J. Phys. Chem. C 2010, 114, 7299–7301
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Antenna Sensing of Surface Phonon Polaritons† Frank Neubrech,‡ Daniel Weber,‡ Dominik Enders,§ Tadaaki Nagao,§ and Annemarie Pucci*,‡ Kirchhoff Institute for Physics, UniVersity of Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany, and World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan ReceiVed: September 15, 2009; ReVised Manuscript ReceiVed: January 7, 2010
Resonant coupling between the plasmonic excitation of gold nanoantennas and phonon polaritons of a dielectric substrate-surface layer of about 3 nm thickness produces significant Fano-type signals from that layer in the infrared extinction spectrum of the fundamental antenna resonance. Compared to infrared transmittance spectra without gold antennas, the vibrational signal from the layer is about 1900 times enhanced and it is observed at surface phonon polariton frequencies of the layer, not at the transverse optical phonon frequency. Antenna-like nanostructures are of great interest for sensing applications due to their fascinating optical properties, in particular the resonant electromagnetic near-field enhancement.1,2 It can be utilized for surface-enhanced Raman scattering (SERS),3-5 enhanced fluorescence,6 surface enhanced infrared spectroscopy,7-9 and much more. As shown recently, fundamental antenna resonances of metallic nanowires can be used for surface-enhanced infrared (IR) spectroscopy with extraordinary huge signal enhancement. The precondition is a good match between the energy of the plasmonic excitation and that of the IR active vibration of the molecular species of interest.9 In this contribution we will apply this approach to enhance vibration signals from a SiO2 layer with nanometer thickness beneath the nanoantennas. The obtained results offer interesting perspectives for a sensitive sensing method of ultrathin surface layers. The phonon polariton excitations measured in that way feature a Fano-type signal, whose line shape and intensity are related to the tuning of the antenna resonance relative to the vibration frequency. It is important to note that, different to weakly infrared active vibrations of molecular adsorbate layers like the CH2 stretching vibrations in octadecanethiol (ODT) where the enhanced Fano-type signal appears at its vibration frequency on the antenna resonance curve, the situation is more complex in the case of an ionic material with strong oscillators giving negative values in the real part of the dielectric function. On surfaces and interfaces of such materials the respective phonon polaritons can be excited under suitable conditions.10 Using scanning near-field infrared microscopy for example, surface phonon polaritons can be excited locally, since the additional momentum, which is required for the excitation of surface phonon polariton, is given by the scattering of IR light at the probing tip as shown in refs 11 and 12. Such surface phonon polaritons have frequencies within the reststrahl region of the respective vibration type. It will be a subject of future work to explain theoretically the observed frequency value in cases of interaction with metal nanoantennas. In this communication, as a first step, we would like to show the experimental proof of the enhancement of thin-film phonon †
Part of the “Martin Moskovits Festschrift”. * Corresponding author:
[email protected]; tel, +49 6221 549863; fax, +496221549869. ‡ Kirchhoff Institute for Physics, University of Heidelberg. § World Premier International Research Center for Materials Nanoarchitectonics, National Institute for Materials Science.
polaritons by nanoantennas excited in resonance. Furthermore, from the viewpoint of an experimentalist who wants simply to detect some specific signal from a thin layer, we will give an estimated enhancement factor for the vibrational signal from the thin film without consideration of line shape changes. The gold nanowires were fabricated by electron beam lithography at the MANA Foundry station at NIMS. The electron beam resist (ZEP520A) was spin-coated onto a Si(111) floating zone wafer (resistivity of about 20 Ωm) covered with a natural SiO2 layer and the nanorod structures were drawn by using an Elionix ELS7500 writer with 50 keV beam energy. Then the resist layer was developed with ZED-N50. Gold was deposited by sputtering deposition with a rate of 1 Å s-1, and finally the resist layer was removed in a ZDMAC solution. Gold nanowire arrays with wire-to-wire distances d of 1 and 5 µm in both directions and different wire lengths from 480 to 1730 nm were produced in this way. Width w and height h both are 100 nm for the 5 µm array and 60 nm for the 1 µm array, respectively. A 5 nm thick titanium layer was employed as an adhesive layer between the natural oxide of the Si wafer and the gold nanowires. (As we separately measured for different Ti layer thickness in the range up to 10 nm, neither the antenna resonance curve nor the enhanced vibration features show remarkable changes with Ti thickness.) The thickness of the natural SiO2 layer on the used wafers was optically measured with ellipsometry and determined to be within a range from 3.2 to 2.5 nm depending on the lateral position on the wafer. The spectroscopic measurements were performed with an IR microscope (Bruker Hyperion 1000) coupled to an FourierTransform IR spectrometer (Bruker Tensor 27 with a LN2 cooled mercury-cadmium-telluride detector, optical path purged with dried air). IR transmittance spectra were taken at near normal incidence, see Figure 1, and a numerical aperture NA ) 0.52 was used. Therefore, certainly no light scattered by the antenna arrived at the detector. The circular measurement spot on the sample had a diameter of about 33 µm. Related to the wirearray geometry a certain number of wires thus contributes to the measured IR spectrum. The IR spectra were normalized to that of a bare Si wafer area taken at least 50 µm away from any wire, which means that almost all background features and wafer absorptions including the weak SiO2 layer absorption peak at the transverse optical (TO) frequency (at about 1065 cm-1, see for example ref 13) are eliminated. All spectra were acquired
10.1021/jp908921y 2010 American Chemical Society Published on Web 02/02/2010
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Figure 1. Scheme of sample geometry (side view) showing the wavevector k of the incident IR radiation and the polarization of the electric field E parallel to the gold-wire antenna with length L and height h. The wire-to-wire distance in x direction (dx) is shown. Beneath the antenna, on the silicon surface, there is a SiO2 layer of about 3 nm thickness.
Figure 3. Comparison of extinction cross sections σext as calculated from the IR transmittance spectra per nanowire of the array and normalized to the geometric cross section σgeo ) Lw. The nanowires of the two panels have similar lengths (L ≈ 1500 nm) but different widths w and heights h (as given in the figure). In the respective arrays, the wire-to-wire distances (dx and dy) in both directions are also different. Figure 2. Relative transmittance spectra of approximately 27 gold nanowires (length L ) 1520 nm, width w, and height h ≈ 100 nm) prepared on a Si wafer with a natural SiO2 layer. Due to the resonant near-field enhancement of the gold nanowires, the SiO2 signal is enhanced for parallel polarization (solid line), whereas for perpendicular polarization (dashed line) no SiO2 signal is observed. The inset shows a zoom to the SiO2 phonon range.
with at least 5000 scans and a resolution of 4 cm-1 in the spectral range from 700 to 7000 cm-1. An IR polarizer was inserted in the optical beam path in front of the sample. The relative transmittance spectra in Figure 2 point out the influence of polarization of the electric field on the signal enhancement of the SiO2 layer. Due to a good match between phonon polariton excitation in the SiO2 layer and antenna resonance, a strong Fano-type signal appears on the antenna curve for parallel polarization. The signal corresponds to a frequency-dependent decrease of resonant antenna extinction. In the case of polarization of the electric field perpendicular to the array, the SiO2 signal is absent, since the nanoantenna is not excited in resonance and therefore the near-field is not enhanced. Apart from this it should be noticed that due to screening effects of the substrate the antenna signal on silicon wafers is much lower than signals observed on substrates with low refractive index; see for example refs 14 and 15. As proven by the polarization dependence and by the Fanotype behavior including the disappearance of the signal at the fully detuned situation, the appearance of the polaritonic signal on the antenna-resonance curve is due to resonant near-field enhancement that, in the case of IR nanoantennas, is confined to a small volume at the wire tip ends.16,17 Thus, only SiO2 within this volume beneath the ends of the nanoantennas contributes to the signal. For an estimate on how much the sensitivity to phonon-related excitations of an ultrathin film is increased, we compared the enhanced polaritonic signal I (difference between minimum and maximum transmittance of the polaritonic feature) to the signal strength of the transverse optical phonon signal that could be seen in the same experimental setup without nanoantennas. Please note that a direct comparison of signal strengths of phonon polaritons is not possible, since in pure far-field transmission geometries the polariton cannot be excited without a nanoantenna since there is a mismatch between the dispersion curves of light and polaritons. In the following we will use the term sensitivity enhancement instead of signal enhancement to emphasize the different nature of the respective
signals. Nevertheless, since both signals originate from the SiO2 layer, they can be used to characterize the detection sensitivity. The calculation of the normal transmittance spectrum of a 3 nm thick SiO2 layer on a Si wafer (dielectric constant εSi ) 11.69, normalized to the transmittance of a pure Si wafer) was done using the software package SCOUT18 and the dielectric function of SiO2 as given in ref 13. In the calculation, we found the strongest absorption peak at the TO phonon frequency at about 1065 cm-1 with a signal strength of Icalc ) 0.011. This is in good accord to our ultrahigh vacuum experiments on silicon oxide growth on silicon. With the resonant gold wires, the enhanced polaritonic signal should mainly originate from the SiO2 area Asignal beneath the tip ends of the nanowires, which can be estimated as Asignal ) 2w2. Comparing this area to A0, the area illuminated by IR radiation, and taking into account the number N of nanowires inside the spot, we obtain the sensitivity enhancement IA0/(IcalcNAsignal) ≈ 1500 for nanowires with w of about 100 nm (wire-to-wire distance 5 µm, I ≈ 0.01) and approximately 1900 in the case of 60 nm thick nanowires (wire-to-wire distance 1 µm, I ≈ 0.056), respectively. Figure 3 shows the extinction cross section per nanowire related to its geometric one14 for both kinds of arrays with different wireto-wire distances and wire cross sections. Obviously, the signal strength of the antiresonance features in both spectra is similar despite the fact that for smaller wire cross sections less SiO2 is in contact with the antenna and the thinner antenna is slightly detuned which should result in a smaller SiO2 signal strength (see discussion below). The result indicates a higher near-field enhancement for smaller wire cross sections. However, in the IR the lightning rod effect leads to pronounced near-field confinement between nanowires up to rather large gaps in wire direction,19 which produces some additional near-field enhancement for 1 µm gaps compared to 5 µm gap size. We will experimentally explore the detailed effects of gap size and wire cross section on near-field enhancement in our future experiments. Figure 4c depicts the effect of antenna-length tuning on the intensity and line shape of the SiO2 signal. In the case of the fully detuned antenna, no SiO2 signal appears. In the three other cases shown here, a Fano-type signal is observed. The line shape of the signal is related to the phase shift between the plasmonic excitation and the vibrational one. Concerning the signal frequency, the situation is different to that of a weak molecular vibration oscillator where the vibration signal appears as antiresonance at the vibration frequency.9 The SiO2 signal, also
Antenna Sensing of Surface Phonon Polaritons
J. Phys. Chem. C, Vol. 114, No. 16, 2010 7301 frequency. Nevertheless, a detailed consideration needs elaborated theoretical work. In summary, we have shown the coupling between plasmonic antenna resonances and surface phonon polariton excitations of a thin dielectric substrate-surface layer. This resonant coupling produces a strongly increased sensitivity in IR detection of surface layers, which offers interesting perspectives for sensitive sensing methods. Acknowledgment. The authors would like to thank the Deutsche Forschungsgemeinschaft (DFG Pu 193/9), the Japanese Science Foundation (Strategic International Cooperative Program), and Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) for financial support. A part of this work was supported by the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, MEXT, Japan. D.W. acknowledges a scholarship by the Heidelberg Graduate School of Fundamental Physics. Additional support for the project by the Excellence Initiative of the German federal and state governments via the Innovation Fund FRONTIER of the University of Heidelberg is gratefully acknowledged. We acknowledge also help from the MANA foundry station (especially Akihiko Ohi) for gold nanowire fabrication.
Figure 4. (a) Imaginary and real part of the dielectric function of amorphous SiO2 according to the data from ref 13. (b) Calculated energy loss function Im[-1/ε(ω)] from these data and Im[-1/(ε(ω) + 1)]. (c) Experimental IR transmission spectra for wires with different lengths and similar widths and heights. The SiO2 signal is observed in the range between the maxima of Im [-1/ε(ω)] and Im [-1/(ε(ω) + 1)] (i.e., in the frequency range of the Fuchs-Kliewer surface phonon polariton).
at the best match, appears not at the typical vibration frequency (i.e., the TO frequency, maximum of the imaginary part of the dielectric function, see Figure 4a). Instead, the signal is observed in the range between the maxima of Im[-1/ε(ω)] at 1251 cm-1 and Im[-1/(ε(ω) + 1)] at 1161 cm-1, which is the frequency range of the Fuchs-Kliewer surface phonon polariton.20,21 In general, surface phonon polariton’s dispersion relations of thin dielectric layers on a substrate have two branches, one excitation at the surface ω+ and another one at the interface ω-.22 The surface polariton approaches the Berreman mode (the maximum of Im[-1/ε(ω)])23 in the limit of long waves and wavevector parallel to the surface layer). The other limit is the surface phonon polariton frequency of a dielectric half-space; in the electrostatic approximation it is at the maximum of Im[-1/(ε(ω) + 1)].10,22 The interface polariton (ω-) is influenced by the substrate dielectric function and has lower frequencies than the surface polariton. Its field intensity at the surface decays with layer thickness.24,25 Therefore, electron energy loss spectra of a 2.5 nm thick SiO2 layer on silicon only show one polariton feature in the SiO2 stretching range.26 Because of its frequency range and its stronger field at the surface, we attribute the observed vibration feature to the surface phonon polariton (ω+) of the SiO2 stretching mode of the thin oxide layer on silicon. The necessary wavevector component parallel to the surface could be provided by the near-field scattering process of the antenna, which is in agreement with the observations made in refs 11 and 12. In these scattering near-field infrared microscopic studies, the near-field of the probing tip, which in fact acts as an antenna, locally excites surface phonon polaritons yielding a resonance in the scattered field close to the resonance frequency of the surface phonon polariton and not at the TO
References and Notes (1) Levin, C. S.; Kundu, J.; Barhoumi, A.; Halas, N. J. Analyst 2009, 134, 1745–1750. (2) Pucci, A.; Neubrech, F.; Aizpurua, J.; Cornelius, T.; Lamy de la Chapelle, M. In One-Dimensional Nanostructures; Wang, Z., Ed.; Springer: New York, 2008. (3) Nie, S. M.; Emory, S. R. Science 1997, 275, 1102. (4) Grand, J.; Lamy de la Chapelle, M.; Bijeon, J.-L.; Adam, P.-M.; Vial, A.; Royer, P. Phys. ReV. B 2005, 72, 033407. (5) Billot, L.; Lamy de la Chapelle, M.; Grimault, A.-S.; Vial, A.; Barchiesi, D.; Bijeon, J.-L.; Adam, P.-M.; Royer, P. Chem. Phys. Lett. 2006, 422, 303. (6) Fort, E.; Gresillon, S. J. Phys. D 2008, 41, 013001. (7) Enders, D.; Pucci, A. Appl. Phys. Lett. 2006, 88, 184104. (8) Wang, H.; Kundu, J.; Halas, N. J. Angew. Chem. 2006, 46, 9040. (9) Neubrech, F.; Pucci, A.; Cornelius, T.; Karim, S.; Garcı´a-Etxarri, A.; Aizpurua, J. Phys. ReV. Lett. 2008, 101, 157403. (10) Cottam, M. G.; Tilley, D. R. Introduction to surface and superlattice excitations; Cambridge University Press: Cambridge, 1989. (11) Hillenbrand, R.; Taubner, T.; Keilmann, F. Nature 2002, 418, 159. (12) Ocelic, N.; Hillenbrand, R. Nat. Mater. 2004, 3, 606. (13) Gunde, M. K. Physica B 2000, 292, 286–295. (14) Neubrech, F.; Kolb, T.; Lovrincic, R.; Fahsold, G.; Aizpurua, J.; Cornelius, T.; Toimil-Molares, M. E.; Neumann, R.; Karim, S.; Pucci, A. Appl. Phys. Lett. 2006, 89, 253104. (15) Neubrech, F.; Weber, D.; Lovrincic, R.; Pucci, A.; Lopes, M.; Toury, T.; Lamy de La Chapelle, M. Appl. Phys. Lett. 2008, 93, 163105. (16) Pelton, M.; Aizpurua, J.; Bryant, G. W. Laser Photonics ReV. 2008, 2, 136. (17) Aizpurua, J.; Bryant, G. W.; Richter, L. J.; García de Abajo, F. J.; Kelley, B. K.; Mallouk, T. Phys. ReV. B 2008, 71, 235420. (18) SCOUT software package; M. Theis Hard- and Software, Aachen. (19) Le, F.; Brandl, D. W.; Urzhumov, Y. A.; Wang, H.; Kundu, J.; Halas, N. J.; Aizpurua, J.; Nordlander, P. ACS Nano 2008, 2, 707. (20) Kliewer, K. L.; Fuchs, R. Phys. ReV. 1966, 144, 495. (21) Fuchs, R.; Kliewer, K. L. Phys. ReV. 1966, 150, 574. (22) Ibach, H.; Mills, D. L. Electron energy loss spectroscopy and surface Vibrations; Academic Press: New York, 1982. (23) Harbecke, B.; Heinz, B.; Grosse, P. Appl. Phys. A 1985, 38, 263– 267. (24) Senet, P.; Lambin, Ph.; Lucas, A. A. Phys. ReV. Lett. 1995, 74, 570. (25) Gao, W.; Fujikawa, Y.; Saiki, K.; Koma, A. Solid State Commun. 1993, 87, 1013–1015. (26) Ibach, H.; Bruchmann, H. D.; Wagner, H. Appl. Phys. A: Mater. Sci. Process. 1982, 29, 113–124.
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