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Kang, Manil,Kim, Sok Won,Ryu, Ji-Wook American Institute of Physics 2015 Journal of Applied Physics Vol.118 No.3
<P>In order to reveal the electron-electron correlation (interaction) effect in a Mott insulator VO2, we measured the temperature dependence of the thermopower, Hall effect, and Raman spectra in VO2 films and extracted the diverging effective mass from the thermoelectric behavior in the metal-insulator transition (MIT) region, using a combination of the thermopower formula for two-dimensional electron system and the extended Brinkman-Rice (EBR) model explaining the correlation effect. The temperature dependence of the measured diverging thermoelectric power is closely fitted by the band-filling (rho) dependent effective mass m*/m = 1/(1-rho(4)) given in the EBR model. The diverging behavior is the effect of measurement and analyzed as the percolation phenomenon increasing the metal region with increasing temperature. The true effective mass in the metal region is regarded as constant regardless of rho. The true correlation strength kappa(BR) in the BR model deduced by extrapolation near rho = 1 is evaluated as kappa(BR) > 0.92. The Raman spectra showed decoupling between the MIT and the structural phase transition associated with lattice distortion induced by the electron-phonon interaction. Thus, we conclude the MIT in VO2 as a first-order Mott MIT induced by strong correlation regardless of the electron-phonon interaction. (C) 2015 AIP Publishing LLC.</P>
Kang, Manil,Kim, Sok Won,Ryu, Ji-Wook The Korean Vacuum Society 2014 Applied Science and Convergence Technology Vol.23 No.5
${\alpha}-V_2O_5$ nanorods were grown by means of electron beam irradiation and thermal oxidation methods and the visible emission properties of the nanorods grown by both methods were investigated. The growth and crystallinity of the nanorods were greatly enhanced by the insertion of a buffer layer. The emission spectra of the nanorods grown by thermal oxidation and electron beam irradiation showed a peak centered at 710~720 nm, which is believed to be due to oxygen vacancies introduced during the growth process. Also, the emission peak centered at 530 nm observed in the $V_2O_5$ nanorods grown by electron beam irradiation was considered to be due to the band edge transition as a result of the enhanced crystallinity.
Optical properties of TiO<sub>2</sub> thin films with crystal structure
Kang, Manil,Kim, Sok Won,Park, Hyo Yeol Elsevier 2018 The Journal of physics and chemistry of solids Vol.123 No.-
<P><B>Abstract</B></P> <P>The optical properties of TiO<SUB>2</SUB> thin films with polymorphs were investigated by spectroscopic ellipsometry (SE) and transmittance measurements. Films were prepared by RF sputtering and e-beam evaporation and post-annealing, and their microstructures were examined by SEM and XRD. The results reveal that the crystal structure of the TiO<SUB>2</SUB> film is strongly affected by the post-annealing temperature. The optical constant spectra of the films showed a gradual change above 3.5 eV. As the crystal structure of the film changes from amorphous phase to rutile phase, the extinction coefficient spectra are enhanced and shift toward lower energy. The direct band gap energies obtained by SE measurements were well matched with those obtained by transmittance measurements. The SE analysis was supported by comparison of the measured and calculated transmittances of the films, and both spectra showed good agreement over the entire spectral range.</P> <P><B>Highlights</B></P> <P> <UL> <LI> TiO<SUB>2</SUB> films were prepared by using an rf sputtering method and post-annealing. </LI> <LI> The crystal structure of TiO<SUB>2</SUB> film is strongly affected by post-annealed temperature. </LI> <LI> The optical constants of TiO<SUB>2</SUB> polymorph films showed a gradual change above 3.5 eV. </LI> <LI> The direct band-gap energies obtained by SE had higher values compared with those gained by transmittance. </LI> </UL> </P>
Visible Emission Properties of V₂O<SUB>5</SUB> Nanorods Prepared by Different Growth Methods
Manil Kang,Sok Won Kim,Ji-Wook Ryu 한국진공학회(ASCT) 2014 Applied Science and Convergence Technology Vol.23 No.5
α-V₂O? nanorods were grown by means of electron beam irradiation and thermal oxidation methods and the visible emission properties of the nanorods grown by both methods were investigated. The growth and crystallinity of the nanorods were greatly enhanced by the insertion of a buffer layer. The emission spectra of the nanorods grown by thermal oxidation and electron beam irradiation showed a peak centered at 710∼720 nm, which is believed to be due to oxygen vacancies introduced during the growth process. Also, the emission peak centered at 530 nm observed in the V₂O? nanorods grown by electron beam irradiation was considered to be due to the band edge transition as a result of the enhanced crystallinity.
정주호(Juho Jeong),강만일(Manil Kang),김석원(Sok Won Kim) 한국진공학회(ASCT) 2013 Applied Science and Convergence Technology Vol.22 No.4
본 연구에서는 sol-gel법을 이용해 제작된 VO2 박막의 광전변환 특성을 연구하기 위해 광 조사 유무에 의해 생성되는 전류를 bias voltage에 따라 온도별로 측정하였다. 그 결과, 광 조사 유무에 따라 박막에서 생성되는 전류가 변화되는 것을 알 수 있었으며, bias voltage가 증가할수록 전류가 일정하게 늘어나는 것을 알 수 있었다. 특히, 광에 의해 생성되는 전류는 VO₂에서의 MIT 온도보다 낮은 50℃에서 최대였다. 이러한 결과는 VO₂에서 photo-voltaic 효과가 발생함을 의미하며, 따라서 VO₂ 박막을 광전변환 소자로 응용할 수 있을 것으로 기대된다. In order to investigate the photo-electric property in VO₂ film grown by a sol-gel method, the currents generated by the light irradiation and nonirradiation were measured as functions of the bias voltage and the temperature. From the result, the generated current in the film changed with the light irradiation and nonirradiation, and it gradually increased with the bias voltage. In particular, the maximum current was generated at 50℃ under the light irradiation; the temperature is lower comparing the MIT (metal-insulator transition) temperature in VO₂. This result indicates that VO₂ shows the photo-voltaic effect, and so that, it is expected that the VO₂ film is applied for a photo-voltaic device.