Ordered arrays of nanocrystals : synthesis, properties and applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Vita.Includes bibliographical references.Nanoscale materials, including nanocrystals and carbon nanotubes, exhibit an appealing array of physical properties, and provide an interesting prospect for research both from a fundamental as well as a technological perspective. The current emerging themes in nanoscale research are: controlled synthesis with well defined sizes and geometries; unraveling their fundamental physical properties; and assembly of these nanoscale building blocks into functional devices. Although several approaches for producing the nanoparticles have been reported in the past decade, a general, large scale method for controlled synthesis of well-defined nanoparticles in the 1-5 nm size regimes is yet to be found. A general method that enables both syntheses of nanoparticles and their assembly on substrates is critical towards furthering technological applications. The work described here involved developing a method that utilized principles of self assembly in conjunction with inorganic and organic synthetic chemistry for the controlled synthesis of ordered arrays of nanocrystals. A unique attribute of this technique is it combined themes one and three, aforementioned, into a single step. First, uniform arrays of various mono- and hetero-bimetallic nanoparticles with sizes in the range of 1-5 nm were synthesized on various substrates using PS-P4VP block copolymer (BCP) templates. These arrays of monodisperse nanoparticles were employed as catalysts for the diameter-controlled growth of SWNTs.(cont.) Comparisons on their catalytic activities provided valuable insight on the catalyst-assisted growth of SWNTs. Alternate ways to improve the catalytic yield of nanotubes employing bi-metallic nanoparticles as well as novel catalysts for nanotube growth are also being reported for the first time. Importantly, a combinatorial approach involving BCPs and gas phase reactions was designed that enabled us in addressing some of the long standing problems associated with the syntheses of semiconductor III-Nitride nanocrystals. Finally, versatility of this synthesis method was further demonstrated by syntheses of ternary nitrides as well as rare earth ions doped GaN. While the investigations on the latter aspects are still in there infancy, initial results show significant promise and pave an exciting prospect for future studies.by Sreekar Bhaviripudi.Ph.D

Growth of large-area single- and bi-layer graphene by controlled carbon precipitation on polycrystalline Ni surfaces

We report graphene films composed mostly of one or two layers of graphene grown by controlled carbon precipitation on the surface of polycrystalline Ni thin films during atmospheric chemical vapor deposition(CVD). Controlling both the methane concentration during CVD and the substrate cooling rate during graphene growth can significantly improve the thickness uniformity. As a result, one- or two- layer graphene regions occupy up to 87% of the film area. Single layer coverage accounts for 5-11% of the overall film. These regions expand across multiple grain boundaries of the underlying polycrystalline Ni film. The number density of sites with multilayer graphene/graphite (>2 layers) is reduced as the cooling rate decreases. These films can also be transferred to other substrates and their sizes are only limited by the sizes of the Ni film and the CVD chamber. Here, we demonstrate the formation of films as large as 1 in2. These findings represent an important step towards the fabrication of large-scale high-quality graphene samples

Selective Molecular Transport through Intrinsic Defects in a Single Layer of CVD Graphene

We report graphene composite membranes with nominal areas more than 25 mm² fabricated by transfer of a single layer of CVD graphene onto a porous polycarbonate substrate. A combination of pressure-driven and diffusive transport measurements provides evidence of size-selective transport of molecules through the membrane, which is attributed to the low-frequency occurrence of intrinsic 1–15 nm diameter pores in the CVD graphene. Our results present the first step toward the realization of practical membranes that use graphene as the selective material