Metallic nanoparticles (NPs) embedded in dielectrics display unique structures and new interesting properties, such as peculiar optical properties , size-dependent electronic and magnetic properties , and so on. Owing to their distinct properties, materials composed of discrete metallic NPs have a wide range of potential applications, and thus have been received much more interest in the last decade. A variety of ways have now been developed to synthesize metallic NPs in dielectrics, such as magnetron sputtering, ion implantation, metal-organic chemical vapor deposition, molecular beam epitaxy, sol-gel deposition, etc. Among them, ion implantation is one of the most promising fabrication methods because it allows reaching a high metal filling factor in an irradiated matrix beyond the equilibrium limit of metal solubility and provides controllable synthesis of NPs at various depths under the substrate surface. Indeed, various metallic NPs, such as Au, Ag, Ga, etc., have been successfully fabricated in silica glass, sapphire and Si 3 N 4 by ion implantation .
Single crystal Al 2 O 3 has been widely used in microelectronics for photo-electronic devices or as window materials owing to its excellent mechanical and optical properties. Concerning the nonlinear optical properties for nonlinear optics, Cu has high concentration of the conductive electrons, and thus is the most preferable material . However, a few groups have devoted themselves to investigate ion synthesis of Cu NPs in Al 2 O 3 . Through these researches, it has been demonstrated that the feasibility of creating composites could be achieved in the Al 2 O 3 by low-energy (<120 keV) implantation of Cu ions . While no evidence for the formation of Cu NPs has been found at higher energies ($130 keV to 2.4 MeV) . Additionally, the thermal evolution of the formed Cu NPs has also been studied, especially for high temperature annealing . Although these earlier works have provided a lot of knowledge for the understanding of ion synthesis of Cu NPs in Al 2 O 3 crystal and also of their evolution during annealing, few of them have been contributed to the possible annealing atmosphere effects on the growth and transformation of Cu NPs, especially in the low temperature range (<700 8C).
In the present work, Cu NPs were synthesized in Al 2 O 3 crystal by high dose Cu ion implantation, and then subjected to furnace annealing in two different atmospheres, i.e. vacuum and oxygen, respectively. By using various analytical techniques, quite different evolution behaviors of Cu NPs have been well revealed.
2. Experimental
Optical polished purity Al 2 O 3 (0 0 0 1) wafers of 0.5 mm in thickness were implanted with 45 keV Cu ions at a dose of