A natural indirect-to-direct band gap transition in artificially fabricated MoS₂ and MoSe₂ flowers

Twisted bilayer (tB) transition metal dichalcogenide (TMD) structures formed from two pieces of a periodic pattern overlaid with a relative twist manifest novel electronic and optical properties and correlated electronic phenomena. Here, twisted flower-like MoS₂ and MoSe₂ bilayers were artificially fabricated by the chemical vapor deposition (CVD) method. Photoluminescence (PL) studies demonstrated that an energy band structural transition from the indirect gap to the direct gap happened in the region away from the flower center in tB MoS₂ (MoSe₂) flower patterns, accompanied by an enhanced PL intensity. The indirect-to-direct-gap transition in the tB-MoS₂ (MoSe₂) flower dominantly originated from a gradually enlarged interlayer spacing and thus, interlayer decoupling during the spiral growth of tB flower patterns. Meanwhile, the expanded interlayer spacing resulted in a decreased effective mass of the electrons. This means that the charged exciton (trion) population was reduced and the neutral exciton density was increased to obtain the upgraded PL intensity in the off-center region. Our experimental results were further evidenced by the density functional theory (DFT) calculations of the energy band structures and the effective masses of electrons and holes for the artificial tB-MoS₂ flower with different interlayer spacings. The single-layer behavior of tB flower-like homobilayers provided a viable route to finely manipulate the energy band gap and the corresponding exotic optical properties by locally tuning the stacked structures and to satisfy the real requirement in TMD-based optoelectronic devices.