Self-Assembled Monolayers with Embedded Dipole Moments for Work Function Engineering of Oxide Substrates

Abstract

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.

Charge Transport Properties of Single-Component and Binary Aromatic Self-Assembled Monolayers with Methyl and Trifluoromethyl Tail Groups

“…45 Similar experimental procedures to those in the previous experiments on dipolar and mixed SAMs were applied; the details can be found elsewhere. 28,32 The binding energy scale of the XPS spectra was referenced to the Au 4f 7/2 peak at 84.0 eV. 46 The photon energy scale of the NEXAFS spectra was referenced to the pronounced π* resonance of highly oriented pyrolytic graphite (HOPG) at 285.38 eV.…”

“…Synchrotron-based XPS and NEXAFS spectroscopy measurements were carried out at the HE-SGM beamline (bending magnet) of the synchrotron storage ring BESSY II in Berlin using a custom-designed experimental station . Similar experimental procedures to those in the previous experiments on dipolar and mixed SAMs were applied; the details can be found elsewhere. , The binding energy scale of the XPS spectra was referenced to the Au 4f 7/2 peak at 84.0 eV . The photon energy scale of the NEXAFS spectra was referenced to the pronounced π* resonance of highly oriented pyrolytic graphite (HOPG) at 285.38 eV …”

“…Controlling and understanding the charge tunneling rate across a molecular framework and, in an ideal case, variation of this rate by external stimuli represent major challenges of molecular electronics. − The great synthetic freedom allows us to vary both chemical composition of potential “device” molecules and their chemical and physical properties, such as, e.g., the proneness to reduction, oxidation, or protonation; possibility of quantum interference; and the availability of a certain dipole moment. − The effect of the latter parameter on the charge transport properties of molecular systems represents a controversial issue, discussed in different contexts, such as its influence on the tunneling rate, molecule–electrode coupling, tuning of the transition voltage, dipole-induced rectification, etc. − The standard way to vary the dipole moment for a molecule, assembled on an electrode and coupled to it via a suitable anchor, is its decoration with a dipolar tail group, − even though the embedding of dipolar groups into the molecular backbone became increasingly popular recently. − A simplest version of a dipolar tail group is represented by a single atom, such as a halogen one, possessing intrinsic electronegativity and polarizability. Indeed, the work function (WF) of both aliphatic and aromatic , self-assembled monolayers (SAMs) composed of such molecules can be varied via a single polarizable terminal atom, viz., F, Cl, Br, and I.…”

Rationalizing energy level alignment by characterizing Lewis acid/base and ionic interactions at printable semiconductor/ionic liquid interfaces

“…2E, F S ) that is significantly shifted downward from the initial (precontact) metal potential (F M ). From the extensive body of work on dipolar self-assembled monolayers on both metals and metal oxides, [34][35][36][37][38][39][40][41][42][43] it is understood that these work function changes result from a linear combination of dipolar fields. We can conclude that the initial IL layer on Au must be at least somewhat oriented with the highest positive charge density component (EMIM + ) dominating at the vacuum interface away from the Au/IL heterojunction, as shown in Fig.…”

Pronounced Solvent Effect on the Composition of Binary Self-Assembled Monolayers with Embedded Dipole Moments

“…Such a termination can, however, affect the morphology of the adjacent layer, diminishing, in an unfavorable case, the performance of the entire device. An alternative strategy decouples the dipole and morphology control by the introduction of a dipolar group into the backbone of the SAM-forming molecules, allowing, thus, an independent optimization of the surface chemistry by selection of a most suitable (in the context of morphology) tail group. − …”