Abstract
Self-assembled monolayers (SAMs) of nitrile-substituted oligo(phenylene ethynylene) thiols (NC-OPEn) with a variable chain length n (n ranging from one to three structural units) on Au(111) were studied by synchrotron-based high-resolution X-ray photoelectron spectroscopy and near-edge absorption fine-structure spectroscopy. The experimental data suggest that the NC-OPEn molecules form well-defined SAMs on Au(111), with all the molecules bound to the substrate through the gold–thiolate anchor and the nitrile tail groups located at the SAM–ambient interface. The packing density in these SAMs was found to be close to that of alkanethiolate monolayers on Au(111), independent of the chain length. Similar behavior was found for the molecular inclination, with an average tilt angle of ~33–36° for all the target systems. In contrast, the average twist of the OPEn backbone (planar conformation) was found to depend on the molecular length, being close to 45° for the films comprising the short OPE chains and ~53.5° for the long chains. Analysis of the data suggests that the attachment of the nitrile moiety, which served as a spectroscopic marker group, to the OPEn backbone did not significantly affect the molecular orientation in the SAMs.
Highlights
IntroductionVery efficient and compact, are being pushed to their physical limits in terms of further miniaturization with associated issues such as electrical leakage and heat dissipation, and this is driving consideration of entirely new types of platforms
Current semiconductor microelectronics devices, very efficient and compact, are being pushed to their physical limits in terms of further miniaturization with associated issues such as electrical leakage and heat dissipation, and this is driving consideration of entirely new types of platforms
The doublet is the only feature in the spectra of NC-OPE2/Au and NC-OPE3/Au suggesting that all the molecules in these films are bound to the substrate in the self-assembled monolayer (SAM) fashion, i.e., through the thiolate–gold anchor
Summary
Very efficient and compact, are being pushed to their physical limits in terms of further miniaturization with associated issues such as electrical leakage and heat dissipation, and this is driving consideration of entirely new types of platforms. The most frequently used group in this regard is thiol, which allows SAM-like assembly of the molecules on coinage metal and various semiconductor substrates, for example Au and GaAs, respectively Another essential element of the experiments is the variation of the length of the molecular wire [3,4,12,14,18,20], which allows further insight into the mechanism of conductance, described as nonresonant superexchange tunneling in most cases [21], and gives the capability to determine essential characteristic parameters, most importantly the attenuation factor describing the trend of exponential tunnelling current versus molecular length. The interpretations of these types of results depend crucially on the actual physical and structural characteristics of the molecules in the SAMs, for example, packing density, molecular orientations, and molecular conformations; and yet in many cases these characteristics are neither precisely controlled nor measured, but assumed to be similar to those of other types of molecules and that they do not vary with different lengths of oligomers in the same series
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