A Resonance-Sensitive Ultralow-Frequency Raman Mode in Twisted Bilayer Graphene.

Twisted bilayer graphene (tBLG) possesses intriguing physical properties including unconventional superconductivity, enhanced light-matter interaction due to the formation of van Hove singularities (vHS), and a divergence of density of states in the electronic band structures. The vHS energy band gap provides optical resonant transition channels that can be tuned by the twist angle and interlayer coupling. Raman spectroscopy provides rich information on the vHS structure of tBLG. Here, we report the discovery of an ultralow-frequency Raman mode at ∼49 cm-1 in tBLG. This mode is assigned to the combination of ZA (an out-of-plane acoustic phonon) and TA (a transverse acoustic phonon) phonons, and the Raman scattering is proposed to occur at the so-called mini-valley. This mode is found to be particularly sensitive to the change in vHS in tBLG. Our findings may deepen the understanding of Raman scattering in tBLG and help to reveal vHS-related electron-phonon interactions in tBLG.

Graphdiyne Enabled Nitrogen Vacancy Formation in Copper Nitride for Efficient Ammonia Synthesis.

The electrocatalytic reduction of nitrate is promising for sustainable ammonia synthesis but suffers from slow reduction kinetics and multiple competing reactions. Here, we report a catalyst featuring copper nitride (Cu3N) anchored on a novel graphdiyne support (termed Cu3N/GDY), which is used for electrocatalytic reduction of nitrate to produce ammonia. The GDY absorbed hydrogen and enabled nitrogen (N) vacancy formation in Cu3N for the fast nitrate reduction reaction (NO3RR). Further, the distinct absorption sites formed by GDY and N vacancy enabled the excellent selectivity and stability of NO3RR. Notably, the Cu3N/GDY catalyst achieved a high ammonia yield (YNH3) up to 35280 µg h-1 mgcat.-1 and a high Faradaic efficiency (FE) of 98.1% using 0.1 M NO3- at -0.9 V versus a reversible hydrogen electrode (RHE). Using electron paramagnetic resonance (EPR) technology and in situ X-ray absorption fine structure (XAFS) spectroscopy measurement, we visualized the N vacancy formation in Cu3N and electrocatalytic NO3RR enabled by GDY. These findings show the promise of GDY in sustainable ammonia synthesis and highlight the efficacy of Cu3N/GDY as a catalyst.

Deep-Ultraviolet and Helicity-Dependent Raman Spectroscopy for Carbon Nanotubes and 2D Materials.

Recent progress of Raman spectroscopy on carbon nanotubes and 2D materials is reviewed as a topical review. The Raman tensor with complex values is related to the chiral 1D/2D materials without mirror symmetry for the mirror in the propagating direction of light, such as chiral carbon nanotube and black phosphorus. The phenomenon of complex Raman tensor is observed by the asymmetric polar plot of helicity-dependent Raman spectroscopy using incident circularly-polarized lights. First-principles calculations of resonant Raman spectra directly give the complex Raman tensor that explains helicity-dependent Raman spectra and laser-energy-dependent relative intensities of Raman spectra. In deep-ultraviolet (DUV) Raman spectroscopy with 266 nm laser, since the energy of the photon is large compared with the energy gap, the first-order and double resonant Raman processes occur in general k points in the Brillouin zone. First-principles calculation is necessary to understand the DUV Raman spectra and the origin of double-resonance Raman spectra. Asymmetric line shapes appear for the G band of graphene for 266 nm laser and in-plane Raman mode of WS2 for 532 nm laser, while these spectra show symmetric line shapes for other laser excitation. The interference effect on the asymmetric line shape is discussed by fitting the spectra to the Breit-Wigner-Fano line shapes.

Observing Axial Chirality of Chiral Single-Wall Carbon Nanotubes by Helicity-Dependent Raman Spectra.

Helicity-dependent Raman spectra of an isolated, chiral, single-wall carbon nanotube (SWNT) are reported using circularly polarized light. A polar plot of polarized Raman intensity for the radial breathing mode (RBM), which is excited by left-handed or right-handed circularly polarized light, shows asymmetric angle dependence relative to the nanotube axis direction, which reflects the axial chirality of a SWNT. The asymmetry in the polar plot of the RBM can be analyzed by a complex Raman tensor. The complex phase of each component of the Raman tensor has a maximum at chiral angle θ = 15° of a SWNT which is between two achiral SWNTs, that is, zigzag (θ = 0°) and armchair (θ = 30°) SWNTs. Considering the interaction between the chiral SWNT and the circularly polarized light, we discuss the origin of the complex phases excited by the opposite helicity of the circularly polarized light.

Hydrogen Radical-Induced Electrocatalytic N2 Reduction at a Low Potential.

Realizing efficient hydrogenation of N2 molecules in the electrocatalytic nitrogen reduction reaction (NRR) is crucial in achieving high activity at a low potential because it theoretically requires a higher equilibrium potential than other steps. Analogous to metal hydride complexes for N2 reduction, achieving this step by chemical hydrogenation can weaken the potential dependence of the initial hydrogenation process. However, this strategy is rarely reported in the electrocatalytic NRR, and the catalytic mechanism remains ambiguous and lacks experimental evidence. Here, we show a highly efficient electrocatalyst (ruthenium single atoms anchored on graphdiyne/graphene sandwich structures) with a hydrogen radical-transferring mechanism, in which graphdiyne (GDY) generates hydrogen radicals (H•), which can effectively activate N2 to generate NNH radicals (•NNH). A dual-active site is constructed to suppress competing hydrogen evolution, where hydrogen preferentially adsorbs on GDY and Ru single atoms serve as the adsorption site of •NNH to promote further hydrogenation of NH3 synthesis. As a result, high activity and selectivity are obtained simultaneously at -0.1 V versus a reversible hydrogen electrode. Our findings illustrate a novel hydrogen transfer mechanism that can greatly reduce the potential and maintain the high activity and selectivity in NRR and provide powerful guidelines for the design concept of electrocatalysts.

Arrays of horizontal carbon nanotubes of controlled chirality grown using designed catalysts.

The semiconductor industry is increasingly of the view that Moore's law-which predicts the biennial doubling of the number of transistors per microprocessor chip-is nearing its end. Consequently, the pursuit of alternative semiconducting materials for nanoelectronic devices, including single-walled carbon nanotubes (SWNTs), continues. Arrays of horizontal nanotubes are particularly appealing for technological applications because they optimize current output. However, the direct growth of horizontal SWNT arrays with controlled chirality, that would enable the arrays to be adapted for a wider range of applications and ensure the uniformity of the fabricated devices, has not yet been achieved. Here we show that horizontal SWNT arrays with predicted chirality can be grown from the surfaces of solid carbide catalysts by controlling the symmetries of the active catalyst surface. We obtained horizontally aligned metallic SWNT arrays with an average density of more than 20 tubes per micrometre in which 90 per cent of the tubes had chiral indices of (12, 6), and semiconducting SWNT arrays with an average density of more than 10 tubes per micrometre in which 80 per cent of the nanotubes had chiral indices of (8, 4). The nanotubes were grown using uniform size Mo2C and WC solid catalysts. Thermodynamically, the SWNT was selectively nucleated by matching its structural symmetry and diameter with those of the catalyst. We grew nanotubes with chiral indices of (2m, m) (where m is a positive integer), the yield of which could be increased by raising the concentration of carbon to maximize the kinetic growth rate in the chemical vapour deposition process. Compared to previously reported methods, such as cloning, seeding and specific-structure-matching growth, our strategy of controlling the thermodynamics and kinetics offers more degrees of freedom, enabling the chirality of as-grown SWNTs in an array to be tuned, and can also be used to predict the growth conditions required to achieve the desired chiralities.

Bifacial Raman Enhancement on Monolayer Two-Dimensional Materials.

Understanding the charge interaction between molecules and two-dimensional (2D) materials is essential for the design of functional devices. Here, we report the bifacial Raman enhancement of molecules on monolayer graphene and hexagonal boron nitride ( h-BN). Taking advantage of the atomically thick layered structure, we show that both surfaces of 2D materials can interact with molecules and simultaneously enhance their Raman scattering. Different enhancement features were observed for monolayer graphene and h-BN. The intensity decrease of particular Raman modes of copper phthalocyanine (CuPc) on both surfaces of h-BN suggests that z-dipoles exist and are partially canceled out between the two interfaces, while the twice Raman intensities of the characteristic Raman modes of CuPc on both surfaces of graphene compared to that on one surface evidenced the charge transfer process. These results provide an approach to modify 2D materials by bifacial adsorption of molecules, and the findings can inspire the design of functional 2D material-based devices.

Synthesis of Hydrogen-Substituted Graphyne Film for Lithium-Sulfur Battery Applications.

Graphyne (GY) is a new type of carbon allotrope, which is viewed as a rapidly rising star in the carbon family referred to as 2D carbon allotropes due to its extraordinary properties. Considering the dynamic nature of the alkyne metathesis reaction, a hydrogen-substituted graphyne (HsGY) film is successfully synthesized on a gas/liquid interface using 1,3,5-tripynylbenzene (TPB) as the precursor. The synthesized HsGY film is used as a sulfur host matrix to be applied in lithium-sulfur batteries (LSBs). The HsGY@S electrode is prepared using S8 as sulfur source and presents excellent electrochemical performance.

Spotting the differences in two-dimensional materials - the Raman scattering perspective.

Two-dimensional (2D) layered materials have attracted tremendous attention and led to a prosperous development in both fundamental investigation and device applications in various fields, such as nanoelectronics, flexible devices, sustainable energy and catalysts. The precise characterization of the structure and properties of 2D materials is in urgent need. Raman scattering spectroscopy is one of the most popular characterization tools that is convenient, rapid and non-invasive. It provides information on both the lattice structure from the frequency of phonon modes and the electronic band structure through the intensity due to electronic resonance Raman scattering. Although a few morphological characterization tools can image 2D materials with atomic resolution, Raman scattering measurements are more tolerant to the conditions of sample preparation such as the substrate and less technically demanding, and have been one of the routine tools for the characterization of 2D materials. In this review, we focus on the characterization of 2D materials using Raman scattering spectroscopy, in particular, the revealing of differences from primitive 2D materials, such as defects, doping effects, van der Waals heterostructures and the interaction with molecules. The characteristic Raman features of such differences and the corresponding interpretation will be discussed. We hope that this review will be useful for wide research communities of materials, physics, chemistry and engineering.

Fast Growth of Strain-Free AlN on Graphene-Buffered Sapphire.

We study the roles of graphene acting as a buffer layer for growth of an AlN film on a sapphire substrate. Graphene can reduce the density of AlN nuclei but increase the growth rate for an individual nucleus at the initial growth stage. This can lead to the reduction of threading dislocations evolved at the coalescence boundaries. The graphene interlayer also weakens the interaction between AlN and sapphire and accommodates their large mismatch in the lattice and thermal expansion coefficients; thus, the compressive strain in AlN and the tensile strain in sapphire are largely relaxed. The effective relaxation of strain further leads to a low density of defects in the AlN films. These findings reveal the roles of graphene in III-nitride growth and offer valuable insights into the efficient applications of graphene in the light-emitting diode industry.

Anisotropic Strain Relaxation of Graphene by Corrugation on Copper Crystal Surfaces.

Corrugation is a ubiquitous phenomenon for graphene grown on metal substrates by chemical vapor deposition, which greatly affects the electrical, mechanical, and chemical properties. Recent years have witnessed great progress in controlled growth of large graphene single crystals; however, the issue of surface roughness is far from being addressed. Here, the corrugation at the interface of copper (Cu) and graphene, including Cu step bunches (CuSB) and graphene wrinkles, are investigated and ascribed to the anisotropic strain relaxation. It is found that the corrugation is strongly dependent on Cu crystallographic orientations, specifically, the packed density and anisotropic atomic configuration. Dense Cu step bunches are prone to form on loose packed faces due to the instability of surface dynamics. On an anisotropic Cu crystal surface, Cu step bunches and graphene wrinkles are formed in two perpendicular directions to release the anisotropic interfacial stress, as revealed by morphology imaging and vibrational analysis. Cu(111) is a suitable crystal face for growth of ultraflat graphene with roughness as low as 0.20 nm. It is believed the findings will contribute to clarifying the interplay between graphene and Cu crystal faces, and reducing surface roughness of graphene by engineering the crystallographic orientation of Cu substrates.

Solar Transparent Radiators by Optical Nanoantennas.

Architectural windows are a major cause of thermal discomfort as the inner glazing during cold days can be several degrees colder than the indoor air. Mitigating this, the indoor temperature has to be increased, leading to unavoidable thermal losses. Here we present solar thermal surfaces based on complex nanoplasmonic antennas that can raise the temperature of window glazing by up to 8 K upon solar irradiation while transmitting light with a color rendering index of 98.76. The nanoantennas are directional, can be tuned to absorb in different spectral ranges, and possess a structural integrity that is not substrate-dependent, and thus they open up for application on a broad range of surfaces.

In-Plane Uniaxial Strain in Black Phosphorus Enables the Identification of Crystalline Orientation.

Identification of the crystalline axis of anisotropic black phosphorus (BP) is important for investigating its physical properties, as well as for optical and electronic applications. Herein, it is showed that by applying in-plane uniaxial strain and measuring the changes of the Raman shifts, the crystalline axis of BP can be reliably determined. The strain effects on the Raman shifts are angle-dependent, and they can be expressed as a combination of the Raman responses under zigzag and armchair strain. Differing from previous polarized optical spectroscopic methods where the Raman intensity is analyzed, the proposed method uses the Raman frequency shift, which is less affected by laser polarization, excitation wavelength, the sample thickness, and the substrate. The effective strain applied on BP from the stretched substrate is estimated, and the results show that only 20 to 40% of the strain can be effectively transferred to BP flakes from a polyethylene terephthalate substrate. Our method provides not only an effective and robust approach to identify the crystalline orientation of layered BP, but it is also a model to extract additional information in strain-related studies. It can also be extended to other 2D anisotropic materials.

Optical Anisotropy of Black Phosphorus in the Visible Regime.

The striking in-plane anisotropy remains one of the most intriguing properties for the newly rediscovered black phosphorus (BP) 2D crystals. However, because of its rather low-energy band gap, the optical anisotropy of few-layer BP has been primarily investigated in the near-infrared (NIR) regime. Moreover, the essential physics that determine the intrinsic anisotropic optical property of few-layer BP, which is of great importance for practical applications in optical and optoelectronic devices, are still in the fancy of theory. Herein, we report the direct observation of the optical anisotropy of few-layer BP in the visible regime simply by using polarized optical microscopy. On the basis of the Fresnel equation, the intrinsic anisotropic complex refractive indices (n-iκ) in the visible regime (480-650 nm) were experimentally obtained for the first time using the anisotropic optical contrast spectra. Our findings not only provide a convenient approach to measure the optical constants of 2D layered materials but also suggest a possibility to design novel BP-based photonic devices such as atom-thick light modulators, including linear polarizer, phase plate, and optical compensator in a broad spectral range extending to the visible window.

Birefringence-Directed Raman Selection Rules in 2D Black Phosphorus Crystals.

The incident and scattered light engaged in the Raman scattering process of low symmetry crystals always suffer from the birefringence-induced depolarization. Therefore, for anisotropic crystals, the classical Raman selection rules should be corrected by taking the birefringence effect into consideration. The appearance of the 2D anisotropic materials provides an excellent platform to explore the birefringence-directed Raman selection rules, due to its controllable thickness at the nanoscale that greatly simplifies the situation comparing with bulk materials. Herein, a theoretical and experimental investigation on the birefringence-directed Raman selection rules in the anisotropic black phosphorus (BP) crystals is presented. The abnormal angle-dependent polarized Raman scattering of the Ag modes in thin BP crystal, which deviates from the normal Raman selection rules, is successfully interpreted by the theoretical model based on birefringence. It is further confirmed by the examination of different Raman modes using different laser lines and BP samples of different thicknesses.

Diameter-Specific Growth of Semiconducting SWNT Arrays Using Uniform Mo2C Solid Catalyst.

Semiconducting single-walled nanotube (s-SWNT) arrays with specific diameters are urgently demanded in the applications in nanoelectronic devices. Herein, we reported that by using uniform Mo2C solid catalyst, aligned s-SWNT (∼90%) arrays with narrow-diameter distribution (∼85% between 1.0 and 1.3 nm) on quartz substrate can be obtained. Mo2C nanoparticles with monodisperse sizes were prepared by using molybdenum oxide-based giant clusters, (NH4)42[Mo132O372(H3CCOO)30(H2O)72]·10H3CCOONH4·300H2O(Mo132), as the precursor that was carburized by a gas mixture of C2H5OH/H2 during a temperature-programmed reduction. In this approach, the formation of volatile MoO3 was inhibited due to the annealing and reduction at a low temperature. As a result, uniform Mo2C nanoparticles are formed, and their narrow size-dispersion strictly determines the diameter distribution of SWNTs. During the growth process, Mo2C selectively catalyzes the scission of C-O bonds of ethanol molecules, and the resultant absorbed oxygen (Oads) preferentially etches metallic SWNTs (m-SWNTs), leading to the high-yield of s-SWNTs. Raman spectroscopic analysis showed that most of the s-SWNTs can be identified as (14, 4), (13, 6), or (10, 9) tubes. Our findings open up the possibility of the chirality-controlled growth of aligned-SWNTs using uniform carbide nanoparticles as solid catalysts for practical nanoelectronics applications.

Enhanced Raman Scattering on In-Plane Anisotropic Layered Materials.

Surface-enhanced Raman scattering (SERS) on two-dimensional (2D) layered materials has provided a unique platform to study the chemical mechanism (CM) of the enhancement due to its natural separation from electromagnetic enhancement. The CM stems from the charge interactions between the substrate and molecules. Despite the extensive studies of the energy alignment between 2D materials and molecules, an understanding of how the electronic properties of the substrate are explicitly involved in the charge interaction is still unclear. Lately, a new group of 2D layered materials with anisotropic structures, including orthorhombic black phosphorus (BP) and triclinic rhenium disulfide (ReS2), has attracted great interest due to their unique anisotropic electrical and optical properties. Herein, we report a unique anisotropic Raman enhancement on few-layered BP and ReS2 using copper phthalocyanine (CuPc) molecules as a Raman probe, which is absent on isotropic graphene and h-BN. According to detailed Raman tensor analysis and density functional theory calculations, anisotropic charge interactions between the 2D materials and molecules are responsible for the angular dependence of the Raman enhancement. Our findings not only provide new insights into the CM process in SERS, but also open up new avenues for the exploration and application of the electronic properties of anisotropic 2D layered materials.

Recent advances in plasmonic sensors.

Plasmonic sensing has been an important multidisciplinary research field and has been extensively used in detection of trace molecules in chemistry and biology. The sensing techniques are typically based on surface-enhanced spectroscopies and surface plasmon resonances (SPRs). This review article deals with some recent advances in surface-enhanced Raman scattering (SERS) sensors and SPR sensors using either localized surface plasmon resonances (LSPRs) or propagating surface plasmon polaritons (SPPs). The advances discussed herein present some improvements in SERS and SPR sensing, as well as a new type of nanowire-based SPP sensor.

Chiral Stacking Identification of Two-Dimensional Triclinic Crystals Enabled by Machine Learning.

Chiral materials possess broken inversion and mirror symmetry and show great potential in the application of next-generation optic, electronic, and spintronic devices. Two-dimensional (2D) chiral crystals have planar chirality, which is nonsuperimposable on their 2D enantiomers by any rotation about the axis perpendicular to the substrate. The degree of freedom to construct vertical stacking of 2D monolayer enantiomers offers the possibility of chiral manipulation for designed properties by creating multilayers with either a racemic or enantiomerically pure stacking order. However, the rapid recognition of the relative proportion of two enantiomers becomes demanding due to the complexity of stacking orders of 2D chiral crystals. Here, we report the unambiguous identification of racemic and enantiomerically pure stackings for layered ReSe2 and ReS2 using circular polarized Raman spectroscopy. The chiral Raman response is successfully manipulated by the enantiomer proportion, and the stacking orders of multilayer ReSe2 and ReS2 can be completely clarified with the help of second harmonic generation and scanning transmission electron microscopy measurements. Finally, we trained an artificial intelligent Spectra Classification Assistant to predict the chirality and the complete crystallographic structures of multilayer ReSe2 from a single circular polarized Raman spectrum with the accuracy reaching 0.9417 ± 0.0059.

Observation of Chirality Transfer in Twisted Few-Layer Graphene.

Chiral materials are the focus of research in a variety of fields such as chiroptical sensing, biosensing, catalysis, and spintronics. Twisted two-dimensional (2D) materials are rapidly developing into a class of atomically thin chiral materials that can be effectively modulated through interlayer twist. However, chirality transfer in chiral 2D materials has not been reported. Here, we show that the chirality from the twist interface of graphene can directly transfer to achiral few-layer graphene and lead to a strong chiroptical response probed with circularly polarized Raman spectroscopy. Distinct Raman optical activity (ROA) for the interlayer shear modes in achiral few-layer graphene is observed, with the degree of polarization reaching as high as 0.5. These findings demonstrate the programmability of chiroptical response through stacking and twist engineering in 2D materials and offer insights into the transfer of chirality in atomically thin chiral materials for optical and electronic applications.