Electrostatically Gated Graphene-Zinc Oxide Nanowire Heterojunction.

You, Xueqiu,Pak, James Jungho American Scientific Publishers 2015 Journal of Nanoscience and Nanotechnology Vol.15 No.3

<P>This paper presents an electrostatically gated graphene-ZnO nanowire (NW) heterojunction for the purpose of device applications for the first time. A sub-nanometer-thick energy barrier width was formed between a monatomic graphene layer and electrochemically grown ZnO NWs. Because of the narrow energy barrier, electrons can tunnel through the barrier when a voltage is applied across the junction. A near-ohmic current-voltage (I-V) curve was obtained from the graphene-electrochemically grown ZnO NW heterojunction. This near-ohmic contact changed to asymmetric I-V Schottky contact when the samples were exposed to an oxygen environment. It is believed that the adsorbed oxygen atoms or molecules on the ZnO NW surface capture free electrons of the ZnO NWs, thereby creating a depletion region in the ZnO NWs. Consequentially, the electron concentration in the ZnO NWs is dramatically reduced, and the energy barrier width of the graphene-ZnO NW heterojunction increases greatly. This increased energy barrier width reduces the electron tunneling probability, resulting in a typical Schottky contact. By adjusting the back-gate voltage to control the graphene-ZnO NW Schottky energy barrier height, a large modulation on the junction current (on/off ratio of 10(3)) was achieved.</P>

Flexible and Conductive Graphene-Poly (diallyldimethylammoniumchloride) Buckypaper.

You, Xueqiu,Pak, James Jungho American Scientific Publishers 2015 Journal of Nanoscience and Nanotechnology Vol.15 No.3

<P>This paper describes the fabrication and characterization of flexible, conductive reduced graphene oxide (rGO)-poly(diallyldimethylammoniumchloride) (PDDA) buckypaper (BP). PDDA acts as a reducing agent to prepare an rGO-PDDA nanosheet dispersion from graphite oxide. The incorporation of PDDA as a 'glue' molecule successfully binds rGO nanosheets into BPs with strong interlayer binding. The resulting BPs were characterized by scanning electronic microscopy (SEM), Raman, energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and resistivity measurements. The sp2 structure was greatly restored by the PDDA-induced reduction. Moreover, rGO was chemically doped from the adsorbed PDDA, which causes the Raman G band to shift from ~1585 to ~1610 cm(-1). This chemical doping substantially increased the density of the free charge carriers in rGO and thereby further enhanced the electrical conductivity of the rGO-BP. Good inter-layer connection in the rGO percolating network was obtained after thermal annealing at higher than ~250 °C. The resulting rGO-PDDA-BPs exhibited an isotropic sheet resistance as low as ~100 곽/sq, which indicates a reduction by six orders of magnitude compared to the GO-BPs resistance before annealing. This PDDA-induced reduction with a low-temperature annealing process preserved the BPs' structural integrity and mechanical flexibility, thus overcoming the fragility problems with high-temperature annealing.</P>

투과전자현미경의 집속 전자빔을 이용한 고체 나노구멍 제작

김태헌,Xueqiu You,박정호 대한전기학회 2012 대한전기학회 학술대회 논문집 Vol.2012 No.7

Role of Graphene in Water-Assisted Oxidation of Copper in Relation to Dry Transfer of Graphene

Luo, Da,You, Xueqiu,Li, Bao-Wen,Chen, Xianjue,Park, Hyo Ju,Jung, Minbok,Ko, Taeg Yeoung,Wong, Kester,Yousaf, Masood,Chen, Xiong,Huang, Ming,Lee, Sun Hwa,Lee, Zonghoon,Shin, Hyung-Joon,Ryu, Sunmin,Kwak American Chemical Society 2017 Chemistry of materials Vol.29 No.10

<P>The process of oxidation of a copper surface coated by a layer of graphene in water-saturated air at 50 degrees C was studied where it was observed that oxidation started at the graphene edge and was complete after 24 h. Isotope labeling of the oxygen gas and water showed that the oxygen in the formed copper oxides originated from water and not from the oxygen in air for both Cu and graphene-coated Cu, and this has interesting potential implications for graphene as a protective coating for Cu in dry air conditions. We propose a reaction pathway where surface hydroxyl groups formed at graphene edges and defects induce the oxidation of Cu. DFT simulation shows that the binding energy between graphene and the oxidized Cu substrate is smaller than that for the bare Cu substrate, which facilitates delamination of the graphene. Using this process, dry transfer is demonstrated using poly(bisphenol A carbonate) (PC) as the support layer. The high quality of the transferred graphene is demonstrated from Raman maps, XPS, STM, TEM, and sheet resistance measurements. The copper foil substrate was reused without substantial weight loss to grow graphene (up to 3 cycles) of equal quality to the first growth after each cycle. It was found that dry transfer yielded graphene with less Cu impurities as compared to methods using etching of the Cu substrate. Using PC yielded graphene with less polymeric residue after transfer than the use of poly(methyl methacrylate) (PMMA) as the supporting layer. Hence, this dry and clean delamination technique for CVD graphene grown on copper substrates is highly advantageous for the cost-effective large-scale production of graphene, where the Cu substrate can be reused after each growth.</P>

Power Generating Characteristics of Zinc Oxide Nanorods Grown on a Flexible Substrate by a Hydrothermal Method

Jae-hoon Choi,Xueqiu You,Chul Kim,Jungil Park,James Jungho Pak 대한전기학회 2010 Journal of Electrical Engineering & Technology Vol.5 No.4

This paper describes the power generating property of hydrothermally grown ZnO nanorods on a flexible polyethersulfone (PES) substrate. The piezoelectric currents generated by the ZnO nanorods were measured when bending the ZnO nanorod by using I-AFM, and the measured piezoelectric currents ranged from 60 to 100 pA. When the PtIr coated tip bends a ZnO nanorod, piezoelectrical asymmetric potential is created on the nanorod surface. The Schottky barrier at the ZnO-metal interface accumulates elecntrons and then release very quickly generating the currents when the tip moves from tensile to compressed part of ZnO nanorod. These ZnO nanorods were grown almost vertically with the length of 300-500 ㎚ and the diameter of 30-60 ㎚ on the Ag/Ti/PES substrate at 90℃ for 6 hours by hydrothermal method. The metal-semiconductor interface property was evaluated by using a HP 4145B Semiconductor Parameter Analyzer and the piezoelectric effect of the ZnO nanorods were evaluated by using an I-AFM. From the measured I-V characteristics, it was observed that ZnO-Ag and ZnO-Au metal-semiconductor interfaces showed an ohmic and a Schottky contact characteristics, respectively. ANSYS finite element simulation was performed in order to understand the power generation mechanism of the ZnO nanorods under applied external stress theoretically.

Power Generating Characteristics of Zinc Oxide Nanorods Grown on a Flexible Substrate by a Hydrothermal Method

Choi, Jae-Hoon,You, Xueqiu,Kim, Chul,Park, Jung-Il,Pak, James Jung-Ho The Korean Institute of Electrical Engineers 2010 Journal of Electrical Engineering & Technology Vol.5 No.4

This paper describes the power generating property of hydrothermally grown ZnO nanorods on a flexible polyethersulfone (PES) substrate. The piezoelectric currents generated by the ZnO nanorods were measured when bending the ZnO nanorod by using I-AFM, and the measured piezoelectric currents ranged from 60 to 100 pA. When the PtIr coated tip bends a ZnO nanorod, piezoelectrical asymmetric potential is created on the nanorod surface. The Schottky barrier at the ZnO-metal interface accumulates elecntrons and then release very quickly generating the currents when the tip moves from tensile to compressed part of ZnO nanorod. These ZnO nanorods were grown almost vertically with the length of 300-500 nm and the diameter of 30-60 nm on the Ag/Ti/PES substrate at $90^{\circ}C$ for 6 hours by hydrothermal method. The metal-semiconductor interface property was evaluated by using a HP 4145B Semiconductor Parameter Analyzer and the piezoelectric effect of the ZnO nanorods were evaluated by using an I-AFM. From the measured I-V characteristics, it was observed that ZnO-Ag and ZnO-Au metal-semiconductor interfaces showed an ohmic and a Schottky contact characteristics, respectively. ANSYS finite element simulation was performed in order to understand the power generation mechanism of the ZnO nanorods under applied external stress theoretically.