Orientation of luminescent excitons in layered nanomaterials

In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties1,2,3,4,5,6. Directional optical properties can be exploited to enhance the performance of optoelectronic devices7,8,9, optomechanical actuators10 and metamaterials11. In layered materials, optical anisotropies may result from in-plane and out-of-plane dipoles associated with intra- and interlayer excitations, respectively. Here, we resolve the orientation of luminescent excitons and isolate photoluminescence signatures arising from distinct intra- and interlayer optical transitions. Combining analytical calculations with energy- and momentum-resolved spectroscopy, we distinguish between in-plane and out-of-plane oriented excitons in materials with weak or strong interlayer coupling—MoS2 and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively. We demonstrate that photoluminescence from MoS2 mono-, bi- and trilayers originates solely from in-plane excitons, whereas PTCDA supports distinct in-plane and out-of-plane exciton species with different spectra, dipole strengths and temporal dynamics. The insights provided by this work are important for understanding fundamental excitonic properties in nanomaterials and designing optical systems that efficiently excite and collect light from exciton species with different orientations. Energy-momentum spectroscopy is used to identify distinct intra- and interlayer exciton species in MoS2 and a perylene derivative.