117 research outputs found
Theory of highly excited semiconductor nanostructures including Auger coupling: exciton-bi-exciton mixing in CdSe nanocrystals
We present a theory of highly excited interacting carriers confined in a
semiconductor nanostructure, incorporating Auger coupling between excited
states with different number of excitations. The Coulomb matrix elements
connecting exciton, bi-exciton and tri-exciton complexes are derived and an
intuitive picture of breaking neutral multi-exction complexes into positively
and negatively charged multi-exciton complexes is given. The general approach
is illustrated by analyzing the coupling of biexciton and exciton in CdSe
spherical nanocrystals. The electron and hole states are computed using
atomistic tight binding Hamiltonian including an effective crystal
field splitting and surface passivation. For each number of electron-hole pairs
the many-body spectrum is computed in the configuration-interaction approach.
The low-energy correlated biexciton levels are broken into charged complexes: a
hole and a negatively charged trion and an electron and a positively charged
trion. Out of a highly excited exciton spectrum a subspace coupled to
bi-exciton levels via Auger processes is identified. The interaction between
correlated bi-exciton and exciton states is treated using exact diagonalization
techniques. This allows to extract the spectral function of the biexciton and
relate its characteristic width and amplitude to the characteristic amplitude
and timescale of the coherent time evolution of the coupled system. It is shown
that this process can be described by the Fermi's Golden Rule only if a fast
relaxation of the excitonic subsystem is accounted for.Comment: 9 figure
The Complete In-Gap Electronic Structure of Colloidal Quantum Dot Solids and Its Correlation with Electronic Transport and Photovoltaic Performance
Epitaxial Metal Halide Perovskites by InkjetâPrinting on Various Substrates
Metalâhalideâperovskites revolutionized the field of thinâfilm semiconductor technology, due to their favorable optoelectronic properties and facile solution processing. Further improvements of perovskite thinâfilm devices require structural coherence on the atomic scale. Such perfection is achieved by epitaxial growth, a method that is based on the use of highâend deposition chambers. Here epitaxial growth is enabled via a â1000 times cheaper device, a single nozzle inkjet printer. By printing, singleâcrystal microâ and nanostructure arrays and crystalline coherent thin films are obtained on selected substrates. The heteroâepitaxial structures of methylammonium PbBr3 grown on lattice matching substrates exhibit similar luminescence as bulk single crystals, but the crystals phase transitions are shifted to lower temperatures, indicating a structural stabilization due to interfacial lattice anchoring by the substrates. Thus, the inkjetâprinting of metalâhalide perovskites provides improved material characteristics in a highly economical way, as a future cheap competitor to the highâend semiconductor growth technologies.DFG, 404984854, Bleifreie Perovksite fĂŒr die RöntgendetektionDFG, 399073171, GRK 2495: Energiekonvertierungssysteme: von Materialien zu Bauteile
Guidelines for reliable urea detection in electrocatalysis
Electrochemical urea synthesis offers a promising green alternative to fossil fuel-based approaches to support the increasing global demand for nitrogen fertilizers. Unfortunately, the conventional urea detection methods produce inconsistent and unreliable results, hindering progress in this field. Both false positives and negatives can be observed depending on environmental factors and interfering chemicals. In this regard, we propose new rigorous 13C NMR and LC-MS protocols for accurate urea detection to analyze catalyst activity in an accessible and reproducible manner
Fine structure and size dependence of exciton and biexciton optical spectra in CdSe nanocrystals
Theory of electronic and optical properties of exciton and biexciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic sp3d5s* tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum is organized in shells (s , p, . . .), while the valence hole spectrum is composed of four low-lying, doubly degenerate states separated from the rest by a gap. As a result, the biexciton and exciton spectrum is composed of a manifold of closely lying states, resulting in a fine structure of exciton and biexciton spectra. The quasidegenerate nature of the hole spectrum results in a correlated biexciton state, which makes it slowly convergent with basis size. We carry out a systematic study of the exciton and biexciton emission spectra as a function of the nanocrystal diameter and find that the interplay of repulsion between constituent excitons and correlation effects results in a change of the sign of biexciton binding energy from negative to positive at a critical nanocrystal size.Peer reviewed: YesNRC publication: Ye
In situ cathode-electrolyte interphase enables high cycling stability of Co-free Li-rich layered cathodes
Despite the extensive research in Li-rich layered oxides (LLOs), which are promising candidates for high-energy density cathodes, their cycle life still cannot meet the real-world application requirements. The poor cycle performance arises from the electrolyte decomposition at high voltage, resulting in damage and subsequent surface-initiated conversion of the cathode from layered to spinel phase. This problem is even more challenging for Co-free LLO cathodes. Here, we report a one-pot synthesis of in situ carbonate-coated nanostructured Co-free LLO (Li2CO3@LLO) through a polyol-assisted method. This inorganic coating suppresses oxygen release, provides good Liâion transport, and protects the cathode from adverse reactions with the electrolyte. The obtained material exhibits excellent long-term stability, with 76% capacity retention after 1000 cycles at a 0.2 C rate without any Co addition, demonstrating a path forward for using LLOs as a next-generation Liâion battery cathode
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