Competition of Photo-Excitation and Photo-Desorption Induced Positive and Negative Photoconductivity Switch in Te Nanowires.

The photocurrent in tellurium nanowire (Te NW) exhibits a subtle influence by many extrinsic factors. Herein, we fabricate Te NW devices and explore their photoresponse properties in detail. It is observed that the current increases greatly at low environmental relative humidity (RH) under light illumination, demonstrating an evident positive photoconductivity (PPC). However, the photocurrent reduces at high RH, yielding a typical negative photoconductivity (NPC). In addition, when exposed to a proper relative humidity, Te NW devices show PPC immediately and then transfer to NPC gradually under illumination, exhibiting the RH sensitive PPC/NPC switch. It is proposed that the competition between photo-excitation and photo-desorption is responsible for this subtle switch of PPC/NPC. On the one hand, the adsorbed water molecules on the surface of Te nanowires, acting as electron acceptors, lead to an increase of conductance, exhibiting the PPC phenomenon. On the other hand, the photo-desorption of water molecules from the surface results in a decreased carrier concentration in the Te nanowires, yielding the NPC phenomenon. The in-depth understanding of such charge transfer processes between the absorbed water molecules and Te nanowires provides an effective route to modulate the carrier densities and control the PPC/NPC switch, which will accelerate the design and application of novel optoelectronic nanodevices.