Strain-Tunable Opto-electronics in PdS$_2$ Monolayer: the Role of Band Nesting and Carrier-Phonon Scattering

TL;DR

This study investigates how biaxial strain tunes the optoelectronic response of a PdS2 monolayer using first-principles methods. It identifies a robust band nesting between the highest valence and lowest conduction bands as the origin of a continuously redshifting optical absorption peak under strain. It additionally shows that carrier mobility rises under strain not only from reduced effective mass and deformation potential but primarily due to a strain-induced suppression of carrier-phonon scattering, evidenced by narrower carrier linewidths. The results provide a physical mechanism-driven path to design continuously tunable flexible optoelectronic devices in 2D semiconductors with similar band-structure features.

Abstract

Strain engineering is a powerful strategy for tuning the optoelectronic properties in two-dimensional materials, yet the underlying mechanisms governing their strain response are often not fully elucidated. In this work, our first-principle calculations show that the penta-orthorhombic PdS$_2$ monolayer exhibits two key strain-tunable properties: a continuous redshift of its main optical absorption peak from $\sim$2.0 to $\sim$1.6~eV and enhancement in carrier mobility, with a more than threefold increase for electron under 0--4\% biaxial tensile strain. Subsequent analysis reveals that the tunable optical response originates from a robust band nesting feature between the highest valence and lowest conduction bands, which is preserved across the Brillouin zone under biaxial strain. For the carrier transport, deformation potential theory predicts mobility increasing with strain, strongly correlating with the reduction of carrier effective mass. Our first-principles calculations show a strain-induced monotonic decrease in carrier linewidths near the band edges, indicating suppressed carrier-phonon scattering and longer carrier lifetime as the origin of the mobility enhancement. Our work establishes a pathway for engineering the optoelectronic response in 2D semiconductors where strong band nesting governs the optical properties and paves the way for the rational design of continuously tunable flexible optoelectronic devices.

Figures (6)

• Figure 1: (a) The crystal structure of PdS$_2$ monolayer. Top view (upper panel) and side view (lower panel). The PdS$_2$ monolayer exhibits a penta-orthorhombic structure with lattice parameters of $a$ = 5.47 Å and $b$ = 5.57 Å. (b) Phonon dispersion of the PdS$_2$ monolayer without imaginary frequencies, confirming dynamical stability.
• Figure 2: The electronic band structure (left panel) and projected density of states (right panel) of PdS$_2$ monolayer. The band structure highlights the lowest conduction band (CB, green), the highest valence band (VB, blue), and the second-highest valence band (VB-1, orange). Nearly constant CB-VB separation along the $k$-paths reveals band nesting, while the projected density of states (PDOS) indicates that the states close to the VB and CB are mainly derived from the $d$ orbitals of Pd atoms (black line) and the $p$ orbitals of S atoms (red line).
• Figure 3: Imaginary part of the dielectric function $\mathrm{Im}(\varepsilon_{2})$ (left vertical axis) and joint density of states (JDOS, right vertical axis) between the highest valence band and lowest conduction band.
• Figure 4: Band energy difference map between the highest valence band (VB) and lowest conduction band (CB) of PdS$_2$ monolayer across the Brillouin zone under (a) 0 and (b) 4%, with high symmetry points in the first Brillouin zone labeled. (c) Band energy difference along high-symmetry paths for transitions from the VB (solid lines) and VB-1 (dashed lines) to the CB. Red and blue curves correspond to 0% and 4% strain, respectively. (d) Peak energy (horizontal axis position) of \ref{['fig:epsilon']} of the imaginary part of the dielectric function $\mathrm{Im}(\varepsilon_{2})$ and joint density of states (JDOS) versus strain.
• Figure 5: Plotted for both the $a$ and $b$ directions are: (a) carrier effective mass, and (b) the resulting carrier mobility as a function of biaxial tensile strain. The elastic constant and deformation potential under biaxial tensile strain are shown in Fig. S5 and Fig. S6, respectively.