Giant excitonic exchange splitting in Si nanowires: First-principles calculations

Abstract

The size and doping dependence of the electron-hole exchange interaction in Si nanowires is investigated from first principles. In pure Si nanowires we found excitonic exchange splittings in very good agreement with the experimental results for porous silicon. For n-doped Si nanowires a giant singlet-triplet splitting, three order of magnitude bigger than in bulk silicon, is predicted as due to the dramatic enhancement of the electron and the hole probability of being in the same place at the same time.

Excitons and optical spectra of phosphorene nanoribbons

“…As a consequence, the triplet excitons have a long lifetime (in the range of a mile-second to infinite if the total spin is a good quantum number). ∆ S−T in phosphorene and PNRs are really large in comparing with other nanostructures such as doped silicon-nanowires (∼ 100 meV [15]), graphyne (∼ 150 meV [39]) and Carbon nanotubes (∼ 20 meV [40]) which are known as materials to impressive spin singlettriplet splitting. The giant singlet-triplet splitting are interesting in photovoltaic, biomedical, photoluminescent and quantum information applications [34,39].…”

Excitons and excitonic fine structures in Si nanowires: Prediction of an electronic state crossover with diameter changes

“…20,34 These properties are strongly dependent on the wire size, morphology and surface passivation. 14,19,22,23 In this situation theoretical studies on excitonic properties of Si wires were usually compared to experimental results for porous Si, 29,30 which has a yet poorly understood morphology and interface. Here we describe via explicit atomistic pseudopotential calculations a comprehensive analysis of Si nanowires along the [001] and [011] growth direction on all three energy scales.…”

“…4 shows the excitonic-gap dependence of the singlet-triplet splitting with no SO, ∆ ST , compared with previous GW+BSE results. 30 The wire diameter dependence of ∆ ST is found to scale as 1/D 27 and thus their electronic and excitonic properties are affected more by H-passivation than those of [011] wires. The Si-H bonding is known to modify Si enormously and H-passivation has been reported to result in much bigger band gaps than those with other passivations.…”

“…In the ∼10 −1 eV energy scale (ii) the Bethe-Salpeter Equation (BSE) within the GW method was used to describe excitonic absorption, 29,30 yet GW-BSE calculations are computationally rather expensive, so large-diameter wires (>1.5 nm) were not considered and the spin-orbit (SO) coupling effect was generally ignored. 29,30 In the ∼10 −3 eV energy scale (iii), the understanding is still dominated by simple exchange singlet-triplet splitting model based on two single-particle levels. 30,31 Furthermore, in an indirect-gap system such as Si the dark states are not only exchange-induced, but are affected by the symmetry of band-edge states and by intervalley coupling in the conduction bands.…”

“…29,30 In the ∼10 −3 eV energy scale (iii), the understanding is still dominated by simple exchange singlet-triplet splitting model based on two single-particle levels. 30,31 Furthermore, in an indirect-gap system such as Si the dark states are not only exchange-induced, but are affected by the symmetry of band-edge states and by intervalley coupling in the conduction bands.…”