Correlative force and tip-enhanced Raman microscopy

It is now possible to establish the chemical identity of a substance at the ultimate detection limit of a single molecule, i.e. the sensitivity required to probe 1.66 yoctomoles (1/NA), using surface-enhanced Raman scattering (SERS). It is also possible to image within an individual molecule, all while retaining chemical selectivity, using tip-enhanced Raman scattering (TERS). The potential applications of ultrasensitive SERS and TERS in chemical and biological detection and imaging are evident, and have attracted significant attention over the past decade. Rather than focusing on conventional single/few-molecule SERS and TERS experiments, where the objective is ultrasensitive spectroscopy and nanoscale chemical imaging, herein we consider greatly informative, yet significantly underrated, signatures of single molecules. Namely, we review recent efforts ultimately aimed at probing different aspects of a molecule’s local environment through a detailed analysis of its SERS and TERS spectra and images. Particular attention is devoted to local electric field imaging using TERS; we describe how the vector components and absolute magnitude of the local electric field may be inferred from molecular Raman signatures. We also propose experiments that can potentially be used to cross-check the insights gained from the described SERS and TERS measurements. The ultimate goal of this review is to demonstrate that there is much more to single-molecule Raman scattering than mere ultrasensitive chemical analysis.

Tip-enhanced Raman scattering (TERS)-based nanoimaging is gaining ever-increasing popularity for nanoscale spectroscopic imaging of 2D materials due to its much improved sensitivity and spatial resolution compared to conventional confocal Raman microscopy. In this work we report a successful application of TERS for spectroscopic imaging of the WS2–WSe2 vertical heterostructures transferred to gold. Our approach affords the unambiguous identification of the composition of vertical heterobi- and heterotrilayers. Collecting TERS maps of the same area with various excitation wavelengths showed that 671 and 785 nm excitations yield a strong enhancement of Raman bands for both WS2 and WSe2 sublayers in vertical heterostructures. On the other hand, 638 nm excitation seems insensitive to the WSe2 component in vertical heterobilayers. In this context, the use of a single wavelength for the TERS characterization of heterobilayers leads to wrong conclusions when it comes to the composition of this important class of low-dimensional quantum materials. Additionally, in TERS spectra recorded from monolayer WS2 collected with 671 nm excitation, we observe the appearance of a strong resonant band at 324 cm–1, which was absent in TERS spectra collected with 638 and 785 nm excitations. This observation further bolsters the importance of varied excitation wavelengths to match the electronic/excitonic resonances of 2D materials probed through TERS.

Step, ridge, and crack submicro/nanostructures of epitaxial graphene on 4H-SiC (0001̅) were characterized using tip-enhanced Raman scattering (TERS) spectroscopy. The nanostructures were created during graphene synthesis due to a difference in the thermal expansion coefficient of graphene and SiC. These structures are a distinctive property of epitaxial graphene, together with other desirable properties, such as large graphene sheet and minimal defects. The results of this study illustrate that the exceptional spatial resolution of TERS allows spectroscopic measurements of individual nanostructures, a feat which normal Raman spectroscopy is not capable of. By analyzing TERS spectra, the change of local strain on the nanoridge and decreased graphene content in the submicrometer crack were detected. Using G′ band positions in the TERS spectra, the strain difference between the ridge center and flat area was calculated to be 1.6 × 10–3 and 5.8 × 10–4 for uniaxial and biaxial strain, respectively. This confirms the proposed mechanism in previous researches that nanoridges on epitaxial graphene form as a relief against compressive strain. With this study, we demonstrate that TERS is a powerful technique for the characterization of individual local nanostructures on epitaxial graphene.

Reflection-mode tip-enhanced Raman scattering (TERS) has the advantage to characterize any sample, particularly opaque, bulk, and multilayered samples. However, the background signal in reflection-mode TERS is huge due to large focus spots associated with an objective lens that has a long working distance. Moreover, for a multilayered and bulk sample, the Raman signal from the bulk layer interferes with the Raman signal on a thin surface layer. This unwanted bulk background signal reduces the sensitivity of the measurement and makes it difficult to get a high-contrast TERS image in the reflection mode. Here, we demonstrate two techniques to suppress the far-field Raman signals coming from the focus area and bulk silicon germanium substrate. First, we reduce the far-field signal by controlling the polarization state of the incident and scattered Raman as well as manipulating the well-defined polarization of a crystalline sample, which strongly depends on the polarization and propagation of the incident light. We used Raman tensor analysis to quantify the far-field Raman intensity at different polarization states of the polarizer and analyzer at varying sample azimuth. The results of the surface-enhanced Raman spectroscopy experiments were in good agreement with the far-field Raman tensor calculation. The polarizer, analyzer, and sample azimuth combination that gives minimum far-field background signals with high contrast in SERS was utilized in the TERS experiment to obtain a high-contrast near-field Raman signal. Both the field enhancement effect and depolarized detection were considered to obtain a high signal-to-noise TERS signal. We found that, for Raman active and thin crystalline samples assembled in bulk materials, the depolarization effect outweighs the field enhancement effect in getting a high-contrast Raman signal.

Tip-enhanced Raman scattering (TERS) is a powerful near-field spectroscopic tool to measure Raman spectra of materials with nanometer spatial resolution and even with sensitivity down to single-molecule level [1]. The optical enhancement in TERS relies on lightning-rod effect and plasmonic coupling of input electric field into sharp metal tips. In general, the presence of strong electric field component along the tip axis (i.e., longitudinal field) is required for efficient coupling and several coupling schemes (e.g., use of radial polarization) have been suggested to enhance the interaction [1, 2].We propose a new approach for coupling light efficiently into the tip structures, which relies on coupling two metal tips together with a metal bridge. Due to the bridge, no strong longitudinal field is anymore needed for efficient coupling. The structures are manufactured using UV-nanoimprint lithography (UV-NIL) combined with electron-beam evaporation, which makes the technique fast, simple and relatively cheap [3]. In addition, large arrays of structures can be manufactured e.g. for lab-on-a-chip or optical tweezing applications.We measured Raman spectra from aqueous solution of crystal violet molecules using arrays of bridged and single gold and silver nanocones [4]. Almost ten-fold increase in the Raman signal for the bridged nanocones was observed, demonstrating the enhanced coupling of light.

Frequency-modulation atomic force microscopy (FMAFM) has proven to be a powerful method for imaging surfaces with true atomic resolution. However, the tip–sample forces are not directly accessible by FMAFM. Here, an algorithm to recover the tip–sample forces from the frequency shift curve is introduced and demonstrated with experimental data. Also, an intuititive connection between frequency shift Δf and tip–sample force gradient kts that simplifies the calculation of FMAFM images is established: Δf is a convolution of kts with a semispherical weight function.

While surface‐enhanced Raman scattering (SERS) is an established technique to significantly enhance the Raman signal from samples bound to metal surfaces, a quantitative interpretation is very difficult because of the rough and arbitrary nature of the substrate. A distinction between concentration and enhancement effects is virtually impossible. Tip‐enhanced Raman scattering (TERS), a scattering type of near‐field optical microscopy technique, circumvents the problem of random enhancement by concentrating the roughness to only a small region directly at the apex of a scanning probe tip. In this paper, TERS spectra of DNA pyrimidine bases have been collected and compared with standard SERS and Raman measurements of the nucleotides and the pure bases. The results are discussed in terms of electromagnetic and chemical effects of the SERS enhancement of the TERS probes. Copyright © 2006 John Wiley & Sons, Ltd.

Quartz tuning fork-based conductive atomic force microscope with glue-free solid metallic tips

Quartz tuning fork (QTF) sensors offer an attractive alternative to traditional silicon microcantilevers for sensing applications in dynamic atomic force microscopy (DAFM). The QTF sensor consists of two identical, weakly-coupled tines with a sharp tip affixed to the distal end of one tine. The fundamental anti-phase mode of the QTF achieves a stable resonant frequency with a high Quality factor making it ideal for DAFM applications in which a small shift in the resonant frequency is linked to a tip-sample force. The addition of the tip-sample force also breaks the symmetry of the QTF leading to a classic eigenvalue veering scenario. The eigenvalue veering and accompanying mode localization phenomena violate the standard DAFM modeling assumptions which treat the addition of the tip-sample force as a small perturbation to a single-degree-of-freedom oscillator. We find that the eigenvalue veering can contribute a systematic error in force measurements on the order of 20%. Methodology for correcting the systematic error caused by eigenvalue veering is proposed.

The energy spectra of single-wall carbon nanotubes (CNTs) with chiralities of (10, 10), (11, 9), and (12, 8) are calculated in the static fluctuation approximation for the Hubbard model. The choice of these systems for investigation was dictated by the fact that these systems are the most typical of heterogeneous samples obtained by synthesis. It is shown that in the chosen model and approximation, the type of conduction is independent of chirality, which contradicts the generally accepted opinion concerning the critical dependence of the CNT conduction current on the chirality indices. The optical absorption spectra of the CNTs under investigation are calculated using the resultant energy spectra. The shape of the optical absorption spectrum averaged over the known weight composition is in good agreement with experimental data. The results of investigation suggest that the rule according to which CNTs exhibit metal-type conductivity when the difference in the chirality indices is a multiple of three; otherwise, they have the semiconductor-type conductivity, is in all probability not general, but has applicability limits that can be established by rigorous calculations based on the Hubbard model.

This work describes how a frequency modulation atomic force microscope (AFM) using a hanging fiber force probe made from a quartz tuning fork provides local measurements on liquid-liquid interfaces. After detailing the manufacture and calibration of the force probe, we provide evidence that this AFM is suitable for quantitative measurements at the interface between two liquids. The repeatability of the measurements allows a poly-dimethylsiloxane-water moving interface to be monitored over several hours. The evaporation of a water droplet immersed in poly-dimethylsiloxane is observed, and its interfacial tension evolution over time is measured. Deformation of the interface is also observed. These capabilities, and preliminary results for the interface between two immiscible liquids, pave the way for interface manipulation and study of complex fluid-fluid interfaces.

Recent advances in Raman spectroscopy for characterizing graphene, graphite, and carbon nanotubes are reviewed comparatively. We first discuss the first-order and the double-resonance (DR) second-order Raman scattering mechanisms in graphene, which give rise to the most prominent Raman features. Then, we review phonon-softening phenomena in Raman spectra as a function of gate voltage, which is known as the Kohn anomaly. Finally, we review exciton-specific phenomena in the resonance Raman spectra of single-wall carbon nanotubes (SWNTs). Raman spectroscopy of SWNTs has been especially useful for understanding many fundamental properties of all sp2 carbons, given SWNTs can be either semiconducting or metallic depending on their geometric structure, which is denoted by two integers (n,m).

Quartz tuning forks (QTFs) are being more frequently adopted instead of micromachined cantilevers as force sensors in scanning probe microscopes due to their mechanical stability, high quality factors, and wide availability at low costs. Mass-produced QTFs are typically housed in vacuumsealed canisters, and this necessitates the removal of the vacuum canisters to make necessary modifications such as attaching a tip or an optical fiber. Commonly, the decrease in the quality factor after the removal of the canister is attributed to an increase in air damping with no further investigation of other possible causes. We carried out several experiments with QTFs before and after the canisters were removed. Despite a vacuum level below 10 −5 Torr, all investigated QTFs exhibited decreased quality factors once the protective canisters had been removed. We present possible causes for this observation by analyzing optical microscope images. Also, complete mechanical and electrical characterizations of the QTFs are presented, taking advantage of the analogies that exist between mechanical and electrical systems.

We report a sub-terahertz scattering-type scanning near-field microscope (sub-THz s-SNOM) which uses a 6 mm long metallic tip driven by a quartz tuning fork as the near-field probe. Under continuous-wave illumination by a 94 GHz Gunn diode oscillator, terahertz near-field images are obtained by demodulating the scattered wave at both the fundamental and the second harmonic of the tuning fork oscillation frequency together with the atomic-force-microscope (AFM) image. The terahertz near-field image of a gold grating with a period of 2.3 µm obtained at the fundamental modulation frequency agrees well with the AFM image. The experimental relationship between the signal demodulated at the fundamental frequency and the tip-sample distance is well fitted with the coupled dipole model indicating that the scattered signal from the long probe is mainly contributed by the near-field interaction between the tip and the sample. This near-filed probe scheme using quartz tuning fork can adjust the tip length flexibly to match the wavelength over the entire terahertz frequency range and allows for operation in cryogenic environment.

The Raman spectra of single-walled carbon nanotubes (SWNTs) produced by the catalytic decomposition of hydrocarbons have been measured in the range of 100–7500 cm−1. The tangential C–C stretching modes show an unusual resonant enhancement process that results from the one-dimensional quantum confinement of the electrons in the SWNT, and its intensity of the tangential C–C stretching mode is about 100 times as strong as that of highly oriented pyrolytic graphite. Because of the resonant enhancement, high-order Raman bands up to fifth order have been revealed and assigned to the corresponding fundamentals.