Controlled Synthesis Of Low Dimentional Transition Metal Dichalcogenides Based On Chemical Vapor Deposition

As a family of layered materials, two-dimensional transition metal dichalcogenides crystals (TMDCs) have attracted considerable attention due to the dramatic and unique properties different from their 3D bulk phases. Semiconducting TMDCs such as MoS2consummate the system of two-dimensional materials which includes metallic graphene and insulating h-BN. When further reducing 2D materials to one dimension, quantum confinement effect together with size effect result in unique propertiesto the one-dimensional materials. Controlled synthesis of low dimensional materials is critical for both functional explorations and applications of these materials. Among various preparation approaches, chemical vapor deposition (CVD) method holds the potential for synthesizing high quality low dimensional materials. This dissertation is focused on the preparation ofTMDCs with CVD method. The studies in this dissertation can be divided into three parts:1. Facile synthesis and phase transition of V2O3nanobelts. The V2O3 nanobelts were reduced from the hydrothermally synthesized V2O5 nanobelts, which were used as both precursor and template. The phase transition temperature was also studied. Major conclusions include:(1) The morphology of nanobelts was well-maintained and the V2O5 nanobelts werecompletely reduced to V2O3 nanobelts by using carefully chosen reducing agents of H2 and S vapor as indicated by TEM, SAED, XPS and XRD measurements.(2) The obtained V2O3 nanobelts exhibited metal-to-insulator transition at ～150 K estimated by variable temperature electrical measurements, similar to that of bulk V2O3. VT-Raman measurements were also utilized for characterizing the phase transition of the nanobelts by tracking the chemical reactivity of V2O3 nanobelts at varied temperatures.(3) The magnetic phase transition of V2O3nanobelts measured with superconducting quantum interference device (SQUID) was found to be different from the bulk materials, indicating that the paramagnetic to antiferromagnetic transition at low temperature was suppressed which is likelydue to the finite size effect of nanomaterials.This study allows for the controlled synthesis of V2O3 nano-materials and reveals the phase transition behavior of 1D V2O3. VT-Raman spectroscopy offers a new way to investigate the phase transition by tracking the chemical reactivity.2. Controlled synthesis of two-dimensional WS2 by CVD. WO3precursors with varied compositions and sizes were prepared and patterned to decrease the sublimationtemperature of WO3. To facilitate the sublimation of WO3, we attempted three growth precursors:V-W-O microflakes, WO3 nanorods and patterned WO3 nanorods.Following results were obtained:(1) Synthesis of WS2 nanoflakes using V-W-O microflakes as precursors. Optical microscope images demonstrated the suppression of crystal plane during the growth process of WS2 nanoflakes. V was not doped in the WS2asconfirmedby energy dispersive X-ray spectrum (EDX), Raman mapping and time of flight secondary ion mass spectrometry (TOF-SIMS).(2) Synthesis of single or bi-layeredWS2 nanoflakes using WO3 nanorods. WS2 nanoflakes were grownat a high yield on the growth substrate and their size and morphology were uniform. Atomic force microscope (AFM) and Raman mapping verified the etchingof WS2 by MoO3 at the center of WS2 flakes. Meanwhile, WO3 nanorods werefound in a core-shell (MoO3-WO3) structure.(3) Synthesis of plenty or large size of WS2 nanoflakesusing patterned WO3 nanorods. Plenty of WS2 nanoflakes were deposited between the parallel lines of patterned WO3. Large and independent WS2 flakes in triangle shape were obtained by optimizing suitable supplying time of S vapor.This study demonstrates that how the rational design of precursors can produce high quality 2D TMDCs in a controlled manner.3. Metallic and ferromagnetic MoS2nanobelts with vertically aligned edges. The approach for synthesizing MoS2 nanobelts terminated with vertically aligned edges was fabricated by sulfurizing hydrothermally synthesized MoO3 nanobelts through a kinetically driven processand investigated the electrical and magnetic properties of these metastable materials.Following results are obtained:(1) SAED, TEM and AFM studies showed that the morphology of nanobelts was remained but the crystallinity was changed from single crystal to polycrystalline. The nanobelts surface became less uniform due to the vertically aligned layered MoS2.(2) The MoO3 nanobelts were completely sulfurized to MoS2, which was characterized with Raman spectroscopy, XPS and XRD. The intensity ratio changes of MoS2 and the broadened diffractionpeaks in XRD spectrum indicated the existence of these vertically aligned edges. With different annealing temperature, the exposed edges were proved to be metastable state. They could be obtained at moderate sulfurization temperature through kinetically driven process.(3) The obtained MoS2 nanobelts were metallic and ferromagnetic, which was dramatically different from the semiconducting and nonmagnetic 2D and 3D 2H-MoS2 materials. The transition in the electronic and magnetic properties of MoS2was recognized to originatefrom the vertically aligned edgeson the surface of MoS2 nanobelts. The MoS2 nanobelts could be oxidized to water-solublemolybdenates by the Joule heating during electrical measurements in air and their magnetic susceptibility coulddegrade with time, which demonstrated the weak stability of these vertically aligned edges in air. Inductively coupled plasma mass spectrometry (ICP-MS) was fabricated to exclude the influence on magnetic susceptibility from the magnetic impurities.Thispart of the dissertation verifies the theoretical calculations and allows for better understanding on the edge effect of low dimensional materials.Thanks for the supports from the National Natural Science Foundation of China (21322303,51372134,21373127)...