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Secondary Crystal Growth on a Cracked Hydrotalcite-Based Film Synthesized by the Sol–Gel Method
Lee, Wooyoung,Lee, Chan Hyun,Lee, Ki Bong American Chemical Society 2016 Inorganic Chemistry Vol.55 No.9
<P>The sol-gel synthesis method is an attractive technology for the fabrication of ceramic films due to its preparation simplicity and ease of varying the metal composition. However, this technique presents some limitations in relation to the film thickness. Notably, when the film thickness exceeds the critical limit, large tensile stresses occur, resulting in a cracked morphology. In this study, a secondary crystal growth method was introduced as a post-treatment process for Mg/Al hydrotalcite-based films synthesized by the sol-gel method, which typically present a cracked morphology. The cracked hydrotalcite-based film was hydrothermally treated for the secondary growth of hydrotalcite crystals. In the resulting film, hydrotalcite grew with a vertical orientation, and the gaps formed during the sol-gel synthesis were filled with hydrotalcite after the crystal growth. The secondary crystal growth method provides a new solution for cracked ceramic films synthesized by the sol-gel method.</P>
Lee, Hwijong,Kim, Gwansik,Lee, Byunghun,Kim, Jeongmin,Choi, Soon-Mok,Lee, Kyu Hyoung,Lee, Wooyoung Elsevier 2017 Scripta materialia Vol.135 No.-
<P><B>Abstract</B></P> <P>Polycrystalline bulks of Si-content tuned Ge-doped higher manganese silicides (HMSs) were fabricated to elucidate the effects of Si content on the phase formation behavior and thermoelectric properties. The phase formation and electronic transport characteristics of HMSs were significantly dependent on Si content. Improved power factor was obtained at higher Si contents because of an enhanced Seebeck coefficient due to the increase in density of states effective mass, maintaining electrical conductivity. Furthermore, the lattice thermal conductivity was reduced through Si-content tuning, which suppressed the formation of secondary phases. Thus, a maximum <I>ZT</I> of 0.61 at 823K was obtained in MnSi<SUB>1.77</SUB>Ge<SUB>0.027</SUB>.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Lee, Eunsongyi,Lee, Jun Min,Koo, Ja Hoon,Lee, Wooyoung,Lee, Taeyoon Elsevier 2010 International journal of hydrogen energy Vol.35 No.13
<P><B>Abstract</B></P><P>We report the fabrication of a novel hydrogen sensor that utilizes the electrical resistance changes in the palladium thin films with nanometer thicknesses. The sensing mechanism is based on transitory absorption of hydrogen atoms into the palladium layer, which leads to the reversible alteration of the electrical resistance. In concentrated hydrogen ambient, the excess hydrogen absorption process leads to mechanical deformation on the surface of the palladium films, corresponding to the phase transition from α-phase to β-phase. The reversible sensing process results in a hysteresis curve for resistive properties, of which the height (sensitivity) could be controlled by manipulating the thickness of the palladium layers. The peel-off phenomena on the surface of the palladium film were suppressed by decreasing the thickness of the film. At the thickness of 20nm, a hysteresis curve of resistance was obtained without any structural change in the palladium thin film. These results provide a significant insight to the fundamental understanding of the relationship between the electrical sensitivity of pure Pd thin films and related structural deformation, which is essential to develop robust H-sensors with high sensibility.</P>
Effects of surface roughness on hydrogen gas sensing properties of single Pd nanowires.
Lee, Jun Min,Lee, Wooyoung American Scientific Publishers 2011 Journal of Nanoscience and Nanotechnology Vol.11 No.3
<P>We report on the effects of surface roughness resulting from an ion milling technique on the hydrogen gas sensing performance of a single Pd nanowire grown by electrodeposition into nanochannels in anodized aluminum oxide templates. A combination of electron beam lithography and a lift-off process was utilized to fabricate four-terminal devices based on individual Pd nanowires. These results are the first demonstration of the effect of ion milling on the response time in a single Pd nanowire used as a hydrogen sensor. The response time of the single Pd nanowire surface-treated by ion milling was 20 times faster than that of a sample without surface treatment. The faster response time was due to the surface roughness effects of the surface treatment, an increase in the surface-to-volume ratio of the ion-milled nanowire.</P>
Lee, Seunghyun,Ham, Jinhee,Jeon, Kyejin,Noh, Jin-Seo,Lee, Wooyoung IOP Pub 2010 Nanotechnology Vol.21 No.40
<P>We have systematically investigated the semimetal-to-semiconductor transition of individual single-crystalline Bi nanowires. For this work, we developed a technique to reduce the diameter of Bi nanowires grown by our unique on-film formation of nanowires (OFF-ON) method. Cooling down the substrate temperature during Bi film deposition by use of liquid nitrogen, film structures with small-sized grains were obtained. Through thermal annealing of these fine-granular Bi films, single-crystalline Bi nanowires can be produced with minimum diameter of ∼ 20 nm. Elaborative nanofabrication techniques were employed to shape state-of-the-art four-probe devices based on the individual small diameter Bi nanowires. Diameter-dependent transport measurements on the individual Bi nanowires revealed that the semimetal-to-semiconductor transition really occurred at about <I>d</I><SUB>w</SUB> = 63 nm. Moreover, band structure calculations supported this occurrence of the semimetal-to-semiconductor transition. </P>
Lee, Jun Min,Park, Ji-eun,Kim, Seri,Kim, Sol,Lee, Eunyoung,Kim, Sung-Jin,Lee, Wooyoung Elsevier 2010 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY - Vol.35 No.22
<P><B>Abstract</B></P><P>We have investigated the fabrication of hydrogen gas sensors based on networks of Pd nanoparticles (NPs) deposited tin dioxide nanowires (NWs). SnO<SUB>2</SUB> NWs with tin NPs attached on the surface were obtained by a simple thermal evaporation of SnO crystalline powders. The tin dioxide NWs were decorated with Pd NPs by the reduction process in Pd ion solution. The sensors showed ultra-high sensitivity (∼1.2 × 10<SUP>5</SUP>%) and fast response time (∼2 s) upon exposure to 10,000 ppm H<SUB>2</SUB> at room temperature. These sensors were also found to enable a significant electrical conductance modulation upon exposure to extremely low concentrations (40 ppm) of H<SUB>2</SUB> in the air. Our fabrication method of sensors combining with Pd NPs, Sn NPs and n-type semiconducting SnO<SUB>2</SUB> NWs allows optimized catalytic and depletion effect and results the production of highly-sensitive H<SUB>2</SUB> sensors that exhibit a broad dynamic detection range, fast response times, and an ultra-low detection limit.</P>