In recent years a significant interest has been developed in the study of the optical properties of noble metallic nanoparticles (NPs) due to their applications in photonics, electronics and biosensors. As matter of fact, it is possible to control their optical response, for instance the surface plasmon resonances (SPRs). It is well known, that particle size, shape and composition, as well as the local dielectric environment of NPs have strong impact on these SPR. Spatial-resolved electron energy loss spectroscopy (SR-EELS) is a very appropriate and powerful technique for providing very rich information of the optical properties, at the (sub-)nanometer level. Indeed, the improvements developed the last two decades in transmission electron microscopes, allowing ~ 100 meV energy resolutions have offered the possibility to expand this kind of these studies.
One of the studies that we have carried out on this topic concerns the investigation of bimetallic nanostructures, which offer the possibility of tuning the plasmonic response. We have combined those SR-EELS studies with other TEM works using different techniques as electron tomography (Coll. O. Ersen (Strasbourg U.-CNRS, Strasbourg, Fr.), high-resolution scanning TEM (HRSTEM) and EDS, see Fig. 1. The combination of all these studies provided a deep knowledge of the synthesized NP at the sub-nanometer (atomic) scale. We concluded that there are two different kind of NP having the same bipyramidal shape: pure gold and Au-Ag core-shell NP. In the case of the latest ones, the shell of Ag could only be few nanometers thick, Fig. 1.
Once the morphology and composition of the NP have been fully characterized, we turned to the study of their optical properties via EELS measurements in the low loss region (below 50 eV). The SPR were excited by a monochromated electron beam, using the STEM-EELS spectrum-image (SPIM) acquisition mode. Figure 2 depicts the EELS analyses of the SPR modes (0 – 3.5 eV) carried out for pure gold and Au-Ag core-shell bipyramids having the same size. Typical EEL spectra (sum of 16 spectra) are plotted in Figures 2 (a) and (d). They have been extracted from different areas of the 2 SPIMs (Figure 2 (c) and (f)) as marked on the HAADF images (Figure 2 (b) and (e)). We have interpreted all these modes helped by discrete dipolar approximation calculations (coll. L. Henrard (U. Namur, Belgium)). In addition we have found interesting phenomena related to the composition and nature of these nano-objects.
On the other hand, we have also studied the local dielectric/optoelectronic properties of other nano-materials as for instance N-doped C-NT by EELS. In order to deeply investigate these properties, we will combine these studies with very detailed analysis of the atomic configuration, spatial distribution and concentration of dopants via spatial-resolved EELS, see Fig. 3. These N-doped nanotubes (CNx-NT) present different morphologies as a function of N concentration and configuration [I. Florea, O. Ersen, R. Arenal, et al., J. Amer. Chem. Soc., 134, 9672 (2012).]. Thus, we have investigated the local dielectric properties considering these aspects that we have also analyzed in parallel. In this sense, Fig. 3 (A-D) correspond to the chemical and structural analysis of a multi-walled CNx-NT. Fig. 3 (E) shows the low-loss EELS studies developed on this NT. The energy shifts of the different modes observed in this graph are related to these aspects as well as the acquisition geometry, the thickness, the structural defects… All these results have been compared to works carried out in pure C-NT under the same conditions. Thus, the present study improves our knowledge of the dielectric/optoelectronic properties of CNx-NT and provides further insight into the potential applications of these materials.
R. Arenal, O. Stephan, M. Kociak, D. Taverna, A. Loiseau, C. Colliex, Electron Energy Loss Spectroscopy Measurement of the Optical Gaps on Individual Boron Nitride Single-Walled and Multiwalled Nanotubes, Phys. Rev. Lett. 95, 127601 (2005).
R. Arenal, O. Stephan, M. Kociak, D. Taverna, A. Loiseau, C. Colliex, Optical gap measurements of Boron Nitride NTs by EELS, Micros. and Microanalysis 11, 274 (2008).
R. Arenal, Low-Loss Measurements on metallic and insulating nanostructures using a monochromatic electron beam, Micros. and Microanalysis 17, 768 (2011).
R. Arenal, L. Henrard, L. Roiban, O. Ersen, J. Burgin, M. Treguer-Delapierre, Local Plasmonic Studies on Individual Core-Shell Gold-Silver and Pure Gold Nano-Bipyramids, submitted.
R. Arenal, Spectroscopic Studies on Nitrogen-doped Multi-Walled Carbon Nanotubes Using Monochromated STEM-EELS at Low-Voltage, Micros. and Microanalysis 19, 1230 (2013).