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

Coherence length () of the Raman scattering process in graphene as a function of Fermi energy is obtained with spatially coherent tip-enhanced Raman spectroscopy. decreases when the Fermi energy is moved into the neutrality point, consistent with the concept of the Kohn anomaly within a ballistic transport regime. Since the Raman scattering involves electrons and phonons, the observed results can be rationalized either as due to unusually large variation of the longitudinal optical phonon group velocity , reaching twice the value for the longitudinal acoustic phonon, or due to changes in the electron energy uncertainty, both properties being important for optical and transport phenomena that might not be observable by any other technique.

Studying 2D materials with advanced Raman spectroscopy: CARS, SRS and TERS.

Raman spectroscopy has been established as a valuable tool to study and characterize two-dimensional (2D) systems, but it exhibits two drawbacks: a relatively weak signal response and a limited spatial resolution. Recently, advanced Raman spectroscopy techniques, such as coherent anti-Stokes spectroscopy (CARS), stimulated Raman scattering (SRS) and tip-enhanced Raman spectroscopy (TERS), have been shown to overcome these two limitations. In this article, we review how useful physical information can be retrieved from different 2D materials using these three advanced Raman spectroscopy and imaging techniques, discussing results on graphene, hexagonal boron-nitride, and transition metal di- and mono-chalcogenides, thus providing perspectives for future work in this early-stage field of research, including similar studies on unexplored 2D systems and open questions.

Nanofabrication of plasmon-tunable nanoantennas for tip-enhanced Raman spectroscopy.

Plasmon-tunable tip pyramids (PTTPs) are reproducible and efficient nanoantennas for tip-enhanced Raman spectroscopy (TERS). Their fabrication method is based on template stripping of a segmented gold pyramid with a size-adjustable nanopyramid end, which is capable of supporting monopole localized surface plasmon resonance (LSPR) modes leading to high spectral enhancement when its resonance energy is matched with the excitation laser energy. Here, we describe in detail the PTTP fabrication method and report a statistical analysis based on 530 PTTPs' and 185 ordinary gold micropyramids' templates.

Probing Spatial Phonon Correlation Length in Post-Transition Metal Monochalcogenide GaS Using Tip-Enhanced Raman Spectroscopy.

The knowledge of the phonon coherence length is of great importance for two-dimensional-based materials since phonons can limit the lifetime of charge carriers and heat dissipation. Here we use tip-enhanced Raman spectroscopy (TERS) to measure the spatial correlation length of the A and A phonons of monolayer and few-layer gallium sulfide (GaS). The differences in values are responsible for different enhancements of the A modes, with A always enhancing more than the A, independently of the number of GaS layers.

Raman evidence for pressure-induced formation of diamondene.

Despite the advanced stage of diamond thin-film technology, with applications ranging from superconductivity to biosensing, the realization of a stable and atomically thick two-dimensional diamond material, named here as diamondene, is still forthcoming. Adding to the outstanding properties of its bulk and thin-film counterparts, diamondene is predicted to be a ferromagnetic semiconductor with spin polarized bands. Here, we provide spectroscopic evidence for the formation of diamondene by performing Raman spectroscopy of double-layer graphene under high pressure.

Passive near-field imaging with pseudo-thermal sources.

We demonstrate experimentally that spurious effects caused by interference can be eliminated in passive near-field imaging by implementing a simple random illumination. We show that typical imaging artifacts are effectively eliminated when the radiation emitted by a pseudo-thermal source illuminates the sample and the scattered field is collected by an aperture probe over essentially all angles of incidence. This novel pseudo-thermal source can be easily implemented and significantly enhances the performance of passive near-field imaging.

Characterization of Few-Layer 1T' MoTe by Polarization-Resolved Second Harmonic Generation and Raman Scattering.

We study the crystal symmetry of few-layer 1T' MoTe using the polarization dependence of the second harmonic generation (SHG) and Raman scattering. Bulk 1T' MoTe is known to be inversion symmetric; however, we find that the inversion symmetry is broken for finite crystals with even numbers of layers, resulting in strong SHG comparable to other transition-metal dichalcogenides. Group theory analysis of the polarization dependence of the Raman signals allows for the definitive assignment of all the Raman modes in 1T' MoTe and clears up a discrepancy in the literature.

Giant and Tunable Anisotropy of Nanoscale Friction in Graphene.

The nanoscale friction between an atomic force microscopy tip and graphene is investigated using friction force microscopy (FFM). During the tip movement, friction forces are observed to increase and then saturate in a highly anisotropic manner. As a result, the friction forces in graphene are highly dependent on the scanning direction: under some conditions, the energy dissipated along the armchair direction can be 80% higher than along the zigzag direction.

Observing the Angular Distribution of Raman Scattered Fields.

In conventional optical spectroscopy, lenses are used to focus light on the sample and to collect light scattered from the sample. Focusing increases the signal intensity, but it amounts to angular (k-space) averaging and leads to information loss. In this issue of ACS Nano, Budde and collaborators record radiation patterns of Raman scattering from a single layer of graphene, revealing the angular distribution of the scattered field.

Near-field Raman spectroscopy of nanocarbon materials.

Nanocarbon materials, including sp(2) hybridized two-dimensional graphene and one-dimensional carbon nanotubes, and sp(1) hybridized one-dimensional carbyne, are being considered for the next generation of integrated optoelectronic devices. The strong electron-phonon coupling present in these nanocarbon materials makes Raman spectroscopy an ideal tool to study and characterize the material and device properties. Near-field Raman spectroscopy combines non-destructive chemical, electrical, and structural specificity with nanoscale spatial resolution, making it an ideal tool for studying nanocarbon systems.

Strain Discontinuity, Avalanche, and Memory in Carbon Nanotube Serpentine Systems.

This work addresses the problem of how a nano-object adheres to a supporting media. The case of study are the serpentine-like structures of single-wall carbon nanotubes (SWNTs) grown on vicinal crystalline quartz. We develop in situ nanomanipulation and confocal Raman spectroscopy in such systems, and to explain the results, we propose a dynamical equation in which static friction is treated phenomenologically and implemented as cutoff for velocities, via Heaviside step function and an adhesion force tensor.

Enhanced Mechanical Stability of Gold Nanotips through Carbon Nanocone Encapsulation.

Gold is a noble metal that, in comparison with silver and copper, has the advantage of corrosion resistance. Despite its high conductivity, chemical stability and biocompatibility, gold exhibits high plasticity, which limits its applications in some nanodevices. Here, we report an experimental and theoretical study on how to attain enhanced mechanical stability of gold nanotips.

Tuning Localized Surface Plasmon Resonance in Scanning Near-Field Optical Microscopy Probes.

A reproducible route for tuning localized surface plasmon resonance in scattering type near-field optical microscopy probes is presented. The method is based on the production of a focused-ion-beam milled single groove near the apex of electrochemically etched gold tips. Electron energy-loss spectroscopy and scanning transmission electron microscopy are employed to obtain highly spatially and spectroscopically resolved maps of the milled probes, revealing localized surface plasmon resonance at visible and near-infrared wavelengths.

Tip-enhanced Raman mapping of local strain in graphene.

We demonstrate local strain measurements in graphene by using tip-enhanced Raman spectroscopy (TERS). We find that a single 5 nm particle can induce a radial strain over a lateral distance of ∼170 nm. By treating the particle as a point force on a circular membrane, we find that the strain in the radial direction (r) is ∝ r−(2 3),in agreement with force-displacement measurements conducted on suspended graphene flakes.

Raman characterization of defects and dopants in graphene.

In this article we review Raman studies of defects and dopants in graphene as well as the importance of both for device applications. First a brief overview of Raman spectroscopy of graphene is presented. In the following section we discuss the Raman characterization of three defect types: point defects, edges, and grain boundaries.

Spatial coherence in near-field Raman scattering.

Inelastic light scattering in crystals has historically been treated as a spatially incoherent process, giving rise to incoherent optical radiation. Here we demonstrate that Raman scattering can be spatially coherent, in which case it depends on the dimensionality and symmetry of the scatterer. Using near-field spectroscopy, we measure a correlation length of ∼30  nm for the optical phonons in graphene, the results varying with vibrational symmetries and spatial confinement of the phonons.

Perspectives on Raman spectroscopy of graphene-based systems: from the perfect two-dimensional surface to charcoal.

Raman spectroscopy has been already established as a powerful tool for characterizing the different types of carbon nanostructures, ranging from the highly ordered two-dimensional graphene and one-dimensional nanotubes, down to disordered materials, like nanographite and charcoal. Here we focus on the recent advances of Raman spectroscopy within carbon nanoscience. We discuss in situ nano-manipulation and Raman imaging for addressing controlled perturbations; multi-technique work for the development of nanometrology; crossing the diffraction limit with near-field optics for high resolution imaging.

Raman signature of graphene superlattices.

When two identical two-dimensional periodic structures are superposed, a mismatch rotation angle between the structures generates a superlattice. This effect is commonly observed in graphite, where the rotation between graphene layers generates Moiré patterns in scanning tunneling microscopy images. Here, a study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented.

Low temperature raman study of the electron coherence length near graphene edges.

We developed a novel optical defocusing method for studying spatial coherence of photoexcited electrons and holes near edges of graphene. Our method is applied to measure the localization l(D) of the disorder-induced Raman D band (∼1350 cm(-1)) with a resolution of a few nanometers. Raman scattering experiments performed in a helium bath cryostat reveal that as temperature is decreased from 300 to 1.