Computational Investigation of Coverage-Dependent Behavior on

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Computational Investigation of Coverage-Dependent Behavior on Functionalization of the Semiconductor X (100)-2 1 Surface (X = C, Si, and Ge) by Cycloaddition of Transition Metal Oxides Bao-Zhen Sun,† Wen-Kai Chen,† and Yi-Jun Xu*,†,‡ † ‡

College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, People’s Republic of China College of Chemistry and Chemical Engineering & Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, People’s Republic of China ABSTRACT: By means of density functional theory in conjunction with periodic slab models, coverage-dependent behavior for chemical functionalization of the semiconductor X (100) (X = C, Si, and Ge) surface by traditional cycloaddition of transition metal oxides (OsO4 and RuO4) has been investigated. We found that the transition-metal-mediated oxygen transfer reaction for cycloaddition of OsO4 and RuO4 onto X (100) is quite favorable. The adsorption energies decrease as the coverage is increased, and the band gap is widened or reduced depending on both the type of model molecules and the coverage. Furthermore, it is feasible to form organic layer films of transition metal oxides onto the semiconductor X (100) surface as reflected by the high adsorption energies at the saturated coverage, which could lead to new hybrid multifunctional materials. In addition, interestingly, two distinct and competitive monolayer structures are simultaneously observed on both the Si (100) and the Ge (100) surfaces, but only one stable monolayer geometry is found on the C (100) surface, indicating the flexibility of controlling the self-assembled molecular binding configuration using transition metal oxides on the semiconductor X (100) surface. It is hoped that our results may help to seek appropriate chemical modification methods to widen the application fields of the group IV semiconductor-based hybrid materials.

1. INTRODUCTION Semiconductor-based materials, including diamond (C), silicon (Si), and germanium (Ge), play a dominant role because of their foundation role in much of modern technology, especially in microelectronic technology.1-6 Because of the huge market for semiconductors, methods that can be used to tailor their surface properties become increasingly important. Organic modification is one means of providing new functionality to the semiconductor surface by depositing layers of organic molecules on the surface. The incorporation of organic materials on the surface offers great flexibility in designing and creating unique molecular properties by tuning the nature of the attached organic groups.1-19 Such a molecular integration of chemical functionalities of organic molecules with the semiconductor surfaces can be very useful for numerous technological applications, including molecular electronics, drug delivery, nonlinear optics, and biological sensors.7-19 Until now, much abundant and fascinating organic-semiconductor surface chemistry has been achieved, especially on the (100) surface of single-crystalline C, Si, and Ge. Investigations have shown that the (100) crystal faces of these semiconductors are comprised of CdC, SidSi, and GedGe structural units that resemble the CdC bond of organic alkenes.13,14,17 In fact, the semiconductor X (100) r 2011 American Chemical Society

(X = C, Si, and Ge) surface, fullerene, and carbon nanotube all feature a bonding motif similar to that of alkenes, as displayed in Scheme 1. Therefore, all of these materials may react toward small organic molecules very much like a molecular carbon-carbon double bond of alkenes. To date, many well-known reactions including Diels-Alder addition,20-33 [2 þ 2] cycloaddition,34-49 and 1,3-dipolar addition50 on the X (100) (X = C, Si, and Ge) surface have been demonstrated both experimentally and theoretically. In addition, we have predicted, by effective cluster models, that the cycloaddition reactions of carbenes and nitrenes to alkenes, the epoxidation of alkenes by dioxiranes in organic chemistry, and cycloaddition of transition metal oxides can be used as new types of surface reaction to functionalize the X (100) surface.51-57 In organic chemistry, the concerted [3 þ 2] cycloaddition of transition metal oxides (such as OsO4 and RuO4) to alkenes represents a prominent paradigm for transition-metal-mediated oxygen transfer reaction, and its catalytic asymmetric form has proven to be a powerful method for enantioselective Received: November 18, 2010 Revised: January 22, 2011 Published: March 04, 2011 5800

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The Journal of Physical Chemistry C Scheme 1. Materials Featuring a Bonding Motif Similar to That of Alkenes

synthesis.58-63 Our recent cluster model calculations reveal that analogous chemical reaction could also be successfully extended to functionalize X(100) surfaces (X = C, Si, and Ge).57 However, it should be stressed that small clusters only provide a testing ground for quick assessment of possibility of a given reaction on the X (100) surface, and only deal with the very finite system. Therefore, studying the coverage dependence of various characteristic properties is hardly feasible in the framework of cluster models. As a result, it is still not known how different coverages of transition metal oxides (OsO4 and RuO4) affect the structural and electronic properties of the semiconductor surface of X (100) (X = C, Si, and Ge). Furthermore, from the viewpoint of practical applications, it is quite essential to understand the structure and electronic properties of organic layer on the semiconductor surfaces because the interface between active organic layers and the substrate plays a crucial role in improving the performance of semiconductors-based devices. Filler et al. have also pointed out that coverage will be an exciting topic for future study in the field of organic chemistry functionalization of semiconductors.8 Notably, coverage effects on phenol adsorption on Si (100),64 1-propanol adsorption on Si (001),65 and electronic structures of thiophene on Ge (100)66 have been investigated as examples. These studies show that the adsorption configuration of phenol is different, the band gap of the Si/1propanol film increases, and the bonding states of thiophene species change with the increased coverage. Most recently, we have investigated the coverage-dependent behavior of X (100) (X = C, Si, and Ge) surface by cycloadditions of carbenes and nitrenes (CH2, SiH2, GeH2, and NH).67 It is found that there is a promising flexibility for engineering the semiconductor X (100) surface by tuning not only the coverage but also the type of organic molecules. Thus, we anticipate that the coverage of transition metal oxides would play an important role in tailoring the chemical properties of X (100) semiconductor surface, which may show the different coverage-dependent chemistry from that of carbenes and nitrenes. Toward this end, we herein report a systematic study on the coverage-dependent behavior for chemical functionalization of the X (100)-2 1 surface with transition metal oxides, using OsO4 and RuO4 as examples, by using slab models with periodic boundary conditions. In particular, the variation of the adsorption structure and

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energies, density of states (DOS), and the band gap with the degree of coverage have been investigated in detail. Our results demonstrate that the structure and electronic properties are affected by the degree of coverage of transition metal oxides. Furthermore, analogy and difference between the coveragedependent behavior for addition of transition metal oxides onto the semiconductor X (100) surface and that of carbenes and nitrenes have been compared. The results strongly confirm a promising flexibility for engineering the semiconductor C (100), Si (100), and Ge (100) surfaces by tuning both the coverage and the type of organic molecules.

2. COMPUTATIONAL DETAILS AND MODELS The X (100) (X = C, Si, and Ge) surfaces have been represented by a (4 4) supercell with vacuum regions of 13 Å. This supercell has two dimer rows consisting of four dimers each. All spin-unrestricted periodic density functional theory (DFT) calculations were performed using the program package DMol3,68,69 in which wave functions are expanded in terms of numerical basis sets. The selected basis set is DNP, a doublenumerical with polarization functions basis set. It is one of the most complete sets available in the code. The size of the DNP set is comparable to Gaussian basis set 6-31G**. The use of numerical basis sets minimizes the basis set super position error (BSSE).70 The exchange-correlation functional was the generalized gradient approximation (GGA) of Perdew and Wang (PW91).71 During the computation, for C, Si, Ge, O, and H atoms, the all-electron basis set was used, while the density functional semicore pseudopotential (DSPP) was used for Os and Ru transition metal atoms. Brillouin-zone integrations were performed using the 2 2 1 Monkhorst-Pack grid and a Fermi-smearing of 0.005 hartree. The tolerances of energy, gradient, and displacement convergence were 1 10-5 Ha, 2 10-3 Ha/Å, and 5 10-3 Å, respectively. To facilitate exploration of coverage effect, we performed calculations on models with 1, 2, 4, and 8 molecules, corresponding to coverage θ = 0.125, 0.25, 0.5, and 1 ML. We choose a five-layer slab to simulate the C (100) surface and a six-layer slab for the Si (100) and Ge (100) surfaces. The bottom layer’s dangling bonds are saturated with H atoms. During the geometry optimization, for the C (100) surface, we relaxed the first four layers while constraining the bottom layer and the H atoms. For Si (100) and Ge (100), however, we optimized the geometries of the Si and the Ge surfaces starting from symmetric dimers and allowing the first four layers to fully relax. The others were fixed in their relaxed positions. The optimized X (100) surface with the reconstructed XdX (X = C, Si, and Ge) dimer is displayed in Figure 1. As shown in Figure 1, we achieve the symmetric C (100) dimer surface and the asymmetric Si (100) and Ge (100) dimer surfaces. In particular, our calculated CdC bond length (1.380 Å) for the clean C(100) surface is in agreement with the result calculated by Kr€uger and Pollmann.72 For the Si (100) and Ge (100) surfaces, we found the SidSi and GedGe bond lengths are 2.364 and 2.571 Å, respectively, and their buckling angles are 18° and 19°, respectively. As we know, the GGA procedure tends to overestimate the bond length, so that our calculated values are slightly larger than the results of the LDA calculations by Kr€uger and Pollmann,72 Northrup,73 Uchiyama,74 Ramstad,75 Pollmann et al.,76 and Spiess et al.77 Yet the error is not large, which is in the range of 3.23-5.54% for the SidSi 5801

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The Journal of Physical Chemistry C

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Figure 1. Side view of the reconstructed semiconductor X (100)-2 1 (X = C, Si, and Ge) surface. For simplicity, only the top five layers are shown.

Figure 2. Top and side views of optimized geometries (units in Å for bond length) for the additions of OsO4 and RuO4 onto the C (100) surface at four coverage levels. The five top layers are shown, and the pink atoms represent CdC dimers in the topmost layers.

bond length and 2.43-6.68% for the GedGe bond length. Moreover, our calculated values are in agreement with the results of the GGA calculation by Wang et al.:78 dSidSi = 2.30 Å and dGedGe = 2.51 Å. In particular, our calculated SidSi bond length is also in agreement with the experimental result:79 dSidSi = 2.24 ( 0.08 Å. Actually, such a slab model has been used successfully for the coverage effect on the addition of carbenes and nitrenes on the X (100) surface (X = C, Si, and Ge).67 For the adsorbate-substrate systems upon the adsorption of transition metal oxides (OsO4 and RuO4), all geometrical parameters of the topmost four layers as well as the adsorbate were allowed to fully relax. The adsorption energy per adsorbate is described as: Eads ¼ ðEsubstrate þ N 3 Eadsorbate - Esystem Þ=N where N is the number of the model molecules adsorbed. Esystem and Esubstrate are the total energies of the system with adsorbates and the bare X (100) reconstructed surface, and Eadsorbate is the energy of the gas-phase model molecule.

3. RESULTS AND DISCUSSION 3.1. Coverage Effect on the Structure and Energy. Figures 2-4 list the optimized geometries after the additions of OsO4 and RuO4 onto the X (100) surface (X = C, Si, and Ge) at various coverages, respectively. For the additions of OsO4 and RuO4 onto C (100), as shown in Figure 2, the as-formed surface structure is very similar when the adsorption coverage varies from 0.125 to 0.5 ML. Two oxygen atoms of transition metal oxides are bonded to the surface CdC dimer symmetrically, and finally a closed five-member ring is formed. When the coverage reaches 1 ML, analogous to the saturated adsorption structures of SiH2 and GeH2 onto C (100),67 a half segment of OsO4 or RuO4 molecules are bonded to the surface CdC dimer, while the other half segment of OsO4 or RuO4 molecules are bonded to the surface carbon atoms across the CdC dimer. Such a specific adsorption structure at the full coverage resulted from the strong molecular repulsion of neighboring transition metal oxides. When OsO4 or RuO4 attaches to Si (100) or Ge (100), as shown in Figures 3 and 4, somewhat differently from the case on C (100), two distinct adsorption geometries are simultaneously 5802



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Figure 3. Top and side views of optimized geometries (units in Å for bond length) for the additions of OsO4 and RuO4 onto the Si (100) surface at four coverage levels. The five top layers are shown, and the pink atoms represent SidSi dimers in the topmost layers. [Note: 1a represents the saturated adsorption structure in which a half segment of OsO4 or RuO4 molecules are bonded to the surface SidSi dimer, while the other half segment of OsO4 or RuO4 molecules are bonded to the surface silicon atoms across the SidSi dimer; 1b indicates the saturated adsorption structure in which all transition metal-oxide molecules are bonded to SidSi dimer symmetrically. The same representation is applied to Figure 4.]

Figure 4. Top and side views of optimized geometries (units in Å for bond length) for the additions of OsO4 and RuO4 onto the Ge (100) surface at four coverage levels. The five top layers are shown, and the pink atoms represent GedGe dimers in the topmost layers.

found at saturated coverage. One (denoted as 1a) is similar to what is observed for the additions of OsO4 or RuO4 on C (100) at the coverage of 1 ML. In this structure, as illustrated in the top view of Figures 2-4, the distance of XdX dimers bonding to transition metal oxides in an unsymmetrical manner is significantly larger than that of XdX dimers bonding to transition

metal oxides in a symmetry manner. For example, for the addition of OsO4 on C (100), the CdC distance in the former case is 1.701 Å and that in the latter case is 1.618 Å. The larger increment of XdX distance in the former case is fit to bond with transition metal oxides. Also, such a trend becomes more pronounced for the additions of transition metal oxides on Si 5803



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Figure 5. Plot of the adsorption energy per adsorbate as a function of adsorption coverage for the additions of OsO4 and RuO4 onto the X (100) (X = C, Si, and Ge) surfaces.

(100) and Ge (100). Because of the larger SidSi or GedGe intradimer distance of Si (100) or Ge (100) than the CdC dimer of C (100), the other structure (denoted as 1b), in which all transition metal-oxide molecules are bonded to SidSi or GedGe dimer symmetrically, can also be found on the Si (100) and Ge (100) surfaces. Furthermore, upon the additions of OsO4 and RuO4 onto Si (100) or Ge (100), the asymmetric buckled SidSi or GedGe dimer will become symmetric for all coverages. In Figure 5, we plot the adsorption energy per adsorbate as a function of coverage. The large adsorption energy at high coverage indicates that it is possible for the formation of monolayer transition metal oxides on the semiconductor X (100) (X = C, Si, and Ge) surface. From Figure 5, three common trends are shown. First, the adsorption energy progressively decreases when the coverage is increased. In particular, the adsorption energy for the C (100) case shows the most pronounced change trend. For example, regarding the addition of OsO4 onto C (100), the adsorption energy is 535.99, 414.37, and 346.76 kJ/mol, corresponding to that with coverage 0.125, 0.25, and 0.5 ML, respectively. At the saturated coverage, the adsorption energy drastically decreases to 298.77 kJ/mol, indicating the existence of a strong repulsion between OsO4 molecules. In contrast to the case on C (100), the coverage dependence of adsorption energy on both Si (100) and Ge (100) is much weaker. This may be related to the fact that Si (100) and Ge (100) have longer XdX intradimer distances than C (100). Second, the adsorption energy of RuO4 on X (100) (X = C, Si, and Ge) is much larger than that of OsO4 on X (100), suggesting that the oxidation power of RuO4 is always stronger than OsO4. Third, the difference in the adsorption energy between 1a and 1b structures is minimal, thereby suggesting that the two structures are highly competitive with each other at saturated coverage. On the basis of the above discussion, it is feasible to form organic layer films of OsO4 and RuO4 onto the semiconductor X (100) (X = C, Si, and Ge) surface, which experimentally may well be the first step in the atomic layer deposition growth of transition metal-oxide films on the X (100) surface.80 One may hypothesize that the as-formed hybrid functional materials could have specific electronic properties differing from the

pristine X (100) surface, which could have promising potential for establishing novel semiconductor-based electronic devices. To examine the validity of this hypothesis, coverage effects on electronic properties of X (100) will be considered in the following section. 3.2. Coverage Effects on Electronic Properties. To understand the coverage effect on electronic properties of the X (100) surface (X = C, Si, and Ge) upon attachment of OsO4 and RuO4, the total density of states (TDOS) and the partial density of states (PDOS) of all model systems at different coverage levels are calculated and shown in Figures 6-8. To facilitate the comparison, the corresponding DOS of clean surface and free adsorbates is also presented. The partial DOS of the chosen moiety are obtained by first selecting the radio button labeled Partial, then checking the checkboxes for s, p, d, and Sum, and finally clicking on View in the DMol3 Analysis dialogue. The partial DOS projected on the adsorbates is much lower in magnitude than the DOS of the total slab. Therefore, to show its features manifestly, we have rescaled this distribution (vertical axis), and the rescale factor is 3 for the C (100) case while it is 10 for the Si (100) and Ge (100) cases. We consider the DOS with special emphasis on the peaks around the Fermi level and some characteristic peaks. As shown in Figures 6-8, the partial DOS projected on the substrates has almost the same profile as that of the DOS of the total slab, which indicates that the total DOS is dominated by the X (100) surface states (X = C, Si, and Ge). To discuss the change in DOS of functionalized X (100) surface by additions of OsO4 and RuO4 at different coverages, we take the OsO4/C (100) model system for example. From Figure 6a2, we can see that the bands of free OsO4 are highly localized. Also, its bands located at -4.2, 3.6, and 5.5 eV mainly belong to the hybridization of the Os5d and O2p orbitals, while the peaks located at -2.1, -0.7 eV, and the Fermi level are dominated by the O2p orbitals. Upon addition, all the bands of the adsorbed OsO4 shift downward, indicating that the adsorbed OsO4 molecules accept the electrons from the C (100) surface. Also, the magnitude of electron transfer is largest as OsO4 coverage reaches 1 ML. Meanwhile, it can also be observed that the electron on the Os5d and O2p orbitals of the adsorbed OsO4 molecules is spatially distributed in wider range in comparison 5804



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Figure 6. Total density of states (DOS) and partial DOS projected on the substrate and the adsorbates for the C (100) surface functionalized by OsO4 and RuO4 at different coverages. The Fermi level is set to zero and indicated by a dashed line. [Note: For clarity, the distribution in vertical direction of partial DOS projected on the adsorbates is 3 times what it was before.]

Figure 7. Total density of states (DOS) and partial DOS projected on the substrate and the adsorbates for the Si (100) surface functionalized by OsO4 and RuO4 at different coverages. The Fermi level is set to zero and indicated by a dashed line. [Note: For clarity, the distribution in vertical direction of partial DOS projected on the adsorbates is 10 times what it was before.]

with that of free OsO4 molecule. This distribution exhibits more evident as increasing coverage and induces a larger overlap between the Os5d orbital and the O2p orbital at high coverage. Therefore, increasing coverage contributes to transition-metalmediated oxygen transfer reaction for the cycloaddition of OsO4 and RuO4 onto X (100). In addition, according to Figure 6a2, the intensity of the peaks of the adsorbed OsO4 molecule is lowered from low coverage to high coverage, suggesting the adsorption of OsO4 molecules at high coverage is weaker than that at low coverage. This result is faithfully consistent with the observation of adsorption energy as shown in Figure 5a. That is, the adsorption energy for OsO4 addition onto C (100) at high coverage is lower than that at low coverage. On the other hand, analysis of Figure 6a1 shows that the main components of the

peaks ranging from -12 to 2 eV of the partial DOS projected on C (100) are traced back to 2p orbital of the carbon atoms. On further analysis of Figure 6a1, we can see that increasing OsO4 coverages make the adsorption states in the TDOS located below the Fermi energy level by about 1 eV stronger. By carrying out partial DOS analysis of Figures 6a1 and a2, we establish that the dominant interaction between OsO4 and C (100) derives from the overlap between (Os5d þ O2p) hybrid orbitals of OsO4 and π orbital of C (100). For other cases (see Figures 6b1 and b2, 7, and 8), the change trend in DOSs and the bonding mechanism are similar to the case of C (100) functionalized by OsO4. Now, we turn to discuss the change trend in the band gap. As compared to the pristine C (100) surface, Figure 6a1 clearly shows that one gap state appears in the midgap in the OsO4/C 5805



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Figure 8. Total density of states (DOS) and partial DOS projected on the substrate and the adsorbates for the Ge (100) surface functionalized by OsO4 and RuO4 at different coverages. The Fermi level is set to zero and indicated by a dashed line. [Note: For clarity, the distribution in vertical direction of partial DOS projected on the adsorbates is 10 times what it was before.]

Figure 9. Plot of the band gap as a function of adsorption coverage for the additions of OsO4 and RuO4 onto the X (100) (X = C, Si, and Ge) surface.

(100) model system, which is from the hybridization Os5d and O2p states, as shown in Figure 6a2, and leads to the band gap reduction. The increased coverage from 0.125 to 0.25 ML does not shift the valence band maximum (VBM) but lowers the gap state, resulting in band gap narrowing. Next, when the coverage increases from 0.25 to 0.5 ML, the VBM moves to the low energy region, and meanwhile the gap state shifts to the high energy region, leading to band gap widening. Finally, when the coverage increases from 0.5 to 1 ML, the VBM and the gap state both shift upward, but the VBM raises more (about 0.29 eV) than the gap state (about 0.23 eV). Therefore, the net result is that the band gap is slightly reduced. The change trend of band gap for OsO4/ C (100) model system as a function of coverage is fully consistent with the result from Figure 9a. That is, the band gap of functionalized C (100) by OsO4 is much smaller than that of pristine C (100); when the coverage varies, the band gap for 1 ML is slightly smaller than those for 0.125 and 0.5 ML, but slightly larger than that for 0.25 ML. For C (100) functionalized by RuO4, it follows the same trend, except that the reduction degree of the band gap is even more pronounced as compared to pristine C (100). For the cases of Si (100), Figure 9b shows opposing behavior in band gap modification: the band gap is enlarged for Si (100) functionalized by OsO4, while reduced for Si (100)

functionalized by RuO4. In addition, the band gap for the former case varies little below 0.25 ML and sharply increases beyond 0.25 ML, while that for the latter case generally decreases when the coverage varies from 0.125 to 1 ML. In particular, at the full coverage of 1 ML, the band gap modifies most significantly. Such an alteration in the band gap for the two cases can be explained by the DOS analysis of Figure 7. For the former case (see Figure 7a), the DOS shows that at 0.5 ML, the VBM displays no movement and the conduction band minimum (CBM) raises about 0.2 eV with respect to that below 0.25 ML, resulting in a band gap increase. As coverage reaches 1 ML, the CBM further raises, thereby inducing the most pronounced enlargement of the band gap at the saturated coverage. For the latter case, on the contrary, the CBM moves to the low energy region, which leads to some degree of reduction in the band gap (see Figure 7b). The band gaps in Figure 9c show a common trend for the functionalized Ge (100) surface by the additions of OsO4 and RuO4. That is, when the coverage is increased, a progressive enlargement of the band gap is observed, and the band gap for full coverage is considerably increased to 0.843 eV after the additions of OsO4 to the Ge (100) surface. Also, the addition of OsO4 induces a larger increase in band gap of Ge (100) with respect to that of RuO4. The increased band gap is attributed to the upward shift of the CBM as compared to that of the pristine Ge (100). In 5806


The Journal of Physical Chemistry C addition, parts b and c of Figure 9 show that the difference in the band gap between 1a and 1b structures is small or even zero. In conclusion, the band gaps for the functionalized X (100) (X = C, Si, and Ge) surfaces can be changed in different directions depending on the surface coverage and the type of the adduct. The widened band gap may be helpful to inhibit quenching the excited states, thus suggesting that functionalized Si (100) by OsO4 and functionalized Ge (100) by OsO4 and RuO4 could be used in heterogeneous photocatalysis. In parallel, the reduced band gap indicates an improved electronic conductance. As a result, C (100) functionalized by OsO4 and RuO4 and Si (100) functionalized by RuO4 is more suitable for a good electronic device. 3.3. Comparison with Carbenes and Nitrenes Cases. In comparison with the results from our previous study of X (100) (X = C, Si, and Ge) functionalized by carbenes and nitrenes at different coverages,67 we find that they share the same trend in the bonding mechanism and the change of the adsorption energy. That is, the dominant contribution to the bonding derives from the X (100) surface states (X = C, Si, and Ge), and the adsorption energy decreases with increasing coverage. Meanwhile, it is found that the asymmetric buckled SidSi or GedGe dimer will become symmetric in the cases of X (100) functionalized by OsO4 and RuO4. Similar behavior can also be observed in the cases of Si (100) functionalized by carbene and nitrene and in the cases of Ge (100) functionalized by SiH2 and GeH2. Yet differently, there is still some degree of surface buckling for the cases of Ge (100) functionalized by CH2 and NH. On the other hand, the band gap is generally widened as the coverage is increased for X (100) functionalized by carbene and nitrene. Likewise, the band gap of Si (100) functionalized by OsO4 and Ge (100) functionalized by both OsO4 and RuO4 is also enlarged with increasing coverage. However, RuO4 modification narrows the band gap of Si (100) when the coverage increases. When C (100) is decorated by OsO4 and RuO4, the band gap at 0.125 and 0.5 ML is larger than that at 0.25 and 1 ML. Furthermore, the band gap for X (100) functionalized by OsO4 and RuO4 is less sensitive to surface coverage as compared to that for X (100) functionalized by carbene and nitrene. Therefore, there is a wide scope to controlling the self-assembled molecular binding configuration and electronic properties of X (100) by tuning both the coverage and the type of organic molecule. 3.4. Comparison with Previous Cluster Model Results. Finally, we want to compare the present periodic results with our previous cluster model results.57 From the viewpoint of structure, the as-formed surface structures at low coverage herein in general resemble eariler ones.57 The five-member ring is formed; the Os-O (or Ru-O) bond length based on periodic calculations is slightly larger than that based on cluster model calculations, whereas the adsorption bond lengths and the XdX bond lengths in this work are both close to those in previous cluster model results. With regard to adsorption energy, similarly, the adsorption energy of OsO4 is always larger than that of RuO4, but slightly different. In our previous work based on cluster model calculations, the adsorption strength for OsO4 and RuO4 is in the order: Si (100) > C (100) > Ge (100). However, in the present work, the sequence below 0.25 ML is C (100) > Si (100) > Ge (100), whereas the order above 0.25 ML is the same as that of the cluster result. Although the cluster model calculations can effectively evaluate the feasibility and the mechanism of the reaction, it cannot be used to answer the coverage effect. Therefore, the periodic calculations herein are essential to help

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understand the coverage-dependent behavior for organic functionalization of X (100).

4. CONCLUSION While organic functionalization of the semiconductor X (100) (X = C, Si, and Ge) surface has been extensively studied theoretically and experimentally, only few theoretical attempts have been made to explore the coverage-dependent behavior systematically. In this work, we present a periodic density functional theory study of coverage-dependent behavior for organic functionalization of X (100) by cycloadditions of OsO4 and RuO4. The effect of coverage on the structural, energetic, and electronic properties of the X (100) surface has been examined. We found that it is possible for the formation of organic layer films of transition metal oxides onto the semiconductor X (100) surfaces due to the large adsorption energies for all the model systems. The adsorption energies of OsO4 and RuO4 on X (100) decrease with increasing coverage due to enhanced intermolecular repulsion. Such behavior is most pronounced on the C (100) surface due to its smaller surface size than the Si (100) and Ge (100) surfaces. Interestingly, at the saturated coverage, there simultaneously exist two distinct and competitive monolayer structures on Si (100) and Ge (100), in which their band gaps are very similar or even identical. DOS analysis reveals that, for C (100) functionalized by OsO4, the Os5d and O2p orbitals of the adsorbed OsO4 are greatly delocalized at high coverage, which leads to a larger overlap of (Os5d þ O2p) orbitals. As such, increasing coverage contributes to the transition-metal-mediated oxygen transfer reaction for the cycloaddition of OsO4 and RuO4 onto the semiconductor X (100) surface. Analogous distribution is also found for other model systems. Moreover, the functionalization of X (100) by cycloaddition of transition metal oxides results in either a band gap reduction or a band gap enlargement, depending on the coverage and the type of the adduct. For example, the band gap for Si (100) functionalized by OsO4 strongly increases with the coverage above 0.25 ML, whereas that for Si (100) functionalized by RuO4 generally decreases with increasing coverage. It is hoped that our work may provide new insights for the seeking of appropriate chemical modification methods to widen the application fields of the group IV semiconductor-based hybrid materials. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Award Program for Minjiang Scholar Professorship, the National Natural Science Foundation of China (20903023), Program for Returned High-Level Overseas Scholar of Fujian Province, and Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0818). W.-K.Chen is thankful for the financial support by the NCETFJ (HX2006-103) and the FSKLCC0814. ’ REFERENCES (1) Meyer zu Heringdorf, F. J.; Reuter, M. C.; Tromp, R. M. Nature 2001, 412, 517. (2) Hamers, R. J. Nature 2001, 412, 489. 5807


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