The ternary alloy CdS<sub>x</sub>Se<sub>1-x</sub> has the physical properties of CdS and CdSe, and its band gap can be adjusted by changing the component ratios of the elements. The alloy has excellent photoelectric properties and has potential application in optoelectronic devices. Although people have made some progress in the CdSSe-based photodetectors, their performance are still far from the commercial requirements, so how to improve the performance of the device is the focus of current research. In this paper, a single crystal CdS<sub>0.42</sub>Se<sub>0.58</sub> nanobelt device was firstly prepared by thermal evaporation. Under 550 nm illumination and 1 V bias, the ratio of photocurrent to dark current of the device was 1.24×10<sup>3</sup>, the responsivity reached 60.1 A/W, and the external quantum efficiency reached 1.36×10<sup>4</sup>%, the detectivity is 2.16×10<sup>11</sup> Jones. Its rise/fall time is about 41.1/41.5 ms. Secondly, after the CdSSe nanobelt was decorated by Au nanoislands, the optoelectronic performance of the device is significantly improved. Under 550 nm illumination and 1 V bias, the I<sub>light</sub>/I<sub>dark</sub> ratio, responsivity, external quantum efficiency and detectivity of the device are increased by 5.4, 11.8, 11.8 and 10.6 times, respectively, and the rise/fall time are both reduced to half of that of single CdSSe nanobelt. Finally, the microscopic physical mechanism of the enhanced optoelectronic performance of the device is explained based on localized surface plasmon resonance of Au nanoislands. After the combination of gold nanoislands and CdSSe nanobelt, the difference in Fermi levels between them results in the transfer of electrons from CdSSe nanobelt to Au nanoislands, thus forming an internal electric field at the interface, which was directed from CdSSe nanobelt to Au nanoislands. Under illumination, the electrons in the Au nanoislands acquire enough energy to jump over the Schottky barrier because of localized surface plasmon resonance. These photoexcited hot electrons were trapped and stored in extra energy levels above the conduction band minimum, and then were cool down to the band edge, thus realizing the electron transfer from Au nanoislands to CdSSe nanobelt. Moreover, the internal electric field also greatly promoted the transfer of hot electrons from Au nanoislands to CdSSe nanobelt and inhibited the recombination of carriers at the interface, resulting in large photocurrent. Our works provide an effective strategy for the fabrication of high-performance photodetectors without increasing the device area.
