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Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe 2 -WSe 2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. We find that their energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of B1.8 ns, an order of magnitude longer than intralayer excitons in monolayers. Our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.

Abstract:Single quantum emitters (SQEs) are at the heart of quantum optics 1 We assign this fine structure to two excitonic eigen-modes whose degeneracy is lifted by a large ~0.71 meV coupling, likely due to the electron-hole exchange interaction in presence of anisotropy 8 . Magneto-optical measurements also reveal an exciton g-factor of ~8.7, several times larger than that of delocalized valley excitons 9-12 . In addition to their fundamental importance, establishing new SQEs in 2D quantum materials could give rise to practical advantages in quantum information processing, such as efficient photon extraction and high integratability and scalability. Here, we report the first observation of photon antibunching from localized SQEs in tungsten-diselenide (WSe2) monolayers. WSe2 monolayers are grown on top of a 300 nm SiO2 on silicon substrate by physical vapor transport 26 , a scalable synthesis approach (see Methods).For the optical experiments, the monolayers are held in vacuum inside a cryostat at 4.2 K, where a magnetic field is applied perpendicular to the sample plane (Faraday geometry).Experiments are performed in the reflection geometry where a confocal microscope allows 3 for both laser excitation with a beam focal spot of ~1 µm and collection of the emission (see Methods and Supplementary Fig. S1).The WSe2 monolayer is excited using a continuous-wave (cw) laser at a wavelength of 637 nm. Figure 1a shows the emergence of sharp spectral lines, which are red shifted by ~40-100 meV from the PL of the delocalized valley excitons (see right inset of Fig. 1a). With an excitation power of 6 µW, the peak intensity of the sharp lines are ~500 times stronger than the delocalized valley excitons. The left inset of Fig. 1a shows the fine structure of the highestintensity line (we call SQE1), which is composed of a doublet. The red lines are Lorentzian fits which yield linewidths of ~112 µeV and ~122 µeV (FWHM) and a splitting of 0.68 meV.A statistical histogram on 92 randomly localized emitters from 15 different monolayers is presented in Fig. 1b, yielding linewidths ranging from 58 µeV to 500 µeV, with an average spectral linewidth of 130 µeV, roughly two orders of magnitude smaller than the linewidth of the delocalized exciton PL. The linewidth of these emitters increases dramatically when the temperature is increased (see Supplementary Fig. S2).The sharp lines are highly spatially localized. Figure 1c illustrates a scanning confocal microscope image of the PL from the emission line centered at 1.7186 eV. The isolated bright spots show that the emission is from localized sites, which are likely excitons bound to atomic defects. These sharp lines show strong saturation behavior as a function of laser power. We investigate the power dependence of the SQE1 peak at 1.7156 eV (left inset of Fig. 1a) as an example. Figure 1d shows the integrated counts as a function of excitation power, demonstrating a pronounced saturation behavior similar to an atom-like two-level system.Under excitation with a 3-ps pulsed laser at ...