Electronic transport properties of individual 4,4′-bis(mercaptoalkyl)-biphenyl derivatives measured in STM-based break junctions

Abstract

Electronic transport measurements of single, systematically varied 4,4'-bis(mercaptoalkyl)-biphenyl derivatives (MABP) are performed in a controlled test-device. The molecules are composed of a central biphenyl unit (BP) carrying two mercaptoalkyl substituents with different chain lengths (m, n = number of CH(2)-units), in the para-position of the BP unit. The total length of both spacers is m + n = 10. The molecular conductance of these individual MABPs deposited on Au (111) substrates is studied using STM-based break junctions. It is shown that the molecular conductance depends on the relative position of the BP unit within the molecule. In the case of the symmetric derivative 5BP5 a value of 0.07 +/- 0.01 nS is obtained, while for 1BP9 the molecular conductance is doubled and a value of 0.17 +/- 0.03 nS results. This relatively high value of conductance for the single Au(tip)-1BP9-Au(substrate) junction is attributed to an increased coupling of the BP unit to the adjacent electrode, i.e. the STM-tip or the Au-substrate. We address the role of the specific contact situation (-S-Au) and of the position of the electrically active molecular moiety and thus come to a deeper understanding of the electronic transport properties of 4,4'-bis(mercaptoalkyl)biphenyl derivatives.

Directed Immobilization of Janus-AuNP in Heterometallic Nanogaps: a Key Step Toward Integration of Functional Molecular Units in Nanoelectronics

“…In order to examine the possible influence of the molecular capping layer on the experimental conductance through the corresponding AuNP devices, we estimated for a first approximation the tunneling current through single molecules, MPA and MOA, respectively, connected to two electrodes applying the single-channel Landauer formula for conductance ( G molecule ): ,, G molecule = I U = G 0 .25em exp ( − β d ) T normalL T normalR where G 0 = quantum of conductance, β = decay constant, d = molecular length, and T L/R = transmission coefficients for left metal–molecule and right metal–molecule contact. We applied the molecular length like indicated in Figure and values for the decay constants and transmission coefficients taken from literature: ,− β phen = 4.6 nm –1 , β alk = 7.6 nm –1 , β COOH = 11.3 nm –1 , T Au–S = 0.81, T AuPd‑COOH ∼ T Au‑COOH = 0.08, T Pt‑COOH = 0.1, and T Pt–N = 0.29. While the theoretical conduction through MPA connected to a Pt- and a Au-electrode amounts to G MPA = 1.100 μS, the theoretical conductance through a MOA connected to two Au-electrodes can be estimated to G MOA = 0.208 nS which is a difference of 4 orders of magnitude.…”

Tuning electronic transport via hepta-alanine peptides junction by tryptophan doping

“…W-7, though, is exceptional: its low « 0 does not follow the trend of the UPS or DFT results, or of the « 0 values for W-1 and W-4. We postulate that for W-7 contact/overlap between the Au's spillover electron density and the indole ring's π-electrons is sufficient to decrease the effective transport barrier (63). Such sensitivity of the energy levels to mere physical proximity to the metallic contact was not observed for thiol-terminated polyphenyls (62), from another hand indicating that actual chemical bonding can act to buffer coupling, rather than enhance it, possibly via electron localization in the bond (64,65).…”

Resonant transport and electrostatic effects in single-molecule electrical junctions

“…(1)], 1.52 nm −1 . Since the only structural change in the molecules xPhx is the length of the alkyl chains, it is pertinent to compare the β values with those found for alkanedithiols in related STM-based experiments; reported values range 7-8.2 nm −1 for all three conductance groups [17,33,34]. Since the Fermi energy of the contacts has to fall between the HOMO and LUMO of the molecule in the junction, for molecules conducting via a tunneling mechanism one expects the β value to depend upon the HOMO-LUMO separation, with more conjugated molecules having a smaller HOMO-LUMO gap and hence a smaller β.…”