Conference-Handai Nano 2007-Atom-by-Atom Chemical Coordination Effect Observed in Noncontact AFM Topography of Pb / Si ( 111 )-( √ 3 × √ 3 ) Mosaic Phase

Using noncontact atomic force microscopy (NC-AFM), we analyzed the height distribution of individual atoms by atomically resolved topography of Pb/Si(111)-( √ 3× √ 3) mosaic phase surfaces, discriminated Pb, and substituted Si adatoms atom-by-atom. This successfully revealed the atom-by-atom chemical coordination effect in the height distribution of individual atoms. We found that increasing the number of surrounding nearest-neighbor Si adatoms increases the height of Si adatoms but rapidly decreases the height of Pb adatoms. These coordination effects were qualitatively independent of the tip–sample surface distance. This result agreed with previous results on Sn/Si(111)-( √ 3 × √ 3) mosaic phase for only Si adatoms. Pb adatoms, however, showed a strong but opposite dependence on the number of surrounding Si adatoms, and the Sn adatom proved independent of the number of surrounding Si adatoms. Such differences in the chemical coordination in NC-AFM height distribution among the fourth period elements of Si, Sn, and Pb atoms will disclose to differences in charge transfer and related phenomena between intermixed heterogeneous atoms, due to differences in electronegativity and/or their metallic nature. [DOI: 10.1380/ejssnt.2008.79]

Many surface core-level shifts [29][30][31][32] and STM [32][33][34][35][36][37][38][39] experiments have been performed for investigating the large charge transfer effects on (Pb or Sn)/Si(111) and (Pb or Sn)/Ge(111) surfaces. Especially, Jemander et al. quantitatively disclosed the chemical coordination effect in the height distribution of individual Sn atoms on a-Sn/Si(111)-( √ 3 × √ 3) surface as a function of the number of surrounding Si atoms using STM [38]. Then Kaminski et al. presented a quantitative theoretical analysis of the atomic and electronic structure, and also STM images of Sn atoms with surrounding Sisubstitutional defects on a-Sn/Si(111)-( √ 3 × √ 3) surface using a first-principles density functional theory (DFT) * This paper was presented at Handai Nanoscience and Nanotechnology International Symposium ( [18]. Strong chemical coordination effects in Si adatom height have been reported, with a decrease of about 10 pm for each increase in the number of surrounding Sn adatoms. In contrast to STM [38,39], however, no appreciable chemical coordination effect in height of a Sn adatom coexisting with substituted Si adatoms has been reported. For NC-AFM and STM measurements, bond order in the atomic force interaction, and local electronic density of state in the tunneling current, can be changed by the charge transfer between heterogeneous adatoms, respectively. Therefore, in the present study, we investigated the chemical coordination effects in Pb adatom height (another fourth period element) and the coexisting Si adatom height, using the Pb/Si(111)-( √ 3 × √ 3) mosaic phase with NC-AFM.

II. EXPERIMENTAL
We used a home-built, room-temperature (RT) ultrahigh-vacuum (UHV) NC-AFM, equipped with an optical interferometer for detecting cantilever deflection. Using NC-AFM, the frequency shift of the mechanical resonance of the cantilever, due to the interaction force between the tip apex and sample surface atoms, is detected by frequency modulation (FM) detection [40]. Contamination and the oxide layer of the Si tip were carefully removed by Ar + ion sputtering in UHV. The nominal values of the spring constant, mechanical resonance frequency, and tipapex radius of the cantilever were ca. 35 N/m, ca. 160 kHz, and 510 nm, respectively. The Pb/Si(111)-( √ 3× √ 3) mosaic phase surfaces were prepared as follows. After the Si(111)-(7 × 7) surface was prepared, Pb was evaporated on the surface, followed by annealing at 500 • C.

III. RESULTS AND DISCUSSIONS
To investigate the chemical coordination effects in NC-AFM topography, we first intermixed Pb and Si adatoms, as shown in Figs Here, we set the z position of the Pb adatom with three nearest-neighbor Si adatoms as a base line (zero relative height). As shown in Figs. 2(b)-(d), at the short tip-sample surface distance, the distributions of Pb and Si adatom relative heights did not separate into two groups. However, at the relatively large distance shown in Fig. 2(a), the relative height distribution separated into two groups, with a nearly equal number of atoms in each group, consistent with the mosaic phase, where the number of Pb and Si atoms is nearly equal. Further, by increasing the Pb content, we  Fig. 2(a), at a relatively large distance, the relative height distribution separated into two groups with a nearly equal number of atoms, consistent with the mosaic phase. Further, we found an increase in the number of brighter spots with higher corrugation by increasing the Pb contents. Therefore, we determined the group with higher corrugation in Fig. 2(a) (red) to be Pb adatoms, while the group with lower corrugation (black) was Si adatoms.
found an increase in the number of the brighter spots with higher corrugation [41]. Therefore, we determined the group with higher corrugation in Fig. 2(a) (red) to be Pb adatoms, while the other group with lower corrugation in Fig. 2(a) (black) were Si adatoms. We attributed the individual adatoms in Figs. 2(b)-(d) to Pb (red) and Si (black) adatoms by comparing the corresponding adatom sites in Fig. 1(a) one-by-one with those in Figs. 1(b)-(d), for the same region. Figures 2(a)-(d) clearly show that decreasing the tip-sample surface distance causes the relative height distribution between Pb and Si adatoms to overlap. This means that decreasing the tip-sample surface distance causes the chemical contrast between Pb and Si adatoms to vanish. This agrees with the previous experimental result on Sn/Si(111)-( √ 3 × √ 3) surfaces [18]. We investigated the chemical coordination effects by plotting the relative height of Pb (Si) adatoms as a function of the surrounding nearest-neighbor Si (Pb) adatoms, as shown in Figs. 3(a)-(d). Figures 3(a)-(d) clearly show that increasing the number of nearest-neighbor Si adatoms increases the relative height of Si adatoms, while that of Pb adatoms decreases rapidly. In other words, by increasing the number of nearest-neighbor heterogeneous adatoms, the relative height of both Si and Pb adatoms decreases. This result agreed with previous results of Sn/Si(111)-( √ 3 × √ 3) mosaic phase surfaces [18], as shown in Fig. 4, for Si adatoms. Pb adatoms, however, show a strong but opposite dependence on the number of surrounding Si adatoms. Further, the relative height of the Sn adatom is nearly independent of the number of surrounding Si adatoms, within the experimental error bar, as shown in Fig. 4. Precisely, Fig. 4    the original experimental result [18]. We investigated the reproducibility of Fig. 4 and concluded that within the experimental error bar, the relative height of the Sn adatom is nearly independent of the number of surrounding Si adatoms. A possible driving mechanism of chemical coordination in NC-AFM height distribution is the charge transfer effect due to differences in electronegativity (1.90 for Si, 1.96 for Sn, 2.33 for Pb) [42] and/or the metallic nature, which induces orbital rehybridization [37] and changes the bond order for the atomic force interaction between the tip apex and sample surface atoms. Further, the charge transfer may be compensated through additional back bonding (rebonding) [37]. Thus, the several mechanisms seem to compete with each other. Therefore, ab initio calculation based on the first principle theory is crucial to understand the origin of atom-by-atom chemical coordination in NC-AFM height distribution, dependent on fourth period elements such as Si, Sn, and Pb adatoms.

is different from
At a relatively large distance, the relaxation of tipapex and sample surface atoms [19] is negligibly small. At a short distance, however, the relaxation of tip-apex and sample surface atoms becomes very large because of the strong short-range interaction force. As shown in Figs. 3(a)-(d), however, the relaxation effect does not change the qualitative feature of chemical coordination in NC-AFM height distribution, even at a short distance, but only degrades and eliminates the chemical contrast, as shown in the relative heights in Figs. 2(a)-(d) and 3(a)-(d). We confirmed a similar tip-sample surface dependence of chemical contrast [18] and chemical coordination in NC-AFM height distribution on the Sn/Si(111)- Si adatom with two nearest-neighbor Si adatoms (four nearest-neighbor Pb adatoms) in Pb/Si(111)-( √ 3 × √ 3) mosaic phase surface, by first principle ab initio calculation. From Figs. 3(a)-(d), we experimentally deduced the corresponding relative height differences in NC-AFM topography between Pb and Si adatoms as ca. 0.37Å, at a relatively large distance ( Fig. 3(a)) and ca. 0.26Åat a short distance (Figs. 3(b)-(d)). Therefore, the relative height differences in NC-AFM topography obtained at the relatively large distance are nearly equal to the corresponding theoretical value of atomic structure, within experimental error. Strictly speaking, however, the value obtained by NC-AFM topography is different from that of atomic structure. Therefore, ab initio calculation based on first-principle theory for NC-AFM topography, including the chemical coordination effect, is also needed.

IV. CONCLUSIONS
Pb/Si(111)-( √ 3 × √ 3) and Sn/Si(111)-( √ 3 × √ 3) mosaic phase surfaces have intermixed Pb (Sn) and Si adatoms and order preferentially into kinked lines, hence are called a "mosaic phase." Pb/Si(111)-( 3) pure phase surfaces, however, ideally have no Si adatoms. In mosaic phase surfaces, therefore, substituted heterogeneous Si adatoms will change the atom height of intermixed Pb (Sn) adatoms and also the Si adatom itself, through orbital rehybridization by charge transfer between heterogeneous adatoms, and in return, through rebonding by additional back bonding. For NC-AFM and STM measurements, bond order in the atomic force interaction, and local electronic density of state in the tunneling current, can be changed by the charge transfer between heterogeneous adatoms, respectively, and also affects the atomic corrugation height. Therefore, such atom-by-atom chemical coordination effects in NC-AFM and STM measurements can directly reveal the structural and electronic-structures of individual atoms surrounded by substituted heterogeneous atoms. Until now, however, the dependence of the relative height of individual atoms on chemical coordination, as a function of the number of surrounding nearest-neighbor atoms, was reported only for Sn/Si(111)-( √ 3× √ 3) mosaic phase surfaces using NC-AFM. In the present experiment, we carefully investigated the atom-by-atom chemical coordination effect of Pb/Si(111)-( √ 3 × √ 3) mosaic phase surfaces using NC-AFM, and compared them with the previous result on Sn/Si(111)-( √ 3× √ 3) mosaic phase surfaces. We found that increasing the number of surrounding nearest-neighbor Si adatoms increases the height of Si adatoms, but rapidly decreases the height of Pb adatoms. These coordination effects were qualitatively independent of the tip-sample surface distance. This result agreed with previous results on Sn/Si(111)-( √ 3 × √ 3) mosaic phase surfaces for Si adatoms. Pb adatoms, however, showed a strong but opposite dependence on the number of surrounding Si adatoms, and was different than the Sn adatom, which was independent of the number of surrounding Si adatoms.

Acknowledgments
This work was supported by The Grant-in-Aid for Scientific Research (19053006, 17101003, 18860046, & 19360017) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), Japan Science and Technology Agency (JST), Handai FRC, 21th Century COE and Global COE programs, and The project "Atomic Technology" funded by MEXT. A part of work of A.O. was supported by the Advance Communication Expert Training Program (adopted in the Initiatives for Attractive Education in Graduate Schools). The work of Y.S. is supported by The Frontier Research Base for Global Young Researchers, Osaka University through the Program of Promotion of Environmental Improvement to Enhance Young Researchers Independence, and "Special-Coordination Funds for Promoting Science and Technology", from MEXT.