2D Materials

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EISSN : 2053-1583
Published by: IOP Publishing (10.1088)
Total articles ≅ 1,472
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Published: 26 October 2021
Modern low-voltage scanning transmission electron microscopes (STEM) have been invaluable for the atomic scale characterization of two-dimensional (2D) materials. Nevertheless, the observation of intrinsic structures of semiconducting and insulating 2D materials with 60 kV-microscopes has remained problematic due to electron radiation damage. In recent years, ultralow-voltage microscopes have been developed with the prospects of minimizing radiation damage of such 2D materials, however, to date only ultralow-voltage TEM investigations of semiconducting and insulating 2D materials have been reported, but similar results using STEM, despite being more widely adopted, are still missing. Here we report a quantitative analysis of radiation damage and beam-induced defect dynamics in semiconducting 2D WS2 during 30 kV and 60 kV-STEM imaging, particularly by recording atomic resolution electrostatic potential movies using integrated differential phase contrast to visualize both the light sulfur and heavy tungsten atoms. Our results demonstrate that electron radiation damage of 2D WS2 aggravates by a factor of two when halving the electron beam energy from 60 keV to 30 keV, from which we conclude electronic excitation and ionization to be the dominant mechanism inducing defects and damage during low-voltage STEM imaging of semiconducting 2D materials.
Huatian Xiong, Haiyuan Zou, Weifeng Rong, Hao Dai, Yongsong Wang, Yongfei Ji,
Published: 13 October 2021
Metal sub-nanoclusters (SNCs) inherit the metrics of unsaturated active sites and ultrahigh metal utilization from single-atom catalysts (SACs), and they can drive the reactions involving multiple adsorbates by their enriched metal cofactors that beyond SACs. However, the current synthetic strategy offers limited versatility to prepare SNCs due to their subnanometric feature and high active surface. Herein, we demonstrate a universal and facile one-pot reaction to construct wide assortments of metal SNCs with the size of 2−3 nm on graphdiyne (GDY), denoted as M-SNCs/GDY (M = Co, Ni, Cu, Ag, Pd, Rh, Au, Ir, and Pt). Systematic investigations reveal that the correlated metal SNCs formation undergone the nucleation and growth process, during which the metal single-atoms were first anchored and then served as nuclei to grow SNCs confined on GDY. The electrochemical CO2 reduction reaction (eCO2RR) catalyzed by Cu-SNC/GDY and Cu-SAC/GDY was investigated to demonstrate the advantages of SNCs over SACs in manipulating the multicomponent reaction. Cu-SNC/GDY exhibited promoted faradic efficiency (FE) of carbon products and suppressed competing hydrogen evolution reaction compared to the Cu-SAC/GDY. Benefiting from the function of multiple active centers, a C2+ FE of 31.6% was achieved over the Cu-SNC/GDY at −0.7 V versus reversible hydrogen electrode, which is 11-fold higher than that of Cu-SAC/GDY. In-situ infrared spectroelectrochemistry confirmed that Cu-SNC/GDY could adsorb more eCO2RR intermediates over Cu-SAC/GDY. This study delivers a single-step strategy for preparing metal SNCs on GDY and expands the scope of SNCs.
, Cora Pravda Bartus, Ákos Szamosvölgyi, , ,
Published: 12 October 2021
In this study, we evaluate the variation of the work function of phosphorene during thermal oxidation at different temperatures. The ultraviolet photoelectron spectroscopy results show an N-shaped behavior that is explained by the oxidation process and the dangling-to-interstitial conversion at elevated temperatures. The exfoliation degree and X-ray photoelectron spectroscopy confirm the formation of native oxides in the top-most layer that passivates the material. Ex-situ XPS reveals the full oxidation of monolayers at temperatures higher than 140 °C, but few-layer phosphorene withstands the thermal oxidation even up to 200 °C with slight modifications of the A2g/A1g and A2g/B2g vibrational mode ratios and a weak fluorescence in the Raman spectra of the heat-treated samples.
Hongwei Liu, Wanzhen He, Zhenjing Liu, Irfan Abidi, Yao Ding, Patrick Ryan Galligan, Mohsen Tamtaji, Jingwei Li, Yuting Cai, Ting Kang, et al.
Published: 9 October 2021
We demonstrate the structure evolution of hexagonal boron nitride (hBN) flakes grown on molten Cu in atmospheric pressure chemical vapor deposition (APCVD) 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 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 temperature, where 80 ℃ is found to be optimal for largest hBN grain size. Our TEM study shows that a misalignment of coalescing grains occurs on such molten Cu substrate, deviated from those observed on molten Au. Our results provide a new tool for the shape and grain size control of two-dimensional materials and the understanding of their growth kinetics for large scale production.
Qingyan Pan, Xinsheng Chen, Hao Li, Siqi Chen, Xuhan Zheng, Hui Liu, Bo Li,
Published: 8 October 2021
We report a crystalline two-dimensional (2D) benzotrithiophene-based graphdiyne (BTT-GDY), which features diacetylene as linkages and benzotrithiophene as the functionalized core. The well-defined 2D structure of BTT-GDY with a unique pore was well characterized by selected area electron diffraction (SAED) due to its good crystallinity. BTT-GDY films show some semiconductor characteristics including a bandgap of 2.38 eV, the conductivity of 2×10-3 S m-1 at room temperature. The hole mobility and the electron mobility of BTT-GDY films were determined to be 2.34 cm2 V-1 S-1 and 3.35×10-2 cm2 V-1 S-1 by the Space-Charge-Limited-Current (SCLC) method.
Alois Arrighi, Elena Del Corro, Daniel Navarro Urrios, , , , Takashi Taniguchi, Jose A. Garrido, , Clivia M Sotomayor Torres, et al.
Published: 8 October 2021
State-of-the-art fabrication and characterisation techniques have been employed to measure the thermal conductivity of suspended, single-crystalline MoS2 and MoS2/hBN heterostructures. Two-laser Raman scattering thermometry was used combined with real time measurements of the absorbed laser power. Measurements on MoS2 layers with thicknesses of 5 and 14 nm exhibit thermal conductivity in the range between 12 and 24 Wm-1K-1. Additionally, after determining the thermal conductivity of a selected MoS2 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 third of the thickness of the MoS2 layer, the heterostructure showed an almost eight-fold increase in the thermal conductivity, being able to dissipate more than 10 times the laser power without any visible sign of damage. These results are consistent with a high thermal interface conductance G between MoS2 and hBN and an efficient in-plane heat spreading driven by hBN. Indeed, we estimate G ⁓ 70 MW·m−2K−1 for hBN layer thermal conductivity of 450 Wm-1K-1 which is significantly higher than previously reported values. Our work therefore demonstrates that the insertion of hBN layers in potential MoS2–based devices holds the promise for efficient thermal management.
Maria Gabriela Sales, Shelby Fields, Samantha Jaszewski, Sean Smith, Takanori Mimura, , Sina Najmaei, ,
Published: 6 October 2021
Direct integration of transition metal dichalcogenides (TMDCs) 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 WSe2 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 WSe2 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 component fully incorporated within the HZO lattice. Measurement of the electrical properties of the HZO films did not provide evidence that the incorporated Se was detrimental to the functionality of the HZO as a ferroelectric layer.
, , Wang Yao
Published: 5 October 2021
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 investigate the evolution of the excitonic mini-bands and the optical dipoles of the bright states inside the light cones with the decrease of the moiré periodicity, upon which the excitonic wave functions evolve from localized wave packets to the extended Bloch states. Furthermore, moiré exciton properties under the interplay of twisting and heterostrain are also discussed.
, Colin Yule, Yifei Yu, Yan Joe Lee, , Linyou Cao,
Published: 5 October 2021
We study suspended membranes of atomically thin WSe2 as hosts of suspended excitons. We perform optical reflectance measurements to probe the exciton physics and obtain the peak energies for the 1s, 2s, and 3s states of the A exciton in suspended WSe2 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) WSe2. Based on the experimental results, we calculate the excitonic binding energies by employing the recently developed quantum electrostatic heterostructure model and the commonly employed Rytova-Keldysh potential model. We see that the binding energy of the ground state A exciton increases from about 0.3 eV (on a substrate) to above 0.4 eV (suspended). We also exploit the tunability of the excitons in suspended samples via mechanical strain. By applying external gas pressure of 2.72 atm to a 1L suspended over a circular hole of 8 μm diameter, we strain the WSe2 and obtain a reversible 0.15 eV redshift in the exciton resonance. The linewidth of the A exciton decreases by more than half, from about 50 meV to 20 meV under 1.5% biaxial strain at room temperature. This line narrowing is due to the suppression of intervalley exciton-phonon scattering. By making use of the observed strain-dependent optical signatures, we infer the two-dimensional (2D) elastic moduli of 1L and 2L WSe2. Our results exemplify the use of suspended 2D materials as novel systems for fundamental studies, as well as for strong and dynamic tuning of their optical properties.
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