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We reported photochromism and largely enhanced visible two-photon luminescence (TPL) of Ag-TiO2 granular composite films by using ps/fs laser at the wavelength of 800 nm. Three types of photochromism spectra were observed when the Ag atom fraction are less than, comparable to and larger than the percolation threshold. The strong surface-plasmon-resonance enhanced visible TPL emissions near Ag2O transition band from the photoactivated Ag-TiO2 samples were also observed. Furthermore, we found that the TPL intensity saturatedly increased while the absorbance at 800 nm exponentially decreased with the same rate as the increasing of photoactivation time, which means that both photochromism and TPL of Ag-TiO2 composite films are originated from the photo-oxidation of Ag to Ag+. These observations exhibit the multifunctional features of Ag nanoparticle materials.
©2007 Optical Society of America
1. Introduction
Noble metal nanoparticles have been the subject of extensive research. Silver nanoparticles and the composite materials exhibit various promising optical properties, such as surface-plasmon-resonance (SPR) effect [1,2], photoluminescence in the visible region [3–6], and reversible multicolor photochromism [7–10], which open a wide range of possible applications from biosensors to optical storage and information processes [11–13]. Excitation of SPRs of noble nanoparticles can create strong local optical field, which leads to various optical enhancement effects, such as surface-enhanced Raman scattering [14–16], enhanced third-order optical nonlinearity [17–20], and enhanced luminescence [21–25]. Several methods have been proposed to adjust the SPR band of Ag nanoparticle materials in a wide region from visible to near infrared (NIR) wavelength [16,26–31]. It is well known that Ag+ can be photo-reduced with UV light or thermal-reduced to Ag [4,32,33], and Ag nanoparticles can also be thermal-oxidated or photo-oxidated to Ag+ with visible light [3]. The strong luminescence from Ag-Ag2O unit with an appropriate ratio of Ag/Ag2O has been reported by using a visible excitation source [4,33].
Here we reported photochromism and visible two-photon luminescence (TPL) of Ag nanoparticles embedded in TiO2 films by using ps/fs laser pulses in near infrared range 800~900 nm. We also investigated the changes of the Ag particle size and shape, transmittance and reflectivity of the photoactivated Ag-TiO2 film samples, comparatively analyzed the decreasing of absorbance and the increasing of TPL intensity as a function of photoactivated time, revealed the originated relationship of the photochromism effect and the TPL emissions of Ag-TiO2 granular composite films.
2. Experimental
A series of Ag-TiO2 nanocomposite films with Ag volume fraction q Ag 0.26~0.82 were prepared by co-sputtering technique with a Ti target (Φ 100 mm, purity 99.999%) attached with several pieces of pure Ag symmetrically. The Ag granular composite films were deposited onto the glass substrate in Ar atmosphere at the pressure of 3.0×10-2 Torr, and the base pressure of the sputtering chamber was 1.1×10-4 Torr. The value of q Ag was controlled by adjusting the area ratio of Ti and Ag and checked by EDAX. The morphologies of the samples were examined by transmission electron microscopy (TEM). The absorbance spectra were recorded by UV-VIS-NIR spectrophotometer (Cary 5000, Varian). The photochromism and two-photon luminescence were investigated by a Ti:Sapphire laser (Mira 900, Coherent) using pulse duration of ~2.5 ps and 130 fs, respectively. The TPL of the samples was collected in reflective mode and the photoluminescence spectra were recorded by a LN cooled CCD detector (SPEC-10, Princeton) through a monochromator (Spectrapro 2500i, Acton).
3. Results and Discussion
The size and the fraction of Ag nanoparticles in the granular composites prepared by sputtering technique were adjusted in a large range. The size of the Ag nanoparticles in the as-deposited Ag-TiO2 composite film with atom fraction q Ag=0.34 is about 20~50 nm estimated from TEM image (see Fig. 1(a)), which decreases by ~7% in average after the photoactivation by ps pulses for 2300 s with irradiation intensity I irr=0.21 MW/cm2 (see Fig. 1(b)). The ratio of the long axis to the short axis of the photoactivated Ag nanoparticles decreases by ~8% in average. The electron-diffraction pattern of the selected area of the Ag-TiO2 film shows the diffraction rings attributed to the Ag particles.
Fig. 1. Nanostructure and optical absorption of Ag-TiO2 composite films. (a) TEM image of as-deposited Ag-TiO2 granular composite film with q Ag=034. The size of Ag nanoparticles is in the range of 20 ~ 50 nm. (b) TEM image of the same sample photoactivated by ps laser pulses at the wavelength of 800 nm for 2300 s with intensity I irr=0.21 MW/cm2. The average size of the photoactivated Ag particles decreases by about 7%. (c) Optical absorbance spectra of the samples with q Ag=0.28, 0.48 and 0.64 with (dash lines) and without (solid lines) photoactivated.
Figure 1(c) shows three types of photochromism spectra of Ag-TiO2 composite films with the Ag atom fraction q Ag 0.34, 0.48 and 0.64, which are less than, comparable to and larger than the percolation threshold q c=0.42, respectively. For the sample with q Ag=0.34 (<q c), the absorbance of photoactivated Ag-TiO2 film decreased in the long wavelength range 600 nm ~ 1200 nm and increased in the short wavelength 360 nm ~ 600 nm. For the sample with q Ag=0.48 (≈q c), the absorbance of photoactivated film decreased in the whole recorded wavelength range 380 ~ 1200 nm. For the sample with q Ag=0.64 (>q c), the absorbance decreased in the range 450 ~ 1200 nm and did not changed significantly in the range 360 nm ~ 450 nm. The decreasing of absorbance in the broad band wavelength range including the photoactivating laser source 800 nm observed in all Ag samples is caused by the photo-oxidation of Ag to Ag+ [7,9,10]. The increasing of SPR enhanced absorbance near 450 nm observed in the sample with small Ag fraction may be caused by the decreasing of aspect ratio of larger Ag nanoparticles, the increasing of smaller spherical Ag nanoparticles and the formation of Ag-Ag2O core-shell structure generated in the photoactivation process.
The photochromism of Ag-TiO2 was carried out by using laser pulses with pulsewidth ~2.5 ps at the wavelength of 800 nm (the reverse processes with UV light was not involved in this study). Fig. 2(a) is the dependence of the transmittance T and reflectivity R on photoactivating time t irr at λ=800 nm. It clearly shows that the transmittance T increases and the reflectivity R decreases with the increasing of photoactivatimg time t irr. As a result, the pure absorbance (α= -ln(T/(1-R))) decreases rapidly from 1.46 to 1.16 at t irr=30 s (see Fig. 2(b)). The relation α~t irr can be well fitted by the two-component exponential decay form,
The decreasing of absorbance is caused by the oxidation of Ag to Ag+. The fast process is attributed to the photo-oxidation and the slow process perhaps is caused by the photo-thermal effect with ps pulses. The slow process described in Eq. (1) is not observed in the photoactivation with fs pulses.
Fig. 2. Photochromism dynamics of Ag-TiO2 films photoactivated by ps laser at the wavelength of 800 nm. (a) Dependence of transmittance and reflectivity at 800nm on photoactivating time t irr. (b) Dependence of pure absorbance at 800nm on t irr.
The Ag-TiO2 samples were also photoactivated by fs pulses at the wavelength of 800 nm. The photoactivated samples excited by the same fs laser at the same wavelength generated strong visible two-photon luminescence (TPL) near 552 nm. To further investigate the dynamics of photoactivation of Ag nanoparticles and the underlying physical and chemical mechanism, we recorded the TPL as a function of photoactivating time t irr. The TPL emissions were collected in reflective mode (see Fig. 3(a)). The TPL of Ag nanoparticles without photoactivation (t irr=0) is very weak, and it increases with increasing of photoactivating time t irr and reaches the saturation (see Fig. 3(b) and 3(c)). The TPL intensity increases from 150 to about 2900 by 200s photoactivation with peak irradiating intensity I irr=17.3 MW/cm2. The recorded I TPF ~ t irr curves are well fitted by
where I s is the saturation value of TPL, which is approximately proportional to I 2 irr. t s is the saturable increasing constant, which is inverse proportional to irradiating intensity I irr as shown in Fig. 3(c). The TPL intensity I TPL increased fast and reached rapidly to the saturation with large photoactivating intensity I irr.
Fig. 3. Photoactivation and two-photo luminescence (TPL) of Ag-TiO2 films photoactivated by fs laser at the wavelength of 800 nm. (a) Illumination of the setup for the TPL recording. (b) The TPL peak intensity of the film with q Ag=0.48 as a function of photoactivating time t irr. The photoactivating intensity I irr equal to 10.4 MW/cm2, 14.4 MW/cm2 and 17.3 MW/cm2, respectively. (c) The saturable increasing constant t s of the TPL as a function of I irr, t s is inverse proportional to I irr.
The emission peaks of TPL as shown in Fig. 4(a) are very similar to the observations of one-photon luminescence (OPL) in Ag-exchanged glass. The emissions around 552 nm were explained by the band-to-band radiative transition in Ag2O with the band gap 2.25 eV[3,4]. The full width at half maximum (HMFW) of TPL in Ag-TiO2 is about 90 nm, which is about 30 nm smaller than the OPL in Ag-exchanged glass due to the depressing of emissions from small Ag nanoparticles centered at 637 nm (1.95 eV)[4].
Fig. 4. Two-photo luminescence (TPL) of Ag-TiO2 films photoactivated by fs laser at the wavelength of 800 nm. (a) TPL spectra of the film with q Ag=0.48 recorded after the photoactivating time t irr= 450 s. The photoactivating intensity I irr is equal to 17.3 MW/cm2. (b) The dependence of the TPL intensity I TPL on the excitation power I irr.
The dependence of the TPL intensity I TPL on the excitation power I irr is shown in Fig. 4(c). The slope of ln(I TPL) ~ ln(I irr) is about 2.2, which indicates that the recorded photoluminescence from photoactivated Ag nanoparticles is generated by the two-photon absorbance processes.
Considering that the exciton absorbance of Ag2O was not observed during the photoactivation and the TPL emission from pure Ag2O and pure Ag were too weak, we thought that a portion of metallic Ag nanoparticles was photo-oxidized to Ag2O and the luminescence unit Ag-Ag2O is generated during the photoactivation process, the up-levels of photoactivated Ag and Ag2O were involved in the two-photon excitation processes, and the excited electrons on the up-levels relaxes to the lower-level of Ag2O, then the radiative transitions in Ag2O emit photons near 552 nm.
The TPL of a series of Ag-TiO2 samples with Ag fraction q Ag in the range 0.28 ~ 0.64 were recorded and shown in Fig. 5(a) and 5(b), it reached the maximum at q Ag=0.42. For the two-photon induced luminescence, the emission intensity I TPL and the peak excitation intensity I exc=I irr has the relationship
where A is a proportional constant and f is localized SPR enhancement factor, ω emi and ω irr represent the circular frequency of emission and excitation light, respectively. The previous studies on the third-order optical susceptibility χ (3) have revealed that the enhancement factor ∣f(ω irr;q Ag)∣ reaches the maximum near the percolation threshold q c[30,34], which leads to a efficient TPL emission of Ag-TiO2 granular composites near the percolation threshold q Ag=0.42. For comparison, the TPL of Ag-SiO2 granular composite was also recorded. The TPL intensity of Ag-TiO2 is about 50 times stronger than that of Ag-SiO2 due to the stronger local field enhancement.
Fig. 5. TPL of Ag-TiO2 samples. (a) Normalized TPL spectra of the samples with q Ag=0.28, 0.34, 0.42 and 0.48, which is excited by fs laser at the wavelength of 800 nm. (b) Normalized I TPF as a function of q Ag with excitation intensity I irr=16.5 MW/cm2.
4. Conclusions
Ag-TiO2 granular composite exhibits a large decreasing of absorbance in a wide wavelength range and three types of photochromism spectra with the photoactivation of ultrafast laser pulses at 800 nm. The photoactivated Ag nanoparticles in the TiO2 film generate strong photoluminescence with two-photon absorbance. The TPL intensity is largely enhanced by the SPR absorbance located around the excitation and emission wavelengths. As the photoactivation time increases, the absorbance of Ag-TiO2 films decrease exponentially and the TPL intensity increases saturatedly with the same rate, which indicates that both photoactivation process in TPL emission and the photochromism are attributed to the photo-oxidation of Ag nanoparticles.
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