Research on hexagonal boron nitride (hBN) has been intensified recently due to the application of hBN as a promising system of single-photon emitters. To date, the single photon origin remains under debate even though many experiments and theoretical calculations have been performed. We have measured the pressure-dependent photoluminescence (PL) spectra of hBN flakes at low temperatures by using a diamond anvil cell device. The absolute values of the pressure coefficients of discrete PL emission lines are all below 15 meV/GPa, which is much lower than the pressure-induced 36 meV/GPa redshift rate of the hBN bandgap. These PL emission lines originate from atom-like localized defect levels confined within the bandgap of the hBN flakes. Interestingly, the experimental results of the pressure-dependent PL emission lines present three different types of pressure responses corresponding to a redshift (negative pressure coefficient), a blueshift (positive pressure coefficient), or even a sign change from negative to positive. Density functional theory calculations indicate the existence of competition between the intralayer and interlayer interaction contributions, which leads to the different pressure-dependent behaviors of the PL peak shift.

Few-layer molybdenum disulfide (MoS2) is advantageous for application in next-generation electronic and optoelectronic devices. For monolayer MoS2, it has been established that both the conduction band minimum (CBM) and the valence band maximum (VBM) occur at the K point in the Brillouin zone. For bilayer MoS2, it is known that the VBM occurs at the Γ point. However, whether the K valley or the Λ valley forms the CBM and the energy difference between them remain disputable. Theoretical calculations have not provided a conclusive answer. In this paper, we demonstrate that a direct K-K to an indirect Λ-K interband transition in bilayer MoS2 can be optically detected by tuning the hydrostatic pressure. A changeover of the CBM from the K valley to the Λ valley is observed to occur under a pressure of approximately 1.5 GPa. The experimental results clearly indicate that the K valley forms the CBM under zero strain, while the Λ valley is approximately 89 ± 9 meV higher in energy.

The energy band structures and related room temperature exciton transitions of monolayer and bilayer tungsten diselenide (WSe2) are investigated using photoluminescence (PL) spectra under hydrostatic pressure up to 5.42 GPa. For monolayer WSe2, it is found that the conduction band Λ valley is 70 ± 30 meV higher than the K valley at zero pressure, and the K-Λ valley crossover happens at a pressure of approximately 2.25 GPa. The PL peak of exciton related to the direct K-K interband transition in monolayer and bilayer WSe2 shows a pressure-induced blue-shift at the rates of 31.5 ± 0.6 and 27 ± 1 meV GPa(-1), respectively. The indirect Λ-K interband transition for monolayer and bilayer WSe2 exhibits a distinctly different pressure response. The pressure coefficient is as small as -3 ± 6 meV GPa(-1) for monolayer, but a much larger value of -22 ± 1 meV GPa(-1) for bilayer WSe2, indicating that the interlayer coupling has a strong effect on the electronic states at the Λ valley.