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

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 absorption Kabs, 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-Al 2 O 3 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.


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  (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][11][12][13][14][15] with determination of selected properties of NPs [16,17] and laser processing of metallic NPs in nanotechnology [18][19][20][21][22][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][26][27][28][29][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 .Thermooptical analysis and selection of the properties of metal NPs for laser applications in nanotechnology were carried out in [9,16,17,[25][26][27][28][29][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 2 Journal of Nanomaterials with dielectric material and by changing the NP geometrical parameters.
Recently, in addition to pure metal NPs also metaldielectric (dielectric-metal) core-shell NPs are studied for the improvement and manipulation of the plasmon resonances of NP properties.For example, SiO 2 -gold and gold-SiO 2 coreshell NPs are widely investigated and applied in experiments [10,16,[31][32][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][32][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].
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-Al 2 O 3 , Zn-ZnO NPs using a computational method.

Numerical Results and Discussions
We numerically calculated the efficiency factors of absorption  abs , scattering  sca , and extinction  ext of radiation with wavelength  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-Al 2 O 3 , and Zn, Zn-ZnO.Values of optical indexes of refraction and absorption of metals, oxides, and surrounding media were used from [46][47][48][49].Figures 1-4 presented below describe the dependencies of efficiency factors of absorption  abs , scattering  sca , and extinction  ext 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.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.
The parameter  1 is used for the description of the optical properties of NPs: The parameter  1 describes the correlation between absorption and scattering of radiation by NP. Figure 1 presents the dependencies of the efficiency factors of absorption  abs , scattering  sca , and extinction  ext of radiation and the parameter  1 on wavelength  for spherical homogeneous Al NPs with radii  0 = 10, 25, and 50 nm; for two-layered core-shell Al-Al 2 O 3 NPs with shell thicknesses Δ 1 = 5 nm and core radii  0 = 5, 20, and 45 nm and  1 for core-shell NP radii  1 = 10, 25, and 50; for Al-Al 2 O 3 NPs with Δ 1 = 5 nm,  0 = 10, 25, and 50 nm, and  1 for  1 = 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 Figures 1(e)-1(h).The influence of the formation of oxide shell on metal NP with equal radii  0 = 10 nm =  0 + Δ 1 = 10 nm 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  0 = const in chemical gaseous atmosphere.It leads to a decrease of factors  max abs for  0 = 10 nm and small influence for all optical factors for  0 = 25, 50 nm.The values of the parameter  1 decrease with increasing  0 ( 1 ) and increase with increasing wavelength bigger than 300 nm.The formation of oxide shell on metal NP leads to decreasing  1 in the spectral interval 150-300 nm for all values of  0 .The formation of oxide shell on metal core with  0 = 10 nm leads to significant decreasing of the values of  1 for all spectral interval 150-1000 nm (Figure 1(l)).The increase of  0 for homogeneous and coreshell NPs and increase of oxide shell thickness leads to increase of  sca ,  ext in comparison with  abs .
The dependencies of efficiency factors of  abs ,  sca , and  ext on  for fixed values of homogeneous radii and core radii  0 and shell thickness Δ have complicated forms (Figures 1-4).In the case of homogeneous NPs of Al with  0 = 10 nm       (Figure 1(a)) there are no maxima in dependencies  abs (),  sca (), and  ext () in the considered region of wavelengths 150-1000 nm.Formally the maximal values of  max abs are placed at  max abs < 150 nm.However, for two-layered NP Al+Al 2 O 3 (Figure 1(e)) with equal total NP size of  1 = 10 nm, consisting of the aluminum core  0 = 5 nm and aluminum oxide shell with thickness Δ 1 = 5 nm, maximums of  max abs ,  max sca , and  max ext arise.Values of  max abs ,  max sca , and  max ext are placed at the same  max abs ∼ 215 nm.It follows that thin oxide shell influences optical properties of two-layered NP.As for homogeneous aluminum NPs with larger radii  0 = 25 (Figure 1(b)) and 50 nm (Figure 1(c)) appearance of aluminum oxide shell thickness (Δ 1 = 5 nm) on the NP surface (Figures 1(f) and 1(g)) leads to small shift of location of  max abs ,  max sca , and  max ext in the direction of bigger (increasing) wavelengths up to (no more than) 10 ÷ 25 nm.The values of  max abs for two-layered NPs increase with ∼50 ÷ 75%, and the values of  max ext increase with ∼5 ÷ 10%.The formation of oxide shell thickness for  0 = 45, 50 nm leads to formation of sharp peak oscillated dependencies with "plato" from wavelength value 150 nm till 400-450 nm.
Figure 2 presents the dependencies of the efficiency factors of absorption  abs , scattering  sca , and extinction  ext of radiation and the parameter  1 on wavelength  for spherical homogeneous Al NPs with radii  0 = 10, 25, and 50 nm [30]; for two-layered core-shell Al-Al 2 O 3 NPs with shell thicknesses Δ 1 = 5 nm, core radii  0 = 5, 20, and 45 nm, and  1 for core-shell NP radii  1 = 10, 25, and 50; for Al-Al 2 O 3 NPs with Δ 1 = 5 nm,  0 = 10, 25, and 50 nm, and  1 for  1 = 15, 30, and 55 nm.NPs are placed in water.The data concerning spherical homogeneous Al NPs with radii  0 = 10, 25, and 50 nm, published in [30], are presented here for direct comparison with the results for core-shell Al-Al 2 O 3 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 of  max abs decreases for  0 = 10 nm, but for  0 = 25 and 50 nm there is no decrease in  max abs with formation of oxide shell thickness.
The formation of oxide shell with thickness Δ 1 = 5 nm on core with radius  0 = 5 nm leads to significant decrease of the values of  abs ,  sca , and  ext in comparison with homogeneous metal NP with  0 = 10 nm and core-shell NP with  0 = 10 nm and Δ 1 = 5 nm.
In Figures 3(a)-3(l) the dependencies of efficiency factors of  abs ,  sca , and  ext on  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  abs (),  sca (), and  ext () in the considered region of wavelengths for homogeneous Zn NP with radius 10 nm (Figure 3 sca and  max ext increase by ∼30 ÷ 100%.In the case of Zn homogeneous and twolayered NPs Zn+ZnO for  0 = 50 nm  max abs and  max sca are practically the same, and  max ext increases no more than 5 ÷ 10%. In Figures 3(a)-3(l) the dependencies of efficiency factors of  abs ,  sca , and  ext on  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  abs (),  sca (), and  ext () 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 ( 0 = 5 nm) and Zn oxide shell with thickness (Δ 1 = 5 nm), maxima in the dependencies of absorption, scattering, and extinction on  arise.Values of  max abs ,  max sca , and  max ext are placed at the same  max abs ∼ 415 nm.As for homogeneous Zn NPs with larger radii  0 = 25 (Figure 3(b)) and 50 nm (Figure 3(c)) appearance of Zn oxide shell thickness (Δ 1 = 5nm) on the NP surface (Figures 3(f) and 3(g)) leads to shift of location of  max abs ,  max sca , and  max ext in the direction of increasing wavelength by 80 nm for  0 = 25 nm and by 30 nm for  0 = 50 nm.The values of  max abs for Zn homogeneous and two-layered NPs Zn+ZnO for  0 = 25 nm decrease by ∼20%, and the values of  max sca and  max ext increase by ∼30 ÷ 100%.In the case of Zn homogeneous and two-layered NPs Zn+ZnO for  0 = 50 nm  max abs and  max sca are practically the same, and  max ext increase no more than 5 ÷ 10%.
The dependencies of efficiency factors of  abs ,  sca , and  ext on  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  abs (),  sca (), and  ext () 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 ( 0 = 5 nm) and Zn oxide shell with thickness (Δ 1 = 5 nm), maxima in the dependencies of absorption, scattering, and extinction on  are shifted more than 100 nm to increase wavelength.Values of  max abs ,  max sca , and  max ext are placed at the same  max abs ∼ 425 nm.As for homogeneous Zn NPs with larger radii  0 = 25 (Figure 4(b)) and 50 nm (Figure 4(c)) appearance of Zn oxide shell thickness (Δ = 5 nm) on the NP surface (Figures 3(f) and 3(g)) leads to shift of location of  max abs ,  max sca , and  max ext in the direction of increasing wavelength by ∼50 nm for  0 = 25 nm and by ∼30 ÷ 50 nm for  0 = 50 nm.The values of  max abs for Zn homogeneous and two-layered NPs Zn+ZnO for  0 = 25 nm increase by ∼10%, and the values of  max sca and  max ext decrease by ∼20 ÷ 100%.In the case of Zn homogeneous and twolayered NPs Zn+ZnO for  0 = 50 nm  max abs and  max sca are practically the same, and  max ext 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 of  max abs increases for  0 = 10 nm, but for  0 = 25 and 50 nm there are no essential changes of  max abs with formation of oxide shell thickness.It is seen from Figures 1-4 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.
(a)).But for two-layered NP Zn+ZnO (Figure 3(e)) with equal total NP size of 10 nm, consisting of the Zn core ( 0 = 5 nm) and Zn oxide shell with thickness (Δ = 5 nm), maxima in the dependencies of absorption, scattering, and extinction on  arise.Values of  max abs ,  max sca , and  max ext are placed at the same  max abs ∼ 415 nm.As for homogeneous Zn NPs with larger radii  0 = 25 (Figure 3(b)) and 50 nm (Figure 3(c)) appearance of Zn oxide shell thickness (Δ 1 = 5 nm) on the NP surface (Figures 3(f) and 3(g)) leads to shift of location of  max abs ,  max sca , and  max ext in the direction of increasing wavelength by 80 nm for  0 = 25 nm and by 30 nm for  0 = 50 nm.The values of  max abs for Zn homogeneous and two-layered NPs Zn+ZnO for  0 = 25 nm decrease by ∼20%, and the values of  max The positions  max abs ,  max sca , and  max ext of maximum values of efficiency factors of  max abs ,  max sca , and  max ext on  axis are denoted in Figures 1-4 by different vertical lines; locations  max abs of maximum value of absorption factor  max abs on axis  are denoted by solid lines,  max sca , dashed lines and  max ext , dasheddotted lines in the case of different values of  max abs ,  max sca , and  max ext .