Oscillatory Band Gap Behavior in Small Diameter Si-Clathrate Nanowires Inna Ponomareva* Department of Physics, UniVersity of Arkansas, FayetteVille, Arkansas 72701 Ernst Richter DaimlerChrysler AG FT3/SA, Wilhelm-Runge-Strasse 11, 89081 Ulm, Germany Antonis N. Andriotis Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, P.O. Box 1527, Heraklio, Crete, Greece 71110 Madhu Menon Department of Physics and Astronomy, and Center for Computational Sciences, UniVersity of Kentucky, Lexington, Kentucky 40506 Received August 3, 2007; Revised Manuscript Received October 2, 2007 ABSTRACT Electronic structure analysis of small cagelike silicon nanowires is carried out and reveals many surprising features. The band gap values for all the nanowires are found to be smaller than their bulk counterparts. The most intriguing aspect appears to be the alternating sequence of direct and indirect band gaps as the diameter changes. This is attributed to the type of surface geometry. We illustrate this with two well- known clathrate forms as well as a new hexagonal clathrate structure with a direct band gap in the optical region. Silicon nanowires (Si-NWs) have established themselves as one of the most promising candidates in nanotechnology applications. In particular, their useful device applications have been demonstrated in recent works. 1-5 The device properties, however, could depend sensitively on the precise structural configurations of these nanowires. The structural predictions for small diameters Si-NWs were first made by Menon and Richter 6 and Marsen and Sattler. 7 Subsequently, theoretical structural determinations of the Si- NWs were made by other groups revealing a wide variety of possible structures containing single-crystalline, clathrate- like and polycrystalline forms. 8-16 The single crystalline Si- NWs can be carved out from the stable bulk crystalline forms of Si, the latter including tetrahedral as well as clathrate or cagelike forms. 10 The clathrate forms, while possessing the same coordination as tetrahedral forms, show deviations in bond angles from the ideal tetrahedral case. 17,18 Although there are many bulk clathrate forms of Si possible, two are known to be the most stable with cohesive energies very close to the bulk tetrahedral Si. These are (i) Si clathrate structure consisting of a face-centered cubic lattice with a 34 atom basis (Si 34 -Clath) and (ii) a simple cubic lattice with a 46 atom basis (Si 46 -Clath). 17,18 The bulk Si 34 -Clath structure can be visualized as three-dimensional (3D) periodic arrangement of Si 20 and Si 28 cages with shared faces. Similarly, the bulk Si 46 -Clath structure can be visual- ized as 3D periodic arrangement of Si 24 cages with shared faces interpenetrated by Si 20 cages. The interstitial spaces are occupied by Si 2 dimers. Experiments have unequivocally demonstrated the 4-fold coordination of the crystalline bulk in nanowires. The rich morphology of nanowires observed in experiments include even polycrystalline types. 19 In the case of single crystalline Si-NWs, however, the experiments have not conclusively determined their crystalline bulk to be tetrahedral to the exclusion of cagelike. This is because cagelike Si-NWs also contain a 4-fold coordinated core as in tetrahedral cases. In fact, the only distinguishing feature between the two types is the bond angle. Given the fact that the bulk clathrate forms are very close in energy to the tetrahedral form of Si, one could expect that in Si-NWs the corresponding energy difference could be even smaller due to the natural surface reconstruction afforded in cagelike forms. * To whom correspondence should be addressed. E-mail: iponoma@uark.edu. NANO LETTERS 2007 Vol. 7, No. 11 3424-3428 10.1021/nl071907z CCC: $37.00 © 2007 American Chemical Society Published on Web 10/18/2007