Remote epitaxy of single-crystal rhombohedral WS2 bilayers.

Compared to transition metal dichalcogenide (TMD) monolayers, rhombohedral-stacked (R-stacked) TMD bilayers exhibit remarkable electrical performance, enhanced nonlinear optical response, giant piezo-photovoltaic effect and intrinsic interfacial ferroelectricity. However, from a thermodynamics perspective, the formation energies of R-stacked and hexagonal-stacked (H-stacked) TMD bilayers are nearly identical, leading to mixed stacking of both H- and R-stacked bilayers in epitaxial films. Here, we report the remote epitaxy of centimetre-scale single-crystal R-stacked WS2 bilayer films on sapphire substrates. The bilayer growth is realized by a high flux feeding of the tungsten source at high temperature on substrates. The R-stacked configuration is achieved by the symmetry breaking in a-plane sapphire, where the influence of atomic steps passes through the lower TMD layer and controls the R-stacking of the upper layer. The as-grown R-stacked bilayers show up-to-30-fold enhancements in carrier mobility (34 cm2V-1s-1), nearly doubled circular helicity (61%) and interfacial ferroelectricity, in contrast to monolayer films. Our work reveals a growth mechanism to obtain stacking-controlled bilayer TMD single crystals, and promotes large-scale applications of R-stacked TMD.

Stamped production of single-crystal hexagonal boron nitride monolayers on various insulating substrates.

Controllable growth of two-dimensional (2D) single crystals on insulating substrates is the ultimate pursuit for realizing high-end applications in electronics and optoelectronics. However, for the most typical 2D insulator, hexagonal boron nitride (hBN), the production of a single-crystal monolayer on insulating substrates remains challenging. Here, we propose a methodology to realize the facile production of inch-sized single-crystal hBN monolayers on various insulating substrates by an atomic-scale stamp-like technique. The single-crystal Cu foils grown with hBN films can stick tightly (within 0.35 nm) to the insulating substrate at sub-melting temperature of Cu and extrude the hBN grown on the metallic surface onto the insulating substrate. Single-crystal hBN films can then be obtained by removing the Cu foil similar to the stamp process, regardless of the type or crystallinity of the insulating substrates. Our work will likely promote the manufacturing process of fully single-crystal 2D material-based devices and their applications.

Influence of respondents' Differentiation of subjective response on water knowledge stock test scale: Evaluation based on two-parameter-multidimensional IRT model.

Insufficient awareness of water issues is a crucial bottleneck restricting the sustainable utilization of water resources. To accurately measure citizens' water knowledge stock and overcome the differences between scales and respondents' characteristic levels on test results, the research focuses on developing and evaluating water knowledge stock test scales. The mechanism for identifying indicators is designed based on the grounded theory, and as a result, the water knowledge stock test indicator system is derived. The data was collected by the form of survey questionnaire developed with the test indicator system. A two-parameter multidimensional item response theoretical model is constructed based on item parameter estimation, data model fitting, and item information function. The survey data and optimization model are used to optimize the water knowledge stock test scale and verify the fitting degree with the characteristics of the respondents. The test information function and standard error function indicate that the scale is most informative for individuals with characteristic levels ranging from -2 to 3, resulting in a highly reliable test effect. The research has established a measurement indicator system, methodology, and presented results that serve as a foundation for measuring the stock of water knowledge.

Universal epitaxy of non-centrosymmetric two-dimensional single-crystal metal dichalcogenides.

The great challenge for the growth of non-centrosymmetric 2D single crystals is to break the equivalence of antiparallel grains. Even though this pursuit has been partially achieved in boron nitride and transition metal dichalcogenides (TMDs) growth, the key factors that determine the epitaxy of non-centrosymmetric 2D single crystals are still unclear. Here we report a universal methodology for the epitaxy of non-centrosymmetric 2D metal dichalcogenides enabled by accurate time sequence control of the simultaneous formation of grain nuclei and substrate steps. With this methodology, we have demonstrated the epitaxy of unidirectionally aligned MoS2 grains on a, c, m, n, r and v plane Al2O3 as well as MgO and TiO2 substrates. This approach is also applicable to many TMDs, such as WS2, NbS2, MoSe2, WSe2 and NbSe2. This study reveals a robust mechanism for the growth of various 2D single crystals and thus paves the way for their potential applications.