Spin-Phonon Coupling in Iron-Doped Ultrathin Bismuth Halide Perovskite Derivatives.

Spin in semiconductors facilitates magnetically controlled optoelectronic and spintronic devices. In metal halide perovskites (MHPs), doping magnetic ions is proven to be a simple and efficient approach to introducing a spin magnetic momentum. In this work, we present a facile metal ion doping protocol through the vapor-phase metal halide insertion reaction to the chemical vapor deposition (CVD)-grown ultrathin Cs3BiBr6 perovskites. The Fe-doped bismuth halide (Fe:CBBr) perovskites demonstrate that the iron spins are successfully incorporated into the lattice, as revealed by the spin-phonon coupling below the critical temperature Tc around 50 K observed through temperature-dependent Raman spectroscopy. Furthermore, the phonons exhibit significant softening under an applied magnetic field, possibly originating from magnetostriction and spin exchange interaction. The spin-phonon coupling in Fe:CBBr potentially provides an efficient way to tune the spin and lattice parameters for halide perovskite-based spintronics.

Spatially Resolved Anion Diffusion and Tunable Waveguides in Bismuth Halide Perovskites.

Bismuth halide perovskites are widely regarded as nontoxic alternatives to lead halide perovskites for optoelectronics and solar energy harvesting applications. With a tailorable composition and intriguing optical properties, bismuth halide perovskites are also promising candidates for tunable photonic devices. However, robust control of the anion composition in bismuth halide perovskites remains elusive. Here, we established chemical vapor deposition and anion exchange protocols to synthesize bismuth halide perovskite nanoflakes with controlled dimensions and variable compositions. In particular, we demonstrated the gradient bromide distribution by controlling the anion exchange and diffusion processes, which is spatially resolved by time-of-flight secondary ion mass spectrometry. Moreover, the optical waveguiding properties of bismuth halide perovskites can be modulated by flake thicknesses and anion compositions. With a unique gradient anion distribution and controllable optical properties, bismuth halide perovskites provide new possibilities for applications in optoelectronic devices and integrated photonics.

Orientation and morphology control in acid-catalyzed covalent organic framework thin films.

As thin films of semiconducting covalent organic frameworks (COFs) are demonstrating utility for ambipolar electronics, channel materials in organic electrochemical transistors (OECTs), and broadband photodetectors, control and modulation of their thin film properties is paramount. In this work, an interfacial growth technique is utilized to synthesize imine TAPB-PDA COF films at both the liquid-liquid interface as well as at the liquid-solid interface on a Si/SiO2 substrate. The concentration of acetic acid catalyst in the aqueous phase is shown to significantly influence the thin film morphology of the liquid-solid growth, with concentrations below 1 M resulting in no film nucleation, concentrations of 1-4 M enabling smooth film formation, and concentrations greater than 4 M resulting in films with a higher density of particulates on the surface. Importantly, while the films grown at the liquid-liquid interface are mixed-orientation, those grown directly at the liquid-solid interface on the Si/SiO2 surface have highly oriented COF layers aligned parallel to the substrate surface. Moreover, this liquid-solid growth process affords TAPB-PDA COF thin films with p-type charge transport having a transconductance of 10 µS at a gate voltage of -0.9 V in an OECT device structure.

Superior mechanical properties of multilayer covalent-organic frameworks enabled by rationally tuning molecular interlayer interactions.

Two-dimensional (2D) covalent-organic frameworks (COFs) with a well-defined and tunable periodic porous skeleton are emerging candidates for lightweight and strong 2D polymeric materials. It remains challenging, however, to retain the superior mechanical properties of monolayer COFs in a multilayer stack. Here, we successfully demonstrated a precise layer control in synthesizing atomically thin COFs, enabling a systematic study of layer-dependent mechanical properties of 2D COFs with two different interlayer interactions. It was shown that the methoxy groups in COFTAPB-DMTP provided enhanced interlayer interactions, leading to layer-independent mechanical properties. In sharp contrast, mechanical properties of COFTAPB-PDA decreased significantly as the layer number increased. We attributed these results to higher energy barriers against interlayer sliding due to the presence of interlayer hydrogen bonds and possible mechanical interlocking in COFTAPB-DMTP, as revealed by density functional theory calculations.

Microwave Facilitated Covalent Organic Framework/Transition Metal Dichalcogenide Heterostructures.

Organic/inorganic heterostructures present a versatile platform for creating materials with new functionalities and hybrid properties. In particular, junctions between two dimensional materials have demonstrated utility in next generation electronic, optical, and optoelectronic devices. This work pioneers a microwave facilitated synthesis process to readily incorporate few-layer covalent organic framework (COF) films onto monolayer transition metal dichalcogenides (TMDC). Preferential microwave excitation of the monolayer TMDC flakes result in selective attachment of COFs onto the van der Waals surface with film thicknesses between 1 and 4 nm. The flexible process is extended to multiple TMDCs (MoS2, MoSe2, MoSSe) and several well-known COFs (TAPA-PDA COF, TPT-TFA-COF, and COF-5). Photoluminescence studies reveal a power-dependent defect formation in the TMDC layer, which facilitates electronic coupling between the materials at higher TMDC defect densities. This coupling results in a shift in the A-exciton peak location of MoSe2, with a red or blue shift of 50 or 19 meV, respectively, depending upon the electron donating character of the few-layer COF films. Moreover, optoelectronic devices fabricated from the COF-5/TMDC heterostructure present an opportunity to tune the PL intensity and control the interaction dynamics within inorganic/organic heterostructures.

3D-printed silica with nanoscale resolution.

Fabricating inorganic materials with designed three-dimensional nanostructures is an exciting yet challenging area of research and industrial application. Here, we develop an approach to 3D print high-quality nanostructures of silica with sub-200 nm resolution and with the flexible capability of rare-earth element doping. The printed SiO2 can be either amorphous glass or polycrystalline cristobalite controlled by the sintering process. The 3D-printed nanostructures demonstrate attractive optical properties. For instance, the fabricated micro-toroid optical resonators can reach quality factors (Q) of over 104. Moreover, and importantly for optical applications, doping and codoping of rare-earth salts such as Er3+, Tm3+, Yb3+, Eu3+ and Nd3+ can be directly implemented in the printed SiO2 structures, showing strong photoluminescence at the desired wavelengths. This technique shows the potential for building integrated microphotonics with silica via 3D printing.

Perovskite-Derivative Valleytronics.

Halide perovskites are revolutionizing the renewable energy sector owing to their high photovoltaic efficiency, low manufacturing cost, and flexibility. Their remarkable mobility and long carrier lifetime are also valuable for information technology, but fundamental challenges like poor stability under an electric field prevent realistic applications of halide perovskites in electronics. Here, it is discovered that valleytronics is a promising route to leverage the advantages of halide perovskites and derivatives for information storage and processing. The synthesized all-inorganic lead-free perovskite derivative, Cs3 Bi2 I9 , exhibits strong light-matter interaction and parity-dependent optically addressable valley degree of freedom. Robust optical helicity in all odd-layer-number crystals with inversion symmetry breaking is observed, indicating excitonic coherence extending well beyond 11 layers. The excellent optical and valley properties of Cs3 Bi2 I9 arise from the unique parallel bands, according to first principles calculations. This discovery points to new materials design principles for scalable valleytronic devices and demonstrates the promise of perovskite derivatives beyond energy applications.

A Low-Cost and High-Efficiency Integrated Device toward Solar-Driven Water Splitting.

Achieving the spontaneous evolution of fuel from integrated devices by solar-driven water splitting is an attractive method for renewable energy conversion. However, their widespread implementation is hindered by their immature architectures and inferior performances. Here, we propose a real integrated device consisting of two series-connected perovskite solar cells (PSCs) and two CoP catalyst electrodes, which can be immersed into the aqueous solution directly for solar-driven water splitting. Benefiting from the low-cost and facile encapsulation technique, this integrated device possesses a compact structure and well-connected circuits for the process of charge carriers generation, transfer, and storage. Moreover, although all expensive components in this integrated device are eliminated, the two series-connected carbon-based PSCs still exhibit a high solar-to-electric efficiency of 10.6% as well as the integrated devices display a solar-to-hydrogen efficiency of as high as 6.7%. This integrated device serves as a model architecture toward future development and optimization of the integrated device that can be immersed into the aqueous solution directly for water splitting.

Cobalt-Modulated Molybdenum-Dinitrogen Interaction in MoS2 for Catalyzing Ammonia Synthesis.

Dinitrogen conversion to ammonia via electrochemical reduction with over 10% Faradaic efficiency is demonstrated in this work. Co-doped MoS2-x polycrystalline nanosheets with S vacancies as the catalysts are loaded onto carbon cloth by hydrothermal growth from Mo, Co, and S precursors. A sulfur vacancy on the MoS2-x basal plane mimicking the natural Mo-nitrogenase active site is modified by Co doping and exhibits superior dinitrogen-to-ammonia conversion activity. Density-functional simulation reveals that the free energy barrier, which can be compensated by applied overpotential, is reduced from 1.62 to 0.59 eV after Co doping. Meanwhile, dinitrogen tends to be chemically adsorbed to defective MoS2-x, which effectively activates the dinitrogen molecule for the dissociation of the N≡N triple bond. This process is further accelerated by Co doping, resulting from the modulation of Mo-N bonding configuration.

Defect-Engineering-Enabled High-Efficiency All-Inorganic Perovskite Solar Cells.

The emergence of cesium lead iodide (CsPbI3 ) perovskite solar cells (PSCs) has generated enormous interest in the photovoltaic research community. However, in general they exhibit low power conversion efficiencies (PCEs) because of the existence of defects. A new all-inorganic perovskite material, CsPbI3 :Br:InI3 , is prepared by defect engineering of CsPbI3 . This new perovskite retains the same bandgap as CsPbI3 , while the intrinsic defect concentration is largely suppressed. Moreover, it can be prepared in an extremely high humidity atmosphere and thus a glovebox is not required. By completely eliminating the labile and expensive components in traditional PSCs, the all-inorganic PSCs based on CsPbI3 :Br:InI3 and carbon electrode exhibit PCE and open-circuit voltage as high as 12.04% and 1.20 V, respectively. More importantly, they demonstrate excellent stability in air for more than two months, while those based on CsPbI3 can survive only a few days in air. The progress reported represents a major leap for all-inorganic PSCs and paves the way for their further exploration in order to achieve higher performance.

Lead-Free Double Perovskite Cs2 SnX6 : Facile Solution Synthesis and Excellent Stability.

Long-term instability and possible lead contamination are the two main issues limiting the widespread application of organic-inorganic lead halide perovskites. Here a facile and efficient solution-phase method is demonstrated to synthesize lead-free Cs2 SnX6 (X = Br, I) with a well-defined crystal structure, long-term stability, and high yield. Based on the systematic experimental data and first-principle simulation results, Cs2 SnX6 displays excellent stability against moisture, light, and high temperature, which can be ascribed to the unique vacancy-ordered defect-variant structure, stable chemical compositions with Sn4+ , as well as the lower formation enthalpy for Cs2 SnX6 . Additionally, photodetectors based on Cs2 SnI6 are also fabricated, which show excellent performance and stability. This study provides very useful insights into the development of lead-free double perovskites with high stability.

Room-Temperature Magnetic Order in Air-Stable Ultrathin Iron Oxide.

Manual assembly of atomically thin materials into heterostructures with desirable electronic properties is an approach that holds great promise. Despite the rapid expansion of the family of ultrathin materials, stackable and stable ferro/ferri magnets that are functional at room temperature are still out of reach. We report the growth of air-stable, transferable ultrathin iron oxide crystals that exhibit magnetic order at room temperature. These crystals require no passivation and can be prepared by scalable and cost-effective chemical vapor deposition. We demonstrate that the bonding between iron oxide and its growth substrate is van der Waals-like, enabling us to remove the crystals from their growth substrate and prepare iron oxide/graphene heterostructures.

Ultrafast Charge Transfer in Perovskite Nanowire/2D Transition Metal Dichalcogenide Heterostructures.

Mixed-dimensional van der Waals (vdW) heterostructures between one-dimensional (1D) perovskite nanowires and two-dimensional (2D) transition metal dichalcogenides (TMDCs) hold great potential for novel optoelectronics and light-harvesting applications. However, the ultrafast carrier dynamics between the 1D perovskite nanowires and 2D TMDCs are currently not well understood, which is critical for related optoelectronic applications. Here we demonstrate vdW heterostructures of CsPbBr3 nanowire/monolayer MoS2 and CsPbBr3 nanowire/monolayer WSe2 and further present systematic investigations on their charge transfer dynamics. We show that CsPbBr3/MoS2 and CsPbBr3/WSe2 are type-I and type-II heterostructures, respectively. Both electrons and holes transfer from CsPbBr3 to MoS2 with an efficiency of 71%. As a contrast, holes transfer from CsPbBr3 to WSe2 with a carrier transfer efficiency of 70% and electrons transfer inversely within 7 ps. The ultrafast and efficient charge transfer in the 1D/2D perovskite-TMDC heterostructures suggest great promise in light emission, photodetector, and photovoltaic devices.

All-Inorganic Perovskite Nanowires-InGaZnO Heterojunction for High-Performance Ultraviolet-Visible Photodetectors.

Inorganic cesium lead halide perovskites have attracted intense interest in optoelectronic applications due to the relatively stable performance in air. However, most reported inorganic perovskite-based optoelectronic devices exhibit low photosensitivity, which greatly hinders their further applications. Here, we first demonstrate a hybrid optical structure, which combines the n-type thin-film InGaZnO and the all-inorganic perovskite nanowires of CsPbBr3 together. By this way, excellent optical and electrical properties such as a low dark current of 10-10 A (at -5 V), a high Iph/Idark of 1.2 × 104, a response time of 2 ms and photoresponsivity of 3.794 A/W have been obtained. It is also found that the photodetector shows good stability in air ambient for 2 months with little reduction in performance. Moreover, such hybrid photodetectors exhibit enhanced photocurrent and Iph/Idark in high-temperature environment. This work paves a new way for high-performance photodetectors and points out the possible application of the inorganic cesium lead halide perovskites in harsh environment.

Two-dimensional metallic tantalum disulfide as a hydrogen evolution catalyst.

Two-dimensional metallic transition metal dichalcogenides are emerging as prototypes for uncovering fundamental physical phenomena, such as superconductivity and charge-density waves, as well as for engineering-related applications. However, the batch production of such envisioned transition metal dichalcogenides remains challenging, which has hindered the aforementioned explorations. Herein, we fabricate thickness-tunable tantalum disulfide flakes and centimetre-sized ultrathin films on an electrode material of gold foil via a facile chemical vapour deposition route. Through temperature-dependent Raman characterization, we observe the transition from nearly commensurate to commensurate charge-density wave phases with our ultrathin tantalum disulfide flakes. We have obtained high hydrogen evolution reaction efficiency with the as-grown tantalum disulfide flakes directly synthesized on gold foils comparable to traditional platinum catalysts. This work could promote further efforts for exploring new efficient catalysts in the large materials family of metallic transition metal dichalcogenides, as well as exploiting their applications towards more versatile applications.Metallic transition metal dichalcogenides are important materials for catalysis, but scalable and controllable preparation methods are scarce. Here, the authors synthesize 2H-TaS2 as centimetre-scale films of tunable thickness and show they are an efficient catalyst for hydrogen evolution.

Van der Waals Epitaxial Growth of 2D Metallic Vanadium Diselenide Single Crystals and their Extra-High Electrical Conductivity.

2D metallic transition-metal dichalcogenides (MTMDs) have recently emerged as a new class of materials for the engineering of novel electronic phases, 2D superconductors, magnets, as well as novel electronic applications. However, the mechanical exfoliation route is predominantly used to obtain such metallic 2D flakes, but the batch production remains challenging. Herein, the van der Waals epitaxial growth of monocrystalline, 1T-phase, few-layer metallic VSe2 nanosheets on an atomically flat mica substrate via a "one-step" chemical vapor deposition method is reported. The thickness of the VSe2 nanosheets is precisely tuned from several nanometers to several tenths of nanometers. More significantly, the 2D VSe2 single crystals are found to present an excellent metallic feature, as evidenced by the extra-high electrical conductivity of up to 106 S m-1 , 1-4 orders of magnitude higher than that of various conductive 2D materials. The thickness-dependent charge-density-wave phase transitions are also examined through low-temperature transport measurements, which reveal that the synthesized 2D metallic 1T-VSe2 nanosheets should serve as good research platforms for the detecting novel many-body states. These results open a new path for the synthesis and property investigations of nanoscale-thickness 2D MTMDs crystals.

Metallic Vanadium Disulfide Nanosheets as a Platform Material for Multifunctional Electrode Applications.

Nanothick metallic transition metal dichalcogenides such as VS2 are essential building blocks for constructing next-generation electronic and energy-storage applications, as well as for exploring unique physical issues associated with the dimensionality effect. However, such two-dimensional (2D) layered materials have yet to be achieved through either mechanical exfoliation or bottom-up synthesis. Herein, we report a facile chemical vapor deposition route for direct production of crystalline VS2 nanosheets with sub-10 nm thicknesses and domain sizes of tens of micrometers. The obtained nanosheets feature spontaneous superlattice periodicities and excellent electrical conductivities (∼3 × 103 S cm-1), which has enabled a variety of applications such as contact electrodes for monolayer MoS2 with contact resistances of ∼1/4 to that of Ni/Au metals, and as supercapacitor electrodes in aqueous electrolytes showing specific capacitances as high as 8.6 × 102 F g-1. This work provides fresh insights into the delicate structure-property relationship and the broad application prospects of such metallic 2D materials.

Direct Chemical Vapor Deposition Growth and Band-Gap Characterization of MoS2/h-BN van der Waals Heterostructures on Au Foils.

Stacked transition-metal dichalcogenides on hexagonal boron nitride (h-BN) are platforms for high-performance electronic devices. However, such vertical stacks are usually constructed by the layer-by-layer polymer-assisted transfer of mechanically exfoliated layers. This inevitably causes interfacial contamination and device performance degradation. Herein, we develop a two-step, low-pressure chemical vapor deposition synthetic strategy incorporating the direct growth of monolayer h-BN on Au foil with the subsequent growth of MoS2. In such vertical stacks, the interactions between MoS2 and Au are diminished by the intervening h-BN layer, as evidenced by the appearance of photoluminescence in MoS2. The weakened interfacial interactions facilitate the transfer of the MoS2/h-BN stacks from Au to arbitrary substrates by an electrochemical bubbling method. Scanning tunneling microscope/spectroscopy characterization shows that the central h-BN layer partially blocks the metal-induced gap states in MoS2/h-BN/Au foils. The work offers insight into the synthesis, transfer, and device performance optimization of such vertically stacked heterostructures.

Temperature-Mediated Selective Growth of MoS2 /WS2 and WS2 /MoS2 Vertical Stacks on Au Foils for Direct Photocatalytic Applications.

A growth-temperature-mediated two-step chemical vapor deposition strategy is designed to synthesize MoS2 /WS2 and WS2 /MoS2 stacks on Au foils. Predominantly A-A stacked MoS2 /WS2 and A-B stacked WS2 /MoS2 are selectively achieved and confirmed. Relative enhancements or reductions in photocatalytic activities of MoS2 /WS2 or WS2 /MoS2 are observed under illumination, because the type-II band alignment enables directional electron flow from electrode to active site.