Strain-Enhanced Large-Area Monolayer MoS2 Photodetectors

Abstract

In this study, we show a direct correlation between the applied mechanical strain and an increase in monolayer MoS₂ photoresponsivity. This shows that tensile strain can improve the efficiency of monolayer MoS₂ photodetectors. The observed high photocurrent and extended response time in our devices are indicative that devices are predominantly governed by photogating mechanisms, which become more prominent with applied tensile strain. Furthermore, we have demonstrated that a nonencapsulated MoS₂ monolayer can be used in strain-based devices for many cycles and extensive periods of time, showing endurance under ambient conditions without loss of functionality. Such robustness emphasizes the potential of MoS₂ for further functionalization and utilization of different flexible sensors.

Keywords: MoS2; PL spectroscopy; atomic force microscopy; photocurrent spectroscopy; photodetector; strain; strain sensor.

Figure 1

Uniformity characterization in the device channel. (a) Optical micrograph of the MoS₂ monolayer transferred on top of the PC sheet with a prefabricated pair of Au electrodes. The MoS₂ area over electrodes is indicated with a dashed red lines. (b) Microreflectance spectrum of the MoS₂ monolayer in the channel. Exciton A and B Lorentzian fits and the cumulative fit are indicated with red, green, and blue lines, respectively. (c) PL map of the A exciton resonance of the device channel. Values taken as characteristic A exciton energies for MoS₂ in the channel at the PC, MoS₂ on the edge of the electrode, and MoS₂ at Au electrodes are indicated with 1, 2, and 3, respectively. (d) AFM topographic image of the sample with no visible cracks in a 30 μm × 30 μm sized region. The blue arrows indicate transfer-induced contaminations. (e) Channel line profile taken along the white dashed arrow in (d).

Figure 2

Piezoresistive effect in the MoS₂ monolayer. (a) I–V characterization under different levels of applied ε up to 1%. (b) Evolution of current versus voltage during 20 bending cycles, where each cycle consists of 21 steps in which strain increases from 0% up to 0.6% and back to 0%, all in 0.06% increments, i.e., decrements. The applied voltage is shown on the Y-axis, a cycle number on the X-axis, and a measured current as a color scale. The red/blue color indicates the current measured under positive/negative bias, respectively. Darker shades of red and blue indicate the highest current (lowest resistance, under 0.6% of applied ε), while brighter corresponds to the lowest current (highest resistance, under 0% of applied ε). (c) Resistance vs cycle number, with an indicated green rectangle that shows the first three cycles after which prestrain is partially released. The maximum resistance in each cycle corresponds to 0% of applied ε while minimum to 0.6%. (d) GF_P for each new half cycle over the 20 subsequent cycles of bending.

Figure 3

Strain-enhanced MoS₂ photoresponse. (a) Optical microscopy image of the device under exposure to light. (b) Photoresponse of MoS₂ at a light illumination of 645 nm, with the light turned on for 45 s and switched off afterward. Measured current and corresponding double exponential fits are shown for 0.0, 0.3, and 0.9% of tensile strain. (c) Photocurrent spectroscopy of the MoS₂ monolayer sheet by exposure to the LED light with different wavelengths at 0.0, 0.03, and 0.09% of tensile strain. (d) Higher-resolution photocurrent spectroscopy of the MoS₂ monolayer sheet with exposure to a continuous light source with different wavelengths at 0.0, 0.3, and 0.9% of tensile strain. Data show the energy range in which B (604 nm) and A (645 nm) exciton peaks are found and fitted with two Lorentzians.