Monitoring Phase Transformation in Single Layer 1T’-WS2 Utilizing In-Situ Time-Resolved 4D STEM in SEM

The method of 4-dimensional scanning transmission electron microscopy, performed in Scanning Electron Microscope (4D STEM in SEM) is an emerging analytical/imaging technique inspired by 4D STEM in Transmission Electron Microscope (TEM). Compared with its TEM-based cousin, 4D STEM in SEM offers lower voltage (kV) capability suitable for non-destructive inspection of 2D or beam-sensitive materials. Moreover, various lamellae prepared in SEM equipped with a Focused Ion Beam (FIB) could be directly analyzed by this methodology right after the lift-out procedure. Especially thin, electron-transparent samples could benefit from the described analytical approach. Various in-situ observations of chemical reactions have been performed both in SEM [1],[2] and TEM [3],[4]. In these studies, the effect of external stimuli (temperature, strain, voltage, etc.) was observed typically using visual information (secondary or transmitted electron images) or exploiting Selected Area Electron Diffraction (SAED). Recently, in-situ 4D STEM experiments were conducted in TEM [5],[6]. In these, either temperature or strain is controlled, while several 4D STEM datasets are acquired at different stages of the process (e.g., grain boundary formation, annealing, phase transformation). The result is then a series of several virtual images. Pixelated detectors suffer from inherently lower speeds compared to standard SEM/TEM detectors. No true “5D STEM dataset” – a continuous movie of 4D STEM data covering the full duration of a chemical or phase transformation has been acquired, until now. WS2 is a prominent member of the transition metal dichalcogenide family [7]. It is a layered compound, where tungsten atoms are sandwiched by layers of sulfur atoms. The arrangements of the atoms in this compound lead to various electronic properties. Importantly, the most stable 2H hexagonal phase is semiconducting, showing direct and indirect bandgaps based on the single or multilayer character of the sample, respectively. Recently, the single-layered metallic phase of WS2 was prepared as stable, water-based printable ink [8]. Its structural arrangement corresponds to the metastable monoclinic 1T’ phase, where lines of tungsten atoms underwent Peierl’s transition [9]. Upon the heating, the metallic 1T’ phase switches into the stable semiconducting 2H arrangement [10]. Here, the temperature-induced phase transformation of single-layer monoclinic 1T’ WS2 to hexagonal 2H arrangement is followed by time- and momentum-resolved in-situ experiments. Nanosheets of 1T’ single layered-WS2 were drop-casted on a heating chip and inserted into the µReactor (described in detail in [11],[12]) in a Scios SEM. Underneath the chip, a Timepix3 electron counting hybrid pixelated detector was installed at a camera length of 37 mm [13]. Timepix3 works in event-based readout mode (time resolution of 1.6 ns), where every electron hit is read out, digitized, and streamed to the processing PC. The sample was continually heated from 65° C to 430° C while exposed to an electron beam with an acceleration voltage of 30 kV. The sample was observed continuously by standard imaging and by 4D STEM acquisition in event-based mode. The scan size of 768x512 px, dwell time 50 ns and frame time of 50 ms (incl. scanning overhead) ensured that the reaction progress was captured with sufficient temporal and spatial resolution. The 1T’ → 2H phase transformation is observable as a disappearance of diffraction spots corresponding to the monoclinic phase (Fig. 1) and occurs during several seconds. Thus, its recording onto a 5D STEM dataset is impossible with standard frame-based detectors. For a 1000 frames/s detector readout, the above-defined image would take 6.5 minutes, completely missing the fast reaction changes. The dataset was analyzed using a custom set of virtual apertures. A time series of 4D STEM images using a mask, sensitive to the phase transition (1T’→2H), was transformed into a motion picture. There, the creation of the 2H phase was detected, taking over the initial 1T’ phase. The phase change appears as the spreading of a dark area due to the diminishing intensity in the monoclinic spots. Two snapshots from the process are displayed in Figure 2 (before and after transformation). Conclusively, the phase transformation of a single layer 1T’ WS2 into the stable hexagonal 2H phase was observed in-situ. The process was monitored by unprecedented methodology allowing the formation of a 5D STEM-in-SEM dataset. In the current example, the transformation was induced by temperature. However, any physical (strain, voltage, magnetic field) or chemical stimuli that lead to changes observable in the diffraction plane can be similarly followed. Electron diffraction patterns of (A), monoclinic 1T’ and (B), hexagonal 2H phase of WS2 2D material acquired at the start and end of an in-situ heating experiment. Acceleration voltage = 30 kV, scalebar = 10 nm-1. Virtual 4D STEM images of µReactor apertures with WS2 flakes. The images are the result of a custom virtual detector mask, sensitive to monoclinic → hexagonal phase change. See changes in dashed ROI. A – at the start of the reaction (T=378 °C), B – at the end of the reaction (T=430 °C). Scalebar = 1 µm.