This study reports the observation of long-lived interlayer excitons in monolayer MoSe₂-WSe₂ heterostructures. Using photoluminescence and photoluminescence excitation spectroscopy, the researchers found that the energy and luminescence intensity of interlayer excitons are highly tunable by an applied vertical gate voltage. They measured an interlayer exciton lifetime of approximately 1.8 ns, which is an order of magnitude longer than that of intralayer excitons in monolayers. This work demonstrates optical pumping of interlayer electric polarization, which may lead to new applications in two-dimensional lasers, light-emitting diodes, and photovoltaic devices. Monolayer transition metal dichalcogenides (TMDs) have been studied extensively due to their unique electronic and optical properties. The MoSe₂-WSe₂ heterostructure, with type-II band alignment, allows for the formation of interlayer excitons, where electrons and holes are localized in different layers. These interlayer excitons have been intensely pursued in bilayer graphene for possible exciton condensation, but direct optical observation has been challenging due to the lack of a sizable bandgap in graphene. Monolayer TMDs with bandgaps in the visible range provide the opportunity to optically pump interlayer excitons, which can be directly observed through photoluminescence (PL) measurements. The researchers directly observed interlayer excitons in vertically stacked monolayer MoSe₂-WSe₂ heterostructures. They found that interlayer exciton PL is enhanced under optical excitation resonant with the intralayer excitons in isolated monolayers, consistent with the interlayer charge transfer resulting from the underlying type-II band structure. They demonstrated the tuning of the interlayer exciton energy by applying a vertical gate voltage, which is consistent with the permanent out-of-plane electric dipole nature of interlayer excitons. Moreover, they found a blue shift in PL energy at increasing excitation power, a hallmark of repulsive dipole-dipole interactions between spatially indirect excitons. Finally, time-resolved PL measurements yielded a lifetime of 1.8 ns, which is at least an order of magnitude longer than that of intralayer excitons. The study shows that monolayer semiconducting heterostructures are a promising platform for exploring new optoelectronic phenomena. The results suggest that interlayer excitons are not defect-related but rather result from the type-II band alignment of the MoSe₂-WSe₂ heterostructure. The interlayer coupling yields the lowest energy bright exciton in the heterostructure, consistent with the temperature dependence of interlayer exciton PL. The long-lived interlayer excitons may lead to new optoelectronic applications, such as photovoltaics and 2D heterostructure nanolasers.This study reports the observation of long-lived interlayer excitons in monolayer MoSe₂-WSe₂ heterostructures. Using photoluminescence and photoluminescence excitation spectroscopy, the researchers found that the energy and luminescence intensity of interlayer excitons are highly tunable by an applied vertical gate voltage. They measured an interlayer exciton lifetime of approximately 1.8 ns, which is an order of magnitude longer than that of intralayer excitons in monolayers. This work demonstrates optical pumping of interlayer electric polarization, which may lead to new applications in two-dimensional lasers, light-emitting diodes, and photovoltaic devices. Monolayer transition metal dichalcogenides (TMDs) have been studied extensively due to their unique electronic and optical properties. The MoSe₂-WSe₂ heterostructure, with type-II band alignment, allows for the formation of interlayer excitons, where electrons and holes are localized in different layers. These interlayer excitons have been intensely pursued in bilayer graphene for possible exciton condensation, but direct optical observation has been challenging due to the lack of a sizable bandgap in graphene. Monolayer TMDs with bandgaps in the visible range provide the opportunity to optically pump interlayer excitons, which can be directly observed through photoluminescence (PL) measurements. The researchers directly observed interlayer excitons in vertically stacked monolayer MoSe₂-WSe₂ heterostructures. They found that interlayer exciton PL is enhanced under optical excitation resonant with the intralayer excitons in isolated monolayers, consistent with the interlayer charge transfer resulting from the underlying type-II band structure. They demonstrated the tuning of the interlayer exciton energy by applying a vertical gate voltage, which is consistent with the permanent out-of-plane electric dipole nature of interlayer excitons. Moreover, they found a blue shift in PL energy at increasing excitation power, a hallmark of repulsive dipole-dipole interactions between spatially indirect excitons. Finally, time-resolved PL measurements yielded a lifetime of 1.8 ns, which is at least an order of magnitude longer than that of intralayer excitons. The study shows that monolayer semiconducting heterostructures are a promising platform for exploring new optoelectronic phenomena. The results suggest that interlayer excitons are not defect-related but rather result from the type-II band alignment of the MoSe₂-WSe₂ heterostructure. The interlayer coupling yields the lowest energy bright exciton in the heterostructure, consistent with the temperature dependence of interlayer exciton PL. The long-lived interlayer excitons may lead to new optoelectronic applications, such as photovoltaics and 2D heterostructure nanolasers.