Excitons in few-layer hexagonal boron nitride: Davydov splitting and surface localization

Paleari, Fulvio; Galvani, Thomas; Amara, Hakim; Ducastelle, F.; Molina-Sánchez, Alejandro; Wirtz, Ludger

doi:10.48550/arxiv.1803.00982

Cited by 4 publications

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“…[55]). These findings are in agreement with earlier studies of this h-BN phase [7,75,87]. The 2D character of the first bright exciton is closely related to the electronic structure of the corresponding arrangements.…”

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confidence: 93%

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Dimensionality of excitons in stacked van der Waals materials: The example of hexagonal boron nitride

et al. 2018

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With the example of hexagonal boron nitride, we demonstrate how the character of electron-hole (e-h) pairs in van der Waals bound low-dimensional systems is driven by layer stacking. Four types of excitons appear, with either a two-or three-dimensional spatial extension. Electron and hole distributions are either overlapping or exhibit a charge-transfer nature. We discuss under which structural and symmetry conditions they appear and they are either dark or bright. This analysis provides the key elements to identify, predict, and possibly tailor the character of e-h pairs in van der Waals materials.Two-dimensional (2D) systems and layered weaklybound structures thereof are considered the materials of the 21st century. Their wealth of intriguing properties is widely explored from a fundamental scientific point of view but also in view of a plethora of possible applications [1,2]. Hexagonal boron nitride (h-BN) is one of these materials, consisting of covalently bound sheets that are held together by van der Waals (vdW) forces [3]. h-BN is a wide-gap semiconductor, exhibiting pronounced excitonic effects in its optical excitations that are present irrespective of the material's dimensionality [4][5][6][7][8][9][10][11]. Owing to the flat geometry of its in-plane hexagonal lattice, h-BN is often chosen as a building block in vdW heterostructures [12][13][14][15]. Combining different 2D systems, quantum confinement effects allow for tailoring their opto-electronic properties [16][17][18]. This not only concerns level alignment at the interface [19][20][21][22][23][24][25][26] but also the way the system interacts with light, i.e., quantum efficiency, as well as the character and spatial distribution of electron-hole (e-h) pairs [14,15,[27][28][29][30][31][32].In this Rapid Communication, we show that the nature and dimensionality of excitons can also be governed in a single vdW-bound bulk material, taking h-BN as an example. This seems surprising at a first glance as e-h pairs in this material have been found to exhibit basically the same character and extension in bulk [6][7][8], monolayers [10], as well as in interfaces with graphene [14]. Here we demonstrate how strongly stacking impacts the optical excitations of a vdW crystal at the absorption onset and beyond. By varying the arrangement of individual h-BN layers, we find in total four types of electron-hole pairs, of a two-dimensional, three-dimensional (3D), and charge-transfer character, and discuss their appearance by symmetry considerations. We focus on the five structures that are obtained by including four inequivalent atoms in the unit cell, allowing only a rigid translation by one bond length and/or exchanged positions between the two atomic species. We employ density-functional theory [33-37] and many-body perturbation theory (MBPT, including G 0 W 0 [38,39] and the Bethe-Salpeter equation [40][41][42][43]), implemented in the all-electron framework of the exciting code [44][45][46].Prototypes of the four kinds of excitations that can appear in h-BN ar...

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confidence: 93%

“…In the following, we discuss how this structure-property relation determines the conditions under which different types of e-h pairs are formed. Alternative approaches to extract this information are based on model Hamiltonians [10,74,75], analytic envelopefunction modeling [76], and double-Bader analysis [77].…”

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Dimensionality of excitons in stacked van der Waals materials: The example of hexagonal boron nitride

et al. 2018

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“…It is important to remind that excitons are extremely compact in hBN. The in-plane extent of the exciton Bohr radius is only a few lattice constants [21]. The EEA process in hBN probably involves an exciton diffusion step as discussed in WSe 2 [22], black phosphorus [23] and organic semiconductors [24][25][26].…”

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confidence: 99%

Exciton-exciton annihilation in hBN

Plaud

Schue

Watanabe³

et al. 2019

Applied Physics Letters

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Known as a prominent recombination path at high excitation densities, exciton-exciton annihilation (EEA) is evidenced in bulk hexagonal boron nitride (hBN) by cathodoluminescence at low temperature. Thanks to a careful tune of the the exciton density by varying either the current or the focus of the incident electron beam, we could estimate an EEA rate of 2×10 −6 cm 3 .s −1 at T = 10 K, the highest reported so far for a bulk semiconductor. Expected to be even stronger in nanotubes or atomic layers, EEA probablly contributes to the luminescence quenching observed in low-dimensionality BN materials.

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“…[28][29][30] ) on the electronic and optical properties of both the bulk and the single layer. These studies, supported by theoretical investigations, [31][32][33][34][35][36] have shown that h-BN is an indirect band gap insulator whose optical absorption spectrum is dominated by strongly bound excitons. Then, the electronic structures of the h-BN monolayer and of the bulk structure are fairly well known-which is convenient for the tight-binding model.…”

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confidence: 81%

“…The simplest extension of the 2D TB model to bulk h-BN consists in introducing transfer integrals between nearest neighbours along the stacking axis. 10,36…”

Section: B Tight-binding Modelmentioning

confidence: 99%

Two-photon absorption in two-dimensional materials: The case of hexagonal boron nitride

et al. 2018

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We calculate the two-photon absorption in bulk and single layer hexagonal boron nitride (h-BN) both by an ab initio real-time Bethe-Salpeter approach and by a the real-space solution of the excitonic problem in tight-binding formalism. The two-photon absorption obeys different selection rules from those governing linear optics and therefore provides complementary information on the electronic excitations of h-BN. Combining the results from the simulations with a symmetry analysis we show that two-photon absorption is able to probe the lowest energy 1s states in the single layer h-BN and the lowest dark degenerate dark states of bulk h-BN. This deviation from the "usual" selection rules based on the continuous hydrogenic model is explained within a simple model that accounts for the crystalline symmetry. The same model can be applied to other two-dimensional materials with the same point-group symmetry, such as the transition metal chalcogenides. We also discuss the selection rules related to the inversion symmetry of the bulk layer stacking. arXiv:1803.10959v1 [cond-mat.mtrl-sci]

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Excitons in few-layer hexagonal boron nitride: Davydov splitting and surface localization

Cited by 4 publications

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Dimensionality of excitons in stacked van der Waals materials: The example of hexagonal boron nitride

Dimensionality of excitons in stacked van der Waals materials: The example of hexagonal boron nitride

Exciton-exciton annihilation in hBN

Two-photon absorption in two-dimensional materials: The case of hexagonal boron nitride

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