Master of Science (MS), Bowling Green State University, 2024, Physics
In recent years, nanomaterials have made significant progress. Among these, PbS
nanomaterials, especially those in the infrared spectrum, show great potential for applications
such as fiber optic communication, solar cells, LEDs, night vision, and sensors. This study aims
to elucidate the energy levels of PbS nanosheets and conduct comprehensive analysis across
different sizes using cyclic voltammetry. This electrochemical method, known for its sensitivity,
simplicity, and cost-effectiveness, is used to measure the absolute energy levels of charge carriers
in the nanosheets.
In our research, we used nanosheets synthesized at temperatures ranging from 70 °C to
130 °C, with thicknesses from 2.0 nm to 4.2 nm. We employed mercaptopropionic acid (MPA)
for ligand exchange with oleic acid. In the cyclic voltammogram, we observed only the reduction
potential and calculated the LUMO (Lowest Unoccupied Molecular Orbital) levels relative to a
vacuum. To estimate the HOMO (Highest Occupied Molecular Orbital) level, we measured the
absorption spectra using an integrating sphere to determine the energy gap. The absorption edge
shifted towards longer wavelengths for nanosheets synthesized at higher temperatures. For
accurate energy gaps, suppressing exciton effects, we fitted the absorption spectra using a model
that includes both exciton and continuum contributions. The HOMO level was then calculated by
subtracting the energy gap from the LUMO level.
Notably, the LUMO and HOMO levels of PbS nanosheets increase as their size decreases
due to the quantum confinement effect. We also estimated the electron affinity of the PbS
nanosheet to be -4.68 eV, consistent with literature values. This information helps researchers
optimize energy level alignment in optoelectronic devices based on nanosheets, enhancing their
performance.
Committee: Liangfeng Sun Ph.D. (Committee Chair); Marco Nardone Ph.D. (Committee Member); Alexey Zayak Ph.D. (Committee Member)
Subjects: Physics