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

Abstract

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.

Recent advances in large-scale assembly of semiconducting inorganic nanowires and nanofibers for electronics, sensors and photovoltaics

“…8,10,[59][60][61][62][63][64][65][66][67][68][69] The applied electric fields can be used effectively to attract and align NWs normally by the dielectrophoresis forces, which are exerted on the dielectric NWs through the induced dipoles. 60,61 Up to now, various nanostructures, such as Au NWs, 60 Rh NWs, 9 Ag NWs, 65 Si NWs, 9 Se NWs, 65 axially modulated pn Si NWs, 67 InP NWs, 8 ZnO NWs, 62,64 CdSe NWs, 63 as well as polymer nanofibers 68 have been assembled by this approach. Fig.…”

Polarization features of optically pumped CdS nanowire lasers

“…Cadmium sulfide (CdS) nanowire lasers bridge the green spectral region around 2.4 eV, provide excellent optical mode confinement [20] and form quasi 1D optical systems with diameters between 150 nm -500 nm. Optical excitation anisotropy was already observed in a spontaneous excitation regime for ultrathin II-VI semiconductor NWs with diameters below 50 nm [21,22] and in photoconductivity studies on ZnO semiconductor NWs [23]. Hence, in optical one dimensional CdS NW laser resonators the excitation polarization should influence the absorption cross section and therefore also the pumping efficiencies, which might help to adjust and to modulate the laser emission directed out of the NW laser end facet.…”

Carrier recombination dynamics in individual CdSe nanowires

“…[15][16][17][18][19][20][21][22][23] This includes reports on spatially resolved NW emission spectra 20 , theoretical and experimental measurements of their ensemble and single wire absorption cross sections 21,22,24 as well as their polarization anisotropies. 23,25 Solution-grown CdSe NWs also exhibit other features that add complexity (and richness) to their optical properties. Namely, they exhibit zinc blende (ZB) and wurtzite (W) sequences within a given wire 26 due to the low energy difference between the two phases.…”