Photonic crystals in biology

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Poster Session, Tuesday, June 15 Theme A1 - B702 Morphological and electronic properties of Moiré patterns on graphene and graphitic layers on HOPG 1 , Tansu Ersoy 1 and 1 * 1 stanbul Technical University, Department of Physics, Maslak, 34469, stanbul, Turkey Abstract - Super periodicities can be observed upon shifting of similar periodic structures on each other. Such structures are named as Moiré patterns. When a graphene layer composing the highly oriented pyrolytic graphite (HOPG) crystal is slightly moved from its original position hexagonal or linear super periodicities form. We investigate the ways to produce these patterns on HOPG crystals and study their physical properties using scanning tunneling microscopy and spectroscopy. Graphite is a widely used and taken for guaranteed material in daily life as solid lubricant or as pencil. Its derivatives (such as highly oriented pyrolytic graphite (HOPG)) are also used extensively in advanced scientific studies such as calibration samples for scanning tunneling microscopy (STM) systems or also as inert substrates for investigation of nano scale structures. It is so much popular in surface science because it has quite a smooth surface. HOPG is composed of stacked two dimensional hexagonal lattices with carbon atoms, popularly named as graphene layers (figure.1). However, this property may disturb that almost perfect smoothness. (a) (b) (a) (b) Figure 2. (a) Moire pattern due to two slightly rotated, 2D hexagonal lattices. (b) STM image of a graphene flake on HOPG surface showing Moire pattern (I=0.8nA, V=1.3V, size 91 nm x 91 nm). Figure 1. (a) HOPG crystal model showing graphene layers. (b) STM image of HOPG (I=0.7nA, V=60mV, size 3.2 nmx 3.2 nm). Atoms in the graphene layers are strongly bonded but the layers are not bonded so strongly with each other. That is why it is easy to exfoliate one atom thick graphene layer from the HOPG surface even with sticky tape. Additionally, the layers can shift on each other creating local Moiré patterns. When a graphene layer is placed on a regular HOPG surface with a slight shift/rotation from its expected position or some broken graphene layer is replaced on the HOPG surface, Moiré patterns due to the hexagonal lattice may form (figure.2a) [1,2]. These patterns on HOPG surfaces can be observed using scanning tunneling microscopy (STM). We use scanning tunneling microscopy and spectroscopy (STM and STS) to observe morphological and electronic properties of graphene Moiré patterns on HOPG. There are several methods mentioned in the literature for producing these patterns on HOPG surfaces but the methodology is not well established. Moreover, in our study we investigate the types of the patterns (with respect to their periodicities and shapes (hexagonal or linear)) and look for certain selection rules as there are infinitely many possibilit ies for the periodicities. Also the correlation between the patterns and the procedures applied to attain those patterns are looked upon. Furthermore, we investigate the effect of these patterns on the local electronic structure due to the change in the surface periodicities. *Corresponding author: 2Tgurlu@itu.edu.tr [1] . BWing-Tat Pong and Colm Durkan (2005), J. Phys. D: Appl. Phys. 38 R329–R355. [2] Yongfeng Wang, Yingchun Ye and Kai Wu (2006) , Surface Science 600 pp.729–734. 6th Nanoscience and Nanotechnology Conference, zmir, 2010 391
Poster Session, Tuesday, June 15 Theme A1 - B702 The Hydrogen Sensing Properties of Pd Nanowires ennik 1 *, Necmettin K 1 and Zafer Ziya Öztürk 1, 2 1 Gebze Institute of Technology, Science Faculty, Department of Physics, Kocaeli 41400, Turkey 2 TUBITAK Marmara Research Centre, P.O. Box 21, 41470 Gebze Kocaeli, Turkey Abstract- In this study, Pd nanowires are successfully fabricated directly on Highly Oriented Pyrolytic Graphite (HOPG) depending on time using palladium nitrate solution at room temperature and then the hydrogen sensing properties of the fabricated these structures are investigated in the various concentrations of H 2 . Nanowires with diameters approximately 100 nm are obtained by electrodeposition onto the step edges on the surface of a ZYH quality of HOPG. The sensitivity is observed approximately 2 % for 1000 ppm H2 at room temperature. Hydrogen has numerous applications in industry, such as chemical production, fuel cell technology, and rocket engines [1]. However, the flammable and explosive properties of hydrogen gas make its detection an important issue [2]. That’s why nanotechnology has been used and become more vogues day by day. Nanostructures such as nanotubes [3] and nanowires [4] have been fabricated for H 2 gas sensing. Pd plating solutions were used: aqueous 2 mM Pd(NO3) 2 , 0.1 M HClO 4 . These solutions were prepared using nanopure water (r ~ experiment. The surface of highly oriented pyrolytic graphite (HOPG) crystal was cleaved using adhesive tape immediately prior to use. The growth of metal nanowires was carried out using the 3 - pulse potential program. The values for the oxidation potential, E ox , the nucleation potential, E nuc , and the growth potential, E grow are 0.8 V, - 0.8 V and 0.3 V, respectively. The oxidation potential was 5 s for activating the electrodeposition. The nucleation potential was chosen for 0.01 s and this potential determined the forming of nanowires through growth potential and growth time that are called deposition potential and deposition time, respectively. Following deposition, the graphite was removed from the plating solution, rinsed with water, and air dried prior to characterization using SEM. sensor were purged with 200 sccm high purity N 2 . As shown in Figure 2, the resistance of the sensor was increased by the increasing concentrations of H 2 . Figure 2. The resistance versus the concentrations of hydrogen for the Pd nanowire sensor [4] In conclusion, electrochemical step edge decoration (ESED) is very general method for preparing nanowires that are size and shape uniformity. Pd nanowires fabricated on HOPG were used as H 2 gas sensor. The results of gas sensing measurements have shown that Pd nanowires sensor is abundant at room temperature. This work was supported by TUBITAK under Grant No. TBAG106T546. *Corresponding author: zozturk@gyte.edu.tr Figure 1. Pd Nanowires Electrodeposition for 600 s (scale: 2 [4] Pd nanowires were fabricated on HOPG for 600 s. These nanowires were approximately 100 nm in diameter, as seen in figure 1 [4]. After the fabrication of Pd nanowires, they were transferred fro m the HOPG surface to a glass slide droped with cyanoacrylate. The cyanoacrylate film was hardened for 8 hours, and then an array of gold (Au) electrode was coated by the evaporation using a mask. The device was prepared for H 2The gas sensing. device was exposed to various concentrations of H 2 in a home made measurement cell. After reaching saturation the [1] Bevenot X, Trouillet A, Veillas C, Gagnaire H, Clement M. Hydrogen leak detection using an optical fibre sensor for aerospace applications. Sensors and Actuators B: Chemical 2000;67:57–67. [2] Yazdi N, Ayazi F, Najafi K. Micromachined inertial sensors. Proceedings of the IEEE 1998;86:1640–59. [3] Erdem Sennik, Zeliha Colak, Ozturk, Synthesis of highly-ordered TiO 2 nanotubes for a hydrogen sensor, International Journal of Hydrogen Energy (doi:10.1016/j.ijhydene.2010.01.100) 2010. dependent H2 gas sensing properties of fabricated Pd nanowires using HOPG submitted to International Journal of Hydrogen Energy 6th Nanoscience and Nanotechnology Conference, zmir, 2010 392
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