Scanning tunneling microscopy investigation of electronic properties of cobalt nanoislands supported by Cu-9at.%Al(111)

► Co islands with different thicknesses were grown on Cu-9at%Al(111). ► Surface electronic states of Co islands were investigated by STM. ► Energies of surface states vary with the thicknesses and stacking manners. ► The origins of surface states were clarified by DFT calculations.