Electronic bands of nanoporous networks and one-dimensional covalent polymers assembled on metal surfaces

Abstract

Complex molecular layers self-assembled on surfaces with engineered architectures and tailored properties, are expected to play an important role in the development of future devices at the nanoscale. The reversibility of non-covalent interactions such as hydrogen bonds or metal ligand interactions, allows error correction processes in the formed structures. Such elimination of defective structures can give rise to almost defect-free, long-range ordered formations. Metal-organic networks grown on metallic surfaces fall into such self-healing structures and show novel magnetic properties, catalytic effects, oxidation states, exotic tesellation patterns and even bear the prospect of exhibiting topological band structures. Nanoporous networks featuring long-range order belong to error corrected noncovalent structures. The recent finding of electron confinement of the two-dimensional electron gas (2DEG) within the nanopores of self-assembled supramolecular nanoporous networks, is an experimental demonstration of a quantum box effect. This is an effect which may play a crucial role in engineering future molecular devices. By using scanning tunneling microscopy and spectroscopy (STM/STS), in a similar fashion to quantum corrals, it is possible to probe such localized electronic states at the single pore or quantum dot (QD) level. However, studies on the long-range ordered and robust 3deh-DPDI metal-organic network on Cu(111), revealed that nanopores are rather imperfect or leaky confining entities, leading to significant coupling to neighboring nanopores. The periodicity of the highly-ordered supramolecular network induces the formation of Bloch-wave states that result into new electronic bands that can be observed by spatially averaging angle-resolved photoemission spectroscopy (ARPES). The well-established control of the structures of porous networks, together with its characteristic degree of coupling between ad-molecules and the surface state, is our starting point for the fabrication and investigation of coupled electronic systems with tailored band structures. Based on the concepts of Supramolecular Chemistry on surfaces, by choosing suitable molecular constituents (functional groups and/or carbon backbone size) and guided by reversible, non-covalent bonding mechanisms, we are able to generate six different long-range ordered nanoporous networks on (111)-terminated coinage metal surfaces in ultra-high vacuum (UHV). Such nanoporous structures are analogous to QD arrays on surfaces, bearing distinct sizes, barrier separations and scattering strengths. As a result, with each particular nanoporous system grown, we not only engineer the local confinement properties at each QD, but also modulate the coherent electronic band structure steming from the overall array. We observe changes in its fundamental energy, band dispersion, effective mass, zone boundary gaps and Fermi surface contour. Our experimental findings are supported by the electron boundary elements method in combination with the electron plane wave expansion (EBEM/EPWE) modelling, density functional theory (DFT) calculations, and the phase accumulation model (PAM). In this way, we disentangle the repulsive scattering potential landscape of each nanoporous network and delve into subtle surface-organic overlayer interactions, such as hybridization and geometry induced effects, which are altogether responsible for the confinement effects and distinct electronic band modulations. Our findings envision the engineering of 2D electronic metamaterials, in analogy to the well-established optical metamaterials. The studied electronic structure from nanoporous networks correspond to the modified substrate’s surface state, which is

independent of the molecular states. However, low-dimensional organic electronic states, such as the one obtained in graphene nanoribbons (GNRs) and oligophenylene chains are currently very attractive to the Scientific Community based on their industrial prospects. These one-dimensional polymeric structures have been extensively studied as simple, appealing nanostructures leading to distinct electronic features, such as gap opening and peculiar edge states. Their quantum confinement origin can be readily tuned through their width, shape, and edge terminations. The rapidly progressing on-surface chemistry is a highly versatile bottom-up tool for the controlled-synthesis of such atomically precise, graphene-based nanostructures. This achievement has paved the way towards the precise mapping of their intriguing electronic structures with ARPES and STS, making them promising candidates for the realization of exotic graphene-based nanodevices. In this thesis, we engineer the electronic band structure of the well-known poly-(para-phenylene) (PPP), namely the Nα = 3 armchair GNR, by introducing periodically spaced meta-junctions into its conductive path. We synthesize and macroscopically align a saturated film of cross-conjugated oligophenylene zigzag chains on a vicinal Ag(111) surface. We find that these atomically precise chains, hosting periodically spaced meta-junctions, remain sufficiently decoupled from each other and from the substrate. ARPES reveals weakly dispersing one-dimensional electronic bands along the chain direction, which is reproduced by DFT and EPWE. In addition, STS shows a significantly larger frontier orbital bandgap than PPP chains and that straight segments are able to confine electrons. These weakly interacting QDs confirm that periodically spaced meta-junctions constitute strong scattering centers for the electrons. These findings corroborate the important effects that the conductive path topology of a molecular wire has on its electronic states, which are responsible for defining its chemical, optical and electronic properties. Such arrays of semiconducting QDs hold potential for designing future oligophenylene-based quantum devices such as electrically driven, telecom-wavelength, room-temperature singlephoton sources.