The adsorption of pyridine onto the Ge(100) surface has been studied using both real-time scanning tunneling microscopy (STM) and ab initio pseudopotential density functional calculations. The results show that pyridine molecules adsorb on the electron-deficient down-Ge atoms of the Ge=Ge dimers via Ge-N dative bonding, with the pyridine ring tilted to the surface. The electron-rich up-Ge atoms remaining after adsorption of pyridine induce an asymmetric dimer row, which is mainly reconstructed to the c(4 x 2) structure. At pyridine coverage of 0.25 ML, the adsorbed pyridine molecules form a perfectly ordered monolayer. The entire Ge substrate underlying this organic monolayer rearranges into the c(4 x 2) structure.
We have performed ab initio pseudopotential calculations in order to investigate the atomic and electronic structure of pyridine adsorbed on the Ge(100) surface. A large number of pyridine/Ge(100) adsorption configurations possibly resulting from cycloadditions and Lewis acid−base reactions are presented. The configuration having the Ge−N linkage formed by dative bonding with adsorbed pyridine molecules tilted is the most stable, which explains the experimental STM images well. The dative bonding character is investigated by comparing the charge densities for the clean and pyridine-adsorbed Ge(100) surfaces. Finally the difference between the Ge(100) and Si(100) surfaces is discussed.
The atomic-scale structural evolution of Ge͑100͒ surfaces etched by H͑g͒ and D͑g͒ at T s ϭ400 K is studied using scanning tunneling microcopy ͑STM͒ and field emission-scanning electron microscopy ͑FE-SEM͒. The STM investigation reveals that etching of the Ge͑100͒ by H͑g͒ and D͑g͒ proceeds initially via the production of single atom vacancies ͑SV͒, dimer vacancies ͑DV͒, and subsequently, line defects along the Ge dimer rows. It is also observed that D͑g͒ etches the Ge͑100͒ surface eight times faster than H͑g͒ does. After extensive exposures of the surface to H͑g͒, the FE-SEM images show square etch pits with V-groove shapes, indicating that H͑g͒ etching of the Ge͑100͒ surface proceeds anisotropically.
The kinetics of H2 (D2) desorption from a Ge(100)-2×1:H (D) surface was studied using scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). Inspection of STM images of surfaces at the saturation coverage of H (D) (θH(D)≃1.0 ML) revealed a 2×1 monohydride (monodeuteride) phase in which most H (D) atoms were paired on Ge-dimers. By counting the sites of H2 (D2) desorption in STM images taken after desorption of H2 (D2) at temperatures in the range Ts=500−550 K, the desorption of H2 (D2) was found to follow first order kinetics with an activation energy of Ed=1.65±0.1 eV (1.65±0.1 eV) and a pre-exponential factor of νd=(2.7±0.5)×1013 s−1 [(1.2±0.5)×1013 s−1]. These values of Ed and νd were used to simulate TPD spectra for the desorption of H2 (D2) from a Ge(100)-2×1:H (D) surface. The simulated spectra were in good agreement with the experimental TPD spectra. In contrast to the surfaces with saturated H coverage, which are characterized by pairs of H atoms on Ge-dimers, at the low H coverage of θH≃0.05 ML unpaired H atoms as well as paired H atoms were observed on the Ge-dimers on the surface, causing the desorption process to follow second order kinetics. At Ts∼300 K, the singly occupied dimers (SODs) appear to be favored over doubly occupied dimers (DODs). However, upon increasing the temperature (Ts) from 300 to 500 K, most SODs were rapidly converted into the thermodynamically favored DODs by the migration of H atoms. On the other hand, it is observed that even above Ts∼500 K, the onset temperature for H2 desorption from DODs, a non-negligible number of SODs remain on the surface due to the large entropic barrier to pairing. These results suggest that H adsorption in the low coverage is strongly influenced by the energetics of the pairing of H atoms.
We studied the atomic scale surface evolution of Ge͑100͒ exposed at 300 K to gas-phase hydrogen atoms, H͑g͒. Surface H͑g͒ uptake created a 2ϫ1:H phase, quickly reaching ϳ1 monolayer H coverage. However, in contrast to the Si͑100͒ surface, dangling bonds of the Ge͑100͒ surface could never be completely removed by H͑g͒ due to their regeneration by highly efficient surface H abstraction. This, together with the instability of surface dihydrides, GeH 2 (a), inhibited the large-scale formation of 3ϫ1:H and 1ϫ1:H phases. Short GeH 2 (a) rows, present in small metastable 3ϫ1:H domains formed near defect sites, were etched selectively by H͑g͒, producing line defects.
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