Cyclopentene (c-C5H8) adsorption on Si(0 0 1)-2 × 1 and on dimer vacancies on the surface: A theoretical study of the electronic structure and chemical bonding

Computational Materials Science 49 (2010) 888–894

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Cyclopentene (c-C5H8) adsorption on Si(0 0 1)-2  1 and on dimer vacancies on the surface: A theoretical study of the electronic structure and chemical bonding E. Germán, I. López-Corral, A. Juan, G. Brizuela * Departamento e Instituto de Física, Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahía Blanca, Argentina

a r t i c l e

i n f o

Article history: Received 8 April 2010 Received in revised form 28 June 2010 Accepted 30 June 2010 Available online 23 July 2010 Keywords: Cyclopentene Si(0 0 1) DFT Dimer vacancies Bonding

a b s t r a c t In the present work we analyzed the geometry and the chemical interactions of c-C5H8 after adsorption on a semiconductor surface: Si(0 0 1), by density functional theory calculations (DFT). Using a slab model, we studied the changes in the atomic and orbital interactions corresponding to the system. We examined overlap population values for specifics bonds during the adsorption. We considered two cases, the cyclopentene adsorption on dimers on the surface and on dimer vacancies on Si(0 0 1). We found an average @CASi distance of 1.96 Å on dimer SiASi; and an average HASi, ACASi, and @CASi distances of 1.59 Å, 1.83 Å, and 1.57 Å respectively on dimer vacancies. We also studied the density of states (DOS) and the crystal orbital overlap populations (OPDOS) corresponding to CAC, CASi, CAH, and SiASi bonds. The main contribution to the adsorption are the C@C double bond in both cases and the H’s belonging to this double bonds in the case of adsorption on dimer vacancies (DV). The orbital contribution includes participation of the 2pz orbitals corresponding to C atoms and 3pz orbitals corresponding to Si in the case of adsorption on Si(0 0 1)-2  1 surface, and 2px, 2py, and 2pz orbitals corresponding to C atoms, and the 3px, 3py, and 3pz orbitals corresponding to Si for cyclopentene adsorption on DV. The adsorption of cyclopentane on Si(0 0 1)-2  1 produce several vibrational frequencies similar to that of c-C5H10, a molecule with sp3 hybridization. After C5H8 adsorption no frequencies coming from a C@C bond are computed which also support the idea of re-hybridization. On DV the SiAC frequency increase 35 cm1, indicating a stronger bond with the surface. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction In the last few years there has been growing interest in the adsorption of organic molecules onto Si surfaces with a view to developing a practical methodology to exploit molecular electronics. The interaction of small unsaturated hydrocarbon molecules on Si(0 0 1) provides a simple model system to explore the relevant underlying physics and chemistry [1]. Several reactions and techniques for forming bonds between organic molecules and a silicon surface are already well established and have been used in the past few years to modify silicon semiconductor materials [2–5]. The Si(1 0 0) surface undergoes 2  1 reconstruction where two silicon atoms pair together into dimers. The bonding within these dimers has been described in terms of a strong r bond and a weak p bond and is therefore electronically similar to C@C or Si@Si double bonds of alkenes or disilylenes [6]. The reactivity of these silicon dimers with unsaturated bonds has been known for several years [7–9]. Recent experimental studies have reported the fabrication and characterization of ordered, self-assembled monolayers of organics * Corresponding author. Tel./fax: +54 291 4595142. E-mail address: [email protected] (G. Brizuela). 0927-0256/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2010.06.042

on Si(0 0 1) [10–13]. The homogeneous structure of these monolayer-thin organic films, as well as their ability to form strong covalent bonds with the underlying Si substrate, make them extremely promising for integration with silicon microelectronics technology. Saraireh et al. showed that among the wide range of organic molecules, unsaturated hydrocarbons with one or more C@C double bonds can readily interact with the Si(0 0 1) surface dangling bonds via typical [2 + 2] or [4 + 2] cycloaddition reactions [14]. Scanning tunneling microscopy and infrared vibrational spectroscopy have shown that cyclopentene (c-C5H8) can form monolayers on Si(0 0 1), with each molecule oriented along a single Si dimer [12]. This orientation is consistent with a cycloaddition mechanism for alkenes on Si(0 0 1) where the alkene p bond breaks to form two new r CASi bonds with the Si dimer atoms. Machida et al. have investigated adsorbed states of cyclopentene on Si(1 0 0)(2  1) by means of low energy electron diffraction (LEED), valence and Si 2p photoelectron spectroscopy (PES) and high-resolution electron energy loss spectroscopy (HREELS) [15]. The atomic structure of an ordered cyclopentene monolayer (ML) on Si(0 0 1) has also been examined using density functional theory (DFT) [16–18]. The clean Si(0 0 1) surfaces usually contain many microscopic flaws. The smallest of these ‘‘native” surface point defects are the so-called A-defects. They correspond to missing dimers and are

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Fig. 1. (a) Schematic top and (b) side view of cyclopentene [2 + 2] adsorption on Si(0 0 1) and on 2DV. Each adsorption case was computed separately having coverage of about h2+2 = 0.08 ML and h2DV = 0.17 ML. For the sake of clarity only four Si layers are shown.

Fig. 2. (a) Bond distances in cyclopentene [2 + 2] cycloaddition adsorption on Si(0 0 1). (b) Bond lengths in c-C5H8 adsorption on Si(0 0 1) with DV.

Table 1 Orbital electron occupations and net charge.

HOMO-1 HOMO LUMO LUMO + 1 Net charge on c-C5H8 unit

[2 + 2] Cycloaddition

Adsorption on DV

Isolated c-C5H8

c-C5H8/Si(0 0 1)

Isolated c-C5H8

c-C5H8/Si(0 0 1)

2 2 0 0 0

1.998 1.606 1.510 0.012 0.900

2 2 0 0 0

1.837 1.879 1.775 0.230 0.263

also known as dimer vacancies (DV). A few per cent of surface dimers are missing on surfaces prepared in a standard way. These dimer vacancies often come together in complexes ranging from vacancy pairs to real vacancy ‘‘islands”. The simplest and usually numerous complexes are dimer vacancy pairs (2DV), with two neighboring dimers missing in the same row. In certain circumstances, some of the ‘‘dimer vacancies” may be in fact adsorb H2O molecules. DV supersaturation on clean Si(0 0 1) may be achieved by bombardment with rare gas ions or by etching [19]. Recent high resolution scanning tunneling microscopy (STM) images of the Si(1 0 0) surface have shown that single missing di-

mers are the dominant defect structure at the surface [20–22]. A higher concentration of random dimer defects can be generated by removing individual atoms from the surface using the ion sputtering technique, yielding multiple missing dimers such as craters of missing dimers [23,24]. An interesting observation is that with annealing of the highly defected surface at 600 °C, the randomly distributed DV migrate to form line defects perpendicular to the surface dimer rows although the starting Si(1 0 0)-(2  1) surface was remarkably free from any metal contamination which could give rise to line defects [25–28]. This DV line is energetically favorable at low vacancy concentrations, whereas at higher vacancy concentrations is aligned parallel [29]. The objective of this work is to model and compare the adsorption of cyclopentene (c-C5H8) on Si (0 0 1)-2  1 reconstructed surface ([2 + 2] cycloaddition) and on the DV of the Si(0 0 1) using a DFT methodology and to study the changes in atomic orbitals, chemical bonding in the molecule and the surface and electronic structure of the system. 2. The adsorption model and the computational methods The adsorption geometries and the electronic densities were calculated by means of the ADF-BAND2000 program (Amsterdam

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Density Functional) [30] program using the Kohn–Sham Hamiltonian with the gradient-corrected Becke exchange [31] potential together with the correlation potential of Perdew and the unrestricted scheme to obtain spin-polarized wave functions [32,33]. Full electron Slater basis sets of triple-n quality contained in the ADF-BAND package were used. The basis set of Si consisted of 3p- and 3s- orbitals. The k-points set were generated according to the geometrical method of Ramirez and Böhm [34,35]. In order

to increase the computational efficiency, the innermost atomic shells of electrons are kept frozen for every atom except hydrogen, since the internal electrons do not contribute significantly to the bonding. We used a supercell containing 316 atomic sites in a diamond like lattice to model the Si surface by a two dimensional slab of finite thickness, so as to better simulate the semi-infinite nature of the semiconductor surface. A 12-layer slab was employed as a compromise between computational economy and reasonable

Fig. 3. (a) Total DOS of c-C5H8 on Si(0 0 1)-2  1. (b) Total DOS of bare 2  1 reconstructed Si(0 0 1). (c) Total DOS of c-C5H8 on Si(0 0 1) with DV. (d) Total DOS of bare DV containing Si(0 0 1) surface. The tick marks on (a) indicates the molecular levels of c-C5H8 before adsorption. The shaded areas are the projections of c-C5H8 DOS.

Fig. 4. Frontier orbitals contribution to DOS curves of adsorbed c-C5H8. (a) HOMO-1, (b) HOMO, (c) LUMO and (d) LUMO + 1 for [2 + 2] adsorption. (e) HOMO-1, (f) HOMO, (g) LUMO and (h) LUMO + 1 for adsorption on DV. The black peaks indicate the orbital positions in the isolated c-C5H8 molecule.

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accuracy. We also included pseudo hydrogen’s for passivating the down Si atom. The molecule was adsorbed on one side of the slab with a 4  4  4 reciprocal space grid in the supercell Brillouin zone [36–38]. We considered two cases, the adsorption following a [2 + 2] cycloaddition in which c-C5H8 locates above the SiASi dimer interacting with them, and the cyclopentene adsorption on the simplest defects complexes, dimer vacancy pairs (2DV) with two neighboring dimers missing in the same row where the molecule is adsorbed on the surface. The adsorption energies have been calculated with the following total energy difference:

DEads ¼ Eðc-C5 H8 =SiÞ  Eðc-C5 H8 Þ  EðSiÞ

Table 2 Percent change in OP and OP of selected bonds formed after [2 + 2] cycloaddition on Si(0 0 1)-2  1. Bond C1@C2 Si1ASi2 Si1ASi3 Si4ASi5 Si5ASi6 C1ASi1 C2ASi2

Si slab

c-C5H8/[2 + 2]-Si(0 0 1)

0.879 0.879 0.879

29.8% 21.2% 6.1% 2.4% 1.5% 0.503 0.503

where E(c-C5H8/Si), E(c-C5H8), and E(Si) are the molecular energy on the slab, the molecular energy and the bare slab total energy, respectively [39]. The density of states (DOS) and the orbital overlap population (OPDOS) curves between atoms and orbitals of both c-C5H8 and Si surface were calculated in order to analyze the adsorbate–substrate interactions. The DOS curve is a plot of number of orbitals per unit volume per unit energy. The OPDOS curve is a plot of the overlap population weighted DOS versus energy. Integration of the OPDOS curve up to the Fermi level (EF) gives the total overlap population of the bond specified [40]. Looking at the OPDOS, we may analyze the extent to which specific states contribute to a bond between atoms or orbitals. The overlap population (OP) shows the degree of bonding of two specified atoms. A positive number means a bonding interaction while a negative number means an antibonding interaction. When computing the DOS and OPDOS, the system is divided into two fragments consisting of the surface and adsorbate respectively. This enables us to compare the changes between the bare surface, the adsorbate, and the composite adsorbed system [41]. Integration of the OPDOS curve up to the Fermi level gives the total OP. 3. Results and discussion The electronic structure of an isolated c-C5H8 molecule can be described taking into account 15 valence orbitals, 14 of them normally occupied. All the occupied molecular orbitals are of closedshell structure [42]. The molecule-surface distance and the molecule geometry were optimized to get the minimum total energy in both c-C5H8 on

Table 3 Overlap populations for selected bonds before and after adsorption on DV sites. Bond

C5H8 monolayer

C1AC2 C2AC3 C3AC4 C4AC5 C1AC5 C1AH1 C1AH2 C2AH3 C2AH4 C3AH5 C3AH6 C4AH7 C5AH8 Si1ASi6 Si1ASi5 Si2ASi5 C1ASi1 C2ASi1 C3ASi1 C4ASi1 C5ASi1 C2ASi6 C1ASi6 H8ASi3 C5ASi3 H2ASi6 C4ASi5 H6ASi5 H7ASi2 C3ASi4

0.737 0.738 0.776 1.261 0.776 0.800 0.801 0.814 0.814 0.800 0.801 0.808 0.808

H2, H3, and H6 down. H1, H4, and H5 up.

Si slab

0.879 0.880 0.879

C5H8/DV–Si(0 0 1) 0.439 0.467 0.466 0.574 0.476 0.722 0.559 0.731 0.726 0.742 0.638 0.513 0.591 0.049 0.010 0.562 0.436 0.419 0.405 0.347 0.460 0.089 0.161 0.312 0.045 0.111 0.506 0.005 0.341 0.025

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Si(0 0 1)-2  1 and c-C5H8 on dimer vacancies (2DV). The studied surface and the adsorbed molecules are shown in Fig. 1. A detailed view of the adsorption geometries and bond lengths is shown in Fig. 2a and b for both situations. When cyclopentene is adsorbed on reconstructed Si(0 0 1) the molecule is placed tilted up on the SiASi dimer with a @CASi distance of 1.96 Å. On dimer vacancies the molecule adopts a semi planar ring position, with an average HASi, ACASi, and @CASi distances of 1.59 Å, 1.83 Å, and 1.57 Å respectively. This results agrees with previous experimental and DFT studies [43–46]. When we analyze the chemisorption system, we can see that the Fermi energy of the whole system moves up because of the finite thickness of the slab and electron transfer between the semiconductor surface and the adsorbate. This fact is confirmed by the net charge on the c-C5H8 unit of 0.900 and 0.263, lost at expenses of the silicon surface (Table 1).

Table 4 Percent change in atomic orbital occupations for the c-C5H8/[2 + 2]-Si(0 0 1) chemisorption system and for the c-C5H8/DV–Si(0 0 1) chemisorption system. [2 + 2] Cycloaddition

s(%)

C1 C2 Si1 Si2

3.35 3.44 8.44 8.44

px(%) 3.20 3.49 0.35 0.37

py(%) 15.21 22.39 5.47 5.47

pz(%) 15.51 15.80 38.15 38.10

DV H1 H2 H3 H4 H5 H6 H7 H8 C1 C2 C3 C4 C5 Si1 Si2 Si5 Si6

1.26 15.84 6.32 5.13 0.37 15.41 11.03 9.38 2.81 3.11 1.23 4.33 1.55 61.88 5.29 11.33 11.66

6.52 14.18 10.80 8.19 5.01 78.14 37.34 13.30 27.28

14.57 11.55 14.30 6.17 5.85 79.62 1.82 41.96 14.70

16.08 4.88 4.82 53.96 81.19 76.70 2.37 31.33 37.33

Previous works reported adsorption energies for cyclopentene on Si(0 0 1)-2  1 calculated by DFT: Ferraz and Miotto found an adsorption energy of 1.52 eV/molecule [47]; Quek et al. found energies between 1.22 and 2.14 eV/molecule [48]. Cho and Kleinman calculated adsorption energy of cyclopentene on Si(0 0 1) of 1.63 eV with a coverage of 0.5 ML [49]. Our present calculations correspond to a very low coverage case obtaining an adsorption energy of 1.60 eV/molecule on Si(0 0 1)-2  1 and an adsorption energy of 2.58 eV/molecule on 2DV. Fig. 3 shows the DOS plots for c-C5H8/Si (0 0 1) systems. The tick marks (Fig. 3a) show the electronic levels of the isolated molecular specie before adsorption. Fig. 3a and c shows the whole system after chemisorptions, c-C5H8 on the Si (0 0 1) slab for both cases. The shaded area shows the contribution of the states from the chemisorbed c-C5H8. Fig. 3b and d shows the DOS curve of the bare Si surface slab model with and without DV. Comparing the DOS curves between 25 and 5 eV in Fig. 3a and b, we can see the bands corresponding to c-C5H8 spread out after adsorption due to the interaction between orbitals from the molecule and the surface. In Ref. [15] analyzing PES spectra the authors conclude that the adsorbed cyclopentene molecule has cyclopentane-like electronic structure, and the assignments of observed peaks are referred to those of free cyclopentane. This agrees with our calculation in Fig. 3a and c. More information is obtained from the adsorbate frontier orbitals analysis, looking at each molecular orbital of c-C5H8 upon adsorption. The contributions to the DOS of each of the c-C5H8 frontier orbitals are shown in Fig. 4 for both studied cases. The horizontal lines show the energy of the molecular orbitals in the isolated molecular species, from HOMO-1 orbital to LUMO + 1 orbital. Almost all the molecular orbitals of c-C5H8 interact with the semiconductor surface, except those lying lower in energy. After adsorption, the whole HOMO band is spread over ca. 10 eV. Due to the strong interaction with the Si surface, we can clearly see the dispersion corresponding to the bands. Table 1 contains the main frontier orbitals electron occupation and the resulting net charge for isolated and adsorbed cyclopentene. The more visible effects for both cases are the HOMO electron depopulation and partial LUMO electron occupation after chemisorption, initially empty for the isolated molecule. Our results indicate that c-C5H8 interacts very strongly with this surface. High resolution photo-

Fig. 5. Orbitals contributions to DOS before and after (shaded) [2 + 2] adsorption on Si(0 0 1). (a) 2pz orbital contribution for C1. (b) 3pz orbital contribution for Si1.

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electron spectroscopy indicates that the surface Si atoms react with cyclopentene molecules and that new interface bonds are formed upon cyclopentene adsorption [15]. For the bonding analysis we collect the OP values for selected bonds in Tables 2 and 3 (see the inserted schemes for atom description). Analyzing the data in Table 2, we can see that cyclopentene is adsorbed to the Si(0 0 1) surface bonded mainly to the SiASi dimer by its unsaturated carbon atoms (C1 and C2). The OP value of the C@C bond decrease approximately 30% and the Si1ASi2 bond decrease about 21% after adsorption process. In the case of adsorption on 2DV of the Si(0 0 1) surface, we collect the OP values for selected bonds in Table 3; thus, we can clearly see that c-C5H8 is adsorbed to the surface bonded mainly by its unsaturated carbons (C4 and C5) and its hydrogen atoms H7 and H8. The OP value of the CAC, C@C,@CAH, and ACAH bonds decrease approximately 39%, 54%, 32% and 15% respectively after adsorption, indicative of a weakening of these bonds that interacts directly with Si atoms. The strong adsorbate–surface bonding requires that both molecular and SiASi intrinsic bonding strength decrease. The SiASi surface bonds are weakened between 36% and 99% (see Table 3). Tables 2 and 3 also show that after c-C5H8 adsorption the SiASi bond is more weakened (Si1ASi6 and Si1ASi5) in the case of DV than in the 2  1 dimer adsorption (Si1ASi2). The OP analysis of the CASi bond present two sharp peaks below the Fermi level (between 25 and 15 eV) corresponding to the s orbitals of c-C5H8 interacting with the surface.

The C1ASi1 interactions, in both cases, and C5ASi1 on DV are stronger bonds because the OP values are mainly bonding (see Tables 2 and 3). To obtain further information about the adsorbate–surface interaction we also studied the contribution to chemisorption of the individual atomic orbitals of the H, C, and Si atoms. The corresponding atomic orbital occupations are displayed in Table 4 for adsorption on reconstructed surface Si(0 0 1)-2  1 and on 2DV. In the [2 + 2] cycloaddition, we can see a significant participation of the 3pz orbitals corresponding to SiASi dimer, the olefinic carbon atoms interact with 2pz orbital and 2py orbital. Fig. 5 confirms this behavior. In the case of adsorption on DV we can see a significant decrease of Si 3pz, an increase of H 1s and C 2pz orbitals OP values after c-C5H8 adsorption. Fig. 6 confirms this behavior where orbitals contributions to total DOS of Si1 3pz, H2 1s, and C1 2pz are shown. The intensity of the orbital band decrease due its participation in the adsorption process. Cyclopentene is not dissociated on Si(0 0 1)-2  1, and it is adsorbed molecularly. The hybridization states of carbon atoms in chemisorbed cyclopentene are in sp3. Therefore, it is concluded that cyclopentene is di-r bonded to the Si(0 0 1)-2  1 surface without dissociation. These results are consistent with previous experimental results of valence PES and Si 2p PES [15]. Table 5 shows the computed and experimental vibration frequencies of vapor phase c-C5H8 and c-C5H10 and c-C5H8 adsorbed on Pt and Si. For the clean Si surface, the computed SiASi frequency

Fig. 6. Orbitals contributions to DOS before and after (shaded) adsorption on DV of Si(0 0 1). (a) 1s orbital contribution for H2. (b) 2py orbital contribution for C1. (c) 3pz orbital contribution for Si1.

Table 5 Assignments of the Loss Peaks in HREELS for cyclopentane on Si and Pt and experimental and calculated IR frequencies (cm1). Normal mode

IR (vapor) [50]

DFT [51]

CH stretching ACH2 stretching C@C stretching ACH2 def. ACH2 wagging CH bending ACH2 twist ACH2 rocking Ring PtAC SiAC Si phonon

3078 s, 3068 s 2903 s, 2873 s, 2860 s 1623 m 1471 vw, 1445 m 1290 m 1353 m, 1101 w, 695 s 1207 m 1047 s, 793 1037 w, 962 w, 933 w, 900 m

3068, 2937, 1649 1481, 1303, 1358, 1209 1052, 1015,

3044 2924, 2881 1459 1296, 1205 1111, 702 875, 803 955, 900, 885

Pt (1 1 1) [52]

Pt (1 1 1) [53]

Si(0 0 1) 2  1 [15]

This work (2  1)

C5H10 [54]

3050 2910

3063 2996

1460 1275 975 1200 865 932

1454

2970, 2890

2944

2991, 2896

1460 1295, 1230

1392 1280 968

1455 1279

888, 840 1090, 880 450

1191, 1111 607 1023, 919

392 775 487, 406

765 467

942

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at 467 cm1 is attributed to surface phonons. The adsorption of cyclopentane on Si(0 0 1)-2  1 produce several vibration losses in the HREEL spectra [15], those losses are similar to that of c-C5H10 [54] a molecule that has only sp3 hybridization. The ACH and ACH2 stretching modes are computed at 2910 cm1, in good agreement with vapor phase cycloalkanes [54] and DFT and HREEL measurements on Pt [53] and on Si(0 0 1) [15]. After C5H8 adsorption no vibrational frequencies coming from a C@C bond are computed which also support the idea of re-hybridization. Studying the cycloaddition chemistry and formation of ordered organic monolayers on Si(0 0 1)-2  1, Hovis et al. do not report any IR frequency in the region 3000–3080 cm1 that correspond to the CAH stretching for alkenes [12]. In the case of C5H8 adsorption on dimers, the SiAC stretching mode is found at 765 cm1. On DV this frequency increase 35 cm1, indicating a stronger bond with the surface. An additional SiAH stretching is computed at 2147 cm1 (this is possible for the more parallel adsorption, see also the OP value for SiAH in Table 3). 4. Conclusions In this study we analyzed and compared the electronic structure changes and bonds formations during c-C5H8 adsorption on Si (0 0 1)-2  1 surface and on DV on the same surface by DFT calculations. On dimer vacancies, the c-C5H8 adsorbs with its ring parallel to the surface while on SiASi dimers the adsorption geometry is tilted up. The molecule binds to the surface mainly through unsaturated carbons, and in the case of the adsorption on dimer vacancies its hydrogen atoms too. A decrease of the bond strength in C@C, CAH, and SiASi bonds, and the formation of @CASi, and HASi bonds is observed, describing the basic interactions during chemisorption. We also found that Si 3px, 3py, and 3pz orbitals, C 2px and 2py orbitals, and H 1s orbital play an important role in the bonding between c-C5H8 and the surface for the adsorption on DV, as well as 2pz orbitals corresponding to double bond C atoms and 3pz orbitals of Si are responsible for the adsorption on Si(0 0 1)-2  1. The computed IR frequencies are in good agreement with previous theoretical and experimental determinations in the case of cyclopentane adsorption on Si dimers. Acknowledgements Our work was supported by SGCyT-UNS-Física, PICT 1186/2006, 560/2007 and PIP785. E. Germán and I. López Corral are fellows of CONICET. A. Juan and G. Brizuela are members of that Institution. We acknowledge the useful comments of the reviewers. References [1] [2] [3] [4]

J.T. Yates Jr., Science 279 (1998) 335. J.M. Buriak, Chem. Rev. 102 (2002) 1271. S.F. Bent, Surf. Sci. 500 (2002) 879. R.J. Hamers, Nature 412 (2001) 489.

[5] R.A. Wolkow, Annu. Rev. Phys. Chem. 50 (1999) 413. [6] J.A. Appelbaum, G.A. Baraff, D.R. Hamann, Phys. Rev. B. 14 (1976) 588. [7] R.J. Hamers, S.K. Coulter, M.D. Ellison, J.S. Hovis, D.F. Padowitz, M.P. Schwartz, C.M. Greenlie, J.N. Russel, J. Acc. Chem. Res. 33 (2000) 617. [8] H. Neergaard Waltenburg, J.T. Yates Jr., Chem. Rev. 95 (1995) 1589. [9] G.T. Wang, C. Mui, C.B. Musgrave, S.F. Bent, J. Phys. Chem. 103 (1999) 6803. [10] J.S. Hovis, S. Lee, H.B. Liu, R.J. Hamers, J. Vac. Sci. Technol. B 15 (1997) 1153. [11] J.S. Hovis, H. Liu, R.J. Hamers, Appl. Phys. A 66 (1998) S553. [12] J.S. Hovis, H. Liu, R.J. Hamers, Surf. Sci. 402–404 (1998) 1. [13] S.W. Lee, J.S. Hovis, S.K. Coulter, R.J. Hamers, C.M. Greenlief, Surf. Sci. 462 (2000) 6. [14] S.A. Saraireh, P.V. Smith, M.W. Radny, S.R. Schofield, B.V. King, Surf. Sci. 602 (2008) 3484. [15] S. Machida, K. Hamaguchi, M. Nagao, F. Yasui, K. Mukai, Y. Yamashita, J. Yoshinobu, H.S. Kato, H. Okuyama, M. kawai, J. Phys. Chem. 106 (2002) 1691. [16] J.H. Cho, L. Kleinman, Phys. Rev. B 64 (2001) 235420. [17] K. Akagi, S. Tsuneyuki, Y. Yamashita, K. Hamaguchi, J. Yoshinobu, Appl. Surf. Sci. 234 (2004) 162. [18] W.C. Lu, W.G. Schmidt, J. Bernholc, Phys. Rev. B 68 (2003) 115327. [19] J. Dabrowski, H.-J. Müssig, Silicon Surfaces and Formation of Interfaces, World Scientific Publishing Co. Pte. Ltd., Singapore, 2000. [20] N. Kitamura, M.G. Lagally, M.B. Webb, Phys. Rev. Lett. 71 (1993) 2082. [21] R.J. Hamers, U.K. Koehler, J. Vac. Sci. Technol. A 7 (1989) 2854. [22] J. Wang, T.A. Arias, J.D. Joannopoulos, Phys. Rev. B 47 (1993) 10497. [23] H. Feil, H.J.W. Zandvliet, M.-H. Tsai, J.D. Dow, I.S.T. Tsong, Phys. Rev. Lett. 69 (1992) 3076. [24] H.J.W. Zandvliet, H.B. Elswijk, E.J. Van Loenen, I.S.T. Tsong, Phys. Rev. B 46 (1992) 7581. [25] F.-K. Men, A.R. Smith, K.-J. Chao, Z. Zhang, C.-K. Shih, Phys. Rev. B 52 (1995) R8650. [26] A.R. Smith, F.-K. Men, K.-J. Chao, J. Vac. Sci. Technol. B 14 (1996) 909. [27] A.R. Smith, F.-K. Men, K.-J. Chao, J. Vac. Sci. Technol. B 14 (1996) 914. [28] P. Bedrossian, T. Klitsner, Phys. Rev. Lett. 68 (1992) 646. [29] E. Kim, C. Chen, T. Pang, Y.H. Lee, Phys. Rev. B 60 (1999) 8680. [30] Amsterdam Density Functional Package Release, 2001 (Vrije Universiteit, Amsterdam). [31] A.D.J. Becke, Phys. Rev. A 38 (1988) 3098. [32] J.P. Perdew, Phys. Rev. B 33 (1986) 8822. [33] G. te Velde, E.J. Baerens, Phys. Rev. B 44 (1991) 7888. [34] R. Ramirez, M.C. Böhm, Int. J. Quantum Chem. 30 (1986) 391. [35] R. Ramirez, M.C. Böhm, Int. J. Quantum Chem. 34 (1988) 571. [36] P. Hohenberg, W. Kohn, Phys. Rev. 136 (1964) B864. [37] W. Kohn, L.J. Sham, Phys. Rev. 140 (1965) A1133. [38] R.G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules, Oxford University Press, New York, 1989. [39] G. Brizuela, N.J. Castellani, Surf. Sci. 401 (1998) 297. [40] R. Hoffmann, Solids and Surfaces: A Chemist’s View of Bonding in Extended Structures, VCH, New York, 1988. [41] A.W. Edith Chan, R. Hoffmann, J. Chem. Phys. 92 (1990) 699. [42] E. Germán, I. López Corral, A. Juan, G. Brizuela, J. Mol. Catal. A: Chem. 290 (2008) 23. [43] F. Ample, C. Joachim, Surf. Sci. 602 (2008) 1563. [44] R. Miotto, A.C. Ferraz, G.P. Srivastava, Surf. Sci. 507–510 (2002) 12. [45] Y. Apeloig, M. Karni, J. Am. Chem. Soc. 106 (1984) 6676. [46] H.S. Gutowsky, J. Chen, P.J. Hajduk, J.D. Keen, T. Emilsson, J. Am. Chem. Soc. 111 (1989) 1901. [47] A.C. Ferraz, R. Miotto, Surf. Sci. 566 (2004) 713. [48] S. Quek, J. Neaton, M. Hybertsen, E. Kaxiras, Phys. Stat. Sol. (b) 243 (2006) 2048. [49] J.H. Cho, L. Klienman, Phys. Rev. B 67 (2003) 115314. [50] J.R. Villarreal, J. Laane, S.F. Bush, W.C. Harris, Spectrochim. Acta A 35 (1979) 331. [51] A.-S. Abdulaziz, J. Laane, J. Mol. Struct. 830 (2007) 46. [52] N.R. Avery, Surf. Sci. 146 (1984) 363. [53] C. Becker, F. Delbecq, J. Breitbach, G. Hamm, D. Franke, F. Jager, K. Wandelt, J. Phys. Chem. B 108 (2004) 18960. [54] D.W. Ball, J. Mol. Struct.: THEOCHEM 417 (1997) 107.