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Nanoplasmonic materials are intensively studied due to the advantages they bring in various applied fields such as photonics, optoelectronics, photovoltaics and medicine. However, their large-scale fabrication and tunability are still a challenge. One of the promising ways of combining these two is to use the self-organization mechanism and after-growth engineering as annealing for tuning the properties. This paper reports the development of a bulk nanoplasmonic, Bi2O3-Ag eutectic-based metamaterial with a tunable plasmonic resonance between orange and green wavelengths. The material, obtained by a simple growth technique, exhibits a silver nanoparticle-related localized surface plasmon resonance (LSPR) in the visible wavelength range. We demonstrate the tunability of the LSPR (spectral position, width and intensity) as a function of the annealing temperature, time and the atmosphere. The critical role of the annealing atmosphere is underlined, annealing in vacuum being the most effective option for a broad control of the LSPR. The various potential mechanisms responsible for tuning the localized surface plasmon resonance upon annealing are discussed in relation to the nanostructures of the obtained materials.
© 2015 Optical Society of America
1. Introduction
One of the most utilized properties of plasmonic nanomaterials is the localized surface plasmon resonance (LSPR) [1, 2], which due to the local field enhancement, enhanced absorption and scattering provides novel functionalities or enables improvement of the existing ones. LSPR can be observed when an incident electromagnetic wave, E0, has the same frequency as the resonant electron oscillation frequency in a plasmonic nanoparticle. Resonance occurs when nanoparticle polarizability (α) and the induced field inside the nanoparticle (EIn), are maximal [3]. For a spherical nanoparticle, the induced field is described by the quasi-static approximation:
where εm is the dielectric function of the dielectric medium in which the nanoparticle is embedded. The nanoparticle is characterized by the dielectric function ε (λ),λ being the wavelength in vacuum of the incoming light. The polarizability of the nanoparticle is defined as:From the above Eqs. (1) and (2) it is shown that the phenomenon of resonance (the polarizability and induced field are maximal) appears when the following condition is fulfilled:The LSPR will be stronger, stronger electromagnetic fields will be generated inside and around the nanoparticle, if condition (3) is closer to zero.The ideal material for fabricating plasmonic nanoparticles should have a dielectric function ε(λ) with a negative real part and a low imaginary part (small losses) at the wavelength of interest. In the visible wavelength range, the best and most widely used plasmonic materials are Ag and Au, due to the relatively small losses and negative values of the real parts of their dielectric function [4, 5]. In the ultraviolet wavelength range Al is the preferred low loss material. However there are many materials other than metals which have been recently studied as potential plasmonic materials [6] exhibiting advantages for some applications due to e.g. their smaller in magnitude negative dielectric function or their capability of presenting a low loss plasmonic response in the mid infrared range [6, 7].
The LSPR spectral features (wavelength λSPR, intensity, shape/width) of an ensemble of plasmonic nanoparticles depends on various parameters, where the intrinsic properties of the material out of which these nanoelements are made plays a key role. The other important parameters are the shape and size of the nanoparticles; the interactions between them; and the surrounding dielectric environment [8, 9]. Thus all these parameters can be controlled to design a plasmonic nanocomposite with LSPR at the desired wavelength for a particular application: (i) Shape - for Ag spherical nanoparticles in water the LSPR appears at blue wavelengths; for pentagonal nanoparticles it appears at green wavelengths; and for triangular nanoparticles it appears at red wavelengths [10, 11]. Upon tailoring the nanoparticles so that their shape is significantly different from the spherical shape, their LSPR can be tuned in a broad wavelength range, up to 1000 nm and more than one peak may be present. In the case of rod-like nanoparticles, two LSPRs can be excited at different wavelengths, which represent transverse and longitudinal modes [12, 13]. (ii) Size - for bigger nanoparticles LSPR shifts to longer wavelengths no matter what kind of nanoparticle it is, for instance spherical Ag nanoparticles or hollow Au nanospheres [14, 15]. (iii) The surrounding environment has a strong influence: the higher the dielectric function of the medium, the more the LSPR is red-shifted. For small Ag spherical nanoparticles embedded in air (εm = 1) λLSPR = 357 nm; for oil (εm = 2,25), λLSPR = 414 nm [16].
In recent years, the fabrication of tailor-made materials based on assemblies of plasmonic nanoparticles has been reported. The control of their properties was achieved by design [17–19] of the material, or by post-treatments such as thermal annealing with controlled temperature, time and atmosphere. The previous reports about synthesis of nanocomposites with tunable LSPR upon annealing include: (i) Au–ZnO nanocomposites with tunable LSPR from 505 to 615 nm with formation of Au nanoparticles on ZnO nanorods with increasing annealing temperature from 200 to 600°C in Ar. The red-shift is a result of the increase in the ZnO refractive index as well as the increase of the Au NP diameters [20]; (ii) Core shell Au–Si nanoparticles embedded in a silica matrix in which LSPR was tuned from 500 to 583 nm by annealing in an argon atmosphere at temperatures from 500 to 600°C. Increasing the annealing temperature caused the formation of thicker Si nanoshells on the Au NPs cores [21], (iii) a Ag–VO2 material with LSPR shifting reversibly between 980 and 720 nm in the range of temperatures from 30 to 80°C. This behavior was achieved due to the thermochromic character of the VO2 medium, which causes the change in dielectric function of VO2 with temperature. With increasing temperature, the LSPR shifts to shorter wavelengths; when temperature decreases, the resonance shifts to longer wavelengths [22], (iv) Silver nanoparticles covered with a thermally responsive organic coating can show a 20 nm change of LSPR resonance achieved by heating [23].
Yet, the self-assembly fabrication of volumetric metamaterials with tunable plasmonic properties is required for applications at a large scale. The eutectic self-organization has been proposed as a new approach to manufacturing of volumetric metamaterials and plasmonic materials [24–28]. In a recent work [29] we reported the synthesis of a nanoplasmonic volumetric metamaterial based on a Bi2O3–Ag eutectic by a simple crystal growth method. This metamaterial exhibits a LSPR in the visible range thanks to the formation of Ag nanoparticles in the Bi2O3 matrix.
Here we demonstrate annealing-controlled tunable LSPR in the visible wavelengths range in this volumetric Bi2O3–Ag eutectic-based nanoplasmonic metamaterial [29]. Changing the annealing conditions (atmosphere, temperature, time), the LSPR is blue-shifted or red-shifted in the wavelength range from 575 to 603 nm, and the LSPR spectral width and intensity are also tuned. As a consequence, the transmitted color of the material under excitation with a Xe lamp can be turned from orange to violet. We discuss the origin of these results in relation to changes in the size and volume fraction of Ag nanoparticles, as well as the dielectric function of the matrix.
2. Experimental
In this work, the Bi2O3–Ag composite was grown from pure starting materials of bismuth oxide powder (Alfa Aesar, 99.99% purity) and Ag (Alfa Aesar, 99.95% purity). The materials were mixed with isopropanol in an alumina mortar to a composition of 84.5 mol% Bi2O3 and 15.4 mol% Ag. This composition was calculated to give a 7.8 vol% volume fraction of Ag in the eutectic-based composite.
The as-grown material was obtained by the micro-pulling down method [30, 31] in an N2 atmosphere. The micro-pulling down method has been previously used for the growth of single crystalline fibres [32], and eutectic-based photonic-crystal-like materials [24, 33], metamaterials [25] and bulk nanoplasmonic materials obtained by direct doping of glass matrices with plasmonic nanoparticles [34].
After growth, samples of the as-grown material were subjected to annealing treatment in three atmospheres using furnaces available at ITME: (i) in air at 600°C for 10-60 hours; (ii) in H2 (99,99%) at temperatures ranging from 200 to 400°C for 30 minutes; (iii) in high vacuum (pressure 6 x 10−6Torr) at 200-600°C for 60 minutes. For each annealing treatment a separate sample was used. Samples were not annealed above 600°C due to the limit of their melting points, which for the Bi2O3–Ag eutectic is at ca. 680°C, depending on the annealing atmosphere [35].
The optical absorbance spectra and photos of the surface (in transmitted light) of Bi2O3-Ag samples were performed at room temperature using a CRAIC microspectrophotometer available in the Institute of Electronic Materials Technology. The size of the sampling area was 21.1 μm x 21.1 μm. The plasmon-related extinction coefficient was defined as the height at the maximum of the baseline corrected differential extinction spectrum and then the full-width-at-half-maximum (FWHM) was calculated. The extinction measurements were collected at 5-7 places for each sample and an average value of the plasmon-related extinction coefficient was calculated.
Optical extinction simulations were performed in the quasi-static dipolar approximation (in this regime, the extinction is dominated by absorption) using textbook relations [36] for the extinction cross-section, which was normalized to the geometrical cross-section of the nanoparticles.
3. Results and discussion
3.1 Effect of annealing on LSPR position, intensity, width and sample coloration
Recently, we have shown [29] that the Bi2O3-Ag eutectic-based metamaterial exhibits LSPR at 595 nm, due to the formation of small Ag nanoparticles in the volume of the material, after annealing it in an oxidizing atmosphere. Here we show that the extinction band of the annealed composite can be easily tuned by controlling the annealing conditions like atmosphere, time and temperature. By such means, the tunability of the LSPR intensity, position and width is achieved in the annealed Bi2O3-Ag composite.
In Fig. 1(a) the tunability of the Bi2O3-Ag LSPR peak upon annealing in an air atmosphere is shown, together with the images of the investigated samples. With the increase of annealing time from 10 to 24 and 60 h the LSPR wavelength is red-shifted, and its intensity is significantly increased (by a factor 2 from 10 h to 60 h) while its width seems to vary very weakly. The 600°C annealing temperature has been applied in all cases since for the lower temperatures the amplitude of extinction coefficient is much smaller. All the investigated samples show blue coloration [Fig. 1(a)] in the transmitted light, which also confirms the occurrence of LSPR centered at yellow wavelengths, the yellow color being thus absorbed.
Fig. 1 Tunability of the LSPR in Bi2O3–Ag composite dependent on annealing shown via images of the samples observed in transmitted light and its extinction coefficients: a) air atmosphere, 600 °C for 10 h, 24 h, and 60 h; b) the hydrogen atmosphere, 30 min at 200°C, 300°C, and 400°C; c) vacuum for 60 min, 200°C, 400°C, and 600°C. The blue arrows in (b) and (c) highlight the “blue regions” containing Ag nanoparticles. The extinction coefficients measured at the selected areas of samples, are shown with the square with the same color.
In Fig. 1(b) the tunability of the Bi2O3-Ag LSPR peak upon annealing in an H2 atmosphere is shown, together with the images of the investigated samples. The temperature varied from 200 to 400°C, while the applied time was 30 minutes. Annealing in H2 causes significant decrease of the blue areas (highlighted by arrows in Fig. 1(b) which contain the Ag nanoparticles. For measurements performed in the areas containing the few blue centres it could be observed that higher annealing temperatures lead to lower LSPR peak intensities and shorter LSPR wavelengths. The LSPR width varies very weakly. For the measurements performed outside of the blue centres the extinction peak related with the presence of nanoparticles was not observed. We also tried annealing the samples at 600°C and for 30 minutes, but this led to cracking and partial destruction of the sample. Hydrogen in general is not a favorable atmosphere due to its potentially destructive influence on oxide materials. On the other hand, a vacuum can also provide slightly reducing conditions and annealing in vacuum is rather harmless for the oxide materials and it may improve the crystal lattice.
Figure 1(c) demonstrates the tunability of the Bi2O3-Ag LSPR peak upon annealing in vacuum at various temperatures, together with images of the investigated samples. The LSPR wavelength is blue-shifted, its intensity is significantly increased (one order of magnitude from 200°C to 600°C) and its width is strongly reduced when the temperature increases from 200 to 600°C with an annealing time of 60 minutes. Annealing in vacuum is much more effective than annealing in the air atmosphere: it causes the change in the samples in a much shorter time and the resulting LSPR peaks are more intense. As can be seen in Fig. 1(c), annealing in vacuum causes formation of the blue centres in the samples. Furthermore, annealing at 600°C causes the change of the colour to blue-violet, which is a result of the resonance shift and of the increase in its intensity.
3.2 Origin of the annealing effect on the resonance tunability
Using different annealing parameters we can control the spectral position and the intensity of the plasmonic resonance in the eutectic-based Bi2O3-Ag nanoplasmonic metamaterial. There can be several processes behind that, such as the change of: (i) the Ag nanoparticles size, (ii) their volume fraction, (iii) the quality of nanoparticle/matrix interfaces, and (iv) the dielectric function of the matrix εm.
In order to discuss the contribution of these different processes, additional information can be extracted from the optical spectra. In particular, the width of the LSPR band is known to be related to the nanoparticle optical diameter D by the following relation [37] (4):
where w1/2 is the full-width-at-half-maximum (FWHM) of the LSPR band expressed as an angular frequency, d is the average nanoparticle geometrical diameter, vfis the Fermi velocity of an electron which is equal to 1.39x106 m/s for Ag [37], and A is a phenomenological coefficient that depends on the nature of the matrix and the matrix-NP interface [38]. If we assume, as a first approximation, that A is unchanged upon the annealing processes done in this work, the evolution of the optical diameter D of the nanoparticles gives direct information on the geometrical diameter d.Table 1 lists all the parameters of investigated LSPR peaks, shown in Fig. 1, including the peak positions, FWHM and calculated nanoparticle optical diameters. The parameters shown in the table are the average parameters obtained from Gaussian fitting of the extinction bands in several places for each of the investigated samples.
Table 1. The annealing parameters and optical characteristics of the Bi2O3-Ag samples after the annealing treatments.
3.2.1 Ag nanoparticle sizes and volume fractions
Figures 2(a), 2(b) shows the values of the optical diameter D of the nanoparticles as a function of annealing temperature for hydrogen and vacuum, and as a function of annealing time in the air atmosphere. In all cases with increasing the time or temperature of annealing the D value increases, with the largest change observed for annealing in vacuum. Figures 2(c), 2(d) shows the LSPR peak intensities as a function of the same annealing parameters. As explained in the previous section, the LSPR peak intensity increases with the annealing time and temperature in the case of the air and vacuum annealing atmospheres, respectively. This increase is much more significant in the case of the vacuum atmosphere. In contrast, annealing in H2 induces a decrease in the LSPR peak intensity as temperature increases.
Fig. 2 The evolution of the optical diameter D of Ag nanoparticles embedded in a Bi2O3 matrix, in the Bi2O3-Ag eutectic nanocomposite, and the LSPR peak intensity. The change of D as a function of a) annealing time in the air atmosphere, and b) annealing temperature in hydrogen and vacuum. The change of the LSPR peak intensity as a function of c) annealing time in the air atmosphere, and d) annealing temperature in hydrogen and vacuum.
There are several explanations for the above results, based on the assumption that the A parameter does not change significantly, but the nanoparticle geometrical diameter does. In this context, Figs. 2(a) and 2(b) suggest that the nanoparticle geometrical diameter increases with the increase of the annealing temperature or time whatever the annealing conditions. This means that Ag diffuses into the matrix, either in the form of ions, clusters or small nanoparticles, or following an Ostwald ripening mechanism [39]. As the optical diameter increases faster under vacuum or H2 atmosphere, reducing conditions may enhance such processes compared to annealing in an oxidizing atmosphere. These processes can occur rather easily in the Bi2O3 matrix, since it is a very good ionic conductor [40, 41].
The very strong increase in the LSPR intensity observed after annealing in vacuum suggests that the volume fraction of Ag nanoparticles has increased, i.e. efficient diffusion of silver and nucleation from Ag ions and clusters present in the matrix followed by the growth has occurred. The same can be observed for annealing under an air atmosphere, although in a much more moderate way. Under the H2 atmosphere, the blue regions disappear and the LSPR intensity in the blue regions decreases, suggesting that Ag nanoparticles disappear.
To sum up: the annealing activates diffusion of silver in the matrix. Reducing conditions are more efficient than oxidizing conditions, but annealing in H2 is destructive and an important amount of Ag nanoparticles is lost. Thus annealing in vacuum is the preferred and the most effective option for the LSPR tuning.
3.2.2 Ag nanoparticles surrounding medium
In Fig. 3 the tunability of the spectral position of the LSPR peak is represented together with the change of optical NP diameter, for annealing in various atmospheres, times and temperatures.
Fig. 3 The tunability of the spectral position of the LSPR peak upon the change of the NPs optical diameter, D, occurring upon annealing.
Longer annealing times in the air atmosphere cause slight increase of the optical NP diameter and the red-shift of the LSPR peak from ~579 nm to ~594 nm. On the other hand, in the case of annealing in hydrogen and vacuum atmospheres, the nanoparticles also increase with the increase of the annealing temperatures. However, a shift of the peak towards shorter wavelengths is observed, from ~592 nm to ~575 nm for the hydrogen atmosphere and from ~603 nm to ~576 nm for vacuum.
Change in the dielectric function of the matrix: Bi2O3 phase transition. There are several possible explanations why we observed opposite shifts of the LSPR in the oxidizing and reducing atmospheres with increasing optical NP diameters. Among them, one is that the shift of the LSPR wavelength can originate from the change of the dielectric function εmof the matrix. By decreasing εm, the LSPR peak should shift to the shorter wavelengths. This is exemplified in Fig. 4(a), which presents simulations of the extinction cross-section spectra of a spherical Ag nanoparticle (d = 5 nm) embedded in a transparent dielectric medium, as a function of its dielectric function εm described using the Cauchy law [42]: εm = (Am + Bmλ−2)2. At near infrared wavelengths where Bmλ−2→ 0, εm takes a wavelength-independent value εm, infrared = Am2. In the simulations, the value of Am has been varied so that εm, infrared ranges from 4.84 to 7.29. The used εm, infrared values are typical of Bi2O3 materials with low porosity. In these conditions the simulated LSPR shifts linearly from 528 nm to 616 nm [Fig. 4(b)]. From this Fig., it can be estimated that a 30 nm blue-shift of the LSPR, as that observed upon annealing in vacuum, would require a decrease of 0.8 in εm (or of 0.15 in the corresponding refractive index of the matrix).
Fig. 4 a) Simulated quasi-static extinction cross-section spectra of a spherical Ag nanoparticle embedded in a transparent medium of dielectric function εm, representing Bi2O3. The wavelength dependence of εm has been calculated using the Cauchy law, with εm = (Am + 0.01λ−2)2, where Am is a wavelength-independent term. The spectra have been computed using different values of Am and thus of εm,infrared = Am2, which is the asymptotic limit of εm in the near infrared where 0.01λ−2→ 0. b) LSPR wavelength for different values of εm, infrared. The dielectric function of Ag was taken from the Palik database and corrected for classical finite size effects using A = 1.
The dielectric function of Bi2O3 can be affected in various ways, where one of them is structural change occurring via the phase transitions upon annealing/cooling of this material. Besides, another important factor is a relatively easy change of stoichiometry of Bi2O3, probably also enabled by its high ionic conductivity [40, 41].
Bismuth oxide has four polymorphs: α, β, δ, γ [43], with the phase transitions occurring upon cooling from α-Bi2O3 to δ-Bi2O3 at ~730°C, then from δ-Bi2O3 to metastable γ-Bi2O3 at ~640°C which transforms at ~500-650°C to α-Bi2O3 [44, 45]. There have been previous studies of the thermal stability of Bi2O3 upon annealing in various atmospheres. Fan et al. [46] reported the transition from the δtoβ and then the α phase under annealing from 200°C to 500°C in the air atmosphere. Klinkova et. al. [47] reported expansion of the unit cell during annealing in the air atmosphere at 600°C (24 h) with no transition observed; while the annealing of α-Bi2O3 in vacuum at temperatures 780–800°C caused the formation of γ-Bi2O3.
In the Bi2O3-Ag eutectic phase Bi2O3 transitions should be observed at lower temperatures due to the lower melting point of the eutectic composition than of its component phases. In the as-grown Bi2O3-Ag eutectic we have observed two polymorphs of bismuth oxide, α-Bi2O3 and γ-Bi2O3 and the δ phase can also not be excluded [29].
Annealing of the bismuth oxide can affect the dielectric function in the visible and the corresponding refractive index [48–50]. For the oxygen-rich Bi2O3 phases (such as α-Bi2O3) the refractive index in the visible should be higher; for the oxygen-deficient phases (such as δ, γ) [51], the opposite should be true.
Therefore, it is possible that annealing in vacuum, by promoting the formation of the δ or/and γ-Bi2O3 phases (lower dielectric function) to the detriment of the α-Bi2O3 phase (higher dielectric function), induces the blue-shift of the LSPR. On the contrary, the red-shift observed after annealing in air may result from the formation of the α-Bi2O3 phase to the detriment of the δ or/and γ-Bi2O3 phases.
Growth of Bi nanoshells around the Ag nanoparticles. Another possible cause for the blue-shift of the LSPR under annealing in vacuum or hydrogen atmospheres could be the formation of Ag-Bi core-shell nanoparticles. Depending on the composition of the core and shell and their thicknesses, the position of the LSPR(s) of core-shell nanoparticles shifts to shorter or longer wavelengths. Depending on the thickness of the core and shell, two peaks have even been observed in the Au-Ag core-shell system: (i) a peak originating from the core which shifts to the shorter wavelengths while reducing its intensity with increasing the thickness of the shell; (ii) at higher thickness of the shell, another peak appears which is connected to the shell and for thicker shells it shifts to longer wavelengths. With an Au core of 13 nm and a Ag shell thickness varying from 0 to 10 nm, the optical properties change according to the shell thickness. Without an Ag shell only a LSPR connected with the Au NPs is observed at around 520 nm and when the thickness of the Ag shell increases, this LSPR shifts to shorter wavelengths, simultaneously reducing its intensity. Upon increase in the shell thickness to 1 nm another LSPR peak appears at 330 nm which originates from Ag. This peak shifts to longer wavelengths and its intensity increases with the increase of the shell thickness. For shell thicknesses up to 7 nm the Ag LSPR moves to 400 nm while, for even thicker Ag shells, the optical properties of the core-shell NP start to resemble the properties of a pure Ag nanoparticle [52].
The Ag-Bi core-shell system has been little studied. Annealing in an air atmosphere has been reported to cause the red-shift of the LSPR wavelength [53]. In order to give insights into the effect of the formation of a Bi shell around a Ag nanoparticle, we have performed quasi-static calculations of the optical extinction of a single core-shell Ag-Bi nanoparticle, in which the Bi shell thickness has been varied (from 0 to 2 nm) at constant Ag core diameter (5 nm).The corresponding simulations are shown in Fig. 5. A single extinction peak can be seen [Fig. 5(a)]. The spectral position and intensity are blue-shifted and decreased, respectively, upon increase of the Bi shell thickness. As seen in Fig. 5(b), the observed shift is very sensitive to the shell thickness. The formation of a 1 nm-thick Bi shell provokes a 40 nm blue shift of the resonance. Based on these simulations, a contribution of the formation of a thin Bi shell around the Ag nanoparticles upon annealing in reducing conditions should be taken into consideration when explaining the observed blue shift of the LSPR. However, further investigation of the core-shell Ag-Bi system is required, since the ultraviolet-visible dielectric function of Bi in the ultrathin thickness range remains unexplored.
Fig. 5 a) Simulated quasi-static extinction cross-section spectra of a spherical Ag-Bi core-shell nanoparticle embedded in a transparent medium of dielectric function εm. The wavelength dependence of εm has been calculated using the Cauchy law, with: εm = (2.5 + 0.01λ−2)2. b) Resulting Ag-Bi core-shell LSPR wavelength as a function of Bi shell thickness. The core diameter was 5 nm and the shell thickness was varied between 0 and 2 nm. The dielectric function of Ag was taken from the Palik database (bulk Ag) and corrected for classical finite size effects using A = 1. The dielectric function of Bi was taken from [54].
The formation of Ag-Bi core-shell nanoparticles seems to be probable due to simultaneous existence of the liquid metals above 271°C based on the phase diagram of Ag-Bi [55]. Moreover, analyzing the phase diagram of Bi-O, it is seen that the solid Bi2O3 also coexists with a liquid of Bi and Bi2O3 above 271°C, which makes formation of Ag-Bi core-shell nanoparticles possible [56]. Bismuth exhibits a smaller surface energy than silver (Ag 0.553 eV/atom for the Ag (111) plane, and 0.356 eV/atom for Bi(100), and 0.507 eV/atom for Bi(110)), according to previous studies [57].While increasing the temperature during the annealing treatment, it is thus probable that a Bi phase can be formed on a Ag phase due to its smaller surface energy [57, 58]. The lattice constant of bulk Ag and Bi are 0.408 nm [59] and a = 0.454 nm, c = 1.186 nm [60] respectively, which can cause a slight lattice mismatch to the formation of Ag-Bi core-shell NPs. Due to similar d spacing of Ag and Bi [29], the change from core to shell will be difficult to observe in TEM measurements, as was reported in the case of Au/Bi core-shell nanocrystals [61]. It has also been reported that Bi3+ can be reduced to Bi0 in the presence of colloidal Ag and can then be deposited on the Ag nanoparticles as a shell, resulting in the blue-shift of the Ag LSPR [62].
4. Conclusions and outlook
In summary, we have demonstrated a tunable LSPR in a nanoplasmonic, volumetric, self-organized eutectic-based Bi2O3-Ag metamaterial. The resonance tuning (spectral position, spectral width and intensity) is achieved by thermal annealing and is controlled by the annealing conditions (time, temperature and atmosphere). The critical role of the annealing atmosphere is underlined. We identify annealing in vacuum as the most efficient way to achieve a broad control of the LSPR properties without deterioration. Potentially, the wavelength range of the LSPR tunability can be increased by further changing the annealing conditions.
The changes occurring in the material during annealing can have various origins. Whatever the atmosphere, annealing likely triggers the formation/growth of the silver nanoparticles. Increasing annealing time and temperature makes the diameter of the Ag nanoparticles increase, thus leading to the observed decrease in the LSPR spectral width. The increase in the LSPR intensity observed upon increasing annealing time and temperature in air and vacuum, respectively, might be a consequence of the increase in the volume fraction of Ag nanoparticles. In contrast, annealing in hydrogen should provoke the out diffusion of the Ag nanoparticles, thus destroying the LSPR.
Additional effects which might play a role are the change of the refractive index of Bi2O3 upon annealing, as well as the potential formation of the Bi-Ag core-shell nanoparticles. Both are expected to be sensitive to redox processes triggered by the annealing and to affect the spectral position of the LSPR.
Therefore, it seems that bismuth oxide is excellent as a matrix for plasmonic nanoparticles to form tunable nanoplasmonic materials due to: (i) its large ionic conductivity, which enables easy diffusion of silver ions or aggregates through its structure, and (ii) change of its refractive index upon annealing enabling additional tuning of the plasmonic resonance wavelength. Bi2O3 is characterized by other useful optical properties, such as (iii) low phonon energy, good for optically active materials due to increasing the probability of radiative optical processes, and (iv) high refractive index enabling stronger nonlinear properties.
Acknowledgments
The authors thank the Maestro Project 2011/02/A/ST5/00471 and the Preludium Project 2012/07/N/ST5/02428 from the National Science Centre, and the U.S. Air Force Office of Scientific Research under Grant FA9550-14-1-0061 for support of this work. The authors thank Dr. Sian Howard for her help with preparing the manuscript.
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