Interface Characteristics at an Organic/Metal Junction: Pentacene on Cu Stepped Surfaces

The adsorption of pentacene on Cu(221), Cu(511) and Cu(911) is investigated using density functional theory (DFT) with the self-consistent inclusion of van der Waals (vdW) interactions. Cu(211) is a vicinal of Cu(111) while Cu(511) and (911) are vicinals of Cu(100). For all the three surfaces, we found pentacene to prefer to adsorb parallel to the surface and near the steps. The addition of vdW interactions resulted in an enhancement in adsorption energies, with reference to the PBE functional, of around 2 eV. With vdWs inclusion, the adsorption energies were found to be 2.98 eV, 3.20 eV and 3.49 eV for Cu(211), Cu(511) and Cu(911) respectively. These values reflect that pentacene adsorbs stronger on (100) terraces with a preference for larger terraces. The molecule tilts upon adsorption with a small tilt angle on the (100) vicinals (about a few degrees) as compared to a large one on Cu(221) where the tilt angle is found to be about 20o. We find that the adsorption results in a net charge transfer to the molecule of ~1 electron, for all surfaces.Comment: 11 pages, 4 figure

A Theoretical and Experimental Investigation of the Physical and Chemical Properties of Solid Nanoscale Interfaces

With the emerging interest in nanoscale materials, the fascinating field of surface science is rapidly growing and presenting challenges to the design of both experimental and theoretical studies. The primary aim of this dissertation is to shed some light on the physical and chemical properties of selected nanoscale materials at the interface. Furthermore, we will discuss the effective application of cutting edge theoretical and experimental techniques that are invaluable tools for understanding the systems at hand. To this effect, we use density functional theory (DFT) with the inclusion of van der Waals (vdW) interactions to study the effect of long-range interactions on the adsorption characteristics of various organic molecules (i.e. benzene, olympicene radical, and sexithiophene) on transition metal surfaces. Secondly, the detailed analysis of x-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements will be presented. These investigations will be dedicated to the study of (i) the effect of pre-treatment on the coarsening behavior of Pt nanoparticles (NPs) supported on ?-Al2O3 and (ii) deconvoluting the intrinsic (size effects) and extrinsic (ligand effects) physical and electronic properties of Au NPs encapsulated by polystyrene 2-vinylpiridine ligands

On the role of long range interactions for the adsorption of sexithiophene on Ag(110) surface

The adsorption characteristics of the sexithiophene (6T) molecule on Ag(110) are studied using density functional theory with the inclusion of van der Waals (vdW) interactions. The stable adsorption configurations on 6T on Ag(110) as well as the nature of bonding the Ag substrate are evaluated. We also assess the performance of the vdW-DF method in describing the adsorption, energetics, heights, as well as the interface characteristics with the Ag(110) surface. We find two lowest adsorption energy configurations, at which the 6T molecule aligns with its molecular long axis parallel and perpendicular to the [001] direction, to be energetically close to each other, suggesting that they may coexist. Our findings indicate a significant increase in the 6T adsorption energies upon the inclusion of vdW interactions with the highest increase obtained using the opt-type functionals, in particular with the optB86b-vdW functional. The revPBE-vdW and rPW86-vdW2 functionals lead to less enhancement in adsorption energies that is attributed to the strong repulsive nature of these functionals, in agreement with earlier predictions. Upon adsorption of the 6T molecule, the changes in the atomic and electronic structures of the 6T molecule and Ag surface are found to be negligible; there is no charge transfer, and no interface state is observed. The work function is reduced upon adsorption with the largest change is similar to 0.6 eV obtained using the optB88-vdW functional. The results are in good agreement with the available experimental observations of the adsorption configurations and the work function changes. Based on our results, we conclude that the nature of bonding for 6T on Ag(110) can be classified as strong physisorption

Understanding the role of adenosine A2AR heteroreceptor complexes in neurodegeneration and neuroinflammation

Adenosine is a nucleoside mainly formed by degradation of ATP, located intracellularly or extracellularly, and acts as a neuromodulator. It operates as a volume transmission signal through diffusion and flow in the extracellular space to modulate the activity of both glial cells and neurons. The effects of adenosine are mediated via four adenosine receptor subtypes: A1R, A2AR, A2BR, A3R. The A2AR has a wide-spread distribution but it is especially enriched in the ventral and dorsal striatum where it is mainly located in the striato-pallidal GABA neurons at a synaptic and extrasynaptic location. A number of A2AR heteroreceptor complexes exist in the striatum. The existence of A2AR-D2R heteroreceptor complexes with antagonistic A2AR-D2R interactions in the striato-pallidal GABA neurons is well-known with A2AR activation inhibiting Gi/o mediated signaling of D2Rs. A2AR-mGluR5 heteroreceptor complexes were also found in with synergistic receptor-receptor interactions enhancing the inhibition of the D2R protomer signaling. They are located mainly in extrasynaptic regions of the striato-pallidal GABA neurons. Results recently demonstrated the existence of brain A2AR-A2BR heteroreceptor complexes, in which A2BR protomer constitutively inhibited the function of the A2AR protomer. These adenosine A2AR heteroreceptor complexes may modulate alpha-synuclein aggregation and toxicity through postulated bidirectional direct interactions leading to marked increases in A2AR signaling both in nerve cells and microglia. It is of high interest that formation of A2AR-A2ABR heteroreceptor complexes provides a brake on A2AR recognition and signaling opening up a novel strategy for treatment of A2AR mediated neurodegeneration. KEYWORDS: G protein-coupled receptor; Parkinson's diseases; adenosine A2A receptor; adenosine receptor; heteroreceptor complexes; neurodegeneration; neuroinflammation; oligomerizatio

VARIABILIDADE SAZONAL DA QUALIDADE DE ÁGUA EM LAGOAS URBANAS NO NORDESTE DO BRASIL: Seazonal variability of water quality in urbans lagoons in Northeast Brazil

The Pituaçú, Abaeté, Tororó and Paixão lagoons, in the city of Salvador-BA (Brazil) have in common the insertion of a lentic ecosystem in the urban environment of a developing city. This research describes the pattern of seasonal variation of non-conservative and biological physical-chemical parameters during the years 2017 and 2018 in 16 points spatially distributed in the four lagoons during the rainy season (from April to July) and dry season (August to November). Water quality was determined in accordance with the Water Quality Index (WQI) and application of the Trophic State Index (TSI), in addition to the CONAMA ordinancy 357/2005. The results showed that the four monitored lagoons exhibit some parameters outside the conformities established by CONAMA ordinances, in addition they are strongly eutrophic, but in a contrasting way, with good quality ratings considering their use, with the exception of Lagoa de Pituaçú. It is concluded that systematic assessments should be considered more frequently with the aim of collecting greater and more accurate information on the quality conditions of these ecosystems, and studying their quality changes as a reflection of the urban condition on their conservation.As Lagoas de Pituaçú, Abaeté, Tororó e Paixão, no município de Salvador-BA (Brasil) apresentam em comum a inserção de um ecossistema lêntico no meio urbano de uma cidade em desenvolvimento. Esta pesquisa descreve o padrão de variação sazonal de parâmetros físico-químicos não conservativos e biológicos durante os anos de 2017 e 2018 em 16 pontos distribuídos espacialmente nas quatro lagoas durante o período chuvoso (compreendido entre os meses de abril a julho) e seco (de agosto a novembro). A qualidade da água foi determinada em acordo com o índice de Qualidade da Água (IQA) e aplicação do Índice de Estado Trófico (IET), além das classes de Resolução CONAMA 357/2005. Os resultados evidenciaram que as quatro lagoas monitoradas exibem alguns parâmetros fora das conformidades estabelecidas pelo CONAMA, além disso apresentam-se fortemente eutrofizadas, mas de forma contrastante, com boas classificações de qualidade considerando seu uso, com exceção da Lagoa de Pituaçú. Conclui-se que avaliações sistemáticas devem ser consideradas com maior frequência com o objetivo de colher maiores e mais precisas informações sobre as condições de qualidade destes ecossistemas, e estudar suas alterações de qualidade como reflexo do condicionante urbano sobre sua conservação

Inhibition of Tyrosinase by 4H-Chromene Analogues: Synthesis, Kinetic Studies and Computational Analysis

Inhibition of mushroom tyrosinase was observed with synthetic dihydropyrano[3,2-b]chromenediones. Among them, DHPC04 displayed the most potent tyrosinase inhibitory activity with a Ki value of 4μM, comparable to the reference standard inhibitor kojic acid. A kinetic study suggested that these synthetic heterocyclic compounds behave as competitive inhibitors for the L-DOPA binding site of the enzyme. Furthermore, molecular modeling provided important insight into the mechanism of binding interactions with the tyrosinase copper active site

Interface Characteristics At An Organic/Metal Junction: Pentacene On Cu Stepped Surfaces

The adsorption of pentacene on Cu (2 2 1), Cu (5 1 1) and Cu (9 1 1) is investigated using density functional theory (DFT) with the self-consistent inclusion of van der Waals (vdW) interactions. Cu (2 1 1) is a vicinal of Cu (1 1 1) while Cu (5 1 1) and (9 1 1) are vicinals of Cu (1 0 0). For all the three surfaces, we found pentacene to prefer to adsorb parallel to the surface and near the steps. The addition of vdW interactions resulted in an enhancement in adsorption energies, with reference to the PBE functional, of around 2 eV. With vdWs inclusion, the adsorption energies were found to be 2.98 eV, 3.20 eV and 3.49 eV for Cu (2 2 1), Cu (5 1 1) and Cu (9 1 1) respectively. These values reflect that pentacene adsorbs stronger on (1 0 0) terraces with a preference for larger terraces. The molecule tilts upon adsorption with a small tilt angle on the (1 0 0) vicinals (about a few degrees) as compared to a large one on Cu (2 2 1) where the tilt angle is found to be about 20°. We find that the adsorption results in a net charge transfer to the molecule of ∼1 electron, for all surfaces

Role Of Long-Range Interactions For The Structure And Energetics Of Olympicene Radical Adsorbed On Au(111) And Pt(111) Surfaces

We report on the results of the van der Waals (vdW) inclusive density functional theory (DFT) calculations for the adsorption characteristics of olympicene radical (C19H11) on Au(111) and Pt(111) surfaces. The nature of bonding between the olympicene radical and Au(111) and Pt(111) surfaces is evaluated along with the effects of vdW interactions, and the chemical characteristics of the surfaces, on the adsorption. Our results show a significant increase in the adsorption energies with the inclusion of vdW interactions, with the largest enhancement is obtained with the optimized exchange functionals, while a smaller enhancement is found using the rPW86-vdW2 functional, in agreement with the trends obtained in our earlier observations for other molecular adsorption on similar surfaces. The adsorption of olympicene radical on Au(111) leads to negligible change in the geometric structures of olympicene and the surface layer, while adsorption on Pt(111) reveals significant structural changes. This observation suggests a strong interaction with the Pt(111) surface, while the interaction with the Au(111) surface is rather weak. While the bonding of olympicene on Pt(111) is governed by a combination of the covalent bonding and the vdW forces, on Au(111), the bonding is mostly governed by the vdW forces. Upon adsorption on both surfaces, no interface state is formed, while a charge transfer of ∼0.6 e- is obtained. We also find the surface work function to be reduced upon adsorption on both surfaces, with the largest change observed on Pt(111) surface, over 1 eV. On the basis of the results obtained using the exchange functionals, we conclude that on Au(111), there is a transition in the nature of bonding from weak physisorption to strong physisorption, while the nature of bonding on Pt(111), is chemisorption, and it is enhanced with the inclusion of vdWs interactions. This is different from the adsorption on Cu(111) surface, for which we observed a transition from weak physisorption to strong chemisorption upon inclusion of vdW forces

Benzene Adsorption on (110) Surfaces of Transition Metals: Role of Van Der Waals Interaction and Substrate

The characteristics of Benzene adsorption on metals and their alloy surfaces, and the interface features have been the subject of many experimental and theoretical studies. With the availability of the new vdW functionals, we revisit this organic molecule/metal system to assess the influence of vdW interactions on the adsorption as well as to examine the performance of these vdW functionals. We will present the adsorption geometries, adsorption energies and heights, the characteristics of interface electronic structure, and the charge transfer for Benzene adsorption on the (110) surfaces of seven transition metals; Au, Ag, Cu, Pd, Pt, Rh, and Ni. The calculations are carried out using PBE and vdW-DF family functionals implemented in the VASP package. We will provide comparisons with the available experimental and theoretical studies on the adsorption geometries and energies, and the effect introduced by varying surface chemistries. We will also provide comparisons with the recent study for Benzene adsorption on the (111) surfaces of the same metal substrates [1]. [1] H. Yildirim, T. Greber, and A. Kara J. Phys. Chem. C 2013, 117, 20572. This work is funded by the U.S. Department of Energy Basic Energy Science under Contract No DE-FG02-11ER16243</jats:p

Adsorption Characteristics of Thiophene on Cu and Ni(100): Role of Van Der Waals

We apply density functional theory, with and without the inclusion of self-consistent van der Waals (vdWs) interactions (optB86, optB88, optPBE, revPBE, rPW86), to study the adsorption of thiophene (C4H4S) on Cu(100) and Ni(100). Our calculations reveal that the C4H4S molecule adsorbs, on either substrate, with its molecular plane parallel to the surface with the sulfur close to the bridge site. The inclusion of vdWs interactions results in a significant increase in the binding energy of thiophene on Cu(100) (from 0.12 eV to up to 0.77 eV), while the adsorption height is also modified from 3.2 A down to, at most, 2.38 A, depending on the functional used. The Ni(100) case presents a similar behavior for the binding energy (enhancement from 1.56 eV to up to 2.34 eV), but the adsorption heights increase from 2.12 Å up to 2.32 Å. In addition to adsorption geometry and energetics, we present the results and analysis of the electronic properties (charge transfer, changes in the d-band of the substrate, and change in the work function) of these two systems to complement our understanding of the molecule-substrate bonding. Our results suggest that the adsorption characteristics are dependent on the type of functional used; opt-type functionals (optB86, optB88, optPBE) are found to produce stronger bonding as compared to PBE, revPBE and rPW86. This work is funded by the U.S. Department of Energy Basic Energy Science under Contract No DE-FG02-11ER16243</jats:p