A closer look at the effects of oxygen on the photoluminescence properties of CdSe/CdS quantum dots.

We present a detailed study on the effects of oxygen on the photoluminescence properties of CdSe/CdS quantum dots (QDs). We investigated the role of oxygen by performing confocal measurements on thin films as well as on single particles while rapidly exchanging the gaseous environment between oxygen and an inert gas atmosphere. We found that the deionization of negatively charged particles by oxygen depends on both the excitation power and the shell thickness of the QDs.

3D and Multimodal X-Ray Microscopy Reveals the Impact of Voids in CIGS Solar Cells.

Small voids in the absorber layer of thin-film solar cells are generally suspected to impair photovoltaic performance. They have been studied on Cu(In,Ga)Se cells with conventional laboratory techniques, albeit limited to surface characterization and often affected by sample-preparation artifacts. Here, synchrotron imaging is performed on a fully operational as-deposited solar cell containing a few tens of voids.

Cation Exchange during the Synthesis of Colloidal Type-II ZnSe-Dot/CdS-Rod Nanocrystals.

Cation exchange is known to occur during the synthesis of colloidal semiconductor heteronanoparticles, affecting their band gap and thus altering their optoelectronic properties. It is often neglected, especially when anisotropic heterostructures are discussed. We present a study on the role of cation exchange inevitably occurring during the growth of anisotropic dot-in-rod structures consisting of a spherical ZnSe core enclosed by a rod-shaped CdS shell.

Two-Dimensional Superstructures from the Gas Phase: Directed Assembly of Copper-Sulfide Nanoplatelets.

We report on a novel plasma-assisted approach for the deposition of free-standing two-dimensional superstructures via directed assembly of copper-sulfide nanoplatelets in the gas phase. For this, the copper-organic complex bis-[bis(,-diethyldithiocarbamato)-copper(II)] is thermally evaporated and transported into a capacitively coupled rf plasma to form two-dimensional nanoplatelets upon fragmentation. On a substrate, the highly anisotropic platelets are attached in a directed edge-to-edge configuration.

Role of Magnetic Coupling in Photoluminescence Kinetics of Mn-Doped ZnS Nanoplatelets.

Mn-doped semiconductor nanocrystals with tuned location and concentration of Mn 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 Mn concentration and the optical properties is still challenging because of the complex interactions of Mn ions and the host and between the Mn ions. Here, atomically flat ZnS nanoplatelets (NPLs) with uniform thickness were chosen as matrixes for Mn doping.

Erratum: Ossig, C., et al. Four-Fold Multi-Modal X-ray Microscopy Measurements of a Cu(In, Ga)Se Solar Cell. 2021, , 228.

In the original version of our article [...

Four-Fold Multi-Modal X-ray Microscopy Measurements of a Cu(In,Ga)Se Solar Cell.

Inhomogeneities and defects often limit the overall performance of thin-film solar cells. Therefore, sophisticated microscopy approaches are sought to characterize performance and defects at the nanoscale. Here, we demonstrate, for the first time, the simultaneous assessment of composition, structure, and performance in four-fold multi-modality.

Specific binding and internalization: an investigation of fluorescent aptamer-gold nanoclusters and cells with fluorescence lifetime imaging microscopy.

Fluorescent gold nanoclusters show promising properties for biological applications. We biofunctionalized fluorescent 11-mercaptoundecanoic-acid stabilized gold nanoclusters (AuNCs) with an aptamer to target the interleukin-6-receptor expressed on BaF3 cells specifically. Although the fluorescence emission of the AuNCs (535 nm) is in the same wavelength region as the autofluorescence of the cell, we are able to distinguish between nanoclusters and cells using the fluorescence decay time, which is much longer for the AuNCs (100 ns) than for the autofluorescence.

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 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 allows 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.

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 SiO spheres. The major drawbacks of this method, when applied for silica encapsulation of anisotropic nanorods (NRs), are spatially unequal silica growth and long reaction times (24 h at least). In this work, various tetraalkoxysilanes [tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), and tetrapropyl orthosilicate (TPOS)] with different alkyl-chain lengths were used as silica precursors in attempt to tune the silanization behavior of CdSe/CdS NRs in a microemulsion system.

Controlling the Spontaneous Emission Rate of Quantum Wells in Rolled-Up Hyperbolic Metamaterials.

We experimentally demonstrate the enhancement of the spontaneous emission rate of GaAs quantum wells embedded in rolled-up metamaterials. We fabricate microtubes whose walls consist of alternating Ag and (In)(Al)GaAs layers with incorporated active GaAs quantum-well structures. By variation of the layer thickness ratio of the Ag and (In)(Al)GaAs layers we control the effective permittivity tensor of the metamaterial according to an effective medium approach.

Solution-Grown Nanowire Devices for Sensitive and Fast Photodetection.

Highly sensitive and fast photodetector devices with CdSe quantum nanowires as active elements have been developed exploiting the advantages of electro- and wet-chemical routes. Bismuth nanoparticles electrochemically synthesized directly onto interdigitating platinum electrodes serve as catalysts in the following solution-liquid-solid synthesis of quantum nanowires directly on immersed substrates under mild conditions at low temperature. This fast and simple preparation process leads to a photodetector device with a film of nanowires of limited thickness bridging the electrode gaps, in which a high fraction of individual nanowires are electrically contacted and can be exposed to light at the same time.

Insight into strain effects on band alignment shifts, carrier localization and recombination kinetics in CdTe/CdS core/shell quantum dots.

The impact of strain on the optical properties of semiconductor quantum dots (QDs) is fundamentally important while still awaiting detailed investigation. CdTe/CdS core/shell QDs represent a typical strained system due to the substantial lattice mismatch between CdTe and CdS. To probe the strain-related effects, aqueous CdTe/CdS QDs were synthesized by coating different sized CdTe QD cores with CdS shells upon the thermal decomposition of glutathione as a sulfur source under reflux.

Quantum-confined emission and fluorescence blinking of individual exciton complexes in CdSe nanowires.

One-dimensional semiconductor nanostructures combine electron mobility in length direction with the possibility of tailoring the physical properties by confinement effects in radial direction. Here we show that thin CdSe quantum nanowires exhibit low-temperature fluorescence spectra with a specific universal structure of several sharp lines. The structure strongly resembles the pattern of bulk spectra but show a diameter-dependent shift due to confinement effects.

Fluorescence spectroscopy of individual semiconductor nanoparticles in different ethylene glycols.

The optical properties of single colloidal semiconductor nanoparticles (NPs) are considerably influenced by the direct environment of the NPs. Here, the influence of different liquid and solid glycol matrices on CdSe-based NPs is investigated. Since the fluorescence of individual NPs varies from one NP to another, it is highly desirable to study the very same individual NPs in different matrices.

The influence of temperature on the photoluminescence properties of single InAs quantum dots grown on patterned GaAs.

We report the temperature-dependent photoluminescence of single site-controlled and self-assembled InAs quantum dots. We have used nanoimprint lithography for patterning GaAs(100) templates and molecular beam epitaxy for quantum dot deposition. We show that the influence of the temperature on the photoluminescence properties is similar for quantum dots on etched nanopatterns and randomly positioned quantum dots on planar surfaces.

Fluorescence modulation of single CdSe nanowires by charge injection through the tip of an atomic-force microscope.

We demonstrate a direct correlation between the charge state and photoluminescence (PL) intensity of individual CdSe nanowires by actively charging them and performing electrostatic force microscopy and PL measurements simultaneously. While the injection of positive charges leads to an immediate PL quenching, a small amount of injected electrons can lead to an increase of the PL intensity. We directly observed the migration of excess charges into the substrate, which leads to a recovery of the PL.

Diameter scaling of the optical band gap in individual CdSe nanowires.

The diameter dependence of the optical band gap of single CdSe nanowires (NWs) is investigated by a combination of atomic force microscopy, scanning fluorescence microscopy, and transmission electron microscopy. We find a good congruence of the experimental data to calculations within the effective mass approximation taking into account quantization, exciton Coulomb interaction, and dielectric mismatch. The experimental data are furthermore compared to different theoretical approaches.

Laser-induced charge separation in CdSe nanowires.

A combination of electrostatic force microscopy and optical microscopy was used to investigate the charge state of individual CdSe nanowires upon local illumination with a focused laser beam. The nanowires were found to be positively charged at the excitation spot and negatively charged at the distant end(s). For high laser powers, the amount of accumulated charges increases logarithmically with the laser power.

Fluorescence enhancement, blinking suppression, and gray states of individual semiconductor nanocrystals close to gold nanoparticles.

The optical properties of nanocrystals are drastically changed by the interaction with adjacent metal nanoparticles. By time-resolved photoluminescence spectroscopy, we investigate CdSe multishell nanocrystals coupled to self-assembled films of Au nanoparticles. The distance between emitter and metal is adjusted by coating the nanocrystals with silica shells.

Optical modes excited by evanescent-wave-coupled PbS nanocrystals in semiconductor microtube bottle resonators.

We report on optical modes in rolled-up microtube resonators that are excited by PbS nanocrystals filled into the microtube core. Long ranging evanescent fields into the very thin walled microtubes cause strong emission of the nanocrystals into the resonator modes and a mode shift after a self-removal of the solvent. We present a method to precisely control the number, the energy and the localization of the modes along the microtube axis.

Nonhydrolytic synthesis of high-quality anisotropically shaped brookite TiO2 nanocrystals.

A surfactant-assisted nonaqueous strategy, relying on high-temperature aminolysis of titanium carboxylate complexes, has been developed to access anisotropically shaped TiO2 nanocrystals selectively trapped in the metastable brookite phase. Judicious temporal manipulation of precursor supply to the reaction mixture enables systematic tuning of the nanostructure geometric features over an exceptionally wide dimensional range (30-200 nm). Such degree of control is rationalized within the frame of a self-regulated phase-changing seed-catalyzed mechanism, in which homogeneous nucleation, on one side, and heterogeneous nucleation/growth processes, on the other side, are properly balanced while switching from the anatase to the brookite structures, respectively, in a continuous unidirectional crystal development regime.

Direct observation of confined acoustic phonons in the photoluminescence spectra of a single CdSe-CdS-ZnS core-shell-shell nanocrystal.

We report on the direct observation of confined acoustic phonons in the photoluminescence spectra of single CdSe-CdS-ZnS nanocrystals, whose ligands were exchanged to poly(ethylene oxide) (PEO) before they were embedded in a PEO matrix. Modeling a nanocrystal as an elastic sphere, the confined acoustic modes can be assigned to purely radial vibrations: the breathing mode and its two first radial harmonics. In addition to acoustic modes, we also observe longitudinal optical modes of the core material and, remarkably, also of both shell materials.

Confined optical phonon modes in aligned nanorod arrays detected by resonant inelastic light scattering.

We investigated the optical phonon excitations of laterally aligned nanorod arrays by resonant Raman scattering. We observed a strong suppression of the surface-optical phonon modes in the closely packed aligned arrays and a small asymmetry in the longitudinal-optical phonon peak with respect to the rod alignment orientation. These observations can be explained by the spatial distribution of the potential of the different phonon modes derived from the first principles calculations.