Maral Aminpour

The role of van der Waals interaction in the tilted binding of amine molecules to the Au(111) surface

We present the results of ab initio electronic structure calculations for the adsorption characteristics of three amine molecules on Au(111), which show that the inclusion of van der Waals interactions between the isolated molecule and the surface leads in general to good agreement with experimental data on the binding energies. Each molecule, however, adsorbs with a small tilt angle (between -5 and 9°). For the specific case of 1,4-diaminobenzene (BDA) our calculations reproduce the larger tilt angle (close to 24°) measured by photoemission experiments, when intermolecular (van der Waals) interactions (for about 8% coverage) are included. These results point not only to the important contribution of van der Waals interactions to molecule-surface binding energy, but also that of intermolecular interactions, often considered secondary to that between the molecule and the surface, in determining the adsorption geometry and pattern formation.

Heterogeneous Metal-Free Hydrogenation over Defect-Laden Hexagonal Boron Nitride

Catalytic hydrogenation is an important process used for the production of everything from foods to fuels. Current heterogeneous implementations of this process utilize metals as the active species. Until recently, catalytic heterogeneous hydrogenation over a metal-free solid was unknown; implementation of such a system would eliminate the health, environmental, and economic concerns associated with metal-based catalysts. Here, we report good hydrogenation rates and yields for a metal-free heterogeneous hydrogenation catalyst as well as its unique hydrogenation mechanism. Catalytic hydrogenation of olefins was achieved over defect-laden h-BN (dh-BN) in a reactor designed to maximize the defects in h-BN sheets. Good yields (>90%) and turnover frequencies (6 × 10–5–4 × 10–3) were obtained for the hydrogenation of propene, cyclohexene, 1,1-diphenylethene, (E)- and (Z)-1,2-diphenylethene, octadecene, and benzylideneacetophenone. Temperature-programmed desorption of ethene over processed h-BN indicates the formation of a highly defective structure. Solid-state NMR (SSNMR) measurements of dh-BN with high and low propene surface coverages show four different binding modes. The introduction of defects into h-BN creates regions of electronic deficiency and excess. Density functional theory calculations show that both the alkene and hydrogen-bond order are reduced over four specific defects: boron substitution for nitrogen (BN), vacancies (VB and VN), and Stone–Wales defects. SSNMR and binding-energy calculations show that VN are most likely the catalytically active sites. This work shows that catalytic sites can be introduced into a material previously thought to be catalytically inactive through the production of defects.

An MoSx Structure with High Affinity for Adsorbate Interaction

MoS2 is an intriguing material: although its basal plane is quite inert, it is the key catalyst for petrochemical hydrodesulfurization (and hydrodenitrogenation) processes. Dow/ Union Carbide developed an MoS2-based catalyst [1] for the formation of higher alcohols from syngas, an application which is gaining increased importance with the emergence of biofuels. In these applications, MoS2 is used as a fine powder; cobalt or nickel (or mixtures thereof) activate the powder through incorporation into edges of the MoS2 [2] structures. Further promotion is achieved by alkali doping with carbon typically serving as the support. Quite recently, MoS2 has attracted increasing interest as an exfoliatable monolayer material for (opto-)electronic applications, and as a surface material for electrochemical reactions, among other applications. Several studies have succeeded in growing MoS2 on various substrates and have shown that its catalytic activity may be ascribed to a metallic electronic state at the brim of MoS2 triangular clusters, which can be readily identified in scanning tunneling microscopy (STM). We have recently developed a technique for growing MoS2—by evaporating molybdenum on a sulfur-preloaded Cu(111) surface—that leads to epitaxial MoS2 islands of sizes ranging from approximately 1 to 100 nm and for which we have confirmed the presence of the brim state. Herein, we demonstrate that another novel MoSx structure, reproducibly formed in the same fashion as in the growth of MoS2 we recently performed, is stable in the entire temperature range of our experiments (25 K to 650 K) and reverts to its pristine form after exposure to oxygen-containing adsorbates upon annealing. More importantly, this structure interacts far more strongly with these adsorbates than MoS2. Analysis of STM images and related electronic structure calculations confirm the metallic nature of this monolayer material, which we rationalize below to have the composition Mo2S3. We chose anthraquinone (AQ) as test adsorbate, because it is large and rigid enough that we can directly image its adsorption geometry, from which we can derive insight into the interaction of the surface with the adsorbate. The sample preparation described in the Supporting Information gives two thermally stable MoSx patterns (Figure 1a). The patches formed by both patterns are capable of extending across substrate steps. Regions not covered with an MoSx patch exhibit the well-known ffiffiffi

Friedel oscillations responsible for stacking fault of adatoms: The case of Mg ( 0001 ) and Be ( 0001 )

We perform a first-principles study of Mg adatom and adislands on the Mg(0001) surface, and Be adatom on Be(0001), to obtain further insights into the previously reported energetic preference of the fcc faulty stacking of Mg monomers on Mg(0001). We first provide a viewpoint on how Friedel oscillations influence ionic relaxation on these surfaces. Our three-dimensional charge-density analysis demonstrates that Friedel oscillations have maxima which are more spatially localized than what one-dimensional average density or two-dimensional cross sectional plots could possibly inform: The well-known charge-density enhancement around the topmost surface layer of Mg(0001) is strongly localized at its fcc hollow sites. The charge accumulation at this site explains the energetically preferred stacking fault of the Mg monomer, dimer and trimer. Yet, larger islands prefer the normal hcp stacking. Surprisingly, the mechanism by which the fcc site becomes energetically more favorable is not that of enhancing the surface-adatom bonds but rather those between surface atoms. To confirm our conclusions, we analyze the stacking of Be adatom on Be(0001) - a surface also largely influenced by Friedel oscillations. We find, in fact, a much stronger effect: The charge enhancement at the fcc site is even larger and, consequently, the stacking-fault energy favoring the fcc site is quite large, 44 meV.