Cubic and oriented Ag nano-particles by high vacuum evaporation D. Berner * , L. Zuppiroli ** , M. Caumont *** and W.S. Bacsa *** * CFG SA, 1110 Morges, Switzerland ** EPFL-STI-IMX-LOMM, Station 3, CH-1015 Lausanne Switzerland *** LPST, IRSAMC-CNRS, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse France, bacsa@lpst.ups-tlse.fr ABSTRACT Ag cubes of uniform size (70nm) and self-assembled into two dimensional arrays are formed by physical vapor deposition. The formation of the Ag cubes depends critically on the substrate composition. While heat dissipation can control the crystallization of Ag islands on surfaces, the thermal conductivity and infrared optical property of the substrate can be tuned by a suitable substrate composition. While evaporation on native SiO 2 on a Si waver leads to the formation of flat islands, the evaporation on a thick SiO 2 / Ag/Si substrate leads to the formation of oriented Ag cubes of narrow size distribution (80-100nm). Keywords: nano-particles, Ag cubes, physical vapor deposition, photonics, scanning probe microscopy 1 INTRODUCTION Optical properties of metal nanoparticles depend strongly on size and shape. Controlling these parameters implies improving the control of their optical properties. Metal nanoparticles find applications in photonics, biological labeling, photography and catalysis. In case of Ag nanopaticles there has been considerable interest for their plasmon resonances in surface enhanced Raman spectroscopy. Nanoparticles can be produced in large variety of processes such as thermal evaporation and gas condensation, laser evaporation, plasma processes, hot spray pyrolysis and other techniques [1-4]. The nanoparticles are often produced in agglomerated form and in macroscopic volumes. The coalescent particles often form a secondary fractal structure and the dispersion of the particle and the stabilization implies often wet chemical processing. When scaling down to smaller volumes and smaller linear dimensions it becomes important to control size, shape and position on a surface and at the same time keep the fabrication process sufficiently simple and cost effective. The self-assembly process is a very efficient process in biology and is particularly attractive to be used to fabricate arrays of nanoparticles on surfaces. Metal particles with submicron size can be fabricated using spherical colloid particles as templates [2]. The metal deposition and subsequent chemical removal the template leads to submicron sized metal particles. A number of metal nanoparticles can be fabricated within a narrow size distribution and shape through solution-phase methods. Cubic silver nanoparticles, 175nm in size have been fabricated using a solution phase method and using the crystalline silver cubes as template nano-boxes of gold have been synthesized [5,6]. We show here that metal nanoparticles with a particular shape can be obtained directly on a surface by physical vapor deposition. Deposition of a few nanometers of metal on an insulator leads in general to the formation of flat islands with circular shape which can at times be uniform in size and be arranged in a trigonal lattice. The formation of metal islands is related to the mobility of the deposited metal species on the insulating surface. The mobility and density of islands of the deposited metal depends on the temperature of the substrate and interaction with the substrate surface. At lager metal deposition the island grow in size and coalesce to form a uniform film. Island growth and shape depend on the cooling rate or heat dissipation between the deposited species and the substrate. Controlling the temperature gradient may allow to control the size and shape of the metal islands. While the cooling rate for an insulating substrate is too large to control the island shape, change of the substrate composition may help to reduce the temperature gradient to form more ordered islands or transforms the shape of the islands. 2 EXPERIMENTAL We prepared a sample with a SiO 2 layer and a metal layer below to reduce cooling by radiation. The two layers combine a layer with low heat conductivity with a layer with high conductivity and high infrared reflectivity. We evaporated 3.5nm Ag using a resistive heating (10 -6 mbar) on a silicon substrate with a thin natural occurring oxide layer (1.5nm) and a silicon substrate with thick Ag layer (1200 nm) and a thick SiO 2 layer (800nm, figure 1). The Ag (3.5nm) was deposited by heating a crucible in a vacuum (10 -5 torr) at a distance of 18 cm and at an evaporation rate of 0.1nm/s. The substrate was clamped on a holder and kept at room temperature. Figure 2 shows the scanning force microscopy image (CP instrument Veeco Inc) of the substrate with the thick SiO 2 layer. Cubes in the size range of 60-120nm are observed. We verified that the orientation of the Ag cubes does not depend on the scan direction. We observe that the cubes are oriented along the substrate edge. 329 NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-6-5 Vol. 1, 2006