Crossover from weakly indirect to direct excitons in atomically thin films of InSe

Abstract

We perform a k • p theory analysis of the spectra of the lowest energy and excited states of the excitons in few-layer atomically thin films of InSe taking into account in plane electric polarizability of the film and the influence of the encapsulation environment. For the thinner films, the lowest-energy state of the exciton is weakly indirect in momentum space, with its dispersion showing minima at a layer-number-dependent wave number, due to an inverted edge of a relatively flat topmost valence band branch of the InSe film spectrum, and we compute the activation energy from the momentum dark exciton ground state into the bright state. For the films with more than seven In 2 Se 2 layers, the exciton dispersion minimum shifts to point.

Tunable spin-orbit coupling in two-dimensional InSe

“…The data shown in circles ( ) were obtained by exact diagonalization of the 14-band Hamiltonian in Ref. 55 and compared with the perturbation theory results obtained by Löwdin partitioning ( ) (note that for N=1, α = 0 for ne = 0 × 10 12 cm -2 ). Inset shows the usual configuration of a single-gated FET device.…”

“…In Fig. 13(b) we show the p z -dependence (around the Apoint) of the linear in k x , k y spin-orbit coupling computed by DFT for bulk InSe using QSGW approach 55,59 , to compare with the SOC form in Eq. (26).…”

“…Further to the DFT calculations for the fewlayer case, in Fig. 3(b) we use previous quasiparticle selfconsistent GW (QSGW) calculations for bulk γ-InSe 55,59 to extract the k z -dependence of the coefficient of the linear component of SOC for small in-plane momentum near k x = k y = 0, for both the conduction and valence bands. This shows that as k z approaches the bulk band edge (located at k z = π/a z ) the SOC strength increases, implying that as k z is restricted by confinement in thin films of InSe, the SOC strength can be expected to decrease from its bulk value, with smaller strengths for thinner films.…”

Angle‐Dependent Raman Spectra of Crystal Polymorphs of GaO: A Computational Study

“…[34] There is a transition from weakly indirect to direct excitons in atomically thin films of InSe. [35] What's more, time-dependent density functional theory (TDDFT) reveals that in-plane excitons are responsible for the normal carrier lifetimes at lower probe photon energy while surface-bound excitons are responsible for the anomalous size-independent carrier duration at higher probe photon energies. [36] The above excellent properties indicate that the monolayers of group III monochalcogenides have great potential in the application of optoelectronic materials.…”

Scaleability of dielectric susceptibility $ε_{zz}$ with the number of layers and additivity of ferroelectric polarization in van der Waals semiconductors

“…It characterizes how a material is polarized under an external electric field, which is relevant for modeling field-effect transistors [1,2], capacitors [3], and ferroelectrics based memristors [4,5]. In layered materials, dielectric permittivity reflects [6][7][8][9] a strong anisotropy of crystalline and electronic properties, which is particularly strong in van der Waals (vdW) layered crystals such as graphite, black phosphorus, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs). Because of the layered nature of these compounds, all of them had already been implemented as components in various field-effect transistor devices, where electrostatics is determined by the out-of-plane component of the dielectric permitivitty tensor, zz .…”