Length-Dependent Nanotransport and Charge Hopping Bottlenecks in Long Thiophene-Containing π-Conjugated Molecular Wires

Abstract

Self-assembled conjugated molecular wires containing thiophene up to 6 nm in length were grown layer-by-layer using click chemistry. Reflection-absorption infrared spectroscopy, ellipsometry and X-ray photoelectron spectroscopy were used to follow the stepwise growth. The electronic structure of the conjugated wires was studied with cyclic voltammetry and UV-vis spectroscopy as well as computationally with density functional theory (DFT). The current-voltage curves (±1 V) of the conjugated molecular wires were measured with conducting probe atomic force microscopy (CP-AFM) in which the molecular wire film bound to a gold substrate was contacted with a conductive AFM probe. By systematically measuring the low bias junction resistance as a function of length for molecules 1-4 nm long, we extracted the structure dependent tunneling attenuation factor (β) of 3.4 nm(-1) and a contact resistance of 220 kΩ. The crossover from tunneling to hopping transport was observed at a molecular length of 4-5 nm with an activation energy of 0.35 eV extracted from Arrhenius plots of resistance versus temperature. DFT calculations revealed localizations of spin densities (polarons) on molecular wire radical cations. The calculations were employed to gauge transition state energies for hopping of polarons along wire segments. Individual estimated transition state energies were 0.2-0.4 eV, in good agreement with the experimental activation energy. The transition states correspond to flattening of dihedral angles about specific imine bonds. These results open up possibilities to further explore the influence of molecular architecture on hopping transport in molecular junctions, and highlight the utility of DFT to understand charge localization and associated hopping-based transport.

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.…”

A direct investigation of photocharge transfer across monomolecular layer between C60 and CdS quantum dots by photoassisted conductive atomic force microscopy

“…23 In the conductive tip-alkyl chain-metal substrate junctions, the current was dominated by the noresonant tunneling through alkyl chains. 17,[24][25][26][27] Figure 2 The resistance of the junction can be summarized as…”

“…In the recent years, conductive atomic force microscopy (C-AFM) has an outstanding advantage, which can provide the surface structure and property at the same time. 17 Also light can be introduced to the C-AFM system for the development of PC-AFM. 18,19 By using this technique, the local photoinduced charge transport and current can be investigated expediently.…”

Click-chemistry approaches to π-conjugated polymers for organic electronics applications

“…Moreover, very recently, Demissie et al 40 and Smith et al 41 reported the successful synthesis of π-conjugated molecular wires on Au surfaces using imine click-like (condensation) reaction to ensure high yields (∼99%) in a simple sequential monomer addition process. The reaction schematic in Scheme 11 demonstrates the alternate addition of benzene-1,4-dicarbaldehyde 22 (ref.…”

“…The reaction schematic in Scheme 11 demonstrates the alternate addition of benzene-1,4-dicarbaldehyde 22 (ref. 40) (or thiophene-2,5-carboxyaldehyde 23 ) 41 and 1,4-diaminobenzene 24 monomers to give O6 and O7 -type oligomers. The wires ranged in length from 0.6 to 5.5 nm.…”