Constructing Ultra-Shallow Near-Edge States for Efficient and Stable Perovskite Solar Cells.

Electronic band structure engineering of metal-halide perovskites (MHP) lies at the core of fundamental materials research and photovoltaic applications. However, reconfiguring the band structures in MHP for optimized electronic properties remains challenging. This article reports a generic strategy for constructing near-edge states to improve carrier properties, leading to enhanced device performances. The near-edge states are designed around the valence band edge using theoretical prediction and constructed through tailored material engineering. These states are experimentally revealed with activation energies of around 23 milli-electron volts by temperature-dependent time-resolved spectroscopy. Such small activation energies enable prolonged carrier lifetime with efficient carrier transition dynamics and low non-radiative recombination losses, as corroborated by the millisecond lifetimes of microwave conductivity. By constructing near-edge states in positive-intrinsic-negative inverted cells, a champion efficiency of 25.4% (25.0% certified) for a 0.07-cm2 cell and 23.6% (22.7% certified) for a 1-cm2 cell is achieved. The most stable encapsulated cell retains 90% of its initial efficiency after 1100 h of maximum power point tracking under one sun illumination (100 mW cm-2) at 65 °C in ambient air.

Catalytic selectivity of nanorippled graphene.

Experiments have shown that nanoscale ripples in a graphene membrane exhibit unexpectedly high catalytic activity with respect to hydrogen dissociation. Nonetheless, the catalytic selectivity of nanorippled graphene remains unknown, which is an equally important property for assessing a catalyst's potential and its fit-for-purpose applications. Herein, we examine the catalytic selectivity of nanorippled graphene using a model reaction of molecular hydrogen with another simple but double-bonded molecule, oxygen, and comparing the measurement results with those from splitting of hydrogen molecules. We show that although nanorippled graphene exhibits a high catalytic activity toward hydrogen dissociation, the activity for catalyzing the hydrogen-oxygen reaction is quite low, translating into a strong catalytic selectivity. The latter reaction involves the reduction of oxygen molecules by the dissociated hydrogen adatoms, which requires additional energy cost and practically determines the selectivity. In this sense, the well-established information about reactions in general of atomic hydrogen with many other species in the literature could potentially predict the selectivity of nanorippled graphene as a catalyst. Our work provides implications for the catalytic properties of nanorippled graphene, especially its selectivity. The results would be important for its extension to a wider range of reactions and for designer technologies involving hydrogen.

Direct growth of single-chiral-angle tungsten disulfide nanotubes using gold nanoparticle catalysts.

Transition metal dichalcogenide (TMD) nanotubes offer a unique platform to explore the properties of TMD materials at the one-dimensional limit. Despite considerable efforts thus far, the direct growth of TMD nanotubes with controllable chirality remains challenging. Here we demonstrate the direct and facile growth of high-quality WS2 and WSe2 nanotubes on Si substrates using catalytic chemical vapour deposition with Au nanoparticles. The Au nanoparticles provide unique accommodation sites for the nucleation of WS2 or WSe2 shells on their surfaces and seed the subsequent growth of nanotubes. We find that the growth mode of nanotubes is sensitive to the temperature. With careful temperature control, we realize ~79% WS2 nanotubes with single chiral angles, with a preference of 30° (~37%) and 0° (~12%). Moreover, we demonstrate how the geometric, electronic and optical properties of the synthesized WS2 nanotubes can be modulated by the chirality. We anticipate that this approach using Au nanoparticles as catalysts will facilitate the growth of TMD nanotubes with controllable chirality and promote the study of their interesting properties and applications.

Engineering Atomic-Scale Patterning and Resistive Switching in 2D Crystals and Application in Image Processing.

The ultrathin thickness of 2D layered materials affords the control of their properties through defects, surface modification, and electrostatic fields more efficiently compared with bulk architecture. In particular, patterning design, such as moiré superlattice patterns and spatially periodic dielectric structures, are demonstrated to possess the ability to precisely control the local atomic and electronic environment at large scale, thus providing extra degrees of freedom to realize tailored material properties and device functionality. Here, the scalable atomic-scale patterning in superionic cuprous telluride by using the bonding difference at nonequivalent copper sites is reported. Moreover, benefitting from the natural coupling of ordered and disordered sublattices, controllable piezoelectricity-like multilevel switching and bipolar switching with the designed crystal structure and electrical contact is realized, and their application in image enhancement is demonstrated. This work extends the known classes of patternable crystals and atomic switching devices, and ushers in a frontier for image processing with memristors.

Optoelectronic properties and applications of two-dimensional layered semiconductor van der Waals heterostructures: perspective from theory.

Van der Waals heterostructures (vdWHs) which combine two different materials together have attracted extensive research attentions due to the promising applications in optoelectronic and electronic devices, the investigations from theoretical simulations can not only predict the novel properties and the interfacial coupling, but also provide essential guidance for experimental verification and fabrications. This review summarizes the recent theoretical studies on electronic and optical properties of two-dimensional semiconducting vdWHs. The characteristics of different band alignments are discussed, together with the optoelectronic modulations from external fields and the promising applications in solar cells, tunneling field-effect transistors and photodetectors. At the end of the review, the further perspective and possible research problems of the vdWHs are also presented.

Interface characteristics of graphene/ZnS hybrid-dimensional heterostructures.

Graphene/ZnS hybrid-dimensional heterostructure is an excellent combination to regulate and improve the conductivity and sensitivity of components, in which the interface effects have crucial impacts on the performance of devices. In this work, we investigate the interface characteristics of Graphene/ZnS 2D/3D heterostructures. X-ray photoelectron spectra show that the ZnS binding energy shifts to lower energy by 0.3 eV after forming heterojunction with graphene. The fluorescence and absorption spectra confirm the luminescence enhancement and blue-shift of the absorbance edge of ZnS caused by graphene. The composition of Graphene/ZnS heterostructure facilitates separation and transfer of spatial charges, resulting in rapid electron transport.

Arsenic Monolayers Formed by Zero-Dimensional Tetrahedral Clusters and One-Dimensional Armchair Nanochains.

One-dimensional (1D) arsenene nanostructures are predicted to host a variety of interesting physical properties including antiferromagnetic, semiconductor-semimetal transition and quantum spin Hall effect, which thus holds great promise for next-generation electronic and spintronic devices. Herein, we devised a surface template strategy in a combination with surface-catalyzed decomposition of molecular As4 cluster toward the synthesis of the superlattice of ultranarrow armchair arsenic nanochains in a large domain on Au(111). In the low annealing temperature window, zero-dimensional As4 nanoclusters are assembled into continuous films through intermolecular van der Waals and molecule-substrate interactions. At the elevated temperature, the subsequent surface-assisted decomposition of molecular As4 nanoclusters leads to the formation of a periodic array of 1D armchair arsenic nanochains that form a (2 × 3) superstructure on the Au(111) surface. These ultranarrow armchair arsenic nanochains are predicted to have a small bandgap of ∼0.50 eV, in contrast to metallic zigzag chains. In addition, the Au-supported arsenic nanochains can be flipped to form a bilayer structure through tip indentation and manipulation, suggesting the possible transfer of these nanochains from the substrate. The successful realization of arsenic nanostructures is expected to advance low-dimensional physics and infrared optoelectronic nanodevices.

Ultrathin ferrite nanosheets for room-temperature two-dimensional magnetic semiconductors.

The discovery of magnetism in ultrathin crystals opens up opportunities to explore new physics and to develop next-generation spintronic devices. Nevertheless, two-dimensional magnetic semiconductors with Curie temperatures higher than room temperature have rarely been reported. Ferrites with strongly correlated d-orbital electrons may be alternative candidates offering two-dimensional high-temperature magnetic ordering. This prospect is, however, hindered by their inherent three-dimensional bonded nature. Here, we develop a confined-van der Waals epitaxial approach to synthesizing air-stable semiconducting cobalt ferrite nanosheets with thickness down to one unit cell using a facile chemical vapor deposition process. The hard magnetic behavior and magnetic domain evolution are demonstrated by means of vibrating sample magnetometry, magnetic force microscopy and magneto-optical Kerr effect measurements, which shows high Curie temperature above 390 K and strong dimensionality effect. The addition of room-temperature magnetic semiconductors to two-dimensional material family provides possibilities for numerous novel applications in computing, sensing and information storage.

General synthesis of 2D rare-earth oxide single crystals with tailorable facets.

Two-dimensional (2D) rare-earth oxides (REOs) are a large family of materials with various intriguing applications and precise facet control is essential for investigating new properties in the 2D limit. However, a bottleneck remains with regard to obtaining their 2D single crystals with specific facets because of the intrinsic non-layered structure and disparate thermodynamic stability of different facets. Herein, for the first time, we achieve the synthesis of a wide variety of high-quality 2D REO single crystals with tailorable facets via designing a hard-soft-acid-base couple for controlling the 2D nucleation of the predetermined facets and adjusting the growth mode and direction of crystals. Also, the facet-related magnetic properties of 2D REO single crystals were revealed. Our approach provides a foundation for further exploring other facet-dependent properties and various applications of 2D REO, as well as inspiration for the precise growth of other non-layered 2D materials.

Strain regulated interlayer coupling in WSe2/WS2heterobilayer.

Strain engineering can effectively modify the materials lattice parameters at atomic scale, hence it has become an efficient method for tuning the physical properties of two-dimensional (2D) materials. The study of the strain regulated interlayer coupling is deserved for different kinds of heterostructures. Here, we systematically studied the strain engineering of WSe2/WS2heterostructures as well as their constituent monolayers. The measured Raman and photoluminescence spectra demonstrate that the strain can evidently modulate the phonon energy and exciton emission of monolayer WSe2and WS2as well as the WSe2/WS2heterostructures. The tensile strain can tune the electronic band structure of WSe2/WS2heterostructure, as well as enhance the interlayer coupling. It is further revealed that the photoluminescence intensity ratio of WS2to WSe2in our WSe2/WS2heterobilayer increases monotonically with tensile strain. These findings can broaden the understanding and practical application of strain engineering in 2D materials with nanometer-scale resolution.

Structure-Composition-Property Relationships in Antiperovskite Nitrides: Guiding a Rational Alloy Design.

The alloy strategy through the A- or X-site is a common method for experimental preparation of high-performance and stable lead-based perovskite solar cells. As one of the important candidates for lead-free and stable photovoltaic absorbers, the inorganic antiperovskite family has recently been reported to exhibit excellent optoelectronic properties. However, the current reports on the design of antiperovskite alloys are rare. In this work, we investigated the previously overlooked electronic property (e.g., conduction band convergence), static dielectric constant, and exciton binding energy in inorganic antiperovskite nitrides by first-principles calculations. Then, we revealed a linear relationship between the tolerance factor and various physical quantities. Guided by the established structure-composition-property relationship in six antiperovskite nitrides X3NA (X2+ = Mg2+, Ca2+, Sr2+; A3- = P3-, As3-, Sb3-, Bi3-), for the first time, we designed a promising antiperovskite alloy Mg3NAs0.5Bi0.5 with a quasi-direct band gap of 1.402 eV. Finally, we made a comprehensive comparison between antiperovskite nitrides and conventional halide perovskites for pointing out the future direction for device applications.

2D GaN for Highly Reproducible Surface Enhanced Raman Scattering.

Surface-enhanced Raman scattering (SERS) based on 2D semiconductors has been rapidly developed due to their chemical stability and molecule-specific SERS activity. High signal reproducibility is urgently required towards practical SERS applications. 2D gallium nitride (GaN) with highly polar Ga-N bonds enables strong dipole-dipole interactions with the probe molecules, and abundant DOS (density of states) near its Fermi level increases the intermolecular charge transfer probability, making it a suitable SERS substrate. Herein, 2D micrometer-sized GaN crystals are demonstrated to be sensitive SERS platforms with excellent signal reproducibility and stability. Strong dipole-dipole interaction between the dye molecule and 2D GaN enhances the molecular polarizability. Furthermore, 2D GaN benefits its SERS enhancement by the combination of increased DOS and more efficient charge transfer resonances when compared with its bulk counterpart.

WS2 nanosheets@ZIF-67-derived N-doped carbon composite as sodium ion battery anode with superior rate capability.

Devising novel composite electrodes with particular structural/electrochemical characteristics becomes an efficient strategy to advance the performance of rechargeable battery. Herein, considering the homogeneous transition metal sulfide with N-doped carbon derived from zeolitic imidazolate framework-67 (ZIF-67) and WS2 with large interlayer spacing, a laurel-leaf-like Co9S8/WS2@N-doped carbon bimetallic sulfide (Co9S8/WS2@NC) is engineered and prepared via a step-by-step method. As an electrode material for sodium ion batteries (SIBs), Co9S8/WS2@NC composite delivers high capacities of 480 and 405 mA h g-1 at 0.1 and 1.0 A g-1, respectively. As the current density increases from 0.1 to 5.0 A g-1, it provides specific capacity of 359 mA h g-1 with a capacity retention rate of 78.0%, which is higher than that of Co9S8@NC (63.5%) and WS2 (58.6%). The Co9S8/WS2@NC composite anode maintains a stable specific capacity (354 mA h g-1 at 2.0 A g-1). It also exhibits a high capacitive contribution ratio of 90.8% at 1.0 mV s-1. This study provides a new and reliable insight for designing bimetallic sulfide with two-dimensional nanostructure for energy storage.

Vertically stacked GaN/WX2 (X = S, Se, Te) heterostructures for photocatalysts and photoelectronic devices.

Tremendous attention has been paid to vertically stacked heterostructures owing to their tunable electronic structures and outstanding optical properties. In this work, we explore the structural, electronic and optical properties of vertically stacked GaN/WX2 (X = S, Se, Te) heterostructures using density functional theory. We find that these stacking heterostructures are all semiconductors with direct band gaps of 1.473 eV (GaN/WTe2), 2.102 eV (GaN/WSe2) and 1.993 eV (GaN/WS2). Interestingly, the GaN/WS2 heterostructure exhibits a type-II band alignment, while the other two stackings of GaN/WSe2 and GaN/WTe2 heterostructures have type-I band alignment. The optical absorption of GaN/WX2 heterostructures is very efficient in the visible light spectrum. Our results suggest that GaN/WX2 heterostructures are promising candidates for photocatalytic water splitting and photoelectronic devices in visible light.

Thermally-driven gold@poly(N-isopropylacrylamide) core-shell nanotransporters for molecular extraction.

HYPOTHESIS: Molecular extraction efficiency can be boosted with the assistance of nanoparticles (NPs). It is based on adsorption of the extractants in one phase and desorption in another phase, which requires a reversible phase transfer of the NPs. EXPERIMENTS: We synthesized the gold@poly(N-isopropylacryamide) (Au@PNIPAM) NPs via an interfacial self-assembly method enhanced by post-polymerization. We adopted Rhodamine 6G (R6G) as the model molecule for the extraction test. In comparison, UV-Vis extinction spectra were recorded to monitor the extraction processes with or without the Au@PNIPAM NPs. We further analyzed theoretically with thermodynamics and first-principle calculations. FINDINGS: The hybrid Au@PNIPAM NPs show a reversible phase transfer between the interface and chloroform phases. The Au NPs assisted extraction efficiency of R6G shows 5 times higher than that without Au NPs. The thermodynamic analysis of the nanotransportation system agrees well with the ab initio density functional theory calculations. This nanoparticle-assisted molecular transportation modifies the extraction kinetics significantly, which will provide further implications for biphasic catalysis, pollutant treatment and drug delivery.

Novel pectin based composite hydrogel derived from grapefruit peel for enhanced Cu(II) removal.

Novel biochar/pectin/alginate hydrogel beads (BPA) derived from grapefruit peel were synthesized and used for Cu(II) removal from aqueous solution. FTIR, SEM-EDS, XRD, TGA and XPS, etc. were applied for characterization analysis. The synergistic reinforcing effect of polymer matrix and biochar fillers improved the adsorptive, mechanical and thermostabilized performance of BPA. Factors like component contents of biochar and pectin, pH, contact time, Cu(II) concentration and coexisting inorganic salts or organic ligands were systematically investigated in batch mode. The adsorption isotherms were fitted well by the Freundlich model and the experimental maximum adsorption capacity of optimized BPA-9 beads (mass ratio of pectin to alginate = 10:1) with 0.25% biochar, was ∼80.6 mg/g at pH 6. Kinetic process was well described by the pseudo-second-order model and film diffusion primarily governed the overall adsorption rate, followed by intraparticle diffusion. Thermodynamics analysis suggested spontaneous feasibility and endothermic nature of adsorption behavior. Moreover, BPA also showed better environmental adaptability in the presence of NaCl, MgCl2, CaCl2, EDTA-2Na and CA as well as good adsorption potential for other heavy metal [e.g. Pb(III)]. Crucially, the BPA beads showed good regeneration ability after five cycles. All these results indicated the potential of BPA for removing heavy metal from water.

A ternary SnS1.26Se0.76 alloy for flexible broadband photodetectors.

Layered two-dimensional (2D) materials often display unique functionalities for flexible 2D optoelectronic device applications involving natural flexibility and tunable bandgap by bandgap engineering. Composition manipulation by alloying of these 2D materials represents an effective way in fulfilling bandgap engineering, which is particularly true for SnS2x Se2(1-x) alloys showing a continuous bandgap modulation from 2.1 eV for SnS2 to 1.0 eV for SnSe2. Here, we report that a ternary SnS1.26Se0.76 alloy nanosheet can serve as an efficient flexible photodetector, possessing excellent mechanical durability, reproducibility, and high photosensitivity. The photodetectors show a broad spectrum detection ranging from visible to near infrared (NIR) light. These findings demonstrate that the ternary SnS1.26Se0.76 alloy can act as a promising 2D material for flexible and wearable optoelectronic devices.

Type-I Transition Metal Dichalcogenides Lateral Homojunctions: Layer Thickness and External Electric Field Effects.

Transition metal dichalcogenide (TMD) heterostructures have been widely explored due to the formation of type-II band alignment and interlayer exciton. However, the studies of type-I TMD heterostructures are still lacking, which limit their applications in luminescence devices. Here, the 1L/nL MX2 (n = 2, 3, 4; M = Mo, W; X = S, Se) lateral homojunction based on the layer-dependent band gaps of TMD nanosheets is theoretically simulated. The studies show that the TMD homojunction presents with high thermal stability and type-I band alignment. The band offset and quantum confinement of carriers can be easily tuned by controlling the thickness of the multilayer region. Moreover, the electric field can decrease the band gaps of 1L/3L and 1L/4L homojunctions linearly. Interestingly, for the 1L/2L MX2 homojunction, the gap value is robust to the weak electric field, while it drops sharply under a strong electric field. This study sheds light on the physical pictures in the TMD lateral homojunction, and provides a practicable and general approach to engineer a type-I homojunction based 2D semiconductor materials.

Two-dimensional transition-metal dichalcogenides-based ferromagnetic van der Waals heterostructures.

The lack of ferromagnetic (FM) van der Waals (vdW) heterostructures hinders the application of two-dimensional (2D) materials in spintronics, information memories and storage devices. Herein, we find theoretically that 2D transition-metal dichalcogenides-based vdW heterostructures, such as MoS2/VS2 and WS2/VS2, possess excellent characteristics of stable stacking configurations, FM semiconducting ground states, high Curie temperatures, staggered band alignment and a large band offset. Fortunately, 100% spin-polarized currents at the Fermi level can be achieved under certain positive external electric fields, which can filter the current into a single spin channel. Moreover, the majority channel undergoes the transition from type-II to type-I (type-III) band alignment under the negative (positive) electric field; while the band alignment of the minority channel is robust to the electric field. Our results provide a feasible way to realize 2D TMDs-based FM semiconducting heterostructures for spintronic devices.

Band engineering of the MoS2/stanene heterostructure: strain and electrostatic gating.

In a fast developing field, it has been found that van der Waals heterostructures can overcome the weakness of single two-dimensional layered materials and extend their electronic and optoelectronic applications. Through first-principles methods, the studied MoS2/stanene heterostructure preserves high-speed carrier characteristics and opens the direct band gap. Simultaneously, the band alignment shows that the electrons transfer from stanene to MoS2, which forms an internal electric field. As an effective strategy, the out-of-plane strain remarkably changes the band gaps of the heterostructure and enhances its carrier concentration. In addition, the combined effects of the internal and external electric fields can further open the band gaps and induce a direct-to-indirect gap transition in the heterostructure. More interestingly, when the external electric field is equal to the reverse internal one, the heterostructure regains a Dirac cone. Our results show that the MoS2/stanene heterostructure has potential applications in high-speed optoelectronic devices.