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This work presents a first-principles study of the optoelectronic properties of vertically-stacked bilayer heterostructures composed of 2D transition-metal dichalcogenides (TMDs). The calculations are performed using the density-functional theory (DFT) and many-body perturbation theory within $G_0W_0$-BSE methodology. We aim to propose these TMD heterostructures for solar cell applications. The TMD monolayers comprising the heterojunctions considered in this work are $MoS_2$, $WS_2$, $MoSe_2$, and $WSe_2$ due to their favorable band gaps, high carrier mobility, robust absorption in the visible region, and excellent stability. These four TMD monolayers provide the basis for six heterostructures. Consequently, we have examined the structural, electronic, and optical properties of six heterostructures ($WS_2/MoS_2$, $MoSe_2/MoS_2$, $MoSe_2/WS_2$, $WSe_2/MoS_2$, $WSe_2/MoSe_2$, and $WSe_2/WS_2$). At the DFT level, all the six considered TMD heterostructures meet the essential criterion of type II band alignment, a critical factor in extending carrier lifetime. However, according to $G_0W_0$ results, $MoSe_2/WS_2$ does not exhibit the type II band alignment, instead it shows type I band alignment. The large quasiparticle gaps obtained from $G_0W_0$ approximation suggest the presence of strong electron-correlation effects. The quality of these heterojunction solar cells is estimated by computing their power conversion efficiencies (PCE). The PCEs are calculated at both the HSE06 and $G_0W_0$ levels, and the maximum PCE predicted by HSE06 calculations on our designed solar cells can reach up to 19.25% for the $WSe_2/WS_2$ heterojunction. In addition, all six TMD heterostructures are examined for their potential applications in photocatalysis for hydrogen evolution reaction, and the three of them, namely, $WS_2/MoS_2$, $MoSe_2/MoS_2$, and $WSe_2/MoS_2$ heterostructures qualify for the same., Comment: 49 pages (40 pages Manuscript and 9 pages Supporting Material), 9 figures and 11 tables in the manuscript, and 7 figures and 6 tables in the Supporting Material

The present study investigates the catalytic performance of a $hg-C_{3}N_{4}$ quantum dot aimed at enhancing electrochemical water splitting, using the first-principles density functional theory. The size of the considered quantum dot lies within the range reported experimentally (2nm - 4nm) [Zhou et al. ACS Nano 9, 12480 (2015)]. The nitrogen vacancies are created in the considered $hg-C_{3}N_{4}$ structure to simulate the realistic scenario, as the presence of nitrogen and carbon defects are reported in the synthesized $hg-C_{3}N_{4}$ quantum dots. First, the structural and vibrational properties are computed to ensure the stability of the nitrogen deficient $hg-C_{3}N_{4}$ quantum dots, and subsequently, their electronic and hydrogen evolution reaction (HER) properties are investigated. The calculate HER parameters, i.e., adsorption energies, Gibbs free energies, and overpotentials demonstrate that the considered $hg-C_{3}N_{4}$ quantum dot with nitrogen vacancies can be used as a moderately effective electrocatalyst for HER performance. We also considered the quantum dot to be dissolved in water and ethanol, and find that the overpotential gets drastically reduced to 16 mV for the alcohol dissolved quantum dot, while some significant reduction is seen for the aqueous solution also. As a result, this study suggests that the nitrogen-deficient $hg-C_{3}N_{4}$ quantum dots dissolved in ethanol are excellent candidates for catalysis aimed at sustainable hydrogen production via electrochemical water splitting., Comment: 36 pages, 9 figures

In this work, we have systematically investigated the structural, electronic, vibrational and optical properties of the edge-functionalized hg-C3N4 quantum dots with the aim of exploring their possible applications in solar cells and other optoelectronic devices such as light-emitting diodes. The functional groups considered in this work are methyl (-CH$_3$), fluorine (-F), and oxygenated groups such as aldehyde (-CHO), carboxyl (-COOH), ketone (-COCH$_3$), and hydroxyl (-OH) groups. The edge-functionalization resulted in significant tuning of the electronic, vibrational, and optical properties. Thus, their structural fingerprints are present in both their vibrational and optical properties, thereby allowing their detection both in the Raman as well as optical spectroscopies. It is observed that edge functionalization broadens the energy range of optical absorption, leading to coverage of most of the ultraviolet and visible regions. This implies that the edge-functionalization of hg-C$_3$N$_4$ quantum dots can be used in a variety of optoelectronic devices such as solar cells and light emitting diodes., Comment: 46 pages, 17 figures (manuscript 30 pages, 5 figures + supporting information 16 pages, 12 figures), Journal of Material Chemistry C (in press)

In this paper we present a thorough first-principles based density functional theory study of the structural stability, electronic, magnetic, and optical properties of pristine and doped gallium phosphide (GaP) monolayers. The pristine GaP monolayer is found to have a periodically buckled structure, with an indirect band gap of 2.15 eV. The doping by X (B, Al, In, C, Si, Ge, Sn, Zn, Cd) at the Ga site, and Y (N, As, Sb, O, S, Se, Te, Zn, Cd) at the P site is considered, and an indirect to direct transition is observed after doping by In at the Ga site. For several cases, significant changes in the band gap are seen after doping, while system becomes metallic when O is substituted at the P site. The spin-polarized band structures are calculated for the monolayers with doping-induced magnetism, and we find that for some cases a direct band gap appears for one of the spin orientations. For such cases, we investigate the intriguing possibility of spin-dependent optical properties. Furthermore for several cases, the band gap is very small for one of the spin orientations, suggesting the possibility of engineering half metallicity by doping. For the layers with direct band gaps, the calculated optical absorption spectra are found to span a wide energy range in the visible and ultraviolet regions. The calculated formation energies of both the pristine and doped structures are quite small, indicating that the laboratory realization of such structures is quite feasible. On the whole, our results suggest that the doped GaP monolayer is a material with potentially a wide range of applications in nanoelectronics, spintronics, optoelectronics, solar cells, etc., Comment: 39 pages, 17 figures