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Magnetic Sensors based on Two-Dimensional Materials/Si

Periodic Reporting for period 1 - 2DSi (Magnetic Sensors based on Two-Dimensional Materials/Si)

Reporting period: 2015-08-03 to 2017-08-02

Due to severe shortage of accessible clean water, the world is looking at alternate technologies/mechanisms by which efficient separation of ions and molecules at the smallest length scale can be realized. The importance of membranes with good ion/water separation capabilities is ever increasing. Reducing the size of capillaries/channels to match the hydrated size of the ions is one of the approaches. Graphene oxide (GO), the oxidized graphite has been widely explored for this purpose; however swelling of capillaries when immersed in water makes it not very useful. The swelling is due to the incorporation of 2 to 3 layers of water molecules inside interlayer spaces in GO provided by oxidized and un-oxidized regions of GO. As a result, the size of the capillaries increases from 0.8 to 1.3 nm when immersed in water, which is much larger than the size of salt ions in water, making it harder to filter out ions. One of the first problems addressed in this project was to devise methods that either completely elliminates or minimize swelling. Further, we challenged ourselves to tune the size of the capillaries so that the ions can be stopped from passing through essentially permitting only water to pass through. The aim was to expose graphene oxide to different humidity conditions and then mechanically glue the GO under these conditions so that filling of water layers can be adjusted.
Controlling the capillary/channel dimensions in a flexible and controllable way is still a dream as the demonstrated nanochannels are made using nanotubes, nanopores or graphene oxide, which lack the scalability required for large scale nanofluidic applications. To remove this bottleneck, we have taken lessons from the recent discovery of graphene which was produced by peeling monolayers from graphite crystals. To create scalable nanofluidic channels, we have aimed to utilize the mechanical peeling capability of van der Waals crystals and lithographic techniques. The aim is to fabricate atomically thin fluidic channels so as to have these membranes either performing desalination or isolate individual ions from a mixture of ions, for example, industrial waste.
To avoid swelling, we have mechanically confined graphene oxide capillaries to Å-scale dimensions, which is a great step forward for the actual implementation of GO for desalination plants. We studied ion transport through these capillaries and found that while ion permeation is exponentially decreased when the size of the capillaries are diminished; water permeation is weakly affected [1], making it very promising for water purification. Though GO is promising, it lacks the integration capabilities required for nano-fluidic circuits. Recently, we reported [2] a novel approach of utilizing the atomic flatness of van der Waals layered materials for the fabrication of Å-scale channels, beating the surface roughness limit traditionally encountered in lithographically fabricated systems. This technique utilizes a combination of van der Waals assembly of layered materials and lithography to create fluidic channels of 1 atom thick ‘at will’ and ‘choice’, pushing the boundaries of nanofluidics to the next level. We systematically studied the water [2] and ion transport through these highly confined channels. The multifunctional capabilities of these artificial channels are still being explored in detail. The ion transport studies through 0.67 nm channels showed that trivalent ions, for example, Al3+ move slower at least by 10 times with respect to K+ ions, which could be exploited for ion separations from a complex mixture [3].
The research work has published in journals such as nature technology [1] and nature [2]. The result of ion transport studies is under minor revision in science magazine [3]. The dissemination of the work has been undertaken through presentations at National graphene institute, Manchester and at graphene conference 2017 held in Barcelona, Spain. The work on graphene oxide has been widely publicised through various media including BBC news, reaching millions of people about European commission and Marie Sklodowska-Curie fellowship. This work also has been publicised through the facebook page of European commission with the ‘fellow of the week, April 14’ recognition. The work has been included in the latest ESN story of successful projects, which is about to publish.
[1] J. Abraham, K. S. Vasu, C. D. Williams, K. Gopinadhan, Y. Su, C. T. Cherian, J. Dix, E. Prestat, S. J. Haigh, I. V. Grigorieva, P. Carbone, A. K. Geim, R. R. Nair, Tunable sieving of ions using graphene oxide membranes. Nat. Nanotechnol. 12, 546-550 (2017).
[2] B. Radha, A. Esfandiar, F. C. Wang, A. P. Rooney, K. Gopinadhan, A. Keerthi, A. Mishchenko, A. Janardanan, P. Blake, L. Fumagalli, M. Lozada-Hidalgo, S. Garaj, S. J. Haigh, I. V. Grigorieva, H. A. Wu, A. K. Geim, Molecular transport through capillaries made with atomic-scale precision. Nature 538, 222-225 (2016).
[3] A. Esfandiar, B. Radha, F. C. Wang, Q. Yang, S. Hu, S. Garaj, R. R. Nair, A. K. Geim, K. Gopinadhan, Size effect in ion transport through angstrom-scale slits, Science (Accepted) (2017).
(a) Schematic of lithographically fabricated nanochannels, (b) the ion conductance of various salts