Fabrication of suspended Graphene microchannels and its application in studying graphene-liquid interface
Graphene is a single layer of sp2 hybridized carbon atoms having extraordinary mechanical, optical, and electrical properties. These exotic properties make them attractive for applications in all fields of science and technology ranging from flexible electronics to multifunctional biomedical devices. Most of these applications require residue-free, and large-area graphene. Commonly, graphene is grown on copper using a self-limiting chemical vapor deposition process and then transferred to the required substrate. But there are still several fabrication challenges in realizing graphene-based devices. Both process steps cause corrugations and stress related defects in the transferred graphene. Further, graphene obtained on substrates suffer from many drawbacks like deteriorated electron mobility. Thus, platforms with suspended graphene has been pursued for its immense potential. Graphene based devices has been primarily for applications in optical and electronic domains. Graphene based mechanical resonators have been demonstrated for several sensing applications. However, use of graphene in microfluidic applications has been limited. The use of graphene and its derivatives (graphene oxide) based layers have also been limited to biochemical sensing where these layers have been used as functionalised electrode surfaces. Flow of fluids with dissolved ions over graphene has been proposed as a technique for harvesting energy. However, there seems to be a lack of agreement between different experiments. This is primarily due to two reasons. Firstly, most studies have been performed on graphene transferred on various surfaces. Variability in underlying surface across studies effects the overall behaviour. Devices with graphene suspended over microfluidic channels have not been achieved yet due to fabrication challenges. Secondly, due to absence of these devices, studies pertaining to fundamental interaction of liquid with free graphene surface has been missing. This work addresses both these issues by first developing a process flow to suspend graphene over microchannels. Then, these devices are used to probe the fundamental nature of interaction between graphene and liquid. Conventionally, polymer assisted transfer has been used by researchers to solve issues with stress and corrugation. The process has been modified with techniques like the inverted floating method or gradual solvent replacement to obtain suspended graphene on microcavities. But all these processes used multiple wetting and drying steps which alleviate the quality of graphene. Also, these techniques are detrimental in obtaining suspended graphene on microchannels due to the presence of liquid on both sides of graphene. To solve these challenges a novel modified direct transfer (MDT) process was developed. We eliminated multiple wetting and drying step and used a softer substrate to obtain suspended graphene over PDMS microchannels. This is a first demonstration of suspending graphene over polymer microchannels. It is a cleaner and gentler approach leading to good yield. The process was optimized, and graphene was repeat ably obtained on PDMS channels with 5 μm width and 10 μm depth. The suspended graphene was characterized using scanning electron microscopy (SEM) and Raman spectroscopy. Suspended graphene was confirmed to be single layer using Raman spectroscopy. Defects such as corrugations, holes, cracks, wrinkles, and rolled up edges were characterized in suspended graphene. The stress in graphene due to the transfer process was analyzed using vector analysis of G and 2D peaks. The direct transfer process was used to suspend graphene on Si/SiO2 microchannels. Graphene suspended on PDMS microchannels using MDT process was compared with graphene suspend on Si/SiO2 microchannels. Less defects were observed on graphene suspended using MDT process, thus better quality. Graphene-liquid interaction is a controversial problem and there have been multiple studies with contradictory results. Our platform can help us in gaining a better insight into such issues. We have demonstrated two key studies that can be done using our platform. The first study probes the graphene-liquid interface using dynamic atomic force microscopy. In this study, open microchannel with suspended graphene was filled with liquid using capillary forces. This led to a configuration with graphene interacting with liquid on one side and air on another side. Such a configuration allows us to mechanically probe the graphene-liquid interface using an AFM. We have probed this interaction and imaged the suspended graphene with water underneath graphene. As compared to suspended graphene, imaging with water underneath provides better quality due to the apparent physical support from the underlying water. Phase imaging was used to clearly distinguish the corrugated area and non-corrugated area in graphene. It is a unique and novel study that help us to understand the wettability of graphene. Even though there have been several studies pertaining to the static contact angle of liquids on pristine graphene, contact line dynamics on graphene has not been studied before. It is not known whether, contact line dynamics shows conventional stick slip behavior on suspended graphene. The second study measures the dynamics of the contact line on suspended graphene. Microchannels with suspended graphene were filled using capillary wetting. Evaporation leads to drying. Liquid meniscus moves across the suspended graphene while drying. The meniscus gets pinned at various defects. The liquid dynamics were captured using a high-speed camera. The strength of various defects was compared using pinning time and meniscus speed. Apart from suspended graphene on PDMS microchannels, a fabrication technique was developed to fabricate graphene sensors in microchannels. This is a unique method to transfer graphene on a flexible and soft substrate that eliminates the need for any polymer support or any process involving multiple wetting and drying processes. The copper on which graphene was grown were used as connection for electrical read out. The electrical read-out was used as etch stop for graphene transfer process. The same platform can be used as solution-gated graphene-based fields effect transistor.
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