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This group is part of the Condensed Matter and Materials Physics group in the department of Physics and Astronomy at University College London.
Our research focusses on the development and application of theoretical methodologies for calculations on defects in solids and at surfaces, with particular emphasis on embedded cluster techniques. A significant part of these efforts were focused on atomistic theory of the geometric and electronic structure of point defects in insulators and semiconductors, and on the understanding of the mechanisms of electronic and ionic processes in ionic crystals.
Theories have been developed for the mechanisms of photo-induced processes at ionic surfaces and models of self-trapped excitons and hole polarons in solids and at surfaces, especially in relation to ultrafast processes induced by electronic excitation. During the last several years we have been strongly involved in the development of theoretical models of atomic force microscopy on ionic surfaces.
Recent Highlights
Two-Dimensional Polaronic Behavior in the Binary Oxides m-HfO2 and m-ZrO2
Keith P. McKenna, Matthew J. Wolf, Alexander L. Shluger, Stephan Lany, Alex Zunger
Physical Review Letters, March 2012
In the recent work published in Physical Review Letters, Alex Shluger, Keith McKenna, Matthew Wolf and colleagues have simulated the behaviour of polarons formed by holes in m-HfO2 and m-ZrO2 oxides. These materials contains two types of oxygen anions, three and four coordinated, that are separated from each other and respectively arranged in 2D layers within the crystal lattice.
Using improved versions of density functional theory they showed that holes self-trap only at three-coordinated oxygen anions and that the material exhibits 2D polaronic mobility. Such effects were previously thought only to occur in more complex oxide materials, such as high-temperature superconducting oxides, and at surfaces or interfaces. Investigation of the correlated dynamics and interaction of holes confined in these types of material may reveal interesting effects such as superconductivity, hole crystallisation, or magnetism which may deepen our understanding of these interesting and important phenomena.
Atom-resolved imaging of ordered defect superstructures at individual grain boundaries
Zhongchang Wang, Mitsuhiro Saito, Keith P. McKenna, Lin Gu, Susumu Tsukimoto, Alexander L. Shluger & Yuichi Ikuhara
Nature, November 2011
Most solid materials, both those formed naturally and those fabricated for technological applications, are polycrystalline. In other words, they consist of a complex arrangement of grains within which atoms form a highly ordered structure. Grain boundaries are the extended defects formed at the interfaces between these grains, and they play a crucial role in determining the mechanical and electrical properties of materials. For this reason, there has been a great deal of scientific research directed towards understanding their atomic structure.
The research team constructed a single grain boundary in the ceramic material magnesium oxide by precisely orienting and bonding two crystals together. The resulting bi-crystal was then characterized using a range of advanced electron microscopy techniques complemented by theoretical simulations. The use of high energy electrons to probe the structure of the materials allows for spatial resolution, down to the scale of atoms (of the order ten billionths of a centimeter). Combined with theoretical modeling, these techniques revealed the chemical identities of all atoms inside the boundary, which form a complex and ordered defect superstructure involving calcium and titanium impurities and atomic vacancy defects (see Figure). These results offer new insights into the complex interactions between defects and grain boundaries in ceramics and demonstrate that atomic-scale analysis of complex multicomponent structures in materials is now becoming possible.
Chemical Resolution at Ionic Crystal Surfaces Using Dynamic Atomic Force Microscopy with Metallic Tips
G. Teobaldi, K. Lämmle, T. Trevethan, M. Watkins, A. Schwarz, R. Wiesendanger, and A. L. Shluger
Physical Review Letters, May 2011
The ability to characterize insulating surfaces and control surface processes down to the atomic scale is extremely important for numerous applications in chemistry, catalysis, nanoscience, and nanotechnology and can be achieved only using AFM. The exact chemical nature and structure of the AFM tip apex as well as the identity of the foremost tip atom are ultimately responsible for the formation of atomic-scale contrast, but are notoriously difficult to control. We demonstrate that well prepared and characterized Cr tips can provide atomic resolution on the bulk NaCl(001) surface with dynamic atomic force microscopy in the noncontact regime at relatively large tip-sample separations. At these conditions, the surface chemical structure can be resolved yet tip-surface instabilities are absent. Our calculations demonstrate that chemical identification is unambiguous, because the interaction is always largest above the anions. This conclusion is generally valid for other polar surfaces, and can thus provide a new practical route for straightforward interpretation of atomically resolved images.
Mechanism of Contrast Formation in Atomic Force Microscopy in Water
M. Watkins, A. L. Shluger
Physical Review Letters, November 2010
In solution, the direct electrostatic interaction between tip and surface is screened by the high dielectric constant of water. A water molecule has a diameter of approximately 0.3 nm, and one or two water molecules would be expected to be present between tip and surface at the distances typical to imaging in ultrahigh vacuum. Thus, in solution, the direct interaction of the tip with the surface that gives rise to the image contrast in vacuum may be of lesser importance and the disruption, or otherwise, of the water structure above the surface, and around the tip, may play a dominant role in image contrast. An understanding of the relevant energy scales for these processes and their net effect in terms of an effective tip-surface force is one of the key aims of this study.
We use computer modelling to investigate the mechanism of atomic-scale corrugation in frequency modulation atomic force microscopy imaging of inorganic surfaces in solution. Molecular dynamics simulations demonstrate that the forces acting on a model microscope tip result from the direct interaction between a tip and a surface, and forces entirely due to the water structure around both tip and surface. The observed force is a balance between largely repulsive potential energy changes as the tip approaches and the entropic gain when water is sterically prevented from occupying sites near the tip and surface. Only extremely sharp tips are likely to measure direct tip-surface interactions. An investigation into the dynamics of water confined between tip and surface shows that water diffusion can be slowed by at least two orders of magnitude compared to its rate in bulk solution.
Functionalized Truxenes: Adsorption and Diffusion of Single Molecules on the KBr(001) Surface
Bartosz Such, Thomas Trevethan, Thilo Glatzel, Shigeki Kawai, Lars Zimmerli, Ernst Meyer, Alexander L. Shluger, Catelijne H. M. Amijs, Paula de Mendoza, and Antonio M. Echavarren
ACS Nano, May 2010
The adsorption and diffusion of organic molecules on surfaces are integral to many areas of surface science and nanotechnology, such as self-assembly and growth of new surface structures, catalysis, coatings, corrosion inhibition, tribology, and molecular electronics. The advent of scanning probes sparked an explosion of interest in studying the behavior and reactions of individual molecules at surfaces. In this work we have studied the adsorption and diffusion of large functionalized organic molecules on an insulating ionic surface at room temperature using a noncontact atomic force microscope (NCAFM) and theoretical modeling. Custom designed syn-5,10,15-tris(4-cyanophenylmethyl)truxene molecules are adsorbed onto the nanoscale structured KBr(001) surface at low coverages and imaged with atomic and molecular resolution with the NC-AFM. The molecules are observed rapidly diffusing along the perfect monolayer step edges and immobilized at monolayer kink sites. Extensive atomistic simulations elucidate the mechanisms of adsorption and diffusion of the molecule on the different surface features. The results of this study suggest methods of controlling the diffusion of adsorbates on insulating and nanostructured surfaces.
Electron Trapping Polycrystalline Materials with Negative Electron Affinity
Keith McKenna and Alex Shluger
Nature Materials, November 2008
Insulating materials, such as MgO, are widely used as substrates for thin films and metallic clusters and employed as insulating barriers in spintronic devices. In most cases they are polycrystalline yet the ability of boundaries between crystallites to trap electrons is not well understood. In this letter we investigate the electron trapping properties of grain boundaries in MgO and alkali halides by first principles calculations. Our results show that conduction band electrons, which may be introduced by an applied electrical voltage or irradiation, can be trapped at grain boundaries in MgO, NaCl and LiF. We find that the nature of the electron trapping in these negative electron affinity (NEA) materials is unusual in that the electron is confined in the empty space inside the dislocation cores rather than associated with interfacial ions (see figure). We demonstrate that this surprising effect can be explained using a simple model which should be applicable to other NEA materials, such as boron nitride and alumina. These grain boundaries represent novel examples of systems that are capable of confining electrons and may be probed experimentally by transport measurements or using scanning probes. These effects can also lead to electrons escaping from thin films grown on MgO substrates and reduced tunnel barrier heights in magnetic tunnel junctions affecting the performance of electronic devices.
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