Accurate measurement of absolute experimental inelastic mean free paths and EELS differential cross-sections

Role of Convergence and Collection Angles in the Excitation of Long- and Short-Wavelength Phonons with Vibrational Electron Energy-Loss Spectroscopy

Current generation electron monochromators employed as attachments to scanning transmission electron microscopes (STEM) offer the ability to obtain vibrational information from materials using electron energy-loss spectroscopy (EELS). We show here that in crystals, long- and short-wavelength phonon modes can be probed simultaneously with on-axis vibrational STEM EELS. The long-wavelength phonons are probed via dipole scattering, while the short-wavelength modes are probed via impact scattering of the incident electrons. The localized character of the short-wavelength modes is demonstrated by scanning the electron beam across the edge of a hexagonal boron nitride nanoparticle. It is found that employing convergence angles that encompass multiple Brillouin zone boundaries enhances the short-wavelength phonon contribution to the vibrational energy-loss spectrum much more than that achieved by employing collection angles that encompass multiple Brillouin zone boundaries. Probing short-wavelength phonons at high spatial resolution with on-axis vibrational STEM EELS will help develop a fundamental connection between vibrational excitations and bonding arrangements at atomic-scale heterogeneities in materials.

Strain effects in core-shell PtCo nanoparticles: a comparison of experimental observations and computational modelling

Strain in Pt nanoalloys induced by the secondary metal has long been suggested as a major contributor to the modification of catalytic properties. Here, we investigate strain in PtCo nanoparticles using a combination of computational modelling and microscopy experiments. We have used a combination of molecular dynamics (MD) and large-scale density functional theory (DFT) for our models, alongside experimental work using annular dark field scanning transmission electron microscopy (ADF-STEM). We have performed extensive validation of the interatomic potential against DFT using a Pt₅₆₈Co₁₈ nanoparticle. Modelling gives access to 3 dimensional structures that can be compared to the 2D ADF-STEM images, which we use to build an understanding of nanoparticle structure and composition. Strain has been measured for PtCo and pure Pt nanoparticles, with MD annealed models compared to ADF-STEM images. Our analysis was performed on a layer by layer basis, where distinct trends between the Pt and PtCo alloy nanoparticles are observed. To our knowledge, we show for the first time a way in which detailed atomistic simulations can be used to augment and help interpret the results of ADF-STEM strain mapping experiments, which will enhance their use in characterisation towards the development of improved catalysts.

Electron inelastic mean free path in water

Liquid phase transmission electron microscopy (LPTEM) is rapidly developing as a powerful tool for probing processes in liquid environments with close to atomic resolution. Knowledge of the water thickness is needed for reliable interpretation and modelling of analytical studies in LPTEM, and is particularly essential when using thin liquid layers, required for achieving the highest spatial resolutions. The log-ratio method in electron energy-loss spectroscopy (EELS) is often applied in TEM to quantify the sample thickness, which is measured relative to the inelastic mean free path (λIMFP). However, λIMFP itself is dependent on sample material, the electron energy, and the convergence and divergence angles of the microscope electronoptics. Here, we present a detailed quantitative analysis of the λIMFP of water as functions of the EELS collection angle (β) at 120 keV and 300 keV in a novel nanochannel liquid cell. We observe good agreement with earlier studies conducted on ice, but find that the most widely used theoretical models significantly underestimate λIMFP of water. We determine an adjusted average energy-loss term Em, water, and characteristic scattering angle θE, water that improve the accuracy. The results provide a comprehensive knowledge of the λIMFP of water (or ice) for reliable interpretation and quantification of observations in LPTEM and cryo-TEM studies.

Inelastic mean free path measurement by STEM-EELS technique using needle-shaped specimen

Inelastic mean free path (IMFP) was determined by electron energy loss spectroscopy (EELS) to estimate the accurate thickness of TEM thin foil using needle-shaped specimen. From EELS measurements performed for 99.99% Al, Si wafer and 99.99% Fe, linear relationships were confirmed between the thickness of the TEM thin foil and the ratio of the total intensity of EELS spectrum to the total intensity of zero-loss spectrum for all samples. By weighted least-square fitting, the IMFP was estimated to be 143-150 nm for Al, 159-165 nm for Si with amorphous layer, and 92-94 nm for Fe with the collection semi-angle β = 11.9 - 35.7 mrad, and accelerated voltage of 200 kV. Thus, the dependence of IMFP on β is not dominant. The accuracy depends on the roundness of the cross-section of the needle-shaped specimen, and is observed to be low in terms of percentage in this work.

EELS at very high energy losses

Electron energy-loss spectroscopy (EELS) has been investigated in the range from 2 to >10 keV using an optimized optical coupling of the microscope to the spectrometer to improve the high loss performance in EELS. It is found that excellent quality data can now be acquired up until about 5 keV, suitable for both energy loss near edge structure (ELNES) studies of oxidation and local chemistry, and potentially useful for extended energy loss fine structure (EXELFS) studies of local atomic ordering. Examples studied included oxidation in Zr, Mo and Sn, and the ELNES and EXELFS of the Ti-K edge. It is also shown that good quality electron energy-loss spectroscopy can even be performed for losses above 9.2 keV, the energy loss at which the collection angle becomes 'infinite', and this is demonstrated using the tungsten L3 edge at about 10.2 keV.

Spectrum imaging of complex nanostructures using DualEELS: II. Absolute quantification using standards

Nanometre-sized Ti_xV_(1-x)C_yN_z precipitates in an Fe20%Mn steel matrix with a thickness range from 14 to 40 nm are analysed using DualEELS. Their thicknesses, volumes and compositions are quantified using experimental binary standards and the process used to give robust results is described. Precisions of a few percent are achieved with accuracies that are estimated to be of a similar magnitude. Sensitivities are shown to be at 0.5-1 unit cells range in the thinnest matrix region, based on the assumption that a sub-lattice is fully populated by the element. It rises to the 1-2 unit cell range for the metals and 2-3 unit cells for the non-metal in the thickest matrix region. The sensitivities for Ti and N are greater than those for V and C respectively because the O K-edge from surface oxide needs to be separated from the V L_2,3-edge, and the C K-edges from C in the matrix and amorphous C on the surface have to be separated from the C in the precipitate itself. Separation of the contributions from the bulk and the surface is demonstrated, showing that there is significant and detectable C in the matrix but no O, while there is significant O but little C in the surface oxide. Whilst applied to precipitates in steel in this work, the approach can be adapted to many multi-phase systems.