In this work, we report a new nonadiabatic molecular dynamics methodology that incorporates many-body (MB) effects in the treatment of electronic excited states in extended atomistic systems via linear-response time-dependent density functional theory (TD-DFT). The nonradiative dynamics of excited states in Si 75 H 64 and Cd 33 Se 33 nanocrystals is studied at the MB (TD-DFT) and single-particle (SP) levels to reveal the role of MB effects. We find that a MB description of the excited states qualitatively changes the structure of coupling between the excited states, leading to larger nonadiabatic couplings and accelerating the dynamics by a factor of 2−4. The dependence of excited state dynamics in these systems on the surface hopping/decoherence methodology and the choice of the dynamical basis is investigated and analyzed. We demonstrated that the use of special "electron-only" or "hole-only" excitation bases may be advantageous over using the full "electron−hole" basis of SP states, making the computed dynamics more consistent with the one obtained at the MB level.
In
this work, we report a new methodology for nonadiabatic molecular
dynamics calculations within the extended tight-binding (xTB) framework.
We demonstrate the applicability of the developed approach to finite
and periodic systems with thousands of atoms by modeling “hot”
electron relaxation dynamics in silicon nanocrystals and electron–hole
recombination in both a graphitic carbon nitride monolayer and a titanium-based
metal–organic framework (MOF). This work reports the nonadiabatic
dynamic simulations in the largest Si nanocrystals studied so far
by the xTB framework, with diameters up to 3.5 nm. For silicon nanocrystals,
we find a non-monotonic dependence of “hot” electron
relaxation rates on the nanocrystal size, in agreement with available
experimental reports. We rationalize this relationship by a combination
of decreasing nonadiabatic couplings related to system size and the
increase of available coherent transfer pathways in systems with higher
densities of states. We emphasize the importance of proper treatment
of coherences for obtaining such non-monotonic dependences. We characterize
the electron–hole recombination dynamics in the graphitic carbon
nitride monolayer and the Ti-containing MOF. We demonstrate the importance
of spin-adaptation and proper sampling of surface hopping trajectories
in modeling such processes. We also assess several trajectory surface
hopping schemes and highlight their distinct qualitative behavior
in modeling the excited-state dynamics in superexchange-like models
depending on how they handle coherences between nearly parallel states.
The aim of this investigation was to enhance the biological behavior of NiTi shape memory alloy while preserving its super-elastic behavior in order to facilitate its compatibility for application in human body. The surfaces of NiTi samples were bombarded by three different nitrogen doses. Small-angle X-ray diffraction was employed for evaluating the generated phases on the bombarded surfaces. The electrochemical behaviors of the bare and surface-modified NiTi samples were studied in simulated body fluid (SBF) using electrochemical impedance and potentio-dynamic polarization tests. Ni ion release during a 2-month period of service in the SBF environment was evaluated using atomic absorption spectrometry. The cellular behavior of nitrogen-modified samples was studied using fibroblast cells. Furthermore, the effect of surface modification on super-elasticity was investigated by tensile test. The results showed the improvement of both corrosion and biological behaviors of the modified NiTi samples. However, no significant change in the super-elasticity was observed. Samples modified at 1.4E18 ion cm(-2) showed the highest corrosion resistance and the lowest Ni ion release.
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