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A single-point diamond turning machine was used to make grooves on (111) p-type single-crystal silicon wafers at room temperature. Scratch tests have been performed with both sharp (Vickers and conical) diamond tools, and a spherical (Rockwell) diamond tool. Our results showed that material removal mechanisms differed between these tools. Pressure-induced metallization of Si allows the ductile regime mechanical micromachining of wafer surfaces. Raman microspectroscopy and electron microscopy were used to determine the machining parameters that do not introduce cracking or other types of damage. The surface of the groove, after machining, was covered by a mixture of metastable, high-pressure silicon phases and amorphous silicon. Further, these phases can be transformed into cubic silicon by annealing. The maximum depth of cut in the ductile regime has been determined for the given scratch test conditions and tools. The developed technique can be used to machine Ge, GaAs and other semiconductors. Applications drawing from this research are many. For example, channels for microfluidic devices can be engraved with a channel cross-section that is determined by the shape of the tool, which allows patterns that cannot be produced using etching. There are no limitations on the channel length or direction, and the channel width can vary from potentially a few nanometres to several micrometres.

Abstract. We investigate the optimal accuracy of the streamline diffusion finite element method applied to convection-dominated problems. For linear/bilinear elements the theoretical order of convergence given in the literature is either O(h 3/2 ) for quasi-uniform meshes or O(h 2 ) for some uniform meshes. The determination of the optimal order in general was an open problem. By studying a special type of meshes, it is shown that the streamline diffusion method may actually converge with any order within this range depending on the characterization of the meshes.

The atomic structure of recently synthesized thiolate-protected Au 76 cluster is theoretically predicted via a simple structural rule summarized from the crystal structures of thiolateprotected Au 44 (SR) 28 , Au 36 (SR) 24 , and Au 52 (SR) 32 clusters. We find that Au 76 (SR) 44 (N = 7) and recently reported Au 52 (SR) 32 (N = 4), Au 44 (SR) 28 (N = 3), Au 36 (SR) 24 (N = 2), and Au 28 (SR) 20 (N = 1) belong to a family of homologous Au 20+8N (SR) 16+4N clusters whose Au cores follow a one-dimensional polytetrahedral growth pathway. The Au 76 (SR) 44 cluster is predicted to contain an anisotropic facecentered-cubic (fcc) Au core, which can be viewed as combination of two helical tetrahedra Au 4 chains and is remarkably different from the well-known spherical Au core in ligand-protected gold clusters in the size region of 1−2 nm. The intense near-infrared (NIR) absorption of Au 76 (SR) 44 is attributed to the synergistic effect of anisotropic Au core structure and ligand protections. A plausible cluster-to-cluster transformation mechanism is further suggested.

In this article, we analyze the local superconvergence property of the streamline-diffusion finiteelement method (SDFEM) for scalar convection-diffusion problems with dominant convection. By orienting the mesh in the streamline direction and imposing a uniformity condition on the mesh, the theoretical order of pointwise convergence is increased from O(h"'*llog 111) to O(h'1log 111). Numerical tests show that this result cannot be extended to arbitrary quasi-uniform meshes. 0 1996 John Wiley & Sons. Inc.