The Raman tensor of a crystal is the derivative of its polarizability tensor and is dependent on the symmetries of the crystal and the Raman-active vibrational mode. The intensity of a particular mode is determined by the Raman selection rule, which involves the Raman tensor and the polarization configurations. For anisotropic two-dimensional (2D) layered crystals, polarized Raman scattering has been used to reveal the crystalline orientations. However, due to its complicated Raman tensors and optical birefringence, the polarized Raman scattering of triclinic 2D crystals has not been well studied yet. Herein, we report the anomalous polarized Raman scattering of 2D layered triclinic rhenium disulfide (ReS2) and show a large circular intensity differential (CID) of Raman scattering in ReS2 of different thicknesses. The origin of CID and the anomalous behavior in polarized Raman scattering were attributed to the appearance of nonzero off-diagonal Raman tensor elements and the phase factor owing to optical birefringence. This can provide a method to identify the vertical orientation of triclinic layered materials. These findings may help to further understand the Raman scattering process in 2D materials of low symmetry and may indicate important applications in chiral recognition by using 2D materials.Keywords: birefringence; circular intensity differential; polarized Raman scattering; rhenium disulfide; triclinic;

β-Graphdiyne (β-GDY) is a member of 2D graphyne family with zero band gap, and is a promising material with potential applications in energy storage, organic electronics, etc. However, the synthesis of β-GDY has not been realized yet, and the measurement of its intrinsic properties remains elusive. In this work, β-GDY-containing thin film is successfully synthesized on copper foil using modified Glaser–Hay coupling reaction with tetraethynylethene as precursor. The as-grown carbon film has a smooth surface and is continuous and uniform. Electrical measurements reveal the conductivity of 3.47 × 10−6 S m−1 and the work function of 5.22 eV. TiO2@β-GDY nanocomposite is then prepared and presented with an enhancement of photocatalytic ability compared to pure TiO2.

AbstractThe decontamination of water polluted with heavy metal ions is of worldwide concern. Among various treatment approaches to remove metal ions from water, adsorption is regarded as an efficient method, and a variety of materials have been applied as adsorbents for the removal of metal ions from polluted water. Recently, carbon nanomaterials have been examined as alternative adsorbents due to their high specific surface areas, high removal efficiency, and strong interactions with metal ions. Graphdiyne, a new kind of carbon allotrope composed of sp- and sp2-hybridized carbon atoms, has attracted great interest due to its impressive properties. Graphdiyne is considered to be a promising candidate for the adsorption of heavy metal ions as the acetylenic links in graphdiyne strongly interact with metal ions. Herein, graphdiyne is used as an adsorbent to remove lead ions from water. The interaction between lead ions and graphdiyne is explored by X-ray photoelectron spectroscopy and Raman spectroscopy. The maximum adsorption capacity calculated by the Langmuir isotherm model is 470.5 mg g−1. Graphdiyne is also synthesized on copper foam and used as a filter to eliminate lead ions from water. The filter shows high performance with a removal efficiency of 99.6% and can be recovered through treatment with acidic solution.

Due to the single-molecule sensitivity and the capability of chemical fingerprints recognition, surface-enhanced Raman scattering (SERS) has been an attractive analytical technique used in various fields. However, SERS sensing still suffers from several problems, including the heterogeneous adsorption of molecules on SERS substrates, the spectral fluctuation of molecules, the photo/chemical reactions of molecules in direct contact with metal, and the continuum spectral background originated from fluorescence or photocarbonization. Such problems greatly hinder its practical applications, in particular, in SERS quantification. Graphene, the star of the two-dimensional (2D) materials family, can be used for Raman enhancement, termed as graphene-based surface-enhanced Raman scattering (G-SERS). In this review, we will introduce the discovery of graphene-enhanced Raman scattering (GERS), the chemical enhancement, and its extension to other 2D materials beyond graphene. Then we will concentrate on graphene-based SERS toward analytical applications—from graphene-veiled SERS to G-SERS tape for quantitative analysis.

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.

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.