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

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 -COO-Au 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 -COO-Au 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.

Electrical characterization of single molecule and Langmuir–Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative

“…The study of single-molecule junctions has enormously contributed to our ability to understand and control charge and heat transport phenomena at the molecular scale [10–21]. Complementary studies of larger area metal–molecular monolayer–metal junctions play a further crucial role in understanding the effect of intermolecular interactions, for example, van der Waals interactions and polarization effects in electronic transport properties [22–24]. In addition, planar-sandwiched monolayer structures are more closely aligned with practical electronic applications.…”

“…LB films also offer the possibility of exploration of a large number of metal–organic interfaces involving either physi- or chemi-sorbed films [31], and also permits the fabrication of directionally oriented monolayers when the molecule contains two different terminal groups that each have affinity for the substrate [30]. In particular, LB films have been used to analyze different properties and explore potential applications including molecular switching behavior [35–36], rectifying molecular junctions [37–38], exciton migration control [39], top-contact metallization [24,40–41], optical and opto-electronic applications [42–43], modulation of the electrical properties of the junction [24], inclusion of a metal atom in the organic structure of a molecular wire [44], and electrical measurements of both molecular ensembles and single molecules in the constrained environment of the film [24,30,45]. …”

Nanofabrication techniques of highly organized monolayers sandwiched between two electrodes for molecular electronics

“…At a single molecule level other contacts on metals have also been studied including acids (R-COOH) [23,85,86], carbodithioates (R-CS 2 H) [87], dithiocarboxylic acids [87], pyridyl (R-C 5 H 4 N) [88,89], isothiocyanates (R-NCS) [90], dimethylphosphine (R-PMe 2 ) [91], 4-(methylthio)phenyl [92], dihydrobenzo[b]thiophene [93], thienyl ring [94] or diphenylphosphine (R-PPh 2 ) [95]. However, their electrical properties have hardly been studied in SAMs.…”

“…In the LS method a substrate, situated in a horizontal position, is slowly put in contact with the liquid surface and subsequently lifted off the liquid surface. Different characterization techniques to determine the organization of the Langmuir films can be used including surface pressure vs. area per molecule isotherms (π-A), surface potential vs. area per molecule iso- properties of the junction by controlling the fabrication conditions (pH of the subphase) [23].…”

“…The study of single-molecule junctions has enormously contributed to understand and control the electronic transport phenomena at the molecular level [6,[10][11][12][13][14][15][16][17][18][19][20]. However, molecular-monolayer junctions offer an advantage over single-molecule junctions due to their crucial role in understanding the effect of intermolecular interactions, e.g., van der Waals interactions and polarization effects in electronic transport properties [21][22][23]. In addition, working with planar-sandwiched geometries is of large interest in electronic applications since two dimensional (2D) monomolecular structures up to 10 15 molecules·cm -2 can be fabricated.…”