arXiv subjects

Christian Strelow

Publications and source records attributed to Christian Strelow.

10 recordsLinked to original sources

arXiv2024

Purcell-enhanced single-photon emission from InAs/GaAs quantum dots coupled to broadband cylindrical nanocavities

On-chip emitters that can generate single and entangled photons are essential building blocks for developing photonic quantum information processing technologies in a scalable fashion. Semiconductor quantum dots (QDs) are attractive candidates that emit high-quality quantum states of light on demand, however at a rate limited by their spontaneous radiative lifetime. In this study, we utilize the Purcell effect to demonstrate up to a 38-fold enhancement in the emission rate of InAs QDs by coupling them to metal-clad GaAs nanopillars. These cavities, featuring a sub-wavelength mode volume of 4.5x10-4 ({\lambda}/n)3 and low quality factor of 62, enable Purcell-enhanced single-photon emission across a large bandwidth of 15 nm. The broadband nature of the cavity eliminates the need for implementing tuning mechanisms typically required to achieve QD-cavity resonance, thus relaxing fabrication constraints. Ultimately, this QD-cavity architecture represents a significant stride towards developing solid-state quantum emitters generating near-ideal single-photon states at GHz-level repetition rates.

quant-ph↗

arXiv2024

CdSe-Dot/CdS-Rod/PbS-Dot Nanocrystals by Partial Cation Exchange Reaction

Dual-emissive nanorods with fluorescence in both, the visible and infrared range are prepared by a combination of a CdSe-nanocrystal-seeded growth of CdS nanorods, and a successive partial Cd-to-Pb cation exchange. We show that the exchange reaction, which involves Pb halides in oleylamine, starts at the tip of the rods, leading to the formation of CdSe-dot/CdS-rod/PbS-dot nanocrystals (DRDs). Besides these DRDs, the reaction product also contains shorter nanorods and spherical quantum dots. Their fraction strongly depend on the amount of lead halide precursor and the reaction time. The reaction mechanism is investigated in detail, such that by carefully adjusting the reaction conditions, it is possible to synthesize DRDs of distinct PbS-dot sizes with yields over 95%. The resulting DRDs are crystalline and show a CdSe-fluorescence band in the visible range at 600 nm, and also a fluorescence band in the NIR at 1440 nm, resulting from the PbS part of the rods.

cond-mat.mtrl-sci↗

arXiv2024

Exciton-Phonon Coupling in Single Band-Gap Engineered ZnCdSe-Dot/CdS-Rod Nanocrystals

Exciton-phonon coupling limits the homogeneous emission linewidth of nanocrystals. Hence, a full understanding of it is crucial. In this work, we statistically investigate exciton-phonon coupling by performing single-particle spectroscopy on Zn$_{1-x}$Cd$_{x}$Se/CdS dot-in-rod nanocrystals at cryogenic temperatures ($T\approx 10~\rm{K}$). In situ cation exchange enables us to analyze different band alignments and thereby different charge-carrier distributions. We find that the relative intensities of the longitudinal optical S- and Se-type phonon replicas correlate with the charge-carrier distribution. Our experimental findings are complemented with quantum mechanical calculations within the effective mass approximation that hint at the relevance of surface charges.

cond-mat.mes-hall↗

arXiv2022

Role of Magnetic Coupling in Photoluminescence Kinetics of Mn2+-doped ZnS Nanoplatelets

Mn2+-doped semiconductor nanocrystals with tuned location and concentration of Mn2+ ions can yield diverse coupling regimes, which can highly influence their optical properties such as emission wavelength and photoluminescence (PL) lifetime. However, investigation on the relationship between the Mn2+ concentration and the optical properties is still challenging because of the complex interactions of Mn2+ ions and the host and between the Mn2+ ions. Here, atomically flat ZnS nanoplatelets (NPLs) with uniform thickness were chosen as matrixes for Mn2+ doping. Using time-resolved (TR) PL spectroscopy and density functional theory (DFT) calculations, a connection between coupling and PL kinetics of Mn2+ ions was established. Moreover, it was found that the Mn2+ ions residing on the surface of a nanostructure produce emissive states and interfere with the change of properties by Mn2+/Mn2+ coupling. In a configuration with suppressed surface contribution to the optical response we show the underlying physical reasons for double and triple exponential decay by DFT methods. We believe that the presented doping strategy and simulation methodology of the Mn2+-doped ZnS system is a universal platform to study dopant location- and concentration-dependent properties also in other semiconductors.

cond-mat.mtrl-sci↗

arXiv2021

Colloidal manganese doped ZnS nanoplatelets and their optical properties

Manganese (Mn)-doped ZnS nanocrystals (NCs) have been extensively explored for optical applications with the advantages of low toxicity, large Stokes shifts, and enhanced thermal and environmental stability. Although numerous studies on Mn-doped ZnS dots, rods, and wires have been reported, the literature related to Mn-doped ZnS nanoplatelets (ZnS:Mn NPLs) is scarce. Here, we present the first example of direct doping of Mn2+ ions into ZnS NPLs via the nucleation-doping strategy. The resulting ZnS:Mn NPLs exhibit Mn luminescence, indicative for successful doping of the host ZnS NPLs with Mn2+ ions. The energy transfer from the ZnS NPLs to the Mn2+ ions was observed by employing spectroscopic methods. Furthermore, the impact of the Mn concentration on the optical properties of ZnS:Mn NPLs was systematically investigated. As a result of Mn-Mn interaction, tunable Mn emission and shortened photoluminescence (PL) lifetime decay were observed and rationalized by means of electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). Finally, we show that the initially low dopant PL quantum yield (QY) of ZnS:Mn NPLs can be dramatically enhanced by passivating the surface trap states of the samples. The presented synthetic strategy of ZnS:Mn NPLs opens a new way to synthesize further doped systems of two-dimensional (2D) NPLs.

cond-mat.mtrl-sci↗

arXiv2019

Micron-size two-dimensional methylammonium lead halide perovskites

Hybrid lead halide perovskites with 2D stacking structures have recently emerged as promising materials for optoelectronic applications. We report a method for growing 2D nanosheets of hybrid lead halide perovskites (I, Br and Cl), with tunable lateral sizes ranging from 0.05 to 8 microns, and a structure consisting of n stacked monolayers separated by long alkylamines, tunable from bulk down to n=1. The key to obtaining such a wide range of perovskite properties hinged on utilizing the respective lead halide nanosheets as precursors in a hot-injection synthesis that afforded careful control over all process parameters. The layered, quantum confined (n small than 4) nanosheets were comprised of major and minor fractions with differing n. Energy funneling from low to high n (high to low energy) regions within a single sheet, mediated by the length of the ligands between stacks, produced photoluminescent quantum yields as high as 49 percent. These large, tunable 2D nanosheets could serve as convenient platforms for future high efficiency optoelectronic devices.

physics.app-ph↗

arXiv2019

Influence of interface-driven strain on the spectral diffusion properties of core/shell CdSe/CdS dot/rod nanoparticles

By combining an atomistic valence-force field approach and calculations based on the effective-mass approximation we investigate the influence of strain effects on the band alignment and general excitonic properties of core/shell CdSe/CdS dot/rod nanoparticles. We find that the inclusion of strain effects leads to an increase in exciton energy as well as to a decrease in electron and hole wave function overlap. Importantly, the native type-I band alignment of the CdSe/CdS material system is preserved and does not change into an quasi-type-II or even type-II band offset for the nanoparticles. Furthermore, we analyze the impact of strain on a model in which the spectral diffusion of the fluorescence emission of these nanoparticles is explained by migrating surface charges. Our calculations show that the addition of strain effects leads to increased energy shifts as well as larger changes in the squared electron and hole wave function overlap, while the correlation of both also exhibits a steeper slope than for the unstrained system. For a given CdSe core size, an increase in CdS-shell thickness decreases the possible ranges of energy shift and squared wave function overlap without changing the slope of their correlation. On the other hand, for a given nanoparticle overall thickness, dot/rod systems with a small CdSe core exhibit the strongest influenceability by surface charges.

cond-mat.mes-hall↗

arXiv2017

Surface Charges on CdSe-Dot/CdS-Rod Nanocrystals: Measuring and Modeling the Diffusion of Exciton-Fluorescence Rates and Energies

By performing spectroscopic single-particle measurements at cryogenic temperatures over the course of hours we study %By performing spectroscopic longtime single-particle measurements at cryogenic temperatures we study both the spectral diffusion as well as the diffusion of the decay rates of the fluorescence emission of core/shell CdSe/CdS dot/rod nanoparticles. A special analysis of the measurements allow for a correlation of data for single neutral excitons only, undisturbed by the possible emission of other excitonic complexes. We find a nearly linear dependency of the fluorescence decay rate on the emission energy. The experimental data is compared to self-consistent model calculations within the effective-mass approximation, in which migrating point charges set onto the surface of the nanoparticles have been assumed to cause the temporal changes of optical properties. These calculations reveal a nearly linear relationship between the squared electron-hole wave function overlap, which is linked to the experimentally determined fluorescence rate, and the exciton emission energy. Within our model single migrating surface charges are not sufficient to fully explain the measured rather broad ranges of emission rates and energies, while two -- and in particular negative -- surface charges close to the core of the DR induce large enough shifts. Importantly, for our nanoparticle system, the surface charges more strongly affect the hole wave function than the electron wave function and both wave functions are still localized within the dot-like core of the nanoparticle, showing that the type-I character of the band alignment between core and shell is preserved.

cond-mat.mes-hall↗

arXiv2017

Ultrathin and highly passivating silica shells for luminescent and water-soluble CdSe/CdS nanorods

Microemulsion (water-in-oil) methods enable the encapsulation of individual nanoparticles into SiO2 spheres. The major drawbacks of this method, when applied for silica encapsulation of anisotropic nanorods (NRs), are a spatially unequal silica growth and long reaction times (24 h at least). In this work, various tetra-alkoxysilanes (tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS) and tetrapropyl orthosilicate (TPOS)) with different alkyl-chain lengths were used as the silica precursors in attempt to tune the silanization behavior of CdSe/CdS NRs in a microemulsion system. We find an enhanced spatial homogeneity of the silica growth with decreasing alkyl-chain length of the tetra-alkoxysilanes. In particular, by using TMOS as the precursor, NRs can be fully encapsulated in a continuous thin ($\le$ 5 nm) silica shell within only 1 h reaction time. Surprisingly, the thin silica shell showed a superior shielding ability to acidic environment even compared to the 30 nm thick shell prepared by using TEOS. Our investigations suggest that the lower steric hindrance of TMOS compared to TEOS or TPOS strongly promotes the homogeneous growth of the silica shells while its increased hydrolysis rate leads decreases the porosity of these shells.

physics.chem-ph↗

arXiv2016

From dots to stripes to sheets - Shape control of lead sulfide nanostructures

Controlling anisotropy in nanostructures is a challenging but rewarding task since confinement in one or more dimensions influences the physical and chemical properties of the items decisively. In particular, semiconducting nanostructures can be tailored to gain optimized properties to work as transistors or absorber material in solar cells. We demonstrate that the shape of colloidal lead sulfide nanostructures can be tuned from spheres to stripes to sheets by means of the precursor concentrations, the concentration of a chloroalkane co-ligand and the synthesis temperature. All final structures still possess at least one dimension in confinement. Electrical transport measurements complement the findings.

cond-mat.mtrl-sci↗