# Influence of Shell Parameters on Optical Properties of Spherical Metallic Core-Oxide Shell Nanoparticles

Authors: Victor K. Pustovalov; Liudmila G. Astafyeva
Journal: Journal of Nanomaterials
 (2015)

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## Abstract

Different metal homogeneous nanoparticles have been extensively studied in recent years due to their wide range of potential applications.  It is very interesting to investigate core-shell nanoparticles with oxide shell from core metal. The formation of oxide shell on metallic nanoparticles can be achieved by different chemical and physical methods including also natural oxidation of pure metallic nanoparticles in gaseous or liquid media, containing oxygen components (air, water, etc.). We numerically calculated efficiency factors of absorptionKabs, scattering Ksca, and extinction Kext of radiation with wavelength λ in the spectral interval 150–1000 nm by spherical homogeneous metallic and two-layered (metal core – oxide metal shell) nanoparticles: Al, Al-Al2O3 and Zn, Zn-ZnO with core radii in the range 5–50 nm and shell thickness 5 nm. Analysis of presented results has been carried out.


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## Body

## 1. Introduction

Recent advances in photothermal nanotechnology based on the use of nanoparticles (NPs) and optical (laser) radiation have been demonstrated their great potential. In recent years the absorption and scattering of radiation energy by NP have become a great interest and an increasingly important topic in photothermal nanotechnology [1–27] (also see the references in these papers). There are many reasons for this interest in nanophotonics including applications of NPs in different fields, such as catalysis [1, 2], nanoelectronics [3, 4], nanooptics and nonlinear optics [5, 6], and energetic nanotechnology (e.g., photovoltaics [7] and light-to-heat conversion [8, 9]). Laser applications in nanotechnology include laser nanobiomedicine [10–15] with determination of selected properties of NPs [16, 17] and laser processing of metallic NPs in nanotechnology [18–23]. Metallic NPs are mostly interesting for different nanotechnologies among other NPs.In recent years, the optical properties of metal NPs have been under extensive research mainly due to their unique optical properties arising from the localized surface plasmon resonance (LSPR) [25–30]. The LSPR causes a relatively narrow absorption peak, which leads to high optical selectivity. Most of the abovementioned technologies rely on the position and strength of the surface plasmon on a nanosphere and successful applications of NPs in nanophotonics are based on appropriate plasmonic and optical properties of NPs. High absorption of radiation by NPs can be used for conversion of absorbed energy into NP thermal energy, heating of NP itself and ambient medium, and following photothermal phenomena in laser and optical nanotechnology and nanomedicine. High scattering of radiation is used as a powerful tool in optical diagnostics and biological and molecular imaging.Different metal (gold, silver, platinum, zinc, etc.) NPs have been extensively studied in recent years due to their wide range of potential applications [1–30]. Thermooptical analysis and selection of the properties of metal NPs for laser applications in nanotechnology were carried out in [9, 16, 17, 25–30]. Metal NPs have their LSPR in the ultraviolet and visible spectral intervals of optical radiation. The possibility of controllably tuning the LSPR wavelength through the visible to near infrared region is very important and promising for the technological applications. Possible effective way of adjusting NP optical properties and shifting the LSPR peak position to near-infrared wavelength is to combine the metal NPs with dielectric material and by changing the NP geometrical parameters.Recently, in addition to pure metal NPs also metal-dielectric (dielectric-metal) core-shell NPs are studied for the improvement and manipulation of the plasmon resonances of NP properties. For example, SiO2-gold and gold-SiO2 core-shell NPs are widely investigated and applied in experiments [10, 16, 31–33]. The position of the LSPR for such core-shell NPs is strongly influenced by the presence of the geometrical characteristics: the core radius, the thickness of the oxide layer, and the ratio between them [10, 16, 31–33].But in many cases oxide (dielectric) shells are formed on the surface of metal NPs and we investigate core-shell NPs with oxide shell from core metal. The formation of oxide shell on metallic NP can be achieved by different chemical [33, 34] and physical [35] methods. The formation of thick oxide shell promotes the use of core-shell metal-oxide NPs in chemical nanotechnology. The presence of oxide shell on metallic NP surface can prevent the origination of chemical reaction on NP surface in chemical reactive atmosphere and further consequences that can be used in some technologies.Natural oxidation of pure metallic NPs in gaseous or liquid media, containing oxygen components (air, water, etc.), leads to the formation of thin oxide shell with thicknesses of about 3–5 nm on metallic NPs and core-shell two-layered metallic-oxide NPs during short period of time. The action of intensive optical (laser) radiation and NP heating can cause oxidation of surface layer of metallic NP and the formation of oxide shell on NP. The laser processing of metallic NPs in air atmosphere can cause simultaneously increasing of oxide shell thickness on particle surface and its evaporation [35].The fabrication and investigation of core-shell NPs formed by a metal core and its own oxide shell were carried out in [36–46]. For example, Ag-Ag2O NPs were investigated by physical and chemical methods in [36–39] and Al-Al2O3 NPs were experimentally investigated in [40–42]. Determination of the oxide layer thickness in core-shell zerovalent iron NPs was made in [43]. Investigation of microstructure control of Zn-ZnO core-shell NPs was carried out in [44]; the surface plasmon resonance of Cu-Cu2O core-shell NPs was studied in [45].On the other side, a comparative analysis of the optical parameters of different metal-oxide NPs for using them as agents in laser nanotechnology is still missing. In this paper, we study systematically influence of shell parameters on optical properties of spherical metallic core-oxide shell Al-Al2O3, Zn-ZnO NPs using a computational method.


## 2. Numerical Results and Discussions

We numerically calculated the efficiency factors of absorptionKabs, scattering Ksca, and extinctionKext of radiation with wavelength λ by spherical homogeneous and two-layered NPs on the base of Mie theory [26]. Numerical results are presented for cases of homogeneous metallic and two-layered (metal core, oxide metal shell) NPs: Al, Al-Al2O3, and Zn, Zn-ZnO. Values of optical indexes of refraction and absorption of metals, oxides, and surrounding media were used from [46–49]. Figures 1–4 presented below describe the dependencies of efficiency factors of absorption Kabs, scattering Ksca, and extinctionKext for homogeneous (Figures 1(a)–1(c), 2(a)–2(c), 3(a)–3(c), and 4(a)–4(c)) and two-layered NPs (Figures 1(e)–1(l), 2(e)–2(l), 3(e)–3(l), and 4(e)–4(l)) on radiation wavelength, NP core radii, and shell thickness. The positions λabsmax⁡, λscamax⁡, and λextmax⁡ of maximum values of efficiency factors of Kabsmax⁡, Kscamax⁡, and Kextmax⁡ on λ axis are denoted in Figures 1–4 by different vertical lines; locations λabsmax⁡ of maximum value of absorption factor Kabsmax⁡ on axis λ are denoted by solid lines, λscamax⁡, dashed lines and λextmax⁡, dashed-dotted lines in the case of different values of λabsmax⁡, λscamax⁡, and λextmax⁡. In some cases solid lines denote the simultaneous locations of all maximums of efficiency factors. We investigated two situations, when NPs were placed into two different surrounding media, air and water.Figure 1
Dependencies of the efficiency factors of absorptionKabs (solid), scattering Ksca (dashed), and extinctionKext (dashed-dotted) of radiation and parameter P1 (solid) on wavelength λ for spherical homogeneous Al NPs with radii r0=10 (a), 25 (b), and 50 (c) nm, P1 for r0=10 (1), 25 (2), and 50 (3) nm (d), for two-layered core-shell Al-Al2O3 NPs with shell thicknesses Δr1=5 nm, r0=5 (e), 20 (f), and 45 (g) nm, P1 for core-shell NP radii r1=10 (1), 25 (2), and 50 (3) nm (h); for Al-Al2O3 NPs with Δr1=5 nm and r0=10 (i), 25 (j), and 50 (k) nm, and P1 for r1=15 (1), 30 (2), and 55 (3) nm (l). NPs are placed in air.Figure 2
Dependencies of the efficiency factors of absorptionKabs (solid), scattering Ksca (dashed), and extinctionKext (dashed-dotted) of radiation and parameter P1 (solid) on wavelength λ for spherical homogeneous Al NPs with radii r0=10 (a), 25 (b), and 50 (c) nm, P1 for r0=10 (1), 25 (2), and 50 (3) nm (d), for two-layered core-shell Al-Al2O3 NPs with shell thicknesses Δr1=5 nm, r0=5 (e), 20 (f), and 45 (g) nm, P1 for core-shell NP radii r1=10 (1), 25 (2), and 50 (3) nm (h), for Al-Al2O3 NPs with Δr1=5 nm and r0=10 (i), 25 (j), and 50 (k) nm, and P1 for r1=15 (1), 30 (2), and 55 (3) nm (l). NPs are placed in water.Figure 3
Dependencies of the efficiency factors of absorptionKabs (solid), scattering Ksca (dashed), and extinctionKext (dashed-dotted) of radiation and parameter P1 (solid) on wavelength λ for spherical homogeneous Zn NPs with radii r0=10 (a), 25 (b), and 50 (c) nm, P1 for r0=10 (1), 25 (2), and 50 (3) nm (d), for two-layered core-shell Zn-ZnO NPs with shell thicknesses Δr1=5 nm, r0=5 (e), 20 (f), and 45 (g) nm, P1 for core-shell NP radii r1=10 (1), 25 (2), and 50 (3) nm (h), for Zn-ZnO NPs with Δr1=5 nm and r0=10 (i), 25 (j), and 50 (k) nm, and P1 for r1=15 (1), 30 (2), and 55 (3) nm (l). NPs are placed in air.Figure 4
Dependencies of the efficiency factors of absorptionKabs (solid), scattering Ksca (dashed), and extinctionKext (dashed-dotted) of radiation and parameter P1 (solid) on wavelength λ for spherical homogeneous Zn NPs with radii r0=10 (a), 25 (b), and 50 (c) nm, P1 for r0=10 (1), 25 (2), and 50 (3) nm (d), for two-layered core-shell Zn-ZnO NPs with shell thicknesses Δr1=5 nm, r0=5 (e), 20 (f), and 45 (g) nm, P1 for core-shell NP radii r1=10 (1), 25 (2), and 50 (3) nm (h), for Zn-ZnO NPs with Δr1=5 nm and r0=10 (i), 25 (j), and 50 (k) nm, P1 for r1=15 (1), 30 (2), and 55 (3) nm (l). NPs are placed in water.The parameterP1 is used for the description of the optical properties of NPs:(1)P1=KabsKsca.The parameter P1 describes the correlation between absorption and scattering of radiation by NP.Figure1 presents the dependencies of the efficiency factors of absorption Kabs, scattering Ksca, and extinctionKext of radiation and the parameter P1 on wavelength λ for spherical homogeneous Al NPs with radii r0=10, 25, and 50 nm; for two-layered core-shell Al-Al2O3 NPs with shell thicknesses Δr1=5 nm and core radii r0=5, 20, and 45 nm and P1 for core-shell NP radii r1=10, 25, and 50; for Al-Al2O3 NPs with Δr1=5 nm, r0=10, 25, and 50 nm, and P1 for r1=15, 30, and 55 nm. NPs are placed in air.The formation of oxide shell on NP with substitution of surface metal layer by oxide layer with approximately equal thickness because of natural oxidation in air atmosphere is presented in Figures1(e)–1(h). The influence of the formation of oxide shell on metal NP with equal radii r0=10nm=r0+Δr1=10nm leads to next consequences. The plasmon maxima are created and shifted to bigger values of the wavelength. Figures 1(i)–1(l) present the influence of the increasing oxide shell thickness on metal core with r0 = const in chemical gaseous atmosphere. It leads to a decrease of factors Kabsmax⁡ for r0=10 nm and small influence for all optical factors for r0=25, 50 nm. The values of the parameter P1 decrease with increasing r0 (r1) and increase with increasing wavelength bigger than 300 nm. The formation of oxide shell on metal NP leads to decreasing P1 in the spectral interval 150–300 nm for all values of r0. The formation of oxide shell on metal core with r0=10 nm leads to significant decreasing of the values of P1 for all spectral interval 150–1000 nm (Figure 1(l)). The increase of r0 for homogeneous and core-shell NPs and increase of oxide shell thickness leads to increase of Ksca,Kext in comparison with Kabs.The dependencies of efficiency factors ofKabs, Ksca, andKext on λ for fixed values of homogeneous radii and core radii r0 and shell thickness Δr have complicated forms (Figures 1–4). In the case of homogeneous NPs of Al with r0=10 nm (Figure 1(a)) there are no maxima in dependencies Kabs(λ), Ksca(λ), andKext(λ) in the considered region of wavelengths 150–1000 nm. Formally the maximal values of Kabsmax⁡ are placed at λabsmax⁡<150 nm. However, for two-layered NP Al+Al2O3 (Figure 1(e)) with equal total NP size of r1=10 nm, consisting of the aluminum core r0=5 nm and aluminum oxide shell with thickness Δr1=5 nm, maximums of Kabsmax⁡, Kscamax⁡, and Kextmax⁡ arise. Values of Kabsmax⁡, Kscamax⁡, and Kextmax⁡ are placed at the same λabsmax⁡~215 nm. It follows that thin oxide shell influences optical properties of two-layered NP. As for homogeneous aluminum NPs with larger radii r0=25 (Figure 1(b)) and 50 nm (Figure 1(c)) appearance of aluminum oxide shell thickness (Δr1=5 nm) on the NP surface (Figures 1(f) and 1(g)) leads to small shift of location of Kabsmax⁡, Kscamax⁡, and Kextmax⁡ in the direction of bigger (increasing) wavelengths up to (no more than) 10÷25 nm. The values of Kabsmax⁡ for two-layered NPs increase with ~50 ÷ 75%, and the values of Kextmax⁡ increase with ~5 ÷ 10%.The formation of oxide shell thickness forr0=45, 50 nm leads to formation of sharp peak oscillated dependencies with “plato” from wavelength value 150 nm till 400–450 nm.Figure2 presents the dependencies of the efficiency factors of absorption Kabs, scattering Ksca, and extinctionKext of radiation and the parameter P1 on wavelength λ for spherical homogeneous Al NPs with radii r0=10, 25, and 50 nm [30]; for two-layered core-shell Al-Al2O3 NPs with shell thicknesses Δr1=5 nm, core radii r0=5, 20, and 45 nm, and P1 for core-shell NP radii r1=10, 25, and 50; for Al-Al2O3 NPs with Δr1=5 nm, r0=10, 25, and 50 nm, and P1 for r1=15, 30, and 55 nm. NPs are placed in water. The data concerning spherical homogeneous Al NPs with radii r0=10, 25, and 50 nm, published in [30], are presented here for direct comparison with the results for core-shell Al-Al2O3 NPs and determination of the changes contributed by the formation of oxide shells on the surface of metal cores.The substitution of surrounding medium (air to water) leads to formation of plasmon peaks for homogeneous metal NP at wavelength ~200 nm and more pronounced peaks for core-shell NPs with oxide shell. We have to note the shifting of the placements of all optical factors to bigger values of wavelength. The factor ofKabsmax⁡ decreases for r0=10 nm, but for r0=25 and 50 nm there is no decrease in Kabsmax⁡ with formation of oxide shell thickness.Figures3 and 4 present dependencies of efficiency factors of absorption Kabs, scattering Ksca, and extinctionKext of radiation by spherical homogeneous Zn NPs with radii r0=10, 25, and 50 nm [30], two-layered core-shell NPs Zn+ZnO with Δr1=5 nm, r0=5, 20, and 45 nm, and P1 for r0=5, 20, 45 (h); Zn+ZnO NPs with Δr1=5 nm and r0=10, 25, and 50 nm and P1 for r0=10, 25, and 50 (l) on wavelength λ. NPs are placed in air (Figure 3) and in water (Figure 4). The data concerning spherical homogeneous Zn NPs with radii r0=10, 25, and 50 nm, published in [30], are presented here for direct comparison with the results for core-shell Zn+ZnO NPs and determination of the changes contributed by the formation of oxide shells on the surface of metal cores.The formation of oxide shell with thicknessΔr1=5 nm on core with radius r0=5 nm leads to significant decrease of the values of Kabs, Ksca, andKext in comparison with homogeneous metal NP with r0=10 nm and core-shell NP with r0=10 nm and Δr1=5 nm.In Figures3(a)–3(l) the dependencies of efficiency factors of Kabs, Ksca, andKext on λ are shown for homogeneous NPs of Zn and two-layered NP Zn+ZnO, placed in air. As in the case of homogeneous aluminum NP with radius 10 nm, there are no maxima in dependencies Kabs(λ), Ksca(λ), andKext(λ) in the considered region of wavelengths for homogeneous Zn NP with radius 10 nm (Figure 3(a)). But for two-layered NP Zn+ZnO (Figure 3(e)) with equal total NP size of 10 nm, consisting of the Zn core (r0=5 nm) and Zn oxide shell with thickness (Δr=5 nm), maxima in the dependencies of absorption, scattering, and extinction on λ arise. Values of Kabsmax⁡, Kscamax⁡, and Kextmax⁡ are placed at the same λabsmax⁡~415 nm. As for homogeneous Zn NPs with larger radii r0=25 (Figure 3(b)) and 50 nm (Figure 3(c)) appearance of Zn oxide shell thickness (Δr1=5 nm) on the NP surface (Figures 3(f) and 3(g)) leads to shift of location of Kabsmax⁡, Kscamax⁡, and Kextmax⁡ in the direction of increasing wavelength by 80 nm for r0=25 nm and by 30 nm for r0=50 nm. The values of Kabsmax⁡ for Zn homogeneous and two-layered NPs Zn+ZnO for r0=25 nm decrease by ~20%, and the values of Kscamax⁡ and Kextmax⁡ increase by ~30 ÷ 100%. In the case of Zn homogeneous and two-layered NPs Zn+ZnO for r0=50 nm Kabsmax⁡ and Kscamax⁡ are practically the same, and Kextmax⁡ increases no more than 5 ÷ 10%.In Figures3(a)–3(l) the dependencies of efficiency factors of Kabs, Ksca, andKext on λ are shown for homogeneous NPs of Zn and two-layered NP Zn+ZnO, placed in air. As in the case of homogeneous aluminum NP with radius 10 nm, there are no maxima in dependencies Kabsλ, Kscaλ, andKext(λ) in the considered region of wavelengths for homogeneous Zn NP with radius 10 nm (Figure 3(a)). But for two-layered NP Zn+ZnO (Figure 3(e)) with equal total NP size of 10 nm, consisting of the Zn core (r0=5 nm) and Zn oxide shell with thickness (Δr1=5 nm), maxima in the dependencies of absorption, scattering, and extinction on λ arise. Values of Kabsmax⁡, Kscamax⁡, and Kextmax⁡ are placed at the same λabsmax⁡~415 nm. As for homogeneous Zn NPs with larger radii r0=25 (Figure 3(b)) and 50 nm (Figure 3(c)) appearance of Zn oxide shell thickness (Δr1=5 nm) on the NP surface (Figures 3(f) and 3(g)) leads to shift of location of Kabsmax⁡, Kscamax⁡, and Kextmax⁡ in the direction of increasing wavelength by 80 nm for r0=25 nm and by 30 nm for r0=50 nm. The values of Kabsmax⁡ for Zn homogeneous and two-layered NPs Zn+ZnO for r0=25 nm decrease by ~20%, and the values of Kscamax⁡ and Kextmax⁡ increase by ~30 ÷ 100%. In the case of Zn homogeneous and two-layered NPs Zn+ZnO for r0=50 nm Kabsmax⁡ and Kscamax⁡ are practically the same, and Kextmax⁡ increase no more than 5 ÷ 10%.The dependencies of efficiency factors ofKabs, Ksca, andKext on λ for homogeneous NPs of Zn and two-layered NP Zn+ZnO, placed in water are shown in Figures 4(a)–4(l). As in the case of homogeneous aluminum NP in water with radius 10 ÷ 50 nm, there are maxima in dependencies Kabs(λ), Ksca(λ), andKext(λ) in the considered region of wavelengths for homogeneous Zn NP. For two-layered NP Zn+ZnO (Figure 4(e)) with equal total NP size of 10 nm, consisting of the Zn core (r0=5 nm) and Zn oxide shell with thickness (Δr1=5 nm), maxima in the dependencies of absorption, scattering, and extinction on λ are shifted more than 100 nm to increase wavelength. Values of Kabsmax⁡, Kscamax⁡, and Kextmax⁡ are placed at the same λabsmax⁡~425 nm. As for homogeneous Zn NPs with larger radii r0=25 (Figure 4(b)) and 50 nm (Figure 4(c)) appearance of Zn oxide shell thickness (Δr=5 nm) on the NP surface (Figures 3(f) and 3(g)) leads to shift of location of Kabsmax⁡, Kscamax⁡, and Kextmax⁡ in the direction of increasing wavelength by ~50 nm for r0=25 nm and by ~30 ÷ 50 nm for r0=50 nm. The values of Kabsmax⁡ for Zn homogeneous and two-layered NPs Zn+ZnO for r0=25 nm increase by ~10%, and the values of Kscamax⁡ and Kextmax⁡ decrease by ~20 ÷ 100%. In the case of Zn homogeneous and two-layered NPs Zn+ZnO for r0=50 nm Kabsmax⁡ and Kscamax⁡ are practically the same, and Kextmax⁡ decreases no more than 10 ÷ 40%.The substitution of surrounding medium air to water leads to formation of plasmon peaks for homogeneous metal NP at wavelength ~300 nm and more pronounced peaks for core-shell NPs with oxide shell. We have to note the shifting of the placements of all maxima of optical factors to bigger values of wavelength. The factor ofKabsmax⁡ increases for r0=10 nm, but for r0=25 and 50 nm there are no essential changes of Kabsmax⁡ with formation of oxide shell thickness.It is seen from Figures1–4 that the changes contributed by the appearance and the presence of thin metallic oxide shells on the surface of metallic NPs are essential for small aluminum NPs and all zinc NPs from considered metallic ones. Our results allow estimating the influence of oxide shells appearing on the surface of metallic nanoparticles on absorption, scattering, and extinction of radiation by NPs and influence of ambient properties for their photonic and technological applications.



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