Structural characterization of Cu nanocrystals formed in SiO ₂ by high-energy ion-beam synthesis

Cu nanocrystals (NCs) were produced by multiple high-energy ion implantations into 5-μm -thick amorphous silica (Si O2) at liquid-nitrogen temperature. The Cu-rich Si O2 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 (EXAFS) 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 andor low annealing temperature contain a significant fraction of Cu oxides, as either oxidized NCs or Cu bonding to O in the Si O2 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 Cu2 O albeit in a disordered form.