Application of ab initio theory to the chemistry of ultrathin films.

In this work, we investigate a number of important nanosheets, e.g., graphene, haeckelites, and titanium disulfide for the expressed purpose of tuning the electronic ground state properties. We employ condensed matter techniques to interrogate real- ized and theoretically postulated ultrathin films to mine ground state properties that may bolster established, or nascent nanotechnologies. In this regard, a number of ultrathin films are tuned to induce new material properties that are not intrinsic to the original crystal. We show that chemical modification with extrinsic substitutional pnictogen dopants placed within the crystal lattice of graphene can functionalize the basal plane of graphene to obtain potentially catalytic properties. Furthermore, an alternative doping strategy, less intensive than pnictogenic substitutions, including halogen diatomic molecules were introduced as adsorbates on monolayer, bilayer, and multilayer graphenes of different polymorphism to influence the ground state of the graphitic nanosheets. We observed the induction of a band gap of controllable size as a function of halogen and polymorphism. Consequently, the semimetallic graphene sys- tems formed a p-type semiconductor, which enables field-dependent control of Dirac carriers within the ultrathin films. Each of these studies take advantage of the orbital and lattice degrees of freedom enabling tunability of this monoelemental nanosheet. However, the authors postulate theorized ultrathin films dubbed Archimedean ultra- thin films. These nanosheets form a unique semiregular polygonal (4,8)-tessellated configuration. This configuration was extended to bulk crystals where we show the potential for forming ultrathin films that contain this unique symmetry. Two groups were studied: the boron pnictides, and the aluminum pnictides. The ground states featured indirect band gap semiconductors, where it was discovered that the boron- pnictides, in particular the planar configurations, possessed a double band gap. Sub- sequently, the optical response of the boron pnictides were revealed within linear response time-dependent density functional theory, which showed that the planar ul- trathin films displayed strong optical response from the UV to the IR. Finally, the electronic ground state of 1T-TiS2 was mechanically strained to induce phase tran- sitions converting this nanosheet into a direct band gap semiconductor. Hence, we demonstrate the tunability of material properties for a series of ultrathin films, whose material properties could provide or support existing and nascent nanotechnologies for the 21st-century.