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ON THE INTERPRETATION OF GRAPHITE IMAGES OBTAINED BY STM ON THE INTERPRETATION OF GRAPHITE IMAGES OBTAINED BY STM Constantinos Zeinalipour-Yazdi 1 , Jose Gonzalez 2 , Karen I. Peterson 2 , and David P. Pullman 2 . (1)Department of Chemistry, UC San Diego & San Diego State University, San Diego, CA 92182-1030, [email protected]. (2) Department of Chemistry, San Diego State University. Abstract: Highly Oriented Pyrolytic Graphite (HOPG) has been used as a standard for STM calibration for over a decade because of the relative ease of imaging in air and vacuum coupled with the known carbon-carbon distances. Most images show only three of the six carbon atoms in a given six- membered ring. This observation has been rationalized in several ways, although no entirely satisfactory explanation exists yet. In this work, a new interpretation of the graphite STM image is proposed. Introduction: 2 kind of Carbon atoms,A and B,B has a neighbor carbon in the 2 nd layer whereas A does not. A and B sites of Graphite have a different LDOS near the Fermi Level. Protrusions in STM image correspond to B sites. Literature cited: (1)Park S.S.,J.Phys.Chem.,1998,102,6020-4. (2)GAMESS, M.W. Schmidt,J.Comput.Chem.,1992, 14,1347-1363. (3)Tersoff, D.R.Hamann,Phys.Rev.B,1985,31,805. (4)C.S.Frampton,J.Molec.struc.,2000,520,29-32. (5)A. Charrier,J.Appl.Phys.,2002,92(5),2479. (6)T.A. Land,J.Phys.Chem.,1992,97(9),6774. Discussion and conclusion: We believe that the anomalous corrugation seen in STM images of HOPG may not be associated with any interaction of subsurface graphitic layers. Our argument is based on STM images of monolayer Graphite islands formed from thermal decomposition of Silicon carbide 5 or heteroepitaxially grown from ethylene on Pt(111) 6 have shown features identical to those of HOPG. The large interlayer distance of 3.354 A estimated from single crystal X-ray diffraction and the weak Van der Waals interaction makes this argument even stronger. Our simulated STM images show that it is possible to have a pi-localized molecular orbitals as a result of H atoms chemically bonded at the periphery of graphitic layers. The simulated STM images suggests that this localization would in fact produce the features we observe in experimental STM images of HOPG. Calculations at higher level and on larger systems are under way to better mimic HOPG STM images. Model: In order to test this hypothesis we did ab-initio electronic structure calculations 2 using Density Functional Theory of several polycyclic aromatic and antiaromatic hydrocarbons. 0 130 260 390 520 distance in a.u. I t Greater number of spots in Y direction than in X direction. Hypothesis: Maybe the protrusion that appear in HOPG STM images do not correspond to Carbon positions but to enhanced electron density near the Fermi level that is a result of pi-localized molecular orbitals. This pi-localization that would correspond to a structure of higher energy might be a result of hydrogen atoms chemisorbed at the periphery of graphitic layers in such a manner that promotes pi-localization rather than delocalization. X direction Y direction Observation: The elliptical shape of the bright spots that we and other groups have seen in our experimental STM images of HOPG suggests the possibility of alternative explanations. We also simulated the STM image of these structures using Tersoff-Hamann 3 treatment for the tunneling current. The tunneling current at small bias voltage under the spherical tip approximation is: Atomic resolution STM images of Graphite Image of Graphite based on the crystal structure close packed hexagonal structure honeycomb like structure Interpretation given by Park et al 1 : I= 8e V D E R m R m rE rE = r E E t 2 3 t F 2 e e o F o F o v F 3 2 2 2 2 exp , , v v eqn.1 eqn.2 C 48 H 20 C 30 H 16 C 16 H 12 C 48 H 18 C 30 H 14 C 16 H 10 H H H H H H H H H H H H -6 -4 -2 -0 2 4 6 x axis in bohrunits -5 -4 -3 -2 -1 -0 1 2 3 4 5 y axis in bohrunits Only MO’s that belong to B 2g B 3g A u & B 1u irreducible representations and had significant contribution in the center of the molecule were included to avoid edge effects. We did not evaluate the first term of eqn.1, because it effects only the contrast of the STM image. Only the second term (eqn.2) which is the LDOS near the fermi level affects the shape of the spots in the simulated STM image. C-C # Average XRD B3LYP/6-31G bond length bond length 1 1.341 A 1.365 A 2 1.428 A 1.431 A 3 1.423 A 1.434 A 4 1.399 A 1.407 A 5 1.438 A 1.440 A 6 1.383 A 1.397 A 1 2 3 4 5 6 Results: We used the B3LYP functional and the 6-31G basis set which gave very good agreement with the structure of pyrene obtained from low temperature X-ray diffraction 4 . -6 -4 -2 -0 2 4 6 x axis in bohr units -5 -4 -3 -2 -1 -0 1 2 3 4 5 y axis in bohr units 0 . 0 4 6 0 . 0 4 6 0 . 0 4 6 0.046 0 . 0 4 6 0 . 0 4 6 0 . 0 7 8 0 . 0 7 8 0.078 0 . 0 7 8 0. 078 0 . 0 7 8 0 . 0 7 8 0 . 0 7 8 0 . 1 1 0 0.110 0 . 1 1 0 0 . 1 1 0 0 . 11 0 0 . 1 1 0 0.142 0.142 0.142 0. 1 42 0 . 1 4 2 0.142 0 . 1 4 2 0 . 17 4 0.174 0.174 0.174 0.174 0.17 4 0.174 0.174 We simulated the constant height STM images of both C 48 H 20 and C 48 H 18 . The tip surface distance of the former was 1.3 A. The constant current STM simulation is still running !#$%*)?. B B A A B A B A A B 1 st layer 2 nd layer
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