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Correlating the 3D atomic structure of metal anisotropic nanoparticles with their optical properties

Periodic Reporting for period 1 - SOPMEN (Correlating the 3D atomic structure of metal anisotropic nanoparticles with their optical properties)

Período documentado: 2018-06-01 hasta 2020-05-31

The interaction of materials with light is of fundamental importance in a variety of applications such as photovoltaics or photocatalysis. Among promising materials for efficient light interaction are metal nanoparticles (NPs) due to their localized surface plasmon resonances (LSPRs), which cause strong light absorption and scattering, a phenomenon that is exploited in many fields ranging from physics to biology and medicine. The wavelength and strength at which the interaction happens can be tuned by changing the shape, size and metal of the NPs. In particular, anisotropic shapes enhance the interaction and are thus intriguing systems to study. Next to NPs with only one material, NPs with two metals, i.e. bimetallic NPs, offer an additional way of tuning the functionality and LSPR resonance.

Because the parameter space is large, understanding the delicate interplay between particle morphology, composition and optical properties is of utmost importance in optimizing particle design for the desired applications. Optical properties can be measured by e.g. luminescence or scattering measurements using lasers or white light lamps as excitation sources but also by using high energetic electrons in electron energy loss spectroscopy. The connection to their structure is often done by scanning electron microscopy or transmission electron microscopy (TEM) performed on similar particles from the same sample batch. However, for a full understanding of how the morphology and composition of a metal NP is related to its optical properties, the characterization of optical and structural properties needs to happen on the same single NP. Moreover, electron microscopy generally yields 2D information but for complex anisotropic NPs, a 2D projection does not yield enough information on the morphology.

To overcome these challenges, the overall objective of this project is to perform optical and TEM measurements on the same single monometallic and bimetallic NPs to directly correlate the composition, size and morphology to their optical properties. As the focus lies on anisotropic NPs, electron tomography is used to obtain the full 3D structure of the NP, down to atomic resolution. With that methodology, three key questions in nanoscience are tackled: 1) In how far does the deviation from a perfect atomic crystal influence the optical properties of metal NPs? 2) How does the 3D morphology and composition of anisotropic bimetallic NPs dictate their optical properties? 3) What are the differences between exciting metal NPs by light or electrons?
The work was divided in three work packages (WPs) to tackle the three questions above. WP 1 was aimed at measuring the influence of crystal defects in metal NPs on their optical properties. Specifically, the optical properties of single metal NPs as well as their corresponding 3D geometry and internal crystal structure were determined for the same NP. WP 2 focused on relating the 3D composition and morphology of bimetallic NPs to their optical properties. For that, the fellow performed correlative measurements on a variety of different bimetallic NPs with different shapes and compositions. In addition, simulations were performed to explore a large parameter space. WP 3 was designed to compare the optical properties of metallic NPs when excited by light or the electron beam. In this WP, the fellow measured optically excited luminescence and electron energy loss spectra on the same NPs.

The work in the described WPs was performed in two different countries: in my host institution (EMAT at the University of Antwerp, Belgium), where all electron microscopy experiments were performed, and in my seconding institution (MoNOS, Leiden University, The Netherlands). Working in two groups was essential in achieving the challenging objectives in this proposal but it also led to efficient knowledge transfer between the groups. Moreover, the research resulted in more exciting international collaborations bridging chemistry and physics group.

The work performed throughout the project resulted in numerous publications, in which the research is reported. In total, the work will bring forward 13 publications to share the knowledge with peers all over the world. 6 of those manuscripts are already published, 4 are currently under peer review and 3 more papers are forthcoming. In addition, the work was presented at 7 international conferences with 6 of these presentations being invited talks, where the fellow was invited by the organizers to present the work. Moreover, the fellow co-organized and contributed by a lecture and practical sessions to the 2019 EMAT Workshop on Transmission Electron Microscopy to share practical knowledge with peers in the electron microscopy community. In addition, the research was present to a more general audience during the 2019 Faculty Research Day of the University of Antwerp, the participation in the 2018 Flemish Day of Science and the 2018 Pint of Science in Antwerp.
The results of this projects have pushed the frontiers of nanoscience further. In WP 1, it was discovered that a single crystal lattice defect in a metal NP can significantly broaden the LSPR of the particle. Such a broadening leads to a decrease in the plasmonic light interaction and needs to be taken into account for optoelectronic applications. Moreover, the fellow found out how laser excitation of single NPs can lead to deformation and restructuring of surface facets, the exact nature of which is crucial in several applications like photocatalysis.

For WP 2, the fellow worked closely together with chemistry groups performing state-of-the-art synthesis of bimetallic NPs. On the one hand, the morphology of a Ag coating grown on a Au NP was seen to determine the plasmonic properties with enhanced LSPR resonances for increasing Ag amount. On the other hand, the addition of Pd led to a decrease in the plasmonic properties but could significantly enhance the thermal stability of the NP as shown in the attached figure. While better plasmonic properties are directly relevant in sensing applications, the enhanced thermal stability is important in (photo)catalysis and photothermal applications such as localized hyperthermic cancer treatment. Moreover, the WP produced a technique to study the 3D composition and shape of NPs in a much faster way than generally done. It was demonstrated that this technique can be used to follow local changes in composition upon external stimuli in a single bimetallic NP. The developed method will have a long-term impact in the field of nanoscience.

For WP 3, I performed correlative measurements on the same NPs, which gave rise to intriguing insights into plasmonic resonances and into the correlation of optically and electronically excited LSPRs as well as quantum phenomena in closely located NPs. While the results are fundamentally interesting from a physics point of view, the unique skill set that the fellow acquired during the MSCA fellowship and specifically for this WP will be very useful in future personal endeavors. This new approach will be beneficial for fellow researchers to explore properties of nanomaterials with a large amount of details, and hence going beyond the sample-averaged structure-property correlation.
Plasmonic properties of AuPd octopods.