Achieving nanoscale strain fields mapping in intricate van der Waals (vdW) nanostructures, like twisted flakes and nanorods, presents several challenges due to their complex geometry, small size, and sensitivity limitations. Understanding these strain fields is pivotal as they significantly influence the optoelectronic properties of vdW materials, playing a crucial role in a plethora of applications ranging from nanoelectronics to nanophotonics. Here, a novel approach for achieving a nanoscale-resolved mapping of strain fields across entire micron-sized vdW nanostructures using four-dimensional (4D) scanning transmission electron microscopy (STEM) imaging equipped with an electron microscope pixel array detector (EMPAD) is presented. This technique extends the capabilities of STEM-based strain mapping by means of the exit-wave power cepstrum method incorporating automated peak tracking and K-means clustering algorithms. This approach is validated on two representative vdW nanostructures: a two-dimensional (2D) MoS₂ thin twisted flakes and a one-dimensional (1D) MoO₃/MoS₂ nanorod heterostructure. Beyond just vdW materials, the versatile methodology offers broader applicability for strain-field analysis in various low-dimensional nanostructured materials. This advances the understanding of the intricate relationship between nanoscale strain patterns and their consequent optoelectronic properties.

Twisted 2D materials present an enticing platform for exploring diverse electronic properties owning to the tunability of their bandgap energy. However, the intricate relationship between local heterostrain fields, thickness, and bandgap energy remains insufficiently understood, particularly at the nanoscale. Here, it presents a comprehensive nanoscale study elucidating the remarkable sensitivity of the bandgap energy to both thickness and heterostrain fields within twisted WS₂ nanostructures. This approach integrates electron energy-loss spectroscopy (EELS) enhanced by machine learning with 4D scanning transmission electron microscopy (STEM). Through this synergistic methodology, enhancements up to 20% in the bandgap energy is unveiled depending on the specimen thickness. This phenomenon is traced back to sizable deformation angles present within individual layers, which can be directly linked to distinct variations in local heterostrain fields. The findings represent a significant advancement in comprehending the electronic behavior of twisted 2D materials and introduce a novel methodological framework with far-reaching implications for twistronics and the investigation of other materials within the nanoscience domain.

Chromium dioxide (CrO₂) nanowires with their half-metallic ferromagnetic properties have shown great promise in spintronics applications. However, growth of such wires remains challenging. We used the Selective Area growth method to fabricate high quality epitaxial CrO₂ wires on a TiO₂ substrate, using trenches oriented both along the substrate [001] c-axis and along the [010] b-axis, which are the magnetically easy and hard axis of the wire, respectively. We investigated the morphology of the wires by high-resolution transmission electron microscopy (TEM) and measured their physical properties, in particular magnetoresistance (MR) and the Anomalous Hall Effect (AHE). TEM images showed that the morphology of the wires grown along the two axes are very different. MR data show very sharp switching for c-axis grown wires (the easy axis), even for quite large wire widths. The AHE is found to be different for c-axis wires and b-axis wires, which we argue to be due to a different wire morphology on the nanoscale.

The fabrication of 2D materials, such as transition metal dichalcogenides (TMDs), in geometries beyond the standard platelet-like configuration exhibits significant challenges which severely limit the range of available morphologies. These challenges arise due to the anisotropic character of their bonding van der Waals out-of-plane while covalent in-plane. Furthermore, industrial applications based on TMD nanostructures with non-standard morphologies require full control on the size-, morphology-, and position on the wafer scale. Such a precise control remains an open problem of which solution would lead to the opening of novel directions in terms of optoelectronic applications. Here, a novel strategy to fabricate position-controlled Mo/MoS₂ core–shell nanopillars (NPs) is reported on. Metal-Mo NPs are first patterned on a silicon wafer. These Mo NPs are then used as scaffolds for the synthesis of Mo/MoS₂ core/shell NPs by exposing them to a rich sulfur environment. Transmission electron microscopy analysis reveals the core/shell nature of the NPs. It is demonstrated that individual Mo/MoS₂ NPs exhibits significant nonlinear optical processes driven by the MoS₂ shell, realizing a precise localization of the nonlinear signal. These results represent an important step towards realizing 1D TMD-based nanostructures as building blocks of a new generation of nanophotonic devices.

Visualizing charge carrier flow over interfaces or near surfaces meets great challenges concerning resolution and vastly different time scales of bulk and surface dynamics. Ultrafast or four-dimensional scanning electron microscopy (USEM) using a laser pump electron probe scheme circumvents the optical diffraction limit, but disentangling surface-mediated trapping and ultrafast carrier dynamics in a single measurement scheme has not yet been demonstrated. Here, we present lock-in USEM, which simultaneously visualizes fast bulk recombination and slow trapping. As a proof of concept, we show that the surface termination on GaAs, i.e., Ga or As, profoundly influences ultrafast movies. We demonstrate the differences can be attributed to trapping-induced surface voltages of approximately 100-200 mV, which is further supported by secondary electron particle tracing calculations. The simultaneous visualization of both competing processes opens new perspectives for studying carrier transport in layered, nanostructured, and two-dimensional semiconductors, where carrier trapping constitutes a major bottleneck for device efficiency.

Transition metal dichalcogenides such as MoS2 represent promising candidates for building blocks of ultra-thin nanophotonic devices. For such applications, vertically-oriented MoS2 (v-MoS2) nanosheets could be advantageous as compared to conventional horizontal MoS2 (h-MoS2) given that their inherent broken symmetry would favor an enhanced nonlinear response. However, the current lack of a controllable and reproducible fabrication strategy for v-MoS2 limits the exploration of this potential. Here we present a systematic study of the growth of v-MoS2 nanosheets based on the sulfurization of a pre-deposited Mo-metal seed layer. We demonstrate that the sulfurization process at high temperatures is driven by the diffusion of sulfur from the vapor-solid interface to the Mo seed layer. Furthermore, we verify an enhanced nonlinear response in the resulting v-MoS2 nanostructures as compared to their horizontal counterparts. Our results represent a stepping stone towards the fabrication of low-dimensional TMD-based nanostructures for versatile nonlinear nanophotonic devices.

Layered materials (LMs) such as graphene or MoS2 have attracted a great deal of interest recently. These materials offer unique functionalities due to their structural anisotropy characterized by weak van der Waals bonds along the out-of-plane axis and covalent bonds in the in-plane direction. A central requirement to access the structural information on complex nanostructures built upon LMs is to control the relative orientation of each sample prior to their inspection, e.g., with transmission electron microscopy (TEM). However, developing sample preparation methods that result in large inspection areas and ensure full control over the sample orientation while avoiding damage during the transfer to the TEM grid is challenging. Here, we demonstrate the feasibility of deploying ultramicrotomy for the preparation of LM 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.