Search

First-principles calculations based on a truncated Coulomb hybrid functional were used to elucidate Y 2 O 3 doping of HfO 2. We calculated the formation enthalpies as well as density of states of nearly 1200 defective structures of two phases of HfO 2 : room-temperature monoclinic and high-temperature cubic structures. For dilute doping, the monoclinic phase is retained and electron trapping states are introduced near the conduction band minimum. For doping concentrations near to 12.5 at. %, the cubic phase is stabilized and the gap is free from charge trapping defect states, making it a suitable high-dielectric constant material for complementary metal-oxide semiconductor applications. [ABSTRACT FROM AUTHOR]

The authors highlight the importance of transient configurations of atoms on the surface of nanocrystallites, and present methodologies for their investigation. A Monte Carlo method has been developed and is used to simulate the thermodynamic equilibrium of nanometer sized Au nanocrystallites, both free and supported on a MgO(100) surface. The authors find that appreciable numbers of atoms transiently occupy adatom positions on Au(111) facets, even at room temperature. This type of dynamically appearing site is usually neglected in relation to catalysis but may have a significant activity (for CO oxidation, for example). They also observe a complex solid-solid roughening transition which involves a variety of transient local atom configurations on the surface of nanocrystallites. [ABSTRACT FROM AUTHOR]

We present a first principles based investigation into the nucleation and growth of metal-rich precipitates in substoichiometric hafnium dioxide with relevance to applications in resistive switching memories. We identify an optimal HfOx stoichiometry, with x in the range 1.50–1.75, for efficient nucleation and growth of stable Hf-rich clusters which may serve as precursors for the growth of conductive filaments during forming. We also show that filaments with sub-nm diameter posses remarkably metallic character. These results provide invaluable atomistic insight into forming in resistive switching films and demonstrate that precise regulation of stoichiometry is essential in order to ensure uniformity in the nucleation and subsequent growth of a conductive filament during the forming process. [ABSTRACT FROM AUTHOR]

A quantum equation of motion method is applied to simulate conduction electron spin-relaxation and transport in the presence of the spin-orbit interaction and disorder. A spin-relaxation time of 25ps is calculated for Cu with a realistic low temperature resistivity of 3.2 μΩ cm – corresponding to a spin-diffusion length of about 0.4 μm. Spin-relaxation in a finite nanocrystallite of Cu is also simulated and a short spin-relaxation time (0.47 ps) is calculated for a crystallite with 7% surface atoms. The spin-relaxation calculated for bulk Cu is in good agreement with experimental evidence, and the dramatic nanocrystallite effect observed has important implications for nano-spintronic devices. [ABSTRACT FROM AUTHOR]

Bournonite (CuPbSbS3) and enargite (Cu3AsS4) have recently been used as absorber layers in thin-film photovoltaic devices due to their ideal bandgap and ferroelectric properties. An understanding of the ferroelectric domain structure in these materials is required so that the benefits of the internal depolarizing electric fields can be fully exploited. Here, the atomic structure and electronic properties of domain walls (DWs) are elucidated through a combined aberration-corrected scanning transmission electron microscopy and density functional theory study. ∼90° and 180° DWs are observed in bournonite. As the 180° DW is charge neutral, it cannot contribute to the anomalous photovoltaic effect that leads to high open circuit voltages. The ∼90° DW shows a slight offset across the boundary, but the contributions of this to the anomalous photovoltaic effect are negligible. The DWs are also electrically passive, i.e., they do not result in significant recombination and do not block charge carrier transport. A high density of stacking faults (SF) was, however, observed in enargite. The SFs have a large number of defect states within the bandgap, which would lower the device efficiency through Shockley–Read–Hall recombination. [ABSTRACT FROM AUTHOR]

Polycrystalline materials are ubiquitous in technology, and grain boundaries have long been known to affect materials properties and performance. First principles materials modeling and electron microscopy methods are powerful and highly complementary for investigating the atomic scale structure and properties of grain boundaries. In this review, we provide an introduction to key concepts and approaches for investigating grain boundaries using these methods. We also provide a number of case studies providing examples of their application to understand the impact of grain boundaries for a range of energy materials. Most of the materials presented are of interest for photovoltaic and photoelectrochemical applications and so we include a more in depth discussion of how modeling and electron microscopy can be employed to understand the impact of grain boundaries on the behavior of photoexcited electrons and holes (including carrier transport and recombination). However, we also include discussion of materials relevant to rechargeable batteries as another important class of materials for energy applications. We conclude the review with a discussion of outstanding challenges in the field and the exciting prospects for progress in the coming years. [ABSTRACT FROM AUTHOR]

We analyze the mechanism of laser-assisted field evaporation from insulators triggered by photoinduced hole accumulation at the tip apex. The effects of hole accumulation on the electric flux line and the activation barrier for field evaporation are analyzed by the boundary element method with a mesoscopic model and atomistic ab initio calculations. The ab initio calculations indicate that a certain amount of hole accumulation reduces the activation barrier for field evaporation substantially. We propose the following scenario for laser-assisted field evaporation from insulators: 1) a laser pulse creates holes in the valence band of oxide through electronic excitation, 2) the holes accumulate at the tip apex due to the external direct-current electric field, 3) the positive charge at the apex reduces the activation barrier for cation evaporation, 4) the cation evaporates from the apex, and 5) the next laser pulse creates holes again and triggers subsequent field evaporations. [ABSTRACT FROM AUTHOR]