Tip-Enhanced Raman Scattering from Nanopatterned Graphene and Graphene Oxide

Nanoscale insights into graphene oxide reduction by tip-enhanced Raman spectroscopy

Graphene oxide (GO) has attracted substantial interest for its tunable properties and as a possible intermediate for the bulk manufacture of graphene. GO and its reduced derivatives display electronic and optical properties that depend strongly on their chemical structure, and with proper functionalization, GO can have a desirable bandgap for semiconductor applications. However, its chemical activity leads to a series of unclear chemical changes under ambient conditions, resulting in changes in color and solubility upon exposure to light. In this paper, we study the properties of fresh and spontaneously reduced GO under ambient conditions using tip-enhanced Raman spectroscopy (TERS) to map its nanometer scale chemical and structural heterogeneity. We observe different types of defect sites on reduced GO (rGO) by spatially mapping the D to G band peak ratio and D and G band spectral positions. The higher spatial resolution and out-of-plane polarization compared to conventional micro-Raman spectroscopy enables us to resolve unusual features, including D-band shifting on rGO. Based on statistical analysis of the spatial variations in modes and theoretical calculations for different functional groups, we conclude the reduction mechanism of GO is a self-photocatalytic reduction with the participation of water and visible light, in which the rate determining step is electron transport through the metal substrate and ion diffusion on the GO surface. These results demonstrate that TERS can reveal structural and chemical details elucidating reduction mechanisms, through the examination of samples at different time points.

Advances in Surface-Enhanced and Tip-Enhanced Raman Spectroscopy, Mapping and Methods Combined with Raman Spectroscopy for the Characterization of Perspective Carbon Nanomaterials

Surface-enhanced Raman spectroscopy (SERS) is based on the effect of the plasmonic enhancement of intensity of the Raman scattering of molecules in cases when they are adsorbed on a substrate [...].

A novel design for the combination of electrochemical atomic force microscopy and Raman spectroscopy in reflection mode for in situ study of battery materials

The emergence of functional materials, especially energy materials made up of various structures with different properties, requires the development of complementary or integrated characterization technologies. The combination of atomic force microscopy and Raman spectroscopy (AFM-Raman) offers a powerful technique for the in situ characterization of physical properties (AFM) and chemical composition (Raman) of materials simultaneously. To further extend the potential application in the battery's field, we here present an electrochemical AFM-Raman (EC-AFM-Raman) in the reflection mode, developed by designing a novel structure including water-immersion objective lens-based optics for high-sensitivity Raman excitation/collection, optical level detection for AFM imaging in the solution, and a dual-cell for electrochemical reaction. EC-AFM imaging and Raman measurement can be realized simultaneously. Dynamic morphologic evolution and phase transition of the LiMn₂O₄ particles during cyclic voltammetry measurement were successfully observed. This technique will provide the possibility of probing physicochemical phenomena of the battery materials and other surface/interface processes such as the formation of the solid electrolyte interphase layer.

Coupling nanobubbles in 2D lateral heterostructures

Two-dimensional transition metal dichalcogenides provide flexible platforms for nanophotonic engineering due to their exceptional mechanical and optoelectronic properties. For example, continuous band gap tunability has been achieved in 2D TMDs by elastic strain engineering. Localized elastic deformations in nanobubbles behave as "artificial atoms" with a spatially varying band gap resulting in funnelling of excitons and photocarriers. Here we present a new method of nanobubble fabrication in monolayer 2D lateral heterostructures using high temperature superacid treatment. We fabricated MoS₂ and WS₂ nanobubbles and performed near-field imaging with nanoscale resolution using tip-enhanced photoluminescence (TEPL) spectroscopy. TEPL nanoimaging revealed the coupling between MoS₂ and WS₂ nanobubbles with a large synergistic PL enhancement due to the plasmonic tip, hot electrons, and exciton funnelling. We investigated the contributions of different enhancement mechanisms, and developed a quantum plasmonic model, in good agreement with the experiments. Our work opens new avenues in exploration of novel nanophotonic coupling schemes.

Tip-Enhanced Raman Scattering Imaging of Single- to Few-Layer Ti3C2Tx MXene

MXenes are among the most widely researched materials due to a unique combination of high electronic conductivity and hydrophilic surface, confined in a 2D structure. Therefore, comprehensive characterization of individual MXene flakes is of great importance. Here we report on nanoscale Raman imaging of single-layer and few-layer flakes of Ti₃C₂T_x MXene deposited on a gold substrate using tip-enhanced Raman scattering (TERS). TERS spectra of MXene monolayers are dominated by an intense peak at around 201 cm^-1 and two well-defined peaks at around 126 and 725 cm^-1. Absolute intensities of these peaks decrease with increasing number of layers, though the relative intensity of the 126 and 725 cm^-1 bands as compared to the 201 cm^-1 band increases. The peak positions of the main MXene bands do not significantly change in flakes of different number of layers, suggesting weak coupling between the MXene layers. In addition, we observed stiffening of the 201 cm^-1 vibration over the wrinkles in MXene flakes. Using TERS for nanoscale spectroscopic characterization of Ti₃C₂T_x allows fast Raman mapping with deep subdiffraction resolution at the laser power density on the sample about an order of magnitude lower as compared to confocal Raman measurements. Finally, we demonstrate very high environmental stability of stoichiometric single-layer MXenes and show that the intensity of TERS response from the single- and few-layer flakes of Ti₃C₂T_x can be used to track early stages of degradation, well before significant morphological changes appear.

Comparing Commercial Metal-Coated AFM Tips and Home-Made Bulk Gold Tips for Tip-Enhanced Raman Spectroscopy of Polymer Functionalized Multiwalled Carbon Nanotubes

Tip-enhanced Raman spectroscopy (TERS) combines the high specificity and sensitivity of plasmon-enhanced Raman spectroscopy with the high spatial resolution of scanning probe microscopy. TERS has gained a lot of attention from many nanoscience fields, since this technique can provide chemical and structural information of surfaces and interfaces with nanometric spatial resolution. Multiwalled carbon nanotubes (MWCNTs) are very versatile nanostructures that can be dispersed in organic solvents or polymeric matrices, giving rise to new nanocomposite materials, showing improved mechanical, electrical and thermal properties. Moreover, MWCNTs can be easily functionalized with polymers in order to be employed as specific chemical sensors. In this context, TERS is strategic, since it can provide useful information on the cooperation of the two components at the nanoscale for the optimization of the macroscopic properties of the hybrid material. Nevertheless, efficient TERS characterization relies on the geometrical features and material composition of the plasmonic tip used. In this work, after comparing the TERS performance of commercial Ag coated nanotips and home-made bulk Au tips on bare MWCNTs, we show how TERS can be exploited for characterizing MWCNTs mixed with conjugated fluorene copolymers, thus contributing to the understanding of the polymer/CNT interaction process at the local scale.

Evolution of Graphene Patterning: From Dimension Regulation to Molecular Engineering

The realization that nanostructured graphene featuring nanoscale width can confine electrons to open its bandgap has aroused scientists' attention to the regulation of graphene structures, where the concept of graphene patterns emerged. Exploring various effective methods for creating graphene patterns has led to the birth of a new field termed graphene patterning, which has evolved into the most vigorous and intriguing branch of graphene research during the past decade. The efforts in this field have resulted in the development of numerous strategies to structure graphene, affording a variety of graphene patterns with tailored shapes and sizes. The established patterning approaches combined with graphene chemistry yields a novel chemical patterning route via molecular engineering, which opens up a new era in graphene research. In this review, the currently developed graphene patterning strategies is systematically outlined, with emphasis on the chemical patterning. In addition to introducing the basic concepts and the important progress of traditional methods, which are generally categorized into top-down, bottom-up technologies, an exhaustive review of established protocols for emerging chemical patterning is presented. At the end, an outlook for future development and challenges is proposed.

Nanoindentation-enhanced tip-enhanced Raman spectroscopy

We combine nanoindentation, herein achieved using atomic force microscopy-based pulsed-force lithography, with tip-enhanced Raman spectroscopy (TERS) and imaging. Our approach entails indentation and multimodal characterization of otherwise flat Au substrates, followed by chemical functionalization and TERS spectral imaging of the indented nanostructures. We find that the resulting structures, which vary in shape and size depending on the tip used to produce them, may sustain nano-confined and significantly enhanced local fields. We take advantage of the latter and illustrate TERS-based ultrasensitive detection/chemical fingerprinting as well as chemical reaction imaging-all using a single platform for nano-lithography, topographic imaging, hyperspectral dark field optical microscopy, and TERS.

Interactions Between Epitaxial Graphene Grown on the Si- and C-Faces of 4H-SiC Investigated Using Raman Imaging and Tip-Enhanced Raman Scattering

Interactions between epitaxial graphene grown on Si- and C-faces were investigated using Raman imaging and tip-enhanced Raman scattering (TERS). In the TERS spectrum, which has a spatial resolution exceeding the diffraction limit, a D band was observed not from graphene surface, but from the edges of the epitaxial graphene ribbons without a buffer layer, which interacts with SiC on the Si-face. In contrast, for a graphene micro-island on the C-face, the D band disappeared even on the edges where the C atoms were arranged in armchair configurations. The disappearance of the edge chirality via combination between the C atoms and SiC on the C-face is responsible for this phenomenon. The TERS signals from the C-face were weaker than those from the Si-face without the buffer layer. On the Si-face with a buffer layer, the graphene TERS signal was hardly observed. TERS enhancement was suppressed by interactions on the edges or by the buffer layer between the SiC and graphene on the C- or Si-face, respectively.

Vibrational spectra and chemical imaging of cyclo[18]carbon by tip enhanced Raman spectroscopy

Vibrational modes and tip enhanced Raman spectroscopy (TERS) of a new carbon allotrope, cyclo[18]carbon (C18), were studied by density functional theory. A silver cluster tip was used to probe the interaction with C18, which is dependent on the distance and the atomically resolved positions. The TERS images show the position of the C[triple bond, length as m-dash]C bonds, as observed in a recent experimental report.

Single-molecule resonance Raman effect in a plasmonic nanocavity

Tip-enhanced Raman spectroscopy (TERS) is a versatile tool for chemical analysis at the nanoscale. In earlier TERS experiments, Raman modes with components parallel to the tip were studied based on the strong electric field enhancement along the tip. Perpendicular modes were usually neglected. Here, we investigate an isolated copper naphthalocyanine molecule adsorbed on a triple-layer NaCl on Ag(111) using scanning tunnelling microscope TERS imaging. For flat-lying molecules on NaCl, the Raman images present different patterns depending on the symmetry of the vibrational mode. Our results reveal that components of the electric field perpendicular to the tip should be considered aside from the parallel components. Moreover, under resonance excitation conditions, the perpendicular components can play a substantial role in the enhancement. This single-molecule study in a well-defined environment provides insights into the Raman process at the plasmonic nanocavity, which may be useful in the nanoscale metrology of various molecular systems.

Graphene Oxide: A Smart (Starting) Material for Natural Methylxanthines Adsorption and Detection

Natural methylxanthines, caffeine, theophylline and theobromine, are widespread biologically active alkaloids in human nutrition, found mainly in beverages (coffee, tea, cocoa, energy drinks, etc.). Their detection is thus of extreme importance, and many studies are devoted to this topic. During the last decade, graphene oxide (GO) and reduced graphene oxide (RGO) gained popularity as constituents of sensors (chemical, electrochemical and biosensors) for methylxanthines. The main advantages of GO and RGO with respect to graphene are the easiness and cheapness of synthesis, the notable higher solubility in polar solvents (water, among others), and the higher reactivity towards these targets (mainly due to - interactions); one of the main disadvantages is the lower electrical conductivity, especially when using them in electrochemical sensors. Nonetheless, their use in sensors is becoming more and more common, with the obtainment of very good results in terms of selectivity and sensitivity (up to 5.4 × 10-10 mol L-1 and 1.8 × 10-9 mol L-1 for caffeine and theophylline, respectively). Moreover, the ability of GO to protect DNA and RNA from enzymatic digestion renders it one of the best candidates for biosensors based on these nucleic acids. This is an up-to-date review of the use of GO and RGO in sensors.

Numerical investigations on the electromagnetic enhancement effect to tip-enhanced Raman scattering and fluorescence processes

In the present work, we theoretically study the electromagnetic (EM) enhancement of the Raman and fluorescence signals for a molecule placed in a nanocavity formed by a metallic tip and substrate that mimics a tip-enhanced Raman scattering (TERS) setup using three-dimensional finite element method calculations. The influence of tip size and tip-molecule distance on the EM enhancements of the incident field as well as the radiative and non-radiative decay rates of the molecule are systematically investigated. Simulation results show that the maximum EM enhancement to the incident light as provided by the localized surface plasmon resonance in the nanocavity can reach  ∼285 for the configuration considered in the present work. Meanwhile, it was found that, at the classical limit, decreasing the apex radius or the tip-molecule distance can both reduce the spatial distribution (as characterized by the full width at half maximum) of the Raman enhancement in a linear fashion. Moreover, simulation results show that the nonlocal dielectric response of the tip and the substrate plays a key role to the fluorescence quantum yield of the molecule. However, it was found that the strong EM excitation enhancement is the dominating factor for the tip enhanced fluorescence (TEF) effect and stronger fluorescence enhancement has been found when increasing the apex radius or reducing the tip-molecule distance with an incident wavelength of 532 nm. The best TERS and TEF enhancements were found to be  ∼[Formula: see text] and  ∼[Formula: see text], respectively, with the tip-molecule distance around 1 nm.

Imaging the Optical Fields of Functionalized Silver Nanowires through Molecular TERS

We image 4-mercaptobenzonitrile-functionalized silver nanowires (∼20 nm diameter) through tip-enhanced Raman scattering (TERS). The enhanced local optical field-molecular interactions that govern the recorded hyperspectral TERS images are dissected through hybrid finite-difference time-domain density functional theory simulations. Our forward simulations illustrate that the recorded spatiospectral profiles of the chemically functionalized nanowires may be reproduced by accounting for the interaction between orientationally averaged molecular polarizability derivative tensors and enhanced incident/scattered local fields polarized along the tip axis. In effect, we directly map the enhanced optical fields of the nanowire in real space through TERS. The simultaneously recorded atomic force microscopy (AFM) images allow a direct comparison between our attainable spatial resolution in topographic (13 nm) and TERS (5 nm) imaging measurements performed under ambient conditions. Overall, our described protocol enables local electric field imaging with few nm precision through molecular TERS, and it is therefore generally applicable to a variety of plasmonic nanostructures.