Surface-Enhanced Infrared Absorption by Optical Phonons in

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Surface-enhanced IR Absorption by Optical Phonons in Nanocrystal Monolayers on Au Nanoantenna Arrays Alexander G. Milekhin, Sergei A. Kuznetsov, Larisa L. Sveshnikova, Tatyana A. Duda, Ilya A. Milekhin, Ekaterina E. Rodyakina, Alexander V. Latyshev, Volodymyr M. Dzhagan, and Dietrich R. T. Zahn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11431 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 18, 2017

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The Journal of Physical Chemistry

Surface-enhanced IR Absorption by Optical Phonons in Nanocrystal Monolayers on Au Nanoantenna Arrays A.G. Milekhin1,2, S.A. Kuznetsov1,3, L.L. Sveshnikova1, T.A. Duda1, I.A. Milekhin1,2, E.E. Rodyakina1,2, A.V. Latyshev1,2, V.M. Dzhagan4, and D.R.T. Zahn4 1

Novosibirsk State University, Pirogov 2, 630090, Novosibirsk, Russia

2

Rzhanov Institute of Semiconductor Physics RAS, Lavrentiev Ave. 13, 630090,

Novosibirsk, Russia 3

Rzhanov Institute of Semiconductor Physics RAS, Novosibirsk Branch “TDIAM”,

Lavrentiev Ave. 2/1, Novosibirsk, 630090, Russia 4

Semiconductor Physics, Technische Universität Chemnitz, Chemnitz, Germany

*e-mail of the corresponding author: [email protected]

Abstract We report on a study of surface-enhanced infrared absorption (SEIRA) by optical phonons in monolayers (MLs) of CdSe, CdS, and PbS nanocrystals (NCs) deposited on arrays of linear nanoantennas the optimized structural parameters of which allow coupling between the localized surface plasmon resonance (LSPR) and diffraction modes in the far-infrared spectral region. The Langmuir-Blodgett technique was used for homogeneous deposition of the NCs. The structural parameters of the arrays and the NC MLs were determined by scanning electron microscopy. According to the 3D electrodynamic simulations of the electromagnetic field distribution around the antennas, the maximal SEIRA enhancement is realized for an array period of about 15 µm when the energy of a diffraction mode coincides with that of the LSPR mode. SEIRA experimental results are in perfect quantitative agreement with the simulation. The maximal SEIRA enhancement is observed for the nanoantenna length and transverse periodicity predicted by the simulations. The frequency positions of the absorption features reveal that only the NC surface optical phonons are activated in the SEIRA spectra.

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Introduction Studying the vibrational spectra of semiconductor nanocrystals (NCs) in ensembles with a low areal density usually faces the problem of negligible optical response from NC vibrational states when probed by conventional spectroscopic techniques, such as IR absorption and Raman scattering1,2. This problem is commonly solved via applying resonant conditions enabling to enhance the optical signal. In particular, the intrinsic electronic resonances in NCs are widely employed in resonant Raman scattering which requires coincidence of the probing energy with that of interband electronic transitions in NCs1-10. Alternatively, external resonances, such as localized surface plasmon resonances (LSPRs) in metal nanoclusters, are also capable of enhancing effectively the optical response of NCs located in the near-field of the metal nanoclusters11-15. In the latter case, surfaceenhanced Raman scattering (SERS) and surface enhanced infrared absorption (SEIRA) are the most advanced, complementary methods based on LSPR which allow the vibrational states of different symmetry to be probed16-18. As a rule, SERS deals with NCs located in the close vicinity of metal nanoclusters having LSPRs in the visible spectral range, wherein the resonance between the excitation laser energy and the LSPR can be easily achieved. SEIRA experiments can be realized for NCs placed near elongated metal nanoclusters (nanorods or nanoantennas) with a high aspect ratio (length-to-width ratio)18. Such nanoantennas exhibit two LSPR modes polarized perpendicular and along the nanoantenna axis. The former has a LSPR energy in the visible spectral range and can be utilized for SERS, while the latter is at much longer wavelengths and well suited for SEIRA experiments: its LSPR energy can be tuned from near- to far-infrared (or terahertz) via increasing the nanoantenna length. SERS by confined and surface optical phonons in semiconductor nanostructures including CdS11,13,15, CdSe12,15,19-21, CuxS15,22-24, ZnO14,25 NCs and GaN15, AlN15, and ZnO14,15 nanorods was observed and investigated by different groups. A strong SERS enhancement by surface optical (SO) modes with the gain factor up to 104 was reported for ZnO nanostructures14,25. The vibrational response from localized optical phonons in CdSe NC arrays of a low areal density down to a few NCs per µm2 was also probed by SERS15,22. Despite the significant progress in SERS of NC arrays of a low areal density, the plasmonic enhancement of IR absorption by NCs has not been extensively examined to 2 ACS Paragon Plus Environment

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date. Surprisingly enough, only in 2015 A. Toma et al.26 published a pioneering report on observing the spectroscopic signature for a monolayer of CdSe NCs deposited on Au nanoantenna arrays ascribed to a Froehlich mode. In their experiments the absorption enhancement induced by nanoantennas was estimated to be as high as 1⋅106. Au nanoantenna arrays can support various excitations including LSPR and diffraction modes27 which can be simultaneously used for effective SEIRA by an analyte (such as CdSe NCs) placed in the vicinity of nanoantennas26. While the LSPR energy depends predominantly on the nanoantenna length, the energy of the diffraction modes is specified by the antenna array periodicity. Therefore, a proper combination of the structural parameters for nanoantenna arrays can promote further enhancement of the analyte response. In this paper, we report on observing the enhanced phonon response from monolayers of CdSe, CdS, and PbS NCs deposited on periodic arrays of Au nanoantennas having LSPR energies at the frequency of surface optical phonons in corresponding NCs. We show that the NC response can be maximized due to superposing plasmonic and diffraction resonances in the periodic nanoantenna structures.

Experimental The uniform periodic Au nanoantenna arrays with the overall dimensions of 3x3 mm2 differed in nanoantenna lengths and lateral periodicities were fabricated on Si(001) substrates by direct electron beam writing (Raith-150, Germany) as described in24. The nanoantenna width B, height t, and the end-to-end spacing A between nanoantennas were fixed as 50, 50, and 100 nm, respectively (see Fig. 1a). The length L and transverse period Gy of the nanoantennas were adjusted to maximize their near-field enhancement in the spectral domain of phonon modes in CdSe, CdS, and PbS NCs (170-270 cm-1) and ranged between: L=5710-9780 nm and Gy=7690-23080 nm. The calculated structural parameters of the fabricated nanoantennas along with the corresponding LSPR and diffraction mode frequencies are listed in Table 1. The technological reproducibility for the structural parameters of the nanoantennas upon their fabrication was as high as about ±10 nm, while the accuracy in the parameter determination was estimated to be ±5 nm originating from statistical fluctuations in the size of gold grains relative to its mean value of 10 nm. 3 ACS Paragon Plus Environment


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Table 1. The calculated structural parameters of nanoantennas for given LSPR and diffraction mode frequencies n

length L,

transverse Calculated diffraction period Gy, mode/LSPR

nm

nm

frequency, cm-1

1

8360

15385

190/190

2

8500

18460

158/190

3

9160

12310

228/190

4

8770

23080

127/190

5

9780

7690

380/190

6

5710

10700

270/270


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Free-standing monolayers (MLs) CdS and PbS NCs were fabricated using the Langmuir-Blodgett (LB) technique as described earlier15,28. The size of the NCs determined from SEM experiments amounts to (4.5±1.5) nm and (7±3) nm, respectively (see Suppl.materials). Monolayers of colloidal CdSe NCs with a size of (5.0±0.3) nm purchased from Lumidot were homogeneously deposited on specially prepared plasmonic substrates by means of the modified LB technique as described in21. LB is a well-proven technique for fabricating both highly ordered organic films29 and NCs with controlled areal density20,21 on solid substrates. The size, shape, and areal density of CdSe NCs, as well as structural parameters of nanoantennas (length, width, period) were determined by SEM using the same Raith150 system at 10 kV acceleration voltage, 30 µm aperture, and 6 mm working distance. The LSPR energy in Au nanoantenna arrays was determined from FIR transmission measurements carried out in the spectral range of 100- 600 cm-1 using a Bruker Vertex 80v Fourier transform spectrometer. For further evaluation the ratio of the transmission spectra corresponding to the light polarization along the nanoantenna axis and perpendicular to it was calculated and analysed. Non-polarised Raman spectra were measured using a Labram spectrometer equipped with an Olympus BX40 Raman microscope (the laser beam was focused to a spot with a size of at least 5 µm in diameter) operating in backscattering geometry at room temperature. A Diode-pumped solid-state (DPSS) laser (Cobolt) was used as an excitation source at the wavelength of 514.7 nm (2.41 eV). A laser power of less than 100 µW at the sample surface was used to avoid possible effects of local heating. The spectral resolution was better than 2.5 сm-1 in the whole spectral range.

Results and Discussion In order to determine the optimal nanoantenna length L and transverse periodicity Gy for linear nanoantenna arrays with a required LSPR energy and maximal SEIRA enhancement, 3D full-wave simulations using ANSYS HFSS™ electromagnetic software were carried out and the field distribution near the Au nanoantenna on a silicon 5 ACS Paragon Plus Environment


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substrate was computed as described in30. The dielectric function of Si used in the simulations was taken from31, while gold was modeled as a Drude medium having linear plasma and damping frequencies of 72500 cm-1 and 216 cm-1, respectively30- 32. Fig. 1a illustrates the unit cell geometry for the nanoantenna array. For realistic imitation of the manufactured structures, the nanoantennas were modeled as rectangular rods with chamfered edges (from the vacuum side) with a fillet radius of 15 nm. Fig. 1b, c show typical distributions of the middle vertical plane XZ (b) and the electric field magnitude at the Si surface (c) at the frequency of the longitudinal LSPR under normal illumination (the incident wave is polarized linearly along the nanoantenna axis: E0 || X). The field magnitude is normalized to the value when the nanoantennas are removed from Si surface, thereby yielding the relative E-field amplification distribution. The presented distributions are computed for a LSPR frequency of 190 cm-1 and correspond to values L=8360 nm and Gy=15385 nm providing the maximal E-field amplification among alternative {L, Gy} combinations, as elucidated below. It can be seen from Fig. 1b,c that the field is localized predominantly at the nanoantenna edges near the Si surface, while decaying rapidly in space. Depending on the tracking trajectory, the decay length in vacuum at 50% reduction of a local E-field maximum ranges from 14 to 21 nm. This conclusion is deduced from Fig. 1d where the normalized E-field magnitudes calculated along the middle in-plane (OQ) and out-of-plane (OP) lines are plotted. Since the NC size lies in the range of 4-10 nm (see Suppl.materials), such a decay length ensures SEIRA enhancement predominantly occurring for the first few monolayers of NCs deposited on the antennas and in the gap between them.


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Fig.1 a) Geometry definition for the nanoantenna array (the unit cell is shown). b) Distribution of the normalized LSPR electric field magnitude in the middle vertical plane (XZ) simulated for the structural parameters: L=8360 nm, Gy=15385 nm, A=100 nm, B=50 nm, t=50 nm. c) Similar Efield distribution at the Si surface. d) Spatial behaviour of the E-field calculated along the lines OP and OQ passing at 1 nm distance from the antenna’s butt-end and Si surface, respectively. Normal excitation, E0 || X.

As predicted earlier33, the experimentally determined LSPR wavelength depends linearly on the antenna length in a wide spectral range28,33,34. Such behaviour originates from the dipole resonance phenomenon, which states that linear nanoantennas effectively couple to the incident electromagnetic waves polarized along the nanoantenna axis when their wavelength become close to the doubled antenna length34. This coupling also depends on the dielectric function of the surrounding medium that makes the LSPR energy different for nanoantennas backed by a bare Si substrate and when an additional layer is deposited on or beneath the nanoantennas30. In general, this circumstance should be taken into account in the nanoantenna sensor design as even a thin layer of the analyte may noticeably shift the LSPR26, thereby making non-optimal the structural parameters of the nanoantenna array initially optimized without analyte overlayer. In the case of NCs, however, due to the lack of information on their dielectric function necessary for accurately modeling the nanoantennas with deposited NCs, the LSPR simulations in this work were carried out 7 ACS Paragon Plus Environment


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in the absence of NCs. Such an approximation is acceptable when probing NCs with monolayer thickness quantity of which is too small to appreciably affect the LSPR. Our analysis shows that among different combinations of the structural parameters L and Gy providing the required value of the LSPR wavelength, the maximal E-field enhancement is achieved only for a specific pair of {L, Gy}. This optimal case is realized when the plasmonic resonance coincides with the onset of the first diffraction lobe referred to the nanoantenna substrate (Si). Under normal incidence the dispersion relation for the higher order diffraction harmonics (Floquet modes) of the nanoantenna array as a 2D periodic structure arranged on a rectangular lattice reads35: 2

2

k

2 z ,mn

ω2  2 πm   2 πn   − = ε 2 −  , c0  G x   G y 

(1)

where k z , mn is the z-component for the wavevector of the Floquet harmonics with the indices (m, n), ω is the radiation angular frequency, c0 is the speed of light in vacuum, ε is the dielectric permittivity of the medium where the harmonics propagates. The nanoantenna arrays are normally exploited in the diffractionless regime, i.e. when all the non-zero Floquet modes remain evanescent: k z2,mn < 0 , ∀ m + n ≠ 0 . Implying G x < G y , that is typically satisfied for antennas closely spaced in the X-direction (A=(Gx–L)

Surface-Enhanced Infrared Absorption by Optical Phonons in

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