J. Phys. Chem. B 1998, 102, 2403-2405
2403
Differentiating between H and F or H and CN on C(111) or Si(111) Surfaces Charles W. Bauschlicher, Jr.* STC-230-3, NASA Ames Research Center, Moffett Field, California 94035
Marzio Rosi Department of Chemistry, UniVersity of Perugia, I-06100, Perugia, Italy ReceiVed: October 28, 1997; In Final Form: January 21, 1998
Molecule-surface interaction energies are computed at the B3LYP level of theory. The C(111) and Si(111) surfaces, with H, F, or CN covalently bonded to the surface, are studied. The incoming molecule simulates the tip of a probe that should be able to differentiate between the atoms or molecules on the surface. A Sc-tipped probe molecule yields a larger difference for the probe-surface H versus probe-surface F interaction energies than our previously studied, electron-rich pyridine (C5H5N) and (CH3)3PO probes. However, it is not always possible to differentiate between the surface H and F atoms because the Sc probe interacts too strongly with the neighboring surface atoms. The difference in the probe-H and probe-F interaction energies is smaller for Si(111) than C(111), making it more difficult to differentiate between these two atoms on Si(111). The larger lattice constant for Si(111) significantly reduces the surface atom-surface atom interaction energy as well as the probe-neighbor interaction energies. This means that the H/CN system, which is not practical for C(111) due to the CN-CN repulsion, is possible for Si(111). The difference in the probe-H and probe-CN interaction energies is very large for the H/CN data storage system, making this the best system studied to date.
1. Introduction We recently proposed1,2 a system to store computer data using covalently bonded atoms (or molecules) on a diamond(111) surface; we used H and F as the two data atoms. We envisioned reading these data with an atomic force microscope (AFM) where the sensitivity to H versus F was enhanced by bonding the appropriate probe molecule to the tip of the AFM. In our first study,1 we used only a 1-D model of the surface and considered probe molecules with an electron-deficient boron atom or with an electron-rich (lone-pair) atom at the tip. As expected, the boron-tipped probe was attracted to F and repelled by H, while the inverse was true for the lone-pair probe molecules. However, the differences in the probe-data atom interaction energies for H and F were too small to “read” the data for the boron-tipped probe molecules. For pyridine (C5H5N) and (CH3)3PO, the differences in the probe-F and probe-H interaction energies were sufficiently large that reading the data appeared possible. Thus only these lone-pair probes were considered in a 2-D model of the surface. This second study2 showed that the probe-nearest neighbor interaction energies were sufficiently large that the difference between the weakest probe-H attraction and weakest probe-F repulsion was sufficiently small that correctly reading the data might be very difficult. Clearly this is unacceptable; it must be possible to differentiate between F and H, regardless of the type of neighboring data atoms. In previous work2 we also considered H versus CN as a storage mechanism, but the neighbor CNCN interaction was sufficiently large that this system was not practical. We should note that we have performed some studies3 on writing the data. This involves removing an H atom with an electric field, as demonstrated by Avouris and co-workers,4 and
subsequently depositing a F atom at the radical site. Additional work is required to make this practical, but the work to date suggests that it should be possible to selectively replace H atoms with other species. In this work, we extend our studies to Si(111), where the nearest neighbor distance is larger than for diamond (2.352 versus 1.5445 Å) and thus the probe-neighbor interaction is smaller. We reconsider H versus CN, since the CN-CN repulsion will be decreased on Si relative to C, since the Si lattice constant is larger. In addition, we consider a probe molecule with a Sc tip atom, to see if a transition metal tip will lead to a larger difference in the interaction energies between the probe and the two data atoms. This also allows us to check if the probe-neighbor interaction problem is smaller for electron-deficient tips than for lone-pair tips. 2. Model Our model of the surface is shown in Figure 1; it contains seven surface atoms (data storage sites) and six subsurface atoms. The dangling bonds on the sides and bottom of the cluster are tied off with H atoms. The probe molecules considered in this work are pyridine and (CH3)3PO, which are those used in our previous studies,1,2 and ScC4H7, which is shown in Figure 2. The dangling bonds, where the probe molecules are covalently bonded to the AFM tip, are tied off by H atoms, as is done for the surface model. For pyridine, this involves the three H atoms at the top (see Figure 1), while for (CH3)3PO and ScC4H7 all C-H bonds are simulating bonds to the AFM tip. 3. Methods The hybrid5 B3LYP6 approach is used in conjunction with the 6-31G* basis set7 for H, C, N, F, and Si. For Sc we use a
S1089-5647(97)03474-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/10/1998
2404 J. Phys. Chem. B, Vol. 102, No. 13, 1998
Bauschlicher and Rosi TABLE 1: Summary of B3LYP/6-31G* Results for the ScC4H7 Probe and Diamond(111). Interaction Energies Are in kcal/mol central H
central F
neighbors
ra
E
r
E
∆E
6H 6F
3.68 3.61
2.6 19.0
3.68 3.68
18.8 30.4
16.2 11.4
a
The optimal Sc-surface C distance, in Å.
The interaction energy of the probe molecule with a data atom is computed as a function of the probe-surface distance, with all other parameters frozen. The probe molecule approaches the central data atom with the C2 axis of pyridine and with the C3 axes of (CH3)3PO and ScC4H7 aligned parallel to the surface-data atom bond. 4. Results and Discussion
Figure 1. C13H20F2 model of diamond(111) surface with a central H data atom, four neighboring H data atoms, and two F (the two filled circles) neighboring data atoms. A pyridine probe molecule is approaching the central surface H atom. The structure of Si(111) is similar, but the Si-Si distance is significantly (2.352 vs 1.5445 Å) larger than the C-C distance.
Figure 2. Sc-tipped probe molecule.
(14s11p6d)/[8s6p4d] contraction of the Wachters8 plus Hay9 basis set. Previous work showed1 that increasing the size of the basis set or substituting the MP2 approach10 for the B3LYP approach did not significantly affect the results. All calculations are performed using Gaussian 94.11 The geometries of the probe molecule and surface model are fully optimized. As discussed previously,2 fully optimizing the cluster eliminates the possibility of some input errors. In addition, a large distortion of the cluster probably indicates a poor model of the surface. Comparing the fully optimized surface model with an idealized structure yields the strength of the data atom-data atom nearest neighbor interaction.
4.1. Sc-Tipped Probe with Diamond(111) Surface. Sc has a 3d14s2 occupation and can therefore form three covalent bonds, which are the Sc-C bonds in our probe. The empty 3d orbitals on Sc should form a dative bond with the lone pairs on F. The Sc-F interaction energy should be much larger than the previously studied boron-tipped probe. The calculations confirm this expectation; see Table 1. The Sc tip is attracted to H, but by much less than for F. While the Sc-F interaction is much stronger than the Sc-H interaction, the H atom is much smaller than the F atom, and as a result, the Sc-surface distance is very similar for both data atoms. The probe-neighbor interaction is an even more serious problem than found for the lonepair probes; the probe-H interaction energy with six F neighbors is actually larger than the probe-F interaction energy for six H neighbors. Thus we conclude that while the Sc tip increases the difference in the interaction energy between H and F, the probe-neighbor interaction problem is as severe as for the lone-pair probes. 4.2. Si Surface. Table 2 summarizes our work for the Si(111) surface. We also include our previous results2 for C(111) for comparison. We first note that the differences in the probe-H and probe-F interaction energies are smaller for Si(111) than C(111). This means that it will be even more difficult to differentiate between H and F on Si(111) than C(111) using pyridine or (CH3)3PO as the probe molecule. While this appears to make Si(111) less interesting than C(111), the calculations show that the neighboring data atoms have a smaller effect on the surface-probe interaction energy, due to the larger lattice constance of Si than C. We next consider using H and CN to store the data. We fully optimized the Si13H15(CN)7 cluster. This represents the Si(111) surface with a CN molecule at the central data site surrounded by six CN molecules at the neighboring data sites. We then form an idealized Si13H15(CN)7 cluster, where we start from the optimal structure for the Si13H21CN cluster (i.e., CN at the central data site) and replace the six neighboring H atoms with CN molecules, which are perpendicular to the surface (parallel to the central CN) with Si-C and C-N distances taken from the Si13H21CN cluster. The idealized structure is 4.8 kcal/ mol above the fully optimized cluster. For C(111) the idealized structure was 145 kcal/mol above the fully optimized structure, because the CN-CN repulsion was so large. Clearly the larger lattice for Si has dramatically reduced the CN-CN repulsion. Therefore, while the H/CN system was not practical for C(111), the CN-CN repulsion does not exclude the H/CN storage system for Si(111).
H, F, and CN on C(111) and Si(111) Surfaces
J. Phys. Chem. B, Vol. 102, No. 13, 1998 2405
TABLE 2: Summary of B3LYP/6-31G* Results. Interaction Energies Are in kcal/mol C(111) neighbors
∆E
H
F
∆E
1.57 2.40
-6.62 -4.42
8.19 6.82
0.41 1.20
-3.17 -2.43
3.58 3.63
2.56 -1.59
-12.57 -13.57
15.13 11.98
1.27 1.77
-7.38 -6.40
8.65 8.17
probea
C5H5N 6H 6F (CH3)3PO probeb 6H 6F
Si(111)
F
H
Si(111) neighbors
H
CN
∆E
0.41 3.71c
-112.18 -99.51d
112.59 103.22
1.27 6.10e
-576.38 -491.85
577.65 497.95
a
C5H5N probe 6H 6 CN (CH3)3PO probeb 6H 6 CN
a The surface-N distance is optimized for X H , i.e., an H data 13 22 atom with six surrounding H atoms, and this distance is used in all other calculations. The values are 3.8 Å for C(111) and 4.3 Å for Si(111). b The surface-O distance is optimized for X13H22, i.e., an H data atom with six surrounding H atoms, and this distance is used in all other calculations. The values are 3.5 Å for C(111) and 3.9 Å for Si(111). c The optimal N-Si distance is 3.8 Å, which increases the interaction energy to 4.74 kcal/mol. For the full optimized structure the interaction energy is 5.17 kcal/mol. d The value is -95.18 kcal/ mol for the idealized surface. e The optimal O-Si distance is 3.4 Å, which increases the interaction energy to 8.81 kcal/mol.
The H/CN on Si(111) results are also summarized in Table 2. At the optimal probe-Si distance for a hydrogen data atom, the probe-CN interaction is very repulsive. For pyridine, the ∆E (difference in the probe-data interaction energies) is more than 100 kcal/mol for H/CN versus about 4 kcal/mol for H/F. Even more encouraging is the fact that the effect of the neighbors is a much smaller percentage of ∆E. Replacing the surrounding six H neighbors by six CN neighbors increases the probe-H interaction energies and decreases the probe-CN repulsion. This is consistent with neighboring CN withdrawing charge from the cluster, thus reducing the charge on the central data CN. We also compute the interaction energy of the pyridine probe with the idealized Si13H15(CN)7 cluster and find that the probe-CN repulsion is reduced by 4.33 kcal/mol. While idealizing the cluster brings the neighbor CN molecules closer to the probe, the Mulliken population shows that it also slightly reduces the charge on the data CN molecule and, hence, reduces the repulsion with the probe. Optimizing the surface-N and surface-O distances for central data H surrounded by six CN molecules causes the interaction energies to increase slightly, 3.71 to 4.74 kcal/mol for pyridine and 6.10 to 8.81 kcal/mol for (CH3)3PO; the surface-N and surface-O distances contract somewhat, 4.3 to 3.8 Å for pyridine and 3.9 to 3.4 Å for (CH3)3PO. The studies show that, regardless of any uncertainties in our model, which might affect the results by 5 or 10 kcal/mol, it should be possible to differentiate between H and CN for all choices of neighbors. While we have considered the case of constant height, the energy differences are so large that it seems
clear that the approach of determining the probe-surface height for a constant pressure would also be able to differentiate H and CN regardless of the neighbors. 5. Conclusions The use of a Sc-tipped probe molecule leads to a sizable difference in the probe-H and probe-F interaction energies. In fact the difference is even larger than for the electron-rich probes such as pyridine and (CH3)3PO. Therefore it appears that either electron-rich or electron-poor probes can be used. Unfortunately for C(111) surfaces, both types of probes suffer from a sizable interaction with the neighboring data atoms, making it very difficult or impossible to correctly read the data for all choices of neighboring data atoms. If C(111) is to be used for the surface, it appears that each data site will have to be surrounded with a hydrogen fence so that the probe molecule will always see the same neighboring data atoms. Since the hydrogen fences are shared, this represents using one-quarter of the surface sites for data storage. Using Si(111) instead of C(111) reduces the difference in the probe-H and probe-F interaction energies, making it more difficult to differentiate between H and F. However, the larger lattice constant for Si than C reduces the probe-neighboring data atom interaction and, more importantly, significantly reduces the data atom-data atom interaction. This allows the use of H/CN as a data storage system on Si(111), which was shown to be unusable for C(111). The difference in the probe-H and probe-CN interaction energies is very large, suggesting that this is a much better choice for a data storage system than H/F. We should also note that while H/CN is not viable for C(111) if every surface site is used for data storage, it should work if the H fences are used on C(111). Acknowledgment. This work was done while M.R. held a NATO Fellowship. References and Notes (1) Bauschlicher, C. W.; Ricca, A.; Merkle, R. Nanotechnology 1997, 8, 1. (2) Bauschlicher, C. W.; Rosi, M. Theor. Chem. Acc. 1997, 97, 213. (3) Thu¨mmel, H. T.; Bauschlicher, C. W. J. Phys. Chem. A 1997, 101, 1188. (4) Avouris, Ph.; Walkup, R. W.; Rossi, A. R.; Akpati, H. C.; Nordlander, P.; Shen, T.-C.; Abeln, G. C.; Lyding, J. W. Surf. Sci. 1996, 363, 368. (5) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (6) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (7) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265 and references therein. (8) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (9) Hay, P. J. J. Chem. Phys. 1977, 66, 4377. (10) Pople, J. A.; Binkley, J. S.; Seeger, R. Int. J. Quantum Chem. Symp. 1976, 10, 1. (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; HeadGordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision D.1; Gaussian, Inc.: Pittsburgh, PA, 1995.
