Our research is focussed on the development and application of theoretical methodologies for calculations of defects and defect related processes in solids and at interfaces and grain boundaries.
- We develop atomistic theories of the geometric and electronic structure of point defects and the mechanisms of electronic and ionic processes in insulators and semiconductors, in particular for microelectronics applications.
- We create models of trapped excitons and electron and hole polarons in solids and at surfaces and develop mechanisms of photo-induced processes at ionic surfaces and in metal nanofilms.
- We develop theoretical methods for modelling of Atomic Force Microscopy imaging of surfaces and the mechanisms of contrast formation in AFM images in vacuum and in liquids.
- We model adsorption and diffusion of organic molecules at insulating surfaces and mechanisms of formation of molecular super-structures and self-assembled monolayers.
Determination of transient atomic structure of laser-excited materials from time-resolved diffraction data
Yvelin Giret, Nobuyasu Naruse, Szymon L. Daraszewicz, Yoshie Murooka, Jinfeng Yang, Dorothy M. Duffy, Alexander L. Shluger, and Katsumi Tanimura
Applied Physics Letters, December 2013
In collaboration with the experimental group of Prof Katsumi Tanimura at the University of Osaka we have studied the time evolution of the Bragg peaks of photo-excited gold nanofilms using transmission ultrafast electron diffraction (UED) with 3.0MeV electron pulses. The corresponding structure evolution is calculated using two-temperature molecular dynamics (2TMD). The good agreement obtained between the measured and calculated Bragg peaks, over the full experimental timescale, enables the lattice temperature effects and the structural changes to be disentangled for the first time. This agreement demonstrates that 2T-MD is a reliable method for solving the inverse problem of structure determination of laser irradiated metals in UED measurements. The results reveal the transition between slow heterogeneous melting of Au films at low absorbed photon fluence to rapid homogeneous melting at higher fluence and nonthermally driven melting at very high fluence.
The figure shows the structure (centrosymmetry parameter) (top), density (middle), and lattice temperature (bottom) evolutions of the irradiated gold nanofilm for the absorbed laser fluence of = 4.5 mJ cm^2. The sample is superheated at 6 ps and the melting has already started from the surfaces. However, the middle of the sample remains largely crystalline until it locally approaches the crystal stability temperature (after 12 ps) when the homogenously distributed molten sites rapidly grow and coalesce completing the melting process at 20 ps. The time frames are chosen in order to include the beginning and the end of the melting process.
Determining Adsorption Geometry, Bonding, and Translational Pathways of a Metal-Organic Complex on an Oxide Surface: Co-Salen on NiO(001)
Alexander Schwarz, David Z. Gao, Knud Lammle, Josef Grenz, Matthew B. Watkins, Alexander L. Shluger, and Roland Wiesendanger
Journal of Physical Chemistry C, January 2013
In collaboration with the experimental group of Alexander Schwarz at the University of Hamburg we studied the adsorption of Co-Salen, a small chiral paramagnetic metal?organic complex, on the NiO(001) surface. Individual molecules were imaged with noncontact atomic force microscopy (NCAFM) using metallic Cr coated tips. Experimentally, both the molecule and the individual surface ions were resolved simultaneously. Images recorded at low temperatures show that the Co-Salen molecules are aligned slightly away from the <110> directions of the surface and that the Co center of the molecule is located above a bright spot in atomically resolved images of the surface.
Our density functional theory calculations predict that the molecule adsorbs with the central Co atom on top of an oxygen ion and is in its lowest energy configuration aligned either + or - 4 degrees away from the <110> directions, dependent on the chirality of the molecule. Combining theoretical predictions and experimental data allowed us to identify bright spots in NC-AFM images as oxygen sites on NiO(001) and hence determine the exact adsorption geometry and position of the molecule. Additionally, we observed tip-induced translations of the Co-Salen molecules along <110> directions on the substrate, which corresponds to the lowest energy pathway for diffusion.
Effects of atomic scale roughness at metal/insulator interfaces on metal work function
Sanliang Ling, Matthew B. Watkins and Alexander L. Shluger
Physical Chemistry Chemical Physics, September 2013
We evaluated the performance of different van der Waals (vdW) corrected density functional theory (DFT) methods in predicting the structure of perfect interfaces between the LiF(001), MgO(001), NiO(001) films on the Ag(001) surface and the resulting work function shift of Ag(001). The results demonstrate that including the van der Waals interaction is important for obtaining accurate interface structures and the metal work function shift. The work function shift results from a subtle interplay of several effects strongly affected by even small changes in the interface geometry. Most of the existing van der Waals corrected functionals are not particularly suited for studying metal/insulator interfaces. This makes the accuracy of theoretical methods insufficient for predicting the shift values better than within 0.2 eV.
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. Lammle, 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.