Characterization of SAMs

Subtle changes in the SAM packing structure can have profound effects on their properties, especially inside tunneling junctions. Therefore we characterize our SAMs in great detail with lab-based techniques (FTIRUV-Visellipsometry, electrochemistry, etc.), and the SINS beamline at the Singapore Synchrotron Light Source (SSLS) make it possible to record on a single sample angle resolved X-ray & ultraviolet photoelectron spectroscopy (ARXPS and ARUPS), and near edge X-ray absorption fine structure spectroscopy spectra (NEXAFS).

Deatiled characterization studies allow us to obtain detailed information of the electronic and supramolecular structures of the SAMs. The figure below shows the odd-even effect in the tilt angle of the Fc units with respect to the surface normal measured by NEXAFS of series of SCnFc SAMs with n = 6 - 15. Our data fit MD models by Damien Thompson very well (see Nat. Nanotechnol. 20138, 113-118  and J. Phys. Chem. C 2015,119, 17910–17919 ).

Fig 1: Comparison between NEXAFS and MD results. (a) Typical NEXAFS data for n= 10, 11(b) Odd-even effect in tilt angle α measured by NEXAFS (c) Typical model used in MD simulations (done by Damien Thompson) (d) Odd-even effect in tilt angle α from MD simulations

Likewise, we studied the SAM packing structure as a function of anchoring group, roughness of the metal surface that supports the SAMs, position of functional groups within the SAMs, and others. For example, we found that thin liquid-like SAMs form better tunneling barriers on defective metal surfaces than thick crystal-like SAMs because liquid like SAMs allow for self-repair (see  Nano Lett. 201515, 6643–6649).
We made use of soft X-ray characterisation techniques to correlate deviations from ideal electrochemical behaviour of SAMs with their supramolecular structure by investigating a series of SAMs of SCnFc with n = 0−15. We found that the origin of the peak broadening and the presence of multiple redox waves can be ascribed to Fc units in different microenvironments which depend on the intermolecular Fc−Fc, Fc−Cn, and Cn−Cn interactions and the nature (i.e., covalent or noncovalent) of the Fc−Au interactions (see J. Phys. Chem. C 20152,1348–1354). We also used SSLS based core-hole clock (CHC) spectroscopy to study orbital dependent charge transfer dynamics between the electrode and Fc groups (see J. Phys. Condens. Matter 201628, (9), 094006).

Fig 2: Schematic illustrations of interfacial CT from Fc moiety to Au substrate for SCnFcC13−n (n = 4) on Au and electronic processes involved in the CHC measurements: (i) a core level electron is photo-excited to one of the Fc-derived unoccupied molecular orbitals, (ii) CT of the excited electron from the respective molecular orbitals to the conduction band (CB) of the substrate, and (iii) following normal Auger decay process.