Growth and Optical Properties of Self‐Organized Au₂Si Nanospheres Pea‐Podded in a Silicon Oxide Nanowire

Abstract

There is a great impetus in the research on semiconductorbased nanostructures due to their unusual optoelectronic properties that result from quantum size effects.[1] The recent success of silicon-based nanostructures [2±6] with interesting optical [7,8] and electronic [9] properties has renewed interest in this relevant field of study. Exotic structures of silicon-based nanowires with self-organized silicon nanospheres have also been reported [10] furthering the ªbottom±upº paradigm for device fabrication. The formation of silicon-based nanowhiskers, following the vapor±liquid±solid (VLS) growth mechanism and using a metal catalyst has been described in all these reports.[2±10] Metals play a major role in the catalytic action, and a detailed study of the metal±silicon interaction is a subject of considerable interest [11] with regard to both fundamental studies and technological applications. However, except for the formation of CoSi 2 /Si and NiSi 2 /Si nanocomposites, and possible implications in the electrical contact formation for silicon nanowhiskers, [4] very little attention has been given to the possible manifestations of the metal±silicon interaction during the growth of nanowires. Gold is the most widely used catalyst in the VLS growth of silicon-based nanowhiskers, as the Au±Si system is a simple eutectic with the eutectic point at 363 C at about 19 at.-% silicon. [12] Over the last three decades, the Au±Si system has been the most well-studied system, with research concentrating o its possible applications in microelectronic devices. Different metastable phases within the composition ranges of Au 2 Si to Au 7 Si, with different structural and electronic properties, have been reported. [12] Despite the early setback of metallization with gold in silicon, as the device property is degraded by a high diffusion rate of gold in silicon, it still has an advantage in bipolar and radio frequency power transistors, as well as for chip capacitors used for internal matching.[13]We report here the growth and optical properties of a unique structure comprised of silicon-based nanowires with self-assembled gold silicide nanospheres embedded in it. High-resolution transmission electron microscopy (HRTEM), along with analytical electron microscopic (AEM) analyses and selected-area electron diffraction (SAED) studies have been used to characterize the nanowire and the pea-podded nanoclusters. Morphological studies aiding the description of the growth mechanism have been performed using field emission scanning electron microscopy (FESEM). The optical photoluminescence (PL) properties of the noble silicon-based nanowire were studied using the 532 nm line of a Nd:YAG laser in the backscattering configuration of a micro-PL setup.A morphological study showed (Fig. 1a) a noble type of one-dimensional nanocomposite with nanocrystals of mostly spherical (5±30 nm), and a few cylindrical, shapes embedded in the nanowires. These nanocomposites were obtained by thermal annealing of gold/amorphous-silicon (Au/a-Si) bilayers at 1...

Templated control of Au nanospheres in silica nanowires

“…Similar Au sphere in silica structures have been previously observed 16,[27][28][29] and with microwave synthesis methods, "pea-pod" structures have been demonstrated with other metals and insulators. 16 Under ultra-high vacuum conditions, Au migration on Si nanowires was observed to play an important role in the growth and resulting structure of the nanowire.…”

“…We hypothesize a nanosphere formation process similar to that first suggested by Wu et al 27 , but initiated sequentially near the reservoir by the process illustrated in Fig. 5.…”

“…The process independence of this result explains the remarkable geometrical consistency of the Au "pea-pod" structures observed by several recent workers for a wide range of growth conditions. 16,[27][28][29] Once the Au/Si alloy in has been isolated from the reservoir by oxidation by the formation of a highly-curved, low surface-to-volume ratio oxide cap as described above, then the isolated volume continues to evolve to become a Au sphere, as illustrated in Fig. 5(b).…”

“…Here, however, the very low melting temperature of Ga (30 °C, vs.1064 °C for Au) allowed the entire tube to be filled with liquid Ga during growth. 40 Wu et al 27 and Hu et al 16 have presented qualitative models and evidence for Au nanosphere formation in silica nanowires. Wu et al first envisioned nanospheres developing by a two-stage process in which a cylindrical Au silicide core is first embedded in a Si nanotube, which subsequently oxidizes.…”

Supported plasma-made 1D heterostructures: perspectives and applications

“…Ag@TiO 2 core@shell NFs have been prepared by TiO 2 deposition at 130 ºC over a silver metal foil that had been previously exposed to a plasma of oxygen [46,47]. Formation of gold-nanoparticle-embedded dielectric nanowires and other related "peapod" nanostructures is an active area of research because of their multiple applications in various fields [37,61,62]. In our previous works by plasma deposition it was shown that oxygen plasma treatments provoke the development of silver surface nanostructures and a high accumulation of mechanical stress at its surface because of the higher specific volume of silver oxide as compared with metal silver.…”