On the structure of crystalline Au nanoparticles formed on SiO 2 /(100) Si substrates E. Piscopiello1, P. Paiano2,3, P. Prete2, N. Lovergine3 , M. Vittori Antisari4, L. Tapfer1 1. ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, R. C. Brindisi, S.S. 7, km 706, 72100 Brindisi, Italy. 2. IMM-CNR, Unità di Lecce, Via Monteroni, I-73100 Lecce, Italy. 3. Università del Salento, Dipartimento di Ingegneria dell’Innovazione, Via Monteroni, 73100 Lecce, Italy. 4. ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, R. C. Casaccia, Via Anguillarese 301, 00123 Rome, Italy.
[email protected] Keywords: Au nanoparticles, SiO 2 , electron microscopy.
In nanotechnology research, manipulation and self-assembling of nanoparticles on surfaces is a crucial point to produce new heterosystems with peculiar properties. In this context, the understanding of surface interaction phenomena at the micro/nano level are a prerequisite for advanced technological applications. As an example, the optical properties of Au NPs deposited onto suitable solid surfaces can be used to detect hybridization processes of specific DNA sequences with the purpose of fabricate solid phase biosensors devices [1]. The behaviour of Au NPs on Si substrates is of particular interest for the integration of NP based sensors into microelectronic devices. The present study aims to describe the synthesis of Au NP on oxidized Si surfaces. Au nanocrystals were synthesized by self-assembling on SiO 2 /(100)Si substrates. To this purpose, substrates were first degreased in isopropanol vapours for 1h before the loading into the UHV chamber of a Joule evaporator for the deposition of Au film with a thickness between 2 and 4nm at a rate of 0.02nm/s. As deposited samples were annealed into the quartz chamber of a horizontal tubular furnace for 20 min at 814°C under a 130 cm3/min pure N 2 flow. This thermal treatment induces a dense and uniform array of Au NP by self-assembling. The morphology, size, and crystalline structure of Au nanoparticles are characterized by SEM (JEOL JSM 6500F), HRTEM (TECNAI F30 TEM operating at 300kV) and by X-ray diffraction (Philips PW1880, 3 kW generator) measurements performed in both Bragg-Brentano (θ-2θ scan) and glancing incidence (GIXRD) geometrical configuration schemes. Figure 1a, taken on a 4nm thin as deposited Au film, shows irregularly-shaped islands, likely due to the Volmer-Weber growth during evaporation. After annealing at 814°C, the same sample shows the morphology reported in figure1b with well separated spherical NPs with a density of about 6 1010 cm-2. Upon further annealing the islands are observed to grow with a density decrease. A TEM overview of this last sample is reported in the TEM cross-sectional bright field image of figure 2a where the Si-substrate, the 150nm thick SiO 2 layer and the spherical Au nanoparticles on top of it can be easily distinguished. Lattice resolved images, an example of which is reported in figure 2b, allow to establish the amorphous nature of the oxide substrate and the poly-crystalline nature of the Au nanoparticles (twinning planes are evidenced). The Au nanoparticles appear randomly oriented without any particular texture. It is interesting to notice that the particles are partially (of about 1/3) embedded into the Si oxide substrate. Further insights on the Au NPs structure are obtained by performing XRD measurements on annealed samples. Grazing incidence (ω i = 0.5° kept constant during the measurement) X-ray diffraction (GIXRD) patterns of a 2nm thin Au film annealed at 814°C are recorded in a wide angular range from 10°
