Dielectrophoresis is an effective method for capturing nanoparticles and assembling them into nanostructures. The frequency of the dielectrophoretic alternating current (ac) electric field greatly influences the morphology of resultant nanoparticle assemblies. In this study, frequency regimes associated with specific gold nanoparticle assembly morphologies were identified. Gold nanoparticles suspended in water were captured by microelectrodes at different electric field frequencies onto thin silicon nitride membranes. The resultant assemblies were examined by transmission electron microscopy. For this system, the major frequency-dependent influence on morphology appears to arise not from the Clausius-Mossotti factor of the dielectrophoretic force itself, but instead from ac electroosmotic fluid flow and the influence of the electrical double layer at the electrode-solution interface. Frequency regimes of technological interest include those forming one-dimensional nanoparticle chains, microwires, combinations of microwires and nanoparticle chains suitable for nanogap electrode formation, and dense three-dimensional assemblies with very high surface area.
A DNA sequencing device which integrates transverse conducting electrodes for the measurement of electrode currents during DNA translocation through a nanopore has been nanofabricated and characterized. A focused electron beam (FEB) milling technique, capable of creating features on the order of 1 nm in diameter, was used to create the nanopore. The device was characterized electrically using gold nanoparticles as an artificial analyte with both DC and AC measurement methods. Single nanoparticle/electrode interaction events were recorded. A low-noise, high-speed transimpedance current amplifier for the detection of nano to picoampere currents at microsecond time scales was designed, fabricated and tested for future integration with the nanopore device.
The mechanism by which an intense electron beam can produce holes in thin films of silicon nitride has been investigated using a combination of in situ electron energy loss spectrometry and electron microscopy imaging. A brief review of electron beam interactions that lead to material loss in different materials is also presented. The loss of nitrogen and silicon decreases with decreasing beam energy and although still observable at a beam energy of 150keV ceases completely at 120keV. The linear behavior of the loss rate coupled with the energy dependency indicates that the process is primarily one of direct displacement, involving the sputtering of atoms from the back surface of the specimen with the rate controlling mechanism being the loss of nitrogen.
Nanopore DNA sequencing devices show promise for rapidly decoding genetic information, although single nucleotide detection appears to be beyond the sensitivity of present transmembrane current measurement schemes. In this paper, a nanodevice combining nanoelectrodes with a nanopore was made to measure the nucleotide dependent transverse tunneling currents during the translocation of DNA through the nanopore. A custom high speed, low noise, integrated circuit current amplifier was designed and fabricated to measure the tunneling current. This nanodevice was characterized electrically using 2nm to 20nm gold nanoparticles as an artificial construct for DNA using both DC and AC detection schemes. Measured transverse tunneling currents were found to be quantized, probably according to the number of nanoparticles residing within the pore.
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