Date of this Version
Sumit Beniwal, Ph.D. Thesis, Low-dimensional materials for organic electronic applications, University of Nebraksa - Lincoln, 2016
This thesis explores the self-assembly, surface interactions and electronic properties of functional molecules that have potential applications in electronics. Three classes of molecules - organic ferroelectric, spin-crossover complex, and molecules that assemble into a 2D semiconductor, have been studied through scanning tunneling microscopy and surfacesensitive spectroscopic methods. The scientific goal of this thesis is to understand the self-assembly of these molecules in low-dimensional (2D) configurations and the influence of substrate on their properties. First, a H-bonded organic ferroelectric, the 3-Hydroxyphenalenone, is studied on two noble metal substrates. It is demonstrated how a variety of different assemblies including 1D chains, p-p stacked structures and chiral network can be fabricated using the substrate as a growth parameter. Especially 1D chains are interesting as they still exhibit the structural motif that is the origin of their ferroelectric behavior in bulk, namely the coupling between the H-bonds and the molecular p electron system. Second, the self-assembly of Fe(II) spin crossover complex is studied on Au(111) substrate. This organic complex can be reversibly switched between paramagnetic high-spin (S=2) and diamagnetic low-spin state (S=0) in the bulk. The magnetic and electronic properties of this complex were found to be drastically influenced by the substrate. Interestingly, the reversible spin-state transition is suppressed in the interfacial molecules, likely due to a conformational change these molecules experience when in contact with the substrate. Third, the 2D boron containing semiconductors were synthesized by covalent linking of boron based precursor molecules. Two molecules, bis-BN cyclohexane and m-carborane- 9-thiol are studied on Ir(111) and Au(111) substrates, respectively. In the first case, the covalent bonding between de-hydrogenated bis-BN cyclohexane rings led to the formation of a new 2D B, C, and N containing material. This material is isostructural to graphene and h-BN, but with the useful band gap of 0.9 eV. In the second case, the m-carborane monolayer films were synthesized on Au(111). A microscopic study of the effect of the electroninduced crosslinking of the carborane cages and the resulting change in the HOMO-LUMO gap is presented. The significance of this study is in the relation it establishes between the structure and properties of these molecular systems on metal substrates through basic surface science, which will potentially enable multi-functional organic electronics applications.
Advisor: Axel Enders