Atomic electrostatic maps of 1D channels in 2D semiconductors using 4D scanning transmission electron microscopy

ARTICLE https://doi.org/10.1038/s41467-019-08904-9 OPEN Atomic electrostatic maps of 1D channels in 2D semiconductors using 4D scanning transmission electron microscopy 1234567890():,; Shiang Fang1, Yi Wen2, Christopher S. Allen Efthimios Kaxiras1,6 & Jamie H. Warner2 2,3, Colin Ophus4, Grace G.D. Han5, Angus I. Kirkland 2,3, Defects in materials give rise to fluctuations in electrostatic fields that reflect the local charge density, but imaging this with single atom sensitivity is challenging. However, if possible, this provides information about the energetics of adatom binding, localized conduction channels, molecular functionality and their relationship to individual bonds. Here, ultrastable electronoptics are combined with a high-speed 2D electron detector to map electrostatic fields around individual atoms in 2D monolayers using 4D scanning transmission electron microscopy. Simultaneous imaging of the electric field, phase, annular dark field and the total charge in 2D MoS2 and WS2 is demonstrated for pristine areas and regions with 1D wires. The in-gap states in sulphur line vacancies cause 1D electron-rich channels that are mapped experimentally and confirmed using density functional theory calculations. We show how electrostatic fields are sensitive in defective areas to changes of atomic bonding and structural determination beyond conventional imaging. 1 Department of Physics, Harvard University, Cambridge, MA 02138, USA. 2 Department of Materials, University of Oxford, 16 Parks Road, Oxford OX1 3PH, UK. 3 Electron Physical Sciences Imaging Center, Diamond Light Source Ltd., Didcot, Oxfordshire OX11 0DE, UK. 4 National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley 94720 CA, USA. 5 Department of Chemistry, Brandeis University, Waltham 02453 MA, USA. 6 John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA. These authors contributed equally: Shiang Fang, Yi Wen. These authors jointly supervised this work: Efthimios Kaxiras, Jamie H. Warner. Correspondence and requests for materials should be addressed to E.K. (email: ) or to J.H.W. (email: ) NATURE COMMUNICATIONS | (2019)10:1127 | https://doi.org/10.1038/s41467-019-08904-9 | www.nature.com/naturecommunications 1 ARTICLE 4D NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-08904-9 scanning transmission electron microscopy (4DSTEM) is gaining momentum for probing materials at sub-Angstrom resolution with the full electron −atom scattering interactions recorded in a convergent beam electron diffraction pattern (CBED)1–4. This has been revolutionized by high-speed electron detectors, either in the form of 2D pixelated cameras (2D-PCs) or as segmented detectors5,6. These have enabled strain maps across samples with picometer precision, and deep sub-Angstrom spatial resolution using ptychographic reconstruction methods7,8. Direct collection of CBED patterns on 2D-PCs provides rich information about phase and momentum transfer from the electron beam interactions with the samples’ electrostatic fields9,10. Phase data can be reconstructed using pytchographic methods, together with simultaneously recorded ADF-STEM images11. The intensity fluctuations of the CBED pattern are used to produce differential phase contrast images that relate to momentum transfer to the electron beam as it propagates through the samples’ electrostatic fields12,13. Atomic resolution images of electrostatic fields and charge distributions have been recorded for bulk crystals, such as GaN, where beam damage does not limit the long acquisition times3. Using 2D-PCs, this is done by measuring the intensity center of mass, while for quadrant detectors, the differential signal between opposite quadrants is used. Translating 4D STEM to the single atom level is more difficult because of the rapid sample damage at time scales faster than the acquisition speed and hence low beam dose is essential13. Furthermore, mapping features around single atoms in defects is challenging due to the low signal to noise14,15. However, 2D materials do offer a thin volume for direct interpretation in electron microscopy16–18, and to study fluctuations of electrostatics around single atoms. For semiconducting 2D monolayers, transition metal dichalcogenides (TMDs), such as MoS2 and WS2, form ultralong 1D channels by S sputtering at high temperature19. Density functional theory (DFT) calculations show that as the width of the S line vacancies increases from 1S to 2S, the band gap narrows from 1.9 to <0.1 eV, and becomes metallic at 4S width20. Theory suggests that these 1D conduction channels are due to the metal-rich bonding areas that form within the larger vacancy sections, but experimental verification of this has yet to be achieved with sufficient resolution to identify charge variations in regions of single atomic bonds. These W−W bonds create 1D sub-nm conduction channels in the 2D semiconductors with potential use in nanoscale electronics and devices. However, the detailed atomic structure of the ultralong 2S and 3S line vacancies is complex and difficult to accurately determine using only ADFSTEM or phase contrast images. Therefore, the multicomponent images obtained from 4D STEM, including total charge maps, are crucial to gaining a better understanding of the structure −property correlations. Furthermore, by using first principle calculations, we can determine the predicted electric fields and total charge values in these monolayer systems and quantitatively compare it to the experimental values. Prior work has primarily used image simulation methods to compare to experimental 4D STEM results. Here, we show that 4D STEM can directly image electrostatic fields, total charge and phase maps with atomic resolution in monolayer MoS2 and WS2 2D crystals with qualitative agreement to the predicted values from DFT calculations. Experimental values are quantitatively half of the DFT predicted values and this stimulates further investigations. Measurements are performed on sulfur line vacancies that form 1D channels at high temperature through vacancy diffusion into ordered lines. Metal−metal bonding is present in the S line vacancies and is shown to lead to electron-rich channels that act as in-gap states for 1D conduction. 2 More complex line vacancies with wider S vacancy regions are studied and show significant modulation of electric fields around atoms. Using a combination of ADF-STEM, phase imaging, electric field and total charge images, we are able to deduce the atomic structure of complex defective regions with a higher degree of certainty than using just one form of imaging contrast alone. The high sensitivity of the electric field maps to atomic bond coordination provides spatial information about nearest neighbor atoms that is not easily extracted from ADF-STEM images or phase maps. Results 4D STEM of pristine 2D MoS2 and WS2 monolayers. Figure (...truncated)