• Proceedings of the Korean Vacuum Society Conference
• /
• 2014.02a
• /
• pp.196.1-196.1
• /
• 2014

VO2 exhibits metal-insulator transition (MIT), of which critical temperature (TC) is about 340 K. There have been many reports that MIT can be induced by UV light as well as heat. Clear mechanism regarding such photo-induced MIT has not been clarified. We have compared the MIT behaviors of VO2 thin film during heating-cooling cycles with and without light. We tried several light sources with different wavelengths (red, blue, and UV). Tc and hysteresis width of the resistance change were influenced by the illumination of the samples. We performed Kelvin probe force microscopy (KPFM) studies, which can reveal the evolution of the local sample work function. In this presentation, we will discuss possible physical origins for the photo-induced effects on the MIT behaviors of the VO2 samples.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.18 no.1
• /
• pp.1-4
• /
• 2008

The growth of composite-structured Nd:$GdVO_4$ single crystal rods by the double die EFG method is reported. Two crucibles are combined with an outer and inner die for ascending of different melt. The composite-structured Nd:$GdVO_4$ single crystal rods with a length of 50 mm and an outer diameter of 5 mm including of inner Nd-doped core region with diameter 3 mm were grown successfully. Nd distribution in the, radial direction has graded profile from result of EPMA. Absorption coefficient in the core region at 808 nm was $42cm^{-1}$. Finally, we demonstrated the laser oscillation using our composite crystal and 2-W output was obtained.

• Proceedings of the KSME Conference
• /
• 2008.11b
• /
• pp.2007-2010
• /
• 2008

Vanadium pentoxide ($V_2O_5$) powder was prepared and mixed with Molybdenum Oxides ($MoM_3$) to form Mo-doped and -undoped $VO_2$ films by a sol-gel method on graphite conductive substrates. X-Ray diffraction (XRD) and scanning electron microscopy (SEM) was used to investigate the chemical compositions and microstructures of the Mo-doped and -undoped $VO_2$ films. The variation of electrical resistance was measured as a function of temperature and stoichiometric composition between vanadium and molybdenum. In this study, it was found that Mo-doped and -undoped $VO_2$ shows the typical negative temperature coefficient (NTC) behavior. As the amount of the molybdenum increases, the electrical resistance of Modoped $VO_2$ film gets reduced under the transition temperature and a linear decrease in the transition temperature is observed. From these experimental results, we can conclude that the electrical resistance behavior with temperature change of $VO_2$ films can be utilized as a self-heating source with the electrical current flowing through the graphite substrate.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2015.08a
• /
• pp.169.2-169.2
• /
• 2015

$VO_2$ is a well-known a metal-to-insulator-transition (MIT) material, accompanied with a first order structural phase transition near room temperature. Because of the structural phase transition and the MIT occur near a same temperature, there is an ongoing argument whether the MIT is induced by the structural phase transition. $VO_2$ exhibits a relatively weak anti-oxidization ability and can be oxidized to higher-valence oxides (e.g., $V_4$ $O_7$ or $V_2$ $O_5$) when annealed at a high temperature in an oxygen-rich atmosphere. We fabricated $VO_2$ films on $Al_2$ $O_3$ (0001) substrates using a DC magnetron sputtering deposition process with carefully control the $O_2$ percentage in an atmosphere. X-ray diffraction measurements from the films showed only (0l0) peaks with no extra peaks, indicating b-oriented films. The temperature-dependent local structural properties of $VO_2$ films were investigated by using in-situ X-ray absorption fine structure (XAFS) measurements at the V K edge. XAFS revealed that the structural phase transition was occurred nearly $70^{\circ}C$ for heating process and reproducible. Resistance measurements as a function of temperature (R-T) demonstrated that the resistance of $VO_2$ films was changed by a factor of 4 near $75^{\circ}C$ which was higher than $68^{\circ}C$ reported from a $VO_2$ bulk. We will discuss the MIT of $VO_2$ films, comparing with the local structural properties determined by XAFS measurements.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2012.02a
• /
• pp.572-572
• /
• 2012

Growth of metal oxides on graphene may lead to a better understanding of delicate effects of their growth habits on their underlying physics. The vanadium dioxide ($VO_2$) is well known for its metal-to-insulator transition accompanied by a reversible first order structural phase transition at 340 K. This transition makes $VO_2$ a potentially useful material for applications in electrical and optical devices. We report a successful growth of $VO_2$ nanostructures on a graphene substrate via a vapor-solid transport route. As-grown $VO_2$ nanostructures on graphene were systematically characterized by field emission scanning electron microscopy, x-ray diffraction, Raman spectroscopy, FT-IR spectroscopy and high resolution transmission electron microscopy. These results indicate that the strain between $VO_2$ and graphene layers may be easily controlled by the number of underlying graphene layer. We also found that the strain in-between $VO_2$ and graphene layer affected its metal-to-insulator transition characteristics. This study demonstrates a new way for synthesizing $VO_2$ in a desired phase on the transparent conducting graphene substrate and an easy pathway for controlling metal-to-insulator phase transition via strain.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
• /
• v.29 no.4
• /
• pp.11-17
• /
• 2015

While most switching devices are based on PN junctions, a single layer can realize a switching device in the case of vanadium dioxide($VO_2$) thin films. In this paper, bidirectional current triggering(switching) is demonstrated in a two-terminal planar device based on a $VO_2$ thin film by illuminating the film with an infrared laser at 1550nm. To begin with, a two-terminal planar device, which had a $30{\mu}m$-wide $VO_2$ conducting layer and an electrode separation of $10{\mu}m$, was fabricated. A specific bias voltage range for stable bidirectional laser triggering was experimentally obtained by measuring the current-voltage characteristics of the fabricated device in a current-controlled mode. Then, by constructing a test circuit composed of the device, a standard resistor, and a DC voltage source, connected in series, the transient response of laser-triggered current and its response time were investigated with a DC bias voltage, included in the above specific bias voltage range, applied to the device. In the test circuit with a DC voltage source of 3.35V and a $10{\Omega}$ resistor, bidirectional laser triggering could be realized with a maximum on-state current of 15mA and a switching contrast of ~78.95.

• Rapid Communication in Photoscience
• /
• v.4 no.4
• /
• pp.82-85
• /
• 2015

Photoelectrochemical cell (PEC) is one of the attractive ways to produce clean and renewable energy. However, solar to hydrogen production via PEC system generally requires high external bias, because of material's innate electronic band potential relative to hydrogen reduction potential and/or charge separation issue. For spontaneous photo-water splitting, here, we design dye-sensitized solar cell (DSSC) and their monolithic tandem cell incorporated with a $BiVO_4$ photoanode. $BiVO_4$ has high conduction band edge potential and suitable band gap (2.4eV) to absorb visible light. To achieve efficient $BiVO_4$ photoanode system, electron and hole mobility should be improved, and we demonstrate a tandem cell in which $BiVO_4/WO_3$ film is connected to cobalt complex based DSSC.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
• /
• v.29 no.11
• /
• pp.59-65
• /
• 2015

By harnessing the thermal hysteresis behavior of vanadium dioxide($VO_2$), we demonstrated multi-resistance states in a two-terminal electronic device based on a $VO_2$ thin film by using a 966nm infrared laser diode as an excitation light source for resistance modulation. Before stimulating the device using 966nm laser pulses, the thermal hysteresis behavior of the device resistance was measured by using a temperature chamber. After that, the $VO_2$ device was thermally biased at ${\sim}71.6^{\circ}C$ so that its temperature fell into the thermal hysteresis region of the device resistance. Six multi-states of the device resistance could be obtained in the fabricated $VO_2$ device by five successive laser pulses with equal 10ms duration and increasing power. Each resistance states were maintained while the temperature bias was applied. And, the resistance fluctuation level was within 2.2% of the stabilized resistance and decreased down to less than 0.9% of the stabilized resistance 5s after the illumination.

• Current Optics and Photonics
• /
• v.7 no.3
• /
• pp.254-262
• /
• 2023

Novel thermo-optically focus-switchable Fresnel zone plates based on phase-change metafilms are designed and analyzed at a visible wavelength (660 nm). By virtue of the large thermo-optic response of vanadium dioxide (VO₂) thin film, a phase-change material, four different plasmonic phase-change absorbers are numerically designed as actively tunable Gires-Tournois Al-VO₂ metafilms in two and three dimensions. The designed phase-change metafilm unit cells are used as the building blocks of actively focus-switchable Fresnel zone plates with strong focus switching contrast (40%, 83%) and high numerical apertures (1.52, 1.70). The Fresnel zone plates designed in two and three dimensions work as cylindrical and spherical lenses in reflection type, respectively. The coupling between the thermo-optic effect of VO₂ and localized plasmonic resonances in the Al nanostructures offer a large degree of freedom in design and high-contrast focus-switching performance based on largely tunable absorption resonances. The proposed method may have great potential in photothermal and electrothermal active optical devices for nonlinear optics, microscopy, 3D scanning, optical trapping, and holographic displays over a wide spectral range including the visible and infrared regimes.

• Journal of the Korean Vacuum Society
• /
• v.22 no.2
• /
• pp.98-104
• /
• 2013

$VO_2$ is an attractive thermochromic material, in which its electrical and optical properties can be switched by the structural phase-transition about $68^{\circ}C$. Recently, graphene is also a rising material which is researched as a transparent electrode because of its superior electrical and optical characteristics. In this respect, we try to fabricate the hybridized films using $VO_2$ and graphene on transparent sapphire substrate and then we investigate a structure and characterize an optical property for the samples as a function of temperature. According to the result of IR-transmittance analysis of $VO_2$ films as a function of temperature, the graphene-supported sapphire substrates are better about 10% than the bare sapphire substrates. The mean phase transition temperatures are also decreased as the number of graphene-layers increased and the hysteresis of phase transitions are narrowed.