A demonstration is presented of how significant improvements in all-2D photodetectors can be achieved by exploiting the type-II band alignment of vertically stacked WS /MoS semiconducting heterobilayers and finite density of states of graphene electrodes. The photoresponsivity of WS /MoS heterobilayer devices is increased by more than an order of magnitude compared to homobilayer devices and two orders of magnitude compared to monolayer devices of WS and MoS , reaching 10 A W under an illumination power density of 1.7 × 10 mW cm . The massive improvement in performance is due to the strong Coulomb interaction between WS and MoS layers. The efficient charge transfer at the WS /MoS heterointerface and long trapping time of photogenerated charges contribute to the observed large photoconductive gain of ≈3 × 10 . Laterally spaced graphene electrodes with vertically stacked 2D van der Waals heterostructures are employed for making high-performing ultrathin photodetectors.
Heterophase homojunction formation in atomically thin 2D layers is of great importance for next-generation nanoelectronics and optoelectronics applications. Technologically challenging, controllable transformation between the semiconducting and metallic phases of transition metal chalcogenides is of particular importance. Here, we demonstrate that controlled laser irradiation can be used to directly ablate PdSe2 thin films using high power, or trigger the local transformation of PdSe2 into a metallic phase PdSe2-x using lower laser power. Such transformations are possible due to the low decomposition temperature of PdSe2 compared to other 2D transition metal dichalcogenides. Scanning transmission electron microscopy is used to reveal the laser-induced Se-deficient phases of PdSe2 material. The process sensitivity to the laser power allows patterning flexibility for resist free device fabrication. The laser patterned devices demonstrate that a laser-induced metallic phase PdSe2-x is stable with increased conductivity by a factor of about 20 compared to PdSe2. These findings contribute to 2 the development of nanoscale devices with homojunctions and scalable methods to achieve structural transformations in 2D materials.
Two-dimensional materials are being increasingly studied, particularly for flexible and wearable technologies because of their inherent thickness and flexibility. Crucially, one aspect where our understanding is still limited is on the effect of mechanical strain, not on individual sheets of materials, but when stacked together as heterostructures in devices. In this paper, we demonstrate the use of Kelvin probe microscopy in capturing the influence of uniaxial tensile strain on the band-structures of graphene and WS (mono- and multilayered) based heterostructures at high resolution. We report a major advance in strain characterization tools through enabling a single-shot capture of strain defined changes in a heterogeneous system at the nanoscale, overcoming the limitations (materials, resolution, and substrate effects) of existing techniques such as optical spectroscopy. Using this technique, we observe that the work-functions of graphene and WS increase as a function of strain, which we attribute to the Fermi level lowering from increased p-doping. We also extract the nature of the interfacial heterojunctions and find that they get strongly modulated from strain. We observe that the strain-enhanced charge transfer with the substrate plays a dominant role, causing the heterostructures to behave differently from two-dimensional materials in their isolated forms.
Two-dimensional (2D) materials hold great promise in flexible electronics, but the weak van der Waals interlayer bonding may pose a problem during bending, where easy interlayer sliding can occur. Furthermore, thin films of rigid materials are often observed to delaminate from soft substrates during straining. Here, we study the influence of substrate strain on some of the heterostructure configurations we expect to find in devices, composed of three common 2D materials: graphene, tungsten disulfide, and boron nitride. We used photoluminescence (PL) spectroscopy to measure changes in the heterostructures with strain applied in situ. All heterostructures were fabricated directly on polymer substrates, using materials synthesized by chemical vapor deposition. We observed an inhomogeneous release of strain in all structures, leading to a nonrecoverable broadening of the PL peak and shift of the bandgap. This suggests the need for preconditioning devices before service to ensure stable behavior. A gradual time-dependent relaxation of strain between strain cycles was characterized using time-dependent measurementsan effect which could lead to drift of device behavior during operation. Furthermore, possible degradation was assessed by performing the strain and relax the cycle up to 200 times, where we found little further change after the initial shifts had stabilized. These results have important ramifications for devices fabricated from these and other 2D materials, as they suggest extra processing steps and considerations that must be taken to achieve consistent and stable properties.
We study the in situ electro-conductance in nanoscale electronic devices composed of suspended monolayer WS with metal electrodes inside an aberration-corrected transmission electron microscope. Monitoring the conductance changes when the device is exposed to the electron beam of 80 keV energy reveals a reversible decrease in conductivity with increasing beam current density. The response time of the electro-conductance when exposed to the electron beam is substantially faster than the recovery time when the beam is turned off. We propose a charge trap model that accounts for excitation of electrons into the conduction band and localized trap states from energy supplied by inelastic scattering of incident 80 keV electrons. These results show how monolayer transition metal dichalcogenide 2D semiconductors can be used as transparent direct electron detectors in ultrathin nanoscale devices.
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