Browsing by Subject "Seebeck effect"

Now showing 1 - 2 of 2

Open Access
Optoelectronic and thermal properties of metallic transition metal dichalcogenides
(2020-11) Mehmood, Naveed
After the successful isolation of graphene monolayer from its bulky counterpart, there has been tremendous advancement in the field of 2D material. Transition metal dichalcogenides(TMDCs) is family of 2D materials comprising of a transition metal atom sandwiched between two chalcogen atoms. Photoresponse of semiconducting TMDCs has been studied extensively in literature. However, photoresponse from metallic TMDCs is unprecedented and hence has not been studied to explore which mechanism might prevail. Among our findings, we discovered that photocurrent generation through metallic TMDCs is possible and has a photo-thermal origin. Using scanning photo-current microscopy, we were able to obtain spatial photocurrent maps for both, zero biased and biased samples. At zero applied bias, the photocurrent generation is localized to metal-metal junction and governed by Seebeck effect. At finite applied bias, photocurrent from the whole crystal is observed and is due to photobolometric effect. As Photo-bolometric effect relies on photo-thermally induced resistance change of the material, we extended our study to extract thermal conductivity of metallic TMDCs via bolometric effect. As contact of crystal with substrate act as a heat sink, we used suspended crystals over a hole to thermally isolate it from any heat sink. Resistance change via laser induced heating is experimentally measured at the center of the suspended part of crystal. Measured resistance change is matched with expected resistance change which is calculated using thermal conductivity(κ) as a fitting parameter via commercially available finite element method package(COMSOL). This way, thermal conductivity of the metallic TMDCs is calculated with very high accuracy and precision.
Open Access
Photocurrent generation in a metallic transition-metal dichalcogenide
(American Physical Society, 2018) Mehmood, Naveed; Rasouli, Hamid Reza; Çakıroǧlu, Onur; Kasırga, T. Serkan
Photocurrent generation is unexpected in metallic 2D layered materials unless a photothermal mechanism is prevalent. Yet, typical high thermal conductivity and low absorption of the visible spectrum prevent photothermal current generation in metals. Here, we report photoresponse from two-terminal devices of mechanically exfoliated metallic 3R-NbS2 thin crystals using scanning photocurrent microscopy (SPCM) both at zero and finite bias. SPCM measurements reveal that the photocurrent predominantly emerges from metal/NbS2 junctions of the two-terminal device at zero bias. At finite biases, along with the photocurrent generated at metal/NbS2 junctions, now a negative photoresponse from all over the NbS2 crystal is evident. Among our results, we realized that the observed photocurrent can be explained by the local heating caused by the laser excitation. These findings show that NbS2 is among a few metallic materials in which photocurrent generation is possible.