• Korean Journal of Optics and Photonics
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• v.26 no.5
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• pp.255-260
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• 2015

We have fabricated an optoacoustic ultrasound generator based on nanostructured germanium (Ge). Ge thin films were deposited via e-beam evaporation and then etched using a metal-assisted chemical (MAC) method to form nanostructured Ge films. The measured intensity of ultrasound from the nanostructured Ge covered with PDMS was about 3 times stronger than that of 100-nm-thick chromium (Cr). When the nanostructured Ge was embedded in the PDMS, the intensity of ultrasound became 8.5 times as strong compared to the 100-nm-thick Cr.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.30 no.3
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• pp.185-191
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• 2017

Cadmium compounds with one dimension (1D) nanostructures have attracted attention for their excellent electrical and optical properties. In this study, vertically aligned CdTe-Si nanostructures with high density were synthesized by several simple chemical reactions. First, l D Te nanostructures were synthesized by silver assisted chemical Si wafer etching followed by a galvanic displacement reaction of the etched Si nanowires. Nanowire length was controlled from 1 to $25{\mu}m$ by adjusting etching time. The Si nanowire galvanic displacement reaction in $HTeO_2{^+}$ electrolyte created hybrid 1D Te-branched Si nanostructures. The sequential topochemical reaction resulted in $Ag_2Te-Si$ nanostructures, and the cation exchange reaction with the hybrid 1D Te-branched Si nanostructures resulted in CdTe-Si nanostructures. Wet chemical processes including metal assisted etching, galvanic displacement, topochemical and cation exchange reactions are proposed as simple routes to fabricate large scale, vertically aligned CdTe-Si hybrid nanostructures with high density.

• Journal of the Korean institute of surface engineering
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• v.49 no.3
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• pp.301-306
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• 2016

The effects of nanotexturing and post-etching on the reflection and quantum efficiency properties of diamond wire sawn (DWS) multicrystalline silicon (mc-Si) solar cell have been investigated. The chemical solutions, which are acidic etching solution (HF-$HNO_3$), metal assisted chemical etching (MAC etch) solutions ($AgNO_3$-HF-DI, HF-$H_2O_2$-DI) and post-etching solution (diluted KOH at $80^{\circ}C$), were used for micro- and nano-texturing at the surface of diamond wire sawn (DWS) mc-Si wafer. Experiments were performed with various post-etching time conditions in order to determine the optimized etching condition for solar cell. The reflectance of mc-Si wafer texturing with acidic etching solution showed a very high reflectance value of about 30% (w/o anti-reflection coating), which indicates the insufficient light absorption for solar cell. The formation of nano-texture on the surface of mc-Si contributed to the enhancement of light absorption. Also, post-etching time condition of 240 s was found adequate to the nano-texturing of mc-Si due to its high external quantum efficiency of about 30% at short wavelengths and high short circuit current density ($J_{sc}$) of $35.4mA/cm^2$.

• Korean Journal of Materials Research
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• v.32 no.12
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• pp.522-527
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• 2022

Among efforts to improve techniques for the chemical vapor deposition of large-area and high-quality graphene films on transition metal substrates, being able to reliably transfer these atomistic membranes onto the desired substrate is a critical step for various practical uses, such as graphene-based electronic and photonic devices. However, the most used approach, the wet etching transfer process based on the complete etching of metal substrates, remains a great challenge. This is mainly due to the inevitable damage to the graphene, unintentional contamination of the graphene layer, and increased production cost and time. Here, we report the systematic study of an H₂ bubbling-assisted transfer technique for graphene films grown on Cu foils, which is nondestructive not only to the graphene film but also to the Cu substrate. Also, we demonstrate the origin of the graphene film tearing phenomenon induced by this H₂ bubbling-assisted transfer process. This study reveals that inherent features are produced by rolling Cu foil, which cause a saw-like corrugation in the poly(methyl methacrylate) (PMMA)/graphene stack when it is transferred onto the target substrate after the Cu foil is dissolved. During the PMMA removal stage, the graphene tearing mainly appears at the apexes of the corrugated PMMA/graphene stack, due to weak adhesion to the target substrate. To address this, we have developed a modified heat-press-assisted transfer technique that has much better control of both tearing and the formation of residues in the transferred graphene films.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.380-381
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• 2012

Bioinspired sea urchin-like structures were fabricated on silicon by inductively coupled plasma (ICP) etching using lens-like shape hexagonally patterned photoresist (PR) patterns and subsequent metal-assisted chemical etching (MaCE) [1]. The lens-like shape PR patterns with a diameter of 2 ${\mu}m$ were formed by conventional lithography method followed by thermal reflow process of PR patterns on a hotplate at $170^{\circ}C$ for 40 s. ICP etching process was carried out in an SF6 plasma ambient using an optimum etching conditions such as radio-frequency power of 50 W, ICP power of 25 W, SF6 flow rate of 30 sccm, process pressure of 10 mTorr, and etching time of 150 s in order to produce micron structure with tapered etch profile. 15 nm thick Ag film was evaporated on the samples using e-beam evaporator with a deposition rate of 0.05 nm/s. To form Ag nanoparticles (NPs), the samples were thermally treated (thermally dewetted) in a rapid thermal annealing system at $500^{\circ}C$ for 1 min in a nitrogen environment. The Ag thickness and thermal dewetting conditions were carefully chosen to obtain isolated Ag NPs. To fabricate needle-like nanostructures on both the micron structure (i.e., sea urchin-like structures) and flat surface of silicon, MaCE process, which is based on the strong catalytic activity of metal, was performed in a chemical etchant (HNO3: HF: H2O = 4: 1: 20) using Ag NPs at room temperature for 1 min. Finally, the residual Ag NPs were removed by immersion in a HNO3 solution. The fabricated structures after each process steps are shown in figure 1. It is well-known that the hierarchical micro- and nanostructures have efficient light harvesting properties [2-3]. Therefore, this fabrication technique for production of sea urchin-like structures is applicable to improve the performance of light harvesting devices.

• Current Photovoltaic Research
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• v.2 no.3
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• pp.130-134
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• 2014

We have fabricated the selective emitter solar cell using double textured nanowires structure. The $40{\times}40mm2$-sized silicon substrates were textured to form the pyramid-shaped surface and the nanowires were fabricated by metal assisted chemical etching process using Ag nanoparticles, subsequently. The heavily doped and shallow emitters for selectiv eemitter solar cells were prepared through the thermal $POCl_3$ diffusion and chemical etch-back process, respectively. The front and rear electrodes were prepared following conventional screen printing method and the widths of fingers have been optimized. The selective emitter solar cell using double textured nanowires structure achieved a conversion efficiency of 17.9% with improved absorption and short circuit current density.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.90.2-90.2
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• 2013

Vertically-aligned silicon nanostructure arrays (SNAs) have been drawing much attention due to their useful electrical properties, large surface area, and quantum confinement effect. SNAs are typically fabricated by chemical vapor deposition, reactive ion etching, or wet chemical etching. Recently, metal-assisted chemical etching process, which is relatively simple and cost-effective, in combination with nanosphere lithography was recently demonstrated for vertical SNA fabrication with controlled SNA diameters, lengths, and densities. However, this method exhibits limitations in terms of large-area preparation of unperiodic nanostructures and SNA geometry tuning independent of inter-structure separation. In this work, we introduced the layerby- layer deposition of polyelectrolytes for holding uniformly dispersed polystyrene beads as mask and demonstrated the fabrication of well-dispersed vertical SNAs with controlled geometric parameters on large substrates. Additionally, we present a new means of building in vitro neuronal networks using vertical nanowire arrays. Primary culture of rat hippocampal neurons were deposited on the bare and conducting polymer-coated SNAs and maintained for several weeks while their viability remains for several weeks. Combined with the recently-developed transfection method via nanowire internalization, the patterned vertical nanostructures will contribute to understanding how synaptic connectivity and site-specific perturbation will affect global neuronal network function in an extant in vitro neuronal circuit.

• Tribology and Lubricants
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• v.39 no.5
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• pp.197-202
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• 2023

Recently, research on experimental and analytical techniques utilizing microfluidic devices has been pursued. For example, lab-on-a-chip devices that integrate micro-devices onto a single chip for processing small sample quantities have gained significant attention. However, during sample preparation, unnecessary gases can be introduced into the internal channels, thus, impeding device flow and compromising specific function efficiency, including that of analysis and separation. Several methods have been proposed to mitigate this issue, however, many involve cumbersome procedures or suffer from complexities owing to intricate structures. Recently, some approaches have been introduced that utilize hydrophobic device structures to remove gases within channels. In such cases, the permeability of gases passing through the structure becomes a crucial performance factor. In this study, a method involving the deposition and sintering of diluted Ag-ink onto a silicon wafer surface is presented. This is followed by unstructured nano-pattern creation using a Metal Assisted Chemical Etching (MACE) process, which yields a nanostructured surface with unstructured pillar shapes. Subsequently, gas permeability in the spaces formed by these surface structures is investigated. This is achieved by experiments conducted to incorporate a pressure chamber and measure gas permeability. Trends are subsequently analyzed by comparing the results with existing theories. Finally, it can be confirmed that the significance of this study primarily lies in its capability to effectively evaluate gas permeability through unstructured pillar-like nanostructures, thus, providing quantitative values for the appropriate driving pressure and expected gas removal time in practical device operation.

• Journal of Electrochemical Science and Technology
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• v.2 no.3
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• pp.157-162
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• 2011

We report a simple route for synthesizing multi-dimensional structured silicon anode materials from commercially available bulk silicon powders via metal-assisted chemical etching process. In the first step, silver catalyst was deposited onto the surface of bulk silicon via a galvanic displacement reaction. Next, the silver-decorated silicon particles were chemically etched in a mixture of hydrofluoric acid and hydrogen peroxide to make multi-dimensional silicon consisting of one-dimensional silicon nanowires and micro-scale silicon cores. As-synthesized silicon particles were coated with a carbon via thermal decomposition of acetylene gas. The carbon-coated multi-dimensional silicon anodes exhibited excellent electrochemical properties, including a high specific capacity (1800 mAh/g), a stable cycling retention (cycling retention of 89% after 20 cycles), and a high rate capability (71% at 3 C rate, compared to 0.1 C rate). This process is a simple and mass-productive (yield of 40-50%), thus opens up an effective route to make a high-performance silicon anode materials for lithiumion batteries.

• Bulletin of the Korean Chemical Society
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• v.33 no.7
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• pp.2333-2337
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• 2012

We report the synthesis and thermal annealing of Si-core/ZnO-shell nanorods using a two-step process comprising the metal-assisted electroless etching of Si and the sputter deposition of ZnO. Transmission electron microscopy and X-ray diffraction analysis showed that the cores of the annealed core-shell nanorods were single crystal diamond cubic-type Si, whereas the shells of the annealed core-shell nanorods were single crystal wurtzite-type ZnO. The PL spectra of Si nanorods consisted of a broad red emission band and a weaker blue emission band. The major emission band of Si nanorods was shifted from 700 nm (in the red region) to 440 nm (in the violet region) by ZnO coating. The violet emission of the core-shell nanorods was enhanced in intensity considerably by annealing in an oxidizing atmosphere. The origin of the PL enhancement by annealing is also discussed.