High Energy-resolution Electron Energy-loss Spectroscopy Research Articles

Local defect and mid-gap state analysis of GaN using monochromated EELS combined with nanodiffraction and atomic-resolution imaging

Defects in semiconductor materials significantly impact their inherent properties, making the evaluation of local defects and their energy levels crucial for controlling device performance. With advancements in monochromators, electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM) has emerged as a promising approach for assessing crystal and band structures of semiconductor materials at the nanoscale. However, there is limited experimental research investigating the relationship between actual defective structures in local regions and mid-gap states. In this study, we conducted high-energy resolution EELS (HR-EELS) measurements with an energy resolution of less than 100 meV to detect the mid-gap states of GaN. Various defects were intentionally induced through Ga-ion implantation, and the defect concentration varied in the depth direction. To understand the origin of the mid-gap states, we performed 4D-STEM analysis and atomic-resolution STEM observations. The HR-EELS measurements provided insights into the depth-dependent valence-loss spectra, revealing that the intensities corresponding to mid-gap states gradually increased toward the surface, whereas the slope at the onsets corresponding to interband transition decreased. Furthermore, local structural analysis unveiled the presence of structural disorder and defective structures, indicating the existence of extended defects such as stacking faults and domain boundaries. Observably, these defective structures were abundant near the surface and less pronounced in deeper regions. Based on these experimental findings, we concluded that the variations in valence-loss spectra can be utilized to qualitatively evaluate the crystal imperfections at the nanoscale.

Electron irradiation effects on lithium peroxide

In this study, electron irradiation effects on lithium peroxide (Li2O2), which is an important discharge product of Li–air (or Li–O2) batteries, were investigated using selected-area electron diffraction (SAED) and high-energy resolution electron energy-loss spectroscopy (EELS). The results obtained show that Li2O2 to Li2O transformation occurs with 80 and 300 keV incident electrons under high electron dose rates at 20 and −183 °C. The Li2O2 to Li2O transformation rate for 300 keV was 1/5 of that for 80 keV with the irradiation taking place at −183 °C. We also present a series of the EELS spectra that can be used as a criterion to judge the molar ratio of Li2O to Li2O2 in the general systems where Li2O2 and Li2O coexist.

Dielectric Properties of Photo-Luminescent CdSe/CdS Mono-Shell and CdSe/CdS/ZnS Multi-Shell Nanocrystals Studied by TEM-EELS

Semiconductor nanocrystals (NCs) of CdSe/CdS (core/shell) mono-shell and CdSe/CdS/ZnS (core/shell/shell) multi-shell having bright and stable luminescence were investigated using high energy-resolution electron energy-loss spectroscopy (EELS) based on transmission electron microscopy to evaluate their dielectric properties in the visible light energy region. EELS spectra of the mono-shell and multi-shell NCs showed two-step structures attributed to interband transitions near band gaps of individual CdSe core and CdS shell. EELS simulations based on the classical dielectric theory revealed that the two-step structures were results of dielectric response from CdSe core and CdS shell. The intensity profiles of the two-step structures depend on a thickness of CdS shells and mono/multi-shell structures of NCs. Besides, the quantum confinement effect was observed in the spectra of mono- and multi-shell NCs, where onset energy of the CdSe core was larger than that of bulk CdSe. The EELS spectra of multi-shell NCs, however, did not present an evident onset due to a bandgap of ZnS outer-shell suggesting defects or amorphous state in ZnS outer-shell. Such dielectric properties of NCs obtained from EELS analysis could provide valuable information to evaluate the crystallinity of individual core and shells in NCs.

Vanadium Substitution in Li2MnSiO4/C as Positive Electrode for Li Ion Batteries

Vanadium substitution is an interesting approach to manipulate the properties of the poor electronic and ionic conducting lithium transition metal orthosilicates. Especially, if incorporated on the Si site, it could alter the highly insulating character of the SiO4 framework. This study addresses the feasibility and limitations of V substitution in Li2MnSiO4. Nominal compositions of Li2Mn1–xVxSiO4 (0 ≤ x ≤ 0.2) and Li2MnSi1–xVxO4 (0 ≤ x ≤ 0.3) were synthesized by a sol–gel method, and the structural evolution was analyzed by X-ray diffraction and transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). While the solid solubility of V on tetrahedral Mn sites was shown to be limited, substantial amounts of V entered the structure when intended to substitute Si. Elemental mapping by TEM showed that V was highly inhomogeneously distributed, and high energy resolution EELS demonstrated that the majority of V was present in a tetravalent state. The nominal compositions Li2Mn...

Electronic structures of three-dimensional C60 polymers studied by high-energy-resolution electron energy-loss spectroscopy based on transmission electron microscopy

Electronic structures of two three-dimensional (3D) C60 polymers, which are Immm (3D C60-Immm) and Fcc (3D C60-Fcc) crystal symmetry, were investigated by high energy-resolution electron energy-loss spectroscopy based on transmission electron microscopy. Dielectric functions of 3D C60-Immm and 3D C60-Fcc were derived from valence electron excitation spectra by Kramers–Kronig analysis. The K-shell excitation spectrum of 3D C60-Immm specimen showed a step-like structure at the onset, which might correspond to Fermi edge. Therefore, metallic property of 3D C60-Immm could be confirmed. On the other hand, Fermi edge could not be observed for 3D C60-Fcc, suggesting to be a semiconductor.

High energy-resolution electron energy-loss spectroscopy study of the dielectric properties of multi-shell nanoparticles.

Nanoparticles, which have multi-shell structure, are expected to be stable and high efficient for the light-emitting devices. The efficiency of luminescence is considered to be affected by the multi-shell structure. In order to understand the mechanism of high efficiency luminescence, it is necessary to evaluate the multi-shell structure and dielectric properties from each particle. High energy-resolution electron energy-loss spectroscopy (HR-EELS) based on TEM is a powerful tool for this purpose. By comparing between the experimental and the simulated results, it is possible to evaluate the effect of the size and physical property of each shell material on the dielectric properties of multi-shell nanoparticles. In this study, simulations of EELS spectra of multi-shell nanoparticle (core: CdSe, inner shell: CdS, outer shell: ZnS) and mono-shell nanoparticle (core: CdSe, shell: CdS) were conducted by the dielectric continuum theory[1].Figure1 shows calculated EELS spectra of multi and mono shell nanoparticles. The spectra are calculated from dielectric functions of single CdSe, CdS and ZnS crystals, which were experimentally derived from HR-EELS spectra by using Kramers-Kronig analysis. The radius of 6.9 nm for the nanoparticle in the simulation corresponds to the average size of actual synthesized nanoparticles. Energy positions of arrows in the inset correspond to band gap energies of CdSe, CdS and ZnS[2]. In the spectrum of multi-shell nanoparticle, the intensity corresponding to interband transition near band gap of CdSe is suppressed comparing with that of the mono shell nanoparticle. This result indicates that ZnS outer shell affects the intensity profile of EELS spectrum near band gap. This effect should be sensitive for the thickness of the shells. Thus, there is a possibility that the effect of size and thickness of each core and shell on dielectric properties of multi-shell nanoparticles could be evaluated by using HR-EELS technique.jmicro;63/suppl_1/i18/DFU039F1F1DFU039F1Fig. 1.Calculated EELS spectra.

Optical properties of Group X-XII intermetallic compounds studied by HR-EELS.

Electronic structure of d orbital states in transition metals is a key factor for their physical properties and chemical functions. Copper and intermetallic compound PdZn have good catalysis function for the methanol steam reforming reaction. Tsai et al. showed that from results of XPS measurements the d electronic structure of PdZn was similar with that of copper, and the catalysis function should be related to the d electron states [1]. This similarity of d electronic states leads to another view point of the mechanism for coloring the intermetallic compounds. It is well-known that the characteristic red color of copper is caused by interband transition from the d electrons. Therefore, PdZn and Group X-XII intermetallic compounds are expected to be colored and the optical properties should depend on the d electronic states. In this study, the relations between optical properties and d electron states of Group X-XII intermetallic compounds were investigated by using high energy-resolution electron energy-loss spectroscopy (HR-EELS) based on transmission electron microscopy (TEM). From the relation between optical properties and d electronic states, the mechanism of colored intermetallic compounds will be discussed.Figure shows the optical reflectivity of NiZn, PdZn and PtZn, which were derived from EELS spectra by Kramers-Kronig analysis. Intensity drops (arrows) of the reflectivity were observed in visible energy region. These are caused by the interband transitions from d electronic states. The energy positions of the reflectivity drops have tendency of shifting to higher energy side with increasing atomic number of Group X elements (Ni → Pd → Pt). This indicates that the transition energies of d electrons become larger with the atomic number of the elements. First principle calculations (WIEN2k) confirmed that the interband transitions of d electronic states were excitations from bonding d states to hybrid states of anti-bonding s, p, and d states of Group X elements. The bonding anti-bonding energy split increase with the atomic numbers because of increasing crossover of wave function. This implies the intermetallic compounds should be colored and the color should be changed gradually depending on the atomic number of Group X elements.

High energy-resolution electron energy-loss spectroscopy and soft-x-ray emission spectroscopy studies of amorphous diamond transformed from neutron-irradiated graphite

High energy-resolution electron energy-loss spectroscopy measurements were performed for amorphous diamond (am-DIA), which was synthesized from neutron-irradiated graphite by shock compression, by using a monochromator transmission electron microscope (TEM). Soft-X-ray emission spectroscopy (SXES) measurements were performed by a wavelength-dispersive type spectrometer attached to the monochromator TEM. A volume plasmon peak of am-DIA is observed at 32.5 eV, which is a little smaller than that of crystalline diamond (c-DIA). The smaller plasmon energy of am-DIA indicates a smaller valence electron density of am-DIA than that of c-DIA. From the onset energies of the K-shell excitation and the SXES spectra, the band gap energy of am-DIA is estimated to be 4 eV, which is similar to the value, 3.9 eV, from the valence electron excitation spectrum and reported values, 3.5-4.5 eV, for the am-DIA synthesized from C60 fullerene by shock compression.

High-Energy Resolution Electron Energy-Loss Spectroscopy Study of Interband Transitions Characteristic to Single-Walled Carbon Nanotubes

An electron energy-loss spectroscopic (EELS) study using a monochromator transmission electron microscope was conducted for investigating the dielectric response of isolated single-walled carbon nanotubes (SWCNTs) owing to interband transitions characteristic to chiral structures. Individual chiral structures of the SWCNTs were determined by electron diffraction patterns. EELS spectra obtained from isolated SWCNTs showed sharp peaks below π plasmon energy of 5 eV, which were attributed to the characteristic interband transitions of SWCNTs. In addition, unexpected shoulder structures were observed at the higher energy side of each sharp peak. Simulations of EELS spectra by using the continuum dielectric theory showed that an origin of the shoulder structures was because of the surface dipole mode along the circumference direction of the SWCNT. It was noticed that the electron excitation energies obtained by EELS were slightly higher than those of optical studies, which might be because of the inelastic scattering process with the momentum transfers. To interpret the discrepancy between the EELS and optical experiments, it is necessary to conduct more accurate simulation including the first principle calculation for the band structure of SWCNTs.

Characteristic chemical shifts of quasicrystalline Zn–Mg–Zr alloys studied by EELS and SXES

Chemical shifts of the constituent atoms of primitive icosahedral quasicrystal (P-QC), face-centred icosahedral quasicrystal (F-QC) and 1/1-approximant (1/1-AP) of F-QC Zn–Mg–Zr alloys were investigated for the first time using high energy-resolution electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Among Zn M-shell and Mg L-shell excitation EELS spectra of P-QC, F-QC and 1/1-AP alloys, only the quasicrystalline alloys showed a chemical shift towards the larger binding energy side. In Zn-L and Zr-L emission SXES spectra, the P-QC and F-QC alloys showed a chemical shift towards larger binding energy side. The magnitudes of the shifts in the Zn-L emission spectra of the quasicrystalline alloys were almost the same as for ZnO. These results strongly suggest a decrease in valence charge in quasicrystalline states. Therefore, it should be concluded that bonding in quasicrystalline states involves a characteristic increase in covalency compared with bonding in corresponding approximant and standard metal crystals.

High energy-resolution electron energy-loss spectroscopy study on the near-infrared scattering mechanism of Cs0.33WO3 crystals and nanoparticles

The optical and dielectric properties of cesium-doped hexagonal tungsten trioxide (Cs0.33WO3) bulk crystal and nanoparticles, which are employed in solar heat-shielding filter, were investigated by high energy-resolution electron energy-loss spectroscopy (HR-EELS). The EELS spectrum of a Cs0.33WO3 crystal showed a prominent peak at 1.2 eV and a subpeak at 1.7 eV. On the basis of the dielectric function derived from the EELS spectrum, the peaks at 1.2 eV and 1.7 eV were assigned to the volume plasmon excitation of the carrier electrons and electronic excitation, respectively. The EELS spectra of the edge region of individual Cs0.33WO3 nanoparticles exhibited a surface dipole plasmon peak at 0.88 eV and a shoulder structure at 1.4 eV, which corresponded with the optical scattering spectrum. Therefore, it was confirmed that the optical scattering of the filter in the near-infrared region resulted from a surface dipole mode of the plasmon oscillation of the nanoparticles. The peak energies in the experimental spectra of the nanoparticle could not be reproduced by the derived dielectric function of the Cs0.33WO3 crystal. On the basis of the EELS and annular dark-field scanning transmission electron microscopy observations, this inconsistent result was attributed to Cs atom deficiency in the surface region of the nanoparticles.

High energy-resolution electron energy-loss spectroscopy analysis of dielectric property and electronic structure of hexagonal diamond

Electron energy-loss spectroscopy measurements of a hexagonal diamond (h-DIA) were performed using a transmission electron microscope equipped with a monochromator. From the measurements, the dielectric function of h-DIA was derived using Kramers–Kronig analysis (KKA) for the first time. The results of KKA showed that the band gap energy of h-DIA was 4.2eV, which is 1.3eV smaller than that of a cubic diamond (c-DIA). The peak energies that appeared in the imaginary part of the dielectric function were assigned to the interband transition energies predicted by theoretical calculations. The K-shell excitation spectrum of h-DIA, which was apparently different from that of c-DIA, also showed good agreement with the reported theoretical prediction.

High energy-resolution electron energy-loss spectroscopy study of the dielectric properties of bulk and nanoparticle LaB6 in the near-infrared region

The dielectric properties of LaB(6) crystals and the plasmonic behavior of LaB(6) nanoparticles, which have been applied to solar heat-shielding filters, were studied by high energy-resolution electron energy-loss spectroscopy (HR-EELS). An EELS spectrum of a LaB(6) crystal showed a peak at 2.0 eV, which was attributed to volume plasmon excitation of carrier electrons. EELS spectra of single LaB(6) nanoparticles showed peaks at 1.1-1.4 eV depending on the dielectric effect from the substrates. The peaks were assigned to dipole oscillation excitations. These peak energies almost coincided with the peak energy of optical absorption of a heat-shielding filter with LaB(6) nanoparticles. On the other hand, those energies were a smaller than a dipole oscillation energy predicted using the dielectric function of bulk LaB(6) crystal. It is suggested that the lower energy than expected is due to an excitation at 1.2 eV, which was observed for oxidized LaB(6) area.

Characteristic chemical shift of quasicrystalline alloy Al53Si27Mn20 studied by EELS and SXES

Chemical shifts of all constituent atoms for amorphous (Am), quasicrystalline (QC) and crystalline (Cryst) alloys of Al53Si27Mn20 were investigated for the first time by high energy-resolution electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Among Al L-shell excitation EELS spectra of Am, QC and Cryst alloys, only QC alloy showed an apparent chemical shift to the larger binding energy side by 0.4 eV. In Al-Kα and Si-Kα emission SXES spectra of these alloys, only QC alloy showed a chemical shift to the larger binding energy side by 4 eV for Al-Kα and 6 eV for Si-Kα. These chemical shift values are comparable to those of corresponding metal oxides. This indicates a smaller amount of valence charge at Al and Si atomic sites in QC alloy. On the other hand, Mn-L SXES spectra did not show any chemical shift. Therefore, the decreased charge from Al and Si sites should be distributed between atomic sites, indicating the presence of covalent bonding nature for QC ordered alloy.

Comparison of EFTEM and STEM EELS plasmon imaging of gold nanoparticles in a monochromated TEM

We present and compare two different imaging techniques for plasmonic excitations in metallic nanoparticles based on high energy-resolution electron energy-loss spectroscopy in a monochromated transmission electron microscope. We demonstrate that a recently developed monochromated energy-filtering (EFTEM) approach can be used in addition to the well established scanning technique to directly obtain plasmon images in the energy range below 1 eV. The EFTEM technique is described in detail, and a double experiment performed on the same, triangular gold nanoparticle compares equivalent data acquired by both techniques, respectively.

準結晶研究の最近の展開 ４．準結晶のはたらき（準結晶の物性と応用）電子線を用いた準結晶の電子状態研究―準結晶の特徴は何か―

Al L-shell excitation spectra of Al-based quasicrystals of Al75Cu15V10, Al73Co27 and Al70Pd20Mn10 and related crystals were obtained from specified single crystalline areas by using a high energy-resolution electron energy-loss spectroscopy microscope. Intensity distributions of the spectra of not only the quasicrystalline materials but also related crystalline materials showed a presence of pseudogap at around the Fermi level. Onset energies of the spectra of the quasicrystals showed shifts to the higher energy side by 0.3 eV for Al75Cu15V10 and Al73Co27 and 0.8 eV for Al70Pd20Mn 10 compared to that of aluminum. Those results show that a decrease of valence charge at Al sites is characteristic for Al-based quasicrystalline materials. It may be attributed to an increase of covalency in bonding nature of the quasicrystalline materials.

Electronic structure analyses of BN network materials using high energy‐resolution spectroscopy methods based on transmission electron microscopy

Electronic structures of boron-nitride (BN) nanotubes and a BN cone-structure material were studied by using a high energy-resolution electron energy-loss spectroscopy (EELS) microscope. A trial of the whole electronic structure study of hexagonal BN (h-BN), which consists of flat BN honeycomb layers, was conducted by a combination of EELS and X-ray emission spectroscopy (XES) based on transmission electron microscopy (TEM) (TEM-EELS/XES). The pi and pi+sigma plasmon energies of BN nanotubes (BNT) were smaller than those of h-BN. The pi+sigma energy was explained by the surface plasmon excitation. The spectrum of a two-wall BNT of 2.7 nm in diameter showed a new spectral onset at 4 eV. The valence electron excitation spectra obtained from the tip region of the BN cone with an apex angle of 20 degrees showed similar intensity distribution with those of BNTs. The B K-shell electron excitation spectra obtained from the bottom edge region of the BN cone showed additional peak intensity when compared with those of h-BN and BNT. The B K-shell electron excitation spectra and B K-emission spectra of h-BN were compared with a result of a LDA band calculation. It showed that high symmetry points in the band diagram appear as peak and/or shoulder structures in the EELS and XES spectra. Interband transitions appeared in the imaginary part of the dielectric function of h-BN experimentally obtained were assigned in the band diagram. The analysis also presented that the LDA calculation estimated the bandgap energy smaller than the real material by an amount of 2 eV. Those results of TEM-EELS/XES analysis presented that high energy-resolution spectroscopy methods combined with TEM is a promising method to analyze whole electronic structures of nanometer scale materials.

High energy-resolution electron energy-loss spectroscopy study of the electric structure of double-walled carbon nanotubes

Electron energy-loss spectra were obtained from two double-walled carbon nanotubes (DWCNTs) with an energy resolution of 85 meV. The spectra showed multiple peak structures between 2 and 3 eV. However, peak positions are different for these two DWCNTs. The chiral indices of CNT layers of the two DWCNTs were determined to be (29,4)(in) (17,8)(out) and (46,6)(out) (26,21)(in), respectively, by comparing experimental electron diffraction patterns with simulated ones. The spectra were also compared with simulated joint density of states, which were derived from the determined chiral indices. It was confirmed that the peak structures in the spectra are due to interband transitions intrinsic for tubular structures of graphitic sheets.

High energy-resolution electron energy-loss spectroscopy study of the electronic structure of C 60 polymer crystals

Electronic structures of C 60 polymer crystals (dimerized C 60: dimer-C 60, one-dimensionally polymerized C 60: 1D-C 60 and two-dimensionally polymerized C 60: 2D-C 60) were studied by using a high energy-resolution electron energy-loss spectroscopy microscope. Spectra were obtained from single crystalline specimen areas of 180 nm in diameter. The dielectric functions of the materials were derived from valence electron excitation spectra by using Kramers–Kronig analysis. The onset energies of the imaginary parts of the dielectric functions ɛ 2 (bandgap energies) were 1.6 eV (dimer-C 60), 1.4 eV (1D-C 60) and 1.5 eV (2D-C 60). Those energies are smaller than that of the monomer C 60 (mono-C 60) crystal of 1.7 eV. Energies of interband transition peaks observed in each ɛ 2 of polymerized C 60 material were smaller by amounts of 0.5 eV than those of corresponding peak energies of mono-C 60. Excitation spectra of 1s electrons to the conduction band showed that the intensity of 1s → π * transitions compared to that of 1s → σ* transitions was smaller than that of mono-C 60. It may due to that a part of π-bonds of a C 60 cluster changed to σ-bonds to form covalent bonds with neighboring C 60 clusters in the polymerization process.

High energy-resolution electron energy-loss spectroscopy study of the electronic structures of Li- and Mg-doped -rhombohedral boron

Electron-doped alpha-rhombohedral boron (alpha-r-B) crystals were synthesized by Li- and Mg-dopings. The doped alpha-r-B particles showed a shiny metallic colour, though non-doped alpha-r-B crystals showed a dark red colour (red boron). The electronic structures of doped alpha-r-B single crystals were examined by a high energy-resolution electron energy-loss spectroscopy microscope. Boron 1s electron-excitation spectra, which show the density of states of the conduction bands, of Li-doped alpha-r-B showed a chemical shift toward a smaller binding-energy side, indicating a charge transfer from the doped atoms to alpha-r-B. These spectra also showed a sharp intensity increase at the onset with a width of an energy resolution of the experiment. The sharp intensity increase may be assigned to a Fermi edge produced by the electron doping. The spectra of Mg-doped alpha-r-B showed a chemical shift indicating an electron doping, but did not show the presence of a Fermi edge.