Out-of-plane orientation of luminescent excitons in two-dimensional indium selenide

Abstract

Van der Waals materials offer a wide range of atomic layers with unique properties that can be easily combined to engineer novel electronic and photonic devices. A missing ingredient of the van der Waals platform is a two-dimensional crystal with naturally occurring out-of-plane luminescent dipole orientation. Here we measure the far-field photoluminescence intensity distribution of bulk InSe and two-dimensional InSe, WSe 2 and MoSe 2 . We demonstrate, with the support of ab-initio calculations, that layered InSe flakes sustain luminescent excitons with an intrinsic out-of-plane orientation, in contrast with the in-plane orientation of dipoles we find in two-dimensional WSe 2 and MoSe 2 at room-temperature. These results, combined with the high tunability of the optical response and outstanding transport properties, position layered InSe as a promising semiconductor for novel optoelectronic devices, in particular for hybrid integrated photonic chips which exploit the out-of-plane dipole orientation.

Out-of-plane emission of trions in monolayer WSe2 revealed by whispering gallery modes of dielectric microresonators

“…The nice match observed between experimental and simulation results summarized in Figure S1), although the microsphere used in this experience was slightly bigger (of 4.80 µm in diameter). These results support previous works reporting the OP orientation of radiative free excitons in InSe, 11 and allow us to conclude, by the relatively simple method used here, that radiative recombination processes of OP free excitons conform the whole room-temperature PL spectrum of InSe nanosheets. The dominant OP orientation of dipoles responsible for the room-temperature PL signal of InSe contrasts to these observed for ML WSe2 (Figure 3b).…”

“…31 Nevertheless, unlike 2D TMDs, 2D layers of InSe have revealed to sustain luminescent free excitons with an intrinsic OP orientation. 14 This panorama puts into evidence the necessity of identify, among the different excitonic complexes existing in 2D semiconductors, those with a suitable dipole orientation for their optimal application in the emerging field of integrated photonic circuits based on 2D materials. [32][33][34][35][36][37] This question is particularly relevant for the development of novel devices based on monolayer (ML) TMDs, since strong spin-orbit coupling makes these materials offer, apart from bright and long-lived dark excitons, 38,39 high-order charge-complexes with unexplored dipolar characteristics, such as bound excitons, trions, [40][41][42][43][44] and biexcitons.…”

Guide to optical spectroscopy of layered semiconductors

“…56 ) and InSe 2 (reF. 57 ), for example. Taking into account also optical transitions that rely on phonon absorption or emission or spin-mixing of the different electronic states results in a large number of possible bright and dark optical transitions for a specific material.…”

Design of van der Waals interfaces for broad-spectrum optoelectronics

“…At the Γ-point, the edge of the InSe conduction band and WS 2 valence band consist of atomic orbitals whose angular momentum component in direction perpendicular to the plane is zero. [36,37] Theoretically this prescribes [38] that the polarization of the photons emitted in the interlayer transition should be perpendicular to the interface. To check if this expectation is satisfied we fabricated dedicated devices -by cutting into a lamella configuration a block of 2 µm x 20 µm x 0.7 µm out of h-BN encapsulated 2L-WS 2 /6L-InSe using a focused ion beam (see On the basis of experimental observations similar to those just presented for 2L-InSe/2L-WS 2 interfaces, we conclude that also all other interfaces that we have investigated, based on combinations of thicker WS 2 and InSe multilayers, exhibit direct interlayer transition at Γ.…”