Uncovering the atomic structure of substitutional platinum dopants in MoS$_2$ with single-sideband ptychography

TL;DR

This work addresses atomic-scale control and verification of substitutional Pt doping in MoS2 by first creating vacancies with low-energy He irradiation and then depositing Pt to fill $V_{1S}$, $V_{2S}$, or $V_{Mo}$ sites. The authors employ single-sideband ptychography on 4D-STEM data, complemented by ab initio simulations, to achieve nearly linear phase contrast that distinguishes both vacancy types and Pt dopants with high dose efficiency, outperforming conventional HAADF imaging in resolving these configurations. They demonstrate that Pt incorporated into sulfur vacancies can occupy multiple lattice sites, with diffusion pathways and binding energies indicating room-temperature stability and a preference for the sulfur vacancy sites, while DFT and IAM simulations support the qualitative interpretation of the SSB signals. The results establish a scalable pathway for controlled substitutional doping in MoS2 and highlight the potential of SSB ptychography for atomic-scale characterization of dopant-modified 2D materials, albeit with considerations for charge redistribution effects and increased data processing requirements. These insights pave the way for targeted defect engineering in 2D catalysts and electronics, and suggest extending the approach to other dopants and 2D hosts.

Abstract

We substitute individual Pt atoms into monolayer MoS$_2$ and study the resulting atomic structures with single-sideband (SSB) ptychography supported by ab initio simulations. We demonstrate that while high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging provides excellent Z-contrast, distinguishing some defect types such as single and double sulfur vacancies remains challenging due to their low relative contrast difference. However, SSB with its nearly linear Z-contrast and high phase sensitivity enables reliable identification of these defect configurations as well as various Pt dopant structures at significantly lower electron doses. Our findings uncover the precise atomic placement and highlight the potential of SSB ptychography for detailed structural analysis of dopant-modified 2D materials while minimizing beam-induced damage, offering new pathways for understanding and engineering atomic-scale features in 2D systems.

Figures (5)

• Figure 1: a) Schematic illustration of the sample manipulation chamber used for the study. In step 1 the sample is subjected to the ion beam, in step 2 the sample is in the field of view of the e-beam evaporator. b) Schematic illustration of the defect-engineering process. The inset shows the beam energy profile of the He ions. (The full dI/dV curve can be found in Supplemental Materials Fig. S1.) c) Schematic illustration of the single atom evaporation process. d) HAADF-STEM image of a clean MoS_2 area before modification steps (not the same area as in the following images). e) HAADF-STEM image MoS_2 after 10 min irradiation with He ions. The red and turquoise arrows in the inset indicate $\mathrm{V_{1S}}$ (red) and $\mathrm{V_{Mo}}$ (turquoise) and defect sites that will be filled with Pt atoms, the yellow arrows mark the same defect features before and after Pt evaporation. f) HAADF-STEM image of roughly the same area and field-of-view as in panel e (see yellow arrows); the sites filled with Pt atoms are indicated with the red and turquoise arrows in the inset.
• Figure 2: a) HAADF-STEM images (field of view ca. 1 nm) of defect structures without and with Gaussian blurring, atomic models of the imaged structures, SSB reconstructions of the phase information at the same location as well as simulations of the SSB images corresponding to experimental parameters. The last column contains simulations of the SSB images under perfect conditions (unlimited dose, no residual aberrations). b) Relative occurrence of different defect types in the defect-engineered MoS_2 based on SSB and HAADF images. The uncertainty in the columns is based on the variation between observed images. c) Histograms of HAADF intensities at the Mo and S$\mathrm{_2}$ sublattice sites. The type of the S vacancies (no vacancy, $\mathrm{V_{1S}}$, $\mathrm{V_{2S}}$) are determined using the SSB intensity at the respective S sublattice site. d) Histograms of SSB phase values at the same atomic sites as in c), together with Gaussian fits of the phase distribution of all structures. The $\mu$ in the legends indicates the center of the Gaussian fits, the $\sigma$ is the standard deviation of the respective Gaussian. N is the number of cases for each histogram.
• Figure 3: a-d) Diffusion paths of Pt atoms on the surface calculated by the nudged elastic band method. The x-axis in the energy diagrams are given in relative atomic mass-weighted distances (normalized reaction coordinates NRC). The black dots in the energy diagram match the (semi-transparent) gray circles on the atomic model. Start and end positions of the diffusion process are marked with silver dots. a) Diffusion from the top of a Mo site to the top of another Mo site over the metastable position on top of a S site (marked by the red dot). b) Diffusion from the surface to a $\mathrm{V_{1S}}$ site, c) to a $\mathrm{V_{2S}}$ site and d) to a $\mathrm{V_{Mo}}$ site.
• Figure 4: a) Line profiles of Gaussian blurred HAADF-STEM images of dopant structures, shown alongside the blurred images as well as HAADF image simulations, atomic models of the imaged structures, SSB reconstructions of the phase information at the same location, and realistic simulations of the SSB images. From top to bottom, the rows contain data of Pt@V$\mathrm{_{1S}}$, Pt@V$\mathrm{_{2S}}$, Pt@V$\mathrm{_{Mo}}$ and Pt adatom columns. b) Histograms of HAADF intensities at Pt@V$\mathrm{_{Mo}}$, Pt@V$\mathrm{_{2S}}$ and Pt@V$\mathrm{_{1S}}$ sites and Gaussian fits of the distributions. The types of the Pt substitutions at the S$\mathrm{_2}$ sublattice are determined using the SSB phase at the respective locations. c) Histograms of SSB phase values at the same atomic sites as in b), together with Gaussian fits of the phase distribution of the structures. The violet and turquoise lines represent the distribution of the Mo and S$\mathrm{_2}$ phase values, respectively. The $\mu$ in the legends indicates the center of the Gaussian fits, the $\sigma$ is the respective standard deviation. d) Relative occurrence of dopant structures obtained from large scale HAADF imaging. As Pt@V$\mathrm{_{1S}}$ and Pt@V$\mathrm{_{2S}}$ are barely distinguishable in HAADF images both types are counted together and the tip of the Pt$\mathrm{@V_{1S+2S}}$ column is shaded in red to mark the approximate ratio of Pt@V$\mathrm{_{2S}}$ based on the ratio between $\mathrm{V_{1S}}$ and $\mathrm{V_{2S}}$. The uncertainty in the columns is based on the variation between observed images and rounded up to the next integer percentage. N is the number of cases for each histogram.
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