Andrew Pratt

Enhanced oxidation of nanoparticles through strain-mediated ionic transport

Geometry and confinement effects at the nanoscale can result in substantial modifications to a materials 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 microscopy 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.

Ultrathin graphene-based membrane with precise molecular sieving and ultrafast solvent permeation

Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation. However, their use is limited to aqueous solutions because GO membranes appear impermeable to organic solvents, a phenomenon not yet fully understood. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from large (10-20 μm) flakes. Without modification of sieving characteristics, these membranes can be made exceptionally thin, down to ∼10 nm, which translates into fast water and organic solvent permeation. We attribute organic solvent permeation and sieving properties to randomly distributed pinholes interconnected by short graphene channels with a width of 1 nm. With increasing membrane thickness, organic solvent permeation rates decay exponentially but water continues to permeate quickly, in agreement with previous reports. The potential of ultrathin GO laminates for organic solvent nanofiltration is demonstrated by showing >99.9% rejection of small molecular weight organic dyes dissolved in methanol. Our work significantly expands possibilities for the use of GO membranes in purification and filtration technologies.

The development of an optically accessible, high-power combustion test rig.

This work summarizes the development of a gas turbine combustion experiment which will allow advanced optical measurements to be made at realistic engine conditions. Facility requirements are addressed, including instrumentation and control needs for remote operation when working with high energy flows. The methodology employed in the design of the optically accessible combustion chamber is elucidated, including window considerations and thermal management of the experimental hardware under extremely high heat loads. Experimental uncertainties are also quantified. The stable operation of the experiment is validated using multiple techniques and the boundary conditions are verified. The successful prediction of operating conditions by the design analysis is documented and preliminary data are shown to demonstrate the capability of the experiment to produce high-fidelity datasets for advanced combustion research.

Adsorbate-induced spin-polarization enhancement of Fe3O4(0?0?1)

Using a spin-polarized metastable helium beam, we have investigated the remanent spin polarization at the surface of clean, hydrogen-terminated and benzene-adsorbed Fe3O4(0?0?1) thin films prepared on MgO(0?0?1) substrates. For the clean surface, a small negative asymmetry is detected in the ejected electron yields for helium-electron spins aligned parallel and anti-parallel to the sample magnetization direction. This confirms earlier experimental results from less surface-sensitive techniques which show that the spin polarization at the surface of Fe3O4(0?0?1) is much reduced from the bulk value and, furthermore, implies that majority-spin states actually dominate at the Fermi level. However, when hydrogen-terminated, the asymmetry is considerably enhanced and positive highlighting the important role that surface adsorption and passivation will play in the development of spintronic materials. This is further demonstrated for the adsorption of the simplest ?-conjugated molecule, benzene, at the clean Fe3O4(0?0?1) surface where a similar enhancement in the spin polarization is observed, an effect that could prove beneficial to the design and fabrication of organic spintronic devices.

First-principles study of the structural and magnetic properties of graphene on a Fe/Ni(1?1?1) surface

The structure and spin-resolved electronic states of a graphene-adsorbed Fe/Ni(1 1 1) surface are investigated and compared with a graphene/Ni(1 1 1) surface using first-principles calculations. Nine possible geometries are studied with Fe and C atoms at different sites with respect to the topmost Ni atoms. Geometries with one C atom located on top of an Fe atom (C1) and one at a hollow (fcc or hcp) site (C2) are the most energetically favourable. The electronic states of graphene are significantly modified by the interaction with the Fe/Ni(1 1 1) surface. The dominant π states of the C2 atom are drastically shifted towards the Fermi level and become highly positive-spin-polarized due to the corresponding spin-down states located above the Fermi level. The level shift is very small for the spin-up π states of the C1 atom but obvious for the spin-down states due to spin splitting induced by Fe atoms, resulting in a negative spin polarization at shallow levels and a positive one at deeper levels. The adsorption of graphene on Fe/Ni(1 1 1) is stronger than that on the clean Ni(1 1 1) surface.

Magnetic moment enhancement and spin polarization switch of the manganese phthalocyanine molecule on an IrMn(100) surface

The geometric, electronic, and magnetic structures of a manganese phthalocyanine (MnPc) molecule on an antiferromagnetic IrMn(100) surface are studied by density functional theory calculations. Two kinds of orientation of the adsorbed MnPc molecule are predicted to coexist due to molecular self-assembly on the surface-a top-site geometry with the Mn-N bonds aligned along the ⟨100⟩ direction, and a hollow-site orientation in which the Mn-N bonds are parallel to the ⟨110⟩ direction. The MnPc molecule is antiferromagnetically coupled to the substrate at the top site with a slight reduction in the magnetic moment of the Mn atom of the MnPc molecule (Mnmol). In contrast, the magnetic moment of the Mnmol is enhanced to 4.28 μB at the hollow site, a value larger than that in the free MnPc molecule (3.51 μB). Molecular distortion induced by adsorption is revealed to be responsible for the enhancement of the magnetic moment. Furthermore, the spin polarization of the Mnmol atom at around the Fermi level is found to change from negative to positive through an elongation of the Mn-N bonds of the MnPc. We propose that a reversible switch of the low/high magnetic moment and negative/positive spin polarization might be realized through some mechanical engineering methods.

New multichannel electron energy analyzer with cylindrically symmetrical electrostatic field

This article discusses an electron energy analyzer with a cylindrically symmetrical electrostatic field, designed for rapid Auger analysis. The device was designed and built. The best parameters of the analyzer were estimated and then experimentally verified.

Chapter 7 – Environmental Applications of Magnetic Nanoparticles

Abstract The inherent properties of magnetic nanoparticles (MNPs) offer enormous benefits to many technologies which are focused on improving the quality of the environment in which we live. As reflected in this chapter, the majority of research to date incorporating MNPs for environmental applications has been in the treatment of water, whether in the remediation of groundwater or through the magnetic separation and/or sensing of contaminants present in various aqueous systems. MNPs couple the modified behaviour uniquely associated with the nanoscale, for example enhanced reactivities arising from enormous surface-to-volume ratios, with inherent magnetic phenomena such as superparamagnetism. As such, they offer precise selectivity and ultrahigh sensitivity in the detection, remediation and removal of micropollutants whilst functionalization further enables targeting of specific chemical and biological contaminants. This chapter aims to show how this powerful combination can greatly benefit a number of environmental applications including groundwater remediation, wastewater treatment and environmental sensing, amongst others. After an introduction and overview of the properties relevant to these environmental settings, the methods used to produce and functionalize MNPs are discussed. Various applications of MNPs are then considered, taking into account detrimental factors such as aggregation, deactivation and contaminant rerelease, before the chapter ends with a summary and discussion of future prospects.

Spin polarization study of graphene on the Ni(111) surface by density functional theory calculations with a semiempirical long-range dispersion correction

The geometric and spin-resolved electronic structure of a graphene-adsorbed Ni(111) surface has been investigated by density functional theory (DFT) calculations without and with a semiempirical long-range dispersion correction (DFT-D). DFT calculations with generalized gradient approximation (GGA) functional cannot predict well about the adsorption properties of graphene to the Ni(111) surface. While DFT-D calculations with the same GGA functional give reasonable values of the adsorption energy and layer distance from graphene to the substrate. The geometry of top_fcc is the most energetically favorable in all geometries. Strong hybridization of graphene with the ferromagnetic Ni substrate induces significant shift partially in graphene π states towards the Fermi level yielding spin polarization. The spin polarization is positive at the shallow levels of modified π states and slightly negative at the deeper levels of fundamental π states, which is indicated by the calculated spin density distributions an...

Energy-level alignment at the Alq3/Fe3O4(001) interface

We have used the technique of metastable de-excitation spectroscopy to probe the interfacial electronic structure of the organic semiconductor (OSC) Alq3 deposited onto clean Fe3O4(001) substrates. We have measured shifts in the low-energy secondary electron cutoff and energetic onset of the highest occupied molecular orbital (HOMO) of Alq3 as the coverage increases from the sub-ML range to multilayer formation. We find that the presence of an interfacial dipole induces a uniform decrease in the valence band electronic states by 1.2 eV with respect to the vacuum level and modifies the position of the HOMO energetic onset to 1.8 eV below the substrate Fermi level. The strong intrinsic dipole moment of Alq3 is suggested as the origin for these changes in accordance with previous studies of Alq3 deposited onto various substrates.