Xingxing Zhang

2D Janus Monolayers

In article number 2106222, Mete Atature, Sefaattin Tongay, and co-workers use an in situ deterministic plasma technique to enable the synthesis of high-quality excitonic grade 2D SWSe, SMoSe, and other 2D Janus layers. Integrated spectrometers allow for the collection of structural, optical, and phononic properties during the growth. Through time-resolved studies, the team offers the first insights into the growth process and minute control provides the first with excitonic grade Janus layers.

A controllable Fe doping strategy is developed in centimeter-sized monolayer MoS₂ films with ultralow contact resistance. Excellent device performance featured with ultrahigh electron mobility and on/off current ratio is achieved, thanks to the ultralow electron effective mass. Unidirectional Fe-MoS₂ domains are prepared on 2 in. commercial c-plane sapphire, suggesting the feasibility of synthesizing wafer-scale single-crystal semiconductors with outstanding device performance.

Abstract

2D semiconductors are emerging as plausible candidates for next-generation “More-than-Moore” nanoelectronics to tackle the scaling challenge of transistors. Wafer-scale 2D semiconductors, such as MoS₂ and WS₂, have been successfully synthesized recently; nevertheless, the absence of effective doping technology fundamentally results in energy barriers and high contact resistances at the metal–semiconductor interfaces, and thus restrict their practical applications. Herein, a controllable doping strategy in centimeter-sized monolayer MoS₂ films is developed to address this critical issue and boost the device performance. The ultralow contact resistance and perfect Ohmic contact with metal electrodes are uncovered in monolayer Fe-doped MoS₂, which deliver excellent device performance featured with ultrahigh electron mobility and outstanding on/off current ratio. Impurity scattering is suppressed significantly thanks to the ultralow electron effective mass and appropriate doping site. Particularly, unidirectionally aligned monolayer Fe-doped MoS₂ domains are prepared on 2 in. commercial c-plane sapphire, suggesting the feasibility of synthesizing wafer-scale 2D single-crystal semiconductors with outstanding device performance. This work presents the potential of high-performance monolayer transistors and enables further device downscaling and extension of Moore's law.

By structurally-functionally modularizing Cu₂Te with multiple point defects, mosaic nanograins, and crystal-amorphicity duality, high thermoelectric performance is obtained near the Mott–Ioffe–Regel limit. This work presents a case of paradigm shift from the band edge to the mobility edge in thermoelectric materials research.

Abstract

To date, thermoelectric materials research stays focused on optimizing the material's band edge details and disfavors low mobility. Here, the paradigm is shifted from the band edge to the mobility edge, exploring high thermoelectricity near the border of band conduction and hopping. Through coalloying iodine and sulfur, the plain crystal structure is modularized of liquid-like thermoelectric material Cu₂Te with mosaic nanograins and the highly size mismatched S/Te sublattice that chemically quenches the Cu sublattice and drives the electronic states from itinerant to localized. A state-of-the-art figure of merit of 1.4 is obtained at 850 K for Cu₂(S_0.4I_0.1Te_0.5); and remarkably, it is achieved near the Mott–Ioffe–Regel limit unlike mainstream thermoelectric materials that are band conductors. Broadly, pairing structural modularization with the high performance near the Mott–Ioffe–Regel limit paves an important new path towards the rational design of high-performance thermoelectric materials.

Highly oriented and long graphene folds are spontaneously formed in chemical-vapor-deposition (CVD)-grown single-crystal graphene film on a Cu(111) foil surface, and this is triggered by the formation of highly ordered bunched Cu steps under the graphene. Such folds are fractured into “back-and-forth” subfold patterns at bunched Cu step edge regions, with cracks propagating along zigzag or armchair directions.

Abstract

A single-crystal graphene film grown on a Cu(111) foil by chemical vapor deposition (CVD) has ribbon-like fold structures. These graphene folds are highly oriented and essentially parallel to each other. Cu surface steps underneath the graphene are along the <110> and <211> directions, leading to the formation of the arrays of folds. The folds in the single-layer graphene (SLG) are not continuous but break up into alternating patterns. A “joint” (an AB-stacked bilayer graphene) region connects two neighboring alternating regions, and the breaks are always along zigzag or armchair directions. Folds formed in bilayer or few-layer graphene are continuous with no breaks. Molecular dynamics simulations show that SLG suffers a significantly higher compressive stress compared to bilayer graphene when both are under the same compression, thus leading to the rupture of SLG in these fold regions. The fracture strength of a CVD-grown single-crystal SLG film is simulated to be about 70 GPa. This study greatly deepens the understanding of the mechanics of CVD-grown single-crystal graphene and such folds, and sheds light on the fabrication of various graphene origami/kirigami structures by substrate engineering. Such oriented folds can be used in a variety of further studies.

2D materials have spectacular electrical and optical properties but their device fabrication and integration are arduous and expensive. Optical modification methods offer less detrimental processes that can often be done in one optical setup, sometimes even simultaneously with one another, saving time and money. Additionally, the extreme locality of lasers can be utilized in designable laser direct writing processes.

Abstract

2D materials are under extensive research due to their remarkable properties suitable for various optoelectronic, photonic, and biological applications, yet their conventional fabrication methods are typically harsh and cost-ineffective. Optical modification is demonstrated as an effective and scalable method for accurate and local in situ engineering and patterning of 2D materials in ambient conditions. This review focuses on the state of the art of optical modification of 2D materials and their applications. Perspectives for future developments in this field are also discussed, including novel laser tools, new optical modification strategies, and their emerging applications in quantum technologies and biotechnologies.

By adopting a novel plasma-assisted doping-triggered phase-transition engineering, stabilized P-doped metallic phase selenide molybdenum (MoSe₂) nanoflower composites (P-1T-MoSe₂ NFs) with expanded interlayer spacing, metallic electronic conductivity, facilitated Na⁺ adsorption, and reduced Na⁺ diffusion barrier are fabricated for high-performance sodium storage. The underlying mechanism analysis provides informative guide for designing advanced materials for next-generation energy-storage systems.

Abstract

Metallic-phase selenide molybdenum (1T-MoSe₂) has become a rising star for sodium storage in comparison with its semiconductor phase (2H-MoSe₂) owing to the intrinsic metallic electronic conductivity and unimpeded Na⁺ diffusion structure. However, the thermodynamically unstable nature of 1T phase renders it an unprecedented challenge to realize its phase control and stabilization. Herein, a plasma-assisted P-doping-triggered phase-transition engineering is proposed to synthesize stabilized P-doped 1T phase MoSe₂ nanoflower composites (P-1T-MoSe₂ NFs). Mechanism analysis reveals significantly decreased phase-transition energy barriers of the plasma-induced Se-vacancy-rich MoSe₂ from 2H to 1T owing to its low crystallinity and reduced structure stability. The vacancy-rich structure promotes highly concentrated P doping, which manipulates the electronic structure of the MoSe₂ and urges its phase transition, acquiring a high transition efficiency of 91% accompanied with ultrahigh phase stability. As a result, the P-1T-MoSe₂ NFs deliver an exceptional high reversible capacity of 510.8 mAh g⁻¹ at 50 mA g⁻¹ with no capacity fading over 1000 cycles at 5000 mA g⁻¹ for sodium storage. The underlying mechanism of this phase-transition engineering verified by profound analysis provides informative guide for designing advanced materials for next-generation energy-storage systems.

Ultra-thin GeSe/WS₂ vertical heterojunctions are fabricated by a site-controllable transfer method and subsequent GeSe layer thinning. Excellent rectifying behavior and photoresponse characteristics are demonstrated, including high on-off ratio (10³), high photoresponsivity (1.1 A W⁻¹), considerable specific detectivity (1.3×10¹⁰ Jones), and high external quantum efficiency (214.8 %), which reveals the great potential of the GeSe/WS₂ vertical heterojunction for future optoelectronic applications.

Abstract

Heterostructural engineering of atomically thin 2D materials offers an exciting opportunity to fabricate atomically sharp interfaces for optoelectronic devices. Herein, GeSe/WS₂ heterojunction devices composed of 2D WS₂ (n-type) and few-layer GeSe (p-type), are fabricated by transferring mechanically exfoliated GeSe to chemical vapor deposition (CVD)-grown WS₂. Excellent rectification behavior is observed from the I−V characteristics of the GeSe/WS₂ heterojunction devices. The reverse photocurrent increases more rapidly than the forward photocurrent under a 635 nm laser illumination, indicating an effective separation of the photogenerated carriers under a minus bias. A large photocurrent on-off ratio of 10³ at −5 V bias, a high responsivity (R _λ) of 1.1 A W⁻¹, a considerable specific detectivity (D*) of 1.3×10¹⁰ Jones, and a high external quantum efficiency (EQE) of 214.8%, are obtained. Owing to the large built-in potential of the heterojunction, efficient charge transfer is achieved from the abrupt interfaces even though vastly different materials are used in the van der Waals (vdW) heterostructure. A convenient route is demonstrated for the preparation of ultra-thin GeSe/WS₂ vdW heterojunctions. The results reveal great potential of the present GeSe/WS₂ vertical heterojunction for future applications in optoelectronics.

To overcome the high exciton binding energy, the efficiency of WSe₂/MoS₂ heterojunction phototransistors is improved by forming periodic arrayed nanopore structures for bandgap engineering. In particular, enhanced carrier lifetime and increased photocurrent are observed with a modulated charge carrier balance of the WSe₂/MoS₂ heterojunction phototransistor in the active area upon illumination.

Abstract

While 2D transition metal dichalcogenides (TMDs) are promising building blocks for various optoelectronic applications, limitations remain for multilayered TMD-based photodetectors: an indirect bandgap and a short carrier lifetime by strongly bound excitons. Accordingly, multilayered TMDs with a direct bandgap and an enhanced carrier lifetime are required for the development of various optoelectronic devices. Here, periodically arrayed nanopore structures (PANS) are proposed for improving the efficiency of multilayered p-WSe₂/n-MoS₂ phototransistors. Density functional theory calculations as well as photoluminescence and time-resolved photoluminescence measurements are performed to characterize the photodetector figures of merit of multilayered p-WSe₂/n-MoS₂ heterostructures with PANS. The characteristics of the heterojunction devices with PANS reveal an enhanced responsivity and detectivity measured under 405 nm laser excitation, which at 1.7 × 10⁴ A W⁻¹ and 1.7 × 10¹³ Jones are almost two orders of magnitude higher than those of pristine devices, 3.6 × 10² A W⁻¹ and 3.6 × 10¹¹ Jones, respectively. Such enhanced optical properties of WSe₂/MoS₂ heterojunctions with PANS represent a significant step toward next-generation optoelectronic applications.

The very recent advances in controlled production of two-dimension transition metal carbides and nitrides (MXenes) crystals by chemical vapor deposition, included several kinds of MXenes crystals and MXenes heterostructures.

Abstract

As a novel family of 2D materials, MXenes have drawn intensive interests owing to its fascinating property profile. The ability to grow high-quality MXenes in a controllable way would in turn further promote the development of fabrication techniques and expand wide advanced applications. Then 2D MXenes crystals are highly desirable and many approaches have been explored to realize the mass production. Chemical vapor deposition (CVD) provides compelling benefits over other alternatives in controllability, uniformity and scalability. In this review, the recent advances in growth of MXenes crystals by CVD method will comprehensively present. Several typical kinds of MXenes crystals are demonstrated to be fabricated with a precise control in terms of size, morphology and thickness. Further, a series of MXenes heterostructures are constructed including vertical and lateral spatial orientations. Then, the properties and applications of MXenes crystals are exhibited, of which superconductivity and electrochemical catalysts will be mainly emphasized. Finally, the authors put forward views on the future development in the synthesis of MXenes. With continuous efforts devoted, a bright future of MXenes crystals prepared by CVD is expected.

2D amorphous MoS₃-on-rGO heterostructure is constructed via an isotropic growth process for beyond-lithium-ion batteries (Na⁺, K⁺, and Zn²⁺) with outstanding electrochemical performance and superior cyclic stability. The amorphous MoS₃-on-rGO exhibits low strain and fast reaction kinetics during cycling.

Abstract

Beyond-lithium-ion storage devices are promising alternatives to lithium-ion storage devices for low-cost and large-scale applications. Nowadays, the most of high-capacity electrodes are crystal materials. However, these crystal materials with intrinsic anisotropy feature generally suffer from lattice strain and structure pulverization during the electrochemical process. Herein, a 2D heterostructure of amorphous molybdenum sulfide (MoS₃) on reduced graphene surface (denoted as MoS₃-on-rGO), which exhibits low strain and fast reaction kinetics for beyond-lithium-ions (Na⁺, K⁺, Zn²⁺) storage is demonstrated. Benefiting from the low volume expansion and small sodiation strain of the MoS₃-on-rGO, it displays ultralong cycling performance of 40 000 cycles at 10 A g⁻¹ for sodium-ion batteries. Furthermore, the as-constructed 2D heterostructure also delivers superior electrochemical performance when used in Na⁺ full batteries, solid-state sodium batteries, K⁺ batteries, Zn²⁺ batteries and hybrid supercapacitors, demonstrating its excellent application prospect.

A Na₂Te (K₂Te) protection layer with high ionic conductivity, low electronic conductivity, and high mechanical stability on a Na (K) metal anode is designed and prepared. The Na@Na₂Te (K@K₂Te) realizes excellent electrochemi cal performance for sodium-metal (potassiummetal) batteries of both symmetrical cells and full batteries in widely used carbonate electrolytes.

Abstract

The sodium (potassium)-metal anodes combine low-cost, high theoretical capacity, and high energy density, demonstrating promising application in sodium (potassium)-metal batteries. However, the dendrites’ growth on the surface of Na (K) has impeded their practical application. Herein, density functional theory (DFT) results predict Na₂Te/K₂Te is beneficial for Na⁺/K⁺ transport and can effectively suppress the formation of the dendrites because of low Na⁺/K⁺ migration energy barrier and ultrahigh Na⁺/K⁺ diffusion coefficient of 3.7 × 10⁻¹⁰ cm² s⁻¹/1.6 × 10⁻¹⁰ cm² s⁻¹ (300 K), respectively. Then a Na₂Te protection layer is prepared by directly painting the nanosized Te powder onto the sodium-metal surface. The Na@Na₂Te anode can last for 700 h in low-cost carbonate electrolytes (1 mA cm⁻², 1 mAh cm⁻²), and the corresponding Na₃V₂ (PO₄)₃//Na@Na₂Te full cell exhibits high energy density of 223 Wh kg⁻¹ at an unprecedented power density of 29687 W kg⁻¹ as well as an ultrahigh capacity retention of 93% after 3000 cycles at 20 C. Besides, the K@K₂Te-based potassium-metal full battery also demonstrates high power density of 20 577 W kg⁻¹ with energy density of 154 Wh kg⁻¹. This work opens up a new and promising avenue to stabilize sodium (potassium)-metal anodes with simple and low-cost interfacial layers.

Stable doping by modulating the thickness is realized in 2D layered materials. The decreasing thickness-induced lattice deformation makes defects in PtSSe transit from Pt vacancies in thicker PtSSe to anion vacancies in thinner PtSSe, which leads to controllable doping. Thickness-modulated doping shows great potential in novel electronics and optoelectronics, especially including diodes and photodetectors.

Abstract

For each generation of semiconductors, the issue of doping techniques is always placed at the top of the priority list since it determines whether a material can be used in the electronic and optoelectronic industry or not. When it comes to 2D materials, significant challenges have been found in controllably doping 2D semiconductors into p- or n-type, let alone developing a continuous control of this process. Here, a unique self-modulated doping characteristic in 2D layered materials such as PtSSe, PtS_0.8Se_1.2, PdSe₂, and WSe₂ is reported. The varying number of vertically stacked-monolayers is the critical factor for controllably tuning the same material from p-type to intrinsic, and to n-type doping. Importantly, it is found that the thickness-induced lattice deformation makes defects in PtSSe transit from Pt vacancies to anion vacancies based on dynamic and thermodynamic analyses, which leads to p- and n-type conductance, respectively. By thickness-modulated doping, WSe₂ diode exhibits a high rectification ratio of 4400 and a large open-circuit voltage of 0.38 V. Meanwhile, the PtSSe detector overcomes the shortcoming of large dark-current in narrow-bandgap optoelectronic devices. All these findings provide a brand-new perspective for fundamental scientific studies and applications.

Polycrystalline SnSe is prepared from surfactant-free particles prepared in solution. Ionic species adsorbed on the surface compensate for the particles′ charge and remain during processing. The adsorbates affect the nano/microstructure of the final material and its transport properties. In the case of sodium, it remains as a dopant and forms secondary phases that induce energy-filtering effects.

Abstract

Solution synthesis of particles emerges as an alternative to prepare thermoelectric materials with less demanding processing conditions than conventional solid-state synthetic methods. However, solution synthesis generally involves the presence of additional molecules or ions belonging to the precursors or added to enable solubility and/or regulate nucleation and growth. These molecules or ions can end up in the particles as surface adsorbates and interfere in the material properties. This work demonstrates that ionic adsorbates, in particular Na⁺ ions, are electrostatically adsorbed in SnSe particles synthesized in water and play a crucial role not only in directing the material nano/microstructure but also in determining the transport properties of the consolidated material. In dense pellets prepared by sintering SnSe particles, Na remains within the crystal lattice as dopant, in dislocations, precipitates, and forming grain boundary complexions. These results highlight the importance of considering all the possible unintentional impurities to establish proper structure–property relationships and control material properties in solution-processed thermoelectric materials.

There is a strong interest in developing phase-controlled metal nanocrystals to enhance their catalytic activity. In this work, template-directed growth is used to synthesize Ru@Pd nanocrystals with the Pd shells in either an fcc or hcp phase, and their catalytic properties are compared.

Abstract

A relatively unexplored aspect of noble-metal nanomaterials is polymorphism, or their ability to crystallize in different crystal phases. Here, a method is reported for the facile synthesis of Ru@Pd core–shell nanocrystals featuring polymorphism, with the core made of hexagonally close-packed (hcp)-Ru while the Pd shell takes either an hcp or face-centered cubic (fcc) phase. The polymorphism shows a dependence on the shell thickness, with shells thinner than ≈1.4 nm taking the hcp phase whereas the thicker ones revert to fcc. The injection rate provides an experimental knob for controlling the phase, with one-shot and drop-wise injection of the Pd precursor corresponding to fcc-Pd and hcp-Pd shells, respectively. When these nanocrystals are tested as catalysts toward formic acid oxidation, the Ru@Pd _hcp nanocrystals outperform Ru@Pd _fcc in terms of both specific activity and peak potential. Density functional theory calculations are also performed to elucidate the origin of this performance enhancement.

Gradient alloyed CdSe/Zn _x Cd_{1− x} S/ZnS and CdSe/CdS/ZnS core/shell/shell quantum-rod downconverters showing high-efficacy light-emitting diodes (LEDs) covering a wide color gamut are reported. The engineered shell thickness suppresses energy transfer and Auger recombination and maintains the high quantum yield in the solid-state (81%). These LEDs show luminous efficacy of 149 lm W⁻¹ and wide color gamut.

Abstract

Efficient white light-emitting diodes (LEDs) with an efficacy of 200 lm W⁻¹ are much desirable for lighting and displays. The phosphor-based LEDs in use today for display applications offer poor color saturation. Intensive efforts have been made to replace the phosphor with quantum-dot-based downconverters, but the efficiency and stability of these devices are still in their infancy. Quantum rods (QRs), nanoparticles with an elongated shape, show superior properties such as relatively larger Stokes shifts, polarized emission, and high light out-coupling efficiency in the solid-state. However, these QRs usually suffer from poor optical quality for PL wavelengths < 550 nm. Herein, a gradient alloyed CdSe/Zn _x Cd_{1− x} S/ZnS and CdSe/CdS/ZnS core/shell/shell QR downconverters showing high efficacy LEDs covering a wide color gamut are reported. These QRs show high stability and a precisely tunable photoluminescence peak. The engineered shell thickness suppresses energy transfer and thus maintains the high quantum yield in the solid-state (81%). These QR-based LEDs attain an efficacy of 149 lm W⁻¹ (@10mA) and wide color gamut (118% NTSC), which is exceedingly higher than state-of-the-art quantum dots and phosphor-based on-chip LEDs.

Bottom-Up Fabrication

In article number 2103708, Joon-Ho Oh, Ka-Hyun Kim, and co-workers present the unprecedented fabrication of crystalline silicon wafers and solar cells by a fully bottom-up growth method without wasting substrate, in contrast to conventional technologies that sacrifice large amounts of raw material. A plasma-assisted epitaxially grown silicon seed layer with a self-organized nanogap is key for the realization of the fully bottom-up process. The results represent a technological breakthrough in advanced silicon microelectronics and photovoltaics.

A large tunneling magnetoresistance is observed in all van der Waals spin valves based on a Fe₃GeTe₂/InSe/Fe₃GeTe₂ (ferromagnet/semiconductor/ferromagnet) heterojunction. Devices with the same thickness of the InSe layer reveal either a tunneling or a metallic transport behavior due to pinholes. This finding suggests opportunities for further development of this spin valve as a platform for nonvolatile memories and spin-logic applications.

Abstract

2D layered chalcogenide semiconductors have been proposed as a promising class of materials for low-dimensional electronic, optoelectronic, and spintronic devices. Here, all-2D van der Waals vertical spin-valve devices, that combine the 2D layered semiconductor InSe as a spacer with the 2D layered ferromagnetic metal Fe₃GeTe₂ as spin injection and detection electrodes, are reported. Two distinct transport behaviors are observed: tunneling and metallic, which are assigned to the formation of a pinhole-free tunnel barrier at the Fe₃GeTe₂/InSe interface and pinholes in the InSe spacer layer, respectively. For the tunneling device, a large magnetoresistance (MR) of 41% is obtained under an applied bias current of 0.1 µA at 10 K, which is about three times larger than that of the metallic device. Moreover, the tunneling device exhibits a lower operating bias current but a more sensitive bias current dependence than the metallic device. The MR and spin polarization of both the metallic and tunneling devices decrease with increasing temperature, which can be fitted well by Bloch's law. These findings reveal the critical role of pinholes in the MR of all-2D van der Waals ferromagnet/semiconductor heterojunction devices.

With the assistance of the transfer medium that has poor wettability on the corrugated surface of graphene, the graphene wrinkle arrays are successfully fabricated without altering the crystalline orientation of entire graphene films, which is promising for numerous applications. This work opens up a new way for periodically modifying the surface properties of graphene for epitaxial growth of AlN films.

Abstract

Formation of graphene wrinkle arrays can periodically alter the electrical properties and chemical reactivity of graphene, which is promising for numerous applications. However, large-area fabrication of graphene wrinkle arrays remains unachievable with a high density and defined orientations, especially on rigid substrates. Herein, relying on the understanding of the formation mechanism of transfer-related graphene wrinkles, the graphene wrinkle arrays are fabricated without altering the crystalline orientation of entire graphene films. The choice of the transfer medium that has poor wettability on the corrugated surface of graphene is proven to be the key for the formation of wrinkles. This work provides a deep understanding of formation process of transfer-related graphene wrinkles and opens up a new way for periodically modifying the surface properties of graphene for potential applications, including direct growth of AlN epilayers and deep ultraviolet light emitting diodes.

By introducing electron-injection enhancement layers of indium, a synaptic device of indium (In)/molybdenum disulfide (MoS₂) shows an ultralow power consumption of 68.9 aJ, short-term plasticity (STP), long-term plasticity (LTP), and good capability of transition between STP and LTP. Additionally, a biomimetic eye with a 5 × 5 array of In/MoS₂ synaptic devices is implemented, exhibiting excellent image sensing and learning functions.

Abstract

Biomimetic eyes, with their excellent imaging functions such as large fields of view and low aberrations, have shown great potentials in the fields of visual prostheses and robotics. However, high power consumption and difficulties in device integration severely restrict their rapid development. In this study, an artificial synaptic device consisting of a molybdenum disulfide (MoS₂) film coated with an electron injection enhanced indium (In) layer is proposed to increase the channel conductivity and reduce the power consumption. This artificial synaptic device achieves an ultralow power consumption of 68.9 aJ per spike, which is several hundred times lower than those of the optical artificial synapses reported in literature. Furthermore, the multilayer and polycrystalline MoS₂ film shows persistent photoconductivity performance, effectively resulting in short-term plasticity, long-term plasticity, and their transitions between each other. A 5 × 5 In/MoS₂ synaptic device array is constructed into a hemispherical electronic retina, demonstrating its impressive image sensing and learning functions. This research provides a new methodology for effective control of artificial synaptic devices, which have great opportunities used in bionic retinas, robots, and visual prostheses.

2D ferromagnetism Cr₅Te₈ single crystals with high crystallinity are successfully synthesized via a tube-in-tube chemical vapor deposition growth. The as-grown Cr₅Te₈ nanosheet shows air-stable and thickness-tunable ferromagnetic properties with strong out-of-plane spin polarization, which open up new prospects for exploring 2D magnetism and spintronic device applications.

Abstract

2D magnetic materials have aroused widespread research interest owing to their promising application in spintronic devices. However, exploring new kinds of 2D magnetic materials with better stability and realizing their batch synthesis remain challenging. Herein, the synthesis of air-stable 2D Cr₅Te₈ ultrathin crystals with tunable thickness via tube-in-tube chemical vapor deposition (CVD) growth technology is reported. The importance of tube-in-tube CVD growth, which can significantly suppress the equilibrium shift to the decomposition direction and facilitate that to the synthesis reaction direction, for the synthesis of high-quality Cr₅Te₈ with accurate composition, is highlighted. By precisely adjusting the growth temperature, the thickness of Cr₅Te₈ nanosheets is tuned from ≈1.2 nm to tens of nanometers, with the morphology changing from triangles to hexagons. Furthermore, magneto-optical Kerr effect measurements reveal that the Cr₅Te₈ nanosheet is ferromagnetic with strong out-of-plane spin polarization. The Curie temperature exhibits a monotonic increase from 100 to 160 K as the Cr₅Te₈ thickness increases from 10 to 30 nm and no apparent variation in surface roughness or magnetic properties after months of exposure to air. This study provides a robust method for the controllable synthesis of high-quality 2D ferromagnetic materials, which will facilitate research progress in spintronics.

Porous single-layer graphene membranes for gas separation are synthesized by one-step chemical vapor deposition (CVD). Highly dense gas-sieving pores are created in graphene by tuning the CVD parameters. The resulting graphene membranes exhibit record-high H₂/CH₄ separation performance to date: H₂/CH₄ selectivity > 2000 while H₂ permeance > 4000 GPU.

Abstract

Single-layer graphene containing molecular-sized in-plane pores is regarded as a promising membrane material for high-performance gas separations due to its atomic thickness and low gas transport resistance. However, typical etching-based pore generation methods cannot decouple pore nucleation and pore growth, resulting in a trade-off between high areal pore density and high selectivity. In contrast, intrinsic pores in graphene formed during chemical vapor deposition are not created by etching. Therefore, intrinsically porous graphene can exhibit high pore density while maintaining its gas selectivity. In this work, the density of intrinsic graphene pores is systematically controlled for the first time, while appropriate pore sizes for gas sieving are precisely maintained. As a result, single-layer graphene membranes with the highest H₂/CH₄ separation performances recorded to date (H₂ permeance > 4000 GPU and H₂/CH₄ selectivity > 2000) are fabricated by manipulating growth temperature, precursor concentration, and non-covalent decoration of the graphene surface. Moreover, it is identified that nanoscale molecular fouling of the graphene surface during gas separation where graphene pores are partially blocked by hydrocarbon contaminants under experimental conditions, controls both selectivity and temperature dependent permeance. Overall, the direct synthesis of porous single-layer graphene exploits its tremendous potential as high-performance gas-sieving membranes.

Taking g-C₃N₄ as a representative example, it is theoretically found that twisted bilayers are endowed with good charge separation, ultrafast interlayer charge transfer, and enhanced visible-light absorption. Due to the induced large moiré potentials, the overpotentials of HER and OER on the twisted bilayers are significantly reduced. Therefore, twist is better and of great potential in catalysis.

Abstract

Moiré pattern superlattice formed by 2D van der Waals layered structures have attracted great attention for diverse applications. In experiments, the enhancement of catalytic performance in twisted bilayer systems is reported while its mechanism remains unclear. From high-accuracy first-principles and time-dependent ab initio nonadiabatic molecular dynamics calculations, ultrafast interlayer charge transfer within 120 fs, excellent charge separation, improved visible-light absorption, and satisfactory overpotentials for the hydrogen evolution and oxygen evolution reactions in twisted graphitic carbon nitride (g-C₃N₄) bilayers are found, which are beneficial to photocatalytic, photo-electrocatalytic, or electrocatalytic water splitting. This work provides insightful guidance to advanced nanocatalysis based on twisted layered materials.

Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting in understanding thermoelectric energy conversion. TlCuSe exhibiting intrinsically ultralow thermal conductivity (0.25 W m^–1 K^–1), a high power factor (11.6 μW cm^–1 K^–1), and a high figure of merit ZT (1.9) at 643 K is described.

Abstract

The entanglement of lattice thermal conductivity, electrical conductivity, and Seebeck coefficient complicates the process of optimizing thermoelectric performance in most thermoelectric materials. Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting and practically important for energy conversion. Herein, an intrinsic p-type semiconductor TlCuSe that has an intrinsically ultralow thermal conductivity (0.25 W m⁻¹ K⁻¹), a high power factor (11.6 µW cm⁻¹ K⁻²), and a high figure of merit, ZT (1.9) at 643 K is described. The weak chemical bonds, originating from the filled antibonding orbitals p-d* within the edge-sharing CuSe₄ tetrahedra and long TlSe bonds in the PbClF-type structure, in conjunction with the large atomic mass of Tl lead to an ultralow sound velocity. Strong anharmonicity, coming from Tl⁺ lone-pair electrons, boosts phonon–phonon scattering rates and further suppresses lattice thermal conductivity. The multiband character of the valence band structure contributing to power factor enhancement benefits from the lone-pair electrons of Tl⁺ as well, which modify the orbital character of the valence bands, and pushes the valence band maximum off the Γ-point, increasing the band degeneracy. The results provide new insight on the rational design of thermoelectric materials.

A systematic study of electric transport in Sb_{2− x} V _x Te₃ films demonstrates the coexistence of topological surface states and long-range ferromagnetism evidenced by Shubnikov–de Haas oscillations. Vanadium doping acts as an efficient tuning parameter for the Fermi surface size. This opens up a new route for the investigation of the crossover between different topological states based on this material class.

Abstract

An effective way of manipulating 2D surface states in magnetic topological insulators may open a new route for quantum technologies based on the quantum anomalous Hall effect. The doping-dependent evolution of the electronic band structure in the topological insulator Sb_{2− x} V _x Te₃ (0 ≤ x ≤ 0.102) thin films is studied by means of electrical transport. Sb_{2− x} V _x Te₃ thin films were prepared by molecular beam epitaxy, and Shubnikov–de Hass (SdH) oscillations are observed in both the longitudinal and transverse transport channels. Doping with the 3d element, vanadium, induces long-range ferromagnetic order with enhanced SdH oscillation amplitudes. The doping effect is systematically studied in various films depending on thickness and bottom gate voltage. The angle-dependence of the SdH oscillations reveals their 2D nature, linking them to topological surface states as their origin. Furthermore, it is shown that vanadium doping can efficiently modify the band structure. The tunability by doping and the coexistence of the surface states with ferromagnetism render Sb_{2− x} V _x Te₃ thin films a promising platform for energy band engineering. In this way, topological quantum states may be manipulated to crossover from quantum Hall effect to quantum anomalous Hall effect, which opens an alternative route for the design of quantum electronics and spintronics.

Air-stable large-area 2D-In _{x Ga 1−x} alloys with tunable composition and no evidence of phase segregation are realized by confinement heteroepitaxy. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

Abstract

Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air-stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large-area 2D-In _x Ga_{1− x} alloys are synthesized byConfinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

Multiscale organic semiconductors (OSs) are widely used as active components in practical devices. The intrinsic properties of OSs can be regulated through chemical structure design and self-assembly process control.

Abstract

Currently, organic semiconductors (OSs) are widely used as active components in practical devices related to energy storage and conversion, optoelectronics, catalysis, and biological sensors, etc. To satisfy the actual requirements of different types of devices, chemical structure design and self-assembly process control have been synergistically performed. The morphology and other basic properties of multiscale OS components are governed on a broad scale from nanometers to macroscopic micrometers. Herein, the up-to-date design strategies for fabricating multiscale OSs are comprehensively reviewed. Related representative works are introduced, applications in practical devices are discussed, and future research directions are presented. Design strategies combining the advances in organic synthetic chemistry and supramolecular assembly technology perform an integral role in the development of a new generation of multiscale OSs.