High spatial resolution mapping of surface plasmon resonance modes in single and aggregated gold nanoparticles assembled on DNA strands
© Diaz-Egea et al.; licensee Springer. 2013
Received: 8 May 2013
Accepted: 19 July 2013
Published: 26 July 2013
We present the mapping of the full plasmonic mode spectrum for single and aggregated gold nanoparticles linked through DNA strands to a silicon nitride substrate. A comprehensive analysis of the electron energy loss spectroscopy images maps was performed on nanoparticles standing alone, dimers, and clusters of nanoparticles. The experimental results were confirmed by numerical calculations using the Mie theory and Gans-Mie theory for solving Maxwell's equations. Both bright and dark surface plasmon modes have been unveiled.
PACS: 78.67.Bf; 61.46.Df; 87.64.Ee
KeywordsPlasmonics Surface plasmon resonance Gold nanoparticles Electron energy loss spectroscopy DNA Assembly
The field of plasmonics has become a topic of major interest in the last years due to its property of showing an enhancement of the electromagnetic field at a sub-wavelength dimension . This phenomenon is especially noticeable when there is plasmon coupling between metallic nanoparticles that are separated by nanometric gaps . As a result of the overlap of the electromagnetic fields, there are near-field interactions that allow propagation of light . In this effort for designing plasmonic circuits by metal nanoparticle paths, the control of the location of the nanoparticles and the exact separation between them has been achieved, among other procedures, by means of biomolecular nanolithography using deoxyribonucleic acid (DNA) as scaffolds for the gold nanoparticles . With this technique, the inter-particle separation is controlled by the ligand shell allowing angstrom-level precision . To fully characterize such systems, electron energy loss spectroscopy (EELS) has demonstrated to be a very powerful tool since it can probe the local density of states for plasmonic nanoparticles , and it has the advantage over optical measurements that it provides information about bright and dark modes.
In this work, we analyze the plasmonic properties of gold nanoparticles attached through DNA strands to a silicon nitride substrate. Individual nanoparticles as well as clusters of them were analyzed by EELS. Spectrum imaging (SI) maps are presented showing dark and bright plasmon modes in these assembled nanoparticles. Analytical calculations based on the Mie theory and Gans-Mie theory for solving Maxwell's equations were performed showing excellent agreement with the experimental results.
Energy-filtered transmission electron microscopy and scanning transmission electron microscopy (STEM) EELS SI are two TEM techniques that have been proven to be very powerful when performing plasmonic analysis in small metallic nanoparticles such as silver nanoprisms , gold nanoprisms , silver nanorods , and nanowire dimers . Both techniques present advantages and disadvantages . The intensity of the LSPR peaks for small nanoparticles (the ones analyzed here have diameters between 5 and 25 nm) is very low, making EELS in STEM the best choice allowing both, very high spatial resolution and fine sampling of the energy loss spectrum.
For the work presented here, the SI maps were acquired using the Zeiss sub-electronvolt-sub-angstrom-microscope operated at 200 kV. This equipment is located at the Stuttgart Center for Electron Microscopy (Stuttgart, Germany). It is equipped with a Schottky field emitter, an electrostatic monochromator, and the high-dispersion and high-transmissivity in-column MANDOLINE filter . The spectrometer dispersion was set to 0.01377 eV per channel for the 2,048 channels with an exposure time of 0.2 s per spectrum. The spatial sampling used was in the range of 1.9 to about 2.6 nm per pixel giving a total acquisition time of between 10 and 20 min for every single SI. The energy resolution achieved, measured as the full width at half maximum of the zero loss peak, was between 138 and 151 meV. Before and after the SI acquisition, high-angle annular dark-field (HAADF) images were taken in the selected area to control spatial drift.
Using the peak at zero energy loss, the SI is realigned in energy to correct energy shifts from one pixel to the other. To mitigate the noise in the spectra, principal component analysis (PCA) was used to decompose the entire map and reconstruct it without the very high-order components . The zero loss peak (ZLP) removal was performed using a power-law function. For every localized surface plasmon resonance (LSPR) peak, one Gaussian function was fitted to the curve by nonlinear least squares fit algorithm. The energy loss maps and the amplitude maps were created using the center of the fitted Gaussian function and its amplitude, respectively.
For the case of a single nanoparticle standing alone, theoretical calculations were done to support the results. The calculations were performed using routines based on the MATLAB toolbox MNPBEM . To estimate the LSPR response of one gold nanosphere, the Mie theory was used to solve the Maxwell equations using both the quasistatic approximation and solving the full Maxwell equations. In that way, the light extinction of such a sphere was used to match the energy loss results acquired at the microscope. In the same way, for the case of one single-standing nanoellipsoid, the Mie-Gans theory was used using the quasistatic approximation.
To fabricate the samples used for this work, DNA strands were deposited on a silicon nitride grid surface. These DNA strands were used as biomolecular templates for the self-assembly of gold nanoparticles . These samples were acquired from Dune Sciences (Eugene, OR, USA). The fabrication process was described elsewhere, and it is not included here because this process is not the aim of this work.
Results and discussion
Both the energy map and the spectrum labeled in red as (curve i) show a very distinct peak at 2.4 eV, this is the typical value for a dipolar LSPR mode in a gold nanoparticle of this size in air [15, 16]. To validate the results, the Mie theory has been used to solve the Maxwell equations using both the quasistatic approximation and solving the full Maxwell equations. A 26-nm gold sphere standing in vacuum was considered yielding both approximations a result of 2.44 eV for the extinction of light with the absorption as the main contribution over scattering which corresponds for a metal nanoparticle of this size . The influence of the silicon nitride substrate explains the slight blueshift of the resonance peak.
The bulk plasmon resonance can also be seen in the energy map showing values between 2.45 and 2.55 eV. One of these spectra marked with the blue dot and labeled as (cuve ii) is shown for display. It clearly shows a resonance peak at 2.5 eV, that resonance peak is broader and less intense than that of the LSPR. Similar results have recently been reported for silver nanoparticles with comparable sizes .
The HAADF image reveals that the nanoparticle is not perfectly symmetrical. There is intensity decay along the long axis of the nanoparticle from top to bottom indicating a higher volume of gold on the top part of the nanoparticle. Profiles of the nanoparticle perpendicular to the longitudinal axis also reveal that this one is slightly thicker on the top and a little bit sharper at the bottom. This shape is confirmed by the energy and intensity maps where an asymmetry can be seen between top and bottom of the nanoparticle. The energy at the top corresponds to 2.15 eV, while at the bottom, a red shift down to 2.1 eV and below is visible. However, the main characteristic of the sharper part of a nanoparticle is that it presents a higher intensity of the field, this can be seen in both the intensity map (c) and the energy-filtered map (d).
Similar to the sphere calculations, the Mie-Gans theory was used to validate the findings using the quasistatic approximation for non-spherical particles. An ellipsoid was modeled estimating its axis to be 21, 11, and 11 nm. It was assumed to be surrounded by vacuum. Two modes for extinction of light at 2.47 and 2.33 eV are found. Both modes seem to be red-shifted with respect to the experimental results which are possibly attributable to the effect of the substrate.
One way to explain the depicted modes is to assume the dimer as a big nanoparticle of 35 nm × 27 nm. One such nanoparticle would behave in the same way as the one analyzed in Figure 2 with a low-energy mode along the long axis and a high-energy one perpendicular to it. The former would correspond to the areas marked as (curves i and ii) and the last to the areas labeled as (curves iii and iv). The symmetry of each of these two global modes is broken by the irregular shapes of the individual nanoparticles.
In order to describe the plasmonic behavior of the structure, three main areas are highlighted. The most intense surface plasmon mode is located in the bottom-right corner of the map. It is represented by the red spectrum (curve ii), the orange area in (b), and the light blue zone in (c). The energy values for this area are close to 1.9 eV. There is a second plasmonic mode of about 1.6 eV which is located both at the top and at the bottom of the cluster. It is displayed using blue colors in the energy map (b), and it corresponds to the lowest energy curve in the blue line (curve i) shown in (a). There is a third area with energy values of 2.3 eV that is located at the upper part of the cluster at its left and at its right. It can be identified with the yellow and orange colors in the map labeled (d). This mode coexists with the lowest energy one in the area selected with (curve i), and that is why it is shown as the highest energy value curve in the spectrum (a). It can be seen in both the intensity map (e) and the energy filtered map (h) that, although this mode is more intense at the right side of the structure, it also exists almost symmetrically at the left side.
In the same way as we did for the dimer of nanoparticles, we can consider the cluster to be a big nanoparticle with a vertical long axis and a horizontal short one. The area in the extreme of the long axis would be the one with blue colors in Figure 4b. This area is again the one with the lowest energy while the high-energy area is the one with orange colors in the same figure. However, for such a complex cluster, these two modes are significantly modified by the irregular shapes of the individual nanoparticles creating areas with two modes like the one depicted by (curve i). It is remarkable that the areas with the highest intensity for the resonance peaks are the sharper areas of the cluster.
In conclusion, the plasmonic properties of gold nanoparticles assembled on DNA strands were investigated at nanometer scale by EELS. This analysis was done for isolated nanoparticles, dimers, and clusters. The results were compared to analytical calculations showing good agreement. It was shown that the LSPR peak appears at 2.44 eV (508 nm) which is typical for isolated spherical gold nanoparticles. For an elongated particle, two modes are identified, a longitudinal and a transversal mode. A dimer of nanoparticles was analyzed with the result of complex modes being exposed. The dimer as a whole seems to present longitudinal and transversal modes behaving similarly to an elongated nanoparticle with the size and shape of the dimer but with the modes shifted by the irregular shapes of the individual nanoparticles. A cluster of six nanoparticles was analyzed with similar results. The use of EELS unveiled bright and dark plasmon modes. The low-energy ones are located on the extremes of the long axis and the high-energy ones on the short axis. The sharper areas of the cluster present higher intensity in the resonance peak. The results presented in this manuscript contribute to the design of plasmonic circuits by metal nanoparticle paths.
CDE is a Ph. D. student at the Universidad de Cádiz. WS is a Research scientist at the Stuttgart Center for Electron Microscopy (StEM), Max Plank Institute for intelligent systems, PAvA is head of the Stuttgart Center for Electron Microscopy (StEM), Max Planck Institute for intelligent systems. SIM is a full professor at the Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Universidad de Cádiz.
Electron energy loss spectroscopy
Transmission electron microscopy
Scanning transmission electron microscopy
High-angle annular dark-field
Principal component analysis
Zero loss peak
Localized surface plasmon resonance.
This work was supported by the Spanish MINECO (projects TEC20011-29120-C05-03 and CONSOLIDER INGENIO 2010 CSD2009-00013) and the Junta de Andalucía (PAI research group TEP-946 INNANOMAT). We would like to thank Giovanni Scavello for helping us on the layout of the figures.
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