- Nano Express
- Open Access
Red-Shift Effect and Sensitive Responsivity of MoS2/ZnO Flexible Photodetectors
© Hsiao et al. 2015
- Received: 7 October 2015
- Accepted: 11 November 2015
- Published: 16 November 2015
The optoelectronic characteristics of molybdenum disulfide (MoS2)/ZnO flexible photodetectors are investigated. A red-shift effect and improved photocurrent properties of the flexible devices are demonstrated. MoS2 doping improved the photocurrent properties and conductivity. The photocurrent/dark current ratios of pure ZnO and MoS2/ZnO flexible photodetectors were 103 and 104, respectively. The responsivity of MoS2/ZnO increased, and the wavelength was red-shifted.
- Photo-induced response
Molybdenum disulfide (MoS2) is a promising candidate for optoelectronic sensors because of its unique semiconducting channel when used as a phototransistor . MoS2 phototransistors have recently been integrated with conventional semiconductor circuitry . Bulk MoS2 is an indirect-gap semiconductor with a bandgap of 1.2 eV , whereas a single-layer MoS2 is a direct-gap semiconductor with a bandgap of 1.8 eV . The photodetector (PD) has a broad spectral range, with photocurrent that monotonously increases as the wavelength of incident light is decreased from 680 to 400 nm. Two-dimensional and single-layer ultrasensitive MoS2 PDs have a photoresponsivity that is 106 better than that of the first graphene PDs (~0.5 mA W−1) . In addition, a high-performance complementary inverter and selective gas sensing based on MoS2 field-effect transistors was studied [6–8].
There are various ways of synthesizing MoS2 nanostructures including electrochemical/chemical synthesis , laser ablation , solution-based exfoliation , and chemical vapor deposition (CVD) . Another method, the rapid vibro-milling technique, was employed for investigating the potentiality of obtaining nano-sized powders. MoS2 nanoparticles obtained using vibro-milling, which can be applied at an industrial scale, have good solubility and biocompatibility. However, few applications of MoS2 synthesized by rapid vibro-milling have been reported. The rapid vibro-milling process was employed for investigating its potential for obtaining nanometer-sized powders . The electro-optical properties of ZnO compounds have been studied extensively . To our knowledge, MoS2/ZnO films have not been thoroughly investigated, which is the motivation for this research. The morphology and photoresponsivity properties of the MoS2 nanocrystals on ZnO film are studied and discussed.
Nanocrystalline MoS2 (from Alfa Aesar, 325 mesh, 99 %) was prepared using a high-energy ball-milling method. MoS2 was milled in ceramic milling vials (zirconia) using zirconia balls for 10, 20, and 40 h. The ball-to-powder weight ratio was 2:1 to produce at least 2 g of nanopowder. The mechanical milling was performed in a horizontal oscillatory mill (Retsch, PM 400) operating at 25 Hz. The as-synthesized materials were characterized by X-ray diffraction (XRD, Rigaku Dmax-33). The morphology and microstructure were examined using atomic force microscopy (AFM, Bruker) and transmission electron microscopy (TEM, Hitachi HF-2000).
Figure 1b shows an optical image of the flexible PDs with MoS2 coated on the ZnO/PEN substrate. The PDs exhibited a transmission of above 80 % and high bending strength. The bending curvature radius was larger than 10 mm. The Ag electrode pattern is shown in the inset of Fig. 1b. The interdigital electrodes have eight fingers with a fixed length of 2000 μm and a width of 50 μm. The spin-coated 5 wt% MoS2 nanocrystals on the ZnO/PEN substrate were also characterized using AFM to better understand the morphological properties with a large area of 100 × 100 μm, as shown in Fig. 1c. The pure ZnO film has a root-mean-square (rms) roughness of 13.2 nm, and the spin-coated MoS2 on the ZnO fim has that of 84.9 nm. In this study, the particle size of MoS2 was around 20~50 nm. During the spin-coating of MoS2, the nanocrystals had a uniform morphology and monodispersity. They were deposited on the ZnO/PEN substrate due to the gravitational force, causing the high roughness on the ZnO/PEN substrate.
where D is the average grain size, k is a constant (equal to 0.89), λ is the X-ray wavelength (equal to 0.1542 nm), θ is the (002) peak angle, and β is half the peak width. The average grain size of powders milled for 20 h was 28.4 nm. The line broadening of the nanocrystalline samples is due to the small grain size and strain-induced response . Figure 2b shows the XRD patterns of ZnO and ZnO films with 5 wt% MoS2 (~28 nm) coating layer. A (002) peak at 34.5° along with a strong (103) peak was observed for the ZnO thin films, indicating the polycrystalline nature of the thin films. The peak position of ZnO remains almost unchanged because the bottom ZnO layers were controlled to have a thickness of 100 nm. Therefore, the nanocrystalline MoS2 can be observed on the ZnO thin film by grazing incidence XRD.
MoS2 nanopowder was deposited on flexible devices using high-energy ball-milling method. Flexible ZnO and MoS2/ZnO MSM PDs were investigated. The results show that the photocurrent/dark current ratios of pure ZnO and MoS2/ZnO flexible PDs were 103 and 104, respectively. The responsivity increased and the wavelength was red-shifted when a 5 wt% MoS2 layer was used. There was a significant improvement in the photo-induced properties.
This work was partially supported by the Ministry of Science and Technology of Taiwan under grants MOST 103-2221-E-151-001-MY3 and MOST 103-2221-E-151-007-MY3.
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