Department of Materials Science & Metallurgy: Research papers of the month

Department of Materials Science & Metallurgy

Research papers of the month

February 2016

 

Two Dimensional Ice from First Principles: Structures and Phase Transitions

Scientists at UCL and Cambridge predict new two-dimensional ice structures on the basis of state-of-the-art computer simulations.

A systematic computer simulation study has led to predictions about how water molecules freeze into a single layer of ice. These simulations, published in Physical Review Letters, reveal several models for 2D ice, including a hexagonal, a Cairo tiling pentagonal, a square and a rhombic structure. The new 2D ice structures, obtained on the basis of first principles simulations and unbiased structure search methods, extend the knowledge of ice in nature and are potentially important in understanding phenomena such as cloud microphysics and tribology.

The authors also predict a sequence of phase transitions that happens as a function of pressure and confinement, leading to the determination of a phase diagram of 2D ice. Overall this work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures formed to the confining pressure and confinement width. The observation of the flat square structure supports recent experimental observations of square ice confined within graphene sheets. The authors also discuss how other structures such as the Cairo tiling pentagonal structure may be observed by slightly altering the conditions used so far in experiments.

Figure:  The structure of two-dimensional ice built purely from water pentagons. This space filling structure is made possible by arranging water molecules in a so-called Cairo tiling (CT) pattern.

J. Chen, G. Schusteritsch, C.J. Pickard, C.G. Salzmann, and A. Michaelides, "Two Dimensional Ice from First Principles: Structures and Phase Transitions", Phys. Rev. Lett. 116, 025501 – Published 13 January 2016

DOI: 10.1103/PhysRevLett.116.025501

 

Perovskite solar cells: observation of the degradation in situ in the TEM

The lack of thermal stability of perovskite solar cells is hindering their progress towards adoption in the consumer market. In this paper we produced devices according to four well established recipes, and characterised their photovoltaic performance as they are heated within the operational range. Using in situ heating in the transmission electron microscope, we identified mechanisms for structural and chemical changes, such as iodine and lead migration, which appear to be correlated to the synthesis conditions. In particular, we determined a correlation between exposure of the perovskite layer to air during processing, and elemental diffusion during thermal treatment.

Figure:  STEM cross-sections of perovskite solar cells manufactured with different approaches (A –in vacuum, B – in glovebox, C – in air, D – single step in glovebox) as they are heated in situ at different temperatures. Degradation of the layers is observable, as well as the nucleation of particles from Pb and I migrating from the perovskite layer).

G. Divitini, S. Cacovich, F. Matteocci, L. Cinà, A. Di Carlo, C. Ducati, “In situ observation of heat-induced degradation of perovskite solar cells”, Nature Energy 15012 (2016).

DOI: 10.1038/nenergy.2015.12

 

Shape-shifting process opens new opportunities in micro- and nanoscience

Slowly cooling oil droplets in a soapy solution, have been found to shape-shift a variety of different regular geometric forms, from octahedrons and hexagons to triangles and fibres. The process is reversible and the usual spherical droplets are obtained on warming up. Stoyan Smoukov, Head of the Active and Intelligent Materials Lab, together with researchers at the University of Sofia, Bulgaria, have identified a new mechanism that the change in this first bottom-up method for producing a large number of geometric shapes by a phase transition.

The dynamics are on the seconds to the minutes timescales, and though the system is artificial and very simple, it often resembles the symmetry-breaking morphogenesis behaviour of complex living things in nature.  In contrast to many previous top-down methods, this bottom-up method for assembling complex forms is both material- and energy-efficient and does not require expensive equipment to make them.

The results, reported in the journal Nature, could have potential applications in pharmaceuticals, paints, and consumer products such as shampoo. The fundamentals of the shape generation and could enable novel approaches for construction of complex structures from simple components. They are also expected to have far-reaching implications for active research fields, such as elastocapillarity, phase changes in confinement, and surface and bulk nucleation.

The researchers are exploring links between dynamic processes in the biological and the non-biological world.  The discovery of the physical principles behind this complex behaviour in a simple system with only three components allows one to better understand the mechanisms of symmetry breaking and further engineer this process of artificial morphogenesis.

Figure: Click on the image to see the full animated clip.

N. Denkov, S. Tcholakova, I. Lesov, D. Cholakova & S.K. Smoukov, "Self-shaping of oil droplets via the formation of intermediate rotator phases upon cooling", Nature 528, 392 (2015)

DOI: 10.1038/nature16189