Structural characterization of Cu nanocrystals formed in SiO2 by high-energy ion-beam synthesis

Abstract

Cu nanocrystals ͑NCs͒ were produced by multiple high-energy ion implantations into 5-m-thick amorphous silica ͑SiO 2 ͒ at liquid-nitrogen temperature. The Cu-rich SiO 2 films were subsequently annealed to reduce irradiation-induced damage and promote NC formation. The NC size distribution and structure were studied utilizing a combination of Rutherford backscattering spectroscopy, x-ray diffraction, cross-sectional transmission electron microscopy, and extended x-ray-absorption fine-structure ͑EXAFS͒ spectroscopy. We present results derived from all four techniques, focussing on EXAFS measurements to study the local atomic structure surrounding Cu atoms, and comparing NC samples with bulk standards. Using a unique sample preparation method, we drastically improve the signal-to-noise ratio to extract high-quality EXAFS data to enable the determination of a non-Gaussian bond length distribution via the third-order cumulant. We quantify subtle concentration-and annealing-temperature-dependent changes in the Cu NC short-range order and relate such changes to NC size. Relative to a bulk Cu standard, enhanced structural disorder is observed in addition to both a suppressed coordination number and bond length contraction. Deviations from bulklike structure increase as the NC size decreases. Samples of low Cu concentration and/or low annealing temperature contain a significant fraction of Cu oxides, as either oxidized NCs or Cu bonding to O in the SiO 2 matrix. EXAFS and x-ray-absorption near-edge structure analyses demonstrate Cu in an oxidized form exhibits an oxidation state and local coordination similar to crystalline Cu 2 O albeit in a disordered form.

Ion Beam Formation and Modification of Cobalt Nanoparticles

“…For semiconductor NPs embedded in SiO 2 , simulations [44] and experiments [45] have demonstrated that the interfacial atom environment is highly distorted to accommodate material and bonding differences across the interface. Similar trends have been reported for other embedded elemental metal NPs [46][47][48][49]. Temperature-dependent, phase-corrected FT EXAFS spectra are presented in Figure 5 for bulk HCP Co and both 5.2 nm and 2.8 nm diameter HCP Co NPs.…”

“…The SHII-induced changes in these structural parameters with ion fluence are attributed to (i) and the increasing SBR for both the elongated and small NPs. Furthermore, the CN calculated from EXAFS is also sensitive to the fraction of atoms dispersed in the matrix as dimers/trimers [72] or in an oxidised state (ii) [46,124,125]. The fraction of oxidised atoms cannot be determined from the XTEM and SAXS analysis and are instead determined from the XANES analysis.…”

Role of Thermodynamics in the Shape Transformation of Embedded Metal Nanoparticles Induced by Swift Heavy-Ion Irradiation

“…For such calculations, we used the thermodynamic parameters of the bulk metals. Finitesize effects in elemental metal NPs can induce changes in the structural, vibrational and thermodynamic properties relative to bulk material including differences in bond length [20][21][22][23], Einstein temperature [23,24] and melting temperature [25]. However, these effects are typically significant only for NPs of diameter considerably less than those used in this report and, as a consequence, any error introduced through our use of bulk values was considered minimal.…”

Structural and elastic characterization of Cu-implanted SiO2 films on Si(100) substrates

“…Comparing the two spectra, negligible Cu diffusion ͑or loss of material͒ is apparent upon annealing, consistent with previous reports. 4 Structural characterization was performed by x-ray methods ͓x-ray diffraction ͑XRD͒, pole figures͔ and transmission electron microscopy ͑TEM͒. The XRD measurements were carried out using Cu K␣ radiation from a Rigaku D/Max 2200V-PC diffractometer operating at 40 kV/40 mA and configured for the Bragg-Bretano focusing geometry.…”

“…Crystallization of Cu in the asimplanted sample was somewhat unexpected, but is not unprecedented for high dose ͑ϳ10 17 cm −2 ͒ implantations. 4 It is also worth noting that the absence of peaks other than the Cu ͑111͒ peak in the XRD spectra ͓there is no Cu ͑200͒ peak, for instance, observed at the expected position of 2 = 50.6°͔ suggests that there is a preferred orientation for Cu in SiO 2 on Si͑100͒. Figure 3͑b͒ shows the size distribution ͑obtained from cross-sectional TEM data͒ of the nanocrystals in the implanted and annealed sample.…”