High-Quality Ultrathin Gold Layers with an APTMS Adhesion for

Research Article www.acsami.org

High-Quality Ultrathin Gold Layers with an APTMS Adhesion for Optimal Performance of Surface Plasmon Polariton-Based Devices J. Sukham,† O. Takayama,† A. V. Lavrinenko,† and R. Malureanu*,†,‡ †

Department of Photonics Engineering, Technical University of Denmark, Ørsteds Plads, Building 345V, DK-2800 Kongens Lyngby, Denmark ‡ National Centre for Micro- and Nano-Fabrication, Technical University of Denmark, Ørsteds Plads, Building 347, DK-2800 Kongens Lyngby, Denmark S Supporting Information *

ABSTRACT: A low-absorption adhesion layer plays a crucial role for both localized and propagating surface plasmons when ultrathin gold is used. To date, the most popular adhesion layers are metallic, namely, Cr and Ti. However, to the best of our knowledge, the influence of these adhesion layers on the behavior of propagating plasmon modes has not been thoroughly investigated nor reported in the literature. It is therefore important to study the effect of these few- to several-nanometers-thick adhesion layers on the propagating plasmons because it may affect the performance of plasmonic devices, in particular, when the Au layer is not much thicker than the adhesion layers. We experimentally compared the performances of the ultrathin gold films to show the pivotal influence of adhesion layers on highly confined propagating plasmonic modes, using Cr and 3-aminopropyl trimethoxysilane (APTMS) adhesion layers and without any adhesion layer. We show that the gold films with the APTMS adhesion layer have the lowest surface roughness and the short-range surface plasmon polaritons supported on the Au surface exhibit properties close to the theoretical calculations, considering an ideal gold film. KEYWORDS: adhesion layer, ultrathin gold film, surface plasmon polaritons, hyperbolic metamaterials

■

film thinner.19 In this article, we use the term “confinement” to describe the expansion of the field away from the Au layer, in a direction normal to the Au layer. Because our plasmonic modes are propagating, in the direction along the Au layer, the mode is not confined but decays due to absorption in the system. It is this absorption that is greatly influenced by the adhesion layers and, as such, the role of the adhesion layers in fabrication of ultrathin, ultrasmooth Au films becomes critical. Different adhesion layers ranging from metallic to polymers have been used in deposition of thin Au films. By far, the most commonly used adhesion layers are Ti20,21 and Cr.22,23 Other options such as benzocyclobutene polymers,24 organosilane compounds like 3-aminopropyl trimethoxysilane (APTMS),25 and mercatopsilane26 have been tested. The reported roughness of the metal films ranges from 0.26 to 1 nm.27 However, even after successful fabrication of ultrathin films, introduction of these typically 2 nm thick metallic adhesion layers affects the optical performance of the structures by introducing extra absorption and scattering of the SPPs.28 Au-based plasmonic nanostructures, such as split rings,29 nanorods,30 and nanodisks,31,32 with the presence of a Ti adhesion layer showed a red shift in the

INTRODUCTION Plasmonics is playing a key role in the field of nanophotonics having a wide range of applications in nanofocussing,1 optical biosensing,2,3 enhanced emission of light,4 and on-chip communications.1,2,5 This is due to surface plasmon polaritons (SPPs), collective oscillations of electron plasma in the metal that have the ability to focus electromagnetic waves at nanoscale in metal−dielectric structures. Nanofocusing via plasmons can be realized by using tapered metallic guiding structures to support propagating modes far beyond the diffraction limit.1,6,7 Plasmonic nanoantennas are used for molecular sensing due to the resonant enhancement of light extraction from dye molecules.8,9 Optical biosensors based on plasmonic nanostructures, such as Au nanorods,10 nanodiscs,11 nanoholes,12 and nanoparticles,13,14 are used to detect colon cancer, bacteria, and immune response. In the visible to near-infrared wavelength regimes, silver (Ag) and gold (Au) are known to be the best plasmonic materials with Ag possessing the better characteristics. However, because of the poor chemical stability of silver toward oxidization, Au is heavily used in the field of plasmonics.15−18 To improve the localization of plasmonic modes, Au films can be patterned to provide structures that support localized plasmonic modes.9−14 For propagating plasmonic modes, especially short-range SPPs (SR-SPPs), confinement can be improved by making the Au © 2017 American Chemical Society

Received: May 21, 2017 Accepted: July 6, 2017 Published: July 6, 2017 25049

DOI: 10.1021/acsami.7b07181 ACS Appl. Mater. Interfaces 2017, 9, 25049−25056

Research Article

ACS Applied Materials & Interfaces

Figure 1. Influence of adhesion layers on bulk plasmons in HMMs. (a) FOM for HMMs made of 10 nm thick Au and SiO2 multilayers with 1 nm thick APTMS, FOMAPTMS, and that of 1 nm Cr, FOMCr, adhesion layers below and above. The inset is normalized FOMAPTMS/FOMCr. (b) Normalized FOMs for different thicknesses of Au layers are plotted in terms of wavelengths. The volume fraction of the SiO2 layer is kept 0.5 for both cases.

Figure 2. Calculated confinement and normalized wavevector of SR-SPPs for various Au thicknesses. (a) Confinement, Γ, and (b) normalized wavevector for SR-SPPs in a semi-infinitely thick SiO2−Au−SiO2 structure in terms of wavelength. Γ=dm + 2ΓSiO2 , where dm is the thickness of the Au film (dm = 6, 8, 10, 12, and 18 nm) and ΓSiO2 = 1/2 Im(NSR ‐ SPP).

resonance frequency together with a drastic decrease in the field enhancement and increase in the decaying time. The metallic adhesion layers with thicknesses higher than 5 nm for Au-based plasmonic nanoantennas are shown to undermine the antenna performance.33 One of the applications of ultrathin Au films is as a constituent component in hyperbolic metamaterials (HMMs). The HMMs act as a dielectric in some directions and as a metal in another. One of their distinguishable features is that they support propagation of bulk and surface waves with wavevectors much exceeding those of light. The presence of large wavevectors in HMMs was used for applications in biosensing,18 spontaneous emission engineering,34−36 and nanoimaging.37−39 They can be realized by periodic arrangement of subwavelength layers or pillars of metal and dielectric, and they typically exhibit extreme anisotropy.40,41 Au-based HMMs with layer thicknesses below 20 nm have been realized and experimentally demonstrated.42−47 Thinner layers support a higher figure of merit (FOM) for the short-range surface plasmon propagation and also allow the use of effective medium theory for a broader wavelength range. 48−50

Fabricating reproducible ultrathin and ultrasmooth films with thicknesses below 10 nm is still challenging as the films become discontinuous and very rough. Therefore, the quest for thinner and smoother Au layers is crucial not only for surface plasmon photonics but also for developing HMMs. In this work, we show that by using APTMS as the adhesion layer of Au we can achieve a high quality of ultrathin gold films required to support the SR-SPPs with large wavevectors.19 We compare their characteristics with Au layers with a Cr adhesion layer and without any adhesion layer experimentally and numerically. It appears that the FOM for HMMs with APTMS is twice that for HMMs with Cr adhesion layers. Thus, such an approach in gold layer deposition allows for superior performance of plasmonic components and HMMs.

■

RESULTS AND DISCUSSION Theoretical Study. Lossy metallic adhesion layers become critical for bulk plasmons that propagate through metal− dielectric multilayer HMMs. To compare the influence of the adhesion layers on bulk plasmons in HMMs, we calculated the FOM for HMMs with APTMS and Cr adhesion layers (Figure 25050


Research Article


Figure 3. Roughness characteristics in different configurations. AFM images of the surface of the 8 nm thick Au films with 1 nm APTMS (a), with 1 nm Cr (b), and without any adhesion layer (c).

Figure 4. Schematic illustration of the structure under consideration. Excitation and detection of SR-SPPs using the Otto configuration. Symmetric plasmonic waveguide structures are made of SiO2−adhesion layer−Au nanofilm−adhesion layer−SiO2. The adhesion layers under consideration are either 1 nm thick APTMS or 1 nm thick Cr layers, or there is no adhesion layer.

adhesion layers becomes more significant due to a higher volume fraction of Cr relative to that of Au. Because bulk plasmons in HMMs are formed as collective modes by coupling of all SR-SPPs in individual metal layers,48,49 the properties of SR-SPP in each metal layer determine the properties of HMMs. Here, we theoretically investigate the influence of adhesion layers on SR-SPPs supported in a single Au film. The SR-SPPs are highly confined around the Au film in the dimension of a few hundreds of nanometers especially for shorter-wavelength ranges, as shown in Figure 2. We also show the real part of the normalized wavevectors of SR-SPPs (NSR‑SPP) with an APTMS adhesion layer versus the Au thickness in Figure 2b to illustrate the highk character of SR-SPPs. As the thickness of the Au film decreases, the normalized wavevector becomes significantly larger. Therefore, we assume that the properties of SR-SPPs when the Au layer thickness decreases are deteriorated due to the high confinement of fields and, as a result, the increased absorption in the adhesion layer. Shorter wavelengths exhibit a higher confinement level (Figure 2a), implying that the SRSPPs with the Cr adhesion layer suffer higher damping in this part of the spectrum. As the thickness of Au layers becomes even thinner, down to 3 nm54 or even down to monolayer,55 novel phenomena may be shielded by the extremely high

1). The FOM is the ratio of the real and imaginary parts of the wavevector perpendicular to the multilayer and is a measure of wavevector as opposed to loss in HMMs made of infinite period multilayers. An HMM with a large FOM can generate a large wavevector for a wave propagating in the HMM for a given loss.51 The calculation scheme of the FOM for each of the HMMs (FOMHMM) is explained in Numerical Simulations section of Methods section. The HMMs under consideration are composed of SiO2 and Au layers with either 1 nm APTMS or 1 nm Cr adhesion layers for each Au−SiO2 interface. The APTMS adhesion layer, which is a silane-based coupling agent having optical properties similar to those of SiO2 with a refractive index of approximately 1.4652,53 in the wavelength range of interest, eliminates the need for a metallic adhesion layer between the Au film and the oxide layers. With the APTMS adhesion layer, the FOM of HMM is approximately twice bigger as opposed to that of the Cr-based counterpart (Figure 1a). In other words, using only a 1 nm adhesion layer at each Au−dielectric interface significantly deteriorates the performance of HMMs. Such an improvement for HMMs with APTMS adhesion layers over the Cr case is more significant when reducing the Au thickness (Figure 1b). This can be intuitively understood because damping in the Cr 25051


Research Article


Figure 5. Dispersion diagram of SR-SPPs on ZnSe prism−SiO2 (200 nm)−adhesion layer (1 nm)−Au−adhesion layer (1 nm)−SiO2 (8000 nm)−Si substrate structures. Simulations (a−d) and measurements with APTMS adhesion layers (e−h), 1 nm Cr adhesion layers (i−l), and without the adhesion layer (m−p), for the thicknesses of Au of 8, 10, 12, and 18 nm, respectively. The insets in (e), (i), (m), and (n) are the scanning electron microscopy (SEM) images of Au surfaces for corresponding Au thicknesses. The white dashed lines in (a)−(d) are theoretical dispersion of SR-SPPs on infinitely thick SiO2−Au−SiO2 layers. The critical angle between the ZnSe prism and the SiO2 layer divides the high- and low-reflectance regions.

We conducted a series of reflection measurements to excite and detect SR-SPPs by using the Otto configuration, as illustrated in Figure 4, and present the results in Figure 5. The structures are composed of ZnSe prism−SiO2 (200 nm)− adhesion layer (1 nm)−Au−adhesion layer (1 nm)−SiO2 (8000 nm)−Si substrate. We used ellipsometry measurements to determine the thickness of APTMS. The measured thickness was 1.3 nm, with an error bar of 0.3 nm. These results are in good agreement with refs56,57 where the thickness of APTMS can vary between 1 and 2.5 nm. We chose to assume 1 nm and not 1.3 nm thickness because the refractive index of APTMS is close to that of the silica one. Thus, the precise thickness of APTMS is not of paramount importance. However, for a good comparison, the thickness of the Cr adhesion layer should be similar, and in this case, a thicker Cr adhesion layer would further degrade the SPP properties. On the basis of this reasoning, we considered the APTMS layer to be 1 nm thick. The measured dispersion diagrams for SR-SPPs are obtained for three different sets of samples, 1 nm thick APTMS as adhesion layers between Au and SiO2 (Figure 5e−h), 1 nm thick Cr adhesion layers (Figure 5i−l), and with no adhesion layers (Figure 5m−p), and are compared with theoretical

damping in the adhesion layer, thus making them unfeasible. In what follows, we study the properties of SR-SPPs in a broad wavelength range for different adhesion layers and thicknesses of the Au layers. Experimental Study. To characterize the performance of the gold films with different adhesion layers, four sets of Au nanofilms with thicknesses of 8, 10, 12, and 18 nm were fabricated in three configurations: with an APTMS adhesion layer, with a Cr adhesion layer, and without any adhesion layer. The fabrication process of the films is discussed in Au Nanofilm Fabrication section in Methods section. The atomic force microscopy (AFM) images in Figure 3 reveal that the Au films with the APTMS adhesion layer are the smoothest, with a rootmean-square (RMS) surface roughness of 0.30 nm, followed by the Au films with the Cr adhesion layer with a RMS surface roughness of 0.40 nm. The films without any adhesion layer have the highest roughness with RMS surface roughness of about 1.2 nm for an 8 nm thick Au layer. We can assume that in this case the homogenous Au film is not fully formed. Table S1 in Supporting Information summarizes the RMS roughness of each nanofilm. 25052



Research Article

■

simulations considering an ideal uniform Au film embedded in SiO2 (Figure 5a−d). Each set of samples has four different thicknesses of the Au layer: 8, 10, 12, and 18 nm. The lowreflection region beneath 35° is due to the nonfulfilment of the total internal reflection (TIR) conditions below the critical angle between the ZnSe prism and the SiO2 layer. Throughout the article, unless otherwise noted, we take the influence of Au film’s thickness on its permittivity into account as explained in the Numerical Simulation section of the Methods section. The measurement and normalization procedures are described in Otto Configuration section of the Methods section. Note that the oscillations visible in the measurement spectra above 1000 nm are due to Fabry−Pérot interference in the setup and are not related to the sample response. The noise close to 1064 nm visible in the experimental dispersion diagram is due to normalization errors arising from the spectral characteristics of the light source. For the 8 nm thick Au film, it can be seen from Figure 5e that the experimental dispersion curve for APTMS as the adhesion layer is much more pronounced and reflection dips associated with SR-SPPs are more visible compared to those in the one with the Cr adhesion layer, where the reflectance dip associated with SR-SPPs cannot be observed below 1600 nm wavelength at all (Figure 5i). For large angles of incidence, in other words large in-plane wavevectors, SR-SPP modes are highly confined and suffer the extra damping introduced by the Cr adhesion layers. As a result, SR-SPPs for larger angles of incidence are not supported. Moreover, there is no any reflection dip visible for the case of the bare 8 nm Au film (Figure 5m). This can be understood in terms of the percolation threshold of the Au film on the SiO2 layer, further sustained by the Au layer RMS roughness of 1.2 nm, as shown in Figure 3c and in the SEM image inset of Figure 5m. At 8 nm, we are below this threshold and therefore there is no SR-SPP supported on the film. In the case of no adhesion layer, the plasmons propagate when the layers start to be continuous (gold thickness is 10 nm and more; see Figure 5n and the inset). For each set of samples, as the thickness of the film is increased, the reflection dips are shifted down toward smaller angles of incidence. The role of the adhesion layers becomes more visible when the thickness of the gold film is below 10 nm because the volume ratio of Cr and Au is high and the SR-SPP is better confined for a thinner metal film. Because of the addition of the lossy Cr adhesion layer, the optical properties of the Au film are deteriorated, which can be clearly observed by comparing Figure 5e,f,i,j. The dispersion curves of ultrathin Au films with APTMS adhesion layer have, compared to the other ones studied, better agreement with the simulated ones. For example, in the case of 12 nm Au film with Cr, the plasmon dips are less pronounced. From this point of view, it is clear that the Au film with APTMS follows more closely the optical characteristics of the ideal Au film. Especially, for the thickness of the gold film below 10 nm, the choice of the adhesion layer makes a distinct difference for the existence of SR-SPP modes for the shorter-wavelength range and large wavevector region. Even in the case of an 18 nm Au film, the observed SR-SPP for the APTMS case is closer to the simulated results (Figure 5h,l,p). Note that the disagreement between the experiment and simulation mainly stems from the air gap between the ZnSe prism and 200 nm thick SiO2 layers, rather than from the permittivities of APTMS or Si substrate, as shown in Figures S4 and S5.

CONCLUSIONS We conducted comprehensive studies on the influence of adhesion layers on propagating plasmonic modes, SR-SPPs, in Au films, which plays a key role in biosensing and HMM properties. We found that using Cr as an adhesion layer greatly influences the structural properties of the Au layers themselves and plasmonic properties of SR-SPPs. In a certain wavelength range, the plasmonic waveguide with the Cr adhesion layers cannot support any of the SR-SPP modes with high confinement due to the additional damping. Also, the FOM of HMMs when using Cr adhesion layers is half or even less as compared to that when using the APTMS adhesion layer. For the Au film with the thickness of 8 nm deposited with the APTMS adhesion layers, the SR-SPP dispersion follows closely the one theoretically calculated for completely flat pure Au layers, showing that APTMS is the better adhesion layer between oxide and Au if one aims to support highly confined propagating plasmon modes.

■

METHODS

Numerical Simulation. The normalized FOM in Figure 1 is calculated by FOMs of HMMs (FOMHMM) with APTMS and Cr adhesion layers, assuming infinite period of multilayer HMMs. FOMHMM = Re(k⊥)/Im(k⊥) = Re(k0n⊥)/Im(k0n⊥),58 where n⊥ = (ε⊥)1/2 can be calculated by the effective media approximation (EMA),59 ε⊥ = ( f m/εm + (1 − f m)/εd)−1, f m and εm are the volume fraction and permittivity of the Au film, respectively, and εd is the permittivity of SiO2 taken from Malitson et al.60 For the HMM with the Cr adhesion layer, we calculate ε⊥ using EMA approximation N formula 1/ε⊥ = 1/D(∑ j = 1 djnj−2),43,61−63 where D is the thickness of the HMM, N is the number of layers, nj is the refractive index of the jth layer, and dj is the thickness of the jth layer. The Drude−Lorentz model of the Au film is expressed by εm = 1 − (G0ωp2 /(ω2 + Γ 20)) + j(G0 Γ0ωp2 /(ω(ω2 + Γ 20))), which 5

+ ∑m = 1 GmΩm2 /(ωm2 − ω2 + jω Γm) is used for all numerical analyses. Here, we introduced the metal’s thickness correction term as Γ0 = Γ∞ + AVF/dm,64,65 where A = 1 and VF = 1.39 × 106 m/s. All Drude−Lorentz parameters are listed in Table S2.66 We modeled the permittivity of Cr as a Drude−Lorentz metal as well.67 We assume that the permittivity of the APTMS adhesion layers below and above the Au nanofilm is the same as that of SiO2, 1.462,52,53 thus neglecting the presence of the adhesion layers in simulation in Figures 1, 2, and 5a−d. The fractuation of APTMS’s permittivity was analyzed between 1.42 and 1.52, showing that its presence has little effect (Figure S5a−c). Moreover, we analyzed the influence of the permittivity of the Si substrate between 32 and 42, resulting in no influence (Figure S5d,e). To calculate the normalized wavevector of SR-SPPs in Figure 2b, we used the following equation68 NSR ‐ PP ≈ =

k SR ‐ SPP k0 ⎛ ε ⎞2 εd + (εd − εm)⎜ d ⎟ tan h−2(0.5k 0dm εd − εm ) ⎝ εm ⎠

where kSR‑SPP is the wavevector of SR-SPPs, k0 is the wavevector in vacuum, k0 = 2π/λ, and dm is the thickness of the Au layer. The confinement of SR-SPP, Γ, for Figure 2b is calculated by Γ = dm + 2ΓSiO2 , where ΓSiO2 = 1/2 Im(NSR ‐ SPP). The simulations in Figure 5a−d were conducted by the scattering matrix formalism to calculate the reflectance from the multilayer structures.69 The model structure consists of five layers: ZnSe prism−SiO2 (200 nm thick)−Au nanofilm (8, 10, 12, 18 nm)−SiO2 (8000 nm)−Si substrate. The permittivity of the Si substrate is taken from Salzberg et al.70 The reflectance simulation was conducted with TM-polarized incident light 25053


ACS Applied Materials & Interfaces with the magnetic field parallel to the plane of the interface and for the same range of wavelengths and the angle of incidence as in the experiment. Au Nanofilm Fabrication. For the SR-SPP measurements, all of the samples are fabricated on Si wafers with thick SiO2 (8 μm). The wafers are cleaned in piranha solution (70% H2SO4 and 30% H2O2) for 20 min. For the deposition of APTMS, the cleaned wafers are immersed into 2.5% APTMS in IPA solution for 3 h.25 In this manner, we are depositing a monolayer of APTMS as the adhesion layer between Au and SiO2 layers. For 1 nm thick layers of Cr as adhesion layers, we used e-beam deposition. Au layers with thicknesses of 8, 10, 12, and 18 nm are sputtered onto the samples with a deposition rate of 10 Å/s. For symmetry reasons, we deposited another 1 nm thick layer of APTMS or Cr respectively on top of the Au films. Then, the 200 nm SiO2 layer was deposited on top of the Au films using sputtering to maintain the symmetry of the mode and also to allow for excitation of plasmons using the Otto configuration. The SEM pictures of the Au surface with APTMS and Cr adhesion layers and without adhesion layers are shown in Figure S1, as well as the inset of Figure 5. Otto Configuration. The spectroscopic reflection measurements were performed using a ZnSe semicylinder as a high-refractive-index prism to excite SR-SPPs. The incidence angle was varied between 33 and 55° with a 2° interval by rotating the mechanical stage as depicted in Figure S2. We chose the 2° interval because our beam has a solid angle of ±0.6°, so we can avoid aliasing effects. The light source was a supercontinuum broadband laser (SuperK; NKT Photonics A/S) allowing for measurements between 600 and 1750 nm wavelengths. Each incidence angle was measured 10 times: 5 times when increasing the incidence angle and 5 times when decreasing the angle to eliminate the mechanical stage uncertainties. The reference reflection spectrum is the measure of the reflected beam at 55° incidence angle with no sample, ensuring TIR. The reflectance spectra in Figure 5 are interpolated from the measured reflectance of Figure S3. The critical angle between ZnSe and SiO2 is approximately 35°, as shown in Figure 5a.

■

■

ABBREVIATIONS

■

REFERENCES

APTMS, 3-aminopropyl trimethoxysilane; SR-SPPs, shortrange surface plasmon polaritons; AFM, atomic force microscopy; IPA, isopropanol alcohol; TIR, total internal reflection; UV, ultraviolet

(1) Issa, N. A.; Guckenberger, R. Optical Nanofocusing on Tapered Metallic Waveguides. Plasmonics 2007, 2, 31−37. (2) Brigo, L.; Gazzola, E.; Cittadini, M.; Zilio, P.; Zacco, G.; Romanato, F.; Martucci, A.; Guglielmi, M.; Brusatin, G. Short and Long Range Surface Plasmon Polariton Waveguides for Xylene Sensing. Nanotechnology 2013, 24, No. 155502. (3) Vala, M.; Etheridge, S.; Roach, J. A.; Homola, J. Long-Range Surface Plasmons for Sensitive Detection of Bacterial Analytes. Sens. Actuators, B 2009, 139, 59−63. (4) Lozano, G.; Louwers, D. J.; Rodríguez, S. R. K.; Murai, S.; Jansen, O. T. A.; Verschuuren, M. A.; Gómez Rivas, J. Plasmonics for SolidState Lighting: Enhanced Excitation and Directional Emission of Highly Efficient Light Sources. Light: Sci. Appl. 2013, 2, No. e66. (5) Fang, Y.; Sun, M. Nanoplasmonic Waveguides: Towards Applications in Integrated Nanophotonic Circuits. Light: Sci. Appl. 2015, 4, No. e294. (6) Zenin, V. A.; Andryieuski, A.; Malureanu, R.; Radko, I. P.; Volkov, V. S.; Gramotnev, D. K.; Lavrinenko, A. V.; Bozhevolnyi, S. I. Boosting Local Field Enhancement by on-Chip Nanofocusing and Impedance-Matched Plasmonic Antennas. Nano Lett. 2015, 15, 8148− 8154. (7) Verhagen, E.; Polman, A.; Kuipers, L. K. Nanofocusing in Laterally Tapered Plasmonic Waveguides. Opt. Express 2008, 16, 45− 57. (8) Muskens, O. L.; Giannini, V.; Sánchez-Gil, J. A.; Gómez Rivas, J. Strong Enhancement of the Radiative Decay Rate of Emitters by Single Plasmonic Nanoantennas. Nano Lett. 2007, 7, 2871−2875. (9) Kinkhabwala, A.; Yu, Z.; Fan, S.; Avlasevich, Y.; Müllen, K.; Moerner, W. E. Large Single-Molecule Fluorescence Enhancements Produced by a Bowtie Nanoantenna. Nat. Photonics 2009, 3, 654−657. (10) Aćimović, S. S.; Ortega, M. A.; Sanz, V.; Berthelot, J.; GarciaCordero, J. L.; Renger, J.; Maerkl, S. J.; Kreuzer, M. P.; Quidant, R. LSPR Chip for Parallel, Rapid, and Sensitive Detection of Cancer Markers in Serum. Nano Lett. 2014, 14, 2636−2641. (11) Soler, M.; Mesa-Antunez, P.; Estevez, M.-C.; Ruiz-Sanchez, A. J.; Otte, M. A.; Sepulveda, B.; Collado, D.; Mayorga, C.; Torres, M. J.; Perez-Inestrosa, E.; Lechuga, L. M. Highly Sensitive Dendrimer-Based Nanoplasmonic Biosensor for Drug Allergy Diagnosis. Biosens. Bioelectron. 2015, 66, 115−123. (12) Yanik, A. A.; Huang, M.; Kamohara, O.; Artar, A.; Geisbert, T. W.; Connor, J. H.; Altug, H. An Optofluidic Nanoplasmonic Biosensor for Direct Detection of Live Viruses from Biological Media. Nano Lett. 2010, 10, 4962−4969. (13) Ma, X.; Truong, P. L.; Anh, N. H.; Sim, S. J. Single Gold Nanoplasmonic Sensor for Clinical Cancer Diagnosis Based on Specific Interaction between Nucleic Acids and Protein. Biosens. Bioelectron. 2015, 67, 59−65. (14) Nguyen, A. H.; Sim, S. J. Nanoplasmonic Biosensor: Detection and Amplification of Dual Bio-Signatures of Circulating Tumor DNA. Biosens. Bioelectron. 2015, 67, 443−449. (15) Yang, J.; You, J.; Chen, C.-C.; Hsu, W.-C.; Tan, H.; Zhang, X. W.; Hong, Z.; Yang, Y. Plasmonic Polymer Tandem Solar Cell. ACS Nano 2011, 5, 6210−6217. (16) Liu, Q.; Cui, Y.; Gardner, D.; Li, X.; He, S.; Smalyukh, I. I. SelfAlignment of Plasmonic Gold Nanorods in Reconfigurable Anisotropic Fluids for Tunable Bulk Metamaterial Applications. Nano Lett. 2010, 10, 1347−1353. (17) Lyvers, D. P.; Moon, J. M.; Kildishev, A. V.; Shalaev, V. M.; Wei, A. Gold Nanorod Arrays as Plasmonic Cavity Resonators. ACS Nano 2008, 2, 2569−2576.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07181. SEM images of 8 nm Au layers with and without adhesion layers; experimental setup schematics; raw experimental data; supplementary simulations to address the influence of the substrate, APTMS refractive index and air gap variation; RMS roughness for various layers; Drude−Lorentz parameters for the Au layer (PDF)

■

Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

R. Malureanu: 0000-0002-6093-5030 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was supported by Villum Fonden “DarkSILD project No. 11116” and Direktør Ib Henriksens Fond, Denmark. Notes

The authors declare no competing financial interest. The raw measurement data as well as the Matlab scripts used for analyzing them can be obtained from the corresponding author. 25054


Research Article

ACS Applied Materials & Interfaces (18) Sreekanth, K. V.; Alapan, Y.; ElKabbash, M.; Ilker, E.; Hinczewski, M.; Gurkan, U. A.; De Luca, A.; Strangi, G. Extreme Sensitivity Biosensing Platform Based on Hyperbolic Metamaterials. Nat. Mater. 2016, 15, 621. (19) Han, Z.; Bozhevolnyi, S. I. Radiation Guiding with Surface Plasmon Polaritons. Rep. Prog. Phys. 2013, 76, No. 016402. (20) Lahiri, B.; Dylewicz, R.; De La Rue, R. M.; Johnson, N. P. Impact of Titanium Adhesion Layers on the Response of Arrays of Metallic Split-Ring Resonators (SRRs). Opt. Express 2010, 18, 11202− 11208. (21) Petrovic, A.; Bratina, G. The Role of Ti Adhesion Layer in Electric Charge Transport in Pentacene Organic Thin Film Transistors. Eur. Phys. J. B 2010, 73, 341−346. (22) Majni, G.; Ottaviani, G.; Prudenziati, M. Interdiffusion of Thin Cr and Au Films Deposited on Silicon. Thin Solid Films 1976, 38, 15− 19. (23) Hugall, J. T.; Finnemore, A. S.; Baumberg, J. J.; Steiner, U.; Mahajan, S. Solvent-Resistant Ultraflat Gold Using Liquid Glass. Langmuir 2012, 28, 1347−1350. (24) Leosson, K.; Ingason, A. S.; Agnarsson, B.; Kossoy, A.; Olafsson, S.; Gather, M. C. Ultra-Thin Gold Films on Transparent Polymers. Nanophotonics 2013, 2, 3−11. (25) Leandro, L.; Malureanu, R.; Rozlosnik, N.; Lavrinenko, A. Ultrathin, Ultrasmooth Gold Layer on Dielectrics without the Use of Additional Metallic Adhesion Layers. ACS Appl. Mater. Interfaces 2015, 7, 5797−5802. (26) Kossoy, A.; Simakov, D.; Olafsson, S.; Leosson, K. Determining Surface Coverage of Ultra-Thin Gold Films from X-Ray Reflectivity Measurements. Thin Solid Films 2013, 536, 50−53. (27) Malureanu, R.; Lavrinenko, A. Ultra-Thin Films for Plasmonics: A Technology Overview. Nanotechnol. Rev. 2015, 4, 259−275. (28) Novikov, S. M.; Frydendahl, C.; Beermann, J.; Zenin, V. A.; Stenger, N.; Coello, V.; Mortensen, N. A.; Bozhevolnyi, S. I. White Light Generation and Anisotropic Damage in Gold Films near Percolation Threshold. ACS Photonics 2017, 4, 1207−1215. (29) Jeppesen, C.; Mortensen, N. A.; Kristensen, A. The Effect of Ti and ITO Adhesion Layers on Gold Split-Ring Resonators. Appl. Phys. Lett. 2010, 97, No. 263103. (30) Habteyes, T. G.; Dhuey, S.; Wood, E.; Gargas, D.; Cabrini, S.; Schuck, P. J.; Alivisatos, A. P.; Leone, S. R. Metallic Adhesion Layer Induced Plasmon Damping and Molecular Linker as a Nondamping Alternative. ACS Nano 2012, 6, 5702−5709. (31) Debu, D. T.; Ghosh, P. K.; French, D.; Herzog, J. B. Surface Plasmon Damping Effects due to Ti Adhesion Layer in Individual Gold Nanodisks. Opt. Mater. Express 2017, 7, 73. (32) Chang, W.-S.; Wen, F.; Chakraborty, D.; Su, M.-N.; Zhang, Y.; Shuang, B.; Nordlander, P.; Sader, J. E.; Halas, N. J.; Link, S. Tuning the Acoustic Frequency of a Gold Nanodisk through Its Adhesion Layer. Nat. Commun. 2015, 6, No. 7022. (33) Aouani, H.; Wenger, J.; Gérard, D.; Rigneault, H.; Devaux, E.; Ebbesen, T. W.; Mahdavi, F.; Xu, T.; Blair, S. Crucial Role of the Adhesion Layer on the Plasmonic Fluorescence Enhancement. ACS Nano 2009, 3, 2043−2048. (34) Krishnamoorthy, H. N. S.; Jacob, Z.; Narimanov, E.; Kretzschmar, I.; Menon, V. M. Topological Transitions in Metamaterials. Science 2012, 336, 205−209. (35) Lu, D.; Kan, J. J.; Fullerton, E. E.; Liu, Z. Enhancing Spontaneous Emission Rates of Molecules Using Nanopatterned Multilayer Hyperbolic Metamaterials. Nat. Nanotechnol. 2014, 9, 48− 53. (36) Kim, J.; Drachev, V. P.; Jacob, Z.; Naik, G. V.; Boltasseva, A.; Narimanov, E. E.; Shalaev, V. M. Improving the Radiative Decay Rate for Dye Molecules with Hyperbolic Metamaterials. Opt. Express 2012, 20, 8100−8116. (37) Liu, Z.; Lee, H.; Xiong, Y.; Sun, C.; Zhang, X. Far-Field Optical Hyperlens Magnifying. Science 2007, 315, 1686. (38) Li, J.; Fok, L.; Yin, X.; Bartal, G.; Zhang, X. Experimental Demonstration of an Acoustic Magnifying Hyperlens. Nat. Mater. 2009, 8, 931−934.

(39) Rho, J.; Ye, Z.; Xiong, Y.; Yin, X.; Liu, Z.; Choi, H.; Bartal, G.; Zhang, X. Spherical Hyperlens for Two-Dimensional Sub-Diffractional Imaging at Visible Frequencies. Nat. Commun. 2010, 1, No. 143. (40) Poddubny, A.; Iorsh, I.; Belov, P.; Kivshar, Y. Hyperbolic Metamaterials. Nat. Photonics 2013, 7, 948−957. (41) Ferrari, L.; Wu, C.; Lepage, D.; Zhang, X.; Liu, Z. Hyperbolic Metamaterials and Their Applications. Prog. Quantum Electron. 2015, 40, 1−40. (42) Liu, F.; Xiao, L.; Ye, Y.; Wang, M.; Cui, K.; Feng, X.; Zhang, W.; Huang, Y. Integrated Cherenkov Radiation Emitter Eliminating the Electron Velocity Threshold. Nat. Photonics 2017, 11, 289−292. (43) Sreekanth, K. V.; De Luca, A.; Strangi, G. Experimental Demonstration of Surface and Bulk Plasmon Polaritons in Hypergratings. Sci. Rep. 2013, 3, No. 3291. (44) Jacob, Z.; Kim, J. Y.; Naik, G. V.; Boltasseva, A.; Narimanov, E. E.; Shalaev, V. M. Engineering Photonic Density of States Using Metamaterials. Appl. Phys. B: Lasers Opt. 2010, 100, 215−218. (45) Shalaginov, M. Y.; Ishii, S.; Liu, J.; Liu, J.; Irudayaraj, J.; Lagutchev, A.; Kildishev, A. V.; Shalaev, V. M. Broadband Enhancement of Spontaneous Emission from Nitrogen-Vacancy Centers in Nanodiamonds by Hyperbolic Metamaterials. Appl. Phys. Lett. 2013, 102, No. 173114. (46) Jacob, Z.; Smolyaninov, I. I.; Narimanov, E. E. Broadband Purcell Effect: Radiative Decay Engineering with Metamaterials. Appl. Phys. Lett. 2012, 100, No. 181105. (47) Luk, T. S.; Kim, I.; Campione, S.; Howell, S. W.; Subramania, G. S.; Grubbs, R. K.; Brener, I.; Chen, H. T.; Fan, S. H.; Sinclair, M. B. Near-Infrared Surface Plasmon Polariton Dispersion Control with Hyperbolic Metamaterials. Opt. Express 2013, 21, 11107−11114. (48) Zhukovsky, S. V.; Kidwai, O.; Sipe, J. E. Physical Nature of Volume Plasmon Polaritons in Hyperbolic Metamaterials. Opt. Express 2013, 21, 14982−14987. (49) Kidwai, O.; Zhukovsky, S. V.; Sipe, J. E. Effective-Medium Approach to Planar Multilayer Hyperbolic Metamaterials: Strengths and Limitations. Phys. Rev. A: At., Mol., Opt. Phys. 2012, 85, No. 053842. (50) Cortes, C. L.; Newman, W.; Molesky, S.; Jacob, Z. Quantum Nanophotonics Using Hyperbolic Metamaterials. J. Opt. 2012, 14, No. 063001. (51) Hoffman, A. J.; Alekseyev, L.; Narimanov, E. E.; Gmachl, C.; Sivco, D. L.; Howard, S. S.; Franz, K. J.; Wasserman, D.; Podolskiy, V. A. Negative Refraction in Mid-Infrared Semiconductor Metamaterials. Nat. Mater. 2007, 6, 946−950. (52) Terracciano, M.; Rea, I.; Politi, J.; De Stefano, L. Optical Characterization of Aminosilane-Modified Silicon Dioxide Surface for Biosensing. J. Eur. Opt. Soc. 2013, 8, 1−6. (53) De Stefano, L.; Oliviero, G.; Amato, J.; Borbone, N.; Piccialli, G.; Mayol, L.; Rendina, I.; Terracciano, M.; Rea, I. Aminosilane Functionalizations of Mesoporous Oxidized Silicon for Oligonucleotide Synthesis and Detection. J. R. Soc., Interface 2013, 10, No. 20130160. (54) Qian, H.; Xiao, Y.; Liu, Z. Giant Kerr response of ultrathin gold films from quantum size effect. Nat. Commun. 2016, 7, No. 13153. (55) Manjavacas, A.; García de Abajo, F. J. Tunable Plasmons in Atomically Thin Gold Nanodisks. Nat. Commun. 2014, 5, No. 3548. (56) Zhu, M.; Lerum, M. Z.; Chen, W. How To Prepare Reproducible, Homogeneous, and Hydrolytically Stable AminosilaneDerived Layers on Silica. Langmuir 2012, 28, 416−423. (57) Heiney, P. A.; Gru, K.; Fang, J.; Dulcey, C.; Shashidhar, R. Structure and Growth of Chromophore-Functionalized (3-Aminopropyl) Triethoxysilane Self-Assembled on Silicon. Langmuir 2000, 16, 2651−2657. (58) Hoffman, A. J.; Alekseyev, L.; Howard, S. S.; Franz, K. J.; Wasserman, D.; Podolskiy, V. A.; Narimanov, E. E.; Sivco, D. L.; Gmachl, C. Negative refraction in semiconductor metamaterials. Nat. Mater. 2007, 6, 946−950. (59) Guo, Y.; Newman, W.; Cortes, C. L.; Jacob, Z. Applications of Hyperbolic Metamaterial Substrates. Adv. OptoElectron. 2012, 2012, No. 452502. 25055


Research Article

ACS Applied Materials & Interfaces (60) Malitson, I. H. Interspecimen Comparison of the Refractive Index of Fused Silica. J. Opt. Soc. Am. 1965, 55, 1205−1209. (61) Lavrinenko, A. V.; Zhilko, V. Application of Coordinate-Free Effective Medium Theory to Periodically Plane-Stratified Anisotropic Media. Microwaves: Opt. Technol. Lett. 1997, 15, 54−57. (62) Agranovich, V. M. Dielectric Permeability and Influence of External Fields on Optical Properties of Superlattices. Solid State Commun. 1991, 78, 747−750. (63) Zhukovsky, S. V.; Andryieuski, A.; Takayama, O.; Shkondin, E.; Malureanu, R.; Jensen, F.; Lavrinenko, A. V. Experimental Demonstration of Effective Medium Approximation Breakdown in Deeply Subwavelength All-Dielectric Multilayers. Phys. Rev. Lett. 2015, 115, No. 177402. (64) Gordon, J. A.; Ziolkowski, R. W. The Design and Simulated Performance of a Coated Nano-Particle Laser. Opt. Express 2007, 15, 2622−2653. (65) Lysenko, O.; Bache, M.; Lavrinenko, A. Third-Order Susceptibility of Gold for Ultrathin Layers. Opt. Lett. 2016, 41, 317−320. (66) OptiFDTD − Optiwave. OptiFDTD Technical Background and Tutorials, version 8.1, 2009; p 27. (67) Rakic, A. D.; Djurisic, A. B.; Elazar, J. M.; Majewski, M. L. Optical Properties of Metallic Films for Vertical-Cavity Optoelectronic Devices. Appl. Opt. 1998, 37, 5271−5283. (68) Bozhevolnyi, S. I.; Søndergaard, T. General Properties of SlowPlasmon Resonant Nanostructures: Nano-Antennas and Resonators. Opt. Express 2007, 15, 10869−10877. (69) Ko, D. Y. K.; Sambles, J. R. Scattering Matrix Method for Propagation of Radiation in Stratified Media: Attenuated Total Reflection Studies of Liquid Crystals. J. Opt. Soc. Am. A 1988, 5, 1863−1866. (70) Salzberg, C. D.; Villa, J. J. Infrared Refractive Indexes of Silicon Germanium and Modified Selenium Glass. J. Opt. Soc. Am. 1957, 47, 244−245.

25056


High-Quality Ultrathin Gold Layers with an APTMS Adhesion for

Recommend Documents