Excitons and Trions in MoS2 Quantum Dots: The Influence of the Dispersing Medium

Single-layer MoS₂ has been reported to exhibit strong excitonic and trionic signatures in its photoluminescence (PL) spectra. Here, we report that the emission spectra of MoS₂ QDs strongly depend on the dielectric constant of the solvent and the relative difference in the electronegativity between the solvent and QDs. Due to the difference in electronegativity, electrons are either added to the QD or withdrawn from it. Consequently, depending upon the dielectric permittivity and the electronegativity of the surrounding medium, the signature peaks of excitons and trions exhibit a significant change in the PL spectra of MoS₂ QDs. Our findings are helpful to understand the effect of the surrounding environment on the optical properties of QDs and the importance of the selection of solvent since MoS₂ QDs are potential candidates for valleytronics applications.

Exfoliation Mechanism of Graphite Cathode in Ionic Liquids

Graphene has been successfully electrochemically exfoliated by electrolysis of cathode graphite in the aluminum-ion battery with ionic liquid electrolyte comprising AlCl₃ and 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl). The AlCl₄^-, Al₂Cl₇^-, etc., intercalation into graphite flakes in ionic liquid of the aluminum-ion battery by different electrolysis processes to exfoliate graphite has been researched in detail. As a result of the enhanced structural flexibility, the intercalant gallery height increases in the less than five-layer graphene film, providing more free space for AlCl₄^-, Al₂Cl₇^-, etc. transport. Therefore, a quantity of 3-5 layers rather than 1-2 layers of graphene can be obtained. The results clearly demonstrate that graphene has been produced in the graphite cathode in AlCl₃/EMImCl ionic liquids, which is significantly meaningful for accelerating the theoretical research and industrialized application of graphene. Meanwhile, it has a vitally important role for promoting the recycling Al-ion batteries.

Controlled Doping in Graphene Monolayers by Trapping Organic Molecules at the Graphene-Substrate Interface

We report controlled doping in graphene monolayers through charge-transfer interaction by trapping selected organic molecules between graphene and underneath substrates. Controllability has been demonstrated in terms of shifts in Raman peaks and Dirac points in graphene monolayers. Under field effect transistor geometry, a shift in the Dirac point to the negative (positive) gate voltage region gives an inherent signature of n- (p-)type doping as a consequence of charge-transfer interaction between organic molecules and graphene. The proximity of organic molecules near the graphene surface as a result of trapping is evidenced by Raman and infrared spectroscopies. Density functional theory calculations corroborate the experimental results and also indicate charge-transfer interaction between certain organic molecules and graphene sheets resulting p- (n-)type doping and reveals the donor and acceptor nature of molecules. Interaction between molecules and graphene has been discussed in terms of calculated Mulliken charge-transfer and binding energy as a function of optimized distance.

The effect of surfactants and their concentration on the liquid exfoliation of graphene

We investigated the effect of surfactants and their concentration on the final graphene concentration via the liquid-phase exfoliation method. Ionic surfactant and non-ionic surfactant have different mechanisms in the exfoliation process.

Dielectric screening of excitons in monolayer graphene

Excitonic transitions in graphene monolayers embedded in different dielectric environments have been investigated using combined absorption and transmission spectroscopy techniques. To vary the dielectric environment, graphene monolayer has been exfoliated in liquid medium. It has been shown that exciton binding energy decreases with increase in the dielectric constant of exfoliating solvents due to the screening of electron-electron and electron-hole interactions in graphene. The typical line shape of the excitonic peak in the absorption spectra is explained by the Fano resonance between the excitonic state and band continuum. Further it has been shown that, there exists a scaling relationship between the dielectric constant of the liquid and the exciton binding energy.