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Our strategy involves a two-step sol-gel process and the co-condensation of the resulting alkoxysilane-capped CdS QDs with other alkoxysilanes.
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Miao Feng,a and Hongbing Zhan*a 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x To realize their practical and operable application as a potential optical limiting (OL) material, quantum dots (QDs) need to have good processability by incorporating them into optical-quality matrices. This work reports a facile route for the room-temperature preparation of large, stable transparent monolithic CdS nanocomposites which can be easily extended to allow the introduction of acid-sensitive functional molecules/nanoparticles into a silica network by sol-gel chemistry. Our strategy involves a two-step solgel process (acid-catalyst hydrolysis and basic-catalyst condensation) and the co-condensation of the resulting alkoxysilane-capped CdS QDs with other alkoxysilanes, which allows the CdS QDs to become part of the silica covalent network. The degradation and agglomeration of CdS QDs were thereby effectively restrained, and large monolithic transparent CdS-silica gel glass was obtained. Using Z-scan theory and the resulting open-aperture Z-scan curves, the nonlinear extinction coefficient of the CdSsilica nanocomposite gel glass was calculated to be 1.02 × 10–14 cm·W–1, comparable to that of the parent CdS QD dispersion, indicating their promise for OL applications.
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Chemically synthesized colloidal quantum dots (QDs) have attracted extensive scientific and industrial interest because of their promising applications in the fabrication of novel optoelectronic devices. This is because of their broadly tunable linear and nonlinear optical properties, which are influenced by their size, shape, and composition.1-4 Recently, QDs have also been studied as potential optical limiting (OL) materials,5-8 which are strongly needed for the development of optical limiters. OL devices should display high transmittance at low intensity, as well as low transmittance of high-intensity laser pulse input, so that they can protect optical sensors and the eyes from potential damage induced by intense laser irradiation. Strong OL effects have been observed for a variety of materials such as fullerenes,9 carbon nanotubes,10 carbon black suspensions,11 metallophthalocyanines,12 metallo-porphyrins,13 metal nanoparticles,14 and most recently, graphene dispersions.15 To realize practical and operable applications, QDs need to have good processability by incorporating them into opticalquality matrices. Furthermore, a rigid solid state environment may provide more opportunities for researching their underlying optoelectronic mechanism. Polymers are chemically compatible with organically capped QDs, so they are helpful in preventing QD agglomeration during nanocomposite preparation and achieve uniform dispersion.16-18 However, polymers suffer from a limited thermal range and low laser damage threshold, and thus are not suitable matrices for OL materials, which must endure high intensity and long laser irradiation. In contrast, inorganically based sol-gel materials, in particular, SiO2 gel glasses, are This journal is © The Royal Society of Chemistry 2013
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considered a better option for the host material owing to their excellent optical properties and inherent physical and chemical stabilities.19-25 Several OL functional materials such as carbon nanotubes, metallo-phthalocyanines, and metal nanoparticles, among others, have been incorporated into silica gel glasses, and the resultant nanocomposites demonstrated retained OL behavior.26-30 Three main approaches have been developed for the sol-gel preparation of QD-based bulk nanostructured materials: (1) In situ synthesis of QDs through a hydrolysis and condensation process,24-26 (2) incorporation of premade QDs with sol-gel precursors,19-23 and (3) most recently, self-assembly of QDs through the hydrolysis and condensation of capping ligands.31,32 Among these, the second approach provides the most convenient and complete control over the size and shape of both the guest QDs and the host matrices, as well as the inter-surface chemistry between the host and the guest. However, owing to the inherent incompatibilities between the surface chemistry of QDs and the sol-gel process itself, as well as the degradation of QDs in the usually adopted harsh acid-catalyst (pH=2) hydrolysis conditions, it is difficult to produce large, stable transparent monolithic QD nanocomposites. In this work, we report a novel, facile route for the preparation of large, stable transparent monolithic CdS nanocomposites, which can be widely used to introduce acidsensitive functional molecules/nanoparticles into a silica network by sol-gel chemistry. Previous work has studied the nonlinear optical properties of in-situ synthesized CdS quantum dots embedded in sol-gel derived Na2O-B2O3-SiO2 glass matrix.26 In comparison to that work, our room temperature one-pot method Nanoscale, 2013, [vol], 00–00 | 1
Nanoscale Accepted Manuscript
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Facile preparation of transparent and dense CdS-silica gel glass nanocomposites for optical limiting applications
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DOI: 10.1039/C3NR05424A
Fig. 1 Schematic synthesis of the transparent CdS-silica gel glass monolithic nanocomposites.
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To confirm the capping effect of MPS to CdS QDs, FT-IR characterization was performed (Fig. 2). The appearance of the Si-C band at 1243 cm-1, together with the -CH2 symmetric (2856 cm-1) and asymmetric stretching vibrations (2921 cm-1) related to the propyl chain of the MPS, supports the covalent linkage of MPS to CdS QDs. The low-frequency signal at 683 cm-1 can be assigned to the tether of S-C groups onto the surface of CdS NPs. The characteristic vibration peaks of the Cd-S bond 33 at 265 and 405 cm-1 are not detected in our experiments since the former is beyond our detection limit, and the latter is too weak to be resolved. While the HRTEM image (Fig. 3a) shows that the assynthesized individual CdS QDs are in regular round shape and give an average diameter of 4.5 nm, TEM image (Fig. 3b) indicates some extent of agglomeration, which leads to the
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Fig. 2 FT-IR spectrum of MPS-capped CdS QDS.
irregular shape and a broad size distribution ranging from several to tens of nanometers. This agglomeration is probably resulted from the partial hydrolysis and condensation of the capping MPS ligands of the CdS QDs, due to the presence of traces of water in the colloidal solution. The resultant gel glass nanocomposites were transparent, dense, and crack-free monolith with a diameter of ca. 35 mm and a thickness around 2 mm (Fig. 3c), which meet the requirement of optical measurements without further processing. As shown in Fig. 4a, the UV-vis absorption and photoluminescent (PL) spectra of the colloidal CdS QDs show that the CdS QDs have a shallow absorption around 375 nm, corresponding to the first optically allowed transition between the electron state in the conduction band and the hole state in the valence band.34 Upon light excitation at 375 nm, the CdS QDs exhibit luminescence with a strong emission at 570 nm and a weak emission at 440 nm, which can be attributed to trap (defect) emission and electron-hole recombination (intrinsic emission), respectively.35 The fluorescence quantum yield of CdS NPs is determined with the value of 16% by using Rhodamine B as a standard.36 Fig. 4b gives the UV-vis transmittance and PL spectra of the resultant gel glass nanocomposites. The nanocomposite glass demonstrates a very good transparency, which is higher than 90% at most of the visible light wavelength region. The zero transmittance in the near ultraviolet region is caused by the strong scattering of the microporous structure of the gel glass, together with the absorption of the CdS QDs. As compared to the emission of the corresponding dispersion, the incorporated CdS QDs present an intensive emission around 490 nm and a very weak
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has the advantage of being highly efficient and convenient. Our strategy involves a two-step sol-gel process (acid-catalyst hydrolysis and basic-catalyst condensation) and the cocondensation of the resulting alkoxysilane-capped CdS QDs with other alkoxysilanes, which allows the CdS QDs to become a part of the covalent network (Fig. 1). Two kinds of alkoxysilanes, tetraethoxysilane (Si(OC2H5)4, TEOS) and 3-glycidoxypropyltrimethoxysilane (CH2OCHCH2O(CH2)3Si(OCH3)3, GPTMS), were chosen as precursors. The long organic chains in GPTMS help improve the elasticity and flexibility of the resultant gel glass, which results in a dense structure. 3-mercaptopropyltrimethoxysilane (MPS) was employed as both the capping ligand of the CdS QDs and the crosslinker between the CdS QDs and the silica network. 3-aminopropyltriethoxysilane (NH2(CH2)3Si(OC2H5)3, APTES) acts as a weak organic basic catalyst for condensation by performing three roles. First, unlike inorganic basic ammonia, for example, which usually causes inhomogeneous coagulation of the silica granules and results in an opaque final product, APTES helps yield a transparent monolith with a highly branched silica network. Second, the use of a basic catalyst leads to rapid gelation, which helps shorten the gelation period from several weeks to several minutes, depending on the amount of APTES introduced. Finally, the neutralization of the mixture by APTES provides a safe environment for the incorporation of CdS QDs by protecting them from degradation.
Transmittance (a.u.)
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Fig. 3 (a) HRTEM and (b) TEM images of the as-synthesized CdS QDs; (c) photograph of the CdS-silica gel glass nanocomposite. The inset in (b) is the SAED pattern taken from the area designated by the rectangle.
emission around 620 nm, along with the disappearance of the peak at 440 nm. To accounted for this, we assume that due to the close packing of the CdS QDs in the composites, particle growth, as well as the removal of some surface defects by perhaps the APTES (Fig 1), what we can actually see is a red shift of the emission and strength inversion of the emission peaks. The peak at 440 nm is now the dominant peak and has red shifted by ~50 nm while the peak at 570 nm has been reduced in intensity due to surface defect removal (by the APTES) and is now at ~620 nm, again red shifting by 50 nm. To investigate the OL property of the samples, an openaperture Z-scan method was used to conduct the fluencedependent light transmittance measurements. Fig. 5 shows the variation in normalized transmittance as a function of input fluence (J cm -2 ) for the CdS QD dispersion and gel glass nanocomposite. Both exhibited a gradually reduced transmittance with increasing input fluence at the investigated 532 nm laser wavelength, indicating an OL effect. For these samples, the energy transmittance at light fluences less than 1 J cm-2 was kept constant, while the transmittance decreased as the incident fluence increased in excess of 1 J cm-2. The OL threshold (Fth),
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defined as the input fluence at which the transmittance falls to 50% of the normalized linear transmittance, is of almost the same value (around 4 J cm-2) for the two samples, indicating a comparable OL effect at a low input fluence. However, the transmittance decreased more rapidly for the glass nanocomposite in comparison to the CdS dispersion, implying an enhanced OL effect at a high input fluence. The OL effect of the CdS QD dispersion and gel glass nanocomposite is much better than noble metal nanoparticles,27 and comparable to benchmarked carbon nanotubes28 and graphene.30 However, it should be noted that, like some noble metal nanoparitcles,37 semi-conductor QDs are also not stable under extreme conditions. At high light intensities, they are susceptible to damage, leading to photofragmentation, ligand desorption, etc. To make them stable at extreme conditions, it is necessary to protect them with stable and chemically inert materials such as oxides.30,38 Our experiments show that after being encapsulated in silica gel glass matrix, no sign of laserinduced damage was observed for the QDs, illustrating the effective protective effect of silica matrix. This makes it possible to fabricate/process materials in the form of thin films and disks for applications. Several OL mechanisms, particularly nonlinear absorption (NLA, two-photon absorption, free-carrier absorption, reverse saturable absorption, etc.), nonlinear scattering (NLS), and nonlinear refraction (NLR) have been found to dominate different Nanoscale, 2013, [vol], 00–00 | 3
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Fig. 6 Z-scan curves of (a) CdS dispersion and (b) CdS-silica gel glass nanocomposite. ● contribution of NLA; ■ contribution of NLA and NLS.
kinds of OL materials.39 For QDs, however, the generally accepted underlying OL mechanisms are two-photon and/or multiphoton absorption for pico-femtosecond laser pulses and free-carrier absorption and/or NLS for nanosecond laser pulses.5-8 To separate the contributions of NLA and NLS, we use a Z-scan setup following Jourdier and Rao,39,40 and the results are shown in Fig. 6. The intensity variation along the beam propagation direction z in a medium having a third-order nonlinearity is described as dI /dz = −α0I−α2I2, where α0 is the linear absorption coefficient, and α2 is the total nonlinear extinction coefficient. The coefficient α2 is a sum of contributions from both NLA (α2A) and NLS (α2S): α2=α2A +α2S. The position dependent transmittance (the Z-scan curve) is given by
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where z is the sample position with respect to the focus, and q(z) = α2 I0L/[1+(z / z0)2], with I0 being the peak intensity at the focal point. L is the effective length of the sample given by [1−exp(α0l) /α0], where l is the sample length, and z0 = πω02/λ is the Rayleigh range, where ω0 is the beam waist radius at focus, and λ is the wavelength of light. The value for α2 is obtained by fitting the T (z) equation to the experimental data. In the context of our experiment, the Z-scan curve (b) gives α2, and curve (a) is fit with α2A. Thus, α2S can be extracted.41,42 The values of α2, α2A, and α2S were calculated to be 1.99, 1.61, and 0.38; 1.02, 0.98, and 0.04 (×10-14 cm·W-1), respectively, for QD dispersion and the gel glass nanocomposite at input pulses of 1.60 mJ. These values do not change significantly for other input optical energies. It is worth noting that the α2S of the gel glass nanocomposite is almost negligible in comparison to that of the dispersion. This result can be well explained from the mechanism of NLS and the different environment of the liquid and solid state matrices. NLS signals originated from the photoinduced excitation, ionization, and heating of the QDs, giving rise to their expansion and making them the scattering centers. Furthermore, energy transfer to the solvent will result in the formation of microbubbles that act as secondary centers and lead to a further reduction in transmittance with increasing input laser fluence.43 The latter is indeed the 4 | Nanoscale, 2013, [vol], 00–00
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In summary, we have provided a simple and effective method for the preparation of a highly transparent CdS-silica gel glass in densely monolithic nanocomposites. Owing to the synthesis strategy, the incorporated CdS QDs remain stable during the whole process and disperse homogeneously in the final products by forming a covalent bond with the silica network. This facile route can be applied to the bulk preparation of other nanostructures, particularly acid-sensitive nanomaterials. Openaperture Z-scan experiments showed that the resultant gel glass nanocomposites retained the OL effect in comparison to the parent CdS QD dispersion, which makes them practical for OL applications.
4 Experimental and computational methods MPS in N-N’-dimethylformamide (DMF) (0.02 mol L-1, 120 mL) was added to an aqueous solution of cadmium chloride (CdCl2·2.5H2O, 3 mmol, 1.5 mL) and was stirred for 15 min. Aqueous sodium sulfide (Na2S, 1.6 mmol, 1.5 mL) was added dropwise to the solution, which was then stirred for 4 h at room temperature in the dark. The yellow reaction mixture was slowly dehydrated by distillation under reduced pressure while adding fresh DMF to maintain the concentration of the CdS QDs in the suspension. NaCl and large aggregated particles were removed by centrifugation three times, yielding CdS-MPS QDs in DMF. TEOS (0.036 mol, 8.0 mL), GPTMS (0.015 mol, 3.5 mL), ethanol (0.205 mol, 12.1 mL), and H2O (0.205 mol, 3.7 mL) were mixed under magnetic stirring. A small amount of hydrochloric acid was added dropwise to promote hydrolysis (pH=2). After the mixture was stirred for 2 h, APTES was added dropwise to neutralize the mixture to pH=7. Then 9 mL of the as-synthesized DMF solution of the CdS-MPS QDs was gradually introduced, and the mixture was kept on stirring for an additional 15 min. After which, it was divided into several equal volume parts, cast into polystyrene cells individually, sealed, and left to age and dry for 8 weeks. Using the method reported by Peng, 44 we estimated the concentration of CdS QDs in the dispersion to be around 2 mg/mL, and therefore the loading level of CdS QDs in the resultant gel glass composite was calculated to be 0.6 wt%. The transmission electron microscopy (TEM) images of the CdS QDs were obtained on a JEOL JEM-2010 microscope. The samples were dispersed in DMF, and a drop of the solution was placed on a copper grid and then dried before it was transferred into the TEM sample chamber. The scanning electron microscopy (SEM) images of the gel glass nanocomposites were obtained on a Philips Nova Nano SEM 230. UV-vis absorption spectral measurements were carried out with a Shimadzu UV-2450 spectrophotometer. PL spectra were measured on a Deinburgh FL/FS TCSPC 920 spectrofluorophotometer. A modified open aperture Z-scan system was used to measure the OL properties of the materials using 8 ns Gaussian pulses from a Q-switched Nd:YAG laser. The beam was spatially This journal is © The Royal Society of Chemistry 2013
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filtered to remove the higher-order modes and was focused with a lens with a focal length of 30 cm. The laser was operated at 532 nm with a pulse repetition rate of 1 Hz. The energy levels of a single pulse were 0.24, 1.09, and 1.60 mJ. The laser beam waist was approximately 14.5 μm. All measurements were conducted at room temperature. The CdS QD suspension was contained in 5 mm-thick quartz cells, while the CdS–silica gel glass nanocomposite was fixed vertically using a clamp. Each sample was mounted on a computer-controlled translation stage that translated the sample along the Z-axis. The linear transmittance of the CdS QD suspension was adjusted to 70%, whereas that of the CdS–silica gel glass nanocomposite was measured to be around 45%. The OL curves, which can be plotted as normalized transmission versus input fluence, were calculated from the open aperture Z-scan data.
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This research was supported by the National Natural Science Foundation of China (No. 51172045), Research Fund for the Doctoral Program of Higher Education of China (No.20113514120006) and Natural Science Foundation of Fujian Province (No. 2012J0511).
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College of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian 350108, China, E-mail: [email protected]
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Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: M. Feng and H. Zhan, Nanoscale, 2013, DOI: 10.1039/C3NR05424A.
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This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication.
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Volume 2 | Number 1 | January 2010 | Pages 1–156
Nanoscale
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Pages 1–156
COVER ARTICLE Graham et al. Mixed metal nanoparticle assembly and the effect on surface-enhanced Raman scattering
REVIEW Lin et al. Progress of nanocrystalline growth kinetics based on oriented attachment
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Our strategy involves a two-step sol-gel process and the co-condensation of the resulting alkoxysilane-capped CdS QDs with other alkoxysilanes.
Nanoscale Accepted Manuscript
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DOI: 10.1039/C3NR05424A
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Cite this: DOI: 10.1039/c0xx00000x
PAPER
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Miao Feng,a and Hongbing Zhan*a 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x To realize their practical and operable application as a potential optical limiting (OL) material, quantum dots (QDs) need to have good processability by incorporating them into optical-quality matrices. This work reports a facile route for the room-temperature preparation of large, stable transparent monolithic CdS nanocomposites which can be easily extended to allow the introduction of acid-sensitive functional molecules/nanoparticles into a silica network by sol-gel chemistry. Our strategy involves a two-step solgel process (acid-catalyst hydrolysis and basic-catalyst condensation) and the co-condensation of the resulting alkoxysilane-capped CdS QDs with other alkoxysilanes, which allows the CdS QDs to become part of the silica covalent network. The degradation and agglomeration of CdS QDs were thereby effectively restrained, and large monolithic transparent CdS-silica gel glass was obtained. Using Z-scan theory and the resulting open-aperture Z-scan curves, the nonlinear extinction coefficient of the CdSsilica nanocomposite gel glass was calculated to be 1.02 × 10–14 cm·W–1, comparable to that of the parent CdS QD dispersion, indicating their promise for OL applications.
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Chemically synthesized colloidal quantum dots (QDs) have attracted extensive scientific and industrial interest because of their promising applications in the fabrication of novel optoelectronic devices. This is because of their broadly tunable linear and nonlinear optical properties, which are influenced by their size, shape, and composition.1-4 Recently, QDs have also been studied as potential optical limiting (OL) materials,5-8 which are strongly needed for the development of optical limiters. OL devices should display high transmittance at low intensity, as well as low transmittance of high-intensity laser pulse input, so that they can protect optical sensors and the eyes from potential damage induced by intense laser irradiation. Strong OL effects have been observed for a variety of materials such as fullerenes,9 carbon nanotubes,10 carbon black suspensions,11 metallophthalocyanines,12 metallo-porphyrins,13 metal nanoparticles,14 and most recently, graphene dispersions.15 To realize practical and operable applications, QDs need to have good processability by incorporating them into opticalquality matrices. Furthermore, a rigid solid state environment may provide more opportunities for researching their underlying optoelectronic mechanism. Polymers are chemically compatible with organically capped QDs, so they are helpful in preventing QD agglomeration during nanocomposite preparation and achieve uniform dispersion.16-18 However, polymers suffer from a limited thermal range and low laser damage threshold, and thus are not suitable matrices for OL materials, which must endure high intensity and long laser irradiation. In contrast, inorganically based sol-gel materials, in particular, SiO2 gel glasses, are This journal is © The Royal Society of Chemistry 2013
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considered a better option for the host material owing to their excellent optical properties and inherent physical and chemical stabilities.19-25 Several OL functional materials such as carbon nanotubes, metallo-phthalocyanines, and metal nanoparticles, among others, have been incorporated into silica gel glasses, and the resultant nanocomposites demonstrated retained OL behavior.26-30 Three main approaches have been developed for the sol-gel preparation of QD-based bulk nanostructured materials: (1) In situ synthesis of QDs through a hydrolysis and condensation process,24-26 (2) incorporation of premade QDs with sol-gel precursors,19-23 and (3) most recently, self-assembly of QDs through the hydrolysis and condensation of capping ligands.31,32 Among these, the second approach provides the most convenient and complete control over the size and shape of both the guest QDs and the host matrices, as well as the inter-surface chemistry between the host and the guest. However, owing to the inherent incompatibilities between the surface chemistry of QDs and the sol-gel process itself, as well as the degradation of QDs in the usually adopted harsh acid-catalyst (pH=2) hydrolysis conditions, it is difficult to produce large, stable transparent monolithic QD nanocomposites. In this work, we report a novel, facile route for the preparation of large, stable transparent monolithic CdS nanocomposites, which can be widely used to introduce acidsensitive functional molecules/nanoparticles into a silica network by sol-gel chemistry. Previous work has studied the nonlinear optical properties of in-situ synthesized CdS quantum dots embedded in sol-gel derived Na2O-B2O3-SiO2 glass matrix.26 In comparison to that work, our room temperature one-pot method Nanoscale, 2013, [vol], 00–00 | 1
Nanoscale Accepted Manuscript
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Facile preparation of transparent and dense CdS-silica gel glass nanocomposites for optical limiting applications
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Nanoscale
DOI: 10.1039/C3NR05424A
Fig. 1 Schematic synthesis of the transparent CdS-silica gel glass monolithic nanocomposites.
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To confirm the capping effect of MPS to CdS QDs, FT-IR characterization was performed (Fig. 2). The appearance of the Si-C band at 1243 cm-1, together with the -CH2 symmetric (2856 cm-1) and asymmetric stretching vibrations (2921 cm-1) related to the propyl chain of the MPS, supports the covalent linkage of MPS to CdS QDs. The low-frequency signal at 683 cm-1 can be assigned to the tether of S-C groups onto the surface of CdS NPs. The characteristic vibration peaks of the Cd-S bond 33 at 265 and 405 cm-1 are not detected in our experiments since the former is beyond our detection limit, and the latter is too weak to be resolved. While the HRTEM image (Fig. 3a) shows that the assynthesized individual CdS QDs are in regular round shape and give an average diameter of 4.5 nm, TEM image (Fig. 3b) indicates some extent of agglomeration, which leads to the
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Fig. 2 FT-IR spectrum of MPS-capped CdS QDS.
irregular shape and a broad size distribution ranging from several to tens of nanometers. This agglomeration is probably resulted from the partial hydrolysis and condensation of the capping MPS ligands of the CdS QDs, due to the presence of traces of water in the colloidal solution. The resultant gel glass nanocomposites were transparent, dense, and crack-free monolith with a diameter of ca. 35 mm and a thickness around 2 mm (Fig. 3c), which meet the requirement of optical measurements without further processing. As shown in Fig. 4a, the UV-vis absorption and photoluminescent (PL) spectra of the colloidal CdS QDs show that the CdS QDs have a shallow absorption around 375 nm, corresponding to the first optically allowed transition between the electron state in the conduction band and the hole state in the valence band.34 Upon light excitation at 375 nm, the CdS QDs exhibit luminescence with a strong emission at 570 nm and a weak emission at 440 nm, which can be attributed to trap (defect) emission and electron-hole recombination (intrinsic emission), respectively.35 The fluorescence quantum yield of CdS NPs is determined with the value of 16% by using Rhodamine B as a standard.36 Fig. 4b gives the UV-vis transmittance and PL spectra of the resultant gel glass nanocomposites. The nanocomposite glass demonstrates a very good transparency, which is higher than 90% at most of the visible light wavelength region. The zero transmittance in the near ultraviolet region is caused by the strong scattering of the microporous structure of the gel glass, together with the absorption of the CdS QDs. As compared to the emission of the corresponding dispersion, the incorporated CdS QDs present an intensive emission around 490 nm and a very weak
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has the advantage of being highly efficient and convenient. Our strategy involves a two-step sol-gel process (acid-catalyst hydrolysis and basic-catalyst condensation) and the cocondensation of the resulting alkoxysilane-capped CdS QDs with other alkoxysilanes, which allows the CdS QDs to become a part of the covalent network (Fig. 1). Two kinds of alkoxysilanes, tetraethoxysilane (Si(OC2H5)4, TEOS) and 3-glycidoxypropyltrimethoxysilane (CH2OCHCH2O(CH2)3Si(OCH3)3, GPTMS), were chosen as precursors. The long organic chains in GPTMS help improve the elasticity and flexibility of the resultant gel glass, which results in a dense structure. 3-mercaptopropyltrimethoxysilane (MPS) was employed as both the capping ligand of the CdS QDs and the crosslinker between the CdS QDs and the silica network. 3-aminopropyltriethoxysilane (NH2(CH2)3Si(OC2H5)3, APTES) acts as a weak organic basic catalyst for condensation by performing three roles. First, unlike inorganic basic ammonia, for example, which usually causes inhomogeneous coagulation of the silica granules and results in an opaque final product, APTES helps yield a transparent monolith with a highly branched silica network. Second, the use of a basic catalyst leads to rapid gelation, which helps shorten the gelation period from several weeks to several minutes, depending on the amount of APTES introduced. Finally, the neutralization of the mixture by APTES provides a safe environment for the incorporation of CdS QDs by protecting them from degradation.
Transmittance (a.u.)
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Fig. 3 (a) HRTEM and (b) TEM images of the as-synthesized CdS QDs; (c) photograph of the CdS-silica gel glass nanocomposite. The inset in (b) is the SAED pattern taken from the area designated by the rectangle.
emission around 620 nm, along with the disappearance of the peak at 440 nm. To accounted for this, we assume that due to the close packing of the CdS QDs in the composites, particle growth, as well as the removal of some surface defects by perhaps the APTES (Fig 1), what we can actually see is a red shift of the emission and strength inversion of the emission peaks. The peak at 440 nm is now the dominant peak and has red shifted by ~50 nm while the peak at 570 nm has been reduced in intensity due to surface defect removal (by the APTES) and is now at ~620 nm, again red shifting by 50 nm. To investigate the OL property of the samples, an openaperture Z-scan method was used to conduct the fluencedependent light transmittance measurements. Fig. 5 shows the variation in normalized transmittance as a function of input fluence (J cm -2 ) for the CdS QD dispersion and gel glass nanocomposite. Both exhibited a gradually reduced transmittance with increasing input fluence at the investigated 532 nm laser wavelength, indicating an OL effect. For these samples, the energy transmittance at light fluences less than 1 J cm-2 was kept constant, while the transmittance decreased as the incident fluence increased in excess of 1 J cm-2. The OL threshold (Fth),
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defined as the input fluence at which the transmittance falls to 50% of the normalized linear transmittance, is of almost the same value (around 4 J cm-2) for the two samples, indicating a comparable OL effect at a low input fluence. However, the transmittance decreased more rapidly for the glass nanocomposite in comparison to the CdS dispersion, implying an enhanced OL effect at a high input fluence. The OL effect of the CdS QD dispersion and gel glass nanocomposite is much better than noble metal nanoparticles,27 and comparable to benchmarked carbon nanotubes28 and graphene.30 However, it should be noted that, like some noble metal nanoparitcles,37 semi-conductor QDs are also not stable under extreme conditions. At high light intensities, they are susceptible to damage, leading to photofragmentation, ligand desorption, etc. To make them stable at extreme conditions, it is necessary to protect them with stable and chemically inert materials such as oxides.30,38 Our experiments show that after being encapsulated in silica gel glass matrix, no sign of laserinduced damage was observed for the QDs, illustrating the effective protective effect of silica matrix. This makes it possible to fabricate/process materials in the form of thin films and disks for applications. Several OL mechanisms, particularly nonlinear absorption (NLA, two-photon absorption, free-carrier absorption, reverse saturable absorption, etc.), nonlinear scattering (NLS), and nonlinear refraction (NLR) have been found to dominate different Nanoscale, 2013, [vol], 00–00 | 3
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dominant contribution to the observed NLS. For the gel glass nanocomposite, the rigid solid environment obviously precludes the formation of microbubbles and therefore results in the elimination of NLS in this context.
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Fig. 6 Z-scan curves of (a) CdS dispersion and (b) CdS-silica gel glass nanocomposite. ● contribution of NLA; ■ contribution of NLA and NLS.
kinds of OL materials.39 For QDs, however, the generally accepted underlying OL mechanisms are two-photon and/or multiphoton absorption for pico-femtosecond laser pulses and free-carrier absorption and/or NLS for nanosecond laser pulses.5-8 To separate the contributions of NLA and NLS, we use a Z-scan setup following Jourdier and Rao,39,40 and the results are shown in Fig. 6. The intensity variation along the beam propagation direction z in a medium having a third-order nonlinearity is described as dI /dz = −α0I−α2I2, where α0 is the linear absorption coefficient, and α2 is the total nonlinear extinction coefficient. The coefficient α2 is a sum of contributions from both NLA (α2A) and NLS (α2S): α2=α2A +α2S. The position dependent transmittance (the Z-scan curve) is given by
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where z is the sample position with respect to the focus, and q(z) = α2 I0L/[1+(z / z0)2], with I0 being the peak intensity at the focal point. L is the effective length of the sample given by [1−exp(α0l) /α0], where l is the sample length, and z0 = πω02/λ is the Rayleigh range, where ω0 is the beam waist radius at focus, and λ is the wavelength of light. The value for α2 is obtained by fitting the T (z) equation to the experimental data. In the context of our experiment, the Z-scan curve (b) gives α2, and curve (a) is fit with α2A. Thus, α2S can be extracted.41,42 The values of α2, α2A, and α2S were calculated to be 1.99, 1.61, and 0.38; 1.02, 0.98, and 0.04 (×10-14 cm·W-1), respectively, for QD dispersion and the gel glass nanocomposite at input pulses of 1.60 mJ. These values do not change significantly for other input optical energies. It is worth noting that the α2S of the gel glass nanocomposite is almost negligible in comparison to that of the dispersion. This result can be well explained from the mechanism of NLS and the different environment of the liquid and solid state matrices. NLS signals originated from the photoinduced excitation, ionization, and heating of the QDs, giving rise to their expansion and making them the scattering centers. Furthermore, energy transfer to the solvent will result in the formation of microbubbles that act as secondary centers and lead to a further reduction in transmittance with increasing input laser fluence.43 The latter is indeed the 4 | Nanoscale, 2013, [vol], 00–00
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In summary, we have provided a simple and effective method for the preparation of a highly transparent CdS-silica gel glass in densely monolithic nanocomposites. Owing to the synthesis strategy, the incorporated CdS QDs remain stable during the whole process and disperse homogeneously in the final products by forming a covalent bond with the silica network. This facile route can be applied to the bulk preparation of other nanostructures, particularly acid-sensitive nanomaterials. Openaperture Z-scan experiments showed that the resultant gel glass nanocomposites retained the OL effect in comparison to the parent CdS QD dispersion, which makes them practical for OL applications.
4 Experimental and computational methods MPS in N-N’-dimethylformamide (DMF) (0.02 mol L-1, 120 mL) was added to an aqueous solution of cadmium chloride (CdCl2·2.5H2O, 3 mmol, 1.5 mL) and was stirred for 15 min. Aqueous sodium sulfide (Na2S, 1.6 mmol, 1.5 mL) was added dropwise to the solution, which was then stirred for 4 h at room temperature in the dark. The yellow reaction mixture was slowly dehydrated by distillation under reduced pressure while adding fresh DMF to maintain the concentration of the CdS QDs in the suspension. NaCl and large aggregated particles were removed by centrifugation three times, yielding CdS-MPS QDs in DMF. TEOS (0.036 mol, 8.0 mL), GPTMS (0.015 mol, 3.5 mL), ethanol (0.205 mol, 12.1 mL), and H2O (0.205 mol, 3.7 mL) were mixed under magnetic stirring. A small amount of hydrochloric acid was added dropwise to promote hydrolysis (pH=2). After the mixture was stirred for 2 h, APTES was added dropwise to neutralize the mixture to pH=7. Then 9 mL of the as-synthesized DMF solution of the CdS-MPS QDs was gradually introduced, and the mixture was kept on stirring for an additional 15 min. After which, it was divided into several equal volume parts, cast into polystyrene cells individually, sealed, and left to age and dry for 8 weeks. Using the method reported by Peng, 44 we estimated the concentration of CdS QDs in the dispersion to be around 2 mg/mL, and therefore the loading level of CdS QDs in the resultant gel glass composite was calculated to be 0.6 wt%. The transmission electron microscopy (TEM) images of the CdS QDs were obtained on a JEOL JEM-2010 microscope. The samples were dispersed in DMF, and a drop of the solution was placed on a copper grid and then dried before it was transferred into the TEM sample chamber. The scanning electron microscopy (SEM) images of the gel glass nanocomposites were obtained on a Philips Nova Nano SEM 230. UV-vis absorption spectral measurements were carried out with a Shimadzu UV-2450 spectrophotometer. PL spectra were measured on a Deinburgh FL/FS TCSPC 920 spectrofluorophotometer. A modified open aperture Z-scan system was used to measure the OL properties of the materials using 8 ns Gaussian pulses from a Q-switched Nd:YAG laser. The beam was spatially This journal is © The Royal Society of Chemistry 2013
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filtered to remove the higher-order modes and was focused with a lens with a focal length of 30 cm. The laser was operated at 532 nm with a pulse repetition rate of 1 Hz. The energy levels of a single pulse were 0.24, 1.09, and 1.60 mJ. The laser beam waist was approximately 14.5 μm. All measurements were conducted at room temperature. The CdS QD suspension was contained in 5 mm-thick quartz cells, while the CdS–silica gel glass nanocomposite was fixed vertically using a clamp. Each sample was mounted on a computer-controlled translation stage that translated the sample along the Z-axis. The linear transmittance of the CdS QD suspension was adjusted to 70%, whereas that of the CdS–silica gel glass nanocomposite was measured to be around 45%. The OL curves, which can be plotted as normalized transmission versus input fluence, were calculated from the open aperture Z-scan data.
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This research was supported by the National Natural Science Foundation of China (No. 51172045), Research Fund for the Doctoral Program of Higher Education of China (No.20113514120006) and Natural Science Foundation of Fujian Province (No. 2012J0511).
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College of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian 350108, China, E-mail: [email protected]
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