Ultrafast Dynamics and Interactions During Growth at Surfaces

Abstract

Helium-3 spin-echo (3HeSE) is a helium atom scattering technique capable of studying nanoscopic surface dynamics in thermal equilibrium. The energy resolution, which greatly exceeds that of alternative methods, makes it possible to probe motion on picosecond to nanosecond timescales. The present thesis is concerned with 3HeSE measurements and modelling of ultrafast dynamics in surface systems with interadsorbate interactions that induce the formation of islands. To this end, we have described investigations into both O/Ru(0001) and S/Ni(111) at elevated temperatures (850 K and 400 K, respectively). In the former, although the attractive forces are weak, 3HeSE is able to resolve their influence. By optimising kinetic Monte Carlo (kMC) simulations against the experimental data and analysing the results, we are able to comment on quantities such as the island lifetime as a function of size. Such information is important for exploring the growth dynamics of substrate-supported 2D materials. S/Ni(111), by contrast, demonstrates much stronger islanding. The system is important in catalysis, as sulfur is a common poison in industrial processes. The periodicity of the clusters is different from that of Ni(111), due to the strong chemisorption of the adatoms causing a reconstruction of the first nickel layer. Even so, the islands have a clear impact on the diffusion signature, demonstrating that they are not static and thus that a dynamic equilibrium is established between the clusters and their mobile precursors. We have also studied O/Ru(0001) at high coverages (above 0.5 ML), when repulsion dominates and three-body interactions are relevant. The work constitutes the first attempt at interpreting 3HeSE measurements in terms of many-body forces, and reveals the state of the surface at the onset of oxidation. In the second half of the thesis, we turn to instrumentation. We first discuss the design and manufacture of new 3HeSE spin precession solenoids with a trapezoidal profile. The coils will dramatically improve the instrument resolution, by a factor of approximately four for the same beam energy, when combined with new power supplies. However, the aberrations of the field - defined as the deviation of the off-axis field from the on-axis value - lead to a reduction in polarisation which increases rapidly as the current rises. Much of the corresponding chapter is therefore devoted to modelling this `depolarisation', using analytic calculations and numerical simulations. The desire for an improved beam resolution was inspired by our measurements of islanding, which would have benefited from the ability to study slow cluster decay. On a similar note, we also describe and model a new design for the spin rotator coils of the instrument, which reorient the 3He spins to account for the scattering geometry. The principal improvement is a change in their position, from the centre of the scattering chamber to the incoming arm, which will facilitate the study of high-energy phonons. The upgraded spectrometer will therefore be capable of measuring the growth dynamics in a wide array of 2D materials, and able to characterise their phonons, which provide an important indication of surface quality. Finally, we describe a semi-analytic method to calculate the attenuation of the beam in the source chamber of the 3HeSE instrument, which is typical of that in many atom scattering machines. There are two components, due to (i) the background gas and (ii) atoms back-scattered from the skimmer and mount. We demonstrate that at room temperatures, the former dominates, suggesting that the design of the skimmer has an insignificant impact on room temperature beams. We derive a number of scaling relationships and analytic formulae which enable us to comment on the design of future source chambers. Our results will allow skimmer interference to be readily incorporated into future calculations of e.g. centreline intensities.