1,721,031 research outputs found
Ab initio optical absorption in conjugated polymers: the role of dimensionality,
No full textThe optical behavior of poly(para-phenylene vinylene) is explored through an ab initio scheme based on the Bethe-Salpeter equation, where the electron-hole interaction is included on top of a density functional theory calculation. Results for different solid-state packings are reviewed, demonstrating that the details of crystalline arrangement dramatically alter the optical properties and lead to a rich excitonic structure, where also charge-transfer states appear (electron and hole on different chains). Moreover, for a typical herringbone packing the excitonic state of the isolated molecule splits in two direct components (with electron and hole on the same chain), one for each non-translationally-invariant chain in the unit cell, and the optical inactivity of the lowest component can crucially quench the luminescence efficiency. Besides the far-field absorption spectra and the description of the excitonic states, a formalism to simulate the near-field spectra is presented that allows one to detect also excitonic states that are dipole-forbidden in the far-field spectra
Transparent Conductive Oxides as Near-IR Plasmonic Materials: The Case of Al-Doped ZnO Derivatives
No full textUsing first-principles calculations, we investigate the origin of near infrared plasmonic activity in Al:ZnO transparent conducting oxides. Our results predict realistic values for the plasma frequency and the free electron density as a function of the Al doping and in agreement with recent experimental results. We also provide a microscopic insight on the formation of surface-plasmon polaritons at the Al:ZnO/ZnO interfaces in terms of characteristic lengths that can be measured by experiments. The direct comparison with standard plasmonic metals underlines the promising capabilities of transparent conducting oxides as compact and low-loss plasmonics materials for optoelectronic applications and telecommunications
Optoelectronic properties and color chemistry of native point defects in Al:ZnO transparent conductive oxide
Full text linkWe present a first principles study on the effect of native point defects in Al:ZnO transparent conductive oxide. Our results indicate that oxygen and zinc vacancies play two completely different roles:the former maintain the electrical properties while worsening the transparency of native Al:ZnO. The latter are strong electron acceptors that can destroy the metal-like conductivity of the system. While the percentage of doping amount is not really relevant, the compensation ratio between Zn vacancies and Al dopants is crucial for the final electrical properties of the system. H impurities always act as electron donors and generally improve the characteristics of the transparent conductor. Finally, we show how the chemistry of the defects affects the color of Al:ZnO samples, in agreement with experimental results. Our results pave the way to defect engineering for the growth of high performance transparent conductive oxides
Surface Effects on Catechol/Semiconductor Interfaces
No full textWe present a density functional investigation of the surface effects on the catechol sensitization of selected hexagonal semiconductors (SiC, GaN, InN, CdS, CdSe). The atomic relaxation, the ionicity, and the reactivity, which characterize the selected substrates, are found to crucially influence both the bonding geometry and the electronic level alignment at the interface. Our results indicate that surface effects must be considered in order to obtain a correct picture of the optoelectronic response of the system. Our findings pave the way to the fundamental understanding and future design of hybrid catecholate materials for optoelectronic and biomedical applications
Dynamics of electrons in a 2D region coming from a point-contact
No full textBallistic and quasi-ballistic transport in mesoscopic systems is, nowadays, a fundamental tool for the investigation of electronic processes in semiconductors. In this work we present some results concerning a numerical simulation of electrons entering a 2D mesoscopic region from a point contact; a magnetic field is applied perpendicular to the structure and influences the electron dynamics. The simulation is performed through a numerical solution of the Schroedinger equation in a finite-difference scheme. It includes a magnetic field and an arbitrary potential V(r). In this way, the quantum effects of impurities on the conductance of the system have been analysed. The resul shows that each impurity configuration characterizes, in a particular way, the transport properties
Surface nano-patterning through styrene adsorption on Si(100)
No full textWe present an ab initio study of the electronic properties of styrene molecules adsorbed on the dimerized Si(100) surface, ranging from the single molecule to the full monolayer (ML). The adsorption mechanism primarily involves the vinyl group via a [2+2] cycloaddition process that leads to the formation of covalent SiC bonds and a local surface derelaxation, while it leaves the phenyl group almost unperturbed. The investigation of the functionalized surface as a function of the coverage (e.g., 0.5–1 ML) and of the substrate reconstruction reveals two major effects. The first results from Si dimer-vinyl interaction and concerns the controlled variation of the energy band gap of the interface. The second is associated to phenyl-phenyl interactions, which give rise to a regular pattern of electronic wires at surface, stemming from the π-π coupling. These findings suggest a rationale for tailoring the nanopatterning of the surface in a controlled way
Ab initio exploration of rearrangement reactions: Intramolecular hydrogen scrambling processes in acetone
No full textThe recently developed metadynamics method is applied to the intramolecular hydrogen migration reactions of acetone in the gas phase. Comparison of different sets of collective coordinates allows efficient description of the underlying free energy surface. The simulations yielded numerous reactions: the enol-oxo tautomerism, the decomposition of acetone to various products, and rearrangement reactions. On the basis of the calculated activation barriers it is concluded that the enol-oxo tautomerism is the most frequent intramolecular proton-exchange process the acetone undergoes in the gas phase.The recently developed metadynamics method is applied to the intramolecular hydrogen migration reactions of acetone in the gas phase. Comparison of different sets of collective coordinates allows efficient description of the underlying free energy surface. The simulations yielded numerous reactions: the enol-oxo tautomerism, the decomposition of acetone to various products, and rearrangement reactions. On the basis of the calculated activation barriers it is concluded that the enol-oxo tautomerism is the most frequent intramolecular proton-exchange process the acetone undergoes in the gas phase. © 2006 American Chemical Society
Tailoring the electronic properties of silicon with cysteine: A first principle study
Full text linkWe discuss the electronic structure modifications induced on the dihydride-terminated Si(001) surface upon cysteine adsorption by means of ab initio calculations: several stable functionalization schemes are presented, providing different routes for biological recognition, surface nanostructuring, and biomolecular electronics applications. The resulting hybrid systems are discussed and compared in terms of stability, structural, and electronic properties. Based on our results, we propose STM and photoemission experiments to determine unambiguously the adsorption mechanism involved and the attached functional group
Ab Initio Simulation of Optical Limiting: The Case of Metal-Free Phthalocyanine
Full text linkWe present a fully ab initio, nonperturbative description of the optical limiting properties of a metal-free phthalocyanine by simulating the effects of a broadband electric field of increasing intensity. The results confirm reverse saturable absorption as the leading mechanism for optical limiting phenomena in this system and reveal that a number of dipole-forbidden excitations are populated by excited-state absorption at more intense external fields. The excellent agreement with the experimental data supports our approach as a powerful tool to predict optical limiting in view of applications
- …
