Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Chemisorption of Ethanol on Ge(100) Surface Sung-Soo Bae,† A-Reum Lee,‡ Sehun Kim,† and Do Hwan Kim*,‡ †
Molecular-Level Interface Research Center, Department of Chemistry, KAIST, Daejeon 34141, Republic of Korea Division of Science Education and Institute of Fusion Science, Chonbuk National University, Jeonbuk 54896, Republic of Korea
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ABSTRACT: Chemical reaction of ethanol (CH3CH2OH) with Ge(100) surface has been investigated using scanning tunneling microscopy (STM) observation and density functional theory (DFT) calculations. At low coverage, highresolution STM images showed that ethanol dissociatively adsorbed on a single Ge dimer. The adsorption features included bright protrusions assigned to Ge−OCH 2 CH 3 structure formed via O−H dissociation of ethanol. Real-time STM observations revealed that the molecular chain of ethanol increased gradually via successive adsorption along the dimer row direction following increased exposure to ethanol. DFT calculation results showed that the adsorption of ethanol on Ge(100) was dominated by kinetic control at room temperature. Thus, an integrated study of experimental and theoretical approaches coherently confirmed that ethanol reacts with Ge(100) via O−H dissociative adsorption and the final structure has the H−Ge−Ge−OCH2CH3 geometry on a single dimer of Ge(100).
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INTRODUCTION The study of the adsorption of organic molecules on semiconductor surfaces has been an active area of research investigation with a number of technological applications, including chemical sensor, microelectronics, semiconductor processing, and etching procedures.1−7 However, many fundamental questions related to the specific properties of such hybrid systems at the atomic or molecular level remain unresolved. Unsaturated species carrying multiple bonds or Lewis bases containing lone pairs of electrons form stable adsorption structures on semiconductor surfaces via interaction with the dangling bonds of silicon (Si) and germanium (Ge) surface dimers. Organic molecules, such as ethylene, 8 amines,9−11 or alcohol,12,13 exhibit characteristic adsorption properties on different group IV semiconductor surfaces, in addition to providing different degrees of surface reconstructions. Among the various organic molecules, O-containing molecules adsorbed on semiconductor surface are important in the semiconductor industry because of their relationship with device fabrication, such as growth of oxide films by etching or wet oxidation.14 The reaction of ethanol (CH3CH2OH) with Si(100) in ultrahigh vacuum (UHV) conditions has been studied, both as a prototype of organic functionalization of semiconductor surface and as a technologically feasible reaction for the growth of silicon dioxide layers.15−18 Eng et al. studied the adsorption of ethanol on Si(100) using surface infrared absorption spectroscopy and ab initio calculations and concluded that ethanol adsorbs dissociatively to form surfacebound hydrogen and ethoxy (C2H5O) groups near room temperature.15 The adsorption of ethanol on Si(100) at room © XXXX American Chemical Society
temperature was also investigated by Casaletto et al. using highresolution synchrotron radiation photoemission.16 The results support O−H dissociative adsorption and suggest the absence of C−O bond cleavage. Zhang et al. studied the reaction of ethanol with the Si(100)-2 × 1 using Auger electron spectroscopy, thermal desorption spectroscopy, and ab initio computations.18 Their calculations showed that ethanol initially interacts with the Si surface via barrierless formation of a dative bond. They found that each of the reaction paths out of the dative bond has significant energy barriers to reaction, except for O−H bond dissociation, which has a small barrier (0.03 eV). The energy level of the transition state was calculated to be below the energy of the reactants (free ethanol and clean Si surface). They concluded that although the O−H bond dissociation was kinetically favored, the C−O bond cleavage was thermodynamically the most stable final product. Contrary to the extensive work on the reaction of ethanol on Si(100), the reaction of ethanol with Ge(100) has received relatively little attention. The chemical reactions of organic molecules on Ge(100) surfaces have been shown to produce more selective and characteristic adsorption structures,6,19 compared to those on Si(100) surfaces. Chemisorption of methanol on Ge(100) surface produced adsorption structures similar to that observed on Si(100), specifically an O−H dissociative adsorption structure.12,13,18,20 This adsorption behavior of methanol is somewhat different from that observed for ethylamine, which dissociatively adsorbs on Si(100)9 but Received: March 29, 2018 Revised: June 12, 2018 Published: June 17, 2018 A
DOI: 10.1021/acs.jpcc.8b02973 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Schematic illustration of possible surface reaction of ethanol on Ge(100): (a) molecular adsorption via Ge−O dative bonding and (b, c) dissociative adsorption via (b) O−H cleavage and (c) C−O cleavage.
exhibits dative bonding on Ge(100).10 The reaction of ethanol on Ge(100) was studied by Kachian and Bent using Fourier transform infrared spectroscopy and DFT calculations of the adsorption of ethanol on Ge(100) as a part of their study of the adsorption behavior of sulfur- versus oxygen-containing organic molecules.21 They concluded that ethanol undergoes intradimer O−H dissociative adsorption at room temperature. Their DFT calculations were based on a cluster model and focused on O−H dissociative adsorption via either an intradimer or interdimer pathway. Meanwhile, we elucidated the reaction of methanol on Ge(100).13,22 Ethanol is similar to methanol with respect to chemical properties, such as acidity (pKa, 15.5 for methanol and 15.9 for ethanol), chemical structures (simple alkyl groups and a single hydroxyl group with a lone pair of electrons), and chemical reactivity (nucleophilicity by electronegative oxygen atom). Thus, the effect of alkyl group on the surface reactivity of alcohols can be investigated via a comparative study of the adsorption behavior of ethanol and methanol. Possible adsorption reactions of ethanol on Ge(100) surfaces are shown in Figure 1: (a) O-dative bonding of lone pairs of electrons on oxygen atoms of ethoxy group to electron-deficient down Ge atom, (b) OH-dissociative adsorption to form O−Ge and H−Ge bonding, and (c) CO-dissociative adsorption to form O−Ge and C−Ge bonding. In this paper, the chemisorption of ethanol on Ge(100) surface was analyzed using STM observation and DFT calculations. The high-resolution STM results recorded during real-time dosing of ethanol on Ge(100) present coverage dependence of the adsorption structures. The study found O−H dissociative adsorption structure as the most favorable adsorption geometry of ethanol on Ge(100), and the dissociated fragments of ethanol were bonded to two Ge atoms of a dimer.
investigate the adsorption structure. A sample was introduced into the chamber through a load-lock transfer system. The Ge(100) sample used was n-type (Sb-doped; R = 0.01−0.39 Ω cm) and cut into 2 × 10 mm2 pieces for STM measurement. The clean surface was prepared by sputtering with Ar+ ion (11.0 μA of 1.0 kV) at 700 K for 30 min and then annealing at 900 K for 10 min. The production of clean and ordered Ge(100) surface was confirmed by STM. Ethanol (CH3CH2OH; anhydrous) with >99.5% purity was purchased from Aldrich and transferred to a sample vial (KOVAR) in a nitrogen-purged glovebox. Ethanol was further purified via several freeze−pump−thaw cycles with liquid nitrogen to remove all of the dissolved gases prior to dosing onto clean Ge(100). The purity of ethanol was checked by an in situ mass spectrometer in the vacuum chamber. Ethanol was dosed onto Ge(100) at room temperature via a direct doser controlled by a variable leak valve. The direct doser with a sevencapillary array was used to reduce the chamber background pressure and interactions with the equipment. All STM measurements were performed at room temperature using electrochemically etched tungsten tips with subsequent annealing in a vacuum. Constant-current STM images were obtained with a tunneling current of It = 0.05−0.10 nA. Sample bias voltages used to obtain images of ethanol on Ge(100) were in the range of Vs = −1.0 to −2.0 V. Detailed observations of the ethanol adsorption were performed recording real-time STM images during the dosing of ethanol onto the Ge(100).
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THEORETICAL CALCULATIONS
To investigate the chemisorption of ethanol on Ge(100), we performed DFT calculations using the Vienna ab initio simulation package.23 Generalized gradient approximation in the form of Perdew−Burke−Ernzerhof functional was employed for the exchange−correlation energy.24 The ionic pseudopotentials were described using the project-augmented wave method.25,26 The DFT-D2 functional was used to consider the long-range dispersion forces.27 Plane waves were included up to an energy level of 400.0 eV. Theoretically determined lattice
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EXPERIMENTAL DETAILS The experiments were performed in an ultrahigh vacuum (UHV) condition with base pressure below 1.0 × 10−10 Torr. The UHV chamber equipped with an OMICRON variabletemperature scanning tunneling microscope was used to B
DOI: 10.1021/acs.jpcc.8b02973 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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initial coverage of 0.012 ML ethanol at room temperature. Unreacted Ge dimers were imaged as either bean-shaped protrusions or zigzag chains. The bean-shaped protrusions correspond to the 2 × 1 symmetric dimer structures, whereas the zigzag chains are the c(4 × 2) structures according to the buckling of dimers. Ethanol adsorption on the clean Ge(100) has depressed dark features, denoted as A and B. The feature A corresponds to the adsorption of a single molecule on the single dimer. Similar dark features were previously detected in STM images of methanol adsorbed on Ge(100).13 Feature B shows a large dark area comprising three to four sequential dimers in the same dimer row and is supposed to originate from the sequential adsorption of ethanol on the neighboring dimers. The line profile of the feature A is presented in Figure 2b. This line profile analysis along the α−α′ arrow line reveals that the feature A consists of asymmetric protrusion with a bright shallow region and a dark deep region and the height of feature A is lower than an unreacted dimer. In the observation of the adsorption of ethanol on Ge(100), we expected to find three distinct features resulting from the possible reaction pathways on a single dimer, shown in Figure 1: molecular adsorption via the Ge−O dative bonding, C−O dissociative adsorption with CH3CH2−Ge−Ge−OH linkage, and O−H dissociative adsorption with H−Ge−Ge−O− CH2CH3 linkage. First, we consider the molecular adsorption via Ge−O dative bonding. If the lone pair of electrons of an oxygen atom of hydroxyl group is donated to a down Ge atom, the remaining electronic density on the down Ge atom is transferred to the unreacted up Ge atom to stabilize the dativebonded structure. Therefore, the dangling bond related to the unreacted Ge atom should be filled and appear somewhat brighter in filled-state STM observation than the unreacted dimers, as in pyridine on Ge(100).19 However, we could not find such a feature in STM observation. Thus, we excluded molecular adsorption as a final structure. Dative-bonded structure may be a precursor state with subsequent O−H bond or C−O bond dissociation. Considering the findings of the previous study of methanol on Ge(100),22 the adsorption configuration arising from C−O dissociation on a single dimer would be expected to appear as two brightly protruding regions due to Ge−CH2CH3 and Ge−OH moieties on a single dimer in STM images; our simulated STM images are presented in the following section. In the line profile, two peaks corresponding to Ge−OH and CH3CH2−Ge are expected around the bonded Ge dimer, which is inconsistent with experimental line profile in Figure 2c. Thus, we excluded the C−O dissociative adsorption with a CH3− CH2−Ge−Ge−OH linkage. On the other hand, if O−H dissociation occurs on a dimer, the STM feature related to the adsorption configuration will consist of a bright region due to the charge density of OCH2CH3 and a dark region due to the passivation of the dangling bond arising from the Ge−H bond formation. This adsorption feature is confirmed in Figure 2b, which will also be confirmed later with simulated STM images. In the line profile of Figure 2c, a peak appears around the bonded dimer, corresponding to Ge−OCH2CH3, which explains the experimental result. Thus, the dark deep and the bright shallow regions correspond to H−Ge and Ge−O−CH2CH3, respectively, from O−H dissociative adsorption on a single dimer. To investigate the adsorption behavior of ethanol at different adsorbate coverages, a series of STM images (20 nm × 20 nm, Vs = −1.6 V, and It = 0.1 nA) of the same area were recorded during the real-time dosing of ethanol onto Ge(100) at room temperature (Figure 3). The images were obtained at specific
constant of germanium was 5.664 Å, comparable to the 5.658 Å value determined experimentally.28 The ethanol-adsorbed Ge(100) surface was modeled by a slab composed of six germanium layers and adsorbed ethanol molecules. The bottom germanium layer was passivated by two hydrogen atoms per germanium atom. The four topmost Ge layers and the adsorbed ethanol molecules were allowed to relax according to the calculated Hellmann−Feynman forces, and the remaining germanium layers were frozen during the structural optimization. The surface structure was relaxed until the equilibrium in which Hellman−Feynman force was less than 0.02 eV/Å. We used a p(4 × 2) or c(4 × 2) unit cell with c(4 × 2) surface symmetry. In the p(4 × 2) unit cell, 48 Ge atoms were included along the two adjacent dimer rows in addition to ethanol molecule and passivating H atoms. The dimension of the p(4 × 2) unit cell was 16.3 Å × 8.2 Å × 23.1 Å. The Brillouinzone integration was performed using 4 × 4 × 1 or 2 × 4 × 1 grids in the Monkhorst−Pack special point scheme, depending upon the unit cell. Using self-consistent Kohn−Sham eigenvalues and wave functions, the constant-current STM images at different bias voltages (Vs) were simulated within the Tersoff−Hamann scheme.29,30 The reaction pathways were investigated using the climbing nudged elastic band method.31 Four states were initially calculated along the reaction pathway from the initial state to the final product, and additional states were tested to determine the exact transition state. The corresponding structure of each intermediate state was relaxed until the Hellmann−Feynman forces were less than 0.05 eV/Å.
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RESULTS AND DISCUSSION STM Observation. Figure 2a shows a filled-state STM image (18.4 nm × 18.4 nm, Vs = −1.5 V, It = 0.1 nA) of Ge surface at an
Figure 2. (a) Filled-state STM image (18.4 nm × 18.4 nm, Vs = −1.5 V, It = 0.1 nA) at 0.012 ML ethanol on Ge(100) at room temperature. Adsorption features are denoted as A and B. (b) High-resolution filledstate STM image (6.8 nm × 6.8 nm, Vs = −1.5 V, and It = 0.1 nA) of the feature A. (c) The line profile of adsorption feature A along α−α′ arrow line. C
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or ethylene on Si(100).34 The intermolecular hydrogen bonding drives another incoming ethanol molecule to the adjacent site of the dimer, which reacted with ethanol. Together with the buckling of neighboring site of reacted dimer, hydrogen bonding may be another driving force for the formation of molecular chains by successive ethanol adsorption on Ge(100), as in the formation of hydrogen-bonded two-dimensional network of histidine on Ge(100).35 Dark features were observed next to the buckled dimer, confirming that ethanol adsorbs via dissociative adsorption rather than dative bonding. If dative bonding was the main mode of adsorption, the molecule next to the buckled dimer would be imaged bright, as in the adsorption of pyrimidine on Ge(100).6 Figure 4 shows STM images at two sample bias voltages (Vs = −1.6 and −1.0 V), obtained after exposure to 0.45 ML of
Figure 3. Sequential filled-state STM images (20 nm × 20 nm, Vs = −1.6 V, and It = 0.1 nA) obtained at the same surface region of the Ge(100) while exposing ethanol molecule: (a) 0.30 ML, (b) 0.32 ML, (c) 0.40 ML, and (d) 0.43 ML. Figure 4. Filled-state STM images of the same surface area (20 nm × 20 nm, It = 0.1 nA) of Ge(100) after exposure to 0.45 ML of ethanol: (a) Vs = −1.6 V and (b) Vs = −1.0 V.
coverages increasing from the initial coverage (0.012 ML) of ethanol in Figure 2. The point denoted by S is a reference point. In the images at increased coverages, the depressed dark features of dimer row resulting from the ethanol adsorption increase on the surface. These features are similar to the dissociative adsorption features of methanol at high coverage on Ge(100).13 In the sequential STM images of Figure 3, ethanol molecules continuously chemisorbed along the direction of dimer rows upon increasing the coverage suggesting that the adsorption of ethanol tends to occur on every Ge−Ge dimer along the same dimer row, which suggests the formation of molecular chain induced by successive adsorption of ethanol along the dimer row. The Ge surface covered with ethanol consisted of many bundles of molecular chains of adsorbed ethanol. The growth of the molecular chain along the dimer row is similar to the behavior of acetic acid on Ge(100), which was explained by the enhanced stability in paired structures related to the surface electronic properties.32 Upon increasing the coverage of ethanol, the buckling of unreacted neighboring Ge dimer is enhanced. Comparing the region indicated by the arrow α in Figure 3b with 3a, we found that the symmetric 2 × 1 dimer structures were altered to the c(4 × 2) buckled dimer structures. The p(2 × 2) dimer structures also changed to the c(4 × 2) dimer structures as indicated by the arrow β. Occasionally, conformational change of buckled dimer structure into symmetric dimer structure was locally observed, as shown in Figure 3. This phase transition of the dimer randomly appeared after the increase in the ethanol coverage. Figure 3 demonstrates sequential adsorption of ethanol on neighboring buckled dimers in the same dimer rows following increased coverage of ethanol, suggesting that the buckled structure of neighboring dimer provides new reactive site for subsequent adsorption of incoming molecule. This behavior may be due to the electronic density changes of neighboring unreacted dimers via adsorption of molecules, such as benzoic acid on Ge(100)33
ethanol. At Vs = −1.6 V, the dimer, which reacted with ethanol, was imaged as a dark region and was not clearly identifiable. However, it was clearly imaged as a bean shape with a central node at Vs = −1.0 V, as shown in Figure 4b. The region around this reacted dimer was discriminated at low bias voltage (Vs = −1.0 V). After reaction with the ethoxy group and hydrogen atom, the up and down Ge atoms of buckled dimer formed Ge− H and Ge−OCH2CH3 geometries and the reacted dimer changed into a symmetric structure showing a central node in the STM images. Therefore, the STM image at low bias voltage suggests that ethanol undergoes dissociative adsorption on a single dimer. On the other hand, the unreacted dimer in Figure 4 was imaged as a bright spot. If the ethanol exposure increases to a nearly perfect saturation coverage (θ ≃ 0.50 ML), these unreacted dimers will eventually disappear following the continuous reaction of ethoxy group and H atoms of ethanol with the surface Ge atoms and a well-ordered array of saturated monolayer will be formed, as in methanol on Ge(100).13 DFT Calculations. We performed DFT calculations to determine the most stable adsorption structures among all of the possible configurations and to confirm the adsorption configurations suggested in the experimental results. Several optimized binding geometries are shown in Figure 5, and the calculated adsorption energies for the structures are summarized in Table 1. The adsorption energy (Eads) is defined by Eads = E(ethanol/Ge) − E(clean) − E(ethanol)
where E(ethanol/Ge), E(clean), and E(ethanol) represent the total energies of the ethanol-adsorbed surface, the clean surface, and the ethanol molecule, respectively. In our DFT calculations, D
DOI: 10.1021/acs.jpcc.8b02973 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. Frontal views of the optimized geometries of ethanol on Ge(100): (a, b) O-dative-bonded structure with linkage of Ge−OHCH2CH3, (c) O−H dissociative adsorption structure containing Ge−H and Ge−OCH2CH3 linkages, and (d) C−O dissociative adsorption structure with Ge− CH2CH3 and Ge−OH bonding. In (a), ethyl group is located between two dimer rows, whereas it is confined within a dimer row in (b). Dark cyan, red, and gray balls represent Ge, O, and C atoms, respectively, and H atoms are represented by white balls.
Table 1. Calculated Adsorption Energies (Eads), in eV/ Molecule, for the Adsorption Structures of Ethanol on Ge(100) within c(4 × 2) Unit Cella reaction mode Ge−O dative bonding O−H bond dissociation C−O bond dissociation
adsorption structure
Eads
Ge−Ge−OHCH2CH3 (DA1) Ge−Ge−OHCH2CH3 (DA2) H−Ge−Ge−OCH2CH3 (DI-OH) CH3CH2−Ge−Ge−OH (DI-CO)
−1.16 −1.02 −1.42 −2.11
a
The notation for each structure follows that in Figure 5.
1 ML indicates one ethanol molecule per surface Ge atom, which is consistent with the definition used in the STM experiment. The dative-bonded structures with oxygen atom bonded to electron-deficient down Ge atom of a Ge−Ge single dimer are presented in Figure 5a,b. The structures arise in two different configurations depending on the spatial orientation of the C−O bond of ethanol: parallel (DA1) or perpendicular (DA2) to the Ge surface. Figure 5c,d is formed via specific bond dissociation in ethanol. Further detailed theoretical results about other structures, including the migration of the dissociated fragments, are not shown here, and will be reported later. In Table 1, the adsorption geometries via O−H or C−O dissociation have the lower adsorption energies than the molecular adsorption structure with Ge−O dative bonding. Furthermore, the adsorption geometry via C−O bond dissociation (DI-CO), shown in Figure 5d, has lower energy than the structure with O−H bond dissociation (DI-OH) shown in Figure 5c. The most favorable geometry expected on the basis of thermodynamic stability of the final products contains Ge−OH and Ge−CH2CH3, which is different from the geometry suggested by the STM results. Thus, to obtain a more reasonable insight into stable adsorption geometry, it is necessary to consider the transition state and activation barrier in the entire reaction process. Figure 6 shows the potential energy diagram for the possible two reaction pathways yielding either Ge−H/Ge−OCH2CH3 or Ge−CH 2 CH 3 /Ge−OH geometries via the Ge− OHCH2CH3. The energy of each state is presented as the relative value to the initial state (the isolated clean Ge surface + the gas-phase ethanol). First, an ethanol adsorbs molecularly on a dimer of the Ge(100) with no barrier and the stabilization energy (absolute value of the adsorption energy) is either 1.16 eV/molecule (DA1) or 1.02 eV/molecule (DA2), respectively. Subsequently, the molecularly adsorbed ethanol dissociates to CH3CH2O + H or to CH3CH2 + OH and the activation energies of these reactions were calculated to be 0.44 and 1.67 eV/ molecule, respectively. Since the transition states in these reactions lie 0.72 eV/molecule below and 0.65 eV/molecule above the energy of the initial state, respectively, the O−H
Figure 6. Potential energy diagram of the reaction pathways for the adsorption of ethanol on Ge(100). The solid and dotted lines indicate the adsorption of ethanol via C−O bond and O−H bond dissociations, respectively.
dissociative reaction has a small barrier to overcome and will occur under milder conditions than the C−O dissociation. Considering that the final product of the C−O dissociative reaction is more stable than that of the O−H dissociative reaction, the C−O dissociative product will be favored only if the activation barrier is overcome. Finally, it is concluded that the H−Ge−Ge−OCH2CH3 geometry is more likely to be the main adsorption configuration under mild conditions, whereas the CH3 CH2−Ge−Ge−OH geometry is favored at higher temperature. Although the calculated adsorption energies imply that the C−O dissociative adsorption structure is the most stable geometry, considerations of the transition state and the activation energy reveal that ethanol adsorption via O−H dissociative adsorption is kinetically more favorable than that of the C−O dissociation at room temperature suggesting that the reaction of ethanol on Ge(100) depends on the reaction conditions and is dominated by kinetic rather than thermodynamic control at room temperature. The energy diagram derived from our calculations differs somewhat from that obtained previously using a cluster model.21 In the cluster model calculation with Ge15H16, the transition state for O−H dissociative intradimer adsorption lies 0.12 eV/molecule (2.8 kcal/mol) above the energy of the initial state and the activation energy is 0.68 eV/molecule (15.8 kcal/mol).21 In the current study, by contrast, the transition state lies 0.72 eV/molecule below the initial state and the activation energy is 0.44 eV/ E
DOI: 10.1021/acs.jpcc.8b02973 J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C molecule, a situation that clearly favors O−H dissociative adsorption. Considering that the transition state in the reaction pathway to C−O dissociative adsorption lies 0.65 eV/molecule above the energy of the reactant, the possibility of C−O dissociative adsorption is very low at room temperature. Thus, the calculation results clarify that the adsorption feature of ethanol on Ge(100) observed in STM results reflects the electronic structure around Ge−OCH2CH3 and Ge−H via the O−H dissociative reaction at room temperature. Figure 7 shows the simulated STM images (Vs = −1.6 V) for (a) H−Ge−Ge−OCH2CH3 and (b) CH3CH2−Ge−Ge−OH
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CONCLUSIONS
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AUTHOR INFORMATION
The chemisorption of ethanol on Ge(100) surface has been studied by STM observation and DFT calculations. At low coverage, the high-resolution STM image shows ethanol adsorption on the single Ge−Ge dimer of Ge(100). The adsorption feature is characterized by a bright shallow site and a dark deep site on the dimer, indicating H−Ge−Ge−OCH2CH3 geometry via O−H bond dissociation of ethanol on the Ge(100). The real-time STM analysis reveals that the molecular chain of ethanol increases gradually by successive adsorption along the dimer row following increased exposure to ethanol. In the Ge(100) surface saturated with molecular chains of ethanol, the STM image at a low bias voltage shows that the reacted dimer changes into a symmetric structure with a central node, indicating dissociative adsorption of ethanol. Theoretical results show that, although the adsorption product resulting from the C−O dissociation represents the most stable geometry thermodynamically, the O−H dissociation is kinetically more favorable than the C−O dissociation. From the detailed analysis of the experimental data and comparison with the theoretical results, we conclude that the adsorption feature observed in the STM study consists of the H−Ge−Ge−OCH2CH3 geometries via O−H bond dissociation on the single dimer of Ge(100) at room temperature, suggesting that the reaction of ethanol on Ge(100) is dominated by kinetic control at room temperature.
Figure 7. Theoretically simulated filled-state STM images showing the (a) H−Ge−Ge−OCH2CH3 and (b) CH3CH2−Ge−Ge−OH configurations at 0.063 ML ethanol coverage on the Ge(100) surface.
at an ethanol coverage of 0.063 ML. A bright protrusion was found around the oxygen atoms in the two simulated images, probably due to the lone pairs of electrons of oxygen atoms. The simulated filled-state STM image in Figure 7a has two wellseparated sites on a single dimer, as observed in the experimental STM image in Figure 2b. The sites consist of a dark region due to H−Ge bonding and a bright region due to Ge−OCH2CH3 bonding. The bonding orbital of Ge−OCH2CH3 is less bright than the dangling bond orbital of an unreacted Ge dimer atom, which agrees well with the experimental results. The simulated STM image of C−O dissociative adsorption structure contains two bright regions around CH3CH2−Ge bonding, and the locations of the two bright spots are inconsistent with line profile analysis of the feature A in Figure 2c. Thus, the simulated STM image from C−O dissociative adsorption structure does not adequately explain the experimental STM result. On the basis of the comparison between the experimental and theoretical STM images, we conclude that the adsorption of ethanol on Ge(100) at room temperature has a H−Ge−Ge−OCH2CH3 configuration on a single dimer. Compared with previous findings regarding the adsorption of ethanol on Ge(100),21 we observed changes in the adsorption configuration depending on the ethanol coverage by real-time STM observations and the growth of a molecular chain along the dimer row. We also elucidated the competition between O−H and C−O dissociative adsorption from both kinetics and thermodynamics standpoint and successfully explained the existence of O−H dissociative structures at room temperature by determining the exact energy level of the transition state of O−H dissociative adsorption. The chemisorption of ethanol on Ge(100) may be compared to that of methanol. Consequently, these studies provide important information to the adsorption and reaction studies of other simple aliphatic alcohols on Ge(100). Especially, formation and spread of molecular chains by dissociative adsorption of ethanol are notable for application of organic functionalized Ge(100).
Corresponding Author
*E-mail:
[email protected]. Tel: +82-63-270-2816. Fax: +8263-270-2811. ORCID
Do Hwan Kim: 0000-0002-2976-6873 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program (NRF-2015R1D1A1A09058844) through the National Research Foundation of Korea (NRF), funded by the Ministry of Education. This research was also supported by the Research Base Construction Fund Support Program funded by Chonbuk National University in 2018.
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DOI: 10.1021/acs.jpcc.8b02973 J. Phys. Chem. C XXXX, XXX, XXX−XXX
