Nanoplasmonic Spectroscopy for Materials and Surface Science

Nanomaterials bring along fascinating technological opportunities due to properties and functionalities that are uniquely different from their corresponding bulk counterparts. For example, the typical size of a nanomaterial is of the same scale as the critical size for numerous physical phenomena.

As a consequence, electronic, magnetic, optical, chemical, and biological processes become often nanomaterial size/thickness and shape dependent. To understand and efficiently utilize these materials and the novel properties present at the nanoscale is a prime driving force of nanoscience and nanotechnology in general, and of nanomaterials science in particular. Therefore the development of efficient experimental in situ probes to help understand, develop and utilize the functionality of nanomaterials is of high priority but also a major challenge in this area.

This project focuses on the development and exploitation of nanoplasmonic sensing principles to probe a specific process in a functional nanomaterial. The key benefits of using localized surface plasmon resonances (LSPRs) as in situ probes fur this purpose include the flexibility of the utilized sensing principles in terms of applicability to very different material systems, and the simplicity of the necessary optical and general experimental “hardware” (usually simple small measurement cells to control the sample environment are enough) in combination with high local sensitivity at the nanoscale. Moreover, LSPR-based probing of materials employs low-power optical readout, which basically makes it non-invasive with minimal impact on the studied processes.

The “workhorse” in this project is our Indirect Nanoplasmonic Sensing (INPS) platform that is schematically illustrated in Figure 1. It is comprised of an amorphous array of non-interacting, identical Gold (Au) nanodisks on a glass support. These plasmonic Au nanosensors are covered with a thin (few tens of nm) film of a dielectric material, which renders the INPS platform very versatile. The dielectric spacer layer can i) exert a purely protective function, may ii) be used to provide a desired surface chemistry, and in that role iii) be either an inert substrate for the nanomaterials to be studied, or iv) participate actively in the process under study, e.g in spillover effects during a catalytic reaction. We are using the INPS platform to study processes like catalyst nanoparticle sintering, particle size-dependent catalytic activity, hydride formation, nanoparticle oxidation and reduction under various conditions, diffusion processes in thin films and mesoporous structures for, e.g., dye sensitized solar cell applications.

One of the many exciting prospects of LSPR-based in situ spectroscopy of functional nanomaterials is the possibility to probe single functional entities. The latter relates to an inherent problem in nanomaterials science, namely unwanted “artifacts” in the response signal and averaged responses due to inhomogeneous sample material. Such effects are always present in data obtained from nanoparticle ensembles and/or due to the lack of sensitivity or spatial resolution of the used probes, and they are mainly caused by heterogeneous size-distributions, differences in the local chemistry and structure of nano-entities in the ensemble, as well as local temperature or mass-transport gradients in large sample volumes. Thus they can hide crucial information linked to the individual characteristics of the studied objects. Single particle INPS (Figure 3) facilitates probing of single functional nanoparticles and is thus expected to alleviate significantly the aforementioned detrimental inhomogeneous sample material related effects in nanomaterials science. Therefore we pursue increasing research efforts in this direction.