Materials with significant porosity, generally termed cellular solids, exhibit unique properties unachievable by their solid counterparts. These characteristics, which may include ultra-low density, high surface area per unit volume, and/or improved impact absorption, are greatly influenced by both the degree of open porosity and the physical arrangement of the solid material within the cellular structure. Ordered cellular structures generally exhibit superior stiffness and peak strength relative to random cellular configurations by changing the mode of deformation within the microstructure during elastic loading.[ [6] However, these techniques are not well suited for fabricating mesoscale structures [7] with feature sizes ranging from tens to hundreds of micrometers. Here we present a new class of cellular structures formed from a three-dimensional interconnected pattern of self-propagating polymer waveguides. In contrast to existing lithographic techniques, [3][4][5][8][9][10][11] , this self-propagating effect enables the rapid formation (< 1 min) of thick (> 5 mm) three-dimensional open-cellular structures from a single two-dimensional exposure surface. The process also affords significant flexibility and control of the geometry and configuration of the resulting cellular structure, which in turn, provides control of the bulk physical and mechanical properties.A self-propagating polymer waveguide can be formed from a single point exposure of light in a suitable photomonomer and can yield a high-aspect-ratio polymer fiber (length/diameter > 100) in seconds with approximately constant cross-section over its entire length. [12,13] This self-propagating phenomenon is a result of a self-focusing effect caused by a change in the index of refraction between the liquid photomonomer and solid polymer during the polymerization reaction. [12][13][14] Upon exposure of light in the appropriate wavelength range -typically UV for most photosensitive monomers -polymerization begins at the point of exposure and the subsequent incident light is trapped in the polymer because of internal reflection, as in optical fibers. This self-trapping effect tunnels the light towards the far end of the already-formed polymer, further propagating the polymerization front within the liquid monomer.[15] The diameter of the waveguide is dependent on the exposed area, and the length is primarily dependent on the incident energy of the light and the photo-absorption properties of the polymer. [16] Eventually, the polymer itself will absorb enough light in the critical wavelength range to terminate waveguide propagation. Previous studies on waveguide formation utilized a fiber optic, lens apparatus, or focusing mask to create a point source of light which initiated the formation of the self-propagating polymer fiber through the monomer. [12][13][14][15][16] However, asshown in the present work, this effect can be achieved using a broad spectrum collimated light source (generated from a mercury arc lamp) directed through a mask with a simple circu...

We report a new stress-induced kinetically driven morphological instability for driven systems. The effect of stress on the interfacial mobility couples to stress variations along a perturbed planar growth front. Comparison of theory and experiment for solid phase epitaxy at a corrugated Si(001) interface, with no free parameters, indicates that the new mechanism is required to account for the observed growth of the corrugation amplitude. This mechanism operates in conjunction with known diffusional and elastic strain energy-driven instabilities in determining morphological evolution.[S0031-9007(98)

The stability of different surface reconstructions on InAs͑001͒ is investigated theoretically and experimentally. Density-functional theory calculations predict four different surface reconstructions to be stable at different chemical potentials. The two dominant reconstructions are the ␤2 ͑2ϫ4͒ for high As, and the ␣2 ͑2ϫ4͒ for low As overpressure. This trend is confirmed by scanning tunneling microscopy of carefully annealed InAs͑001͒ surfaces. A similar behavior is predicted for GaAs͑001͒.

We have determined the structure of AlSb and GaSb (001) surfaces prepared by molecular beam epitaxy under typical Sb-rich device growth conditions. Within the range of flux and temperature where the diffraction pattern is nominally ͑1 3 3͒, we find that there are actually three distinct, stable ͑4 3 3͒ surface reconstructions. The three structures differ from any previously proposed for these growth conditions, with two of the reconstructions incorporating mixed III-V dimers within the Sb surface layer. These heterodimers appear to play an important role in island nucleation and growth. PACS numbers: 68.35.Bs, 61.16.Ch, 73.61.Ey, 81.15.Hi The surface reconstruction on a semiconducting material is the starting point for understanding the mechanisms of growth from the vapor. The steric and energetic landscape of the surface reconstruction determines the kinetic factors for adsorption, diffusion, and desorption, and provides the template for island nucleation [1]. These factors are critical to our understanding of growth and the formation of interfaces between materials, particularly for the case of III-V semiconductor quantum heterostructures, which are key components in a wide range of optical and high frequency electronic devices under development. Many of the most promising applications require extremely thin layers, so that even submonolayer variations in film thickness and interfacial roughness can dramatically affect the ultimate device performance [2,3]. To achieve the level of morphological control needed to reproducibly fabricate optimized III-V devices, a detailed understanding of the relevant surface reconstructions and the mechanisms by which epitaxy proceeds is essential.The structure of III-As and III-P (001) surfaces under typical device growth conditions (V͞III flux . 1) has been generally established [4,5]. For the much-studied case of GaAs, the extensive experimental and theoretical knowledge accumulated about the surface structure has led to significant progress in understanding the atomistic mechanisms of growth during molecular beam epitaxy (MBE) [6,7]. In contrast, the III-Sb device surfaces are poorly understood, despite their demonstrated potential for a variety of advanced electronic and optoelectronic applications [8]. Although the surface structures have been determined for atypical, extreme Sb-rich conditions-InSb and AlSb have the c͑4 3 4͒ structure common to the arsenides, and GaSb reconstructs into unusual, metallic ͑n 3 5͒ structures [9,10]-even after more than 20 years of study [11], the atomic-scale structures under more typical growth conditions have yet to be resolved. During device growth both GaSb and AlSb usually exhibit a ͑1 3 3͒-like reflection high-energy electron diffraction (RHEED) pattern, with the GaSb RHEED further delineated into distinct c͑2 3 6͒ and ͑1 3 3͒ (higher temperature/lower Sb flux) regimes [12,13]. Simple Sb-dimer row models for these reconstructions have been proposed [13,14], but scanning tunneling microscopy (STM) studies of the III-Sb(001) surfaces...