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Effect of thermal post-treatment on surface plasmon resonance characteristics of gold nanoparticles formed in glass by UV laser irradiation
Journal of Alloys and Compounds 803 (2019) 354e363
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Effect of thermal post-treatment on surface plasmon resonance characteristics of gold nanoparticles formed in glass by UV laser irradiation Vasiliy V. Srabionyan a, Maximilian Heinz b, Sviatoslav Yu Kaptelinin a, Leon A. Avakyan a, Galina B. Sukharina a, Anna V. Skidanenko a, VasiliyV. Pryadchenko a, Kamal G. Abdulvakhidov c, AlexeyS. Mikheykin a, VeniaminA. Durymanov a, € rg Meinertz d, Juergen Ihlemann d, Manfred Dubiel b, Lusegen A. Bugaev a, * Jo a
Department of Physics, Southern Federal University, Zorge Str. 5, RU-344090, Rostov-on-Don, Russia Institute of Physics, Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 3, D-06120, Halle (Saale), Germany c The Smart Materials Research Center, Southern Federal University, Sladkova 178/24, RU-344090, Rostov-on-Don, Russia d €ttingen e.V., Hans-Adolf-Krebs-Weg 1, D-37077, Go €ttingen, Germany Laser-Laboratorium Go b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 April 2019 Received in revised form 18 June 2019 Accepted 21 June 2019 Available online 22 June 2019
Effects of thermal post treatment and of thickness of the initial gold film coated the silicate glass surface, on localized surface plasmon resonance (SPR) of gold nanoparticles formed in glass by UV laser irradiation were studied to develop effective technique for production of gold nanoparticles and their arrays with required and stable characteristics of SPR. Optical spectra of the obtained Au/glass samples showed that SPR of gold nanoparticles formed by irradiation of glasses sputter coated with gold films of thickness 6e10 nm depends of T-treatment, while irradiation of glass coated with gold film ~70 nm leads to SPR stable of temperature. The origin of such dependences was studied by the structural analysis of gold nanoparticles in Au/glass samples before and after thermal treatment using XRD, EXAFS methods, TEMEDX images and direct calculations of optical spectra. It was revealed that the changes or stability of SPR parameters under thermal post-treatment of Au/glass samples depend respectively of the presence or absence of tin atoms in the near-surface region (shell) of the gold particles. Thus, if tin atoms are presented in the shell of gold particles (as in “as prepared” Au/glass samples initially coated with gold films of 6e10 nm) then the leave of tin atoms from the particle's volume during the heating leads to partial dissolution of the particles shell and consequently, to decreasing of the particles size, which in turn leads to the observed changes in SPR. The most probable explanation of the presence of tin atoms in the shell of gold particles formed by irradiation of samples initially coated with gold films of thickness 6e10 nm and their absence in the case of sample with film ~70 nm was proposed, basing on the difference in the effects of the first laser pulse actions on these films on the tin-bath side of the glass samples. The sizes of coherent scattering region in gold nanoparticles before and after thermal treatment of Au/glass samples were determined and the dependence of these sizes upon the thickness of the gold film coating the sample's surface before laser irradiation was revealed. © 2019 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials UV laser processing Thermal post-treatment Surface plasmon resonance Optical spectroscopy XRD EXAFS
1. Introduction Gold and silver nanoparticles in dielectric media are extensively studied mainly because of their ability to localize electro-magnetic
* Corresponding author. E-mail address: [email protected] (L.A. Bugaev). https://doi.org/10.1016/j.jallcom.2019.06.263 0925-8388/© 2019 Elsevier B.V. All rights reserved.
field on nano-scale distances due to the phenomenon of surface plasmon resonance (SPR), providing various valuable applications like i) light-heat energy transfer [1e5], ii) SPR sensors for biomedicine and nano-applications [6e13] and iii) in catalysis [14e16]. Depending on the specific application, different requirements to SPR characteristics are needed, which can be achieved by constructing the nanoparticles with tunable parameters of SPR peak, such as its wavelength position, full width at half
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maximum (FWHM) of SPR peak and combining the SPR properties of single particles with the effects of their space arrangement, in particular, within the lattice plasmon resonance (LPR) [17e20]. The promising approach for creating single plasmonic particles, their submicron line patterns and arrays with wavelength position and shapes of SPR tuned in a wide ranges, is the synthesis of bimetallic gold-silver nanoparticles with alloy or core-shell architecture [21] by UV ArF-excimer laser irradiation (193 nm) below the ablation threshold of the glass, varying the number of pulses at different fluences. The suggested experimental technique for the synthesis of such bimetallic single nanoparticles and their arrays was based on the results of preliminary study of the formation of gold nanoparticles under laser irradiation [22], as the possible substrates or nuclei for precipitation of silver atoms on them. In these papers the initial gold film of thickness 70 nm on the glass surface was used which provided the highest concentration of the generated Au particles in the near-surface region. To propose a reliable and effective technique for production of plasmonic nanoparticles with the required and stable during operation optical properties, in this paper we present the results of the study i) of thermal post-treatment of the Au/glass samples after UV laser irradiation and ii) of the effect of thickness of the initial gold film on the glass surface on SPR characteristics of the formed gold particles. With this aim, the glass samples coated by gold films of thickness 6, 10, 70 nm, irradiated by UV laser and then heat treated, were prepared (Sec 2). Analysis of optical spectra of “as prepared” and obtained after heat treatment samples was performed (Sec 3.1) which confirms the formation of gold particles and allowed to determine the changes in their mean size. The results of XRD study of gold particles in the samples are presented in Sec. 3.2. Finally, the processing of Au L3-EXAFS spectra (Sec 3.3) gave the structural parameters of the atomic bonds Au-M (M ¼ Au, Sn, Mg, Na) in the shell and in the outer coverage of the gold nanoparticles, which allowed to reveal the dependencies of their atomic composition upon thickness of the initial gold film on the glass surface and thermal post-treatment in accordance with the observed changes in SPR of experimental optical spectra. 2. Experimental methods
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each laser pulse was measured simultaneously using a beam splitter, and the fluence of the first laser pulse was ensured to be equal to 140 mJ/cm2 for each studied sample. As a next step, the laser irradiated areas of size 6 6 cm2 were cleaned with acetone to remove gold nanopartices, which were not implanted into the glass. Finally, the laser irradiated areas of each sample were broken into two pieces. One piece has been thermally treated at 300 C for a duration of 48 h, whereas the other piece (named “as prepared” or briefly - “asp” sample) was used for the reference measurements without thermal post-treatment. The glass samples before and after thermal post-treatment, prepared by UV laser irradiation of glasses coated with gold films of different thickness are notated respectively as: Au-6nm/glass_asp, Au-6nm/glass_T, Au-10nm/ glass_asp, Au-10nm/glass_T, Au-70nm/glass_asp, Au-70nm/glass_T.
2.2. Experimental measurements Optical spectra have been measured by Lambda 900 UV/Vis/NIR spectrometer (PerkinElmer) in the wavelength range from 300 nm to 800 nm. To reduce the spot size to the laser irradiated areas a circular aperture with a diameter of 3 mm has been used. Au L3-edge X-ray absorption spectra were measured at the KMC-2 beamline of the BESSY-II Synchrotron Radiation Facilities (Berlin, Germany). All spectra were measured at room temperature. Due to the low concentration of the nanoparticles and their location in the surface layer of the glass, the spectra were collected in fluorescence yield mode. The photon energy scanning steps were adjusted to dE ¼ 1.0 eV and dk ¼ 0.05 Å-1in XANES and EXAFS region respectively, where E is the energy of incident radiation and k is the corresponding photoelectron wavenumber. X-ray diffraction (XRD) measurements were carried out using diffractometer Bruker D2 PHASER at Southern Federal University (Rostov-on-Don, Russia) with Cu K a radiation (wavelength of l ¼ 1.54 Å) and Scintillation counter 1-dimensional LYNXEYE detector in Bragg-Brentano geometry. The experiment was performed using Bragg reflection angles range from 30 to 50 with step 2q ¼ 0.01 and exposure time 10 s at each point.
2.1. Samples preparation The experimental techniques and the study of the formation and implantation of gold nanoparticles in the near-surface region of soda-lime silicate float glass by UV laser radiation (193 nm) was performed in Ref. [22]. In this work, implanted gold nanoparticles were additionally annealed by subsequent thermal post-treatment. The commercial float glass with thickness of 1 mm had the following composition (the values are given in weight %): 72.3% SiO2, 0.5% Al2O3, <0.02% Fe2O3, 13.3% Na2O, 8.8% CaO, 0.4% K2O, 4.3% MgO. In addition, one side of the glass contains 0.02 wt % of SnO2, this side will be further referred as tin-bath side. The opposite side (without tin) will be referred as air-side. At the first step, the tinbath sides of the glass samples were sputter coated with gold by means of a K550 Sputter Coater system (manufactured by Quorum Technologies) using the plasma current of 20 mA and Ar pressure of 101 mbar. By this procedure, gold films of thickness 6 nm, 10 nm and 70 nm were produced on the tin-bath glass surface. As a next step, 193 nm ArF-excimer laser irradiation (LPX 315 laser, manufactured by Lambda Physik) with 20 ns pulse duration and 10 laser pulses with a repetition rate of 1 Hz was applied, which leads to the formation and implantation of gold nanoparticles into the glass surface (see Ref. [22]). The average laser fluence was 140 mJ/cm2. As the fluence of the first laser pulse was shown to affect significantly the nanoparticle formation process [22], the actual fluence of
3. Results and discussion 3.1. SPR in optical extinction spectra of the studied samples Comparison of experimental optical spectrum of “as prepared” sample with the spectrum of this sample after thermal posttreatment is presented in Fig. 1, respectively for the samples with thicknesses 6, 10 and 70 nm of the initial gold films on the glass surface, which were used for Au nanoparticles preparation by UV laser irradiation. The starting point of the analysis of experimental optical spectra of Au/glass samples was the analysis using Mie theory, which allows to determine size distribution of the particles in assumption of their spherical shape, homogeneity, and absence of plasmon interaction between them [23]. The calculation of extinction spectra of individual nanoparticle was performed using Sander's code [24] and the theoretical spectra of particles ensemble was obtained by summation of these spectra with weights determined by log-normal distribution of particles with parameters m and s [25] varied during the fit. Additionally, the background contribution and the scale multiplier N0, which is proportional to the total number of nanoparticles, were varied. The minimized residual function was calculated as:
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V.V. Srabionyan et al. / Journal of Alloys and Compounds 803 (2019) 354e363 Table 1 Results of the fitting of optical spectra of the samples using Mie theory. Sample
Fit quality, c2
N0
m
s
D(max
Au-6nm/glass_asp Au-6nm/glass_T Au-10nm/glass_asp Au-10nm/glass_T Au-70nm/glass_asp Au-70nm/glass_T
0.001 0.001 0.007 0.001 0.010 0.009
0.05 0.06 0.06 0.05 0.11 0.09
4.1 3.9 4.6 4.5 4.1 3.8
0.2 0.3 0.1 0.2 0.9 0.8
59 42 84 68 25 23
in SD),
nm
so that the above mentioned misfits are probably originated from ignoring of the plasmon coupling between nanoparticles. The last column of Table 1 shows the values of D(max in SD) e size, corresponding to the position of maximum in particles size distributions (SD) presented in Fig. 3, which compares the size distributions for the studied samples according to the results of the fit by the approach of Mie theory. The narrow distributions for the samples synthesized using thin gold film (Au-6nm/glass_asp and Au-10nm/glass_asp) became broader and are shifted towards smaller particle sizes after thermal post-treatment. Together with almost unchanging particles density N0 (Table 1) this indicates on dissolution of some of the particles in the glass. However, for the particles synthesized using gold film of 70 nm (sample Au-70nm/ glass_asp) the initial size distribution is very broad, and the effect of thermal post-treatment is weak.
3.2. X-ray diffraction study Fig. 4 shows the diffraction patterns of three series of the studied glass samples with gold coating before and after thermal post-treatment, where each series consists of pair of samples: Au6nm/glass_asp and Au-6nm/glass_T, Au-10nm/glass_asp and Au10nm/glass_T, and Au-70nm/glass_asp and Au-70nm/glass_T. Diffraction patterns of all samples demonstrate Bragg peak (111) corresponding to fcc lattice (Fm3m). XRD analysis was performed using FULLPROFF program complex [28]. The profile of analyzing peak was approximated using pseudo-Voight function which is linear combination of Gaussian (G) and Lorentzian (L) functions:
FPV ¼ h,L þ ð1 hÞ,G Fig. 1. Comparison of optical spectra of “as prepared” samples (black solid curves) with thicknesses 6 (a), 10 (b) and 70 nm (c) of the initial gold film on the glass surface with the corresponding spectra of these samples after thermal post-treatment (red dashed curves). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
c2 ¼
X
exp 2
I theor Ii i
(1)
i
where Iexp and Itheor are the experiential and calculated spectra respectively. Index i numbers the wavelengths l in the interval 400 < li < 1000 nm. The dielectric function of bulk gold [26] was used in the calculations, which is valid for nanoparticleы bigger than 10 nm. The results of the fit using Mie theory are summarized in Table 1 and illustrated in Fig. 2. The fit quality for the samples Au-6nm/ glass_asp, Au-6nm/glass_T and Au-10nm/glass_T is high, then slightly worse for sample Au-10nm/glass_asp due to misfit in the left shoulder of the peak, and poorer for samples Au-70nm/ glass_asp and Au-70nm/glass_T. Our previous studies [22,27] of the samples analogues to sample Au-70nm/glass_asp demonstrated the presence of nanoparticles agglomerates in the sample,
(2)
where parameter h (0 < h < 1) corresponds to the Lorentzian contribution in pseudo-Voight function. The size of the coherent scattering region in the volume of nanoparticle can be estimated from the widths of the peaks using Sherrer equation [29,30], which is a suitable approach for the structural study of nanoparticles with sizes > 2 nm [31,32]. Instrumental broadening resolution function was calibrated by Al2O3 reference sample according to the procedure [33]. Table 2 contains the structural parameters of the samples obtained from diffraction data: mean sizes (D) of Au nanoparticles and unit cell parameters (a). The last appeared to be the same for all the considered samples. As can be seen, the thermal post-treatment leaded to the particles size increasing in contrast to the particles sizes obtained by optical data which is illustrated in Fig. 5. The possible reason of this contradiction as well as the reason of strong difference in the values of mean sizes D obtained from optical spectra and from XRD is that the particles size obtained from XRD corresponds to the coherent scattering region in the volume of gold nanoparticles, while the particles size obtained from the analysis of optical spectra includes not only the ordered internal region of the particle, but also its shell containing structural distortions, defects and impurities. It is revealed that the tin atoms are strongly penetrated into the volume of gold particles (Sec. 3.4) and as a
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Fig. 2. Illustration of the fits quality of experimental optical spectra of the studied samples using Mie theory: a, b e samples Au-6nm/glass_asp and Au-6nm/glass_T; c, d e samples Au-10nm/glass_asp and Au-10nm/glass_T; e, f e samples Au-70nm/glass_asp and Au-70nm/glass_T. Experiment spectra e red dots, fit results e blue solid curves, background contributions e green dashed curves.
result, the size of the particle's coherent scattering region may differ strongly from the full particles size. One more explanation of the obtained contradiction is the possible polycrystalline structure of nanoparticles [30,34]. In this case, the “optical size” corresponds to the sizes of polycrystalline particles, while “XRD size" is the size of each of the crystalline regions of particle, which coherently scatters X-rays.
3.3. Analysis of Au L3-EXAFS The structural characterization of gold nanoparticles in the glass samples before and after thermal post-treatment was performed by Au L3-edge EXAFS. Fig. 6 compares the magnitudes of the Fourier transforms jF(R)j of the experimental Au L3-edge EXAFS spectra of the gold foil e reference compound (Fig. 6a) and of the glass samples before and after thermal post-treatment with different thickness of the initial gold film: Au-6nm/glass_asp and Au-6nm/ glass_T (Fig. 6a), Au-10nm/glass_asp and Au-10nm/glass_T (Fig. 6b), Au-70nm/glass_asp and Au-70nm/glass_T (Fig. 6c). As can be seen, there are significant differences, especially in the magnitudes of the two main peaks A and B, in jF(R)j of experimental spectra between samples Au-6nm/glass_asp (A:B ¼ 1.79) and Au6nm/glass_T (A:B ¼ 1.47) and between Au-10nm/glass_asp
(A:B ¼ 1.72) and Au-10nm/glass_T (A:B ¼ 1.59), while the value of this ratio in the reference Au-foil A:B ¼ 1.52. At the same time, the changes in jF(R)j between samples Au-70nm/glass_asp and Au70nm/glass_T (Fig. 6c) are negligible and the ratio A:B ¼ 1.55 is quite close to that in the reference Au-foil. To elucidate the origin of these differences in the observed behavior of jF(R)j before and after thermal treatment for the samples with different thicknesses of the initial gold layers on the glass surface, the processing of experimental Au L3-EXAFS spectra was performed. The oscillatory parts c(k) of the experimental EXAFS spectra (k e photoelectrons wave number) were processed using the original technique for reducing the effect of correlations among the fitting parameters on the obtaining values of structural parameters [35]. The application of the used approach in the structural analysis of monometallic nanoparticles enables to determine the fraction (C) of atoms in the internal (usually structurally ordered) region of a mean nanoparticle of their total number in it and to obtain the average values of structural parameters, characterizing the near-surface region of particle [36,37]. In the studied glass samples, the experimental EXAFS signal is averaged over the different sites of gold atoms with different types of local atomic structure in each sample. The simplest structural model of nanoparticle which can be used contains the two
358
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Fig. 3. Comparison of gold particles size distributions for the studied samples before (blue solid curves) and after (green dashed curves) thermal post-treatment obtained as a result of the fit of experimental optical spectra using Mie theory. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
structural states of the absorbing gold atom [22]: Au(1) e atoms in the internal or core region of the nanoparticle; and Au(2) e atoms of the near-surface region of particle, that usually contains distortions, defects and impurities. The applicability of this model for the studied glass samples is provided by Fig. 6a, which shows that in the extended R-range (up to ~ 5.5 Å), all the peaks in jF(R)j of the reference gold foil are presented in the jF(R)j of the samples in Fig. 6(aec), but with reduced magnitudes. This indicates the presence of an fcc like structure for at least part of the gold atoms, which are located in the ordered core region of the gold nanoparticles. Such atoms are denoted in the following as Au(1) and according to Ref. [22] the oscillatory part c(k) of Au L3-EXAFS in the studied samples can be written as:
cmodel ðkÞ ¼ C cAuð1Þ ðkÞ þ ð1 CÞcAuð2Þ ðkÞ
(3)
where C is the fraction of Au(1) atoms and cAu(2)(k) is the contribution of gold atoms attributed to the near-surface region of nanoparticle and denoted as Au(2). To reduce the number of fitting variables and correlations among them, providing the stability of
Fig. 4. X-ray diffraction patterns of the samples before (red curves 1) and after (blue curves 2) thermal post-treatment (300 C, 48 h): (a) e Au-6nm/glass_asp and Au-6nm/ glass_T; (b) e Au-10nm/glass_asp and Au-10nm/glass_T; (c) e Au-70 nm/glass_asp and Au-70nm/glass_T. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 2 Structural parameters of the studied samples obtained by the processing of XRD data: mean nanoparticles size (D) and unit cell parameter (a). Samples
D±3 (nm)
a ± 0.01 (Å)
Au-6nm/glass_asp Au-6nm/glass_T Au-10nm/glass_asp Au-10nm/glass_T Au-70nm/glass_asp Au-70nm/glass_T
27 29 24 27 12 15
4.08
̊
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Fig. 5. Mean sizes of Au nanoparticles in the “as prepared” samples (black squares) and in the samples after thermal post-treatment (red circles) obtained from optical spectra (a) and XRD (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
the obtained values of structural parameters, the term cAu(1)(k) in (3) was constructed according to Ref. [35] as cAuð1Þ ðkÞ ¼ cexperim ðkÞ,expð 2 ,Ds2Auð1Þ k2 Þ. In this expression, the function Aufoil experim cAufoil ðkÞ was extracted from experimental Au L3-edge EXAFS of the gold foil and hence, contained the exact contribution from the first and more distant shells of the Au(1) atoms. The factor expð 2 , Ds2Auð1Þ k2 Þ takes into account the difference in structural order in the coordination shells of a gold atom in a nanoparticle with that of one in the foil. As a result, only two variable parameters are used for the first term in (1) in the fit of F(R): global parameter C and Ds2Auð1Þ . For the term cAu(2)(k) in (3), the approximation of photoelectron scattering processes on one nearest atoms was used according to Ref. [35] to account for the contribution of the particle's near-surface region. The reduction factor S20 ðAu AuÞ was fixed to its value in the gold foil: 0.89 for Au(2) atoms. Fourier analysis based on expression (3) for the gold local structures in nanoparticle reproduces all the features of the experimental jF(R)j in the extended R-range (up to ~ 5.5 Å) for “as prepared” sample Au-70nm/glass_asp, which in agreement with [22], and for this sample after thermal treatment Au-70nm/glass_T, 2 giving the following values of parameters: Ds2Auð1Þ ¼ 0:0003 A and C ¼ 48%. At the same time, the same fit procedure based on expression (3) didn't allow to describe the differences in the magnitudes of the two main peaks A and B of jF(R)j between samples before and after thermal post-treatment. This indicates on the presence of more complicated structures and compositions of the near-surface region (shell) of gold particles, which depend upon
359
Fig. 6. FT magnitudes jF(R)j of k c(k), obtained by Dk interval with kmin ¼ 3.0 Å1 and kmax ¼ 9.5 Å 1 for the experimental Au L3-edge EXAFS in the gold foil (solid black curve in (a)), and in the “as prepared” (dashed red curves) and after thermal treatment (doted blue curves) samples: (a) e for 6 nm thickness of initial gold-film; (b) - for 10 nm thickness of gold-film; (c) - for 70 nm thickness of gold-film. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
the thicknesses of the initial gold film on the glass surface and the applied thermal post-treatment. Therefore, a suitable structural model for the gold particle's shell must include the possible penetration of other metal atoms of the glass matrix into this shell and accounting for the possible oxidation of these metal atoms which are adsorbed on the particle's surface. To suggest such a model, we used TEM results of [22], which showed that the gold nanoparticles are formed within the thin (<30 nm) near-surface region of the tin-bath side of the glass, where the concentration of Sn atoms is the highest. The results of STEM-EDX analysis of sample Au-10nm/glass_asp presented in Fig. 7 show that with high probability Au atoms of nanoparticle are interacting with Sn, Mg, Na atoms. The thickness of the scanning region is of ~100 nm and therefore one can conclude that concentration of Si atoms (EDX for Si e part (f)) in the region of gold nanoparticle is observably lower than in the surrounding glass, while the EDX for Sn (part (d)) shows almost similar concentration of Sn atoms both in the region of gold nanoparticle with size ~ 50 nm and in the glass matrix. This indicates that the Sn atoms are penetrated rather deeply into the volume of gold particle. From EDX images which correspond to Mg, Na atoms one can see the outlines of the gold particle and hence, Mg and Na atoms can be
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Fig. 7. TEM images of sample Au/glass_asp: (a) e STEM high-angle annular dark-field image; (b)e(f) e results of STEM-EDX analysis.
located both on the particle's surface and in its volume. The higher oxidizing ability of Mg, Na compared to Sn [38], prevents their penetration into the volume of gold nanoparticles and suggests that Mg and Na should be dominantly located on the surface of nanoparticles, which is confirmed below by the values of the structural parameters of AueSn, AueMg,Na bonds, obtained from the processing of Au L3-EXAFS spectra. To account for the presence of Sn, Mg, Na atoms in the volume or on the surface of gold particle, these atoms were included into the fit model for experimental Au L3-EXAFS as the nearest neighbors of the absorbing Au(2) atom. Basically, this can be done by the inclusion of corresponding interatomic interactions as separate terms into the second term cAu(2)(k) in expression (3). However, the fit which is based on such complicated expression for cAu(2)(k) requires a large number of varied parameters, which significantly exceeds the allowed number of variable parameters [39]. Therefore, the following approximations were used, which enabled to reduce the number of terms in the expression for cAu(2)(k), and as a consequence, to reduce the number of fit variables:
respectively. Fig. 8 illustrates the goodness of the fit performed by the model of Au local structures based on expression (3) with the term cAu(2)(k) determined by expression (4), for the samples Au6nm/glass_asp and Au-6nm/glass_T. As can be seen, all the features of experimental jF(R)j functions are reproduced in details in the extended R-range (up to ~ 5.5 Å) by the suggested model of Au local structures. The obtained values of structural parameters are presented in Tables 3 and 4 which correspond to the samples Au-6nm/glass and Au-10nm/glass respectively. Changes of the values of parameter C in Tables 3 and 4 show the decreasing of the fraction of the core atoms in mean gold nanoparticle after thermal post-treatment of samples Au-6nm/glass_asp and Au-10nm/glass_asp. Simultaneously with the decreasing of parameters N2 (number of AueSn bonds) and N3 (number of
e the higher oxidation ability of Mg, Na ions comparing to Sn leads to less probability of their diffusion penetration into the nearsurface region of gold particle and therefore, in this particle's region some of the absorbing Au(2) atoms have Sn as the first neighbors while Mg or Na atoms are located on the surface of gold particles and are surrounded by oxygen atoms. This allows to neglect the interaction of Au(2) atoms of the particle's surface with the surrounding oxygen atoms of the glass matrix e phases and amplitudes of photoelectron backscattering in atomic pairs AueMg and AueNa differ mainly from each other by the energy shift and therefore the distinguishing of quite close contributions of AueMg and AueNa bonds from experimental Au L3-EXAFS is rather complicated from the one side and from the other e it is not needed for the aims of our study. Therefore, these contributions are combined in the following within the single term noted as cAu(2)-Mg,Na(k) and the backscattering amplitudes for the fit were taken for Mg atoms. According to these approximations, the second term cAu(2)(k) in expression (3) can be presented by the following expression:
cAuð2Þ ðkÞ ¼ N1 cAuð2ÞAuð2Þ ðkÞ þ N2 cAuð2ÞSn ðkÞ þ N3 cAuð2ÞMg;
Na ðkÞ
(4)
where cAu(2)-Au(2)(k), cAu(2)-Sn(k), cAu(2)-Mg,Na(k) e are the contributions of corresponding atomic pairs Au(2)eAu(2), Au(2)-Sn, Au(2)-Mg,Na and N1, N2, N3 are the numbers of these atomic pairs
Fig. 8. Comparison of the FT magnitudes jF(R)j of kc(k) of experimental Au L3-edge EXAFS (solid black curves) in as prepared sample Au-6nm/glass_asp (a) and in sample after thermal treatment Au-6nm/glass_T (b) with the theoretical functions obtained by the fit based on the expression (3), (4) for c(k) (dashed blue curves). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Table 3 Parameters of local structure of Au atoms in gold nanoparticles in the samples Au-6nm/glass before and after thermal treatment, obtained from Au L3-edge EXAFS by the fit based on expressions (3) and (4). Sample
Au-6nm/glass_asp Au-6nm/glass_T
Core of Au NP: Au(1) atoms C
Ds2Auð1Þ ,
0.69 0.58
0.0004 0.0003
Shell of Au NP: Au(2)eAu(2) Å
2
Au(2)-Sn 2
2
R, Å
N1
s ,Å
2.78 2.83
7.9 10.0
0.0086 0.0113
Au(2)-Mg,Na 2
2
R, Å
N2
s ,Å
3.11 2.95
1.5 0.9
0.0086 0.0113
R, Å
N3
s2, Å2
3.32 3.54
0.6 0.4
0.0089 0.011
Table 4 Parameters of local structure of Au atoms in gold nanoparticles in the samples Au-10nm/glass before and after thermal treatment, obtained from Au L3-edge EXAFS by the fit based on expressions (3) and (4). Sample
Au-10nm/glass_asp Au-10nm/glass_T
Core of Au NP: Au(1) atoms
Shell of Au NP: Au(2)eAu(2)
Au(2)-Sn
Au(2)-Mg,Na
C
Ds2Auð1Þ , Å2
R, Å
N1
s2, Å2
R, Å
N2
s2, Å2
R, Å
N3
s2, Å2
0.69 0.62
0.0004 0.0002
2.85 2.87
6.5 6.9
0.0051 0.0065
3.12 2.91
1.1 0.7
0.0055 0.0065
3.31 3.35
0.4 0.2
0.0112 0.0095
AueMg,Na bonds) this indicates that the mean nanoparticles size decreases after thermal treatment, according to Ref. [37]. Otherwise (if particles size non-decreases), the decreasing of C should have led to the increasing values of N2, N3. This conclusion agrees with the changes of gold particles sizes after thermal post-treatment obtained by the analysis of optical spectra (Table 1), but contradicts with XRD results, which possible reasons were discussed in Sec.3.2. 3.4. Discussion The structural parameters presented in Tables 3 and 4 provide the quantitative evidences for the presence of bonds Au(2)-Sn and Au(2)-Mg,Na in the near-surface region of gold nanoparticles in samples Au-6nm/glass and Au-10nm/glass before and after thermal post-treatment. In contrast, the processing of Au L3-EXAFS in samples Au-70nm/glass_asp and in Au-70nm/glass_T gave the high quality of the fit by expression (3) without using AueSn and AueMg,Na contributions, and enabled to obtain the values of AueAu structural parameters, which agree with those obtained in Ref. [22]. The revealed differences in the number of bond types for Au(2) atom with its nearest neighbors e differences in the number of terms in expression (4), which contribute into Au L3-EXAFS spectra in samples Au-6nm/glass, Au-10nm/glass, in comparison with (3) for Au-70nm/glass enabled to conclude that composition and structure of the obtained gold particles near-surface region, as well as their sizes, depend not only on the thermal treatment, but strongly depend upon the thickness of the initial gold films, which coated the surface of glass before laser irradiation. Most probably, the origin of these dependences is the difference in the results of the action of the first laser pulse on the initial “thin” gold films of thickness 6e10 nm on the tin-bath side of samples Au-6nm/glass, Au-10nm/glass, in comparison with its action on the film of thickness 70 nm on sample Au-70nm/glass. The matter is that as was shown in Ref. [22], irradiation of the gold film of 70 nm by the first laser pulse leads to the strong evaporation of Au atoms resulting in the residual gold film of thickness ~ 6 nm on sample Au-70nm/ glass, while the Au nanoparticles are forming beginning from the second pulse only. Due to this evaporation of Au atoms, the surface of the glass and the gold film on it in sample Au-70nm/glass should be less heated, compared with irradiated by first pulse initial films of 6e10 nm on samples Au-6nm/glass or Au- 10nm/glass, in which the formation and implantation of nanoparticles begins with the
first laser pulse. Less heating of the residual gold film and the adjacent glass surface in sample Au-70nm/glass should lead to a decrease in the thermal diffusion of atoms and, consequently, to a difficult penetration of Sn, Mg, Na atoms into the gold film and, as a result, into the volume of the forming gold nanoparticles. Formation of gold nanoparticles from the initial gold films of different thicknesses is schematically illustrated in Fig. 9(aec). As was revealed in Ref. [22] for the sample Au-70nm/glass, the shell of Au NPs consists of Au atoms only and hence, one can conclude that Mg, Na and Sn atoms, probably in oxidized state which prevents their penetration into the gold particles, are dominantly located on the surface of particle as its outer coverage. In contrast, for the samples Au-6nm/glass and Au-10nm/glass, EXAFS derived parameters of AueSn and AueMg,Na bonds of Tables 3 and 4 show that Sn atoms (due to the above discussed reason and their less oxidizing ability comparing to Mg, Na) are penetrated into the shell of gold particle while Mg, Na-atoms, in oxidized state are adsorbed on the particle's surface, forming the outer coverage of the gold particles (N2 > N3). Due to such a complicated composition of particles shell and coverage, the effective sizes of the obtained nanoparticles in the samples Au6nm/glass_asp and Au- 10nm/glass_asp are larger than in the sample Au-70nm/glass_asp which is confirmed by the results of the estimates performed by XRD and optical extinction spectra. The differences in the gold particles surface structures in the studied “as prepared” samples leaded to the differences in the gold particles obtained after thermal post-treatment of long duration in the samples Au-6nm/glass_T, Au-10nm/glass_T in comparison to Au-70nm/glass_T: - the presence of Sn atoms in the shell of gold nanoparticles in the samples Au-6nm/glass_asp, Au-10nm/glass_asp leaded to the less stable surface of the particles during thermal treatment and consequently e to the dissolution of the particles surface (Tables 3 and 4 e parameters C and N2, N3 are less in samples after thermal post-treatment). As a result, the size of the gold particles after thermal treatment became smaller in agreement with their estimates by optical extinction spectra (Fig. 5, Table 1); - the presence of the outer coverage containing the oxidized Sn, Mg, Na on the surface of “as prepared” gold particles in the sample Au-70nm/glass_asp prevents its dissolution during
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Fig. 9. Schematic illustration of the formation of gold nanoparticles from the initial gold films of different thicknesses: (a) e initial gold film of thickness 70 nm on sample Au-70nm/ glass; (b) e gold films of 6e10 nm on samples Au-6nm/glass, Au-10nm/glass (top part of (b)) in comparison with the gold film of ~6 nm remained on the sample Au-70nm/glass after first laser pulse (bottom part of (b)); (с) e Au nanoparticles in “as prepared” samples obtained after laser irradiation by 10 pulses; (d) e Au nanoparticles in samples after thermal post-treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
thermal treatment and as a result, the size of these particles doesn't change significantly in the sample Au-70nm/glass_T. e According to the above made conclusions, the formation of nanoparticles in the studied samples before and after thermal posttreatment is shown schematically in Fig. 9. In addition, it must be noted that the process of dissolution of the gold nanoparticles shell in the samples Au-6nm/glass_asp and Au-10nm/glass_asp during the thermal post-treatment is different and has its own features. To analyze them, one must consider that the dissolved Au atoms also have their contribution into the function cAu(2)(k). Table 3 shows that Sn atoms in gold nanoparticles of sample Au-6nm/glass_asp partially leave the dissolving shell (N2 decreases) and partially remain on the particles surface (presence of AueSn contributions) in oxidized state, which protect the nanoparticles from further dissolution. The significant increase of parameter N1 after thermal post-treatment enables to conclude that Sn atoms leave the shell of nanoparticles and the remaining Au atoms of the shell have a significant contribution into cAu(2)(k) after thermal post-treatment comparing to the contribution of the dissolved Au atoms (N1 is quite large). In the case of gold nanoparticles in the sample Au-10nm/glass_asp, the parameter N1 is almost unchanged (Table 4), whereas it should increase if Sn atoms leave this shell. Therefore, it is reasonable to assume that the Au atoms dissolved during thermal post-treatment of sample Au-10nm/ glass_asp have significant contribution to the function cAu(2)(k). These atoms are probably re-deposited into more smaller clusters of gold. The discussed differences in the shell of gold nanoparticles in the samples Au-6nm/glass_T and Au-10nm/glass_T explain the differences in the A:B ratios in FT of Au L3-EXAFS of these samples before and after T-treatment.
e
e
e
the stable gold core, shell consisting of Au and Sn atoms and an outer coverage of Sn, Mg, Na oxides gold particles formed by laser irradiation of sample, initially coated with gold film of thickness ~70 nm, had a stable gold core, shell consisting of Au atoms only and an outer coverage of Sn, Mg, Na oxides most probably, the origin of such difference in composition of shell of the obtained gold particles (presence or absence of tin atoms in it) depending upon the thickness of the initial gold film coating the surface of glass before laser irradiation, is the difference in the results of the action of the first laser pulse on the gold films of thickness 6e10 nm on the tin-bath side of the glass samples in comparison with its action on the film of thickness 70 nm the revealed differences in the composition and structure of gold particles shell in the studied “as prepared” samples leaded to the differences in the gold particles obtained after thermal post-treatment of long duration, reflecting in the observed changes of SPR. Thus, the AueSn composition of the gold particles shell in “as prepared” samples, initially coated with gold films of 6e10 nm, makes this shell and the coverage on it less stable to the heating and consequently, leads to a partial destruction of the oxidized metals coverage and substantial dissolution of this AueSn shell during thermal post-treatment, reducing the particles size in corresponding samples, which is reflected in SPR parameters in contrast, homogeneous composition of the gold particles shell in glass sample initially coated with film of thickness 70 nm, prevents dissolution of particles under heating providing stable particles size and hence, the stability of SPR characteristics before and after the thermal treatment, which is confirmed by optical spectra.
4. Conclusions Acknowledgements The performed study of the effects of thermal post-treatment and of thickness of the initial gold film coating silicate glass surface, on SPR characteristics and atomic structure of gold nanoparticles prepared in glass by UV laser irradiation enabled to make the following conclusions: e irradiation of the glass samples coated with gold films of thickness 6e10 nm leaded to formation of gold particles with
The work was supported by Southern Federal University and DFG project (DFG No DU 214/14-1). V.V.S., S. Yu.K. acknowledge €tz RFBR project 18-32-00818 mol_а. Authors would like to thank Go Schuck for the help during XAFS measurements at KMC-2 beamlines at BESSY II (Berlin, Germany), proposal 181-06412, Christian Patzig from Fraunhofer Institute for Microstructure of Materials and Systems IMWS for providing TEM-EDX images, and Igor Leontyev
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of thermal post-treatment on surface plasmon resonance characteristics of gold nanoparticles formed in glass by UV laser irradiation Vasiliy V. Srabionyan a, Maximilian Heinz b, Sviatoslav Yu Kaptelinin a, Leon A. Avakyan a, Galina B. Sukharina a, Anna V. Skidanenko a, VasiliyV. Pryadchenko a, Kamal G. Abdulvakhidov c, AlexeyS. Mikheykin a, VeniaminA. Durymanov a, € rg Meinertz d, Juergen Ihlemann d, Manfred Dubiel b, Lusegen A. Bugaev a, * Jo a
Department of Physics, Southern Federal University, Zorge Str. 5, RU-344090, Rostov-on-Don, Russia Institute of Physics, Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 3, D-06120, Halle (Saale), Germany c The Smart Materials Research Center, Southern Federal University, Sladkova 178/24, RU-344090, Rostov-on-Don, Russia d €ttingen e.V., Hans-Adolf-Krebs-Weg 1, D-37077, Go €ttingen, Germany Laser-Laboratorium Go b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 April 2019 Received in revised form 18 June 2019 Accepted 21 June 2019 Available online 22 June 2019
Effects of thermal post treatment and of thickness of the initial gold film coated the silicate glass surface, on localized surface plasmon resonance (SPR) of gold nanoparticles formed in glass by UV laser irradiation were studied to develop effective technique for production of gold nanoparticles and their arrays with required and stable characteristics of SPR. Optical spectra of the obtained Au/glass samples showed that SPR of gold nanoparticles formed by irradiation of glasses sputter coated with gold films of thickness 6e10 nm depends of T-treatment, while irradiation of glass coated with gold film ~70 nm leads to SPR stable of temperature. The origin of such dependences was studied by the structural analysis of gold nanoparticles in Au/glass samples before and after thermal treatment using XRD, EXAFS methods, TEMEDX images and direct calculations of optical spectra. It was revealed that the changes or stability of SPR parameters under thermal post-treatment of Au/glass samples depend respectively of the presence or absence of tin atoms in the near-surface region (shell) of the gold particles. Thus, if tin atoms are presented in the shell of gold particles (as in “as prepared” Au/glass samples initially coated with gold films of 6e10 nm) then the leave of tin atoms from the particle's volume during the heating leads to partial dissolution of the particles shell and consequently, to decreasing of the particles size, which in turn leads to the observed changes in SPR. The most probable explanation of the presence of tin atoms in the shell of gold particles formed by irradiation of samples initially coated with gold films of thickness 6e10 nm and their absence in the case of sample with film ~70 nm was proposed, basing on the difference in the effects of the first laser pulse actions on these films on the tin-bath side of the glass samples. The sizes of coherent scattering region in gold nanoparticles before and after thermal treatment of Au/glass samples were determined and the dependence of these sizes upon the thickness of the gold film coating the sample's surface before laser irradiation was revealed. © 2019 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials UV laser processing Thermal post-treatment Surface plasmon resonance Optical spectroscopy XRD EXAFS
1. Introduction Gold and silver nanoparticles in dielectric media are extensively studied mainly because of their ability to localize electro-magnetic
* Corresponding author. E-mail address: [email protected] (L.A. Bugaev). https://doi.org/10.1016/j.jallcom.2019.06.263 0925-8388/© 2019 Elsevier B.V. All rights reserved.
field on nano-scale distances due to the phenomenon of surface plasmon resonance (SPR), providing various valuable applications like i) light-heat energy transfer [1e5], ii) SPR sensors for biomedicine and nano-applications [6e13] and iii) in catalysis [14e16]. Depending on the specific application, different requirements to SPR characteristics are needed, which can be achieved by constructing the nanoparticles with tunable parameters of SPR peak, such as its wavelength position, full width at half
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maximum (FWHM) of SPR peak and combining the SPR properties of single particles with the effects of their space arrangement, in particular, within the lattice plasmon resonance (LPR) [17e20]. The promising approach for creating single plasmonic particles, their submicron line patterns and arrays with wavelength position and shapes of SPR tuned in a wide ranges, is the synthesis of bimetallic gold-silver nanoparticles with alloy or core-shell architecture [21] by UV ArF-excimer laser irradiation (193 nm) below the ablation threshold of the glass, varying the number of pulses at different fluences. The suggested experimental technique for the synthesis of such bimetallic single nanoparticles and their arrays was based on the results of preliminary study of the formation of gold nanoparticles under laser irradiation [22], as the possible substrates or nuclei for precipitation of silver atoms on them. In these papers the initial gold film of thickness 70 nm on the glass surface was used which provided the highest concentration of the generated Au particles in the near-surface region. To propose a reliable and effective technique for production of plasmonic nanoparticles with the required and stable during operation optical properties, in this paper we present the results of the study i) of thermal post-treatment of the Au/glass samples after UV laser irradiation and ii) of the effect of thickness of the initial gold film on the glass surface on SPR characteristics of the formed gold particles. With this aim, the glass samples coated by gold films of thickness 6, 10, 70 nm, irradiated by UV laser and then heat treated, were prepared (Sec 2). Analysis of optical spectra of “as prepared” and obtained after heat treatment samples was performed (Sec 3.1) which confirms the formation of gold particles and allowed to determine the changes in their mean size. The results of XRD study of gold particles in the samples are presented in Sec. 3.2. Finally, the processing of Au L3-EXAFS spectra (Sec 3.3) gave the structural parameters of the atomic bonds Au-M (M ¼ Au, Sn, Mg, Na) in the shell and in the outer coverage of the gold nanoparticles, which allowed to reveal the dependencies of their atomic composition upon thickness of the initial gold film on the glass surface and thermal post-treatment in accordance with the observed changes in SPR of experimental optical spectra. 2. Experimental methods
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each laser pulse was measured simultaneously using a beam splitter, and the fluence of the first laser pulse was ensured to be equal to 140 mJ/cm2 for each studied sample. As a next step, the laser irradiated areas of size 6 6 cm2 were cleaned with acetone to remove gold nanopartices, which were not implanted into the glass. Finally, the laser irradiated areas of each sample were broken into two pieces. One piece has been thermally treated at 300 C for a duration of 48 h, whereas the other piece (named “as prepared” or briefly - “asp” sample) was used for the reference measurements without thermal post-treatment. The glass samples before and after thermal post-treatment, prepared by UV laser irradiation of glasses coated with gold films of different thickness are notated respectively as: Au-6nm/glass_asp, Au-6nm/glass_T, Au-10nm/ glass_asp, Au-10nm/glass_T, Au-70nm/glass_asp, Au-70nm/glass_T.
2.2. Experimental measurements Optical spectra have been measured by Lambda 900 UV/Vis/NIR spectrometer (PerkinElmer) in the wavelength range from 300 nm to 800 nm. To reduce the spot size to the laser irradiated areas a circular aperture with a diameter of 3 mm has been used. Au L3-edge X-ray absorption spectra were measured at the KMC-2 beamline of the BESSY-II Synchrotron Radiation Facilities (Berlin, Germany). All spectra were measured at room temperature. Due to the low concentration of the nanoparticles and their location in the surface layer of the glass, the spectra were collected in fluorescence yield mode. The photon energy scanning steps were adjusted to dE ¼ 1.0 eV and dk ¼ 0.05 Å-1in XANES and EXAFS region respectively, where E is the energy of incident radiation and k is the corresponding photoelectron wavenumber. X-ray diffraction (XRD) measurements were carried out using diffractometer Bruker D2 PHASER at Southern Federal University (Rostov-on-Don, Russia) with Cu K a radiation (wavelength of l ¼ 1.54 Å) and Scintillation counter 1-dimensional LYNXEYE detector in Bragg-Brentano geometry. The experiment was performed using Bragg reflection angles range from 30 to 50 with step 2q ¼ 0.01 and exposure time 10 s at each point.
2.1. Samples preparation The experimental techniques and the study of the formation and implantation of gold nanoparticles in the near-surface region of soda-lime silicate float glass by UV laser radiation (193 nm) was performed in Ref. [22]. In this work, implanted gold nanoparticles were additionally annealed by subsequent thermal post-treatment. The commercial float glass with thickness of 1 mm had the following composition (the values are given in weight %): 72.3% SiO2, 0.5% Al2O3, <0.02% Fe2O3, 13.3% Na2O, 8.8% CaO, 0.4% K2O, 4.3% MgO. In addition, one side of the glass contains 0.02 wt % of SnO2, this side will be further referred as tin-bath side. The opposite side (without tin) will be referred as air-side. At the first step, the tinbath sides of the glass samples were sputter coated with gold by means of a K550 Sputter Coater system (manufactured by Quorum Technologies) using the plasma current of 20 mA and Ar pressure of 101 mbar. By this procedure, gold films of thickness 6 nm, 10 nm and 70 nm were produced on the tin-bath glass surface. As a next step, 193 nm ArF-excimer laser irradiation (LPX 315 laser, manufactured by Lambda Physik) with 20 ns pulse duration and 10 laser pulses with a repetition rate of 1 Hz was applied, which leads to the formation and implantation of gold nanoparticles into the glass surface (see Ref. [22]). The average laser fluence was 140 mJ/cm2. As the fluence of the first laser pulse was shown to affect significantly the nanoparticle formation process [22], the actual fluence of
3. Results and discussion 3.1. SPR in optical extinction spectra of the studied samples Comparison of experimental optical spectrum of “as prepared” sample with the spectrum of this sample after thermal posttreatment is presented in Fig. 1, respectively for the samples with thicknesses 6, 10 and 70 nm of the initial gold films on the glass surface, which were used for Au nanoparticles preparation by UV laser irradiation. The starting point of the analysis of experimental optical spectra of Au/glass samples was the analysis using Mie theory, which allows to determine size distribution of the particles in assumption of their spherical shape, homogeneity, and absence of plasmon interaction between them [23]. The calculation of extinction spectra of individual nanoparticle was performed using Sander's code [24] and the theoretical spectra of particles ensemble was obtained by summation of these spectra with weights determined by log-normal distribution of particles with parameters m and s [25] varied during the fit. Additionally, the background contribution and the scale multiplier N0, which is proportional to the total number of nanoparticles, were varied. The minimized residual function was calculated as:
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Fit quality, c2
N0
m
s
D(max
Au-6nm/glass_asp Au-6nm/glass_T Au-10nm/glass_asp Au-10nm/glass_T Au-70nm/glass_asp Au-70nm/glass_T
0.001 0.001 0.007 0.001 0.010 0.009
0.05 0.06 0.06 0.05 0.11 0.09
4.1 3.9 4.6 4.5 4.1 3.8
0.2 0.3 0.1 0.2 0.9 0.8
59 42 84 68 25 23
in SD),
nm
so that the above mentioned misfits are probably originated from ignoring of the plasmon coupling between nanoparticles. The last column of Table 1 shows the values of D(max in SD) e size, corresponding to the position of maximum in particles size distributions (SD) presented in Fig. 3, which compares the size distributions for the studied samples according to the results of the fit by the approach of Mie theory. The narrow distributions for the samples synthesized using thin gold film (Au-6nm/glass_asp and Au-10nm/glass_asp) became broader and are shifted towards smaller particle sizes after thermal post-treatment. Together with almost unchanging particles density N0 (Table 1) this indicates on dissolution of some of the particles in the glass. However, for the particles synthesized using gold film of 70 nm (sample Au-70nm/ glass_asp) the initial size distribution is very broad, and the effect of thermal post-treatment is weak.
3.2. X-ray diffraction study Fig. 4 shows the diffraction patterns of three series of the studied glass samples with gold coating before and after thermal post-treatment, where each series consists of pair of samples: Au6nm/glass_asp and Au-6nm/glass_T, Au-10nm/glass_asp and Au10nm/glass_T, and Au-70nm/glass_asp and Au-70nm/glass_T. Diffraction patterns of all samples demonstrate Bragg peak (111) corresponding to fcc lattice (Fm3m). XRD analysis was performed using FULLPROFF program complex [28]. The profile of analyzing peak was approximated using pseudo-Voight function which is linear combination of Gaussian (G) and Lorentzian (L) functions:
FPV ¼ h,L þ ð1 hÞ,G Fig. 1. Comparison of optical spectra of “as prepared” samples (black solid curves) with thicknesses 6 (a), 10 (b) and 70 nm (c) of the initial gold film on the glass surface with the corresponding spectra of these samples after thermal post-treatment (red dashed curves). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
c2 ¼
X
exp 2
I theor Ii i
(1)
i
where Iexp and Itheor are the experiential and calculated spectra respectively. Index i numbers the wavelengths l in the interval 400 < li < 1000 nm. The dielectric function of bulk gold [26] was used in the calculations, which is valid for nanoparticleы bigger than 10 nm. The results of the fit using Mie theory are summarized in Table 1 and illustrated in Fig. 2. The fit quality for the samples Au-6nm/ glass_asp, Au-6nm/glass_T and Au-10nm/glass_T is high, then slightly worse for sample Au-10nm/glass_asp due to misfit in the left shoulder of the peak, and poorer for samples Au-70nm/ glass_asp and Au-70nm/glass_T. Our previous studies [22,27] of the samples analogues to sample Au-70nm/glass_asp demonstrated the presence of nanoparticles agglomerates in the sample,
(2)
where parameter h (0 < h < 1) corresponds to the Lorentzian contribution in pseudo-Voight function. The size of the coherent scattering region in the volume of nanoparticle can be estimated from the widths of the peaks using Sherrer equation [29,30], which is a suitable approach for the structural study of nanoparticles with sizes > 2 nm [31,32]. Instrumental broadening resolution function was calibrated by Al2O3 reference sample according to the procedure [33]. Table 2 contains the structural parameters of the samples obtained from diffraction data: mean sizes (D) of Au nanoparticles and unit cell parameters (a). The last appeared to be the same for all the considered samples. As can be seen, the thermal post-treatment leaded to the particles size increasing in contrast to the particles sizes obtained by optical data which is illustrated in Fig. 5. The possible reason of this contradiction as well as the reason of strong difference in the values of mean sizes D obtained from optical spectra and from XRD is that the particles size obtained from XRD corresponds to the coherent scattering region in the volume of gold nanoparticles, while the particles size obtained from the analysis of optical spectra includes not only the ordered internal region of the particle, but also its shell containing structural distortions, defects and impurities. It is revealed that the tin atoms are strongly penetrated into the volume of gold particles (Sec. 3.4) and as a
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Fig. 2. Illustration of the fits quality of experimental optical spectra of the studied samples using Mie theory: a, b e samples Au-6nm/glass_asp and Au-6nm/glass_T; c, d e samples Au-10nm/glass_asp and Au-10nm/glass_T; e, f e samples Au-70nm/glass_asp and Au-70nm/glass_T. Experiment spectra e red dots, fit results e blue solid curves, background contributions e green dashed curves.
result, the size of the particle's coherent scattering region may differ strongly from the full particles size. One more explanation of the obtained contradiction is the possible polycrystalline structure of nanoparticles [30,34]. In this case, the “optical size” corresponds to the sizes of polycrystalline particles, while “XRD size" is the size of each of the crystalline regions of particle, which coherently scatters X-rays.
3.3. Analysis of Au L3-EXAFS The structural characterization of gold nanoparticles in the glass samples before and after thermal post-treatment was performed by Au L3-edge EXAFS. Fig. 6 compares the magnitudes of the Fourier transforms jF(R)j of the experimental Au L3-edge EXAFS spectra of the gold foil e reference compound (Fig. 6a) and of the glass samples before and after thermal post-treatment with different thickness of the initial gold film: Au-6nm/glass_asp and Au-6nm/ glass_T (Fig. 6a), Au-10nm/glass_asp and Au-10nm/glass_T (Fig. 6b), Au-70nm/glass_asp and Au-70nm/glass_T (Fig. 6c). As can be seen, there are significant differences, especially in the magnitudes of the two main peaks A and B, in jF(R)j of experimental spectra between samples Au-6nm/glass_asp (A:B ¼ 1.79) and Au6nm/glass_T (A:B ¼ 1.47) and between Au-10nm/glass_asp
(A:B ¼ 1.72) and Au-10nm/glass_T (A:B ¼ 1.59), while the value of this ratio in the reference Au-foil A:B ¼ 1.52. At the same time, the changes in jF(R)j between samples Au-70nm/glass_asp and Au70nm/glass_T (Fig. 6c) are negligible and the ratio A:B ¼ 1.55 is quite close to that in the reference Au-foil. To elucidate the origin of these differences in the observed behavior of jF(R)j before and after thermal treatment for the samples with different thicknesses of the initial gold layers on the glass surface, the processing of experimental Au L3-EXAFS spectra was performed. The oscillatory parts c(k) of the experimental EXAFS spectra (k e photoelectrons wave number) were processed using the original technique for reducing the effect of correlations among the fitting parameters on the obtaining values of structural parameters [35]. The application of the used approach in the structural analysis of monometallic nanoparticles enables to determine the fraction (C) of atoms in the internal (usually structurally ordered) region of a mean nanoparticle of their total number in it and to obtain the average values of structural parameters, characterizing the near-surface region of particle [36,37]. In the studied glass samples, the experimental EXAFS signal is averaged over the different sites of gold atoms with different types of local atomic structure in each sample. The simplest structural model of nanoparticle which can be used contains the two
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Fig. 3. Comparison of gold particles size distributions for the studied samples before (blue solid curves) and after (green dashed curves) thermal post-treatment obtained as a result of the fit of experimental optical spectra using Mie theory. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
structural states of the absorbing gold atom [22]: Au(1) e atoms in the internal or core region of the nanoparticle; and Au(2) e atoms of the near-surface region of particle, that usually contains distortions, defects and impurities. The applicability of this model for the studied glass samples is provided by Fig. 6a, which shows that in the extended R-range (up to ~ 5.5 Å), all the peaks in jF(R)j of the reference gold foil are presented in the jF(R)j of the samples in Fig. 6(aec), but with reduced magnitudes. This indicates the presence of an fcc like structure for at least part of the gold atoms, which are located in the ordered core region of the gold nanoparticles. Such atoms are denoted in the following as Au(1) and according to Ref. [22] the oscillatory part c(k) of Au L3-EXAFS in the studied samples can be written as:
cmodel ðkÞ ¼ C cAuð1Þ ðkÞ þ ð1 CÞcAuð2Þ ðkÞ
(3)
where C is the fraction of Au(1) atoms and cAu(2)(k) is the contribution of gold atoms attributed to the near-surface region of nanoparticle and denoted as Au(2). To reduce the number of fitting variables and correlations among them, providing the stability of
Fig. 4. X-ray diffraction patterns of the samples before (red curves 1) and after (blue curves 2) thermal post-treatment (300 C, 48 h): (a) e Au-6nm/glass_asp and Au-6nm/ glass_T; (b) e Au-10nm/glass_asp and Au-10nm/glass_T; (c) e Au-70 nm/glass_asp and Au-70nm/glass_T. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 2 Structural parameters of the studied samples obtained by the processing of XRD data: mean nanoparticles size (D) and unit cell parameter (a). Samples
D±3 (nm)
a ± 0.01 (Å)
Au-6nm/glass_asp Au-6nm/glass_T Au-10nm/glass_asp Au-10nm/glass_T Au-70nm/glass_asp Au-70nm/glass_T
27 29 24 27 12 15
4.08
̊
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Fig. 5. Mean sizes of Au nanoparticles in the “as prepared” samples (black squares) and in the samples after thermal post-treatment (red circles) obtained from optical spectra (a) and XRD (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
the obtained values of structural parameters, the term cAu(1)(k) in (3) was constructed according to Ref. [35] as cAuð1Þ ðkÞ ¼ cexperim ðkÞ,expð 2 ,Ds2Auð1Þ k2 Þ. In this expression, the function Aufoil experim cAufoil ðkÞ was extracted from experimental Au L3-edge EXAFS of the gold foil and hence, contained the exact contribution from the first and more distant shells of the Au(1) atoms. The factor expð 2 , Ds2Auð1Þ k2 Þ takes into account the difference in structural order in the coordination shells of a gold atom in a nanoparticle with that of one in the foil. As a result, only two variable parameters are used for the first term in (1) in the fit of F(R): global parameter C and Ds2Auð1Þ . For the term cAu(2)(k) in (3), the approximation of photoelectron scattering processes on one nearest atoms was used according to Ref. [35] to account for the contribution of the particle's near-surface region. The reduction factor S20 ðAu AuÞ was fixed to its value in the gold foil: 0.89 for Au(2) atoms. Fourier analysis based on expression (3) for the gold local structures in nanoparticle reproduces all the features of the experimental jF(R)j in the extended R-range (up to ~ 5.5 Å) for “as prepared” sample Au-70nm/glass_asp, which in agreement with [22], and for this sample after thermal treatment Au-70nm/glass_T, 2 giving the following values of parameters: Ds2Auð1Þ ¼ 0:0003 A and C ¼ 48%. At the same time, the same fit procedure based on expression (3) didn't allow to describe the differences in the magnitudes of the two main peaks A and B of jF(R)j between samples before and after thermal post-treatment. This indicates on the presence of more complicated structures and compositions of the near-surface region (shell) of gold particles, which depend upon
359
Fig. 6. FT magnitudes jF(R)j of k c(k), obtained by Dk interval with kmin ¼ 3.0 Å1 and kmax ¼ 9.5 Å 1 for the experimental Au L3-edge EXAFS in the gold foil (solid black curve in (a)), and in the “as prepared” (dashed red curves) and after thermal treatment (doted blue curves) samples: (a) e for 6 nm thickness of initial gold-film; (b) - for 10 nm thickness of gold-film; (c) - for 70 nm thickness of gold-film. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
the thicknesses of the initial gold film on the glass surface and the applied thermal post-treatment. Therefore, a suitable structural model for the gold particle's shell must include the possible penetration of other metal atoms of the glass matrix into this shell and accounting for the possible oxidation of these metal atoms which are adsorbed on the particle's surface. To suggest such a model, we used TEM results of [22], which showed that the gold nanoparticles are formed within the thin (<30 nm) near-surface region of the tin-bath side of the glass, where the concentration of Sn atoms is the highest. The results of STEM-EDX analysis of sample Au-10nm/glass_asp presented in Fig. 7 show that with high probability Au atoms of nanoparticle are interacting with Sn, Mg, Na atoms. The thickness of the scanning region is of ~100 nm and therefore one can conclude that concentration of Si atoms (EDX for Si e part (f)) in the region of gold nanoparticle is observably lower than in the surrounding glass, while the EDX for Sn (part (d)) shows almost similar concentration of Sn atoms both in the region of gold nanoparticle with size ~ 50 nm and in the glass matrix. This indicates that the Sn atoms are penetrated rather deeply into the volume of gold particle. From EDX images which correspond to Mg, Na atoms one can see the outlines of the gold particle and hence, Mg and Na atoms can be
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Fig. 7. TEM images of sample Au/glass_asp: (a) e STEM high-angle annular dark-field image; (b)e(f) e results of STEM-EDX analysis.
located both on the particle's surface and in its volume. The higher oxidizing ability of Mg, Na compared to Sn [38], prevents their penetration into the volume of gold nanoparticles and suggests that Mg and Na should be dominantly located on the surface of nanoparticles, which is confirmed below by the values of the structural parameters of AueSn, AueMg,Na bonds, obtained from the processing of Au L3-EXAFS spectra. To account for the presence of Sn, Mg, Na atoms in the volume or on the surface of gold particle, these atoms were included into the fit model for experimental Au L3-EXAFS as the nearest neighbors of the absorbing Au(2) atom. Basically, this can be done by the inclusion of corresponding interatomic interactions as separate terms into the second term cAu(2)(k) in expression (3). However, the fit which is based on such complicated expression for cAu(2)(k) requires a large number of varied parameters, which significantly exceeds the allowed number of variable parameters [39]. Therefore, the following approximations were used, which enabled to reduce the number of terms in the expression for cAu(2)(k), and as a consequence, to reduce the number of fit variables:
respectively. Fig. 8 illustrates the goodness of the fit performed by the model of Au local structures based on expression (3) with the term cAu(2)(k) determined by expression (4), for the samples Au6nm/glass_asp and Au-6nm/glass_T. As can be seen, all the features of experimental jF(R)j functions are reproduced in details in the extended R-range (up to ~ 5.5 Å) by the suggested model of Au local structures. The obtained values of structural parameters are presented in Tables 3 and 4 which correspond to the samples Au-6nm/glass and Au-10nm/glass respectively. Changes of the values of parameter C in Tables 3 and 4 show the decreasing of the fraction of the core atoms in mean gold nanoparticle after thermal post-treatment of samples Au-6nm/glass_asp and Au-10nm/glass_asp. Simultaneously with the decreasing of parameters N2 (number of AueSn bonds) and N3 (number of
e the higher oxidation ability of Mg, Na ions comparing to Sn leads to less probability of their diffusion penetration into the nearsurface region of gold particle and therefore, in this particle's region some of the absorbing Au(2) atoms have Sn as the first neighbors while Mg or Na atoms are located on the surface of gold particles and are surrounded by oxygen atoms. This allows to neglect the interaction of Au(2) atoms of the particle's surface with the surrounding oxygen atoms of the glass matrix e phases and amplitudes of photoelectron backscattering in atomic pairs AueMg and AueNa differ mainly from each other by the energy shift and therefore the distinguishing of quite close contributions of AueMg and AueNa bonds from experimental Au L3-EXAFS is rather complicated from the one side and from the other e it is not needed for the aims of our study. Therefore, these contributions are combined in the following within the single term noted as cAu(2)-Mg,Na(k) and the backscattering amplitudes for the fit were taken for Mg atoms. According to these approximations, the second term cAu(2)(k) in expression (3) can be presented by the following expression:
cAuð2Þ ðkÞ ¼ N1 cAuð2ÞAuð2Þ ðkÞ þ N2 cAuð2ÞSn ðkÞ þ N3 cAuð2ÞMg;
Na ðkÞ
(4)
where cAu(2)-Au(2)(k), cAu(2)-Sn(k), cAu(2)-Mg,Na(k) e are the contributions of corresponding atomic pairs Au(2)eAu(2), Au(2)-Sn, Au(2)-Mg,Na and N1, N2, N3 are the numbers of these atomic pairs
Fig. 8. Comparison of the FT magnitudes jF(R)j of kc(k) of experimental Au L3-edge EXAFS (solid black curves) in as prepared sample Au-6nm/glass_asp (a) and in sample after thermal treatment Au-6nm/glass_T (b) with the theoretical functions obtained by the fit based on the expression (3), (4) for c(k) (dashed blue curves). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Table 3 Parameters of local structure of Au atoms in gold nanoparticles in the samples Au-6nm/glass before and after thermal treatment, obtained from Au L3-edge EXAFS by the fit based on expressions (3) and (4). Sample
Au-6nm/glass_asp Au-6nm/glass_T
Core of Au NP: Au(1) atoms C
Ds2Auð1Þ ,
0.69 0.58
0.0004 0.0003
Shell of Au NP: Au(2)eAu(2) Å
2
Au(2)-Sn 2
2
R, Å
N1
s ,Å
2.78 2.83
7.9 10.0
0.0086 0.0113
Au(2)-Mg,Na 2
2
R, Å
N2
s ,Å
3.11 2.95
1.5 0.9
0.0086 0.0113
R, Å
N3
s2, Å2
3.32 3.54
0.6 0.4
0.0089 0.011
Table 4 Parameters of local structure of Au atoms in gold nanoparticles in the samples Au-10nm/glass before and after thermal treatment, obtained from Au L3-edge EXAFS by the fit based on expressions (3) and (4). Sample
Au-10nm/glass_asp Au-10nm/glass_T
Core of Au NP: Au(1) atoms
Shell of Au NP: Au(2)eAu(2)
Au(2)-Sn
Au(2)-Mg,Na
C
Ds2Auð1Þ , Å2
R, Å
N1
s2, Å2
R, Å
N2
s2, Å2
R, Å
N3
s2, Å2
0.69 0.62
0.0004 0.0002
2.85 2.87
6.5 6.9
0.0051 0.0065
3.12 2.91
1.1 0.7
0.0055 0.0065
3.31 3.35
0.4 0.2
0.0112 0.0095
AueMg,Na bonds) this indicates that the mean nanoparticles size decreases after thermal treatment, according to Ref. [37]. Otherwise (if particles size non-decreases), the decreasing of C should have led to the increasing values of N2, N3. This conclusion agrees with the changes of gold particles sizes after thermal post-treatment obtained by the analysis of optical spectra (Table 1), but contradicts with XRD results, which possible reasons were discussed in Sec.3.2. 3.4. Discussion The structural parameters presented in Tables 3 and 4 provide the quantitative evidences for the presence of bonds Au(2)-Sn and Au(2)-Mg,Na in the near-surface region of gold nanoparticles in samples Au-6nm/glass and Au-10nm/glass before and after thermal post-treatment. In contrast, the processing of Au L3-EXAFS in samples Au-70nm/glass_asp and in Au-70nm/glass_T gave the high quality of the fit by expression (3) without using AueSn and AueMg,Na contributions, and enabled to obtain the values of AueAu structural parameters, which agree with those obtained in Ref. [22]. The revealed differences in the number of bond types for Au(2) atom with its nearest neighbors e differences in the number of terms in expression (4), which contribute into Au L3-EXAFS spectra in samples Au-6nm/glass, Au-10nm/glass, in comparison with (3) for Au-70nm/glass enabled to conclude that composition and structure of the obtained gold particles near-surface region, as well as their sizes, depend not only on the thermal treatment, but strongly depend upon the thickness of the initial gold films, which coated the surface of glass before laser irradiation. Most probably, the origin of these dependences is the difference in the results of the action of the first laser pulse on the initial “thin” gold films of thickness 6e10 nm on the tin-bath side of samples Au-6nm/glass, Au-10nm/glass, in comparison with its action on the film of thickness 70 nm on sample Au-70nm/glass. The matter is that as was shown in Ref. [22], irradiation of the gold film of 70 nm by the first laser pulse leads to the strong evaporation of Au atoms resulting in the residual gold film of thickness ~ 6 nm on sample Au-70nm/ glass, while the Au nanoparticles are forming beginning from the second pulse only. Due to this evaporation of Au atoms, the surface of the glass and the gold film on it in sample Au-70nm/glass should be less heated, compared with irradiated by first pulse initial films of 6e10 nm on samples Au-6nm/glass or Au- 10nm/glass, in which the formation and implantation of nanoparticles begins with the
first laser pulse. Less heating of the residual gold film and the adjacent glass surface in sample Au-70nm/glass should lead to a decrease in the thermal diffusion of atoms and, consequently, to a difficult penetration of Sn, Mg, Na atoms into the gold film and, as a result, into the volume of the forming gold nanoparticles. Formation of gold nanoparticles from the initial gold films of different thicknesses is schematically illustrated in Fig. 9(aec). As was revealed in Ref. [22] for the sample Au-70nm/glass, the shell of Au NPs consists of Au atoms only and hence, one can conclude that Mg, Na and Sn atoms, probably in oxidized state which prevents their penetration into the gold particles, are dominantly located on the surface of particle as its outer coverage. In contrast, for the samples Au-6nm/glass and Au-10nm/glass, EXAFS derived parameters of AueSn and AueMg,Na bonds of Tables 3 and 4 show that Sn atoms (due to the above discussed reason and their less oxidizing ability comparing to Mg, Na) are penetrated into the shell of gold particle while Mg, Na-atoms, in oxidized state are adsorbed on the particle's surface, forming the outer coverage of the gold particles (N2 > N3). Due to such a complicated composition of particles shell and coverage, the effective sizes of the obtained nanoparticles in the samples Au6nm/glass_asp and Au- 10nm/glass_asp are larger than in the sample Au-70nm/glass_asp which is confirmed by the results of the estimates performed by XRD and optical extinction spectra. The differences in the gold particles surface structures in the studied “as prepared” samples leaded to the differences in the gold particles obtained after thermal post-treatment of long duration in the samples Au-6nm/glass_T, Au-10nm/glass_T in comparison to Au-70nm/glass_T: - the presence of Sn atoms in the shell of gold nanoparticles in the samples Au-6nm/glass_asp, Au-10nm/glass_asp leaded to the less stable surface of the particles during thermal treatment and consequently e to the dissolution of the particles surface (Tables 3 and 4 e parameters C and N2, N3 are less in samples after thermal post-treatment). As a result, the size of the gold particles after thermal treatment became smaller in agreement with their estimates by optical extinction spectra (Fig. 5, Table 1); - the presence of the outer coverage containing the oxidized Sn, Mg, Na on the surface of “as prepared” gold particles in the sample Au-70nm/glass_asp prevents its dissolution during
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Fig. 9. Schematic illustration of the formation of gold nanoparticles from the initial gold films of different thicknesses: (a) e initial gold film of thickness 70 nm on sample Au-70nm/ glass; (b) e gold films of 6e10 nm on samples Au-6nm/glass, Au-10nm/glass (top part of (b)) in comparison with the gold film of ~6 nm remained on the sample Au-70nm/glass after first laser pulse (bottom part of (b)); (с) e Au nanoparticles in “as prepared” samples obtained after laser irradiation by 10 pulses; (d) e Au nanoparticles in samples after thermal post-treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
thermal treatment and as a result, the size of these particles doesn't change significantly in the sample Au-70nm/glass_T. e According to the above made conclusions, the formation of nanoparticles in the studied samples before and after thermal posttreatment is shown schematically in Fig. 9. In addition, it must be noted that the process of dissolution of the gold nanoparticles shell in the samples Au-6nm/glass_asp and Au-10nm/glass_asp during the thermal post-treatment is different and has its own features. To analyze them, one must consider that the dissolved Au atoms also have their contribution into the function cAu(2)(k). Table 3 shows that Sn atoms in gold nanoparticles of sample Au-6nm/glass_asp partially leave the dissolving shell (N2 decreases) and partially remain on the particles surface (presence of AueSn contributions) in oxidized state, which protect the nanoparticles from further dissolution. The significant increase of parameter N1 after thermal post-treatment enables to conclude that Sn atoms leave the shell of nanoparticles and the remaining Au atoms of the shell have a significant contribution into cAu(2)(k) after thermal post-treatment comparing to the contribution of the dissolved Au atoms (N1 is quite large). In the case of gold nanoparticles in the sample Au-10nm/glass_asp, the parameter N1 is almost unchanged (Table 4), whereas it should increase if Sn atoms leave this shell. Therefore, it is reasonable to assume that the Au atoms dissolved during thermal post-treatment of sample Au-10nm/ glass_asp have significant contribution to the function cAu(2)(k). These atoms are probably re-deposited into more smaller clusters of gold. The discussed differences in the shell of gold nanoparticles in the samples Au-6nm/glass_T and Au-10nm/glass_T explain the differences in the A:B ratios in FT of Au L3-EXAFS of these samples before and after T-treatment.
e
e
e
the stable gold core, shell consisting of Au and Sn atoms and an outer coverage of Sn, Mg, Na oxides gold particles formed by laser irradiation of sample, initially coated with gold film of thickness ~70 nm, had a stable gold core, shell consisting of Au atoms only and an outer coverage of Sn, Mg, Na oxides most probably, the origin of such difference in composition of shell of the obtained gold particles (presence or absence of tin atoms in it) depending upon the thickness of the initial gold film coating the surface of glass before laser irradiation, is the difference in the results of the action of the first laser pulse on the gold films of thickness 6e10 nm on the tin-bath side of the glass samples in comparison with its action on the film of thickness 70 nm the revealed differences in the composition and structure of gold particles shell in the studied “as prepared” samples leaded to the differences in the gold particles obtained after thermal post-treatment of long duration, reflecting in the observed changes of SPR. Thus, the AueSn composition of the gold particles shell in “as prepared” samples, initially coated with gold films of 6e10 nm, makes this shell and the coverage on it less stable to the heating and consequently, leads to a partial destruction of the oxidized metals coverage and substantial dissolution of this AueSn shell during thermal post-treatment, reducing the particles size in corresponding samples, which is reflected in SPR parameters in contrast, homogeneous composition of the gold particles shell in glass sample initially coated with film of thickness 70 nm, prevents dissolution of particles under heating providing stable particles size and hence, the stability of SPR characteristics before and after the thermal treatment, which is confirmed by optical spectra.
4. Conclusions Acknowledgements The performed study of the effects of thermal post-treatment and of thickness of the initial gold film coating silicate glass surface, on SPR characteristics and atomic structure of gold nanoparticles prepared in glass by UV laser irradiation enabled to make the following conclusions: e irradiation of the glass samples coated with gold films of thickness 6e10 nm leaded to formation of gold particles with
The work was supported by Southern Federal University and DFG project (DFG No DU 214/14-1). V.V.S., S. Yu.K. acknowledge €tz RFBR project 18-32-00818 mol_а. Authors would like to thank Go Schuck for the help during XAFS measurements at KMC-2 beamlines at BESSY II (Berlin, Germany), proposal 181-06412, Christian Patzig from Fraunhofer Institute for Microstructure of Materials and Systems IMWS for providing TEM-EDX images, and Igor Leontyev
V.V. Srabionyan et al. / Journal of Alloys and Compounds 803 (2019) 354e363
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