As to substrates not suitable for a direct transfer, the team also determined that PMMA can be used either as a surface modifier or as a glue to ensure a successful graphene transfer. First, we needed to see if our hydrophobicity prediction was correct – and PMMA was a very convenient choice, since its hydrophobic, Araujo explains, and. hydrophobic substrates worked very well for the transfer. We therefore asked if we could turn a hydrophilic substrate, with which the transfer had previously failed, into one that is hydrophobic substrate. The answer was yes – and we can could use PMMA, since it is soft (meaning that they it could potentially achieve the needed glass temperature transition) and hydrophobic. However, he adds, this led us to another question: If we spincoat PMMA over the hydrophilic substrate, will the transfer work? Testing showed that it did, enabling graphene transfer onto cloth and paper. Regarding the teams demonstration that multilayers enable large area conducting sheets to be placed on most substrates they studied, Kong acknowledges this step was straightforward from the point of view of direct transfer. Since graphene is hydrophobic, and assuming that the first transfer was successful, we could perform multiple transfers successfully, he points out. The hardest part was capturing scanning electron microscope images of multiple-graphenes over the flexible substrates. Being insulators, the substrates get electrically charged very easily, which prevented us from seeing the substrate/graphene set. Also, sheet resistance measurements were tricky, since the fragile substrates are very often damaged by the probes. In addressing these challenges, Araujo says that the key insight came from thinking about the critical factors in the interaction between graphene and PMMA/thermal tapes. The great innovation was certainly to show that, for most commercial substrates, we do not need to use any intermediate membrane to transfer graphene to the flexible substrates. The absence of the intermediate membranes provides a clean transfer which greatly improves the quality of the transferred material. Finally, in my point of view, its fantastic to show that we can transfer graphene onto cloth or paper by treating then with a PMMA membrane which offers the environment necessary to make the transfer work – a method can easily be described as a new technique to transfer graphene into this class of substrates. In the near future, Kong says that there will be a strong need for alternative ways of harvesting energy. In this context, she explains, the ability to adequately synthesize and manipulate and transfer relevant materials from the growth station to the target platforms is a major problem, since these steps will determine the quality of the final product. The growth of graphene is already quite advanced – and what were offering with this research is a simple recipe to make multiple transfers of materials while avoiding contaminants brought along with standard glue-based procedures. Araujo sees this advance leading to a new era of high quality flexible touch screens, flexible light emitting diodes, flexible sensors, gas filters and solar cells. Furthermore, he notes that with the emergent interest in new layered materials – for example, boron nitride, transition metal dichalcogenides, and oxides – it will become possible to fabricate heterostructures by intercalating different materials. The different ways in which one intercalates the layered materials provide a whole new class of applications involving electronics, spintronics, superconductivity and optoelectronics, Araujo says, adding that the residue-free transfer procedure might also represent an advance to building high-quality heterostructures. In terms of the planned next steps in our research, Kong continues, the extension of our methodology should be tested with other layered materials such, including the boron nitride, transition metal dichalcogenides and oxides those mentioned above as well as other substrates. A more thorough study regarding the temperatures determining the hot/cold transfer should be conducted as well. Another challenge Araujo cites is the structural quality of the transferred material. Even though weve demonstrated the residue-free transfer concept and addressed reasons for a successful transfer, the continuity of the transferred film is still not at state-of-the-art. The lack of continuity is welcome for some applications, such as filters – but its undesirable in the production of, for example, high-quality touch screen devices. Also, he concludes, the extension of this technique to perform this residue-free transfer to rigid substrates is still a challenge – and its worth remembering that even though the technological appeal of flexible devices is high, many applications involving, for example, logic circuits, are still strongly connected to rigid substrates. Read more at: phys.org/news/2013-11-faster-simpler-depositing-graphene-flexible
Posted on: Tue, 05 Nov 2013 16:30:59 +0000
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