| Due to the large surface-to-volume ratio and size confinement effects, low-dimensional nanomaterials exhibit electronic, photonic, mechanic, and magnetic properties which are quite different from their bulk materials. Therefore, they have been in the forefront of the hottest fundamental materials researches in recent years. Discovery of carbon nanotubes in the early 1990s has inspired a great interest in exploiting the promising potentials of groups III-V and II-VI semiconductor nanomaterials in new electronic and optoelectronic device applications based on their specific geometries and distinct properties. ZnS is an important wide-bandgap semiconductor. The nanoscale morphologies of ZnS have gained a tremendous amount of attention in recent years because of their fascinating properties, such as the blue-shift of absorption peak, enhanced field-emission properties, the promotion of catalytic activities, the change of magnetical correlation etc. Low dimensional ZnS nanomaterials are thus promising candidates for novel diverse applications, including nonlinear optical devices, light-emitting diodes, field emitters, sensors, photocatalyst, and so forth.ZnS nanomaterials have been proven to possess a variety of morphologies. It is shown that the properties of ZnS nanomaterials are highly size-and shape-dependent, and they can also be enhanced via component modulation. However, some key issues in ZnS nanomaterials, such as the relationship among the stable morphology, the electronic property, and the size; phase transition; the effect of surface states on the electronic structure, etc., remain unclear. Theoretical investigations on size-dependent stable configurations, energetics, and electronic properties of ZnS nanomaterials are expected to provide vital information for understanding the size confinement effects, surface effects, the growth mechanism, as well as useful guidance to experimental synthesis of these nanomaterials, and are thus highly desirable. On this basis, design of ZnS-based hetero-nanomaterials with new or enhanced performances is valuable. Therefore, these issues become the choice of present dissertation theme.The first-principles calculations on the basis of density functional theory (DFT) combined with molecular dynamics simulations (MDSs) have been proven to be a useful theoretical method in revealing the structures and properties of nanomaterials. The morphologies and electronic structures, magnetism, mechanic properties, optoelectronic absorption and excitation, dynamics of chemical reaction, etc., can be reasonably predicted using this theoretical scheme. In this dissertation, we performed first-principles calculations to study the size-dependent stable configurations, energetic, and electronic properties of ZnS nanomaterials, as well as the tuning of electronic properties via formation of ZnS-based hetero-nanomaterials. The main conclusions are summarized as follows:(1) The size-dependent stable configurations, energetic, and electronic properties of ZnS nanomaterials, including (ZnS)n (n=6-48) bubble clusters, (ZnS)60 double bubble cluster, single-walled ZnS nanotubes (SW-ZnSNTs), faceted double-and triple-walled ZnS nanotubes (DW-, TW-ZnSNTs), faceted ZnS nanowires (ZnSNWs), and (1010) ZnS nanosheets (ZnSNSs), were systematically studied by using first-principles calculations. We found that the formation energy of faceted ZnSNTs is proportional to the inverse of wall thickness, irrespective of the diameter. If the wall thickness of a faceted ZnSNT is close to the diameter (thickness) of a ZnSNW (ZnSNS), they will have close formation energies. The ZnSNWs and faceted ZnSNTs are energetically more favorable than SW-ZnSNTs with round cross sections. The formation energy of (ZnS)n bubble clusters is proportional to the inverse of the cluster size, n-1, especially for large-sized clusters. Both of the (ZnS)60 double bubble cluster and DW-ZnSNT are energetically more favorable than the (ZnS)n bubble clusters and SW-ZnSNTs, among which DW-ZnSNT is energetically the most favorable. The highly strained 4-atom rings involved in (ZnS)n bubble and double bubble clusters are energetically disadvantageous for them compared with ZnSNTs. All of the ZnSNWs and ZnSNTs are wide band gap semiconductors, each of which has a direct band gap (energy gap) at the?point. The surface states lie in below the valence band maximum (VBM) of ZnSNWs, which is different from those in AINNWs. The band gaps (energy gaps) of these ZnS nanostructures decrease with the increase of size or dimension, indicating that ZnS nanomaterials with smaller size or dimension have stronger size confinement effect.(2) To explore the stablization mechanism of the polar ZnS (0001)/(0001) surfaces and possible phases in the growth of (0001)/(0001) nanofilms (NFs), the stable configurations and electronic properties of (0001)/(0001)-surfaces-derivated ZnS NFs were investigated. The size-dependent stable configurations of these NFs are characterized by a graphitic-like structure (G-NF), a film terminated by (0001)/(0001) surfaces, and a new phase composed of quadrilateral-octagon network (QO-NF), respectively. Our DFT calculations showed that the stable configurations of ultrathin ZnS NFs are size-dependent, and exhibit phase transition with the increase of film thickness. For the NFs containing 2 bilayers, the G-NF is energetically the most favorable. When the film thickness is in the range of 2.60 A-66 A, the QO-NFs are more stable than the polar (0001)/(0001) NFs because of the partially removed surface dipoles. Surface metallization occurs on the S-terminated surface of the (0001)/(0001) NFs, while other NFs are direct-band-gap semiconductors with band gaps wider than w-ZnS crystal. Both electron transfer and surface reconstruction are responsible for the stabilization of the (0001)/(0001) NFs.(3) The metallization of Si surfaces via the formation of heterointerfaces is an interesting topic from the viewpoint of fundamental physics and potential applications in new-generated electronic devices. We proposed, using first-principles method, that the electronic structures of Si (111) substrate can be tuned from semiconducting to metallic by depositing ZnSNFs with different thicknesses. Both of the S/Si and Zn/Si interfaces are considered. We found that both of the S/Si and Zn/Si interfaces are free from defects and only slightly distorted, with the latter being energetically more preferable. As the ZnSNFs contain more than 2 bilayers, the ZnS/Si heteronanofilms become metallic independent of interface structures. Metallization can be achieved in the Si(ZnS)n heterostructures with n> 1, in which highly dispersive bands cross the Fermi level. The metallization of Si(SZn)n with n> 2 is derived from electron transfer from ZnS surface to the interface Si atoms, forming two-dimensional electron gas systems confined in the region near the S/Si interface.(4) ZnS/ZnO hetero-nanostructures with different morphologies have been synthesized experimentally, which show tremendous potentials in new electronic and optoelectronic device applications. However, due to the large lattice-mismatch between ZnS and ZnO, defects, such as lattice distortion and stacking faults, were often observed at the interfaces of the hetero-nanostructures, making theoretical simulations of them impracticable. For simplification, we performed first-principles calculations to study the interface geometry, built-in field, and electronic properties of ZnS/ZnO heteronanotubes with small strain. Both of the axial and core-shell heteronanotubes were considered. The results showed that the axial (5,5) and (6,0) ZnS/ZnO heteronanotubes have smooth and defect-free interfaces. The charge redistribution in the region near the interfaces brings about a built-in field, resulting in steplike and sawtooth-like static potential profiles in (5,5) and (6,0) heteronanotubes, respectively. The band scheme diagram of the (5,5) heteronanotube displays the characteristic of type-I band alignment. The ZnO (5,0)@ZnS (13,0) core-shell heteronanotube, rather than the ZnS (5,0)@ZnO (15,0) one, has a type-II band alignment, and the weak interaction makes them possible candidates as UV optically active materials, which are required for applications in photonics and optical communication. |
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