Adsorption of alanine on Cu(001) studied by scanning tunneling microscopy

The adsorption of L- and DL-alanine on the Cu(001) surface under an ultrahigh vacuum environment has been studied by means of scanning tunneling microscopy (STM). From both the as-adsorbed and the annealed surface it has been found that L-alanine can form only the c(2×4) superstructure on the Cu(001) terraces while it can induce step faceting to 〈310〉 directions as well as further bunching of 〈310〉 steps to form {3 1 17}1×1 facets. It has also been found in the case of DL-alanine adsorption that D- and L-alanine tend to segregate from each other to form their own c(2×4) domains, indicating that hydrogen-bond formation is the driving force behind formation of the superstructures. On the basis of our STM observations, the value of the binding energy of alanine on Cu(001) is estimated as ∼1.7 eV, and that of the diffusion activation energy is ∼0.83 eV. These values suggest that alanine molecules adsorb on the Cu(001) surface in the anionic form, and in turn enable us to propose detailed models for both the Cu(001)c(2×4)–L-alanine and Cu(3 1 17)1×1–L-alanine superstructures for further investigation.