Accumulated recent evidence reveals a little effect of chemical modifications on net charge transport across molecular junctions. It is attributed to hybridization between the molecular energy levels and those of the contacts, leading to their broadening and shifting. Using a combined computational – experimental approach, we have explored how the chemistry of a tether group affects the energetics of a distal aromatic group (Ar, mostly a phenyl ring), and the net charge transport across metal/monolayer-Si junctions. For monolayers of Ar-X-Si, where X is a hetero-atom, such as O or N, there is a drastic hybridization of the π orbitals with Si states, which is facilitated by the lone-pair electrons on X. [1] Such coupling is efficiently blocked when binding is done via a saturated tether as in Ar-R-Si with R being an alkyl chain. The number of carbons in the saturated R spacer, n = 1-5, has a minor effect on the coupling. However, there is a clear odd-even effect of n on the orientation of the phenyl ring, with a prominent effect on barrier height for transport. [2] Finally, we studied a combined system including both X and R, i.e., phenyl-X-(CH2)2-Si, with X = O, S or CH2. Interestingly, in such systems, the nature of X has a little effect on the induced surface dipole, yet the HOMO of the bonded monolayer varies by almost 1 eV depending on X. [3] Overall, this study does show a fine chemical tuning of electronic properties, yet it is a combined result of various factors; saturated spacers are especially effective in buffering the substrate effect. [1] T. Toledano et al., J. Electron Spectr. 2015, ASAP. [2] T. Toledano, et al., Langmuir 2014, 30, 13596. [3] H. Alon, et al., in preparation.

Phenyl ring as an electronic design motif: Orientation and coupling

251st National Meeting (2016)

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