title: Electrical characterization of single molecule and Langmuir-Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative.
abstract: Monolayer Langmuir-Blodgett (LB) films of 1,4-bis(pyridin-4-ylethynyl)benzene (1) together with the "STM touch-to-contact" method have been used to study the nature of metal-monolayer-metal junctions in which the pyridyl group provides the contact at both molecule-surface interfaces. Surface pressure vs area per molecule isotherms and Brewster angle microscopy images indicate that 1 forms true monolayers at the air-water interface. LB films of 1 were fabricated by deposition of the Langmuir films onto solid supports resulting in monolayers with surface coverage of 0.98 × 10(-9) mol·cm(-2). The morphology of the LB films that incorporate compound 1 was studied using atomic force microscopy (AFM). AFM images indicate the formation of homogeneous, monomolecular films at a surface pressure of transference of 16 mN·m(-1). The UV-vis spectra of the Langmuir and LB films reveal that 1 forms two dimensional J-aggregates. Scanning tunneling microscopy (STM), in particular the "STM touch-to-contact" method, was used to determine the electrical properties of LB films of 1. From these STM studies symmetrical I-V curves were obtained. A junction conductance of 5.17 × 10(-5) G 0 results from the analysis of the pseudolinear (ohmic) region of the I-V curves. This value is higher than that of the conductance values of LB films of phenylene-ethynylene derivatives contacted by amines, thiols, carboxylate, trimethylsilylethynyl or acetylide groups. In addition, the single molecule I-V curve of 1 determined using the I(s) method is in good agreement with the I-V curve obtained for the LB film, and both curves fit well with the Simmons model. Together, these results not only indicate that the mechanism of transport through these metal-molecule-metal junctions is non-resonant tunneling, but that lateral interactions between molecules within the LB film do not strongly influence the molecule conductance. The results presented here complement earlier studies of single molecule conductance of 1 using STM-BJ methods, and support the growing evidence that the pyridyl group is an efficient and effective anchoring group in sandwiched metal-monolayer-metal junctions prepared under a number of different conditions.
authors.affiliation: Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, 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 Microscopias Avanzadas (LMA) C/Mariano Esquilor s/n, Campus Rio Ebro, 50018 Zaragoza, Spain.
authors.forename: Henrry Marcelo
authors.initials: HM
authors.lastname: Osorio
authors.affiliation: Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain ; Instituto de Ciencia de Materiales de Aragón (ICMA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain.
authors.forename: Santiago
authors.initials: S
authors.lastname: Martín
authors.affiliation: Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain ; Instituto de Nanociencia de Aragón (INA), Edificio I+D, Campus Rio Ebro, Universidad de Zaragoza, C/Mariano Esquillor s/n, 50017 Zaragoza, Spain.
authors.forename: María Carmen
authors.initials: MC
authors.lastname: López
authors.affiliation: Department of Chemistry, University of Durham, Durham DH1 3LE, United Kingdom.
authors.forename: Santiago
authors.initials: S
authors.lastname: Marqués-González
authors.affiliation: Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Higgins
authors.affiliation: Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Nichols
authors.affiliation: Department of Chemistry, University of Durham, Durham DH1 3LE, United Kingdom ; School of Chemistry and Biochemistry, University of Western Australia, Crawley 6009, WA, Australia.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Low
authors.affiliation: Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, 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 Microscopias Avanzadas (LMA) C/Mariano Esquilor s/n, Campus Rio Ebro, 50018 Zaragoza, Spain.
authors.forename: Pilar
authors.initials: P
authors.lastname: Cea
keywords: Langmuir–Blodgett films; STM touch-to-contact method; molecular electronics