Influence of the Characteristics of the STM-tip on the Electroluminescence Spectra

We analyze the influence of the characteristics of the STM-tip (applied voltage, tip radius) on the electroluminescence spectra from an STM-tip-induced quantum dot taking into account the many-body effects. We find that positions of electroluminescence peaks, attributed to the electron-hole recombination in the quantum dot, are very sensitive to the shape and size of the confinement potential as determined by the tip radius and the applied voltage. A critical value of the tip radius is found, at which the luminescence peak positions as a function of the tip radius manifest a transition from decreasing behavior for smaller radii to increasing behavior for larger radii. We find that this critical value of the tip radius is related to the confinement in the lateral and normal direction.Comment: 15 pages, 5 figure

Licht uit nul-dimensionale objecten

No abstract

Licht uit nul-dimensionale objecten

No abstract

Mössbauer studies of trimethyl and triphenyl tin chloride adsorbed on grafoil

Trimethyl tin chloride [(CH3)3SnCl] and triphenyl tin chloride [(C6H5)3SnCl] adsorbed on grafoil were studied by the Mössbauer resonance in 119Sn. The temperature dependence of the Mössbauer fraction and the anisotropy ratio of the quadrupole transitions in (CH3)3SnCl measured at two different angles supplied information on the structure of the substrate and on the anisotropy of the recoilless fraction. The relative orientation of (C6H5)3SnCl adsorbed on grafoil was found to be with the Sn–Cl axis perpendicular to the exposed basal planes of grafoil, contrary to (CH3)3SnCl, where the molecules have their symmetry axes parallel to the grafoil plane

Experimentally Calibrated Kinetic Monte Carlo Model Reproduces Organic Solar Cell Current-Voltage Curve

Kinetic Monte Carlo (KMC) simulations are a powerful tool to study the dynamics of charge carriers in organic photovoltaics. However, the key characteristic of any photovoltaic device, its current-voltage ($J$-$V$) curve under solar illumination, has proven challenging to simulate using KMC. The main challenges arise from the presence of injecting contacts and the importance of charge recombination when the internal electric field is low, i.e., close to open-circuit conditions. In this work, an experimentally calibrated KMC model is presented that can fully predict the $J$-$V$ curve of a disordered organic solar cell. It is shown that it is crucial to make experimentally justified assumptions on the injection barriers, the blend morphology, and the kinetics of the charge transfer state involved in geminate and nongeminate recombination. All of these properties are independently calibrated using charge extraction, electron microscopy, and transient absorption measurements, respectively. Clear evidence is provided that the conclusions drawn from microscopic and transient KMC modeling are indeed relevant for real operating organic solar cell devices.Comment: final version; license update

Exchange interaction in p-type GaAs/Al_{x}Ga_{1-x}As heterostructures studied by magnetotransport

Low-temperature magnetotransport experiments have been performed on a p-type GaAs/AlxGa1-xAs quantum well. From activation measurements on Shubnikov–de Haas conduction minima it was found that exchange interactions can be of great importance for both odd and even filling factors and strongly influence the observed periodicity. Furthermore, it was found that the temperature dependence of Shubnikov–de Haas oscillations in the low-magnetic-field regime could not be explained within a single-particle model based on a solution of the full Luttinger Hamiltonian in a magnetic field. Numerical simulations of Shubnikov–de Haas spectra, based on a model that treats hole exchange interactions in a simplified manner, show unambiguously that exchange driven enhancement of hole "spin" splittings are extremely important at magnetic fields as low as 1.5 T. Also, the inclusion of a valence-band warping in the calculations is shown to be essential. Qualitatively, most experimental observations could be described within the presented model. Our results imply that, in any hole system, the effective masses obtained from temperature-dependent SdH measurements are to be treated with extreme care as they can deviate from their single-particle value by as much as a factor of 2

Full capacitance matrix of coupled quantum dot arrays: static and dynamical effects

We numerically calculated the full capacitance matrices for both one-dimensional (1D) and two-dimensional (2D) quantum-dot arrays. We found it is necessary to use the full capacitance matrix in modeling coupled quantum dot arrays due to weaker screening in these systems in comparison with arrays of normal metal tunnel junctions. The static soliton potential distributions in both 1D and 2D arrays are well approximated by the unscreened (1/r) coulomb potential, instead of the exponential fall-off expected from the often used nearest neighbor approximation. The Coulomb potential approximation also provides a simple expression for the full inverse capacitance matrix of uniform quantum dot arrays. In terms of dynamics, we compare the current-voltage (I-V) characteristics of voltage biased 1D arrays using either the full capacitance matrix or its nearest neighbor approximation. The I-V curves show clear differences and the differences become more pronounced when larger arrays are considered.Comment: 8 pages preprint format, 3 PostScript figure

Contested water rights in post-apartheid South Africa: The struggle for water at catchment level

The National Water Act (1998) of South Africa provides strong tools to redress inequities inherited from the past. However, a decade after the introduction of the Act, access to water is still skewed along racial lines. This paper analyses the various ways in which the Water Act is contested, based on empirical data detailing the interactions between smallholder farmers and commercial farmers in a case-study catchment in KwaZulu-Natal Province. The paper argues that the legacy of the apartheid era still dominates the current political and economical reality and shows how the redistribution of water resources is contested by the elite. The paper identifies several issues that prevent the smallholder farmers from claiming their rights, including the institutional arrangements in former homelands, the ‘community approach’ of Government and NGOs, the disconnect between land and water reform processes, and historically-entrenched forms of behaviour of the various actors. The paper concludes that the difficulties encountered in the water reform process are illustrative for what is happening in the society at large and raises the question as to what price is being paid to maintain the current status quo in the division of wealth?Keywords: natural resources, water reform, water rights, smallholder farmers, legal pluralism, South Afric

Dimensionality of charge transport in organic field-effect transistors

Application of a gate bias to an organic field-effect transistor leads to accumulation of charges in the organic semiconductor within a thin region near the gate dielectric. An important question is whether the charge transport in this region can be considered two-dimensional, or whether the possibility of charge motion in the third dimension, perpendicular to the accumulation layer, plays a crucial role. In order to answer this question we have performed Monte Carlo simulations of charge transport in organic field-effect transistor structures with varying thickness of the organic layer, taking into account all effects of energetic disorder and Coulomb interactions. We show that with increasing thickness of the semiconductor layer the source-drain current monotonically increases for weak disorder, whereas for strong disorder the current first increases and then decreases. Similarly, for a fixed layer thickness the mobility may either increase or decrease with increasing gate bias. We explain these results by the enhanced effect of state filling on the current for strong disorder, which competes with the effects of Coulomb interactions and charge motion in the third dimension. Our conclusion is that apart from the situation of a single monolayer, charge transport in an organic semiconductor layer should be considered three-dimensional, even at high gate bias