仲云雷

Author(s): Florie Mesple, Ahmed Missaoui, Tommaso Cea, Loic Huder, Francisco Guinea, Guy Trambly de Laissardière, Claude Chapelier, and Vincent T. Renard

The moiré of twisted graphene bilayers can generate flat bands in which charge carriers do not possess enough kinetic energy to escape Coulomb interactions with each other, leading to the formation of novel strongly correlated electronic states. This exceptionally rich physics relies on the precise ...

[Phys. Rev. Lett. 127, 126405] Published Fri Sep 17, 2021

The 2D WSe₂ based memtransistor with high choice freedom can not only mimic homosynaptic plasticity by applying spikes on either the drain or gate terminal, but also can simulate heterosynaptic plasticity and has numerous merits: 1) no need for adding an extra modulation terminal, 2) versatile reconfigurability of excitatory and inhibitor, and 3) selectivity of input.

Abstract

The mimicking of both homosynaptic and heterosynaptic plasticity using a high-performance synaptic device is important for developing human-brain–like neuromorphic computing systems to overcome the ever-increasing challenges caused by the conventional von Neumann architecture. However, the commonly used synaptic devices (e.g., memristors and transistors) require an extra modulate terminal to mimic heterosynaptic plasticity, and their capability of synaptic plasticity simulation is limited by the low weight adjustability. In this study, a WSe₂-based memtransistor for mimicking both homosynaptic and heterosynaptic plasticity is fabricated. By applying spikes on either the drain or gate terminal, the memtransistor can mimic common homosynaptic plasticity, including spiking rate dependent plasticity, paired pulse facilitation/depression, synaptic potentiation/depression, and filtering. Benefitting from the multi-terminal input and high adjustability, the resistance state number and linearity of the memtransistor can be improved by optimizing the conditions of the two inputs. Moreover, the device can successfully mimic heterosynaptic plasticity without introducing an extra terminal and can simultaneously offer versatile reconfigurability of excitatory and inhibitory plasticity. These highly adjustable and reconfigurable characteristics offer memtransistors more freedom of choice for tuning synaptic weight, optimizing circuit design, and building artificial neuromorphic computing systems.

Dimensionality-driven anomalous phase transition of MoTe₂ is demonstrated. The thinner MoTe₂ has a higher 2H-to-T_d phase transition temperature with distinct temperature differences. Vertical and lateral phase-patterning is achieved by modulating the thickness via stacking and insertion of graphene. By using dimensionality-driven phase transition, seamless T_d contacts for 2H-MoTe₂ transistors are fabricated, leading to low contact resistance and high mobility.

Abstract

Phase transition in nanomaterials is distinct from that in 3D bulk materials owing to the dominant contribution of surface energy. Among nanomaterials, 2D materials have shown unique phase transition behaviors due to their larger surface-to-volume ratio, high crystallinity, and lack of dangling bonds in atomically thin layers. Here, the anomalous dimensionality-driven phase transition of molybdenum ditelluride (MoTe₂) encapsulated by hexagonal boron nitride (hBN) is reported. After encapsulation annealing, single-crystal 2H-MoTe₂ transformed into polycrystalline T_d-MoTe₂ with tilt-angle grain boundaries of 60°-glide-reflection and 120°-twofold rotation. In contrast to conventional nanomaterials, the hBN-encapsulated MoTe₂ exhibit a deterministic dependence of the phase transition on the number of layers, in which the thinner MoTe₂ has a higher 2H-to-T_d phase transition temperature. In addition, the vertical and lateral phase transitions of the stacked MoTe₂ with different crystalline orientations can be controlled by inserted graphene layers and the thickness of the heterostructure. Finally, it is shown that seamless T_d contacts for 2H-MoTe₂ transistors can be fabricated by using the dimensionality-driven phase transition. The work provides insight into the phase transition of 2D materials and van der Waals heterostructures and illustrates a novel method for the fabrication of multi-phase 2D electronics.

Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting in understanding thermoelectric energy conversion. TlCuSe exhibiting intrinsically ultralow thermal conductivity (0.25 W m^–1 K^–1), a high power factor (11.6 μW cm^–1 K^–1), and a high figure of merit ZT (1.9) at 643 K is described.

Abstract

The entanglement of lattice thermal conductivity, electrical conductivity, and Seebeck coefficient complicates the process of optimizing thermoelectric performance in most thermoelectric materials. Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting and practically important for energy conversion. Herein, an intrinsic p-type semiconductor TlCuSe that has an intrinsically ultralow thermal conductivity (0.25 W m⁻¹ K⁻¹), a high power factor (11.6 µW cm⁻¹ K⁻²), and a high figure of merit, ZT (1.9) at 643 K is described. The weak chemical bonds, originating from the filled antibonding orbitals p-d* within the edge-sharing CuSe₄ tetrahedra and long TlSe bonds in the PbClF-type structure, in conjunction with the large atomic mass of Tl lead to an ultralow sound velocity. Strong anharmonicity, coming from Tl⁺ lone-pair electrons, boosts phonon–phonon scattering rates and further suppresses lattice thermal conductivity. The multiband character of the valence band structure contributing to power factor enhancement benefits from the lone-pair electrons of Tl⁺ as well, which modify the orbital character of the valence bands, and pushes the valence band maximum off the Γ-point, increasing the band degeneracy. The results provide new insight on the rational design of thermoelectric materials.

A universal strategy for preparing metal phosphides embedded with in situ-formed amorphous phosphates to function as buffer materials is proposed. The existence of amorphous phosphates can effectively reduce the volume expansion of metal phosphide electrodes and improve the solid electrolyte interphase and cycling stability when used as anodes for potassium-ion batteries in the KFSI-based electrolyte.

Abstract

As anodes for metal-ion batteries, metal phosphides usually suffer from severe capacity degradation because of their huge volume expansion and unstable solid electrolyte interphase (SEI), especially for potassium-ion batteries (PIBs). To address these issues, this study proposes amorphous phosphates acting as buffer materials. Ten types of metal phosphide composites embedded with in situ-formed amorphous phosphates are prepared by one-step ball milling using red phosphorus (RP) and the corresponding metal oxides (MOs) as starting materials. A zinc phosphide composite is selected for further study as a PIB anode. Benefitting from the effective suppression of volume variation, a KF-rich SEI is formed on the electrode surface in the KFSI-based electrolyte. The zinc phosphide composite exhibits a high reversible specific capacity and outstanding long-term cycling stability (476 mAh g⁻¹ over 350 cycles at 0.1 A g⁻¹ after going through a rate capability test and 177 mAh g⁻¹ after 6000 cycles at 1.0 A g⁻¹) and thus achieves the best cycling performance among all reported metal phosphide-based anodes for PIBs. This study highlights a low-cost and effective strategy to guide the development of metal phosphides as high-performance anodes for PIBs.

Taking g-C₃N₄ as a representative example, it is theoretically found that twisted bilayers are endowed with good charge separation, ultrafast interlayer charge transfer, and enhanced visible-light absorption. Due to the induced large moiré potentials, the overpotentials of HER and OER on the twisted bilayers are significantly reduced. Therefore, twist is better and of great potential in catalysis.

Abstract

Moiré pattern superlattice formed by 2D van der Waals layered structures have attracted great attention for diverse applications. In experiments, the enhancement of catalytic performance in twisted bilayer systems is reported while its mechanism remains unclear. From high-accuracy first-principles and time-dependent ab initio nonadiabatic molecular dynamics calculations, ultrafast interlayer charge transfer within 120 fs, excellent charge separation, improved visible-light absorption, and satisfactory overpotentials for the hydrogen evolution and oxygen evolution reactions in twisted graphitic carbon nitride (g-C₃N₄) bilayers are found, which are beneficial to photocatalytic, photo-electrocatalytic, or electrocatalytic water splitting. This work provides insightful guidance to advanced nanocatalysis based on twisted layered materials.

Author(s): Tao Yu, Dante M. Kennes, Angel Rubio, and Michael A. Sentef

Recent measurements of the resistivity in magic-angle twisted bilayer graphene near the superconducting transition temperature show twofold anisotropy, or nematicity, when changing the direction of an in-plane magnetic field [Cao et al., Science 372, 264 (2021)]. This was interpreted as strong evide...

[Phys. Rev. Lett. 127, 127001] Published Mon Sep 13, 2021

The atomic bonding in the van der Waals crystal In₂Se₃ is studied using picosecond ultrasonics and density functional theory. A significant difference in the sound velocities in the direction perpendicular to the van der Waals layers of α- and β-In₂Se₃ is revealed and assigned to the different elastic properties of the two polytypes.

Abstract

The interplay between the strong intralayer covalent-ionic bonds and the weak interlayer van der Waals (vdW) forces between the neighboring layers of vdW crystals gives rise to unique physical and chemical properties. Here, the intralayer and interlayer bondings in α and β polytypes of In₂Se₃ are studied, a vdW material with potential applications in advanced electronic and optical devices. Picosecond ultrasonic experiments are conducted to probe the sound velocity in the direction perpendicular to the vdW layers. The measured sound velocities are different in α- and β-In₂Se₃, suggesting a significant difference in their elastic properties. Density functional theory and an effective spring model are used to calculate the elastic stiffness of the layer and vdW gap in α- and β-In₂Se₃. The calculated elastic moduli show good agreement with experimental values and reveal the dominant contribution of interlayer atomic bonding to the different elastic properties of the two polytypes. The findings show the power of picosecond ultrasonics for probing the fundamental elastic properties of vdW materials. The data and analysis also provide a reliable description of the intra- and interlayer forces in complex crystal structures, such as the polytype phases of In₂Se₃.

Porous single-layer graphene membranes for gas separation are synthesized by one-step chemical vapor deposition (CVD). Highly dense gas-sieving pores are created in graphene by tuning the CVD parameters. The resulting graphene membranes exhibit record-high H₂/CH₄ separation performance to date: H₂/CH₄ selectivity > 2000 while H₂ permeance > 4000 GPU.

Abstract

Single-layer graphene containing molecular-sized in-plane pores is regarded as a promising membrane material for high-performance gas separations due to its atomic thickness and low gas transport resistance. However, typical etching-based pore generation methods cannot decouple pore nucleation and pore growth, resulting in a trade-off between high areal pore density and high selectivity. In contrast, intrinsic pores in graphene formed during chemical vapor deposition are not created by etching. Therefore, intrinsically porous graphene can exhibit high pore density while maintaining its gas selectivity. In this work, the density of intrinsic graphene pores is systematically controlled for the first time, while appropriate pore sizes for gas sieving are precisely maintained. As a result, single-layer graphene membranes with the highest H₂/CH₄ separation performances recorded to date (H₂ permeance > 4000 GPU and H₂/CH₄ selectivity > 2000) are fabricated by manipulating growth temperature, precursor concentration, and non-covalent decoration of the graphene surface. Moreover, it is identified that nanoscale molecular fouling of the graphene surface during gas separation where graphene pores are partially blocked by hydrocarbon contaminants under experimental conditions, controls both selectivity and temperature dependent permeance. Overall, the direct synthesis of porous single-layer graphene exploits its tremendous potential as high-performance gas-sieving membranes.

Systematic research on highly reversible lithium storage behavior in crystalline C₆₀ nano particles as anode material of lithium-ion batteries is demonstrated. The involved new phase generation and transformation of Li _x C₆₀ during the lithiation/de-lithiation process are monitored unprecedently and proved through advanced in situ X-ray diffraction, high-resolution transmission electron microscopy, and first-principles calculations.

Abstract

Li⁺ intercalates into a pure face-centered-cubic (fcc) C₆₀ structure instead of being adsorbed on a single C₆₀ molecule. This hinders the excess storage of Li ions in Li-ion batteries, thereby limiting their applications. However, the associated electrochemical processes and mechanisms have not been investigated owing to the low electrochemical reactivity and poor crystallinity of the C₆₀ powder. Herein, a facile method for synthesizing pure fcc C₆₀ nanoparticles with uniform morphology and superior electrochemical performance in both half- and full-cells is demonstrated using a 1 m LiPF₆ solution in ethylene carbonate/diethyl carbonate (1:1 vol%) with 10% fluoroethylene carbonate. The specific capacity of the C₆₀ nanoparticles during the second discharge reaches ≈750 mAh g⁻¹ at 0.1 A g⁻¹, approximately twice that of graphite. Moreover, by applying in situ X-ray diffraction, high-resolution transmission electron microscopy, and first-principles calculations, an abnormally high Li storage in a crystalline C₆₀ structure is proposed based on the vacant sites among the C₆₀ molecules, Li clusters at different sites, and structural changes during the discharge/charge process. The fcc of C₆₀ transforms tetragonal via orthorhombic Li _x C₆₀ and back to the cubic phase during discharge. The presented results will facilitate the development of novel fullerene-based anode materials for Li-ion batteries.

Nature Communications, Published online: 07 September 2021; doi:10.1038/s41467-021-25543-1

Nature Materials, Published online: 06 September 2021; doi:10.1038/s41563-021-01074-4

A novel sulfide electrode system allows sulfur-vacancy self-healing in 2D MoS₂. Ultrathin CuS electrode heals defects in the MoS₂ channel spontaneously upon mild thermal annealing. The self-healed CuS/MoS₂ transistors and phototransistors show impressive device performance.

Abstract

Contact engineering for monolayered transition metal dichalcogenides (TMDCs) is considered to be of fundamental challenge for realizing high-performance TMDCs-based (opto) electronic devices. Here, an innovative concept is established for a device configuration with metallic copper monosulfide (CuS) electrodes that induces sulfur vacancy healing in the monolayer molybdenum disulfide (MoS₂) channel. Excess sulfur adatoms from the metallic CuS electrodes are donated to heal sulfur vacancy defects in MoS₂ that surprisingly improve the overall performance of its devices. The electrode-induced self-healing mechanism is demonstrated and analyzed systematically using various spectroscopic analyses, density functional theory (DFT) calculations, and electrical measurements. Without any passivation layers, the self-healed MoS₂ (photo)transistor with the CuS contact electrodes show outstanding room temperature field effect mobility of 97.6 cm² (Vs)⁻¹, On/Off ratio > 10⁸, low subthreshold swing of 120 mV per decade, high photoresponsivity of 1 × 10⁴ A W⁻¹, and detectivity of 10¹³ jones, which are the best among back-gated transistors that employ 1L MoS₂. Using ultrathin and flexible 2D CuS and MoS₂, mechanically flexible photosensor is also demonstrated, which shows excellent durability under mechanical strain. These findings demonstrate a promising strategy in TMDCs or other 2D material for the development of high performance and functional devices including self-healable sulfide electrodes.