Atom-by-atom condensation in and electronic modification of 2D quantum box arrays

In this thesis the influence of adsorbates on quantum states and vice versa is investigated. The approach taken is that regular porous networks have been produced by means of on-surface self-assembly. Each pore contains a characteristic confined state derived from substrate electrons, thus constituting a quantum box. Importantly, the boxes are electronically coupled with each other, creating quantum box arrays. Here it is shown for the first time that the quantum boxes can be used as nano-beakers offering insight into the condensation atom-by-atom (Chapter [[1]]) and that the quantum states in the boxes can be controllably modified locally as well as globally by adding adsorbates (Chapter [[2,3]]). The structural characterization of those systems with sub-atomic/sub-molecular resolution was performed by scanning tunnelling microscopy (STM), whereas the electronic properties were investigated by complementary techniques: locally by scanning tunneling spectroscopy (STS) and globally by angle-resolved photoemission spectroscopy (ARPES).
In Chapter [[1]] the condensation behaviour of xenon in quantum boxes has been investigated in depth. For these studies a quantum box array, formed by a metal-coordinated network of perylene-derived molecules self-assembled on Cu(111), was exposed to xenon. STM studies revealed, that xenon atoms adsorb in the quantum boxes and each occupancy from 1 to 12 is observed. In this way the condensation of xenon has been monitored in an atom-by-atom manner. The analysis of the condensates’ structure revealed different sets of ‘hierarchic filling rules’ governing the condensation in low and high occupancy regimes. It was concluded that the condensation is governed by the subtle interplay of weak interactions occurring with (1) the underlying substrate (registry), (2) the border of the quantum box and (3) the electronic quantum box state. Condensation events in the low occupancy regime, i.e. from one to six xenon atoms per pore, occur solely at the border of the pore. In combination with xenon repositioning sequences conclusion has been drawn about the repulsive interaction between xenon and the quantum box state. The occupancy histogram spectacularly revealed the existence of more frequently observed, thus particularly stable occupancies, i.e. ‘magic’ condensates. The detailed evolution of the xenon clustering in the quantum boxes also constitutes a system to evaluate different models for first principle calculations.
In the second study (Chapter [[2]]) the influence of adsorbates on a quantum box state was investigated by dosing and controlled removal of xenon in quantum box arrays formed by a metal-coordinated network of perylene-derived molecules self-assembled on Cu(111). Thus the opposite dependence, i.e. how adsorbates influence electronic quantum box state, compared to the first study (Chapter [[1]]) has been studied with the same model system. The occupancy of xenon in the quantum boxes was controlled here by STM repositioning of individual xenon atoms. Remarkably decrementing the xenon occupancy discretely changes the ground state energy of the quantum box, i.e. shifts it towards higher binding energies, due to the reduced Pauli repulsion. In this way the electronic states embedded in the array could be configured: xenon-by-xenon. Another important feature investigated is the interaction between neighbouring quantum boxes. By analysing the electronic states in specific patterns of empty and xenon-filled pores unambiguous evidence has been provided that the electronic state in a given quantum box depends also on the electronic states of the surrounding boxes. On the basis of these pioneering investigations and results, a quantum box array can be viewed as a quantum breadboard opening up the possibility of configuring electronic quantum states by combining different adsorbates exerting distinct influence on these states.
Another important property of a quantum box array is the transmission coefficient of the confining barrier describing in this case the probability of an electron to tunnel between the neighbouring quantum boxes, as it determines the strength of the inter-box coupling. In the third study (Chapter [[3]]) it is demonstrated that the transmission coefficient of the barrier can be also modified by adsorbates. Here the investigated quantum array was formed by a porphyrin network held by C−H···F−C interactions self-assembled on Ag(111). This array exhibits the quantum box state above the Fermi level, contrary to the array formed from the metal-coordinated network of perylene-derived molecules self-assembled on Cu(111) (Chapter [[2]]). The existence of the inter-box coupling in the porphyrin array was also confirmed by combined STS/ARPES measurements. Notably, these are the first ARPES studies of a molecular array featuring its confined state above the Fermi level. The porphyrin building block exhibits two different barrier regions, i.e. the porphyrin macrocycle and the pentafluorophenyl substituents, which interact differently with the quantum states. To modify the transmission probability of the barrier regions two adsorbates exhibiting characteristically different electronic properties were investigated, i.e. xenon and C60 fullerene. The STM revealed two xenon atoms adsorbed on the porphyrin macrocycle and one or two C60 molecules adsorbed in the pore. Xenon interacting with the barrier by weak van der Waals forces did not detectably affect the transmission probability, while the strong electron acceptor C60 reduced the transmission probability significantly. These results demonstrate that the interaction between neighbouring quantum boxes can be tuned by the appropriate selection of adsorbates.
This work established a radically new approach to engineer coupled quantum states in quantum boxes embedded in on-surface porous networks. Moreover, it is shown that the quantum boxes can be used as nano-beakers, offering real-space access to the condensation proceeding under the interplay of weak interactions in an atom-by-atom way.