Volume 19, Issue 17 p. 5352-5363
Full Paper

Controlling the Structural and Electrical Properties of Diacid Oligo(Phenylene Ethynylene) Langmuir–Blodgett Films

Dr. Luz Marina Ballesteros 

Dr. Luz Marina Ballesteros 

Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza 50009 (Spain)

Instituto de Nanociencia de Aragón (INA), Edificio i+d Campus Rio Ebro, Universidad de Zaragoza, C/Mariano Esquillor, s/n, 50017 Zaragoza (Spain)

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Dr. Santiago Martín 

Dr. Santiago Martín 

Instituto de Ciencia de Materiales de Aragón (ICMA), Universidad de Zaragoza-CSIC, Departamento de Física de la Materia Condensada, 50009 Zaragoza (Spain)

Laboratorio de Microscopías Avanzadas (LMA), C/Mariano Esquilor s/n Campus Rio Ebro, 50018 Zaragoza (Spain)

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Javier Cortés 

Javier Cortés 

Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza 50009 (Spain)

Instituto de Nanociencia de Aragón (INA), Edificio i+d Campus Rio Ebro, Universidad de Zaragoza, C/Mariano Esquillor, s/n, 50017 Zaragoza (Spain)

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Santiago Marqués-González

Santiago Marqués-González

Department of Chemistry, University of Durham, Durham DH1 3LE (UK)

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Prof. Simon J. Higgins

Prof. Simon J. Higgins

Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD (UK)

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Prof. Richard J. Nichols

Prof. Richard J. Nichols

Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD (UK)

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Prof. Paul J. Low

Prof. Paul J. Low

Department of Chemistry, University of Durham, Durham DH1 3LE (UK)

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Dr. Pilar Cea 

Corresponding Author

Dr. Pilar Cea 

Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza 50009 (Spain)

Instituto de Nanociencia de Aragón (INA), Edificio i+d Campus Rio Ebro, Universidad de Zaragoza, C/Mariano Esquillor, s/n, 50017 Zaragoza (Spain)

Laboratorio de Microscopías Avanzadas (LMA), C/Mariano Esquilor s/n Campus Rio Ebro, 50018 Zaragoza (Spain)

Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza 50009 (Spain)Search for more papers by this author
First published: 27 February 2013
Citations: 15

Graphical Abstract

Modulation of conductance in Langmuir–Blodgett (LB) films of a symmetrically acid-terminated oligomeric phenylene ethynylene derivative was achieved by means of the pH value of the aqueous subphase from which the corresponding Langmuir films were transferred. Current–voltage curves of the LB films on a gold substrate (see figure), recorded with an STM tip positioned just above the monolayer, and good fits to the Simmons model indicate that charge flow through the metal |molecule|metal junction occurs by a non-resonant tunnelling mechanism.

Abstract

The preparation, characterization and electrical properties of Langmuir–Blodgett (LB) films composed of a symmetrically substituted oligomeric phenylene ethynylene derivative, namely, 4,4′-[1,4-phenylenebis(ethyne-2,1-diyl)]dibenzoic acid (OPE2A), are described. Analysis of the surface pressure versus area per molecule isotherms and Brewster angle microscopy reveal that good-quality Langmuir (L) films can be formed both on pure water and a basic subphase. Monolayer L films were transferred onto solid substrates with a transfer ratio of unity to obtain LB films. Both L and LB films prepared on or from a pure water subphase show a red shift in the UV/Vis spectrum of about 14 nm, in contrast to L and LB films prepared from a basic subphase, which show a hypsochromic shift of 15 nm. This result, together with X-ray photoelectron spectroscopic and quartz crystal microbalance experiments, conclusively demonstrate formation of one-layer LB films in which OPE2A molecules are chemisorbed onto gold substrates and consequently COOAu junctions are formed. In LB films prepared on a basic subphase the other terminal acid group is also deprotonated and associates with an Na+ counterion. In contrast, LB films prepared from a pure water subphase preserve the protonated acid group, and lateral H-bonds with neighbouring molecules give rise to a supramolecular structure. STM-based conductance studies revealed that films prepared from a basic subphase are more conductive than the analogous films prepared from pure water, and the electrical conductance of the deprotonated films also coincides more closely with single-molecule conductance measurements. This result was interpreted not only in terms of better electron transmission in COOAu molecular junctions, but also in terms of the presence of lateral H-bonds in the films formed from pure water, which lead to reduced conductance of the molecular junctions.