J. Phys. Chem. C 2009, 113, 14311–14315
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Structural Properties of Norbornene Monolayers on Ge(100) Ah-young Kim,† Junghun Choi,† Do Hwan Kim,*,‡ and Sehun Kim*,† Department of Chemistry, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea, and DiVision of Science Education, Daegu UniVersity, Gyeongbuk 712-714, Republic of Korea ReceiVed: February 13, 2009; ReVised Manuscript ReceiVed: May 30, 2009
The adsorption structures of norbornene monolayers on a Ge(100) surface were investigated by using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. At a coverage of ∼0.05 ML, most norbornene molecules adsorb on top of Ge dimers (OT). As the coverage increases, new features located between two neighboring Ge dimers appear: (i) single end bridge (SEB) and (ii) paired end bridge (PEB). Of these three configurations, the OT configuration is the most stable, according to the adsorption energies obtained with DFT calculations. 1. Introduction Organic dielectric materials such as polyimides, benzocyclobutenes, and polynorbornene have attracted much attention because of their relatively lower dielectric constants than SiO2 and facile processing in a metal-insulator-semiconductor (MIS) structure.1,2 Among the dielectric materials, polynorbornene was developed for use in multichip modules.3 To develop polymeric dielectrics that can adapt to the miniaturization of devices, detailed study of the behavior of the monomer of the polymer (e.g., norbornene, see Figure 1a) on semiconductor surfaces is required. Besides, formation of well-ordered organic monolayers on a semiconductor surface is technologically important toward the study of electron transport in MIS systems.4 Norbornene (or bicyclo[2.2.1]hept-2-ene or C7H10) is a bicyclic olefin with ring strain, so it contains a reactive double bond. The adsorption of alkenes on group IV semiconductor surfaces has been extensively studied because the double bond of alkenes easily reacts with a surface dimer on Si(100)5-11 and Ge(100).12-14 The dissimilarity of surface dimers on Si(100) and Ge(100) can be summarized as follows.15 First, they have different bond lengths; thus adsorbed alkenes feel a different ring strain. Second, the strength of the Si-C bond is nearly 10 kcal/mol stronger than that of the Ge-C bond. Figure 1b-d shows three possible bonding configurations of norbornene on the Ge(100) surface, which are classified as (i) on-top (OT in Figure 1b), (ii) single end bridge (SEB in Figure 1c), and (iii) paired end bridge (PEB in Figure 1d) configurations. In a study of ethylene (CH2dCH2) on Si(100),5-8 ethylene molecules were found to adsorb on top of Si dimers. Even in the case of cycloalkenes with one double bond, such as cyclopentene (C5H8)5,9,10 and cyclohexene (C6H10),11 on Si(100) the on top (OT) configuration is predominantly observed. On the other hand, in the case of ethylene on Ge(100),12 which was investigated with scanning tunneling microscopy (STM), paired end bridge (PEB) configurations are also observed. Norbornene on Ge(100) is expected to exhibit similar configurations (i.e., OT in Figure 1b and PEB in Figure 1d) to those found in the case of ethylene on Ge(100)12 because the adsorption of * To whom correspondence should be addressed. E-mail: (Sehun Kim)
[email protected]; (Do Hwan Kim)
[email protected]. † Korea Advanced Institute of Science and Technology. ‡ Daegu University.
Figure 1. Chemical structure (a) of norbornene and the possible adsorption configurations of norbornene on Ge(100): (b) on-top (OT); (c) single end bridge (SEB); (d) paired end bridge (PEB). Purple balls represent surface Ge dimers; orange ones represent subsurface Ge atoms. Gray and white ones represent C and H atoms of norbornene, respectively.
norbornene on Ge(100) is driven by interaction between the double bond of norbornene and Ge dimers. Besides, it is expected that norbornene has a reactivity that is different from that of noncycloalkenes on Ge(100) because of the presence of ring strain. However, the adsorption of bicycloalkenes such as norbornene on Ge(100) has not previously been investigated. In this study, the adsorption structures of monolayers of norbornene on the Ge(100) surface were investigated using STM. The optimized adsorption structures and energies of norbornene on Ge(100) were also determined by carrying out density functional theory (DFT) calculations. 2. Experimental and Theoretical Methods A. Experimental Details. All experiments were performed in an ultrahigh vacuum chamber equipped with a STM. The base pressure of the chamber was maintained below 1.0 × 10-10 Torr. This pressure is sufficiently low to maintain a clean surface for several hours.
10.1021/jp901360g CCC: $40.75 2009 American Chemical Society Published on Web 07/17/2009
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A single-crystalline Ge(100) wafer (n type, Sb doped, F < 0.4 Ω cm) was cleaved to a size of 2 × 10 mm2 and mounted between two tantalum foil clips in the sample holder for resistive heating. The Ge(100) substrate was cleaned by carrying out several cycles of Ar+ sputtering (4.0 µA of 1.0 keV) for 20 min at 700 K, followed by annealing at 900 K for 10 min, and then slow cooling at -2 K/s. This treatment produced large, well-ordered, defect-free terraces. The cleanliness of the Ge(100) surface was confirmed with STM. The sample temperature was monitored with an infrared optical pyrometer and calibrated by using a chromel-alumel (type-K) thermocouple glued to the back of the sample. Norbornene (C7H10, Aldrich, 99% purity) was directly dosed onto the Ge(100) substrate at room temperature through a doser with a seven-capillary array controlled by a variable leak valve after purification with the freeze-pump-thaw method. All STM measurements were performed by using an OMICRON VT-STM with an electrochemically etched W-tip with subsequent annealing in a vacuum chamber. All STM images were recorded in constant current mode with a tunneling current of 0.1 nA. A sample bias voltage of Vs ) -1.96 V was employed in order to image the organic molecules effectively. B. Theoretical Calculations. To investigate the adsorption configurations of norbornene on the Ge(100) surface, ab initio calculations within the generalized gradient approximation (GGA) using the Vienna ab initio simulation package (VASP)16,17 were performed. Plane waves up to an energy of 211.3 eV were included to expand the wave functions. Our slab model for the norbornene adsorbed Ge(100) surface consisted of adsorbed norbornene molecules and six Ge atomic layers. The Ge bottom layer was passivated with two H atoms per Ge atom. We used a p(4 × 4) surface unit cell with a c(4 × 2) surface symmetry. The topmost four layers of the slab, as well as the adsorbed molecules, were allowed to relax under the influence of the calculated Hellmann-Feynman force, while the remaining two bottom Ge layers and the H layer were kept frozen during the structure optimization. The surface structure was considered to be in equilibrium when the Hellmann-Feynman force was found to be smaller than 0.02 eV/Å. A Gaussian broadening with a width of 0.02 eV was used to accelerate the convergence in the k-point sum. 3. Results and Discussion A. Coverage Dependence. Figures 2a-d show the STM images for various coverages recorded during real-time dosing of norbornene onto the Ge(100) surface at room temperature. All four images were recorded over the same area. Unreacted Ge dimers on the clean Ge(100) surface are imaged either as bean-shaped protrusions due to symmetric dimers or as zigzag chains due to asymmetric dimers, resulting in (2 × 1) or c(4 × 2) reconstructions as shown in Figure 2a. When norbornene molecules adsorb on asymmetric Ge dimers via stepwise [2 + 2] cycloaddition reactions, the neighboring Ge dimers become symmetric. As the norbornene coverage increases, three different adsorption configurations appear, which are classified as (i) ontop (OT), (ii) single end bridge (SEB), and (iii) paired end bridge (PEB) configurations as shown in Figure 2b,c. At coverages under 0.05 ML, most norbornene molecules adsorb on top of Ge dimers, denoted OT in Figure 2b. The OT configurations can easily be identified because of their bright protrusions. They are distributed randomly on the surface, with neither preferential adsorption on steps nor line growth of the adsorbate at initial low coverages. In other words, the diffusion of adsorbed norbornene is negligible at room temperature.
Kim et al.
Figure 2. STM images (15 × 15 nm2) of norbornene on Ge(100) at (a) 0, (b) 0.04, (c) 0.17, and (d) 0.62 ML (saturation coverage). [ML] ) Adsorbed molecules/Ge dimers. (e) Populations of the adsorption configurations of norbornene as functions of coverage.
However, as the coverage increases, the OT features are observed to form short chains across Ge dimer rows, which indicate that the attractive interaction between adsorbed norbornene molecules across the Ge dimer rows is larger than that along the Ge dimer row direction. It is also expected that there is a repulsive interaction along the Ge dimer row direction between two adsorbed norbornene molecules due to the size of the molecules. At higher coverages (>0.05 ML), two additional features are observed. As can be seen in Figure 2c, one protrusion (SEB, yellow circles) or two protrusions (PEB, red boxes) are located between two Ge dimer rows. The SEB and PEB configurations appear as smaller and less bright protrusions than the OT configurations. Whereas the OT configurations are randomly distributed on the surface, the SEB configurations are preferentially found next to OT configurations. Further, the PEB configuration appears as a pair of SEB features. Figure 2d shows the saturation coverage (approximately 0.62 ML) of a well-ordered norbornene monolayer on the Ge(100) surface. If the Ge(100) surface is covered with only norbornene molecules in OT configurations adsorbed at every second Ge dimer, the saturation coverage will be 0.5 ML. In the case of ethylene (CH2dCH2) on Si(100),5-8 it has been reported that ethylene only adsorbs in the OT configuration; neither SEB nor PEB configurations were observed with STM. Even in the case of cycloalkenes with one double bond, such as cyclopentene
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Figure 3. Calculated optimized adsorption structures and energies (Eads, eV/molecule) of the configurations: (a) on-top (OT), (b) single end bridge (SEB), (c) paired end bridge (PEB). Cyan balls represent Ge atoms.
(C5H8)5,9,10 and cyclohexene (C6H10),11 on Si(100) only OT configurations are observed. Although norbornene on Si(100) has not been studied, it is expected that this system will have similar properties to those mentioned above. On the other hand, in the case of ethylene on Ge(100), OT and PEB configurations were observed with STM, while the SEB configuration was not.12 The difference between the adsorption configurations in these cases originates from the dissimilarity between the Si(100) and Ge(100) surfaces as mentioned in above. Based on this result, it is expected that the SEB and PEB configurations of norbornene will arise only on Ge(100). If so, norbornene on Ge(100) and Si(100) will have a different coverage due to the existence of SEB and PEB configurations. Figure 2e shows the variations of the populations of the configurations with increases in the coverage of norbornene. The populations of the OT and PEB configurations increase linearly with increasing coverage up to the saturation coverage of 0.62 ML, and the OT configuration dominates over the entire range of norbornene coverages. However, the population of the SEB configuration appears constant because it is the initial state of the PEB configuration that results from sequential adsorption of two norbornene molecules. At the saturation coverage, ∼60% of the adsorbed molecules are in the OT configuration, ∼32% in the PEB configuration, and ∼8% in the SEB configuration. B. The Adsorption Configurations. To understand the atomic-level adsorption geometries of norbornene on Ge(100) observed in the STM experiments, the DFT calculations for many possible adsorption structures were performed. Three relevant structures and their adsorption energies are shown in Figure 3. The calculated adsorption energy, Eads, is defined by the formula: Eads ) -[Etot(adsorbed) - Etot(clean) - E(norbornene)], where Etot(adsorbed), Etot(clean), and E(norbornene) are the total energies of the norbornene-adsorbed surface, the clean p(4 × 4) surface, and the free norbornene molecule, respectively.
Figure 3a presents the results of the DFT calculations for the OT configuration. The adsorption energy of OT configuration is 1.06 eV/molecule. Note that the cyclopentane ring of norbornene is slanted with respect to the neighboring Ge dimer, as shown in the side view in Figure 3a. This configuration is energetically more favorable than the other configuration, which is that the cyclohexane ring of norbornene is tilted to the surface. Figure 4a shows an enlarged STM image of the OT configuration. In this image, the bright protrusion of the OT configuration is not located exactly on top of the reacted single Ge dimer, which means that the molecule leans toward neighboring Ge dimers. The black arrows in Figure 4a indicate the direction of leaning. The line profile of the OT configuration in Figure 4d indicates that the maximum height of the OT configuration is located between the two dimers, not on top of the reacted single dimer. These experimental observations agree with the DFT calculation results. Figures 3b and 4b show the DFT calculation results and an enlarged STM image, respectively, of the SEB configuration (indicated by yellow arrows). In the SEB configuration, the protrusion is located between two Ge dimer rows, which means that the norbornene molecule inclines toward the neighboring Ge dimer row, as shown in Figure 3b. In the same Ge dimer row, the protrusion lies between two Ge dimers (i.e., bridge adsorption). The different adsorption geometries of the OT and SEB configurations result in a difference between the heights of the features, as the line profile in Figure 4e illustrates. According to our calculations, the height difference between the two configurations is about 0.78 Å, which agrees with the result of 0.74 Å obtained from the line profile (c to c′ in Figure 4b) of the two configurations. The SEB configuration was found to have a lower adsorption energy (0.74 eV/molecule), which means it is less stable than the OT configuration (1.06 eV), as shown in Figure 3. When we consider the adsorption geometry of the SEB configuration, there are two radicals on the Ge dimer atoms that are not bonded to the norbornene molecule. The
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Figure 4. Enlarged STM images of (a) OT (6.72 × 6.72 nm2), (b) SEB (6.60 × 6.60 nm2), and (c) PEB (10.08 × 10.08 nm2) configurations on Ge(100). The positions of the Ge atoms on the dimers are represented by intersections between two perpendicular white lines. Line profiles of the (d) OT, (e) SEB, and (f) PEB configurations in panels a, b, and c, respectively.
presence of these radicals explains why the SEB configuration is less stable than the OT configuration. This phenomenon is also related to the properties of the SEB configuration. As mentioned above, an SEB configuration is always located next to an OT configuration. There are two possible explanations. First, when norbornene adsorbs in the OT configuration, it affects the neighboring dimers in the specific direction in which the cyclopentane ring is slanted. As a result of this process, adsorption in the SEB configuration is promoted. In other words, the neighboring SEB configuration is stabilized by the OT configuration. This effect was confirmed by comparing the adsorption energies. When the additional norbornene molecule adsorbs in SEB configuration next to the OT configuration, total adsorption energy is increased to about 0.79 eV by the SEB configuration. This value is 0.05 eV higher than that of the isolated SEB configuration. Second, the SEB configuration arises only after the coverage increases above 0.05 ML. As the coverage increases, the strengths of the repulsive interactions between adsorbed molecules also increase. These repulsive interactions are expected to affect the appearance of the SEB configuration. However, in the case of the adsorption of ethylene,5-8,12 cyclopentene,5,9,10,13 and cyclohexene,11,13 the SEB configuration was not observed on either Si(100) or Ge(100) surfaces. Thus the presence of the SEB configuration in the case of norbornene on Ge(100) might be due to the influence of the slanted bicyclic ring.
Kim et al. Figure 4c shows an enlarged STM image of the PEB configuration (indicated by green arrows). The PEB configuration is formed by the sequential adsorption of two norbornene molecules in SEB configurations. The adsorption energy of PEB configuration per molecule is calculated to be 0.98 eV. Even though the radicals on the Ge dimer atoms are bonded to another norbornene molecule in the PEB configuration, the PEB configuration is still less stable than the OT configuration by ∆Eads ) 0.08 eV. The subtle calculated energy difference between the PEB and OT configurations is meaningful because it is consistent with our STM results. In our previous study on styrene on Ge(100), the energy difference between the cis-PEB and the trans-PEB configurations is calculated to be 0.09 eV, which is in good agreement with the STM observation.18 Figure 4f shows the line profile of the single protrusions of the PEB and OT configurations. As in the comparison of the SEB and OT configurations, there is a height difference between the PEB and OT configurations. Our calculations indicate that the height difference is approximately 0.96 Å, which agrees with the result of 0.97 Å from the line profile (d to d′ in Figure 3c) of the two configurations obtained from the STM results. In the PEB configuration, two norbornene molecules adsorb on two neighboring Ge dimers, resulting in a repulsive interaction between the two norbornene molecules. Thus the norbornene molecules in the PEB configuration have a lower apparent height than those in the SEB configuration. Repeatedly, it is emphasized that the sequence of the population of each structure (OT > PEB > SEB) for norbornene on Ge(100) is different from that (PEB > OT > SEB) for ethylene12 or styrene on Ge(100).18 The PEB configurations of styrene can be divided into trans-PEB and cis-PEB configurations according to the orientation of the phenyl rings of the adsorbed styrene molecules. Both PEB configurations of styrene appear more frequently than the OT configuration. In norbornene, the adsorption energy for PEB configuration of norbornene is smaller than that for the OT configuration by 0.08 eV, and the PEB configuration is less frequent in STM observations. It is considered that this difference arises from the presence of the bicyclic ring of norbornene. The steric hindrance between two norbornene molecules destabilizes PEB configuration for norbornene on Ge(100). 4. Conclusions The adsorption structures of norbornene monolayers on a Ge(100) surface were investigated by using STM. At low coverages less than 0.05 ML, most norbornene molecules adsorb in the OT configuration. At higher coverages, the SEB and PEB configurations are also observed. The population of the OT configuration is found to dominate over the entire range of norbornene coverage. This result is in agreement with the adsorption energies obtained with DFT calculations. It is confirmed that the OT configuration is most stable among these three configurations. The saturation coverage of norbornene molecules on the Ge(100) surface was found to be approximately 0.62 ML. Acknowledgment. This work was supported by Korea Research Foundation Grant No. KRF-2006-312-C00566 and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant No. 20090067346). The calculations were supported by the KISTI through the Eleventh Strategic Supercomputing Support Program.
Norbornene Monolayers on Ge(100) References and Notes (1) Maex, K.; Baklanov, M. R.; Shamiryan, D.; Iacopi, F.; Brongersma, S. H.; Yanovitskaya, Z. S. J. Appl. Phys. 2003, 93, 8793. (2) Maier, G. Prog. Polym. Sci. 2001, 26, 3. (3) McDougall, W. C.; Farling, S.; Shick, R.; Glukh, S.; Jayaraman, S. K.; Rhodes, L. F.; Vicari, R.; Kohl, P.; Bidstrup-Allen, S. A.; Chiniwalla, P. Proc. SPIE 1999, 3830, 17 (International Conference on High Density Packaging and MCMs, 1999). (4) Salomon, A.; Boecking, T.; Chan, C. K.; Amy, F.; Girshevitz, O.; Cahen, D.; Kahn, A. Phys. ReV. Lett. 2005, 95, 266807. (5) Wolkow, R. A. Annu. ReV. Phys. Chem. 1999, 50, 413. (6) Mayne, A. J.; Avery, A. R.; Knall, J.; Jones, T. S.; Briggs, G. A. D.; Weinberg, W. H. Surf. Sci. 1993, 284, 247. (7) Shimomura, M.; Munakata, M.; Iwasaki, A.; Ikeda, M.; Abukawa, T.; Sato, K.; Kawawa, T.; Shimizu, H.; Nagashima, N.; Kono, S. Surf. Sci. 2002, 504, 19. (8) Chung, C. H.; Jung, W. J.; Lyo, I. W. Phys. ReV. Lett. 2006, 97, 116102. (9) Hovis, J. S.; Liu, H.; Hamers, R. J. Surf. Sci. 1998, 402, 1.
J. Phys. Chem. C, Vol. 113, No. 32, 2009 14315 (10) Hamers, R. J.; Hovis, J. S.; Lee, S.; Liu, H.; Shan, J. J. Phys. Chem. B 1997, 101, 1489. (11) Yoshinobua, J.; Yamashitaa, Y.; Yasuia, F.; Mukaia, K.; Akagia, K.; Tsuneyuki, S.; Hamaguchi, K.; Machida, S.; Nagao, M.; Sato, T.; Iwatsuki, M. J. Electron Spectrosc. Relat. Phenom. 2001, 114, 383. (12) Kim, A.; Choi, D. S.; Lee, J. Y.; Kim, S. J. Phys. Chem. B 2004, 108, 3256. (13) Lee, S. W.; Hovis, J. S.; Coulter, S. K.; Hamers, R. J. Surf. Sci. 2000, 462, 6. (14) Hwang, Y. J.; Kim, A.; Hwang, E.; Kim, S. J. Am. Chem. Soc. 2005, 127, 5016. (15) Bent, S. F. J. Phys. Chem. B 2002, 106, 2830. (16) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15. (17) Kresse, G.; Hafner, J. J. Phys.: Condens. Matter 1994, 6 8245. (18) Hwang, Y. J.; Hwang, E.; Kim, D. H.; Kim, A.; Hong, S.; Kim, S. J. Phys. Chem. C 2009, 113, 1426.
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