Editorial for nanoscale themed issue “Recent Advance in Semiconductor Nanowire Research”

Semiconductor nanowires and heterostructures have drawn considerable attention in the last few years.¹ A variety of unprecedented properties has been discovered along with the flourishing discoveries of rationally synthetic tunabilities. Recent applications of nanowires have been focused largely in optoelectronics and energy, including photodetectors, light-emitting diodes, photovoltaics, photocatalysis, nanogenerators and optopiezotronics.^2–5 All these applications require a solid understanding of the optical and electrical property of the nanowires and the rational engineering of structure or the composition of the nanowires. For instance, tunable bandgaps via doping, defect engineering or alloying, is demanded in nanomaterials towards applications in photodetection and photovoltaics in the visible spectral regime. One particular advantage of nanowires is the fast electron transport which can be even further tuned and controlled because of additional engineering knobs attributed to nanowires and heterostrutctures. For example, core–shell/multishell nanowire heterostructures have been shown as effective ways to confine carriers at the interface, leading to one dimensional electron or hole gas with considerably higher mobility than bulk and high performance nanoelectronic circuits.^6,7 Additionally, further nanowire branching out is recently explored to achieve large surface area and efficient solar energy harvesting towards the third generation photovolatics and high efficient photocatalysis.^8–11

This themed issue highlights the recent advances in synthesis of nanowires and heterostructures, electrical and optical properties, and the applications particularly in the emergent solar energy harvesting such as photovoltaics and photocatalysis. This issue consists a collection of two review articles, two featured articles and a few communications and articles that are contributed from the International Conferences on Materials for Advanced Technologies 2011 (ICMAT 2011) held in Singapore in June 2011.

The review article by the Huang group overviews the synthesis of metal silicide nanowires by controllable solid state reaction, by which a sub-10 nm silicide–silicon–silicide abrupt junction can be formed. Both the growth kinetics and high performance field effect transistors have been elaborated in great detail. The review article from the Xiong group presents a comprehensive review to cover the synthesis and optical properties of II–VI nanowires and nanobelts (e.g., ZnO and cadmium chalcogenides), elaborating the fundamentals of optical properties, such as exciton and exciton complex formation, optical transitions involved in the exciton dynamics and phonon properties. The feature article by the Soci group discusses the photonic properties of III–V nanowires. The anisotropic absorption and emission properties are elaborated in the context of two common crystallographic phases: zinc-blende and wurtzite, supplemented with theoretical calculations. Nanowire and nanotube arrays offer the advantages of faster electron transport, lower recombination loss and facile large scale synthesis compared with nanoparticle-based dye-sensitized solar cells (DSSC). The feature article by the Cao group highlights their recent progress in ZnO nanowire and TiO₂ nanotube arrays for solar cell applications, particularly as photoelectrodes in DSSC. An inverted structure polymer solar cell is discussed with a higher performance due to faster electron transport from polymer to electron collection electrodes.

The contributed communications and articles covering three main research directions: (1) nanowire growth mechanism (3 papers); (2) optical and electrical properties (3 papers); and (3) photochemical water splitting (4 papers). These three groups are summarized as follows.

The article by Russo-Averchi et al. discusses that the multiple seed formation and 3-D twinning at the initial growth stage determine the orientation of nanowires in self-catalyzed GaAs nanowires grown on silicon. The ratio of V/III plays a crucial role, in which a high V/III ratio can yield 100% vertical nanowires. Ortrikov et al. discuss the plasma effect in the synthesis of semiconductor nanowire. Plasma generates high energy radicals which increase the nucleation and growth rates. Pan and co-authors report an interesting chain-like Si@SiSe₂ heterostructures via CVD and explain the growth mechanism based on a self-organization model.

In the 2nd category, Wong et al. present a study of tunable electroluminescence from randomly distributed ternary n-CdS_xSe_1−x nanowires on p-SiC substrates. The variation of the Se doping concentration may tune the electroluminescence in a large range between 520–720 nm. Kao et al. report high yield synthesis of ZnO via a multi-step hydrothermal method, leading to improved overall shape and length control. The defect emission due to surface and functional groups is discussed. Single and multiple nanowire UV photodetectors have also been demonstrated. Huang et al. report a low temperature synthesis of tin oxide nanorod arrays by plasma-enhanced CVD. Improved field emission properties are recorded after Ni nanocrystal decoration on the nanorod tips.

Semiconductor nanowires and heterostructures exhibit considerable promise towards high efficient solar energy absorption towards photocatalytic water splitting due to their excellent light harvesting, fast electron transport and tunable absorption edge due to expanded toolbox of compositional and structural tunability. Four contributed articles describe a variety of nanowire and heterostructures for photocatalytic water splitting applications. Sun et al. present a 3-D branched tree-like ZnO–Si nanowire heterostructures as the photocathode. Chu et al. describe the aligned ZnO nanowires for the photocalytic activity by photodegradation of methylene blue. Wang et al. discuss the improved photoelectrochemical performance of CdS-sensitized TiO₂ nanowire arrays by thermal treatment in ammonia atmosphere or under vacuum. Zhang et al. present a two-step formation of vertical-aligned ZnO-ZnGa₂O₄ core–shell nanowire array, and find that the core–shell structure enhances the anti-corrosion stability of ZnO during the photoelectrochemical reactions.

To put it in perspective, research on semiconductor nanowires and heterostructures will continue to be driven by various functionalities in optoelectronics and energy harvesting. It is still ultimately important to achieve low-cost, high yield, and large scale synthesis with controllable properties and structural complexity to engineer electrons, phonons and their interactions with light. In addition, novel device architecture and design, processing and packaging are highly demanded towards high performance optoelectronic and energy conversion devices, which is much under-exploited compared to the knowledge we gained in the synthetic controls.

References

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2. P. D. Yang, R. X. Yan and M. Fardy, Nano Lett., 2010, 10, 1529–1536.
3. B. Tian, X. Zheng, T. J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang and C. M. Lieber, Nature, 2007, 449, 885–889.
4. Z. L. Wang, Nano Today, 2010, 5, 540–552.
5. M. D. Kelzenberg, S. W. Boettcher, J. A. Petykiewicz, D. B. Turner-Evans, M. C. Putnam, E. L. Warren, J. M. Spurgeon, R. M. Briggs, N. S. Lewis and H. A. Atwater, Nat. Mater., 2010, 9, 239–244.
6. W. Lu, J. Xiang, B. P. Timko, Y. Wu and C. M. Lieber, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 10046–10051.
7. S. Nam, X. C. Jiang, Q. H. Xiong, D. Ham and C. M. Lieber, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 21035–21038.
8. C. Cheng, B. Liu, H. Yang, W. Zhou, L. Sun, R. Chen, S. F. Yu, J. Zhang, H. Gong, H. Sun and H. J. Fan, ACS Nano, 2009, 3, 3069–3076.
9. M. J. Bierman and S. Jin, Energy Environ. Sci., 2009, 2, 1050–1059.
10. I. S. Cho, Z. B. Chen, A. J. Forman, D. R. Kim, P. M. Rao, T. F. Jaramillo and X. B. Zheng, Nano Lett., 2011, 11, 4978–4984.
11. J. Z. Zhang, MRS Bull., 2011, 36, 48–55.

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Footnote

† This article was submitted as part of a collection highlighting papers on the ‘Recent Advances in Semiconductor Nanowires Research' from ICMAT 2011.

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