Jiuxiang Dai

Jamie Geng and co-workers quantitatively characterize the optical properties of 𝛾-InSe. InSe is discovered to exhibit both weak absorption and strong photoluminescence quantum yield, an unusual combination of properties. The emission efficiency and bandgap may be tuned with both flake thickness and temperature. Alternating-current electroluminescence devices are fabricated and demonstrated.

Abstract

Bulk γ-InSe has a direct bandgap of 1.24 eV, which corresponds to near infrared wavelengths (λ = 1.0 µm) useful in optoelectronic applications from biometric detectors to silicon photonics. However, its potential for optoelectronic applications is largely untapped due in part to the lack of quantitative studies of its optical properties. Here, the unusually low absorptance and high photoluminescence quantum efficiency of single-crystalline InSe flakes with thickness in the hundreds of nanometers are studied. InSe emits brightly at room temperature from its direct bandgap with a peak photoluminescence quantum yield (PLQY) of 20%, despite displaying indirect bandgap like low absorption coefficient due to the symmetry of its crystal structure. By performing pump-dependent PLQY measurements, the radiative and nonradiative recombination coefficients are extracted, including the Shockley-Read-Hall and Auger coefficients. Finally, a proof-of-concept alternating current electroluminescent device at low temperature is demonstrated to show the promise of InSe in optoelectronic technology such as highly transparent, bright NIR light sources.

The strong coordination interaction between 3AMPY²⁺ and 3D perovskite components as well as the introduced nucleation sites by 3AMPYSnI₄ crystals adjust the phase distribution of 2D and 3D perovskite phase, accompanied by the suppressed Sn²⁺ oxidation and self-p-doping, resulting in lower trap density and non-radiative recombination loss, faster carrier extraction and transfer, and higher stability for 2D-3D Sn-based PSCs.

Abstract

Various popular large organic cations have been extensively used as the essential additives in the perovskite precursor solution due to their satisfactory passivation effect but may produce the low-n value (n ≤ 2) 2D perovskite phases with undesired distribution. Meanwhile, the remaining easy oxidation of Sn²⁺ and the p-type self-doping in the perovskites are also detrimental to the ultimate photovoltaic properties and stability of tin (Sn)-based perovskite solar cells (PSCs). Here, 3AMPYSnI₄ crystals (3AMPY = 3-(aminomethyl)pyridinium)) are designed and applied to adjust the crystallization process and the phase distribution of the Sn-based perovskite. Consequently, the strong coordination interaction between 3AMPY²⁺ and 3D perovskite components and the introduced nucleation sites by 3AMPYSnI₄ crystals not only decreases the low-n value 2D phase and increases 3D perovskite phase, but also inhibits the oxidation of Sn²⁺ and the self-p-doping in the Sn-based perovskites, resulting in lower trap density and non-radiative recombination loss, faster carrier extraction and transfer, and higher stability for 2D-3D Sn-based PSCs. As a result, the optimized devices deliver an increased power conversion efficiency from an initial 10.91% to 13.28% and retain 96.0% of their original performance for more than 3000 h in the nitrogen (N₂) atmosphere.

Due to the anisotropy of the ultra-wide bandgap semiconductor β-Ga₂O₃, it can be prepared as a solar-blind UV detector and is intrinsically sensitive to polarized light without the need of polarizers. This β-Ga₂O₃-based detector has an ultra-high polarization ratio and is commercially viable. This suggests a new strategy for solar-blind ultraviolet polarization detection and also provides promising solar-blind optical communication.

Abstract

Solar-blind ultraviolet (UV) detection plays a critical role in imaging and communication due to its low-noise background, high signal-to-noise ratio, and strong anti-interference capabilities. Detecting the polarization state of UV light can enhance image information and expand the communication dimension. Although polarization detection is explored in visible and infrared light, and applied in fields such as astrophysics and submarine seismic wave detection, solar-blind UV polarization detection remains largely unreported. This is primarily due to the challenge of creating UV polarizers with high transmittance, high extinction ratio, and strong resistance to UV radiation. In this study, it is discovered that the space symmetry breaking of the β-Ga₂O₃’s b–c plane results in a significant optical absorption dichroic ratio. Leveraging β-Ga₂O₃’s high solar-blind UV response, a lensless solar-blind UV polarization-sensitive photodetector, circumventing the challenges associated with solar-blind UV polarizers is designed. This photodetector exhibits an exceptionally high intrinsic polarization ratio under 254 nm linearly polarized light, approximately two orders of magnitude higher than other reported nanomaterial-based polarization-sensitive photodetectors. Additionally, it demonstrates significant advantages in solar-blind UV imaging and light communication. This work introduces a novel strategy for solar-blind ultraviolet polarization detection and offers a promising approach for solar-blind light communication.

This work demonstrates a universal strategy for robust growth of high-quality transition metal dichalcogenide (TMD) vertical heterostructures and a novel design concept for the fabrication of high mobility and high responsivity photodetectors based on 2D TMD vertical heterostructures, which holds promise for the next-generation photodetectors.

Abstract

Two dimension (2D) transition metal dichalcogenides (TMD) heterostructures have opened unparalleled prospects for next-generation electronic and optoelectronic applications due to their atomic-scale thickness and distinct physical properties. The chemical vapor deposition (CVD) method is the most feasible approach to prepare 2D TMD heterostructures. However, the synthesis of 2D vertical heterostructures faces competition between in-plane and out-of-plane growth, which makes it difficult to precisely control the growth of vertical heterostructures. Here, a universal and controllable strategy is reported to grow various 2D TMD vertical heterostructures through an ammonium-assisted CVD process. The ammonium-assisted strategy shows excellent controllability and operational simplicity to prevent interlayer diffusion/alloying and thermal decomposition of the existed TMD templates. Ab initio simulations demonstrate that the reaction between NH₄Cl and MoS₂ leads to the formation of MoS₃ clusters, promoting the nucleation and growth of 2D MoS₂ on existed 2D WS₂ layer, thereby leading to the growth of vertical heterostructure. The resulting 2D WSe₂/WS₂ vertical heterostructure photodetectors demonstrate an outstanding optoelectronic performance, which are comparable to the performances of photodetectors fabricated from mechanically exfoliated and stacked vertical heterostructures. The ammonium-assisted strategy for robust growth of high-quality vertical van der Waals heterostructures will facilitate fundamental physics investigations and device applications in electronics and optoelectronics.

A surfactant self-assembly growth mechanism is proposed to controllably synthesize the metal indium sulfide atomic layers. Eleven types of metal indium sulfide atomic layers with tunable compositions, thickness, and defect concentrations are successfully achieved. 2D metal indium sulfides provide a unique platform to explore the correlation between surface atomic structure, electronic state, and photocatalytic performance.

Abstract

The metal indium sulfides have attracted extensive research interest in photocatalysis due to regulable atomic configuration and excellent optoelectronic properties. However, the synthesis of metal indium sulfide atomic layers is still challenging since intrinsic non-van-der-Waals layered structures of some components. Here, a surfactant self-assembly growth mechanism is proposed to controllably synthesize metal indium sulfide atomic layers. Eleven types of atomic layers with tunable compositions, thickness, and defect concentrations are successfully achieved namely In₂S₃, MgIn₂S₄, CaIn₂S₄, MnIn₂S₄, FeIn₂S₄, ZnIn₂S₄, Zn₂In₂S₅, Zn₄In₁₆S₃₃, CuInS₂, CuIn₅S₈, and CdIn₂S₄. The typical CaIn₂S₄ shows a defect-dependence activity for CO₂ photoreduction. The designed S vacancies in CaIn₂S₄ can serve as catalytic centers to activate CO₂ molecules via localized electrons for π-back-donation. The engineered S vacancies tune the non-covalent interaction with CO₂ and intermediates, manages to tune the free energy, and lower the reaction energy barrier. As a result, the defect-rich CaIn₂S₄ displays 2.82× improved reduction rate than defect-poor CaIn₂S₄. Meantime, other components also display promising photocatalytic performance, such as Zn₂In₂S₅ with a H₂O₂ photosynthesis rate of 292 µmol g⁻¹ h⁻¹ and CuInS₂ with N₂−NH₄ ⁺ conversion rate of 54 µmol g⁻¹ h⁻¹. This work paves the way for the multidisciplinary exploration of metal indium sulfide atomic layers with unique photocatalysis properties.

2D to 3D magnetic transition in KFe₃[FeGe₃]O₁₀(OH)₂ synthetic mica is observed. This occurs thanks to the intralayer tetrahedral Fe, which couples antiferromagnetically with the otherwise frustrated octahedra layer.

Abstract

Fe-based mica minerals usually display two opposing magnetic ground states, either they behave as spin-glasses or as layered ferrimagnets. No definite reason has been proposed as an explanation for this duality. This conundrum is unraveled by comparing the synthetic micas KFe₃[MGe₃]O₁₀ X ₂ with M═Fe and Ga, X═OH⁻ and F⁻. Neutron diffraction demonstrates a 2D to 3D magnetic transition in KFe₃[FeGe₃]O₁₀(OH)₂ while just hints or no order at all are observed for the fluorides with M═Fe and Ga respectively. The 3D transition is triggered by the presence of iron in the intralayer tetrahedra. DFT+U calculations show that the magnetic exchange couplings between the previously believed solely magnetic octahedral layers would otherwise be frustrated without this intralayer iron.

On-surface synthesis of high-spin S=2 and S=3 nanographenes achieved through covalent linking of triangulenes via 1,3- and 1,3,5-phenylene spacers is reported. Using scanning tunneling spectroscopy, magnetic evidence of their degenerate ground states and an effective ferromagnetic inter-triangulene exchange is established. A spin model using a single coupling parameter describes magnetic excitations in both, the S=2 and S=3 nanographenes.

Abstract

In the pursuit of high-spin building blocks for the formation of covalently bonded 1D or 2D materials with controlled magnetic interactions, π ${\pi }$ -electron magnetism offers an ideal framework to engineer ferromagnetic interactions between nanographenes. As a first step in this direction, we explore the spin properties of ferromagnetically coupled triangulenes—triangular nanographenes with spin S=1 ${S = 1}$ . By combining in-solution synthesis of rationally designed molecular precursors with on-surface synthesis, we successfully achieve covalently bonded S=2 ${S = 2}$ triangulene dimers and S=3 ${S = 3}$ trimers on Au(111). Starting with the triangulene dimer, we meticulously characterize its low-energy magnetic excitations using inelastic electron tunneling spectroscopy (IETS). IETS reveals conductance steps corresponding to a quintet-to-triplet excitation, and a zero-bias peak resulting from higher-order spin-spin scattering of the five-fold degenerate ferromagnetic ground state. The Heisenberg model captures the key parameters of inter-triangulene ferromagnetic exchange, and its successful extension to the larger S=3 ${S = 3}$ system validates the model's accuracy. We anticipate that incorporating ferromagnetically coupled building blocks into the repertoire of magnetic nanographenes will unlock new possibilities for designing carbon nanomaterials with complex magnetic ground states.

Flexible crystals with unique mechanical properties have presented enormous applications in optoelectronics, soft robotics and sensors. However, there have been no reports of low-temperature-resistant flexible crystals with second-order nonlinear optical properties (NLO). Here, we report the flexible chiral Schiff-base crystals capable of efficient second harmonic generation (SHG). Both enantiomers and racemic modifications of these crystals are mechanically flexible in two directions at both room temperature and at -196 °C, although their mechanical responses differ. The enantiomers display SHG with an intensity of up to 12 times that of potassium dihydrogenphosphate (KDP) when pumped at 980 nm, and they also have high laser-induced damage thresholds (LDT). Even when bent, the crystals retain strong second harmonic generation, although with a different intensity distribution depending on the polarization, compared to when they are straight. This work describes the first instance of flexible organic crystal with NLO properties and lays the foundation for the development of mechanically flexible organic NLO materials.

The hydrolytic resistance of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors can be improved by sintering the powder to be the ceramic type. The initial luminescence intensity of the obtained ceramic reaches 8200 mcd m⁻² by optimizing the Eu, Dy, and H₃BO₃ concentrations. The prepared ceramics can be excited under 365 nm UV light and X-ray and show potential applications in information storage.

Abstract

SrAl₂O₄:Eu²⁺, Dy³⁺ (SAED) is one of the most popular materials for information storage and night display applications because it has a wide excitation range and long afterglow duration. However improving the hydrolytic resistance of SAED remains a challenge. In this study, the SAED is presented to be the ceramic type by a solid-state reaction in a vacuum ambiance. The achieved SAED ceramic has 8200 mcd m⁻² initial luminescence intensity. It can also obtain long-persistent luminescence after UV light irradiation even soaking in water for more than 30 days. This ceramic demonstrates sustained imaging capability irradiated under 365 nm UV light and X-ray, as well as erasability and reproducibility of storage by lighting and heating. These results indicate the promising applications of SAED ceramics in optical information storage and night display in complex environments. The phase evolution in the SAED ceramic is identified directly in the micromorphology for the first time based on scanning electron microscopy coupled with a cathodoluminescence system.

Self-hybridized exciton-polaritons are achieved in van der Waals magnet CrSBr crystals free from external cavities up to room temperature. The ultrastrong exciton-photon coupling effectively suppresses emission processes related to donors, phonons, and defects. Moreover, adjustments in the magnetic field, temperature, and CrSBr crystal thickness can precisely tailor the exciton-polariton dispersion and emission behaviors.

Abstract

2D van der Waals (vdW) layered materials exhibit significant exciton binding energy and versatile stacking options, making them ideal for room-temperature exciton-polariton devices used in low-threshold lasing, nonlinear optical switching, and quantum computing. However, most existing systems depend on external optical microcavities coupled with single monolayers, leading to limited controllability and increased costs. Here, external cavity-free vdW magnet CrSBr crystals are presented that feature magnetically controllable self-hybridized exciton-polaritons that remain stable up to room temperature. The ultrastrong exciton-photon coupling suppresses donor-, phonon-, and defect-related emissions. Furthermore, the exciton-polariton dispersion and emission spectra can be effectively controlled by adjusting the magnetic field, temperature, and CrSBr thickness. This vdW exciton-polariton material platform, demonstrating remarkable magnetic responsiveness in open cavity configurations under ambient conditions, paves the way for the development of compact, fast, and low-loss spin, quantum, and magneto-photonic devices.

Nature Materials, Published online: 19 September 2024; doi:10.1038/s41563-024-02002-y

Pt oxides are essential catalysts in many critical reactions, but are typically unstable and prone to evaporation above 700 K. A two-dimensional layered Pt oxide with exceptional thermal stability is introduced, capable of surviving at high temperatures.

After the rapid growth of nucleation sites to form triangular MoS₂ assisted by Na atoms, the Na atoms adsorb on the basal sulfur atoms to promote the formation of sulfur vacancies, and defect formation during the cooling. The introduced basal defects are uniformly distributed on the MoS₂ surface, which significantly improves the SERS detection performance of MoS₂ on R6G.

Abstract

Two-dimensional molybdenum disulfide (2D MoS₂) shows great promise as a surface-enhanced Raman scattering (SERS) substrate due to its strong exciton resonance. However, the inert basal plane limits the performance of SERS. In this work, a strategy is proposed for the one-step synthesis of atomically basal defect-rich MoS₂. The study first reveals that NaCl plays a two-stage role in the growth process, where NaCl initially promotes the rapid growth of large MoS₂ as previously reported, and then promotes the formation of atomic basal defects dominated by single sulfur vacancies. Additionally, spectral changes induced by modulation of experimental parameters and density function theory calculation show that defect generation occurs during cooling. Meanwhile, the ratio of E2g1${\mathrm{E}}_{{\mathrm{2g}}}^{\mathrm{1}}$ to A_1g in defect-rich MoS₂ exhibits different variation trends compared with pristine MoS₂ in power-dependent Raman, and the ratio increases with increasing basal defects. In SERS tests, the limit of detection for rhodamine 6G reached 10⁻⁹  m, which is comparable to the performance of conventional noble metal SERS substrate. The activation strategy of the inert basal plane is applicable to other 2D transition metal dichalcogenides, and further has the potential to enhance performance in other domains, such as SERS and hydrogen evolution reactions.

Nature Electronics, Published online: 18 September 2024; doi:10.1038/s41928-024-01254-5

Two-dimensional transistors heat up

Nature Electronics, Published online: 18 September 2024; doi:10.1038/s41928-024-01245-6

A high-κ dielectric ceramic, magnesium niobate, can be epitaxially grown on a mica substrate and then transferred to form the gate dielectric in molybdenum disulfide transistors, providing van der Waals interfaces and high robustness to temperature and voltage.

Nature, Published online: 18 September 2024; doi:10.1038/s41586-024-07941-9

Superconducting transmon qubits have been fabricated in a 300 mm complementary metal–oxide–semiconductor (CMOS) pilot line using industrial fabrication methods, achieving relaxation and coherence times exceeding 100 μs.

a) The neuron-inspired ferroelectric memtransistor (van der Waals heterosynaptic memtransistor, vdW-HT) with b) full vdW functional layers. c) The visualized matrix with brain-like learning behaviors. d) Identified grabbing and directional movement of the robotic arm integrated with the neuromorphic system based on vdW-HT synapses.

Abstract

The emergence of 2D van der Waals (vdW) materials, owing to highly tunable electrical conductivity, remarkably free stackability, and excellent compatibility for heterojunction integration, has provided abundant research diversity of artificial synapses. Here, an innovative 2D vdW heterosynaptic memtransistor (vdW-HT) synapse is proposed with a ferroelectric CuInP₂S₆ inserted layer. Neuromorphic synaptic weight change of the vdW-HT synapse in this work are modulated by the synergistic effects of interlayer coupling and ferroelectric polarization reversal. It is the first time to evaluate the required initial energy consumption of ferroelectric vdW-HT synapses with matrixed biomimetic characteristics of “Learning–Forgetting–Re-learning–Memorizing.” The required initial energy consumption is only ≈3.06 pJ, which provided supporting evidence to indicate the promising potential of vdW-HT synapses in exploring neuromorphic applications. A vdW-HT computing system with artificial recognition capability for intelligent automobiles is established and demonstrated outstanding recognition abilities for multiple targets in various environments. The highest recognition accuracy for pedestrians is 98%. In addition, the excellent recognition property is integrated with a robotic arm, successfully achieving high-precision grasping behavior and designated position transmission for identified targets. These results provide promising strategies for the integrated development of emerging neuromorphic electronics and industrial applications.

Tailoring electrocatalytic activity in 2D materials through doping effect is fascinating. Herein, Ni-doped MoS₂ (Ni-MoS₂) nanosheets exhibit enhanced performance in NO₃ ⁻RR, achieving a remarkable NH₃ FE of 92.3% at −0.3 V _RHE with excellent stability. The Ni dopants facilitate the adsorption of *NO₃ and the stabilization of *N intermediates, offering new insights into the regulation of active sites in 2D materials.

Abstract

The electrocatalytic nitrate reduction reaction (NO₃ ⁻RR) presents a promising pathway for achieving both ammonia (NH₃) electrosynthesis and water pollutant removal simultaneously. Among various electrocatalysts explored, 2D materials have emerged as promising candidates due to their ability to regulate electronic states and active sites through doping. However, the impact of doping effects in 2D materials on the mechanism of NO₃ ⁻RR remains relatively unexplored. Here, Ni-doped MoS₂ (Ni-MoS₂) nanosheets are investigated as a model system, demonstrating enhanced NO₃ ⁻RR performance compared to undoped counterparts. By controlling the doping concentration, the Ni-MoS₂ nanosheets achieve a remarkable faradic efficiency (FE) of 92.3% for NH₃ at −0.3 V _RHE with excellent stability. The mechanistic studies reveal that the elevation of the NO₃ ⁻RR performances originates from the generation of more active hydrogen and the acceleration of the reaction from nitrite (NO₂ ⁻) to NH₃ facilitated by Ni doping. Combining the experimental observations and theoretical calculations it is revealed that the appropriate Ni doping level in MoS₂ can enhance *NO₃ adsorption strength, thereby facilitating subsequent electrocatalytic steps. Together with the demonstration of Zn−NO₃ ⁻ and Zn−NO₂ ⁻ battery devices, the work provides new insights into the design and regulation of the active sites in 2D material catalysts for efficient NO₃ ⁻RR.

As an emerging p-type van der Waals semiconductor, tellurium (Te) exhibits great potential in advancing future electronics and optoelectronics. This review provides a comprehensive overview of the fundamental properties and synthesis strategies of Te nanostructures, followed by a detailed discussion of the state-of-the-art progress in their application across various advanced devices, including electronics, optoelectronics, sensors, and large-scale circuits.

Abstract

As a true 1D system, group-VIA tellurium (Te) is composed of van der Waals bonded molecular chains within a triangular crystal lattice. This unique crystal structure endows Te with many intriguing properties, including electronic, optoelectronic, thermoelectric, piezoelectric, chirality, and topological properties. In addition, the bandgap of Te exhibits thickness dependence, ranging from 0.31 eV in bulk to 1.04 eV in the monolayer limit. These diverse properties make Te suitable for a wide range of applications, addressing both established and emerging challenges. This review begins with an elaboration of the crystal structures and fundamental properties of Te, followed by a detailed discussion of its various synthesis methods, which primarily include solution phase, and chemical and physical vapor deposition technologies. These methods form the foundation for designing Te-centered devices. Then the device applications enabled by Te nanostructures are introduced, with an emphasis on electronics, optoelectronics, sensors, and large-scale circuits. Additionally, performance optimization strategies are discussed for Te-based field-effect transistors. Finally, insights into future research directions and the challenges that lie ahead in this field are shared.

Nature Materials, Published online: 16 September 2024; doi:10.1038/s41563-024-02003-x

Second-order superlattices emerge from the interference between moiré superlattices of comparable periodicities. Direct real-space visualization reveals their rich structural diversity and extreme sensitivity to external parameters such as strain and twist angle.

Nature Reviews Materials, Published online: 12 September 2024; doi:10.1038/s41578-024-00711-z

To meet the physical demands of a new environment, organisms evolve morphological and behavioural adaptations that specialize their locomotor performance to that niche. This Perspective discusses how robots can emulate — and perhaps even exceed — biological levels of adaptability through shape-morphing mechanisms and complementary control strategies to achieve compressed, rapid and reversible ‘evolution on demand’.

npj 2D Materials and Applications, Published online: 16 September 2024; doi:10.1038/s41699-024-00498-1

Piezoelectricity in NbOI₂ for piezotronics and nanogenerators