Synthesis and characterization of single-crystal CdS nanosheet for high-speed photodetection

Abstract

One-dimensional nanostructures have several unique advantages over bulk material and thin films, which can be exploited for high-speed photodetection. Furthermore, as bulk CdS has a high photosensitivity and quantum efficiency, there is considerable potential for the use of CdS nanostructures in advanced devices. In this study, single-crystal CdS nanosheets were grown by thermal evaporation and fully characterized to determine their potential for application in high-speed photodetectors. A high-quality nanosheet was confirmed to have a smooth surface with no extraneous particles and a strong orientation to the (110) plane of the wurtzite (hexagonal) phase of CdS. The Cd/S ratio was found to be nearly stoichiometric at 1.09. Photoluminescence measurement of a single-crystal CdS nanosheet showed a high emission intensity at a wavelength of 493 nm. The current–voltage characteristics of the CdS nanosheet on Al thin film indicated an Ohmic contact in dark and under illumination by ambient, 365-nm, 405-nm, and 460-nm light. The light responsivity showed a peak at 460 nm. Under 365-nm, 405-nm, and 460-nm chopped light, at a bias voltage of 1, 3, and 5 V, the photocurrent rise and decay times were investigated. The device showed faster response times for 460-nm light. This fast response was attributed to the high quality of the single crystal, the absence of defect states, and the high surface/volume ratio. The device showed a high quantum efficiency of 22.3×10³% when it was illuminated by 365-nm light under a bias of 5 V; this efficiency increased to 36.3×10³% and 40.5×10³% when the device was illuminated by 405-nm and 460-nm light, respectively.

Introduction

Visible light detection is of considerable importance in a wide range of applications, including environmental monitoring, space research, and optical communications [1]. Further, in many of these applications, a high processing speed of the device is critical, and so there is a strong demand for devices with a fast response. Moreover, in the quest for new materials and device structures with such speed, high sensivity and high gain are also necessary. In this light, compared with conventional bulk phase or thin films devices, one-dimensional (1-D) semiconductors such as nanowires, nanorods, and nanoribbons have several unique advantages, including high crystallinity, self-assembly, high surface-to-volume ratio, quantum confinement effects, as well as slow electron–hole recombination [2], [3]. Thus there has been a gradual shift from conventional photo detectors made usually from thin films or bulk material towards respective nanostructured materials [4]. Nanosheets or nanoribbons have just one dimension in nanoscale range that making them easier to use in nanoscale optoelectronic devices fabrication rather than nanowires.

Group II–VI semiconducting compounds are frequently utilized in optoelectonic applications due to their wide direct band gap [5], [6], [7]. Cadmium sulfide (CdS), in particular, with a direct band gap of 2.4 eV, is a very attractive material for optoelectronics devices such as solar cell, gas and optical sensors because of its high photosensitivity and quantum efficiency [8], [9], [10], [11]. CdS in the bulk or film phase usually has a 4–5 order of magnitude increase in photocurrent under optimum illumination. Furthermore, CdS nanoribbons were found to have significantly higher photosensitivity [4]. To further explore this potential, in this study, single-crystal CdS nanosheets were prepared by thermal evaporation and its photoelectric properties were characterized in Al/CdS photodetector devices.