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)Se2 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. By measuring operando current and X-ray excited optical luminescence, the local electrical and optical performance in the proximity of the voids are estimated, and via ptychographic tomography, the depth in the absorber of the voids is quantified. Besides, the complex network of material-deficit structures between the absorber and the top electrode is highlighted. Despite certain local impairments, the massive presence of voids in the absorber suggests they only have a limited detrimental impact on performance.

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. The material combination exhibits a type-II band alignment. Two reactions are compared: the shell-growth reaction of CdS on ZnSe and an exchange-only reaction of ZnSe cores to CdSe. Transmission electron microscopy and a comprehensive set of optical spectroscopy data, including linear and time-resolved absorption and fluorescence data, prove that cation exchange from ZnSe to CdSe is the dominant process in the initial stages of the shell-growth reaction. The degree of cation exchange before significant shell growth starts was determined to be about 50%, highlighting the importance of cation exchange during the heteronanostructure growth.

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(N,N-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. We found that a high substrate temperature of 400 °C is necessary for the 2D vertical growth of copper sulfide. Using plasma reinforces the directional assembly and leads to nanowalls which are several micrometers high with the thickness of a single nanoplatelet. The morphology and crystallographic composition of the emerging superstructures were extensively investigated via scanning and transmission electron microscopy as well as electron diffraction. The data reveal the (010) plane to be the preferred axis for the arrangement of the nanoplatelets.

Growth and Self-Assembly of CsPbBr3 Nanocrystals in the TOPO/PbBr2 Synthesis as Seen with X-ray Scattering.

Despite broad interest in colloidal lead halide perovskite nanocrystals (LHP NCs), their intrinsic fast growth has prevented controlled synthesis of small, monodisperse crystals and insights into the reaction mechanism. Recently, a much slower synthesis of LHP NCs with extreme size control has been reported, based on diluted TOPO/PbBr2 precursors and a diisooctylphosphinate capping ligand. We report new insights into the nucleation, growth, and self-assembly in this reaction, obtained by in situ synchrotron-based small-angle X-ray scattering and optical absorption spectroscopy. We show that dispersed 3 nm Cs[PbBr3] agglomerates are the key intermediate species: first, they slowly nucleate into crystals, and then they release Cs[PbBr3] monomers for further growth of the crystals. We show the merits of a low Cs[PbBr3] monomer concentration for the reaction based on oleate ligands. We also examine the spontaneous superlattice formation mechanism occurring when the growing nanocrystals in the solvent reach a critical size of 11.6 nm.

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 is 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 (ZnS:Mn) system is a universal platform to study dopant location- and concentration-dependent properties also in other semiconductors.

Encapsulation of Gold Nanoparticles into Redesigned Ferritin Nanocages for the Assembly of Binary Superlattices Composed of Fluorophores and Gold Nanoparticles.

Nanomaterials with a defined composition and structure can be synthesized by exploiting natural templates or biomolecular matrices. In the present work, we use protein nanocages derived from human ferritin as a nanoscale building block for the assembly of gold nanoparticles and fluorescent molecules in the solid state. As a generalizable strategy, we show that prior to material synthesis, the cargo can be encapsulated into the protein nanocages using a dis- and reassembly approach. Toward this end, a new ligand system for gold nanoparticles enables efficient encapsulation of these particles into the nanocages. The gold nanoparticle-loaded protein nanocages are co-assembled with fluorophore-loaded protein nanocages. Binary superlattices are formed because two oppositely charged ferritin nanocages are used as templates for the assembly. The binary crystals show strong exciton-plasmon coupling between the encapsulated fluorophores and gold nanoparticles, which was spatially resolved with fluorescence lifetime imaging. The strategy outlined here offers a modular approach toward binary nanomaterials with highly ordered building blocks.

Impact of Ligands on Structural and Optical Properties of Ag29 Nanoclusters.

A ligand exchange strategy has been employed to understand the role of ligands on the structural and optical properties of atomically precise 29 atom silver nanoclusters (NCs). By ligand optimization, ∼44-fold quantum yield (QY) enhancement of Ag29(BDT)12-x(DHLA)x NCs (x = 1-6) was achieved, where BDT and DHLA refer to 1,3-benzene-dithiol and dihydrolipoic acid, respectively. High-resolution mass spectrometry was used to monitor ligand exchange, and structures of the different NCs were obtained through density functional theory (DFT). The DFT results from Ag29(BDT)11(DHLA) NCs were further experimentally verified through collisional cross-section (CCS) analysis using ion mobility mass spectrometry (IM MS). An excellent match in predicted CCS values and optical properties with the respective experimental data led to a likely structure of Ag29(DHLA)12 NCs consisting of an icosahedral core with an Ag16S24 shell. Combining the experimental observation with DFT structural analysis of a series of atomically precise NCs, Ag29-yAuy(BDT)12-x(DHLA)x (where y, x = 0,0; 0,1; 0,12 and 1,12; respectively), it was found that while the metal core is responsible for the origin of photoluminescence (PL), ligands play vital roles in determining their resultant PLQY.

Superionic phase transition in individual silver selenide nanowires.

Silver selenide (Ag2Se) is a promising material for applications as a solid-state electrolyte, with a superionic phase transition at 133 °C. Here, we studied the temperature dependent transport properties of single Ag2Se nanowires in a transistor geometry, which allowed us to determine charge carrier type, concentration, and mobility below and above the superionic phase transition temperature. We found the majority charge carriers to be n-type in the temperature range of 30-150 °C. Across the superionic phase-transition, we observed a sudden increase in conductivity by about 30%, which was accompanied by an increase in charge carrier density by about 200% and a decrease in mobility by about 45%. Interestingly, the size dependent shift of the transition temperatures to below 100 °C in our wires is much more pronounced than for nanocrystals of comparable size. This surprising and potentially useful effect could be caused by changes in crystal structure arising from the synthesis process.

Erratum: Ossig, C., et al. Four-Fold Multi-Modal X-ray Microscopy Measurements of a Cu(In, Ga)Se2 Solar Cell. Materials 2021, 14, 228.

In the original version of our article [...].

Four-Fold Multi-Modal X-ray Microscopy Measurements of a Cu(In,Ga)Se2 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. Using scanning X-ray microscopy of a Cu(In,Ga)Se2 (CIGS) solar cell, we measured the elemental distribution of the key absorber elements, the electrical and optical response, and the phase shift of the coherent X-rays with nanoscale resolution. We found structural features in the absorber layer-interpreted as voids-that correlate with poor electrical performance and point towards defects that limit the overall solar cell efficiency.

Nanocrystal Aerogels with Coupled or Decoupled Building Blocks.

The influence of interparticle contact in nanoparticle-based aerogel network structures is investigated by selectively connecting or isolating the building blocks inside of the network, thereby coupling and decoupling them in regards to their optical and electronic properties. This is achieved by tuning the synthesis sequence and exchanging the point of shell growth and the point of particle assembly, leading to two distinctly different structures as examined by electron microscopy. By thorough examination of the resulting optical properties of the generated structures, the clear correlation between nanoscopic/microscopic structure and macroscopic optical properties is demonstrated. Temperature-dependent measurements and effective mass approximation calculations support our findings.

Ligand density on nanoparticles: A parameter with critical impact on nanomedicine.

Nanoparticles modified with ligands for specific targeting towards receptors expressed on the surface of target cells are discussed in literature towards improved delivery strategies. In such concepts the ligand density on the surface of the nanoparticles plays an important role. How many ligands per nanoparticle are best for the most efficient delivery? Importantly, this number may be different for in vitro and in vivo scenarios. In this review first viruses as "biological" nanoparticles are analyzed towards their ligand density, which is then compared to the ligand density of engineered nanoparticles. Then, experiments are reviewed in which in vitro and in vivo nanoparticle delivery has been analyzed in terms of ligand density. These results help to understand which ligand densities should be attempted for better targeting. Finally synthetic methods for controlling the ligand density of nanoparticles are described.

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. After a first short incubation period we detected AuNCs specifically bound to the cell membrane by using two fluorescence lifetime imaging microscopy (FLIM) methods: gated and direct FLIM. After a second incubation period the previously bound AuNCs are internalized by the cells, as could be resolved solely by the direct FLIM. This proves the superior sensitivity of this method compared to gated FLIM. We find that the optical properties of AuNCs do not change upon binding to the cells, but exhibit a change when internalized into the cells, induced by an interaction between the AuNCs and cells.

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. The experimental data are 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.

Size dependent targeted delivery of gold nanoparticles modified with the IL-6R-specific aptamer AIR-3A to IL-6R-carrying cells.

The delivery of gold nanoparticles (AuNPs) to specific cells strongly depends on the properties e.g. the size of the particles and is of great interest for a large variety of biomedical applications. Here we investigated the size dependence of the receptor-ligand mediated AuNP delivery to cells by comparing very small "molecular" Au-clusters of only 2 nm to larger 7 nm and 36 nm AuNPs with a distinct surface plasmon resonance. Since the molecular weight in this range changes by almost three orders of magnitude, we show how the amount of gold relates to the number of delivered AuNPs. We attached small interleukin-6 receptor (IL-6R) specific aptamer molecules (AIR-3A) in different amounts to the particles and investigated the specificity of the delivery to IL-6R-carrying cells. To reduce unspecific interaction the particles were additionally covered with polyethylene glycol (PEG). Besides particle size and concentration we varied additional parameters such as aptamer surface coverage as well as incubation time and temperature. We found that in particular, small particles with diameters of less than 2 nm show an up to six times higher delivery rate for the aptamer-conjugated AuNPs compared to untargeted PEG-coated AuNPs. The specificity reduces with a decreasing aptamer/PEG ratio, and also with an increase in particle size where the unspecific uptake is much higher. In addition we also compared the delivery efficiency of this aptamer-mediated delivery system with an antibody-mediated system targeting the same receptor to validate the performance of this approach.

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 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. We find enhanced spatial homogeneity of silica growth with decreasing alkyl-chain length of the tetraalkoxysilanes. In particular, by use of TMOS as the precursor, NRs can be fully encapsulated in a continuous thin (≤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 use of TEOS. Our investigations suggest that the lower steric hindrance of TMOS compared to TEOS or TPOS strongly promotes homogeneous growth of the silica shells, while its increased hydrolysis rate decreases the porosity of these shells.

Investigations of ion transport through nanoscale polymer membranes by fluorescence quenching of CdSe/CdS quantum dot/quantum rods.

Detailed steady-state and time-resolved fluorescence quenching measurements give deep insight into ion transport through nanometer thick diblock copolymer membranes, which were assembled as biocompatible shell material around CdSe/CdS quantum dot in quantum rods. We discuss the role of polymer chain length, intermolecular cross-linking and nanopore formation by analysing electron transfer processes from the photoexcited QDQRs to Cu(II) ions, which accumulate in the polymer membrane. Fluorescence investigations on single particle level additionally allow identifying ensemble inhomogeneities.

A universal approach to ultrasmall magneto-fluorescent nanohybrids.

Seeded emulsion polymerization is a powerful universal method to produce ultrasmall multifunctional magnetic nanohybrids. In a two-step procedure, iron oxide nanocrystals were initially encapsulated in a polystyrene (PS) shell and subsequently used as beads for a controlled assembly of elongated quantum dots/quantum rods (QDQRs). The synthesis of a continuous PS shell allows the whole construct to be fixed and the composition of the nanohybrid to be tuned. The fluorescence of the QDQRs and magnetism of iron oxide were perfectly preserved, as confirmed by single-particle investigation, fluorescence decay measurements, and relaxometry. Bio-functionalization of the hybrids was straightforward, involving copolymerization of appropriate affinity ligands as shown by immunoblot analysis. Additionally, the universality of this method was shown by the embedment of a broad scale of NPs.

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. The high sensitivity of the photodetector device can be expressed by its on/off ratio or its photosensitivity of more than 10(7) over a broad wavelength range up to about 700 nm. The specific detectivity and responsivity are determined to D* = 4 × 10(13) Jones and R = 0.32 A/W, respectively. The speed of the device reflects itself in a 3 dB frequency above 1 MHz corresponding to rise and fall times below 350 ns. The remarkable combination of a high sensitivity and a fast response is attributed to depletion regions inside the nanowires, tunnel-junction barriers between nanowires, and Schottky contacts at the electrodes, where all of these features are strongly influenced by the number of photogenerated charge carriers.