Subnanometer Electron Probes Research Articles

Hyperspectral imaging of exciton confinement within a moiré unit cell with a subnanometer electron probe

Electronic and optical excitations in two-dimensional systems are distinctly sensitive to the presence of a moiré superlattice. We used cryogenic transmission electron microscopy and spectroscopy to simultaneously image the structural reconstruction and associated localization of the lowest-energy intralayer exciton in a rotationally aligned WS2-WSe2 moiré superlattice. In conjunction with optical spectroscopy and ab initio calculations, we determined that the exciton center-of-mass wave function is confined to a radius of approximately 2 nanometers around the highest-energy stacking site in the moiré unit cell. Our results provide direct evidence that atomic reconstructions lead to the strongly confining moiré potentials and that engineering strain at the nanoscale will enable new types of excitonic lattices.

Functional Group Mapping by Electron Beam Vibrational Spectroscopy from Nanoscale Volumes.

Vibrational spectroscopies directly record details of bonding in materials, but spatially resolved methods have been limited to surface techniques for mapping functional groups at the nanoscale. Electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope presents a route to functional group analysis from nanoscale volumes using transmitted subnanometer electron probes. Here, we now use vibrational EELS to map distinct carboxylate and imidazolate linkers in a metal-organic framework (MOF) crystal-glass composite material. Domains <100 nm in size are observed using vibrational EELS, with recorded spatial resolution <15 nm at interfaces in the composite. This nanoscale functional group mapping is confirmed by correlated EELS at core ionization edges as well as X-ray energy dispersive spectroscopy for elemental mapping of the metal centers of the two constituent MOFs. These results present a complete nanoscale analysis of the building blocks of the MOF composite and establish spatially resolved functional group analysis using electron beam spectroscopy for crystalline and amorphous organic and metal-organic solids.

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.

Multipolar Plasmonic Resonances in Silver Nanowire Antennas Imaged with a Subnanometer Electron Probe

We detect short-range surface plasmon-polariton (SR-SPP) resonances setup in individual silver nanoantenna structures at high-spatial resolution with a scanning, subnanometer electron probe. Both even and odd multipolar resonant modes are resolved up to sixth order, and we measure their spatial distribution in relation to nanoantenna structures at energies down to 0.55 eV. Fabry-Perot type SR-SPP reflection phase shifts are calculated from direct measurements of antinode spacings in high-resolution plasmonic field maps. We observe resonant SR-SPP antinode bunching at nanoantenna terminals in high-order resonant modes, and antinode shifts in nonhomogeneous local environments. Finally, we achieve good agreement of our experimental SR-SPP maps with numerical calculations of photon excited near fields, using a novel integrated photon excitation geometry.

Optical Gap Measurements on Individual Boron Nitride Nanotubes by Electron Energy Loss Spectroscopy

Electromagnetic response of individual boron nitride nanotubes (BNNTs) has been studied by spatially resolved electron energy loss spectroscopy (EELS). We demonstrate how dedicated EELS methods using subnanometer electron probes permit the analysis of local dielectric properties of a material on a nanometer scale. The continuum dielectric model has been used to analyze the low-loss EEL spectra recorded from these tubes. Using this model, we demonstrate the weak influence of the out-of-plane contribution to the dielectric response of BNNTs. The optical gap, which can be deduced from the measurements, is found to be equal to 5.8 +/- 0.2 eV, which is close to that of the hexagonal boron nitride. This value is found to be independent of the nanotubes configuration (diameter, helicity, number of walls, and interaction between the different walls).

Long-period ordered structure in a high-strength nanocrystalline Mg-1 at% Zn-2 at% Y alloy studied by atomic-resolution Z-contrast STEM

The microstructure of a nanocrystalline Mg 97Zn 1Y 2 (at%) bulk alloy prepared by warm extrusion of rapidly solidified powders has been investigated by a combination of techniques, such as conventional high-resolution transmission electron microscopy (HRTEM), atomic-resolution high-angle annular dark field scanning-TEM (HAADF-STEM) with Z-contrast and energy-dispersive X-ray spectroscopy (EDS) with a sub-nanometer electron probe. We show that a novel long-period ordered structure is formed in the alloy, whose unit cell is composed of six close-packed planes of the magnesium crystal with a stacking sequence of ABCBCB′ where A and B′ layers are significantly enriched by Zn and Y. The lattice is distorted from an ideal hexagonal lattice of 6H-type (ABCBCB), which is probably due to an asymmetry of the chemical order with respect to the 6H-type stacking order. The present results demonstrate that the additional elements of a few atomic percent to Mg lead to formation of a long-period chemical-ordered as well as stacking-ordered structure, as directly revealed by a unique Z-contrast method.

Evaluation of Methods for Extracting Chemical Information from Eels Line-Spectra Across Interfaces

EELS fine structures on characteristic core edges are closely related to the bonding type and structural environment around the excited atoms. Consequently, they constitute useful hints for investigating the electronic properties at interfaces. One major difficulty then lies in the extraction of the information relevant to the involved atoms, from that due to neighbouring atoms on both sides of the selected interface seen end on by the impinging electrons. We have investigated possible ways to reach this goal, using the spectrum-line method, which consists in acquiring sequences of spectra at well defined increments (0.3 nm) while scanning a subnanometer electron probe (0.7 nm) across the interface. This approach constitutes one step forward with respect to the spatial-difference (SD) method which processes spectra acquired on two adjacent areas, one encompassing the interface, the other one on a reference area nearby. A normalized version (NSD) has been introduced to extend its field of application from grain boundaries to heterophase interfaces in order to enhance their specific ELNES contribution

Nanoscale EELS analysis of dielectric function and bandgap properties in GaN and related materials

The low loss region of an EEL spectrum (<50 eV) contains information about excitations of outer shell electrons and thus the electronic structure of a specimen which determines its optical properties. In this work, dedicated electron energy loss spectroscopy (EELS) methods for the experimental acquisition and analysis of spectra are described, which give improved information about the electronic structure near the bandgap region at a spatial resolution in the range of nanometers. For this purpose, we made use of a cold field emission scanning transmission electron microscope (STEM) equipped with a dedicated EELS system. This device provides a subnanometer electron probe and offers an energy resolution of 0.35 eV. Application of suitable deconvolution routines for removal of the zero loss peak extracts information on the bandgap region while the Kramers–Kronig transformation deduces the dielectric properties from the measured energy loss function. These methods have been applied to characterize the optical properties of wide-bandgap materials for the case of III-nitride compounds, which are currently the most promising material for applications on optoelectronic devices working in the blue and ultraviolet spectral range. The obtained results are in excellent agreement with experimental measurements by synchrotron ellipsometry and theoretical studies. The potential of the superior spatial resolution of EELS in a STEM is demonstrated by the analysis of dielectric properties of individual layers of heterostructures and individual defects within wurtzite GaN.

EELS Characterization of Electronic Structure in Group III - Nitrides Compared With Synchrotron Ellipsometry Results

Abstract EELS in the low loss region of the spectra (&amp;lt; 50eV) provides information on excitations of outer shell electrons and thus the electronic structure of a specimen material which determines its optical properties. In this work dedicated EELS methods for the experimental acquisition and analysis of spectra are described which give improved information about the electronic structure near the bandgap region at a spatial resolution in the range of nanometers. For this purpose we made use of a cold field emission STEM equipped with a dedicated EELS system. This device provides a subnanometer electron probe and offers an energy resolution of 0.35 eV. Application of suitable deconvolution routines for removal of the zero loss peak extracts information on the closest bandgap region while Kramers-Kronig transformation deduces the dielectric properties from the measured energy loss function. These methods have been applied to characterize the optical properties of wide-bandgap materials for the case of group Ill-nitride compounds which are currently the most promising material for applications on optoelectronic devices working in the blue and ultraviolet spectral range.

Probing the local dielectric/optical properties of group III-Nitrides by spatially resolved EELS on the nanometer scale

Usually, information on the dielectric-optical properties of semiconductors is obtained from synchrotron spectroscopic ellipsometry to cover the desired spectral range between 2 and 25 eV; but such measurements lack spatial resolution. In this work the local electronic structure of (In, Ga, Al) N heterostructures has been investigated by electron energy loss spectroscopy. Using subnanometer electron probes the spatial resolution of the measurements depends on the physical localization of the scattering process itself, typically in the range of nanometers. The low-loss region of the energy spectra reveals information on plasmon excitations and transitions across the band gap and the characteristic shape of the joint density of states. From these results the local dielectric/optical properties can be deduced via Kramers–Kronig transformation. The results obtained are in excellent agreement with theoretical studies and ellipsometry measurements.

Subnanometer electron probes and their capabilities for internal interface characterization

Internal interfaces in materials like e.g. semiconductor heterostructures get more and more interest not only under aspects of basic research but as well under aspects of new electronic and optoelectronic devices. The interface properties often govern the device performance. Thus, the evaluation of individual heterointerfaces with respect to chemical composition and crystal structure requires characterisation techniques which offer the necessary high spatial resolution. The fine focused electron probe (&lt; 0.3 nm at 100 keV) in a field-emission STEM (Scanning Transmission Electron Microscope) allows the application of special imaging and analytical techniques to cross-sectional specimens of semiconductor heterostructures. Qualitative information on the chemical composition is provided by atomic number (Z) contrast imaging with atomic resolution. The same fine probe can be used to analyse subnanometer areas by both spectroscopic and diffraction techniques. Quantitative compositional information is provided by electron energy-loss spectroscopy (EELS) which allows the detection of concentrations of specific elements.

Characterization of MOVPE grown InGaAsP superlattices for modulators by electron diffraction, X-ray diffraction and Z-contrast imaging

MOVPE grown ternary and quaternary GaxIn1−xAszP1−z heterostructures on InP substrates are increasingly used for the fabrication of, e.g. optoelectronic devices like modulators. Two of the key parameters for the performance of such devices are the crystalline quality and especially the variation of the tetragonal distortion on a nanometer scale as well as abrupt interfaces. We investigated cross-sectional specimens from nominally strain compensated quaternary superlattices (SL) of GaxIn1−xAszP1−zGayIn1−yAszP1−z on InP substrates by convergent beam electron diffraction (CBED) and atomic number (Z)-contrast imaging using subnanometer electron probes in a field-emission scanning transmission electron microscope (STEM). While CBED allows the measurement of local strain, Z-contrast imaging combines qualitative chemical information with high spatial resolution. Additionally, X-ray diffraction (XRD) was used to characterize the layer stacks. By CBED and Z-contrast imaging we found that even small deviations from design parameters of the SL like slightly degraded interfaces and asymmetries in thicknesses and strain balance of the SL layers can give rise to degradation in device performance. The MOVPE growth process was successfully optimized to obtain abrupt interfaces in the SL, as demonstrated by XRD and Z-contrast imaging. Photocurrent spectra of this type of SL give clear evidence for the Wannier-Stark effect. So-called “oblique” transitions of the order up to 2 are observed, which demonstrates the high quality of the SL and shows the applicability as active layers in optoelectronic modulators operating at 1.55 μm.

Characterization of III - V semiconductor interfaces by Z-contrast imaging, EELS and CBED

The application of scanning transmission electron microscopy (STEM)-based techniques, atomic number (Z)-contrast imaging, electron energy-loss spectroscopy (EELS) and convergent beam electron diffraction (CBED) allows determination of chemical compositions at internal interfaces of semiconductor heterostructures as well as determination of local crystalline properties such as strain, relaxation effects or ordering with high lateral spatial resolution. Z-contrast images recorded at internal heterostructure interfaces exhibit atomic spatial resolution in combination with qualitative chemical information. EELS can be used to record the chemical composition quantitatively but with slightly decreased spatial resolution compared to Z-contrast imaging. However, EELS results can be used to calibrate the Z-contrast. Thus, the combination of both techniques can give quantitative information on the chemical composition at interfaces from monolayer to monolayer. The interpretation of Z-contrast imaging is further supported by Z-contrast simulations. Examples demonstrating the performance of Z-contrast imaging (and simulation) and EELS are given for technically relevant III - V heterostructure interfaces. Additionally, we used CBED in order to investigate the crystalline properties of cross sectional specimens from ternary and quaternary heterostructures of on InP or GaAs substrates. Even when using subnanometer electron probes, the quality of the obtained CBED patterns is sufficient to perform local strain measurements with 1 nm spatial resolution and with a sensitivity of . This is proved by a CBED linescan across an alternately strained quaternary superlattice. CBED patterns recorded at interfaces directly exhibit symmetry violations, which are not yet understood satisfactorily. Therefore, further simulations are necessary for a detailed quantitative understanding of CBED patterns from internal interfaces. The combination of Z-contrast imaging, EELS and CBED allows the extensive quantitative characterization of semiconductor heterostructures and interfaces with the necessary lateral spatial resolution down to the monolayer range. STEM-based techniques are therefore an important tool for heterostructure and device development.

The structural sensitivity of electron energy-loss near-edge structure (ELNES)

Calculations are reported using the multiple scattering, Green function method for the transmission electron energy-loss near-edge structure (ELNES) of inner shell excitations. These reveal a high degree of sensitivity to the local crystallographic environment. A plot of the reciprocal of the ejected crystal electron wavevector for a particular feature in the ELNES against nearest-neighbor distance is found to be linear over a small range. This suggests a method for the quantitative measurement of local strain using ELNES obtained with a subnanometer electron probe. An experimental K-edge spectrum from beryllium is used to demonstrate the removal of plasmon multiple inelastic scattering artifact from ELNES by the method of logarithmic deconvolution, without adjustable parameters.