Influence of Exciton Localization on the Emission and Ultraviolet Photoresponse of ZnO/ZnS Core–Shell Nanowires

Abstract

The structural and optical properties of ZnO and ZnO/ZnS core-shell nanowires grown by a wet chemical method are investigated. The near-bandgap ultraviolet (UV) emission of the ZnO nanowires was enhanced by four times after coating with ZnS. The enhanced emission was attributed to surface passivation of the ZnO nanowires and localized states introduced during ZnS growth. The emission of the ZnO and ZnO/ZnS core-shell nanowires was attributed to neutral donor-bound excitons and localized excitons, respectively. Localized states prevented excitons from diffusing to nonradiative recombination centers, so therefore contributed to the enhanced emission. Emission from the localized exciton was not sensitive to temperature, so emission from the ZnO/ZnS core-shell nanowires was more stable at higher temperature. UV photodetectors based on the ZnO and ZnO/ZnS core-shell nanowires were fabricated. Under UV excitation, the device based on the ZnO/ZnS core-shell nanowires exhibited a photocurrent approximately 40 times higher than that of the device based on the ZnO nanowires. The differing photoresponse of the detectors was consistent with the existence of surface passivation and localized states. This study provides a means for modifying the optical properties of ZnO materials, and demonstrates the potential of ZnO/ZnS core-shell nanowires in UV excitonic emission and detection.

Investigation of localized and delocalized excitons in ZnO/ZnS core-shell heterostructured nanowires

“…As shown in Figure 5B, the curves of FWHM show the "valley-shape" features that gradually disappear after thermal treatments. The "S-shape" and "valley-shape" characteristics obtained from the samples' emission have further confirmed the existence of localized states, which is consistent with previous reports [1,2,[6][7][8].…”

“…It has been confirmed that the localized effect can prevent the generated carriers from defusing to nonradioactive recombination centers, thus enhancing the emission and the response range of light. Therefore, at present, the localized states in the semiconductor nanomaterial have stimulated intensive research attention for high-performance excitonic optoelectronic devices, such as light-emitting diodes (LEDs), lasers, solar cells, and photodetectors [6][7][8][9][10][11][12].…”

“…Recently, in our group, ZnO/ ZnS core-shell NWs were successfully synthesized by wet chemical method, in which localized exciton phenomena were found after the ZnS growth on ZnO NWs. The ZnO/ ZnS nanocomposite structures exhibit remarkable UV photoresponse and enhanced electroluminescence from LEDs, which have been ascribed to the existence of localized states in the material system [6,8].…”

“…In this paper, through the growth of ZnO/ ZnS core-shell heterostructured NWs, we have introduced localized states in the core-shell interface between the ZnO and ZnS. After subsequent thermal treatments, the optical properties of the localized excitons have been discussed based on laser spectroscopy [1][2][3][4][5][6][7][8]. The underlying physical mechanism of the change of localized excitons after thermal treatments has also been proposed and discussed in detail.…”

Ultraviolet Electroluminescence from ZnS@ZnO Core–Shell Nanowires/p-GaN Introduced by Exciton Localization

“…13,[19][20][21][22][23] Thus, various approaches such as quantum dots sensitization, introducing of localized surface plasmon or localized excitons have been adopted to optimize the performance of ZnO nanostructures. 21,[22][23][24][25][26] Herein, we propose the ZnS@ZnO core-shell nanowires/p-GaN structure heterojunctions. The strategy of using core/shell structure not only allows the efficient use of individual ZnO nanowires but also enhances the charge carrier recombination by localized excitons.…”