Establishing the excitation field in tip-enhanced Raman spectroscopy to study nanostructures within two-dimensional systems

Abstract

The optical field generated by a nanoplasmonic probe is revealed in tip-enhanced Raman spectroscopy – TERS – experiments. The TERS intensity profile of nano-objects smaller than the probe’s apex has a donut-like shape which resembles the magnitude of the field generated by a point-dipole source, being well described by the Dyadic Green’s function. Having prior knowledge on the excitation field generated by the TERS probe, we measured the width of shear solitons caused by lattice reconstruction in low-angle twisted bilayer graphene, a prominent platform for twistronics, and the extend of defect-induced light emission from graphene edges.

Spatially Coherent Tip-Enhanced Raman Spectroscopy Measurements of Electron–Phonon Interaction in a Graphene Device

“…Given the laser energy of 1.96 eV, these obtained values are in agreement with the Δ E = 40 meV predicted in ref . However, assigning L c as due to the electron coherence length is also troublesome considering that the observed coherent TERS enhancements depend on phonon symmetry, and considering that other experimental evidence ,− and theoretical considerations , describe the electron coherence length in the Raman process to be in the 3–4 nm range, which is 1 order of magnitude lower than what has been measured (see further discussion on the Supporting Information).…”

“…In the electron interpretation, our results imply that the electronic energy uncertainty Δ E is not constant, but it rather varies with doping, probably due to electron–electron interaction, as suggested previously based on 2D-band analysis . The values obtained from Δ E = v F ℏ/ L c are still debatable, being consistent with some works in the literature and inconsistent with others. ,− …”

Nano-Raman spectroscopy of 2D materials

“…Remarkably, at spatial resolutions of ten(s) of nm, the TERS field distribution for such 10-20 nm graphene flake in xy is effectively captured by a straightforward model that considers a dipole positioned at the center of the circle circumscribed in the tip apex (refer to figure 2(c)). The asymmetry observed in the experimental image (figure 2(b)) can be attributed to the irregular shape of the flake [35]. However, when delving into resolutions smaller than 1 nm, TERS demands a more intricate modeling approach, as elucidated in the subsequent discussion.…”

“…While the intensity of a spectral feature may appear localized atop an atom, it does not necessarily imply motion of that atom. Rather, what is observed is the deformation of electron density surrounding In these plots, the tip apex measures 15 nm, z is set to 5 nm below z0, and the excitation laser wavelength λ is 632.8 nm [35]. Reproduced from [35].…”

Review on infrared nanospectroscopy of natural 2D phyllosilicates

“…One of the most efficient ways to access a material structure and chemistry is to probe its natural vibrational activity [10,[19][20][21][22]. Thus, a systematical way to study the vibrational assignments of minerals in their FL form, which have a low response to excitation, is to use tip-enhanced techniques [23][24][25][26]. Since the small amount of material results in a low signal response in techniques with resolution at the microscale such as standard Fourier-transform infrared (FTIR) and Raman spectroscopy, scattering-type scanning near-field optical microscopy (s-SNOM) is a powerful alternative to overcome this diffraction limit [26][27][28] and, therefore, a tool able to characterize 2D materials composition and optical response at the nanometer scale [29].…”