Investigations of Optical Functions and Optical Transitions of 2D Semiconductors by Spectroscopic Ellipsometry and DFT.

Optical functions and transitions are essential for a material to reveal the light-matter interactions and promote its applications. Here, we propose a quantitative strategy to systematically identify the critical point (CP) optical transitions of 2D semiconductors by combining the spectroscopic ellipsometry (SE) and DFT calculations. Optical functions and CPs are determined by SE, and connected to DFT band structure and projected density of states via equal-energy and equal-momentum lines. The combination of SE and DFT provides a powerful tool to investigate the CP optical transitions, including the transition energies and positions in Brillouin zone (BZ), and the involved energy bands and carries. As an example, the single-crystal monolayer WS2 is investigated by the proposed method. Results indicate that six excitonic-type CPs can be quantitatively distinguished in optical function of the monolayer WS2 over the spectral range of 245-1000 nm. These CPs are identified as direct optical transitions from three highest valence bands to three lowest conduction bands at high symmetry points in BZ contributed by electrons in S-3p and W-5d orbitals. Results and discussion on the monolayer WS2 demonstrate the effectiveness and advantages of the proposed method, which is general and can be easily extended to other materials.

Giant in-plane optical and electronic anisotropy of tellurene: a quantitative exploration.

Tellurene's giant in-plane optical anisotropy brings richer physics and an extra degree of freedom to regulate its optical properties for designing novel and unique polarization-sensitive devices. Here, we quantitatively evaluate the in-plane optical anisotropy of tellurene and further reveal its physical origins by combining imaging Mueller matrix spectroscopic ellipsometry (MMSE) and first-principles calculations. The anisotropic complex refractive indices and dielectric functions, as well as the derived giant birefringence (|Δn|max = 0.48) and dichroism (Δk > 0.4), are accurately determined by imaging MMSE to quantitatively evaluate the in-plane optical anisotropy of tellurene. With density functional theory (DFT), tellurene's optical anisotropy is connected to its low-symmetry lattice structure with electrical anisotropy (including the anisotropic effective mass, partial charge density, and carrier mobility), leading to anisotropic electric polarization and ultimately optical anisotropy. This work provides a general and quantitative way to explore the optical anisotropy and also helps to comprehend the connection between the lattice structure and the optical anisotropy of tellurene and even other emerging low-symmetry materials, which will further promote their polarization-sensitive optical applications.

Layer-dependent dielectric and optical properties of centimeter-scale 2D WSe2: evolution from a single layer to few layers.

Two-dimensional (2D) materials usually exhibit interesting layer-dependent dielectric and optical properties, which play important roles in structure optimization and performance improvement of related devices. Recently, 2D WSe2 has attracted considerable attention in atomically thin electronics and optoelectronics, due to its exotic photoelectric properties. In this paper, high-quality, continuous, and centimeter-scale 2D WSe2 with different layers on a sapphire substrate are prepared by an ultrafast ambient-pressure chemical vapor deposition method. We comprehensively investigate the evolution of the layer-dependent dielectric and optical properties of 2D WSe2 from a single layer to five layers by spectroscopic ellipsometry over an ultra-broad energy range (0.73-6.42 eV). We identify the critical points (CPs) in the dielectric function spectra of 2D WSe2 with different layers, and reveal physical origins of the corresponding optical transitions at these CPs by the CP analysis method and first-principles calculations. Results demonstrate that the center energies of these CPs exhibit intriguing layer-dependencies, which can be interpreted as the alternative domination of the decreasing exciton binding energy and the striking band renormalization. For the first time, we found that the imaginary part of the dielectric function of WSe2 at these CPs exhibits a valley-like shape versus the layer number, and the bottom appears at 3-layers. This non-monotonic evolution is explained as a competition between the layer-dependent decrease of the exciton effect and the layer-dependent increase of the joint density of states.

Complex Optical Conductivity of Two-Dimensional MoS2: A Striking Layer Dependency.

The complex optical conductivities of two-dimensaionl (2D) materials are fundamental for extended applications of related optoelectronic devices. Here, we systematically investigate the layer-dependent evolutions in the complex optical conductivities of 1-6 layer 2D MoS2 over an ultrawide spectral range (0.73-6.42 eV) by spectroscopic ellipsometry. We identify five feature peaks (A-E) in the optical conductivity spectra, which present interesting layer dependencies due to the scaling effect. Results suggest that the center energies of peaks A and B are nearly layer-independent, while those of peaks C and D exhibit redshifts as the layer increases. We interpret these layer-dependent evolutions as the competition between the decreasing exciton effect and the prominent band shrinkage with the increasing layer number. Additionally, the applicability of the classical slab model and the surface current model in evaluating the optical conductivities of 2D MoS2 with different layers is discussed from an experimental perspective.

Competency standard of school health teachers under Japanese health education system and its implications / 中国学校卫生

Abstract@#Health teacher system in Japan has a history of more than 70 years. At present, it has formed a comprehensive teacher education and certification system at three levels including junior college, bachelor and graduate degree. In 2017, Japan Association of Nursing Programs in University developed competency standard system for school health teachers, which covers 7 major dimensions, 19 core competence indicators and 87 academic standards according to requirements from Ministry of Education. This paper introduces and interprets school health teacher training system in Japan, and the core competence indicators and academic standards of the system, offering some basic reference for the construction of school health teacher training system in China.