Seminar of Physics of Wrocław University of Technology
PWr, bud. A1, sala 322
Tip-enhanced signals of 2D materials: elastically and inelastically scattered light
dr Iris Niehues
Institute of Physics, University of Münster
Tip-enhanced (TE) near-field spectroscopy techniques are scanning probe methods based on an atomic force microscope (AFM). The metallic AFM tip is illuminated with either visible (VIS), infrared (IR) or terahertz (THz) light and converts it into a strongly concentrated near field at the tip apex (nanofocus). This induces scattering that contains optical information on the sample area reached by the nanofocus, with a spatial resolution determined by the apex radius (usually around 20 nm). On the one hand, by recording the elastically scattered light alongside topography, nanoscaled optical amplitude and phase images can be obtained (s-SNOM), which encode information about the local dielectric function of the sample [1]. On the other hand it is possible to detect inelastically scattered light, such as photoluminescence (PL) and Raman leading to TEPL and TERS signals with nanoscale resolution.
In the first part of my talk I will show how we can use elastically scattered light to measure variations of the charge carrier density on the nanoscale. Intercalation of van der Waals layered semiconductors with molecules can drastically change their electric, optical, and magnetic properties. Recently, we found that MoS2 bulk crystals become superconducting when intercalated with Tetraethylammonium (TEA) molecules [2]. To get a deeper understanding of the molecule distribution in the material we performed IR and THz s-SNOM measurements to map the local carrier density induced by the TEA intercalation. Both the Drude-like spectral response and the spatially inhomogeneous signal distributions confirmed spatial variations of the carrier concentration and therefore of the conductivity.
In the second part of my talk I will focus on TEPL measurements of color centers in hexagonal boron nitride (hBN) which have emerged as important quantum light sources due to their stable and bright single-photon emission at room temperature. We employ the near-field optical microscope in tapping mode to characterize the impact of the metallic AFM tip on the detected PL signals of quantum emitters in metalorganic vapor phase epitaxy grown hBN [3]. On the one hand, we demonstrate direct near-field optical excitation and emission through interaction with the nanofocus of the tip resulting in a sub-diffraction limited TEPL hotspot. On the other hand, we observe a more pronounced ‘arc’ pattern around the hotspot. We explain this feature by constructive interference between direct beams to/from the color center and those scattered from the AFM tip (indirect beams) leading to a significant increase of the recorded PL intensity. We apply this tip-assisted PL (TAPL) method to map the in-plane dipole orientations of the hBN color centers on the nanoscale.
References
[1] N. Ocelic et al., Appl. Phys. Lett. 89, 101124 (2006)[2] J. M. Pereira et al., Adv. Funct. Mater. 32, 208761 (2022)
[3] I. Niehues et al., Nanophotonics 14, 335-342 (2025).
