Synthesis of novel materials by pulsed laser deposition and chemical vapor deposition: Part I: Energetic deposition and stability of carbon nitride thin films. Part II: Catalytic growth of one dimensional materials and devices.

摘要

The discovery and synthesis of new materials are often the driving forces for revolutionary scientific and technological advances. In this thesis, we report two aspects of our effort toward the synthesis of novel materials: the energetic deposition of carbon nitride thin films by pulsed laser deposition (PLD) and the catalytic synthesis of one dimensional materials and devices, including nanowires, nanotubes, and nanowire/nanotube hetero-junctions.; In the first part of the thesis, we discuss the synthesis and stability of sp3-hybridized carbon nitride thin films. We developed a novel process to synthesize carbon nitride thin films based on the mechanism of energetic deposition by using a combination of PLD and atomic nitrogen plasma. We first studied the mechanism of energetic deposition of diamond-like-carbon (DLC) by PLD, using the combination of TOF-MS and EELS measurements. These studies serve as the foundation for the synthesis of sp3-hybridized carbon nitride.; We then studied the deposition of carbon nitride thin films. The TOF-MS studies on laser plume in N₂/He plasma showed that the concept of energetic deposition is applicable in our experiments and the kinetic energy of C+ should be high enough to produce highly sp3-hybridized carbon nitride. However, carbon nitride films with ∼40% nitrogen always contain essentially pure sp2-bonded carbon. Subsequent systematical studies on the effect of nitrogen on the local bonding revealed that there is a nitrogen-driven sp3 to sp2 relaxation when the nitrogen concentration increases from 11 to 17%. Theoretical studies on model structures using HF and DF theory calculations showed that there is a strong energetic preference for sp2 vs. sp3-bonded structure when the nitrogen concentration is larger than 12%.; In the second part of the thesis, we report the catalytic growth of one-dimensional (ID) materials and devices. We developed a general laser-ablation/chemical-vapor-deposition (LA-CVD) approach to synthesize nanowires and nanotubes. In this approach, nano-sized particles are generated by laser ablation first, and subsequently act as catalytic sites in the reactive background gases for nanowires or nanotubes to growth. Single crystalline Si nanowires (SiNWs) with diameters of several to tens of nanometers and lengths larger than 10 μm have been produced by ablating Au in silane gas. Structural properties of silicon nanowires and size-controlled growth are discussed. Single-walled nanotubes (SWNTs) and metal-filled multi-walled nanotubes (MWNTs) have also been produced by ablating Fe, Ni, Co, or iron oxide in ethylene gas. The experimental conditions for producing high yield, pure nanotubes are discussed.; We further used the catalytic approach in two different methods to synthesize nanojunctions of Si nanowires and carbon nanotubes. In the first method, a common iron based catalyst is used to grow SiNWs first by using LA-CVD, and then to grow MWNTs by using CVD method. Nanometer sized SiNW/MWNT junctions both with or without catalyst clusters at the joint are observed with TEM. In the second method, SiNWs are grown from a catalyst attached at the end of a nanotube tip. The electrical properties of the NT/NW junctions made by this method are measured by using the Ga-In liquid metal alloy to make contact to the free end of the junction. We found that the junctions exhibit rectifying behavior characteristic of a M/S Schottky diode.