NANO LETTERS
3D Quantum Dot Lattice Inside Mesoporous Silica Films
2002 Vol. 2, No. 4 409-414
Sophie Besson,†,‡ Thierry Gacoin,† Christian Ricolleau,§ Catherine Jacquiod,‡ and Jean-Pierre Boilot*,† Groupe de Chimie du Solide, Laboratoire de Physique de la Matie` re Condense´ e, UMR CNRS 7643, Ecole Polytechnique, 91128 Palaiseau, France, Laboratoire CNRS/Saint-Gobain “Surface du Verre et Interfaces”, UMR CNRS 125, 39 quai Lucien Lefranc, 93303 AuberVilliers, France, and Laboratoire de Mine´ ralogie Cristallographie de Paris, UMR CNRS 7590, UniVersite´ s Paris VI et Paris VII, 4 place Jussieu, 75252 Paris Cedex 05, France Received November 29, 2001; Revised Manuscript Received January 23, 2002
ABSTRACT We report the synthesis of CdS quantum dots inside 3D hexagonal mesoporous silica films using a very simple chemical route that can be generalized to other II−VI semiconductors. X-ray diffraction and electron microscopy characterizations unambiguously demonstrate that the mesoporous structure allows to control the particle size and the organization of CdS nanoparticles. This leads to the growth of a 3D quantum dot lattice at a large scale.
The synthesis of nanosized materials has become an important research field since the past decade, as these materials have specific optical and electronic properties, which differ significantly from the bulk material. Physical properties, such as enhanced local field effects for metals,1 quantum confinement for semiconductors,2 and superparamagnetism for magnetic compounds,3 are observed for nanocrystals below 10 nm. As the physical properties of nanocrystals are highly dependent on their size, their shape, and their surface state, it is very important to control these parameters. Moreover, the arrangement of nanoparticles is crucial because it can determine the physical properties of nanosized materials for applications in different domains such as nonlinear optics, optoelectronics, biology, or catalysis.4 In this field, the production of 3D periodic arrays of nanoparticles represents the ultimate challenge for the synthesis of new materials. Periodic mesoporous materials are potential candidates to support nanocrystals. These materials are synthesized via the polymerization of inorganic species (generally silica) around a periodic organic template, which could be surfactant micelles, or copolymers. After a thermal treatment, a porous material is obtained whose pores are the perfect replica of the organic species. Since the discovery of MCM-41 in 1992,5 a lot of mesostructures with different pore sizes (from 2 to 30 nm), shapes, and periodicities have been synthesized * Corresponding author. E-mail:
[email protected] † Groupe de Chimie du Solide, Laboratoire de Physique de la Matie ` re Condense´e. ‡ Laboratoire CNRS/Saint-Gobain “Surface du Verre et Interfaces”. § Laboratoire de Mine ´ ralogie Cristallographie de Paris. 10.1021/nl015685v CCC: $22.00 Published on Web 02/07/2002
© 2002 American Chemical Society
by varying the synthesis conditions and the surfactant molecules.6,7 These mesoporous materials can be used as molds to synthesize nanocrystals with controlled size and shape. They can also act as templates for the synthesis of periodic 2D or 3D arrays of nanoparticles, which is a key point to study collective effects.8,9 In addition, the use of ordered porous structures is the ideal route to optimize the space filling by nanocrystals in a solid porous matrix. To date, a lot of studies have concerned the growth of nanoparticles inside mesoporous powders.10-17 But, in these materials, particles are generally randomly distributed within the porous matrix and their size is not well controlled. In fact, only a few studies have succeeded in synthesizing periodic arrays of nanoparticles18,19 but with rather small domains, except in the promising works of Ryoo et al. who have synthesized the carbon20,21 and platinum22,23 replicas of MCM-48 and SBA-15. Some papers have reported the synthesis of nanowires by using mesoporous materials with cylindrical pores.19,24,25 However, for several optical applications, it would be better to dispose of coatings filled with nanoparticle arrays instead of powders. To our knowledge, only Tang et al.,26 Dag et al.,27 and Plyuto et al.28 have reported the synthesis of nanoparticle-loaded mesoporous films. In the first case, SiGe was deposited by MBE on a mesoporous film, but the width of penetration inside was not clearly characterized. It is the same problem in the second example, where silicon clusters were grown by CVD in an uncalcined free-standing film. In the last case, silver nanoparticles were grown inside
a mesoporous film but without any control of the particle size and organization. We report here the synthesis of CdS nanoparticles inside 3D hexagonal mesoporous silica films. Films were characterized by UV-visible absorption spectroscopy, X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). We used the following general strategy to precipitate metal sulfides within mesoporous matrices. A preliminary point concerns the use of highly organized and textured mesoporous films as matrices. Recently, we have reported the synthesis of strongly textured mesoporous films by spincoating on glass plates with an organization process that takes place over the whole film thickness.29,30 They have a 3D hexagonal structure which is oriented with the c-hexagonal axis perpendicular to the film plane. These films exhibit open porosity allowing impregnation by different solutions, which is necessary to fill the pores with nanoparticles. The first step of our chemical process concerns the homogeneous adsorption of cationic species on the silica walls that form the pore surface. This implies the impregnation by well-controlled pH aqueous solutions with soluble cationic species. The presence of complexing agents is generally required to avoid the precipitation of hydroxides. For example, concerning cadmium ions, the pH of the solution should be controlled in the 9-10 range because the adsorption on silica is optimal above pH ) 9 and the silica walls are partially soluble when the pH is above 10.31 In this pH range, soluble cadmium species are obtained by using citrate and ammonia ligands. The impregnated film should be then washed to eliminate reactive in excess, and above all to avoid the accumulation of cationic species at the film-air interface. In most cases, and especially for cadmium ions, interactions with Si-Ogroups are so intense that no significant leaching is observed during the rinsing stage. The last step of the process is the homogeneous formation of nanoparticles via the precipitation of the ionic species. This requires the use of gaseous hydrogen sulfide, which rapidly diffuses into the film leading to simultaneous precipitation within all the pores. Some heterogeneity, due to the formation of isolated porous domains closed by nanoparticles, is then avoided in the films. Finally, the precipitation of CdS nanoparticles regenerates the silanols at the pore surface and thus allows a second adsorption of cadmium ions. To fully fill the mesoporous film with CdS, several cycles of impregnation-precipitation were repeated until the film saturation was obtained. This contrasts with the previously reported techniques developed for the formation of CdS nanoparticles within MCM-41 powders which used a single impregnation step procedure.32-34 Mesoporous matrices were synthesized using a previously described procedure.29 The polymeric silica sol was prepared under acidic conditions by mixing TEOS (Si(OC2H5)4), water (pH ) 1.25) and ethanol in the 1:5:3.8 molar ratio, and aged 1h at 60 °C. The CTAB was then dissolved into the sol with the CTAB/TEOS molar ratio equal to 0.1. The obtained solution was diluted with ethanol (1:1) and deposited on 410
Pyrex slides by spin-coating at 3000 rpm. The films were then calcined in air at 450 °C to remove the surfactant molecules. In these conditions, highly ordered films of about 300 nm in thickness were obtained, with a 3D hexagonal structure throughout, oriented with the c-axis perpendicular to the substrate. Cadmium ions were introduced within the mesoporous film by impregnation with a basic aqueous solution of cadmium nitrate. The solution was prepared by addition to a 0.1 M cadmium nitrate aqueous solution of 1 equiv ammonia and 1 equiv sodium citrate (1 M in water), completed with ammonia up to pH ) 9.5. The film was then washed in deionized water to eliminate the cations in excess and have a proper surface. The resulting film was put under vacuum, and gaseous H2S was injected slowly until PH2S ) Patm. These two steps (impregnation and precipitation) were repeated until the film saturation was obtained. The Cd2+ impregnated film was colorless. After the first H2S treatment, the film was light yellow, and the color intensity increased during the following impregnations. Secondary ion-mass spectrometry analyses (SIMS) show a homogeneous distribution of cadmium as a function of the depth, for both the Cd2+ impregnated film and the CdS saturated one. The evolution of the absorption spectrum of the film after each impregnation-precipitation step is shown in Figure 1a. The saturation is observed after nine cycles, and the presence of excitonic transitions due to the quantum size effect shows that the particle size distribution is narrow. As deduced from the gap-size correlation curves,35 the average particle diameter increases from 2.2 nm after one impregnation to 3.6 nm at the saturation (Figure 1b). These results suggest that the growth of particles is controlled by the pore size of the mesoporous matrix.36 This is confirmed by preliminary results obtained with another porous film templated with a triblock copolymer. This matrix also exhibits a highly ordered 3D periodic structure but has larger spherical pores.37 Figure 1c displays the evolution of the film absorption spectrum during the filling by CdS. Compared to the spectra obtained with the CTAB film, the absorption edge is shifted to higher wavelengths, which means that the particles are larger. They grow from 3.3 nm after the first impregnation to 5.8 nm at the saturation (Figure 1d). Excitonic transitions are no longer present in the spectra after the second impregnation, as the quantum confinement is weak for sizes above 5 nm.35 These experiments clearly suggest that the particle size can be tuned by using the appropriate matrix. Films were also characterized by X-ray diffraction (CuKR radiation) using an X’Pert Philips diffractometer in the Bragg Brentano geometry. In this configuration, only the 0002 peak of the 3D hexagonal structure and its harmonic are observed on the diffraction pattern, because of the texture of the film with the c-axis perpendicular to the film plane.29 The evolution of the intensity of the 0002 peak after each treatment is represented in Figure 2. After adsorption of cadmium ions on the silica walls and in comparison with the initial calcined film, the peak is twice less intense and shifted to larger 2θ values. This corresponds to a slight Nano Lett., Vol. 2, No. 4, 2002
Figure 1. Evolution of the UV-visible absorption spectrum of the film and of the CdS particle size during the impregnation-precipitation cycles within CTAB (a and b) and copolymer (c and d) templated matrices.
Figure 2. Evolution of the X-ray diffraction pattern of the film during the impregnation-precipitation cycles.
decrease of the c parameter, from 6.9 to 6.8 nm, which can be due to a silica condensation in walls related to the high pH of the solution used for the impregnation. After the first H2S treatment, the intensity is dramatically lowered, then it increases progressively until the film is saturated. These intensity modifications can be explained by an inversion of the scattering contrast. First, the electronic density increases in pores due to the adsorption of Cd2+ ions on silica walls. This decreases the scattering contrast and consequently the intensity of the 0002 peak. By reaction with H2S and growth of CdS particles, the electronic density gradually increases in pores and becomes superior to the silica wall scattering due to the high scattering atomic factors of Cd and S atoms compared with Si and O atoms. This leads to the disappearance of the 0002 peak followed by a progressive increase of its intensity when the CdS particles grow. This inversion of scattering contrast, from silica-air to silica-CdS, clearly shows Nano Lett., Vol. 2, No. 4, 2002
that the pores are progressively filled with CdS nanoparticles. Moreover, no significant change of the 0002 peak width is associated with these intensity modifications, indicating that the size of ordered regions remains constant. This suggests that the order is not affected by the particle growth and that this growth is homogeneous throughout the film thickness. In addition, the figure shows a peak shift to lower 2θ values, corresponding to an increase of the c-hexagonal parameter from 6.8 nm before the first H2S treatment to 7.2 nm for the saturated film. This can be explained by the full filling of the pores, which slightly deforms them. This effect has also been observed for mesoporous powders filled with InP by MOCVD.10 Both UV-visible spectra and XRD patterns show that CdS particles grow inside the pores without altering the structure, and are therefore periodically distributed inside the film. Moreover, taking as reference the maximum of the absorbance of a CdS colloidal solution with the same average size of 3.5 nm,38 one can deduce a CdS volumic fraction of 13% in the saturated film. This can be compared with the mesoporous volumic fraction of 15% deduced from cristallographic data (two mesopores by hexagonal unit cell), showing that the mesoporous space filling is about 85% in the saturated film. The saturated film was also studied by transmission electron microscopy using a Topcon 002B electron microscope operating at 200 kV with a point-to-point resolution of 0.18 nm. The HRTEM images were slightly underfocused in order to enhance the contrast of the particles within the silica film. The organization of the CdS nanoparticles inside the film was studied by examining the film in cross section 411
Figure 3. (a) HRTEM image in cross section of the CdS saturated film. (b) HRTEM image in cross section of a calcined mesoporous film before impregnation. (c) Enlargement of the cross section (a) showing the periodic arrangement of the CdS particles. (d) Power spectrum of the image (a) showing the 3D hexagonal structure.
(Figure 3). In Figure 3a, the CdS quantum dots are imaged as black dots over a white background. At the opposite, in Figure 3b, which shows the empty structure, the dots corresponding to the pores are imaged as white dots over a black background. This inversion of contrast is in very good agreement with the inversion of scattering contrast observed in XRD experiments. In high resolution transmission electron microscopy, the contrast of the image depends on numerous parameters such as zone axis orientation, crystal thickness, objective lens focus value, accelerating voltage. Assuming the crystal thickness as well as the experimental conditions were the same from one sample to the other and the underfocused value of the objective lens being exactly identical, we can conclude that the contrast inversion is due to the inversion of the scattering factor in the filled material (CdS/SiO2) by comparison to the empty material (air/SiO2). Consequently, it can be concluded from Figure 3a that the film is totally filled with CdS nanoparticles (only a few white points near the film-substrate interface correspond to empty pores) and that the structure is preserved. The periodic arrangement of the particles is shown in Figure 3c with some clusters exhibiting 111 lattice fringes of the blende-type structure, which is well-known to occur for this size range of CdS quantum dots.39 In fact, the contrast observed on this 412
micrograph arises from nanoparticles that have different orientations with respect to the electron beam: the darkest particles are in zone axis orientation (i.e., they are in a strong Bragg diffraction position) which is not the case for the others. This figure is a representation of the 3D super-lattice of CdS particles in projection, with a small disorientation of the [112h0] direction of the 3D hexagonal structure relative to the electron beam. Due to a superimposition effect, this gives rise to a continuous gray contrast between the nanoparticles, which is observed in some areas of the Figure 3c. The power spectrum () |Fourier transform|2) of the cross section is shown in Figure 3d. This pattern is similar to the one obtained in cross section for a 3D hexagonal mesoporous film30 (see Figure 2c in ref 30) with the space group P63/ mmc. The lattice parameters are approximately a ) 6.0 nm and c ) 6.8 nm, the c/a ratio is equal to 1.13, which is in agreement with the parameters of an empty mesoporous film. These HRTEM observations demonstrate that the film is homogeneously filled with CdS nanoparticles and that the porous structure enables to control the particle size and organization. Photoluminescence and excitation spectra were performed using a Hitachi F-4500 fluorescence spectrophotometer. To study the properties of the CdS loaded material, a mesopoNano Lett., Vol. 2, No. 4, 2002
Acknowledgment. This work was supported by Saint Gobain Recherche. We thank T. Cretin for SIMS analyses. References
Figure 4. Excitation (dashed lines) and luminescence spectra (solid lines) of CdS nanoparticles in a mesoporous silica film. Spectra are taken at room temperature. Upper curves correspond to spectra recorded after the first H2S treatment. Lower curves show spectra performed after the following impregnation in the cadmium solution.
rous film was deposited on a silicon wafer to avoid the Pyrex slide luminescence. A PL spectrum was taken after the first H2S treatment. Only a weak and broad emission band around 640 nm is observed, which can be attributed to surface defect luminescence. After the following impregnation in the cadmium solution, the surface emission band at 640 nm is enhanced due to the apparition of sulfur vacancies at the nanoparticle surface.40 Another sharp band is observed at 450 nm, which corresponds to direct recombination from the bound exciton (Figure 4). This signal is usually observed in CdS nanoparticles with a passivated surface. In our case, this passivation may result from surface complexation by ammonia or hydroxyl groups.40,41 After the second H2S treatment and the other impregnations, the emission decreases and the bound exciton luminescence disappears. This could be explained by the particle growth which limits the accessibility of their surface and increases the interactions with the silica walls. It has been already shown that such interactions are unfavorable for the direct exciton recombination.42 We have succeeded in filling a 300 nm thick mesoporous film with CdS nanoparticles. The size and the 3D organization of nanocrystals are directly governed by the starting ordered pore structure. Using this template, the 3D order of nanoparticles can be easily extended at a large scale on glass substrates. This contrasts with the assembly of quantum dots produced by the self-organization of colloids, which only leads to small ordered regions.9,43 In addition, this result demonstrates that the 3D hexagonal mesoporous structure is open to the exterior with interconnected pores. The control of nanoparticle size and organization is achieved thanks to the high degree of the film order and to its ability to support several cycles of impregnationprecipitation without structural deterioration. This latter point is essential to saturate the film, whereas in most other studies, only one impregnation-precipitation is realized, which is not sufficient to completely fill the porous material. Finally, our synthesis method offers new opportunities in the elaboration of nanoparticle periodic arrays as it is very simple and could be generalized to other sulfides or selenides, provided that the cation could be anchored to the silica surface. Nano Lett., Vol. 2, No. 4, 2002
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