Tuning the molecular order of C₆₀-based self-assembled monolayers in field-effect transistors

Abstract

The control of order in organic semiconductor systems is crucial to achieve desired properties in electronic devices. We have studied the order in fullerene functionalized self-assembled monolayers by mixing the active molecules with supporting alkyl phosphonic acids of different chain length. By adjusting the length of the molecules, structural modifications of the alignment of the C 60 head groups within the SAM can be tuned in a controlled way. These changes on the sub-nanometre scale were analysed by grazing incidence X-ray diffraction and X-ray reflectivity. To study the electron transport properties across these layers, self-assembled monolayer field-effect transistors (SAMFETs) were fabricated containing only the single fullerene monolayer as semiconductor. Electrical measurements revealed that a high 2D crystalline order is not the only important aspect. If the fullerene head groups are too confined by the supporting alkyl phosphonic acid molecules, defects in the crystalline C 60 film, such as grain boundaries, start to strongly limit the charge transport properties. By close interpretation of the results of structural investigations and correlating them to the results of electrical characterization, an optimum chain length of the supporting alkyl phosphonic acids in the range of C 10 was determined. With this study we show that minor changes in the order on the sub-nanometre scale, can strongly influence electronic properties of functional self-assembled monolayers.

Formation of Perfluoroalkyl Fullerene Alkylphosphonic Acid Self-Assembled Monolayers on Aluminum Oxide

“…The measured electrical conductivity in FET devices with TMF-based SAMs was below the leakage current through the dielectric and was thus much lower than in non-fluorinated fullerene SAMs. 15 This decrease in drain current is in accordance with the insulating effects of the fluorous coat on the carbon cage of highly derivatized TMF. With this respect, fullerene-based electron acceptors with fewer substituents on the core will be more promising candidates for further studies directed toward electronic applications.…”

“…In comparison, the non-fluorinated C 60 -SAMs showed a hexagonal crystalline order. 15 Preliminary testing of SAMFET devices using (60-10-3)(PA)-SAMs was carried out. The measured electrical conductivity in FET devices with TMF-based SAMs was below the leakage current through the dielectric and was thus much lower than in non-fluorinated fullerene SAMs.…”

“…15 Preliminary FET testing.-FET devices using (60-10-3)(PA)-SAMs were tested ( Figure SI-4). Several different transistor setups and layouts (top contact, bottom contact, patterned by photolithography/ E-beam lithography, and shadow masks) were fabricated, but in all cases the drain current across the transistor channel did not exceed the leakage current through the SAM.…”

Structural Investigations of Self-Assembled Monolayers for Organic Electronics: Results from X-ray Reflectivity

“…21,22,41 This effect can be easily realized by mixing C 60 functionalized molecules (C 60 C 18 -PA) with corresponding PAs, due to the same anchor group chemistry. The composition of the randomly mixed SAM on the surface is determined by the stoichiometry of the molecules in solution.…”

Simulation of charge transport in organic semiconductors: A time-dependent multiscale method based on nonequilibrium Green's functions

“…This method was previously applied to study charge transport through DNA [6,7,11,12], and it has recently been extended to account for charge relaxation and electric field effects [8]. Here, we apply the methodology in a different setting to study charge transport in significantly larger organic structures, in particular C 60 -based SAMs used in FETs [13,14] (cf. Fig.…”