Dewetted Pt nanostructures on Silicon Carbide surface

Journal of Physics and Chemistry of Solids 140 (2020) 109403

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Dewetted Pt nanostructures on Silicon Carbide surface F. Ruffino *, M. Censabella , M.G. Grimaldi Dipartimento di Fisica e Astronomia “Ettore Majorana” Universit� a di Catania and MATIS CNR-IMM, via S. Sofia 64, 95123, Catania, Italy

A R T I C L E I N F O

A B S T R A C T

Keywords: Silicon carbide Platinum Dewetting Nanostructures Shape

We exploit the thermal-induced dewetting process of a deposited Pt film to produce two-dimensional arrays of Pt nanostructures on SiC surface. After depositing 7.5 nm-thick Pt film on SiC, we performed annealings increasing the temperature from 773 to 1073 K. Energy dispersive X-ray spectroscopy, atomic force microscopy, and scanning electron microscopy an­ alyses were crossed to draw information on the chemical and morphological evolution of the Pt film increasing the annealing temperature. The gradual evolution of the Pt film, due to the ongoing dewetting process, from a holed film to a web of Pt filaments connecting Pt agglomerates and to, finally, isolated shaped nanostructures was observed. The Pt film surface roughness, fraction of uncovered surface area by the metal film, mean height and mean planar size of the dewetted nanostructures were quantified versus the annealing temperature to extract quantitative information on the main parameters characterizing the dewetting process and the nanostructures morphology.

1. Introduction Silicon Carbide (SiC) is a wide band gap ceramic semiconductor whose physical and chemical properties can be exploited in several technological applications ranging from electronics and optoelectronics to sensing and energy [1–3]. These applications rely on SiC properties as high melting temperature, high thermal conductivity, low thermal expansion coefficient, high hardness, chemical stability, low density, wide band gap [1–3]. In particular, metal/SiC Schottky diodes are, usually, used to design and produce high temperature and high power operating electronic devices and extremely sensitive high temperature operating detectors of hydrogen and hydrocarbon gas [1–15]. So, in the last decades, several approaches were investigated for the controlled production of metal/SiC diodes changing the metal layer (or combina­ tion of metal layers or alloyed metallic layers) and studying the resulting properties as barrier height, stability, etc. To date, a lot of metals in contact to SiC were used to produce Schottky contacts for specific functional applications. Recently, in particular, Pt/Schottky contacts attracted large interest in sensing applications providing promising re­ sults for use in high temperature operating gas sensors as H2 and NOx [7, 8,12]. In addition, Pt nanostructures, as nanowires and nanoparticles, show a high catalytic activity if compared to bulk Pt [16–18]. In this regard, the Pt nanostructures/bulk SiC combination could be the base for the fabrication of high-efficiency sensors [10] overcaming, for

example, the typical limitations of the Pt/Si-based sensors related, mainly, to the low operating temperature [7]. On the other hand, the general key characteristic of metal nanostructures, making them useful in technological applications, is the possibility to largely tune their physico-chemical properties by the control of their size and shape. This is particularly true for Pt nanostructures regarding catalytic properties [16–20]. As a consequence, concerning solid-state technological appli­ cations, the main critical issue for the use of metal nanostructures is the development of simple, versatile, and low-cost methodologies for the fabrication of arrays of nanostructures with desired shape and size directly on surface, as the case of Pt nanostructures on SiC substrate [19–26]. Obviously, the detailed understanding of the basic microscopic mechanisms governing the involved process in these methodologies is crucial in assuring the desired nanostructures control. Due to the recent importance acquired by the Pt nanostructures/SiC system, we present here a dewetting-based approach to produce Pt nanostructures directly on SiC surface and a study on their morpho­ logical properties using microscopic analysis as Scanning Electron Mi­ croscopy (SEM), Atomic Force Microscopy (AFM), Energy Dispersive Xray Spectroscopy (EDX). The thermal-induced dewetting of deposited Pt films on non-wettable substrates was, recently, used as an effective approach to produce large area two-dimensional arrays of Pt nano­ structures on functional surfaces [27–34]. In particular, as examples: Strobel et al. [27] reported a study on the dewetting behavior of Pt thin

* Corresponding author. E-mail address: [email protected] (F. Ruffino). https://doi.org/10.1016/j.jpcs.2020.109403 Received 22 October 2019; Received in revised form 27 January 2020; Accepted 8 February 2020 Available online 9 February 2020 0022-3697/© 2020 Elsevier Ltd. All rights reserved.

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films on Si determining how variation of dewetting parameters affects the evolution of film morphology. Depending on the starting Pt film thickness, annealing temperature and annealing duration they observed the restructuration of the Pt film via hole or droplet formation and a transition from an intermediate network structure to separated islands. The formation of hte interfacial Pt–Si phase was, also, monitored by the authors. This study, in addition to elucidate the basic dewetting mech­ anisms for the Pt films on Si, was proposed by the authors as an effective nanostructuring approach for templated nanofabrication. Atiya et al. [28] studied the thermal-induced (fixing 1150 � C annealing tempera­ ture) solid-state dewetting of 50 nm-thick or 100 nm-thick Pt films on SrTiO3 substrate. As typical for a dewetting process, they observed the retraction of the Pt films, however, interestingly, they observed the formation, from the continuous Pt films, of anisotropic rod-shaped nanostructures depleted in Ti. In addition to focus on the dewetting characteristics of the Pt films on SrTiO3, the authors highlighted the potential properties of the Pt dewetted nanostructures/SrTiO3 systems in electronic devices applications. Jahangir et al. [29] reported on the characteristics of the thermal-induced (annealing temperature in the 580–900 � C range, 15 or 30 min annealing time) solid state dewetting and the subsequent morphological changes for 40 nm-thick Pt thin film grown on ZnO buffered silicon substrates. Generally, increasing the annealing temperature, they observed in the Pt film hillock formation, hole formation, and hole growth that lead to formation of a network of Pt ligaments, break up of Pt ligaments to individual islands and subse­ quent Pt islands shape reformation with the possibility, on the basis of the annealing temperature, to tune the metal exposed surface area and the Pt nanostructures sizes. The impact of the ZnO adhesion layer on the morphological, structural, and chemical characteristics of the Pt nano­ structures was studied so as the formation of specific Pt–Si phases. This study was justified by the importance of the Pt–Si contact in electronic applications. Finally, Lee et al. [30–34] reported a series of studies on the thermal-induced dewetting process of Pt films on sapphire so to fabricate two-dimensional arrays of Pt nanostructures on the sapphire surface whose morphological, structural, chemical, optical (plasmonic) properties were in detail studied as a function of several process pa­ rameters. These studies were justified by general potential Pt nano­ structures applications as in optoelectronics, biomedical, catalytical devices. In particular, the authors demonstrated an improvement of the morphological and localized surface plasmon resonance properties of the Pt nanostructures by using a In sacrificial layer between the Pt films and the sapphire substrates during the dewetting process [32–34]. These works clearly show the recent interest on the controllable production of large arrays of Pt nanostructures on suitable functional substrates using the deposited Pt films dewetting-based approach which proved to be simple, versatile, high-throughput. The present work, regarding the formation of two-dimensional arrays of Pt nanostructures on SiC surface by the thermal-induced dewetting process of deposited Pt film and about the study of the specific characteristics of the dewetting mechanism and morphological and chemical characteristics of the Pt nanostructures versus the annealing parameters, is inserted within this general scientific and technological framework. SiC is a ceramic wide-band-gap semi­ conductor which, by the years, was established as the leading material for power electronics applications [1–6] and which, more recently, was considered in several cutting-edge applications as in sensing and opto­ electronics [1–6]. In these applications, the synergistic combination of the SiC properties and metal nanostructures characteristics often proved to enhance the SiC-based devices performances [1–15]. So, the work here reported arises from the fundamental request for a simple, low-cost, versatile approach for the direct production of SiC-supported large ar­ rays of Pt nanostructures with morphological and chemical character­ istics which can be simply and largely tunable by the control of the structuring mechanisms (i. e by the process parameters). In particular, we deposited a thin (7.5 nm-thick) Pt film on 6H–SiC substrate and performed annealing processes increasing the annealing temperature T from 773 K to 1073 K (fixing the annealing time to 30

min). At each annealing stage, SEM, AFM, EDX analysis were performed to image the step-by-step dewetting process of the Pt film towards the formation of Pt nanostructures. In particular, plots of the fraction of the uncovered surface area by the Pt film, of the surface roughness, of the mean planar size and mean height of the Pt nanostructures, versus the annealing temperature were derived. Analyzing the SEM and AFM im­ ages and these plots, quantitative insights on the characteristics of the dewetting process of the Pt film on the 6H–SiC surface and on the morphological evolution of the Pt nanostructures were drawn. As a consequence, we set a general framework connecting the process pa­ rameters (annealing temperature) to Pt nanostructures morphological characteristics. On the basis of these results, a general qualitative picture for the Pt film dewetting process is, finally, proposed. 2. Experimental Concerning the production of the samples, the supporting substrates were n-type 6H–SiC slides (Si-terminated, doping concentration ND � 5.1 � 1017 cm 3). Pt films were grown on the SiC slides by sputter deposition using a RF Emitech K550X Sputter coater employing a Pt (99.999% purity) target and maintaining the SiC substrate at room temperature. The deposition conditions were set so to deposit 7.5 nmthick Pt films on the SiC surface (as checked by subsequent ex-situ Rutherford Backscattering Spectrometry analysis). The samples were thermally processed in nitrogen environment fixing the annealing time t to 1800 s (30 min) and changing the annealing temperature T from 773 K to 1073 K. Atomic Force Microscopy (AFM) studies were carried out by a Bruker-Innova microscope operating in contact mode and employing Si tips with radius of curvature of ~2 nm. Each AFM images was acquired using a scan rate of 0.3 Hz and acquiring 512 � 512 lines. The acquired AFM images were analyzed using the SPMLABANALYSES V7.00 software. Scanning Electron Microscopy (SEM) studies were carried out by a Gemini Field Emission SEM (FE-SEM) Carl Zeiss SUPRA 25 Microscope operating at 5 kV and working distance of 3 mm. The acquired SEM images were analyzed by the Gatan Digital Micrograph software. Energy Dispersive X-ray (EDX) analyses were performed using the Gemini Field Emission SEM (FE-SEM) Carl Zeiss SUPRA 25 Microscope equipped with an EDAX EDX detector. 3. Results and discussions Fig. 1 reports representative SEM images of the starting SiC substrate surface and of the surface of the 7.5 nm-thick Pt film deposited on the SiC surface. In particular, (a) and (b) show images of the surface of the starting SiC substrate with increasing magnification from (a) to (b); (c) and (d) shows SEM images of the surface of the SiC surface after the deposition of the Pt film, with increasing magnification from (c) to (d). The images in (c) and (d) allow to recognize: 1) parallel lines which are characteristic of the starting SiC surface and which are the typical atomic step-terrace structure (with step height in the Å range) of the SiC surface. They are due to the surface energy minimization of the basal planes exposed by the off-cut, which rearrange into lower energy surfaces [1–3,35] (they are not visible in the images in (a) and (b) due to the low contrast, while in the images in (c) and (d) the presence of the very thin Pt film, increasing the images contrast, makes visible the atomic steps); 2) a rough granular surface in (c) and (d) typical, in the late stages of growth, of the Volmer-Weber growth of metal films on non-wetting surface [36–38] as on SiC [39–41]. Generally speaking, vapour-phase metal atoms arriving on a non-wetting surface undergo various ki­ netics and thermodynamic processes including surface diffusion fol­ lowed by dimers and small clusters nucleation [36–41]. The continued atoms deposition results in the growth of the clusters so to form two-dimensional or three-dimensional isolated islands. However, 2

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Journal of Physics and Chemistry of Solids 140 (2020) 109403

Fig. 1. (a) and (b): SEM images of the surface of the starting SiC substrate with increasing magnification from (a) to (b); (c) and (d): SEM images of the surface of the SiC surface after the deposition of the Pt film, with increasing magnification from (c) to (d).

further increasing the amount of deposited atoms, the islands grow, contact each other, and then partially coalesce resulting in the formation of a holed percolative metal film on the substrate surface (i. e. elongated metal islands, with wormlike shape, touching each other forming a web-like network) [36–41]. Finally, as deposition continues, the wormlike structures grow longer and thicker, and the last holes in the film are filled by the continuing arriving holes-filling process continuous rough metal film is formed by a hole-filling process [36–41]. Then, Fig. 2 reports representative SEM images of the surface of the annealed (for 30 min) Pt film on the SiC surface at 773 K (first line, (a) and (b)), 873 K (second line, (c) and (d)), 973 K (third line, (e) and (f)), 1073 K (fourth line, (g) and (h)). The second row shows images ((b), (d), (f), (h)) at higher magnification than the images in the first row ((a), (c), (e), (g)). Considering that in these SEM images the black regions represent the exposed SiC surface and the brighter regions the exposed Pt surface, it is evident the increase of the SiC exposed surface area (starting from zero value in the Pt as-deposited sample since the Pt film completely and uniformly covers, initially, the SiC surface) and the decrease of the covered surface by Pt (starting from a 100% coverage in the Pt as-deposited sample) by increasing the annealing temperature. This is due to a dewetting process of the Pt film on the SiC surface induced by the annealing process which leads to the gradual formation of Pt nanostructures on the SiC surface due to the minimization of the total surface (energy) of the system. In fact, metal films, typically, don’t wet non-metal surface resulting in a thermodynamically unstable con­ dition. As a consequence, the system reaches an energetic stable con­ dition by minimizing the total surface energy: this occurs by the dewetting process which drives the formation of droplets (both in the solid-state or molten-state) from the continuous film if surface diffusion occurs as, for example, activated by a thermal budget [42–49]. The dewetting process of the film proceeds by some subsequent stages as time or temperature increase [42–49]. Considering, in particular, a sputter-deposited metal layer, it is, generally, characterized by a va­ cancy concentration much higher than the equilibrium concentration at the annealing temperature [49,50]. Therefore, the very first stage of dewetting consists in voids agglomeration in the film preferentially aat locations corresponding to film defects (i. e. heterogeneous voids

nucleation) resulting in the stochastic holes formation in the film reaching the substrate surface [42–49]. This very first stage can be recognized for the Pt film on the SiC surface in Fig. 2(a) and (b). After the holes nucleation stage, further increasing the annealing temperature, the hole grow by continuous addition of more distant vacancies since increasing the annealing temperature an increase of the vacancies mobility occurs (i. e. the vacancies diffusion length increases). As the holes in the film grow they can touch and coalesce to form larger holes with volume the sum of the volumes of the coalescing holes but surface area lower than the sum of the surface areas of the coalescing holes (i. e. the coalescence process of the holes is driven by surface energy mini­ mization). This stage can be recognized for the Pt film on the SiC surface in Fig. 2(c) and (d). The holes formation in the film and their coalescence can be regarded, also, as metal film retraction from locations were structural defects are present. However, the mass conservation requires the formation of more and more thick rim at the holes edges as the holes grow in planar size. In the late stage of dewetting, the holed film appears as a web of interconnected metal filaments. These filaments change their length and thick (i. e. diameter if modeled as cylinders) as the holes grow. However, a filament becomes thermodynamically unstable due to Rayleigh-like instability [42–49] when its length becomes larger than its cross-sectional perimeter and, then, it decays into isolated droplets. This last stage can be recognized for the Pt film on the SiC surface in Fig. 2(e) and (f), where Pt shaped nanostructures are clearly visible to arise from web-like filaments which are the original holes rims. Further increase of time or temperature results in the complete fialements breaking with the formation of the shaped isolated nanostructures, as visible for the Pt nanostructures on SiC by Fig. 2(g) and (h). The information extracted by the SEM planar imaging on the Pt film morphology evolution versus the annealing temperature were supported by AFM imaging so to draw three-dimensional insights. As examples, Fig. 3 reports representative three-dimensional AFM images (2 μm � 2 μm) of the Pt film deposited on the SiC substrate and annealed (for a time of 30 min) at 773 K ((a)), 873 K ((b)), 973 K ((c)), 1023 K ((d)), 1073 K ((e)). Observing these images, increasing the annealing temperature, the gradual roughening of the Pt surface can be recognized, so as the for­ mation of the isolated Pt nanostructures (Fig. 3(e)) through a deweeting 3

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Fig. 2. SEM images of the Pt film deposited on the SiC substrates and annealed (for a time of 30 min) at 773 K (first line, (a) and (b)), 873 K (second line, (c) and (d)), 973 K (third line, (e) and (f)), 1073 K (fourth line, (g) and (h)). The second row shows images ((b), (d), (f), (h)) at higher magnification than the images in the first row ((a), (c), (e), (g)).

process which, first, leads to the partial break-up of the film (Fig. 3(b)) and, then, to the formation of the nanostructures by film retreating (Fig. 3(c)), holes coalescence to form filaments which, finally, decay in the isolated shaped nanostructures. We started the quantitative studies by the chemical analysis of the Pt nanostructures using the EDX technique. For each samples correspond­ ing to each annealing temperature, EDX spectra were acquired by focusing the SEM electron beam on several surface nanostructures. For annealing temperature T < 1073 K, the EDX spectra evidenced, solely, the presence of the Pt peak indicating that the surface nanostructure are formed by pure Pt. On the contrary, at the annealing temperature T ¼ 1073 K the situation was different, see Fig. 4: the figure reports in (a) the representative SEM image of the Pt film deposited on the SiC substrate and annealed at 1073 K, in (b) the EDX spectrum acquired on the region (SiC exposed surface) in the SEM image showed in (a) and over which the red spot was superimposed, and in (c) the EDX spectrum acquired on the structure in the SEM image showed in (a) and over which the yellow spot was superimposed. In the EDX spectra the characteristic peaks are

labelled (C-Kα, Si-Kα and Pt-Mα). The SiC exposed surface show the characteristic C (Kα) and Si (Kα) peaks and no Pt presence is recognized. On the other hand, the EDX spectrum acquired on a shaped surface structure, see Fig. 4(c), shows the simultaneous presence of Pt (Pt-Mα peak) and Si (Si-Kα peak) and a very low contamination of C (C-Kα peak) in the surface structure. In particular, the inset of Fig. 4(c) report an enlargement of the region of the spectrum where the Pt characteristic peak is located. So, we conclude that the 1073 K annealing process causes, in addition to the dewetting process, also a Pt–Si interface re­ action leading to the formation of Pt–Si nanostructures. These data are in agreement with previous studies on thermal annealing induced re­ action of Pt films and SiC substrate: Porter et al. [51] after depositing thin Pt films (0.4–100 nm thick) on n-type 6H–SiC substrate, performed series of annealing processes and analyzed the resulting chemical, structural, and electrical characteristics of the samples versus the annealing temperature. In particular, their analysis of the Pt–SiC inter­ face allowed to conclude that Pt–SiC interfacial reaction does not occur at annealing temperature below 1023 K. Above this temperature, 4

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however, the Pt–SiC interface reaction occurs leading to the formation of Pt2Si silicide and C precipitates due to a segregation-driven process. As a further example, Chou [52] needed high temperatures annealing to observe Pt–SiC reaction: he studied Pt film-SiC reaction in the 1173–1373 K range finding, at 1173 K, reaction zones alternating layers of Pt3Si and carbon. Overall, we conclude that the annealing processes performed at temperature T � 1023 K lead, purely, to a dewetting process of the Pt film on the SiC surface, being in this temperature range the Pt–SiC interface unreactive, and, so, the formation of pure Pt nanostructures on the SiC surface. On the other hand, the annealing process performed at T ¼ 1073 K leads, in addition to the further evo­ lution of the dewetting process with the formation of the isolated shaped surface nanostructures, also, to the Pt–SiC reaction with the formation of Pt2Si silicide. In this case, then, the finally obtained isolated and shaped nanostructures on the SiC surface are formed by Pt and Si in the stable Pt2Si phase. Quantitative morphological analysis were perfomed, also, using both AFM and SEM analysis so to draw insights on the dewetting process and characteristics of the surface nanostructures. First of all, using the AFM analysis, we quantified the evolution of the surface roughness σ of the Pt film versus the annealing temperature. Thin films deposited on a sub­ strate do not present perfectly flat surfaces. They present, instead, rough surfaces due to local height fluctuations. Cumulatively, this height fluctuations are, generally, characterized by the Root Mean Square (RMS) width parameter σ. It is, simply, the height standard deviation arising from these fluctuations with respect to a planar reference surface [53–55]: σ¼<[h (x,y)-]2>1/2 being h (x,y) the height function (the point-bay-point height measured, for example, by the AFM anal­ ysis) and h (x,y)- representing the spatial average over a planar reference surface [53–55]. The film surface roughness, often, determines the film quality and establishes film properties as the elec­ trical, optical, and wetting ones [56]. For this reason, we evaluated the samples surface roughness as a function of the annealing temperature. In particular, for each temperature T, we acquired, at least, six 2 μm � 2 μm AFM scans on the sample surface. Using the SPMLABANALYSES V7.00 software (which implements the RMS calculation as described above on 512 � 512 hi values for each scan), the value σi of the RMS corre­ sponding to the i-th image was calculated by. Then, to each sample was associated the corresponding mean value of σ as obtained by the aver­ aging procedure on the (at least) six σi values. The error in σ was derived as the standard deviation arising from the averaging procedure. The results are summarized in the plot in Fig. 5(a). As a reference, the plot reports, also, the roughness value for the as-deposited Pt film (indicated as the zero annealing temperature). So, for the as-deposited Pt film σ ¼ 1.3 nm and then it increases to σ ¼ 7.2 nm for the annealing temperature T ¼ 773 K. Increasing the annealing temperature, σ linearly increases till σ ¼ 18.1 nm for the annealing temperature T ¼ 1073 K. We interpret this behavior as follows, considering the Pt dewetting process: σ is constant and equals to the starting value of the as-deposited film till to a critical value TC for the temperature. For T ¼ TC, σ abruptly increases and for T > TC it linearly increases with temperature. The critical temperature TC is the temperature needed to start the dewetting process which leads to the surface roughening of the metal film. TC is material film-dependent and, also, dependent on the material forming the substrate, since the film-substrate energetic interaction establishes the characteristics of the film dewetting. Using the plot in Fig. 5(a), we estimated a value for TC by the intersection point of a line parallel to the temperature axes and passing through the starting value of σ for the as-deposited Pt film and the line fitting the linear behavior of σ versus T for the annealing tem­ perature 773 K � T � 1073 K. This procedure leads to the rough esti­ mation TC � 600 K. In our description, so, this is the temperature needed to start the dewetting process of the Pt film on the SiC surface, i. e. the temperature needed to start the process of the holes formation in the film. At T > TC, σ abruptly increases with the annealing temperature: the breaking of the film starts and proceeds by roughening of the film sur­ face due to film retreating and the holes formation in the Pt film. Further

Fig. 3. Three-dimensional AFM images (2 μm � 2 μm) of the Pt film deposited on the SiC substrate and annealed (for a time of 30 min) at 773 K ((a)), 873 K ((b)), 973 K ((c)), 1023 K ((d)), 1073 K ((e)).

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Fig. 4. (a): SEM image of the Pt film deposited on the SiC substrate and annealed at 1073 K (for 30 min); (b): EDX spectrum acquired on the region (SiC exposed surface) in the SEM image showed in (a) and over which the red spot was superimposed; (c): EDX spectrum acquired on the structure in the SEM image showed in (a) and over which the yellow spot was superimposed. In (c), the inset shows an enlargement of the region where the Pt-Mα EDX peak is located.

increasing the temperature T above TC results, gradually, in the holes grow, holes coalescence, filaments formation, and filaments decay in droplets. Correspondently, σ increases as T increases. The SEM images were analyzed using the Gatan Digital Micrograph software to extract quantitative information. The analysis procedure was based, first of all, on setting a threshold on the brightness of the SEM images so that the parts in the SEM images corresponding to the brighter regions (intensity value 1) represent the Pt film while the parts in the SEM images corresponding to the darker regions (intensity value 0) represent the SiC exposed substrate. Exploiting this procedure, first of all, we evaluated the fraction of the SiC substrate surface area with respect to the total surface area versus the annealing temperature. In particular, for each SEM image we measured the areas of the dark and bright parts calculating, finally, the ratio Fi between the dark area (the SiC surface area) and the total area (dark plus bright) of the image (Fi ¼ SiC surface area/total surface area in the SEM image). This procedure was repeated for, at least, ten SEM images for each sample corre­ sponding to each annealing temperature. Therefore, we derive, for each annealing temperature T, a mean value for F and the corresponding error as the standard deviation by the averaging procedure on the (at least) ten Fi values. The results are reported in the plot in Fig. 5(b): the fraction of the surface area uncovered by Pt increases by increasing T which is fully consistent with the Pt film dewetting process which is driven by the minimization of the total surface area of Pt and minimi­ zation of the interface area between Pt and SiC (from the continuous Pt films the dewetting process causes the formation of Pt nanostructures in the form of droplets where the stable final stage is the formation of isolated metal dropltes with shape minimizing the exposed surface area). In Fig. 5(c), we plotted F in an Arrhenius plot, i. e. Log(F) versus T 1: interestingly, the experimental data well-follow the Arrhenius plot indicating a thermal-activated behavior for F, i. e. a thermal activated behavior for the dewetting process. In this plot, the experimental data were fitted by the standard Arrhenius function, F¼F0exp (-EA/kT) with F0 the pre-exponential factor, EA the characteristic activation energy, k the Boltzmann constant. In particular, the fit of the experimental data allows to obtain EA¼(57.0 � 0.4) meV. It is meaningful to observe that this activation energy corresponds to the temperature T ¼ EA/K � 660 K which is in-line with the rough estimation of TC � 600 K as the critical temperature to start the Pt film deweting process as estimated by the plot in Fig. 5(a). Now, we focus our attention on the characteristics of the Pt nano­ structures originating from the Pt film by the dewetting process. In this regard, we use in a combined way the SEM and AFM analysis to draw information on their morphological features. First of all, Fig. 6 reports some representative high-magnification SEM and AFM images focusing on the gradual formation of the Pt nanostructures from the breaking film. In particular, (a)-(d) show high magnification SEM images of the Pt

film, on the SiC susbtrate, annealed at 773 K ((a)), 873 K ((b)), 973 K ((c)), 1073 K ((d)) focusing on one or few Pt structures; (e)-(m) report high magnification AFM images (three-dimensional and twodimensional) and section analysis of some representative Pt structures. In particular: (e) three-dimensional AFM image (500 nm � 500 nm) of the Pt film deposited on the SiC substrate and annealed at 773 K, (f) the corresponding two-dimensional AFM image, and (g) cross-sectional line profile along the line indicated in (f) with reported values for the plan size and height of the corresponding Pt structure; (h) three-dimensional AFM image (500 nm � 500 nm) of the Pt film deposited on the SiC substrate and annealed at 973 K, (i) the corresponding two-dimensional AFM image, and (j) cross-sectional line profile along the line indicated in (i) with reported values for the plan size and height of the corresponding Pt structure; (k) three-dimensional AFM image (500 nm � 500 nm) of the Pt film deposited on the SiC substrate and annealed at 1073 K, (l) the corresponding two-dimensional AFM image, and (m) cross-sectional line profile along the line indicated in (l) with reported values for the plan size and height of the corresponding Pt structure. Clearly, the SEM im­ ages show, increasing the annealing temperature, the breaking of the Pt film. In Fig. 6(a) the holes in the Pt film can be clearly recognized and the increase of the holes planar size is evident in Fig. 6(b). As the size of the holes increases, correspondently the planar size of the Pt nano­ structures increases (due to the mass conservation). So, in Fig. 6(b) the Pt shaped nanostructures are visible and these nanostructures are all connected by an underalying Pt web forming filaments from which the nanostructures originate. Further increasing the temperature, see Fig. 6 (c), causes the break-up of the filaments which, further retreating, leads to the further increase of the planar size of the shaped nanostructures. Finally, further increasing the annealing temperature results in the formation of the isolated nanostructures (and the Pt–SiC reaction to form Pt–Si mixed nanostructures). The equilibrium shape of the Pt nanostructures is not, clearly, spherical: the SEM images evidences the formation of faceted nanostructure rather than spherical droplets. In fact, the Pt equilibrium shape is dictated by the minimization of the surface energy in combination with the crystal structure of Pt: this combination of factors leads to the formation of structures exposing the lowest energy crystal planes. The consequence is the formation of truncated octahedrons consisting of {111} and {100} facets [57,58]. The AFM section analysis quantitatively evidences the increase of the planar size of the nanostructures so as an increase of the height of the nanostructures till to a saturation condition above the annealing tem­ perature T ¼ 973 K. To analyse in detail these evolutions, we used the AFM cross-sectional analysis to measure the height H of 200 nano­ structures for each annealing temperature so to construct distributions from which mean values and standard deviations ΔH can be extracted. Similarly, SEM images and AFM images were used to measure the planar size (diameter) D of 200 nanostructures for each annealing 6

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nanostructures versus the annealing temperature T are reported in Fig. 8. It reports, in particular: in (a) the picture of a Pt structure on the SiC substrate indicating the height H and the plan size (diameter) D; in (b) the plot of the mean height H of the Pt structures versus the annealing temperature T; in (c) the plot of the mean diameter D of the Pt structures versus the annealing temperature T; in (d) the plot of the mean aspect ratio AR ¼ / of the Pt structures versus the annealing temperature T. Observing these plots we can conclude that the mean height of the nanostructures abruptly increases from about 45 nm to about 60 nm when the annealing temperature increases from 773 K to 873 K and then it saturates to the constant value of about 60 nm for T � 873 K. On the contrary, increasing the annealing temperature from 773 K to 1073 K, the mean planar size of the nanostructures monotonically increases from, about, 220 nm to, about, 380 nm. The combination of these results suggests the following qualitative picture for the formation and evolution of the nanostructures, as schematically reported in Fig. 9: during the initial stages of the dewetting process, the retreating metal film concentrates, stochastically, in some points forming the nano­ structures. These agglomerates are connected by a web of metal fila­ ments which, increasing the annealing temperature, gradually reduce their width and thickness and, correspondently, the metal agglomerates (i. e. the nanostructures) grow in height and diameter. However, once the temperature for which the metal filaments break-up is reached, the further metal mass diffusion is likely to occur on the SiC surface than on the surface of the metal nanostructures towards the top of the nano­ structures. In this way, the planar enlargement of the nanostructures is favoured respect to the thickening of the nanostructures. Finally, then, the formation of two-dimensional Pt nanostructures is reached with a planar size about six times larger than the height. This should be the consequence of the specific wetting nature of the Pt–SiC system. Generally, a strongly non-wetting behavior of the film on a substrate is indicated by a high (>90� ) contact angle θ leading to the formation of almost-spherical particles and this does not seem the case of the Pt–SiC system. The structures contact angle is cosθ¼(γSiC-γPt/SiC)/γPt with γSiC the SiC surface energy, γPt the Pt surface energy, γPt/SiC the Pt/SiC interface energy. Data for the Pt and SiC surface energy are available as γPt ¼ 2.2 J/m2 [59] and γSiC � 2.7 J/m2 [60] for Si-terminated SiC. Our experimental data suggest that the Pt nanostructures are as two-dimensional islands indicating a slight non-wetting nature of Pt on SiC characterized, roughly, by 0� <θ < 90� , so that we expect 0.5 J/m2<γPt/SiC<2.7 J/m2. In this sense, and more generally, we have to consider the impact of the SiC support surface for the dewetting of the Pt film: a) the previous considerations on the surface and interface energies are important in establishing the non-wetting nature of the SiC surface to the Pt film. This is the key condition for the occurrence of the dewetting process of the film. In fact, it is needed the thermodynamic instability of the deposited film on the surface support for the occurrence of the film dewetting upon surface diffusion. This condition is, for example, quan­ tified by a negative value of the spreading coefficient: S ¼ γSiC-γPt-γPt/SiC<0 means the thermodynamic instability for the Pt film on the SiC surface and is the necessary condition for the occurrence of the dewetting process [44]. Otherwise, the film wets the substrate surface and, upon annealing, the dewetting process does not occur. At least, the lowest possible value of γPt/SiC ¼ 0.5 J/m2 establishes the condition of the negative value for the spreading coefficient for the Pt film on the SiC surface. b) On a perfectly flat surface, the minimization of the total surface and interface energy is the solely driving force for the dewetting process. The situation is different for films deposited on non-flat (i. e. structured) surfaces [61,62]: in this case, the topographic structuration of the supporting surface act as a geometric structuring mean by the introduction of geometric-dependent chemical potential driving, during the film dewetting process, the preferential metal diffusion from surface peaks and ridges to surface valleys. In this way, the supporting surface topographic structuration or patterning can impact both on the dew­ etted metal particles sizes and spatial order. However, this effect is active as a structuring mechanism when the metal film thickness is

Fig. 5. (a): Plot of the Root-Mean-Square (RMS) of the Pt film, on the SiC substrate, versus the annealing temperature T (starting from the un-annealed film), for the fixed annealing time of 30 min. The dashed lines are guides for the eyes allowing to identify the crossing point corresponding to the critical temperature TC � 600 K; (b): Plot of the fraction F of surface area uncovered by the Pt film (i. e. ratio between the area exposed by the SiC substrate over the total surface area where the total surface area is the sum of the area exposed by the SiC substrate and the area covered by Pt) versus the annealing temperature T, for the fixed annealing time of 30 min; the dashed line is only a guide for the eyes; (c) Arrhenius plot of the fraction F (dots) of the area uncovered by the Pt film (i. e. Log(F) versus the inverse of the annealing temperature, 1/T, for the fixed annealing time of 30 min). The full line indicates the exponential fit of the experimental data.

temperature (in this case, the planar size for each nanostructure was evaluated as the diameter of the minimum circle inscribing the nano­ structures) so to construct distributions from which mean values and standard deviations ΔD can be extracted. Examples of some distri­ butions are reported in Fig. 7: it reports distributions for the height H and planar size D of Pt structures on the SiC surface obtained by annealing the deposited Pt film at 873 K ((a) and (b)) and 1073 K ((c) and (d)). The results for the evolution of (mean height), (mean planar size), and mean aspect ratio AR ¼ / of the Pt 7

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Fig. 6. (a)–(d): high magnification SEM images of the Pt film, on the SiC susbtrate, annealed (for a time of 30 min) at 773 K ((a)), 873 K ((b)), 973 K ((c)), 1073 K ((d)) focusing on one or few Pt structures; (e)–(m) report high magnification AFM images (three-dimensional and two-dimensional) and section analysis of some representative Pt structures. In particular: (e) three-dimensional AFM image (500 nm � 500 nm) of the Pt film deposited on the SiC substrate and annealed at 773 K, (f) the corresponding two-dimensional AFM image, and (g) cross-sectional line profile along the line indicated in (f) with reported values for the plan size and height of the corresponding Pt structure; (h) three-dimensional AFM image (500 nm � 500 nm) of the Pt film deposited on the SiC substrate and annealed at 973 K, (i) the corresponding two-dimensional AFM image, and (j) cross-sectional line profile along the line indicated in (i) with reported values for the plan size and height of the corresponding Pt structure; (k) three-dimensional AFM image (500 nm � 500 nm) of the Pt film deposited on the SiC substrate and annealed at 1073 K, (l) the corresponding two-dimensional AFM image, and (m) cross-sectional line profile along the line indicated in (l) with reported values for the plan size and height of the corresponding Pt structure.

the present case of the Pt film dewetting on the SiC surface, we observed, by the SEM images in Fig. 1, a natural structuration of the SiC surface which is characterized by parallel lines arising as the typical atomic step-terrace structure of the SiC surface. On the other hand, however, we quantified the surface roughness of the starting SiC surface by the RMS parameter finding σ ¼ 1.3 nm which is very small compared to the starting thickness of the deposited Pt film in the present experiments (7.5 nm): we can conclude, therefore, that the Pt film dewetting process on the analyzed SiC surface proceeds as on a flat surface without any significant effect of the natural SiC surface step-terrace structure. A final consideration concerns the following aspects: as shown by the AFM and SEM images, the Pt nanostructures obtained by annealing at temperature T < 1073 K are widely connected resulting in network-like structures. At the annealing temperature T ¼ 1073 K are, now, isolated as a result of the breaking of the connecting metal filaments. The resulting isolated structures show just a non-uniform shape and size distribution. This fact is characteristic of the stochastic nature of the dewetting process [44]. However, regarding this aspect (in view of ap­ plications for which uniformly shaped and sized nanostructures could be useful), within the characteristic limitations imposed by the stochastic nature of the dewetting process, some strategies can be effective as the use of lower Pt film thickness [44] or higher annealing temperature and longer annealing duration [44] or the use of some sacrificial layer be­ tween the Pt film and the substrate impacting on the atoms surface diffusion [32–34]. Any of these approaches can results in some im­ provements on the nanostructures size uniformity resulting, however, in some possible drawbacks in relation to the specific designed application

Fig. 7. Representative distributions for the height H and planar size (diameter D) of Pt structures on the SiC surface obtained by the annealing (for 30 min) of the deposited Pt film at 873 K ((a) and (b)) and 1073 K ((c) and (d)).

comparable to the characteristic size of the supporting surface geomet­ rical features. Otherwise, the geometric-dependent chemical potential tends to be zero with negligible effect on the film dewetting process. In 8

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for the nanostructures. The increase of the annealing temperature typically results in larger mean particles size and lower size standard deviation around the mean value [44] as a consequence of the smaller particles dissolution and larger particles growth. The higher tempera­ tures, however, should not worse the properties of the substrate in view of device applications taking advantage of these properties. For example, very high temperature can results in structural modification of the SiC substrate negatively affecting its electrical properties [1–6]. The mean particles size and the corresponding standard deviation increases by increasing the film thickness, typically showing a power-law dependence [44]. This property gives the possibility to largely tune the mean particles size. However, for a fixed annealing temperature and time, higher the film thickness, higher the mean nanoparticles size and higher the size dispersion which often prevents the use of some specific particles (optical, electrical, etc.) property due to an averaging effect of the property over the entire particle population. This can be the case, for example, of the Pt nanostructures-SiC Schottky barrier height to be used in electronic devices [63,64]: metal nanostructures-SiC Schottky barrier height is size-dependent [63,64] and this property can be used to con­ trollably tune the response of SiC-based electronic devices. However, the large size dispersion of the metal nanostructures size in contact to the SiC frustrates this effect. Recently, the use of a sacrificial layer between the metal film and the substrate was proven to impact on the film dewetting process leading to the improvement of several particles characteristics [32–34]: these results were observed for the dewetting process of Pt films on sapphire substrate by using an In sacrificial layer. However, the obtained dewetted nanostructures result as bimetallic Pt–In due to the layers interaction. In general, this approach is, surely, interesting and useful for all those applications which can take advan­ tage of the combined properties of the multi-component particles (as shown for the Pt–In bimetallic particles regarding optical and sensing applications). Otherwise, the sacrificial layer element could result as a dendrimental contamination in others applications for which the metal particles properties are negatively affected by the second undesired element. Concluding, these considerations open future perspectives for the present work as the use of Pt films on the SiC surface with lower thickness (<7.5 nm), higher annealing temperature (>1073 K), longer annealing duration (>30 min), or as the use of a sacrificial layer between the Pt film and the SiC substrate (starting from the testing of the In case as suggested by the notable results in Refs. [32–34]). 4. Conclusions We exploited the annealing induced dewetting process of a 7.5 nmthick Pt deposited film to produce two-dimensional arrays of Pt nano­ structures on SiC surface. In particular, after depositing the Pt film on the SiC surface, annealing processes increasing the annealing tempera­ ture from 773 K to 1073 K (fixing an annealing time of 30 min) were performed and the following chemical and morphological microscopic analysis allowed to draw the following conclusions:

Fig. 8. (a) Picture of a Pt structure on the SiC substrate indicating the height H and the plan size (diameter) D; (b) Plot of the mean height H of the Pt structures versus the annealing temperature T (for the fixed annealing time of 30 min); (c) Plot of the mean diameter D of the Pt structures versus the annealing temper­ ature T (for the fixed annealing time of 30 min); (d) plot of the mean aspect ratio AR ¼ / of the Pt structures versus the annealing temperature T (for the fixed annealing time of 30 min).

a) the annealing processes performed at temperature T � 1023 K lead, purely, to a dewetting process of the Pt film on the SiC surface, being the Pt–SiC surface unreactive in this temperature range, with the consequent formation of pure Pt nanostructures on the SiC surface. On the other hand, the annealing process performed at T ¼ 1073 K leads, in addition to the further evolution of the dewetting process with the formation of the isolated shaped surface nanostructures, also to the Pt–SiC reaction with the formation of Pt2Si silicide; b) the surface roughness of the Pt film linearly increases by increasing the annealing temperature from 773 K to 1073 K. The fraction of the surface area uncovered by Pt increases by increasing the annealing temperature from 773 K to 1073 K showing, as a function of the annealing temperature, a thermal activated behavior characterized by the activation energy EA¼(57 � 0.4) meV;

Fig. 9. Qualitative picture of the evolution of the dewetting of the Pt film, on the SiC surface, (without considering Pt–SiC interaction) increasing the annealing temperature from 773 K to 1073 K.

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c) the dewetting process of the Pt film starts at a critical temperature TC � 660 K with nucleation of pre-existing vacancies in the Pt film. The vacancies nucleation leads to formation of holes which growing causes the retreating of the Pt film from the center of the holes. From the retreating Pt film, agglomerates originate giving place to the formation, in stochastic positions on the SiC surface, of nano­ structures connected by an underlaying web constituted by Pt fila­ ments. Further increasing the annealing temperature, the filaments decrease their width and thickness (and, correspondently, the nanostructures increase their height and planar size) till to break-up. Finally, further increasing the annealing temperature causes a pref­ erential increase of the nanostructures planar size while the nano­ structures height saturates to a constant maximum value; d) increasing the annealing temperature from 773 K to 1073 K, the Pt morphology on SiC can be tuned from a holed film at 773 K, to Pt agglomerates interconnected by Pt filaments at 873 K, and to isolated Pt–Si nanostructures at 1073 K. In this final stage, the Pt–Si nano­ structures are as two-dimensional islands with mean planar size about six times higher than the mean height; e) the Pt/SiC interface energy is roughly estimated in the range from 0.5 J/m2 to 2.7 J/m2.

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Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement F. Ruffino: Conceptualization, Methodology, Validation, Investiga­ tion, Data curation, Writing - original draft. M. Censabella: Data curation, Investigation, Writing - review & editing. M.G. Grimaldi: Supervision, Funding acquisition, Investigation, Conceptualization, Writing - review & editing. Acknowledgements This work was supported by the project -“Materiali innovativi e nano strutturati per microelettronica, energia e sensoristica”Linea di inter­ vento 2 (Univ. Catania, DFA). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpcs.2020.109403. References [1] S.E. Saddow, A. Agarwal, Advances in Silicon Carbide Processing and Applications, Artech House Inc., Norwood, 2004. [2] T. Kimoto, J.A. Cooper, Fundamentals of Silicon Carbide Technology-Growth, Characterization, Devices, and Applications, Wiley, Singapore, 2014. [3] M. Shur, S. Rumyantsev, M. Levinshtein, vol. 1 and vol. 2, SiC Materials and Devices, World Scientific Publishing, Singapore, 2007. [4] L.M. Porter, R.F. Davis, A critical review of ohmic and rectifying contacts for silicon carbide, Mater. Sci. Eng. B 34 (1995) 83–105. [5] K. Zekentes, K. Vasilevskiy, Advancing Silicon Carbide Electronics Technology IMetal Contacts to Silicon Carbide: Physics, Technology, Applications, Materials Research Forum LLC, USA, 2018. [6] P. Friedrichs, T. Kimoto, L. Ley, G. Pensl, Silicon Carbide-Power Devices and Sensors, vol. 2, Wiley, Weinheim, 2010.

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