https://doi.org/10.1007/s10854-018-0439-5
Preparation of composite Ag@Au core–shell nanoparticles and their
linear and nonlinear optical properties
A. Sakthisabarimoorthi1 · S. A. Martin Britto Dhas1 · M. Jose1 Received: 28 August 2018 / Accepted: 23 November 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
We herein report the preparation of composite Ag@Au core–shell nanoparticles (NPs) through galvanic replacement reaction and the investigations of linear and nonlinear optical properties. TEM micrographs envisage the formation of core–shell NPs. EDS elemental mapping images demonstrate the homogeneous distribution of the elements and their chemical composition. The powder XRD pattern confirms the formation of highly crystalline composite Ag@Au core–shell NPs. The UV–Vis absorption spectrum shows contribution of Ag and Au characteristic absorption bands and the slight red shift provides the strong evidence for coating of Au shell over the Ag core. Moreover, the linear optical parameters such as the extinction coefficient, reflectance, refractive index, conductivity, susceptibility and polarizability are studied. z-scan analysis reveals the enhanced nonlinear optical properties of the composite Ag@Au core–shell NPs with nonlinear refraction coefficient (n2) being ten times higher than that of Ag NPs and eight times increase in comparison with that of Au NPs.
1 Introduction
Metal nanoparticles (NPs) have long been the focus of nanoscience and technology due to its remarkable opti-cal properties ever since the pioneering work of Faraday’s experimental relations of colloidal metal NPs to light in the nineteenth century [1, 2]. This leads to exploit metal NPs as building blocks for numerous applications such as cataly-sis, chemical and biological sensing, photonics, electron-ics, optoelectronelectron-ics, and so on [3, 4]. There is an increasing interest to discover novel materials for nonlinear optical (NLO) applications, wherein a core–shell NPs with inor-ganic metallic core layered by an inorinor-ganic metallic shell is widely utilized for their superior optical properties [5–7]. Core–shell NPs is a new class of nanostructures having unique and tailored properties, which is entirely different from their single counterparts and they are widely employed in large number of applications [8]. The interesting and unu-sual optical properties of core–shell metal NPs is originating from the surface plasmon resonance (SPR) and local field enhancement of metals leading to their applications in opto-electronic devices. Several investigations have been made
towards the NLO analysis of core–shell architectures such as, Ag@TiO2 [9], Ag@CdS [10], Cu@Ag [6] and SiO2@ Ag [7]. The reports categorically show that the core–shell architectures have enhanced NLO properties than that of single counterparts. Among various core–shell structured metal NPs, Ag@Au core–shell NPs attracted much more research interest not only owing to its unusual optical prop-erties but also due to their several advantages such as, the extinction coefficient of SPR resonance for Ag NPs is nearly four times larger than those of Au NPs for the same particle size, which allowed to develop the improved physiochemi-cal properties of Ag@Au core–shell NPs. Furthermore, the Au NPs is more stable than that of Ag NPs which prevents from oxidation and conserve the properties of Ag NPs [11]. In general, the Ag@Au core–shell NPs are prepared by two ways, seed mediated shell growth and direct synthesis route. However, seed mediated growth involves multistep process and is more time consuming, while direct synthesis route is an one step synthesis technique and form homogeneous shell over the core and avoid the over growth of shell [12]. The core–shell Ag@Au NPs is largely employed in surface enhanced Raman spectroscopy (SERS) [13], colorimetric sensing [14], biosensing [15] and catalysis [16] etc.
We have prepared composite Ag@Au core–shell NPs through simple galvanic replacement reaction. This protocol was first developed by Xia et al. [17] to prepare core–shell structured NPs in an aqueous medium and subsequently * M. Jose
[email protected]; [email protected]
1 Department of Physics, Sacred Heart College (Autonomous),
employed to create hollow interior core–shell nanostruc-tures [12]. This technique is influenced by the electrochemi-cal potential difference among the Ag and Au; wherein Ag and Au serve as cathode and anode respectively, since the reduction potential of Au is much higher than those of Ag. However, to the best of our knowledge, the linear and NLO investigations of Ag@Au core–shell NPs have not yet been studied. In the present investigation, core–shell structured Ag@Au NPs were prepared by simple galvanic replacement reaction and their NLO response was effectively probed by z-scan technique. Interestingly, we found enhancement of nonlinear absorption coefficient (n2) of Ag@Au NPs in
com-parison with bare Ag and Au NPs. The observed reverse sat-urated absorption behavior is attributed to the excited state absorption and multiphoton absorption of the materials. This interesting result could be attributed to the consequence of core–shell structured NPs formation.
2 Experimental
2.1 Materials
The following chemicals were used for the preparation of composite Ag@Au core–shell NPs: hydrogen tetracholoro-auric acid (HAuCl4), silver nitrate (AgNO3), polyvinylpyr-rolidone (PVP, K30), sodium borohydride (NaBH4) and
dou-ble distilled water. All the reagents were an analytical grade and used as received without further purifications.
2.2 Synthesis of composite Ag@Au core–shell NPs The Ag colloidal NPs were synthesized by the typical route as follows; 10 mL of 0.25 mM PVP aqueous solution was brought to 50 mL of 6 mM AgNO3 aqueous solution under
continuous stirring, the reaction mixture remained colorless, into which 1 mL of 20 mM NaBH4 was slowly added. The
color was turned from colorless to yellow due to the reduc-tion of AgNO3 by NaBH4 that indicates the formation of
Ag NPs. After 30 min continuous stirring, the Ag NPs was harvested from the colloidal medium by the centrifugation process and washed several times with acetone and water for further uses.
The core–shell structured Ag@Au NPs were successfully synthesized according to the preparation route reported in literatures [17, 18]. Briefly, 5 mL of 5 mM HAuCl4 aqueous solution was gradually injected into 10 mL of as synthesized Ag NPs solution. The color of the solution changed from yellow into blue then bluish red due to the formation of Au NPs over the Ag NPs surface, which is further evidenced from the UV–Vis optical investigations.
The Au colloidal NPs were synthesized by the following route, 10 mL of 0.25 mM PVP was added into 20 mL of
5 mM HAuCl4 aqueous solution under vigorous stirring.
After 20 min of stirring, few mL of 20 mM NaBH4 was added drop wise and stirring continued for 1 h, the addition of NaBH4 giving rise to a red color Au colloidal NPs. The
prepared Au colloidal NPs were stable under normal envi-ronmental condition for a month.
2.3 Mechanism of core–shell structured Ag@Au NPs formation
The core–shell structured Ag@Au NPs were prepared by galvanic replacement reaction between Ag NPs and HAuCl4. Initially, the Ag NPs was prepared by simple chemical reduction method, which involves reduction of metal salts of AgNO3 by a strong reductant NaBH4 as represented in rela-tion (1). Then, the gold shell was developed over the silver core via galvanic replacement reaction, since the standard reduction potential of Au is higher than those of Ag (i.e., 0.99 V for Au and 0.80 V for Ag vs. standard hydrogen elec-trode (SHE)] [13]. After the addition of HAuCl4 into the Ag NPs solution, the replacement reaction has commenced, the gold atoms are evenly generated and deposited on the outer surface of the Ag NPs based on the relation (2). The overall chemical interaction among them and core–shell formation is represented in following relations. Every three nulvalent of Ag0 atom sacrificed their electrons for the reduction of
Au3+ ions and produce one Au0 atom and turn into Ag+
ions. This process is continued as long as Ag+ ions are
per-meating and electrons are transfer through the shell and this lead to form the uniform Au layer over the surface of Ag NPs that is further visualized from TEM micrographs. The graphical illustration for the formation of composite Ag@ Au core–shell NPs is shown in Fig. 1.
2.4 Characterization
The optical properties of the as prepared Ag@Au core–shell colloidal NPs in 1 cm path length suprasil quartz cuvette was studied by Cary Varian 50 Ls UV–Vis spectrometer in the wavelength range of 200–800 nm at ambient condition. The structural characterization of the product was investigated by Rigaku Miniflex-II X-ray diffractometer with 2θ ranging from 20° to 90° using Cu Kα radiation. The morphology and elemental mapping of the product were captured by TEM micrographs using Tecni G2 (TF-20) Transmission electron microscope operated at the acceleration voltage of 200 kV and energy dispersive X-ray spectrometry (EDS) using Quanta 200 FEG Scanning electron microscope, (1) AgNO3+ NaBH4→ Ag +1∕2H2+ 1∕2B2H6+ NaNO3
(2) 3Ag0+ AuCl4−→ Au
0
respectively. The NLO property of the product was exam-ined by z-scan technique using continuous wave He–Ne laser at the excitation wavelength of 632.8 nm.
3 Results and discussion
3.1 Morphological properties of Ag@Au core–shell NPs
The TEM images and corresponding SAED pattern of the core–shell structured Ag@Au NPs prepared via gal-vanic replacement reaction is presented in Fig. 2a–d. It
is clearly observable from these images that the particles consist of several fine core–shell structured Ag@Au NPs. Though, few of them are slightly agglomerated which may be due to the shell formed over the core in the absence of any stabilizer, the particles are generally well dispersed in the medium in poly-dispersed manner. Furthermore, such particles are highly packed as core–shell structures with an average particle size of around 20 nm with the shell thickness of about 4 nm. This is visualized from the TEM image of single core–shell structured Ag@Au NPs wherein, the distinct boundary among the core–shell NPs is clearly visible (Fig. 2c).
Fig. 1 Graphical illustration for the formation of composite Ag@Au core–shell NPs
Fig. 2 a–d TEM images and
corresponding SAED pattern of Ag@Au core–shell NPs
The selected area electron diffraction pattern (SAED) taken from the Ag@Au core–shell NPs exhibits the Debye–Scherrer spots corresponding to FCC structure of the product (Fig. 2d). The spots diffraction pattern suggests the single crystalline nature of the product. The interplanar spacing estimated from this pattern is 2.42, 2.11, 1.52, 1.25 and 1.18 Å corresponding to their Bragg’s diffraction peaks (111), (200), (220), (311) and (222), respectively which are consistent with the JCPDF standard value of Au NPs (04-0784). The TEM micrographs evidenced that the each Ag NPs are preferentially coated by Au NPs and formed as core–shell structured Ag@Au NPs which is further substan-tiated by the UV–Vis and powder XRD results.
Moreover, it is hard to interpret the distribution of ele-ments in complex structures like core–shell structure by the analysis of TEM micrographs. Thus the prepared composite Ag@Au NPs are subjected to the EDS elemental mapping analysis (Fig. 3a–e). As seen from Fig. 3b, the distribution of Ag@Au core–shell NPs are accompanied by merely Ag and Au elements without any other chemical species and they are distributed homogeneously, which revealed the purity of the synthesized product is good. Furthermore, it is observed from the mapping images of Au and Ag elements that the Au elements are largely distributed in comparison with Ag elements which suggest the surface of the product is mainly composed of Au elements indicating the formation
Fig. 3 a–e EDS elemental
of Ag@Au core–shell like NPs (Fig. 3c, d). The EDS line scan spectrum (Fig. 3e) also shows high intensity signals for Au than that of Ag. Moreover, the corresponding weight and atomic percentages of Au and Ag are (59.06% and 72.48%) and (40.94% and 27.52%) respectively which is in agreement with the EDS dot mapping results.
3.2 Structural properties of Ag@Au core–shell NPs The crystallinity and core–shell structure of the product was further identified by powder XRD analysis (Fig. 4). The sharp and strong intensity peaks located at the Bragg’s angles at 38.16°, 44.44°, 64.43° and 77.47° correspond to the crystalline planes (111), (200), (220) and (311) respectively. It represents the formation of highly crystalline Ag@Au core–shell NPs. The predominant reflection plane (111) rep-resents the particles are preferentially orientated along (111) growth plane. As can be seen from this diffraction pattern, only one set of reflection planes are obtained for both Ag and Au composition, which is due to the similar lattice constant among them. In general, the Ag and Au are face-centered cubic (FCC) structures and have extremely similar lattice constants (4.086 Å and 4.078 Å for Ag and Au respectively, JCPDF standard: 04-0783 and 04-0784), consequently, the powder XRD pattern gives only one set of reflection peaks. This similar lattice match among them plays a key role in attaining uniform Au coating over the Ag core [19].
3.3 Optical properties of Ag@Au core–shell NPs One of the major advantages of using core–shell metal NPs in NLO applications is their tunable surface plasmon prop-erty, wherein by changing either or both the core and shell thickness, the surface plasmon property can be varied. SPR
of metallic NPs is a collective oscillation of conduction elec-trons over the metal NPs surface, which is excited by an incident electromagnetic radiation. The optical absorption spectra of Ag NPs, Au NPs and core–shell structured Ag@ Au NPs are represented in Fig. 5. The Ag NPs has displayed the characteristic SPR band at 403 nm that is well agreed with the reported literatures [20, 21]. Likewise, the Au NPs has exhibited the characteristic absorption band at 510 nm, which is also in good resemblance to the reported values [22, 23]. The core–shell structured Ag@Au NPs exhibits an intense absorption band at 523 nm that is characteristic band of Au NPs. Moreover, the small hump located at 400 nm is characteristic band of Ag NPs which exemplifies the forma-tion of Au shell over the Ag core with very small thickness. However, the absorption band of Ag@Au core–shell NPs is slightly red shifted as compared with the bare Au NPs, which may be attributed to the formation of core–shell struc-tured NPs. Since, the SPR bands are consequence of parti-cle size, shape and core to shell ratio, even a slight change may also result in an apparent spectral changes. The sym-metric nature of the peak suggests that spherical particles are formed into nearly uniform particles size. These SPR variations in the spectrum strongly suggest the formation of core–shell structured Ag@Au NPs, which is validated from the TEM micrographs.
3.3.1 Linear optical properties
The linear optical properties such as, extinction coefficient, reflectance, refractive index, conductivity, susceptibility and polarizability of the Ag NPs, Au NPs and composite Ag@ Au core–shell NPs obtained from the following empirical relations are represented in Fig. 6a–f. The extinction coeffi-cient (k) of the materials was analysed using the expression,
where λ is the wavelength and α is the absorption coefficient that is gained by the relation,
where T is the transmittance and d is the thickness of the sample. The extinction coefficient (k) is referred as, the amount of absorption loss during the propagation of electro-magnetic radiation through the material, which is presented in Fig. 6a. As can be seen from the figure, the variation of k (3) k= 𝜆𝛼 4𝜋 (4) 𝛼= 2.303 log(1∕T ) d
plotted against photon energy reveals that the k value gradu-ally increases till the photon energy of 3 eV and static up to 5.5 eV, then increases in higher energies. This variation in k values as a function of photon energy may be owing to the weak interaction among the photons and electrons. It can be observed that the k value of Ag@Au core–shell NPs is higher than the Ag NPs, which may due to the formation of core–shell NPs. Furthermore, this behaviour may lead to change in nonlinear absorption cross-section of the mate-rial [24].
The refractive index (n) of the material is derived by the equation, (5) n= � (−R + 1) ±√(−3R2+ 10R − 3)� (2(R − 1))
where R is the reflectance which is obtained as a function of absorption coefficient (α),
Figure 6b represents the change of reflectance (i.e., the fraction of light reflected from the sample) against photon energy. The reflectance of the samples demonstrate a gradual increase with increase in photon energy showing maximum values at higher frequencies, however, the composite Ag@ Au core–shell NPs exhibit low reflectance in comparison with Ag NPs and Au NPs. The rapid decrease and increase in reflectance of the material is due to the excitation of col-lective plasma oscillations of the valence electrons and tran-sitions among filled d-bands below the valence band and empty conduction band states, respectively [25]. It can be observed that the refractive index (Fig. 6c) shows the anoma-lous dispersion at lower photon energy region (< 3.5 eV) and normal dispersion at higher photon energy region (> 3.5 eV) and this dispersion nature of the material is a significant factor in optical communications [26]. Besides, the refrac-tive index gradually increases towards higher photon energy region due to the increase of optical density in UV and vis-ible region [27]. The refractive index of Ag NPs and Au NPs ranges from 0.03 to 0.84 and 0.01 to 0.80 respectively in the photon energy of 1.55–6.24 eV which are closer to the values reported in literature; however it is found to be lesser than that of bulk Ag and Au (i.e., for bulk Ag, it is 0.24–1.07 and for bulk Au, it is 0.92–1.28 in the photon energy from 0.64 to 6.60 eV) [28]. The estimated low refractive index of the material is more suitable for NLO applications, espe-cially in integrated optical devices.
The optical conductivity (σoc) of the material is calculated using the relation,
where c is the velocity of light. The optical conductivity shows an increase in conduction as a function of photon energy (Fig. 6d) that indicates good optical response of the material. In addition, the optical conductivity is anomalously increased to the higher photon energy region and massively decreased in the lower photon energy region, which might be due to the availability of free carriers of photon energy and trapping of free carriers respectively. The high magnitude of optical conductivity (109 s−1) further witnesses the high
photo response of the material [29].
Moreover, the optical susceptibility (χop) and optical polarization (p) are calculated using the relations,
(6) R= 1±√(1 − exp(−𝛼d) + exp(𝛼d) 1+ exp(−𝛼d) (7) 𝜎oc= 𝛼nc 4𝜋 (8) 𝜒op= n2− k2− 𝜀 0 4𝜋
where ε0 is the vacuum permittivity and h is the Planck’s
constant. The polarizability shows very low value (10−36)
that may lead to change in refractive index value of the mate-rial. The low magnitude of extinction coefficient (10−6) and
high optical conductivity (109 s−1) evidences the surface
homogeneity of the particles and high photo response of the material respectively, that is more suitable for the appli-cations in computing and information processing [29]. The composite Ag@Au core–shell NPs show improved values of extinction coefficient, reflectance, refractive index, conduc-tivity, susceptibility and polarizability in comparison with Ag NPs, which may due to the formation of core–shell NPs. 3.4 Nonlinear optical properties of Ag@Au core–
shell NPs
The Ag@Au core–shell NPs is further subjected to z-scan analysis for probing their NLO properties, since they are suitable candidates for NLO devices. To date, a lot of efforts have been devoted to develop novel NLO materials for spatial light modulators and optical limiters based on the mechanisms of nonlinear refraction, nonlinear scatter-ing, saturated absorption, reverse saturated absorption and multiphoton absorption. The size, shape and dielectric func-tion dependant SPR of metallic NPs further enhances NLO properties [22, 30, 31].
Ag NPs, Au NPs and the core–shell structured Ag@Au NPs are investigated for the determination of the NLO prop-erties employing z-scan technique using He–Ne laser with both closed aperture and open aperture profiles (Fig. 7a–f). The closed aperture profile was performed to find out the nonlinear refraction coefficient (n2) of the material. The
results show the analogous trend for both Ag NPs and Ag@ Au core–shell NPs, with the peak followed by valley while the reverse is found for Au NPs. In general, the peak fol-lowed by valley represents the negative nonlinear refrac-tion coefficient behavior that involves self-defocusing effect (n2 < 0), while the valley followed by peak represents the positive nonlinear refraction coefficient behavior that indi-cates self-focusing effect (n2 > 0). Furthermore, the
self-defocusing effect of the Ag@Au core–shell NPs mainly caused by the thermal nonlinearities, wherein high single photon absorption cross sections possibly responsible for this phenomenon [32]. Interestingly, the nonlinear refraction coefficient of Ag@Au core–shell NPs is ten times higher than that of Ag NPs and eight times higher in comparison with the Au NPs, which could be attributed to the enhanced (9)
p= 𝜀0hc𝜒op
SPR due to increase in the molecular density of the mate-rial [9].
The open aperture profile was executed to determine the absorption coefficient (β) of the material. It is observed from the figure; all the three samples exhibit the symmetrical val-ley shape transmittance curves, which represent the reverse saturated absorption behavior of the material. In general, reverse saturated absorption behavior arises from the free carrier absorption such as, excited state absorption and enhanced multiphoton absorption [33]. This reverse satu-rated absorption behavior of the material is more appropriate in spatial light modulators of photonic device applications.
The NLO characteristic parameters of Ag NPs, Au NPs and Ag@Au NPs were estimated according to the empirical rela-tions derived by Shiek-Bahae et al. [34] and listed in Table 1. The results show much better NLO properties of Ag@Au NPs than the values reported for Ag and Au NPs [35, 36] which could be ideal for number of optoelectronic and NLO applications.
4 Conclusions
In summary, the Au coating over the Ag core was carried out via an effective galvanic replacement reaction. The forma-tion of core–shell structured composite Ag@Au NPs was recognized by TEM micrographs with the average particle size of 20 nm and their core and shell thickness are around 16 and 4 nm, respectively. The increased Au signal in EDS elemental mapping clearly demonstrated that the surface of the formed product mainly consisted of Au with homogene-ous distribution. The similar lattice match among the Ag and Au facilitates the formation of highly crystalline Ag@Au core–shell NPs that is evidenced by the powder XRD analy-sis. The linear optical investigations illustrate the improved optical behaviour of Ag@Au core–shell NPs than those of Ag NPs. The Ag@Au core–shell NPs exhibit the enhanced NLO properties and the enhancement of nonlinear refractive index is ten and eight times higher than that of bare Ag and Au NPs respectively, which could be useful for multiple pho-tonic applications including, optical limiting devices, optical telecommunications, optical data storage and information processing.
Acknowledgements The author (Dr. M. Jose) thank the Department of Atomic Energy-Board of Research in Nuclear Sciences (DAE-BRNS), Government of India (Sanction No: 34/14/54/2014- BRNS) for pro-viding Impedance spectroscopy analysis facility in the Department of Physics, Sacred Heart College (Autonomous), Tirupattur, India. The authors also gratefully acknowledge Prof. D. Sastikumar, Department of Physics, National Institute of Technology, Trichy for providing the z-scan facility.
References
1. K. Lance Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107, 668 (2003)
2. H. Karimi-Maleh, M. Hatami, R. Moradi, M.A. Khalilzadeh, S. Amiri, H. Sadeghifar, Microchim. Acta 183, 2957 (2016) 3. P. Dauthal, M. Mukhopadhyay, Ind. Eng. Chem. Res. 55(36), 9557
(2016)
4. H.M. Chen, R.-S. Liu, J. Phys. Chem. C 115, 3513 (2011) 5. E. Kirubha, P.K. Palanisamy, Adv. Nat. Sci. Nanosci.
Nanotech-nol. 5, 045006 (2014)
6. A. Sakthisabarimoorthi, M. Jose, S.A. Martin Britto Dhas, S. Jerome Das, J. Mater. Sci. Mater. Electron. 28, 4545 (2016) 7. A. Sakthisabarimoorthi, S.A. Martin Britto Dhas, M. Jose, Mater.
Sci. Semicond. Process. 71, 69 (2017)
8. R. Ghosh Chaudhuri, S. Paria, Chem. Rev. 112, 2373 (2012) 9. M. Karimipour, M. Ebrahimi, Z. Abafat, M. Molaei, Opt. Mater.
57, 257 (2016)
10. X. Fu, Z. Wu, M. Lei, L. Zhang, H. Chen, W. Tang, Z. Peng, Mater. Lett. 104, 76 (2013)
11. D. Tang, R. Yuan, Y. Chai, Biotechnol. Bioeng. 94, 996 (2006) 12. D.M. Mott, D.T.N. Anh, P. Singh, C. Shankar, S. Maenosono,
Adv. Colloid Interface Sci. 185–186, 14 (2012)
13. Y. Cui, B. Ren, J.-L. Yao, R.-A. Gu, Z.-Q. Tian, J. Phys. Chem. B
110, 4002 (2006)
14. Y. Li, Q. Wang, X. Zhou, C.-y. Wen, J. Yu, X. Han, Z.-f. Yan, J. Zeng, Sens. Actuators B 228, 366 (2016)
15. T. Ghodselahi, S. Arsalani, T. Neishaboorynejad, Appl. Surf. Sci.
301, 230 (2014)
16. D.A. Ferrer, L.A. Diaz-Torres, S. Wu, M. Jose-Yacaman, Catal. Today 147, 211 (2009)
17. Y. Sun, B. Mayers, Y. Xia, Nano Lett. 2, 481 (2002)
18. J. Yang, J.Y. Lee, H.-P. Too, J. Phys. Chem. B 109, 19208 (2005) 19. Y. Wu, P. Jiang, M. Jiang, T.-W. Wang, C.-F. Guo, S.-S. Xie, Z.-L.
Wang, Nanotechnology 20, 305602 (2009) 20. M. Jose, M. Sakthivel, Mater. Lett. 117, 78 (2014)
21. M. Jose, R. Thanal, J. Prince Joshua, S.A. Martin Britto Dhas, S. Jerome Dhas, Superlatt. Microstruct. 89, 68 (2016)
22. E. Xenogiannopoulou, K. Iliopoulos, S. Couris, T. Karakouz, A. Vaskevich, I. Rubinstein, Adv. Funct. Mater. 18, 1281 (2008) 23. R. Philip, P. Chantharasupawong, H. Qian, R. Jin, J. Thomas,
Nano Lett. 12, 4661 (2012)
24. F. Naseri, D. Dorranian, Opt. Quantum Electron. 49, 4 (2017) 25. M. Bass, E.W. Van Stryland, D.R. Williams, W.L. Wolfe,
Hand-book of Optics, 2nd edn. (McGraw-Hill, Inc., USA, 1995)
26. D. Dorranian, Y. Golian, A. Hojabri, J. Theor. Appl. Phys. 6, 1 (2012)
27. S.W. Xue, X.T. Zu, W.L. Zhou, H.X. Deng, X. Xiang, L. Zhang, H. Deng, J. Alloys Compd. 448, 21 (2008)
28. P.B. Johnson, R.W. Christy, Phys. Rev. B 6, 4370 (1972) 29. N.R. Dhineshbabu, V. Rajendran, N. Nithyavathy, R.
Vetumperu-mal, Appl. Nanosci. 6, 933 (2016)
30. L. Polavarapu, N. Venkatram, W. Ji, Q.-H. Xu, ACS Appl. Mater. Interfaces 1(10), 2298 (2009)
31. A. Sakthisabarimoorthi, S.A. Martin Britto Dhas, M. Jose, Mater. Chem. Phys. 212, 224 (2018)
32. M.S. Neo, N. Venkatram, G.S. Li, W.S. Chin, W. Ji, J. Phys. Chem. C 114, 18037 (2010)
33. Y.H. Lee, Y. Yan, L. Polavarapu, Q.-H. Xu, Appl. Phys. Lett. 95, 023105 (2009)
34. M. Sheik-Bahae, A.A. Said, T.-h. Wei, D.J. Hagan, E.W. Van Stry-land, IEEE J. Quantum Electron. 26(4), 760 (1990)
35. M.H. Majles Ara, Z. Dehghani, R. Sahraei, A. Daneshfar, Z. Javadi, F. Divsar, J. Quant. Spectrosc. Radiat. Transf. 113, 366 (2012)
36. M.H. Majles Ara, Z. Javadi, R.S. Sirohi, Optik 122, 1961 (2011)
Table 1 Estimated NLO parameter values of the prepared Ag@Au core–shell NPs Sample Refractive index
(n2) × 10−6 cm2/W
Absorption coefficient
(β) × 103 cm/W Real part of susceptibility (Re χ3) × 10−11 esu ity (Im χImaginary part of susceptibil-3) × 10−4 esu Susceptibility (χ3) × 10−4 esu
Ag NPs 8.10 0.88 0.503 3.404 3.404
Au NPs 10.5 0.72 1.490 4.906 4.906
