Phase Controlled Synthesis and Characterizations of Transition Metal Di- and Tri-chalcogenides Alloys

Two-dimensional (2D) materials offer various novel properties such as direct bandgap, high carrier mobility, room-temperature ferromagnetism and so on, which facilitate the advanced functionalities in many applications. Especially, transition metal dichalcogenide (TMDC) materials have enabled the engineering of physical properties by alloying the chemical compositions and tuning structural phases. Intrigued by tunability established in 2D TMDC materials, the reasonable efforts have been carried out in the exploration of one-dimensional (1D) transition metal trichalcogenide (TMTC) materials. This dissertation presents the study of (1) the phase‐dependent band gap engineering in alloys induced by charge density wave (CDW) phases, (2) enhanced thermal and electrical stabilities of high-entropy alloyed structure, (3) synthesis and characterizations of psedo-1D TMTC alloy with unprecedented multifunctionality in various applications, and (4) thermodynamics and kinetics in the anisotropic growth of high-entropy TMTC alloy. In the first project, a novel form of bandgap engineering involving alloying non‐isovalent cations in a 2D transition metal dichalcogenide (TMDC) is presented. By alloying semiconducting MoSe2 with metallic NbSe2, two structural phases of Mo0.5Nb0.5Se2, the 1T and 2H phases, are produced each with emergent electronic structure. At room temperature, it is observed that the 1T and 2H phases are semiconducting and metallic, respectively. For the 1T structure, scanning tunneling microscopy/spectroscopy (STM/STS) is used to measure band gaps. Electron diffraction patterns of the 1T structure obtained at room temperature show the presence of a nearly commensurate charge density wave (NCCDW) phase with periodic lattice distortions. In the second project, computational study systematically investigates the thermodynamic, thermal and dynamical stability, and electronic properties of 15 different TMDC alloys. The single-phase solid solution of the highest configurational entropy with five-transition metal component (pentanary) consisting of Mo, W, Nb, Ta and V with the sulfur chalcogen has been successfully synthesized. Multi-scale in-situ and ex-situ measurements confirm an exceptionally high thermal stability of this material at elevated temperatures tested up to ~1300 K. Moreover, this material exhibits a very low electrical sheet resistance (~0.7 mΩ.cm) at both thin-film and 2D forms comparable to other state-of-the-art materials but with much higher stability at ambient condition for an extended time tested up to 90 days. It also exhibits excellent electrical stability under cyclic mechanical strains. In the third project, the nanofibers of a solid solution TMTC, Nb1-xTaxS3, has been successfully synthesized with outstanding electrical, thermal, and electrochemical characteristics rivaling the performance of the-state-of-the art materials for each application. This material shows nearly unchanged sheet resistance (~740 Ω/sq) versus bending cycles tested up to 90 cycles, stable sheet resistance in ambient conditions tested up to 60 days, remarkably high electrical breakdown current density of ~ 30 MA cm-2, strong evidence of successive CDW transitions, and outstanding thermal stability up to ~900 K. Additionally, this material demonstrates excellent activity and selectivity for CO2 conversion to CO reaching ~ 350 mA cm-2 at – 0.8 V vs RHE with an outstanding turnover frequency number of 25 for CO formation. It also exhibits an excellent performance in a high-rate Li-air battery with the specific capacity of 3000 mAhg−1 at a high current density of 0.3 mAcm-2. In the last project, an in-depth study of the growth mechanism for equimolar TMTC alloy (NbTaTi)0.33S3 has been taken. The different synthesis temperature and time have been performed to investigate the energetically preferred phase in varying conditions. With the favored growth temperature, the phase evolution has been inspected at a sequence of growth steps, where the favorable growth direction and vapor species have been demonstrated by high-resolution scanning electron microscopy (SEM) imaging of as-synthesized samples. Furthermore, structural phase transition has been revealed in individual (NbTaTi)0.33S3 nanoribbon by both three-dimensional electron diffraction (3DED) and temperature-dependent electrical transport measurement.