Jing Zhang

The Computational 2D Materials Database (C2DB) is a highly curated open database organising a wealth of computed properties for more than 4000 atomically thin two-dimensional (2D) materials. Here we report on new materials and properties that were added to the database since its first release in 2018. The set of new materials comprise several hundred monolayers exfoliated from experimentally known layered bulk materials, (homo)bilayers in various stacking configurations, native point defects in semiconducting monolayers, and chalcogen/halogen Janus monolayers. The new properties include exfoliation energies, Bader charges, spontaneous polarisations, Born charges, infrared polarisabilities, piezoelectric tensors, band topology invariants, exchange couplings, Raman spectra and second harmonic generation spectra. We also describe refinements of the employed material classification schemes, upgrades of the computational methodologies used for property evaluations, as well as signifi...

The channel current measured in the PtSe₂ field-effect transistor under switching light shows positive photoconductivity at low pressure that converts into negative photoconductivity at atmospheric pressure. Experimental observations and density functional theory calculations demonstrate that such behavior is caused by light-induced oxygen desorption.

Abstract

Platinum diselenide (PtSe₂) field-effect transistors with ultrathin channel regions exhibit p-type electrical conductivity that is sensitive to temperature and environmental pressure. Exposure to a supercontinuum white light source reveals that positive and negative photoconductivity coexists in the same device. The dominance of one type of photoconductivity over the other is controlled by environmental pressure. Indeed, positive photoconductivity observed in high vacuum converts to negative photoconductivity when the pressure is raised. Density functional theory calculations confirm that physisorbed oxygen molecules on the PtSe₂ surface act as acceptors. The desorption of oxygen molecules from the surface, caused by light irradiation, leads to decreased carrier concentration in the channel conductivity. The understanding of the charge transfer occurring between the physisorbed oxygen molecules and the PtSe₂ film provides an effective route for modulating the density of carriers and the optical properties of the material.

Interlayer vibrational modes in 2H-MoTe 2 /hBN heterostructures were investigated by low-frequency Raman spectroscopy. A series of low-frequency Raman modes are observed for MoTe 2 thicknesses of 1–4 layers: the shear modes of MoTe 2 persist in the heterostructure with no shift in the frequency, but the breathing modes show dramatic changes in the heterostructures. The number and the frequencies of the breathing modes do not depend on the twist angle between the two materials but strongly vary with the layer thicknesses of both MoTe 2 and hBN layers. The breathing modes were observed for both resonant (1.96 eV) and non-resonant (2.41 eV) excitations. The breathing mode frequencies were analyzed by using the linear chain model, and the interfacial force constant between 2 H-MoTe 2 and hBN is estimated to be 3.77 × 10 19 N m −3 .

2D van der Waals heterojunctions (vdWh) are a novel type of metamaterial developed rapidly in recent years. It has been widely used in the research of improving the performance of nanophotonic devices. This review summarizes the fabrication methods of 2D vdWhs and the research progress of nanophotonic devices based on 2D vdWhs. The critical challenges and future perspectives are discussed.

Abstract

2D van der Waals heterojunctions (vdWhs) are a novel type of metamaterial that are flexible, adjustable, and easy to assemble. Using weak van der Waals forces (vdWfs), layered 2D materials can stack freely to form vdWhs with atomic level flat interfaces. By using different 2D materials and specific stacking methods, their unique properties can be organically combined, to exhibit more abundant optical properties. In fact, nanophotonic devices based on 2D vdWhs have developed rapidly and made significant progress. Therefore, the main progress of 2D vdWhs nanophotonic devices in recent years, including the preparation methods of 2D vdWhs and the performance improvements of various nanophotonic devices, is reviewed. Lastly, the prospects of 2D vdWhs nanophotonic devices are discussed.

A facile self-assembly method via electrostatic attraction is applied to prepare MoS₂/graphene van der Waals heterostructures, which can transform the ion channel construction from pristine interlamination of two MoS₂ monolayers to the interlaminatiof the MoS₂ monolayer with the graphene monolayer, thereby reducing the diffusion energy barrier of ions in favor of introducing Li⁺/Mg²⁺ co-intercalation into the host material.

Abstract

Owing to the low-cost, dendrite-free formation, and high volumetric capacity, rechargeable Li⁺/Mg²⁺ hybrid-ion batteries (LMIBs) have attracted great attention and are regarded as promising energy storage devices. However, due to the strong Coulombic interaction of Mg²⁺ with host materials, the traditional “Daniell Type” LMIBs with only Li⁺ intercalation usually cannot ensure a satisfactory energy density. Herein, graphene monolayers are arranged intercalating into MoS₂ interlamination to construct van der Waals heterostructures (MoS₂/G VH). This operation transforms the construction of ion channels from pristine interlamination of two MoS₂ monolayers to the interlamination of MoS₂ monolayer with graphene monolayer, thereby greatly reducing ion diffusion energy barriers. Compared with pristine MoS₂, the MoS₂/G VH can obviously reduce the migration energy barriers of Li⁺ (from 0.67 to 0.09 eV) and Mg²⁺ (from 1.01 to 0.21 eV). Moreover, it is also demonstrated that MoS₂/G VHs realize Li⁺/Mg²⁺ co-intercalation even at a rate current of 1000 mA g⁻¹. As expected, the MoS₂/G VH exhibits superior electrochemical performance with a reversible capacity of 145.8 mAh g⁻¹ at 1000 mA g⁻¹ after 2200 cycles, suggesting the feasibility of potential applications for high-performance energy storage devices.

A highly ordered organic semiconducting self-assembled monolayer (SAM) with transition metal dichalcogenides is integrated into fully 2D organic–inorganic hybrid van der Waals heterostructures. The ordered nature of the SAM results in efficient exciton dissociation at the interface. The phototransistors show a superior photoresponsivity (475 A W⁻¹), which benefits from the precise confinement of charge in the monolayers and a strong photogating effect.

Abstract

Van der Waals (vdW) heterostructures composing of organic molecules with inorganic 2D crystals open the door to fabricate various promising hybrid devices. Here, a fully ordered organic self-assembled monolayer (SAM) to construct hybrid organic–inorganic vdW heterojunction phototransistors for highly sensitive light detection is used. The heterojunctions, formed by layering MoS₂ monolayer crystals onto organic [12-(benzo[b]benzo[4,5]thieno[2,3-d]thiophen-2-yl)dodecyl)]phosphonic acid SAM, are characterized by Raman and photoluminescence spectroscopy as well as Kelvin probe force microscopy. Remarkably, this vdW heterojunction transistor exhibits a superior photoresponsivity of 475 A W⁻¹ and enhanced external quantum efficiency of 1.45 × 10⁵%, as well as an extremely low dark photocurrent in the pA range. This work demonstrates that hybridizing SAM with 2D materials can be a promising strategy for fabricating diversified optoelectronic devices with unique properties.

Nature Nanotechnology, Published online: 15 July 2021; doi:10.1038/s41565-021-00942-z

Kinetics-controlled van der Waals epitaxy in the near-equilibrium limit by metal–organic chemical vapour deposition enables precise layer-by-layer stacking of dissimilar transition metal dichalcogenides.

Nature Nanotechnology, Published online: 19 July 2021; doi:10.1038/s41565-021-00936-x

The preparation of a diverse set of 2D materials and their co-integration in van der Waals heterostructures enable innovative material design and device engineering. This Review summarizes recent advances in 2D spintronics and opto-spintronics, the underlying physical concepts and future perspectives of the field.

Tuning the particle size of the 2D carbon nitride poly(heptazine imide) enables optimization of photocatalytic hydrogen evolution. It is shown that changes in the particle size affect the overall photocatalytic process in different ways, and the individual contributions of size-related variables on the photocatalytic activity are traced back. This multi-parameter analysis offers design strategies for next generation polymer photocatalysts.

Abstract

The carbon nitride poly(heptazine imide), PHI, has recently emerged as a powerful 2D carbon nitride photocatalyst with intriguing charge storing ability. Yet, insights into how morphology, particle size, and defects influence its photophysical properties are virtually absent. Here, ultrasonication is used to systematically tune the particle size as well as concentration of surface functional groups and study their impact. Enhanced photocatalytic activity correlates with an optimal amount of those defects that create shallow trap states in the optical band gap, promoting charge percolation, as evidenced by time-resolved photoluminescence spectroscopy, charge transport studies, and quantum-chemical calculations. Excessive amounts of terminal defects can act as recombination centers and hence, decrease the photocatalytic activity for hydrogen evolution. Re-agglomeration of small particles can, however, partially restore the photocatalytic activity. The type and amount of trap states at the surface can also influence the deposition of the co-catalyst Pt, which is used in hydrogen evolution experiments. Optimized conditions entail improved Pt distribution, as well as enhanced wettability and colloidal stability. A description of the interplay between these effects is provided to obtain a holistic picture of the size–property–activity relationship in nanoparticulate PHI-type carbon nitrides that can likely be generalized to related photocatalytic systems.

Charge-based transistors greatly suffer from unavoidable heat dissipation. Chiral anomaly current is topologically protected and dissipationless, which is promising for complementing modern electronics. Here, field-effect chiral anomaly devices are demonstrated with Dirac semimetal, which show the analogue output and transfer curves with a more than 10³ ON/OFF ratio. Essential logic functions are realized with electric and magnetic fields as input signals.

Abstract

Charge-based field-effect transistors (FETs) greatly suffer from unavoidable carrier scattering and heat dissipation. Analogous to valley degree of freedom in semiconductors, chiral anomaly current in Weyl/Dirac semimetals is theoretically predicted to be nearly nondissipative over long distances, but still lacks experimental ways to efficiently control its transport. Here, field-effect chirality devices are demonstrated with Dirac semimetal PtSe₂, in which its Fermi level is close to the Dirac point in the conduction band owing to intrinsic defects. The chiral anomaly is further corroborated by the planar Hall effect and nonlocal valley transport measurement, which can also be effectively modulated by external fields, showing robust nonlocal valley transport with micrometer diffusion length. Similar to charge-based FETs, the chiral conductivity in PtSe₂ devices can be modulated by electrostatic gating with an ON/OFF ratio of more than 10³. Basic logic functions in the devices are also demonstrated with electric and magnetic fields as input signals.

Physical pressure can drastically increase the transition temperature (T _c) of many superconductors. This study shows in tetragonal FeSe, a simple 2D van der Waals material, that isovalent atomic substitution can mimic the effect of high-pressure and boost record-high superconductivity under ambient conditions.

Abstract

High pressure has become a powerful platform for creating and controlling novel states of matter, including high temperature (T _c) superconductivity. However, the emergent phenomena generally disappear as high pressure is removed and cloud prospects for future applications. Here, from a distinguishing perspective, FeSe_{1− x} S _x is reported as 2D van der Waals materials with extraordinary high-T _c at ambient pressure, where the superconductivity is boosted by extreme “chemical pressure” inside the materials. Superior to external high pressure, isovalent S substitution in FeSe leads to a much greater compression rate within the superconducting iron-chalcogenide layer, which guarantees an unabridged superconducting dome that peaked at 37.5 K. Density functional theory calculations reveal that the decreased lattice and structural parameters contribute together for the shift of Fe 3d _{x 2− y 2} orbital, which creates a new hole-pocket at the Fermi level that intimately correlated with the enhanced superconductivity. This study demonstrates the design of materials with optimized superconductivity by introducing chemical pressure.

2D ferroelectric field-effect transistors devices are fabricated by epitaxial growth of Bi₂O₂Se on Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃. The devices exhibit ferroelectric polarization-dependent photoresponse upon visible light (λ = 405 nm) and infrared light (IR, λ = 980 nm) illumination. Combining optical stimuli with ferroelectric gating, the devices show not only nonvolatile memory and optoelectronic response, but also coincidence detection of visible and infrared light.

Abstract

Information processing with optoelectronic devices provides an alternative way to efficiently process hybrid optical and electronic signals. Ferroelectric field-effect transistors (FeFETs) can effectively respond to external optical and electrical stimuli by modulating their polarization states. Here, a 2D FeFET is demonstrated by the epitaxial growth of high-quality 2D bismuth layered oxyselenide (Bi₂O₂Se) films on PMN-PT(001) ferroelectric single-crystal substrates. Upon switching the polarization direction of PMN-PT, the authors realize in situ, reversible, and nonvolatile manipulation of the resistance of Bi₂O₂Se thin film (≈877%). The device simultaneously exhibits a polarization-dependent photoresponse through visible light (λ = 405 nm) and infrared light (IR, λ = 980 nm) illumination. Combining optical stimuli with ferroelectric gating, it is demonstrated that the devices not only show nonvolatile memory and optoelectronic responses, but also show coincidence detection of visible and IR light. This work holds great potential in constructing new multiresponse and multifunction 2D-FeFETs.

Nature Nanotechnology, Published online: 05 July 2021; doi:10.1038/s41565-021-00930-3

Author Correction: Ultrafast hole spin qubit with gate-tunable spin–orbit switch functionality

Nature Nanotechnology, Published online: 08 July 2021; doi:10.1038/s41565-021-00943-y

Continuous-time data representation and frequency multiplexing enable the implementation of a scalable massively parallel computing scheme in a nanoscale crossbar array for applications in intelligent edge devices.

Stress transfer has been investigated for exfoliated hexagonal boron nitride (hBN) nanosheets (BNNSs) through the use of Raman spectroscopy. Single BNNSs of different thicknesses of up to 100 nm (300 layers) were deposited upon a poly(methyl methacrylate) (PMMA) substrate and deformed in unixial tension. The Raman spectra from the BNNSs were relatively weak compared to graphene, but the in-plane E 2g Raman mode (the G band) could be distinguished from the spectrum of the PMMA substrate. It was found that G band down-shifted during tensile deformation and that the rate of band shift per unit strain decreased as the thickness of the BNNSs increased, as is found for multi-layer graphene. The efficiency of internal stress transfer between the different hBN layers was found to be of the order of 99% compared to 60%–80% for graphene, as a result of the stronger bonding between the hBN layers in the BNNSs. The reduction in bandshift rate can be related to the effective Young’...