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Seong, Mingi,Lee, Seung-Wook,Joh, Hyungmok,Lee, Woo Seok,Paik, Taejong,Oh, Soong Ju Elsevier 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.698 No.-
<P><B>Abstract</B></P> <P>We fabricated highly conductive and stable silver nanocrystal (Ag NC) thin film electrodes with tunable work functions using an all-solution-based method involving sequential ligand exchange and gold (Au) coating processes. We studied the effect of the Au coating process on the ligand-exchanged Ag NC thin films and successfully demonstrated the formation of a thin Au layer on the surface of the Ag NCs. We investigated the morphological, structural, optical, chemical, and electrical properties of the Ag NC thin films before and after Au coating. The work function of the electrodes was precisely controlled by varying the treatment time and concentration of gold chloride trihydrate (HAuCl<SUB>4</SUB>·3H<SUB>2</SUB>O) solutions. The thermal stability of the Au coated Ag NC thin films in air was greatly enhanced, and an extremely low resistivity of 3.4 μΩcm was achieved. Taking advantage of this solution-based low-temperature process and using a minimal amount of Au for cost reduction, we fabricated all-NC-based high-performance flexible photodetectors.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Ag nanocrystal (NC) thin films were prepared by an all-solution-based process. </LI> <LI> The work function could be tuned from 3.9 to 4.5eV by ligand exchange and Au coating. </LI> <LI> A very low resistivity of 3.4 μΩ cm was achieved from the Ag NC thin films. </LI> <LI> The thermal stability of the thin films in air was significantly enhanced. </LI> <LI> All-NC- and all-solution-process-based flexible photodetectors were fabricated. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Seong, Mingi,Kim, Hoyoung,Woo, Ho Young,Hossain, Md Ashraf,Ahn, Jun Hyuk,Kim, Soo-Kil,Paik, Taejong,Oh, Soong Ju THE KOREAN SOCIETY OF INDUSTRIAL AND ENGINEERING 2019 JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY -S Vol.76 No.-
<P><B>Abstract</B></P> <P>Efficient methods for the controlled syntheses of metal nanocrystals are important for emerging electronics and catalysis applications. In this report, we present a solution-based method for the fabrication of Pt-coated Ag nanocrystal (NC) thin films through the use of ligand-exchange and electroless deposition. We successfully prepared uniform or bumpy Ag@Pt core/shell structures, as well as Ag–Pt alloy structures, through control of the reducing agents. We first report the Ag–Pt alloy structures can be synthesized by low-temperature and solution-based process of post treatment that exhibited high catalytic activity toward the hydrogen evolution reaction.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A solution-processed, conductive Pt-coated Ag nanocrystal thin films was made by ligand exchange and electroless deposition. </LI> <LI> Various structures of Pt-Ag core/shell or alloys were designed through control of the platinum-deposition conditions. </LI> <LI> Pt-Ag alloy structure exhibits high catalytic activity toward the hydrogen evolution reaction. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Kim, Soo-Jung,Seong, Mingi,Yun, Hye-Won,Ahn, Junhyuk,Lee, Heon,Oh, Soong Ju,Hong, Sung-Hoon American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.30
<P>We developed a simple and systematic method to fabricate optically tunable and thermally and chemically stable Au-Ag nanocrystal-based plasmonic metamaterials. An Ag nanocrystal-based metamaterial with desirable optical properties was fabricated via nanoimprinting and ligand-exchange process. Its optical properties were controlled by selectively substituting Ag atoms with Au atoms through a spontaneous galvanic replacement reaction. The developed Au-Ag-based metamaterials provide excellent tunable plasmonic properties required for various applications in the visible and near-infrared regions by controlling the Au-Ag composition according to the conditions of the galvanic displacement. Furthermore, their thermal and chemical stabilities significantly improved because of the protective Au thin layer on the surface. Using this developed process, chemically and thermally stable and flexible plasmonic metamaterials were successfully fabricated on a flexible polyester terephthalate substrate.</P> [FIG OMISSION]</BR>
Lee, Seung-Wook,Joh, Hyungmok,Seong, Mingi,Lee, Woo Seok,Choi, Ji-Hyuk,Oh, Soong Ju American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.30
<P>Ligand exchange is an advanced technique for tuning the various properties of nanocrystal (NC) thin films, widely used in the NC thin-film device applications. Understanding how the NC thin films transform into functional thin-film devices upon ligand exchange is essential. Here, we investigated the process of structural transformation and accompanying property changes in the NC thin films, by monitoring the various characteristics of silver (Ag) NC thin films at each stage of the ligand-exchange process. A transition state was identified in which the ligands are partially exchanged, where the NC thin films showed unexpected electromechanical features with high gauge factors up to 300. A model system was established to explain the origin of the high gauge factors, supported by the observation of spontaneously formed nanocracks and metal-insulator transition from the structural analysis and charge transport study, respectively. Taking advantages of the unique electromechanical properties of the NC thin films, we fabricated flexible strain gauge sensor devices with high sensitivity, reliability, and stability. We introduce a one-step fabrication process, namely, “the time- and spatial-selective ligand-exchange process”, for the design of low-cost and high-performance wearable sensors that effectively detect human motion, such as finger or neck muscle movement. This study provides a fundamental understanding of the ligand-exchange process in NCs, as well as an insight into the functionalities of the NC thin films for technological applications.</P> [FIG OMISSION]</BR>
Kim, Haneun,Lee, Seung-Wook,Joh, Hyungmok,Seong, Mingi,Lee, Woo Seok,Kang, Min Su,Pyo, Jun Beom,Oh, Soong Ju American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.1
<P>With the increase in interest in wearable tactile pressure sensors for e-skin, researches to make nanostructures to achieve high sensitivity have been actively conducted. However, limitations such as complex fabrication processes using expensive equipment still exist. Herein, simple lithography-free techniques to develop pyramid-like metal/insulator hybrid nanostructures utilizing nanocrystals (NCs) are demonstrated. Ligand-exchanged and unexchanged silver NC thin films are used as metallic and insulating components, respectively. The interfaces of each NC layer are chemically engineered to create discontinuous insulating layers, i.e., spacers for improved sensitivity, and eventually to realize fully solution-processed pressure sensors. Device performance analysis with structural, chemical, and electronic characterization and conductive atomic force microscopy study reveals that hybrid nanostructure based pressure sensor shows an enhanced sensitivity of higher than 500 kPa<SUP>-1</SUP>, reliability, and low power consumption with a wide range of pressure sensing. Nano-/micro-hierarchical structures are also designed by combining hybrid nanostructures with conventional microstructures, exhibiting further enhanced sensing range and achieving a record sensitivity of 2.72 × 10<SUP>4</SUP> kPa<SUP>-1</SUP>. Finally, all-solution-processed pressure sensor arrays with high pixel density, capable of detecting delicate signals with high spatial selectivity much better than the human tactile threshold, are introduced.</P> [FIG OMISSION]</BR>
Designing Surface Chemistry of Silver Nanocrystals for Radio Frequency Circuit Applications
Oh, Hanju,Lee, Seung-Wook,Kim, Minsoo,Lee, Woo Seok,Seong, Mingi,Joh, Hyungmok,Allen, Mark G.,May, Gary S.,Bakir, Muhannad S.,Oh, Soong Ju American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.43
<P>We introduce solution-based, room temperature- and atmospheric pressure-processed silver nanocrystal (Ag NC)-based electrical circuits and interconnects for radio frequency (RF)/microwave frequency applications. We chemically designed the surface and interface states of Ag NC thin films to achieve high stability, dc and ac conductivity, and minimized RF loss through stepwise ligand exchange, shell coating, and surface cleaning. The chemical and structural properties of the circuits and interconnects affect the high-frequency electrical performance of Ag NC thin films, as confirmed by high-frequency electromagnetic field simulations. An all solution-based process is developed to build coplanar structures, in which Ag NC thin films are positioned at both sides of the substrates. In addition, we fabricated flexible transmission lines and broadband electrical circuits for resistors, interdigitated capacitors, spiral and omega-shaped inductors, and patch antennas with maximum inductance and capacitance values of 3 nH and 2.5 pF at frequencies up to 20 GHz. We believe that our approach will lead to a cost-effective realization of RF circuits and devices in which sensing and wireless communication capabilities are combined for internet-of-things applications.</P> [FIG OMISSION]</BR>