title: Disorder-derived, strong tunneling attenuation in bis-phosphonate monolayers.
abstract: Monolayers of alkyl bisphosphonic acids (bisPAs) of various carbon chain lengths (C4, C8, C10, C12) were grown on aluminum oxide (AlO(x)) surfaces from solution. The structural and electrical properties of these self-assembled monolayers (SAMs) were compared with those of alkyl monophosphonic acids (monoPAs). Through contact angle (CA) and Kelvin-probe (KP) measurements, ellipsometry, and infrared (IR) and x-ray photoelectron (XPS) spectroscopies, it was found that bisPAs form monolayers that are relatively disordered compared to their monoPA analogs. Current-voltage (J-V) measurements made with a hanging Hg drop top contact show tunneling to be the prevailing transport mechanism. However, while the monoPAs have an observed decay constant within the typical range for dense monolayers, β(mono)  =  0.85  ±  0.03 per carbon atom, a surprisingly high value, β(bis) =  1.40  ±  0.05 per carbon atom, was measured for the bisPAs. We attribute this to a strong contribution of 'through-space' tunneling, which derives from conformational disorder in the monolayer due to strong interactions of the distal phosphonic acid groups; they likely form a hydrogen-bonding network that largely determines the molecular layer structure. Since bisPA SAMs attenuate tunnel currents more effectively than do the corresponding monoPA SAMs, they may find future application as gate dielectric modification in organic thin film devices.
authors.affiliation: Institut für Halbleitertechnik, Technische Universität Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany. Department of Molecular Electronics, Technische Universität München, Theresienstraße 90, 80333 München, Germany.
authors.forename: Anshuma
authors.initials: A
authors.lastname: Pathak
authors.forename: Achyut
authors.initials: A
authors.lastname: Bora
authors.forename: Kung-Ching
authors.initials: KC
authors.lastname: Liao
authors.forename: Hannah
authors.initials: H
authors.lastname: Schmolke
authors.forename: Antje
authors.initials: A
authors.lastname: Jung
authors.forename: Claus-Peter
authors.initials: CP
authors.lastname: Klages
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Schwartz
authors.forename: Marc
authors.initials: M
authors.lastname: Tornow