The control of gold nanocluster sizes in dielectric thin films via ion beam assisted deposition
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The control of gold nanocluster sizes in dielectric thin films via ion beam assisted deposition 1
S. Schiestel, C.A. Carosella, G.K. Hubler, S.B. Qadri, D. Knies and R.M. Stroud George Washington University, Washington, DC 20052 Naval Research Laboratory, Code 6370, Washington, DC 20375, USA 1
ABSTRACT Gold nanoclusters in silica were deposited by co-evaporation of gold and silica in the presence of an argon ion beam. Gold clusters are formed in-situ during the deposition process. The damage induced by the ion beam controls the cluster size, and clusters with a diameter between 15 – 30 Å are obtained. The Au clusters show a highly compressive strain as measured by X-ray diffraction and lattice parameters up to 3.5 % smaller than in bulk gold are observed. Postdeposition annealing in air leads to cluster growth and release of the strain. Annealing near the melting point of gold causes the clusters to grow exponentially up to 180 Å. This is accompanied by a blue shift of the Mie absorption peak. Furthermore the formation of a superlattice was observed by TEM and confirmed by small angle XRD measurements.
INTRODUCTION Ever since nanocluster containing materials gained significant potential for applications in optical-based devices, researchers have attempted to tailor these materials regarding the cluster size, size distribution, texture and fill factors [1]. In the past, we demonstrated the potential of ion beam assisted deposition (IBAD) for the growth of such films [2]. During IBAD many processes occur simultaneously such as introduction of defects, radiation enhanced diffusion and segregation and ballistic mixing [3]. The motivation of this work was to study the influence of damage induced during film deposition on the formation and growth of gold cluster in a silica matrix. The hope, of course, is to control cluster parameters with IBAD. By varying the current and energy of the ion assist, we can vary the damage unit, displacements per atom or dpa. One dpa is the ion dose that causes every atom in the irradiated material, on average, to be knocked from its lattice position once. The damage was calculated by the following equation: Damage (dpa) =
Ar ions x Vacancies / ion Atomic density x depostion rate
(1)
The vacancies/ion were obtained from SRIM [4]. Gold clusters in silica were chosen as a model system because of the ease of phase separation, the crystalline nature of the Au nanoclusters and their resistance to oxidation. X-ray diffraction, UV/VIS spectroscopy and transmission electron microscopy (TEM) were used to study cluster size, strain and texture as well as the cluster growth.
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EXPERIMENTAL Gold clusters in silica were deposited in an ultrahigh vacuum system (8 x 10-9 Torr) by coevaporation of gold and silica under simultaneous bombardment with Ar ions. The arrival ratio Au/SiO2 was kept at 0.27 atoms/molecule, the ion beam energy was varied between 50 and 400 eV, the current density between 34 and 100 µA/cm2, resulting in a dpa range of 0.055 to 1.8. For comparison, samples were deposit
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