Silicon nanowires and biosensors

Over the past several years, exciting advances have been achieved with chemically synthesized silicon nanowires (SiNWs) as both unique system to study novel transport properties in low dimensional structures and versatile building blocks to functional devices. SiNWs possess single crystalline structure with a high degree of material perfection, and low temperature measurement reveals that coherent single charge tunneling in molecular-scale SiNWs can extend over hundreds of nanometers, which leads to greatly improved room-temperature performance of SiNW field effect transistors (FETs) over conventionally lithography-defined structures. Their applications in biomolecule detection offer a general, robust, and powerful platform for both sensitive biomolecule analysis and study of fundamental biophysics.; In this thesis, I first present research efforts focused on fundamental studies of SiNWs and their tunable transport performance as FETs. SiNWs are rationally synthesized via a metal cluster-catalyzed vapor-liquid-solid (VLS) growth mechanism. Low temperature transport studies on molecular scale SiNWs weakly coupled to contacts reveal phase coherent single charge transport through discrete single particle quantum levels with length scales up to several hundred nanometers. Further, first order tunneling in SiNWs with enhanced coupling to contacts can be pseudo transparent and cotunneling processes provide an accurate method to study the energy spectrum in molecular scale SiNWs. Correspondingly, room temperature electrical transport studies carried out on SiNW-FETs show controllable and exceptional device performance.; SINW-FETs modified with surface receptors such as peptide nucleic acids can directly detect DNA molecules in the solution. Significantly, by solid state reaction between silicon and nickel contacts at temperature as low as 400°C, these novel sensors can further shrink to sub-hundred nanometer long and are sensitive enough to measure a single DNA oligonucleotide as short as 10-mer. Statistical analysis of single molecule time trajectories by these ultra short SiNW-FETs reveals binding/unbinding rates of complementary DNA-PNA duplex. In addition, studies of 10-mer DNA containing 0, 1 and 2 mismatches show clearly that these subtle differences can be distinguished both qualitatively and quantitatively from the single-molecule trajectories. This study opens up new opportunities for fundamental biophysical research and genomics.; SiNW-FETs are further fabricated on tens' of nanometer thin suspended silicon nitride membranes of transmission electron microscopy (TEM) grids. Focused-illumination with high resolution TEM reliably/controllably punches nanometer-sized pores through nanowires and the membrane. Conductance versus time measurements with NW-FETs reveal single lambda-DNA translocations driven through nanowires with the nanopore as guidance. Combined with the capability of creating large addressable arrays of these sensors, this study offers promise for cheap, fast, and high throughput DNA sequencing.