Toward direct bandgap silicon via two-dimensional silicane: A joint experimental and computational study

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Ryan, Bradley James
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Panthani, Matthew G
Roling, Luke T
Rossini, Aaron J
Cochran, Eric W
Prozorov, Ruslan
Committee Member
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Chemical and Biological Engineering
Silicon nanosheets (SiNSs) have attracted considerable attention due to their potential as next-generation materials for electronic, optoelectronic, spintronic, and catalytic applications. Even though monolayer SiNSs were first synthesized over 150 years ago, there is a lack of consensus within the literature regarding the structure and optical properties of this material. In this dissertation, we provide conclusive evidence of the structural and chemical properties of SiNSs. In addition, we cover topics including (1) a literature review of SiNSs, (2) theoretical predictions of the geometric, bonding, electronic, and vibrational properties of SiNSs with varying layer thicknesses, (3) a new pathway to synthesize SiNSs, (4) the short- and long-range structure of SiNSs, (5) the feasibility of using SiNSs in optoelectronic devices, (6) the surface chemistry of SiNSs, (7) the effects of defects and functionalization on the optical properties of SiNSs, (8) the energetics of intersheet spacing and translation of SiNSs, and (9) the modeling of the temperature-dependent time-resolved photoluminescence of SiNSs. Other miscellaneous topics include (10) a streamlined approach to simulating vibrational spectra and a convenient method of visualizing vibrational modes associated with structures, (11) theoretically synthesizable 2D III-V nanosheets, and (12) hydrazine carboxylic acid as a complexing agent for d-block metals for functional materials. We find that the structure of SiNSs is comprised of stacks of Si layers with disorder within the intersheet spacing and little disorder within the intersheet registry. The sheets are comprised of a monolayer of hexagonally arranged Si atoms, and the Si framework is corrugated. Each Si atom is connected to three other silicon atoms, though there are pockets of oxidized silicon, whereby oxygen is inserted into the backbone of the nanosheets. The surface of the nanosheet is terminated with ~70% hydrogen, ~20% chlorine, and ~10% hydroxyl groups. These chlorine and hydroxyl groups are mostly randomly dispersed through the surface of the sheets. We find evidence for direct bandgap emission in SiNSs, making them promising for CMOS-compatible optoelectronic applications.