Wire-like nanostructures have attracted extensive interest over the past decade because of their great potential for addressing some basic issues about dimensionality and space-confined transport phenomena as well as applications in nanoelectronics, nanomechanics, and photonics. The vapor-liquid-solid (VLS) method [ 1 ] has been extensively used to grow one dimensional nanostructures, such as nanowires and nanotubes. This process employs catalytic metal liquid droplets to direct the growth direction and determine the diameter of the nanowires. Nanowires of Si, III-V and II-VI compounds, and various oxides (e.g. ZnO and SiO₂) with very good crystallinity have been fabricated by the VLS process.  

Nevertheless, defects such as dislocations, stacking faults, and twinning have also been frequently reported [ 2, 3, 4 ]. To understand formation mechanism of defects, and to develop strategies to control over them become urgent since defects will greatly affect physical properties of nanowires. We will discuss this issue starting with a brief introduction of a recent work on high density lamellar twins in GaP nanowires grown in <111>B direction [ 5 ]. 

III-V nanowires are one type of the extensively studied semiconductor nanowires. Generally <111>B direction has been chosen to grow III-V nanowires, because it is the most favorable direction for wire growth. Unfortunately, the products always incorporate with a great number of twins with the twinning planes normal to the growth direction [ 2, 3 ]. Taken GaP as a model material, Johansson et al. recently studied such high density of twinning common in <111>B-oriented nanowires with the zinc blende structure, which is an important contribution to understanding of defect formation in nanowires[ 5 ]. 

The high resolution transmission electron microscopy (HRTEM) images of the GaP nanowires viewed along [-110] showed that the wires had zig-zag sides, which were correlated to lamellar twin segments with the twin planes normal to the growth direction. The sides became perfectly smooth and the twin planes disappeared when the wires were viewed in the [11-2] direction. From these facts the authors determined that each segment was a truncated {111} octahedron.

Fig. 1 Model of the zig-zag GaP nanowire.

a) an octahedron with 8 equal sized {111} faces acted as the building block; b) formation of a nanowire with faceted side surfaces through consecutive twinning of the octahedral segments; c), d ) a slice cut from the regular octahedron and the zig-zag nanowire built by the twinning rule, which is the better represent for GaP nanowires reported by Johansson et al [ 5 ]. The orientation is tilted away from [110] by 30° for a better view.

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Figure 1 illustrates how to form a GaP nanowire with faceted side faces through consecutive twinning of the octahedron blocks. Shown in Fig. 1a is an octahedron with eight equal sized {111} faces, i.e. a regular octahedron. A (111) twin forms when such an octahedron contacts with another one that rotates 60° by [111], and repeating this process will finally results in the zig-zag nanowire illustrated in Fig. 1b. As the cross-section of their GaP nanowires was hexagonal and the building blocks were rather thin, Johansson et al modeled that the blocks were slices cut from the middle region of the corresponding regular octahedron (Fig. 1c, and see Fig. 1d for the nanowire constructed by twinning of these truncated octahedrons) [ 5 ]. 

They further analyzed the thickness of the twin segments and found out that it had an exponential distribution and was independent to the wire diameter, and higher growth temperature would lead to higher twinning probability. They then drew out from a qualitative calculation that the critical radii for the twinned nucleus and the single crystalline nucleus are 30 nm and 15 nm, respectively. The small difference of the critical radii means that the twin plane energy is very low, so the energy barrier can be easily overcome to result in nanowires with consecutive twinning.  

This finding is noteworthy because it indicates that defects such as twinning are difficult to avoid in growth of nanomaterials.  It has been reported that nanoparticles with the face-centered cubic lattice exist easily as decahedral and icosahedral multiply twinned particles (MTPs), i.e. twinning of 5 or 20 (truncated) {111} tetrahedrons. Despite introducing of twinning plane energies and elastic energies (lattice has to be deformed for space filling), a theoretic approach indicates that MTPs are energetically more favorable than single crystals for smaller nanoparticles [ 6 ]. Another important behavior of nanocrystals is that the existence of dislocations in very small crystal, which is contrary to calculations showing that small crystals are defect-free, which the authors attributed to misoriented contact of single crystals [ 7 ]. 

These findings indicate that the energy difference of the single crystalline phase and the phases with defects (meta phases from the viewpoint of classical theories) is small, and the energy barrier between these minima can be very low that fluctuations during growth are ready to introduce defects such as twinning. However, there are examples that seem contrary to this mechanism. One example is the Zn₂SnO₄ nanowires with periodic twins [ 8 ]. The blocks in this case are regular octahedra, therefore twinning is forced to form for maintaining wire growth. Otherwise, the (111) plane will end in a sharp tip and the growth will either cease or begin in other directions.  

This is similar to the bamboo-like anatase TiO₂ nanowires synthesized by a nonaqueous solution method, wherein the wires were formed by contact of truncated octagonal bipyramids (with {101} side faces and {001} top/bottom faces) on {001} faces [ 9 ]. These nanowires are defect-free single crystalline, and the formation of the beer tun-like segments prevents {001} growth faces from vanishing.

We can see from these examples that the work from Lund University is an important contribution to understanding of defect formation in nanowires, but still we should see that we have little knowledge of the properties of nanomaterials. How to avoid high density defects such as twinning is important for growth of nanowires with good crystallinity. Yet it is equally important to find out strategies to realize controlled formation of twins. For one thing, nanowires with periodic twin planes represent a novel structure that may have specific electronic, phonotic, and other physical properties. For the other, twinning induced branching of nanowires (nanorods) can be employed to fabricate complicated three dimensional structures (see e.g. the octa-twin tetraleg ZnO nanostructures, [ 10 ]), which may have potential perspective applications in nanomechanics and photonics.  

How about other defects then? Here are two examples to answer this question. One is that the growth ends of many nanorods exhibit square or rectangular micro terraces [ 11, 12 ], which probably indicates that the rods are formed by lateral epitaxial growth of thinner rods, and small angle boundaries or dislocations may be presented and hinder thinner rods from merging into a perfect thinker one.  

The other example is the growth of three dimensional WO_3-d nanowire networks [ 12 ]. The growth direction of this material was <001> and the network was formed by continuous branching along other <001> directions. The branching was thought to be initiated by ordered in-plane oxygen vacancies.

We have paid much attention to make single crystal nanowires, now it is time to focus on how to introduce controlled defects such as twinning, stacking faults, and dislocations in nanocrystals. □

*Guobin Ma is in the Physics Department of Nanjing University, Nanjing 210093, CHINA.

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