Au/SiO2 nanocomposite films, studied in this work, were prepared by RF-magnetron sputtering technique on glass substrate at room temperature under two different substrate temperatures (Ts), and subsequent heat treatment. For the deposited sample at TS = 25℃, no apparent surface plasmon resonance peak could be observed. After annealing, optical absorption spectrum of the Au/SiO2 thin films showed a broad absorption band around 500 nm relating to gold nanoparticles without any modification in the position of the SPR and the size of particles. For the series deposited at TS = 400℃, the surface plasmon resonance (SPR) was found at 500 nm. After heat treatment it’s redshift from 500 nm to 503 nm, while the size increases from 2.01 nm to 2.3 nm. We have also shown that, as the AuNPs are embedded in silica films, the small nanoparticles size have a slightly larger expansion coefficient than for bigger one.
Metallic nanoparticles possess unique optical, electronic, chemical, and magnetic properties that are different from those of individual atoms as well as their bulk counterparts. Noble metal nanoparticles embedded in dielectric matrices exhibit a strong absorption band in UV-visible region [
The temperature dependence of optical properties of metal nanoparticles is a precondition for the development of successful and reliable applications and devices. The temperature effects on SPR absorption band in metal nanoparticles were studied e.g. by Kreibig [
In our recent work [
In this paper, we investigate the influence of substrate temperature and thermal annealing on the structural and optical properties of gold/silica composite films grown by RF-magnetron sputtering technique.
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.
Working argon pressure (mbar) | 2 × 10−3 |
---|---|
Initial pressure(mbar) | 1 × 10−6 |
Au target (%) | 2.6 |
Bias (V) | −50 |
Power (W) | 50 |
Substrate Temperature | 25˚C and 400˚C |
Annealing Temperatures | 300˚C - 500˚C |
Sputter deposition, in a radio frequency (13.56 MHz) machine, has been carried out after the chamber reached a base pressure of (1 × 10−6) mbar. Deposition was carried out at fixed argon pressure 2 × 10−3 mbar and at two different substrate temperatures 25˚C and 400˚C. The relevant growth conditions of the films are shown in
Crystalline phase of films were determined by X-ray diffractometry (XRD), using a Siemens D5000 diffractometer, with CuKα radiation (λ = 0.15406 nm) and a Bragg-Brentano geometry. The diffraction patterns were collected over the range 10˚ < θ < 80˚ at room temperature. The identification of Au crystalline phases was done using the JCPDS database cards ( n _ 04 − 0784 ) . Optical absorption measurements of the prepared samples were registered by a Shimadzu UV30101PC spectrophotometer, in near ultra-violet-visible-near infrared (NIV-VIS-NIR) in a range of 200 - 2000 nm wavelength.
From
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 an fcc-like structure.
The crystallite sizes of AuNPs were calculated from the Debye-Scherer’s formula using the FWHM in radians of the Au (111) reflection: D = k λ / β ⋅ cos θ B , where k is a constant (0.9), D is the crystallite size (in nm), l is wavelength (0.15406 nm), β is full width at half maximum (FWHM in radian) and θ B is the Bragg diffraction angle.
In order to promote some structural and optical changes that will be required to tailor the SPR effect, the films were thermally annealed in air. The XRD spectra for two series A1 and A2 of Au/SiO2 nanocomposite films as deposited and annealed at various temperatures are presented in
For all the samples, characteristic peaks representing pure Au were not very prominent and no peak corresponding to SiO2 was observed, indicating that after annealing process, there is no crystallized SiO2 in these films. The X-ray spectra of the nanocomposite films have been deconvoluted in the same manner as previously mentioned. The results are reported in
R ( T ) = R 0 ( 1 + β Δ T ) 1 / 3 (1)
where R 0 is the nanoparticle radius at room temperature. At the increase of temperature, the volume of nanoparticle increases [
V ( T ) = V 0 ( 1 + β Δ T ) (2)
Samples Number | Temperature (˚C) | Bragg’s angle 2θ (degree) | FWHM (degree) | Particle size (nm) |
---|---|---|---|---|
A1 | as deposited | 39.76 | 11.64 | 0.73 |
300˚C | 39.19 | 9.40 | 0.89 | |
400˚C | 39.38 | 8.63 | 0.97 | |
500˚C | 39.39 | 7.77 | 1.08 | |
A2 | as deposited | 39.16 | 7.44 | 1.13 |
300˚C | 39.14 | 6.30 | 1.34 | |
400˚C | 39.40 | 5.86 | 1.44 | |
500˚C | 39.30 | 5.45 | 1.54 |
where Δ T = T − T 0 is the change of temperature from the room one and β is the volume expansion coefficient for gold nanoparticle. The graphical representation of the volume ratio V / V 0 of gold nanoparticle versus the annealing temperature is shown in
The values of β = 1.61 × 10 − 5 / ˚ K and β = 1.13 × 10 − 5 / ˚ K have been obtained for the A1 and A2 series respectively. Note that we consider the thermal
expansion of a nanoparticle, by assuming that it is free. However, the nanoparticle is embedded in the silica matrix. Respectively, since the volume thermal expansion coefficient for silica is smaller ( 1.65 × 10 − 6 / ˚ K for fused silica) than one for gold ( 4.17 × 10 − 5 / ˚ K ) [
deposited and annealed at different temperature for the two series A1 and A2 respectively. It can be noted that after annealing, the films start to exhibit a broad and weak absorption band. Unlike in the A1 series, broad band absorption is observed for the as-deposited sample in A2 series as seen in
In order to explain the absorption curves, a modelling of the spectra has been performed. Taking into account that particles are small compared to the wavelength of incident radiation the dipole approximation was applied. In this approximation, the absorption coefficient α for the medium with particles of volume V and number of particles per unit volume N is given by the following equation [
α ( λ ) = 18 π N V ε m 3 / 2 λ ε 2 ( ε 1 + 2 ε m ) 2 + ε 2 2 (3)
where λ is the wavelength of the absorbing radiation, ε m is the dielectric constant of the surrounding medium, ε 1 and ε 2 are the real and imaginary part of the dielectric function of particles. The dependence of the metal dielectric function on the size of the particles is taken into account using the model presented by Hövel et al. [
ε ( λ , D ) = ε b u l k ( λ ) + ω P 2 ω 2 + i ω γ b u l k − ω P 2 ω 2 + i ω ( γ b u l k + 2 A v F / D ) (4)
where ε b u l k is the bulk gold dielectric constant, ω P , v F and γ b u l k being, the metal plasma frequency, the Fermi velocity and damping constant in the bulk respectively. A is a phenomenological parameter including details of the scattering process. The values of these parameters used in our simulation are those cited in the work [
For A1 series, the average size of Au particles in the samples A1 is in the range of 2.01 - 2.04 nm obtained from the optical absorption spectra, the size effect on the SPR band shift can be ignored. For A2 series, the plasmon peak positions just vary from 500 nm to 503 nm, and the size increases slightly from 2.01 nm to 2.3 nm when heating temperature increases from 25˚C to 500˚C. The changes occurring in the Au nanocrystals upon increasing temperature show, when the gold particle size exceed slightly the well-known critical size (≈2 nm ), superimposed on the background, a broad surface plasmon band around 500 nm occurs characteristic of gold nanoclusters, due to surface plasmon resonance. Similar observations have been reported for other gold-dispersed dielectric materials: Alvarez et al. [
Samples Number | Temperature (˚C) | SPR(nm) | Particle size (nm) |
---|---|---|---|
A1 | as deposited | --- | -- |
300˚C | 500 | 2.01 | |
400˚C | 500 | 2.02 | |
500˚C | 500 | 2.04 | |
A2 | as deposited | 500 | 2.01 |
300˚C | 501 | 2.1 | |
400˚C | 501 | 2.1 | |
500˚C | 503 | 2.3 |
plasmon absorption is damped and blueshifted with decreasing particle size, in the case of gold clusters in the size range 2 - 4 nm, embedded in alumina matrix grown by co-deposition technique using pulsed laser ablation.
The effect of substrate temperature (ambient and 400˚C) and thermal annealing on structural and optical properties of Au/SiO2 nanocomposite films, prepared by RF-sputtering technique, have been investigated. The results of the present study lead to the following conclusions:
For the nanocomposite films grown at room temperature, formation of small gold nanoclusters with size below 2 nm inside the silica matrix was confirmed by XRD and optical absorption measurements. After annealing, the size of AuNPs is slightly larger than the critical size and the plasmon band peak position of gold clusters is around 500 nm.
As the substrate temperature increases to 400˚C, the SPR absorption band begins to appear at 500 nm wavelength indicating formation of gold nanoclusters. After annealing, the size of AuNPs increases slightly and the plasmon band peak position redshifts from 500 nm to 503 nm.
These experimental results show the ability to create and control very small gold clusters inside dielectric films, by a combination of the sputtering deposition parameters and subsequent heat-treatment.
We are grateful to Professor M.J.M. Gomes from the Centre of Physics, University of Minho, Portugal, for the experimental support.
Belahmar, A., Chouiyakh, A. and Fahoume, M. (2017) Structural and Optical Properties Evolution of Au/SiO2 Nanocomposite Films: The Influence of Substrate Temperature and Thermal Annealing. Open Access Library Journal, 4: e3909. https://doi.org/10.4236/oalib.1103909