Size control of silver nanoclusters during ion-assisted pulse-plasma deposition of carbon-silver composite thin films

Journal Pre-proof Size control of silver nanoclusters during ion-assisted pulse-plasma deposition of carbon-silver composite thin films O.A. Streletskiy, I.A. Zavidovskiy, O.Yu. Nischak, A.A. Haidarov PII:

S0042-207X(20)30123-8

DOI:

https://doi.org/10.1016/j.vacuum.2020.109286

Reference:

VAC 109286

To appear in:

Vacuum

Received Date: 3 October 2019 Revised Date:

9 February 2020

Accepted Date: 17 February 2020

Please cite this article as: Streletskiy OA, Zavidovskiy IA, Nischak OY, Haidarov AA, Size control of silver nanoclusters during ion-assisted pulse-plasma deposition of carbon-silver composite thin films, Vacuum, https://doi.org/10.1016/j.vacuum.2020.109286. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Size control of silver nanoclusters during ion-assisted pulse-plasma deposition of carbon-silver composite thin films

O.A. Streletskiy1, I.A. Zavidovskiy1, O.Yu. Nischak1, A.A. Haidarov1 1

M.V. Lomonosov Moscow State University, Department of Physics, Leninskie Gory 1, 119991 Moscow, Russia;

Abstract We present an investigation of carbon-silver composite thin films obtained by ionassisted pulse-plasma sputtering of graphite cathode with silver inclusions. Transmission electron microscopy, X-ray photoelectron spectroscopy were used to analyze the films structural properties. It was shown that the films consist of amorphous carbon matrix and silver nanoclusters. The increase of assisting ions energy from 0 eV to 300 eV resulted in decrease of silver nanoclusters average size and their number density increase. Variation of the films extinction spectra with deposition parameters change was studied. Keywords: pulse-plasma sputtering; carbon-silver composite films; silver nanoclusters; optical transmission spectroscopy; X-ray photoelectron spectroscopy; transmission electron spectroscopy

1. Introduction

Carbon-silver composite coatings have recently deserved significant attention because of wide range of their applications including protective and tribological coatings [1, 2], antimicrobial coatings [3, 4], piezoresistive layers [5] and so on. The properties of silver nanoparticles also gave rise to promising surface-enhanced Raman spectroscopy (SERS) technique [6, 7]. Although numerous ways of isolated silver clusters (island

silver films) deposition and adjusting were introduced, the SERS-active platforms still possess limited longevity, because they suffer from the rapid degradation of polycrystalline silver under ambient atmospheric conditions [8]. It is associated with the formation of chemical bonds suppressing the charge transfer between the analyte and silver nanocluster and thereby decreasing the signal enhancement [9]. This effect may be caused by the interaction of silver nanoclusters with atmospheric oxygen or sulfur compounds [10-13]. Sulfur compounds also form silver sulfide layers or bumps affecting nanocluster plasmonic performance. Contamination can be avoided by encapsulation of silver clusters in an inert amorphous matrix [14, 15]. Consequently, carbon-silver nanocomposites applied as SERS coatings are widely studied experimentally and theoretically [16-18].

Carbon-silver films possess tunable optical properties, which depend not only on the amorphous carbon matrix properties, but on the silver cluster size, shape and silver content [17-19]. Electrophysical effects observed in carbon-silver films, such as field emission, piezoresistivity, and resistive switching, are also sensitive to the nanocomposite structure [20,21]. Size of the nanoclusters is known to affect the SERS resonant wavelength [22-25], antimicrobial properties [26], coatings wettability [27], gas sensing and catalytic performance [28]. Therefore, comprehensive analysis of the composite films is required to understand the deposition parameters influence on the samples structure and applicability.

Numerous techniques can be used to obtain carbon-silver coatings: reactive magnetron sputtering [17], plasma-enhanced chemical vapor deposition [19], pulsed laser deposition [29], simultaneous sputtering of graphite and silver targets [30] etc. The detailed review of various DLC-Ag nanocomposites synthesis techniques is provided in [20]. In our work we investigate the carbon-silver films deposited by pulse-plasma

sputtering. This technique has been widely studied recently because of its outstanding prospects: layer-by-layer deposition and metastable carbon phase synthesis possibilities as well as low substrate heating which makes it possible to deposit coatings on various substrate types [31-34]. The possibility of silver nanoclusters encapsulation in amorphous carbon by pulse-plasma technique was shown by Kolpakov et al. in [35]. The work [35] was aimed at the investigation of the deposition speed influence at the aC:Ag films structure.

In the current paper, we present the result of studies of the carbon-silver composite films deposited by pulse-plasma sputtering of graphite cathode with silver inclusions assisted by the beam of Ar+ ions. We show that ion assistance makes it possible to control the size of the silver nanoclusters encapsulated in amorphous carbon matrix.

2. Materials and methods

The analyzed samples were deposited by pulse-plasma sputtering of a graphite cathode with silver inclusions at the discharge power of 1 kW in Ar atmosphere in a highvacuum chamber (see Fig. 1). The cathode was made of MPG-7 graphite with cylindrical silver (99.99%) inserts of 3-5 mm in diameter. The sputtered area of the silver was about 10% of the total cathode area.

Fig. 1. The scheme of the deposition system

Before the deposition, the chamber was pumped down to the residual pressure of 10-4 Pa. Carbon-silver composite films were deposited at the operating pressure of 10-1 Pa. The discharge pulse length was ~1 ms, the pulse frequency equaled 1 Hz. The energy of the assisting ion beam produced by Hall-effect ion source with cold hollow cathode Klan 53-M (Platar Corp.) varied from 100 eV to 300 eV. The detailed description of the ion source parameters can be found at the manufacturer website [36], while the descriptions of the system and its applications are provided elsewhere [37-39]. Ion current remained constant for all of the films deposited with ion assistance. The sample deposited without ion assistance is indicated as “0 eV” in the pictures. NaCl and cover

glass plates were used as substrates. Surface temperature didn’t exceed 50°С during the deposition process.

The thickness of all the samples was nearly 50 nm. Transmission electron microscopy (TEM) and electron diffraction data were obtained by LEO 912 AB transmission electron microscope with Omega energy filter. The operating acceleration voltage was 120 keV. X-ray photoelectron spectra (XPS) were measured with an Axis Ultra DLD (Kratos, UK) spectrometer using monochromatized Al Kα (1486.6 eV) radiation at the operating power of the X-ray tube 150 W. Optical transmission spectra of the deposited films on cover glass substrates were recorded with custom set-up including Xe lamp, monochromator and photomultiplier tube. The obtained spectra were normalized by the subtraction of the spectrum obtained on non-coated cover glass. The transmittance intensity was converted to extinction. 3. Results and discussion 3.1. Transmission electron microscopy TEM revealed the presence of silver nanoclusters in the obtained film structure. The clusters size varied with the assistance energy change (see Fig. 2).

Fig. 2. TEM images of the samples deposited at different ion assistance energies: (a) 0 eV, (b) 100 eV, (c) 200 eV, (d) 300 eV

The size distribution of silver nanoclusters in the films were calculated for different assistance energies using Gwyddion software [40]. These distributions as well as average silver cluster radius, their number density and their relative volume (the ratio of

120

(a)

100

2

(b)

15 10 5 0

(c)

10

Assistance energy, eV 0 eV 100 eV 200 eV 300 eV

60

10

Content (108 1/cm2)

80

Relative volume (%) Number (x10 1/cm ) Average radius (nm)

the silver to the total volume of the film) are shown in Fig. 3.

40

20

0 0

5

10

15

Nanocluster radius (nm)

20

25

8 6 4 2 0

(d)

8 6 4 2 0 0

100

200

300

Assistance energy (eV)

Fig. 3. (a) Silver nanoclusters size distribution, and the dependence of average nanocluster radius (b), number density (dashed line for theoretical dependence) (c) and relative volume of silver clusters (d) on ion assistance energy

The formation of the silver nanoclusters with broad size distribution and ~15 nm average radius was observed for the sample deposited without ion assistance. The reduction of average nanocluster size takes place with ion assistance energy increase. At 300 eV assistance the average nanocluster radius reduces to 2 nm.

Ion assistance is known to have significant influence on the crystallite size of polycrystalline films [41, 42]. Two effects should be taken into consideration to explain silver nanocluster size variation with ion assistance.

Firstly, point defects are formed in the substrate and in the growing film due to ion assistance. They act as favorable nucleation centers for silver nanoclusters [43]. The concentration of point defects V is varied with ion assistance conditions altering. It can be evaluated as

=

(1 +

), where

is a surface defect concentration in the

absence of ion assistance, β is a recombination coefficient, j is assisting ion current density and E is ion beam energy [44]. The ion beam energy increase at constant current and material-depending coefficient led to the silver clusters number density increase, which was proved by TEM measurements shown in Fig. 3(c).

The sputtering yield of silver irradiated by low-energy argon ions is significantly larger than the yield of carbon, and in 100-300 eV range it increases with ion energy increase [45]. The pulsed discharge parameters, such as discharge power and pulse frequency, remained constant, which allows us to assume that silver flux was constant for all of the films. Thus, assisting energy increase led to the decrease of the silver content in the film, because it was sputtered more efficiently than carbon. It is confirmed by relative silver clusters square decrease shown in Fig. 3(d). Consequently, both the increase of nuclei concentration and the silver fraction decrease resulted in the reduction of the average nuclei size.

3.2. X-ray photoelectron spectroscopy The overview XPS spectrum of 200 eV sample is shown in Fig. 4. This spectrum was characteristic for all the deposited films. To obtain elemental analysis of the films,

fitting with CasaXPS software was performed. For the coatings studied in current paper, the atomic composition after 30 days of air exposure was the following: 87-93% carbon, 4-5% - oxygen, 2-5% - silver, 1-3% - nitrogen and a

negligible amount of

other elements.

Intensity

C1s

Ag3d

CKLL AgMNN OKLL

1200

1000

Ag3p O1s N1s Ag3s

800

600

400

200

0

Binding energy, eV Fig. 4. Characteristic overview XPS spectrum

To study the chemical bonds of the sample, peak fitting of C1s, Ag3d, and O1s lines obtained with high resolution was carried out (see Fig. 5). Deconvolution of C1s line shows C-C(sp2) bonds (284.3-284.5 eV peak for different samples) and C-C(sp3) bonds (285.2.-285.4 eV line) [46] corresponding to samples amorphous matrix. The C-N and C-O lines at 286.3 eV indicate the presence of adsorbed gas layer and the ions embedded into the film during deposition process. C-N and C-O lines are practically indistinguishable, because these lines are characterized by similar energies (286.1-286.7 eV) [47-49]. C=O line is located at 288.8 eV [50, 51].

sp2

C1s 3

Intensity

sp

C-O C-N

282

284

286

C=O

288

290

Binding Energy (eV)

Ag3d

Intensity

Ag(0), Ag(I)

Ag(III)

366

368

370

372

374

376

378

Binding Energy (eV)

O1s

Intensity

C=O

AgO C-O

528

530

532

534

536

538

Binding Energy (eV)

Fig. 5. Deconvolution of C1s, A3d, O1s XPS lines

The Ag3d experiences spin-orbital splitting of Ag 3d5/2 and Ag 3d3/2 levels [52]. The line deconvolution shows lines peaked at 368.3-368.4 eV and 374.4-374.5 eV corresponding to Ag(0) and Ag(I) states for Ag 3d5/2 and Ag 3d3/2, respectively. The 369.2 eV and 375.2-375.3 eV lines are characteristic for Ag(III) state [53]. It is not possible to determine of the ratio of Ag oxidation using Ag 3d line because of a small theoretical shift between metallic and oxidized state, as well as the line position sensitivity to silver nanocluster sizes [54].

The O1s fitting shows three lines. The peak located at 530.5 eV is attributed to O-Ag bond [55], while 532.2 eV and 533.5 eV peaks are attributed to O=C and O-C bonds

[56]. The oxidation ratio of Ag atoms, estimated from the relative intensity of O-Ag line, is 8-10%. The amount of sulfur is negligible. It indicates that our samples haven’t undergone sulfidation characteristic for silver nanoparticles deposited by magnetron sputtering cluster apparatus [12] and electron beam lithography [13]. Therefore, oxidation is found to be predominant effect causing the samples degradation. This factor can affect plasmonic properties of the films. 3.3. Electron diffraction

Fig. 6 shows diffraction patterns of the samples. Narrow lines correspond to silver crystallites, silver crystallographic indexes are shown. The intensity of the silver lines decreases with the ion assistance energy increase, indicating the reduction of the silver content discussed in section 3.1. Broad maxima observed for all the samples are attributed to disordered amorphous carbon. The amorphous carbon spectrum [57] is shown for reference. In this spectrum 1.2 Å line is typical for amorphous carbon structures [58], and 2.1 Å line is characteristic for carbon structures with sp3 hybridization [59].

(111)(200)

(220) (311)(222) (400)(331)

Intensity

300 eV 200 eV 100 eV 2.1 Å line

0,2

0,4

0 eV 1.2 Å line

0,6 0,8 1/d (A-1)

amorphous carbon

1,0

1,2

Fig. 6. Electron diffraction of carbon-silver films deposited at different ion assistance energies and amorphous carbon film

3.4. Optical properties

Optical spectra were measured for three films with various sizes of silver nanoclusters. The normalized extinction peaks are shown in Fig. 7.

Relative extinction

Assistance energy / Nanocluster radius 0 eV / 15 nm 100 eV / 4.8 nm 200 eV / 1.9 nm 300 eV / 1.4 nm

300

400

500

600

700

800

Wavelength, nm Fig. 7. Relative extinction spectra of different samples

For the films deposited with simultaneous ion assistance 70 nm, blueshift of the resonance peak was observed. According to TEM data, ion assistance induced nanoclusters shrinking and silver sputtering, and these effects are known to have significant influence on the films optical properties. The plasmonic peak wavelength of films containing Ag clusters decreases with metal clusters shrinking, which is proved by the dipole approximation of Mie theory [20] and experimental results [20-23]. Decrease of the silver content also blueshifts the plasmonic peak [20, 60, 61].

However, the assistance energy variation hasn’t significantly changed the plasmon energy. The influence of several processes accompanying the samples formation has probably affected its position. For example, optical properties of the amorphous carbon matrix are known to change in different deposition conditions [62, 63], affecting the plasmon energy [64]. The clusters deviation from the spherical shape [65, 66], their oxidation [67, 68], the influence of substrate and overcoating layer [69] etc. can also have impact on plasmon wavelength. Therefore, its variation with ion assistance energy

increase can be caused by several reasons which particular role needs further investigation.

4. Conclusions

Carbon-silver composite films deposited by ion-assisted pulse-plasma sputtering of the graphite cathode with silver inclusions were studied in current paper. The ion assistance energy increase lead to the silver nanocluster size and silver content decrease and clusters number density increase. We showed that two processes affected the nanoclusters formation in amorphous carbon matrix. One of them is generation of point defects acting as nucleation centers for silver nanoclusters, and the second one is silver preferential sputtering. The maximum of the films extinction spectra is attributed to plasmon excitation. Silver content and silver nanocluster size decrease caused by ion assistance application are the most important factors affecting the plasmon peak position. Ion assistance application led to the resonant wavelength blueshift from 500 nm to 410 nm. However, assistance energy variation hasn’t significantly changed the plasmon energy.

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Figure captions Fig. 1. The scheme of the deposition system Fig. 2. TEM images of the samples deposited at different ion assistance energies: (a) 0 eV, (b) 100 eV, (c) 200 eV, (d) 300 eV Fig. 3. (a) Silver nanoclusters size distribution, and the dependence of average nanocluster radius (b), number density (dashed line for theoretical dependence) (c) and relative volume of silver clusters (d) on ion assistance energy Fig. 4. Characteristic overview XPS spectrum Fig. 5. Deconvolution of C1s, A3d, O1s XPS lines Fig. 6. Electron diffraction of carbon-silver films deposited at different ion assistance energies and amorphous carbon film Fig. 7. Relative extinction spectra of different samples

Substrate holder

Loader Gas inlet (Ar)

C

+ Ion source

Pulse-arc source

Silver

Vacuum chamber

Graphite cathode with silver inclusions

Graphite H2O cooling

(a)

100

2

80

(b) 15

10

5

0

(c)

10

2

1/cm )

10

Assistance energy, eV 0 eV

Relative volume (%) Number (x10

8

Content (10

1/cm )Average radius (nm)

120

100 eV

60

200 eV 300 eV

40

20

0 0

5

10

15

Nanocluster radius (nm)

20

25

8 6 4 2 0

(d)

8

6

4

2

0 0

100

200

Assistance energy (eV)

300

Intensity

C1s Ag3d CKLL AgMNN OKLL

1200

1000

Ag3p O1s N1s Ag3s

800

600

400

Binding energy, eV

200

0

2

sp

C1s 3

Intensity

sp

C-O C-N

282

284

286

C=O

288

290

Binding Energy (eV)

Ag3d

Intensity

Ag(0), Ag(I)

Ag(III)

366

368

370

372

374

376

378

Binding Energy (eV)

O1s

Intensity

C=O

AgO C-O

528

530

532

534

Binding Energy (eV)

536

538

(111)(200)

(220)

(311)(222)

(400)(331)

Intensity

300 eV

200 eV

100 eV

0 eV

2.1 Å line

1.2 Å line

amorphous carbon

0,2

0,4

0,6

0,8 -1

1/d (A )

1,0

1,2

Assistance energy / Nanocluster radius

Relative extinction

0 eV / 15 nm

300

100 eV / 4.8 nm 200 eV / 1.9 nm 300 eV / 1.4 nm

400

500

600

W avelength, nm

700

800

HIGHLIGHTS 1. Carbon-silver composite films were obtained by ion-assisted pulse-plasma sputtering 2. Ion assistance energy increase led to encapsulated silver nanoclusters size decrease 3. Ion-induced silver sputtering and point defects formation caused cluster shrinking 4. Ion assistance led to structure rearrangement causing plasmon wavelength shift

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: