Comment on "Linear Scaling of the Exciton Binding Energy versus the Band Gap of Two-Dimensional Materials"

High mobility and enhanced photoelectric performance of two-dimensional ternary compounds NaCuX (X = S, Se, and Te)

Monolayer ternary materials NaCuX (X = S, Se, and Te) show high mobilities and strong optical absorption in the visible region.

Me-graphene: a graphene allotrope with near zero Poisson's ratio, sizeable band gap, and high carrier mobility

The exploration of new two-dimensional (2D) allotropes of carbon has attracted great research attention after graphene, but experiment-feasible graphene allotropes with novel properties are still rare. Here, we predict a new allotrope of graphene, named Me-graphene, composed of both sp2- and sp3-hybridized carbon by topological assembly of C-(C3H2)4 molecules. With a transitional ratio of sp2- and sp3-hybridized carbon atoms (12 : 1) between those of graphene (1 : 0) and penta-graphene (2 : 1), Me-graphene has transition properties between those of graphene and penta-graphene, such as energy, band gap, and Poisson's ratio. Unusually, Me-graphene exhibits a near zero Poisson's ratio of from -0.002 to 0.009 in the xy-plane (or called "anepirretic"), different from that of graphene (0.169) and penta-graphene (-0.068). More importantly, the near zero Poisson's ratio behavior remains in a large strain range, being less than ±0.02 for strain from -15% to +3%. Me-graphene possesses an indirect band gap of 2.04 eV, as a transition of graphene (semimetal) and penta-graphene (wide band gap), and turns into a direct-bandgap semiconductor with an enlarged band gap of 2.62 eV under compressive strain. It possesses high hole mobility of 1.60 × 105 cm2 V-1 s-1 at 300 K. Me-Graphene has potential applications in electronic, photoelectric and high-speed mechatronic devices. The transitional properties related to the ratio of sp2- and sp3-hybridized carbon atoms are inspiring for searching for new graphene allotropes with combinational properties.

Versatile two-dimensional silicon diphosphide (SiP2) for photocatalytic water splitting

The development of two-dimensional (2D) photocatalysts with excellent visible light absorption and favorable band alignment is critical for highly-efficient water splitting. Here we systematically study the structural, electronic and optical properties of an experimentally unexplored 2D Silicon Diphosphide (SiP2) based on density functional theory (DFT). We found that the single-layer SiP2 is highly feasible to obtain experimentally by mechanical cleavage and it is dynamically stable by analyzing its vibrational normal mode. Two dimensional SiP2 possesses a direct band gap of 2.25 eV, which is much smaller than those of more widely studied photocatalysts including titania (3.2 eV) and graphitic carbon nitride (2.7 eV), thus displaying excellent ability for sunlight harvesting. Most interestingly, the positions of the conduction band minimum (CBM) and valence band maximum (VBM) in 2D SiP2 fit perfectly the water oxidation and reduction potentials, making it a potential new 2D material that is suitable as a nanoscale photocatalyst for photo-electrochemical water splitting.

Optically Discriminating Carrier-Induced Quasiparticle Band Gap and Exciton Energy Renormalization in Monolayer MoS_{2}

Optoelectronic excitations in monolayer MoS_{2} manifest from a hierarchy of electrically tunable, Coulombic free-carrier and excitonic many-body phenomena. Investigating the fundamental interactions underpinning these phenomena-critical to both many-body physics exploration and device applications-presents challenges, however, due to a complex balance of competing optoelectronic effects and interdependent properties. Here, optical detection of bound- and free-carrier photoexcitations is used to directly quantify carrier-induced changes of the quasiparticle band gap and exciton binding energies. The results explicitly disentangle the competing effects and highlight longstanding theoretical predictions of large carrier-induced band gap and exciton renormalization in two-dimensional semiconductors.