The irradiation wavelength 800 nm corresponds to energy much smaller than the band gap of silica. Hence, no linear absorption can occur when the matrix is irradiated, thus allowing the laser beam to penetrate deeply inside the material. Hence, it is possible to embed a PbS layer of nanoparticles anywhere in the monolith. However, even at the lowest average power corresponding to tremendous laser peak power, a multi-photon absorption exists. As a result, the impact area is highly localized, confined within the focal volume of the focused beam. The irradiated zone showed brown or black color, depending on the average power of the laser beam. This color is easily visible to the naked eye, as shown in Figure .
The formation of PbS nanocrystals inside the porous silica glass, after laser irradiation, was confirmed by different analysis techniques. According to the different analysis techniques, we have irradiated at different depths: just under the surface for XRD measurements and for the other characterisations, the focused beam was in the middle of the monolith.
TEM analysis has been performed to evidence the formation of PbS nanoparticles. Figure shows a TEM image of the irradiated area at 40 mW. Quasi-spherical nanoparticles, with a size distribution ranging between 5 and 12 nm, have been observed. Figure shows the EDX analysis recorded on the same area as in Figure , confirming that the obtained nanoparticles contained Pb atoms. The Cu peaks at approximately 1 and 8 keV came from the TEM grid. Moreover, Figure presents the HR-TEM of different nanoparticles. The calculation of the atomic interplanar distances has been performed on five zones labeled by circles. The zones 1, 2, and 3 exhibit fringe distances around 0.34 nm, which are attributed to the (111) lattice planes of cubic PbS (d111 = 0.343 nm, JCPDS card, reference code 03-065-0692). While the fringes spacing for the zones 4 and 5 were of about 0.3 nm, which corresponds to the (200) lattice planes of PbS (d200 = 0.297 nm, JCPDS card, reference code 03-065-0692). In addition, Figure presents an electron diffraction pattern taken in a zone filled with nanoparticles. The estimated diffraction radii R1, R2, and R3 correspond well to the PbS lattice planes (200), (220), and (222), respectively.
Figure 2 Results of TEM measurements in the irradiated area with 40 mW: (a) TEM image, (b) EDX spectrum obtained from an ensemble of PbS nanocrystals, (c) HR-TEM image of several PbS nanoparticles, (d) electron diffraction image taken from a zone filled with nanoparticles. (more ...)
Optical absorption spectra have been recorded in the irradiated and in the non-irradiated areas, as shown in Figure (left). The absorption spectrum of the zone irradiated with an average power of 40 mW is red-shifted as compared to the absorption spectra of the non-irradiated zone. Moreover, one can note that the absorbance baseline in the irradiated area is much higher than those of the non-irradiated zone. Such a strong absorption level has been attributed to the formation of PbS nanoparticles in high concentration. The absence of any excitonic absorption structure in the absorption spectra may have two main reasons: first, the weak exciton binding energy because of the strong Coulomb screening in narrow gap semiconductors and second, the existing size distribution of the nanocrystallites.
Figure 3 Left: Optical absorbance spectra of S-doped silica matrix: (a) non-irradiated area, (b) femtosecond laser irradiated area with 40 mW. Right: (c) PL spectrum of a non-irradiated area under excitation at 351 nm, (d) PL spectrum of the irradiated area under (more ...)
Photoluminescence spectra of the area irradiated with a power of 40 mW and in the unirradiated area are shown in Figure (right). Under excitation with the 351 nm laser line, the non-irradiated area emits light in a large band centered on 600 nm, while the excitation of the irradiated area at the same wavelength results in two emission bands: the first one being centered on 650 nm and the second one around 910 nm. Consequently, the bands peaking at 600 and 650 nm could be attributed to the luminescence of PbS precursors and SiO2
host matrix, since the S-doped silica absorption at 350 nm is still strong. On the other hand, the PL band centered at 910 nm is due to the PbS nanoparticles. To confirm our assumption, the same PL spectra were recorded under excitation at 514.5 nm. At this wavelength, the S-loaded matrix absorbs less and as a result, no emission could be observed in the non-irradiated area. However, in the case of the irradiated area, both emission bands are still present, although red-shifted by 20 nm. As far as the PbS nanoparticles are concerned, the red-shift of the 650 nm emission band can be interpreted through a resonant size-selection by the excitation wavelength, as already observed with CdS particles [27
]. The same kind of selection process in the electronic level population may be assumed for the "matrix band" at 650 nm. Meanwhile, the intensity ratio between these two emission bands is inverted when switching from UV to visible excitation. This can easily be understood as the result of a drastic decreasing number of absorbing entities in the non-irradiated zone between 351 and 514 nm wavelengths. The maximum of the emission band located around 930 nm is because of the presence of dominant small PbS nanoparticles (3-5 nm) [28
]. This maximum emission is shifted toward lower wavelengths comparing to the one reported by Hines and Scholes [29
] for a bigger colloidal PbS with a size of 6.5 nm.
In the literature, the mechanism of PbS nanoparticles formation inside a porous silica matrix has been explained by pyrolysis or decomposition of the PbS precursor under heat treatment [21
]. Moreover, Chao et al. [24
] have reported on the mechanism of PbS nanoparticles precipitation inside melting glasses induced by a femtosecond laser: when glasses containing lead and sulfur are submitted to a femtosecond laser irradiation, it can probably induce ion-exchange effects and a redistribution of network modifiers, resulting in the enrichment of the laser-irradiated zone in lead and sulfur atoms. This enrichment makes it possible to decrease the thermal treatment temperature. Hence, the formation of bigger and denser quantum dots in the irradiated area is accelerated after a subsequent and necessary heat treatment. In our case, the nano-crystallization of PbS inside the porous silica occurred in the region of the laser irradiation without any further annealing because of two main processes. First, the multi-photon absorption of the porous silica matrix and of the PbS precursors at 800 nm [30
] leads to the decomposition of these precursors, thus enriching the irradiated zone with lead and sulfur ions. In the same time, the temperature elevation during the laser pulse could be sufficient to aggregate the molecular PbS into nanoparticles [26
The effect of the incident average laser power on the size of the PbS nanocrystallites has been studied. Three S-doped samples were irradiated by the pulsed femtosecond laser at three different incident powers: 10, 25, and 40 mW. XRD measurements were performed on the irradiated samples, as shown in Figure . For each power, the XRD pattern presents reflections for 2θ equal to 26.1°, 29.9°, 42.9°, and 50.8°. These reflexes can be attributed to (111), (200), (220), and (311) reticular planes of the PbS cubic phase, respectively [21
]. One can note an increase in intensity and a decrease in width of the reflexes with increasing the irradiation power. The mean nanocrystal diameter, d
, can be determined from the line width of the XRD pattern by the Scherrer formula [32
XRD patterns of S-doped silica matrices recorded in the irradiated area with different laser powers: (a) 40 mW, (b) 25 mW and (c) 10 mW.
where λ is the X-ray wavelength, B is the full width at half maximum of the diffraction reflex (in radian), and θB is the half angle position of the diffraction reflex on the 2θ scale. The diameter of the PbS nanoparticles, calculated from the Scherrer formula (Equation 1), is summarized in Table .
Calculated sizes of PbS nanoparticles from XRD measurements for different laser average powers
The most important drawback that limits the use of PbS nanoparticles in many applications is their low chemical stability. This is due to their high ability to oxidation, which limits their use under ambient conditions.
To indirectly investigate the stability of PbS nanoparticles precipitated inside the silica host matrix by pulsed laser irradiation, irradiated samples have been left in ambient air a for long period and optical absorption spectrum has been monitored upon time. Figure shows the absorption spectra of two samples irradiated with 10 and 40 mW at three different times after the irradiation. One can note that the absorption edge of the sample irradiated with 40 mW was blue-shifted after 100 days. This blue-shift is attributed to the oxidation of PbS nanoparticles created inside the silica matrix. On the other hand, the absorption edge of the sample irradiated with 10 mW presents no shift after 100 days in comparison with the absorption taken just after preparation.
Figure 5 Optical absorption of PbS nanoparticles created by femtosecond laser at different times. Left: sample irradiated with 40 mW, Right: sample irradiated with 10 mW. (a) 100 days after the initial irradiation, (b) 50 days after the initial irradiation and (more ...)
These preliminary results demonstrate that the stability of PbS nanoparticles, inside the porous host matrix, depends on their size. It should be recalled that the PbS particle mean size, as deduced from the XRD measurements around time t0
, has been evaluated to 4 and 8 nm after irradiations with 10 and 40 mW, respectively (see Table ). In the first case, the crystallite size is thus lower than the average matrix pore size of 5.4 nm, as opposed to the case of 40 mW irradiation. Since the diameter size of PbS nanoparticles created with 40 mW is greater than the pore diameter, it implies that, under the extreme local conditions of the femtosecond laser-induced plasma, the created nanoparticles are capable of breaking the walls of the interconnected pores during their formation. An important amount of oxygen is then allowed to diffuse inside the matrix and to react with PbS nanoparticles, leading to their oxidation in several days. It has been noticed that the oxidation rate seems to decrease versus time. This shift in the absorption band edge could be explained by the formation of core-shell structure with unknown shell phase. Indeed several oxide phases can be observed when PbS is oxidized such as PbO. PbSO4
]. We have already detected this phase when we irradiate the doped monoliths by 514.5 nm continuous laser. On the contrary, no oxidation was observed with a lower laser power of 10 mW, but most of the nanoparticles have a size (4 nm) suited to the pores. In this case, it is likely that the nanoparticles have grown without any modification of the silica matrix and these particles remain protected from oxidation by the silica walls. This effect could explain the stability of PbS nanoparticles, even after 100 days. Hence, both the pore diameter of the host silica matrix and the power of pulsed femtosecond laser irradiation play an important role in the stability of the obtained PbS nanoparticles.