Impact of Metal-Optical Properties on Surface-Enhanced Infrared

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Impact of Metal-Optical Properties on Surface Enhanced Infrared Absorption Michael Tzschoppe, Christian Huck, Jochen Vogt, Frank Neubrech, and Annemarie Pucci J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04209 • Publication Date (Web): 17 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018

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The Journal of Physical Chemistry

Impact of Metal-Optical Properties on Surface Enhanced Infrared Absorption Michael Tzschoppe,† Christian Huck,† ,* Jochen Vogt,† Frank Neubrech,†,‖ and Annemarie Pucci† †

Kirchhoff Institute for Physics, Heidelberg University, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany

‖

4th Physics Institute and Research Center SCoPE, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany

1 ACS Paragon Plus Environment

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ABSTRACT: Surface-enhanced infrared absorption (SEIRA) spectroscopy using resonant metallic nanostructures is increasingly attracting interest during the last decade. Nevertheless, the impact of the metals' intrinsic properties on SEIRA is still little studied. We present an experimental work on this topic, examining the infrared-optical resonance spectra of linear nanoantennas made of five of the most common metals (gold, silver, copper, aluminum, and iron) with respect to the intrinsic and radiation damping. Highly material and size dependent ratios of the two damping contributions were found and discussed. Using layers of organic probe molecules, we obtained SEIRA enhancement factors for the different nanoantennas and experimentally verified the predicted relationship between the plasmonic damping mechanisms and the SEIRA enhancement. The multitude of our experimental data for the ratio between the intrinsic electronic damping and the radiation damping is compared with the measured SEIRA enhancement of the various nanoantennas and so deliver the proof that the best SEIRA enhancement is achieved when both damping mechanisms equally contribute. Furthermore, it is shown that for a given nanoantenna geometry the redshift away from the plasmonic extinction maximum is strongly dependent on material parameters.


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INTRODUCTION Surface enhanced infrared (IR) absorption (SEIRA) spectroscopy by using resonant plasmonic nanostructures is of particular interest since the very weak vibrational signals of molecules and ionic compounds can be enhanced by many orders of magnitude.1,2 IR investigations allow identification and characterization of analytes label-free and without destruction. Linear nanoantennas,3,4 nanoantenna dimers,5,6 nanoantennas on pedestals,7–9 and inverse structures10,11 were intensively studied in order to optimize the enhancement and to pave the way for routine sensing applications. In addition to the variety of different geometries that have been studied in recent years, research was also done on the role of the arrangement of nanoantennas in periodic arrays, leading to optimized configurations with increased signal enhancement.12,13 But an important aspect, the choice of the material of the plasmonic nanostructures, has not been comprehensively studied in the past. The choice of material is interesting in several respects. First, it strongly influences the extinction cross section of the nanostructures and is important for the intrinsic damping of the material. Secondly, the use of different materials opens the door to a variety of surface chemistries with various functional groups,14,15 which is extremely interesting for the application of SEIRA in sensor technologies, because the molecules to be detected must be brought as close as possible to the surface of the nanostructures and need to be stable on the structures.16 Another aspect that may influence the choice of material is from a more practical point of view, namely the chemical stability and the manufacturing process including a possible integration into common electronic devices. E.g., doped semiconductors are promising materials for the on-chip integration17 of plasmonic antennas, sensors,18,19 and detectors.20,21



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Although the underlying material of the nanostructures represents an interesting degree of freedom, most of the investigations so far concentrate on gold nanostructures. This can be explained by the well-known benefits of gold. Gold is a low-loss metal, can be excellently described over a broad spectral range by the Drude model, is biocompatible and does not age. Nevertheless, nanostructures made out of other materials have also been investigated recently. The application of non-precious metals may have big advantages as it was shown by Cerjan et al.22 for aluminum that has a plasma frequency much higher than silver and gold. This work demonstrates that the native oxide layer prevents the complete oxidation of the aluminum nanostructures and that plasmonic enhancement persists. For all plasmonic structures including the plasmonic application of graphene,23 low plasmonic damping is highly desired. In metallic nanostructures an important amount of damping is due to intrinsic damping from electronic scattering.24 The other loss channel is radiation damping. It is present for all kinds of materials and is related to the geometry of the radiating particle and to the oscillator strength of the resonance.25–28 In 2013, Abb et al.29 demonstrated that there is a huge impact of the nanostructures radiation scattering behavior on the coupling among each other (the so-called far-field coupling). Whereas the arrangement of gold nanoantennas dramatically influences the plasmonic response in the mid infrared regime, there is much less influence of the arrangement on indium tin oxide (ITO) nanostructures, which allows the design of high-density arrays for SEIRA applications. The reason for this effect is the much lower oscillator strength for the plasmonic excitations of ITO structures because of the lower free charge carrier density. Therefore, the plasma frequency is in the near infrared.30 So the emitted field strength (thus the radiation damping) is low and so is the electromagnetic coupling in the array.


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Considering commercially viable sensing applications, ways for large area fabrication of nanostructures in reasonable time scales were studied during the last years.13,31,32 Thereby Braun et al.32 raised the question to what extend the metallic optical properties affect SEIRA by offering a way for the preparation of large area substrates of different metals. Due to the growing interest in a larger variety of materials,29,32–34 we carefully study the impact of different metals on SEIRA and investigate the influence of the materials' infraredoptical properties on the plasmonic resonance spectrum of linear nanostructures. For accessing a large variety of metal-optical properties,35 nanoantenna arrays of the metals copper, silver, gold, aluminum, and iron, respectively, were fabricated by using electron beam lithography (see METHODS for details). A thin layer of the tetrafluorinated zinc phthalocyanine complex F4ZnPc (C32H12F4N8Zn, see Figure S1 in the Supporting Information for the structural formula), was used to investigate the SEIRA effect. By the application of a quasi-static model for the absorption and extinction cross sections (with a correction for radiation damping) the plasmonic extinction spectrum in the mid infrared region is well described and the Drude model parameters for each material are extracted.36 For the effective plasma frequency, it should be noticed that a real metallic nanostructure suffers from various defects, for example grain boundaries, which also give rise to a change of the average plasma frequency and not only of the electronic damping.37 Studying the various nanoantenna arrays’ plasmonic properties (before the deposition of the probe molecules) and the associated SEIRA signals after the deposition, enables us to determine the proportion of the intrinsic damping (electronic scattering) and the radiation damping for each metal and antenna geometry. Thereby we experimentally confirm the assertion of Neuman et al.36 that similar proportions of both damping contributions are beneficial to achieve optimal



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SEIRA enhancement, and verify that the optimal geometry of the nanoantennas for SEIRA is clearly determined by the intrinsic scattering rate. For the search for even higher enhancement38 we give recommendations how to design the nanoantenna for a certain metal to achieve the highest possible SEIRA enhancement per molecule. METHODS Metal Nanoantenna Fabrication. Metal nanoantenna arrays were fabricated with electron beam lithography (EBL). A bare CaF2(100) substrate was pretreated with oxygen plasma for 30 s under a pressure of 0.4 mbar with a power of 150 W. A 160 nm thick film of PMMA (Allresist GmbH, 950 K) was spincoated at 1500 rpm for 60 s on the slightly modified CaF2 surface and baked for 3 min at 150°C. Before electron exposure (Zeiss Leo 1530 scanning microscope equipped with a Raith EBL setup) a thin film of aluminum was thermally evaporated on the PMMA covered sample to avoid charging effects. For exposure, an acceleration voltage of 15 kV, a beam current of 12 pA, an aperture with a diameter of 7.5 µm and an exposure dose of 300 µC/cm² was used. The sample was immersed in a NaOH solution to remove the aluminum layer before developing the exposed photoresist using methylisobutylketone and isopropyl alcohol solution with a mixing ratio of 1:3. The developed PMMA was removed with isopropyl alcohol in a subsequent step, resulting in a nanometer sized shadow mask for evaporating of metal. For each sample, a chromium adhesion layer with a thickness of 3 nm was evaporated on the shadow masked CaF2 substrate with an evaporation rate of 1 nm/min and a background pressure below 1 ∙ 10

mbar. The different metals were evaporated directly afterwards, without breaking the

vacuum to avoid oxidation of the chromium layer. The metal films with a thickness of 47 nm were evaporated with an evaporation rate of 1 nm/min, never exceeding a pressure of 3.3 ∙ 10

mbar. Next, the surplus metal and photoresist was removed during the lift-off step by


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shaking the sample in boiling acetone, resulting in well-defined nanostructure arrays (100 µm  100 µm) with different nanoantenna geometries on the substrate. Evaporation of a Thin Film of F4ZnPc. The organic semiconductor F4ZnPc was used as a molecular probe layer in our SEIRA studies. F4ZnPc physisorbs on metals and is highly IRactive in the fingerprint regime. In order to clean the samples before deposition of the probe layer, the samples were treated with hydrogen plasma for 15 s at a pressure of 0.4 mbar with a power of 150 W. Hydrogen plasma treatment was used as alternative to the commonly used oxygen plasma cleaning procedure9,10,39 preventing oxidation of the non-precious metals while removing any residues of the photoresist. A 10 nm thin film of F4ZnPc was thermally evaporated on the nanoantennas with a deposition rate of 0.9 nm/min and a background pressure below 9.9 ∙ 10

mbar. The crucible temperature

was 390°C. The thickness was checked by fitting the IR transmittance of the layer (without antennas) with an already established oscillator model (with optical parameters derived for thicker layers) using the software SCOUT.40 Microscopic IR-Spectroscopy. The IR spectroscopic measurements were performed with a Bruker Hyperion 1000 IR microscope coupled to a Bruker Tensor 27 FTIR spectrometer. An IR polarizer was set into the beam to excite the nanoantennas with the light’s electrical field along the long antenna axis. The transmittance beam path was purged with dry air to minimize atmospheric absorptions of water and carbon dioxide. The signal was detected with a mercury cadmium telluride (MCT) detector kept at liquid nitrogen temperature. An optical microscope was used to locate the array on the substrate. The transmittance measurements were carried out with an aperture of 58.3 µm diameter. For the accumulation of spectra, several hundred scans



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with a spectral resolution of 2 cm-1 were used. The background scans were taken at an unstructured region of the CaF2 substrate as close as possible to the nanoantenna array. In order to disclose degradation effects that differently might modify the plasmonic properties of nanoantennas made of different metals, IR measurements were repeated after ca. six months storage in a desiccator at room temperature. With the organic cover layer, the IR plasmonic signals turned out to be almost unchanged for all metals under investigation as shown in Figure S2 in the Supporting Information.

RESULTS AND DISCUSSION Analysis of the plasmonic resonances Nanoantennas made of different materials were prepared on a pretreated CaF2 substrate using electron beam lithography and subsequent thermal evaporation (see METHODS for details). The structures’ height was set to ℎ

50 nm, including the chromium adhesion layer of 3 nm. The

produced nanoantennas have various widths, 50 nm, 60 nm, 80 nm, 100 nm, 150 nm, and 200 nm. The lattice constants of the arrays are 𝑑x

𝑑y

4000 nm in the direction

perpendicular (𝑑x ) and parallel (𝑑y ) to the antennas (see Figure 1a). The large lattice constants avoid any influence due to collective lattice modes of the nanoantenna arrays13,41 allowing statements of the individual antenna properties. The lengths were tuned to achieve plasmonic resonances in the fingerprint regime.


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Figure 1. (a) SEM image of a gold nanoantenna array with antenna width 𝑤 fabricated on a CaF2 substrate. The periodicities 𝑑x

𝑑y

200 nm,

4000 nm, the antenna length 𝐿 and

the width 𝑤 are indicated. (b) Relative transmittance spectra (stacked) of nanoantenna arrays of the metals iron, aluminum, silver, gold and copper with width 𝑤

50 nm. For the Cu antenna

the maximum extinction is indicated (as 𝛴max . (b) Photon wavelength 𝜆plas (for the antenna width 𝑤

50 nm) at maximum extinction versus antenna length 𝐿. For comparison, the values



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for the resonant wavelengths (2𝜋𝑐/𝜔res , c as the vacuum velocity of light) from the fits with Equation (1) are added (open symbols, position indicated by the grey ellipses). (c) Normalized extinction cross section at maximum extinction versus antenna width 𝑤. The antenna lengths were chosen to match a plasmonic resonance frequency close to the strongest vibrational bands of F4ZnPc.

Figure 1b shows the IR optical response of nanoantenna arrays for selected geometries and different metals. For each metal, an antenna width of 𝑤

50 nm was used whereas the lengths

are chosen to match the plasmonic resonance spectrum to typical vibrational bands of F4ZnPc in the mid IR. The wavelength 𝜆plas at the maximum of the extinction 𝛴 is shown as a function of the antenna length (for the antenna width 𝑤

50 nm) for the five different metals in Figure 1c. We

obtain similar linear9,42,43 slopes for aluminum, copper, gold and silver, whereas iron is resulting in a steeper increase than the other metals. Further, there is an impact of the metals’ plasma frequency on the slope of this curve expected according to 𝜆plas ∝ 𝐿⁄𝜔p .44 Based on the analysis of the slope according to Ref. 44 we conclude that the plasma frequencies of all nano-scaled materials are similar, except of iron. The same is obtained in literature: The values of the materials aluminum45 (77700 cm-1), copper46 (66000 cm-1), gold37 (67900 cm-1) and silver47 (71800 cm-1) are similar, whereas the value for iron46 (35200 cm-1) points out a significantly lower plasma frequency. Note, that for aluminum there is a significant mismatch with other literature values (e.g. 119000 cm-1).35 Our aluminum antennas certainly do not have an ideal homogeneous morphology. This non-ideal morphology may strongly influence the plasma frequency. It is well known that the thermal evaporation of aluminum results in a grainy


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morphology.45 As a result, the antenna consists of an effective conductive medium that for high enough filling factors can be described by a Drude model but with an effective plasma frequency.37,48 The results for the maximum extinction efficiency 𝜎ext ⁄𝜎geo (see Supporting Information for the details on the calculation) are plotted versus the antenna width 𝑤 for each metal in Figure 1d. The plasmonic resonance frequencies are similar for all the data points in that figure (1300 cm-1). The highest values are observed for copper, while the values for gold and silver are similar. First, looking at the antennas with the broadest width, we get almost similar values for the normalized extinction cross section for all metals. In this regime, the geometry dependent radiation damping should be dominant, which explains the almost equal extinction for the various metals. Only iron shows a quite low extinction cross section also for broader antennas, presumably because of the important contribution from electronic damping. With decreasing antenna width, the differences in extinction for the various metals increase, which makes obvious the vanishing importance of the geometry dependent radiation damping and the increase of the electronic scattering rate’s importance. Copper was found to have the smallest contribution of the electronic damping for the smaller widths of this study since it reaches the highest ratio of extinction cross section to geometric cross section. These trends demonstrate highly material dependent plasmonic properties and the crucial role of electronic damping. In case of nanostructures, this damping strongly depends on the size and the manufacturing process (including the adhesion layers) that determines the amount of grain boundaries and impurities.



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Figure 2. Relative transmittance spectra of gold nanoantenna arrays, measured with light polarized along the long antenna axis (see inset). For every width between 𝑤

50 nm and 𝑤

200 nm, the longest antenna length was chosen in order to avoid an overlap with the collective lattice modes, indicated by the black triangle. The spectra were fitted with Equation (1) to extract the contribution of the electronic scattering and radiation damping.

In literature, there are many publications available, reporting Drude parameters (electronic scattering rate 𝜔 , plasma frequency 𝜔p , dielectric background 𝜀 ) of metal films. It was found that for the very same material these parameters dramatically vary for thin films45 as well as for different morphologies37 as it was shown by Fahsold et al.49 in the example of iron. For the analysis of the plasmonic properties of the nanoantenna arrays in the IR it is important that in the range below 2000 cm-1 the metals under investigation can be well described by the Drude model. Deviations to the Drude-type behavior due to interband transitions arise only at higher


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wavenumbers.37,50 According to several previous studies24,36,51 we extract these parameters from the plasmonic resonance spectra. The relation applied is based on an oscillator model,26 and quantitatively allows the determination of the electronic scattering rate 𝜔 and the radiation damping 𝜔rad (see Supporting Information). Applying the relation to the plasmonic extinction to a relative transmittance (Trel) measurement yields 𝑇

1

𝑚∙𝑇∙ res

( 1)

where normal incidence of light and very thin (compared to the IR wavelength) antennas are assumed and a correction for radiation damping is inserted, see below. The parameter 𝑚 contains information about the surrounding refractive index and the area density of the nanostructures. It might be also influenced by non-optimal referencing and the numerical aperture of the IR microscope. Since the same substrate material and antenna density per area was used for all samples and the same experimental setup was used for all measurements, we assumed 𝑚 to have a constant value. The Larmor-time parameter51 𝑇 is mainly determind by the structures’ volume, the surrounding refractive index, and the plasma frequency. The contribution of the radiation damping 𝜔rad

𝜔res 𝑇, is calculated at the plasmonic resonance frequency 𝜔res . Experimental

spectra for gold and their model spectra for the best-fit parameters are shown in Figure 2. The low frequency limit of the fitting range was chosen to be 800 cm-1 (limit given by the spectral range of the experimental set up). The plasmonic resonances’ high frequency side was fitted only up to the wavenumber at one half of the maximum intensity in order to avoid influences of higher harmonics52 and the collective lattice mode13 that is indicated by the black triangle. For all antenna widths and metals under investigation, the fit with Equation (1) well describes the spectral line shape. Slight deviations of the fit from the data points at the high frequency side indicate the tails of a second order (defect related52) antenna resonance or by the collective mode,



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as mentioned above. The results for 𝜔res from the spectral fits determined by using Equation (1) are in accordance with the values of 𝜆res

2𝜋𝑐/𝜔res (𝑐 as the vacuum light velocity) read out at

the maximum of the extinction 𝛴 as shown in Figure 1c (indicated by the grey ellipses). The difference between 𝜔res (which peaks at the maximum near-field of the antenna) and the far-field extinction maximum (𝜔plas

2𝜋𝑐/𝜆plas is too small to show up in this figure. The values for

𝜔 and 𝜔rad , including also the fit results for the other metals, are plotted in Figure 3a-b versus the antenna width. The resulting 𝜔 values deviate from values in a typical data basis,35 which is mainly caused by the different amounts of defect scattering in the polycrystalline material used for the data base and in the metallic nanostructure. Interestingly, for all metals under investigation, 𝜔 decreases with increasing width. This trend is partially due to surface scattering that decreases with the decreasing surface to volume ratio of the antennas (

1⁄ 𝑤

1⁄ ℎ ,

resulting in less electronic surface scattering.24 However, a change of the defect density with increasing width cannot be excluded. For gold, the electronic scattering rate 𝜔 𝑤 196

60 nm

16 cm-1 agrees with the results for the resonance of a similarly shaped gold antenna on

CaF2 which has been studied in Ref 36. Along with the downward trend of 𝜔 , an increase of 𝜔rad is observed upon the increasing antenna width, illustrating the gain in dominance of the radiation damping for larger volumes. Our experimental results reveal the lowest electronic scattering for copper compared to the other metals, equally followed by gold and silver. Aluminum and iron yield higher values. This trend agrees with the trends of the literature data.35,49 To reconcile the statement that SEIRA enhancement for Drude-type metallic antennas is highest when 𝜔

𝜔rad ,36 we present the ratio of these values in Figure 3c. For copper, gold,

silver, and aluminum, a balanced contribution 𝜔 /𝜔rad geometries in the range between 𝑤

50 nm and 𝑤

1 was found for available antenna

200 nm, whereas the electronic scattering


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is dominant for the whole experimental range in the case of iron nanoantennas. Therefore, a comparison of the width for equal damping contributions with the width for optimal SEIRA enhancement is possible for all metals but iron.

Figure 3. Results for the fits with Equation (1) of the nanoantenna arrays with the greatest length for each width and metal: (a) electronic scattering rate 𝜔 and (b) radiation damping rate 𝜔rad , (c) ratio of 𝜔 and 𝜔rad shown in a) and b), respectively, as a function of the antenna width for



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each metal. The iron data are in a different order of magnitude range and shown in the inset. (d) Total damping 𝜔tot

𝜔

𝜔rad as a function of the antenna volume 𝑉. The values are

normalized to the result for the smallest volume.

The total damping 𝜔tot

𝜔

𝜔rad , normalized to the value for smallest volume, is shown

Figure 3d. The radiative part is proportional to the volume28 𝑉 (more precisely, to the volume of the excitation) and an increase of the total damping with increasing 𝑉 (due to an increase in width and/or height of the nanoantenna) should be seen when radiation damping is the dominant contribution and if the effective plasma frequency is not changing with 𝑉. It is important to note that the volume where the plasmonic oscillation is excited should be proportional to the width of our nanostructures (that have the same height of about 50 nm) because the typical penetration depth of radiation in the mid IR is similar to our structures’ thickness.53 For iron, aluminum, and silver there is no increase apparent for increasing width or volume, respectively. In contrast, a slight increase can be seen for the biggest volume for gold. Only for copper, an increase of 𝜔tot can be seen. The different behaviors of these antennas from the other metals indicate also morphological differences for different antenna volumes 𝑉. To characterize the quality of the evaporated metals, the results for 𝜔 of antennas for two widths (𝑤

60 nm and 80 nm) are

compared with selected literature values in Table 1. Literature values for electronic scattering rates very much differ, which is dependent on several factors. These are the analytical model (frequency dependent or not frequency-dependent parameters in a Drude-type model) and type of measurement and of sample. The samples of the literature studies are often polycrystalline and mostly have surface layers modified by polishing procedures and surface species. For thin layers, electronic scattering at the interface can be stronger than on the free surface. Furthermore, there


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are already differences between the phonon contributions to the Drude parameters for zero frequency and IR frequencies, as discussed for example in Ref. 24. In this reference it is also shown that the plasmonic resonances of single crystalline lead antennas correspond to electronic scattering rates much smaller than the bulk values from Refs. 35 and 54. Although, the antennas of his study are not single crystals, the electronic scattering rates obtained are mostly smaller than the literature values. This result indicates the high-quality (regarding morphology and purity) of the metal nanostructures. It might also be a consequence of the different kind of optical excitation in which the charge carriers oscillate along the long axis of a metal rod. In the Supporting Information we evaluated the contributions of intrinsic damping and radiation damping (analogous to the data in Figure 3) from the full width at half maximum. Thereby, we followed the approach proposed by Novo et al.28 and observe results similar to those from the spectral fits with Equation (1).

Table 1. Intrinsic damping 𝝎𝝉 (in the mid-IR) for antennas height of 𝒉 width 𝒘

𝟔𝟎 nm and 𝒘

𝟓𝟎 nm and two

𝟖𝟎 nm in comparison with selected literature data obtained

from IR measurementsa Material

𝜔 (cm-1) 𝑤

60 nm

𝜔lit (cm-1)

𝜔 (cm-1) 𝑤

80 nm

Cu

123

17

112

18

73 (Ref. 35), 300 (Ref. 55)

Ag

174

40

130

17

145 (Ref. 35), 312 (Ref. 47)

Au

196

16

151

18

215 (Ref. 35), 221 (Ref. 37)

Al

288

19

231

21

647 (Ref. 54), 660 (Ref. 35)



Fe

a

655

8

650

7

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500 – 1000 (Refs. 46 and 49)

The error of our data results from the accuracy of the fits. The literature values for copper55 and

iron46,49 were taken from frequency dependent models in the spectral range of the plasmonic resonances. Frequency independent parameters from literature were used for gold,35,37 silver,35,47 and aluminum.35,54 The values for aluminum especially strongly vary for different thicknesses and morphologies,45 respectively. A recent publication56 shows higher near IR reflectance for copper than calculated with older textbook data. Higher near IR reflectance means lower electronic scattering. It is interesting to have a look at plasmonic resonance studies with aluminum nanoparticles. As usually done, FDTD simulations based on several ten years old textbook data are compared to measurements. Interestingly, for the smallest Al particle in Figure 2 of Ref. 57 the experimental line is smaller than the simulated one.


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Figure 4. Upper panel: Relative transmittance spectrum of a 10 nm thin film of F4ZnPc on CaF2, measured on a mm²-sized area (aperture diameter of 5 mm). The strongest well-separated vibrational bands of F4ZnPc (1338 cm-1, 1400 cm-1, 1491 cm-1) are indicated. Middle panel: Relative transmittance spectra of the nanoantenna arrays for the different metals. The respective baselines are shown in black. Lower panel: Baseline corrected SEIRA signals of F4ZnPc. For the purpose of a better visibility, the two lower panels show stacked spectra.

SEIRA enhancement A thin layer (

10 nm) of the probe molecule F4ZnPc was thermally evaporated on top of the

antenna arrays (see METHODS for details). This molecule, used in organic electronics,58,59



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features many vibrational bands in the mid infrared region, equally physisorbs on all metals used, and is quite stable, which makes it ideally suitable for our purposes. For the evaluation of the enhancement, we considered the molecule’s vibrational bands at 1338 cm-1, 1400 cm-1, and 1491 cm-1. Experimental SEIRA spectra are shown in Figure 4. In the upper panel, the relative transmittance of a F4ZnPc layer without antennas is shown for comparison. The vibrational signal strength 𝑆layer (measured with the measurement spot 𝐴layer ) for this measurement without antennas is used to reference the enhanced signal strength 𝑆SEIRA for each of the inspected vibrational bands. The middle panel shows one relative transmittance spectrum of nanoantennas covered with F4ZnPC for each metal. The selected spectra shown for each of the metals are those featuring the highest SEIRA signal. A baseline calculated according to an algorithm proposed by Eilers39,60,61 for each spectrum is also plotted. The bottom panel shows the baseline corrected signal of the molecular vibrations. Depending on the ratio between the resonance frequency and the molecular vibration, the so-called tuning (𝜔vib /𝜔plas ), a more or less asymmetric Fano-type signal is observed. The signal 𝑆SEIRA is read out as peak-to-peak value in each case. To obtain comparable values for the signal enhancement due to a plasmonic resonance we considered three referencing steps in our analysis (see Supporting Information for more details): First, we referenced the respective signal sizes 𝑆SEIRA /𝑆layer as described above. Second, we considered the ratio of the active area in SEIRA (𝐴

) with respect to the area of the reference

measurement 𝐴layer /𝐴SEIRA . Third, the nanostructures’ plasmonic resonance frequency was adapted to a certain value (𝜔plas,ref

1350 cm ) as 𝜔plas /𝜔plas,ref

according to Weber et

al.62, which is of importance to compare nanoantennas with different resonance frequencies. Following this approach, a normalized enhancement factor 𝐸𝐹n is calculated as


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𝐸𝐹n

𝑆SEIRA 𝐴layer 𝜔plas 𝑆layer 𝐴SEIRA 𝜔plas,ref

,

(2)

for various detuning between plasmonic resonance and vibrational frequency, which leads to highly tuning dependent values for the SEIRA enhancement as exemplarily shown for copper in Figure 5a. We found a huge impact of the geometrical antenna width on 𝐸𝐹 as function of the tuning ratio, especially on the maximum enhancement.

Figure 5. (a) Normalized enhancement factor for copper nanoantenna arrays, calculated by using Equation (2), as a function of the tuning. The strongest vibrational bands of F4ZnPc (𝜔 1338 cm-1 and 𝜔

1491 cm-1, in parts also 𝜔

1400 cm-1) are extracted from baseline

corrected SEIRA spectra. The data was sorted by antenna width for variable length. To estimate the maximum enhancement for each width, the data is fitted with Equation (3) (solid lines in the same color as the data points). (b) Maximum of the normalized enhancement factor for each metal as a function of the nanoantenna width 𝑤, as obtained from the fit with Equation (3) as exemplarily shown for copper in (a). The inset shows the values for iron with a smaller scale to



Page 22 of 35

illustrate the curvature of the data. The maximum of each curve is indicated by the triangles of the same color.

The frequency for the maximum enhancement (where the vibrational frequency is equal to the NF of the near-field spectrum) is shifted away from the frequency where maximum position 𝜔max

the plasmonic extinction maximum 𝜔plas matches the spectral position 𝜔vib of the vibrational bands (tuning ratio 𝜔vib /𝜔plas

1 .61,63 We fitted an analytical function to the data for 𝐸𝐹n in

order to get the precise maximum of 𝐸𝐹n (the SEIRA maximum) and its position (in terms of a tuning ratio) for each width. We take a Lorentzian oscillator model26 as the analytical function that is used for the fit, 𝐸𝐹n 𝑥

𝑥

𝑥

𝑚∗ ∙ 𝑇 ∗ 𝑥 𝜔∗

𝑥 𝑇∗

,

(3)

with 𝑥 as the tuning ratio and 𝑥 as the normalized resonance frequency. The model describes the near-field spectrum of an oscillating dipole, as it is underlying the model for the transmittance in this study (Equation (1)). Therefore, the damping parameters marked by a star could be well described by the corresponding parameters in Equation (1), resulting in a very good agreement of the experimental data and the fits, as exemplarily shown for copper in Figure 5a. The electronic scattering 𝜔 (see Figure 3a) nicely agrees with the results for 𝜔∗ ∙ 𝜔plas (

𝜔 ) shown in

Figure S4 (see Supporting Information), which demonstrates the conformance of the applied theory for both, describing the plasmonic excitations’ spectral line shape and the spectral shift between near-field and far-field. Also for 𝑇 ∗ we got reasonable values, which are comparable with the results from fitting the plasmonic resonances. The deviations for these values from 𝑇 in


Page 23 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Equation (1) are caused by the consideration of a different number of antenna lengths (a larger variety of width and length for fitting 𝐸𝐹 𝑥 . The analysis of the maximum (normalized) enhancement as a function of the antenna width is shown in Figure 5b. Following the copper data from 𝑤

200 nm to lower widths, there is a

steep increase of the enhancement observable. Below the optimal width of 𝑤opt Cu

60 nm,

the enhancement decreases for smaller antennas. An equivalent behavior is observed for silver, gold, and aluminum, resulting in broader optimum geometrical widths 𝑤opt Ag 80 nm, and 𝑤opt Al

𝑤opt Au

100 nm. The inset presents the values for iron on a reduced scale,

revealing that there is no maximum observable. Therefore the optimal value for iron was found to be 𝑤opt Fe

200 nm (for the height about 50 nm). Our experimental work points out that

the radiation damping is dominant for large volumes, whereas the influence is negligible for small antennas. In that case, the properties are dominated by the electronic surface and defect scattering of each metal, fully analogous to the results for the normalized extinction cross section shown in Figure 1d. For the bigger volumes, the geometry dependent radiation damping and the SEIRA enhancement approach similar values for the various metals. The optimum widths along with the maximum (normalized) enhancement factors are listed in Table 2 for each metal. Further, the shift between the near- and far-field maximum position for each material, i.e. the metals’ influence on 𝑥 determined. We compare the relative shift 𝛿

𝜔plas

NF 𝜔max /𝜔plas (see Equation (3)) is also NF 𝜔max /𝜔plas for an antenna width of

80 nm because no reliable fits could be performed for iron antennas with smaller width. The results are given in Table 2. The smallest 𝛿 is obtained for copper, which corresponds to the small damping in this nanostructures. Similar results were obtained for gold and silver, which is



Page 24 of 35

consistent with our analysis of the arrays’ plasmonic properties. The value for gold is in agreement with literature.63

Table 2. Optimal width, maximum enhancement, and relative shift 𝛿 between the spectral maximum of the near-field intensity and of the extinction for different nanostructuresb Material

b

𝑤opt (nm)

𝐸𝐹nmax (10³)

𝛿 %

Cu

60

6

51.1

2.6

0.4

0.1

Au

80

5

29.8

1.5

1.1

0.3

Ag

80

5

31.0

1.6

1.2

0.3

Al

100

10.4

0.5

3.2

0.8

Fe

200

0.88

0.05

10.5

2.6

Optimal width 𝑤opt , maximum enhancement 𝐸𝐹nmax , evaluated from Figure 5b, and relative shift

𝛿

𝜔plas

NF 𝜔plas /𝜔plas for an antenna width 𝑤

80 nm. The 𝛿 values were evaluated as the

maximum positions of the fits with Equation (3). For 𝑤opt , we determined the experimental error by studying scanning electron microscopy images of the structures. The errors for 𝐸𝐹nmax and 𝛿 directly result from accuracy of the fits.

CONCLUSIONS Investigating nanoantennas of five different metals, we characterized the influence of the intrinsic and radiation damping on the plasmonic properties and on the SEIRA enhancement. The


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detailed analysis of the extinction cross sections and the plasmonic damping let us identify the geometry ranges where electronic surface scattering or radiation losses prevail, respectively. Clearly, the radiation loss dominates for broader antennas with high volume, which leads to material independent and merely geometry dependent plasmonic properties. When decreasing the antenna width, the electronic scattering gains importance, leading to different plasmonic properties for each metal with additional modification by the crystalline quality. Putting all together we are able to draw a self-contained picture, especially with regard to Ref. 36, where the importance of electronic scattering for SEIRA was predicted. In our experimental study of the plasmonic properties of nanoantennas, we determined the antenna widths for equal contributions of electronic damping and radiation damping for various metals. The dimensions (geometrical widths) that fulfill this condition perfectly agree with our findings on the maximum SEIRA enhancement of the antennas. Obviously, there is a crucial influence of the electronic damping on the maximum enhancement while the plasma frequency seems to be of less importance. This means that narrower antennas are beneficial in case of better metals and thicker ones in case of higher electronic scattering. Furthermore, we found that the highest SEIRA enhancement is achieved for the smallest total damping (containing both intrinsic and radiation damping). Because of the extraordinary good metallic properties of the copper nanoantennas of our production method and their stability (as found for antennas with cover layer), we can recommend copper for reaching highest SEIRA enhancements in the mid IR.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: http://pubs.acs.XXXXXXX.



Page 26 of 35

Structural formula of the organic probe molecule F4ZnPc. Relative transmittance spectra of nanoantenna arrays fabricated from the metals copper, gold, silver, aluminum, and iron with molecular layer, measured as fabricated and after several months. Detailed information regarding the model for describing the data. Full width at half maximum, plotted against the inverse effective length and the volume. Details regarding the normalization of the SEIRA enhancement factor. Electronic scattering rate which results from fitting the tuning dependent enhancement factors with Equation (3). Antenna width dependent shift between spectral maxima of near- and far-field. Corresponding Author *Address correspondence to [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT M.T. acknowledge financial support by the German Science Foundation (DFG) via the collaborative research center SFB 1249. M.T. also acknowledges support by the Heidelberg Graduate School of Fundamental Physics. The F4ZnPc evaporation was done by Sabina Hillebrandt at the clustertool hosted by the InnovationLab, Heidelberg, Germany. Initial sample preparation was done by Xinghui Yin at the 4th Physics Institute of the University of Stuttgart. The authors acknowledge support by the state of Baden-Württemberg through bwHPC for providing compute services of the bwForCluster MLS&WISO (Production) at Heidelberg University and the University of Mannheim.


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Impact of Metal-Optical Properties on Surface-Enhanced Infrared

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