Monolayer graphene-supported free-standing PS-b-PMMA thin film with perpendicularly orientated microdomains

Abstract

A facile way to fabricate robust free-standing BCP thin films with perpendicularly orientated microdomains on a CVD-grown monolayer graphene support is reported. Graphene acts as both the neutral surface to control the assembly of the BCP film and the support of the thin film to provide high mechanical strength. The free-standing BCP films with the nanopattern can be used as a substrate-independent template to facilitate BCP nano-lithography.

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During the past few decades, block copolymer (BCP) thin film based nanopatterning techniques have attracted enormous attention.^1,2 The self-assembly of BCPs, which is driven by the microphase separation of polymer blocks with different properties, can lead to various ordered nanostructures, and act as templates to control the spatial order of other matters. The combined features, such as a tunable nano-scale feature size,³ a designable pattern symmetry,⁴ high throughput,⁵ and great compatibility with the current top-down patterning techniques, make BCP-based patterning techniques a promising solution for the further microelectronic device miniaturization.^6,7 In addition, the applications of BCP patterning techniques for magnetic storage devices,⁸ photovoltaic devices,⁹ microreactors,¹⁰ and porous filtration membranes have been well demonstrated.¹¹

In a typical BCP lithography process, the controllable formation of a perpendicular microdomain orientation in the BCP thin film is a crucial step to facilitate pattern transfer.^12,13 The strategy of surface modification of the substrate, to obtain a “neutral” surface for two immiscible blocks, is widely applied to achieve a perpendicular orientation.^14–16 Nowadays, some methods of surface modification, to control the microdomain orientation, have been developed,¹⁷ yet most of them are subject to specific substrate and/or tedious preparation steps.^18,19 In this regard, a free-standing BCP thin film is highly desirable as a universal substrate-independent template for the nanopattern technology,^20,21 as it can be transferred onto an arbitrary substrate, including a rough surface, a flexible substrate, a substrate with low surface energy, and a heat-labile substrate.²² Unfortunately, although there are a few reported methods to prepare free-standing polymer films by floating films off the substrate,²³ the resulting thin films are not robust enough to keep the BCP nanopatterns intact, and are generally used for characterization only.^24,25 Gopalan et al. recently developed a pre-assembled BCP thin film that can be transferred onto Cu foil or a graphene/Cu substrate.²⁶ The transferrable BCP films show importance for the nanopattern technology, but for more versatile applications, we aspire for free-standing BCP thin films with a higher quality and better mechanical strength.

Previously, the BCP nanolithography technique has been used to pattern graphene to tune the electronic properties of graphene-related materials.^27,28 In these reports, the graphene films are just used as the substrates to be etched, not for free-standing BCP films. Recently, Kim et al. showed that transferable BCP nanopatterns can be assembled on modified graphene films using spin-coated reduced graphene oxide as the substrate.^22,29 However, the thermal or chemical reduced graphene films are obtained from small-flake graphene oxide and therefore have a less uniform film thickness and integrity.³⁰ Compared to reduced graphene, graphene grown by chemical vapor deposition (CVD) holds several intrinsic advantages, such as a large area, high quality, controllable layers, and an unambiguous thickness.^31,32 In particular, the monolayer graphene shows superb mechanical properties, with an intrinsic strength of 130 GPa and a Young’s modulus of 1 TPa, exceeding any other materials.^33,34 The monolayer graphene promises to be an ideal support for robust free-standing BCP films. Herein, we adopt CVD-grown monolayer graphene as a thin but robust support for fragile BCP thin films. Although pristine graphene is hydrophobic with a low surface energy, its surface energy is tunable.^35,36 Via a mild UV/ozone (UVO) based protocol to tune the surface energy of the graphene, we demonstrate that free-standing PS-b-PMMA thin films with perpendicularly orientated microdomains can be fabricated on graphene. The free-standing films show a satisfying mechanical strength, and thus can be transferred and applied as the nanopattern template for other applications.

The overall fabrication process for the free-standing BCP films on graphene with a perpendicularly orientated morphology is schematically illustrated in Fig. 1. Graphene grown by CVD on copper foil is used as the support for the BCP thin film. First, we control the wetting property of graphene by UVO treatment, which is a simple, mild, and controllable oxidation method for graphene.³⁷ Next, cylinder-forming or lamella-forming PS-b-PMMAs, determined by the volume fraction of the BCP blocks, are spin-coated onto the modified graphene. BCP films with a perpendicular orientation morphology are afforded after a proper thermal annealing process to induce the microphase separation. Finally, the well developed technique of graphene transfer is used to fabricate free-standing PS-b-PMMA thin films on graphene, simply by etching the Cu foil. The overall process for producing free-standing BCP films is simple and straightforward, effectively preserving the BCP nanopattern.

Fig. 1 Schematic procedure of producing free-standing PS-b-PMMA thin films with a perpendicular orientation of microdomains on monolayer graphene treated with UVO.

The morphology of the BCP films is governed by the interfacial energy/interaction between the BCP and the substrate. The pristine graphene has a low surface energy and good affinity to the PS blocks. With UVO treatment, graphene films with different oxidation degrees and thus different interfacial energies are obtained. Raman and X-ray photoelectron spectroscopy (XPS) are employed to track the evolution of the structural and chemical information of the graphene films with UVO treatment. The intensity ratio between the 2D (∼2680 cm⁻¹) and G (∼1580 cm⁻¹) bands (I_2D/I_G) gradually decreases with exposure time, and the 2D and G peaks become broad (Fig. 2a). In addition, the intensity ratio between the D (∼1350 cm⁻¹) and G bands (I_D/I_G) increases dramatically and finally stabilizes at ∼1, exhibiting a two-step evolution.³⁸ According to the XPS results (Fig. 2b), after graphene is exposed to UVO for 7 min, a C=O bond (290.0 eV) appears and the relative intensity of the C–O bond (286.4 eV) increases. The Raman and XPS results verify that graphene is gradually oxidized by UVO exposure. The evolution of the static water contact angle (θ_w) of the graphene films further shows that θ_w gradually decreases from pristine 83.6° to 68.5° after an exposure time of 15 min (Fig. 2c). This indicates that with the oxidization by UVO, the graphene carries more polar groups, presumably O-containing functional groups, thereby becoming more hydrophilic. The surface energies of the graphene films exposed to UVO for 5 min and 7 min, measured according to Neumann’s method (detailed data in Table S1, ESI†),³⁹ are 59.0 ± 0.4 mJ m⁻² and 60.1 ± 0.3 mJ m⁻², respectively. The surface energies are between that of graphene and graphene oxide, indicating that the treated graphene films are oxidized to a proper degree.⁴⁰

Fig. 2 (a) Raman spectra of the CVD-grown graphene films exposed to UVO for different times; (b) XPS spectra of the graphene before and after UVO treatment; (c) water contact angles of the graphene films with various UVO treatment time (typical photographs of water drops are shown in the insets).

The modified graphene films with a tunable surface tension/energy are applied to manipulate the morphology of the BCP films. Fig. 3 displays the scanning electron microscopy (SEM) images of the morphology of the cylinder-forming (a–e) and lamella-forming (f–j) PS-b-PMMA on the graphene films that have been treated with UVO for different periods of time. For the cylinder-forming PS-b-PMMA films (Fig. 3a–e), the perpendicularly and parallelly orientated morphologies present hexagonal and fingerprint arrays, respectively; for the lamella-forming PS-b-PMMA films (Fig. 3f–j), the fingerprint array and the featureless morphology are ascribed to the perpendicularly and parallelly orientated morphologies. We have carried out a statistical analysis of p_p (the areal percentage of perpendicularly orientated domains in the whole BCP film) to understand the evolution of the BCP morphology on the wetting property of UVO-treated graphene (Fig. S1, ESI†). For the cylinder-forming films, we can see that on the pristine graphene, the parallel orientation of microdomains dominates, covering almost the whole area of the BCP film (Fig. 3a). With the UVO treatment time increased, p_p increases from ∼59.0% at 3 min to ∼95.6% at 7 min, showing a highly ordered cylinder morphology (Fig. 3b–d). With the UVO treatment time further increased to 10 min, however, only sparse features of the perpendicularly orientated microdomains are observed (Fig. 3e). For the lamella-forming PS-b-PMMA film, the evolution tendency of p_p upon UVO treatment time of the underlying graphene films is similar to the cylinder one. The portion of the fingerprint morphology increases gradually with the oxidization degree of the graphene films (i.e. the UVO treatment time, Fig. 3f–i), and decreases abruptly at 10 min, which shows excessive oxidation (Fig. 3j). The variation of the BCP morphology is considered as the result of the interface energy (E_i) of graphene.^19,29 By controlling E_i of graphene with the UVO treatment, we are able to control the perpendicular orientation of the microdomains in the BCP thin films conveniently. Intriguingly, Kim et al. proved that graphene oxide can also be reduced to a suitable E_i to be a neutralized surface and to induce BCP morphology. In conjunction with their results, our results help to comprehensively reveal the effect of E_i of substrates, from graphene to graphene oxide, on the morphology of the BCP self-assembly. The BCP film on the graphene treated for 7 min exhibits the highest p_p, and is adopted as the optimal film in the following experiment.

Fig. 3 SEM images of the (a–e) cylinder-forming (58.8 nm) and (f–j) lamella-forming (77.5 nm) PS-b-PMMAs self-assembled on graphene films exposed to UVO for various times: (a and f) 0 min, (b and g) 3 min, (c and h) 5 min, (d and i) 7 min, and (e and j) 10 min (scale bar: 200 nm).

In addition to the interfacial energy/tension, the thickness of the BCP films is also an important parameter affecting the morphology. The film thickness window of the cylinder-forming PS-b-PMMA film, in which the structure of the perpendicularly orientated microdomain persists, is 30.0–89.0 nm on the graphene films (Fig. S2, ESI†). This thickness dependence of the perpendicularly orientated microdomains results from the boundary conditions of the substrate and the free surface (see Fig. S2, ESI† for more discussion). The wide thickness window offers more choices of film thickness.

Based on the optimal conditions of the pattern control, we are able to get a large-areal nanopattern on the whole copper foil (cm-scale). Upon formation, the free-standing BCP films on the graphene films can be obtained by etching the Cu substrate in a saturated (NH₄)₂S₂O₈ solution for a short time (Fig. S3a, ESI†). Due to the beneficial mechanical property of the underlying graphene, the large areal free-standing film with a nanopattern is easy to transfer to various substrates, such as solid substrates (ITO and Si), and even flexible and thermal/chemical instable substrates, such as polydimethylsiloxane (PDMS) and poly-ethylene terephthalate (PET) (Fig. S3 and S4, ESI†). It is noteworthy that the conventional method of the interfacial wettability, which is controlled by P(S-r-MMA), is hardly applicable for PDMS and PET. Also, because of the graphene underneath the BCP film (Fig. S5a, ESI†), the BCP pattern is robust and stays intact even if folded (Fig. S5b, ESI†). The BCP nanopattern alone can hardly survive such deformation without graphene.

As one of many possible subsequent patterning applications of the free-standing BCP film, we show the patterned deposition of Au nanoparticles using the BCP template. The procedure is illustrated in Fig. 4a. Firstly, a free-standing lamellar-forming PS-b-PMMA film with a uniform fingerprint morphology on graphene is transferred onto a Si wafer (Fig. 4b). Next, the PMMA blocks and the underneath graphene are removed with brief Ar plasma exposure to form a PS template. The Au nanoparticles are then deposited on the Si substrate via galvanic displacement.⁴¹ Since Au particle deposition can only occur where PMMA and graphene are removed and the silicon is exposed to the solution, the Au nanoparticles are deposited in the intervals of the PS template (Fig. 4c). From the side view SEM image, we can clearly see the Au nanoparticles and the PS template (inset in Fig. 4c). The Au nanoparticles, deposited precisely in the intervals of the PS template, indicate a conformal and defect-free contact between the BCP-graphene films and the substrate. Otherwise, the Au nanoparticles would deposit randomly between the film and the substrate or in the defect region and thus cannot form the patterns. As the interaction between graphene and the Si wafer is weak, the template can be completely peeled off with adhesive tape in a simple way. After that, an Au nanoparticle pattern replicating the PMMA pattern is obtained (Fig. 4d). PS and graphene are no longer observed (inset in Fig. 4d). This easy removal of the template can be also demonstrated by Raman spectroscopy (Fig. 4e). The Raman signals of the graphene films and the PS blocks (Fig. S6, ESI†) can be seen before the peeling off step and are enhanced upon deposition of Au (due to the surface enhanced Raman scattering effect).³⁷ After peeling the PS blocks and graphene off, these signals disappear, indicating a complete removal. The compatibility of the graphene-supported BCP films with an even solution-phase reaction indicates that free-standing BCP films can form a conformal and defect-free contact upon transfer to another substrate. Furthermore, the capability to be easily peeled off after pattern transfer is another good feature. These features result from the high integrity and superb mechanical property of the graphene supports, which also demonstrate the advantages of the free-standing BCP film on monolayer graphene.

Fig. 4 (a) Schematic procedure of the Au nanopattern formation using the free-standing PS-b-PMMA film; SEM images of (b) the lamellar-forming PS-b-PMMA film (77.5 nm in thickness) on the graphene films exposed to UVO for 7 min, which is transferred onto a Si wafer, (c) the Au nanopattern formed in the intervals of the PS template, and (d) the Au nanopattern with the template removed; (the insets show the corresponding side view, scale bar: 200 nm). (e) Raman spectra of the samples shown in b–d.

In summary, we have demonstrated a simple yet effective way to form free-standing PS-b-PMMA thin films with perpendicular microdomain orientation using CVD-grown monolayer graphene as a support. By controlling the UVO treatment time, we tune the surface energy of graphene and achieve a highly ordered and perpendicular microdomain orientation of the BCP film with a large size. Benefited from the support of the monolayer graphene, the free-standing PS-b-PMMA thin film holds unique advantages. (1) Graphene acts as a support as well as a neutralized surface. The large-sized PS-b-PMMA thin film with nanopatterns can be self-assembled, simply by tuning the surface energy of graphene on the Cu foil. The straightforward method greatly simplifies the preparation process and avoids possible damage to the BCP nanopattern. (2) Monolayer graphene provides a strong support for the BCP pattern, resulting in a free-standing BCP film with satisfying mechanical properties. The robust free-standing thin film is independent of the substrate and is thus feasible to be transferred to many substrates, including flexible and thermal/chemical instable substrates, for further nanopatterning. (3) The CVD-grown graphene is of high quality and integrity, which promises a BCP nanopattern with few defects. It also has controllable and homogenous layers/thicknesses, which greatly facilitate the processing during BCP lithography. (4) The BCP nanopattern fabricated on the monolayer graphene also provides possibilities for the generation of nanopatterns using integrated graphene, which promises potential applications in semiconductor devices and in the nanoelectronic field.

Acknowledgements

This work was supported by the National Key Project on Basic Research (Grants 2011CB808700 and 2011CB932300), National Natural Science Foundation of China (21121063 and 21127901), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDB12020100).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and additional figures. See DOI: 10.1039/c4ra12655f

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