(Invited) Momentum-Resolved Dielectric Response of Free Standing Black Phosphorus Down to the Monolayer

Abstract

Atomic layers of Black Phosphorus (BP) have been recently isolated, ten years after graphene. BP 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 photoluminescence (PL) and absorption properties of high quality BP single crystals at 2K [FTIR]. The PL intensity appears comparable to high quality InAs crystals confirming the suitability of BP for infrared applications. Two peaks are evidenced at 0.275 eV and 0.26 eV. The highest energy peak is particularly narrow (FWHM = 3.8meV) and is tentatively attributed to exciton recombination, thanks to the temperature dependence of PL spectra from 2K to 300K. Further, we have measured the gap related energy shift of the PL spectra as a function of the BP thickness in a series of mechanically exfoliated samples with thicknesses down to 10 nm, complementary to the previous measures done on thinner samples [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] C. Chen et al., NanoLett., (2019)[4] F. Fossard et al, Phys. Rev. B 96, 115304 (2017)[5] E. Gaufres et al, Nanoletters (2019); DOI: 10.1021/acs.nanolett.9b03928