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Probing Higher Order Surface Plasmon Modes on Individual Truncated Tetrahedral Gold Nanoparticle Using Cathodoluminescence Imaging and Spectroscopy Combined with FDTD Simulations Pabitra Das,† Tapas Kumar Chini,*,† and James Pond‡ †

Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India Lumerical Solutions Inc., Suite 201-1290 Homer Street, Vancouver, BC V6B 2Y5, Canada

‡

S Supporting Information *

ABSTRACT: We report the spatial maps of the localized surface plasmon resonances associated photon emission in a truncated tetrahedral gold nanoparticle on a silicon substrate. Site-specific cathodoluminescence spectroscopy and imaging in a scanning electron microscope shows stronger photon emission in the visible range near the tips of the particle in contact with the substrate compared to the edges of the particle. Strong local field variations on a length scale as short as 19 nm are resolved. We also perform FDTD simulations of both the spectra and, for the first time, the full cathodoluminescence images. Excellent agreement is obtained with the experimental results, and the detailed information available from the simulated results makes it possible to identify the signature of out-of-plane higher order modes in the truncated tetrahedral gold particle.

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INTRODUCTION In recent years, there has been a revolutionary change in the understanding of light-matter interaction related to some unusual electromagnetic (EM) properties associated with negative refractive index,1 invisibility,2 light-based nanocircuits,3 etc. The resulting interest in materials with unusual EM properties has led to a rapidly growing new field of photonic research termed plasmonics4,5 where surface-bound EM modes, i.e., surface plasmons (SP), play an important role. Surface plasmon (SP) or more specifically surface plasmon polaritons (SPP) are propagating, transverse EM waves coupled to the electron plasma of a conductor at a dielectric interface. Localized surface plasmon (LSP), on the other hand, are nonpropagating excitations of the conduction electrons in metallic nanoparticles. LSP when excited resonantly with a particular wavelength of the exciting light or evanescent wave associated with fast-moving electrons,6 can decay into radiative photons that can be collected into far-field. LSP resonances can show highly localized enhancement of near-field amplitude at the nanostructured metal surface and similar enhancement of the far-field intensity. Mapping the spatial variation7−13 of the photon emission is a direct probe of resonant modes of plasmonic nanostructures and, consequently, provides a direct way to map the local electric fields. Often the local EM field enhancement in the plasmonic structures is confined spatially on length scales of ∼10−50 nm and varies strongly with the morphology and composition of nanoparticles, meaning, ensemble measurement of nanoparticle optical properties is not expected to reflect the spectrum of an individual entity. Consequently, single nanoparticle spectroscopy with imaging © 2012 American Chemical Society

facility is an essential tool to understand the plasmonic properties of nanoparticles. Due to the simplicity of the instrumental setup, a large body of the optical studies of metal nanoparticle so far has been centered around the use of optical dark-field microscopy (DFM).14,15 However, except for a few cases,15 the scattering spectrum obtained from DFM could not be correlated to the exact size and shape of the investigated nanoparticle and due to the poor spatial resolution of the DFM optical imaging, the spatial profile of the plasmon resonances could not be realized. A subdiffraction-limited optical imaging technique,11 such as near-field scanning optical microscopy (NSOM), can achieve a resolution of ∼20 nm or slightly better, but (in the case of NSOM) is constrained by the requirement of fabricating very sharp tips and still may not have the necessary resolution to probe the detailed spatial structure of LSP. Alternatively, electron beam based tools6 such as cathodoluminescence (CL) spectroscopy in a transmission electron microscope (TEM)/ scanning electron microscope (SEM) through the detection of emitted photons or electron energy loss spectroscopy (EELS) in TEM through the detection of energy loss suffered by the inelastically scattered transmitted electrons are shown to constitute an excellent probe of plasmons that allows capturing few-nanometer resolution information in the spatial and spectral domains. CL-SEM is advantageous8,9,16,17 over EELS or CL-TEM because a large sample area on a thick substrate Received: May 16, 2012 Revised: June 22, 2012 Published: June 29, 2012 15610

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and the mixture was stirred for 3 min. An aqueous solution of 100 mL of 2.5 × 10−4 M HAuCl4 was prepared in a 250 mL flask. Then 3.645 g of CTAB was added with continuous stirring until the CTAB was dissolved completely. Two flasks A and B each of 25 mL and another flask C of 100 mL were used. A 4.5-mL sample of the growth solution was added to flasks A and B and 25 μL of 0.1 M ascorbic acid solution was added to each flask. In flask C 45 mL of growth solution was added and mixed with 250 μL of ascorbic acid (0.1 M) and 300 μL of nitric acid (0.1 M). Next, 400 μL of Au seed solution was added to the solution in flask A with stirring for 3 min. Next, 400 μL of solution from flask A was added to the solution in flask B with stirring for 5 s. Then, 4 mL of solution from flask B was added to the solution of flask C. The solution in flask C was kept undisturbed at room temperature for 20 h. Next, the supernatant was discarded and the precipitate was dispersed in 10 mL of fresh DI water. The dispersed solution was centrifuged twice for 20 min each time at 2000 rpm. Each time supernatant was removed and the precipitate was dispersed with fresh DI water. Since our aim was to perform single particle spectroscopy the final solution used was much diluted. It was then deposited on a Si (100) substrate by dropcasting and desiccation. This synthetic approach produces a mixture of nanospheres, triangular prisms, and rods of different sizes scattered throughout the whole dropcasted region. Among the randomly distributed nanoparticles, isolated single nanoprisms were identified in SEM for CL measurements. Si is chosen as the substrate material to suppress background CL in the near-UV and visible wavelength. Cathodoluminescence Measurements. CL or electronbeam-induced radiation emission (EIRE) imaging/mapping on an isolated single TT Au nanoparticle was performed in a ZEISS SUPRA40 SEM equipped with the Gatan MonoCL3 cathodoluminescence system.28 The ZEISS SUPRA40 SEM has a hot Schottky field-emission gun and the attached MonoCL3 system uses a retractable paraboloidal light collection mirror. The parabolic mirror collects light that is emitted from the sample covering 1.42π sr of the full 2π of the upper half sphere and collimates it through a hollow aluminum tube to a 300 mm Czerny-Turner type optical monochromator with a spectral band-pass of approximately 11 nm and finally the signal is fed to a high-sensitivity photomultiplier tube (HSPMT). In the present study, data were recorded with an electron acceleration voltage of 30 kV and beam current of about 15 nA with a beam diameter of approximately 5 nm. The electron beam was directed onto the sample surface through a 1 mm diameter opening in the mirror. To ensure maximum efficiency of light collection, the top surface of the sample is kept at the focal plane of the mirror, which lies approximately 1 mm below the bottom plane of the mirror. The CL system in conjunction with the SEM can be operated in two modes, namely, monochromatic and panchromatic. In the monochromatic mode of CL operation, the focused electron beam is either scanned over the sample or positioned on a desired spot while the emitted light from the sample passing through the monochromator allows the emission spectra to be recorded. Subsequently, the monochromatic photon map is built up at a selected peak wavelength of the EIRE spectrum by scanning the electron beam over the sample. Note that bright pixels in the CL image are due to an electron beam position that strongly excites a resonant plasmonic mode, while the photons emitted and subsequently collected may come from anywhere over the Au TT once a

can be accessed without any stringent requirement of sample preparation, such as maintaining electron transparency ( 2) mode and a net dipole mode along the thickness of the TT gold particle in the present case might be associated with our experimentally observed tip mode at 570 nm. Single particle spectra and imaging of such higher order surface plasmon modes from the TT Au nanoparticle on a high index substrate (such as Silicon) has not been reported so far to the best of our knowledge.

Figure 8. Simulated EIRE spectrum in the wavelength range 500− 1600 nm. Inset: Calculated near-field intensity (|E|2) map at a plane 5 nm above the triangle top surface (XY plane) at λ = 580 and 1390 nm. The scale bar = 200 nm. The green spots in the inset images indicate the e-beam impact position.

several peaks ranging from ∼580 to 1390 nm, an indication of the presence of several higher order modes along with the dipolar one. We have taken the refractive index (n) of the substrate as 2 here, instead of 4 used earlier, mainly because the in-plane dipole mode, which highly depends on the substrate index and aspect ratio of the triangular nanoplatelet, will be redshifted beyond 2000 nm for n = 4, a situation constrained by our computer memory and the requirement of the use of finer mesh to ensure convergence in the simulation. The full spectrum shows peaks at 1390, 875, 750, 700, 650, and 580 nm. 15617

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Extensive theoretical studies by Shuford et al.19 have shown that multipolar excitations may be observed as the triangular prisms become large and thin, and the sharp tips on a perfect triangle give rise to a kind of polarization distortion that favors higher order multipoles. But, due to the presence of rounded edges or other similar imperfections,19 multipolar orders greater than 2 would be very difficult to measure, especially in solution phase. On the other hand, while the in-plane modes dominate for large and thin prisms, out-of-plane excitations are dominant for small and thick nanoprisms. Spatial Resolution. Spatial resolution is conventionally23 expressed in terms of a fraction of the wavelength of the excited lowest energy (highest wavelength) optical mode. However, the lowest energy optical mode for the Au nanoparticle on the Si substrate under present study for 30 kV electron excitation is estimated (from FDTD simulation) to appear beyond the detection range of the detector used in the present experiment and obviously the corresponding photon emission map is inaccessible in our current experimental situation. So, we use the spatial variation in the intensity of light emission in a panchromatic CL image to estimate the spatial resolution, as illustrated in Figure 10. We fit Gaussian functions to the light

vertical sidewalls used in the simulated structures and simulations of sloped sidewall structures will be the subject of our future work. To the best knowledge of the authors this is the first time that cathodoluminescence images have been simulated directly. We have shown how the wide range of different experimental results (spectra from different excitation points of the e-beam, panchromatic images, and monochromatic images) can be used in conjunction with the simulation results to identify individual surface plasmon modes and to study their spatial and spectral properties. Using these methods, we have identified a higher order mode than the diploe mode for the first time in a truncated tetrahedral gold particle. In future, these methods of probing and analyzing near-field plasmonic modes may help improve the design of new technologies, such as plasmon-enhanced silicon solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

Near-field E intensity and vector plot corresponding to the 530 nm peak (Figure SI.1) and vector plot showing the out-of-plane dipole mode corresponding to the FDTD simulated peak at 875 nm (Figure SI.2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91 33 2337 4637. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P. Das and T. K. Chini gratefully acknowledge Prof. M. Kociak of Université Paris Sud, France, for his valuable suggestions and critical comments, Dr. S. Jana and Mr. S. Mandal of SINP, Kolkata, India, for their assistance in sample preparation, Dr. K. H. Fung and Prof. Nicholas X. Fang of the Massachusetts Institute of Technology, USA, for their assistance in initial stages of simulation. Finally, thanks to Prof. N. R. Jana of IACS, Kolkata, India, for introducing us to the chemical route to nanoparticle synthesis.

Figure 10. Variation of the panchromatic CL emission along the line AB joining two side edges of the TT Au particle. The scale bar of the pan CL image = 200 nm.

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emission profiles (Figure 10) obtained along the line joining the edges of the TT shown in the inset of Figure 10. A spatial modulation with the fwhm of ∼19 nm across the bottom edge of the Au particle is clearly detectable and we can conclude that the spatial resolution is better than or equal to 19 nm.

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CONCLUSIONS We have used cathodoluminescence spectroscopy and imaging in a scanning electron microscope to observe the plasmonic behavior of truncated tetrahedral Au nanoparticles for the first time. The imaging can be performed in panchromatic or monochromatic modes and provides an opportunity to probe the near-field mode profiles of the surface plasmon modes supported by the structure, with a spatial resolution of approximately 19 nm. In addition, we have simulated the cathodoluminescence spectroscopy and imaging using the FDTD method and obtained excellent agreement with the experimental spectra and images. The remaining discrepancy between simulated and experimental results is likely due to the 15618

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