Self-assembled nanopillar arrays by simple spin coating from blending systems comprising PC₆₁BM and conjugated polymers with special structure

Abstract

Three conjugated D–A copolymers were found to form well-defined nanopillar arrays through facile spin-casting process when blended with fullerene derivatives. Research results indicate that the presence of large coplanar segments and intramolecular S⋯O attractive interactions in the polymers are both crucial factors for achieving self-assembled nanopatterned pillar arrays.

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The fabrication of periodic nanostructures, such as nanopillar arrays, has attracted great attention due to their potential applications in magnetic storage media, organic semiconductors, plasmonic nanophotonics, and filtration membranes.^1–5 Most of the related researches have focused on inorganic materials, such as SiO₂, Fe₂O₃, ZnO, and TiO₂.^6,7 Recently, there has been increasing interest in manufacturing nanopillar arrays comprising polymer components, especially conjugated polymers, due to their low cost and successful applications in organic light-emitting diodes (OLED) and organic photovoltaics (OPV).^8,9 For example, nanopillar arrays or nanovertical domains in films of poly(3-hexylthiophene) (P3HT, a well-known conjugated polymer) have been developed.^10,11 To obtain polymer-containing periodic nanostructures, a nanotemplate technique is generally indispensable, while the fabrication of a pre-patterned nanotemplate is usually costly and complex.^12–14

Recently, with the booming development of supramolecular chemistry, self-organization has been recognized to be a very powerful and facile alternative to nanofabrication and nanomanipulation.¹⁵ Many fantastic nanomotifs like nanofibers, nanorods and nanospheres have been realized through controlled self-assembly processes.^16–19 With regard to the fabrication of self-assembled nanostructure arrays, diblock copolymers and binary polymers with poor miscibility, e.g., polystyrene (PS) and polymethylmethacrylate (PMMA), are generally utilized. This procedure is often accompanied with the post-processing of either physical dissolution or chemical etching to selectively remove one component, because the effective lateral phase separation in bulk could be triggered during the spin-coating process.^20–22 Spin-coating is a very widely used technique and a simple method for making uniform thin polymer films. Therefore, the self-assembly behavior of diblock copolymers during the spin-coating process has drawn significant attention.^23,24

In this paper, we report our discovery that well-defined polymer-comprising nanopillar arrays could be achieved by a rather simple spin-casting process by blending the systems of [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) (structure shown in Fig. S1, ESI†) and certain conjugated copolymers on quartz substrates, dispensing with any pre- or post-processing requirements. To the best of our knowledge, this is the very first report to achieve nanopillar arrays just by facile spin-coating without further processing. In-depth investigations reveal that the presence of large coplanar polymeric backbones is indispensable for acquiring nanopatterned pillar motifs, while the introduction of long-chain alkyloxy groups into appropriate positions of the polymeric backbones would endow these macromolecules with strong intramolecular S⋯O attractive interactions, resulting in higher molecular weights as well as maintaining planar conformation of the copolymers, and hence, better self-organization capability.

In our previous work exploiting low bandgap D–A-featured (donor–acceptor) copolymers for organic photovoltaic (OPV) applications, we synthesized PT-BOAQ (structure shown in Fig. 1) bearing electron-deficient planar fused aromatic A subunits.²⁵ After spin-casting the PT-BOAQ/PC₆₁BM (1/3) composite solution, we found surprisingly that the surface of this optically active layer displays incredible and charming well-aligned nanopillars (Fig. 2a). This should be a quite valuable discovery, since these nanopillar arrays are achieved by facile spin-coating, dispensing with any pre- or post-processing, i.e., through a mere self-assembly process.

Fig. 1 Molecular structure of the copolymers investigated in this article.

Fig. 2 3D-visualized AFM image and cross section view in height mode of (a) PT-BOAQ/PC₆₁BM blend film and (b) pure PT-BOAQ film. All images span 5.00 × 5.00 μm.

In fact, although considerable attention has been paid to the morphology characterization of spin-coated polymers/PC₆₁BM composite films due to the rapid development of OPV materials and devices, to the best of our knowledge, there have been no related reports declaring the observation of nanopillar arrays on the surface of such film samples. Consequently, we conjectured that the formation of well-aligned nanopillars in this blending system should be dominated by polymers rather than PC₆₁BM species. To validate this hypothesis, a pure PT-BOAQ-based reference film was prepared. As shown in Fig. 2b, although the width and height of the self-assembled domains are far less regular in comparison with those of the PT-BOAQ/PC₆₁BM composite film, obvious pillars are still distinguishable on the surface of this reference film. Therefore, PT-BOAQ should possess an intrinsic self-organization capability, while the presence of PCBM derivative should be beneficial to the reinforcement of the self-assembly process, leading to perfectly arrayed nanopillar structures.

According to literature reports,^26,27 since both the chemical structure and molecular weight of the polymers are essential factors influencing the morphological properties of polymer/PC₆₁BM composite films, we first investigated the relationship between the morphological properties of the blending films and the M_w value of PT-BOAQ. As depicted in Fig. S2 (in the ESI†), in all these samples, distinct nanopillar arrays were discernible despite the drastically different M_w values of the copolymer component. Moreover, with an increasing M_w value of PT-BOAQ from 6900 to 27 900, the surface of the corresponding blending films shows further well-isolated and higher nanopillar domains. Therefore, it should be the chemical structure rather than molecular weight of PT-BOAQ that determines the intrinsic self-assembly capability of the blending films.

Based on the structural analysis, PT-BOAQ could be split into three core segments: (1) 3,3′′-didodecyl-2,2′:5′,2′′-terthiophene; (2) acenaphtho[1,2-b]quinoxaline (AQ); and (3) two octyloxy groups attached to AQ. Although poly(alkylthiophene)s (PAT), one of the most intensively investigated OPV electron-donating materials, bears terthiophene building units similar to those of PT-BOAQ, no such well-aligned nanopatterns have been reported in pristine PAT/PC₆₁BM blend films; therefore, the terthiophene subunit might play a trivial role in the self-organization process of PT-BOAQ.

Therefore, to determine the relationship between the chemical structure and self-assembly capability of copolymers, PT-BOPQ bearing lesser number of planer diphenylquinoxaline A units and PT-AQ lacking two octyloxys on the AQ segment were synthesized as reference polymers (structure shown in Fig. 1). As depicted in Fig. 3a, the PT-BOPQ/PC₆₁BM blending film manifests a fairly smooth surface with RMS of 0.53 nm, implying that the presence of a large coplanar aromatic segment in polymers is indispensable for acquiring nanopillar arrays. On the other hand, for PT-AQ bearing similar AQ subunits but no alkyloxy groups, obvious pillars could be identified in the composite film (Fig. 3b), confirming that the existence of a planar aromatic system will help facilitate the self-assembly process. In fact, 9,10-bis(octyloxy)acenaphtho[1,2-b]quinoxaline, the key constructive moiety of PT-AQ, is observed to form a vertical column crystal in an ethanol solution, as shown in Fig. S3 (in the ESI†), suggesting that the large planar structure would indeed trigger self-organization in the perpendicular direction. Nevertheless, in comparison with the PT-BOAQ/PC₆₁BM film, the domains formed on the surface of the PT-AQ/PC₆₁BM film are far from well-organized along both width and height. Taking into account that PT-AQ has an extremely low molecular weight of 1780, which could not be further enhanced due to its poor solubility, both the lower M_w and the absence of two alkyloxy groups may be responsible for the low quality arrays of the PT-AQ/PC₆₁BM film.

Fig. 3 3D-visualized AFM image and cross section view in height mode of (a) PT-BOPQ/PC₆₁BM blend film and (b) PT-AQ/PC₆₁BM blend film. All images span 5.00 × 5.00 μm.

Recent reports^28,29 have revealed that in a large number of organosulfur compounds, obvious intramolecular nonbonded interactions between sulfur and oxygen atoms exist, which could act as an effective conformational controlling factor. Therefore, to gain deeper insights into the role of two alkyloxy groups in the self-assembly process of PT-BOAQ, density functional theory (DFT) calculations have been performed to investigate the optimized ground state geometries of the repeating units of these copolymers. As shown in Fig. S4 (in the ESI†), in all the repeating units with or without octyloxy groups, the two thiophene units and their neighboring AQ or quinoxaline segments are found to be mutually coplanar. Furthermore, for the repeating units bearing two alkyloxy groups on AQ or quinoxaline moieties, the S⋯O distances between thienyl and alkyloxy groups are calculated to be ∼2.7 Å, which is significantly shorter than the sum of the van der Waals radii for sulfur and oxygen atoms (3.32 Å), indicative of the presence of intramolecular S⋯O attractive interaction between the thienyl and alkyloxy groups. Consequently, the introduction of two octyloxys at the appropriate position of the A units may be beneficial to both the maintenance of planar conformation and the enhancement of M_w of these copolymers, and hence, better self-organization capability.

From all these experimental observations, we can infer that both large planar segments and long-chain alkyloxy substituents may be essential constructional units to acquire copolymers capable of forming well-defined nanopillar arrays. To verify this conjecture, two copolymers (PB-BOAQ and PBDT-BOAQ, structures shown in Fig. 1) bearing similar A units of dioctyloxy-substituted AQ but different D units with PT-BOPQ were synthesized. For PB-BOAQ, a larger planar benzodithiophene (BDT) instead of thiophene segment was inserted between two thienyls; while in PBDT-BOAQ, the BDT segment was further modified by two alkylthienyls, which may result in more twisted conformation of the copolymer, and hence, a weakened self-organization capability. These deductions were confirmed by the DFT calculation results, since the intramolecular S⋯O attractive interactions between thienyl and alkyloxy groups are discernible in the construction units of both PB-BOAQ and PBDT-BOAQ; further, the repeating unit of PB-BOAQ displays a much planar conformation, yet that of PBDT-BOAQ shows drastically decreased planarity due to the twisting of the two thienyls. Clearly evidenced by the AFM images shown in Fig. 4, the PB-BOAQ/PC₆₁BM blending film indeed shows more well-defined nano-patterned pillars than those of the PT-BOAQ/PC₆₁BM film, but the quality of the nanopillars on the surface of the PBDT-BOAQ/PC₆₁BM film is much lower, since most of the pillars are not well-separated. Therefore, the presence of large coplanar π-conjugation segments does contribute significantly toward the enhancement of the self-organization capability of the copolymers, and hence, is beneficial to the fabrication of high quality nanopillar arrays.

Fig. 4 3D-visualized AFM image and cross section view in height mode of (a) PB-BOAQ/PC₆₁BM blend film and (b) PBDT-BOAQ/PC₆₁BM blend film. All images span 5.00 × 5.00 μm.

Besides the relationship between the molecular structure and self-assembly ability, the role that fullerene derivative plays in the formation of nanopillar motifs also deserves elucidation. Inspired by the comment of Sasha Y. Heriot³⁰ and André D. Taylor et al.³¹ that the phase separation in spin-casted polymer blending films correlates closely with the wetting procedure, we deduced that the self-assembled pillar nanomotifs might arise from the surface/interfacial instability between the substrate, PC₆₁BM and the copolymers used. Therefore, the contact angles of the quartz substrate, PC₆₁BM and the five copolymers were measured, and the corresponding surface energies were calculated accordingly. As summarized in Table 1, the surface energy of PC₆₁BM is 28.80 mJ m⁻², while those of PT-BOAQ (11.74 mJ m⁻²), PB-BOAQ (16.48 mJ m⁻²), PBDT-BOAQ (18.96 mJ m⁻²) and PT-AQ (19.45 mJ m⁻²) are lower. Consequently, during the rapid solvent-casting process, phase separation may take place due to the relatively large surface energy difference between PC₆₁BM and these copolymers, which may trigger the self-assembling of these copolymers into nanopillars. However, for PT-BOPQ, since its surface energy (22.40 mJ m⁻²) is closer to that of PC₆₁BM, the demixing of the two components should be less efficient, and hence, no nanoarrays were formed in this blending system. Furthermore, the surface energy of the quartz substrate (61.24 mJ m⁻²) is much larger than those of both PC₆₁BM and these polymers. Taking into account that PC₆₁BM has higher composition and smaller surface energy difference from that of quartz, it should be PC₆₁BM rather than these polymers that adsorbs preferentially on the substrate (Fig. 5b). Nevertheless, the surface energy difference between PC₆₁BM and quartz substrate is still quite large, thus PC₆₁BM might form pre-organized clusters on the surface of the quartz substrate, followed by self-organization of the macromolecular component if lateral phase separation occurs in these blending systems. Finally, nanopillar arrays are formed on the substrate (Fig. 5c).

Table 1 The contact angle and surface energy of all the materials investigated in this article

Materials	Contact angle [deg.]	Surface energy [mJ m^−2]
PT-BOAQ	119.7	11.74
PB-BOAQ	111.1	16.48
PBDT-BOAQ	106.8	18.96
PT-AQ	106.0	19.45
PT-BOPQ	101.1	22.40
PCBM	90.7	28.80
Quartz substrate	36.5	61.24

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Fig. 5 Diagram of the proposed pillar nanomotif formation process of polymer/PC₆₁BM blending systems. (a) Dropping the composite solution on quartz substrates; (b) efficient phase separation of the two components during spin-coating due to their interfacial instability, with PC₆₁BM preferentially adsorbed on quartz substrates; (c) the formation of nanopillar arrays arising from the self-organization of copolymers after evaporation of solvent.

In conclusion, through delicate molecular design on the macromolecular component, well-defined nanopillar arrays could be fabricated by a facile spin-casting process by blending the systems of PC₆₁BM and conjugated copolymers. The presence of large coplanar segments in the polymers is a crucial factor for achieving self-assembled nanopatterned pillar arrays, while the existence of intense intramolecular S⋯O attractive interactions between long-chain alkyloxy and thienyl groups on the polymeric backbones would endow these macromolecules with compromised planar conformations and high molecular weights, and hence, an enhanced self-organization capability. Our results may shed light on the molecular design strategy for achieving periodically nanopatterned optoelectronic active polymeric systems through template-free and simple approaches.

Acknowledgements

The financial support for this work of the National Natural Science Foundation of China (project no. 50803040, 21190031 and 21372168) is acknowledged. We also thank the Analytical & Testing Center of Sichuan University for providing NMR data for the intermediates and objective molecules.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and measuring data. See DOI: 10.1039/c4ra03145h

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