A5079782199- Semiconductor Quantum Structures and Devices
- Quantum Dots Synthesis And Properties
- Quantum and electron transport phenomena
- Chalcogenide Semiconductor Thin Films
- Intermetallics and Advanced Alloy Properties
- Molecular Junctions and Nanostructures
- ZnO doping and properties
- Semiconductor Lasers and Optical Devices
- GaN-based semiconductor devices and materials
- Graphene research and applications
- Advanced Chemical Physics Studies
- Advanced Semiconductor Detectors and Materials
- Boron and Carbon Nanomaterials Research
- 2D Materials and Applications
- Quantum Information and Cryptography
- Rare-earth and actinide compounds
- Perovskite Materials and Applications
- Physics of Superconductivity and Magnetism
- Electronic and Structural Properties of Oxides
- Semiconductor materials and interfaces
- Nanocluster Synthesis and Applications
- Diamond and Carbon-based Materials Research
- Semiconductor materials and devices
- Laser-Ablation Synthesis of Nanoparticles
- Surface and Thin Film Phenomena
Universität Hamburg
2015-2024
Max Planck Institute for the Structure and Dynamics of Matter
2015-2024
Hamburg Centre for Ultrafast Imaging
2015-2024
Hamburg Institut (Germany)
2021
Capital Normal University
2017
Max Planck Institute for Solid State Research
2009-2016
Max Planck Society
1999-2013
National Renewable Energy Laboratory
2003-2009
Lawrence Berkeley National Laboratory
2006
Max Planck Institute for Intelligent Systems
1999-2002
The atomistic pseudopotential method is used to accurately predict the electron-hole exchange-induced fine structure (FS) and polarization anisotropy in million-atom ${\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dots of various shapes compositions. origin FS splittings clarified using a simple model where effects symmetry spin-orbit interaction are separately evident. Remarkably, shown occur, even cylindrically...
The intermediate-band solar cell (IBSC) concept has been recently proposed to enhance the current gain from spectrum whilst maintaining a large open-circuit voltage. Its main idea is introduce partially occupied intermediate band (IB) between valence (VB) and conduction (CB) of semiconductor absorber, thereby increasing photocurrent by additional $\text{VB}\ensuremath{\rightarrow}\text{IB}$ $\text{IB}\ensuremath{\rightarrow}\text{CB}$ absorptions. confined electron levels self-assembled...
Self-assembled quantum dots are often modeled by continuum models (effective mass or $\mathbf{k}∙\mathbf{p}$) that assume the symmetry of dot to be its overall geometric shape. Lens-shaped conical thus assumed have continuous cylindrical ${C}_{\mathrm{\ensuremath{\infty}}v}$, whereas pyramidal ${C}_{4v}$ symmetry. However, considering III--V made atoms arranged on (relaxed) positions a zinc-blende lattice, one would expect highest possible in these structures ${C}_{2v}$. In this group all...
We predict that heterostructure quantum wires and [111] grown dots have a vanishing fine-structure splitting on the grounds of their symmetry, are therefore ideal candidates to generate entangled photon pairs. underpin this proposal by atomistic million-atom many-body pseudopotential calculations realistic structures find is robust against possible variations in morphology.
We show that the piezoelectric effect describes emergence of an electric field in response to a crystal deformation III-V semiconductors such as GaAs and InAs has strong contributions from second-order effects have been neglected so far. calculate tensors using density-functional theory obtain for [111]-oriented In(x)Ga(1-x)As quantum wells realistic dimensions concentration x. find linear quadratic coefficients opposite on field, large strains (large In concentration) terms even dominate....
We study the effect of an external biaxial stress on light emission single InGaAs/GaAs(001) quantum dots placed onto piezoelectric actuators. With increasing compression, blueshifts and binding energies positive trion (X+) biexciton (XX) relative to neutral exciton (X) show a monotonic increase. This phenomenon is mainly ascribed changes in electron hole localization it provides robust method achieve color coincidence X XX, which prerequisite for possible generation entangled photon pairs...
Quantum dots are widely recognized for their advantageous light-emitting properties. Their excitonic fine structure along with the high quantum yields offers a wide range of possibilities technological applications. However, especially case colloidal QDs, there still characteristics and properties which not adequately controlled downgrade performance applications go far beyond simple light emission. Such challenging task is ability to manipulate energetic ordering exciton biexciton emission...
The existence of enormous strain fields in self-assembled quantum dots has led to the expectation dramatic effects piezoelectricity. However, only linear piezoelectric tensors were used all previous calculations. We calculate properties using and quadratic derived from first-principles density functional theory. find that previously ignored term similar magnitude as two terms tend cancel each other. show effect piezoelectricity on electron hole energy levels wave functions well correlated...
Physics at the nanoscale has emerged as a field where discoveries of fundamental physical effects lead to greater understanding solid state. Additionally, is believed have large potential for technological applications, which driven high pace experimental achievements in fabrication and characterization. From side theoretical modeling-so successful state physics general, since emergence density functional theory-we must acknowledge weak connection art realm nanostructures. The cause this...
The excitonic fine structure splitting describes the of bright excitons as a consequence atomistic symmetry lattice and electron-hole exchange interaction. Efforts are underway to eliminate this natural by external constraints in order use quantum dots optics. We show million atom empirical pseudopotential calculations that for realistic structures lower bound exists. underpin our numerical an insightful analysis.
We study the electronic and optical properties of ZnO quantum dots within atomistic empirical pseudopotential framework. The highest occupied molecular orbital (HOMO) is found to be P character for structures larger than 2.6 nm in diameter. identify origin this unconventional situation HOMO state, originating from an even mixture A- B-bands Wurtzite band structure. This situation, however, does not lead orbitally dark exciton ground as one might expect. Coulomb interactions lower bright...
We calculate the height dependence of recombination energies, polarization and radiative lifetimes optical transitions various excitonic complexes: neutral excitons (X^0), negatively- (X^-) positively-charged (X^+) trions, biexcitons (XX^0) in lens-shaped, self-assembled In_0.6Ga_0.4As/GaAs quantum dots. By using an atomistic pseudopotential method combined with configuration-interaction method, we predict following. (i) The energy lowest transition X^- blue-shifts as increases, whereas that...
We present the results of first-principles plane-wave pseudopotential calculations piezoelectric coefficients first and second order for a total nine III-V binary phases (AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb) zinc-blende semiconductors. These are used to calculate fields [111]-oriented quantum wells (QWs) with different well-barrier combinations various dimensions. derive an approximate analytic expression strain tensor in case pseudomorphic growth along arbitrary direction....
We present first-principles calculations of piezoelectric coefficients first and second order for AlN, GaN, InN, ZnO, along with the composition dependence (bowing) linear terms. show that eight are required provide a simple description on how to incorporate effects in calculation polarization fields. demonstrate effect leads corrections 15% quantum wells up 40% epitaxial dots.
In the present work, we studied electronic and optical properties of ZnO quantum dots (QDs) subjected to externally applied hydrostatic pressure. Our single-particle calculations are based on empirical pseudopotential method excitonic effects considered by employing configuration interaction approach. The band gap, Stokes shift, emission polarization have been investigated as a function It is found that pressure causes linear increase in gap. coefficient appears be highly size dependent,...
Fully inorganic, colloidal gold nanoclusters (NCs) constitute a new class of nanomaterials that are clearly distinguishable from their commonly studied metal-organic ligand-capped counterparts. As synthesis by chemical methods is challenging, details about optical properties remain widely unknown. In this work, laser fragmentation in liquids performed to produce fully inorganic and size-controlled NCs with monomodal particle size distributions an fcc-like structure. Results reveal these...
Atomic effective pseudopotentials enable atomistic calculations at the level of accuracy density functional theory for semiconductor nanostructures with up to fifty thousand atoms. Since they are directly derived from ab-initio performed in local approximation (LDA), inherit typical underestimated band gaps and masses. We propose an empirical correction based on modification non-local part pseudopotential demonstrate good performance bulk binary materials (InP, ZnS, HgTe, GaAs) quantum dots...
We show that the origin of antiferromagnetic coupling in spin-1 triangulene chains, which were recently synthesized and measured by Mishra et al. ( Nature 2021, 598, 287−292), originates from a superexchange mechanism. This process, mediated intertriangulene states, opens possibility to control parameters effective bilinear-biquadratic spin model. start derivation an tight-binding model for chains using combination Hartree–Fock methods fitted hybrid density functional theory results. Next,...
Ultrasmall CdSe quantum dots (QDs) with diameters up to 2 nm show broad photoluminescence (PL) spectra presumably due emission from band-edge excitons and defect states. However, the origin of effect defects on is not fully understood. Based spin–orbit density functional theory screened configuration interaction singles, we that Cd-dimer Se form in-gap In comparison experiment, discuss role deep shallow states for PL cover dependence their contributions respect QD size. Further, observe...
We present results of correlated pseudopotential calculations an exciton in a pair vertically stacked InGaAs/GaAs dots. Competing effects strain, geometry, and band mixing lead to many unexpected features missing contemporary models. The first four excitonic states are all optically active at small interdot separation, due the broken symmetry single-particle states. quantify degree entanglement wave functions show its sensitivity separation. suggest ways spectroscopically identify maximize