Phosphorene 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). 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.

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.

Two-dimensional (2D) materials have emerged as promising candidates for miniaturized optoelectronic devices due to their strong inelastic interactions with light. On the other hand, a miniaturized optical system also requires strong elastic light–matter interactions to control the flow of light. Here we report that a single-layer molybdenum disulfide (MoS2) has a giant optical path length (OPL), around one order of magnitude larger than that from a single-layer of graphene. Using such giant OPL to engineer the phase front of optical beams we have demonstrated, to the best of our knowledge, the world’s thinnest optical lens consisting of a few layers of MoS2 less than 6.3 nm thick. By taking advantage of the giant elastic scattering efficiency in ultra-thin high-index 2D materials, we also demonstrated high-efficiency gratings based on a single- or few-layers of MoS2. The capability of manipulating the flow of light in 2D materials opens an exciting avenue towards unprecedented miniaturization of optical components and the integration of advanced optical functionalities. More importantly, the unique and large tunability of the refractive index by electric field in layered MoS2 will enable various applications in electrically tunable atomically thin optical components, such as micro-lenses with electrically tunable focal lengths, electrical tunable phase shifters with ultra-high accuracy, which cannot be realized by conventional bulk solids.

The tightly bound biexcitons found in atomically thin semiconductors have very promising applications for optoelectronic and quantum devices. However, there is a discrepancy between theory and experiment regarding the fundamental structure of these biexcitons. Therefore, the exploration of a biexciton formation mechanism by further experiments is of great importance. Here, we successfully triggered the emission of biexcitons in atomically thin MoSe, via the engineering of three critical parameters: dielectric screening, density of trions, and excitation power. The observed binding energy and formation dynamics of these biexcitons strongly support the model that the biexciton consists of a charge attached to a trion (excited state biexciton) instead of four spatially symmetric particles (ground state biexciton). More importantly, we found that the excited state biexcitons not only can exist at cryogenic temperatures but also can be triggered at room temperature in a freestanding bilayer MoSe. The demonstrated capability of biexciton engineering in atomically thin MoSe provides a route for exploring fundamental many-body interactions and enabling device applications, such as bright entangled photon sources operating at room temperature.