Jiuxiang Dai

The crystal structure of type-II red phosphorus is investigated by various structural characterizations, including 3D electron diffraction, atomic-resolution STEM imaging, and powder X-ray diffraction. A triclinic unit cell with a large volume containing around 250 phosphorus atoms is identified via 3D electron diffraction. The twisted wavy tubular motif, a new variation of building blocks in phosphorus, is also revealed via STEM.

Abstract

Elemental phosphorus exhibits fascinating structural varieties and versatile properties. The unique nature of phosphorus bonds can lead to the formation of extremely complex structures, and detailed structural information on some phosphorus polymorphs is yet to be investigated. In this study, we investigated an unidentified crystalline phase of phosphorus, type-II red phosphorus (RP), by combining state-of-the-art structural characterization techniques. Electron diffraction tomography, atomic-resolution scanning transmission electron microscopy (STEM), powder X-ray diffraction, and Raman spectroscopy were concurrently used to elucidate the hidden structural motifs and their packing in type-II RP. Electron diffraction tomography, performed using individual crystalline nanowires, was used to identify a triclinic unit cell with volume of 5330 ų, which is the largest unit cell for elemental phosphorus crystals up to now and contains approximately 250 phosphorus atoms. Atomic-resolution STEM imaging, which was performed along different crystal-zone axes, confirmed that the twisted wavy tubular motif is the basic building block of type-II RP. Our study discovered and presented a new variation of building blocks in phosphorus, and it provides insights to clarify the complexities observed in phosphorus as well as other relevant systems.

Nature Communications, Published online: 20 July 2023; doi:10.1038/s41467-023-39304-9

The graphene grown on 4 in. Ge(110) substrate is directly peeled from the substrate directly using the hexagonal boron nitride (hBN)-assisted dry transfer method. In electrical transport measurements using the edge-contact Hall device, hBN-encapsulated graphene shows extremely high carrier mobility, resulting in the observation of quantum Hall effects and Shubnikov-de Haas oscillations.

Abstract

The successful synthesis of wafer-scale single crystalline graphene on semiconducting Ge substrate has been considered a significant breakthrough toward the manufacturing of graphene-based electronic and photonic devices; however, the assumed extremely high electrical mobility has not been found yet due to the lack of an adequate characterization method. Herein, state-of-the-art transfer methods are developed to encapsulate the single crystalline graphene, which is grown on semiconducting Ge(110), in two hexagonal boron nitride (hBN) flakes, then acquire its inherent electrical mobility precisely via edge-contact technique. It is found that single crystalline graphene grown on Ge(110) possesses a maximum carrier mobility of over 100 000 cm² V⁻¹ s⁻¹ at low temperatures (2.3 K), which is superior to that obtained from graphene grown on other nonmetal substrates. Due to the extremely high mobility, well-defined quantum Hall effect and Shubnikov-de Haas oscillations can be observed at low temperatures as well. The study suggests that the excellent carrier mobility of graphene grown on Ge(110) may open an avenue to develop the practical graphene-based nanodevices with high performance.

Nature Nanotechnology, Published online: 20 July 2023; doi:10.1038/s41565-023-01445-9

Nature Photonics, Published online: 20 July 2023; doi:10.1038/s41566-023-01242-9

Infrared nanoimaging reveals propagating phonon polaritons on wafer-scale nanometer-thick hexagonal boron nitride grown by chemical vapor deposition. The material quality allows for the fabrication of phonon polariton nanoresonators with quality factors comparable to those fabricated from exfoliated hexagonal boron nitride, paving the way for wafer-scale applications.

Abstract

Polaritons in layered materials (LMs) are a promising platform to manipulate and control light at the nanometer scale. Thus, the observation of polaritons in wafer-scale LMs is critically important for the development of industrially relevant nanophotonics and optoelectronics applications. In this work, phonon polaritons (PhPs) in wafer-scale multilayer hexagonal boron nitride (hBN) grown by chemical vapor deposition are reported. By infrared nanoimaging, the PhPs are visualized, and PhP lifetimes of ≈0.6 ps are measured, comparable to that of micromechanically exfoliated multilayer hBN. Further, PhP nanoresonators are demonstrated. Their quality factors of ≈50 are about 0.7 times that of state-of-the-art devices based on exfoliated hBN. These results can enable PhP-based surface-enhanced infrared spectroscopy (e.g., for gas sensing) and infrared photodetector applications.

A vapor deposition method in combination with a solvent-assisted recrystallization technique is presented to fabricate high-quality larger-area perovskite film. Efficiency of 19.9% is achieved, which is among the highest values ever reported for minimodules based on vapor-deposited perovskite.

Abstract

Vapor deposition is a promising technology for the mass production of perovskite solar cells. However, the efficiencies of solar cells and modules based on vapor-deposited perovskites are significantly lower than those fabricated using the solution method. Emerging evidence suggests that large defects are generated during vapor deposition owing to a specific top-down crystallization mechanism. Herein, a hybrid vapor deposition method combined with solvent-assisted recrystallization for fabricating high-quality large-area perovskite films with low defect densities is presented. It is demonstrated that an intermediate phase can be formed at the grain boundaries, which induces the secondary growth of small grains into large ones. Consequently, perovskite films with substantially reduced grain boundaries and defect densities are fabricated. Results of temperature-dependent charge-carrier dynamics show that the proposed method successfully suppresses all recombination reactions. Champion efficiencies of 21.9% for small-area (0.16 cm²) cells and 19.9% for large-area (10.0 cm²) solar modules under AM 1.5 G irradiation are achieved. Moreover, the modules exhibit high operational stability, i.e., they retain >92% of their initial efficiencies after 200 h of continuous operation.

Abstract

The chirality-induced spin selectivity (CISS) has been found in the antiferromagnetic and paramagnetic chiral inorganic materials with unpaired electrons, while rarely reported in the spin-paired diamagnetic inorganic materials with spin-pairing energy. Here, we report the CISS in the spin-paired diamagnetic BiOBr endowed with three levels of chiral mesostructures. Chiral mesostructured BiOBr films (CMBFs) were fabricated through a sugar alcohol-induced hydrothermal route. The antipodal CMBFs exhibited chirality-dependent, magnetic field-independent magnetic circular dichroism (MCD) signals, which indicates the existence of spin selectivity. The spin selectivity of CMBFs was speculated to be the result of the competing effect between the externally applied magnetic field and the effective magnetic field arisen from the spin electron motions in chiral potential. The chirality-induced effective magnetic field acts on the magnetic moment of electrons, potentially overcoming the spin-pairing energy and producing opposite energy changes for spin-down and spin-up electrons.

Atomically thin 2D oxide crystals have attracted considerable attention owing to their remarkable physical properties and great potential for versatile applications. This review discusses the recent progress in the synthesis of 2D oxide crystals for applications in electronics, optoelectronics, magnetics, and ferroelectric devices. Finally, the challenges and prospects in this exciting field are discussed.

Abstract

Atomically thin two-dimensional (2D) oxide crystals have garnered considerable attention because of their remarkable physical properties and potential for versatile applications. In recent years, significant advancements have been made in the design, preparation, and application of ultrathin 2D oxides, providing many opportunities for new-generation advanced technologies. This review focuses on the controllable preparation of 2D oxide crystals and their applications in electronic and optoelectronic devices. Based on their bonding nature, the various types of 2D oxide crystals are first summarized, including both layered and nonlayered crystals, as well as their current top-down and bottom-up synthetic approaches. Subsequently, in terms of the unique physical and electrical properties of 2D oxides, recent advances in device applications are emphasized, including photodetectors, field-effect transistors, dielectric layers, magnetic and ferroelectric devices, memories, and gas sensors. Finally, conclusions and future prospects of 2D oxide crystals are presented. It is hoped that this review will provide comprehensive and insightful guidance for the development of 2D oxide crystals and their device applications.

The electronic properties and chemical composition at grain boundaries in ferroelectric ErMnO₃ polycrystals are studied. Atom probe tomography shows that the chemical structure at grain boundaries is substantially different from the bulk, revealing the microscopic origin of their distinct electronic responses. The work demonstrates the benefit of quantitative atomic-scale measurements and additional opportunities for property engineering at polar oxide interfaces.

Abstract

Polar discontinuities, as well as compositional and structural changes at oxide interfaces can give rise to a large variety of electronic and ionic phenomena. In contrast to earlier work focused on domain walls and epitaxial systems, this work investigates the relation between polar discontinuities and the local chemistry at grain boundaries in polycrystalline ferroelectric ErMnO₃. Using orientation mapping and scanning probe microscopy (SPM) techniques, the polycrystalline material is demonstrated to develop charged grain boundaries with enhanced electronic conductance. By performing atom probe tomography (APT) measurements, an enrichment of erbium and a depletion of oxygen at all grain boundaries are found. The observed compositional changes translate into a charge that exceeds possible polarization-driven effects, demonstrating that structural phenomena rather than electrostatics determine the local chemical composition and related changes in the electronic transport behavior. The study shows that the charged grain boundaries behave distinctly different from charged domain walls, giving additional opportunities for property engineering at polar oxide interfaces.

Modifications to improve the optical properties of devices based on indirect bandgap transition metal dichalcogenides and their emerging applications in optoelectronic sensors are the focus of this review. Various engineering techniques are discussed to overcome their limitations derived from the indirect bandgap of multilayer transition metal dichalcogenides as well as future directions of these materials for optoelectronic applications.

Abstract

The unique electrical and optical properties of transition metal dichalcogenides (TMDs) make them attractive nanomaterials for optoelectronic applications, especially optical sensors. However, the optical characteristics of these materials are dependent on the number of layers. Monolayer TMDs have a direct bandgap that provides higher photoresponsivity compared to multilayer TMDs with an indirect bandgap. Nevertheless, multilayer TMDs are more appropriate for various photodetection applications due to their high carrier density, broad spectral response from UV to near-infrared, and ease of large-scale synthesis. Therefore, this review focuses on the modification of the optical properties of devices based on indirect bandgap TMDs and their emerging applications. Several successful developments in optical devices are examined, including band structure engineering, device structure optimization, and heterostructures. Furthermore, it introduces cutting-edge techniques and future directions for optoelectronic devices based on multilayer TMDs.

By screening bulk van der Waals (vdW) materials featuring 1D chains, materials with exceptional properties are identified, including large second harmonic generation (SHG) susceptibility, appropriate bandgap, and birefringence. These materials' nonlinear optical (NLO) properties are intricately linked to the stacking prototypes of their chains and the charge difference of ions on the chains.

Abstract

Nonlinear optical (NLO) materials are of great importance for applications in lasers, atomic clocks, free-space communication, etc. Herein, inspired by the recent prediction of excellent second harmonic generation (SHG) performance in van der Waals (vdW) materials with 1D building blocks, 14 new NLO materials are found from 244 bulk crystals constructed with 1D polymers using high-throughput first-principles calculations. Nearly half of the new NLO materials exhibit superior NLO performance with SHG susceptibilities approaching the theoretical upper limit. The 2D form of 11 candidates inherits the NLO property covering UV, visible, and infrared regions. Bader charge analysis reveals that the SHG susceptibility is determined by the charge difference of ions on the chains. Finally, it is proposed that superior NLO materials can be found in materials with proper bandgaps and large charge differences on the chains. This work not only screens out candidates with outstanding NLO performance in vdW materials with 1D building blocks but also provides a guideline for the search and design of NLO vdW 1D polymer patterns with excellent NLO properties.

Abstract

The interlayer friction behavior of two-dimensional transition metal dichalcogenides (TMDCs) as crucial solid lubricants has attracted extensive attention in the field of tribology. In this study, the interlayer friction is measured by laterally pushing the MoTe₂ powder on the MoTe₂ substrate with the atomic force microscope (AFM) tip, and density functional theory (DFT) simulations are used to rationalize the experimental results. The experimental results indicate that the friction coefficient of the 1T′-MoTe₂/1T′-MoTe₂ interface is 2.025 × 10⁻⁴, which is lower than that of the 2H-MoTe₂/2H-MoTe₂ interface (3.086 × 10⁻⁴), while the friction coefficient of the 1T′-MoTe₂/2H-MoTe₂ interface is the lowest at 6.875 × 10⁻⁵. The lower interfacial friction of 1T′-MoTe₂/1T′-MoTe₂ compared to 2H-MoTe₂/2H-MoTe₂ interface can be explained by considering the relative magnitudes of the ideal average shear strengths and maximum shear strengths based on the interlayer potential energy. Additionally, the smallest interlayer friction observed at the 1T′-MoTe₂/2H-MoTe₂ heterojunction is attributed to the weak interlayer electrostatic interaction and reduction in potential energy corrugation caused by the incommensurate contact. This work suggests that MoTe₂ has comparable interlayer friction properties to MoS₂ and is expected to reduce interlayer friction in the future by inducing the 2H-1T′ phase transition.

Nature Electronics, Published online: 17 July 2023; doi:10.1038/s41928-023-00983-3

Nature Electronics, Published online: 17 July 2023; doi:10.1038/s41928-023-00984-2

Nature Synthesis, Published online: 17 July 2023; doi:10.1038/s44160-023-00375-7

Nature Communications, Published online: 18 July 2023; doi:10.1038/s41467-023-39997-y

A monolithic three-dimensional (M3D) design is proposed to construct ultra-flexible complementary metal-oxide-semiconductor electronics with high electrical-performance and integration by stacking n- and p-type transistors vertically and saving inter tier vias used in conventional M3D structure. The ultra-flexible and high-integration circuits enable a wearable light recorder to collect the harmful blue light illuminated into human eyes by attaching the circuits on a contact lens.

Abstract

Flexible electronics based on complementary metal-oxide-semiconductor (CMOS) technology have enabled a smart soft world. However, the trade-off among flexibility, density, and electrical performance has been a long-lasting unresolved issue. Here, a monolithic three-dimensional (M3D) CMOS design is proposed to address this problem and realize ultra-flexible electronics with high electronic-performance and integration. This design utilizes vertically stacked p-type carbon nanotube transistors and n-type indium gallium zinc oxide ones, which share common gates and drains, saving the inter tier vias required in the traditional M3D structure to reduce routing and improve flexibility. With this design, CMOS logic gates, multi-stage circuits, ring oscillators (ROs) and memory modules, are demonstrated. This design enables circuits to save up to 45% of area compared with their planar counterparts. Particularly, inverters exhibit a record-high gain of 191, and 55-stage ROs can operate well even after bending at a 500-µm radius for 50 cycles, exhibiting the highest flexibility among the reported ones. The ultra-flexible and high-integration RO enables a wearable light recorder to collect harmful blue light shining into human eyes by simply attaching the circuits on a contact lens. This integration method provides possibilities for developing complex-function wearable electronics.

Nature Communications, Published online: 18 July 2023; doi:10.1038/s41467-023-40075-6

Nature Communications, Published online: 19 July 2023; doi:10.1038/s41467-023-40020-7

Nature, Published online: 19 July 2023; doi:10.1038/s41586-023-06264-5

Nature, Published online: 19 July 2023; doi:10.1038/s41586-023-06290-3

Nature, Published online: 19 July 2023; doi:10.1038/s41586-023-06107-3

Nature Nanotechnology, Published online: 19 July 2023; doi:10.1038/s41565-023-01472-6

Nature Materials, Published online: 13 July 2023; doi:10.1038/s41563-023-01609-x

The synthesis yield of 2D Na₂Cl crystals with unconventional stoichiometries in a graphene membrane can be significantly improved by applying a negative potential. Such high content can substantially improve the physical performance of the abnormal Na₂Cl crystals. These findings represent a step toward the potential applications of abnormal 2D crystal materials with unique properties.

Abstract

Abnormal salt crystals with unconventional stoichiometries, such as Na₂Cl, Na₃Cl, K₂Cl, and CaCl crystals that have been explored in reduced graphene oxide membranes (rGOMs) or diamond anvil cells, hold great promise in applications due to their unique electronic, magnetic, and optical properties predicted in theory. However, the low content of these crystals, only <1% in rGOM, limits their research interest and utility in applications. Here, a high-yield synthesis of 2D abnormal crystals with unconventional stoichiometries is reported, which is achieved by applying negative potential on rGOM. A more than tenfold increase in the abnormal Na₂Cl crystals is obtained using a potential of −0.6 V, resulting in an atomic content of 13.4 ± 4.7% for Na on rGOM. Direct observations by transmission electron microscopy and piezoresponse force microscopy demonstrates a unique piezoelectric behavior arising from 2D Na₂Cl crystals with square structure. The output voltage increases from 0 to ≈180 mV in the broad 0–150° bending angle regime, which meets the voltage requirement of most nanodevices in realistic applications. Density functional theory calculations reveal that the applied negative potential of the graphene surface can strengthen the effect of the Na⁺–π interaction and reduce the electrostatic repulsion between cations, making more Na₂Cl crystals formed.

A generic strategy to phase-controllable synthesis of 2D single-crystalline spinel-type oxides with thicknesses down to one unit cell is proposed. A unique combination of ferromagnetic and semiconducting properties is demonstrated. Owing to its ultrathin geometry, a Mn₃O₄ nanosheet exhibits a superior ultraviolet detection performance with an ultralow noise power density of 0.126 pA Hz^−1/2.

Abstract

2D magnetic materials have been of interest due to their unique long-range magnetic ordering in the low-dimensional regime and potential applications in spintronics. Currently, most studies are focused on strippable van der Waals magnetic materials with layered structures, which typically suffer from a poor stability and scarce species. Spinel oxides have a good environmental stability and rich magnetic properties. However, the isotropic bonding and close-packed nonlayered crystal structure make their 2D growth challenging, let alone the phase engineering. Herein, a phase-controllable synthesis of 2D single-crystalline spinel-type oxides is reported. Using the van der Waals epitaxy strategy, the thicknesses of the obtained tetragonal and hexagonal manganese oxide (Mn₃O₄) nanosheets can be tuned down to 7.1 nm and one unit cell (0.7 nm), respectively. The magnetic properties of these two phases are evaluated using vibrating-sample magnetometry and first-principle calculations. Both structures exhibit a Curie temperature of 48 K. Owing to its ultrathin geometry, the Mn₃O₄ nanosheet exhibits a superior ultraviolet detection performance with an ultralow noise power density of 0.126 pA Hz^−1/2. This study broadens the range of 2D magnetic semiconductors and highlights their potential applications in future information devices.