Guided Heterostructure Growth of CoFe LDH on Ti3C2Tx MXene for Durably High Oxygen Evolution Activity.

Heterostructures of layered double hydroxides (LDHs) and MXenes have shown great promise for oxygen evolution reaction (OER) catalysts, owing to their complementary physical properties. Coupling LDHs with MXenes can potentially enhance their conductivity, stability, and OER activity. In this work, a scalable and straightforward in situ guided growth of CoFeLDH on Ti3C2Tx is introduced, where the surface chemistry of Ti3C2Tx dominates the resulting heterostructures, allowing tunable crystal domain sizes of LDHs. Combined simulation results of Monte Carlo and density functional theory (DFT) validate this guided growth mechanism. Through this way, the optimized heterostructures allow the highest OER activity of the overpotential = 301 mV and Tafel slope = 43 mV dec-1 at 10 mA cm-2, and a considerably durable stability of 0.1% decay over 200 h use, remarkably outperforming all reported LDHs-MXenes materials. DFT calculations indicate that the charge transfer in heterostructures can decrease the rate-limiting energy barrier for OER, facilitating OER activity. The combined experimental and theoretical efforts identify the participation role of MXene in heterostructures for OER reactions, providing insights into designing advanced heterostructures for robust OER electrocatalysis.

Raman Spectra of Amino Acids and Peptides from Machine Learning Polarizabilities.

Raman spectroscopy is an important tool in the study of vibrational properties and composition of molecules, peptides, and even proteins. Raman spectra can be simulated based on the change of the electronic polarizability with vibrations, which can nowadays be efficiently obtained via machine learning models trained on first-principles data. However, the transferability of the models trained on small molecules to larger structures is unclear, and direct training on large structures is prohibitively expensive. In this work, we first train two machine learning models to predict the polarizabilities of all 20 amino acids. Both models are carefully benchmarked and compared to density functional theory (DFT) calculations, with the neural network method being found to offer better transferability. By combination of machine learning models with classical force field molecular dynamics, Raman spectra of all amino acids are also obtained and investigated, showing good agreement with experiments. The models are further extended to small peptides. We find that adding structures containing peptide bonds to the training set greatly improves predictions, even for peptides not included in training sets.

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.

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.

High-throughput computation of Raman spectra from first principles.

Raman spectroscopy is a widely-used non-destructive material characterization method, which provides information about the vibrational modes of the material and therefore of its atomic structure and chemical composition. Interpretation of the spectra requires comparison to known references and to this end, experimental databases of spectra have been collected. Reference Raman spectra could also be simulated using atomistic first-principles methods but these are computationally demanding and thus the existing databases of computational Raman spectra are fairly small. In this work, we developed an optimized workflow to calculate the Raman spectra efficiently and taking full advantage of the phonon properties found in existing material databases. The workflow was benchmarked and validated by comparison to experiments and previous computational methods for select technologically relevant material systems. Using the workflow, we performed high-throughput calculations for a large set of materials (5099) belonging to many different material classes, and collected the results to a database. Finally, the contents of database are analyzed and the calculated spectra are shown to agree well with the experimental ones.

Highly Selective H2S Gas Sensor Based on Ti3C2Tx MXene-Organic Composites.

Cost-effective and high-performance H2S sensors are required for human health and environmental monitoring. 2D transition-metal carbides and nitrides (MXenes) are appealing candidates for gas sensing due to good conductivity and abundant surface functional groups but have been studied primarily for detecting NH3 and VOCs, with generally positive responses that are not highly selective to the target gases. Here, we report on a negative response of pristine Ti3C2Tx thin films for H2S gas sensing (in contrast to the other tested gases) and further optimization of the sensor performance using a composite of Ti3C2Tx flakes and conjugated polymers (poly[3,6-diamino-10-methylacridinium chloride-co-3,6-diaminoacridine-squaraine], PDS-Cl) with polar charged nitrogen. The composite, preserving the high selectivity of pristine Ti3C2Tx, exhibits an H2S sensing response of 2% at 5 ppm (a thirtyfold sensing enhancement) and a low limit of detection of 500 ppb. In addition, our density functional theory calculations indicate that the mixture of MXene surface functional groups needs to be taken into account to describe the sensing mechanism and the selectivity of the sensor in agreement with the experimental results. Thus, this report extends the application range of MXene-based composites to H2S sensors and deepens the understanding of their gas sensing mechanisms.

Stacking Fault Induced Symmetry Breaking in van der Waals Nanowires.

While traditional ferroelectrics are based on polar crystals in bulk or thin film form, two-dimensional and layered materials can support mechanisms for symmetry breaking between centrosymmetric building blocks, e.g., by creating low-symmetry interfaces in van der Waals stacks. Here, we introduce an approach toward symmetry breaking in van der Waals crystals that relies on the spontaneous incorporation of stacking faults in a nonpolar bulk layer sequence. The concept is realized in nanowires consisting of Se-rich group IV monochalcogenide (GeSe1-xSx) alloys, obtained by vapor-liquid-solid growth. The single crystalline wires adopt a layered structure in which the nonpolar A-B bulk stacking along the nanowire axis is interrupted by single-layer stacking faults with local A-A' stacking. Density functional theory explains this behavior by a reduced stacking fault formation energy in GeSe (or Se-rich GeSe1-xSx alloys). Computations demonstrate that, similar to monochalcogenide monolayers, the inserted A-layers should show a spontaneous electric polarization with a switching barrier consistent with a Curie temperature above room temperature. Second-harmonic generation signals are consistent with a variable density of stacking faults along the wires. Our results point to possible routes for designing ferroelectrics via the layer stacking in van der Waals crystals.

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.

C60Br24/SWCNT: A Highly Sensitive Medium to Detect H2S via Inhomogeneous Carrier Doping.

H2S is a toxic and corrosive gas, whose accurate detection at sub-ppm concentrations is of high practical importance in environmental, industrial, and health safety applications. Herein, we propose a chemiresistive sensor device that applies a composite of single-walled carbon nanotubes (SWCNTs) and brominated fullerene (C60Br24) as a sensing component, which is capable of detecting 50 ppb H2S even at room temperature with an excellent response of 1.75% in a selective manner. In contrast, a poor gas response of pristine C60-based composites was found in control measurements. The experimental results are complemented by density functional theory calculations showing that C60Br24 in contact with SWCNTs induces localized hole doping in the nanotubes, which is increased further when H2S adsorbs on C60Br24 but decreases in the regions, where direct adsorption of H2S on the nanotubes takes place due to electron doping from the analyte. Accordingly, the heterogeneous chemical environment in the composite results in spatial fluctuations of hole density upon gas adsorption, hence influencing carrier transport and thus giving rise to chemiresistive sensing.

Synthesis and Properties of Monolayer MnSe with Unusual Atomic Structure and Antiferromagnetic Ordering.

Transition metal chalcogenides (TMCs) are a large family of 2D materials that are currently attracting intense interest. TMCs with 3d transition metals provide opportunities for introducing magnetism and strong correlations into the material with manganese standing out as a particularly attractive option due to its large magnetic moment. Here we report on the successful synthesis of monolayer manganese selenide on a NbSe2 substrate. Using scanning tunneling microscopy and spectroscopy experiments and global structure prediction calculations at the density functional theory level, we identify the atomic structure and magnetic and electronic properties of the layered Mn2Se2 phase. The structure is similar to the layered bulk phase of CuI or a buckled bilayer of h-BN. Interestingly, our results suggest that the monolayer is antiferromagnetic, but with an unusual out-of-plane ordering that results in two ferromagnetic planes.

Surface Functionalization of 2D MXenes: Trends in Distribution, Composition, and Electronic Properties.

Using a multiscale computational scheme, we study the trends in distribution and composition of the surface functional groups -O, -OH, and -F on two-dimensional (2D) transition metal carbides and nitrides (MXenes). We consider Ti2N, Ti4N3, Nb2C, Nb4C3, Ti2C, and Ti3C2 to explore MXenes with different chemistry and different number of atomic layers. Using a combination of cluster expansion, Monte Carlo, and density functional theory methods, we study the distribution and composition of functional groups at experimentally relevant conditions. We show that mixtures of functional groups are favorable on all studied MXene surfaces. The distribution of functional groups appears to be largely independent of the type of metal, carbon, or nitrogen species and/or number of atomic layers in the MXene. We further show that some properties (e.g., the work function) strongly depend on the surface composition, while others, for example, the electric conductivity, exhibit only a weak dependence.

Formation of Highly Doped Nanostripes in 2D Transition Metal Dichalcogenides via a Dislocation Climb Mechanism.

Doping of materials beyond the dopant solubility limit remains a challenge, especially when spatially nonuniform doping is required. In 2D materials with a high surface-to-volume ratio, such as transition metal dichalcogenides, various post-synthesis approaches to doping have been demonstrated, but full control over spatial distribution of dopants remains a challenge. A post-growth doping of single layers of WSe2 is performed by adding transition metal (TM) atoms in a two-step process, which includes annealing followed by deposition of dopants together with Se or S. The Ti, V, Cr, and Fe impurities at W sites are identified by using transmission electron microscopy and electron energy loss spectroscopy. Remarkably, an extremely high density (6.4-15%) of various types of impurity atoms is achieved. The dopants are revealed to be largely confined within nanostripes embedded in the otherwise pristine WSe2 . Density functional theory calculations show that the dislocations assist the incorporation of the dopant during their climb and give rise to stripes of TM dopant atoms. This work demonstrates a possible spatially controllable doping strategy to achieve the desired local electronic, magnetic, and optical properties in 2D materials.

Structural Phase Transitions to 2D and 3D Oxygen Vacancy Patterns in a Perovskite Film Induced by Electrical and Mechanical Nanoprobing.

Oxygen vacancy migration and ordering in perovskite oxides enable manipulation of material properties through changes in the cation oxidation state and the crystal lattice. In thin-films, oxygen vacancies conventionally order into equally spaced planes. Here, it is shown that the planar 2D symmetry is broken if a mechanical nanoprobe restricts the chemical lattice expansion that the vacancies generate. Using in situ scanning transmission electron microscopy, a transition from a perovskite structure to a 3D vacancy-ordered phase in an epitaxial La2/3 Sr1/3 MnO3- δ film during voltage pulsing under local mechanical straining is imaged. The never-before-seen ordering pattern consists of a complex network of distorted oxygen tetrahedra, pentahedra, and octahedra that, together, produce a corrugated atomic structure with lattice constants varying between 3.5 and 4.6 Å. The giant lattice distortions respond sensitively to strain variations, offering prospects for non-volatile nanoscale physical property control driven by voltage and gated by strain.

First principles study of the stability of MXenes under an electron beam.

Interactions between two-dimensional MXene sheets and electron beams of a (scanning) transmission electron microscope are studied by first-principles calculations. We simulated the knock-on sputtering threshold for Ti3C2 MXene sheets via ab initio molecular dynamics simulations and for five other MXenes (Ti2C, Ti2N, Nb2C, Mo2TiC2, and Ti3CN) approximately from defect formation energies. We evaluated the sputtering cross section and sputtering rates and based on those evaluated the surface composition. We find that at the exit surface and for "low" TEM energies H and F sputter at equal rates, but at "high" TEM energies the F is sputtered most strongly. In the entry surface, H sputtering dominates. The results were found to be largely similar for all studied MXenes, and although the sputtering thresholds varied between the different metal atoms the thresholds were always too high to lead to significant sputtering of the metal atoms. We simulated electron microscope images at the successive stages of sputtering and found that while it is likely difficult to identify surface groups based on the spot intensities, the local contraction of the lattice around O groups should be observable. We also studied MXenes encapsulated with graphene and found them to provide efficient protection from knock-on damage for all surface group atoms except H.

Band Bending and Valence Band Quantization at Line Defects in MoS2.

The variation of the electronic structure normal to 1D defects in quasi-freestanding MoS2, grown by molecular beam epitaxy, is investigated through high resolution scanning tunneling spectroscopy at 5 K. Strong upward bending of valence and conduction bands toward the line defects is found for the 4|4E mirror twin boundary and island edges but not for the 4|4P mirror twin boundary. Quantized energy levels in the valence band are observed wherever upward band bending takes place. Focusing on the common 4|4E mirror twin boundary, density functional theory calculations give an estimate of its charging, which agrees well with electrostatic modeling. We show that the line charge can also be assessed from the filling of the boundary-localized electronic band, whereby we provide a measurement of the theoretically predicted quantized polarization charge at MoS2 mirror twin boundaries. These calculations elucidate the origin of band bending and charging at these 1D defects in MoS2. The 4|4E mirror twin boundary not only impairs charge transport of electrons and holes due to band bending, but holes are additionally subject to a potential barrier, which is inferred from the independence of the quantized energy landscape on either side of the boundary.

Self-organized twist-heterostructures via aligned van der Waals epitaxy and solid-state transformations.

Vertical van der Waals (vdW) heterostructures of 2D crystals with defined interlayer twist are of interest for band-structure engineering via twist moiré superlattice potentials. To date, twist-heterostructures have been realized by micromechanical stacking. Direct synthesis is hindered by the tendency toward equilibrium stacking without interlayer twist. Here, we demonstrate that growing a 2D crystal with fixed azimuthal alignment to the substrate followed by transformation of this intermediate enables a potentially scalable synthesis of twisted heterostructures. Microscopy during growth of ultrathin orthorhombic SnS on trigonal SnS2 shows that vdW epitaxy yields azimuthal order even for non-isotypic 2D crystals. Excess sulfur drives a spontaneous transformation of the few-layer SnS to SnS2, whose orientation - rotated 30° against the underlying SnS2 crystal - is defined by the SnS intermediate rather than the substrate. Preferential nucleation of additional SnS on such twisted domains repeats the process, promising the realization of complex twisted stacks by bottom-up synthesis.

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.

Layer Rotation-Angle-Dependent Excitonic Absorption in van der Waals Heterostructures Revealed by Electron Energy Loss Spectroscopy.

Heterostructures comprising van der Waals (vdW) stacked transition metal dichalcogenide (TMDC) monolayers are a fascinating class of two-dimensional (2D) materials. The presence of interlayer excitons, where the electron and the hole remain spatially separated in the two layers due to ultrafast charge transfer, is an intriguing feature of these heterostructures. The optoelectronic functionality of 2D heterostructure devices is critically dependent on the relative rotation angle of the layers. However, the role of the relative rotation angle of the constituent layers on intralayer absorption is not clear yet. Here, we investigate MoS2/WSe2 vdW heterostructures using monochromated low-loss electron energy loss (EEL) spectroscopy combined with aberration-corrected scanning transmission electron microscopy and report that momentum conservation is a critical factor in the intralayer absorption of TMDC vdW heterostructures. The evolution of the intralayer excitonic low-loss EEL spectroscopy peak broadenings as a function of the rotation angle reveals that the interlayer charge transfer rate can be about an order of magnitude faster in the aligned (or anti-aligned) case than in the misaligned cases. These results provide a deeper insight into the role of momentum conservation, one of the fundamental principles governing charge transfer dynamics in 2D vdW heterostructures.

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.

Alkali Postdeposition Treatment-Induced Changes of the Chemical and Electronic Structure of Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers: A First-Principle Perspective.

The effects of alkali postdeposition treatment (PDT) on the valence band structure of Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers are addressed from a first-principles perspective. In detail, experimentally derived hard X-ray photoelectron spectroscopy (HAXPES) data [ Handick , E. ; ACS Appl. Mater. Interfaces 2015 , 7 , 27414 - 27420 ] of the valence band structure of alkali-free and NaF/KF-PDT CIGSe are directly compared and fit by calculated density of states (DOS) of CuInSe2, its Cu-deficient counterpart CuIn5Se8, and different potentially formed secondary phases, such as KInSe2, InSe, and In2Se3. The DOSs are based on first-principles electronic structure calculations and weighted according to element-, symmetry-, and energy-dependent photoionization cross sections for the comparison to experimental data. The HAXPES spectra were recorded using photon energies ranging from 2 to 8 keV, allowing extraction of information from different sample depths. The analysis of the alkali-free CIGSe valence band structure reveals that it can best be described by a mixture of the DOS of CuInSe2 and CuIn5Se8, resulting in a stoichiometry slightly more Cu-rich than that of CuIn3Se5. The NaF/KF-PDT-induced changes in the HAXPES spectra for different alkali exposures are best reproduced by additional contributions from KInSe2, with some indications that the formation of a pronounced K-In-Se-type surface species might crucially depend on the amount of K available during PDT.