Examination of band bending at buckminsterfullerene (C60)/metal interfaces by the Kelvin probe method

Abstract

This study addresses the question of band bending at the ${\mathrm{C}}_{60}\mathrm{/metal}$ interface. The change in the energy of the vacuum level upon the deposition of ${\mathrm{C}}_{60}$ on various metal substrates (Au, Cu, and Ag) was examined by the Kelvin probe method under ultrahigh vacuum as a function of ${\mathrm{C}}_{60}$ thickness $\mathrm{d.}$ We observed (1) an abrupt shift of the energy of the vacuum level relative to the Fermi level of the metal substrate ${\epsilon }_{\mathrm{vac}}^{\mathrm{F}}$ at $\text{d⩽1\hspace{0.05em}}\mathrm{nm},$ to a uniform value of about 4.65 eV at all of the interfaces examined and (2) a slower shift with further deposition of ${\mathrm{C}}_{60},$ which stopped at $\text{d∼500\hspace{0.05em}}\mathrm{nm}$ at another common value. These abrupt and gradual shifts can be ascribed to the formation of an interfacial dipole layer and to band bending leading to Fermi level alignment, respectively. The value of ${\epsilon }_{\mathrm{vac}}^{\mathrm{F}}$ for the thick region is ascribed to the bulk work function of the specific specimen studied, and it is noted that the values reported for a few monolayers in the literature should not actually be regarded as the bulk work function. These results clearly demonstrate the necessity of detailed measurements of ${\epsilon }_{\mathrm{vac}}^{\mathrm{F}}$ up to a large thickness of the organic film and of using various metal substrates to determine the bulk work function. The convergence of ${\epsilon }_{\mathrm{vac}}^{\mathrm{F}}$ to a common value at a large thickness indicates the existence of remaining impurity. Its concentration was estimated and the effect of purification by sublimation was examined. We note that the bulk work functions obtained as the convergent values in such measurements are characteristic of specific specimens and may still not correspond to the value of the really intrinsic material.