Highly Conducting π-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes

Abstract

Supporting information placeholder ABSTRACT:We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ resulting in formation of a direct covalent sigma bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G 0 = 2e 2 /h). Junctions formed with methylene terminated oligophenyls with two to four phenyl units show a hundred-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling as they exhibit an exponential dependence of conductance with oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show nearresonant transmission with a cross-over to tunneling for the longer oligomers.

High-Conductive Organometallic Molecular Wires with Delocalized Electron Systems Strongly Coupled to Metal Electrodes

“…11 However, it turned out that the transport-limiting barriers shifted from the molecule-metal interfaces onto the molecular backbone, independently of the specific connection scheme to the fullerene. 12 In contrast to fullerenes with many, but weak sp 2 "bonds", the direct C-Au bond showed unprecedented high conductances for oligophenyls up to 0.9 G 0 , 13 (for one phenyl ring) close to the theoretical maximum of 1 G 0 (with G 0 = 2e 2 /h 77 µS the conductance quantum). The C-Au bond can be established either by extrusion of a trimethyltin moiety 13 or post deprotection of a trimethylsilyl moiety.…”

“…12 In contrast to fullerenes with many, but weak sp 2 "bonds", the direct C-Au bond showed unprecedented high conductances for oligophenyls up to 0.9 G 0 , 13 (for one phenyl ring) close to the theoretical maximum of 1 G 0 (with G 0 = 2e 2 /h 77 µS the conductance quantum). The C-Au bond can be established either by extrusion of a trimethyltin moiety 13 or post deprotection of a trimethylsilyl moiety. 14 Currently, the direct C-electrode bond seems to be the most promising coupling scheme also for graphene electrodes 15,16 if polymerization via the free termini can be prevented.…”

“…Due to the good agreement between DFT and experiments for both the 'monomer' and the 'dimer' compounds, we conclude that spontaneous dimerization is most likely the origin for the low-conductance peaks of compounds 4 (and also 5), in agreement with the observation of dimerization in SnMe 3 -capped oligophenyls with C-Au anchors. 13 A dimerization explains further why the contacting traces for molecular signatures are 5-7 times longer for the low-conductance I-Vs compared to the high-conductance I-V s (see SI).When comparing the main peaks in the conductance data at high bias (1.0 V) or low bias (0.2 V, see SI) of 2, 3, 4 and 5 measured at 300 K, a good qualitative agreement with DFT at zero-bias is found as directly compared in Fig. 5 B).…”

Characterizing the Metal–SAM Interface in Tunneling Junctions

“…[6][7][8][9][10][11][12][13][14][15][16][17][18] Recent studies of singlemolecule break junctions have been interpreted to indicate that the presence of covalent Au−C σ-bonds-formed using trimethyltin (-SnMe 3 )-terminated n-alkyl groups, 19,20 and SnMe 3 -terminated aromatics [19][20][21] or trimethylsilyl (TMS)-terminated conjugated systems 22 -increases rates of charge transport across these junctions by approximately a factor of 10-100, relative to amine or thiolate anchoring groups. One possible inference from the increase is that the Au−C σ-bond, and the absence of resistive anchoring heteroatoms, increases "conductivity" (although the meaning of this word is not entirely clear for tunneling junctions).…”

Structure dependent spin selectivity in electron transport through oligopeptides

“…[24][25][26][27] This technique enables the study of several parameters that affect the spin dependent electron transfer or the overall electron transfer, in a relatively simple manner. The same concept was applied in the past both with CP-AFM and STM for investigating length dependent [28][29][30][31][32] and force dependent conduction. 4,[33][34][35][36] The present study demonstrates the use of CP-AFM to study the spin dependent electron transfer through oligopeptide monolayers.…”