Micrometer Dimensions Research Articles

Tuning the Morphology of Nanostructured Peptide Films by the Introduction of a Secondary Structure Conformational Constraint: A Case Study of Hierarchical Self-Assembly.

Peptide self-assembly is ubiquitous in nature. It governs the organization of proteins, controlling their folding kinetics and preserving their structural stability and bioactivity. In this connection, model oligopeptides may give important insights into the molecular mechanisms and elementary forces driving the formation of supramolecular structures. In this contribution, we show that a single residue substitution, that is, Aib (α-aminoisobutyric acid) in place of Ala at position 4 of an -(l-Ala)5-homo-oligomer, strongly alters the aggregation process. In particular, this process is initiated by the formation of small peptide clusters that promote aggregation on the nanometer scale and, through a hierarchical self-assembly, lead to mesoscopic structures of micrometric dimensions. Furthermore, we show that the use of the well-established Langmuir-Blodgett technique represents an effective strategy for coating extended areas of inorganic substrates by densely packed peptide layers, thus paving the way for application of peptide films as templates for biomineralization, biocompatible coating of surfaces, and scaffolds for tissue engineering.

Perfluorooctylbromide nanoparticles for ultrasound imaging and drug delivery

Perfluorooctylbromide nanoparticles (PFOB NPs) are a type of multifunctional nanotechnology that has been studied for various medical applications. Commercial ultrasound contrast agents (UCAs) suffer from the following limitations: short half-lives in vivo, high background signal and restricted distribution in the vascular circulation due to their micrometer dimensions. PFOB NPs are new potential UCAs that persist for long periods in the circulatory system, possess a relatively stable echogenic response without increasing the background signal and exhibit lower acoustic attenuation than commercial UCAs. Furthermore, PFOB NPs may also serve as drug delivery vehicles in which drugs are dissolved in the outer lipid or polymer layer for subsequent delivery to target sites in site-targeted therapy. The use of PFOB NPs as carriers has the potential advantage of selectively delivering payloads to the target site while improving visualization of the site using ultrasound (US) imaging. Unfortunately, the application of PFOB NPs to the field of ultrasonography has been limited because of the low intensity of US reflection. Numerous researchers have realized the potential use of PFOB NPs as UCAs and thus have developed alternative approaches to apply PFOB NPs in ultrasonography. In this article, we review the latest approaches for using PFOB NPs to enhance US imaging in vivo. In addition, this article emphasizes the application of PFOB NPs as promising drug delivery carriers for cancer and atherosclerosis treatments, as PFOB NPs can transport different drug payloads for various applications with good efficacy. We also note the challenges and future study directions for the application of PFOB NPs as both a delivery system for therapeutic agents and a diagnostic agent for ultrasonography.

High precision linear frequency modulation ranging system based on semiconductor laser

Based on the linear frequency modulation (LFM) of semiconductor laser, a laser ranging system is built. The ranging precision of the system is about micron dimension with the repetition frequency of 100 kHz and a distance of 1–10 m. We analyzed the ranging precision experimentally. As the bandwidth increases, the ranging precision becomes higher and higher. When the bandwidth is fixed, as the distance increases, the signal-to-noise ratio (SNR) will decrease, which reduces the precision.

Membrane Nanotubes Increase the Robustness of Giant Vesicles.

Giant unilamellar vesicles (GUVs) provide a direct connection between the nano- and the microregime. On the one hand, these vesicles represent biomimetic compartments with linear dimensions of many micrometers. On the other hand, the vesicle walls are provided by single molecular bilayers that have a thickness of a few nanometers and respond sensitively to molecular interactions with small solutes, biopolymers, and nanoparticles. These nanoscopic responses are amplified by the GUVs and can then be studied on much larger scales. Therefore, GUVs are increasingly used as a versatile research tool for basic membrane science, bioengineering, and synthetic biology. Conventional GUVs have one major drawback, however: they have only a limited capability to cope with external perturbations such as osmotic inflation, adhesion, or micropipette aspiration that tend to rupture the membranes. In contrast, cell membranes tolerate the same kinds of mechanical perturbations without rupture because the latter membranes are coupled to reservoirs of membrane area. Here, we introduce GUVs with membrane nanotubes as model systems that include such area reservoirs. To demonstrate the increased robustness of these tubulated vesicles, we use micropipette aspiration and changes in the osmotic conditions applied to phospholipid membranes doped with the glycolipid GM1. A quantitative comparison between theory and experiment reveals that the response of the GUVs is governed by the membranes' spontaneous tension, a curvature-elastic material parameter that describes the bilayer asymmetry on the nanoscale. Because of their increased robustness, GUVs with nanotubes represent improved research tools for membrane science, in general, with potential applications as storage and delivery systems and as cell-like microcompartments in bioengineering, pharmacology, and synthetic biology.

Tuning metal-insulator transitions in epitaxial V2O3 thin films

We present a study of the synthesis of epitaxial V2O3 films on c-plane Al2O3 substrates by reactive dc-magnetron sputtering. The results reveal a temperature window, at substantially lower values than previously reported, wherein epitaxial films can be obtained when deposited on [0001] oriented surfaces. The films display a metal-insulator transition with a change in the resistance of up to four orders of magnitude, strongly dependent on the O2 partial pressure during deposition. While the electronic properties of the films show sensitivity to the amount of O2 present during deposition of the films, their crystallographic structure and surface morphology of atomically flat terraced structures with up to micrometer dimensions are maintained. The transition temperature, as well as the scale of the metal-insulator transition, is correlated with the stoichiometry and local strain in the films controllable by the deposition parameters.

(Vittorio de Nora Award Address) New Tools for Brain Research

At the system level neuroscience is trying to understand how neural circuits work in learning, memory, multisensory integration, motor coordination and how the electrochemical function of these circuits is compromised in the case of disease. Diseases that involve the nervous system are highly complex and mostly not well understood. The Global Burden of Disease Study 2010 (GBD 2010), estimated that a substantial proportion of the world’s disease burden came from mental, neurological and substance use disorders. We need new tools and methods at the system level to effectively tackle these conditions and to better elucidate brain function and disease. One such tool is a novel cognitive computing platform, IBM's TrueNorth chip, that enables the use of deep learning techniques in an ultra-low power environment. We have used deep learning with a convolutional neural network (CNN) and TrueNorth technologies for real-time analysis of brain-activity data at the point of sensing. This approach has the potential to create true closed-loop insights (analytics) and therapies for next generation of wearable and implantable devices at the intersection of neurobiology and artificial intelligence. Neurotransmitters are small proteins secreted between neurons to facilitate neural communication. These compounds are key to information processing during behavior. However, until recently, this chemical communication had not been characterized because biosensors suitable to monitor sub-second chemical events in micron dimensions were unavailable. Cyclic voltammetry at carbon-fiber microelectrodes provides measurements with sub-second time resolution and has been used to examine the dynamics of neurotransmitter concentrations. To enhance the sensitivity and the selectivity of these measurements, we have developed polymeric coatings for bare carbon electrodes. Dopamine, serotonin and adenosine are neurotransmitters that were measured with this methodology. Another tool that we have developed is a novel nanoscale electrode array with superior sensitivity and improved spatial resolution which can be used in the future to gain improved understanding of dopamine dysregulation. We report on the scalable fabrication of dopamine neurochemical probes of a nanostructured glassy carbon that is smaller than any existing dopamine sensor and arrays of more than 6000 nanorod probes. Compared with a flat glassy carbon surface, the nanostructured glassy carbon nanorods provide about 7X higher signal per unit area (current density) for dopamine sensing. The glassy carbon nanorods were fabricated by pyrolysis of a lithographically defined polymeric nanostructure with an industry standard semiconductor fabrication infrastructure. The scalable fabrication strategy offers the potential to integrate these nanoscale dopamine rods with an integrated circuit control system, with other sensors and with different modalities of neural activation.

Inside the Reaction Layer: Investigation of Electrochemical Reactions By Coupling Electrochemistry and Confocal Fluorescence Microscopy

Electrochemical reactivity studies are usually performed by conventional electrochemical measurements carried out with a single electrode of either macroscopic (∼millimeter) or microscopic (∼micrometer) dimensions. Such kind of measurements provides information which can be time resolved down to very short time scales, but which is typically averaged over the entire electrode area and is thus insensitive to spatial heterogeneities in the x-y plane of the electrode. The measured current also gives access to the transport properties of the involved redox-active species, but this additional information corresponding to the z-direction axial to the electrode is rather indirect. Coupling electrochemistry with confocal fluorescence microscopy (EC-fluorescence) is an expedient way to obtain 3-dimensional spatial information on electrochemical reactions, in-situ and in real time. The optical sectioning enabled by confocal microscopy is particularly useful to map individual chemical species in the diffusion layer, especially for unstable electrogenerated species that further react homogeneously. The present contribution focuses on the properties of the electroactive, fluorogenic molecule resorufin (see Figure), to illustrate the many potentialities of EC-fluorescence. The highly fluorescent resorufin can be generated electrochemically by the irreversible reduction of its N-oxide parent compound resazurin which is slightly fluorescent. It can also be reduced reversibly to the non-fluorescent dihydroresorufin, whose acetyl derivative is actually the popular Amplex® Red that can be oxidized to generate resorufin. This exhaustive electrochemistry covers several electrochemical reaction mechanisms ranging from simple electron transfers to coupled electrochemical-chemical steps, with different degrees of reversibility. A comproportionation scheme is also evidenced for the first time. Each of these reactions is associated with specific characteristics of the reaction-diffusion layer that can be probed with an appropriate spatial resolution by EC-fluorescence. Figure 1

Functionalization of a nanostructured hydroxyapatite with Cu(II) compounds as a pesticide: in situ transmission electron microscopy and environmental scanning electron microscopy observations of treated Vitis vinifera L. leaves.

The present study evaluated a biocompatible material for plant protection with the aim of reducing the amount of active substance applied. We used a synthetic hydroxyapatite (HA) that has been studied extensively as a consequence of its bioactivity and biocompatibility. An aggregation between HA nanoparticles and four Cu(II) compounds applied to Vitis vinifera L. leaves as a pesticide was studied. Formulations were characterized by X-ray diffraction (XRD), dynamic light scattering (DLS) and electron microscopy and applied in planta to verify particle aggregation and efficiency in controlling the pathogen Plasmopara viticola. The XRD patterns showed different crystalline phases dependig on the Cu(II) compound formulated with HA particles, DLS showed that nanostructured particles are stable as aggregates out of the nanometer range and, in all formulations, transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM) microscopy showed large aggregates which were partially nanostructured and were recognized as stable in their micrometric dimensions. Such particles did not show phytotoxic effects after their application in planta. A formulation based on HA and a soluble Cu(II) compound showed promising results in the control of the fungal pathogen, confirming the potential role of HA as an innovative delivery system of Cu(II) ions. The present work indicates the possibility of improving the biological activity of a bioactive substance by modifying its structure through an achievable formulation with a biocompatible material. © 2018 Society of Chemical Industry.

On traveling-wave field-effect flow control for simultaneous induced-charge electroosmotic pumping and mixing in microfluidics: physical perspectives and theoretical analysis

Since its first proposition at the end of the last century (Schasfoort et al 1999 Science 286 942–5), field-effect flow control at micrometer dimensions has attracted tremendous attention from the microfluidic community. Most previous research on this subject has mainly focused on enhancing the electroosmotic pump flow rate by introducing an additional in-phase counterionic charge across the diffusing screening cloud with external gate electrodes of static DC voltages. However, there is a flaw, namely that AC fields, which suppress undesirable electrochemical reactions, result in zero time-averaged flow. Starting from this point, we present herein a brand new approach to traveling-wave field-effect electroosmosis control from a theoretical point of view, in the context of a smart manipulation tool for the stratified liquid content of miniaturization systems. In the configuration of a traveling-wave flow field-effect transistor (TW-FFET), the field-induced out-of-phase Debye screening charge within the thin double layer originates from the forward propagation of a traveling potential wave along a discrete arrangement of external gating electrode arrays, which interacts actively with the horizontal standing-wave electric field imposed across the source–drain terminal. Since the voltage waves and induced free charge are all sinusoidal functions of the observation time, the net ICEO flow component can survive in a broad frequency range. Due to the action of the background AC electric field on the inhomogeneous counterionic charge induced at the solution/sidewall interface, asymmetric ICEO vortex patterns appear above the traveling-wave gate arrays, giving rise to simultaneous induced-charge electroosmotic pumping and mixing of fluidic samples. A mathematical model is then developed to numerically investigate the feasibility of TW-FFETs in electrokinetic microflow manipulation. A prototyping paradigm of fully electrokinetics-driven microfabricated fluidic networks in a cross shape is theoretically erected, with four sets of gating traveling-fields in perpendicular orientations, from which the resulting liquid mixture is obtainable at any one of the three outlet ports. Supported by mathematical analysis, our physical demonstration of the TW-FFET shows it has great potential to advance fully automated electroconvective sample treatment in modern micro total analytical systems.

Electrochemical exfoliation of graphite to stage-III graphite bisulfate flakes in low concentration sulfuric acid solution: A novel synthesis route to completely trilayer graphene suspension

Graphene produced from electrochemical exfoliation of graphite show a scatter in the number of layers. This scatter is a serious drawback for sensor and opto-electronic applications since the electronic properties of graphene change with number of layers. The scatter in the layer number of graphene is caused by formation of the intermediate graphite intercalation compounds (GIC) of different stage numbers as well as simultaneous cleaving of the GICs in the dispersion-unsuitable aqueous environment. In this work, we show the synthesis of stage-III graphite bisulfate (GB) enriched flakes by electrochemical exfoliation of graphite in low concentration (0.1 M) sulfuric acid from two different routes. We further show that the intercalated bisulfate planes provide sites for selective cleaving of the GB particles to trilayer graphene in DMF, a solvent favorable for graphene dispersion. Morphological characterizations show that while GB particles from one of the routes give graphene with a small scatter in the layer numbers, the other route provides a completely trilayer graphene dispersion. TEM and optical micrographs show graphene flakes have linear dimensions of several micrometers and a low aspect ratio suitable for use in sensor applications.

On the Bipolar DC Flow Field-Effect-Transistor for Multifunctional Sample Handing in Microfluidics: A Theoretical Analysis under the Debye⁻Huckel Limit.

We present herein a novel method of bipolar field-effect control on DC electroosmosis (DCEO) from a physical point of view, in the context of an intelligent and robust operation tool for stratified laminar streams in microscale systems. In this unique design of the DC flow field-effect-transistor (DC-FFET), a pair of face-to-face external gate terminals are imposed with opposite gate-voltage polarities. Diffuse-charge dynamics induces heteropolar Debye screening charge within the diffuse double layer adjacent to the face-to-face oppositely-polarized gates, respectively. A background electric field is applied across the source-drain terminal and forces the face-to-face counterionic charge of reversed polarities into induced-charge electroosmotic (ICEO) vortex flow in the lateral direction. The chaotic turbulence of the transverse ICEO whirlpool interacts actively with the conventional plug flow of DCEO, giving rise to twisted streamlines for simultaneous DCEO pumping and ICEO mixing of fluid samples along the channel length direction. A mathematical model in thin-layer approximation and the low-voltage limit is subsequently established to test the feasibility of the bipolar DC-FFET configuration in electrokinetic manipulation of fluids at the micrometer dimension. According to our simulation analysis, an integrated device design with two sets of side-by-side, but upside-down gate electrode pair exhibits outstanding performance in electroconvective pumping and mixing even without any externally-applied pressure difference. Moreover, a paradigm of a microdevice for fully electrokinetics-driven analyte treatment is established with an array of reversed bipolar gate-terminal pairs arranged on top of the dielectric membrane along the channel length direction, from which we can obtain almost a perfect liquid mixture by using a smaller magnitude of gate voltages for causing less detrimental effects at a small Dukhin number. Sustained by theoretical analysis, our physical demonstration on bipolar field-effect flow control for the microfluidic device of dual functionalities in simultaneous electroconvective pumping and mixing holds great potential in the development of fully-automated liquid-phase actuators in modern microfluidic systems.

Far-UVC light: A new tool to control the spread of airborne-mediated microbial diseases

Airborne-mediated microbial diseases such as influenza and tuberculosis represent major public health challenges. A direct approach to prevent airborne transmission is inactivation of airborne pathogens, and the airborne antimicrobial potential of UVC ultraviolet light has long been established; however, its widespread use in public settings is limited because conventional UVC light sources are both carcinogenic and cataractogenic. By contrast, we have previously shown that far-UVC light (207–222 nm) efficiently inactivates bacteria without harm to exposed mammalian skin. This is because, due to its strong absorbance in biological materials, far-UVC light cannot penetrate even the outer (non living) layers of human skin or eye; however, because bacteria and viruses are of micrometer or smaller dimensions, far-UVC can penetrate and inactivate them. We show for the first time that far-UVC efficiently inactivates airborne aerosolized viruses, with a very low dose of 2 mJ/cm2 of 222-nm light inactivating >95% of aerosolized H1N1 influenza virus. Continuous very low dose-rate far-UVC light in indoor public locations is a promising, safe and inexpensive tool to reduce the spread of airborne-mediated microbial diseases.

Cross-Scale Molecular Analysis of Chemical Heterogeneity in Shale Rocks

The organic and mineralogical heterogeneity in shale at micrometer and nanometer spatial scales contributes to the quality of gas reserves, gas flow mechanisms and gas production. Here, we demonstrate two molecular imaging approaches based on infrared spectroscopy to obtain mineral and kerogen information at these mesoscale spatial resolutions in large-sized shale rock samples. The first method is a modified microscopic attenuated total reflectance measurement that utilizes a large germanium hemisphere combined with a focal plane array detector to rapidly capture chemical images of shale rock surfaces spanning hundreds of micrometers with micrometer spatial resolution. The second method, synchrotron infrared nano-spectroscopy, utilizes a metallic atomic force microscope tip to obtain chemical images of micrometer dimensions but with nanometer spatial resolution. This chemically “deconvoluted” imaging at the nano-pore scale is then used to build a machine learning model to generate a molecular distribution map across scales with a spatial span of 1000 times, which enables high-throughput geochemical characterization in greater details across the nano-pore and micro-grain scales and allows us to identify co-localization of mineral phases with chemically distinct organics and even with gas phase sorbents. This characterization is fundamental to understand mineral and organic compositions affecting the behavior of shales.

High temperature thermo-physical properties of SPS-ed W–Cu functional gradient materials

The divertor of a fusion reactor like DEMO requires materials able to withstand high heat fluxes and neutron irradiation for several years. For the water cooling concept of this essential part of the reactor, the most likely plasma facing material will be W, while the heatsink material considered is CuCrZr or an improved version of such a Cu-based alloy. To realize W–Cu alloy joints able to withstand thousands of thermal cycles can be difficult due to the difference between the thermal expansion coefficients of these materials. In this work we investigate the possibility to realize such joints by using W–Cu functional gradient materials (FGMs) produced from nanometric and micrometric metallic powders mixtures and consolidated by spark plasma sintering at about 900 °C. Morphological and thermal properties investigations, performed for typical compositions, shows that the best results are obtained using powders with micrometric dimensions. A resulting 1 mm thick, 3 layers W–Cu FGM produced by this simple method shows a remarkable almost constant thermal conductivity value of 200 W m−1 K−1, from room temperature up to 1000 °C.

Microscale Tests of Cement Paste Performed with FIB and Nanoindentation

This study deals with experimental determination of tensile properties of cement paste hydration products at micro-scale. Cantilever micro-beams with length of about 16 µm and pentagon cross section with micrometer dimensions were fabricated by focused ion beam milling on hydrated cement paste samples. Nanoindentation was used for evaluating elastic properties while tensile properties were derived from beam bending tests. Displacement controlled micro-scale tests give access to both tensile strength and estimates of fracture energy based on the load-displacement curves measured with the nanoindenter. The mean tensile strength and the fracture energy of inner hydration product were assessed as 791 MPa and 16.7 J/m2, respectively. The huge difference of the micro-scale properties when compared to macroscopic values comes from the scaling properties of concrete.

Controlled Electrodeposition of Zinc Oxide on Conductive Meshes and Foams Enabling Its Use as Secondary Anode

Both compact and porous zinc oxide (ZnO) films were electrochemically deposited out of aqueous zinc nitrate solutions under pulsed galvanostatic control onto different three-dimensional (3D) substrates. Gold and carbon meshes as well as copper and nickel foams were used as substrates, all having complex geometries in micrometer dimensions with high surface area. Electrodepositions of ZnO were controlled by regulating the most relevant parameters: the applied current density, the total deposition time, and the length of current pulses and pauses to avoid gas evolution interfering with film growth at the electrodes. Optimization of these parameters for each of the substrates allowed deposition of homogeneous ZnO films of the desired total film thickness. Potential-time curves measured during deposition helped to monitor film growth. Scanning electron microscopy (SEM) micrographs showed that pinhole-free ZnO films were deposited uniformly in a 3D manner on all substrates. X-ray diffraction (XRD) measurements confirmed the formation of highly crystalline ZnO films. Such electrodeposited ZnO films on electronic conductive substrates with high surface area are promising for application in energy storage and conversion systems – especially for secondary batteries with zinc anodes which was shown by cyclic voltammetry (CV).

Hierarchical self-assembly of amphiphilic building blocks into various supramolecular architectures

Molecular self-assembly is a process to obtain a well defined structure from suitable starting materials. The study of self-assembly provides a fundamental understanding of the driving forces and co-operative interaction behind these processes. Among the plenty of molecules, an amphiphilic molecule, containing both hydrophilic and hydrophobic counter parts is one of the interesting building blocks to study the self-assembly processes. In this short review, the hierarchical self-assembly of amphiphilic molecules into various supramolecular architectures such as vesicles, spheres, flowers, tubes and fibrillar networks of nano- to micrometer dimensions and gels have been discussed.

Terpenoids in Early 21st Century: From Renewable Functional Nano-entities to Advanced Materials

Plant metabolites can serve as renewable chemicals for the development of a sustainable society. Terpenoids are the major component of over 500,000 plant secondary metabolites reported till date. Monoterpenoid (C10) to tetraterpenoid (C40) all the terpenoid have recently been shown to have nanometric lengths. Studies on the self-assembly of terpenoids, have drawn the attention of scientific community in recent years due to many of its potential and realized applications in medicine, drug delivery, pollutant capture, metal nanoparticle hybrid material, selective damage of cancer cells, etc. In this review, we have discussed the isolation of seven terpenoids from plants and their self-assembly properties in different liquids, even without functional transformation. The utilization of the resulting supramolecular architectures such as vesicles, spheres, flowers and fibrillar networks of nano- to micro-meter dimensions and gels have also been discussed paving the way for a green, renewable and sustainable world.

Effect of Iron-Containing Intermetallic Particles on Film Structure and Corrosion Resistance of Anodized AA2099 Alloy

The anodizing behavior of iron-containing intermetallic particles in AA2099 aluminum-lithium alloy and their effect on structure and corrosion resistance of the anodic film were investigated using electrochemical measurements and scanning electron microscopy. High-copper-containing Al-Fe-Mn-Cu particles (HCCPs) dissolved preferentially through dealloying at ∼ 0 V (vs. saturated calomel electrode) in a tartaric-sulfuric acid solution, at 22, 37 and 42°C, leading to formation of copper-rich nanoparticle of 50–200 nm diameters. They dissolved completely under normal anodizing conditions, resulting in cavity defects of micrometer dimensions in the anodic film and sunken regions in the alloy substrate immediately beneath the dissolved HCCPs. Immersion testing of the anodized alloy in 3.5% NaCl solution for 24 h indicated that localized corrosion of the anodized alloy predominantly developed at the site containing dissolved HCCPs at the film/alloy interface. It is suggested that HCCPs play a critical role in controlling the corrosion resistance of the anodic film formed on AA2099 aluminum-lithium alloy.

Fluorescence Spectroscopy of AdamBODIPY Single Crystals

AbstractInterest in the fluorescence of organic solids is increasing with the development of nanosensors and research into new molecules with aggregation‐induced fluorescence properties. We have gone beyond the qualitative observation of fluorescence by analyzing the luminescence properties of planar single crystals of a 4,4′‐difluoro‐4‐bora‐(3a,4a)‐diaza‐s‐indacene (BODIPY) derivative with micrometric dimensions. A simple Frenkel exciton model applied to this crystal predicts one band. From time‐resolved fluorescence spectra, three emissions were distinguished. The shortest‐lived one at λ=547 nm was predicted by theory and corresponds to an exciton lifetime of 0.9 ns. A trap with an intermediate emission lifetime of 1.2 ns was found at 569 nm and a final trap at 620 nm has a lifetime of 1.9 ns. These attributions were confirmed by the study of the polarization of these emissions. The 547 nm emission was polarized along the long axis of the crystal as predicted by the Frenkel exciton model. The 569 nm emission was polarized perpendicularly to the plane of the crystal and the 620 nm emission was polarized along the short axis. Thus, the two red‐shifted bands were related to well‐defined defects with specific orientations in the crystal. Fluorescence lifetime imaging measurements showed that the density of these defects is not uniform and that under our synthesis conditions, they are formed in the initial steps of the growth and therefore appear in the center of the crystals.