Effects of Embedded Dipole Layers on Electrostatic Properties of Alkanethiolate Self-Assembled Monolayers

Abstract

Alkanethiolates (ATs) forming self-assembled monolayers (SAMs) on coinage metal and semiconductor substrates have been used successfully for decades for tailoring the properties of these surfaces. Here, we provide a detailed analysis of a highly promising class of AT-based systems, which are modified by one or more dipolar carboxylic acid ester groups embedded into the alkyl backbone. To obtain comprehensive insight, we study nine different embedded-dipole monolayers and five reference non-substituted SAMs. We systematically varied chain lengths, ester group orientations, and number of ester groups contained in the chain. To understand the structural and electronic properties of the SAMs, we employ a variety of complementary experimental techniques, namely infrared reflection absorption spectroscopy (IRS), highresolution X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM), and Kelvin probe (KP) AFM. These experiments are complemented with state-of-the-art electronic band-structure calculations. We find intriguing electronic properties like large and variable SAM-induced work function modifications and dipole induced shifts of the electrostatic potential within the layers. These observations are analyzed in detail by joining the results of the different experimental techniques with the atomistic insight provided by the quantum-mechanical simulations.orientation of this group in the backbone, the work function of the system can be changed by either +0.57 or −0.42 eV relative to a reference oligophenylene SAM. This variation is achieved without changing the chemistry for docking to the substrate or the chemical composition of the SAMambient interface. 28 Tuning the work function is, however, not the only effect of the embedded pyrimidine group in these systems, since it also induces a potential discontinuity inside the monolayer. This effect significantly changes the transport properties of the SAM, shifting the transition voltage and resulting in current rectification. 30,31 The potential discontinuity also shifts the core-level energies in the regions above and below the embedded dipoles relative to each other. These shifts can be observed directly by X-ray photoelectron spectroscopy (XPS), reflected as different binding energies (BEs) for the photoemission peaks associated with both regions. This observation, along with others, [32][33][34][35] lead us to question the generally accepted chemical shift model that assumes that shifts in the core-level BEs in monomolecular films are solely a consequence of different chemical environments of the respective atoms, 36 with the energy referenced to the Fermi level of the substrate. In contrast, it suggests that electrostatic shifts not related to the immediate chemical environment of an atom are similarly important. Generally, such electrostatic shifts are superimposed on the chemical ones and can under certain circumstances even play a dominant role. 37 The respective electrostatic effects in photoemiss...

A dithiocarbamate anchoring group as a flexible platform for interface engineering

“…As a reference, we used a HDT SAM with the work function value set to 4.3 eV according to the literature. 22 The latter value was additionally verified by its referencing to the work function of freshly sputtered gold set to 5.2 eV. 33 Note that the work functions measured for the PPz and PPd SAMs displayed some sample to sample variation.…”

“…Such a comparably high value for the aliphatic chains is presumably related to electrostatic effects, since similar and even higher values were observed for the alkanethiolate SAMs with embedded dipolar groups. 22 The intensity of this peak is comparably high since the signal from the phenyl ring and piperidine is more strongly attenuated. This is also the case for the signal stemming from the C atoms bound to the uppermost N atom.…”

“…a decrease in the work function upon substitution with electrondonating groups (C12N and OMe) and an increase in the work function upon partial or complete substitution of the terminal moiety with electronegative fluorine atoms (FP-Pz). 7,10,19,22 3.4. Quantum-mechanical simulations 3.4.1.…”

“…a docking group that provides anchoring to the substrate, a tail group comprising the ''outer'' SAM-ambient interface, and a spacer that separates the docking and tail groups and drives the self-assembly. 17,18 While all these building blocks can contribute to the joint dipole moment of the molecules constituting the film, it is mostly a dipolar tail group 2,3,5,10,19 or a polar mid-chain moiety [20][21][22][23][24] which is selected to specifically adjust the interfacial dipole. In contrast, the docking group is predominantly chosen based on its affinity to a particular substrate serving as the electrode or, in the context of organic photovoltaics, also as the buffer layer.…”

X-Ray Photoelectron Spectroscopy for Determining Interface Dipoles of Self-Assembled Monolayers

“…[27,28,31,39,40,42,[45][46][47][53][54][55][56][57][58][59][60][61][62] This has an immediate impact on chargetransport through the adsorbed layer [53][54][55][56][57][58][59][60][61][62] and also on the positions of the core levels of the adsorbate. [20][21][22]42,48,49,[63][64][65] These changes in core-level binding energies can be probed by X-ray photoelectron spectroscopy (XPS), [66][67][68][69] which is one of the default techniques for characterizing SAMs, used, for example, for analyzing the quality of self-assembled monolayers and to verify bonding. [70][71][72][73][74][75]…”