Role of Sputter Deposition Rate in Tailoring Nanogranular Gold

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Role of Sputter Deposition Rate in Tailoring Nanogranular Gold Structures on Polymer Surfaces Matthias Schwartzkopf,*,† Alexander Hinz,‡ Oleksandr Polonskyi,‡ Thomas Strunskus,‡ Franziska C. Löhrer,§ Volker Körstgens,§ Peter Müller-Buschbaum,§ Franz Faupel,‡ and Stephan V. Roth†,# †

Photon Science, Deutsches Elektronen-Synchrotron (DESY), Notkestr. 85, D-22607 Hamburg, Germany Lehrstuhl für Materialverbunde, Institut für Materialwissenschaft, Christian Albrechts-Universität zu Kiel, Kaiserstr. 2, D-24143 Kiel, Germany § Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, James-Franck-Str. 1, D-85748 Garching, Germany # KTH Royal Institute of Technology, Department of Fibre and Polymer Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden ‡

S Supporting Information *

ABSTRACT: The reproducible low-cost fabrication of functional polymer−metal interfaces via self-assembly is of crucial importance in organic electronics and organic photovoltaics. In particular, submonolayer and nanogranular systems expose highly interesting electrical, plasmonic, and catalytic properties. The exploitation of their great potential requires tailoring of the structure on the nanometer scale and below. To obtain full control over the complex nanostructural evolution at the polymer−metal interface, we monitor the evolution of the metallic layer morphology with in situ time-resolved grazingincidence small-angle X-ray scattering during sputter deposition. We identify the impact of different deposition rates on the growth regimes: the deposition rate affects primarily the nucleation process and the adsorption-mediated growth, whereas rather small effects on diffusion-mediated growth processes are observed. Only at higher rates are initial particle densities higher due to an increasing influence of random nucleation, and an earlier onset of thin film percolation occurs. The obtained results are discussed to identify optimized morphological parameters of the gold cluster ensemble relevant for various applications as a function of the effective layer thickness and deposition rate. Our study opens up new opportunities to improve the fabrication of tailored metal−polymer nanostructures for plasmonic-enhanced applications such as organic photovoltaics and sensors. KEYWORDS: polymer−metal interfaces, gold cluster morphology, real-time growth kinetics, in situ GISAXS, sputter deposition rates

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INTRODUCTION Supported submonolayer and nanogranular metal thin films have promising applications in optoelectronics, plasmonics, and heterogeneous catalysis.1−3 Their specific device performances arise from the collective properties of the nanoparticle ensemble, which strongly depend on the average sizes, shapes, and distances present.4 To fully exploit the great potential of such nanogranular metal thin films, a tailoring of the structure on the nanometer scale and below is required. It is evident from this objective that sputter deposition stands out as an attractive and versatile routine method in industry and science for the targeted preparation of metal coatings and compounds with specific material properties depending on their application.5,6 Especially in the case of using diverse polymer thin films as substrates, a large variety of different functional morphologies on different length scales can be addressed, e.g., metal © 2017 American Chemical Society

nanoparticles, nanorods, and ramified nanostructures as a coating or embedded in a polymer matrix.5,7 These polymer− metal nanocomposites provide an auspicious range of applications as cost-efficient and flexible organic photovoltaics (OPV),8 organic light-emitting diodes,9 organic field effect transistors,10,11 and sensors.12 In these fields, the use of sputter deposition instead of chemical methods has advantages to reproducibly tailor the average nanoparticle size and distances on a macroscopic area by a controlled variation of process parameters.5 Moreover, sputter deposition provides far better adhesion and facilitates the deposition of any material, even high-melting point metals, ceramics, alloys, and bimetallics.5 Received: November 30, 2016 Accepted: January 20, 2017 Published: January 20, 2017 5629

DOI: 10.1021/acsami.6b15172 ACS Appl. Mater. Interfaces 2017, 9, 5629−5637

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ACS Applied Materials & Interfaces

calculated according to the kinetic freezing model46 for the first time on the basis of real-time experiments and compares very well with values reported in literature.18 Furthermore, a change in cluster aspect ratio (height to radius) was observed during growth.22,42 Starting from these results, a set of molecular dynamics (MD) simulations based on atomistic Langevin equations confirmed the developed model concepts and was able to reconstruct the evolution of geometrically modeled GISAXS data from Au growth on PS during slower RF sputter deposition considering the simplifications inherent in the geometric model.47 Thus, Au on PS serves as a comprehensive model system for exploring the polymer−metal interaction during self-organization processes under controllable and reproducible conditions like sputter deposition.31 Thereby, a basic prerequisite was established to shed more light on the influence of specific sputter deposition process parameters such as deposition rates. The fundamental principles of the formation and growth of an ensemble of clusters and its development into a layer are premised on a feedback loop of various competing surface processes. The resulting complex nonlinear behavior can be partially characterized by specifying critical thresholds. First mathematical descriptions of the atomic mechanisms and dynamics provide the rate equations of the pioneering work of Venables et al.48 The rate equations primarily describe the growth of immobilized clusters in a thermodynamic equilibrium on a perfectly flat surface, which is not the case during sputter deposition on polymers. However, those equations suggest a pronounced influence of the deposition rates on the initial particle density and the cluster growth by direct adsorption of atoms from the gaseous phase. The rate dependence of vapor phase deposition (VPD) processes shows a maximum for homogeneous nucleation (random nucleation) and decreases with increasing number of defects present at the interface, leading to heterogeneous nucleation (preferred nucleation).47,49 The specific nature of the surface-centric defects induced by the impingement of the highly energetic particles is not clearly known. They could be small hollows or hillocks due to local ablation, radical terminal groups due to polymer chain scission, attractive local arrangements due to cross-linking of polymer chains, or formation of oxygen and other reactive functionalities.16,50 During sputter deposition, the defect density increases with higher deposition rates, which should markedly affect the early deposition stages, i.e., condensation (sticking) and nucleation phenomena, and thus the nanostructure of the developing metal film.47,51 Comprehensive real-time observations of the sputter deposition of metals on organic surfaces with respect to effects of the deposition rate on the early stages and the formation of a closed film are still missing hitherto. In the present study, we combined DC-magnetron sputter deposition with time-resolved surface sensitive X-ray scattering (GISAXS) to explore the evolution of gold nanogranular structures on PS thin films in situ and as a function of the sputter deposition rate. We monitored the metallic layer morphology at deposition rates relevant for manufacturing processes in advanced nanotechnology regarding changes in the key scattering features as a function of the effective gold layer thickness from the submonolayer regime toward rough metal nanolayers. Analytical geometric modeling quantified the average cluster radii and distances, which enabled the visualization and interpretation of the gold cluster growth kinetics in terms of nanoscopic processes.22,28 This procedure

The high achievable surface coverage of shell-free nanoparticles is beneficial for an efficient device performance. This holds especially for the efficiency of OPV devices, where supported noble metal nanoparticles enhance the photovoltaic conversion13,14 and act as performance stabilizers to enhance the exciton lifetime or reduce the photodegradation rate.15 Nevertheless, the higher-energy ions and UV irradiation during sputter deposition may provoke chemical modification depending on the polymer composition.16 To manipulate and tailor nanogranular metal structures on polymer substrates using sputter deposition, fundamental investigations of the gold cluster growth kinetics on polystyrene (PS) thin films are essential prerequisites as a widespread metal-dielectric/ insulator model system exhibiting a weak metal−polymer interaction.12,17−23 In this framework, grazing incidence small-angle X-ray scattering (GISAXS) stands out as an advantageous nondestructive experimental technique which provides statistically relevant information on lateral and vertical correlations of the electron density distribution in the near surface regime over a macroscopic sample area.24−26 Especially for the morphological characterization of thin films during fabrication processes such as vapor deposition27−33 or evaporation-mediated self-assembly,34−36 GISAXS was proven to be a very powerful tool for following the growth of thin nanostructured metallic films in situ on a variety of inorganic and organic templates, even with millisecond and subnanometer resolution.22,28−30,37−41 In advanced real-time studies, we followed in situ the morphological evolution of nanostructured Au films on spin-casted PS homopolymer thin films and the related optical properties in the UV−vis regime during radio frequency (RF) sputter deposition by combining GISAXS with specular reflectance spectroscopy measurements.22 Being very surface sensitive, these techniques facilitate the simultaneous study of thin film morphologies and their optical properties, respectively.42−44 In particular, the Au/PS nanocomposite thin films exhibited both significant changes in their structural and optoelectronic properties at different thicknesses, demonstrating a tailoring of visible colors. During sputter deposition, a change in the optical reflectivity of the pristine gray-blue PS film was detected, ranging from dark blue color due to the presence of isolated nanoclusters at the interface to bright red color due to larger Au aggregates.22 An antireflective behavior with its maximum at around 1 nm effective gold thickness was observed, suggesting a promising range for effective antireflective coatings in OPV applications to increase their light harvesting capabilities.45 Moreover, the real-time GISAXS experiment enabled direct observation of four different growth regimes as a function of effective gold layer thickness δ: nucleation, isolated island growth, growth of larger aggregates via partial coalescence, and continuous layer growth.22,27 The individual thresholds of these growth regimes were identified with subnanometer resolution. In an earlier publication,28 a versatile analytical tool based on geometrical assumptions was introduced for processing the extensive real-time GISAXS data sets obtained during gold sputter deposition. The geometrical model facilitates the interpretation and evaluation of changes in the cluster morphology according to size ratios between the model parameters.28 The model was already successfully applied to explain the correlation between nanostructure and Raman scattering enhancement for supported silver clusters in sensor applications.29 On the basis of this approach, the surface diffusion coefficient of Au on PS at room temperature was 5630


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ACS Applied Materials & Interfaces enabled us to identify the impact of different deposition rates on the particle density as well as on the onsets of long-range connectivity of the cluster ensemble during the sputter deposition process. Our study permits a better understanding of the growth kinetics of gold clusters on polymer substrates and opens up the opportunity to improve the fabrication of tailored polymer−metal nanostructures for organic electronics such as photovoltaic applications and plasmonic-based technologies.45 This is in turn beneficial for their low-cost fabrication, device performance, and for an optimized use of noble metals in general.

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EXPERIMENTAL SECTION

GISAXS measurements were performed at the P03/MiNaXS beamline of the PETRA III storage ring at DESY (Hamburg, Germany).52 A dedicated mobile DC sputter deposition chamber was integrated in the GISAXS setup to enable in situ and real-time observations of the morphological evolution. More details about the sputter chamber and photographs of the setup can be found in the Supporting Information. An incident photon energy of 13 keV was used with a beam size of (31 × 24) μm2 at the sample position. The sample to detector distance was set at SDD = (2387 ± 3) mm, and a PILATUS 1 M (Dectris Ltd., Switzerland) with a pixel size of (172 × 172) μm2 was used as the detector. The sputter deposition experiments were performed on (90 ± 5) nm thin polystyrene films (Mw = 270 kg/mol, Polymer Source Inc., Canada) obtained by spin coating from 12.5 g/L polymer solution in toluene onto previously acid-cleaned Si substrates (see Supporting Information). To achieve a good separation between the PS and Au Yoneda peaks, an incident angle of αi = 0.445° was selected during the in situ experiments. Sputter deposition was performed using a plasma-cleaned 99.999% 2 in. Au target (Kurt J. Lesker, United States) at an argon working pressure of 0.52 × 10−2 mbar. The deposition times for the different sputter power of P = 3, 14, 25, 50, and 100 W were calculated to be t = 540.65, 101.60, 57.70, 26.70, and 15.15 s, respectively, by using a quartz crystal microbalance, which can be positioned above the sample by a pneumatic actuator. To avoid possible X-ray beam effects during the in situ GISAXS experiments on the sample, continuous scanning with a velocity faster than the horizontal beam size per frame (0.6 mm/s) along the sample horizontal direction was performed (see Supporting Information, Figure S7).37 Meanwhile, the scattering patterns were continuously recorded at a frame rate of 20 images per second. The final Au effective thickness was determined from a static GISAXS image recorded directly after deposition, which results in an effective deposition rate (see Supporting Information, Table 1). The GISAXS data were analyzed using the DPDAK software package.53

Figure 1. Selected GISAXS patterns illustrate the dependence of the evolution of key scattering features with increasing effective Au film thicknesses δ from left to right: 1, 2, 4, and 8 nm on the sputter power from top to bottom: P = 3, 14, 25, 50, and 100 W, respectively. The coordinate system (qy, qz), corresponding key scattering features, Yoneda peak position of polystyrene (YPS) and gold (YAu), and scale bars are indicated. The vertical dashed white lines indicate the qy,max position at 3 W and are guides for the eye to visualize the shifts in peak positions at higher sputter power. The dark circles correspond to the specular beam stop used to avoid detector saturation. The black horizontal stripes correspond to the detector intermodular gaps.

emerges at large qy values, indicating the presence of a laterally fairly unordered ensemble of nanogranular gold structures on the polymer. In general, the nanoparticle distances and radii grow simultaneously in a disordered arrangement with lognormal size distributions during sputter deposition.54 Thus, form factors being a fingerprint of the clusters’ shapes and sizes become strongly smeared, and the structure factor dominates the horizontal out-of-plane intensity curve.55 Therefore, the side peak is approximated as a pseudo-Bragg peak stemming from scattering at a paracrystalline lattice of gold nanoparticles.24,28 As the deposition proceeds, the side peak becomes narrower and shifts toward smaller qy values, whereas the peak shift significantly decelerates at later sputtering times. The shift toward lower qy is primarily related to an increase in the average center-to-center distance between Au clusters due to coalescence effects.27,28,54 When form factor contributions are disregarded, the width of the side peak is related to the mean cluster distance distribution, and the observed decrease of the fwhm reveals an increase in the order of the nanocluster ensemble. In other words, the observed peak evolution is related to an increase in the average cluster correlation distances via coalescence during the lateral growth of nanostructures with increasing order until the cluster ensemble becomes constrained due to spatial restrictions near the percolation threshold.28,29 At higher deposition rates, the peak position is shifted to higher qy values at comparable effective thicknesses (Figure 1), visualizing significant systematic differences in the nanogranular gold cluster ensemble. Moreover, the intensity distribution along qz shows additional peaks appearing at higher qz and shifting toward lower qz values during the sputter deposition, indicating a vertical growth of the nanostructured thin films. These scattering features are thickness modulations similar to Kiessig fringes above the critical angle resulting from constructive interference due to the refraction of the incident X-rays at the metal surface and subsequent reflection at the polymer−metal interfaces while matching the diffraction condition.56,57 At the later stages

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RESULTS Figure 1 presents selected two-dimensional (2D) GISAXS patterns obtained during DC-magnetron sputter deposition of Au on PS thin films: shown are different effective film thicknesses obtained at different sputter power and deposition rates, respectively. By increasing the sputter power, the ion current to the target is increased, resulting in a higher sputter rate and thus a higher deposition rate on the polymer substrate. For more details about the experimental method, deposition parameters, and the corresponding effective deposition rates, see the Supporting Information and Schwartzkopf et al.28 It becomes obvious that for deposition rates higher than J = 1 nm/s, a high time resolution is mandatory to observe the nanocluster growth under conditions relevant for industrial manufacturing. A remotely controlled, moveable quartz crystal microbalance (QCM) placed above the sample serves as sample shutter and ensures a reproducible quantification of the deposition rates. After starting the sputter deposition process by a fast removal of the QCM shutter, a broad side peak (qy,max) 5631


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ACS Applied Materials & Interfaces of the sputter deposition process (δ > 6 nm), these thickness modulations are more pronounced in intensity, displaying the growth of relatively smoother metal films at higher deposition rates. In addition, a sigmoidal shift of the intensity from the PS Yoneda peak58 (YPS at qz = 0.626 nm−1) toward higher qz values (YAu at qz = 0.9 nm−1) is visible, which is related to an increase in electron density and gold coverage at the polymer surface.22,28 To extract quantitative information from the entire sequences of GISAXS patterns, vertical line cuts, so-called out-of-plane cuts (along qy) at the Yoneda peak position of PS (YPS at qz = 0.626 nm−1) and offset horizontal line cuts, socalled horizontal off-detector cuts (along qz) for 0.187 nm−1 < qy < 2.852 nm−1 were performed. The obtained out-of-plane cuts were directly parametrized using a sharp Gaussian fixed at qy = 0 for modeling the Yoneda intensity and Lorentzian functions constrained to be axially symmetric with respect to the scattering plane for characterizing the side peak development related to cluster growth. Thus, the shift in the side peak position (qy,max) for different deposition rates is extracted and normalized to the effective Au layer thickness. A detailed overview of the temporal evolution of the key scattering features during the growth process at different deposition rates as well as the corresponding fitting procedure are given in the Supporting Information as movie files and contour plots of the out-of-plane and off-detector cuts versus the effective thickness δ of the deposited film. Because GISAXS provides averaged morphological information from all the objects within the X-ray footprint, the utilization of a simplified geometrical model allows for the extraction and comparison of average real space parameters from large sequences of in situ GISAXS data.28 The general validity of the analytical model for metal cluster growth on inorganic and polymeric substrates has been shown by simulating corresponding scattering patterns (see also Supporting Information, Figure S5).22,28,29,38 The model assumes a local hexagonal arrangement of uniform hemispherical clusters during the growth, which are composed of the same amount of material deposited on the unit cell surface area. This leads to the equation:28

R(δ , D) =

3

⎡ ⎤1/3 33/2 2 δ 3/2 D δ ≈ ⎢3 π 2 ⎥ ⎢ 4π qy,max ⎥⎦ ⎣

Figure 2. (a) Evolution of average real space parameters: interparticle distances D(qy,max) (full symbols) and cluster radii R(δ,D) (open symbols) extracted by geometrical modeling in the first 2 nm effective thickness at different sputter powers: 3 W (black rectangle), 14 W (red circle), 25 W (blue up-triangle), 50 W (magenta down-triangle), and 100 W (green diamond). The Roman numbers represent individual growth regimes characterized by the predominant surface process, namely I, nucleation; II, diffusion mediated cluster growth; and III, adsorption mediated cluster growth. The vertical dashed lines indicate thresholds between individual growth regimes. The initial gray area depicts the resolution limit. (b) Evolution of average real space parameters in the first 8 nm effective thickness at different sputter powers. The dashed yellow area depicts the thickness range of panel a.

thickness interval, surface diffusion processes are the predominant driving force for cluster growth.28 Thus, the observed similar slopes or growth kinetics, respectively, imply a negligible effect of the deposition rates on diffusion-mediated growth processes (Figure 2a). Exceeding δII, the slope of the interparticle distance evolution changes significantly, indicating that a critical coverage has been reached, where a higher probability of direct adsorption of Au atoms mediates the cluster growth. Here, the surface diffusion and coalescence of clusters is further affected by size-induced restrictions.59,60 For deposition at lower rates (P = 3 W, 14 W), the clusters tend to form local dimers,60 and the correlation distances D increase much faster than the average radii R. For the high rate deposition, this clear onset of partial coalescence was not observed. Due to a much stronger adsorption-mediated growth of clusters at already smaller interparticle distances, the probability of forming elongated, branched domains instead of dimers is increased.22 From the moment the average clusters tend to touch each other, the distances increase almost linearly, accommodating to lateral spatial restrictions near the percolation threshold.22,27,28 Subsequently, the residual gaps are filled, and a vertical layer growth occurs (Figure 2b). For a better visualization of the lateral dimensions and intrinsic thresholds, Figure 3a shows the ratio of cluster diameter over distance 2R/D. The presence of the partial coalescence is connected to a local maximum around δ = 1.6 nm and can be observed only for the lower deposition rates (P = 3 W, 14 W).18,22 Around this thickness, the majority of

(1)

From the extracted shift of the side peak position toward lower qy values, the relation D ≈ 2π/qy,max permits tracking of the approximate values of the average interparticle distance D throughout the sputter deposition process.24,27,28 In general, an increase in sputter power leads to a decelerated increase of the interparticle distances, which results in a common decrease of the average particle size according to eq 1 (Figure 2). A nonuniform increase was found, indicating the existence of different growth regimes within the first 2 nm effective thickness (Figure 2a). In line with Schwartzkopf et al.,28 a transition from a nucleation dominated growth regime (I) to diffusion-mediated coalescence (II) occurs around δI = 0.27 nm followed by an adsorption-mediated growth of immobilized clusters (III) becoming predominant around δII = 1.30 nm. It can be clearly observed that an increase in the deposition rate affects primarily the nucleation processes, whereby the different cluster ensembles are characterized by a smaller interparticle distance at higher rates between δI and δII. In this effective 5632


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GISAXS pattern, where at higher rates the Kiessig fringes of the Au films along qz are more pronounced, pointing at a reduced roughness (Figure 1). Apart from that, the geometrical modeling also allows for calculating the average particle density ρ from the area covered by the model clusters within the triangular unit cell by28 2 ρ= (2) 3 D2 Figure 3b visualizes the temporal evolution of particle densities at different deposition rates, which follows the same sequence of growth regimes as described in Figure 2. Following the analytical model, higher deposition rates cause higher particle densities and smaller clusters at comparable effective thicknesses.

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DISCUSSION Comparing the results of the five deposition experiments, we clearly observe a significant influence of the sputter deposition rate on the growth morphology at different growth stages. A nonlinear decrease of the percolation thresholds δp as a function of the deposition rate is observed (Figure 4). In Figure 3. Evolution of (a) diameter over distance ratio 2R/D and (b) cluster density ρ at different sputter powers: 3 W (black rectangle), 14 W (red circle), 25 W (blue up-triangle), 50 W (magenta downtriangle), and 100 W (green diamond).

previously isolated islands interconnect to metastable dimers,60 which leads to a release of initially covered surface area and a faster decay in the particle density. This phenomenon has been predicted by Jeffers et al. for the island-to-percolation transition during the growth of metal films on nonwetting amorphous substrates.46 In accordance with Jeffers et al., an adsorptionmediated growth due to deposition of new Au atoms overtakes the wiping due to coalescence and metastable, percolating domains of connected islands are created. The decrease reaches a local minimum around δ = 2.3 nm for P = 3 and 14 W, where 2R/D increases again due to branching and coarsening of the elongated domains.46 For higher sputtering powers, the local maxima and minima in the course of 2R/D ratios are not pronounced anymore. We may speculate that the nanocluster ensemble grown at higher deposition rates has less time to coalesce and rearrange in an energetically more favorable conformation. This would lead to a higher probability to form elongated, branched cluster domains consisting of more than two clusters, which are covering more surface area. Consequently, stronger effects of adsorption processes on the further cluster growth take place, which in turn promotes an earlier onset of long-range connectivity. With further deposition, the ratio 2R/D finally equals 1 at an effective thickness δp, defining the model-based percolation threshold. At this point, all average cluster boundaries touch each other, and a fully percolated ramified gold layer is formed. This definition of percolation is in accordance with electrical conductivity measurements on Au sputter deposited polyethylene-terephtalate (PET) surfaces, where a decrease of more than 10 magnitudes in the sheet resistance was observed at effective Au layer thicknesses ranging from 4 to 6 nm.61 We thus conclude that the higher the deposition rate, the higher the surface filling factors, resulting in smoother, more compact films. This effect can also be monitored in the

Figure 4. Shift of percolation threshold (red circle) and initial particle densities at effective thickness δ = 0.44 nm (blue rectangle) as a function of the effective deposition rate.

addition, the impact of the deposition rates on the initial particle densities is shown in Figure 4 at an effective thickness δ = 0.44 nm, where the corresponding scattering patterns of all five different deposition experiments showed quantifiable intensity distributions. Here, a nonlinear increase in initial particle density is observed. The differently pronounced influence of the deposition rate on percolation as well as nucleation density suggests two distinct regimes with a transition around J = 0.5 nm/s. In the first regime, an increase in the rate affects primarily the onset of percolation but has little influence on the early growth stages, as seen by the initial particle densities. For deposition rates above J = 0.5 nm/s, the shift of the percolation threshold is less affected than the initial particle densities. In general, the higher the deposition rate, the more gold nuclei can be established at the surface in a shorter time period. Following Zaporojtchenko et al.,49 the critical nucleus size for metal evaporation on polymers is a single atom, and the probability for a collision event of two adatoms for homogeneous nucleation increases with the square of the deposition rate. For this so-called random nucleation, an increase in the deposition rate by 1 order of magnitude should result in an increase in the cluster density by a factor of 2 at the same nominal metal-layer coverage.49 This behavior does not 5633


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ACS Applied Materials & Interfaces seem to be the case in the present study on sputter deposition, which suggests a predominant preferred nucleation at defects on the polymer surface. To explain the observed rate dependence, two effects have to be taken into account. On the one hand, the influence of homogeneous nucleation is stronger at high sputter deposition rates, and the nucleus density increases nonlinearly with the rate. On the other hand, an increase in the deposition rate required a higher power feed into the discharge. This results in an increased flux of defect generating species to the substrate. X-ray photoelectron spectroscopy (XPS) data acquired in situ from Au/PS samples with δ = (1.0 ± 0.1) nm directly after deposition at different sputter powers (3W, 25W) showed no significant peaks of oxygen or other reactive species (see Supporting Information, Figure S8). After the samples were exposed to air at ambient conditions, oxygen peaks become visible with slightly higher content at higher deposition rates. This implies the creation of more reactive defects, probably due to increased polymer chain scission at the interface during deposition at higher sputter power.16,50 The increase in defect generation promotes preferred nucleation and can explain the lower rate dependence of initial particle density compared to evaporation. Additionally, the observed less pronounced rate-dependence of the nucleation density during sputter deposition is consistent with atomistic MD simulations of Au cluster growth on PS films considering nucleation at surface defects during sputter deposition.47 They predict an increase by a factor of 2 if the deposition rate is increased by a factor of 30. The simulations also confirm the observed trend that a decrease in defect density directly causes a decrease in initial particle densities during sputter deposition.47 The investigated deposition rates affect primarily the nucleation processes below approximately 0.27 nm, whereas the observed similar morphological evolutions up to about 1.3 nm (diffusion threshold) implies a negligible effect on diffusionmediated growth processes. Exceeding this value, higher deposition rates have a strong effect on the adsorptionmediated growth regime. Average interparticle distances and radii are up to 25% smaller at J = 1.5 nm/s compared to cluster assemblies grown at J = 0.04 nm/s. Consequently, a larger fraction of metal covers the polymer surface at equal effective thicknesses at higher rates, and most incoming atoms are directly adsorbed onto existing clusters. Thus, higher rates directly promote an earlier onset of cluster percolation. In other words, the clusters have less time to agglomerate between ongoing deposition and nucleation events, which results in increased particle densities, and branched domains consisting of smaller clusters occur much earlier for higher deposition rates. Figure 5 schematically depicts the obtained results from geometrical modeling of the real-time GISAXS data about the role of sputter deposition rates on the morphology of nanogranular gold structures on polymer surfaces at different effective film thicknesses. Following the quantitative model-based analysis, it is possible to identify optimized morphological parameters of the gold clusters ensemble relevant for various technical applications as a function of the effective layer thickness and deposition rate. In general, the electrical conductivity should be increased for nanogranular gold cluster ensembles grown at higher deposition rates due to the improvement of single-electron hopping and tunneling processes at their reduced interparticle distances.62 Lamberti et al. reported recently how morphology and properties can be tuned with sputter deposition of Ag at

Figure 5. Schematic representation of the influence of sputter deposition rates on the morphology of nanogranular gold structures on polymer surfaces at different effective film thicknesses.

different rates on polydimethylsiloxane (PDMS) thin films.63 Moreover, the catalytic activity depends on the availability and accessibility of binding sites of the catalyst. Especially the perimeter of a gold cluster creates highly catalytically active binding sites at the interface, enabling, e.g., selectively oxidative conversions.64,65 Consequently, the maximum catalytic activity for a gold cluster ensemble should be accessible for a high particle density of small clusters, which are present at higher deposition rates and very low effective film thicknesses near the nucleation threshold. The highest catalytic activity for gold clusters was reported for a particle diameter distribution of 2−4 nm.66 Furthermore, knowledge of interparticle distances and radii allows an exemplary calculation of field enhancement effects on a theoretical basis of surface plasmons between spherical noble metal particles.67 The enhancement factor for localized surface plasmons is in a simplified approach proportional to the fourth power of the diameter to the interparticle gap ratio. Consequently, the interparticle field enhancement rises drastically near the percolation threshold due to the very small distances between the clusters and would theoretically amplify the vibrational field of a molecule in between the particles more than 1010 times. On the basis of the presented results, enhancement factors should be higher for a nearly closed packed ensemble of bigger particles, which would be favored at lower deposition rates and thicknesses around 4− 6 nm.12,29 Thus, tailoring of optoelectronic and catalytic properties could be ensured solely by means of the deposition rates and the effective layer thickness.

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CONCLUSIONS The combination of sputter deposition of Au on thin PS films with in situ time-resolved GISAXS is very suitable to monitor the evolution of the metallic layer morphology according to changes in the key scattering features and geometrical modeling as a function of effective deposition rates. We identify the impact of different deposition rates on the growth regimes and observed their influence on the gold cluster growth during sputter deposition: the higher the deposition rate, smaller particles in smaller distances grow in shorter time intervals. Thus, the thin metal films become smoother at higher rates. This leads to the summarized effect that, at higher rates, an earlier onset of thin film percolation occurs, and initial particle densities are higher due to an increase in the nucleation probability caused by an enhanced influence of random nucleation at higher rates together with a significant increase of the surface defect density at higher magnetron power. A comprehensive real-time GISAXS study during the early stages of gold sputter deposition on differently plasma-treated PS thin 5634


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ACS Applied Materials & Interfaces film or an additional application of a bias voltage creating an argon plasma in the vicinity of the substrate may shed more light on the influence of plasma-induced defects on the growth morphology.68 Because rates higher than J = 1 nm/s are typically of interest for fast processing in technological applications, this study permits a better understanding of the growth kinetics of gold clusters on polymer substrates during rapid sputter deposition. In line with the structure zone models for characterizing microstructural evolution in polycrystalline thin films in dependence on process parameters,69,70 the schematic shown in Figure 5 presents an important guideline for systematizing the dependence of nanogranular structures on the sputter deposition rates. This issue especially provides the requirements to directly promote the optimization of sputter deposition process parameters and manufacture defined metal− polymer interfaces with tailored properties depending on their foreseen application.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Jan J. Rubeck, Björn Beyersdorff, André Rothkirch, and Sarathlal K. Vayalil for their help with the GISAXS setup and Vivian Waclawek for preparation of PS thin films. The real-time GISAXS experiments in combination with DC sputter deposition were performed at the third generation synchrotron source PETRAIII at DESY in Hamburg, Germany, a member of the Helmholtz Association (HGF).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15172. Further details on GISAXS, the sputter chamber, results from static samples, and the procedures used for data analysis and sample preparation (PDF) GISAXS image sequences of the experiments at 3W (AVI) GISAXS image sequences of the experiments at 14W (AVI) GISAXS image sequences of the experiments at 25W (AVI) GISAXS image sequences of the experiments at 50W (AVI) GISAXS image sequences of the experiments at 100W (AVI)

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ABBREVIATIONS GISAXS, grazing incidence small-angle X-ray scattering PS, polystyrene VPD, vapor phase deposition OPV, organic photovoltaics MD, molecular dynamics simulations RF, radio frequency DC, direct current fwhm, full width at half-maximum QCM, quartz crystal microbalance

REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +4940/8998-3768; E-mail: Matthias.Schwartzkopf@ desy.de. ORCID

Matthias Schwartzkopf: 0000-0002-2115-9286 Peter Müller-Buschbaum: 0000-0002-9566-6088 Stephan V. Roth: 0000-0002-6940-6012 Author Contributions

S.V.R., F.F., and P.M.-B. conceived the experiment and provided materials and supervision. For this experiment at the large scale facility, M.S., A.H., O.P., T.S., F.C.L., V.K., and S.V.R. contributed to the alignment of X-ray optics, setup of the sputter chamber, and ensured stable operation of the sophisticated sample environment at the beamline. Moreover, these authors carried out the time-consuming deposition experiments in several shifts. All authors contributed with valuable comments to the internal revision and discussion of the experimental results and their interpretation. All authors have given approval to the final version of the manuscript. Funding

The authors acknowledge the Deutsche Forschungsgemeinschaft (DFG) for funding under projects RO 4638/1-1, FA 234/23-1, and MU 1487/18-1. 5635


Research Article

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Role of Sputter Deposition Rate in Tailoring Nanogranular Gold

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