We systematically explore chemical functionalization of monolayer black phosphorene via chemisorption of oxygen and fluorine atoms. Using the cluster expansion technique, with vary- ing concentration of the adsorbate, we determine the ground states c
onsidering both single- as well as double- side chemisorption, which have novel chemical and electronic properties. The nature of the bandgap depends on the concentration of the adsorbate: for fluorination the direct bandgap first decreases, and then increases while becoming indirect, with increasing fluorination, while for oxidation the bandgap first increases and then decreases, while mostly maintaining its direct nature. Further we find that the unique anisotropic free-carrier effective mass for both the electrons and holes, can be changed and even rotated by 90 degrees, with controlled chemisorption, which can be useful for exploring unusual quantum Hall effect, and novel electronic devices based on phosphorene.
We explore the collective density oscillations of a collection of charged massive Dirac particles, in one, two and three dimensions and their one dimensional superlattice. We calculate the long wavelength limit of the dynamical polarization function
analytically, and use the random phase approximation to obtain the plasmon dispersion. The density dependence of the long wavelength plasmon frequency in massive Dirac systems is found to be different compared to systems with parabolic, and gapless Dirac dispersion. We also calculate the long wavelength plasmon dispersion of a 1d metamaterial made from 1d and 2d massive Dirac plasma. Our analytical results will be useful for exploring the use of massive Dirac materials as electrostatically tunable plasmonic metamaterials and can be experimentally verified by infrared spectroscopy as in the case of graphene [L. Ju. et. al., Nat. Nanotechnol. 6, 630 (2011)].
We investigate the possibility of band structure engineering in the recently predicted 2D layered form of blue phosphorus via an electric field (E$_z$) applied perpendicular to the layer(s). Using density functional theory, we study the effect of a t
ransverse electric field in monolayer, as well as three differently stacked bilayer structures of blue phosphorus. We find that, for E$_z > 0.2$ V/AA the direct energy gap at the $Gamma$ point, which is much larger than the default indirect band gap of mono- and bilayer blue phosphorus, decreases linearly with the increasing electric field; becomes comparable to the default indirect band gap at E$_z approx 0.45 (0.35)$ V/AA for monolayer (bilayers) and decreases further until the semiconductor to metal transition of 2D blue phosphorus takes place at E$_zapprox 0.7 (0.5)$ V/AA for monolayer (bilayers). Calculated values of the electron and hole effective masses along various high symmetry directions in the reciprocal lattice suggests that the mobility of charge carriers is also influenced by the applied electric field.
Collective charge-density modes (plasmons) of the clean two-dimensional unpolarized electron gas are stable, for momentum conservation prevents them from decaying into single-particle excitations. Collective spin-density modes (spin plasmons) possess
no similar protection and rapidly decay by production of electron-hole pairs. Nevertheless, if the electron gas has a sufficiently high degree of spin polarization ($P>1/7$, where $P$ is the ratio of the equilibrium spin density and the total electron density, for a parabolic single-particle spectrum) we find that a long-lived spin-plasmon---a collective mode in which the densities of up and down spins oscillate with opposite phases---can exist within a pseudo gap of the single-particle excitation spectrum. The ensuing collectivization of the spin excitation spectrum is quite remarkable and should be directly visible in Raman scattering experiments. The predicted mode could dramatically improve the efficiency of coupling between spin-wave-generating devices, such as spin-torque oscillators.
