Optical Properties Of MoS2 Monolayer Research Articles

Optical Modification of Monolayer MoS2: Deterministic Modification of CVD Grown Monolayer MoS2 with Optical Pulses (Adv. Mater. Interfaces 10/2021)

In article number 2002119, Mika Pettersson, Zhipei Sun, and co-workers demonstrate an optical modification process that simultaneously creates topographical structures and alters the optical properties of MoS2 monolayer. This local and controllable optical modification method shows great promise for applications in electronics, photonics, and mechanics.

Impact of thermal annealing in forming gas on the optical and electrical properties of MoS2 monolayer

Technological applications involving 2D MoS2 require transfer of chemical vapor deposition (CVD) grown material from its original substrate and subsequent lithographic processes. Inevitably, those steps contaminate the surface of the 2D material with polymeric residues affecting the electronic and optical properties of the MoS2. Annealing in forming gas is considered an efficient treatment to partially remove such residues. However, hydrogen also interacts with MoS2 creating or saturating sulfur vacancies. Sulfur vacancies are known to be at the origin of n-doping evident in the majority of as-grown MoS2 samples. In this context, investigating the impact of thermal annealing in forming gas on the electronic and optical properties of MoS2 monolayer is technologically important. In order to address this topic, we have systematically studied the evolution of CVD grown MoS2 monolayer using Raman spectroscopy, photoluminescence, x-ray photoelectron spectroscopy and transport measurements through a series of thermal annealing in forming gas at temperatures up to 500 °C. Efficient removal of the polymeric residues is demonstrated at temperatures as low as 200 °C. Above this value, carrier density modulation is identified by photoluminescence, x-ray photoelectron spectroscopy and electrical characterization and is correlated to the creation of sulfur vacancies. Finally, the degradation of the MoS2 single layer is verified with annealing at or above 350 °C through Raman and photocurrent measurements.

Tuning carrier concentration in a superacid treated MoS2 monolayer

The effect of bis(trifluoromethane) sulfonimide (TFSI, superacid) treatment on the optical properties of MoS2 monolayers is investigated by means of photoluminescence, reflectance contrast and Raman scattering spectroscopy employed in a broad temperature range. It is shown that when applied multiple times, the treatment results in progressive quenching of the trion emission/absorption and in the redshift of the neutral exciton emission/absorption associated with both the A and B excitonic resonances. Based on this evolution, a trion complex related to the B exciton in monolayer MoS2 is unambiguously identified. A defect-related emission observed at low temperatures also disappears from the spectrum as a result of the treatment. Our observations are attributed to effective passivation of defects on the MoS2 monolayer surface. The passivation reduces the carrier density, which in turn affects the out-of-plane electric field in the sample. The observed tuning of the carrier concentration strongly influences also the Raman scattering in the MoS2 monolayer. An enhancement of Raman scattering at resonant excitation in the vicinity of the A neutral exciton is clearly seen for both the out-of-plane A′1 and in-plane E′ modes. On the contrary, when the excitation is in resonance with a corresponding trion, the Raman scattering features become hardly visible. These results confirm the role of the excitonic charge state plays in the resonance effect of the excitation energy on the Raman scattering in transition metal dichalcogenides.

Optical spectroscopy of excited exciton states in MoS2 monolayers in van der Waals heterostructures

The optical properties of MoS2 monolayers are dominated by excitons, but for spectrally broad optical transitions in monolayers exfoliated directly onto SiO2 substrates detailed information on excited exciton states is inaccessible. Encapsulation in hexagonal boron nitride (hBN) allows approaching the homogenous exciton linewidth, but interferences in the van der Waals heterostructures make direct comparison between transitions in optical spectra with different oscillator strength more challenging. Here we reveal in reflectivity and in photoluminescence excitation spectroscopy the presence of excited states of the A-exciton in MoS2 monolayers encapsulated in hBN layers of calibrated thickness, allowing to extrapolate an exciton binding energy of about 220 meV. We theoretically reproduce the energy separations and oscillator strengths measured in reflectivity by combining the exciton resonances calculated for a screened two-dimensional Coulomb potential with transfer matrix calculations of the reflectivity for the van der Waals structure. Our analysis shows a very different evolution of the exciton oscillator strength with principal quantum number for the screened Coulomb potential as compared to the ideal two-dimensional hydrogen model.

Localized defect states in MoS2monolayers: Electronic and optical properties

Defects usually play an important role in tuning and modifying various properties of semiconducting or insulating materials. Therefore we study the impact of point and line defects on the electronic structure and optical properties of MoS2 monolayers using density-functional methods. The different types of defects form electronic states that are spatially localized on the defect. The strongly localized nature is reflected in weak electronic interactions between individual point or line defect and a weak dependence of the defect formation energy on the defect concentration or line defect separation. In the electronic energy spectrum the defect states occur as deep levels in the band gap, as shallow levels very close to the band edges, as well as levels in-between the bulk states. Due to their strongly localized nature, all states of point defects are sharply peaked in energy. Periodic line defects form nearly dispersionless one-dimensional band structures and the related spectral features are also strongly peaked. The electronic structure of the monolayer system is quite robust and it is well preserved for point defect concentrations of up to 6%. The impact of point defects on the optical absorption for concentrations of 1% and below is found to be very small. For higher defect concentrations molybdenum vacancies were found to quench the overall absorption and sulfur defects lead to sharp absorption peaks below the absorption edge of the ideal monolayer. For line defects, we did not find a considerable impact on the absorption spectrum. These results support recent experiments on defective transition metal chalcogenides.

Tuning electronic and optical properties of MoS2monolayer via molecular charge transfer

Density functional theory computations were performed to investigate the adsorption of four organic molecules, including tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), tetrathiafulvalene (TTF) and benzyl viologen (BV) on the basal plane of MoS2 monolayer (MoS2ML). There mainly exist non-covalent weak interactions between these organic molecules and MoS2ML with considerable charge transfer. Due to the adsorption of organic molecules, the band gap of MoS2ML can be efficiently reduced as the flat molecular levels lie in the band gap region of MoS2ML. Moreover, the adsorption of TCNQ can significantly enhance the optical absorption of MoS2ML in the infrared region of solar spectrum, whereas the adsorption of other molecules has negligible effect on the optical properties of MoS2ML. Our computations provide a flexible approach towards tuning the electronic and optical properties of MoS2ML.