Quijarroite, Cu6HgPb2Bi4Se12, a New Selenide from the El Dragόn Mine, Bolivia

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Tight-Binding model for semiconductor nanostructures

An empirical $s_cp^3_a$ tight-binding (TB) model is applied to the investigation of electronic states in semiconductor quantum dots. A basis set of three $p$-orbitals at the anions and one $s$-orbital at the cations is chosen. Matrix elements up to the second nearest neighbors and the spin-orbit coupling are included in our TB-model. The parametrization is chosen so that the effective masses, the spin-orbit-splitting and the gap energy of the bulk CdSe and ZnSe are reproduced. Within this reduced $s_cp_a^3$ TB-basis the valence (p-) bands are excellently reproduced and the conduction (s-) band is well reproduced close to the $\Gamma$-point, i.e. near to the band gap. In terms of this model much larger systems can be described than within a (more realistic) $sp^3s^*$-basis. The quantum dot is modelled by using the (bulk) TB-parameters for the particular material at those sites occupied by atoms of this material. Within this TB-model we study pyramidal-shaped CdSe quantum dots embedded in a ZnSe matrix and free spherical CdSe quantum dots (nanocrystals). Strain-effects are included by using an appropriate model strain field. Within the TB-model, the strain-effects can be artifically switched off to investigate the infuence of strain on the bound electronic states and, in particular, their spatial orientation. The theoretical results for spherical nanocrystals are compared with data from tunneling spectroscopy and optical experiments. Furthermore the influence of the spin-orbit coupling is investigated

High-field magnetoexcitons in unstrained GaAs/AlxGa1-xAs quantum dots

The magnetic field dependence of the excitonic states in unstrained GaAs/AlxGa1-xAs quantum dots is investigated theoretically and experimentally. The diamagnetic shift for the ground and the excited states are studied in magnetic fields of varying orientation. In the theoretical study, calculations are performed within the single band effective mass approximation, including band nonparabolicity, the full experimental three-dimensional dot shape and the electron-hole Coulomb interaction. These calculations are compared with the experimental results for both the ground and the excited states in fields up to 50 Tesla. Good agreement is found between theory and experiment

Effect of wetting layers on the strain and electronic structure of InAs self-assembled quantum dots

The effect of wetting layers on the strain and electronic structure of InAs self-assembled quantum dots grown on GaAs is investigated with an atomistic valence-force-field model and an empirical tight-binding model. By comparing a dot with and without a wetting layer, we find that the inclusion of the wetting layer weakens the strain inside the dot by only 1% relative change, while it reduces the energy gap between a confined electron and hole level by as much as 10%. The small change in the strain distribution indicates that strain relaxes only little through the thin wetting layer. The large reduction of the energy gap is attributed to the increase of the confining-potential width rather than the change of the potential height. First-order perturbation calculations or, alternatively, the addition of an InAs disk below the quantum dot confirm this conclusion. The effect of the wetting layer on the wave function is qualitatively different for the weakly confined electron state and the strongly confined hole state. The electron wave function shifts from the buffer to the wetting layer, while the hole shifts from the dot to the wetting layer.Comment: 14 pages, 3 figures, and 3 table

Influence of symmetry and Coulomb-correlation effects on the optical properties of nitride quantum dots

The electronic and optical properties of self-assembled InN/GaN quantum dots (QDs) are investigated by means of a tight-binding model combined with configuration interaction calculations. Tight-binding single particle wave functions are used as a basis for computing Coulomb and dipole matrix elements. Within this framework, we analyze multi-exciton emission spectra for two different sizes of a lens-shaped InN/GaN QD with wurtzite crystal structure. The impact of the symmetry of the involved electron and hole one-particle states on the optical spectra is discussed in detail. Furthermore we show how the characteristic features of the spectra can be interpreted using a simplified Hamiltonian which provides analytical results for the interacting multi-exciton complexes. We predict a vanishing exciton and biexciton ground state emission for small lens-shaped InN/GaN QDs. For larger systems we report a bright ground state emission but with drastically reduced oscillator strengths caused by the quantum confined Stark effect.Comment: 15 pages, 17 figure

Exciton and negative trion dissociation by an external electric field in vertically coupled quantum dots

We study the Stark effect for an exciton confined in a pair of vertically coupled quantum dots. A single-band approximation for the hole and a parabolic lateral confinement potential are adopted which allows for the separation of the lateral center-of-mass motion and consequently for an exact numerical solution of the Schr\"odinger equation. We show that for intermediate tunnel coupling the external electric field leads to the dissociation of the exciton via an avoided crossing of bright and dark exciton energy levels which results in an atypical form of the Stark shift. The electric-field-induced dissociation of the negative trion is studied using the approximation of frozen lateral degrees of freedom. It is shown that in a symmetric system of coupled dots the trion is more stable against dissociation than the exciton. For an asymmetric system of coupled dots the trion dissociation is accompanied by a positive curvature of the recombination energy line as a function of the electric field.Comment: PRB - in prin

Porous Single-Crystal-Based Inorganic Semiconductor Photocatalysts for Energy Production and Environmental Remediation: Preparation, Modification, and Applications

This is the peer reviewed version of the following article: Niu, J., Albero, J., Atienzar, P., García, H., Porous Single-Crystal-Based Inorganic Semiconductor Photocatalysts for Energy Production and Environmental Remediation: Preparation, Modification, and Applications. Adv. Funct. Mater. 2020, 30, 1908984, which has been published in final form at https://doi.org/10.1002/adfm.201908984. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.[EN] Semiconductor photocatalytic and photovoltaic performance depends on crystallinity and surface area to a large extent. One strategy that has recently emergyed to improve semiconductor photoresponse efficiency is their synthesis as porous single crystals (PSCs), therefore providing simultaneously high crystallinity, minimization of grain boundaries, and large specific surface area. Other factors, such as high density of active sites, and enhanced light absorption, also contribute to increased PSC photoresponse with respect to analogous bulk or amorphous materials. This review initially presents the concept and main properties of PSCs. Then, the synthetic routes and the applications as photocatalysts and as photovoltaic devices, mainly in sunlight applications, are summarized. The synthetic procedures have been classified according to the mechanism of pore generation. Applications cover photocatalysis for environmental remediation, solar fuels production, selective photooxidation of organic compounds, and photovoltaic devices. Finally, a summary and views on future developments are provided. The purpose of this review is to show how the use of PSCs is a powerful general methodology applicable beyond metal oxides and can ultimately lead to sufficient photoresponse efficiency, bringing these processes close to commercial application.Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa SEV2016-0683 and RTI2018-89023-CO2-R1) and by the Generalitat Valenciana (Prometeo 2017-083) is gratefully acknowledged. J.N. also gratefully acknowledges financial support from the Fundamental Research Funds for the Central Universities (2019XKQYMS76).Niu, J.; Albero-Sancho, J.; Atienzar Corvillo, PE.; García Gómez, H. (2020). Porous Single-Crystal-Based Inorganic Semiconductor Photocatalysts for Energy Production and Environmental Remediation: Preparation, Modification, and Applications. Advanced Functional Materials. 30(15):1-51. https://doi.org/10.1002/adfm.2019089841513015Lee, B., Yamashita, T., Lu, D., Kondo, J. N., & Domen, K. (2002). 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Comparison of Zn_{1-x}Mn_xTe/ZnTe multiple-quantum wells and quantum dots by below-bandgap photomodulated reflectivity

Large-area high density patterns of quantum dots with a diameter of 200 nm have been prepared from a series of four Zn_{0.93}Mn_{0.07}Te/ZnTe multiple quantum well structures of different well width (4 nm, 6 nm, 8 nm and 10 nm) by electron beam lithography followed by Ar+ ion beam etching. Below-bandgap photomodulated reflectivity spectra of the quantum dot samples and the parent heterostructures were then recorded at 10 K and the spectra were fitted to extract the linewidths and the energy positions of the excitonic transitions in each sample. The fitted results are compared to calculations of the transition energies in which the different strain states in the samples are taken into account. We show that the main effect of the nanofabrication process is a change in the strain state of the quantum dot samples compared to the parent heterostructures. The quantum dot pillars turn out to be freestanding, whereas the heterostructures are in a good approximation strained to the ZnTe lattice constant. The lateral size of the dots is such that extra confinement effects are not expected or observed.Comment: 23 pages, LaTeX2e (amsmath, epsfig), 7 EPS figure