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,

Molybdenum telluride (MoTe2) has emerged as a special member in the family of two-dimensional transition metal dichalcogenide semiconductors, owing to the strong spin-orbit coupling and relatively small energy gap, which offers new applications in valleytronic and excitonic devices. Here we successfully demonstrated the electrical modulation of negatively charged (X(-)), neutral (X(0)), and positively charged (X(+)) excitons in monolayer MoTe2 via photoluminescence spectroscopy. The binding energies of X(+) and X(-) were measured to be ∼24 and ∼27 meV, respectively.The exciton binding energy of monolayer MoTe2 was measured to be 0.58 ± 0.08 eV via photoluminescence excitation spectroscopy, which matches well with our calculated value of 0.64 eV.

The anisotropic nature of the new two-dimensional (2D) material phosphorene 1-9 , in contrast to other 2D materials such as graphene 10 A neutral exciton is a bound quasi-particle state between one electron and one hole through a Coulomb interaction, similar to a neutral hydrogen atom. A trion is a charged exciton composed of two electrons and one hole (or two holes and one electron), analogous to H -(or H2 + ) 20 . Trions have been of considerable interest for the fundamental studies of many-body interactions, such as carrier multiplication and Wigner crystallization 21 . In contrast to the exciton, a trion has an extra charge with nonzero spin, which can be used for spin manipulation 22,23 . More importantly, the density of trions can be electrically tuned by the gate voltage, enabling remarkable optoelectronic applications [18][19][20]24,25 . For these purposes, a large trion binding energy is critical in order to overcome the room-temperature thermal fluctuations as well as to widen the spectral tuning range. The dimensional confinement is the dominating factor that determines the binding energy of trions. In quasi-2D quantum wells, the trion binding energy is only 1-5 meV, and trions are highly unstable, except at cryogenic temperatures 16,17 . The complete separation of the exciton and trion emission peaks was observed at room temperature 16,17 . However, the application of 1D carbon nanotubes for practical optoelectronic devices is intrinsically limited by their small cross-sections. The overall optical responses of such 1D lines are extremely weak. The diverse distribution of the chirality in carbon nanotubes also makes it impossible to assemble a large-size film with uniform optoelectronic responses.While the reduced dimensionality leads to far more attractive exciton and trion properties, the trade-off between the cross-section and the dimensional confinement has hindered the development of useful excitonic optoelectronic devices.Here, we show that phosphorene presents an intriguing platform to overcome the aforementioned trade-off. We observed quasi-1D trions with ultra-high binding energies up to This new type of material, few-layer phosphorene, is unstable and does not survive well in many standard nanofabrication processes. To overcome the challenge of the instability, we designed special fabrication and characterization techniques. We used mechanical exfoliation to drily transfer 26 a phosphorene flake onto a SiO2/Si substrate (275 nm thermal oxide on n + -doped silicon). The phosphorene was placed near a gold electrode that was pre-patterned on the substrate. Another thick graphite flake was similarly transferred to electrically bridge the phosphorene flake and the gold electrode, forming a MOS device (Figure 1). This fabrication procedure kept the phosphorenes free from chemical contaminations by minimizing the postprocesses after the phosphorene flake was transferred. In the measurement, the gold electrode is grounded, and the n + -doped Si substrate functions as a back gate providing uniform...

The control of exciton and triondynamics in bilayer MoS2 is demonstrated, via the comodulations by both temperature and electric field. The calculations here show that the band structure of bilayer MoS2 changes from indirect at room temperature toward direct nature as temperature decreases, which enables the electrical tunability of the K-K direct PL transition in bilayer MoS2 at low temperature.

We investigate the anisotropic hybrid plasmon-SO phonon dispersion relations in monolayer and double-layer phosphorene systems located on the polar substrates, such as SiO2, h-BN and Al2O3. We calculate these hybrid modes with using the dynamical dielectric function in the RPA by considering the electron-electron interaction and long-range electric field generated by the substrate SO phonons via Fröhlich interaction. In the long-wavelength limit, we obtain some analytical expressions for the hybrid plasmon-SO phonon dispersion relations which represent the behavior of these modes akin to the modes obtaining from the loss function. Our results indicate a strong anisotropy in plasmon-SO phonon modes, whereas they are stronger along the light-mass direction in our heterostructures. Furthermore, we find that the type of substrate has a significant effect on the dispersion relations of the coupled modes. Also, by tuning the misalignment and separation between layers in double-layer phosphorene on polar substrates, we can engineer the hybrid modes.