Seminar

Graduate Student Gil Repa

Thursday, November 5, 2020 - 10:45am
https://lehigh.zoom.us/j/99299496643?pwd=RkRERjNsQUlUY0J3ZC96RHl1Q2ROUT09

"Beyond Linear Response Theory: Modelling Ion Beam Induced Defects by Real Time TD-DFT"

The interaction of projectile ions with target materials has long attracted the attention of researchers with beam techniques now becoming a critical means for fabrication and analysis. Despite such attention, a fundamental understanding of the electronic interactions that occur when an energetic particle moves through material is lacking. This issue is well-suited for computational study utilizing time dependent density functional theory (TD-DFT), which can exceed the temporal and spatial resolutions afforded by current experimental techniques. However, the time-dependent Kohn-Sham equations that arise in the derivation of TD-DFT can be solved in two different ways. Linear Response TD-DFT (LR TD-DFT), is the more traditional of these methods, but intrinsic assumptions may limit its applicability when modelling material-ion beam interactions. An alternative approach, Real Time TD-DFT (RT TD-DFT)1 is able to bypass some of the shortcomings of LR TD-DFT. Additionally, RT TD-DFT can be used in conjunction with nonadiabatic semi-classical Ehrenfest Dynamics (ED) to transcend the Born Oppenheimer approximation and explicitly couple the motion of nuclei and electrons. Taken together, this strategy provides a powerful method to study material-ion beam interactions.

This talk will provide a brief overview of the RT TD-DFT/ED formalism, followed by a review of its application to three materials of different dimensionalities to probe the chemistry of particle irradiation. First, a mechanism for defect formation in 2-dimensional molybdenum disulfide sheets undergoing low energy electron irradiation2 will be reviewed. Next, bulk materials bombarded with heavy ions will be considered in the case of self-irradiated silicon to examine the roles of semi-core electrons, particle trajectory, charge, and initial kinetic energy on the nature and dynamics of projectile equilibration.3 Finally, an intermediate case will be presented of thin aluminum sheets undergoing proton irradiation to shed light on pre-equilibration dynamics, as well as rationalization for deviation from bulk behavior via additional excitation channels.4 The implications of the data discussed here hold great importance for understanding the fundamental chemistry of material-ion beam interactions, and will lead to improved prediction of defects, materials design, and ion beam techniques.

References:
(1) Lopata, K.; Govind N. J. Chem. Theory Comput. 2011, 7, 1344-1355.
(2) Kretschmer S.; Lehnert T.; Kaiser U.; Krasheninnikov A.V. Nano. Lett. 2020, 20, 2865-2870.
(3) Lee C.W.; Stewart J.A.; Dingreville R.; Foiles S.; Schleife A. Phys. Rev. B. 2020, 102, 024107.¬¬
(4) Kononov A.; Schleife A. Phys. Rev. B. 2020, 102, 165401.