Large enhancement of red upconversion luminescence in beta Ba2Sc0.67Yb0.3Er0.03AlO5 phosphor via Mn2+ ions doping for thermometry.

Here, this study reports single-band red upconversion emission in ß-Ba2ScAlO5: Yb3+/Er3+ phosphor by doping Mn2+. The optimum concentration of Mn2+ ions in ß-Ba2ScAlO5: Yb3+/Er3+ phosphor was 0.20. The intensity of red and green emissions is increased by 27.4 and 19.3 times, respectively. Compared with the samples without Mn2+ ions, the red-green integral strength ratio of ß-Ba2ScAlO5: Yb3+/Er3+/Mn2+ sample was significantly increased by 28.4 times, reaching 110.9. The UCL mechanism was explored by analyzing the down-conversion luminescence spectra, absorption spectra, UCL spectra, and upconversion fluorescence lifetime decay curves of Yb3+/Er3+/Mn2+ co-doped ß-Ba2ScAlO5. The enhancement of upconversion red light is achieved through energy transfer between defect bands and Er3+ ions, as well as energy transfer between Mn2+ ions and Er3+ ions. In addition, the Mn2+ doped ß-Ba2ScAlO5: Yb3+/Er3+ red UCL phosphors have great potential for ambient temperature sensing in the 298-523 K temperature range. The maximum sensitivity of ß-Ba2ScAlO5: Yb3+/Er3+/Mn2+ phosphor as a temperature sensor at 523 K is 0.0247 K-1.

Intuitive and Efficient Approach to Determine the Band Structure of Covalent Organic Frameworks from Their Chemical Constituents.

The optical, electronic, and (photo) catalytic properties of covalent organic frameworks (COFs) are largely determined by their electronic structure and, specifically, by their Frontier conduction and valence bands (VBs). In this work, we establish a transparent relationship between the periodic electronic structure of the COFs and the orbital characteristics of their individual molecular building units, a relationship that is challenging to unravel through conventional solid-state calculations. As a demonstration, we applied our method to five COFs with distinct framework topologies. Our approach successfully predicts their first-principles conduction and VBs by expressing them as a linear combination of the Frontier molecular orbitals localized on the COF fragments. We demonstrate that our method allows for the rapid exploration of the impact of chemical modifications on the band structures of COFs, making it highly suitable for further application in the quest to discover new functional materials.

Epitaxial substitution of metal iodides for low-temperature growth of two-dimensional metal chalcogenides.

The integration of various two-dimensional (2D) materials on wafers enables a more-than-Moore approach for enriching the functionalities of devices1-3. On the other hand, the additive growth of 2D materials to form heterostructures allows construction of materials with unconventional properties. Both may be achieved by materials transfer, but often suffer from mechanical damage or chemical contamination during the transfer. The direct growth of high-quality 2D materials generally requires high temperatures, hampering the additive growth or monolithic incorporation of different 2D materials. Here we report a general approach of growing crystalline 2D layers and their heterostructures at a temperature below 400 °C. Metal iodide (MI, where M = In, Cd, Cu, Co, Fe, Pb, Sn and Bi) layers are epitaxially grown on mica, MoS2 or WS2 at a low temperature, and the subsequent low-barrier-energy substitution of iodine with chalcogens enables the conversion to at least 17 different 2D crystalline metal chalcogenides. As an example, the 2D In2S3 grown on MoS2 at 280 °C exhibits high photoresponsivity comparable with that of the materials grown by conventional high-temperature vapour deposition (~700-1,000 °C). Multiple 2D materials have also been sequentially grown on the same wafer, showing a promising solution for the monolithic integration of different high-quality 2D materials.

Multi-target free-space laser communication system based on a rotating double prism with an RF signal beacon.

Traditional free-space laser communication systems use beacon and signal lights for target detection and alignment. However, these approaches are inaccurate owing to signal dispersion errors. To overcome this difficulty, we propose a new method using transient radio frequency (RF) signals to achieve highly accurate target detection and alignment. To validate the feasibility of our proposed method, we built an experimental multi-target space-laser communication system based on a rotating double prism and applied it to achieve multi-target space-laser communication. The results demonstrate the efficiency of the proposed method to capture multi-target positions in the field of view using wireless RF signals and a rotating double prism. In addition, we show that the system is capable of rapid scanning and accurate pointing as well as establishing a one-way stable communication with multiple targets. When the target is 36 cm away, the pointing accuracy of the system motor is less than 0.8°, the pointing time is 1.2 s, and the average pointing lateral error is 0.666 mm.

An investigation of the role of transition metal ions impurities in CdO: Local structure and electronic properties.

According to the high-order perturbation formulae of 3d5 (Mn2+ ) and 3d9 (Cu2+ ) ions in octahedron, the local structures and electron paramagnetic resonance (EPR) parameters (g factors and hyperfine structure constants A) for Cu2+ and Mn2+ in CdO are theoretically studied in a consistent way. Due to the Jahn-Teller effect, both the substituted sites of Cu2+ and Mn2+ show the tetragonally elongated distortion with different elongation τ. Meanwhile, the crystal field and covalency around doped Cu2+ and Mn2+ are obtained, which can account for the electronic properties in doped CdO. In order to make further investigation of the potential optical and electrical properties, the band structure and density of states (DOS) of pure and transition metal ions (TMs) doped CdO are comparably calculated through density functional theory (DFT). The results show that the band gap of Mn2+ - and Cu2+ -doped CdO can be effectively reduced, due to the improved covalency between the central ions and ligand ions.

Visualizing Van der Waals Epitaxial Growth of 2D Heterostructures.

Understanding the growth mechanisms of 2D van der Waals (vdW) heterostructures is of great importance in exploring their functionalities and device applications. A custom-built system integrating physical vapor deposition and optical microscopy/Raman spectroscopy is employed to study the dynamic growth processes of 2D vdW heterostructures in situ. This allows the identification of a new growth mode with a distinctly different growth rate and morphology from those of the conventional linear growth mode. A model that explains the difference in morphologies and quantifies the growth rates of the two modes by taking the role of surface diffusion into account is proposed. A range of material combinations including CdI2 /WS2 , CdI2 /MoS2 , CdI2 /WSe2 , PbI2 /WS2 , PbI2 /MoS2 , PbI2 /WSe2 , and Bi2 Se3 /WS2 is systematically investigated. These findings may be generalized to the synthesis of many other 2D heterostructures with controlled morphologies and physical properties, benefiting future device applications.

Multiple-Dimensionally Controllable Nucleation Sites of Two-Dimensional WS2/Bi2Se3 Heterojunctions Based on Vapor Growth.

Two-dimensional (2D) heterojunctions have attracted great attention due to their excellent optoelectronic properties. Until now, precisely controlling the nucleation density and stacking area of 2D heterojunctions has been of critical importance but still a huge challenge. It hampers the progress of controlled growth of 2D heterojunctions for optoelectronic devices because the potential relation between numerous growth parameters and nucleation density is always poorly understood. Herein, by cooperatively controlling three parameters (substrate temperature, gas flow rate, and precursor concentration) in modified vapor deposition growth, the nucleation density and stacking area of WS2/Bi2Se3 vertical heterojunctions were successfully modulated. High-quality WS2/Bi2Se3 vertical heterojunctions with various stacking areas were effectively grown from single and multiple nucleation sites. Moreover, the potential nucleation mechanism and efficient charge transfer of WS2/Bi2Se3 vertical heterojunctions were systematically studied by utilizing the density functional theory and photoluminescence spectra. This modified vapor deposition strategy and the proposed mechanism are helpful in controlling the nucleation density and stacking area of other heterojunctions, which plays a key role in the preparation of electronic and optoelectronic nanodevices.

Universal Precise Growth of 2D Transition-Metal Dichalcogenides in Vertical Direction.

Two-dimensional transition-metal dichalcogenides (TMDs) have been one of the hottest focus of materials due to the most beneficial electronic and optoelectronic properties. Up to now, one of the big challenges is the synthesis of large-area layer-number-controlled single-crystal films. However, the poor understanding of the growth mechanism seriously hampers the progress of the scalable production of TMDs with precisely tunable thickness at an atomic scale. Here, the growth mechanisms in the vertical direction were systemically studied based on the density functional theory (DFT) calculation and an advanced chemical vapor deposition (CVD) growth. As a result, the U-type relation of the TMD layer number to the ratio of metal/chalcogenide is confirmed by the capability of ultrafine tuning of the experimental conditions in the CVD growth. In addition, high-quality uniform monolayer, bilayer, trilayer, and multilayer TMDs in a large area (8 cm2) were efficiently synthesized by applying this modified CVD. Although bilayer TMDs can be obtained at both high and low ratios of metal/chalcogenide based on the suggested mechanism, they demonstrate significantly different optical and electronic transport properties. The modified CVD strategy and the proposed mechanism should be helpful for synthesizing and large-area thickness-controlled TMDs and understanding their growth mechanism and could be used in integrated electronics and optoelectronics.

CuFe2O4/MoS2 Mixed-Dimensional Heterostructures with Improved Gas Sensing Response.

Mixed-dimensional (2D + nD, n = 0, 1, and 3) heterostructures opened up a new avenue for fundamental physics studies and applied nanodevice designs. Herein, a novel type-II staggered band alignment CuFe2O4/MoS2 mixed-dimensional heterostructures (MHs) that present a distinct enhanced (20-28%) acetone gas sensing response compared with pure CuFe2O4 nanotubes are reported. Based on the structural characterizations and DFT calculation results, the tentative mechanism for the improvement of gas sensing performance of the CuFe2O4/MoS2 MHs can be attributed to the synergic effect of type-II band alignment and the MoS2 active sites.

Studies on the local structures for the trigonal Ni3+ centers in cathode materials LiAly Co1-y O2 (y = 0, 0.1, 0.5, and 0.8).

The local angular distortions Δθ are theoretically studied for the various Ni3+ centers in LiAly Co1-y O2 at different Al concentrations (y = 0, 0.1, 0.5, and 0.8) based on the perturbation calculations of electron paramagnetic resonance g factors for a trigonally distorted octahedral 3d7 cluster with low spin (S = 1/2). Due to the Jahn-Teller effect, the [NiO6 ]9- clusters are found to experience the local angular distortions (Δθ ≈ 5°-9°) along the C3 axis. The variation trend of Δθ with y is in accordance with that of anisotropy (Δg = g||  - g⊥ ). As the substitutions can weaken bond strengths between transition metal and oxygen and the structural stability plays an important role in cathode performances, detailed investigations on the structural properties of the cathode materials LiAly Co1-y O2 can be practically helpful to understand the performances of these materials. The oxy-redox properties of LiAly Co1-y O2 systems are comprehensible in the framework of Ni3+ /Ni4+ couples, and the trigonally compressed octahedral [NiO6 ]9- clusters are applicable to the clarification of the electrochemical properties of lithium nickel oxide batteries. It appears that LiAl0.8 Co0.2 O2 with the largest Al concentration may correspond to the smallest distortion among the mixing systems.

Sesquiterpene amino ether and cytotoxic phenols from Dendrobium wardianum Warner.

A new bibenzyl derivative, dendrocandin V (1) and a new sesquiterpene amino ether, wardianumine A (2), together with eleven known compounds, including phenanthrenes (denbinobin (3), 9,10-dihydro-denbinobin (4), mostatin (5), loddigesiinols A (6)), bibenzyls (moscatilin (7), 5-hydroxy-3,4'-dimethoxybibenzyl (8), 3,4-dihydroxy-5,4'-dimethoxy bibenzyl (9), dendrocandin A (10), gigantol (11), dendrocandin U (12)) and an alkaloids (dihydroshihunine, 13) were isolated from the EtOH extraction of stems of Dendrobium wardianum Warner. Isolation of the new compound 2 indicated that N,N-dimethylethanolamine as the key adduction in the synthesis of dendroxine and its analogs in Dendrobium species. The hypothetical biosynthetic pathway of 2 was then postulated. Inspired by literature and traditional usage of the herbal medicine, some compounds were sent for cytotoxic activity and the results indicated that compounds 1, 3, 4, 5 showed cytotoxic activities against five human cancer cell lines (HL-60, A-549, SMMC-7721, MCF-7, and SW-480) with IC50 from 2.33-38.48µM. Among those compounds, 3 and 4 showed cell line selectivity with strong activity comparable to DDP.

[Rapid identification of Dendrobium plants based on near infrared diffuse reflection spectroscopy].

Near infrared diffuse reflection spectra of 15 species' 171 samples of Dendrobium combined with chemometrics statistical analysis were used to build prediction model, in order to discriminate different species of Dendrobium quickly and nondestructively. Hotelling T2 was applied to stability analysis of spectrum of 5 random drawing samples, and the results showed that the samples spectrum possessed good stability. Orthogonal test L24 (2 x 4 x 3 x 8) was designed to optimize optical path type, spectral band, derivative and smooth. The result of orthogonal test was analyzed by principal component analysis, which revealed that when 6500-4000 cm(-1) spectral band was applied, and with multiplicative scatter correction, second derivative, Norris smooth, and the number of principal components 7, the spectrum distinguishing accuracy was 100%. With the optimized condition of orthogonal test as the input value of partial least squares discriminant analysis and random drawing 123 samples as calibration set to establish the prediction model, and the rest 48 samples as prediction set were use to assess the property of the prediction model, the results indicated that the accumulating contribution rate of the first 3 principal components of the model was 99.36%, the identification of the standard deviation was +/- 0.1, and the correct recognition rate of the model was 97.92%. The results were satisfied. The method provided a new way for the rapid identification of different species of Dendrobium, and also supplied a reference for the authentication of medicinal plants.