Elsevier

Vacuum

Volume 204, October 2022, 111353
Vacuum

Polydimethylsiloxane as protecting layer to improve the quality of patterns on graphene oxide

https://doi.org/10.1016/j.vacuum.2022.111353Get rights and content

Highlights

  • The polydimethylsiloxane as a coating of material improves the quality of the patterns realized by the ion lithography.

  • Good quality of reduced graphene oxide (rGO) due to its permeability to oxygen.

  • Better homogeneity of the distribution of ions current during the patterning process.

  • Lessening of the defects generation and the relaxation effects of the GO during the patterning by 5 MeV energy of heavy ions.

  • Better performance of the capacitance prepared by ion-beam writing on GO covered with PDMS.

Abstract

Graphene oxide (GO) foils have been covered with poly(dimethylsiloxane) (PDMS) via spin coating and then heated in oven to strengthen the bond of the polymerized composite. Micro ion beam has been used to directly pattern the realized film and to promote the localized reduction in the ion irradiated GO areas in vacuum. The lessening of the disorder in the carbon crystal structure and the defects production are demanding in all the graphene-based materials applications. Micro beams of 5 MeV C3+ ions with a spot of 1.5 μm × 10 μm (focused beam) and unfocused beams with size of 3 × 3 mm2 have been employed to irradiate the produced composites on a small localized region and wide areas respectively. The poly(dimethylsiloxane) foil, used to cover GO, confers flexibility to the created patterns and allows the oxygen degassing from GO. The quality and the fidelity of the patterns have been investigated by Atomic Force Microscopy (AFM). The ion beam induced structural changes in the coated and uncoated graphene oxide foils have been studied by Raman spectroscopy confirming that PDMS can be used as protecting layer to improve the quality of patterns obtained on graphene oxide by the 5.0 MeV C3+ ions beam writing.

Introduction

In the recent years, the demand of wearable flexible electronic microdevices as microsensors for monitoring, evaluating and managing the state of human wellness through the online collection of health-related information has grown exponentially. This research line is strongly connected to the production of a reliable power source with miniaturized sizes. A feasible solution could be the fabrication of micro-capacitors able to store and use energy, to respond to external stimuli as voltage, temperature, light and electrochemical reduction and oxidation processes [[1], [2], [3]]. Another sector of growing and current interest concerns the possibility of realizing graphene oxide (GO)-based micro dosimeters for ionizing radiations whose composition depends on the absorbed dose in vacuum because it reduces the amount of functional oxygen groups. The reading of the so obtained reduction level with various analysis techniques (X-ray, Raman and optical spectroscopies) allows to evaluate the absorbed dose with good linearity and sensitivity. However, the dosimetry information is lost if the irradiated GO is left in the air for a long time and/or exposed to temperatures above about 50 °C. Therefore, the coating of the GO dosimeter with a PDMS film could allow the data protection of the dosimeter itself.

Among the plenty of materials proposed for the realization of supercapacitors, graphene-based materials [4] have gathered the attention of the materials science community [5] due to their optical properties [6], mechanical strength [7], charge carrier mobility [8] related to the unique Dirac cone band structure near the Fermi level [9] and remarkable thermal conductivity [10].

Graphene oxide (GO), a precursor [11] for the synthesis of graphene, is obtained from the chemical oxidation of graphite into graphite oxide and its subsequent exfoliation into graphene oxide. The bonding configuration in GO is described as hydroxyl, epoxide, carboxyl and carbonyl groups separated from the most of the sp2 hybridized carbon atoms arranged in a two-dimensional honeycomb lattice; the hydroxyl and epoxide groups are mostly located at the basal plane while the carbonyl and carboxyl groups at the edges. The presence of these functionalities makes GO soluble in several solvents as water, and simultaneously decreases the electrical conductivity making GO an insulator [12]. Accordingly, the removal of the oxygen functional groups in vacuum turns GO into reduced graphene oxide (rGO) restoring the sp2 bonded carbon atoms and the band gap of graphene. Moreover, as reported in Refs. [[13], [14], [15]] the dielectric properties of GO and rGO, are strongly connected to the air humidity, to the presence of CO2 and toxic gases such as NO2, NH3, CO and others. This suggests that, thanks to both its high specific surface and its chemical functionalization, GO appears a good candidate for applications as micro-capacitors [16], sensors [17], dosimeters [18] and for the basic research [19]. The GO features are strongly affected by differences in the starting graphite material as well as in the preparation methods and the vacuum conditions.

Typically, the carbon–oxygen ratio in GO ranges between 1.35 and 1.98 depending on the oxidation process and on the used analysis technique [20]. Notwithstanding in GO the presence of oxygen functional groups is an asset when the anchoring of nanoparticles or other chemical group are required, nevertheless, the GO oxidation degree is responsible for the disruption of the electronic π delocalization.

Several methods have been adopted to turn GO into rGO as well as the exposition of GO to hydrogen plasma, thermal heating, laser radiation [21] UV, X-ray, electrophoretic deposition [22] and particle irradiation. All these methods lead to the sp2/sp3 ratio modification, to the band gap reduction and to the production of rGO with higher electronic conductivity but also with greater structural damage and defect formation which affect also the mechanical strength of the material. The ion implantation [23] compared to the other conventional approaches seems to be the most promising and effective method to modify GO in a cost-effective way, to produce in vacuum large-scale GO based materials and to obtain high quality and good reproducibility patterns in controlled way without the use of harsh reducing agents and/or high temperatures. After reduction treatments [24,25], in rGO the sp2 domains remain still partially disrupted, strongly affecting the electrochemical behavior of the material [26]. During the GO reduction processing structural defects and impurities are created due to contaminations. Better control of the reduction, improvement of the wanted conductivity and the defect-free graphene formation are highly demanded as the more rGO is close to the graphene structure the more it will be free from defects. During the GO reduction, Chen et al. [27] reported deoxygenation, amorphization and loss of the sheet like structure leading to deterioration of the electronic conductivity of the so-formed rGO. To overcome these undesirable outcomes, in the literature several routes were examined: R. Ariati et al. [28] reported on the mixing of GO with polydimethylsiloxane (PDMS) [29] to improve the modulus of elasticity, the mechanical strength, the stretchability of the matrix; in Ref. [29] it was pointed out that the structures directly written in polydimethylsiloxane (PDMS) by the ion irradiation exhibit high quality ratio when low ion current and low fluence were used, while in ref [30] the use of PDMS as GO coating was encouraged because of both the permeability of the PDMS in the oxygen degassing from GO during its patterning by 2.0 MeV protons and the improvement of the quality of the obtained patterns.

The present study is a prolongation of our previous contribution [16] related to the realization of micro capacitors made of GO for application as flexible and miniaturized energy sources strongly demanded in microsensors. In Ref. [16] the fabrication of a 4 pF micro capacitor by the localized C3+ ion irradiation of GO foil is described in detail. The irradiation of 5 MeV carbon ions in GO has led, due to the dominant electronic stopping power, to its deoxygenation, dehydrogenation and carbonization [16]. The performance of as-prepared micro capacitors depends beside on the preparation method of native GO and on the ion irradiation conditions also on the not constant distance between electrodes patterned on GO. The GO shrinkage is responsible for the partial reduction of GO in the areas between the realized electrodes with the following reduction of the distance between electrodes. Herein a micro beam system is used to investigate the efficiency of a polydimethylsiloxane (PDMS) [29] coating in the improvement of the micro capacitor performance. In fact, the spatially localized irradiation by controlled high doses of high energy heavy ions improves the so obtained rGO quality enabling oxygen and water degassing, reducing the defects generation and the relaxation effects of the carbon-based matrix.

Section snippets

Materials

The GO foil was synthesized from a Graphene Oxide Water Dispersion (GOWD) at the 0.4 wt % concentration containing GO flakes with the size of 600 nm produced by Graphenea. GOWD was deposited with a glass dropper at room temperature on substrates constituted of poly-tetra-fluoro-ethylene (PTFE, Teflon) with a surface of about 4 × 4 cm2, and located on a rotating disc of a spin coating system (from 1000 to 3000 rpm), to obtain uniform GO thin foils on flat substrates. Then the so-obtained foils

Results

For all the ion irradiated materials, the quality of the patterns progressively deteriorates increasing the ion fluence. Fig. 2 shows an x4 magnified optical micrographs overview relative to patterns written with a 5.0 MeV C3+ ions beam focused to a spot of 1.5 μm × 10 μm at the fluencies of at 1.93·1014 ions/cm2, 3.88·1014 ions/cm2 and 5.81·1014 ions/cm2 on PDMS (a) (b), (c); on PDMS_GO (d), (e), (f) and on GO (g), (h), (i). At the highest fluence in PDMS the initially straight lines become

Discussion

The bombardment of material by ions is an established method for the controlled modification of its original structures. Over the possible modifications induced during the ion irradiation it is possible to name the structural changes, damage, electron excitation, cross-linking, breaking of weak bonds, deoxygenation and radical creation are expected in the bombarded PDMS and GO materials. Likewise, during the proton irradiations, the ions cross both PDMS and GO foils and the electronic stopping

Conclusions

The present study follows-up our research focused on the validation of the PDMS use as GO coating to improve the quality of both the patterning and the obtained rGO under the irradiation of energetic (5.0 MeV) carbon ions.

To optimize the quality of the pattern avoiding the defect generation and relaxation effects of the GO foil, the applicability of a PDMS coating on it during its localized reduction through the patterning by micro ion beam irradiation has been investigated using the AFM and

Funding

The research has been carried out at the CANAM (Center of Accelerators and Nuclear Analytical Methods) infrastructure LM 2015056. This publication was supported by OP RDE, MEYS, Czech Republic under the project CANAM OP, CZ.02.1.01/0.0/0.0/16_013/0001812 and by the Czech Science Foundation (GACR No. 22-10536S) and by CIMA project of INFN, Catania section (Italy).

CRediT authorship contribution statement

Mariapompea Cutroneo: Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Data curation. Vladimir Havranek: Writing – review & editing, Formal analysis. Lorenzo Torrisi: Validation. Anna Mackova: Writing – review & editing, Resources. Petr Malinsky: Formal analysis. Barbara Fazio: Formal analysis. Petr Slepicka: Formal analysis. Dominik Fajstavr: Formal analysis. Letteria Silipigni: Writing – review & editing, Writing – original draft,

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Mackova Anna reports financial support was provided by Czech Science Foundation.

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