Determination of nano-size effect on conduction and valence band edge positions in ZnO and Al-doped ZnO nanostructures by Kelvin probe force microscopy

Particle size and doping affect the surface energetics of ZnO nanoparticles and their electronic energy-level alignment. This study investigates size-dependent work functions in pristine ZnO and Al-doped ZnO nanoparticles and estimates the corresponding conduction- and valence-band-edge positions. Atomic force microscopy and Kelvin probe force microscopy were used to correlate particle size with local work function calculated from calibrated contact potential difference measurements. Optical band gaps estimated from ultraviolet visible absorption spectra using Tauc analysis were 3.050 eV for ZnO and 3.185 eV for Al-doped ZnO. Twelve nanoparticles were examined per material, with AFM measurements showing sizes of 5.7–29.3 nm for ZnO and 7.1–25.2 nm for Al-doped ZnO. Work function decreased approximately linearly with increasing particle size, with slopes of −1.871 and −3.912 meV nm -1 and coefficients of determination of 0.956 and 0.961, respectively. The Al-doped ZnO sample exhibited a slope magnitude approximately 2.09 times that of pristine ZnO. Assuming that the Fermi level coincides with the conduction-band minimum, combining the optical gaps with the work-function measurements yielded estimated band edges that shifted toward less negative energies with increasing particle size. The study provides a quantitative particle-level comparison of size-dependent work functions and estimated band edge trends in ZnO and Al-doped ZnO nanoparticles. These findings establish a correlation between nanoparticle size and work function in the investigated ZnO and Al-doped ZnO samples and provide a framework for estimating size dependent conduction and valence band edge positions.