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.

Chirality plays a significant role in the physical properties and biological functions of many biological materials, e.g., climbing tendrils and twisted leaves, which exhibit chiral growth. However, the mechanisms underlying the chiral growth of biological materials remain unclear. In this paper, we investigate how the Towel Gourd tendrils achieve their chiral growth. Our experiments reveal that the tendrils have a hierarchy of chirality, which transfers from the lower levels to the higher. The change in the helical angle of cellulose fibrils at the subcellular level induces an intrinsic torsion of tendrils, leading to the formation of the helical morphology of tendril filaments. A chirality transfer model is presented to elucidate the chiral growth of tendrils. This present study may help understand various chiral phenomena observed in biological materials. It also suggests that chirality transfer can be utilized in the development of hierarchically chiral materials having unique properties. Many biological materials, such as climbing plant tendrils 1,2 , flower petals of Paphiopedilum dianthum 3 , and snail shells 4 , exhibit chiral growth. In these natural materials, there are a number of distributed chiral elements, e.g., biomacromolecules, which lead to the formation of various chiral morphologies and surface patterns at the macroscopic scale. For example, helices and twisted belts are often observed to form in growing biological materials. These chiral morphologies generally tend to a specific handedness, either right or left 5 . A well-known example is climbing tendrils, which help some climbing plants attach to trees or other objects and to provide supporting forces 2 . The investigation of the mechanisms underlying the chiral growth of biological materials is a fundamental and important issue in not only developmental biology but also materials science and technology.The chiral shapes of some biological and artificial materials may origin from the asymmetry of growth, deswelling 6,7 , surface stresses 8,9 and other physical quantities associated with anisotropic atomic or molecular structures 10 . For example, the formation of twisting belts of Bauhinia seed pods 6 and chiral polymer lamellae 3 has been attributed to the anisotropy of material properties and surface stresses, respectively. From the view point of chirality transfer, the chiral morphologies of many biological materials originate from the chirality of their microscopic constituent building blocks. Typical examples include the flagellar filaments of bacteria [11][12][13] , the flower petals of Paphiopedilumdilum dianthum 3 , and the stork's bill awns 14,15 . Their chiral growth is caused by the helical arrangements of protein lattices, cortical microtubules, and tilted celluloses at the micro scale, respectively. The helical tendrils of climbing plants have attracted the interest of many scientists since the pioneering work of Charles Darwin 2 . A recent study suggested that Cucumber tendril coiling might be induced by the asymmetric contra...

The oscillatory behavior of a double-walled carbon nanotubes with a rotating inner tube is investigated using molecular dynamics simulation. In the simulation, one end of the outer tube is assumed to be fixed and the other is free. Without any prepullout of the rotating inner tube, it is interesting to observe that self-excited oscillation can be triggered by nonequilibrium attraction of the ends of two tubes. The oscillation amplitude increases until it reaches its maximum with decrease of the rotating speed of the inner tube. The oscillation of a bitube is sensitive to the gap between two walls. Numerical results also indicate that the zigzag/zigzag commensurate model with a larger gap of >0.335 nm can act as a terahertz oscillator, and the armchair/zigzag incommensurate model plays the role of a high amplitude oscillator with the frequency of 1 GHz. An oblique chiral model with a smaller gap of <0.335 nm is unsuitable for the oscillator because of the steep damping of oscillation.

The lipid bilayer plays a crucial role in gating of mechanosensitive (MS) channels. Hence it is imperative to elucidate the rheological properties of lipid membranes. Herein we introduce a framework to characterize the mechanical properties of lipid bilayers by combining micropipette aspiration (MA) with theoretical modeling. Our results reveal that excised liposome patch fluorometry is superior to traditional cell-attached MA for measuring the intrinsic mechanical properties of lipid bilayers. The computational results also indicate that unlike the uniform bilayer tension estimated by Laplace's law, bilayer tension is not uniform across the membrane patch area. Instead, the highest tension is seen at the apex of the patch and the lowest tension is encountered near the pipette wall. More importantly, there is only a negligible difference between the stress profiles of the outer and inner monolayers in the cellattached configuration, whereas a substantial difference (∼30%) is observed in the excised configuration. Our results have farreaching consequences for the biophysical studies of MS channels and ion channels in general, using the patch-clamp technique, and begin to unravel the difference in activity seen between MS channels in different experimental paradigms.MscL | azolectin | electrophysiology | finite element modeling L iposome reconstitution has been used for both functional and structural studies of bacterial mechanosensitive (MS) channels (MscL and MscS) for many years (1-4) and also more recently for the study of eukaryotic MS channels (5-8). Most frequently azolectin, a crude extract of phospholipids from animal and plant tissue consisting mainly of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol (PI) (9), has been used for liposome preparations. Given the frequent use of azolectin bilayers for the study of MS channels and the importance of protein-lipid interactions in MS channel gating it is imperative to consider the best way to accurately calculate and assess its material properties. This is also essential for studies of voltage-and ligand-gated ion channels as the activity of these proteins can also be modulated by membrane tension (6-8, 10-12).The mechanical properties of different types of lipids have been studied extensively using the micropipette aspiration (MA) approach (13-18). These experiments have shown that the mechanical properties of lipids differ considerably and that lipid bilayers exhibit elastic behavior (14,15,18,19). The MA technique is versatile and has the advantage of exerting a wide range of aspiration pressures on a specific portion of a bilayer (20-23).Here we use this technique in combination with finite-element (FE) simulation to examine in depth the mechanical properties of azolectin bilayers in both cell-attached and excised patch configurations (SI Materials and Methods). It is widely known that lipid behavior is vastly different at low (<0.5 mN/m) compared with high tensions, due to thermal shape fluctuations (24). Given that the m...