DETECTOR MATERIALS: Economical nanocrystal fabrication demonstrated

Researchers at the University of California (Irvine, CA) have developed a method of semiconductor nanocrystal deposition on graphite surfaces that has potential to replace more-costly current fabrication techniques. The first crystals produced with the method, formed of copper iodide, have potential for efficient, more-economical ultraviolet (UV) detectors. Project head Reginald Penner describes the method as a hybrid electrochemical-chemical (E-C) process that can be done in less than 30 minutes.

The three-step E-C process produces the microscopic particles, containing as few as 100 atoms, on the smooth surface of a graphite crystal, which is one of the electrodes in an electrochemical cell. Penner notes, "We use about $10 worth of materials with laboratory equipment costing just $20,000" in fabricating copper iodide crystals. Conventional semiconductor nanocrystals are formed using $1 million molecular beam epitaxy (MBE) machines.

In producing these crystals, the graphite electrode functions as a cathode by being subjected to negative potential, allowing deposition of copper nanocrystals out of a water solution of copper ions. Next, with a positive voltage on the graphite, the electrode functions as an anode to oxidize the copper, producing copper oxide. These two steps take only about a minute. Finally, the graphite is flushed with water and then immersed in a potassium iodide solution in which the oxygen is exchanged for iodine, resulting in copper iodide nanocrystals.

Penner says the material has excellent luminescence properties when exposed to ultraviolet UV light. "The crystals are all very similar in size, and they align with each other on the graphite surface," he explains. Potential applications for these nanocrystals include forming sensitization layers for UV detectors.

The researchers are investigating the range of the E-C technique and are now working on fabricating II-VI materials. They have produced nanocrystals of cadmium sulfide, cadmium selenide, and zinc sulfide. In the future they are looking to make nanocrystals of III-V materials, such as gallium arsenide. The E-C development was partially funded by the Office of Naval Research and the National Science Foundation.