Jiuxiang Dai

This work demonstrates the feasibility of synthesizing stable aliovalent alloys of transition metal dichalcogenide materials in the single-layer limit throughout the entire alloy composition range. Furthermore, an experimental and theoretical atomic-scale characterization of the evolution of the electronic ground state (both the electronic structure and collective electronic phases) of a 2D Ising superconductor (single-layer NbSe₂) with structural disorder is reported.

Abstract

Transition metal dichalcogenides offer unprecedented versatility to engineer 2D materials with tailored properties to explore novel structural and electronic phase transitions. In this work, the atomic-scale evolution of the electronic ground state of a monolayer of Nb_{1− δ} Mo _δ Se₂ across the entire alloy composition range (0 < δ < 1) is investigated using low-temperature (300 mK) scanning tunneling microscopy and spectroscopy (STM/STS). In particular, the atomic and electronic structure of this 2D alloy throughout the metal to semiconductor transition (monolayer NbSe₂ to MoSe₂) is studied. The measurements enable extraction of the effective doping of Mo atoms, the bandgap evolution and the band shifts, which are monotonic with δ. Furthermore, it is demonstrated that collective electronic phases (charge density wave and superconductivity) are remarkably robust against disorder and further shown that the superconducting T _C changes non-monotonically with doping. This contrasting behavior in the normal and superconducting state is explained using first-principles calculations. Mo doping is shown to decrease the density of states at the Fermi level and the magnitude of pair-breaking spin fluctuations as a function of Mo content. These results paint a detailed picture of the electronic structure evolution in 2D TMD alloys, which is of utmost relevance for future 2D materials design.

npj 2D Materials and Applications, Published online: 04 March 2022; doi:10.1038/s41699-022-00287-8

Coexisting charge density wave (CDW) and nontrivial topology is predicted in monolayer NbTe₂. In its CDW phase, monolayer NbTe₂ changes from a quantum spin Hall insulator to a quantum anomalous Hall insulator with increasing electronic correlations. This finding establishes NbTe₂ as a promising candidate to explore exotic quantum states at the confluence of nontrivial topology, electronic correlation, and CDW.

Abstract

The combination of nontrivial topology and charge density wave (CDW) has been proposed as a powerful resource for realizing novel quantum phenomena such as axion electrodynamics and the anomalous Hall effect. Hence, topological materials with CDW states attract great interest, yet they are still very rare, particularly in the 2D limit. Here, it is predicted that monolayer NbTe₂ in its high-symmetry 1T phase stabilized by anharmonicity at room temperature exhibits nontrivial topology sensitive to electronic interactions: it changes from a quantum spin Hall (QSH) state to a quantum anomalous Hall (QAH) state when the Hubbard potential U exceeds a critical value. At low temperature, a 4 × 4 CDW order emerges and coexists with the nontrivial topology. Meanwhile, the critical U increases because CDW reduces density of states at Fermi level. More interestingly, in contrast to the high-symmetry structure that actually is a topologically nontrivial metal, the CDW structure shows an insulating nontrivial gap either in the QSH or the QAH phase, indicating CDW is an effective means to modulate the topological state for developing new functions and devices. These discoveries establish NbTe₂ as a promising candidate to explore exotic quantum states at the confluence of nontrivial topology, electronic correlation, and CDW.

Merits of tunable narrow bandgap, high environmental stability, and lower thermal conductivity have endowed 2D tellurene and tellurides significant potential in optoelectronics. This review gives an intriguing bottom-to-top organization from important electronic and optical properties of such materials and extends to their practical photodetector applications, and some challenges and opportunities for 2D Te-based photodetectors are also concluded.

Abstract

As with all stylish 2D functional materials, tellurene and tellurides possessing excellent physical and chemical properties such as high environmental stability, tunable narrow bandgap, and lower thermal conductivity, have aroused the great interest of the researchers. These properties of such materials also form the basis for relatively newfangled scholarly fields involving advanced topics, especially for broadband photodetectors. Integrating the excellent properties of many 2D materials, tellurene/telluride-based photodetectors show great flexibility, higher frequency response or faster time response, high signal-to-noise ratio, and so on, which make them leading the frontier of photodetector research. To fully understand the excellent properties of tellurene/tellurides and their optoelectronic applications, the recent advances in tellurene/telluride-based photodetectors are maximally summarized. Benefiting from the solid research in this field, the challenges and opportunities of tellurene/tellurides for future optoelectronic applications are also discussed in this review, which might provide possibilities for the realization of state-of-the-art high-performance tellurene/telluride-based devices.

Atomically thin 2D film, the semimetallic TaSe₂, is fabricated with high performance for surface-enhanced Raman scattering (SERS). As a plasmon-free substrate, it exhibits great efficiency in jaundice diagnosis by detecting the bilirubin in serum and urine. And this sheds light on the clinical application of the SERS technique without noble metals.

Abstract

An atomically thin TaSe₂ sample, approximately containing two to three layers of TaSe₂ nanosheets with a diameter of 2.5 cm is prepared here for the first time and applied on the detection of various Raman-active molecules. It achieves a limit of detection of 10⁻¹⁰ m for rhodamine 6G molecules. The excellent surface-enhanced Raman scattering (SERS) performance and underlying mechanism of TaSe₂ are revealed using spectrum analysis and density functional theory. The large adsorption energy and the abundance of filled electrons close to the Fermi level are found to play important roles in the chemical enhancement mechanism. Moreover, the TaSe₂ film enables highly sensitive detection of bilirubin in serum and urine samples, highlighting the potential of using 2D SERS substrates for applications in clinical diagnosis, for example, in the diagnosis of jaundice caused by excess bilirubin in newborn children.

Highly active metal-free bifunctional ORR-OER catalyst fabricated by high-flux plasma enhanced chemical vapor deposition with in situ plasma diagnostics, thus enabling high-performance Zn-air batteries outperforming those using catalysts based on rare and precious metals.

Abstract

Rechargeable zinc–air batteries (ZABs) have attracted great interests for emerging energy applications. Nevertheless, one of the major bottlenecks lies in the fabrication of bifunctional catalysts with high electrochemical activity, high stability, low cost, and free of precious and rare metals. Herein, a high-performance metal-free bifunctional catalyst is synthesized in a single step by regulating radicals within the recently invented high-flux plasma enhanced chemical vapor deposition (HPECVD) system equipped with in situ plasma diagnostics. Thus-derived (N, O)-doped vertical few-layer graphene film (VGNO) is of high areal population with perfect vertical orientation, tunable catalytic states, and configurations, thus enabling significantly enhanced electrochemical kinetic processes of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with reference to milestone achievements to date. Application of such VGNO to aqueous ZABs (A-ZABs) and flexible solid-state ZABs (S-ZABs) exhibited high discharge power density and excellent cycling stability, which remarkably outperformed ZABs using benchmarked precious-metal based catalysts. The current work provides a solid basis toward developing low-cost, resource-sustainable, and eco-friendly ZABs without using any metals for outstanding OER and ORR catalysis.

Abstract

Lead sulfide (PbS), a typical functional semiconductor material, has attracted serious attention due to its great potential in optoelectronics applications. However, controllable growth of PbS single-crystal film still remains a great challenge. Here, we report heteroepitaxial growth of large-scale highly crystalline PbS films on alkali salt (NaCl and KCl) substrates via chemical vapor deposition (CVD). Structural characterizations demonstrate that the as-grown PbS films exhibit an atomically sharp interface with the underlying substrates. The epitaxial relationships between the epilayers and substrates were determined to be PbS (100)//NaCl (100) or KCl (100), PbS [010]//NaCl [010] or KCl [010]. Owing to the high solubility of alkali salt, the epitaxial PbS films can be rapidly released from the underlying substrates and transferred to other substrates of interest while maintaining good integrity and crystallinity, the process of which is particularly attractive in the fields of electronics and optoelectronics. Furthermore, photodetectors based on the transferred PbS films were fabricated, exhibiting a high photoresponsivity of 7.5 A/W, a detectivity of 1.44 × 10¹² Jones, and a rapid response time of approximately 0.25 s. This work sheds light on the batch production, green transfer, and optoelectronic application of PbS films.

The exchange bias effect in ferro-/antiferromagnetic Fe₃GeTe₂ (FGT)/CrOCl heterostructure is thoroughly studied through anomalous Hall effect and reflective magnetic circular dichroism (RMCD) measurements. The bias field (H_EB ) reveals anomaly increase with the presence of an oxidized layer in FGT. The larger H_EB in the RMCD measurements can be related to the distribution of uncompensated spins.

Abstract

The exchange bias effect is extremely expected in 2D van der Waals (vdW) ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures due to the high-quality interface. CrOCl possesses strong magnetic anisotropy at 2D limit, and is an ideal antiferromagnet for constructing FM/AFM heterostructures to explore the exchange bias effect. Here, the exchange bias effect in Fe₃GeTe₂ (FGT)/CrOCl heterostructures through both anomalous Hall effect (AHE) and reflective magnetic circular dichroism (RMCD) measurements is studied. In the AHE measurements, the exchange bias field (H_EB ) at 3 K exhibits a distinct increase from ≈150 Oe to ≈450 Oe after air exposure, and such variation is attributed to the formation of an oxidized layer in FGT by analyzing the cross-sectional microstructure. The H_EB is successfully tuned by changing the FGT/CrOCl thickness and the cooling field. Furthermore, a larger H_EB of ≈750 Oe at 1.7 K in FGT/CrOCl heterostructure through RMCD measurements is observed, and it is proposed that the larger H_EB in RMCD measurements is related to the distribution of uncompensated spins at the interface. This work reveals several intriguing phenomena of the exchange bias effect in 2D vdW magnetic systems, which paves the way for the study of related spintronic devices.

Nature Energy, Published online: 03 March 2022; doi:10.1038/s41560-022-00985-z

Nature Electronics, Published online: 03 March 2022; doi:10.1038/s41928-022-00722-0

Interlayer Coupling

In article number 2106053, Kyung-Tae Ko, Kimoon Lee, and co-workers unveil the crucial role of the Pd 4d orbital distribution on the strong interlayer interaction, which is attributed to the combined effect of local symmetry and unique occupancy. The cover image illustrates that there exists a strong degree of interlayer interaction in 2D PdSe₂ originating from the directional orbitals even under a large interlayer distance.

Dative epitaxy represents the Goldilocks’ principle of epitaxy: it takes advantage of dative bonding for fixing the atomic registry and crystal orientation, while ensuring the full flexibility of van der Waals epitaxy. The globally commensurate Cr₅Te₈/WSe₂ moiré supercrystal is distinctly different from conventional incommensurate moiré superlattices or local commensurate domains.

Abstract

Realizing van der Waals (vdW) epitaxy in the 1980s represents a breakthrough that circumvents the stringent lattice matching and processing compatibility requirements in conventional covalent heteroepitaxy. However, due to the weak vdW interactions, there is little control over film qualities by the substrate. Typically, discrete domains with a spread of misorientation angles are formed, limiting the applicability of vdW epitaxy. Here, the epitaxial growth of monocrystalline, covalent Cr₅Te₈ 2D crystals on monolayer vdW WSe₂ by chemical vapor deposition is reported, driven by interfacial dative bond formation. The lattice of Cr₅Te₈, with a lateral dimension of a few tens of micrometers, is fully commensurate with that of WSe₂ via 3 × 3 (Cr₅Te₈)/7 × 7 (WSe₂) supercell matching, forming a single-crystalline moiré superlattice. This work establishes a conceptually distinct paradigm of thin-film epitaxy, termed “dative epitaxy”, which takes full advantage of covalent epitaxy with chemical bonding for fixing the atomic registry and crystal orientation, while circumventing its stringent lattice matching and processing compatibility requirements; conversely, it ensures the full flexibility of vdW epitaxy, while avoiding its poor orientation control. Cr₅Te₈ 2D crystals grown by dative epitaxy exhibit square magnetic hysteresis, suggesting minimized interfacial defects that can serve as pinning sites.

Ultrathin membranes have undergone rapid development for fast and precise separations in the past 15 years. Here, a review is given on the assembly, structure, materials, and transport of sub-50 nm membranes, with a focus on their fabrication-structure-transport relationships. Applications for liquid and gas separations are analyzed and compared to provide insights for future advances in high-performance membranes.

Abstract

Ultrathin membranes are at the forefront of membrane research, offering great opportunities in revolutionizing separations with ultrafast transport. Driven by advanced nanomaterials and manufacturing technology, tremendous progresses are made over the last 15 years in the fabrications and applications of sub-50 nm membranes. Here, an overview of state-of-the-art ultrathin membranes is first introduced, followed by a summary of the fabrication techniques with an emphasis on how to realize such extremely low thickness. Then, different types of ultrathin membranes, categorized based on their structures, that is, network, laminar, or framework structures, are discussed with a focus on the interplays among structure, fabrication methods, and separation performances. Recent research and development trends are highlighted. Meanwhile, the performances and applications of current ultrathin membranes for representative separations (gas separation and liquid separation) are thoroughly analyzed and compared. Last, the challenges in material design, structure construction, and coordination are given, in order to fully realize the potential of ultrathin membranes and facilitate the translation from scientific achievements to industrial productions.

Topological Insulators

In article number 2106172, Tianxiao Nie, Jimin Zhao, Xiaojun Wu, and co-workers report the successful realization of room-temperature 2D ferromagnetism in topological-insulator-induced van der Waals Fe₃GeTe₂, evidenced by ultrafast terahertz emission spectroscopy results. Furthermore, the direction of the external magnetic field is rotated and the sample's front surface reversed to probe the terahertz radiation symmetry.