We report for the first time, the fabrication of novel two-dimensional (2D) p-WS2/n-Si vertical heterostructures with superior junction and photoresponse characteristics. Few layer WS2 has been synthesized by a lithium-ion intercalation technique in hexane and coated on Si substrates for realization of CMOS compatible devices. Atomic force microscopy and Raman spectroscopy have been used to confirm the 2D nature of WS2 layers. Sharp band-edge absorption and emission peaks have indicated the formation of mono-to-few-layers thick direct band gap WS2 films. The electrical and optical responses of the heterostructures have exhibited superior properties revealing the formation of an abrupt heterojunction. The fabricated photodetector device depicts a peak responsivity of 1.11 A W(-1) at -2 V with a broadband spectral response of 400-1100 nm and a moderate photo-to-dark current ratio of ∼10(3). The optical switching characteristics have been studied as a function of applied bias and illuminated power density. A comparative study of the reported results on 2D transition metal chalcogenides indicates the superior characteristics of WS2/n-Si heterostructures for future photonic devices.
Strong light-matter interactions in layered transition metal dichalcogenides (TMDs) open up vivid possibilities for novel excitonic quasiparticle-based devices. The optical properties of TMDs are dominated mostly by the tightly bound excitons and more complex quasiparticles, the biexcitons. Instead of physically exfoliated monolayers, the solvent-mediated chemical exfoliation of these 2D crystals is a cost-effective, large-scale production method suitable for substantial practical implications. Here, we explore the ultrafast excitonic phenomena in layered WS2 (mono-to-quad) dispersion using broadband (350–750 nm) femtosecond pump-probe spectroscopy at room temperature (300 K) which are inaccessible to the steady-state absorption or emission spectroscopy. The transient absorption spectra (TAS) suggest that the mono-to-quad layered dispersion of WS2 has similar spectral features as monolayer WS2 in terms of saturation absorptions (SA) and excited state absorptions (ESA). Similar to monolayer TMDs, we are able to identify excitons and biexcitons in multi-layered 2D stratum of WS2 as well as calculate the biexciton binding energies ( 69 meV and 66 meV), which are in excellent agreement with earlier theoretical predictions. Furthermore, using many-body physics, we demonstrate that the excitons in layered WS2 behave like Wannier–Mott excitons and explain their origins via first-principles calculations. Our detailed time-resolved investigation provides ultrafast radiative and non-radiative lifetimes of the excitons and biexcitons in layered WS2. Indeed, our results unravel the complex optical response of layered TMDs, which should lead to numerous technological applications for developing excitonic quasiparticle-based valleytronic devices and ultrafast biexciton lasers at room temperature.
Chemical doping and plasmonic enhanced photoresponsivity of two dimensional (2D) n-WS/p-Si heterojunctions are demonstrated for the first time. Novel PVP coated Ag intercalation induced synthesis has led to the formation of impurity-free, chemically doped few-layer n-WS with reversed conductivity following the Maxwell-Wagner-Sillars interfacial effect. The resultant composite film exhibits excellent stability and tunable plasmonic absorption due to silver nanoparticles of different sizes. A sharp band-edge absorption of the hybrid material indicates the presence of spin-orbit coupled direct band gap transitions in WS layers, in addition to a broader plasmonic peak attributed to Ag nanoparticles. Stabilized Ag-nanoparticle (∼4-6 nm) embedded electron rich n-WS has been used to fabricate plasmon enhanced, silicon compatible heterojunction photodetectors. The detectors exhibited superior properties, possessing a photo-to-dark current ratio of ∼10, a very high responsivity (8.0 A W) and an EQE of 2000% under 10 V bias with a broad spectral photoresponse in the wavelength range of 400-1100 nm. The results provide a new paradigm for intercalant impurity-free metal nanoparticle assisted exfoliation of n-type few-layer WS, with the nanoparticles playing a dual role towards the realization of 2D materials based broadband heterojunction optoelectronic devices by inducing chemical doping as well as tunable plasmon enhanced absorption.
Enhanced light–matter interactions by integrating plasmonic Au nanostructures as a light harvester on two-dimensional (2D) MoS2 carrier sink layers are reported, leading to broadband optical absorption and significantly enhanced Raman scattering intensity. The calculations of electronic band structure using density functional theory analysis and optical simulations elucidate the metal induced doping in MoS2 and the enhancement of electromagnetic field through localized surface plasmon resonance at the Au/MoS2 interface by forming a number of hot spots, corroborating the spectroscopic results. The ultrafast time-domain results reveal a 200-fold enhancement in the carriers’ lifetime for nanohybrids, as compared to the control sample, which is attributed to the efficient transfer of hot electrons from Au to MoS2. Fabricated metal–semiconductor–metal photodetectors using hybrid nanostructures exhibit a 20-fold enhancement of photoresponsivity (∼1.5 A/W at 640 nm) as compared to pristine MoS2 and a remarkably high peak detectivity (∼4.75 × 1013 Jones at 3 V), which are promising for broadband and multicolor photodetection, making them attractive for large area 2D materials-based nanophotonic devices.
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