Zeolite Y Films as Ideal Platform for Evaluation of Third-Order Nonlinear Optical Quantum Dots

Zeolites are ideal host material for generation and stabilization of regular ultrasmall quantum dots (QDs) array with the size below 1.5 nm. Quantum dots (QDs) with high density and extinction absorption coefficient have been expected to give high level of third-order nonlinear optical (3rd-NLO) and to have great potential applications in optoelectronics. In this paper, we carried out a systematic elucidation of the third-order nonlinear optical response of various types of QDs including PbSe, PbS, CdSe, CdS, ZnSe, ZnS, Ag 2 Se, and Ag 2 S by manipulation of QDs into zeolites Y pores. In this respect, we could demonstrate that the zeolite offers an ideal platform for capability comparison 3rd-NLO response of various types of QDs with high sensitivities.


Introduction
Materials with large and fast nonlinear responses are highly essential for practical optical devices, owing to the high thirdorder nonlinear optical (denoted as 3rd-NLO) responses of the NLO materials [1].These properties endorse them as promising candidate materials for all-optical switches [2] and waveguides [3], which are key components in high-speed alloptical telecommunication [4] and optical limiters [5], as well as high-performance saturable absorbers [6] in mode-locked laser system.Therefore, there has been considerable interest lately in the development of new materials with higher 3rd-NLO response.
Along this line, it has been proposed that zeoliteintercalated QDs would give very high 3rd-NLO responses [24,25].This speculation is on the basis of the following expectations: the size of the intercalated QDs will be much smaller (<1.5 nm) than that of the QDs in other matrices; QDs of highly uniform size, when arranged in a periodic way into the zeolite crystalline host, will give rise to increased concentration of QDs per constant matrices volume.For the first time, Kim and Yoon reported the extraordinarily high level of 3rd-NLO responses of zeolite-intercalated PbS QDs [26].Thus, the nonlinear refractive () and absorption () coefficients of the zeolite-intercalated PbS QDs, the two important properties of 3rd-NLO materials, are ∼20 to 330 times higher than the highest values obtained from other QD-dielectric matrix systems [12][13][14][15][16][17][18][19][20][21][22][23] at 532 and 1064 nm, respectively.Moreover, very recently, Lee and Kim [27] demonstrated that the electric field strength, framework donor strength, and cation acceptor strength in zeolites as the QD host could be sensitively and systematically changed via varying cation species in the zeolite framework.This is one of the unique advantages of using zeolites as the hosts for QDs, given the fact that the conventional dielectric matrices cannot easily and systematically change the environment of generated QDs.These results demonstrated that the zeolite could be an ideal host not only for the generation of ultrasmall QDs organized into regular arrays but also for facilitating a perfect condition to enhance the 3rd-NLO response by systemically changing the surrounding environment of generated QDs.

Experimental Section
2.1.Preparation of Zeolite Y Film on ITO Glass.Zeolite Y seed crystals (80 nm, 1 g) were prepared according to the literature procedure [28] and dispersed in DDW (0.1 w%).Independently, several pieces of ITO glass (30 × 30 mm) were cleaned with ultrasound sonication in mucosal solution (3%) for 10 min and subsequently washed with copious amounts of water.The aqueous suspension of the seed crystals was spin-coated twice onto the ITO side at the speed of 2,000 rpm.The seeded ITO glasses were then kept for 30 min in an oven preheated to 100 ∘ C for adhesion.After cooling to room temperature, the seeded ITO glasses were placed vertically on a Teflon support.The support was then immersed into a synthesis gel with the molar composition of 14Na 2 O : Al 2 O 3 : 10SiO 2 : 720H 2 O, and the reactor was placed for 12 h in an oven preheated to 100 ∘ C. The average thickness of the resulting zeolite Y films was 2.5 m.

Preparation of MS or MSe
Quantum Dot (M = Pb, Cd, Ag, and Zn)-Containing Zeolite Y Films Supported on ITO Glass.Zeolite Y films (denoted as Y f ) were immersed into each 25 mL aliquot of various types of metal ion source solution (0.1 mM) such as Cd(NO 3 ) 2 , Pb(NO 3 ) 2 , AgNO 3 , and Zn(NO 3 ) 2 for 10 min at room temperature.The desired ionexchanged Y f films supported on ITO glass were washed with copious amounts of distilled deionized water and subsequently dried under a stream of N 2 and were introduced into a Schlenk tube and dried at 200 ∘ C for 12 h under vacuum (< 10 −5 Torr).After cooling to the room temperature, dry H 2 S gas was introduced into the Schlenk tube, and the tube was kept for 30 min at the room temperature to generate desired MS (M = Pb, Cd, Ag, and Zn)-QDs in zeolite Y.The remaining unreacted H 2 S gas was removed by evacuation.Instead of H 2 S, when dry H 2 Se gas was introduced into the reaction, MSe (M = Pb, Cd, Ag, and Zn)-QDs in zeolite Y films were obtained.

Instrumentation.
The -2 scans of Y f films were performed on a Rigaku Ultima 4 with the power of 40 kV and 40 mA.The wavelength of the monochromated X-ray beam was 1.5406 nm generated from Cu K radiation.Diffuse reflectance UV-vis spectra of the samples were recorded on a Varian Cary 5000 UV-Vis-NIR spectrophotometer equipped with an integrating sphere.Barium sulfate (BaSO 4 ) was used as the reference.The diffuse reflectance spectra were converted into the Kubelka-Munk (K-M) formalism.
Measurement of 3rd-NLO response of various types of quantum dots was conducted according to the literature [25,26].

Results and Discussion
In general, metal sulphide (MS) or metal selenide (MSe) QDs could be produced inside zeolite Y (in Scheme 1) via the following simple chemical reaction [28].In the typical process, dried and fully different M + or M 2+ -exchanged zeolite Y is treated with dry H 2 S gas according to where [ ]Y represents zeolite Y, (PbS)QD represents PbS QDs, and 2H + represent 2H + ions charge-balancing two negative Al centres in zeolite Y framework instead of a Pb 2+ ion.Zeolite, as the host for QDs, offers an opportunity to compare the 3rd-NLO responses of various MS or MSe QDs under identical conditions, especially in terms of the volume fraction of QDs relative to matrices, size, and matrix environment.In the present study, we prepared various metal chalcogenide QDs such as PbSe, PbS, CdSe, CdS, ZnSe, ZnS, Ag 2 Se, and Ag 2 S intercalated into zeolite Y.To the best of our knowledge, for the first time in this study, we evaluated and compared 3rd-NLO responses of various types of QDs at different wavelengths, namely, 532 nm and 1065 nm, under exactly constant circumstances.
Zeolite Y films (denoted as Y f ) grown on the indium-tinoxide-coated glass plates were used in this study.This was particularly chosen because the films remain firmly bonded to the substrates even during subsequent chemical treatments, such as fully ion exchange with various heavy metal cations including Pb 2+ , Cd 2+ , Zn 2+ , and Ag + ions, drying, and formation of metal chalcogenides MS and MSe via treatment with H 2 S and H 2 Se.Several Y f s loaded with various metal chalcogenide QDs, such as PbSe, PbS, CdSe, CdS, ZnSe, ZnS, Ag 2 Se, and Ag 2 S, were prepared from rigorously dried M + (M = Ag) and M 2+ (M = Pb, Cd, and Zn)-exchanged Y f s treated with dry H 2 S and H 2 Se, respectively.All the numbers of generated MS and MSe units in Y f s were almost 32 per unit cell (the maximum amount for this zeolite is 32) as ]-Y f s surfaces [28].
The photographs of [(MS or MSe)]-Y f are shown in the right panel in Scheme 1.As is seen, the colour progressively changes from white to dark red, which could be attributed to the decrease in the band gap of loaded MS and MSe, respectively.The diffuse reflectance and absorption spectra of [(MS or MSe)]-Y f s are shown in Figure 1, respectively.As is seen, the absorptions are distinctively blue shift when compared to those of bulk MS or MSe, which is due to the quantum size effects.The order of bulk band gap of each MS or MSe is as follows: Ag 2 Se (0.15 eV) < PbSe (0.27 eV) < PbS (0.37 eV) < Ag 2 S (0.9 eV) < CdSe (1.7 eV) < CdS (2.4 eV) < ZnSe (2.6 eV) < CdS (2.4 eV) < ZnS (3.6 eV).Similarly, the order of optical band gap of [(MS or MSe)]-Y f s estimated from the absorption spectrum shown in SI-2 in Supplementary Information indicated similar trend as follows: Ag 2 Se (0.15 eV) < PbSe (0.27 eV) < PbS (0.37 eV) < Ag 2 S (0.9 eV) < CdSe (1.7 eV) < CdS (2.4 eV) < ZnSe (2.6 eV) < ZnS (3.6 eV).Among the different QDs considered in this study, PbSe, PbS, Ag 2 Se, Ag 2 S, and CdSe intercalated Y f s with smaller band gap show strong absorption intensity at 532 nm.This is called resonance.Particularly, the order of absorption intensity at 532 nm, which is the nonlinear excitation wavelength, was PbSe < PbS < Ag 2 Se < Ag 2 S < CdSe.On the other hand, CdS, ZnSe, and ZnS intercalated Y f s present no absorption at 532 nm, or, in other words, they were nonresonant.Furthermore, at the other nonlinear excitation wavelength of 1064 nm, PbSe, PbS, Ag 2 Se, and Ag 2 S intercalated Y f s show weak absorption.This implies that the wavelength 1064 nm is near-resonant.However, CdSe, ZnSe, CdS, and ZnS show no absorption, which implies that these QDs are nonresonant at the wavelength of 1064 nm.These three distinct wavelengths, namely, resonant, near-resonant, and nonresonant, depending on absorption spectrums, lead to huge differences in the 3rd-NLO responses values.
The two 3rd-NLO responses,  (nonlinear refractive index) and  (nonlinear absorption) coefficients at 532 and 1064 nm, respectively, of [MS or MSe]-Y f were obtained from the -scan system as shown in Figure 2 using modelocked picosecond laser pulses with the pulse width of 50 ps.The validity of measurement system was confirmed by performing calibration measurement with CS 2 liquid as wellknown standard material [26].The nonlinear refractive index of CS 2 was determined to be 3.5 × 10 6 cm 2 /GW, which is in accordance with the standard value measured under similar conditions [26].Besides, the -scan was also conducted for the bare Y f without any QDs.No transmittance change was observed.This confirms that the 3rd-NLO effects originate from the intercalated QDs.
The measured values of nonlinear refractive coefficient, , and nonlinear absorption coefficient, , of each of [MS or MSe]-Y f s at 532 and 1064 nm are listed in Table 1.The order of increase in the obtained  and  values at 532 nm is as follows: PbSe ( = −3.35× 10 −10 cm 2 /W;  = 6118 cm/GW) < PbS ( = −2.84× 10 −10 cm 2 /W;  = 5680 cm/GW) < Ag 2 Se ( = −2.53× 10 −10 cm 2 /W;  = 4550 cm/GW) < Ag 2 S ( = −2.27× 10 −10 cm 2 /W;  = 3880 cm/GW) < CdSe ( = −1.79× 10 −10 cm 2 /W;  = 2822 cm/GW) < ZnSe ( = −0.68 × 10 −10 cm 2 /W;  = 1206 cm/GW) < CdS ( = −0.50× 10 −10 cm 2 /W;  = 1050 cm/GW) < ZnS ( = −0.37 × 10 −10 cm 2 /W;  = 708 cm/GW).Depending on whether or not the QDs have linear absorption at 532 nm as nonlinear excitation wavelength, they can be divided into two distinct categories as follows: QDs having resonant nonlinearity,  such as PbSe, PbS, Ag 2 Se, and Ag 2 S and CdSe, and those having nonresonant nonlinearity, such as ZnSe, CdS, and ZnS QDs.We plot the 3rd-NLO response,  and  coefficients, against absorption coefficient at 532 nm, as shown in Figure 3. Interestingly, each  and  value coincides with that of the absorption intensity of [MS or MSe]-Y f s at 532 nm.Besides, the 3rd-NLO activity values of QDs that exhibit absorption at 532 nm are larger than those of QDs that do not show absorption.This trend originates from the fact that the resonant excitation greatly enhances the 3rd-NLO response by giving rise to diverse processes, such as electronic polarization effect and nuclear and thermal processes, which may contribute to 3rd-NLO activity.We could confirm that the linear absorption intensity at the measured wavelength is predominantly affected by the resonant nonlinear optical properties.In addition, we could observe the relation between nonresonant 3rd-NLO response and exciton Bohr radius.This can be achieved by plotting the  and  values against the exciton Bohr radius, respectively, in cases of ZnSe, CdS, and ZnS QDs in Y f s, which do not exhibit linear absorption at 532 nm.As shown in Figure 3, an increase in exciton Bohr radius results in an increase in nonresonant 3rd-NLO response.
At the excitation wavelength corresponding to 1064 nm, similar to 532 nm, we could classify two distinct categories of QDs, namely, those having near-resonant nonlinearity, such as PbSe, PbS, Ag 2 Se, and Ag 2 S, with absorption band edge near 1064 nm, and those having nonresonant nonlinearity, such as CdSe, ZnSe, CdS, and ZnS QDs, with no absorption at 1064 nm.Also, under the same conditions of excitation at 1064 nm, the obtained near-resonant nonlinear coefficient values of  and  from PbSe, PbS, Ag 2 Se, and Ag 2 S QDs in Y f s are larger than the nonresonant nonlinear coefficient values of  and  from CdSe, ZnSe, CdS, and ZnS QDs in Y f s.This could be explained on the basis of the abovementioned reasons from the case of 532 nm.
Furthermore, the effect of exciton Bohr radius on the 3rd-NLO activities of the intercalated QDs was observed by plotting the  and  values against the exciton Bohr radius in both cases of near-resonant and nonresonant nonlinearity under 1064 nm excitation (SI-3 in Supplementary Information).These plots depict a remarkably good correlation between the 3rd-NLO activities and the exciton Bohr radius.These results confirm that the 3rd-NLO activities increase with the exciton Bohr radius in case of near-resonant or nonresonant nonlinearity.
Among the various intercalated QDs prepared and analysed in this study, we could observe remarkably high 3rd-NLO activities in zeolite-intercalated PbSe QDs.In particular, PbSe is expected to be an attractive material for 3rd-NLO among the other semiconductor QDs due to the larger quantum confinement effect than PbS and other semiconductors.Actually, the highest values of nonlinear refraction () and absorption () coefficients measured from the PbSe sample were −1.35 × 10 −10 cm 2 /W and 2118 cm/GW at 532 nm and 1.47 × 10 −10 cm 2 /W and 1550 cm/GW at 1064 nm, respectively.In particular, the magnitude of the absorption coefficient  of [PbSe]-Y f at 1064 nm is as large as ∼7 times higher than the highest magnitude ever obtained.Furthermore, the  and  values of [PbSe]-Y f at 532 nm and 1064 nm represent the values first ever measured from any PbSe QD/matrix composites.We observed that the significant increase in cation size, as we go from H + to Rb + , is the key factor underlying the remarkably high 3rd-NLO responses of the PbSe QDs, which is in accordance with previous reports (SI-4 in Supplementary Information).The highest values of nonlinear refraction () and absorption () coefficients measured from the above samples were −7.95 × 10 −10 cm 2 /W and 9118 cm/GW at 532 nm and 9.52 × 10 −10 cm 2 /W and 6550 cm/GW at 1064 nm, respectively, after ion exchanged with Rb + (SI-4 in Supplementary Information).To the best of our knowledge, this is the highest value that has ever been reported in zeolite QD composite and QDs in other matrices.

Conclusion
In summary, we intercalated various metal chalcogenide QDs, such as PbSe, PbS, CdSe, CdS, ZnSe, ZnS, Ag 2 Se, and Ag 2 S, into transparent zeolite films and compared their 3rd-NLO responses at different wavelengths, namely, 532 nm and 1065 nm.Results indicated that the zeolite provides an ideal platform for the tuneable 3rd-NLO QDs materials with high sensitivities, as evidenced from the 3rd-NLO responses of the intercalated QDs in zeolite.In addition, results suggest that the 3rd-NLO responses are greatly affected by not only the absorption intensity at the excited wavelength but also the exciton Bohr radius.

Scheme 1 :
Scheme 1: Schematic representation of QDs encapsulated in zeolite Y and photograph images of zeolite Y films containing the QDs.

Figure 1 :
Figure 1: Absorption spectra of [MSe or MS]-Y f s as indicated.

Figure 2 :
Figure 2: Measured -scan data (symbols) and the theoretically fitted curves (solid curves) of [(PbSe)]-Y f obtained under the (a) closed-and (b) open-aperture conditions at 532 nm and under the (c) closed-and (d) open-aperture conditions at 1064 nm.The input irradiance at the focal point ( 0 ) was 0.10 and 0.18 GW/cm 2 at 532 nm and 0.27 and 0.47 GW/cm 2 at 1064 nm.