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Effect of oxidation of copper nanoparticles on absorption spectra of DLC:Cu nanocomposites
Diamond & Related Materials 99 (2019) 107538
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Effect of oxidation of copper nanoparticles on absorption spectra of DLC:Cu nanocomposites
T
⁎
I. Yaremchuka, , Š. Meškinisb, T. Bulavinetsa, A. Vasiliauskasb, M. Andrulevičiusb, V. Fitioa, Ya. Bobitskia,c, S. Tamulevičiusb a
Department of Photonics, Lviv Polytechnic National University, S. Bandera St. 12, Lviv 79013, Ukraine Institute of Materials Science of Kaunas University of Technology, Baršausko St. 59, LT-51423 Kaunas, Lithuania c Faculty of Mathematics and Natural Sciences, University of Rzeszow, Pigonia St.1, 35959 Rzeszow, Poland b
A R T I C LE I N FO
A B S T R A C T
Keywords: Diamond like carbon Copper nanoparticles Nanoshells Nanocomposite Surface plasmon resonance
In the present work diamond like carbon (DLC) films with embedded Cu nanoparticles were researched. The spectral characteristics of Cu nanoparticles, Cu-CuO and Cu-Cu2O nanoshells and nanocomposites on their base were studied including effects of size of nanoparticles. These studies included experimental research and theoretical simulations based on Bruggeman effective medium theory and dipole equivalence method. It is shown that experimentally observed relatively small redshift of the absorption peak of the DLC:Cu, having place for a wide range of Cu atomic concentration and Cu nanoparticle size, is mostly due to the oxidation of embedded copper nanoparticles.
1. Introduction The development of modern optoelectronic and photonic technologies has steadily led to the need to study, create and use nanoscale objects and nanocomposite structures. The work of a number of modern optoelectronic devices, namely, filters, antennas, sensors, solar cells and others is based on the resonant phenomena of the interaction of electromagnetic radiation with such elements. The localized surface plasmon resonance (LSPR) is the most pronounced optical effect for metallic nanostructures, which manifests itself in the collective vibration of conduction electrons excited by an electromagnetic field [1–3]. The most widely used plasmonic nanoparticles are the noble metals Au, Ag [4]. However, the use of other metals such as Cu can reduce production costs and expand the spectral range of the effect [2]. Moreover, considerable interest has been focused on copper and copper oxide-based materials due to their applications in semiconductor devices [3], gas sensors [5,6], photovoltaics [7], energy storage [8,9], catalysts [10–12]. Antimicrobial applications should be mentioned as well [13,14]. Nanocomposite materials with copper nanoparticles as filler also are widely researched. For example, nanocomposites based on silica matrix [15,16], fluoropolymer matrix [2], cellulose [17], diamond like carbon (DLC) matrix [18–21] should be mentioned. Diamond like carbon as a matrix material in this case has some advantages. This amorphous allotrope of carbon consisting of the sp3
⁎
and sp2 bonded carbon atoms as well as 0–40 at.% of hydrogen is known for very interesting combination of mechanical, optical and electrical properties [22,23]. Particularly, room temperature deposition, ultrasmoothness, high hardness and wear resistance, optical transparency, possibility to change refractive index in a broad range, chemical inertness, biocompatibility make this material very attractive for many applications [22,23]. Formation of Au, Ag, Cu nanoparticles embedded into the DLC matrix and appearance of characteristic plasmonic peak in the absorption spectra of nanocomposites takes place when atomic concentration of the metals mentioned exceeds some threshold value (usually several atomic percents) (see review [4] and references therein). It is interesting to note that in the case of DLC:Cu nanocomposites, only very low shift of absorption peak in a wide range of copper concentration has been observed [21,24]. It was detected, despite of increase of copper concentration in DLC:Cu film from 20 at.% up to 60 at.% and significantly increased size of copper nanoparticles due to agglomeration. On the other hand, in [16] it was shown that surface plasmon resonance extinction peak of passivated Cu nanoparticle array is red-shifted by more than 200 nm with the increase of copper nanoparticle's size. Such experimentally observed behavior of DLC:Cu nanocomposites probably can be explained by oxidation of copper nanoparticles and as a result creation of core-shell structure. Such mechanism is well known and can be met during fast oxidation of Cu nanoparticles at ambient conditions [25]. In the present work the
Corresponding author. E-mail address: [email protected] (I. Yaremchuk).
https://doi.org/10.1016/j.diamond.2019.107538 Received 27 June 2019; Received in revised form 13 August 2019; Accepted 7 September 2019 Available online 09 September 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.
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Significant changes of the nanoparticle size and shape were reported in [24]. High-resolution XPS spectra of oxygen, carbon and copper for DLC:Cu films were acquired to study chemical composition of the samples and possible presence of the Cu and O bonds. The spectra were measured before and after extra Ar+ surface sputtering to eliminate possible influence of the surface adsorbates. Significant increase of copper content on the surface after the applied sputtering procedure was found [24].Changes of chemical bonds on the surface of the samples containing different amount of copper concentrations before and after ion sputtering procedure were investigated analyzing high resolution spectra (Figs. 3–6). Fig. 2 presents XPS high resolution carbon C 1s spectra acquired from untreated and sputtered samples. Four peaks situated at approximately 284.3 eV, 284.8 eV, 286.2 eV and 288.8 eV we assigned to C]C (sp2), CeC (sp3), CeO and O-C=O bonds respectively [28–30]. The peaks at 286.2 eV and 288.8 eV almost totally disappeared after sputtering process, thus indicating reduction of carbon and oxygen bonds. Therefore it can be assumed that CeO and OC=O peaks belong mostly to the atmospheric contaminants and are removed during sputtering process [31,32]. High-resolution oxygen O1s spectrum of the untreated and ion sputtered samples (Fig. 3) shows four peaks situated at approximately 529.5 eV, 530.4 eV, 531.7 eV and 533.1 eV. Positions of these peaks are in good agreement with the binding energy values reported in the literature for CuO [33,34], Cu2O [34], O-C=O and C-O/O-H [28–30] bonds respectively. For the most untreated samples the O-C=O peak dominates in O1s spectra and the Cu2O peak dominates for the ion etched surfaces. In analogy to C1s spectra, changes after the sputtering process, disappearance of O-C=O and C-O/O-H peaks indicate successful elimination of the atmospheric contaminants from the surface of the sample. Copper high-resolution Cu2p spectra are presented in Fig. 4 for the treated and ion untreated samples. In this picture positions of the possible bonds are indicated along with the typical satellite peak positions [33,35–37]. In this picture the broadened peak at approximately 932.7 eV for the untreated samples transforms in to narrow peak after the sputtering process. Appearance of additional satellite peak at 946.6 eV after the sputtering process indicates removal of CuO oxide from the surface of the sample. For more precise distinction of copper bonds in the samples, the XRay excited Cu LVV Auger spectra were acquired for the untreated and sputtered samples (Fig. 5). In this picture, one can see that there is no visible peak at 568.9 eV, indicating absence of CuO bonds. Appearance of additional peaks at 568.1 eV and 565.4 eV suggests presence of metallic copper. In the case of the samples containing 59.6, 62.0, and 63.5 at.% of Cu, most of copper is in Cu2O oxide form. For the samples containing 21.7 and 26.2 at.% of copper, Cu peak prevails over Cu2O (Fig. 6). While for sample with 55.4 at.% of Cu, metallic copper and Cu2O peaks are of the similar intensity. Yet in all cases both copper and copper oxide were found in all studied DLC:Cu films. Summarizing the experimental results and the results presented in [16], we can state that with the change in the size of copper nanoparticles, the absorption peak recorded for DLC:Cu is redshifted only by less than 30 nm, despite significant changes of the copper atomic concentration and Cu nanoparticle size. According to the XPS studies, it can be assumed that such behavior may be related to the oxidation of copper nanoparticles and possible subsequent formation of the core shell nanostructure. To check this hypothesis, in the further steps we have performed numerical modelling of optical properties of Cu nanoparticles, Cu nanoparticle with the core shell and finally considered role of DLC on the optical response of such nanoparticles embedded in a DLC matrix.
effect of oxidation of copper nanoparticles on absorption spectra of DLC:Cu nanocomposites has been investigated by combining simulation and experimental study. The spectral characteristics of Cu nanoparticles, Cu-CuO and Cu-Cu2O nanoshells and nanocomposites on their base were studied including effects of size of nanoparticles. 2. Experimental techniques In the present study, diamond like carbon films with embedded copper nanoparticles were deposited by using reactive high power pulse magnetron sputtering (HIPIMS) technique. The diameter of the copper target was 3″. Argon was used as a sputtering gas and acetylene (C2H2) was used as a source of carbon and hydrogen. Base pressure was 5 × 10−4 Pa and work pressure (4 ± 1) × 10−1 Pa was maintained throughout the deposition process. In all experiments the substrate–target gap was set at 0.1 m and the substrates were grounded. The thickness of the deposited films in all cases was in the range of 50–100 nm. More information on the deposition conditions and structure of the deposited samples can be found in [24]. In all cases Raman scattering spectra of the samples were typical for diamond like carbon [24]. Morphology of DLC:Cu nanocomposite films on silicon substrates was investigated employing helium ion microscope (ORION NanoFab, Zeiss), which enables high spatial resolution imaging using focused ion beam. Transmission and reflection spectra of DLC:Cu nanocomposites on quartz substrates were obtained using a stabilized halogen light source (SLS301, ThorLabs), integrating sphere (RTC-060-SF, Labsphere) and high sensitivity spectrometer (Maya 2000Pro, Ocean Optics). The spectra were recorded in a wavelength range of 360–1100 nm (limited by the light source and spectrometer). The absorption (A) was calculated as A = 1 – T – R, where T is the transmission and R is the reflectivity in relative units. High-resolution X-ray photoelectron spectroscopy (XPS) spectra of oxygen, carbon and copper for DLC:Cu films were acquired before and after extra Ar+ surface sputtering for the surface composition analysis and possible chemical bonds detection. Cu atomic concentration estimated after the ion beam etching was used in subsequent analysis of the possible correlation between the optical properties and structure of the nanocomposite films vs composition. A Thermo Scientific ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, East Grinstead, UK) with a monochromatic Al Kα radiation (hν = 1486.6 eV) was used for the XPS measurements. The base pressure in the analytical chamber was lower than 2 × 10−7 Pa. The 40 and 20 eV pass energy values of a hemispherical electron energy analyzer were used for the survey and for the recording of high-resolution spectra, respectively. The energy scale of the system was calibrated with respect to Au 4f7/2, Ag 3d5/2, and Cu 2p3/2 peak positions. The modelling was performed using a dipole equivalent method and Bruggeman theory of effective medium in the wavelength range from 400 to 700 nm [26,27]. 3. Results and discussion 3.1. Experimental: optical properties, structure and composition of DLC:Cu The absorption spectra of DLC:Cu films are characterized by absorption peak that corresponds to the surface plasmon resonance (Fig. 1). In spite of significant changes of the copper concentration (from 21.7 to 63.5 at.%), shape of the absorption spectra in all cases was very similar. Position of the plasmonic peak maximum (noted in the insert of Fig. 1a) was rather similar as well. In all cases the peak was in 594–620 nm range (Fig. 1b). Analysis of morphology of the DLC:Cu nanocomposite films deposited on silicon substrates indicated that average copper nanoparticle size increased with the increase of Cu atomic concentration (see Fig. 2).
3.2. Simulations As a starting point, the first simplified simulations of absorption for 2
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Fig. 1. Absorption spectra of DLC:Cu nanocomposite films containing different copper concentrations (adapted from [24]) (a) and plasmonic peak position vs Cu atomic concentration (b).
Fig. 2. High-resolution XPS C1s spectra of untreated and ion sputtered samples. Fig. 3. High-resolution XPS O1s spectra of untreated and ion sputtered samples.
3
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Fig. 4. High-resolution XPS Cu2p spectra of untreated and ion sputtered samples.
the Cu nanoparticles without any amorphous carbon matrix were performed. Simulation for the particles of different radii from 10 nm to 50 nm was carried out (see Fig. 6) to study the influence of the size of copper nanoparticles on their absorption spectrum. Fig. 6 indicates that plasmonic peak position shifts to the long-wave region when nanoparticles size increases - from 575 nm (for nanoparticle having radius10 nm) to 775 nm (for nanoparticle with radius 50 nm). Moreover, the absorption peak becomes broader and plasmonic peak intensity decreases with the radius of the nanoparticle. It is known that interaction of copper nanoparticles with air and moisture can cause its oxidation. As a result, the copper oxide layer can be in two semiconducting phases, namely, cupric oxide CuO and cuprous oxide (Cu2O). Copper oxide nanoparticles of sufficient size (200 and 240 nm) are characterized by the peak of plasmonic absorption as it was stated in [8]. However according to our simulations (Fig. 7), it can be seen that the smaller copper oxide nanoparticles do not have plasmonic absorption peak.
Fig. 5. Cu LVV spectra of untreated and ion sputtered samples.
Concerning behavior of Cu nanoparticles in dielectric matrix it should be mentioned that in our previous XPS study of DLC:Cu films, relatively high concentration (13–20 at.%) of oxygen was found in the nanocomposites containing 22–63 at.% of Cu [24]. Analysis of the XPS peaks of nanocomposite films presented in this study clearly revealed presence of the copper oxide related peaks, even after the ion beam cleaning. Thus, one can assume that the copper nanoparticles dispersed in DLC film are oxidized also. Taking into account amount of copper and oxygen in the films, it can be assumed that no pure CuO and Cu2O nanoparticles are present in the studied DLC:Cu film. The influence of the DLC matrix parameters on the optical characteristics of copper nanoparticles was studied assuming that these particles are spherical as it was reported in Spherical nanoparticles were chosen for the calculation, because in ref. [24], where for DLC:Cu
4
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Fig. 6. Influence of the size of Cu nanoparticles on the position and shape of the absorption peak. Fig. 9. The absorption spectra of DLC films with embedded copper (Cu) nanoparticles with diameter of 20 nm. Optical constants of DLC films simulated in Fig. 8 were used as a DLC matrix: sample n1, k1 (DLC1), n2, k2 (DLC2), n3, k3 (DLC3), n4, k4 (DLC4), n5, k5 (DLC5), n6, k6 (DLC6), n7, k7 (DLC7) and n8, k8 (DLC8).
samples containing about 60 at.% of copper, spherical Cu nanoparticles were observed. The refractive index of copper nanoparticles was calculated according to the analytical formula presented in [38]. It is known that refractive index of DLC reported may vary in a wide range from 1.4 to 2.8 [39]. Thus, the set of different experimentally measured DLC refractive indices presented in ref. [40, 41] were used for calculations. The real and imaginary parts of the DLC refractive indices are presented in Fig. 8. The absorption spectra of spherical copper nanoparticles with a diameter of 20 nm embedded to these 8 different refractive indices DLC matrices were calculated and the results are shown in Fig. 9. One can see that shape of the spectra, amplitude and position of the absorption peak is different in all eight considered cases. Dependences the plasmonic peak position, plasmonic peak intensity and peak width (full width at half maximum - FWHM) on the DLC refractive index are
Fig. 7. The simulated absorption spectra Cu, CuO and Cu2O nanoparticles of diameter 20 nm in air.
Fig. 8. The real (red) and imaginary (blue) parts of the DLC refractive indices. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5
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It is well known, that the absorption spectrum of such nanoshells depends primarily on the thickness of the core and the thickness of the shell, as well as the medium in which they are located. Therefore, at the next step of the study, Fig. 11 presents the dependences of the absorption spectrum for the nanoshells Cu-CuO and the Cu-Cu2O at the constant shell thickness of 2 nm on the core thickness for the nanoshells Cu-CuO and the Cu-Cu2O at the constant shell thickness of 2 nm for embedded to DLC with the refractive index of the DLC represented by Eq. (1)were obtained (see Fig. 12). It can be seen, that absorption spectra of Cu-CuO and Cu-Cu2O nanoshells in DLC are very similar. When the core thickness increases, the absorption peak is shifted to the short-wave region. The absorption peaks for Cu-CuO and Cu-Cu2O nanoshells with the core size of 20 nm are near 598 nm, and with the core size of 50 nm near to the wavelength of 589 nm. It should be noted that this shift of the plasmonic peak in case of Cu-CuO or Cu-Cu2O nanoshells is rather small in comparison to pure Cu nanoparticles (about 20 nm and nearly 200 nm respectively, as shown in Figs. 6 and 11). In the next step of our research, the simulation of the absorption spectra of Cu-CuO and Cu-Cu2O nanoshells embedded into the DLC matrix was done. Fig. 12 presents the simulation results of the dependences of the absorption spectra of the CuO and Cu2O shells embedded in DLC on the thickness of the shell at the constant radius core thickness of the CuO and Cu2O shells, were researched. The simulation results are presented in Fig. 13. One can see that the plasmonic absorption peak redshifts from 600 nm to 647 nm when the shell thicknesses changes from 1 nm to 5 nm. The intensity of absorption decreases, when the shell thickness increases. It should be noted that the full width at half maximum of the absorption spectra increases also when the shell thickness increases. Since, the Cu prone to oxidation at ambient conditions and the dominating product is Cu2O not CuO, oxidation occurs only on the surface of the nanoparticle, the nanostructures such as the core-shell DLC:Cu-Cu2O are formed. Therefore, next our research were carried out only for DLC:Cu-Cu2O. The simulations of optical properties of DLC films with embedded Cu core and Cu2O shell nanoparticles were carried out. The Bruggeman effective medium theory [42] has been used for calculation of the effective refractive index of the diamond like carbon nanocomposite with embedded oxidized Cu nanoparticles:
Table 1 Dependences of the plasmonic peak position, plasmonic peak intensity and FWHM on DLC refractive index (presented in Fig. 9). DLC refractive index
Plasmonic peak position, nm
Plasmonic peak intensity a.u.
Plasmonic peak width, nm
n1, n2, n3, n4, n5, n6, n7, n8,
586.9 564.7 645.4 571.4 585.5 647.3 594.2 580.8
0.00242 0.03506 0.00351 0.78289 0.01812 0.00561 0.06083 0.00284
25.134 7.68 22.342 1.396 9.077 18.152 4.887 22.342
k1 k2 k3 k4 k5 k6 k7 k8
summarized in Table 1. Particularly, plasmonic absorption peak position can be changed by more than 80 nm just by changing the DLC matrix optical properties. Concerning behavior of Cu nanoparticles in dielectric matrix it should be mentioned that in our previous XPS study of DLC:Cu films, relatively high concentration (13–20 at.%) of oxygen was found in the nanocomposites containing 22–63 at.% of Cu [24]. Analysis of the XPS peaks of nanocomposite films presented in this study clearly revealed presence of the copper oxide related peaks, even after the ion beam cleaning. Thus, one can assume that the copper nanoparticles dispersed in DLC film are oxidized also. Taking into account amount of copper and oxygen in the films, it can be assumed that no pure CuO and Cu2O nanoparticles are present in the studied DLC:Cu film. It can be supposed that the core-shell nanoparticles consisting of the core Cu and the CuO or Cu2O shell are formed. The possible structures of these nanoshells are presented in Fig. 10. In the following steps, only one refractive index value of DLC was used for study of oxidation effects. It can be explained by fact that plasmonic peak position in the experimental absorption spectra of copper nanoparticles for the studied films was found in a region from 550 to 650 nm for all the DLC refractive indices considered. The two criteria in the selection of right optical constants of DLC were used. According to the first criterion – the calculated plasmonic Cu peak position should fit within the experimental peak position data defined in our experimental studies. In addition, it should be relatively broad and similar to the experimental ones. According to this criterion, the wider simulated peak is closer to the experimental data than the narrow one. The dependence of the real and imaginary parts of the selected DLC refractive index on the wavelength (λ) are described as follows:
εBG − εh ε − εh =f i , εBG + 2εh εi + 2εh
(2)
where εBG is the effective dielectric permittivity of nanocomposite, εh is the dielectric permittivity of the host matrix, εi is the dielectric permittivity of inclusions. Consider the exactly-solvable case for two-component mixture; the two solutions are available in this case:
n = 1.001 + 5.914λ − 13.517⋅10−5λ2 + 14.071λ3 − 6, 982λ4 + 1.357λ5, k = 0.243 + 1.862λ − 8.155λ2 + 10.660λ3 − 5.503λ4 + 0.906λ5. (1) According to the curves presented in Fig. 10, the chosen refractive index values described by Eq. (1) correspond to the spectra of the sample noted by n1 and k1, and absorption spectrum of Cu nanoparticles with a diameter of 20 nm in DLC matrix with such a refractive index is shown in Fig. 9.
εBG =
b±
8ε1 ε2 + b2 , 4
(3)
where
b = (2f1 − f2 ) ε1 + (2f2 − f1 ) ε2
(4)
where ε1, ε2, f1, f2 are the dielectric permittivity and filling factor of DLC, respectively; ε2, f2 are the effective dielectric permittivity and filling factors of Cu-CuO2 respectively. The effective dielectric permittivity of Cu-CuO2 is calculated using dipole equivalence method [43] as follows:
ε2 = εCuO2
1− Fig. 10. A schematic representation of the structure core-Cu and CuO- or Cu2Oshell.
3 RCu αCu − CuO2 Cu − CuO2 3 RCu αCu − CuO2 3 RCu − CuO2
1 + 2R3
, (5)
where εCuO2 is the dielectric permittivity of shell, RCu and RCu−CuO2 are radii of core and core + shell respectively, αCu−CuO2 is the average 6
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Fig. 11. Dependences of the absorption of Cu-CuO (a) and Cu-Cu2O (b) nanoshells on the radius of the core at the constant shell thickness of 2 nm in the DLC calculated by Eq.(1).
Fig. 12. Dependences of the absorption of Cu-CuO (a) and Cu-Cu2O (b) nanoshells on the thickness of the shell at the constant radius core of 10 nm in the DLC calculated by Eq. (1).
permittivities of such nanoshells were calculated using Eqs. (5), (6). In the next, the effective refractive indices of the three DLC:Cu-Cu2O nanocomposites with copper concentrations 26.2%, 55.4% and 63.5% were simulated using Eqs. (1)–(4). The simulation results are presented in Fig. 13 and the dependences of the absorption coefficient on the wavelength for these samples are presented in Fig. 14. It can be seen from Fig. 13 that absorption spectra are shifted into a short-wave region from 583 nm to 565 nm, when concentration increases from 26.2% to 63.5%. These results are not in good agreement with the experimental ones, since redshift of the absorption peak was detected.
polarization. The average polarization can be expressed as follows:
αCu − CuO2 =
εCu − εCuO2 εCu + 2εCuO2
(6)
Average size of nanoparticles for each value of the concentration of nanoparticles (filling factor) was estimated from Fig. 1b, namely for 21.7% the diameter of Cu-Cu2O nanoshell is of 20 nm, for 26.2% is 26 nm, for 55.4% is 38 nm, for 59.6% is 46 nm, for 62.0% is 52 nm and for 63.5% is 55 nm. It is assumed that the thickness of the oxide layer remains unchanged and is equal to 2 nm. The effective dielectric
Fig. 13. Real and imaginary parts of the DLC:Cu-Cu2O nanocomposite refractive index at different copper concentrations 26.2% (a), 55.4% (b) and 63.5% (c) at constant thickness of shell of 2 nm. 7
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Fig. 14. Absorption spectra of DLC:Cu-Cu2O nanocomposite at different concentrations 26.2% (a), 55.4% (b) and 63.5% (c) at constant thickness (2 nm) of the shell.
Using the obtained effective refractive index spectra of DLC:CuCu2O thin film the corresponding absorption spectra were calculated for different Cu-Cu2O concentration that are presented in Fig. 16. One can see that the absorption peaks related to the surface plasmon resonance are red-shifted with the increase in Cu-Cu2O concentration. The plasmonic peak widths of the simulated absorption and experimentally measured are different. However, the theoretically obtained plasmonic peak positions provide reasonable fit of the experimental curves. Moreover, the intensity of absorption spectra decreases with the increase of concentration of nanoparticles. It can be explained by the fact that in the calculations it was considered that the size of nanoparticles increases when the concentration of filler increases. The increase in the size of nanoparticles leads to a widening of the absorption peak and decrease in its amplitude. In summary it can be supposed that, on one hand, the increase in the size of the nanoparticle causes the shift of absorption peak to the longwave region, but on the other hand, formation of the nanoshells causes the shift of absorption peak to the short-wave region. As a result, the absorption peak is shifted only slightly, even concentration of copper nanoparticles and their size increase substantially.
Some discrepancies between the experimental and simulation results can be explained by the fact that copper nanoparticles were oxidized differently. It is confirmed by the XPS analysis results (see Figs. 4, 5) where it is shown that most of copper is in Cu2O oxide form in the case of the samples with concentrations 62.0, 59.6 and 63.5%. Copper peak prevails over Cu2O for the samples with concentrations 21.7 and 26.2%. In case of the sample with concentration 55.4% the metallic copper and Cu2O peaks are of the similar intensity. Therefore it can be assumed that oxide shell thickness increased with the increase of concentration. Taking into account the results presented in Figs. 11 and 12 and results of XPS analysis the following sizes of nanoparticles have been used for the next simulation: the radius of core was of 7 nm and thickness of the shell was 3 nm for concentration 21.7%, radius of core was of 9 nm and thickness of shell was 4 nm for 26.2%, radius of core was of 11 nm and thickness of shell was 8 nm for 55.4%, radius of core was of 13 nm and thickness of shell is 10 nm for 59.6%, radius of core is of 15 nm and thickness of shell was 11 nm for 62.0% and for 63.5% radius of core was of 15 nm and thickness of shell was 13 nm. The calculated effective refractive indices for all these samples are presented in Fig. 15.
Fig. 15. Real and imaginary parts of the refractive index of DLC:Cu-Cu2O nanocomposite at different concentrations 21.7% (a), 26.2% (b), 55.4% (c), 59.6% (d), 62.0% (e), and 63.5% (f). 8
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Fig. 16. Absorption spectra of DLC:Cu-Cu2O nanocomposite at different concentrations 21.7, 26.2, 55.4, 59.6, 62.0 and 63.5%.
4. Conclusions [2]
The diamond like carbon films with embedded Cu nanoparticles were researched. The effect of oxidation of copper nanoparticles on absorption spectra of DLC:Cu nanocomposites has been studied by combination of the simulation and experiment. The simulations revealed that increase of pure Cu nanoparticle size leads to significant shift plasmonic absorption peak position to longwave region. However, only relatively small shift in the plasmonic absorption peak position and practically constant plasmonic peak intensity in the DLC:Cu film experimental spectra was experimentally observed, despite significant increase in Cu concentration and copper nanoparticle size. Such a discrepancy can be explained by the oxidation of copper nanoparticles. As the Cu prone to oxidation at ambient conditions and the dominating product is Cu2O (and not CuO), oxidation occurs only on the surface of the nanoparticle, the nanostructures such as the core-shell DLC:Cu-Cu2O are formed. Plasmonic peak position of nanoshells also shifts to the long-wave region but it is small in comparison to the one due to Cu nanoparticles. Comparison of the simulated and measured absorption spectra revealed that the nanoparticles were oxidized differently, and oxide shell thickness increased when concentration of nanoparticles in sample increased. Having experimentally measured absorption spectrum of DLC:Cu-Cu2O and the size of the nanoparticles, one can estimate the magnitude of oxidation of copper nanoparticles.
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
Acknowledgements This research was funded by a grant (proposal No. P-MIP-17-243, contract No. S-MIP-17-82) from the Research Council of Lithuania, project title “Plasmonic Carbon Nanocomposite Based Self Saturable Absorber Mirrors for Fiber Lasers”. Financial support of Ministry of Education and Science of Ukraine should be acknowledged (DB/ Fotonika No. 0117U007176, DB/MEV No. 0118U000267).
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Effect of oxidation of copper nanoparticles on absorption spectra of DLC:Cu nanocomposites
T
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I. Yaremchuka, , Š. Meškinisb, T. Bulavinetsa, A. Vasiliauskasb, M. Andrulevičiusb, V. Fitioa, Ya. Bobitskia,c, S. Tamulevičiusb a
Department of Photonics, Lviv Polytechnic National University, S. Bandera St. 12, Lviv 79013, Ukraine Institute of Materials Science of Kaunas University of Technology, Baršausko St. 59, LT-51423 Kaunas, Lithuania c Faculty of Mathematics and Natural Sciences, University of Rzeszow, Pigonia St.1, 35959 Rzeszow, Poland b
A R T I C LE I N FO
A B S T R A C T
Keywords: Diamond like carbon Copper nanoparticles Nanoshells Nanocomposite Surface plasmon resonance
In the present work diamond like carbon (DLC) films with embedded Cu nanoparticles were researched. The spectral characteristics of Cu nanoparticles, Cu-CuO and Cu-Cu2O nanoshells and nanocomposites on their base were studied including effects of size of nanoparticles. These studies included experimental research and theoretical simulations based on Bruggeman effective medium theory and dipole equivalence method. It is shown that experimentally observed relatively small redshift of the absorption peak of the DLC:Cu, having place for a wide range of Cu atomic concentration and Cu nanoparticle size, is mostly due to the oxidation of embedded copper nanoparticles.
1. Introduction The development of modern optoelectronic and photonic technologies has steadily led to the need to study, create and use nanoscale objects and nanocomposite structures. The work of a number of modern optoelectronic devices, namely, filters, antennas, sensors, solar cells and others is based on the resonant phenomena of the interaction of electromagnetic radiation with such elements. The localized surface plasmon resonance (LSPR) is the most pronounced optical effect for metallic nanostructures, which manifests itself in the collective vibration of conduction electrons excited by an electromagnetic field [1–3]. The most widely used plasmonic nanoparticles are the noble metals Au, Ag [4]. However, the use of other metals such as Cu can reduce production costs and expand the spectral range of the effect [2]. Moreover, considerable interest has been focused on copper and copper oxide-based materials due to their applications in semiconductor devices [3], gas sensors [5,6], photovoltaics [7], energy storage [8,9], catalysts [10–12]. Antimicrobial applications should be mentioned as well [13,14]. Nanocomposite materials with copper nanoparticles as filler also are widely researched. For example, nanocomposites based on silica matrix [15,16], fluoropolymer matrix [2], cellulose [17], diamond like carbon (DLC) matrix [18–21] should be mentioned. Diamond like carbon as a matrix material in this case has some advantages. This amorphous allotrope of carbon consisting of the sp3
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and sp2 bonded carbon atoms as well as 0–40 at.% of hydrogen is known for very interesting combination of mechanical, optical and electrical properties [22,23]. Particularly, room temperature deposition, ultrasmoothness, high hardness and wear resistance, optical transparency, possibility to change refractive index in a broad range, chemical inertness, biocompatibility make this material very attractive for many applications [22,23]. Formation of Au, Ag, Cu nanoparticles embedded into the DLC matrix and appearance of characteristic plasmonic peak in the absorption spectra of nanocomposites takes place when atomic concentration of the metals mentioned exceeds some threshold value (usually several atomic percents) (see review [4] and references therein). It is interesting to note that in the case of DLC:Cu nanocomposites, only very low shift of absorption peak in a wide range of copper concentration has been observed [21,24]. It was detected, despite of increase of copper concentration in DLC:Cu film from 20 at.% up to 60 at.% and significantly increased size of copper nanoparticles due to agglomeration. On the other hand, in [16] it was shown that surface plasmon resonance extinction peak of passivated Cu nanoparticle array is red-shifted by more than 200 nm with the increase of copper nanoparticle's size. Such experimentally observed behavior of DLC:Cu nanocomposites probably can be explained by oxidation of copper nanoparticles and as a result creation of core-shell structure. Such mechanism is well known and can be met during fast oxidation of Cu nanoparticles at ambient conditions [25]. In the present work the
Corresponding author. E-mail address: [email protected] (I. Yaremchuk).
https://doi.org/10.1016/j.diamond.2019.107538 Received 27 June 2019; Received in revised form 13 August 2019; Accepted 7 September 2019 Available online 09 September 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.
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Significant changes of the nanoparticle size and shape were reported in [24]. High-resolution XPS spectra of oxygen, carbon and copper for DLC:Cu films were acquired to study chemical composition of the samples and possible presence of the Cu and O bonds. The spectra were measured before and after extra Ar+ surface sputtering to eliminate possible influence of the surface adsorbates. Significant increase of copper content on the surface after the applied sputtering procedure was found [24].Changes of chemical bonds on the surface of the samples containing different amount of copper concentrations before and after ion sputtering procedure were investigated analyzing high resolution spectra (Figs. 3–6). Fig. 2 presents XPS high resolution carbon C 1s spectra acquired from untreated and sputtered samples. Four peaks situated at approximately 284.3 eV, 284.8 eV, 286.2 eV and 288.8 eV we assigned to C]C (sp2), CeC (sp3), CeO and O-C=O bonds respectively [28–30]. The peaks at 286.2 eV and 288.8 eV almost totally disappeared after sputtering process, thus indicating reduction of carbon and oxygen bonds. Therefore it can be assumed that CeO and OC=O peaks belong mostly to the atmospheric contaminants and are removed during sputtering process [31,32]. High-resolution oxygen O1s spectrum of the untreated and ion sputtered samples (Fig. 3) shows four peaks situated at approximately 529.5 eV, 530.4 eV, 531.7 eV and 533.1 eV. Positions of these peaks are in good agreement with the binding energy values reported in the literature for CuO [33,34], Cu2O [34], O-C=O and C-O/O-H [28–30] bonds respectively. For the most untreated samples the O-C=O peak dominates in O1s spectra and the Cu2O peak dominates for the ion etched surfaces. In analogy to C1s spectra, changes after the sputtering process, disappearance of O-C=O and C-O/O-H peaks indicate successful elimination of the atmospheric contaminants from the surface of the sample. Copper high-resolution Cu2p spectra are presented in Fig. 4 for the treated and ion untreated samples. In this picture positions of the possible bonds are indicated along with the typical satellite peak positions [33,35–37]. In this picture the broadened peak at approximately 932.7 eV for the untreated samples transforms in to narrow peak after the sputtering process. Appearance of additional satellite peak at 946.6 eV after the sputtering process indicates removal of CuO oxide from the surface of the sample. For more precise distinction of copper bonds in the samples, the XRay excited Cu LVV Auger spectra were acquired for the untreated and sputtered samples (Fig. 5). In this picture, one can see that there is no visible peak at 568.9 eV, indicating absence of CuO bonds. Appearance of additional peaks at 568.1 eV and 565.4 eV suggests presence of metallic copper. In the case of the samples containing 59.6, 62.0, and 63.5 at.% of Cu, most of copper is in Cu2O oxide form. For the samples containing 21.7 and 26.2 at.% of copper, Cu peak prevails over Cu2O (Fig. 6). While for sample with 55.4 at.% of Cu, metallic copper and Cu2O peaks are of the similar intensity. Yet in all cases both copper and copper oxide were found in all studied DLC:Cu films. Summarizing the experimental results and the results presented in [16], we can state that with the change in the size of copper nanoparticles, the absorption peak recorded for DLC:Cu is redshifted only by less than 30 nm, despite significant changes of the copper atomic concentration and Cu nanoparticle size. According to the XPS studies, it can be assumed that such behavior may be related to the oxidation of copper nanoparticles and possible subsequent formation of the core shell nanostructure. To check this hypothesis, in the further steps we have performed numerical modelling of optical properties of Cu nanoparticles, Cu nanoparticle with the core shell and finally considered role of DLC on the optical response of such nanoparticles embedded in a DLC matrix.
effect of oxidation of copper nanoparticles on absorption spectra of DLC:Cu nanocomposites has been investigated by combining simulation and experimental study. The spectral characteristics of Cu nanoparticles, Cu-CuO and Cu-Cu2O nanoshells and nanocomposites on their base were studied including effects of size of nanoparticles. 2. Experimental techniques In the present study, diamond like carbon films with embedded copper nanoparticles were deposited by using reactive high power pulse magnetron sputtering (HIPIMS) technique. The diameter of the copper target was 3″. Argon was used as a sputtering gas and acetylene (C2H2) was used as a source of carbon and hydrogen. Base pressure was 5 × 10−4 Pa and work pressure (4 ± 1) × 10−1 Pa was maintained throughout the deposition process. In all experiments the substrate–target gap was set at 0.1 m and the substrates were grounded. The thickness of the deposited films in all cases was in the range of 50–100 nm. More information on the deposition conditions and structure of the deposited samples can be found in [24]. In all cases Raman scattering spectra of the samples were typical for diamond like carbon [24]. Morphology of DLC:Cu nanocomposite films on silicon substrates was investigated employing helium ion microscope (ORION NanoFab, Zeiss), which enables high spatial resolution imaging using focused ion beam. Transmission and reflection spectra of DLC:Cu nanocomposites on quartz substrates were obtained using a stabilized halogen light source (SLS301, ThorLabs), integrating sphere (RTC-060-SF, Labsphere) and high sensitivity spectrometer (Maya 2000Pro, Ocean Optics). The spectra were recorded in a wavelength range of 360–1100 nm (limited by the light source and spectrometer). The absorption (A) was calculated as A = 1 – T – R, where T is the transmission and R is the reflectivity in relative units. High-resolution X-ray photoelectron spectroscopy (XPS) spectra of oxygen, carbon and copper for DLC:Cu films were acquired before and after extra Ar+ surface sputtering for the surface composition analysis and possible chemical bonds detection. Cu atomic concentration estimated after the ion beam etching was used in subsequent analysis of the possible correlation between the optical properties and structure of the nanocomposite films vs composition. A Thermo Scientific ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, East Grinstead, UK) with a monochromatic Al Kα radiation (hν = 1486.6 eV) was used for the XPS measurements. The base pressure in the analytical chamber was lower than 2 × 10−7 Pa. The 40 and 20 eV pass energy values of a hemispherical electron energy analyzer were used for the survey and for the recording of high-resolution spectra, respectively. The energy scale of the system was calibrated with respect to Au 4f7/2, Ag 3d5/2, and Cu 2p3/2 peak positions. The modelling was performed using a dipole equivalent method and Bruggeman theory of effective medium in the wavelength range from 400 to 700 nm [26,27]. 3. Results and discussion 3.1. Experimental: optical properties, structure and composition of DLC:Cu The absorption spectra of DLC:Cu films are characterized by absorption peak that corresponds to the surface plasmon resonance (Fig. 1). In spite of significant changes of the copper concentration (from 21.7 to 63.5 at.%), shape of the absorption spectra in all cases was very similar. Position of the plasmonic peak maximum (noted in the insert of Fig. 1a) was rather similar as well. In all cases the peak was in 594–620 nm range (Fig. 1b). Analysis of morphology of the DLC:Cu nanocomposite films deposited on silicon substrates indicated that average copper nanoparticle size increased with the increase of Cu atomic concentration (see Fig. 2).
3.2. Simulations As a starting point, the first simplified simulations of absorption for 2
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Fig. 1. Absorption spectra of DLC:Cu nanocomposite films containing different copper concentrations (adapted from [24]) (a) and plasmonic peak position vs Cu atomic concentration (b).
Fig. 2. High-resolution XPS C1s spectra of untreated and ion sputtered samples. Fig. 3. High-resolution XPS O1s spectra of untreated and ion sputtered samples.
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Fig. 4. High-resolution XPS Cu2p spectra of untreated and ion sputtered samples.
the Cu nanoparticles without any amorphous carbon matrix were performed. Simulation for the particles of different radii from 10 nm to 50 nm was carried out (see Fig. 6) to study the influence of the size of copper nanoparticles on their absorption spectrum. Fig. 6 indicates that plasmonic peak position shifts to the long-wave region when nanoparticles size increases - from 575 nm (for nanoparticle having radius10 nm) to 775 nm (for nanoparticle with radius 50 nm). Moreover, the absorption peak becomes broader and plasmonic peak intensity decreases with the radius of the nanoparticle. It is known that interaction of copper nanoparticles with air and moisture can cause its oxidation. As a result, the copper oxide layer can be in two semiconducting phases, namely, cupric oxide CuO and cuprous oxide (Cu2O). Copper oxide nanoparticles of sufficient size (200 and 240 nm) are characterized by the peak of plasmonic absorption as it was stated in [8]. However according to our simulations (Fig. 7), it can be seen that the smaller copper oxide nanoparticles do not have plasmonic absorption peak.
Fig. 5. Cu LVV spectra of untreated and ion sputtered samples.
Concerning behavior of Cu nanoparticles in dielectric matrix it should be mentioned that in our previous XPS study of DLC:Cu films, relatively high concentration (13–20 at.%) of oxygen was found in the nanocomposites containing 22–63 at.% of Cu [24]. Analysis of the XPS peaks of nanocomposite films presented in this study clearly revealed presence of the copper oxide related peaks, even after the ion beam cleaning. Thus, one can assume that the copper nanoparticles dispersed in DLC film are oxidized also. Taking into account amount of copper and oxygen in the films, it can be assumed that no pure CuO and Cu2O nanoparticles are present in the studied DLC:Cu film. The influence of the DLC matrix parameters on the optical characteristics of copper nanoparticles was studied assuming that these particles are spherical as it was reported in Spherical nanoparticles were chosen for the calculation, because in ref. [24], where for DLC:Cu
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Fig. 6. Influence of the size of Cu nanoparticles on the position and shape of the absorption peak. Fig. 9. The absorption spectra of DLC films with embedded copper (Cu) nanoparticles with diameter of 20 nm. Optical constants of DLC films simulated in Fig. 8 were used as a DLC matrix: sample n1, k1 (DLC1), n2, k2 (DLC2), n3, k3 (DLC3), n4, k4 (DLC4), n5, k5 (DLC5), n6, k6 (DLC6), n7, k7 (DLC7) and n8, k8 (DLC8).
samples containing about 60 at.% of copper, spherical Cu nanoparticles were observed. The refractive index of copper nanoparticles was calculated according to the analytical formula presented in [38]. It is known that refractive index of DLC reported may vary in a wide range from 1.4 to 2.8 [39]. Thus, the set of different experimentally measured DLC refractive indices presented in ref. [40, 41] were used for calculations. The real and imaginary parts of the DLC refractive indices are presented in Fig. 8. The absorption spectra of spherical copper nanoparticles with a diameter of 20 nm embedded to these 8 different refractive indices DLC matrices were calculated and the results are shown in Fig. 9. One can see that shape of the spectra, amplitude and position of the absorption peak is different in all eight considered cases. Dependences the plasmonic peak position, plasmonic peak intensity and peak width (full width at half maximum - FWHM) on the DLC refractive index are
Fig. 7. The simulated absorption spectra Cu, CuO and Cu2O nanoparticles of diameter 20 nm in air.
Fig. 8. The real (red) and imaginary (blue) parts of the DLC refractive indices. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5
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It is well known, that the absorption spectrum of such nanoshells depends primarily on the thickness of the core and the thickness of the shell, as well as the medium in which they are located. Therefore, at the next step of the study, Fig. 11 presents the dependences of the absorption spectrum for the nanoshells Cu-CuO and the Cu-Cu2O at the constant shell thickness of 2 nm on the core thickness for the nanoshells Cu-CuO and the Cu-Cu2O at the constant shell thickness of 2 nm for embedded to DLC with the refractive index of the DLC represented by Eq. (1)were obtained (see Fig. 12). It can be seen, that absorption spectra of Cu-CuO and Cu-Cu2O nanoshells in DLC are very similar. When the core thickness increases, the absorption peak is shifted to the short-wave region. The absorption peaks for Cu-CuO and Cu-Cu2O nanoshells with the core size of 20 nm are near 598 nm, and with the core size of 50 nm near to the wavelength of 589 nm. It should be noted that this shift of the plasmonic peak in case of Cu-CuO or Cu-Cu2O nanoshells is rather small in comparison to pure Cu nanoparticles (about 20 nm and nearly 200 nm respectively, as shown in Figs. 6 and 11). In the next step of our research, the simulation of the absorption spectra of Cu-CuO and Cu-Cu2O nanoshells embedded into the DLC matrix was done. Fig. 12 presents the simulation results of the dependences of the absorption spectra of the CuO and Cu2O shells embedded in DLC on the thickness of the shell at the constant radius core thickness of the CuO and Cu2O shells, were researched. The simulation results are presented in Fig. 13. One can see that the plasmonic absorption peak redshifts from 600 nm to 647 nm when the shell thicknesses changes from 1 nm to 5 nm. The intensity of absorption decreases, when the shell thickness increases. It should be noted that the full width at half maximum of the absorption spectra increases also when the shell thickness increases. Since, the Cu prone to oxidation at ambient conditions and the dominating product is Cu2O not CuO, oxidation occurs only on the surface of the nanoparticle, the nanostructures such as the core-shell DLC:Cu-Cu2O are formed. Therefore, next our research were carried out only for DLC:Cu-Cu2O. The simulations of optical properties of DLC films with embedded Cu core and Cu2O shell nanoparticles were carried out. The Bruggeman effective medium theory [42] has been used for calculation of the effective refractive index of the diamond like carbon nanocomposite with embedded oxidized Cu nanoparticles:
Table 1 Dependences of the plasmonic peak position, plasmonic peak intensity and FWHM on DLC refractive index (presented in Fig. 9). DLC refractive index
Plasmonic peak position, nm
Plasmonic peak intensity a.u.
Plasmonic peak width, nm
n1, n2, n3, n4, n5, n6, n7, n8,
586.9 564.7 645.4 571.4 585.5 647.3 594.2 580.8
0.00242 0.03506 0.00351 0.78289 0.01812 0.00561 0.06083 0.00284
25.134 7.68 22.342 1.396 9.077 18.152 4.887 22.342
k1 k2 k3 k4 k5 k6 k7 k8
summarized in Table 1. Particularly, plasmonic absorption peak position can be changed by more than 80 nm just by changing the DLC matrix optical properties. Concerning behavior of Cu nanoparticles in dielectric matrix it should be mentioned that in our previous XPS study of DLC:Cu films, relatively high concentration (13–20 at.%) of oxygen was found in the nanocomposites containing 22–63 at.% of Cu [24]. Analysis of the XPS peaks of nanocomposite films presented in this study clearly revealed presence of the copper oxide related peaks, even after the ion beam cleaning. Thus, one can assume that the copper nanoparticles dispersed in DLC film are oxidized also. Taking into account amount of copper and oxygen in the films, it can be assumed that no pure CuO and Cu2O nanoparticles are present in the studied DLC:Cu film. It can be supposed that the core-shell nanoparticles consisting of the core Cu and the CuO or Cu2O shell are formed. The possible structures of these nanoshells are presented in Fig. 10. In the following steps, only one refractive index value of DLC was used for study of oxidation effects. It can be explained by fact that plasmonic peak position in the experimental absorption spectra of copper nanoparticles for the studied films was found in a region from 550 to 650 nm for all the DLC refractive indices considered. The two criteria in the selection of right optical constants of DLC were used. According to the first criterion – the calculated plasmonic Cu peak position should fit within the experimental peak position data defined in our experimental studies. In addition, it should be relatively broad and similar to the experimental ones. According to this criterion, the wider simulated peak is closer to the experimental data than the narrow one. The dependence of the real and imaginary parts of the selected DLC refractive index on the wavelength (λ) are described as follows:
εBG − εh ε − εh =f i , εBG + 2εh εi + 2εh
(2)
where εBG is the effective dielectric permittivity of nanocomposite, εh is the dielectric permittivity of the host matrix, εi is the dielectric permittivity of inclusions. Consider the exactly-solvable case for two-component mixture; the two solutions are available in this case:
n = 1.001 + 5.914λ − 13.517⋅10−5λ2 + 14.071λ3 − 6, 982λ4 + 1.357λ5, k = 0.243 + 1.862λ − 8.155λ2 + 10.660λ3 − 5.503λ4 + 0.906λ5. (1) According to the curves presented in Fig. 10, the chosen refractive index values described by Eq. (1) correspond to the spectra of the sample noted by n1 and k1, and absorption spectrum of Cu nanoparticles with a diameter of 20 nm in DLC matrix with such a refractive index is shown in Fig. 9.
εBG =
b±
8ε1 ε2 + b2 , 4
(3)
where
b = (2f1 − f2 ) ε1 + (2f2 − f1 ) ε2
(4)
where ε1, ε2, f1, f2 are the dielectric permittivity and filling factor of DLC, respectively; ε2, f2 are the effective dielectric permittivity and filling factors of Cu-CuO2 respectively. The effective dielectric permittivity of Cu-CuO2 is calculated using dipole equivalence method [43] as follows:
ε2 = εCuO2
1− Fig. 10. A schematic representation of the structure core-Cu and CuO- or Cu2Oshell.
3 RCu αCu − CuO2 Cu − CuO2 3 RCu αCu − CuO2 3 RCu − CuO2
1 + 2R3
, (5)
where εCuO2 is the dielectric permittivity of shell, RCu and RCu−CuO2 are radii of core and core + shell respectively, αCu−CuO2 is the average 6
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Fig. 11. Dependences of the absorption of Cu-CuO (a) and Cu-Cu2O (b) nanoshells on the radius of the core at the constant shell thickness of 2 nm in the DLC calculated by Eq.(1).
Fig. 12. Dependences of the absorption of Cu-CuO (a) and Cu-Cu2O (b) nanoshells on the thickness of the shell at the constant radius core of 10 nm in the DLC calculated by Eq. (1).
permittivities of such nanoshells were calculated using Eqs. (5), (6). In the next, the effective refractive indices of the three DLC:Cu-Cu2O nanocomposites with copper concentrations 26.2%, 55.4% and 63.5% were simulated using Eqs. (1)–(4). The simulation results are presented in Fig. 13 and the dependences of the absorption coefficient on the wavelength for these samples are presented in Fig. 14. It can be seen from Fig. 13 that absorption spectra are shifted into a short-wave region from 583 nm to 565 nm, when concentration increases from 26.2% to 63.5%. These results are not in good agreement with the experimental ones, since redshift of the absorption peak was detected.
polarization. The average polarization can be expressed as follows:
αCu − CuO2 =
εCu − εCuO2 εCu + 2εCuO2
(6)
Average size of nanoparticles for each value of the concentration of nanoparticles (filling factor) was estimated from Fig. 1b, namely for 21.7% the diameter of Cu-Cu2O nanoshell is of 20 nm, for 26.2% is 26 nm, for 55.4% is 38 nm, for 59.6% is 46 nm, for 62.0% is 52 nm and for 63.5% is 55 nm. It is assumed that the thickness of the oxide layer remains unchanged and is equal to 2 nm. The effective dielectric
Fig. 13. Real and imaginary parts of the DLC:Cu-Cu2O nanocomposite refractive index at different copper concentrations 26.2% (a), 55.4% (b) and 63.5% (c) at constant thickness of shell of 2 nm. 7
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Fig. 14. Absorption spectra of DLC:Cu-Cu2O nanocomposite at different concentrations 26.2% (a), 55.4% (b) and 63.5% (c) at constant thickness (2 nm) of the shell.
Using the obtained effective refractive index spectra of DLC:CuCu2O thin film the corresponding absorption spectra were calculated for different Cu-Cu2O concentration that are presented in Fig. 16. One can see that the absorption peaks related to the surface plasmon resonance are red-shifted with the increase in Cu-Cu2O concentration. The plasmonic peak widths of the simulated absorption and experimentally measured are different. However, the theoretically obtained plasmonic peak positions provide reasonable fit of the experimental curves. Moreover, the intensity of absorption spectra decreases with the increase of concentration of nanoparticles. It can be explained by the fact that in the calculations it was considered that the size of nanoparticles increases when the concentration of filler increases. The increase in the size of nanoparticles leads to a widening of the absorption peak and decrease in its amplitude. In summary it can be supposed that, on one hand, the increase in the size of the nanoparticle causes the shift of absorption peak to the longwave region, but on the other hand, formation of the nanoshells causes the shift of absorption peak to the short-wave region. As a result, the absorption peak is shifted only slightly, even concentration of copper nanoparticles and their size increase substantially.
Some discrepancies between the experimental and simulation results can be explained by the fact that copper nanoparticles were oxidized differently. It is confirmed by the XPS analysis results (see Figs. 4, 5) where it is shown that most of copper is in Cu2O oxide form in the case of the samples with concentrations 62.0, 59.6 and 63.5%. Copper peak prevails over Cu2O for the samples with concentrations 21.7 and 26.2%. In case of the sample with concentration 55.4% the metallic copper and Cu2O peaks are of the similar intensity. Therefore it can be assumed that oxide shell thickness increased with the increase of concentration. Taking into account the results presented in Figs. 11 and 12 and results of XPS analysis the following sizes of nanoparticles have been used for the next simulation: the radius of core was of 7 nm and thickness of the shell was 3 nm for concentration 21.7%, radius of core was of 9 nm and thickness of shell was 4 nm for 26.2%, radius of core was of 11 nm and thickness of shell was 8 nm for 55.4%, radius of core was of 13 nm and thickness of shell is 10 nm for 59.6%, radius of core is of 15 nm and thickness of shell was 11 nm for 62.0% and for 63.5% radius of core was of 15 nm and thickness of shell was 13 nm. The calculated effective refractive indices for all these samples are presented in Fig. 15.
Fig. 15. Real and imaginary parts of the refractive index of DLC:Cu-Cu2O nanocomposite at different concentrations 21.7% (a), 26.2% (b), 55.4% (c), 59.6% (d), 62.0% (e), and 63.5% (f). 8
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Fig. 16. Absorption spectra of DLC:Cu-Cu2O nanocomposite at different concentrations 21.7, 26.2, 55.4, 59.6, 62.0 and 63.5%.
4. Conclusions [2]
The diamond like carbon films with embedded Cu nanoparticles were researched. The effect of oxidation of copper nanoparticles on absorption spectra of DLC:Cu nanocomposites has been studied by combination of the simulation and experiment. The simulations revealed that increase of pure Cu nanoparticle size leads to significant shift plasmonic absorption peak position to longwave region. However, only relatively small shift in the plasmonic absorption peak position and practically constant plasmonic peak intensity in the DLC:Cu film experimental spectra was experimentally observed, despite significant increase in Cu concentration and copper nanoparticle size. Such a discrepancy can be explained by the oxidation of copper nanoparticles. As the Cu prone to oxidation at ambient conditions and the dominating product is Cu2O (and not CuO), oxidation occurs only on the surface of the nanoparticle, the nanostructures such as the core-shell DLC:Cu-Cu2O are formed. Plasmonic peak position of nanoshells also shifts to the long-wave region but it is small in comparison to the one due to Cu nanoparticles. Comparison of the simulated and measured absorption spectra revealed that the nanoparticles were oxidized differently, and oxide shell thickness increased when concentration of nanoparticles in sample increased. Having experimentally measured absorption spectrum of DLC:Cu-Cu2O and the size of the nanoparticles, one can estimate the magnitude of oxidation of copper nanoparticles.
[3]
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Acknowledgements This research was funded by a grant (proposal No. P-MIP-17-243, contract No. S-MIP-17-82) from the Research Council of Lithuania, project title “Plasmonic Carbon Nanocomposite Based Self Saturable Absorber Mirrors for Fiber Lasers”. Financial support of Ministry of Education and Science of Ukraine should be acknowledged (DB/ Fotonika No. 0117U007176, DB/MEV No. 0118U000267).
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