Kelvin by Kelvin Controlled Diffusion, Nucleation, Growth and Coarsening Within and Across Different Compartments of an On-Surface Supramolecular Network

Noble gases, with their close-shell structure and their predominantly physisorptive interaction with metals serve as models to understand adsorption, diffusive mass transport and nanocluster nucleation and growth. Our present work roots in stepping up the complexity of the adsorbent surface while keeping noble gas atoms as ‘probes’ for the on-surface assembly and dynamics of nucleates. Atomically precise on-surface architectures in the form of 2D coordination networks of a N-substituted perylene linked by Cu adatoms have been assembled on Cu(111) single crystal surfaces. By the step-wise increase of the sample temperature in our Ultra High Vacuum (UHV) Low Temperature (LT) Scanning Tunneling Microscope, we gained insight into the unexpected complexity of the adsorption physics and the thermodynamic equilibration between higher binding energy and lower binding energy adsoption sites. As we increase the thermal energy, kT, we clearly observe Xe diffusion between energetically different adsorption sites. Interestingly, the nodes of the network are occupied with higher probability after dosing of Xe into the system, BUT nevertheless ‘fall’ into the pores of the networks as we increase the thermal energy. This apparent paradox can be attributed to the presence of a high electron confined surface state which effectively repels individual Xe atoms impinging from the source and directs them to the energetically less favourable node sites. With increasing ambient temperature, the system equilibrates and surprisingly allows for Xe atoms in the less-energetically favoured positions to migrate to minimum energy sites where they nucleate. In our studies we have in particular observed that Xe nucleates of small (n&lt;7 Xe atoms) ‘evaporate’ and fill up those pockets which already host more than 7 Xe atoms. This leads to a ‘perfect’ coarsening transition in the form that the pores are either completely full (n=12) or remain completely empty. Thereby the added complexity of an on-surface coordination network modifies the surface energy landscape significantly and we can trace the complex re-distribution/re-nucleation and coarsening/growth phenomena.<br/>Within individual pores of the network we show that adsorbates and clusters can be evaporated/re-captured into/from ‘gas phases’ or ‘lattice gas phases’. The participation of different thermodynamic reservoirs to host diffusing Xe is observed to be changing in many steps between 5K to 40K. While only very few Xe atoms are already mobile at 5K after surface adsorption hopping motion between different confinements (0D) and then also along the boundaries of the network (1D) and across the whole network backbone (2D) becomes thermally activated. This gradual activation of different reservoirs for thermodynamically activated diffusion also leads to gradual re-equilibration of Xe atoms which are originally confined in more Energy minimal states of the complex surface potential. After heating to ~40K and cooling back to 5K the Xe atoms are almost exclusively found in the thermally minimal state of 12 Xe atoms filling one pore, a process which we assign to the analogous of Ostwald ripening and coarsening transition which is in the case of these nanometers confined porous network clearly limited in size. By the capability to design on-surface architectures complex functions may be realizable, far beyond the demonstrated capability to adsorb 12 Xe atoms per pore. It may be envisioned that the specific environment of a pore containing specific electronic states is not only a capable absorber, but also a very specific catalytic reaction environment that can be loaded/unloaded via the diffusion mechanisms, in the specific case for trapping atoms or molecules. Low Temperature Scanning Tunneling Microscopy (LT-STM) studies enriched our knowledge how rare gases behave at atomic level.