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in the symmetry-breaking property of a membrane filter upon which the nanospheres reside and represents a significant step forward in the fabrication of complex nanocrystal architecture. The membrane serves as a filter for the larger nanospheres capturing them at the surface while allowing the smaller nanoparticles to pass through. As the nanoparticles flow past the nanospheres, they stick to amine groups that coat the nanospheres. The nanoparticles do not adhere to the underneath of the nanospheres during filtration. The researchers stress the importance of controlling the number of nanospheres retained at the surface of the membrane filter to ensure a sufficient number of unblocked pores so that filtration occurs efficiently. The material allows for the construction of new types of nanoscale architecture; for example, gold nanoparticles on one face of a nanosphere serve as selective binding sites for more gold nanoparticles. The repeated chemisorption of gold nanoparticles results in the templated assembly of multilayers of gold nanoparticles on one half of individual nanospheres. This research represents an attempt directed toward building complex nanoparticle architecture and the researchers are quick to point out that, for this to be useful, functionality will be necessary. In this regard, their future efforts will demonstrate the utility of similar materials as biochemical tags, templates for the synthesis of new nanostructures, and potential display elements. YUE HU

sis on a single-crystal Si(111) substrate. The substrates were cleaned and then Arion etched. A 100–200-nm-thick adhesion layer of Cr was then deposited, followed by a thin layer consisting of a mixture of Cr and Bi deposited in an atmosphere of Ar and N2. The total thickness of the latter films ranged from 1.5 µm to 2.5 µm. The researchers varied the Bi concentration from 1.5 at.% to 4.3 at.% by changing the power on the Bi target while maintaining constant power on the Cr target. Analysis by scanning electron microscopy (SEM) confirmed that the Bi nanowires were formed by compressive stress. Furthermore, SEM revealed a very high growth

rate of ~5 µm/s. By using wafer curvature measurements and Stoney’s equation, the researchers measured the compressive stress of ~470 MPa in a Bi-CrN thin film deposited on a silicon substrate. The research team concluded that the high compressive stress that accompanies the formation of thin films can be used for the formation of metallic nanowires. They reported, “When the high compressive stress is present in a composite thin film consisting of a low melting point material [like Bi, with a melting point of 271°C] and a high melting point matrix material [such as CrN], the low melting point material may be extruded from the

Compressive Stress Drives Formation of Bismuth Nanowires The properties of bismuth have attracted significant interest because of its highly anisotropic Fermi surface, low carrier densities, small carrier effective mass, and long carrier mean free path. Bi nanowires have previously been p