Optically active quantum dots in monolayer WSe2

Abstract

*Semiconductor quantum dots have emerged as promising candidates for the implementation of quantum information processing, because they allow for a quantum interface between stationary spin qubits and propagating single photons [1][2][3] . In the meantime, transition-metal dichalcogenide monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large bandgap with degenerate valleys and non-zero Berry curvature 4 . Here, we report the observation of zero-dimensional anharmonic quantum emitters, which we refer to as quantum dots, in monolayer tungsten diselenide, with an energy that is 20-100 meV lower than that of two-dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host transition-metal dichalcogenide. The large ∼1 meV zero-field splitting shows that the quantum dots have singlet ground states and an anisotropic confinement that is most probably induced by impurities or defects. The possibility of achieving electrical control in van der Waals heterostructures 5 and to exploit the spin-valley degree of freedom 6 renders transitionmetal-dichalcogenide quantum dots interesting for quantum information processing.Advances in semiconductor-based quantum information processing have been made on two disjoint fronts. While optically active self-assembled quantum dots with deep electron and hole confinement allow for the realization of highly efficient single-photon sources 7 , all-optical manipulation of confined spins 8,9 and a spinphoton quantum interface 3,10 , the random nature of their growth seems to be the biggest hindrance to their use in scalable quantum information processing. In contrast, electrically defined single 11 or double quantum dots 12 hosting one or two excess electrons have been shown to exhibit long spin coherence times together with a clear path towards integrated scalable devices. However, weaker confinement has precluded the possibility to reliably transfer quantum information from spins to photons in these systems. Quantum dots in monolayer transition-metal dichalcogenides (TMDs) have the potential to combine the desirable features of both optically active and electrically defined quantum dots. Although we report tungsten diselenide (WSe 2 ) quantum dots that appear due to uncontrolled impurity-or defect-induced traps, the two-dimensional nature of these materials makes it easier to electrically control the local potentials on a scale of a few tens of nanometres. More importantly, strong electron-hole binding in TMDs suggests that it would be possible to obtain a quantized optical excitation spectrum due to trapping of excitons or trions in large electric field gradients induced by external gates 13 .The samples we s...

Single photon emission from deep-level defects in monolayerWSe2

“…1,10 It can be assumed to be a result of the competition between different kinds of radiative recombination. The 2D excitons and weakly bounded excitons are more easily to be trapped by strong localization centers newly created by hydrostatic pressure.…”

“…[1][2][3][4][5][6][7][8][9] Local strain gradients which occur at the edges are considered to modulate the electronic states of the localized excitons, 2, 9-11 resulting in spatially and spectrally isolated single photon emission. This means that strain engineering is an effective approach to obtain spatially and spectrally isolated quantum emitters in two-dimensional (2D) semiconductors.…”

Optical manipulation of valley pseudospin

“…Our scheme for valley pseudospin manipulation should also extend to the recently discovered quantum-confined states of excitons in TMDC layers [29][30][31][32][33] . For these localized excitonic states, the valley pseudospin degree of freedom is expected to be well preserved 34 , but their lifetimes are far longer than for the free excitons examined in this work.…”

Two-Dimensional MoS2-Graphene-Based Multilayer van der Waals Heterostructures: Enhanced Charge Transfer and Optical Absorption, and Electric-Field Tunable Dirac Point and Band Gap

“…In recent five years, strong interlayer coupling and its effects on novel physical phenomena and diverse potential applications of vdWs heterostructures built from 2D materials has become a hot topic in various disciplines 2,15,16,18,19,[24][25][26][27][28] . The multilayer vdW heterostructures can be experimentally prepared by using state-of-the-art atomically thin 2D materials synthesis techiques, such as, chemical vapour deposition (CVD) to directly grow through the vertical layer-by-layer growth mode 29,30 , and exfoliation techniques with roll-to-roll assembly 2 and co-segregation method 31 .…”