Hydrothermal synthesis of MoS₂ nanosheets for multiple wavelength optical sensing applications

Highlights

• MoS₂ nanosheets synthesized by facile and economical hydrothermal process.

• Nanosheets characterized by XRD, SEM, TEM, FTIR, UV–Vis , RAMAN and XPS studies.

• MoS₂ based multi-wavelength optical sensor.

• Stable and reproducible optical sensing response.

Abstract

In the present work, MoS₂ nanosheets were synthesized by a hydrothermal method for optical sensing application. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Raman and UV–vis spectroscopy were carried out to characterize the synthesized MoS₂ nanosheets. SEM and TEM images showed the morphology of ultrathin nanosheets of MoS₂. Optical sensor of MoS₂ nanosheets was fabricated and thoroughly studied using various laser excitation wavelengths (λe_x): 440 nm (indigo); 460 nm (blue); 550 nm (green); 570 nm (yellow); 635 nm (red) and 785 nm (infra-red). The excellent photoresponsivity was observed in the visible range and maximum was found to be 23.8 μA/W for λe_x: 635 nm (red illumination). The mechanism for photoresponse was proposed and correlated with the absorption spectroscopy. The response and recovery times for 635 nm were 2.5 and 3.2 s, respectively. The photoresponsive characteristics of MoS₂ nanosheets were also explored as a function of optical power density.

Introduction

Graphene is the rising material in the field of nanotechnology and it has provided researchers with skills and tools to forward studies of other two-dimensional (2D) layered materials. In a 2D layered material, there is a strong covalent bonds interaction between the atoms of crystalline lattice structure and atoms interact with weak van der Waals forces between the layers of layered material. As compared to graphene, transition metal dichalcogenides (TMDs) possess remarkable electrical, optical and electronic properties [[1], [2], [3]]. MoS₂ comes from the family of layered TMD’s (MX₂, M = Mo, W; X = S, Se, Te). Its atomic structure is a hexagonal arrangement of Mo and S atoms that layered on top of each other and form a trigonal prismatic arrangement. The layered spacing of MoS₂ nanosheets is 6.5 Å [4]. Bulk MoS₂ is an indirect bandgap semiconductor with a bandgap of 1.29 eV, whereas monolayered MoS₂ has a direct bandgap of 1.8 eV due to quantum confinement effect and hybridization change in d orbital of Mo atoms and p_z of S-atom [5]. Thus, 2D MoS₂ is an optically active and interesting material for device applications such as chemical sensors, electronic device, photodetectors etc. [[6], [7], [8], [9], [10]].

In this work, MoS₂ nanosheets were prepared by a facile hydrothermal method. This method is far better than other conventional and non-conventional process of synthesis due to its low cost, simplicity and lesser number of precursors [11]. In hydrothermal process, sample is directly precipitated from the solution, thus regulated rate and uniformity of nucleation, control over growth, ageing, size of the nanoparticles and morphology is achieved that is not possible with many other synthesis routes [12,13]. It is reported that this method can be hybridized with other processes and do not need any catalyst, seed, expensive and harmful surfactant or template, promising for large scale production with quality crystals [[12], [13], [14]]. The unique temperature-pressure interaction of the hydrothermal solution allows the preparation of different phases of MoS₂ [14].

In the present work, deep and comprehensive study is carried out on MoS₂ nanosheets for optical sensing applications as a function of laser wavelength and power. Different lasers were used as an illumination source with wavelengths (λex): 440 nm (indigo); 460 nm (blue); 550 nm (green); 570 nm (yellow); 635 nm (red) and 785 nm (infra-red). The optical sensing parameter viz. photoresponsivity is evaluated for all laser sources. Researchers reporting studies on optical sensing, have so far calculated photoresponsivity only for selected wavelength of 650 nm by W. Zhang et al., and for 405 nm by Caiyun Chen et al. [15,16] with no measurement over wide spectra of light [[17], [18], [19]]. This work is to find the applicability of the MoS₂ nanosheets based optical sensor in the visible light of spectrum and its sensing activity as a function of power density. The photoresponse was studied as a function of power density (0.88–3.53 mW/mm²) for fixed laser wavelength of 635 nm. The dependence of photoresponsivity over power density was used to study power law and it signifies the complex process of electron–hole generation, trapping, recombination rate trap states as well as the efficient quantum efficiency which is the significance of conducting the experiment [[20], [21], [22], [23], [24], [25]]. The reported optical sensor produced a large value of photo-to-dark current ratio with high switching speed. The optical sensing response is tested for number of cycles to confirm the reproducibility and stability of the optical sensor.