Theoretical characterization of the electronic properties of heterogeneous vertical stacks of 2D metal dichalcogenides containing one doped layer.

The rise of van der Waals hetero-structures based on transition metal dichalcogenides (TMDs) opens the door to a new generation of optoelectronic devices. A key factor controlling the operation and performance of such devices is the relative alignment of the band edges of the components. The electronic properties of the layers can be further modulated by chemical doping, typically leading to the introduction of gap states. However, it is not clear whether the impact of doping in a given layer is preserved when building vertical stacks incorporating it. This has motivated the present study aiming at shedding light by means of first-principles calculations on the electronic properties of heterogeneous bilayers containing one doped layer. Doping has been achieved based on the experimental literature by inserting the dopants by substitution in the 2D layer, by covalently attaching adatoms or functional groups on the surface, or by physisorbing electroactive molecules. Interestingly, very different scenarios can be encountered depending on the two materials present and the nature of doping. The impact of doping is preserved when the trap levels associated with the dopants lie in the bandgap of the bilayer. On the other hand, the pristine neutral layer can get doped to an extent depending on how its electrons can fill the trap levels associated with the other component. Altogether, the present theoretical work demonstrates that the properties of the bilayers are not simply defined by additive rules of the components.

Tuning the Optical and Electrical Properties of Few-Layer Black Phosphorus via Physisorption of Small Solvent Molecules.

Black phosphorus (BP) is recently becoming more and more popular among semiconducting 2D materials for (opto)electronic applications. The controlled physisorption of molecules on the BP surface is a viable approach to modulate its optical and electronic properties. Solvents consisting of small molecules are often used for washing 2D materials or as liquid media for their chemical functionalization with larger molecules, disregarding their ability to change the opto-electronic properties of BP. Herein, it is shown that the opto-electronic properties of mechanically exfoliated few-layer BP are altered when physically interacting with common solvents. Significantly, charge transport analysis in field-effect transistors reveals that physisorbed solvent molecules induce a modulation of the charge carrier density which can be as high as 1012 cm-2 in BP, i.e., comparable to common dopants such as F4 -TCNQ and MoO3 . By combining experimental evidences with density functional theory calculations, it is confirmed that BP doping by solvent molecules not only depends on charge transfer, but is also influenced by molecular dipole. The results clearly demonstrate how an exquisite tuning of the opto-electronic properties of few-layer BP can be achieved through physisorption of small solvent molecules. Such findings are of interest both for fundamental studies and more technological applications in opto-electronics.

Band offset engineering at C2N/MSe2 (M = Mo, W) interfaces.

Stacking layered two-dimensional materials in a type-II band alignment block has provided a high-performance method in photocatalytic water-splitting technology. The key parameters in such heterostructure configurations are the valence and conduction band offsets at the interface, which determine the device performance. Here, based on density functional theory calculations, the bandgap and band offsets at C2N/MSe2 (M = Mo, W) interfaces have been engineered. The main findings demonstrate that the C2N monolayer interacts with both MoSe2 and WSe2 monolayers through weak van der Waals interactions. These heterostructures possess a narrower indirect bandgap and a typical type-II heterostructure feature, being suitable for promoting the separation of photogenerated electron-hole pairs. The calculated Gibbs free energy of hydrogen adsorption demonstrates a reduction in the overpotential, towards the hydrogen evolution reaction, upon forming heterostructures. To further tune the bandgap values and band offsets of heterostructures, the external perturbations are included through a vertical strain and finite electric field. It is found that both the vertical strain and electric field strongly modulate the bandgap values and the magnitude of the band offsets, while the typical type-II band alignment remains preserved. It is noticeable that the band offset magnitudes of the C2N/MoSe2 and C2N/WSe2 heterostructures are more sensitive to an external electric field than to a vertical interlayer strain.

Interlayer Bonding in Two-Dimensional Materials: The Special Case of SnP3 and GeP3.

Stacked two-dimensional (2D) heterostructures are evolving as the "next-generation" optoelectronic materials because of the possibility of designing atomically thin devices with outstanding characteristics. However, most of the existing 2D heterostructures are governed by weak van der Waals interlayer interactions that, as often is the case, exert limited impact on the resulting properties of heterostructures relative to their constituting components. In this work, we investigate the optoelectronic properties of a novel class of 2D MP3 (M = Ge and Sn) materials featuring strong interlayer interactions, applying a robust theoretical framework combining density functional theory and many-body perturbation theory. We demonstrate that the remarkable intrinsic vertical strain (of ∼40% relative to the monolayers) promotes the exfoliation of these materials into bilayers and profoundly impacts their electronic structure, charge transport, and optical properties. Most strikingly, we observe that the strong interlayer hybridization indicates continuous optical absorption across the entire visible range that, together with high charge carrier mobility, makes these 2D MP3 heterostructures attractive for photoconversion applications.

Carbon microspheres derived from walnut shell: Rapid and remarkable uptake of heavy metal ions, molecular computational study and surface modeling.

Carbon microspheres were synthesized under nitrogen flow from walnut shells (WS) and then used as an adsorbent for the removal of Pb (II), Cu(II), Cr(III) and Cd(II) metals. The prepared material was characterized using X-ray photoelectron spectroscopy, scanning electron microscopy, Brounauer-emett-teller surface, Fourier transform infrared and Raman spectroscopy. SEM micrographs showed homogenous sphere-like structure with an average diameter of 4.55 µm. The prepared carbon microspheres exhibit selective and rapid removal of hazardous metals from synthetic water samples. The effects of solution pH, contact time and temperatures on the removal process have been systematically investigated. The material used in this present work present the highest adsorption capacities ever reported for Cr(III), Pb(II), Cd(II) and Cu(II) at an optimum pH of 5, the adsorption capacities reached 792, 638, 574 and 345 mg g-1 for Cr(III), Pb(II), Cd(II) and Cu(II), respectively. Density functional calculations (DFT) showed an agreement with the adsorption process results, Cr(III) had stronger binding ability to the OH and/or COOH functional groups followed by Pb, Cu and Cd. The adsorption mechanism was discussed based on the experimental and theoretical results. Finally, the response surface methodology was used to optimize the adsorption conditions.

Doping of Monolayer Transition-Metal Dichalcogenides via Physisorption of Aromatic Solvent Molecules.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) recently emerged as novel materials displaying a wide variety of physicochemical properties that render them unique scaffolds for high-performance (opto)electronics. The controlled physisorption of molecules on the TMD surface is a viable approach for tuning their optical and electronic properties. Solvents, made of small aromatic molecules, are frequently employed for the cleaning of the 2D materials or as a "dispersant" for their chemical functionalization with larger (macro)molecules, without considering their potential key effect in locally modifying the characteristics of 2D materials. In this work, we demonstrate how the electronic and optical properties of a mechanically exfoliated monolayer of MoS2 and WSe2 are modified when physically interacting with small aromatic molecules of common solvents. Low-temperature photoluminescence (PL) spectra recorded at 78 K revealed that physisorbed benzene derivatives could modulate the charge carrier density in monolayer TMDs, hence affecting the switching between a neutral exciton and trion (charged exciton). By combining experimental evidence with density functional theory calculations, we confirm that charge-transfer doping on TMDs depends not only on the difference in chemical potential between molecules and 2D materials but also on the thermodynamic stability of physisorption. Our results provide unambiguous evidences of the great potential of optical and electrical tuning of monolayer MoS2 and WSe2 by physisorption of small aromatic solvent molecules, which is highly relevant for both fundamental studies and device application purposes.