Simultaneous topographical, electrical and optical microscopy of optoelectronic devices at the nanoscale

Visualizing nanoscale heterogeneity in perylene thin films via tip-enhanced photoluminescence with unsupervised machine learning

We investigate the properties of ultrathin 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) films using a combination of tip-enhanced photoluminescence and unsupervised machine learning. We expose nanoscale spectral heterogeneities that can be understood on the basis of the interplay between vibronic effects, intermolecular excitons, and intramolecular excitons in PTDCI films.

Nanoscale Chemical Analysis of Thin Film Solar Cell Interfaces Using Tip-Enhanced Raman Spectroscopy

Interfacial regions play a key role in determining the overall power conversion efficiency of thin film solar cells. However, the nanoscale investigation of thin film interfaces using conventional analytical tools is challenging due to a lack of required sensitivity and spatial resolution. Here, we surmount these obstacles using tip-enhanced Raman spectroscopy (TERS) and apply it to investigate the absorber (Sb2Se3) and buffer (CdS) layers interface in a Sb2Se3-based thin film solar cell. Hyperspectral TERS imaging with 10 nm spatial resolution reveals that the investigated interface between the absorber and buffer layers is far from uniform, as TERS analysis detects an intermixing of chemical compounds instead of a sharp demarcation between the CdS and Sb2Se3 layers. Intriguingly, this interface, comprising both Sb2Se3 and CdS compounds, exhibits an unexpectedly large thickness of 295 ± 70 nm attributable to the roughness of the Sb2Se3 layer. Furthermore, TERS measurements provide compelling evidence of CdS penetration into the Sb2Se3 layer, likely resulting from unwanted reactions on the absorber surface during chemical bath deposition. Notably, the coexistence of ZnO, which serves as the uppermost conducting layer, and CdS within the Sb2Se3-rich region has been experimentally confirmed for the first time. This study underscores TERS as a promising nanoscale technique to investigate thin film inorganic solar cell interfaces, offering novel insights into intricate interface structures and compound intermixing.

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.

Nanoscale chemical analysis of 2D molecular materials using tip-enhanced Raman spectroscopy

Two-dimensional (2D) molecular materials have attracted immense attention due to their unique properties, promising a wide range of exciting applications. To understand the structure-property relationship of these low-dimensional materials, sensitive analytical tools capable of providing structural and chemical characterisation at the nanoscale are required. However, most conventional analytical techniques fail to meet this challenge, especially in a label-free and non-destructive manner under ambient conditions. In the last two decades, tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful analytical technique for nanoscale chemical characterisation by combining the high spatial resolution of scanning probe microscopy and the chemical sensitivity and specificity of surface-enhanced Raman spectroscopy. In this review article, we provide an overview of the application of TERS for nanoscale chemical analysis of 2D molecular materials, including 2D polymers, biomimetic lipid membranes, biological cell membranes, and 2D reactive systems. The progress in the structural and chemical characterisation of these 2D materials is demonstrated with key examples from our as well as other laboratories. We highlight the unique information that TERS can provide as well as point out the common pitfalls in experimental work and data interpretation and the possible ways of averting them.

Nanoscale Chemical Imaging of Human Cell Membranes Using Tip-Enhanced Raman Spectroscopy

Lack of appropriate tools for visualizing cell membrane molecules at the nanoscale in a non-invasive and label-free fashion limits our understanding of many vital cellular processes. Here, we use tip-enhanced Raman spectroscopy (TERS) to visualize the molecular distribution in pancreatic cancer cell (BxPC-3) membranes in ambient conditions without labelling, with a spatial resolution down to ca. 2.5 nm. TERS imaging reveals segregation of phenylalanine-, histidine-, phosphatidylcholine-, protein-, and cholesterol-rich BxPC-3 cell membrane domains at the nm length-scale. TERS imaging also showed a cell membrane region where cholesterol is mixed with protein. Interestingly, the higher resolution TERS imaging revealed that the molecular domains observed on the BxPC-3 cell membrane are not chemically "pure" but also contain other biomolecules. These results demonstrate the potential of TERS for non-destructive and label-free imaging of cell membranes with nanoscale resolution.

Tip-enhanced Raman spectroscopy with amplitude-controlled tapping-mode AFM

Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for analyzing chemical compositions at the nanoscale owing to near-field light localized at a metallic tip. In TERS, atomic force microscopy (AFM) is commonly used for tip position control. AFM is often controlled under the contact mode for TERS, whereas the tapping mode, which is another major operation mode, has not often been employed despite several advantages, such as low sample damage. One of the reasons is the low TERS signal intensity because the tip is mostly away from the sample during the tapping motion. In this study, we quantitatively investigated the effect of the tapping amplitude on the TERS signal. We numerically evaluated the dependence of the TERS signal on tapping amplitude. We found that the tapping amplitude had a significant effect on the TERS signal, and an acceptable level of TERS signal was obtained by reducing the amplitude to a few nanometers. We further demonstrated amplitude-controlled tapping-mode TERS measurement. We observed a strong dependence of the TERS intensity on the tapping amplitude, which is in agreement with our numerical calculations. This practical but essential study encourages the use of the tapping mode for further advancing TERS and related optical techniques.

Ultrastable tip-enhanced hyperspectral optical nanoimaging for defect analysis of large-sized WS2 layers

Optical nanoimaging techniques, such as tip-enhanced Raman spectroscopy (TERS), are nowadays indispensable for chemical and optical characterization in the entire field of nanotechnology and have been extensively used for various applications, such as visualization of nanoscale defects in two-dimensional (2D) materials. However, it is still challenging to investigate micrometer-sized sample with nanoscale spatial resolution because of severe limitation of measurement time due to drift of the experimental system. Here, we achieved long-duration TERS imaging of a micrometer-sized WS₂ sample for 6 hours in a reproducible manner. Our ultrastable TERS system enabled to reveal the defect density on the surface of tungsten disulfide layers in large area equivalent to the device scale. It also helped us to detect rare defect-related optical signals from the sample. The present study paves ways to evaluate nanoscale defects of 2D materials in large area and to unveil remarkable optical and chemical properties of large-sized nanostructured materials.

Visualizing Surface Phase Separation in PS-PMMA Polymer Blends at the Nanoscale

Phase-separated polymer blend films are an important class of functional materials with numerous technological applications in solar cells, catalysis, and biotechnology. These technologies are underpinned by the precise control of phase separation at the nanometer length-scales, which is highly challenging to visualize using conventional analytical tools. Herein, we introduce tip-enhanced Raman spectroscopy (TERS), in combination with atomic force microscopy (AFM), confocal Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), as a sensitive nanoanalytical method to determine lateral and vertical phase-separation in polystyrene (PS)-poly(methyl methacrylate) (PMMA) polymer blend films. Correlative topographical, molecular, and elemental information reveals a vertical phase separation of the polymers within the top ca. 20 nm of the blend surface in addition to the lateral phase separation in the bulk. Furthermore, complementary TERS and XPS measurements reveal the presence of PMMA within 9.2 nm of the surface and PS at the subsurface of the polymer blend. This fundamental work establishes TERS as a powerful analytical tool for surface characterization of this important class of polymers at nanometer length scales.

Probing coke formation during the methanol-to-hydrocarbon reaction on zeolite ZSM-5 catalyst at the nanoscale using tip-enhanced fluorescence microscopy

The deactivation mechanism of the widely used zeolite ZSM-5 catalysts remains unclear to date due to the lack of analytical techniques with sufficient sensitivity and/or spatial resolution. Herein, a combination of hyperspectral confocal fluorescence microscopy (CFM) and tip-enhanced fluorescence (TEFL) microscopy is used to study the formation of different coke (precursor) species involved in the deactivation of zeolite ZSM-5 during the methanol-to-hydrocarbon (MTH) reaction. CFM submicron-scale imaging shows a preferential formation of graphite-like coke species at the edges of zeolite ZSM-5 crystals within 10 min of the MTH reaction (i.e., working catalyst), whilst the amount of graphite-like coke species uniformly increased over the entire zeolite ZSM-5 surface after 90 min (i.e., deactivated catalyst). Furthermore, TEFL nanoscale imaging with ∼35 nm spatial resolution revealed that formation of coke species on the zeolite ZSM-5 surface is non-uniform and a relatively larger amount of coke is formed at the crystal steps, indicating a higher initial catalytic activity.

Inhomogeneities in coke formation during methanol-to-hydrocarbon reaction on the zeolite ZSM-5 catalyst are imaged with ∼35 nm spatial resolution using tip-enhanced fluorescence microscopy.

Molecular Perturbation Effects in AFM-Based Tip-Enhanced Raman Spectroscopy: Contact versus Tapping Mode

Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for nondestructive and label-free surface chemical characterization at nanometer length scales. However, despite being considered nondestructive, the interaction of the TERS probe used in the analysis can alter the molecular organization of the sample. In this study, we investigate the role of the atomic force microscopy (AFM) feedback (contact mode and tapping mode) on molecular perturbation in TERS analysis of soft samples using a self-assembled monolayer (SAM) of 2-chloro-4-nitrobenzene-1-thiol (Cl-NBT) as a test sample. Surprisingly, the tapping mode shows a consistently higher TERS signal resulting from a minimal perturbation of the Cl-NBT SAM compared to the contact mode. This study provides novel insights into the choice of the correct AFM-TERS operation mode for nanoscale chemical analysis of soft and delicate samples and is expected to expedite the growing application of TERS in this area.

Nanoscale characterization of plasma functionalized graphitic flakes using tip-enhanced Raman spectroscopy

The chemical functionalization of graphene nanomaterials allows for the enhancement of their properties for novel functional applications. However, a better understanding of the functionalization process by determining the amount and location of functional groups within individual graphene nanoplatelets remains challenging. In this work, we demonstrate the capability of tip-enhanced Raman spectroscopy (TERS) to investigate the degree and spatial variability of the appearance of disorder in graphitic nanomaterials on the nanoscale with three different levels of nitrogen functionalization. TERS results are in excellent agreement with those of confocal Raman spectroscopy and chemical analysis, determined using x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, of the functionalized materials. This work paves the way for a better understanding of the functionalization of graphene and graphitic nanomaterials at the nano-scale, micro-scale, and macro-scale and the relationship between the techniques and how they relate to the changes in material properties of industrial importance.

In Situ Nanoscale Investigation of Catalytic Reactions in the Liquid Phase Using Zirconia-Protected Tip-Enhanced Raman Spectroscopy Probes

Tip-enhanced Raman spectroscopy (TERS) is a promising technique that enables nondestructive and label-free topographical and chemical imaging at the nanoscale. However, its scope for in situ characterization of catalytic reactions in the liquid phase has remained limited due to the lack of durable and chemically inert plasmonically active TERS probes. Herein, we present novel zirconia-protected TERS probes with 3 orders of magnitude increase in lifetime under ambient conditions compared to unprotected silver-coated probes, together with high stability in liquid media. Employing the plasmon-assisted oxidation of p-aminothiophenol as a model reaction, we demonstrate that the highly robust, durable, and chemically inert zirconia-protected TERS probes can be successfully used for nanoscale spatially resolved characterization of a photocatalytic reaction within an aqueous environment. The reported improved lifetime and stability of probes in a liquid environment extend the potential scope of TERS as a nanoanalytical tool not only to heterogeneous catalysis but also to a range of scientific disciplines in which dynamic solid-liquid interfaces play a defining role.

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.

Nanoscale Chemical Imaging of a Single Catalyst Particle with Tip-Enhanced Fluorescence Microscopy

Determining the active site in real-life solid catalysts remains an intellectual challenge and is crucial for exploring the road towards their rational design. In recent years various micro-spectroscopic methods have revealed valuable structure-activity data at the level of a single catalyst particle, even under reaction conditions. Herein, we introduce Tip-Enhanced FLuorescence (TEFL) microscopy as a novel and versatile characterization tool for catalysis research. This has been achieved using a Fluid Catalytic Cracking (FCC) catalyst as showcase material. Thin sectioning of industrially used FCC particles together with selective staining of Brønsted acidity has enabled high-resolution TEFL mapping of different catalyst regions. Hyperspectral information gained via TEFL microscopy reveals a spatial distribution of Brønsted acidity within individual zeolite domains in different regions of the FCC catalyst particle. Comparison of TEFL measurements from different FCC particles showed significant intra- and inter-particle heterogeneities both in zeolite domain size and chemical reactivity.

Nanoscale chemical imaging of solid-liquid interfaces using tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for non-destructive and label-free surface molecular mapping at the nanoscale. However, to date nanoscale resolution chemical imaging in a liquid environment has not been possible, in part due to the lack of robust TERS probes that are stable when immersed in a liquid. In this work, we have addressed this challenge by developing plasmonically-active TERS probes with a multilayer metal coating structure that can be successfully used within a liquid environment. Using these novel TERS probes, we have compared the plasmonic enhancement of TERS signals in air and water environments for both gap mode and non-gap mode configurations and show that in both cases the plasmonic enhancement decreases in water. To better understand the signal attenuation in water, we have performed numerical simulations that revealed a negative correlation between the electric field enhancement at the TERS probe-apex and the refractive index of the surrounding medium. Finally, using these robust probes we demonstrate TERS imaging with nanoscale spatial resolution in a water environment for the first time by employing single-wall carbon nanotubes as a model sample. Our findings are expected to broaden the scope of TERS to a range of scientific disciplines in which nanostructured solid-liquid interfaces play a key role.

Nanoscale mapping of newly-synthesised phospholipid molecules in a biological cell using tip-enhanced Raman spectroscopy

Non-destructive & label-free nanoscale mapping of newly-synthesised phospholipid molecules inside a biological cell is demonstrated using tip-enhanced Raman spectroscopy for the first time.