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The influence of the atom source operating parameters on the structural and optical properties of In xGa 1 − xN/GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy is investigated. Electron microscopy and photoluminescence spectroscopy reveal a change of the NW tip morphology and an enhancement of the local indium incorporation with increasing nitrogen flux. Tuning the density ratio of atomic-to-excited molecular nitrogen to lower values minimizes the point defect density, which results in a decrease of the non-radiative recombination rate as demonstrated by a combination of continuous wave and time-resolved photoluminescence spectroscopy.The influence of the atom source operating parameters on the structural and optical properties of In xGa 1 − xN/GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy is investigated. Electron microscopy and photoluminescence spectroscopy reveal a change of the NW tip morphology and an enhancement of the local indium incorporation with increasing nitrogen flux. Tuning the density ratio of atomic-to-excited molecular nitrogen to lower values minimizes the point defect density, which results in a decrease of the non-radiative recombination rate as demonstrated by a combination of continuous wave and time-resolved photoluminescence spectroscopy.

Two-dimensional (2D) materials, such as graphene1, boron nitride2, and transition metal dichalcogenides (TMDs)3-5, have sparked wide interest in both device physics and technological applications at the atomic monolayer limit. These 2D monolayers can be stacked together with precise control to form novel van der Waals heterostructures for new functionalities2,6-9. One highly coveted but yet to be realized heterostructure is that of differing monolayer TMDs with type II band alignment10-12. Their application potential hinges on the fabrication, understanding, and control of bonded monolayers, with bound electrons and holes localized in individual monolayers, i.e. interlayer excitons. Here, we report the first observation of interlayer excitons in monolayer MoSe2-WSe2 heterostructures by both photoluminescence and photoluminescence excitation spectroscopy. The energy and luminescence intensity of interlayer excitons are highly tunable by an applied vertical gate voltage, implying electrical control of the heterojunction band-alignment. Using time resolved photoluminescence, we find that the interlayer exciton is long-lived with a lifetime of about 1.8 ns, an order of magnitude longer than intralayer excitons13-16. Our work demonstrates the ability to optically pump interlayer electric polarization and provokes the immediate exploration of interlayer excitons for condensation phenomena, as well as new applications in 2D light-emitting diodes, lasers, and photovoltaic devices., With Supplementary Materials

Coupling degrees of freedom of distinct nature plays a critical role in numerous physical phenomena. The recent emergence of layered materials provides a laboratory for studying the interplay between internal quantum degrees of freedom of electrons. Here, we report experimental signatures of new coupling phenomena connecting real spin with layer pseudospins in bilayer WSe2. In polarization-resolved photoluminescence measurements, we observe large spin orientation of neutral and charged excitons generated by both circularly and linearly polarized light, with a splitting of the trion spectrum into a doublet at large vertical electrical field. These observations can be explained by locking of spin and layer pseudospin in a given valley. Because up and down spin states are localized in opposite layers, spin relaxation is substantially suppressed, while the doublet emerges as a manifestation of electrically induced spin splitting resulting from the interlayer bias. The observed distinctive behavior of the trion doublet under circularly and linearly polarized light excitation further provides spectroscopic evidence of interlayer and intralayer trion species, a promising step toward optical manipulation in van der Waals heterostructures through the control of interlayer excitons., Comment: Original submitted version. To appear in Nature Physics