Fig. 1. Geometry and electronic structure of the MoS2–GaN heterostructure calculated from the first principles. (a) Top view and (b) side view of the most stable interface structure (II). The unit cell is marked as a blue dashed parallelogram in panel (a). The dashed bonds in (b) denote their mostly van der Waals character. (c) Relative band edge positions of bulk GaN and four interface structures. The bands of the isolated MoS2 monolayer are not aligned, and the panel indicates only the value of the band gap. More details are provided in Fig. 7 in Appendix A. (d) Electronic structure of the interface calculated within the semi-infinite surface model and projected on MoS2 (red) and GaN states (blue).

Fig. 2. (a), (b), (c) Raman spectrum of MoS2 on quartz, GaN, and MoS2–GaN interface, respectively, showing the active Raman modes. There are various Raman modes activated at the MoS2–GaN interface. (d) Steady-state absorption spectrum of MoS2 on quartz (black) and MoS2–GaN interface (red). (e) The imaginary part of permittivity of a freestanding MoS2 layer (black) and MoS2–GaN interface (red). The insets in (d) and (e) show the corresponding spectrum of GaN.

Fig. 3. (a) Transient absorption spectrum of MoS2 showing the effect of the GaN layer and the effect of excitation energy on excitonic absorption bands. The black, red, and blue colors represent the MoS2 on quartz with 2.33 eV pump excitation, MoS2 on GaN with 2.33 eV pump excitation, and MoS2 on GaN with 3.54 eV pump excitation, respectively. The inset shows the schematics of the interface phonon coupling and the charge transfer at the interface. (b) Power dependence of the transient absorption spectrum of MoS2 on GaN with 2.33 eV. The black, red, blue, and pink colors represent the spectrum at pump fluence of 93.75, 187.5, 281, and 375  μJ/cm2, respectively.

Fig. 4. (a) Decay kinetics showing the recovery of probe absorption at the (a) A, (b) B, and (c) C excitonic bands of MoS2 showing the effect of the GaN layer and the effect of excitation energy on excitonic absorption bands. The black, red, and blue colors represent the MoS2 on quartz with 2.33 eV pump excitation, MoS2 on GaN with 2.33 eV pump excitation, and MoS2 on GaN with 3.54 eV pump excitation, respectively.

Fig. 5. PL emission spectrum. (a) The emission spectrum heterostructure showing the MoS2 and GaN emission bands. PL emission band of MoS2 on (b) quartz substrate and (c) GaN substrate. The fitted peaks represent the contribution due to trion (blue) and exciton (cyan) recombination. (d) GaN band-edge emission from the GaN (black) and MoS2–GaN (red) heterostructures. The inset shows the defect band emission after heterostructure formation. PL emission characteristics at 30 K showing the LO phonon replica of GaN in (e) GaN and (f) MoS2–GaN.

Fig. 6. Top and side views of different stacking configurations I–IV of the MoS2–GaN heterostructure. The numbers in the bottom are the energy differences with respect to the most stable structure II.

Fig. 7. Calculated energy band alignment diagram of 2D MoS2 and bulk GaN. The valence band levels are aligned with respect to the vacuum, and the valence band offset is calculated by choosing as a reference the most stable MoS2–GaN heterostructure (model II in Fig. 1).

Fig. 8. (a) AFM image of MoS2 on GaN substrate. (b) Height profile of MoS2 layer along the line in (a).

Fig. 9. Power-dependent recovery of probe absorption at the A excitonic band of MoS2 on GaN. The black, red, blue, and pink colors represent the spectrum at pump fluence of 93.75, 187.5, 281, and 375  μJ/cm2, respectively.