Silicon nanowires (SiNWs) have attracted intense interest in recent years [1] because they are not only important for fundamental studies on various size-dependent phenomena, but they are also potential building blocks for nanoscale electronic and optoelectronic devices. [1±2] Various growth methods, including laser ablation, [3±6] thermal evaporation, [6±9] and chemical vapor deposition, [10±11] have been used to synthesize SiNWs. One important target of the synthetic approaches is to achieve a high degree of control of the diameter, growth orientation, and morphology of the SiNWs. A good understanding of the growth mechanism is obviously the key to obtaining SiNWs with desired properties required for various novel applications. While recent efforts have substantially advanced our understanding on the growth mechanisms of SiNWs, [6] many fundamental issues remain unclear. [3] Although SiO 2 -clad SiNWs appear generally circular in crosssection as shown by transmission electron microscopy (TEM) images, the cross-section of the Si crystalline core has been neither observed nor investigated. The cross-sectional study of SiNWs is expected to provide structural information about the boundary between the crystalline core and the oxide sheath, which, in turn, may shed light on the growth mechanism.In the present work, we have developed a unique samplepreparation technique to prepare the cross-sectional samples of SiNWs for TEM examinations. Using this technique, we successfully prepared and studied the cross-sections of SiNWs, which revealed that the Si core is bounded by well-defined facets of low-index crystallographic planes. We also obtained statistical data to elucidate the growth directions of the SiNWs synthesized by the OAG approach. We point out that the developed technique should be generally applicable to preparing the cross-sectional TEM samples of one-dimensional nanomaterials including nanowires and nanotubes.The as-grown SiNWs are randomly oriented with a length of more than 10 lm and a typical diameter of about 20 nm, as shown in Figure 1a. It is clearly revealed that SiNWs have different shapes. Some have a uniform size along the length of COMMUNICATIONS

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Energetic materials, including explosives, pyrotechnics, and propellants, are widely used in mining, demolition, automobile airbags, fireworks, ordnance, and space technology. Nanoenergetic materials (nEMs) have a high reaction rate and high energy density, which are both adjustable to a large extent. Structural control over nEMs to achieve improved performance and multifunctionality leads to a fascinating research area, namely, nanostructured energetic materials. Among them, core–shell structured nEMs have gained considerable attention due to their improved material properties and combined multiple functionalities. Various nEMs with core–shell structures have been developed through diverse synthesis routes, among which core–shell structured nEMs associated with explosives and metastable intermolecular composites (MICs) are extensively studied due to their good tunability and wide applications, as well as excellent energetic (e.g., enhanced heat release and combustion) and/or mechanical properties. Herein, the preparation methods and fundamental properties of the abovementioned kinds of core–shell structured nEMs are summarized and the reasons behind the satisfactory performance clarified, based on which suggestions regarding possible future research directions are proposed.

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A new type of GdAl 2 nanocapsule with single-phase intermetallic compound GdAl 2 as the core and amorphous Al 2 O 3 as the shell has been synthesized by the arc-discharge technique with modified strategies. Meanwhile, novel three-dimensional coral-like hierarchical branching macro-aggregates were self-assembled by disordered nanocapsules synthesized simultaneously in the arc-discharge process. The GdAl 2 nanocapsules display superparamagnetic properties between their blocking temperature of 100 K and Curie temperature of 162 K. The magnetocaloric effect of the GdAl 2 nanocapsules was measured between 5 and 165 K. The absolute value of the change of magnetic entropy of the GdAl 2 nanocapsules sharply increases with decreasing temperature and reaches 14.5 J kg −1 K −1 at 5 K in magnetic fields varying from 0 to 50 kOe. As a result, this new type of nanocapsule can prospectively be applied in cryogenic magnetic refrigerator devices.

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