(545b) Adsorption of Heterobifunctional 4-Nitrophenol On the Ge(100) Surface

Group IV semiconductor materials play a crucial role in integrated circuits. Recently, high carrier mobility has drawn attention to Ge as an alternative channel material. However its surface requires modification to be utilized in devices, since the native oxide of germanium is much less stable compared to SiO₂. The (100) surface of Ge under vacuum reconstructs to a 2 × 1 pattern after proper preparation. Each dimer possesses partial double bond character so that alkene-like reactions occur, and also charge separation within each dimer allows acid/base-like reactions. The ordered dimers that form rows separated by trenches serve as the local reaction sites. On the other hand, organic functionalization of surfaces could be interesting since that way the versatility of the organic chemistry could be coupled with the current semiconductor technologies. Especially, adsorption of bifunctional molecules renders possibilities of having one terminus of the molecule available for further chemical modification of the surface. Most studies on the adsorption of bifunctional molecules conducted so far have focused on homobifunctional molecules, where the two functionalities are identical; consequently, dual reaction of a single molecule with the surface was found in many cases, reducing the number of available reaction sites. A heterobifunctional molecule may be useful by undergoing an asymmetric reaction where one terminus would preferentially react with the surface, keeping the other unreacted.

In this study, adsorption of a heterobifunctional molecule, 4-nitrophenol, on the Ge(100) surface was explored using multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy and  X-ray photoelectron spectroscopy (XPS) experiments combined with density functional theory (DFT) calculations. Since the two functional groups are both reactive with Ge(100), there are three possible adsorbate structures: (1) singly bound through the O-H dissociation of the hydroxyl group, (2) singly bound through the cycloaddition of the nitro group, and (3) dually bound with both functionalities. DFT calculations show all these products have large enough adsorption energies to be found at room temperature as final products, and that conversion to (3) from (1) would be feasible while (2) is unlikely to be transformed to (3) because of a high activation barrier. The Ge(100) surface was exposed to increasing amounts of nitrophenol to test the coverage dependence of adsorption experimentally. The IR spectrum of the physisorbed molecules shows that the presence of the free nitro group could be identified with its strong characteristic absorption bands. While the –NO₂ bands are clearly discernible in saturated chemisorption spectra at room temperature, they are not observed at low surface coverage. Therefore structure (1) can be understood to appear only at higher coverage. Also, multiple peaks overlapping at the ring stretch region even at low coverage suggest that a mixture of products is formed, most likely by structures (2) and (3). In the XPS spectra, the existence of both unbound –NO₂ and unbound –OH are suggested. The free nitro group is most apparent close to saturation, confirming the findings from IR. Moreover, through XPS we could quantify the fraction of each adsorbate at saturation. In conclusion, we show that 4-nitrophenol adsorbs on Ge(100) through both of its functionalities at low coverage, but forms singly adsorbed products through either the –NO₂ or –OH group as saturation coverage is approached.