Elsevier

Carbon

Volume 129, April 2018, Pages 63-75
Carbon

Letter to the editor
Reductive nanometric patterning of graphene oxide paper using electron beam lithography

https://doi.org/10.1016/j.carbon.2017.11.067Get rights and content

Abstract

Electron beam lithography (EBL) was used for preparing nanostructured reduced patterns on the GO paper surface, while preserving its mechanical resistance and flexibility. Different EBL parameters, like dose and time of exposure for patterning were tested. SEM analysis showed the consequent increase of contrast of the reduced stripes on the patterned regions due to the increase of electron beam doses. Moreover, surface potential microscopy experiments also exhibited a clear contrast between the patterned and non-patterned regions. Structural analysis of the patterned paper through X-ray diffraction and nanoindentation showed that the interlayer distance between GO sheets decreases after reduction allowing the increase of the Hardness and Young modulus that makes this material able to be manipulated and integrated on different devices. Furthermore, we also observe that exposed areas to electron beam reduction process show an increase in the electrical conductivity up to 3 × 104 times. The developed flexible GO films can have interesting applications such as biosensors or templates for inducing tissue regeneration, by providing a surface with differently patterned cues with contrasting electron mobility. Preliminary in vitro studies with L929 fibroblasts support the cytocompatible nature of this patterned GO paper.

Introduction

Nowadays, the wet chemical exfoliation of graphite is one of the most used methodologies to prepare graphene derivatives, with the advantage of large scale production, technological simplicity, high efficiency and low cost of the procedure. [1] [2].

Graphene oxide (GO) results when the exfoliation is done in aqueous oxidative medium, then retaining a high degree of oxidation, with a C/O ratio achieving the maximum value of 2.5, usually designated as the threshold limit [3]. This particular feature of GO provides it with a high hydrophilic character allowing the preparation of highly stable aqueous colloidal suspensions [4]. The chemical structure of GO is generally described by the model of Lerf-Klinowski that defines an atomic layer of carbon atoms in a hexagonal structure with hydroxyl and epoxy groups on the plane and carbonyl and carboxylic groups on the periphery [5]. Recently, Dimiev et al. proposed that the GO structure is dynamic in aqueous solution suggesting that GO is a metastable nanomaterial at room conditions [6]. Kim et al. observed that a quasi-equilibrium of GO structure was reached after a relaxation time of a month, resulting in a chemical structure rich in hydroxyl groups and devoid of epoxide groups [7]. The reduction of GO can be promoted through chemical [8] or thermal [9] treatments that remove most of the oxygen functional groups and allow the partial restauration of sp2 carbon bonds [10]. However, when compared to graphene, the quality standard of this graphene derivative is low, since it contains a high density of structural defects and a remaining percentage of oxygen functional groups.

However, the structural heterogeneity of GO can be an advantage for certain applications. In fact, the high versatility of GO arises from its chemical diversity based on the combination of aromatic and oxidized domains in a single atomic sheet. These characteristics allow its functionalization through several chemical and physical approaches using small molecules, nanoparticles or structures building-up new hybrid materials. Also, GO nanosheets have the ability to interact between them allowing their self-assembly into macroscopic structures like films or three dimensional microporous architectures, like foams [11], [12]. The heterogeneity of GO chemical structure allows the establishment of hydrogen bonds between the oxygen functional groups or/and by the assembly of hydrophobic domains. A controlled assembly of the GO sheets through the interactions of functional groups in the basal planes leads to an ordered stacking, forming robust films [13]. However, if the interactions are preferentially achieved through the oxygen functional groups located at the edges of GO sheets, the creation of three-dimensional microporous architectures is favoured [14]. This new class of materials has been recently designated as solvated graphene, a new emerging area with enormous challenges and opportunities in multidisciplinary fields [15].

The first successful attempt to prepare GO films, usually designated as a GO paper, consisted on the simple flow-directed assembly of individual GO sheets [13]. The horizontally ordered assembly of solvated GO was guaranteed by the constant flow of the liquid from the suspension, which avoids aggregation allowing the individual sheets to reach an equilibrium between interconnecting forces (hydrogen bridges and π-π stacking) and repulsive colloidal forces (hydration and electrostatic forces), at the filter surface. After drying, the resultant membranes showed high flexibility and stiffness. This approach is still commonly used for the preparation of GO paper nanocomposites with ordered structure and high performance for several applications [16], [17], [18], [19], [20].

Interestingly, a multilayer GO film is considered a metastable material at room conditions because it continuously undergoes chemical and structural changes, which result in a persistent decrease of the C/O ratio [7]. The metastable state provides the opportunity to enhance the transformation of several functional groups by applying soft physical treatments, such as low temperature treatment processes [21], in order to modulate the final properties of GO. On the other hand, when carbon nanomaterials are exposed to electrons or ions, their microstructure evolves through the creation of defects [22]. Krashnenniniko et al. reported how these effects can be used to tailor the properties of carbon nanomaterials [23], [24]. In the case of GO, exposure to direct sunlight radiation leads to the elimination of oxygen in the form of water vapour [25].

The patterning based on reduction of GO has been obtained in the literature by a variety of techniques. The existing reports are collected in Table 1. Several techniques are based on local heating. It is assumed that thermal reduction starts occurring in a range from 100 °C [26] to 180 °C [27], and it is considered that 200–500 °C is an effective temperature range leading to reduction [28], [29] by elimination of carboxyl groups [30]. Higher local temperatures lead to ablation [29]. The use of a scanning focused laser light is a common way to obtain local heating, as GO is a light absorber, absorbing 63% of incident light within 1 μm [31] in average over the visible range, which leads to a rapid decrease of temperature along the depth. This decrease will be steeper for shorter wavelengths [32], leading to different patterning performance with different wavelengths used in thicker films and papers. Besides direct scanning, other patterning strategies include shadow masks, used with either photographic flash [31], [33] or laser irradiation [34], and also the use of interference patterns [35], [36], [37]. Some authors combine several techniques to achieve patterns, such as laser ablation and chemical reduction [28], or pulsed laser deposition followed by thermal annealing [38]. Another thermally-based technique was the use of a heated AFM probe, leading to an extremely small lateral resolution of 12 nm [26]. Besides thermal processes, a photochemical reduction of GO [39] and other carbon materials can occur where water dissociation provides protons which bind to oxygen-rich functional groups, leading to their removal from the carbon backbone [40]. This phenomenon occurs for photon energies above a threshold of 3.2 eV [30] and up to ∼300.0 eV [39], while using higher energy laser lines caused the breaking of carbon-carbon bonds [40]. GO surface patterning was also performed using an extreme-UV (EUV) radiation source showing high efficiency on the reduction process, with very high resolution (nanometre range) and absence of photothermal effects [41]. The same authors showed that GO photoreduction patterning could be significantly improved with the use of high-performance light sources like vacuum UV (VUV) synchrotron radiation [42]. In parallel, others reported the reductive effect of electron beams [43], and the lithographic writing of reduced patterns was reported using a 20 keV electron beam [32]. More recently, devices made of GO with micrometer scale reduced patterns have been produced using electron exposure at 25 keV [44]. Furthermore, it was observed that electron irradiation at 10 MeV can induce the chemical bonding between the adjacent reduced GO sheets by sp3 carbon formation, increasing the mechanical hardness and electrical conductivity of the paper [45].

In the context of biomedical applications, EBL has been explored as an attractive technique for the fabrication of submicron (down to the nanoscale) topographies on a diverse palette of materials, including protein patterns for multiplexed cytokine detection [46], substrates with grooved topographies to modulate cell differentiation [47], and membranes for in vitro models of barrier tissue [48]. However, its application to graphene-based materials has been scarcely investigated, despite the tremendous biological interest of these materials due to their attractive physico-chemical properties and versatile nature for functionalization. For example, graphene-based materials are under study to be used as part of engineered devices to promote cell growth and alignment [49], to interface native tissues [50], and to modulate cell differentiation processes [51], which could benefit from the application of versatile patterning techniques, such as EBL. Although the compatibility of graphene based materials in contact with biological systems remains a matter of open debate [52], published results to date support the existence of a safe range of conditions in which these materials display biocompatible behaviour and do not initiate any toxic responses [53].

The majority of the studies previously mentioned on the patterning of GO, focused on GO thin films obtained by spin-coating on hard substrates, scarce work has been done on self-sustaining GO paper. Herein, we report the development of self-stained GO paper where direct-write EBL was used to create micrometer-scale conductive lines, using a 100 keV electron beam. To the best of our knowledge no studies were reported about GO reduction in the energy range used in this study, previous works used 20 keV [32] and 25 keV [44] for GO films, and 5 MeV [54] and 10 MeV [45] for GO paper. The GO paper prepared by self-assembly of the GO sheets was reduced according to a free pattern chosen to be a series of parallel lines, using different electron doses and beam step sizes in order to effectively obtain a reduced pattern at the GO paper surface. A discussion of the characterization results of the GO patterned paper obtained for the different working conditions is presented. Finally, in order to elucidate the biocompatible nature of the patterned GO paper for future biological uses, we carried out cytocompatibility tests with murine L929 fibroblasts. Specifically, cell adhesion, growth and morphology were investigated both on patterned and non-patterned areas of the GO paper by confocal laser scanning microscopy (CLSM) and SEM.

Section snippets

Material

GO was supplied by Graphenea with a concentration of 4mg/mL. The GO was repeatedly washed with ultrapure water by centrifugation until reaching neutral pH. Chemical reagents and antibodies were purchased from Sigma-Aldrich and used as received, unless otherwise indicated. Cell culture media components were obtained from Lonza.

GO paper preparation

The preparation of the GO paper was based on the method described by Dikin et al. [13] Briefly it consists of the filtration of 20 mL of GO aqueous colloidal solution (3

Reducing patterning of GO paper by electron beam lithography

The reducing patterning of GO paper was performed under increasing EBL doses using an apparatus as can be visualized in Fig. 1a). Five identical patterns with 999 lines of 200 nm width and 1 μm center-to-center pitch were made. The five patterns were performed with doses successively higher, increasing in factors of 10, starting with the dose normally used for polymethylmethacrylate (PMMA) exposures (800 μC/cm2) and up to 8C/cm2. The complete exposure parameters are shown in Table 1. After

Conclusions

We explored the patterning design of GO paper substrates using direct-write EBL to create nanoscale conductive lines, using a 100 keV electron beam. This new approach revealed to be very effective and versatile for the design of reduced patterns on the GO papers surface, allowing for free pattern choice and use of different electron doses and beam step sizes. The contrast of the reduced stripes in the patterned regions increased with the increase of electron beam doses. Structural analysis of

Acknowledgements

Gil Gonçalves thanks the Fundação para a Ciência e a Tecnologia for the PostDoc grant (SFRH/BDP/84419/2012).

P.A.A.P.M. acknowledge the FCT/MCTES for a research contract under the Program Investigator 2013 (IF/00917/2013/CP1162/CT0016) and TEMA – Centre for Mechanical Technology and Automation (UID/EMS/00481/2013), financed by national funds through the FCT/MEC. I.B. wish to acknowledge the Portuguese Foundation for Science and Technology for the financial support (grant IF/00582/2015).

H·I·S.N.

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