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석윤지 ( Yoon Ji Seok ),송은지 ( Eun Ji Song ),차인태 ( In Tae Cha ),이현진 ( Hyunjin Lee ),노성운 ( Seong Woon Roh ),정지영 ( Ji Young Jung ),이유경 ( Yoo Kyung Lee ),남영도 ( Young Do Nam ),서명지 ( Myung Ji Seo ) 한국미생물생명공학회(구 한국산업미생물학회) 2016 한국미생물·생명공학회지 Vol.44 No.2
최근 빙하의 융해로 인해 빙하 해안 지역에 다양한 토양 미생물과 초목들이 드러나고 있다. 본 연구에서는 북극 스발바르 군도 중앙로벤 빙하 해안 지역으로부터 Ion Torrent Personal Genome Machine(PGM)을 활용한 메타지놈 분석을 통해 세균(bacteria), 고균(archaea), 및 진핵생물(eukaryotes)를 포함하는 다양한 미생물 군집을 분석하였다. 연구에 사용된 토양시료는 빙하 후퇴에 따른 토양의 노출 시기에 따라 2개 지역(ML4 및 ML7)으로부터 수집하였다. ML4 및 ML7 시료의 메타지놈 염기서열을 기반으로 총 2,798,108 및 1,691,859 reads가 각각 미생물 군집 분석에 활용되었다. Domain (계) 수준에서 미생물 군집의 상대 빈도를 분석한 결과 2개 시료 모두 세균(86-87%)이 높은 반면 고균과 진핵생물은 1% 미만으로 존재하는 것으로 나타났다. 또한 약 12%의 염기서열은 기존에 분류되지 않은(unclassified) 서열로 분석되었다. 세균의 경우 Proteobacteria(40.3% for ML4 and 43.3%for ML7)와 Actinobacteria(22.9% and 24.9%)가 우점하는 것으로 분석되었다. 고균의 경우에는 Euryarchaeota(84.4% and 81.1%)및 Crenarchaeota(10.6% and 13.1%), 그리고 진핵생물의 경우에는 Ascomycota(33.8% and 45.0%)가 우점하는 것으로 분석되었다. 본 연구를 통해 Ion Torrent PGM 플랫폼을 활용한 메타지놈 분석이 북극의 중앙로벤 빙하 해안 지역의 전체 미생물 군집 구조를 파악하는데 충분히 적용될 수 있을 것으로 사료된다. Recent succession of soil microorganisms and vegetation has occurred in the glacier foreland, because of glacier thawing. In this study, whole microbial communities, including bacteria, archaea, and eukaryotes, from the glacier foreland of Midtre Lovenbreen in Svalbard were analyzed by metagenome sequencing, using the Ion Torrent Personal Genome Machine (PGM) platform. Soil samples were collected from two research sites (ML4 and ML7), with different exposure times, from the ice. A total of 2,798,108 and 1,691,859 reads were utilized for microbial community analysis based on the metagenomic sequences of ML4 and ML7, respectively. The relative abundance of microbial communities at the domain level showed a high proportion of bacteria (about 86-87%), whereas archaeal and eukaryotic communities were poorly represented by less than 1%. The remaining 12% of the sequences were found to be unclassified. Predominant bacterial groups included Proteobacteria (40.3% from ML4 and 43.3% from ML7) and Actinobacteria (22.9% and 24.9%). Major groups of Archaea included Euryarchaeota (84.4% and 81.1%), followed by Crenarchaeota (10.6% and 13.1%). In the case of eukaryotes, both ML4 and ML7 samples showed Ascomycota (33.8% and 45.0%) as the major group. These findings suggest that metagenome analysis using the Ion Torrent PGM platform could be suitably applied to analyze whole microbial community structures, providing a basis for assessing the relative importance of predominant groups of bacterial, archaeal, and eukaryotic microbial communities in the Arctic glacier foreland of Midtre Lovenbreen, with high resolution.
<P>For transition metal dichalcogenides, the fluctuation of the channel current due to charged impurities is attributed to a large surface area and a thickness of a few nanometers. To investigate current variance at the interface of transistors, we obtain the low-frequency (LF) noise features of MoTe2 multilayer field-effect transistors with different dielectric environments. The LF noise properties are analyzed using the combined carrier mobility and carrier number fluctuation model which is additionally parametrized with an interfacial Coulomb-scattering parameter (a) that varies as a function of the accumulated carrier density (Nacc) and the location of the active channel layer of MoTe2. Our model shows good agreement with the current power spectral density (PSD) of MoTe2 devices from a low to high current range and indicates that the parameter a exhibits a stronger dependence on Nacc with an exponent -gamma, of -1.18 to approximately -1.64 for MoTe2 devices, compared with -0.5 for Si devices. The raised Coulomb scattering of the carriers, particularly for a low-current regime, is considered to be caused by the unique traits of layered semiconductors such as interlayer coupling and the charge distribution strongly affected by the device structure under a gate bias, which completely change the charge screening effect in MoTe2 multilayer. Comprehensive static and LF noise analyses of MoTe2 devices with our combined model reveal that a chemical-vapor deposited h-BN monolayer underneath MoTe2 channel and the Al2O3 passivation layer have a dissimilar contribution to the reduction of current fluctuation. The three-fold enhanced carrier mobility due to the h-BN is from the weakened carrier scattering at the gate dielectric interface and the additional 30% increase in carrier mobility by Al2O3 passivation is due to the reduced interface traps.</P>
Ji, Hyunjin,Ghimire, Mohan Kumar,Lee, Gwanmu,Yi, Hojoon,Sakong, Wonkil,Gul, Hamza Zad,Yun, Yoojoo,Jiang, Jinbao,Kim, Joonggyu,Joo, Min-Kyu,Suh, Dongseok,Lim, Seong Chu American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.32
<P>The transport behaviors of MoS<SUB>2</SUB> field-effect transistors (FETs) with various channel thicknesses are studied. In a 12 nm thick MoS<SUB>2</SUB> FET, a typical switching behavior is observed with an <I>I</I><SUB>on</SUB>/<I>I</I><SUB>off</SUB> ratio of 10<SUP>6</SUP>. However, in 70 nm thick MoS<SUB>2</SUB> FETs, the gating effect weakens with a large off-current, resulting from the screening of the gate field by the carriers formed through the ionization of S vacancies at 300 K. Hence, when the latter is dual-gated, two independent conductions develop with different threshold voltage (<I>V</I><SUB>TH</SUB>) and field-effect mobility (μ<SUB>FE</SUB>) values. When the temperature is lowered for the latter, both the ionization of S vacancies and the gate-field screening reduce, which revives the strong <I>I</I><SUB>on</SUB>/<I>I</I><SUB>off</SUB> ratio and merges the two separate channels into one. Thus, only one each of <I>V</I><SUB>TH</SUB> and μ<SUB>FE</SUB> are seen from the thick MoS<SUB>2</SUB> FET when the temperature is less than 80 K. The change of the number of conduction channels is attributed to the ionization of S vacancies, which leads to a temperature-dependent intra- and interlayer conductance and the attenuation of the electrostatic gate field. The defect-related transport behavior of thick MoS<SUB>2</SUB> enables us to propose a new device structure that can be further developed to a vertical inverter inside a single MoS<SUB>2</SUB> flake.</P> [FIG OMISSION]</BR>
<P>Compared to the silicon device whose performance is severely degraded due to the pin-holes and channel inactive space when the channel thickness is less than 1 nm, despite monolayer transition-metal dichalcogenides being the most stable structure to be used as a two-dimensional semiconductor material, precise analysis of the double-gate (DG) field-effect transistor (FET) device structure has hardly been performed thus far. Hence, we analyzed the device operation characteristics of single-gate and DG sweeps in a monolayer MoS<SUB>2</SUB> DG FET structure, where the interfacial carrier behavior is distinguished from both gates by the different gate dielectric materials at the top and bottom. The synchronized DG sweep operation with biasing of <I>V</I> <SUB> <I>TG</I> </SUB> and <I>V</I> <SUB> <I>BG</I> </SUB> (=10 <I>V</I> <SUB> <I>TG</I> </SUB>) increased the carrier mobility by a factor of 4.85 compared with the independent DG sweep. Direct-current analysis and low-frequency noise modeling indicate that the device performance improves under equivalent gate voltages from both sides, because the device operates in a low vertical electric field and the interfacial carrier fluctuation effect is significantly reduced.</P>
<P>Direct current (DC) and low-frequency (LF) noise analyses of a chemical vapor deposition (CVD)-grown monolayer MoS2 field effect transistor (FET) indicate that time-varying carrier perturbations originate from gas adsorbates. The LF noise analysis supports that the natural desorption of physisorbed gas molecules, water and oxygen, largely reduces the interface trap density (<I>N</I>ST) under vacuum conditions (∼10<SUP>−8</SUP> Torr) for 2 weeks. After a longer period of 8 months under vacuum, the carrier scattering mechanism alters, in particular for the low carrier density (<I>N</I>acc) region. A decrease of both <I>N</I>ST and the scattering parameter <I>α</I>SC with desorption of surface adsorbates from MoS2, explains the enhanced carrier mobility and the early turn-on of the device. The stabilized carrier behavior is verified with <I>γ</I> = 0.5 in the formula <I>α</I>SC ∝ <I>N</I>acc<SUP>−γ</SUP>, as in Si-MOSFETs. Our results support that the gas adsorbates work as charged impurities, rather than neutral ones.</P>
<P><B>Abstract</B></P> <P>In this work, a methanol steam reforming (MSR) reactor was operated using an indirect heating method. A thermal circuit was constructed between the MSR reactor and the electrical heater to supply the heat required for the endothermic reaction, and deionized water was used as the heat transfer medium (HTM). The MSR reactors featured a shell-and-tube type design to operate at high pressures. A Cu/Zn catalyst was installed on the tube side, and HTM was supplied to the shell side. To improve the heat transfer performance, the heat transfer area between the shell and tube was increased from 598 to 1117 cm<SUP>2</SUP>. Because the MSR reactor had a sufficient heat exchange area corresponding to the catalytic reaction rate, the heat exchange area had little effect on methanol conversion. However, the heat exchange area had a greater effect on the performance because the operating temperature of reactor was lower. Under the same operating temperature conditions, the MSR reactor operated under the indirect heating method showed relatively higher methanol conversion than the MSR reactor operated in an electric furnace because of the effective heat transfer by the latent heat of saturated steam. The MSR reactor based on the indirect heating method was continuously operated at 250 °C for 72 h to verify characteristic start-up and operation. The results showed that the MSR reactor could be operated at a constant temperature; however, low methanol conversion at low operating temperatures led to slow catalyst degradation. In addition, the MSR reactor required more than 2 h for initial start-up and for restart after emergency shutdown because the HTM needed to be evaporated and pressurized to the target pressure.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The methanol steam reforming (MSR) reactor featured a shell-and-tube type design. </LI> <LI> The MSR reactor was operated between 250 and 280 °C by indirect heating method. </LI> <LI> The saturated steam used as the heat transfer medium supplied heat to MSR reactor. </LI> <LI> The MSR reactor operated under the indirect heating method showed relatively higher methanol conversion. </LI> <LI> The MSR reactor based on the indirect heating method was continuously operated at 250 °C for 72 h. </LI> </UL> </P>
<P><B>Abstract</B></P> <P>The purpose of this work is to suggest a steam-to-carbon ratio (SCR) control strategy for the start-up and operation of a fuel processor and to experimentally verify this strategy. To overcome ambient temperature variability and manufacturing deviations, a controlled SCR method (CSM) is suggested. The CSM controls the water flow rate independently through heat exchangers (HEXs) to maintain a constant inlet temperature of the reactors. To consistently satisfy the target SCR value, the remaining water after control is fed to the last HEX used as a buffer. To verify the CSM, seven gasoline fuel processors (GFPs) were constructed. The GFPs consisted of an autothermal reformer (ATR), hydrodesulphurization (HDS), a high-temperature shift reactor (HTS), a medium-temperature shift reactor (MTS), a preferential oxidation reactor (PROX), a HEX, and an exhaust gas burner. Water was individually supplied to HEX #1 ∼ HEX #4 as a cool-side fluid. One of the GFPs was operated at a low (−32 °C) and a high (50 °C) temperature. The CSM maintained a constant inlet temperature of the reactors; only the inlet temperature of the PROX was affected by the ambient temperature thanks to the CSM. Temperature results for the other six GFPs showed that manufacturing deviations appeared only in the inlet temperature of the PROX by the CSM. To confirm the effect of the CSM on durability, 38 start–stop cycles were performed over 314 h of operation. The results showed that the repeated use of the CSM led to a slow degradation of efficiency, while the temperatures of the reformer and reactor remained steady during cycling testing.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A steam-to-carbon ratio (SCR) control strategy for a fuel processor is suggested. </LI> <LI> The heat exchanger and preferential oxidation reactor were used as a control buffer. </LI> <LI> The inlet temperatures of the reactors were maintained regardless of disturbances. </LI> <LI> Only the preferential oxidation reactor was affected by disturbance. </LI> <LI> We tested seven fuel processors to confirm the usefulness of the control strategy. </LI> </UL> </P>