Phase Segregation Mechanisms in Mixed-Halide CsPb(BrxI1-x)3 Nanocrystals in Dependence of Their Sizes and Their Initial [Br]:[I] Ratios.

Phase segregation in inorganic CsPb(BrxI1-x)3 nanoparticles (NPs) exhibiting originally a homogeneous [Br]:[I] mixture was investigated by means of in situ transmission electron microscopy (TEM) and evaluated by using multivariate analyses. The colloidal synthesis of the NPs offers good control of the halide ratios on the nanoscale. The spatially resolved TEM investigations were correlated with integral photoluminescence measurements. By this approach, the halide-segregation processes and their spatial distributions can be described as being governed by the interaction of three partial processes: electron- and photon-irradiation-induced iodide oxidation, local differences in band gap energy, and intrinsic lattice strain. Since the oxidation can be induced by both electron-beam and light irradiation, both irradiation types can induce phase segregation in CsPb(BrxI1-x)3 compounds. This makes in situ TEM a valuable tool to monitor phase transformation in corresponding NPs and thin films on the sub-nm scale.

On the non-bonding valence band and the electronic properties of poly(triazine imide), a graphitic carbon nitride.

Graphitic carbon nitrides are covalently-bonded, layered, and crystalline semiconductors with high thermal and oxidative stability. These properties make graphitic carbon nitrides potentially useful in overcoming the limitations of 0D molecular and 1D polymer semiconductors. In this contribution, we study structural, vibrational, electronic and transport properties of nano-crystals of poly(triazine-imide) (PTI) derivatives with intercalated Li- and Br-ions and without intercalates. Intercalation-free poly(triazine-imide) (PTI-IF) is corrugated or AB stacked and partially exfoliated. We find that the lowest energy electronic transition in PTI is forbidden due to a non-bonding uppermost valence band and that its electroluminescence from the π-π* transition is quenched which severely limits their use as emission layer in electroluminescent devices. THz conductivity in nano-crystalline PTI is up to eight orders of magnitude higher than the macroscopic conductivity of PTI films. We find that the charge carrier density of PTI nano-crystals is among the highest of all known intrinsic semiconductors, however, macroscopic charge transport in films of PTI is limited by disorder at crystal-crystal interfaces. Future device applications of PTI will benefit most from single crystal devices that make use of electron transport in the lowest, π-like conduction band.

Poly(ionic liquid) Nanovesicle-Templated Carbon Nanocapsules Functionalized with Uniform Iron Nitride Nanoparticles as Catalytic Sulfur Host for Li-S Batteries.

Poly(ionic liquid)s (PIL) are common precursors for heteroatom-doped carbon materials. Despite a relatively higher carbonization yield, the PIL-to-carbon conversion process faces challenges in preserving morphological and structural motifs on the nanoscale. Assisted by a thin polydopamine coating route and ion exchange, imidazolium-based PIL nanovesicles were successfully applied in morphology-maintaining carbonization to prepare carbon composite nanocapsules. Extending this strategy further to their composites, we demonstrate the synthesis of carbon composite nanocapsules functionalized with iron nitride nanoparticles of an ultrafine, uniform size of 3-5 nm (termed "FexN@C"). Due to its unique nanostructure, the sulfur-loaded FexN@C electrode was tested to efficiently mitigate the notorious shuttle effect of lithium polysulfides (LiPSs) in Li-S batteries. The cavity of the carbon nanocapsules was spotted to better the loading content of sulfur. The well-dispersed iron nitride nanoparticles effectively catalyze the conversion of LiPSs to Li2S, owing to their high electronic conductivity and strong binding power to LiPSs. Benefiting from this well-crafted composite nanostructure, the constructed FexN@C/S cathode demonstrated a fairly high discharge capacity of 1085 mAh g-1 at 0.5 C initially, and a remaining value of 930 mAh g-1 after 200 cycles. In addition, it exhibits an excellent rate capability with a high initial discharge capacity of 889.8 mAh g-1 at 2 C. This facile PIL-to-nanocarbon synthetic approach is applicable for the exquisite design of complex hybrid carbon nanostructures with potential use in electrochemical energy storage and conversion.

Surface-Functionalized Au-Pd Nanorods with Enhanced Photothermal Conversion and Catalytic Performance.

Bimetallic nanostructures comprising plasmonic and catalytic components have recently emerged as a promising approach to generate a new type of photo-enhanced nanoreactors. Most designs however concentrate on plasmon-induced charge separation, leaving photo-generated heat as a side product. This work presents a photoreactor based on Au-Pd nanorods with an optimized photothermal conversion, which aims to effectively utilize the photo-generated heat to increase the rate of Pd-catalyzed reactions. Dumbbell-shaped Au nanorods were fabricated via a seed-mediated growth method using binary surfactants. Pd clusters were selectively grown at the tips of the Au nanorods, using the zeta potential as a new synthetic parameter to indicate the surfactant remaining on the nanorod surface. The photothermal conversion of the Au-Pd nanorods was improved with a thin layer of polydopamine (PDA) or TiO2. As a result, a 60% higher temperature increment of the dispersion compared to that for bare Au rods at the same light intensity and particle density could be achieved. The catalytic performance of the coated particles was then tested using the reduction of 4-nitrophenol as the model reaction. Under light, the PDA-coated Au-Pd nanorods exhibited an improved catalytic activity, increasing the reaction rate by a factor 3. An analysis of the activation energy confirmed the photoheating effect to be the dominant mechanism accelerating the reaction. Thus, the increased photothermal heating is responsible for the reaction acceleration. Interestingly, the same analysis shows a roughly 10% higher reaction rate for particles under illumination compared to under dark heating, possibly implying a crucial role of localized heat gradients at the particle surface. Finally, the coating thickness was identified as an essential parameter determining the photothermal conversion efficiency and the reaction acceleration.

Unraveling electronic band structure of narrow-bandgap p-n nanojunctions in heterostructured nanowires.

The electronic band structure of complex nanostructured semiconductors has a considerable effect on the final electronic and optical properties of the material and, ultimately, on the functionality of the devices incorporating them. Valence electron energy-loss spectroscopy (VEELS) in the transmission electron microscope (TEM) provides the possibility of measuring this property of semiconductors with high spatial resolution. However, it still represents a challenge for narrow-bandgap semiconductors, since an electron beam with low energy spread is required. Here we demonstrate that by means of monochromated VEELS we can study the electronic band structure of narrow-gap materials GaSb and InAs in the form of heterostructured nanowires, with bandgap values down to 0.5 eV, especially important for newly developed structures with unknown bandgaps. Using complex heterostructured InAs-GaSb nanowires, we determine a bandgap value of 0.54 eV for wurtzite InAs. Moreover, we directly compare the bandgaps of wurtzite and zinc blende polytypes of GaSb in a single nanostructure, measured here as 0.84 and 0.75 eV, respectively. This allows us to solve an existing controversy in the band alignment between these structures arising from theoretical predictions. The findings demonstrate the potential of monochromated VEELS to provide a better understanding of the band alignment at the heterointerfaces of narrow-bandgap complex nanostructured materials with high spatial resolution. This is especially important for semiconductor device applications where even the slightest variations of the electronic band structure at the nanoscale can play a crucial role in their functionality.

Quantifying the data quality of focal series for inline electron holography.

Inline electron holography, the recovery of amplitude and phase of an electron wave function having passed through a thin specimen from a focal series recorded in a transmission electron microscope is being applied in many labs worldwide. At medium range magnification (i.e. typically ≥0.8 nm where the lattice of small unit cell crystals such as silicon is not resolved), where the defocus needs to be varied over a rather large range of several hundred nm or even µm, the retrieval of low spatial frequency information is severely affected by the choice of experimental parameters as well as the way of data normalization. Methods to quantitatively analyze the reliability of phase maps obtained by inline electron holography are presented, and data recorded and processed in different ways are compared. While, even under optimized conditions, the phase reconstructed from an experimental focal series still lacks very low spatial frequency components, regularization schemes exist and are demonstrated to effectively hide artifacts associated with this lack of information.

In Situ TEM Monitoring of Phase-Segregation in Inorganic Mixed Halide Perovskite.

Photoinduced phase separation, which limits the available band gap energies for photovoltaic applications, was reported for a range of mixed-halide perovskites. A microscopic understanding of the phase separation mechanism is still lacking but may be beneficial to rationalize limitations as well as enable the design of phase-stable perovskite semiconductors. In this letter, electron-beam-induced phase separations and transformations were investigated in a small crystallite of CsPb(Br0.8I0.2)3 by means of in situ high-resolution imaging in a transmission electron microscope. The acquired time series was evaluated using principal and independent component analysis to classify the structural change during the illumination by the electron beam. A more iodine-rich phase with the approximate composition of CsPb(Br0.6I0.4)3 was found to form at the edges of the particle, while a ternary pure bromide phase of CsPbBr3 remained at its center. These results provide an atomistic picture of in-grain phase segregation into iodide-rich phases at grain boundaries and bromide-rich phases in the interior of the grain.

Multi-resolution convolutional neural networks for inverse problems.

Inverse problems in image processing, phase imaging, and computer vision often share the same structure of mapping input image(s) to output image(s) but are usually solved by different application-specific algorithms. Deep convolutional neural networks have shown great potential for highly variable tasks across many image-based domains, but sometimes can be challenging to train due to their internal non-linearity. We propose a novel, fast-converging neural network architecture capable of solving generic image(s)-to-image(s) inverse problems relevant to a diverse set of domains. We show this approach is useful in recovering wavefronts from direct intensity measurements, imaging objects from diffusely reflected images, and denoising scanning transmission electron microscopy images, just by using different training datasets. These successful applications demonstrate the proposed network to be an ideal candidate solving general inverse problems falling into the category of image(s)-to-image(s) translation.

Design and application of a relativistic Kramers-Kronig analysis algorithm.

Low-loss electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope probes the valence electron density and relevant optoelectronic properties such as band gap energies and other band structure transitions. The measured spectra can be formulated in a dielectric theory framework, comparable to optical spectroscopies and ab-initio simulations. Moreover, Kramers-Kronig analysis (KKA), an inverse algorithm based on the same name relations, can be employed for the retrieval of the complex dielectric function. However, spurious contributions traditionally not considered in this framework typically impact low-loss EELS modifying the spectral shapes and precluding the correct measurement and retrieval of the dielectric information. A relativistic KKA algorithm is able to account for the bulk and surface radiative-loss contributions to low-loss EELS, revealing the correct dielectric properties. Using a synthetic low-loss EELS model, we propose some modifications on the naive implementation of this algorithm that broadens its range of application. The robustness of the algorithm is improved by regularization, applying previous knowledge about the shape and smoothness of the correction term. Additionally, our efficient numerical integration methodology allows processing hyperspectral datasets in a reasonable amount of time. Harnessing these abilities, we show how simultaneous relativistic KKA processing of several spectra can share information to produce an improved result.

Multi-focus TIE algorithm including partial spatial coherence and overlapping filters.

The transport of intensity equation (TIE) relates the variation of intensity of a wave-front along its mean direction of propagation with its phase. In experimental application, characteristic artefacts may affect the retrieved phase. These originate from inadequacies in estimating the axial derivative and the amplification of noise in the inversion of the TIE. To tackle these issues, images recorded at multiple planes of focus can be integrated into a multi-focus TIE (MFTIE) solution. This methodology relies on the efficient sampling of phase information in the spatial-frequency domain, typically by the definition of band pass filters implemented as a progression of sharp spatial frequency cut-offs. We present a convenient MFTIE implementation which avoids the need for recording images at very specific planes of focus and applies overlapping cut-offs, greatly simplifying the experimental application. This new approach additionally also accounts for partial spatial coherence in a flux-preserving framework. Using simulated data as well as experimental data from optical microscopy and electron microscopy we show that the frequency response of this MFTIE algorithm recovers efficiently a wide range of spatial frequencies of the phase that can be further extended by simple iterative refinement.

Quantitative parameters for the examination of InGaN QW multilayers by low-loss EELS.

We present a detailed examination of a multiple InxGa1-xN quantum well (QW) structure for optoelectronic applications. The characterization is carried out using scanning transmission electron microscopy (STEM), combining high-angle annular dark field (HAADF) imaging and electron energy loss spectroscopy (EELS). Fluctuations in the QW thickness and composition are observed in atomic resolution images. The impact of these small changes on the electronic properties of the semiconductor material is measured through spatially localized low-loss EELS, obtaining band gap and plasmon energy values. Because of the small size of the InGaN QW layers additional effects hinder the analysis. Hence, additional parameters were explored, which can be assessed using the same EELS data and give further information. For instance, plasmon width was studied using a model-based fit approach to the plasmon peak; observing a broadening of this peak can be related to the chemical and structural inhomogeneity in the InGaN QW layers. Additionally, Kramers-Kronig analysis (KKA) was used to calculate the complex dielectric function (CDF) from the EELS spectrum images (SIs). After this analysis, the electron effective mass and the sample absolute thickness were obtained, and an alternative method for the assessment of plasmon energy was demonstrated. Also after KKA, the normalization of the energy-loss spectrum allows us to analyze the Ga 3d transition, which provides additional chemical information at great spatial resolution. Each one of these methods is presented in this work together with a critical discussion of their advantages and drawbacks.

3D Visualization of the Iron Oxidation State in FeO/Fe3O4 Core-Shell Nanocubes from Electron Energy Loss Tomography.

The physicochemical properties used in numerous advanced nanostructured devices are directly controlled by the oxidation states of their constituents. In this work we combine electron energy-loss spectroscopy, blind source separation, and computed tomography to reconstruct in three dimensions the distribution of Fe(2+) and Fe(3+) ions in a FeO/Fe3O4 core/shell cube-shaped nanoparticle with nanometric resolution. The results highlight the sharpness of the interface between both oxides and provide an average shell thickness, core volume, and average cube edge length measurements in agreement with the magnetic characterization of the sample.

Density Functional Theory Modeling of Low-Loss Electron Energy-Loss Spectroscopy in Wurtzite III-Nitride Ternary Alloys.

In the present work, the dielectric response of III-nitride semiconductors is studied using density functional theory (DFT) band structure calculations. The aim of this study is to improve our understanding of the features in the low-loss electron energy-loss spectra of ternary alloys, but the results are also relevant to optical and UV spectroscopy results. In addition, the dependence of the most remarkable features with composition is tested, i.e. applying Vegard's law to band gap and plasmon energy. For this purpose, three wurtzite ternary alloys, from the combination of binaries AlN, GaN, and InN, were simulated through a wide compositional range (i.e., Al x Ga1-x N, In x Al1-x N, and In x Ga1-x N, with x=[0,1]). For this DFT calculations, the standard tools found in Wien2k software were used. In order to improve the band structure description of these semiconductor compounds, the modified Becke-Johnson exchange-correlation potential was also used. Results from these calculations are presented, including band structure, density of states, and complex dielectric function for the whole compositional range. Larger, closer to experimental values, band gap energies are predicted using the novel potential, when compared with standard generalized gradient approximation. Moreover, a detailed analysis of the collective excitation features in the dielectric response reveals their compositional dependence, which sometimes departs from a linear behavior (bowing). Finally, an advantageous method for measuring the plasmon energy dependence from these calculations is explained.

Retrieving the electronic properties of silicon nanocrystals embedded in a dielectric matrix by low-loss EELS.

In this work we apply low-loss electron energy loss spectroscopy (EELS) to probe the structural and electronic properties of single silicon nanocrystals (NCs) embedded in three different dielectric matrices (SiO2, SiC and Si(3)N(4)). A monochromated and aberration corrected transmission electron microscope has been operated at 80 kV to avoid sample damage and to reduce the impact of radiative losses. We present a novel approach to disentangle the electronic features corresponding to pure Si-NCs from the surrounding dielectric material contribution through an appropriate computational treatment of hyperspectral datasets. First, the different material phases have been identified by measuring the plasmon energy. Due to the overlapping of Si-NCs and dielectric matrix information, the variable shape and position of mixed plasmonic features increases the difficulty of non-linear fitting methods to identify and separate the components in the EELS signal. We have managed to solve this problem for silicon oxide and nitride systems by applying multivariate analysis methods that can factorize the hyperspectral datacubes in selected regions. By doing so, the EELS spectra are re-expressed as a function of abundance of Si-NC-like and dielectric-like factors. EELS contributions from the embedded nanoparticles as well as their dielectric surroundings are thus studied in a new light, and compared with the dielectric material and crystalline silicon from the substrate. Electronic properties such as band gaps and plasmon shifts can be obtained by a straightforward examination. Finally, we have calculated the complex dielectric functions and the related electron effective mass and density of valence electrons.

EELS tomography in multiferroic nanocomposites: from spectrum images to the spectrum volume.

Electron Energy Loss Spectroscopy (EELS) in a transmission electron microscope offers the possibility of extracting high accuracy maps of composition and electronic properties through EELS spectrum images (EELS-SI). Acquiring EELS-SI for different tilt angles, a 3D tomographic reconstruction of EELS information can be achieved. In the present work we show that an EELS spectrum volume (EELS-SV), a 4D dataset where every voxel contains a full EELS spectrum, can be reconstructed from the EELS-SI tilt series by the application of multivariate analysis. We apply this novel approach to characterize a nanocomposite material consisting of CoFe2O4 nanocolumns embedded in a BiFeO3 matrix grown on a LaNiO3 buffered LaAlO3 (001) substrate.

Structural and compositional properties of Er-doped silicon nanoclusters/oxides for multilayered photonic devices studied by STEM-EELS.

High resolution scanning transmission electron microscopy with an aberration corrected and monochromated instrument has been used for the assessment of the silicon-based active layer stack for novel optoelectronic devices. This layer contains a multilayer structure consisting of alternate thin layers of pure silica (SiO2) and silicon-rich silicon oxide (SRO, SiOx). Upon high temperature annealing the SRO sublayer segregates into a Si nanocluster (Si-nc) precipitated phase and a SiO2 matrix. Additionally, erbium (Er) ions have been implanted and used as luminescent centres in order to obtain narrow emission at 1.54 µm. Our study exploits the combination of high angle annular dark field (HAADF) imaging with a sub-nanometer electron probe and electron energy loss spectroscopy (EELS) with an energy resolution below 0.2 eV. The structural and chemical information is obtained from the studied multilayer structure. In addition, the instrumental techniques for calibration, deconvolution, fitting and analysis of the EELS spectra are explained in detail. The spatial distribution of the Si-nanoclusters (Si-ncs) and the SiO2 barriers is accurately delimited in the multilayer. Additionally, the quality of the studied multilayer in terms of composition, roughness and defects is analysed and discussed. Er clusterization has not been observed; even so, blue-shifted plasmon and interband transition energies for silica are measured, in the presence of Er ions and sizable nanometer-size effects.

Insight into the compositional and structural nano features of AlN/GaN DBRs by EELS-HAADF.

III-V nitride (AlGa)N distributed Bragg reflector devices are characterized by combined high-angle annular dark-field (HAADF) and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope. Besides the complete structural characterization of the AlN and GaN layers, the formation of AlGaN transient layers is revealed using Vegard law on profiles of the position of the bulk plasmon peak maximum. This result is confirmed by comparison of experimental and simulated HAADF intensities. In addition, we present an advantageous method for the characterization of nano-feature structures using low-loss EELS spectrum image (EEL-SI) analysis. Information from the materials in the sample is extracted from these EEL-SI at high spatial resolution.The log-ratio formula is used to calculate the relative thickness, related to the electron inelastic mean free path. Fitting of the bulk plasmon is performed using a damped plasmon model (DPM) equation. The maximum of this peak is related to the chemical composition variation using the previous Vegard law analysis. In addition, within the context of the DPM, information regarding the structural properties of the material can be obtained from the lifetime of the oscillation. Three anomalous segregation regions are characterized, revealing formation of metallic Al islands.

EEL spectroscopic tomography: towards a new dimension in nanomaterials analysis.

Electron tomography is a widely spread technique for recovering the three dimensional (3D) shape of nanostructured materials. Using a spectroscopic signal to achieve a reconstruction adds a fourth chemical dimension to the 3D structure. Up to date, energy filtering of the images in the transmission electron microscope (EFTEM) is the usual spectroscopic method even if most of the information in the spectrum is lost. Unlike EFTEM tomography, the use of electron energy-loss spectroscopy (EELS) spectrum images (SI) for tomographic reconstruction retains all chemical information, and the possibilities of this new approach still remain to be fully exploited. In this article we prove the feasibility of EEL spectroscopic tomography at low voltages (80 kV) and short acquisition times from data acquired using an aberration corrected instrument and data treatment by Multivariate Analysis (MVA), applied to Fe(x)Co((3-x))O(4)@Co(3)O(4) mesoporous materials. This approach provides a new scope into materials; the recovery of full EELS signal in 3D.