Enhancing Chemical and Optical Stability of Silver Nanostructures by

Article pubs.acs.org/JPCC

Enhancing Chemical and Optical Stability of Silver Nanostructures by Low-Temperature Hydrogen Atoms Processing Maria Losurdo,*,† Iris Bergmair,‡ Maria M. Giangregorio,† Babak Dastmalchi,§ Giuseppe V. Bianco,† Christian Helgert,∥ Ekaterina Pshenay-Severin,∥ Matthias Falkner,∥ Thomas Pertsch,∥ Ernst-Bernhard Kley,∥ Uwe Huebner,⊥ Marc A. Verschuuren,# Michael Muehlberger,‡ Kurt Hingerl,§ and Giovanni Bruno† †

Institute of Inorganic Methodologies and of Plasmas-CNR, Via Orabona, 4, 70126 Bari, Italy Functional Surfaces and Nanostructures, PROFACTOR GmbH, Im Stadtgut A2, 4407 Steyr-Gleink, Austria § Center for Surface- and Nanoanalytics, Johannes Kepler University Linz, Altenbergerstr. 69, 4040 Linz, Austria ∥ Institute for Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Max Wien Platz 1, 07743 Jena, Germany ⊥ Institute of Photonic Technology, A. Einstein Str. 9, 07745 Jena, Germany # Philips Research, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands ‡

ABSTRACT: A large variety of applications ranging from plasmonic sensing to plasmonic enhanced solar cells, photonics, and optics can benefit from a reliable method to enhance chemical and time stability of silver-based plasmonic nanostructures and metamaterials. Therefore, here we demonstrate and discuss the effectiveness of a low-temperature (100 °C) hydrogen atom processing of silver to inhibit its oxidation and stabilize surface plasmon resonances in silver nanostructure suitable for plasmonics, metamaterials, sensing, and photovoltaics. Interestingly, no dielectric overlayer encapsulating Ag is used to protect the silver nanostructure, differently from the common approach, because these overlayers typically lead to a red shift of the optical resonances due to their refractive index. Conversely, we demonstrate that the silver deoxidation by the hydrogen treatment results in a slight blue shift of resonances, which is useful for preserving resonances in the visible range. The chemical mechanism rationalizing the validity of this processing is discussed. The optical properties of the fabricated samples were measured by means of transmission, reflection, and ellipsometry spectroscopies. Theoretical support to the interpretation of the optical properties demonstrates the advantages of this advanced processing. Therefore, this work is an important step toward novel and breakthrough applications of stable silver-based nanostructures for plasmonics and metamaterials exploiting visible light. stability. Furthermore, a more fundamental hurdle is finding and optimizing the plasmonic materials to enable effective applications. The challenge to provide suitable low-loss material candidates for plasmonic nanostructures at optical wavelengths is of rather fundamental nature;12 in particular, there is also a need to provide a convincing and reliable technology for lowloss NIMs in the visible regime of the electromagnetic spectrum. The electric permittivity of the noble metal silver exhibits formally the lowest imaginary part and in addition a highly negative real part at optical frequencies.13 The former criterion is essential to enable a minimum of losses for corresponding plasmonic nanostructures and metamaterials made of silver.

1. INTRODUCTION The unique optical versatility of the surface plasmon resonance (SPR)1 of metal thin films2 and nanostructures3 is attracting increasing widespread interest in fundamental science to improve knowledge of light manipulation at the nanoscale and to develop novel synthesis and processing methodologies for nanomaterials as well as in applications that can span from chemistry and medicine to energetic and photonics. SPR exploitation includes biochemical sensing,4 surface-enhanced Raman spectroscopy,5 solar cells,6 optical storage,7 nanoscale waveguiding, and artificial metamaterials.8 Among a variety of resonant plasmonic nanostructures, negative index materials (NIMs) have also gained interest due to their potential exploitation for perfect lenses9 and magnification tools.10 Although the field of plasmonics and metamaterials is developing rapidly,11 the effective transfer of pivotal prototypes into large scale applications requires corresponding advancements in nanofabrication and material © 2012 American Chemical Society

Received: August 9, 2012 Revised: October 6, 2012 Published: October 12, 2012 23004

dx.doi.org/10.1021/jp307936k | J. Phys. Chem. C 2012, 116, 23004−23012

The Journal of Physical Chemistry C

Article

Figure 1. Cross-sectional scanning electron microscopy (SEM) images (left) and schematic drawing of different process steps (right). (a,d) Imprint into Ormocere on LOR1A layer. (b,e) Imprint after etching to achieve recessed sidewalls due to different etching rates of the resists. (f) Deposition of Ag, SiO2, Ag. (c,g) Final sample after lift-off.

Here we demonstrate a dry low-temperature (100 °C) methodology using hydrogen atoms for the effective passivation of silver unpatterned films, wires, and periodic fishnet nanostructures with different dimensions in the nanometer regime fabricated by an adapted nanoimprint lithography (NIL) method.22 It is shown that our method is able to reduce the silver oxide, inhibit silver reoxidation, and keep the optical performance of the samples stable over time. We address the important aspect of the hydrogenation to reduce silver oxide and clean silver regions, passivate grain boundaries, and stabilize Ag nanostructures. The peculiarities of this lowtemperature (100 °C)23 processing are: (i) it provides atomic hydrogen in the ground state ready to interact chemically with AgO, overcoming the limitation of molecular H2 that does not chemisorb dissociatively on the Ag low index plane surfaces.24 (ii) It provides selective localized heating of the Ag sites by the heterogeneous surface recombination of H atoms, necessary to gain the reaction heat to reduce silver oxide, desorb the reaction product, and activate mild crystallization of silver. (iii) It successfully decouples the chemical effect of AgO reduction by the Ag crystallization unavoidable during thermal annealing. A comparison between our low-temperature hydrogen-atomsbased passivation process and the commonly used N2 annealing is also discussed to stress the limits of conventional approaches. With respect to their optical functionality, we show that our low-temperature hydrogen atoms passivation is suitable to stabilize silver plasmonic and metamaterials, with feature sizes below 100 nm and a high aspect ratio. The optical properties of the fabricated silver nanostructures are discussed, also demonstrating their long temporal stability. Experimental results are corroborated by rigorous numerical simulations.

Furthermore, its high-energy interband transitions (at >4 eV) make it suitable for plasmonic applications in the visible range. The main problem of silver is that it suffers from oxidation and corrosion14 and forms different silver oxides depending on the environment, Ag2O and AgO being most common ones.15 Furthermore, oxygen atoms have been reported to diffuse subsurface and reside there even under reductive ambient conditions.16,17 Accordingly, the optical properties of silverbased nanostructures are anything but time-stable under ambient laboratory conditions.18 Fabrication routes of silver-based optical metamaterials have so far circumvented this issue by passivating the nanostructured silver surfaces with dielectric layers.19,20 Although the oxidation and corrosion is prevented by this method, the resulting larger refractive index of the surrounding dielectric layers shifts the designed plasmonic resonance frequencies to longer wavelengths, ultimately limiting high-frequency operation of silverbased photonic devices. Therefore, the stabilization of silver nanostructures and consequently their plasmonic resonances without additional passivation layers is important for a plethora of plasmonic applications, which span from photonics to photovoltaic and sensing devices. Previous work on silver passivation has shown that annealing may improve the optical quality of thin metal films.21 However, thermal annealing does not always work for metal nanostructures. Specifically, annealing does not improve the quality of high aspect ratio nanostructures, such as nanostrips with a width less than 80 nm.21 Indeed, previous attempts of annealing21 show a complex dependence on particle size, aspect ratio, and material. Therefore, optimal annealing and passivation conditions for specific sample types must be tailored to the individual parameters of the sample. Furthermore, a limitation of the thermal annealing in processing high aspect ratio nanostructures is given by the fact that the increase in the annealing temperature causes crystallization, affecting not only grain boundaries effects and, consequently, the electron relaxation time and the plasmon resonance width,21 but also damaging the high aspect ratio geometry of nanostructure. This requires control of annealing temperature to tailor and reduce grain boundaries effects; that is, the development of lowtemperature processes is preferable.

2. EXPERIMENTAL METHODS 2.1. Silver Nanostructures Fabrication. For fabrication of the silver NIMs herein discussed, NIL masters were fabricated on a 4″ silicon wafer by e-beam lithography. A diluted Ormocere layer is spin-coated on top of a lift-off resist (LOR) and structured using NIL (Figure 1a,d). The Ormocere contains not only organic but also inorganic materials, resulting in a much lower etching rate than the used LOR layer underneath. Therefore, when using this material combination, 23005



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where Ψ = |rp/rs| represents the ratio of the absolute value of the change of amplitude of the reflected polarized light beam with respect to the linearly polarized incident beam, whereas Δ = δp − δs is the phase change between the two reflected polarizations. Ellipsometric spectra were acquired using a phase-modulated spectroscopic ellipsometer (UVISEL, Horiba Jobin Yvon) in the 180−1700 nm spectral range with 1 nm resolution.

recessed sidewalls are achieved in a simple RIE etching step (Figure 1b,e) without the need for a wet chemical development step of the LOR layer as in previous experiments.22 These recessed sidewalls are crucial to have a working lift-off, especially for higher aspect ratios. After the RIE etching step, the different layers needed for a NIM material are deposited. Either one layer, that is, Ag, three layers, that is, Ag/SiO2/Ag (Figure 1f), or five layers, Ag/SiO2/Ag/SiO2/Ag, were deposited, and the resist was removed by immersing the sample in the developer such that only the deposited layer stack remained on the substrate (Figure 1g). With this method, we achieved ∼2 × 2 cm2 structured area and high aspect ratios (line width to height). 2.2. Hydrogen Atoms Processing. Hydrogen atoms were generated in a 4 cm i.d. quartz tube equipped with external electrodes to activate a radiofrequency (rf) discharge operating at 13.56 MHz.23 This plasma tube was positioned at a distance of 10 cm from the sample, which can therefore be considered in a post-discharge position. Therefore, the peculiarity of this passivation process was that only neutral hydrogen atoms interacted with the sample, whereas electron and ion bombardment, which could possibly cause radiative damage, were avoided. For plasma generation of hydrogen atoms, the pressure was 1 Torr, the rf power level was 80 W, and the H2 gas flow rate was 1000 sccm. Despite the use of this source in the present paper, hydrogen atoms were required, which means that the method was extendable by also using Hoffman or filament sources of atomic hydrogen. The substrate holder during the optimized processing was kept at 100 °C. For the investigation of the temperature effect on the silver grain size, the temperature was changed in the range 100−700 °C. 2.3. Characterization and Modeling. The effects of the topographical characteristics of the silver nanostructures were monitored by field-emission scanning electron microscope (FESEM) and atomic force microscope (AFM) investigations. Morphology analysis was carried out with an AFM (AutoProbeCP ThermoMicroscope) using a gold-coated Si tip with a resonant frequency of 80 kHz. Chemical analysis was carried out by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer, PHY); survey scans were acquired to detect element and verify sample purity; the C1s, O1s, Ag3d photoelectron core levels were acquired in high resolution (pass energy of 17.9 eV) to identify the chemical oxidation states. Optical analysis was carried out by reflection and normal incidence transmission measurements and also by spectroscopic ellipsometry. Spectroscopic ellipsometry25 recorded the corresponding plasmonic response of the fishnet structures by directly recording the pseudodielectric function = + i, which is related to the extinction coefficient, k, and to the refractive index, n, by the following equation

3. RESULTS AND DISCUSSION To extend the validity of the developed low-temperature (∼100 °C) processing, it has been tested on silver unpatterned thin film and 1-D and 3-D fishnet nanostructures. The lowtemperature (100 °C) hydrogen atoms processing is compared with the conventional annealing in N2 at 400 °C.26,27 Optimizing annealing is also important for thin silver films because rough surface results in a significant surface plasmon polariton scattering loss, which exposes a major impediment to achieving higher optical quality and lower-loss plasmonic devices. Inhomogeneity of the surface morphology can sensitively affect the SPR at the metal dielectric interface, giving rise to performance degradation.28 Therefore, to state the problem of annealing silver and its impact even on the roughness/morphology, we started from analyzing an unpatterned 50 nm silver thin film deposited on SiO2/Si to measure the roughness and average grain size upon annealing; the results are summarized in Figure 2. The as-deposited film is

Figure 2. 5 μm × 5 μm AFM images (a) of as-deposited 50 nm Ag film and (b) after its processing by hydrogen atoms at 100 °C compared with thermal annealing at 400 °C in (c) H2 and (d) N2. Treatment time is 15 min in all cases. For the continuous films (a,b), the grain size as estimated by the line profiles of AFM images is also indicated. The surface root-mean-square roughness (RMS) values are also reported. The z-scale is 0−21.68 nm for (a,b), 0−145.6 nm for (c) and 0−591.3 nm for (d).

⟨ε⟩ = ⟨ε′⟩ + i⟨ε″⟩ ⎡ (1 − ρ)2 ⎤ = sin 2 ϕ⎢1 + tan 2 ϕ ⎥ ⎣ (1 + ρ)2 ⎦ = ⟨(n + ik)2 ⟩

characterized by a surface roughness (root-mean-square roughness, RMS) of 1 nm, which is drastically changed upon annealing at 400 °C in N2, leading to clusters of Ag whose height and RMS are higher than the initial 50 nm thickness of the film because of its clustering. This disaggregation of the dense film into particles and clusters is a process driven by the minimization of surface energy and denoted as solid-state dewetting.29 The phenomenon is reduced when H2 is used

(1)

where ϕ is the angle of incidence fixed at 70° and ρ is the complex reflection coefficient for the parallel, rp, and perpendicular, rs, polarizations, defined as rp ρ= = tan Ψ·eiΔ rs (2) 23006



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instead at the same temperature of 400 °C, although the film dewetting starting at surface fluctuations at grain or twin boundaries results in the growth of holes, leading to high roughness. A similar phenomenon has also been reported recently for annealing at 400 °C of ultrathin layers of gold,30 putting in evidence that reducing processing temperature is important for various ultrathin plasmonic metal layers. Continuity of the Ag film with smooth morphology is preserved when thin films are processed by hydrogen atoms at 100 °C, resulting in an almost unchanged RMS of 1 nm. The observed changes in morphology upon thermal dewetting affect the optical properties of the Ag layers, as shown in Figure 3. Specifically, Figure 3 shows ellipsometric

roughness. Therefore, the advantage of operating effectively the hydrogen atoms treatment at a temperature of 100 °C is the removal of oxygen, as shown from XPS spectra also reported in Figure 4, yielding a grain size smaller than the width of the strip, therefore preserving their periodicity. From the chemical point of view, the different behaviors observed by annealing in N2 and H2 environments can be explained considering that the thermal decomposition of silver oxides occurs in the range 330−500 °C by the reactions26 2AgO → Ag 2O +

Ag 2O → 2Ag +

1 O2 2

1 O2 2

(T > 200 °C)

(T > 400 °C)

(3) (4)

However, at these temperatures, the silver product of reaction somehow melts, leading to the disruption of nanostructure and periodicity.30 During H-atoms reduction of the oxide at low temperature, melting is prevented, the thermal decomposition mechanism is ruled out and only the chemical selective oxygen removal and silver reduction occur according to reactions

Figure 3. Spectra of the real, , and imaginary, , parts of the 50 nm Ag films pseudodielectric function (dots) also after H2 (triangles) and N2 (crosses) annealing at 400 °C for 15 min. For comparison, the Ag dielectric function from ref 13 is also shown (black line).

2AgO + 2H → Ag 2O + H 2O

(5)

Ag 2O + 2H → 2Ag + H 2O

(6)

The effective silver oxide reduction by H atoms is consistent with the much lower activation energy for reaction 6 of 24.8 kcal/mol with respect to the activation energy of 96.6 kcal/mol determined for the thermal decomposition of silver oxide in N2.30 A chemical aspect that has to be considered when a metal like silver interacts with H-atoms is the H-atoms heterogeneous recombination on the metal surface leading to the formation of desorbing H2 molecules, which can be generally schematized as

spectra of the real, , and imaginary, , parts of the pseudodielectric function for the drastic annealing at 400 °C, showing a broad band for the N2 annealing associated to the round nanoparticles, whereas oscillations present in the H2-400 °C annealed samples are from the roughening and partial depolarization in addition to development of resonances from the nonhomogeneous void texture of the silver film. Therefore, the observed morphological and optical modifications rule out application of thermal annealing in H2, N2, or mixtures of both for deoxidation of thin silver films. Moving to nanostructures, Figure 4 shows the advantage of the low-temperature hydrogen atoms processing on the morphology of silver lines with a height of ∼65 nm and a width of 600 nm compared with the conventional N2 thermal annealing reported in literature. The morphology and periodicity are preserved after the hydrogen atoms processing; the same average height of 65 nm is still measured by AFM line profiles after the processing, indicating that no etching of the nanostructure occurs under the present conditions. Conversely, the N2 annealing clearly results in a disruption of the silver lines, and the higher peak-to-valley height, Rp-v, measured indicates mobility and reaggregation of silver. Chemically, the XPS spectra of the Ag3d core level show the effectiveness of the hydrogen treatment in reducing the silver oxides to metallic silver. Furthermore, the AFM images also put in evidence a crystallization of the silver by the hydrogen atoms treatment. We also run the H2 imposing to the substrate various temperatures in the range of 100−700 °C, and of course by increasing the temperature, the thermal annealing results in increasing grain diameter, as shown in Figure 5 and consequently in disrupting structure periodicity and increasing

H + H + surface → H 2(v) + surface

(7)

where H2(v) indicates molecular hydrogen vibrationally excited. Indeed, in the case of Ag, the H-atoms recombination occurs according to the Eley−Rideal recombination mechanism; that is, chemisorptions of H atoms on an Ag surface site, Ag(s), occurs forming silver−hydrogen (Ag−H) complexes/hydrides at the Ag surface site (Ag−H(s))31,32 Ag(s) + H → Ag − H(s)

(8)

33

Eley and Rossington showed that the heat of adsorption of hydrogen on silver foils is 2.5 kcal/mol, whereas gold does not chemisorb hydrogen, and they explained that this difference has a consequence of higher density of defects in Ag yielding vacancies in d bands formed from the d orbitals of unsaturated Ag atoms near the surface and/or at grain boundaries. The sites Ag−H are subsequently involved in the surface recombination of hydrogen according to34 Ag − H(s) + H → Ag(s) + H 2

(10)

The peculiarity of silver (as well as gold) is that the H2 desorbing from its surface is not vibrationally excited,35 implying that all of the energy released by the heterogeneous recombination of H-atoms is actually dissipated into the Ag surface at the site reaction,36 providing local heating. Furthermore, among various metals, silver is the one with the highest energy accommodation coefficient for hydrogen; that is., Ag accommodates very well the recombination energy by 23007



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Figure 4. AFM topography images on different sizes of 5 μm × 5 μm and 2 μm × 2 μm of Ag gratings with line width of 600 nm and height of ∼65 nm, respectively, as fabricated, after N2 annealing at 400 °C, and after hydrogen atoms processing at 100 °C; all processes were run for 15 min. The peak-to-valley height (Rp-v) values from AFM are also reported. The line profiles as indicated in the 5 μm × 5 μm AFM scan are also shown. For the three cases, typical XPS spectra of the Ag3d photoelectron core level are also shown. The XPS spectra were acquired at a takeoff angle of 90° with a monochromatic Al Ka X-ray source (1486.6 eV) and pass energy during the high-resolution scans of 11.75 eV.

crystallization of silver, even at bias temperature as low as 100 °C. Because the hydrogen recombination coefficient for Ag is γH = 2.8 × 10−3 at the low bias temperature of 100 °C (where the present experiments are run), some Ag−H(s) still remains at the Ag surface, passivating grain boundaries and grain internal defects, which are primarily contributing to loss factor.21 During the H2 reduction of oxide simultaneous crystallization occurs, and because of the different density of the oxides and metallic silver, silver atoms are mobile and rearrange to form an Ag(111) surface (confirmed by XRD measurements), which is energetically favorable to form and the most dense surface orientation. The sticking coefficient of oxygen on Ag(111) is much lower (i.e., ∼10−6) than on any other silver surface (e.g., for Ag(110), it is ∼10−3).38,39 Thus, those factors explain the passivation effect of the low-temperature atomic hydrogen treatment on silver, which is, therefore, less prone to oxidation. Applying these findings to functional plasmonic silver nanostructures to verify their usefulness for the corresponding optical properties, we have chosen the well-known fishnet NIM40 as a referential design. Figure 6 shows the morphology of a silver fishnet before and after hydrogen atoms processing. It is noteworthy that the fishnet structure preserves its geometry by applying the present 100 °C treatment, which avoids decomposition, large grain crystallization, or both, and a smoothing of the surface roughness of the structure is also achieved (as seen also by the line profiles). To characterize the optical response, we rely on a comparison of the SE spectra before and after treatment as

Figure 5. Dependence of the Ag grain size induced by processing the Ag gratings at various temperatures in the range 100−700 °C; the process time was fixed at 15 min in all cases. Typical AFM 2 μm × 2 μm images are also shown for some samples to put in evidence Ag grain size.

lattice rearrangement.37 Thus, the energy coming from the hydrogen recombination enables the selective localized 23008



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ωp = 9.17 eV, ωL = 5.27 eV,

γ = 0.021 eV,

f = 2.2,

Γ = 1.14 eV

where ωp and γ (ωL and Γ) are, respectively, the plasma frequency and collision frequency in the Drude (Lorentz) term. α and β are free parameters, with α being the relaxation rate factor for the Drude term. α = β = 1 results in the best fit to the silver dielectric function of Johnson and Christy.13 The Lorentz term in the expression of the dielectric function accounts for the interband transitions of silver. Loss in the silver is reflected in the collision frequency, which is inversely proportional to the carrier lifetime, τ: γ=

well as the consequent changes in the retrieved effective properties. Optical analysis is performed by comparing the measured SE data to simulations obtained by a rigorous coupled wave analysis (RCWA) solver for Maxwell equations of light propagation through the metamaterial. For structured samples, with a corrugation length on the order of the wavelength, the correct reflection and transmission coefficients cannot be derived any more by the Fresnel formula, which requires a homogeneous stratified layer. Because the layers have diffracting structures, we employ the RCWA numerical technique, with which the complex reflectances (and transmittances) for s- as well as for p-polarized light can be exactly derived. As input parameters for the RCWA calculations we use the geometrical values of the fishnet (line width, period along both in plane axis, height, in certain cases the inclination angle for side walls) as well as the dielectric functions of each material. In the simulations geometrical parameters are determined from SEM pictures of the fabricated sample, i.e., we consider two silver layers separated by a 35 nm SiO2 spacer; the thickness of top silver layer is ∼45 nm, and the thickness of the bottom layer is 55 nm. Layers are perforated with square holes of 265 nm wide with round corners; the unitcell size is 365 nm. The dielectric functions of the SiO2 spacer and substrate were previously determined from ellipsometry fitting of their experimental spectra, whereas the dielectric function of the silver is approximated by a Drude−Lorentz model21 ε( ω , α , β ) = 1 −

ω(ω − iαγ )

+

Figure 7. (a) Comparison of measured and simulated transmission spectra before and after hydrogen atoms treatment measured in the range of the negative refraction resonance of the Ag fishnet in Figure 6. (The fishnet was designed to have a negative refraction resonance in the visible.) Sharpening of the resonance is due to hydrogen processing and blue shift is a consequence of reduced AgO. (b) Real, ε′, and imaginary, ε″, parts of the dielectric function of silver used for ab initio simulations before treatment (with α = 3.2, see eq 8 in the text) and after the hydrogen atoms treatment (with α = 1.5, see text). For comparison the one measured for AgO is also shown.

fωL2 ωL2

(12)

where vf is the Fermi velocity and l is the carrier mean free path. Scattering from surfaces, lattice defects and grain boundaries are limiting factors for the carrier lifetime in a metallic thin film. Surface scattering affects electrons in a range closer than mean free path to the film surfaces and should not differ for the preand postprocessed samples. Only the grain boundaries are significantly affected by the annealing process to form larger grains, which increases mean free path and consequently the carrier lifetime. Figure 7a shows the simulation and the normal incidence measurement of the sample measured 4 days after fabrication and soon after the atomic hydrogen treatment. Equation 11 is

Figure 6. (a,b) 2 μm × 2 μm AFM topography images of 365 nm Ag fishnet structures as fabricated (a) and after hydrogen atoms cleaning/ passivation at 100 °C (b). Rp-v values are also indicated. The A dimension (line width) is 200 ± 5 nm and B dimension is 210 ± 10 nm.(c,d) Corresponding 1 μm × 1 μm 3D images are also shown, together with representative line profiles (e,f).

ωp2

vf 1 = l τ

− ω 2 + iβ ΤLω (11) 23009



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used for the silver dielectric function of bottom layer, whereas for the top layer an effective medium approximation of eq 11 and the dielectric function of oxidized silver are used. By iteratively varying the percentage of silver oxide and the free parameter α, the best fit to the measured transmission curve for the as-fabricated sample is achieved for α = 3.2 and including a volume percentage of 50% of silver oxide in the top metallic layer. We monitored in transmission a pronounced resonance sharpening and a slight blue shift induced by the hydrogen atoms treatment (red lines in Figure 7a). Optical response of the sample can be reproduced by calculations by using the same geometry but different silver parameters. Applying the Drude− Lorentz model to obtain a best fit with this data, we have found α = 1.5 for the hydrogen-atoms-processed sample as a consequence of grain size enhancement. The blue shift of the resonance is reproduced by eliminating from the model the volume fraction of silver oxide and assuming only the silver dielectric function and therefore can be associated with the removal of silver oxide for the hydrogen atoms processed, consistently with the XPS data discussed in this manuscript. The observed blue shift and sharpening of the resonance at ∼0.77 μm upon hydrogen atoms processing ascribed to silver oxide reduction is also corroborated by previous findings reporting red shift, broadening, and damping of resonance peaks by the formation of an overlayer (which could be the oxide) on the metal.41 Figure 7b shows the retrieved dielectric function of as fabricated silver, hydrogen-atoms processed silver according to eq 11, and silver oxide. The decrease (becoming more negative) of the real part of the dielectric function, ε′, for H-processed Ag with respect to that of AgO is indicative of a larger Drude metallic behavior, consistently with the hydrogen atoms operated reduction of the silver oxide. The optical response of fishnet metamaterials40 for normal light incidence is characterized in terms of effective material properties such as the magnetic permeability μ (permeability is the degree of magnetization that a material obtains in response to an applied magnetic field) and the electric permittivity ε (permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium, and is determined by the ability of a material to electrically polarize in response to the field), or, as an additional quantity, the effective refractive index n. Making use of a well-established retrieval algorithm whose details can be found in ref 42, we have calculated the main parameters for the fishnet with a period of 365 nm, and the results are summarized in Figure 8 in the spectral region of the magnetic resonance around 770 nm. We note the positive impact of reduced losses on the magnetic resonance around 770 nm for the hydrogen-atoms-processed sample, which shows a sharper and blue-shifted resonance in μeff and neff. Here we also point out that in a RCWA calculation of a perfectly periodic sample there is no depolarization. Depolarization occurs only due to angular spread, wavelength bandwidth, or statistical variations in the geometry. So, variations of the parameters above will lead to a slight depolarization of the reflected beam due to statistical roughness. For our samples, however, depolarization due to statistical variations has only a minor effect due to the rather sophisticated structuring technique. Finally, from the optical point of view, the effectiveness of the 100 °C hydrogen atoms treatment has been demonstrated by monitoring as a function of time of air exposure the stability of the optical resonance. Spectroscopic ellipsometry was recently

Figure 8. Retrieved effective parameters of the real (continuous lines) and imaginary (dashed lines) parts of the complex refractive index, neff, permeability, μeff, and permittivity, εeff, for the Ag fishnet design of Figure 6, for the as fabricated structure (measured after 4 days of air exposure) (black curves). and for the hydrogen atoms treated (also measured after 4 days of air exposure from the treatment) (red curves).

established as a valuable tool for the time-dependent monitoring of optical properties of plasmonic thin films, which was described in detail in a recent review.43 Plasmonic modes are excited by the p-polarized component of the linearly polarized light incident field, whereas the s-polarized component undergoes minor changes in the explored range, causing the resonances to appear as a deep in the ratio of the Fresnel reflection coefficient rp /r s and, hence, in the ellipsometric angle Ψ; the peaks in Ψ correspond primarily to the peaks in rp.43 Therefore, Figure 9a shows the ellipsometric Ψ spectra for the fabricated fishnet after 4 days of air exposure without any treatment and therefore partially oxidized and after the H-processing. The hydrogen atoms treatment results in a blue shift of ∼10 nm and an enhancement of the amplitude of resonance peak of more than 4° in the Ψ ellipsometric parameter. Calculations using the previously described approach (shown in Figure 9b) confirm that these variations are ascribed to the reduction of silver oxide (as confirmed by XPS). Specifically, increased grain size and consequently smaller loss for the hydrogen-atoms-processed sample enhances resonances and results in a more pronounced minimum in the Ψ spectra (as indicated by the variation of the α-parameter; see eq 11), whereas the blue shift of the resonance can be associated with the reduction of silver oxide. Most notably, the ellipsometric spectra of the H2passivated fishnet change only slightly over more than a month of air exposure, as indicated by the stability of the Ψ amplitude in time also shown in Figure 9c. For comparison, a similar fishnet structure not treated and passivated by hydrogen shows over 2 weeks of air exposure a red shift a damping of the resonance due to oxidation. It is noteworthy that our long-term stability of the hydrogen-atoms-processed fishnet is comparable to the temporal stability reported for AgFON (silver film over nanosphere of polystyrene) substrates used for SERS.44 By the time this paper has been reviewed and published, the stability 23010



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and as a result we always observed a reproducible blue shift and sharpening of the resonances due to deoxidation and passivation of silver.

4. CONCLUSIONS In conclusion, we have demonstrated a low-temperature (100 °C) hydrogen atoms processing effective in passivating and stabilizing silver nanostructures in time under ambient conditions. The proposed processing does not need an additional heating of the structure that can lead to uncontrolled huge enlargement of silver grain size and thus avoids disturbing effects on the original topography of silver nanostructures. The passivation effect is explained on the basis of a localized Langmuir annealing that exploits the surface recombination of atomic hydrogen on silver. The method is easily applicable and was shown to be very efficient in stabilizing the metallic nanostructures, preventing any form of silver oxidation and tarnishing, and preserving their electromagnetic/optical properties. Our method lifts the previous constraint of silver-based plasmonic nanostructures and optical metamaterials, which strongly degraded in time, effectively ruling out the usage of silver under ambient conditions. We anticipate that our process will enlarge the possibilities of using silver-based nanostructures in opto-electronic and sensing devices and pave the way for low-loss plasmonic structures and metamaterials at high frequencies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding by the European Community’s seventh Framework Programme under grant agreement no. 228637 NIM_NIL (www.nimnil.org). The Austrian authors acknowledge the Austrian NANO Initiative (FFG and bmvit) and bmvit for funding this work partially by the NILmeta project within the NILaustria project cluster (www.NILaustria. at). Work at Ames Laboratory was supported by the Department of Energy (Basic Energy Sciences) under contract no. DE-AC02-07CH11358. We are grateful for the possibility to use the equipment of the Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz.

Figure 9. (a) Ellipsometric spectra of the angle Ψ for the fabricated 365 nm Ag fishnet structure (red curve) after the hydrogen atoms treatment (black curve) and its subsequent exposure to air for 4 weeks (blue curve). (b) Calculated curves assuming the same oxidized fishnet (red curve) and Ag with different values of the α parameter. (See text eq 1.) (c) Specific variation of the amplitude of the peak at the 720 nm over 1 month of air exposure.

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REFERENCES

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has been checked again, and we found that the value shown in Figure 9c for a month represents almost a saturation value for the reoxidation. We also verified that most of the observed variation is actually due to adsorption of carbon contaminant on the structure, and after few seconds again of the hydrogen processing the initial clean, deoxidized state is easily reobtained without significant aging of the Ag fishnet and fully restoring of its “clean” corresponding optical properties. Furthermore, it has also to be said that the same hydrogen atoms treatment has been applied meanwhile on a large number of silver nanoparticles and fishnet structures with different periodicity and different numbers of stacked Ag layers, 23011



Article

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