Computational simulation of the excited states dynamics of azobenzene in solution

Azobenzene and its derivatives are molecules very often used to construct photomodulable materials and molecular devices. The main characteristic of this kind of molecules is the efficient and reversible trans → cis photoisomerization, that occurs in either sense, without secondary processes. Using the appropriate wavelength, one can convert either isomer into the other one. The photoisomerization mechanism of azobenzene has been debated, during the last decades, because of the peculiar wavelength dependence of the quantum yields and because at least two standard possibilities exist: N=N double bond torsion and N inversion. Our research group has performed simulations of the photodynamics of azobenzene molecule by mixed quantum-classical methods. Such simulations have been successful in explaining the dependence of the quantum yield on the excitation wavelength. However, these simulations have been conducted on the isolated azobenzene molecule, while almost all the experimental data have been obtained in condensed phase. In particular, Diau's group, from Taiwan, has shown a strong dependence of the excited states dynamics on the solvent viscosity. The general aim of this work is to study the excited state dynamics of azobenzene in solution, in order to obtain its transient spectra and to produce data directly comparable with the experiments. In particular, we have studied the quantum yields, the isomerization mechanism and the reorientation of the transition dipole moment during the excited state relaxation, in order to understand the time resolved fluorescence anisotropy measurements obtained by Diau and collaborators. This research will also permit to study the reorientation of the whole molecule, which leads to alignment of an azobenzene sample in a polarized laser field. A basic issue for the interpretation of the fluorescence anisotropy and of the orientation of azobenzene samples in polarized light is related with the direction of the transition dipole vector for the forbidden n-π* transition of trans-azobenzene. Therefore, we have carried out a preliminary ab initio study of the n-π* transition dipole moment, considering the vibrational motions that contribute to the oscillator strength, and focusing on the most effective ones, i.e. those of lowest frequency. The most effective coordinate in promoting this transition is the symmetric torsion of the phenyl groups. Other important coordinates are the antisymmetric phenyl torsion and the torsion of the N=N double bond. The transition dipole vector turns out to lie essentially in the molecular plane, almost parallel to the N-C bonds and to the longest axis of the molecule. Semiempirical calculations are in sufficiently good agreement with those obtained by ab initio methods. The main part of the thesis work has been devoted to the simulation of the dynamics of the photoisomerization process of azobenzene in solution. We have made use of a mixed quantum-classical method of the surface hopping family. The electronic energies and wavefunctions are computed on the fly, by a semiempirical method modified by our group. A reparameterization of the semiempirical AM1 Hamiltonian has been carried out, considering new ab initio results used as reference values, in order to improve the accuracy of the semiempirical PES. The solvent effects have been introduced in a preliminary way by brownian dynamics, simulating two different solvent viscosities, and then explicitly, with a QM/MM approach. In this approach, the solvent itself is represented by a Molecular Mechanics force-field (OPLS) and the QM/MM interactions are made of electrostatic and Lennard-Jones terms. We have first determined, by ab initio calculations, the solute-solvent interaction potential between azobenzene and two simple molecules, methane and methanol (representatives of non-polar and of protic compounds). In this way we have obtained the necessary QM/MM interaction parameters, and we have run simulations with two solvents used in the experiments, methanol and ethylene glycol (simulations with n-hexane are in progress). We obtain very good results for the dependence of the quantum yields on the solvent viscosity, and in this way we can confirm that the photoisomerization mechanism is dominated by the torsion of the N=N double bond. The simulations also provide the necessary information to compute the time-resolved fluorescence spectra and anisotropy, i.e. for a complete reproduction of the experimental results. We have obtained a good agreement with the measured time-dependent intensities and anisotropies, but our explanation of the mechanism partly differs from that put forward in the experimental work

Growth-Induced Strain in Chemical Vapor Deposited Monolayer MoS2: Experimental and Theoretical Investigation

Monolayer molybdenum disulphide (MoS$_2$) is a promising two-dimensional (2D) material for nanoelectronic and optoelectronic applications. The large-area growth of MoS$_2$ has been demonstrated using chemical vapor deposition (CVD) in a wide range of deposition temperatures from 600 {\deg}C to 1000 {\deg}C. However, a direct comparison of growth parameters and resulting material properties has not been made so far. Here, we present a systematic experimental and theoretical investigation of optical properties of monolayer MoS$_2$ grown at different temperatures. Micro-Raman and photoluminescence (PL) studies reveal observable inhomogeneities in optical properties of the as-grown single crystalline grains of MoS$_2$. Close examination of the Raman and PL features clearly indicate that growth-induced strain is the main source of distinct optical properties. We carry out density functional theory calculations to describe the interaction of growing MoS$_2$ layers with the growth substrate as the origin of strain. Our work explains the variation of band gap energies of CVD-grown monolayer MoS$_2$, extracted using PL spectroscopy, as a function of deposition temperature. The methodology has general applicability to model and predict the influence of growth conditions on strain in 2D materials.Comment: 37 pages, 6 figures, 10 figures in supporting informatio

Ultra Low Specific Contact Resistivity in Metal-Graphene Junctions via Atomic Orbital Engineering

A systematic investigation of graphene edge contacts is provided. Intentionally patterning monolayer graphene at the contact region creates well-defined edge contacts that lead to a 67% enhancement in current injection from a gold contact. Specific contact resistivity is reduced from 1372 {\Omega}m for a device with surface contacts to 456 {\Omega}m when contacts are patterned with holes. Electrostatic doping of the graphene further reduces contact resistivity from 519 {\Omega}m to 45 {\Omega}m, a substantial decrease of 91%. The experimental results are supported and understood via a multi-scale numerical model, based on density-functional-theory calculations and transport simulations. The data is analyzed with regards to the edge perimeter and hole-to-graphene ratio, which provides insights into optimized contact geometries. The current work thus indicates a reliable and reproducible approach for fabricating low resistance contacts in graphene devices. We provide a simple guideline for contact design that can be exploited to guide graphene and 2D material contact engineering.Comment: 26 page

Simulation of contact resistance in patterned graphene

While trying to exploit graphene in Radio Frequency applications, the reduction of the contact resistance (Rc) is probably one of the most challenging technological issues to be solved. Graphene patterning under the metal has been demonstrated to be a promising solution, leading to a reduction of Rc by up to a factor of 20, probably due to an increased conductivity at the borders of the patterns of graphene. This technology is still at the early stage and a complete understanding of the physical mechanisms at play is lacking. To this purpose we propose a multi- scale approach based on first-principle calculations, and the solution of the continuity equation to compute Rc in the considered patterned contacts

Vertical Heterostructures between Transition-Metal Dichalcogenides -- A Theoretical Analysis of the NbS2_2/WSe2_2 junction

Low-dimensional metal-semiconductor vertical heterostructures (VH) are promising candidates in the search of electronic devices at the extreme limits of miniaturization. Within this line of research, here we present a theoretical/computational study of the NbS$_2$/WSe$_2$ metal-semiconductor vertical hetero-junction using density functional theory (DFT) and conductance simulations. We first construct atomistic models of the NbS$_2$/WSe$_2$ VH considering all the five possible stacking orientations at the interface, and we conduct DFT and quantum-mechanical (QM) scattering simulations to obtain information on band structure and transmission coefficients. We then carry out an analysis of the QM results in terms of electrostatic potential, fragment decomposition, and band alignment. The behavior of transmission expected from this analysis is in excellent agreement with, and thus fully rationalizes, the DFT results, and the peculiar double-peak profile of transmission. Finally, we use maximally localized Wannier functions, projected density of states (PDOS), and a simple analytic formula to predict and explain quantitatively the differences in transport in the case of epitaxial misorientation. Within the class of Transition-Metal Dichalcogenide systems, the NbS$_2$/WSe$_2$ vertical heterostructure exhibits a wide interval of finite transmission and a double-peak profile, features that could be exploited in applications.Comment: 22 pages main text, 11 pages supplementar

Almeida garrett e o Romantismo em Portugal Almeida Garrett, Fra' Luís de Sousa - traduzione

traduzione a sei mani di un dramma teatrale di Garrett

Almeida garrett e o Romantismo em Portugal Almeida Garrett, Fra' Luís de Sousa - traduzione

traduzione a sei mani di un dramma teatrale di Garrett

Trajectory integration with potential energy discontinuities

Many approximate methods of quantum chemistry yield potential energy surfaces with discontinuities. While clearly unphysical, such features often fall within the typical error bounds of the method, and cannot be easily eliminated. The integration of nuclear trajectories when the potential energy is locally discontinuous is obviously problematic. We propose a method to smooth out the discontinuities that are detected along a trajectory, based on the definition of a continuous function that fits locally the computed potential, and is used to integrate the trajectory across the discontinuity. With this correction, the energy conservation error can be reduced by about one order of magnitude, and a considerable improvement is obtained in the energy distribution among the internal coordinates