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As seen in Fig. 2a, for the nanowires with diameter less than 1 nm, the heats of formation for the different directions are close to each other, showing no preferential growth direction in this size range. As wire diameter further increases, the preferential growth direction becomes clear. Formation of Si nanowires along the [100] direction costs the most energy, whereas the [111] and [112] directions correspond to the smallest heat of formation. This is in agreement with experimental observations [7,14] and explains why [100] growth direction was found to be less favorable than the [111] and [112] directions. Indeed, the most commonly reported growth directions in literatures are [111], [11 2] and [110] [1¨C4,7,14]. The thinnest hydrogenated SiNWs in experiments, with 1 nm diameter, are oriented along the [112] direction [7]. In addition to the [100] nanowires with quadrangular cross-section and four (110) lateral surfaces shown in Fig. 1, we also considered a series of [10 0] nanowires with the octagonal cross-section, which have four (11 0) and four (100) lateral surfaces, to further examine the effect of morphology on the nanowires stability. After full optimization, the heats of formation for the octagonal [10 0] nanowires with 1.152 and 1.736 nm in diameters are 0.09 and 0.07 eV/atom, respectively, higher than those of the quadrangular [100] nanowires with the same diameter by 0.066 and 0.058 eV/atom, respectively. The heat of formation per atom for the octagonal [100] nanowire with 2.11 nm in diameter is 0.06 eV, higher than that of the quadrangular [100] nanowires with a diameter of 2.0 nm by 0.05 eV. Thus, the [100] nanowires with the quadrangular cross-section seems to be energetically favorable even with four sharp edges. |
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thorny_rose
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2Â¥2012-05-09 02:25:25














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