ME PhD Oral Examination - Dong Rip Kim (1pm, 10/28/2011, Nano Bldg. 232)

Dong Rip Kim dongrip at stanford.edu
Mon Oct 24 10:34:25 PDT 2011


Ph.D. Dissertation Defense

Characterization and Optimization of Silicon Nanowires

for Biosensing and Photovoltaic Applications 

Dong Rip Kim

Department of Mechanical Engineering

Advisor: Prof. Xiaolin Zheng

Time: Friday, 28th October 2011, 1pm (Refreshments at 12:45pm)

Venue: Center for Nanoscale Science and Engineering (Nano Building)

Conference room # 232

Research Abstract

One-dimensional nanowires, due to their excellent electrical, optical, and chemical properties, and large surface to volume ratios, offer the promise of producing revolutionary advances in many areas of technology. This study investigates two application examples of Si nanowires (SiNWs) in the areas of biosensing and solar cells, respectively.

First, SiNWs have been highlighted as real-time, label-free, multiplexing, and femtomolar level accuracy biosensors. However, SiNWs are frequently placed on the bottom wall of a microfluidic channel in the sensing process where the convection velocity is almost zero such that the performance of SiNW biosensors is restrained by the analyte transport. Moreover, although the sensing is carried out in a microfluidic environment where the charged analytes coexist with charged ions of the electrolytic solution, efforts are lacking in understanding the effects of charged ions in the electrolytic solution on the sensing performance of SiNWs. Herein, this study focuses on the characterization and optimization of microfluidic sensing environment for SiNW biosensors in terms of the response time, sensitivity, and selectivity. Our numerical studies suggest that the diffusive analyte transport currently limits the response time of SiNW biosensors. Through microfluidic optimizations, about 17.4 times faster response time and doubled sensitivity are achievable in SiNW biosensing. In addition, our experimental investigation shows that SiNWs can also detect ionic movements and distributions other than targeted species inside the microfluidic channel, which can affect the selectivity of SiNW biosensors and lead to false signals.

Second, vertically aligned radial junction Si wire array solar cells can offer the opportunity to use lower grade material to produce efficient solar cells, due to the decoupling of light absorption and charge-carrier collection directions, and enhanced light trapping. Nevertheless, radial junction Si wire array solar cells still face critical challenges such as large junction and surface recombination losses of photogenerated charge carriers due to their inherent large surface area, which is one of the primary reasons responsible for the gap between reported experimental efficiencies (typically below 10%) and the estimated 17% theoretical efficiency. Therefore, we experimentally investigated junction and surface passivation strategies to effectively suppress large junction and surface recombinations of vertically aligned radial junction Si wire array solar cells. The inclusion of intrinsic polycrystalline Si layer between the p-n junction layers increases the efficiency by about 30% by reducing the dark current, and the top amorphous silicon nitride layer improves the efficiency by about 20% due to its combined surface passivation and antireflection effects. With the combination of both passivation layers, the maximum efficiency of our solar cells is improved from 7.2% to 11.0% under AM 1.5G illumination.
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