Nanoscale Study of the Tarnishing Process in Electron Beam

J. Phys. Chem. C , 2016, 120 (42), pp 24314–24323. DOI: 10.1021/acs.jpcc.6b03963. Publication Date (Web): September 30, 2016. Copyright © 2016 Amer...
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Nanoscale Study of the Tarnishing Process in Electron Beam Lithography-Fabricated Silver Nanoparticles for Plasmonic Applications Mario Scuderi,† Marco Esposito,‡,§ Francesco Todisco,‡,§ Daniela Simeone,‡ Iolena Tarantini,§ Luisa De Marco,⊥ Milena De Giorgi,‡ Giuseppe Nicotra,*,† Luigi Carbone,‡ Daniele Sanvitto,‡ Adriana Passaseo,‡ Giuseppe Gigli,‡,§ and Massimo Cuscunà*,‡ †

CNR-IMM Istituto per la Microelettronica e Microsistemi, Zona Industriale Strada VIII, Catania I-95121, Italy CNR-NANOTEC Istituto di Nanotecnologia, Via Monteroni, Lecce I-73100, Italy § Dipartimento di Matematica e Fisica Ennio De Giorgi, Università del Salento, Lecce I-73100, Italy ⊥ Istituto Italiano di Tecnologia, Center for Biomolecular Nanotechnologies, Via Barsanti, Arnesano I-73010, Italy ‡

S Supporting Information *

ABSTRACT: Silver is the ideal material for plasmonics because of its low loss at optical frequencies, though it is often replaced by a lossier metal, gold. This is because of silver’s tendency to tarnish, an effect which is enhanced at the nanoscale due to the large surface-to-volume ratio. Despite chemical tarnishing of Ag nanoparticles (NPs) has been extensively studied for decades, it has not been well understood whether resulted by sulfidation or oxidation processes. This intriguing quest is herein rationalized by studying the atmospheric corrosion of electron beam lithography-fabricated Ag NPs, through nanoscale investigation performed by high-resolution transmission electron microscopy (HRTEM) combined with electron energy loss (EEL) and energy dispersive X-ray (EDX) spectroscopies. We demonstrate that tarnishing of Ag NPs upon exposure to indoor air of an environment located inside a rural site, not particularly influenced by naturally and human-made sulfur sources, is caused by chemisorbed sulfur-based contaminants rather than via an oxidation process. Furthermore, we show that the sulfidation occurs through the formation of crystalline Ag2S bumps onto Ag surface in place of a homogeneous growth of a silver sulfide film. From a single 2D Z-contrast scanning transmission electron microscopy image, a method for 3D reconstruction of silver nanoparticles with extremely high spatial resolution has been derived thus establishing the preferential nucleation of Ag2S bumps in proximity of lattice defects located on the NP surface. Finally, we also provide a straightforward and low-cost solution to achieve stable Ag NPs by passivating them with a self-assembled monolayer of hexanethiols. The sulfidation mechanism inhibition allows to prevent the increased material damping and scattering losses.

1. INTRODUCTION Localized surface plasmon resonances (LSPR) are collective electronic oscillations in metallic nanoparticles and can be resonantly excited by external electric fields.1 They have been explored extensively in terms of their fundamental properties as well as for a plethora of possible applications in, e.g., optoelectronic devices,2,3 optical metamaterials,4,5 sensors,6,7 and solar cells.8 During the past decade’s rapid development of plasmonics into a subarea of nanotechnology, the focus was almost entirely on Au and Ag as nanoplasmonic metals. This preference was motivated by the distinct dielectric properties of Au and Ag in the vis-NIR spectral range. In particular, the low intrinsic losses through intraband excitations at energies just below the interband absorption threshold, which are related to © XXXX American Chemical Society

the position of the d bands with respect to the Fermi level, make Ag and Au very attractive plasmonic metals. However, Ag has the higher optical cross section, more than four times that of Au9 and more than 2 orders of magnitude higher enhancement in surface enhanced Raman spectroscopy (SERS),10 which are very important characteristics for high sensitivity detection.11−14 In addition, silver price is about 60 times lower than that of gold, and over 70 times more abundant, making it the most effective alternative for visible light plasmonic-related applications. Received: April 19, 2016 Revised: September 28, 2016

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focus the research of corrosive elements within the single NP, reducing the possibility of detecting spurious signals from an ensemble of them; they analyze the entire volume of the single nanoparticle, not only the surface as occurring in surfacesensitive techniques; most of times it is possible to map with high accuracy the distribution of corrosive elements within the single nanoparticle. Sachan et al.29 carried out chemical studies of Ag NP tarnishing using nanoscale investigation. In that work, Ag nanoparticles were synthesized by pulsed laser dewetting route and subsequently stored in indoor air for 50 days at room temperature. By means of scanning TEM-EELS they find tarnishing phenomena consisting in a morphology change with bump formation. However, the tarnishing effect was ascribed to oxidation phenomena relying on a very poor signal provided by the oxygen elemental map that might be confused with noise. Therefore, such results are not enough to gain the explanation of the real reason for silver NP corrosion and further nanoscale investigation has to be undertaken. Herein, we present our findings on the tarnishing of Ag ordered NP arrays fabricated by EBL, when exposed to indoor air at ambient conditions. Our work represents the first TEM investigation of EBL-fabricated NPs. EBL has become a widely used technique in plasmonic research because it allows the fabrication of nanostructures with exquisitely controlled shape, size, and interparticle spacing. In particular, we investigate ordered nanodisk (ND) arrays, which provide narrow sizedistribution minimizing the inhomogeneous broadening of the LSPR. We demonstrate that tarnishing of Ag NDs upon exposure to indoor air of an environment located inside a rural site, not particularly influenced by naturally and human-made sulfur sources, is caused by chemisorbed sulfur-based contaminants rather than via an oxidation process.23,26,30−32 The nanostructure sulfidation does not occur uniformly on the silver surface, but rather forms crystalline Ag2S bumps in proximity of silver defects. Such bumps grow larger over the course of time, leading to changes in the shape and chemical composition of Ag nanostructures. Along with our findings regarding the Ag ND sulfidation, we adopt a straightforward route to achieve stable EBL-fabricated Ag nanostructures after being exposed to indoor air at ambient conditions, based on a self-assembled monolayer (SAM) of hexanethiol (C6H13SH) passivation. Thiols have been largely used in wet-chemical synthesis processes as capping ligands with the aim at leading to size monodisperse nanoparticles and preventing their aggregation in solution.33 The coverage of Ag with thiol molecules dramatically slows down corrosion process preserving the NP plasmonic properties. Silver NP passivation with a few nanometers of dielectric layers as TiO234,35 or the formation of bimetallic nanoparticles29,36 were attempted to inhibit the degradation of silver after indoor air exposure. Although the silver instability is prevented by those methods, the resulting larger refractive index of the surrounding media shifts the designed plasmonic resonance frequencies to longer wavelengths, limiting high-frequency operation of silver-based plasmonic devices. On the other hand, silver stability after 4 days of air exposure was obtained by a hydrogen plasma treatment, free of further passivating materials.37 Nevertheless, such a method could induce detachment from the substrate of silver nanostructures with very small size, ranging from hundreds down to tens of nanometers, used for application into the UV region of the spectrum, which are often realized on glass substrate where Ag poor adhesion is well-known. In addition, plasma treatment has to be performed in rather

However, Ag suffers from serious chemical degradation, i.e., tarnishing upon exposure to atmosphere, an effect which is enhanced at the nanoscale due to the large surface-to-volume ratio. The latter precludes its use for plasmonic-based devices, due to a significant broadening and weakening of the resonances and a shift of the LSPR wavelength.15,16 Chemical tarnishing of Ag NPs has been extensively studied over decades ascribing it either to sulfidation17,18 or oxidation.15,19−21 Indeed, a few studies showed that carbonyl sulfide (OCS) and hydrogen sulfide (H2S), commonly dispersed in the atmosphere were involved in creation of Ag2S on the surface of bulk silver.22−25 High humidity in the atmosphere also served as a primary factor to enhance the sulfidation processes.26 McMahon et al.17 reported that bare Ag NPs evaporated by pulsed laser deposition with an average size of 65 nm, are rapidly sulfidized in ambient laboratory air after 8 days. The sulfidation rate, measured by Auger spectroscopy, was found to be 7.5 times higher than that of bulk silver under the same ambient conditions.25 Cao et al.18 studied the chemical and morphological stability of Ag NPs with squared shape of 200 nm side, fabricated by electron beam lithography (EBL), aged up to 12 weeks at room temperature in air. They found a high sulfur concentration in the nanoparticle by means of scanning electron microscope (SEM) combined with EDX spectroscopy. On the contrary, a number of recent studies have linked the silver NP tarnishing to oxidation phenomena. Qi et al.21 investigated the influence of the Ag NPs size distribution on the oxidation process. Ag NPs were formed via the electroless silver plating process and e-beam evaporation and then exposed to indoor air up to 12 days. Compositional results achieved by SEM-EDX spectroscopy indicated a faster oxidation process for Ag NPs with smaller diameter ( {110} > {100} due to differences in the binding energies and in the number of dangling bonds exposed by Ag facets.33 In this manner, thiol ligands contribute to preserve Ag {111} surfaces which are generally the most favored in thermally evaporated silver (Figure 7c), as well as the facets onto which molecular oxygen tends to be adsorbed most.53 Atmospheric oxygen, for its side, is expected to promote Ag tarnishing at room temperature, according to the following chemical reaction: 2Ag + H2S + 1/2O2 = Ag2S + H2O when sulfur-based contaminants are present.54

Figure 8. SEM images of hexanethiol-coated Ag NDs. (a) Freshly passivated. (b) The same NDs after being exposed 96 h to indoor air at ambient conditions. Scale bars are 200 nm.

4. CONCLUSIONS Transmission electron microscopy of Ag NDs fabricated by EBL demonstrates that tarnishing of Ag nanoparticles, upon exposure to indoor air of an environment located inside a rural site, not particularly influenced by naturally and human-made sulfur sources, is caused by chemisorbed sulfur rather than via an oxidation process. Nanoscale investigation, based on a 3D reconstruction of Ag nanoparticles from a single 2D Z-contrast STEM image, demonstrates that tarnishing does not occur uniformly on the Ag surface, but rather forms crystalline Ag2S bumps in proximity of ND surface defects as grain boundaries. Therefore, the presence of randomly distributed defects, favored by conventional thin film deposition techniques, can sensibly trigger an inhomogeneous bump distribution on neighbor Ag nanostructures. Such bumps become larger over time of exposure to indoor air, degrading the whole nanostructure in terms of surface morphology and chemical composition. These lead to significant changes in the optical response over a period of few days, as indicated by substantial plasmon resonances degradation of ND arrays after 48 h of exposure to air, when Ag2S bumps start to decorate the ND. Surface passivation of Ag NDs with a self-assembled monolayer of hexanethiols demonstrates that sulfidation can be prevented by effectively blocking diffusion of sulfur compounds present in the atmosphere. Hexanethiol passivation does not affect the designed plasmonic resonance frequencies, unlike to what occurs with dielectric passivation layer. Hence, our understanding of Ag nanostructure sulfidation and its hindering via passivation with a self-assembled monolayer of hexanethiols open the way for a disrupting improvement of Agbased LSPR sensors and applications.

The hexanethiol molecules-based passivation of Ag NDs represents a straightforward and very low-cost treatment, which can significantly slow down metal nanostructure sulfidation. The optical spectra of the hexanethiol-coated Ag ND arrays fabricated on a glass substrate, were measured over the course of 96 h of indoor air exposure to study how well hexanethiol passivation retained the plasmonic properties of Ag. Figure 9 reports the temporal evolution of the extinction efficiency for Ag NDs with average diameters of 90, 130, and

Figure 9. Temporal evolution of the extinction efficiency over a 96 h period for hexanethiol-passivated Ag ND arrays with three different diameters.

170 nm, respectively. Over a 4 days of air exposure, NDs showed a robust preservation of the initial plasmonic properties in terms of resonant peak intensity, LSPR wavelength and line width, independently of ND dimension. Also, a similar stabilizing behavior was clear for metal nanostructure size below 100 nm. Furthermore, the postfabrication coating with hexanethiols on top of silver surface does not significantly influence the resonant peak position (Figures 1a and 9). Hence, hexanethiol passivation is able to prevent a red-shift of designed plasmonic resonance frequencies, unlike to what occurs with

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03963. H

DOI: 10.1021/acs.jpcc.6b03963 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Electron microscopy characterization of Ag nanodisk before ambient sulfidation; morphology changes of Ag nanodisk array after ambient sulfidation; chemical characterization of Ag nanodisk after ambient sulfidation; selected-area electron diffraction analysis on ensemble of NDs; and hexanethiols treatment of Ag nanodisk arrays (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partially founded by the ERC project POLAFLOW (Grant No. 308136), the national projects “Molecular Nanotechnologies for Health and Environment” (MAAT, PON02_00563_3316357 and CUP B31C12001230005) and “Beyond-Nano” (PONa3_00363) for financial support.

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