Effect of strain and sulfur vacancies on the luminescence and valley polarization properties of CVD grown monolayer MoS2_2 films

Using temperature dependent photoluminescence (PL), polarization resolved PL and Raman spectroscopy, we investigate the effect of in situ vacuum annealing as well as the relaxation of strain on the luminescence and the valley polarization properties of large area strictly monolayer (1L)-MoS$_2$, grown on sapphire and SiO$_2$/Si substrates by a microcavity based chemical vapor deposition (CVD) technique. The study shows that the strain as well as the physisorption of air molecules at the sulfur vacancy ($V_S$) sites play key roles in governing the optical quality of CVD grown 1L-MoS$_2$. Removal of air molecules from the $V_S$ sites enhances the relative strength of the A-exciton/trion transition as compared to the broad luminescence (BL) band arising from those defects at low temperatures. It has also been found that such removal helps in improving the valley polarization property of the film. Relaxation of biaxial tensile strain, which has been achieved by post growth transferring of 1L-MoS$_2$ film from the sapphire to a SiO$_2$/Si substrate by a polystyrene assisted transfer process, is also found to be helpful to get back the high polarization character ($\sim$80%) of the valleys. The study further shows that the transfer process not only facilitates the removal of physisorbed air molecules from the $V_S$ sites but also puts in place a long lasting capping layer on MoS$_2$ that shields the film from reacting with air and hence enhances the relative yield of A-exciton/trion transition by suppressing the BL transition. The study thus creates an opportunity to use CVD grown large area 1L-MoS$_2$ for the development of optoelectronic as well as valleytronic devices for practical applications for the future

Influence of Defects on the Valley Polarization Properties of Monolayer MoS2_{2} Grown by Chemical Vapor Deposition

Here, the underlying mechanisms behind valley de-polarization is investigated in chemical vapor deposited 1L-MoS$_{2}$. Temperature dependent polarization resolved photoluminescence spectroscopy was carried out on as-grown, transferred and capped samples. It has been found that the momentum scattering of the excitons due to the sulfur-vacancies attached with air-molecule defects has a strong influence on the valley de-polarization process. Our study reveals that at sufficiently low densities of such defects and temperatures, long range electron-hole exchange mediated intervalley transfer due to momentum scattering via Maialle-Silva-Sham (MSS) mechanism of excitons is indeed the most dominant spin-flip process as suggested by T. Yu et al. The rate of momentum scattering of the excitons due to these defects is found to be proportional to the cube root of the density of the defects. Intervalley transfer process of excitons involving $\Gamma$-valley also has significance in the valley de-polarization process specially when the layer has tensile strain or high density of $V_S$ defects as these perturbations reduce $K$ to $\Gamma$-energy separation. Band-structural calculations carried out within the density functional theory framework validate this finding. Experimental results further suggest that exchange interactions with the physisorbed air molecules can also result in the intervalley spin-flip scattering of the excitons, and this process gives an important contribution to valley depolarization, specially at the strong scattering regime

Ni cluster embedded (111)NiO layers grown on (0001)GaN films using pulsed laser deposition technique

(111) NiO epitaxial layers embedded with crystallographically oriented Ni-clusters are grown on c-GaN/Sapphire templates using pulsed laser deposition technique. Structural and magnetic properties of the films are examined by a variety of techniques including high resolution x-ray diffraction, precession-electron diffraction and superconducting quantum interference device magnetometry. The study reveals that the inclusion, orientation, shape, size, density and magnetic properties of these clusters depend strongly on the growth temperature (TG). Though, most of the Ni-clusters are found to be crystallographically aligned with the NiO matrix with Ni(111) parallel to NiO(111), clusters with other orientations also exist, especially in samples grown at lower temperatures. Average size and density of the clusters increase with TG . Proportion of the Ni(111) parallel to NiO(111) oriented clusters also improves as TG is increased. All cluster embedded films show ferromagnetic behaviour even at room temperature. Easy-axis is found to be oriented in the layer plane in samples grown at relatively lower temperatures. However, it turns perpendicular to the layer plane for samples grown at sufficiently high temperatures. This reversal of easy-axis has been attributed to the size dependent competition between the shape, magnetoelastic and the surface anisotropies of the clusters. This composite material thus has great potential to serve as spin-injector and spinstorage medium in GaN based spintronics of the future

An electroplating-based plasmonic platform for giant emission enhancement in monolayer semiconductors

Two dimensional semiconductors have attracted considerable attention owing to their exceptional electronic and optical characteristics. However, their practical application has been hindered by the limited light absorption resulting from their atomically thin thickness and low quantum yield. A highly effective approach to manipulate optical properties and address these limitations is integrating subwavelength plasmonic nanostructures with these monolayers. In this study, we employed electron beam lithography and electroplating technique to fabricate a gold nanodisc (AuND) array capable of enhancing the photoluminescence (PL) of monolayer MoS$_2$ giantly. Monolayer MoS$_2$ placed on the top of the AuND array yields up to 150-fold PL enhancement compared to that on a gold film. We explain our experimental findings based on electromagnetic simulations

Effect of manganese incorporation on the excitonic recombination dynamics in monolayer MoS2_2

Using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy techniques we investigate the incorporation of Manganese (Mn) in monolayer (1L)-MoS$_2$ grown on sapphire substrates by microcavity based chemical vapor deposition (CVD) method. These layers are coated with different amount of Mn by pulsed laser deposition (PLD) technique and temperature dependent photo-luminescence (PL) spectroscopic study has helped us in understanding how such deposition affects the dynamics of excitonic recombination in this system. The study further reveals two distinctly different Mn-incorporation regimes. Below a certain critical deposition amount of Mn, thin Mn-coating with large area coverage is found on MoS$_2$ layers and in this regime, substitution of Mo ions by Mn is detected through XPS. Dewetting takes place when Mn-deposition crosses the critical mark, which results in the formation of Mn-droplets on MoS$_2$ layers. In this regime, substitutional incorporation of Mn is suppressed, while the Raman study suggests an enhancement of disorder in the lattice with the Mn-deposition time. From PL investigation, it has been found that the increase of the amount of Mn-deposition not only enhances the density of non-radiative recombination channels for the excitons but also raises the barrier height for such recombination to take place. The study attributes these non-radiative transitions to certain Mo related defects (either Mo-vacancies or distorted Mo-S bonds), which are believed to be generated in large numbers during Mn-droplet formation stage as a result of the withdrawal of Mn ions from the Mo-substitutional sites.Comment: Page 1-19 main text with 6 figures. Page 20-24 supplementary material with 4 figure