Phonon-phonon anharmonic effects have a strong influence on the phonon spectrum; most prominent manifestation of these effects are the softening (shift in frequency) and broadening (change in FWHM) of the phonon modes at finite temperature. Using Raman spectroscopy, we studied the temperature dependence of the FWHM and Raman shift of E 1 2g and A1g modes for single-layer and natural bilayer MoS2 over a broad range of temperatures (8 100 K, whereas they become independent of temperature for T < 100 K. Using first-principles calculations, we show that three-phonon anharmonic effects intrinsic to the material can account for the observed temperaturedependence of the line-width of both the modes. It also plays an important role in determining the temperature-dependence of the frequency of the Raman modes. The observed evolution of the linewidth of the A1g mode suggests that electron-phonon processes are additionally involved. From the analysis of the temperature-dependent Raman spectra of MoS2 on two different substrates-SiO2 and hexagonal boron nitride, we disentangle the contributions of external stress and internal impurities to these phonon-related processes. We find that the renormalization of the phonon mode frequencies on different substrates is governed by strain and intrinsic doping. Our work establishes the role of intrinsic phonon anharmonic effects in deciding the Raman shift in MoS2 irrespective of substrate and layer number.

We report the room temperature tuning of excitonic lifetime in pristine and hole-doped monolayer MoS2 based field effect transistor (FET) devices by systematically controlling the free carrier density. We observed that in pristine MoS2 devices, with intrinsic electron doping, an exciton dominant regime with an exciton lifetime of 3 ns exists, when doped electrostatically with holes. Interestingly we observe a sharp decrease in exciton lifetime and population with an increase of the electron density by electrostatic doping, with a corresponding increase in negative trion population. With increased hole doping by a chemical method, the exciton lifetime decreases, but it remains almost constant with electrostatic carrier density tuning. This decrease in lifetime, compared to that of the pristine case, might be due to the exciton–exciton annihilation mechanism which is proposed to be existent in a high exciton density regime. Further hole doping by a chemical method leads to a transition to a positive trion dominated regime, in which the exciton lifetime decreases further due to nonradiative energy transfer to the positive trions. We observe a slight increase in exciton lifetime due to partial neutralization of positive trions at high electrostatic electron doping and a corresponding increase in the probability of excitons. We suggest that when calculating the lifetime of excitons, the exciton-to-trions formation and exciton–exciton annihilation mechanisms should be considered. These fine-tunings of excitons in monolayer MoS2 can provide a platform for probing the excitonic physics and photonic applications.

In this paper, we show experimentally that for van der Waals heterostructures (vdWh) of atomically-thin materials, the hybridization of bands of adjacent layers is possible only for ultra-clean interfaces. This we achieve through a detailed experimental study of the effect of interfacial separation and adsorbate content on the photoluminescence emission and Raman spectra of ultra-thin vdWh. For vdWh with atomically-clean interfaces, we find the emergence of novel vibrational Raman-active modes whose optical signatures differ significantly from that of the constituent layers. Additionally, we find for such systems a significant modification of the photoluminescence emission spectra with the appearance of peaks whose strength and intensity directly correlate with the inter-layer coupling strength. Our ability to control the intensity of the photoluminescence emission led to the observation of detailed optical features like indirect-band peaks. Our study establishes that it is possible to engineer atomically-thin van der Waals heterostructures with desired optical properties by controlling the inter-layer spacing, and consequently the inter-layer coupling between the constituent layers.

Non-radiative processes like energy and charge transfer in 0D-2D semiconductor quantum dot (QD)-transition metal dichalcogenide (TMD) and other two dimensional layered materials, like graphene and analogs, leading to strong quenching...