Metal nanoparticles embedded in dielectric matrices in the form of nanocomposite films have gained significant research interest due to their multifunctional properties appropriate for various applications ranging from solar cells to targeted drug delivery     . The plasmonic properties of the nanocomposite films mainly depend upon the type of nanoparticles (Au or Ag), their morphology and the dielectric constant of the embedding matrix      . One of the most important aspects at the nanoscale is that the noble metals like silver and gold exhibit strong absorption band in visible range. The origin of this absorption is attributed to their collective oscillation of conduction band electron in response to the electrical field of the electromagnetic radiation of light   . Silicon dioxide (SiO2), one of the most abundant materials on earth, has broadly been used in various fields such as passivation layers of electronic devices, protection layers of magnetic or optical discs and anti-reflective coatings because of their excellent chemical stability and optical transmittance with low refractive index   . Several dielectric matrices, such as SiO2 have been utilized to fabricate different multifunctional nanocomposites for different applications   . Generally, the main motivation behind the use of an insulating matrix is to maintain the necessary separation between metal nanoparticles, thereby preventing an agglomeration of the metallic nanoparticles. Nanocomposite films consisting of metal particles such as gold embedded in a silica matrix have recently been the subject of many studies  -  . A large number of methods have been used to obtain AuNPs embedded in SiO2 films, such ion implantation   , sol-gel  , plasma enhanced chemical vapor deposition (PECVD)  , hybrid techniques combining pulsed-DC sputtering and PECVD, which is used for simultaneous Au sputtering and SiO2 deposition   , and RF magnetron sputtering  -  . The flexibility and easy fabrication of diverse composite films are the advantages of sputtering method. The important factors to influence the formation of AuNPs are the working distance between the target and the substrate, rf-power, sputtering time, the substrate temperature, applied voltage, and working pressure. The purpose of this work is to investigate the influence of substrate temperature on the structural and optical properties of gold/silica composite films grown by RF-magnetron sputtering technique. The as-deposited films were characterized by X-ray diffraction and optical absorption spectroscopy.
2. Experimental Methods
The samples, consisting of gold/silica composite thin films, were prepared by conventional radio-frequency magnetron sputtering method using an Alcatel SCM 650 apparatus. The target consisted of pure (99.99%) metal Au chips on top of a 50 mm diameter silica disc placed 60 mm away from the substrates, is shown Figure 1.
Sputter deposition, in a radio frequency (13.56 MHz) machine, has been car-
Figure 1. Schematic top view of the target for sputtering of the Au chips and a SiO2 disc.
ried out after the chamber reached a base pressure of mbar. Deposition was carried out at two argon pressures and two substrate temperatures 25˚C - 400˚C. The relevant growth conditions of the films are shown in Table 1. Under these conditions, four sets of samples are prepared and they are denoted .
X-ray experiments were performed in a Philips PW 1710 spectrometer using radiation and a Bragg-Brentano geometry. The diffraction patterns were collected over the range at room temperature. The identification of Au crystalline phases was done using the JCPDS database cards (n_04-0784). Optical absorption spectra, of Au/SiO2 composite films, were registered by a Shimadzu UV 30101 PC spectrometer, in near ultra- violet-visible-near infra-red range (NIV-VIS-NIR) from 200 to 2000 nm.
3. Results and Discussion
3.1. Structural Analysis
Figure 2(a) and Figure 2(b) presents the XRD patterns of the samples deposited at two working argon pressure mbar and at two substrate temperatures 25˚C, 400˚C. X-ray diffractogram of gold thin film with a cubic structure, presented as a reference, is also reported in Figure 2. From Figure 2(a), it is evident that there are no Bragg reflections that are clearly visible in the spectra, due to the small AuNPs. Also, it well known that the peak centered on , in the spectra of all the samples, is attributed to the amorphous silica. It can be expected that the measured spectra of the composite films results from the superposition of two diffractograms, assigned to small gold particles and the amorphous silica matrix. The XRD spectra of the series and have the same appearance. The effect of substrate temperature on these two series may be neglected. On the other hand, for the series and , the intensity and the full width at half maximum of the diffraction peak corresponding to Au(111) orientation decrease with increase in deposition temperature. This may be due to the improvement in the crystallinity of the AuNPs at higher deposition temperatures and increases the mobility of added atoms which facilitate the grain growth  .
Figure 2. XRD patterns of gold thin films and Au/SiO2 composite deposited with two Argon pressures, two substrate temperatures.
Table 1. Films growth conditions.
As shown in Figure 2, it is difficult to determine crystalline phases and size. However, using a commercial software program available on our computer, the XRD patterns were deconvoluted, assuming pseudo-Voigt functions in order to obtain the peak position intensity and the preferential growth of the and full width at half maximum (FWHM). Note that the purpose of the deconvolution is to fit the measured XRD spectrum in well-defined peaks to which a physical meaning can be attributed. For more details see the works    . Figure 3 presents the curve fitting of the XRD spectrum of serie. Outside the peak assigned to amorphous silica film, the diffraction peaks resulting from the fitting are attributed to the crystal planes of Au(111), Au(200) and Au(220). The peak positions are in agreement with the well known data: JCPDS-04-04784 characteristic of the FCC cubic structure, indicating that the small gold particles should adopt a fcc-like structure. Table 2 summarizes the fitting parameters determined from the Au(111) orientation plane for all the samples.
The average crystallite size (D) can be estimated from the Debye?Scherrer Equation  .
Where, is the Scherrer constant and is equal to 0.9, is the Bragg’s angle, is the wavelength of the CuKα radiation line and is the full width at half maximum (FWHM) of peak in radian. The average crystallite size was calculated and it was found in the range between 0.56 nm and 0.6 nm
Figure 3. XRD diffraction patterns of serie and their curve fitting.
Table 2. Results of the curves fitting of the experimental diffractograms of the four series.
and 1.15 - 1.23 nm at and argon pressure respectively corresponding to prominent (111) peak. The variation of nanoparticle size with respect to substrate temperature is presented in Table 2. It is observed that the grain size increases with an increase of substrate temperature. Also, the larger grain sizes were obtained with lower argon pressure deposition.
From XRD results, the various structural parameters like dislocation density , strain and lattice distortion , which are commonly used to describe the structural analysis, were calculated using the following equations and evaluated data are presented in Table 3  .
where, D is crystalline size, β is full-width at half maximum in radians, θ is the Bragg’s angle.
Table 3 demonstrates that the strain and dislocation density of the material of the film decrease with an increase of substrate temperature. This may due to the increase in grain size  . Further, the decrease in strain indicates a decrease of lattice constant with an increase of substrate temperature  .
3.2. Optical Studies
Figure 4 presents the measured optical transmission spectra of all the samples. It is observed that the transmittance increases in the visible range and small increasing of the transmittance maxima for wavelength larger than 1200 nm, when the substrate temperature changes from 25˚C to 400˚C for and series. No significant change was observed in the case of and series. This observation is confirmed by the obtained from XRD measurements. Moreover, the transmittance curves denote a pronounced blue-shift of the absorption edge with the enhancement of substrate temperature. The Blue-shift in the absorption edge is ascribed to an enlargement of the optical band gap suggesting a strong effect of the size AuNPs on the electronic properties. Therefore, the position of the absorption edge can be controlled in both the near UV and whole visible band. Moreover, from the optical transmittance measurements we may calculate the absorption coefficient, α, of the films using the formula:
where , is the transmittance and , is the film thickness.
From the absorbance curves presented in Figure 5, we note the absence of the typical SPR extinction peaks, in the A1, A2 and A3 series. This is an indication that the AuNPs should be incorporated in silica films with size lower than 2 nm. For the A4 series, Surface plasmon resonance broad absorption peak is observed
Table 3. Micro structural parameters of the series A1, A2, A3 and A4.
Figure 4. Transmittance spectra of Au/SiO2 nanocmposite films sputtered at two substrate tempertures and argon pressues: (a) at ; (b) at .
in the absorption spectra around 500 nm. These results suggest that Au/SiO2 samples have a semi-conductor character.
According to the Tauc relation, the absorption coefficient α for direct band gap material is given by  ,
where A is a constant, is the optical gap expressed in eV and is the photon energy in . Figure 6 shows the plot of vs. the photon energy ( ) of the Au/SiO2 nanocomposite thin films with different substrate temperature and argon pressures. The ( ) deduce values of A1 and A2 series varied from 3.9 eV to 4.07 eV, while the ( ) values of A3 and A4 varied from 3.85 eV from 4.1 eV when the substrate temperature increases from 25˚C to 400˚C.
The obtained nanocomposite materials have lower absorbance and higher transmittance in the visible region can be used as a suitable antireflection layer in solar cells working mainly in the visible region  .
Au/SiO2 nanocomposite films have been prepared by RF-sputtering technique.
Figure 5. Optical absorption spectra of Au/SiO2 nanocomposite films sputtered at two substrate temperatures and argon pressures: (a) at ; (b) at .
Figure 6. Plot of
versus photon energy
The effect of argon pressure and substrate temperature on the structural and optical properties of the composite films was investigated. XRD analysis shows that the size of the Au NPs increases, the strain, lattice distortion and dislocation density decrease with the increasing the substrate temperature. The transmittance measurements were taken at room temperature in the wavelength range 200 - 2000 nm. The absorption coefficient and optical gap energy were deduced from the transmittance measurements. The increasing of the substrate temperature increases the optical gap energy. The best transparency of composite thin films is obtained at low argon pressure and at higher substrate temperature. These results can give a design guide how to control the properties of composite films by metal nanoparticles.
We are grateful to Professor M.J.M. Gomes from the Centre of Physics, University of Minho, Portugal, for the experimental support.
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