The effects of electron–hole separation on the photoconductivity of individual metal oxide nanowires

Abstract

The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron-hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron-hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron-hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

Facile integration of ordered nanowires in functional devices

“…ZnO is one of the most promising materials for optoelectronic applications in the ultraviolet range [12], with remarkably high sensitivity when shaped in the form of nanowires [25][26][27].…”

“…On the other hand and as far as photodetection applications go, metal oxide nanowires show high intrinsic photoconductive gain values [12,25] mainly linked to the high crystallinity and ultimately caused by the electron-hole separation processes at the surface [26]). Additionally, in contrast with structures based on randomly oriented materials, parallel nanowire arrangements enable uniform light absorption through all the photoconductor material.…”

Persistent ion beam induced conductivity in zinc oxide nanowires

“…As the surface influence is increased in nanowires, greatly enhanced sensitivity is expected from nanowire devices [1,6]. Although the conductivity of ZnO can easily be increased by three orders of magnitude by UV excitation, the current only returns to the relaxed value very slowly due to the known Persistant Photo Conduction (PPC) effect [6,[8][9][10].…”

“…To contribute to a persistent current increase, excited charge carriers have to populate distinct, stable states. According to Prades et al [6] these are surface-oxygen bound states. Further discussion on the origin and properties of PPC can be found in literature [1, 6-10, 12, 13].…”

Enhancement of the photoelectrochemical properties of Cl-doped ZnO nanowires by tuning their coaxial doping profile

“…Considering a typical barrier height of 0.55 eV, 27 the width of the calculated depletion layer increases from 7 to 230 nm when reducing the carrier concentration from 10 19 cm À3 to 10 16 cm À3 . [27][28][29] In this scenario, the growth of an undoped ZnO shell on the surface of ZnO:Cl NWs offers an effective way to adjust the depletion region without modifying the NW's bulk conductivity (Fig. 4(d)).…”