Optical interband transitions in strained phosphorene

A multi-factor adjustable PtSe2/GaN van der Waals heterostructure with enhanced photocatalytic performance

Two-dimensional van der Waals heterostructures with multiple tunable approaches in electronic and optical properties are highly superior for photocatalysis and novel devices. By applying first-principles calculations, we systematically studied the electronic structure, optical absorption, carrier mobility and solar-to-hydrogen efficiency of PtSe2/GaN heterostructures, which are affected by different thicknesses, varying directions of polarization of the GaN nanosheets and applied mechanical strain of the whole system. The results indicate that these heterostructures exhibit thermodynamic stability at room-temperature (300 K), and most configurations have type-II band alignment, among which the heterostructure consisting of GaN trilayers and PtSe2 shows high visible-light absorption (1.71 × 105 cm-1) and ultra-wide range of pH values (pH = 0-14) for the photocatalytic water splitting reaction and exceedingly high overpotential for the hydrogen evolution reaction (3.375 eV). Simultaneously, being two of the most significant parameters of photocatalysis and devices, the carrier mobility and solar-to-hydrogen efficiency have also been calculated, respectively, reaching up to 1601 cm2 V-1 s-1 and 17.2%. Moreover, the photoelectrical properties can be highly tuned through further biaxial strain engineering; especially, the visible-light absorption can be enhanced to 2.85 × 105 cm-1 by applying 6% compression strain. Thus, the PtSe2/GaN heterostructure we proposed shows a broad prospect for photocatalytic water splitting.

Systematic competition between strain and electric field stimuli in tuning EELS of phosphorene

The strongly anisotropic properties of phosphorene makes it an attractive material for applications in deciding the specific direction for different purposes. Here we have particularly reported the competition between strain and electric field stimuli in evaluating the band gap and electron energy loss spectrum (EELS) of single-layer black phosphorus using the tight-binding method and the Kubo conductivity. We construct possible configurations for this competition and evaluate the interband optical excitations considering the corresponding band gap variations. The band gap increases with the individual electric field, while it increases (decreases) with tensile (compressive) uniaxial in-plane strain. Contrary to the in-plane strains, the uniaxial out-of-plane strain shows a critical strain at which the system suffers from a phase transition. Furthermore, the presence of these stimuli simultaneously results in an extraordinary band gap engineering. Based on the EELS response in the electromagnetic spectrum, the armchair (zigzag) direction is classified into the infrared and visible (ultraviolet) region. We report that the electric field gives rise to the blue shift in the interband optical transitions along the armchair direction, while the compressive/tensile (tensile/compressive) in-plane/out-of-plane strain provides a red (blue) shift. Moreover, we observe an inverse behavior of EELS response to the individual and combined effects of electric field and strains compared to the band gap behavior except at critical out-of-plane strain for which the physical theory of interband excitation is simply violated. Our results provide a new perspective on the applicability of phosphorene in stimulated optical applications.

Linear interband optical refraction and absorption in strained black phosphorene

Strain effects have been widely addressed in monolayer black phosphorus (MBP) due to its significant influence on the orbital hybridization of atoms. In this theoretical contribution, we use the tight-binding model, the Harrison rule and the Kubo formula to describe the optical refraction and absorption of MBP in detail. The analytical study of the band gap in strained MBP demonstrates electronic phase transitions from semiconductor-to-semimetal/metal and semiconductor-to-insulator, in which both the compressive and tensile strains act linearly on the band gap alterations. The critical strains corresponding to these phase transitions are fully characterized as well. Our calculations show that the variation of the refraction inflections and absorption peaks depends on the strained band gap, however; the band gap changes under out-of-plane strains are different than the in-plane ones. The conditions under which this discrepancy is significant and/or negligible are investigated. Moreover, the dedication of minimal/maximal optical refraction and/or absorption in MBP to both in-plane and out-of-plane strains are fully addressed. Our theoretical results clarify the strain-induced interplay between the band gap and optical properties to propose a wide range of applications in nano-optoelectronics.

Band alignment and optical features in Janus-MoSeTe/X(OH)2 (X = Ca, Mg) van der Waals heterostructures

van der Waals heterostructures can be effectively used to enhance the electronic and optical properties and extend the application range of two-dimensional materials.