Jing Zhang

Two-dimensional (2D) palladium ditelluride (PdTe 2 ) and platinum ditelluride (PtTe 2 ) are two Dirac semimetals, which have fascinating quantum properties such as superconductivity, magnetism and topological order and show the promising applications in future nanoelectronics and optoelectronics. However, the synthesis of PdTe 2 and PtTe 2 monolayers (MLs) is hindered by a strong interlayer coupling and orbital hybridization. In this study, we demonstrate an efficient synthesis of PdTe 2 and PtTe 2 MLs Large-area and high quality MLs of PdTe 2 and patterned PtTe 2 were epitaxially grown on the Pd(111) and Pt(111) surfaces by direct telurization in ultra-high vacuum. This was confirmed by x-ray photoelectron spectroscopy, low energy electron diffraction and scanning tunnelling microscopy. PdTe 2 ML demonstrated high thermal stability showing no decomposition sign after annealing at 470 °C. A w...

We investigate the optical properties of interlayer excitons in heterobilayer transition metal dichalcogenides where moiré pattern is introduced by heterostrain, in comparison with that introduced by twisting (and/or lattice mismatch). Besides being a cause of the moiré texture, strain also effectively introduces a constant gauge potential on either electron or hole, which shifts the dispersion of kinetic energy with respect to the excitonic crystal momenta in the moiré superlattices. This leads to distinct exciton mini-band dispersions and light coupling properties from the twisting induced moiré, even if the excitonic moiré superlattice potentials have the similar real-space profile for the two cases. For strain that breaks the three-fold rotational symmetry at the atomic scale, the exciton wave packets trapped at the superlattice potential minima have elliptically polarized valley optical selection rules, in contrast to the circularly polarized ones in the twisting moiré. We ...

A fabric electrode and double-side-contact TENG (DSC-TENG) based on a fabric-assisted micropatterning method on a highly negative MXene/silicone nanocomposite surface (charge generating) with MXene layer (charge trapping) for self-powered sensors and wearable electronics applications. The intentionally designed DSC-TENG is waterproof, safe from humidity, and exhibits stable and durable output performance, soft, and comfortable wearing.

Abstract

Surface modification of triboelectric negative layers is an essential factor for boosting the output performance of triboelectric nanogenerators (TENGs). Herein, a novel scalable surface modification method is introduced using a fabric-assisted micropatterning technique on a highly negative MXene/silicone nanocomposite surface (charge generating) with MXene layer (charge trapping) for self-powered sensors and wearable electronics. The microstructured surface is fabricated directly from a fabric template requiring no surface-active agent, high-pressure equipment, or high vacuum. To boost the proposed double-side-contact TENG (DSC-TENG) output performance, different parameters of the fabric textures are tested and optimized for the roughened microstructures, namely the MXene layer and relative humidity. Under optimal conditions, the fabricated DSC-TENG improves the voltage and peak current density by factors of 9.8 and 20, respectively, regarding flat silicone. It exhibits a maximum peak power density of 55.47 W m⁻² at load resistance of 0.18 MΩ, and a corresponding decrease in resistance by 75% using MXene content of 3 mg cm⁻². Also, DSC-TENG-based smart home control of electrical appliances, theft protection, self-powered electronic devices, password authentication, and human motion monitoring via smartphone for the IoT are demonstrated. The proposed method can be implemented for different types of polymers, thereby enabling the large-scale fabrication of high-performance TENGs in industrial applications.

Nature, Published online: 20 October 2021; doi:10.1038/s41586-021-04121-x

Evidence for unconventional superconductivity in twisted bilayer graphene

A 1D van der Waals Nb₂Pd₃Se₈ is synthesized by a chemical vapor transport reaction. Field-effect transistors are fabricated on mechanically exfoliated Nb₂Pd₃Se₈ nanowires, displaying electron mobility and I _on/I _off ratio values of 31 cm² V⁻¹ s⁻¹ and ≈10⁴, respectively. It is confirmed that Nb₂Pd₃Se₈ field effect transistors can form a stable ohmic contact with an extremely low Schottky barrier for the gold electrode.

Abstract

In this work, high-quality 1D van der Waals (vdW) Nb₂Pd₃Se₈ is synthesized, showing an excellent scalability from bulk to single-ribbon due to weakly bonded repeating unit ribbons. The calculation of electronic band structures confirmed that this novel Nb₂Pd₃Se₈ is a semiconducting material, displaying indirect-to-direct bandgap transition with decreasing the number of unit-ribbons from bulk to single. Field effect transistors (FETs) fabricated on the mechanically exfoliated Nb₂Pd₃Se₈ nanowires exhibit n-type transport characteristics at room temperature, resulting in the values for the electron mobility and I _on/I _off ratio of 31 cm² V⁻¹ s⁻¹ and ≈10⁴, respectively. Through transport measurements at various temperatures from room temperature down to 90 K, it is confirmed that Nb₂Pd₃Se₈ FETs can achieve negligible Schottky barrier height (SBH) for the Au contacts at the temperature range, displaying clear ohmic contact characteristics. Furthermore, top-gated FETs fabricated with the Al₂O₃ dielectric layer are studied simultaneously with back-gated FETs.

Nature Nanotechnology, Published online: 08 October 2021; doi:10.1038/s41565-021-00990-5

A decade of R2R graphene manufacturing

Nature, Published online: 06 October 2021; doi:10.1038/s41586-021-03849-w

Bilayer graphene states are observed at anomalously small magnetic fields and show magnetic hysteresis, providing evidence for a quantum anomalous Hall effect driven by orbital magnetism.

Nature, Published online: 29 September 2021; doi:10.1038/s41586-021-03867-8

Extremely anisotropic thermal conductors based on large-area van der Waals thin films with random interlayer rotations are reported here.

Nature, Published online: 29 September 2021; doi:10.1038/s41586-021-03874-9

So far, only indirect evidence of Wigner crystals has been reported, but a specially designed scanning tunnelling microscope is used here to directly image them in a moiré heterostructure.

Nature Physics, Published online: 30 September 2021; doi:10.1038/s41567-021-01347-4

In addition to the broken time-reversal symmetry that typifies Chern insulators, twisted bilayer graphene hosts a set of topological states with broken translational symmetry.

The ability to create highly efficient and stable bifunctional electrocatalysts, capable of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the same electrolyte, represents an important endeavor toward high-performance zinc-air batteries (ZABs). Herein, we report a facile strategy for crafting wrinkled MoS2/N-doped carbon core/shell nanospheres interfaced with single...

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.

Nature Nanotechnology, Published online: 09 September 2021; doi:10.1038/s41565-021-00966-5

Low-temperature ultraclean integration of large-area MoS2 thin-film transistors with nitride micro-LEDs through a back end of line process enables the demonstration of displays with high resolution and uniformity.

The photophysics of 2D perovskites is studied upon variation of the constituent metal and halide anions. All compounds exhibit persistent luminescence of hot excitons that violate Kasha's rule and a complex set of low-energy transitions involving defects.

Abstract

2D perovskites offers a rich playing field to explore exciton physics and they possess a great potential for a variety of opto-electronic applications. Whilst their photophysics shows intricate interactions of excitons with the lattice, most reports have so far relied on single compound studies. With the exception of variations of the organic spacer cations, the effect of constituent substitution on the photophysics and the nature of emitting species, in particular, have remained largely under-explored. Here PEA₂PbBr₄, PEA₂PbI₄, and PEA₂SnI₄ (where PEA stands for phenylethylammonoium) are studied through a variety of optical spectroscopy techniques to reveal a complex set of excitonic transitions at low temperature. Weak high-energy features are attributed to vibronic transitions breaking Kasha's, for which the responsible phonons cannot be accessed through simple Raman spectroscopy. Bright peaks at lower energy are due to two distinct electronic states, of which the upper is a convolution of the free exciton and a localized dark state and the lower is attributed to recombination involving shallow defects. This study offers deeper insights into the photophysics of 2D perovskites through compositional substitution and highlights critical limits to the communities’ current understanding of processes in these compounds.

A patterning method for van der Waals materials is introduced to pattern the materials via highly intensive light at multiple scales, high throughput, and high resolution. The method has scalability in types of materials and substrates and does not result in accompanying polymeric residues on the surface of the patterned materials unlike conventional photolithography. Simulations confirm that the intensive light allows high resolution patterning.

Abstract

Advanced patterning techniques are essential to pursue applications of 2D van der Waals (vdW) materials in electrical and optical devices. Here, the direct optical lithography (DOL) of vdW materials by single-pulse irradiation of high-power light through a photomask is reported. The DOL exhibits large-scale patterning with a sub-micrometer resolution and clean surface, which can be applied to various combinations of vdW materials and substrates. In addition, the thermal profile during DOL is investigated using the finite element method, and the ideal conditions of DOL according to the materials and substrates are determined.

The novel designed tri-layer HfO₂/BiFeO₃(BFO)/HfO₂ memristor shows excellent resistive switching performance. Essential synaptic functions can be successfully realized. The pattern recognition accuracy is more than 91.2%. The sufficient oxygen vacancies in the inserting BFO thin film play a crucial role in adjusting the growth of Hf CFs, which lead to the promising synaptic and enhanced RS behavior for neuromorphic computing.

Abstract

Neuromorphic devices are among the most emerging electronic components to realize artificial neural systems and replace traditional complementary metal–oxide semiconductor devices in recent times. In this work, tri-layer HfO₂/BiFeO₃(BFO)/HfO₂ memristors are designed by inserting traditional ferroelectric BFO layers measuring ≈4 nm after thickness optimization. The novel designed memristor shows excellent resistive switching (RS) performance such as a storage window of 10⁴ and multi-level storage ability. Remarkably, essential synaptic functions can be successfully realized on the basis of the linearity of conductance modulation. The pattern recognition simulation based on neuromorphic network is conducted with 91.2% high recognition accuracy. To explore the RS performance enhancement and artificial synaptic behaviors, conductive filaments (CFs) composed of Hafnium (Hf) single crystal with a hexaganal lattice structure are observed using high-resolution transmission electron microscopy. It is reasonable to believe that the sufficient oxygen vacancies in the inserting BFO thin film play a crucial role in adjusting the morphology and growth of Hf CFs, which leads to the promising synaptic and enhanced RS behavior, thus demonstrating the potential of this memristor for use in neuromorphic computing.