Impurities Enhance Semiconductor Nanocrystal Performance

The electronic properties of free-standing semiconductor nanocrystals can be tuned by diffusing metallic impurities into them. The semiconductor industry annually spends billions of dollars deliberately adding atomic impurities, called dopants, into very pure semiconductors. Dopants can make devices run faster by increasing the number of negatively (n) or positively (p) charged mobile carriers. They can also determine the predominant type of charge carrier: electrons in n-type semiconductors, “holes” in p-type semiconductors (the dopant atom accepts an electron, resulting in the formation of a “hole”). Without doping, the fabrication of key transistor components such as p-n junctions would not be possible ( 1 ). Doping has had less impact on lower-cost devices in which semiconductor nanocrystals may be used, such as solar cells, printable low-power devices, and light-emitting diodes ( 2 ). Our knowledge of the doping effects on the electronic properties of semiconductor nanocrystals has been incomplete because of a lack of robust synthetic methods for doping free-standing nanocrystals (as opposed to thin films of nanocrystals). On page 77 of this issue, Mocatta et al. ( 3 ) report a solution-phase synthesis of metallically doped, free-standing indium arsenide (InAs) nanocrystals. They present strong evidence that both n- and p-type nanocrystals were formed, as well as insights into the electronic and optical effects of doping small nanocrystals (less than 10 nm in diameter).