Using vdW technique, we have first fabricated the nanochannel devices which comprise the top, bottom and space layers. Typically, a nanochannel device contains a single or multiple (50 to 100) channels with channel height varied from 5 to 15 nm, length of 10 µm and width of 200 nm. During this project, electrical conductivity and streaming measurements were performed to investigate the electrokinetic properties of the nanochannel devices made of graphite and hBN. Further, we have performed fluorescence-based flow measurements to establish a completely novel and first direct experimental demonstration of nanojet emerging out of 2D nanochannels. In a standard fluorescence-based flow experiment, we have fixed the nanochannel device between two reservoirs where one reservoir contains salt solution and the other reservoir contains the corresponding cation sensitive fluorophore. In our experiments we have used calcium sensitive Fluo-4ff Penta Potassium salt as a fluorophore. We have further analyzed the experimental results and extracted the slip lengths on graphitic surfaces.
Recent simulation studies have shown high mobility for the physisorbed hydroxyl ions leading to mobile surface charge on the pristine carbon surfaces. We have performed electrical conductivity and streaming measurements at different pH along with diffusio-osmotic (DO) measurements for understanding the effect of mobile surface charge contribution to the electrokinetics in the pristine graphitic nanochannels. Our experiments on pristine graphitic nanochannel devices have shown enhanced conductivity at each salt concentration. We have further developed a generalized theoretical fame work to rationalize the slip and surface charge effects on enhanced ionic transport in 2D nanochannels. Our experiments together with detailed theoretical analysis have shown the concentration dependent surface charge on pristine graphitic surfaces in the order of mC/m2 which can be attributed to the physisorbed hydroxyl ions on pristine graphitic surfaces.
We have also developed a method to fabricate nanochannel devices with functionalized channel wall surface. The nanochannel device made in this method contains only two graphite crystals i.e. top and bottom layers and, the nanochannel and passage for the molecules to enter the nanochannel were made on bottom layer graphite crystal using electron beam induced etching (EBIE). These devices and the pristine graphitic nanochannel devices with moderate confinement (5 to 15 nm) were used to further disentangle surface effects from the bulk effects for the ionic transport and, characterize the surface properties such as surface charge and slip length. The nanochannel devices made using EBIE have shown potential to produce a single pore power density in the order of kW/m2 under salinity gradients. This work is currently under review in Nat. Mater.
