Tuning the Molecular Order of C₆₀ Functionalized Phosphonic Acid Monolayers

Abstract

Mixed self-assembled monolayers (SAM) of alkyl phosphonic acids and C(60) functionalized octadecyl phosphonic acids (C(60)C(18)-PA) are deposited on alumina substrates from solution and are shown to form well-ordered structures with an insulating layer of alkyl chains and a semiconducting layer that comprises mainly C(60). Such an ordered structure is a necessity for the application of SAMs in organic transistors but is difficult to obtain since C(60)C(18)-PA without additional support do self-assemble in dense packaging but not in a well-ordered fashion. To avoid disordering of the SAM and to gain a better control of the interfacial properties we have investigated the stabilizing effects of fluorinated dodecyl phosphonic acids (FC(12)-PA) on the C(60)C(18)-PA monolayer. Vibrational sum-frequency (SFG) spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, and electrical measurements were applied to study the mixed monolayers. Here, we make use of the differently labeled PA to determine surface coverages and molecular properties of the two species independently. Adsorption of FC(12)-PA gives rise to vibrational bands at 1344 cm(-1) and 1376 cm(-1) in SFG spectra, while a pronounced vibrational band centered at 1465 cm(-1) is attributable to C(60) vibrations. The coexistence of the bands is indicative for the presence of a mixed monolayer that is composed of both molecular species. Furthermore, a pronounced maximum in SFG intensity of the C(60) band is observed for SAMs, which are deposited from solutions with ~75% C(60)C(18)-PA and ~25% FC(12)-PA. The intensity maximum originates from successful stabilization of C(60) modified C(60)C(18)-PA by FC(12)-PA and a significantly improved molecular order. Conclusions from SFG spectra are corroborated by electric measurements that show best performance at these concentrations. Our results provide new information on the morphology and composition of C(60) modified SAMs and establish a route to fabricate well-defined layers for molecular scale organic electronics.

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

“…For self-assembly of mixed monolayers, the solutions of alkyl-PA and C 60 C 18 -PA were mixed in a 1 : 1-ratio, which is expected to result in roughly the same mixing ratio in the SAM. 24 For structural investigations, monolayers of respective mixtures of phosphonic acids were self-assembled on aluminium oxide (AlO x ) films of around 10 nm, grown by atomic layer deposition (ALD) on silicon substrates. These substrates offer an anchor surface which is very similar to the AlO x in the SAMFET devices which is made by plasma oxidation of Al layers, yet much smoother which is crucial for the structural investigations.…”

“…SAMs of functionalized fullerenes have been used in organic photovoltaics, 13,21,22 memory transistors, 9 light sensors 23 and SAMFETs. 18,24,25 In all of these devices the order of the SAM is of crucial importance for electronic properties. Therefore, ways to access information about and actually control the morphology of these films can enhance the device performance.…”

N‐Type Self‐Assembled Monolayer Field‐Effect Transistors and Complementary Inverters

“…The result-a mixed monolayer of both species-will however always be a necessary trade-off between interfacial order and surface coverage. [ 15 ] The lack of reliable n-type SAMFET materials hampers the realization of self-assembled CMOS circuits. High performance and reliable n-type SAMFETs will enable a huge step forward in a bottom-up approach towards robust self-assembled complementary organic circuits.…”

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

“…[4][5][6] We found that selectivity of a Bingel-Hirsch reaction with phosphonic acid malonatic ester greatly depended on the TMF substrate. Furthermore, the number of phosphonic ester (PE) additions and isomers formed varied significantly, depending on the TMF substrate.…”

“…Tunable electron acceptors have been attached to ZnO nanorods in the form of self-assembled monolayers (SAMs) in order to tune the electrical properties of the ZnO nanostructures. [4][5][6] The Halik group has shown that a SAM with C 60 -acceptor units can accept and accumulate electronic charges when incorporated into organic thin-film memory transistors. 5 The C 60 derivatives that differed by alkyl linkers of different chain lengths were synthesized and formed SAMs on the surface of ZnO nanorods with different insulating properties to tune the electrical properties of ZnO nanorods.…”