Graphene and other two-dimensional materials, such as transition metal dichalcogenides, have rapidly established themselves as intriguing building blocks for optoelectronic applications, with a strong focus on various photodetection platforms. The versatility of these material systems enables their application in areas including ultrafast and ultrasensitive detection of light in the ultraviolet, visible, infrared and terahertz frequency ranges. These detectors can be integrated with other photonic components based on the same material, as well as with silicon photonic and electronic technologies. Here, we provide an overview and evaluation of state-of-the-art photodetectors based on graphene, other two-dimensional materials, and hybrid systems based on the combination of different two-dimensional crystals or of two-dimensional crystals and other (nano)materials, such as plasmonic nanoparticles, semiconductors, quantum dots, or their integration with (silicon) waveguides.

operation, and good internal quantum efficiency. ╪These authors contributed equally to this work *

Moreover, owing to the unique band structure and exceptional electronic properties of graphene, high speed photodetectors with an ultra-wide operational wavelength range at least from 300 nm to 6 µm 10, 11 can be realized using this fascinating material.

Two-dimensional (2D) atomic crystals 1 , such as graphene 2 and atomically thin transition metal dichalcogenides 3, 4 (TMDCs), are currently receiving a lot of attention. They are crystalline, and thus of high material quality, even so, they can be produced in large areas and are bendable, thus providing opportunities for novel applications. Here, we report a truly 2D p-n junction diode, based on an electrostatically doped 5 tungsten diselenide (WSe 2 ) monolayer. As p-n diodes are the basic building block in a wide variety of optoelectronic devices, our demonstration constitutes an important advance towards 2D optoelectronics. We present applications as (i) photovoltaic solar cell, (ii) photodiode, and (iii) light emitting diode. Light power conversion and electroluminescence efficiencies are ≈ 0.5 % and ≈ 0.1 %, respectively. Given the recent advances in large-scale production of 2D crystals 6, 7 , we expect them to profoundly impact future developments in solar, lighting, and display technologies.* Email: thomas.mueller@tuwien.ac.at 2 Most of today's electronic devices rely on bulk semiconductor crystals. However, their rigidity, heavy weight, and high costs of production hinder seamless integration into everyday objects. Therefore other, non-crystalline, materials are currently investigated, with organic and thin-film semiconductors 8,9 being the most prominent ones. These are, however, notoriously known for their low material quality and degradation over time. 2D atomic crystals, on the other hand, are crystalline, yet they can (potentially) be produced at low cost and in large areas, making them attractive for applications such as solar cells or display panels. P-n junction diodes are an integral part of many optoelectronic devices. P-n junctions have previously also been formed in graphene 10 , but did not show diode-like rectification behavior, due to Klein-tunneling 11 . Although graphene can be employed for photodetection 12 , it does not produce a sizable photovoltage because of its zero bandgap.For the same reason, graphene p-n junctions would not produce any electrically driven light emission. Graphene has extensively been explored for solar and display applications, but only in conjunction with other materials 6,[13][14][15] . More recently, other 2D crystals, such as MoS 2 and WSe 2 , have gained increasing attention 3,4,[16][17][18][19] , as these materials have a bandgap. Bulk MoS 2 and WSe 2 are indirect semiconductors, whereas their monolayers exhibit a direct gap 20, 21 , making them attractive for optoelectronics.Very recently, a p-n junction diode has been realized 22 in ionic liquid gated bulk MoS 2 .However, to our knowledge, a monolayer p-n diode has not yet been demonstrated in any 2D crystal.In our device, electrostatic doping is used to form a monolayer WSe 2 lateral p-n junction diode. As schematically illustrated in Figure 1a, split gate electrodes couple to 3 two different regions of a WSe 2 flake (crystal structure in Figure 1b). Biasing one gate with a negative and the other with...

Semiconductor heterostructures form the cornerstone of many electronic and optoelectronic devices and are traditionally fabricated using epitaxial growth techniques. More recently, heterostructures have also been obtained by vertical stacking of two-dimensional crystals, such as graphene and related two-dimensional materials. These layered designer materials are held together by van der Waals forces and contain atomically sharp interfaces. Here, we report on a type-II van der Waals heterojunction made of molybdenum disulfide and tungsten diselenide monolayers. The junction is electrically tunable, and under appropriate gate bias an atomically thin diode is realized. Upon optical illumination, charge transfer occurs across the planar interface and the device exhibits a photovoltaic effect. Advances in large-scale production of two-dimensional crystals could thus lead to a new photovoltaic solar technology.