A new two-dimensional material: Phosphorene

Abstract

Phosphorene [1,2] is a new family member of two dimensional materials. We observed strong and highly layer dependent photoluminescence in few-layer phosphorene (two to five layers) (Fig. 1). The results confirmed the theoretical prediction that few-layer phosphorene has a direct and layer-sensitive band gap. We also demonstrated that few-layer phosphorene is more sensitive to temperature modulation than graphene and MoS2 in Raman scattering. The anisotropic Raman response in few-layer phosphorene has enabled us to use an optical method to quickly determine the crystalline orientation without tunneling electron microscopy or scanning tunneling microscopy. Our results provide much needed experimental information about the band structures and exciton nature in few-layer phosphorene [3]. Two-dimensional (2D) layered materials, including semi-metallic graphene, semiconducting transition metal dichalcogenides (TMDs) and insulating hexagonal boron nitride (hBN), have been heavily investigated in past decade. Compared with the gapless graphene, most recently investigated TMD semiconductor MoS2 has energy gap in the range of 1.3 eV (bulk) to 1.8 eV (monolayer). MoS2, an indirect band gap material in its bulk form, becomes a direct band gap semiconductor when thinned to a monolayer, enabling significantly enhanced photoluminescence in monolayer MoS2. Black phosphorous (termed as phosphorene) has become a new class of 2D layered material, with predicted layer-dependent band gap ranging from 0.3 eV (bulk) to 1.5 eV (monolayer). Particularly, few-layer phosphorene with narrow band gaps ranging from mid-infrared to near-infrared wavelengths can fill the space between the gapless graphene and the comparably large gap TMD semiconductors. The predicted direct band gap nature in few-layer phosphorene will also enable high-performance optoelectronic devices, compared with the indirect band gap behavior in most few-layer TMD semiconductors. However, so far there has been very little experimental data to confirm the theoretical prediction in few-layer phosphorene.