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Adsorptions of Tetrafluorotetracyanoquinodimethane on Entirely and Partially Hydrogenated C(100)-2×1 Surfaces G. X. Jia,† C. S. Guo,† R. Q. Zhang,*,† W. J. Zhang,† I. Bello,† and Th. Fraueheim‡ Centre of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, China, and Bremen Center for Computational Material Science, UniVersity Bremen, 28334 Bremen, Germany ReceiVed: January 2, 2009; ReVised Manuscript ReceiVed: March 30, 2009
We studied the adsorptions of tetrafluorotetracyanoquinodimethane (F4-TCNQ) on entirely and partially hydrogenated C(100)-2×1 surfaces using density-functional tight-binding and first-principle methods. The results indicate that F4-TCNQ could form hydrogen bonding with surface H atoms of the entirely hydrogenated C(100)-2×1 surface, and chemically react with the bare carbon atoms of the partially hydrogenated C(100)2×1 surface. The adsorption of F4-TCNQ induces charge transfer from the diamond surface to F4-TCNQ and the holes left in the diamond surface could increase the conductivity of the diamond surface. It is shown that electron transfer is facilitated by the higher valence band maximum of the diamond surface compared with the energy level of the lowest unoccupied molecular orbital of F4-TCNQ. 1. Introduction Diamond, a wide-band gap semiconductor, has attracted considerable interest in the design of high-power electronic devices and field-effect transistors because of its unique properties, such as high thermal conductivity and high breakdown field strength. The surface conductivity (SC) of diamond has also received much attention since Landstrass et al. observed that undoped, hydrogenated diamond showed p-type SC when exposed to air.1 Subsequently, Maier et al.2 and Larsson et al.3 found that an H2/H+ electrochemical reaction occurred at the interface between the hydrogenated diamond surface and air. However, Chakrapani et al.4,5 thought that at the interface the electrochemical reaction O2 + 4H+ + 4e- ) 2H2O occurred, which contributed to the p-type SC, because the Fermi level of the hydrogenated diamond was higher than the chemical potential of the air involving O2. For the usual surface transfer doping of the diamond, Ristein6 considered that the energy difference ∆ of the LUMO of the dopants and the valence band maximum (VBM) of the diamond determined whether electrons were transferred from the diamond to the dopants. Fortunately, the exceptionally low ionization energy of the hydrogenated diamond surface leads to the negative or rather small ∆ value. Therefore, the doping of the fullerenes7,8 and fluorofullerene molecules9 on hydrogenated diamond surfaces made holes left in the hydrogenated diamond. A recent experiment10 found that the deposition of F4-TCNQ, an organic molecule, on the monohydrogenated (100) diamond (C(100)-2×1:H) surface increased the areal hole density in diamond. This value was substantially larger than the intrinsic boron doping level of the diamond sample, and comparable to the maximum hole density obtained by using the fluorofullerene. However, no detailed theoretical study of the involved mechanism has been reported so far. Therefore, for deeper insight into the mechanisms at electronic and atomic levels, we * To whom correspondence should be addressed. E-mail: aprqz@ cityu.edu.hk. † City University of Hong Kong. ‡ University Bremen.
systematically investigated the effects of the organic molecule F4-TCNQ doping on the geometrical and electronic properties of diamond surfaces. In this work, the monohydrogenated diamond surface was chosen to study the charge transfer mechanism at the molecular adsorption since it is more stable under conventional conditions. In addition, it has a significant negative electron affinity and an exceptionally low ionization potential,11-13 which enables easy electron transfer from the monohydrogenated diamond surface to the adsorbates. On the contrary, F4-TCNQ has a strong electron accepting capability14 and has been successfully used for controlled p-type doping on metal surface15 and single-walled carbon nanotubes.16 Therefore, after the adsorption of F4-TCNQ on the monohydrogenated diamond surface, electrons will be easily transferred between these respective surfaces. Considering the weak interaction of the inactive, monohydrogenated diamond surface and the adsorbates, we only chose the (100) surface as a representative of the three low-index (100), (111), and (110) surfaces to explore the mechanism of the interaction between the monohydrogenated diamond surface and the F4-TCNQ molecule. Hydrogen atoms were used to passivate the reconstructed diamond (100) (C(100)-2×1) surface. After removing hydrogen atoms at selected surface areas, the enhanced reactivity of the resulting dangling bonds can be used to produce templates. The surface not fully covered by H atoms (defined as partially hydrogenated C(100)-2×1 surface in this work) could be produced by photon irradiation, atomic hydrogen exposure of the clean surface,17 or selective desorption of H from an entirely hydrogenated C(100)-2×1 surface using a scanning tunneling microscope.18-20 Thus, in addition to the F4-TCNQ adsorption on the entirely hydrogenated C(100)-2×1 surface, the study on the partially hydrogenated C(100)-2×1 surface is also of scientific significance. 2. Computational Details Our calculations were performed under generalized gradient approximation (GGA)21 in the form of Perdew, Burke, and Ernzerhof (PBE),22 which is implemented in the SIESTA
10.1021/jp9000187 CCC: $40.75 2009 American Chemical Society Published on Web 04/23/2009
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Figure 1. The entirely hydrogenated C(100)-2×1 surface, and the F4-TCNQ molecule and its geometric parameters (bond length in Å). Possible adsorption models of F4-TCNQ on the entirely hydrogenated C(100)-2×1 surface (M1-M7). Values in parentheses are the calculated binding energies from the DFTB method.
method.23-25 Nonlocal pseudopotentials that were constructed by using the Trouiller and Martins scheme26-28 were used to describe core electrons, while a localized linear combination of numerical atomic orbital basis sets was adopted to describe valence electrons. A double ζ plus polarization function was used for all the atoms. The numerical integrals were determined by using a real space grid with an equivalent cutoff of 100 Ry. Considering that the interaction system of the hydrogenated C(100)-2×1 surface and the F4-TCNQ contains a large number of atoms, we first employed the density functional-based tightbinding (DFTB) method29-33 to obtain the stable structures, then used the SIESTA method for further finer optimization. In the SIESTA calculations, atomic coordinates were optimized by utilizing a conjugate gradient algorithm until each component of the stress tensors was below 0.5 GPa and the atomic forces were under 0.04 eV/Å, and a 6 × 6 × 1 Monkhorst-Pack k-point grid was adopted. In modeling the hydrogenated C(100)-2×1 surface, a supercell was employed. The supercell included a slab consisting of five carbon atomic layers with 24 C atoms per layer and one hydrogen atomic layer with 48 H atoms to saturate the bottom dangling bonds, plus 24 H atoms for the entirely hydrogenated C(100)-2×1 surface, or 24 - n (n is an integer less than 24) H atoms for the partially hydrogenated C(100)-2×1 surface on the top diamond surface (Figure 1). A slab of diamond with infinite extent in the horizontal direction was formed. The slab was separated by vacuum gaps with a width of at least 10 Å in the vertical direction. All atoms were allowed to freely move during geometrical optimization by using the DFTB method. After relaxation, the three bottom layers of C atoms and a bottom layer of H atoms were fixed in space in the subsequent calculations. The calculated lattice constant of the diamond determined by the DFTB method is 3.562 Å, which is consistent with the experimental value of 3.567 Å.34 The F4-TCNQ molecule contains N and F atoms and a large π bonding that allowed it to form N · · · H-C, F · · · H-C, or π · · · H-C hydrogen bonding with the top H atoms of the entirely hydrogenated C(100)-2×1 surface. We considered many possible adsorption models (M1 to M7) (Figure 1) in which F4TCNQ is parallel or perpendicular to the diamond surface. The
calculated binding energies (values in parentheses) for different adsorption systems from the DFTB method show that parallel adsorptions are more favorable than perpendicular ones, because N and F atoms and π electrons of F4-TCNQ can interact with more H atoms anchored on the diamond surface. For stable parallel adsorption models M1 to M3, the largest difference between their binding energies is not greater than 0.02 eV. Thus we conclude that the F4-TCNQ molecule can freely move over the diamond surface, because the entirely hydrogenated C(100)2×1 surface is full of H atoms and the π electrons of the F4TCNQ molecule disperse throughout the whole molecule. In the following calculations, we only selected the M1 model as a representative to investigate the geometric and electronic structures of the adsorption systems using the SIESTA method. Similarly, in the adsorption of F4-TCNQ molecules on the partially hydrogenated C(100)-2×1 surface, we first used the DFTB method to obtain the stable structures of the adsorption systems. Then starting from these structures, we chose several models as representative samples for further optimization and study of electronic properties using the SIESTA code. The binding energy, Eb, was calculated by using the following equation
Eb ) E(doped diamond) - E(F4-TCNQ) - E(diamond) where E(doped diamond) is the total energy of the system for F4-TCNQ doped on the diamond surface, and E(F4-TCNQ) and E(diamond) are the energies of the isolated F4-TCNQ molecule and the pure hydrogenated C(100)-2×1 surface, respectively. 3. Results and Discussion 3.1. Hydrogen Bonding Interaction of F4-TCNQ and the Entirely Hydrogenated C(100)-2×1 Surface. 3.1.1. Geometric Structures. The most characteristic property of the dehydrogenated C(100)-2×1 surface is the dimer bond of 1.397 Å. When every carbon atom at the surface is saturated by one H atom, the monohydrogenated C-C dimer bond is 1.602 Å (slightly larger than a single C-C bond in a hydrocarbon molecule (1.55 Å)), which is in agreement with other theoretical results35,36 and the value of 1.60 ( 0.05 Å determined by low-energy electron
Adsorption of F4-TCNQ on C(100)-2×1 Surfaces
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Figure 2. Charge difference for the M1 model. Red shows electron gaining and green shows electron losing.
TABLE 1: Calculated Charge (in electrons) of the Separate Surface Atomic Layers of the Stable Systems no. of electron charges after adsorption
models
before adsorption
M1
M2
M3
F4-TCNQ 4F 4 C≡N groups surface H layer first C layer second C layer third C layer
0 -0.7 -0.2 +1.6 -2.4 +0.4 +0.6
-0.5 -0.8 -0.4 +2.0 -2.4 +0.3 +0.6
-0.5 -0.8 -0.4 +2.0 -2.4 +0.3 +0.6
-0.5 -0.8 -0.4 +2.0 -2.4 +0.3 +0.6
diffraction. A distinct feature of the structure is the buckling of the atomic layer, being consistent with other calculated and experimental results.35,37 When F4-TCNQ is adsorbed on the entirely hydrogenated C(100)-2×1 surface, its whole geometric structure bends toward the surface. Bending the structure is invoked by the interaction of the terminated N or F atoms of F4-TCNQ and the top H atoms of the diamond surface, which also involve slight change of the corresponding bond lengths of F4-TCNQ (Figure 1). In the M1 model, the distance from the center of F4-TCNQ and the diamond surface is 2.36 Å, and the closest distance from F4-TCNQ to the diamond surface is 2.26 Å between the N and H atoms. This distance is significantly shorter, by 18%, than the sum of van der Waals radii of 2.75 Å, implying the formation of N · · · H-C and π · · · H-C hydrogen bonds. For other possible adsorption models, such as M1-M3, N · · · H-C, F · · · H-C, and π · · · H-C hydrogen bonding is also formed between the diamond surface and F4-TCNQ. These account for the structure after the interaction of the entirely hydrogenated C(100)-2×1 surface and F4-TCNQ, and can provide the supplement to photoemission spectroscopy (PES) of the N 1s and C 1s core levels.10 3.1.2. Charge Redistribution. We obtained the charge difference of the M1 adsorption system by subtracting the charge densities of the entirely hydrogenated C(100)-2×1 surface and F4-TCNQ from the M1, as shown in Figure 2. The F4-TCNQ molecule extracts electrons from the top H atoms of the diamond. Experimentally, Qi10 attributed the obtained electrons of F4-TCNQ to its CtN groups using PES analysis of the N 1s core level. Our Mulliken analysis from the DFTB method involving self-consistent-charge calculations (Table 1) shows that the F4-TCNQ extracts more than 0.5 electron from the
diamond surface. These electrons mostly originate from the top H atom of the diamond. Almost a half of the 0.5 electron is transferred to four CtN groups while the other half of electron is transferred to the other part of the F4-TCNQ, leaving holes in the diamond behind. Other atomic layers, except the top H layer of the diamond, have no obvious charge gain or loss. Therefore, charge redistribution confirms that only the atoms at the surface layer would contribute to the increase of conductivity of the diamond surface after the adsorption of F4TCNQ, supporting the experimental results.10 In the adsorption system, since the electrons at the surface region are transferred from the diamond surface to the dopant F4-TCNQ, the work function of the system should be increased. Note that in the experiment the work function of the diamond surface increased with the deposition thickness of the F4-TCNQ molecule by about 1.5 eV.10 3.1.3. Analysis of Partial Density of States. Further insight into the mechanism of charge transfer between the diamond and F4-TCNQ could be obtained from the analysis of the partial density of states (PDOS) of the system. The entirely hydrogenated C(100)-2×1 surface has a band gap of 3.8 eV, being in agreement with the calculated results of other authors.11,35 Like the Furthmu¨ller’s PDOS result,11 at the top region of the valence band there is no state from the top H atoms of the diamond surface, but there are states from the C atomic layers (Figure 3a). When the F4-TCNQ is deposited on the diamond surface (M1 model), the Fermi level moves toward the valence band, and new states from the LUMO of F4-TCNQ appear at 0 eV (Figure 3c), which ensures that electrons are transferred from the diamond to F4-TCNQ. Hence it is expected that the conductivity of the diamond surface increases after the adsorption of F4-TCNQ, which is coincident with the experimental result.10 It has been reported10 that the surface transfer doping moved the VBM to the energy level of 0.2 eV above the Fermi level at the diamond surface region, thus, the estimated hole areal density in diamond was about 1.6 × 1013 cm-2. Our results, as shown in Figure 3c, obtained by the Mulliken analysis signify that about 0.5 electron from the diamond fills up the LUMO of F4-TCNQ, which give raise to the hole areal density of about 5 × 1013 cm-2 in the diamond surface. These theoretical findings are also consistent with the experimental data. For a better understanding of the distribution of the DOS in the vicinity of the Fermi level, we plotted the located density of states (LDOS) (Figure 4) ranging from -0.5 to 0.5 eV and 2.4 to 3.7 eV. At the bottom of the conduction band, electronic density only localizes on F4-TCNQ, unlike the top of the valence band where a diffuse electronic cloud running through the whole adsorption system exists. 3.1.4. Interaction Mechanism. After the calculations about the electrostatic potential of the entirely hydrogenated C(100)2×1 surface and F4-TCNQ, we obtained the corresponding vacuum energy level values and set them to 0 eV. The calculated results show that the energy level value of the LUMO of F4TCNQ is -5.87 eV, which is 2.95 eV lower than the VBM of the entirely hydrogenated C(100)-2×1 surface, which supports the experimental results in ref 10. With regard to the configuration of the energy levels, electrons are driven from diamond to the F4-TCNQ molecule until the VBM of the diamond and the energy level of the LUMO of F4-TCNQ are aligned. The interaction mechanism adopts the usual surface transfer doping of the diamond,6 like the deposition of the fullerenes7,8 and fluorofullerene molecules9 on hydrogenated diamond surfaces.
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Figure 3. PDOS of the entirely hydrogenated C(100)-2×1 surface before (a) and after (c) the adsorption of F4-TCNQ (M1) and DOS of pure F4-TCNQ (b). In all figures Fermi energy is set to 0 eV.
Figure 4. LDOS of the M1 model at the energy range of -0.5 to 0.5 eV (left) corresponding to the top of the valence band and 2.4 to 3.7 eV (right) corresponding to the bottom of the conduction band.
3.2. Chemical Adsorption of F4-TCNQ on the Partially Hydrogenated C(100)-2×1 Surface. Further, the adsorption of F4-TCNQ on the partially hydrogenated C(100)-2×1 surface was studied since this type of defect may be induced in production or post treatment procedures of diamond. The aim of this work is thus to reveal the effect of the adsorption on the geometric and electronic structures of diamond, and study the difference between adsorptions on the partially hydrogenated diamond surface and that on the entirely hydrogenated one. 3.2.1. Geometric Structures. As noted in the Introduction, the partially hydrogenated diamond surface can be obtained by selectively removing H atoms. If a single H atom is removed from an entirely hydrogenated C(100)-2×1 surface, the M8 model is obtained (Figure 5). Removing two adjacent H atoms along the x and y axes gives the M9 and M10 models (Figure 5), respectively, whereas abstraction of four adjacent H atoms along the x axis leads to the M11 model as illustrated in Figure
5. The geometric optimization shows that the functional groups of F4-TCNQ and the bare C atoms at the surface form chemical bonds. The M8 model is endothermic because one H atom separates from the diamond surface and connects with an N atom of the F4-TCNQ molecule. All the others indicated that interactions are exothermic. The bare C-C bonds participating in the bonding at the diamond surface are lengthened. For example, the C-C bond of the M9 model changes from the original 1.399 Å (1.400 Å from the SIESTA calculation) to 1.587 Å (1.607 Å from the SIESTA calculation). The other studied models are characteristic with similar signatures (see Figure 5 for details). As a result, the coplanar character of the F4-TCNQ molecule is abolished by the adsorption of F4-TCNQ on the diamond surface. 3.2.2. Charge Redistribution. The M9 model, due to its stable nature (the binding energy values in parentheses of Figure 5), is a suitable example to demonstrate the charge difference by
Adsorption of F4-TCNQ on C(100)-2×1 Surfaces
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Figure 5. The optimized geometric structures of representative adsorption models M8-M11 of F4-TCNQ on the partially hydrogenated C(100)2×1 surface (bond length in Å). Values in parentheses are the calculated binding energies for different adsorption systems from the DFTB method.
Figure 6. Charge difference for the M9 model. Red shows electron gaining and green shows electron losing.
TABLE 2: Calculated Charges (in electrons) of the Separate Surface Atomic Layers of the Stable Systems no. of electron charges after adsorption
models
before adsorption
M9
M10
F4-TCNQ 4F 4N surface H layer first C layer second C layer third C layer
0 -0.8 -0.7 +1.5 -2.5 +0.6 +0.6
-0.17 -1.0 -1.0 +1.8 -2.5 +0.4 +0.6
-0.5 -0.8 -1.2 +1.6 -2.0 +0.4 +0.5
subtracting the charge densities of the partially hydrogenated C(100)-2×1 surface and F4-TCNQ from the M9, as shown in Figure 6. The bare CdC bonds of the diamond surface and the coplanar six-membered ring of F4-TCNQ are damaged to form two new CsC bonds. Because of weak interactions between the F or N atoms, or π electrons of F4-TCNQ and the top H atoms of the diamond, the top H atoms loses electrons to the F4-TCNQ molecules (see Table 2). So, the electron donation ability of the diamond surface does not change by removing several surface H atoms. Like the adsorption of F4-TCNQ on the entirely hydrogenated diamond surface, the formation of holes at the partially hydrogenated diamond surface results in an increase of the conductivity of the diamond surface. As seen from the M9 mode (Table 2), no electrons are transferred from
the first C layer since newly formed CsC bonds originate in CdC functional groups of F4-TCNQ and the bare CdC bonds at the diamond surface. However, in the M10 model, the first C layer loses many more electrons to F4-TCNQ because of the newly formed CsN bonds (Table 2). 3.2.3. Analysis of Partial Density of States. In this study, the M9 model is again regarded as a suitable representative model, because of its good stability, to analyze the PDOS before and after the adsorption. The pure partially hydrogenated C(100)-2×1 surface for the M9 model has a band gap, 0.9 eV, smaller than that of the entirely hydrogenated C(100)-2×1 surface. This discrepancy is caused by the contribution of the bare carbon atoms at the surface to the bottom region of the conduction band in the range of 0.5 to 1.4 eV (Figure 7a). Meanwhile, new states of the bare C atoms also appear at the top region of the valence band. Similar characteristics were reported for the dehydrogenated C(100)-2×1 surface.38 In analogy to the PDOS of the entirely hydrogenated C(100)-2×1 surface, the states induced by the top H atoms are absent at the top region of the valence band, and the states originating in the C atomic layers are present. Adsorption of a F4-TCNQ molecule on the partially hydrogenated C(100)-2×1 surface leads to moving the Fermi level toward the valence band, and new states from the F4-TCNQ molecule are at 0 eV (Figures 7b). Such a reconstruction of the energy levels then will also increase the conductivity of the partially hydrogenated diamond surface. Figure 7b illustrates that after the bare C atoms of the partially hydrogenated diamond surface are saturated by the F4-TCNQ molecule, their original DOS near the Fermi level decreases, which indicates the strong interaction between the F4-TCNQ and bare C atoms of the diamond surface. Therefore, the original orbital distribution of F4-TCNQ is not observed in the PDOS. The visualized LDOS (Figure 8) in the range of -0.5 to 0.5 eV and 2.0 to 3.0 eV shows that from 2.0 to 3.0 eV of the conduction band, the electronic density only localizes on F4TCNQ. This contrasts with the LDOS ranging from -0.5 to 0.5 eV of the valence band, where a diffuse electronic cloud running through the whole adsorption system is identified. A similar situation is observed at F4-TCNQ adsorption on the entirely hydrogenated C(100)-2×1 surface. 3.2.4. Comparison to the F4-TCNQ Adsorption on the Entirely Hydrogenated C(100)-2×1 Surface. If we compare the F4-TCNQ adsorption on the partially hydrogenated C(100)2×1 surface to that on the entirely hydrogenated C(100)-2×1 surface, the following conclusions could be drawn. First, the former involves a strong chemical interaction besides the very
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Figure 7. DOS of the partially hydrogenated C(100)-2×1 surface before (a) and after (b) (for the M9 model) the adsorption of F4-TCNQ.
Figure 8. LDOS of the M9 model at the energy range of the -0.5 to 0.5 eV (left) corresponding to the top of the valence band and 2.0 to 3.0 eV (right) corresponding to the bottom of the conduction band.
weak hydrogen bonding, while the latter is only a weak hydrogen bonding interaction. Second, the strong chemical adsorption of the former results in the severe breakage of the molecular orbital of F4-TCNQ; however, the latter only causes the energy level shift of the molecular orbital of F4-TCNQ. Finally, both adsorptions will lead to the increase of the diamond surface conductivity because of the charge redistribution. 4. Conclusion The F4-TCNQ molecule inclines to be physiadsorbed on the entirely hydrogenated C(100)-2×1 surface, and it forms hydrogen bonding with the top H atoms of the diamond surface. The greater the number of H atoms of the diamond surface that interact with F4-TCNQ is, the larger the binding energy. For the partially hydrogenated C(100)-2×1 surface, the N atoms or CdC and CtN functional groups of F4-TCNQ directly link with the bare carbon atoms of the surface, forming chemical bonds. The calculated charge difference shows that the adsorptions of F4-TCNQ on the entirely and partially hydrogenated C(100)2×1 surfaces induce electron transfer from the diamond to F4-
TCNQ, leaving holes in the diamond behind, which increases the diamond conductivity. The PDOS after the adsorption of the F4-TCNQ on the entirely and partially hydrogenated C(100)-2×1 surfaces shows that the Fermi level moves toward the valence band, and new states induced by F4-TCNQ are formed at 0 eV. This observation adequately confirms that the effect of the charge transfer between the diamond and F4-TCNQ increases the diamond conductivity. And the bottom of the conduction band only originates from F4-TCNQ, besides the bare C atoms of the partially hydrogenated C(100)-2 × 1 surface. These findings are in agreement with the results of the visualized LDOS analysis in the corresponding range. The VBM of the entirely hydrogenated C(100)-2×1 surface is higher than the energy value of the LUMO of the F4-TCNQ, which drives the electron transfer from the diamond to the F4TCNQ, adopting the usual surface transfer doping of the diamond, like the deposition of the fullerenes and fluorofullerene molecules on hydrogenated diamond surfaces. Acknowledgment. The work described in this paper is supported by CAS-Croucher Funding Scheme for Joint Labo-
Adsorption of F4-TCNQ on C(100)-2×1 Surfaces ratories [project No. 9500004], Research Grants Council of Hong Kong SAR [project No. CitgU 123607], and by the Centre for Applied Computing and Interactive Media (ACIM). References and Notes (1) Landstrass, M. I.; Ravi, K. V. Appl. Phys. Lett. 1989, 55, 975. (2) Maier, F.; Riedel, M.; Mantel, B.; Ristein, J.; Ley, L. Phys. ReV. Lett. 2000, 85, 3472. (3) Larsson, K.; Ristein, J. J. Phys. Chem. B 2005, 109, 10304. (4) Chakrapani, V.; Eaton, S. C.; Anderson, A. B.; Tabib-Azar, M.; Angus, J. C. Electrochem. Solid-State Lett. 2005, 8, E4. (5) Chakrapani, V.; Angus, J. C.; Anderson, A. B.; Wolter, S. D.; Stoner, B. R.; Sumanasekera, G. U. Science 2007, 318, 1424. (6) Ristein, J. Science 2006, 313, 1057. (7) Strobel, P.; Riedel, M.; Ristein, J.; Ley, L. Nature (London) 2004, 430, 439. ¨ berg, S.; Briddon, P. R. J. Mater. Sci.: (8) Sque, S. J.; Jones, R.; O Mater. Electron. 2006, 17, 459. (9) Strobel, P.; Riedel, M.; Ristein, J.; Ley, L.; Boltalina, O. Diamond Relat. Mater. 2005, 14, 451. (10) Qi, D.; Chen, W.; Gao, X.; Wang, L.; Chen, S.; Loh, K. P.; Wee, A. T. S. J. Am. Chem. Soc. 2007, 129, 8084. (11) Furthmu¨ller, J.; Hafner, J.; Kresse, G. Phys. ReV. B 1996, 53, 7334. (12) Himpsel, F. J.; Knapp, J. A.; VanVechten, J. A.; Eastman, D. E. Phys. ReV. B 1979, 20, 624. (13) Zhang, Z.; Wensell, M.; Bernholc, J. Phys. ReV. B 1995, 51, 5291. (14) Gao, W. Y.; Kahn, A. Org. Electron. 2002, 3, 53. (15) Koch, N.; Duhm, S.; Rabe, J. P.; Vollmer, A.; Johnson, R. L. Phys. ReV. Lett. 2005, 95, 4. (16) Lu, J.; Nagase, S.; Yu, D.; Ye, H.; Han, R.; Gao, Z.; Zhang, S.; Peng, L. Phys. ReV. Lett. 2004, 93, 116804. (17) Bobrov, K.; Comtet, G.; Dujardin, G.; Hellner, L.; Bergonzo, P.; Mer, C. Phys. ReV. B 2001, 63, 165421.
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