Excitation energies and radiative lifetimes of Ge1-xSix nanocrystals: alloying versus confinement effects

Photochemical Ring Opening of Oxirane Modeled by Constrained Density Functional Theory

A constrained density functional theory/classical trajectory surface hopping study of the photochemical dissociation of oxirane (CH₂)₂O is presented. The calculations confirm the Gomer-Noyes mechanism for the initial reaction and agree largely with experimental photolysis data including reaction yields. The calculated yields, however, depend both on temperature and its modeling. The timescales of the various reaction steps are well below 100 fs, similar to previous time-dependent density functional calculations. At variance with those, however, the present calculations obey Kasha's rule, i.e., the photoreaction is initiated in the energetically lowest excited state.

Tuning Hydrogenated Silicon, Germanium, and SiGe Nanocluster Properties Using Theoretical Calculations and a Machine Learning Approach

There are limited studies available that predict the properties of hydrogenated silicon-germanium (SiGe) clusters. For this purpose, we conducted a computational study of 46 hydrogenated SiGe clusters (Si _xGe _yH _z, 1 < X + Y ≤ 6) to predict the structural, thermochemical, and electronic properties. The optimized geometries of the Si _xGe _yH _z clusters were investigated using quantum chemical calculations and statistical thermodynamics. The clusters contained 6 to 9 fused Si-Si, Ge-Ge, or Si-Ge bonds, i.e., bonds participating in more than one 3- to 4-membered rings, and different degrees of hydrogenation, i.e., the ratio of hydrogen to Si/Ge atoms varied depending on cluster size and degree of multifunctionality. Our studies have established trends in standard enthalpy of formation, standard entropy, and constant pressure heat capacity as a function of cluster composition and structure. A novel bond additivity correction model for SiGe chemistry was regressed from experimental data on seven acyclic Si/Ge/SiGe species to improve the accuracy of the standard enthalpy of formation predictions. Electronic properties were investigated by analysis of the HOMO-LUMO energy gap to study the effect of elemental composition on the electronic stability of Si _xGe _yH _z clusters. These properties will be discussed in the context of tailored nanomaterials design and generalized using a machine learning approach.

Amorphous Ge quantum dots embedded in crystalline Si: ab initio results

We study amorphous Ge quantum dots embedded in a crystalline Si matrix through structure modeling and simulation using ab initio density functional theory including spin-orbit interaction and quasiparticle effects. Three models are generated by replacing a spherical region within diamond Si by Ge atoms and creating a disordered bond network with appropriate density inside the Ge quantum dot. After total-energy optimisations of the atomic geometry we compute the electronic and optical properties. We find three major effects: (i) the resulting nanostructures adopt a type-I heterostructure character; (ii) the lowest optical transitions occur only within the Ge quantum dots, and do not involve or cross the Ge-Si interface. (iii) for larger amorphous Ge quantum dots, with diameters of about 2.0 and 2.7 nm, absorption peaks appear in the mid-infrared spectral region. These are promising candidates for intense luminescence at photon energies below the gap energy of bulk Ge.

Temperature dependence of the radiative lifetimes in Ge and Si nanocrystals

The effect of finite temperature on the optical properties of nanostructures has been a longstanding problem for their theoretical description and its omission presents serious limits on the validity of calculated spectra and radiative lifetimes. Most ab initio calculations have been carried out neglecting temperature effects altogether, although progress has been made recently. In the present work, the temperature dependence of the intrinsic radiative lifetimes of excited electron-hole pairs in Ge and Si nanocrystals due to classical temperature effects is calculated using ab initio molecular dynamics. Fully hydrogen-saturated Ge and Si nanocrystals without surface reconstructions show opposite behavior: the very short lifetimes in Ge increase with temperature, while the much longer ones in Si decrease. However, the temperature effect is found to be strongly dependent on the surface structure: surface reconstructions cause partial localization of the wave functions and override the difference between Si and Ge. As a consequence, the temperature dependence in reconstructed nanocrystals is strongly attenuated compared to the fully saturated nanocrystals. Our calculations are an important step towards predictive modeling of the optical properties of nanostructures.

Vegard's law-like behavior for Mn(m)Tc(n) alloy clusters: a first-principles prediction

With a view to gaining an understanding of the alloying tendency of bimetallic nanoalloy clusters of isoelectronic constituents, we studied the structural and mixing behavior of MnmTcn alloy clusters with m + n = 13 for all possible compositions, using first-principles electronic structure calculations. Our study reports a favorable mixing tendency for the alloy clusters. The average bond lengths of the minimum energy structures show an overall linear variation with concentration, indicating a Vegard's law-like variation for the nanoalloy clusters, though the optimized structures undergo a structural transition from a closed and compact structure for the Mn-rich alloy clusters to an open layered-like structure for the Tc-rich alloy clusters. We work out a continuous and smooth interplay between hybridization and magnetization properties of the alloy clusters, which plays a vital role in the Vegard's law-like variation in their average bond lengths.

Optical absorption and emission of α-Sn nanocrystals from first principles

We investigate the optical properties of hydrogenated α-Sn nanocrystals up to diameters of 3.6 nm in the framework of an ab initio pseudopotential method including spin-orbit interaction (SOI) and the repeated supercell approximation. The fundamental absorption and emission edges are determined including quasiparticle effects and electron-hole interaction. The atomic geometries in the ground and excited electronic states follow from total energy optimization. We discuss the oscillator strengths of the optical absorption near the fundamental gaps for the most important transitions. We demonstrate that the spectra can only be correctly described including SOI. The strongly size-dependent Stokes shifts between optical absorption and emission are shown to be mainly a consequence of the different atomic geometries.

Ground- and excited-state properties of DNA base molecules from plane-wave calculations using ultrasoft pseudopotentials

We present equilibrium geometries, vibrational modes, dipole moments, ionization energies, electron affinities, and optical absorption spectra of the DNA base molecules adenine, thymine, guanine, and cytosine calculated from first principles. The comparison of our results with experimental data and results obtained by using quantum chemistry methods show that in specific cases gradient-corrected density-functional theory (DFT-GGA) calculations using ultrasoft pseudopotentials and a plane-wave basis may be a numerically efficient and accurate alternative to methods employing localized orbitals for the expansion of the electron wave functions.