Laser Power Dependent Optical Properties of Mono- and Few-Layer MoS2 (original) (raw)
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Photoluminescence from Chemically Exfoliated MoS 2
Nano Letters, 2011
A tomically thin sheets of layered inorganic compounds such as metal chalcogenides and oxides represent an emerging class of materials with prospects for a range of applications. 1À5 Layered transition-metal dichalcogenides (LTMDs) form a large family of materials with interesting properties that have been studied for decades. 6 Crystals of LTMDs can be easily cleaved along the layer plane due to weak van der Waals forces between the layers similar to graphite. Individual monolayers of LTMDs can be isolated via micromechanical cleavage or the "Scotch tape method" used to obtain graphene from graphite. 1 The two-dimensional crystals of LTMDs are an inorganic analogue of graphene and represent the fundamental building blocks for other low-dimensional nanostructures such as inorganic nanotubes and fullerene. Molybdenum disulfide (MoS 2 ), a widely known LTMD, is a solid state lubricant and catalyst for hydrodesulfurization and hydrogen evolution. 8 It is an indirect band gap semiconductor with an energy gap of ∼1.2 eV in the bulk form 9 and has also attracted interest as photovoltaic and photocatalytic materials. 10, The band gap of MoS 2 increases with decreasing crystal thickness below 100 nm due to quantum confinement 12 and calculations predict it to reach 1.9 eV for a single monolayer. In addition to the increase in its size, the nature of the band gap also changes from indirect to direct when the thickness reaches a single monolayer. 13 Recent success in isolating monolayers of MoS 2 has allowed the observation of strong photoluminescence that can be attributed to the direct gap electronic structure of monolayer MoS 2 (refs 14À16). The indirect-to-direct gap transition results in giant enhancement (∼10 4 ) in photoluminescence quantum yield, highlighting the distinguishing feature of the monolayer compared to multilayer counterpart. 15 Large inplane carrier mobility of around 200À500 cm 2 /(V s) (ref 17) and robust mechanical properties of MoS 2 also make it an attractive material for flexible field-effect transistors (FETs). Recently Radisavljevic et al. 20 demonstrated high-performance FET fabricated using monolayer MoS 2 as the channel material with mobilities comparable to those of bulk crystals.
Laser Power Dependent Optical Properties of Mono- and Few-Layer MoS2
We report on the exponential decay of the red-shift of the photoluminescence A-exciton peak in monolayer molybdenum disulfide (MoS2) with the excitation laser power. The linear relationship found for the thermal variation of the same peak suggests that the laser power effect goes beyond the exciton dynamics associated to temperature variations. Laser exitation power effect on the broadening and red-shifting of the A1g and E12g phonon peaks observed by Raman spectroscopy reflect the damping of vibration due local thermal heating induced by the laser. Our results point out the laser excitation power dependence on the photoluminescence properties of monolayer MoS2.
A direct comparison of CVD-grown and exfoliated MoS2using optical spectroscopy
Semiconductor Science and Technology, 2014
MoS2 is a highly interesting material system, which exhibits a crossover from an indirect band gap in the bulk crystal to a direct gap for single layers. Here, we perform a direct comparison between large-area MoS2 films grown by chemical vapor deposition (CVD) and MoS2 flakes prepared by mechanical exfoliation from natural bulk crystal. Raman spectroscopy measurements show differences between the in-plane and out-of-plane phonon mode positions in CVD-grown and exfoliated MoS2. Photoluminescence (PL) mapping reveals large regions in the CVD-grown films that emit strong PL at room temperature, and low-temperature PL scans demonstrate a large spectral shift of the A exciton emission as a function of position. Polarization-resolved PL measurements under near-resonant excitation conditions show a strong circular polarization of the PL, corresponding to a valley polarization.
Investigation of Optical Properties of layered MoS 2
2012
Few atomic layers of molybdenum disulphide (MoS 2) have been prepared using chemical exfoliation techniques. Optical properties of few atomic layers of MoS 2 were investigated experimentally using Raman and Photoluminescence spectroscopy. Ab-initio density functional theory has been used to understand the electronic structure and lattice dynamics of monolayered and bilayered MoS2.
ACS Applied Nano Materials
Monolayer (ML) MoS 2 has become a very promising two-dimensional material for photorelated applications, potentially serving as the basis for an ultrathin photodetector, switching device, or transistors because of its strong interaction with light in ambient conditions. Establishing the impact of individual ambient gas components on the optical properties of MoS 2 is a necessary step toward application development. By using in situ Raman microspectroscopy with an environmentcontrolled reaction cell, the photoluminescence (PL) intensity of chemical vapor deposition (CVD)-grown MoS 2 MLs is monitored at different intralayer locations under ambient and controlled gas environments, such as N 2 , O 2 , and H 2 O. This new approach enables us to monitor the optical properties of MoS 2 at different locations on the flakes and separate the role of photoreaction of various gases during laser irradiation. Upon mild photoirradiation in ambient conditions, the PL intensity in the interior of the ML MoS 2 flakes remains unchanged, while the PL intensity at the edge region increases drastically. Photoirradiation in controlled gas environments reveals that O 2 is necessary to increase the PL intensity at the MoS 2 flake edges, attributed to the charge transfer of chemisorbed O 2. N 2 or H 2 O and N 2 environments induce decreasing PL intensity upon repetitive laser irradiation. However, the H 2 O and O 2 gas mixture, a combination designed to mimic ambient conditions, is necessary to maintain the PL intensity at the interior of the ML MoS 2 flakes. Our study demonstrates that photoreactions with the gaseous environment on the MoS 2 ML flakes should be taken into consideration even upon mild photoirradiation because they strongly impact the flakes' optical properties.
Nature and origin of unusual properties in chemically exfoliated 2D MoS2
APL Materials, 2020
MoS2 in its two-dimensional (2D) form is known to exhibit many fundamentally interesting and technologically important properties. One of the most popular routes to form extensive amount of such 2D samples is the chemical exfoliation route. However, the nature and origin of the specific polymorph of MoS2 primarily responsible for such spectacular properties has remained controversial with claims of both T and T' phases as well as metallic and semiconducting natures. We show that a comprehensive scrutiny of the available literature data of Raman spectra from such samples allow little scope for such ambiguities, providing overwhelming evidence for the formation of the T' phase as the dominant metastable state in all such samples. We also explain that this small band-gap T' phase may attain substantial conductivity due to thermal and chemical doping of charge-carriers, explaining the contradictory claims of metallic and semiconducting nature of such samples, thereby attaining a consistent view of all reports available so far.
Optoelectronic response and excitonic properties of monolayer MoS2
Journal of Applied Physics, 2016
Ab initio, electronic energy bands of MoS2 single layer are reported within the local density functional approximation. The inclusion of spin orbit coupling reveals the presence of two excitons A and B. We also discuss the change of physical properties of MoS2 from multilayer and bulk counterparts. The nature of the band gap changes from indirect to direct when the thickness is reduced to a single monolayer. The imaginary and real dielectric functions are investigated. Refractive index and birefringence are also reported. The results suggest that MoS2 is suitable for potential applications in optoelectronic and photovoltaic devices. The ab initio study is essential to propose the crucial parameters for the analytical model used for A-B exciton properties of the monolayer MoS2. From a theoretical point of view, we consider how the exciton behavior evolves under environmental dielectrics.
Sonochemical exfoliation and photodetection properties of MoS2 Nanosheets
Materials Science in Semiconductor Processing, 2019
Atomically thin layers of transition metal dichalcogenides (TMDCs) have shown great promise for the applications in the field of optoelectronics. TMDCs have drawn huge attention of researchers in recent past due to their unique layer dependent properties. Here, we report the high yield synthesis of atomically thin MoS 2 Nanosheets. The Nanosheets are synthesized by sono-chemical exfoliation technique. The exfoliated MoS 2 have 2H-hexagonal lattice structure with P6 3 /mmc space group. The Raman spectrum shows the resonances corresponding to A 1g and E 2g vibrational modes of 2H-MoS 2. The lateral morphology of MoS 2 Nanosheets is observed by transmission electron microscopy. The SAED pattern depicts the highly crystalline nature of Nanosheets. The absorption spectrum of MoS 2 Nanosheets shows A and B excitonic resonances corresponding to direct inter-band transition at 1.82 eV and 2.0 eV, respectively. Moreover, the photo detector based on MoS 2 Nanosheets is fabricated and studied under periodic 670 nm illumination of power intensity 3 mW/cm 2. The fast response with enhanced photo responsivity of 16.93 mA/W and specific detectivity of 5.12 × 10 8 Jones is observed. Overall, the present findings demonstrate high yield synthesis of Nanosheets with good quality to be used for photodetector.
2D-MoS2 goes 3D: transferring optoelectronic properties of 2D MoS2 to a large-area thin film
npj 2D Materials and Applications, 2021
The ongoing miniaturization of electronic devices has boosted the development of new post-silicon two-dimensional (2D) semiconductors, such as transition metal dichalcogenides, one of the most prominent materials being molybdenum disulfide (MoS2). A major obstacle for the industrial production of MoS2-based devices lies in the growth techniques. These must ensure the reliable fabrication of MoS2 with tailored 2D properties to allow for the typical direct bandgap of 1.9 eV, while maintaining large-area growth and device compatibility. In this work, we used a versatile and industrially scalable MoS2 growth method based on ionized jet deposition and annealing at 250 °C, through which a 3D stable and scalable material exhibiting excellent electronic and optical properties of 2D MoS2 is synthesized. The thickness-related limit, i.e., the desired optical and electronic properties being limited to 2D single/few-layered MoS2, was overcome in the thin film through the formation of encapsulat...