Jiuxiang Dai

The atomic structure of single-monolayer Na₂S film on metal surfaces is first disclosed, which depends on the underlying metals and thermal annealing. These structures are well supported by DFT simulation based on the corresponding Na₂S up, down, and standing models. Further, a wide bandgap of Na₂S single-monolayer film is measured, which decreases from Au(111) to Ag(111) and Cu(111).

Abstract

Na₂S is a key material in sodium-sulfur batteries and its single-monolayer film may also be a new 2D material. Herein, the results are first reported by STM, XPS, and DFT simulation that the Na₂S single-monolayer film shows different atomic structures depending on the underlying metals, and these structures can be changed by thermal annealing. All structures are well supported by DFT simulation on corresponding Na₂S up, down, and standing models, as well as the NaS in-plane model. Further, a wide bandgap of Na₂S single-monolayer film is measured, which decreases from Au(111) to Ag(111) and Cu(111) due to the different Na₂S interactions with metal substrates. Finally, it is demonstrated that dibenzo 18-crown-6-ether molecules can grow on phase VI of Na₂S film at room temperature, as compared with preferred metal surfaces and Na₂S phase III structure.

The first experimental observation of interlayer exciton emission in telecom C-band based on a CVD-synthesized MoTe₂/MoS₂ heterobilayer is reported, and further comprehensive electrical and magnetic control over the polarization, wavelength, and intensity of these interlayer excitons are demonstrated. The ability to integrate all these functions in a single excitonic device can pave the way for novel silicon photonics and all-optical telecommunications.

Abstract

Excitonic devices based on interlayer excitons in van der Waals heterobilayers are a promising platform for advancing photoelectric interconnection telecommunications. However, the absence of exciton emission in the crucial telecom C-band has constrained their practical applications. Here, this limitation is addressed by reporting exciton emission at 0.8 eV (1550 nm) in a chemically vapor-deposited, strictly aligned MoTe₂/MoS₂ heterobilayer, resulting from the direct bandgap transitions of interlayer excitons as identified by momentum–space imaging of their electrons and holes. The decay mechanisms dominated by direct radiative recombination ensure constant emission quantum yields, a basic demand for efficient excitonic devices. The atomically sharp interface enables the resolution of two narrowly-splitter transitions induced by spin–orbit coupling, further distinguished through the distinct Landé g-factors as the fingerprint of spin configurations. By electrical control, the double transitions coupling into opposite circularly-polarized photon modes, preserve or reverse the helicities of the incident light with a degree of polarization up to 90%. The Stark effect tuning extends the emission energy range by over 150 meV (270 nm), covering the telecom C-band. The findings provide a material platform for studying the excitonic complexes and significantly boost the application prospects of excitonic devices in silicon photonics and all-optical telecommunications.

Ferrimagnetic and ferroelectric order–disorder transition temperatures of 2D ɛ-Fe₂O₃ up to 800 K are experimentally demonstrated for the first time. Unexpectedly, the hexagonal polycrystal nanosheets composed of single crystal domains exhibit easily switchable ferroelectric polarizations comparable to that of rectangle single crystals. The existence of grain boundary does not hinder the switching and retention of ferroelectric polarization.

Abstract

2D single-phase multiferroic materials with the coexistence of electric and spin polarization offer a tantalizing potential for high-density multilevel data storage. One of the current limitations for application is the scarcity of the materials, especially those combine ferromagnetism and ferroelectricity at high temperatures. Here, robust ferrimagnetism and ferroelectricity in 2D ɛ-Fe₂O₃ samples with both single-crystalline and polycrystalline form are demonstrated. Interestingly, the polycrystalline nanosheets also exhibit easily switchable ferroelectric polarizations comparable to that of single crystals. The existence of grain boundary does not hinder the switching and retention of ferroelectric polarization. Furthermore, the ɛ-Fe₂O₃ nanosheets show ferrimagnetic and ferroelectric Curie temperatures up to 800 K, which reaches record highs in 2D single-phase multiferroic materials. This work provides important progress in the exploration of 2D high-temperature single-phase multiferroics for potentially compact high-temperature information nanodevices.

This review focuses on the emerging reversible crystalline-to-crystalline phase transitions in phase-change materials. It delves into the atomic structures and switching mechanisms of these atypical transitions, provides insights into their thermodynamic and kinetic features, and offers an outlook on the opportunities for developing novel phase-change memory devices and on challenges that need to be addressed.

Abstract

Phase-change random access memory (PCRAM) is one of the most technologically mature candidates for next-generation non-volatile memory and is currently at the forefront of artificial intelligence and neuromorphic computing. Traditional PCRAM exploits the typical phase transition and electrical/optical contrast between non-crystalline and crystalline states of chalcogenide phase-change materials (PCMs). Currently, traditional PCRAM faces challenges that vastly hinder further memory optimization, for example, the high-power consumption, significant resistance drift, and the contradictory nature between crystallization speed and thermal stability, nearly all of them are related to the non-crystalline state of PCMs. In this respect, a reversible crystalline-to-crystalline phase transition can solve the above problems. This review delves into the atomic structures and switching mechanisms of the emerging atypical crystalline-to-crystalline transitions, and the understanding of the thermodynamic and kinetic features. Ultimately, an outlook is provided on the future opportunities that atypical all-crystalline phase transitions offer for the development of a novel PCRAM, along with the key challenges that remain to be addressed.

A ubiquitous stripe structure adsorbate that has previously been observed on graphene and graphitic surfaces is examined within twisted graphene-graphene interfaces. This adsorbate is observed to be stable during sliding of layers in this interface and does not destroy the low friction (superlubricious) contact at the interface under ambient conditions.

Abstract

Van der Waals heterostructures formed by stacked 2D materials show exceptional electronic, mechanical, and optical properties. Superlubricity, a condition where atomically flat, incommensurate planes of atoms result in ultra-low friction, is a prime example enabling, for example, self-assembly of optically visible graphene nanostructures in air via a sliding auto-kirigami process. Here, it is demonstrated that a subtle but ubiquitous adsorbate stripe structure found on graphene and graphitic surfaces in ambient conditions remains stable within the interface between twisted graphene layers as they slide over each other. Despite this contamination, the interface retains an exceptional superlubricious state with an estimated upper bound frictional shear strength of 10 kPa, indicating that direct atomic incommensurate contact is not required to achieve ambient superlubricity for 2D materials. The results suggest that any phenomena depending on 2D heterostructure interfaces such as exotic electronic behavior may need to consider the presence of stripe adsorbate structures that remain intercalated.

The development of a true random number generator (TRNG) based on stochastic ferroelectric polarization switching in 2D CuInP₂S₆ materials, is reported. The TRNG exhibits exceptional stochastic variability, surpassing conventional entropy metrics, and passes the National Institute of Standards and Technology (NIST) randomness tests. Moreover, TRNG's unclonability is demonstrated through device-to-device variation, providing resilience against machine learning (ML) attacks.

Abstract

True random number generators (TRNGs), which create cryptographically secure random bitstreams, hold great promise in addressing security concerns regarding hardware, communication, and authentication in the Internet of Things (IoT) realm. Recently, TRNGs based on nanoscale materials have gained considerable attention for avoiding conventional and predictable hardware circuitry designs that can be vulnerable to machine learning (ML) attacks. In this article, a low-power and low-cost TRNG developed by exploiting stochastic ferroelectric polarization switching in 2D ferroelectric CuInP₂S₆ (CIPS)-based capacitive structures, is reported. The stochasticity arises from the probabilistic switching of independent electrical dipoles. The TRNG exhibits enhanced stochastic variability with near-ideal entropy, uniformity, uniqueness, Hamming distance, and independence from autocorrelation variations. Its unclonability is systematically examined using device-to-device variations. The generated cryptographic bitstreams pass the National Institute of Standards and Technology (NIST) randomness tests. This nanoscale CIPS-based TRNG is circuit-integrable and exhibits potential for hardware security in edge devices with advanced data encryption.

In this study, the doping of deep-level element sulfur (S) is synergized with the device architecture incorporating a blocked impurity band (BIB) to fabricate an advanced infrared detector. The S-doped Si (Si:S) BIB detector exhibited excellent short-wave infrared detection performance. The multi-scenario application demonstrations further confirmed its potential for practical use.

Abstract

Silicon (Si)-based photodetectors are cost-effective, eco-friendly, and compatible with on-chip complementary metal-oxide-semiconductor (CMOS) technology. However, expanding their photoresponse into the short-wavelength-infrared region beyond 1.1 µm remains challenging because of the intrinsic bandgap of Si. In this study, an ion implantation sulfur-doped Si-based infrared photodetector featuring a blocked impurity band (BIB) structure is investigated. The detector achieves an extended sub-bandgap infrared response up to 2 µm through impurity band transitions, facilitated by the artificial creation of a deep-level impurity band within the Si bandgap. The device exhibited a competitive photodetection effect with a high responsivity of 107.3 mA W⁻¹ and an external quantum efficiency of 10.2%. Notably, the detector yielded an enhanced response speed with a raising time of 46 µs and a decay time of 135 µs at 1310 nm under room temperature. Finally, the versatile applications of this detector is presented in a multitude of scenarios encompassing material composition identification, multi-band imaging, and optical communication. The proposed investigation not only proposed a method for Si-based infrared detectors but also constituted a contribution to the advancement of silicon photonics.

An exceptional pH-universal electrocatalyst with multifunctional phosphorus sites and multilevel interfaces is constructed from in situ hybridization, which demands ultrasmall overpotentials of 115, 140, and 290 mV to drive 500 mA cm⁻² in basic, acidic, and neutral media, prominently outperforming most of non-noble catalysts. As confirmed by theoretical calculations and in situ XPS analysis, Ni₃N/CoP interface acts as a water dissociation promoter, while Co₃N/CoP serves as a hydrogen acceptor, thereby synergistically promoting hydrogen evolution kinetics in different pH electrolytes.

Abstract

Hydrogen evolution reaction (HER) in neutral or alkaline electrolytes is appealing for sustainable hydrogen production driven by water splitting, but generally suffers from unsatisfied catalytic activities at high current densities owing to extra kinetic energy barriers required to generate protons through water dissociation. In response, here, a competitive Ni₃N/Co₃N/CoP electrocatalyst with multifunctional interfacial sites and multilevel interfaces, in which Ni₃N/CoP performs as active sites to boost initial water dissociation and Co₃N/CoP accelerates subsequent hydrogen adsorption process as confirmed by density functional theory calculations and in situ X-ray photoelectron spectroscopy analysis, is reported. This hybrid catalyst possesses extraordinary HER activity in base, featured by extremely low overpotentials of 115 and 142 mV to afford 500 and 1000 mA cm⁻², respectively, outperforming most ever-reported metal phosphides-based catalysts. This catalyst presents an ultrahigh current density of 3545 mA cm⁻² by a factor of 4.96 relative to noble Pt/C catalysts (715 mA cm⁻²) at 0.2 V. Assembled with Fe(PO₃)₂/Ni₂P anode, industrial-level current densities of 500/1000 mA cm⁻² at ultralow cell voltages of 1.62/1.66 V for overall water electrolysis with outstanding long-term stability are actualized. More interestingly, this hybrid catalyst also performs well in acidic, neutral freshwater, and seawater requiring relatively low overpotentials of 140, 290, and 331 mV to reach 500 mA cm⁻². Particularly, this catalyst can withstand electrochemical corrosion without obvious activity decay at the industrial-level current densities for over 100 h in base. This work provides a cornerstone for the construction of advanced catalysts operated in different pH environments.

Integrating low-dimensional InAs-based materials with van der Waals systems advances electronics, optics, and magnetics, promoting miniaturization per Moore's Law. However, progress lags due to synthesis challenges and surface state effects. This review addresses experimental advances in the vdW epitaxy of InAs, theoretical system design achievements, and proposes novel growth techniques and deep learning integration to overcome bottlenecks.

Abstract

The strategic integration of low-dimensional InAs-based materials and emerging van der Waals systems is advancing in various scientific fields, including electronics, optics, and magnetics. With their unique properties, these InAs-based van der Waals materials and devices promise further miniaturization of semiconductor devices in line with Moore's Law. However, progress in this area lags behind other 2D materials like graphene and boron nitride. Challenges include synthesizing pure crystalline phase InAs nanostructures and single-atomic-layer 2D InAs films, both vital for advanced van der Waals heterostructures. Also, diverse surface state effects on InAs-based van der Waals devices complicate their performance evaluation. This review discusses the experimental advances in the van der Waals epitaxy of InAs-based materials and the working principles of InAs-based van der Waals devices. Theoretical achievements in understanding and guiding the design of InAs-based van der Waals systems are highlighted. Focusing on advancing novel selective area growth and remote epitaxy, exploring multi-functional applications, and incorporating deep learning into first-principles calculations are proposed. These initiatives aim to overcome existing bottlenecks and accelerate transformative advancements in integrating InAs and van der Waals heterostructures.

The intrinsic nature of transport is presented by hole carriers in 2D copper without grain boundaries. Twin boundaries almost do not scatter electrons. The hidden hole-like properties of 2D copper can only be unmasked by thorough removal of grain boundaries. The existence of hole carriers is revealed in 2D copper through angle-resolved photoemission spectroscopy and nonlinear Hall effect measurements.

Abstract

In 2D noble metals like copper, the carrier scattering at grain boundaries has obscured the intrinsic nature of electronic transport. However, it is demonstrated that the intrinsic nature of transport by hole carriers in 2D copper can be revealed by growing thin films without grain boundaries. As even a slight deviation from the twin boundary is perceived as grain boundaries by electrons, it is only through the thorough elimination of grain boundaries that the hidden hole-like attribute of 2D single-crystal copper can be unmasked. Two types of Fermi surfaces, a large hexagonal Fermi surface centered at the zone center and the triangular Fermi surface around the zone corner, tightly matching to the calculated Fermi surface topology, confirmed by angle-resolved photoemission spectroscopy (ARPES) measurements and vivid nonlinear Hall effects of the 2D single-crystal copper account for the presence of hole carriers experimentally. This breakthrough suggests the potential to manipulate the majority carrier polarity in metals by means of grain boundary engineering in a 2D geometry.

This study presents edge passivation for p-type monolayer WSe₂ nanoribbon transistors using nanolithography and a controlled remote O₂ plasma process. The edge passivation material consists of amorphous WO_xSe_y. Compared to microribbons, nanoribbons with passivated edges exhibit reduced defect density and increased p-doping, whereas nanoribbons with open edges exhibit higher defect densities and reduced p-doping.

Abstract

The ongoing reduction in transistor sizes drives advancements in information technology. However, as transistors shrink to the nanometer scale, surface and edge states begin to constrain their performance. 2D semiconductors like transition metal dichalcogenides (TMDs) have dangling-bond-free surfaces, hence achieving minimal surface states. Nonetheless, edge state disorder still limits the performance of width-scaled 2D transistors. This work demonstrates a facile edge passivation method to enhance the electrical properties of monolayer WSe₂ nanoribbons, by combining scanning transmission electron microscopy, optical spectroscopy, and field-effect transistor (FET) transport measurements. Monolayer WSe₂ nanoribbons are passivated with amorphous WO_xSe_y at the edges, which is achieved using nanolithography and a controlled remote O₂ plasma process. The same nanoribbons, with and without edge passivation are sequentially fabricated and measured. The passivated-edge nanoribbon FETs exhibit 10 ± 6 times higher field-effect mobility than the open-edge nanoribbon FETs, which are characterized with dangling bonds at the edges. WO_xSe_y edge passivation minimizes edge disorder and enhances the material quality of WSe₂ nanoribbons. Owing to its simplicity and effectiveness, oxidation-based edge passivation could become a turnkey manufacturing solution for TMD nanoribbons in beyond-silicon electronics and optoelectronics.

Buckled AB honeycomb lattice with C_3v symmetry belongs to polar space group P3m ₁, is proposed to be a promising candidate hosting topological Dirac fermions and out-of-plane polarity. Here, the successful realization of a IV-V binary compound on Ag (111) surface via molecular beam epitaxy, is reported. Strikingly, polarity and metallicity are found to coexist in the 2D buckled SiP monolayer.

Abstract

Structural topology and symmetry of a two-dimensional (2D) network play pivotal roles in defining its electrical properties and functionalities. Here, a binary buckled honeycomb lattice with C_3v symmetry, which naturally hosts topological Dirac fermions and out-of-plane polarity, is proposed. It is successfully achieved in a group IV-V compound, namely monolayer SiP epitaxially grown on Ag(111) surface. Combining first-principles calculations with angle-resolved photoemission spectroscopy, the degeneration of the Dirac nodal lines to points due to the broken horizonal mirror symmetry is elucidated. More interesting, the SiP monolayer manifests metallic nature, which is mutually exclusive with polarity in conventional materials. It is further found that the out-of-plane polarity is strongly suppressed by the metallic substrate. This study not only represents a breakthrough of realizing intrinsic polarity in 2D metallic material via ingenious design but also provides a comprehensive understanding of the intricate interplay of many exotic low-dimensional quantum phenomena.

Pristine van der Waals homojunctions consisting of 2H-MoTe₂ layers with asymmetric thickness are constructed to eliminate heterogenous interfaces and obtain clean boundaries. The layer-engineered energy bands of 2H-MoTe₂ layers induce a built-in electric field at the interface, enabling self-powered photodetection. Furthermore, a 10 × 10 array based on 2H-MoTe₂ homojunction devices realize zero-power consumption imaging functions.

Abstract

Van der Waals junctions hold significant potentials for various applications in multifunctional and low-power electronics and optoelectronics. The multistep device fabrication process usually introduces lattice mismatch and defects at the junction interfaces, which deteriorate device performance. Here the layer engineering synthesis of van der Waals homojunctions consisting of 2H-MoTe₂ with asymmetric thickness to eliminate heterogenous interfaces and thus obtain clean interfaces is reported. Experimental results confirm that the homostructure nature gives rise to the formation of pristine van der Waals junctions, avoiding chemical disorders and defects. The ability to tune the energy bands of 2H-MoTe₂ continuously through layer engineering enables the creation of adjustable built-in electric field at the homojunction boundaries, which leads to the achievement of self-powered photodetection based on the obtained 2H-MoTe₂ films. Furthermore, the successful integration of 2H-MoTe₂ homojunctions into an image sensor with 10 × 10 pixels, brings about zero-power consumption and near-infrared imaging functions. The pristine van der Waals homojunctions and effective integration strategies shed new insights into the development of large-scale application for two-dimensional materials in advanced electronics and optoelectronics.

The pressure induced re-entrant superconductivity (SC-II) in a typical charge-density-wave (CDW) material TiSe₂ is characterized by using high-pressure electrical transport, X-ray diffraction, and Raman spectroscopy measurements, which unveil that a pressure-induced structure transition can account for the re-entrant superconductivity.

Abstract

Transition metal dichalcogenide TiSe₂ exhibits a superconducting dome within a low pressure range of 2–4 GPa, which peaks with the maximal transition temperature T_c of ≈1.8 K. Here it is reported that applying high pressure induces a new superconducting state in TiSe₂, which starts at ≈16 GPa with a substantially higher T_c that reaches 5.6 K at ≈21.5 GPa with no sign of decline. Combining high-throughput first-principles structure search, X-ray diffraction, and Raman spectroscopy measurements up to 30 GPa, It is found that TiSe₂ undergoes a first-order structural transition from the 1T phase under ambient pressure to a new 4O phase under high pressure. Comparative ab initio calculations reveal that while the conventional phonon-mediated pairing mechanism may account for the superconductivity observed in 1T-TiSe₂ under low pressure, the electron-phonon coupling of 4O-TiSe₂ is too weak to induce a superconducting state whose transition temperature is as high as 5.6 K under high pressure. The new superconducting state found in pressurized TiSe₂ requires further study on its underlying mechanism.

A novel ultra-low power consumption strain sensor by using dendritic bilayer MoS₂ with reduced contact resistance (5.4 kΩ μm). The sensor offers high sensitivity, high resolution (0.04%), and minimal power consumption (33 pW), making it ideal for wearable and flexible electronics in health monitoring applications.

Abstract

Two-dimensional transition metal dichalcogenides (2D TMDs) are promising as sensing materials for flexible electronics and wearable systems in artificial intelligence, tele-medicine, and internet of things (IoT). Currently, the study of 2D TMDs-based flexible strain sensors mainly focuses on improving the performance of sensitivity, response, detection resolution, cyclic stability, and so on. There are few reports on power consumption despite that it is of significant importance for wearable electronic systems. It is still challenging to effectively reduce the power consumption for prolonging the endurance of electronic systems. Herein, we propose a novel approach to realize ultra-low power consumption strain sensors by reducing the contact resistance between metal electrodes and 2D MoS₂. A dendritic bilayer MoS₂ has been designed and synthesized by a modified CVD method. Large-area edge contact has been introduced in the dendritic MoS₂, resulting in decreased the contact resistance significantly. The contact resistance can be down to 5.4 kΩ μm, which is two orders of magnitude lower than the conventional MoS₂ devices. We fabricate a flexible strain sensor, exhibiting superior sensitivity in detecting strains with high resolution (0.04%) and an ultra-low power consumption (33.0 pW). This study paves the way for future wearable and flexible sensing electronics with high sensitivity and ultra-low power consumption.

Nature Electronics, Published online: 17 July 2024; doi:10.1038/s41928-024-01198-w

Nature Electronics, Published online: 22 July 2024; doi:10.1038/s41928-024-01223-y

Nature Electronics, Published online: 22 July 2024; doi:10.1038/s41928-024-01221-0

Nature Communications, Published online: 18 July 2024; doi:10.1038/s41467-024-50452-4

Nature Communications, Published online: 22 July 2024; doi:10.1038/s41467-024-50535-2

Nature Nanotechnology, Published online: 18 July 2024; doi:10.1038/s41565-024-01743-w

Highlights

• Plasma-assisted phosphorization has been used to prepare defect-rich metal phosphides.

• The p-NiCoP/NCF@CC shows high activities and excellent stability for the hydrogen evolution reaction and the oxygen evolution reaction.

• The p-NiCoP/NCF@CC shows great potential for overall water splitting.