Historical PerspectiveGraphene inks for printed flexible electronics: Graphene dispersions, ink formulations, printing techniques and applications
Graphical abstract
Introduction
For the last decade, the world of consumer electronics has experienced massive improvements in the manufacturing techniques toward the production of smaller, faster and better efficiency devices for everyday use. However, the use of traditional solid-state technology poses some limitation on the flexibility of the device, the environmental concerns and the processing cost. In recent years, a remarkable transition is taking place in the world of consumer electronics: devices are becoming thinner, flexible and wearable. Printing of flexible electronics has showed a promising alternative for the traditional fabrication of inorganic materials due to their numerous advantages [[1], [2], [3]]. By offering a low-cost, simple and scalable method for the production of devices with high flexibility and stretchability [[4], [5], [6]], printing of conductive inks on flexible substrates are facilitating to enable this transition.
Until now, a number of mass printing techniques have been developed for the ultimate goal to achieve a fabrication process with high-performance, stable, low-cost, and zero-waste of materials. Such processing technologies include ink-jet [7], screen [8], and gravure printing [9]. These technologies are associated with liquid-phase inks with markedly different physical properties including the concentration of fillers present, viscosity and surface tension of the solution. The screen printing technique is compatible with a variety of inks, the gravure printing requires the use of a low shear viscosity ink suspension, while the ink-jet printing needs a high surface tension and diluted ink solution [10]. In the fabrication of printed electronics, different techniques possess their own pros and cons, but they all aim to provide rapid and efficient approaches in marking conductive traces on the flexible substrates. Printing technology is undergoing a rapid development and expected to bring a transformational change in the manufacturing of flexible electronics.
Multiple types of conductive inks have been developed for printed flexible circuits including metal-based inks [[11], [12], [13]], conductive polymers [7,14], and carbon complexes [[15], [16], [17], [18], [19]]. Among them, metal-based inks particularly Ag and Cu [11,20,21] are widely used due to their high conductivity and their previous conversant use in solid-state electronics. Silver is an attractive material for conductive ink due to its excellent electrical properties [22]. However, silver is of high-cost and showed unstable performance by migrating into device layers [23,24], making copper a good alternative for its abundance and fairly high conductivity. However, copper poses oxidation issues under ambient condition, which can be facilitated by the high processing temperature, and reducing its electrical conductivity [25,26]. Besides, the high sintering temperature of metal-based inks is also limiting their widespread use with papers and other flexible plastic substrates [27,28]. It was also reported that the use of these metals is not environmentally friendly and might cause serious problems including water toxicity, cytotoxicity and genotoxicity [29]. Thus, there is a critical need for the further development of a low-cost, stable, and environmentally benign conductive ink, which can solve the above-mentioned disadvantages.
Graphene, a two-dimensional carbon lattice, has received tremendous attention due to its excellent mechanical, thermal, and electrical properties [[30], [31], [32], [33], [34], [35]]. With an exceptional carrier mobility of up to 2 × 105 cm2 V−1 cm−2 [[36], [37], [38]], graphene has become a golden candidate for printed flexible electronics. However, the absence of a mass production method for high quality graphene prevents its practical application in inks for conductive patterns. Among the available graphene synthesis methods, oxidative-exfoliation of graphite can potentially be used for the production of large quantity of graphene [[39], [40], [41], [42], [43], [44]]. This route basically allows for the oxidation of graphite into graphite oxide, which can be easily exfoliated as individual sheet of graphene oxide (GO). Subsequently, the exfoliated GO sheets can be subjected to a suitable reduction process to remove the oxygen-based functional groups to form reduced-graphene oxide (rGO) sheets, a graphene-like material [[45], [46], [47]]. However, the resulting graphene-like sheets are significantly damaged with holes and vacancies, which compromise their outstanding electrical conductivity [48,49]. For this reason, high quality graphene free from any defect is preferred for its use as conductive materials, especially as inks for printed flexible electronics. In recent years, research efforts have been focused on the direct exfoliation of pristine graphene in liquid media, which can be directly used for printing of conductive patterns [[50], [51], [52], [53]]. The development of proper conducting inks for printing on flexible substrate is still facing certain challenges, which include the aggregation of graphene sheets in the suspension, the unsuitable viscosity and surface tension, and the lack of adhesion of the inks to the substrate. Enormous efforts have been devoted to overcome these issues, to bring graphene inks closer to practical applications. Currently, research on graphene inks has received intensive attention and entered an exploded growth phase (Fig. 1), where many applications and fundamental understanding of their formulations have been recently discovered. In the near future, it is no doubt that graphene inks can potentially replace the traditional solid-state-electronics and open up a whole new manufacturing process for low-cost, thinner, light weight, and flexible electronic devices.
In this review, we aim to guide the readers through recent advances in graphene inks for printed flexible electronics, specifically focusing on the synthesis of graphene-based inks and the techniques that have been developed for printing on flexible substrates, followed by several recent practical applications of graphene inks used in their realization.
Section snippets
Formulation of graphene inks
Graphene can be obtained using two distinct strategies, respectively (i) the bottom-up and (ii) the top-down [54,55]. The bottom-up approach is based on the growth of carbon atoms into two-dimensional carbon layers using chemical vapor deposition (CVD) [56,57], which can produce very high quality graphene sheets on metal substrates. However, practical use of this approach is limited due to its high cost, complexity in the transfer process and difficult to scale-up. Alternatively, the top-down
Printing techniques
Printing technology is one of the foremost inventions for advanced lean manufacturing. Various printing techniques have been developed for fabrication of flexible electronics, which are associated with roll-to-roll processing. The printing techniques that will mainly be discussed in this review are mass printing techniques, which are feasible and cost-effective toward roll-to-roll fabrication.
We can classify the available printing technologies into two distinct models, conventional and digital
Flexible conductive circuits
Due to its outstanding electrical and mechanical properties, graphene ink is promising for printing of flexible conductive circuits. In 2013, Hyun et al. [138] prepared foldable paper-based electronic circuits by a simple selective transfer process with a pen. Graphene membranes with controlled thicknesses were prepared by vacuum filtration of graphene nanoplates dispersions (a type of chemically-derived graphene ink). Then, foldable electronic circuits were fabricated by selective transfer of
Conclusion and future outlook
In this report, most recent work on the formulation of graphene inks, their printing techniques and applications for printed flexible electronic devices have been reviewed. The preparation of proper graphene inks, which allows for mass printing in a cost-effective manner is desired for its commercial adoption. It has been clearly demonstrated that pristine graphene inks are highly conductive, but they show poor solubility in common solvents. It appears that the chemically-derived graphene (GO)
Conflict of interest
The authors declare no conflict of interest.
Acknowledgements
The authors acknowledge the School of Graduate Research, RMIT University for supporting the PhD scholarship of T.S. Tran. The research has been supported by the Australian Research Council (ARC) Industry Transformation research hub (IH 150100003) funding.
Tuan Sang Tran obtained his Bachelor's degree in Chemical Engineering from Hochiminh University of Industry, Vietnam, in 2014, and the Master's degree in Nanoscience from Gachon University, South Korea, in 2016. Mr. Tran is currently a PhD student at the Department of Chemical Engineering, RMIT University, Australia. His research interests include carbon nanomaterials for energy applications and graphene-based inks for printed electronics.
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Tuan Sang Tran obtained his Bachelor's degree in Chemical Engineering from Hochiminh University of Industry, Vietnam, in 2014, and the Master's degree in Nanoscience from Gachon University, South Korea, in 2016. Mr. Tran is currently a PhD student at the Department of Chemical Engineering, RMIT University, Australia. His research interests include carbon nanomaterials for energy applications and graphene-based inks for printed electronics.
Naba Kumar Dutta is a Professor at School of Engineering, RMIT University. He performs research at the interface cutting across the disciplines of Nanomaterials Engineering, Structural Biology and Chemical Engineering. He has wide-ranging research interests and published widely in the areas of Polymers, Advanced materials for energy, Carbon-nanomaterias and Structure-property relationship in multi-componenthetero-structures with specific focus on the role of the interfaces.
Namita Roy Choudhury is a Professor at School of Engineering, RMIT University. She received her PhD from IIT, Kharagpur and subsequently did her post-doctoral research at CNRS, Mulhouse, France. Choudhury's research interest spans from Hybrid polymers to Biomimetic polymers to Graphene based inks for renewable energy and advanced manufacturing.