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By Ernie Fontes - July 9, 2010

In the race to make electronic devices smaller and more efficient, monolayer transistors consisting of only a single sheet of molecules are under intense scrutiny. The monolayer geometry confines electronic transport to two dimensions, causing devices to be extraordinarily sensitive to structural imperfections like voids or grain boundaries, as well sensitive to challenges injecting and extracting charge from the single monolayer at contact boundaries. A team of researchers from Philips Laboratory in Eindhoven, the Netherlands, and the Technical University of Graz, Austria, recently reported that an organic field-effect transistor can be realized by just a single functionalized self-assembled monolayer (SAM-FET) [1]. The molecules forming the monolayer have a silane end group to attach to the oxide layer of a silicon wafer, an alkane chain flexible linker and a functional quinquethiophene end group. While the substrate oxide coating is disordered, the flexible linker permits the quaterthiophene moieties to attain crystalline order, as was shown with grazing-incidence diffraction results obtained at CHESS G2 station. Scattering under grazing incidence reduces the penetration of the beam into the substrate and thus provides a good signal-to-noise ratio for the weak surface scattering above the diffuse scattering from the bulk of the substrate. G2 station has monolayer sensitivity and is compatible with radiation-sensitive samples [2]. The Philips researchers were able to detect three monolayer peaks (figure), and thus determine the lattice constants of the self-assembled monolayer. The lattices constants closely resemble the lattice spacing in thiophene thin films, and hence the diffraction signal can be related to ordering in the quinquethiophene moieties. The favorable transistor properties determined for the SAM-FET were ascribed to the order in the active layer [1]. The authors also begin a case describing how partially covered SAM-FETs could be well suited for use as chemical sensors. The mechanism for sensing would depend upon the strong dependence of the in-plane electron mobility on the film perfection. Any interaction between external molecules and the SAM-FET impairs percolation paths and creates an easily detected resistivity.

Grazing-incidence x-ray diffraction from a SAM-FET revealing the in-plane ordering of the quinquethiophene moieties.
Grazing-incidence x-ray diffraction from a SAM-FET revealing the in-plane ordering of the quinquethiophene moieties (see inset).

References: 

[1] Simon G. J. Mathijssen, Edsger C. P. Smits, Paul A. van Hal, Harry J. Wondergem, Sergei A. Ponomarenko, ArminMoser, Roland Resel, Peter A. Bobbert, Martijn Kemerink, René A. J. Janssen and Dago M. de Leeuw, Monolayer coverage and channel length set the mobility in self-assembled monolayer field-effect transistors, Nature Nanotechnology 4, 674 - 680 (2009).

[2] Christine L. McGuiness, Daniel Blasini, John P. Masejewski, Sundararajan Uppili, Oando M. Cabarcos, Detlef Smilgies, and David L. Allara: "Molecular Self-Assembly at Bare Semiconductor Surfaces: Characterization of a Homologous Series of n-Alkanethiolate Monolayers on GaAs(001)", ACS-Nano 1, 30–49 (2007).