Evolution of silver in a eutectic-based Bi2O3–Ag metamaterial
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The development of novel manufacturing techniques of nano-/micromaterials, especially metallodielectric materials, has enabled dynamic development of such fields as nanoplasmonics. However, the fabrication methods are still mostly based on time-consuming and costly top-down techniques limited to two-dimensional materials. Recently, directional solidification has been proposed and utilized for manufacturing of volumetric nanoplasmonic materials using the example of a Bi2O3–Ag eutectic-based nanocomposite. Here, we explain the evolution of silver in this composite, from the crystal growth through the post-growth annealing processes. Investigation with tunneling electron microscopy shows that silver initially enters the composite as an amorphous AgBiO3 phase, which is formed as a wetting layer between the grains of Bi2O3 primary phase. The post-growth annealing leads to decomposition of the amorphous phase into Bi2O3 nanocrystals and intergranular Ag nanoparticles, providing the tunable localized surface plasmon resonance at yellow light wavelengths.
KeywordsBi2O3 Localize Surface Plasmon Resonance Bismuth Oxide Localize Surface Plasmon Resonance Peak Eutectic Composite
Plasmonics [1, 2] is currently one of the rapidly developing fields due to its role in enhancing optical properties, which makes it useful for application in solar cell efficiency enhancement [3, 4], cancer treatment , improved hard disks , lasers  and home-used diagnostics . To achieve plasmonic effects on local field enhancement, localized surface plasmon resonance (LSPR) or surface plasmon propagation, an interface between two media—plasmonic (metallic-like) and dielectric—is needed . Due to the collective oscillations of free charges, which are responsible for negative real permittivity, metals like silver and gold are currently the most used materials in plasmonics . However, though other plasmonic materials have been considered [11, 12], it is still silver in the visible wavelengths, which is most widely used due to its lowest optical losses  and highest electrical conductivity at room temperature .
Recently, eutectic solidification [15, 16] has been proposed as one of the most promising bottom-up methods for manufacturing of metamaterials [17, 18, 19, 20], plasmonic materials [21, 22] and photonic crystals [23, 24]. Eutectic solidification enables crystallization of a two-phase solid, often with an interesting self-organized micro-/nanostructure (typical are rod-like and lamellar structures) from a miscible liquid phase at a certain temperature . Eutectic composites have been investigated for various applications such as solar energy conversion [26, 27, 28], power generation gas turbines , scintillators [30, 31] or second-harmonic generators . The plasmonic effect was presented in a eutectic composite for the first time with a Bi2O3–Ag composite [21, 22]. After annealing the Bi2O3–Ag eutectic material, metal nanoparticles (silver and bismuth) are formed, which are responsible for the occurrence of plasmonic resonance in the visible wavelength range, at ~595 nm. Using different annealing conditions such as the atmosphere, time and temperature, it is possible to control the peak frequency of the LSPR . However, the development of metallic silver in this material is not yet well understood. Here, we demonstrate the evolution of silver in a Bi2O3–Ag eutectic starting from the crystal growth, and formation of the microstructure, through the influence of the post-growth annealing of the samples on its micro-/nanostructure and thus the optical properties. The analysis is based on high-resolution transmission electron microscopy (HRTEM) and selected area diffraction (SAD).
Materials and methods
The Bi2O3–Ag material was obtained by the micro-pulling down method [33, 34] in a nitrogen atmosphere from pure powders of bismuth oxide (Alfa Aesar, 99.99% purity) and silver (Alfa Aesar, 99.95% purity). Detailed growth and preparation methods have been described elsewhere . After growth, the samples were annealed in vacuum at 600 °C for 60 min . The obtained Bi2O3–Ag composites were characterized by the following methods: high-resolution transmission electron microscopy (HRTEM) connected with selected area electron diffraction and with scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDX). Samples for transmission electron microscope analysis were prepared by a focused ion beam (FIB) lift-out technique using a 30-keV Ga+ ion beam in an AURIGA CrossBeam Workstation (Carl Zeiss) equipped with a Canion FIB column (Orsay Physics). TEM investigations were conducted with the use of a Tecnai F20ST (FEI) TEM-STEM microscope with field emission gun and electron beam energy of 200 keV and coupled to a high-angle annular detector (HAADF) and a EDAX X-ray energy-dispersive spectrometer (EDX). The software “TE Imaging & Analysis” (EI) was used to provide standardless semiquantitative analyses of the EDX spectra. These analyses take into account the thickness and the chemical elements of the thin foil (ZAF corrections) and use preregistered K factors. Differential scanning calorimetry/thermogravimetry (DSC/TG) measurements were taken on an STA 449 (NETZSCH) with a platinum furnace under argon flow with different amounts of purge oxygen. Stoichiometric portions of powders were measured out and mixed manually with minor addition of isopropyl alcohol until a homogeneous color was achieved. The mixture was subsequently dried at 80 °C to remove the alcohol. The signals were measured with a Pt–Rh thermocouple using platinum crucibles in a temperature range from room temperature to 1173 K with a heating/cooling rate of 10 K min−1. Bi2O3–Ag powder mixtures were heated to 1173 K and cooled to 923 K. The process was repeated three times in each atmosphere to ensure homogeneity of the melt. The solidus temperatures were extracted from the heating curves as onset values, whereas liquidus temperatures were extracted from the cooling curves as the end temperatures. The influence of oxygen content on the phase diagram was investigated using powder samples of mixed bismuth oxide (99.9% purity) with silver (99.95% purity) in the range of 0–10 mol% in steps of 1 mol%.
Results and discussion
The as-grown Bi2O3–Ag composite is characterized by a three-dimensional micro-/nanostructure of silver-containing phase in a Bi2O3 matrix. Silver is located in a second phase wetting the Bi2O3 grain boundaries and at triple points where it adopts a triangular shape. Between two Bi2O3 grains, silver is in the form of plates with lengths of several tens of microns and thicknesses of a few nanometers.
Due to a strong dependence of the eutectic point on the content of oxygen in the atmosphere, it is not clear whether the composition of the Bi2O3–Ag composite we have grown is shifted to a higher abundance of Bi2O3 or Ag, as shown in Fig. 1. From the phase diagram of Bi2O3–Ag , if the composition is shifted from the eutectic point to a higher abundance of Bi2O3, it is the Bi2O3 phase that should crystallize first in contact with a liquid phase. While, if the composition is shifted toward a higher abundance of Ag, the Ag phase should crystallize first, and then, the eutectic will be made of Bi2O3 and Ag. The geometry of the Ag–Bi–O phase placed in between the grains of Bi2O3 is typical of a liquid phase that solidified after the main grains, though potentially it could be a Ag-containing phase in which the grains were squeezed by growing Bi2O3 grains. The AgBiO3 phase is probably formed in the following reactions : 4AgO + Bi2O3 → 2AgBiO3 + Ag2O, and Ag2O + Bi2O3 → 2AgBiO2.
In summary, an explanation of the evolution of silver in a Bi2O3–Ag eutectic composite from the crystal growth through the post-growth annealing processes has been presented. In the as-grown samples, silver is inserted in an amorphous phase enclosing AgBiO3 clusters, which is formed as intergranular films and triple points wetting the grains of Bi2O3 primary phase. After the post-growth annealing, the AgBiO3 phase decomposes into Bi2O3 nanocrystals and intergranular Ag. These studies give us an explanation for the origin of silver nanoparticles in the Bi2O3–Ag eutectic-based material after the annealing procedure and thus the localized surface plasmon resonance at yellow light wavelengths.
The authors thank the Preludium Project 2012/07/N/ST5/02428 and Maestro Project 2011/02/A/ST5/00471 from the National Science Centre and the US Air Force Office of Scientific Research under Grant FA9550-14-1-0061 for support of this work.
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