Advanced Transmission Electron Microscopy

Electron microscopy to accelerate the understanding of two-dimensional materials. In our group we also exploit state-of-the-art electron microscopy and related techniques, including high-resolution scanning transmission electron microscopy (HR-STEM) and electron energy-loss spectroscopy (EELS), to unambiguously ascertain the underlying physical mechanisms leading to the remarkable phenomena emerging in quantum materials. Learn more about our nanofabrication research here. For the complete list of publications from our lab, please check here.

Strain field map calculated in MoS2 flakes indicating the presence of strong tensile strain in the transition region between different layers, and the EELS spectra taken along this region.

Here, we quantify the local strain field that arises at the edges of MoS2 flakes by combining aberration-corrected transmission electron microscopy with the geometrical-phase analysis method. We also provide further insight on the possible effects of such edge strain on the resulting electronic behavior by means of electron energy loss spectroscopy measurements. Our results reveal that the two-dominant edge structures, ZZ and AC, induce the formation of different amounts of localized strain fields. We also show that by varying the free edge curvature from concave to convex, compressive strain turns into tensile strain.

Here, we demonstrate the feasibility of deploying ultramicrotomy for the preparation of layered materials samples in TEM analyses. We show how ultramicrotomy leads to the reproducible large-scale production of both in-plane and out-of-plane cross-sections, with bulk vertically oriented MoS2 and WS2 nanosheets as a proof of concept. The robustness of the prepared samples is subsequently verified by their characterization by means of both high-resolution TEM and Raman spectroscopy measurements. Our approach is fully general and should find applications for a wide range of materials as well as of techniques beyond TEM, thus paving the way to the systematic large-area mass-production of cross-sectional specimens for structural and compositional studies.

Robust fabrication of large-area in- and out-of-plane cross-section samples of layered materials with ultramicrotomy”, M. O. Cichocka, M. Bolhuis, S. E. van Heijst, and S. Conesa-Boj, ACS Applied Materials and Interfaces, 12, 15867 (2020)

(a,b) BF- and ADF-STEM images respectively of WS2 flakes. c) Spatially-resolved EELS map of the area indicated with a grey rectangle in (b). (d–g) Intensity maps of the EELS signals integrated for different energy-loss windows. (h,i) Individual EEL spectra taken at different locations along the horizontal petals as indicated by the black and blue arrows in (b).

In this work, mixed 2H/3R free-standing WS2 nanostructures displaying a flower-like configuration are fingerprinted by means of state-of-the-art transmission electron microscopy. Their rich variety of shape-morphology configurations is correlated with relevant local electronic properties such as edge, surface, and bulk plasmons. Machine learning is deployed to establish that the 2H/3R polytype displays an indirect bandgap. The findings of this work represent a stepping stone towards an improved understanding of TMD nanomaterials based on mixed crystalline phases.

“Illuminating the Electronic Properties of WS2 Polytypism with Electron Microscopy“, S. E. van Heijst, M. Mukai, E. Okunishi, H. Hashiguchi, L. I. Roest, L. Maduro, J. Rojo, and S. Conesa-Boj, Ann. Phys. (Berlin) 533, 2000499 (2021)