• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.197.1-197.1
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• 2014

VO2 has intensively investigated for several decades due to its interesting physical properties, including metal-insulator transition (MIT), thermochromic and thermoelectric properties, near the room temperature. And also gas and photo sensing properties of VO2 nanowires have attracted increasing research interest due to the high sensitivity and multi-sensing capability. We studied the light-induced resistance change of VO2 nanowires. In particular, we have investigated plasmonic enhancement of the photo-sensing properties of the VO2 nanowires. To select proper wavelength, we performed finite-difference time-domain simulations of electric field distribution in the VO2 nanowires attached with Ag nanoparticles. Localized surface plasmon resonance (LSPR) is expected at wavelength of 560 nm. The photo-sensitivity was carefully examined as a function of the sample temperature. In the presentation, we will discuss physical origins of the photo-induced resistance change in VO2.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1988.10a
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• pp.33-35
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• 1988

From the study of mechanism of electrical conduction of film which is made from Polyethylene Terephthalate at very high temperature which is larger than low electric field and glass transition point, we find that there is a extraordinary non ohmic region (I∝V$^n$, 0

• Journal of information and communication convergence engineering
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• v.1 no.1
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• pp.31-34
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• 2003

Sol-gel processing of $BaTiO_{3}$ ceramics doped with La (0.01∼1.00 at.%) were prepared from metal barium, titanium n-butoxide and lanthanum isopropoxide. Characterization of the sol-gel-derived powder using XRD, SEM is also reported. The obtained results suggested that insulator to semiconductor transition for highly donor-doped barium titanate was closely related to the incorporation of donor into the grains and to the resultant grain size, which were significantly affected by the sinterability of $BaTiO_{3}$ powders and sintering conditions used.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2003.05c
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• pp.287-290
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• 2003

Sol-gel processing of $BaTiO_3$ ceramics doped with La(0.01~1.00 at.%) were prepared from metal barium, titanium n-butoxide and lanthanum iso-propoxide. Characterization of the sol-gel-derived powder using XRD, SEM is also reported. The obtained results showed that insulator to semiconductor transition for highly donor-doped barium titanate was closely related to the incorporation of donor into the grains and to the resultant grain size, which were significantly affected by the sinterability of $BaTiO_3$ powders and sintering conditions used.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.11a
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• pp.241-242
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• 2008

The Velocity Autocorrelation Function (VAF) of the sodium ions is calculated for a range of temperature from 250K to 1000K and converted into the linear ac-conductivity and ac-susceptibility response via Fourier transformation. A peak is found in the conductivity around $6\times10^{12}$ Hz that has some of the character of a Poley absorption. Here it is shown to be due to an harmonically coupled site vibrations of the sodium atoms, which extend only over a limited range. At frequencies below the peak the conductivity tends towards a constant i.e. dc value corresponding to a constant flow of ions through the simulation cell. At high temperatures the conductivity due to this ion transport process behaves like a metal with an insulator to metal transition occurring around a specific temperature.

• Proceedings of the KIEE Conference
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• 2008.07a
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• pp.2299-2300
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• 2008

The Velocity Autocorrelation Function (VAF) of the sodium ions is calculated for a range of temperature from 250K to 1000K and converted into the linear ac-conductivity and ac-susceptibility response via Fourier transformation. A peak is found in the conductivity around $6{\times}10^{12}$ Hz that has some of the character of a Poley absorption. Here it is shown to be due to an harmonically coupled site vibrations of the sodium atoms, which extend only over a limited range. At frequencies below the peak the conductivity tends towards a constant i.e. dc value corresponding to a constant flow of ions through the simulation cell. At high temperatures the conductivity due to this ion transport process behaves like a metal with an insulator to metal transition occurring around a specific temperature.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.379-380
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• 2008

Molecular Dynamics (MD) simulations have been used to model the dynamics of the charge-compensating sodium ions in the non-stoichiometric hollandite Nax$(Ti_{8-x}Cr_x)O_{16}$. These interstitial ions reside in 'tunnels' in the crystal structure and move under the forces of both the ions making up the cage structure and the many body interactions of the other sodium ions in the tunnel. The Velocity Autocorrelation Function (VAF) of the sodium ions is calculated for a range of temperature from 250K to 1000K and converted into the linear ac-conductivity and ac-susceptibility response via Fourier transformation. A peak is found in the conductivity around $6\times10^{12}$ Hz that has some of the character of a Poley absorption. Here it is shown to be due to an harmonically coupled site vibrations of the sodium atoms, which extend only over a limited range. At frequencies below the peak the conductivity tends towards a constant i.e. dc value corresponding to a constant flow of ions through the simulation cell. At high temperatures the conductivity due to this ion transport process behaves like a metal with an insulator to metal transition occurring around a specific temperature.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.361.2-361.2
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• 2014

Transition metal dichalcogenides (MoS2, WS2, WSe2, MoSe2, NbS2, NbSe2, etc.) are layered materials that can exhibit semiconducting, metallic and even superconducting behavior. In the bulk form, the semiconducting phases (MoS2, WS2, WSe2, MoSe2) have an indirect band gap. Recently, these layered systems have attracted a great deal of attention mainly due to their complementary electronic properties when compared to other two-dimensional materials, such as graphene (a semimetal) and boron nitride (an insulator). However, these bulk properties could be significantly modified when the system becomes mono-layered; the indirect band gap becomes direct. Such changes in the band structure when reducing the thickness of a WS2 film have important implications for the development of novel applications, such as valleytronics. In this work, we report for the controlled synthesis of large-area (~cm2) single-, bi-, and few-layer WS2 using a two-step process. WOx thin films were deposited onto a Si/SiO2 substrate, and these films were then sulfurized under vacuum in a second step occurring at high temperatures ($750^{\circ}C$). Furthermore, we have developed an efficient route to transfer these WS2 films onto different substrates, using concentrated HF. WS2 films of different thicknesses have been analyzed by optical microscopy, Raman spectroscopy, and high-resolution transmission electron microscopy.

• Korean Journal of Materials Research
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• v.19 no.4
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• pp.186-191
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• 2009

Thermoelectric power and resistivity are measured for the perovskite $LaNi_{1-x}Ti_xO_3$ ($x{\leq}0.5$) in the temperature range 77 K - 300 K. The measured thermoelectric power of $LaNi_{1-x}Ti_xO_3$ ($x{\leq}0.5$) increases linearly with temperature and is represented by A + BT. The x = 0.1 sample showed metallic behavior, the x = 0.3 showed metal and insulating transition around 150 K, and x = 0.5 showed insulating behavior the over the whole temperature range. The electrical resistivity of x = 0.1 shows linear temperature dependence over the whole temperature range and $T^2$ dependence. On the other hand, the electrical resistivity of x = 0.3 shows a linear relation between $ln{\rho}$ and $T^{-1/4}$ (variable range hopping mechanism) in the range of 77 K to 150 K. For x = 0.5, the temperature dependence of resistivity is characteristic of insulating materials; the resistivity data was fitted to an exponential law, such as ln(${\rho}/T$) and $T^{-1}$, which is usually attributed to a small polaron hopping mechanism. These experimental results are interpreted in terms of the spin polaron (x = 0.1) and variable range hopping (x = 0.3) or small polaron hopping (x = 0.5) of an almost localized $Ni^{3+}$ 3d polaron.

• Korean Journal of Materials Research
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• v.20 no.3
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• pp.137-141
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• 2010

The $(1-x)La_{0.7}Sr_{0.3}MnO_3(LSMO)/xZnFe_2O_4$(ZFO) (x = 0, 0.01, 0.03, 0.06 and 0.09) composites were prepared by a conventional solid-state reaction method. We investigated the structural properties, magnetic properties and electrical transport properties of (1-x)LSMO/xZFO composites using X-ray diffraction (XRD), scanning electron microscopy (SEM), field-cooled dc magnetization and magnetoresistance (MR) measurements. The XRD and SEM results indicate that LSMO and ZFO coexist in the composites and the ZFO mostly segregates at the grain boundaries of LSMO, which agreed well with the results of the magnetic measurements. The resistivity of the samples increased by the increase of the ZFO doping level. A clear metal-to-insulator (M-I) transition was observed at 360K in pure LSMO. The introduction of ZFO further downshifted the transition temperature (350K-160K) while the transition disappeared in the sample (x = 0.09) and it presented insulating/semiconducting behavior in the measured temperature range (100K to 400K). The MR was measured in the presence of the 10kOe field. Compared with pure LSMO, the enhancement of low-field magnetoresistance (LFMR) was observed in the composites. It was clearly observed that the magnetoresistance effect of x = 0.03 was enhanced at room temperature range. These phenomena can be explained using the double-exchange (DE) mechanism, the grain boundary effect and the intrinsic transport properties together.