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Electronic Structure and Binding Geometry of Tetraphenylporphyrin-Derived Molecules Adsorbed on Metal and Metal Oxide Surfaces

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Electronic Structure and Binding Geometry of Tetraphenylporphyrin-Derived Molecules Adsorbed on Metal and Metal Oxide Surfaces ELECTRONIC STRUCTURE AND BINDING GEOMETRY OF TETRAPHENYLPORPHYRIN-DERIVED MOLECULES ADSORBED ON METAL AND METAL OXIDE SURFACES BY SENIA COH A dissertation submitted to the Graduate School|New Brunswick Rutgers, The State University of New Jersey in partial fulfillment of the requirements for the degree of Doctor of Philosophy Graduate Program in Physics and Astronomy Written under the direction of Professor Robert A. Bartynski and approved by New Brunswick, New Jersey October, 2012 ABSTRACT OF THE DISSERTATION Electronic structure and binding geometry of tetraphenylporphyrin-derived molecules adsorbed on metal and metal oxide surfaces by Senia Coh Dissertation Director: Professor Robert A. Bartynski Tetraphenylporphyrin (TPP)-derived molecules have been studied extensively as efficient pho- tosensitizers when chemisorbed on the metal oxide substrates in dye-sensitized solar cells. Still, many fundamental electronic properties of the dye/oxide interface are not understood and need careful consideration. In this thesis we present a comprehensive study of the electronic structure, energy level alignment and the adsorption geometry of the TPP-derived dye molecules adsorbed on TiO2(110), ZnO(1120) and Ag(100) single crystal surfaces using ultra-high vacuum (UHV) based surface sensitive techniques. The alignment of the molecular energy levels with respect to the TiO2 and ZnO band edges for all TPP-derived molecules we studied was found to be insensitive to either the nature of the functional groups located on the phenyl rings, presence of zinc as a central metal ion and different binding geometry of the molecules. Binding geometry, molecule-molecule interaction and the aggregation effects in the adsorbed layer, that were observed in the UV-visible spectra of the molecules adsorbed on ZnO substrate were not observed in the ultraviolet photoemission ii (UPS) and inverse photoemission (IPS) spectra of the occupied and unoccupied molecular states. Using near edge X-ray absorption fine structure (NEXAFS) and scanning tunneling microscopy (STM), binding geometry of the two representative TPP-derivatives was directly determined to be upright, with the porphyrin ring under large angle with respect to the surface for the p-ZnTCPP molecules and with the porphyrin ring parallel to the surface for the m-ZnTCPP molecules. We observe that the energies and the energy level alignment of the ZnTPP molecular lev- els measured in UPS and IPS depend on the substrate on which the molecules are adsorbed (Ag(100) or TiO2(110) single crystal surfaces). The differences are attributed to different charge screening properties of these two materials. Image charges created in the substrates during the measurement affect both the ground state electronic structure and the electronic excitations in the molecules causing the transport gap, the optical gap and the exciton binding energy of the molecules to decrease as the thickness of the film decreases. As measured in STM, the molecules in the first layer adsorb with the porphyrin rings parallel to the surface, while the phenyl rings are essentially upright on both surfaces. iii Acknowledgements These past six years at Rutgers had been such a wonderful time and there are so many people I would like to thank for that. First and foremost, I'm deeply grateful to my advisor, Bob Bartynski on his guidance and kindness, especially when challenges arose. His knowledge and scientific intuition have always amazed me. I would like to thank Sylvie for her guidance and patience throughout my Ph.D. and for teaching me that there is always a different perspective. My past and present lab mates were absolutely great and I would like to thank them for their support. Thank you Eric, Levan, Ryan, Chaz and Grant. The lab was a great place to work with you around. I'm grateful to Boris, Wenhua, Quantong and Leszek for their support and for being so welcoming and helpful, especially whenever I needed to borrow some ultra-high vacuum parts. I would like to thank Gwen for her support, warmheartedness and for being such great secretary with great organizational skills. I'm thankful that we have such skilled team of machinists in the machine shop at the Physics Department. Thank you Bill, Arvid, Eric and Ernie. It was always enjoyable working with you. I would like to thank our collaborators in Elena Galoppini's group at Chemistry Department at Rutgers, Newark for interesting discussions that were very helpful in understanding many chemistry-related details that came along the way. I'm grateful to my committee members, Premi Chandra, Misha Gershenson and Terry Matilsky and to Ron Ransome for their guidance, especially during the hardest time in my Ph.D. when I had to switch my advisors. My Ph.D. would absolutely wouldn't be anything close to what it was if it wasn't for my friends. Thank you Aatish, Adina, Andrei, Alexey, Andy, Anil, Anindya, Anthony, Am- ruta, Ashlee, Bumsu, Beth, Daniel, Darakhshan, Deepak, Can, Chen, Chioun, Eammon, Erin, iv George, Hang Dong, Hyowon, Ilya, James, Jiyeon, John, Kasturi, Kshitij, Kyoo, Lisa, Lu- cia, Lizzie, Madel, Manjul, Marietta, Maryam, Patrick, Qibin, Ray, Rebecca, Ritvik, Robin, Roberto, Sung Po, Sushmita, Takeshi, Vandana, Vijay, Yanan, Yi, Yuval, Xueyun. I would especially like to thank Chuck for being such a great friend and support during both easy and hard times during my Ph.D.. I would like to thank my mom and my dad for being always there for me and being supportive of what I do and to all of my family members. Thank you for your unconditional love. And last but not least, to the two most important men in my life. Thank you, Sinisa for your never ending love and support throughout my Ph.D.. Thank you for staying home and putting your research on hold so I can finish my thesis. Thank you, Benjamin for bringing smiles to our lives and for reminding us where is the greatest joy of all. Love you both with all my heart. v Dedication To my family vi Table of Contents Abstract :::::::::::::::::::::::::::::::::::::::::::: ii Acknowledgements ::::::::::::::::::::::::::::::::::::: iv Dedication ::::::::::::::::::::::::::::::::::::::::::: vi List of Tables :::::::::::::::::::::::::::::::::::::::::: x List of Figures ::::::::::::::::::::::::::::::::::::::::: xi 1. Introduction and motivation :::::::::::::::::::::::::::::: 1 1.1. Organic photovoltaics: a general scope . 1 1.2. Dye-sensitized solar cells . 3 1.3. Interfaces that contain organic molecules . 13 1.4. Thesis Outline . 25 2. Experimental and theoretical methods :::::::::::::::::::::::: 27 2.1. Introduction . 27 2.2. Ultra High Vacuum (UHV) . 28 2.3. Photoemission Spectroscopy . 31 2.4. Inverse Photoemission Spectroscopy . 41 2.5. Scanning Tunneling Microscopy . 50 2.6. UV-visible Absorption Spectroscopy . 52 2.7. Reflection Electron Energy Loss Spectroscopy . 57 2.8. Near-Edge X-ray Absorption Fine Structure . 63 2.9. Computational methods . 68 vii 3. Electronic structure, energy alignment and molecular packing of zinc(II)te- traphenylporphyrin derivatives adsorbed on TiO2(110) and ZnO(1120) surfaces 70 3.1. Introduction . 70 3.2. Experimental . 73 3.3. Results and discussion . 75 3.4. Conclusion . 93 4. Electronic structure and energy level alignment of free-base and zinc tetraphenyl- porphyrin molecules on ZnO(1120) :::::::::::::::::::::::::::: 95 4.1. Introduction . 95 4.2. Experimental Methods and Sample Preparation . 99 4.3. Computational Methods . 100 4.4. Results and discussion . 100 4.5. Conclusion . 116 5. Electronic structure, adsorption geometry and energy level alignment of zinc(II)tetraphenylporphyrin molecules evaporated onto Ag(100) and TiO2(110) surfaces ::::::::::::::::::::::::::::::::::::::::::::: 117 5.1. Introduction . 117 5.2. Experimental methods and sample preparation . 120 5.3. Computational . 121 5.4. Results and discussion . 122 5.5. Conclusion . 151 Bibliography :::::::::::::::::::::::::::::::::::::::::: 154 Appendix A. UPS spectrum calibration and work function measurement ::: 167 A.1. UPS spectrum calibration . 167 A.2. Obtaining the work function and the ionization potential using UPS . 169 viii Appendix B. Inverse photoemission spectrum calibration :::::::::::::: 173 Appendix C. UV-visible absorption spectrum of the organic molecules ::::: 176 C.1. Ground and excited states of the organic molecules . 176 C.2. Aggregation and molecular exciton coupling model . 179 Appendix D. Gouterman four-orbital model :::::::::::::::::::::: 181 Appendix E. Molecular orientation determination using NEXAFS :::::::: 187 Appendix F. Density functional theory basics and basis sets :::::::::::: 193 F.1. The many-body problem . 193 F.2. Basis sets in quantum chemistry calculations . 199 Vita ::::::::::::::::::::::::::::::::::::::::::::::: 203 ix List of Tables 2.1. Vacuum types and corresponding pressure ranges . 29 3.1. Calculated frontier orbital energies in eV and their respective symmetries. 78 3.2. Position of HOMO-1/HOMO and LUMOs centroids extracted from the experi- mental spectra measured on sensitized TiO2(110) and resulting energy gap. 84 4.1. Calculated frontier orbitals energies in eV and their respective symmetry. 110 4.2. HOMO-1/HOMO and LUMOs centroids extracted from the experimental spectra and resulting peak to peak energy gap of the four TCPP considered in this study. 115 5.1. Energy positions of features in UPS and IPS spectra of ZnTPP/Ag(100) with annealing temperature in eV . 138 5.2. Energy positions of features in UPS and IPS spectra of ZnTPP/TiO2(110) with annealing temperature in eV . 140 5.3. Transport and optical gaps and the exciton binding energies in eV for ZnTPP/Ag(100)151 5.4. Transport and optical gaps and the exciton binding energies in eV for ZnTPP/TiO2(110)151 x List of Figures 1.1. Schematic of the dye-sensitized solar cell structure. 4 1.2. Schematic of the charge transfer processes in dye-sensitized solar cell. 5 1.3. Chemical structures of (a) porphyrin, (b) zinc porphyrin, (c) ZnTPP, (d) p- ZnTCPP, (e) ZnTMP-Ipa, (f) ZnTXPSCA molecules. 9 1.4. Schematic of the binding modes of the COOH anchoring group to the metal oxide surface. 11 1.5. Chemical structure of the (a) p-H2TCPP, (b) p-ZnTCPP and (c) N3 dye molecules. 11 1.6.
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