Titanium Self-Intercalation Induced Formation of Orthogonal (1 × 1) Edge/Surface Reconstruction in 1T-TiSe2: Atomic Scale Dynamics and Mechanistic Study.

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.

Concentration Phase Separation of Substitution-Doped Atoms in TMDCs Monolayer.

The density and spatial distribution of substituted dopants affect the transition metal dichalcogenides (TMDCs) materials properties. Previous studies have demonstrated that the density of dopants in TMDCs increases with the amount of doping, and the phenomenon of doping concentration difference between the nucleation center and the edge is observed, but the spatial distribution law of doping atoms has not been carefully studied. Here, it is demonstrated that the spatial distribution of dopants changes at high doping concentrations. The spontaneous formation of an interface with a steep doping concentration change is named concentration phase separation (CPS). The difference in the spatial distribution of dopants on both sides of the interface can be identified by an optical microscope. This is consistent with the results of spectral analysis and microstructure characterization of scanning transmission electron microscope. According to the calculation results of density functional theory, the chemical potential has two relatively stable energies as the doping concentration increases, which leads to the spontaneous formation of CPS. Understanding the abnormal phenomena is important for the design of TMDCs devices. This work has great significance in the establishment and improvement of the doping theory and the design of the doping process for 2D materials.

Visualizing the Structure-Property Nexus of Wide-Bandgap Perovskite Solar Cells under Thermal Stress.

Wide-bandgap perovskite solar cells (PSCs) toward tandem photovoltaic applications are confronted with the challenge of device thermal stability, which motivates to figure out a thorough cognition of wide-bandgap PSCs under thermal stress, using in situ atomic-resolved transmission electron microscopy (TEM) tools combing with photovoltaic performance characterizations of these devices. The in situ dynamic process of morphology-dependent defects formation at initial thermal stage and their proliferations in perovskites as the temperature increased are captured. Meanwhile, considerable iodine enables to diffuse into the hole-transport-layer along the damaged perovskite surface, which significantly degrade device performance and stability. With more intense thermal treatment, atomistic phase transition reveals the perovskite transform to PbI2 along the topo-coherent interface of PbI2/perovskite. In conjunction with density functional theory calculations, a mutual inducement mechanism of perovskite surface damage and iodide diffusion is proposed to account for the structure-property nexus of wide-bandgap PSCs under thermal stress. The entire interpretation also guided to develop a thermal-stable monolithic perovskite/silicon tandem solar cell.

Edge Reconstruction-Dependent Growth Kinetics of MoS2.

Understanding the growth mechanisms of multielement two-dimensional (2D) crystals is challenging because of the unbalanced stoichiometry and possible reconstruction of their edges. Here, we present a systematic theoretical study on the chemical vapor deposition (CVD) growth mechanism of MoS2. We found that the growth kinetics of MoS2 highly depends on its edge reconstruction determined by concentrations of Mo and S in the growth environment. Based on the calculated energies of nucleation and propagation of various MoS2 edges, we predicted the transition of a MoS2 island growth from a regime of a triangle enclosed by Mo-terminated zigzag edges that are passivated by 50% S (Mo-II edges), to a regime of continuous evolution within a triangle, hexagon, and inverted triangle with 75%-S-terminated edges (S-III edges) and Mo-II edges, and finally to a regime of triangles with Mo-terminated zigzag edges that are passivated by 100% S (Mo-III edges) by tuning the growth condition from Mo-rich to S-rich, which provides a reasonable explanation to many experimental observations. This study provides a general guideline on theoretical studies of 2D crystals' growth mechanisms, deepens our understanding on the growth mechanism of multielement 2D crystals, and is beneficial for the controllable synthesis of various 2D crystals.

Atomistic Insight into the Epitaxial Growth Mechanism of Single-Crystal Two-Dimensional Transition-Metal Dichalcogenides on Au(111) Substrate.

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., ∼88% for MoS2 and 90% for MoSe2, nucleate on surface terraces, with the rest (i.e., ∼12% for MoS2 and 10% for MoSe2) on surface steps. Moreover, within the latter case, step-associated nucleation, ∼64% of them are terminated with a Mo-zigzag edge in connection with the Au surface steps, with the rest (∼36%) 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 (∼2.4 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 ∼3.0% for ML-MoS2/MoSe2 domains on Au(111). The present study aims to understand the growth mechanism of 2D TMD wafers.

Interlayer Coupling Dependent Discrete H → T' Phase Transition in Lithium Intercalated Bilayer Molybdenum Disulfide.

In this work, the interlayer coupling dependent lithium intercalation induced phase transition in bilayer MoS2 (BL-MoS2) was investigated using an atomic-resolution annual dark-field scanning transmission electron microscope (ADF-STEM). It was revealed that the lithiation induced H → T' phase transition in BL-MoS2 strongly depended on the interlayer twist angle; i.e., the H → T' phase transition occurred in well-stacked H phase BL-MoS2 (with a twist angle of θt = 0°) but not for θt ≠ 0° BL-MoS2. The lithiated BL-MoS2 appeared in homophase stacking, either T'/T' or H/H (locally, no phase transformation) stacking, without any heterophase stacking such as H/T' or T'/H observed. This finding indicated the H → T' phase transition occurred via a domain-by-domain mode rather than layer-by-layer. Up to 15 types of stacking orders were experimentally identified locally in lithiated bilayer T'-MoS2, and the formation mechanism was attributed to the discrete interlayer translation with a unit step of (m/6a, n/6b) (m, n = 0, 1, 2, 3), where a and b were the primitive lattice vectors of T'-MoS2. Our experimental results were further corroborated by ab initio density functional theory (DFT) calculations, where the occurrence of different stacking orders can be quantitatively correlated with the variation of intercalated lithium contents into the BL-MoS2. The present study aids in the understanding of the phase transition mechanisms in atomically thin 2D transition metal dichalcogenides (TMDCs) and will also shed light on the precisely controlled phase engineering of 2D materials for memory applications.

Lateral and Vertical MoSe2-MoS2 Heterostructures via Epitaxial Growth: Triggered by High-Temperature Annealing and Precursor Concentration.

Atomically thin transition-metal dichalcogenide (TMDC) heterostructures have attracted increasing attention because of their unprecedented potential in the fields of electronics and optoelectronics. However, selective growth of either lateral or vertical TMDC heterostructures remains challenging. Here, we report that lateral and vertical MoS2/MoSe2 epitaxial heterostructures can be successfully fabricated via a one-step growth strategy, which includes triggering by the concentration of sulfur precursor vapor and a high-temperature annealing process. Vertically stacked MoS2/MoSe2 heterostructures can be synthesized via control of the nucleation and growth kinetics, which is induced by high sulfur vapor concentration. The high-temperature annealing process results in the formation of fractured MoSe2 and in situ epitaxial growth of lateral MoSe2-MoS2 heterostructures. This study has revealed the importance of sulfur vapor concentration and high-temperature annealing processes in the controllable growth of MoSe2-MoS2 heterostructures, paving a new route for fabricating two-dimensional TMDC heterostructures.

Ultra-sensitive NH3 sensor based on flower-shaped SnS2 nanostructures with sub-ppm detection ability.

Layered metal dichalcogenides (LMDs) semiconducting materials have recently attracted tremendous attention as high performance gas sensors due to unique chemical and physical properties of thin layers. Here, three-dimensional SnS2 nanoflower structures assembled with thin nanosheets were synthesized via a facile solvothermal process. When applied to detect 100ppm NH3 at 200°C, the SnS2 based sensor exhibited high response value of 7.4, short response/recovery time of 40.6s/624s. Moreover, the sensor demonstrated a low detection limit of 0.5ppm NH3 and superb selectivity to NH3 against CO2, CH4, H2, ethanol and acetone. The excellent performance is attributed to the unique thin layers assembled flower-like nanoarchitecture, which facilitates both the carrier charge transfer process and the adsorption/desorption reaction. More importantly, it was found that the sensor response enhanced with increasing oxygen content in background and was improved by 3.57 times with oxygen content increasing from 0 to 40%. The increased response is owing to the enhanced binding energies between SnS2 and NH3 moleculers. Theoretically, density functional theory was employed to reveal the NH3 adsorption mechanism in different background oxygen contents, which opens a new horizon for LMD based structures applied in various gas sensing fields.

Enhanced Room Temperature Oxygen Sensing Properties of LaOCl-SnO2 Hollow Spheres by UV Light Illumination.

In this paper, a facile and elegant Green Chemistry method for the synthesis of SnO2 based hollow spheres has been investigated. The influences of doping, crystallite morphology, and operating condition on the O2 sensing performances of SnO2 based hollow-sphere sensors were comprehensively studied. It was indicated that, compared with undoped SnO2, 10 at. % LaOCl-doped SnO2 possessed better O2 sensing characteristics owing to an increase of specific surface area and oxygen vacancy defect caused by LaOCl dopant. More importantly, it was found that O2 sensing properties of the 10 at. % LaOCl-SnO2 sensor were significantly improved by ultraviolet light illumination, which was suited for room-temperature O2 sensing applications. Besides, this sensor also had a better selectivity to O2 with respect to H2, CH4, NH3, and CO2. The remarkable increase of O2 sensing properties by UV light radiation can be explained in two ways. On one hand, UV light illumination promotes the generation of electron-hole pairs and oxygen adsorption, giving rise to high O2 response. On the other hand, UV light activates desorption of oxygen adsorbates when exposed to pure N2, contributing to rapid response/recovery speed. The results demonstrate a promising approach for room-temperature O2 detection.