Tip-enhanced Raman Spectroscopy Research Articles

Nanostructured gold-coated AFM tips generated by potentiostatic electrodeposition for tip-enhanced Raman spectroscopy

Potentiostatic electrodeposition was used to manufacture gold-coated atomic force microscopy (AFM) tips for applications in tip-enhanced Raman spectroscopy (TERS). The growth of the gold film was found to be initiated by the instantaneous nucleation mechanism on the doped-silicon AFM tip taken as substrate. This cost-effective method allowed robust AFM-TERS probes with localized surface plasmon resonances in the visible range to be produced with 53 % yield. These probes enable TERS mapping of a graphene oxide flake and a carbon nanotube to be performed with lateral spatial resolution below 20 nm, which was uncorrelated with the curvature radius of the metallized tip apex.

SiOx coated graphite with inorganic aqueous binders as high-performance anode for lithium-ion batteries

Inorganic aqueous binders (IAB) are an emerging class of aqueous binders. They offer exceptional physicochemical properties like intrinsic ionic conductivity, high thermal stability (>1000 °C), and environmental benignity making them attractive. In a previous study, we found that graphite anode shows improved electrochemical performance with these binders as compared to conventional PVDF binder for lithium-ion batteries (LIB). However, the cyclic performance of graphite-IAB at a higher rate (e.g., 1C) showed a declining trend. We attributed it to the poor binding strength between graphite and IAB due to insufficient functional groups in graphite. Therefore, in this report SiOx-based surface coatings of graphite are employed to improve its rate capability with silicate-based IAB by providing functional silicon oxide polymorphs on the coated graphite as an intermediate layer. The nature and structural arrangement of these coatings are investigated by tip-enhanced Raman spectroscopy (TERS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Optimized SiOx-coated graphite (GS) with sodium metasilicate binder leads to excellent cyclic stability with a capacity retention of >90 % at 20C for >4000 cycles. A high specific capacity of >315 mAhg−1 at 2C, stable for over 1000 cycles, is achieved for GS with IAB. The improved performance of the coated graphite is attributed to ameliorated binding with IAB as well as stable solid electrolyte interphase. We propose inorganic aqueous binders in combination with SiOx-coated graphite as an approach to realize a stable anode for LIB.

Probing the Vertical Resolving Ability of Tip-Enhanced Raman Spectroscopy

Probing the Vertical Resolving Ability of Tip-Enhanced Raman Spectroscopy

Superhydrophobic Surface Modification of Polymer Microneedles Enables Fabrication of Multimodal Surface-Enhanced Raman Spectroscopy and Mass Spectrometry Substrates for Synthetic Drug Detection in Blood Plasma.

Microneedles are widely used substrates for various chemical and biological sensing applications utilizing surface-enhanced Raman spectroscopy (SERS), which is indeed a highly sensitive and specific analytical approach. This article reports the fabrication of a nanoparticle (NP)-decorated microneedle substrate that is both a SERS substrate and a substrate-supported electrospray ionization (ssESI) mass spectrometry (MS) sample ionization platform. Polymeric ligand-functionalized gold nanorods (Au NRs) are adsorbed onto superhydrophobic surface-modified polydimethylsiloxane (PDMS) microneedles through the control of various interfacial interactions. We show that the chain length of the polymer ligands dictates the NR adsorption process. Importantly, assembling Au NRs onto the micrometer-diameter needle tips allows the formation of highly concentrated electromagnetic hot spots, which provide the SERS enhancement factor as high as 1.0 × 106. The micrometer-sized area of the microneedle top and high electromagnetic field enhancement of our system can be loosely compared with tip-enhanced Raman spectroscopy, where the apex of a plasmonic NP-functionalized sharp probe produces high-intensity plasmonic hot spots. Utilizing our NR-decorated microneedle substrates, the synthetic drugs fentanyl and alprazolam are analyzed with a subpicomolar limit of detection. Further analysis of drug-molecule interactions on the NR surface utilizing the Langmuir adsorption model suggests that the higher polarizability of fentanyl allows for a stronger interaction with hydrophilic polymer layers on the NR surface. We further demonstrate the translational aspect of the microneedle substrate for both SERS- and ssESI-MS-based detection of these two potent drugs in 10 drug-of-abuse (DOA) patient plasma samples with minimal preanalysis sample preparation steps. Chemometric analysis for the SERS-based detection shows a very good classification between fentanyl, alprazolam, or a mixture thereof in our selected 10 samples. Most importantly, ssESI-MS analysis also successfully identifies fentanyl or alprazolam in these same 10 DOA plasma samples. We believe that our multimodal detection approach presented herein is a highly versatile detection technology that can be applicable to the detection of any analyte type without performing any complicated sample preparation.

Capturing Nano‐Scale Inhomogeneity of the Electrode Electrolyte Interface in Sodium‐Ion Batteries Through Tip‐Enhanced Raman Spectroscopy

AbstractA prime challenge in the development of new battery chemistries is the fundamental understanding of the generation of the electrode–electrolyte interface (EEI) and its evolution upon cycling. Tip‐enhanced Raman spectroscopy (TERS) under an inert gas atmosphere is employed to study the chemical components of the anode/cathode electrolyte interface in a sodium‐ion battery. After the first cycle, TERS reveals that the EEI mostly consists of organic carbonate/dicarbonate, oligoethylene oxides, α,β‐unsaturated vinyl ketones/acetates, and inorganic species ClO4−, ClO3−, and Na2CO3. Whereas after 5× cycling, the EEI composition has evolved to contain long chain monodentate or bridging/bidentate carboxylates and alkoxides. The TERS map reveals the nano‐scale heterogeneity present in the EEI layers and elucidates a multilayered nano‐mosaic coating structure. The sheer volume of Raman signature present in the TERS signal can completely unravel the mysteries regarding the chemical composition and may shed light to the physicochemical behavior of the EEI.

Visualizing Ribbon-to-Ribbon Heterogeneity of Chemically Unzipped Wide Graphene Nanoribbons by Silver Nanowire-Based Tip-Enhanced Raman Scattering Microscopy.

Graphene nanoribbons (GNRs), a quasi-one-dimensional form of graphene, have gained tremendous attention due to their potential for next-generation nanoelectronic devices. The chemical unzipping of carbon nanotubes is one of the attractive fabrication methods to obtain single-layered GNRs (sGNRs) with simple and large-scale production. The authors recently found that unzipping from double-walled carbon nanotubes (DWNTs), rather than single- or multi-walled, results in high-yield production of crystalline sGNRs. However, details of the resultant GNR structure, as well as the reaction mechanism, are not fully understood due to the necessity of nanoscale spectroscopy. In this regard, silver nanowire-based tip-enhanced Raman spectroscopy (TERS) is applied for single GNR analysis and investigated ribbon-to-ribbon heterogeneity in terms of defect density and edge structure generated through the unzipping process. The authors found that sGNRs originated from the inner walls of DWNTs showed lower defect densities than those from the outer walls. Furthermore, TERS spectra of sGNRs exhibit a large variety in graphitic Raman parameters, indicating a large variation in edge structures. This work at the single GNR level reveals, for the first time, ribbon-to-ribbon heterogeneity that can never be observed by diffraction-limited techniques and provides deeper insights into unzipped GNR structure as well as the DWNT unzipping reaction mechanism.

(Invited) Multiscale Interfacial Heterogeneity Explored by Advanced Spectroscopy and Imaging for Batteries Beyond Lithium-Ion

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.

(Invited) Multiphysics Model of the sSNOM Spectroscopy: Can Near-Field Probes Report the Spectrum of Nanomaterials?

Last decade has been celebrated with a substantial increase of publications on imaging of nanomaterials with near-field probes. The latter include Tip-Enhanced Raman and/or Photoluminescence Spectroscopy (TERS/TEPL), various near-infrared (NIR) Atomic Force Microscopy (AFM) based methods, and several scattering Scanning Near-field optical methods (sSNOM). Using various coherent sources, often tunable in terms of excitation, these methods produce high spatial resolution images – near-field scanning maps, in terms of a pixel size claimed to be limited only by the size of the scanning tip.By design, TERS/TEPL detects a spectral response at each point of a map, which is, naively speaking, gives us a local probe of optical material properties within a single pixel (at the fixed excitation wavelength). NIR-AFM and sSNOM (leaving aside the FTIR mode) allow to obtain quasi-spectral information by tweaking the excitation. In this case, the collected (elastic) signal is restricted to the wavelength of the excitation.The essence of the near-field method is in breaking the laws of a plane-wave reflection from the sample, which requires a sub-wavelength separation between the tip and the sample and a strong enough near-field coupling between those. Still, it is routinely assumed that the tip is an “ideal probe” and its role is to “channel” the information about the local optical response to the detector. Up to date, little is known about the spectral response of the tip-sample system from the perspective of quantum physics.In this work, several aspects of the problem: mechanical motion of the scanning tip, electrodynamics of the excitation/scattered light, and quantum solid state description of the sample and the tip – are combined to obtain, within a simple model, a full picture of the near-field detection. The results, though not general and limited by the simplicity of model used for the sake of clarity of derivation, shed the light on the spectral response of a typical near-field microscope and describe effects, earlier called “artifacts”, as natural response for coupled tip-sample elementary excitations. Lastly, an application of the model for an analysis of spectra of nBN heteronanotubes (Y. Feng, et.al, ACS Nano 15, 5600, 2021) will be presented. Acknowledgement: This work has been partially supported by NSF MRSEC (DMR-2011839).Fig. 1. (left) Amplitude and phase of the near-field optical signal vs. the optical frequency: S(ω). (right) Same optical amplitude and phase vs. ω and the phase of mechanical motion of an AFM tip. Figure 1

In situ plasmonic tip preparation and validation techniques for scanning tunneling microscopy

Among the many parts constituting a scanning tunneling microscope, the metallic tip is the component that directly interacts with the specimen and plays a critical role in visualizing the physical quantity of interest. While tip materials such as W and Pt–Ir are commonly used for topographic imaging and their preparation is well-documented, the preparation of plasmonic materials such as Ag for tip-enhanced Raman spectroscopy is relatively less standardized. Here, we present several in situ Ag tip preparation and validation techniques for the microscopist to use depending on their intended application, including atomic resolution imaging, scanning tunneling spectroscopy (STM), and tip-enhanced Raman spectro-microscopy in ultrahigh vacuum. Besides optical applications, these methods are not limited to Ag but also applicable to other STM tip materials.

Conventional Raman, SERS and TERS Studies of DNA Compounds

DNA identification is possible by detecting its components through vibrational spectroscopy. Conventional Raman, Surface-enhanced Raman spectroscopy (SERS) and Tip-enhanced Raman spectroscopy (TERS) have shown a high capacity for the exploration of different molecules and materials (semi-conducting material, carbon nanotubes and biologicals molecules as DNA, proteins). Their applications extended to biological systems and brought significant information to this field. This review summarizes a high number of studies and research conducted with conventional Raman, SERS and TERS on every DNA component starting from the four different nucleic acids in their different forms (nucleosides, deoxyribonucleosides, deoxyribonucleotides) to their biological interaction to form one and double DNA strands. As SERS has an advantage on conventional Raman by exploiting the optical properties of metallic nanostructures to detect very small quantities of molecules, it also clarifies the DNA structure’s orientation in addition to its composition. It also clarifies the influence of different parameters, such as the presence of a spacer or a mutation in the strand on the hybridization process. TERS was shown as a relevant tool to scan DNA chemically and to provide information on its sequence.

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

Coherence length (Lc) of the Raman scattering process in graphene as a function of Fermi energy is obtained with spatially coherent tip-enhanced Raman spectroscopy. Lc 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 vg, 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.

A Full Quantum Mechanical Approach Assessing the Chemical and Electromagnetic Effect in TERS.

Tip-enhanced Raman spectroscopy (TERS) is a valuable method for surface analysis with nanometer to angstrom-scale resolution; however, the accurate simulation of particular TERS signals remains a computational challenge. We approach this challenge by combining the two main contributors to plasmon-enhanced Raman spectroscopy and to the high resolution in TERS, in particular, the electromagnetic and the chemical effect, into one quantum mechanical simulation. The electromagnetic effect describes the sample's interaction with the strong, highly localized, and inhomogeneous electric fields associated with the plasmonic tip and is typically the thematic focus for most mechanistic studies. On the other hand, the chemical effect covers the different responses to the extremely close-range and highly position-sensitive chemical interaction between the apex tip atom(s) and the sample, and, as we could show in previous works, plays an often underestimated role. Starting from a (time-dependent) density functional theory description of the chemical model system, comprised of a tin(II) phthalocyanine sample molecule and a single silver atom as the tip, we introduce the electromagnetic effect through a series of static point charges that recreate the electric field in the vicinity of the plasmonic Ag nanoparticle. By scanning the tip over the molecule along a 3D grid, we can investigate the system's Raman response on each position for nonresonant and resonant illumination. Simulating both effects on their own already hints at the achievable signal enhancement and resolution, but the combination of both creates even stronger evidence that TERS is capable of resolving submolecular features.

Unveiling localized electronic properties of ReS2 thin layers at nanoscale using Kelvin force probe microscopy combined with tip-enhanced Raman spectroscopy

Electronic properties of two-dimensional (2D) materials can be strongly modulated by localized strain. The typical spatial resolution of conventional Kelvin probe force microscopy (KPFM) is usually limited in a few hundreds of nanometers, and it is difficult to characterize localized electronic properties of 2D materials at nanoscales. Herein, tip-enhanced Raman spectroscopy (TERS) is proposed to combine with KPFM to break this restriction. TERS scan is conducted on ReS2 bubbles deposited on a rough Au thin film to obtain strain distribution by using the Raman peak shift. The localized contact potential difference (CPD) is inversely calculated with a higher spatial resolution by using strain measured by TERS and CPD-strain working curve obtained using conventional KPFM and atomic force microscopy. This method enhances the spatial resolution of CPD measurements and can be potentially used to characterize localized electronic properties of 2D materials.

Micro‐ and Nano‐Raman Spectroscopy Characterization of Exfoliated Graphene with Helium‐Ion Microscope Patterned Line Defects

Material´s patterned modifications are crucial for device fabrication, and their evolution from the micro‐ to the nanoscale depends on the development of modification and characterization techniques. Herein, a graphene sample cut by a He‐ion beam is characterized using Raman spectroscopy. What can be obtained from micro‐Raman spectroscopy as compared to nano‐Raman spectroscopy is analyzed, the latter implemented in the tip‐enhanced Raman spectroscopy (TERS) configuration. Local sputtering, inhomogeneous distributions of defects, strain and doping are only observed in the higher‐resolution nano‐Raman‐mode characterization.

Atomic-Scale Insights into the Interlayer Characteristics and Oxygen Reactivity of Bilayer Borophene.

Bilayer (BL) two-dimensional boron (i.e., borophene) has recently been synthesized and computationally predicted to have promising physical properties for a variety of electronic and energy technologies. However, the fundamental chemical properties of BL borophene that form the foundation of practical applications remain unexplored. Here, we present atomic-level chemical characterization of BL borophene using ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS). UHV-TERS identifies the vibrational fingerprint of BL borophene with angstrom-scale spatial resolution. The observed Raman spectra are directly correlated with the vibrations of interlayer boron-boron bonds, validating the three-dimensional lattice geometry of BL borophene. By virtue of the single-bond sensitivity of UHV-TERS to oxygen adatoms, we demonstrate the enhanced chemical stability of BL borophene compared to its monolayer counterpart by exposure to controlled oxidizing atmospheres in UHV. In addition to providing fundamental chemical insight into BL borophene, this work establishes UHV-TERS as a powerful tool to probe interlayer bonding and surface reactivity of low-dimensional materials at the atomic scale.

Revealing Intramolecular Isotope Effects with Chemical-Bond Precision.

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.

Polarized tip-enhanced Raman spectroscopy at liquid He temperature in ultrahigh vacuum using an off-axis parabolic mirror.

Tip-enhanced Raman spectroscopy (TERS) combines inelastic light scattering well below the diffraction limit down to the nanometer range and scanning probe microscopy and, possibly, spectroscopy. In this way, topographic and spectroscopic as well as single- and two-particle information may simultaneously be collected. While single molecules can now be studied successfully, bulk solids are still not meaningfully accessible. It is the purpose of the work presented here to outline approaches toward this objective. We describe a home-built, liquid helium cooled, ultrahigh vacuum TERS. The setup is based on a scanning tunneling microscope and, as an innovation, an off-axis parabolic mirror having a high numerical aperture of ∼0.85 and a large working distance. The system is equipped with a fast load-lock chamber, a chamber for the in situ preparation of tips, substrates, and samples, and a TERS chamber. Base pressure and temperature in the TERS chamber were ∼3 × 10-11 mbar and 15K, respectively. Polarization dependent tip-enhanced Raman spectra of the vibration modes of carbon nanotubes were successfully acquired at cryogenic temperature. The new features described here including very low pressure and temperature and the external access to the light polarizations, thus the selection rules, may pave the way toward the investigation of bulk and surface materials.

Probing the Role of Environmental and Sample Characteristics in Gap Mode Tip-Enhanced Raman Spectroscopy.

Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful analytical tool for nondestructive and label-free molecular characterization at the nanoscale. However, the influence of environmental factors and sample characteristics on the occurrence of spurious signals, enhancement of TERS signals, and longevity of TERS probes is not well understood yet. Herein, we present a detailed investigation of the influence of oxygen, humidity, and atmospheric carbon contaminants on scanning tunneling microscopy-TERS (STM-TERS) measurements of self-assembled monolayer systems in ambient and inert environments. Our results reveal a consistent increase of TERS signals, significant reduction of spurious signals, and drastically improved longevity of TERS probes in the inert environment. Additionally, sample characteristics such as molecular packing, chemisorption behavior, and hydrophilicity are found to have a direct impact on signal enhancement in the TERS measurements of molecular self-assembled monolayers (SAMs). The novel insights gained in this study are expected to pave the way for a more robust data analysis and improved experimental design in the future gap mode STM- and atomic force microscopy-TERS (AFM-TERS) studies.

Resonance Raman spectroscopy characterization of linear carbon chains encapsulated by multi-walled carbon nanotubes

The unique electronic and vibrational properties of linear carbon chains (LCCs) have attracted close attention from the scientific community in recent years. Raman spectroscopy addressing the LCC spectral signature around 1850 cm−1 has been widely used to identify the LCC and probe electronic and vibrational properties as a function of carbon length. Despite the number of works available in literature, some aspects of the LCC's aforementioned properties remain unclear. Using a combination of confocal and Tip-enhanced Raman Spectroscopy (TERS), along with different laser lines, this work addresses important aspects of the optical resonance window of LCCs encapsulated by multi-walled carbon nanotubes (MWCNTs) (i.e. LCC@MWCNT) as well as an elusive Raman signature around 1637 cm−1, which is assigned to the LCC's longitudinal acoustic (LA) phonon mode at the X point (zone edge), which becomes Raman active likely due to disorder effects. First-principles calculations endorse our conclusions.

Regioselective Tip-Enhanced Raman Spectroscopy of Lipid Membranes with Sub-Nanometer Axial Resolution.

Noninvasive and label-free analysis of cell membranes at the nanoscale is essential to comprehend vital cellular processes. However, conventional analytical tools generally fail to meet this challenge due to the lack of required sensitivity and/or spatial resolution. Herein, we demonstrate that tip-enhanced Raman spectroscopy (TERS) is a powerful nanoanalytical tool to analyze dipalmitoylphosphatidylcholine (DPPC) bilayers and human cell membranes with submolecular resolution in the vertical direction. Unlike the far-field Raman measurements, TERS spectra of the DPPC bilayers reproducibly exhibited a uniquely shaped C-H band. These unique spectral features were also reproducibly observed in the TERS spectrum of human pancreatic cancer cells. Spectral deconvolution and DFT simulations confirmed that the TERS signal primarily originated from vibrations of the CH3 groups in the choline headgroup of the lipids. The reproducible TERS results obtained in this study unequivocally demonstrate the ultrahigh sensitivity of TERS for nanoanalysis of lipid membranes under ambient conditions.