Jing Zhang

Graphene‐templated monolayer to few‐multilayers of Pt, synthesized as contiguous 2D films by room temperature electrochemical methods, exhibits a {100} structure where the surface Pt atoms are more stable than their bulk Pt {100} or Pt {111} counterparts. This supra‐bulk stability of the 2D Pt/graphene results from a combination of inter‐planar PtC covalent bonding and inter/intraplanar metallic bonding.

Abstract

The nature of the atomic configuration and the bonding within epitaxial Pt‐graphene films is investigated. Graphene‐templated monolayer/few‐multilayers of Pt, synthesized as contiguous 2D films by room temperature electrochemical methods, is shown to exhibit a stable {100} structure in the 1–2 layer range. The fundamental question being investigated is whether surface Pt atoms rendered in these 2D architectures are as stable as those of their bulk Pt counterparts. Unsurprisingly, a single layer Pt on the graphene (Pt_1ML/GR) shows much larger Pt dissociation energy (−7.51 eV) than does an isolated Pt atom on graphene. However, the dissociation energy from Pt_1ML/GR is similar to that of bulk Pt(100), −7.77 eV, while in bi‐layer Pt on the graphene (Pt_2ML/GR), this energy changes to −8.63 eV, surpassing its bulk counterpart. At Pt_2ML/GR, the dissociation energy also slightly surpasses that of bulk Pt(111). Bulk‐like stability of atomically thin Pt–graphene results from a combination of interplanar PtC covalent bonding and inter/intraplanar metallic bonding. This unprecedented stability is also accompanied by a metal‐like presence of electronic states at the Fermi level. Such atomically thin metal‐graphene architectures can be a new stable platform for synthesizing 2D metallic films with various applications in catalysis, sensing, and electronics.

Via systematic linear polarization‐ and helicity‐resolved Raman measurements, the fingerprint and chirality of phonons in 2D itinerant ferromagnet Fe₃GeTe₂ are elucidated for the first time. Importantly, the spin–phonon coupling is determined through analysis of temperature‐dependent phonon energies and lifetimes. Such spin–phonon coupling significantly enhances the Raman susceptibility. Finally, the Raman fingerprints associated with the degradation of Fe₃GeTe₂ are uncovered.

Abstract

Fe₃GeTe₂ has emerged as one of the most fascinating van der Waals crystals due to its 2D itinerant ferromagnetism, topological nodal lines, and Kondo lattice behavior. However, lattice dynamics, chirality of phonons, and spin–phonon coupling in this material, which set the foundation for these exotic phenomena, have remained unexplored. Here, the first experimental investigation of the phonons and mutual interactions between spin and lattice degrees of freedom in few‐layer Fe₃GeTe₂ is reported. The results elucidate three prominent Raman modes at room temperature: two A_1g(Γ) and one E_2g(Γ) phonons. The doubly degenerate E_2g(Γ) mode reverses the helicity of incident photons, indicating the pseudoangular momentum and chirality. Through analysis of temperature‐dependent phonon energies and lifetimes, which strongly diverge from the anharmonic model below Curie temperature, the spin–phonon coupling in Fe₃GeTe₂ is determined. Such interaction between lattice oscillations and spin significantly enhances the Raman susceptibility, allowing to observe two additional Raman modes at the cryogenic temperature range. In addition, laser radiation‐induced degradation of Fe₃GeTe₂ in ambient conditions and the corresponding Raman fingerprint is revealed. The results provide the first experimental analysis of phonons in this novel 2D itinerant ferromagnet and their applicability for further fundamental studies and application development.

Nanoassembly growth model of low‐symmetry 2D materials is revealed to understand the formation mechanism of grain boundary and subdomain in CVD‐grown 1T′ ReS₂. The controlled construct of diverse grain boundary structures combined with their novel properties will open up new prospects for the grain boundary‐mediated engineering of material properties and applications.

Abstract

Grain boundaries (GBs) significantly affect the electrical, optical, magnetic, and mechanical properties of 2D materials. An anisotropic 2D material like ReS₂ provides unprecedented opportunities to explore novel GB properties, since the reduced lattice symmetry offers greater degrees of freedom to build new GB structures. Here the atomic structure and formation mechanism of unusual multidomain and diverse GB structures in the vapor phase synthesized ReS₂ atomic layers are reported. Using high‐resolution electron microscopy, two major categories of GBs are observed in each ReS₂ domain, namely, the joint GB including three structures, and the GBs formed from a reconstruction of Re4‐chains including seven different structures. Based on the experimental observations, a novel “nanoassembly growth model” is proposed to elucidate the growth process of ReS₂, where three types of Re4‐chain reconstruction give rise to a multidomain structure. Moreover, it is shown that by controlling the thermodynamics of the growth process, the structure and density of GB in the ReS₂ domain can be tailored. First‐principles calculations point to interesting new properties resulting from such GBs, such as a new electron state or ferromagnetism, which are highly sought after in the construction of novel 2D devices.

Nanohybrids with antagonistic properties (high capacitance and good conductivity) like pMoS₂‐nSNrGO are demonstrated among excellent electrode materials for ionic actuators. With a 670% bending improvement at a low voltage of 0.5 V and the ability to perform fast bending up to 15 Hz, the pMoS₂‐nSNrGO‐based actuators successfully act as soft fingers to touch fragile surfaces of smartphones to switch the flashlight.

Abstract

Future smart mobile electronics and wearable robotics that can perform delicate activities controlled by artificial intelligence can require rapid motion actuators working at low voltages with acceptable safety and improved energy efficiency. Accordingly, ionic soft actuators can have great potential over other counterparts because they exhibit gentle movements at low voltages, less than 2 V. However, these actuators currently show deficient performances at sub‐1 V voltages in the high‐frequency range because of the lack of electrode materials with the vital antagonistic properties of high capacitance and good conductivity. Herein, a mutually exclusive nanohybrid electrode (pMoS₂‐nSNrGO) is reported consisting of oxide‐doped p‐type molybdenum‐disulfide and sulfur‐nitrogen‐codoped n‐type reduced‐graphene‐oxide. The pMoS₂‐nSNrGO electrode derives high capacitance from MoS₂ and good charge transfer between the two components from p‐n nano‐junctions, resulting in excellent actuation performances (670% improvement compared with rGO electrode at 0.5 V and 1 Hz, together with fast responses up to 15 Hz). With such excellent performances, these actuators can be successfully applied to realize an artificial soft robotic finger system for delicately touching the fragile surfaces of smartphones and tablets. The mutually exclusive pMoS₂‐nSNrGO electrode can open a new way to develop high‐performance soft actuators for soft robotic applications in the future.

Transition metal dichalcogenides (TMDs) promise to revolutionize optoelectronic applications. While monolayer exfoliation and vapor phase growth produce extremely high quality 2D materials, direct fabrication at wafer scale remains a significant challenge. Here, we present a method that we call ‘lateral conversion’, which enables the synthesis of patterned TMD structures, with control over the thickness down to a few layers, at lithographically predefined locations. In this method, chemical conversion of a metal-oxide film to TMD layers proceeds by diffusion of precursor propagating laterally between silica layers, resulting in structures where delicate chalcogenide films are protected from contamination or oxidation. Lithographically patterned WS 2 structures were synthesized by lateral conversion and analyzed in detail by hyperspectral Raman imaging, scanning electron microscopy and transmission electron microscopy. The rate of conversion was investigated as a functi...

Two-dimensional (2D) layered materials-based field-effect transistors (FETs) are promising for ultimate scaled electron device applications because of the improved electrostatics to atomically thin body thickness. However, compared with the typical thickness of ~5 nm for Si-on-insulator (SOI), the advantage of the ultimate thickness limit of monolayer for the device performance has not been fully proved yet, especially for the on-state at the accumulation region. Here, we present much stronger quantum-mechanical effect at the accumulation region based on the C – V analysis for top-gate MoS 2 FETs. The self-consistent calculation elucidated that the electrons are confined in the monolayer thickness, unlike in the triangle potential formed by the electric field for SOI, the gate-channel capacitance is ideally maximized to the gate insulator capacitance since the capacitive contribution of the channel can be neglected due to the negligible channel thickness. T...

Nature Nanotechnology, Published online: 30 September 2019; doi:10.1038/s41565-019-0547-2

Engineering multiple moiré patterns within a boron nitride–graphene–boron nitride heterostructure enables tunable crystal symmetry and strong modification of the graphene band structure.

Surface plasmons in 2-dimensional electron systems with narrow Bloch bands feature an interesting regime in which Landau damping (dissipation via electron–hole pair excitation) is completely quenched. This surprising behavior is made possible by strong coupling in narrow-band systems characterized by large values of the “fine structure” constant α=e2/ℏκvF. Dissipation quenching...

Among two-dimensional (2D) transition metal dichalcogenides (TMDs), platinum diselenide (PtSe 2 ) stands at a unique place in the sense that it undergoes a phase transition from type-II Dirac semimetal to indirect-gap semiconductor as thickness decreases. Defects in 2D TMDs are ubiquitous and play crucial roles in understanding and tuning electronic, optical, and magnetic properties. Here we investigate intrinsic point defects in ultrathin 1T-PtSe 2 layers grown on mica through the chemical vapor transport (CVT) method, using scanning tunneling microscopy and spectroscopy (STM/STS) and first-principles calculations. We observed five types of distinct defects from STM topography images and obtained the local density of states (LDOS) of the defects. By combining the STM results with the first-principles calculations, we identified the types and characteristics of these defects, which are Pt vacancies at the topmost and next monolayers, Se vacancies in the topmos...

Transition metal dichalcogenide (TMDC) monolayers have attracted great attention due to their unique electronic properties, which promise their applications especially in optoelectronics and valleytronics. A simple and reliable way to fabricate the monolayers is highly desirable for wide applications. Herein, we report a photo-induced exfoliation method for the controllable fabrication of monolayer TMDCs proceeded with the oxidation reaction of TMDCs by the photo-generated holes. The commonly used microscope halogen lamp is sufficient to initiate the exfoliation in pure water. A bulk MoS 2 flake with a surface area of 10 000 µ m 2 and a thickness of 100 nm can be directly exfoliated down to monolayer within four seconds under a 94 nW µ m −2 660 nm laser illumination and by applying 0.1 V potential electrochemically, achieving an astonishing exfoliation speed and efficiency. This method is demonstrated to be applicable also to other semic...

We study the effect of electric stress, gas pressure and gas type on the hysteresis in the transfer characteristics of monolayer molybdenum disulfide (MoS 2 ) field effect transistors. The presence of defects and point vacancies in the MoS 2 crystal structure facilitates the adsorption of oxygen, nitrogen, hydrogen or methane, which strongly affect the transistor electrical characteristics. Although the gas adsorption does not modify the conduction type, we demonstrate a correlation between hysteresis width and adsorption energy onto the MoS 2 surface. We show that hysteresis is controllable by pressure and/or gas type. Hysteresis features two well-separated current levels, especially when gases are stably adsorbed on the channel, which can be exploited in memory devices.

A hybrid photodetector consisting of a 2D semiconductor (MoS₂) integrated with multiple grating metallic stripes (Mo₂C) demonstrates high sensitivity and broad spectral detection of light, overcoming the inherent weakness of conventional 2D photodetectors and opening up possibilities for next‐generation photoelectric device technology by providing new functionalities for high sensitivity and effective broad‐spectrum photodetection.

Abstract

A novel hybrid phototransistor consisting of molybdenum carbide (Mo₂C) and molybdenum disulfide (MoS₂) is proposed. By exploiting the interface properties of MoS₂ and Mo₂C, a highly sensitive and broad‐spectral response photodetector is fabricated. The underlying mechanism of the enhanced performance is the efficient hot carrier injection from Mo₂C to MoS₂. The strong coupling of MoS₂ and Mo₂C at the interface provides the significantly low Schottky barrier height (≈70 meV), which gives rise to the significantly efficient hot carrier transfer from Mo₂C to MoS₂. The grating of metallic Mo₂C produces plasmonic resonance, which provides hot carriers to the MoS₂ channel. By adjusting the grating period of Mo₂C (400–1000 nm), the optimal photoresponse of light can be controlled, from visible to NIR. By integrating various Mo₂C multigrating periods (400–1000 nm) with MoS₂, a novel photodetector is demonstrated with high responsivity (R > 10³ A W⁻¹) and light‐to‐dark current ratio (>10²) over a broad spectral range (405–1310 nm). The proposed novel hybrid photodetector, 2D semiconductors with multigrating 2D metallic stripes, exhibits high sensitivity and broad spectral detection of light and can overcome the inherent weakness of conventional 2D photodetectors, paving the way forward for next‐generation photoelectric devices.

In this study, a MoS₂/HfO₂/silicon‐on‐insulator field effect phototransistor with wavelength distinguishing ability with resolution up to 2 nm is presented, where the photogating effect can be simultaneously formed in the top MoS₂ gate and bottom Si substrate gate. This device may find prospective applications in future optoelectronic devices, such as intensity‐independent filterless color sensing.

Abstract

Photogating detectors based on 2D materials attract significant research interests. However, most of these photodetectors are only sensitive to the incident intensities and lack the ability to distinguish different wavelengths. Color imaging based on these detectors usually requires additional optical filter arrays to collect red, green, and blue (RGB) colors in different photodetectors to restore the true color of one pixel. In this study, an MoS₂/HfO₂/silicon‐on‐insulator field effect phototransistor with wavelength distinguishing ability is presented, where the photogating effect can be simultaneously formed in the top MoS₂ gate and bottom Si substrate gate. These two individual photogating effects can reduce and increase the read current in the middle 12 nm Si channel, respectively. Thus, by tuning the applied voltages on these two gates, the device can be used to obtain tunable ambipolar photoresponsivity from +7000 A W⁻¹ (Si bottom gate dominated) to 0 A W⁻¹ (balanced), and finally to −8000 A W⁻¹ (MoS₂ gate dominated). In addition, the experimental results show that the corresponding top gate voltage to the zero responsivity (0 A W⁻¹) point can be positively shifted by the increasing of incident wavelength with high resolution up to 2 nm and is insensitive to the incident intensity.

Nanohybrids with antagonistic properties (high capacitance and good conductivity) like pMoS₂‐nSNrGO are demonstrated among excellent electrode materials for ionic actuators. With a 670% bending improvement at a low voltage of 0.5 V and the ability to perform fast bending up to 15 Hz, the pMoS₂‐nSNrGO‐based actuators successfully act as soft fingers to touch fragile surfaces of smartphones to switch the flashlight.

Abstract

Future smart mobile electronics and wearable robotics that can perform delicate activities controlled by artificial intelligence can require rapid motion actuators working at low voltages with acceptable safety and improved energy efficiency. Accordingly, ionic soft actuators can have great potential over other counterparts because they exhibit gentle movements at low voltages, less than 2 V. However, these actuators currently show deficient performances at sub‐1 V voltages in the high‐frequency range because of the lack of electrode materials with the vital antagonistic properties of high capacitance and good conductivity. Herein, a mutually exclusive nanohybrid electrode (pMoS₂‐nSNrGO) is reported consisting of oxide‐doped p‐type molybdenum‐disulfide and sulfur‐nitrogen‐codoped n‐type reduced‐graphene‐oxide. The pMoS₂‐nSNrGO electrode derives high capacitance from MoS₂ and good charge transfer between the two components from p‐n nano‐junctions, resulting in excellent actuation performances (670% improvement compared with rGO electrode at 0.5 V and 1 Hz, together with fast responses up to 15 Hz). With such excellent performances, these actuators can be successfully applied to realize an artificial soft robotic finger system for delicately touching the fragile surfaces of smartphones and tablets. The mutually exclusive pMoS₂‐nSNrGO electrode can open a new way to develop high‐performance soft actuators for soft robotic applications in the future.

Nature Nanotechnology, Published online: 16 September 2019; doi:10.1038/s41565-019-0533-8

Mechanically and electronically stable graphene/molecule/graphene devices can be fabricated by combining a covalent binding of the molecules to the substrate with an optimized intermolecular π–π interaction.

Nature Nanotechnology, Published online: 02 September 2019; doi:10.1038/s41565-019-0536-5

Few-layer micas show proton permeation across the sheet, exceeding that of graphene and hBN by one to two orders of magnitude.

Nature Nanotechnology, Published online: 23 September 2019; doi:10.1038/s41565-019-0543-6

van der Waals materials enable the realization of an electrically driven polariton LED operating at room temperature.

Physics of the quantum critical point is one of the most perplexing topics in current condensed-matter physics. Its conclusive understanding is forestalled by the scarcity of experimental systems displaying novel aspects of quantum criticality. We present comprehensive experimental evidence of a magnetic field-tuned tricritical point separating paramagnetic, antiferromagnetic, and metamagnetic...