Review of Raman spectroscopy of two-dimensional magnetic van der Waals materials

Ultrathin van der Waals (vdW) magnets provide a possibility to access magnetic ordering in the two-dimensional (2D) limit, which are expected to be applied in the spintronic devices. Raman spectroscopy is a powerful characterization method to investigate the spin-related properties in 2D vdW magnets, including magnon and spin-lattice interaction, which are hardly accessible by other optical methods. In this paper, the recent progress of various magnetic properties in 2D vdW magnets studied by Raman spectroscopy is reviewed, including the magnetic transition, spin-wave, spin-lattice interaction, symmetry tuning induced by spin ordering, and nonreciprocal magneto-phonon Raman scattering.     

Raman spectroscopy of graphene and carbon nanotubes

This paper reviews progress that has been made in the use of Raman spectroscopy to study graphene and carbon nanotubes. These are two nanostructured forms of sp² carbon materials that are of major current interest. These nanostructured materials have attracted particular attention because of their simplicity, small physical size and the exciting new science they have introduced. This review focuses on each of these materials systems individually and comparatively as prototype examples of nanostructured materials. In particular, this paper discusses the power of Raman spectroscopy as a probe and a characterization tool for sp² carbon materials, with particular emphasis given to the field of photophysics. Some coverage is also given to the close relatives of these sp² carbon materials, namely graphite, a three-dimensional (3D) material based on the AB stacking of individual graphene layers, and carbon nanoribbons, which are one-dimensional (1D) planar structures, where the width of the ribbon is on the nanometer length scale. Carbon nanoribbons differ from carbon nanotubes is that nanoribbons have edges, whereas nanotubes have terminations only at their two ends.     

Layer-number determination of two-dimensional materials by optical characterization

Initiated by graphene, two-dimensional(2D) layered materials have attracted much attention owing to their novel layer-number-dependent physical and chemical properties. To fully utilize those properties, a fast and accurate determination of their layer number is the priority. Compared with conventional structural characterization tools, including atomic force microscopy, scanning electron microscopy, and transmission electron microscopy, the optical characterization methods such as optical contrast, Raman spectroscopy, photoluminescence, multiphoton imaging, and hyperspectral imaging have the distinctive advantages of a high-throughput and nondestructive examination. Here, taking the most studied 2D materials like graphene, MoS_2, and black phosphorus as examples, we summarize the principles and applications of those optical characterization methods. The comparison of those methods may help us to select proper ones in a cost-effective way.     

Local probe of the interlayer coupling strength of few-layers SnSe by contact-resonance atomic force microscopy

The interlayer bonding in two-dimensional (2D) materials is particularly important because it is not only related to their physical and chemical stability but also affects their mechanical, thermal, electronic, optical, and other properties. To address this issue, we report the direct characterization of the interlayer bonding in 2D SnSe using contact-resonance atomic force microscopy (CR-AFM) in this study. Site-specific CR spectroscopy and CR force spectroscopy measurements are performed on both SnSe and its supporting SiO₂/Si substrate comparatively. Based on the cantilever and contact mechanic models, the contact stiffness and vertical Young’s modulus are evaluated in comparison with SiO₂/Si as a reference material. The interlayer bonding of SnSe is further analyzed in combination with the semi-analytical model and density functional theory calculations. The direct characterization of interlayer interactions using this non-destructive methodology of CR-AFM would facilitate a better understanding of the physical and chemical properties of 2D layered materials, specifically for interlayer intercalation and vertical heterostructures.     

Orientation of sp2 carbon nanoclusters in ion‐implanted polymethylmethacrylate as revealed by polarized Raman spectroscopy

Raman spectroscopy is widely used for the characterization of bonding type in carbon‐based materials, including carbonized surface layer in ion‐implanted polymers. Studies of the polarization properties of Raman scattering from amorphous carbonaceous materials, however, are very scarce. In this paper, we investigate the polarized Raman spectra of polymethylmethacrylate (PMMA) implanted with 50‐keV Si⁺ ions at fluences in the range 3.2 × 10¹⁴–1.0 × 10¹⁷ ions/cm² and for different visible excitation wavelengths. The spectra of the implanted samples are dominated by the D‐ and G‐bands of sp² carbon, which evidence strong carbonization of the ion‐modified layer. The multiwavelength excitation allowed us to resonantly probe the depolarization ratios for sp² clusters of different sizes. We established that the depolarization ratio ρ_G of the G‐band correlates with the sp² cluster size approaching the random orientation limit of 0.75 for the smallest clusters and a limiting value of 0.41 for the largest clusters. The experimental findings give evidence for a preferable orientation of the larger size clusters with their hexagonal planes perpendicular to the surface of the sample. A plausible explanation for such an arrangement is that the sp² clusters form tile‐like arrangements along the ion tracks. This finding may give clues for understanding of the strong transconductance of the ion‐modified layer, and open prospects for the application of polarized Raman spectroscopy as a characterization tool for surface morphology in ion‐implanted materials. Copyright © 2010 John Wiley & Sons, Ltd.     

共振光谱技术在分析空间非均质物质上的应用(英文)

pacc:0765Thispaperdemonstratesthreevibrational spectroscopictechniquesandtheirapplicationsin characterizationofspatiallyheterogamousmateri als,namelystep-scanphasemodulationFTIR photoacousticspectroscopy(S2MPAS),visible(confocal)Ramanmicroscopy(CRM)andF…     

Stress‐dependent correlations for resonant Raman bands in graphite with defects

We have studied the Raman features characteristics of defects generated in graphite under high stress conditions. Defects are generated in pristine highly oriented pyrolytic graphite by squeezing the samples in a high‐pressure anvil cell and monitored in situ by Raman spectroscopy. On the basis of our Raman measurements and existing literature correlations, we conclude that vacancies and grain boundaries are generated during compression–decompression cycles, being the defects mostly generated during the decompression stage. Our results demonstrate that the relative intensities of the D, D′, and (D + D′) bands are strongly correlated. Which is important for practical application of Raman spectroscopy in the characterization of carbon materials is that such correlations are essentially constant over the whole stress range covered in the experiments (~7 GPa). An additional interesting result concerns the relative intensities of the denoted 2D₁ and 2D₂ contributions, which are correlated with the intensity of the G band; the intensity ratio between both features is modified by stress indicating that the stress affects the stacking order of pristine graphite. Finally, we find that the decrease in intensity of the 2D₂ band with decreasing crystallite size found in existing studies on unstrained graphite remains under stress conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental Synthesis of Strained Monolayer Silver Arsenide on Ag(111)Substrates

Two-dimensional(2 D) materials are playing more and more important roles in both basic sciences and industrial applications. For 2 D materials, strain could tune the properties and enlarge applications. Since the growth of 2 D materials on substrates is often accompanied by strain, the interaction between 2 D materials and substrates is worthy of careful attention. Here we demonstrate the fabrication of strained monolayer silver arsenide(AgAs) on Ag(111) by molecular beam epitaxy, which shows one-dimensional stripe structures arising from uniaxial strain.The atomic geometric structure and electronic band structure are investigated by low energy electron diffraction,scanning tunneling microscopy, x-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy and first-principle calculations. Monolayer AgAs synthesized on Ag(111) provides a platform to study the physical properties of strained 2 D materials.     

Dimensional crossover tuned by pressure in layered magnetic NiPS_3

The physical properties of most 2D materials are highly dependent on the nature of their interlayer interaction. In-depth studies of the interlayer interaction are beneficial to the understanding of the physical properties of 2D materials and permit the development of related devices. Layered magnetic NiPS_3 has unique magnetic and electronic properties. The electronic band structure and corresponding magnetic state of NiPS_3 are expected to be sensitive to the interlayer interaction, which can be tuned by external pressure. Here, we report an insulator-metal transition accompanied by the collapse of magnetic order during the 2D-3D structural crossover induced by hydrostatic pressure. A two-stage phase transition from a monoclinic(C2/m) to a trigonal(P31m)lattice is identified via ab initio simulations and confirmed via high-pressure X-ray diffraction and Raman scattering; this transition corresponds to a layer-by-layer slip mechanism along the a-axis. Temperature-dependent resistance measurements and room temperature infrared spectroscopy under different pressures demonstrate that the insulator-metal transition and the collapse of the magnetic order occur at ~20 GPa, which is confirmed by low-temperature Raman scattering measurements and theoretical calculations. These results establish a strong correlation between the structural change, electric transport, and magnetic phase transition and expand our understanding of layered magnetic materials. Moreover, the structural transition caused by the interlayer displacement has significance for designing similar devices at ambient pressure.     

Signal-to-noise ratio of Raman signal measured by multichannel detectors

Raman spectroscopy has been widely used to characterize the physical properties of two-dimensional materials(2 DMs). The signal-to-noise ratio(SNR or S/N ratio) of Raman signal usually serves as an important indicator to evaluate the instrumental performance rather than Raman intensity itself. Multichannel detectors with outstanding sensitivity, rapid acquisition speed and low noise level have been widely equipped in Raman instruments for the measurement of Raman signal. In this mini-review, we first introduce the recent advances of Raman spectroscopy of 2 DMs. Then we take the most commonly used CCD detector and IGA array detector as examples to overview the various noise sources in Raman measurements and analyze their potential influences on SNR of Raman signal in experiments. This overview can contribute to a better understanding on the SNR of Raman signal and the performance of multichannel detector for numerous researchers and instrumental design for industry, as well as offer practical strategies for improving spectral quality in routine measurement.     

Raman evidence of the interaction between multiwalled carbon nanotubes and nanostructured TiO2

Nanocomposites of carbon nanotubes and titanium dioxide (TiO₂) have attracted much attention due to their photocatalytic properties. Although many examples in the literature have visualized these nanocomposites by electron microscopic images, spectroscopic characterization is still lacking with regard to the interaction between the carbon nanotube and TiO₂. In this work, we show evidence of the attachment of nanostructured TiO₂ to multiwalled carbon nanotubes (MWNTs) by Raman spectroscopy. The nanostructured TiO₂ was characterized by both full‐width at half‐maximum (FWHM) and the Raman shift of the TiO₂ band at ca 144 cm⁻¹, whereas the average diameter of the crystallite was estimated as approximately 7 nm. Comparison of the Raman spectra of the MWNTs and MWNTs/TiO₂ shows a clear inversion of the relative intensities of the G and D bands, suggesting a substantial chemical modification of the outermost tubes due to the attachment of nanostructured TiO₂. To complement the nanocomposite characterization, scanning electronic microscopy and X‐ray diffraction were performed. Copyright © 2011 John Wiley & Sons, Ltd.     

Characterization of carbon nanotube filters and other carbonaceous materials by Raman spectroscopy—II: study on dispersion and disorder parameters

Recently, we have reported on the characterization of various carbonaceous materials including multiwalled carbon nanotube (MWCNT) filters, which have specific molecular filtering capabilities and good mechanical strength and can be produced in bulk as highly aligned arrays of bundles of CNTs. We have extended our studies using Fourier transform‐Raman spectroscopy with 1064 nm excitation wavelength and a rotating sample holder in the region 1000–2800 cm⁻¹, in addition to 532 and 785 nm, which were used for Raman excitation in our previous study. Raman spectra were analyzed for band positions and line shape with special emphasis on the D‐, G‐ and G′‐ bands. For the single‐walled species, Carbotrap and graphite spectra were also recorded with 488 nm excitation. A dispersion study has been made from the Raman data available with the different excitation wavelengths. Slight band shifts and band broadening could be observed under the two sample conditions, one with the stationary sample and the other with sample rotation. The spectral changes are related to the excessive heating caused in a stationary sample by laser irradiation. Based on our findings in this study combined with our earlier study, we can state that only a careful line shape analysis and study of intensity pattern of the D‐ and G‐Raman bands under well‐defined measurement conditions lends itself as a good measure of degree of alignment in the MWCNT bundles. Copyright © 2010 John Wiley & Sons, Ltd.     

Nanostructured Au/Si substrate for organic molecule SERS detection

Nanoparticles of noble metals, such as gold and silver, exhibit unique and tunable optical properties on account of their surface plasmon resonance. In particular, gold nanoparticles on silicon substrates are attractive for future nanoscale sensors and optical devices due to their resistance to oxidation and due to their electrical and optical properties. In this study, we developed a nanostructured gold/macroporous silicon (Au/PS) substrate capped with 11-mercaptoundecanoic acid (11-MUA) with ultra-sensitive detection properties achieved in characterization, an approach based on surface-enhanced Raman scattering (SERS). Surface-enhanced Raman scattering allows us to detect substances at a low concentration level and to observe structural details of a thiol molecule bonded to small film thicknesses. Raman measurements were carried out at 514 nm and 785 nm. In order to emphasize the effect of the Si microstructuration on the efficiency of this new substrate (Au/PS) proposed for SERS experiments, the same molecule (11-MUA) was adsorbed on it as well as on gold/atomically flat silicon (Au/Si) and on commercial Klarite (Mesophotonics) substrates. Systematic studies realized by Raman spectroscopy, electron microscopy, and X-ray spectroscopy show the influence of silicon substrate texturing and metallic deposition conditions, including time and temperature on the optical phenomena.     

Enhancement of lattice defect signatures in graphene and ultrathin graphite using tip‐enhanced Raman spectroscopy

Understanding the role of defects in graphene is the key to tailoring the properties of graphene and promoting the development of graphene‐based devices. Defects can affect the electronic properties of a device while also offering a means by which to functionalize the local properties. Using tip‐enhanced Raman spectroscopy (TERS), heightened defect sensitivity was demonstrated on graphene edges, folds, and overlapping regions. Measurements confirm that TERS can provide simultaneous structural and spectral information on a localized scale, hence offering defect characterization on a scale that is not obtainable using conventional Raman spectroscopy. This study observed preferential enhancement of the D band signal on multilayered graphene and ultrathin graphite; in addition, other key defect signatures were also enhanced and detected. We present our findings in relation to theoretical predictions of graphene defect signatures and an analysis of the sensitivity of TERS in measuring two‐dimensional structures. Copyright © 2013 John Wiley & Sons, Ltd.     

集成光谱学和显微学技术的多域高分辨表征平台

本文研究了最近建立的集成光谱学和显微学技术的多域(能量、空间、时间)高分辨表征平台. 它提供了一种前所未有的方式来分析材料的光谱(能量)在时间域和空间域方面的演变. 展示了在此多域分辨平台上所采集的几种实验结果,其中包括原位拉曼成像(高分辨率拉曼、偏振拉曼、低波数拉曼)、时间分辨光致发光成像和光电性能成像等等. 新建立的多域高分辨表征平台将在材料科学的光化学领域发挥重要的作用.     

Different angle-resolved polarization configurations of Raman spectroscopy: A case on the basal and edge plane of two-dimensional materials

Angle-resolved polarized Raman(ARPR) spectroscopy can be utilized to assign the Raman modes based on crystal symmetry and Raman selection rules and also to characterize the crystallographic orientation of anisotropic materials.However, polarized Raman measurements can be implemented by several different configurations and thus lead to different results. In this work, we systematically analyze three typical polarization configurations: 1) to change the polarization of the incident laser, 2) to rotate the sample, and 3) to set a half-wave plate in the common optical path of incident laser and scattered Raman signal to simultaneously vary their polarization directions. We provide a general approach of polarization analysis on the Raman intensity under the three polarization configurations and demonstrate that the latter two cases are equivalent to each other. Because the basal plane of highly ordered pyrolytic graphite(HOPG) exhibits isotropic feature and its edge plane is highly anisotropic, HOPG can be treated as a modelling system to study ARPR spectroscopy of twodimensional materials on their basal and edge planes. Therefore, we verify the ARPR behaviors of HOPG on its basal and edge planes at three different polarization configurations. The orientation direction of HOPG edge plane can be accurately determined by the angle-resolved polarization-dependent G mode intensity without rotating sample, which shows potential application for orientation determination of other anisotropic and vertically standing two-dimensional materials and other materials.     

Tip-Enhanced Raman Spectroscopy and Mapping of Graphene Sheets

Abstract: Single graphene sheets, a few graphene layers, and bulk graphite, obtained via both micromechanical cleavage of highly oriented pyrolytic graphite and carbon vapor deposition methods, were deposited on a thin glass substrate without the use of any chemical treatment. Micro-Raman spectroscopy, tip-enhanced Raman spectroscopy (TERS), and tip-enhanced Raman spectroscopy mapping (TERM) were used for characterization of the graphene layers. In particular, TERM allows for the investigation of individual graphene sheets with high Raman signal enhancement factors and allows for imaging of local defects with nanometer resolution. Enhancement up to 560% of the graphene Raman band intensity was obtained using TERS. TERM (with resolution better than 100 nm) showed an increase in the number of structural defects (D band) on the edges of both graphene and graphite regions.     

Optically probing the interaction between monolayer MoS<Subscript>2</Subscript> and single-wall carbon nanotube

Owing to outstandingly tunable optoelectronic properties, hybrid materials consisting of atomic scale thickness of two dimensional (2D) transition metal dichalcogenides (TMDs) and one dimensional (1D) nanowires have been attracting steady interests over the last several years. In this research for the first time we report optically probing the interaction between monolayer MoS₂ and single-wall carbon nanotube (SWCNT). By using Raman and photoluminescence measurements, we found the charge transfer between MoS₂ and SWCNT is sensitive to the intensity of light field. We also demonstrate that SWCNT acts as p-type dopants at physical contact with monolayer MoS₂. Our study gives new insight into the interaction between monolayer MoS₂ and SWCNT, which may allow new phenomena and ideas for novel low dimensional hybrid materials.     

Monitoring of the microstructure of ion‐irradiated nuclear ceramics by in situ Raman spectroscopy

Raman spectroscopy is an efficient technique for studying the evolution of microstructure of materials under irradiation. For that purpose, a Raman spectrometer has been recently installed at the JANNUS‐Saclay platform. In this paper, we describe the new setup for in situ experiments. These in situ experiments allowed following the microstructural evolution of different materials (SiC, ZrO₂ and B₄C) as a function of ion fluence on a single sample (either single crystal or polycrystalline ceramics) under the same irradiation conditions. Our results show that Raman spectroscopy is a versatile non‐contact technique for studying on‐line crystalline phase changes or amorphization of irradiated iono‐covalent solids. A detailed analysis of Raman spectra is provided for the three materials (SiC, ZrO₂ and B₄C) investigated in this study, revealing quite different behaviors upon irradiation. Basically, Raman spectroscopy gives insight on these evolutions at the level of bonds given by specific phonon modes, in good agreement with Rutherford backscattering channeling (RBS/C), X‐ray diffraction (XRD) or transmission electron microscopy (TEM) data, which provide information at a long‐range scale. Copyright © 2015 John Wiley & Sons, Ltd.