(The) adsorption structure of purine on the Ge(100) surface

Abstract

The adsorption of purine on Ge(100) has been investigated using Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), scanning tunneling microscopy (STM) and the density functional theory (DFT) calculation. From the ratio of C(KLL) / Ge(MNN) Auger peak intensities, we find that the surface coverage of purine molecules increases with increasing the exposure and reached saturation after the dosing of 8 minutes (θ = 0.25 ML). In TPD spectra, both chemisorbed and physisorbed purine molecules are observed to be present at the temperature of 110 K. Chemisorbed purine molecularly desorbs at 620 K while physisorbed purine at 140 K. It is also found that Purine molecularly desorbed following the first order kinetics, and the desorption energy is 38.67 kcal/mol. The STM image shows that the adsorbed purine molecules reside between the the buckled dimmer rows and bind to the down Ge atoms of the dimers forming local c(4×2) and p(2×2) reconstructions. Accordiong to the DFT calculation based on a four-dimer Ge cluster, the energetcially most stable adsorption geometry is that purine is chemisorbed on Ge(100) making two dative bonds between two N-atoms of purine and down atoms in Ge dimer. These results are in good agreement with the the experimental results.