Functionalization of metal-oxide nanostructured substrates with host-guest complexes and functionalization of rubrene single crystal
Creators: name: Ma, Xiuyuan
Publisher: name: No Publisher Supplied
Year: 2019

description: Molecular functionalization of the metal-oxide (MOn, with M = Ti, Zn, Zr, etc.) interface with chromophores and redox active compounds enables the applications of metal-oxide semiconductors in many fields, such as dye sensitized solar cells (DSSCs) and photocatalysts. Functionalization of MOn interface with host-guest complexes provide an alternative way to bind chromophores and redox compounds via the anchor groups of the host, rather than synthetically attaching a functional group directly onto the chromophore. In this thesis, we have developed two methods to bind host-guest complexes to the MOn interfaces. The first one is the traditional one-step method, in which an organic molecule is encapsulated into a molecular host, and the host-guest complex is attached to the surface of a MOn nanoparticle. The second one is a stepwise approach, in which the nanostructured MOn surface is decorated with a layer of hosts and then this functionalized MOn surface is able to capture guest molecules forming host-guest complexes on the nanostructured MOn surface.In Chapter A, we studied five host-guest systems both in solution and on MOn surfaces, they are 1-phenylethynylpyrene@β-cyclodextrin (1-PhEPy@β-CD), 1,4-di(1-pyrenyl)-butadiynylene@β-/γ-cyclodextrin (PyEEPy@β-/γ-CD), azulene@β-cyclodextrin (Az@ β-CD), azulene@(octa acid)2 (Az2@OA2) and guaiazulene@(octa acid)2 (GAz@OA2). The encapsulation of a chromophore into a host prevented the formation of aggregates, and improved photostability by shielding the guest from the heterogeneous interface. Both the one-step method and the stepwise method were applied to bind the complexes to MOn interface. The stepwise method always led to more effective binding compared to the one-step method. In this chapter, wide band-gap semiconductor TiO2 and insulator ZrO2 nanoparticle films were used as our MOn nanostructured materials. A comparison of binding behaviors on these two MOn materials indicated that charge transfer from encapsulated chromophore to TiO2 happened when bound Az@host complexes to TiO2 surface, and the quenching of fluorescence emission signals were observed; whereas insulator ZrO2 did not quench the signal.In Chapter B, we studied the functionalization of organic semiconductor rubrene single crystal by vapor deposition with silanes and by solution deposition with aryl diazonium salts. It has been shown that formation of self-assembled monolayer (SAM) of alkyl-silanes on the surfaces of rubrene crystals results in an increase of several orders of magnitude of the surface p-conductivity of the material, which is important for OFETs applications. The key requirement for this kind of behavior is the formation of a molecular self-assembled monolayer (SAM) on the rubrene surface. We studied the functionalization of rubrene single crystals with nine SAM precursors by vapor deposition to understand the mechanism and to rationalize the features that affect the monolayer growth.Rubrene has a strong p-carrier mobility, but impurities (defects) in the rubrene crystal generate trap states, decreasing the crystal conductivity. It was reported that the functionalization of rubrene crystal preferentially happens at impurity sites, such as sites where the rubrene framework has reacted with oxygen. Similar phenomena were observed for other polycyclic aromatic hydrocarbon (PAH) crystals, such as pentacene and tetracene. To understand the nature of such impurities, and to investigate the mechanism that impact the conductivity of crystals, we proposed the structure of impurity candidates of pentacene, rubrene and tetracene and studied them by using density functional theory (DFT) calculations in Chapter C. After our two-step computation, 11 molecules were identified to be hole-trap impurities that can decrease the hole conductivity in PAH materials.