Electrochromic Nanocrystals

Nanotechnology is the science of creating devices at the atomic or molecular scale. At this scale, the nanometer (10-9 m) is the most common unit used to measure distance. Nanotechnology is still in it's infancy but it hold may hold the key to unlock incredible advances in medicine and computer technology.

Most tiny devices are made by paring down larger materials into the desired size and shape. For instance, the transistors used in most computers are carved out of silicon with ultraviolet light. This is known as the "top down" approach to creating nanometer size devices. Top down construction is rapidly approaching its limit. Many scientists are now persuing a "bottom up" approach that involves building up the desired devices from molecular components. Furthermore, materials at this elusive nanoscale exhibit unique and novel behavior. It is this behavior that specifically interests Chicago researchers Professor Philippe Guyot-Sionnest and graduate students Congjun Wang and Moonsub Shim.

The crystals they work with range from about 2-20 nm in diameter and have unique size dependant electrical and optical properties. The vials in Figure 1 are all filled with crystals made from the same compound; the size of the crystals gives them their different colors.

These nanocrystals are so small that adding a single electron or taking one away can significantly change the crystal's properties. The Guyot-Sionnest team here in Chicago has successfully injected electrons into various crystals to give them a negative charge. One of the changes observed when injecting electrons into CdSe nanocrystals is a complete bleach in the nanocrystal's visible absorption and the appearance of a new and unique infrared absorption as shown in Figure 2.

Recently the team has found a way to reverse the process to bring the nanocrystals back to their original state. The changes are significant enough to be visible to the naked eye. In Figure 3 you can see that these CdSe/ZnS core/shell nanocrystals are normally bright under ultraviolet light (A) but they lose their florescence when a -1.5 V potential is applied (B) and they return to normal after the potential is reset to 0 V (C). The on/off nature of this change means this semiconducting nanocrystal might be useful in creating displays of nanocrystals. At the moment, it takes about 20 minutes for the nanocrystal to turn back on after it's been tuned off do this process does not have technical applications yet. However, it is possible that this technique can be perfected as the scientists here at the Materials Center continue to experiment with different nanocrystals.