Scanning Transmission Electron Microscopy Cathodoluminescence Research Articles

Preparation of silver-decorated Soluplus® nanoparticles and antibacterial activity towards S. epidermidis biofilms as characterized by STEM-CL spectroscopy

Biofilm infections present a serious problem because antibacterial drugs are not effective against mature biofilms or biofilms formed by drug-resistant bacteria. To address this issue, we developed a drug delivery system based on metal-decorated polymeric particles. Polyvinyl caprolactam–polyvinyl acetate–polyethylene glycol graft copolymer (Soluplus®) is an amphiphilic polymer used in biomedical formulations, while silver nanoparticles are widely acknowledged to have high antibacterial activity. We prepared silver-decorated Soluplus® micelle nanoparticles with high antibacterial activity using the emulsion solvent diffusion method. Decoration of Soluplus® micelles with silver nanoparticles was found to increase their antibacterial activity. Scanning transmission electron microscopy-cathodoluminescence (STEM-CL) spectroscopy allows imaging of the spatial distribution of labeled targets and the chemical identification of materials. However, STEM-CL spectroscopy of fragile polymer materials is challenging. We optimized the STEM-CL spectroscopy technique to determine the distribution of silver nanoparticles in Soluplus® micelles. Additionally, the surface plasmon properties of the silver nanoparticles were successfully characterized without deactivation. The developed silver-decorated Soluplus® nanoparticles were effective against biofilm infections and have the potential to be applied for other biofilm-related diseases. Additionally, the optimized STEM-CL spectroscopy technique is expected to contribute to the analysis and imaging of fragile polymer materials, as well as other soft materials such as cells and tissues.

Photoluminescence of ZnO Nanowires: A Review.

One-dimensional ZnO nanostructures (nanowires/nanorods) are attractive materials for applications such as gas sensors, biosensors, solar cells, and photocatalysts. This is due to the relatively easy production process of these kinds of nanostructures with excellent charge carrier transport properties and high crystalline quality. In this work, we review the photoluminescence (PL) properties of single and collective ZnO nanowires and nanorods. As different growth techniques were obtained for the presented samples, a brief review of two popular growth methods, vapor-liquid-solid (VLS) and hydrothermal, is shown. Then, a discussion of the emission process and characteristics of the near-band edge excitonic emission (NBE) and deep-level emission (DLE) bands is presented. Their respective contribution to the total emission of the nanostructure is discussed using the spatial information distribution obtained by scanning transmission electron microscopy−cathodoluminescence (STEM-CL) measurements. Also, the influence of surface effects on the photoluminescence of ZnO nanowires, as well as the temperature dependence, is briefly discussed for both ultraviolet and visible emissions. Finally, we present a discussion of the size reduction effects of the two main photoluminescent bands of ZnO. For a wide emission (near ultra-violet and visible), which has sometimes been attributed to different origins, we present a summary of the different native point defects or trap centers in ZnO as a cause for the different deep-level emission bands.

Measurement of nanoscale local stress distribution in phase-separated glass using scanning transmission electron microscopy-cathodoluminescence

Phase-separated glass (PSG) is being used widely for manufacturing scientific apparatus and kitchenware, owing to its advantages of high hardness and heat resistance, which can be attributed to its sea- and island-like structures on the nanoscale. There is a growing demand in the glass industry for methods for evaluating and visualizing the local stress distribution in such glass materials at the nanoscale in order to elucidate their fracture mechanism and to obtain more information to develop even harder glass materials. In this study, we clarified the correlation between the local stress in this type of glass material and its associated nanoscale structure using a nanoelectron probe based on scanning transmission electron microscopy and cathodoluminescence. The peak shift of an emission line corresponding to an intentionally doped rare-earth element was measured based on the piezospectroscopic effect. We found that the sea- and island-like structures, whose origins can be attributed to the difference in the thermal expansion coefficients of the two phases, mutually push each other. This result was confirmed by a simple finite-element method simulation. The present result suggests that the local mechanical properties of PSG can be well described by a coherent elastic continuum consisting of the phases that evolve during thermal treatment.

Mapping the Origins of Luminescence in ZnO Nanowires by STEM-CL.

In semiconductor nanowires, understanding both the sources of luminescence (excitonic recombination, defects, etc.) and the distribution of luminescent centers (be they uniformly distributed, or concentrated at structural defects or at the surface) is important for synthesis and applications. We develop scanning transmission electron microscopy-cathodoluminescence (STEM-CL) measurements, allowing the structure and cathodoluminescence (CL) of single ZnO nanowires to be mapped at high resolution. Using a CL pixel resolution of 10 nm, variations of the CL spectra within such nanowires in the direction perpendicular to the nanowire growth axis are identified for the first time. By comparing the local CL spectra with the bulk photoluminescence spectra, the CL spectral features are assigned to internal and surface defect structures. Hyperspectral CL maps are deconvolved to enable characteristic spectral features to be spatially correlated with structural features within single nanowires. We have used these maps to show that the spatial distribution of these defects correlates well with regions that show an increased rate of nonradiative transitions.

Effect of nano‐porous SiNx interlayer on propagation of extended defects in semipolar ‐orientated GaN

Semipolar orientations of GaN have attracted considerable attention as potential platforms for high brightness light‐emitting devices. The heteroepitaxy of semipolar GaN layers results in high densities of threading dislocations (TDs) and basal‐plane stacking faults (BSFs). Various defect reduction methods have been employed to improve the crystal quality of the semipolar substrates, among which the in situ epitaxial lateral overgrowth (ELO) is of great importance due to its relatively simple procedure, low cost, and high effectiveness in reducing density of TDs. We have shown that in situ deposited nano‐porous SiNx could act as a blocking layer preventing both TDs and BSFs from penetrating to overgrown semipolar GaN layers. In this contribution, we present microscopic study of interaction of extended defects (TDs and BSFs) with nano‐porous SiNx in ‐oriented GaN layers grown by MOCVD on m‐sapphire substrates. Cross‐sectional scanning transmission electron microscopy‐cathodoluminescence (STEM‐CL) reveals reduction of BSF density by about 50% for dual insertion of SiNx interlayers. It is also shown that the integrated CL intensity is enhanced by about two orders of magnitude indicating that the majority of TDs are blocked by the interlayers. The interaction of TDs and BSFs with nano‐porous SiNx and defect distribution in GaN nuclei are briefly discussed.

Nanoscale Imaging of Structural and Optical Properties Using Helium Temperature Scanning Transmission Electron Microscopy Cathodoluminescence of Nitride Based Nanostructures

Nanoscale Imaging of Structural and Optical Properties Using Helium Temperature Scanning Transmission Electron Microscopy Cathodoluminescence of Nitride Based Nanostructures

Investigating the Origin of Luminescence in Zinc Oxide Nanostructures With STEM-Cathodoluminescence

ZnO nanostructures display luminescence in the UV and across the visible spectrum, and show promise as future nanoscale electronic, optoelectronic, and sensing devices [1]. The visible luminescence arises from surface or bulk states at energies inside the ZnO bandgap, however, a fundamental understanding of the luminescent sources is still lacking. The assignment of particular defects to different visible emission peaks is a highly controversial and active area of current research [1,2]. Here, we perform the first spatially resolved scanning transmission electron microscopy cathodoluminescence (STEM-CL) measurements on ZnO nanostructures, and show the emergence of CL spectral peaks associated with morphological changes in ZnO nanorods. Further studies using parallel techniques in the TEM sensitive to intrinsic and extrinsic defects (e.g. HRTEM, atomic resolution HAADF-STEM, EELS, and EDS) will likely conclusively reveal the origin of emission in ZnO and other technologically relevant, luminescent nanostructures.

Direct nano-scale correlation of structural and optical properties of lattice matched AlInN/AlGaN DBRs using helium temperature scanning transmission electron microscopy cathodoluminescence

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Direct imaging of GaN Pyramids covered by InGaN Single Quantum Well using nano-scale Scanning Transmission Electron Microscopy Cathodoluminescence

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

First step of degradation mechanisms in AlGaAs/GaAs laser-like structures

We analyse the first step of artificial degradation experiments performed on Al 0.4Ga 0.6As/GaAs/Al 0.17Ga 0.83As samples. Aluminium content fluctuations in the AlGaAs layer deposited on the GaAs were deduced, in regions sensitive to laser damage, from local scanning transmission electron microscopy cathodoluminescence spectra recorded at 30 K. No fluctuation could be detected in areas where degradation could not be initiated. Even if the lattice mismatch is very low for such epitaxial layers, such composition fluctuations can be seen as being one possible origin of the first step of degradation mechanisms of optical devices. The fluctuations induce stress variations of the order of 1 MPa, which are large enough to induce photoplastic glide of dislocations. The optical contribution to dislocation glide is roughly estimated to be 0.9 eV. It is much more likely to correspond to electronic transitions at a dislocation reconstruction defect than to transitions related to dislocation intrinsic bands.