Si nanotubes and nanospheres with two-dimensional polycrystalline walls† Paola Castrucci,‡ * a Marco Diociaiuti,‡ b Chiti Manohar Tank,‡ c Stefano Casciardi, d Francesca Tombolini, d Manuela Scarselli, a Maurizio De Crescenzi, a Vikas Laxman Mathe c and Sudha Vasant Bhoraskar c Received 16th April 2012, Accepted 23rd May 2012 DOI: 10.1039/c2nr30910f We report on the characteristics of a new class of Si-based nanotubes and spherical nanoparticles synthesized by the dc-arc plasma method in a mixture of argon and hydrogen. These two nanostructures share common properties: they are hollow and possess very thin, highly polycrystalline and mainly oxidized walls. In particular, we get several hints indicating that their walls could constitute only one single Si oxidized layer. Moreover, we find that only the less oxidized nanotubes exhibit locally atomic ordered, snakeskin-like areas which possess a hexagonal arrangement which can be interpreted either as an sp 2 or sp 3 hybridized Si or Si–H layer. Their ability to not react with oxygen seems to suggest the presence of sp 2 configuration or the formation of silicon–hydrogen bonding. Introduction In the last few decades, intense efforts have been directed at studies of the structure and properties of nanotubes, nano- particles and single layered nanostructures. This interest has been in part stimulated by the discovery of carbon nanotubes (CNTs) in the early ’90s. 1 More recently, hollow quasi-one-dimensional structures of a variety of materials have been investigated due to their often unusual physical and chemical properties, making them suitable for several technological applications. Among non- carbon low-dimensional nano-objects, silicon nanotubes 2–30 and silicene nanoribbons 30–40 have attracted great attention from an applicative and fundamental point of view. On the one hand, they appear to be compatible with the silicon-based microelec- tronics and might become, prospectively, the most versatile building materials for nanoelectronic devices. On the other hand, though silicon and carbon lie in the same group of the periodic table and have a similar electronic structure, the former is not expected to arrange in an sp 2 configuration which is usually necessary to give rise to the rolled-up graphite-like sheets based on the atomic structure of CNTs. In fact, Si atoms favourably form sp 3 hybridization thus promoting the growth of silicon nanowires, crystalline silicon nanotubes (SiNTs) 2,4,5,10–14 and hollow nanoparticles 41 with very thick walls, generally covered by a silicon oxide layer. Therefore, a number of theoretical calculations have been devoted to clarify if Si atoms could arrange in single wall nanotubes similar to the conventional CNTs. 15–30 As a result, several authors found that single wall SiNTs are energetically viable in sp 2 hybridization 15,20,21,30 or in an sp 2 –sp 3 mixed configuration. 18,23 Alternatively, other theo- retical reports showed that also sp 3 hybridization can be considered a source of energetically stable SiNTs in the form of small nanotubes with faceted geometry, 24,28 larger tubes assuming a smooth or a corrugated lateral surface 17,19,22,25 or SiH and SiO x nanotubes. 16,26,27,30 However, these theoretical SiNTs are rather artificial and hypothetical. In fact, several synthesis routes have been reported to form SiNTs but, generally, the resulting nanotubes present walls with a diamond-like structure no thinner than 5 nm, mostly covered by silicon oxide. 2,4,5,10–14 In 2005 and 2006, we succeeded in growing almost non-oxidized SiNTs with very thin walls (<1 nm) by using the arc-discharge method in an Ar atmosphere. 7,8 In the same period, two papers appeared showing that Si multiwall nanotubes, very similar to the carbon nanotubes, can be also obtained by particularly high non-equilibrium synthesis processes. 6,9 These last experimental works all gave hints to the existence of the sp 2 configuration for silicon atoms. The thermal plasma assisted synthesis of such Si nanostructures has recently been reviewed. 42 As far as hollow silicon and silica nanospheres are concerned, their formation has been reported by several researchers mainly stimulated by the high number of excellent properties making these nanostructures potential candidates as electrodes for lithium-ion batteries 41 and for novel encapsulation technology, 43–47 respectively. Nonethe- less, the smallest reported skin of the former hollow nano- particles is about 25 nm (ref. 41) while that of the latter is no lower than 5 nm. 46 In addition, they are generally amorphous. a Dipartimento di Fisica, Universit  a Roma Tor Vergata and Unit  a CNISM, via della Ricerca Scientifica 1, 00133 Roma, Italy. E-mail: castrucci@ roma2.infn.it b Dipartimento di Tecnologie e Salute, Istituto Superiore di Sanit  a, 00161 Roma, Italy. E-mail: marco.diociaiuti@iss.it c Department of Physics, University of Pune, 411007 Pune, India. E-mail: svb@physics.unipune.ac.in d INAIL ex ISPESL, Dipartimento di Igiene del Lavoro, via Fontana Candida 1, 00040 Monte Porzio Catone (Roma), Italy. E-mail: stefano. casciardi@ispesl.it † Electronic supplementary information (ESI) available. See DOI: 10.1039/c2nr30910f ‡ These authors have to be considered as co-first authors. This journal is ª The Royal Society of Chemistry 2012 Nanoscale, 2012, 4, 5195–5201 | 5195 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2012, 4, 5195 www.rsc.org/nanoscale PAPER