High Temperature Structural Phase Transition Phenomena in CVD Grown MoTe2

Among all known transition metal dichalcogenides (TMDs), molybdenum ditelluride (MoTe2) has attracted intensive scientific attention due to rich polymorphic phenomenon, i.e., various structural configurations with different electrical and optical properties. It is the most attractive candidate for the development of phase change devices and low-resistance contacts due to the low energy difference (i.e., 30-60 meV/f.u.) between semiconductor-2H and semimetal-1T’ phases. Also, monolayer 1T’ is predicted to be a 2D topological and large-gap quantum spin Hall insulator (QSHI), with important implications for spintronic and quantum applications, and recently it was demonstrated to exhibit superconductivity in the monolayer limit. In the few-layer/bulk limit, the 1T′ phase undergoes a structural phase transition to the orthorhombic Td phase upon cooling. The 1T′ phase is monoclinic with the stacking angle of ∼93.9°, while the Td phase is orthorhombic and a type-II Weyl semimetal. Monolayer 1H phase has a near-infrared (NIR) band gap (1.1 eV), strong spin-orbit coupling (SOC) and ambipolar behavior, that make it a promising candidate for spintronic, valleytronic, electronic, and NIR optoelectronic applications. In the few-layer/bulk limit, 2H or 3R phases can be obtained via stacking 1H single layers. In the 2H structure, the second layer is rotated by 180o around the c-axis with respect to the first layer, while in the 3R structure all layers are stacked parallel and 1/3 shifted along the armchair (AC) direction with respect to each other. Both configurations find implementation in nonlinear optical, spin- and valleytronic devices. While in the past most experiments have been carried out on mechanically exfoliated flakes, nowadays, it has become possible to synthesize different polymorphs of MoTe2, i.e., 2H, 1T’, Td, 3R and also new defective metallic forms such as 3D-Mo3Te4, 2D-Mo5Te8 (v1H) and 1D-Mo6Te6 nanowires. However, in the monolayer limit of MoTe2 only the 1H, v1H and 1T’ phases exist. Understanding the phenomenon of polymorphism between these structural configurations and achieving controllable and reversible 1H-1T’ transformation is of great scientific and practical importance. While 1H-MoTe2 is a well-studied material, 1T’-MoTe2 experimental research lags behind due to the extreme air instability of this material, which rapidly degrades upon air exposure, with a measured lifetime in the minutes range. In our laboratory, we have developed approaches to synthesize both the 1H and the 1T’ phases via chemical vapor deposition (CVD) and to stabilize them via scalable encapsulation. This encapsulation method makes possible spectroscopic and microscopic studies on 1T’ (and 1H) monolayer MoTe2, thus enabling investigations until recently unviable. This thesis work focus is study phase transition phenomena in CVD-grown MoTe2 occurring at high temperatures. We investigate spontaneous 1H-1T’ polymorphism taking place during CVD growth of MoTe2 (under a complex chemical environment and caused by 1H-1T’ heterocontact formation) as well as that induced by high temperature annealing in ultra-high vacuum (UHV) conditions. We pay great attention to unveiling the mechanisms behind the pristine H-T’ phase transition, especially at the monolayer limit. The latter case has been investigated in a limited manner to date, because of the inherent difficulty in working with monolayer materials. We complement experimental results with density functional theory (DFT) modeling to gain a deeper understanding of such a complex topic. Our experiments also allow us to explore Mo-Te compounds with different stoichiometry which are generated upon high temperature annealing of MoTe2. Notably, we show that in some cases it is possible to control the degradation of the original MoTe2 material so to obtain novel uniform low-dimensional materials with interesting properties (e.g., 2D-Mo6Te6). The experimental and theoretical studies presented in this work do not only strongly contribute to the understanding of high temperature polymorphisms in MoTe2, but also open interesting applicative prospects by demonstrating the possibility to realize scalable semiconductor-semimetal(metal) junctions and novel materials whose interesting properties could be of use in a variety of applications ranging from quantum technology to electronics, optoelectronics and photonics. The most relevant findings of this work are: 1) We reveal that during CVD growth in presence of a 1H/1T’ heterocontact, a semiconductor (1H) to semimetal (1T’) phase transition takes place in monolayer MoTe2 at a critical temperature (730°C) significantly lower than that (1075 oC) reported for exfoliated annealed 1H-MoTe2 crystals. By adopting microscopic and spectroscopic techniques and by implementing DFT simulations we explain the role of the heterocontact and of atomic defects in favoring or preventing the phase transition propagation; 2) We demonstrate reversible phase transition in graphene-encapsulated monolayer MoTe2 (either starting from CVD-grown 1T’ and 1H single crystals) in ex-situ UHV annealing experiments. Critical temperatures for 1H to 1T’ and 1T’ to 1H transformations are found to be ~ 1090 oC (in line with what reported for exfoliated MoTe2 flakes) and ~ 900 oC, respectively. This is the first time that a robust reversible phase transition (with all Raman-active modes of the 1T’ phase detected) is demonstrated in monolayer MoTe2; 3) We report an approach to reduce the critical temperature of the 1T’ to 1H transformation from ~ 900 oC to ~ 650 oC by adopting graphene-encapsulated MoTe2 vertical heterostructures and UHV annealing. We show the results are reproducible both for as-grown and van der Waals (vdW)-assembled heterostructures. Moreover, we find 2 new stacking configurations of bilayer 2H-MoTe2 by high-resolution transmission electron microscopy (HRTEM), namely 2Hr (bilayer form of 3R stacking) and 2Hs (sliding or shifted stacking of parallel 1H layers), synthesized via thermal annealing of vdW-assembled bilayer 1H/1T’-MoTe2; 4) We show that µm-scale homogenous monolayer 2D-Mo6Te6 can be realized in defective graphene-encapsulated monolayer MoTe2 upon high-temperature (> 700 oC) UHV annealing. We predict 8 different phases of 2D-Mo6Te6 via DFT and experimentally verify the existence of 3 of them and calculate their electronic properties. The synthesized monolayer 2D-Mo6Te6 is the first realization of paradigmatic 2D Su-Schrieffer-Heeger (SSH) model in a real material. For layered MoTe2, we report transformation into 3D-Mo6Te6 nanowires and 2D-Tellurium at > 800 oC. We study the Raman spectroscopy response of 2D- and 3D- Mo6Te6 and identify the representative Raman-active modes for this material by combining DFT simulations and linearly polarized Raman spectroscopy. We report wavelength-dependent polarity conversion, measured via linearly polarized Raman spectroscopy, and observe nontrivial absorption properties in 3D-Mo6Te6.