Raman tensor of layered MoS2

Layer-dependent excitonic valley polarization properties in MoS2-WS2 heterostructures

In this work, we investigate the polarization of the excitonics valley in MoS₂-WS₂ heterostructures using circular polarization-resolved photoluminescence. The valley polarization is the largest (≈28.45%) in the 1L-1L MoS₂-WS₂ heterostructure and the polarizability of A_WS2 decreases as the number of WS₂ layers increases. We further observed a redshift of exciton XMoS₂- in MoS₂-WS₂ heterostructures with the increase of WS₂ layers, which is attributed to the displacement of the MoS₂ band edge, indicating the layer-sensitive optical properties of the MoS₂-WS₂ heterostructure. Our findings shed light on the understanding of exciton behavior in multilayer MoS₂-WS₂ heterostructures that may promote their potential applications in optoelectronic devices.

A multifunctional Fe2O3@MoS2@SDS Z-scheme nanocomposite: NIR enhanced bacterial inactivation, degradation antibiotics and inhibiting ARGs dissemination

To fight the flourishment of drug-resistant bacteria caused by antibiotics and the dissemination of antibiotic resistance genes (ARGs), it is of great urgency to develop multifunctional non-antibiotic agents with residual antibiotics elimination, and ARGs dissemination inhibition properties. Herein, sodium dodecyl sulfate (SDS) was modified onto the surface of Fe2O3 @MoS2 by ultrasonic method to obtain the Z-scheme, multifunctional Fe2O3 @MoS2 @SDS nanocomposites. The Fe2O3 @MoS2 @SDS (weight ratio of Fe2O3 @MoS2 and SDS was 1:1) was selected as the optimal agent. Under NIR irradiation, the Fe2O3 @MoS2 @SDS had a photothermal conversion efficiency of 45.96%, and could generate plenty of reactive oxygen species (ROS) at the same time. Under the synergy of photothermal and photodynamic, the antibacterial efficiency of Fe2O3 @MoS2 @SDS to E. coli, MRSA and P. aeruginosa could reach 99.95%, 99.97% and 99.58%, respectively, indicating excellent photothermal-photodynamic therapy (PPT) effect. The Fe2O3 @MoS2 @SDS also displayed photocatalytic activity in degradation of tetracycline (TC). The degradation rate of TC could reach 92.3% after 2 h of visible light irradiation. The obtained results indicated that a promising Fe2O3 @MoS2 @SDS composite based multifunctional nanoplatform could be constructed for NIR induced bacterial inactivation, antibiotics degradation and ARGs dissemination inhibition.

Polarized Raman spectroscopy in low-symmetry 2D materials: angle-resolved experiments and complex number tensor elements

In this perspective review, we discuss the power of polarized Raman spectroscopy to study optically anisotropic 2D materials, belonging to the orthorhombic, monoclinic and triclinic crystal families. We start by showing that the polarization dependence of the peak intensities is described by the Raman tensor that is unique for each phonon mode, and then we discuss how to determine the tensor elements from the angle-resolved polarized measurements by analyzing the intensities in both the parallel- and cross-polarized scattering configurations. We present specific examples of orthorhombic black phosphorus and monoclinic 1T'-MoTe₂, where the Raman tensors have null elements and their principal axes coincide with the crystallographic ones, followed by a discussion on the results for triclinic ReS₂ and ReSe₂, where the axes of the Raman tensor do not coincide with the crystallographic axes and all elements are non-zero. We show that the Raman tensor elements are, in general, given by complex numbers and that phase differences between tensor elements are needed to describe the experimental results. We discuss the dependence of the Raman tensors on the excitation laser energy and thickness of the sample within the framework of the quantum model for the Raman intensities. We show that the wavevector dependence of the electron-phonon interaction is essential for explaining the distinct Raman tensor for each phonon mode. Finally, we close with our concluding remarks and perspectives to be explored using angle-resolved polarized Raman spectroscopy in optically anisotropic 2D materials.

Angle-resolved polarized Raman spectra of the basal and edge plane of MoS2

Angle-resolved polarized (ARP) Raman spectroscopy can be utilized to characterize the Raman modes of two-dimensional layered materials based on crystal symmetry or crystal orientation. In this paper, the polarization properties of E ¹ _2g and A_1g modes on the basal plane and edge plane of high purity 2H-MoS₂ bulk crystal grown by chemical vapor transport (CVT) method were investigated by ARP Raman spectroscopy. The I and II type ARP Raman spectroscopy with four kinds of polarization configurations: αY, αX, βY, and βX were used to explore the intensity dependence of E ¹ _2g and A_1g modes at different planes on the polarization direction of incident/scattered light. The results show that the E ¹ _2g and A_1g modes exhibit different polarization properties dependent on the polarization of the incident laser and the in-plane rotation of the sample at different planes. The experimental results were confirmed and analyzed through theoretical calculation. Our work sheds light on the intriguing effect of the subtle atomic structure in stacked MoS₂ layers on the resulting ARP Raman properties. This provides a reference for the study of other two-dimensional layered crystalline materials by ARP Raman spectroscopy.

Polarized Raman Spectroscopy for Determining Crystallographic Orientation of Low-Dimensional Materials

Raman spectroscopy is a fast and nondestructive characterization technique, which has been widely used for the characterization of the composition and structure information of various materials. The symmetry-dependent Raman tensor allows the detection of crystallographic orientation of materials by using polarization information. In this Perspective, we discuss polarized Raman spectroscopy as a powerful tool for determination of the crystallographic orientation of various materials. First, we introduce the basic principles of polarized Raman spectroscopy and the corresponding experimental setups; the determination of crystallographic orientation of two-dimensional (2D) materials with in-plane isotropy and in-plane anisotropy using linearly polarized Raman scattering are then discussed. Furthermore, we discuss that using circularly polarized Raman spectroscopy, the azimuthal angle of materials in three dimensions (3D) can be characterized. In the final section, we show that the orientation distribution of nanomaterial assemblies can be measured using polarized Raman spectroscopy by introducing the orientation distribution function.

Laser Tuning in Layered h-BN Crystals

Integrated optics shows great potential in the current optical communication systems, sensor technology, optical computers, and other fields. Tunable laser technology within a certain range is the key to achieving on-chip optical integration; to realize which, Raman scattering is a competitive method that can effectively transfer incident laser energy to optical phonons due to the photon-phonon interaction. Here, we take hexagonal boron nitride as the energy conversion medium, and based on the angle-resolved polarized Raman spectroscopy, it is found that when laser polarization vector e_i ⊥ c axis, the spectrum obtains maximal scattering across the cross section and a minimal depolarization ratio. At room temperature, h-BN obtains an output signal with a wavelength of 522.8 nm and a full-width at half-maximum of 0.24 nm under the excitation of 488 nm pump laser, and the depolarization ratio is 0.09 (theoretically, it is 0, and this difference is due to experimental errors). And then, within the temperature range of 80∼420 K, the scattered light wavelength shows a high-precision shift of 0.006 nm/25 K, indicating that continuous wavelength tuning has been successfully achieved in h-BN.

2D van der Waals Molecular Crystal β-HgI2 : Economical, Rapid, and Substrate-Free Liquid-Phase Synthesis and Strong In-Plane Optical Anisotropy

2D materials have a great potential for wide-range applications due to their adjustable bandgap characteristics and special crystal structures. β-HgI₂ is a new 2D van der Waals inorganic molecular crystal material with a wide bandgap of 4.03 eV, on whose preparation and properties there are few relevant reports due to the feature of instability of molecular crystals. Here, an economical method to control the synthesis of large-size 2D β-HgI₂ single crystal by using a mineralizer-assisted solution is reported. According to angle-resolved polarization Raman spectroscopy and first-principles optical absorption calculation, 2D β-HgI₂ flake has a strong in-plane anisotropic light scattering characteristic and high optical absorption dichroism (a_z /a_y  = 3.4), which is due to a low in-plane symmetry of the orthorhombic structure of β-HgI₂ . More importantly, due to the molecular crystal structure of β-HgI₂ , its sensitivity to temperature is less than that of 2D materials such as MoS₂ , which has been confirmed by temperature-dependent Raman spectroscopy. In the work, more 2D inorganic molecular crystals are studied in the aspect of growth, which provides a theoretical basis for 2D molecular crystal optoelectronic devices' potential applications.

Raman Tensor of Layered SnS2

Recently, tin disulfide (SnS₂) has become a hot research focus in various fields due to its advantages of a high transistor switching ratio, an adjustable band gap in visible light range, excellent Li storage performance, sensitive gas recognition, and efficient photocatalytic capability. However, at present, studies of its basic structure mostly stay on the regulation related to the number of layers. To maximize the value of SnS₂ in the application design, this paper analyzes the angle-resolved polarized Raman spectra of SnS₂ crystals grown under high-temperature sealing systems. Under the parallel scattering configuration test of both the sample basal plane and the cross plane, we observed that how the Raman scattering intensity of the two test planes varies with the polarization angle is different. Combining this experimental result with theory support allows us to reach a conclusion that the differential polarizability of the phonon vibration mode along the z-axis of the cross plane of SnS₂ is proven to be the strongest. This finding is expected to provide favorable support for the application of structural regulation of SnS₂ and work as a reference for studying other van der Waals layered materials with greater potential.

Raman Tensor of van der Waals MoSe2

Molybdenum selenide (MoSe₂) is a van der Waals layered crystal with both the anisotropic light absorption and light scattering outside its surface. At present, the study of the Raman tensor of MoSe₂, which affects and even determines the inelastic light scattering's anisotropy, is not sufficient. In this research, with the aim of studying the out-of-plane anisotropy, we performed systematic angle-resolved polarized Raman (APR) spectroscopy and abstracted complete Raman tensors both experimentally and theoretically. In addition, according to first-principles calculations, in different conditions of laser excitation, MoSe₂ has various Raman tensor forms, of which the phase difference between Raman tensor elements of the A_1g mode is a particular one. By studying the anisotropic optical absorption properties, we confirmed that it is the dispersion and absorption of MoSe₂ under different pump light that lead to the photon-energy-dependent phase differences.