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Sangdan Kim,Su Hyung Jang KOREAN SOCIETY OF CIVIL ENGINEERS 2007 KSCE JOURNAL OF CIVIL ENGINEERING Vol.11 No.4
<P>A new stochastic model for the propagation analysis of fluctuations in rainfall to soil water dynamics is proposed with major focus on its probabilistic structure. This model is derived by using cumulant expansion theory from a stochastic differential equation with stochastic rainfall forcing, and has the advantage of providing the probabilistic solution in the form of a probability distribution function, from which one can obtain the ensemble average behavior of the system. Steady state probability distribution function for soil water is obtained analytically and analyzed for different climate and soil conditions.</P>
Probabilistic Solution to Soil Water Evaporated Flow Equation
Sangdan Kim 대한토목학회 2006 KSCE journal of civil engineering Vol.10 No.1
A new stochastic model for one-dimensional evaporated soil water flow is proposed with major focus on its probabilistic structure. The newly developed model has the form of the Fokker-Planck equation, and its validity as a model for the probabilistic evolution of the nonlinear stochastic unsaturated flow process is investigated under a stochastic soil-related parameter (i.e., saturated hydraulic conductivity). This model is based on a parabolic type of stochastic partial differential equation, and has the advantage of providing the probabilistic solution in the form of a probability distribution function, from which one can obtain the ensemble average behavior of the flow system. The comparison results with Monte Carlo simulations show that the proposed model can reproduce well the vertically varying soil water wetting front depth. Overall, the ensemble averaging approach using the cumulant expansion method shows good promise for the stochastic modeling of nonlinear hydrologic processes.
Threshold Voltage Control for SnO2 Nanowire Transistors by Gas Treatment
Sangdan Kim,주상현 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.8
In order to apply nanowire transistors to next-generation electronic devices, it is crucial to control their characteristics. In particular, threshold voltage (V<sub>th</sub>) must be controlled, since it determines the driving voltage of the transistors. In this research, we were able to control threshold voltage (Vth) characteristics in both the negative and positive directions by performing H<sub>2</sub> and O<sub>2</sub> gas treatments at room temperature; this control was achieved since oxygen vacancies on the surface of oxide nanowire channel regions could be controlled with the treatments. The H<sub>2</sub> treatment changed Vth in the negative direction with the processing time (-0.6 V at 10 min; -0.9 V at 30 min; and -1.4 V at 60 min), whereas the O<sub>2</sub> treatment changed V<sub>th</sub> in the positive direction regardless of the processing time (+0.73 V at 10 min; +0.85 V at 30 min; +0.84 V at 60 min). This method can be applied to display-pixel and digital-logic circuits, especially those that employ nanowire transistors and require V<sub>th</sub> control.
Kim, Sangdan,Han, Suhee Wiley (John WileySons) 2010 Water environment research Vol.82 No.1
<P>Most related literature regarding designing urban non-point-source management systems assumes that precipitation event-depths follow the 1-parameter exponential probability density function to reduce the mathematical complexity of the derivation process. However, the method of expressing the rainfall is the most important factor for analyzing stormwater; thus, a better mathematical expression, which represents the probability distribution of rainfall depths, is suggested in this study. Also, the rainfall-runoff calculation procedure required for deriving a stormwater-capture curve is altered by the U.S. Natural Resources Conservation Service (Washington, D.C.) (NRCS) runoff curve number method to consider the nonlinearity of the rainfall-runoff relation and, at the same time, obtain a more verifiable and representative curve for design when applying it to urban drainage areas with complicated land-use characteristics, such as occurs in Korea. The result of developing the stormwater-capture curve from the rainfall data in Busan, Korea, confirms that the methodology suggested in this study provides a better solution than the pre-existing one.</P>
Hybrid nanowire–multilayer graphene film light-emitting sources
Kim, Sangdan,Choi, Hongkyw,Jung, Mi,Choi, Sung-Yool,Ju, Sanghyun IOP Pub 2010 Nanotechnology Vol.21 No.42
<P>We report a versatile hybrid device consisting of one-dimensional ZnS and Te-doped ZnS (ZnS:Te) nanowires (NWs) upon two-dimensional multilayer graphene films (MGFs). Single-crystalline ZnS and ZnS:Te NWs were grown directly on a MGF without a catalyst, and exhibited blue-green and blue emission peaks of ∼ 503 and ∼ 440 nm. A field emission light emitter using ZnS:Te NWs on a MGF was demonstrated, and it indicates excellent contact properties between the NWs and MGFs. The resulting hybrid devices are promising candidates for potential applications as building blocks for the development of highly functional and efficient electroluminescent devices and field-emitting devices including flexible and/or transparent display devices. </P>
Fabrication of reliable semiconductor nanowires by controlling crystalline structure
Kim, Sangdan,Lim, Taekyung,Ju, Sanghyun IOP Pub 2011 Nanotechnology Vol.22 No.30
<P>One-dimensional SnO<SUB>2</SUB> nanomaterials with wide bandgap characteristics are attractive for flexible and/or transparent displays and high-performance nano-electronics. In this study, the crystallinity of SnO<SUB>2</SUB> nanowires was regulated by controlling their growth temperatures. Moreover, the correlation of the crystallinity of nanowires with optical and electrical characteristics was analyzed. When SnO<SUB>2</SUB> nanowires were grown at temperatures below 900 °C, they showed various growth directions and abnormal discontinuity in their crystal structures. On the other hand, most nanowires grown at 950 °C exhibited a regular growth trend in the direction of [100]. In addition, the low temperature photoluminescence measurement revealed that the higher growth temperatures of nanowires gradually decreased the 500 nm peak rather than the 620 nm peak. The former peak is derived from the surface defect related to the shallow energy level and affects nanowire surface states. Owing to crystallinity and defects, the threshold voltage range (maximum–minimum) of SnO<SUB>2</SUB> nanowire transistors was 1.5 V at 850 °C, 1.1 V at 900 °C, and 0.5 V at 950 °C, with dispersion characteristics dramatically decreased. This study successfully demonstrated the effects of nanowire crystallinity on optical and electrical characteristics. It also suggested that the optical and electrical characteristics of nanowire transistors could be regulated by controlling their growth temperatures in the course of producing SnO<SUB>2</SUB> nanowires. </P>