Enhancement of W nanoparticles synthesis by injecting H 2 in a magnetron sputtering gas aggregation cluster source operated in Ar

Enhancement of W nanoparticles synthesis by injecting H 2 in a magnetron sputtering gas aggregation cluster source operated in Ar | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Enhancement of W nanoparticles synthesis by injecting H 2 in a magnetron sputtering gas aggregation cluster source operated in Ar Tomy Acsente, Silviu Daniel Stoica, Cristina Craciun, Bogdana Mitu, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4349303/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 11 You are reading this latest preprint version Abstract Synthesis of W nanoparticles by magnetron sputtering combined with gas aggregation operated in Ar suffers from a continuous decrease of the synthesis rate, ceasing in a finite time interval, in the range of minutes to tens of minutes. Experimentally, we noticed that the addition of small amounts of H 2 to Ar (5–20%) increases the synthesis rate, which remains constant over time, at a value dependent on the amount of injected hydrogen. Mass spectrometry investigations revealed, in the hydrogen presence, a dominance of the ArH + ions over the Ar + ones, associated also with an increased number of W + and WH + species in plasma, sustaining a substantial increase in the nucleation rate. gas aggregation magnetron sputtering nanoparticles mass spectrometry dimer synthesis rate of nanoparticles Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Magnetron Sputtering combined with Gas Aggregation (MSGA) is presently a well-established physical method for the preparation of nanoparticles of various materials like metals [1], their compounds (oxides[2], nitrides [3]), and even polymers [4]. In addition, this method is used for the preparation of core-shell nanoparticles [5, 6], or multi-component ones [7, 8]. Significant advantages of the method are obvious due to the mono-disperse dimension of the particles, and the tunability of the particles shape and dimension via the process parameters [9]. Still, the MSGA method presents, for some target materials, the drawback of the synthesis rate reduction down to zero in a finite time interval. The duration of this time interval (when a decrease in the synthesis rate is observed) may last for minutes to hours, being dependent on the initial vacuum conditioning of the cluster source walls [10]. As such, it is supposed that the residual gases present in the aggregation chamber play a significant role in the cluster seeds formation [11]. The gradual consumption of these gases in the nucleation process leads to the depletion of the number of nucleation centers until their total absence. Moreover, it was observed that the deliberate addition of some gases in the aggregation chamber leads to the revitalisation of the synthesis of nanoparticles. For example, the nucleation of Ti or Co NPs depends on the presence of O 2 or N 2 trace, the formation of nucleation centers based on metal oxide or nitride being more efficient than that of pure metal atoms originating from the sputtering [12]. A similar effect was observed by us during the synthesis of W nanoparticles (W NPs) by MSGA in an Ar atmosphere: the synthesis rate, as indicated by the amount of NPs collected on a substrate, decreases in time [13]. Apart to the vacuum conditioning of the aggregation chamber, we noted that this duration depends also on the degree of target erosion (tens of minutes for a new target, respective tens of seconds for a highly eroded one) [14]. In both cases, the W NPs synthesis process is revitalized after venting the MSGA chamber with ambient air for at least one hour; this fact suggests that the nucleation of the W NPs is dependent on the atmospheric residual gases (like H 2 , He, O 2 , N 2 , CO 2 , H 2 O vapours) present in the MSGA. Aiming to clarify this behaviour, we looked at the effects of mixing small amounts of these gases with Ar in MSGA, immediately after the deposition rate ceased. We identified by trials that hydrogen (H 2 ) is the only gas sustaining the continuous synthesis of the tungsten nanoparticles in an MSGA process [15]. In the present study, we describe the behaviour of the W NPs production rate when small amounts of H 2 (up to 20%) are mixed with Ar in the MSGA aggregation chamber. Weighing the particles deposited on collectors and quartz crystal microbalance measurements were used to assess the deposition rates of W NPs. The description of the ionic species present in discharge during the W NPs synthesis is performed by mass spectrometry. This technique is frequently used for the characterization and optimization of plasma processes, like in the synthesis of carbonic nanostructures,[16, 17] functionalization of nanoparticles[18] and generation of dusty plasma in methane-based discharges [19]. Moreover, mass spectrometry investigations have shown that small amounts of H 2 injected in sputtering discharges play a critical role in increasing the deposition rate of gold films [20]. Also, a recent mass spectrometry study presents the role of dimmers directly sputtered from the target in the efficient growth of Cu or Ag nanoparticles in gas aggregation cluster sources [21]. The present study benefits from our recent works[22, 23] presenting a novel method for processing mass spectra and the species identified in sputtering plasmas generated in Ar and Ar/H 2 gases in contact with W surfaces. Synthesis of the W NPs in such experimental conditions is relevant for applications where tungsten material is in contact with hydrogen plasmas, specifically in fusion reactors and in nanotechnology [15]. Experimental setups and methods Figure 1 a presents the schematic of the MSGA cluster source attached to a particle collection chamber. The cluster source was previously presented in [13] and consists of a small cylindrical vacuum chamber (100 mm x 300 mm), axially housing a 2” magnetron sputtering gun (MS), and ending with a small diameter (2 mm) exit aperture. The W target is already eroded from previous experiments, allowing an initial deposition of W NPs in Ar no longer than one minute. The distance between the target and the exit aperture (the aggregation length, where nanoparticles growth takes place) is d TN =7 cm. In between the pumping system and the MSGA chamber is mounted a bypass valve (nominated R). Every process starts with pumping both chambers down to 10 − 4 Pa and with target cleaning by pre-sputtering for around 20 minutes. The cleaning plasma is generated in Ar (at 5 sccm mass flow rate) at low pressure (0.2 Pa in both chambers), applying P RF = 80 W to the magnetron cathode. During this process, the bypass valve R is opened. By closing the valve, the pressure increases up to 80 Pa in the aggregation chamber and 0.8 Pa in the collection chamber, values adequate for the initiation of the nanoparticle synthesis process [13]. Results 3.1 Evidence for the enhancement of the nanoparticle production rate by hydrogen addition After target cleaning in low-pressure Ar plasma, the particle synthesis is triggered in Ar by closing the bypass valve, thus leading to the pressure increase in the aggregation chamber to an established value of 80 Pa. Figure 2 presents an image of the particles deposited on the moving collector translated across the nanoparticle beam, where the position scale was converted to time scale (1mm corresponds to 5 s). The onset of the synthesis is noted with point a and it corresponds to the moment when the valve R is closed, leading to the pressure increase up to 80 Pa. The seed-like shape of the deposit (region ab on the substrate) demonstrates the decrease of the synthesis rate in time until the region bc , characterized by a hard-to-see track of nanoparticles, is reached. The injecting of a small amount of H 2 in the discharge (5%) leads to a sudden increase in the deposition rate (point c), which is maintained at a high value over a long period (region cd ) until the process is stopped by switching off the RF power. A similar evolution of the deposition rate is revealed by QCM measurements (see Fig. 2. b). First, only Ar is injected in the aggregation chamber (between 1500–3500 s), and at around 3500 s H 2 (5%) is injected in the discharge. We note at the beginning of the chart (at around 1500 s) a short time interval (~ 48 s) presenting a peak of the QCM rate signal (60 mHz/s ), compatible with the time length of the seed-like spot (region a-b of the substrate) presented in Fig. 2. a. Further on, the QCM signal presents a much smaller, still constant (~ 7 mHz/s) value of the synthesis rate. This region is associated with the region bc of the substrate (Fig. 2a). After H 2 injection in the discharge, the deposition rate suddenly increases, also presenting an oscillatory behaviour (~ 10 s period and amplitudes of around 50 mHz/s). It deserves commenting that besides the QCM oscillations, also the self-bias voltage starts to oscillate during dust synthesis (Fig. 2.b). Similar oscillations were reported also in [24], suggesting the presence of a significant amount of dust in the aggregation chamber. The effect of increasing the amount of injected H 2 on the W NPs deposition rate (obtained by weighting of stationary substrates) is presented in Fig. 3. We note that in the range of the H 2 /Ar ratios used in these experiments the deposition rate increases with the H 2 content, even if the amount of Ar (which is the sputtering gas) was slightly decreased to maintain a constant pressure of 80 Pa in the MSGA cluster source (see ratios H 2 /Ar denoted as insets in Fig. 3). 3.2 Behaviour of ion species in Ar plasmas in contact with the W surfaces during H 2 injection General mass spectra obtained in Ar and Ar/H 2 plasmas in contact with W surfaces were discussed in detail in our previous works [22, 23]. In the low-mass region of these spectra, the ionic species of hydrogen (H + , H 2 + , and H 3 + ), and those related to argon (Ar 2+ , Ar +, ArH + ) are noticed, with some signatures of impurities-related ions (containing oxygen, nitrogen, carbon). In the high-mass region (bigger than 180 amu) the ions related to tungsten are observed. As commented in [23], the interpretation of W-related mass spectra is not straightforward because of the peaks superposition, which is caused by the closed positions of W isotopes ( 180 W with natural abundance NA of 0,12%, 182 W with NA 26.5%, 183 W with NA 14.3%, 184 W with NA 30.6%, 186 W with NA 28.4%) combined with the limited resolution of the mass spectrometer. Nevertheless, by using the fitting procedure developed in[22] we succeeded in featuring in the spectra, not only the W + peaks but also the tungsten molecular species formed in the presence of hydrogen and gas impurities, among which WH + is presenting the highest signal. We mention that the measuring range of our spectrometer (0-300 amu) does not permit the detection of W-W dimers. To exemplify, Figs. 4a, b, and c present in more detail the mass spectra in the mass regions of Ar and W, recorded from Ar plasmas with hydrogen content of 0%, 5%, and 10%. In these figures are observed the peaks corresponding to Ar + (40 amu), ArH + (41 amu), W + (180; 182; 183; 184; 186 amu), and respective WH + (at 181; 183; 184; 185; 187 amu). The peaks corresponding to 180 W + and 180 WH + are negligible due to the low natural abundance of the 180 W isotope (0.12%). We underline that, when both W + and WH + ions are present, the 182 amu peak originates only from W + ions (i.e. 182 W + ) while 185 and 187 amu peaks only from WH + ions (i.e. 184 WH + and 186 WH + ). Figure 4a shows that feeding the discharge with Ar only leads to a strong signal associated with Ar + , accompanied by a hardly detectable signature of ArH + , originating from the inherent residual hydrogen. In the high masses region, one observes the presence of all mentioned W + ions, while the peaks related only to WH + ions (185 and 187 amu) are missing. Adding a small amount (5%) of H 2 in discharge (Fig. 4b), we note a reversal in the amplitude of the signals corresponding to Ar + and ArH + ions, with the last one becoming dominant; a slight increase in the signals associated only to WH + ions (185 and 187 amu) is also noted. At 10% H 2 in discharge (Fig. 4c), the Ar + signal is practically absent, indicating that all available Ar is consumed in generating ArH + ions; in the same time, the peaks of WH + are enhanced. Figure 5 presents the temporal evolution of the H 2 + , Ar + and ArH + ions intensities in a time-dependent experiment like those discussed in Fig. 2a,b. The plasma was initially fed only with Ar and subsequently (t = 2000 s), H 2 in the amount of 5% was injected. We note in the first 2000 s that the residual H 2 is gradually decreasing, presumably consumed in the formation of ArH + ions (ArH + having however a smaller intensity than Ar + , similar to Fig. 4a). When H 2 is admitted in the discharge, the signal from ArH + becomes dominant over that of Ar + (similar with Fig. 4b), being canceled immediately after H 2 suppression. These results prove that injection in the discharge of even small amounts of H 2 induces the dominant presence of ArH + ions in the discharge, overcoming the number of Ar + ions. Figure 6 presents the relative amounts of some relevant species for the sputtering process in the magnetron discharge. They resulted from the experimental spectra fitting and deconvolution, considering the areas corresponding to each peak. In the W masses range, the peaks corresponding to W + and WH + ions are overlapping and the deconvolution procedure previously presented by us in[22] was applied. Figure 6a shows that when H 2 exceeds 10%, the ArH + becomes dominant among the sputtering species Ar + and ArH + . Concerning the metal-related peaks, the relative amount of WH + ions increases when H 2 increases (Fig. 6b). Finally, Fig. 6c shows the behavior of the ratio (W + +WH + )/(Ar + +ArH + ) when the amount of H 2 added to the discharge increases. This ratio can be taken as a rough indication of the number of tungsten species released in the discharge per available sputtering (Ar + or ArH + ) ion. Discussion The above-presented results demonstrate higher nanoparticle production rates in the presence of low amounts of hydrogen injected in the Ar sputtering discharge. Several causes can be considered for this effect, the most important being an increase in the sputtering rates as suggested by the data from Fig. 6c. The modification of the sputtering rates can be related to the energies, nature, and the number of ions impinging on the target surface, but also to the state of the surface. Adding H 2 changes the Ar discharge properties, which is a vast and complex subject. An overview of the relevant processes leading to the formation of species may be found in [25, 26], where the occurrence of different ionized species like Ar + , ArH + , H + , H 2 + , H 3 + , sputtered metal atoms and the corresponding ions are presented. All these types of species were observed also by us, as shown above. The first aspect, referring to the energy of ions, is related to the electrical properties of the discharge, which are changed during H 2 addition. The RF magnetron discharge used in MSGA is highly asymmetric, proceeding between the small area target acting as an RF electrode and the larger area of the grounded aggregation chamber wall. Consequently, a negative self-bias appears on the RF electrode [27], responsible for the positive ions acceleration towards it. Figure 2b presents the time dependence of the self-bias dependence upon hydrogen content, showing that inserting hydrogen in the Ar based discharge leads to an increase of the self-bias value. It results that the positive ions are accelerated to higher energies while crossing the cathode sheath, being a reason for the enhancement of the sputtering rate in the presence of hydrogen; a similar effect of increasing the sputtering rate is reported [20] for gold thin films. On the other hand, the target self-bias voltage oscillates at hydrogen injection, similar to the deposition rates measured by QCM (Fig. 2) during dust synthesis. Another aspect is related to the discharge chemistry, manifested by the variation of Ar + and/or ArH + ions number and the appearance of W molecular ionic species for example WH + (see Fig. 4 and Fig. 5 ). In pure Ar discharges, the cathode atoms are sputtered both by the fast Ar + ions[28, 29] and by the fast Ar neutrals produced by symmetrical charge transfer (SCT) processes[30]. In a SCT event, the charge and part of the kinetic energy of the Ar + ion are transferred to a neutral Ar atom. The SCT process is efficient at higher pressures (due to higher probability of collisions), comparable with that used in the MSGA processes. The SCT process limits the upper energies of the Ar + ions arriving at the target: in a collision of particles with the same mass about half of the kinetic energy of the incoming particle is transferred to the collided particle [30]. Therefore, the energies of the ion and neutral populations tend to equilibrate, with the energy of ions at a lower value and that of neutral at higher values than those corresponding to the missing SCT situation. The addition of H 2 in the discharge leads to the efficient generation of the ArH + ions (see Figs. 4, 5 and 6a). As these ions do not have a neutral[20, 31] counterpart they are not affected by SCT and can acquire higher kinetic energy during traversing the cathode sheath. Such as the addition of H 2 in the discharge will increase the target sputtering rate, as was also pointed in [20]. In addition, the state of the surface affects the sputtering rate. In the presence of hydrogen plasma, the tungsten surfaces may become supersaturated with hydrogen, leading to the formation on the target surface of a thin surface layer with a high content of H, a phenomenon which can appear even at low energies of H + ions [32]. This can lead to chemical assistance of the physical sputtering (like that reported in [33]), contributing to the higher release of W-species in plasma, in particular at larger amounts of H 2 in discharge. Another aspect related to discharge chemistry is the presence of W molecular species. It is generally known that the growth of nanoparticles is conditioned by the initial formation of dimers, on which subsequent attachment of atoms takes place. These can be of Me-Me or Me-E, E being an impurity or residual gas (such a situation is presented in [12], where Ti-O and Co-N dimmers are the nucleation centres for Ti and Co NPs synthesis by MSGA processes). The Me-Me dimmers are generated through two channels: directly from the target (due to sputtering) and in plasma volume following a three-body collision process (Me + Me + Ar→Me-Me + Ar) [34, 35]. The value of the bond dissociation energy (BDE) of the dimmers depends on metal nature. Dimmers with lower BDE (for example Cu-Cu with BDE = 2.08 eV [12]) have great chances of being destructed in the intense magnetron discharge regions, and only the surviving ones will act as nucleation centers for further growth of nanoparticles. In the literature, it is noted that for metals with higher binding energy (like Nb, BDE = 5.3 eV [36]) the dimmers originating from the target are not destroyed by discharge [9]. In the case of the W target, due to the mass spectrometry system limitation (max 300 a.m.u), the W-W dimers cannot be observed. Nonetheless, considering the value of W-W dimer BDE reported in the literature (5. 22 eV[37] ) which is in the same range as that of Nb, one can consider that the W-W dimers originating from the target will survive in plasma with high probability. Another mechanism which contributes to the entire process is related to the three-body collision, responsible for W-W dimers formation in the plasma volume. It is expected that the dilution of the Ar with H 2 will lead to a decrease in the frequency of the three-body collision (W + W + Ar), therefore to the decrease in the number of W-W dimers formed in the gas-phase. It remains that the number of W species released (W atoms, W-W dimmers) by direct sputtering, which is more efficient when H 2 is present in discharge, will compensate for the decrease of the three-body reaction rate and will contribute substantially to the W NPs synthesis. Still, the literature does not consider the possibility that higher mass tungsten species containing hydrogen or other gas impurities (oxygen, nitrogen, inherent in discharges [23]) may be involved as nucleation centres for nanoparticle formation, this one remaining an open subject of future research. Considering these arguments, we conclude that the W NPs deposited on the a-b region of the substrate, (Fig. 3 ) originate from the W atoms and W-W dimmers sputtered by the fast Ar neutrals, Ar + and ArH + ions (the latest originating from the residual H 2 present in the vacuum chamber). The initial decrease of the deposition rate (see Fig. 2 and Fig. 3 ) is correlated with the gradual consumption of the residual H 2 and the decrease of the ArH + ions signal (see Fig. 6); such as in short time (around 50 s) the relative amount of Ar + is becoming higher than ArH + . After total consumption of the residual H 2 (around 150 s in Fig. 6.), the Ar + becomes dominant and the sputtering process and W NPs synthesis will be supported only by the fast Ar neutrals and Ar + ions. This corresponds to the region b-c of the substrate (see Fig. 2), where a hardly visible trace of W NPs is present; corroborating the QCM measurements that show a small but constant deposition rate (see Fig. 3 ). In fact, deposition in sole Ar take place only in the region c-d of the substrate, after complete residual H 2 consumption. Immediately after deliberate H 2 injection in the discharge (H 2 ON), we note a sudden increase of the ArH + signal, while the Ar + signal is strongly decreasing near zero (see Fig. 5 and Fig. 6); in the same time, the deposition rate of W NPs increases substantially. Conclusions We investigate the behaviour of the synthesis rate of W nanoparticles when a small amount of H 2 (5–20%) is added to an Ar magnetron discharge. The enhancement of the W NPs synthesis rate is noticed and the enhanced synthesis rate remains constant over time, its value increasing with the increase of the H 2 content. The positive ions mass spectrometry investigations revealed that when the discharge is fed with sole Ar, some ArH + ions, originating from the residual H 2 , participate together with Ar ions and fast neutrals to the target sputtering. After the residual H 2 consumption, the sputtering will be performed only by Ar + ions and fast Ar neutrals resulting in a strong decrease of the W NPs synthesis rate. The nanoparticles obtained at this low rate are produced in pure Ar, without residual hydrogen influence. Still, this synthesis rate is extremely low, being inadequate for practical applications. The deliberate injection of H 2 in discharge, in the range of 5–20%, at constant pressure, leads to a substantial increase in the deposition rate (around 10 times). With over 10% H 2 content, the discharge is dominated by the ArH + ions. As the ArH + ions, compared to Ar + ones, are not affected by the symmetrical charge transfer, they are accelerated toward the target at much higher energies. Consequently, the sputtering process is more efficient in Ar/H 2 plasmas. We conclude that the increase of the synthesis rate in the presence of H 2 in discharge is due to the substantial increase of W species sputtered from the target. Declarations Author Contribution T.A. performed the experiment and contributed to the paper writing; S.D.S performed the Mass Spectrometry experiments; C.C. performed mass spectra processing and interpretation; M.B. contributed to the mass spectrometry data interpretation and text revision; G. D. contributed to the manuscript writing, discussion and interpretation of the results. Acknowledgement This work was supported by the Ministry of Research, Innovation and Digitalization, under Romanian National Core Program LAPLAS VII – contract no. 30N/2023 References Haberland H, Karrais M, Mall M (1991) A new type of cluster and cluster ion source. Zeitschrift für Physik D Atoms, Molecules and Clusters 20:413–415. https://doi.org/10.1007/BF01544025 Shelemin A, Krtous Z, Baloukas B, et al (2023) Fabrication of Plasmonic Indium Tin Oxide Nanoparticles by Means of a Gas Aggregation Cluster Source. 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Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 30 Jun, 2024 Reviews received at journal 18 Jun, 2024 Reviews received at journal 04 Jun, 2024 Reviewers agreed at journal 04 Jun, 2024 Reviews received at journal 02 Jun, 2024 Reviewers agreed at journal 31 May, 2024 Reviewers agreed at journal 30 May, 2024 Reviewers invited by journal 30 May, 2024 Submission checks completed at journal 01 May, 2024 Editor assigned by journal 01 May, 2024 First submitted to journal 30 Apr, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4349303","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":299054794,"identity":"d518bec7-ab94-4198-8165-4fb2246b008c","order_by":0,"name":"Tomy Acsente","email":"","orcid":"","institution":"National Institute for Laser Plasma and Radiation Physics","correspondingAuthor":false,"prefix":"","firstName":"Tomy","middleName":"","lastName":"Acsente","suffix":""},{"id":299054797,"identity":"1649054b-4a89-46ab-ba26-9cc189186967","order_by":1,"name":"Silviu Daniel Stoica","email":"","orcid":"","institution":"National Institute for Laser Plasma and Radiation Physics","correspondingAuthor":false,"prefix":"","firstName":"Silviu","middleName":"Daniel","lastName":"Stoica","suffix":""},{"id":299054801,"identity":"57e65c03-7506-43d8-b5c0-36cdd0763ee1","order_by":2,"name":"Cristina Craciun","email":"","orcid":"","institution":"National Institute for Laser Plasma and Radiation Physics","correspondingAuthor":false,"prefix":"","firstName":"Cristina","middleName":"","lastName":"Craciun","suffix":""},{"id":299054804,"identity":"5d07a492-23b2-4a62-b427-dc8e03521cda","order_by":3,"name":"Bogdana Mitu","email":"","orcid":"","institution":"National Institute for Laser Plasma and Radiation Physics","correspondingAuthor":false,"prefix":"","firstName":"Bogdana","middleName":"","lastName":"Mitu","suffix":""},{"id":299054807,"identity":"c3a6d29d-aea7-47bc-8695-e01c7cb02405","order_by":4,"name":"Gheorghe Dinescu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAwklEQVRIie3QMQqDMBSA4SeuMa4GL/FEsJQWvYri0KXQCzikeICu7TkKzpYHdhF6hXqDdtOtodg56VZo/uVlyBfIA7DZfjGap+8CPPKviKjBOZqRdp7YKmIk+NW7DGNF6Vkd5LCFna8jgngZs47KhnghiwaWJ6khSCwJHUllQmxxVwSx1ZPFNCkS1yySpiQBT1KKrikRxOKQdZs8oDcJUOj+wm999ByrVeYf+mg/NWvUbuxTMb8dmAKAzPyqzWaz/V0vsXw+3nRXBngAAAAASUVORK5CYII=","orcid":"","institution":"National Institute for Laser Plasma and Radiation Physics","correspondingAuthor":true,"prefix":"","firstName":"Gheorghe","middleName":"","lastName":"Dinescu","suffix":""}],"badges":[],"createdAt":"2024-04-30 13:08:36","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4349303/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4349303/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":55977837,"identity":"a60ab167-63dd-4c0f-bbbd-5606b803d284","added_by":"auto","created_at":"2024-05-07 05:58:54","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":184057,"visible":true,"origin":"","legend":"\u003cp\u003ea) The experimental setups used for: \u0026nbsp;a) synthesis of W NPs, and b) investigation of the species present in the clustering plasma at the exit aperture position [22].\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4349303/v1/f4de5675b0d8390ce591b9f1.png"},{"id":55977839,"identity":"32591ac0-f7fd-4149-a929-b38ed5f7adc2","added_by":"auto","created_at":"2024-05-07 05:58:54","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":154644,"visible":true,"origin":"","legend":"\u003cp\u003ea) Image of the particles collected on the collector translated across the nanoparticle beam; b) evolution of the deposition rate as revealed by QCM measurements and of the target DC self-bias voltage.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4349303/v1/10ba6de240d08d1951b90b08.png"},{"id":55977822,"identity":"07e7841f-eaf9-4427-b49e-fc2b19f4cf2d","added_by":"auto","created_at":"2024-05-07 05:58:54","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":105883,"visible":true,"origin":"","legend":"\u003cp\u003eDependence of the WNPs deposition rates over the H\u003csub\u003e2\u003c/sub\u003e contents in discharge; the flow rates in sccm of H\u003csub\u003e2\u003c/sub\u003e/Ar\u0026nbsp; are represented for every point.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4349303/v1/a86e8289d7f43e8f2f6edcae.png"},{"id":55977820,"identity":"02468cc7-23ee-41e0-9194-2fdc5bd5cb9f","added_by":"auto","created_at":"2024-05-07 05:58:54","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":210479,"visible":true,"origin":"","legend":"\u003cp\u003eMass spectra of the ions corresponding to Ar (left column) and W (right column) regions for different H\u003csub\u003e2\u003c/sub\u003e content in discharge: \u0026nbsp;a) 0% H\u003csub\u003e2\u003c/sub\u003e; b) 5 % H\u003csub\u003e2\u003c/sub\u003e; c) 10 % H\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4349303/v1/609b8d51d582e1948eb18485.png"},{"id":55978449,"identity":"e9d29ed4-7ac5-4cb2-a4ef-37e79b752137","added_by":"auto","created_at":"2024-05-07 06:06:54","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":257375,"visible":true,"origin":"","legend":"\u003cp\u003eTime evolution of the ionic species originating from H\u003csub\u003e2\u003c/sub\u003e and Ar detected by mass spectrometry.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4349303/v1/2b39b8fd2eb6e761fd47f85f.jpg"},{"id":55978448,"identity":"06715eeb-5ae2-4858-914e-28dec3d3385c","added_by":"auto","created_at":"2024-05-07 06:06:54","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":112317,"visible":true,"origin":"","legend":"\u003cp\u003eRelative amounts of ionized species detected by mass spectrometry: a) ArH\u003csup\u003e+\u003c/sup\u003e ions compared to the gaseous ions; b) WH\u003csup\u003e+\u003c/sup\u003e compared to all metallic ions and c) metallic ions in relation to the Ar\u003csup\u003e+\u003c/sup\u003e and ArH\u003csup\u003e+\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4349303/v1/0451031500aa5f4307836512.png"},{"id":55978462,"identity":"028c6da5-404c-446a-9e98-efe1d251ba55","added_by":"auto","created_at":"2024-05-07 06:07:00","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1248500,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4349303/v1/4e8b7f95-e811-406a-8eb0-fb561250027c.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Enhancement of W nanoparticles synthesis by injecting H 2 in a magnetron sputtering gas aggregation cluster source operated in Ar","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMagnetron Sputtering combined with Gas Aggregation (MSGA) is presently a well-established physical method for the preparation of nanoparticles of various materials like metals [1], their compounds (oxides[2], nitrides [3]), and even polymers [4]. In addition, this method is used for the preparation of core-shell nanoparticles [5, 6], or multi-component ones [7, 8]. Significant advantages of the method are obvious due to the mono-disperse dimension of the particles, and the tunability of the particles shape and dimension via the process parameters [9]. Still, the MSGA method presents, for some target materials, the drawback of the synthesis rate reduction down to zero in a finite time interval. The duration of this time interval (when a decrease in the synthesis rate is observed) may last for minutes to hours, being dependent on the initial vacuum conditioning of the cluster source walls [10]. As such, it is supposed that the residual gases present in the aggregation chamber play a significant role in the cluster seeds formation [11]. The gradual consumption of these gases in the nucleation process leads to the depletion of the number of nucleation centers until their total absence. Moreover, it was observed that the deliberate addition of some gases in the aggregation chamber leads to the revitalisation of the synthesis of nanoparticles. For example, the nucleation of Ti or Co NPs depends on the presence of O\u003csub\u003e2\u003c/sub\u003e or N\u003csub\u003e2\u003c/sub\u003e trace, the formation of nucleation centers based on metal oxide or nitride being more efficient than that of pure metal atoms originating from the sputtering [12]. A similar effect was observed by us during the synthesis of W nanoparticles (W NPs) by MSGA in an Ar atmosphere: the synthesis rate, as indicated by the amount of NPs collected on a substrate, decreases in time [13]. Apart to the vacuum conditioning of the aggregation chamber, we noted that this duration depends also on the degree of target erosion (tens of minutes for a new target, respective tens of seconds for a highly eroded one) [14]. In both cases, the W NPs synthesis process is revitalized after venting the MSGA chamber with ambient air for at least one hour; this fact suggests that the nucleation of the W NPs is dependent on the atmospheric residual gases (like H\u003csub\u003e2\u003c/sub\u003e, He, O\u003csub\u003e2\u003c/sub\u003e, N\u003csub\u003e2\u003c/sub\u003e, CO\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003eO vapours) present in the MSGA. Aiming to clarify this behaviour, we looked at the effects of mixing small amounts of these gases with Ar in MSGA, immediately after the deposition rate ceased. We identified by trials that hydrogen (H\u003csub\u003e2\u003c/sub\u003e) is the only gas sustaining the continuous synthesis of the tungsten nanoparticles in an MSGA process [15].\u003c/p\u003e \u003cp\u003eIn the present study, we describe the behaviour of the W NPs production rate when small amounts of H\u003csub\u003e2\u003c/sub\u003e (up to 20%) are mixed with Ar in the MSGA aggregation chamber. Weighing the particles deposited on collectors and quartz crystal microbalance measurements were used to assess the deposition rates of W NPs. The description of the ionic species present in discharge during the W NPs synthesis is performed by mass spectrometry. This technique is frequently used for the characterization and optimization of plasma processes, like in the synthesis of carbonic nanostructures,[16, 17] functionalization of nanoparticles[18] and generation of dusty plasma in methane-based discharges [19]. Moreover, mass spectrometry investigations have shown that small amounts of H\u003csub\u003e2\u003c/sub\u003e injected in sputtering discharges play a critical role in increasing the deposition rate of gold films [20]. Also, a recent mass spectrometry study presents the role of dimmers directly sputtered from the target in the efficient growth of Cu or Ag nanoparticles in gas aggregation cluster sources [21]. The present study benefits from our recent works[22, 23] presenting a novel method for processing mass spectra and the species identified in sputtering plasmas generated in Ar and Ar/H\u003csub\u003e2\u003c/sub\u003e gases in contact with W surfaces. Synthesis of the W NPs in such experimental conditions is relevant for applications where tungsten material is in contact with hydrogen plasmas, specifically in fusion reactors and in nanotechnology [15].\u003c/p\u003e"},{"header":"Experimental setups and methods","content":"\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea presents the schematic of the MSGA cluster source attached to a particle collection chamber. The cluster source was previously presented in [13] and consists of a small cylindrical vacuum chamber (100 mm x 300 mm), axially housing a 2\u0026rdquo; magnetron sputtering gun (MS), and ending with a small diameter (2 mm) exit aperture. The W target is already eroded from previous experiments, allowing an initial deposition of W NPs in Ar no longer than one minute. The distance between the target and the exit aperture (the aggregation length, where nanoparticles growth takes place) is d\u003csub\u003eTN\u003c/sub\u003e=7 cm. In between the pumping system and the MSGA chamber is mounted a bypass valve (nominated R). Every process starts with pumping both chambers down to 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e Pa and with target cleaning by pre-sputtering for around 20 minutes. The cleaning plasma is generated in Ar (at 5 sccm mass flow rate) at low pressure (0.2 Pa in both chambers), applying P\u003csub\u003eRF\u003c/sub\u003e = 80 W to the magnetron cathode. During this process, the bypass valve R is opened. By closing the valve, the pressure increases up to 80 Pa in the aggregation chamber and 0.8 Pa in the collection chamber, values adequate for the initiation of the nanoparticle synthesis process [13].\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv\u003e\n\u003ch2\u003e3.1 Evidence for the enhancement of the nanoparticle production rate by hydrogen addition\u003c/h2\u003e\n\u003cp\u003eAfter target cleaning in low-pressure Ar plasma, the particle synthesis is triggered in Ar by closing the bypass valve, thus leading to the pressure increase in the aggregation chamber to an established value of 80 Pa. Figure\u0026nbsp;2 presents an image of the particles deposited on the moving collector translated across the nanoparticle beam, where the position scale was converted to time scale (1mm corresponds to 5 s). The onset of the synthesis is noted with point \u003cem\u003ea\u003c/em\u003e and it corresponds to the moment when the valve R is closed, leading to the pressure increase up to 80 Pa. The seed-like shape of the deposit (region \u003cem\u003eab\u003c/em\u003e on the substrate) demonstrates the decrease of the synthesis rate in time until the region \u003cem\u003ebc\u003c/em\u003e, characterized by a hard-to-see track of nanoparticles, is reached. The injecting of a small amount of H\u003csub\u003e2\u003c/sub\u003e in the discharge (5%) leads to a sudden increase in the deposition rate (point c), which is maintained at a high value over a long period (region \u003cem\u003ecd\u003c/em\u003e) until the process is stopped by switching off the RF power. A similar evolution of the deposition rate is revealed by QCM measurements (see Fig.\u0026nbsp;2. b). First, only Ar is injected in the aggregation chamber (between 1500\u0026ndash;3500 s), and at around 3500 s H\u003csub\u003e2\u003c/sub\u003e (5%) is injected in the discharge. We note at the beginning of the chart (at around 1500 s) a short time interval (~\u0026thinsp;48 s) presenting a peak of the QCM rate signal (60 mHz/s ), compatible with the time length of the seed-like spot (region \u003cem\u003ea-b\u003c/em\u003e of the substrate) presented in Fig.\u0026nbsp;2. a. Further on, the QCM signal presents a much smaller, still constant (~\u0026thinsp;7 mHz/s) value of the synthesis rate. This region is associated with the region \u003cem\u003ebc\u003c/em\u003e of the substrate (Fig.\u0026nbsp;2a). After H\u003csub\u003e2\u003c/sub\u003e injection in the discharge, the deposition rate suddenly increases, also presenting an oscillatory behaviour (~\u0026thinsp;10 s period and amplitudes of around 50 mHz/s). It deserves commenting that besides the QCM oscillations, also the self-bias voltage starts to oscillate during dust synthesis (Fig.\u0026nbsp;2.b). Similar oscillations were reported also in [24], suggesting the presence of a significant amount of dust in the aggregation chamber. The effect of increasing the amount of injected H\u003csub\u003e2\u003c/sub\u003e on the W NPs deposition rate (obtained by weighting of stationary substrates) is presented in Fig.\u0026nbsp;3. We note that in the range of the H\u003csub\u003e2\u003c/sub\u003e/Ar ratios used in these experiments the deposition rate increases with the H\u003csub\u003e2\u003c/sub\u003e content, even if the amount of Ar (which is the sputtering gas) was slightly decreased to maintain a constant pressure of 80 Pa in the MSGA cluster source (see ratios H\u003csub\u003e2\u003c/sub\u003e/Ar denoted as insets in Fig.\u0026nbsp;3).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.2 Behaviour of ion species in Ar plasmas in contact with the W surfaces during H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003einjection\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eGeneral mass spectra obtained in Ar and Ar/H\u003csub\u003e2\u003c/sub\u003e plasmas in contact with W surfaces were discussed in detail in our previous works [22, 23]. In the low-mass region of these spectra, the ionic species of hydrogen (H\u003csup\u003e+\u003c/sup\u003e, H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, and H\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e), and those related to argon (Ar\u003csup\u003e2+\u003c/sup\u003e, Ar\u003csup\u003e+,\u003c/sup\u003e ArH\u003csup\u003e+\u003c/sup\u003e) are noticed, with some signatures of impurities-related ions (containing oxygen, nitrogen, carbon). In the high-mass region (bigger than 180 amu) the ions related to tungsten are observed. As commented in [23], the interpretation of W-related mass spectra is not straightforward because of the peaks superposition, which is caused by the closed positions of W isotopes (\u003csup\u003e180\u003c/sup\u003eW with natural abundance NA of 0,12%, \u003csup\u003e182\u003c/sup\u003eW with NA 26.5%, \u003csup\u003e183\u003c/sup\u003eW with NA 14.3%, \u003csup\u003e184\u003c/sup\u003eW with NA 30.6%, \u003csup\u003e186\u003c/sup\u003eW with NA 28.4%) combined with the limited resolution of the mass spectrometer. Nevertheless, by using the fitting procedure developed in[22] we succeeded in featuring in the spectra, not only the W\u003csup\u003e+\u003c/sup\u003e peaks but also the tungsten molecular species formed in the presence of hydrogen and gas impurities, among which WH\u003csup\u003e+\u003c/sup\u003e is presenting the highest signal. We mention that the measuring range of our spectrometer (0-300 amu) does not permit the detection of W-W dimers. To exemplify, Figs.\u0026nbsp;4a, b, and c present in more detail the mass spectra in the mass regions of Ar and W, recorded from Ar plasmas with hydrogen content of 0%, 5%, and 10%. In these figures are observed the peaks corresponding to Ar\u003csup\u003e+\u003c/sup\u003e (40 amu), ArH\u003csup\u003e+\u003c/sup\u003e (41 amu), W\u003csup\u003e+\u003c/sup\u003e (180; 182; 183; 184; 186 amu), and respective WH\u003csup\u003e+\u003c/sup\u003e (at 181; 183; 184; 185; 187 amu). The peaks corresponding to \u003csup\u003e180\u003c/sup\u003eW\u003csup\u003e+\u003c/sup\u003e and \u003csup\u003e180\u003c/sup\u003eWH\u003csup\u003e+\u003c/sup\u003e are negligible due to the low natural abundance of the \u003csup\u003e180\u003c/sup\u003eW isotope (0.12%). We underline that, when both W\u003csup\u003e+\u003c/sup\u003e and WH\u003csup\u003e+\u003c/sup\u003e ions are present, the 182 amu peak originates only from W\u003csup\u003e+\u003c/sup\u003e ions (i.e. \u003csup\u003e182\u003c/sup\u003eW\u003csup\u003e+\u003c/sup\u003e) while 185 and 187 amu peaks only from WH\u003csup\u003e+\u003c/sup\u003e ions (i.e. \u003csup\u003e184\u003c/sup\u003eWH\u003csup\u003e+\u003c/sup\u003e and \u003csup\u003e186\u003c/sup\u003eWH\u003csup\u003e+\u003c/sup\u003e).\u003c/p\u003e\n\u003cdiv\u003e\u0026nbsp;\u003c/div\u003e\n\u003cp\u003eFigure\u0026nbsp;4a shows that feeding the discharge with Ar only leads to a strong signal associated with Ar\u003csup\u003e+\u003c/sup\u003e, accompanied by a hardly detectable signature of ArH\u003csup\u003e+\u003c/sup\u003e, originating from the inherent residual hydrogen. In the high masses region, one observes the presence of all mentioned W\u003csup\u003e+\u003c/sup\u003e ions, while the peaks related only to WH\u003csup\u003e+\u003c/sup\u003e ions (185 and 187 amu) are missing. Adding a small amount (5%) of H\u003csub\u003e2\u003c/sub\u003e in discharge (Fig.\u0026nbsp;4b), we note a reversal in the amplitude of the signals corresponding to Ar\u003csup\u003e+\u003c/sup\u003e and ArH\u003csup\u003e+\u003c/sup\u003e ions, with the last one becoming dominant; a slight increase in the signals associated only to WH\u003csup\u003e+\u003c/sup\u003e ions (185 and 187 amu) is also noted. At 10% H\u003csub\u003e2\u003c/sub\u003e in discharge (Fig.\u0026nbsp;4c), the Ar\u003csup\u003e+\u003c/sup\u003e signal is practically absent, indicating that all available Ar is consumed in generating ArH\u003csup\u003e+\u003c/sup\u003e ions; in the same time, the peaks of WH\u003csup\u003e+\u003c/sup\u003e are enhanced.\u003c/p\u003e\n\u003cp\u003eFigure\u0026nbsp;5 presents the temporal evolution of the H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, Ar\u003csup\u003e+\u003c/sup\u003e and ArH\u003csup\u003e+\u003c/sup\u003e ions intensities in a time-dependent experiment like those discussed in Fig.\u0026nbsp;2a,b. The plasma was initially fed only with Ar and subsequently (t\u0026thinsp;=\u0026thinsp;2000 s), H\u003csub\u003e2\u003c/sub\u003e in the amount of 5% was injected. We note in the first 2000 s that the residual H\u003csub\u003e2\u003c/sub\u003e is gradually decreasing, presumably consumed in the formation of ArH\u003csup\u003e+\u003c/sup\u003e ions (ArH\u003csup\u003e+\u003c/sup\u003e having however a smaller intensity than Ar\u003csup\u003e+\u003c/sup\u003e, similar to Fig.\u0026nbsp;4a). When H\u003csub\u003e2\u003c/sub\u003e is admitted in the discharge, the signal from ArH\u003csup\u003e+\u003c/sup\u003e becomes dominant over that of Ar\u003csup\u003e+\u003c/sup\u003e (similar with Fig.\u0026nbsp;4b), being canceled immediately after H\u003csub\u003e2\u003c/sub\u003e suppression. These results prove that injection in the discharge of even small amounts of H\u003csub\u003e2\u003c/sub\u003e induces the dominant presence of ArH\u003csup\u003e+\u003c/sup\u003e ions in the discharge, overcoming the number of Ar\u003csup\u003e+\u003c/sup\u003e ions.\u003c/p\u003e\n\u003cdiv\u003e\u0026nbsp;\u003c/div\u003e\n\u003cp\u003eFigure\u0026nbsp;6 presents the relative amounts of some relevant species for the sputtering process in the magnetron discharge. They resulted from the experimental spectra fitting and deconvolution, considering the areas corresponding to each peak. In the W masses range, the peaks corresponding to W\u003csup\u003e+\u003c/sup\u003e and WH\u003csup\u003e+\u003c/sup\u003e ions are overlapping and the deconvolution procedure previously presented by us in[22] was applied. Figure\u0026nbsp;6a shows that when H\u003csub\u003e2\u003c/sub\u003e exceeds 10%, the ArH\u003csup\u003e+\u003c/sup\u003e becomes dominant among the sputtering species Ar\u003csup\u003e+\u003c/sup\u003e and ArH\u003csup\u003e+\u003c/sup\u003e. Concerning the metal-related peaks, the relative amount of WH\u003csup\u003e+\u003c/sup\u003e ions increases when H\u003csub\u003e2\u003c/sub\u003e increases (Fig.\u0026nbsp;6b). Finally, Fig.\u0026nbsp;6c shows the behavior of the ratio (W\u003csup\u003e+\u003c/sup\u003e+WH\u003csup\u003e+\u003c/sup\u003e)/(Ar\u003csup\u003e+\u003c/sup\u003e+ArH\u003csup\u003e+\u003c/sup\u003e) when the amount of H\u003csub\u003e2\u003c/sub\u003e added to the discharge increases. This ratio can be taken as a rough indication of the number of tungsten species released in the discharge per available sputtering (Ar\u003csup\u003e+\u003c/sup\u003e or ArH\u003csup\u003e+\u003c/sup\u003e) ion.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe above-presented results demonstrate higher nanoparticle production rates in the presence of low amounts of hydrogen injected in the Ar sputtering discharge. Several causes can be considered for this effect, the most important being an increase in the sputtering rates as suggested by the data from Fig.\u0026nbsp;6c.\u003c/p\u003e\n\u003cp\u003eThe modification of the sputtering rates can be related to the energies, nature, and the number of ions impinging on the target surface, but also to the state of the surface. Adding H\u003csub\u003e2\u003c/sub\u003e changes the Ar discharge properties, which is a vast and complex subject. An overview of the relevant processes leading to the formation of species may be found in [25, 26], where the occurrence of different ionized species like Ar\u003csup\u003e+\u003c/sup\u003e, ArH\u003csup\u003e+\u003c/sup\u003e, H\u003csup\u003e+\u003c/sup\u003e, H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, H\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, sputtered metal atoms and the corresponding ions are presented. All these types of species were observed also by us, as shown above.\u003c/p\u003e\n\u003cp\u003eThe first aspect, referring to the energy of ions, is related to the electrical properties of the discharge, which are changed during H\u003csub\u003e2\u003c/sub\u003e addition. The RF magnetron discharge used in MSGA is highly asymmetric, proceeding between the small area target acting as an RF electrode and the larger area of the grounded aggregation chamber wall. Consequently, a negative self-bias appears on the RF electrode [27], responsible for the positive ions acceleration towards it. Figure\u0026nbsp;2b presents the time dependence of the self-bias dependence upon hydrogen content, showing that inserting hydrogen in the Ar based discharge leads to an increase of the self-bias value. It results that the positive ions are accelerated to higher energies while crossing the cathode sheath, being a reason for the enhancement of the sputtering rate in the presence of hydrogen; a similar effect of increasing the sputtering rate is reported [20] for gold thin films. On the other hand, the target self-bias voltage oscillates at hydrogen injection, similar to the deposition rates measured by QCM (Fig.\u0026nbsp;2) during dust synthesis.\u003c/p\u003e\n\u003cp\u003eAnother aspect is related to the discharge chemistry, manifested by the variation of Ar\u003csup\u003e+\u003c/sup\u003e and/or ArH\u003csup\u003e+\u003c/sup\u003e ions number and the appearance of W molecular ionic species for example WH\u003csup\u003e+\u003c/sup\u003e (see Fig.\u0026nbsp;4 and Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e). In pure Ar discharges, the cathode atoms are sputtered both by the fast Ar\u003csup\u003e+\u003c/sup\u003e ions[28, 29] and by the fast Ar neutrals produced by symmetrical charge transfer (SCT) processes[30]. In a SCT event, the charge and part of the kinetic energy of the Ar\u003csup\u003e+\u003c/sup\u003e ion are transferred to a neutral Ar atom. The SCT process is efficient at higher pressures (due to higher probability of collisions), comparable with that used in the MSGA processes. The SCT process limits the upper energies of the Ar\u003csup\u003e+\u003c/sup\u003e ions arriving at the target: in a collision of particles with the same mass about half of the kinetic energy of the incoming particle is transferred to the collided particle [30]. Therefore, the energies of the ion and neutral populations tend to equilibrate, with the energy of ions at a lower value and that of neutral at higher values than those corresponding to the missing SCT situation. The addition of H\u003csub\u003e2\u003c/sub\u003e in the discharge leads to the efficient generation of the ArH\u003csup\u003e+\u003c/sup\u003e ions (see Figs.\u0026nbsp;4, \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e and 6a). As these ions do not have a neutral[20, 31] counterpart they are not affected by SCT and can acquire higher kinetic energy during traversing the cathode sheath. Such as the addition of H\u003csub\u003e2\u003c/sub\u003e in the discharge will increase the target sputtering rate, as was also pointed in [20]. In addition, the state of the surface affects the sputtering rate. In the presence of hydrogen plasma, the tungsten surfaces may become supersaturated with hydrogen, leading to the formation on the target surface of a thin surface layer with a high content of H, a phenomenon which can appear even at low energies of H\u003csup\u003e+\u003c/sup\u003e ions [32]. This can lead to chemical assistance of the physical sputtering (like that reported in [33]), contributing to the higher release of W-species in plasma, in particular at larger amounts of H\u003csub\u003e2\u003c/sub\u003e in discharge.\u003c/p\u003e\n\u003cp\u003eAnother aspect related to discharge chemistry is the presence of W molecular species. It is generally known that the growth of nanoparticles is conditioned by the initial formation of dimers, on which subsequent attachment of atoms takes place. These can be of Me-Me or Me-E, E being an impurity or residual gas (such a situation is presented in [12], where Ti-O and Co-N dimmers are the nucleation centres for Ti and Co NPs synthesis by MSGA processes). The Me-Me dimmers are generated through two channels: directly from the target (due to sputtering) and in plasma volume following a three-body collision process (Me\u0026thinsp;+\u0026thinsp;Me\u0026thinsp;+\u0026thinsp;Ar\u0026rarr;Me-Me\u0026thinsp;+\u0026thinsp;Ar) [34, 35]. The value of the bond dissociation energy (BDE) of the dimmers depends on metal nature. Dimmers with lower BDE (for example Cu-Cu with BDE\u0026thinsp;=\u0026thinsp;2.08 eV [12]) have great chances of being destructed in the intense magnetron discharge regions, and only the surviving ones will act as nucleation centers for further growth of nanoparticles. In the literature, it is noted that for metals with higher binding energy (like Nb, BDE\u0026thinsp;=\u0026thinsp;5.3 eV [36]) the dimmers originating from the target are not destroyed by discharge [9]. In the case of the W target, due to the mass spectrometry system limitation (max 300 a.m.u), the W-W dimers cannot be observed. Nonetheless, considering the value of W-W dimer BDE reported in the literature (5. 22 eV[37] ) which is in the same range as that of Nb, one can consider that the W-W dimers originating from the target will survive in plasma with high probability.\u003c/p\u003e\n\u003cp\u003eAnother mechanism which contributes to the entire process is related to the three-body collision, responsible for W-W dimers formation in the plasma volume. It is expected that the dilution of the Ar with H\u003csub\u003e2\u003c/sub\u003e will lead to a decrease in the frequency of the three-body collision (W\u0026thinsp;+\u0026thinsp;W\u0026thinsp;+\u0026thinsp;Ar), therefore to the decrease in the number of W-W dimers formed in the gas-phase. It remains that the number of W species released (W atoms, W-W dimmers) by direct sputtering, which is more efficient when H\u003csub\u003e2\u003c/sub\u003e is present in discharge, will compensate for the decrease of the three-body reaction rate and will contribute substantially to the W NPs synthesis. Still, the literature does not consider the possibility that higher mass tungsten species containing hydrogen or other gas impurities (oxygen, nitrogen, inherent in discharges [23]) may be involved as nucleation centres for nanoparticle formation, this one remaining an open subject of future research.\u003c/p\u003e\n\u003cp\u003eConsidering these arguments, we conclude that the W NPs deposited on the a-b region of the substrate, (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e) originate from the W atoms and W-W dimmers sputtered by the fast Ar neutrals, Ar\u003csup\u003e+\u003c/sup\u003e and ArH\u003csup\u003e+\u003c/sup\u003e ions (the latest originating from the residual H\u003csub\u003e2\u003c/sub\u003e present in the vacuum chamber). The initial decrease of the deposition rate (see Fig.\u0026nbsp;2 and Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e) is correlated with the gradual consumption of the residual H\u003csub\u003e2\u003c/sub\u003e and the decrease of the ArH\u003csup\u003e+\u003c/sup\u003e ions signal (see Fig.\u0026nbsp;6); such as in short time (around 50 s) the relative amount of Ar\u003csup\u003e+\u003c/sup\u003e is becoming higher than ArH\u003csup\u003e+\u003c/sup\u003e. After total consumption of the residual H\u003csub\u003e2\u003c/sub\u003e (around 150 s in Fig.\u0026nbsp;6.), the Ar\u003csup\u003e+\u003c/sup\u003e becomes dominant and the sputtering process and W NPs synthesis will be supported only by the fast Ar neutrals and Ar\u003csup\u003e+\u003c/sup\u003e ions. This corresponds to the region b-c of the substrate (see Fig.\u0026nbsp;2), where a hardly visible trace of W NPs is present; corroborating the QCM measurements that show a small but constant deposition rate (see Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). In fact, deposition in sole Ar take place only in the region c-d of the substrate, after complete residual H\u003csub\u003e2\u003c/sub\u003e consumption. Immediately after deliberate H\u003csub\u003e2\u003c/sub\u003e injection in the discharge (H\u003csub\u003e2\u003c/sub\u003e ON), we note a sudden increase of the ArH\u003csup\u003e+\u003c/sup\u003e signal, while the Ar\u003csup\u003e+\u003c/sup\u003e signal is strongly decreasing near zero (see Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e and Fig.\u0026nbsp;6); in the same time, the deposition rate of W NPs increases substantially.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eWe investigate the behaviour of the synthesis rate of W nanoparticles when a small amount of H\u003csub\u003e2\u003c/sub\u003e (5\u0026ndash;20%) is added to an Ar magnetron discharge. The enhancement of the W NPs synthesis rate is noticed and the enhanced synthesis rate remains constant over time, its value increasing with the increase of the H\u003csub\u003e2\u003c/sub\u003e content. The positive ions mass spectrometry investigations revealed that when the discharge is fed with sole Ar, some ArH\u003csup\u003e+\u003c/sup\u003e ions, originating from the residual H\u003csub\u003e2\u003c/sub\u003e, participate together with Ar ions and fast neutrals to the target sputtering. After the residual H\u003csub\u003e2\u003c/sub\u003e consumption, the sputtering will be performed only by Ar\u003csup\u003e+\u003c/sup\u003e ions and fast Ar neutrals resulting in a strong decrease of the W NPs synthesis rate. The nanoparticles obtained at this low rate are produced in pure Ar, without residual hydrogen influence. Still, this synthesis rate is extremely low, being inadequate for practical applications. The deliberate injection of H\u003csub\u003e2\u003c/sub\u003e in discharge, in the range of 5\u0026ndash;20%, at constant pressure, leads to a substantial increase in the deposition rate (around 10 times). With over 10% H\u003csub\u003e2\u003c/sub\u003e content, the discharge is dominated by the ArH\u003csup\u003e+\u003c/sup\u003e ions. As the ArH\u003csup\u003e+\u003c/sup\u003e ions, compared to Ar\u003csup\u003e+\u003c/sup\u003e ones, are not affected by the symmetrical charge transfer, they are accelerated toward the target at much higher energies. Consequently, the sputtering process is more efficient in Ar/H\u003csub\u003e2\u003c/sub\u003e plasmas. We conclude that the increase of the synthesis rate in the presence of H\u003csub\u003e2\u003c/sub\u003e in discharge is due to the substantial increase of W species sputtered from the target.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eT.A. performed the experiment and contributed to the paper writing; S.D.S performed the Mass Spectrometry experiments; C.C. performed mass spectra processing and interpretation; M.B. contributed to the mass spectrometry data interpretation and text revision; G. D. contributed to the manuscript writing, discussion and interpretation of the results.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was supported by the Ministry of Research, Innovation and Digitalization, under Romanian National Core Program LAPLAS VII \u0026ndash; contract no. 30N/2023\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eHaberland H, Karrais M, Mall M (1991) A new type of cluster and cluster ion source. Zeitschrift f\u0026uuml;r Physik D Atoms, Molecules and Clusters 20:413\u0026ndash;415. https://doi.org/10.1007/BF01544025\u003c/li\u003e\n\u003cli\u003eShelemin A, Krtous Z, Baloukas B, et al (2023) Fabrication of Plasmonic Indium Tin Oxide Nanoparticles by Means of a Gas Aggregation Cluster Source. ACS Omega 8:6052\u0026ndash;6058. https://doi.org/10.1021/acsomega.2c08070\u003c/li\u003e\n\u003cli\u003eXu Y-H, Hosein S, Judy JH, Wang J-P (2005) Iron nitride nanoparticles by nanocluster deposition. 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J Chem Phys 131:. https://doi.org/10.1063/1.3187525\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"plasma-chemistry-and-plasma-processing","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Plasma Chemistry and Plasma Processing](https://www.springer.com/journal/11090 ","snPcode":"11090","submissionUrl":"https://mc.manuscriptcentral.com/pcpp","title":"Plasma Chemistry and Plasma Processing","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"gas aggregation, magnetron sputtering, nanoparticles, mass spectrometry, dimer, synthesis rate of nanoparticles","lastPublishedDoi":"10.21203/rs.3.rs-4349303/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4349303/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSynthesis of W nanoparticles by magnetron sputtering combined with gas aggregation operated in Ar suffers from a continuous decrease of the synthesis rate, ceasing in a finite time interval, in the range of minutes to tens of minutes. Experimentally, we noticed that the addition of small amounts of H\u003csub\u003e2\u003c/sub\u003e to Ar (5\u0026ndash;20%) increases the synthesis rate, which remains constant over time, at a value dependent on the amount of injected hydrogen. Mass spectrometry investigations revealed, in the hydrogen presence, a dominance of the ArH\u003csup\u003e+\u003c/sup\u003e ions over the Ar\u003csup\u003e+\u003c/sup\u003e ones, associated also with an increased number of W\u003csup\u003e+\u003c/sup\u003e and WH\u003csup\u003e+\u003c/sup\u003e species in plasma, sustaining a substantial increase in the nucleation rate.\u003c/p\u003e","manuscriptTitle":"Enhancement of W nanoparticles synthesis by injecting H 2 in a magnetron sputtering gas aggregation cluster source operated in Ar","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-07 05:58:49","doi":"10.21203/rs.3.rs-4349303/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-06-30T11:21:49+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-18T10:10:50+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-04T12:29:40+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"107487568727276561770479881270112924813","date":"2024-06-04T12:02:36+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-02T14:31:45+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"172337647249620825617973390977826800566","date":"2024-05-31T07:36:07+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"154122125652270740891281267733286617279","date":"2024-05-30T14:10:59+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-05-30T13:36:42+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-05-01T05:14:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-01T05:14:44+00:00","index":"","fulltext":""},{"type":"submitted","content":"Plasma Chemistry and Plasma Processing","date":"2024-04-30T13:07:12+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"plasma-chemistry-and-plasma-processing","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Plasma Chemistry and Plasma Processing](https://www.springer.com/journal/11090 ","snPcode":"11090","submissionUrl":"https://mc.manuscriptcentral.com/pcpp","title":"Plasma Chemistry and Plasma Processing","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"202abd04-ff54-4ee7-9bae-c95b008e1084","owner":[],"postedDate":"May 7th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2024-08-14T13:06:21+00:00","versionOfRecord":[],"versionCreatedAt":"2024-05-07 05:58:49","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4349303","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4349303","identity":"rs-4349303","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}