Fig. 4.

Excitation with 3.1 eV also leads to the formation of interlayer excitons. a Spectra of MoSe₂/graphene heterostructure and graphene monolayer at 16 ps after excitation with 3.1 eV photons detected in the frequency range 1280–1380 cm⁻¹ below the excitonic 1s-2p transition frequency. Both signals are zero. b Spectra of MoSe2/graphene heterostructure, MoSe₂, and graphene at 16 ps after excitation with 3.1 eV photons detected in the frequency range 1900–2230 cm⁻¹ covering the excitonic 1s-2p transition frequency. Both graphene and MoSe₂ spectra are flat, whereas that of heterostructure is a peak with a much higher intensity centered at 2156 cm⁻¹, with a Lorentzian width 278 cm⁻¹. c Spectra of MoSe₂/graphene heterostructure and graphene monolayer at 16 ps after excitation with 3.1 eV photons detected in the frequency range 2450–2800 cm⁻¹, above the excitonic 1s-2p transition frequency. The graphene signal is zero, and that of the heterostructure is a nonzero line because of the transition to higher bound and unbound states12. d Waiting time-dependent normalized transient IR signals detected at 2185 cm⁻¹ of MoSe₂/graphene heterostructure, MoSe₂, and graphene. The dynamics of heterostructure is the slowest. The initial absolute intensity ratio of the three samples is 3.4/1.8/1=heterostructure/graphene/MoSe₂. Dots are data, and lines are theoretical calculations. e The electronic dynamics in graphene of heterostructure. Dots are calculations and the line is fitting. f The interlayer excitonic signal in the heterostructure. Dots are experimental data and the line is kinetic calculation. g Illustration of electron/hole gas transition in the heterostructure. An electron/hole pair in ellipse represents an exciton. Excitation with photo energy (3.1 eV) higher than MoSe2 bandgap creates free carriers in both MoSe₂ and graphene. The carriers transfer between the two layers. The carriers collide with each other and transfer energy and momenta so that phonon motions are not necessary for the ultrafast formation of interlayer excitons. Because of the band alignment, more electrons are on the graphene side and more holes are on the MoSe₂ side