Ultrafast charge transfer pathways through a prototype amino-carboxylic molecular junction

The chemical affinity between different functional groups represents a powerful mechanism for driving the assembly of complex hetero-organic architectures on surfaces. The improvement of the charge transport (CT) properties of these systems is the key issue for their efficient employment as prototypes of organic- based electronic devices. In this view, we report here the study of the CT in a molecular junction based on the amino-carboxylic (A-C) interaction. The system consists of a Self-Assembled Monolayer (SAM) of a small carboxylic molecule (Benzoic Acid, BA) grown on top of the Au(111) surface, previously functionalized by an amino-terminated alkanethiol (cysteamine, CA). The growth of the vertically stacked hetero-junction is driven by the A-C recognition process, which establishes a hydrogen bonding scheme between the functional groups of the two molecular species. We describe the CT properties of the system by means of resonant photoemission spectroscopy (RPES) and the core-hole clock (CHC) method. In particular, we show how the A-C junction differently affects the electron delocalization dynamics on the different unoccupied molecular orbitals (UMOs) of the BA molecule.
The experiments were performed at the ALOISA beamline by researchers from the University of Ljubljana, University of Trieste, CNR-ISM and CNR-IOM.
We first investigated the nature of the BA UMOs closer to the vacuum level, which is important for the presentation of the RPES maps. The C K-edge NEXAFS spectrum of BA taken in gas phase, shown in Figure 1, presents two separated spectral features at ~285 eV and ~288 eV. DFT calculations assigned them to transitions to the π*1-3 orbitals, from carbon atoms of the aromatic ring and of the carboxylic group respectively. We then measured the RPES spectra on both the BA/CA and BA gas-phase systems.

Figure 1. Carbon K-edge NEXAFS of BA. Upper panel: experimental data in the gas phase (filled circles) compared to DFT calculated spectrum (light blue curve). Inset: sketch of the BA molecule, with carbon site labels (C1-C7) and calculated LUMO (isosurface plot). Lower panel: spectral decomposition of DFT calculated spectrum according to core-hole excitation site (C1-C7).Reprinted with permission from 10.1021/acs.nanolett.5b05231. Copyright 2016 American Chemical Society.

Figure 2 reports the RPES map of the BA/CA system, obtained as a series of valence band spectra taken with photon energy tuned across the C K-edge in steps of 0.1 eV and represented in two-dimensional like intensity map, I(EK,hn) with the electron kinetic energy and the photon-energy scales on the horizontal and vertical axis, respectively. On the bottom, the resonant spectra taken on the two main features are compared with the BA-gas phase ones. We note that resonances on the phenyl excitation (at hn ~285 eV) show almost no quenching with respect to the BA-gas phase, indicating that delocalization dynamics over empty orbitals of the phenyl ring exceeds the upper limit of the CHC method (tphenyl> 10·tch= 60 fs). On the contrary the resonances on the carboxyl group (ht~288 eV) of the BA/CA/Au are quenched yielding an estimate of the delocalization time tcarboxyl≤ 20 fs, which we attribute to the specific intermolecular coupling of the amino-carboxylic molecular anchor. We extended our study to the case of BA molecules adsorbed directly on the bare Au(111) surface. Interestingly, in this case an opposite situation is obtained with respect to the A-C system: the BA/Au interaction promotes an ultra-fast CT on the phenyl group of the BA molecules, whereas the dynamics on the UMOs localized on the carboxylic group appears unaffected by the interaction. In conclusion, our study shows that the A-C recognition at a hetero-organic interface can be exploited to promote ultrafast CT over empty molecular orbitals involved in the coupling scheme. Moreover, we demonstrated how the Resonant Photoemission CHC method can successfully be applied to investigate the CT properties of molecular interfaces based on the hydrogen bond, alternatively to STM based recognition methods.


Figure 2. Carbon K-edge RPES map for BA/CA/Au. Upper panel: Resonant photoemission intensity I(Ek, hn). Lower panel: single VB spectra taken at resonance excitations from the C atoms of the phenyl (black) and carboxyl groups (red), and at photon energies indicated by the arrows. The respective spectra of the BA in gas phase are also reported (gray and red fill-to-zero lines). The obtained CT times are indicated. Reprinted with permission from 10.1021/acs.nanolett.5b05231. Copyright 2016 American Chemical Society.

Publication and authors:

G. Kladnik, M. Puppin, M. Coreno, M.de Simone, L. Floreano, A. Verdini, A. Morgante, D. Cvetko, A. Cossaro, “Ultrafast Charge Transfer Pathways Through A Prototype Amino-Carboxylic Molecular Junction”, Nano Letters (2016) DOI: 10.1021/acs.nanolett.5b05231