RESUMEN
Edges and surfaces play indispensable roles in affecting the chemical-physical properties of materials, particularly in two-dimensional transition metal dichalcogenides (TMDCs) with reduced dimensionality. Herein, we report a novel edge/surface structure in multilayer 1T-TiSe2, i.e., the orthogonal (1 × 1) reconstruction, induced by the self-intercalation of Ti atoms into interlayer octahedral sites of the host TiSe2 at elevated temperature. Formation dynamics of the reconstructed edge/surface are captured at the atomic level by in situ scanning transmission electron microscopy (STEM) and further validated by density functional theory (DFT), which enables the proposal of the nucleation mechanism and two growth routes (zigzag and armchair). Via STEM-electron energy loss spectroscopy (STEM-EELS), a chemical shift of 0.6 eV in Ti L3,2 is observed in the reconstructed edge/surface, which is attributed to the change of the coordination number and lattice distortion. The present work provides insights to tailor the atomic/electronic structures and properties of 2D TMDC materials.
RESUMEN
Development of low energy cost membranes for separating helium from natural gas is highly desired. Using van der Waals-corrected first-principles density functional theory (DFT) calculations, we theoretically investigate the helium separation performance of divacancy-defective germanene. The 555 777 divacancy-defective germanene presents a 0.53 eV energy barrier for helium, which is slightly larger than the energy threshold value of gas molecule penetration of a membrane (0.5 eV). Thus, the 555 777 divacancy-defective germanene is difficult for helium to permeate, except under high temperature or pressure. However, the 585 divacancy-defective germanene presents a surmountable energy barrier (0.27 eV) for helium, and it shows extremely high helium selectivities relative to other studied gas molecules. Especially, the He/Ne selectivity can be as high as 1 × 104 at room temperature. Together with the acceptable permeance for helium, the 585 divacancy-defective germanene can be used for helium separation with remarkably good performance.
RESUMEN
A mechanistic understanding of interactions between atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDs) and their growth substrates is important for achieving the unidirectional alignment of nuclei and seamless stitching of 2D TMD domains and thus 2D wafers. In this work, we conduct a cross-sectional scanning transmission electron microscopy (STEM) study to investigate the atomic-scale nucleation and early stage growth behaviors of chemical vapor deposited monolayer (ML-) MoS2 and molecular beam epitaxy ML-MoSe2 on a Au(111) substrate. Statistical analysis reveals the majority of as-grown domains, i.e., â¼88% for MoS2 and 90% for MoSe2, nucleate on surface terraces, with the rest (i.e., â¼12% for MoS2 and 10% for MoSe2) on surface steps. Moreover, within the latter case, step-associated nucleation, â¼64% of them are terminated with a Mo-zigzag edge in connection with the Au surface steps, with the rest (â¼36%) being S-zigzag edges. In conjunction with ab initio density functional theory calculations, the results confirm that van der Waals epitaxy, rather than the surface step guided epitaxy, plays deterministic roles for the realization of unidirectional ML-MoS2 (MoSe2) domains on a Au(111) substrate. In contrast, surface steps, particularly their step height, are mainly responsible for the integrity and thickness of MoS2/MoSe2 films. In detail, it is found that the lateral growth of monolayer thick MoS2/MoSe2 domains only proceeds across mono-Au-atom high surface steps (â¼2.4 Å), but fail for higher ones (bi-Au atom step and higher) during the growth. Our cross-sectional STEM study also confirms the existence of considerable compressive residual strain that reaches â¼3.0% for ML-MoS2/MoSe2 domains on Au(111). The present study aims to understand the growth mechanism of 2D TMD wafers.
RESUMEN
Epidermal electronic systems that simultaneously provide physiological information acquisition, processing, and storage are in high demand for health care/clinical applications. However, these system-level demonstrations using flexible devices are still challenging because of obstacles in device performance, functional module construction, or integration scale. Here, on the basis of carbon nanotubes, we present an epidermal system that incorporates flexible sensors, sensor interface circuits, and an integrated flash memory array to collect physiological information from the human body surface; amplify weak biosignals by high-performance differential amplifiers (voltage gain of 27 decibels, common-mode rejection ratio of >43 decibels, and gain bandwidth product of >22 kilohertz); and store the processed information in the memory array with performance on par with industrial standards (retention time of 108 seconds, program/erase voltages of ±2 volts, and endurance of 106 cycles). The results shed light on the great application potential of epidermal electronic systems in personalized diagnostic and physiological monitoring.
RESUMEN
Vanadium diselenide (VSe2), a member of the transition metal dichalcogenides (TMDs) family, is emerging as a promising two-dimensional (2D) candidate for the electronic and spintronic device with exotic properties including charge/spin density wave and ferromagnetism. The bulk crystal VSe2 exists in a crystallographic form of 1T-phase with metallic behavior. In this paper, we report a structural phase transition of multilayer VSe2 from 1T to 2H through annealing at 650 K, accompanying a metal-insulator transition. We observe that the 2H-phase is more thermodynamically favorable than the 1T-phase at 2D.