Native Vacancy Defects in MXenes at Etching Conditions

Two-dimensional MXenes have recently received increased attention due to their facile synthesis process and extraordinary properties suitable for many different applications. During the wet etching synthesis of MXenes, native defects, such as metal and carbon or nitrogen vacancies, are produced, but the underlying defect formation processes are poorly understood. Here, we employ first-principles calculations to evaluate formation energies of Ti, C, and N vacancies in Ti3C2 and Ti2N MXenes under etching conditions. We carefully account for the mixed functionalization of the surfaces as well as the chemical environment in the solution (pH and electrode potential). We observe that the formation energies of the metal vacancies differ significantly for different types of surface functionalization as well as for different local and global environments. We attribute these differences to electrostatic interactions between vacancies and the surrounding functional groups. We predict that Ti vacancies will be prevalent on bare or OH-functionalized surfaces but not on O-functionalized ones. In contrast, C and N vacancies are more prevalent in O-functionalized surfaces. In addition, our results suggest that the pH value of the etching solution and the electrode potential strongly affect vacancy formation. In particular, the predicted conditions at which abundant vacancy formation is expected are compared to experiments and found to coincide with conditions at which MXenes oxidize readily. This suggests that Ti vacancy formation is a crucial step in initiating the oxidation process

Identification of Material Dimensionality Based on Force Constant Analysis

Identification of low-dimensional structural units from the bulk atomic structure is a widely used approach for discovering new low-dimensional materials with new properties and applications. Such analysis is usually based solely on bond-length heuristics, whereas an analysis based on bond strengths would be physically more justified. Here, we study dimensionality classification based on the interatomic force constants of a structure with different approaches for selecting the bonded atoms. The implemented approaches are applied to the existing database of first-principles calculated force constants with a large variety of materials, and the results are analyzed by comparing them to those of several bond-length-based classification methods. Depending on the approach, they can either reproduce results from bond-length-based methods or provide complementary information. As an example of the latter, we managed to identify new non-van der Waals two-dimensional material candidates

pH-Dependent Distribution of Functional Groups on Titanium-Based MXenes

MXenes are a new rapidly developing class of two-dimensional materials with suitable properties for a broad range of applications. It has been shown that during synthesis of these materials the surfaces are usually functionalized by O, OH, and F and further suggested that controlling the surface allows controlling the material properties. However, a proper understanding of the surface structure is still missing, with a significant discrepancy between computational and experimental studies. Experiments consistently show formation of surfaces with mixed terminations, whereas computational studies point toward pure terminated surfaces. Here, we explain the formation of mixed functionalization on the surface of titanium-based two-dimensional carbides, Ti2C and Ti3C2, using a multiscale modeling scheme. Our scheme is based on calculating Gibbs free energy of formation by a combination of electronic structure calculations with cluster expansion and Monte Carlo simulations. Our calculations show formation of mixtures of O, OH, and F on the surface with the composition depending on pH, temperature, and the work function. On the other hand, our results also suggest a limited stable range of compositions, which challenges the paradigm of a high tunability of MXene properties

Lateral Integration of SnS and GeSe van der Waals Semiconductors: Interface Formation, Electronic Structure, and Nanoscale Optoelectronics

The emergence of atomically thin crystals has allowed extending materials integration to lateral heterostructures where different 2D materials are covalently connected in the plane. The concept of lateral heterostructures can be generalized to thicker layered crystals, provided that a suitably faceted seed crystal presents edges to which a compatible second van der Waals material can be attached layer by layer. Here, we examine the possibility of integrating multilayer crystals of the group IV monochalcogenides SnS and GeSe, which have the same crystal structure, small lattice mismatch, and similar bandgaps. In a two-step growth process, lateral epitaxy of GeSe on the sidewalls of multilayer SnS flakes (obtained by vapor transport of a SnS2 precursor on graphite) yields heterostructures of laterally stitched crystalline GeSe and SnS without any detectable vertical overgrowth of the SnS seeds and with sharp lateral interfaces. Combined cathodoluminescence spectroscopy and ab initio calculations show the effects of small band offsets on carrier transport and radiative recombination near the interface. The results demonstrate the possibility of forming atomically connected lateral interfaces across many van der Waals layers, which is promising for manipulating optoelectronics, photonics, and for managing charge- and thermal transport

Which Transition Metal Atoms Can Be Embedded into Two-Dimensional Molybdenum Dichalcogenides and Add Magnetism?

As compared to bulk solids, large surface-to-volume ratio of two-dimensional (2D) materials may open new opportunities for postsynthesis introduction of impurities into these systems by, for example, vapor deposition. However, it does not work for graphene or h-BN, as the dopant atoms prefer clustering on the surface of the material instead of getting integrated into the atomic network. Using extensive first-principles calculations, we show that counterintuitively most transition metal (TM) atoms can be embedded into the atomic network of the pristine molybdenum dichalcogenides (MoDCs) upon atom deposition at moderate temperatures either as interstitials or substitutional impurities, especially in MoTe2, which has the largest spacing between the host atoms. We further demonstrate that many impurity configurations have localized magnetic moments. By analyzing the trends in energetics and values of the magnetic moments across the periodic table, we rationalize the results through the values of TM atomic radii and the number of (s + d) electrons available for bonding and suggest the most promising TMs for inducing magnetism in MoDCs. Our results are in line with the available experimental data and should further guide the experimental effort toward a simple postsynthesis doping of 2D MoDCs and adding new functionalities to these materials

Composition Dependence of the Band Gaps of Semiconducting GeS<i>_x</i>Se_1–<i>x</i> van der Waals Alloys

Alloying of two-dimensional (2D)/layered chalcogenide semiconductors by forming ternaries with properties that span the range between the binary constituents allows tuning of the electronic and optical properties and achieving the full potential of these materials. While the focus so far has been on transition-metal dichalcogenides, alloying in layered group IV chalcogenidespromising for optoelectronics, photovoltaics, ferroelectrics, etc.remains less understood. Here, we investigate alloying in the GeSe–GeS system and its effect on the fundamental band gap. We synthesize single-crystalline layered GeSxSe1–x alloy micro- and nanowires whose compositions are tunable over the entire range of S content, x, via the GeS and GeSe precursor temperatures. Cathodoluminescence in scanning transmission electron microscopy is used to investigate the composition dependence of the band gaps of GeSxSe1–x alloy micro- and nanowires. The band gaps of bulk-like microwires increase systematically with the sulfur content of the alloys, thereby covering the entire range between GeSe (1.27 eV) and GeS (1.6 eV). The composition dependence of the fundamental band gap is close to linear with a bowing coefficient b = 0.173 eV. Density functional theory calculations support the isomorphous behavior of GeSe–GeS solid solutions and demonstrate that the band gaps are indirect and have similar small bowing as determined experimentally. Finally, we establish pronounced size effects in GeSxSe1–x alloy nanowires that provide access to higher-energy optoelectronic transitions than can be realized in bulk alloys of the same composition. Our results support applications of germanium monochalcogenide alloys in areas such as optoelectronics and photovoltaics

Electronic Origin of Enhanced Selectivity through the Halogenation of a Single Mn Atom on Graphitic C₃N₄ for Electrocatalytic Reduction of CO₂ from First-Principles Calculations

The electrochemical reduction of CO2 to valuable products is a critical process that can potentially address energy and environmental challenges. Single-metal atom catalysts have gained significant attention because of their high efficiency and potential to mitigate the challenges associated with traditional-metal nanocatalysts. In particular, non-precious-metal-based catalysts are of great interest because of their low cost and abundance in the Earth’s crust. This work is a comprehensive study to reveal the role of halogen X (X = F, Cl, Br, and I) in improving the CO2 reduction activity and selectivity of single manganese atom-based active sites on a graphitic carbon nitride (g-C3N4) monolayer. Although previous experiments prove that halogenation improves the selectivity of a single Mn-atom-based catalyst, our calculations reveal the reason for the selectivity of the catalyst. The halogen-modified MnN6 active site on g-C3N4 has a high hydrogen evolution reaction (HER) tolerance. Hence, the selectivity due to the increased electronic stability originated from half-filled d orbitals of the Mn atom stabilized on g-C3N4. Also, we present the Gibbs free energy profile, onset potential (UMin), and overpotential (η) for various C1 products (CO, HCOOH, CH3OH, and CH4) at the active sites with and without halogenation. These results suggest that the MnN6 active site of Mn-X-decorated g-C3N4 is a highly efficient and selective electrocatalyst for the CO2RR against the HER. Our study provides directions for the design of a new CO2RR catalyst with improved selectivity and efficiency

1D Germanium Sulfide van der Waals Bicrystals by Vapor–Liquid–Solid Growth

Defects in two-dimensional and layered materials have attracted interest for realizing properties different from those of perfect crystals. Even stronger links between defect formation, fast growth, and emerging functionality can be found in nanostructures of van der Waals crystals, but only a few prevalent morphologies and defect-controlled synthesis processes have been identified. Here, we show that in vapor–liquid–solid growth of 1D van der Waals nanostructures, the catalyst controls the selection of the predominant (fast-growing) morphologies. Growth of layered GeS over Bi catalysts leads to two coexisting nanostructure types: chiral nanowires carrying axial screw dislocations and bicrystal nanoribbons where a central twin plane facilitates rapid growth. While Au catalysts produce exclusively dislocated nanowires, their modification with an additive triggers a switch to twinned bicrystal ribbons. Nanoscale spectroscopy shows that, while supporting fast growth, the twin defects in the distinctive layered bicrystals are electronically benign and free of nonradiative recombination centers