Sivakumar et al. Nanoscale Research Letters 2011, 6:78 http://www.nanoscalereslett.com/content/6/1/78 N A N O E X P R E S S Open Access Characterization of MHz pulse repetition rate femtosecond laser-irradiated gold-coated silicon surfaces Manickam Sivakumar1,3*, Krishnan Venkatakrishnan2, Bo Tan1 Abstract In this study, MHz pulse repetition rate femtosecond laser-irradiated gold-coated silicon surfaces under ambient condition were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), and X-ray photoelectron spectroscopy (XPS). The radiation fluence used was 0.5 J/cm2 at a pulse repetition rate of 25 MHz with 1 ms interaction time. SEM analysis of the irradiated surfaces showed self- assembled intermingled weblike nanofibrous structure in and around the laser-irradiated spots. Further TEM investigation on this nanostructure revealed that the nanofibrous structure is formed due to aggregation of Au-Si/ Si nanoparticles. The XRD peaks at 32.2°, 39.7°, and 62.5° were identified as (200), (211), and (321) reflections, respectively, corresponding to gold silicide. In addition, the observed chemical shift of Au 4f and Si 2p lines in XPS spectrum of the irradiated surface illustrated the presence of gold silicide at the irradiated surface. The generation of Si/Au-Si alloy fibrous nanoparticles aggregate is explained by the nucleation and subsequent condensation of vapor in the plasma plume during irradiation and expulsion of molten material due to high plasma pressure. Introduction Nanostructures of Au, Au-Si alloy, and Si have been employed in micro and nanoelectromechanical systems [1], biosensors [2], and photonics [3,4]. The field- emission property [5] of Au-Si alloy structures is used for the fabrication of panels and displays. Au-Si alloy nanoparticles are increasingly relevant as they are used as catalysts in the growth of nanowires [6]. Recently, femtosecond lasers have proven to be a powerful tool for nanostructuring of bulk metals [7-10]. Femtosecond laser pulses have also been used for precise nanostruc- turing of thin films with minimal thermal side effects [11-13]. The ultrafast excitation of materials controls the deposited energy in the material with femtosecond pulses. As a result, nanostructures with spatial resolu- tion smaller than the wavelength of radiation can be generated. Although interaction of laser radiation with gold, gold-silicon thin films that lead to the formation of microbumps/nanojet structures have been studied [11,14,15], investigations on the Au-Si/Si fibrous * Correspondence: r.m.sivakumar@gmail.com 1Department of Aerospace Engineering, Ryerson University, 350 Victoria Street, Toronto, ON M5B 2K3, Canada. Full list of author information is available at the end of the article nanoparticles aggregate formation using femtosecond laser radiation under ambient condition have not been reported. In the previous studies, synthesis of self- assembled weblike fibrous nanoparticles aggregate structures, nanofibers, and nanoscale tips with bulk semiconductor, metallic, and dielectric materials using femtosecond laser radiation under ambient condition was reported [8,16-18]. The fibrous structure generation is explained by nucleation and condensation of plasma plume generated during the irradiation process. It was comprehended that generation of nanofibrous structures can significantly be controlled by laser radiation fluence, laser interaction time, and pulse repetition rate [16]. This study is aimed to investigate the synthesis of Si/ Au-Si alloy nanoparticles aggregate with femtosecond laser irradiation of Au-coated silicon samples and the influence of laser interaction time on composition of structures. The plausible mechanism underlying Au-Si alloy nanoparticles aggregate generation will also be dis- cussed. The irradiated sample surfaces are characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction analy- sis (XRD). The chemical composition of nanoparticles © 2011 Sivakumar et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.