Model dielectric function for 2D semiconductors including substrate screening

2020

Sci Rep

Self Cite

Photoexcited intralayer excitons in van der Waals heterostructures (vdWHs) with type-II band alignment have been observed to tunnel into interlayer excitons on ultrafast timescales. Such interlayer excitons have sufficiently long lifetimes that inducing dissociation with external in-plane electric fields becomes an attractive option of improving efficiency of photocurrent devices. In the present paper, we calculate interlayer exciton binding energies, Stark shifts, and dissociation rates for six different transition metal dichalcogenide (TMD) vdWHs using a numerical procedure based on exterior complex scaling (ECS). We utilize an analytical bilayer Keldysh potential describing the interaction between the electron-hole pair, and validate its accuracy by comparing to the full multilayer Poisson equation. Based on this model, we obtain an analytical weak-field expression for the exciton dissociation rate. The heterostructures analysed are MoS 2 /MoSe 2 , MoS 2 /WS 2 , MoS 2 /WSe 2 , MoSe 2 /WSe 2 , WS 2 /MoSe 2 , and WS 2 /WSe 2 in various dielectric environments. For weak electric fields, we find that WS 2 /WSe 2 supports the fastest dissociation rates among the six structures. We, furthermore, observe that exciton dissociation rates in vdWHs are significantly larger than in their monolayer counterparts.Naturally occurring layered materials held together by van-der-Waals-like forces have been intensely studied in recent years. Peeling graphite layer-by-layer and forming graphene 1 , a task thought impossible, turned researchers on to two-dimensional materials. Realizing that these layers may be used as building blocks of artificially layered materials, so-called van der Waals heterostructures, provides an endless array of possible combinations 2, 3 . The extraordinary electronic and optical properties of TMDs 4-11 have made them one of the most interesting classes of building blocks for vdWHs. Potential electro-optical applications of TMDs include photodetectors 12-14 , light-emitting diodes 15-17 , and solar cells 16,18,19 . It is well known that the optical properties of TMD monolayers are dominated by excitons 2,3,6,8,20 , as such two-dimensional excitons can have giant binding energies 6,8,[21][22][23] . Strongly bound excitons, in turn, make generation of photocurrents difficult, as excitons must first be dissociated into free electrons and holes. This is one of the challenges facing the use of monolayer TMDs in efficient photocurrent devices. A further complication is the fast recombination rates of excitons in these monolayers [24][25][26] . Without inducing dissociation in some way, excitons will typically recombine before they are dissociated. Applying an in-plane electric field to the excitons, however, enhances generation of photocurrents for two reasons: (i) the electric field counteracts recombination by pulling electrons and holes in opposite directions, and (ii) the electric field assists dissociation of excitons [27][28][29][30] .When two TMD monolayers are brought together with a type-II band align...

Section: Interlayer Excitonsmentioning

confidence: 99%

Interlayer excitons in van der Waals heterostructures: Binding energy, Stark shift, and field-induced dissociation

Kamban

2020

Sci Rep

Self Cite

“…where 2π/q is the 2D Fourier transform of 1/r. The resulting interaction is given by the Keldysh [23,24] form…”

Section: Tmd Exciton In Electrostatic Fieldmentioning

confidence: 99%

Field-induced dissociation of two-dimensional excitons in transition metal dichalcogenides

Kamban

2019

Phys. Rev. B

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Generation of photocurrents in semiconducting materials requires dissociation of excitons into free charge carriers. While thermal agitation is sufficient to induce dissociation in most bulk materials, an additional push is required to induce efficient dissociation of the strongly bound excitons in monolayer transition-metal dichalcogenides (TMDs). Recently, static in-plane electric fields have proven to be a promising candidate. In the present paper, we introduce a numerical procedure, based on exterior complex scaling, capable of computing field-induced exciton dissociation rates for a wider range of field strengths than previously reported in literature. We present both Stark shifts and dissociation rates for excitons in various TMDs calculated within the Mott-Wannier model. Here, we find that the field induced dissociation rate is strongly dependent on the dielectric screening environment. Furthermore, applying weak-field asymptotic theory (WFAT) to the Keldysh potential, we are able to derive an analytical expression for exciton dissociation rates in the weakfield region. arXiv:1905.02571v1 [cond-mat.mes-hall]

“…However, the screening is not properly included in the MFA and, hence, it is introduced phenomenologically [45]. In the present work, we use the Keldysh potential for the direct Coulomb interaction, which is a widely accepted form of the screened potential for 2D materials [21,26,[46][47][48]. In real space, the Keldysh potential is given by…”

Section: Numerical Resultsmentioning

confidence: 99%

Gauge invariance of excitonic linear and nonlinear optical response

Taghizadeh

2018

Phys. Rev. B

We study the equivalence of four different approaches to calculate the excitonic linear and nonlinear optical response of multiband semiconductors. These four methods derive from two choices of gauge, i.e., length and velocity gauges, and two ways of computing the current density, i.e., direct evaluation and evaluation via the time-derivative of the polarization density. The linear and quadratic response functions are obtained for all methods by employing a perturbative density-matrix approach within the mean-field approximation. The equivalence of all four methods is shown rigorously, when a correct interaction Hamiltonian is employed for the velocity gauge approaches. The correct interaction is written as a series of commutators containing the unperturbed Hamiltonian and position operators, which becomes equivalent to the conventional velocity gauge interaction in the limit of infinite Coulomb screening and infinitely many bands. As a case study, the theory is applied to hexagonal boron nitride monolayers, and the linear and nonlinear optical response found in different approaches are compared.