UC Riverside

Polyaromatic carbon-based materials, including polyaromatic hydrocarbons (PAH), graphene, and carbon nanotubes (CNT), are envisioned to be widely used in next-generation electronics for their great properties, richness in richness source materials, adaptability in applications, and scalability. Morphological engineering of materials’ shape, size, surface, and orientation is necessary to control their mesoscopic properties and incorporate them into practical electronic devices. This dissertation explores different strategies to control the morphology of polyaromatic carbon-based materials and also investigates the accompanied morphological effect that changes materials’ properties.PAH monomers have many novel excitonic properties. By packing them orderly, monomers can be assembled into molecular crystals suitable for solid-state organic electronics. However, a general method to control the shape of molecular crystals after growth is still lacking. One of the intuitive ways to shape an object is using a knife to cut or sculpt it. Therefore, in Chapter 3, a hypothetical knife made of a focused ion beam (FIB) will be used to shape PAH microcrystals. A stream of high-energy Ga+ ions are used to mill organic perylene crystals coated with a chemically removable gold layer. A programmable FIB blade can machine crystals into arbitrary shapes. The cutting resolution of FIB is found to be about 130 nm. Perylene crystals retain 90% of its original fluorescence after the whole shaping process. This experiment provides a general way to control the shape and size of polyaromatic crystals in a top-down fashion. Another problem limiting the potential of PAHs is that bare molecular crystals can be easily corroded by solvents, moisture, heat, and vacuum. Even if we manage to control crystals' bulk shape, these surface corrosions can still exclude PAHs from many practical applications. So, in Chapter 4, an impermeable atomic blanket—graphene is draped on molecular crystals to passivate their surfaces and protect them from the environment. Graphene successfully encapsulates perylene microcrystals grown on glasses, protecting the sample from solvent dissolution and sublimation at high temperatures. No detectable impact is found on the fluorescence lifetime of underneath thick crystals with thicknesses around 150 nm. In Chapter 5, the quenching effect at the graphene/PAH interface is further investigated by interfacing graphene with dye-doped emitting polymer films with various thicknesses. Time-resolved PL measurements are done on these samples. A quenching radius of 14.6 nm is extracted from the fitting of a series of PL decay data. Therefore, graphene encapsulation is still not a perfect solution since graphene can quench the molecules near the interface. An insulating few-layer hexagonal boron nitride (h-BN) is then used as an alternative protective layer to further eliminate the quenching effect at the interface. h-BN also successfully protects perylene crystals from solvent and heat. Surprisingly, wet-transferred h-BN is not totally inert but has a quenching radius of 2.9 nm. This quenching effect can be avoided by the dry transfer method that minimizes the lattice distortion and breakage from liquids (etching solutions and solvents). The work demonstrates the possibility of protecting molecular crystals by interfacing them with 2D materials. The investigated charge transfer dynamics provides guidance for the design of perspective organic/2D heterostructure devices Similar to PAH molecules, individual CNTs can also be assembled into macroscopic solid CNT networks by the filtration method. However, the randomly oriented network structure deteriorates films' conductivity because of inter-tube junctions that hinder the carrier transport. Thus, in Chapter 6, the filtration method is modified to produce aligned CNT thin films. The film's conductivity is improved in the alignment direction because of a reduction in the number of junction sites. Chromium surface functionalization that connects adjacent CNTs is then applied to reduce the junction resistance further. Cr atoms are inserted between CNTs by photochemistry to provide electrical channels at junctions for carriers to pass through. A reversible switch is also built to demonstrate the reversibility of this Cr functionalization.