Spontaneous Wetting Dynamics in Perylene Diimide n-Type Thin

Article pubs.acs.org/JPCC

Spontaneous Wetting Dynamics in Perylene Diimide n‑Type Thin Films Deposited at Room Temperature by Supersonic Molecular Beam Fabio Chiarella,*,†,‡ Federico Chianese,‡,† Mario Barra,†,‡ Loredana Parlato,‡,† Tullio Toccoli,§ and Antonio Cassinese‡,† †

SPIN-CNR Division of Naples, Piazzale Tecchio, 80, I-80125 Naples, Italy Physics Department, University of Naples, “Federico II”, Piazzale Tecchio, 80, I-80125 Naples, Italy § IMEM-CNR Division of Trento, Via alla Cascata 56/C, I-38123 Povo, Italy ‡

S Supporting Information *

ABSTRACT: During the last few years, the perylene tetracarboxylic diimide derivative PDIF-CN2 has been identified as one of the most interesting compounds to achieve air-stable n-type organic thin-film transistors. In this work, a detailed investigation on the properties of PDIF-CN2 thin films deposited by supersonic molecular beam deposition technique on HMDS-treated Si/SiO2 surfaces is reported. Our efforts have been addressed to analyze deeply the role played by the growth rate (R) on the thin film properties, working with the substrates kept at room temperature and using a kinetic energy of the impinging molecules of about 17 eV. In this way, we could observe that while, just after the deposition, the layers were not compact, being composed of small and mostly separated islands with a rate-dependent density, an unconventional spreading effect of the condensate took place over time until the formation of a continuous film. According to our results, this spontaneous wetting process occurs over very long time scales (i.e., from days to months) typical of a viscous phase and exhibits a dynamics also related to the initial growth rate. These remarkable morphological transitions are coherently accompanied by a several orders of magnitude increase in the field-effect charge carrier mobility, achieving maximum values of about 0.1 cm2 V−1s−1 in a specific R range. We have also found that a 1 h postannealing procedure at only 50 °C guarantees the rapid complete substrate wetting and the structural reassembly of the investigated films, providing comparable high mobility values.

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INTRODUCTION The control of the basic mechanisms ruling the condensation process of organic molecules from the vapor to the solid phase is fundamental for the deposition of films with improved morphological, structural, and electrical features. In conventional evaporation-based techniques, the deposition rate, substrate temperature, and the surface functionalization are typically the only accessible parameters to modify the molecule−surface and molecule−molecule interactions and, in such a way, to strongly influence the related adsorption and condensation phenomena.1 With reliance on an alternative approach where molecules are seeded in a free-expanding highpressure gas stream, the supersonic molecular beam deposition (SuMBD) technique is instead able to provide additional degrees of freedom in driving the film formation. This technique makes it possible to significantly increase the molecular kinetic energy (Ek) and to decouple the control of Ek from the deposition rate (R). Unconventional deposition regimes combining low R and high Ek energies are thus achievable. © XXXX American Chemical Society

Along with the theoretical predictions, over the few last years, several experimental works confirmed that SuMBD is a helpful approach to grow organic thin films with improved morphological and structural quality.2 This powerful technique was indeed exploited to get highly performing p-type organic thin-film transistors (OTFT) based on a variety of molecules such as pentacene,3 picene,4 and α-quaterthiophene5 as well as to control the deposition of different polymorphic phases in titanyl pthalocyanine layers.6 Very recently, the SuMBD approach was also applied to the growth of thin films of N,N′-1H,1H-perfluorobutyl-dicyanoperylenecarboxydiimide (PDIF-CN2), and the possibility to deposit n-type organic layers displaying high charge field effect mobility (μ) values even by keeping the substrate at room temperature was successfully demonstrated.7 Received: July 21, 2016 Revised: October 7, 2016 Published: October 28, 2016 A

DOI: 10.1021/acs.jpcc.6b07310 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

increase by orders of magnitude of the device charge mobility was recorded over time until maximum values were obtained that were comparable with the best ones achievable using ordinary effusive techniques and heated substrates. This unconventional spreading effect and the related phenomenology were systematically analyzed for a period of about one year.

In this regard, it should be remembered that PDIF-CN2, a perylene tetracarboxylic diimide derivative bearing two fluorinated (CH2-C3F7) side chains at imide positions, is presently one of the most-studied compounds for the fabrication of air-stable n-type organic transistors,8−10 exhibiting proper working operation even in a liquid environment11 and low sensitivity to the bias stress phenomenon.12 On the other hand, it is well-known that the electrical response of the PDIF-CN2-based OTFT is strongly affected by the specific surface selected for the film growth and by the related wettability properties. A suitable way to get highly performing OTFT is the reduction of the substrate critical surface tension (i.e., the increase in the hydrophobic surface power) through the application of specific self-assembled monolayers or insulating polymers. For PDIF-CN2 devices based on SiO2 dielectric barriers in particular, the surface treatment with hexamethyldisiloxane (HMDS) is able to produce a considerable increase in the extracted μ values in comparison with those of the transistors fabricated on untreated SiO 2 substrates.13 Considering the equation of spreading coefficient S = γs − (γs/o + γo), where γs/o, γo, and γs are the surface tensions of interface, organic molecules, and substrate, respectively, HMDS treatment actually reduces the γs value, favoring a 3D growth mode expressed by a negative S value. The main route to enhance the surface molecular diffusivity and promote a layer-by-layer growth with the occurrence of more effective crystallization processes is the increase in the substrate temperature. In particular, for PDIF-CN2 OTFT based on bottom-gate architectures and fabricated by a conventional effusive process, mobility values were found to rise by several orders of magnitude, up to 0.6 cm2V−1s−1, when the substrate temperature (Tsub) ranged from room temperature to 120 °C.9 Furthermore, Weitz and coauthors reported14 that PDIF-CN2 films deposited by Knudsen cells exhibit structural polymorphs for Tsub ∼ 140 °C, while the related mobility values result in being significantly lowered. This last phenomenon was explained in terms of a partial dewetting of the material from the surface which is likely ascribable exactly to the polymorphism presence. As a whole, all of these experimental findings clarify that the realization of PDIF-CN2-based thin-film devices with optimized response requires the achievement of a critical compromise among several competing growth mechanisms. In this work, PDIF-CN2 thin films were deposited by hyperthermal seeded supersonic beams on hydrophobic SiO2 surfaces for the fabrication of OTFT. During the film growth, the substrates were kept at room temperature, the molecular kinetic energy (Ek) was 17 eV, and the growth rate was finely tuned from about 0.01 to 0.22 nm/min. We found that, in the analyzed conditions where high kinetic energies and very low deposition rates are combined, the film growth mode has a largely dominant 3D character. Indeed, immediately after the deposition, the condensate was not compact, being formed by rounded islands with a typical diameter of about 150 nm. The surface density of these islands changes with the growth rate, which consequently affects the related degree of interconnectivity. Quite surprisingly, however, we also observed the occurrence during the postdeposition period of a spontaneous slow wetting process of the condensate that ends with the formation of a continuous film. This exotic phenomenon evolves with unusually long time scales which are also dependent on the deposition rate used for the film growth. As a consequence of the film morphology modification, an

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EXPERIMENTAL SECTION PDIF-CN2 (Polyera ActivInk N1100) powder was purchased from Polyera Corporation and used without any further purification. Bottom-contact bottom-gate transistors were fabricated by multilayered structures consisting of a 500 μmthick highly doped silicon substrate, a 200 nm-thin SiO2 dielectric barrier suitably functionalized by application of a hexamethyldisiloxane (HMDS) monolayer (contact angle of 110°, see ref 15 for a detailed description of the complete procedure), and source/drain gold electrodes (channel width/ length (w/L) ratio was fixed to 550) with interdigitated layout. As final step, a nominally 20 nm-thick film of PDIF-CN2 was deposited on the top of the stacked structure by SuMBD. The organic precursor was seeded in a supersonic jet of He used as carrier gas with stagnation pressure value of about 2000 mbar, resulting in an Ek value of approximately 17 eV for the impinging PDIF-CN2 molecules. This last value represents the saturation limit of the process of momentum transfer from the He carrier gas to the molecule for the sublimation temperature of the raw material. The deposition rate parameter (R) was finely tuned through the control of the sublimation temperature within the effusive cell containing the organic powder. In this work, temperatures between 203 and 220 °C were used, corresponding to deposition rates going from 0.01 to 0.22 nm/ min. In this range of temperatures, the maximum reachable final value of Ek is practically the same. The film growth rate was measured in situ, intercepting the molecular beam just before and after the deposition process by a thickness monitor. The values estimated in this way were then systematically checked ex-situ by performing AFM measurements. For all of the films analyzed here, the substrate was kept at room temperature (around 25 °C) during the deposition. In the case of postannealing experiments, samples were heated ex situ at 50 °C in vacuum for 1 h. Morphological characterizations were performed in air by a Park Instruments XE-100 atomic force microscope equipped with silicon-doped cantilevers (PPPNCHR) in true noncontact mode. Image analysis with the extraction of relevant morphological parameters was accomplished by Gwyddion open source software. X-ray diffraction spectra were recorded using a Rigaku diffractometer equipped with a conventional Cu Kα source and a graphite monochromator. The electrical response of the PDIF-CN2 OTFT was assessed in vacuum and darkness by a Keithley 2612A dual-channel system source-meter instrument connected to a Janis cryogenic probe station. Mobility values were extracted from the transfer curves in saturation regime by applying the standard MOSFET model.

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RESULTS

In this study, with the goal to investigate the growth of PDIFCN2 thin films by fully exploiting the potentialities of the SuMBD technique, the kinetic energy of the molecular beam was set (with few exceptions, as discussed in the following text) at a saturation value of about 17 eV. B


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Figure 1. AFM images (5 × 5 μm) of PDIF-CN2 films deposited by SuMBD at different deposition rates (along the rows) acquired at different times (in columns). The white marker is a reference scale of 1 μm. Panel h reports an AFM phase signal image zoom of image b (1 × 1 μm) with relative color scale in degrees. The red marker is a reference scale of 0.2 μm.

Table 1. Statistical Parameters Extracted by AFM Images for Different Deposition Rates at Different Timesa a few hours

1 month −2

1 year

percolation parameters

R (nm/min)

ξ (nm)

Φ (%)

N (μm )

ξ (nm)

Φ (%)

ξ (nm)

Φ (%)

τ (days)

0.03 ± 0.01 0.08 ± 0.01 0.22 ± 0.01

38.5 ± 1.0 33.5 ± 0.8 37.6 ± 0.9

50 ± 4 75 ± 3 89 ± 2

38.4 ± 0.1 42.8 ± 0.1 48.7 ± 0.1

37.7 ± 0.8 77 ± 1 58 ± 3

44 ± 4 99 ± 2 96 ± 2

37.5 ± 0.6 77 ± 1 77.0 ± 0.5

45 ± 4 99 ± 2 99 ± 2

unappr. 30 ± 3 132 ± 2

Δt (days)

μs (cm2 V−1 s−1)

4±1 5±1

7 × 10−7 0.07 ± 0.02 0.004 ± 0.002

a Deposition rate (R), correlation length (ξ), surface coverage (Φ), and density of the islands (N). This last parameter is reported just for the asgrown morphologies. ξ was estimated by fitting autocorrelation data extracted by numerical calculations from AFM images with the function C(r) = σ2 exp(−(r/ξ)2), where σ is the root mean square roughness and r is the radial position. μ0 is not reported, being a very low value comparable to zero for our experimental setup. The right side of the table (Percolation parameters) shows the parameters of sigmoidal Boltzmann fitting function for the data of the mobility versus time (Figure 2b). For R = 0.03 nm/min, τ was indicated as “unappr.” (unappreciable). In this case, we assume that the time needed for the formation of a percolation path is greater than the available experimental timing.

For all of the deposited films, the substrate was kept at room temperature, and the effect of the deposition molecular rate R was investigated in the range 0.01−0.22 nm/min, as explained in detail in the Experimental Section. It should be mentioned that the analyzed R values are, in any case, considerably lower than those commonly employed (∼1 nm/min) for the growth of PDIF-CN2 films with effusive sources.9 The first outcome of our experiments was the observation that, when the growth rate R was lower than 0.02 nm/min, films were not deposited. This finding highlights the relevant

molecule tendency to the desorption for the conditions investigated here.16 Figure 1 instead shows AFM images of the surface of films grown with R = 0.03, 0.08, or 0.22 nm/min. These three rate values were selected because they are representative of the whole phenomenology hereafter described. The AFM images in Figures 1a−c (for 0.03, 0.08, and 0.22 nm/min, respectively) were taken few hours after the end of the deposition process and reveal that the as-grown films are not continuous, being composed of mostly separated islands with a rounded shape C


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Figure 2. (a) Semi-log plot of the transfer curves measured in the saturation regime (VDS = 50 V). The curves were acquired at different times and are referred to a device achieved with a deposition rate of 0.08 nm/min. (b) Mobility time dependence extracted from the transfer curves in the saturation regime using the related MOSFET equation. Continuous lines are the sigmoidal Boltzmann fitting curves (see text).

being dependent on the R values. Indeed, while for R = 0.08 nm the phenomenon was finalized in few weeks (Figure 1e), it took several months for the layers deposited at the higher rate (see the mixed morphology in Figure 1f after 1 month). It should also be mentioned that the final continuous layers achieved with R = 0.08 and 0.22 nm/min showed different surface roughness values (3.5 and 4.3 nm, respectively) as a further confirmation that the complex phenomenon here observed is strongly affected by the initial growth conditions. As clearly shown in the graph of Figure 2a, the morphological modifications of the film structure were directly translated into remarkable changes of the electrical performances of the related OTFT. Herein, transfer curves in the saturation regime acquired at different times for a device based on a film grown at R = 0.08 nm/min are reported. Figure S2 certifies that these curves follow the MOSFET model quite closely, predicting the linear behavior of the square root of the measured I DS. Transfer curves in saturation are indeed commonly considered more reliable than those recorded in the linear regime, where the effect of the contact resistances cannot be neglected in the response of perylene-diimide transistors.18 The mobility increase evaluated for the different analyzed devices is presented in Figure 2b, confirming that the final mobility value is about 0.1 cm2 V−1 s−1 in the case of optimized R. In the theoretical framework of IDS current flow driven by percolation mechanisms, a fit of mobility behavior over time was performed using the Sigmoidal Boltzmann function:

and average diameter of about 150 nm. The partial coverage of the substrate can be inferred even more clearly from the strong contrast in the AFM phase image as reported in Figure 1h. For the morphological images in Figure 1, correlation length (ξ), island density (N), and coverage (Φ) were estimated by standard autocorrelation function and grain watershed analysis, as summarized in Table 1. N extracted from Figures 1a−c was found to increase with R following the classical law N∝ Rα, where α is a coefficient usually related to the critical nuclei size.2,16 While Φ was also enhanced at increasing R, ξ exhibits a peculiar not-monotonous behavior with a minimum at R ∼ 0.08 nm. This specific feature will be further discussed in the following text. Focusing the attention on the structure of the single domains (see Figures S1a and b in the Supporting Information), AFM images acquired on a 1 × 1 μm area underline that these islands, on their turn, are composed of a varying number of smaller adherent subdomains. Moreover, Xray θ−2θ diffractograms exhibiting a clear diffraction peak at about 4.6° (Figure S1c) confirm that the islands of the asgrown layers present a crystalline character and an out-of-plane order following what is reported in ref 7. After the deposition process, the PDIF-CN2 films were stored in air and darkness and periodically characterized both in terms of morphological and electrical features. The idea of analyzing systematically the film properties over time was inspired by previous observations, indicating occasionally the occurrence of progressive improvements of the electrical response, in open contradiction with the usual aging phenomenon typically affecting samples stored in air.17 In any case, we observed also that, by keeping the deposited films in the vacuum chamber (in darkness and without any electrical polarization) for an entire week, the hereafter described phenomenology did not change substantially, suggesting that the wetting process is not activated by the exposure to ambient agents or by the atmospheric pressure. Figures 1d and g reveal that the basic structure of the films deposited with R = 0.03 nm/min did not change significantly over time, while the situation appeared deeply different for layers grown at increasing R values. In particular, for R = 0.08 and 0.22 nm, a spontaneous change in the wetting properties of the film−substrate interface from partial to complete wetting occurred until the film morphology appeared restructured and substrate surface was fully covered (Figures 1e and i). The dynamics of this radical change, moreover, resulted in also

μ(t ) = μs + (μ0 − μs )(1 + e(t − τ)/ Δt )−1

In this equation, μ0 and μs are the subcritical and supercritical mobility values, respectively, while τ and Δt represent the mean value and the width of the distribution of activation timing of the percolation paths. The values of the data fitting are summarized in Table 1. According to this model, it was possible to assume τ as characteristic time for the wetting dynamics. From the fitting curves, we derived that τ values were considerably higher (by a factor 4) at 0.22 nm/min in comparison with 0.08 nm/min, while the mobility value μs followed the opposite behavior. This issue was further examined by extracting the mobility values after 1 year from the film deposition for a wide number of devices realized at different rate values (Figure 3). D



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DISCUSSION Summarizing the reported results, it is possible to state that, on HMDS-treated SiO2 surface kept at room temperature, PDIFCN2 films deposited by SuMBD follow a peculiar 3D growth mode, which is basically promoted by the specific energetic conditions in the nonequilibrium hyperthermal condensation process. In particular, while the very low deposition rates and the high kinetic energy tend to reduce the density of the nucleation centers, the room temperature of the substrate during the deposition depresses the molecular diffusivity, disfavoring the formation of larger islands.2,16,19 The most important aspect determining the PDIF-CN2 film growth mode, however, seems to be related to the strong imbalance between the molecule−molecule and molecule− surface interactions. In this regard, the presence of HMDS selfassembled monolayer favors the cohesion strengths with respect to those of the adhesion ones, and as is well-known, three-dimensional growth modes are mainly supported by a weaker molecule−substrate interaction.20 All of these features in combination with the observed low sticking coefficient (a desorption rate of about 0.02 nm/min was estimated on HMDS/SiO2 surface at Ek ∼ 17 eV) produce the 3D nanostructured morphology of the films, as depicted in Figure 1. It is also important to outline that the film growth process observed here displays distinctive aspects which cannot be simply rationalized in the classical Volmer−Webber (VW) growth model.20 In particular, even if the nucleation density was found to obey the classical power law with the deposition rate (N∝ Rα), the estimated exponent α ≅ 1/8 is quite lower than that expected for films nucleating according to the pure VW mechanism. This discrepancy can be justified by hypothesizing that, under the molecular flow, the islands can diffuse on the substrate, producing a partial aggregation. This characteristic appears in very good agreement with the morphological details of the as-grown films, suggested by the AFM magnified image reported in Figure 1h. When the flow of molecules stopped, a spontaneous spreading phenomenon was observed. This wetting process, over a long time scale, brings a complete surface coverage (see the Φ parameter in Table 1) where the progressive formation of a more homogeneous film is accompanied by the continuous enhancement of the OTFT electrical response. As already shown in Figures 1−3, the evolution over time of morphological and electrical properties is strongly correlated to the initial deposition rate and hence to the starting distribution of deposited islands. The dependence of the spreading phenomenon from the rate immediately jumps out from Table 1, linking the correlation length of the as-grown morphologies, the final mobility values measured after one year, and the different timing of the wetting dynamics (see the values of ξ, μs, and τ parameters reported in Table 1). As shown, the minimum value of ξ achieved for R ∼ 0.08 nm/min corresponds to the maximum value for μs and to a faster kinetics (lower τ) of the spreading phenomenon. This finding can be understood if we keep in mind that, in our case, ξ represents a measure of the average size of the flat uncovered zones of the substrate, being practically proportional to the minimum distance between two neighboring islands. The occurrence of a non-monotonous behavior of the ξ parameter, different from what happens for N and Φ, suggests that if the volume of the deposited materials ends up being distributed in

Figure 3. Semi-log plot of the mobility values extracted in the saturation regime one year after the deposition for PDIF-CN2 devices fabricated at different growth rates. The red dashed line is a guide for the eyes.

In the lowest range of the explored rate window (∼0.03 nm/ min), the film morphology appeared basically stable over one year. Consequently, the device mobility was invariant, remaining very close to the minimum resolution limit of our experimental setup. For R values higher than 0.05 nm/min, mobility was instead demonstrated to rise to a maximum value of about 0.1 cm2 V−1 s−1 at an R value of around 0.07−0.08 nm/min. A further increase in the deposition rate, on the other hand, revealed that the mobility value measured after one year tends to be slightly lower in comparison with the best value. As a whole, this analysis definitively clarified that, in the case of SuMBD deposition, the range around 0.08 nm/min is the best option to get PDIF-CN2 films with an optimized electrical response. In this regard, we also analyzed the effect of a postannealing procedure (as described in the Experimental Section) on devices fabricated at various R values. Hence, we found that a 1 h annealing at only 50 °C noticeably accelerates the time scale of the wetting process. In particular, in the case of devices realized with the optimized rate (∼0.08 nm/min), a full transition from the initial structure with isolated islands to a continuous well-shaped morphology was observed after annealing (Figure S3a). A maximum mobility in saturation of 0.1 cm2 V−1 s−1 was thus rapidly achieved (Figure S3b). Finally, in an attempt to deepen the described phenomenology, a further set of complementary experiments was performed. First, the effect of the kinetic energy on the film growth was assessed by fixing the deposition rate in the optimized range and reducing the kinetic energy Ek value of the impinging molecules down to 13 eV. In this case, a weak enlargement in the time scale of the wetting process proportional to the Ek reduction was observed. A further lowering of Ek pushed the system toward conventional growth conditions, i.e., toward the stable amorphous phase of the condensate, which is usually observed when the substrate is at room temperature.7 Second, PDIF-CN2 films were deposited at the optimized conditions of Ek and R on untreated (bare) SiO2 surfaces with a water contact angle of about 60°. In this case, the as-grown layer was continuous and composed of small grains (Figure S4), and no wetting dynamics was observed. The related OTFT consequently displayed a time invariant response with low mobility values in the range between 10−6 and 10−5 cm2 V−1 s−1. This finding specifically highlights the central role of the reduced surface tension induced by HMDS treatment in the above-described spreading phenomenon. E


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ditions bring the system in a mesophase, where the fluorinated functional group of the PDIF-CN2 should play a crucial role. For PDIF-CN2 molecules deposited at room temperature with Ek ∼ 17 eV and rate of about 0.08 nm/min, the process of the condensate spreading on the substrate was completed in about 2 weeks, and good quality OTFTs were obtained. To overcome the long relaxation time, we also observed that 1 h postannealing at low temperature (50 °C) is able to guarantee the rapid complete wetting and structural reassembly of the film, providing mobility values up to 0.1 cm2 V−1 s−1.

spherical caps on the surface, the islands do not keep the same aspect ratio for each R value.21 In this context, it is necessary to keep in consideration that the island−island interaction could influence the system energetics, growth dynamics, and island shape,22 inducing the non-monotonic behavior of the ξ parameter in the as-grown morphologies and influencing all the subsequent phenomenology of the spreading and surface wetting dynamics observed. Beyond the related morphological evolution, another element to be discussed is the driving force of the spreading phenomenon. Molecules deposited by SuMBD on the HMDStreated surface could be arranged in a metastable polymorph, as recently discussed by Jones et al.23 in the framework of substrate-induced polymorphic phases. Molecules could condensate with a mechanism of cross-nucleation between polymorphs, stabilized by the hyperthermal nonequilibrium conditions induced by the supersonic molecular beam. In the unconventional deposition conditions considered here, it is possible that, due to the high kinetic energy of the impinging molecules, the fluorinated chains result in being chaotically interdigitated, thus producing a disorder along the direction perpendicular to the substrate (i.e., along the molecule− molecule interlayer direction) as already observed for other perylene derivatives with longer alkyl side chains in liquid crystalline phase.24,25 According to this scenario, after the deposition, due to a slow dynamics transition of the molecules from the metastable to a more stable polymorph, the system relaxes, and the balance between the molecule−molecule interlayer and intralayer cohesion energies and the surfacemolecule adhesion energy gradually changes, inducing macroscopic reorganization. The postdeposition annealing procedure at 50 °C speeds this transition, revealing the thermally activated mechanism. As a whole, we can assume that PDIF-CN2 molecules deposited with high kinetic energy at low rates on hydrophobic surface kept at room temperature condensate not in the classical solid phase but rather in a mesophase such as a liquid crystal, where the side fluorinated chains likely play a key role in determining the complex phenomenology described here. More specifically, for these PDIF-CN2 films, the presence of the substrate should trigger a spreading phenomenon toward a smectic phase following terraced wetting dynamics, similar to what was already observed and discussed for liquid polymers26,27 and liquid crystal28,29 drops on solid surfaces. Deeper investigation and theoretical modeling are required to further understand the complex phenomenology illustrated in this work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07310. Additional morphological (AFM images), structural (Xray diffractograms), and electrical (transfer curves) characterization of as-grown, annealed, and deposited bare substrate films (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +390817382423. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Dr. A. Carella (University of Naples) for the HMDS treatment of the transistor substrates. Financial support from INFN-CNR national project (PREMIALE 2012) EOS “Organic Electronics for Innovative research instrumentation” is gratefully acknowledged.

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REFERENCES

(1) Virkar, A. A.; Mannsfeld, S.; Bao, Z.; Stingelin, N. Organic Semiconductor Growth and Morphology Considerations for Organic Thin-Film Transistors. Adv. Mater. 2010, 22, 3857−3875. (2) Iannotta, S.; Toccoli, T. Supersonic Molecular Beam Growth of Thin Films of Organic Materials: A Novel Approach to Controlling the Structure, Morphology, and Functional Properties. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2501. (3) Wu, Y.; Toccoli, T.; Koch, N.; Iacob, E.; Pallaoro, A.; Rudolf, P.; Iannotta, S. Controlling the Early Stages of Pentacene Growth by Supersonic Molecular Beam Deposition. Phys. Rev. Lett. 2007, 98, 076601. (4) Gottardi, S.; Toccoli, T.; Iannotta, S.; Bettotti, P.; Cassinese, A.; Barra, M.; Ricciotti, L.; Kubozono, Y. Optimizing Picene Molecular Assembling by Supersonic Molecular Beam Deposition. J. Phys. Chem. C 2012, 116, 24503. (5) Toccoli, T.; Tonezzer, M.; Bettotti, P.; Coppedè, N.; Larcheri, S.; Pallaoro, A.; Pavesi, L.; Iannotta, S. Supersonic Molecular Beams Deposition of α-Quaterthiophene: Enhanced Growth Control and Devices Performances. Org. Electron. 2009, 10, 521−526. (6) Coppedè, N.; Castriota, M.; Cazzanelli, E.; Forti, S.; Tarabella, G.; Toccoli, T.; Walzer, K.; Iannotta, S. Controlled Polymorphism in Titanyl Phthalocyanine on Mica by Hyperthermal Beams: A MicroRaman Analysis. J. Phys. Chem. C 2010, 114, 7038−7044. (7) Chiarella, F.; Toccoli, T.; Barra, M.; Aversa, L.; Ciccullo, F.; Tatti, R.; Verucchi, R.; Iannotta, S.; Cassinese, A. High Mobility n-Type Organic Thin-Film Transistors Deposited at Room Temperature by Supersonic Molecular Beam Deposition. Appl. Phys. Lett. 2014, 104, 143302. (8) Jones, B. A.; Ahrens, M. J.; Yoon, M. H.; Facchetti, A.; Marks, T. J.; Wasielewski, M. R. High-Mobility Air-Stable n-Type Semi-

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CONCLUSIONS In this paper, the growth of PDIF-CN2 films by the SuMBD technique on hydrophobic HMDS-treated SiO2 substrates kept at room temperature was investigated as a function of the deposition rate. We found that, by maximizing the molecular kinetic energy (up to 17 eV) with this hyperthermal molecular beam technique, stable thin films composed of isolated nanostructured islands can be achieved for rate values between 0.02 and 0.04 nm. When the rate was increased (up to 0.22 nm/min), a terraced wetting dynamics finally providing a continuous film and displaying very long relaxation times (up to several weeks) typical of a viscous phase was instead observed. The specific combination of surface hydrophobic functionalization, molecular structure, and hyperthemal deposition conF


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Spontaneous Wetting Dynamics in Perylene Diimide n-Type Thin

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