Study of ice nucleation on surfaces focusing on the effect of surfaces on heterogeneous nucleation and ice growth at ambient conditions. (AV)

Ice is central to climate, geology and life. Understanding its behavior is essential for predicting the future of our planet. On average, 7% of the ocean’s surface is frozen; this alters ocean currents and limits the exchange of gases with seawater. Ice and snow coat 10% of the land permanently and up to half of the Northern Hemisphere in midwinter. Ice in clouds concentrate airborne chemicals and are sites were atmospheric chemistry takes place. Above the poles, clouds of ice grains host ozone-depleting reactions, forming holes in the stratospheric ozone layer at high latitudes that expose millions of people to increased ultraviolet radiation. Chemical reactions in snow on the ground can produce ozone and other environmental pollutants.  Yet the molecular mechanisms underlying these processes remain largely unknown.

Without knowing more about ice formation, it is impossible to build snow or ice-cloud modules for atmospheric and climate models or to extrapolate laboratory studies to environmental conditions with enough confidence. A few years ago it was established the ten questions that science needs to study to gain this knowledge. The first question raised was to understand how ice nucleation occurs on solid surfaces. Ice often forms easily on solid surfaces through heterogeneous nucleation. This happens at higher temperatures than homogeneous nucleation (i.e. with pure water only) and it’s directly responsible of most of the rain. To understand why that happens, the molecular bases of the interaction of water molecules with such surfaces need to be studied. The goal of this research line is to study ice nucleation induced by surfaces to be able to obtain the knowledge needed for the modification of natural surfaces or the design of artificial surfaces with controlled ice nucleation properties.

Study of the interaction of water with ferroelectric surfaces and its role in surface charge screening using SPM and AP-XPS techniques. (AV)

Ferroelectric materials are polar materials with a permanent electric dipole below a certain transition temperature. This polarization can be controlled and switched by externally applied electric fields, for example to build thin-film ferroelectric memories. At the surface of ferroelectric materials, polarization suffers from serious fundamental and practical challenges such as depolarization fields and surface charge screening. Uncompensated surface charges due to the discontinuity of the normal polarization component result in depolarization fields that strongly affect polarization states. The ultimate stability of ferroelectric phases is determined by a balance between bulk thermodynamics and the screening mechanisms for polarization, which can be internal (domain formation or charge carriers migration within the bulk) or external (chemical environment or adsorbates). Understanding the interplay between ferroelectric phase stability, screening, and atomistic processes at the surface is key to control low-dimensional ferroelectricity. The interplay between polarization and surface adsorbates works in both directions: adsorbates influence polarization, but the orientation of the polarization also determines the type of adsorbates that bond at the surfaces, and it has been demonstrated that ferroelectric surfaces with opposite polarity can have different behavior toward molecules adsorption.

In this research line we focus our interest in studying surface screening mechanisms on ferroelectric materials using Piezoresponse Force Microscopy (PFM) and Ambient Pressure Photoelectron Spectroscopy (AP-XPS). AP-XPS allow us to obtain direct information of the different chemical species that form at the ferrolectric surface in contact with water vapor and other gases present in the ambient were devices containing ferrolectric materials are expected to work. 


Development of new SPM modes based in multifrequency dynamic Atomic Force Microscopy (AFM) to study wetting, ice nucleation and identification of chemical groups at the nanoscale. (AV)

Water/solid interfaces are of fundamental interest in various fields including geology, metrology, biology, and chemistry. Despite its simple molecular structure the structure and interactions of water with surfaces, which determines wetting and reactivity remain unsolved. The knowledge of this structure at the nanoscale is crucial to understand key properties that determine corrosion, dissolution, and electrochemical processes. In this research line we use Scanning Probe  (SPM) techniques to study water/solid interfaces at the nanoscale. One of the differences of SPM from other techniques is the locality of the information. SPM uses a probe tip to scan over the surface, and obtain structural information together with, e.g., electronic, mechanical, and vibrational properties. Because it is not an averaged information over a wide area, as in the case of all the other techniques listed above, detailed investigations of how atomic steps, kinks, and defects residing on the surface influence on the adsorption of molecules are possible.  However using SPM to study water/solid interfaces is not a straightforward technique because of the perturbations that the SPM probe can cause on the water films during measurements. Non-contact SPM techniques to avoid direct interaction between the SPM probe and water have been developed in the group. Those techniques are being used now to go a step forward and try to create new SPM modes that could allow to identify at the nanoscale between different organic chemical groups.