Abstract text (incl. figure legends and references)

HRTEM, enables the investigation of electron beam-induced structural transformations, properties and single atom dynamics in two-dimensional (2D) materials. Furthermore, electron irradiation has been shown to lead to local transformations in single-layer MoTe₂; from the semiconducting 2H to the metallic 1T" phase [1].

Here, we report about the in-situ formation of Mo₆Te₆ nanowires by heating and under electron irradiation in freestanding 2H-MoTe₂ in TEM, as well as graphene-encapsulated samples. The transformation is specifically analysed in freestanding single- and few-layer samples.

HRTEM images were acquired at the Cc/Cs-corrected Sub-Ångström Low-Voltage Electron microscope (SALVE) at an electron accelerating voltage of 80kV [2]. 2H-MoTe₂ single-layers were prepared by mechanical exfoliation [3] onto Quantifoil grids and by stamping flakes with the help of a PMMA layer [4] onto a ThermoFisher MEMS-chip (see Fig. 1 (a-c)). Graphene-encapsulated MoTe₂ was prepared by transferring three separate flakes to a TEM grid by iterative mechanical exfoliation.

By increasing the annealing temperature under constant electron irradiation (Figs. 1 (d-f)), the structural evolution from layered 2H-MoTe₂ to one-dimensional (1D) Mo₆Te₆ chains [5] can be observed and analysed. An experimentally acquired freestanding nanowire formed within a single-layer region at a temperature of about 600°C is shown in (f). Pure electron irradiation showed in contrast to heating, nanowires are formed more slowly and at the edges of holes within the single-layer, thus the electron beam enables the precise formation of Mo₆Te₆. Figs. 2 (a-c) shows experimental images of a single-layer MoTe₂ bombarded by electrons forming holes and nanowires. The supreme electric and thermal conduction [6], as well as the strong intralayer covalent bonding (mechanical strength) [7] makes graphene an excellent candidate for protecting 2D materials in TEM. Fig. 2 (d) shows a schematic depiction of the solid-to-solid transformation in planar 1H-MoTe₂ (encapsulated between Graphene) which transforms to 1D Mo₆Te₆ nanowires under electron irradiation. A hole in one of the graphene layers, driven by the interaction with the electron beam, enables Te atoms to escape the graphene-encapsulation and form nanowires.

Our results demonstrate the controlled step by step transformation of freestanding single- and few-layers 2H-MoTe₂ to 1D Mo₆Te₆ nanowires and thus provide a comprehensive picture of the response of MoTe₂ crystals to heat and to electron irradiation. Furthermore, we compare the effect of electron irradiation and annealing on the formation of nanowires.

Fig. 1. (a-c) llustrates the sample preparation of 2H-MoTe₂ onto a MEMS-chip. (d-f) show the formation of freestanding Mo₆Te₆ at high temperatures.

Fig. 2. (a-c) Single-layer MoTe₂ under constant electron irradiation at 80kV. (d) Illustrates the formation of one-dimensional Mo₆Te₆ nanowires encapsulated between graphene under electron bombardment.

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[2] M. Linck, et al., Phys. Rev. Lett., 117, 076101, 2016.

[3] P. Blake, et al., Applied physics letters 91.6: 063124, 2007.

[4] M. Ibrahim, et al., Sensors 20.6: 1572, 2020.

[5] H. Kim, et al., Small 16.47: 2002849, 2020

[6] A. A. Balandin, Nature materials, 10(8), 569-581, 2011.

[7] C. Lee, et al., science, 321(5887), 385-388, 2008.