Study of chromium states in silica glass implanted with Cr+ ions to high fluences
Introduction
Composite materials formed by nanometer-sized transition metal (TM) clusters embedded in dielectric matrices exhibit particular optical and magnetic properties, and are promising candidates for application in several fields. Metal nanocluster composite glasses (MNCGs), also indicated as metal quantum-dot composites (MQDCs), exhibit an enhanced optical Kerr susceptibility, whose real part is related to the n2 coefficient in the intensity-dependent refraction index, defined as n=n0+n2I, where n0 and I are the linear index of refraction and the intensity of the light, respectively. The optical Kerr effect is due to both dielectric and quantum-confinement effects [1], [2], and it is the most important process for applications in all-optical switching devices: typical values of n2 exhibited by MNCGs can be orders of magnitude larger than those of the untreated glass matrices (e.g., pure silica exhibits an n2=10−16 cm2/W [3]). Moreover, MNCGs synthesized with TMs are important for their magnetic properties: in the nanometer range of size, the magnetic order can be replaced by some other magnetic state, such as superparamagnetism [4]. When the nanometer-sized magnetic particles are inside a non-magnetic matrix, surface effects are dominant and affect the magnetic properties significantly, in terms of both oxidation and anisotropy effects.
Among the different MNCG synthesis techniques, ion implantation has a great scientific interest due to its inherent versatility, which allows a fine tuning of both the characteristics and the physical properties of the final composite system [5]: the ion implantation enables to overcome thermodynamic restrictions, and to control both concentration and depth distribution of the dopant atoms, resulting in material properties unattainable by many other synthesis processes. Implantation of TM ions into glass can originate the formation of colloidal metallic particles in the implanted layers, depending on the reactivity among dopant and host substrate atoms [6]. The formation of several kinds of metallic colloids embedded in silica glass has been already obtained by ion implantation; however, many types of implanted ions cannot form elementary colloids in silica [6].
Electron paramagnetic resonance (EPR) spectroscopy is effectively applied to the study of silica glasses implanted with TM ions [7], [8], [9], [10], [11], [12], [13]. This method yields information concerning valence state and local environment of implanted TM ions, as well as the nature of structural defects induced in the substrate by the implantation. EPR is also very sensitive to interactions among TM ions, and to the formation of both TM clusters and fine crystalline inclusions containing TM atoms.
EPR spectra of silica glass implanted with Cr+ ions at E=160 keV, fluences ranging from 5×1015 to 6×1016 ions/cm2 have been reported in Ref. [9]. Three types of paramagnetic defects (E′-center, E′-type center, and peroxyradical, POR) have been identified and their fluence and depth dependence have been measured by EPR. An absorption with line width of ∼3 mT has been detected near g=1.97. Its intensity was more than one order of magnitude smaller than that due to structural defects, and decreased monotonically with implantation fluence. This absorption has been assigned to paramagnetic state of implanted Cr ions. No discussion about this signal was given in Ref. [9]. Preliminary results of an EPR study of silica glass implanted with Cr+ have been reported recently [14].
Several informations concerning the chemical compounds formed inside the solid substrate by TM implanted atoms can be obtained by means of X-ray photoelectron and X-ray-excited Auger electron spectroscopies (XPS and XE-AES, respectively). The XPS and XE-AES study of Cr-implanted silica glasses [15], [16] have shown the formation of chromium oxides in all the samples. Moreover, in the high fluence implanted samples (up to 11×1016 ions/cm2, E=35 keV) the formation of chromium silicide compounds of mean stoichiometry Cr5Si3 has been also detected. Transmission electron microscopy (TEM), selected area electron diffraction, and energy dispersive spectroscopic X-ray microanalysis have shown that chromium silicide compounds appear as amorphous nanoclusters embedded in the silica substrate [16], whose diameters range from 10 to 20 nm. No presence of metallic chromium has been revealed, nor of crystalline chromium oxides.
Optical absorption spectra in the range from 1 to 7 eV in silica glass implanted with Cr+ have been extensively studied [9], [12], [17]: no absorption bands related to Cr ions were detected. The absorption bands observed in UV and Vis–UV ranges were due to silica structural defects induced by Cr+ implantation [9], [12], [17]. However, in Ref. [14] an absorption band around 625 nm, associated to Cr3+ ions, has been reported for silica glass implanted with Cr+ ions.
Section snippets
Experimental
Disks 1 mm thick of Soviet commercial silica glasses of KV type, obtained by gas-flame fusion and contained several hundreds of ppm of OH groups, were irradiated with Cr+ ions at 200 keV, and fluences F1=2×1017 (sample #1), F2=3×1017 (sample #2) or F3=5×1017 ions/cm2 (sample #3). Current densities were about 0.5 μA/cm2. The temperature of substrates during the implantation process measured by a thermocouple was about 330 K. Mean projected range, Rp, and range straggling, ΔRp, calculated by TRIM
Results
Fig. 1(a) shows the EPR spectrum recorded at RT and microwave power P=40 mW for the sample #1 implanted with Cr+ to a fluence F1=2×1017 ions/cm2. The spectrum consists of three signals. One of them can be identified as POR based on its g-value (g1=2.066; g2=2.0081 and g3=2.0010) and line shape. It has a low-field shoulder that is characteristic of POR [7], [12], [22]. The second signal with g∼2.0020 and width of ∼0.8 mT is presumably due to E′-type center [9], [12], [23]. The third signal is
Discussion
The structural defects (E′-center, E′-type center and POR) in ion-implanted silica glasses have been discussed in many papers. Here, we consider only EPR signals around g∼1.97–1.98 which can be associated with Cr ions [9]. The narrow asymmetric signals at g∼1.97–1.98 (similar to Cr-1 line) can be attributed both to Cr5+ and to Cr3+ ions [26].
As follows from the inset of Fig. 1(b), the absorption band around 625 nm is observed in optical spectrum of the sample #1. Similar band has been reported
Conclusion
In EPR spectra of silica glasses implanted with Cr+ at energy E=200 keV and fluences F>1017 ions/cm2, in addition to the well-known paramagnetic defects (E′-center, E′-type center and POR), the signals due to Cr3+ ions are found. One of them can be attributed to few isolated Cr3+ ions in slightly distorted octahedral environment (less than 1% of the nominal chromium atoms inside the samples). Unusual temperature dependence of the other signal indicates the formation of crystalline Cr2O3
Acknowledgements
We are grateful to Professor E. Tondello, director of “Centro di Studio sulla Stabilità e Reattività dei Composti di Coordinazione, CNR- Padova, Italy”, for the use of the XPS facilities.
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