Angularly Resolved Tip‐Enhanced Raman Spectroscopy

Recent developments in tip-enhanced Raman spectroscopy (TERS) have demonstrated its capabilities in lateral resolution down to Ångström scale and chemical identification of adjacent molecules at the submolecular level. However, most studies are showcased only on the planar molecules lying flat on surfaces, and the resolving ability of TERS in the vertical direction still remains elusive, especially regarding its ability to resolve vertical heteromolecular structures. In this work, we artificially construct a bilayer-molecular system with one molecule at the tip apex and the other on the substrate to quantitatively investigate the vertical resolving ability of the TERS technique. The vertical detection depth is first measured using a pristine Ag tip by tracking the spectral evolution upon tip retraction away from a C60 molecule on the substrate until the TERS signals become practically vanished, which yields a detection depth around 500 pm. Consistently, when the tip is decorated by a C60 molecule at the tip apex, no TERS signals from the CuPc molecule on the substrate can be observed due to the large size of the decorated C60 molecules of ∼700 pm in diameter. However, when the Ag-tip apex is decorated with a single flat-lying CuPc molecule with a thickness of ∼280 pm, the TERS signals of the C60 molecule on the substrate can be detected clearly, demonstrating the capability of TERS to resolve vertical heteromolecular structures. Furthermore, the presence of the molecule at the tip apex is found to influence the local plasmonic field and extend the vertical detection depth up to ∼580 pm due to the hot-spot transfer effect. Our results not only provide a quantitative study on the vertical resolving ability of the TERS technique but also are instructive for potential applications in resolving subsurface chemical constituents and stereochemical structures.

Imaging technique with high-sensitivity and high-spatial-resolution at single molecular level is always a desired aspect to the researchers. In association with scanning probe microscopy and Surface-Enhanced Raman Spectroscopy (SERS), a new technology has shown the light of hope to achieve what was thought as unachievable earlier, named Tip-Enhanced Raman Spectroscopy (TERS). An essential tip is used in TERS to disseminate the information of the morphology of the target molecule by scanning probe technique and concurrently can magnify the Raman signal largely without any special sample preparation. Between this metal tip and surface, a ‘hot-spot’ is formed when the laser beam is applied. TERS has many applications in nanotechnology, biophotonics, and sensing etc. Generally, noble materials are used in the operation of TERS. Recently, 2D materials are showing great potential in parallel with the noble materials in producing sensors, measurement devices, conductive films, printed electronics and many more due to their extraordinary properties like strength, thermal conductivity, high electrical conductivity etc. In this paper, we will compare the enhancement factor of noble and 2D materials at the vicinity of the metallic tip of TERS. Finite-Difference Time-Domain (FDTD) simulation method has been opted to find out the enhancement factor. For this simulation, tip radius was varied from 5 nm to 30 nm with 5 nm interval and distance between tip and substrate was varied from 1 nm to 5 nm with 1 nm interval and incident wavelength was kept constant at visible wavelength (300nm–700nm).

Depending on each nitrogen atom of adenine molecule to which a silver atom of a metallic tip approaches, tip-enhanced near-field Raman spectroscopy may show a potential to achieve atomic site-selective detection sensitivity. Molecular vibrational calculations show that silver atoms and adenine molecule create several isomers generating specific vibrational modes of each isomer that are shifted or not observable in isolated adenine molecule itself. Here, the authors observe the specific vibrational modes and spectral shifts of isomers experimentally and are in good agreement with their calculations.

Raman scattering is a powerful probe of local bonding, strain, temperature, and other properties of materials via their influence on vibrational modes or optical phonons. Tip-enhanced Raman spectroscopy (TERS), in which plasmonic modes are excited at the apex of a metal-coated scanning probe tip, enables Raman scattering signals to be detected from nanoscale volumes with precise positional control. We discuss the application of TERS to characterize a variety of semiconductor nanostructures. In studies of Ge-SiGe core-shell nanowires, we measure spatially resolved Raman spectra along the length of a tapered nanowire to demonstrate the ability to measure local strain distributions with nanoscale spatial resolution. In tip-induced resonant Raman spectroscopy of monolayer and bilayer MoS 2 , we observe large enhancements in Raman signal levels measured for MoS 2 associated with excitation of plasmonic gap modes between an Au-coated probe tip and Au substrate surface onto which MoS 2 has been transferred. Transitions in B exciton photoluminescence intensity between monolayer and bilayer regions of MoS 2 are observed and discussed. Significant differences in nanoscale Raman spectra between monolayer and bilayer MoS 2 are also observed. The origins of specific resonant Raman peaks, their dependence on Mo S 2 layer thickness, and spatial resolution associated with the transition in Raman spectra between monolayer and bilayer regions are described.

As a kind of new nano-materials, carbon nanotubes (CNTs) are of the particular properties, in the aspects of physics, mechanics, electronics, optics and so on. They indicate the remarkable potential applications in the nano-electronic devices, composite materials, scanning probe microscopy (SPM), field emission, hydrogen storage and environment protection. The research on CNTs has become a hot spot in the fields of nano-science and nano-technology, especially in nano-materials (Iijima, 1991; Yu et al., 2000; Frank et al., 1998; Chiang et al., 2001). The commonly used methods for the measurement and characterization of CNTs include the scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning probe microscopy (SPM), Raman spectroscopy, infrared spectroscopy and so on. Electron microscopy and SPM are commonly used in nowadays research on nano-science. And both of them provide direct characterization of the topography and nano-structure of CNTs with comparatively high spatial resolution. However, they fail to provide the corresponding specimens' chemical information, which is also required in the research and identifications of nano-materials and nano-structures. Raman spectroscopy is one of the spectral analysis methods used to detect vibrational, rotational, and other low-frequency modes information of the specimen structures or molecular structures, and is usually employed to obtain the structural information of CNTs (Rao et al., 1997; Dresselhaus et al., 2002; Dresselhaus et al., 2005). However, this spectral method is subject to two obstacles. One is the diffraction-limited spatial resolution, and the other is its inherent small Raman cross section and weak signal. As facing the challenges of nano-scale measurement, a new characterization method, which can simultaneously obtain topography characteristics with nanometer resolution and spectral information from nanometer localized specimen surface is desiderated. Tip-Enhanced Raman Spectroscopy (TERS) is an emerging technology developed to realize the aim mentioned above in the recent years (Stockle et al, 2000; Hayazawa et al., 2000; Hartschuh et al., 2003; Pettinger et al., 2004). It spans the main obstacles of conventional Raman spectroscopy by tactfully combining the near-field optics advantage of nanometric spatial resolution and spectroscopy of obtaining chemical information. Also the Raman signal from nanometer localized specimen surface is considerably enhanced with the tipenhanced optical technology (TEOT) and the signal-to-noise ratio (SNR) is consequently

Materials and their interfaces are the core for the development of a large variety of fields, including catalysis, energy storage and conversion. In this case, tip-enhanced Raman spectroscopy (TERS), which combines scanning probe microscopy with plasmon-enhanced Raman spectroscopy, is a powerful technique that can simultaneously obtain the morphological information and chemical fingerprint of target samples at nanometer spatial resolution. It is an ideal tool for the nanoscale chemical characterization of materials and interfaces, correlating their structures with chemical performances. In this review, we begin with a brief introduction to the nanoscale characterization of materials and interfaces, followed by a detailed discussion on the recent theoretical understanding and technical improvements of TERS, including the origin of enhancement, TERS instruments, TERS tips and the application of algorithms in TERS. Subsequently, we list the key experimental issues that need to be addressed to conduct successful TERS measurements. Next, we focus on the recent progress of TERS in the study of various materials, especially the novel low-dimensional materials, and the progresses of TERS in studying different interfaces, including both solid-gas and solid-liquid interfaces. Finally, we provide an outlook on the future developments of TERS in the study of materials and interfaces.

Advances in tip-enhanced Raman spectroscopy (TERS) have demonstrated ultrahigh spatial resolution so that the vibrational modes of individual molecules can be visualized. The spatial resolution of TERS is determined by the confinement of the plasmon-induced field in the junction; however, the conditions necessary for achieving the high spatial confinement required for imaging individual molecules are not fully understood. Here, we present a systematic theoretical study of TERS imaging of single molecules, using a hybrid atomistic electrodynamics-quantum mechanical method. This approach provides a consistent treatment of the molecule and the plasmonic near field under conditions where they cannot be treated separately. In our simulations, we demonstrate that TERS is capable of resolving intricate molecule vibrations with atomic resolution, although we find that TERS images are extremely sensitive to the near field in the junction. Achieving the atomic resolution requires the near field to be confined within a few ångstroms in diameter and the near-field focal plane to be in the molecule plane. Furthermore, we demonstrate that the traditional surface selection rule of Raman spectroscopy is altered due to the significant field confinement that leads to significant field-gradient effects in the Raman scattering. This work provides insights into single-molecule imaging based on TERS and Raman scattering of molecules in nanojunctions with atomic dimensions.

Plasmon-enhanced photocatalytic coupling reactions have been used as model systems in surface-enhanced Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS) research for decades. However, the role of reactive arrangement on efficiency of these model reactions has remained largely unknown to date often leading to conflicting interpretations of experimental results. Herein, we use an interdisciplinary toolbox of nanoscale TERS imaging in combination with molecular-resolution ambient scanning tunnelling microscopy (STM) and density functional theory (DFT) modeling to investigate the role of reactive arrangement in photocatalytic coupling of 4-nitrobenzenethiol (4-NTP) to p,p'-dimercaptoazobisbenzene on single-crystal and polycrystalline Au surfaces for the first time. TERS imaging with 3 nm resolution clearly revealed a significantly higher catalytic efficiency inside a kinetically driven disordered phase of the 4-NTP adlayer on Au compared to the thermodynamically stable ordered phase. Furthermore, molecular level details of the self-assembled structures in the disordered and ordered phases obtained using ambient high-resolution STM enabled an unambiguous structure-reactivity correlation of photocatalytic coupling. Finally, quantitative mechanistic insights obtained from DFT modeling based on the accurate parameters determined from STM imaging emphatically confirmed that a combination of steric hindrance effect and energetic barrier leads to a lower reaction efficiency in the ordered phase of the 4-NTP adlayer. This fundamental study establishes the first direct structure-reactivity correlation in photocatalytic coupling and highlights the critical role of reactive arrangement in the efficiency of on-surface coupling reactions in heterogeneous catalysis at large.

Keywords: solid-liquid interface, solid-state battery, surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS), neutron tomography, sulfide solid-state electrolyte, Raman imagingThe ever-growing needs for energy storage systems require batteries with even higher power and energy density with extended life and enhanced safety beyond the current Li-ion battery technologies. Fulfilling these needs requires new battery chemistries, including high-energy-density electrode materials, solid-state electrolytes, and efficient interphases to mitigate the side reactions between the electrodes and the electrolytes.Charge transport across the electrode and electrolyte interface is believed to be one of the charge/discharge rate-limiting steps. Although the composition, morphology, and structure of solid-electrolyte interphase (SEI) have been extensively studied, probing its evolution at the nanoscale is challenging to a large extent. Plasmon-enhanced Raman spectroscopy (PERS) is promising to solve this challenge. PERS is the resonant oscillation of conduction electrons at the interface between negative and positive permittivity molecules stimulated by incident light. It includes surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). In the first part of this talk, I will show that gap-mode SERS is ideal to probe salt solvation structure in the immediate vicinity (< 20 nm) of the electrode/electrolyte interface. SERS lacks the nanoscale resolution in the sample plane. This can be compensated by TERS. TERS analysis on cycled high-energy silicon anodes indicates that the nanometer scale SEI "islands" are unevenly distributed. Even for the same SEI "island", the composition is different from point to point with inter-point distance smaller than 10 nm.All-solid-state lithium metal batteries (SSBs) promise specific energy >500 Wh/kg. A solid-state electrolyte (SSE) plays an irreplaceable role in reaching such an energy-density goal. Sulfide-based SSEs have emerged as a prominent class of soft ionic conductors that have comparable room-temperature ionic conductivity to their liquid electrolyte counterparts. However, when paring with a high voltage cathode, such as lithium nickel manganese cobalt oxide (NMC), the (electro)chemical instability of the sulfide SSE at the electrode/SSE interfaces becomes a major challenge. The interfacial instability can result in >50% initial capacity loss in a Li/sulfide SSE/NMC battery, thereby keeping the sulfide SSEs from commercialization. In the second part of this talk, I will show how we synergistically use neutron computed tomography and in situ Raman imaging, with clustering analysis to track lithium displacement at the interface of a sulfide-based SSB. Acknowledgment This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is supported by Asst. Secretary, Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office (VTO) through the Advanced Battery Materials Research (BMR) Program. This research used resources at High Flux Isotope Reactor, a DOE Office of Science User Facilities operated by the Oak Ridge National Laboratory.

We investigate the compositional homogeneity of InGaN thin films with a high In content grown by migration-enhanced plasma-assisted metal-organic chemical vapor deposition. Micro-Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS) are used to analyze the local InGaN composition on the micro- and nanoscale. Based on conventional micro-Raman mapping, the InGaN composition for all samples appears uniform but shows indications for intrinsic phase separations. TERS, a nanoscopic technique with a high spatial resolution far below the diffraction limit, verifies the formation of nanoscale compositional inhomogeneities. The dimensions of these compositional fluctuations observed in TERS are confirmed by scattering-type scanning near-field infrared nanoscopy (s-SNIN). In contrast to s-SNIN, we show that TERS furthermore enables the quantification of the In content in the different compositional regions and even allows the identification of InN nanoclusters near the surface of the epitaxial films.

Tip-enhanced Raman spectroscopy (TERS) offers one of the best techniques for analysis and imaging of molecule structures at nanoscale spatial resolution. An important issue in TERS is to improve the detection sensitivity of inherently weak Raman scattering so as to observe varieties of materials. For enhancement of the Raman signal, fully metallized tips are utilized in TERS, which enhance signals through plasmon oscillation at the tip apex. However, length of metal along the tip axis is on the order of a few to a few tens of micrometers, which means the plasmon resonant wavelength is much longer than the wavelength of the visible light used in TERS. From that point, if the tip has a metallic nanostructure on the apex, it would give better enhancement in the visible range compared with fully metallized tips. In this research, we employed photoreduction as a new fabrication method to grow a metallic nanostructure at the tip apex. We found a particular property of photoreduction that it occurs selectively at sharp corners, such as the tip apex of silicon cantilevers. Through this property, we succeeded in growing silver nanoparticles selectively at the tip apex. One of the advantages of the photoreduction is that the size of metal nanostructures is well controlled by optimizing various parameters. We controlled the size of silver nanoparticles from 100 to 400 nm by changing the laser exposure time. Furthermore, we obtained an order of magnitude higher enhancement from our fabricated tip compared with fully metalized tips through TERS measurements.

Tip-enhanced Raman spectroscopy (TERS) is an emerging tool to characterize low-dimensional materials requiring high spatial resolution beyond the light diffraction limit. The most extreme example of a spatially localized scatterer is that of a zero-dimensional system, e.g., a single molecule, a nanoparticle, or an individual defect within a crystal. Here, using an analytical model for near-field Raman scattering from point-like structures in the presence of a plasmonic metal probe, we investigate the TERS patterns of representative single molecules with arbitrary orientation, for a range of Raman tensor symmetries. The results can be used to determine the orientation of single molecules as well as to differentiate molecules with diverse Raman tensors or, conversely, Raman modes with different Raman tensors from the same molecule. To demonstrate that the model is of general applicability to any spatially localized Raman modes, we further apply it to TERS imaging of individual point defects in graphene, unv...

Tip-enhanced Raman Spectroscopy – Its status, challenges and future directions

Evanescent-wave-coupled emission has been used for reverse Kretschmann fluorescence and Raman spectroscopies with high collection efficiency. However, it has a negative effect on the common surface-enhanced Raman spectroscopy and tip-enhanced Raman spectroscopy without the reverse Kretschmann configuration because the coupling of a large fraction of light power into the substrate impairs the Raman signal backscattered in air. A rough core layer can significantly weaken evanescent-wave-coupled emission, which is conducive to enhancing the backscattered Raman signal. In this work, we theoretically investigate the surface-plasmon-coupled emission and its effects on surface Raman spectroscopy.

Isotope substitution of a molecule not only changes its vibrational frequencies but also changes its vibrational distributions in real-space. Quantitatively measuring the isotope effects inside a polyatomic molecule requires both energy and spatial resolutions at the single-bond level, which has been a long-lasting challenge in macroscopic techniques. By achieving ångström resolution in tip-enhanced Raman spectroscopy (TERS), we record the corresponding local vibrational modes of pentacene and its fully deuterated form, enabling us to identify and measure the isotope effect of each vibrational mode. The measured frequency ratio νH/νD varies from 1.02 to 1.33 in different vibrational modes, indicating different isotopic contributions of H/D atoms, which can be distinguished from TERS maps in real-space and well described by the potential energy distribution simulations. Our study demonstrates that TERS can serve as a non-destructive and highly sensitive methodology for isotope detection and recognition with chemical-bond precision.