The ability to detect light over a broad spectral range is central for practical optoelectronic applications, and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS 2 . However, polarization sensitivity within such a photodetector remains elusive. Here we demonstrate a linear-dichroic broadband photodetector with layered black phosphorus transistors, using the strong intrinsic linear dichroism arising from the in-plane optical anisotropy with respect to the atom-buckled direction, which is polarization sensitive over a broad bandwidth from about 400 nm to 3750 nm. Especially, a perpendicular built-in electric field induced by gating in the transistor geometry can spatially separate the photo-generated electrons and holes in the channel, effectively reducing their recombination rate, and thus enhancing the performance for linear dichroism photodetection. This provides practical functionality using anisotropic layered black phosphorus, thereby enabling novel optical and optoelectronic device applications. Corresponding author: hyhwang@stanford.edu, yicui@stanford.edu. 2Confined electronic systems in layered two-dimensional (2D) crystals are host to many emerging electronic, spintronic and photonic phenomena, 1, 2, 3 including quantum Hall and Dirac electrons in graphene 4, 5, 6 and topological surface states in topological insulators 7, 8 . Experimentally identifying new functionalities of two-dimensional materials is a challenging and rewarding frontier, enabled by recent advances in materials and device fabrication. One example is the valley polarization control using circularly polarized light in the non-centrosymmetric MoS 2 monolayer and resulting potential valleytronics applications. 9, 10,11 Other examples include recent demonstrations of novel electronic and optoelectronic applications of the well-known layered material black phosphorus (BP), such as high-mobility field effect transistors and linear-polarization dependent optical absorption. 12,13,14 Therefore, further discovering new properties and functionalities utilizing known layered materials is of practical importance and great current interest. 14,15,16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 As a potential functionality for layered 2D materials, linear dichroism (LD) is an electromagnetic spectroscopy probing different absorption of light polarized parallel or perpendicular to an orientation axis. It directly depends on the conformation and orientation of material/device structures, where they are either intrinsically oriented in an anisotropic crystal structure 27, 28 or extrinsically oriented in anisotropic device patterns 29, 30 . Compared to the hexagonal in-plane lattice in other 2D materials such as graphene and MoS 2 , which are insensitive to the linear polarization of incident light, the layered BP crystal with a rectangular in-plane lattice has a highly-anisotropic structure along the x and y directions (defined in Fig. 1a), where every two rows of P atoms alternatel...

Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 10, along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.