Minhee Yun

Alumina nanotemplate fabrication on silicon substrate

Alumina nanotemplates integrated on silicon substrate with pore diameters of 12?100?nm were prepared by galvanostatic (constant current) anodization. High current density (e.g.?100?mA?cm?2) promoted a highly ordered hexagonal pore structure with fast formation rate independent of anodizing solution. Alumina formation rates of 2000 and 1000?nm?min?1 were achieved at current densities of 100 and 50?mA?cm?2, respectively. These rates were approximately two orders of magnitude greater than other reports in the literature. Different electrolytes of sulfuric acid (1.8?7.2?M), oxalic acid (0.3?M) and mixed solutions of sulfuric and oxalic acids were evaluated as anodizing solutions. At fixed current density, sulfuric acid promoted smaller pore diameter with lower porosity than mixed acids and oxalic acid. The I?V characteristics of aluminium anodization show the measured voltages at given current densities strongly depend on solution composition, operating temperature, and bath agitation. The pore diameter of the silicon-integrated alumina nanotemplate varied linearly with measured voltage with a slope of 2.1?nm?V?1, which is slightly smaller than reported data.

Nanowire growth for sensor arrays

A design concept for nanowire-based sensors and arrays is described. The fabrication technique involves electrodeposition to directly grow nanowires between patterned thin film contact electrodes. To prove our concept, we have electrodeposited 1-μm diameter Pd single wires and small arrays. To demonstrate nanowire sensors, we have electrochemically grown metal (Pd, Au, Pt), metal oxide (Sb2O3), and conducting polymer (polyaniline) bundled nanowires. Using Pt bundled nanowires surface modified with glucose oxidase, we have demonstrated glucose detection as a demonstration of a biomolecular sensor.

Single metal and conducting polymer nanowire sensors for chemical and DNA detections

Due to the small size, sensitivity, real time detection, and ultra-low power demands, nanowire sensors are being investigated for detection of a wide range of chemical and biochemical species. However, techniques used to fabricate these nanowire sensors have drawbacks of limited controllability and manufacturability. Reliable and controllable nanowire fabrication remains a significant challenge. In this work, we have developed a fabrication technique that is potentially capable of producing arrays of individually addressable nanowire sensors with controlled dimensions, positions, alignments, and chemical compositions and are in the process of fabricating sensor arrays to detect gases, and biochemicals. Fabrication of single Pd nanowires with diameters from 70 nm to 300 nm and up to 7 µm in length will be presented. These nanowires are used to sense hydrogen gas at concentrations as low as 0.02% and show a response time of 300 msec with an operating power of 350 nW. We also recently demonstrated the feasibility of fabricating single polypyrrole and polyaniline nanowires and their application as DNA sensor (1 nM). Currently, we are investigating single nanowire field effect transistors (SNWFET) for possible applications which include label-free DNA detection, early detection of disease signatures, and environmental monitoring.

The controlled growth of single metallic and conducting polymer nanowires via gate-assisted electrochemical deposition

The fabrication of nanowires with well-controlled lengths and diameters is the basis of the application of one-dimensional nanostructures in more sophisticated electronic and biomolecular device systems. A wide variety of materials, including metals and conducting polymers, have been utilized in nanowire arrays as building blocks for chemical or biomolecular sensors. Thus far, the cheapest and most effective way of nanowire synthesis is electrochemical deposition. In this work, we investigate a new method of electrochemical deposition using two-dimensional electric fields instead of the conventional one-directional electric field between working electrodes. Reproducible fabrication of metallic (palladium) and conducting polymer (polypyrrole) single nanowires with diameters down to 30-50 nm is achieved by application of a vertical gate electric field in addition to the lateral one between the two working electrodes. Diameters and lengths of the nanowires can be easily controlled by varying the dimensions of the nanochannels in which the nanowires are grown. A good ohmic contact between the nanowire and gold electrodes is also obtained, indicating the feasibility of electronic devices based on the single nanowires synthesized via this method. In conjunction with experimental findings of nanowire growth mechanism under two-dimensional electric field, molecular dynamic simulations are employed to further understand the deposition process. This improved electrochemical deposition is applicable for controlled and simple fabrication of a wide range of metallic and conducting polymeric nanowires with small diameters.

Microfabrication of silicon–nitride micromesh bolometric detectors for planck high frequency instrument

The high frequency instrument (HFI) on the National Aeronautics and Space Administration/European Space Agency Planck Surveyor, scheduled for launch in 2007, will map the entire sky in six frequency bands ranging from 100 to 857 GHz to probe cosmic microwave background anisotropy and polarization with angular resolution ranging from 9[prime] to 5[prime]. The HFI focal plane will contain 48 silicon–nitride micromesh bolometers operating from a 100 mK heat sink. Four detectors in each of the six bands will detect unpolarized radiation. An additional four pairs of detectors will provide sensitivity to linear polarization of emissions at 143, 217, and 353 GHz. We have fabricated and developed sensitive Si3N4 micromesh spider-web bolometers for submillimeter observation using microelectromechanical system techniques. The spiderweb architecture in this research provides high infrared absorption with minimal heat capacity and volume.

Electrochemically grown single-nanowire sensors

We report a fabrication technique that is potentially capable of producing arrays of individually addressable nanowire sensors with controlled dimensions, positions, alignments, and chemical compositions. The concept has been demonstrated with electrodeposition of palladium wires with 75 nm to 350 nm widths. We have also fabricated single and double conducting polymer nanowires (polyaniline and polypyrrole) with 100nm and 200nm widths using electrochemical direct growth. Using single Pd nanowires, we have also demonstrated hydrogen sensing. It is envisioned that these are the first steps towards nanowire sensor arrays capable of simultaneously detecting multiple chemical species.