Abstract
We report on the discovery of asymmetries of Raman scattering along one scanning direction, between two scanning directions, and by structure variation of the sample in space. Asymmetry of Raman shift along the x direction, and the asymmetry of Raman shift and linewidth between the two scanning directions (x and y) are found for a 1210 nm diameter silica particle. The observed asymmetries are confirmed by further 2D Raman scanning of the same particle. To further explore the asymmetry of Raman scattering, glass fibers of three diameters (0.53, 1.00, and 3.20 μm) are scanned along two directions. The asymmetry of Raman shift along each direction, the asymmetry of linewidth along the y direction, and the asymmetry of Raman shift and linewidth between the two scanning directions are discovered. Additionally, 11 nm-thick MoSe2 nanosheets on silicon are used to discover whether an asymmetry of Raman scattering exists at the edge of the nanosheets. One edge of the nanosheet is scanned in four directions and the asymmetry of Raman scattering caused by the step variation is also detected. All the observed Raman scattering asymmetries are explained soundly by the Raman signal diffraction and image shift on the CCD detector arrays of the Raman spectrometer. In practice, to use scanning Raman for surface structure study, great measure has to be taken to consider the structure-induced asymmetries to uncover the real Raman wave number variation by intrinsic material structure. We propose a signal processing method by averaging the scanning points along four directions to eliminate the interference of the edge. This method works well to significantly suppress the asymmetries of Raman properties and uncover the real Raman signal change by structure variation.
Highlights
Raman spectroscopy is a technique specialized in measuring the frequency shift of inelastic scattered light from the sample when the photon from the incident light strikes a molecule and produces a scattered photon [1,2,3,4]
In summary, asymmetries of Raman scattering along one scanning direction, between two scanning directions, and by structure variation of sample in space were discovered
Silica microparticles with a diameter of 1210 nm, glass fibers of different diameters, and MoSe2 were deposited on the silicon substrate to explore the effect of the asymmetry on the experimental results
Summary
Raman spectroscopy is a technique specialized in measuring the frequency shift of inelastic scattered light from the sample when the photon from the incident light strikes a molecule and produces a scattered photon [1,2,3,4]. TERS combines the high spatial resolution of scanning probe microscopy (SPM) with the chemical, structural, and temperature information obtained through Raman scattering [11,12]. The increase of the Raman signal is due to a strong field enhancement at the SPM tip apex related to the singular behavior of the electromagnetic field (akin to the lightning-rod effect) and localized surface plasmon resonance (LSPR) excitation for certain tip materials [13] This technique has been used to study scientific problems in biology [14], photovoltaics [15], catalysis [16], semiconductors [17], carbon nanotubes [18], graphene [19] and single molecule detection [20]. This technique could be very promising for exploring the structure variation in space for different kinds of nanomaterials and for studying the phonon-structure interaction of different materials
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