Mixing of non- and fluorine-substituted mercaptobiphenyls in binary monolayers not only leads to work function variation but also electrostatic effects in photoemission and tunable charge tunneling rates across the films.

We studied charge tunneling rates across the single-component and binary self-assembling monolayers (SAMs) composed of 4-methyl-4′-mercaptobiphenyl (CH3-BPT) and 4-trifluoromethyl-4′-mercaptobiphenyl (CF3-BPT) on Au(111). The composition of the binary SAMs could be flexibly tuned, accompanied by gradual variation of the work function between ∼4.45 eV (CH3-BPT) and ∼5.5 eV (CF3-BPT). The tunneling rate across the single-component CH3-BPT SAM was found to be notably higher (by 1.5–2 orders of magnitude) than in the CF3-BPT case, while that across the binary SAMs varied progressively with their composition, between the values for the single-component monolayers and could, consequently, be fine-tuned. The observed behavior was tentatively explained by the higher projected density of states at the position of the terminal tail groups in the CH3-BPT case compared to CF3-BPT and by the appearance of an internal electrostatic field in the SAMs, leading to a change and renormalization of the energy-level alignments within the junction upon contact of the SAMs to the top EGaIn electrode. The extent of the latter effect depends primarily on the characteristics of the strongly confined two-dimensional (2D) sheet of dipolar tail groups at the SAM/top electrode interface. The height of the respective injection barrier is, however, unaffected by these characteristics, as follows from the values of the transition voltage, which do not change notably with the SAM composition. Analysis of the presented as well as literature data suggests that the position of a dipolar group in SAM-forming molecules has a significant impact on the performance of the respective SAM in the context of molecular electronics.

Thermal stability of several representative self-assembled monolayers (SAMs) with phosphonic acid (PA) anchoring group on Al 2 O 3 substrates was studied by in situ high resolution X-ray photoelectron spectroscopy, complemented by thermogravimetric analysis of the relevant bulk materials and theoretical simulations. The range of thermal stability and the degradation pathways were found to be dependent on the chemical composition of the SAM forming molecules, with backbone-specific "weak links". Whereas the anchoring groups were hardly affected by annealing up to 773 K (highest temperature applied for most of the samples), temperature-induced breakup of the bond between the backbone and anchoring group or a specific bond within the backbone was observed. The former is the case for nonsubstituted alkyl backbone, which is cleaved at temperatures above 673−773 K (depending on the backbone length), with the subsequent desorption of the released fragments. The latter is the case for the substituted (pentafluorophenoxy) or partly fluorinated alkyl backbone, with the bond cleavage occurring at temperatures above 523 K, either between the substitution and alkyl linker or between the fluorinated and nonfluorinated segments of the backbone. The above results suggest a higher robustness and better thermal stability of PA monolayers as compared to other types of SAMs, such as thiolates on coinage metal substrates. It is however advisible to test thermal stability of a specifically designed, functional PA SAM in context of possible "weak links" as far as this is important for a particular application.

Self-assembled monolayers (SAMs) are frequently used for work function (WF) engineering of different materials. For this, typically dipolar groups are attached to the molecule terminus at the SAM–ambient interface, which also influences its chemistry. WF engineering and interface chemistry can, however, be decoupled from one another using embedded dipolar groups, as has been demonstrated before for thiolate SAMs on metals. Herein, we extend this concept to oxide substrates. For this, a series of biphenyl-based molecules with a phosphonic acid (PA) anchoring group was synthesized, with one of the nonpolar phenyl units exchanged for a polar pyrimidine moiety, the dipole moment of which is oriented either toward (“down”) or away (“up”) to/from the PA group and, consequently, to/from the substrate. SAMs of these molecules formed on indium tin oxide (ITO), a frequently used and application-relevant oxide substrate, feature a uniform molecular configuration, dense molecular packing, and an upright molecular orientation. These SAMs exhibit pronounced electrostatic effects associated with the embedded dipolar groups, viz. shifts of the characteristic peaks in the C 1s X-ray photoelectron spectra and WF variations. The latter values were found to be 3.9, 4.85, and 4.4 eV for the up, down, and nonpolar reference SAM-engineered ITO, respectively. Consequently, these SAMs can serve as a powerful tool to monitor WF engineering effects in a variety of device assembles, decoupling these effects from the interface chemistry. The comparably low WF value for the up SAM is particularly important since it extends a rather limited variety of SAMs capable of lowering the WF of ITO.