Core-Shell Nanowire Structures

The fundamental physical properties of nanowire materials can be improved even more to surpass their bulk counterpart using precisely engineered NW heterostructures. It has been recently demonstrated that Si/Ge/Si core/shell nanowires exhibit electron mobility surpassing that of state-of-the-art technology.46 Group III-V nitride core/shell NWs of multiple layers of epitaxial structures with atomically sharp interfaces have also been demonstrated with well-controlled and tunable optical and electronic properties.47,48 Together, the studies demonstrate that semiconductor nanowires represent one of the best-defined nanoscale building block classes, with well-controlled chemical composition, physical size, and superior electronic/optical properties, and therefore, that they are ideally suited for assembly of more complex functional systems. 

Yang, W. L., Z. Gao, J. Ma, X. M. Zhang, J. Wang, and J. Y. Liu. 2014. Hierarchical NiCo204 NiO core-shell hetero-structured nanowire arrays on carbon cloth for a high-performance flexible all-solid-state electrochemical capacitor. Journal of Materials Chemistry A 2 1448-1457. 

OA) and OAm and heated to 90 °C to produce a CdS shell of desired thickness. Then the reaction mixture was heated to 130 °C to obtain CdSe/ CdS core/shell nanowires of ca. 35 nm core and ca. 4nm shell thickness. The mechanism of the formation of CdSe/CdS consists of two steps nucleation and growth of the CdS nanorods over CdSe NWs followed by the CdS intrarod ripening to form ribbon-like structures. 

The advances in nanotechnology and synthesis methods have enabled nanomaterials to be produced in various shapes and structures. Coating of a luminescent layer activated by lanthanide ions on nanoparticles such as SiC>2 or AI2O3 is one of such approaches to develop new nanophosphors. In section 6, we review recent work on interesting spectroscopic features and luminescence dynamics of lanthanide ions in other novel low-dimensional nanostructures including core-shell, one-dimensional (ID) nanowires and nanotubes, two-dimensional (2D) nanofilms, hollow nanospheres, 2D nanosheet and nanodisk which have also attracted extensive attention. 

Pistol ME, Pryor CE (2008) Band structure of core-shell semiconductor nanowires. Phys Rev B 78 115319... 

In a further extension of LCG growth, Lieber and coworkers and Yang and coworkers independently demonstrated the preparation of nanowires with structurally complex radial or axial heterostructures. Radial or core-shell heterostructure nanowires were formed by depositing layers on a core nanowire (Fig. 6A). Using this approach, homoepitaxial growth of B-doped Si shells on intrinsic Si and heteroepitaxial... 

Most recently, Pt-based electrocatalysts with novel nanostructures such as nanowire, nanotube, hollow, core-shell, and nanodendrite structures have been investigated [71-74, 101]. One-dimensional ternary PtRuM (M = Ni, Co, and W) nanowire catalysts were synthesized, and these catalysts outperform Pt-Ru commercial catalyst and have a low noble-metal content due to the incorporation of an Earth-abundant element [101]. 

Over the past few years, research efforts have focused on the use of nano-structured anode materials (nanoparticles, nanocrystals, nanowires, nanorods etc., including complex core-shell and composite structures) in order to mitigate the effects of volume change upon Li uptake. Nanostructured materials present the add-on advantage of shorter diffusion distances for Li species, thus offering the possibility to increase charging and discharging rates (i.e., battery power). This chapter will essentially concentrate on such recent developments. 

Fig. 12. TEM and SAED analyses of the BeNx/BN nanostructures, (a) TEM image showing the coaxial core-shell structure. (b,c) SAED patterns correspond to the electron diffractions from [100] and [122] zone axes of BeN rhombohedral unit cell, respectively. The weak halfrings (corresponding to a separation of 0.33 nm) in the SAED patterns are from the electron diffraction of hexagonal BN (0002) atomic layers in the nanowire shells.

HRTEM studies highlight that the smooth nanowires consist of a single-crystalline core sheathed with a few graphitic walls with an interlayer spacing of 0.33 nm [Fig. 13]. The lattice images from the crystalline nanowire cores match the structure of rhombohedral B6N. The shell layers can be designated to hexagonal BN (0002) planes based on chemical analysis (see below) and TEM studies. 

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