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In the early 19th century, a series of engineering and scientific breakthroughs by Nicolas Léonard Sadi Carnot, James Watt and many others led to the foundations of thermodynamics and a new pedigree of mechanical designs that reset the standards of engineering efficiency. The result was the industrial revolution. In optical- and electronics- based nanotechnology research, a similarly subtle bargain is being made; we cannot alter the fact that systems have a finite response to external excitations, but what we can do is enhance that response. The promising attributes of ZnO have long been recognised; its large band gap and high exciton binding energy lend it to a number of applications from laser diodes, LEDs, optical waveguides and switches, and acousto-optic applications to sun cream. When this material is grown into nanowires and nanorods, the material gains a whole new dimension, as quantum confinement effects come into play. Discovery of the enhanced radiative recombination, which has potential for exploitation in many optical and opto-electronic applications, drove intensive research into investigating these structures and into finding methods to synthesise them with optimised properties. This research revealed further subtleties in the properties of these materials. One example is the work by researchers in the US reporting synthesis procedures that produced a yield—defined as the weight ratio of ZnO nanowires to the original graphite flakes—of 200%, and which also demonstrated, through photoluminescence analysis of nanowires grown on graphite flakes and substrates, that graphite induces oxygen vacancies during annealing, which enhances the deep-level to near-band-edge emission ratio [1].Other one-dimensional materials that provide field emission enhancements include carbon nanotubes, and work has been performed to find ways of optimising the emission efficiency from these structures, such as through control of the emitter density [2]. One of the advantages of ZnO nanowires for field emission devices has been greater control over the electronic properties. Alternative morphologies of ZnO nanostructures have also been explored for field emission enhancements, such as urchin structures, which provide field enhancement factors of 1239, but with the additional benefit of greater stability [3].Theoretical investigations to understand the mechanisms behind these field enhancements have also grown increasingly more sophisticated, through both analytical techniques and finite theorems. Results from a comparison of these two approaches in the form of Mie theory and the finite element method, using a dipole oscillator as the excitation source, were reported recently by researchers from Duke University, USA [4]. The work found excellent agreement in terms of amplitude, plasmon resonance peak position and full width at half-maximum.These field enhancements lend themselves to a range of technological applications, such as the demonstrated potential of plasmonic interactions in DNA sensing arrays [5]. As well as plasmon resonances, Bragg diffraction in nanoparticles also has the potential to provide enhanced system responses. Researchers in Taiwan have shown enhancements in the acceptance angle as well as the photoresponsivity of n-ZnO/p-si photodiodes with the use of a monolayer of silica nanoparticles [6].In this issue, researchers in Italy and Japan report work on enhancing the cathodoluminescence from SiC-based systems. They investigate the role of a shell of amorphous silica in core/shell 3C-SiC/SiO<sub>2</sub> nanowires and observe a shell-induced enhancement of the SiC near-band-edge emission, which is attributed to carrier diffusion from the shell to the core, promoted by the alignment of the SiO<sub>2</sub> and SiC bands in a type I quantum well [7]. Their research is another demonstration of how nanostructures provide enhancements to system responses through a wide range of mechanisms, a breadth of creativity that is mirrored in the approaches to investigating and exploiting these structures. References[1] Banerjee D, Lao J Y, Wang D Z, Huang J Y Steeves D, Kimball B and Ren Z F 2004 Nanotechnology 15 4040–9[2] Nilsson L, Groening O, Emmenegger C, Kuettel O, Schaller E, Schlapbach L, Kind H, Bonard J-M and Kern K 2000Appl. Phys. Lett. 76 2071–3[3] Jiang H, Hu J, Gu F and Li C 2009 Nanotechnology 20 055706[4] Khoury C G, Norton S J and Vo-Dinh T 2010 Nanotechnology 21 315203[5] Le Moal E, Lévéque-Fort S, Potier M-C and Fort E 2009 Nanotechnology 20 225502[6] Chen C-P, Lin P-H, Chen L-Y, Ke M-Y, Cheng Y-W and Huang J-J 2009 Nanotechnology 20 245204[7] Fabbri F, Rossi F, Attolini G, Salviati G, Iannotta S, Aversa L, Verucchi R, Nardi M, Fukata N, Dierre B and Sekiguchi T 2010 Nanotechnology 21 345702 |
