English

• 0.   Introduction

  Currently, the performances of nanomaterials comprising only one substance are no longer sufficient to meet the needs of an increasingly developed society. A variety of nanocomposites with excellent properties have become prevalent in the field of materials^[1-2].

  Nanosilica materials have become crucial carrier materials on account of their large specific surface area, good chemical stability, nontoxicity, hydrophilicity and easy functionalization^[3-4]. For these reasons, silica is the optimal material for nanocomposite matrices, and the morphology and dispersion of its materials are innovations of researchers.

  Quantum dots (QDs) are ideal fluorophores for bio-imaging and broadly serve the major field of biomedical sciences with their fluorescent labeling ability and chemical properties^[5-6]. They can emit or absorb light at specific frequencies. Researchers can precisely control these frequencies by changing the size, shape and type of QDs to select the specific wavelengths required. Furthermore, Au nanoparticles (NPs) have inestimable potential in terms of light, electricity and magnetism due to their surface effects and quantum size effects^[7]. In addition to their special electronic and optical properties, Au NPs also exhibit interesting nonlinear optics and strong surface plasmon resonance properties, which are exactly what we need to develop^[8]. Effectively regulating and utilizing the surface plasmon resonance properties of Au NPs can promote related research. Moreover, the surface plasmon resonance properties of particles can promote related research, and it is beneficial to develop the practical application of composite nanomaterials.

  Many researchers have reported the preparation and application of SiO₂ and CdTe QDs or SiO₂ and Au NPs combinations. Ge et al. synthesized SiO₂@CdTe NPs by hydrothermal method on mercapto capped silica NPs (SH-SiO₂). SiO₂@CdTe NPs have good fluorescence preservation properties and are used for the detection of H₂O₂ in chemistry and biology^[9]. Pan and Jie et al. used EDC/NHS cross-linking to connect CdTe QDs on the surface of amino SiO₂ to form CdTe functionalized SiO₂, which has a great application prospect in clinical diagnosis^[10-11]. Wang et al. reported a method of sound chemical auxiliary seed growth on the surface of SiO₂ self-assembly cationic polyethyleneimine (PEI). The cationic thin layer with lots of primary amine groups formed, which is easy to absorb density of Au seed, and the preparation of SiO₂@Au NPs used highly uniform size and surface-enhanced Raman scattering (SERS) active, which are ideal SERS tags for SERS-based immunoassay^[12]. Yang et al. prepared gold-embedded silica by in-situ deposition. Au(OH)₃ was formed on the surface of amines coated silica NPs, which was used as the nuclear site to form gold clusters, and SiO₂@Au core-shell NPs were obtained, which could control heat generation at the nanometer level^[13].

  In the field of materials, it is common that silica combines with CdTe QDs or Au NPs. Because of the complex preparation process and other issues, it rarely occurs in nanocomposites comprised of three materials. In order to get ternary composite NPs, Liu and Jiang et al. coated gold particles with silica by amino functionalization, and then the composite was covalently bonded with CdTe QDs through EDC/NHS crosslinking to form core-shell NPs for biosensors and biomarkers^[14-15]. Zhou et al. synthesized gold nanorods @SiO₂@CdTe QDs hybrid nanostructures, and photoluminescence (PL) spectra showed that the quenching effect of gold nanorods is reduced through the isolation of silica layer^[16]. Wang et al. put CdTe encapsulated into SiO₂, and synthesized bovine serum albumin (BSA) modified gold cluster (Au@BSA). Then the composite was covalently connected by amino and formed a new type of satellite core CdTe/Silica/Au replication hybrid NPs which were used as dual emission ratio fluorescent probes for Cu^{2+ [17]}. Although these nanocomposites have been prepared, the preparation processes are complex and expensive.

  Therefore, we have developed nanocomposites to overcome these shortcomings and improve material performance. Three basic nanomaterials were combined into one material to complement each other. This new material not only retained the advantages of the three original materials but also improved the nonlinear optical properties of the final product. Adding a charged polymer solution to the surface of each material can promote the equilibrium of positive and negative phase adsorption so that the silica, QDs and Au NPs firmly adsorbed layer-by-layer. More unexpectedly, the layer- by-layer method allowed the polymer to be more compact without affecting the other layers or destroying its own properties. The nondispersibility and noninfluence of this method bring great benefits of the preparation of nanocomposites.

  The paper describes the tight layer-by-layer adsorption of QDs and Au NPs on silica spheres. Au NPs were supported on the matrix material, effectively utilizing the advantages of both, improving the optical properties and stability of the composite. This phenom- enon was accompanied by the unique fluorescent properties of the QDs, making the material a refined nanocomposite. In addition, the prepared SiO₂@CdTe and SiO₂@CdTe@Au nonlinear absorption coefficients were measured by a Z-scan device to investigate the beneficial nonlinear optical properties.

  1.   Experimental

  1.1   Synthetic procedures

  The formation process of SiO₂@CdTe@Au NPs is shown in Fig. 1. First, we used an improved Stöber method to prepare SiO₂ NPs^[18]. Phase A (150 mL ammonia and 880 mL ethanol) was added to a 2 L three-necked flask and heated in a water bath to 30 ℃ with rapid mechanical stirring at 280 r·min^-1. At the same time, another 1 L three-necked flask was prepared, and phase B (36 mL tetraethylorthosilicate and 880 mL ethanol) was likewise heated in a water bath to 30 ℃. When the temperature of both samples reached 30 ℃, phase B was quickly added to phase A, and the reaction was stirred for 12 h. SiO₂ NPs of different particle sizes were prepared by varying the reaction time and temperature. HAuCl4, sodium citrate, and deionized water were used to prepare Au NPs according to the literature^[19], and aqueous CdTe QD solutions were prepared according to a previous report^[20]. Subsequently, we adopted the properties of the positive and negative charge phases of the electrolyte solution to synthesize the final composite sample. We added 0.4 mL of the positively charged polymer solution P+ (100 μL 3.5% poly(diallyldimethylammonium chloride) (PDADMAC) solution with 1 mol·L^-1 NaCl solution as base solution) while stirring. The adsorption time was at least 30 min, and the mixture was centrifuged three times. CdTe QDs were added while stirring continuously. After reacting for 30 min, 0.4 mL of the negatively charged polymer solution P- (10 mg·mL^-1 P- solution: poly(sodium 4-styrenesulfonate) (PSS) with 1 mol·L^-1 NaCl solution as base solution) was added. After centrifugation, 0.4 mL of solution P+ was added. Then, after sufficient reaction for 30 min, sodium citrate-modified Au NPs were added by centrifugation. The nanocomposite particle solution obtained after sufficient reaction time was centrifuged and stored. The above method for coating CdTe QDs and Au NPs layer by layer on SiO₂ spheres is referred to as the layer-by-layer (LBL) method^[21].

  Figure 1

  

  Figure 1.  Process of forming SiO₂@CdTe@Au composite NPs with LBL method

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  1.2   Characterization

  The morphology required for each prepared sample was characterized by scanning electron microscopy (SEM, S-4800SEM, HITACHI, Japan) and transmis- sion electron microscopy (TEM, FEI Tecnai F20, FEI, American). The accelerating voltage of SEM was 5 kV, and the working distance was 8.9 mm. The accelerating voltage of TEM was 220 kV. The elemental mapping of the product was gained by using a Quanta 200 FEG scanning electron microscope (FEI, American) to capture energy dispersive X-ray spectroscopy (EDS). The corresponding crystals were studied by X-ray diffraction (XRD, D8 advance, Bruker, German) with Cu Kα (λ=0.154 nm). The tube current was 100 mA and the tube voltage was 50 kV, and the scanning range (2θ) was 5°~80°, with a scanning rate of 5 (°) ·min^-1. The UV-Vis spectra of Au NPs, CdTe QDs and SiO₂@ CdTe@Au composite NPs were measured by a UV-Vis spectrophotometer (TU-1901, Puxi Instruments, China). A continuous wave laser at 405 nm (MLL-Ⅲ, CNI) served as the excitation source for steady-state laser excitation. Steady-state fluorescence spectra were collected by a spectrometer (IHR550, HORIBA, Japan) with a CCD (Synapse, HORIBA Jobin Yvon, Japan).

  1.3   Z-scan technique

  The third-order nonlinear absorption properties were detected and measured by open-aperture Z-scan according to a previous report^[22].

  The laser source used in this experiment was generated by a Q-switched Nd∶YAG laser (Surelite Ⅱ, Continuum) with a pulse width of 4 ns (full width at half maxima, FWHM) at 532 nm and 4.8 μJ single-pulse energy. A low repetition rate was used under all conditions (10 Hz in nanosecond pulses) to eliminate spurious cumulative effects originating from thermally induced nonlinearities. A water solution with a linear transmittance of 56% measured in a 2 mm quartz cell was used as the sample. A nonlinear absorptive back-ground from the solvent was removed from the data. The nonlinear absorption properties of both SiO₂@CdTe and SiO₂@CdTe@Au were tested with an open aperture Z-scan device.

  2.   Results and discussion

  The method we have adopted to combine SiO₂ with CdTe and Au is obviously different from the existing methods. In order to get the ternary-composite NPs, the gold particles were coated with silica and then covalently bonded with CdTe QDs or CdTe QDs was encapsulated into SiO₂, then covalently bonded with gold^[14-17]. The main purpose of these preparation methods is to avoid contact which leads to gold quenching. Although these nanocomposites have been prepared, the preparation processes are complex. The LBL method we adopted is relatively simple and it can protect Au NPs from quenching.

  As shown in Fig. 2, SiO₂ nanoparticle samples with a particle size of approximately 120 nm were obtained by a modified Stöber method. SiO₂ NPs exhibited a uniform spherical shape in the SEM image, which was consistent with the TEM image (Fig. 3). The adsorption process of Au NPs from less to more is also shown in the related figures.

  Figure 2

  

  Figure 2.  SEM images of (a) SiO₂ NPs, (b) SiO₂@CdTe, (c) SiO₂@CdTe@Au(less) and (d) SiO₂@CdTe@Au(more)

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  Figure 3

  

  Figure 3.  TEM images of (a) SiO₂ NPs, (b) SiO₂@CdTe, (c) SiO₂@CdTe@Au(less) and (d) SiO₂@CdTe@Au(more)

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  Consequently, according to the TEM image, we successfully prepared the product. Obviously, CdTe QDs and Au NPs were adsorbed on the SiO₂ spheres. In addition, Au NPs were uniform in size and dispersed, and the diameters of these particles were 5~20 nm.

  Fig. 4 shows an EDS mappings, and it is confirmed again that (a) Si, (b) Cd, (c) Te and (d) Au nanoparticles are present in SiO₂@CdTe@Au. The XRD patterns of various samples prepared in the experiment are shown in Fig. 5. The pattern of CdTe QDs in the observation showed three distinct diffraction peaks at 25.61°, 42.61° and 50.31°. In contrast to the standard card (PDF No. 15-0700), these nanocrystals have cubic sphalerite structures. The diffraction peaks of synthesized SiO₂ and SiO₂@CdTe were relatively dissimilar; three diffraction peaks that were obvious in the XRD pattern of CdTe QDs were not present, but a new peak of 21.09° (2θ) appeared. Although the peak positions of some samples were not obvious, the TEM image and fluorescence spectrum can totally prove the presence of CdTe QDs, despite of the low diffraction peak of CdTe QDs. For the composite nanomaterial, we measured the XRD diffraction peak of the sample with a small number of Au NPs, and then we measured that of the sample containing a large number of Au NPs. Both of their diffraction peaks were unclear compared to the images of Au NPs themselves.

  Figure 4

  

  Figure 4.  EDS-elemental mappings of SiO₂@CdTe@Au

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  Figure 5

  

  Figure 5.  XRD patterns for as-prepared samples: CdTe QDs, Au NPs, SiO₂, SiO₂@CdTe, SiO₂@CdTe@Au(less) and SiO₂@CdTe@Au(more)

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  Fig. 6 shows that the maximum absorption peak for CdTe was at 516 nm. Yu′s team^[23] summarized the relationship between the size of QDs and the first absorption peak. The size of CdTe was approximately 2.2 nm. Au NPs had a characteristic surface plasmons peak at 520 nm. SiO₂@CdTe, SiO₂@CdTe@Au(less) and SiO₂@CdTe@Au(more) nanocomposites showed the phenomenon of red shift, which is due to the increase of the sample size resulting from the adsorption of QDs and gold NPs. The absorption spectra of these nanocomposite materials were not as strong as those of the corresponding monomer CdTe QDs. This is because SiO₂ NPs also absorb light, which partially covers the absorption spectrum of QDs. In the fluorescence spectra (Fig. 7), as the layers of the polymer were added, the sample gradually redshifted. Due to the size effects of QDs, their sizes have a great relationship with their own absorption and emission.

  Figure 6

  

  Figure 6.  UV-Vis absorption spectra of as-prepared samples

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  Figure 7

  

  Figure 7.  Fluorescence spectra of as-prepared samples

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  As the particle size of the nanocomposite material with adsorbed QDs and gold increased, the corresponding fluorescence spectrum also shifted to the long wavelength direction, showing a clear redshift phenomenon. In the images and characterization experiments of all SiO₂@CdTe@Au composite nanoparticles described above, it is apparent that CdTe QDs and Au NPs were adsorbed on SiO₂ spheres to form nanocomposite particles.

  The nonlinear absorption characteristics (Fig. 8) exhibited by SiO₂@CdTe are derived from CdTe QDs. The QD material is a three-dimensionally constrained quantum system. When its size is close to the Bohr radius, its energy level structure changes with the size, which in turn leads to a change in nonlinear absorption characteristics. The wavelength of the incident light of the laser was 532 nm, and the single photon absorption of CdTe QDs was responsible for the saturated absorption of the sample. In addition, Au NPs are attracting attention due to their surface plasmon resonance; they exhibit a distinct nonlinear optical response while excited near surface plasmon resonance. As a result, the local field resonance in the NPs is strengthened; this phenomenon is called the local field effect. At the near-plasma resonance excitation frequency, an enhancement phenomenon occurs^[24].

  Figure 8

  

  Figure 8.  Open aperture Z-scan normalized transmittance curves of SiO₂@CdTe and SiO₂@CdTe@Au NPs at 4 ns

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  Desirable results have been further obtained by fitting the open-aperture Z-scan experimental data to a theoretical formula^[22]. The nonlinear absorption transmittance of the Z-scan experimental medium can be expressed by Eq.1, and the nonlinear absorption coefficient β can be calculated^[22]:

  $ T\left( z \right) = 1 - \frac{{\beta {I_0}{L_{{\rm{eff}}}}}}{{\sqrt 8 (1 + {z^2}/z_0^2)}} $

  (1)

  where I₀ is the maximum light intensity at the laser focus; L_eff is the effective length of the sample, and L_eff=(1-T₀)l/(-ln T₀); T₀ is the linear transmittance of the solution; l is the sample pool thickness; z₀ is the diffraction length of Rayleigh diffraction, and z₀=πω₀²/λ; λ is the laser incident wavelength; ω₀ is the radius of the laser beam waist at the focal point. By the calculation of Eq.1, we can obtain the nonlinear absorption coefficients of SiO₂@CdTe and SiO₂@CdTe@Au, as shown in Table 1.

  Table 1

  

  Table 1.  Nonlinear absorption coefficients of SiO₂@CdTe and SiO₂@CdTe@Au

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  Sample	β/(m·W-1)
  SiO2@CdTe	-8.00×10-10
  SiO2@CdTe@Au	-1.60×10-10

  Fig. 8 indicated that the nonlinear optical absorption properties increased with increasing coatings and that the materials exhibited saturated absorption, which means that the transmittance of the laser became larger as the light intensity increased. Various experimental results have proven that the nonlinear absorption characteristics of SiO₂@CdTe@Au are stronger than those of SiO₂@CdTe. We speculate that this is all due to the unique surface plasmon resonance phenomenon of Au NPs. On this basis, we are confident that the nanocomposite material has promising nonlinear optical absorption properties.

  3.   Conclusions

  Herein, nanocomposites were prepared by taking advantage of the positive and negative charge properties of the electrolyte solution to adsorb CdTe quantum dots and Au nanoparticles firmly onto SiO₂ nano-spheres by layer-by-layer adsorption to prepare SiO₂@CdTe and SiO₂@CdTe@Au nanocomposite materials. Through a variety of characterization techniques and from different perspectives, the morphologies of the materials were obtained, and the synthesis of the nanocomposites was verified. Finally, the nonlinear optical absorption characteristics of SiO₂@CdTe and SiO₂@CdTe@Au nanocomposites were studied comparatively under the action of nanosecond and picosecond laser pulses using Z-scan technology. The experimental results show that nanocomposite SiO₂@CdTe@Au experiences a nonlinear optical property enhancement compared to SiO₂@CdTe. The main reason for this enhancement is the surface plasmon resonance enhancement effect of Au nanoparticles.

  
  
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• 

  Figure 1  Process of forming SiO₂@CdTe@Au composite NPs with LBL method

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  Figure 2  SEM images of (a) SiO₂ NPs, (b) SiO₂@CdTe, (c) SiO₂@CdTe@Au(less) and (d) SiO₂@CdTe@Au(more)

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  Figure 3  TEM images of (a) SiO₂ NPs, (b) SiO₂@CdTe, (c) SiO₂@CdTe@Au(less) and (d) SiO₂@CdTe@Au(more)

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  Figure 4  EDS-elemental mappings of SiO₂@CdTe@Au

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  Figure 5  XRD patterns for as-prepared samples: CdTe QDs, Au NPs, SiO₂, SiO₂@CdTe, SiO₂@CdTe@Au(less) and SiO₂@CdTe@Au(more)

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  Figure 6  UV-Vis absorption spectra of as-prepared samples

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  Figure 7  Fluorescence spectra of as-prepared samples

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  Figure 8  Open aperture Z-scan normalized transmittance curves of SiO₂@CdTe and SiO₂@CdTe@Au NPs at 4 ns

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  Table 1.  Nonlinear absorption coefficients of SiO₂@CdTe and SiO₂@CdTe@Au

  Sample	β/(m·W-1)
  SiO2@CdTe	-8.00×10-10
  SiO2@CdTe@Au	-1.60×10-10

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