Radial heterojunction based on single ZnO-CuxO core-shell nanowire for photodetector applications

Abstract

ZnO-Cu x O core-shell radial heterojunction nanowire arrays were fabricated by a straightforward approach which combine two simple, cost effective and large-scale preparation methods: (i) thermal oxidation in air of a zinc foil for obtaining ZnO nanowire arrays and (ii) radio frequency magnetron sputtering for covering the surface of the ZnO nanowires with a Cu x O thin film. The structural, compositional, morphological and optical properties of the high aspect ratio ZnO-Cu x O core-shell nanowire arrays were investigated. Individual ZnO-Cu x O core-shell nanowires were contacted with Pt electrodes by means of electron beam lithography technique, diode behaviour being demonstrated. Further it was found that these n-p radial heterojunction diodes based on single ZnO-Cu x O nanowires exhibit a change in the current under UV light illumination and therefore behaving as photodetectors.

Photodetecting properties of single CuO–ZnO core–shell nanowires with p–n radial heterojunction

“…Using these equations, the key parameters of a photodetector (R λ , EQE and D * ) were estimated to be: 4.84 A/W, 11.59% and 22.52 × 10 9 Jones for 520 nm and 8.74 A/W, 26.56% and 40.5 × 10 9 Jones for 405 nm, respectively. The values obtained for the R λ , EQE and D * parameters are in agreement with data reported in the literature for photodetectors based on core–shell nanowires 16 , 59 – 61 .…”

“…It can be noticed a rise and decay time of 35 s for both wavelengths and a photocurrent gain of 2.23 nA at 520 nm and 4.01 nA at 405 nm. Comparing to our previous study 16 , it can be remarked and improvement in terms of photocurrent gain and rise and decay time at 405 nm, most probably due to the improved geometry of the single CuO–ZnO core–shell nanowire contacted with Pt directly on the CuO core and with Ti–Au on the ZnO shell. Furthermore, it was observed an increase of the photocurrent in time under illumination due to the effect of heating produced through the illumination.…”

“…The next step was to dissolve a part of the ZnO shell at the other end of the single CuO–ZnO core–shell nanowire to be able to connect only the CuO core with a Pt contact that leads to the Ti–Pt interdigitated electrodes. In this way, comparing to our previous study 16 , proper electrical contacts are provided to the single p–n radial heterojunction nanowire diode. Thus, considering that ZnO at the nanoscale dimension can be dissolved in water-based solutions 39 , 49 , a second EBL process was performed for controlled opening of a well-defined area at the other end of the single CuO–ZnO core–shell nanowire in a new PMMA layer.…”

“…In this type of heterojunction, charge separation region is distributed over the radius of the nanowire, which is significantly smaller than its length leading to an improved collection at the electrodes 14 . Thus, these radial core–shell heterostructure nanowires can be regarded as “smart materials” with controlled interfaces and enhanced multi-functionalities that can lead to the development of the next-generation high-performance optoelectronic devices 14 – 16 . Hence, radial core–shell nanowire arrays based on ZnO–Cu x O 16 , ZnO/CuCrO 2 17 , ZnO/WS 2 18 , ZnO–Cu 2 O 19 , ZnO–Co 3 O 4 20 , ZnO/ZnS 21 , CdSe/ZnTe 22 , etc.…”

“…By combining the two materials, CuO and ZnO, into a radial core–shell architecture as CuO–ZnO nanowires, a type II band alignment is obtained between the two semiconductors. The radial heterostructure architecture leads to an improved charge carrier collection efficiency due to the formation of a specific, high surface area junction which separates charges with enhanced efficiency and suppresses their recombination 16 , 40 . Consequently, such radial core–shell heterostructure can lead to an enhancement of the photocurrent produced under illumination, fundamental in photodetectors 14 , these optoelectronic devices being in the spotlight of the researchers due to their applications in a wide range of areas like advanced communications devices, flame detectors, ozone sensors, missile warning system devices, healthcare related sensors, etc.…”