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.

3D Printable Hybrid Gel Made of Polymer Surface-Modified Cellulose Nanofibrils Prepared by Surface-Initiated Controlled Radical Polymerization (SI-SET-LRP) and Upconversion Luminescent Nanoparticles

A cellulose nanofibril-based hybrid gel material was developed by grafting the polymerized stearyl acrylate (PSA) and upconversion nanoparticles (UCNPs) onto cellulose nanofibrils (CNFs) via Cu⁰-mediated radical polymerization (SET-LRP) to create a highly cross-linked CNF system. A two-step strategy was exploited to surface-exchange the ligand of the UCNPs from a hydrophobic ligand (oleic acid) to a hydrophilic small-molecule ligand (2-acrylamido-2-methyl-1-propanesulfonic acid, AMPS) and therefore be suitable for SET-LRP. The characteristics and properties of the hybrid material (UCNP-PSA-CNF) were monitored by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), rheology, X-ray diffraction (XRD), and microscopic analysis. Those characterization techniques prove the efficient modification of the CNF, with the presence of 1.8% UCNPs. The luminescence measurement was carried out using a homebuilt confocal microscope with a 980 nm laser source. The nanostructure of UCNPs and their incorporated CNF species were measured by small-angle X-ray scattering (SAXS). In addition, this CNF-based hybrid gel has decisive rheological properties, such as good viscoelasticity (loss tangent was below 0.35 for the UCNP-PSA-CNF gel, while the PSA-CNF gel reached the highest value of 0.42), shear-thinning behavior, and shape retention, and was successfully applied to three-dimensional (3D) gel printing throughout various 3D print models.

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, Ag_29-yAu_y(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.

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.

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 μm 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 ≤ 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%. These large, tunable 2D nanosheets could serve as convenient platforms for future high-efficiency optoelectronic devices.

Monitoring the death of single BaF3 cells under plasmonic photothermal heating induced by ultrasmall gold nanorods

Morphological changes and trypan-blue staining are temporally tracked in single cells via optical microscopy after plasmonic photothermal heating.

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.

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.

Metal-Semiconductor Nanoparticle Hybrids Formed by Self-Organization: A Platform to Address Exciton-Plasmon Coupling

Hybrid nanosystems composed of excitonic and plasmonic constituents can have different properties than the sum of of the two constituents, due to the exciton-plasmon interaction. Here, we report on a flexible model system based on colloidal nanoparticles that can form hybrid combinations by self-organization. The system allows us to tune the interparticle distance and to combine nanoparticles of different sizes and thus enables a systematic investigation of the exciton-plasmon coupling by a combination of optical spectroscopy and quantum-optical theory. We experimentally observe a strong influence of the energy difference between exciton and plasmon, as well as an interplay of nanoparticle size and distance on the coupling. We develop a full quantum theory for the luminescence dynamics and discuss the experimental results in terms of the Purcell effect. As the theory describes excitation as well as coherent and incoherent emission, we also consider possible quantum optical effects. We find a good agreement of the observed and the calculated luminescence dynamics induced by the Purcell effect. This also suggests that the self-organized hybrid system can be used as platform to address quantum optical effects.

Clustering of CdSe/CdS Quantum Dot/Quantum Rods into Micelles Can Form Bright, Non-blinking, Stable, and Biocompatible Probes

We investigate clustered CdSe/CdS quantum dots/quantum rods, ranging from single to multiple encapsulated rods within amphiphilic diblock copolymer micelles, by time-resolved optical spectroscopy. The effect of the clustering and the cluster size on the optical properties is addressed. The clusters are bright and stable and show no blinking while retaining the fundamental optical properties of the individual quantum dots/quantum rods. Cell studies show neither unspecific uptake nor morphological changes of the cells, despite the increased sizes of the clusters.

Synthesis of Carbon Nanowalls and Few-Layer Graphene Sheets on Transparent Conductive Substrates

We report on the growth of carbon nanowalls and few-layer graphene sheets on glass substrates coated with different transparent conductive oxides or gold. The growth is accomplished by a capacitively-coupled radio-frequency plasma-enhanced chemical vapor deposition setup with a gas mixture of C2H2, H2, and Ar or He. This system allows the synthesis of thin vertical carbon nanosheets without catalyst at mild reaction conditions due to the low plasma density, which is preferable for sensitive substrate materials. In fact, the electrical properties of the transparent conductive indium tin oxide and fluorine-doped tin oxide stay intact upon carbon growth. Carbon nanowalls and few-layer graphene sheets on transparent conductive oxides are promising candidates for solar-cell applications while these nanostructures on gold coated glass can act as catalyst support material.

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. Also the fluorescence shows a pronounced complex blinking behavior with very different blinking dynamics of different emission lines in one and the same spectrum. Time- and space-resolved optical spectroscopy are combined with high-resolution transmission electron microscopy of the very same quantum nanowires to establish a detailed structure-property relationship. Extensive numerical simulations strongly suggest that excitonic complexes involving donor and acceptor sites are the origin of the feature-rich spectra.

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. The photoluminescence properties indicate that the prepatterning does not degrade the radiative recombination rate for the site-controlled quantum dots.

Large array of single, site-controlled InAs quantum dots fabricated by UV-nanoimprint lithography and molecular beam epitaxy

We present the growth of single, site-controlled InAs quantum dots on GaAs templates using UV-nanoimprint lithography and molecular beam epitaxy. A large quantum dot array with a period of 1.5 µm was achieved. Single quantum dots were studied by steady-state and time-resolved micro-photoluminescence experiments. We obtained single exciton emission with a linewidth of 45 µeV. In time-resolved experiments, we observed decay times of about 670 ps. Our results underline the potential of nanoimprint lithography and molecular beam epitaxy to create large-scale, single quantum dot arrays.

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. We discuss the influence of alternating wurtzite and zinc blende segments along the NWs on their optical properties.

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.