Nanowires composed of direct-gap semiconductors have the potential to function as transistors in which light gates the flow of charge from one end of the nanowire to the other. In such devices, light with energy greater than the bandgap is absorbed by the nanowire producing mobile electrons and holes that increase the conductivity of the nanowire. The photoconductivity performance of the nanowires investigated to date, however, has revealed an unexpected limitation: The conductivity of photoexcited nanowires requires between seconds and minutes to decay in the dark. In the case of single crystalline ZnO nanowires, dark photoconductivity decays of 50s to 15,000s (4 hours) have been attributed to slow surface oxygen photochemistry coupled with a surface space charge. Slow photocurrent decays with time constants of 1-1000 s have also been reported for nanowires composed of GaN, SnO₂ and In₂O₃ but the reasons for these slow photoconductivity dark decays are unclear. We are unaware of published photoconductivity responses faster than those reported in these papers.

Shown above is the two-step electrochemical/chemical (E/C) method employed here for obtaining CdS HSNW arrays (Scheme 1) involves the preparation of cadmium (Cd) nanowires by electrochemical step edge decoration (ESED) on HOPG electrodes in the first step, according to the reaction:

Cd(EDTA)^2- + 2e^- Cd + EDTA^4-

In the second step, Cd nanowires were converted to CdS by exposure to flowing H₂S at 280-300 ^oC according to the reaction:

Cd + H₂S CdS + H₂

The starting point for this synthesis are cadmium nanowires, which are highly crystalline as shown below.

After conversion to CdS in flowing H₂S at 300 ^oC for 50 minutes, these wires increase in diameter, and become nanocrystalline. And something else happens that is not obvious from these images - they become hollow (just as indicated in Scheme 1). This radical geometry change is driven by disparate diffusion coefficients for sulfur and cadmium atoms in the nacsent CdS layer that forms initially during the conversion process; the diffusion of cadmium is much faster.

Surprisingly, these nanocrystalline CdS nanowires show bandedge photoluminescence with almost no trap state emission at all, and most importantly, are highly photoconductive. In contrast to the nanowire system studies up until now, the recovery of a low conductivity state in the dark (after illumination) is fast with these materials - fast enough that we can not distinguish the "turn off" rate from the "turn on" rate. Certainly, more work on this fascinating material is justified!