Sivakumar et al. Nanoscale Research Letters 2011, 6:78 http://www.nanoscalereslett.com/content/6/1/78 aggregate is further analyzed using X-ray photoelectron spectroscopy (XPS). Experimental methods The laser source used (l = 1030 nm) is a direct-diode pumped Yb-doped fiber oscillator/amplifier system cap- able of delivering a maximum of 15 W average power with a pulse repetition rate ranging from 200 kHz to 25 MHz. The beam profile is Gaussian and the spot dia- meter is (10 μm) measured at 1/e2. The samples used were gold-coated (thickness 200 to 400 nm) silicon wafers. The laser beam is focused on the sample surface with a lens of focal length of 71 mm and scanned using a computer-controlled galvanometer to produce arrays of spots. The experiments were carried out in air at atmospheric pressure. The radiation fluence used was 0.5 J/cm2 at a pulse repetition rate of 25 MHz with an interaction time of 1 ms and with the pulse width of 214 fs. Three sets of samples were prepared with same experimental conditions. One set was used for SEM ana- lysis. The other set was used for XRD analysis followed by TEM investigations. Third set was used for XPS ana- lysis. XRD measurements were performed with a Cu Ka radiation (l = 0.154184 nm). The diffractograms were recorded using Bruker detector from 20° to 70°. To transfer the nanostructures to TEM grids, the samples were sonicated in isopropanol solution. A drop of the nanoparticles aggregate dispersed solution was placed on the copper grid and allowed to dry in air. XPS mea- surements were carried out on a Thermo Scientific K-Alpha XPS spectrometer. A monochromatic Al Ka X-ray source was used, with a spot area (on a 90° sample) of 400 μm. Results and discussion SEM micrographs of the sample surfaces around the laser-irradiated spots are shown in Figure 1a. Weblike fibrous nanoparticles aggregate with certain degree of porosity is observed in and around the laser-irradiated Page 2 of 5 spots with all processing parameters used. SEM/EDX analysis of the nanostructure shows the presence of gold, silicon, and oxygen. In addition to Au-Si, nanopar- ticles of amorphous silicon which are comparatively smaller are also observed (Figure 1b). The size of Au-Si nanoparticles in the fibrous nanoparticles aggregate structure is bigger than that of Si particles as evidenced from TEM analysis (Figure 1b, c). Moreover, the differ- ent nanoparticles are intermixed with each other in the aggregate. TEM/EDX analysis of the nanoparticles aggregate revealed the composition Au-Si nanoparticles. The relative amount of silicon and gold in Au-Si nano- particle varies with different particles (Figure 2). X-ray diffractograms of both treated and untreated samples were performed with Cu Ka radiation (l = 0.1541848 nm) [19] (Figure 3). The peaks 32.2°, 39.7°, and 62.5° were identified as (200), (211), and (321) reflections, respectively, corresponding to JCPD file for Au81Si19 (JCPD 39-0735). Further peaks at 38.2°, 44.38°, and 64.6° were identified to originate from (111), (200), and (220) planes of Au (JCPD 04-0784), respec- tively. The average Au-Si particles size is calculated from the full-width at half-maximum (FWHM) of the diffraction peaks using the Debye-Scherrer formula [20] D = kl/bcosθ, where D is the mean grain size, k is a geometric factor (= 0.89), l is the X-ray wavelength, b is the FWHM of diffraction peak, and θ is the diffraction angle. The grain sizes of Au-Si calculated from the peaks 32.2° and 39.7° are 26 and 55 nm, respectively. The chemical state of Au, Si, and oxygen atoms for both untreated and laser-treated samples was investi- gated by XPS. The correction of the XPS spectra for the charge accumulation was performed using C 1s peak (BE = 284.6 eV), which can be ascribed to contaminant hydrocarbons. Figure 4 shows XPS spectra of Au 4f line obtained for the untreated and laser-treated samples. For the untreated samples, the peaks of Au 4f7/2 and Au 4f5/2 lines are located at BE = 83.82 and 87.4 eV, which correspond to elemental gold. Laser-irradiated samples Figure 1 SEM and TEM micrographs of the sample surface irradiated at laser radiation fluence (0.5 J/cm2) with interaction time (1 ms) and the nanoparticles aggregate respectively. (a) surface featuring weblike nanoparticles around the laser spot, (b, c) Si/Au-Si nanoparticles in the aggregate structure.