Nanoscale Analysis

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Tip-enhanced Raman spectroscopy (TERS) is a technique for optical spectroscopy and chemical imaging of surfaces on the nanoscale.  It combines the spatial resolution of scanning probe microscopy with the chemical selectivity of Raman spectroscopy. Our team is focusing on the further development of the technique, with the aim to turn it into a versatile analytical tool for nanoscience. [NOTE: this is NOT nanomaterial/nanoparticle based analytical chemistry]

Raman spectroscopy is a powerful analytical technique providing detailed vibrational fingerprints from the molecules under investigation. However, Raman scattering is relatively weak compared to, e.g., fluorescence. Surface-enhanced Raman scattering (SERS) has helped to overcome this problem, offering signal enhancements of several orders of magnitude over conventional Raman scattering. However, the (often random) heterogeneity of SERS substrates leads to high variations in electromagnetic field enhancement across the sample surface, which limits its range of application. Reducing the enhancing substrate to a single local 'hot spot' at the end of a very sharp tip, which can be accurately positioned on the sample surface by a scanning probe microscope (SPM), circumvents this issue. This approach is called tip-enhanced Raman spectroscopy (TERS) and was pioneered in our laboratory. It gives the possibility to perform spectroscopic imaging with sub-diffraction spatial resolution (<50 nm) by scanning the local 'hot spot' at the end of the tip across the sample surface.

A typical TERS instrument working principle combines a confocal laser scanning microscope (CLSM) with a scanning probe microscope. The confocal microscope allows for rapid optical alignment of the laser focus with a customized SPM tip and requires a movable sample stage to scan the sample relative to the prealigned laser-to-tip configuration. Systems can be based on an inverted or upright microscope (both available in our laboratory, see picture section) and in most cases the same objective is used for illumination and detection. Figure 1 schematically shows the basic working principle of a TERS setup combining an inverted Raman microscope with an atomic fore microscope (AFM).

TERS working principle with a microscope objective underneath the sample. A modified (e.g. silver coated) AFM tip enhances the signal.  
Figure 1: Basic working principle of a tip-enhanced Raman spectroscopy (TERS) setup based on an inverted Raman microscope and an atomic force microscope (AFM). (a) No enhanced Raman scattering (b) Enhanced Raman scattering at the tip apex

The general aim of our research is to turn TERS into a versatile analytical technique for surface science, with a particular emphasis on reproducibility of TERS imaging.  It is difficult to acquire high-resolution ambient-TERS maps due to technical obstacles, including the fragility and reactivity of TERS probes.  We focus on tackling those challenges, and perform stable TERS imaging of new materials, like individual peptide nanotapes shown in figure 2.

TERS results with high resolution. smaller than 250 nm fibril nanotapes are shown  
Figure 2: STM (a) and TERS (b,c) images of individual beta-amyloid(1-40) fibril nanotapes. Nanotapes 1 and 2, the circle and the square are markers for differences between the TERS and the STM images (structural and/or chemical heterogeneity)

Current Scientific Projects

TERS imaging of molecular thin films and nanostructured surfaces. We pursue reproducible TERS imaging on a broad range of substrates, including self-assembled monolayers, 2D polymers, supported lipid layers, carbon nanotubes, photovoltaic materials, DNA, amyloid fibrils and more.  Our aim is to turn TERS imaging into a versatile technique, applicable for routine characterization of molecular thin films and nanostructured surfaces.

Delicate 2-dimensional molecular systems. We aim, for the first time, to monitor nanoscale defects and heterogeneities in delicate 2-dimensional layers, for example lipid rafts in native membranes, which have never been observed directly. Sensitive 2D structures such as synthetic 2D-polymers and biological membranes and their components (lipids, proteins, glycans) are notoriously difficult to study by TERS, because extended laser irradiation and intense local fields lead to their decomposition. Conducting TERS in a pulsed fashion, carrying out TERS experiments in liquids (the natural environment for membranes) and using protected tips are the proposed key methodological innovations. We are looking for several PhD students, one post-doctoral researcher and one senior scientist (see open positions).

Protection and functionalization of TERS tips. The quality and durability of the metal or metallized tips are often key to a successful TERS experiment. We develop methods to protect TERS tips from chemical degradation, mechanical wear and unwanted adsorbates. Moreover, functionalized tips are used to probe specific interaction with the analyte.

Spectral features of TERS. TER spectra are often comprised of fewer bands than the corresponding confocal Raman spectra.  Examples include the amide I mode in peptides, or the base stacking mode in DNA.  We systematically study the spectroscopic information delivered by TERS, to understand it better and facilitate its comparison with data from confocal Raman spectroscopy and SERS.

New scanning modes for TERS. The speed of TERS mapping and the available scan range are limited by the small step size between the pixels, and by the dwell time for acquisition of a spectrum at each individual pixel.  Those limitations hamper the application of TERS as a standard tool for spectroscopic imaging of surfaces.  We work on the development of new scanning modes for TERS, enabling a trade-off between image size and spatial resolution.

Staff: Feng Shao, Jacek Szczerbinski, Liqing ZhengDr. Ewelina Wioletta Lipiec

 
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26.06.2017
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