A study on novel layered nanomaterials : synthesis, structural engineering and applications
Date of Issue2017-04-04
Interdisciplinary Graduate School (IGS)
Energy Research Institute @ NTU
Preparation of novel layered materials and engineering their composition and structure is fascinating to generate new functions for a diverse range of applications. In the light of this, the aim of this thesis is to synthesize novel layered materials and engineer their nanostructures through oxygen hybridization and size reduction, and then explore their potential applications in electronic devices such as light-emitting diodes (LEDs) and memory devices. First, I demonstrated a facile method to oxidize the exfoliated MoS2 nanosheets, forming MoS2-MoO3 hybrid nanomaterials. The oxidization process was realized through the in-situ oxidation process in air during the film preparation utilizing heat-assisted spray-coating process. After thermal annealing, the MoS2-MoO3 hybrid nanomaterials composed of (100)-dominated MoS2 and (021)-dominated -MoO3 were formed. Importantly, the MoS2-MoO3 hybrid film can be formed on several solid substrates, such as SiC, Si, glass or quartz. Moreover, the obtained MoS2-MoO3 exhibited p-type conductivity. Then LEDs were fabricated by using the heterojunction composed of p-type MoS2-MoO3 (p-MoS2-MoO3) film and n-type 4H-SiC (n-SiC) substrate. The LEDs exhibited broad spectra with four sub-wavelength emissions. The electroluminance mechanism was investigated through experiments and theoretical calculations. Second, I decreased the lateral size of transition metal dichalcogenide (TMD) materials and prepared a series of TMD nanodots (NDs), including MoS2, MoSe2, WS2, WSe2, ReS2, TaS2, and NbSe2. All these TMD NDs have a small size less than 10 nm and good dispersity in N-methyl-2-pyrrolidone (NMP) solution. Moreover, I also developed a method to isolate these NDs from the solvent of NMP, i.e., post-treatment of these NDs with hexane and chloroform. As a proof-of-concept application, the TMD NDs (e.g., MoSe2, NbSe2 or WS2) were mixed with organic polymer (e.g., polyvinylpyrrolidone (PVP)) and used as active layers for fabrication of memory devices. These devices display nonvolatile write-once-read-many (WORM) memory behavior with high ON/OFF current ratio and good stability. We believe that these TMD NDs can show more potential in optoelectronics, solar cells, catalysis, and biomedicine. Then, I further extended the size reduction protocol to fabricate other novel layered NDs and prepared black phosphorus quantum dots (BPQDs) from the bulk black phosphorus crystal, which is an emerging layered material. The obtained BPQDs have a lateral size of 4.9±1.6 nm and thickness of 1.9±0.9 nm (ca. 4±2 layers). As a proof of concept, a flexible memory device based on polyethylene terephthalate (PET) substrate, using BPQDs mixed with PVP as the active layer, have been successfully fabricated, which shows nonvolatile rewritable memory behavior with high ON/OFF current ratio of more than 6.0×104 and good stability.