Gold-coated AFM tips for tip-enhanced Raman spectroscopy: theoretical calculation and experimental demonstration

Proposed Quantum Twisting Scanning Probe Microscope over Twisted Bilayer Graphene

Twisted bilayer graphene (TBG) has the natural merits of tunable flat bands and localized states distributed as a triangular lattice. However, the application of this state remains obscure. By density functional theory (DFT) and pz orbital tight-binding model calculations, we investigate the tip-shaped electrostatic potential of top valence electrons of TBG at half filling. Adsorption energy scanning of molecules above the TBG reveals that this tip efficiently attracts molecules selectively to AA-stacked or AB-stacked regions. Tip shapes can be controlled by their underlying electronic structure, with electrons of low bandwidth exhibiting a more localized feature. Our results indicate that TBG tips offer applications in noninvasive and nonpolluting measurements in scanning probe microscopy and theoretical guidance for 2D material-based probes.

d-Band Holes React at the Tips of Gold Nanorods

Reactive hot spots on plasmonic nanoparticles have attracted attention for photocatalysis as they allow for efficient catalyst design. While sharp tips have been identified as optimal features for field enhancement and hot electron generation, the locations of catalytically promising d-band holes are less clear. Here we exploit d-band hole-enhanced dissolution of gold nanorods as a model reaction to locate reactive hot spots produced from direct interband transitions, while the role of the plasmon is to follow the reaction optically in real time. Using a combination of single-particle electrochemistry and single-particle spectroscopy, we determine that d-band holes increase the rate of gold nanorod electrodissolution at their tips. While nanorods dissolve isotropically in the dark, the same nanoparticles switch to tip-enhanced dissolution upon illimitation with 488 nm light. Electron microscopy confirms that dissolution enhancement is exclusively at the tips of the nanorods, consistent with previous theoretical work that predicts the location of d-band holes. We, therefore, conclude that d-band holes drive reactions selectively at the nanorod tips.

Rapid theoretical method for inverse design on a tip-enhanced Raman spectroscopy (TERS) probe

Tip-enhanced Raman spectroscopy (TERS) can provide correlated topographic and chemical information at the nanoscale, with great sensitivity and spatial resolution depending on the configuration of the TERS probe. The sensitivity of the TERS probe is largely determined by two effects: the lightning-rod effect and local surface plasmon resonance (LSPR). While 3D numerical simulations have traditionally been used to optimize the TERS probe structure by sweeping two or more parameters, this method is extremely resource-intensive, with computation times growing exponentially as the number of parameters increases. In this work, we propose an alternative rapid theoretical method that reduces computational loading while still achieving effective TERS probe optimization through the inverse design method. By applying this method to optimize a TERS probe with four free-structural parameters, we observed a nearly 1 order of magnitude improvement in enhancement factor (|E/E₀|²), in contrast to a parameter sweeping 3D simulation that would take ∼7000 hours of computation. Our method, therefore, shows great promise as a useful tool for designing not only TERS probes but also other near-field optical probes and optical antennas.

Apex-Confined Plasmonic Tip for High Resolution Tip-Enhanced Raman Spectroscopic Imaging of Carbon Nanotubes

This paper reports a handy technical scheme to decorate atomic force microscopy (AFM) tips toward tip-enhanced Raman spectroscopy (TERS) applications. The major attraction of these homemade tips lies in that silver decoration can be confined at the apex of commercial tips by the means of an AFM-controlled electrochemical reaction. The reduction of Ag⁺ occurs in a highly sealed environment to secure the metal coating efficiency. Key factors include silver nitrate solution to provide Ag⁺, ambient relative humidity and temperature in a humidity cell, electric potential bias, and tip-surface distance. Subsequently, these silver-coated tips are evaluated for TERS measurement of carbon nanotubes (CNTs) so that both morphological and chemical characteristics of CNTs are concurrently obtained. The Raman spectra reveal that our plasmonic tip competently possesses an ∼30-fold local field signal increase and the corresponding TERS image laterally resolves at the single-pixel level.

Effect of the focused gap-plasmon mode on tip-enhanced Raman excitation and scattering

As a powerful molecular detection approach, tip-enhanced Raman scattering (TERS) spectroscopy has the advantages of nanoscale spatial resolution, label-free detection and high enhancement factor, therefore has been widely used in fields of chemistry, materials and life sciences. A TERS system enhanced by the focused gap-plasmon mode composed of Surface Plasmon Polariton (SPP) focus and the metal probe has been reported, however, its underlying enhancement mechanism for Raman excitation and scattering remains to be deeply explored. Here, we focus on the different performances of optical focus and SPP focus in the TERS system, and verify that the cooperation of these two focuses can produce maximum enhancement in a local electromagnetic field. Further, the Purcell effect on sample scattering in such a system is studied for the enhancement of Raman scattering collection in the far field. Finally, the local field enhancement and the sample far-field scattering enhancement are combined to show a full view of the whole process of TERS enhancement. This research can be applied to optimize the excitation and collection of Raman signals in TERS systems, which is of great value for the research and development of TERS technology.

Machine learning ensures rapid and precise selection of gold sea-urchin-like nanoparticles for desired light-to-plasmon resonance

Sustainable energy strategies, particularly solar-to-hydrogen production, are anticipated to overcome the global reliance on fossil fuels. Thereby, materials enabling the production of green hydrogen from water and sunlight are continuously designed, e.g., ZnO nanostructures coated by gold sea-urchin-like nanoparticles, which employ the light-to-plasmon resonance to realize photoelectrochemical water splitting. But such light-to-plasmon resonance is strongly impacted by the size, the species, and the concentration of the metal nanoparticles coating on the ZnO nanoflower surfaces. Therefore, a precise prediction of the surface plasmon resonance is crucial to achieving an optimized nanoparticle fabrication of the desired light-to-plasmon resonance. To this end, we synthesized a substantial amount of metal (gold) nanoparticles of different sizes and species, which are further coated on ZnO nanoflowers. Subsequently, we utilized a genetic algorithm neural network (GANN) to obtain the synergistically trained model by considering the light-to-plasmon conversion efficiencies and fabrication parameters, such as multiple metal species, precursor concentrations, surfactant concentrations, linker concentrations, and coating times. In addition, we integrated into the model's training the data of nanoparticles due to their inherent complexity, which manifests the light-to-plasmon conversion efficiency far from the coupling state. Therefore, the trained model can guide us to obtain a rapid and automatic selection of fabrication parameters of the nanoparticles with the anticipated light-to-plasmon resonance, which is more efficient than an empirical selection. The capability of the method achieved in this work furthermore demonstrates a successful projection of the light-to-plasmon conversion efficiency and contributes to an efficient selection of the fabrication parameters leading to the anticipated properties.

Plasmonic Enhanced SERS in Ag/TiO2 Nanostructured Film: An Experimental and Theoretical Study

In this work, we present a new study on the electromagnetic (EM) enhancement properties generated by Ag/TiO₂ toward the finger print of methylene blue (MB) molecules deposited on the surface of Ag nanostructures. SERS intensity generated by MB molecules reflects the interaction between the local electric field and their bonds. A power-dependent SERS study in order to reveal the magnitude effect of a local electric field on the vibration behavior of molecular bonds of MB was performed. A theoretical study using finite element (COMSOL Multiphysics) was performed in order to understand the effect of interparticle distance of Ag nanoparticles on the enhancement properties.

Plasmon-enhanced Raman spectroscopy of two-dimensional semiconductors

Two-dimensional (2D) semiconductors have grown fast into an extraordinary research field due to their unique physical properties compared to other semiconducting materials. The class of materials proved extremely fertile for both fundamental studies and a wide range of applications from electronics/spintronics/optoelectronics to photocatalysis and CO₂reduction. 2D materials are highly confined in the out-of-plane direction and often possess very good environmental stability. Therefore, they have also become a popular material system for the manipulation of optoelectronic properties via numerous external parameters. Being a versatile characterization technique, Raman spectroscopy is used extensively to study and characterize various physical properties of 2D materials. However, weak signals and low spatial resolution hinder its application in more advanced systems where decoding local information plays an important role in advancing our understanding of these materials for nanotechnology applications. In this regard, plasmon-enhanced Raman spectroscopy has been introduced in recent time to investigate local heterogeneous information of 2D semiconductors. In this review, we summarize the recent progress of plasmon-enhanced Raman spectroscopy of 2D semiconductors. We discuss the current state-of-art and provide future perspectives on this specific branch of Raman spectroscopy applied to 2D semiconductors.

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.

Nanoscale Magnetization and Current Imaging Using Time-Resolved Scanning-Probe Magnetothermal Microscopy

Magnetic microscopy that combines nanoscale spatial resolution with picosecond scale temporal resolution uniquely enables direct observation of the spatiotemporal magnetic phenomena that are relevant to future high-speed, high-density magnetic storage and logic technologies. Magnetic microscopes that combine these metrics has been limited to facility-level instruments. To address this gap in lab-accessible spatiotemporal imaging, we develop a time-resolved near-field magnetic microscope based on magnetothermal interactions. We demonstrate both magnetization and current density imaging modalities, each with spatial resolution that far surpasses the optical diffraction limit. In addition, we study the near-field and time-resolved characteristics of our signal and find that our instrument possesses a spatial resolution on the scale of 100 nm and a temporal resolution below 100 ps. Our results demonstrate an accessible and comparatively low-cost approach to nanoscale spatiotemporal magnetic microscopy in a table-top form to aid the science and technology of dynamic magnetic devices with complex spin textures.

Influence of nonlocal dielectric response on the Au tip-enhanced fluorescence effect

Tip-enhanced fluorescence (TEF) with ultra-high detection sensitivity and spatial resolution has been a powerful characterization technique in the study of surface science and life science. Herein, a systematically theoretical investigation in the visible range had been performed to study TEF properties of a single molecule located inside a nanogap formed by Au tip and substrate. In the strong localized surface plasmon coupling effect, the contribution of nonlocal dielectric response to the fluorescence quantum yield as well as radiative and energy dissipated decay rates were calculated. It is found that the nonlocal dielectric effects become comparable to the radiative and energy dissipated decay rates with the increasing of the tip-molecule distance, as a result, the nonlocal dielectric effect significantly suppresses the fluorescence process. The huge excitation enhancement at the shorter tip-molecule distance can efficiently compensate the low quantum yield, leading to the great fluorescence enhancement. The results show that the maximum enhancement obtained from the calculations can reach as high as four orders of magnitude by optimizing the tip-molecule distance. These results are not only helpful to our understanding of the TEF mechanism but also valuable for its further applications.

Simple Preparation of Ceramic-Like Materials Based on 1D-Agx(x=0, 5, 10, 20, 40 mM)/TiO2 Nanostructures and Their Photocatalysis Performance

Vertical Agx/TiO2 nanorods were successfully grown by a simple oxidation method of a Ti-Ag coating. The samples were grown in the phase of ceramic-like materials, which can be reusable for many cycles for photocatalysis applications. These ceramic-like Agx/TiO2 nanostructures were prepared by the spin-coating of silver nitrate onto Ti sheets. The presence of silver on the surface of the Ti sheet during the oxidation process helped in the growth of one-dimensional nanostructures. The physical properties of the fabricated ceramic-like nanostructures were studied by varying the concentration of silver on the Ti-sheet before the oxidation. One-dimensional nanostructures with an average size varying within the range of 200–500 nm were grown. The presence of silver made the nanostructure vertically directed. The nanorods were dense at the low and medium concentrations of 5, 10, and 20 mM of silver in contrary to high silver concentrations, where the nanorods were very sparse at 40 mM. Structural analysis showed the anatase and rutile structure of pure TiO2 with distinguishing diffraction lines A(101) and R(110); however, Agx/TiO2 showed a dominant orientation of A(101), confirming the 1D growth. Raman spectra confirmed the presence of TiO2 via the observation of its corresponding phonon modes. The photocatalysis properties of the fabricated ceramic-like nanostructures were performed on methylene blue (MB) as a known target dye. The low- and medium-silver-concentration samples showed a high photocatalytic activity compared to the pure and high-silver-concentration samples.

Fabrication of a Biocompatible Mica/Gold Surface for Tip-Enhanced Raman Spectroscopy

Tip‐enhanced Raman spectroscopy (TERS) is a promising technique for structural studies of biological systems and biomolecules, owing to its ability to provide a chemical fingerprint with sub‐diffraction‐limit spatial resolution. This application of TERS has thus far been limited, due to difficulties in generating high field enhancements while maintaining biocompatibility. The high sensitivity achievable through TERS arises from the excitation of a localized surface plasmon resonance in a noble metal atomic force microscope (AFM) tip, which in combination with a metallic surface can produce huge enhancements in the local optical field. However, metals have poor biocompatibility, potentially introducing difficulties in characterizing native structure and conformation in biomolecules, whereas biocompatible surfaces have weak optical field enhancements. Herein, a novel, biocompatible, highly enhancing surface is designed and fabricated based on few‐monolayer mica flakes, mechanically exfoliated on a metal surface. These surfaces allow the formation of coupled plasmon enhancements for TERS imaging, while maintaining the biocompatibility and atomic flatness of the mica surface for high resolution AFM. The capability of these substrates for TERS is confirmed numerically and experimentally. We demonstrate up to five orders of magnitude improvement in TERS signals over conventional mica surfaces, expanding the sensitivity of TERS to a wide range of non‐resonant biomolecules with weak Raman cross‐sections. The increase in sensitivity obtained through this approach also enables the collection of nanoscale spectra with short integration times, improving hyperspectral mapping for these applications. These mica/metal surfaces therefore have the potential to revolutionize spectromicroscopy of complex, heterogeneous biological systems such as DNA and protein complexes.

Controllable plasmon-induced catalytic reaction by surface-enhanced and tip-enhanced Raman spectroscopy

The controllable catalytic reaction plays a pivotal role in heterogeneous catalysis. Surface-enhanced Raman scattering (SERS) and tip enhanced Raman spectroscopy (TERS) are considered promising techniques for the study of catalytic reactions due to the highly localized sensitivity of SERS and the nanoscale spatial resolution of TERS. Herein, Ag/Au composite films were employed as catalyst for in situ monitoring of the catalytic reaction of 4‑nitrobenzenethiol (4NBT) to p, p'‑dimercaptoazobenzene (DMAB). The catalytic reaction of 4NBT adsorbed on Au film can be manipulated at the nanoscale using TERS by controlling the height between the tip-apex and the sample surface in Ag tip-Au substrate geometry. According to finite difference time domain (FDTD) simulations, the 'hot electron' induced by the localized surface plasmon is sufficient for promoting the catalytic reaction. These findings provide a novel way for controllable graph drawing of molecules at the nanoscale level.

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.

Advances in surface-enhanced Raman spectroscopy (SERS) substrates for lipid and protein characterization: sensing and beyond

Surface-enhanced Raman spectroscopy (SERS) has become an essential ultrasensitive analytical tool for biomolecular analysis of small molecules, macromolecular proteins, and even cells. SERS enables label-free, direct detection of molecules through their intrinsic Raman fingerprint. In particular, protein and lipid bilayers are dynamic three-dimensional structures that necessitate label-free methods of characterization. Beyond direct detection and quantitation, the structural information contained in SERS spectra also enables deeper biophysical characterization of biomolecules near metallic surfaces. Therefore, SERS offers enormous potential for such systems, although making measurements in a nonperturbative manner that captures the full range of interactions and activity remains a challenge. Many of these challenges have been overcome through advances in SERS substrate development, which have expanded the applications and targets of SERS for direct biomolecular quantitation and biophysical characterization. In this review, we will first discuss different categories of SERS substrates including solution-phase, solid-supported, tip-enhanced Raman spectroscopy (TERS), and single-molecule substrates for biomolecular analysis. We then discuss detection of protein and biological lipid membranes. Lastly, biophysical insights into proteins, lipids and live cells gained through SERS measurements of these systems are reviewed.

Nanoscale chemical imaging using tip-enhanced Raman spectroscopy

Confocal and surface-enhanced Raman spectroscopy (SERS) are powerful techniques for molecular characterization; however, they suffer from the drawback of diffraction-limited spatial resolution. Tip-enhanced Raman spectroscopy (TERS) overcomes this limitation and provides chemical information at length scales in the tens of nanometers. In contrast to alternative approaches to nanoscale chemical analysis, TERS is label free, is non-destructive, and can be performed in both air and liquid environments, allowing its use in a diverse range of applications. Atomic force microscopy (AFM)-based TERS is especially versatile, as it can be applied to a broad range of samples on various substrates. Despite its advantages, widespread uptake of this technique for nanoscale chemical imaging has been inhibited by various experimental challenges, such as limited lifetime, and the low stability and yield of TERS probes. This protocol details procedures that will enable researchers to reliably perform TERS imaging using a transmission-mode AFM-TERS configuration on both biological and non-biological samples. The procedure consists of four stages: (i) preparation of plasmonically active TERS probes; (ii) alignment of the TERS system; (iii) experimental procedures for nanoscale imaging using TERS; and (iv) TERS data processing. We provide procedures and example data for a range of different sample types, including polymer thin films, self-assembled monolayers (SAMs) of organic molecules, photocatalyst surfaces, small molecules within biological cells, single-layer graphene and single-walled carbon nanotubes in both air and water. With this protocol, TERS probes can be prepared within ~23 h, and each subsequent TERS experimental procedure requires 3-5 h.

Controllable Fabrication of Au-Coated AFM Probes via a Wet-Chemistry Procedure

Tip-enhanced Raman spectroscopy (TERS), which offers a spatial resolution far beyond the limitations of the optical diffraction and detection sensitivity down to a single molecular level, has become one of the powerful techniques applied in current nanoscience and technology. However, the excellent performance of a TERS system is very much dependent on the quality of metallized probes used in TERS characterization. Thus, how to prepare higher-quality probes plays a vital role in the development and application of TERS technique. In this work, one simple wet-chemistry procedure was designed to fabricate atomic force microscopy-based TERS (AFM-TERS) probes. Through the controlled growth of a gold film on a commercial silicon AFM probe, TERS probes with different apex diameters were prepared successfully. A series of TERS results indicated that the probes with the apex size of 50~60 nm had the maximum TERS enhancement, and the Raman enhancement factor was in the range of 106 to 107. Compared with those prepared by other fabrication methods, our TERS probes fabricated by this wet-chemistry method have the virtues of good stability, high reproducibility, and strong enhancement effect.

Quantitative comparison of plasmon resonances and field enhancements of near-field optical antennae using FDTD simulations

Plasmon resonances and electric field enhancements of several near-field optical antennae with plasmonic nanostructures engineered at their apices were quantitatively compared using finite difference time domain simulations. Although many probe designs have been tested experimentally, a systematic comparison of field enhancements has not been possible, due to differences in instrument configuration, reporter mechanism, excitation energy, and plasmonic materials used. For plasmonic nanostructures attached to a non-plasmonic support (e.g., a nanoparticle functionalized AFM tip), we find that the complex refractive index of the support material is critical in controlling the overall plasmonic behavior of the antenna. Supports with strong absorption at optical energies (Pt, W) dampen plasmon resonances and lead to lower enhancements, while those with low absorption (SiO₂, Si₃N₄, Si) boost enhancement by increasing the extinction cross-section of the apex nanostructure. Using a set of physically realistic constraints, probes were optimized for peak plasmonic enhancement at common near-field optical wavelengths (633-647 nm) and those with focused ion-beam milled grooves near the apex were found to give the largest local field enhancements (~30x). Compared to unstructured metal cones, grooved probes gave a 300% improvement in field strength, which can boost tip-enhanced Raman spectroscopy (TERS) signals by 1-2 orders of magnitude. Moreover, grooved probe resonances can be easily tuned over visible and near-infrared energies by varying the plasmonic metal (Ag or Au) and groove location. Overall, this work shows that probes with strong localized surface plasmon resonances at their apices can be engineered to provide large field enhancements and boost signals in near-field optical experiments.

Atomic Force Microscopy Based Tip-Enhanced Raman Spectroscopy in Biology

Most biological phenomena occur at the nanometer scale, which is not accessible by the conventional optical techniques because of the optical diffraction limitation. Tip-enhanced Raman spectroscopy (TERS), one of the burgeoning probing techniques, not only can provide the topography characterization with high resolution, but also can deliver the chemical or molecular information of a sample beyond the optical diffraction limitation. Therefore, it has been widely used in various structural analyses pertaining to materials science, tissue engineering, biological processes and so on. Based on the different feedback mechanisms, TERS can be classified into three types: atomic force microscopy based TERS system (AFM-TERS), scanning tunneling microscopy based TERS system (STM-TERS) and shear force microscopy based TERS system (SFM-TERS). Among them, AFM-TERS is the most widely adopted feedback system by live biosamples because it can work in liquid and this allows the investigation of biological molecules under native conditions. In this review, we mainly focus on the applications of AFM-TERS in three biological systems: nucleic acids, proteins and pathogens. From the TERS characterization to the data analysis, this review demonstrates that AFM-TERS has great potential applications to visually characterizing the biomolecular structure and crucially detecting more nano-chemical information of biological systems.

Rational fabrication of silver-coated AFM TERS tips with a high enhancement and long lifetime

Tip-enhanced Raman spectroscopy (TERS), known as nanospectroscopy, has received increasing interest as it can provide nanometer spatial resolution and chemical fingerprint information of samples simultaneously. Since Ag tips are well accepted to show a higher TERS enhancement than that of gold tips, there is an urgent quest for Ag TERS tips with a high enhancement, long lifetime, and high reproducibility, especially for atomic force microscopy (AFM)-based TERS. Herein, we developed an electrodeposition method to fabricate Ag-coated AFM TERS tips in a highly controllable and reproducible way. We investigated the influence of the electrodeposition potential and time on the morphology and radius of the tip. The radii of Ag-coated AFM tips can be rationally controlled at a few to hundreds nanometers, which allows us to systematically study the dependence of the TERS enhancement on the tip radius. The Ag-coated AFM tips show the highest TERS enhancement under 632.8 nm laser excitation and a broad localized surface plasmon resonance (LSPR) response when coupled to a Au substrate. The tips exhibit a lifetime of 13 days, which is particularly important for applications that need a long measuring time.

Effect of the gold crystallinity on the enhanced luminescence signal of scanning probe tips in apertureless near-field optical microscopy

The effect of gold tip crystallinity on their spectral amplification characteristics, monitored through the luminescence enhanced by surface plasmon resonance (SPR), is investigated experimentally. As the tip radius increases, the grains composing polycrystalline tips become larger, resulting in a blueshift of the emission while a redshift of the SPR was predicted for monocrystalline gold. This reveals that the effect of the grain size, a parameter that has not been considered so far, is dominant over that of the tip radius. This study is significant to apertureless scanning near-field optical microscopy, where the gold tip emission defines the spectral antenna range.

Tunable, Liquid Resistant Tip Enhanced Raman Spectroscopy Probes: Toward Label-Free Nano-Resolved Imaging of Biological Systems

Tip enhanced Raman spectroscopy (TERS) has been established as a powerful, noninvasive technique for chemical identification at the nanoscale. However, difficulties, including the degradation of probes, limit its use in liquid systems. Here TERS probes for studies in aqueous environments have been demonstrated using titanium nitride coatings with an alumina protective layer. The probes show enhancement in signal intensity as high as 380% in liquid measurements, and the probe resonance can be tuned by varying deposition conditions to optimize performance for different laser sources and types of samples. This development of inexpensively produced probes suited for studies in aqueous environments enables its wider use for fields such as biology and biomedicine in which aqueous environments are the norm.

Spatially distinct Raman scattering characteristics of individual ZnO nanorods under controlled polarization: intense end scattering from forbidden modes

In this study, we characterize incident/scattered polarization-specific and NR position-resolved Raman scattering behaviors of individual zinc oxide nanorods (ZnO NRs). We quantify Raman signals from the five key ZnO phonon modes of E_2L, E_2H-2L, A_1T, E_1T, and E_2H, and reveal the NR position-dependent Raman scattering characteristics of the phonon modes per given light-matter interaction geometry. We then present Raman intensity maps and elucidate Raman behaviors consistent and incongruous with Raman selection rules. In particular, we identify an intriguing Raman scattering phenomenon from the forbidden modes, distinctively occurring at the two NR ends. Their unexpectedly strong and localized scattering signals at the NR termini are contrasted by the scattering behaviors from the rest of the NR positions agreeing with the selection predictions. By carrying out control measurements on isotropic ZnO microparticles (MPs), we ascertain that the unique NR position-specific Raman responses observed on ZnO NRs originate from their high shape anisotropy. Owing to the superior optical property coupled with reduced dimensionality and high geometric anisotropy, ZnO NRs have gained much attention recently for use in optoelectronic, photonic, and biosensor technologies. Raman scattering has been increasingly exploited as a noninvasive and sensitive analytical tool to investigate NR properties pertinent to these applications. Hence, our endeavors, explicitly providing the spatially distinct, polarized Raman scattering behaviors from individual ZnO NRs, will be central to the correct interpretation of Raman data of both the individual and ensemble NRs as well as to the accurate correlation of the measurement outcomes to their chemical/physical/optical properties. Our efforts may also promote novel applications for polarized Raman scattering whose optical outputs on the various positions along the ZnO NRs can be selectively modulated.

Tip-enhanced Raman spectroscopy for surfaces and interfaces

TERS offers the high spatial resolution to establish structure-function correlation for surfaces and interfaces.

Electromagnetic theories of surface-enhanced Raman spectroscopy

A fundamental theoretical understanding of SERS, and SERS hotspots, leads to new design principles for SERS substrates and new applications in nanomaterials and chemical analysis.

Single-Molecule Chemistry with Surface- and Tip-Enhanced Raman Spectroscopy

Single-molecule (SM) surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) have emerged as analytical techniques for characterizing molecular systems in nanoscale environments. SERS and TERS use plasmonically enhanced Raman scattering to characterize the chemical information on single molecules. Additionally, TERS can image single molecules with subnanometer spatial resolution. In this review, we cover the development and history of SERS and TERS, including the concept of SERS hot spots and the plasmonic nanostructures necessary for SM detection, the past and current methodologies for verifying SMSERS, and investigations into understanding the signal heterogeneities observed with SMSERS. Moving on to TERS, we cover tip fabrication and the physical origins of the subnanometer spatial resolution. Then, we highlight recent advances of SMSERS and TERS in fields such as electrochemistry, catalysis, and SM electronics, which all benefit from the vibrational characterization of single molecules. SMSERS and TERS provide new insights on molecular behavior that would otherwise be obscured in an ensemble-averaged measurement.

Simulation of fluorescence enhancement by an AFM tip on a gold particle quenched emitter

The local field enhancement in proximity of metallic nanostructures can strongly modify the excitation and emission behaviors for the nearby fluorophore. In this paper, Maxwell's time-dependent curl equations are solved by the finite-difference time-domain method to investigate the electric field enhancement around an atomic-force microscopy (AFM) tip and a Au nanosphere (NS). To lower the background fluorescence signal, we proposed to induce the fluorescence quenching by placing the emitter at an optimized position that is 2 nm away from the Au NS. The AFM tip is thereby moved to the vicinity of the emitter quenched by the Au NS. The fluorescence enhancement factor (FEF) increases rapidly when the tip approaches the Au NS. A maximum FEF of 1500-fold is obtained when their separation is 4 nm. By laterally scanning the tip over the Au NS at a constant height, the full width at half-maximum of fluorescence's signal peak with respect to tip position is around 20 nm. This high sensitivity of the FEF on the relative position of the tip and Au NS provides valuable information to guide future experiments on high-resolution optical imaging and fluorescence enhancement for high quantum yield emitters.

A Nanoplasmonic Strategy for Precision in-situ Measurements of Tip-enhanced Raman and Fluorescence Spectroscopy

We theoretically investigate an optimized tip-film system that supports in-situ measurement of tip-enhanced Raman spectroscopy (TERS) and tip-enhanced fluorescence (TEF) of dye molecules. A scanning tunneling microscope (STM) is proposed to precisely control the tip-film distance, and thus in-situ measurement of TERS and TEF can be realized utilizing the specific surface plasmon resonance (SPR) properties of the tip-film system. Our calculations show that the optimized tip-film distance of 2 nm suggests a possibility of efficient acquisition of TERS and TEF in-situ. The calculated spatial resolution of TERS and spectral resolution of TEF can be down to 6.5 nm and 10 nm, respectively. Our theoretical results may find promising application in developing multiple functional nano-spectroscopy through which Raman and fluorescence can be measured in-situ at the nanoscale level.

Rational fabrication of a gold-coated AFM TERS tip by pulsed electrodeposition

Reproducible fabrication of sharp gold- or silver-coated tips has become the bottleneck issue in tip-enhanced Raman spectroscopy, especially for atomic force microscopy (AFM)-based TERS. Herein, we developed a novel method based on pulsed electrodeposition to coat a thin gold layer over atomic force microscopy (AFM) tips to produce plasmonic TERS tips with high reproducibility. We systematically investigated the influence of the deposition potential and step time on the surface roughness and sharpness. This method allows the rational control of the radii of gold-coated TERS tips from a few to hundreds of nanometers, which allows us to systematically study the dependence of the TERS enhancement on the radius of the gold-coated AFM tip. The maximum TERS enhancement was achieved for the tip radius in the range of 60-75 nm in the gap mode. The coated gold layer has a strong adhesion with the silicon tip surface, which is highly stable in water, showing the great potential for application in the aqueous environment.

Tip-enhanced Raman spectroscopy: tip-related issues

After over 15 years of development, tip-enhanced Raman spectroscopy (TERS) is now facing a very important stage in its history. TERS offers high detection sensitivity down to single molecules and a high spatial resolution down to sub-nanometers, which make it an unprecedented nanoscale analytical technique offering molecular fingerprint information. The tip is the core element in TERS, as it is the only source through which to support the enhancement effect and provide the high spatial resolution. However, TERS suffers and will continue to suffer from the limited availability of TERS tips with a high enhancement, good stability, and high reproducibility. This review focuses on the tip-related issues in TERS. We first discuss the parameters that influence the enhancement and spatial resolution of TERS and the possibility to optimize the performance of a TERS system via an in-depth understanding of the enhancement mechanism. We then analyze the methods that have been developed for producing TERS tips, including vacuum-based deposition, electrochemical etching, electrodeposition, electroless deposition, and microfabrication, with discussion on the advantages and weaknesses of some important methods. We also tackle the issue of lifetime and protection protocols of TERS tips which are very important for the stability of a tip. Last, some fundamental problems and challenges are proposed, which should be addressed before this promising nanoscale characterization tool can exert its full potential. Graphical Abstract ᅟ.