zemin zheng

The evaporation of Au or the sputtering of Cr on 2D layered hexagonal boron nitride (h-BN) produces local atomic defects in its structure, specially at its interfaces, which increases the leakage current across it. It is found that the deposition of metal on hexagonal boron nitride (h-BN) using inkjet printing does not introduce any defect, and maintains its layered structure free of defects.

Abstract

2D materials have many outstanding properties that make them attractive for the fabrication of electronic devices, such as high conductivity, flexibility, and transparency. However, integrating 2D materials in commercial devices and circuits is challenging because their structure and properties can be damaged during the fabrication process. Recent studies have demonstrated that standard metal deposition techniques (like electron beam evaporation and sputtering) significantly damage the atomic structure of 2D materials. Here it is shown that the deposition of metal via inkjet printing technology does not produce any observable damage in the atomic structure of ultrathin 2D materials, and it can keep a sharp interface. These conclusions are supported by abundant data obtained via atomistic simulations, transmission electron microscopy, nanochemical metrology, and device characterization in a probe station. The results are important for the understanding of inkjet printing technology applied to 2D materials, and they could contribute to the better design and optimization of electronic devices and circuits.

Antisite defects in monolayer WS₂ are selectively formed with a nonequilibrium synthesis strategy that can reduce their formation energy via W-poor growth conditions achieved by limiting the available surface W concentration on a W/Au alloy. A high density of S_W and S2_W antisite defects can be obtained and identified in monolayer WS₂ that are sufficient to induce localized defect states in the bandgap.

Abstract

Defects are ubiquitous in 2D materials and can affect the structure and properties of the materials and also introduce new functionalities. Methods to adjust the structure and density of defects during bottom-up synthesis are required to control the growth of 2D materials with tailored optical and electronic properties. Here, the authors present an Au-assisted chemical vapor deposition approach to selectively form S_W and S2_W antisite defects, whereby one or two sulfur atoms substitute for a tungsten atom in WS₂ monolayers. Guided by first-principles calculations, they describe a new method that can maintain tungsten-poor growth conditions relative to sulfur via the low solubility of W atoms in a gold/W alloy, thereby significantly reducing the formation energy of the antisite defects during the growth of WS₂. The atomic structure and composition of the antisite defects are unambiguously identified by Z-contrast scanning transmission electron microscopy and electron energy-loss spectroscopy, and their total concentration is statistically determined, with levels up to ≈5.0%. Scanning tunneling microscopy/spectroscopy measurements and first-principles calculations further verified these antisite defects and revealed the localized defect states in the bandgap of WS₂ monolayers. This bottom-up synthesis method to form antisite defects should apply in the synthesis of other 2D materials.

Nature Communications, Published online: 01 November 2021; doi:10.1038/s41467-021-26342-4

CrI3 is a popular van der Waals magnet that exhibits anomalous magnetic properties between bulk and thin layers due to different crystal symmetry. Here, the authors report the coexistence of different magnetostructural phases over the entire range of temperatures, solving a long-standing puzzle.

Nature Communications, Published online: 02 November 2021; doi:10.1038/s41467-021-26708-8

The evolution of superconductivity in LiFeAs with respect to pressure or strain remains elusive. Here, the authors observe different response of superconducting states due to different orientations of local wrinkles on the surface of LiFeAs.

Two dimensional van der Waals ferromagnets with honeycomb structures are expected to host the bosonic version of Dirac particles in their magnon excitation spectra. Using inelastic neutron scattering, we study spin wave excitations in polycrystalline CrCl 3 , which exhibits ferromagnetic honeycomb layers with antiferromagnetic stackings along the c -axis. For comparison, polycrystal samples of CrI 3 with different grain sizes are also studied. We find that the powder-averaged spin wave spectrum of CrCl 3 at T  = 2 K can be adequately explained by the two dimensional spin Hamiltonian including in-plane Heisenberg exchanges only. The observed excitation does not exhibit noticeable broadening in energy, which is in remarkable contrast to the substantial broadening observed in CrI 3 . Based on these results, we conclude that the ferromagnetic phase of CrCl 3 hosts massless Dirac magnons and is thus not topological.

2D all-inorganic NbWO₆ perovskite nanosheets with thicknesses down to 1.5 nm are synthesized for the fabrication of gas sensors, and the fabricated few-layer NbWO₆-based semiconducting sensor exhibits fast H₂S sensing speed with high selectivity and sensitivity at low temperature.

Abstract

Hydrogen sulfide (H₂S) detection with high selectivity and low working temperature is of great significance due to its strong toxicity both to the environment and to humans and also as an endogenous signaling molecule existing in various physiological processes. 2D perovskites with high carrier mobility are promising candidates for gas sensing; however, the development of stable and nontoxic 2D perovskites nanosheets still remains a challenge. Herein, 2D all-inorganic NbWO₆ perovskite nanosheets with thicknesses down to 1.5 nm are synthesized by liquid exfoliation, and the gas-sensing performance based on these ultrathin nanosheets is investigated. A few-layer NbWO₆-based sensor exhibits fast H₂S sensing speed (<6 s) with high selectivity and sensitivity (S = 12.5 vs 50 ppm) at low temperature (150 °C). A small variation of H₂S concentration (<0.5 ppm) can be detected with a fully reversible resistance signal. This work sheds light on the development of high-performance gas sensors working in ambient conditions based on low-dimensional, nontoxic, and wide-bandgap perovskite semiconductors. The high carrier mobility, ultrathin structure, and soft nature make this type of 2D perovskite semiconductor an ideal material candidate for the fabrication of flexible, transparent, and wearable sensing devices in the future.

We study suspended membranes of atomically thin WSe 2 as hosts of excitons. We perform optical reflectance measurements to probe the exciton physics and obtain the peak energies for the 1 ##IMG## [http://ej.iop.org/images/2053-1583/9/1/015002/tdmac2d15ieqn1.gif] {$s$} , 2 ##IMG## [http://ej.iop.org/images/2053-1583/9/1/015002/tdmac2d15ieqn2.gif] {$s$} , and 3 ##IMG## [http://ej.iop.org/images/2053-1583/9/1/015002/tdmac2d15ieqn3.gif] {$s$} states of the ##IMG## [http://ej.iop.org/images/2053-1583/9/1/015002/tdmac2d15ieqn4.gif] {$A$} exciton in suspended WSe 2 and consider supported membranes as a reference. We find that elimination of the influence of the dielectric environment enables a strong electron–hole interaction and a concomitant increase in the exciton binding energy in suspended monolayer (1L) WSe 2 . Based on the experimental results, we calculate the exciton...

Direct integration of transition metal dichalcogenides on a ferroelectric such as hafnium zirconium oxide (HZO) using an industrially scalable technique is important for realizing various ferroelectric-based device architectures. The interface formed due to the processing conditions during direct deposition is the focus of the current study. In this work, molecular beam epitaxy (MBE) is used to directly deposit WSe 2 on HZO substrates, and the effects of the MBE growth conditions, specifically high temperature and a high Se flux, are examined. Anneals of HZO under a Se flux, which serve to replicate the conditions during actual WSe 2 deposition, result in the crystallization of amorphous as-deposited HZO substrates and incorporation of Se into the HZO. The crystallinity and composition of the HZO substrates affect the degree of Se incorporation. Some of the Se found in the HZO is an adsorbed layer that can be thermally desorbed, but it also has a chemisorbed ...

We demonstrate the structure evolution of hexagonal boron nitride (hBN) flakes grown on molten Cu in atmospheric pressure chemical vapor deposition by regulating the flux of precursor. We found that under lower precursor flux, tuned by temperature that controls the sublimation rates, the hBN grains change from triangle to truncated triangle shape with additional B-terminated edges, which could be understood through kinetic Wulff construction, while under higher flux, they form circular shape following deposition-controlled growth and predicted by a phase field modeling. In addition to the monolayer morphology from a single nucleation, adlayer patterns with centered aggregation and diffusive features at high precursor flux are observed and simulated by a two-dimensional (2D) diffusion-reaction model, where the random diffusion and deposition are revealed to be the dominating kinetics. The nucleation density and growth velocity could also be modulated by the ammonia borane heating...

State-of-the-art fabrication and characterisation techniques have been employed to measure the thermal conductivity of suspended, single-crystalline MoS 2 and MoS 2 /hBN heterostructures. Two-laser Raman scattering thermometry was used combined with real time measurements of the absorbed laser power. Measurements on MoS 2 layers with thicknesses of 5 and 14 nm exhibit thermal conductivity in the range between 12 Wm −1 K −1 and 24 Wm −1 K −1 . Additionally, after determining the thermal conductivity of the latter MoS 2 sample, an hBN flake was transferred onto it and the effective thermal conductivity of the heterostructure was subsequently measured. Remarkably, despite that the thickness of the hBN layer was less than a hal of the thickness of the MoS 2 layer, the heterostructure showed an almost eight-fold increase in the thermal conductivity, being able to dissipate more than ten times the ...

By utilizing time-resolved magneto-optical Kerr effect technique, we investigated both demagnetization and remagnetization processes of a typical two-dimensional (2D) van der Waals (vdW) ferromagnetic (FM) semiconductor Cr 2 Ge 2 Te 6 (CGT) and two comparative Te-based crystals, i.e. three-dimensional (3D) FM metal Cr 3 Te 4 (CT) and 2D vdW FM metal Fe 3 GeTe 2 (FGT). Different from 3D CT showing one-step laser-induced demagnetization, two-steps demagnetization behavior is dominated in 2D vdW FGT and CGT, which indicates that the 2D vdW magnets have relatively weaker electron-spin coupling. Of particular interest, we discovered an ultra-long remagnetization behavior in the 2D vdW CGT flake. The laser pumping induced spin demagnetization would not recover even in the time scale of 3500 ps (experimental setup limitation). To the best of our knowledge, this is the longest remagnetization process observed so far and i...