Large Area Growth of Single Phase Transition Metal Tellurides Nanosheets Through Simulation Guided AP-CVD Tellurization

Among transition metal dichalcogenides, transition metal mono- and di-tellurides (TMTs) are the subject of a significant attention due to their intriguing polymorphic nature from semiconductor to metal and topological Weyl semimetal. To date, considerable efforts have been devoted to synthesizing single phase TMTs nanosheets with superior quality and lateral extension suitable for device integration in nanotechnologies and fundamental investigations. Ambient pressure chemical vapor deposition (AP-CVD) is an easy and cost effective method for synthesizing TMTs nanosheets up to the centimeter scale, due to its high flexibility. Here we report on AP-CVD large area growth and<br/>selective phase tuning strategies combined with finite element simulations for obtaining single phase TMTs nanosheets by design via tellurization of a predeposited metallic thin film by e-beam evaporation. In particular, we demonstrate the capability to obtain MoTe₂ in pure 1T’ and 2H phase, PtTe₂ in 1T phase and NiTe₂ in 1T phase. In all cases, the AP-CVD tellurization makes use of Te solid powder as source for the generation of Te vapors and Me (Me: Mo, Pt, Ni) thin films of variable nominal thickness from 4 to 10 nm, on SiO₂/Si substrates.<br/>Considering MoTe₂ case as an example, we also show a detailed study of the heterogeneous vapor-solid reaction between the pre-deposited Mo film and Te vapor optimizing the scalability and quality of the MoTe₂ nanosheets. Specifically, with simulation based on finite element method, for a given growth temperature, we show MoTe₂ structure and morphology are kinetically dictated by the distribution of the Te concentration gradient on the reaction site with varying geometric configurations inside the CVD reactor. Further, the role of temperature and Te concentration is elucidated for the other cases to achieve PtTe₂ and NiTe₂ growth. We characterize the so-grown nanosheets by using several techniques, such as Raman visible spectroscopy, atomic force microscopy, X-ray diffraction and transmission electron microscopy. By combining the results we could retrieve the structure of the different TMTs nanosheets in terms of their crystalline phase and crystallinity, thickness (number of layers), morphology, RMS roughness and domain (grain) size. In synthesis, we obtain nanosheets in pure single phase, with a polycrystalline character related to the pristine metallic thin film, with RMS roughness limited to few nm and a grain size in the tenth to the hundreds nm range.<br/>Our study elucidates a methodological approach that could be considered as an efficient, scalable, and relatively simple solution to enable the controlled growth and optimize the characteristics of TMTs, and in extension TMDs, over large areas, whenever a tellurization (chalcogenization) CVD based process is considered. Furthermore, this concept provides a pivot scheme for enabling scalable integration of TMTs exhibiting single phase in potential applications for novel micro- and nano-electronics, spintronics, photonics, and thermoelectric devices.