Alexander Shluger Group

Research Overview

• 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.

Recent Highlights

Electron Ptychography for Atom-by-Atom Quantification of 1D Defect Complexes in Monolayer MoS₂

Loh, L., Ning, S., Kieczka, D., Chen, Y., Yang, J., Wang, Z., Pennycook, S.J., Eda, G., Shluger, A.L., Bosman, M.

ACS Nano, February 2025

Defect complexes can induce beneficial functionalities in two-dimensional (2D) semiconductors. However, understanding their formation mechanism with single-atom sensitivity has proven to be challenging for light elements using conventional transmission electron microscopy (TEM) techniques. Here, we demonstrate the atom-resolved formation of various one-dimensional (1D) defect complexes─consisting of rhenium dopants, sulfur interstitials, and sulfur vacancies─in monolayer MoS₂ by applying electron ptychography to our four-dimensional scanning transmission electron microscopy (4D-STEM) data sets. Our image resolution of 0.35 Å and a spatial precision of 2 pm allow us to achieve accurate matching between experimental structures and density functional theory (DFT) simulations at the atomic level. Additionally, we utilize out-of-focus ptychography to observe defect formation processes at dose rates comparable to those used in conventional TEM imaging, while maintaining a large field of view. This study demonstrates the systematic application of electron ptychography to extensive 4D-STEM data sets for quantitative defect imaging in 2D materials. We provide direct, atomically precise evidence that critical defect densities govern the formation of extended 1D defect complexes. For instance, we show that sulfur single-vacancy lines form when the vacancy density reaches 5 × 10¹³ cm⁻² and transform into double-vacancy lines beyond 8 × 10¹³ cm⁻². Rhenium-dopant lines emerge at a dopant concentration higher than 3 × 10¹³ cm⁻², where metastable sulfur interstitial-vacancy lines also form as the cumulative electron dose reaches 3 × 10⁵ e/Ų, initiating a local nucleation of the 1T′ phase. Our results highlight the potential of electron ptychography for high-precision defect characterization and engineering in ultrathin 2D materials.

Structure and Migration Mechanisms of Oxygen Interstitial Defects in β-Ga₂O₃

Kaewmeechai C., Strand, J., Shluger A.L.

physica status solidi (b), February 2025

Geometric configurations of oxygen interstitial (Oᵢ) defects in β-Ga₂O₃ are studied using an intensive search strategy and optimized with density functional theory using PBEsol and PBE0-TC-LRC functionals. The results show that Oᵢ atoms can form O–O dimers at three different O sites in β-Ga₂O₃ as oxygen split-interstitial defects in 0 and +1 charge states. At higher Fermi levels above 1 eV from the conduction band minimum, trapping of extra electrons can break the O–O dimer, forming bulk Oᵢ¹ and Oᵢ² configurations. The investigation of O–O dimer migration mechanisms in β-Ga₂O₃ reveals that dimers move between rings through bond-breaking and rotation processes with activation barriers of ≈1 and 0.3 eV, respectively. Oᵢ² can migrate to neighboring rings with a much lower barrier of 0.15 eV. These results provide a detailed map of the potential energy surface of Oᵢ in β-Ga₂O₃ and demonstrate how interstitial O can migrate between different rings.

Impact of Confined Water on the Electronic Structure of the SiO₂ and WS₂ Interface

Milton, K.L., Shluger, A.L.

ACS Applied Materials & Interfaces

WS₂/SiO₂ heterostructures are commonly used in the production of field effect transistors, sensors and other devices. It is difficult to remove the water present on the SiO₂ surface during the fabrication process. Using density functional theory simulations, we investigate how confinement between SiO₂ and WS₂ layers affects the water properties and how the presence of water affects the properties of WS₂ film. Using ab initio molecular dynamics simulations we found that the confined water remains mobile but is structured by the interaction with WS₂ and SiO₂ with water protons drawn closer to both surfaces. The presence of 1–3 water layers does not significantly affect the band alignment between SiO₂ and WS₂ and the electronic properties of the WS₂ monolayer. However, in-gap states caused by the dynamic rearrangement of water molecules could cause reduction of electron and hole mobility in the WS₂ layers. This study provides insights into the impact of water on the electronic properties of WS₂ under different environmental conditions.

Structure and Dynamics of Water Confined at the SiO₂/WS₂ Interface

Milton, K.L., Hargreaves, L., Shluger, A.L.

The Journal of Physical Chemistry C, February 2025

The WS₂/SiO₂ interface is of interest to a variety of research communities due to the electronic properties of WS₂ and the ubiquity of SiO₂ as a dielectric substrate. Due to the hydrophilic nature of silanol groups on the surface of SiO₂, water is difficult to remove at the surface, leading to confined water between WS₂ and SiO₂. Understanding the properties of confined water is important both fundamentally and for their effects on the interfacing materials. We investigated the structure and dynamics of confined water between WS₂ and SiO₂ using density functional theory and ab initio molecular dynamics, comparing it to adsorbed water on the surfaces of WS₂ and SiO₂. The results show that confined water becomes increasingly structured, with its orientation influenced by hydrogen bonding to the silanol groups as well as by the partial reorientation of water molecules to face WS₂ in an H-up configuration. The presence of silanol groups disrupts the hydrogen bonding network of water at monolayer coverage for both confined and unconfined water. For all interfaces explored, changes in both structural and dynamic properties are dependent on the number of water layers present.

Electronic structure and properties of trapped holes in crystalline and amorphous Ga₂⁢O₃

Kaewmeechai C., Strand, J., Shluger A.L.
Physical Review B, January 2025

Structure and electronic properties of self-trapped holes were studied in both crystalline and amorphous Ga₂O₃ using density functional theory (DFT) and the nonlocal PBE0-TC-LRC density functional. Amorphous (a)-Ga₂O₃ structures were generated using classical molecular dynamics and the melt-quench technique and further optimized using DFT. They exhibit an average density of 4.84 g/cm³ and a band gap around 4.3 eV. Calculations predict deep hole trapping in crystalline and amorphous phases, with the hole-trapping energies in the amorphous structures being deeper than those found in the crystalline structure. In a-Ga₂O₃, trapped holes are localized around low-coordinated oxygen atoms (two- or three-coordinated). We predict the formation of stable hole bipolarons in both the crystalline and amorphous phases, facilitated by the formation of O–O bonds, with binding energies of about 0.2 eV.

Dielectric Breakdown of Oxide Films in Electronic Devices

Padovani, A., La Torraca, P., Strand, J., Larcher, L., Shluger, A.L.

Nature Reviews Materials, August 2024

Dielectric breakdown is a sudden and catastrophic increase in the conductivity of an insulator caused by electrical stress. It is one of the major reliability issues in electronic devices using insulating flms as gate insulators and in energy and memory capacitors. Despite extensive studies, our understanding of the physical mechanisms driving the breakdown process remains incomplete, and atomistic models describing the dielectric breakdown are controversial. This Review surveys the enormous amount of data and knowledge accumulated from experimental and theoretical studies of dielectric breakdown in different insulating materials, focusing on describing phenomenological models and novel computational approaches.

Modeling the Effects of Varying the Ti Concentration on the Mechanical Properties of Cu–Ti Alloys

Fotopoulos, V., O’Hern, C.S., Shattuck, M., Shluger, A.L.

ACS Omega, January 2024

The mechanical properties of CuTi alloys have been characterized extensively through experimental studies. However, a detailed understanding of why the strength of Cu increases after a small fraction of Ti atoms are added to the alloy is still missing. In this work, we address this question using density functional theory (DFT) and molecular dynamics (MD) simulations with the modified embedded atom method (MEAM) interatomic potentials. First, we performed calculations of the uniaxial tension deformations of small bicrystalline Cu cells using DFT static simulations. We then carried out uniaxial tension deformations on much larger bicrystalline and polycrystalline Cu cells by using MEAM MD simulations. In bicrystalline Cu, the inclusion of Ti increases the grain boundary separation energy and the maximum tensile stress. The DFT calculations demonstrate that the increase in the tensile stress can be attributed to an increase in the local charge density arising from Ti. MEAM simulations in larger bicrystalline systems have shown that increasing the Ti concentration decreases the density of the stacking faults. This observation is enhanced in polycrystalline Cu, where the addition of Ti atoms, even at concentrations as low as 1.5 atomic (at.) %, increases the yield strength and elastic modulus of the material compared to pure Cu. Under uniaxial tensile loading, the addition of small amounts of Ti hinders the formation of partial Shockley dislocations in the grain boundaries of Cu, leading to a reduced level of local deformation. These results shed light on the role of Ti in determining the mechanical properties of polycrystalline Cu and enable the engineering of grain boundaries and the inclusion of Ti to improve degradation resistance.