Percolation conductivity in amorphous carbon films modified with palladium nanoparticles

Journal of Non-Crystalline Solids 532 (2020) 119876

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Percolation conductivity in amorphous carbon films modified with palladium nanoparticles

T

⁎

A.P. Ryaguzova, R.R. Nemkayevaa, N.R. Guseinova, A.R. Assembayevaa,b, , S.I. Zaitsevc a

NNLOT al-Farabi KazNU, Almaty, Republic of Kazakhstan Satbayev University, Almaty, Republic of Kazakhstan c IMT-RAS, Chernogolovka, Russia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Amorphous carbon films Modification Palladium nanoparticles Raman spectroscopy Percolation conductivity

The paper considers a question of modifying thin amorphous carbon (a-C) films with palladium (Pd) nanoparticles (a-C) synthesized by magnetron ion-plasma co-sputtering of a combined target. Raman studies revealed the effect of the Pd concentration on the structure of amorphous carbon films. The dependence of the electronic properties of a-C films on the Pd concentration is shown. It was detected that the variation of Pd content from 0.175 to 1 at.% changes the conductivity of the films by 108.

1. Introduction The discovery of new allotropic forms of carbon illustrated its difference in structure and electronic properties [1–4]. For instance, nanotubes, depending on the structure and chirality, have semiconductor or metallic types of conductivity. This specificity of carbon modifications has attracted the special attention of researchers that consider the possibility of their wide application in various thin-film electronic devices, circuits of opto- and nanoelectronics. Nowadays, non-crystalline structures, particularly amorphous diamond-like carbon (DLC), have become of particular interest. Due to their unique properties, such as high hardness, low friction, wear resistance and chemical inertness, diamond-like carbon films are widely used as durable and anti-friction coatings for various electronic devices and instruments [5–7]. Carbon inertness to form bonds with atoms of other chemical elements is of no less interest. In this regard, current research is aimed at the modification of the electronic properties of DLC films through the variation of synthesis conditions and doping with non-carbide-forming elements [8–10]. One of the non-carbide-forming elements is palladium. The features of the electronic configuration and chemical inertness under normal conditions make palladium a promising dopant for DLC films. Palladium is the only element from the group of transition metals that has a completely filled outer shell of 18 electrons. This explains its chemical inertness under normal conditions. At the same time, it has the properties of a catalyst in a number of chemical processes. At temperatures below 300°C, palladium atoms do not form a chemical bond with oxygen and carbon atoms.

⁎

Properties of palladium embedded in the carbon matrix have been little studied so far. Earlier, studies were carried out on the synthesis of palladium nanoparticles in nanoporous carbon structures prepared using the methods of powder technology from boron carbide (B4C) and silicon carbide C (SiC) [11]. Obtaining of nanoporous bulk carbon structures with palladium nanoparticles using the method described in this report [11] is a long and expensive process. Modern technologies tend to minimize the production time and use inexpensive methods for the synthesis of thin films. At the same time, knowledge of the properties of modified diamond-like carbon films is important for modern technologies in the field of nanostructures. The study of the synthesis of DLC films modified with palladium nanoparticles can be an interesting object in the physics of low-dimensional structures. The introduction of Pd nanoparticles into the carbon matrix makes it possible to obtain new composite materials with new properties. 2. Experimental In this paper, the synthesis of a-C films was carried out by the method of magnetron ion-plasma co-sputtering of a combined target of carbon (99.999%) and palladium (99.9%) in an argon atmosphere (99.999%). The direct current (DC) power of the ion-plasma discharge equals to 15.75 W at an argon gas pressure of 0.7 Pa in the working chamber. The substrate temperature did not exceed 50°C. a-C films were synthesized on silicon (100) and quartz plates simultaneously. The concentration of palladium (XPd) in a-C films on silicon

Corresponding author. E-mail address: [email protected] (A.R. Assembayeva).

https://doi.org/10.1016/j.jnoncrysol.2019.119876 Received 20 June 2019; Received in revised form 18 December 2019; Accepted 23 December 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.

Journal of Non-Crystalline Solids 532 (2020) 119876

A.P. Ryaguzov, et al.

1555 cm−1. As shown in [13] using the example of a three-stage model, the shift of the G peak to the low-frequency region is caused by an increase in the content of sp3 hybridized bonds and the formation of clusters of diamond-like structures. According to the studies performed in [14,15], when the G peak is located at a frequency of 1555 cm−1, the concentration of sp3 sites in the film reaches 50–70%. Films with such characteristics are commonly referred to diamond-like carbon films. For a more detailed detection of the effect of palladium nanoparticles on the structure of an amorphous carbon film, the Raman spectrum was decomposed using Gauss method. Studies have shown that the Raman spectra of a–C films with Pd concentrations up to 0.24 at.% are decomposed into three Gaussian peaks (Fig. 3(a), (b), (c), (d)). In this case, the reliability with which the resulting Gaussian curve (red line) describes the experimental curve is >0.999. At a Pd concentration of more than 0.24 at.%, the increase of shoulder occurs in the lowfrequency region, which makes it possible to perform the decomposition only into 2 peaks. According to the distribution of the phonon density of states in diamond [13], the first Gaussian peak at a frequency of 1260 cm−1 corresponds to the oscillations of sp3 sites in diamondlike clusters. Gaussian peaks at frequencies of 1394 cm−1 and 1400 cm−1 correspond to the D peak, which indicates the existence of hexagonal rings in the matrix of the carbon film. The increase in the number of palladium nanoparticles prevents the further formation of diamond-like clusters. In Fig. 3 it can be seen that the intensity of the first peak decreases, and its position shifts to the low-frequency region, this is due to the increase of sp2 sites in the composition of the carbon matrix. As can be seen from Fig. 3(e), (f), with further increase of the Pd concentration the peak in the frequency range of 1260 cm–1 does not manifest itself, which indicates the disappearance of diamond-like clusters. In the matrix, the proportion of hexagonal rings from sp2 clusters increases, while the sp3 sites possibly serve as connecting bridges between graphite clusters of sp2 sites. The appearance of graphite nanoclusters composed of sp2 sites is confirmed by the shift of the Gaussian G peak to the high-frequency region to the position of 1568 cm−1 (Fig. 3(e) and (f)). Thus, at concentrations of Pd more than 1.0 at.%, a-C film consists of palladium nanoparticles and graphite nanoclusters in an amorphous carbon matrix. In the paper [16], the size of clusters of sp2 and sp3 sites in a–C films were evaluated. It was determined, that the size of sp2 and sp3 nanoclusters at a synthesis temperature of 50°C can reach 10 Å. A similar picture with palladium concentrations not exceeding 0.24 at.% is observed in this work. At a palladium concentration of more than 0.24 at.

Table 1 The values of palladium concentration in a-C films (at.%). №

1

2

3

4

5

6

7

8

9

10

11

XPd, at.%

0.0

0.09

0.19

0.24

0.41

0.48

0.59

0.97

1.16

1.44

1.88

wafers was determined by energy dispersive X-ray spectroscopy (EDS) on the EDAX device (AMETEC Materials Analysis Division, USA) and varied from 0.09 at.% (atomic percent) up to 1.88 at.% (Table 1). As can be seen in Fig. 1, the atoms of other substances were not detected. The thickness of the films was determined on a fresh cleavage of a silicon wafer on a Quanta 200i 3D raster electron microscope (FEI Company, USA) and varied from 45 to 75 nm, depending on the palladium concentration. Fig. 2(a) shows an example of determining the film thickness. In addition, scanning the surface with an electron microscope (Fig. 2(b)) showed that Pd particles have a spherical shape and a maximum size of ~60 nm. To study the local structure of the carbon matrix of a-C films, Raman spectroscopy (RS) studies were carried out using NTegra Spectra installation (NT-MDT, Russia) with 473 nm laser. The transmission and reflection spectra were studied on a UV3600 spectrophotometer (Shimadzu, Japan). All measurements were performed on samples annealed at 145 ± 3°C.

3. Results and discussions 3.1. Structural properties of а–С films Typical Raman spectra (RS) of amorphous carbon films are shown in Fig. 3. The spectra are characterized by one peak and a shoulder in the low-frequency region. It should be noticed that RS of graphite and diamond are determined by the main sharp peaks at frequencies of 1580 cm−1 and 1332 cm−1, respectively [12]. The graphite peak is usually denoted by the letter G (graphite). In disordered structures, the main G peak shifts to the low-frequency region and, with the rise of the number of broken bonds, an additional D (disordered) peak appears in the frequency range 1350–1400 cm−1 characterizing the breathing mode of the C6 molecule [13]. Fig. 3 presents the Raman spectra for six a-C films for the studied range of Pd concentrations. As can be seen from Fig. 3, an increase in the concentration of palladium leads to a decrease in the intensity of the G peak, but its position remains at a frequency of

Fig. 1. Determination of Pd concentration in а-С films by the EDS method: a) XPd = 0 at.%, b) XPd = 1.44 at.%. 2

Journal of Non-Crystalline Solids 532 (2020) 119876

A.P. Ryaguzov, et al.

Fig. 2. SEM images a) of the freshly cleaved profile of a silicon wafer with a-C film and b) film surface with Pd nanoparticles ХPd = 1.44 at.%.

to a change in the ratio of sp3 and sp2 sites, which, in turn, is the reason for the formation of C6 molecules. At a Pd concentration of more than 0.4 at.%, the activation energy of charge carriers is less than 0.1 eV. Such a significant change in Eσ can be associated with an increase in the number and size of Pd nanoparticles, as well as clusters of sp2 sites, which provide significantly higher conductivity.

% we observe an increase of peak D, which indicates an increase of the size and concentration of clusters of sp2 sites. 3.2. Electronic properties of a-C films Optical spectra were studied in the range from 190 nm to 1100 nm. The absorption coefficient α was determined from the transmission and reflection spectra. The calculation of optical band gap (Eg) was carried out according to a standard procedure in the region of α~105 сm−1 and α•d~1, where d is the film thickness [17]. From Fig. 4(a) it can be seen that the value of the optical band gap Eg varies nonlinearly from 1.53 eV to 0.07 eV with increasing Pd concentration. The error of energy band gap estimation is about 0.01 eV. At Pd concentration of more than 0.5 at.%, the films can be attributed to narrow-band semiconductors with Eg<0.4 eV. Such a change in the band gap is associated with an increase in sp2 sites and the formation of graphite clusters, which increase the density of allowed states within the band gap. In addition, the formation of the band structure is strongly influenced by the electron density of palladium nanoparticles, which further reduces the value of Eg. The studies of thermo-electromotive force (thermoEMF) showed that the main carriers in the films are electrons. Studies of the temperature dependence of conductivity were carried out in the temperature range from 40°C to 145°C. The activation energy (Eσ) of charge carriers was determined. As can be seen from Fig. 4(b), the change in Еσ values correlates with the changes in Еg (Fig. 4(а)). Palladium nanoparticles contribute to the change in the values of the activation energy and the width of the bandgap. Comparison of the values of Eσ and Еg showed a significant difference Еg/2>>Eσ. Pure a-C film demonstrates a pseudo-doping and the formation of impurity centers in the band gap above the Fermi level due to sp2 sites. Palladium nanoparticles in an amorphous carbon matrix with C–C sp 2sites form a density of states in the band gap, that is closer to the bottom of the conduction band, which leads to an additional decrease in the activation energy. Fig. 3 shows that up to 0.24 at.% of palladium concentration, no significant changes in the Raman spectra are observed. The position of the decomposed Gaussian peaks does not change, which indicates the stability of cluster structures made of sp3 sites; this is confirmed by the values of band gap, which are larger than 1.0 eV. As shown in Fig. 3(g), the intensity and position of 1 and 2 Gaussian peaks change, this is due

3.3. Percolation conductivity in a-C films The values of specific conductivity at room temperature (σk) were determined. The σk(XPd) dependence (Fig. 5) revealed a change of σk by 108 in the concentration range from 0.175 at.% to 1 at.%. From Fig. 5 it can be seen that at Pd concentrations up to 0.175 at.%, the a-C films have a conductivity of ~2 × 10–6(S/cm), which indicates their high specific resistance. When the palladium concentration is greater than 0.175 at.%, a significant increase in conductivity occurs. The concentration value, which determines a significant increase in conductivity, is called a percolation threshold (xc). The percolation threshold in disordered structures with a matrix and fillers that differ in electrical properties is associated with the possibility of opening a certain conduction channel of charge [18,19]. As the concentration of the conducting phase increases, at x>xc, the proportion of conducting channels increases. The moment comes when the rise of concentration of sp2 sites and Pd nanoparticles across the entire film volume form an infinite cluster along which the charge flows [18–20]. The beginning of the increase in conductivity occurs in diamondlike samples, as DLC structure in the films remains up to palladium concentrations of 0.2 at.%. The conduction channel between Pd nanoparticles is carried out through sp2 sites in the DLC structure and the concentration of these sites can reach 40–60%. Increasing the concentration of palladium increases the number of sp2 sites, which leads to a transition from the DLC structure to a graphite-like structure. In addition, an increase in the number of palladium nanoparticles and sp2 sites lowers the potential barrier of charge flow between nanoparticles and significantly increases conductivity. At values of concentration x
Journal of Non-Crystalline Solids 532 (2020) 119876

A.P. Ryaguzov, et al.

Fig. 3. Raman spectra of a-C films (Gaussian decomposition): a) XPd = 0.0 at.%, b) XPd = 0.09 at.%, c) XPd = 0.19 at.%, d) XPd = 0.24 at.%, e) XPd = 0.97 at.%, f) XPd = 1.44 at.%.

percolation threshold, R describes the dimensions of the regions where primary percolation channels form in the carbon matrix. As the Pd concentration increases to values of the percolation threshold xc, the distance between the palladium nanoparticles within the carbon film

When x < xc the correlation radius determines the size of the conducting cluster before the percolation threshold. However, when x> xc the correlation radius indicates the size of the non-conducting phase or voids in the carbon matrix. Thus, at Pd concentrations below the

4

Journal of Non-Crystalline Solids 532 (2020) 119876

A.P. Ryaguzov, et al.

Fig. 4. Effect of Pd concentration in a-C films on a) optical band gap; b) activation energy of charge carriers (lines are drawn as guides to the eye).

decreases, and this leads to an increase in the average diameter of the conducting regions. In cases when the Pd concentration is higher than 0.175 at.%, the correlation radius characterizes the dimensions of the nonconducting regions in the amorphous carbon matrix. In our case Fig. 6), the size of a nonconducting cluster at which we observe the start of increase of percolation is comparable with the thickness of the filmsbeing studied. As shown in Fig. 6, the correlation radius decreases rapidly. This is due to the amount of conductive filler, which becomes enough to transform the carbon film with individual conductive particles into a single conductive system with an infinite number of flow channels. In this case, the dimensions of the non-conducting phase and voids in the carbon matrix are significantly reduced and can be determined by the formula ((1)

R=

Ro (x − x c )

(1)

where R0 is the average size of the structural elements/sites of the matrix, ν is the critical index of the correlation radius. As can be seen from Fig. 6, a change of the Pd concentration in the range from 0.175 at.% to 0.24 at.% leads to a significant change in the correlation radius and these changes also correlate with the conductivity of a-C films (Fig. 5). At the concentrations above and below the percolation threshold, conductivity is determined by the formulas (2) and (3), respectively:

Fig. 5. Dependence of conductivity on Pd concentration in a-C films at room temperature (line is drawn as guide to the eye).

σ (x ) = σM (x − x c )t

(2)

σ (x ) = σD (x c − x )−q

(3)

Conductivity near or at the percolation transition point is described by expression (4)

σ σ (x c ) = σM ⎛ D ⎞ ⎝ σM ⎠ ⎜

s

⎟

(4)

where x is the concentration of conductive elements, xc is the concentration of conductive elements at which the percolation threshold occurs, σM is the conductivity of conductive elements, σD is the conductivity of the diamond-like carbon matrix. The indices t, q, s are called critical indices or the critical conductivity indices, when x xc or at the percolation transition point, respectively. These indices are considered as universal parameters since their valuesdepend only on the dimension (d) of the space [18]. The values of the critical electrical conductivity t for d = 3 are greater than t for d = 2. As shown in [18], the results of computer and

Fig. 6. Dependence of the correlation radius on the palladium concentration in a-C films at room temperature and x >xc (line is drawn as guide to the eye). 5

Journal of Non-Crystalline Solids 532 (2020) 119876

A.P. Ryaguzov, et al.

T

ν

S

q

Methodology, Software, Supervision. N.R. Guseinov: Software. A.R. Assembayeva: Methodology, Writing - original draft. S.I. Zaitsev: Data curation.

2.8

1.4

0.84

0.58

Declaration of Competing Interest

Table 2 Values of critical conductivity indices of a-C films. Critical indices Critical indices values

хc, at.% 0.175

σD, S/cm 2.28⋅10

−6

σ(хc), S/cm −5

3.43⋅10

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

model representations do not contradict their universality and for the three-dimensional spacet~1.6 and for the two-dimensional space t~1.3. In our work, the index t was determined from the slope of the straight line from the logarithmic dependence of the specific conductivity ln(σx) on the difference in concentrations ln (x-xc). The value of t is given in Table 2. It should be noted that the critical index t does not depend on the conductivity of conductive elements. The value of s for two-dimensional space is 0.5; for 0.5
q=

t −t s

Acknowledgements The work was carried out within the framework of grant financing No. AP05131495 from Science Committee of Ministry of Education and ScienceRK. References [1] J.C. Angus, C.C. Hayman, Low-Pressure, metastable growth of diamond and "Diamond-like" phases, Science 241 (1988) 913. [2] The Properties of Natural and Synthetic Diamond, in: J.E. Field (Ed.), The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992. [3] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, London, 1996. [4] K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A.K. Geim, Two-Dimensional atomic crystal, Proc. Natl. Acad. Sci. U.S.A. 102 (2005) 10451. [5] F.G. Celii, J.E. Butler, Diamond chemical vapor deposition, Annu. Rev. Phys. Chem. 42 (1991) 643–684. [6] A. Gangopadhyay, Mechanical and tribological properties of amorphous carbon films, Tribol. Lett. 5 (1998) 25–39. [7] P.W. Shum, Z.F. Zhou, K.Y. Li, C.Y. Chan, Mechanical and tribological properties of amorphous carbon filmsdeposited on implanted steelsubstrates, Thin Solid Films 458 (2004) 203–211. [8] R. Paul, S. Hussain, A.K. Pal, Characterization of nanocrystalline gold/DLC composite films synthesized by plasma CVD technique, Appl. Surf. Sci. 255 (2009) 8076–8083. [9] Sh. Sarsembinov, O. Yu. Prihodko, A.P. Ryaguzov, S. Ya.Maksimova, Ye. A. Daineko, F.A. Mahmoud, Electronic properties of diamond–like carbon films modified by silver nanoclusters, Phys. Status Solidi C. 7 (2010) 805–807. [10] O. Prikhodko, N. Manabaev, N. Guseinov, S. Maksimova, E. Muhametkarimov, S. Mikhailova, E. Daineko, Plasmon resonance in A-С:H films modified with platinum nanoclusters, J. Nano–and Electron. Phys. 6 (2014) 03067. [11] A.M. Danishevskii, R.N. Kiutt, A.A. Sitnikova, B.D. Shanina, D.A. Kurdiukov, S.K. Gordeev, Palladium clusters in nanoporous carbon samples: structural properties, Solid State Phys. 51 (2009) 604–608. [12] D.S. Knight, W.B. White, Characterization of diamond films by Raman spectroscopy, J. Mater. Res 4 (1989) 385–393. [13] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (2000) 14095. [14] A.C. Ferrari, J. Robertson, Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond, Phil. Trans. R. Soc. Lond. A. 362 (2004) 2477–2512. [15] A. Modabberasl, P. Kameli, M. Ranjbar, H. Salamati, R. Ashiri, Fabrication of DLC thin films with improved diamond-like carbon character by the application of external magnetic field, Carbon N Y 94 (2015) 485–493. [16] A.P. Ryaguzov, G.A. Yermekov, T.E. Nurmamytov, R.R. Nemkayeva, N.R. Guseinov, R.K. Aliaskarov, Visible Raman spectroscopy of carbon films synthesized by ionplasma sputtering of graphite, J. Mater. Res. 31 (2016) 127–136. [17] J. Tauc, Optical properties of semiconductors in the visible and ultraviolet ranges, Prog. Semicond. (1965) 9. [18] A.L. Efros, B.I. Shklovski, Conduction of nanostructured metal insulator, Phys. Status solidi 76 (1976) 475–485. [19] V.I. Roldughin, V.V. Vysotskii, Percolation properties of metal–filled polymer films, structure and mechanisms of conductivity, Prog.Org. Coat. 39 (2000) 81–100. [20] V.I. Ivanov-Omski, A.B. Lodygin, S.G. Yastrebov, The structure of copper–doped amorphous hydrogenated carbon films, Fizika tverdogo tela. 37 (1995) 1693–1697 [in Russian]. [21] B.I. Shklovski, A.L. Efros, Percolation theory and conductivity of strongly inhomogeneous media, Adv. Phys. Sci. 117 (1975) 24–433.

(5)

This formula illustrates the relationship between critical conductivity indices in cases when x xc, and at the transition point. It is also convenient to use this formula when we have a set of experimental values of x corresponding to x> xc, and it is difficult to decide which value of σ(x) to take for calculations. According to the calculated parameters of the critical indices shown in Table 2, the synthesized thin films a-C can be attributed to three-dimensional composite materials. Conductive clusters of sp2 sites and palladium nanoparticles in thin amorphous carbon films represent bulk structural configurations and spherical particles in an amorphous carbon matrix, respectively. This is confirmedby the findings from the consideration of electronand Raman spectroscopy. 4. Conclusions According to the results obtained from the studies of Raman and electronic spectra and optical properties, it can be concluded that the structure and electronic properties of amorphous carbon can be significantly changed by modifying them with palladium nanoparticles. In this case, the structure of thin carbon films varies from diamond-like to graphite-like, depending on the palladium concentration. Palladium nanoparticles contribute to the appearance of additional sp2 sites, which increase the density of states in the band gap, which in turn leads to a decrease in the band gap of amorphous carbon. It is also shown that at a certain value of Pd concentration in DLC films, a "conduction channel" appears. This “channel” corresponds to the percolation threshold, at which a percolation transition is observed. A change in the palladium concentration in the range from 0.17 at.% to 1 at.% increases the conductivity from~10−6 to 102 S/cm. Such significant changes in properties make it possible to expand the field of application of amorphous carbon films in the creation of new sensors and nanoelectronic devices. CRediT authorship contribution statement A.P. Ryaguzov: Conceptualization, Data curation, Formal analysis, Funding acquisition, Writing - review & editing. R.R. Nemkayeva:

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