From Tunneling to Hopping: A Comprehensive Investigation of Charge Transport Mechanism in Molecular Junctions Based on Oligo(p-phenylene ethynylene)s

Abstract

The charge transport mechanism of oligo(p-phenylene ethynylene)s with lengths ranging from 0.98 to 5.11 nm was investigated using modified scanning tunneling microscopy break junction and conducting probe atomic force microscopy methods. The methods were based on observing the length dependence of molecular resistance at single molecule level and the current-voltage characteristics in a wide length distribution. An intrinsic transition from tunneling to hopping charge transport mechanism was observed near 2.75 nm. A new transitional zone was observed in the long length molecular wires compared to short ones. This was not a simple transition between direct tunneling and field emission, which may provide new insights into transport mechanism investigations. Theoretical calculations provided an essential explanation for these phenomena in terms of molecular electronic structures.

Fabrication of carbon nanotube nanogap electrodes by helium ion sputtering for molecular contacts

“…20 We observe an average resistance almost three orders of magnitude higher, with values spread over roughly two orders of magnitude. We ascribe this to a varying imperfect attachment of the molecules' anchor groups to the CNT electrodes and conformational freedom of molecules in the junction; both profoundly deteriorating the conductance.…”

Oligo(p‐phenyleneethynylene) (OPE) Molecular Wires: Synthesis and Length Dependence of Photoinduced Charge Transfer in OPEs with Triarylamine and Diaryloxadiazole End Groups

“…[24,25] Electronic conduction through OPE wires anchored in metal-molecule-metal junctions has also been reported, with length dependent tunneling-to-hopping transitions observed. [26,27] In the present investigation we focus on a new series of five D-B-A complexes 1-5…”

Electrical characteristics of conjugated self-assembled monolayers in large-area molecular junctions

“…2 Studies on various -conjugated molecules have been performed. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The molecules, with delocalized electrons due to sp 2 hybridized carbon atoms, exhibit a smaller energy gap between the highest occupied molecular orbital ͑HOMO͒ and the lowest unoccupied molecular orbital ͑LUMO͒. For -conjugated molecules longer than ϳ3 nm hopping conduction was observed.…”

“…For -conjugated molecules longer than ϳ3 nm hopping conduction was observed. 12,19,20 For molecules shorter than ϳ3 nm the transport is by tunneling and an exponential dependence of the resistance on molecular length was observed. Values for ␤ range from 0.04-0.61 Å −1 and are significantly lower than for alkanes.…”