J. Phys. Chem. B 2000, 104, 11847-11852
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ARTICLES Morphological Changes of Silver Nanoparticle Distributions in Glass Induced by Ultrashort Laser Pulses M. Kaempfe,† H. Hofmeister,‡ S. Hopfe,‡ G. Seifert,*,† and H. Graener† Fachbereich Physik, Martin-Luther-UniVersita¨ t Halle-Wittenberg, D-06099 Halle, Germany, and Max-Planck-Institut fu¨ r Mikrostrukturphysik, D-06120 Halle, Germany ReceiVed: June 5, 2000; In Final Form: August 8, 2000
Persistent form modifications of silver nanoparticles in glass and the resulting dichroitic color changes induced by irradiation of a single intense femtosecond laser pulse were studied by means of high-resolution electron microscopy and spatially resolved optical transmission spectroscopy. Electron microscopy reveals a variety of morphological changes of the silver particles, in particular irregularly shaped particles surrounded by halolike structures consisting of very small silver clusters. The spectral analysis suggests that at lower intensities preferably desorption of silver into the glass matrix occurs producing isotropic spectral changes, while at higher intensities anisotropic spectra caused by particle deformation are observed. An analysis of the effects produced by different laser wavelengths allows one to correlate these findings with the variations of particle sizes depending on their penetration depth.
1. Introduction During the past decades, the physical and in particular the linear and nonlinear optical properties of metallic nanoparticles in glasses have been investigated using numerous experimental and theoretical approaches.1-5 In recent years the use of improved experimental equipment such as femtosecond laser studies6-8 or near-field optical microscopy9 led to quite detailed knowledge of the ultrafast dynamics of the surface plasmons (SPs) of this type of particles. In the particular case of silver nanoparticles in glass, in recent years a lot of information was obtained about the processes of cluster formation upon ion exchange and successive annealing in air or hydrogen atmosphere.10,11 Also, permanent changes of the optical properties after laser irradiation were observed, which have been attributed to morphological and chemical changes of clusters and matrix.12,13 Recently, permanent color changes and, particularly, dichroism were observed upon irradiating glass samples containing silver nanoparticles with a single intense femtosecond laser pulse.14 The observed spectral changes were interpreted to be due to an ultrafast reshaping of the particles. Meanwhile electron microscopy studies were performed revealing the nanoscopic morphology in this type of samples after laser irradiation. The results of these experiments are presented in this work. Taking into account the electron microscopy results, an extended analysis, as compared to our previous work,6 of the intensity dependence of the observed optical changes, is discussed. Additionally the inhomogeneity of the SP absorption band was studied optically by using different laser wavelengths. * Corresponding author. Fax: +49/345/55 27 221. E-mail: g.seifert@ physik.uni-halle.de. † Martin-Luther-Universita ¨ t Halle-Wittenberg. ‡ Max-Planck-Institut fu ¨ r Mikrostrukturphysik.
Similar studies on a different type of samples containing silver particles with a rather narrow size distribution will be published separately.15 2. Experiments The experiments were carried out on commercial flat glass of usual chemical composition.16 Silver nanoparticles were prepared in this samples by 5 min of Ag+/Na+ ion exchange in a mixed melt of AgNO3/NaNO3 (0.2 wt % AgNO3) at 330 °C and subsequent 5 h annealing of the sample in hydrogen atmosphere at a temperature slightly below the glass transformation temperature Tg ≈ 535 °C. By this procedure the silver ions in the sample are reduced and spherical silver particles are formed in a thin surface near region of about 30 µm thickness. The sample was analyzed by transmission electron microscopy (see below for experimental details) at distances of approximately 2, 5, and 15 µm below the original surface yielding particle diameters of ≈10, 50-60, and ≈100 nm in these depths. The silver volume concentration can be estimated to be e10-3 in the particle containing layer, where the maximum concentration is found for the smallest particles, i.e., close to the surface. This type of sample, which is the same as used in our previous investigation,14 is of particular technological interest, because the preparation of silver particles can be done easily with nearly any type of sodium silicate glass device at any time after its production. These samples were irradiated with single, linearly polarized laser pulses from an amplified frequency-doubled Ti: sapphire laser (pulse energy typically 100 µJ, pulse duration 150 fs, nearly Gaussian intensity profile with 100 µm full width at half-maximum (fwhm)). Wavelengths between 380 and 420 nm were used in this investigation. These wavelengths cover the maximum extinction and the high-frequency wing of the plasmon resonance band of the sample centered at 2.97 eV (417
10.1021/jp002039w CCC: $19.00 © 2000 American Chemical Society Published on Web 11/28/2000
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Figure 1. Extinction spectra of the original glass sample (straight line) and after exposure to a single laser pulse, measured with the light polarized parallel (p-pol, dotted line) and perpendicular (s-pol, dashed line) to the laser polarization.
nm). The laser pulse produces significant color changes in the irradiated area. These spots were analyzed by optical spectroscopy using a microscope spectrophotometer (MPM 800 D/UV; Zeiss). For this purpose series of spectra were recorded for areas of 10 × 10 µm2 along a symmetry axis of a spot. Using the known intensity profile of the laser beam, the position information of the individual local spectra was converted to an intensity dependence of the induced spectral changes. To analyze the dichroism generated, at each position two spectra were taken with the polarization of the analyzing light either parallel or perpendicular to the linear polarization of the laser pulses. The two types of spectra will be distinguished by the notations p-pol (for parallel) and s-pol (for perpendicular) in the following. Figure 1 compares the initial spectrum of a hydrogen annealed sample (solid line) with the spectra measured in the center of an irradiated spot. Both spectra, s-pol and p-pol, are red-shifted and broadened as compared to the original line but with different magnitudes. It is important to note that s-pol shows a larger red shift than p-pol (see erratum to ref 14). For structural characterization by electron microscopy, an array of single spots, 3 × 3 mm2 in size, was produced on the samples, the spots being separated from each other by about 250 µm. To look for different particle sizes, layers of different thickness were removed from the top surface of the sample by an etching process, the thickness of the removed layer ranging from 2 to 20 µm. Then the glass was thinned from the backside by mechanical polishing, dimple grinding, and finally Ar ion beam etching until a small breakthrough was formed. During the final ion thinning step the samples were cooled to prevent any heating that could modify the laser-induced structural changes. The hole formed by the thinning procedure was of such a size that its edge passes through at least one region irradiated by the laser pulses as well as a region where no irradiation occurred. However, due to this rather challenging preparation, it is not possible to assign results from irradiated areas to a distinct local intensity. Transmission electron microscopy (TEM) was done by means of a JEM 100C operating at 100 kV. For structural characterization down to the atomic scale by high-resolution electron microscopy (HREM) a JEM 4000EX operating at 400 kV was used. 3. Results and Discussion 3.1. Electron Microscopy. The TEM micrographs shown in Figure 2 were taken at three different distances from the original glass surface. Figure 2a-c corresponds to approximately 2, 5, and 15 µm, respectively. In all three cases the image on the top was taken from a region between the laser-irradiated spots; i.e.,
Figure 2. TEM micrographs recorded at different distances from the original glass surface: (a) ≈2 µm; (b) ≈5 µm; (c) ≈15 µm. The images on the left-hand side originate from an area which was not irradiated by laser light; those on the right side from an irradiated area.
it represents the initial situation in the sample. As expected, nearly spherical silver particles of rather different sizes are found at different depths: the particle diameters measured from the images are 10 nm (a), 56 nm (b), and 110 nm (c), respectively. Using the above-mentioned maximum silver volume concentration of 10-3, average particle distances of g80, >450, and >900 nm can be estimated, in reasonable agreement with the shown TEM micrographs. From Figure 2a also significantly smaller distances could be deduced. This may be due, at least partially, to the projection to the image plane of particles located within a certain sample thickness (which is typically in the range of 50-100 nm to be suitable for TEM studies). Nevertheless, it should be noted that interactions of the SPs of closely neighbored silver clusters cannot be neglected completely. The lower images of Figure 2 give examples found in laser-irradiated regions of the same depths as specified above. In Figure 2a significant changes can be seen: instead of the initial spherical particles, smaller, not necessarily spherical central, particles are found surrounded by distinctly smaller clusters of about 2 nm average size. Sometimes the core is missing completely and only the halolike arrangement is observed. HREM imaging allows one to analyze the crystal lattice spacings of such clusters, revealing that the halo particles also consist of metallic silver. Figure 2b shows an example for the situation after a laser pulse in 5 µm depth. Again, quite dramatic changes occur as compared to the initial situation: like in Figure 2a a halo of very small particles can be seen around a larger core, but in this case both features exhibit a clearly nonspherical shape. For the largest particles present in the sample, no such irregular deformations were found. Instead, structures as given in Figure 2c were observed frequently, where a more or less unchanged silver particle of nearly spherical shape was surrounded by a shell of about 10 nm radial spacing containing very small clusters of sizes comparable to the other halo clusters. In summary for the TEM study, the following general statements can be made about the femtosecond laser pulse induced changes: a common feature for silver particles of all sizes is the formation of a halo of small clusters having diameters of about 2 nm. In most cases this shell surrounds a larger core particle, which may still have nearly spherical shape for initial particles of about 100 nm diameter but can also have nonspherical and even very irregular shapes for particles e60 nm. Sometimes, the core may even be missing as it was observed for particles of about 10 nm size. It should be mentioned that
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due to their obviously rather low concentration only very few of the largest particles could be localized at all, so a nonspherical shape cannot be excluded for this type of particles. In general, particles having a somehow elongated shape could explain the observed dichroism provided the anisotropy has a common direction. Due to the complexity of the preparation for electron microscopy such a common direction could not definitely be verified for the studied polydisperse sample but was proven unambiguously for another type of sample containing nearly monodisperse silver particles. There, uniformly oriented ellipsoidal silver particles with halo have been located; the corresponding results will be discussed in a forthcoming paper.15 Because of the variety of the observed morphological changes induced by laser irradiation, it seems impossible to model the corresponding spectral changes on the basis of Mie’s theory in a satisfactory way. Nevertheless, additional information can be obtained by analyzing the intensity dependence of the induced spectral changes. This should facilitate an (at least qualitative) interpretation of the characteristic interactions between the intense laser pulse and the silver nanoparticles. This analysis, being similar to that of ref 14, is given in the next section under consideration of the results revealed by electron microscopy. 3.2. Optical Spectroscopy. In general, the total optical density (OD) due to surface plasmon resonances of a sample of embedded silver nanoparticles of varying size can be written as a sum of individual bands due to the different species of particles having normalized spectral band shapes Li(ν) and maximum extinctions i. If, in particular, spectra are recorded with polarized light, additionally parallel (p-pol) and perpendicular (s-pol) polarization must be distinguished:
ODP,S(ν) ) -log(TP,S(ν)) )
∑i iP,S‚LiP,S(ν)
(1)
Here, TP,S means the sample transmission for either polarization. If the distribution of particles and their individual line shapes are known, this formula can be directly used to model the obtained spectra. For the samples described in this work this is possible, in principle, for the initial situation. However, after laser irradiation a variety of both spherical and nonspherical morphological features can be found, the extinction bands of which are not known in detail. So, motivated by the electron microscopy results, we define a few spectral components in a very general sense and trace their behavior as a function of (local) intensity, instead of trying to simulate the spectral changes. In particular, the polarization independent band caused by residual silver particles of initial (spherical) shape will be called OD1. The variety of deformed particles is decomposed into their isotropic (non-polarization-dependent) part called OD2 and their anisotropic components, OD3P and OD3S, respectively. The superscripts P and S again refer to p-pol and s-pol. While OD2 is thought to cover all core and halo particles of still nearly spherical shape and also the isotropic spatial arrangement of halo clusters, OD3 refers to any nonspherical particles and anisotropic spatial arrangement. Obviously OD3P and OD3S can only be different when some kind of preferential orientation of these features exists. OD2 and OD3 will be termed isotropic and anisotropic product component in the following. Using this nomenclature the total optical density of an irradiated sample region can be written as
ODP,S(ν) ) OD1(ν) + OD2(ν) + OD3P,S(ν)
(2)
The band shapes of the so defined components cannot be expected to be Lorentzian-like in the case of monodisperse
Figure 3. Illustration of the spectral analysis for differential spectra at a position of intermediate intensity within an irradiated spot: (a) difference between the experimental s-pol and p-pol spectra ODS ODP (thin solid line), two-line Gauss fit (solid curve), and its components s-pol (dashed) and p-pol (dotted); (b) difference between the s-pol component of the optical density change ∆ODS and the fitted s-pol component OD3S (solid line), Gaussian fits for the isotropic hole ∆OD1 (dotted), and the isotropic product band ∆OD2 (dash-dotted); (c) s-pol component of the optical density change ∆ODS (thin solid line), three-line Gauss fit (solid curve), and its components (dashed, dash-dotted, and dotted).
spherical or spheroidal silver particles but will strongly depend on the variation of particle shapes and sizes. For the purpose of the fit routine described below, Gaussian line shapes rather than Lorentzian were used, since they yielded a better agreement with the experimental data. If an experimental series of spectra can be fitted with good quality by the components defined above, the parameters frequency position, bandwidth, and band area of all components of OD represent a full characterization of the experimental spectra. The routine applied to extract these parameters in dependence on the laser intensity is illustrated in Figure 3, which shows the effects of irradiation with a single fs pulse at 400 nm wavelength. The spectra shown here were taken at intermediate intensity and separated into the two polarization directions, namely s-pol and p-pol. The routine is as follows: first the difference (ODS - ODP) of the experimental spectra is calculated, which is obviously identical to (OD3S - OD3P), because all isotropic components are canceling out (see Figure 3a). Since the extinction cannot take negative values, the positive band in this spectrum can definitely be assigned to OD3S and the negative one to OD3P, and after fitting (ODS - ODP) with the sum of two Gaussian lines (shown as dashed and dotted curves, respectively), the obtained two bands can be identified directly with OD3S and OD3P. These components (more specifically their Gaussian fits) are now used to extract OD1 and OD2, i.e., the isotropic parts of the spectral changes induced by the laser irradiation: for this purpose first the differences ∆ODP,S of the spectra taken after and before the irradiation are calculated, and then the appropriate OD3P,S is subtracted from ∆ODP and ∆ODS, respectively. This is correct, because OD3 is obviously zero before laser irradiation. The resulting spectral differences are identical for p-pol and s-pol within experimental accuracy, indicating that the Gaussian fits are reasonably good. As an example this difference is shown for s-pol in Figure 3b.
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Figure 5. Intensity dependence of the center frequency of the isotropic hole ∆OD1 for three different laser wavelengths 380 nm (squares), 400 nm (circles), and 420 nm (triangles).
Figure 4. Intensity-dependent parameters of the spectral analysis: (a) center frequencies; (b) band areas; (c) bandwidths of the isotropic hole ∆OD1, the isotropic product ∆OD2, and the two anisotropic product components ∆OD3P and ∆OD3S.
As the initial spectrum has been subtracted, the negative component seen in Figure 3b can only be due to the decreased number of spherical particles of initial size (isotropic hole). The positive contribution must be caused by a polarization independent extinction from deformed particles (isotropic product). Fitting finally these spectra again with two Gaussian lines (dashed and dotted curves in Figure 3b), the obtained bands can be interpreted as a good approximation for the laser induced changes of OD1 and OD2. Figure 3c uses the example of s-pol to illustrate that the sum of the obtained three Gaussian components (dashed and dash-dotted curves) fits the corresponding measured spectral changes very well. A comparably good agreement between experimental spectra and Gaussian fit is obtained for p-pol, when it is subjected to the corresponding procedure (not shown here). Analyzing the measured intensity dependence in this way provides center frequency, bandwidth, and band area of the components OD1, OD2, OD3P, and OD3S as a function of relative intensity. Figure 4 shows an example for the evaluation of a single spot after irradiation with a laser pulse at λ ) 400 nm: the maximum intensity in the center of the laser spot corresponds to about 2 × 1012 W/cm2. At low intensities (I/Imax < 0.3) the difference ODS - ODP was found to be zero within experimental accuracy, so only the isotropic components are obtained in this intensity region. The parameters of the isotropic hole (OD1) are very similar to those published previously14: with increasing intensity a slight red shift (
