Fig. 3

Ultrafast measurements reveal the formation of interlayer excitons. Waiting time-dependent spectra of a graphene monolayer and b MoSe₂/graphene heterostructure excited by 1.03 eV photons. The maximum intensity at each waiting time is set to be 1. Thus, the figures only reflect the changes of detection frequency dependence rather than decay dynamics. Each contour represents 10% intensity change. c Spectra of MoSe₂/graphene heterostructure and graphene monolayer at 16 ps after excitation with 1.03 eV photons. The graphene signal (both dots and curve) is already zero, but the heterostructure signal is at peak. The peak is fit with a Lorentzian centered at 2185 cm⁻¹ with a width 280 cm⁻¹. Dots are data and the curve is calculation. d Waiting time-dependent transient IR signals detected at 2185 cm⁻¹ of monolayer graphene, MoSe₂/graphene heterostructure, and MoSe₂ monolayer with 1.03 eV excitation. The dynamics of the heterostructure is apparently slower than graphene. Dots are experimental data, and lines are theoretical calculations. The negligibly small signal of MoSe₂ is normalized to the maximum intensity of the graphene signal. e Enlarged waiting time-dependent transient IR signals after 3 ps in Fig. 3d, which illustrates the nonzero tail of the heterostructure signal. f Waiting time-dependent transient IR signals detected at 1860 cm⁻¹ of the monolayer graphene and MoSe₂/graphene heterostructure with 1.03 eV excitation, and MoSe₂ monolayer with 3.1 eV excitation. Different from the 2185 cm⁻¹ detection, within experimental uncertainty the dynamics of graphene and heterostructure are the same. Both are slower than the free-carrier dynamics in MoSe₂ monolayer with 3.1 eV excitation. g Calculated interlayer exciton energy levels of MoSe₂/graphene heterostructure with graphene’s Fermi level at -0.17 eV with a 2D model. The calculations show a binding energy of about 0.3 eV for the interlayer excitons. h Calculated waiting time dependence of electronic dynamics (navy) for graphene with Fermi level at −0.17 eV, and interlayer exciton signal (red). Lines are kinetic analyses. i Calculated huge population difference between excitons and free carrier after 1 ps in Fig. 3h, when the concentration of free carriers is close to 0