Date of Award
Doctor of Philosophy
PLE presents an alternative or complementary probe-based tool to DPN, PPL, and NFL. Unlike most scanning probe techniques, where patterning by deposition is usually employed, PLE is unique because it is capable of deposition and removal in one or multiple steps. Therefore, PLE allows rectification of patterning errors, and it can be employed for both additive and subtractive patterning through molecular deposition and chemical and electrochemical etching, respectively. PLE is a technique that exploits the intrinsic porosity of hydrogels like agarose and polyacrylamide. The probes are made by polymerizing a liquid mixture of agarose or acrylamide monomers in a conical or pyramidal master. The polymeric probe is hydrated in deionized water or ink of interest after polymerization. For deposition, PLE has shown promising results in the selective deposition of fluorescent inks on bare or functionalized glass substrates. Erasing via PLE has been done in two ways: the first method involves selectively erasing the fluorescent molecules using a probe loaded with deionized water by bringing the probe in contact with the area of interest. Thus, solvation and transportation of the molecules into the polymeric probes render selective removal of materials (fluorescent inks) from a substrate. On the other hand, erasing or removal of metals deposited on a substrate was demonstrated using redox reactions. Here, the probe is loaded with an etchant, which is selectively delivered onto the substrate by bringing the probe close to or in contact with the surface. Thus, the etchant molecules passively diffuse from the probe to the substrate through a meniscus formed at the probe-substrate interface. Removal of molecules occurs after the redox reaction between the ink, and the substrate is completed. Many in-length microscale complex patterns can be easily made by translocating the probe over the substrate while the probe’s tip is in contact with the surface. Since the probes used in PLE are made of polymers, the probe-substrate contacting area can be easily modulated, and damage to the substrate by the probe is minimum. Moreover, it has been shown that the probes can be used multiple times, a hurdle frequently faced by probes made of hard materials such as silicon-based probes. We explored the capabilities of a polymeric probe made of PAAM to selectively deliver and remove (erase) material deposited on a surface. PLE, pioneered by our group, takes advantage of the hydrophilic and porous nature of polyacrylamide. In addition, the conformability of PAAM hydrogels was exploited to make patterns of various sizes and to the pattern on non-planar surfaces. The main advantage of PLE is removing materials from various substrates. Additionally, selective delivery of material to planar and non-planar substrates was demonstrated. Whereas DPN and sister techniques require multiple steps for patterning through the etching process, PLE can perform etching in one step. Therefore, using PLE, microscale patterning on surfaces can save considerable time, labor, and cost. Further, chemical and supplies waste are minima in PLE. Notably, the deposition and etching at the microscale level can be simultaneously achieved in one single step, providing an extremely high throughput patterning rate (on the order of 1000 mm2/s). The PLE patterning rate is two to three orders larger than DPN-based patterning. However, PLE inherently deposits and removes materials with features much larger (microscale) than that can be achieved with DPN (sub-nanoscale). Therefore, PLE is an alternative to DPN, PPL, and related probe-based deposition and erasing techniques, and in some cases, PLE provides enhanced capabilities than its contemporary techniques. In this dissertation, I intend to demonstrate the potential of PLE for fabricating working devices at a lower cost as an alternative to contemporary fabrication. Chapter 2 involves the fabrication of micro-electrodes on rigid and flexible substrates by selectively removing copper and ITO from a glass and a PET substrate. As proof of concept, substrates coated with the PLE patterned surfaces were used to fabricate a photodetector, and LEDs were assembled on the electrodes made on ITO-PET substrates. Chapter 3 describes a series of experiments involving the evaluation of ink withholding capacity, large area patterning, and the effect of modification of substrate surface energy on PLE patterning. These experiments an increased understanding of processes involved in PLE editing and microscale patterning. A potential pitfall of PLE-based etching was also observed in these experiments, where a thin layer of material was left behind after subtractive editing with a PLE probe. EDS analysis indicated that the material was composed of iron, chlorine, and copper ─ components of the etchant solution and the copper film. The ring structure was attributed to the coffee-ring effect pinning the water meniscus to the substrate. By understanding the potential causes of the formation of the coffee-ring possible solutions to this problem were formulated. Chapter 4 describes the physical and mechanical properties of the hydrogel PAAM probes at the nanoscale. ESEM and AFM were employed to investigate the structural and mechanical properties of the probes after impregnation with metal etchants of various concentrations. The effect of local RH on PLE patterns was also investigated. More importantly, these experiments show critical structural differences of PAAM hydrogels composed of various monomer and crosslinker concentrations. ESEM showed the significant influence exerted by RH on meniscus size and its interaction with the substrate. The behavior of the water meniscus observed in ESEM shows that large RH promotes water spreading on the substrate generating larger patterning features. Chapter 5 describes the capability of PLE to selectively deliver metallic inks on a non-linear curved substrate to fabricate a microscale battery. PLE was used to deposit silver nitrate onto a non-planar flexible substrate which was used to grow a thin electrically conductive copper film via copper electroless deposition. Electrodeposition of zinc on the copper substrate was accomplished. By coupling a zinc electrode to a manganese oxide-graphite composite cathode, we demonstrated a working Zn-MnO2 aqueous microscale battery.
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