arXiv subjects

Faiha Mujeeb

Publications and source records attributed to Faiha Mujeeb.

7 recordsLinked to original sources

arXiv2026

Nanoscale mapping of stacking-dependent work function and local photoresponse in CVD-grown MoS2 bilayers by KPFM

Stacking order in bilayers of transition metal dichalcogenides (TMDs) controls structural symmetry and layer-to-layer interactions, offering a direct route to tune their electronic properties and enable optoelectronic applications. The work function is a key parameter that determines the electronic and optoelectronic device performance. However, a comprehensive understanding of the influence of stacking order on work function of TMDs remains limited. Herein, we employ Kelvin Probe Force Microscopy (KPFM) to probe spatial variations in surface potential and thereby determine the work function of AA'- and AB-stacked MoS2 bilayers grown using NaCl-assisted chemical vapor deposition (CVD) technique. The work function increases with layer number in both AA'- and AB-stacked MoS2, with a larger work function difference in AB-stacked layers, reflecting their stronger interlayer coupling. KPFM measurements clearly resolve local electronic heterogeneities arising from carrier trapping at residual surface particulates from CVD growth. Photoinduced surface potential variations imply n-type doping in MoS2 due to enhanced photogating from trapped holes and Na+ ions at the MoS2/SiO2 interface. Our study demonstrates the competing effects of interlayer coupling, substrate-induced photogating, and carrier trapping by surface particulates in determining the localized optoelectronic response of MoS2 bilayers. Correlative atomic force microscopy measurements in lateral force microscopy and force modulation microscopy modes probe the nanomechanical response to electronic variations. These findings provide new insights into the localized optoelectronic response of CVD-grown AA'- and AB-stacked MoS2, with significant implications for the design and reliability of optoelectronic devices.

cond-mat.mes-hall↗

arXiv2025

High-energy electron-beam induced defect engineering of monolayer MoS2 for tunable optical properties

Structural defects in 2D-transition metal dichalcogenides are critical in modulating their optical and electrical behavior. Nevertheless, precise defect control within the monolayer regime poses a significant challenge. Herein, a high-energy (1MeV) electron beam irradiation strategy is harnessed to induce defects in monolayer MoS2. Controlled variation of electron-beam irradiation time tunes the defect density, as reflected by the evolution of defect-mediated photoluminescence characteristics. The optically active defect emission appearing at approx. 200-300meV below the A exciton at 85K exhibits a systematic increase in intensity with prolonged exposure and saturates at higher laser excitation power. Circular polarization-resolved photoluminescence spectroscopy reveals strong suppression of valley polarization of the A exciton after irradiation. Complementary x-ray photoelectron spectroscopy identifies enhanced Mo-O bonding signatures in MoS2 following irradiation. Kelvin probe force microscopy indicates the transition to p-type doping behaviour. A detailed temperature and power-dependent photoluminescence measurements further elucidate the optical behaviour of these defect states. Density functional theory calculations using these configurations establish that the transition between the conduction band and acceptor states within the bandgap accounts for the defect emission. This work presents a tunable route for defect engineering in monolayer TMDs, enabling controlled tailoring of their structural and optical properties for optoelectronic, electronic and valleytronic applications.

cond-mat.mes-hall↗

arXiv2025

Experimental demonstration of two distinct pathways of trion generation in monolayer MoS2

Excitation power and energy dependent photoluminescence (PL) and transient absorption spectroscopy (TAS) studies are carried out on chemical vapour deposition (CVD) grown 1L-MoS2 films to understand the process of trion formation. The study shows that the excitation with sufficiently low photon energy results in the creation of trions directly in the K/K' valleys through photon absorption followed by phonon scattering events. On the other hand, excitation energy sufficiently larger than the band-gap can generate the carriers away from the K/K' valleys. Dissimilarity in the rates of relaxation of the photo-excited electrons and the holes to the bottom of the K/K' valleys results in the transformation of the excitons residing there into trions. Our TAS study clearly demonstrates a temporary increase of the trion population in the K/K' valleys. Moreover, excitation intensity dependent PL spectroscopy performed under above-band-gap excitation, also suggests the coexistence of both the pathways of trion generation in this material. This conclusion is further validated by a rate equation model. Our findings provide valuable insight into the formation of trions in monolayer transition metal dichalcogenides (TMDC), which could be crucial in designing valleytronic devices based on trions.

cond-mat.mtrl-sci↗

arXiv2024

Controlled Growth of large area bilayer MoS$_2$ films on SiO$_2$ substrates by chemical vapour deposition technique

Bilayer (2L) transition metal dichalcogenides (TMD) have the ability to host interlayer excitons, where electron and hole parts are spatially separated that leads to much longer lifetime as compared to direct excitons. This property can be utilized for the development of exciton-based logic devices, which are supposed to be superior in terms of energy efficiency and optical communication compatibility as compared to their electronic counterparts. However, obtaining uniformly thick bilayer epitaxial films with large area coverage is challenging. Here, we have engineered the flow pattern of the precursors over the substrate surface to obtain large area (mm2) covered strictly bilayer MoS$_2$ films on SiO$_2$ by chemical vapour deposition (CVD) technique without any plasma treatment of the substrate prior to the growth. Bilayer nature of these films is confirmed by Raman, low-frequency Raman, atomic force microscopy (AFM) and photoluminescence (PL) studies. The uniformity of the film has been checked by Raman peak separation and PL intensity map. High resolution transmission electron microscopy (HRTEM) reveals that crystalline and twisted bilayer islands coexist within the layer. Back gated field-effect transistor (FET) structures fabricated on the bilayers show on/off ratio of 10^6 and subthreshold swings (SS) of 2.5 V/Decade.

cond-mat.mtrl-sci↗

arXiv2023

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.

physics.optics↗

arXiv2022

Influence of Defects on the Valley Polarization Properties of Monolayer MoS$_{2}$ 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 $Γ$-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 $Γ$-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.

physics.optics↗

arXiv2022

Effect of strain and sulfur vacancies on the luminescence and valley polarization properties of CVD grown monolayer MoS$_2$ 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.

cond-mat.mtrl-sci↗