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Hybrid systems, which take advantage of the low material dimensionality, have great potential for designing nanoscale devices. Quantum dots can be combined with two-dimensional (2D) monolayers to achieve success in photovoltaics and water splitting. In such colloidal systems, ligand molecules play an important role in stabilizing the nanostructures, but their role in heterostructure device performance is still poorly understood. In this study, time-dependent density functional theory is employed to explore how the cysteine ligand affects the charge transfer across the CdS–MoS2heterostructure, at the ultrafast time scale. We show that the cysteine ligand enhances charge transfer, not only by coupling the CdS and MoS2electronic states across the junction but also through enhanced electron–phonon coupling, where the carrier energy is quickly dissipated to high-frequency local vibrational modes arising from the lighter ligand atoms. This enhanced electron–phonon mechanism associated with the ligand is expected to be broadly applicable to most solution-based nanodevices.

The advent of halide perovskites in recent years has opened an avenue for redeveloping perovskite materials as semiconductors. In the quest for semiconducting perovskites, chalcogenides, which exhibit higher stability than their halide siblings and often direct band gaps for optoelectronics, have attracted more and more attention. So far, functional chalcogenide perovskites have been exclusively sulfides. Here, employing first-principles calculations and the criterion of phase stability in addition to the commonly used thermodynamic and dynamical criteria, we precisely predict the existence of LaScSe3as a thermodynamically stable selenide perovskite, which is validated by our experimental synthesis. Combining hybrid functional and many-body quasi-particle (G0W0and Bethe–Salpeter equation) calculations, we predict that LaScSe3is a direct-gap semiconductor having the band gap in the green-to-blue region and capable of p- and n-type bipolar doping, potentially for optoelectronic applications.

Chalcogenide perovskites (CPs) exhibiting lower band gaps than oxide perovskites and higher stability than halide perovskites are promising materials for photovoltaic and optoelectronic applications. For such applications, the absence of deep defect levels serving as recombination centers (dubbed defect tolerance) is a highly desirable property. Here, using density functional theory (DFT) calculations, we study the intrinsic defects in BaZrS3, a representative CP material. We compare Hubbard-Uand hybrid functional methods, both of which have been widely used in addressing the band gap problem of semi-local functionals in DFT. We find that tuning the Uvalue to obtain experimental bulk band gap and then using the obtained Uvalue for defect calculations may result in over-localization of defect states. In the hybrid functional calculation, the band gap of BaZrS3can be accurately obtained. We observe the formation of small S-atom clusters in both methods, which tend to self-passivate the defects from forming mid-gap levels. Even though in the hybrid functional calculations several relatively deep defects are observed, all of them exhibit too high formation energy to play a significant role if the materials are prepared under thermal equilibrium. BaZrS3is thus expected to exhibit sufficient defect tolerance promising for photovoltaic and optoelectronic applications.

The emergence of two-dimensional (2D) materials launched a fascinating frontier of flatland electronics. Most crystalline atomic layer materials are based on layered van der Waals materials with weak interlayer bonding, which naturally leads to thermodynamically stable monolayers. We report the synthesis of a 2D insulator composed of a single atomic sheet of honeycomb structure BeO (h-BeO), although its bulk counterpart has a wurtzite structure. The h-BeO is grown by molecular beam epitaxy (MBE) on Ag(111) thin films that are also epitaxially grown on Si(111) wafers. Using scanning tunneling microscopy and spectroscopy (STM/S), the honeycomb BeO lattice constant is determined to be 2.65 Å with an insulating band gap of 6 eV. Our low-energy electron diffraction measurements indicate that the h-BeO forms a continuous layer with good crystallinity at the millimeter scale. Moiré pattern analysis shows the BeO honeycomb structure maintains long-range phase coherence in atomic registry even across Ag steps. We find that the interaction between the h-BeO layer and the Ag(111) substrate is weak by using STS and complementary density functional theory calculations. We not only demonstrate the feasibility of growing h-BeO monolayers by MBE, but also illustrate that the large-scale growth, weak substrate interactions, and long-range crystallinity make h-BeO an attractive candidate for future technological applications. More significantly, the ability to create a stable single-crystalline atomic sheet without a bulk layered counterpart is an intriguing approach to tailoring 2D electronic materials.

ZnO-based catalysts have been widely used in industrial reactions involving syngas conversions. Hydrogen dynamics on the surface of the catalyst is an essential process for understanding the mechanisms of such reactions. As a polar material, however, the role of ZnO surface polarity on hydrogen dynamics has not been studied under operando conditions. Here, we investigate the behavior of polar (0001) and (0001̅) surfaces of single-crystal ZnO under different H2pressures using near-ambient-pressure X-ray photoelectron spectroscopy. We found that the (0001̅) surface shows a monotonic and irreversible band bending with increasing H2pressure. In contrast, the (0001) surface shows two opposite responses depending on H2pressure, which are reversible in pressure cycles. This polarity- and pressure-dependent hydrogen dynamics is clearly understood with the assistance from first-principles calculations. In particular, the amphoteric behavior of atomic H is identified to play a key role.

Semiconducting monolayers of a 2D material are able to concatenate multiple interesting properties into a single component. Here, by combining opto-mechanical and electronic measurements, we demonstrate the presence of a partial 2H–1T′ phase transition in a suspended 2D monolayer membrane of MoS2. Electronic transport shows unexpected memristive properties in the MoS2membrane, in the absence of any external dopants. A strong mechanical softening of the membrane is measured concurrently and may only be related to the 2H–1T′ phase transition, which imposes a 3% directional elongation of the topological 1T′ phase with respect to the semiconducting 2H. We note that only a few percent 2H–1T′ phase switching is sufficient to observe measurable memristive effects. Our experimental results combined with first-principles total energy calculations indicate that sulfur vacancy diffusion plays a key role in the initial nucleation of the phase transition. Our study clearly shows that nanomechanics represents an ultrasensitive technique to probe the crystal phase transition in 2D materials or thin membranes. Finally, a better control of the microscopic mechanisms responsible for the observed memristive effect in MoS2is important for the implementation of future devices.

As a two-dimensional direct band gap semiconductor with unique electronic and magnetic properties, monolayer chromium triiodide (CrI3) has a great potential for spintronic applications. Under the guidance of the defect theory, we adopt first-principles calculation to determine the structural, thermodynamic, electronic, and magnetic properties of point defects of monolayer CrI3. Twenty kinds of point defects, including vacancies, interstitial, substitution, and bond-rotation defects have been investigated. Either the metalloid or metallic single atom vacancies is energetically stable depends on the chemical potential of iodide, leading to the intrinsic n-type and p-type dopings, respectively. More interestingly, a ferromagnetic to antiferromagnetic transition of the magnetic phase can be induced for most type of point defects, which can be explained by a simple Heisenberg model. The content of present investigation contributes to the atomic-scale understanding of the electric and magnetic properties of monolayer CrI3.

Octahedron rotation (OR) is a unique structural feature in most perovskite materials. During dimensional reduction from bulk to two-dimensional (2D) perovskites, how the OR pattern evolves is still an open question. Here, we employ an adapted Glazer's notation from bulk perovskites to systematically study the structural evolution of 2D perovskites by fully considering the possible OR patterns. Applying this approach to the 2D Ba–Zr–S system, we establish the relationship between the OR pattern and slab thickness. It is found that as the thickness decreases, the OR pattern undergoes a transition by suppressing out-of-plane rotations. The OR in 2D chalcogenide perovskites could result in an anti-confinement effect, i.e., reducing the band gap to even below that of the bulk by countering the quantum confinement effect. In addition, we show that the Ba–Zr–S 2D perovskites exhibit reasonable electron mobility of ∼150 cm2V−1s−1and large exciton binding energy of ∼0.9 eV. Combining the slab thickness and strain as effective knobs for widely tuning the electronic structure, we suggest the 2D chalcogenide perovskites as promising optoelectronic materials.

We show that non-equilibrium dynamics plays a central role in the photoinduced 2H-to-1T′ phase transition of MoTe2. The phase transition is initiated by a local ordering of Te vacancies, followed by a 1T′ structural change in the original 2H lattice. The local 1T′ region serves as a seed to gather more vacancies into ordering and subsequently induces a further growth of the 1T′ phase. Remarkably, this process is controlled by photogenerated excited carriers as they enhance vacancy diffusion, increase the speed of vacancy ordering, and are hence vital to the 1T′ phase transition. This mechanism can be contrasted to the current model requiring a collective sliding of a whole Te atomic layer, which is thermodynamically highly unlikely. By uncovering the key roles of photoexcitations, our results may have important implications for finely controlling phase transitions in transition metal dichalcogenides.

InSe, a newly rediscovered two-dimensional (2D) semiconductor, possesses superior electrical and optical properties as a direct-band-gap semiconductor with high mobility from bulk to atomically thin layers and is drastically different from transition-metal dichalcogenides, in which the direct band gap only exists at the single-layer limit. However, absorption in InSe is mostly dominated by an out-of-plane dipole contribution, which results in the limited absorption of normally incident light that can only excite the in-plane dipole at resonance. To address this challenge, we have explored a unique geometric ridge state of the 2D flake without compromising the sample quality. We observed the enhanced absorption at the ridge over a broad range of excitation frequencies from photocurrent and photoluminescence (PL) measurements. In addition, we have discovered new PL peaks at low temperatures due to defect states on the ridge, which can be as much as ∼60 times stronger than the intrinsic PL peak of InSe. Interestingly, the PL of the defects is highly tunable through an external electrical field, which can be attributed to the Stark effect of the localized defects. InSe ridges thus provide new avenues for manipulating light–matter interactions and defect engineering that are vitally crucial for novel optoelectronic devices based on 2D semiconductors.

Polycrystalline Cu(InGa)Se2(CIGS) thin film is a well-known light absorber material for photovoltaic solar cells. However, the mechanism of the benignity of its grain boundary (GB) remains in dispute. By using a combination of experimental techniques, we have systematically investigated the compositions, structures, and properties of the grain interior (GI) and GB of the polycrystalline CIGS films at nanometer scale. A definitive Cu deficient layer with thickness depending on the overall Cu content of the films was observed near the non Σ3 GBs with distinctly different characteristics from GIs. The band alignment between GB and GI was discovered to be of type II with downward offset for both conduction and valence bands at GB, well correlating to the local copper deficiency and structure. Our findings expressively support the type inversion and large hole barrier in this GB layer, and establish a comprehensive mechanism for the benignity of the CIGS grain boundaries towards carrier recombination.

Electronic applications require the ability to dope a material with a controllable amount of impurities. However, current understanding of the doping mechanism in colloidal–synthesized quantum dots (QDs) is still limited. This is in contrast with bulk semiconductors for which first-principles-based theories have been well established. Using prototype CdSe as an example, here we propose an atomistic theory for the doping of colloidal-synthesized QDs. The key in our theory is the evaluation of atomic chemical potential inside the solution, whose range can deviate considerably from the bulk value due to the presence of solvent. This theory, coupled to first-principles calculations and ab initio molecular dynamics, is able to explain the difference of doping limit in Mn (or Co)-doped CdSe QDs and their bulk counterparts. It also explains the doping behavior of a number of other 3d transition-metal impurities in CdSe QDs in contrast with the solid case.

We report the synthesis and characterization of a novel class of ionic semiconductor materials – inorganic chalcogenide perovskites. Several different compounds including BaZrS3, CaZrS3, SrTiS3and SrZrS3were synthesized by high temperature sulfurization of their oxide counterparts. Their crystal structures were identified by XRD and composition by EDX. UV–vis and photoluminescence measurements confirmed that they are direct gap semiconductors with band gap values consistent with theoretical predictions. By adopting an anion alloying approach, we demonstrate widely tunable band gap from 1.73eV to 2.87eV. These strongly ionic semiconductors provide a new avenue for engineering the semiconducting properties for applications such as energy harvesting, solid state lighting and sensing.

Magnetic doping is the most common method for breaking time-reversal-symmetry surface states of topological insulators (TIs) to realize novel physical phenomena and to create beneficial technological applications. Here we present a study of the magnetic coupling of a prototype magnetic TI, that is, Cr-doped Bi2Se3, in its ultrathin limit which is expected to give rise to quantum anomalous Hall (QAH) effect. The high quality Bi2–xCrxSe3epitaxial thin film was prepared using molecular beam epitaxy (MBE), characterized with scanning transimission electron microscopy (STEM), electrical magnetotransport, and X-ray magnetic circularly dichroism (XMCD) techniques, and the results were simulated using density functional theory (DFT) with spin–orbit coupling (SOC). We observed a sizable spin moment mspin= (2.05 ± 0.20) μB/Cr and a small and negative orbital moment morb= (−0.05 ± 0.02) μB/Cr of the Bi1.94Cr0.06Se3thin film at 2.5 K. A remarkable fraction of the (CrBi–CrI)3+antiferromagnetic dimer in the Bi2–xCrxSe3for 0.02 < x< 0.40 was obtained using first-principlessimulations, which was neglected in previous studies. The spontaneous coexistence of ferro- and antiferromagnetic Cr defects in Bi2–xCrxSe3explains our experimental observations and those based on conventional magnetometry which universally report magnetic moments significantly lower than 3 μB/Cr predicted by Hund’s rule.

Graphene oxide (GO), the functionalized graphene with oxygen-containing chemical groups, has recently attracted resurgent interests because of its superior properties such as large surface area, mechanical stability, tunable electrical and optical properties. Moreover, the surface functional groups of hydroxyl, epoxy and carboxyl make GO an excellent candidate in coordinating with other materials or molecules. Owing to the expanded structural diversity and improved overall properties, GO and its composites hold great promise for versatile applications of energy storage/conversion and environment protection, including hydrogen storage materials, photocatalyst for water splitting, removal of air pollutants and water purification, as well as electrode materials for various lithium batteries and supercapacitors. In this review, we present an overview on the current successes, as well as the challenges, of the GO-based materials for energy and environmental applications.

Chemical reactions involve competing pathways. Here, two pathways for H desorption from MgH2are identified by first-principles calculations: One involves H diffusion in bulk Mg, while the other involves H vacancy diffusion at the MgH2surface. The latter is sensitive to the size of the reactant MgH2and self-terminates in bulk material as dehydrogenation eventually eliminates exposed MgH2. However, this surface vacancy pathway can maximize the catalyst effect of Pd by decoupling the kinetics of H desorption from that of H diffusion. When the surface-to-bulk ratio is large as in the case of MgH2nanostructures, H desorption will take place primarily via the low-barrier surface vacancy pathway. Our picture attributes the experimentally observed size effect of MgH2on H desorption, beyond the quantum size regime, to a synergy between the nanosize of the reactant and the catalyst.

Ein Puzzlespiel: Ein einfaches Modell für die Stabilität konjugierter Kohlenstoff‐Hybridmaterialien wird vorgestellt und rechnerisch verifiziert. Auf dieser Grundlage wird eine einfache „Puzzle“‐Strategie für das Design solcher Hybridmaterialien vorgeschlagen (siehe Bild; C grün, N blau, H weiß).

The magnetic properties of Cr-doped wurtzite ZnS nanowires were studied through first-principles computations. At a low doping concentration of 2.08% (one Cr atom in a 96-atom supercell), the Cr atom prefers to substitute for the surface four-coordinated Zn atom in the groove of the ZnS nanowire with a diameter of 1.2 nm; at a higher doping concentration of 4.17% (two Cr atoms in the 96-atom supercell), the Cr atoms preferably substitute for the surface four-coordinated Zn atom and its neighboring surface three-coordinated Zn atom. Irrespective of the sites that Cr atoms occupy, Cr atoms in ZnS nanowires are always ferromagnetically coupled to each other and antiferromagnetically coupled to the mediating S atom; moreover, the robust ferromagnetism is not sensitive to surface passivation. The ferromagnetism in Cr-doped ZnS nanowires is attributed to a double-exchange mechanism.

Organometallic nanomaterials hold the promise for molecular hydrogen (H2) storage by providing nearly ideal binding strength to H2for room-temperature applications. Synthesizing such materials, however, faces severe setbacks due to the problem of metal clustering. Inspired by a recent experimental breakthrough (J. Am. Chem. Soc.2008, 130, 6992), which demonstrates enhanced H2binding in Ti-grafted mesoporous silica, we propose combining the graphene oxide (GO) technique with Ti anchoring to overcome the current synthesis bottleneck for practical storage materials. Similar to silica, GO contains ample hydroxyl groups, which are the active sites for anchoring Ti atoms. GO can be routinely synthesized and is much lighter than silica. Hence, higher gravimetric storage capacity can be readily achieved. Our first-principles computations suggest that GO is primarily made of low-energy oxygen-containing structural motifs on the graphene sheet. The Ti atoms bind strongly to the oxygen sites with binding energies as high as 450 kJ/mol. This is comparable to that of silica and is indeed enough to prevent the Ti atoms from clustering. Each Ti can bind multiple H2with the desired binding energies (14−41 kJ/mol-H2). The estimated theoretical gravimetric and volumetric densities are 4.9 wt % and 64 g/L, respectively.

Hydrogen storage: A family of Ti‐substituted boranes (see figure) having optimum electronic structures and the ability to absorb hydrogen has been designed computationally. Substantial binding energies and gravimetric densities of hydrogen storage show their potential as hydrogen storage materials. The computational study invites experimental synthesis of the novel borane family and offers a guide to searching for new hydrogen storage materials.

We report a first-principle study of the tunable hydrogenation of endohedral metallofullerenes MC60and M2C60, where M Li, Be, Mg, Ca, Al, and Sc. The interaction between the encapsulated metal atoms and the C60cage leads to a tuning of the hydrogen binding in a desired manner as the hydrogenation proceeds. At lower H densities, when H atoms are too strongly bound to pure C60, the endohedral dopants weaken the binding. The dopants also enhance the hydrogen-binding energy at higher coverages and enable the degree of hydrogenation to be substantially increased relative to that available with empty-cage C60. Overall, the encapsulated metals increase the capacity and improve the energy efficiency for hydrogen storage. We identify materials capable of storing 6.1 wt % hydrogen and elucidate a subtle interplay between reactivity and structure which will be important for designing the next-generation hydrogen-storage materials.

Owing to its superior visible light absorption and high chemical stability, chalcogenide perovskite barium zirconium sulfide (BaZrS3) has attracted significant attention in the past few years as a potential alternative to hybrid halide perovskites for optoelectronics. However, the high processing temperatures of BaZrS3thin films at above 1000 ℃ severely limits their potential for device applications. Herein, we report the synthesis of BaZrS3thin films at temperatures as low as 500 ℃, by changing the chemical reaction pathway. The single phase BaZrS3thin film was confirmed by X-ray diffraction and Raman spectroscopy. Atomic force microscopy and scanning electron microscopy show that crystalline size and surface roughness were consistently reduced with decreasing annealing temperature. The lower temperatures further eliminate sulfur vacancies and carbon contaminations associated with high temperature processing. The ability to synthesize chalcogenide perovskite thin films at lower temperatures removes a major hurdle for their device fabrication. The photodetectors demonstrate fast response and an on/off ratio of 80. The fabricated field effect transistors show an ambipolar behavior with electron and hole mobilities of 16.8 cm2/Vs and 2.6 cm2/Vs, respectively.

Lead iodide (PbI2) as a layered material has emerged as an excellent candidate for optoelectronics in the visible and ultraviolet regime. Micrometer-sized flakes synthesized by mechanical exfoliation from bulk crystals or by physical vapor deposition have shown a plethora of applications from low-threshold lasing at room temperature to high-performance photodetectors with large responsivity and faster response. However, large-area centimeter-sized growth of epitaxial thin films of PbI2with well-controlled orientation has been challenging. Additionally, the nature of grain boundaries in epitaxial thin films of PbI2remains elusive. Here, we use mica as a model substrate to unravel the growth mechanism of large-area epitaxial PbI2thin films. The partial growth leading to uncoalesced domains reveals the existence of inversion domain boundaries in epitaxial PbI2thin films on mica. Combining the experimental results with first-principles calculations, we also develop an understanding of the thermodynamic and kinetic factors that govern the growth mechanism, which paves the way for the synthesis of high-quality large-area PbI2on other substrates and heterostructures of PbI2on single-crystalline graphene. The ability to reproducibly synthesize high-quality large-area thin films with precise control over orientation and tunable optical properties could open up unique and hitherto unavailable opportunities for the use of PbI2and its heterostructures in optoelectronics, twistronics, substrate engineering, and strain engineering.

Exploring two-dimensional (2D) van der Waals (vdW) systems is at the forefront of materials of physics. Here, through molecular beam epitaxy on graphene-covered SiC(0001), we report successful growth of AlSb in the double-layer honeycomb (DLHC) structure, a 2D vdW material which has no direct analogue to its 3D bulk and is predicted to be kinetically stable when freestanding. The structural morphology and electronic structure of the experimental 2D AlSb are characterized with spectroscopic imaging scanning tunneling microscopy and cross-sectional imaging scanning transmission electron microscopy, which compare well to the proposed DLHC structure. The 2D AlSb exhibits a band gap of 0.93 eV versusthe predicted 1.06 eV, which is substantially smaller than the 1.6 eV of bulk. We also attempt the less-stable InSb DLHC structure; however, it grows into bulk islands instead. The successful growth of a DLHC material here demonstrates the feasibility for the realization of a large family of 2D DLHC traditional semiconductors with characteristic excitonic, topological, and electronic properties.

BaZrS3is a prototypical chalcogenide perovskite, an emerging class of unconventional semiconductor. Recent results on powder samples reveal that it is a material with a direct band gap of 1.7–1.8 eV, a very strong light-matter interaction, and a high chemical stability. Due to the lack of quality thin films, however, many fundamental properties of chalcogenide perovskites remain unknown, hindering their applications in optoelectronics. Here we report the fabrication of BaZrS3thin films, by sulfurization of oxide films deposited by pulsed laser deposition. We show that these films are n-type with carrier densities in the range of 1019-1020 cm−3. Depending on the processing temperature, the Hall mobility ranges from 2.1 to 13.7 cm2/Vs. The absorption coefficient is > 105 cm−1 at photon energy >1.97 eV. Temperature dependent conductivity measurements suggest shallow donor levels. By assuring that BaZrS3is a promising candidate, these results potentially unleash the family of chalcogenide perovskites for optoelectronics such as photodetectors, photovoltaics, and light emitting diodes.

We present a comparison based on first-principles calculations of the electronic structure of nonalkylated and alkylated dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene: DNTT and DNTT-C10. The calculations show that the addition of alkyl chains decreases intermolecular distances, in agreement with experiments. Calculations indicate that effective masses are reduced by the addition of alkyl chains, and within a simple deformation potential model, this translates into higher mobility for DNTT-C10. The shorter intermolecular distances found in DNTT-C10are attributed to van der Waals interactions between alkyl chains.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

We have developed a first-principles generalized one dimensional Ising model to study the band structures and stabilities of two types of ternary zinc-blende-based polytype series: type-a GaInP2and type-b CuInSe2. We predict that the CuAu-like phase can coexist in nominally chalcopyrite CuInSe2. We also predict that type-II band alignment can exist between different ordered type-a GaInP2polytypes.