Strain-Enhanced Large-Area Monolayer MoS2 Photodetectors

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.

The PDMS stamp, bought from the "Gel Pak" company, was cut into a rectangle and placed on CVD-grown MoS2 on a Si/SiO2 wafer that was purchased from the "2D Semiconductors" company, as illustrated in Figure S1(a).To increase the speed of the next step, PDMS is left without contact on one of the edges of the Si wafer, which can be ensured by placing a carbon fibre or any similar object on the edge of the wafer before placing PDMS on top.Figure S1(b) shows an illustration of the pick-up step during which a PDMS / MoS2 / Si wafer is placed on top of the solution of 100 ml DI water with a few drops of ammonia (100 μL), which is gently stirred in start to ensure the flow of the solution.The solution slowly intercalates between PDMS and Si wafer, separating 2D materials from the Si wafer.After the complete separation in a few minutes, Si wafer sinks into the solution, and PDMS with MoS2 is left floating on top of the solution, as shown in Figure S1(c).Ammonia is known to dissolve alkali-like metals and is often used during the intercalation of 2D materials. 1It is added into DI water to improve the removal of any ions that could dope 2D materials afterwards, but pure DI water could also be used for this step. 2Figure S1(d) shows the step during which PDMS with MoS2 is placed onto the patterned PC sheet.A pair of Ti/Au electrodes were previously patterned by e-beam evaporation and shadow mask lithography using a metal stencil from the company "Ossila".
During placing, the PDMS stamp is gently pressed with an ear stick or simply by hand to ensure adequate adhesion over the whole substrate.This step is monitored by an optical microscope or camera and controlled with a set of micromanipulators with which one can precisely control the position and separation of PDMS and substrate. 3To ensure MoS2 has higher adhesion with the substrate, PDMS is slowly separated in small steps with a z-axis manipulator, as shown in   Additionally, more minor cracks in the channel could be present, which are not distinguishable in Figure S2(b) due to the high RMSq roughness ≈ of 8 nm inside the channel. 4,5 is important to note that these defects can be further minimized with annealing treatments or previously reduced by optimized transfer protocols such as heat application or general usage of the glove box during the transfer.Most importantly, we have used various PDMS separation front speeds, as described in Figure S1(e), while a slower and constant speed would result in lower transfer-induced defects and contaminations.

Figure S3. Continuous measurement of MoS2 sheet current as a function of time with cyclic
exposure to light, where each subsequent peak corresponds to illumination by light with a 5 nm S6 higher wavelength than the previous peak.Measurements for 0.0%, 0.3% and 0.9% of strain are shown, where the first wavelength was 555 nm, while cut-off wavelengths are marked correspondingly for each strain value.
Figure S3 shows photocurrent measurement by a continuous light source for 0.0%, 0.3%, and 0.9% of induced ε, with the first peak of each curve corresponding to 550 nm and each subsequent peak to 5nm increase of light source wavelength.Even on unprocessed data, it can be seen that the total current increases with an increase in strain.Due to persistent photoconductivity (PPC), the device's conductance was enhanced after the illumination was terminated and remained in a high-conductivity state during the whole measurement. 6This delayed phenomenon is governed by the relaxation of the excess electrons in the conduction band to hole-trapped sites. 7It can be seen that at higher amounts of the applied strain, the PPC effect is also increased, which results in the larger photo gain at higher ε.(b) GFA and GFB derived from photocurrent spectroscopy under strain up to 0.9%.
Figure S4 shows GFs derived from strain-dependent characterizations.GFP was derived from strain-dependent resistance measurement illustrated in Figure 2(a), while GFA and GFB were derived from strain-dependent photocurrent spectroscopy shown in Figure 3(d).
Table S1 compares device properties from our work with prior publications on MoS2-based flexible photodetectors and strain sensors.In contrast to earlier publications, devices in this work were based on a MoS2 monolayer sheet with a lateral size of over 1 cm and a device channel width of 250 μm.Notably, most research on MoS2 monolayers was conducted with exfoliated samples, while CVD monolayers were usually multilayers or chemically modified samples.Although we did not optimize our devices' performances (e.g.annealing, encapsulating, gating, smaller channel dimensions, higher applied voltage and laser power), we achieved performance comparable to prior reported values.It is important to note that we investigated both photodetectors and strain sensors, while previous publications mainly focused on a single type of device and the improvement of its properties.
Table S1.Comparison of device properties from our work with previous publications.

Figure S1 .
Figure S1.Schematic MoS2 transfer over patterned PC sheet.(a) PDMS layer is placed on top

Figure
Figure S1(e).After the complete separation of PDMS, MoS2 is left on top of the new substrate,

Figure S2 .
Figure S2.AFM topographfs of MoS2.(a) Image taken over a flat area of the PC sheet with

Figure
Figure S2(a) shows the AFM topography of the MoS2 monolayer sheet after transfer on PC

Figure S4 .
Figure S4.Determined GFs.(a) GFP derived from resistance measurements up to 1% of strain.
Park et al.