Localized Surface Plasmon Coupling between Mid-IR-Resonant ITO

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Localized Surface Plasmon Coupling between Mid-IR Resonant ITO Nanocrystals Min Xi, and Bjoern M. Reinhard J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01283 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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The Journal of Physical Chemistry

Localized Surface Plasmon Coupling between

Mid-IR Resonant ITO Nanocrystals Min Xi and Björn M. Reinhard*

Department of Chemistry and the Photonics Center, Boston University, Boston, Massachusetts 02215, United States.

Corresponding Author *E-mail: [email protected] *Tel: (617) 353 – 8669

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Abstract

Sn-doped indium oxide (ITO) nanocrystals (NC) provide tunable localized surface plasmon resonances (LSPR) in the mid-IR. To evaluate the applicability of these n-doped plasmonic semiconductors in field-enhanced spectroscopies, it is necessary to assess how the, compared to metal nanoparticles (NP) low, free electron density affects E-field localization and plasmon coupling in NC films. In this manuscript, we investigate plasmon coupling between approximate 6 nm diameter ITO NC on the collective resonance, and quantify the effect of the electromagnetic field enhancement on the absorbance signal of surface attached ligands in NC films and monolayers with different ratios of doped and undoped indium oxide (IO) NC.


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Introduction Localized surface plasmon resonances (LSPRs) are coherent surface charge density oscillations that can result in large optical cross-sections and greatly enhanced electromagnetic fields.1 Metal nanoparticles (NP) provide strong plasmon resonances in the visible, but true nanoscale materials with LSPRs in the infrared (IR) are of high interest for many applications. The Fröhlich resonance condition for the LSPR in any spherical NP, Re(ε) = -2, establishes a clear requirement for the real part of the dielectric function εd at which the resonance occurs.1,2 Considering the free electron gas dielectric function with = 1 −

, where γ is the

electron collision frequency and the plasma frequency, it is clear that materials with lower plasma frequencies than metals present opportunities to red-shift the LSPR. For the free electron gas the plasma frequency is given as =

∗

, where m* is the effective mass, ε0 is the

vacuum permittivity, e is the elementary charge, and N is the free electron carrier concentration.3–8 Doped metal oxides have plasmon resonances in the IR whose resonances can be systematically shifted through control of the carrier density by doping.3–7 A series of beneficial properties, including low price, CMOS capability, and high thermal and mechanical stability make doped metal oxides, for instance tin (Sn) n-type doped indium oxide (ITO), interesting plasmonic materials in the IR.3,4,9,10 ITO is already used in various optical applications, including thermal imaging,11 nonlinear optics,12 telecommunications,13 and solar thermal-photovoltaics14 to name only a few. A particularly interesting application of ITO NC lies in the area of surface enhanced infrared spectroscopy (SEIRS) for label-free molecular sensing in the mid-IR.15–17 In this manuscript, we investigate the signal enhancement provided by colloidal ITO NC in surface enhanced IR spectroscopy of surface attached ligands. In particular, we 3 ACS Paragon Plus Environment


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evaluate how electromagnetic coupling between ITO NC enhances the signal measured from NC films. Unlike in the case of noble metal NP, for which the coupling of localized plasmons has been studied in great detail,18–30 the effect is less well characterized for ITO NC and other nonmetallic plasmonic materials. Electromagnetic simulations confirm significant coupling between ITO NC,5 but additional experimental characterization remains necessary. We investigate the effect of plasmon coupling on the ITO NC far-field spectrum and the SEIRS signal of surfacebound ligands.

Methods and Materials Indium (III) acetylacetonate (In(acac)3, 99.99%), tin (IV) bis(acetylacetonate) dichloride (Sn(acac)2Cl2), 1-dodecanol (DDL), oleic acid (OA), and octadecene (ODE) were purchased from Sigma Aldrich and used as received. Oleylamine (OAm) was purchased from Fisher Scientific and used as received. Calcium Fluoride (CaF2) windows (with 10 mm (diameter) × 0.5 mm (thickness)) were purchased from Crystran, and used without further treatment. ITO NC synthesis. ITO NC were synthesized following established procedures with some modifications.31 All synthetic steps were carried out under nitrogen atmosphere using standard Schlenk line techniques and magnetic stirring. Typically, for the synthesis of 5.0 at. % Sn doped In2O3 (ITO) NC, 3.42 mmol In(acac)3, 0.18 mmol Sn(acac)2Cl2, 21 mL ODE, 3 mL OA, and 6 mL DDL were first added to a 100 mL three-neck flask, and the resulting solution was degassed at 140 °C for 30 min. Then, the solution was heated to 250 °C within 1 h. After reaching the desired temperature, the mixture was kept at 250 °C for 3 h for NC growth. After cooling down to room temperature, the resulting NC were purified by adding 7.5 mL of hexane and subsequent precipitation with 37.5 mL of ethanol and centrifugation. The separated NC were re-dispersed in 4 ACS Paragon Plus Environment

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30 mL hexane in the presence of OAm (1 mL) and OA (1 mL). 45 mL ethanol was added to flocculate the NC, which were then separated by centrifugation. The process was repeated once more, and the final product was dispersed in ~ 15 mL of chloroform for storage. ITO NC with other doping levels were obtained with a similar procedure by adjusting the molar ratio of In and Sn precursors. The actual Sn content in all samples was validated by elemental analysis through inductively coupled mass spectrometry (ICP-MS). TEM images were taken with a FEI Tecnai Osiris 200 KV, and were used to determine the NC diameters. Spincoating of ITO NC Films. NC were washed by precipitation with ethanol and subsequent resuspension in chloroform twice to remove the excessive free ligand in the solution. To generate films containing ITO and undoped indium oxide (IO) on CaF2 windows, we first determined the NC concentration in solution by measuring the mass of solid crystal contained in 1 mL colloidal solution. After the last washing step, mixtures of defined ratios of ITO and IO NC were suspended in a chloroform/chlorobenzene mix (v/v = 1:1) at a concentration of ~ 100 mg/mL for the spincoating process. Spin-coating was performed with the following parameters: 1500 r/min2, 2000 r/min for 45 s. For monolayer films, the NC concentration was diluted to ~20 mg/mL. SEM images of the NC films were taken with a Zeiss Supra 55VP after gold sputtering. NC samples intended for ellipsometry measurements were dispersed in a chloroform/chlorobenzene mix (v/v = 1:1) at a concentration of ~ 40 mg/mL and then deposited on a 3 cm × 3 cm silicon wafer through spincoating. The films were subsequently UV light (UV transilluminator, 8W, 302 nm UV) irradiated for 6 h to completely decompose the surface ligands as described previously.32,33 Ellipsometry and Drude Model Fitting. Ellipsometry measurements were performed on a V-Vase Spectroscopic Ellipsometer (J. A. Woollam). Measurements were taken at an angle of incidence 5 ACS Paragon Plus Environment


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of 65o, 70o and 75o in the wavelength range from 500 nm to 2000 nm. The acquired raw data Φ and ∆ shown in Figure S1 (A) – (D) are used to determine the optical constant of the sample film, and the layer thicknesses in the extracted model are summarized in Table S1. The experimental measured permittivity of the UV treated ITO nanocrystal film, which is composed of ITO nanocrystals and air-filled gaps, was then fitted with a Maxwell Garnett approximation: =

34,35 ,

where εeff is the effective experimental dielectric function of the

film, εm the optical constant of the surrounding (here air), εi the dielectric function of ITO NC, and δi the filling faction of 68%. The experimental dielectric function of ITO was then expressed by a Drude model, with the real and imaginary parts as: = −

/

, and =

. The resulting fitted parameters are summarized in Table S2.

Fourier Transform Infrared Spectroscopy (FTIR) Measurements. FTIR spectra of spincoated films were acquired using a Bruker Vertex 70 FTIR with a liquid nitrogen cooled mercury cadmium telluride detector. The spectral resolution of FTIR was 2 cm-1. Each sample was measured at 5 random spots in transmission mode with 1 mm aperture size, 200 spectra were accumulated per spot. Surface Profiler measurement. Surface profiler measurement was performed with ITO NC films on Si with a Zygo NewView 6300 optical profilometer at an optical magnification of 25 ×. The measurement results are shown in Figure S2. Electromagnetic simulations. All structures were designed using the electromagnetic design module in COMSOL Multiphysics. For single NC extinction cross section simulation, the structures were surrounded by a sphere representing the surrounding medium of air, and enclosed 6 ACS Paragon Plus Environment

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by a perfect matching layer, which prevented unwanted reflections from the outside boundary. As shown in Figure S3, ITO/IO NC films containing NC separated by nanoscale gaps were modeled as a core-shell structure (ITO/IO core with diameter of 6 nm, ligands shell with thickness of 0.75 nm). The ligand shell was modeled as a dielectric with optical constants of = 2, = 0. The core-shell particles were embedded in an IO matrix with constant dimensions of 22.5 nm × 20.5 nm × 7.5 nm. We applied the fitted dielectric function of IO to this cubic matrix. The IO was surrounded by a sphere of vacuum and a perfect matching layer. We used a variable meshing with mesh sizes down to 0.35 nm in the immediate vicinity of the NC. We validated in test computations that even finer meshes did not result in increased E-field intensities (Figure S4). In all simulations the electromagnetic field was incident normal to the sample and polarized in the plane of the NC film.

Results and Discussion Figure 1(A) shows representative TEM images of synthesized ITO NC with nominal Sn concentrations (atom %) of 0%, 1.5%, 3.0%, and 5.0%. The Sn content in the synthesized ITO NC was independently validated with inductively coupled plasma mass spectroscopy, ICP-MS (Table S2). Figure 1(B) shows the size distributions of the synthesized NC. The TEM analysis reveals that the NC were highly uniform in size with average diameters ± standard deviation of (in order of increasing dopant concentration) 6.09 ± 0.67 nm, 6.11 ± 0.61 nm, 6.17 ± 0.62 nm, and 6.32 ± 0.63 nm. These NC were cast into films on different substrates through spin-coating. The film thicknesses as determined by ellipsometry are given in Table S1. High and low magnification SEM images of a representative ITO NC film on a CaF2 substrate are shown in Figure 1(C) and its inset, respectively. The micrographs confirm the formation of an ITO NC 7 ACS Paragon Plus Environment


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Figure 1. (A1) – (A4) Representative TEM images of ITO NC with Sn doping ratio of 0%, 1.5%, 3.0% and 5.0%, respectively (scale bar = 5 nm); (B) Distribution of ITO NC and resulting averages ± standard dev. (C) SEM image of ITO film on CaF2 at high magnification (300k), scale bar = 20 nm; the upper right inset SEM image is ITO film on CaF2 at low magnification (1k), scale bar = 10 µm. (D) Measured real (solid line) and imaginary (dashed line) components of the Maxwell Garnett effective dielectric function of films assembled from ITO NC with different doping ratios. (E) Drude model of the real and imaginary part of the ITO dielectric function with different Sn doping levels in the wavelength range 1-12 µm.; (F) Measured FTIR absorbance spectra for ITO NC films with defined Sn doping ratios. film that exhibits some structural roughness under higher magnification. The thickness of the film was determined by surface profiling as ~100 nm, and the roughness was measured to be Ra (roughness average) = 0.003 µm and Rrms (root mean square) = 0.008 µm (Figure S2).36


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We characterized the optical properties of “neat” ITO NC films obtained after removal of the surface ligands through intense UV light irradiation for 6 h by ellipsometry and Fourier transform infrared spectroscopy (FTIR). The experimental permittivity was measured through ellipsometry in the wavelength range from 500 nm - 2000 nm and the resulting real and imaginary components are plotted as solid and dashed lines, respectively, in Figure 1(D). The dielectric function was then fit with a Maxwell Garnett effective medium assuming that the film consists of air separated ITO NC. The ITO dielectric function was expressed by a Drude free electron model. The extracted carrier density, plasma frequency, and damping for the ITO NC films are summarized in Table S2. The ITO dielectric function is plotted in the wavelength range of 1 – 12 µm in Figure 1(E). The increase in the Sn doping from 0% to 5.0% results in an increase in the bulk plasma frequency from 2161.7 cm-1 to 12702.0 cm-1, an increase in the carrier density from 2.1×1019 cm-3 to 7.2×1020 cm-3, and an increase in the damping coefficient from 583.7 cm-1 to 1934.7 cm-1. The plots in Figure 1(E) show that with increasing free carrier concentration, the absolute value of both the real and imaginary part of the dielectric function increase in the IR. The NC films become conductive and lossy for wavelengths > 2 µm, which is characteristic of metallic behavior. Overall, the observed trends for our ITO NC films are consistent with previous studies of ITO NC films.37,38 Our data indicate a slightly lower carrier density when compared with sputtered and annealed ITO films,12 which may be related to details of the processing of the film. Figure 1(F) shows representative FTIR spectra of NC films of ITO NC with different Sn doping levels. The spectra of the NC films peak at 3385 cm-1, 3625 cm-1 and 4306 cm-1 for doping levels of 1.5%, 3.0%, and 5.0%. Importantly, indium oxide (IO) NC without doping show almost no absorbance, confirming that the FTIR spectrum is a reliable observable of the LSPR. The additional features in the experimental spectra at around 2900 cm-1 9 ACS Paragon Plus Environment


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Figure 2. (A1) and (A2) Simulated E-field intensity maps for ITO (Sn 3.0%) NC clusters evaluated at 2900 cm-1 with the incident light polarization pointing along the x- and y- axis, respectively. (B) Simulated absorption cross sections of different series of ITO/IO NC clusters (average of x- and y polarization). The solid curves highlight the monomer and octamer in each series. (C) Normalized absorption cross sections (solid curve) and normalized (Emax/E0)2 for clusters of 8 ITO NC with specified Sn doping levels. The ITO NC clusters were embedded in an ITO ambient medium. (D), (E), (F) and (G) are the simulated peak absorption cross-section and wavenumber, full width at half maximum (FWHM), and (Emax/E0)2 at 2900 cm-1for ITO NC clusters as function of cluster size and Sn concentration. are assigned to alkyl CH stretch vibrations from attached ligands, primarily oleylamine (OAm). Some contribution from residual oleic acid (OA) can also not be excluded considering the weak spectral features at around 1600 cm-1, which indicates the existence of –COOH groups. Next, we simulated the effect of plasmon coupling on the near- and far-field response of clusters of gap-separated (1.5 nm) ITO NC of different sizes embedded in a background of IO. The gap size of 1.5 nm was estimated based on TEM images (Figure 1(A)). Figure 2(A1) and Figure 2(A2) show representative near-field intensity maps of the investigated cluster geometries for two orthogonal light polarizations. We evaluated the near-field at 2900 cm-1 as this wavelength overlaps with the CH stretch mode of the NC ligands. The maps vividly illustrate the localization of the enhanced E-field in the gaps between the NC. The simulated absorption 10 ACS Paragon Plus Environment

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spectra (averaged over 2 orthogonal polarizations) are plotted in Figure 2(B). The calculated absorption spectra confirm the tunability of the NC spectra in the mid-IR through choice of the Sn content. The simulated absorption spectra of the largest investigated ITO/IO NC cluster (n = 8, solid curve) peak at 2950 cm-1, 3590 cm-1, and 4410 cm-1 for ITO doping ratios of 1.5%, 3.0%, and 5.0%, respectively. Both the peak wavelength and the relative intensities of the simulated spectra are in general agreement with the experimental FTIR absorbance spectra in Figure 1(F), and the observed trends are consistent with an increase in carrier concentration and resulting increase in NC polarizability as function of increasing Sn doping. Figure 2(C) compares the normalized absorption spectra of the largest investigated ITO/IO NC cluster (n = 8, solid curve) for different doping levels and their normalized (Emax/E0)2 spectra (dashed curve). The near-field enhancement spectrum is systematically red-shifted by 560 cm-1 to 650 cm-1 when compared with the absorption spectrum. The damped harmonic oscillator model of a localized plasmon predicts this red-shift between the near- and far-field spectrum as direct consequence of damping.39 As we will elaborate below, this spectral shift has immediate consequences for the design of plasmonic substrates that enhance infrared vibrational signals.40 Figure 2(D) summarizes the peak absorption cross-sections of the spectra in Figure 2(B) as function of number of nanocrystals per cluster. The fitted peak wavelengths (averaged over two orthogonal polarizations) as function of cluster size in Figure 2(E) show a gradual red-shift with increasing cluster size. The full width at half-maximum (FWHM) initially increases with increasing Sn doping for the simulated cluster spectra (Figure 2(F)). For cluster sizes n > 4, the FWHM, however, no longer systematically increase. This mirrors the trends observed for the peak E-field intensity shown in Figure 2(G). The peak E-field intensity enhancement (averaged over two polarizations) initially increases for small cluster sizes but then converges at around 11 ACS Paragon Plus Environment


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Figure 3(A) – (C) Experimental absorbance spectra of ITO/IO NC films with 0%, 20%, 40%, 60%, 80% and 100% of ITO NC with specified Sn doping levels. Spectra are averages of multiple measurements at different spots on the same film. Insets show the magnifications of the CH stretch mode at 2920 cm-1; (D) UV-Vis spectrum of ITO/IO mixed solution in chloroform containing 0%, 20%, 40%, 60%, 80% and 100% of ITO NC with 5.0% Sn doping level. (E), (F), (G) are the measured peak absorbance, wavenumber, and FWHM of the FTIR spectra of mixed ITO/IO films as function of ITO NP volume fraction for ITO NC with different Sn doping levels. (H) SEIRS signal enhancement factor of the 2920 cm-1 CH stretch mode as function of film composition and Sn doping level. four NC per cluster. This behavior is remarkably similar to what was experimentally observed before for gold NP and can be attributed to the dominance of nearest neighbor interactions in shaping the spectral response of plasmon coupled NP.41,42 According to the simulation results in Figure 2(G), we noticed that at the resonance of the monitored molecular vibration at 2900 cm-1, Sn 5.0% showed a similar enhancement as Sn 3.0 %, which is higher than that of Sn 1.5 %. As shown in Figure 2(C) the near-field spectrum of ITO NC clusters with 3.0 % Sn doping peaks closest to 2900 cm-1, whereas the maxima in the near-field spectra of ITO NC with 1.5% and 5.0% Sn doping are shifted off-resonance. In the case of 5.0% Sn doping, this de-tuning can be compensated through an overall higher E-field associated with a higher carrier density. Even if the molecular resonance does not co-incide with the peak E-field intensity enhancement, the 12 ACS Paragon Plus Environment

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overall higher E-field intensity enhancement across the width of the plasmon still results in an enhancement at 2900 cm-1 that is comparable to that of Sn 3.0%. For 1.5 % Sn doping, the spectral de-tuning of the plasmon resonance is not compensated by a higher peak E-field intensity enhancement, and the E-field enhancement at 2900 cm-1 is, consequently, considerably lower. The broadening of the simulated spectra with cluster size in Figure 2(F) results i.) from a larger number of available cluster modes, and ii.) an increase in E-field intensity and effective polarizability due to plasmon coupling in NC clusters that increase absorption and scattering losses. To experimentally validate the simulated behavior for the clusters, we generated mixed NC films containing different volume ratios of ITO and undoped IO on CaF2 windows. The average separation of doped NC decreases with increasing loading ratio, allowing a systematic study of the effect of increasing interparticle interactions on the spectral response of the film. Based on the results from the NC simulations in Figure 2(G), we expect that the near-field coupling increases with the ITO NC content as with growing filling fraction the probability of electromagnetically coupled NC increases. Figure 3(A) – (C) shows the experimental absorbance spectra of ITO/IO NC films with different contributions from doped/undoped NC ratios and Sn doping levels of 1.5%, 3.0%, and 5.0%. Intriguingly, the absorbance increases with growing ITO NC contribution (Figure 3(E)). At low ITO NC concentrations, the film contains individual ITO NC dispersed in the sea of undoped IO NC and no or weak coupling effects can be observed. As the contribution from doped NC increases, coupling effects become more prevalent and achieve a spectral red-shift of the FTIR spectrum (Figure 3(F)). The maximum shifts were 65.3 cm-1, 94.5 cm-1 and 153.8 cm-1 for doping levels of 1.5%, 3.0%, and 5.0%, respectively, which compares with predictions based on our cluster simulations (Figure 2(E)) of 103.6 cm-1, 137.3 13 ACS Paragon Plus Environment


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cm-1, 144.7 cm-1. The systematic difference between simulation and experiment can be attributed to the fact that the films used in the simulations have void areas and defects with larger interparticle separation (Figure 1(C)), whereas the clusters are idealized structures of strong coupling. The red-shift observed for increasing Sn doping is accompanied by a systematic spectral broadening. As shown in Figure 3(G) the experimental FWHM of the FTIR spectra are in the range of ~1300 cm-1 to ~1500 cm-1 for the investigated NC films, and all doping levels show a systematic broadening as the fraction of doped NC in the film increases from 20% to 100%. The relative spectral broadening with increasing ITO NC filling fraction is consistent with increasing NC clustering as observed in the numerical simulations in Figure 2(F). In Figure 3(D), we included UV-Vis spectra of solutions of mixtures of ITO and IO NC in chloroform containing 0%, 20%, 40%, 60%, 80% and 100% ITO NC with 5.0% Sn doping. The solution spectra show neither a systematic red-shift of the peak resonance, which remained constant at approximately 4680 cm-1, nor a broadening of the spectral width (FWHM ≅ 1502 cm1

), confirming the absence of near-field coupling effects in the solution phase, where the

interparticle distances are much longer than those in the film. To evaluate the effect of plasmon coupling on the absorbance signal of the CH stretch mode in the different ITO/IO NC films, SEIRS signals were obtained by subtracting a linear baseline fit from the measured FTIR absorption spectra in the 2600 cm-1 – 3200 cm-1 wavenumber range (as shown in the insets in Figure 3(A) – (C)). We determined the mass ratios of bound organic ligands for the different NC by thermal gravity analysis (TGA) (Figure S5) as 1.55 : 1.20 : 1.0 : 1.05 for Sn 0%, Sn 1.5%, Sn 3.0% and Sn 5.0%, respectively. We used these ratios to correct the signal intensities measured at 2900 cm-1 for differences in the amount of bound ligands. The resulting signal enhancements (relative to the IO NC film signal) are plotted in Figure 3(H). 14 ACS Paragon Plus Environment

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Figure 4 (A) FTIR absorption spectra of IO and ITO NC monolayers with different Sn doping levels. (B) Average SEIRS signals enhancements of the CH stretch mode for the investigated IO and ITO monolayers. (C) CH stretch signal measured on an ITO (3.0% Sn) NC monolayer film before (black) and after (red) ligand removal through 6 h of UV irradiation, and after binding of additional OA (blue). (D) SEM image of ITO NC monolayer on silicon wafer. Films assembled from 3% and 5% Sn containing NC show overall higher signal enhancements than films containing NC with 1.5% Sn, which is in good agreement with the predicted E-field intensity enhancements in discrete clusters discussed above and suggests an electromagnetic enhancement mechanism. Intriguingly, for all doping ratios, the plots show a systematic increase as function of ITO NC filling fraction. Especially for films containing 5.0% and 1.5% Sn doping, the signal enhancement as function of ITO NC content increases in a non-linear fashion, with a gradual slope at low ITO NC concentrations that becomes steeper at higher concentrations. This behavior is consistent with an electromagnetic enhancement of the molecular absorption signal through near-field coupled ITO NC whose probability increases with growing ITO NC film content. In our analysis so far, we have explored films of NC that consist of several layers of NC. To test if monolayers of these materials provide a measurable SEIRS signal enhancement, we 15 ACS Paragon Plus Environment


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generated monolayer of ITO NC with different doping levels, and measured their FTIR spectra (Figure 4(A)). We determined the signal enhancement relative to the undoped IO NC monolayer as 1.80, 2.61 and 2.57 for monolayers of ITO NC containing 1.5%, 3.0% or 5.0% Sn (Figure 4(B)). In Figure 4(C), we show spectra of the CH vibrational feature recorded on ITO NC films before and after ligand removal through UV treatment and after binding of additional OA (20mM OA solution in chloroform was spincoated onto the ITO monolayer after UV treatment). The CH signal disappears after the UV treatment, indicative of a complete removal of the ligands, but re-emerges with a similar intensity as before the UV treatment after addition of new OA. The SEM image in Figure 4(D) was taken from a 5.0% Sn ITO NC sample and confirms that the film represents a monolayer with a thickness of approximately 5 nm as determined by ellipsometry. Although the average signal enhancement of the CH stretch mode provided by the ITO NC film is moderate when compared to what is reported for top-down fabricated mid-IR resonant gold rod antennas, the integrated signal intensity can reach the same level as obtained with the antennas.43,44 This apparent contradiction is related to the fact that the much smaller size of the NC allows a much higher hot spots density. For instance, a mid-IR resonant gold rod antenna has a typical footprint of 1.8 µm × 0.8 µm and achieves an enhancement factor in the range of 7.2×104.45 Assuming a closed packed monolayer, 3.8×104 ITO NC fit into the footprint of the same gold antenna. Given this large number, it is clear that even an average signal enhancement of only 2 per NC results in a comparable integrated signal intensity than for the gold NR. This result is consistent with previous theoretical studies by Kutznetsov who has demonstrated that the smaller volume of IR-resonant plasmonic semiconductor NC allows for a higher integration density when compared with resonant metal antennas, and that this effect can offset a weaker 16 ACS Paragon Plus Environment

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peak E-field intensity enhancement by the semiconductor NC.46 These theoretical predictions were independently validated by Abb et al. using top-down fabricated ITO nanorods.17 Although this line of argument can be questioned for low analyte concentration conditions when not all available sensor sites are occupied, a low density of highly efficient hot-spots provided by gold antennas is equally disadvantageous under these conditions as the target molecule has to “find” a hot-spot to experience any signal enhancement. One potential solution to this dilemma can be homogenously warm electromagnetic surfaces that achieve a compromise between peak signal enhancement and hot-spot density. Larger ITO crystals that generate higher E-field enhancements than the 6 nm diameter NC investigated in this work but that are still much smaller than resonant gold antennas might be a promising strategy to achieve this goal. The optimization of the ITO NC size and density for maximum SEIRS signal intensity warrants further investigation. In summary, we have synthesized and characterized ITO NC with Sn doping levels between 1.5% and 5.0% with plasmon resonances in the mid-IR and investigated the spectral response of mixed films of IO/ITO NC. The dielectric constants of ITO NC were determined experimentally through ellipsometry. Spectral shifts, broadening of the plasmon resonance, and the enhancement of the absorption of the CH stretch mode at 2900 cm-1 for increasing ITO NC concentrations in the films are consistent with an increasing near-field enhancement due to growing electromagnetic coupling. Our results confirm that even for the small ITO NC investigated here, with an average diameter of around 6 nm, plasmon coupling has measurable effects on the near- and far-field response of the NC in the mid-IR and results in a measurable SEIRS signal enhancement.



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Supporting Information Ellipsometry measurement raw data and fitting parameter, dielectric constant fitting parameters , surface profiler measurement, simulation mesh plot and convergence study, and TGA analysis.

Acknowledgement This work was partially supported by the National Institutes of Health through grant 5R01CA138509 and the National Science Foundation through grant CHE-1609778.

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6 000

5 000

40 00 3 00 0 Wavenumber ( cm -1 )

2 00 0


Localized Surface Plasmon Coupling between Mid-IR-Resonant ITO

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