Eunpa Kim

Low‐Cost Facile Fabrication of Flexible Transparent Copper Electrodes by Nanosecond Laser Ablation

Low-cost Cu flexible transparent conducting electrodes (FTCEs) are fabricated by facile nanosecond laser ablation. The fabricated Cu FTCEs show excellent opto-electrical properties (transmittance: 83%, sheet resistance: 17.48 Ω sq(-1)) with outstanding mechanical durability. Successful demonstration of a touch-screen panel confirms the potential applicability of Cu FTCEs to the flexible optoelectronic devices.

Site Selective Doping of Ultrathin Metal Dichalcogenides by Laser-Assisted Reaction.

Laser-assisted phosphorus doping is demonstrated on ultrathin transition-metal dichalcogenides (TMDCs) including n-type MoS2 and p-type WSe2 . Temporal and spatial control of the doping is achieved by varying the laser irradiation power and time, demonstrating wide tunability and high site selectivity with high stability. The laser-assisted doping method may enable a new avenue for functionalizing TMDCs for customized nanodevice applications.

On demand shape-selective integration of individual vertical germanium nanowires on a Si(111) substrate via laser-localized heating.

Semiconductor nanowire (NW) synthesis methods by blanket furnace heating produce structures of uniform size and shape. This study overcomes this constraint by applying laser-localized synthesis on catalytic nanodots defined by electron beam lithography in order to accomplish site- and shape-selective direct integration of vertically oriented germanium nanowires (GeNWs) on a single Si(111) substrate. Since the laser-induced local temperature field drives the growth process, each NW could be synthesized with distinctly different geometric features. The NW shape was dialed on demand, ranging from cylindrical to hexagonal/irregular hexagonal pyramid. Finite difference time domain analysis supported the tunability of the light absorption and scattering spectra via controlling the GeNW shape.

Large-area nanoimprinting on various substrates by reconfigurable maskless laser direct writing

Laser-assisted, one-step direct nanoimprinting of metal and semiconductor nanoparticles (NPs) was investigated to fabricate submicron structures including mesh, line, nanopillar and nanowire arrays. Master molds were fabricated with high-speed (200 mm s(-1)) laser direct writing (LDW) of negative or positive photoresists on Si wafers. The fabrication was completely free of lift-off or reactive ion etching processes. Polydimethylsiloxane (PDMS) stamps fabricated from master molds replicated nanoscale structures (down to 200 nm) with no or negligible residual layers on various substrates. The low temperature and pressure used for nanoimprinting enabled direct nanofabrication on flexible substrates. With the aid of high-speed LDW, wafer scale 4 inch direct nanoimprinting was demonstrated.

On demand-direct synthesis of Si and Ge nanowires on a single platform by focused laser illumination

Laser irradiation can incur spatially confined and rapid heating that enables precisely controlled nucleation and subsequent growth of nanomaterials. This localization of the laser-driven growth can realize on-demand, direct synthesis of nanowires composed of multiple elements on a single platform. In this study, silicon and germanium nanowires are grown by laser-induced vapor-liquid-solid mechanism in a hetero-array configuration by simply switching the reactant gas precursors as the growth of nanowires is limited within the heat-affected zone induced by the laser. Energy dispersive x-ray and Raman spectroscopies were performed to observe the elemental composition and crystallinity of as-grown nanowires, respectively.

Time-resolved analysis of thickness-dependent dewetting and ablation of silver films upon nanosecond laser irradiation

Nanosecond pulsed laser dewetting and ablation of thin silver films is investigated by time-resolved imaging. Laser pulses of 532 nm wavelength and 5 ns temporal width are irradiated on silver films of different thicknesses (50 nm, 80 nm, and 350 nm). Below the ablation threshold, it is observed that the dewetting process does not conclude until 630 ns after the laser irradiation for all samples, forming droplet-like particles in the spot central region. At higher laser intensities, ablative material removal occurs in the spot center. Cylindrical rims are formed in the peripheral dewetting zone due to the solidification of transported matter at about 700 ns following the laser pulse exposure. In addition to these features, droplet fingers are superposed upon irradiation of 350-nm thick silver films with higher intensity.

In situ TEM Raman spectroscopy and laser-based materials modification

We present a modular assembly that enables both in situ Raman spectroscopy and laser-based materials processing to be performed in a transmission electron microscope. The system comprises a lensed Raman probe mounted inside the microscope column in the specimen plane and a custom specimen holder with a vacuum feedthrough for a tapered optical fiber. The Raman probe incorporates both excitation and collection optics, and localized laser processing is performed using pulsed laser light delivered to the specimen via the tapered optical fiber. Precise positioning of the fiber is achieved using a nanomanipulation stage in combination with simultaneous electron-beam imaging of the tip-to-sample distance. Materials modification is monitored in real time by transmission electron microscopy. First results obtained using the assembly are presented for in situ pulsed laser ablation of MoS2 combined with Raman spectroscopy, complimented by electron-beam diffraction and electron energy-loss spectroscopy.

Incubation behavior of silicon nanowire growth investigated by laser-assisted rapid heating

We investigate the early stage of silicon nanowire growth by the vapor-liquid-solid mechanism using laser-localized heating combined with ex-situ chemical mapping analysis by energy-filtered transmission electron microscopy. By achieving fast heating and cooling times, we can precisely determine the nucleation times for nanowire growth. We find that the silicon nanowire nucleation process occurs on a time scale of ∼10 ms, i.e., orders of magnitude faster than the times reported in investigations using furnace processes. The rate-limiting step for silicon nanowire growth at temperatures in the vicinity of the eutectic temperature is found to be the gas reaction and/or the silicon crystal growth process, whereas at higher temperatures it is the rate of silicon diffusion through the molten catalyst that dictates the nucleation kinetics.

Cutting-Processed Single-Wall Carbon Nanotubes with Additional Edge Sites for Supercapacitor Electrodes

Carbon nanotubes are frequently selected for supercapacitors because of their major intrinsic properties of mechanical and chemical stability, in addition to their excellent electrical conductivity. However, electrodes using carbon nanotubes suffer from severe performance degradation by the phenomenon of re-stacking during fabrication, which hinders ion accessibility. In this study, short single-wall carbon nanotubes were further shortened by sonication-induced cutting to increase the proportion of edge sites. This longitudinally short structure preferentially exposes the active edge sites, leading to high capacitance during operation. Supercapacitors assembled using the shorter-cut nanotubes exhibit a 7-fold higher capacitance than those with pristine single-wall nanotubes while preserving other intrinsic properties of carbon nanotubes, including excellent cycle performance and rate capability. The unique structure suggests a design approach for achieving a high specific capacitance with those low-dimensional carbon materials that suffer from re-stacking during device fabrication.

Tuning the optical and electrical properties of MoS2 by selective Ag photo-reduction

Two-dimensional transition metal dichalcogenides have demonstrated potential for advanced electrical and optoelectronic applications. For these applications, it is necessary to modify their electrical or optoelectronic properties. Doping is one of the most prevalent techniques to modify the band structure of semiconductor materials. Herein, we report the p-type doping effect on few-layer and multi-layer MoS2 that are selectively decorated with Ag nanoparticles via laser-assisted direct photoexcitation of MoS2 exposed in AgNO3 solution. This method can control the doping level by varying the duration of the laser irradiation, which is confirmed by the observed gradual rise of MoS2 device channel resistance and photoluminescence spectra enhancement. This study demonstrated a simple, controllable, and selective doping technique using laser-assisted photo-reduction.Two-dimensional transition metal dichalcogenides have demonstrated potential for advanced electrical and optoelectronic applications. For these applications, it is necessary to modify their electrical or optoelectronic properties. Doping is one of the most prevalent techniques to modify the band structure of semiconductor materials. Herein, we report the p-type doping effect on few-layer and multi-layer MoS2 that are selectively decorated with Ag nanoparticles via laser-assisted direct photoexcitation of MoS2 exposed in AgNO3 solution. This method can control the doping level by varying the duration of the laser irradiation, which is confirmed by the observed gradual rise of MoS2 device channel resistance and photoluminescence spectra enhancement. This study demonstrated a simple, controllable, and selective doping technique using laser-assisted photo-reduction.