Flexible 2D material hetero-structure photodetectors with high responsivities, tunable wavelengths and short response times

Abstract By using graphene as the carrier transport layer and transition metal dichalcogenides (TMDs, WS 2 , MoS 2 , and WSe 2 ) as the light absorption layer, wavelength-tunable photodetectors can be fabricated on sapphire substrates. With the short carrier transport time in the graphene layer, the hole accumulation in the TMD layers would result in the conduction change in the graphene layer. Therefore, high responsivity values of 1521.9, 6077.9 and 3977.8 A W −1 at 630, 660 and 750 nm are observed for the WS 2 , MoS 2 and WSe 2 devices, respectively. With the high responsivity values, high detectivity values 6.9 (WS 2 ), 17.6 (MoS 2 ) and 9.2 (WSe 2 ) × 10 10 Jones are also observed for the three devices. Following the similar film transferring procedure and device fabrication procedure, photodetectors with mono-layer graphene on different mono-layer TMDs are fabricated on polyethylene terephthalate (PET) substrates. High responsivity values (10 2 –10 3 A W −1 ) are still observed for the devices fabricated on flexible substrates. Although a relatively larger responsivity reduction is observed for the graphene/MoS 2 device under bending conditions, the similar responsivity values observed for the other two devices (graphene/WS 2 and graphene/WSe 2 ) still indicate that the thin-body nature of 2D materials is advantageous for the fabrication of flexible devices. With increasing WSe 2 layer numbers, even higher responsivities and significantly shortened rise/fall times 4.3/13.0 ms are observed for the mono-layer graphene/tri-layer WSe 2 photodetector, which has demonstrated that the excess electron storage in multi-layer 2D materials would result in fast hole accumulation/relaxation in the WSe 2 light absorption layer. The easy replacement of 2D materials with different bandgap values for wavelength-tunable detections also indicate an alternate application other than electronic devices of 2D materials in weak light detections.