Large area tunable arrays of graphene nanodots fabricated using diblock copolymer micelles

Graphene is a two-dimensional planar material that is one atom thick with a honeycomb lattice of carbon atoms. Since its simple preparation by mechanical exfoliation was reported by Geim et al [1], a large number of studies have revealed significant features of graphene such as quantum relativistic phenomena [2, 3], ultrahigh mobility of charge carriers [4], and superior mechanical strength [5]. Recently graphene forms with a specific nanostructure such as graphene nanoribbons [6], graphene quantum dots [7], and graphene nanomeshes [8–10] have been fabricated, and extraordinary properties of nanostructured graphenes, which pristine graphene does not show, have been widely explored. For example, graphene nanoribbons where charge carriers can be confined to a quasi-one-dimensional system were theoretically expected to have a band gap which depends on the width and crystallographic orientation [11], although pristine graphene is generally known not to have a band gap close to the so-called Dirac point [12]. Kim and co-workers have experimentally proven the semiconducting nature of nanostructured graphenes by fabricating a field-effect transistor based on graphene nanoribbons [13]. Moreover, Novoselov and co-workers demonstrated that graphene quantum dots less than 100 nm in diameter exhibit chaotic quantum phenomena represented in terms of Dirac billiards, envisaging the feasibility of molecular-scale electronics based on graphene quantum dots [7]. Recently nanodevices were also fabricated by combining nanosized graphenes and state-of-the-art lithographic techniques [14–16].

A considerable number of graphene-based devices have been fabricated, chiefly by electron-beam lithography, a feasible top-down technique for producing arbitrary nanostructures. However, the nature of the serial process in electron-beam lithography has severe limitations as regards mass production of large area nanostructured materials in addition to sophisticated processes required, the low productivities, and the high costs. In contrast, bottom-up approaches utilizing molecular self-assemblies have immense attraction because they effectively enable the fabrication of large area nanostructured materials or devices; they have also been employed for producing nanostructured graphenes recently. For example, graphene nanodisc arrays were produced by the self-assembly of nanosized colloidal particles, used as an etching mask [17], although graphene nanodiscs larger than 100 nm in diameter were obtained due to the size limitation of colloid particles.

The block copolymer approach, a promising bottom-up technique for generating large area nanostructures and nanopatterns of various materials, can be applied in the effective creation of nanostructured graphenes. Diblock copolymers composed of two covalently bonded polymer chains can spontaneously form a variety of periodic nanometre-sized morphologies such as spherical, cylindrical, and lamellar structures by microphase separation. These nanostructures depend on the molecular weight, the relative volume fraction, and the Flory–Huggins interaction parameter of copolymers. Nanostructured thin films of diblock copolymers show great promise for controlled large area nanopatterning of diverse materials [18, 19]. Recently the fabrication of graphene nanomeshes by thin films of diblock copolymers was introduced [8, 9]. By varying the molecular weights of diblock copolymer, the periodicity and neck size of holes in nanomeshes were controlled, leading to adjustment of the electronic properties of the graphene nanomeshes. To achieve appropriate etching contrast in the fabrication of nanomeshes with diblock copolymers, a two-step procedure was employed. First, a nanopattern of diblock copolymers was transferred onto the SiO₂ layer deposited onto graphene. Then, the patterned SiO₂ layer was used as an etching mask to generate graphene nanomeshes by plasma treatment.

In addition, diblock copolymers can form nanometre-sized micelles with a soluble corona and an insoluble core in a selective solvent that dissolves only one of the blocks. The size of the copolymer micelles can be determined by the molecular weight of copolymers and the interactions between the blocks and the solvent [20, 21]. By coating a single layer of copolymer micelles on a substrate, we can fabricate a large area hexagonal array of nanosized micelles which can be applied to prepare nanoarrays of diverse materials.

Since precursors of various nanoparticles can be selectively loaded in nanosized cores of diblock copolymer micelles, two-dimensional arrays of metal or oxide nanoparticles can be effectively prepared by plasma treatment, which allows the localized synthesis of nanoparticles in the core areas with the removal of copolymers [22–27]. An array of nanoparticles synthesized by using diblock copolymer micelles can be further used as an etching mask to transfer the hexagonal pattern to the substrate for the generation of nanodots or vertically oriented nanorods [28]. Furthermore, the size of the diblock copolymer micelles can be easily scaled below 100 nm through the molecular weight of the copolymers for facile production of hexagonally ordered nanomaterials.

In this study, we demonstrate the fabrication of large area arrays of graphene nanodots with tunable sizes and inter-distances by the diblock copolymer micellar approach. Plasma treatment on hexagonally ordered micelles containing Au precursors coated on graphene enabled the synthesis of Au nanoparticles in the micellar cores and the removal of all graphenes and copolymers except the graphene under synthesized Au nanoparticles, resulting in an array of graphene nanodots. The size and inter-distance of graphene nanodots were effectively controlled by adjusting the molecular weight of the copolymers.

2.1. The preparation of graphene

2.2. Diblock copolymer micelles with Au precursors

2.3. Arrays of graphene nanodots

2.4. Characterization

Figure 1 shows an overall schematic diagram of the fabrication of an array of graphene nanodots. We first prepared graphene on SiO₂/Si substrates from graphite powders by mechanical exfoliation (figure 1(a)); it was identified by optical microscopy (OM; supplementary data, figure S1(a), available at stacks.iop.org/Nano/23/125301/mmedia). Generally graphene in a single layer on a Si wafer with a 300 nm thick oxide layer appears pale purple due to the light. A few layers of graphene look bluish purple (supplementary data, figure S1(a), available at stacks.iop.org/Nano/23/125301/mmedia) [1, 29, 30].

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We also investigated the graphene by atomic force microscopy (AFM). Graphene is on the left of the white dotted line in figure 2(a). The height profile along the white solid line in the image shows that the graphene is 4.5 nm thick, indicating approximately 12 graphene layers [31]. We mainly employed a few layers of graphene like this graphene, because of its clear visualization by OM and AFM. We also used single layer of graphene for the Raman analysis of graphene nanodots (figure 4) to confirm that our approach works readily with a single layer of graphene.

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A monolayer of PS–P4VP micelles containing HAuCl₄, a precursor of Au nanoparticles, in the P4VP cores was fabricated on mechanically exfoliated graphene by spin-coating as shown in the schematic diagram of figure 1(b). Amphiphilic PS–P4VP copolymers in toluene, a selective solvent for the PS block, form spherical micelles on the nanometre scale with PS coronas and P4VP cores [21, 22]. By controlling the concentration and the spinning speed for the spin-coating process, a single-layered micellar film with hexagonal order can be fabricated with the preservation of spherical morphology of the micelles due to the fast evaporation of the solvent [32]. This micellar film has periodic variation of the thickness because of the spherical form of the micelles, which can be directly applied to an etching mask [33]. However, this pristine micellar film would not be an effective etching mask for patterning carbon-based graphene because of there being no appreciable difference in etching rates between PS coronas and P4VP cores, as well as the small thickness variation in the micellar film. Since we selectively loaded HAuCl₄ into the P4VP cores, the etching contrast was dramatically enhanced because Au nanoparticles were synthesized in the core area during the oxygen plasma treatment. When Au nanoparticles were synthesized, as illustrated in figure 1(c), all other materials, including the copolymers and graphene, were etched out except the graphenes under the Au nanoparticles which are graphene nanodots.

A monolayer of PS–P4VP (51k–18k) containing HAuCl₄ in the P4VP cores was coated on the entire substrate including the part covered with mechanically exfoliated graphene (supplementary data, figure S2, available at stacks.iop.org/Nano/23/125301/mmedia). The plasma treatment of the micellar monolayer resulted in the formation of an array of Au nanoparticles on the entire substrate with the preservation of the micellar order as shown in figure 2(b). We do not discuss the detailed characterization of Au nanoparticles here because the synthesis of Au nanoparticles by combining PS–P4VP micelles containing HAuCl₄ precursors with oxygen plasma treatment has been well reported [23, 26–28].

Finally, to obtain a pure array of graphene nanodots, we selectively removed Au nanoparticles using aqua regia, as shown in the schematic diagram of figure 1(d). The complete removal of Au nanoparticles by aqua regia was confirmed by the disappearance of the Au(4f) peaks in the x-ray photoelectron spectra (XPS; figure 3). We note that a weak peak around 90 eV in figure 3(a) is associated with gold oxides. However, gold oxides are thermodynamically unstable at room temperature, so they turn into gold after a few days [34].

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After the removal of Au nanoparticles, the area covered with graphene nanodots no longer shows bluish purple in the OM image (supplementary data, figure S1(b), available at stacks.iop.org/Nano/23/125301/mmedia). The disappearance of the light interference implies that this region can no longer be continuous two-dimensional graphene. Figure 2(c) shows an AFM image of graphene nanodots which are only observed on the left of the white dotted line where graphene was initially located. In contrast, we find the bare substrate to the right, because Au nanoparticles synthesized directly on the substrate (figure 2(b)) were completely removed by the aqua regia. The height of the graphene nanodots is about 4.5 nm, identical to the thickness of the original graphene. Thus, we can assert the fabrication of an array of graphene nanodots over the entire area of mechanically exfoliated graphene.

The radius and inter-distance of fabricated graphene nanodots have close relationships with those of PS–P4VP (51k–18k) micelles and Au nanoparticles. They are 12 nm (±3.0 nm), core only, and ∼47 nm for the micelles, 11 nm (±3.4 nm) and ∼47 nm for the Au nanoparticles, and 11 nm (±4.3 nm) and ∼44 nm for the graphene nanodots, respectively (supplementary data, figure S3, available at stacks.iop.org/Nano/23/125301/mmedia). Thus, we confirm that the size and order of the micelles were successfully transferred to the graphene nanodots.

Graphene nanodots from PS–P4VP (51k–18k) micelles were analysed by Raman spectroscopy. In particular, we prepared a single layer of graphene by mechanical exfoliation and used it for the generation of graphene nanodots. In the Raman spectrum of the graphene employed in this case (figure 4(a)), we can find the G band (1584 cm⁻¹) and the 2D band (2681 cm⁻¹). Their intensity ratio (I_2D/I_G) is 0.32. In addition, the 2D band consists of one symmetric sharp peak. These observations are directly related to the characteristics of single-layered graphene [35, 36].

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We fabricated an array of graphene nanodots from this single-layered graphene as before. Figure 4(b) shows the Raman spectrum of the graphene dots, whose AFM image is also included. The intensity of the G band decreased significantly and the 2D band nearly disappeared, implying a decrease of the number of sp²-bonded carbons in graphene nanodots compared to pristine continuous graphene. In contrast, we can find a new D band (1350 cm⁻¹) which is one of the disorder-activated Raman bands of graphene, indicating the generation of abundant edge states in graphene nanodots [36, 37]. Thus, we confirm that the micellar approach can be applied to fabricate graphene nanodots in single layers as well as a few layers. We note that the Raman spectra of graphene nanodots with different sizes were not very different from that shown in figure 4(b) because of the abundant edge states in graphene nanodots. A well-controlled edge state would be necessary for studying size-dependent properties of graphene dots, which would not be easily obtained with the current method.

Since we verified that the size and order of PS–P4VP micelles can be transferred to graphene nanodots, we also controlled the size and inter-distance of graphene nanodots by varying the molecular weight of the PS–P4VP copolymers. Figure 5(a) is a magnified AFM image of figure 2(c) for graphene nanodots from PS–P4VP (51k–18k) micelles with the radius distribution of about 800 nanodots. The average radius is 11 nm (±4.3 nm). By the fast Fourier transformation of the image (inset in figure 5(a)), the inter-distance of graphene nanodots is obtained as ∼44 nm. When we employed the larger molecular weights for both the core and corona blocks, i.e. PS–P4VP (75k–25k), the size and inter-distance of the nanodots were increased to 14 nm (±4.1 nm) and ∼50 nm, respectively (figure 5(b)). Furthermore, by employing PS–P4VP (109k–27k) which has a similar size core block but a larger corona block compared to PS–P4VP (75k–25k), we were able to increase just the inter-distance (∼85 nm), keeping the radius (15 nm ± 4.9 nm) almost unchanged, as shown in figure 5(c). Thus, we confirm that the radius of graphene nanodots can be controlled through the size of the micellar core and their inter-distance through the size of the micellar corona.

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By utilizing a monolayer of PS–P4VP micelles, we successfully fabricated an array of graphene nanodots with tunable size and inter-distance. Au nanoparticles synthesized in the P4VP cores effectively worked as shielding nanostructures during the process of etching of the graphene using oxygen plasma. By removing Au nanoparticles, we obtained an array of graphene nanodots whose size and inter-distance have close relationships with those of the copolymer micelles employed. Thus, the size and inter-distance of the graphene nanodots were effectively controlled through the molecular weight of PS–P4VP copolymers. Since the diblock copolymer micellar approach demonstrated here for generating the graphene nanodots can be applied to a single layer as well as a few layers of graphene, we suggest that this method could be utilized for the effective fabrication of a large area tunable array of graphene nanodots. One of the potential applications of an array of graphene nanodots is as an effective catalytic template for synthesizing semiconductor nanorods. For example, ZnO nanostructures grown along the edges of graphene have been reported [38, 39]. We are currently working on the fabrication of an array of vertical ZnO nanorods using the graphene nanodots demonstrated here.

This work was supported by a grant (Code No. 2011-0031635) from the Centre for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Educational Science and Technology, Korea.