Resonant Raman Scattering of 4‐Nitrothiophenol

Thiophenol‐based molecules are commonly used reporter molecules for various experiments, especially within the scope of surface‐ and tip‐enhanced Raman spectroscopy. Due to their molecular structure, they bind covalently to noble metals and have a huge Raman scattering cross section. Herein, the widely uncharted optical properties of the frequently used probe molecule 4‐nitrothiophenol (p‐NTP or 4‐NTP) are analyzed by resonant Raman spectroscopy. Based on the three different types of samples, it is demonstrated that the molecule exhibits two intrinsic resonances at specific wavelengths. For a wide range of experiments, this is an important information since intrinsic resonances may give rise to an enhancement of the Raman intensity at these specific excitation wavelengths. The Raman cross section of p‐NTP in resonance at 1.9 eV (650 nm) to be 6 × 10−26 cm2 per molecule is also measured

Carbon nanotubes for the optical far-field readout of processes that are mediated by plasmonic near-fields

As science progresses at the nanoscopic level, it becomes more and more important to comprehend the interactions taking place at the nanoscale, where optical near-fields play a key role. Their phenomenology differs significantly from the propagative light we experience at the macroscopic level. This is particularly important in applications such as surface-enhanced spectroscopies for single-molecule detection, where often the optimization of the plasmonic structures and surfaces relies on far-field characterizations. The processes dominating in the far-field picture, though, are not the same dominating in the near-field. To highlight this, we resort to very simple metallic systems: isolated gold nanorods in solution. We show how single-walled nanotubes can be exploited to read out processes occurring at the near-field level around metallic nanoparticles and make the information accessible in the far-field region. This is implemented by monitoring the spectral profile of the enhancement of the photoluminescence and Raman signal of the nanotubes for several excitation wavelengths. Through this excitation-resolved study, we show that the far-field optical readout detects the transversal and longitudinal dipolar plasmonic oscillations of gold nanorods, whereas the near-field readout through the nanotubes reveals other mechanisms to dominate. The spectral position of the maximum enhancement of the optical near-field mediated signals are located elsewhere than the far-field bands. This dichotomy between near-field and far-field response should be taken into account when optimizing plasmonic nanostructures for applications such as surface-enhanced spectroscopies

Carbon nanotubes for the optical far-field readout of processes that are mediated by plasmonic near-fields

As science progresses at the nanoscopic level, it becomes more and more important to comprehend the interactions taking place at the nanoscale, where optical near-fields play a key role. Their phenomenology differs significantly from the propagative light we experience at the macroscopic level. This is particularly important in applications such as surface-enhanced spectroscopies for single-molecule detection, where often the optimization of the plasmonic structures and surfaces relies on far-field characterizations. The processes dominating in the far-field picture, though, are not the same dominating in the near-field. To highlight this, we resort to very simple metallic systems: isolated gold nanorods in solution. We show how single-walled nanotubes can be exploited to read out processes occurring at the near-field level around metallic nanoparticles and make the information accessible in the far-field region. This is implemented by monitoring the spectral profile of the enhancement of the photoluminescence and Raman signal of the nanotubes for several excitation wavelengths. Through this excitation-resolved study, we show that the far-field optical readout detects the transversal and longitudinal dipolar plasmonic oscillations of gold nanorods, whereas the near-field readout through the nanotubes reveals other mechanisms to dominate. The spectral position of the maximum enhancement of the optical near-field mediated signals are located elsewhere than the far-field bands. This dichotomy between near-field and far-field response should be taken into account when optimizing plasmonic nanostructures for applications such as surface-enhanced spectroscopies

Asymmetry of resonance Raman profiles in semiconducting single-walled carbon nanotubes at the first excitonic transition

Carbon nanotubes are one-dimensional nanoscale systems with strongly pronounced chirality-dependent optical properties with multiple excitonic transitions. We investigate the high-energy G mode of semiconducting single-walled nanotubes of different chiralities at first excitonic transition by applying resonant Raman spectroscopy. The G mode intensity dependence on excitation energy yielded asymmetric resonance Raman profiles similar to ones we reported for the second excitonic transition. We find the scattering efficiency to be strongest at the incoming Raman resonance. Still, the degree of asymmetry is different for the first and second transitions and the first transition profiles provide a narrower line shape due to longer exciton lifetimes. The overall scattering efficiency is up to a factor of 25 times more intense at first excitonic transition, compared to the second transition. The fifth-order perturbation theory, with implemented phonon scattering pathways between excitonic states, excellently reproduced experimental data

Probing the local dielectric function of WS2 on an Au substrate by near field optical microscopy operating in the visible spectral range

The optoelectronic properties of nanoscale systems such as carbon nanotubes (CNTs), graphene nanoribbons and transition metal dichalcogenides (TMDCs) are determined by their dielectric function. This complex, frequency dependent function is affected by excitonic resonances, charge transfer effects, doping, sample stress and strain, and surface roughness. Knowledge of the dielectric function grants access to a material’s transmissive and absorptive characteristics. Here we use the dual scanning near field optical microscope (dual s-SNOM) for imaging local dielectric variations and extracting dielectric function values using a pre-established mathematical inversion method. To demonstrate our approach, we studied a monolayer of WS2 on bulk Au and identified two areas with differing levels of charge transfer. The experiments highlight a further advantage of the technique: the dielectric function of contaminated samples can be measured, as dirty areas can be easily identified and excluded for the calculation, being important especially for exfoliated 2D materials (Rodriguez et al., 2021). Our measurements are corroborated by atomic force microscopy (AFM), Kelvin force probe microscopy (KPFM), photoluminescence (PL) intensity mapping, and tip enhanced photoluminescence (TEPL). We extracted local dielectric variations from s-SNOM images and confirmed the reliability of the obtained values with spectroscopic imaging ellipsometry (SIE) measurements

Moving beyond the electromagnetic enhancement theory

The electromagnetic enhancement theory describes surface-enhanced Raman scattering (SERS) as a Raman effect that takes place in the near-field of a plasmonic nanostructure. The theory has been very successful in explaining the fundamental properties of SERS, modelling the performance of different metals as enhancing materials and optimizing SERS hotspots for strongest possible enhancement. Over the last decade, a number of carefully designed experimental studies have examined predictions of the electromagnetic theory like the size and shape of SERS hotspots, the absolute magnitude of the enhancement and the width of the SERS resonance. Although the overall picture was quite satisfactory, the theory failed to predict key aspects of SERS, for example, the absolute magnitude of the plasmonic enhancement. We scrutinize these experiments and review them focusing on the results that require going beyond the electromagnetic enhancement theory. We argue that the results of these experiments create the need to develop the theory of SERS further, especially the exact role of plasmonic enhancement in inelastic light scattering

Quantum Nature of Plasmon-Enhanced Raman Scattering

We report plasmon-enhanced Raman scattering in graphene coupled to a single plasmonic hotspot measured as a function of laser energy. The enhancement profiles of the G peak show strong enhancement (up to $10^5$) and narrow resonances (30 meV) that are induced by the localized surface plasmon of a gold nanodimer. We observe the evolution of defect-mode scattering in a defect-free graphene lattice in resonance with the plasmon. We propose a quantum theory of plasmon-enhanced Raman scattering, where the plasmon forms an integral part of the excitation process. Quantum interferences between scattering channels explain the experimentally observed resonance profiles, in particular, the marked difference in enhancement factors for incoming and outgoing resonance and the appearance of the defect-type modes.Comment: Keywords: plasmon-enhanced Raman scattering, SERS, graphene, quantum interferences, microscopic theory of Raman scattering. Content: 22 pages including 5 figures + 11 pages supporting informatio

a resonant Raman study

We report resonant Raman scattering (RRS) by the TO, LO, and 2 LO modes of single wurtzite and zinc-blende GaAs nanowires. The optical band gap of wurtzite GaAs is 1.460eV ± 3meV at room temperature, and 35 ± 3meV larger than the GaAs zinc-blende band gap. Raman measurements using incoming light polarized parallel and perpendicular to the wire c axis allowed us to investigate the splitting of heavy Γ9 and light-hole Γ7 band at the Γ point of 65 ± 6meV

Photoluminescence Modulation of Graphene/MoS2 Heterostructures Separated by Laser-Induced Functionalization

Tuning the optoelectronic properties of monolayer MoS2 (1L-MoS2) is highly desired for optoelectronic applications. Gaining profound insights into the fundamental mechanisms that govern optoelectronic properties is of utmost significance. Here, we demonstrate that the photoluminescence (PL) of 1L-MoS2 can be modulated by photochemically functionalized graphene (F-G), which is covalently modified by oligophenyl groups. More importantly, the layer stacking sequence of F-G and 1L-MoS2 brings different interface structures, resulting in a significant difference in the PL enhancement. MoS2 supported by F-G (F-G/MoS2) has a 5-fold PL enhancement, while it only shows a 1.8-fold PL enhancement if stacked underneath F-G (MoS2/F-G). Accordingly, the results indicate that the oligophenyl groups in F-G/MoS2 not only have a p-doping effect on MoS2 but also largely prevent electron donation from the graphene basal plane with an enlarged interlayer distance of 8 nm. Consequently, the PL enhancement is lost with the thermal defunctionalization of F-G. Thus, we conclude that the functional groups can be considered as separate molecular components with the vertical arrangement in the functionalized heterostructure system. The photoactive graphene acts as a template for perpendicular molecular alignment in the heterointerface construction. The F-G/MoS2 heterostructures bring new perspectives to the design and investigation of optoelectronic devices