Wavelength encoding spectral imaging based on the combination of deeply learned filters and an RGB camera.

Hyperspectral imaging is a critical tool for gathering spatial-spectral information in various scientific research fields. As a result of improvements in spectral reconstruction algorithms, significant progress has been made in reconstructing hyperspectral images from commonly acquired RGB images. However, due to the limited input, reconstructing spectral information from RGB images is ill-posed. Furthermore, conventional camera color filter arrays (CFA) are designed for human perception and are not optimal for spectral reconstruction. To increase the diversity of wavelength encoding, we propose to place broadband encoding filters in front of the RGB camera. In this condition, the spectral sensitivity of the imaging system is determined by the filters and the camera itself. To achieve an optimal encoding scheme, we use an end-to-end optimization framework to automatically design the filters' transmittance functions and optimize the weights of the spectral reconstruction network. Simulation experiments show that our proposed spectral reconstruction network has excellent spectral mapping capabilities. Additionally, our novel joint wavelength encoding imaging framework is superior to traditional RGB imaging systems. We develop the deeply learned filter and conduct actual shooting experiments. The spectral reconstruction results have an attractive spatial resolution and spectral accuracy.

Elimination of varying chromatic aberrations based on diffractive optics.

In telescopic systems consisting of Alvarez lenses, chromatic aberrations vary with the magnifications and the fields of view. Computational imaging has developed rapidly in recent years, therefore we propose a method of optimizing the DOE and the post-processing neural network in 2 stages for achromatic aberrations. We apply the iterative algorithm and the gradient descent method to optimize the DOE, respectively, and then adopt U-Net to further optimize the results. The results show that the optimized DOEs improve the results, the gradient descent optimized DOE with U-Net performs the best and has a very robust and good performance in the case of simulated chromatic aberrations. The results also verify the validity of our algorithm.

Snapshot hyperspectral imaging based on equalization designed DOE.

Hyperspectral imaging attempts to determine distinctive information in spatial and spectral domain of a target. Over the past few years, hyperspectral imaging systems have developed towards lighter and faster. In phase-coded hyperspectral imaging systems, a better coding aperture design can improve the spectral accuracy relatively. Using wave optics, we post an equalization designed phase-coded aperture to achieve desired equalization point spread functions (PSFs) which provides richer features for subsequent image reconstruction. During the reconstruction of images, our raised hyperspectral reconstruction network, CAFormer, achieves better results than the state-of-the-art networks with less computation by substituting self-attention with channel-attention. Our work revolves around the equalization design of the phase-coded aperture and optimizes the imaging process from three aspects: hardware design, reconstruction algorithm, and PSF calibration. Our work is putting snapshot compact hyperspectral technology closer to a practical application.

Tailored modifications of the electronic properties of g-C3N4/C2N-h2D nanoribbons by first-principles calculations.

The electronic structure of g-C3N4/C2N-h2D nanoribbons was investigated by first-principles calculations. As a splice structure, we first computed the three magnetic coupled states of g-C3N4/C2N-h2D nanoribbons. After self-consistent calculations, both the antiferromagnetic and paramagnetic coupling states become ferromagnetic coupling states. It was proved that the ferromagnetic coupling state is the most stable state. Thermodynamic stability was subsequently verified based on the ferromagnetic coupling state. It had a steady electron spin polarization, with a magnetic moment of 1 µB for each primitive cell. It changed from a direct band-gap semiconductor to an indirect band-gap semiconductor and exhibited the properties of a narrow band gap semiconductor through the analysis of the energy band and charge density. To transform the electronic structure, we adsorbed different transition metals in g-C3N4/C2N-h2D nanoribbons. We investigated the electronic structure of g-C3N4/C2N-h2D nanoribbons adsorbed by different transition metals. It was shown that the electronic structure of g-C3N4/C2N-h2D nanoribbons could be regulated by the adsorption of different transition metal atoms. Moreover, the adsorption of Fe and Ni can generate a 100% polarized current in the Fermi surface, which will provide more application potential for spintronics devices.

The metal atomic substitution induced half-metallic properties, metallic properties and semiconducting properties in X-N4 nanoribbons.

Armchair X-N4 nanoribbons (X-AN4NRs) and zigzag X-N4 nanoribbons (X-ZN4NRs) were calculated using first-principles calculations. Ferromagnets (FM) were found to be the most stable among the initial magnetic structures. Furthermore, nanoribbons were found to be thermodynamically stable through molecular dynamics simulations. It can be found that when the temperature and total energy of X-AN4NRs and X-ZN4NRs change with time, they have a small oscillation range, which confirms the dynamic stability of X-AN4NRs and X-ZN4NRs under realistic experimental conditions. Subsequently, the magnetic moment analysis of the X-AN4NRs and X-ZN4NRs revealed that the magnetic moment of the X-AN4NRs is significantly smaller than that of X-ZN4NRs. The band structure and density of states (DOS) of the X-AN4NRs and X-ZN4NRs were also computed, which indicate different properties for different transition metal nanoribbons. The results suggest that different edge structures and transition metals can influence the electronic structure of the nanoribbons. Moreover, based on the band structure and DOS, it was found that Mn-AN4NRs and Fe-ZN4NRs exhibit half-metallic properties. They can generate 100% polarized currents at the Fermi level, providing valuable information for developing spintronic devices. Our study has a positive value for regulating the properties of the nanoribbons by metal atom substitution.

Fast method for designing broadband achromatic diffractive optical elements.

Diffractive optical elements play a crucial role in the miniaturization of the optical systems, especially in correcting achromatic aberration. Considering the rapidity and validity of the design method, we propose a fast method for designing broadband achromatic diffractive optical elements. Based on the direct binary search algorithm, some improvements have been made including the selection of the initial height map to mitigate the uncertainty, the reduction of the variations to accelerate the optimization and the increase of sampling rate to deal with the large operation bandwidth. The initial height map is calculated instead of random initial value. Due to different regions of the height map contributing to point spread functions differently, the variations are reduced to speed up the optimization. The large operation bandwidth is solved by increasing the sampling rate at unfitted wavelengths instead of setting weighting coefficients. We demonstrate via simulations that our method is effective through several examples. The design of broadband achromatic diffractive optical elements can be quickly achieved by our method.

Doping atom improves photocatalytic performance in a new metal-free organic photocatalyst for water splitting.

Photocatalytic water splitting, using solar energy to obtain hydrogen, is an ideal technology for producing new energy. In the process of photocatalysis, the improvement of the catalytic performance of the catalysts used is a matter of great concern to scientists. So far, there are many problems preventing improvements in photocatalytic performance. In this paper, we propose an atom-doping method, which is an effective method to improve the catalytic performance. We present a type of graphene-like carbon nitride material, whose primitive cell is composed of 12 carbon atoms and 14 nitrogen atoms, so it is denoted as g-C12N14. The energy band, density of states, and optical absorption spectrum of g-C12N14 have been studied using first-principles calculations. From the characteristics of these properties, it is concluded that g-C12N14 can be used as a photocatalyst, but its catalytic performance is low. To improve the catalytic performance, atom doping has been used, which can change the electronic state of the surface to enhance the activity of the photocatalyst. It was calculated that the doped phosphorus and boron system improved the optical absorption range, thus improving the photocatalytic efficiency. To make the results more accurate, all the calculations used the computationally-large HSE06 hybrid functional method.

Investigation of Electron Momentum Density in Carbon Nanotubes Using Transmission Electron Microscopy.

Valence Compton profiles (CPs) of multiwall (MWCNTs) and single-wall carbon nanotubes (SWCNTs) were obtained by recording electron energy-loss spectra at large momentum transfer in the transmission electron microscope, a technique known as electron Compton scattering from solids (ECOSS). The experimental MWCNT/SWCNT results were compared with that of graphite. Differences between the valence CPs of MWCNTs and SWCNTs were observed, and the SWCNT CPs indicate a greater delocalization of the ground-state charge density compared to graphite. The results clearly demonstrate the feasibility and potential of the ECOSS technique as a complementary tool for studying the electronic structure of materials with nanoscale spatial resolution.

Tensile strain induced half-metallicity in graphene-like carbon nitride.

Polymeric graphitic carbon nitride materials exhibit exotic properties superior to graphene which are promising for applications in energy conversion, environment protection, and spintronics devices. We propose a two-dimensional (2D) framework of graphene-like carbon nitride composing of C9N7 units connected by nitrogen atoms. From first-principles, we demonstrate that this 2D carbon nitride has a spin-polarized ground state and exhibits metallic electronic properties, in contrast to commonly studied graphitic carbon nitrides which are nonmagnetic semiconductors. Additionally, half-metallicity can be achieved in this framework by applying tensile strain. The realization will be beneficial for spintronics as a candidate material for a spin-current generator. More importantly, this provides a feasible way to realize half-metallicity in experiments.

Synthesize magnetic ZnFe2O4@C/Cd0.9Zn0.1S catalysts with S-scheme heterojunction to achieve extraordinary hydrogen production efficiency.

Suppressing the electron-hole recombination rate of catalyst legitimately is one of the effective strategies to improve photocatalytic hydrogen evolution. Herein, carbon-coated metal oxide, ZnFe2O4@C (ZFO@C), nanoparticles were synthesized and employed to couple with quadrupedal Cd0.9Zn0.1S (CZS) via an ordinary ultrasonic self-assembly method combined with calcination to form a novel ZFO@C/CZS catalyst with step-scheme (S-scheme) heterojunction. The photocatalytic hydrogen evolution reaction (HER) was conducted to verify the enhanced photoactivity of ZFO@C/CZS. The optimal ZFO@C/CZS exhibits an extraordinary photocatalytic HER rate of 111.3 ± 0.9 mmol g-1 h-1 under visible-light irradiation, corresponding to an apparent quantum efficiency as high as (76.2 ± 0.9)% at 450 nm. Additionally, the as-synthesized ZFO@C/CZS composite exhibits high stability and recyclability. The excellent photocatalytic hydrogen evolution performance should arise from the formed S-scheme heterojunction and the unique ZFO@C core-shell structure, which inhibit electron hole recombination as well as provide more reactive sites. The pathway of S-scheme charge transfer was validated through density functional theory calculations and electrochemical measurements. This work provides a rational strategy for the synthesis of unique magnetic S-scheme heterojunction photocatalysts for water splitting under visible light irradiation.

Negative Differential Resistance of n-ZnO Nanorods/p-degenerated Diamond Heterojunction at High Temperatures.

In the present study, an n-ZnO nanorods (NRs)/p-degenerated diamond tunneling diode was investigated with regards to its temperature-dependent negative differential resistance (NDR) properties and carrier tunneling injection behaviors. The fabricated heterojunction demonstrated NDR phenomena at 20 and 80°C. However, these effects disappeared followed by the occurrence of rectification characteristics at 120°C. At higher temperatures, the forward current was increased, and the turn-on voltage and peak-to-valley current ratio (PVCR) were reduced. In addition, the underlying mechanisms of carrier tunneling conduction at different temperature and bias voltages were analyzed through schematic energy band diagrams and semiconductor theoretical models. High-temperature NDR properties of the n-ZnO NRs/p-degenerated diamond heterojunction can extend the applications of resistive switching and resonant tunneling diodes, especially in high-temperature, and high-power environments.

Combined study of the ground and excited states in the transformation of nanodiamonds into carbon onions by electron energy-loss spectroscopy.

The electron momentum density and sp2/sp3 ratio of carbon materials in the thermal transformation of detonation nanodiamonds (ND) into carbon nano-onions are systematically studied by electron energy-loss spectroscopy (EELS). Electron energy-loss near-edge structures of the carbon K-ionization in the electron energy-loss spectroscopy are measured to determine the sp2 content of the ND-derived samples. We use the method developed by Titantah and Lamoen, which is based on the ability to isolate the π* spectrum and has been shown to give reliable and accurate results. Compton profiles (CPs) of the ND-derived carbon materials are obtained by performing EELS on the electron Compton scattering region. The amplitude of the CPs at zero momentum increases with increasing annealing temperature above 500 °C. The dramatic changes occur in the temperature range of 900-1300 °C, which indicates that the graphitization process mainly occurs in this annealing temperature region. Our results complement the previous work on the thermal transformation of ND-derived carbon onions and provide deeper insight into the evolution of the electronic properties in the graphitization process.

Compton profile of few-layer graphene investigated by electron energy-loss spectroscopy.

In this paper, acquisition of the valence Compton profile of few-layer graphene using electron energy-loss spectroscopy at large scattering angle is reported. The experimental Compton profile is compared with the corresponding theoretical profile, calculated using the full-potential linearized augmented plane wave method based on the local-density approximation. Good agreement exists between the theoretical calculation and experiment. The graphene profile indicates a substantially greater delocalization of the ground state charge density compared to that of graphite.

The charge regulation of electronic structure and optical properties of graphitic carbon nitride under strain.

The electronic structure of the graphitic carbon nitride (g-C6N6) under strain was obtained using the hybrid density functional HSE06 with a larger computational workload. The g-C6N6 could withstand 12% of the applied tensile strain. The electronic structure of g-C6N6 could be changed effectively under the tensile force. The band gap changed from direct to indirect under the strain and could be tuned in the range of 3.16 eV to 3.75 eV. At approximately 4% of the applied strain, there was a transition of the valence band maximum (VBM). A wider range of light absorption could be obtained under the strain. Our results provide a prospect for the future applications of two-dimensional materials in electronic and optoelectronic devices.

Improved electrical transport properties of an n-ZnO nanowire/p-diamond heterojunction.

A heterojunction of n-ZnO nanowire (NW)/p-B-doped diamond (BDD) was fabricated. The rectifying behavior was observed with the turn on voltage of a low value (0.8 V). The forward current at 5 V is 12 times higher than that of a larger diameter n-ZnO nanorod (NR)/p-BDD heterojunction. The electrical transport behaviors for the comparison of n-ZnO NWs/p-BDD and n-ZnO NRs/p-BDD heterojunctions are investigated over various bias voltages. The carrier injection process mechanism for ZnO NWs/BDD is analyzed on the basis of the proposed equilibrium energy band diagrams. The ZnO NWs/BDD heterojunction displays improved I-V characteristics and relatively high performance for the electrical transport properties.

Forming heterojunction: an effective strategy to enhance the photocatalytic efficiency of a new metal-free organic photocatalyst for water splitting.

Photocatalytic water splitting is a new technology for the conversion and utilization of solar energy and has a potential prospect. One important aspect of enhancing the photocatalytic efficiency is how to improve the electron-hole separation. Up to now, there is still no ideal strategy to improve the electron-hole separation. In this article, for metal-free organic photocatalysts, we propose a good strategy- forming heterojunction, which can effectively improve the electron-hole separation. We provide a metal-free organic photocatalyst g-C12N7H3 for water splitting. The stability of g-C12N7H3 has been investigated, the X-ray diffraction spectra has been simulated. Using first-principles calculations, we have systematically studied the electronic structure, band edge alignment, and optical properties for the g-C12N7H3. The results demonstrated that g-C12N7H3 is a new organocatalyst material for water splitting. In order to enhance the photocatalytic efficiency, we provided four strategies, i.e., multilayer stacking, raising N atoms, forming g-C9N10/g-C12N7H3 heterojunction, and forming graphene/g-C12N7H3 heterojunction. Our research is expected to stimulate experimentalists to further study novel 2D metal-free organic materials as visible light photocatalysts. Our strategies, especially forming heterojunction, will substantially help to enhance the photocatalytic efficiency of metal-free organic photocatalyst.