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RISS 인기검색어
- 검색키워드
- 저자 : Odongo Francis Ngome Okello (검색결과 4 건)
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Odongo Ngome, Francis Okello,Kim, Young-Tae,Lee, Hyeon-Dong,Kim, Young-Hoon,Lee, Tae-Woo,Park, Chan-Gyung The Royal Society of Chemistry 2017 Journal of Materials Chemistry C Vol.5 No.37
<▼1><P>We successfully control the Ir(ppy)3 needle-like aggregates in small-molecule based emitting layers by solvent mixture for improving the properties of OLEDs.</P></▼1><▼2><P>The morphology of emitting layers (EMLs) plays a vital role in determining the overall performance of solution processed phosphorescent organic light emitting diodes (PhOLEDs). Herein, the morphology of undoped small molecule binary blend EMLs prepared by blending tris(4-carbazoyle-9-ylphenyl)amine (TCTA) hole transport material (HTM) with a series of electron transport materials (ETMLs) was studied using a transmission electron microscope (TEM). Experimental results show that phase separation of the binary blend EMLs significantly depends on the polarity of the host. The binary blend ELMs were further doped with tris(2-phenylpyridine)iridium(iii) (Ir(ppy)3) to form ternary blend EMLs and the resulting morphology was examined using scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS). The results report for the first time the existence of Ir(ppy)3 needle-like aggregates in small molecule ternary blend EMLs. By comparing the size of the aggregates formed in small molecule ternary blend EMLs with those formed in polymer–small molecule blends, our results showed that small molecule blend EMLs exhibit minimal Ir(ppy)3 aggregates with a low aspect ratio in contrast to polymer–small molecule blends. The effect of mixed solvent on the distribution of the aggregates was also examined using a TEM and an atomic force microscope (AFM). The disappearance of the aggregates with varying solvent mixture ratios signifies that solvent mixture is an effective way to control homogeneous distribution of Ir(ppy)3 in the emitting layers of PhOLEDs. This was further evidenced by an improvement in light emitting efficiency and current efficiency of OLED devices fabricated using a mixed solvent.</P></▼2>
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TEM sample preparation using micro-manipulator for in-situ MEMS experiment
이현종,Odongo Francis Ngome Okello,김기엽,송경,최시영 한국현미경학회 2021 Applied microscopy Vol.51 No.1
Growing demands for comprehending complicated nano-scale phenomena in atomic resolution has attracted in-situ transmission electron microscopy (TEM) techniques for understanding their dynamics. However, simple to safe TEM sample preparation for in-situ observation has been limited. Here, we suggested the optical microscopy based micro-manipulating system for transferring TEM samples. By adopting our manipulator system, several types of samples from nano-wires to plate-like thin samples were transferred on micro-electro mechanical systems (MEMS) chip in a single step. Furthermore, the control of electrostatic force between the sample and the probe tip is found to be a key role in transferring process.
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All-Dry Transfer of Graphene Film by van der Waals Interactions
Yang, Seong-Jun,Choi, Shinyoung,Odongo Ngome, Francis Okello,Kim, Ki-Jeong,Choi, Si-Young,Kim, Cheol-Joo American Chemical Society 2019 NANO LETTERS Vol.19 No.6
<P>We report a method that uses van der Waals interactions to transfer continuous, high-quality graphene films from Ge(110) to a different substrate held by hexagonal boron nitride carriers in a clean, dry environment. The transferred films are uniform and continuous with low defect density and few charge puddles. The transfer is effective because of the weak interfacial adhesion energy between graphene and Ge. Based on the minimum strain energy required for the isolation of film, the upper limit of the interfacial adhesion energy is estimated to be 23 meV per carbon atom, which makes graphene/Ge(110) the first as-grown graphene film that has a substrate adhesion energy lower than that of typical van der Waals interactions between layered materials. Our results suggest that graphene on Ge can serve as an ideal material platform to be integrated with other material systems by a clean assembly process.</P> [FIG OMISSION]</BR>
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Epitaxial van der Waals Contacts between Transition-Metal Dichalcogenide Monolayer Polymorphs
Lee, Chang-Soo,Oh, Seung Jae,Heo, Hoseok,Seo, Seung-Young,Kim, Juho,Kim, Yong Hyeon,Kim, Donghwi,Ngome Okello, Odongo Francis,Shin, Hocheol,Sung, Ji Ho,Choi, Si-Young,Kim, Jun Sung,Kim, Jong Kyu,Jo, M American Chemical Society 2019 Nano letters Vol.19 No.3
<P>We have achieved heteroepitaxial stacking of a van der Waals (<I>vdW</I>) monolayer metal, 1T’-WTe<SUB>2</SUB>, and a semiconductor, 2H-WSe<SUB>2</SUB>, in which a distinctively low contact barrier was established across a clean epitaxial <I>vdW</I> gap. Our epitaxial 1T’-WTe<SUB>2</SUB> films were identified as a semimetal by low temperature transport and showed the robust breakdown current density of 5.0 × 10<SUP>7</SUP> A/cm<SUP>2</SUP>. In comparison with a series of planar metal contacts, our epitaxial <I>vdW</I> contact was identified to possess intrinsic Schottky barrier heights below 100 meV for both electron and hole injections, contributing to superior ambipolar field-effect transistor (FET) characteristics, i.e., higher FET mobilities and higher on-off current ratios at smaller threshold gate voltages. We discuss our observations around the critical roles of the epitaxial <I>vdW</I> heterointerfaces, such as incommensurate stacking sequences and absence of extrinsic interfacial defects that are inaccessible by other contact methods.</P> [FIG OMISSION]</BR>
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