(Invited) Spectroscopic Properties of Black Phosphorus Crystals and Layers

Black Phosphorus stands out in the 2D-materials panorama by its unique semiconducting properties: direct bandgap which can be tuned by the layer number in a wide range of wavelengths from visible (monolayer) to midinfrared (bulk) [1]. This tunability combined the anisotropy of the structure therefore offers promising perspectives in various fields such as electronics and photonics. However, the fast photooxidation in ambient condition, coupled to a high sensitivity to quantum confinement and dielectric environment in ultrathin BP, make very difficult the investigations on its intrinsic optical properties [2]. Further, as screening effects may strongly affect electronic and spectroscopic properties of 2D materials, it is highly desirable to investigate intrinsic properties of free-standing layers as well the ones of the bulk material which remain poorly known. To start with, we have investigated the infrared photoluminescence of BP single crystals at very low temperature [3]. Near-band-edge recombinations are observed at 2 K, including dominant excitonic transitions at 0.276 eV and a weaker one at 0.278 eV. The free-exciton binding energy is calculated with an anisotropic Wannier-Mott model and found equal to 9.1 meV. On the contrary, the PL intensity quenching of the 0.276 eV peak at high temperature is found with a much smaller activation energy, attributed to the localization of free excitons on a shallow impurity. This analysis leads us to attribute respectively the 0.276 eV and 0.278 eV PL lines to bound excitons (I°X) and free excitons (X) in BP. As a result, the value of bulk BP electronic bandgap is refined to 0.287 eV at 2K, to serve as reference for future work on thin BP layers [3]. As far as the thinnest layers are concerned, which cannot manipulated in air, we have shown that Angular resolved Electron energy loss spectroscopy implemented in Transmission Electron Microscopy (Ar-EELS-TEM) offers a unique way to investigate dielectric response of free-standing layers related to valence band and plasmon excitations with the advantage to get access to their q dispersion and their symmetry properties [4]. By combining this technique with suitable ab initio calculations, we have studied the dielectric response of free-standing BP layers as a function of the number of layers. We found optical bandgap values of 1.9 eV, 1.4 eV and 1.1 eV for the mono- bi- and trilayer respectively. Moreover, by combining our results with a simple variational model, we correlate the exciton energy with the dielectric screening. We hence demonstrate that the variations of the electronic gap are sizeably larger than the variations of the binding energy. Finally, we probe and analyze the volume and surface plasmons dispersion as a function of momentum for the 1-3 BP layers and bulk and highlight a deviation and linearization of the parabolic dispersion with strong anisotropic fingerprints [5]. [1] G. Zhang et al., Nat. Com., 8, 14071, (2017) [2] A. Favron, E. Gaufres et al Nature Mat. 14 (2015) 826. [3] E. Carré et al, 2D Materials (2020) :doi.org/10.1088/2053-1583/abca81 [4] F. Fossard et al, Phys. Rev. B 96, 115304 (2017) [5] E. Gaufres et al, Nanoletters 19, 8303 (2019); DOI: 10.1021/acs.nanolett.9b03928