• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.175.1-175.1
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• 2014

Despite growing interest in Ge as a possible alternative to Si, reliable data on Ge surface has been relatively scarce. Using low temperature scanning tunneling microscopy (STM), we investigate band-bending effects of localized charge traps at Ge(001) surface at 78 K. For this investigation, we prepared nearly defect-free Ge(001) surface by keeping the background pressure to < $1{\times}10^{-10}$ mbar during outgassing. Ge(001) surfaces this obtained exhibit a flat-band condition, and deposition of charge traps induce a distinct, sharp boundary between pinned and depinned surface area in the constant current mode STM images. We will show the tip-surface interaction plays an essential role in producing the boundary, and discuss about the conditions that enable the pinning effect.

• Bulletin of the Korean Chemical Society
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• v.33 no.12
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• pp.4155-4160
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• 2012

We characterized the physisorption of p-iodo-phenyl octadecyl ether molecules (I-POE) onto superlattice regions of graphite surfaces using scanning tunneling microscopy (STM). The formation of self-organized I-POE monolayers does not affect the overall structures of moir$\acute{e}$ patterns and their modulation periods. However, the packing density of the I-POE monolayer and the orientations of lamella structures were sensitive to the underlying superlattice structure. Depending on the bias voltage, the STM images selectively showed moir$\acute{e}$ pattern, I-POE layer, or both. Reflecting the local density of states at a certain energy level, the STM images thereby revealed the relative energy level scale of the superlattice with respect to the molecular orbitals of I-POE.

• Korean Chemical Engineering Research
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• v.47 no.2
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• pp.163-168
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• 2009

Negative differential resistance(NDR) behaviors of Keggin-type and Wells-Dawson-type heteropolyacids with cation, heteroatom, and polyatom substitutions were investigated by scanning tunneling microscopy. A reliable correlation between NDR peak voltage and reduction potential of heteropolyacid catalysts was established. It was found that more reducible heteropolyacid catalyst showed NDR behavior at less negative voltage, regardless of the structural difference. Thus, NDR peak voltage of heteropolyacid catalyst could be utilized as a single correlating parameter for the reduction potential of heteropolyacid catalyst.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.368-368
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• 2010

Monolayers of graphite can be grown by fine controlled surface graphitization on the surfaces of various metallic and semiconducting materials. Epitaxial graphene grown on polished silicon carbide crystal surfaces has drawn much attention due to well known vacuum annealing procedures from surface analysis methods, especially scanning tunneling microscopy(STM) and scanning tunneling spectroscopy(STS). In this study, we have grown single layer and few layer graphene on silicon terminated 6H-SiC(0001) crystals. The growth of graphene layers were observed by low energy electron diffraction(LEED) patterns. Scanning tunneling microscopy and spectroscopy measurements were performed to illustrate the electronic structure which may display some clue on the influence of the underlying structure. Spatially resolved STS results acquired at the edges of epitaxial graphene show in detail the electron density of states, which is compared to theoretical calculations. STM measurements were also done on graphene films grown by chemical vapor deposition(CVD) and transferred onto a SiC(0001) crystal. These observations may provide a hint for the understanding of carrier scattering at the edges.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.140.2-140.2
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• 2016

Despite the growing interest in GeTe as a archetypal displacive ferroelectric material as well as the basis of related materials used in data-storage applications, atom-resolved study of single crystalline GeTe surface been lacking. Using low temperature scanning tunneling microscopy (STM) and spectroscopy (STS), we investigated as-cleaved and annealed surfaces of GeTe. We found that as-cleaved GeTe(111) surface is composed of at least two kinds of terraces at 78 K. While two terraces show metallic characteristics, they also exhibit distinctive I-V spectra and imaging conditions, with each being attributed to Ge-terminated, and Te-terminated surfaces respectively. GeTe(111) surfaces annealed at moderately elevated temperature introduces intricate networks of extended defect structures. We will present these data and discuss the role of vacancies in the formation of these structures.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.125-125
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• 2012

Graphene was epitaxially grown on a 6H-SiC(0001) substrate by thermal decomposition of SiC under ultrahigh vacuum conditions. Using scanning tunneling microscopy (STM), we monitored the evolution of the graphene growth as a function of the temperature. We found that the evaporation of Si occurred dominantly from the corner of the step rather than on the terrace. A carbon-rich $(6{\sqrt{3}}{\times}6{\sqrt{3}})R30^{\circ}$ layer, monolayer graphene, and bilayer graphene were identified by measuring the roughness, step height, and atomic structures. Defect structures such as nanotubes and scattering defects on the monolayer graphene are also discussed. Furthermore, we confirmed that the Dirac points (ED) of the monolayer and bilayer graphene were clearly resolved by scanning tunneling spectroscopy (STS).

• Journal of the Korean Electrochemical Society
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• v.3 no.2
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• pp.104-108
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• 2000

It is very important to know the real roughness of electrode surface in electrochemistry. But it is impossible to know absolute roughness of electrode surface for various reasons. In this work, we compared the roughnesses of polycrystalline gold electrode often used in electrochemistry calculated from the images of scanning tunneling microscopy (STM) and cyclic voltammetry with those of Au (111) and HOPG. The roughness of polycrystalline gold calculated from STM image was $1.1(\pm0.1)$, that from adsorption-desorption of oxygen was $2.4(\pm0.7)$ and that from adsorption of N-docosyl-N'-methyl viologen was $1.6(\pm0.1)$.

• Journal of the Institute of Electronics Engineers of Korea TE
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• v.39 no.2
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• pp.29-33
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• 2002

Various nanometer-scale structures are fabricated on hydrogen-passivated p-type Si(100) surface by scanning probe microscopy(SPM). The hydrogen-passivation is performed by dipping the samples in diluted 10% HF solution for one min.. Pt alloy wires are used for tips and the tips are made by cutting the wires at 45$^{\circ}$ slanted. Various line features are fabricated in various bias voltage. The optimal structure is the line of about 30 nm width on 1.7V bias voltage and 1 nA tunneling current.  

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2003.11a
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• pp.108-111
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• 2003

Currently, molecular devices are reported utilizing active self-assembled monolayers containing the nitro group as the active component, which has active redox centers[1]. We confirm the electrical properties of 4,4-di(ethynylphenyl)-2'-nitro-1-benzenethiolate. To deposit the SAM layer onto gold electrode, we transfer the prefabricated Au(111) substrates into a 1mM self-assembly molecules in THF solution. Au(111) substrates were prepared by ion beam sputtering method of gold onto the silicon wafer. As a result, we measured current-voltage curve using ultra high vacuum scanning tunneling microscopy (UHV STM), I-V curve also clearly shows several current peaks between the negative bias region (-0.3958V) and the positive bias region (0.4658V), respectively.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.12-12
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• 2010

By using scanning tunneling microscopy/spectroscopy (STM/S), we can make images of various physical properties in nanometer-scale spatial resolutions. Here, I demonstrate imaging of two electron-correlated subjects; screening and superconductivity by STM/S. The electrostatic potential around a charge is described with the Coulomb potential. When the charge is located in a metal, the potential is modified because of the free electrons in the host. The potential modification, called screening, is one of the fundamental phenomena in the condensed matter physics. Using low-temperature STM we have developed a method to measure electrostatic potential in high spatial and energy resolutions, and observed the potential around external charges screened by two-dimensional surface electronic states. Characteristic potential decay and the Friedel oscillation were clearly observed around the charges [1]. Superconductivity of nano-size materials, whose dimensions are comparable with the coherence length, is quite different from their bulk. We investigated superconductivity of ultra-thin Pb islands by directly measuring the superconducting gaps using STM. The obtained tunneling spectra exhibit a variation of zero bias conductance (ZBC) with a magnetic field, and spatial mappings of ZBC revealed the vortex formation [2]. Size dependence of the vortex formation will be discussed at the presentation.