Dependence of nonlinear optical properties of Ag2S@ZnS core-shells on Zinc precursor and capping agent M. Dehghanipour, M. Khanzadeh*, M. Karimipour and M. Molaei Department of Physics, Faculty of Sciences, Vali-e-Asr University of Rafsanjan , 77188-97111 Rafsanjan, Iran.
Abstract: In this research, four different types of Ag2S@ZnS core-shells were synthesized and their nonlinear optical (NLO) properties were investigated using a Z-scan technique by a 532 nm laser diode. Here, Ag2S and ZnS nanoparticles were also synthesized and their NLO properties were compared with Ag2S@ZnS core-shells. It was observed that the NLO properties of Ag2S@ZnS quantum dots significantly increased by increasing the values of Zn(NO3)2 and thioglycolic acid (TGA). It was also observed that the NLO properties of Ag2S@ZnS core-shells for 0.1 g of Zn(NO3)2 and 7000 µl TGA is higher than sole Ag2S and ZnS nanoparticles. In open aperture Z-scan curve of ZnS sample, a saturable absorption peak was observed and this peak was seen also in type of Ag2S@ZnS nanoparticles which the value of Zn(NO3)2 much more. 1. Introduction Today, various materials as core-shells structure are synthesized and their linear optical properties are studied. For example Ag@SnO2 [1], CuInS2/ZnS [2], CdS@TiO2 and ZnS@TiO2 [3], Ag@TiO2 [4], CdS@ZnSe [5], Au-AgCdS [6], and Au@Ag [7]. But the nonlinear optical (NLO) properties of a few of these materials have been investigated [8,9]. In recent years, the nonlinear optics and nonlinear optical properties have attracted great attentions for many researchers [10-14]. In many studies, researchers are looking for ways to increase the NLO properties of materials. For example, in some studies to improve the NLO properties of graphene oxide which have many applications in today's world, core-shell structures such as Ag@TiO2 and Pt@TiO2 were combined with this matter and observed that the NLO properties of graphene
*
Corresponding author. E-mail address:
[email protected] (M. Khanzadeh).
oxide in combination with these structures substantially increases [15,16]. Our previous study also showed that the NLO properties of graphene oxide could be increased using simply mixing with Ag2S@ZnS quantum dots and it was observed that with increase of sonication time, the NLO properties of the mixture increased accordingly [17]. According to the description given, if at first as far as possible be the NLO properties of a core-shell structure increased and also the effective ways to increase these properties studied. Then core-shell structure can be used optimally to increase the NLO properties of a material such as graphene oxide. In this study, the NLO properties of Ag2S@ZnS core-shells structure using a Z-scan method by continuous laser diode with 532 nm wavelength were evaluated and compared with Ag2S and ZnS nanoparticles (NPs) in addition to that, the factors affecting in the NLO properties of Ag2S@ZnS core-shells were fully studied. 2. Synthesis 2.1. Synthesis of zinc sulfide (ZnS) and Silver sulfide (Ag2S) First 0.05 g of Zinc nitrate Zn(NO3)2 was dissolved in 50 ml of deionized water then a 1M thioglycolic acid (TGA) (0.5 ml) was added as capping agent to this solution as it was stirred and then ammonia was added for transparency. Then separately 0.64 g of thiosulfate as a sulfur source (Na2S2O3) was dissolved in 50 ml of deionized water and was added to first solution and the resulting solution was stirred for 15 minutes. The prepared transparent solution was exposed to microwave irradiation at 450 W for 5 min. For the synthesis of Ag2S, 0.1 g silver nitrate was dissolved in 50 ml DI water and a separately 50 ml solution containing 0.64 g Na 2S2O3 predissolved was added to it and this solution stirred for 10 min and exposed to microwave irradiation for 12 min. 2.2. Preparation of Ag2S@ZnS core-shells Ag2S@ZnS core-shells were prepared using a previously reported approach [18]. First 0.1 g of silver nitrate was dissolved in 50 ml of DI water, then different values of 7000 µl TGA were added to the solution and ammonia was used for pH adjustment to 7.5 and solution was stirred for 15 minutes subsequently, 0.64 g thiosulfate as a sulfur source was dissolved in 50 ml of DI water and was added to silver nitrate solution and then exposed to microwave irradiation for 2 min for the nucleation of silver sulfide cores. Also 0.05 g and 0.1 g Zinc acetate hexahydrate was
dissolved in 50 ml of deionized water and was added to the solution of silver sulfide and samples were named S1 and S2, respectively. Finally, the solution was placed in microwave with the power of 450 for 18 minutes. After completion of the reaction, solution was precipitated and centrifuged. The resulting nanoparticles were dried in a vacuum oven for 24 hours. The same procedure was re-synthesis, just 4000 µl TGA as capping agent was added to the solution and samples were named S3 and S4, respectively. 2.3. Characterizations ofAg2S@ZnS core-shells Optical absorption and photoluminescence (PL) measurements were carried out using an Avantes spectrometer (AvaSpec-2048 TEC). X-ray diffraction (XRD) was conducted on the centrifuged and extracted particles using an Advanced d8-Bruker system. Transmission Electron Microscope (TEM) images were recorded using a JEOL 2010 system. Scanning electron microscope (SEM) images have been obtained using LEO 1450 VP system. 3. Z-scan measurement In this research to measure the NLO properties of samples, Z-scan technique was employed. Samples with a concentration of
1mg 3ml
were prepared and poured into a quartz cell with a
thickness of 1 mm. Here, a continuous 532 nm laser diode as the light source used which its beam was focused on to the samples through a 10 mm focal length lens and the respective beam waist size at focus by a knife-edge diffraction experiment to be 10 µm at power of 29 mW and then the quartz cell with an accuracy of 10 µm was shifted by a stepper motor axis along the propagation axis of focused laser beam while its transmission was recorded at each position. The layout of close aperture (CA) and open aperture (OA) Z-scan experimental apparatus are the same as that used in our previous research (Fig. 1) [17]. With these two methods, the nonlinear refractive (NLR) index and nonlinear absorption (NLA) coefficient are obtained. The Sheik-Bahae model’s is used [19] to analyze the curves obtained from CA Z-scan measurements of samples, as follow: TN (z, ∆φ0 ) = 1 −
(x 2
4x∆φ0 + 9)(x 2 + 1)
(1)
Where, TN is the normalized transmittance for the CA/OA curve, x = z⁄𝑧0 is the normalized distance from the focus, ∆φ0 is the on axis nonlinear phase shift at the focus, and z0 is the Rayleigh length. The NLR index (n2 ) can be measured given by: n2 =
∆φ0 KI0 Leff
Where K = Leff =
(2)
2𝜋
is the wave vector, λ is the laser wavelength, I0 is the laser intensity at focus,
𝜆
1−e−α0 L α0
is the effective length of the sample (L denotes its thickness), and α0 is the linear
absorption coefficient. In order to compute the NLA coefficient, OA Z-scan results with flip of saturable absorption around the focal are analyzed by a model related to both saturable absorption (SA) and reverse saturable absorption (RSA) which yielding the total absorption coefficient as [17]: α0 α(I) = + βI (3) 1 + I⁄I s Where I is laser radiation intensity, Is is saturation intensity, and β is positive NLA coefficient. The normalized transmittance for the OA curve (TOA ) using Eq. 3 can be written as: +∞
TOA
Q(z) ∫ Ln [1 + q(z)exp(−τ2 )] dτ = ⁄ √πq(z) −∞
Where Q(z) = exp (
(4)
α0 LI βI L ⁄I + I ) and q(z) = 0 eff . z 2 ⁄ s (1 + (z ) ) 0
It is worth noting, that OA Z-scan curves which shows pure RSA behavior, α(I) is expressed as: α(I) = α0 + βI
(5)
And the TOA is given by [20]: ∞
TOA = ∑ m=0
[−
βI0Leff m z2
1+ 2 z
]
0
(m + 1)3/2
(6)
Finally, as regards NLR index (n2 ) and NLA coefficient (β) are measured at the same intensity, the absolute value of the third-order susceptibility (χ(3)) can be obtained given by following equation [21]: 2
2 1 ⁄2
|χ(3) | = [(Re(χ(3) )) + (Im(χ(3) )) ] Where Re(χ(3) ) =
(7)
10−4 ε0 c 2 n20 n2⁄ 10−2 ε0 c 2 n20 λβ⁄ (3) (3) π is the real part of χ , and Im(χ ) = 4π2
is imaginary part of χ(3). ε0 is the permittivity of free space, n0 is the linear refractive index of the medium, and c is the velocity of light in vacuum. 4. Results and discussion As seen in Fig. 2-a, monoclinic structure of Ag2S with characteristic planes (111), (-112), (-121), (-103), (031), (200), (113), (-212), and (-213) in the XRD spectra specified. The peak observed at 2θ = 49.47° and 2θ = 26.28° belongs to (220) and (210) the hexagonal phase of ZnS. With increasing TGA from 2500 µl up to 7000 µl as can be seen the peaks undergo a broadening due to the shrinkage of particles size such that an amorphous background is appeared and overshadows the ZnS diffraction pattern as well such that it is not observable. On the whole, with the increase of TGA the estimated size of particles changes from 12 nm up to 3.5 nm thus for the comparison of shell growth, samples with different Zinc acetate content of 0.05 and 0.1 g using 2500 µL were prepared (Fig.2-right) showing the clear growth of ZnS Bragg reflections and a naive broadening of Ag2S peaks pattern [18]. Transmission electron microscopy images were obtained to investigate the microstructure of nanoparticles. Fig. 3-a &b shows the effect of TGA increase on the size of nanoparticles which is 15 nm for the case of 2500 µL and 4-5 nm for the case of 5000 µL TGA which is in good agreement with XRD analyses [18]. Fig.3-c&d shows the effect of Zinc acetate on the formation of ZnS shell and the shrinkage of particles that with the increase of Zinc precursor, the size of core decreases and the shell could be observed (Fig.3-d inset).
Linear optical properties of the samples examined using UV-Vis spectroscopy. In Fig. 4 the absorption spectra of the samples Ag2S and ZnS are shown. Fig. 5 also shows the absorption spectrum of the synthesized core-shell structures that the absorption edge of the Ag 2S quantum dots and the exciton recombination peak at 810 nm wavelength also is observable. At first, in order to remove the contribution of solvent on the nonlinear response of the samples, deionized water is being tested under the same conditions. No signal was observed in Z-scan curves of solvent. The results of the Z-scan experiment for Ag2S and ZnS samples are shown in Fig. 6. In OA Z-scan curve of Ag2S nanoparticles (Fig. 6b) a valley related to RSA behavior is observed which is ascribed to excited state absorption (ESA). Actually in Ag2S nanoparticles at first electron is excited from valance band (VB) to conduction band (CB) and then through a second photon the excited electron is induced from the lower CB to the higher CB states. In OA Z-scan curve of ZnS nanoparticles (Fig. 6d) a peak is initially observed and then a shallow valley related to SA behavior which is imputed to the ground state bleaching. In detail, when the laser beam is absorbed by an absorbing medium, if the absorption cross section of the excited state is smaller than that of the ground state, the transmission of the system will be increased when the system is highly excited, this process is called SA. Also, according to Fig. 6a and Fig. 6c peak-valley difference in CA Z-scan curve of Ag2S nanoparticles is higher than ZnS sample which actually show the NLR index of Ag2S is larger than ZnS nanoparticles (Table 1). In the following, the NLO properties of core-shell structures meaning samples S1, S2, S3 and S4 were also studied and the experimental data of Z-scan measurements were represented in Figs .7 and 8. As it is shown in Fig. 7, the highest and lowest peak-valley difference related to S2 and S3, respectively. Also the results obtained from fitting the Z-scan curves of samples (Table 1) show that the highest and lowest values of NLR index related to S2 and S3, respectively. In OA Z-scan curves of samples (Fig. 8) the deepest valley belongs to S2 sample. Due to the OA Z-scan curves form of Ag2S and S1 samples, it is clear that the NLO response of these samples is pure RSA, while in other samples, a combination of SA and RSA processes participate in nonlinear optical response. Also the CA Z-scan curves of all samples exhibit a peak-valley signature, indicating the self-defocusing effect. This effect is related to thermal phenomena resulting from absorption of a tightly focused CW laser beam propagating through an absorbing medium and causes a spatial distribution of temperature in the sample solution. It is noteworthy to indicate that the value of NLA coefficient and NLR index of S1 sample are approximately 10 and 2 times
more than sole Ag2S, respectively. Also the NLA coefficient of S1 and S4 compared with pure Ag2S about 130% and 225% increased, respectively. According to the results listed in Table 2, third-order susceptibility of S2 nanoparticles compared with Ag2S increased approximately 230%. As it is seen in Fig. 5, for samples with the same TGA value the linear absorption (α0 ) in visible region is increased with the increase of Zn(NO 3)2 value and consequently this phenomenon enhances the NLO properties of Ag 2S@ZnS core-shells. For this reason the NLO properties of S2 were obtained higher than S1 and also the NLO properties of S4 were achieved higher than S3. As it is shown in Fig. 8, when the value of Zn(NO3)2 in Ag2S@ZnS core-shell is increased in OA Z-scan curve of structure a peak before valley is appeared which is related to SA behavior. This phenomenon seems quite logical because in OA Z-scan curve of ZnS nanoparticles (Fig. 6d) a peak was observed. In the other words, with increase the value of Zinc precursor, SA behavior in Ag2S@ZnS core-shells increased. The results of curves fitting (Table 1) show that the value of TGA in Ag2S@ZnS core-shell also influences on its NLO properties and this causes the NLO properties of samples S1 and S2 are higher than S3 and S4. As previously was described and shown in Fig. 1 with the increase of TGA the size of core decreases and leads to better shell growth around Ag 2S cores [18]. The effect of the quality of core-shell structure on nonlinear optical properties previously has been reported by Karimipour et al. [9]. The NLA coefficient and NLR index of these core-shells in this research were obtained (2.29 − 36.9) × 10−5
cm W
and −(0.93 − 9.16) × 10−9
cm2 W
,
respectively which are comparable to the reported NLO coefficients of pure and doped CdSe[11]. Also the NLA coefficient of CdSe-PMMA hybrid nanocomposite and potassium dichromate single crystals (KDC) reported in Refs. 22 and 23 are smaller than the obtained results [22,23]. It is noteworthy to indicate that in our previous study, S2 sample as the best Ag2S@ZnS core-shells structure was mixed with graphene oxide [17]. Fig. 9 shows the suggested mechanism for the increase of nonlinear absorption of Ag 2S@ZnS core-shells. A real TPA occurs from valance band (VB) to conduction band (CB) of ZnS via a virtual state. An excited state absorption (ESA) from CB of Ag2S core to CB of ZnS shell and there is also a RSA mechanism due to the relaxation of electrons in the opposite direction of ESA mechanism. It is interesting to notice that the ESA and RSA mechanisms in Ag 2S@ZnS nanoparticles increased with increase the values of Zn(NO3)2 and thioglycolic acid (TGA). According to the results can
be claimed that to increase the NLO properties of a semiconductor such as Ag 2S, this semiconductor could be covered with another semiconductor which has a larger energy gap. To do this should be a high-quality core-shell prepared and is suggested according to this research the values of shell or capping agentbe modified to achieve the best results. 5. Conclusion The NLO properties of Ag2S@ZnS core-shells were investigated using a Z-scan technique by a 532 nm laser diode and compared with Ag2S and ZnS nanoparticles. The results showed that by changing the values of Zn(NO3)2 and TGA, the NLO properties of the Ag2S@ZnS structure could be improved. Also, obtained results indicated that the NLO properties of Ag2S@ZnS structure is very flexible and these properties could be tailored to its application. The improved performance is due to a synergistic effect arising from the combination of nonlinear absorption originating from Ag2S and ZnS NPs and efficient charge transfer between ZnS shells and Ag2S cores.
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Fig. 1. Schematic of (a) Close aperture Z-scan (b) Open aperture Z-scan.
Fig. 2. XRD plots for samples prepared with different TGA values (left) with different Zinc(Ac)2 values and 2500 µl TGA (right).(reprinted with permission from Elsevier)
(a) (b)
(d)
(c)
Fig. 3. TEM images of samples prepared with (a) 2500 and (b) 5000 µl TGA. And TEM of samples with (c) 0.05 g and (d) 0.1 g Zn(Ac)2 (The inset picture is a single core-shell particle showing a non-uniform shell of ZnS, the scale bar is 40 nm). (reprinted with permission from Elsevier)
Fig. 4. UV–vis spectra of (a) Ag2S (b) and ZnS nanoparticles.
Fig. 5. UV–vis spectra of Ag2S@ZnS nanoparticles with various synthesis.
Fig. 6. Z-scan measurements results of Ag2S and their corresponding fitted curve (a) Closed aperture (b) Open aperture, and Z-scan results of ZnS and their corresponding fitted curve (c) Closed aperture (d) Open aperture.
Fig. 7. Closed aperture Z-scan of samples and their corresponding fitted curve (solid line) (a) S1, (b) S2, (c) S3, (d) S4.
Fig. 8. Open aperture Z-scan of our samples (a) S, (b) S2, (c) S3, (d) S4. Here dots are related to the experimental results and the solid line is the curve fitting.
Fig. 9. Schematic of the processes involved in increasing the nonlinear optical properties of Ag2S@ZnS nanoparticles.
Table 1 The results of Z-Scan curve fitting of our samples. Sample α0 (cm−1 ) I0 (KW⁄ 2 ) n (cm2⁄ ) 2 cm GW Ag2S ZnS S1 S2 S3 S4
1.953 0.296 0.722 2.724 0.226 0.757
25.092 25.092 25.063 25.063 25.063 25.063
−4.26 −1.39 −3.51 −9.16 −0.93 −2.88
β(cm⁄MW)
Is (KW⁄cm2 )
32.23 7.57 41.93 369 22.9 72.6
− 3.65 − 19.64 13.45 12.25
Table 2 The result of third-order susceptibility measurements for samples. Sample n0 Re(χ(3) )(esu) Im(χ(3) )(esu) −7 Ag2S 1.45 −2.271 × 10 7.27 × 10−9 −8 ZnS 1.37 −6.59 × 10 1.52 × 10−9 S1 1.47 −1.92 × 10−7 9.73 × 10−8 −7 S2 1.49 −5.16 × 10 8.80 × 10−8 S3 1.40 −4.63 × 10−8 4.81 × 10−9 −7 S4 1.41 −1.45 × 10 1.55 × 10−8
|χ(3) |(esu) 2.272 × 10−7 6.59 × 10−8 1.93 × 10−7 5.23 × 10−7 4.65 × 10−8 1.46 × 10−7
