Jiuxiang Dai

A universal strategy of nonepitaxial synthesizing wafer-scale single-crystal 2D materials on arbitrary insulating substrates is presented. The metal foil in a nonadhered metal–insulator substrate system is almost melted by a brief high-temperature treatment, thereby pressing the as-grown 2D layers to well attach onto the insulators. The findings of this study provide a universal epitaxial platform for single-crystal 2D material production.

Abstract

Next-generation nanodevices require 2D material synthesis on insulating substrates. However, growing high-quality 2D-layered materials, such as hexagonal boron nitride (hBN) and graphene, on insulators is challenging owing to the lack of suitable metal catalysts, imperfect lattice matching with substrates, and other factors. Therefore, developing a generally applicable approach for realizing high-quality 2D layers on insulators remains crucial, despite numerous strategies being explored. Herein, a universal strategy is introduced for the nonepitaxial synthesis of wafer-scale single-crystal 2D materials on arbitrary insulating substrates. The metal foil in a nonadhered metal–insulator substrate system is almost melted by a brief high-temperature treatment, thereby pressing the as-grown 2D layers to well attach onto the insulators. High-quality, large-area, single-crystal, monolayer hBN and graphene films are synthesized on various insulating substrates. This strategy provides new pathways for synthesizing various 2D materials on arbitrary insulators and offers a universal epitaxial platform for future single-crystal film production.

A versatile multi-patterning approach is introduced as a tool to support researchers in studying 2D materials integration into nanoscaled electronic devices. The critical dimension of the self-aligned process does not depend on the diffraction limit but on a bottom-up expansion step. This method supports scalable 2D materials-based electronics at high quality for future studies in the field.

Abstract

Two-dimensional (2D) materials are promising successors for silicon transistor channels in ultimately scaled devices, necessitating significant research efforts to study their behavior at nanoscopic length scales. Unfortunately, current research has limited itself to direct patterning approaches, which limit the achievable resolution to the diffraction limit and introduce unwanted defects into the 2D material. The potential of multi-patterning to fabricate 2D materials features with unprecedented precision and low complexity at large scale is demonstrated here. By combining lithographic patterning of a mandrel and bottom-up self-expansion, this approach enables pattern resolution one order of magnitude below the lithographical resolution. In-depth characterization of the self-expansion double patterning (SEDP) process reveals the ability to manipulate the critical dimension with nanometer precision through a self-limiting and temperature-controlled oxidation process. These results indicate that the SEDP process can regain the quality and morphology of the 2D material, as shown by high-resolution microscopy and optical spectroscopy. This approach is shown to open up new avenues for research into high-performance, ultra-scaled 2D materials devices for future electronics.

Nature Nanotechnology, Published online: 15 December 2023; doi:10.1038/s41565-023-01566-1

Graphene multistacks with four or five layers show the signature of correlated electronic states both in and out of a magnetic field.

Nature Materials, Published online: 15 December 2023; doi:10.1038/s41563-023-01750-7

By forming a heterostructure interface, and by judicious choice of crystallographic orientation, piezoelectrics are developed that show expansion or contraction along all axes on application of an electric field.

Cobalt telluride with stoichiometric degree of freedom, recently emerges as a promising low-dimensional magnet for fundamental studies and device applications. This mini-review summarized the synthesis strategies of different cobalt telluride that exhibits fantastic electrical, magnetic, quantum, and topological properties especially in device environments, attempting to offer useful guidance for boosting novel devices and technologies.

Abstract

Cobalt telluride, with tunable magnetism and ferromagnetism that hinged upon the stoichiometric ratio, has emerged as a new member of 2D materials in the last decade. Metallic doping by sodium (Na) and platinum (Pt) atoms below critical concentrations is found to enhance the magnetism of cobalt telluride. After thinning cobalt telluride down to few-layer, the saturation magnetism is improved by two order of magnitudes because of the oxidation state of cobalt (Co) and reduced coordination number of the surface atoms. In 2D limit, cobalt di-telluride possesses Dirac band structure with many nodal lines that correlate with quantum criticality and other fantastic physics. In this mini-review, the crystal and band structures of cobalt telluride categorized by stoichiometric ratio are overviewed after briefing the introduction. Both top-down and bottom-up methods are then discussed to offer optimum solutions for each specified application scenario. Afterward, emerging magnetic and electronic properties and credit enhancements are launched accompanied with their key advances. Beyond these, the faced challenges and possible directions of future research are also provided, attempting to boost both fundamental physics and device applications based on 2D cobalt telluride.

The power conversion efficiency (PCE) as a function of the photovoltaic bandgap energy (Eg) is shown as summarized by the yearly emerging PV reports (e-PVr) published in Advance Energy Materials over the last 4 years, as well as in the emerging-pv.org website and database.

Abstract

Following the 3rd release of the “Emerging PV reports”, the best achievements in the performance of emerging photovoltaic (e-PV) devices in diverse e-PV research subjects are summarized, as reported in peer-reviewed articles in academic journals since August 2022. Updated graphs, tables, and analyses are provided with several performance parameters, such as power conversion efficiency, open-circuit voltage, short-circuit current density, fill factor, light utilization efficiency, and stability test energy yield. These parameters are presented as a function of the photovoltaic bandgap energy and the average visible transmittance for each technology and application, and are put into perspective using, for example, the detailed balance efficiency limit. The 4th installment of the “Emerging PV reports” discusses the “PV emergence” classification with respect to the “PV technology generations” and “PV research waves” and highlights the latest device performance progress in multijunction and flexible photovoltaics. Additionally, Dale-Scarpulla's plots of efficiency-effort in terms of cumulative academic publication count are also introduced.

A chemical vapor deposition-based sulfur-engineering strategy is reported for the fabrication of copper-based single-atom nanozyme containing unique Cu−N₁S₂ sites. Due to the enhanced adsorption and activation process of H₂O₂ substrate by Cu−N₁S₂ sites, they exhibit significantly high enzyme-like catalytic performance. As a proof-of-concept, the Cu−N₁S₂ sites achieve effective tumor killing through the synergy of catalytic therapy and oxygen-dependent phototherapy.

Abstract

Single-atom nanozymes (SAzymes), with well-defined and uniform atomic structures, are an emerging type of natural enzyme mimics. Currently, it is important but challenging to rationally design high-performance SAzymes and deeply reveal the interaction mechanism between SAzymes and substrate molecules. Herein, this work reports the controllable fabrication of a unique Cu−N₁S₂-centred SAzyme (Cu-N/S-C) via a chemical vapor deposition-based sulfur-engineering strategy. Benefiting from the optimized geometric and electronic structures of single-atom sites, Cu-N/S-C SAzyme shows boosted enzyme-like activity, especially in catalase-like activity, with a 13.8-fold increase in the affinity to hydrogen peroxide (H₂O₂) substrate and a 65.2-fold increase in the catalytic efficiency when compared to Cu-N-C SAzyme with Cu−N₃ sites. Further theoretical studies reveal that the increased electron density around single-atom Cu is achieved through electron redistribution, and the efficient charge transfer between Cu-N/S-C and H₂O₂ is demonstrated to be more beneficial for the adsorption and activation of H₂O₂. The as-designed Cu-N/S-C SAzyme possesses an excellent antitumor effect through the synergy of catalytic therapy and oxygen-dependent phototherapy. This study provides a strategy for the rational design of SAzymes, and the proposed electron redistribution and charge transfer mechanism will help to understand the coordination environment effect of single-atom metal sites on H₂O₂-mediated enzyme-like catalytic processes.

Advanced Materials, Volume 36, Issue 4, January 25, 2024.

Ultrafast microwave plasma is developed to synthesize halogen elements-doped Ru/RuP₂ (X-Ru/RuP₂). The Br-doped electrocatalyst (Br-Ru/RuP₂) exhibits excellent catalytic performances for hydrogen evolution reaction with 34 mV in 1.0 m KOH to achieve 10 mA cm⁻².

Abstract

Anionic modification engineering is a crucial approach to develop highly efficient electrocatalysts for hydrogen evolution reaction. Herein, halogen elements (X = Cl, Br, and I)-modified Ru-based nanosheets (X-Ru/RuP₂) are designed by rapid and eco-friendly microwave-phosphide plasma approach within 60 s. Experimental and density functional theory calculations verify that the introduced halogen element, especially Br, can optimize the surface intermediates adsorption. Specially, the designed Br-Ru/RuP₂ favors the water dissociation and following hydrogen adsorption/desorption process. Then, the as-synthesized Br-Ru/RuP₂ exhibits low overpotential of 34 mV to reach 10 mA cm⁻² coupled with small Tafel slope of 27 mV dec⁻¹ in alkaline electrolyte with excellent long-term stability. Moreover, the electrocatalytic performances in acid and neutral media are also boosted via Br element modification. This work paves a novel way to regulate the electronic structure of Ru-based compounds, and then can boost the electrocatalytic kinetics.

Advanced Materials, Volume 36, Issue 3, January 18, 2024.

Transition metal dichalcogenides (TMDC) in 2D exfoliated layers are promising materials for realizing field-effect transistor electronic devices at the extreme limit of miniaturization. Here, a versatile 2D TMDC material is designed to combine features of both a metal and a semiconductor and is predicted to enable engineering an electronic nanodevice with ideal performances.

Abstract

A chlorine-doped ultrathin phase of hafnium disulfide (HfS₂) is proposed as an ideal candidate material for 2D field-effect transistor (FET) device applications, down to the extreme sub-5 nm miniaturization limit. This transition metal dichalcogenide 2D material is designed to combine features of both a metal and a semiconductor, exhibiting a high electric conductivity comparable with ordinary metals, that can be abruptly cut down via gating due to an energy gap immediately below the Fermi level and its anomalous metallic properties. These unique features enable realizing an alternative design of a FET device in which electrode and channel are made of the same Cl-doped ML HfS₂ phase, a potential breakthrough bypassing all issues associated with electronic (Schottky) and structural dis-homogeneities or low conductivity that have hindered progress in this field. This material/design combination shall lead to a FET device with purely ohmic behavior, high metallic conductance, no interfacial contact resistance, and facile gating with extremely high on/off ratio.

Engineering the microstructure across multi-length scales in potassium sodium niobate enhances the functionality of piezoelectric acoustic sensors. This pioneering approach sets a precedent for utilizing lead-free ferroelectric films in MEMS devices, opening avenues in diverse applications encompassing noise monitoring, nondestructive detection, and hearing aids technologies.

Abstract

With increasing concerns about noise pollution, the pursuit of highly dependable piezoelectric acoustic sensors for real-time noise monitoring has come to the forefront of scientific research. Lead-based perovskite piezoelectric films, exemplified by lead zirconate titanate Pb(Zr,Ti)O₃ (PZT), surpass traditional piezoelectric materials such as ZnO and AlN in their piezoelectric properties, promising substantial advancements in next-generation acoustic sensor technologies. However, the toxic nature of lead in PZT materials poses formidable environmental and human health risks. In an unprecedented breakthrough, it presents the pioneering development of an environmentally benign lead-free piezoelectric Micro-Electro-Mechanical System (MEMS) acoustic sensor based on potassium sodium niobate (K,Na)NbO₃ (KNN) film. High-quality <001> textured 3 µm-thick KNN film is successfully integrated into commercially used Si substrate, rendering exceptional piezoelectricity (transverse piezoelectric coefficients e ₃₁ ^* of ≈8.5 C m⁻²) with satisfactory thermal stability. The atomic-scale Z-contrast imaging and piezoresponse force microscopy characterizations reveal that the outstanding piezoresponse originates from the local coexistence of multiple phases and the enhancement of extrinsic piezoelectric contributions from in-plane polarization anisotropy. Finite element simulation is employed to design the triangular cantilever structure and annular diaphragm structure, each corresponding to different operating bandwidths. The resultant MEMS acoustic sensors stand out with outstanding acoustic performance (the high sensitivity and expansive receiving field of view), which are attributed to the microstructural engineering at multi-length scales for the excellent piezoelectric properties of KNN film. These features enable sensitive acoustic monitoring in various environments, including large-scale power grids and urban traffic.

Single propagating electron states were generated and controlled in monolayer graphene.

Nature Protocols, Published online: 14 December 2023; doi:10.1038/s41596-023-00916-6

The detailed design and fabrication of small-scale magnetic soft-bodied robots with multimodal locomotion capability, including the processes required for locomotion control and optimization.

A heterojunction of van der Waals (vdW) SnS with Ga₂O₃ transparent photovoltaic (TPV) device shows a 360^o wFoV with bifacial onsite power production and its real-time application in visual-speech photocommunication aiming to help visually and auditory impaired individuals, promising an environmental-friendly sustainable future. The device shows an ultra-sensitive photoinduced defect-modulated heterojunction and photo capacitance, suggesting photon-flux-driven charge diffusion

Abstract

Omnidirectional photosensing is crucial in optoelectronic devices, enabling a wide field of view (wFoV) and leveraging potential applications for the Internet of Things in sensors, light fidelity, and photocommunication. The wFoV helps overcome the limitations of line-of-sight communication, and transparent photodetection becomes highly desirable as it enables the capture of optical information from various angles. Therefore, developing a photoelectric device with a 360° wFoV, ultra sensitivity to photons, power generation, and transparency is of utmost importance. This study utilizes a heterojunction of van der Waals SnS with Ga₂O₃ to fabricate a transparent photovoltaic (TPV) device showing a 360° wFoV with bifacial onsite power production. SnS/Ga₂O₃ heterojunction preparation consists of magnetron sputtering and is free from nanopatterning/nanostructuring to achieve the desired wFoV window device. The device exhibits a high average visible transmittance of 56%, generates identical power from bifacial illumination, and broadband fast photoresponse. Careful analysis of the device shows an ultra-sensitive photoinduced defect-modulated heterojunction and photocapacitance, revealed by the impedance spectroscopy, suggesting photon-flux driven charge diffusion. Leveraging the wFoV operation, the TPV embedded visual and speech photocommunication prototype demonstrated, aiming to help visually and auditory impaired individuals, promising an environmental-friendly sustainable future.

Pulsed laser deposition (PLD) is an impactful method for wafer-scale growth of 2D films, owing to target-maintained stoichiometry, high growth rate, and efficiency. In this review, the recent advances on the PLD fabrication of 2D films, including the growth mechanisms, strategies, and materials classification are summarized. First, efficacious strategies of PLD growth are highlighted. Then, the growth, characterization, and device applications of various 2D films, like graphene, h-BN, MoS₂, BP, oxide, perovskite, semi-metal etc, are presented. Finally, the potential challenges and further research directions of PLD technique is envisioned.

Abstract

2D thin films, possessing atomically thin thickness, are emerging as promising candidates for next-generation electronic devices, due to their novel properties and high performance. In the early years, a wide variety of 2D materials are prepared using several methods (mechanical/liquid exfoliation, chemical vapor deposition, etc.). However, the limited size of 2D flakes hinders their fundamental research and device applications, and hence the effective large-scale preparation of 2D films is still challenging. Recently, pulsed laser deposition (PLD) has appeared to be an impactful method for wafer-scale growth of 2D films, owing to target-maintained stoichiometry, high growth rate, and efficiency. In this review, the recent advances on the PLD preparation of 2D films are summarized, including the growth mechanisms, strategies, and materials classification. First, efficacious strategies of PLD growth are highlighted. Then, the growth, characterization, and device applications of various 2D films are presented, such as graphene, h-BN, MoS₂, BP, oxide, perovskite, semi-metal, etc. Finally, the potential challenges and further research directions of PLD technique is envisioned.

Scanning transmission electron microscopy (STEM) is used to study cross-sections of epitaxial VO₂ films grown using pulsed laser deposition on three substrates – c-sapphire, TiO₂ (101), and TiO₂ (001). The macroscale differences in the metal–insulator transition characteristics of the films are linked to the structures on the nanoscale, where oxygen atoms are also resolved using advanced STEM imaging.

Abstract

The metal–insulator transition (MIT) observed in vanadium dioxide has been a topic of great research interest for past decades, with the underlying physics yet not fully understood due to the complex electron interactions and structures involved. The ability to understand and tune the MIT behavior is of vital importance from the perspective of both underlying fundamental science as well as potential applications. In this work, scanning transmission electron microscopy (STEM) is used to investigate cross-section lamella of the VO₂ films deposited using pulsed laser deposition on three substrates: c-cut sapphire, TiO₂(101) and TiO₂(001). Advanced STEM imaging is performed in which also the oxygen atom columns are resolved. The overall film quality and structures on atomic and nanoscale are linked to the electrical transition characteristics. Relatively poor MIT characteristics are observed on c-sapphire due to the presence of very small domains with six orientation variants, and on TiO₂ (001) due to the presence of cracks induced by stress relaxation. However, the MIT on TiO₂ (101) behaves favorably, despite similar stress relaxation which, however, only leads to domain boundaries but no cracks.

Nature Communications, Published online: 12 December 2023; doi:10.1038/s41467-023-44119-9

The enhanced Coulomb interaction in two dimensions leads to not only tightly bound excitons but also many-particle excitonic complexes: excitons interacting with other quasiparticles, which results in improved and even new exciton properties with better controls. Here, we summarize studies of excitonic complexes in monolayer transition metal dichalcogenides and their moiré heterojunctions, envisioning how to utilize them for exploring quantum many-body physics.

Optogenetically engineered Escherichia coli (E. coli/eLightOn-smCSE) that biosynthesize CdS quantum dots (QDs) in response to light can be utilized for dynamic information encryption. By harnessing the advantage of living materials, this approach introduces three layers of security: the information remains invisible under natural light, becomes readable only with specific substrates, and auto-deletes after access.

Abstract

Microbial biosynthesis, as an alternative method for producing quantum dots (QDs), has gained attention because it can be conducted under mild and environmentally friendly conditions, distinguishing it from conventional chemical and physical synthesis approaches. However, there is currently no method to selectively control this biosynthesis process in a subset of microbes within a population using external stimuli. In this study, we have attained precise and selective control over the microbial biosynthesis of QDs through the utilization of an optogenetically engineered Escherichia coli (E. coli). The recombinant E. coli is designed to express smCSE enzyme, under the regulation of eLightOn system, which can be activated by blue light. The smCSE enzymes use L-cysteine and Cd²⁺ as substrates to form CdS QDs. This system enables light-inducible bacterial biosynthesis of QDs in precise patterns within a hydrogel for information encryption. As the biosynthesis progresses, the optical characteristics of the QDs change, allowing living materials containing the recombinant E. coli to display time-dependent patterns that self-destruct after reading. Compared to static encryption using fluorescent QD inks, dynamic information encryption based on living materials offers enhanced security.

By revealing tunable photoluminescence behavior under varying UV excitations and demonstrating efficient second-harmonic generation response at 1064 nm, homochiral d¹⁰-metal coordination polymers exemplify unique crystal structures driven by intricate intermolecular interactions between two kinds of ligands, positioning them as promising candidates for multifunctional optical materials.

Abstract

A series of homochiral coordination polymers (HCPs), [M₂(SIAP)₂(bpy)₂] [M(S)] and [M₂(RIAP)₂(bpy)₂] [M(R)] (M = Zn or Cd, SIAP or RIAP = (S,S)- or (R,R)- 2,2′-(isophthaloylbis(azanediyl))di-propionic acid, bpy = 4,4′-bipyridine), is successfully synthesized through solvothermal reactions, self-assembling d¹⁰ metal cations, chiral dicarboxylic ligands, and π-conjugated bipyridyl ligands. The HCPs crystallize in the extremely rare triclinic chiral space group, P1, and present 3D framework structures attributed to the strong intermolecular interactions, such as hydrogen bonds and π–π stacking. Due to the unique crystal structures, the title compounds reveal efficient photoluminescence emission across a broad visible range, with significant brightness and color tuning by varying the excitation wavelength. Moreover, they exhibit efficient phase-matched second-harmonic generation (SHG) with very high laser-induced damage thresholds, essential for high-power nonlinear optical (NLO) applications. Intriguingly, the title compounds exhibit a measurable contrast in the SHG response under right- and left-handed circularly polarized excitation, thereby providing a unique case of SHG circular dichroism from the chiral centers of SIAP²⁻ or RIAP²⁻ ligand packed in the noncentrosymmetric environment. These exceptional attributes position these HCPs as promising candidates for multifunctional materials, with potential applications ranging from NLO devices to tailored luminescent systems with polarization control.

In this study, a bi-oriented step-guided strategy is theoretically proposed for the nucleation and epitaxial growth of twist bilayer graphene, where the twist angle can be predetermined by the orientations of adjacent steps. The presence of bi-oriented steps can greatly reduce the free energy of the tBLG with a matched twist angle, thus promoting the nucleation priority of tBLG.

Abstract

Twist bilayer graphene (tBLG) with a small magic angle deviated from trivial stacking exhibits fantastic electronic properties. However, the growth of large-area tBLG with precisely controlled twist angle remains a grand challenge due to the thermodynamically unfavorable nucleation. Here, a bi-oriented step-guided strategy is theoretically proposed for the nucleation and epitaxial growth of tBLG, where the twist angle can be predetermined by the orientations of adjacent steps owing to the covalent alignment between the steps and graphene zigzag edges. The presence of bi-oriented steps can greatly reduce the free energy of the tBLG with matched twist angle, thus promoting the nucleation priority of tBLG. Importantly, it is shown that the 28 sets of bi-oriented steps with twist angles of 1.5°–30° can be intentionally constructed via two-step miscutting from bulk crystal of catalytic metals by exploring all potential combinations of bi-steps on different epitaxial surfaces. The employment of either low-temperature CVD growth with certain precursors or the high-melting-point metal substrates is suggested to against the reconstruction of bi-oriented steps during growth. This work demonstrates an efficient scheme for the epitaxial growth of large-area tBLG with pre-designed twist angle, which can be potentially extended to the growth of other twist 2D materials.

Benefiting from the high photogenerated electron–hole separation efficiency and the inherent mass transfer characteristics of MoS₂ confined nanotubes, the photo-assisted Zn–air battery delivers a high power density (70 mW cm⁻²), and obtains a Li–O₂ battery with excellent rate performance, which fully proves the universality of this confined structure to achieve simple, efficient and fast photogenerated carrier separation dynamics.

Abstract

Applying solar energy into energy storage battery systems is challenging in achieving green and sustainable development, however, the efficient progress of photo-assisted metal–air batteries is restricted by the rapid recombination of photogenerated electrons and holes upon the photocathode. Herein, a 1D-ordered MoS₂ nanotube (MoS₂-ONT) with confined mass transfer can be used to extend the lifetime of photogenerated carriers, which is capable of overcoming the challenge of rapid recombination of electron and holes. The tubular confined space cannot only promote the orderly separation and migration of charge carriers but also realize the accumulation of charge and the rapid activation of oxygen molecules. The concave surface of MoS₂-ONT can improve the carrier separation ability and prolong the carrier lifetime. Meanwhile, the ordered tubular confined space can effectively realize the rapid transfer of charge, ion, and oxygen. Under light irradiation, a fast oxygen reduction reaction kinetics of 70 mW cm⁻² for photo-assisted Zn–air battery is achieved, which is the highest value reported for photo-assisted Zn–air batteries. Significantly, the photo-assisted Li–O₂ battery based on MoS₂-ONT also shows superior rate capability and other exciting battery performance. This work shows the universality of the confined carrier separation strategy in photo-assisted metal–air batteries.

Graphite is tailored into subnanometer graphene by an all-physical top–down method. Compared with graphene nanosheets and quantum sheets, subnanometer graphene demonstrates comprehensively and extremely enhanced performances in photoluminescence and nonlinear saturation absorption.

Abstract

Within the intersection of materials science and nanoscience/technology, extremely downsized (including quantum-sized and subnanometer-sized) materials attract increasing interest. However, the effective and controllable production of extremely downsized materials through physical strategies remains a great challenge. Herein, an all-physical top–down method for the production of sub-1 nm graphene with completely broken lattice is reported. The graphene subnanometer materials (GSNs) with monolayer structures and lateral sizes of ≈0.5 nm are obtained. Compared with their bulk, nanosheets, and quantum sheets, the intrinsic GSNs present extremely enhanced photoluminescence and nonlinear saturation absorption performances, as well as unique carrier behavior. The non-equilibrium states induced by the entirely exposed and broken, intrinsic lattices in sub-1 nm graphene can be determinative to their extreme performances. This work shows the great potential of broken lattice and provides new insights toward subnanometer materials.