Material Design And Computational Investigation On Graphene Nanoribbons And Silicon Carbide Nanoribbons

The family of carbon-based nanomaterials, including fullerene, carbon nanotubes,and graphene nanoribbons, has become the crucial intersection linkinginterpdisciplinary researches of chemistry, physics, biology, and material science fromboth the experimental and theoretical viewpoints ever since the successful fabricationof graphene in experiment. Amongst, graphene, along with its one-dimensionalallotrope graphene nanoribbons, is undoubtedly the brightest star instantly. Driven bythe more advanced computational methods and more upgraded computer capacity, thegeometric structures and physicochemical properties of more practicallow-dimensional nanostructures can be designed and studied at atomic level moreprecisely right now. With employing systematic density functional theory (DFT)computations, here, we have investigated the geometry, electronic and magneticfeatures, as well as the stabilities for graphene nanoribbons and the inorganicanalogue silicon carbide nanoribbons under different surface-modification strategies,where the effects of the formation of some typical defects on these performances havebeen explored, aiming at further facilitating the practical application potentials ofrelevant nanomaterials in the future multi-functional and spintronic nanodevices.Initially, we identified a new noncovalent surface-modification strategy toconveniently and effectively modulate the electronic and magnetic properties ofzigzag graphene nanoribbons (zGNRs). Taking advantage of the excellent delocalizedπ–conjugated backbone with single/multiple C≡C bonds bridging twopolydiacetylene (PDA) chains, the modified ladder-structure PDA derivatives canphysisorb stably on zGNRs via π π interactions, and more interestingly, break theenergetically degenerated edge states in the original band structures of zGNRs by thedipole field induced through the charge transfer between different acceptor/donor groups that decorate the PDA chains, rendering them half-metallic or metallic. Byaltering the width of zGNRs or the number of C≡C bond in the linking bridge ofPDA, the effect of the induced dipole field can be altered, leading to the abundantelectronic transition of spin gapless semiconductor (SGS) half-metal metal alongwith the magnetic conversion of antiferromagnetic ferromagnetic. Even with the57-reconstruction at the zGNR edges, multiple transformation of antiferromagneticSGS ferromagnetic half-metal antiferromagnetic metal nonmagnetic metal can alsobe observed with the PDA functionalization. Further, hydrogenation at the57-reconstructed edges of zGNRs can eliminate the effect of the edge-reconstructionand generally recover their properties to those based on the corresponding perfectnarrow H-terminated zGNRs representing the remaining pristine zGNRs inside,realizing similar SGS half-metal transition for zGNRs with a series of widths.Inspired by the accumulated extensive researches in graphene and rapidlyadvanced cognition in the inorganic nanomaterials, subsequently, we mainlyaddressed how the electronic and magnetic properties of silicon carbide nanoribbons(SiCNRs), one inorganic counterpart of graphene nanoribbons, may be affected underthe functionalization of hydrogenation one typical covalent surface-modificationstrategy and the formation of Stone-Wales (SW) defects one typical topologicaldefect, respectively, focusing on elaborating the effects of different hydrogenationpatterns/ratios and different defect orientations/sites on the relevant performances.Initially, the computed results reveal that the fully hydrogenated SiCNRs favor thechair configuration over boat and stirrup, where they are all nonmagneticsemiconductors with a descending trend of their band gaps as a function of the ribbonwidth, regardless of the edge chirality. With hydrogenation starting from the Si edge,C edge, and both edges of zSiCNRs, respectively, a series of substantial electronic andmagnetic behaviors can be precisely achieved as increasing the hydrogenation ratio:half-metal, SGS, metal, wide-band-gap semiconductor, magnetic, and nonmagneticfeatures. There even exist some structures among the partially hydrogenated zSiCNRsexhibiting almost the same electronic and magnetic behaviors as that of the remainingpristine zSiCNRs without hydrogenation, which may provide some useful insights into producing “narrow” SiCNRs in experiment. As inferred from the computedformation energies and binding energies per hydrogen atom to the SiCNRs, all ofthese hydrogenated SiCNRs present high thermodynamic stability, stronglysuggesting the feasibility to realize these structures by hydrogenating the pristineSiCNRs experimentally.It’s well-known that the formation of various defects is inevitable during thegrowth, fabrication, and processing of nanomaterials, amongst, SW defects is onetypical class of topological defects, created through rotating one bond of the pristinestructure by90°. It’s inferred from the computed results that when SW defects occurin SiCNRs (According to the different orientations of Si C bond relative to theperiodical direction of the nanoribbon, SW-1and SW-2are labeled to differentiate theSW defects with parallel/vertical and slanted orientations, respectively), the band gapsof aSiCNRs can be significantly reduced due to the inpurity band states introduced bythe defects, independent of the defect orientation, yet the original nonmagnetic andsemiconducting features are not affected; similarly, the SW-defective zSiCNRs stillexhibit energetically degenerated ferromagnetic and antiferromagnetic states with thecorresponding metallic and half-metallic behaviors, respectively, even half-metallicitymay be observed in both states, simultaneously. This can be advantageous for theprotection of the intriguing half-metallicity, inhibiting the drawback of beingvulnerable or even removed under small extra disturbances. Meanwhile, it’s foundthat the formation energies of SW defects in SiCNRs have defect orientation-andsite-dependency. Regardless of the edge chirality, the formation energies of SWdefects in the center of SiCNRs increase as widening the ribbon; comparatively, SWdefects at the edges of zSiCNRs generally present lower formation energies than thosein the center, yet the formation of SW-2is always energetically more favored thanSW-1. Moreover, we infer from the kinetic process of the formation of SW defects inSiCNRs that its barrier is remarkably lower than that computed for SW defects ingraphite, which has already been observed experimentally. Along with the largereverse energy barriers to stabilize the formed SW defects, these results can furthervalidate the possibility of the existence of SW defects in SiCNRs.