Abstract
Two-dimensional (2D) materials exhibit a number of improved mechanical, optical, and electronic properties compared to their bulk counterparts. The absence of dangling bonds in the cleaved surfaces of these materials allows combining different 2D materials into van der Waals heterostructures to fabricate p-n junctions, photodetectors, and 2D-2D ohmic contacts that show unexpected performances. These intriguing results are regularly summarized in comprehensive reviews. A strategy to tailor their properties even further and to observe novel quantum phenomena consists in the fabrication of superlattices whose unit cell is formed either by two dissimilar 2D materials or by a 2D material subjected to a periodic perturbation, each component contributing with different characteristics. Furthermore, in a 2D material-based superlattice, the interlayer interaction between the layers mediated by van der Waals forces constitutes a key parameter to tune the global properties of the superlattice. The above-mentioned factors reflect the potential to devise countless combinations of van der Waals 2D material-based superlattices. In the present feature article, we explain in detail the state-of-the-art of 2D material-based superlattices and describe the different methods to fabricate them, classified as vertical stacking, intercalation with atoms or molecules, moiré patterning, strain engineering and lithographic design. We also aim to highlight some of the specific applications of each type of superlattices.
Highlights
In their Communication (1970), IBM researchers Esaki and Tsu envisioned theoretically the realization of a novel semiconductor structure,[1] either by a periodic variation of the doping level in a single material or by a periodic variation of two dissimilar materials
Through the fabrication of optoelectronic nanodevices based on 2D materials, heterostructures and superlattices we probe the electronic, mechanical and optical properties of these nanomaterials
The dimensionality (m layers of material 1 and n layers of material 2 per unit cell, i.e. their ratio m/n) of these materials is taken into account to control the properties of both materials: the electrical transport,[39,40,41] the charge transfer between the layers[42,43] or the presence/strength of a charge density wave state.[44]. Another strategy followed to modulate the electrical properties is by substitution or doping in specific sites of the layers.[45,46,47]. Both misfit layer compounds and ferecrystals present a high potential in thermoelectric applications because their structure is ideal to develop phonon glass/electron crystal responses, where dichalcogenide layers with high electrical mobility constitute the electron crystal component while the mismatch or turbostratic disorder along the c-axis lowers the lattice thermal conductivity, acting as the phonon glass.[36,48,49]
Summary
In their Communication (1970), IBM researchers Esaki and Tsu envisioned theoretically the realization of a novel semiconductor structure,[1] either by a periodic variation of the doping level in a single material or by a periodic variation of two dissimilar materials. As a result of these features, 2D materials have shown extraordinary physical and chemical properties This has motivated the synthesis of novel 2D nanomaterials such as ultrathin metal–organic framework layers,[11] transition metal oxides and hydroxides[12] or covalent-organic framework layers,[13,14] in order to obtain enhanced chemical reactivity and device performance in comparison to their 3D, covalent homologues. Each type of superlattice (1 to 5) and its potential applications will be discussed in detail and the corresponding subsections
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