Partial restoration of aromaticity of pentacene-5,7,12,14-tetrone on Cu(111).

The π-conjugation of organic molecules can be strongly influenced when functional groups are added to a molecule, for example when pentacene is converted into pentacene-5,7,12,14-tetrone (P4O) by substitution of four H-atoms with four O-atoms, leading to four CO double bonds. In fact, although free P4O resembles the parent hydrocarbon pentacene structurally at a first glance, its electronic properties differ drastically and can be more accurately described by three benzene units connected via four carbonyl groups. If P4O is deposited onto Cu(111), the electronic interaction across the interface has previously been reported to fully restore the π-conjugation through a weakening of the CO double bonds and a redistribution of electrons, both of which have been explained with the model of surface-induced aromatic stabilization. Here, we observe for the case of P4O on Cu(111) that the molecule does not exhibit full π-conjugation upon interaction with the surface, likely because of the special electronic nature of the hybridized P4O on Cu(111). Our results are derived from CO-functionalized noncontact atomic force microscopy measurements in combination with dispersion-corrected density functional theory calculations yielding bond lengths and molecular geometries. To characterize the aromaticity, we apply the harmonic oscillator model of aromaticity.

Structural and electronic properties of MoS2 and MoSe2 monolayers grown by chemical vapor deposition on Au(111).

The exceptional electronic and photonic properties of the monolayers of transition metal dichalcogenides including the spin-orbit splitting of the valence and conduction bands at the K points of the Brillouin zone make them promising for novel applications in electronics, photonics and optoelectronics. Scalable growth of these materials and understanding of their interaction with the substrate is crucial for these applications. Here we report the growth of MoS2 and MoSe2 monolayers on Au(111) by chemical vapor deposition at ambient pressure as well as the analysis of their structural and electronic properties down to the atomic scale. To this aim, we apply ultrahigh vacuum surface sensitive techniques including scanning tunneling microscopy and spectroscopy, low-energy electron diffraction, X-ray and angle-resolved ultraviolet photoelectron spectroscopy in combination with Raman spectroscopy at ambient conditions. We demonstrate the growth of high-quality epitaxial single crystalline MoS2 and MoSe2 monolayers on Au(111) and show the impact of annealing on the monolayer/substrate interaction. Thus, as-grown and moderately annealed (<100 °C) MoSe2 monolayers are decoupled from the substrate by excess Se atoms, whereas annealing at higher temperatures (>250 °C) results in their strong coupling with the substrate caused by desorption of the excess Se. The MoS2 monolayers are strongly coupled to the substrate and the interaction remains almost unchanged even after annealing up to 450 °C.

Atomically Thin Metal-Dielectric Heterostructures by Atomic Layer Deposition.

Heterostructures increasingly attracted attention over the past several years to enable various optoelectronic and photonic applications. In this work, atomically thin interfaces of Ir/Al2O3 heterostructures compatible with micro-optoelectronic technologies are reported. Their structural and optical properties were determined by spectroscopic and microscopic techniques (XRR, XPS, HRTEM, spectroscopic ellipsometry, and UV/vis/NIR spectrophotometry). The XRR and HRTEM analyses reveal a layer-by-layer growth mechanism of Ir in atomic scale heterostructures, which is different from the typical island-type growth of metals on dielectrics. Alongside, XPS investigations imply the formation of Ir-O-Al bonding at the interfaces for lower Ir concentrations, in contrast to the nanoparticle core-shell structure formation. Precisely tuning the ratio of the constituents ensures the control of the dispersion profile along with a transition from effective dielectric to metallic heterostructures. The Ir coating thickness was varied ranging from a few angstroms to films of about 7 nm in the heterostructures. The transition has been observed in the structures containing individual Ir coating thicknesses of about 2-4 nm. Following this, we show epsilon-near-zero metamaterials with tunable dielectric constants by precisely varying the composition of such heterostructures. Overall, a comprehensive study on structural and optical properties of the metal-dielectric interfaces of Ir/Al2O3 heterostructures was addressed, indicating an extension of the material portfolio available for novel optical functionalities.

Correlation between two- and three-dimensional crystallographic lattices for epitaxial analysis. II. Experimental results.

While the crystal structure of the polymorph phase can be studied in three dimensions conveniently by X-ray methods like grazing-incidence X-ray diffraction (GIXD), the first monolayer is only accessible by surface-sensitive methods that allow the determination of a two-dimensional lattice. Here, GIXD measurements with sample rotation are compared with distortion-corrected low-energy electron diffraction (LEED) experiments on conjugated molecules: 3,4;9,10-perylenetetracarboxylic dianhydride (PTCDA), 6,13-pentacenequinone (P2O), 1,2;8,9-dibenzopentacene (trans-DBPen) and dicyanovinyl-quaterthiophene (DCV4T-Et2) grown by physical vapor deposition on Ag(111) and Cu(111) single crystals. For these molecular crystals, which exhibit different crystallographic lattices and crystal orientations as well as epitaxial properties, the geometric parameters of the three-dimensional lattice are compared with the corresponding geometry of the first monolayer. A comparison of the monolayer lattice from LEED investigations with the multilayer lattices determined by rotated GIXD experiments reveals a correlation between the first monolayer and the epitaxial growth of three-dimensional crystals together with lattice distortions and re-alignment of molecules. The selected examples show three possible scenarios of crystal growth on top of an ordered monolayer: (i) growth of a single polymorph, (ii) growth of three different polymorphs; in both cases the first monolayer serves as template. In the third case (iii) strong lattice distortion and distinct molecular re-alignments from the monolayer to epitaxially grown crystals are observed. This is the second part of our work concerning the correlation between two- and three-dimensional crystallographic lattices for epitaxial analysis. In the first part, the theoretical basis has been derived which provides a mathematical relationship between the six lattice parameters of the three-dimensional case and the three parameters obtained for the two-dimensional surface unit cell, together with their orientation to the single-crystalline substrate. In this work, a combined experimental approach of GIXD and LEED is introduced which can be used to investigate the effect of the epitaxial monolayer on the structural properties of molecular crystals grown on top.

Hybridization vs decoupling: influence of an h-BN interlayer on the physical properties of a lander-type molecule on Ni(111).

2D materials such as hexagonal boron nitride (h-BN) are widely used to decouple organic molecules from metal substrates. Nevertheless, there are also indications in the literature for a significant hybridization, which results in a perturbation of the intrinsic molecular properties. In this work we study the electronic and optical properties as well as the lateral structure of tetraphenyldibenzoperiflanthene (DBP) on Ni(111) with and without an atomically thin h-BN interlayer to investigate its possible decoupling effect. To this end, we use in situ differential reflectance spectroscopy as an established method to distinguish between hybridized and decoupled molecules. By inserting an h-BN interlayer we fabricate a buried interface and show that the DBP molecules are well decoupled from the Ni(111) surface. Furthermore, a highly ordered DBP monolayer is obtained on h-BN/Ni(111) by depositing the molecules at a substrate temperature of 170 °C. The structural results are obtained by quantitative low-energy electron diffraction and low-temperature scanning tunneling microscopy. Finally, the investigation of the valence band structure by ultraviolet photoelectron spectroscopy shows that the low work function of h-BN/Ni(111) further decreases after the DBP deposition. For this reason, the h-BN-passivated Ni(111) surface may serve as potential n-type contact for future molecular electronic devices.

Effect of an electric field during the deposition of silicon dioxide thin films by plasma enhanced atomic layer deposition: an experimental and computational study.

The growth, chemical, structural, mechanical, and optical properties of oxide thin films deposited by plasma enhanced atomic layer deposition (PEALD) are strongly influenced by the average-bias voltage applied during the reaction step of surface functional groups with oxygen plasma species. Here, this effect is investigated thoroughly for SiO2 deposited in two different PEALD tools at average-bias voltages up to -300 V. Already at a very low average-bias voltage (< -10 V), the SiO2 films have significantly lower water content than films grown without biasing together with the formation of denser films having a higher refractive index and nearly stoichiometric composition. Substrate biasing during PEALD also enables control of mechanical stress. The experimental findings are supported by density functional theory and atomistic simulations. They demonstrate that the application of an electric field during the plasma step results in an increased energy transfer between energetic ions and the surface, directly influencing relevant surface reactions. Applying an electric field during the PEALD process leads to SiO2 thin films with significantly improved properties comparable to films grown by ion beam sputtering.

Fraternal twins: distinction between PbPc and SnPc by their switching behaviour in a scanning tunnelling microscope.

In this contribution, we compare the optical absorbance behaviour and the structural properties of lead(II)-phthalocyanine (PbPc) and tin(II)-phthalocyanine (SnPc) thin films. To this end, we employ a Ag(1 1 1) substrate terminated with a monolayer of 3,4,9,10-perylene tetracarboxylic dianhydride constituting an internal interface whose main effect is an electronic decoupling of the phthalocyanine adlayer from the metal surface. As deduced from low-energy electron diffraction and scanning tunnelling microscopy (STM) measurements, the epitaxial relations and unit cell compositions of the prevailing PbPc monolayer and multilayer domains are confusingly similar to those of SnPc on PTCDA/Ag(1 1 1). However, SnPc and PbPc can be readily distinguished by their STM-induced switching behaviours: while the former is capable of reversible configurational changes, no effect on the latter could be achieved by us under comparable conditions. This corroborates earlier theoretical predictions and even renders the chemical identification of individual shuttlecock-shaped metal-phthalocyanines feasible.

Fully Atomistic Understanding of the Electronic and Optical Properties of a Prototypical Doped Charge-Transfer Interface.

The current study generates profound atomistic insights into doping-induced changes of the optical and electronic properties of the prototypical PTCDA/Ag(111) interface. For doping K atoms are used, as KxPTCDA/Ag(111) has the distinct advantage of forming well-defined stoichiometric phases. To arrive at a conclusive, unambiguous, and fully atomistic understanding of the interface properties, we combine state-of-the-art density-functional theory calculations with optical differential reflectance data, photoelectron spectra, and X-ray standing wave measurements. In combination with the full structural characterization of the KxPTCDA/Ag(111) interface by low-energy electron diffraction and scanning tunneling microscopy experiments (ACS Nano 2016, 10, 2365-2374), the present comprehensive study provides access to a fully characterized reference system for a well-defined metal-organic interface in the presence of dopant atoms, which can serve as an ideal benchmark for future research and applications. The combination of the employed complementary techniques allows us to understand the peculiarities of the optical spectra of K2PTCDA/Ag(111) and their counterintuitive similarity to those of neutral PTCDA layers. They also clearly describe the transition from a metallic character of the (pristine) adsorbed PTCDA layer on Ag(111) to a semiconducting state upon doping, which is the opposite of the effect (degenerate) doping usually has on semiconducting materials. All experimental and theoretical efforts also unanimously reveal a reduced electronic coupling between the adsorbate and the substrate, which goes hand in hand with an increasing adsorption distance of the PTCDA molecules caused by a bending of their carboxylic oxygens away from the substrate and toward the potassium atoms.

Self-Assembly of Tetraphenyldibenzoperiflanthene (DBP) Films on Ag(111) in the Monolayer Regime.

Tetraphenyldibenzoperiflanthene (DBP) is a promising candidate as a component of highly efficient organic photovoltaic cells and organic light-emitting diodes. The structural properties of thin films of this particular lander-type molecule on Ag(111) were investigated by complementary techniques. Highly ordered structures were obtained, and their mutual alignment was characterized by means of low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) images reveal two slightly different arrangements within the first monolayer (ML), both describable as specific herringbone patterns with two molecules per unit cell whose dibenzoperiflanthene framework is parallel to the surface. In contrast, single DBP molecules in the second ML were imaged with much higher intramolecular resolution, resembling the shape of the frontier orbitals in the gas phase as calculated by means of density functional theory (DFT). Further deposition leads to the growth of highly ordered bilayer islands on top of the first ML with identical unit cell dimensions and orientation but slightly inclined molecules. This suggests that the first ML acts as a template for the epitaxial growth of further layers. Simultaneously, a significant number of second-layer molecules mainly located at step edges or scattered over narrow terraces do not form highly ordered aggregates.

Complex Stoichiometry-Dependent Reordering of 3,4,9,10-Perylenetetracarboxylic Dianhydride on Ag(111) upon K Intercalation.

Alkali metal atoms are frequently used for simple yet efficient n-type doping of organic semiconductors and as an ingredient of the recently discovered polycyclic aromatic hydrocarbon superconductors. However, the incorporation of dopants from the gas phase into molecular crystal structures needs to be controlled and well understood in order to optimize the electronic properties (charge carrier density and mobility) of the target material. Here, we report that potassium intercalation into the pristine 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) monolayer domains on a Ag(111) substrate induces distinct stoichiometry-dependent structural reordering processes, resulting in highly ordered and large KxPTCDA domains. The emerging structures are analyzed by low-temperature scanning tunneling microscopy, scanning tunneling hydrogen microscopy (ST[H]M), and low-energy electron diffraction as a function of the stoichiometry. The analysis of the measurements is corroborated by density functional theory calculations. These turn out to be essential for a correct interpretation of the experimental ST[H]M data. The epitaxy types for all intercalated stages are determined as point-on-line. The K atoms adsorb in the vicinity of the oxygen atoms of the PTCDA molecules, and their positions are determined with sub-Ångström precision. This is a crucial prerequisite for the prospective assessment of the electronic properties of such composite films, as they depend rather sensitively on the mutual alignment between donor atoms and acceptor molecules. Our results demonstrate that only the combination of experimental and theoretical approaches allows for an unambiguous explanation of the pronounced reordering of KxPTCDA/Ag(111) upon changing the K content.

Identification of vibrational excitations and optical transitions of the organic electron donor tetraphenyldibenzoperiflanthene (DBP).

Tetraphenyldibenzoperiflanthene (DBP) attracts interest as an organic electron donor for photovoltaic applications. In order to assist in the analysis of vibrational and optical spectra measured during the formation of thin films of DBP, we have studied the vibrational modes and the electronic states of this molecule. Information on the vibrational modes of the electronic ground state has been obtained by IR absorption spectroscopy of DBP grains embedded in polyethylene and CsI pellets and by calculations using density functional theory (DFT). Electronic transitions have been measured by UV/vis absorption spectroscopy applied to DBP molecules isolated in rare-gas matrices. These measurements are compared with the results of ab initio and semi-empirical calculations. Particularly, the vibrational pattern observed in the S1 ← S0 transition is interpreted using a theoretical vibronic spectrum computed with an ab initio model. The results of the previous experiments and calculations are employed to analyze the data obtained by high-resolution electron energy loss spectroscopy (HREELS) applied to DBP molecules deposited on a Au(111) surface. They are also used to examine the measurements performed by differential reflectance spectroscopy (DRS) on DBP molecules deposited on a muscovite mica(0001) surface. It is concluded that the DBP molecules in the first monolayer do not show any obvious degree of chemisorption on mica(0001). Regarding the first monolayer of DBP on Au(111), the HREELS data are consistent with a face-on anchoring and the absence of strong electronic coupling.