The research reported in this thesis is about the fabrication and characterization of three kinds of Angstrom-scale nanomaterials embedded in linear channels of the AlPO₄-5 (AFI) zeolite crystals. In particular, we focus on the 4 Å carbon nanotubes, aluminum nanowires and silicon nanowires.

Four Ångstrom carbon nanotubes were fabricated by chemical vapor deposition (CVD), in which carbon nanotubes formed inside the pores of AFI crystals. Up to now the pore filling factor has been very low (around 35%) which can be evidenced by the weight percentage of carbon obtained in thermal gravimetric analysis (TGA) measurements. In order to increase the pore filling factor, we have systematically studied the influence of carbon source, gas pressure, heating temperature and heating time...[ Read more ]

The research reported in this thesis is about the fabrication and characterization of three kinds of Angstrom-scale nanomaterials embedded in linear channels of the AlPO₄-5 (AFI) zeolite crystals. In particular, we focus on the 4 Å carbon nanotubes, aluminum nanowires and silicon nanowires.

Four Ångstrom carbon nanotubes were fabricated by chemical vapor deposition (CVD), in which carbon nanotubes formed inside the pores of AFI crystals. Up to now the pore filling factor has been very low (around 35%) which can be evidenced by the weight percentage of carbon obtained in thermal gravimetric analysis (TGA) measurements. In order to increase the pore filling factor, we have systematically studied the influence of carbon source, gas pressure, heating temperature and heating time in the fabrication of 4 Å carbon nanotubes by CVD. On this basis, by using a new type of micro-platelet AFI crystals as the template, combined with using ethylene and methane in successive CVD processes, we improved the TGA to 22.5wt%, which translates to a pore filling factor of 91%. This is much better than the previous results. Two types of superconductivity behavior in 4 Å carbon nanotubes were observed in different samples. The first type is the 1D fluctuation superconductivity, which is unaffected by the applied magnetic field up to 9 T. The resistance exhibits a smooth drop with decreasing temperature, initiating at 60-70 K, and the differential resistance exhibits a dip at zero bias current, and smoothly increases with increasing bias current. The dip in the differential resistance disappears above the initiation temperature. Such fluctuation superconductivity, arising from thermally activated phase slips, can be explained by the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory. The second type is the 1D to 3D superconducting crossover transition which can be suppressed by a magnetic field below 9 T. The temperature dependence of resistance and differential resistance also give clear evidence of superconductivity. This manifestation can be attributed to the Berezinskii-Kosterlitz-Thouless (BKT)-like transition which establishes quasi-long-range order in the ab plane perpendicular to the c-axis of the carbon nanotubes.

Aluminum nanowires were also fabricated by CVD using AFI as the template. The existence of aluminum inside pores of AFI is verified by both energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis. The Raman spectra indicate the aluminum nanowires to be in a core-shell structure with a metallic core and oxide coating. We measured the transport property of aluminum nanowires and observed 1D fluctuation superconductivity with a transition temperature around 40 K. This enhancement of transition temperature can originate from the alumina coating that have a high Debye temperature.

Silicon nanowires were grown by CVD with silane as the silicon source. The X-ray photoelectron spectroscopy (XPS) and Raman spectra analysis confirmed the existence of silicon nanowires inside the pores of AFI crystals. We found silicon nanowires to have strong photoconductance effect and photoluminescence (PL) effect. With increasing temperature, PL intensity becomes smaller and PL peak exhibits a blue shift. Transport measurement shows that the resistance of silicon nanowires was proportional to the logarithm of temperature, analogues to the Kondo effect. This phenomenon can be attributed to two-level impurity scattering.