Charge transport properties of a vertically stacked organic heterojunction based on the amino-carboxylic (A-C) hydrogen bond coupling scheme are investigated by means of X-ray resonant photoemission and the core-hole clock method. We demonstrate that hydrogen bonding in molecular bilayers of benzoic acid/cysteamine (BA/CA) with an A-C coupling scheme opens a site selective pathway for ultrafast charge transport through the junction. Whereas charge transport from single BA layer directly coupled to the Au(111) is very fast and it is mediated by the phenyl group, the interposition of an anchoring layer of CA selectively hinders the delocalization of electrons from the BA phenyl group but opens a fast charge delocalization route through the BA orbitals close to the A-C bond. This evidences that hydrogen bonding established upon A-C recognition can be exploited to spatially/orbitally manipulate the charge transport properties of heteromolecular junctions.
COBISS.SI-ID: 29261863
We investigate bidirectional femtosecond charge transfer dynamics using the core–hole clock implementation of resonant photoemission spectroscopy from 4,4′-bipyridine molecular layers on three different surfaces: Au(111), epitaxial graphene on Ni(111), and graphene nanoribbons. We show that the lowest unoccupied molecular orbital (LUMO) of the molecule drops partially below the Fermi level upon core– hole creation in all systems, opening an additional decay channel for the core–hole, involving electron donation from substrate to the molecule. Furthermore, using the core–hole clock method, we find that the bidirectional charge transfer time between the substrate and the molecule is fastest on Au(111), with a 2 fs time, then around 4 fs for epitaxial graphene and slowest with graphene nanoribbon surface, taking around 10 fs. Finally, we provide evidence for fast phase decoherence of the coreexcited LUMO* electron through an interaction with the substrate providing the first observation of such a fast bidirectional charge transfer across an organic/graphene interface.
COBISS.SI-ID: 2895460
Advanced molecular electronic components remain vital for the next generation of miniaturized integrated circuits. Thus, much research effort has been devoted to the discovery of lossless molecular wires, for which the charge transport rate or conductivity is not attenuated with length in the tunneling regime. Herein, we report the synthesis and electrochemical interrogation of DNA-like molecular wires. We determine that the rate of electron transfer through these constructs is independent of their length and propose a plausible mechanism to explain our findings. The reported approach holds relevance for the development of high-performance molecular electronic components and the fundamental study of charge transport phenomena in organic semiconductors.
COBISS.SI-ID: 3014500
The self-assembly of the naphthylmethylamine molecules (NMA) on the Au(111) surface is investigated by a combined experimental and theoretical approach. Three well-defined phases are observed upon different thermal treatments at the monolayer stage. The role played by the methylamine termination is evidenced in both the molecule–molecule and molecule–substrate interactions. The autorecognition process of the amino groups is identified as the driving factor for the formation of a complex hydrogen bonding scheme in small molecular clusters, possibly acting also as a precursor of a denitrogenation condensation process induced by thermal annealing.
COBISS.SI-ID: 29338407
Electronic structure of 4-hydroxycyanobenzene in the gas phase, thick films, and single crystals has been investigated by X-ray photoemission spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). We have used resonant photoemission spectroscopy (RESPES) to identify the symmetry and atomic localization of the occupied and unoccupied molecular orbitals for the free molecule. Upon condensation into a thick film, we find XPS energy shifts in opposite directions for the oxygen and nitrogen core levels, consistent with the formation of an intermolecular hydrogen bond. This interaction is also accompanied by a significant spatial distortion of the lowest unoccupied molecular orbital that is displaced from the nitrogen atom, as indicated by the RESPES measurements. Thick films and single crystals display the same dichroism in polarization dependent NEXAFS, indicating that the intermolecular hydrogen bonding also steers the molecular assembly into a preferred molecular orientation.
COBISS.SI-ID: 2765412