Zinc oxide has attracted extensive interests in the ultraviolet photo-detection field owing to its outstanding optoelectronic properties. In recent years, the ZnO based photodetectors have achieved a fast response speed by different methods. In this work, we develop a simple method to achieve the simultaneous improvement in both response speed and responsivity by CdSe quantum dots (QDs) modified ZnO microwires (MWs). Due to the built in electric field formed between ZnO and CdSe accelerates the separation of photogenerated electron-hole pairs and significantly increases the photocurrent. At the same time, ZnO and CdSe form a type-II band structure, which can accelerate the carrier transport, resulting in a significant reduction in response time. Furthermore, with the increase of temperature, the carriers trapped by ZnO surface states escape from the surface trap states, which reduces the influence of surface states, improves response speed, and has been confirmed by the temperature-dependent current-voltage curves. As a result, the CdSe QDs modified ZnO MWs photodetector exhibits a rise time of 1.4 s, almost one order of magnitude smaller than that of the ZnO photodetector. These excellent optoelectronic properties demonstrate that the modification in the CdSe QDs modified ZnO MWs photodetector is a promising way to achieve fast response.

In this study, we prepare the CdSe QDs modified ZnO MWs photodetectors. First, the ZnO MWs are grown by chemical vapor deposition at 1050 ℃ in Ar and O₂ atmospheres, and the CdSe quantum dots are prepared by the hydrothermal method. Then, the CdSe quantum dots dispersed in toluene are added to the surface of ZnO MWs to prepare the CdSe QDs modified ZnO MWs. An electrode is prepared at both ends of the ZnO MWs to form a photodetector. Finally, the morphologies of the CdSe modified ZnO MWs and the pure ZnO MWs are characterized by scanning electron microscope. The absorption and luminescence properties of CdSe QDs modified ZnO MWs are characterized by the ultraviolet-visible (UV-Vis) and fluorescence spectrometers. The photo-response performance of the ZnO MWs photodetector is characterized by the UV-Vis response system.

The prepared CdSe QDs modified ZnO MWs have excellent photoelectric properties, the photo-response is increased by 3 times, and the rise time and fall time are reduced to 1.4 s and 6.8 s, respectively. A type II band structure is formed between CdSe QDs and ZnO MWs, and a built-in electric field is generated at the interface. Under the action of the built-in electric field, the electrons migrate to the surface of ZnO MWs, and the holes migrate to the CdSe QDs, meaning that the separation of carriers speeds up. The rapid separation of photogenerated carriers increases the lifetimes of electrons and holes, resulting in the increase of photocurrent. At the same time, the CdSe QDs modified ZnO MWs can also enhance light absorption of the detector and significantly increase the photocurrent of the detector (Fig. 3). The type II band structure between ZnO MWs and CdSe QDs also improves the photo-response of the material. Meanwhile, due to the passivation effect of CdSe QDs on the surface of ZnO MWs, the oxygen content adsorbed on the surface of ZnO MWs is reduced, and the interaction between photogenerated carriers and oxygen molecules adsorbed on the ZnO surface is minimized (Fig. 5). Under the combined action of these two factors, the light response speed of the material is significantly shortened.

We have demonstrated a fast speed ultraviolet photodetector based on CdSe QDs modified ZnO MWs. The built-in electric field between ZnO and CdSe promotes the separation of photogenerated electron-hole pairs, which makes photocurrent increase significantly from 1.59×10^-6 A to 4.3×10^-6 A at 1 V bias. Meanwhile, due to the influence of the type-II band structure and surface passivation, the rise time of CdSe QDs modified ZnO MW photodetector decreases from 11.4 s to 1.4 s, and the fall time decreases from 28.6 s to 6.8 s. Finally, after thermal annealing, the materials show high photocurrent and a fast carrier separation rate. The effect of CdSe QDs on the photoelectric properties of ZnO is further confirmed. This work provides an efficient method for the fabrication of ZnO MW photodetectors with a fast photoelectric response.