Letter to the Editor Surface morphology of silica nanowires at the nanometer scale Cornelia Rodenburg a , Xiong Liu b , Mark A.E. Jepson c , Stuart A. Boden d , Gilberto Brambilla e, ⁎ a Department of Engineering Materials, University of Sheffield, Mappin Street S1 3JD, UK b Carl Zeiss NTS, Carl-Zeiss-Straße 56 73447, Oberkochen Germany c Department of Materials, Loughborough University, Loughborough LE11 3TU, UK d Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK e Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK article info Article history: Received 12 November 2010 Received in revised form 8 March 2011 Available online 12 April 2011 Keywords: Surface; Silica; Defects; Optical fiber Introduction Surface quality has been recognized to be a fundamental limitation to mechanical strength and optical transmission, particularly for sub- micrometric structures. Mechanical properties of nanowires are known to improve significantly for decreasing diameters [1–9]. Materials with high specific ultimate strength σ ρ (ratio between strength and density) are highly desirable because they could have radical implications in extreme engineering applications like aerospace structural materials, wind turbines, powerboats, suspended bridges or even more futuristic applications like the space elevator. The presence of defects degrades the overall σ ρ : a single missing atom in an otherwise perfect CNT would decrease its tensile strength by 20% [10]. This is a major drawback since the great majority of nanostructures is crystalline and can be fabricated flawlessly only for few millimeters. In contrast, a silica glass does not have any long-distance atomic periodicity, thus it can accommodate sub-nanometer defects, preserving a relatively high σ ρ . Silica nanowires with diameters smaller than 50 nm and surface smoothness at the atomic value have been manufactured [11] from optical fibers. Yet, the strength of silica nanowires is affected by abrupt changes in diameter and surface uniformity over a long range is the single most crucial factor in their performance. For optical applications, photonic bandgap fibers (PBGFs) have been proposed as the means to reach the ultimate low loss [12] and to change radically data transmission over long distances. While the minimum transmission loss (~0.15 dB km -1 ) observed so far in conventional fibers [13] is determined by fundamental scattering and absorption processes in silica, in PBGFs over 99% of the light can propagate in air [12] and avoid these loss mechanisms. Optimized silica PBGFs could have a transmission loss as little as 0.03 dB km -1 [12] if the only surface roughness were of intrinsic nature [14] and came from ripplons (surface capillary waves). Optimized PBGFs would have a hollow core bordered by very thin silica strands with thickness of the order of ~ 100 nm, the surface quality of which will be the single most important factor to minimize the transmission loss. In this manuscript we analyze the surface morphology of silica nanowires with unprecedented resolution and discuss the origin of defects which can be as large as 20 nm. Nanowires used in these experiments were manufactured from commercial optical fibers by the “microheater brushing technique”,a top–down technique originally developed to manufacture nanowires from glasses with low processing temperature [15]. A commercial telecom optical fiber (SMF-1300/1550-9/125-0.25-10641, manufac- tured from Oz Optics), was held by two clamps mounted on translational stages (WSX-300 provided by Rockwell Automation) and heated by a ceramic microheater (manufactured by NTT-AT) with a bore 20 mm long and 2 mm wide (Fig. 1a) in air. The fiber was pulled Journal of Non-Crystalline Solids 357 (2011) 3042–3045 ⁎ Corresponding author. Tel.: + 44 23 8059 3588; fax: + 44 23 8059 3149. E-mail address: gb2@orc.soton.ac.uk (G. Brambilla). LETTER TO THE EDITOR 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.03.005 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol