Solution-based stoichiometric control over charge transport in nanocrystalline CdSe devices.

Using colloidal CdSe nanowire (NW) field-effect transistors (FETs), we demonstrated the dependence of carrier transport on surface stoichiometry by chemically manipulating the atomic composition of the NW surface. A mild, room-temperature, wet-chemical process was devised to introduce cadmium, selenium, or sulfur adatoms at the surface of the NWs in completed devices. Changes in surface composition were tested for by energy dispersive spectroscopy and inductively coupled plasma-atomic emission spectroscopy and through the use of the vibrational reporter thiocyanate. We found that treatment with cadmium acetate enhances electron currents, while treatment with sodium selenide or sodium sulfide suppressed them. The efficacy of doping CdSe NWs through subsequent thermal diffusion of indium was highly dependent on the surface composition. While selenium-enriched CdSe NW FETs were characterized by little to no electron currents, when combined with indium, they yielded semimetallic devices. Sulfur-enriched, indium-doped devices also displayed dramatically enhanced electron currents, but to a lesser extent than selenium and formed FETs with desirable ION/IOFF >10(6). The atomic specificity of the electronic behavior with different surface chalcogens suggested indium was bound to chalcogens at the NW surface, indicating commonalities with and implications for indium-containing CdSe nanocrystal films. Low temperature measurements of indium-doped CdSe NW FETs showed no evidence of impurity scattering, further supporting the existence of an indium-chalcogen interaction at the surface rather than in the core of the NW.