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.
Calculating the Entropy Loss on Adsorption of Organic Molecules at Insulating Surfaces
Julian Gaberle, David Z. Gao, Matthew B. Watkins, and Alexander Shluger
Journal of Computational Chemistry, January 2016
Understanding the adsorption of organic molecules at insulating surfaces at room and elevated temperatures is important for many fields from catalysis to molecular electronics. However, at such temperatures the entropic contributions to adsorption become significant and have to be considered.
We compared the adsorption of two different functionalized organic molecules on the KCl (001) surface using density functional theory (DFT) and molecular dynamics (MD) simulations. The accuracy of the van der Waals corrected DFT-D3 was benchmarked using Möller-Plesset perturbation theory calculations and classical force fields were then parameterized for both the TCB and CDB molecules on the KCl (001) surface. These force fields were used to perform potential of mean force (PMF) calculations on adsorption of individual molecules and extract information on the entropic contributions to adsorption energy.
Our comparative study revealed two main results. Firstly, using DFT-D3 overestimates the vdW contribution to the adsorption enthalpy of functionalized organic molecules by only about 20% and is therefore a computationally efficient way of studying their adsorption. Secondly the results reveal that losses in entropy significantly lower the adsorption energy at elevated temperatures and even at relatively low temperatures (e.g. 400K) can match the enthalpic contribution to adsorption free energy, thus leading to desorption.
Efficient Parametrization of Complex Molecule-Surface Force Fields
David Z. Gao, Filippo Federici Canova, Matthew B. Watkins, and Alexander Shluger
Journal of Computational Chemistry, April 2015
An understanding of the dynamic properties and growth of functional molecules on surfaces can be achieved through the use of classical force fields and large scale molecular dynamics (MD) simulations. However, a major challenge that arises is the lack of complete classical interaction models for these kind of systems; the parametrization of such classical models requires a significant investment of resources. We present an efficient scheme for parametrizing complex molecule–surface force fields from ab initio data.
The cost of producing a sufficient fitting library is mitigated using a 2D periodic embedded slab model made possible by the quantum mechanics/molecular mechanics scheme in CP2K. These results were then used in conjunction with genetic algorithm (GA) methods to optimize the large parameter sets needed to describe such systems. The derived potentials are able to well reproduce adsorption geometries and adsorption energies calculated using density functional theory. Finally, we discuss the challenges in creating a sufficient fitting library, determining whether or not the GA optimization has completed, and the transferability of such force fields to similar molecules.
Hydrogen Induced Rupture of Strained Si-O Bonds in Amorphous Silicon Dioxide
Al-Moatasem El-Sayed, Matthew Benjamin Watkins, Tibor Grasser, Valery Afanas'ev, and Alexander Shluger
Physical Review Letters, March 2015
The interaction of hydrogen with amorphous silicon dioxide (a-SiO2) is important for many applications and has been the subject of a number of experimental and theoretical studies. However, the involvement of atomic hydrogen in silica network degradation mechanisms is still poorly understood.
In a collaboration with Tibor Grasser from TU Wien and Valeri Afanas'ev from KU Leuven, we demonstrate that H atoms can break strained Si-O bonds in continuous a-SiO2 networks. This results in a new defect consisting of a 3-coordinated Si atom with an unpaired electron facing a hydroxyl group, shown in the figure. These results clearly demonstrate that the presence of strained Si-O bonds in a-SiO2 gives rise to an additional channel of interaction of H atoms with a-SiO2 networks, predicting the formation of what we call a hydroxyl E' centre. The energy barriers to form this defect from interstitial H atoms range between 0.5 and 1.3 eV.
With the current trend in technology to lower fabrication processing temperatures, extreme bonding geometries in the oxide are expected to become more abundant and increase the influence of strain, ranging from ultra-thin oxides sandwiched between electrodes to porous low-k insulators intrinsically strained by re-bonding reactions. Hence this discovery of unexpected reactivity of atomic hydrogen may have significant implications for the future of silica based device processing.
Using Metallic Noncontact Atomic Force Microscope Tips for Imaging Insulators and Polar Molecules: Tip Characterization and Imaging Mechanisms
David Zhe Gao, Josef Grenz, Matthew Benjamin Watkins, Filippo Federici Canova, Alexander Schwarz, Roland Wiesendanger, and Alexander Shluger
ACS Nano, May 2014
In collaboration with Alexander Schwarz and Josef Grenz at the University of Hamburg we demonstrated that using metallic tips for noncontact atomic force microscopy (NC-AFM) imaging at relatively large (>0.5 nm) tipâ€“surface separations provides a reliable method for studying molecules on insulating surfaces with chemical resolution. Furthermore, this greatly reduces the complexity of interpreting experimental data. The experimental NC-AFM imaging and theoretical simulations were carried out for the NiO(001) surface as well as adsorbed CO and Co-Salen molecules using Cr-coated Si tips.
Our experimental results and density functional theory calculations confirmed that metallic tips possess a permanent electric dipole moment with its positive end oriented toward the sample. By analyzing the experimental data, we could directly determine the dipole moment of the Cr-coated tip. A model representing the metallic tip as a point dipole was described and shown to produce NC-AFM images of individual CO molecules adsorbed onto NiO(001) in good quantitative agreement with experimental results. Finally, we discussed methods for characterizing the structure of metal-coated tips and the application of these tips to imaging dipoles of large adsorbed molecules.
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.