Properties Of Few-layer Black Phosphorus Research Articles

Surface coordination modification and electrical properties of few-layer black phosphorus exfoliated by the liquid-phase method

Black phosphorus (BP) is a novel multifunctional two-dimensional semiconductor material, which has wide application prospects in nano-electronic devices and optoelectronic devices. However, its poor environmental stability hinders its industrial applications. In this work, liquid exfoliation method was employed to isolate few-layer BP in N-methyl-2-pyrrolidone (NMP), and a titanium sulfonate ligand (TiL4) was synthesized to coordinate with BP to prevent oxidation in air and water. We have shown that TiL4-modified BP nanoplates present the excellent stability against water and oxygen due to the passivation of lone pair electrons in P atoms. The current-voltage curves, which do not change with temperature, indicates that surface coordination modification may passivate the defects or impurities on surfaces of few-layer BP and inhibits the production of carriers from the ionization of these defects or impurities, and the carrier migration mobility is also greatly affected. These results indicated a possible method to tailor the electrical properties of few-layer BP by the surface coordination coverage of titanium sulfonate ligand.

Low-Temperature Associated Interface Influence on the Black Phosphorus Nanoflakes.

The temperature-dependent structure properties of few-layer black phosphorus (BP) nanoflakes supported on the Si substrate and suspended on the holes are investigated using the Raman spectroscopy in the range of 77-293 K. The temperature coefficients of three main Raman modes for supported BP are larger than those of suspended BP because of the interface effects, mainly the tensile strain and the different thermal expansion coefficients (TEC) between the substrate and BP nanoflakes. The out-of-plane vibration is more apt to be affected by the initial strain, whereas the influence on the in-plane vibrations is negligible. During the varied temperature circulation, the temperature coefficients in the heating process are much larger than those in the cooling process because of the interface influences. Thus, the interfacial interaction between BP and the substrate plays an important role in the thermal-mechanical properties of BP. These findings pave a way for the further study of thermal-mechanical related applications of flexible BP nanodevices.

Environmental effects in mechanical properties of few-layer black phosphorus

We report on the mechanical properties of few-layer black phosphorus (BP) nanosheets, in high vacuum and as a function of time of exposure to atmospheric conditions. BP flakes with thicknesses ranging from 4 to 30 nm suspended over circular holes are characterized by nanoindentations using an atomic force microscope tip. From measurements in high vacuum an elastic modulus of 46 ± 10 GPa and breaking strength of 2.4 ± 1 GPa are estimated. Both magnitudes are independent of the thickness of the flakes. Our results show that the exposure to air has substantial influence in the mechanical response of flakes thinner than 6 nm but small effects on thicker flakes.

Weak localization in few-layer black phosphorus

We have conducted a comprehensive investigation into the magneto-transport properties of few-layer black phosphorus in terms of phase coherence length, phase coherence time, and mobility via weak localization measurement and Hall-effect measurement. We present magnetoresistance data showing the weak localization effect in bare p-type few-layer black phosphorus and reveal its strong dependence on temperature and carrier concentration. The measured weak localization agrees well with the Hikami–Larkin–Nagaoka model and the extracted phase coherence length of 104 nm at 350 mK, decreasing as ∼T−0.513+−0.053 with increased temperature. Weak localization measurement allows us to qualitatively probe the temperature-dependent phase coherence time τϕ, which is in agreement with the theory of carrier interaction in the diffusive regime. We also observe the universal conductance fluctuation phenomenon in few-layer black phosphorus within moderate magnetic field and low temperature regime.

Quasiparticle energies, excitons, and optical spectra of few-layer black phosphorus

We report first-principles GW–Bethe–Salpeter-equation (BSE) studies of excited-state properties of few-layer black phosphorus (BP) (phosphorene). With improved GW computational methods, we obtained converged quasiparticle band gaps and optical absorption spectra by the single-shot (G0W0) procedure. Moreover, we reveal fine structures of anisotropic excitons, including the series of one-dimensional like wave functions, spin singlet–triplet splitting, and electron–hole binding energy spectra by solving BSE. An effective-mass model is employed to describe these electron–hole pairs, shedding light on estimating the exciton binding energy of anisotropic two-dimensional semiconductors without expensive ab initio simulations. Finally, the anisotropic optical response of BP is explained by using optical selection rules based on the projected single-particle density of states at band edges.

Significant effect of stacking on the electronic and optical properties of few-layer black phosphorus

The effect of the number of stacking layers and the type of stacking on the electronic and optical properties of bilayer and trilayer black phosphorus are investigated by using first principles calcula- tions within the framework of density functional theory. We find that inclusion of many body effects (i.e., electron-electron and electron-hole interactions) modifies strongly both the electronic and opti- cal properties of black phosphorus. While trilayer black phosphorus with a particular stacking type is found to be a metal by using semilocal functionals, it is predicted to have an electronic band gap of 0.82 eV when many-body effects are taken into account within the G0W0 scheme. Though different stacking types result in similar energetics, the size of the band gap and the optical response of bilayer and trilayer phosphorene is very sensitive to the number of layers and the stacking type. Regardless of the number of layers and the type of stacking, bilayer and trilayer black phosphorus are direct band gap semiconductors whose band gaps vary within a range of 0.3 eV. Stacking arrangments different from the ground state structure in both bilayer and trilayer black phosphorus significantly modify valence bands along the zigzag direction and results in larger hole effective masses. The optical gap of bilayer (trilayer) black phosphorus varies by 0.4 (0.6) eV when changing the stacking type. Due to strong interlayer interaction, some stackings obstruct the observation of the optical excitation of bound excitons within the quasi-particle band gap. In other stackings, the binding energy of bound excitons hardly changes with the type of stacking and is found to be 0.44 (0.30) eV for bilayer (trilayer) phosphorous.

Scaling laws for the band gap and optical response of phosphorene nanoribbons

We report the electronic structure and optical absorption spectra of monolayer black phosphorus (phosphorene) nanoribbons (PNRs) via first-principles simulations. The band gap of PNRs is strongly enhanced by quantum confinement. However, differently orientated PNRs exhibit distinct scaling laws for the band gap vs the ribbon width (w). The band gaps of armchair PNRs scale as 1/(w^2), while zigzag PNRs exhibit a 1/w behavior. These distinct scaling laws reflect a significant implication of the band dispersion of phosphorene: electrons and holes behave as classical particles along the zigzag direction, but resemble relativistic particles along the armchair direction. This unexpected merging of classical and relativistic properties in a single material may produce novel electrical and magnetotransport properties of few-layer black phosphorus and its ribbon structures. Finally, the respective PNRs host electrons and holes with markedly different effective masses and optical responses, which are suitable for a wide range of applications.