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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