We demonstrate the imaging capabilities of energy-filtered transmission electron microscopy at high-energy resolution in the low-energy-loss region, reporting the direct image of a surface plasmon of an elongated gold nanoparticle at energies around 1 eV. Using complimentary model calculations performed within the boundary element method approach we can assign the observed results to the plasmon eigenmodes of the metallic nanoparticle

Andreas Trügler

Bernhard Schaffer

Ferdinand Hofer

H. Atwater

J. D. Jackson

L. Novotny

R. Egerton

Ulrich Hohenester

English

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High-resolution surface plasmon imaging of gold nanoparticles by energy-filtered transmission electron microscopy

Schaffer, B.

Hohenester, U.

Trugler, A.

Hofer, F.

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       Schaffer, B. and Hohenester, U. and Trugler, A. and Hofer, F. (2009) High-resolution surface plasmon imaging of gold nanoparticles by energy-filtered transmission electron microscopy. Physical Review B, 79 (4). 041401. ISSN 1098-0121   http://eprints.gla.ac.uk/6522/  Deposited on: 24 July 2009   Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk High-Resolution Surface Plasmon Imaging of Gold Nanoparticlesby Energy Filtered Transmission Electron MicroscopyBernhard Schaﬀer,1, 2, 3 Ulrich Hohenester,4 Andreas Tru¨gler,4 and Ferdinand Hofer11Institute for Electron Microscopy and Fine Structure Research,Graz University of Technology, 8010 Graz, Austria2SuperSTEM Laboratory, STFC Daresbury,Keckwick Lane, WA4 4AD, Warrington, United Kingdom3Department of Physics and Astronomy,University of Glasgow, G12 8QQ, Glasgow, United Kingdom4Institut fu¨r Physik, Karl–Franzens–Universita¨t Graz,Universita¨tsplatz 5, 8010 Graz, Austria(Dated: December 2, 2008)AbstractWe demonstrate the imaging capabilities of energy-ﬁltered transmission electron microscopy athigh energy resolution in the low energy-loss region, reporting the ﬁrst direct image of a surfaceplasmon of an elongated gold nanoparticle at energies around 1 eV. Using complimentary modelcalculations performed within the boundary element method approach we can assign the observedresults to the plasmon eigenmodes of the metallic nanoparticle.PACS numbers: 73.20.Mf, 79.20.Uv, 68.37.Og, 78.20.Bh1Plasmonics is an emerging ﬁeld with numerous applications [1, 2], ranging from opticaldata processing [3, 4] over negative-refraction materials [5, 6] to bio-sensors [7, 8]. Surfaceplasmons, the workhorse of plasmonics, allow to bridge between the micro- and nanome-ter length scales of conventional optics and nano-devices. This is achieved by binding orconverting light to coherent electron charge oscillations, conﬁned to the surface of metallicnanostructures. When optical emitters, such as molecules or quantum dots, are placed inthe vicinity of metallic nanoparticles, they couple strongly to the evanescent ﬁelds of thesurface plasmons, and the light-matter coupling becomes signiﬁcantly enhanced. This isexploited in surface enhanced Raman scattering [9] or surface enhanced ﬂuorescence [10], aswell as in single-plasmon generation [11].The electromagnetic enhancement is particularly strong at the so-called “hot spots”,with typical sizes of a few nanometers, which are usually located in the gaps between twonanoparticles or at the sharp edges of single particles. While enhancement factors of sev-eral orders of magnitude are predicted [12], the analysis of the experimental data is oftencumbersome and controversial due to the limited spatial resolution provided by optical far-and nearﬁeld techniques, as well as due to the uncertainties regarding the precise loca-tion of the optical emitters. An alternative experimental technique, which allows to bypassthese diﬃculties, is electron energy-loss spectroscopy (EELS) in combination with scanningtransmission electron microscopy (STEM) [13]. Here, a high-energy electron beam, witha transversal extension on the sub-nanometer scale, passes in the vicinity or through thenanoparticle, and some electrons lose energy through plasmon excitations, which are subse-quently monitored. By raster scanning the beam over the metallic nanoparticle, one obtainsinformation about the photonic local density of states of the metallic nanoparticles [13–17].The spatial resolution of the maps is limited to the nanometer range by delocalization ofthe inelastic scattering [18, 19].The drawbacks of the EELS technique have so far been the energy cutoﬀ in the low-energy regime and the limited spatial resolution in large-area mappings. On the one hand,the so-called zero-loss peak (ZLP) of the electron beam usually masks the low-energy part ofthe spectrum. This hinders the direct observation of surface plasmons in the red and infra-red regime, which is particularly appealing for (bio)sensors or applications at the telecomwavelengths. On the other hand, as STEM EELS data are sequentially acquired point-by-point, the sampling of the spatial dimensions is limited by keeping the total acquisition time2low, which leads to usually rather coarse maps.In this paper we show that energy-ﬁltered transmission electron microscopy (EFTEM)as an alternative acquisition technique enables us to directly image surface plasmons at lowenergies. As a parallel imaging technique, which gathers images using scattered electronsof a speciﬁc energy-loss range only, it achieves high spatial sampling easily and fast, whileproviding the same information content as STEM EELS [20, 21]. The energy resolution ofthe EFTEM technique is typically in the range of a few eV, but can be pushed to the sub-eV range if a small energy-ﬁltering slit width and appropriate data correction schemes areused [22]. This enabled us to directly monitor surface plasmons at energies around or below1 eV without employing any deconvolution procedure. We achieved image sizes of 512× 512pixels for exposure times on the minute scale.Experiment.—In our experiments we used a Wien-ﬁlter monochromated FEI Tecnai F20microscope with a High Resolution Gatan Imaging Filter and an UltraScan CCD camera [23].The investigated specimen was a rod-shaped gold nanoparticle synthesized by a modiﬁcationof the conventional citrate reduction of gold salt in water [24]. A holey carbon ﬁlm was usedas the sample support grid. Figure 1(a) displays the particle in a TEM bright ﬁeld image.With the monochromated beam, the condenser lens was adjusted such that a homogeneousillumination of the particle could be achieved. An EFTEM image series was acquired via ourcustomized acquisition script, which adapts exposure times and multiple frame read outs foreach energy step to the encountered intensity situation [25]. We used an energy selectingslit width of 0.3 eV, and recorded the images in the energy range from 1.0 to 4.5 eV in0.1 eV steps. The spatial sampling of the images was 512 × 512 pixels for a ﬁeld-of-viewof 456 × 456 nm, and the total acquisition time was 17 minutes. The raw data were thenprocessed to correct for spatial drift, energy drift, and non-isochromaticity [22].We ﬁrst extracted, from single pixels of the EFTEM image series, energy-loss spectraat the three positions indicated in Fig. 1(a). Fig. 2(a) shows the corresponding spectra.For comparison, we also measured three EELS point spectra at approximately the samepositions, which are shown in panel (b). All data are shown without background subtrac-tion. In the spectra we observe three distinct peaks in the energy range below 3 eV, whichwe tentatively ascribe to the surface plasmon eigenmodes of the gold nanorod. This inter-pretation is supported by the EFTEM images of Fig. 1(b–d), taken at the energies of thepeak maxima. The mode at 0.97 eV has its intensity maxima at the top and bottom end3FIG. 1: (Color online) (a) TEM bright ﬁeld image of rod-shaped gold nanoparticle, with approxi-mately 400 nm length and 75 nm diameter. The markers indicate pixel positions at which EELSspectra in Fig. 2 have been extracted. (b–d) EFTEM images at given energy-loss values. Imageintensities have been normalized with respect to the maximum intensity of panel (b). We usepseudocolor for better visibility. The EFTEM maps show the electromagnetic eigenmodes of thegold nanorod.of the rod, reminiscent of a dipolar excitation, while the map depicted in panel (c) showsan additional peak in the central region of the nanorod, similar to the ﬁrst excited modeof a linear antenna. Finally, the mode at 2.27 eV, panel (d), vaguely shows two maximaat approximately one and two thirds of the long nanorod axis. In all cases we observe a4Intensity(arb.units)pos 1pos 2pos 3Energy loss (eV)-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5Intensity(arb.units)pos 1pos 2pos 3a)b)EFTEMSTEM EELSE = 0.43 eVE = 0.17 eVFIG. 2: (Color online) (a) Single-pixel extracted EELS spectra from EFTEM data at positionsindicated in Fig. 1(a). (b) STEM EELS spectra acquired at the same sample positions. Thearrows in the lower parts of the panels indicate the energy resolution of the data.strongly reduced intensity at the sides of the carbon support, in comparison to the vacuumside, which we attribute to absorption losses of the electron beam in the carbon.Theory.—To unambiguously interpret our experimental results, we additionally per-formed simulations using the boundary element method (BEM) approach [26, 27]. In oursimulations we assume that a beam of 200 keV electrons passes in the vicinity of the metal-lic nanoparticle, as depicted in Fig. 3(a). For simplicity no penetrating trajectories wereconsidered [13]. We model the gold nanoparticle as a rod with 400 nm length and 75 nmdiameter, in accordance to the particle shape depicted in Fig. 1(a). The dielectric functionof gold was extracted from optical data [28] and the refractive index of the surrounding5FIG. 3: (Color online) Results of BEM simulation. (a) Triangulated surface of rod-shaped nanopar-ticle and EFTEM maps as computed according to the prescription of Ref. [26]. The maps at theenergies of 1.08 eV, 1.85 eV, and 2.29 eV have been displayed in pseudocolor scaled to match thedata range of the 1.08 eV map. The loss rates for the 2.29 eV map have been multiplied by a factorof 5 for better visibility. The lines indicate the direction of the electron beam. (b) Energy-lossprobability at the positions indicated in panel (a).medium was set to one. The external excitation due to the electron beam is described bythe Lie`nard-Wiechert potentials [29]. We compute the dielectric response of the nanoparticlewithin the BEM framework by means of auxiliary surface charges and currents [26], whichallow us to compute for a given frequency ω the induced electric ﬁeld Eind[re(t), ω] at the6positions re(t) of the electron beam, and to express the energy loss probabilities accordingto [26]Γ(ω) =1πω∫Re{e−iωtv ·Eind[re(t), ω]}dt . (1)In our computational approach, we discretize the surface of the nanoparticle by a set oftriangles, as shown in Fig. 3(a), and match the electromagnetic potentials at the trianglecenters [27, 30]. For spherical nanoparticles we compared the loss probability Γ(ω) with theanalytic results of Mie theory [31], and found excellent agreement between the numericaland analytic results.Results of our BEM simulation are shown in Fig. 3. Plasmon peak maxima are found atthe energy positions of 1.08 eV, 1.85 eV, and approximately 2.29 eV. The spatial intensitydistribution of the plasmonic modes ﬁts the experimental ﬁndings very well, giving evidencethat the detected peaks in the EELS spectrum can be interpreted as the resonance modesof the plasmon oscillations of the gold nanoparticle. We attribute the diﬀerences betweenthe energetic positions of the peak maxima in theory and experiment to our neglect of theholey carbon ﬁlm in the simulations, which is expected to red-shift the peak maxima and tointroduce an anisotropy in the simulated EFTEM maps. As regarding the peak at 2.29 eV,we ﬁnd that it is strongly broadened. This is attributed to interband scatterings in gold,that take place above a threshold of approximately 2 eV. We also found that the EFTEMmap of the 2.29 eV peak is smeared out when we compute the loss probabilities not at asingle energy, but integrate over an energy window of 0.4 eV to simulate the eﬀect of thelimited energy resolution in the experiments.Discussion.—The energy resolutions of both EFTEM and EELS is governed by the widthof the ZLP. Surface plasmons of silver nanoparticles have been mapped by EELS in an energyregime above 1.70 eV, using cold ﬁeld emission guns together with a deconvolution procedureapplied to the raw data [13]. Surface plasmons of gold particles, which lie closer to the ZLP,are more diﬃcult to observe and may need the resolution improvements of a monochromatedsystem [32]. A monochromated beam has a truly symmetric energy distribution and nopronounced ZLP tails towards the low-loss regime, giving profound advantages over coldﬁeld emission guns in this energy range [33]. An analysis of the full width at half maximumof the ZLP, extracted from the data in the vacuum area, reveals for our setup an EFTEMenergy resolution of 0.43 eV, to be compared with the EELS resolution of 0.17 eV. Thishigher spectral resolution of EELS is also apparent from Fig. 2, although in both cases7the diﬀerent plasmonic eigenmodes, whose spectral widths are usually of the same order ofmagnitude, can be easily discriminated.The advantage of EFTEM lies in its high spatial resolution over large ﬁeld-of-views. Itis important to realize that any attempt to obtain mapped EELS images with comparablespatial sampling is not simply more cumbersome or time-consuming, but is probably doomedto failure for the following reason. A diﬃculty for extreme low-loss mapping is posed by thestrong intensity diﬀerence between the ZLP and the adjacent low-loss region, which chal-lenges the dynamic range of the detector. The situation is even worse for a monochromatedsource, as the ZLP maximum becomes stronger with decreasing peak width. While it ispossible to choose a setup which truncates the most intense parts of the ZLP by shiftingthe spectrum just across the edge of the detector, this technique relies on the stability ofthe system with respect to energy oﬀsets over the time of acquisition. Considering the pixeldwell time of about 0.6 sec, which includes both the exposure time of 0.1 sec and the datareadout of the detector, a spatial map of 512 × 512 data points would have taken close to36 hours, during which energy drift must not have exceeded 0.5 eV. Such long time stabilitycan hardly be achieved in any modern system. In contrast to the scanning beam approach,within EFTEM the energy dimension is sampled sequentially and the dynamic range prob-lem close to the ZLP can be overcome by especially adapted acquisition routines [25]. Asecond advantage of EFTEM is its ability to rapidly image large areas at lower magniﬁcation,without tediously adjusting the electron beam to coarse sampling conditions. For instance,imaging the spatial distribution of the 1 eV peak alone would have required only 1 minute,thus demonstrating the unique capability of mapping large ﬁeld-of-views with spatial resolu-tion on the nanometer scale. Furthermore, in a scanned beam approach spatial drift duringacquisition can lead to map distortions, which can not be corrected or even unambiguouslyidentiﬁed from the data alone, whereas spatial consistence of EFTEM images is immediatelyevident.In conclusion, we have shown that EFTEM imaging can be used to directly and eﬃcientlyvisualize the electromagnetic eigenmodes of surface plasmons in metallic nanoparticles. Thetechnique surpasses the spatial resolution of optical measurements by more than one or-der of magnitude and still allows for large ﬁeld-of-views to be investigated within severalminutes. Therefore, detailed studies of assembled particles or structured devices becomeavailable, bridging the gap between highly resolved local measurements, such as STEM8EELS, and optical microscopy techniques, which average over larger areas and smear outinhomogeneities on a nanometer scale. The technique can also be applied for fast screeningof many particles with slightly diﬀerent resonance frequencies, or to localize “hot spots” ofstrongest resonance in a device, prior to detailed local analysis. It could also be advanta-geous for beam sensitive samples, in those cases where high dose rates rather than the doseitself harms the specimen. 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High-resolution surface plasmon imaging of gold nanoparticles by energy-filtered transmission electron microscopy

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