Electron Energy Loss Spectroscopy Peak Research Articles

Direct Visualization of Subnanometer Variations in the Excitonic Spectra of 2D/3D Semiconductor/Metal Heterostructures

The integration of metallic contacts with two-dimensional (2D) semiconductors is routinely required for the fabrication of nanoscale devices. However, nanometer-scale variations in the 2D/metal interface can drastically alter the local optoelectronic properties. Here, we map local excitonic changes of the 2D semiconductor MoS2 in contact with Au. We utilize a suspended and epitaxially grown 2D/metal platform that allows correlated electron energy-loss spectroscopy (EELS) and angle resolved photoelectron spectroscopy (nanoARPES) mapping. Spatial localization of MoS2 excitons uncovers an additional EELS peak related to the MoS2/Au interface. NanoARPES measurements indicate that Au-S hybridization decreases substantially with distance from the 2D/metal interface, suggesting that the observed EELS peak arises due to dielectric screening of the excitonic Coulomb interaction. Our results suggest that increasing the van der Waals distance could optimize excitonic spectra of mixed-dimensional 2D/3D interfaces and highlight opportunities for Coulomb engineering of exciton energies by the local dielectric environment or moiré engineering.

Introduction of recombinant fullerene-PbI2 core-shell and investigation of its use as surface modifier of biosensors

For improving function of sensors, one can modify their surface with new fullerene-based nanostructure or nanocomposite, especially through hosting. In this work, Fullerene/Lead Iodide (C60/PbI2) recombinant was introduced and its use was investigated as modifier of biosensors. Results show that C60/PbI2 has bandgap-free, p symmetry and paramagnetic properties. Charge carriers were highly mobile specifically for hole. The electron energy loss spectroscopy peak showed the existence of C60/PbI2 on the modified nanobiosensors. High energy radiation did not propagate by orbitals҆ electrons. Electromagnetic radiation with energy value of 1–6 eV was absorbed with less than 20% of reflectivity. The refractive index for C60/PbI2 corresponded to yellow light. Based on results, this structure is good for surface modification of sensors to be used in biomaterial sensing applications.

Invited: Surface Modification of Positive Electrode and its Effect to Bulk

Improvement technique of the life of Li-ion batteries has been intensively studied. Modification of the surface of positive electrode material including coating is effective to this purpose. In that process, it is very important to compare the bulk and surface structure of the positive electrode with/without surface modification to understand this effect.For basic information concerning the effect of surface modification, authors have applied Transmission Electron Microscopy (TEM) to visualize the structural distribution inside the positive electrode particles after degradation [1]. As a model material, we synthesized sub-micron size particle of positive electrode such as LiCoO2 by Pechini sol-gel method [2, 3] that could provide the suitable sample for TEM observation without damages from sample preparation like ion-milling or FIB. Analytical TEM measurements by scanning transmission microscopy (STEM) with electron energy-loss spectroscopy (EELS) equipped with monochromator were applied. Li-K EELS peak around 62 eV was clearly found and this peak intensity changed with Li charge-discharge state. The EELS mapping from the spectrum imaging method using this Li-K region spectra showed a distribution of Li in the particle or among particles during the charge discharge cycle. We discuss precisely the change of chemical states at the surface and Li ion distribution inside particles of positive electrode material with/without surface modification using Si, Zr, Al, Mg oxide during charge-discharge cycle.

Chemical-state imaging of Li using scanning Auger electron microscopy

The demand for measurement tools to detect Li with high spatial resolution and precise chemical sensitivity is increasing with the spread of lithium-ion batteries (LIBs) for use in a wide range of applications. In this work, scanning Auger electron microscopy (SAM) is used to image chemical states of a partially oxidized Li surface on the basis of the Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) data obtained during an oxidation process of a metal Li. We show that distribution of metallic and oxidized states of Li is clearly imaged by mapping the intensity of the corresponding AES and EELS peaks. Furthermore, a tiny difference in the extent of oxidation can be distinguished by comparing the elemental map of an AES peak with that of an EELS peak owing to the different behaviors of those signals to the chemical states of Li.

Direct measurement of the nitrogen content by XPS in self‐passivated TaNx thin films

AbstractThe nitrogen content in tantalum nitride (TaNx) thin films, where x indicates that TaNx is not generally stoechiometric, can be measured directly by XPS. This is the purpose of the present study. However, the XPS spectra of TaNx present electron energy loss spectroscopy (EELS) peaks that lead to a complex peak fitting, particularly for self‐passivated thin films. A complete peak fitting procedure based upon Tougaard's background, the Doniach‐Sunjic Function and EELS peaks, is presented. It is applied to two self‐passivated TaNx thin films elaborated by reactive sputtering and presenting a different nitrogen content. The physical properties of these surfaces are interpreted in terms of Ta 4f7/2 chemical states directly dependent on the nitrogen content. The main results are discussed and improvements are proposed to the method. Copyright © 2008 John Wiley & Sons, Ltd.

Re-Evaluating Neptunium in Uranyl Phases Derived from Corroded Spent Fuel

Interest in mechanisms that may control radioelement release from corroded commercial spent nuclear fuel (CSNF) has been heightened by the selection of the Yucca Mountain site in Nevada as the repository for high-level nuclear waste in the United States. Neptunium is an important radionuclide in repository models owing to its relatively long half-life and its high aqueous mobility as neptunyl [Np(V)O2+]. The possibility of neptunium sequestration into uranyl alteration phases produced by corroding CSNF would suggest a process for lowering neptunium concentration and subsequent migration from a geologic repository. However, there remains little experimental evidence that uranyl compounds will, in fact, serve as long-term host phases for the retention of neptunium under conditions expected in a deep geologic repository. To directly explore this possibility, we examined specimens of uranyl alteration phases derived from humid-air–corroded CSNF by X-ray absorption spectroscopy to better determine neptunium uptake in these phases. Although neptunium fluorescence was readily observed from as-received CSNF, it was not observed from the uranyl alteration rind. We establish upper limits for neptunium incorporation into CSNF alteration phases that are significantly below previously reported concentrations obtained by using electron energy loss spectroscopy (EELS). We attribute the discrepancy to a plural-scattering event that creates a spurious EELS peak at the neptunium-MV energy.

On the electronic structure of Cu ultra-thin film on α-Al 2O 3(0001) surfaces

Electron energy-loss spectroscopy (EELS) and uv photoelectron spectroscopy (UPS) were used to study the electronic structure of a Cu thin film on sapphire, Al 2O 3(0001) and on Al 2O 3Al(111). The surfaces were characterized by Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). The results indicate that a Cu(i)I state is formed on the Al 2O 3Al(111) and on the α-Al 2O 3(0001) surface when the Cu film is less than 2 Å thick. Upon further growth Cu(O) (metallic copper) is formed as nuclei. For Cu on α-Al 2O 3(0001), the EELS peak at 5.5 eV for coverages corresponding to less than 2 Å of Cu layer thickness is probably derived from an electronic transition from antibonding molecular orbitals of energies consisting of Cu(3d) and O (2p) near the top of the Al 2O 3 valence band to energy levels below conduction band minimum.

Location and structure of coke deposits on alumina-supported platinum catalysts by EELS associated with electron microscopy

The location and structure of carbonaceous deposits formed upon coking Pt Al 2O 3 catalysts with cyclopentane have been studied by conventional TEM and by electron energy loss spectroscopy (EELS) associated with high resolution scanning transmission electron microscopy. The TEM investigation shows patches of amorphous coke covering the support surrounding each metal particle. This is substantiated by EELS showing that the coke coverage can extend as far as 20 nm from a given particle. From the absence of fine structure on the EELS peak of carbon and from the comparison with reference compounds (graphite, coronene, and pentacene), it is concluded that the local structure of coke is not graphitic or pregraphitic but may consist in a disordered arrangement of polyaromatic molecules. After partial combustion at 563 K, the coke covering the metal particles is removed but the coke on the support is still present. This confirms previous temperature-programmed Oxidation results.

Initial stage of Pd adsorption on Si(111)7 × 7 surface studied by AES and EELS

The initial stage of adsorption of Pd on a Si(111)7 × 7 surface has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and surface work-function change. For Pd deposition at room temperature (RT) the Si(LVV) Auger signal intensity decays in a broken linear line. The structure factor, defined as the intensity ratio of the subpeak to the main one in Si(LVV) Auger spectra, increases up to a maximum around one monolayer coverages. In EELS spectra two peaks, characteristics of Pd, appear at the completion of the first Pd layer. Pd atoms deposited on Si(111) at RT form initially flat layers of a few monolayers height without mixing with substrate Si atoms. For Pd deposition at a moderately high temperature (MT) of about 300°C, however, the structure factor for Si(LVV) Auger spectra does not change. EELS peaks, characteristic of Si substrate, remain clearly even beyond one monolayer coverage. Pd atoms deposited at MT are unstable and easily diffuse into the bulk. We present evidences to support the “screening” model for the bond-breaking mechanism at the Pd/Si interface.

Auger and core level electron energy loss studies of (Y2O3)m(ZrO2)1−m single crystal surfaces

Core level electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES) were used to study yttria stabilized cubic zirconia (YSCZ) substrate surfaces subjected to various treatments before Si epitaxy for silicon-on-insulator (SOI) applications. The Y and Zr M4,5 EELS peaks are shown to be quite sensitive to chemical state differences. In the case of YSCZ these peaks exhibit subtle differences with varying yttria content. These differences are thought to be related to oxygen vacancy concentration and its effect on local density of states. Because of Y and Zr AES peak overlap problems, no changes in peak shape with yttria content were observed in corresponding AES specta. Both AES and EELS data were used to quantify the yttria concentration at YSCZ surfaces and to identify the elements remaining on YSCZ surfaces following various chemical treatments.

Chemisorption of nitrogen on Cr(110)

Abstract The chemisorbed state of nitrogen on a Cr(110) surface at 300 K has been investigated by means of electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES) and work function change measurement (Δφ). The dissociative state is suggested by our observations that the EELS spectrum and the N(KLL) Auger line shape of the Cr(110) surface after N 2 adsorption at 300 K are practically the same as those of the nitride-covered surface formed by NH 3 exposure at 800 K. From the Auger uptake curve, the initial sticking probability and the saturation coverage at 500 L N 2 are estimated respectively to be 0.09 and 0.36. Both the Auger uptake and the work function decrease (saturation value: -0.27 eV) show an abrupt change at about 10 L. The clean-surface EELS peaks due to the interband transitions in the low energy range of 2–20 eV have merely decreased and the high-energy side tailing of the loss peaks at 5.0 and 9.0 eV is observed after N 2 adsorption. Although the nitrogen-induced EELS peaks are not distinctly resolved in the low-energy range at less than 30 eV, the shift of the loss peak at 41.5 eV due to the Cr 3p core-level excitation is clearly observed at about 10 L in accordance with the AES and Δφ results. The above results strongly indicate the onset of transformation of the chemisorbed phase to the surface nitride phase on Cr(110) surface at about 10 L N 2 exposure even at 300 K.

Chemisorption of nitrogen on Cr(110)

The chemisorbed state of nitrogen on a Cr(110) surface at 300 K has been investigated by means of electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES) and work function change measurement (Δφ). The dissociative state is suggested by our observations that the EELS spectrum and the N(KLL) Auger line shape of the Cr(110) surface after N 2 adsorption at 300 K are practically the same as those of the nitride-covered surface formed by NH 3 exposure at 800 K. From the Auger uptake curve, the initial sticking probability and the saturation coverage at 500 L N 2 are estimated respectively to be 0.09 and 0.36. Both the Auger uptake and the work function decrease (saturation value: −0.27 eV) show an abrupt change at about 10 L. The clean-surface EELS peaks due to the interband transitions in the low energy range of 2–20 eV have merely decreased and the high-energy side tailing of the loss peaks at 5.0 and 9.0 eV is observed after N 2 adsorption. Although the nitrogen-induced EELS peaks are not distinctly resolved in the low-energy range at less than 30 eV, the shift of the loss peak at 41.5 eV due to the Cr 3p core-level excitation is clearly observed at about 10 L in accordance with the AES and Δφ results. The above results strongly indicate the onset of transformation of the chemisorbed phase to the surface nitride phase on Cr(110) surface at about 10 L N 2 exposure even at 300 K.

CHaracterization of CO binding sites on Rh{111} and Rh{331} surfaces by XPS and LEED: Comparison to EELS results

Changes in the nature of the binding site of chemisorbed CO on the Rh{111} and Rh{331} single crystal surfaces during adsorption and desorption have been monitored by X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). Two bonding states of molecular CO have been identified from the O 1s photoemission line. These states are assigned as atop and bridge-bonded species and are observed to be coverage and temperature dependent. On both surfaces atop sites are populated first and at higher CO coverages bridge sites are filled. On Rh{111} the bridge sites are filled at a CO coverage of θ CO ∼ 0.50 and their presence is correlated with a change in the LEED pattern. The presence of the step atoms on the Rh{331} surface markedly influenced the sequential filling of binding sites in comparison to that observed on the Rh{111} surface. A comparison of our data to previous Electron Energy Loss Spectroscopy (EELS) work on Rh{111} is in remarkable quantitative agreement with EELS peak heights.

A comparison between electron energy loss spectroscopy and Auger electron spectroscopy sputtering depth profiles of Ta 2O 5/Ta

Electron energy loss spectroscopy (EELS) is used simultaneously with Auger electron spectroscopy for profiling an anodic Ta 2O 5 layer 30 nm thick on tantalum using 3 keV Ar + ions. Plotted as a function of the sputtering time, the 16 eV EELS peak intensity correlates fairly well with the O(510 eV) Auger peak intensity but shows a different depth resolution at the interface.

Oxygen chemisorption and initial oxidation of Cr(110)

The initial stages of the interaction of oxygen with a Cr(110) surface have been investigated at 300 K by LEED, AES, electron energy loss spectroscopy (ELS), secondary electron emission spectroscopy (SES) and work-function change measurement (Δφ). In the exposure region up to 2 L, the clean-surface ELS peaks due to interband transition weakened and then disappeared, while the ∼5.8 and 10 eV loss peaks attributed to the O 2p → Cr 3d transitions appeared, accompanied with a work-function increase ( Δφ = +0.19 eV at2L). In the region 2–6 L the work function decreased to below the original clean-surface value ( Δφ min = −0.24 eV at6L), and five additional ELS peaks were observed at ∼2, 4, 11, 20 and 32 eV: the 2 and 4 eV peaks are ascribed to the ligand-field d → d transitions of a Cr 3+ ion, the 11 eV peak to the O 2p → Cr 4s transition, the 20 eV peak to the Cr 3d → 4p transition of a Cr 3+ ion and the 32 eV peak probably to the Cr 3d → 4f transition. A new SES peak at 6.1 eV, being attributed to the final state for t he 11 eV ELS peak, was observed at above 3 L and identified as due to the unfilled Cr 4s state caused by charge transfer from Cr to oxygen sites in this region. In the region 6–15 L the work function increased again ( Δφ max = +0.32 eV at15 L), the 33 and 46 eV Auger peaks due to respectively the M 2,3(Cr)L 2,3(O)L 2,3(O) cross transition and the M 2,3VV transition of the oxide appeared and the 26 eV ELS peak due to the O 2s → Cr 4s transition was also observed. Above 10 L, the ELS spectra were found to be practically the same as that of Cr 2O 3. Finally, above 15 L, the work function decreased slowly ( Δφ = +0.13 eV at40L). From these results, the oxygen interaction with a Cr(110) surface can be classified into four different stages: (1) dissociative chemisorption stage up to 2 L, (2) incorporation of O adatoms into the Cr selvedge between 2–6 L, (3) rapid oxidation between 6–15 L leading to the formation of thin Cr 2O 3 film, and (4) slow thickening of Cr 2O 3 above 15 L. The change in the Cr 3p excitation spectrum during oxidation was also investigated. The oxide growth can be interpreted on the basis of a modified coupled current approach of low-temperature oxidation of metals.

Oxygen chemisorption and initial oxidation of Cr(110)

Abstract The initial stages of the interaction of oxygen with a Cr(110) surface have been investigated at 300 K by LEED, AES, electron energy loss spectroscopy (ELS), secondary electron emission spectroscopy (SES) and work-function change measurement (Δφ). In the exposure region up to 2 L, the clean-surface ELS peaks due to interband transition weakened and then disappeared, while the ∼5.8 and 10 eV loss peaks attributed to the O 2p → Cr 3d transitions appeared, accompanied with a work-function increase ( Δφ = +0.19 eV at2L). In the region 2–6 L the work function decreased to below the original clean-surface value ( Δφ min = −0.24 eV at6L), and five additional ELS peaks were observed at ∼2, 4, 11, 20 and 32 eV: the 2 and 4 eV peaks are ascribed to the ligand-field d → d transitions of a Cr 3+ ion, the 11 eV peak to the O 2p → Cr 4s transition, the 20 eV peak to the Cr 3d → 4p transition of a Cr 3+ ion and the 32 eV peak probably to the Cr 3d → 4f transition. A new SES peak at 6.1 eV, being attributed to the final state for t he 11 eV ELS peak, was observed at above 3 L and identified as due to the unfilled Cr 4s state caused by charge transfer from Cr to oxygen sites in this region. In the region 6–15 L the work function increased again ( Δφ max = +0.32 eV at15 L), the 33 and 46 eV Auger peaks due to respectively the M 2,3 (Cr)L 2,3 (O)L 2,3 (O) cross transition and the M 2,3 VV transition of the oxide appeared and the 26 eV ELS peak due to the O 2s → Cr 4s transition was also observed. Above 10 L, the ELS spectra were found to be practically the same as that of Cr 2 O 3 . Finally, above 15 L, the work function decreased slowly ( Δφ = +0.13 eV at40L). From these results, the oxygen interaction with a Cr(110) surface can be classified into four different stages: (1) dissociative chemisorption stage up to 2 L, (2) incorporation of O adatoms into the Cr selvedge between 2–6 L, (3) rapid oxidation between 6–15 L leading to the formation of thin Cr 2 O 3 film, and (4) slow thickening of Cr 2 O 3 above 15 L. The change in the Cr 3p excitation spectrum during oxidation was also investigated. The oxide growth can be interpreted on the basis of a modified coupled current approach of low-temperature oxidation of metals.

Electron energy loss spectroscopy studies of the Si-SiO2 interface

We have performed, for the first time, measurements of the electronic structure of the Si-SiO2 interface with electron energy loss spectroscopy (ELS) in connection with argon-ion sputtering. We have measured the depth profiles of both the 5.1- and 7.2-eV ELS peaks, which have been previously observed for both the Si surface with oxygen adsorbed and SiO2 with defects present. We have found that the intensities of these peaks, especially that of the 5.1-eV peak, show a maximum at the Si-SiO2 interface. This suggests that these ELS peaks come from special bonding configurations characteristic of the connective layer between Si and SiO2 at the interface.