Light-matter interactions in two-dimensional nanomaterial Phosphorene




Zhang, Shuang

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Phosphorene is a new member of the family of two-dimensional (2D) materials. Compared with other 2D materials, phosphorene exhibits a layer-dependent direct bandgap in the range of mid-infrared to near-infrared wavelengths, which can bridge the gap between relatively large bandgap transition metal dichalcogenide semiconductors and gapless graphene. The predicable direct bandgap in few-layer phosphorene will facilitate the delivery of high-performance optoelectronic devices. Besides, the high surface-volume ratio in few-layer phosphorene enables strong light-matter interactions, making phosphorene very promising for applications in different optical components. In this work, the inelastic and elastic light-matter interactions in few-layer phosphorene were investigated. For the study of inelastic light-matter interactions in phosphorene, photoluminescence (PL) and Raman effects were investigated to explore photon-electron and photon-phonon energy transfers respectively. Strong and highly layer-dependent PL was observed in few-layer phosphorene (2-5 layers). The results confirmed the theoretical prediction (by other researchers) that few-layer phosphorene had a direct and layer-sensitive bandgap. The work also demonstrated by Raman scattering that few-layer phosphorene was more sensitive to temperature modulation than graphene and MoS2, which could be due to the superior mechanical flexibility of phosphorene originating from its unique puckered crystal structure. The anisotropic Raman response in few-layer phosphorene enabled the use of a pure optics method to quickly determine crystalline orientation without a tunnelling electron microscope or a scanning tunnelling microscope. The results obtained provided much-needed experimental information about the band structures and exciton nature in few-layer phosphorene, paving the way for various optoelectronic and electronic applications. Further study in terms of inelastic light-matter interaction was undertaken regarding PL emission from excitons and trions in few-layer phosphorene. Electrostatic modulation was applied to tune the density of trions. Thereby, a trion binding energy of 162 meV was found in few-layer phosphorene at room temperature. Such a large binding energy had previously only been observed in truly 1D materials such as carbon nanotubes, whose optoelectronic applications had been severely hindered by their intrinsically small optical cross-sections. Phosphorene offers an elegant way to overcome this hurdle by enabling quasi-1D excitonic and trionic behaviours in a large 2D area, allowing optoelectronic integration. Moreover, the quasi-1D nature of excitonic and trionic dynamics in phosphorene was validated experimentally and theoretically. The implications of the extraordinarily large trion binding energy in a higher-than-one-dimensional material are far-reaching. In addition to inelastic light-matter interactions, elastic light-matter interactions in few-layer phosphorene were also studied by optical path length (OPL) and micro-lens. The giant OPL achieved in few-layer phosphorene was more than 20 times the physical thickness achieved in this few-layer phosphorene. Based on the layer-dependent OPL information, the number of layers could be more quickly and accurately identified compared with the conventional method for identifying number of layers. Black-phosphorus-based micro-lens optical properties were also studied to obtain information about elastic light-matter interactions in black phosphorus and its oxides.



Light-matter interactions, Phosphorene




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