Ado Jorio

Compared to bulk solids, defects in low-dimensional materials and, specifically, 2D systems are expected to have a stronger effect, detrimental or beneficial, on their properties. Owing to their geometry, defects in 2D materials can easily be formed due to the interaction with the environment or under impacts of energetic particles, such as ions and electrons. At the same time, many concepts of defect production under irradiation in bulk systems are not applicable for 2D materials or require substantial modifications. Various aspects of the physics and chemistry of defects in 2D materials have been addressed, and the results of these investigations are presented in hundreds of research papers and review articles. However, the challenges and open questions that still remain in the field have received relatively little attention. These topics were recently addressed at the symposium “Defect-mediated engineering of nanomaterials for energy and quantum applications” organized by the Beilstein-Institut. Following the discussions at the symposium, here, we present the challenges and open questions in our understanding of the behavior of defective 2D materials, interaction of energetic particles with low-dimensional targets, and defect-mediated engineering of the properties of 2D systems. We further discuss possible solutions to these problems or suggest “work-arounds”, which should accelerate the progress in the field.

Tip‐enhanced Raman spectroscopy (TERS) typically relies on high‐numerical‐aperture (NA) excitation to generate a strong axial field at the tip apex, limiting working distance and constraining sample geometries. Here, we show that a plasmonic bi‐cavity tip, the plasmon‐tunable tip pyramid (PTTP), supports a hybrid antenna‐cavity response when the nanopyramid length and plateau length are co‐tuned. In this configuration, an in‐plane surface‐plasmon‐polariton cavity supported by the plateau hybridizes with the longitudinal antenna mode of the nanopyramid, efficiently funneling energy to the apex under radially polarized, on‐axis excitation. Finite‐element simulations identify as a design‐critical parameter governing a Fabry‐Pérot‐like SPP resonance, with co‐tuning of producing a sequence of geometrical resonances associated with maximal apex . Experiments on monolayer graphene confirm robust near‐field enhancement using a dry objective with . By relaxing the high‐NA requirement, this approach increases working distance and compatibility with constrained environments, advancing practical implementations of nano‐Raman spectroscopy.

Two-dimensional (2D) semiconductors such as monolayer WSe₂ have attracted significant interest for their quantum properties and potential as scalable single-photon emitters. However, conventional microphotoluminescence (μ-PL) techniques are fundamentally limited by optical diffraction, hindering access to critical nanoscale features such as strain gradients and localized quantum confinement. In this study, we utilize tip-enhanced photoluminescence (NanoPL) with a spatial resolution of ≈10 nm to directly image the emission landscape of monolayer WSe₂ on top of nanopillars at room temperature. Our results reveal two distinct localization regimes associated with leading theoretical models for single-photon activation and provide guidelines for deterministic nanoengineering of quantum light sources.

Tip-enhanced Raman spectroscopy (TERS) combines Raman spectroscopy with scanning probe microscopy to overcome the spatial resolution limitation imposed by light diffraction, offering a primary optical technique for the comprehensive study of two-dimensional (2D) materials. In this work, we investigate an anomalous decay profile of the TERS intensity of the graphene 2D band as the tip-sample separation changes, observations enabled by high TERS efficiency and accuracy in tip-approach and tip-retract procedures. The anomalous results can be properly described by the addition of an ad hoc deformation to the effective tip-sample distance, rationalized here as due to the presence of a liquid meniscus formed via capillary forces.

Tip-Enhanced Raman Spectroscopy (TERS) combines Raman spectroscopy with scanning probe microscopy to overcome the spatial resolution limitation imposed by light diffraction, offering a primary optical technique for the comprehensive study of two-dimensional (2D) materials. In this work, we investigate an anomalous decay profile of the TERS intensity of the graphene 2D band as the tip-sample separation changes, observations enabled by high TERS efficiency and accuracy in tip-approach and tip-retract procedures. The anomalous results can be properly described by the addition of an ad hoc deformation to the effective tip-sample distance, rationalized here as due to the presence of a liquid meniscus formed via capillary forces.

Tellurium‐based nanowires are interesting structures for applications in low‐dimensional electronics. This work investigates the vibrational dynamics of trigonal tellurium (t‐Te) nanowires through tip‐enhanced Raman spectroscopy (TERS), achieving spatial resolution in the 50‐nm range. The analysis revealed a dependence of the Raman active phonon mode frequency on nanowire diameter. Besides the observation of different frequencies for isolated nanowires with different diameters, we also observed changes of the mode frequency along the axis of a single nanowire that are consistent with diameter changes measured by in situ atomic force microscopy (AFM). Furthermore, we demonstrate that the mode frequency also changes along a single nanowire species due to local strain likely induced by the presence of amorphous material deposited on top. Our work thus provides detailed insights into the nanoscale vibrational and structural properties of t‐Te nanowires, highlighting the capabilities of TERS to study of Raman shifts along single nanowires.

Some biological systems exhibit nanoscale constructions to produce optical effects. This study utilises Atomic Force Microscopy (AFM) and Tip‐Enhanced Raman Spectroscopy (TERS) to study the complex bionanometric structure of cicada wings. Topographical irregularities of the wings due to ‐chitin nanopillars hinder the probe's approach to the sample, a crucial step in overcoming the light diffraction limit in TERS measurements. To mitigate this issue, graphene was deposited, promoting surface smoothing and ensuring a reliable TERS measurement. Combined analyses of AFM and TERS mapping revealed a significant enhancement of the graphene 2D band, particularly in the regions surrounding the nanometric pillars, while the characteristic ‐chitin Raman peaks are evident on top of the pillars, clarifying details of how light passed through the material.

The process in which Raman scattering produces correlated Stokes and anti‐Stokes radiation is known as Stokes–anti‐Stokes (SaS) scattering. It has been shown recently that this process can generate entangled photon pairs, making it a promising tool for quantum optical technologies, but a proper quantum theoretical description was lacking. In this article, a fully quantum derivation of the electric polarization in a medium with vibrational Raman response, with quantized electromagnetic fields, is developed. Using quantum perturbation theory for Heisenberg operators, the solution for the material electric polarization is found and it is shown that a four‐wave mixing‐like correlated SaS scattering appears in the first order of perturbation and completely characterizes the nonresonant SaS photon pair production. It is also discussed how to construct the third‐order nonlinear optical susceptibility for the SaS scattering from the quantum formalism and show that it coincides with the one derived for classical fields in stimulated Raman.