Condensation and T-dependent mobility of different vdW adsorbates within and across quantum confinements

Abstract

In this thesis a number of important and fundamental surface phenomena have been investigated in an unprecedented way: These involve the condensation of atomic (Xe, Ar) and molecular (cycloalkanes C5H10 to C8H16) gases within surface supported network architectures; the observation of surface phase transitions for the condensates in confinements and the diffusion of adsorbates across a complex nanostructured surface. Uniquely, all these phenomena are investigated under the Scanning Tunneling Microscope and with different adsorbates which predominantly interact by non-directional van der Waals interactions. The substrate for all these investigations is provided by a complex surface architecture formed by a regular porous metal-coordinated network of perylene-derived molecules self-assembled on Cu(111). Each pore contains a characteristic confined state derived from substrate electrons, thus constituting a quantum confinement. Condensation of Xe is observed in the larger pores and on the smaller nodes of the network, as well as next to the network on the free metal Cu(111) surface. Xe, the first ‘van der Waals’ gas forms condensates comprising a different number of Xe atoms in different pores. The structural transitions of these condensates containing 1-9 Xe atoms in their hosting confinements have been investigated first (Chapter [[1]]). These transitions e.g. between the ‘solid’ condensed and the ‘fluid’ phase of a minimal amount of matter are attributed to different ‘phase transition temperatures’ and have been also induced locally by electric field excitation. In the second part of the thesis the unique and complex hierarchy of the Xe atoms’ diffusion pathways within and across the surface nano-architecture is revealed with respect to their temperature dependent activation. The inter-pore diffusion at higher temperatures leads to a ‘coarsening’ of the condensates in that the lower populated ones disappear to the benefit of the larger condensates, in particular the 12 fold occupied ‘full’ pore (Chapter [[2]]). A unique chemical object has been identified in the form of a linear trimer which we attribute to the Xe3 or the Xe3+ condensate (Chapter [[3]]). The last chapter discusses the condensation of the significantly larger cyclo-alkanes as they form aggregates with sizes incrementing from one to a max value which depends both on the size and also on the different possibilities for the stacking of the cycloalkanes in the nanopore confinements (Chapter [[4]]). This work establishes a radically new approach to induce phase transition in minimal amount of matter in confinements embedded in on-surface porous networks. Moreover, it is shown that the quantum confinements can be used as nano-traps, offering real-space access to the phase transition and condensation proceeding under the influence of van der walls forces in an atom-by-atom and molecule-by-molecule way.