Valence Electron Energy Loss Spectroscopy Research Articles

Footprints of atomic-scale features in plasmonic nanoparticles as revealed by electron energy loss spectroscopy.

We present a first-principles theoretical study of the atomistic footprints in the valence electron energy loss spectroscopy (EELS) of nanometer-size metallic particles. Charge density maps of excited plasmons and EEL spectra for specific electron paths through a nanoparticle (Na380 atom cluster) are modeled using ab initio calculations within time-dependent density functional theory. Our findings unveil the atomic-scale sensitivity of EELS within this low-energy spectral range. Whereas localized surface plasmons (LSPs) are particularly sensitive to the atomistic structure of the surface probed by the electron beam, confined bulk plasmons (CBPs) reveal quantum size effects within the nanoparticle's volume. Moreover, we prove that classical local dielectric theories mimicking the atomistic structure of the nanoparticles reproduce the LSP trends observed in quantum calculations, but fall short in describing the CBP behavior observed under different electron trajectories.

The atomic and electronic band structures in Ca/Si-doped and Ca/Si/Ti-doped Al2O3 grain boundaries

Grain boundary (GB) segregation plays a crucial role in determining structures, chemistries and resultant properties of polycrystalline ceramic materials. Although GB segregation behaviors of single-element dopants have been extensively investigated, GB segregation in practical polycrystalline ceramics often involves multiple elements. In this study, we conducted experimental investigations to explore the atomic and electronic band structures of Ca/Si-doped and Ca/Si/Ti-doped Al2O3 Σ745̅10 GBs. By employing atomic-resolution scanning transmission electron microscopy combined with density functional theory calculations, we elucidated the interactions between different elements at the GBs. Our findings reveal that Ca and Si were synergistically co-segregated in both GBs, maintaining charge neutrality within the GB core, while the site competition between Ca and Ti occurred in the Ca/Si/Ti-doped GB. Furthermore, valence electron energy loss spectroscopy measurements provided insights into the band gaps of doped GBs, demonstrating that they are dependent on doping species but independent of interactions between different elements.

Clustering in gallium ion beam sputtered compound materials driven by bond strength and interstitial/vacancy reaction

The investigation of chemical reactions during ion irradiation is a frontier for the study of the ion–material interaction. In order to probe the chemistry of ion produced nanoclusters, valence electron energy loss spectroscopy (VEELS) was exploited to investigate Ga+ ion damage in Al2O3, InP, and InGaAs, where each target material has been shown to react differently to the interaction between impinging ions, recoil atoms, and vacancies: metallic Ga, ternary InGaP clusters, and metallic In clusters are formed in Al2O3, InP, and InGaAs, respectively. Supporting simulations based on Monte Carlo and crystal orbital Hamiltonian calculations indicate that the chemical constitution of cascade induced nano-precipitates is a result of a competition between interstitial/vacancy consumption rates and preferential bond formation due to differing bond strengths.

Differentiating the bonding states in calcium carbonate polymorphs by low-loss electron-energy-loss spectroscopy

Calcium carbonate is one of the important building components in organisms, especially the two most common polymorphs, calcite and aragonite. Here, to understand the difference in bonding state, the two polymorphs are characterized by valence (low-loss) electron energy loss spectroscopy. It is found that the difference in Ca M23 edge originating from 3p to 3d states is consistent with the change of Ca-O bonds in the two studied polymorphs. Surprisingly, the measured Ca M23 edge is in qualitative agreement with the calculated partial density of states (PDOS) of Ca-d states in contrast to their L edges (from 2p to 3d states) which are strongly influenced by atomic multiplet effect (spin-orbit coupling). This is because the atomic multiplet effect is much reduced for the Ca 3p orbital, which permits the corresponding Ca M23 edge to be compared with the PDOS results. Our findings show insights that PDOS can potentially be used to interpret the M23 edge of lighter 3d transition metals such as scandium, titanium, vanadium and chromium when such interpretation may not be achieved for their L edges.

Single-dislocation ultraviolet light emission

Micro-nano ultraviolet light sources have recently attracted great attention due to their promising applications in future quantum display, photoelectric detection sensors, biomedical devices, and so on. Design of ultraviolet light sources with smaller size, stronger emission intensity, and higher integration, are the key aspects for the development of high-performance micro-nano ultraviolet devices. Here, we demonstrate that unidirectional threading dislocation arrays in an AlN thin film can emit ultraviolet light with wavelength of ∼ 317 nm. The density of AlN dislocations is ∼ 4 × 1010 cm−2, and the light emission intensity of AlN dislocations is comparable to the intrinsic emission of the AlN thin film. By combining aberration-corrected transmission electron microscopy, atomic-resolution valence electron energy-loss spectroscopy, and first-principles calculations, the atomic and electronic structures and the band gaps of the AlN threading dislocations are determined. It is found that the AlN threading dislocations are comprised of edge, screw, and mixed dislocations. All the AlN dislocations exhibit smaller band gaps compared to the AlN bulk due to the increase of the Al-N bond length at the dislocation cores. The dangling bonds of Al and N at the edge dislocations, Al-Al and NN bonds at the screw dislocations cause the defect states in the band gaps. This study not only has clarified the atomic origin of dislocation nano-optics, but also will encourage the development of microelectronics and optoelectronic devices based on dislocation ultraviolet light emission.

Structural properties, bandgap, and complex dielectric function in Bi2Te3 thermoelectric by Valence Electron Energy Loss Spectroscopy (VEELS) analysis

This article describes the results and discussion performed to analyze the bandgap, optoelectronic properties, and complex dielectric function of Bi2Te3 thermoelectric material, using Valence Electron Energy Loss Spectroscopy. A key aspect of the report presented in this article is that the properties described above have not been analyzed and reported using the Valence Electron Energy Loss Spectroscopy technique. The material’s structure was analyzed using Rietveld and High-Resolution Transmission Electron Microscopy techniques. The band gap obtained value is 0.91 eV by Valence Electron Energy Loss Spectroscopy. Whereas, values obtained by other techniques are between 0.26 and 0.92 eV for Bi2Te3. The real part (ε1) and imaginary part (ε2) of the complex dielectric function (ε * ) were measured. For ε1, the principal peak at 1.03 eV and a second peak at 1.4 eV were observed. Furthermore, the optical dielectric constant is 8.6 for (ε0 = 0 eV), which is related to electronic contributions and is given by the refractive index squared (n2).

Relationships between Compositional Heterogeneity and Electronic Spectra of (Ga1–xZnx)(N1–xOx) Nanocrystals Revealed by Valence Electron Energy Loss Spectroscopy

Many ternary and quaternary semiconductors have been made in nanocrystalline forms for a variety of applications, but we have little understanding of how well their ensemble properties reflect the properties of individual nanocrystals. We examine electronic structure heterogeneities in nanocrystals of (Ga1–xZnx)(N1–xOx), a semiconductor that splits water under visible illumination. We use valence electron energy loss spectroscopy (VEELS) in a scanning transmission electron microscope to map out electronic spectra of (Ga1–xZnx)(N1–xOx) nanocrystals with a spatial resolution of 8 nm. We examine three samples with varying degrees of intraparticle and interparticle compositional heterogeneity and ensemble optical spectra that range from a single band gap in the visible to two band gaps, one in the visible and one in the UV. The VEELS spectra resemble the ensemble absorption spectra for a sample with a homogeneous elemental distribution and a single band gap and, more interestingly, one with intraparticle compositional heterogeneity and two band gaps. We observe spatial variation in VEELS spectra only with significant interparticle compositional heterogeneity. Hence, we reveal the conditions under which the ensemble spectra reveal the optical properties of individual (Ga1–xZnx)(N1–xOx) particles. More broadly, we illustrate how VEELS can be used to probe electronic heterogeneities in compositionally complex nanoscale semiconductors.

Direct Determination of Band Gap of Defects in a Wide Band Gap Semiconductor.

Crystal defects play an important role in the degradation and failure of semiconductor materials and devices. Direct determination of band gap of defects is a critical step for clarifying how the defects affect the physical properties of semiconductors. Here, high-quality aluminum nitride (AlN) thin films were grown epitaxially on single-crystal Al2O3 substrates via pulsed laser deposition. The atomic structure and band gap of three types of inversion domain boundaries (IDBs) in AlN were determined using aberration-corrected transmission electron microscopy and atomic-resolution valence electron energy-loss spectroscopy. It was found that the band gap of all of the IDBs reduces evidently compared to that of the bulk AlN. The maximum band gap reduction of the IDBs is 1.0 eV. First-principles calculations revealed that the band gap reduction of the IDBs is mainly due to the rise of pz orbital at the valence band maximum, which originates from the elongated Al-N bonds along the [0001] direction at the IDBs. The successful band gap determination of defects paves an avenue for quantitatively evaluating the effect of defects on the performance of semiconductor materials and devices.

Sub-nanometer mapping of strain-induced band structure variations in planar nanowire core-shell heterostructures

Strain relaxation mechanisms during epitaxial growth of core-shell nanostructures play a key role in determining their morphologies, crystal structure and properties. To unveil those mechanisms, we perform atomic-scale aberration-corrected scanning transmission electron microscopy studies on planar core-shell ZnSe@ZnTe nanowires on α-Al2O3 substrates. The core morphology affects the shell structure involving plane bending and the formation of low-angle polar boundaries. The origin of this phenomenon and its consequences on the electronic band structure are discussed. We further use monochromated valence electron energy-loss spectroscopy to obtain spatially resolved band-gap maps of the heterostructure with sub-nanometer spatial resolution. A decrease in band-gap energy at highly strained core-shell interfacial regions is found, along with a switch from direct to indirect band-gap. These findings represent an advance in the sub-nanometer-scale understanding of the interplay between structure and electronic properties associated with highly mismatched semiconductor heterostructures, especially with those related to the planar growth of heterostructured nanowire networks.

Plasmon spectroscopy for the determination of Ti3C2T x MXene few layer stacks architecture

Like many 2D materials, numerous properties of MXene multilayers, and especially the most popular one Ti3C2T x , have been shown to significantly depend on their architecture, i.e. the number of layers and interlayer distance. These structural parameters are thus key elements to be characterized for the analysis of MXene properties. Focusing on valence electron energy-loss spectroscopy (VEELS) as performed in a transmission electron microscope (TEM), and using density functional theory (DFT) simulations, we here analyze the layer dependent large changes in the VEEL spectra of Ti3C2T x multilayers as a probe of their total thickness, and emphasize the bulk plasmon energy sensitivity to interlayer distance. Together these findings allow to directly quantify the absolute number of layers in a Ti3C2T x stack up to ∼10 nm thickness and give access to interlayer distance modifications with sub-angström sensitivity, evidencing VEELS as a powerful method for the characterization of MXene multilayers on the nanometer scale. We expect these results to be relevant for the study of structure/properties correlations in this class of materials, especially with the development of in situ or environmental TEM experiments.

Probing the Optical Response and Local Dielectric Function of an Unconventional Si@MoS2 Core-Shell Architecture.

Heterostructures of optical cavities and quantum emitters have been highlighted for enhanced light-matter interactions. A silicon nanosphere, core, and MoS2, shell, structure is one such heterostructure referred to as the core@shell architecture. However, the complexity of the synthesis and inherent difficulties to locally probe this architecture have resulted in a lack of information about its localized features limiting its advances. Here, we utilize valence electron energy loss spectroscopy (VEELS) to extract spatially resolved dielectric functions of Si@MoS2 with nanoscale spatial resolution corroborated with simulations. A hybrid electronic critical point is identified ∼3.8 eV for Si@MoS2. The dielectric functions at the Si/MoS2 interface is further probed with a cross-sectioned core-shell to assess the contribution of each component. Various optical parameters can be defined via the dielectric function. Hence, the methodology and evolution of the dielectric function herein reported provide a platform for exploring other complex photonic nanostructures.

Complex dielectric function, Cole-Cole, and optical properties evaluation in BiMnO3 thin-films by Valence Electron Energy Loss Spectrometry (VEELS) analysis

Valence Electron Energy Loss Spectrometry-Transmission Electron Microscopy (VEELS-TEM) analyses were found to provide important information on optical properties and the dielectric function of BiMnO3 thin films grown, via radio frequency (RF) magnetron sputter-deposition on SrTiO3 substrates. The research described in this article involved using Kramer's–Krӧnig analysis to determine bandgap, complex dielectric function, and static dielectric constant, ε* = 4.68 of the grown BiMnO3 thin films. The reflection coefficient Ґl of the BiMnO3 thin films was used to elucidate the reflection loss in the BiMnO3 thin films. The bandgap energy of the films was determined using a polynomial fit in the energy loss function (ELF) plot with an Eg = 1.63 eV. The scope of the Cole-Cole plot demonstrates the bandgap and the plasmon region in the BiMnO3 films, which is reported here for the first time. A key contribution of this article relates to describing formulations used for transforming the experimentally obtained dielectric constant components of the BiMnO3 films to produce Cole-Cole plots and applying Kramers-Krӧnig analysis to transform a single scattering energy loss function, to enable determination of the complex dielectric function ε′andε″ and structure of BiMnO3 films.

Band gap, complex dielectric function, and optical properties of Sb-doped lead-free KNN piezo-ceramics analyzed by VEELS-TEM

Sb-doped KNN (Na0.9K0.1Nb0.97Sb0.03O3, KNNS) is an environment-friendly lead-free ferroelectric and it has high piezoelectric response. A band gap energy Eg = 3.58 eV for KNNS was evaluated by valence electron energy loss spectroscopy (VEELS), which has not been reported in the literature. The zero loss peak (ZLP) was extracted by the reflected tail model, results were gotten by the Fourier-log method. Single scattering dispersion (SSD) was used to execute the Kramers-Krӧnig analysis, thence: complex dielectric function (ε1 and ε2) based on KNNS, and optical properties were obtained; The static refractive index n(0)(nKNNS) resulted to be 2.21, which mathematically is about a square root of dielectric constant, coinciding with that obtained experimentally. Also, a peak at n1=4.1 eV and n2=9.42 eV located in the near ultraviolet was observed. Understanding the different applications of KNNS in optoelectronic devices is vital, also it is worth mentioning that many articles overlook the relativistic effects in the band gap, dielectric function, and the optical properties measurements.

Atomic and electronic band structures of Y-doped Al<sub>2</sub>O<sub>3</sub> grain boundaries

An Y-doped Σ7{4510}[0001] Al2O3 grain boundary was fabricated by the bicrystal method. The grain boundary segregation structure was studied using atomic-resolution scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. It was found that the atomic structure of the Y-doped grain boundary differs from that of the non-doped grain boundary, indicating that Y doping induced the structural transformation from asymmetrical shape to symmetrical shape in the grain boundary core. Apart from Y, impurities of Ca were also segregated at the grain boundary. Both Y3+ and Ca2+ are segregated at the same atomic columns in the grain boundary, mainly due to their larger ionic size compared with Al3+. In addition, valence electron energy loss spectroscopy measurements indicate that the bandgap energy of the doped grain boundary is about 0.5 eV smaller than that of the pristine grain boundary.

Observation of grain boundary plasmon and associated deconvolution techniques for low-loss electron energy-loss (EEL) spectra acquired from grain boundaries

Spatially resolved valence electron energy-loss spectroscopy (VEELS) was used to acquire low-loss EEL spectra from Al grain boundaries (GBs) with different GB energies. The loss signal from the GB is highly delocalized and is mixed with the bulk loss, therefore requiring separation. Three different separation techniques, i.e., Fourier-log, Fourier-ratio deconvolution and direct subtraction, were employed to extract the GB response from the low-loss spectra and produced similar results. The GB response consists of a positive intensity peak from the excitation of GB plasmons (GBP) and a negative intensity begrenzungs (Bgs) peak from reduced scattering from bulk oscillations. Also, lower electron density at the GB reduces the inelastic scattering of the bulk plasmon. The intensity of GBP scattering and begrenzungs peak is found to increase toward the GBs, with maximum intensity when the electron probe is positioned on the GB, connecting the begrenzungs effect with the creation of a GBP.

Nanoscale Investigation of Local Thermal Expansion at SrTiO3 Grain Boundaries by Electron Energy Loss Spectroscopy.

The presence of grain boundaries (GBs) has a great impact on the coefficient of thermal expansion (CTE) of polycrystals. However, direct measurement of local expansion of GBs remains challenging for conventional methods due to the lack of spatial resolution. In this work, we utilized the valence electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to directly measure the CTE of Σ5 and 45°GBs of SrTiO3 at a temperature range between 373 and 973 K. A CTE that was about 3 times larger was observed in Σ5 GB along the direction normal to GB plane, while only a 1.4 time enhancement was found in the 45° GB. Our result provides direct evidence that GBs contribute to the enhancement of CTE in polycrystals. Also, this work has revealed how thermodynamic properties are varied in different GB structures and demonstrated the potential of EELS for probing local thermal properties with nanometer-scale resolution.

Evaluation of relaxation process in FeMo14C15B6Erx (x = 0–2) bulk metallic glass by valence electron energy loss spectroscopy

Relaxation processes during heating of both ribbon and bulk FeMo14C15B6Erx(x = 0∼2) metallic glasses were examined using valence electron energy loss spectroscopy (VEELS). The plasmon energy was decreased with increasing temperature due to thermal expansion and chemical reordering in the glasses. The chemical reordering stimulates a specific solute cluster formation; M23(CB)6 solute clusters began to form above about 200 °C in both ribbon and bulk FeMo14C15B6Erx (x = 0, 0.5, 1) metallic glasses. The M23(CB)6 solute clusters were only formed above 400 °C in the ribbon FeMo14C15B6Er2 metallic glass, indicating inhibition of the M23(CB)6 formation occurred owing to the Er-(C, B) complexes/clusters formation. The Er-(C, B) complexes/clusters were formed in the cooling process of the sample fabrication. In contrast to the ribbon sample, the M23(CB)6 formation did not occur in the bulk FeMo14C15B6Er2 metallic glass during heating. The amount of the Er-(C, B) complexes/clusters in the bulk sample was larger than that in the ribbon sample because of the slow cooling rate in the bulk sample fabrication. Because the solute carbon and boron atoms were mostly consumed by forming Er-(C, B) complexes/clusters in the bulk FeMo14C15B6Er2 metallic glass, the M23(CB)6 formation was suppressed.

Nanoscale Growth Initiation as a Pathway to Improve the Earth-Abundant Absorber Zinc Phosphide.

Growth approaches that limit the interface area between layers to nanoscale regions are emerging as a promising pathway to limit the interface defect formation due to mismatching lattice parameters or thermal expansion coefficient. Interfacial defect mitigation is of great interest in photovoltaics as it opens up more material combinations for use in devices. Herein, an overview of the vapor–liquid–solid and selective area epitaxy growth approaches applied to zinc phosphide (Zn3P2), an earth-abundant absorber material, is presented. First, we show how different morphologies, including nanowires, nanopyramids, and thin films, can be achieved by tuning the growth conditions and growth mechanisms. The growth conditions are also shown to greatly impact the defect structure and composition of the grown material, which can vary considerably from the ideal stoichiometry (Zn3P2). Finally, the functional properties are characterized. The direct band gap could accurately be determined at 1.50 ± 0.1 eV, and through complementary density functional theory calculations, we can identify a range of higher-order band gap transitions observed through valence electron energy loss spectroscopy and cathodoluminescence. Furthermore, we outline the formation of rotated domains inside of the material, which are a potential origin of defect transitions that have been long observed in zinc phosphide but not yet explained. The basic understanding provided reinvigorates the potential use of earth-abundant II–V semiconductors in photovoltaic technology. Moreover, the transferrable nanoscale growth approaches have the potential to be applied to other material systems, as they mitigate the constraints of substrate–material combinations causing interface defects.

The Advantage of Nanowire Configuration in Band Structure Determination (Adv. Funct. Mater. 41/2021)

Band Structure Determination In article number 2105426, Mirjana Dimitrievska, Anna Fontcuberta i Morral, and co-workers demonstrate how nanowires can be effectively used for direct determination of the band structure by means of valence electron energy-loss spectroscopy. This work opens the path for characterizing the band structure of new compounds in a non-destructive and facile manner, strengthening the route of new materials discovery.

Near band edge and defect emissions in wurtzite Cd0.025Mg0.10Zn0.875O nanocrystals

We report on near band edge and local defects emissions in Cd0·025Mg0·10Zn0·875O (CdMgZnO) nanoparticles (NPs) as a function of temperature, where a strong temperature-dependent near-infrared emission around 1.7 eV (~730 nm) has been observed. The NPs were synthesized by a modified sol-gel method and were annealed at 750 °C after growing. The crystallographic parameters have been determined by 2-dimensional synchrotron x-ray diffraction (XRD) and conventional XRD analysis, confirming their growth within the wurtzite phase with a preferred orientation along the (101) plane and an apparent crystallite size of 52.72 ± 0.18 nm. This apparent crystallite size is consistent with the nearly hexagonal particle shape (50.15 ± 0.38 nm) obtained from high resolution transmission electron microscopy. Valence Electron Energy Loss Spectroscopy (VEELS) has been conducted to gain information on the band structure and interband transitions. From the VEELS spectrum, an onset bandgap of 3.2 ± 0.3 eV was attributed to Cd/Mg co-doped ZnO. The temperature-dependent photoluminescence properties and enhancement of the optically active local defects emission have been analyzed.