Atomically-resolved exciton emission from single defects in MoS$_2$

Abstract

Understanding how atomic defects shape the nanoscale optical properties of two-dimensional (2D) semiconductors is essential for advancing quantum technologies and optoelectronics. Using scanning tunneling spectroscopy (STS) and luminescence (STML), we correlate the atomic structure and optical fingerprints of individual defects in monolayer MoS$_2$. A bilayer of hexagonal boron nitride (hBN) effectively decouples MoS$_2$ from the graphene substrate, increasing its band gap and extending the defect charge state lifetime. This enables the observation of sharp STML emission lines from MoS$_2$ excitons and trions exhibiting nanoscale sensitivity to local potential fluctuations. We identify the optical signatures of common point defects in MoS$_2$: sulfur vacancies (Vac$_\text{S}^-$), oxygen substitutions (O$_\text{S}$), and negatively charged carbon-hydrogen complexes (CH$_\text{S}^-$). While Vac$_\text{S}^-$ and O$_\text{S}$ only suppress pristine excitonic emission, CH$_\text{S}^-$ generate defect-bound exciton complexes ($A^-X$) about 200\,meV below the MoS$_2$ exciton. Sub-nanometer-resolved STML maps reveal large spectral shifts near charged defects, concurrent with the local band bending expected for band-to-defect optical transitions. These results establish an atomically precise correlation between structure, electronic states, and optical response, enabling deterministic engineering of quantum emitters in 2D materials.

Figures (5)

• Figure 1: Electronic property of MoS$_2$ on ultrathin hBN decoupling layers.a, Sketch of electrically-stimulated MoS$_2$(1L)/hBN(2L)/Gr heterostructure. Both quasi-freestanding epitaxial graphene on SiC and few-layer graphene on hBN/SiO$_2$/Si have been used as substrates, denoted as Gr for simplicity. b, STM topography of bilayer hBN on Gr moiré pattern. c, Large scale STM topography of MoS$_2$(1L)/hBN(2L)/Gr heterostructure with low density of intrinsic isovalent oxygen substitutions (O$_\text{S,top/bottom}$, see inset) and an ion beam-induced negatively charged sulfur vacancy (Vac$_\text{S,top}^-$). d, STS of multilayer (6L) hBN, bilayer hBN, monolayer MoS$_2$/hBN(2L), and epitaxial graphene on SiC. e, Comparison of band gap and band alignment of MoS$_2$/Gr and MoS$_2$/hBN(2L)/Gr. f, STM image of Vac$_\text{S,top}^-$ on MoS$_2$/hBN(2L)/Gr with STS locations from (g) indicated. g, Comparison of Vac$_\text{S,top}^-$ defect states on MoS$_2$/Gr (light red) and MoS$_2$/hBN(2L)/Gr (dark red), respectively. h, Z (height) spectroscopy ($V = -1\,$V) on Vac$_\text{S,top}^-$/MoS$_2$/hBN(2L)/Gr exhibiting current saturation behavior.
• Figure 2: Exciton and trion emission of pristine MoS$_2$.a, STML emission from monolayer MoS$_2$ supported by mono- (blue) and bilayer (purple) hBN, respectively (150 l/mm grating). The spectra location are indicated in the STM topography inset. b, STML emission from pristine MoS$_2$(1L)/hBN(2L)/Gr with a 600 l/mm grating. The exciton $X$ and three trion $X_{1-3}^-$ emission energies extracted from Pseudo-Voigt fits are indicated. c, Voltage dependence of STML emission (150 l/mm grating). d, Valence band edge from d$I$/d$V$ spectra overlaid with the exciton emission intensity (red crosses). e, Current dependence of STML emission (600 l/mm grating). f, Exciton peak energy shift and emission intensity as a function of excitation current. The dashed lines represent linear fits. g, STML emission spectra measured on a 4$\times$4 grid near defects (spectra locations in inset) exhibit variations in the exciton-to-trion emission ratio (600 l/mm grating). The data shown in c-g are recorded on MoS$_2$(1L)/hBN(2L)/Gr.
• Figure 3: Suppressed STML emission at Vac$_{\text{S}}^{-}$ and O$_{\text{S}}$.a, STS spectra on top of the O$_\text{S}$ defect (orange) and pristine MoS$_2$(1L)/hBN(2L)/Gr (blue) revealing a resonant defect state at -2 V, but no in-gap defect state. The STM topography inset illustrates the locations of STS measurements and STML spectra shown in panel (b). b, STML emission recorded directly above and adjacent to the O$_\text{S}$ defect, highlighting suppression of pristine emission due to the defect. c, STS spectra of Vac$^-_{\text{S}}$ and pristine MoS$_2$(1L)/hBN(2L)/Gr, at the positions indicated in the inset. d, Spatially resolved STML emission along a line across Vac$^-_{\text{S}}$, showing a reduction in the pristine emission intensity as the tip approaches the defect. A broadband plasmonic emission is observed when the tip is positioned directly above Vac$^-_{\text{S}}$.
• Figure 4: Defect-bound exciton emission at negatively charged CH$^-_\text{S}$ defect.a, STML spectra of pristine MoS$_2$ (blue) and negatively charged CH$^-_\text{S}$ defect (orange) recorded at -2.8 V and 5 nA. b, STS spectra of CH$^-_\text{S}$ (red) and pristine MoS$_{2}$ (blue). The left inset shows the locations of the STML and STS measurements. The identity of the CH$^-_\text{S}$ defect is verified by cleaving off the H with a voltage pulse (4.5 V, 8 nA), creating C$^-_\text{S}$ with its characteristic vibronic spectrum (right inset) cochraneSpindependentvibronicresponse2021c. c,d, STML line spectra off-center (c) and on-center (d) across CH$^-_\text{S}$ (-2.8 V and 7.5 nA), at the location indicated on the top right. While the MoS$_2$ exciton and trion energies stay constant, the CH$^-_\text{S}$ defect-bound excitons, labelled 1s, 2p, 2s, blue-shift significantly near the defect center. e, 3D plot of X (orange), 2s (purple), 2p (green) and 1s (blue) exciton peaks as a function of energy and spatial coordinate with relative peak intensity indicated by the colorscale. The shaded plane and black lines mark the cross-section shown in (d). f-i, STML intensity maps of the MoS$_2$ exciton ($X$, f) and CH$^-_\text{S}$ excitons (g-i). The map shows the emission peak intensity at each point corrected for shifts in the emission line. Scale bar: 1 nm. Colorscale: kcts/nC/eV.
• Figure 5: Electroluminescence mechanism for pristine MoS$_2$ and defect-bound excitons localized at CH$_\text{S}^-$.a, Schematic level diagram of the MoS$_2$ quasi-particle bands and exciton $X$ and trion $X_{1-3}^-$ energies as derived from STS and STML spectra. $E_\text{B}$ is the exciton binding and $E_\text{opt}$ the emission energy, respectively. b, Many-body level diagram of the pristine MoS$_2$ STML emission via hole attachment to the valence band. c, Corresponding STM double barrier tunneling junction indicating hole attachment (1), neutralization via electron tunneling from the substrate into the excitonic state (2), and radiative recombination (3). d, Schematic level diagram at a negatively charged CH$_\text{S}^-$ defect, with the band bending, band-to-band (blue) and band-to-defect transitions (orange) indicated. e, Many-body level diagram of the $X$ and defect-bound exciton emission $A^{-}X$ via transient hole attachment. f, Corresponding tunneling junction model.