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Electrical control of nanoscale functionalization in graphene by the scanning probe technique
Byun, Ik-Su,Kim, Wondong,Boukhvalov, Danil W.,Hwang, Inrok,Son, Jong Wan,Oh, Gwangtaek,Choi, Jin Sik,Yoon, Duhee,Cheong, Hyeonsik,Baik, Jaeyoon,Shin, Hyun-Joon,Shiu, Hung Wei,Chen, Chia-Hao,Son, Young Nature Publishing Group (NPG) ; Tokyo Institute of 2014 NPG Asia Materials Vol.6 No.-
Functionalized graphene is a versatile material that has well-known physical and chemical properties depending on functional groups and their coverage. However, selective control of functional groups on the nanoscale is hardly achievable by conventional methods utilizing chemical modifications. We demonstrate electrical control of nanoscale functionalization of graphene with the desired chemical coverage of a selective functional group by atomic force microscopy (AFM) lithography and their full recovery through moderate thermal treatments. Surprisingly, our controlled coverage of functional groups can reach 94.9% for oxygen and 49.0% for hydrogen, respectively, well beyond those achieved by conventional methods. This coverage is almost at the theoretical maximum, which is verified through scanning photoelectron microscope measurements as well as first-principles calculations. We believe that the present method is now ready to realize 'chemical pencil drawing' of atomically defined circuit devices on top of a monolayer of graphene.
Shin, Min-Su,Sekora, Michael,Byun, Yong-Ik Blackwell Publishing Ltd 2009 Monthly notices of the Royal Astronomical Society Vol.400 No.4
<P>ABSTRACT</P><P>We present a new framework to detect various types of variable objects within massive astronomical time series data. Assuming that the dominant population of objects is non-variable, we find outliers from this population by using a non-parametric Bayesian clustering algorithm based on an infinite Gaussian mixture model (GMM) and the Dirichlet process. The algorithm extracts information from a given data set, which is described by six variability indices. The GMM uses those variability indices to recover clusters that are described by six-dimensional multivariate Gaussian distributions, allowing our approach to consider the sampling pattern of time series data, systematic biases, the number of data points for each light curve and photometric quality. Using the Northern Sky Variability Survey data, we test our approach and prove that the infinite GMM is useful at detecting variable objects, while providing statistical inference estimation that suppresses false detection. The proposed approach will be effective in the exploration of future surveys such as Gaia, Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) and Large Synoptic Survey Telescope (LSST), which will produce massive time series data.</P>
EFFICIENT PERIOD SEARCH FOR TIME SERIES PHOTOMETRY
SHIN MIN-SU,BYUN YONG-IK The Korean Astronomical Society 2004 Journal of The Korean Astronomical Society Vol.37 No.2
We developed an algorithm to identify and determine periods of variable sources. With its robustness and high speed, it is expected to become an useful tool for surveys with large volume of data. This new scheme consists of an initial coarse. process of finding several candidate periods followed by a secondary process of much finer period search. With this multi-step approach, best candidates among statistically possible periods are produced without human supervision and also without any prior assumption on the nature of the variable star in question. We tested our algorithm with 381 stars taken from the ASAS survey and the result is encouraging. In about $76\%$ cases, our results are nearly identical as their published periods. Our algorithm failed to provide convincing periods for only about $10\%$ cases. For the remaining $14\%$, our results significantly differ from their periods. We show that, in many of these cases, our periods are superior and much closer to the true periods. However, the existence of failures, and also periods sometimes worse than manually controlled results, indicates that this algorithm needs further improvement. Nevertheless, the present experiment shows that this is a positive step toward a fully automated period analysis for future variability surveys.
Shin, Min-Su,Yi, Hahn,Kim, Dae-Won,Chang, Seo-Won,Byun, Yong-Ik American Institute of Physics 2012 The Astronomical journal Vol.143 No.3
<P>We present variability analysis of data from the Northern Sky Variability Survey (NSVS). Using the clustering method, which defines variable candidates as outliers from large clusters, we cluster 16,189,040 light curves having data points at more than 15 epochs as variable and non-variable candidates in 638 NSVS fields. Variable candidates are selected depending on how strongly they are separated from the largest cluster and how rarely they are grouped together in eight-dimensional space spanned by variability indices. All NSVS light curves are also cross-correlated with IRAS, AKARI, Two Micron All Sky Survey, Sloan Digital Sky Survey (SDSS), and GALEX objects, as well as known objects in the SIMBAD database. The variability analysis and cross-correlation results are provided in a public online database, which can be used to select interesting objects for further investigation. Adopting conservative selection criteria for variable candidates, we find about 1.8 million light curves as possible variable candidates in the NSVS data, corresponding to about 10% of our entire NSVS sample. Multi-wavelength colors help us find specific types of variability among the variable candidates. Moreover, we also use morphological classification from other surveys such as SDSS to suppress spurious cases caused by blending objects or extended sources due to the low angular resolution of the NSVS.</P>
Light Effects on the Bias Stability of Transparent ZnO Thin Film Transistors
Jae-Heon Shin,Ji-Su Lee,황치선,박상희,Woo-Seok Cheong,Minki Ryu,Chun-Won Byun,Jeong-Ik Lee,Hye Yong Chu 한국전자통신연구원 2009 ETRI Journal Vol.31 No.1
We report on the bias stability characteristics of transparent ZnO thin film transistors (TFTs) under visible light illumination. The transfer curve shows virtually no change under positive gate bias stress with light illumination, while it shows dramatic negative shifts under negative gate bias stress. The major mechanism of the bias stability under visible illumination of our ZnO TFTs is thought to be the charge trapping of photo-generated holes at the gate insulator and/or insulator/channel interface.
Engineering ferromagnetic lines in graphene by local functionalization using AFM lithography
Bae Ho Park,Ik-Su Byun,Danil W. Boukhvalov,Duk Hyun Lee,Wondong Kim,Jaeyoon Baik,Hyun-Joon Shin,Young-Woo Son 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
Monolayer graphene with sp<sup>2</sup>-carbon-atom network is a promising platform for next-generation spintronic devices due to its high carrier mobility and long spin relaxation length. For implementation of practical and high-density graphene-based spintronic devices, we need to define nanoscale areas with ferromagnetic properties on graphene. Up to now, conventional ferromagnetic metal electrodes accompanied by barrier insulators have been used for injection and detection of polarized spins in graphene-based spintronic devices. If graphene-based materials show ferromagnetic behaviors, they will become ideal candidates for spin injectors and detectors, because they structurally, chemically, and electrically match well with pristine graphene. In this presentation, I will report on local magnetic characteristics of nanoscale graphene oxidized and hydrogenated by atomic force microscope (AFM) lithography without conventional sources of surface contamination and chemical agents. By using AFM lithography, we can selectively control functional groups and their coverages on the nanoscale at the surface of graphene. By performing magnetic force microscope (MFM) measurement, we can clearly distinguish local magnetic signal of selectively oxidized or hydrogenated graphene from that of surrounding pristine graphene which does not produce ferromagnetic signal. The nanoscale oxidized and hydrogenated graphene show experimental evidences for room-temperature ferromagnetism. From x-ray magnetic circular dichroism photoemission electron microscope (XMCD-PEEM) measurement, we also identified remarkable asymmetry in carbon K edge XMCD spectra, which strongly indicates that the observed ferromagnetic order in functionalized graphene layers is intrinsic.