Semiconducting two-dimensional transition-metal dichalcogenides (TMDs) have garnered a great deal of interest owing to their large surface-to-volume ratios compared to traditional three-dimensional (3D) semiconductors. One such TMD is tungsten diselenide (WSe2), which at monolayer (1L) exhibits a direct bandgap and tunable charge transport behavior. This, in combination with its large surface-to-volume ratio, has made WSe2 a heavily researched material, including for gas sensing applications. However, the synthesis of monolayer WSe2 has proven to be challenging. In this study, we demonstrate the halide-assisted low-pressure chemical vapor deposition (HA-LPCVD) of high crystalline-quality, monolayer WSe2. The average synthesized WSe2 crystallite possessed an edge length of ~ 30 microns. After synthesis, the properties of the WSe2 membranes were characterized using Raman and photoluminescence spectroscopy. For device fabrication, Cr contacts were deposited atop the WSe2 membranes using photolithography with a dark-field mask and a metal lift-off process with electron-beam evaporation. After device fabrication, we then proceeded to use these devices for gas sensing applications, where two terminal electronic transport measurements allowed us to monitor the modulation in the WSe2 transport characteristics with incoming gas flow. The I-V response to target gases N2 and CO2 were recorded to investigate the effects of surface adsorption mechanisms of extrinsic molecules on our material’s conducting behavior.
Monolayers of transition metal dichalcogenides (TMDCs) have caught the interest of post-silicon electronics and optoelectronics researchers due to their exceptional electronic and optoelectronic properties which stem from their unique two-dimensional (2D) layered structure. In recent years, there has been a focus on exploring van der Waals (vdW) heterojunctions with TMDCs, including with lower dimensionality materials such as zero-dimensional (0D) systems. Integrating 0D-2D assemblies together provides an opportunity to configure a diverse array of material stacks towards optoelectronics and electronics applications. In this study, we synthesized 0D-2D vdW heterostructure by spin coating C60 molecules on halide-assisted-low-pressure chemical-vapor-deposition (HA-LPCVD) produced monolayer WSe2 flakes. Raman and photoluminescence spectroscopy allowed us to assess the charge carrier exchange at the vdW interface. We found that after C60 deposition, the photodetector figures of merit for WSe2 − C60 hybrids improved, and investigations were conducted as a function of illumination power. Our studies reveal that WSe2 − C60 hybrid system is an appealing choice for next generation optical sensing devices.
High-performance hybrid graphene photodetectors were prepared with fullerenes and metallofullerenes deposited on graphene using electrophoretic deposition technique. Fullerene C60 which acts as an electron acceptor, and metallofullerene Sc3N@C80 which act as an electron acceptor and electron donor respectively, were used to decorate the surface of the graphene. Raman spectroscopy analysis of C60 film after electrophoretic deposition and annealing reveals no change in bonding or chemical composition. Both Raman area map and AFM topography scans reveal a continuous film like deposition of C60 between electrodes during electrophoretic deposition. The charge transfer between graphene and fullerene/metallofullerene and consequent photocurrent generation contribute to a high photoresponsivity ℛ in our hybrid graphene detectors. The responsivity ℛ of the C60 - graphene and Sc3N@C80 – graphene hybrids were measured to be ~ 109 A/W, while the detectivity and external quantum efficiency were also exceptional, ~ 1015 Jones and ~ 109 %, respectively. The exceptional performance gains achieved with graphene–0D hybrid structures confirm the potential of endohedrals and metallofullerenes to dope graphene for high performance optoelectronic devices using a facile and scalable fabrication process.
Transition metal dichalcogenides (TMDCs) have received a great deal of attention from the scientific community since the advent of graphene. Tungsten diselenide (2H-WSe2) has particularly drawn-out attention of researchers because of its broadband spectral detection range. In this work, we have reported a halide assisted chemical vapor deposition (HA-LPCVD) technique for synthesis of large crystallites of 2H-WSe2 with high crystalline perfection. The average crystallite size of synthesized 2H-WSe2 was in the order of ~20 μm. We have reported device 2H-WSe2 device fabrication using poly (methyl methacrylate) and electron beam lithography process to define titanium (Ti) metal contacts. A temperature (T) dependent analysis of the electronic transport reported here reveals a T-dependent conduction process existing at the interface of Ti and 2H-WSe2 and an interfacial barrier height of ~ 0.35 eV was calculated at the thermionic emission regime. From the reported optoelectronic characterization, an on-off ratio (Ion/Ioff) of ~9 was calculated. Furthermore, a responsivity (ℜ) of ~ 242 A/W was calculated for our 2H-WSe2 based photodetector under broadband light excitation. The reported photodetector figures of merit will open avenues for use of monolayer 2H-WSe2 with Ti metal contacts for high performance photodetection.
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