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J. Phys. Chem. C 2010, 114, 22230–22236
Adsorption Behavior of Bifunctional Molecules on Ge(100)-2 × 1: Comparison of Mercaptoethanol and Mercaptamine Jessica S. Kachian and Stacey F. Bent* Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305-5025, United States ReceiVed: September 8, 2010; ReVised Manuscript ReceiVed: NoVember 9, 2010
The adsorption behavior of mercaptamine and mercaptoethanol at the Ge(100)-2 × 1 surface has been studied under ultrahigh vacuum conditions to elucidate the effect of a second functional group on selective attachment of bifunctional molecules with one common and one distinct functional group. Mercaptamine and meraptoethanol each consist of a thiol group separated by a two-carbon alkyl chain from amine and hydroxyl groups, respectively. X-ray photoelectron spectroscopy, infrared spectroscopy and density functional theory results show that both mercaptamine and mercaptoethanol adsorb on Ge(100)-2 × 1 via S-H dissociation, with no S dative bonding observed. Whereas all mercaptoethanol adducts also react via O-H dissociation, most mercaptamine adducts do not undergo N-H dissociation. Rather, the major product of mercaptamine adsorption has a free amine group; the S-H dissociated, N dative-bonded adducts and dual S-H, N-H dissociated adducts comprise minor products. The results elucidate the importance of each moiety in a bifunctional molecule in controlling the product distribution of adsorption at the surface. I. Introduction Over the past two decades, considerable attention has been directed toward organic functionalization of group IV semiconductor surfaces due to its potential applications in the development of molecular-scale devices.1-3 Combining the tailorability of organic materials with the precise interface control provided by atomically clean surfaces can potentially impart novel functionalities to nanoscale devices. Realizing this vision requires a detailed understanding of the chemistry taking place at the semiconductor surface. In particular, interest in the attachment of bifunctional molecules at the semiconductor surface is driven by the potential to create versatile multilayer structures and films. As a starting point in the development of a sequential molecular layer deposition process for growth of ultrathin organic films,4-7 molecular design rules must first be determined to successfully engineer a bifunctional molecule in which one moiety reacts with the surface while the other functional group remains available for attachment to a subsequent layer. Recent studies have shown that analogies between the organic functionalization of (100)-2 × 1 group IV semiconductor surfaces and classic solution-phase organic chemistry constitute a means for characterizing and understanding these surface reactions.8-11 The 2 × 1 reconstructed Ge(100) and Si(100) surfaces are comprised of rows of surface dimers possessing a strong σ bond and a weak π bond. The tilting of these dimers creates an uneven distribution of charge within the dimer, resulting in an electron-rich, nucleophilic up atom and an electron-deficient, electrophilic down atom.12 Consequently, the up and down atom of a dimer can function as a Lewis base and Lewis acid, respectively.8 This chemical description is invoked in our present study of the adsorption of mercaptoethanol and mercaptamine (shown in Figure 1) on the clean Ge(100)-2 × 1 surface. These molecules contain a thiol group and either an amine or hydroxyl * To whom correspondence should be addressed. Phone: 650-723-0385. Fax: 650-723-9780. E-mail:
[email protected].
Figure 1. Two molecules studied, mercaptoethanol and mercaptamine.
group, each of which contains a heteroatom (S, O, N) with at least one lone pair of electrons. Consequently, each of these atoms can form a dative bond to the germanium surface by donating a lone pair of electrons to the electron-deficient down atom of the germanium dimer. The dative-bonded adducts may react further by a dissociation event in which a nucleophilic up atom of the germanium surface abstracts a proton from the heteroatom atom through which the adsorbate is dative-bonded to the surface. For example, earlier studies have shown that, at room temperature, ethanethiol and ethanol adsorb on Ge(100)2 × 1 via S-H and O-H dissociation respectively, with S-H dissociation being thermodynamically and kinetically more favorable than O-H dissociation.13 On the other hand, methylamine is found to datively bond to the Ge(100)-2 × 1 surface through the nitrogen atom at room temperature, with no evidence for N-H dissociation.14 The reactivity of these monofunctional analogs suggests that at room temperature, mercaptoethanol will adsorb at Ge(100)-2 × 1 via S-H dissociation and, if the ring strain is not excessive, by O-H dissociation as well. Mercaptamine, on the other hand, is predicted to react by S-H dissociation with possible secondary interaction with the surface through N dative bonding. Although the results of the monofunctional analogs are useful, they are not complete, because the behavior of a functional group may differ when present within a polyfunctional molecule.8 This can be illustrated by the results for ethylenediamine (ED)15 on Ge(100)-2 × 1. Unlike methylamine, the bifunctional ED has been reported to react with Ge(100)-2 × 1 at room temperature via N-H dissociation of both amino groups at low surface coverages, purportedly due to the stabilization gained when two amino groups interact with the surface.15 At higher
10.1021/jp1085894 2010 American Chemical Society Published on Web 11/30/2010
Comparison of Mercaptoethanol and Mercaptamine coverages, ED reacts via N-Ge dative bond formation.15 Consequently, it is possible that in addition to S-H dissociated, N dative-bonded mercaptamine adducts, there will also be a fraction of dual S-H and N-H dissociated adducts at room temperature. In addition, because electronic effects in dativebonded amino groups can limit the surface coverage, as reported for amines on Si(100)-2 × 1,16,17 some S-H dissociated mercaptamine adducts may have free amine groups especially at higher coverages. In the present work, we use a combination of experimental and theoretical methods to investigate the adsorption products of mercaptamine and mercaptoethanol on the Ge(100)-2 × 1 surface. Our results will show that, at room temperature, both molecules adsorb on Ge(100)-2 × 1 via S-H dissociation, with no S dative bonding observed. However, whereas all mercaptoethanol adducts adsorb through dual S-H and O-H dissociation, only a modest fraction of mercaptamine adducts adsorb through dual S-H and N-H dissociation, with a smaller fraction adsorbed via S-H dissociation and N dative bonding, and the largest fraction adsorbed through S-H dissociation and not interacting with the surface through the amine group. Consequently, in examining adsorption of two molecules with a common functionality (-SH) and spacer (-CH2CH2-), we demonstrate the significant effect of a second functionality, -NH2 versus -OH, on the product distribution at the Ge(100)-2 × 1 surface. II. Experimental and Computational Details Infrared experiments were completed under ultrahigh vacuum conditions (UHV) in a previously described reaction chamber18 with a base pressure of less than 1 × 10-10 Torr. Briefly, the UHV chamber was paired with a BioRad FTS-60A Fourier transform infrared (FTIR) spectrometer equipped with a liquid nitrogen-cooled narrow-band mercury-cadmium-telluride (MCT) detector. A trapezoidally shaped Ge(100) crystal (19 × 14 × 1 mm, 45° beveled edges) designed for multiple internal reflection (MIR) experiments was conductively heated by a resistive tungsten heater and cooled by heat exchange with a copper braid connected to a liquid nitrogen reservoir. The temperature of the sample was monitored using a thermocouple directly attached to the crystal. Ge(100) surface cleaning via two cycles consisting of sputtering (0.5 keV Ar+ ions, 20 min, room temperature) followed by annealing (900 K, 5 min) resulted in the reconstructed surface, as confirmed by a 2 × 1 low-energy electron diffraction (LEED) pattern; and levels of carbon, oxygen, and nitrogen were below the detection limit of our Auger electron spectrometer following this cleaning procedure. The unpolarized beam from the FTIR spectrometer entered the chamber through a CaF2 viewport, was focused onto the beveled edge of a germanium MIR crystal (Umicore), and then exited through another CaF2 viewport at a right angle to the first viewport. The beam path was purged by nitrogen gas to eliminate H2O and CO2 spectral features. The spectral range of the collected infrared data is limited by absorption by the CaF2 windows, resulting in a low-frequency cutoff of ∼1050 cm-1. To record infrared spectra of unreacted molecules, several multilayers were condensed on the surface of the sample at low temperature (
