No abstract

Recent years have seen tremendous improvement of our understanding of high resolution reachable in TERS experiments, forcing us to re-evaluate our understanding of the intrinsic limits of this field, but also exposing several inconsistencies. On the one hand, more and more recent experimental results have provided us with clear indications of spatial resolutions down to a few nanometres or even on the subnanometre scale. Moreover, lessons learned from recent theoretical investigations clearly support such high resolutions, and vice versa the obvious theoretical impossibility to evade high resolution from a purely plasmonic point of view. On the other hand, most of the published TERS results still, to date, claim a resolution on the order of tens of nanometres that would be somehow limited by the tip apex, a statement well accepted for the past 2 decades. Overall, this now leads the field to a fundamental question: how can this divergence be justified? The answer to this question brings up an equally critical one: how can this gap be bridged? This review aims at raising a fundamental discussion related to the resolution limits of tip-enhanced Raman spectroscopy, at revisiting our comprehension of the factors limiting it both from a theoretical and an experimental point of view and at providing indications on how to move the field ahead. It is our belief that a much deeper understanding of the real accessible lateral resolution in TERS and the practical factors that limit them will simultaneously help us to fully explore the potential of this technique for studying nanoscale features in organic, inorganic and biological systems, and also to improve both the reproducibility and the accuracy of routine TERS studies. A significant improvement of our comprehension of the accessible resolution in TERS is thus critical for a broad audience, even in certain contexts where high resolution TERS is not the desired outcome.

The ability to characterize individual electrospun fibers is essential in order to understand and control this complex process. In this paper, we demonstrate that confocal Raman microscopy is a powerful method to quantify molecular orientation and structure at the individual fiber level using poly(ethylene terephthalate) as a model system. Highly reproducible polarized spectra with an excellent signal-to-noise ratio were measured in 1 min or less for fibers with a diameter as little as 500 nm. The orientation of smaller fibers can also be probed using a calibration procedure. Our results reveal a very broad distribution of molecular orientation and structure within the samples: some individual fibers are completely isotropic and amorphous while others present a ⟨P 2 ⟩ orientation parameter as large as 0.75. The development of this large orientation is accompanied by a gauche-to-trans structural conversion into the mesomorphous phase. Even the most highly oriented fibers only present a very small degree of crystallinity.

The physical properties of polymers are strongly affected by their molecular orientation. In this paper, we demonstrate for the first time a new and improved Raman spectroscopy method to characterize this key parameter. In recent years, Raman spectroscopy has emerged as an indispensable tool for this purpose, but its widespread use is still largely restricted by the experimental complexity and the limitations imposed by the standard quantification procedure, referred in this article as the depol constant (DC) method. We have very recently proposed and established theoretically a simplified quantification approach that is based on the most probable orientation distribution (MPD method). Herein, we demonstrate its experimental validity and its wide applicability by studying a series of samples from three highly dissimilar polymers (HDPE, PET, and PS), and covering the full possible orientation range. We show that the new MPD method overcomes the experimental and theoretical difficulties faced with the current DC method and that it leads to more accurate orientation values. We expect that this method will greatly extend the accessibility of Raman spectroscopy for molecular orientation studies of polymer systems.