In this thesis the physical properties of water confined between two-dimensional materials are studied.
‘We performed experiments that challenge the physical limits,’ Pantelis Bampoulis says. ‘To that extent graphene, a two-dimensional material, proved a rich instrumental tool, and allowed us to investigate confined water structures.’
Scanning probe techniques were used to directly visualize and measure water structures, confined between graphene and a variety of supporting substrates. Information regarding the influence of the interface structure and wettability, environmental humidity, temperature, pressure and the presence of foreign species on the structure and dynamics of confined water, could be experimentally accessed, in situ and real time.
‘In nanofluidics the interaction with the walls is always a major issue,’ Pantelis says. ‘The relevance of my work also extends to fundamental issues in physics, as well as to macro environmental issues for example in the dynamics of glaciers.’
First in this PhD work, fractal growth between mica and graphene was studied. ‘Based on our scanning tunneling spectroscopy data, we provided evidence that these fractals are 2D ice,’ Pantelis says.
The fractals grow while they are in material contact with the atmosphere at 20 °C without significant thermal contact to the ambient, and at low relative humidity. ‘Confined between a hydrophilic and hydrophobic material, the structure of water changes dramatically,’ Pantelis says. ‘The real-time processes were recorded via atomic force microscopy (AFM). These are available on YouTube, and shows the relevant molecular dynamics.’
Not the local availability of water molecules, but rather them having the locally required orientation, is the key factor for incorporation into the 2D ice nanocrystal, Pantelis concluded.
‘This result showed that we were on the right track with our line of research,’ he says. ‘We managed to isolate parameters, like temperature and pressure, individually in our experimental setups and on the nanoscale. This helped us to understand the dynamics of confined water structures at extremely well-defined conditions.’
Pantelis studied also the exact details of the graphene-ice-mica interface, He found that the distribution and exact lateral organization of Potassium (K+) ions, proved to play a major role on the ice structure. The K+ ions form row-like structures as well as small domains due to repulsive forces acting between neighboring ions.
‘Our results shed light on the local distribution of ions on air-cleaved mica, solving a long-standing enigma,’ Pantelis says. ‘They also provide a detailed understanding of charge transfer from the ionic domains towards graphene.’
Also the classic regelation experiment of William Thomson in the 1850’s was discussed, dealing with cutting an ice cube by the application of an external pressure, followed by refreezing when the pressure is released.
Pantelis and co-workers managed to reproduce this experiment in two-dimensions and decoupled by thermal effects. In AFM experiments, thermal isolation from the environment was achieved and studied for water confined between graphene and muscovite mica. ‘The phase transition appeared completely reversible: refreezing occurred when the applied force was lifted.’
The cutting was attributed to pressure induced melting, but has been challenged continuously. Only lately consensus emerges by understanding that compression shortens the O:H non-bond and lengthens the H-O bond simultaneously. This H-O elongation leads to energy loss and lowers the melting point.
The main findings of his work Pantelis was happy to publish in high-impact journals such as: ASC Nano Letters, Journal of Chemical Physics and Applied Physics Letters.
Also the dynamics of alcohol-water mixtures (‘nano-vodka’) were studied. AFM images revealed that the adsorbed molecules were segregated into faceted alcohol-rich islands on top of an ice layer on mica, surrounded by a preexisting multilayer water-rich film.
‘Sometimes counterintuitive results were obtained during the PhD project,’ Pantelis shares. ‘For example, between graphene and MoS2 – both hydrophobic materials – the confined water acted in a (quasi) hydrophobic manner,’ he says.
In his thesis he concludes: ‘Additional water condensation leads to either lateral expansion of the ice layers or to the formation of three-dimensional water droplets on top or at the edges of the two-layer ice, indicating that water does not wet these planar ice films. The hydrophobic character arises from the lack of dangling bonds on either surface of the ice film because of their nontetrahedral bonding geometry. The unusual geometry of these ice films is of great importance in biological systems with water in direct contact with hydrophobic surfaces.’
During his PhD work Pantelis became more critical on several facets of scientific research as when he started.
‘At the beginning my attitude was more naïve,’ he says. ‘Now I pay much more attention to the reproducibility of research in publications. Also I am more challenged to find even better interpretations for the phenomena introduced and described. My passion for research has grown. In my future career my choice is for academics, as I like fundamental research working on issues in an independent way, always hoping to find new results.’
After his Defense Pantelis will be working as a post-doc, both at Mesa+ and at Leibniz University Hannover.
‘In Hannover a 4-probe scanning tunneling microscope is available providing good chances to valuable experimental results,’ he says. ‘During the PhD my experimental skills progressed gradually. By ruling out parameters that can go wrong in an early stage, one works more efficiently. Also you learn to make the most out of the available experimental equipment. And, most importantly, I learned to never work alone, and to always collaborate with expert colleagues. I am much more confident in sharing my knowledge and views, and on finding support from experts by being open-minded. At the moment I am working on a research proposal allowing me to continue my research.