- Nano Express
- Open Access
Annealing Effects on Structure and Optical Properties of Diamond-Like Carbon Films Containing Silver
© Meškinis et al. 2016
Received: 30 November 2015
Accepted: 7 March 2016
Published: 15 March 2016
In the present study, diamond-like carbon films with embedded Ag nanoparticles (DLC:Ag) were deposited by reactive magnetron sputtering. Structure of the films was investigated by Raman scattering spectroscopy. Atomic force microscopy was used to define thickness of DLC:Ag films as well as to study the surface morphology and size distribution of Ag nanoparticles. Optical absorbance and reflectance spectra of the films were studied in the 180–1100-nm range. Air annealing effects on structure and optical properties of the DLC:Ag were investigated. Annealing temperatures were varied in the 180–400 °C range. Changes of size and shape of the Ag nanoclusters took place due to agglomeration. It was found that air annealing of DLC:Ag films can result in graphitization following destruction of the DLC matrix. Additional activation of surface-enhanced Raman scattering (SERS) effect in DLC:Ag films can be achieved by properly selecting annealing conditions. Annealing resulted in blueshift as well as significant narrowing of the plasmonic absorbance and reflectance peaks. Moreover, quadrupole surface plasmon resonance peaks appeared. Modeling of absorption spectra of the nanoclusters depending on the shape and surrounding media has been carried out.
Surface plasmons are collective oscillations of the free electrons localized at surfaces of metal structures . At a certain excitation frequency, these oscillations will be in resonance with the incident light, resulting in significantly increased intensity of oscillation of the surface electrons . This effect is called localized surface plasmon resonance (LSPR) . Surface plasmon resonance effect results in resonant optical absorbance, transmittance, scattering, and/or reflectance spectra of the plasmonic materials [1, 2]. The most often used plasmonic materials are the group IB metal (gold, silver, copper) nanoparticles. Position of the nanoparticle surface plasmon resonance peak can be controlled by setting the appropriate nanoparticle shape, size, and interparticle distance . Among all of the group IB metals, silver has some advantages over Au and Cu as a plasmonic material [3, 4]. Particularly, the stronger surface plasmon resonance effect should be mentioned [3, 4].
An interesting class of plasmonic materials is plasmonic nanocomposites . These nanomaterials consist of a dielectric matrix with the embedded metal nanoparticles . Using such nanocomposites, Ag nanoparticle oxidation problems can be controlled or even avoided [6–8]. It provides additional possibility to control the optical properties of the plasmonic material by using a dielectric matrix material of the appropriate dielectric permittivity . The prospective dielectric matrix material is diamond-like carbon (DLC). DLC is an amorphous allotrope of carbon consisting of the sp3-bonded and sp2-bonded carbon atoms and containing 0–40 at.% of hydrogen [9, 10]. Hardness of these films is as high as up to 80 % of the diamond hardness [9, 10]. Wear and corrosion resistance as well as biocompatibility can be mentioned, too [9, 10]. Optical and electrical properties of DLC films can be changed in a broad range [9, 10]. DLC films containing silver (DLC:Ag) are growing in the form of the nanocomposites with embedded Ag nanoparticles when the silver atomic concentration in the film is more than 1–2 at.% [11–13]. Surface plasmon resonance effect was reported for the DLC:Ag films .
Optical properties of the plasmonic nanocomposites and other plasmonic nanomaterials can be additionally changed by using annealing [15–20]. In such a way, increased intensity of the surface plasmon resonance absorption [15, 18, 19] and reflectance  peaks as well as control of the plasmonic peak position [17, 19] can be achieved. Using such approach, new interesting nanostructures such as ZnO nanorods with embedded Au nanoparticles can be fabricated . However, annealing can result in detrimental effects on optical properties of the plasmonic nanocomposites, too [16, 17]. Thus, in every case, the annealing process should be optimized, taking into account properties of the dielectric matrix. However, there are no studies on the influence of annealing on optical properties of the plasmonic DLC nanocomposites with embedded group IB metal nanoparticles. Therefore, in the present research, annealing effects on the structure and optical properties of DLC:Ag were studied. Modeling and experimental research were combined to elucidate processes taking place during the annealing.
In the present study, DLC:Ag were deposited by using reactive direct current unbalanced magnetron sputtering of the silver target. The diameter of the magnetron was 3 in. In most cases, silica substrates were used. Samples for the study of the Raman scattering spectra were grown on monocrystalline silicon substrates. Mixture of the hydrocarbons (acetylene) and argon gas was used in the reactive magnetron sputtering system. Ar gas flux was 80 sccm and C2H2 gas flux was 7.8 sccm. In all the experiments, the substrate-target gap was set at 10 cm, magnetron target current was 0.1 A, base pressure was 5 × 10−4 Pa, and work pressure was (4 ± 1) × 10−1 Pa. The thickness of the deposited films was about 50 nm. No additional bias during deposition was used, and the substrates were grounded. For more information on deposition conditions as well as chemical composition, structure, and optical properties of DLC:Ag films, please see .
After the deposition, DLC:Ag films were annealed in air at 140, 200, 300, and 400 °C temperatures. In all cases, annealing time was 3 min. Annealing was conducted using a muffle furnace. It was preheated to the necessary temperature before the DLC:Ag films were placed inside. Subsequent temperature stabilization took place in a 30–60-s time interval. Afterwards, annealing was carried out for 3 min. After the annealing, the samples were immediately taken from the furnace and were cooled to room temperature (~20 °C) for 5–10 min. After that, the films were investigated.
The morphology of the thin film surface was analyzed by the atomic force microscope (AFM) NanoWizard® 3 (JPK, Germany) working in AC mode. Silicon probes with a reflective backside Al coating (ACTA-10, AppNano, USA) with a resonance frequency of 200–400 kHz and force constant of 13–77 N/m were used. Nominal tip radius was less than 10 nm. The scanning rate of 0.8 Hz was selected.
Thickness of the thin films was evaluated by fabrication of the DLC:Ag film-based diffraction grating and measurement of the grating step height by using AFM.
Raman scattering measurements were performed using the Raman microscope inVia (Renishaw) with a 532-nm excitation. Integration time was 100 s, power was 0.3 mW, and grating groove density was 2400 grooves/mm.
The Martens hardness of the nanocomposite films was measured by using the micro-indenter Fischerscope HM2000 (Helmut Fischer GmbH, Germany). A Berkovich-type diamond indentor was used. In all cases, a 0.4-mN load was applied. Ten repeat indentations were made for each sample, and hardness value is given as an average of all the measurements.
Optical properties of the DLC:Ag in the 180–1100-nm range were investigated by using the fiber optic spectrometer AvaSpec-2048 (Avantes). The spectrometer is based on the AvaBench-75 symmetrical Czerny-Turner design and is equipped with a 2048-pixel CCD detector array (resolution 1.4 nm).
The electrodynamic dipole approaches were used for simulation of the optical properties of plasmonic nanocomposites.
Results and Discussion
Further increase of the annealing temperature up to 400 °C resulted in disappearance of the Raman scattering peaks in the investigated spectral range. It seems that at this temperature, a structure typical of the DLC is destructed or the DLC matrix is burned out (vaporized). This permission is in good accordance with the decreased thickness of the film as a result of the annealing (Figs. 1 and 2) as well as increased surface roughness (Fig. 4). In addition, shift of the G peak position to the higher wave numbers should be mentioned. According to , it means decreased sp3/sp2 carbon bond ratio (graphitization of the amorphous carbon matrix). This fact is in good accordance with the data reported by other studies on annealing of undoped DLC films [23, 24]. In , annealing of undoped hydrogenated DLC films in oxygen-containing ambient resulted in mass loss, decrease of thickness, and sp3/sp2 carbon bond ratio decrease of the hydrogenated DLC film.
The Martens hardness of the DLC:Ag films before the annealing was 8.407 ± 1.495 N/mm2. It should be mentioned that similar hardness was reported in  for DLC:Ag nanocomposite films containing ~13 at.% Ag. Film annealing resulted in the decreased hardness, and for example, after annealing at 140 °C temperature, it was decreased twice and was equal to 4.115 ± 0.689 N/mm2. This decrease is in good accordance with the decrease of the sp3/sp2 carbon bond ratio mentioned above.
As it was indicated, the larger Ag nanoclusters along with the smaller ones appear as a result of the annealing (Fig. 3). The shape of these nanoclusters plays a crucial role in the absorption spectra, since we assign the peak in the absorption spectra to the plasmon excited in the clusters of certain size. It is known that for a spheroid-shaped metallic nanoparticle, the plasmon band splits into two bands corresponding to the oscillation of the free electrons along (dipole plasmon) and perpendicular (quadrupole plasmon) to the long axis of the spheroid [26, 27]. For small nanoparticles, only dipole resonance modes can be excited, whereas for larger nanoparticles, quadrupole resonance modes can also be excited because of the onset of the electromagnetic retardation effect . Moreover, the similar dipole and quadrupole plasmon resonant wavelengths already have been observed experimentally for Ag-SiO2 nanostructures in .
Usually, the decreased nanoparticle size results in blueshift of the plasmonic peak [1, 14, 21]. On the other hand, in our case when we are dealing with the nanocomposite, the decreased thickness of the DLC matrix should be taken into account, too. It should result in an effect similar to that of the decreased refractive index of the media surrounding Ag nanoparticles. For example, the decreased refractive index in such a case results in blueshift of the plasmonic peak , too. So blueshift of the plasmonic peak observed is in good accordance with this assumption [1, 24].
where ε i and ε h are dielectric permittivity of nanoparticles and the surrounding medium, respectively.
where e is nanoparticle eccentricity (two signs correspond to prolate or oblate shape).
According to the calculations, nanoparticles placed in air have two narrow absorption peaks near 300 and 360 nm for the prolate shape and near 320 and 390 nm for the oblate. Two peaks in the absorption spectra of nanoclusters placed in the DLC are shifted to the long-wavelength region—380 and 520 nm for the prolate and 420 and 590 nm for the oblate. Moreover, the peaks are broadened.
One can see that the absorption peaks are sensitive to the dielectric constant of DLC—peaks (Fig. 8b) are different in value of absorption, peak positions, and bandwidth.
Having the size of nanoclusters from AFM (Fig. 3) and experimental value of DLC refractive index (measured at 632 nm), we fitted eccentricity of the formed nanoclusters after annealing at different temperatures. In this way, we have obtained optimal fitting of the experimental and calculated data for annealing: at 140 °C, e = 0.89 (Fig. 9a); at 200 °C, e = 1.0 (Fig. 9b); and at 300 and 400 °C, e = 1.2 (Fig. 9c, d, respectively). The results show that with the increase in annealing temperature, the nanoclusters became more and more asymmetrical. A large discrepancy between the bandwidth of the calculated and experimental curves presented in Fig. 9c, d can be explained by the dominant role of large nanoclusters in the absorption spectrum. According to our calculations, the nanoclusters obtained after annealing by 300 and 400 °C have the same asymmetry; however, the refractive index of DLC is slightly changed, resulting in two well-expressed absorption peaks.
In summary, change of the absorption spectra of the DLC:Ag film as a result of the annealing is caused by changes of the size of nanoparticles due to their coalescence and change of the relative permittivity of the surrounding host media.
It should be mentioned that annealing-induced appearance of the second plasmonic peak can be beneficial in different applications. Particularly in SERS applications, it can result in simultaneous enhancement at both the excitation and the specific Raman-Stokes lines . Double plasmon resonance can be used as well for fabrication of the double wavelength chemical sensors . Narrowing of the plasmonic peak width results in the increased resolution of the sensor . Thus, by setting appropriate annealing conditions, modification of the optical properties advantageous for the sensor applications takes place.
Thermal air annealing in the 140–400 °C temperature range resulted in the decreased thickness of DLC:Ag films and increased surface roughness. Decrease of sp3/sp2 carbon bond ratio (graphitization of the amorphous carbon matrix) took place. Air annealing at 400 °C resulted in destruction of the DLC matrix. The size of Ag nanoparticles embedded into the DLC:Ag matrix increased with the annealing temperature. Their shape has been changed from spherical to the prolated. This effect was explained by agglomeration of the Ag nanoparticles.
Annealing at 140 and 200 °C temperatures resulted in increased intensity of the Raman scattering spectra and G peak particularly. Further increase of the annealing temperature resulted in significant decrease of the intensity of carbon-related Raman scattering peaks and even their disappearance.
Annealing at 140 and 200 °C temperatures resulted in blueshift of the surface plasmon resonance peak and substantially decreased their width. The second absorption peak at a <400-nm wavelength appeared. Increase of the annealing temperature to 300 and 400 °C resulted in redshift of the main surface plasmon resonance peak. A similar behavior was observed in the case of the reflectance spectra. Shift of the absorption spectra and appearance of the additional quadrupole peaks were studied by using a combination of the modeling and experimental studies. The observed regularities were explained by the increased size of the nanoparticles, formation of the prolated nanoparticles instead of spheroidal ones, and change of the relative permittivity of the surrounding media.
This research was funded by the European Social Fund under the Global Grant measure (project No. VP1-3.1-ŠMM-07-K-03-057). Financial support from Ministry of Education and Science of Ukraine should be acknowledged (grant No M/124-2015, DB/Tekton No 0115U000427). One of the authors (ST) would like to thank the support from the Research Council of Lithuania (Lithuanian-Ukrainian research collaboration program, grant TAP LU 04/2014).
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