of effective thermal exchange process is responsible for the enhancement of phase transition speed of small VO2 microcrystals. Our findings not only enhance the understanding of VO2 intrinsic properties but also contribute to the development of innovative electronic devices.

Investigation on onset voltage and conduction channel temperature in voltage-induced metal-insulator transition of vanadium dioxide

The characteristics of onset voltages and conduction channel temperatures in the metal-insulator transition (MIT) of vanadium dioxide (VO2) devices are investigated as a function of dimensions and ambient temperature. The MIT onset voltage varies from 18 V to 199 V as the device length increases from 5 to 80 μm at a fixed width of 100 μm. The estimated temperature at local conduction channel increases from 110 to 370 °C, which is higher than the MIT temperature (67 °C) of VO2. A simple Joule-heating model is employed to explain voltage-induced MIT as well as to estimate temperatures of conduction channel appearing after MIT in various-sized devices. Our findings on VO2 can be applied to micro- to nano-size tunable heating devices, e.g., microscale scanning thermal cantilevers and gas sensors.

Metal insulator transition characteristics of macro-size single domain VO2 crystals

The metal insulator transition (MIT) characteristics of macro-size single-domain VO2 crystal were investigated. At the MIT, the VO2 crystal exhibited a rectangular shape hysteresis curve, a large change in resistance between the insulating and the metallic phases, in the order of ∼105, and a small transition width (i.e. temperature difference before and after MIT) as small as 10−3°C. These MIT characteristics of the VO2 crystals are discussed in terms of phase boundary motion and the possibility of controlling the speed of the phase boundary, with change in size of crystal, is suggested.

Characterizing EEG during mental activity using non-linear measures: the more concentration, the higher correlation dimension

We have investigated a method of discriminating EEG changes during emotional and cognitive states in healthy volunteers who responded to three designed states, in which the subjects were relaxing with ease with closed eyes, listening to music and computing a simple subtraction with closed eyes. Especially, we estimated EEG dimensional complexity by Skinners point-wise correlation dimension (PD2) method for each mental state. It was found that the subjects, who responded that they had concentrated more during the arithmetic task, show higher PD2 in their non-linear EEG measures, in comparison with the subjects who responded that they had not concentrated during the task. This high PD2 is also significant in statistical analysis. A subject who had the highest score in evaluating the degree of induced emotion during emotional task shows significantly lower PD2 in statistical analysis than other subjects who had low scores. For these results, linear spectral analyses were performed; however, they did not show any significant difference. Only non-linear dynamical analysis shows the significant difference of the mental states.