Tiziana Cesca*,
Boris Kalinic,
Chiara Maurizio,
Niccolò Michieli,
Carlo Scian and
Giovanni Mattei
University of Padova, Department of Physics and Astronomy, Nanostructures Group, via Marzolo 8, I-35131 Padova, Italy. E-mail: tiziana.cesca@unipd.it
First published on 10th October 2016
Finding effective strategies for improving the luminescence emission performance of Er3+ ions in silica is of paramount importance for the realization of efficient Er-based nanophotonic devices. To this aim, in the present work the sensitization mechanism of room temperature Er3+ luminescence by AuN quantum clusters, upon annealing in a reducing atmosphere, was investigated by photoluminescence characterization of Er–Au co-implanted samples in which Au ions were implanted at a low fluence of 3.4 × 1014 Au+ per cm2. The role of the annealing atmosphere was unveiled by comparing the emission properties of samples thermally treated in reducing and inert atmospheres. A significant amplification of the Er3+ sensitization efficiency (up to a 6× enhancement of the photoluminescent intensity) was revealed upon annealing in a reducing atmosphere and the possibility to get efficient room temperature Er3+ luminescence from Er–Au co-implanted samples with only a limited amount of sensitizing agents was demonstrated and discussed.
Gold nanoparticles have been widely investigated during recent years for their plasmonic properties, with applications in many different fields such as photovoltaics,17,18 nonlinear optics,19,20 or sensing.21,22 Nonetheless, in the present case, due to their sub-nanometric size the AuN clusters incorporated in the samples do not exhibit any plasmonic behavior. No surface plasmon resonance peak can be detected in the absorbance spectra of the samples. Instead, these ultra-small AuN clusters have a “pre-plasmonic” or molecule-like character which definitely triggers their efficiency as nanoantennae for Er3+ luminescence. Indeed, it was demonstrated that such ultra-small AuN clusters exhibit discrete electronic energy levels which are resonant with corresponding absorption levels of the Er3+ ions and this represents the preferential path for the transfer of electromagnetic energy from the AuN clusters to the Er3+ ions.23 The process was proved to occur non-radiatively16 via short-range coupling with interaction distances in the range 0.4–0.8 nm.14 Moreover, since it is mediated by the formation of discrete energy levels related to the AuN clusters, the energy transfer process is strongly controlled by the clusters’ size in the sub-nanometer range. Furthermore, recent studies on the sensitization efficiency of Er–Au co-doped silica systems implanted with Au+ ions at different fluences and thermally treated in inert (N2) atmosphere have shown the beneficial effect of incorporating gold at low concentrations in the samples.14,24 In this way the kinetics of the cluster growth is slowed down and the annealing temperature to get AuN clusters in the optimal size range (of about N = 10–20 atoms) for the Er sensitization shifts to higher values, allowing also for a better recovery of the silica matrix from implantation damage, which is further advantageous for efficient Er3+ emission.14 Nonetheless, for annealing treatments in an inert atmosphere a lower limit for the Au concentration needed for the efficient Er3+ sensitization is observed, below which the efficiency of the energy transfer process drops drastically.14 The present work is therefore aimed at addressing the role of the annealing atmosphere (inert or reducing) on the sensitization efficiency of the Er3+ luminescence in Er–Au co-implanted silica samples in which Au is incorporated at low fluence (3.4 × 1014 Au+ per cm2). In particular, we demonstrated that a significant boost of the Er3+ emission properties can be obtained by thermal treatments in a reducing atmosphere with respect to analogous samples heated in an inert atmosphere, and a very efficient Er3+ sensitization is achieved in spite of the low Au content. These results are extremely valuable since the capability to get a strong amplification of the Er3+ luminescence sensitization and the understanding of the mechanisms that control this process have important technological implications for the effective employment of AuN quantum clusters as sensitizers of the Er3+ luminescence in photonic devices.
X-Ray Absorption Spectroscopy (XAS) measurements were performed to get structural information on the samples. The experiment was performed at the Au L3-edge at ID26 beamline of the European Synchrotron ESRF, Grenoble (F). A High-Energy Resolution Fluorescence Detection (HERFD) setup was used, with an energy resolution of the crystal analyzer of about 2 eV. In this way, a full insight on the Au site was achieved by combining the analysis of the high-resolution near-edge part of the X-ray absorption spectrum with the analysis of the full EXAFS (Extended X-ray Absorption Spectroscopy) spectrum. Details on the measurements and data analysis are reported in ref. 24.
Photoluminescence (PL) measurements (both integrated and time-resolved) of all the samples were performed at room temperature using a mechanically chopped cw multi-line Ar laser as the excitation source. The different laser lines are selected by interference filters. The laser line at 488 nm was used for the measurements to get resonant Er excitation conditions (matching the Er3+ absorption transition 4I15/2 → 4F7/2). The spectral PL emission was detected by a single-grating monochromator coupled to a N2-cooled photomultiplier tube. The photon flux was changed by varying the pumping power and the area of the beam spot at the sample position. For all the measurement configurations the spot-size was determined by the knife-edge method.26 The spectra were recorded with a lock-in amplifier using the chopper frequency as a reference. Time-resolved PL measurements were carried out by fixing the detected wavelength and collecting the PL intensity evolution as a function of time with a transient digitizer.
A simple phenomenological model can be introduced to describe the observed evolution of the PL emission as a function of the annealing temperature.28 The basic idea is that the Er luminescent emission is proportional to the population of Er3+ ions that are optically active in the samples and competitive mechanisms occurring during the thermal treatments (as recovery of matrix defects or diffusion of quenching species) give rise to the net active Er3+ population, NEr,A. Assuming an initial population of Er ions, NEr, and defining as NEr,D the population of Er ions that can be eventually de-activated upon the annealing process due to quenching phenomena, and thus do not contribute to the PL emission, the evolution of NEr,A and NEr,D during the annealing duration (1 h in the present case) can be described by the following coupled rate-equations:
![]() | (1a) |
![]() | (1b) |
RA and RD are, respectively, the rates of optical activation and de-activation of Er ions in the samples and they can be expressed by Arrhenius equations as:
RA(T) = R0,Ae(−EA/kBT) | (2a) |
RD(T) = R0,De(−ED/kBT) | (2b) |
As a first step, this approach has been used to fit the evolution of the PL intensity versus annealing temperature of the Er-implanted silica samples (see inset in Fig. 1). For both sets of samples (treated in inert and reducing atmosphere) we get EA = (1.37 ± 0.01) eV and ED = (2.63 ± 0.01) eV, with pre-factors R0,A ≈ 4 × 103 s−1 and R0,D ≈ 4 × 107 s−1. For Er-implanted silica systems, it is known that the progressive recovery of the SiO2 matrix from implantation damage (and the removal of irradiation defects that may act as nonradiative recombination centers),29 as well as the restoration of the full octahedral coordination of oxygen atoms around the Er3+ ions, is beneficial for the Er luminescence giving rise to an increase of the emitted intensity.30 The activation energy for molecular oxygen diffusion in silica was found to be in the range of 1.1–1.3 eV,31,32 in agreement with the values of EA determined by the fits. Concerning the de-activation mechanisms, instead, the PL intensity drop observed upon annealing at T > 900 °C can be attributed to possible Er clustering.30 An activation energy of 5.3 eV was recently determined for Er diffusion in silica33 that was significantly higher than the estimated ED. Nonetheless, possible structural rearrangements of the implanted silica matrix have to be considered that could speed up the clustering process. Moreover, upon annealing treatment, hydroxyl groups (OH−) can penetrate the Er-containing layer (from the external environment or the silica substrate itself). OH− groups are known to be efficient quenching species for the Er3+ luminescence34 and an activation energy of about 0.82 eV was found for their diffusion in amorphous silica upon annealing in the temperature range explored in the present work,35 thus giving rise to a lower effective activation energy for the de-activation process, as estimated in the present case.
Concerning the Er–Au co-implanted samples (main panel in Fig. 1) the fitting procedure yielded EA = (1.7 ± 0.2) eV and ED = (2.2 ± 0.3) eV for the samples annealed in N2 atmosphere, and very similar values, EA = (1.71 ± 0.02) eV and ED = (1.96 ± 0.01) eV, for those thermally treated in Ar + H2; in both cases the pre-factors were R0,A ≈ 4 × 104 s−1 and R0,D ≈ 4 × 107 s−1. For this class of systems, the Er3+ luminescence is controlled by the energy-transfer from the AuN quantum clusters, formed upon annealing, to the Er3+ ions and the most effective sensitization is obtained when the AuN cluster size is of the order of N = 10–20 atoms per cluster.14,15 The growth of AuN clusters with larger size produces instead a decrease of the sensitization efficiency and thus of the Er3+ PL emission. Within this framework, the activation energies EA and ED determined by the fits can be related to processes involving Au diffusion and nucleation. An activation energy of 2.14 eV is reported in the literature for Au diffusion in silica upon thermal annealing,36 but a much lower value was demonstrated for radiation damage assisted diffusion processes. As an example, for Au clustering in Au-implanted silica samples (at a fluence of 3 × 1016 Au+ per cm2) an activation energy of 1.17 eV was measured, which was interpreted taking into account the role of oxygen in promoting gold diffusivity and assuming a thermodynamic interaction between gold and oxygen.37,38 Furthermore, it is interesting to note that in the present case the same values of EA and ED have been obtained for the two sets of samples treated in inert or reducing atmospheres. This result is consistent with the findings of a structural study on the AuN cluster formation performed on these samples by HERFD-XAS analyses,24 as discussed below.
Structural characterization of the Er–Au co-implanted samples was performed by HERFD-XAS measurements at the Au L3 absorption edge. Due to its unique capability of revealing the aggregation of Au atoms in few-atom clusters since the very early stages of nucleation, the HERFD-XAS technique allowed us to follow the Au cluster growth upon annealing even in the ultra-low size range demonstrated for the samples investigated in the present work. All the details of the experimental conditions of the HERFD-XAS measurements and the data analysis are reported in ref. 24. In Table 1 we reported the main HERFD-XAS results obtained for the samples of the two series investigated in the present work. Ultra-small AuN clusters with the same size (number of atoms per cluster, N) and size evolution upon annealing are formed in both sample series, independently of the annealing atmosphere. Moreover, the HERFD-XANES (X-ray Absorption Near-Edge Structure) analysis reported in ref. 24 indicates that the Au clusters can be sketched as an effective core–shell system in which the Au atoms in the ‘core’ develop a metallic character, whereas the Au atoms in the ‘shell’ retain a partially covalent bond with O atoms of the matrix. Particularly, the Au–O coordination is favored for smaller cluster size and lower annealing temperature. Interestingly, a net difference between the two sample sets investigated in the present work is observed in the Debye–Waller (DW) factor (see Table 1) that is lower in the samples annealed in reducing atmosphere with respect to those treated in an inert atmosphere, especially for the samples annealed at 600 °C. The higher DW factor was related to a higher degree of configurational disorder of Au clusters induced by the defective matrix upon annealing in inert atmosphere.24 Indeed, while nitrogen permeation and diffusion in silica is very poor in this range of temperatures, H atoms from the reducing atmosphere can enter into the matrix and contribute to passivate Si dangling bonds. This favors the most stable Au cluster configurations, thus resulting in a lower structural disorder of Au sites and a smaller DW factor upon annealing in a reducing atmosphere than in the inert one.24
Samples Er–Au | Ann. T (°C) | dAu–Au (Å) | N (atoms) | DW (×10−4 Å2) |
---|---|---|---|---|
N2 | 400 | 2.61 ± 0.03 | 2.2 ± 0.7 | 70 ± 30 |
600 | 2.69 ± 0.02 | 5.7 ± 1.2 | 131 ± 6 | |
800 | 2.74 ± 0.01 | 12.7 ± 2.4 | 70 ± 2 | |
Ar + H2 | 400 | 2.62 ± 0.02 | 2.5 ± 0.8 | 49 ± 6 |
600 | 2.68 ± 0.03 | 5.0 ± 1.9 | 61 ± 4 | |
800 | 2.74 ± 0.01 | 12.7 ± 2.4 | 62 ± 3 |
Structural information on the samples was also obtained by investigation of their PL emission spectra. In Fig. 2, we report the room temperature PL spectra in the wavelength range 600–1700 nm of the Er–Au co-implanted samples annealed at the different indicated temperatures. Panel (a) refers to the samples annealed in a reducing atmosphere (Ar + H2), while panel (b) to those treated in an inert atmosphere (N2). The spectrum of the Er800 reference sample is also reported for comparison in both panels (black curve). The measurements have been taken under cw resonant Er3+ excitation at 488 nm. To increase the readability of the plots, different vertical scales have been used for the wavelengths shorter (left-hand scale) and longer (right-hand scale) than 1400 nm, and the spectra have been vertically shifted of a constant value. The peak visible in the wavelength region 1400–1700 nm is the characteristic luminescence band at 1540 nm of Er3+ ions in silica whose temperature evolution upon annealing is shown in Fig. 1. About the spectral features visible in the wavelength range 600–1400 nm (named as bands A, B and C in Fig. 2), it has been recently demonstrated on similar Er–Au co-implanted silica samples that such features are unambiguously related to the presence of ultra-small AuN quantum clusters in the samples.23 Owing to their sub-nanometric size, indeed, these clusters behave as quantum-dots and efficient radiative relaxation can be obtained even at room temperature.39,40 Particularly, bands A and C were attributed respectively to interband (d-sp) and intraband (sp) electronic transitions of the AuN quantum clusters, while band B was related to the formation of electronic surface states at the clusters. Specifically, the formation of this band, which is resonant with a corresponding Er3+ absorption band at 980 nm, was proved to be the channel for the transfer of electromagnetic energy from the Au clusters to the Er3+ ions, giving rise to the Er3+ luminescence sensitization.23 In order to highlight how these bands evolve as a function of the annealing temperature and the role of the annealing atmosphere, we have performed multiple Gaussian peak deconvolution of the spectra in the energy domain. As an example in Fig. 3a we have plotted the spectra and the fitting results of the Er–Au co-implanted samples annealed at 600 °C in the two atmospheres. Three main bands have been resolved, with peaks at about 1.7 eV (band A), 1.3 eV (band B) and 1 eV (band C). In Fig. 3b the amplitude of each Gaussian peak, as resulting from the fits, has been reported as a function of the annealing temperature. A similar trend with the temperature is observed for the three bands. Particularly, the amplitude of each band decreases for annealing at T > 600 °C in agreement with the growth of the AuN clusters that progressively loose their “quantum-dot” character. Furthermore, it is interesting to note that above 600 °C the amplitude of each band is higher for thermal treatment in a reducing atmosphere with respect to annealing in an inert atmosphere. This behavior is consistent with the higher degree of configurational disorder revealed by HERFD-XAS (higher DW factor) in the samples treated in N2 than in Ar + H2, which may induce the damping of the luminescence bands due to inhomogeneous broadening.
From a structural point of view, all the experimental findings revealed strong similarities in the behavior of the two classes of samples, independent of the annealing atmosphere. In an attempt to explain the amplified sensitization demonstrated in the samples for thermal treatment in a reducing atmosphere (see Fig. 1), thus we addressed the issue from a different perspective: not only from the point of view of the sensitizing agents (AuN clusters), but also from that of the emitters (Er3+ ions). To do so, time resolved PL characterization of the samples has been performed to determine the lifetime of the Er3+ emission, and its evolution upon annealing in the different environments. As an example, in Fig. 4a we report the temporal decay curves of the PL emission at 1540 nm of the two Er–Au samples annealed at 700 °C. For both sample series, they represent the samples at which the measured Er3+ PL intensity is the highest (Fig. 1). The PL decay of the Er800 reference sample is also reported for comparison. Stretched exponential fits to the experimental decays have been performed, with the lifetime, τ, and the stretching parameter, β, as free parameters. Then the effective lifetime (τeff) of the PL emission has been calculated through eqn (3), as described in ref. 15:
![]() | (3) |
In Fig. 4b and c the values of τeff and of the stretching parameters β estimated from the PL decays of the different samples have been reported as a function of the annealing temperature. In both panels the horizontal dashed line indicates the results of the Er800 reference sample: a single exponential decay (β = 1) was measured in this case (see Fig. 4a) and the lifetime is τ* = (10.7 ± 0.1) ms. A progressive increase of τeff by increasing the annealing temperature is observed for both samples sets, in agreement with the progressive recovery upon annealing of implantation-induced defects that may act as non-radiative recombination centers.30 Nonetheless, the emission lifetime of the Er800 reference sample cannot be recovered even for thermal treatments at the highest temperatures. This effect has been ascribed to the presence of the AuN clusters in the samples and the opening up of the energy-transfer paths to the Er ions,14 while the formation of non-radiative de-excitation channels as a consequence of the implantation process has been instead ruled out.30 The change of the local dielectric environment around the Er ions due to the incorporation of the AuN quantum clusters has been also taken into account as a possible further effect for the observed lifetime shortening in the Er–Au co-implanted samples.41,42 However, due to the structural characteristics of the samples (ultra-small Au cluster size and size distribution) a quantitative estimation of this effect was not possible and its role can only be guessed. By comparing the data of the two sets of samples in Fig. 4b and c, we observe that slightly longer lifetimes and larger values of the stretching parameter are obtained for the samples annealed in Ar + H2 with respect to those thermally treated in a N2 atmosphere. This suggests that a better local environment is created around the Er3+ ions if the annealing treatments are performed in a reducing atmosphere, with a reduction of the non-radiative decay rate due to the possible passivation of non-radiative de-excitation paths.
To investigate more deeply the effect of the annealing atmosphere on the Er3+ luminescence emission in the Er–Au co-implanted samples, we measured the evolution of the PL intensity emitted at 1540 nm as a function of the photon flux. This study has been done on the samples of both sets that showed the highest PL emission upon low pumping flux (in both cases such “best-performing” samples are those annealed at 700 °C, see Fig. 1). The data have been analyzed according to the phenomenological model reported in ref. 14, which allowed us to determine quantitatively the main photo-physical parameters that control the Er3+ emission properties of the samples, as the effective sensitization cross-section (σeff) and the fraction of Er3+ ions sensitized by the AuN clusters (fs), and the luminescence quantum yield of the Er emission transition at 1540 nm (Q). Fig. 5 shows the trend of the PL intensity at 1540 nm as a function of the photon flux of the two “best-performing” samples resonantly excited at 488 nm. According to the proposed model, for resonant Er3+ excitation the PL emitted intensity at 1540 nm of the Er–Au co-implanted samples can be written as a function of the photon flux by the following expression:
![]() | (4) |
The first term in the right-hand side of eqn (4) represents the contribution to the PL signal given by the fraction fs of optically active Er ions (NEr,A) indirectly sensitized by the AuN clusters, while the second term accounts for the direct excitation by the laser beam of the residual fraction fd (fs + fd = 1). The constant K is used to take into account the collection efficiency of the PL setup. σ represents the intrinsic emission cross-section of the 4I13/2 → 4I15/2 Er3+ transition and it was estimated by applying the Fuchtbauer–Ladenberg approach43 to the analysis of the Er3+ emission spectrum, obtaining σ = 4.4 × 10−21 cm2,14 in agreement with the values determined with different methods for various Er-implanted photonic materials.2 A summary of the main results determined by the analysis of the two “best-performing” samples according to this model is reported in Table 2. A comment is needed concerning the parameter reported in the last column, . As discussed in detail in ref. 14, by analyzing the data with the proposed model it is not possible to decouple the quantum yield Q and the parameter ξ = NEr,A/NEr, and the product of these two terms
= ξQ is indeed obtained by the fits. ξ represents the fraction of optically active Er ions in the Er–Au co-implanted samples with respect to the population in the reference sample Er800, and it is introduced in the model to account for all the possible effects that may reduce such population (thus giving ξ ≤ 1), as for example residual damage due to Au implantation not completely recovered by the thermal treatment. By comparing the results in Table 2 we can observe that both samples are characterized by a similar sensitization cross-section (about 3 orders of magnitude larger than the intrinsic emission cross-section, σ) and sensitized fraction (fs < 1%). Conversely, very different values of the effective quantum yield
have been determined, whose ratio is: (
)Ar+H2/(
)N2 = 2.8 ± 0.2. With the definition of
given above we can write:
![]() | (5) |
Samples | τeff (ms) | σeff (×10−18 cm2) | fs (%) | ![]() |
---|---|---|---|---|
Er1Au0.5–H700 | 5.76 ± 0.04 | 2.2 ± 0.2 | 0.50 ± 0.06 | 73 ± 5 |
Er1Au0.5–N700 | 5.04 ± 0.04 | 2.9 ± 0.8 | 0.39 ± 0.08 | 26 ± 1 |
The second equality in eqn (5) is obtained by writing the luminescence quantum yield in the form Q = τeff/τr and assuming the same radiative lifetime (τr) for the two samples, i.e., independently of the annealing atmosphere. Considering the very strong structural similarities demonstrated for the two classes of samples, this hypothesis is quite reasonable. Therefore, by inserting in eqn (5) the values of τeff obtained for the two samples annealed at 700 °C (see Fig. 4b) we can provide an estimation of the ratio between the population of optically active Er ions in the two samples, that results: (NEr,A)Ar+H2/(NEr,A)N2 = 2.5 ± 0.2. Thus, upon annealing in an Ar + H2 atmosphere a much larger population of Er3+ ions (by a factor of 2.5) are optically active in the Er–Au co-implanted sample with respect to the sample thermally treated in N2 atmosphere. This is indeed the dominant effect, responsible for the improved performance demonstrated for the Er–Au co-implanted samples annealed in a reducing atmosphere with respect to those heated in an inert atmosphere, that allowed us to get a very efficient Er3+ luminescence emission in spite of the very limited amount of sensitizing agents (AuN clusters) incorporated in the samples.
By taking into account all the experimental findings, the following scenario can be tentatively depicted. When the Er–Au co-implanted samples are annealed in a reducing (Ar + H2) atmosphere, hydrogen passivation of Si dangling bonds around the AuN clusters may occur24 which reduces the degree of configurational disorder of the gold clusters, as it is testified by the smaller Debye–Waller factor estimated by XAS analysis and the higher amplitude of Au-related PL bands. At the same time, such a passivation limits the possibility of formation of Si–O bonds. In this way, a higher amount of oxygen atoms becomes available for coordinating with Er ions in the octahedral configuration, which is optimal for their optical activation, thus increasing the population of optically active Er ions (NEr,A) in close proximity with the Au clusters, which is, in turn, the population of Er3+ ions that can be effectively sensitized by short-range energy transfer from the Au clusters. Moreover, it is worth noting that, upon passivation, a certain amount of H atoms are immobilized, thus reducing also the probability of formation of quenching species for the Er emission which is further beneficial for the Er3+ luminescence. Indeed, as previously discussed, hydrogen is known to act as a quenching species for the Er3+ luminescence in silica and different quenching mechanisms can be considered: H2 molecules can penetrate the silica network forming hydroxyl (OH−) groups, which act as quenchers of the Er3+ emission2 or, as it has been recently demonstrated,44 the deactivation of excited Er3+ ions can occur through the energy transfer to H2 molecular vibrations. Such quenching mechanisms can explain instead the slightly lower PL emission which was detected from the samples implanted with Er only (without gold) and annealed in a reducing atmosphere, with respect to those thermally treated in an inert atmosphere (as shown in the inset in Fig. 1).
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