Transition from Tunneling to Hopping Transport in Long, Conjugated Oligo-imine Wires Connected to Metals

Abstract

We report the electrical transport characteristics of conjugated oligonaphthalenefluoreneimine (ONI) wires having systematically varied lengths up to 10 nm. Using aryl imine addition chemistry, ONI wires were built from gold substrates by extending the conjugation length through imine linkages between highly conjugated building blocks of alternating naphthalenes and fluorenes. The resistance and current-voltage characteristics of ONI wires were measured as a function of molecular length, temperature, and electric field using conducting probe atomic force microscopy (CP-AFM). We have observed a transition in direct current (DC) transport from tunneling to hopping near 4 nm as previously established for oligophenyleneimine (OPI) wires. Furthermore, we have found that long ONI wires are less resistive than OPI wires. The single-wire conductivity of ONI wires is approximately 1.8 +/- 0.1 x 10(-4) S/cm, a factor of approximately 2 greater than that of OPI wires, and consistent with the lower transport activation energy ( approximately 0.58 eV versus 0.65 eV or 13 versus 15 kcal/mol). Quantum chemical calculations reveal that charge is preferentially localized on the fluorene subunits and that the molecules are substantially twisted. Overall, this work confirms that imine addition chemistry can be used to build molecular wires long enough to probe the hopping transport regime. The versatility of this chemistry, in combination with CP-AFM, opens up substantial opportunities to probe the physical organic chemistry of hopping conduction in long conjugated molecules.

Room-temperature air-stable spin transport in bathocuproine-based spin valves

“…The room-temperature current scales linearly with voltage in the low bias regime which is consistent as well with an ohmic hopping conduction (see Fig. 5a) 30,31 . In our experimental data, we can observe three distinct features of the temperaturedependent I-V traces of the devices with leaky AlO x barrier that are typical of amorphous organic layers with hopping transport 21 .…”

“…5a). These two different regimes following a power law at low and high bias correspond to an ohmic conduction and a field emission regime, respectively 30,31 . The room-temperature current scales linearly with voltage in the low bias regime which is consistent as well with an ohmic hopping conduction (see Fig.…”

“…The voltage at which the barrier changes its shape appears as an inflection point V trans in a plot of ln(I/V 2 ) versus 1/V (see Fig. 5c) [30][31][32] . As shown in Fig.…”

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.…”

Imine-linked conjugated organic polymer bearing bis(imino)pyridine ligands and its catalytic application in C–C coupling reactions

“…The successful design and synthesis of these polymers linked by B-O [3][4][5], C-C [6,7], C-N [8], and C=N [9][10][11][12] bonds have been reported. Recent studies have shown that conjugated organic polymers (COPs) with extended π-conjugation possess high thermal stability and structure diversity [13][14][15].…”