Sivakumar et al. Nanoscale Research Letters 2011, 6:78 http://www.nanoscalereslett.com/content/6/1/78 Page 4 of 5 Figure 5 XPS spectra of laser treated samples for Si 2p system. lattice takes place with a time limit of up to 50 ps [25]. During irradiation, first few pulses alter the gold film and significantly increasing the absorption of this mod- ified surface for the subsequent pulses [9]. At 25 MHz pulse repetition rate, the pulse separation time is 40 ns, Au film reaches high temperature due to accumula- tion of heat from successive pulses. Laser pulse repeti- tion rate plays a significant role in generating these structures due to cumulative heating. At MHz laser pulse repetition rate, the delay between successive pulses is comparable to the critical time of nucleation [8]. Besides, repetition rate helps to sustain the molten liquid thereby maintaining the plasma and nanoparti- cles agglomeration. Taking into consideration the dif- ferent plume components, formation of gold nanoparticles with various sizes under femtosecond laser irradiation is explained by nucleation and con- densation of vapor in the plasma plume and explosion of molten material due to high plasma pressure [17]. The melting point of Au is 1063°C while for Si it is 1414°C. Although for an alloy which contains 81% Au and 19% Si, the melting point is 359°C, called the eutectic point. The formation of Au-Si alloy catalyst is explained via a Vapor-Liquid-Solid (VLS) process [6] during the synthesis of Si nanowires. In this experiment, since the laser radiation fluence (0.5 J/cm2) used is much above the ablation threshold of Au (0.2 J/cm2) with multiple pulses [26], gold may diffuse into the silicon substrate to form an alloy at the interface [27]. This alloy layer at the interface started melting due to cumulative heating by the subsequent laser pulses. The expulsion of molten alloy material results in the formation of alloy nanoparticles. Once the alloy layer is depleted further irradiation ablates the underlying silicon substrate and generates the plasma. At this point, the silicon nanoparticles are generated by nucleation and condensation of vapor in the plasma [8] and agglomerates with Au-Si alloy nanoparticles to form weblike nanofibrous structure. Although irradiation of molten alloy nanoparticles by subsequent laser pulses may increase their temperature, it is not supporting the growth of silicon nanowires [27]. The relative proportion of alloy nanoparticles in the aggregate nanostructure is mainly determined by the laser interaction time. This is implicitly understood from the SEM/EDX analysis, which shows the atomic percent of gold and silicon as a function of interaction time in the fibrous aggregate structure. In contrast to normal VLS process [6], where the source of semiconductor is supplied as vapor phase, in this case both gold and silicon are originated from the gold-coated solid silicon substrate. Upon laser irradiation, molten Au- Si alloy layer is formed and transforms into molten nano- particles and agglomerates as solid weblike self-assembled fibrous structure. In other words, the fibrous nanoparti- cles aggregate process can be regarded as Solid-Liquid- Solid [27] process. In this experiment, Si/Au-Si fibrous nanoparticles aggregate is generated in a single step under ambient condition. The existence of Au-Si alloy in the nanoparticles aggregate is corroborated from XRD, TEM/EDX, and XPS analysis. Moreover, the size of alloy nanoparticles calculated from the peaks of XRD diffracto- gram matches well with the size observed from TEM micrographs. Conclusions A simple method of generating Si/Au-Si alloy nanoparti- cles aggregate using MHz pulse repetition rate femtose- cond laser radiation under ambient conditions is reported. The generation of nanoparticles aggregate is explained by nucleation and condensation of vapor in the plasma plume and expulsion of molten material due to high plasma pressure. This technique could be extended to generate other metal-semiconductor alloy nanostructures. Further studies are required to find the influence of film thickness and laser processing para- meters on nanostructure generation. Abbreviations SEM: scanning electron microscopy; TEM: transmission electron microscopy; VLS: vapor-liquid-solid; XPS: X-ray photoelectron spectroscopy; XRD: X-ray diffraction analysis. Acknowledgements This study was funded by the Natural Science and Engineering Research Council of Canada. Author details 1Department of Aerospace Engineering, Ryerson University, 350 Victoria Street, Toronto, ON M5B 2K3, Canada. 2Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, ON M5B 2K3, Canada. 3On leave from Department of Sciences, Amrita School of Engineering, Amrita Vishwa Vidyapeetham, Ettimadai, Coimbatore 641105, India.