Fully Solution Processed Method to Fabricate ZnO Nanoparticles Devices for Sensing Application

Abstract

This thesis presents a cost-effective, solution-based methodology for the fabrication of zinc oxide nanoparticles (ZnO NPs) devices aimed at pH sensing applications. ZnO NPs were dispersed in methanol and subsequently spin-coated onto glass substrates. Experimental results indicate that ZnO deposited directly from the solution is susceptible to defects, evidenced by a high sheet resistance of 10^10 Ω/□ in the absence of light, as well as sensitivity to environmental changes, low electrical stability, and vulnerability to water etching.
To mitigate these limitations, a novel processing technique involving exposure of ZnO to ultraviolet (UV) vacuum heating (UVVH) was developed. In pursuit of long-term stability, various polymers were evaluated as passivation materials, including polyvinyl alcohol/polydimethylsiloxane (PVA/PDMS) bilayers, ethylene-vinyl acetate (EVA), and ethylene-vinyl alcohol (EVOH).
The intrinsic high resistivity of ZnO NPs is attributed to ionized oxygen molecules adsorbed on the nanoparticle surface. These adsorbed oxygen molecules act to trap free electrons within the nanoparticle, serving as scattering centers that decrease both carrier concentration and mobility, thereby increasing film resistivity. The UVVH process effectively removes the ionized adsorbed oxygen molecules, while the passivation layer prevents re-adsorption, maintaining a low resistivity level for the ZnO NPs. This enhanced process yields resistivity values comparable to those achieved through traditional physical and chemical vapor deposition techniques.
The proposed method involves exposing the solution-processed ZnO NP film to 365 nm UV light and encapsulating it with an 80 μm layer of EVOH under vacuum conditions. The manufacturing process reaches a maximum temperature of 190 °C, making it suitable for flexible substrates such as polyimide. The resulting ZnO film exhibits a sheet resistance of 2.5 × 104 Ω/□ and a thickness of 2.35 μm. The EVOH passivation layer significantly enhances film stability, demonstrating resilience upon exposure to ambient conditions, with resistivity remaining consistent after 60 days.
Various thicknesses of EVOH passivation were also applied to evaluate their effects on the electrical stability of the nanoparticle films. The implementation of the UVVH process facilitates the production of a highly sensitive ZnO NP semiconductor pH sensor, wholly realized through a solution-based process that eschews high-energy consumption methods. The sensor exhibits diode-like electrical characteristics, with an increase in threshold voltage corresponding to decreases in the pH of the measured solution, yielding a sensitivity of 360 mV/pH. Maximum processing temperature is recorded at 120 °C, utilizing the UVVH technique.
The waterproof and oxygen-isolating polymer EVOH effectively passivates the ZnO NPs. The operational principle of the sensor, alongside its diode-like attributes, is closely linked to the adsorption of ionized oxygen molecules on the surface of the ZnO NPs. Furthermore, a mathematical model derived from the adsorption isotherm closely correlates with the experimental data. This fully solution-based manufacturing approach holds significant promise for applications in medical sensing and flexible wearable electronics.

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Identifiers

ORCID for Mengyang Qu: orcid.org/0009-0003-3685-8658

ORCID for Stephen Beeby: orcid.org/0000-0002-0800-1759

ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

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