Perspective pubs.acs.org/JPCL
Understanding Plasmonic Properties in Metallic Nanostructures by Correlating Photonic and Electronic Excitations Vighter Iberi, Nasrin Mirsaleh-Kohan, and Jon P. Camden* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States ABSTRACT: A large number of optical phenomena rely on the excitation of localized surface plasmon resonances (LSPR) in metallic nanostructures. Electron-energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has emerged as a technique capable of mapping plasmonic properties on length scales 100 times smaller than optical wavelengths. While this technique is promising, the connection between electron-driven plasmons, encountered in EELS, and photondriven plasmons, encountered in plasmonic devices, is not well understood. This Perspective highlights some of the contributions that have been made in correlating optical scattering and STEM/EELS from the exact same nanostructures. The experimental observations are further elucidated by comparison with theoretical calculations obtained from the electron-driven discrete dipole approximation, which provides a method to calculate EEL spectra for nanoparticles of arbitrary shape. Applications of plasmon mapping to the electromagnetic hot-spots encountered in single-molecule surface-enhanced Raman scattering and electron beam induced damage in silver nanocubes are discussed. It is anticipated that the complementarity of both techniques will address issues in fundamental and applied plasmonics going forward.
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nm by using light of different wavelengths. Super-resolution imaging21 was recently applied to plasmonic nanostructures to pinpoint the location of light emission well below the diffraction limit.22,23 Another commonly used method for plasmon mapping is near-field scanning optical microscopy (NSOM), which has demonstrated spatial resolutions of ∼10 nm,24 while local field distributions in plasmonic nanostructures25−30 have been observed using apertureless NSOM (ANSOM). In contrast to these all-optical techniques, electron microscopy can easily image features on the nanometer scale, with the resolution of individual atoms being routinely achieved.31,32 Until recently, however, both experimental and theoretical challenges had limited the application of electron microscopy to plasmon mapping. Several techniques that utilize swift electrons in exciting plasmon modes exist. For example, cathodoluminescence (CL), in a scanning electron microscope (SEM), has been used to map LSPRs in plasmonic nanostructures with a spatial resolution of ∼10 nm.33−35 However, CL is limited in applicability due to its inability to map plasmon modes that do not radiate into the far field (dark modes). More recently, scanning transmission electron microscopy (STEM) has emerged as a highly promising method for plasmon imaging. When a material is exposed to an energetic beam of electrons, such as those in a STEM, the majority of the interacting electrons are elastically scattered; however, a smaller fraction of these electrons are inelastically scattered (Figure 1). An analysis
he unique optical properties demonstrated by noble metal nanoparticles of silver and gold have been utilized since ancient times. For instance, the fourth century Roman Lycurgus cup, which contains a mixture of silver and gold nanoparticles, looks red when light is transmitted and green in reflected light. The optical phenomena manifested by these coinage metals are governed by the excitation of plasmons, which are the collective oscillations of the conduction band electrons in metals1 and whose frequencies are influenced by the size, shape, and surrounding dielectric environment.2,3 When the size of the metal nanoparticle is much smaller than the incident electromagnetic radiation, localized surface plasmon resonances (LSPRs) are generated. On the other hand, surface plasmon polaritons (SPPs) have the ability to couple with incident radiation and propagate on planar surfaces.4 The excitation of the LSPR and the associated near-field enhancement are the basis for emerging applications such as photonic circuits,5 plasmonic solar cells,6 biomedicine,7 and plasmon-assisted catalysis.8,9 These are in addition to traditional applications such as surface-enhanced spectroscopies, both linear10−12 and nonlinear,13−15 plasmonic waveguides,16,17 and photodynamic therapy of tumor cells.18 Despite the fact that most applications of plasmonic particles rely on the large near-field enhancements, it remains quite challenging to directly image these near-field enhancements with the appropriate spatial (∼1 nm) and energy resolution (∼0.1−0.3 eV). While all-optical methods have made impressive progress toward subdiffraction limited resolution, atomic imaging still remains out of reach. Stochastic optical reconstruction microscopy (STORM),19,20 for example, can locate individual fluorescent molecules with a resolution of ∼20 © 2013 American Chemical Society
Received: December 21, 2012 Accepted: March 14, 2013 Published: March 14, 2013 1070
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has mapped plasmons in Ag nanoprisms44 and Au nanorods.45 Yang and co-workers have applied STEM/EELS plasmon imaging to lithographically prepared nanostructures.46,47 A pioneering work by Dionne and co-workers48 utilized STEM/ EELS and analytical quantum mechanical theory to examine the plasmonic properties of ultrasmall (3 eV) that is well removed from the energy of the Raman laser (2.3 eV). Placing the electron beam on the edge of the nanoaggregate, however, leads to a net bonding arrangement of the induced polarization vectors and a strong loss feature at 2.3 eV. Further, in this geometry, a capacitive electric field is localized in the junction, suggesting that the electromagnetic hot spot located between the gaps of nanoparticles can indeed be excited when the electron beam is positioned at the periphery of the nanoaggregate.
energy of the Raman laser (532 nm) used in the SMSERS experiment. Far-field plane-wave excitation of the SMSERSactive nanocluster yields an intense electromagnetic hot spot in the gap region (Figure 4, right), but no intense EEL probability is observed in the same region (Figure 4, middle), in agreement with the experimental findings (Figure 4, left). Insight into this finding was gained by investigation of the polarization induced in the SMSERS-active aggregate at 2.3 eV at different electron beam positions, which are presented in Figure 5. The results demonstrate that positioning the electron beam in the junction of the nanoparticles yields a net antibonding arrangement of the target’s polarization vectors, leading to a 1074
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monochromators mentioned earlier, deconvolution methods that remove the tail contribution of the ZLP due to the point spread function of the detector can be routinely applied to STEM/EELS.78 Going forward, the newest generation of aberration-corrected microscopes are achieving spectacular energy resolutions, with ∼15 meV (∼120 cm−1) being recently demonstrated (unpublished results). To put this in perspective, the vibrational spectrum of a molecule typically spans 370 meV (0−3000 cm−1), and the normal line width of a molecular vibrational transition is 3 meV (20 cm−1). This suggests that the vibrational signatures of molecules may be probed using STEM/EELS in the near future and promises an exciting method to probe the coupling of molecules and plasmonic structures. Our knowledge of the electromagnetic field enhancement factor (EF) from regions between the nanoparticles derives essentially from theoretical simulations such as DDA and finite element method (FEM),79,80 and also experimental approximations incorporating the ratio of the SERS intensity to the normal Raman intensities.68,81−83 Currently, there are no viable techniques that can directly measure the intensity of electromagnetic enhancements, and it remains to be seen whether STEM/EELS is capable of quantifying the intensity of the electromagnetic field. A direct measurement of the electromagnetic field intensity will unambiguously quantify the EF pertinent to SERS experiments and find application to many other plasmon-enhanced processes.
While STEM/EELS provides unprecedented advantages in plasmon imaging, there are still some challenges that hamper its robustness across different platforms. For instance, during STEM/EELS experiments, a buildup of charge on the specimen may occur as the electron beam is rastered over the material. This effect is more evident when thick specimens (with thickness dimensions greater than the mean free path of the electron) are deposited on insulating TEM substrates such as silicon nitride (SiNx) membranes. As the nanoparticle’s thickness increases, some of the incident electrons are slowed down due to multiple scattering events and end up being trapped in the underlying insulating substrate. These trapped charges in the substrate eventually create electrostatic instabilities that yield distorted images. Furthermore, charging can be exacerbated by the longer acquisition times (>12 min) required for high-resolution spatial maps of plasmon modes. Contamination, another effect that is prevalent in STEM/ EELS, can occur when the electron beam polymerizes mobile hydrocarbons present in the sample.76 Electron beam-induced contaminations modify the LSPR of nanoparticles due to a change in the surrounding dielectric environment. We have investigated this effect by measuring the resonance-Rayleigh scattering of a silver nanocube before and after exposure to the electron beam (Figure 6). As is evident, two distinct peaks are observed, and these peaks are shifted after exposing the cube to the electron beam. The first peak (at lower energy) shows a red shift, and the second peak (at higher energy) shifts to a higher energy. Previous silver nanocube studies by Sherry et al.53 and Zhang et al.77 have reported red shifts for both peaks in the LSPR spectra when the refractive index of the substrate is increased. Our observation not only demonstrates blue and red shifts in the LSPR spectra, but also a modification of the LSPR spectral shape induced by the electron beam. This suggests that even when no structural changes are induced in the particle itself, the modification of the dielectric environment by the electron beam might be complicated and further studies are warranted. Several approaches can be employed to alleviate contamination buildup during STEM/EELS experiments, but caution must be exercised based on the unique properties of the sample being examined. For instance, while heating the sample with an electric lamp in air76 is a common approach, it can anneal nanoparticles with regular shapes, which makes shape-dependent STEM/EELS studies impossible. Another approach involves using an anticontaminator to maintain the microscope vacuum at cryogenic temperatures. Any water vapor present in the microscope may condense on the sample and oxidize the hydrocarbons present under the electron beam.76 However, this approach may not be suitable for organic specimens due to erosion. We have previously alluded to high energy resolution being an important factor that must be considered when conducting STEM/EELS experiments. Factors, such as the stability of the high-tension tank, electronic noise, the energy spread of the electron source, and the stability (and resolution) of the spectrometer,78 could affect the FWHM of the ZLP, which determines the energy resolution of a typical STEM/EELS experiment. Due to the intense nature of the ZLP, useful information related to plasmonic excitations that lie in the visible and near-infrared (IR) regions tend to be masked by the tail of the ZLP. Several approaches including improvements in hardware and numerical methods have been developed to reduce the width of the ZLP significantly. In addition to
The extraordinary promise of electron microscopy for the characterization of plasmons, along with its complementarity to alloptical techniques, suggests that it will play an essential role in fundamental and applied studies of plasmonics going forward. In summary, STEM/EELS correlated with optical spectroscopy and theoretical simulations provide the best means of understanding the connection between electron- and photondriven plasmonic processes. We have highlighted progress made in understanding this fundamental connection in metallic nanostructures at the single-particle level. We have also demonstrated the application of STEM/EELS to SMSERS in order to explore the nature of electromagnetic hot spots. Finally, some of the challenges encountered in plasmon imaging with STEM/EELS have been addressed. The extraordinary promise of electron microscopy for the characterization of plasmons, along with its complementarity to alloptical techniques, suggests that it will play an essential role in fundamental and applied studies of plasmonics going forward.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biographies Vighter Iberi received his B.S. from Campbellsville University in 2008 and is currently a Ph.D. student in Professor Jon Camden’s group at 1075
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the University of Tennessee Knoxville. He is currently working on understanding plasmonics in metallic nanostructures using correlated optical microscopy and STEM/EELS. Nasrin Mirsaleh-Kohan received her Ph.D. in Physics from the University of Tennessee under Robert N. Compton, followed by a postdoctoral fellowship with Leon Sanche at the University of Sherbrooke in Canada. She is currently a postdoctoral research associate at the University of Tennessee, and her research interests include surface-enhanced Raman spectroscopy, negative ions, and radiation damage to DNA. Jon P. Camden received his B.S. in chemistry and music from the University of Notre Dame, and his Ph.D. from Stanford University under Richard Zare. After completing postdoctoral research at Northwestern University with George Schatz and Richard Van Duyne, he joined the faculty at the University of Tennessee in 2008. His research interests include STEM/EELS plasmon mapping, surface enhanced nonlinear spectroscopy, sensing, and chemical reaction dynamics.
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ACKNOWLEDGMENTS The University of Tennessee (UT) Office of Research, College of Arts and Sciences, and Department of Chemistry, the UT/ ORNL Joint Institute for Advanced Materials, and the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE-SC0004792 are acknowledged for funding. We thank Drs. David J. Masiello and Stephen J. Pennycook for insightful discussions during the preparation of this manuscript.
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