Deposition of W Nanoparticles by Magnetron Sputtering Gas Aggregation Using Different Amounts of H₂/Ar and Air Leaks

Abstract

This work presents the synthesis of tungsten nanoparticles (W NPs) using a cluster source based on magnetron sputtering combined with gas aggregation (MSGA), operated with up to 81% H₂ in the hydrogen/argon mixture used as a working gas. The results show that, with up to 41% H₂ in discharge, the synthesis rate increases by more than 60 times, rapidly decreasing for over 50% H₂ in discharge. The W dust is still produced for H₂-dominated discharges (81%), and its deposition rate is small but not negligible (0.02 mg/h). The obtained W NPs are isolated, with the diameter decreasing from 50 nm to 15 nm when the amount of H₂ in discharge is smaller than 41%. Over this value, the particles tend to agglomerate, forming structures similar to film-like deposits. Also, the diameter of the dust spots deposited on substrates depends on the H₂ content of the discharge. This allows the efficient coating of substrates up to 26 mm wide by translating them in front of the MSGA cluster source exit aperture. Additionally, for 41% H₂ in discharge, the influence of synthetic air leaks (0%–8.2%) in discharge was investigated. The deposition rate decreases rapidly (ceasing for around 6% air in discharge), and the obtained nanoparticles tend to agglomerate on the substrate (at 3.3% air content, the dust deposit has the aspect of a near-continuous film). Chemical composition investigations show a pronounced tendency for oxidation, nitridation, and oxynitride formation in the presence of air leaks.

1. Introduction

Tungsten presents outstanding properties (higher melting temperature amongst all metals and good thermal conductivity and stability) and is of high interest for domestic, industrial, and advanced technology applications. These include lighting (for light bulb filaments), electrical engineering (like alloys of W-Cu for electrical contacts), electronics (for electron emitters, thin films in microelectronic devices, heat sinks), high-temperature techniques (like heating elements and radiation shields in high-temperature furnaces, thermocouples such as W/W-Re), metal machining (welding electrodes, cutting), X-ray generation (with applications in medicine and other fields), coating technologies (for evaporation boats or sputtering targets), chemical industry (as catalysts, high-temperature components for the production of quartz glass, spring materials in corrosive media), melting metallurgy (for alloy and steel production), space aviation (for ignition tubes in rocket engines, guide nozzles, etc.), aviation (for aircraft balance weight), military applications (for high-kinetic energy penetrators in ammunition, armor plating), and leisure time and sports equipment (like parts of professional darts, golf club components, counterbalance weights in Formula (1) racing cars). This long list is incomplete, and more applications of W material can be found in [1].

Also, tungsten and its compounds find modern applications in nanotechnology. Thin films of W find application in spintronics [2,3] or for superconducting transition-edge sensors used as ultrasensitive thermometers [4]. Moreover, nanostructured-tungsten oxide surfaces (obtained as thin films [5,6] or nanoparticles [7]) react with many gas molecules, making them a perfect choice for developing new types of gas sensors [8]. Also, WO₃-based nanocomposites (with conductive polymers or carbon nanostructures) may present enhanced electrochemical properties [9] or enhanced capacitive performance [10,11].

Tungsten is “the best, if not the only, material to withstand the extraordinary operating conditions in a nuclear fusion reactor divertor” [1]. In this part of the reactor, a large part of the fusion heat is removed, with the heat flux reaching more than 20 MW/m² and the surface temperatures up to 1800 °C [12]. Tungsten’s thermal properties (high melting temperature, high thermal conductivity, and low thermal expansion coefficient) and its low sputtering yield make it suitable for use as an armor material.

Despite its advantages, the use of tungsten in fusion reactors presents challenges, one of them being the formation of tungsten dust due to the plasma-wall interaction. Apart from the decline in fusion plasma performances [13], dust can pose safety hazards related mainly to the accumulation of nuclear fuel in the dust over the permissible limits [14]. Recent investigations into the dust collected from the divertor region of the WEST (W Environment in Steady-state Tokamak) reactor show the presence of two distinct types of dust [15]. The first one is dominated by particles with expected sizes (micrometers up to tens of micrometers), being attributed to the off-normal events (droplets emitted due to high thermal load) or delamination of W coatings. The second class of dust was not expected to appear in tokamaks with W plasma-facing components, and it consists of W particles with dimensions in the range of nanometers. Their formation can be attributed either to the condensation of the supersaturated vapors above molten tungsten droplets or to ion metallic clusters developing in the low-temperature plasma regions in the presence of W sputtering. In this context, it is important to study the synthesis of W dust in laboratory conditions that are as close as possible to those in a fusion device. One of the plasma-based methods for the synthesis of metallic dust is magnetron sputtering gas aggregation (MSGA), and it consists of condensation in an inert gas (argon) flow of the metallic supersaturated vapors produced by a magnetron discharge. MSGA is used for the synthesis of nanoparticles from different metals and their compounds [16,17], polymers [18,19], and core-shell [20,21] or multi-component nanoparticles [22]. Using this method, we successfully produced W nanoparticles with a controlled shape using only Ar as a working gas [23,24]. During these preliminary studies, we noted a rapid decrease in the W dust synthesis rate when only Ar was used as a process gas, and we determined that adding a small amount of H₂ (around 5%) in discharge would increase and sustain the W dust synthesis rate [25,26]. A similar decrease in the synthesis rate when nanoparticles are produced by MSGA was also reported in other works [27,28], where the influence of the vacuum state and impurities as nucleation centers are underlined. Also, [29] presents the enhancement of the synthesis rate of Ti and Co nanoparticles when small amounts of reactive gases (O₂ and N₂) are added to the discharge.

In this work, we present the results of a parametric study regarding the synthesis of W nanometric dust using the MSGA cluster source operated by increasing the amount of H₂ in the discharge (between 0%–81%). At the same time, the pressure is kept constant by progressively decreasing the amount of Ar. This study proves that the MSGA device can produce W dust in atmospheres dominated by H₂, being a valuable tool for generating W dust similar to the nanometric dust detected in the WEST tokamak [15]. The influence of air leaks on the synthesis of the W dust is also presented. This study considers the following properties of the generated dust: the variation in the synthesis rate, dust morphology, and chemical composition. It also assesses the possibility of coating flat surfaces with W nanoparticles.

2. Materials and Methods

Figure 1 presents the experimental setup used for the synthesis of tungsten nanoparticles by magnetron plasma [23,24]. It comprises an MSGA cluster source (left) attached to a collection vacuum chamber (right). The magnetron is equipped with a 2″ diameter and 1/8″ thickness W target (99.95% purity, from Kurt J. Lesker Company, Jefferson Hills, PA, USA). The exit aperture, connecting the cluster source with the deposition chamber, has a diameter of 2 mm, while the length of the aggregation region (d_TA) is 90 mm. Due to the pressure difference between the MSGA chamber (80 Pa) and the deposition chamber (5 Pa), the nanoparticles grown in the aggregation chamber are ejected through the exit aperture and are collected on the substrates. The sputtering process uses an RF (13.56 MHz) generator and an impedance matching box (models AX-600 respective AMV-100-EN, both manufactured by ADTEC Plasma Technology Co., Ltd., Hiroshima, Japan). The applied RF power is 80 W for all deposited samples. Ar alone or mixed with H₂ is used as process gas, presenting a 6.0 grade purity. The influence of H₂ over the W NP synthesis was thoroughly investigated in a parametric study, in which the amount of H₂ injected in discharge was varied between 0% and 81% (i.e., the H₂[sccm]/(Ar[sccm] + H₂[sccm] ratio). During this parametric study, the pressure in the MSGA chamber was kept constant (80 Pa) to eliminate the influence of the increasing pressure over the W NP synthesis process. For this purpose, while the H₂ mass flow rate increased from zero to 9.7 sccm, the Ar mass flow rate was correspondingly diminished to between 5.3 and 2.3 sccm. At 41% hydrogen content in discharge, small amounts of synthetic air (0–0.5 sccm, i.e., 0%–8.2%) were deliberately injected in discharge, keeping the pressure constant by reducing the Ar flow correspondingly. The synthetic air used was a mixture of 79% N₂ and 21% O₂, presenting a 5.0 purity. The deposition duration was adapted for every sample, depending on the deposition rates for individual sets of parameters: 30 min for the samples with high deposition rates and up to 3 h for those with lower deposition rates.

The deposition procedure requires some precautions, which are detailed below. We noted that when using sole Ar as process gas, the deposition process starts with a high value, which decreases to a much smaller value in a short time interval (up to half an hour) [25,26]; this effect is related to the consumption of the residual gases in the aggregation chamber. Following this, the deposition takes place in Ar without any residual gases. Our tests have shown that the only atmospheric residual gas that sustains and enhances the synthesis of W NPs is H₂. Therefore, each time the chamber was vented with atmospheric air, the W NPs were first deposited on a shutter until the deposition rate decreased to near zero. During this preparatory stage, the discharge was fed only with Ar. Only after this was the desired amount of H₂ injected into the discharge. In this way, only the W NPs deposited in conditions relevant to our study were collected on the substrate. The substrates consisted of microscope glass slides (25 mm × 75 mm, from Labbox Labware, Barcelona, Spain) or 011 Si substrates (10 mm × 10 mm, from Neyco, Paris, France) fixed on a holder, which could be translated vertically, perpendicular to the nanoparticle beam, using a vacuum compatible translation stage. The substrates were placed 5 cm from the exit aperture. The dust collection rates were determined by weighing the collectors before and after dust deposition, using an electronic scale with 0.01 mg accuracy. The sample surface morphology was investigated using scanning electron microscopy (SEM). This analysis was conducted using an Apreo FEG High-Resolution Scanning Electron Microscope (HR-SEM), specifically the S LoVac model from Thermo Fisher Scientific Inc. in Hillsboro, OR, USA, and a Gemini 500 apparatus from Zeiss (Oberkochen, Germany). The particle detection from SEM images was made using a Matlab (version R2018b, The MathWorks Company, USA) program we developed [30], which can detect the particles automatically. The program uses a five-step calculation process, and the selection criteria for the particles are defined by the radius range and the sensitivity value of the particle’s boundaries. In this line, the obtained statistics were based on the most detectable particles in the SEM images. The chemical composition of the tungsten nanoparticles was analyzed using X-ray Photoelectron Spectroscopy (XPS). We used a K-Alpha Thermo Scientific (ESCALAB™ XI+, East Grinstead, UK) spectrometer with a 180° double-focusing hemispherical analyzer. To calibrate the peak positions, we used the adventitious C1s peak at 284.8 eV as a reference, indicated by Avantage data software (Thermo Avantage v5 9921, East Grinstead, UK). Surface elemental composition was determined by recording survey spectra at a pass energy of 50 eV. For the evaluation of elemental bonding states, high-resolution spectra for the binding energy regions of C1s, O1s, N1s, and W4f were measured at a pass energy of 20 eV. The acquisition and processing of spectra were performed using the Avantage data software mentioned above.

3. Results and Discussion

The experiments carried out in this study provide information on the rate of W dust (W NPs) deposition, the size and morphology of the deposited spot, and dust chemical composition.

3.1. Deposition Rate Evolution in the Presence of H₂ Injected in Discharge—The Influence of Air Leaks

Figure 2a presents the evolution of the deposition rate and the deposited spot shape when the amount of H₂ in the discharge is increased, keeping the pressure constant (80 Pa). We note that the deposition rate in sole Ar (point A) is low, around 1 mg/h. The deposition rates increase up to 60 times when the H₂ content is increased up to 41% in the discharge, even if the amount of Ar is gradually reduced to keep the pressure in the MSGA chamber constant.

For values of H₂ content over 55%, the deposition rate is noted to decrease. However, W dust is still observed on the substrate when H₂ dominates the discharge (up to 81% H₂). Although the deposition rate is low, it is still not negligible (0.02 mg/h). It is essential to mention that for every experimental point from B to J, the deposition rate remains constant in time, proving that the H₂ present in discharge continuously sustains the synthesis of the W NPs.

Adding low amounts of H₂ to the discharge increases the deposition rates because new species appear in the discharge, namely the ArH⁺ ions, which enhance the target sputtering, leading to an increase in the number of W species (atoms, ions) available for the synthesis of W nanoparticles [25]. The Materials and Methods section mentioned that increasing the H₂ flow rate is done simultaneously with decreasing the Ar flow rate (thereby maintaining the MSGA pressure constant). For this reason, the number of Ar atoms available for reactions producing ArH⁺ ions decreases at high H₂ values (starting at spot G in Figure 2a). Accordingly, the sputtering rate of the target decreases, thus causing the W NP synthesis rate to decrease. A similar evolution of the sputtering rate in variable H₂/Ar mixture is reported for gold in [31].

Figure 2b presents the evolution of the dust synthesis rate in the presence of air leaks (collected dust spots from F1 to F8), compared with that obtained using 41% H₂ in the discharge (spot F in both Figure 2a,b). The increase in air leaks leads to a rapid decrease in the W dust synthesis rate. This happens naturally while the air leaks interact with the target surface, with the resulting chemical compounds presenting a smaller sputtering yield when compared with the metallic W. Again, the deposition rate remains constant in time for every experimental point up to 6.5% air in the discharge (points from F1 to F7). Over 7% air, the deposition rate ceases in around 10 min (point F8 in Figure 2b); this behavior is most probably due to the target poisoning.

One may observe that the W NP spots obtained on the substrate present a maximum diameter of 2.6 cm. It shows that the MSGA method allows the efficient coating of microscope slides by translating them in the front of the MSGA cluster source.

3.2. Morphological Investigations of W NPs—The Effect of Variable Amounts of H₂ Injected into Discharge

The synthesis of tungsten nanoparticles was assessed by examining the surface of Si substrates placed on the collector holder. Figure 3 presents a series of SEM images describing the morphology of the particles at various H₂ amounts in the discharge. It is visible that with increasing H₂ amount in the discharge, the dust appearance changes from individual nanoparticles (in Ar-dominated atmospheres) to agglomerations of smaller nanoparticles (in H₂-dominated atmospheres). Figure 4a presents an example of the particle size distribution obtained from the sample deposited with 17% H₂ content in the discharge (sample D). In this line, the computational strategies were presented in reference [30], and the resulting particle dimensions show an interesting behavior of the deposition process. Herein, using a small amount of H₂, the mean particle size is around 56 nm (spot A), with a particle density of 280 part/μm². Increasing the H₂ content in the discharge highlights a two-step threshold phenomenon. Mainly, by increasing the H₂ content (starting with spot B), the mean size of the tungsten particles starts to decrease, from 41 nm (spot B) down to 15 nm (spot E). This could be considered the first particle mean size threshold. By continuing to increase the H₂ content (spot F), the mean size increases from 15 nm (spot F) up to 41 nm (spot J). This behavior could be considered as the second threshold. The results of this analysis make it possible to identify two distinct domains of H₂ content (see Figure 4b) for W NP growth. Interestingly, the growth tendency of the particles in the second region (from spot F to J) is slow, expressing the nanoparticle tendency to agglomerate and form structures similar to film-like deposits. Usually, the deposits of nanoparticles produced by the MSGA technique operated with Ar consist of individual nanoparticles with a low degree of agglomeration, as is reported in other works [32].

The same characteristics are observed in W NPs samples obtained with low H₂ content. The tendency of nanoparticles to agglomerate at high H₂ content may be associated with the known mechanisms of nanoparticle growth: coagulation (joining together of two clusters), coalescence (growth of large clusters while the small clusters evaporate), and aggregation (solid clusters joined due to their contact, while partially preserving their initial shape) [33]; these processes taking place in the aggregation space of the MSGA chamber. On the other hand, it is not excluded that the agglomeration of the nanoparticles takes place at the substrate level, especially for the higher H₂ content in the discharge (see Figure 3f, for 81% H₂ content in the discharge). This hypothesis requires supplementary investigations and may be relevant for fusion-related studies.

3.3. Morphological Investigations of W NPs—The Effect of Air Leaks in the Discharge

Figure 5 presents a series of SEM images describing the morphology of the particles with different amounts of air leaks injected into the discharge, corresponding to the following spots from Figure 2b: F2 (0.8% air), F4 (1.7% air), and F5 (3.3% air). The SEM image from Figure 3d (spot F in Figure 2a,b corresponding to 41% H₂ in the discharge, deposited in the absence of air leaks) is the reference sample for this experiment. As illustrated in Figure 5a–c, in the presence of air leaks, the NPs tend to agglomerate on the substrate, with the agglomeration dimensions increasing with air content. Moreover, at 3.3% air content, the dust deposit appears as a continuous film. Of course, due to air leaks, its chemical composition must be altered.

3.4. Chemical Composition of the W Dust—Influence of the Air Leaks

A comprehensive analysis of the chemical bonding in tungsten nanoparticles was performed using X-ray photoelectron spectroscopy (XPS) investigations. For this purpose, three samples obtained under different experimental conditions were considered: (i) using only Ar (spot A in Figure 2a); (ii) containing 41% H₂ in the Ar + H₂ mixture (sample F in Figure 2a,b); (iii) adding 0.8% air leak into the discharge (sample F2 in Figure 2b).

Figure 6a presents the survey spectra of these samples, revealing that the primary elements present in the obtained nanoparticles are tungsten, oxygen, and nitrogen. The spectra also show the presence of carbon, an adventitious postdeposition contaminant (proven by the absence of the C-W bond from the C1s high-resolution spectrum, not presented here). Compared to our previous results [23], the presence of N₂ in the sample using only Argon is unexpected, most probably due to the residual nitrogen coming from the gas supply system.

The detection of oxygen and nitrogen suggests the formation of tungsten oxides, nitrides, and oxynitrides, which could result from the incorporation of these compounds during the synthesis of the NPs (in the presence of air leaks or of the residual N₂) or from the NP surfaces due to subsequent exposure to air (for all samples). Figure 6b,c present the high-resolution spectra of all examined samples in the W4f and N1s spectral regions, respectively. The spectra in the W4f region contain the following distinct doublets: metallic tungsten, with peaks at 31.3 eV (W4f_7/2) and 33.4 eV (W4f_5/2), tungsten dioxide WO₂, with peaks at 32.7 eV (W4f_7/2) and 34.8 eV (W4f_5/2), and tungsten trioxide WO₃, with peaks at 35.9 eV (W4f_7/2) and 38.1 eV (W4f_5/2). Additionally, some peaks attributed to W-N and W-N-O bonds are expected to overlap (see Figure 6b) within the W 4f spectrum, thus preventing its proper deconvolution [34]. The following qualitative remarks over the W4f spectral region can be drawn despite this. The spectra in Figure 6b suggest that the sample deposited in sole Ar contains the maximum amount of the metallic W. On the other hand, the contribution of WO₃ bonds increases with the addition of hydrogen and/or synthetic air in the discharge. However, all samples from the plasma synthesis process or post-synthesis nanoparticle exposure in the ambient environment show a common tendency toward oxidation.

On the other hand, in the N1s spectral region, such overlapping does not appear, allowing for the proper decomposition of this spectral region. As an example, Figure 7a presents the deconvolution [34] of the spectrum recorded in the N1s region recorded for the sample F2 (deposited with 0.8% air leak in the discharge): W₂N bond at 397 eV (contribution of 54.3%), WxOyNz bond at 399 eV (contribution of 27%) and nitrogen adsorbed on surfaces at 402 eV (contribution of 19%).

Figure 7b illustrates the relative contributions of different nitrogen bonds to the overall N1s-related XPS signal, depending on the gas used to synthesize tungsten nanoparticles. The processed data reveal that when a mixture of argon, hydrogen, and synthetic air is used as process gas, a pronounced contribution of tungsten nitride (W₂N) is noted, while the amount of N₂ adsorbed on surfaces decreases. However, regardless of the synthesis conditions or exposure to a nitrogen-rich environment after synthesis, all the samples consistently tend to form nitrogen-based chemical bonds.

4. Conclusions

The results presented in this work show that tungsten dust (W NPs) can be produced using the MSGA method operated with entirely different amounts of H₂ injected into the discharge. At low H₂ content (up to 41%) in the discharge, the deposition rate is strongly enhanced (over 60 times), this working regime being useful for applications where high amounts of W NPs with controlled dimensions are necessary (for example, in nanotechnology). The deposition rate of W NPs starts to decrease at over 50% H₂ in the discharge. When the MSGA discharge contains predominantly H₂ (up to 81%), W dust is still produced; this outcome may be relevant for nanometric dust production mechanisms in nuclear fusion devices. An increase in the H₂ amount in the discharge leads to a decrease in the nanoparticle size; at the same time, their agglomeration tendency on the substrate increases, forming structures similar to film-like deposits. In addition, air leaks in discharge lead to a complex change in nanoparticle morphology (leading to their high agglomeration) and chemical composition (with the deposited dust showing a high tendency to form oxygen- and nitrogen-based chemical bonds). Future studies will focus on increasing the amount of H₂ in the discharge and on studies regarding nuclear fuel retention in tungsten dust.