Excitons in two-dimensional (2D) materials are tightly bound and exhibit rich physics. So far, the optical excitations in 2D semiconductors are dominated by Wannier-Mott excitons, but molecular systems can host Frenkel excitons (FE) with unique properties. Here, we report a strong optical response in a class of monolayer molecular J-aggregates. The exciton exhibits giant oscillator strength and absorption (over 30% for monolayer) at resonance, as well as photoluminescence quantum yield in the range of 60–100%. We observe evidence of superradiance (including increased oscillator strength, bathochromic shift, reduced linewidth and lifetime) at room-temperature and more progressively towards low temperature. These unique properties only exist in monolayer owing to the large unscreened dipole interactions and suppression of charge-transfer processes. Finally, we demonstrate light-emitting devices with the monolayer J-aggregate. The intrinsic device speed could be beyond 30 GHz, which is promising for next-generation ultrafast on-chip optical communications.

We report a method for quantitative phase recovery and simultaneous electron energy loss spectroscopy analysis using ptychographic reconstruction of a data set of "hollow" diffraction patterns. This has the potential for recovering both structural and chemical information at atomic resolution with a new generation of detectors.

The roles of formic acid and levulinic acid on the formation and growth of carbonaceous spheres.

In recent years, there have been many significant developments made in scanning transmission electron microscopy (STEM), notably the development of aberration correctors and complementary electron optical components such as monochromators and high brightness guns [1]. These advances have made it possible to obtain a 0.5Å resolution at 300kV for radiation resistant materials. However, spatial resolution is still limited for beam-sensitive specimen such as organics, biological specimens, zeolites and ceramics due to radiation damage. Beam-sensitive specimens have varying tolerances to electron dose due to different damage mechanisms. Although it is possible to reduce the dose in the STEM geometry by decreasing the pixel dwell time or the probe current density, a large pixel array is still needed for atomic resolution imaging and in addition manual adjustment of residual low order aberrations further increases the dose at the sample.Single shot coherent diffraction imaging (CDI) and ptychography has been widely used in light and Xray optics. The advantage of ptychography over traditional CDI is that it does not need prior information about the probe function and also overcomes some of the other issues of CDI, such as non-unique solutions and a limited field of view [2]. In electron microscopy, it has also attracted considerable interest due to its potential for super resolution [3], high phase sensitivity [5], three-dimensional [6] and low-dose [7] capabilities. One of the major developments that has advanced this field is the availability of a new generation of direct detection cameras that are particularly suited to ptychographic data acquisition with new modes of operation, such as electron counting and fast acquisition [8,9]. These new cameras dramatically also increase the detective quantum efficiency (DQE) and hence significantly improve the signal to noise ratio (SNR) of the recorded far field diffraction patterns (DP). Hence lower signals at higher scattering angles can be captured in each DP in a ptychograhic dataset which facilitates higher resolution in ptychographical reconstructions, even within the constraints of low electron dose work as required for beam sensitive samples.In this paper, we will firstly review our previous work on the capabilities of defocused probe ptychography to achieve a 2D phase reconstruction of a nanocrystal at sub-Å resolution [4] and a 3D reconstruction of nanostructured materials [7] using traditional CCD camera. Subsequently we will show results from focused and defocused electron ptychography using a fast direct electron detector to reconstruct the wavefunction of various low dimensional materials under different low dose conditions.

In recent years, scanning transmission electron microscopy (STEM) has become an important research tool for materials characterization. Using an annular detector geometry, electron energy loss spectroscopy (EELS) can be combined with high-angle annular dark field (HAADF) imaging to provide simultaneous structural and compositional information [1]. However, due to the dependence on atomic number of Z contrast, HAADF imaging is relatively insensitive to light elements [2]. By comparison, ptychography, as a phase sensitive imaging technique, has attracted great interest due to its high sensitivity to light elements at atomic resolution, especially given recent developments in fast direct-electron detectors and data-processing algorithms [3][4][5][6]. The phase recovered from the exit wave can simultaneously image light and heavy atoms [4] and further be extended to provide information along the Z-axis [3,5]. Furthermore, as the theoretical resolution is determined by the maximum spatial frequency recorded in diffraction patterns, electron ptychography has the potential to recover super-resolution phase data [6]. However, as entire forward-scattered electrons have to be detected for phase reconstruction, no electrons are allowed to enter an EELS spectrometer. Therefore, a method compatible with quantitative phase recovery and simultaneous EELS analysis has not been demonstrated.In this work, we will briefly demonstrate the capabilities of defocused ptychography to achieve a 2D phase reconstruction of nanocrystals at sub-ångström resolution [4] and 3D reconstruction of nanostructured materials [5]. Subsequently, we propose a new geometry for iterative ptychographic reconstruction from synthesized hollow diffraction patterns using a focused scanned probe [7]. This 5D-STEM imaging method with a hollow direct-electron detector not only fully integrates various traditional STEM imaging modes, including HAADF, annular bright field and differential phase contrast, as well as X-ray energy dispersive spectrometry and EELS mapping, but also provides ptychographic reconstructed phases with higher resolution and contrast for light elements. Fig. 1(a) shows a schematic illustration of the 5D-STEM experimental setup. JEM-ARM300F electron microscope was operated at 80 kV with a convergence semi-angle of 24 mrad. The focused probe was raster-scanned across monolayer MoS2 as shown in Fig. 1(b) with 256 × 256 probe positions and with a step size of 0.204 Å. The images shown in Fig. 2, demonstrate that the phase of monolayer MoS2 reconstructed from hollow diffraction patterns can achieve a resolution of 0.91 Å and that the 2S sites 20

To explore the application feasibility of high-strength steel in skeleton columns of precipitator casing structures, the bearing behavior of axially compressed H-section high-strength steel columns was investigated by the nonlinear finite element method by considering the stressed-skin effect of wallboard. When the column yield strength does not exceed 460 MPa, the column undergoes elasto-plastic interactive buckling, which means the steel strength can be fully utilized. For the column strength of 550 MPa or 690 MPa, the wallboard yield failure occurs, owing to excessive loading of the relatively weak wallboard, and column stress magnitude is usually in the elastic range without the full utilization of steel strength, whereas if the wallboard is stiff enough, columns will still undergo buckling failure. A welding residual stress measuring test was conducted to validate the residual stress generation simulation via the thermal-mechanical coupling finite element method. Concerning the geometrical imperfections and residual stresses, it was found that their influence becomes less severe when the column steel strength increases. The bearing capacity can be improved by increasing the wallboard thickness and stiffener stiffness, or reducing the wallboard width, the stiffener spacing, the width-to-thickness ratio of column flange, the height-to-thickness ratio of column web, and column torsional slenderness ratio. Column material can be fully utilized when column steel strength does not exceed 460 MPa. Hence, employing high-strength steel is reasonable. When the column steel strength is equal to or higher than 550 MPa, wallboard strength should be sufficient to ensure that the column failure occurs before wallboard failure. In such cases, high-strength steel should be used carefully.