HOW MOLECULAR IS MOLECULAR ELECTRONICS? IMPORTANCE OF ORIENTATION AND COUPLING TO SUBSTRATE FOR TRANSPORT ACROSS MOLECULAR JUNCTIONS

Evidence is accumulating that chemical modifications have only a small effect on net charge transport across molecular junctions. This is attributed to hybridization between the molecular energy levels and the very high density of states of the contacts. Using a combined computational – experimental approach we explore 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 drastic hybridization of the π orbitals with Si states, facilitated by the lone-pair electrons on X.¹ Such coupling is efficiently blocked if binding is via a saturated tether as in Ar-R-Si with R = alkyl chain. However the number of carbons in R, n = 1-5, has a clear odd-even effect of n on the Ar orientation, 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 affects the induced surface dipole only slightly, yet the bonded monolayer’s HOMO varies by almost 1 eV, depending on X.³ Overall, effective chemical modification of transport requires altering the frontier orbital energy level(s). At the same time, binding groups and saturated tethers are critical in dictating the degree of delocalization and broadening of the frontier orbitals.

Work done with Cahen, Kronik (WIS); Sukenik (BIU), and Zuilhof (Wageningen)

[1] Toledano et al., J. Electron Spectr. 2015, ASAP;

[2] Toledano et al., Langmuir 2014, 30, 13596.

[3] Alon et al., in prep. 2015.