Morphological Evolution and Ordered Quantum Structure Formation in Heteroepitaxial CoreShell Nanowires Jun-Yan Guo, †,‡ Yong-Wei Zhang, †,‡, * and Vivek B. Shenoy §, * † Department of Materials Science and Engineering, National University of Singapore, Singapore 117546, ‡ Institute of High Performance Computing, Fusionopolis, 1 Fusionopolis Way, #16-16, Connexis, Singapore 138632, and § School of Engineering, Brown University, 182 Hope Street, Providence, Rhode Island 02912 A dvances in synthesis and fabrica- tion of coherently strained semi- conductor heterostructures, such as quantum dots 1-3 (QDs) and core-shell nanowires, 4-7 offer the prospect for accu- rate and scalable device engineering at an atomistic scale for applications in electron- ics, photonics, and biology. 8 Since the opti- cal and electronic properties of low- dimensional nanostructures are sensitive to morphology, dimension, composition, and, especially, strain, they can be “tuned” to very specific requirements. Thus, control of surface morphology and engineering of strain is of significant importance in fabrica- tion of these structures. It has been recog- nized that the strain is crucial to the mor- phological evolution and growth of nanostructures. For example, stressed pla- nar thin films are susceptible to formation of surface undulations or corrugations through surface diffusion driven by elastic strain energy. This phenomenon is often called the Asaro-Tiller-Grinfeld instability. 9-13 A useful consequence of the thin film surface instability is the formation of arrays of coherent 3D islands that spon- taneously appear during the growth of strained films. This growth mode provides a natural route to fabricate quantum dot arrays in a number of different material systems. 14-16 The morphological instability inherent to strained thin films has recently also been observed in strained core-shell nanowires. For example, experiments from Pan et al. 17 and Goldthorpe et al. 18 have revealed the growth of the 3D germanium (or silicon) is- lands around a silicon (or germanium) core during the growth of Si-Ge core-shell nanowires. While the understanding of the factors that control the morphological sta- bility is important for growing smooth core-shell structures on the one hand, on the other hand, quantum dot arrays grown by this process can open up opportunities for novel photonic and electronic devices. For example, new energy levels that arise from the formation of quantum structures could be potentially exploited to allow more efficient absorption of solar energy in nanowire-based solar cells. 19 Most recently, Huang et al. 20 have experimentally obtained pure germanium QDs on both sides of 20 nm thick, 80 nm wide free-standing silicon nanoribbons via the Stranski-Krastanov growth or quantum dot self-assembly. This opens up a new way to create mechano- electronic superlattices. Only a limited amount of work is cur- rently available on the morphological stabil- ity of heteroepitaxial core-shell nanowires. 21,22 Schmidt and co-workers 21 performed an analysis of the stability of coherently strained core-shell nanowires using a first-order perturbation method. Their investigation showed the existence of a parameter range within which surface instabilities arise during growth. Very *Address correspondence to zhangyw@ihpc.a-star.edu.sg, vivek_shenoy@brown.edu. Received for review February 20, 2010 and accepted July 27, 2010. Published online August 3, 2010. 10.1021/nn101218r © 2010 American Chemical Society ABSTRACT We have performed three-dimensional dynamic simulations to study strain-driven morphological evolution and the formation of quantum structures on heteroepitaxial coreshell nanowire surfaces. Our simulations show that depending on geometric and material parameters, such as the radius of the wire, the thickness of the shell, and the mismatch strain, various surface morphologies including smooth coreshell nanowire surfaces, nanoring arrays, nanowire arrays, and ordered quantum dot arrays can be obtained by controlling initial surface configurations through prepatterning. It is also shown that these quantum structures may be trapped in a metastable state and may undergo a series of metastable state transitions during subsequent dynamic evolution. Our results identify possible pathways for fabrication of ordered quantum structures on the epitaxial coreshell nanowire surfaces and provide guidelines for achieving smooth coreshell structures. KEYWORDS: coreshell nanowire · misfit strain · surface morphology · instability · quantum structures ARTICLE www.acsnano.org VOL. 4 ▪ NO. 8 ▪ 4455–4462 ▪ 2010 4455