Article pubs.acs.org/JPCC
Controlling the Growth of Silver Nanoparticles on Thin Films of an n‑Type Molecular Semiconductor Maria Girleanu,†,‡ Giulia Casula,§ Christian Blanck,† Marc Schmutz,† Christophe Contal,† Navaphun Kayunkid,†,∥ Piero Cosseddu,§ Analisa Bonfiglio,§ Ovidiu Ersen,*,‡ and Martin Brinkmann*,† †
Institut Charles Sadron, UPR22CNRS, 23 rue du Loess, BP 84047, Strasbourg 67034 Cedex 2, France Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504 CNRS-Unistra, 23 rue du Loess, BP 43, Strasbourg 67034 Cedex 2, France § Department of Electrical and Electronic Engineering (DIEE), University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy ‡
S Supporting Information *
ABSTRACT: Nucleation and growth of silver nanoparticles were studied on the surface of an n-type organic semiconductor (N,N′-bis(n-octyl)dicyanoperylene-3,4:9,10-bis(dicarboximide) (N1400)) as a function of the deposition rate τ and the substrate temperature Ts. Electron tomography was used to probe the bulk diffusion of Ag in the N1400 layers. No Ag nanoparticles (NPs) are formed in the bulk of N1400 even for high substrate temperatures, Ts = 125 °C, indicating that Ag diffusion in the organic semiconductor is marginal. The NP distribution on the surface of N1400 is essentially determined by the surface roughness of the N1400 films. A transition in the nucleation mode of Ag NPs on N1400 is evidenced as a function of Ts: for Ts ≤ 50 °C, Ag NPs form random patterns, whereas, for Ts ≥ 75 °C, linear arrays of aligned NPs are observed. Such arrays result from step edge decoration of the N1400 terraces. The surface density of Ag NPs is thermally activated, but the activation energy depends on the structure of the N1400 films: the smaller the crystal size of the N1400 grains, the larger the activation energy. morphologies and structures formed in evaporated thin films of OSCs of rodlike (sexithiophene) or discotic (phthalocyanines) molecules have been the subject of numerous studies, and the underlying mechanisms of growth are rather well understood.11,12 Although metal/OSC interfaces are omnipresent in the devices, the early stage of nucleation and growth of metal islands on OSCs has been addressed only marginally. Thermal evaporation of metals such as gold or silver on organic layers leads usually to a Volmer−Weber type of island (or nanoparticle) growth because the metal−metal interactions prevail over the interactions with the organic layer.13,14 Thermal evaporation allows the growth of the metal NPs to be fine tuned by adjusting deposition parameters such as the evaporation rate τ and the substrate temperature Ts. Even though thermal evaporation allows for a precise growth control of metal NPs on surfaces, it can be invasive because of metal diffusion in the OSC. Schreiber and co-workers have demonstrated that the use of high temperatures of the OSC film during metal deposition can result in bulk metal diffusion in the OSC for very low deposition rates (0.03 nm/min).15,16
I. INTRODUCTION Molecular semiconductors are fascinating materials whose unique electronic and optical properties are widely exploited to fabricate devices such as field effect transistors, organic lightemitting diodes, and organic photovoltaic cells.1−3 In the past decade, the combination of organic semiconductors (OSCs) and metal nanoparticles (NPs) opened new perspectives for low-cost fabrication of hybrid devices with improved performances. As an example, incorporation of metal nanoparticles in the OSC layer of photovoltaic devices enhances the power conversion efficiency by improving the light-harvesting capability in the device.4,5 In nonvolatile hybrid memories (NVHMs), the presence of metal NPs in the OSC layer is found to ease the on → off switching of the devices.6,7 It is therefore essential to understand and control precisely the incorporation of such metal NPs in the matrix of OSCs. Numerous strategies were developed to prepare such hybrid films by using either thermal evaporation of the metal and the OSC or solution-processing using soluble semiconductors and functionalized metal nanoparticles.8−10 Most relevant to such devices is the growth of metal NPs by thermal evaporation on the surface of OSCs. In this case, the hybrid layers can be prepared in a sequential mode by successive evaporation of the OSC and the metal NPs. The © XXXX American Chemical Society
Received: April 16, 2015 Revised: May 20, 2015
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DOI: 10.1021/acs.jpcc.5b03646 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C At a substrate temperature of 70 °C, Au diffuses over several tens of nanometers in the matrix of diindenoperylene (DIP) (a p-type OSC) and forms Au NPs in the bulk of the DIP films. Fladischer et al. showed that Ag diffuses in the bulk of tris(8hydroxyquinoline)aluminum (Alq3) but not in crystalline 3,4,7diphenyl-1,10-phenanthroline.17 Along a similar line, Faupel et al. investigated the impact of slow deposition rates and high substrate temperatures on the metal diffusion into polyimide layers, evidencing some clusters in the bulk of various epoxy resins.18−20 This study focuses on the nucleation and growth of Ag NPs on the crystalline layers of N,N′-bis(n-octyl)dicyanoperylene3,4:9,10-bis(dicarboximide) (N1400) (see Scheme 1). N1400 is
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. N1400 (Polyera) was evaporated at a low pressure in the range of 10−5 to 2 × 10−6 mbar from a resistively heated crucible (K. Lesker) using an Auto 306 (Edwards). The substrates of poly(3,4-(ethylenedioxy)thiophene)−poly(styrenesulfonic acid) (PEDOT−PSS; purchased from Heraeus-Clevios) were spin coated at 2000 rpm on clean glass slides. The glass substrates were previously cleaned by sequential sonication in acetone, ethanol, an aqueous solution of Hellmanex (Hellma Analytics), and finally distilled water (3×). Two series of N1400 layers were evaporated at substrate temperatures TsN1400 of 25 and 120 °C. The nominal thickness of the N1400 films was in the range of 20−35 nm and the evaporation rate fixed at 2 nm/min. Thermal evaporation of Ag (Alfa Aesar, purity of 99.999%) was performed at a constant rate in the range of 0.4−13 nm/ min, and the substrate temperature of the films was controlled using a proper substrate heating system. The substrate temperature of N1400 during Ag evaporation is labeled Ts, whereas TsN1400 is the temperature of the PEDOT−PSS substrate during thermal evaporation of N1400. After deposition of the films, the substrate temperature was quenched to room temperature to avoid any coalescence or annealing effect on the film morphology. 2.2. Structural Investigations. The surfaces of both series of N1400 thin films were investigated by atomic force microscopy (AFM) using a multimode Nanoscope V (Bruker) in peak force tapping mode with Si tips (0.4 N/m and λN1400, i.e., for high Ts, Ag adatoms can reach step edges of N1400 terraces where nucleation of NPs preferentially takes place. Of interest is the growth of rather well-defined Ag NP arrays with a characteristic inter-NP distance. Such linear arrays of metal islands or nanoparticles were reported previously in the case of the Co deposition on the reconstructed surface of Au(111) and the deposition of Au on ordered SiO2/Mo(112).35,36 At low Ts, random nucleation of Au islands is observed on the surface of SiO2 terraces, whereas linear arrays of Au NPs are observed at higher Ts. The presence of a characteristic inter-NP distance might be interpreted as a consequence of specific nucleation sites at the step edges or as a consequence of some correlated nucleation between successive NPs along the steps of the terraces. These results underline a general mechanism of step edge nucleation and its dependence on the substrate temperature; i.e., linear arrays of metal NPs are formed upon
bimodal distribution. For the N1400 films prepared at 25 °C, the Ag island size density, NAg, decreases with Ts from 47 × 103 μm−1 at Ts = 50 °C to 12 × 103 μm−1 at Ts = 125 °C. For the N1400 films prepared at 120 °C, the surface density of NPs shows a similar trend but with a smaller amplitude. An Arrhenius representation shown in Figure 5a was used to quantify the Ts dependence of NAg. Such an Arrhenius behavior is indeed expected for a thermally activated nucleation mechanism, such that NAg ∝ N0 exp(−Enucl/kBTs) (where NAg is the surface density of Ag NPs and Enucl is the activation energy for nucleation).33 The calculated activation energies are 70 ± 10 and 20 ± 7 meV for the films of N1400 prepared at 25 and 120 °C, respectively. Although only indicative, these low activation energies illustrate the ease of nucleation of Ag metal NPs on the surface of N1400. The observed difference in activation energy is possibly due to the different surface topographies of the N1400 films. In particular, for TsN1400 = 120 °C, the N1400 films consist of large and extended terraces for which surface diffusion of Ag atoms might be facilitated during the deposition process, which should enhance nucleation. Further differences between the Ag NP assemblies on N1400 can be analyzed using the radially averaged pair correlation function g(r) extracted from bright-field TEM images (see the Experimental Section).34 Parts b and c of Figure 5 show the representative g(r) functions extracted in the regime corresponding to the isotropic Ag NP distribution observed at Ts = 50 °C and for the films showing aligned NPs, i.e., Ts = 125 °C. For Ts = 50 °C, regardless of TsN1400, g(r) shows a marked maximum corresponding to the average interparticle distance: 6.8 nm for TsN1400 = 25 °C and 9.3 nm for TsN1400 = 120 °C. For the Ag films prepared at Ts = 125 °C, several maxima of g(r) are evidenced for the two types of N1400 films. For TsN1400 = 120 °C, g(r) shows three characteristic peaks at 7.1, 12.4, and 17.8 nm. The first two peaks correspond to two characteristic interparticle distances between nearest-neighbor NPs. The short 7.1 nm distance is associated with the small and rather close-lying Ag NPs composing the aligned NP assemblies which are located essentially at the step edges of the N1400 terraces. The longer interparticle distance at 12.4 nm is attributed to the larger Ag NPs distributed in a more isotropic and noncorrelated manner on the surface of the N1400 crystals (see Figure 4). Such large Ag NPs are most probably formed by coalescence. Finally, the last peak at 17.8 nm is possibly related to the distance between next-nearest-neighbor NPs. To summarize, for Ts ≥ 75 °C, peripheral edge decoration of N1400 terraces by Ag NPs is observed, whereas, for Ts ≤ 50 G
DOI: 10.1021/acs.jpcc.5b03646 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C “activation“ of specific nucleation sites on the substrates.36 It is also worth stressing that our interpretation of the observed results is based on a nucleation and growth mechanism of Ag NPs on N1400 and does not consider that the observed NP arrays arise from diffusion of the NPs themselves. Indeed, metal NP diffusion is possible but would require much higher substrate temperatures that are not compatible with the thermal stability of the N1400 layers.
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4. CONCLUSION TEM, SEM, and AFM were used to study the early stage of growth of Ag NPs on polycrystalline films of the n-type semiconductor N1400. The growth of Ag NPs by thermal evaporation is characterized by (i) the absence of substantial bulk diffusion of Ag in the N1400 layers even for substrate temperatures up to 125 °C and (ii) a transition from random to linear arrays of NPs when the substrate temperature increases. This transition is explained by the enhanced surface diffusion of Ag adatoms at higher Ts, which allows them to reach preferential nucleation sites at step edges of N1400 terraces. Moreover, it is shown that the activation energy for NP nucleation is lower for N1400 films with larger terraces. Overall, this study shows that bulk diffusion of metals in organic layers can be analyzed in its early growth stage by TEM tomography, which yields a true 3D representation of the bulk distribution of metal NPs in the organic layers.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Figures showing terraces of N1400 films highlighting several screw dislocations, the impact of the deposition rate τ on the Ag NP size, the growth kinetics of Ag NPs on an N1400 layer, and evolution of the Ag NP size distribution as a function of the substrate temperature. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03646. Corresponding Authors
*Phone: +33 0 3 88 10 70 28. E-mail:
[email protected]. *Phone: +33 0 3 88 41 40 47. E-mail:
[email protected]. Present Address ∥
N.K.: College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang (KMITL), Bangkok 10520, Thailand. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the European Community (FP7, Project HYMEC 263073). E. Gonthier is acknowledged for sample preparation.
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