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 × 10⁴ cm² V^–1 s^–1. 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 Å. 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>_h^*/<i>m</i>_e^*) 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 TiO₂ 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 TiO₂ 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 TiO₂ nanoparticles is nonproblematic

Limits to doping of wide band gap semiconductors

The role of defects in materials is one of the long-standing issues in solid-state chemistry and physics. On one hand, intrinsic ionic disorder involving stoichiometric amounts of lattice vacancies and interstitials is known to form in highly ionic crystals. There is a substantial literature on defect formation and the phenomenological limits of doping in this class of materials; in particular, involving the application of predictive quantum mechanical electronic structure computations. Most wide band gap materials conduct only electrons and few conduct holes, but rarely are both modes of conduction accessible in a single chemical system. The energies of electrons and holes are taken from the vertical ionization potentials and electron affinities; polaronic trapping of carriers is excluded. While the focus here is defect energetics, the atomic and electronic structures have been carefully examined in all cases to ensure physical results were obtained.</p

Limits to doping of wide band gap semiconductors

The role of defects in materials is one of the long-standing issues in solid-state chemistry and physics. On one hand, intrinsic ionic disorder involving stoichiometric amounts of lattice vacancies and interstitials is known to form in highly ionic crystals. There is a substantial literature on defect formation and the phenomenological limits of doping in this class of materials; in particular, involving the application of predictive quantum mechanical electronic structure computations. Most wide band gap materials conduct only electrons and few conduct holes, but rarely are both modes of conduction accessible in a single chemical system. The energies of electrons and holes are taken from the vertical ionization potentials and electron affinities; polaronic trapping of carriers is excluded. While the focus here is defect energetics, the atomic and electronic structures have been carefully examined in all cases to ensure physical results were obtained.</p

Limits to Doping of Wide Band Gap Semiconductors

Limits to Doping of Wide Band Gap Semiconductor