Functionalization of planar and nanoscale gallium phosphide for biosensor applications

Early detection of diseases, especially cancer, can significantly reduce mortality. Semiconductor-based biosensors are proving to be the ideal choice for biomarker detection due to their ultrasensitivity, quick detection time, and label-free methods. In a common semiconductor biosensor architecture, namely the field-effect transistor (FET) architecture, organic molecules must be chemisorbed to the semiconductor surface to introduce new functional groups in a process called functionalization. Functionalization allows for passivation and the immoblization of biomolecules which are all crucial for future device fabrication. Much research has been dedicated to silicon because of its domination of the semiconductor industry and well-characterized properties, however, with the miniaturization of technology and a better understanding of inorganic-organic interactions, silicon is quickly becoming obsolete. This dissertation focuses on the functionalization of gallium phosphide (GaP), a III-V semiconductor with properties better suited for the advancement of biomedical technology. Specifically, three separate functionalization strategies were utilized that involve thiol, terminal alkene, and azide chemistry to covalently bind to the GaP surface. Various surface sensitive techniques such as atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and water contact angle were used to quantify the extent of passivation. Inductively coupled plasma mass spectrometry (ICP-MS) was also used to evaluate passivation by quantifying the amount of gallium leaching into various solutions that mimic physiological conditions. The immobilization of biomolecules (specifically, DNA) was also demonstrated using a technique called Kelvin probe force microscopy (KPFM) that measures surface potential. Finally, since nanoscale architectures are currently more pertinent in the field of biosensors, the functionalization and biomolecule immobilization on GaP nanorods was also demonstrated using KPFM