Designed growth of large bilayer graphene with arbitrary twist angles.

The production of large-area twisted bilayer graphene (TBG) with controllable angles is a prerequisite for proceeding with its massive applications. However, most of the prevailing strategies to fabricate twisted bilayers face great challenges, where the transfer methods are easily stuck by interfacial contamination, and direct growth methods lack the flexibility in twist-angle design. Here we develop an effective strategy to grow centimetre-scale TBG with arbitrary twist angles (accuracy, <1.0°). The success in accurate angle control is realized by an angle replication from two prerotated single-crystal Cu(111) foils to form a Cu/TBG/Cu sandwich structure, from which the TBG can be isolated by a custom-developed equipotential surface etching process. The accuracy and consistency of the twist angles are unambiguously illustrated by comprehensive characterization techniques, namely, optical spectroscopy, electron microscopy, photoemission spectroscopy and photocurrent spectroscopy. Our work opens an accessible avenue for the designed growth of large-scale two-dimensional twisted bilayers and thus lays the material foundation for the future applications of twistronics at the integration level.

Static volumetric three-dimensional display based on an electric-field-controlled two-dimensional optical beam scanner.

Using the quadratic electro-optic effect and the gradient of the composition ratio m [Nb/(Ta+Nb) in mol. %] in a potassium tantalate niobate crystal, we have designed an electric-field-controlled two-dimensional optical beam scanner with a wide wavelength range and fast response. This scanner is used to realize a volumetric display based on a two-frequency, two-step upconversion technique that is used to address the imaging volume. Use of appropriately designed imaging optics and custom-designed software to convert 2D renderings of volumetric images into control signals for the scanner along with appropriate infrared laser source selection allows efficient single-color image generation with a large viewing zone, without flicker and with natural depth cues. The resulting system has the potential to increase image resolution to nearly 61.5×109 with high scanning frequency and to expand to display three-color imagery.

Greatly Enhanced Raman Scattering of Graphene on Metals by a Boron Nitride Film Covering.

Raman spectroscopy, a nondestructive fingerprinting technique, is mainly utilized to identify molecular species and phonon modes of materials. However, direct Raman characterization of two-dimensional materials typically synthesized on catalytic metal substrates is extremely challenging because of the significant electric screening and interfacial electronic couplings. Here, we demonstrate that by covering as-grown graphene with boron nitride (BN) films, the Raman intensity of graphene can be enhanced by two orders of magnitude and is also several times stronger than that of suspended graphene. This great Raman enhancement originates from the optical field amplification by Fabry-Pérot cavity in BN films and the local field plasmon near copper steps. We further demonstrate the direct characterization of the local strain and doping level of as-grown graphene and in situ monitoring of the molecule reaction process by enhanced Raman spectroscopy. Our results will broaden the optical investigations of interfacial sciences on metals, including photoinduced charge transfer dynamics and photocatalysis at metal surfaces.

Electric field control of a Bragg diffraction optical beam splitter based on a cubic K(0.99)Li(0.01)Ta(0.63)Nb(0.37)O3 single crystal.

We have realized an electric field controlled Bragg diffraction optical beam splitter based on a photorefractive Bragg diffraction grating. In our experiments, the splitter was produced by wave coupling (532.0 nm) with a potassium lithium tantalate niobate single crystal. In the process of splitting, the incident beam could be split into multioutput beams by the splitter. The influence of an externally applied electric field was studied, and the results show that the intensity of the Bragg diffraction could be controlled by the electric field. The polarization properties of the splitter are discussed.

Computational Screening of High Activity and Selectivity TM/g-C3N4 Single-Atom Catalysts for Electrocatalytic Reduction of Nitrates to Ammonia.

Electrocatalytic reduction of nitrates (NO3RR) selectively generating ammonia (NH3) opens up a new idea for treating nitrates in wastewater, which not only reduces nitrates but also obtains the valuable product ammonia. By first-principles calculations, we explore the activity and selectivity for NO3RR to NH3 of TM/g-C3N4 single-atom catalysts. Six TM/g-C3N4 catalysts (TM = Ti, Os, Ru, Cr, Mn, and Pt) are selected by a four-step screening method. Ru/g-C3N4 is the most promising of these six TM/g-C3N4 catalysts because of its lowest energy barrier and extraordinary selectivity. The origin of the NO3RR activity of Ru/g-C3N4 is explained from the viewpoint of NO3- adsorption. In addition, the hydrogen evolution reaction has also been implied to be uncompetitive for the poor adsorption on H atoms. This work provides a screening mechanism for finding new catalysts for NO3RR to NH3, promotes the development of NO3RR, and provides a stimulating impetus for further experimental exploration.

Properties of strong anisotropic a-axis single-crystal fiber with an applied electric field.

The effect of an applied electric field on the properties of strongly anisotropic a-axis single-crystal fiber is studied theoretically. We solve the electromagnetic field equations for strongly anisotropic a-axis single-crystal fiber and numerically analyze the mode characteristics of the fiber that conducts only the zeroth-order elementary mode. We discuss the effects that an applied electric field has on the refractive index anisotropy and the mode characteristics of the fiber that conducts only the zeroth-order elementary mode.

Photorefractive real-time phase-coded optical correlation storage using a crossed cylindrical-collimating lens system.

Phase-coded optical correlation storage in photorefractive crystals, using a crossed cylindrical-collimating lens system, has been realized. It possesses the advantages of both storage and correlator. It can perform real time and fast selection of the information correlated to the input information from a great amount of stored information. In Zn:Fe:LiNbO(3)(0.03 wt. % Fe, 3 mol. % Zn), combining this rotational phase-coded multiplexing with angular multiplexing, 36 holograms have been successfully multiplexed and exactly identified in the same crystal volume. The cross talk and angular selectivity of such phase-coded multiplexing are discussed.