| Over the past decade there has been a remarkable surge in the development of nanoscience and nanotechnology because of their ability to achieve novel properties and functions of materials with size reduction to nanoscale. At low dimension, quantum size effects have significant influence on the self-assemble growth process and related electronically, optical, chemical and mechanical properties of materials. Studies on the growth of ultrathin films and nanowires have great potential application in many areas including laser, superconductor and integrated circuit. In this paper, by using first principle calculations based on density functional theory, we systemically investigated the quantum size effects on the growth and properties of two dimensional ultrathin films and one dimensional nanowires. The main results are as follows:1. The stability of prototypical metal-atom wires including Au, Pb, and Ir wires are investigated with respect to the wire length as well as the wire diameter. We reveal that the Friedel oscillation generated due to the edges of wires drives the oscillating behaviors of stability as a function of the wire length, leading to the existence of highly preferred lengths. Such magic lengths are found to change depending on the wire diameter d and the number of valence electrons. Monovalent Au wires exhibit a monotonous increase in the distance between consecutive magic lengths with increasing d, giving rise to a converged value of 4a0 or 5a0(a0 : lattice spacing of each wire) even at d ~ 5.77 ?. By contrast, multivalent Pb and Ir wires exhibit a decrease in l with increasing d, approaching the same value of 2a0 at d ~ 4.97 and 11.4 ?, respectively. These results demonstrate that the stability of nanowires with diameters of less than ~10 ?(equivalently nanosize islands) rapidly converges to that of the corresponding film. We also find that the work function of all the wires oscillates with the same oscillation period of the wire stability. Our findings not only provide an explanation for the magic lengths or magic thicknesses observed from Au and Ir nanowires and Pb nanosize islands, but also shed light on the generic perspectives of quantum size effects in the stability of nanowires, nanosize islands, and ultrathin films.2. The phase shift between surface energy and work function in FCC(111) and HCP(0001) Pb and Pb1–x Bix alloy films has been investigated. Deviating from the previously described π/2 phase mismatch between the surface energy and work functions, an additional phase shift of about one monolayer is identified at small thickness of the Pb and Pb alloy films. The additional phase shift depends on the film thickness and will disappear as thickness increases. Moreover, we give an interpretation of the one-monolayer deviation in the framework of the free electron model, attributing it to the unique structure of the Fermi surface in Pb ultrathin films.3. Our cooperator demonstrated that the Zn O/Ga N hererojunction quality is typically poor despite minimal lattice mismatch between these two. More counter-intuitively, experimental studies demonstrate that a proper introduction of an Al2O3 spacer can drastically improve the overall quality of the heteroepitaxial growth, even though the spacer has substantially larger lattice mismatch with the host system. Using density functional theory calculations, we elucidate the underlying mechanisms involved. We show that the strain energy accumulation in Zn O/Ga N increases linearly with the Zn O thickness, rapidly reaching the critical value for unstable structural rearrangements. In contrast, when Zn O is grown on an initially crystalline Al2O3, the strain energy buildup is much slower, and quickly saturates, implying more stable epitaxial growth. Further detailed analyses reveal that in the latter case, an amorphous spacer can effectively shield the stress propagation across the interface, making the Al2O3 an effective compliant substrate. |
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