Semiconductor Nanocrystals
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Introduction The work I will describe on semiconductor nanocrystals started with the realization that it is possible to precipitate a semiconductor out of an organic liquid. We can precipitate out a semiconductor as a colloid—a very small-sized semiconductor with reduced dimensionality—that will show large, quantum size effects.1'2 A dream at that time was to make an electronic material by such a process in a liquid beaker, by starting with an organic fluid and somehow injecting something into the fluid to make very small particles, which we could use in electronics. The materials we use in electronics today have perfect crystalline order. We are able to put in dopants very specifically, or control precisely their arrangements in space in enormously complicated ways. The level of purity of electronic materials is so high that making an electronic material in a wet chemistry approach seems almost impossible. If, in addition, we specify that the size must be controlled precisely, we recognize the project is a problem for basic research, yet not one ready for applications. Many fundamental problems arise if we try to make semiconductor particles, in a liquid, of such high quality that they can be used as electronic materials. Over a period of time, the nanocrystals have improved in quality. I will present a snapshot of the status of these materials and some of the basic concepts that have emerged in this area. I also hope to illustrate some fundamental problems we have not solved, which prevent us from doing everything we would like. We are perhaps midpoint in a long process of translating a simple idea into an electronic material.
MRS BULLETIN/AUGUST 1995
First I will talk about the shape of nanocrystals, and how their shape changes under pressure when they change crystal structure. I will describe some work on direct- and indirect-gap nanocrystals, and finally some work on electroluminescence using nanocrystals.
Shapes To create crystallites, a set of precursors are injected into a very hot liquid, and upon injection the temperature immediately rises above the nucleation limit so that nucleation occurs, and then the temperature quickly drops. The concentration drops quickly because dilution occurs, resulting in crystallites in a fluid. The crystallites will be encapsulated in a layer of organic material so that they do not run into each other and fuse, and they will be as perfect as one can possibly hope.3 I will now show TEM micrographs. The TEM is, of course, a tremendously powerful tool in this business because it allows us to see the arrangements, sizes, and the important physical characteristics of the crystallites. Figure 1 shows
Figure 1. TEM micrograph showing many particles of CdSe. Taken by Andreas Kadavanich and Paul Alivisatos, using the facilities of the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory.
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Semiconductor Nanocrystals
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