The scientists start with polycarbonate foils (30 µm thick), which are irradiated with swift heavy ions (specific energy=11.4 MeV/nucleon) at GSI Darmstadt's UNILAC facility. The ion tracks are then etched to obtain cylindrical parallel pores. These nanoporous ion track membranes can now be used as templates for electrodepositing the diode nanowires.

When depositing CdTe, due to the characteristics of the electrochemical process, the operator is able to tune the composition just by choosing the electrode potential. Thus, the composition can be either stoichiometric on a certain potential range or present an excess of Cd or Te. A slight excess of Cd will lead to n-type conduction while a slight excess of Te will lead to p-type conduction.

By applying a predefined sequence of potentials, the researchers are able to deposit multisegment nanowires with the structure Cd – CdTe p – CdTe n –Cd. The electrical response of the material shows that these multi-segment nanowires exhibit rectifying IV characteristics and are sensitive to light.

A major advantage of the technique is that it can lead to ultra-high densities of photosensing devices in excess of 109 per centimetre square. The devices are functional as grown, no further manipulation being required.

Full details can be found in the journal Nanotechnology.