Date of Award
Doctor of Philosophy (PhD)
Dr. Steven J. Stuart
Dr. Jason McNeill
Dr. Apparao Rao
Dr. Brian Dominy
Dr. Rachel Getman
An improvement to the AIREBO potential for hydrocarbons is presented in which contribu-tions to the bond order are determined by the local bonding environment around the bond, rather than the average of the environments around the two constituent atoms. This bond-centric approach decreases the errors by ∼80% in the fullerene-type systems for which the original approach leads to the most severe errors. With the newly developed and parameterized method, energy errors are less than 0.7eV for a collection of hydrocarbon molecules not used in the ﬁtting. This modiﬁed AIREBO potential is expected to be more useful not only for the molecular hydrocarbons and fullerene isomers studied here, but also for the full range of carbon and hydrocarbon systems to which the AIREBO potential has been applied.
The Raman intensity changes have been studied using the various armchair CNT(n,n)to understand bond polarizability changes in terms of the diameter, length, and atom number changes of the CNT(n,n). The DFT intensity and frequency calculations are performed at B3LYP/6-31G level. The bond polarizability changes are dependent on the structural distortions of the CNT(n,n), leading to the Raman intensity change. The structural changes from more like sp2 to more like sp3 hybridizations contribute to increasing the Raman intensities. The Raman intensity trends of CNTs are validated by the Raman intensity trends of Cn;n=20,28,60, and 70 isomers that represent the σ-π hybridizations from more like sp2 to sp3 structures. With the ﬁxed length unit(l=2), CNT(n,n)n=5-15 show the similar frequency patterns unlike CNT(n,n);n=below5in∼1100cm−1 and ∼1500cm−1. Additionally, the frequency intervals in the D band as well as the G band are shifted inward when increasing the diameters of the CNT(n,n);n=5-15. Vibrational density of states of CNT(n,n);n=2-10 have also been calculated. Strong VDOS changes are observed in 400-800 cm−1 and in 1200-1700 cm−1 as changing the armchair index(n,n);n=2-10. The VDOS found in ∼2100 cm−1 are considered to be edge eﬀects, having the relation to the small system size. From the VDOS, 3-decomposed vibrational modes can be provided. The decomposed vibrational modes demonstrate that most of the strong vibrational modes below 800 cm−1 are radial, and frequency shifts and the edge mode VDOS in the tangential mode are mainly shown. The edge mode VDOS found in ∼2100 cm−1 can be an interesting vibrational mode to study.
Lithium-ion rechargeable batteries are widely used for portable energy storage, both in con-sumer electronics, and increasingly in the automobile industry. There is still a pressing need for new electrode materials with higher capacity and energy density, however, silicon(Si) and germanium(Ge) are very attractive anode materials to improve the performance of Li-ion batteries, due to their re-markable theoretical capacities(Ge:1600 mAh/g, Si:4200 mAh/g). These high capacities correspond to the maximum lithiations Li22Ge5 and Li22Si5, compared to that of graphite, used in the current technology, with only a theoretical capacity of only 372 mAh/g, corresponding to the maximum lithiation LiC6. However, bulk Si and bulk Ge cause the aggregation and the pulverization during alloying/dealloying cycles. Recently, group IV nanowires such as silicon nanowire(SiNW) and germa-nium nanowire(GeNW) have been utilized to address the low cyclability issue. These nanostructured anode materials do not completely solve the issue, because their low cyclability(limited to about 10 cycles) means capacity fading of the anode still remains a critical problem. The low cyclability may stem from the aggregation encountered with nanosized materials during alloying/dealloying cycles. Consequently, the introduction of carbon nanotubes(CNTs) encapsulating SiNW or GeNW may address the low cyclability issue. Calculations with SnNW encapsulated by CNTs have reported favorable results. The use of SiNW and GeNW encapsulated by CNTs has been newly used for Li-ion battery applications using electronic structures with DFT method. Band structures and den-sity of states for the nanostructured systems have been evaluated to determine the metallicity and semiconducting properties for the anode materials.
Hur, Jaewoong, "Bond-Centric Modifications to Bond-Order Potentials and Ab-initio Evaluation of Nanostructured Materials Properties" (2013). All Dissertations. 2903.