Understanding methods to determine energy levels of quantum dot films for device integration

Knowing the energy levels in quantum dot films is a crucial variable for determining materials to be used for electrodes and other active layers (i.e., hole blocking layer, as well as operation conditions in quantum dot devices. Kelvin Probe Force Microscopy (KPFM), a technique commonly used to determine the contact potential difference between materials, is used to determine the Fermi level position of lead chalcogenide and silver chalcogenide quantum dot films. Choice of capping ligand during film formation is shown to have significant effect on the position of the Fermi level, valence, and conduction bands. In Ag2Se quantum dots films a 0.3 eV variation in Fermi level as a function of capping ligand is observed while 0.45 eV variation is observed in PbSe quantum dot films, with iodide-based ligands showing the highest Fermi level position and oleylamine displaying the lowest. KPFM measurement procedure is outlined, and the current strength and limitations of the technique are discussed.