Nanocellulose is a nascent and promising material with many exceptional properties and a broad spectrum of potential applications; hence, it has drawn increasing research interests in the past decade.  A new type of nanocellulose -- with mono- or bi-layer cellulose molecular sheet thickness -- was synthesized through a combined chemical-mechanical process (TEMPO-mediated oxidation followed by intensive sonication), and this new material was named molecularly thin nanocellulose (MT nanocellulose).  The overarching objective of this study was to understand the formation and supramolecular structure of MT nanocellulose and contribute to the knowledge of native cellulose structure. The research involved four major bodies of study: preparation of MT nanocellulose, characterization of MT nanocellulose, modeling wood pulp-derived cellulose microfibril cross section structure, and a comparative life cycle assessment (LCA) of different nanocellulose fabrication approaches.  The results revealed that MT nanocellulose with mono- to bi-layer sheet thickness (~0.4-0.8 nm), three to six chain width (~2-5 nm), and hundreds of nanometers to several microns length, can be prepared through TEMPO-mediated oxidation followed by 5-240 min intensive sonication.  The thickness, width, and length of MT nanocellulose all decreased with extended sonication time and leveled off after 1 or 2 h sonication.  Crystallinity, hydrogen bonding, and glycosidic torsion angles were evaluated by XRD, FTIR, Raman, and NMR.  These experiments revealed systematic changes to structure with sonication treatments.  A microfibril "cross section triangle scheme" was developed for the microfibril supramolecular modeling process and a 24-chain hexagonal/elliptical hybrid model was proposed as the most credible representation of the supramolecular arrangement for wood pulp-derived cellulose I" microfibril.  Comparative LCA of the fabrication of nanocellulose indicated that nanocellulose presented a significant environmental burden markup on its precursor, kraft pulp, and the environmental hotspot was attributed to the mechanical disintegration process.  Yet, overall nanocellulose still presented a prominent environmental advantage over other nanomaterials like single-walled carbon nanotubes, due to its relative low energy consumption. Overall, this research developed a facile approach to produce a new type of nanocellulose, the MT nanocellulose, provided new insights about the supramolecular structure of cellulose microfibrils, and evaluated the environmental aspects of the fabrication process of nanocellulose. / Ph. D.