Center for Functional Nanomaterials Seminar

Controlling the size and dimensionality of colloidal semiconductor nanocrystals provide powerful means to tune their properties, and accompanied by the wet chemical processing, has also opened the path for their application in various technologies ranging from bio-tagging to displays. Electronic impurity doping in such colloidal nanocrystals, however, still remains an open challenge. From the synthesis side, the introduction of a few impurity atoms into a nanocrystal which contains only a few hundred atoms may lead to their expulsion to the surface or compromise the crystal structure. From a physical viewpoint, impurities inherently create a heavily doped nanocrystal under strong quantum confinement, and the electronic and optical properties in such circumstances are still unresolved.
We developed a solution based method to dope semiconductor nanocrystals with metal impurities providing control of the band gap and Fermi energy (Figure 1).1 Structural studies using Xray Absorption Spectroscopy techniques were used to determine the location of the induced impurities.2,3 A combination of optical measurements, scanning tunnelling spectroscopy and theory revealed the emergence of size dependent band-tailing effects and of a confined impurity pseudo band – signatures of heavily doped semiconductors. Ultrafast measurements investigating the carrier dynamics support the formation of the impurity pseudo band.4
Successful control of doping and its understanding, provide n- and p-doped semiconductor nanocrystals which greatly enhance the potential application of such materials in solar cells, thin-film transistors, and optoelectronic devices prepared by facile bottom-up methods.