CdSe nanowires with illumination-enhanced conductivity: Induced dipoles, dielectrophoretic assembly, and field-sensitive emission

Positive ac dielectrophoresis (DEP) is used to rapidly align ensembles of CdSe semiconductor nanowires (NWs) near patterned microelectrodes. Due to their large geometric aspect ratio, the induced dipole of the wires is proportional to their conductivity, which can be drastically enhanced under super-band-gap illumination by several orders of magnitude, with a corresponding increase in the wire DEP mobility. This optical enhancement of conductivity occurs because of the generation of mobile electrons and holes and is verified by a photocurrent measurement. The linear nanowire alignment exhibits a high degree of fluorescent polarization anisotropy in both absorption and emission. An unexpected observation is a reversible, factor of $∼4$⁠, electric-field-induced, and frequency-dependent enhancement of the nanowire emission near $10Hz$⁠. Such illumination-sensitive, field-enhanced, and frequency-dependent alignment and emission phenomena of NWs suggest an electrical-optical platform for fabricating CdSe nanowire devices for polarization-sensitive photodetection and biosensing applications.