This is the submitted version of the journal article: Bubaš, Matej; Janicki, Vesna; Mezzasalma, Stefano A.; [et al.]. «Tailoring plasmonic resonances in Cu-Ag metal islands films». Applied surface science, Vol. 564 (Oct. 2021), art. 150260. DOI 10.1016/j.apsusc.2021.150260 This version is available at https://ddd.uab.cat/record/271931 under the terms of the license Tailoring plasmonic resonances in Cu-Ag metal islands films Matej Bubaša , Vesna Janickia , Stefano A. Mezzasalmaa,b , Maria Chiara Spadaroc , Jordi Arbiolc,d , Jordi Sancho-Parramona,∗ a Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia b Lund Institute of Advanced Neutron and X-ray Science (LINXS), Lund University, Scheelevägen 19, 22370 Lund, Sweden c Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain d ICREA, Pg. LluÃŋs Companys 23, 08010 Barcelona, Catalonia, Spain Abstract The plasmonic response of Cu-Ag metal islands films is investigated. Films are obtained by subsequent electron beam deposition of Ag and Cu using different fabrication conditions: deposited mass thickness, substrate temper- ature and post-deposition annealing in vacuum. Optical properties of films are investigated by spectroscopic ellipsometry and correlated with the struc- tural characterization results obtained by electron microscopy. It is observed that Ag enhances island growth and increases the percolation threshold of Cu films. The localized surface plasmon resonance of isolated particles shows signatures of both Cu and Ag. Moderate thermal annealing enhances island growth and favours Janus-like morphology, increasing the Ag contribution to the surface plasmon resonance. In case of percolated films, annealing- induced dewetting can lead to the appearance of large and irregular particles with a remarkable absorption peak in the near-infrared range. Composition ∗ Corresponding author. E-mail: jsancho@irb.hr Preprint submitted to Applied Surface Science April 28, 2021 and optical properties of the films can be further modified by Ag partial evaporation upon annealing at high temperatures. The variation of optical properties with aging is related to Cu oxidization and follows different trends depending on the sample morphology. Overall, it is shown that Cu-Ag is- land films are compelling systems for plasmonic applications, as their optical response can be widely and easily tuned by adjusting fabrication conditions. Keywords: surface plasmon resonance, metal islands films, ellipsometry, optical properties, dewetting, electron microscopy, Janus nanoparticles 1. Introduction The large application potential of plasmonics has motivated a quest for alternative materials that can provide better performance and wavelength tuning than silver and gold. Other pure metals, nitrides, oxides, silicides and 2D materials have been proposed as potential replacement for classical plasmonics materials [1, 2]. Although many of these materials have low ohmic losses and cover a broad spectral range of plasmon resonances, they also present a very low plasma frequency that ultimately limits their performance in many standard plasmonic applications [3, 4]. Therefore, the combination of metals as alloys or hybrid nanostructures appears as one of the most prospective directions for tailoring plasmonic resonances [5, 6, 7]. In addition to the search for alternative materials, cost efficient and high throughput nano-structuring fabrication methods are key for practical imple- mentation of plasmonic applications. In this context, well-established thin film physical vapor deposition techniques have a central role. The initial 2 stages of metal condensation on dielectric surfaces follow the Volmer-Weber growth mechanism that results into nearly 2-dimensional ensembles of metal nanoparticles [8, 9], known as metal islands films. Multiple metal combina- tions suitable for plasmonics have been used to produce alloy or hybrid island films [6, 10, 11, 12, 13, 14]. Among them, Cu-Ag systems are of special inter- est due to the immiscibility of these metals enabling the formation of hybrid islands. These systems have been investigated in the context of surface enhanced Raman scattering in samples obtained by simultaneous deposition of Ag and Cu via co-evaporation [15], co-sputtering [16], pulsed laser deposi- tion [17] or dewetting after deposition [18, 19, 20]. In these studies individual islands show well-separated Cu and Ag regions forming heterodimer or Janus particles, i.e., asymmetric structures with two sides having different compo- sition. Janus particles have a localized surface plasmon resonance (LSPR) that show individual signatures of Cu and Ag [21], covering a broader spectral range than pure or alloyed metal particles. On the other hand, subsequent deposition of Cu and Ag can result in core-shell particles [14] that transform into Janus-like particles upon annealing [22]. Although the dynamics of these morphological changes appears to be well understood [23, 24, 25] their effect on the optical response has received little attention. In this work we investigate the plasmonic properties of Cu-Ag metal is- lands films fabricated by sequential deposition of Ag and Cu on glass and silicon substrates. Samples are fabricated using a wide range of conditions (deposited metal thickness, substrate temperature, post-deposition anneal- ing) that produce different island morphology and optical properties. It is observed that Ag reduces the percolation threshold and enhances the Cu 3 island growth. LSPR line-shape depends on film composition and islands morphology, which can be modified upon annealing. Films deposited on sub- strates without pre-heating show percolated structures that can dewet into large particles with strong near-infrared absorption, an unusual feature so far not reported in island films. High temperature annealing induces par- tial Ag evaporation, modifying the LSPR line-shape. Optical properties of samples change with aging due to Cu oxidization but follow different trends depending on the film structure. These results demonstrate that it is pos- sible to broadly tune the optical properties of Cu-Ag islands by controlling the islands morphology through the fabrication process. 2. Materials and methods Metal islands films were deposited on silicon and BK7 glass substrates by subsequent electron beam evaporation of Ag and Cu in a modified Varian chamber with a base pressure of 6 × 10−7 Torr. Quartz crystal monitoring was used to control the deposited mass thickness of Ag (dAg ) and Cu (dCu ). Combinations with dAg = 0, 2, 5, 8 nm and dCu = 5, 10 nm were fabricated. Deposition rate for both metals was ≈ 1 Å/s. All substrates were pre-coated with a 80 nm thick SiO2 in order to insure the same conditions for island growth regardless the substrate. Before deposition, substrates were either not pre-heated (Ts = RT) or pre-heated at Ts = 200 ◦ C. In order to enhance island growth, Ag was deposited before Cu, as Ag has worse wetting properties due to its lower surface energy on dielectric substrates [26]. In recent works it has been shown that the opposite deposition order can be used to quench Ag island growth [27]. After deposition, samples were annealed in vacuum at 450 4 ◦ C and 750 ◦ C for 1 hour to induce particle growth and dewetting of compact films [28]. Generally speaking, morphological and optical properties of metal island films are highly sensitive to the preparation conditions. For this reason, we have investigated a large number of samples in order to determine optical response trends from the characterization of several samples and minimize the effect of potential outliers. We have verified that properties of pure metal island films (dAg or dCu equal to 0) are in very in very good agreement with those previously investigated [29, 30], supporting the reproducibility of our fabrication methodology. Optical properties of samples were investigated by spectroscopic ellip- sometry using a J. A. Woollam V-VASE ellipsometer. Measurements were done on samples deposited on Si substrates in the spectral range 0.57 - 4.5 eV and at angles of incidence of 65◦ and 75◦ . Fitting of ellipsometric data led to retrieval of the Cu-Ag film effective thickness (def f ) and dielectric function (εef f ). Spectral dependence of εef f was modelled using a multiple- oscillator dispersion equation with Gaussian oscillators to account for LSPR and interband transitions and a Drude term to represent connection among islands into a percolated network. In principle, LSPR of isolated small metal particle has a Lorentzian line-shape. However, in random composites like metal island films, particles present a finite size and shape distribution and significant dipole-dipole interaction. In this case, the composite optical re- sponse emerges from a distribution of resonances rather than from a single resonance. If this distribution is broad and has a Gaussian line-shape, it turns out that εef f is better described with Gaussian oscillators than with Lorentzian ones [29]. Overall, it has been shown that the multiple oscillator 5 model is able to provide an accurate description of optical properties for dif- ferent metal island films systems [31, 29, 30]. Transmittance measurements at normal incidence were done in the spectral range 300 - 1100 nm using a Lambda 25 Perkin-Elmer spectrophotometer. These measurements were taken at different time intervals to monitor optical response change upon sample exposition to air. The insights obtained from optical measurements were supported by elec- tron microscopy structural and morphological characterizations on selected samples. Scanning electron microscopy (SEM) plain view images were taken with a field-emission microscope Jeol JSM 7000F at a typical acceleration voltage of 15 kV. Scanning transmission electron microscopy (STEM) inves- tigation was performed on a field emission gun FEI Tecnai F20 microscope. High angle annular dark-field (HAADF) STEM was combined with elec- tron energy loss spectroscopy (EELS) in the Tecnai microscope by using a GATAN QUANTUM filter in order to obtain compositional maps. TEM samples were prepared by both, (1) via preparing cross-sectional views of the as deposited material on Si substrates, by conventional mechanical polishing and ion milling routes and (2) by scratching the surface of the as grown sam- ple and depositing the obtained powders on top of a carbon coated Cu grid for plain view analyses. EELS mapping was performed using the M-edge at 367 eV for Ag and the Cu L-edge at 931 eV for Cu. The O K-edge at 532 eV was used to distinguish the SiO2 film. Simulation of the optical response of single particles was carried out us- ing the MNPBEM boundary element method toolbox [32] with the aim to investigate absorption properties of core-shell and Janus nanoparticles. Op- 6 tical constants of Ag and Cu used in the simulations were taken from the literature [33]. 3. Results and discussion 3.1. Influence of dAg on metal islands deposited on pre-heated substrates We start by investigating the effect of dAg on the optical and structural properties of samples with dCu = 5 nm that are deposited on substrates pre- heated at Ts = 200 ◦ C. The imaginary part of εef f for these films is shown in the top panel of Figure 1. All samples show a major peak in the range between 1.5 and 2 eV that can be assigned to LSPR of isolated particles. This LSPR peak progressively blue-shifts and increases in strength when dAg grows. The peak has a shoulder around 2.5 eV that can be related to the presence of Ag, for it becomes more prominent at larger dAg . In fact, LSPR of small particles takes place for Ag at higher photon energies than for Cu [34]. Presence of Ag is also evident in a progressive strengthening of the peak at 4.2 eV, as it reflects the occurrence of interband transitions at the L point of Ag electronic band structure [35]. On the other hand, the film without Ag shows a nearly constant imaginary part of εef f that is correlated with the broad distribution of electronic interband absorption in Cu above 2.5 eV [33, 35]. Values of def f , shown in the Figure legend, suggest an increase of particle size in the direction perpendicular to the substrate when more Ag is deposited. The isolated-particle morphology of these films is confirmed by scanning electron micrographs of the sample surface (Figure 2). In all cases the shape of nanoparticles is slightly elongated and irregular, and their size distribution 7 Figure 1: Imaginary part of εef f for samples deposited on pre-heated substrates with dCu = 5 nm and different dAg as a function of photon energy. Color lines indicate different Ag seeds: dAg = 0 (red), 2 (orange), 5 (green) and 8 nm (blue). Legend numbers indicate def f values for each sample. Top panel: as-deposited (AD) samples, middle panel: samples annealed at 450 ◦ C, bottom panel: samples annealed at 750 ◦ C. resembles a log-normal distribution, which points to a high surface mobility and coalescence of nanoislands during deposition [36]. Average particle size in the direction parallel to the surface (D) increases with the amount of Ag: 13.5, 14.2, 21.2 and 33.6 nm for dAg = 0, 2, 5 and 8 nm, respectively. It can be expected that particle dimensions increase with the total amount of deposited material (dAg + dCu ). Yet, it should be noted that Volmer- Weber growth mechanism is enhanced by Ag: adding a small amount of Ag seeds (dAg = 2 nm) results in a change of the average particle aspect ratio (estimated as def f /D) from 0.59 to 0.83. A sphericity increase can contribute 8 Figure 2: Scanning electron microscopy of samples with dCu = 5 nm and different dAg deposited on pre-heated substrates. All micrographs are taken at the same magnification. in fact to the large initial LSPR blue shift [37]. A further increase in Ag amount reduces the particles aspect ratio to 0.76 (dAg = 5 nm) and 0.60 (dAg = 8 nm). In addition, the overall particle density decreases from 1700 (dAg = 0 nm) to 470 (dAg = 8 nm) particles/µm2 . Observe that, as the aspect ratio is quite similar in these two samples, it cannot afford an explanation for the LSPR blue- shift in the Ag-rich films. A more promising correlation seems to be rather dictated by the large Ag content and a weaker electromagnetic coupling, as it establishes as a consequence of a larger interparticle distance [30, 31]. 3.2. Annealing effects on samples deposited on pre-heated substrates Annealing of these samples at 450 ◦ C induces a blue shift of LSPR along with an increase of def f (middle panel in Figure 1). The shift is most evident in the sample without Ag seeds, that as-deposited shows the smallest and most closely located islands. In films containing Ag the relative weight of LSPR high-energy shoulder appears to increase. Annealing at higher temper- 9 ature (750 ◦ C, bottom panel in Figure 1) leads to more remarkable changes. Samples with dAg = 0, 2 and 5 nm have very similar optical properties, show- ing a LSPR peak without noticeable Ag contribution. Ag interband transi- tion peak at 4.2 eV is also absent from these samples. This behaviour can be explained by the evaporation of Ag from the film, given that Ag has nearly 3 orders of magnitude higher vapor pressure than Cu at the annealing tem- perature [38]. In addition, values of def f for films with dAg = 2 and 5 nm are smaller than after annealing at 450 ◦ C, suggesting a reduced particle size due to Ag evaporation. It has been proposed that differences in evaporation rates of metals can be used as a way to control the composition and hence physical properties of hybrid nanostructures [39]. Interestingly, Ag evapora- tion is not evident in the film with dAg = 8 nm because the sample annealed at 750 ◦ C shows stronger Ag contribution to LSPR and larger def f than the one annealed at 450 ◦ C. Scanning electron micrographs of the sample with dAg = 8 nm and dCu = 5 nm annealed at 450 ◦ C (left panel in in Figure 3) reveal a structure sim- ilar to the as-deposited sample (D = 31.3 nm and ca. 470 particles/µm2 ). The sample annealed at 750 ◦ C (right panel in Figure 3) shows larger and more isolated particles (D = 44 nm and 270 particles/µm2 ). Besides size and interparticle distance increase with annealing, the nanoislands size distribu- tion still resembles log-normal distribution and their shape remains elon- gated. This suggests that island growth during annealing follows a similar mechanism based on island coalescence and high surface mobility as during deposition. Energy dispersion X-ray spectroscopy of surfaces investigated by SEM indicate that Cu to Ag ratio of weight concentrations is 0.41 for the 10 Figure 3: Scanning electron microscopy of sample with dCu = 5 nm and dAg = 8 nm deposited on pre-heated substrates and annealed at 450 (left) and 750 ◦ C (right). All micrographs are taken at the same magnification. sample annealed at 450 ◦ C and 0.47 for the one annealed at 750 ◦ C. The corresponding ratio for weight concentrations for the as deposited amount of Ag and Cu is 0.42. Thus, a mild Ag evaporation seems to takes place upon annealing at 750 ◦ C. Nonetheless, the remaining amount of Ag influences the film optical properties more strongly than in as-deposited and moderately annealed samples. Island brightness in the micrographs of Figure 3 suggests that islands have an inhomogeneous electron density, many of them showing a distinctive two- region structure. This inhomogeneity is more evident in the sample annealed at 750 ◦ C. In order to get a better insight on the individual island charac- teristics, STEM-EELS maps of metal island film cross section were obtained for the sample annealed at 450 ◦ C (Figure 4). Islands show preferentially Janus-like structures with well-separated Cu and Ag regions. Some pure Ag particles are also observed. Thus, although Cu was initially deposited on Ag, core-shell morphology is not dominant upon annealing [22]. It turns out that annealing primarily results in a redistribution of Ag and Cu atoms within islands, justifying the moderate changes in average particle size and particle density observed by SEM. 11 Figure 4: STEM-EELS map of the sample with dAg = 8 nm and dCu = 5 nm deposited on pre-heated substrate and annealed at 450 ◦ C. Top panel shows islands cross section morphology while lower panels the Ag, Cu and O distribution and the combined elemental distribution. For elucidating whether the islands morphological changes can explain LSPR trends upon annealing, numerical simulations of the optical response of composite ellipsoids were carried out (Figure 5). Two geometries were considered: Ag core-Cu shell structure and Ag-Cu Janus particle, assuming in both cases a 1:1 volume contribution of Ag and Cu. The core-shell ge- ometry is used as an approximation to represent particles where Cu coats Ag seeds. In order to make simulations consistent with experimental obser- vations, the ellipsoid semi-axes were set to a = 35 nm and b = c = 15 nm and the refractive index of the particle environment to 1.25, i.e. a mean of the refractive index of air and SiO2 underlayer. Light polarization parallel and perpendicular to the long semi-axes are considered because, for ran- domly oriented particles laying on a substrate along their major semi-axis, 12 the normal-incidence optical response should average these polarizations. In the core-shell case ellipsoids representing the Ag core and Cu shell are con- centric and have the same aspect ratio. Janus ellipsoid is split in Cu and Ag regions through the symmetry plane perpendicular to the major semi-axes. The optical response core-shell ellipsoid is dominated by a main resonance around 2 eV that can be associated with a longitudinal dipole plasmon mode, as evidenced by the calculated surface charge distribution shown in panel A. The transverse response is much weaker. For the Janus nanoparticle, spec- tra show three dipole-like resonances of comparable strength: a longitudinal resonance around 2 eV related to the Cu region (panel B), a longitudinal response around 2.8 eV related to the Ag region (panel C) and a transverse resonance located around 3.5 eV, also connected with the Ag region (panel D). Transverse resonance associated to Cu region is absent due to the inter- band transitions in the corresponding spectral region [34]. A richer spectral response of Janus ellipsoid is explained by the presence of two different mate- rials at the particle surface. In this way, the Janus configuration enhances the role of Ag in the overall plasmonic response of nanoparticles in comparison with the Ag core - Cu shell structure. The progressive strengthening of the Ag-related shoulder in the LSPR upon annealing therefore can be correlated with island morphological transitions that strengthen materials segregation [40, 22] and favour Janus-like structures. It should be noted that simulations are based on an oversimplified model with equal Ag and Cu volume and shape contributions. A broader distribution of resonances is expected in the actual samples and unveiling individual contributions to the LSPR line-shape might be complex. Thus, although the transversal LSPR Ag contribution is 13 Figure 5: Top panel: computed extinction cross section of core-shell (CS) and Janus ellipsoids for light polarization parallel (k) and perpendicular (⊥) to the major semi-axis as a function of photon energy. Lower panels show the surface charge distribution on the particle surface (at z=0) of the ellipsoid at the plasmon resonance peaks identified in the top panel. not clearly distinguished in the experimental spectra, it can be associated to the shoulder of the high energy LSPR region (3-3.5 eV). 3.3. Influence of dAg and annealing on substrates without pre-heating Films deposited on substrates without pre-heating (Ts = RT) show en- hanced absorption in the low-energy spectral region and no evident signature of LSPR (top panel in Figure 6). Only the sample with dAg = 5 nm has a feature in the imaginary part of εef f at ≈ 1.7 eV, suggesting that the film may be partially formed by isolated particles. Drude contribution term is necessary to accurately model εef f in samples with dAg = 0 and 8 nm only, indicating that these films percolate, as confirmed by SEM (top panel in 14 Figure 7). Optical characterization of samples without Cu (not shown) con- firms that the Ag seeding film with dAg = 12 nm is percolated. Therefore, a moderate amount of Ag (dAg = 2 and 5 nm) does not lead to percolation on substrates with no pre-heating. The large near-infrared absorption of these films that does not follow Drude dispersion law can be explained by clusters of very closely located or touching nanoparticles, as it has been observed in films near the percolation threshold [31, 29, 41]. Values of def f increase with dAg but are smaller than for pre-heated substrates, confirming that now films are more packed. Optical properties of samples annealed at 450 ◦ C (middle panel in Figure 6) show an absorption peak around 2 eV that can be assigned to LSPR of isolated particles. It indicates the formation of islands as a result of film dewetting upon annealing [11, 28]. However, modelling εef f in samples with- out Ag seeds still requires the Drude term, implying that the film is also percolated. Likewise in pre-heated substrates, a shoulder in LSPR high en- ergy side is observed in samples with Ag. Although these films are not percolated, they show a broad absorption peak centered at energies below 1 eV. It should be noted that films with more Ag content (dAg = 5 and 8 nm) have larger def f than those deposited on pre-heated substrates and subjected to the same annealing treatment. Scanning electron micrographs of samples with dAg = 8 nm and annealed at 450 ◦ C (middle panel in Figure 7) show a (two-dimensional) film structure composed of large (' 100 nm) irregular particles and of circular-like particles with size in the range of few to few tens of nm. These two families can explain the double-peak structure of εef f , since large, flat and irregular particles can be responsible of the low-energy 15 Figure 6: Imaginary part of the εef f for samples deposited on substrates without pre- heating (Ts = RT) with dCu = 5 nm and different dAg as a function of photon energy. Color lines indicate different Ag seeds: dAg = 0 (red), 2 (orange), 5 (green) and 8 nm (blue). Legend numbers indicate def f values for each sample. Top panel: as-deposited (AD) samples, middle panel: samples annealed at 450 ◦ C, bottom panel: samples annealed at 750 ◦ C. peak [42]. Cross section and plain view STEM-EELS maps (Figure 8) re- veal that irregular particles contain both Cu and Ag, with Cu preferentially in contact with the SiO2 film surface. Height of these particles is ranging within several tens of nm, justifying the large values of def f in comparison with other samples. Such an irregular particle distribution results from the preferential dewetting of Ag, due to its lower surface energy than Cu. Annealing at the highest temperature returns similar optical response and def f for all the samples (bottom panel in Figure 6). Compared to samples 16 Figure 7: Scanning electron microscopy of sample with dCu = 5 nm and dAg = 8 nm deposited on substrates without pre-heating as deposited (top) annealed at 450 (middle) and at 750 ◦ C (bottom). All micrographs are taken at the same magnification. deposited on pre-heated substrates, even the one with dAg = 8 nm does not show evident Ag contribution to LSPR peak. As mentioned earlier, the Ag seeded film with dAg = 8 nm is percolated. Therefore the SiO2 surface coverage is larger than in case of a film with the same Ag amount but formed by isolated nanoparticles. In this way, Cu deposition leaves a larger portion of Ag exposed to the atmosphere than in pre-heated substrates, facilitating its evaporation upon annealing. EDS analysis of the sample annealed at 750 ◦ C confirmed the absence of Ag. The optical spectra of a Cu particle (not 17 Figure 8: STEM-EELS map of samples with dAg = 8 nm and dCu = 5 nm deposited on substrates at Ts and annealed at 450 ◦ C. Top panel illustrates cross section and lower panel a plain view of the sample surface. Composition maps show the distribution of Cu (red), Ag (blue) and O (green). shown) presents a main LSPR peak broadening that is similar to the case of Janus or core-shell configurations. Thus, LSPR narrowing observed for the sample annealed at higher temperature seems to rather originate from a reduced interparticle electromagnetic coupling, as suggested by the particle spatial distribution evolution (Figure 7). The shape evolution upon annealing of as-deposited samples can be di- vided in two steps: In the first step, which occurs already at 450 ◦ C the interconnected structure begins to break apart into isolated and irregular nanoislands due to Rayleigh-like instability [43]. In the second step, which occurs when annealing at 750 ◦ C, all the silver evaporates and the remain- ing copper nanoislands take a more regular and round shape. A similar morphological evolution of nanoislands following silver evaporation at high 18 temperature in [44] was explained by minimization of surface energy by the remaining nanoislands attaining equilibrium shape. 3.4. Influence of dCu In general terms, when a larger amount of Cu is deposited (dCu = 10 nm) results are qualitatively similar to those described so far. For deposition on pre-heated substrates, larger def f values and red-shifted LSPR are obtained in comparison to films with dCu = 5 nm (left column in Figure 9). In addition, the LSPR high energy shoulder displays a smaller weight due to the decreased Ag fraction in these samples. Moderate annealing (450 ◦ C) results in LSPR blue shift and def f increase, as it happens in samples with smaller Cu amount. Differences in def f for films with dAg 6= 0 annealed at 450 ◦ C and 750 ◦ C are less remarkable than in samples deposited on pre-heated substrates. This observation indicates that Ag evaporates more easily with less Cu, for Ag is more exposed at the film surface and more weakly held by Cu because the two metals have smaller contact surface. Optical properties of as-deposited films with dCu = 10 nm prepared with- out substrate pre-heating show Drude-like absorption, indicating that all films are percolated. Upon annealing at 450 ◦ C, the Drude term contribu- tion does not disappear from any of the samples. Although the films are percolated, they are less packed -as seen by the increase of def f - suggesting that the dewetting process is still at the hole propagation regime [45]. High temperature annealing yields well-defined LSPR peak in all samples. For Ag-containing films, values of def f are moderately lower or even larger than after annealing at 450 ◦ C. In addition, the LSPR high energy shoulder is present even when dAg = 2 nm. These observations imply that the metal 19 island films still contain a significant Ag fraction and, like in the case of de- position on pre-heated substrates, confirm that larger dCu aids to prevent Ag evaporation. Figure 9: Imaginary part of εef f for samples deposited on pre-heated (left column, Ts = RT) and not pre-heated substrates (right column, Ts = 200 ◦ C) with dCu = 10 nm and different dAg as a function of photon energy. Color lines indicate different Ag seeds: dAg = 0 (red), 2 (orange), 5 (green) and 8 nm (blue). Legend numbers indicate def f values for each sample. Top panel: as-deposited (AD) samples, middle panel: samples annealed at 450 ◦ C, bottom panel: samples annealed at 750 ◦ C. 3.5. Aging effects Finally, we investigate the changes in optical properties of as-deposited samples with time inasmuch as Cu is prone to oxidize upon exposure to nor- 20 mal atmosphere [30]. Figure 10 shows transmittance spectra of samples with dAg = 0 and 8 nm deposited onto substrates with and without pre-heating. Due to the progressive oxidization of Cu that quenches the LSPR, samples with no Ag show a transmittance increase with time. In addition, the film deposited on a pre-heated substrate shows LSPR red-shift versus time. This is explained by the gradual increase of the effective refractive index of the medium around the islands as it follows from the high refractive index of copper oxide [46, 47]. This variation cannot be confirmed for the sample deposited without pre-heating because LSPR peak falls out from the mea- sured spectral range. It should be noted a slight decrease of transmittance with time for wavelengths below 400 nm that can be related to the Cu2 O interband transition at 3.5 eV [47]. Optical properties of films containing Ag present a significantly differ- ent time evolution. First, the sample deposited on a substrate without pre-heating shows weaker temporal variation than the other samples here discussed. This sample corresponds to the most packed film and has the smallest fraction of Cu in direct contact with the atmosphere, hence being less affected by Cu oxidization. The observed changes are a slight blue-shift and narrowing of the transmittance dip. These changes suggest that oxidized Cu regions reduce the number of possible electron paths along the film as it becomes, from an electro-optical point of view, a less percolated network. In this way, LSPR-like properties can more clearly emerge from the optical spectra. Lastly, the sample deposited on a pre-heated substrate is the only one whose transmittance decreases with time. As Cu is replaced by its oxide, LSPR becomes dominated by the Ag seeds that have better plasmonic prop- 21 erties than Cu. Alongside, seeds get surrounded by a high refractive index medium (Cu2 O) which further strengthens but also red-shifts LSPR. Figure 10: Transmittance spectra of samples with dCu = 5 nm and dAg = 0 (left column) or 8 nm (right column) and deposited on substrares without pre-heating (top) and pre- heated substrates (bottom) . Measurements were taken at different time intervals after deposition: 5 minutes, 30 minutes, 1 hour, 3 hours, 1 day, 1 week, 2 weeks, 3 weeks and 2 months. Line color continuously varies from the shortest (blue) to the longest time (red) after deposition. 4. Conclusions It has been shown that optical properties of Cu-Ag metal island films deposited by electron beam evaporation can be broadly tailored by control- ling fabrication conditions. The interplay between the amount of deposited materials, substrate deposition temperature and post-deposition annealing enables to modify film structure, islands morphology and composition. As a result, the optical properties can be varied from a metal-like behavior dom- inated by the Drude contribution at low energies to well-defined localized 22 surface plasmon resonances with line-shape depending on the island compo- sition and morphology. It is observed that Ag seeds can quench film percolation and enhance island formation. In this way, pre-deposition of a small amount of Ag can increase the low percolation threshold of Cu islands on dielectric substrates while the film retains Cu-like plasmonic properties. Thermal annealing at moderate temperatures results in island growth and morphological changes that favour Janus-like structures. In case of percolated films, annealing in- duces dewetting that results in large irregular particles with a near-infrared LSPR peak not previously reported in metal island films in addition to smaller particles with a resonance in the visible range. Annealing at high temperature brings to partial Ag evaporation and film properties become primarily dominated by Cu. Time evolution of the optical response shows different trends depending on the film structure. Overall, the present study demonstrates that the optical properties of these samples can be broadly var- ied by adjusting a number of easily controllable deposition parameters thanks to the non-alloying character of the metal combination. 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