Energy Dissipation on Dirac and 2D Material Surfaces

Energy dissipation at material surfaces controls the rate of chemical reactions, the efficiency of novel technology, friction and lubrication, as well as materials growth including nanostructures. The current project seeks to obtain a deeper understanding of how energy dissipates on novel material surfaces, focusing on Dirac and two-dimensional materials. The first Dirac material was graphene, a single layer of carbon atoms followed by the so-called topological insulators and later by an entire class of new two-dimensional materials and even superconductors. The discovery of these materials is so recent that many fundamental questions are still wide open, with a strong potential for the discovery of novel physical and chemical aspects in addition to the materials being promising candidates for future use in technological applications. The first aspect of the project concentrates on how energy dissipates on these novel material surfaces, and the role of the electron-phonon coupling in these complex materials: Electronic transport, i.e. the movement of electrons in a conducting material is coupled to atomic vibrations, so-called phonons. The electron-phonon (e-ph) interaction at surfaces is one of the most important mechanisms for energy dissipation in electronic transport and its understanding is therefore of huge importance for future low-power technologies. It is also at the heart of conventional superconductivity - i.e. in materials where the electrical resistance drops to zero when it is cooled below a certain temperature which corresponds to dissipationless electronic transport. In these superconducting materials, phonons mediate the required attractive interaction between electrons. As a second aspect, the project aims to quantify the role of energy dissipation in the motion and dynamics of molecules at surfaces. Molecular motion is determined by the rate of energy transfer between the molecule and the surface over which it translates. In analogy to macroscopic motion, energy dissipation can be quantified in terms of atomic-scale friction. A central question for this motion is, in what way the molecule dissipates energy to the surface during its motion, which further governs the type of molecular motion and how fast and far the molecule may travel. Following the motion of individual molecules at surfaces is deceptively difficult and direct studies of these elementary events are scarce. Within this project, studies at industrially relevant temperatures will be carried out at the Cambridge atom scattering centre. Finally, by studying example systems from different material families we will learn about general trends i.e. how values such as the e-ph coupling and the atomic-scale friction change and their influence on energy dissipation.

Energy constantly flows across the surfaces of materials. It determines how much power devices consume, how efficiently heat is removed, the rate of chemical reactions and even how materials grow. Yet many of these processes take place on the scale of single atoms and molecules - far beyond what we can see in everyday life. This project set out to uncover the fundamental principles behind these processes, focussing on a new generation of ultra-thin "two-dimensional" materials such as graphene and related layered compounds expected to shape the future of electronics and sensing technologies. Over its duration, the project produced 13 peer-reviewed publications, including journal cover features and editors' highlights, attracted media attention, and trained early-career researchers. Together, these studies provide a consistent picture of how energy is transferred and dissipates at material surfaces. One key outcome concerns the interaction between electrons and atomic vibrations. As electricity flows, electrons transfer part of their energy to these vibrations - a microscopic process that limits efficiency and generates heat. We directly measured this energy exchange and determined how long the vibrations persist, down to trillionths of a second, providing insight crucial for designing faster electronics that waste less energy and manage heat more effectively. We also showed how energy-transfer mechanisms influence the growth of ultra-thin two-dimensional materials. By monitoring growth in real time, we uncovered hidden intermediate stages and the formation of tiny, regularly arranged nanoholes during boron nitride formation, enabling more precise control over the production of high-quality atomically thin materials. Another major result concerns motion at the atomic scale. When molecules move across a surface, they experience friction which slows them down. This friction governs how far and how fast molecules travel - crucial for catalysis, sensors and nanotechnology. Tracking such motion is extremely challenging, especially over atomic distances. We succeeded in following the movement of individual molecules and quantifying the tiny energy losses that control their motion. Water provided a particularly striking example. Although water covers most of our planet, the movement of single water molecules on solid surfaces has rarely been measured. We showed that water "dances" on hexagonal boron nitride, quite different from its behaviour on graphene. As a result, individual water molecules move in fundamentally different ways on these two seemingly similar materials. These differences arise from subtle differences in the materials' physical properties, leading to distinct friction and transport behaviour. Such insights are relevant for applications ranging from water purification and microfluidics to lubrication at the nanoscale. Overall, the project reveals how energy flows and is dissipated at material surfaces - knowledge that underpins more energy-efficient electronics, improved thermal management and better control of chemical and biological processes at the smallest scales.