Laser-induced growth of nanocrystals embedded in porous materials
© Capoen et al.; licensee Springer. 2013
Received: 14 March 2013
Accepted: 24 April 2013
Published: 6 June 2013
Space localization of the linear and nonlinear optical properties in a transparent medium at the submicron scale is still a challenge to yield the future generation of photonic devices. Laser irradiation techniques have always been thought to structure the matter at the nanometer scale, but combining them with doping methods made it possible to generate local growth of several types of nanocrystals in different kinds of silicate matrices. This paper summarizes the most recent works developed in our group, where the investigated nanoparticles are either made of metal (gold) or chalcogenide semiconductors (CdS, PbS), grown in precursor-impregnated porous xerogels under different laser irradiations. This review is associated to new results on silver nanocrystals in the same kind of matrices. It is shown that, depending on the employed laser, the particles can be formed near the sample surface or deep inside the silica matrix. Photothermal and/or photochemical mechanisms may be invoked to explain the nanoparticle growth, depending on the laser, precursor, and matrix. One striking result is that metal salt reduction, necessary to the production of the corresponding nanoparticles, can efficiently occur due to the thermal wrenching of electrons from the matrix itself or due to multiphoton absorption of the laser light by a reducer additive in femtosecond regime. Very localized semiconductor quantum dots could also be generated using ultrashort pulses, but while PbS nanoparticles grow faster than CdS particles due to one-photon absorption, this better efficiency is counterbalanced by a sensitivity to oxidation. In most cases where the reaction efficiency is high, particles larger than the pores have been obtained, showing that a fast diffusion of the species through the interconnected porosity can modify the matrix itself. Based on our experience in these techniques, we compare several examples of laser-induced nanocrystal growth in porous silica xerogels, which allows extracting the best experimental conditions to obtain an efficient particle production and to avoid stability or oxidation problems.
Introduction and background
Linear and nonlinear optical properties of metal [1, 2] and semiconductor [3, 4] nanoparticles are now well-known, and numerous applications [5, 6] have been envisaged for ages. In the past decade, we have witnessed huge advances in chemical synthesis of nanoparticles (NPs), especially metal NPs that have found applications in plasmonics-based biosensors and surface-enhanced Raman spectroscopy (SERS) detectors [7, 8] and biological fluorescent labeling . Up to now, the commercial use of NPs, still limited to colloidal solutions or thin films, is always based on the linear optical properties of metal clusters (the so-called surface plasmon resonance (SPR)) or of semiconducting nanocrystals (tunable exciton light emission). In order to exploit the now demonstrated nonlinear optical properties [10, 11] of such quantum dots and to go further towards photonics applications (lasers, optical fibers), we now need to embed the nanocrystal in vitreous matrices, if possible, in a localized manner. However, in the state of the art, when nanoparticles can be produced in glasses or other transparent matrices, it is essentially without space selectivity. Through photosensitivity effects, the laser techniques have been demonstrated for many years to be efficient in structuring the matter and more particularly in Bragg embodiment in optical waveguides . Either isotropic or anisotropic linear refractive index changes (up to a few 10−3) have been obtained under laser irradiation, due to densification processes or stoichiometric defects in hydrogen-loaded germanosilicate glasses. Furthermore, where pulsed lasers are used with higher fluence or high peak power density, larger densification and even damaging can occur, yielding a large refractive index contrast, a seducing application of which could be imagined in the topical domain of data storage . Finally, at the highest power density, the intense electric field may blast the matter, producing surface corrugation or microbubbles. With regard to the production of NPs using a laser, apart from the now well-known pulsed-laser deposition and laser pyrolysis techniques, a recent method based on laser-induced transfer of molten metal allowed to deposit one unique small gold particle (20 nm diameter) on a surface . All of these techniques are however inappropriate for doping a bulk sample with NPs.
where λ is the radiation wavelength, and NA is the microscope numerical aperture.
Moreover, considering the inevitable atomic diffusion in the glass under high laser power densities, this resolution is finally comparable with that of a phase mask technique (approximately 0.5 μm). Hence, it would be an illusion to believe in achieving the creation of one unique particle (the grail of nanoscience), but at least the wavelength scale can be reached, and more importantly, the number of possible designs is virtually infinite at the micron scale.
Alkoxide-derived inorganic xerogels have been recently shown as a much cheaper alternative to chemical vapor deposition methods for providing pure silica rods. Those porous silica rods could even be easily impregnated with doping solutions, then densified and drawn into microstructured fiber preforms after association with other silica rods . Here, we review the different experimental configurations employed in our group (in Lille) to obtain space-localized metal or semiconductor NP in a bulk xerogel. The main objective is to help the reader compare and choose the best method, together with the adapted precursor for the space-selective growth of NPs. The criteria of this choice could be space resolution, high efficiency, particle size, stability, etc.
Characteristics of materials and lasers
Metal nanoparticles in a xerogel
The localization of metal nanocrystals inside a glass matrix may be desired for applications based on the electric field enhancement, like SERS detection, for their sensitivity to environment  or for their optical nonlinearities .
The silver precursor has a strong influence on the reduction process. To realize this, a more complicated molecule can be used, like silver hexafluoroacetylacetonate (1.5-cyclooctadiene), alias Aghfacac. Contrary to the silver nitrate, this precursor molecule is not entirely broken in the aqueous solution and presents several bonds between Ag and the organic groups. As a consequence, the energy density necessary to produce NP is multiplied by 2.5, and only a slight release of Ag+ ions occurs under the laser irradiation. This is the reason why the optical spectra exhibit a very weak SPR band after irradiation, contrary to the band at 307 nm ascribed to the precursor, which remains almost unchanged (Figure 4d). In other words, a nonnegligible amount of complementary thermal energy is necessary to obtain Ag+ ions from this precursor. This heat quantity, coming from the weak absorption of light by the matrix and by the precursor, is also used to grab electrons from the matrix defects.
Such a scheme is quite different from the one explaining the photoprecipitation of Au-NP in the same kind of samples under CW laser irradiation . The CW irradiation conditions being more or less the same as previously described for Ag-NP and the Au-doped sample being the same as in the fs experiment, the result shown in Figure 5b is the local production of Au-NP at the surface of the sample, with a size distribution between 5 and 15 nm and a rather good space resolution of 20 μm. Although limited to the sample surface, this approach presents two main advantages over the IR fs experiment: firstly, CW lasers are obviously cheaper than a fs laser chain. Secondly, since one-photon absorption generates sufficient energy to extract electrons from the matrix, no carbonate additive is required here. In any case, both growing processes can be qualified as efficient to produce Au-NPs in a porous silica gel.
Semiconductor nanoparticles in a xerogel
Semiconducting nanoparticles (SC-NP) are particularly suited to increase the linear refractive index of a glass, because their own refractive indices are among the highest . For example, Bragg mirrors of high efficiency can be foreseen using a series of PbS-doped and nondoped regions in an optical fiber. Moreover, quantum confinement in SC-NP is the base for the well-known narrow and tunable light emission having a great potential in display and lighting technologies . SC-NPs are also of particular interest for their high nonlinear refractive indices  and absorption coefficients , which depend on particle size too.
Cadmium sulfide nanoparticles
Lead sulfide nanoparticles
An even darker and stronger coloration could be obtained by using a visible CW laser . In this latter case, the high concentration of NP observed in the TEM image of Figure 8b is an indication of the process efficiency, as well as the particle size that overpasses the mean pore size. For the highest doping concentration (precursor solution 0.37 M), the mean NP diameter, estimated using PbS peaks in XRD pattern and Debye-Scherrer equation, seems to reach a maximum around 11 nm, namely about twice the pore size diameter. However, the particle size can be tuned down to 2 or 3 nm by decreasing the doping concentration.
NP size: correlation with photoprocess efficiency
Mean NP size (nm) CW
Mean NP size (nm) ns
Mean NP size (nm) fs
10 to 20, ME
4 to 8, HE
3 to 8a, LE
2 to 3, LE
5 to 15, HE
8 to 11, HE
4 to 8, HE
The authors acknowledge financial supports from the French National Agency (ANR) in the frame of its program in Nanosciences and Nanotechnologies (POMESCO project), the ‘Conseil Régional Nord Pas de Calais Picardie,’ and the ‘Fonds Européen de Développement Economique des Régions’.
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