Quantum Dot Optical Memory.

The image at the right shows recent results obtained in our laboratory. We demonstrate here that under appropriate conditions, a low power laser beam can be used to write luminescent features in a quantum dot film. The same laser beam can then be used to read the stored information at a later time.

Our research centers around nanomaterials. Nanomaterials are materials which are extremely small (measured in billionths of a meter—the prefix "nano-" means one billionth) in at least one dimension. These nanomaterials may be nanoscale in only one dimension (such as a thin polymer film), or they may be nanoscale in all three dimensions (such as nanometer-scale particles). One focus of our research is a particular kind of nanoparticle called a "quantum dot." (What are quantum dots? Click here to find out more.)

Below, we highlight a few nanoscience projects that we’re working on right now.

• Electronic Coupling in Quantum Dot Arrays
  We are developing state-of-the-art synthetic approaches to build well-defined quantum dot arrays, including heterodimers linked by rigid, nanoscale molecular structures. For a description of some of our recent synthetic improvements, see D. G. Wu, M. E. Kordesch, and P. G. Van Patten, Chem. Mater. 2005, 17, 6436-6441. We are using a variety of steady-state and time-resolved spectroscopic methods to perform detailed, systematic measurements on the extent of electronic coupling between the quantum dots in these arrays as a function of the interparticle distance. We hope that these measurements will lead to a greater understanding of how energy, charge, and quantum information may be transferred between semiconductor quantum dots. This new understanding will aid in the development of new quantum dot-based technologies such as solar cells, lasers, and quantum computers.
• Nitride Quantum Dots
  We have discovered several new methods for synthesizing GaN quantum dots. GaN is a technologically important material for the development of optoelectronic devices (such as lasers and light-emitting diodes) operating in the blue and ultraviolet spectral regions. It has good chemical and thermal stability, can be doped both n-type and p-type, and has high luminescence yield. GaN quantum dots build on these advantageous properties by reducing undesirable trap state emission and by allowing bandgap tunability through control of particle size. Unfortunately, GaN quantum dots have historically proven exceedingly difficult to prepare. Our new methods represent a real breakthrough in materials synthesis and enable us to be among the first in the world to study this interesting material. Two manuscripts related to this work have been submitted, and another manuscript is currently in preparation.
• Polyelectrolyte Multilayer Thin Film
  Polyelectrolyte multilayers are a type of thin film containing different materials in different ultrathin layers. The film is built one layer at a time, and each layer is typically between 0.5 and 50 nm thick. This layer-by-layer (LbL) adsorption is a powerful method for including two or more different functional materials into a single film. In some cases, it might be desirable to ensure complete mixing of these different components on the nanometer scale (to make high-strength composite materials, for example). In other cases, it might be preferable to sequester these different materials in different regions of the film. (By doing so, it may be possible to emulate biological systems, which use compartmentalization of individual chemical processes to achieve very complicated overall functions.) Either case is possible with LbL film deposition if the experimental conditions are properly controlled. Although LbL assembly has now been around for more than a decade, certain aspects are just beginning to be understood. Our group is studying fundamental aspects of the deposition process such as specific ion interactions in these films. We have also recently published a study of the effects of temperature on the LbL process (see Tan, H. L.; McMurdo, M. J.; Pan, G.; Van Patten, P. G. Langmuir 2003, 19, 9311-9314, DOI: 10.1021/la035094f). Recently, we have been working in collaboration with Professor Harald Morgner’s group at the University of Leipzig in Germany to develop new approaches to studying these films. The first of these efforts is described in Tan, H. L.; Krebs, T; Andersson, G.; Neff, D.; Norton, M.; Morgner, H. Langmuir 2005, 21, 2598-2604, DOI: 10.1021/la047423p. Figure 1 shows a picture of several polyelectrolyte multilayer films on silicon wafers. The thicknesses of the films are given in the figure. Note that the material from which the films are made are colorless; the perceived color comes from an interference effect that depends upon the film thickness. Strong interference can only be observed if the films are quite smooth.

Figure 1. Polyelectrolyte thin films on Si[100]. These films are constructed of alternating layers of two polymers, PDDA and PSS. The materials are colorless, but the films appear colored due to Fabry-Perot interference effects related to the film thickness.