Surface spin polarization of Fe nanoclusters

The spin polarization at the surface of Fe nanoclusters has been probed using a spin-polarized metastable helium beam. The clusters, produced in a gas aggregation source, display a lognormal size distribution with a peak centered at similar to 11 nm. Varying coverages of both spheroid-and cuboid-shaped particles were concomitantly deposited onto clean Si(111) substrates for investigation with the extremely surface sensitive technique of metastable de-excitation spectroscopy (MDS). A nominal cluster coverage of 8 angstrom yielded a maximum asymmetry of similar to 10% in the ejected electron yield for He spins aligned parallel and anti-parallel to the magnetization direction of the clusters. When compared to values obtained from epitaxial Fe films on various substrates, the measured asymmetry suggests an enhancement in the surface spin polarization, as theoretically proposed. The atomic structure of the clusters and their topography on the Si(111) substrates were studied with transmission and scanning electron microscopy

Enhanced oxidation of nanoparticles through strain-mediated ionic transport

Geometry and confinement effects at the nanoscale can result in substantial modifications to a material’s properties with significant consequences in terms of chemical reactivity, biocompatibility and toxicity. Although benefiting applications across a diverse array of environmental and technological settings, the long-term effects of these changes, for example in the reaction of metallic nanoparticles under atmospheric conditions, are not well understood. Here, we use the unprecedented resolution attainable with aberration-corrected scanning transmission electron microscopy3 to study the oxidation of cuboid Fe nanoparticles. Performing strain analysis at the atomic level, we reveal that strain gradients induced in the confined oxide shell by the nanoparticle geometry enhance the transport of diffusing species, ultimately driving oxide domain formation and the shape evolution of the particle. We conjecture that such a strain-gradient-enhanced mass transport mechanism may prove essential for understanding the reaction of nanoparticles with gases in general, and for providing deeper insight into ionic conductivity in strained nanostructures

Metastable De-excitation Spectroscopy and Density Functional Theory Study of the Selective Oxidation of Crotyl Alcohol over Pd(111)

The extremely surface sensitive technique of metastable de-excitation spectroscopy (MDS) has been utilized to probe the bonding and reactivity of crotyl alcohol over Pd(111) and provide insight into the selective oxidation pathway to crotonaldehyde. Auger de-excitation (AD) of metastable He (23S) atoms reveals distinct features associated with the molecular orbitals of the adsorbed alcohol, corresponding to emission from the hydrocarbon skeleton, the O n nonbonding, and C═C π states. The O n and C═C π states of the alcohol are reversed when compared to those of the aldehyde. Density functional theory (DFT) calculations of the alcohol show that an adsorption mode with both C═C and O bonds aligned somewhat parallel to the surface is energetically favored at a substrate temperature below 200 K. Density of states calculations for such configurations are in excellent agreement with experimental MDS measurements. MDS revealed oxidative dehydrogenation of crotyl alcohol to crotonaldehyde between 200 and 250 K, resulting in small peak shifts to higher binding energy. Intramolecular changes lead to the opposite assignment of the first two MOs in the alcohol versus the aldehyde, in accordance with DFT and UPS studies of the free molecules. Subsequent crotonaldehyde decarbonylation and associated propylidyne formation above 260 K could also be identified by MDS and complementary theoretical calculations as the origin of deactivation and selectivity loss. Combining MDS and DFT in this way represents a novel approach to elucidating surface catalyzed reaction pathways associated with a “real-world” practical chemical transformation, namely the selective oxidation of alcohols to aldehydes

Scanning Field Emission Microscopy with Spin and Energy Analysis

Scanning Field Emission Microscopy with Polarization Analysis was recently introduced to detect the spin polarization of electrons excited in the field emission regime of Scanning Tunnelling Microscopy. In this work, a miniature electron energy analyzer, called Bessel Box, is implemented into the Scanning Field Emission Microscope with Polarization Analysis setup. It is used to filter electrons according to their energy before they reach the spin detector. The Bessel Box allows, e.g., the spin polarization of elastically scattered electrons to be compared with the spin polarization obtained with the full energy spectrum. We use this technology to measure the local in-plane polarization signal as a function of the magnetic field B at room temperature for 10 monolayers Fe deposited on top of a W(011)-single crystal surface through a half mask (half of the surface is covered with Fe, the other half is uncovered). The spin polarization at the Fe-W crossing drops sharply from 9 % above Fe to 0 % above W(011) only if the elastically scattered electrons are selected for spin analysis. The mechanism of signal generation in Scanning Field Emission Microscope with Polarization Analysis including the formation of cascade of inelastically scattered electrons is discussed as an explanation for the different spin polarization profiles observed with and without Bessel Box (energy filtered)

Improving the Detection and the Analysis of Energy Filtered and Spin Polarised Electrons with the implementation of a Miniature Energy Analyser

With the aim of improving detection and analysis of energy filtered electrons in the Scanning Field-Emission Microscope (SFEM) and of the spin polarised electrons in the SFEM with Polarisation Analysis (SFEMPA) tests are performed on a miniature electron detection unit employing a Bessel Box energy analyser. Even in conventional electron microscopes, the detection of low-energy electrons (with kinetic energies of the order of 100eV or lower) is inherently difficult due to the presence of electrostatic (and magnetic) fields in proximity of the beam-target interaction region, inhibiting the escape of these electrons and complicating the interpretation of their detected signal. The reduced dimensions of such a compact energy analyser - with a length of 1&1/2 channeltrons - consent its employment close to the sample surface, thus minimising the aforementioned fields effects. Experimental results demonstrating the capability of this analyser to collect electron spectra are discussed. © 2020 IEEE

Electron energy analysis in Scanning Field Emission Microscopy using a Bessel box energy analyzer

In this study, we use Scanning Field Emission Microscopy (SFEM) combined with a miniature electron energy analyzer known as a Bessel box to measure electron energy spectra emitted from a sample. Previous studies using SFEM have revealed that the work function (ϕ) of the material under study has a significant role to play in the formation of the signal intensity. Hence, in order to understand the role of ϕ in greater detail, a sample of W(110) (ϕ = 5.25 eV) and a sample of Cs deposited on W(110) (ϕ ≈ 1.7 eV) were investigated. STM images show that the Cs covered surface has a speckled appearance indicating small Cs islands. The electron energy loss spectra obtained (which are the first using the Bessel box in SFEM) show differing structure in the elastic peak region. Monte Carlo (MC) simulations including quantum mechanical "bouncing" have been carried out. The results are consistent with MC simulations of the electrons escaping from the tip-sample junction

Metastable De-excitation Spectroscopy and Density Functional Theory Study of the Selective Oxidation of Crotyl Alcohol over Pd(111)

The extremely surface sensitive technique of metastable de-excitation spectroscopy (MDS) has been utilized to probe the bonding and reactivity of crotyl alcohol over Pd(111) and provide insight into the selective oxidation pathway to crotonaldehyde. Auger de-excitation (AD) of metastable He (23S) atoms reveals distinct features associated with the molecular orbitals of the adsorbed alcohol, corresponding to emission from the hydrocarbon skeleton, the O n nonbonding, and C═C π states. The O n and C═C π states of the alcohol are reversed when compared to those of the aldehyde. Density functional theory (DFT) calculations of the alcohol show that an adsorption mode with both C═C and O bonds aligned somewhat parallel to the surface is energetically favored at a substrate temperature below 200 K. Density of states calculations for such configurations are in excellent agreement with experimental MDS measurements. MDS revealed oxidative dehydrogenation of crotyl alcohol to crotonaldehyde between 200 and 250 K, resulting in small peak shifts to higher binding energy. Intramolecular changes lead to the opposite assignment of the first two MOs in the alcohol versus the aldehyde, in accordance with DFT and UPS studies of the free molecules. Subsequent crotonaldehyde decarbonylation and associated propylidyne formation above 260 K could also be identified by MDS and complementary theoretical calculations as the origin of deactivation and selectivity loss. Combining MDS and DFT in this way represents a novel approach to elucidating surface catalyzed reaction pathways associated with a “real-world” practical chemical transformation, namely the selective oxidation of alcohols to aldehydes