Article pubs.acs.org/JPCC
Coverage-Dependent Adsorption of Bifunctional Molecules: Detailed Insights into Interactions between Adsorbates Bonggeun Shong,† Rasmus Y. Brogaard,†,‡,§ Tania E. Sandoval,† and Stacey F. Bent*,† †
Department of Chemical Engineering, Stanford University, 443 Via Ortega, Stanford, California 94305, United States SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
‡
S Supporting Information *
ABSTRACT: Adsorption of bifunctional molecules is important for chemical modification of semiconductor surfaces, since such molecules can be used to alter the terminal functionality. In this work, the effect of coverage in the reaction of catechol (1,2benzenediol) and resorcinol (1,3-benzenediol) with the (100) surface of germanium is investigated by surface spectroscopy experiments and theoretical methods. The benzenediols are site-specifically adsorbed on the Ge surface dimers through dissociative adsorption of either one or two of the −OH groups. Infrared spectroscopy and density functional theory results suggest that the dually tethered products selectively assume certain geometrical configurations at the surface. Infrared and X-ray photoelectron spectroscopies reveal that singly tethered species become increasingly prevalent as the coverage approaches saturation. Monte Carlo simulations that account for specific binding configurations and interadsorbate interactions, identified through density functional theory calculations, quantitatively reproduce the experimentally observed coverage-dependence of singly and dually tethered adsorbates for both benzenediols. Our results indicate that the singly bound adsorbates with unreacted hydroxyls appear on the reactive pristine Ge surface due to a limitation of available adjacent reaction sites, and show that interadsorbate interactions play a major role in determining reaction product distributions.
I. INTRODUCTION Functionalization of semiconductor surfaces with organic molecules has gained increasing attention in recent years.1−5 Its potential applications include modification of organic− inorganic interfaces toward tailored chemical, electronic, optical, and mechanical properties.6−10 The reconstructed low-index surfaces of Si or Ge single crystals prepared under vacuum comprise well-ordered surface reaction sites that can serve as a basis for fundamental adsorption studies. For example, the localized reactions of the surface dimers on 2 × 1 reconstructed Si or Ge (100) surfaces toward organic reagents can be understood by analogy with acid−base or pericyclic reactions, both well-known in organic chemistry.1 Adsorption of bifunctional molecules is especially interesting for their potential to modify the terminal chemical functionality of the surface, as in molecular layer deposition (MLD) processes.11,12 Once a bifunctional molecule is tethered to a solid surface, the reactivity of the second functional group may be changed from that of the free molecule. For example, while uniform products were observed in adsorption of various monofunctional molecules on Ge(100)-2 × 1,6,13−17 their bifunctional analogues retained a significant fraction of the adsorption products with a single functionality (abbreviated as 1A) on Ge(100)-2 × 1,18−26 even when the dual adsorption © 2014 American Chemical Society
product by both of the functionalities (abbreviated as 2A) was thermodynamically favorable. Whether a bifunctional adsorbate reacts dually or singly can be a critical issue, especially in successive reactions such as MLD; 1A leaves one functionality for further chemical modification, while 2A terminates the reactive site and decreases the number of functional moieties.21,27−30 On the other hand, dual tethering of bifunctional molecules may be desirable when passivation of the surface is required.31 Therefore, in order to engineer the properties of organic-functionalized surfaces, it is important to understand why dual or single attachment occurs during chemisorption of bifunctional molecules. One of the factors known to affect the reactivity of bifunctional molecules on semiconductor surfaces is the coverage (θ) of the adsorbates, such that a larger fraction of 1A appears at higher coverages.19,22,26 However, only a few different coverages were explored in most previous vacuumbased studies, providing only rough snapshots of the adsorption phenomena. Similar coverage-dependent behavior has been observed with bifunctional self-assembled monolayers (SAMs) Received: July 22, 2014 Revised: September 18, 2014 Published: October 6, 2014 23811
dx.doi.org/10.1021/jp507349k | J. Phys. Chem. C 2014, 118, 23811−23820
The Journal of Physical Chemistry C
Article
and the 2 × 1 reconstruction was confirmed by low energy electron diffraction (LEED). IR spectra were collected by a Nicolet 6700 FT-IR spectrometer using an external liquid nitrogen cooled HgCdTe detector in multiple internal reflection (MIR) geometry. All spectra were corrected for baseline sloping by manually subtracting spline functions fit to points devoid of spectral features. Absorption by the CaF2 viewports resulted in a low-frequency cutoff of ∼1000 cm−1. XPS experiments of core-level binding energies were conducted in a separate reaction chamber with a base pressure of 99%, Tokyo Chemical Industry) and resorcinol (>99%, Tokyo Chemical Industry) are white solids at room temperature and atmospheric pressure. The precursors were further purified by repeated sublimination and pumping cycles before being dosed. The vapor in equilibrium with the solid sample was leaked into the vacuum chamber by means of a gate valve separating the doser from the chamber. The molecular identities were confirmed by an in situ quadrupole mass spectrometer. Exposures are reported in units of langmuir (L; 1 L = 10−6 Torr s) and are not corrected for ion gauge sensitivity. The DFT calculations were performed using GPAW,48,49 a real-space grid implementation of the projector augmentedwave method50 interfaced with the ASE package.51 Unless otherwise indicated, standard GPAW parameters were used. The calculations employed the BEEF-vdW functional52 taking dispersion interactions into account. A real-space grid spacing of 0.18 Å was used in calculations of adsorbates and isolated molecules. The lattice constant of bulk Ge was determined using the Equation of State module in ASE by calculating total energies with a constant number of grid points corresponding to a grid spacing around 0.15 Å. The resulting computed lattice constant of 5.7909 Å, a close reproduction of the experimental value of 5.6574 Å,53 was used to construct slab models of the Ge(100) surface. The calculations model the surface in a p(2 × 2) configuration, employing a thickness of 8 atomic layers (two unit cell lengths) and constraining the atoms in the two lowermost layers to the bulk positions. The dipole correction was employed, and vacuum was added below (7.5 Å) and above (13 Å) the slabs to ensure meV convergence of total energies with and without periodic boundary conditions in the direction perpendicular to the slab. Two orthorhombic supercells (Figure 2) were used in the calculations, employing 2 × 2 × 1 (Cell I)
on metal surfaces,32−34 which can be explained by reversible phase transition. However, since adsorption of organic molecules on semiconductor surfaces is usually irreversible, an alternative explanation is required for the increasing appearance of 1A at higher coverages. As a clue for the behavior of bifunctional molecules on semiconductor surfaces, it is known that many bifunctional adsorbates selectively assume a specific dual binding geometry even if multiple surface configurations are possible.23,35−39 The interactions among adsorbates on low-index surfaces of group 14 semiconductors can be systematically understood, since the highly ordered arrangements of the atoms on these surfaces allow only a small number of possible two-adsorbate configurations. Such surfaces with well-defined geometries allow construction of simple Monte Carlo simulations that can accurately model the adsorption process.40 Catechol (1,2-benzenediol) and resorcinol (1,3-benzenediol) are bifunctional molecules that are often used as linker groups for applications involving molecular attachment onto surfaces.41−44 The two reactive hydroxyl (−OH) groups in these benzenediol molecules are separated by a phenyl group (Figure 1). The rigid phenyl spacer between the functionalities can be
Figure 1. Positional isomers of benzenediol.
beneficial for their utilization, as previous studies have shown that rigid bifunctional molecules tend to produce more 1A product at the first monolayer23−25 and show less dual reaction in sequential adsorption27−30 compared to analogous flexible bifunctional molecules. The interaction between the Ge(100) substrate and the phenyl ring is much weaker than the interaction of the surface with the hydroxyls17,23,45 so that chemisorption of benzenediols should occur only through the hydroxyl groups. In this study, we investigate the coverage-dependent adsorption of catechol and resorcinol on the Ge(100)-2 × 1 surface. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy experiments were conducted together with density functional theory (DFT) calculations and Monte Carlo (MC) simulations to elucidate the interactions determining the product distribution. Our results show that (1) the geometrical commensurability with the surface is an important factor in the reactivity and configurational selectivity of bifunctional molecules on the surface, and (2) interadsorbate interactions are the main driving force for the emergence of the singly attached adsorbate with a free functionality at higher total coverages.
II. EXPERIMENTAL AND THEORETICAL DETAILS All experiments were conducted under ultrahigh vacuum (UHV) conditions. FTIR spectroscopy experiments were performed in a previously described reaction chamber15 with a base pressure of
