Improvement of nonlinear optical properties of ...

Optical Materials 66 (2017) 664-670

Improvement of nonlinear optical properties of graphene oxide in mixed with Ag2S@ZnS core-shells M. Khanzadeh*, M. Dehghanipour, M.Karimipour, M.Molaei Department of Physics, Faculty of Sciences, Vali-e-Asr University of Rafsanjan , 77188-97111 Rafsanjan, Iran. DOI: 10.1016/j.optmat.2017.03.008

Abstract Nonlinear optical properties including size and sign of nonlinear refractive index and nonlinear absorption coefficient of Graphene Oxide (GO), Ag2S@ZnS quantum dots and GO-Ag2S@ZnS were investigated using a Z-scan technique by laser diode with 532 nm wavelength. Third-order susceptibility of the compounds was calculated and compared with the reported values. By comparing the nonlinear optical (NLO) properties of GO and the mixture of GO-Ag2S@ZnS, it was observed that the NLO properties of GO were increased using simply mixing with Ag2S@ZnS quantum dots. It was also observed that with increase of sonication time, the NLO properties of the mixture increased accordingly. 1. Introduction There are different allotropes of Carbon such as: three-dimensional diamond, twodimensional graphene, one-dimensional carbon nanotubes and dimensionless fullerene [1-3]. Many researches are focusing on carbon nanostructures because of the vast application of onedimensional and dimensionless carbon nanostructures [4-7]. Graphene is known as a new material in science and it has a vast application because of its thin two-dimensional structure that has electronic and mechanical properties [8-13]. Theoretically, other molecules of carbon allotropes can be made from graphene. For example: Carbon nanotubes can be made from rolling up graphene with different layers and fullerene (dimensionless carbon) can be made from wrapping up one layer of graphene [1,14]. Graphene oxide is made from oxidation of graphene and unlike pure graphene, graphene oxide sheets are highly oxygenated, bearing hydroxyl and epoxide functional groups on their basal planes, with carbonyl and carboxyl groups located at the sheet edges. The presence of these functional groups makes the GO sheets strongly hydrophilic, * Corresponding author: [email protected]


which allows GO to readily swell and disperse in water [3]. In contrast to the well-established electronic structure of graphene as a zero-gap semiconductor with sp2 hybridized carbon atoms [15,16], GO is a two-dimensional network consisting of sp2 and sp3 hybridized regions, resulting in a heterogeneous structure that features both state from the sp2 carbon sites and a large energy gap between the states of the sp3 bonded carbons [3,17,18]. Researches have shown that a suitable and reliable method for improving the NLO properties of GO is its combination with materials that have nonlinear optical properties. Therefore, many different compound materials with GO basis were synthesized and studied [19-25]. Ag2S quantum dots (QDs) has also shown many capabilities in different works [26-29]. The aim of this research is to increase the NLO properties of GO in combination with core-shell structure of Ag2S@ZnS QDs. 2. Synthesis method 2.1. Synthesis of GO In this research, GO was synthesized as a raw material by improved hummer method [30]. 1gr Graphite powder was added to Sulfuric acid and phosphoric acid with 80:20 volume ratio. after mixing, the solution should be put in an ice bath and 3 gr Potassium permanganate was added to the solution for better oxidation of graphite. The resulting solution was placed in a vial of oil for 1 hour at 60 ° C. Then, 80 ml of distilled water was added to the solution slowly for 1 hour at 110 ° C. 10 ml of hydrogen peroxide (H2O2) and 100 ml of water was added to the stirring solution. After the solution is filtered and washed twice, the pH would reach 5-6. Then the remaining solution is centrifuged at 8000 rpm. The precipitations are placed in the oven at 40 ° C to be dried. 2.2. Synthesis of Ag2S@ZnS QDs According to the recent work of Karimipour et.al [31] Ag2S@ZnS core-shells were prepared as follows: First 0.1 gr of silver nitrate was dissolved in 50 ml of deionized water. Then 1M, 2500 µl thioglycolic acid (TGA) was added to the solution as a capping agent. Ammonia was added to obtain a clear solution and it was stirred for 15 minutes. Then 0.64 gr thiosulfate was added to 50 ml of deionized water separately as a sulfur source and this solution was added to the clear solution of silver nitrate and TGA. Also 0.023 gr of zinc nitrate was dissolved in 50 ml of deionized water and added to the second solution afterward. Finally, the solution was placed in a 2


microwave with 450 power for 12 minutes. After completion of the reaction, nanoparticles were precipitated using 10 ml acetone. And then the sediment was separated from solution by centrifugation. Finally, the resulting particles were dried in an oven for 24 hours. 2.3. Preparation of GO-Ag2S@ZnS 20 mg of graphene oxide powder was poured in 20 ml of deionized water and was put in the ultrasonic probe for 10 minutes in order to be a perfectly homogeneous solution. Also 10 mg of Ag2S@ZnS powder was dispersed in 30 ml deionized water, separately. Then the graphene oxide solution was put on the stirrer. 250 µl of Ag2S@ZnS QDs solution was added to it each minute. The total amount of 3 ml of colloidal solution of QDs was added to graphene oxide solution and then the resulting solution stirred for another 10 minutes. 3. Z-scan measurement To measure the nonlinear refractive (NLR) index and nonlinear absorption (NLA) coefficient, close aperture (CA) Z-scan and open aperture (OA) Z-scan were employed, respectively. To fit the curves obtained from CA Z-scan measurements of samples Sheik-Bahae model’s is used, as follows [32]: TN (z, ∆𝜑0 ) = 1 − Where, x =

z z0

4x∆𝜑0 (x 2 + 9)(x 2 + 1)

(1)

, ∆𝜑0 is the on-axis phase shift and z0 is the Rayleigh range. The NLR index

(n2 ) can be measured given by: n2 =

∆𝜑0 KI0 Leff

Where K = Leff =

(2)

2𝜋 𝜆

1−e−α0 L α0

is the wave vector, λ is the laser wavelength, I0 is the laser intensity at focus, is the effective length of the sample, α0 is the linear (low intensity) absorption

coefficient which is obtained from Beer’s low for samples and L is the sample length. For as much as in the OA Z-scan curves a flip of saturable absorption around the beam waist has been showed, these curves are analyzed by a model related to both saturable absorption (SA) and reverse saturable absorption (RSA) [33]. This model yielding the total absorption coefficient as: 3

Optical Materials 66 (2017) 664-670 α

α(I) = 1+I0 + βI ⁄I s

(3)

Where the first and second terms regarding to negative NLA such as SA and positive NLA such as two photon absorption (TPA) and/or RSA, respectively. In Eq. 3 α0 is linear absorption, β is positive NLA coefficient, I is laser radiation intensity. [34]. Is is saturation intensity which define as the intensity required to reduce the first term of Eq. 3 to half of α0 value [35,36]. Samples with a concentration of

1mg 1ml

were prepared in deionized water solvent and were

poured in a quartz cell with a thickness of 1 mm. The light source used in this experiment was a continuous laser diode with 532 nm wavelength. Fig. 1 shows the layout of CA and OA Z-scan experimental apparatus. As it is observed the important point is the structures of CA and OA Zscan used in this study is slightly different than the conventional arrangements. In this arrangement, a spatial filter and a beam expander is used. A lens (L1) with a focal length of 75 mm was placed in the path of the laser beam and then a pinhole size of 70 µm was placed exactly at the center of the L1 and then another lens with a focal length of 30 mm (L2) was placed exactly at a distance of 30 mm from the pinhole that this collection is used as a spatial filter in order to eliminate the scattering effects of the laser beam. Then to increase the incident intensity on the sample in the Z-scan test, after the spatial filter, another lens with a focal length of 75 mm (L3) was placed on the path of the beam and another different lens with a focal length of 50 mm (L4) was also placed. In addition, to collimate the divergent beam, these two lenses were confocal, meaning that the two lenses were placed at a distance of 125 mm from each other. Finally, the laser beam was focused onto the samples through a 10 mm focal length lens (L5). The beam waist size at focus using a knife edge diffraction experiment at the power of 29 and 30 mW were 10.5 and 8.73 µm, respectively. Moreover, the quartz cell with an accuracy of 10 microns was shifted by a stepper motor axis along the propagation axis of focused laser beam while its transmission was recorded at each position. As seen in Fig. 1 in layout of OA Z-scan experiment, a lens (L6) instead of aperture is replaced that all the light output of the sample could be revealed and the effects of NLR during measurement of OA Z-scan can be removed. With these two methods, the NLR index and NLA coefficient are obtained. One of the important points in this method is the calculation of NLR and NLA coefficient in the same intensity laser, which is an important parameter to calculate the third-order susceptibility. 4


By calculating the nonlinear refractive index (n2 ) and the nonlinear absorption coefficient (β), the real and imaginary parts of the third order susceptibility (χ(3)) can be calculated according to the following formulas [37]: Re(χ(3) )(esu) =

10−4 ε0 c 2 n20 n2⁄ π

(4)

Im(χ(3) )(esu) =

10−2 ε0 c 2 n20 λβ⁄ 4π2

(5)

By measuring n2 and β at the same intensity, calculation of the third order susceptibility is possible given by [37]: 2

2 1 ⁄2

|χ(3) | = [(Re(χ(3) )) + (Im(χ(3) )) ]

(6)

4. Results and discussion The x-ray diffraction pattern of graphene oxide (Fig. 2) shows a peak at around 2θ = 13.29° which belongs to (002) plane that implies GO sheets have been synthesized with the thickness of 0.66 nm. The broad peak between 2θ = 25-30° is related to a relative reduction of graphene oxide [21]. Fig. 3 shows the scanning electron microscopy (SEM) images of GO sheets, Ag2S@ZnS coreshell structure and the mixture of GO-Ag2S@ZnS. In Fig. 3a, the SEM image of spherical Ag2S@ZnS nanoparticles is presented. The particle size varies from 35.5 nm to 77.6 nm. Fig. 3b shows the large sheet of GO aggregated layers. In Fig. 3c, GO-Ag2S@ZnS mixture sonicated for 6min with characteristic spherical particles of Ag2 S@ZnS sitting on the graphene oxide sheets is depicted. As it is seen in Fig. 3c, graphene oxide sheets are almost completely covered by Ag2S@ZnS nanoparticles. The characteristics of the linear absorption are investigated by using the absorption analysis of UV-Vis. As it is shown in Fig. 4, two peaks are visible in 230 nm and 300 nm in the spectrum of GO absorption. The main absorption peak is at 230 nm regarding to the transition of π → π∗ of the carbon-carbon bond and the shoulder at about 300 nm is attributed to electronic transition of n → π∗ [38]. Fig. 4b shows the spectrum curve of Ag2S@ZnS nanoparticles absorption, where

5


the absorption edge of Ag2S@ZnS QDs and the peak of exciton at the wavelength of 810 nm are obvious [31]. Fig. 5 shows the GO-Ag2S@ZnS mixture absorption spectrum in different ultrasonic times. As it is clear by increasing the sonication time, the absorption of the samples did not degrade as an indication of sample’s stability. Fig. 6 depicts the results obtained from CA Z-scan of samples and optical parameters are listed in Table 1. The results show that with the increase of the sonication time of GOAg2S@ZnS, the NLR coefficient increases. Fig. 7 also demonstrates the OA Z-scan of the samples. The results of these curves (Table 1) also show that with the increase of ultrasonic time, the NLA coefficient of this mixture has increased as its NLR coefficient. It is noteworthy to indicate that only by making a mixture of GO-Ag2S@ZnS the NLR index and NLA coefficient have been enhanced to almost 159% and 245%, respectively. The third-order susceptibility (χ(3)) obtained from Z-scan, are listed in Table 2. The NLR index and NLA coefficient of GO are equal to 10−9 10−6

Cm2 W

cm2 W

and 10−4

and 10−3

Cm W

cm W

, respectively. Ebrahimi et.al

calculated these coefficients

, respectively [21]. In order to justify this difference, it should be

considered that the light intensity, solvent and concentration of GO in Ebrahimi et.al report and current study. The real and imaginary parts of third order susceptibility reported by many researchers are of the scale of 10−12 − 10−13 esu with a nanosecond pulse laser at 532 nm [3942], that are not comparable to the present work which is a continues beam technique. Ebrahimi et al. reported values of 10−4 esu and 10−6 esu for the Re(χ(3) ) and Im(χ(3) ) parts, respectively [21]. Moreover, to the best of our knowledge, there is not such a report of χ(3) values for silver sulfide and/or zinc sulfide quantum dots yet. Our results obtained values of Re(χ(3) ) and Im(χ(3) ) from the order of 10−7 esu and 10−8 esu, respectively. These differences are related to the NLR and NLA coefficients as it has already been explained above. The NLO properties of GO are determined with a combination of process in sp 3 and sp2 areas. The nonlinear optical reactions carrying charge in hybrid areas of sp2 is as saturable absorption (SA) in low intensities, whereas, the nonlinear optical reactions carrying charge in hybrid areas of sp3 is as two photon absorption (TPA) [3]. Table 1 shows that the NLO properties of GO have been increased after mixing with Ag2S@ZnS QDs. It is noticeable that the input intensity for the NLR and NLA values of Ag2S@ZnS is 25.062

W cm2

6

while the input intensity for pure and mixed


GO is 16.773

W cm2

. It is because Ag2S@ZnS did not show any considerable NLA and NLR

values at low input intensities. After mixing with GO, these values are increasing with sonication time and reach the values close to those for Ag 2S@ZnS but obtained at a high intensity. Thus, these results indicate the enhancement of nonlinear optical properties of GO by just mixing with Ag2S@ZnS core-shells. Fig. 8 shows the suggested mechanism for the increase of nonlinear absorption of GO-Ag2S@ZnS mixture. There are two TPA mechanisms involved and also two RSA mechanisms that can enhance NLA coefficient. A real TPA occurs from valance band (VB) to conduction band (CB) of GO via a virtual state. The other TPA mechanism is actually an excited state absorption (ESA) from CB of Ag 2S core to CB of ZnS shell. Also a single ESA mechanism from CB of GO to CB of ZnS occurs that can improve the NLA coefficient in mixture. There are also two RSA mechanisms due to the relaxation of electrons in the opposite direction of ESA mechanisms. The linear absorption (α0 ) of the compound is increased with the sonication time which is confirmed from Fig. 5 and consequently enhances the NLR. Actually, according to Eq. 2 when the α0 is increased, Leff is reduced which causes the enhancement of the NLA coefficient. Also in CA Z-scan curves peak-valley difference is increased slightly with the sonication time which according to Eq. 2 this causes that the enhancement of the NLR index becomes more significant. 5. Conclusion In summary, the nonlinear optical properties of GO, Ag2S@ZnS nanoparticles and the mixture of GO-Ag2S@ZnS have been investigated by using Z-scan method and using a 532 nm continuous wavelength of a laser diode. comparison between the nonlinear optical properties of GO and the mixture of GO-Ag2S@ZnS shows that the nonlinear optical properties of GO has been increased by just mixing with the Ag2S@ZnS core-shells. Furthermore, the NLR and NLA values of GO-Ag2S@ZnS mixture are increasing with the sonication time. References [1]

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11


Fig. 1. Schematic of (a) Close aperture Z-scan (b) Open aperture Z-scan.

12


Fig. 2. XRD spectra of graphene oxide.

13


Fig. 3. SEM image of (a) Ag2S@ZnS nanoparticls (b) Graphene oxide (c) GO-Ag2S@ZnS mixture.

14


Fig. 4. UV–Vis spectra of (a) Graphene oxide (b) Ag2S@ZnS nanoparticles.

15


Fig. 5. UV–Vis spectra of GO-Ag2S@ZnS with various duration of ultrasonic.

16


Fig. 6. Close aperture Z-scan curves for (a) Ag2S@ZnS nanoparticles (b) Graphene oxide (c) GO-Ag2S@ZnS 0min (d) GO-Ag2S@ZnS 2min (e) GO-Ag2S@ZnS 4min (f) GO-Ag2S@ZnS 6min.

17


Fig. 7. Open aperture Z-scan curves for (a) Ag2S@ZnS nanoparticles (b) Graphene oxide (c) GO-Ag2S@ZnS 0min (d) GO-Ag2S@ZnS 2min (e) GO-Ag2S@ZnS 4min (f) GO-Ag2S@ZnS 6min.

18


Fig. 8. A schematic of the process involved in increasing the nonlinear absorption of GOAg2S@ZnS mixture.

19


Table 1 The results of Z-scan curve fitting of our samples. Sample

α0 (

1 ) cm−1

P0 (mW)

KW I0 ( 2 ) cm

cm2 n2 ( ) GW

KW Is ( 2 ) cm

cm β( ) MW

Ag2S@ZnS

2.724

30

25.062

-9.16

19.65

369

GO

1.155

29

16.773

-4.77

19.73

139

GO-Ag2S@ZnS 0min

1.393

29

16.773

-5.27

16.73

211

GO-Ag2S@ZnS 2min

1.807

29

16.773

-6.76

17.93

261

GO-Ag2S@ZnS 4min

2.318

29

16.773

-7.31

27.93

272

GO-Ag2S@ZnS 6min

2.511

29

16.773

-7.59

20.83

341

20


Table 2 The magnitude of third-order susceptibility for samples. Sample

n0

Re(χ(3) )(esu)

Im(χ(3) )(esu)

|χ(3) |(esu)

Ag2S@ZnS

1.49

−5.16 × 10−7

8.79 × 10−8

5.23 × 10−7

GO

1.39

−2.34 × 10−7

2.88 × 10−8

2.35 × 10−7

GO-Ag2S@ZnS 0min

1.42

−2.69 × 10−7

4.57 × 10−8

2.73 × 10−7

GO-Ag2S@ZnS 2min

1.43

−3.50 × 10−7

5.73 × 10−8

3.55 × 10−7

GO-Ag2S@ZnS 4min

1.45

−3.90 × 10−7

6.14 × 10−8

3.94 × 10−7

GO-Ag2S@ZnS 6min

1.47

−4.16 × 10−7

7.91 × 10−8

4.23 × 10−7

21