We report optical scanning measurements on photocurrent in individual Si nanowire field effect transistors (SiNW FETs). We observe increases in the conductance of more than 2 orders of magnitude and a large conductance polarization anisotropy of 0.8, making our SiNW FETs a polarization-sensitive, high-resolution light detector. In addition, scanning images of photocurrent at various biases reveal the local energy-band profile especially near the electrode contacts. The magnitude and polarity of the photocurrent vary depending on the gate bias, a behavior that can be explained using band flattening and a Schottky-barrier-type change. This technique is a powerful tool for studying photosensitive nanoscale devices.
Broad-spectrum analysis of DNA and RNA samples is of increasing importance in the growing field of biotechnology. We show that nanopore measurements may be used to assess the purity, phosphorylation state, and chemical integrity of nucleic acid preparations. In contrast with gel electrophoresis and mass spectrometry, an unprecedented dynamic range of DNA sizes and concentrations can be evaluated in a single data acquisition process that spans minutes. Because the molecule information is quantized and digitally recorded with single-molecule resolution, the sensitivity of the system can be adjusted in real time to detect trace amounts of a particular DNA species.T he purity of a nucleic acid preparation and the chemical integrity of nucleic acid bases affect the efficiency of hybridization procedures, enzymatic reactions, and chemical modifications. These processes dictate the accuracy and reliability of routine biochemical and clinical investigations as well as the expanding field of array technologies. Although traditional techniques of electrophoresis, chromatography, and mass spectrometry can assess DNA or RNA sample purity and chemical integrity, the sensitivity of these methods is limited by the relative size and quantity of contaminating nucleic acids. More importantly, the resolution of these methods decreases with increasing DNA or RNA length. Sample evaluation is difficult for nucleic acids with Ͼ100 nt and is virtually impossible for those with Ͼ1,000 nt. In a process we call single-molecule electrophoresis, we show that a transmembrane nanopore can evaluate polynucleotides with Ͻ100 bases or Ͼ1,000 bases. The ensemble pattern produced by the individual interaction between the polymer and the nanopore reveals DNA sample properties in a manner unparalleled by other detection systems.The nanopore was formed by self-assembly of ␣-hemolysin (1), a robust channel-forming protein that has been used and engineered for stochastic sensing, characterization of small molecules, and detection and discrimination of individual DNA strands (2-7). A single ␣-hemolysin channel was embedded in a lipid membrane that partitioned a conducting solution into two chambers. A voltage bias across the membrane produced an ion current through the ␣-hemolysin and also drove negatively charged single-stranded DNA from the cis to the trans side of the channel. Translocations occurred on the microsecond timescale and were signaled by the partial blockage of ion current to 10-25% of the open pore current value (Fig.
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