Optical Properties Of Monolayer MoS2 Research Articles

Understanding the interplay of defects, oxygen, and strain in 2D materials for next-generation optoelectronics

Two-dimensional transition metal dichalcogenides are leading materials for next-generation optoelectronics, but fundamental problems stand enroute to commercialization. These problems include, firstly, the widely debated defect- and strain-induced origins of intense low-energy broad luminescence peaks (L-peaks) observed at low temperatures. Secondly, the role of oxygen in tuning the properties via chemisorption and physisorption is intriguing but challenging to understand. Thirdly, our physical understanding of the benefits of hexagonal boron nitride (hBN) encapsulation is inadequate. Using a series of samples, we decouple the contributions of oxygen, defects, adsorbates, and strain on the optical properties of monolayer MoS2. The defect origin of the L-peak is confirmed by temperature- and power-dependent photoluminescence (PL) measurements, with a dramatic redshift of ∼130 meV for oxygen-assisted chemical vapour deposition (O-CVD) samples compared with exfoliated samples. Anomalously, the O-CVD samples show high A-exciton PL at room temperature (cf exfoliated), but reduced PL at low temperatures, attributed to the strain-induced direct-to-indirect bandgap crossover in low-defect O-CVD MoS2. These observations are consistent with our density functional theory calculations and are supported by Raman spectroscopy. In the exfoliated samples, the charged O adatoms are identified as thermodynamically favourable defects, and create in-gap states. The beneficial effect of encapsulation originates from the reduction of charged O adatoms and adsorbates. This experimental–theoretical study uncovers the type of defects in each sample, enables an understanding of the combined effect of defects, strain, and oxygen on the band structure, and enriches our understanding of the effects of encapsulation. This work proposes O-CVD as a method for creating high-quality materials for optoelectronics.

Influence of Eu and Nd dopants on the photovoltaic properties of monolayer MoS2: A first-principles study

In this paper, the effects of Eu, Nd doping, and Eu–Nd co-doping on the stability, magnetic and optical properties of monolayer MoS2 are investigated by the density functional theory based on the first principle's calculations. It was found that the Eu–Nd co-doped monolayer MoS2 is more stable than other considered doping structures. After the introduction of Eu and Nd atoms, the band gap are decreased to different degrees, indicating that the doping is favorable to the electronic jump, which might enhances the conductivity of the system. All doped systems exhibit magnetic properties. In addition, the absorption spectra of all doped systems show significant red-shift compared with those of pure monolayer MoS2, indicating that the doping improves the absorption capacity in visible light. These results might provide theoretical guidance for the design of novel monolayer MoS2 semiconductor materials.

Electronic and optical properties of doped monolayer MoS2 under shear deformation

The effect of O-atom doping on the electronic and optical properties of monolayer MoS2 under shear deformation has been systematically investigated using first principles. The results show that shear deformation reduces the structural stability of the doped system. The forbidden band width of the doped system decreases sequentially with increasing shear deformation, while conductivity increases. The density of states of both intrinsic and doped systems is primarily contributed by the 4d and 3p orbitals of the Mo and S atoms, respectively. Analysis of the optical properties reveals that shear deformation enhances the static permittivity of the doped systems, leading to an increased ability to bind charges. Additionally, absorption and reflection peaks of all doped systems occur in the ultraviolet region. Compared to the doped system without shear deformation, absorption peaks of the remaining doped systems shift towards the high energy region, resulting in enhanced utilization of ultraviolet light. In the energy range of 16.7–17.3 eV, peak energy loss of all doped systems decreases sequentially, suggesting that shear deformation can reduce energy loss. These results demonstrate that shear deformation can modulate the optoelectronic properties of O-doped monolayer MoS2 and provide a theoretical foundation for practical applications in semiconductor devices.

In situ doping effect in monolayer MoS2 via laser irradiation

Abstract Two-dimensional (2D) semiconducting materials with a single atomic layer display exceptional structural symmetry and band structures, making them the most promising candidates for investigating the spin-valley coupling effect and fabricating novel optoelectronic devices. Their atomic thinness also makes it easy to adjust their excitonic optical response through plasma treatment or thermal annealing. In this study, we present a simple technique for modifying the optical properties of monolayer MoS2 by briefly exposing it to laser irradiation in ambient conditions. Initially, this exposure resulted in a nearly twofold increase in photoluminescence (PL) intensity, with the neutral exciton intensity increasing while the trion exciton intensity decreased. We propose that oxygen-related functional groups, such as O2 and H2O from the surrounding air, adsorb onto MoS2 and extract extra electrons, which enhances exciton emission while reducing trion emission. In a subsequent stage, both exciton intensities decreased as all extra electrons were depleted. Additionally, any structural distortions or potential damage were found to decrease the PL intensity, and these changes were linked to alterations in the Raman spectra.

Ab-Initio and Experimental Study of the Electronic and Optical Properties of Layered 2D Molybdenum Disulphide

Molybdenum disulphide (MoS2) in its two-dimensional (2D) mono- to few layers is applied in numerous photonic applications owing to its wide bandgap. In this work, the electronic and optical properties of monolayer MoS2 were successfully investigated by ab initio study through density functional theory (DFT) calculation and experimental work. A 2D monolayer MoS2 model was simulated using CASTEP in the framework of plane-wave pseudopotential. The simulated bandgap is 1.7 eV which is interestingly close to those determined bandgap of MoS2 prepared through the liquid-phase exfoliation method. This confirmed the successful formation of 2D layered MoS2 in the proposed fabrication process. The MoS2 model also predict well the experimentally derived absorption range which has a significant peak at around ~600 nm wavelength ascribed to the excitonic property.

Role of gas flow direction on monolayer MoS2 growth on patterned surfaces via CVD

The chemical vapor deposition (CVD) technique has been widely used to grow high quality single layer MoS2 crystals. However, there are still issues that need to be clarified in order to determine the appropriate conditions for the repeatable and scalable process. In this study, we experimentally investigated the effect of flow direction on the size distribution and optical signal of monolayer MoS2 flakes grown on microchannel arrayed surfaces. As an important parameter controlling the CVD process, the carrier gas flow was revealed to be highly sensitive to geometric orientation of the patterned surface. The flow direction parallel to the array yielded larger flakes with narrower size distribution, and enhanced photoluminescence (PL) peak intensities compared to the case with flow normal to the array while the Raman intensities for both the cases are almost the same. The size distributions were obtained as 6 ± 13 and 37 ± 4 µm for the perpendicular and parallel cases, respectively. It was also found that the PL peak intensity increased by approximately 1.5 times when switching to parallel configuration. This study presents an experimental approach to understand the influence of the precursor stream reshaped by the growth surface geometry on the structural and optical properties of monolayer MoS2, which has great potential for use in optoelectronic applications.

Effect of biaxial [110] strain on monolayer MoS2 and its vacancy defect system: A first-principles study

In this paper, the effects of biaxial [110] strain synergistic vacancy defects on the binding energy, electronic structure, and optical properties of monolayer MoS2 were investigated using first-principles based on density functional theory. The results show that the monolayer MoS2 is the most stable in the unstressed state, when it has vacancy defects, the stability decreases slightly, and the binding energy decreases with the increase of the absolute value of the strain. Monolayer MoS2 is a semiconductor with a direct band gap of 1.73 ​eV. When Mo-vacancy appears, the band gap decreases significantly and the metallicity increases. When S-vacancy appears, the direct band gap transforms into an indirect band gap and the forbidden bandwidth decreases. The optical properties of the material are red-shifted by tensile strain, and blue-shifted by compressive strain. The strain coordinated vacancy defects have a significant effect on the regulation of the optical properties of monolayer MoS2. The synergistic strain effect of vacancy provides some theoretical support for the application of monolayer MoS2 in optics and other fields.

Tuning electronic and optical properties of monolayer MoS2 by transition metal and nitrogen co-doping

In this work, using first principles study, we systematically studied the doping site, formation energy, electronic and optical properties of transition metal (TM), nitrogen (N) mono-doped and co-doped MoS2. It is found that TM and N atoms tend to settle at Mo and N sites, respectively. Both mono-doping and co-doping can reduce the band gap of MoS2. Mono-doping and co-doping can introduce impurity levels (ILs) within the band gap of MoS2. Especially, Sc-N co-doping positions the ILs in the middle of the band gap of MoS2, which can better assist the photogenerated electronic transitions. More importantly, the additionally added interband transitions can realize infrared light (IR) photoresponse through two-step optical absorption. This work has guiding significance for promoting the applications of MoS2 in the field of IR photodetectors and photovoltaic devices.

Bandgap Engineering and Near-Infrared-II Optical Properties of Monolayer MoS2: A First-Principle Study.

The fluorescence-based optical imaging in the second near-infrared region (NIR-II, 1,000–1,700 nm) has broad applications in the biomedical field, but it is still difficult to find new NIR-II fluorescence materials in the two dimension. As a crucial characteristic of the electronic structure, the band structure determines the fundamental properties of two-dimensional materials, such as their optical excitations and electronic transportation. Therefore, we calculated the electronic structures and optical properties of different crystalline phases (1T phase and 2H phase) of pure monolayer MoS2 films and found that the 1T phase has better absorption and thus better fluorescence in the NIR-II window. However, its poor stability makes the 1T-phase MoS2 less useful in vivo bioimaging. By introducing vacancy defects and doping with foreign atoms, we successfully tuned the bandgap of the monolayer 2H-MoS2 and activated it in the NIR-II. Our results show that by engineering the vacancy defects, the bandgap of the 2H phase can be tailored to around 1 eV, and there are three candidates of vacancy structures that exhibit strong absorption in the NIR-II.

The First Principles Calculation on the Raman Spectrum and Optical Properties of the Defect Monolayer MoS2

In the process of preparing monolayer MoS2 by chemical vapor deposition, the peak value of Raman spectrum will be different due to the thickness of deposited film. After irradiated by fast neutron, the two modes of E2g and A1g in the Raman spectrum of MoS2 are red shifted and broadened, and a new peak appears near 440–460 cm−1 (Tang. G, Pam. M. E, Zhang. H, et al. Applied Physics Express. 12(5), 056,001 2019). In this paper, two types of defects are constructed and their Raman spectra are calculated by first principles. To compare with the experimental Raman spectra of monolayer MoS2, we confirm the defect configurations in the experiment. Through the calculation of optical properties of monolayer MoS2 with defects, the calculation results show that the imaginary part of the dielectric function of ideal monolayer MoS2 has a high intensity peak near 3.0 eV, and the imaginary part of the dielectric function of the defect monolayer MoS2 also has a peak of the similar shape; due to the localized effect of vacancy defects, the peak value appears a little red shift. A new peak value of the imaginary part of the dielectric function of the monolayer MoS2 with molybdenum vacancy defects appears near 0.6~1.2 eV.

Deterministic Modification of CVD Grown Monolayer MoS2 with Optical Pulses

AbstractTransition metal dichalcogenide monolayers have demonstrated a number of exquisite optical and electrical properties. Here, the authors report the optical modification of topographical and optical properties of monolayer MoS2 with femtosecond pulses under an inert atmosphere. A formation of three‐dimensional structures on monolayer MoS2 with tunable height up to ≈20 nm is demonstrated. In contrast to unmodified monolayer MoS2, these optically modified structures show significantly different optical properties, such as lower photoluminescence intensity and longer fluorescence lifetime. The results suggest a novel way to modify transition metal dichalcogenide materials for mechanic, electronic and photonic applications.

Photoluminescence of monolayer MoS2 modulated by water/O2/laser irradiation.

The low photoluminescence (PL) quantum yields of transition metal dichalcogenide monolayers have been a limiting factor for their optoelectronic applications. Various and even inconsistent mechanisms have been proposed to modulate their PL efficiencies. Herein, we use PL/Raman microspectroscopy and the corresponding in situ mapping, atomic force microscopy, and field-effect transistor (FET) characterization to investigate the changes in the structural and optical properties of monolayer MoS2. Relatively low power density (<4.08 × 105 W cm-2) of laser irradiation in ambient air can cause a slight PL suppression effect on monolayer MoS2, whereas relatively high power density (∼1.02 × 106 W cm-2) of laser irradiation brings significant PL enhancement. Experiments under different atmospheres reveal that the laser-irradiation-induced enhancement only occurs in the atmosphere containing O2 and is more remarkable in pure O2. In addition, physically adsorbed water can also induce PL enhancement of monolayer MoS2. FET devices suggest that the adsorbed water produces a p-doping effect on MoS2, and the laser irradiation in ambient air generates an n-doping effect, and both types of doping can enhance the PL intensity. The island-shaped defects caused by laser irradiation can be stabilized by oxygen atoms and act as trapping centers for excited trions or electrons, thus reducing the non-radiative recombination ratio and enhancing the PL intensity. The physically adsorbed water works in a similar way. A low power density of laser irradiation can sweep away the originally adsorbed H2O on the surface, thus reducing the PL.

Tuning optical properties of monolayer MoS2 through the 0D/2D interfacial effect with C60 nanoparticles

The success of 2D materials reveals that material properties are highly dependent on their dimensionality. In heterostructure engineering, therefore, the dimensionality of interface also affects the coupling and process there. Here, we construct a fresh 0D/2D interface, demonstrating that their optical properties could be tuned due to the rich surface chemistry of nanoparticles. As a preliminary example, C60 nanoparticles are introduced onto monolayer MoS2 films through spin coating featured by simple operation, uniform coverage, wafer-scale fabrication and printing capability. A combined study of morphology characterization, optical measurement and band calculations has been performed on these heterostructures. The blue-shift of MoS2 photoluminescence peak is attributed to the efficient electron transfer from MoS2 to C60. Moreover, the emission behavior of C60/MoS2 heterostructures can evolve over time, which may relate to a non-equilibrium process governed by the kinetics and thermodynamic behavior of C60 nanoparticles upon the continued illumination of laser, with some potential in future dynamic imaging and thermal/light sensing systems. We are optimistic that, by further polishing the fabrication of 0D/2D heterostructures, their interaction effect could be in an artificial-atomic designing towards the flexible optical engineering with mixed 2D and 0D quantum confinements.

Probing the Effect of Chemical Dopant Phase on Photoluminescence of Monolayer MoS2 Using in Situ Raman Microspectroscopy

Understanding the role of chemical dopants is crucial to modulating the optoelectronic properties of monolayer (ML) MoS2 and realizing its optoelectronic applications such as photodetectors, switching devices, and ultrathin transistors. Here, the effect of the dopant phase of the same dopants—liquid and gaseous—on the optical properties of ML MoS2 has been investigated using in situ Raman microspectroscopy with an environment-controlled reaction cell. The results show that the gaseous n-type dopant, that is, pyridine, completely quenched the PL intensity of ML MoS2, while liquid pyridine preserved 50% of the original PL intensity attributed to its less effective charge transfer to MoS2 than the gaseous counterpart. Photoemission of the quenched MoS2 could not be recovered with mild annealing in nitrogen and only partially recovered upon long-term exposure to air. The results show that the dopant phase strongly impacts the PL properties of ML MoS2.

Enhanced two-photon absorption and two-photon luminescence in monolayer MoS2 and WS2 by defect repairing.

In this work, we investigated the nonlinear optical properties of monolayer MoS2 and WS2 modulated by defect engineering via chemical treatment. The results demonstrate that the two-photon luminescence (TPL) and two-photon absorption (TPA) coefficient were remarkably improved after the repair of sulfur vacancies for both monolayer MoS2 and WS2. After the chemical treatment, the nonradiative relaxation path dominant in pristine monolayer MoS2 is significantly alleviated, resulting in enhanced TPL. Our work affords an effective way to tailor the nonlinear absorption, luminescence and relaxation properties of sulfur-based two-dimensional metal dichalcogenides by defect engineering.

Research on the Factors Affecting the Growth of Large-Size Monolayer MoS₂ by APCVD.

The transition-metal chalcogenides (TMDs) are gaining increased attention from many scientists recently. Monolayer MoS2 is an emerging layered TMD material with many excellent physical and electrical properties. It can be widely used in catalysis, transistors, optoelectronics and integrated circuits. Here, the large-sized monolayer MoS2 is grown on the silicon substrate with a 285-nm-thick oxide layer by atmospheric pressure chemical vapor deposition (APCVD) of sulfurized molybdenum trioxide. This method is simple and it does not require vacuum treatment. In addition, the effects of growth conditions, such as sulfur source, molybdenum source, growth temperature, and argon flow rate on the quality and area of MoS2 are further studied systematically. These analysis results help to master the morphology and optical properties of monolayer MoS2. The high quality, excellent performance, and large-size monolayer MoS2 under the optimal growth condition is characterized by optical microscopy, AFM, XPS, photoluminescence, and Raman spectroscopy. The Raman spectrum and PL mapping show that the grown MoS2 is a uniform triangular monolayer with a side length of 100 μm, which can pave the way for the applications of photodetectors and transistors.

Chemical vapor deposition of monolayer MoS2 on sapphire, Si and GaN substrates

Monolayer MoS2 grown on sapphire, Si, and GaN substrates by chemical vapor deposition (CVD) method under the same growth conditions have been demonstrated, and the effects of diverse substrates on the morphological and optical properties of monolayer MoS2 are systematically compared. The variations of the PL and Raman spectra of monolayer MoS2 on the three substrates are mainly attributed to the different lattice mismatch and thermal conductivity between MoS2 and the substrates. Moreover, uniform contrast of the PL and Raman intensity mappings suggest that the as-grown monolayer MoS2 on diverse substrates achieve high quality and uniformity. Accordingly, the ability to grow monolayer MoS2 with high quality to surface corrugation on diverse substrates would open a route toward the synthesis of heteroand composite structures.

Effect of lithium doping on the optical properties of monolayer MoS2

The effect of lithium atoms' evaporation on the surface of monolayer MoS2 grown on SiO2/Si substrate is studied using ultrahigh vacuum (∼10−11 mbar) Raman and circularly polarized photoluminescence spectroscopies, at low lithium coverage (up to ∼0.17 monolayer). With increasing Li doping, the dominant E2g1 and A1g Raman modes of MoS2 shift in energy and broaden. Additionally, non zone-center phonon modes become Raman active. This regards, in particular, to double resonance Raman scattering processes, involving longitudinal acoustic phonon modes at the M and K points of the Brillouin zone of MoS2 and defects. It is also accompanied by a significant decrease in the overall intensity and the degree of circular polarization of the photoluminescence spectrum. The observed changes in the optical spectra are understood as a result of electron doping by lithium atoms and disorder-activated intervalley scattering of electrons and holes in the electronic band structure of monolayer MoS2.

Measurement of the surface susceptibility and the surface conductivity of atomically thin MoS2 by spectroscopic ellipsometry.

We show how to correctly extract from the ellipsometric data the surface susceptibility and the surface conductivity that describe the optical properties of monolayer MoS2. Theoretically, these parameters stem from modelling a single-layer two-dimensional crystal as a surface current, a truly two-dimensional model. Current experimental practice is to consider this model equivalent to a homogeneous slab with an effective thickness given by the interlayer spacing of the exfoliating bulk material. We prove that the error in the evaluation of the surface susceptibility of monolayer MoS2, owing to the use of the slab model, is at least 10% or greater, a significant discrepancy in the determination of the optical properties of this material.

Influence of Mo-vacancy concentration on the structural, electronic and optical properties of monolayer MoS2: A first-principles study

Effects of Mo-vacancy concentration on the structural, electronic and optical properties of monolayer MoS2 have been investigated using the first-principles calculations. Results show that Mo-vacancy is prone to form in monolayer MoS2 under S-rich condition. S atoms around Mo-vacancy exhibit an outward relaxation, whereas Mo atoms around Mo-vacancy show an inward relaxation. At low Mo-vacancy concentration, some localized impurity states are induced in the band gap of monolayer MoS2, coupled with a band gap increment. As the Mo-vacancy concentration increases, the impurity states become delocalized and mix with the upper valence bands, resulting in the band gap decrease. The covalent character of Mo-S bonding is enhanced upon the introduction of Mo-vacancy, and the enhancement is weakened as the Mo-vacancy concentration increases. Optical properties calculations show that the static dielectric constant increases with the increasing Mo-vacancy concentration. The imaginary part of complex dielectric function exhibits a little blue shift for monolayer MoS2 with low Mo-vacancy concentration, whereas the imaginary part of complex dielectric function shows distinct red shift for monolayer MoS2 with high Mo-vacancy concentration.