Exciton and Trion Dynamics in Bilayer MoS₂

PL spectra from bilayer MoS 2 could not be tuned by electric fi eld at room temperature owing to its indirect band gap manner, [ 7 ] which makes the exciton and trion dynamics in bilayer MoS 2 still underexplored. In this paper, we demonstrate the valley control of exciton and trion dynamics in bilayer MoS 2 , via the comodulations by both temperature and electric fi eld. We found that as temperature decreases from 300 to 100 K, the valley of the conduction band at Λ point (named as Λ valley) moves down relatively to the valley at K point (named as K valley) in monolayer MoS 2 , while the Λ valley rises up relatively to the K valley in bilayer MoS 2 ( Figure 1 ). This opposite temperature dependence of the valley movements in monoand bilayer MoS 2 can signifi cantly change the photocarrier relaxation pathways in their PL processes, which leads to more than twice faster increasing of the measured PL intensity from bilayer MoS 2 than that from monolayer MoS 2 as temperature decreases. More importantly, the rising up of the Λ valley in bilayer MoS 2 at low temperature offers the electrical tunability of the K–K direct PL transition, enabling the exploration of the exciton and trion dynamics in bilayer MoS 2 . The trion binding energy of bilayer MoS 2 was fi rstly measured to be 27 meV at 83 K, which is smaller than the measured trion binding energy of 39 meV in monolayer MoS 2 . Our fi ndings provide insight into exciton and trion dynamics in bilayer MoS 2 and enable new applications in photonics and optoelectronics. [ 1,2,17 ] Moreover, the comodulation technique by both temperature and electric fi eld provides a novel method to explore the fundamental phenomena in few-layer 2D semiconductors. We calculated the band structures of monoand bilayer MoS 2 at various temperatures (Figure 1 ) within density functional theory (DFT) molecular dynamics using Perdew– Wang (PW) generalized gradient approximation (GGA) based on a real-space numerical atomic orbital code. [ 18 ] From the simulation results, as temperature decreases from 300 to 100 K, the Γ peak in the valence band (named as Γ peak) of 2L MoS 2 signifi cantly moves down relative to the K peak, which drives the indirect band structure of bilayer MoS 2 at room temperature approaching direct band structure at the temperature range of ≈50–250 K (Figure 1 and Figure S1, Supporting Information). Meanwhile, Λ valley moves down relatively to K valley in monolayer MoS 2 , while Λ valley moves up relatively to K valley in bilayer MoS 2 (Figure 1 a,c), which drives the direct band structure of monolayer MoS 2 at room temperature approaching indirect band structure at the temperature range of ≈50–210 K (Figure 1 and Figures S1 DOI: 10.1002/smll.201501949 MoS 2