Nicholas Prokopuk

Alligator clips to molecular dimensions

Techniques for fabricating nanospaced electrodes suitable for studying electron tunneling through metal-molecule-metal junctions are described. In one approach, top contacts are deposited/placed on a self-assembled monolayer or Langmuir-Blodgett film resting on a conducting substrate, the bottom contact. The molecular component serves as a permanent spacer that controls and limits the electrode separations. The top contact can be a thermally deposited metal film, liquid mercury drop, scanning probe tip, metallic wire or particle. Introduction of the top contact can greatly affect the electrical conductance of the intervening molecular film by chemical reaction, exerting pressure, or simply migrating through the organic layer. Alternatively, vacant nanogaps can be fabricated and the molecular component subsequently inserted. Strategies for constructing vacant nanogaps include mechanical break junction, electromigration, shadow mask lithography, focused ion beam deposition, chemical and electrochemical plating techniques, electron-beam lithography, and molecular and atomic rulers. The size of the nanogaps must be small enough to allow the molecule to connect both leads and large enough to keep the molecules in a relaxed and undistorted state. A significant advantage of using vacant nanogaps in the construction of metal-molecule-metal devices is that the junction can be characterized with and without the molecule in place. Any electrical artifacts introduced by the electrode fabrication process are more easily deconvoluted from the intrinsic properties of the molecule.

RF GaN HEMT Sensors for Detection of Caustic Chemicals

For future wireless sensor network applications, high-speed RF GaN HEMTs (high electron mobility transistor) are investigated for toxic industrial chemical detection, for the first time. RF GaN HEMTs with a Pt-based gate metal show reliable, repeatable, and distinctive responses toward Cl2 gas and HCl vapor at room temperature.

Development of GaN-based micro chemical sensor nodes

Sensors based on III-N technology are gaining significant interest due to their potential for monolithic integration of RF transceivers and light sources and the capability of high temperature operations. We are developing a GaN-based micro chemical sensor node for remote detection of chemical toxins, and present electrical responses of AlGaN/GaN HEMT (high electron mobility transistor) sensors to chemical toxins as well as other common gases. Upon exposure to a chemical toxin, the sensor showed immediate increase in source-drain current (Ids). The electrical response of the sensor was clear, reproducible and characteristic of the concentration of the analyte. This is the first time that electrical responses of chemical toxins are measured with a GaN-based microsensor. Detailed analysis on response time, sensitivity and temperature dependence will be discussed

Nanocrossbar arrays as molecular sensors

Electron tunneling between nanospaced electrodes provides a mechanism for directly transducing the presence of molecular analytes into electrical signals. Crossbar junctions with vertical separations on the order of a few nanometers were fabricated using a combination of electron-beam lithography and selective chemical etching. The current-voltage properties of the nanojunctions are highly sensitive to the chemical environment. The tunneling currents increase over one order of magnitude in response to water and organic vapors diluted with a background of pure nitrogen. The resistance of the junctions is also dependent on the concentration of the analyte. These results demonstrate that tunneling can be used to detect changes in the chemical environment.