Intragrain impurity annihilation for highly efficient and stable perovskite solar cells.

Intragrain impurities can impart detrimental effects on the efficiency and stability of perovskite solar cells, but they are indiscernible to conventional characterizations and thus remain unexplored. Using in situ scanning transmission electron microscopy, we reveal that intragrain impurity nano-clusters inherited from either the solution synthesis or post-synthesis storage can revert to perovskites upon irradiation stimuli, leading to the counterintuitive amendment of crystalline grains. In conjunction with computational modelling, we atomically resolve crystallographic transformation modes for the annihilation of intragrain impurity nano-clusters and probe their impacts on optoelectronic properties. Such critical fundamental findings are translated for the device advancement. Adopting a scanning laser stimulus proven to heal intragrain impurity nano-clusters, we simultaneously boost the efficiency and stability of formamidinium-cesium perovskite solar cells, by virtual of improved optoelectronic properties and relaxed intra-crystal strain, respectively. This device engineering, inspired and guided by atomic-scale in situ microscopic imaging, presents a new prototype for solar cell advancement.

Synthesis of core@shell catalysts guided by Tammann temperature.

Designing high-performance thermal catalysts with stable catalytic sites is an important challenge. Conventional wisdom holds that strong metal-support interactions can benefit the catalyst performance, but there is a knowledge gap in generalizing this effect across different metals. Here, we have successfully developed a generalizable strong metal-support interaction strategy guided by Tammann temperatures of materials, enabling functional oxide encapsulation of transition metal nanocatalysts. As an illustrative example, Co@BaAl2O4 core@shell is synthesized and tracked in real-time through in-situ microscopy and spectroscopy, revealing an unconventional strong metal-support interaction encapsulation mechanism. Notably, Co@BaAl2O4 exhibits exceptional activity relative to previously reported core@shell catalysts, displaying excellent long-term stability during high-temperature chemical reactions and overcoming the durability and reusability limitations of conventional supported catalysts. This pioneering design and widely applicable approach has been validated to guide the encapsulation of various transition metal nanoparticles for environmental tolerance functionalities, offering great potential to advance energy, catalysis, and environmental fields.

Deciphering the ultra-high plasticity in metal monochalcogenides.

The quest for electronic devices that offer flexibility, wearability, durability and high performance has spotlighted two-dimensional (2D) van der Waals materials as potential next-generation semiconductors. Especially noteworthy is indium selenide, which has demonstrated surprising ultra-high plasticity. To deepen our understanding of this unusual plasticity in 2D van der Waals materials and to explore inorganic plastic semiconductors, we have conducted in-depth experimental and theoretical investigations on metal monochalcogenides (MX) and transition metal dichalcogenides (MX2). We have discovered a general plastic deformation mode in MX, which is facilitated by the synergetic effect of phase transitions, interlayer gliding and micro-cracks. This is in contrast to crystals with strong atomic bonding, such as metals and ceramics, where plasticity is primarily driven by dislocations, twinning or grain boundaries. The enhancement of gliding barriers prevents macroscopic fractures through a pinning effect after changes in stacking order. The discovery of ultra-high plasticity and the phase transition mechanism in 2D MX materials holds significant potential for the design and development of high-performance inorganic plastic semiconductors.

Extendable piezo/ferroelectricity in nonstoichiometric 2D transition metal dichalcogenides.

Engineering piezo/ferroelectricity in two-dimensional materials holds significant implications for advancing the manufacture of state-of-the-art multifunctional materials. The inborn nonstoichiometric propensity of two-dimensional transition metal dichalcogenides provides a spiffy ready-available solution for breaking inversion centrosymmetry, thereby conducing to circumvent size effect challenges in conventional perovskite oxide ferroelectrics. Here, we show the extendable and ubiquitous piezo/ferroelectricity within nonstoichiometric two-dimensional transition metal dichalcogenides that are predominantly centrosymmetric during standard stoichiometric cases. The emerged piezo/ferroelectric traits are aroused from the sliding of van der Waals layers and displacement of interlayer metal atoms triggered by the Frankel defects of heterogeneous interlayer native metal atom intercalation. We demonstrate two-dimensional chromium selenides nanogenerator and iron tellurides ferroelectric multilevel memristors as two representative applications. This innovative approach to engineering piezo/ferroelectricity in ultrathin transition metal dichalcogenides may provide a potential avenue to consolidate piezo/ferroelectricity with featured two-dimensional materials to fabricate multifunctional materials and distinguished multiferroic.

Unlocking Liquid Sulfur Chemistry for Fast-Charging Lithium-Sulfur Batteries.

A recent study of liquid sulfur produced in an electrochemical cell has prompted further investigation into regulating Li-S oxidation chemistry. In this research, we examined the liquid-to-solid sulfur transition dynamics by visually observing the electrochemical generation of sulfur on a graphene-based substrate. We investigated the charging of polysulfides at various current densities and discovered a quantitative correlation between the size and number density of liquid sulfur droplets and the applied current. However, the areal capacities exhibited less sensitivity. This observation offers valuable insights for designing fast-charging sulfur cathodes. By incorporating liquid sulfur into Li-S batteries with a high sulfur loading of 4.2 mg cm-2, the capacity retention can reach ∼100%, even when increasing the rate from 0.1 to 3 C. This study contributes to a better understanding of the kinetics involved in the liquid-solid sulfur growth in Li-S chemistry and presents viable strategies for optimizing fast-charging operations.

Uncooled Mid-Infrared Sensing Enabled by Chip-Integrated Low-Temperature-Grown 2D PdTe2 Dirac Semimetal.

Next-generation mid-infrared (MIR) imaging chips demand free-cooling capability and high-level integration. The rising two-dimensional (2D) semimetals with excellent infrared (IR) photoresponses are compliant with these requirements. However, challenges remain in scalable growth and substrate-dependence for on-chip integration. Here, we demonstrate the inch-level 2D palladium ditelluride (PdTe2) Dirac semimetal using a low-temperature self-stitched epitaxy (SSE) approach. The low formation energy between two precursors facilitates low-temperature multiple-point nucleation (∼300 °C), growing up, and merging, resulting in self-stitching of PdTe2 domains into a continuous film, which is highly compatible with back-end-of-line (BEOL) technology. The uncooled on-chip PdTe2/Si Schottky junction-based photodetector exhibits an ultrabroadband photoresponse of up to 10.6 µm with a large specific detectivity. Furthermore, the highly integrated device array demonstrates high-resolution room-temperature imaging capability, and the device can serve as an optical data receiver for IR optical communication. This study paves the way toward low-temperature growth of 2D semimetals for uncooled MIR sensing.

Phase-controllable large-area two-dimensional In2Se3 and ferroelectric heterophase junction.

Memory transistors based on two-dimensional (2D) ferroelectric semiconductors are intriguing for next-generation in-memory computing. To date, several 2D ferroelectric materials have been unveiled, among which 2D In2Se3 is the most promising, as all the paraelectric (ß), ferroelectric (α) and antiferroelectric (ß') phases are found in 2D quintuple layers. However, the large-scale synthesis of 2D In2Se3 films with the desired phase is still absent, and the stability for each phase remains obscure. Here we show the successful growth of centimetre-scale 2D ß-In2Se3 film by chemical vapour deposition including distinct centimetre-scale 2D ß'-In2Se3 film by an InSe precursor. We also demonstrate that as-grown 2D ß'-In2Se3 films on mica substrates can be delaminated or transferred onto flexible or uneven substrates, yielding α-In2Se3 films through a complete phase transition. Thus, a full spectrum of paraelectric, ferroelectric and antiferroelectric 2D films can be readily obtained by means of the correlated polymorphism in 2D In2Se3, enabling 2D memory transistors with high electron mobility, and polarizable ß'-α In2Se3 heterophase junctions with improved non-volatile memory performance.

Phase and polarization modulation in two-dimensional In2Se3 via in situ transmission electron microscopy.

Phase transitions in two-dimensional (2D) materials promise reversible modulation of material physical and chemical properties in a wide range of applications. 2D van der Waals layered In2Se3 with bistable out-of-plane ferroelectric (FE) α phase and antiferroelectric (AFE) ß' phase is particularly attractive for its electronic applications. However, reversible phase transition in 2D In2Se3 remains challenging. Here, we introduce two factors, dimension (thickness) and strain, which can effectively modulate the phases of 2D In2Se3. We achieve reversible AFE and out-of-plane FE phase transition in 2D In2Se3 by delicate strain control inside a transmission electron microscope. In addition, the polarizations in 2D FE In2Se3 can also be manipulated in situ at the nanometer-sized contacts, rendering remarkable memristive behavior. Our in situ transmission electron microscopy (TEM) work paves a previously unidentified way for manipulating the correlated FE phases and highlights the great potentials of 2D ferroelectrics for nanoelectromechanical and memory device applications.

Stable Liquid-Sulfur Generation on Transition-Metal Dichalcogenides toward Low-Temperature Lithium-Sulfur Batteries.

The electrochemical formation of liquid sulfur at room temperature on the basal plane of MoS2 has attracted much attention due to the high areal capacity and rapid kinetics of lithium-liquid sulfur chemistry. However, the liquid sulfur is converted to the solid phase once it contacts the solid sulfur crystals generated from the edge of MoS2. Thus, stable liquid sulfur cannot be formed on the entire MoS2 surface. Herein, we report entire liquid sulfur generation on hydrogen-annealed MoS2 (H2-MoS2), even under harsh conditions of large overpotentials and low working temperatures. The origins of the solely liquid sulfur formation are revealed to be the weakened interactions between H2-MoS2 and sulfur molecules and the decreased electrical polarization on the edges of the H2-MoS2. Progressive nucleation and droplet-merging growth behaviors are observed during the sulfur formation on H2-MoS2, signifying high areal capacities by releasing active H2-MoS2 surfaces. To demonstrate the universality of this strategy, other transition-metal dichalcogenides (TMDs) annealed in hydrogen also exhibit similar sulfur growth behaviors. Furthermore, the H2 annealing treatment can induce sulfur vacancies on the basal plane and partial oxidation on the edge of TMDs, which facilitates liquid sulfur formation. Finally, liquid sulfur can be generated on H2-MoS2 flakes at an ultralow temperature of -50 °C, which provides a possible development of low-temperature lithium-sulfur batteries. This work demonstrates the potential of a pure liquid sulfur-lithium electrochemical system using functionalized two-dimensional materials.

Sub-Nanometer Electron Beam Phase Patterning in 2D Materials.

Phase patterning in polymorphic two-dimensional (2D) materials offers diverse properties that extend beyond what their pristine structures can achieve. If precisely controllable, phase transitions can bring exciting new applications for nanometer-scale devices and ultra-large-scale integrations. Here, the focused electron beam is capable of triggering the phase transition from the semiconducting T'' phase to metallic T' and T phases in 2D rhenium disulfide (ReS2 ) and rhenium diselenide (ReSe2 ) monolayers, rendering ultra-precise phase patterning technique even in sub-nanometer scale is found. Based on knock-on effects and strain analysis, the phase transition mechanism on the created atomic vacancies and the introduced substantial in-plane compressive strain in 2D layers are clarified. This in situ high-resolution scanning transmission electron microscopy (STEM) and in situ electrical characterizations agree well with the density functional theory (DFT) calculation results for the atomic structures, electronic properties, and phase transition mechanisms. Grain boundary engineering and electrical contact engineering in 2D are thus developed based on this patterning technique. The patterning method exhibits great potential in ultra-precise electron beam lithography as a scalable top-down manufacturing method for future atomic-scale devices.

Topological phase change transistors based on tellurium Weyl semiconductor.

Modern electronics demand transistors with extremely high performance and energy efficiency. Charge-based transistors with conventional semiconductors experience substantial heat dissipation because of carrier scattering. Here, we demonstrate low-loss topological phase change transistors (TPCTs) based on tellurium, a Weyl semiconductor. By modulating the energy separation between the Fermi level and the Weyl point of tellurium through electrostatic gate modulation, the device exhibits topological phase change between Weyl (Chern number ≠ 0) and conventional (Chern number = 0) semiconductors. In the Weyl ON state, the device has low-loss transport characteristics due to the global topology of gauge fields against external perturbations; the OFF state exhibits trivial charge transport in the conventional phase by moving the Fermi level into the bandgap. The TPCTs show a high ON/OFF ratio (108) at low operation voltage (≤2 volts) and high ON-state conductance (39 mS/µm). Our studies provide alternative strategies for realizing ultralow power electronics.

Ferroelectricity in untwisted heterobilayers of transition metal dichalcogenides.

Two-dimensional materials with out-of-plane (OOP) ferroelectric and piezoelectric properties are highly desirable for the realization of ultrathin ferro- and piezoelectronic devices. We demonstrate unexpected OOP ferroelectricity and piezoelectricity in untwisted, commensurate, and epitaxial MoS2/WS2 heterobilayers synthesized by scalable one-step chemical vapor deposition. We show d33 piezoelectric constants of 1.95 to 2.09 picometers per volt that are larger than the natural OOP piezoelectric constant of monolayer In2Se3 by a factor of ~6. We demonstrate the modulation of tunneling current by about three orders of magnitude in ferroelectric tunnel junction devices by changing the polarization state of MoS2/WS2 heterobilayers. Our results are consistent with density functional theory, which shows that both symmetry breaking and interlayer sliding give rise to the unexpected properties without the need for invoking twist angles or moiré domains.

Atomically Resolved Electrically Active Intragrain Interfaces in Perovskite Semiconductors.

Deciphering the atomic and electronic structures of interfaces is key to developing state-of-the-art perovskite semiconductors. However, conventional characterization techniques have limited previous studies mainly to grain-boundary interfaces, whereas the intragrain-interface microstructures and their electronic properties have been much less revealed. Herein using scanning transmission electron microscopy, we resolved the atomic-scale structural information on three prototypical intragrain interfaces, unraveling intriguing features clearly different from those from previous observations based on standalone films or nanomaterial samples. These intragrain interfaces include composition boundaries formed by heterogeneous ion distribution, stacking faults resulted from wrongly stacked crystal planes, and symmetrical twinning boundaries. The atomic-scale imaging of these intragrain interfaces enables us to build unequivocal models for the ab initio calculation of electronic properties. Our results suggest that these structure interfaces are generally electronically benign, whereas their dynamic interaction with point defects can still evoke detrimental effects. This work paves the way toward a more complete fundamental understanding of the microscopic structure-property-performance relationship in metal halide perovskites.

Modulating Built-In Electric Field via Variable Oxygen Affinity for Robust Hydrogen Evolution Reaction in Neutral Media.

Work function strongly impacts the surficial charge distribution, especially for metal-support electrocatalysts when a built-in electric field (BEF) is constructed. Therefore, studying the correlation between work function and BEF is crucial for understanding the intrinsic reaction mechanism. Herein, we present a Pt@CoOx electrocatalyst with a large work function difference (ΔΦ) and strong BEF, which shows outstanding hydrogen evolution activity in a neutral medium with a 4.5-fold mass activity higher than 20 % Pt/C. Both experimental and theoretical results confirm the interfacial charge redistribution induced by the strong BEF, thus subtly optimizing hydrogen and hydroxide adsorption energy. This work not only provides fresh insights into the neutral hydrogen evolution mechanism but also proposes new design principles toward efficient electrocatalysts for hydrogen production in a neutral medium.

Unlocking surface octahedral tilt in two-dimensional Ruddlesden-Popper perovskites.

Molecularly soft organic-inorganic hybrid perovskites are susceptible to dynamic instabilities of the lattice called octahedral tilt, which directly impacts their carrier transport and exciton-phonon coupling. Although the structural phase transitions associated with octahedral tilt has been extensively studied in 3D hybrid halide perovskites, its impact in hybrid 2D perovskites is not well understood. Here, we used scanning tunneling microscopy (STM) to directly visualize surface octahedral tilt in freshly exfoliated 2D Ruddlesden-Popper perovskites (RPPs) across the homologous series, whereby the steric hindrance imposed by long organic cations is unlocked by exfoliation. The experimentally determined octahedral tilts from n = 1 to n = 4 RPPs from STM images are found to agree very well with out-of-plane surface octahedral tilts predicted by density functional theory calculations. The surface-enhanced octahedral tilt is correlated to excitonic redshift observed in photoluminescence (PL), and it enhances inversion asymmetry normal to the direction of quantum well and promotes Rashba spin splitting for n > 1.

Self-supporting CoP-C nanosheet arrays derived from a metal-organic framework as synergistic catalysts for efficient water splitting.

Here, a new strategy that combines accessible active sites and multiphase synergy in a simple process is developed for constructing bifunctional electrocatalysts toward overall water splitting. By using metal-organic framework (MOF) nanosheets hydrothermally grown on pre-oxidized nickel foam (denoted by Co2(OH)2(BDC)/NiO/NF) as a precursor, two novel heterogeneous nanosheet arrays including a cobalt phosphide nanoparticle embedded carbon nanotube nanosheet array supported by phosphorized nickel foam (denoted by CoP-CNT/Ni2P/NF) and a cobalt phosphide nanorod decorated carbon nanosheet array supported by oxidized nickel foam (denoted by CoP-C/NiO/NF) are prepared. Both were confirmed to be highly efficient for hydrogen and oxygen evolution reactions. In particular, CoP-C/NiO/NF exhibits higher catalytic activity toward the hydrogen evolution reaction (η100 = -131 mV), promoted by the synergy of oxidized nickel foam. CoP-CNT/Ni2P/NF performs better in the oxygen evolution reaction (η50 = 301 mV), benefiting mainly from its improved electrochemically active surface area. The two catalysts match well in overall water splitting with satisfactory activity (η10 = 1.57 V) and stability when directly applied in a two-electrode cell. This method will bring new inspiration to maximize the electrocatalytic efficiency of MOF-derived catalysts for energy conversion applications in the future.

Ag2S monolayer: an ultrasoft inorganic Lieb lattice.

Lieb lattice, a two-dimensional edge-centered square lattice, has attracted considerable interest due to its exotic electronic and topological properties. Although various optical and photonic Lieb lattices have been experimentally demonstrated, it remains challenging for an electronic Lieb lattice to be realized in real material systems. Here, based on first-principles calculations and tight-binding modeling, a silver sulfide (Ag2S) monolayer is reported as a long-sought-after inorganic electronic Lieb lattice. This Lieb-lattice Ag2S is further found to be ultrasoft, which enables its electronic properties and topological states near the Fermi level to be finely tuned, as evidenced by the strain-induced topologically non-trivial edge states near the valence band edge. These results not only provide an ideal platform to further explore and harvest interesting quantum properties but also pave a way to pursue other inorganic electronic Lieb lattices in a broader material domain.

Large-scale growth of few-layer two-dimensional black phosphorus.

Two-dimensional materials provide opportunities for developing semiconductor applications at atomistic thickness to break the limits of silicon technology. Black phosphorus (BP), as a layered semiconductor with controllable bandgap and high carrier mobility, is one of the most promising candidates for transistor devices at atomistic thickness1-4. However, the lack of large-scale growth greatly hinders its development in devices. Here, we report the growth of ultrathin BP on the centimetre scale through pulsed laser deposition. The unique plasma-activated region induced by laser ablation provides highly desirable conditions for BP cluster formation and transportation5,6, facilitating growth. Furthermore, we fabricated large-scale field-effect transistor arrays on BP films, yielding appealing hole mobility of up to 213 and 617 cm2 V-1 s-1 at 295 and 250 K, respectively. Our results pave the way for further developing BP-based wafer-scale devices with potential applications in the information industry.

Anomalous fracture in two-dimensional rhenium disulfide.

Low-dimensional materials usually exhibit mechanical properties from those of their bulk counterparts. Here, we show in two-dimensional (2D) rhenium disulfide (ReS2) that the fracture processes are dominated by a variety of previously unidentified phenomena, which are not present in bulk materials. Through direct transmission electron microscopy observations at the atomic scale, the structures close to the brittle crack tip zones are clearly revealed. Notably, the lattice reconstructions initiated at the postcrack edges can impose additional strain on the crack tips, modifying the fracture toughness of this material. Moreover, the monatomic thickness allows the restacking of postcrack edges in the shear strain-dominated cracks, which is potentially useful for the rational design of 2D stacking contacts in atomic width. Our studies provide critical insights into the atomistic processes of fracture and unveil the origin of the brittleness in the 2D materials.

Van der Waals Epitaxial Growth of Mosaic-Like 2D Platinum Ditelluride Layers for Room-Temperature Mid-Infrared Photodetection up to 10.6 µm.

Mid-infrared (MIR) photodetection, covering diverse molecular vibrational regions and atmospheric transmission windows, is vital to civil and military purposes. Versatile use of MIR photodetectors is commonly dominated by HgCdTe alloys, InSb, and quantum superlattices, which are limited by strict operation demands, high-cost, and environmental toxicity. Despite the rapid advances of black phosphorus (BP)-based MIR photodetectors, these are subject to poor stability and large-area integration difficulty. Here, the van der Waals (vdW) epitaxial growth of a wafer-scale 2D platinum ditelluride (PtTe2 ) layer is reported via a simple tellurium-vapor transformation approach. The 2D PtTe2 layer possesses a unique mosaic-like crystal structure consisting of single-crystal domains with highly preferential [001] orientation along the normal direction, reducing the influence of interface defects and ensuring efficient out-of-plane carrier transportation. This characteristic, combined with the wide absorption of PtTe2 and well-designed vertical device architecture, makes the PtTe2 /Si Schottky junction photodetector capable of sensing ultra-broadband light of up to 10.6 µm with a high specific detectivity. Also, the photodetector exhibits an excellent room-temperature infrared-imaging capability. This approach provides a new design concept for high-performance, room-temperature MIR photodetection based on 2D layered materials.