Evolution of Ion Beam Synthesized Au Nanoclusters in SiO 2 under Ion Irradiation
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Evolution of Ion Beam Synthesized Au Nanoclusters in SiO2 under Ion Irradiation Bernd Schmidt, Karl-Heinz Heinig, Arndt Mücklich Forschungszentrum Rossendorf, Institut für Ionenstrahlphysik und Materialforschung PO-BOX 510119, 01314 Dresden, Germany
ABSTRACT The evolution of the mean size and the size distribution of Au nanoclusters (NCs) under high-energy ion irradiation has been studied. Au NCs were synthesized in a 480 nm thick SiO2 layer by 330 keV Au+ implantation and subsequent annealing at T = 1000 °C for 1h in dry O2. XTEM images show a 70 nm thick layer of Au NCs, being centered at the projected ion range Rp(330keV) = 100 nm, having a mean NC size of 5 nm at Rp, and resembling the broad Lifshiz-Slyozov-Wagner (LSW) size distribution of diffusion controlled Ostwald ripening. Post-irradiation of the Au NCs by 4.5 MeV gold ions was used in order to tailor their size and size distribution. The high-energy Au+ irradiations were performed at 190...210 °C with a fluence of (0.5...1.0)x1016 cm-2. By the post-irradiation no gold was deposited into the SiO2 layer, the Au+ ions come to rest in the (001)Si substrate at Rp(4.5MeV) = 1 µm. XTEM images of the post-irradiated Au NCs show a strong decrease of their mean size as well as the width of their size distribution. The observed NC evolution under ion irradiation agrees with recent theoretical predictions and kinetic Monte-Carlo simulations.
INTRODUCTION NC fabrication by ion beam synthesis (IBS) requires a thermal treatment for phase separation of implanted impurity atoms and damage annealing. This annealing leads inevitably to a broad NC size distribution of the Lifshitz-Slyosov-Wagner (LSW) type which is typical for conventional Ostwald ripening (COR) [1]. However, for many applications narrower size distributions are required. For example, semiconductor quantum dots having a broad size distribution emit a broad luminescence spectrum instead of a narrow line. Therefore, a well-controlled method for the formation of narrow NC size distributions would be of great interest. From the literature only indirect hints to ion radiation induced re-ensembling of a given NC distribution are known [2-6]. In ref. [2] the ion beam induced formation of Au NCs in a glass matrix during high-energy Si ion irradiation was observed. The authors of ref. [3] report on the formation of a halo of nanometer-size NCs around sub-µm size gold inclusions in SiO2 during 4.5 MeV Au+ ion irradiation. The authors of ref. [3] explained the observed effect by irradiation induced inverse Ostwald ripening (IOR) leading to a monodisperse precipitate distribution. It is interesting to note that inverse coarsening under irradiation has actually been seen before [4-6], but not identified as such. In the present paper ion beam processing far from the thermodynamic equilibrium is proposed for the controlled tailoring of ion beam synthesized NC ensembles in SiO2. It will be shown experimantally that high energy ion irradiation of a given size distribution of Au-NCs results in IOR, i.e. small NCs grow at th
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