A STUDY OF FULLERENE, PORPHYRIN AND THEIR DYAD ON GRAPHENE A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Navin Kumar Kafle August, 2016 A STUDY OF FULLERENE, PORPHYRIN AND THEIR DYAD ON GRAPHENE Navin Kumar Kafle Thesis Approved: Accepted: Advisor Dean of the College Dr. Alper Buldum Dr. John Green Faculty Reader Dean of the Graduate School Dr. Robert R. Mallik Dr. Chand Midha Faculty Reader Date Dr. Graham S. Kelly Department Chair Dr. David Steer ii ABSTRACT Molecular devices employing hybrids of graphene, acting as electron acceptor, with C60 and porphyrin, acting as electron donors are promising for the solar cell. The models of fullerene (C60), zinc-tetraphenylporphyrin (ZnTPP) and their dyad on sin- gle layer graphene (SLG) were designed and their structural and electronic properties were studied using classical and quantum mechanics computational modeling. First- principles density functional theory (DFT) calculations were carried out in quantum calculations. Our study shows that the C60ZnTPP dyad and SLG interact more strongly than the other two molecules with SLG. We also found that van der Waals in- teraction was an important factor for non-covalent interactions between our molecules and SLG. The HOMO-LUMO gap of the ZnTPP-SLG model structure was smaller than C60-SLG model structure. Density of states (DOS) calculations shows that C60ZnTPP-SLG had a high value of DOS at Fermi level and it has number of par- tially filled orbitals above Fermi level. This showed that our C60ZnTPP had metallic character. We believe that such properties of our model structures would encourage experimentalists to examine them for their practical applications in electronic devices and energy conversion. iii ACKNOWLEDGEMENTS My deepest gratitude goes first to Professor Alper Buldum, who expertly guided me throughout my graduate research and gave me the freedom to work independently. It was a pleasure working with him. I would like to thank Dr. Robert R. Mallik and Dr. Graham S. Kelly for being on my thesis defense committee and for reading my thesis and offering great suggestions. Further, I would like to thank Dr. Mahesh Dawadi and Sudip Adhikari whose continuous support and ideas with regards to programming were invaluable to my work. I would also like to thank my family and friends for all their encouragement, and I am especially thankful to my lovely wife Sabita Kafle for her continuous care, support and motivation during my research. iv NOMENCLATURE SLG Single Layer Graphene C60 Fullerene ZnT P P Zinctetraphenylporphyrin DF T Density Functional Theory HOMO Highest Occupied Molecular Orbital LUMO Lowest Occupied Molecular Orbital DOS Density Of States 2D Two Dimentional TPP Tetraphenylporphyrin M − TPP Metallo-Tetraphenylporphyrin H Hamiltonian GP W Gaussian and Planewave MD Molecular Dynamics UFF Universal Force Field R Separation between molecule and SLG C − R Carbon Resonant N − 2 Nitrogen Trigonal v P BE Perdew-Burke-Erznerhof FFT Fast Furier Transform GT H Gredecker-Teter-Hutter GAP W Gaussian and Agumented plane wave ST O Slater Type Orbitals DZ Double Zeta EAd Adsorbtion energy EC60 Energy of optimozed C60 EGr Energy of optimized graphene ET ot Total energy vi TABLE OF CONTENTS Page LIST OF TABLES . ix LIST OF FIGURES . x CHAPTER I. INTRODUCTION . 1 II. BACKGROUND . 3 2.1 Atomic Structure and Properties of Graphene . 3 2.2 Atomic Structure and Properties of Fullerene (C60).......... 7 2.3 Atomic Structure and Properties of Porphyrins . 8 2.4 Computational Methods . 11 2.5 CP2K . 14 2.6 Material Studio . 15 III. MODELS . 17 3.1 C60-SLG Model . 17 3.2 ZnTPP-SLG Model . 18 3.3 C60ZnTPP-SLG Model . 20 IV. CLASSICAL MOLECULAR MECHANICS SIMULATIONS . 21 4.1 Methods . 21 vii 4.2 Result and Discussion . 24 V. QUANTUM MECHANICS SIMULATION . 37 5.1 Method . 37 5.2 Results and Discussion . 40 VI. CONCLUSION . 53 BIBLIOGRAPHY . 54 viii LIST OF TABLES Table Page 4.1 Parameters used in geometrical optimization calculations . 24 4.2 Initial Parameters used for energy minimization and geometrical optimization of C60-SLG. 26 4.3 Parameters used in the energy calculation and geometrical optimiza- tion of ZnTPP-SLG. 30 4.4 Parameters used in the energy calculation and geometrical optimiza- tion of C60ZnTPP-SLG. 33 5.1 The energies of optimized structures. 43 ix LIST OF FIGURES Figure Page 2.1 The allotropes of carbon derived from graphene. Figure extracted from Ref. [13]. 4 2.2 Graphene sheet with the unit cell, where Lx = 2.4592 Aand˚ Ly = 4.26 A.˚ 5 2.3 Structure of buckyball (C60)......................... 8 2.4 Structure of Porphyrin. 9 2.5 Structure of zinc tetraphenylporphyrin (ZnTPP), where cyan repre- sents C atoms, red is Zinc, blue is Nitrogen and orange is Hydrogen. 10 3.1 Side view of the structure of C60-SLG . 18 3.2 Structure of ZnTPP-SLG with (a) a Zn atom of ZnTPP on the top of a carbon atom of SLG (b) a Zn atom of ZnTPP in the middle of a hexagonal ring of SLG . 19 3.3 Side view of the structure of C60ZnTPP-SLG . 20 4.1 Changes in the molecule's geometry and energy stepwise by geome- try optimization from point 1 to 4. 23 4.2 Snapshot of C60-SLG model with periodic boundaries. 25 4.3 The variation of total energy as a function of C60 and SLG separation in C60-SLG. One kilocalorie per mole is 0.0433eV. 27 4.4 Forcite Geometrical Optimization- Convergence, purple; Grad.Norm (kcal/mol/A),˚ blue; Energy Change (kcal/mol). 28 4.5 Forcite Geometrical Optimization- Energy minimization. 28 x 4.6 Snapshot of ZnTPP-SLG model with periodic boundaries . 29 4.7 The variation in total energy of the ZnTPP-SLG with changes the separation between ZnTPP and SLG with (a) a Zn atom of ZnTPP in the middle of hexagonal ring of SLG (b) a Zn atom of ZnTPP on the top of a carbon atom of SLG. 31 4.8 (a) Forcite Geometrical Optimization- Convergence, purple; Grad.Norm (kcal/mol/A),˚ blue; Energy Change(kcal/mol) (b) Forcite Geometrical Optimization- Energy minimization. 32 4.9 Snapshot of the C60ZnTPP-SLG model with periodic boundaries . 34 4.10 The variation in total energy of the C60ZnTPP-SLG with change separation between C60ZnTPP and SLG. 35 4.11 Forcite Geometrical Optimization- Convergence, purple; Grad.Norm (kcal/mol/ A),˚ blue; Energy Change (kcal/mol). 36 4.12 Forcite Geometrical Optimization- Energy minimization. 36 5.1 The variation of total energy as a function of C60 and SLG separation in C60-SLG. One atomic unit of energy is 27.211 eV. 41 5.2 (a) The optimized structure of the fullerene (C60) molecule. (b) The optimized structure of the supercell of graphene monolayer with 240 carbon atoms on it. (c) Side view of the optimized structure of C60-SLG structure. 42 5.3 Energies of five HOMO and five LUMO along with the HOMO- LUMO energy gap. 44 5.4 Electronic profiles of LUMO of C60-SLG structure. The isosurface value is ± 0.01 (distinguished by blue/red) in atomic unit. 45 5.5 The variation of total energy as a function of the separation of ZnTPP and SLG in the ZnTPP-SLG structure. One atomic unit of energy is 27.211 eV. 46 5.6 The variation of total energy as a function of the separation of C60ZnTPP dyad and SLG in the C60ZnTPP-SLG structure. 47 5.7 DOS for the C60-SLG. The Fermi energy is shifted to zero. 49 5.8 DOS for the ZnTPP-SLG. The Fermi energy is shifted to zero. 50 xi 5.9 DOS for the C60ZnTPP-SLG. The Fermi energy is shifted to zero. 51 5.10 DOS for the SLG. The Fermi energy is shifted to zero. 52 xii CHAPTER I INTRODUCTION Carbon nanostructures of low dimensions like fullerenes (C60), carbon nanotubes (CNT) and single layer graphene (SLG) have attracted great research interest in recent years. They offer novel properties and they can be potential building block materials for future nano-electronics structure [1]. Graphene is a mono atomic (2D) sheet of carbon. Due to its outstanding structural, electronic and mechanical proper- ties, it is a promising material for advanced technological applications. By controlling its size, shape, carrier types, one can control graphene's electronic and magnetic prop- erties [2]. It is a zero band gap semiconductor and shows ballistic conductance [3]. 2 C60 is a soccer ball shaped carbon allotrope with sixty sp -hybridized carbon atoms. It can be treated as a semiconductor or conductor depending upon its size [3]. In a broad range of fields like solar cells, superconductors, photosensitizer and molecular electronics C60 is applied [4]. Porphyrins are micro cyclic compounds with four mod- ified pyrrole rings linked through a methine bridge [5]. Metals can be inserted into the central region of a macrocyclic ring. This gives the derivative of porphyrin called Metallo-porphyrin. These are more stable and significant natural compounds that play an important role in respiration, electron transfer and photosynthesis [6]. They are mostly taken as the potential materials for photovoltaic devices [7]. 1 The study and fabrication of hybrid carbon nanostructures has been an in- tense research subject over the past few years. Chemically functionalized graphene with fullerene which has an electron-accepting behavior, has been studied [8]. Ex- perimentally, C60 molecules on graphene are prepared epitaxially on SiC [9]. Energy transfer has been observed between porphyrin-graphene complexes [10]. Here, we designed three hybrid models containing graphene. Models of C60, porphyrin and their dyad on graphene were created.