M.R. Beltrán

Chirality in Bare and Passivated Gold Nanoclusters

Detailed knowledge of the lattice structure, shape, morphology, surface structure, and bonding of bare and passivated gold clusters is fundamental to predict and understand their electronic, optical, and other physical and chemical properties. This information is essential to optimizing their utilization as novel nanocatalysts, 1 and as building-blocks of new molecular nanostructured materials, 2 with potential applications in nanoelectronics 3 and biological diagnostics. 4 An effective theoretical approach to determine gold cluster structures is to combine genetic algorithms and many-body potentials ~to perform global structural optimizations!, and first-principles density functional theory ~to confirm the energy ordering of the local minima!. Using this procedure we recently found many topologically interesting disordered gold nanoclusters with energy near or below the lowestenergy ordered isomer. 5‐ 8 The structures of these clusters

Hybrid DNA-gold nanostructured materials: an ab initio approach

The controlled assembly of metal nanoparticles into macroscopic materials using DNA oligonucleotides has opened new directions of research in nanoscience and nanotechnology. Here, we describe recent ab initio calculations on structural and electronic properties of the subsystems forming these materials: bare and thiol-passivated gold nanoclusters, gold nanowires and fragments of DNA chains. Our results indicate that gold nanoclusters are distorted dramatically by a passivating methylthiol monolayer, that monatomic gold chains are stable in zigzag geometries and that dry acidic λ-DNA is a good insulator. These results provide useful insights towards the complete understanding, design and proper utilization of hybrid DNA-gold nanostructured materials.

Tight-binding description of disordered nanostructures : An application to porous silicon

Abstract We present the calculations of the coefficient of light (photo) absorption in porous silicon (por-Si) using the supercell tight-binding sp3s* model, in which the pores are columns digged in crystalline silicon. The disorder in the pore sizes and the undulation of the silicon wires are taken into account by considering nonvertical interband transitions. The results obtained for 8- and 32-atom supercells show a strong dependence on the pore morphology, i.e., the absorption coefficient changes with the shape and size of the silicon wires even at constant porosity. The absorption spectrum of this model for por-Si is defined by the interplay between the decrease in the indirectness of the material (connected to the absorption processes assisted by the scattering on the pores), which effectively reduces the direct gap, and the increase of the gap due to the quantum confinement.

Isomers of adenine

Abstract Adenine is one of the four principle bases of nucleic acid, the essential molecule of life and evolution. Apparently, only one configuration of adenine exists in nature giving it unique chemical and biological properties. Using a force field type potential model with parameters fitted to the nucleic acid bases, proteins and other biological molecules (involving the sum of the contributions from bond stretching, bond angle bending, torsional angle twisting, and Coulomb and Lennard–Jones terms) we searched the potential energy surface for other stable isomers of adenine. The search was performed using a genetic algorithm, an efficient and global technique. The most interesting of the lowest energy minima found in the global search were relaxed using quantum-mechanical, semiempirical (PM3), and first principles (Hartree–Fock and density functional theory) methods. These calculations gave similar geometries and energy ordering for the new structures. This work formed part of a larger project to study the binding of nanoclusters of gold to segments of nucleic acid for possible application in the emerging field of nano-electronics. The results could also have implications in mutation and transcription of DNA.

Quasi-confinement, localization and optical properties in porous silicon

Abstract The quasi-confinement concept, where electrons can find ways out through the necks between the pores, is discussed and its consequences in the localization and optical properties of porous silicon are analyzed. The polarized light absorption is studied by observing the oscillator strength behaviour. The localization is quantified using the inverse participation ratio (IPR), which gives the number of sites occupied by the wave function. The pore structure is simulated by a supercell model, where a tight-binding Hamiltonian with an sp 3 s * basis is used and empty columns of atoms are produced in an otherwise perfect silicon structure. These columns are passivated with hydrogen atoms. The results show that the bandgap broadens and the conduction band minimum shifts towards the gamma point, as the porosity increases. Likewise, the oscillator strength analysis reveals a significant enlargement of the optically active zone in the k-space, due to the localization of the wavefunction, which has been analyzed by looking at the IPR.

Analysis of the interband transitions in porous silicon

Abstract Porous silicon (PS) presents interesting phenomena such as efficient luminescence and a peculiar transport of carriers. Due to its possible optoelectronic applications, it is important to calculate the dielectric function from interband optical transitions in PS to include quantum effects. In this work, we apply a supercell model for PS within an sp 3 s * tight-binding technique, to analyze the effects of pores on the above-mentioned transitions. The polarized light absorption is studied by observing the oscillator strength behavior within two different schemes, which are applied and compared. We have found a significant enlargement of the optically active zone in the k -space, due to the localization of the wave function. The calculated dielectric functions for crystalline silicon and PS are compared with experimental results, giving the correct energy range and shape.

Optical Absorption in Porous Silicon

The optical properties of porous silicon (p-Si) are calculated from the electronic band structure obtained by means of an sp3s* tight-binding Hamiltonian and a supercell model, in which the pores are columns detched in crystalline silicon (c-Si). The disorder in the pore sizes and the undulation of the silicon wires are considered by the existence of arandom perturbative potential, which produces non-vertical interband transitions, otherwise forbidden. A typical interval around each k-vector (optical window), where non-vertical transitions make an important contribution, depends on the value of the disorder and its order of magnitude is given by l−1, where l is the localization length. The calculated absorption spectra are compared with experiments, showing good agreement.

EFFICIENT NON-VERTICAL INTERBAND TRANSITIONS IN POROUS SILICON

In this work, we study an inhomogeneous material, porous silicon (PS), using a supercell model and an s p3 s∗ tight-binding Hamiltonian. The interband non-vertical transitions are studied in two schemes, which consider different contributions within the intra-atomic approximation. The oscillator strength analysis as a function of the porosity reveals a significant enlargement of the optically active zone in the k-space, due to the localization of the wave function.

Computation of the Porous Silicon Dielectric Function in the Supercell Model and Comparison with Experiment

We present the results for the imaginary part of the dielectric function of porous silicon, which were obtained with the tight-binding 128–atom supercell model for different porosities. The supercells have been chosen to allow the interconnection of the Si skeleton. We have analyzed also the effects of pore morphology. We have found that, at a fixed porosity, the developing of the surface, resulting in the increase of saturating hydrogen atoms, leads to a noticeable blueshift of the absorption edge.

Theoretical Aspects of Porous Silicon

In this work, we study the electronic and optical properties of porous silicon (PS) using a supercell model, where an sp 3 s* tight-binding Hamiltonian is used and empty columns of atoms are produced in an otherwise perfect silicon structure, passivated with hydrogen atoms. As it is considered that quantum confinement is one of the causes of the optoelectronic properties of PS, we perform a detailed analysis of the consequences of confinement on its band structure. Our results show that the band gap broadens and the minimum of the conduction band shifts towards the gamma point as the porosity is increased. The polarized light absorption study shows that the optically active zone in the reciprocal space broadens significantly due to disorder, relaxing the k-wavevector selection rule. We found that introducing non-vertical interband transitions to take into account the PS disordered nature, we get a very good agreement with experimental data.