2 research outputs found
Why Are Polar Surfaces of ZnO Stable?
We probe and rationalize
the complex surface chemistry of wurtzite
ZnO by employing interatomic potential calculations coupled with a
Monte Carlo procedure that sampled over 0.5 million local minima.
We analyze the structure and stability of the (0001) and (0001Ì…)
ZnO surfaces, rationalizing previous patterns found in STM images
and explaining the (1 × 1) periodicity reported by LEED analysis.
The full range of Zn/O surface occupancies was covered for a (5 ×
5) supercell, keeping |<i>m</i><sub>Zn</sub> – <i>m</i><sub>O</sub>|/<i>N</i> ≈ 0.24 where <i>m</i> and <i>N</i> are the numbers of occupied surface
sites and total surface sites, respectively. Our calculations explain
why the (5 × 5) reconstructions seen in some experiments and
highlight the importance of completely canceling the inherent dipole
of the unreconstructed polar surfaces. The experimentally observed
rich reconstruction patterns can be traced from the lowest occupancy,
showing the thermodynamically most stable configurations of both polar
surfaces. Triangular and striped reconstructions are seen, <i>inter alia</i>, on both polar surfaces, and hexagonal patterns
also appear on the O terminated surface. Our results explain the main
experimental structures observed on these complex surfaces. Moreover,
grand canonical simulations of ZnO polar surfaces reveal that disorder
is favored and, thus, configurational entropic factors is the the
cause of their stability
Describing Excited State Relaxation and Localization in TiO<sub>2</sub> Nanoparticles Using TD-DFT
We have investigated the description
of excited state relaxation
in naked and hydrated TiO<sub>2</sub> nanoparticles using Time-Dependent
Density Functional Theory (TD-DFT) with three common hybrid exchange-correlation
(XC) potentials: B3LYP, CAM-B3LYP and BHLYP. Use of TD-CAM-B3LYP and
TD-BHLYP yields qualitatively similar results for all structures,
which are also consistent with predictions of coupled-cluster theory
for small particles. TD-B3LYP, in contrast, is found to make rather
different predictions; including apparent conical intersections for
certain particles that are not observed with TD-CAM-B3LYP nor with
TD-BHLYP. In line with our previous observations for vertical excitations,
the issue with TD-B3LYP appears to be the inherent tendency of TD-B3LYP,
and other XC potentials with no or a low percentage of Hartree–Fock
like exchange, to spuriously stabilize the energy of charge-transfer
(CT) states. Even in the case of hydrated particles, for which vertical
excitations are generally well described with all XC potentials, the
use of TD-B3LYP appears to result in CT problems during excited state
relaxation for certain particles. We hypothesize that the spurious
stabilization of CT states by TD-B3LYP even may drive the excited
state optimizations to different excited state geometries from those
obtained using TD-CAM-B3LYP or TD-BHLYP. Finally, focusing on the
TD-CAM-B3LYP and TD-BHLYP results, excited state relaxation in small
naked and hydrated TiO<sub>2</sub> nanoparticles is predicted to be
associated with a large Stokes’ shift