Robust valley polarization of helium ion modified atomically thin MoS ₂

Abstract

Atomically thin semiconductors have dimensions that are commensurate with critical feature sizes of future optoelectronic devices defined using electron/ion beam lithography. Robustness of their emergent optical and valleytronic properties is essential for typical exposure doses used during fabrication. Here, we explore how focused helium ion bombardement affects the intrinsic vibrational, luminescence and valleytronic properties of atomically thin MoS2. By probing the disorder dependent vibrational response we deduce the interdefect distance by applying a phonon confinement model. We show that the increasing interdefect distance correlates with disorder-related luminscence arising 180 meV below the neutral exciton emission. We perform ab-initio density functional theory of a variety of defect related morphologies, which yield first indications on the origin of the observed additional luminescence. Remarkably, no significant reduction of free exciton valley polarization is observed until the interdefect distance approaches a few nanometers, namely the size of the free exciton Bohr radius. Our findings pave the way for direct writing of sub-10 nm nanoscale valleytronic devices and circuits using focused helium ions. arXiv:1705.01375v2 [cond-mat.mes-hall]

Defect sizing, separation, and substrate effects in ion-irradiated monolayer two-dimensional materials

“…5 (a) and (b) respectively. Both peaks are observed to broaden with increasing disorder and the peak positions shift as expected from previous reports 63 . Broadening begins at a substantially lower dose for Ne + than for He + as expected.…”

Large Spin-Orbit Splitting of Deep In-Gap Defect States of Engineered Sulfur Vacancies in Monolayer WS2

“…As argued in a recent paper [20], chalcogen site defects are abundant but they can be identified as oxygen substituents rather than chalcogen vacancies with a radically different electronic structure [20,24]. Undecorated sulfur vacancies can, however, be generated by annealing or ion bombardment in vacuum as reported previously [4,10,25,26]. Calculations also showed that in vacuum, the chalcogen vacancy has the lowest formation energy of any intrinsic defect in several TMD materials [11,13].…”

Towards the evaluation of defects in MoS2 using cryogenic photoluminescence spectroscopy

“…Room temperature PL spectroscopy reveals some information about the type and density of defects, because the intensity ratio between the A and B excitons depends on the defect density. 25 However, the doping and strain within the MoS 2 layers strongly influence the PL signal 26,27 and the defects can also enhance the PL, 28 owing to the carrier confinement around defects in the TMD layers. Furthermore, the large surface-to-volume ratio of 2D materials causes the chemisorbed 29 and physisorbed 10 molecules to significantly influence the intrinsic properties of these materials.…”

“…As an analogy to the defect-driven phenomena imprinted in the Ramanactive modes, the defect-related PL modes are barely visible at room temperature and are only observable for very defective samples. 28 In contrast, the low-temperature PL spectra change drastically based on the sample quality. At these temperatures, the lattice defects allow an exciton to be more strongly bound to a charged or neutral defect as well as to other quasi-particles to form a bound exciton.…”

“…At these temperatures, the lattice defects allow an exciton to be more strongly bound to a charged or neutral defect as well as to other quasi-particles to form a bound exciton. 28 The temperature determines if the binding energy of a free exciton to a defect (E bind ) is large enough, such that the exciton can be trapped for sufficiently long time, or that the thermal energy is significantly larger and the exciton will be released before a radiative recombination occurs. A recent study on CVD-grown WS 2 showed that the defects consist primarily of mono-and di-sulfur vacancies, which are mostly located on the edges of the flake.…”