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Microstructure and Tribological Properties of Fe NPs @ a-C: H Films by Micromorphology Analysis and Fractal Geometry #tefan ##lu, Miroslaw Bramowicz, S#awomir Kulesza, Azizollah Shafiekhani, Atefeh Ghaderi, Fatemeh Mashayekhi, and Shahram Solaymani Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02449 • Publication Date (Web): 11 Aug 2015 Downloaded from http://pubs.acs.org on August 15, 2015

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Industrial & Engineering Chemistry Research

Microstructure and Tribological Properties of Fe NPs @ a-C: H Films by Micromorphology Analysis and Fractal Geometry The running head: Microstructure and tribological properties of Fe NPs @ a-C: H films Ştefan Ţălu 1, Miroslaw Bramowicz 2, Slawomir Kulesza 3, Azizollah Shafiekhani 4, 5, Atefeh Ghaderi 6, Fatemeh Mashayekhi 7, Shahram Solaymani 6, * 1

Technical University of Cluj-Napoca, Faculty of Mechanical Engineering, Department of AET, Discipline of Descriptive Geometry and Engineering Graphics, 103-105 B-dul Muncii St., ClujNapoca 400641, Cluj, Romania. 2 University of Warmia and Mazury in Olsztyn, Faculty of Technical Sciences, Oczapowskiego 11, 10-719 Olsztyn, Poland. 3 University of Warmia and Mazury in Olsztyn, Faculty of Mathematics and Computer Science, Sloneczna 54, 10-710 Olsztyn, Poland. 4 School of Physics, Institute for Research in Fundamental Sciences, PO Box 19395-5531, Tehran, Iran. 5 Physics Department, Alzahra University, PO Box 1993891167, Tehran, Iran. 6 Young Researchers and Elite Club, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran. 7 Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran

Corresponding author*: Shahram Solaymani Young Researchers and Elite Club, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran. Phone: +989194947717 E-mail: [email protected] Declaration of interest: The authors report no conflict of interests. The authors alone are responsible for the content and writing of the paper. Abstract This paper analyses the three-dimensional (3-D) surface texture of amorphous hydrogenated carbon films with sputtered iron nanoparticles (Fe NPs @ a-C: H) deposited by RF-Plasma Enhanced Chemical Vapor Deposition (RF-PECVD) method on the quartz substrates. The prepared Fe NPs @ a-C: H films were used as research materials. The synthesized samples were deposited at four different pressures of 2.5, 3, 3.35 and 3.5 N/m2 in the acetylene gas atmosphere. The Fe and C content of the thin films was obtained from X-ray photoelectron spectroscopy (XPS). X-ray diffraction profile and electron diffraction pattern indicates that iron nanoparticles with body-centered cubic crystalline structure are formed in these films. Localized 1

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Surface Plasmon Resonance (LSPR) peak that is signature of the existence of the Fe core nanoparticles appears in visible spectra of these films. The sample surface images were recorded using an Atomic Force Microscope (AFM) operating in a non-contact mode and analyzed to reveal statistical, fractal and functional surface properties of prepared samples. The analysis of 3-D surface texture is essential for the correct interpretation of surface topographic features as well as its functional role for the test surface. It also helps to understand the relationship between the surface topography and the functional properties. Key words: Fe NPs @ a-C: H films, RF-sputtering and RF-PECVD, XPS, AFM, Fractal Analysis, 3-D Surface micro morphology. 1. INTRODUCTION Over the last decades, the science and technology of thin films made fundamental progress towards understanding relations between processing, microstructure at the nanometer level, properties, and nanoscale structural performances.1-4 The three dimensional (3-D) surface morphology plays a major role in nanomechanics, tribology, and thermodynamics of thin films and their interfaces, involved at the nanometer-scale resolution and can yield important insights into the applied mathematics, theoretical methods and numerical tools for multi-scale modelling.5-9 The sophisticated surface design and engineering of hierarchical hybrid nanostructures lead to many interdisciplinary researches. These include, among others, comparative studies of surface topography and results of numerical simulations involving complex physics and geometry of thin films, especially when abundant series of data are generated in high-resolution models.10-16 The wide variety of 3-D engineering surface textures can be divided into isotropic (Gaussian or non-Gaussian) or anisotropic.17-19 Most of them are topographically anisotropic, based on an “anisotropy index”, a ratio combining topographic parameters measured along two arbitrary directions on the surface. Usually, a ratio close to unity is specific of isotropic surfaces, whereas below some threshold correspond to anisotropic ones.20 To characterize the 3-D nanoscale intrinsic geometry of thin films from the height images by atomic force microscopy (AFM),21 fractal4, 8, 9, 14, 18, 19 and multifractal 1-3, 10-12 geometry was applied in different studies. Even though surfaces of thin films are self-similar only in a restricted range of the spatial scales,18, 19, 22 fractal geometry is useful for understanding the 3-D nanoscale architecture and phenomena of surface roughness. In past decades, numerous physical and chemical techniques have been developed for the preparation of metal nanoparticles (NPs) on various substrates. These methods include, among others: RF-sputtering, RF-PECVD, hydrothermal reduction, sol gel method etc. All these methods allow fine control for chemical composition and introduction of the lowest possible concentrations of finely dispersed dopants.23-26 Different techniques, including Surface Plasmon Resonance (SPR) absorption1,27 and Power Spectral Density (PSD),28 were applied in order to study the chemistry and nanoscale architecture of continuous thin films with metallic nanoparticles less than 100 nm deposited on various substrates. The objective of this study is to synthesize the Fe NPs @ a-C: H films by co-deposition of RFPECVD and RF-sputtering methods on quartz substrates, and to characterize their 3-D surface 2


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texture using AFM data in connection with the statistical, and fractal analyses, to elucidate the 3D fractal patterns of the heterogeneous components. 2. MATERIALS AND METHODS 2.1. Materials and preparation of thin films Fe NPs @ a-C: H thin films were prepared by a capacitance coupled RF-PECVD system with 13.56 MHz power supply. The reaction chamber consists of two electrodes with different areas: the smaller electrode (5 cm in diameter) was the target made of 99% pure iron, and the larger one (13 cm in diameter) was the substrate. The substrate electrode was grounded via the body of the stainless steel chamber while the distance between two electrodes was 5.5 cm. Then, the deposition was performed on quartz substrate on the grounded electrode at room temperature. The chamber was evacuated to an initial pressure of about 2.5 N/m2 prior to the deposition and then the pressure was raised to an arbitrary amount with acetylene gas flow. Acetylene has been used as a both reactive and bombarding gas in the RF plasma system. The RF power and deposition time were 240 W and 60 min, respectively. Details of prepared samples are given in Table 1. Table 1. Details of prepared samples

ID #1 #2 #3 #4

Sputtering Parameters Initial Pressure Power Deposition 2 [N/m ] [Watt] Time (min) 2.5 240 60 3 240 60 3.35 240 60 3.5 240 60

2.2. Characterization of the thin film properties Iron, carbon, and oxygen atomic content as well as their surface chemical bond structures of films were obtained from XPS spectra using a Gammadat-scienta ESCA 200 hemispherical analyzer equipped with an Al Kα (1486.6 eV) X-ray source. The powder X-ray diffraction pattern on the samples was recorded by X-ray diffractometer using Cu-kα radiation of wave length λ = 1.54 °A for 2Ө range from 20 to 90° by Bruker make (Model D8) operated at 40 kV and 30 mA. The Surface Plasmon Resonance absorption spectra of UV–visible spectrometer, was obtained by Stellar.net (Florida, USA) from 2 mm diameter optical fiber that transfers a non-polarized light beam (400 – 850 nm) through samples to a CCD detector was measured. AFM in non-contact mode was used to obtain the surface topography and roughness of the samples and the average size of iron nanoparticles using a Nanoscope Multimode atomic force microscope (Digital Instruments, Santa Barbara, CA), with a scan speed of 10–20 µm/s. The experiments were carried out at room temperature (24 ± 1 °C) using cantilevers with the nominal 3



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properties for force-distance curve measurements specified in Ref. 29 All images were obtained by scanning over square areas of 1 µm x 1 µm. 2.3. Characterization of the film surface texture AFM measurements of the surface topography produce discrete arrays of heights, however, in the beginning residual surface in the form of a matrix z(x,y) need to be extracted from raw AFM data in order to get rid of the longest wavelength components. Then, the three-dimensional Areal Autocorrelation Function (AACF) R(τx,τy) 30 can be calculated by averaging of the AFM images with their lagged copies (see Fig. 1a) 31: R ( τx , τ y ) =

( z ( x, y ) − z ) ⋅ ( z ( x + τ , y + τ ) − z ) x

y

(1) S where: - denotes the mean value, Sq - is the root-mean-square surface (RMS) roughness, whereas (τx, τy) – vector of a discrete spatial shift (space lag). RMS roughness Sq is simply standard deviation of surface height samples defined as: 2 q

Sq =

2

( z ( x, y ) − z )

(2)

Having that, the surface texture ratio Str can be defined as the ratio of extreme autocorrelation decay lengths τ with which the normalized AACF falls from 1.0 down to 0.2 (see Fig. 1a): τ 0 < Str = a1 ≤1 (3) τa 2 R =1→0.2 Where: a1, and a2 – specify the axes of the fastest and the slowest AACF decays, respectively. 30 For Str > 0.5 the surface is said to be isotropic, while for Str < 0.3 – strongly anisotropic. The plot of the AACF is also useful in determination of the grain dimension. To this end, the half-widths of the AACF curve at its half maximum along these extreme directions are taken as the grain radii Ra1 and Ra2, respectively, the sum of which gives the average grain diameter (see Fig. 1b): dAACF = R a1 + R a 2 (4) Since residual surface is stationary, the Structure Function (SF) can be computed making use of the formula32: S ( τx , τy ) = 2Sq2 1 − R ( τx , τy ) (5)

(

)

By plotting the structure function on a double logarithmic scale, fractal dimension D and corner frequency fc can be derived. Functional parameters can be derived from the Firestone-Abbott curve (see Fig. 1c) also known as the bearing curve. The curve is followed from a series of height data arranged in a descending order and plotted on the percent scale, where 100 % corresponds to the lowest sample in the series. DIN 4776 standard specifies several characteristics that can be useful in describing topographical complexity of the surface that includes (see Fig. 1c): a) kernel roughness depth Sk – thickness of the core at the flattest part of the bearing curve where the largest increase in material exists; b) reduced peak height Spk, reduced valley depth Svk – thickness of the bearing curve above/below the core profile, respectively; 4


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c) upper bearing area Mr1, lower bearing area Mr2 – intersection points of horizontal lines plotted from both ends of the flattest tangent of the bearing curve with that curve that delimit peaks and valleys from the core, respectively; d) surface bearing index Sbi – the ratio of the RMS roughness over the surface height at 5% of the bearing curve; e) core fluid retention index Sci – the ratio of the void volume of the unit sampling area at the core zone over the RMS roughness; f) valley fluid retention index Svi – the ratio of the void volume of the unit sampling area at the valley zone over the RMS roughness.

Fig. 1. (a) Three-dimensional plot of AACF explaining directions of the extreme decays: a1 and a2, (b) plot explaining decays lengths τa1 and τa2 along extreme directions and the estimation of grain radii; (c) The plot of the Firestone-Abbott curve explaining main functional characteristics of the surface. 2.4. Statistical Analysis Statistical analyses were performed using the GraphPad InStat version 3.20 computer software package (GraphPad, San Diego, CA, USA). Analysis of variance (ANOVA), followed by a posthoc Tukey's test, was used to determine if there was a difference in the average parameters values between the different examined surface regions. The differences in the average parameters values for a particular surface region were also analyzed using ANOVA and Tukey's test. A P value of less than 0.05 denoted the presence of a statistically significant difference.

3. Results The representative 3-D AFM images, for scanning square areas of 1 µm x 1 µm, of the Fe NPs @ a-C:H film surfaces deposited on the quartz substrates at: (a) #1, (b) #2, (c) #3, and (d) #4, are presented in Fig. 2 (a-d).

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Fig. 2. 3-D AFM images of the surface of Fe nanoparticles on the surface of a-C:H films deposited on the quartz substrates at: (a) #1, (b) #2, (c) #3, and (d) #4. The surface property of NPs was studied by XPS analysis. Fig. 3 shows the main and the satellite peaks of typical Fe NPs @ a-C: H sample in the range of 0 to 1200 eV.

(a)

(b)

Fig. 3. Typical XPS spectra of (a) the Fe NPs @ a-C: H, and (b) deconvolution of C1s peak Table 2 presents the initial pressure, sp3/sp2 and Fe/C ratios. 6


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Table 2. The results of deconvolution of C1s peak analyzed by XPS. Sample

sp3/sp2

Fe/C

#1 #2 #3 #4

1.799 1.304 1.284 0.824

0.061 0.017 0.014 0.007

The XRD profile is presented in Fig. 4.

Fig. 4. Typical glancing-angle X-ray diffraction profile and electron diffraction (inset) of Fe NPs @ a-C:H. The UV–visible absorption spectra of Fe NPs @ a-C:H with different Fe content are shown in Fig. 5.

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Fig. 5. UV−visible spectra of Fe NPs @ a-C:H with different Fe content. Main statistical and fractal characteristics of prepared samples are given in Table 3. Table 3. Statistical and fractal surface properties of prepared samples that involve: Str – surface anisotropy ratio, D – fractal dimension, K – pseudo-topothesy, fc – corner frequency, Sq – RMS surface roughness, dAACF – grain diameter from the AACF, dGD – grain diameter from the grain density. ID

Str

D

K

#1 #2 #3 #4

0.79 0.94 0.87 0.88

2.55 2.28 2.32 2.26

7.66·10-6 2.65·10-4 4.75·10-4 1.94·10-3

fc [nm] 21 28 34 29

Sq dAACF [nm] [nm] 0.346 19.4 0.888 36.5 1.529 43.6 2.242 39.3

dGD [nm] 18.3 40.8 37.8 44.7

Table 4 summarizes main functional parameters concerning surface texture. Table 4. Functional surface properties of prepared samples that involve: Sk – kernel roughness depth, Spk – reduced peak height, Svk – reduced valley depth, Mr1/Mr2 – upper/lower bearing area, Sbi – surface bearing index, Sci – core fluid retention index, Svi – valley fluid retention index. ID #1 #2 #3 #4

Sk [nm] 0.817 1.963 3.278 5.643

Spk [nm] 3.865 7.531 8.591 6.999

Svk [nm] 0.718 1.506 2.331 4.958

Mr1 [%] 10.63 11.06 12.80 11.06

Mr2 [%] 91.97 91.76 92.41 90.67

Sbi

Sci

Svi

0.702 0.696 0.607 0.592

4.43·10-4 1.646·10-4 1.087·10-4 7.266·10-5

2.536·10-5 9.411·10-6 4.893·10-6 4.706·10-6

8


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4. DISCUSSION At first glance, AFM image of sample #1 (Fig. 2a) exhibits large number of fine grains spread homogeneously across the surface, which are generally not higher than about 1 nm, although few of them might be even 2-3 nm high. Particle density, i.e. the number of particles per square area, appears very large at 3×1011 cm-2. Assuming that the particles are identical circles packed in a dense, regular lattice, particle diameter equals to 18.26 nm. Fig. 2b shows that sample #2 is composed of larger crystallites (4-5 nm high) apart from rarely spotted even thicker grains (up to 10 nm high). Particle density is five times lower with respect to that in sample #1 approaching 6×1010 cm-2, and the grain diameter equals to 40.8 nm. Similar topography can be observed in Fig. 2c (sample #3) and Fig. 2d (sample #4), with the exception that the height of extreme particles approaches 13 nm in both cases. Particle densities equal to about 7×1010 cm-2 and 5×1010 cm-2 in sample #3 and sample #4, respectively, which correspond to particle diameters of 37.8 nm and 44.72 nm, respectively. In Fig. 3, the main peak is at 285 eV and its full width at half maximum (FWHM) is about 1.9 eV, which is larger than FWHM of diamond (1.45 eV) and graphite (1.35 eV). 33-35 Therefore, C1s spectra show a combination of these two structures. Fig. 3b shows deconvolution of C1s peak of XPS spectra. The broad C1s peak was deconvoluted into three peaks which are marked as peaks A, B and C in Fig. 3b. Peaks A, B and C are attributed to the sp2 and sp3 bonds of C atoms and CO, respectively. Thus, the sp3/sp2 ratio of the film was calculated by taking into account the area ratio of the sp3 to sp2 peaks.33-35 The C1s peak at the higher binding energy (about 287.7 eV) indicates that some carbon atoms are bonded to oxygen on the film surface.35-37 The main peak at 284.5 eV is attributed to C_C or C_H bonds. The peaks A_C are related to C_C or C_H, CO, and CO2, respectively.38 Fe NPs could be bound to the C atoms either because dangling bonds around the defective vacancies in carbon films or because the presence of chemical functional groups, such as hydroxyl (–OH) and carboxyl (–COOH) in carbon trace. There is an important role of magnetic exchange interaction in the p–d hybridization between carbons and transition-metal atoms; hence the transition-metal atoms acting as substitution defects can substantially modify the electronic structure. Then the dangling bonds around the defective vacancies can provide active sites for Fe NPs adsorption.39 The oxides in carbon films have mostly carboxylic functions and act as anchors for Fe NPs, presumably through an ion-exchange reaction, to produce material that consists of Fe NPs with average size between 1 and 7 nm.40 Therefore Fe NPs are trapped inside the carbon trace films and will not be agglomerated. The XRD profile presented in Fig. 4 shows the presence of crystallized α-Fe (JCPDS 00-0060696) and Fe2O3 (JCPDS 00-033-0664) and Fe3O4. Since α-Fe NPs with highly surface activity can be oxidized, the compact shell of Fe2O3 generated on the surface of α-Fe particles hinders further oxidation of inner iron core. The micro diffraction pattern, inset part of Fig. 4, indicates that bright circular rings exhibit the crystalline of Fe NPs. These rings correspond to (2 1 1), (2 0 0), (1 1 0) and (3 2 1) plans of Fe nanocrystals.41 In the UV–visible absorption spectra of these films in Fig. 5, it can be observed an absorption peak at about 220 nm. This absorption peak is the signature of the existence of Fe NPs and is due to their localized surface plasmon resonance.42-44 Samples #1 and #2 exhibit sharper peaks compared to #3 and #4 with wide peaks and a small red shift as a function of Fe/C content. This 9



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occurs because of change in size and distribution of NPs 1, which is confirmed by AFM images. For the samples #1 and #2, where the concentration of Fe NPs is higher than other samples, percolation occurs with a small blue shift by increasing the Fe/C ratio. These effects can be explained by delocalization of electrons from the surface of Fe NPs according to Fe/C content.1, 27

In can be noted that in Table 3 the grain diameters estimated from the grain density dGD differ no more than 13 per cent from those obtained using the AACF. It also turns out that sample #1 contains smallest particles with the diameter smaller than 20 nm, whereas particles in remaining samples are approximately twice that size approaching 40 nm. Unlike the Ni nanoparticles, however, which form irregular 100-200 nm wide clusters contributing to higher-order particle arrangement, the Fe nanoparticles are found homogenously distributed over the sample surface. This, together with their regular shape results in very high anisotropy ratio: 0.79 in sample #1, and around 0.9 in remaining samples that points at almost isotropic surface. Fractal parameters describe how the surface shape scales with changes in its linear dimension. Except for the sample #1, fractal dimension D is found quite low in the range 2.26-2.32, which is characteristic of poorly developed surfaces with large, smooth grains on it. Unexpectedly, minute particles in sample #1 contribute to much more developed surface with D equal to 2.55, similar to D value of the entire Ni clusters (2.51-2.81). Obtained fractal dimension, however, does not go hand in hand with the RMS surface roughness Sq, because the latter steadily increases from 0.346 nm in sample #1 up to 2.242 in sample #4. Studied samples are found to exhibit monofractal structure, which means that there is single characteristic scan length, referred to as the corner frequency τ, beyond which power-law scaling behavior disappears. Here, the corner frequency takes the lowest value at 21 nm for the first sample in the series, but then it increases and remains almost constant in the range 28-34 nm. Except for the first sample, corner frequency turns out to be lower compared to the particle diameter dAACF, but even though it follows similar tendency approaching maximum value in sample #3. Data in Table 4 suggest that although the kernel roughness depth Sk is found low, but it continuously increases from 0.817 nm (sample #1) to 5.643 nm (sample #4). In general, Sk defines the working base of the surface responsible for its long-term tribological behavior. In that aspect films with Fe nanoparticles demonstrate higher mechanical resistance and higher loadcarrying capacity during contacting operations compared to those with Ni nanoparticles (Sk around 15 nm). On the other hand, reduced peak height Spk, which defines the film depth that is removed upon touch to other material, varies between 3.865 nm (sample #1) and 8.591 (sample #3). Low Spk is associated with short running-in time and small amount of material exposed to mechanical damage, hence, films with Fe nanoparticles exhibit large potential for tribological applications. The upper bearing area Mr1, which actually specifies the contribution of the peak area to the entire surface height, confirms the above finding. Studied films exhibit Mr1 in the range from 10.63 up to 12.80 per cent with the highest value observed in sample #3. Likewise, reduced valley depth Svk defines the depth of the valleys on the surface that remain after the wearing process serving as a lubrication channel. As seen in Table 4, Svk increases with increased deposition pressure from 0.718 nm (sample #1) to 4.958 nm (sample #4). Large Svk is necessary for good fluid retention, however, results presented in the paper remain low and very close to Sk. Lower bearing index Mr2, which depicts the ratio of the valleys to the entire surface 10


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height, slightly varies between 90.67 and 92.41 per cent, having its maximum value for sample #3. Another functional measure of the surface topography in terms of its bearing properties is the surface bearing index Sbi. For a perfect Gaussian surface it is close to 0.61, whereas studied samples exhibit Sbi in the range from 0.592 to 0.702. The last two parameters: core fluid retention index Sci and valley fluid retention index Svi, take extremely small values as in other grown materials. The former index, Sci, falls down from 4.43·10-4 to 7.27·10-5, whereas in a Gaussian surface it should approach 1.56. The same trend holds for the latter one, Svi, which decreases from 2.54·10-5 to 4.71·10-6, while the Gaussian surface exhibits Svi close to 0.11. After all, obtained results point at good bearing properties in the kernel zone of the films with Fe nanoparticles, among which the flattest bearing curve is demonstrated in sample #1. Even though the films exhibit poorer fluid retention characteristics in the valley zone, their tribological potential is in general higher than similar films with Ni nanoparticles.

5. CONCLUSIONS The objective of this study was the micromorphology analysis of Fe NPs @ a-C: H thin films deposited by RF-PECVD and RF-sputtering method on the quartz substrates. The characterizations were carried out using X-ray diffractometer, UV–visible absorption spectra, AFM, and fractal analyses. LSPR peaks according to deposition parameters are quite sharp for #1 and #2 then widens with a small red shift for #3 and #4 that show the morphology change of samples. XPS analyses determine the percentage of Fe and C. Our results also suggest that AFM, statistical, fractal and functional surface properties of Fe NPs @ a-C: H thin films can contribute to obtain structural information from local regions and to a better understanding of the effects, spatial scaling, and correct interpretation of surface topographic features as well as its functional role for the test surface. Consequently surface micromorphology parameters and fractal analysis may be key elements in designing, processing, measurement, developing and manipulating of optimal surface characteristics of thin films within the science of applied surfaces. Also, the 3-D surface texture of nanostructures estimated by specific parameters can be included in accurate and robust mathematical models to describe 3-D multi-scale modelling of time-dependent and/or highly nonlinear processes involved at nanometer-scale resolution of thin films. These results can be applied in thermodynamics of thin films in study of surface adsorption, electrostatic forces, structural forces and the application of these ideas to elucidate the stability of thin films.

ACKNOWLEDGMENTS The authors are grateful to Mrs A. Mahmoodi for the XRD measurements.

REFERENCES

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(1) Ţălu, Ş.; Stach, S.; Ghodselahi, T.; Ghaderi, A.; Solaymani, S.; Boochani, A.; Garczyk, Ż.; Topographic Characterization of Cu–Ni NPs @ a-C:H Films By AFM and Multifractal Analysis,

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Microstructure and Tribological Properties of FeNPs@a-C:H Films by

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