Photonic crystals in biology

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Poster Session, Tuesday, June 15 Theme A1 - B702 Ab-initio Investigation of Structural Properties of 6H-SiC {0001} Surfaces Ahmet Cicek 1 *, Oguz Gulseren 2 and Bulent Ulug 1 1 Department of Physics, Faculty of Arts and Sciences, Akdeniz University, Antalya 07058, Turkey 2 Department of Physics, Bilkent University, Ankara 06800, Turkey Abstract-Structural properties of unreconstructed 6H-SiC surfaces are studied by using pseudopotential plane-wave calculations based on density functional theory. Surface truncation is observed to lead to relaxation of outermost atoms such that the Si or C atoms at the surface move inwards in the growth direction, while the adjacent atoms of the other species move outwards. Intra- and inter-layer separations are observed to vary more significantly for the topmost bilayer. It is seen that at least 12-bilayer slab structures must be considered for an adequate description of 6H-SiC {0001} surfaces. The {0001} surfaces of hexagonal SiC polytypes, especially 6H, are recently under intense investigation due to ease of epitaxial graphene growth [1-4]. Growth mechanisms and geometry of the reconstructions on 6H SiC surfaces are not easy to treat in theoretical studies, where simple cases such as 3 3R30 reconstruction for the Si-terminated [5-7] and 2x2 reconstruction for the C-terminated face [8] are in itially treated. Surface is defined by a finite number of bilayers (BL) permitting computationally-affordable investigations. Since the introduced stress propagates through the surface and charge separation occurs at the two faces terminated by Si and C atoms, a thorough study must investigate the minimum slab thickness to describe the surface [10]. In this work, structural properties of unreconstructed 6H-SiC {0001} surfaces are investigated by ab-initio calculations under Generalized Gradient Approximation (GGA) and Local Density Approximation (LDA). First, structural properties of the bulk are investigated by geometry optimization [9]. The optimized bulk geometry is utilized as the starting configuration for geometry optimization of the surfaces. The investigated 6H-SiC (0001) and (000 1 ) surfaces are demonstrated in Figure-1. Important entities in the investigations are intra- and inter-layer separations of BLs, denoted by d i and z i in Figure-1(a)., respectively. Figure 1. Unreconstructed 6H-SiC {0001} 1x1 surfaces for geometry optimization computations employing (a) and (c) 6 bilayers, (b) and (d) 12 bilayers. Computations are carried out by employing slabs of 6 and 12 BLs. Vacuum distance between adjacent supercells is 20A 0 and cut-off energy is 50Ry, where 12x12x2 and 12x12x1 Monkhorst-Pack meshes are employed for 6-BL and 12-BL slabs, respectively. For geometry optimization, Broyden- Fletcher-Goldfarb-Shanno (BFGS) algorithm is employed where 1, 2, 3, 4 or 6 topmost BLs are relaxed for 6-BL and 1, 2, 3, 6, 7, 8 or 9 topmost BLs are relaxed for 12-BL slabs. Ideal bulk 6H-SiC is composed of ABCACB… stacking of atoms where neighboring atoms around any atom form a regular tetrahedron with d i=z i /3. However, both LDA and GGA computations reveal that small displacements take place so that d 1 =62.6p m and z 1 =187.1p m for LDA, wh ile d 1 =63.5p m and z 1 =189.8pm for GGA. Relaxed slab geometries show that the outermost surface atom moves towards the surface and the next atom of the other species moves outwards. Displacement of the top Si atoms is around 5pm inwards and the neighboring C atoms move approximately 2.5 p m outwards for the (0001) surface for LDA computations. For GGA, considerably smaller displacements are observed. On the other hand, C atoms of the (000 1 ) surface are displaced more significantly, where LDA results reveal around 17pm shifts inwards. The outward displacement of the Si atoms of the (000 1 ) surface is around 8pm. Again, smaller displacements are observed for GGA. Comparisons of results for 6 and 12 BL slabs show that atomic displacements do not converge as the number of relaxed BLs is increased for the case of 6-BL slabs, whereas convergence is achieved for 12-BL slabs. In summary unreconstructed {0001} surfaces of 6H-SiC, the surface must be described by at least 12 bilayers, where relaxation of 3 or 6 BLs is sufficient for obtaining optimized surface geometry. This study is supported by Akdeniz University Scientific Research Projects Coordination Unit (Project No: 2008.01.0105.010) and by TUBITAK (Grant No: 107T720). *Corresponding author: acicek@akdeniz.edu.tr [1] W. A. de Heer, C. Berger, X. Wu, P. N. First, E. H. Conrad, X. Li, T. Li, M. Sprinkle, J. Hass, M. L. Sadowski, M. Potemski ve G. Martinez, Solid State Comm. 143, 92 (2007). [2] Th. Seyller, A. Bostwick, K. V. Emtsev, K. Horn, L. Ley, J. L. McChesney, T. Ohta, J. D. Riley, E. Rotenberg, ve F. Speck, Phys. Stat. Solidi B 245 (7), 1436 (2008). [3] H. Huang, W. Chen, S. Chen ve A. Thye Shen Wee, 2 (12), 2513 (2008). [4] U. Starke ve C. Riedl, J. Phys.: Condens. Matter 21, 134016 (2009). [5] A. Mattausch ve O. Pankratov, Phys. Rev. Lett. 99, 076802 (2007). [6] A. Mattausch ve O. Pankratov, Phys. Stat. Solidi (b) 245 (7), 1425 (2008). [7] F. Varchon, R. Feng, J. Hass, X. Li, B. Ngoc Nguyen, C. Naud, P. Mallet, J.-Y. Veuillen, C. Berger, E. H. Conrad ve L. Magaud, Phys. Rev. Lett. 99, 126805 (2007). [8] L. Magaud, F. Hiebel, F. Varchon, P. Mallet ve J.-Y. Veuillen, Phys. Rev. B 79, 161405(R) (2009). [9] P. Käckell, B. Wenzien ve F. Bechstedt, Phys. Rev. B 50 (23), 17037 (1994). [10] J. Soltys, J. Piechota, M. Lopuszynski ve S. Krukowski, arXiv: cond-mat.mtrl-sci (30.11.2009). [11] M. Sabisch, P. Krüger ve J. Pollmann, Phys. Rev. B 55 (16) 10561 (1997). 6th Nanoscience and Nanotechnology Conference, zmir, 2010 274
Poster Session, Tuesday, June 15 Theme A1 - B702 The Synthesis and characterizat ion of Zinc Oxide (ZnO) in difficult crystalline structure Alisah Cagatay C 1 , Sule Erten-Ela 1 *, Siddik Icli 1 , 1 Solar Energy Institute, Ege University, Bornova, 35100 Izmir, Turkey Abstract-ZnO nanopowder has been successfully synthesized by a microwave-assisted solution approach and a solution phase reaction. An efficient microwave method and simple solution reaction method are presented to synthesize ZnO nanostructures. We investigated the effects of surfactant, growth temperature, annealing to control the morphologies of ZnO nanostructures. One-dimensional (1D) nanostructures such as nanowires, nanobelts, and nanorods, whose lateral dimensions fall in the range of 1–100 nm, have attracted a lot of interest and have been extensively researched in recent years because of their peculiar and interesting physical properties and potential device applications [1–3]. Among these important materials, zinc oxide (ZnO) has been given considerable interest because of its attractive optical functions based on the large binding energy of excitons and biexcitons (60 and 15 meV, respectively) as well as its multifunctional physical properties. Notable applications of 1D ZnO nanostructures include the fabrication of nanometer scale electronic devices such as light-emitting diodes [4], nanolasers [5,6], gas sensors [7,8], field-effect transistors [9], and dyesensitized solar cells [10]. Semiconductor-assisted photocatalysis has attracted considerable attention among advanced oxidation process (AOP) as a promising tool for implementing the large-scale purification of waste waters at low cost. This methodology exploits the strong reactivity of hydroxyl radicals in driving oxidation processes, ultimately leading to the extensive mineralization of a variety of environmental contaminants[11,12]. ZnO is the one of the most suitable material for photocatalytic degradation in the presence of sunlight. [13,14]. In this work, we report the synthesis, structural characterization of ZnO nanostructures using a microwave method and solution phase method to prepare different morphologies of nanostructures by adjusting the amount of NaOH, growth temperature, surfactant, annealing. Structural characterization of nanostructures were done by scanning electron microscopy (SEM). We acknowledge financial support from Scientific and Technological Research Council of Turkey, TUBITAK. *Corresponding author: sule.erten@ege.edu.tr [1] C.B. Murray, C.R. Kagan, M.G. Bawendi, Science 270 (1995) 1335. [2] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [3] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [4] R. K¨onenkamp, R.C.Word, C. Schlegel, Appl. Phys. Lett. 85 (2004) 6004. [5] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [6] J.-H. Choy, E.-S. Jang, J.-H. Won, J.-H. Chung, D.-J. Jang, Y.-W. Kim, Adv. Mater. 15 (2003) 1911. [7] Q.H. Li, Y.X. Liang, Q.Wan, T.H.Wang, Appl. Phys. Lett. 85 (2004) 6389. [8] X. Wang, J. Zhang, Z. Zhu, Appl. Surf. Sci. 252 (2006) 2404. [9] S. Ju, K. Lee, D.B. Janes, M.-H. Yoon, A. Facchetti, T.J. Marks, Nano Lett. 5 (2005) 2281. [10] M. Law, L. Greene, J.C. Johnson, R. Saykally, P.Yang, Nat. Mater. 4 (2005) 455. [11] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Catal. Today 53 (1999) 51. [12] J.M. Hermann, Catal. Today 53 (1999) 115. [13] F.D. Mai, C.S. Lu, C.W. Wu, C.H. Huang, J.Y. Chen, C.C. Chen, Sep. Purif. Technol. 62 (2008) 423. [14] A.H. Akyol, C. Yatmaz, M. Bayramoblu, Appl. Catal. B: Environ. 54 (2004) 19. Figure 1. SEM images of ZnO nanorods By using microwave method and solution phase method, ZnO nanostructures with different morphologies were successfully synthesized. Effects of the reaction conditions on the morphological characterization were discussed. 6th Nanoscience and Nanotechnology Conference, zmir, 2010 275
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