Showing papers by "Dahl Young Khang published in 2021"

Structuring of Si into Multiple Scales by Metal-Assisted Chemical Etching

Abstract: Structuring Si, ranging from nanoscale to macroscale feature dimensions, is essential for many applications. Metal-assisted chemical etching (MaCE) has been developed as a simple, low-cost, and scalable method to produce structures across widely different dimensions. The process involves various parameters, such as catalyst, substrate doping type and level, crystallography, etchant formulation, and etch additives. Careful optimization of these parameters is the key to the successful fabrication of Si structures. In this review, recent additions to the MaCE process are presented after a brief introduction to the fundamental principles involved in MaCE. In particular, the bulk-scale structuring of Si by MaCE is summarized and critically discussed with application examples. Various approaches for effective mass transport schemes are introduced and discussed. Further, the fine control of etch directionality and uniformity, and the suppression of unwanted side etching are also discussed. Known application examples of Si macrostructures fabricated by MaCE, though limited thus far, are presented. There are significant opportunities for the application of macroscale Si structures in different fields, such as microfluidics, micro-total analysis systems, and microelectromechanical systems, etc. Thus more research is necessary on macroscale MaCE of Si and their applications.

Soft-lithographically line-patterned In-doped ZnO quantum dots with hydrothermally grown ZnO nanocolumns for acetone detection

Abstract: We introduced a new approach to enhance the sensing performance of sensors based on metal oxide semiconductors. We synthesized In-doped ZnO quantum dots (IZO QDs) by a hydrothermal method and fabricated line-patterned IZO QD layers with polydimethylsiloxane (PDMS) molds. Additional hydrothermal growth was conducted to create ZnO nanocolumns (NCs) on the patterned surfaces. Thus, drop-cast and line-patterned sensors and line-patterned samples with NCs grown for 0.5 h (NC(0.5 h)/Line) and 4 h (NC(4 h)/Line) were prepared. Among these four different sensors, the NC(0.5 h)/Line sensor exhibited an excellent response of 26,000 with fast response times of less than 1 s to 10 ppm of acetone. In addition, the detection limit was approximately 0.1 ppm of acetone, and the resistance was almost constant even after repeatability tests. According to UV–vis and X-ray photoelectron spectroscopic analyses, the extraordinary sensing characteristics of NC(0.5 h)/Line were mainly because this sensor had the largest optical band energy and the highest ratio of oxygen vacancies among the tested sensors. On the other hand, the NC(4 h)/Line sensor showed the lowest response of the four sensors. During the long-term growth of NCs, OH groups are produced on the surface of the material, and Zn(OH)2 NCs are formed instead of ZnO NCs, resulting in a large decrease in the carrier concentration and active sites. In conclusion, we developed highly sensitive acetone sensors by constructing special morphologies.

Selective growth of ZnO nanorods by thickness contrast in In-doped ZnO quantum dots seed layer.

Abstract: Selective growth of ZnO nanorods (NRs) have been demonstrated using thickness contrast in In-doped ZnO (IZO) quantum dot (QD) seed layer. The use of IZO QD as a seed layer has enabled the direct growth of ZnO NRs on soft substrates such as polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS). Depending on the annealing temperature, the seed layers show different grain sizes: as the annealing temperature increases, the seed grain size also increases accordingly. Interestingly, the hydrothermal growth of ZnO NRs has been found to depend on the seed grain size: the larger grain seed sample shows earlier start of growth compared to smaller seed grain counterpart. The same growth behavior has been found in the growth of ZnO NRs on seed layers having different thickness, due again to the difference in seed grain size. To advantageously exploit the observed growth behavior, the IZO QDs seed layers have been patterned by soft lithographic technique, which led to the formation of alternating thin/thick region periodically. On this patterned seed surface, the thin regions showed earlier start of NRs growth compared to thick regions, enabling the spatially selective growth of ZnO NRs. When applied for acetone gas sensors, the selectively grown sample showed better performance than the non-selectively grown counterpart. The low resistance in air, due to increased amount of chemisorbed oxygen, has been found to be responsible for the inferior sensor performance with non-selectively grown sample.