Materials science applications of HREELS in near edge structure analysis and low-energy loss spectroscopy

Measurement of dielectric function and bandgap of germanium telluride using monochromated electron energy-loss spectroscopy

Using a monochromator in transmission electron microscopy, a low-energy-loss spectrum can provide inter- and intra-band transition information for nanoscale devices with high energy and spatial resolutions. However, some losses, such as Cherenkov radiation, phonon scattering, and surface plasmon resonance superimposed at zero-loss peak, make it asymmetric. These pose limitations to the direct interpretation of optical properties, such as complex dielectric function and bandgap onset in the raw electron energy-loss spectra. This study demonstrates measuring the dielectric function of germanium telluride using an off-axis electron energy-loss spectroscopy method. The interband transition from the measured complex dielectric function agrees with the calculated band structure of germanium telluride. In addition, we compare the zero-loss subtraction models and propose a reliable routine for bandgap measurement from raw valence electron energy-loss spectra. Using the proposed method, the direct bandgap of germanium telluride thin film was measured from the low-energy-loss spectrum in transmission electron microscopy. The result is in good agreement with the bandgap energy measured using an optical method.

Spatially Resolved Band Gap and Dielectric Function in Two-Dimensional Materials from Electron Energy Loss Spectroscopy

The electronic properties of two-dimensional (2D) materials depend sensitively on the underlying atomic arrangement down to the monolayer level. Here we present a novel strategy for the determination of the band gap and complex dielectric function in 2D materials achieving a spatial resolution down to a few nanometers. This approach is based on machine learning techniques developed in particle physics and makes possible the automated processing and interpretation of spectral images from electron energy loss spectroscopy (EELS). Individual spectra are classified as a function of the thickness with K-means clustering, and then used to train a deep-learning model of the zero-loss peak background. As a proof of concept we assess the band gap and dielectric function of InSe flakes and polytypic WS₂ nanoflowers and correlate these electrical properties with the local thickness. Our flexible approach is generalizable to other nanostructured materials and to higher-dimensional spectroscopies and is made available as a new release of the open-source EELSfitter framework.

Pathak A, Novikov D, Leitus G, Kaplan-Ashiri I, Kolíbal M, Enyashin AN, Houben L, Tenne R. Nanotubes from the Misfit Layered Compound (SmS)1.19TaS2: Atomic Structure, Charge Transfer, and Electrical Properties

Misfit layered compounds (MLCs) MX-TX₂, where M, T = metal atoms and X = S, Se, or Te, and their nanotubes are of significant interest due to their rich chemistry and unique quasi-1D structure. In particular, LnX-TX₂ (Ln = rare-earth atom) constitute a relatively large family of MLCs, from which nanotubes have been synthesized. The properties of MLCs can be tuned by the chemical and structural interplay between LnX and TX₂ sublayers and alloying of each of the Ln, T, and X elements. In order to engineer them to gain desirable performance, a detailed understanding of their complex structure is indispensable. MLC nanotubes are a relative newcomer and offer new opportunities. In particular, like WS₂ nanotubes before, the confinement of the free carriers in these quasi-1D nanostructures and their chiral nature offer intriguing physical behavior. High-resolution transmission electron microscopy in conjunction with a focused ion beam are engaged to study SmS-TaS₂ nanotubes and their cross-sections at the atomic scale. The atomic resolution images distinctly reveal that Ta is in trigonal prismatic coordination with S atoms in a hexagonal structure. Furthermore, the position of the sulfur atoms in both the SmS and the TaS₂ sublattices is revealed. X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and X-ray absorption spectroscopy are carried out. These analyses conclude that charge transfer from the Sm to the Ta atoms leads to filling of the Ta 5d _{z ²} level, which is confirmed by density functional theory (DFT) calculations. Transport measurements show that the nanotubes are semimetallic with resistivities in the range of 10^-4 Ω·cm at room temperature, and magnetic susceptibility measurements show a superconducting transition at 4 K.

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.

Local electronic structure variation resulting in Li 'filament' formation within solid electrolytes

Solid electrolytes hold great promise for enabling the use of Li metal anodes. The main problem is that during cycling, Li can infiltrate along grain boundaries and cause short circuits, resulting in potentially catastrophic battery failure. At present, this phenomenon is not well understood. Here, through electron microscopy measurements on a representative system, Li₇La₃Zr₂O₁₂, we discover that Li infiltration in solid oxide electrolytes is strongly associated with local electronic band structure. About half of the Li₇La₃Zr₂O₁₂ grain boundaries were found to have a reduced bandgap, around 1-3 eV, making them potential channels for leakage current. Instead of combining with electrons at the cathode, Li⁺ ions are hence prematurely reduced by electrons at grain boundaries, forming local Li filaments. The eventual interconnection of these filaments results in a short circuit. Our discovery reveals that the grain-boundary electronic conductivity must be a primary concern for optimization in future solid-state battery design.

Charting the low-loss region in electron energy loss spectroscopy with machine learning

Exploiting the information provided by electron energy-loss spectroscopy (EELS) requires reliable access to the low-loss region where the zero-loss peak (ZLP) often overwhelms the contributions associated to inelastic scatterings off the specimen. Here we deploy machine learning techniques developed in particle physics to realise a model-independent, multidimensional determination of the ZLP with a faithful uncertainty estimate. This novel method is then applied to subtract the ZLP for EEL spectra acquired in flower-like WS₂ nanostructures characterised by a 2H/3R mixed polytypism. From the resulting subtracted spectra we determine the nature and value of the bandgap of polytypic WS₂, finding E_BG=1.6_-0.2^+0.3eV with a clear preference for an indirect bandgap. Further, we demonstrate how this method enables us to robustly identify excitonic transitions down to very small energy losses. Our approach has been implemented and made available in an open source Python package dubbed EELSfitter.

Hierarchical TiO2 Nanoflower Photocatalysts with Remarkable Activity for Aqueous Methylene Blue Photo-Oxidation

This study systematically evaluates the performance of a series of TiO₂ nanoflower (TNF) photocatalysts for aqueous methylene blue photo-oxidation under UV irradiation. TNF nanoflowers were synthesized from Ti(IV) butoxide by a hydrothermal method and then calcined at different temperatures (T = 400-800 °C) for specific periods of time (t = 1-5 h). By varying the calcination conditions, TNF-T-t photocatalysts with diverse physicochemical properties and anatase/rutile ratios were obtained. Many of the TNF-T-1 photocatalysts demonstrated remarkable activity for aqueous methylene blue photo-oxidation at pH 6 under UV excitation (365 nm), with activities following the order TNF-700-1 > TNF-600-1 > TNF-500-1 > TNF-400-1 ∼ P25 TiO₂ ≫ TNF-800-1. The activity of the TNF-700-1 photocatalyst (99% anatase, 1% rutile) was 2.3 times that of P25 TiO₂ at pH 6 and 14.4 times that of P25 TiO₂ at pH 4. Prolonged calcination of the TNFs at 700 °C proved detrimental to dye degradation performance due to excessive rutile formation, which reduced the photocatalyst surface area and suppressed OH^• generation. The outstanding activities of TNF-700-1 and TNF-600-1 are attributed to their hierarchical nanoflower morphology which benefitted UV absorption, a near-ideal anatase crystallite size for efficient charge separation, and their unusually low isoelectric point (IEP = 4.3-4.5).

Thickness-dependent band gap of α-In2Se3: from electron energy loss spectroscopy to density functional theory calculations

α-In₂Se₃ has attracted increasing attention in recent years due to its excellent electrical and optical properties. Especially, attention has been paid to its peculiar ferroelectric and piezoelectric properties which most other two-dimensional (2D) materials do not possess. This paper presents the first measurement of the thickness-dependent band gaps of few-layer α-In₂Se₃ by electron energy loss spectroscopy (EELS). The band gap increases with decreasing film thickness which varies from 1.44 eV in a 48 nm thick area to 1.64 eV in an 8 nm thick area of the samples. Further, by combining the improved exchange-correlation potential and proper screening of the internal electric field in an advanced 2D electronic structure technique, we have been able to obtain the structural dependence of the band gap within density functional theory up to hundreds of atoms. This is also the first calculation of a similar type for 2D ferroelectric materials. Both experiment and theory suggest that the variation of the band gap of α-In₂Se₃ fits well with the quantum confinement model for 2D materials.

Understanding semiconductor nanostructures via advanced electron microscopy and spectroscopy

Transmission electron microscopy (TEM) offers an ample range of complementary techniques which are able to provide essential information about the physical, chemical and structural properties of materials at the atomic scale, and hence makes a vast impact on our understanding of materials science, especially in the field of semiconductor one-dimensional (1D) nanostructures. Recent advancements in TEM instrumentation, in particular aberration correction and monochromation, have enabled pioneering experiments in complex nanostructure material systems. This review aims to address these understandings through the applications of the methodology for semiconductor nanostructures. It points out various electron microscopy techniques, in particular scanning TEM (STEM) imaging and spectroscopy techniques, with their already-employed or potential applications on 1D nanostructured semiconductors. We keep the main focus of the paper on the electronic and optoelectronic properties of such semiconductors, and avoid expanding it further. In the first part of the review, we give a brief introduction to each of the STEM-based techniques, without detailed elaboration, and mention the recent technological and conceptual developments which lead to novel characterization methodologies. For further reading, we refer the audience to a handful of papers in the literature. In the second part, we highlight the recent examples of application of the STEM methodology on the 1D nanostructure semiconductor materials, especially III-V, II-V, and group IV bare and heterostructure systems. The aim is to address the research questions on various physical properties and introduce solutions by choosing the appropriate technique that can answer the questions. Potential applications will also be discussed, the ones that have already been used for bulk and 2D materials, and have shown great potential and promise for 1D nanostructure semiconductors.

Structural and optical properties of individual Zn2GeO4 particles embedded in ZnO

Functionalizing transparent conducting oxides (TCOs) is an intriguing approach to expand the tunability and operation of optoelectronic devices. For example, forming nanoparticles that act as quantum wells or barriers in zinc oxide (ZnO), one of the main TCOs today, may expand its optical and electronic tunability. In this work, 800 keV Ge ions have been implanted at a dose of 1 × 10¹⁶ cm^-2 into crystalline ZnO. After annealing at 1000 °C embedded disk-shaped particles with diameters up to 100 nm are formed. Scanning transmission electron microscopy shows that these are particles of the trigonal Zn₂GeO₄ phase. The particles are terminated by atomically sharp facets of the type {11 [Formula: see text] 0}, and the interface between the matrix and particles is decorated with misfit dislocations in order to accommodate the lattice mismatch between the two crystals. Electron energy loss spectroscopy has been employed to measure the band gap of individual nanoparticles, showing an onset of band-to-band transitions at 5.03 ± 0.02 eV. This work illustrates the advantages of using STEM characterization methods, where information of structure, growth, and properties can be directly obtained.

Electron energy-loss spectroscopy (EELS) with a cold-field emission scanning electron microscope at low accelerating voltage in transmission mode

A commercial electron energy-loss spectrometer (EELS) attached to a high-resolution cold-field emission scanning electron microscope in transmission mode (STEM) is evaluated and its potential for characterizing materials science thin specimens at low accelerating voltage is reviewed. Despite the increased beam radiation damage at SEM voltages on sensitive compounds, we describe some potential applications which benefit from lowering the primary electrons voltage on less-sensitive specimens. We report bandgap measurements on several dielectrics which were facilitated by the lack of Cherenkov radiation losses at 30 kV. The possibility of volume plasmon imaging to probe local composition changes in complex materials was demonstrated using energy-filtered STEM, either via spectrum imaging or elemental mapping using the "three-windows" method. As plasmonic materials are increasing used for energy, electronics or biomedical applications, the ability of reliably evaluate their properties at low accelerating voltage in a SEM is very appealing and is demonstrated. The energy resolution of the spectrometer, taken as the full width at half maximum of the zero-loss peak, was routinely measured at around 0.55 eV and it is demonstrated that t/λ ratios up to 1.5 allowed practical EEL spectroscopy at 30 kV.

Exploring possibilities of band gap measurement with off-axis EELS in TEM

A technique to measure the band gap of dielectric materials with high refractive index by means of energy electron loss spectroscopy (EELS) is presented. The technique relies on the use of a circular (Bessel) aperture and suppresses Cherenkov losses and surface-guided light modes by enforcing a momentum transfer selection. The technique also strongly suppresses the elastic zero loss peak, making the acquisition, interpretation and signal to noise ratio of low loss spectra considerably better, especially for excitations in the first few eV of the EELS spectrum. Simulations of the low loss inelastic electron scattering probabilities demonstrate the beneficial influence of the Bessel aperture in this setup even for high accelerating voltages. The importance of selecting the optimal experimental convergence and collection angles is highlighted. The effect of the created off-axis acquisition conditions on the selection of the transitions from valence to conduction bands is discussed in detail on a simplified isotropic two band model. This opens the opportunity for deliberately selecting certain transitions by carefully tuning the microscope parameters. The suggested approach is experimentally demonstrated and provides good signal to noise ratio and interpretable band gap signals on reference samples of diamond, GaN and AlN while offering spatial resolution in the nm range.

Influence of the Capping Ligand on the Band Gap and Electronic Levels of PbS Nanoparticles through Surface Atomistic Arrangement Determination

Lead sulfide (PbS) nanoparticles were synthesized by chemical methods with different sizes and different capping ligands (oleic acid, myristic acid, and hexanoic acid), avoiding ligand exchange procedures, to study the effect of characteristics of the capping ligands on their energy levels and band gap values. Experimental results (UV-vis-NIR, Fourier transform infrared, and Raman spectroscopies, cyclic voltammetry, transmission electron microscopy, and electron energy loss spectroscopy) showed a marked influence of the capping ligand nature on the electro-optical properties of PbS nanoparticles with a very similar size. Differences were observed in the atomistic arrangement on the nanoparticle surface and phonon vibrations with the different capping ligands. These observations suggest that the electro-optical properties of PbS nanoparticles are not only determined by their size, through quantum confinement effects, but also strongly affected by the atomistic arrangement on the nanoparticle surface, which is determined by the capping ligand nature.

Bandgap measurement of high refractive index materials by off-axis EELS

In the present work Cs aberration corrected and monochromated scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) has been used to explore experimental set-ups that allow bandgaps of high refractive index materials to be determined. Semi-convergence and -collection angles in the µrad range were combined with off-axis or dark field EELS to avoid relativistic losses and guided light modes in the low loss range to contribute to the acquired EEL spectra. Off-axis EELS further supressed the zero loss peak and the tail of the zero loss peak. The bandgap of several GaAs-based materials were successfully determined by simple regression analyses of the background subtracted EEL spectra. The presented set-up does not require that the acceleration voltage is set to below the Čerenkov limit and can be applied over the entire acceleration voltage range of modern TEMs and for a wide range of specimen thicknesses.

Band-Gap Widening at the Cu(In,Ga)(S,Se)2 Surface: A Novel Determination Approach Using Reflection Electron Energy Loss Spectroscopy

Using reflection electron energy loss spectroscopy (REELS), we have investigated the optical properties at the surface of a chalcopyrite-based Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cell absorber, as well as an indium sulfide (InxSy) buffer layer before and after annealing. By fitting the characteristic inelastic scattering cross-section λK(E) to cross sections evaluated by the QUEELS-ε(k,ω)-REELS software package, we determine the surface dielectric function and optical properties of these samples. A comparison of the optical values at the surface of the InxSy film with bulk ellipsometry measurements indicates a good agreement between bulk- and surface-related optical properties. In contrast, the properties of the CIGSSe surface differ significantly from the bulk. In particular, a larger (surface) band gap than for bulk-sensitive measurements is observed, providing a complementary and independent confirmation of earlier photoelectron spectroscopy results. Finally, we derive the inelastic mean free path λ for electrons in InxSy, annealed InxSy, and CIGSSe at a kinetic energy of 1000 eV.

Distinguishing cubic and hexagonal phases within InGaN/GaN microstructures using electron energy loss spectroscopy

3D InGaN/GaN microstructures grown by metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) have been extensively studied using a range of electron microscopy techniques. The growth of material by MBE has led to the growth of cubic GaN material. The changes in these crystal phases has been investigated by Electron Energy Loss Spectroscopy, where the variations in the fine structure of the N K‐edge shows a clear difference allowing the mapping of the phases to take place. GaN layers grown for light emitting devices sometimes have cubic inclusions in the normally hexagonal wurtzite structures, which can influence the device electronic properties. Differences in the fine structure of the N K‐edge between cubic and hexagonal material in electron energy loss spectra are used to map cubic and hexagonal regions in a GaN/InGaN microcolumnar device. The method of mapping is explained, and the factors limiting spatial resolution are discussed.

Advanced and In Situ Analytical Methods for Solar Fuel Materials

In situ and operando techniques can play important roles in the development of better performing photoelectrodes, photocatalysts, and electrocatalysts by helping to elucidate crucial intermediates and mechanistic steps. The development of high throughput screening methods has also accelerated the evaluation of relevant photoelectrochemical and electrochemical properties for new solar fuel materials. In this chapter, several in situ and high throughput characterization tools are discussed in detail along with their impact on our understanding of solar fuel materials.

Silicon carbide embedded in carbon nanofibres: structure and band gap determination

Materials drastically alter their electronic properties when being reduced to the nanoscale due to quantum effects. Concerning semiconductors, the band gap is expected to broaden as a result of the quantum confinement. In this study we report on the successful synthesis of wide bandgap SiC nanowires (with great potential for applications) and the local determination of their band gap. Their value was found to be higher with respect to bulk SiC. The nanowires are grown as a heterostructure, i.e. encapsulated in carbon nanofibres via dc hot-filament Plasma-Enhanced Chemical Vapour Deposition on the Si/SiO2 substrate. The structure of the as-produced carbon nanofibres was characterized by means of aberration-corrected high-resolution transmission electron microscopy. Two different pure SiC polytypes, namely the 3C (cubic) and the 6H (hexagonal) as well as distorted structures are observed. The SiC nanowires have diameters in the range of 10-15 nm and lengths of several hundred nanometers. The formation of the SiC is a result of the substrate etching during the growth of the CNFs and a subsequent simultaneous diffusion of not only carbon, but also silicon through the catalyst particle.

Local band gap measurements by VEELS of thin film solar cells

This work presents a systematic study that evaluates the feasibility and reliability of local band gap measurements of Cu(In,Ga)Se2 thin films by valence electron energy-loss spectroscopy (VEELS). The compositional gradients across the Cu(In,Ga)Se2 layer cause variations in the band gap energy, which are experimentally determined using a monochromated scanning transmission electron microscope (STEM). The results reveal the expected band gap variation across the Cu(In,Ga)Se2 layer and therefore confirm the feasibility of local band gap measurements of Cu(In,Ga)Se2 by VEELS. The precision and accuracy of the results are discussed based on the analysis of individual error sources, which leads to the conclusion that the precision of our measurements is most limited by the acquisition reproducibility, if the signal-to-noise ratio of the spectrum is high enough. Furthermore, we simulate the impact of radiation losses on the measured band gap value and propose a thickness-dependent correction. In future work, localized band gap variations will be measured on a more localized length scale to investigate, e.g., the influence of chemical inhomogeneities and dopant accumulations at grain boundaries.

High-resolution spectroscopy of bonding in a novel BeP2N4 compound

The recently discovered compound BeP2N4 that crystallizes in the phenakite-type structure has potential application as a high strength optoelectronic material. Therefore, it is important to analyze experimentally the electronic structure, which was done in the present work by monochromated electron energy-loss spectroscopy. The detection of Be is challenging due to its low atomic number and easy removal under electron bombardment. We were able to determine the bonding behavior and coordination of the individual atomic species including Be. This is evident from a good agreement between experimental electron energy-loss near-edge structures of the Be-K-, P-L2,3-, and N-K-edges and density functional theory calculations.

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.

Full-scale characterization of UVLED Al(x)Ga(1-x)N nanowires via advanced electron microscopy

III-Nitride semiconductor heterostructures continue to attract a great deal of attention due to the wide range of wavelengths at which they can emit light, and the subsequent desire to employ them in optoelectronic applications. Recently, a new type of pn-junction which relies on polarization-induced doping has shown promise for use as an ultraviolet light emitting diode (UVLED); nanowire growth of this device has been successfully demonstrated. However, as these devices are still in their infancy, in order to more fully understand their physical and electronic properties, they require a multitude of characterization techniques. Specifically, the present contribution will discuss the application of advanced scanning transmission electron microscopy (STEM) to AlxGa1-xN UVLED nanowires. In addition to structural data, chemical and electronic properties will also be probed through various spectroscopy techniques, with the focus remaining on practically applying the knowledge gained via STEM to the growth procedures in order to optimize device peformance.

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.

Plasmonic response of bent silver nanowires for nanophotonic subwavelength waveguiding

We have imaged, with electron energy loss spectroscopy, the plasmonic response of straight and bent silver nanowires for their potential use in nanophotonic circuits. The guided surface plasmon polaritons appear unaffected by the presence of sharp kinks and corners in the nanowires studied, shown by direct imaging of excited Fabry-Perot-type resonances. Nanoscale detection is extended down to 0.17 eV, enabling detailed measurements of the spatial extent and dispersion of guided surface plasmon polaritons at low wave numbers. The experimental measurements are in excellent agreement with calculations, and the results are relevant in the design of integrated nanophotonic circuits and devices.

Optoelectronic properties of InAlN/GaN distributed bragg reflector heterostructure examined by valence electron energy loss spectroscopy

High-resolution monochromated electron energy loss spectroscopy (EELS) at subnanometric spatial resolution and <200 meV energy resolution has been used to assess the valence band properties of a distributed Bragg reflector multilayer heterostructure composed of InAlN lattice matched to GaN. This work thoroughly presents the collection of methods and computational tools put together for this task. Among these are zero-loss-peak subtraction and nonlinear fitting tools, and theoretical modeling of the electron scattering distribution. EELS analysis allows retrieval of a great amount of information: indium concentration in the InAlN layers is monitored through the local plasmon energy position and calculated using a bowing parameter version of Vegard Law. Also a dielectric characterization of the InAlN and GaN layers has been performed through Kramers-Kronig analysis of the Valence-EELS data, allowing band gap energy to be measured and an insight on the polytypism of the GaN layers.

Quantifying the low-energy limit and spectral resolution in valence electron energy loss spectroscopy

While the development of monochromators for scanning transmission electron microscopes (STEM) has improved our ability to resolve spectral features in the 0-5 eV energy range of the electron energy loss spectrum, the overall benefits relative to unfiltered microscopes have been difficult to quantify. Simple curve fitting and reciprocal space models that extrapolate the expected behavior of the zero-loss peak are not enough to fully exploit the optimal spectral limit and can hinder the ease of interpreting the resulting spectra due to processing-induced artifacts. To address this issue, here we present a quantitative comparison of two processing methods for performing ZLP removal and for defining the low-energy spectral limit applied to three microscopes with different intrinsic emission and energy resolutions. Applying the processing techniques to spectroscopic data obtained from each instrument leads in each case to a marked improvement in the spectroscopic limit, regardless of the technique implemented or the microscope setup. The example application chosen to benchmark these processing techniques is the energy limit obtained from a silicon wedge sample as a function of thickness. Based on these results, we conclude on the possibility to resolve statistically significant spectral features to within a hundred meV of the native instrumental energy spread, opening up the future prospect of tracking phonon peaks as new and improved hardware becomes available.

Triple-twin domains in Mg doped GaN wurtzite nanowires: structural and electronic properties of this zinc-blende-like stacking

We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted molecular beam epitaxy. The most significant feature is the presence of triple-twin domains, the density of which increases with increasing Mg concentration. The resulting high concentration of misplaced atoms gives rise to local changes in the crystal structure equivalent to the insertion of three non-relaxed zinc-blende (ZB) atomic cells, which result in quantum wells along the wurtzite (WZ) nanowire growth axis. High resolution electron energy loss spectra were obtained exactly on the twinned (zinc-blende) and wurtzite planes. These atomically resolved measurements, which allow us to identify modifications in the local density of states, revealed changes in the band to band electronic transition energy from 3.4 eV for wurtzite to 3.2 eV in the twinned lattice regions. These results are in good agreement with specific ab initio atomistic simulations and demonstrate that the redshift observed in previous photoluminescence analyses is directly related to the presence of these zinc-blende domains, opening up new possibilities for band-structure engineering.

Controlled modifications in electronic and chemical structures of a nanoscale region of polystyrene by fast electrons

In this study the effect of electron irradiation on properties of a nanosized area of polystyrene was investigated at various doses by an electron-microscope-electron-spectroscopy system. Therefore, changes in electronic and chemical properties by the exposure were monitored from local areas of the conventional polymer using electron energy loss spectroscopy. Also, their spatial extension was measured by a high angle annular dark field detector. From the study it was found that a nanoscale region of polystyrene can modify its electronic and chemical structures dramatically by the irradiation, and the degree of modifications can be tailored by the electron dose. These results may suggest a way of utilizing conventional polymers in nanotechnology.

The impact of surface and retardation losses on valence electron energy-loss spectroscopy

The inelastic scattering of fast electrons transmitting thin foils of silicon (Si), silicon nitride (Si(3)N(4)), gallium arsenide (GaAs), gallium nitride (GaN) and cadmium selenide (CdSe) was analyzed using dielectric theory. In particular, the impact of surface and bulk retardation losses on valence electron energy-loss spectroscopy (VEELS) was studied as a function of the foil thickness. It is shown that for the materials analyzed, surface and retardation losses can cause a systematic, thickness-dependent modulation of the dielectric volume losses, which can hamper the determination of the bulk dielectric data as well as the identification of band-gap and interband transition energies by VEELS. For Si and GaAs, where the dielectric function is strongly peaked with high absolute values, retardation losses lead to additional intensity maxima in the spectrum. For thin films of these materials (below approximately 100 nm), the additional intensity maxima are related to retardation effects due to the finite size of the sample leading to the excitation of guided light modes. For thicker films, exceeding about 200 nm, the intensity maxima are caused by bulk retardation losses, i.e., Cerenkov losses. Although thickness-dependent modulations were observed for Si(3)N(4), GaN and CdSe, the form of the dielectric functions and their lower maxima, means that for TEM samples < 100 nm thick, the band-gap energies of these materials can be accurately identified by VEELS. Guidelines are given that allow for forecasting the impact of surface and retardation losses on VEELS.

The influence of relativistic energy losses on bandgap determination using valence EELS

Since monochromated transmission electron microscopes have become available, the determination of bandgaps and optical properties using electron energy loss spectrometry (EELS) has again attracted interest. The underlying idea is very simple: below the bandgap energy no transitions can contribute to the valence EELS signal. However, the bandgap cannot be directly read out from the recorded data. Therefore the optical properties cannot be determined correctly from the low loss using the Kramers-Kronig relations. We will discuss under which conditions relativistic effects may be suppressed. It is demonstrated that scanning TEM (STEM) geometry is not applicable for most bandgap measurements.

Practical considerations for high spatial and temporal resolution dynamic transmission electron microscopy

Although recent years have seen significant advances in the spatial resolution possible in the transmission electron microscope (TEM), the temporal resolution of most microscopes is limited to video rate at best. This lack of temporal resolution means that our understanding of dynamic processes in materials is extremely limited. High temporal resolution in the TEM can be achieved, however, by replacing the normal thermionic or field emission source with a photoemission source. In this case the temporal resolution is limited only by the ability to create a short pulse of photoexcited electrons in the source, and this can be as short as a few femtoseconds. The operation of the photo-emission source and the control of the subsequent pulse of electrons (containing as many as 5 x 10(7) electrons) create significant challenges for a standard microscope column that is designed to operate with a single electron in the column at any one time. In this paper, the generation and control of electron pulses in the TEM to obtain a temporal resolution <10(-6)s will be described and the effect of the pulse duration and current density on the spatial resolution of the instrument will be examined. The potential of these levels of temporal and spatial resolution for the study of dynamic materials processes will also be discussed.

Preliminary results from the first monochromated and aberration corrected 200-kV field-emission scanning transmission electron microscope

Experimental results from the first monochromated and aberration-corrected scanning transmission electron microscope operated at 200 kV are described. The formation of an electron probe with a diameter of less than 0.2 nm at an energy width significantly under 0.3 eV and its planned application to the chemical analysis of nanometer-scale structures in materials science are described. Both energy and spatial resolution will benefit from this: The monochromator improves the energy resolution for studies of energy loss near edge structures. The Cs corrector allows formation of either a smaller probe for a given beam current or yields, at fixed probe size, an enhanced beam current density using a larger condenser aperture. We also point out another advantage of the combination of both components: Increasing the convergence angle by using larger condenser apertures in an aberration-corrected instrument will enlarge the undesirable chromatic focus spread. This in turn influences spatial resolution. The effect of polychromatic probe tails is proportional to the product of convergence angle, chromatic aberration constant, and energy spread. It can thus be compensated for in our new instrument by decreasing the energy width by the same factor as the beam convergence is increased to form a more intense probe. An alternative in future developments might be hardware correction of the chromatic aberration, which could eliminate the chromatic probe spread completely.

Low-energy-loss electron microscopy of doxorubicin in human breast cancer MCF-7 cells: localization by color

The distribution of the anti-cancer drug doxorubicin (DOX) in human breast cancer MCF-7 cells was imaged directly by low-energy-loss electron microscopy (EM) without specific antibodies or heavy metal stains, using only the electron-induced molecular orbital excitation of the drug. Cells treated with DOX were examined live by confocal fluorescence microscopy and as very thin sections in an electron microscope equipped with an electron energy filter having an energy resolution of 1 eV. The distribution of DOX obtained by EM from pairs of images at energy losses of 3+/-1 eV and 10+/-1 eV agreed with fluorescence microscope observations, but provided much more detail, easily distinguishing localization between nuclear membrane and perimembrane compartments and between vacuolated nucleoli and perinucleolar chromatin. Treatment times up to 1h and DOX concentrations up to 30 microM indicated a progression of DOX ingress from higher concentrations in the nuclear membrane to labeling of the nucleolus. Subsequently DOX moved into perinucleolar chromatin and concentrated in perimembrane chromatin aggregations. Quantification of the DOX signal indicated a decay half-life of 320 e/A2 under electron irradiation, whereas each image at 3000 x required 10 e/A2. The results point to a new field of high resolution microanalysis: color electron microscopy.

First experimental test of a new monochromated and aberration-corrected 200kV field-emission scanning transmission electron microscope

The first 200 kV scanning transmission electron microscope (STEM) with an imaging energy filter, a monochromator and a corrector for the spherical aberration (Cs-corrector) of the illumination system has been built and tested. The STEM/TEM concept with Koehler illumination allows to switch easily between STEM mode for analytical and TEM mode for high-resolution or in situ studies. The Cs-corrector allows the use of large illumination angles for retaining a sufficiently high beam current despite the intensity loss in the monochromator. With the monochromator on and a 3 microm slit in the dispersion plane that gives 0.26 eV full-width at half-maximum (FWHM) energy resolution we have obtained so far an electron beam smaller than 0.20 nm in diameter (FWHM as measured by scanning the spot quickly over the CCD) which contains 7 pA current and, according to simulations, should be around 0.12 nm in true size. A high-angle annular dark field (ADF) image with isotropic resolution better than 0.28 nm has been recorded with the monochromator in the above configuration and the Cs-corrector on. The beam current is still somewhat low for electron energy-loss spectroscopy (EELS) but is expected to increase substantially by optimising the condenser set-up and using a somewhat larger condenser aperture.