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...

Two-dimensional electron gases (2DEG), arising due to quantum confinement at interfaces between transparent conducting oxides, have received tremendous attention in view of electronic applications. Here, we explore the potential of interfaces formed by two lattice-matched wide-gap oxides of emerging interest, i.e., the polar, orthorhombic perovskite LaInO3 and the nonpolar, cubic perovskite BaSnO3, employing first-principles approaches. We find that the polar discontinuity at the interface is mainly compensated by electronic relaxation through charge transfer from the LaInO3 to the BaSnO3 side. This leads to the formation of a 2DEG hosted by the highly dispersive Sn-s-derived conduction band and a 2D hole gas of O-p character, strongly localized inside LaInO3. We rationalize how polar distortions, termination, thickness, and dimensionality of the system (periodic or non-periodic) can be exploited in view of tailoring the 2DEG characteristics, and why this material is superior to the most studied prototype LaAlO3/SrTiO3.

By investigating the optoelectronic properties of prototypical graphene/hexagonal boron nitride (h-BN) heterostructures, we demonstrate how a nanostructured combination of these materials can lead to a dramatic enhancement of light-matter interaction and give rise to unique excitations. In the framework of ab initio many-body perturbation theory, we show that such heterostructures absorb light over a broad frequency range, from the near-infrared to the ultraviolet (UV), and that each spectral region is characterized by a specific type of excitations. Delocalized electron-hole pairs in graphene dominate the low-energy part of the spectrum, while strongly bound electron-hole pairs in h-BN are preserved in the near-UV. Besides these features, characteristic of the pristine constituents, charge-transfer excitations appear across the visible region. Remarkably, the spatial distribution of the electron and the hole can be selectively tuned by modulating the stacking arrangement of the individual building blocks. Our results open up unprecedented perspectives in view of designing van der Waals heterostructures with tailored optoelectronic features.