Peter Rez

Vibrational spectroscopy in the electron microscope

Vibrational spectroscopies using infrared radiation, Raman scattering, neutrons, low-energy electrons and inelastic electron tunnelling are powerful techniques that can analyse bonding arrangements, identify chemical compounds and probe many other important properties of materials. The spatial resolution of these spectroscopies is typically one micrometre or more, although it can reach a few tens of nanometres or even a few ångströms when enhanced by the presence of a sharp metallic tip. If vibrational spectroscopy could be combined with the spatial resolution and flexibility of the transmission electron microscope, it would open up the study of vibrational modes in many different types of nanostructures. Unfortunately, the energy resolution of electron energy loss spectroscopy performed in the electron microscope has until now been too poor to allow such a combination. Recent developments that have improved the attainable energy resolution of electron energy loss spectroscopy in a scanning transmission electron microscope to around ten millielectronvolts now allow vibrational spectroscopy to be carried out in the electron microscope. Here we describe the innovations responsible for the progress, and present examples of applications in inorganic and organic materials, including the detection of hydrogen. We also demonstrate that the vibrational signal has both high- and low-spatial-resolution components, that the first component can be used to map vibrational features at nanometre-level resolution, and that the second component can be used for analysis carried out with the beam positioned just outside the sample—that is, for ‘aloof’ spectroscopy that largely avoids radiation damage.

Site-specific fabrication of Fe particles for carbon nanotube growth.

We report a method for site-specific fabrication of Fe catalyst particles on silica (SiO(2)) substrate by electron beam induced decompositionat 650 (EBID) of iron nonacarbonyl. The unobstructed, atomic level in situ observations of the catalyst particles, recorded degrees C in 8-15 mTorr of acetylene, reveal the structural transformations during reduction, sintering, carburization of Fe nanoparticles and subsequent CNT growth.

Inner shell edge profiles in electron energy loss spectroscopy

Abstract Inner shell edge profiles for K, L and M edges that are most likely to be used in microanalysis have been calculated using Hartree-Slater wave functions and are compared to experimental data. The aim is to identify those features that are not predicted by a one-electron atomic theory and to get some estimate of the accuracy of quantitative analysis using these calculations. In general, the fit between theory and experiment is quite good for those edges which do not have maxima delayed by more than 40 eV. In addition, solid state effects are averaged out if large (100 eV) integration windows are used. Accuracy can be improved in the transition metals and the rare earths by excluding the “white line” portion of the spectrum in any comparison.

Review of methods for calculating near edge structure

Electron energy loss inner shell near edge structures within about 30 eV of threshold can give information on charge redistribution, conduction band changes, coordination and structure changes on a local scale. A theoretical understanding is necessary to go beyond empirical rules based on fingerprinting. All approaches to near edge structure calculation in solids are derived from various band theory methods with varying degrees of approximation. There is a place for theories based on simple physical ideas as well as large sophisticated calculations. In many oxides estimates of the number of peaks and their positions can be based on an extension of single scattering EXAFS theory to include strong second order intrashell scattering. These effects will be illustrated in MgO and NiO. In other cases it is necessary to use band structure calculations to give either projected densities of states or wave functions from which matrix elements can be calculated directly. Examples of this approach will show results of calculations for Si, diamond and SiC using the Cambridge plane wave pseudopotential code (CASTEP).

Electron energy-loss spectrometry on lithiated graphite

Transmission electron energy-loss spectrometry was used to investigate the electronic states of metallic Li and LiC6, which is the Li-intercalated graphite used in Li-ion batteries. The Li K edges of metallic Li and LiC6 were nearly identical, and the C K edges were only weakly affected by the presence of Li. These results suggest only a small charge transfer from Li to C in LiC6, contrary to prior results from surface spectra obtained by x-ray photoelectron spectroscopy. Effects of radiation damage and sample oxidation in the transmission electron microscopy are also reported.

Calculation of near edge structure

Near edge fine structure has the potential to solve problems related to localised electronic states and bonding. Theory and calculation provide the link between electronic or structural properties and features observed in an electron loss spectrum. A hierarchy of approximations for the calculation of near edge structure features is introduced and the importance of using a self-consistent charge density and potential is emphasised. The use of various electronic structure calculation methods and their application to near edge structure calculation is reviewed. Finally, core hole effects are discussed and examples presented for cubic BN showing that the core hole mainly enhances intensity near threshold.

Evaluation of the role of Au in improving catalytic activity of Ni nanoparticles for the formation of one-dimensional carbon nanostructures.

In situ dynamic imaging, using an environmental transmission electron microscope, was employed to evaluate the catalytic activity of Au/SiO(2), Ni/SiO(2), and Au-Ni/SiO(2) nanoparticles for the formation of one-dimensional (1-D) carbon nanostructures such as carbon nanofibers (CNFs) and nanotubes (CNTs). While pure-Au thin-film samples were inactive for carbon deposition at 520 °C in 0.4 Pa of C(2)H(2), multiwalled CNTs formed from Ni thin films samples under these conditions. The number of nanoparticles active for CNF and CNT formation increased for thin films containing 0.1 mol fraction and 0.2 mol fraction of Au but decreased as the overall Au content in thin films was increased above 0.5 mol fraction. Multiwalled CNTs formed with a root growth mechanism for pure Ni samples, while with the addition of 0.1 mol fraction or 0.2 mol fraction of Au, CNFs were formed via a tip growth mechanism at 520 °C. Single-walled CNTs formed at temperatures above 600 °C in samples doped with less than 0.2 mol fraction of Au. Ex situ analysis via high-resolution scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) revealed that catalytically active particles exhibit a heterogeneous distribution of Au and Ni, where only a small fraction of the overall Au content was found in the portion of each particle actively involved in the nucleation of graphitic layers. Instead, the majority of the Au was found to be segregated to an inactive capping structure at one the end of the particles. Using density-functional theory calculations, we show that the activation energy for bulk diffusion of carbon in Ni reduces from ≈1.62 eV for pure Ni to 0.07 eV with the addition of small amounts (≈0.06 mol fraction) of Au. This suggests that the enhancement of C diffusion through the bulk of the particles may be responsible for improving the number of particles active for nucleating the 1-D carbon nanostructures and thereby the yield.

Chemistry and bonding changes associated with the segregation of Bi to grain boundaries in Cu

Grain-boundary embrittlement, caused by the segregation of impurity and alloying elements, occurs in many systems and has been the focus of a large amount of research owing to its technological importance. However, the exact mechanism by which the segregating elements cause embrittlement remains unclear. In this paper the localized changes in the electronic structure in the classical embrittling system of Bi in Cu have been studied. Experimental results were obtained by examining the fine structure in the electron energy loss spectrum which was then compared to calculations using the layer Korringa-Kohn-Rostoker (LKKR) method. A change in the d density of states has been observed for the Cu atoms at the grain boundary, associated with Bi, and an electronic model to explain embrittlement is described.

Nanometer-scale measurements of Fe3+/ΣFe by electron energy-loss spectroscopy: A cautionary note

Abstract The effects of electron-beam damage on the Fe3+/ΣFe (total iron) ratio were measured by electron energy-loss spectroscopy (EELS) with a transmission electron microscope (TEM). Spectra were acquired from crushed and ion-beam-thinned cronstedtite. For fluences below 1 × 104 e/Å2, the Fe3+/ΣFe values from crushed grains range between 0.43 and 0.49, consistent with undamaged material. These measurements were acquired from flakes 180 to 1000 Å thick. With increase influence, samples <400 Å thick become damaged and exhibit Fe3+/ΣFe values >0.5. The critical fluence for radiation damage by 100 kV electrons as defined by Fe3+/ΣFe <0.5 for cronstedtite at 300 K, is 1 × 104 e/Å2. The absorbed dose to the speciman during acquisition of a typical EELS spectrum is large, with values around 2.2 × 1010 Gy (J/kg), equivalent to the deposition of 620 eV/Å3. Cooling to liquid N2 temperature did not significantly slow the damage process. Ion-beam thinning produces an amorphous layer on crystal surfaces. Spectra from the thinnest regions, which are amorphous, exhibit Fe3+/ΣFe >0.7. With increase in sample thickness, the Fe3+/ΣFe values decrease to a minimum, consistent with data from the undamaged material. The increase of Fe3+/ΣFe with respect to electron-beam irradiation is likely caused by loss of H. At low fluences, the loss of H is negligible, thus allowing consistent Fe3+/ΣFe values to be measured. The cronstedtite study illustrates the care required when using EELS to measure Fe3+/ΣFe values. Similar damage effects occur for a range of high-valence and mixed-oxidation state metals in minerals. EELS is the only spectroscopic method that can be used routinely to determine mixed-valence ratios at the nanometer scale, but care is required when measuring these data. Consideration needs to be given to the incident beam current, fluence, fluence rate, and sample thickness.

Bonding in alpha-quartz (SiO2): A view of the unoccupied states

Abstract High-resolution core-loss and low-loss spectra of α-quartz were acquired by electron energyloss spectroscopy (EELS) with a transmission electron microscope (TEM). Spectra contain the Si L1, L2,3, K, and O K core-loss edges, and the surface and bulk low-loss spectra. The core-loss edges represent the atom-projected partial densities of states of the excited atoms and provide information on the unoccupied s, p, and d states as a function of energy above the edge onset. The band structure and total density of states were calculated for α-quartz using a self-consistent pseudopotential method. Projected local densities of Si and O s, p, and d states (LDOS) were calculated and compared with the EELS core-loss edges. These LDOS successfully reproduce the dominant Si and O core-loss edge shapes up to ca. 15 eV above the conduction-band onset. In addition, the calculations provide evidence for considerable charge transfer from Si to O and suggest a marked ionicity of the Si-O bond. The experimental and calculated data indicate that O 2p-Si d π-type bonding is minimal. The low-loss spectra exhibit four peaks that are assigned to transitions from maxima in the valence-band density of states to the conduction band. A band gap of 9.65 eV is measured from the low-loss spectrum. The structures of the surface low-loss spectrum are reproduced by the joint density of states derived from the band-structure calculation. This study provides a detailed description of the unoccupied DOS of α-quartz by comparing the core-loss edges and low-loss spectrum, on a relative energy scale and relating the spectral features to the atom- and angular-momentum-resolved components of a pseudopotential band-structure calculation.