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Youngjun Yun,Ajeong Choi,Suk Gyu Hahm,Jong Won Chung,Yong Uk Lee,Ji Young Jung,Joo-Young Kim,Jeong-Il Park,Sangyoon Lee,Jaewon Jang IEEE 2017 IEEE electron device letters Vol.38 No.5
<P>In this letter, we demonstrated a high performance organic thin-film transistor using thermally evaporated amorphous phase MoO<SUB>x</SUB> as a hole injection layer between metal electrodes and organic semiconductor. The fabricated organic thin-film transistors showed the field-effect mobility of 7 cm<SUP>2</SUP>/Vs in linear and saturation regimes and an ON/OFF current ratio of 10<SUP>7</SUP>. The MoO<SUB>x</SUB> hole injection layers significantly reduced the injection barrier from metal electrode, resulting in the improvement of ohmic contact properties of a synthesized thiophene-rich heteroacene, dibenzothiopheno [6,5-b:6',5'-f] thieno [3,2-b] thiophene p-type organic semiconductor, as compared with those with single metals. Furthermore, high performance organic thin-film transistors can be successfully realized with Al electrode, which is not suitable for p-type organic semiconductors due to its low work function by introducing a 75-nm-thick MoO<SUB>x</SUB> hole injection layer.</P>
Surface Modulation of Graphene Field Effect Transistors on Periodic Trench Structure
Jin, Jun Eon,Choi, Jun Hee,Yun, Hoyeol,Jang, Ho-Kyun,Lee, Byung Chul,Choi, Ajeong,Joo, Min-Kyu,Dettlaff-Weglikowska, Urszula,Roth, Siegmar,Lee, Sang Wook,Lee, Jae Woo,Kim, Gyu Tae American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.28
<P>In this work, graphene field effect transistors (FETs) were fabricated on a trench structure made by carbonized poly(methylmethacrylate) to modify the graphene surface. The trench-structured devices showed different characteristics depending on the channel orientation and the pitch size of the trenches as well as channel area in the FETs. Periodic corrugations and barriers of suspended graphene on the trench structure were measured by atomic force microscopy and electrostatic force microscopy. Regular barriers of 160 mV were observed for the trench structure with graphene. To confirm the transfer mechanism in the FETs depending on the channel orientation, the ratio of experimental mobility (3.6-3.74) was extracted from the current voltage characteristics using equivalent circuit simulation. It is shown that the number of barriers increases as the pitch size decreases because the number of corrugations increases from different trench pitches. The noise for the 140 nm pitch trench is 1 order of magnitude higher than that for the 200 nm pitch trench.</P>
Electromechanical properties of CNT-coated cotton yarn for electronic textile applications
Kang, Tae June,Choi, Ajeong,Kim, Dai-Hong,Jin, Kyoungcheol,Seo, Dong Kyun,Jeong, Dae Hong,Hong, Seong-Hyeon,Park, Yung Woo,Kim, Yong Hyup Institute of Physics Publishing 2011 Smart materials & structures Vol.20 No.1
<P>Smart fabrics have attracted considerable attention due to their potential applications. The essential features of smart fabrics include wearability, weaveability, and stretchability, as well as their sensing/response capability, which is frequently based on electrical measurement. Thus, the electromechanical behavior of these fabrics is considered the most important material property. Here, we report the negative piezoresistance of single-walled carbon nanotube coated cotton yarn (SWNT-CY). The gauge factor (the ratio of the normalized change in piezoresistance to the change in strain) of SWNT-CY is measured to be − 24. It is noteworthy that the factor is negative and an order of magnitude higher than that for conventional metal strain gauges. The negative piezoresistance is due to mechanical contact between fabric fibers, which leads to better electrical paths of SWNT networks. The conduction behavior can be modeled as fluctuation-induced tunneling (FIT) through the contact barriers between conducting regions. The effective barrier strength of strained SWNT-CY is measured to be ∼ 30% lower than that of unstrained SWNT-CY. This characteristic may offer new design opportunities for wearable electronics and has significant implications for sensor applications. </P>
Enhancement of Heating Performance of Carbon Nanotube Sheet with Granular Metal
Im, Hyeongwook,Jang, Eui Yun,Choi, Ajeong,Kim, Wal Jun,Kang, Tae June,Park, Yung Woo,Kim, Yong Hyup American Chemical Society 2012 ACS APPLIED MATERIALS & INTERFACES Vol.4 No.5
<P>A strategy for enhancing the heating performance of freestanding carbon nanotube (CNT) sheet is presented that involves decorating the sheet with granular-type palladium (Pd) particles. When Pd is added to the sheet, the heating efficiency of CNT sheet is increased by a factor of 3.6 (99.9 °C cm<SUP>2</SUP>/W vs 27.3 °C cm<SUP>2</SUP>/W with no Pd). Suppression of convective heat transfer loss attributes to the enhanced heat generation efficiency. However, higher heating response of CNT/Pd sheet was observed compared to CNT sheet, hence suggesting that the electron–lattice energy exchange could be additional heating mechanism in the presence of granular-type particles of Pd having a diameter of 10 nm or less. CNT sheet/Pd is quite stable, retaining its initial characteristics even after 300 cycles of on–off voltage pulses and shows fast thermal responses of the heating and cooling rates being 154 and −248 °C/s, respectively.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2012/aamick.2012.4.issue-5/am300477u/production/images/medium/am-2012-00477u_0003.gif'></P>
Fibers of reduced graphene oxide nanoribbons
Jang, Eui Yun,Carretero-Gonzá,lez, Javier,Choi, Ajeong,Kim, Wal Jun,Kozlov, Mikhail E,Kim, Taewoo,Kang, Tae June,Baek, Seung Jae,Kim, Dae Weon,Park, Yung Woo,Baughman, Ray H,Kim, Yong Hyup IOP Pub 2012 Nanotechnology Vol.23 No.23
<P>Reduced graphene oxide nanoribbon fibers were fabricated by using an electrophoretic self-assembly method without the use of any polymer or surfactant. We report electrical and field emission properties of the fibers as a function of reduction degree. In particular, the thermally annealed fiber showed superior field emission performance with a low potential for field emission (0.7 V µm<SUP>−1</SUP>) and a giant field emission current density (400 A cm<SUP>−2</SUP>). Moreover, the fiber maintains a high current level of 300 A cm<SUP>−2</SUP> corresponding to 1 mA during long-term operation.</P>
Jung, Minwoo,Yoon, Youngwoon,Park, Jae Hoon,Cha, Wonsuk,Kim, Ajeong,Kang, Jinback,Gautam, Sanjeev,Seo, Dongkyun,Cho, Jeong Ho,Kim, Hyunjung,Choi, Jong Yong,Chae, Keun Hwa,Kwak, Kyungwon,Son, Hae Jung American Chemical Society 2014 ACS NANO Vol.8 No.6
<P>We synthesized a series of acceptor–donor–acceptor-type small molecules (SIDPP-EE, SIDPP-EO, SIDPP-OE, and SIDPP-OO) consisting of a dithienosilole (SI) electron-donating moiety and two diketopyrrolopyrrole (DPP) electron-withdrawing moieties each bearing linear <I>n</I>-octyl (O) and/or branched 2-ethylhexyl (E) alkyl side chains. X-ray diffraction patterns revealed that SIDPP-EE and SIDPP-EO films were highly crystalline with pronounced edge-on orientation, whereas SIDPP-OE and SIDPP-OO films were less crystalline with a radial distribution of molecular orientations. Near-edge X-ray absorption fine structure spectroscopy disclosed an edge-on orientation with a molecular backbone tilt angle of ∼22° for both SIDPP-EE and SIDPP-EO. Our analysis of the molecular packing and orientation indicated that the shorter 2-ethylhexyl groups on the SI core promote tight π–π stacking of the molecular backbone, whereas <I>n</I>-octyl groups on the SI core hinder close π–π stacking to some degree. Conversely, the longer linear <I>n</I>-octyl groups on the DPP arms facilitate close intermolecular packing <I>via</I> octyl–octyl interdigitation. Quantum mechanics/molecular mechanics molecular dynamics simulations determined the optimal three-dimensional positions of the flexible alkyl side chains of the SI and DPP units, which elucidates the structural cause of the molecular packing and orientation explicitly. The alkyl-chain-dependent molecular stacking significantly affected the electrical properties of the molecular films. The edge-on oriented molecules showed high hole mobilities in organic field-effect transistors, while the radially oriented molecules exhibited high photovoltaic properties in organic photovoltaic cells. These results demonstrate that appropriate positioning of alkyl side chains can modulate crystallinity and molecular orientation in SIDPP films, which ultimately have a profound impact on carrier transport and photovoltaic performance.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2014/ancac3.2014.8.issue-6/nn501133y/production/images/medium/nn-2014-01133y_0010.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn501133y'>ACS Electronic Supporting Info</A></P>
<i>In situ</i> fabrication of freestanding single‐walled carbon nanotube rope interconnection
Kim, Taewoo,Kang, Tae June,Seo, Dong Kyun,Jang, Eui Yun,Jin, Kyoung Cheol,Choi, Ajeong,Kim, Dae‐,Weon,Park, Yung Woo,Jeong, Dae Hong,Kim, Yong Hyup WILEY‐VCH Verlag 2012 Physica status solidi. PSS. A, Applications and ma Vol.209 No.11
<P><B>Abstract</B></P><P>A single‐walled carbon nanotube rope interconnect is fabricated by utilizing dielectrophoresis and capillary condensation. The rope connects a pair of prefabricated cantilevers with good alignment and outstanding packing density. The diameter and resistance of the rope are readily controlled by adjusting the dielectrophoresis parameters such as applied voltage and frequency. The properties of the rope, including packing density, electron transport, and maximum current density, are investigated. The maximum current density of the rope is measured to be as high as 1 × 10<SUP>7</SUP> A/cm<SUP>2</SUP> in vacuum. </P>