Direct Electronic Property Imaging of a Nanocrystal-Based Photovoltaic Device by Electron Beam-Induced Current via Scanning Electron Microscopy

Comparative study of ion milling techniques for the preparation of ceramic fibers in transmission electron microscopy

Sample preparation, essential to any analysis, remains an under-documented step. Preparation methods for transmission electron microscopy (TEM) are complex and lead to artifacts that need to be identified to avoid wrong conclusions about the sample's microstructure. Ion milling techniques are increasingly becoming the reference techniques to prepare thin foils. The possibilities of different ion beam processes for milling samples will be shown and compared, using ceramic carbon/carbide fibers, a material applied in many industrial applications, as a test specimen. This overview of ion milling preparation techniques will enable us to identify the advantages, disadvantages and parameters in order to best prepare these thin samples. It will be carried out for the community, and will also highlight a new preparation method for ceramic fibers to minimize the artefacts inherent in these techniques.

Effect of indium alloying on the charge carrier dynamics of thick-shell InP/ZnSe quantum dots

Thick-shell InP/ZnSe III-V/II-VI quantum dots (QDs) were synthesized with two distinct interfaces between the InP core and ZnSe shell: alloy and core/shell. Despite sharing similar optical properties in the spectral domain, these two QD systems have differing amounts of indium incorporation in the shell as determined by high-resolution energy-dispersive x-ray spectroscopy scanning transmission electron microscopy. Ultrafast fluorescence upconversion spectroscopy was used to probe the charge carrier dynamics of these two systems and shows substantial charge carrier trapping in both systems that prevents radiative recombination and reduces the photoluminescence quantum yield. The alloy and core/shell QDs show slight differences in the extent of charge carrier localization with more extensive trapping observed in the alloy nanocrystals. Despite the ability to grow a thick shell, structural defects caused by III-V/II-VI charge carrier imbalances still need to be mitigated to further improve InP QDs.

Minority Carrier Transport in Lead Sulfide Quantum Dot Photovoltaics

Lead sulfide quantum dots (PbS QDs) are an attractive material system for the development of low-cost photovoltaics (PV) due to their ease of processing and stability in air, with certified power conversion efficiencies exceeding 11%. However, even the best PbS QD PV devices are limited by diffusive transport, as the optical absorption length exceeds the minority carrier diffusion length. Understanding minority carrier transport in these devices will therefore be critical for future efficiency improvement. We utilize cross-sectional electron beam-induced current (EBIC) microscopy and develop methodology to quantify minority carrier diffusion length in PbS QD PV devices. We show that holes are the minority carriers in tetrabutylammonium iodide (TBAI)-treated PbS QD films due to the formation of a p-n junction with an ethanedithiol (EDT)-treated QD layer, whereas a heterojunction with n-type ZnO forms a weaker n⁺-n junction. This indicates that modifying the standard device architecture to include a p-type window layer would further boost the performance of PbS QD PV devices. Furthermore, quantitative EBIC measurements yield a lower bound of 110 nm for the hole diffusion length in TBAI-treated PbS QD films, which informs design rules for planar and ordered bulk heterojunction PV devices. Finally, the low-energy EBIC approach developed in our work is generally applicable to other emerging thin-film PV absorber materials with nanoscale diffusion lengths.

Core-shell InGaN/GaN nanowire light emitting diodes analyzed by electron beam induced current microscopy and cathodoluminescence mapping

We report on the electron beam induced current (EBIC) microscopy and cathodoluminescence (CL) characterization correlated with compositional analysis of light emitting diodes based on core/shell InGaN/GaN nanowire arrays. The EBIC mapping of cleaved fully operational devices allows to probe the electrical properties of the active region with a nanoscale resolution. In particular, the electrical activity of the p-n junction on the m-planes and on the semi-polar planes of individual nanowires is assessed in top view and cross-sectional geometries. The EBIC maps combined with CL characterization demonstrate the impact of the compositional gradients along the wire axis on the electrical and optical signals: the reduction of the EBIC signal toward the nanowire top is accompanied by an increase of the CL intensity. This effect is interpreted as a consequence of the In and Al gradients in the quantum well and in the electron blocking layer, which influence the carrier extraction efficiency. The interface between the nanowire core and the radially grown layer is shown to produce in some cases a transitory EBIC signal. This observation is explained by the presence of charged traps at this interface, which can be saturated by electron irradiation.

Insights into the physical chemistry of materials from advances in HAADF-STEM

HAADF-STEM provides atomic-resolution real space imaging. Here an image of a single Si dopant atom in a graphene lattice is shown adjacent to a schematic of the instrument. Simultaneous EELS on electrons scattered to low angles can provide chemical identification of the species preset. Differences between the Si L-edge spectra reveal differences in atomic bonding and hybridization for different configurations of Si atoms in graphene.