Simple Method Can Suspend Individual Nanofibers
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from the ohmic behavior that is observed in pure InAs whiskers. Additional details on the electronic properties of these structures are reported in the February 11 issue of Applied Physics Letters. To synthesize Si/SiGe nanowires with diameters ranging from 50 nm to 300 nm, Wu and co-workers periodically introduced Ge vapor through pulsed laser ablation of a Ge target in the presence of a Au nanocluster catalyst. At temperatures ranging from 850°C to 950°C, a Au thin film on a Si substrate forms an alloy with Si and separates into nanometersized droplets. Si continuously deposits into the Au-Si alloy droplets, and growth of the Si nanowire occurs upon supersaturation of the droplets. By turning the laser on, the researchers caused both Ge and Si to be deposited into the droplets, causing the precipitation of SiGe. By continuously switching the laser on and off, a Si/SiGe superlattice was formed in a block-by-block fashion. The researchers showed that the diameter and composition of the highly crystalline Si/SiGe nanowires could be controlled by adjusting the reaction conditions. Specifically, the nanowire diameter was varied from 20 nm to 100 nm by changing the thickness of the Au film from 1 nm to 20 nm. The work performed by these three research teams signifies an important turning point in nanoscale research. Analogous to the way in which twodimensional thin-film heterostructures transformed the planar semiconductor industry, heterostructures created inside nanowires offer the potential for diverse applications such as nanobarcodes, polarized nanoscale LEDs, 1D–0D–1D resonant tunneling devices, and improved thermoelectric devices. STEFFEN K. KALDOR
Simple Method Can Suspend Individual Nanofibers Researchers at Germany’s Max Planck Institute have devised a technique to suspend an individual nanofiber over a Si/SiO2 substrate by using coordinate markers and a sacrificial layer of electronbeam resist. Individual suspended nanofibers are required to study their electromechanical properties. The generality of this method is of key importance because it does not rely on the selectivity of particular etching processes or on the necessity of growing fibers by means of chemical vapor deposition. Gyu-Tae Kim and colleagues report in the March 11 issue of Applied Physics Letters that they have demonstrated their technique by suspending a 2.3-nm-diameter carbon nanotube and measuring its Young’s MRS BULLETIN/APRIL 2002
modulus by using a calibrated atomic force microscope (AFM) tip. To achieve this suspended-fiber configuration, Kim and co-workers prepared a Si/SiO2 substrate with reference marks made by means of electron-beam deposition. They spin-coated this substrate with an electron-beam resist, poly(methyl methacrylate) (PMMA), then spindeposited a solution that contained dispersed nanofibers. The researchers determined the locations of these fibers relative to the underlying coordinate system using AFM. More PMMA was spun on, effectively embedding and fixing the strands parallel to the substrate, much as amber might
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