Magnetron Sputtering of Polymeric Targets: from Thin Films to Heterogeneous Metal/Plasma Polymer Nanoparticles

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Article Magnetron Sputtering of Polymeric Targets: From Thin Films to Heterogeneous Metal/Plasma Polymer Nanoparticles

OndˇrejKylián 1,* , Artem Shelemin 1, Pavel Solaˇr 1, Pavel Pleskunov 1, Daniil Nikitin 1 , Anna Kuzminova 1, Radka Štefaníková 1, Peter Kúš 2, Miroslav Cieslar 3, Jan Hanuš 1, Andrei Choukourov 1 and Hynek Biederman 1 1 Department of Macromolecular Physics, Faculty of Mathematics and Physics, Charles University, V Holešoviˇckách 2, 180 00 Prague 8, Czech Republic 2 Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, V Holešoviˇckách 2, 180 00 Prague 8, Czech Republic 3 Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic * Correspondence: [email protected]

Received: 25 June 2019; Accepted: 23 July 2019; Published: 25 July 2019

Abstract: Magnetron sputtering is a well-known technique that is commonly used for the deposition of thin compact films. However, as was shown in the 1990s, when sputtering is performed at pressures high enough to trigger volume nucleation/condensation of the supersaturated vapor generated by the magnetron, various kinds of nanoparticles may also be produced. This finding gave rise to the rapid development of magnetron-based gas aggregation sources. Such systems were successfully used for the production of single material nanoparticles from metals, metal oxides, and plasma polymers. In addition, the growing interest in multi-component heterogeneous nanoparticles has led to the design of novel systems for the gas-phase synthesis of such nanomaterials, including metal/plasma polymer nanoparticles. In this featured article, we briefly summarized the principles of the basis of gas-phase nanoparticles production and highlighted recent progress made in the field of the fabrication of multi-component nanoparticles. We then introduced a gas aggregation source of plasma polymer nanoparticles that utilized radio frequency magnetron sputtering of a polymeric target with an emphasis on the key features of this kind of source. Finally, we presented and discussed three strategies suitable for the generation of metal/plasma polymer multi-core@shell or core-satellite nanoparticles: the use of composite targets, a multi-magnetron approach, and in-flight coating of plasma polymer nanoparticles by metal.

Keywords: magnetron sputtering; nanoparticles; gas aggregation sources

1. Introduction Sputtering is a deposition process based on the ejection of atoms, molecules, or molecular fragments from a target that is bombarded by energetic particles (mostly ions) and subsequent condensation of emitted particles on adjacent surfaces. Since its discovery in the mid-1800s (for the history of sputtering, please refer to J.E. Green’s excellent recent review [1]), sputtering has become one of the most widely used techniques for film deposition, with thickness reaching from several nm to several µm. Despite the long history of sputter deposition, the introduction of external magnetic fields was a crucial moment that led to a massive spread of sputtering technology. Specifically, a configured magnetic field constrains the plasma to a close proximity to the cathode and enormously increases the deposition rate. Among the different configurations that are altogether termed as “magnetrons”,

Materials 2019, 12, 2366; doi:10.3390/ma12152366 www.mdpi.com/journal/materials Materials 2019, 12, 2366 2 of 16 systems with a planar configuration appear to be the most important [2]. Since their introduction, such planar magnetrons have become irreplaceable tools for the deposition of various conductive, mostly metallic, thin films. In addition, the demand for the production of coatings with enhanced functional properties and deposition of non-metallic/non-conductive thin films triggered the development of novel concepts of magnetron sputtering. These include high-power pulsed magnetron sputtering [3–7], dual-magnetron sputtering [8], and radio frequency (RF) magnetron sputtering of non-conductive targets [9]. Concerning the latter, RF magnetron sputtering was found to be suitable not only for the production of inorganic materials (e.g., glasses, metal-oxides, and nitrides) but also for the deposition of polymer-like coatings [10–12], i.e., the so-called plasma polymers [13–16]. As opposed to conventional polymers, such materials are characterized by considerably higher levels of cross-linking and branching, as well as by an absence of regularly repeating monomer units. Despite their random and inherently complex structure, plasma polymers appear to be a highly valuable class of materials for different applications, including dielectric separation layers, permeation barriers or gas separation membranes, laser facilities, adhesion-promoting coating, and films that enable the fine tuning of wettability and the bio-adhesive/bio-repellent behavior of surfaces [17–38]. The great advantage of RF magnetron sputtering over commonly used plasma-enhanced chemical vapor deposition is the complete lack of gaseous or liquid precursors, which makes RF sputtering a “green” technology. Although much attention has been devoted to the fabrication and characterization of fluorocarbon plasma polymers [10–12,39–47], other polymers were also studied, including polyarylates [48], polyimides [45,49–52], polyethylene [50,53,54], polyetherimide [55], polypropylene [56,57], and Nylon [58]. Regardless of the sputtered material, magnetron-based deposition was primarily applied for the production of thin compact films for a long time. The situation changed in the 1990s, when Haberland and his co-workers introduced the first magnetron-based Gas Aggregation cluster Source (GAS) [59,60], which opened a completely new and highly attractive application field for magnetron sputtering technology. In this type of source, magnetron sputtering serves as a supply of supersaturated vapors that spontaneously nucleate and form clusters or nanoparticles (NPs) in the volume of the aggregation chamber at appropriate conditions (higher pressure). NPs are subsequently transferred by a carrier gas (typically argon) through a small aperture from the aggregation chamber of the GAS to the main deposition chamber, where they are collected on substrates. This deposition strategy offers several key benefits as compared to other methods used for the production of NPs—high purity, possibility to tailor kinetic energy and size distribution of produced NPs, directionality of the deposition process, which is suitable for the production of patterned surfaces, as well as the possibility to deposit NPs on the substrates of virtually any material that is compatible with high vacuum conditions. Furthermore, gas aggregation cluster sources can be easily combined with other vacuum-based deposition techniques and, hence, nanocomposite coatings with different architectures can be fabricated. For instance, our group recently reported on the fabrication of Ag/a-C:H and Cu/a-C:H nanocomposites with metallic NPs randomly distributed in the a-C:H matrix [61,62], metal/plasma polymer sandwich structures [63], gradient coatings [64,65], and multi-layered metal/plasma polymer nanocomposites [66–68]. In analogy to “conventional” magnetron sputtering, magnetron-based GAS systems were initially employed generally for the production of various metallic (e.g., Ag [69–71], Cu [72,73], Al [74], Ti [75–77], Co [78,79], Pt [80,81], Nb [82], Pd [83], W [84], Ni [85], Ru [86]) and metal-oxide NPs [87,88]. However, our group recently showed that GAS systems may also be easily adapted for the production of plasma polymer NPs if the polymeric target is sputtered in the RF mode [89–93]. Lately, the interest in the production and utilization of heterogeneous multi-component NPs led to the development of three principal approaches:

(1) Use of bi-metallic composite targets. In this case, the targets composed of two metals were sputtered, which gave rise, depending on the operational conditions, to the formation of Materials 2019, 12, 2366 3 of 16

heterogeneous NPs with different structures (core@shell, onion-like structure, dumbbell-like structure) [94–96]. (2) Multi-magnetron approach. Up to three individual planar magnetrons were placed into a single aggregation chamber. Depending on the mutual position of the magnetrons, the applied magnetron currents and the sputtered materials, alloy, core-satellite, Janus-like, core@shell or core@shell@shell NPs were produced [97–101]. (3) In-flight coating/modification of NPs. In this case, NPs produced by GAS were modified/coated in-flight in an auxiliary chamber located in between the GAS and the substrate. This method was reported to be effective for oxidation of the surface layer of metallic NPs [102], production of core@shell NPs [103–106], and NPs decorated by other materials (so-called strawberry-like or core-satellite structures) [107].

The above-mentioned strategies were successfully tested by different research groups. However, the majority of the research published to date focused on the production of inorganic multi-component NPs. The aim of this featured article is to demonstrate that all three strategies are also applicable for the production of metal/plasma polymer NPs. To meet this general aim, all the examples involved sputtered Nylon (C:H:N:O) particles only. Nylon was selected on the basis of previous studies of sputter deposition of C:H:N:O plasma polymer thin films, which revealed that such materials are suitable for various bio-medical applications as they can enhance the adhesion and attachment of biomolecules or cells [108,109], and can be utilized for the design of systems for controlled drug-delivery [110]. The article is organized as follows: Section2 briefly presents the deposition setups; Section 3.1 presents the main features for the production of C:H:N:O NPs by the GAS source with the magnetron equipped with the Nylon 6,6 target; Section 3.2, Section 3.3, and Section 3.4 present the first results that were obtained by the use of the composite target, the dual-magnetron system, and the deposition setup for the in-flight deposition of silver onto C:H:N:O NPs; and finally, the results are briefly summarized in Section4.

2. Materials and Methods The system schematically depicted in Figure1a was used for the deposition of plasma polymer C:H:N:O nanoparticles and heterogeneous metal/plasma polymer particles. It was based on a planar water-cooled RF magnetron that was inserted into a water-cooled gas aggregation chamber (102 mm inner diameter). The magnetron was equipped either by the Nylon 6,6 target (Goodfellow, 81 mm in dimeter, 3 mm thick) or by the same target with a strip of a Cu plate (Figure1b). The magnetron was powered by an RF power supply (Dressler, Cesar 133) through an automatic matching network (ADTEC, AMV-1000-EN). If not specified elsewhere in the text, Ar was used as the working gas. The aggregation chamber was terminated by a conical lid with a circular orifice 2.5 mm in diameter. The orifice separated the aggregation chamber from the rest of the deposition system. The deposition rate was measured by a quartz crystal microbalance located in the main deposition chamber. Materials 2019, 12, 2366 4 of 16 Materials 2019, 12, x FOR PEER REVIEW 4 of 16

Figure 1. (a) Basic system for the deposition of plasma polymer nanoparticles (NPs). (b) Image of Figure 1. (a) Basic system for the deposition of plasma polymer nanoparticles (NPs). (b) Image of Nylon 6,6 with Cu strip. (c) Dual-magnetron system. (d) Setup for in-flight modification of C:H:N:O Nylon 6,6 with Cu strip. (c) Dual-magnetron system. (d) Setup for in-flight modification of C:H:N:O NPs by silver. NPs by silver. In addition to the basic configuration, two modifications were tested as well. In the first case, 3. Results a second magnetron equipped with an Ag target (Safina, 3 inch in diameter, with a thickness of 3 mm) was installed into the aggregation chamber perpendicularly to the one equipped with the 3.1. Gas-Phase Fabrication of C:H:N:O Nanoparticles Nylon 6,6 target (Figure1c). This additional magnetron was operated in a direct current (DC) mode (AdvancedThe first energy, step in MDX this study 500). Thewas second to investigat configuratione the properties was used of for C:H:N:O the in-flight NPs coatingproduced of C:H:N:Oby the GASnanoparticles system equipped by silver. with In this the case, Nylon an additional 6,6 target. chamber It was wasfound introduced that the betweenkey parameter the GAS for and the the productionmain deposition of C:H:N:O chamber NPs (Figure was the1d). pressure This part in ofthe the aggregation deposition chamber. system consisted For low ofpressures, 3 inch planar the molecularmagnetron fragments installed emitted perpendicularly from the toNylon the direction target predominantly of the beam of condensed the C:H:N:O on nanoparticlesthe walls of the and aggregationwas ended chamber, by an orifice where (3 mm they in formed diameter). a thin compact film, and no NPs were detected in the main depositionThe morphologychamber. The of formation the produced of thin particles film in was the evaluated aggregation by meanschamber of was a scanning confirmed electron by ellipsometricmicroscopy (SEM,measurements MIRA 3 Tescan, of the Brno,coatings Czech deposite Republic)d on or Si a wafers transmission that were electron introduced microscopy into (TEM, the aggregationJEOL2200FS, chamber Akishima, of the Japan). GAS The at a optical distance properties of approximately of the fabricated 50 mm nanoparticles from the magnetron were determined target. Asby the UV-Vis pressure spectrophotometry in the aggregation (Hitachi chamber U-2910, increased, Tokyo, Japan) the deposition in the spectral rate range of the 325 C:H:N:O nm–800 film nm. gradually decreased and at about 100 Pa, the deposition rate of the C:H:N:O film inside the aggregation3. Results chamber approached zero (Figure 2a). At this moment, the C:H:N:O NPs became detectable by the quartz crystal microbalance (QCM) that was inserted into the main deposition 3.1. Gas-Phase Fabrication of C:H:N:O Nanoparticles chamber. This suggests that starting at a pressure of 100 Pa, the inter-molecular collisions prevented the out-diffusionThe first step of inthe this sputtered study was fragments to investigate away thefrom properties the plasma of C:H:N:Oand forced NPs them produced to recombine by the GAS in thesystem volume equipped of the discharge with the Nylon that gave 6,6 target. rise to It th wase formation found that of the the key plasma parameter polymer for theNPs production [93]. Such of formedC:H:N:O NPs NPs were was afterwards the pressure transported in the aggregation by the chamber. flow of Forthe lowcarrier pressures, gas to the the molecular main deposition fragments chamberemitted and from detected the Nylon by target the QCM. predominantly A further condensedincrease of on the the pressure walls of thesubsequently aggregation led chamber, to an increasedwhere they deposition formed arate thin of compactthe C:H:N:O film, NPs. and noHowever, NPs were as detectedcan be seen in thein Figure main deposition2b, the deposition chamber. rateThe was formation found to of not thin be film temporally in the aggregation stable—instead chamber of a was linear confirmed rise of the by frequency ellipsometric shift measurements of the QCM (theof theshift coatings in the resonant deposited frequency on Si wafers is directly that were proportional introduced to the into mass the aggregationdeposited) with chamber the deposition of the GAS time,at a the distance NPs arrived of approximately to the crystal 50 in mm periodically from the magnetron repeated pulses. target. The As thefrequency pressure of inthese the deposition aggregation burstschamber was increased,approximately the deposition 1 per minute. rate ofSuch the C:H:N:Obehavior filmis well-known gradually decreasedin the field and of atdusty-plasma about 100 Pa, andthe is deposition connected rate to the of the charging C:H:N:O of filmgrowing inside NPs, the the aggregation confinement chamber of negatively approached charged zero (FigureNPs in2 aa). plasmaAt this potential, moment, and the C:H:N:Otheir subsequent NPs became release detectable from the by plasma the quartz bulk crystalas soon microbalance as they reach (QCM) a critical that sizewas [111]. inserted The mono-dispersity into the main deposition of the produced chamber. NPs This (see suggests Figure 3) that suggests starting that at all a pressure the NPs ofreached 100 Pa, the critical size at the same time. Furthermore, the critical size of the NPs was highly sensitive to the operational parameters (plasma density, energy of charged species, gas flow, gas temperature,

Materials 2019, 12, 2366 5 of 16

the inter-molecular collisions prevented the out-diffusion of the sputtered fragments away from the plasma and forced them to recombine in the volume of the discharge that gave rise to the formation of the plasma polymer NPs [93]. Such formed NPs were afterwards transported by the flow of the carrier gas to the main deposition chamber and detected by the QCM. A further increase of the pressure subsequently led to an increased deposition rate of the C:H:N:O NPs. However, as can be seen in Figure2b, the deposition rate was found to not be temporally stable—instead of a linear rise of the frequency shift of the QCM (the shift in the resonant frequency is directly proportional to the mass deposited) with the deposition time, the NPs arrived to the crystal in periodically repeated pulses. The frequency of these deposition bursts was approximately 1 per minute. Such behavior is well-known in the field of dusty-plasma and is connected to the charging of growing NPs, the confinement of negatively charged NPs in a plasma potential, and their subsequent release from the plasma bulk as soon as they reach a critical size [111]. The mono-dispersity of the produced NPs (see Figure3) suggests that all the NPs reached the critical size at the same time. Furthermore, the critical size of the Materials 2019, 12, x FOR PEER REVIEW 5 of 16 NPs was highly sensitive to the operational parameters (plasma density, energy of charged species, gas workingflow, gas gas, temperature, etc.) and therefore working the gas, size etc.) distribution and therefore of the the NPs size distributioncan be finely of tuned the NPs in a can relatively be finely widetuned range in a by relatively adjusting wide these range parameters by adjusting [89,93]. these An parameters example of [ 89this,93 behavior]. An example is presented of this behaviorin Figure is 3,presented where the in NPs Figure produced3, where using the NPseither produced argon or using nitrogen either are argon compared. or nitrogen are compared.

20 25 200 Film growth NPs production

Deposition bursts 20 15 150

10 100 [Hz] 10 [Hz/min]

chamber [nm/min] 122 Pa 5 50 5 100 Pa Deposition rate inside the aggregation Effective deposition rate of C:H:N:O NPs

75 Pa 9 Pa 0 0 Shift in frequency of the quartz crystal microbalance 0 0 50 100 150 0 100 200 300 400 Pressure inside the Time [s] aggregation chamber [Pa] a) b)

Figure 2. (a) Pressure dependences of the deposition rate of C:H:N:O film inside the aggregation Figure 2. (a) Pressure dependences of the deposition rate of C:H:N:O film inside the aggregation chamber and effective deposition rate of C:H:N:O NPs in the main deposition chamber. (b) Pressure chamber and effective deposition rate of C:H:N:O NPs in the main deposition chamber. (b) Pressure dependences of frequency shift on quartz crystal microbalance (QCM) installed into the main deposition dependences of frequency shift on quartz crystal microbalance (QCM) installed into the main chamber. The change in the frequency of quartz crystal is directly proportional to the deposited mass. deposition chamber. The change in the frequency of quartz crystal is directly proportional to the RF power 40 W. deposited mass. RF power 40 W. Naturally, the variation of the working gas led not only to the variation of the mean size of the Naturally, the variation of the working gas led not only to the variation of the mean size of the produced NPs but also to the alteration of their chemical composition, in analogy to the deposition of produced NPs but also to the alteration of their chemical composition, in analogy to the deposition thin films by the RF magnetron sputtering of Nylon [108]. For instance, the substitution of argon by of thin films by the RF magnetron sputtering of Nylon [108]. For instance, the substitution of argon nitrogen resulted in the formation of nitrogen-rich NPs. This is demonstrated in Figure4, where the by nitrogen resulted in the formation of nitrogen-rich NPs. This is demonstrated in Figure 4, where high-resolution C 1s X-Ray Photoelectron Spectroscopy (XPS) spectra and Fourier Transform Infrared the high-resolution C 1s X-Ray Photoelectron Spectroscopy (XPS) spectra and Fourier Transform Infrared Spectroscopy (FT-IR) spectra are compared as measured on the NPs deposited with either argon or nitrogen and in Table 1, where elemental composition of C:H:N:O NPs is presented. Evidently, the substitution of argon by nitrogen caused a substantial decrease of the fraction of C- C/C-H chemical bonds that was accompanied by the significant increase of the number of nitrogen- containing moieties. This is a very important and valuable feature as it allows for the production of NPs not only with different sizes but with altered chemical compositions and related functionalities.

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Spectroscopy (FT-IR) spectra are compared as measured on the NPs deposited with either argon or nitrogen and in Table1, where elemental composition of C:H:N:O NPs is presented. Evidently, the substitution of argon by nitrogen caused a substantial decrease of the fraction of C-C/C-H chemical bonds that was accompanied by the signiﬁcant increase of the number of nitrogen-containing moieties. Materials 2019, 12, x FOR PEER REVIEW 6 of 16 This is a very important and valuable feature as it allows for the production of NPs not only with diﬀerentMaterials sizes 2019 but, 12 with, x FOR altered PEER REVIEW chemical compositions and related functionalities. 6 of 16

Figure 3. Scanning electron microscopy (SEM) images of C:H:N:O NPs deposited in (a) pure Ar or (b) NFigure2. The presented 3. Scanning mean electronsizes of produced microscopy NPs were (SEM) evaluated images from of C:H:N:O the diameters NPs of deposited 300 individual in ( a) pure Ar NPs. Figure 3. Scanning electron microscopy (SEM) images of C:H:N:O NPs deposited in (a) pure Ar or (b) or (b)N . The presented mean sizes of produced NPs were evaluated from the diameters of 300 N22. The presented mean sizes of produced NPs were evaluated from the diameters of 300 individual individualNPs. NPs.

Argon Argon

C-O/C-N C-C Argon Argon C-H C=C, C=O, C=N C-O/C-N C-C OH, NH C=O/N-C=O CHx C-H C=C, C=O, C=N

CHx

Intensity [a.u.] Intensity [a.u.] Intensity OH, NH C=O/N-C=O CHx

CHx

Intensity [a.u.] Intensity O-C=O [a.u.] Intensity C≡N C≡C

O-C=O C≡N 294 292 290 288 286 284 282 280 4000 3500 3000 2500 2000 1500 1000 500 C≡C Binding energy [eV] Wavenumber [cm-1]

294 292 290 288 286 284 282 280 4000 3500 3000 2500 2000 1500 1000 500 Binding energy [eV] Nitrogen Wavenumber [cm-1] Nitrogen C-O/C-N

Nitrogen Nitrogen C-O/C-N

C=C, C=O, C=N C=O/N-C=O C-C C-H O-C=O CH Intensity [a.u.] Intensity [a.u.] Intensity x C=C, C=O, C=N C=O/N-C=O C-C OH, NH C-H O-C=O CH C≡N x CH Intensity [a.u.] Intensity [a.u.] Intensity OH, NH C≡C x

≡ CHx C N ≡ 294 292 290 288 286 284 282 280 4000 3500 3000 2500 2000 1500C C 1000 500 Binding energy [eV] Wavenumber [cm-1] a) b) 294 292 290 288 286 284 282 280 4000 3500 3000 2500 2000 1500 1000 500 Binding energy [eV] Wavenumber [cm-1] Figure 4. (a) High-resolution X-Ray Photoelectron Spectroscopy (XPS) spectra of C 1s peak of NPsb) Figure 4. (a) High-resolution X-Ray Photoelectron Spectroscopya) (XPS) spectra of C 1s peak of NPs deposited in Ar (top) and N (bottom) measured by XPS (Phoibos 100, Specs) with an Al Kα X-ray source deposited in Ar (top) and N2 (bottom)2 measured by XPS (Phoibos 100, Specs) with an Al Kα X-ray source(1486.6 (1486,6Figure eV, 200W, eV, 4. (200W,a) Specs). High-resolution Specs). (b) Fourier(b) Fourier X-Ray Transform Transform Photoelectron Infrared Infrared Spectroscopy Spectroscopy Spectroscopy (XPS) (FT-IR)(FT-IR) spectra spectra of C 1sof of NPspeak NPs of deposited NPs deposited in Ar (top) and N2 (bottom) measured by XPS (Phoibos 100, Specs) with an Al Kα X-ray depositedin pure Arin pure (top) Ar and (top) N 2and(bottom) N2 (bottom) recorded recorded by FT-IR by FT-IR (Bruker (Bruker Equinox Equinox 55) 55) in in a a reﬂectance-absorbance reflectance- absorbancemode usingsource mode gold-plated(1486,6 using eV, gold-plated 200W, silicon Specs). waferssilicon (b )wafers asFourier substrates. as Transform substrates. Infrared Spectroscopy (FT-IR) spectra of NPs deposited in pure Ar (top) and N2 (bottom) recorded by FT-IR (Bruker Equinox 55) in a reflectance- Table 1.absorbance Elemental modecomposition using gold-plated of C:H:N:O silicon NPs wafers deposi asted substrates. using Ar or nitrogen and relative contributions of different bond types, resulting from spectral de-convolution of C 1s peak. Table 1. Elemental composition of C:H:N:O NPs deposited using Ar or nitrogen and relative Workingcontributions Gas Oof different C bond types, N resultin C-C/C-Hg from C-O/C-Nspectral de-convolution C=O/N-C=O of C 1sO-C= peak.O

Working Gas O C N C-C/C-H C-O/C-N C=O/N-C=O O-C=O MaterialsMaterials2019 2019, ,12 12,, 2366 x FOR PEER REVIEW 7 ofof 1616

[at.%] [at. %] [at. %] [%] [%] [%] [%] Table 1. Elemental composition of C:H:N:O NPs deposited using Ar or nitrogen and relative Ar 12 76 12 55 37 6 2 contributions of different bond types, resulting from spectral de-convolution of C 1s peak. N2 6 64 30 23 62 12 2 Working O C N C-C/C-H C-O/C-N C=O/N-C=O O-C=O 3.2. CompositeGas Nylon/Cu[at.%] Target[at. %] [at. %] [%] [%] [%] [%] TheAr first strategy 12 tested with 76 the aim 12 to produce 55 heterogeneous 37 metal/C:H:N:O 6 NPs was 2 based N 6 64 30 23 62 12 2 on the utilization2 of a Cu/Nylon composite target. It was found out that the introduction of the Cu strip onto the Nylon 6,6 target resulted in the formation of composite NPs at a relatively high 3.2.pressure, Composite which Nylon assured/Cu Target the stable production of bare C:H:N:O NPs. However, the supplied RF powerThe had first to strategy be increased tested up with to 80 the W aim to toprovide produce a sufficient heterogeneous supply metal of copper./C:H:N:O As shown NPs was in basedFigure on5a, the NPs utilization produced of in a Cuthis/Nylon way had composite a rather target. complicated It was structure—the found out that NPs the introduction were composed of the of CuCu stripparticles onto with the Nylon different 6,6 targetsizes resulted(from several in the nm formation up to almost of composite 50 nm) NPs that at were a relatively all embedded high pressure, into a whichplasma assured polymer the matrix. stable production Such a structure of bare with C:H:N:O multiple NPs. Cu However, cores enveloped the supplied by a RF shell power of the had plasma to be increasedpolymer upresembles to 80 W tothe provide structure a su ffiof cientNPs supplyformed of wh copper.en the As metallic shown intarget Figure was5a, NPssputtered produced in inthe thisAr/hexamethyldisiloxane way had a rather complicated mixture structure—the[112]. It is assumed NPs werethat the composed multi-core@shell of Cu particles NPs originated with different from sizesthe competing (from several growth nm up toof almostmetallic 50 nm)and thatplasma were allpolymeric embedded NPs, into plasma a plasma polymerization, polymer matrix. phase Such asegregation structure with of multiplethe metal Cu and cores plasma enveloped polymer, by a shell and of thesubsequent plasma polymer coalescence resembles of the the produced structure ofheterogeneous NPs formed whenNPs. the metallic target was sputtered in the Ar/hexamethyldisiloxane mixture [112]. It is assumedThe presence that theof metallic multi-core@shell NPs was also NPs evidence originatedd by from UltraViolet-Visible the competing growth (UV-Vis) of metallicspectroscopy and plasmathat showed polymeric a localized NPs, plasma surfacepolymerization, plasmon resonance phase (LSPR) segregation peak at ofabout the metal600 nm, and which plasma is typical polymer, for andCu NPs subsequent embedded coalescence in plasma-sputtered of the produced nylon heterogeneous [66] (Figure 5b). NPs.

Figure 5. (a) Transmission electron microscopy (TEM) image of multi-core@shell Cu/C:H:N:O NPs producedFigure 5. when(a) Transmission the composite electron target wasmicroscopy used. (b )(TEM) UV-Vis image spectra of ofmulti-core@shell C:H:N:O NPs and Cu/C:H:N:O heterogeneous NPs Cuproduced/ C:H:N:O when NPs. RFthe powercomposite 80 W. target was used. (b) UV-Vis spectra of C:H:N:O NPs and heterogeneous Cu/ C:H:N:O NPs. RF power 80 W. The presence of metallic NPs was also evidenced by UltraViolet-Visible (UV-Vis) spectroscopy that showedHowever, a localized the deposition surface process plasmon was resonance found to (LSPR) be very peak unstable. at about After 600 switching nm, which on is the typical plasma, for Cuthe NPs deposition embedded rate in initially plasma-sputtered fluctuated, nylonwith [a66 frequency] (Figure5 b).close to the one observed for the case in whichHowever, only the the Nylon deposition 6,6 target process was used. was This found chan to beged very after unstable. several minutes After switching of the plasma on the operation plasma, thewhen deposition the production rate initially of heterogeneous ﬂuctuated, with Cu/C:H: a frequencyN:O closeNPs todramatically the one observed decreased for the and case eventually in which onlycompletely the Nylon stopped 6,6 target (Figure was used.6a). The This aforemention changed aftered several temporal minutes evolution of the plasmaof the operationdeposition when rate 2 +− 2 + thefollowed production the same of heterogeneous trend as the Cuintensities/C:H:N:O of NPs Ar dramatically(750 nm) and decreased CN (B Σ andX eventuallyΣ at 388 nm) completely spectral stoppedemission (Figure lines and6a). bands. The aforementionedThe most noteworthy temporal was the evolution substantial of the decrease deposition of the rate intensities followed of these the samespectral trend systems as the after intensities approximately of Ar (750 3 nm) min and of the CN plasma (B2Σ+ Xoperation,2Σ+ at 388 i.e., nm) at spectral the time emission at which lines the − andproduction bands. of The the most NPs noteworthystarted to decrease was the rapidly. substantial In contrast, decrease the of intensity the intensities of Cu spectral of these lines spectral was systemsfound to after significantly approximately increase 3 min 3 min of theafter plasma the plasma operation, ignition, i.e., which at the timesuggests at which enhanced the production sputtering ofof thecopper NPs at started the later to stages decrease of the rapidly. plasma In operat contrast,ion. The the enhanced intensity ofsputtering Cu spectral of Cu lines was was accompanied found to by a partial re-deposition of Cu back onto the target that limited sputtering of its polymeric part.

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Materialssignificantly 2019, 12 increase, x FOR PEER 3 min REVIEW after the plasma ignition, which suggests enhanced sputtering of copper8 of 16 at the later stages of the plasma operation. The enhanced sputtering of Cu was accompanied by a Indeed, the substantial re-deposition of copper was confirmed by a visual inspection of the target partial re-deposition of Cu back onto the target that limited sputtering of its polymeric part. Indeed, after the magnetron operation, which showed that almost all the target’s surface had been covered the substantial re-deposition of copper was confirmed by a visual inspection of the target after the by copper (see Figure 6b). Because of this, the target had to be dismounted and cleaned in order to magnetron operation, which showed that almost all the target’s surface had been covered by copper restart the production of Cu/C:H:N:O NPs, which limits the applicability of this deposition strategy. (see Figure6b). Because of this, the target had to be dismounted and cleaned in order to restart the Furthermore, the high power necessary for the efficient sputtering of copper resulted in the production of Cu/C:H:N:O NPs, which limits the applicability of this deposition strategy. Furthermore, substantial heating of the system at longer plasma durations (The temperature of the target the high power necessary for the efficient sputtering of copper resulted in the substantial heating of the approached 150oC after 5 min of the plasma operation as measured by IR-thermocamera). The system at longer plasma durations (The temperature of the target approached 150 ◦C after 5 min of the elevated temperature subsequently hindered the nucleation of NPs and thus no NPs were formed or plasma operation as measured by IR-thermocamera). The elevated temperature subsequently hindered detected after the prolonged plasma operation. the nucleation of NPs and thus no NPs were formed or detected after the prolonged plasma operation.

Figure 6. (a) Time evolution of intensities of spectral emission lines and bands of Ar, Cu, and CN Figuremeasured 6. (a by) Time emission evolution spectrometer of intensities (AvaSpec of spectral 3648, Avantes)emission togetherlines and with bands the of deposition Ar, Cu, and rate CN of measuredproduced NPs.by emission (b) Photography spectrometer of Nylon (AvaSpec/Cu composite 3648, Avantes) target aftertogether the plasmawith the operation. deposition RF rate power of produced80 W. NPs. (b) Photography of Nylon/Cu composite target after the plasma operation. RF power 80 W. 3.3. System with Two Independent Magnetron 3.3. SystemIn order with to Two avoid Independent the issues Magnetron connected with the gradual covering of the Nylon part of the target by theIn order metal to re-deposit, avoid the twoissues independent connected with magnetrons the gradual were covering installed of inthe the Nylon aggregation part of the chamber target by(Figure the metal1c). In re-deposit, this situation, two bothindependent the high magnetro pressurens and were the installed direction in of the the aggregation gas ﬂow limited chamber the (Figurecontamination 1c). In ofthis the situation, Nylon target both by the the high metallic pressure layer (silverand the in thisdirection case). of As the a result, gas flow the production limited the of contaminationthe NPs became of temporally the Nylon stable.target by Nevertheless, the metallic thelayer synthetized (silver in this NPs case). also exhibited As a result, a multi-core@shell the production ofstructure the NPs (Figure became7) similar temporally to the onestable. observed Neverthele whenss, the the composite synthetized target NPs was also used exhibited (Figure5). a Suchmulti- a core@shellﬁnding was structure expected, (Figure as both 7) the similar plasma to polymerization the one observed and thewhen growth the ofcomposite the NPs weretarget also was running used (Figuresimultaneously 5). Such ina finding this case. was expected, as both the plasma polymerization and the growth of the NPs were also running simultaneously in this case.

Materials 2019, 12, x FOR PEER REVIEW 9 of 16 Materials 2019, 12, 2366 9 of 16

Figure 7. (a) TEM image of NPs produced when the dual-magnetron Gas Aggregation cluster Source (GAS)Figure system 7. (a) TEM was used.image ( bof) UV-VisNPs produced spectra ofwhen produced the dual NPs-magnetron at different Gas DC Aggregation magnetron currentscluster Source used for(GAS) silver system sputtering. was used. RF power (b) UV-Vis 40 W. spectra of produced NPs at different DC magnetron currents used for silver sputtering. RF power 40 W. In contrast to the setup with the composite Cu/Nylon target, the use of two individual magnetrons offered—besidesIn contrast ato better the setup stability with of NPsthe composite production—higher Cu/Nylon flexibility target, the in termsuse of of two the producedindividual materials,magnetrons as offered—besides it allowed for the a independentbetter stability control of NPs of production—higher the sputtering rates flexibility of polymer in terms and metal. of the Inproduced other words, materials, decoupling as it allowed the sputtering for the independen of Nylont control and metal of the made sputtering it possible rates to of regulatepolymer theand metalmetal./plasma In other polymer words, decoupling ratio in the the produced sputtering NPs. of ThisNylon eff andectis metal demonstrated made it possible in Figure to regulate7b, where the UV-Vismetal/plasma spectra polymer are presented ratio in for th thee produced samples thatNPs. were This preparedeffect is demonstrated at a constant RFin powerFigure of7b, Nylon where sputtering,UV-Vis spectra but at are di ffpresentederent DC for magnetron the samples currents that forwere the prepared sputtering at a of constant silver. For RF thepower low of DC, Nylon the UV-Vissputtering, spectra but had at different a shape similarDC magnetron to the spectra currents of thefor C:H:N:Othe sputtering NPs, withof silver. only For a weak the low LSPR DC, peak the ofUV-Vis silver locatedspectra approximatelyhad a shape similar at 450 to nm. the spectra With the of increasing the C:H:N:O DC NPs, magnetron with only current, a weak i.e., LSPR with peak the increasingof silver located amount approximately of sputtered silver, at 450 the nm. intensity With th ofe theincreasing LSPR peak DC alsomagnetron increased, current, which i.e., reflects with the the higherincreasing number amount of Ag of NPssputtered embedded silver, into the theintensity C:H:N:O of the matrix. LSPR In peak addition, also increased, the width which of the reflects silver LSPRthe higher peak number dramatically of Ag broadened NPs embedded dueto into the the wide C:H: sizeN:O distribution matrix. In addition, of the silver the width inclusions of the in silver the “nanocomposite”LSPR peak dramatically NPs. Finally, broadened for the due magnetron to the wide current size of distribution 500 mA, the of UV-Vis the silver spectrum inclusions resembled in the the“nanocomposite” one obtained when NPs. onlyFinally, Ag for NPs the were magnetron produced current with of the 500 RF mA, magnetron the UV-Vis switched spectrum off .resembled In other words,the one the obtained production when of only Ag NPsAg NPs started were to produced dominate with over the the RF production magnetron of switched the plasma off. polymer In other matrixwords, and the theproduction NPs were of mostly Ag NPs metallic. started The to domi absencenate of over the plasmathe production polymer of matrix the plasma/envelope polymer also causedmatrix aand hypsochromic the NPs were shift mostly of the metallic. silver LSPR The peak absence to 370 of nm.the plasma polymer matrix/envelope also caused a hypsochromic shift of the silver LSPR peak to 370 nm. 3.4. In-Flight Coating of C:H:N:O Nanoparticles 3.4. InIn-Flight the two Coating previous of C:H:N:O cases, Nanoparticles i.e., for the situations when either the composite target or the dual-magnetronIn the two previous system was cases, used, i.e., multi-core@shell for the situations NPs when were either produced the composite due to thetarget fact or that the bothdual- themagnetron plasma polymerization system was used, and multi-core@shell the production of NPs NPs were took placeproduced at the due same to time.the fact In orderthat both to fully the decoupleplasma polymerization these processes, and a systemthe prod foruction the in-flight of NPs coatingtook place of NPsat th wase same recently time. developedIn order to [ 113fully]. Indecouple this case, these the gas-phaseprocesses, productiona system for of the C:H:N:O in-flight NPs coating was separatedof NPs was from recently the deposition developed of [113]. metal In (silver)this case, that the was gas-phase performed production in the auxiliary of C:H:N:O “inoculation” NPs was chamber. separated As afrom result, the the deposition final structure of metal of the(silver) NPs dithatffered was significantly; performed in instead the auxiliary of multi-core@shell “inoculation” NPs, chamber. C:H:N:O As a NPs result, decorated the final by structure small Ag of nanoparticlesthe NPs differed were significantly; formed, as depictedinstead of in multi-core@shell Figure8. Such strawberry-like NPs, C:H:N:O structuresNPs decorated arose by as small a result Ag ofnanoparticles the condensation were formed, of supersaturated as depicted silver in Figure vapor 8. on Such the strawberry-like C:H:N:O NPs that structures acted as arose efficient as a result sites forof the nucleationcondensation of silverof supersaturated NPs. Silver silver atoms vapo werer adsorbedon the C:H:N:O onto the NPs surface that acted of the as C:H:N:O efficient NPssites for the nucleation of silver NPs. Silver atoms were adsorbed onto the surface of the C:H:N:O NPs and

Materials 2019, 12, x FOR PEER REVIEW 10 of 16

subsequently formed small Ag NPs in a similar way to the “conventional” sputter deposition of metals onto solid substrates at lower pressures. However, the drawback of this configuration is that Materialsthe volume2019, 12 growth, 2366 of Ag NPs was not fully inhibited, thus purely silver NPs were formed alongside10 of 16 the metal/plasma polymer strawberry-like NPs. Such metallic NPs, which are considerably bigger andthan subsequently those detected formed on the small surface Ag NPs of inthe a similarC:H:N:O way nanoparticles, to the “conventional” are clearly sputter visible deposition in Figure of 8, metalsespecially onto for solid the substrateshigher DC at magnetro lower pressures.n currents However, used for thesilver drawback sputtering. of this conﬁguration is that the volumeThese growthpreliminary of Ag results, NPs was which not fullyshow inhibited, that the strawberry-like thus purely silver metal/plasma NPs were formed polymer alongside NPs can thebe metalprepared/plasma in a polymerfully physical strawberry-like way, are very NPs. promising, Such metallic as this NPs, method which aremay considerably substitute recently bigger thanused thosetechniques detected that on employ the surface wet-chemical of the C:H:N:O synthesis. nanoparticles, Further research are clearly is, however, visible instill Figure needed8, especially in order to forenable the higher the production DC magnetron of nanoparticles currents used with for tailor silver-made sputtering. properties (e.g., amounts of metallic NPs attached to a single plasma polymer NPs).

Figure 8. TEM images of Ag/C:H:N:O NPs produced by in-flight sputter deposition of silver onto C:H:N:OFigure 8. NPs. TEM DC images magnetron of Ag/C:H:N:O current for silverNPs produced deposition by (a )in 100-flight mA sputter and (b) 300deposition mA. RF of power silver 40 onto W. C:H:N:O NPs. DC magnetron current for silver deposition (a) 100 mA and (b) 300 mA. RF power 40 TheseW. preliminary results, which show that the strawberry-like metal/plasma polymer NPs can be prepared in a fully physical way, are very promising, as this method may substitute recently used techniques4. Conclusions that employand Outlook wet-chemical synthesis. Further research is, however, still needed in order to enable the production of nanoparticles with tailor-made properties (e.g., amounts of metallic NPs This featured article summarizes the principles and different strategies that utilize magnetron- attached to a single plasma polymer NPs). based gas aggregation cluster sources for the fabrication of heterogeneous metal/plasma polymer 4.nanoparticles. Conclusions In and comparison Outlook with recent results reported for metal-metal NPs, this article shows that three different strategies may be followed: (i) the use of a metal/polymer composite sputtering target, (ii) aThis multi-magnetron featured article strategy, summarizes and the (iii) principles in-flight and deposition different of strategies metal onto that utilizeplasma magnetron-based polymer NPs. It gaswas aggregation shown that cluster depending sources on for the the followed fabrication strategy, of heterogeneous NPs with different metal/plasma structures polymer can nanoparticles. be produced. InFor comparison the cases in withwhich recent the plasma results polymerization reported for metal-metaland formation NPs, of both this metal article and shows plasma that polymer three diNPsfferent took strategies place at the may same be followed:time, multi-core@shell (i) the use of nanoparticles a metal/polymer were compositeproduced. sputteringFrom this point target, of (ii)view, a multi-magnetron the procedure that strategy, utilizes and two (iii) independent in-flight deposition magnetrons of metal for onto sputtering plasma of polymer metal and NPs. polymer It was showntargets that allows depending for the tailoring on the followed of the properties strategy, NPs of the with formed different nanoparticles, structures can which be produced.is not possible For thewhen cases a single in which composite the plasma target polymerization is used. Furthermor and formatione, the results of both reveal metal that and the plasma complete polymer decoupling NPs tookof plasma place atpolymer the same NPs time, production multi-core@shell and the sputteri nanoparticlesng of metal were opens produced. the way From to thissynthetizing point of view, core- thesatellite procedure NPs, thati.e., utilizesnanoparticles, two independent with a large magnetrons plasma polymer for sputtering core decorated of metal and by polymernumerous targets small allowsmetallic for NPs. the tailoringAlthough of emphasis the properties was placed of the solely formed on nanoparticles,metal/C:H:N:O which NPs in is this not possiblestudy, it whenis worth a singlestressing composite that similar target procedures is used. Furthermore, may be employed the results for revealother combinations that the complete of materials. decoupling Furthermore, of plasma polymerthe physical NPs method production of NPs and production the sputtering reported of metal here opens offers the several way tokey synthetizing benefits as core-satellitecompared to NPs,methods i.e., nanoparticles, based on the chemical with a large synthesis plasma of polymer heterogeneous core decorated NPs and by thus numerous presents small a vivid metallic alternative NPs. Althoughto them. Nevertheless, emphasis was it placed is also solely important on metal to note/C:H:N:O that many NPs challenges in this study, still it have is worth to be stressing faced before that similarthe wider procedures spread of may such be nanomaterials employed for otherin various combinations fields (e.g., of materials.bio-sensing, Furthermore, tissue engineering, the physical drug methoddelivery). of NPsThese production challenges reported are related here to o ffaers better several control key benefitsof the physico-chemical as compared to methodsand/or bio-related based on theproperties chemical of synthesis produced of heterogeneous NPs, which requires NPs and thusnot only presents more a vivid targeted alternative experiments to them. using Nevertheless, different itmaterials/configurations/operational is also important to note that many challenges conditions still but have a better to be facedunderstanding before the widerof the spreadprocesses of such that nanomaterials in various fields (e.g., bio-sensing, tissue engineering, drug delivery). These challenges are related to a better control of the physico-chemical and/or bio-related properties of produced NPs, which requires not only more targeted experiments using different materials/configurations/operational Materials 2019, 12, 2366 11 of 16 conditions but a better understanding of the processes that occur during the formation and growth of NPs. Thus, to conclude, the presented results should be considered as a promising starting point that opens new opportunities for the sputter deposition of functional nanomaterials.

Author Contributions: The author contributions to the paper are as follow: Writing-Original Draft Preparation, O.K.; Methodology, O.K., A.C., and H.B.; Investigation, A.S., P.P., R.Š., A.K., D.N., P.S., J.H., P.K., and M.C.; Writing-Review & Editing, A.C.; Funding Acquisition, A.C. and H.B. Funding: This research was funded from Czech Science Foundation in the frame of grants GACRˇ 17-12994S (investigation of Nylon-like NPs) and GACRˇ 17-22016S (investigation of heterogeneous metal/plasma polymer NPs). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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