Optical tuning of exciton and trion emissions in monolayer phosphorene

Abstract

Monolayer phosphorene provides a unique two-dimensional (2D) platform to investigate the fundamental dynamics of excitons and trions (charged excitons) in reduced dimensions. However, owing to its high instability, unambiguous identification of monolayer phosphorene has been elusive. Consequently, many important fundamental properties, such as exciton dynamics, remain underexplored. We report a rapid, noninvasive, and highly accurate approach based on optical interferometry to determine the layer number of phosphorene, and confirm the results with reliable photoluminescence measurements. Furthermore, we successfully probed the dynamics of excitons and trions in monolayer phosphorene by controlling the photo-carrier injection in a relatively low excitation power range. Based on our measured optical gap and the previously measured electronic energy gap, we determined the exciton binding energy to be ,0.3 eV for the monolayer phosphorene on SiO 2 /Si substrate, which agrees well with theoretical predictions. A huge trion binding energy of ,100 meV was first observed in monolayer phosphorene, which is around five times higher than that in transition metal dichalcogenide (TMD) monolayer semiconductor, such as MoS 2 . The carrier lifetime of exciton emission in monolayer phosphorene was measured to be ,220 ps, which is comparable to those in other 2D TMD semiconductors. Our results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using monolayer phosphorene. Keywords: exciton; monolayer phosphorene; optical injection; two-dimensional materials INTRODUCTION Phosphorene is a recently developed two-dimensional (2D) material that has attracted tremendous attention owing to its unique anisotropic manner 1-6 , layer-dependent direct band gaps 7,8 , and quasi-onedimensional (1D) excitonic nature 9,10 , which are all in drastic contrast with the properties of other 2D materials, such as graphene 11 and transition metal dichalcogenide (TMD) semiconductors [12][13][14] . Monolayer phosphorene has been of particular interest in exploring technological applications and investigating fundamental phenomena, such as 2D quantum confinement and many-body interactions 9,15 . However, such unique 2D materials are unstable in ambient conditions and degrade quickly 8,16 . Particularly, monolayer phosphorene is expected to be much less stable than few-layer phosphorene 16 , hence making its identification and characterization extremely challenging. There is a huge controversy on the identification of very few-layer (one or two layers) phosphorene and thus on their properties [16][17][18] . This controversy was primarily due to the lack of a robust experimental technique to precisely identify the monolayer phosphorene. Consequently, many important fundamental properties of monolayer phosphorene, such as its excitonic nature, remain elusive. In this study, we propose and implement a rapid, noninvasive,

Effective-mass theory for the anisotropic exciton in two-dimensional crystals: Application to phosphorene

“…The agreement is somewhat surprising since this has been measured for phosphorene on a SiO substrate [12]. A more recent experiment, however, reports a smaller value for E X , which is maybe more expected due to the screening of the substrate [17].…”

Direct observation of the layer-dependent electronic structure in phosphorene

“…Previously, photoluminescence (PL) spectroscopy has been used to probe the bandgap of monolayer and few-layer phosphorene 8,[19][20][21][22] . Such studies, however, have their Monolayer and few-layer phosphorene samples are prepared by micromechanical exfoliation of bulk crystals.…”

Field Effect Optoelectronic Modulation of Quantum-Confined Carriers in Black Phosphorus

“…[5][6][7][8][9] Since the first isolation of black phosphorus and demonstration of a field effect device, numerous reports investigating the synthesis and optoelectronic properties of this material have emerged, appropriately summarized in recent reviews. 5,6,[10][11][12] Likewise a number of reports have also appeared on the applications of black phosphorus in fast photodetectors 13 , polarization sensitive detectors, 14 waveguide integrated devices 15 , multispectral photodetectors 16 , visible to near-infrared absorbers 17 and emitters, [18][19][20][21] heterojunction 22 and split gate p-n homojunction photovoltaics 23 , gatetunable van der Waals heterojunctions for digital logic circuits 24,25 and gigahertz frequency transistors in analog electronics 26 . A majority of the studies on both the fundamental optical properties of black phosphorus and applications in optoelectronic devices have explored only the visible frequency range [27][28][29][30] .…”