We used partially fluorinated alkyl and aromatic\ud phosphonates as model systems with similar molecular dipole\ud moments to form self-assembled monolayers (SAMs) on the\ud Zn-terminated ZnO(0001) surface. The introduced surface\ud dipole moment allows tailoring the ZnO work function to tune\ud the energy levels at the inorganic−organic interface to organic\ud semiconductors, which should improve the efficiency of charge\ud injection/extraction or exciton dissociation in hybrid electronic\ud devices. By employing a wide range of surface characterization\ud techniques supported by theoretical calculations, we present a\ud detailed picture of the phosphonates’ binding to ZnO, the\ud molecular orientation in the SAM, their packing density, as\ud well as the concomitant work function changes. We show that\ud for the aromatic SAM the interaction between neighboring molecules is strong enough to drive the formation of a more densely packed monolayer with a higher fraction of bidentate binding to ZnO, whereas for the alkyl SAM a lower packing density was found with a higher fraction of tridentate binding

The technological exploitation of the extraordinary properties of graphene relies on the ability to achieve full control over the production of a high-quality material and its processing by up-scalable approaches in order to fabricate large-area films with single-layer or a few atomic-layer thickness, which might be integrated in working devices. A simple method is reported for producing homogenous dispersions of unfunctionalized and non-oxidized graphene nanosheets in N-methyl-2-pyrrolidone (NMP) by using simple molecular modules, which act as dispersion-stabilizing compounds during the liquid-phase exfoliation (LPE) process, leading to an increase in the concentration of graphene in dispersions. The LPE-processed graphene dispersion was shown to be a conductive ink. This approach opens up new avenues for the technological applications of this graphene ink as low-cost electrodes and conducting nanocomposite for electronics.

Ultraviolet and X-ray photoelectron spectroscopies in combi- nation with density functional theory (DFT) calculations were used to study the change in the work function (Φ) of graphene, supported by quartz, as induced by adsorption of hexaazatriphenylene−hexacarbonitrile (HATCN). Near edge X-ray absorption fine structure spectroscopy (NEXAFS) and DFT modeling show that a molecular-density-dependent reorientation of HATCN from a planar to a vertically inclined adsorption geometry occurs upon increasing surface coverage. This, in conjunction with the orientation- dependent magnitude of the interface dipole, allows one to explain the evolution of graphene Φ from 4.5 eV up to 5.7 eV, rendering the molecularly modified graphene-on-quartz a highly suitable hole injection electrode

The functionality of interfaces in hybrid inorganic/organic (opto)electronic devices is determined by the alignment of the respective frontier energy levels at both sides of the heterojunctions. Controlling the interface electronic landscape is a key element for achieving favourable level alignment for energy and charge transfer processes. Here, it is shown that the electronic properties of polar ZnO surfaces can be reversibly modified using organic photochromic switches. By employing a range of surface characterization techniques combined with density functional theory calculations, it is demonstrated that self-assembled monolayers (SAMs) of photochromic phosphonic acid diarylethenes (PA-DAEs) can be employed to reversibly change the electronic properties of polar ZnO/ SAM structures by light stimuli. The highest occupied molecular orbital level of PA-DAE is raised by 0.7 eV and the lowest unoccupied one lowered by 0.9 eV, respectively, upon illumination by ultraviolet light and the levels shift back to their original position upon illumination by green light. The results thus provide a pathway to tailor hybrid interface electronic properties in a dynamic manner upon simple light illumination, which can be exploited to reversibly tune the electrical properties of photoswitchable (opto)electronic devices.

A combination of ultraviolet and X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and first principle calculations was used to study the electronic structure at the interface between the strong molecular acceptor 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane (F6TCNNQ) and a graphene layer supported on either a quartz or a copper substrate. We find evidence for fundamentally different charge redistribution mechanisms in the two ternary systems, as a consequence of the insulating versus metallic character of the substrates. While electron transfer occurs exclusively from graphene to F6TCNNQ on the quartz support (p-doping of graphene), the Cu substrate electron reservoir induces an additional electron density flow to graphene decorated with the acceptor monolayer. Remarkably, graphene on Cu is n-doped and remains n-doped upon F6TCNNQ deposition. On both substrates, the work function of graphene increases substantially with a F6TCNNQ monolayer atop, the effect being more pronounced (∼1.3 eV) on Cu compared to quartz (∼1.0 eV) because of the larger electrostatic potential drop associated with the long-distance graphene-mediated Cu-F6TCNNQ electron transfer. We thus provide a means to realize high work function surfaces for both p- and n-type doped graphene.

Transition metal dichalcogenides (TMDCs) are layered compounds where layers are held together by weak van der Waals (vdW) interactions and can be cleaved with ease, enabling the exfoliation of a single layer by means of physically thinning down the crystal. [1,2] This transition from 3D to 2D [2,3] is accompanied by a whole set of outstanding new physics of 2D TMDCs. [4-7] This started the quest for high-quality and large-scale monolayers, which fostered the development of different techniques ranging from bottom-up approaches like chemical vapor deposition (CVD) and molecular-beam epitaxy (MBE) [8] to top-down routes via mechanical and liquid exfoliation. [2,8,9] To date, all routes suffer specific drawbacks. Both CVD and MBE can supply large-area monolayers yet crave optimization for each new TMDC composition and bear challenges in terms of subsequent clean transfer off the growth substrate. Liquid-based exfoliations introduce contaminants, thereby limiting studies of intrinsic material properties. Historically, tape-based mechanical exfoliation has largely carried the advance on 2D material research and the study of their intrinsic properties by supplying high-quality materials in an accessible manner. However, it is easily diagnosed with low yield, limiting its use beyond lab-scale experiments. To overcome these limitations scalable exfoliations beyond scotch tape have been investigated. [10] On this note, gold has been investigated as an exfoliation substrate and provides the needed adhesive forces via strong vdW [11] or "covalent-like quasibonding" (CLQB) [12] interactions with layered materials. Several TMDCs have already been exfoliated using gold (Table S1,

The control of the cathode work function (WF) is essential to enable efficient electron injection and extraction at organic semiconductor/cathode interfaces in organic electronic devices. The adsorption of an air-stable molecular donor onto electrodes, compatible with both evaporation and solution processes, is a simple way to reduce the WF. Such a versatile molecule, however, has not been identified yet. In this paper, ultraviolet photoelectron spectroscopy is used to confirm that depositing an ultrathin layer of the moderately air-stable pentamethylrhodocene-dimer onto various conducting electrodes, by either vacuum 2 deposition or drop-casting from solution, substantially reduces their WF to less than 3.6 eV, with 2.8 eV being the lowest attainable value. Detailed measurements of the Rh core levels with X-ray photoelectron spectroscopy reveal that the electron transfer from the molecule to the respective substrates is responsible for the appreciable WF reduction. Notably, even after air-exposure, the WF of the donor-covered electrodes remains below those of typically used clean cathode-metals such as Al and Ag, rendering the approach appealing for practical applications. The WF reduction, together with the observed air-stability of the covered electrodes, demonstrates the applicability of the pentamethylrhodocene-dimer to reduce the WF for a wide range of electrodes used in all-organic or organic-inorganic hybrid devices.

We used aromatic phosphonates with substituted phenyl rings with different molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface in order to engineer the energy-level alignment at hybrid inorganic/organic semiconductor interfaces, with an oligophenylene as organic component. The work function of ZnO was tuned over a wide range of more than 1.7 eV by different SAMs. The difference in the morphology and polarity of the SAMmodified ZnO surfaces led to different oligophenylene orientation, which resulted in an orientation-dependent ionization energy that varied by 0.7 eV. The interplay of SAM-induced work function modification and oligophenylene orientation changes allowed tuning of the offsets between the molecular frontier energy levels and the semiconductor band edges over a wide range. Our results demonstrate the versatile use of appropriate SAMs to tune the energy levels of ZnO-based hybrid semiconductor heterojunctions, which is important to optimize its function, e.g., targeting either interfacial energy-or chargetransfer.