Department of Physics and Astronomy
Condensed Matter and Materials Physics
Alex Shluger's group
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JPC Cover.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.

Recent Highlights

Morphology and Growth Mechanicsms of Self-Assembled Films on Insulating Substrates: Role of Molecular Flexibility and Entropy
Julian Gaberle, David Z. Gao, Alexander L. Shluger et al.
Physical Chemistry C February 2017

We studied the effect of molecular flexibility on the morphology and growth mechanisms of self-assembled films on an insulating substrate using a combination of experimental and theoretical methods. 1,3,5-Tri-(4-cyano-4,4 biphenyl)-benzene (TCB) and 1,4-bis(cyanophenyl)-2,5-bis(decyloxy) benzene (CDB) molecules were deposited on a KCl (001) surface and imaged using noncontact atomic force microscopy (NC-AFM).

Both molecules were designed to contain the same anchoring groups and benzene ring structures, yet CDB self-assembled structures were observed to grow from step edges, while TCB self-assembled structures grew as islands on the clean terrace and from step edges. Density functional theory (DFT) and atomistic molecular dynamics simulations were performed to understand this qualitative difference in growth modes. Calculations of free energies of dimer formation and step adhesion provide an insight into the role played by entropy loss in the morphology and growth modes of self-assembled films.

Role of Hydrogenin Volatile Behaviour of Defects in SiO2-based Electronic Devices
Yannick Wimmer, Al-Moatasem El-Sayed, Wolfgang Gos, Tibor Grasser, Alexander Shluger
Proceedings of the Royal Society A, June 2016

Charge capture and emission by point defects in gate oxides of metal-oxide-semiconductor field-effect transistors (MOSFETs) strongly affect reliability and performance of electronic devices.

Recent advances in experimental techniques used for probing defect properties have led to new insights into their characteristics. In particular, these experimental data show a repeated dis- and reappearance (the so-called volatility) of the defect-related signals.

We use multiscale modelling to explain the charge capture and emission as well as defect volatility in amorphous SiO2 gate dielectrics. We link a multiphonon charge capture model to ab initio calculations that investigate the three most promising defect candidates. Statistical distributions of defect characteristics obtained from ab initio calculations in amorphous SiO2 are compared with the experimentally measured statistical properties of charge traps. This allows us to suggest an atomistic mechanism to explain the experimentally observed volatile behaviour of defects. We conclude that the hydroxyl-E' centre is a promising candidate to explain all the observed features, including defect volatility.

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 Physical 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.

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