Jing Zhang

Nature Materials, Published online: 25 October 2022; doi:10.1038/s41563-022-01383-2

This Review discusses the progress in and potential pathways for incorporating two-dimensional materials into silicon platforms, from integrated devices to monolithic circuits.

Nature Materials, Published online: 25 October 2022; doi:10.1038/s41563-022-01378-z

Twisted bilayer graphene is epitaxially grown between two adjacent Cu(111) surfaces, with the twist angle controlled by the rotation of the Cu foils as designed.

Nature Materials, Published online: 25 October 2022; doi:10.1038/s41563-022-01382-3

Two-dimensional electrons on the surface of an electride are found to exhibit a phase transition from a normal Fermi liquid to an interesting quantum liquid, which is probably a quantum version of an electronic crystal beyond the melting point.

Publication date: February 2023

Source: Progress in Materials Science, Volume 132

Author(s): Shucheng Xing, Jian Zhou, Xuanguang Zhang, Stephen Elliott, Zhimei Sun

In this work, the composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe₅Se₈ and 50% Fe-intercalated monoclinic Fe₃Se₄), via a robust chemical vapor deposition strategy is presented. Intriguingly, it has been revealed that the 2D Fe₅Se₈ exhibits intrinsic room-temperature ferromagnetic property, and the ultrathin Fe₃Se₄ presents a novel metallic feature.

Abstract

Exploring new-type 2D magnetic materials with high magnetic transition temperature and robust air stability has attracted wide attention for developing innovative spintronic devices. Recently, intercalation of native metal atoms into the van der Waals gaps of 2D layered transition metal dichalcogenides (TMDs) has been developed to form 2D non-layered magnetic TMDs, while only succeeded in limited systems (e.g., Cr₂S₃, Cr₅Te₈). Herein, composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe₅Se₈ and 50% Fe-intercalated monoclinic Fe₃Se₄) are firstly reported, via a robust chemical vapor deposition strategy. Specifically, the 2D Fe₅Se₈ exhibits intrinsic room-temperature ferromagnetic property, which is explained by the change of electron spin states from layered 1T'-FeSe₂ to non-layered Fe-intercalated Fe₅Se₈ based on density functional theory calculations. In contrast, the ultrathin Fe₃Se₄ presents novel metallic features comparable with that of metallic TMDs. This work hereby sheds light on the composition-controllable synthesis and fundamental property exploration of 2D self-intercalation induced novel TMDs compounds, by propelling their application explorations in nanoelectronics and spintronics-related fields.

2D superlattice materials combining the advantages of 2D materials and advanced composites provide tunable physical and chemical properties to meet diverse requirements in different applications. This review article summarizes the major fabrication methods for preparing 2D superlattice materials and reviews their applications in electrocatalysis involved in emerging energy devices.

Abstract

Exploring low-cost and high-efficient electrocatalyst is an exigent task in developing novel sustainable energy conversion systems, such as fuel cells and electrocatalytic fuel generations. 2D materials, specifically 2D superlattice materials focused here, featured highly accessible active areas, high density of active sites, and high compatibility with property-complementary materials to form heterostructures with desired synergetic effects, have demonstrated to be promising electrocatalysts for boosting the performance of sustainable energy conversion and storage devices. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the 2D superlattice-based catalysts yet remain ambiguous. In this review, based on the recent progress of 2D superlattice materials in electrocatalysis applications, the rational design and fabrication of 2D superlattices are first summarized and the application of 2D superlattices in electrocatalysis is then specifically discussed. Finally, perspectives on the current challenges and the strategies for the future design of 2D superlattice materials are outlined. This review attempts to establish an intrinsic correlation between the 2D superlattice heterostructures and the catalytic properties, so as to provide some insights into developing high-performance electrocatalysts for next-generation sustainable energy conversion and storage.

A synergistic use of finite element simulations as a guide together with tellurization process by chemical vapor deposition allows to evidence the key role of tilt angle and distribution of Te vapor concentration gradient at the substrate position in the reactor to obtain molybdenum ditelluride few-layer nanosheets on large area growth and phase selectivity, namely pure 1T’ or 2H phase.

Abstract

Among transition metal dichalcogenides, molybdenum ditelluride (MoTe₂) holds significant attention due to its polymorphic nature including semiconducting, metallic, and topological semimetal phases. Considerable efforts are devoted to synthesizing MoTe₂ nanosheets to make them suitable for device integration in nanotechnologies and for fundamental investigations. In this respect, chemical vapor deposition (CVD) via tellurization of a pre-deposited Mo thin film is an easy and flexible way for synthesizing large scale MoTe₂ nanosheets. Here, the study report on the CVD of large-area (up to 4 cm × 1 cm) MoTe₂ nanosheets with pure 1T’ and 2H phase selection by design. Within the tellurization scheme, the vapor-solid reaction between the pre-deposited molybdenum film and tellurium vapor is studied thus optimizing the scalability and quality of the MoTe₂ nanosheets grown on SiO₂/Si substrates. It is demonstrated that the MoTe₂ structure and morphology are kinetically dictated by the tellurium concentration gradient on the reaction site with varying geometric configurations inside the CVD reactor. This study provides a pivot scheme for enabling scalable 1T’ and 2H-MoTe₂ integration in applications for novel micro- and nano-electronics, spintronics, photonics, and thermoelectric devices.

Nature Photonics, Published online: 24 October 2022; doi:10.1038/s41566-022-01084-x

One-micrometre-thick OLEDs with low operating voltages of 5.11 V, 3.55 V and 6.88 V at 1,000 cd cm–2 for red, green and blue devices, respectively, and long lifetimes (55,000 h, 18,000 h and 1,600 h, respectively) are realized.

Nature Photonics, Published online: 17 October 2022; doi:10.1038/s41566-022-01080-1

Researchers show that resonant coupling of light pulses with excitonic transitions affects the optimal time difference between pulses for sum-frequency generation and four-wave mixing in monolayer WSe2.

Nature Communications, Published online: 20 October 2022; doi:10.1038/s41467-022-33965-8

Slow light effects are interesting for telecommunications and quantum photonics applications. Here, the authors use coupled exciton-surface plasmon polaritons (SPPs) in a hybrid monolayer WSe2-metallic waveguide structure to demonstrate a 1300-fold reduction of the SPP group velocity.

Nature Communications, Published online: 21 October 2022; doi:10.1038/s41467-022-33673-3

In conventional materials, charge carriers are electron-like quasiparticles, but topological bands allow for more exotic possibilities. Here, the authors predict that in the Chern-ferromagnet phase of twisted bilayer graphene charge is carried by spin polarons, bound states of an electron and a spin flip.

Nature Communications, Published online: 23 October 2022; doi:10.1038/s41467-022-33822-8

Here, the authors report the observation of room temperature excitons in a single layer of bismuth atoms epitaxially grown on a SiC substrate - a material of non-trivial global topology - with excitonic and topological physics deriving from the very same electronic structure.

Nature Electronics, Published online: 21 October 2022; doi:10.1038/s41928-022-00864-1

Making robots move more like humans

Nature Electronics, Published online: 21 October 2022; doi:10.1038/s41928-022-00867-y

Memristive devices can be used to create energy-efficient hardware implementations of reservoir computing.

The merging state and isolated bound states in the continuum (BIC) can be manipulated in momentum space by tuning carrier density in 2D material or distorting magnitude of the hole in each unit of photonic crystal. In such configuration, the BICs in different high-symmetry axis have different moving scale, thus achieving inversion of topological charge at Γ point.

Abstract

High quality factor (Q-factor) resonance can effectively enhance light–matter interactions, but suffers from radiative losses caused by fabrication imperfections and material impurities. Merging bound states in the continuum (BICs) becomes a promising candidate to address this challenge since they generate a large high-Q region in momentum space against undesired external perturbations. However, it is only achieved by altering the geometric parameters of the structure, which constrains its dynamical tunability once the fabrication is ready, limiting its performance in modulators and photodetectors. Here, a convenient approach is proposed to manipulating the merging BIC by electrically tuning the material properties, aiming at on-chip photonic devices. The asymmetric merging BIC is achieved and tuned by reducing the in-plane structural symmetry from C4Z$C_4^Z$ to C2Z$C_2^Z$, which enables the inversion of topological charge and reconfigurable polarization distribution in the momentum space. Such tunable merging BIC promises various potential applications including vortex generation, optical communication, and nanolasers.

Hybrid semiconductor–superconductor nanowires are promising candidates as quantum information processing devices. Here, the growth of large scale PbTe networks by molecular beam epitaxy is introduced. The high quality of the resulting material is confirmed by Hall bar measurements yielding mobilities up to 5600 cm² (Vs)^-1, and Aharonov–Bohm experiments indicating a low-temperature phase coherence length exceeding 21 µm.

Abstract

Hybrid semiconductor–superconductor nanowires are promising candidates as quantum information processing devices. The need for scalability and complex designs calls for the development of selective area growth techniques. Here, the growth of large scale lead telluride (PbTe) networks is introduced by molecular beam epitaxy. The group IV-VI lead-salt semiconductor is an attractive material choice due to its large dielectric constant, strong spin-orbit coupling, and high carrier mobility. A crystal re-orientation process during the initial growth stages leads to single crystalline nanowire networks despite a large lattice mismatch, different crystal structure, and diverging thermal expansion coefficient to the indium phosphide (InP) substrate. The high quality of the resulting material is confirmed by Hall bar measurements, indicating mobilities up to 5600 cm² (Vs)⁻¹, and Aharonov–Bohm experiments, indicating a low-temperature phase coherence length exceeding 21 µm. Together, these properties show the high potential of the system as a basis for topological networks.

By utilizing the monolayer thickness of graphene in Nanoelectromechanical system (NEMS) switches, sub-1 V switching characteristics are achieved. The irreversible static friction at the switch contact is overcome by employing the weak van der Waals (vdW) bonding of graphene-hexagonal boron nitride. These Graphene NEMS vdW switches show sub-0.5 V switching voltage, 10⁵ ON/OFF ratio, and nearly zero hysteretic window characteristics.

Abstract

The Nanoelectromechanical (NEM) switches are a promising candidate to overcome the physical limitations of the complementary metal-oxide-semiconductor (CMOS) switches due to their quasi-zero leakage behavior, sub-thermal switching, and suitability to operate in harsh environments. The main obstacles affecting NEM switches are their irreversible switch-contact stiction, the large switching voltage, and its hysteretic loop. In this study, the irreversible static friction is overcome by employing the weak van der Waals (vdW) bonding of graphene-hexagonal boron nitride (hBN) contact in the Graphene NEM (GNEM) switches. These vdW switches show sub-0.5 V switching voltage with an ON/OFF ratio higher than 10⁵ and nearly zero hysteretic window characteristics with a high endurance of over 50 000 switching cycles. These remarkable performances are achieved by exploiting graphene's monolayer thickness, high Young's modulus, cubic mechanical restoring force, and low vdW binding energy characteristics. As chemical vapor deposition graphene and hBN are used in these GNEM switches, it exhibits the prospect for large-scale graphene NEM system applications. These GNEM switches can be potentially used in ultralow-power CMOS integrated circuits, hybrid NEM-CMOS systems, logic devices, NEM resonator mass sensing, and single-molecule sensors.

The ZnO NS-based artificial synapses are prepared at low temperature by a simple spin coating process. Combining various characterization techniques and data analysis, the weight transition process in artificial synapses is recognized, In addition, the typical synaptic plasticity and high recognition accuracy are realized and kept stable under a series of bending operations.

Abstract

In neuromorphic computing networks, a flexible synaptic memristor with high recognition accuracy is highly desired. In this study, ZnO nanosheets (ZnO NS) embedded within a polymethyl methacrylate host material are used as the intermediate layer to prepare flexible synaptic memristor at a low-temperature of 80 °C. The device shows excellent switching characteristics with low SET/RESET voltages (−0.4 V/0.4 V) and stable retention characteristic (10⁴ s). By modulating the conductance continuously, the flexible synaptic memristor simulates typical synaptic plasticities, including excitation post-synaptic current, paired-pulse facilitation, and spike-timing dependent plasticity. Especially, the neuromorphic system built from flexible ZnO NS-based memristors achieves a high recognition accuracy up to 97.7% for handwriting digit. Under the influence of 5% Uniform noise and 5% Gaussian noise, recognition accuracies are maintained at 94.6% and 93.7%, respectively. These properties are well maintained even when bending 1000 times at a radius of 5 mm. The flexible ZnO NS-based memristor shows great prospects in wearable devices and neural morphology calculation.

Ultra-Flat Graphene

In article number 2200428, Tongbo Wei, Jingyu Sun, Zhongfan Liu, and co-workers implement direct growth of wafer-level, ultra-flat graphene without any wrinkles and metallic impurities on quartz via the inhibition of textured SiO _x seed, identification of the critical temperature regime, and in-situ flattening of the substrate surface.

Chemical Vapor Deposition

In article number 2203191, Feng Ding and co-authors report the vapor-liquid-solid growth of graphene nanospears and graphene nanoribbons on Cu surface. The vapor-liquid-solid growth of graphene nanostructures is enabled by liquid Si-Cu catalysts and opens a platform for excellent graphene-based electronics.

Special Issue

This special issue organized by Profs. Jin Zhang, Hailin Peng, Hua Zhang, Shigeo Maruyama, and Li Lin, provides a body of high-impact reviews and research articles in the fields of synthesis, engineering, and application of graphene materials. It was proposed in Advanced Functional Materials on the occasion of the 60th birthday of Prof. Zhongfan Liu, a world-famous expert in graphene research. This special issue consists of 16 reviews and 14 research articles, focusing on the latest summary and progress from fundamental research to the broad applications of graphene.

Flexible Optoelectronics

In article number 2203115, Wencai Ren and co-workers review the recent advances and significant developments of chemi cal vapor deposition-grown graphene toward flexi ble optoelectronics. Various prototype devices including photodetectors, organic solar cells, and light emitting diodes are demonstrated. These devices are available to large-area fabrication and have good flexibility, showing great potential to integration of multifunctional wearable systems based on chemical vapor deposition-grown graphene.

Highly conductive, ultraflexible, and multifunctional graphene films are assembled from pristine graphene nanosheets via electrostatic-repulsion aligning where highly electronegative titania nanosheets play a key role. The obtained graphene films present significantly enhanced mechanical properties and electrical properties for electromagnetic interference shielding and thermal-management multifunctional applications in smart and wearable electronics.

Abstract

Assembling pristine graphene into freestanding films featuring high electrical conductivity, superior flexibility, and robust mechanical strength aims at meeting the all-around high criteria of new-generation electronics. However, voids and defects produced in the macroscopic assembly process of graphene nanosheets severely degrade the performance of graphene films, and mechanical brittleness often limits their applications in wide scenarios. To address such challenges, an electrostatic-repulsion aligning strategy is demonstrated to produce highly conductive, ultraflexible, and multifunctional graphene films. Typically, the high electronegativity of titania nanosheets (TiNS) induces the aligning of negatively charged graphene nanosheets via electrostatic repulsion in the film assembly. The resultant graphene films show fine microstructure, enhanced mechanical properties, and improved electrical conductivity up to 1.285 × 10⁵ S m⁻¹. Moreover, the graphene films can withstand 5000 repeated folding without structural damage and electrical resistance fluctuation. These comprehensive improved properties, combined with the facile synthesis method and scalable production, make these graphene films a promising platform for electromagnetic interference (EMI) shielding and thermal-management applications in smart and wearable electronics.

Ferroelectricity is generated in twisted hBN flakes, which are used as a ferroelectric tunneling junction. Spatially resolved investigation by a conductive atomic force microscope tip reveals the unconventional ferroelectricity in this system, including lattice-slide-induced domain change, an unusual relationship between the tunneling resistance and polarization, and highly spatially dependent ferroelectric hysteresis.

Abstract

Robust room-temperature interfacial ferroelectricity has been formed in the 2D limit by simply twisting two atomic layers of non-ferroelectric hexagonal boron nitride (hBN). A thorough understanding of this newly discovered ferroelectric system is required. Here, twisted hBN is used as a tunneling junction and it is studied at the nanometer scale using conductive atomic force microscopy. Three properties unique to this system are discovered. First, the polarization dependence of the tunneling resistance contrasts with the conventional theory. Second, the ferroelectric domains can be controlled using mechanical stress, highlighting the original meaning of the emergent “slidetronics”. Third, ferroelectric hysteresis is highly spatially dependent. The hysteresis is symmetric at the domain walls. A few nanometers away, the hysteresis shifts completely to the positive or negative side, depending on the original polarization. These findings reveal the unconventional ferroelectricity in this 2D system.

The perovskite SrRuO₃ is a prototypical itinerant ferromagnet that has drawn considerable attention due to its strongly correlated physical properties. In this work, a new concept is demonstrated to induce octahedral distortions, magnetism, and reversal of the anomalous Hall effect in SrRuO₃ thin films via hydrogen spillover, revealed by comprehensive in situ experiments and density functional theory calculations.

Abstract

The perovskite SrRuO₃ (SRO) is a strongly correlated oxide whose physical and structural properties are strongly intertwined. Notably, SRO is an itinerant ferromagnet that exhibits a large anomalous Hall effect (AHE) whose sign can be readily modified. Here, a hydrogen spillover method is used to tailor the properties of SRO thin films via hydrogen incorporation. It is found that the magnetization and Curie temperature of the films are strongly reduced and, at the same time, the structure evolves from an orthorhombic to a tetragonal phase as the hydrogen content is increased up to ≈0.9 H per SRO formula unit. The structural phase transition is shown, via in situ crystal truncation rod measurements, to be related to tilting of the RuO₆ octahedral units. The significant changes observed in magnetization are shown, via density functional theory (DFT), to be a consequence of shifts in the Fermi level. The reported findings provide new insights into the physical properties of SRO via tailoring its lattice symmetry and emergent physical phenomena via the hydrogen spillover technique.

An in situ crystallized Sb₂Te₃/GeTe superlattice film is grown by atomic layer deposition on the planar and sidewall memory cell, showing a significantly decreased reset current compared with the homogeneously mixed alloy. The in-plane compressive stress and effective electromigration of the Ge atoms induce a melt-quenching-free amorphization mechanism.

Abstract

Atomic layer deposition (ALD) of Sb₂Te₃/GeTe superlattice (SL) film on planar and vertical sidewall areas containing TiN metal and SiO₂ insulator is demonstrated. The peculiar chemical affinity of the ALD precursor to the substrate surface and the 2D nature of the Sb₂Te₃ enable the growth of an in situ crystallized SL film with a preferred orientation. The SL film shows a reduced reset current of ≈1/7 of the randomly oriented Ge₂Sb₂Te₅ alloy. The reset switching is induced by the transition from the SL to the (111)-oriented face-centered-cubic (FCC) Ge₂Sb₂Te₅ alloy and subsequent melt-quenching-free amorphization. The in-plane compressive stress, induced by the SL-to-FCC structural transition, enhances the electromigration of Ge along the [111] direction of FCC structure, which enables such a significant improvement. Set operation switches the amorphous to the (111)-oriented FCC structure.

Nature Nanotechnology, Published online: 20 October 2022; doi:10.1038/s41565-022-01221-1

A one-step simultaneous van der Waals integration of high-k dielectrics and contacts enables the realization of top-gated transistors with atomically clean and electronically sharp interfaces.