Control of the Exciton Radiative Lifetime in van der Waals Heterostructures

Abstract

Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strength. Encapsulation of monolayers such as MoSe2 in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect. The time-resolved photoluminescence measurements show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizeable enhancement of the exciton radiative decay rate. Understanding the role of these electrodynamical effects allow us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.

Near-Unity Light Absorption in a Monolayer WS2 Van der Waals Heterostructure Cavity

“…Furthermore, the extracted γ r from the two cavities yields 2.2 meV and 70 µeV in the "on" and "off" regions, respectively. This is in agreement with the Purcell effect linewidth modulation obtained by Ref 54 .…”

Measurement of the spin-forbidden dark excitons in MoS2 and MoSe2 monolayers

“…We fabricated high-quality samples by encapsulating MoS 2 and MoSe 2 MLs in hexagonal boron nitride (hBN). The heterostructures are fabricated onto SiO 2 (80 nm)/Si substratesusing a dry stamping technique 12 , 41 . The typical thickness of the top (bottom) hBN layer is ∼10 (200) nm and the typical in-plane size of the ML is ∼10 × 10 μm 2 .…”

Filtering the photoluminescence spectra of atomically thin semiconductors with graphene

“…Our approach is systematically applied to four tungsten and molybdenumbased TMDs and establishes TMD/graphene heterostructures as a unique set of opto-electronic building blocks. Graphene not only endows TMDs monolayers with superior optical performance and enhanced photostability but also provides an excellent electrical contact, suitable for TMDbased electroluminescent systems emitting visible and near-infrared photons at near THz rate with linewidths approaching the lifetime limit.TMD monolayers (thereafter simply denoted TMD), such as MoS 2 , MoSe 2 , WS 2 , WSe 2 are direct-bandgap semiconductors [1,2], featuring short Bohr radii, large exciton binding energy (near 500 meV [3]) and picosecond excitonic radiative lifetimes at low temperature [4][5][6], all arising from their strong 2D Coulomb interactions, reduced dielectric screening and large effective masses [3,7]. Since the first investigations of light emission from TMDs, it has been clear that their lowtemperature spectra was composed of at least two prominent features, stemming from bright neutral excitons (X 0 ) and charged excitons (trions, X ) [8-10] endowed with a binding energy of typically 20 to 40 meV relative to X 0 .…”

“…As a result, X 0 and X emission dominate the PL spectrum of Mo-based TMDs [9], whereas the emission spectra of W-based TMDs display a complex series of lines stemming from X 0 , bi-excitons (XX 0 )[12-15], charged excitonic states (including X [10,16] and charged biexcitons (XX ) [12][13][14][15]), spin-dark excitons [17][18][19], defectinduced emission and exciton-phonon sidebands [20].Considerable progress has been made to determin-istically observe intrinsic TMD emission features. In particular, encapsulation of TMDs in hexagonal boron nitride (BN) films results in narrower neutral exciton linewidth [21,22], approaching the radiative limit [6,23,24], without however, getting rid of the other emission features mentioned above. Even in electrostatically gated devices tuned near the charge neutrality point, sizeable emission sidebands remain observable at energies close to the X feature, suggesting residual charge inhomogeneity [9,10] or intrinsic contributions from longer-lived exciton-phonon replicas [20].The complex emission spectra of TMD stimulate lively scientific debates.…”

“…TMD monolayers (thereafter simply denoted TMD), such as MoS 2 , MoSe 2 , WS 2 , WSe 2 are direct-bandgap semiconductors [1,2], featuring short Bohr radii, large exciton binding energy (near 500 meV [3]) and picosecond excitonic radiative lifetimes at low temperature [4][5][6], all arising from their strong 2D Coulomb interactions, reduced dielectric screening and large effective masses [3,7]. Since the first investigations of light emission from TMDs, it has been clear that their lowtemperature spectra was composed of at least two prominent features, stemming from bright neutral excitons (X 0 ) and charged excitons (trions, X ) [8][9][10] endowed with a binding energy of typically 20 to 40 meV relative to X 0 .…”