LMA

Plasmonic and opto-electronic properties of materials

  Plasmonic and opto-electronic properties of materials

 

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.
Figure 1: Low magnification (LM)-HAADF image of one of the Au bipyramids. (b) EDS spectra recorded at different positions marked in Figure 1 (a). (c) Low magnification HAADF image of an Au-Ag core-shell bipyramidal nanoparticle. (d) EDS spectra recorded in two different regions marked on the LM-HAADF image. (e) 3D model of the nanoparticle obtained by a classical data segmentation procedure from a reconstructed HAADF-STEM tilt series, Au in purple and Ag in grey, respectively. (f) A slice through the XY plane of the chemical sensitive volume showing the Au (red) and Ag (green) distribution at the basis of the pyramid. (R. Arenal, et al., submitted).

Figure 1: Low magnification (LM)-HAADF image of one of the Au bipyramids. (b) EDS spectra recorded at different positions marked in Figure 1 (a). (c) Low magnification HAADF image of an Au-Ag core-shell bipyramidal nanoparticle. (d) EDS spectra recorded in two different regions marked on the LM-HAADF image. (e) 3D model of the nanoparticle obtained by a classical data segmentation procedure from a reconstructed HAADF-STEM tilt series, Au in purple and Ag in grey, respectively. (f) A slice through the XY plane of the chemical sensitive volume showing the Au (red) and Ag (green) distribution at the basis of the pyramid. (R. Arenal, et al., submitted).

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.
Figure 2. (a) and (d) EEL spectra (each of them corresponds to the sum of 16 spectra) extracted from the SPIMs recorded on the Au and Au-Ag core-shell nanoprisms depicted in the HAADF images (the scale bar is 50 nm), Figures 2 (b) and (e), respectively. The dimensions of the nanoparticles are specified in those images, L: length, W: width and σ: aspect ratio (W / L). Inset of Figure 2 (d) corresponds to a zoom of the 2.0-3.5 eV region. (c) and (f) Different intensity maps showing the spatial distribution of the SPR modes of these Au and Au-Ag core-shell nano-bipyramids. These different modes are marked as A–D. (R. Arenal, et al., submitted).

Figure 2. (a) and (d) EEL spectra (each of them corresponds to the sum of 16 spectra) extracted from the SPIMs recorded on the Au and Au-Ag core-shell nanoprisms depicted in the HAADF images (the scale bar is 50 nm), Figures 2 (b) and (e), respectively. The dimensions of the nanoparticles are specified in those images, L: length, W: width and σ: aspect ratio (W / L). Inset of Figure 2 (d) corresponds to a zoom of the 2.0-3.5 eV region. (c) and (f) Different intensity maps showing the spatial distribution of the SPR modes of these Au and Au-Ag core-shell nano-bipyramids. These different modes are marked as A–D. (R. Arenal, et al., submitted).

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.

Figure 3: A)-C) HAADF and HRTEM images of an area (the tip) of one of these NT where spectroscopy data have been recorded (see Fig. C)). D) EEL spectra extracted from the spectrum-image in the areas marked in Fig. C). C- and N-K edges are visible on these spectra. E) Low-Loss EEL spectra extracted from a spectrum-image recorded in the same area of the previous one and they have been extracted from the same areas that those marked in Fig. C), and they correspond to a spectral range of 2-10 eV. (R. Arenal, Micros. and Microanalysis 19, 1230 (2013); R. Arenal, to be submitted).

Highlights:

  • 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).