3 research outputs found
Compressive Straining of Bilayer Phosphorene Leads to Extraordinary Electron Mobility at a New Conduction Band Edge
By means of hybrid DFT calculations
and the deformation potential approximation, we show that bilayer
phosphorene under slight compression perpendicular to its surface
exhibits extraordinary room temperature electron mobility of order
7 × 10<sup>4</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. This is approximately 2 orders of magnitude higher
than is widely reported for ground state phosphorenes and is the result
of the emergence of a new conduction band minimum that is decoupled
from the in-plane acoustic phonons that dominate carrier scattering
Strain and Orientation Modulated Bandgaps and Effective Masses of Phosphorene Nanoribbons
Passivated phosphorene nanoribbons, armchair (a-PNR), diagonal
(d-PNR), and zigzag (z-PNR), were investigated using density functional
theory. Z-PNRs demonstrate the greatest quantum size effect, tuning
the bandgap from 1.4 to 2.6 eV when the width is reduced from 26 to
6 Ã…. Strain effectively tunes charge carrier transport, leading
to a sudden increase in electron effective mass at +8% strain for
a-PNRs or hole effective mass at +3% strain for z-PNRs, differentiating
the (<i>m</i><sub>h</sub><sup>*</sup>/<i>m</i><sub>e</sub><sup>*</sup>) ratio by an order of magnitude in each case.
Straining of d-PNRs results in a direct to indirect band gap transition
at either −7% or +5% strain and therein creates degenerate
energy valleys with potential applications for valleytronics and/or
photocatalysis
Modeling Excited States in TiO<sub>2</sub> Nanoparticles: On the Accuracy of a TD-DFT Based Description
We have investigated the suitability
of Time-Dependent Density
Functional Theory (TD-DFT) to describe vertical low-energy excitations
in naked and hydrated titanium dioxide nanoparticles. Specifically,
we compared TD-DFT results obtained using different exchange-correlation
(XC) potentials with those calculated using Equation-of-Motion Coupled
Cluster (EOM-CC) quantum chemistry methods. We demonstrate that TD-DFT
calculations with commonly used XC potentials (e.g., B3LYP) and EOM-CC
methods give qualitatively similar results for most TiO<sub>2</sub> nanoparticles investigated. More importantly, however, we also show
that, for a significant subset of structures, TD-DFT gives qualitatively
different results depending upon the XC potential used and that only
TD-CAM-B3LYP and TD-BHLYP calculations yield results that are consistent
with those obtained using EOM-CC theory. Moreover, we demonstrate
that the discrepancies for such structures originate from a particular
combination of defects that give rise to charge-transfer excitations,
which are poorly described by XC potentials that do not contain sufficient
Hartree–Fock like exchange. Finally, we consider that such
defects are readily healed in the presence of ubiquitously present
water and that, as a result, the description of vertical low-energy
excitations for hydrated TiO<sub>2</sub> nanoparticles is nonproblematic