Dynamic physicochemical nanoscale imaging at the solid-liquid interface

Raman spectroscopy is an invaluable analytical tool for materials characterisation, providing essential information on the structure, chemical composition and local environment of molecules using the diagnostic fingerprint that the vibrational spectrum uniquely delivers. It is applied almost ubiquitously across the engineering, physical and life sciences, enabling analysis of molecular materials in their native state (regardless of the physical state of matter or environment), in the absence of labels or preparation procedures, in a non-invasive and non-destructive fashion. It does, however, suffer from two significant limitations: (i) low sensitivity, a result of the weakness of the Raman effect, where very few incident photons can be harnessed to generate information on vibrational structure, and; (ii) spatial resolution limited by the laws of optical diffraction. This places fundamental restrictions on the breadth of materials that can be examined, and the absolute precision with which information can be obtained in those that can. Necessitated by the comprehension that the principle functions of materials are governed by characteristics and phenomena that arise in molecular structures at the nanoscale, significant developments in the technology of Raman spectroscopy was warranted and an innovative approach - termed tip-enhanced Raman spectroscopy (TERS) - was consequently established. In TERS, a scanning probe microscopy (SPM) tip is illuminated with a laser at the natural plasmon frequency of the noble metal nanoparticle that resides at the tip apex. This creates a high-intensity electromagnetic field in the immediate vicinity of the SPM tip, and as such provides a new mechanism for high-sensitivity imaging of molecular materials at the nanometre length scale. Yet, to date, and for reasons of prior technical inadequacies of commercial instrumentation, its application has been largely restricted to analysis of solid surfaces in air at standard conditions of temperature and pressure. Thus, whilst TERS has effectively solved the fundamental deficiencies inherent to Raman spectroscopy, it has essentially failed to translate the highly desirable aspects of the parent technique, thus placing a significant barrier to its application for important materials science discoveries.

To address this critical issue, we aim to pioneer an innovative nanoscale imaging capability, comprising optically-coupled SPM and Raman spectroscopy, and uniquely configured for the first time with dual optical access, multiple SPM functionalities and custom-made stages and liquid cells for environmental control. Designated DCI-TERS, our cutting-edge analytical platform will enable chemical fingerprint imaging of molecular materials significantly below the diffraction limit (<10 nm spatial resolution) for sampling both liquids and solids, with near-single-molecule-level sensitivity, along with 3D topographical analysis, acquired simultaneously from a single location (Tip-Enhanced Raman Spectroscopy). The incorporation of full performance SPM modes extends the breadth of surface physical characteristics obtainable from a single nanoscale volume (Correlative Imaging), whilst first-time provisions for environmental control stands to revolutionise in situ and operando investigations of chemical transformations at the gas-solid and liquid-solid interfaces, in response to light, heat and electrical potential (Dynamic Imaging). Thus, DCI-TERS represents an innovative methodology for temporally-resolved and location-correlated imaging of molecular materials and will deliver new fundamental knowledge on surface physicochemical (mechanical, electrical, thermal, structural compositional) properties under relevant conditions and at the nanoscale level, applicable to a broad spectrum of material research programmes, from drug delivery and medical devices to optoelectronics and batteries.