Photochromic molecular motors hold promise for a multitude of potential applications in fields ranging from medicine to communications and structural repair. Yet, it is still a challenge to predict their mechanical efficiency. Here, azobenzene is explored as a representative light-driven nanomotor and estimate its quantum yield of photoisomerization and maximum mechanical efficiency. This is based on first-principles mapping of the 3D potential energy surfaces for the ground and excited states of the trans and cis configurations and identifying the minimum energy pathway for isomerization. A work cycle is devised and identifies force constant as the parameter that resembles temperature in the Carnot heat engine, but with very different efficiencies. The results show that the optomechanical efficiency of azobenzene at constant load is about 5% albeit under ideal conditions. To test the hypothesis, the study also explores the optomechanical efficiency of stilbene and 2-butene and shows that their efficiency does not exceed 5%.

Controllable and reliable doping of cubic boron nitride (cBN) is a critical challenge in its widespread application in power electronics. Recently, progress was made in this regard when an experiment reported successful n-doping of cBN with carbon, reaching carbon concentrations as high as 5%, which is beyond the thermodynamic solubility limit. However, the nature of carbon based defects introduced within the cBN matrix remains unknown thus far. Here, we explore the electronic structure of carbon doped cBN under nitrogen-rich conditions, which are conducive to the formation of donor-type defects (such as carbon substituents replacing boron), and predict several possible arrangements of carbon in clusters at experimentally relevant concentrations of 1.5-7.8%. Our theoretical calculations show that carbon dopants prefer to aggregate into small clusters with local ordering rather than distributing randomly in the bulk. Along with defects that result in easily ionizable defect states, we also report a distribution of carbon dopants, where the structure exhibits a defect-bound small electron polaron. The polaron can be identified via a split-off localized dopant band near the valence band edge. These findings not only shed light on identities of possible carbon-based defects but also predict additional carriers-defect-bound small polarons.

The equilibrium shape of crystals is a fundamental property of both aesthetic appeal and practical importance: the shape and its facets control the catalytic, light-emitting, sensing, magnetic and plasmonic behaviors. It is also a visible macro-manifestation of the underlying atomic-scale forces and chemical makeup, most conspicuous in two-dimensional (2D) materials of keen current interest. If the crystal surface/edge energy is known for different directions, its shape can be obtained by the geometric Wulff construction, a tenet of crystal physics; however, if symmetry is lacking, the crystal edge energy cannot be defined or calculated and thus its shape becomes elusive, presenting an insurmountable problem for theory. Here we show how one can proceed with auxiliary edge energies towards a constructive prediction, through well-planned computations, of a unique crystal shape. We demonstrate it for challenging materials such as SnSe, which is of C 2v symmetry, and even AgNO 2 of C 1 , which has no symmetry at all.

While heterostructures are ubiquitous tools enabling new physics and device functionalities, the palette of available materials has never been richer. Combinations of two emerging material classes, two-dimensional materials and topological materials, are particularly promising because of the wide range of possible permutations that are easily accessible. Individually, both graphene and Pb1-xSnxTe (PST) are widely investigated for spintronic applications because graphene's high carrier mobility and PST's topologically protected surface states are attractive platforms for spin transport. Here, we combine monolayer graphene with PST and demonstrate a hybrid system with properties enhanced relative to the constituent parts. Using magnetotransport measurements, we find carrier mobilities up to 20 000 cm2/(V s) and a magnetoresistance approaching 100%, greater than either material prior to stacking. We also establish that there are two distinct transport channels and determine a lower bound on the spin relaxation time of 4.5 ps. The results can be explained using the polar catastrophe model, whereby a high mobility interface state results from a reconfiguration of charge due to a polar/nonpolar interface interaction. Our results suggest that proximity induced interface states with hybrid properties can be added to the still growing list of behaviors in these materials.

Surface-states of topological insulators are assumed to be robust against non-magnetic defects in the crystal. However, recent theoretical models and experiments indicate that even non-magnetic defects can perturb these states. Our first-principles calculations demonstrate that the presence of Se vacancies in Bi$_2$Se$_3$, has a greater impact than a mere n-doping of the structure, which would just shift the Fermi level relative to the Dirac point. We observe the emergence of a non-linear band pinned near the Fermi level, while the Dirac cone shifts deeper into the valence band. We attribute these features in the bandstructure to the interaction between the surface and defect states, with the resulting hybridization between these states itself depending on the position and symmetry of the Se vacancy relative to the surfaces. Our results bring us a step closer to understanding the exotic physics emerging from defects in Bi$_2$Se$_3$ that remained unexplored in prior studies.

Two-dimensional transition metal dichalcogenides (TMDs) can adopt one of several possible structures, with the most common being the trigonal prismatic and octahedral symmetry phases. Since the structure determines the electronic properties, being able to predict phase-preferences of TMDs from just the knowledge of the constituent atoms is highly desired, but has remained a long-standing problem. In this study, we applied high-throughput quantum mechanical computations with machine learning algorithms to solve this old problem. Our analysis provides insights into determining physiochemical factors that dictate the phase-preference of a TMD, identifying and going beyond the attributes considered by earlier researchers in predicting crystal structures. A knowledge of these underlying physiochemical factors not only helps us to rationalize, but also to accurately predict structural preferences. We show that machine learning algorithms are powerful tools that can be used not only to find new materials with targeted properties, but also to find connections between elemental attributes and the target property/properties that were not previously obvious.

Selenium (Se) vacancies are the most abundant and unavoidable n-type defects in the topological insulator, bismuth selenide (Bi2Se3). A recent study has shown that the surface Se vacancies not only n-dope the system but also result in the splitting of the Dirac cone associated with the surface and the emergence of a nonlinear state pinned at the Fermi level due to the interactions between surface-, defect-, and quantum-well states. In this combined theoretical and experimental work, we show how the defective surfaces of Bi2Se3 slabs can be healed by adsorption of different gases. Depending on the adsorbates, we find that the band structure of Bi2Se3 either reverts back to its pristine form or exhibits localized adsorbate bands near the Fermi level. Notably, our density functional theory calculations show that both atomic and molecular oxygen are isoelectronic to Se, binding strongly to the vacancy position. Along with counterdoping (p-doping) of Bi2Se3 (as reported by earlier studies), oxygen adsorption completely restores the Dirac structure of the surface states. Our experiments confirm that annealing intrinsically n-doped Bi2Se3 samples with oxygen reduces the carrier density by approximate to 6%. This is a reversible process, with the Bi2Se3 slab reverting back to the original carrier concentration on vacuum annealing, thus confirming the healing of vacancies by oxygen. We distinguish the possible features of the adsorbates that can be used to a priori predict their effects on the electronic structure of the Bi2Se3 slab after adsorption. Our results provide a foundation for a general strategy for the in situ engineering of the band structure of theBi(2)Se(3) family of topological insulators by quenching Se vacancies.

Studies on intercalation or substitution of atoms into layered two-dimensional (2D) materials are rapidly expanding and gaining significant consideration due to their importance in electronics, catalysts, batteries, sensors, etc. In this manuscript, we report a straightforward method to create sulphur (S) deficient molybdenum (Mo) sulfide (MoS2-x) structures and substitute them with zerovalent copper (Cu) atoms using a colloidal synthesis method. The synthesized materials were studied using several techniques to understand the proportion and position of copper atoms and the effect of copper functionalization. Specifically, the impact of change in the ratio of Cu : S and the hydrogen evolution reaction (HER) activity of the derived materials were evaluated. This technique paves the way for the synthesis of various functionalized 2D materials with a significant impact on their physical and chemical behavior making them potential candidates for catalysis and several other applications such as energy storage and the development of numerous functional devices.

Green hydrogen production is a vital requirement of the upcoming hydrogen fuel-based locomotion and economy. Water electrolysis facilitated by electricity derived from renewable sources and direct solar-to-hydrogen conversion centred on photochemical and photoelectrochemical water splitting is a promising pathway for sustainable hydrogen production. All these methods require a highly active noble metal catalyst to make the water-splitting process more energy-efficient and in order to make it economical, metal-free hydrogen evolution catalysts such as graphene nanoplatelets (GNPs) are essential. Herein, we report the effect of a range of functionalizations on the catalytic properties of graphene nanoplatelets (GNPs) for the hydrogen evolution reaction (HER). We also account for the effect of functionalization on the strength of the electrical double layer formation on the surface of functionalized GNPs. It is observed that the catalytic activity and the electrical double layer strength are inversely related to each other. Our first-principles-based density functional theoretical (DFT) modelling unravels the origin of the observed electrocatalytic activity and its trend and the strength of the electrical double layers in terms of free energy changes during the ion absorption/desorption events on the electrode surface. Based on our observations, minimizing the electrical double layer strength is identified as an approach to improve the catalytic performance of the catalysts.

The discovery of ferromagnetism in atomically thin layers at room temperature widens the prospects of 2D materials for device applications. Recently, two independent experiments demonstrated magnetic ordering in two dissimilar 2D systems, CrI3 and Cr2 Ge2 Te6 , at low temperatures and in VSe2 at room temperature, but observation of intrinsic room-temperature magnetism in 2D materials is still a challenge. Here a transition at room temperature that increases the magnetization in magnetite while thinning down the bulk material to a few atom-thick sheets is reported. DC magnetization measurements prove ferrimagnetic ordering with increased magnetization and density functional theory calculations ascribe their origin to the low dimensionality of the magnetite layers. In addition, surface energy calculations for different cleavage planes in passivated magnetite crystal agree with the experimental observations of obtaining 2D sheets from non-van der Waals crystals.

As CO2 emissions are sharply increasing, processes for converting CO2 into value-added products are becoming more desirable. Ruthenium-based catalysts are the most active for CO2 methanation; howev...

The equilibrium shape of crystals is a fundamental property of both aesthetic appeal and practical import. It is also a visible macro-manifestation of the underlying atomic-scale forces and chemical makeup, most conspicuous in two-dimensional (2D) materials of keen current interest. If the crystal surface/edge energy is known for different directions, its shape can be obtained by geometric Wulff construction, a tenet of crystal physics. However, if symmetry is lacking, the crystal edge energy cannot be defined or calculated, so its shape becomes elusive, presenting an insurmountable problem for theory. Here we show how, in fact, one can proceed with "latent edge energies" towards constructive prediction of a unique crystal shape, and demonstrate it for challenging material-examples, like ${\rm SnS}$ of ${\rm C_{2v}}$ symmetry and even ${\rm AgNO_2}$ of ${\rm C_1}$-no symmetry at all.

Recent experiments have revealed ripplocations, atomic-scale ripplelike defects on samples of MoS2 flakes. We use quantum mechanical calculations based on density functional theory to study the effect of ripplocations on the structural and electronic properties of single-layer MoS2, and, in particular, the coupling between these extended defects and the most common defects in this material, S-vacancies. We find that the formation of neutral S-vacancies is energetically more favorable in the ripplocation. In addition, we demonstrate that ripplocations alone do not introduce electronic states into the intrinsic bandgap, in contrast to S-vacancies. We study the dependence of the induced gap states on the position of the defects in the ripplocation, which has implications for the experimental characterization of MoS2 flakes and the engineering of quantum emitters in this material. Our specific findings collectively aim to provide insights into the electronic structure of experimentally relevant defects in MoS2 and to establish structure-property relationships for the design of MoS2-based quantum devices.

The physical and chemical properties of polymorphs of iron oxides are utilized for electronic, energy, and biomedical applications. To design a functional material with arresting interplay at the interfaces and boundaries between polymorphs of iron oxide (Fe 3 O 4 – magnetite with Fe 2 O 3 – hematite), two different approaches of synthesis are adopted, namely, mechanical mixing and in situ growth. Unlike mechanically mixed composites, the in situ‐synthesized composites show the development of a highly distinct non‐stoichiometric, Fe 21.34 O 32 phase at the boundary. The atomically diffused composition at boundary is found to govern the fourfold increase in conductivity. By varying the ratio of constituent iron oxide polymorphs, the dielectric constant can be tuned and is found to be highly frequency dependent with minimum loss in tan δ plot. The inherent ferromagnetism of Fe 3 O 4 reveals to be retained in composite samples.

Optical spectroscopy (OS) techniques are often coupled with first-principles density functional theoretical (DFT) calculations for determining the precise influence of defects on the electronic and structural properties of two-dimensional (2D) transition metal dichalcogenides. Such calculations are carried out presuming there is little or no effect of vibrational transitions on the observed electronic spectrum. However, if the effect of change in vibrational energy (Franck Condon (FC) shift) associated with such a transition is large, it could possibly lead to a different origin for the observed peak. One such instance is the attribution of the 0.75 eV cathodoluminescence peak by Fabbri et al (2016 Nat. Commun. 7 13044) to an optical transition from an S vacancy level in the band gap, under the assumption that the FC shift is negligible. Here, by first principles constrained DFT calculations using hybrid HSE06 functional we show that this combined prediction of OS and DFT calculations is valid for 2D MoS2 since the FC shift associated with electronic transitions from a sulfur vacancy is indeed small ~28 meV. Based on our calculations we conclude that it is reasonable to make a direct connection between DFT calculations and optical spectroscopy techniques in this material, hence, establishing a one to one relation between defect related emission bands and electronic transitions from the defect levels.

In dieser Zuschrift wurde die minimale Dicke der hoch dotierbaren Zone fälschlich auf d = 4 gesetzt.Statt dessen ist d = 3 die korrekte Dicke:D ieser Wert ist in Rechnungen konvergent, deutlich grçßer als der Kovalenzdurchmesser von Bor (1.74 ), und Rechnungen mit d = 3 vermeiden eine zufällige Besetzung der Vakuumzustände auch bei hohen Dotierungen von q = 0.5-1 e/Atom.Für q !0.5 e/ Atom wird die v 1/3 -Wabenschicht zum Grundzustand (Abbildung 1).

The trigonal form of selenium (t-Se) has an unusual, quasi-one-dimensional, chiral crystal structure. First-principles calculations have helped us uncover a polar, optic phonon in t-Se that exhibits a diagonal magnetoelectric coupling with the electric field of the incident THz radiation, and induces a parallel magnetic field due to its inherent chirality. We show that this phonon-mediated, magnetoelectric mechanism is predicted to cause optical rotation in t-Se in the 1-3 THz range and is quite distinct from the high frequency (>80 THz) activity due to electronic excitations that has been reported previously. In the second part of the paper, we report our experimental results based on THz time-domain spectroscopy as well as direct measurements of optical rotation that confirm this prediction in an aligned, monocrystalline array of t-Se microrods. The Se microrod array not only exhibits a large birefringence (Delta n = 1.3) but also rotates the polarization of THz radiation by similar to 3 degrees/mm To our knowledge, this is the only elemental solid known to rotate THz polarization, because the currently available THz rotators are based mainly on liquid crystals or metamaterials. The identification of new THz-active materials and a better understanding of the underlying physics are both clearly essential to the development of better sources, detectors and components.

DOI: 10.1002/anie.201705459 In this Communication, the minimum thickness of the jellium slab (to allow for high doping) was set at d = 4 b, which is incorrect. In fact, d = 3 b is the correct thickness limit: it is computationally convergent, is reasonably larger than the covalent diameter of boron (1.74 b), and the calculations with d = 3 b still avoid spurious occupancy of the vacuum states even at doping as high as q = 0.5–1 e/atom. Importantly, for q +0.5 e/ atom, the v1/3 honeycomb sheet emerges as the ground state (Figure 1). Furthermore, at q = 1 e/atom the honeycomb borophene becomes isoelectronic to graphene and Dirac cones appear at the Fermi level (Figure 1c, inset). This honeycomb structure is the same as that of the boron lattice responsible for superconductivity in MgB2.

The absence of inherent magnetism in the family of 2D materials limits its application in magnetoelectric and magnetic storage media. Here, a simple scalable route for the synthesis of magnetic 2D material chromite (chromiteen) via sonication-assisted liquid-phase exfoliation is demonstrated. The (111) plane of the exfoliated chromite is found to be the most stable which is confirmed by its common occurrence in exfoliation. Further, the stability and dispersion are verified by ab initio density functional theoretical simulations. Magnetic measurements over a large temperature range of 4KT300K confirm ferromagnetic/superparamagnetic order with nearly 40 times higher magnetic moment saturation in chromiteen compared to chromite. The results reveal that 2D chromiteen causes a change in the magnetic behavior with respect to chromite which could be ascribed to the increase in the lattice strain as well as a magnetic strain due to high ferro-magnetic fraction in 2D plane.

Despite being only a few atoms thick, single-layer two-dimensional (2D) materials display strong electron-photon interactions that could be utilized in efficient light modulators on extreme subwavelength scales. In various applications involving light modulation and manipulation, materials with strong optical response at different wavelengths are required. Using qualitative analytical modeling and first-principles calculations, we determine the theoretical limit of the maximum optical response such as absorbance ( A) and reflectance ( R) in 2D materials and also conduct a computational survey to seek out those with best A and R in various frequency ranges, from mid-infrared to deep-ultraviolet. We find that 2D boron has broadband reflectance R > 99% for >100 layers, surpassing conventional thin films of bulk metals such as silver. Moreover, we identify 2D monolayer semiconductors with maximum response, for which we obtain quantitative estimates by calculating quasiparticle energies and accounting for excitonic effects by solving the Bethe-Salpeter equation. We found several monolayer semiconductors with absorbances ≳30% in different optical ranges, which are more than half of the maximum possible value, Alim = 1/2, for a freestanding 2D material. Our study predicts 2D materials which can potentially be used in ultrathin reflectors and absorbers for optoelectronic application in various frequency ranges.