The co-adsorption of pre- and post-annealed C60 molecules on Si(100)-2x1 surfaces has been investigated using scanning tunneling microscopy (STM) in ultra-high vacuum (UHV). The images reveal that the adsorption characteristics of the post-annealed adsorbates are different from their pre-annealed counterparts. The post-annealed C60 molecules bond either to the top of the dimer rows or to the missing dimer defects, while the pre-annealed molecules predominantly occupy the four-dimer sites in the troughs. The apparent size and height of post-annealed C60 molecules are smaller than those of the pre-annealed ones. Our observations suggest that the post-annealed C60 molecules are chemisorbed on the surface, because the molecules covalently bond to Si atoms during the sample annealing. In contrast, the adsorption of the pre-annealed C60 molecules can be explained by a dipole-induced dipole interaction between the molecules and the Si(100) surface, i.e. physisorption.
The success of synthesizing large quantities of C60 molecules (1) and related fullerene materials has caused a rapid growth of the study of these materials in recent years. Scanning tunneling microscopy, with its ability to probe the real space surface structural and electronic properties, has been used to investigate the adsorption of fullerene molecules and the growth of fullerene films on a variety of solid surfaces (2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17). Silicon surfaces, due to their scientific interest and industrial importance, have been used intensively as substrates for studying fullerene-semiconductor interactions. The adsorption of C60 molecules on Si(100)-2x1 surfaces at room temperature was first studied using STM by Hashizume et al (10). They observed that the C60 molecules were adsorbed in the trough of dimer rows, and suggested two possible bonding sites: one surrounded by four dimers, and the other aligned with two dimers. They proposed that at room temperature, the C60 molecules form strong bonds with the dangling bonds of the Si dimers on the Si(100)-2x1 surface via a charge transfer from the Si to the C60 molecules. Recently, Chen and Sarid(17) studied the C60/Si(100) system at room temperature and after annealing the sample to 900°. They proposed that the adsorbate-substrate interaction at room temperature is characterized by a relatively weak dipole-induced dipole interaction, rather than by charge transfer from the substrate to the molecules. A strong covalent bond between the carbon atoms of the adsorbates and the Si atoms of the substrate is formed only after annealing the sample. However, in their study, the STM images were taken separately before and after annealing. This might have caused some ambiguity since STM images differ under different tips and tip-sample bias.
In this paper, both the pre-annealed and post-annealed C60 molecules are observed on the same sample. It is unambiguously shown that the adsorbate-substrate bonding before and after the annealing process is different. We conclude that the post-annealed C60 molecules chemisorb onto the Si surface, while the pre-annealed C60 molecules are physisorbed.
The experiments were conducted in an ultra-high vacuum chamber with a base pressure of 8.0x 10^-11 torr. Commercial n-type Si(100) wafers (P-doped, 1.0 ohm-cm, Virginia Semiconductor) were used as sample substrates. The samples were heated with electron beam bombardment from the back. The clean Si(100)-2x1 surface was prepared by outgassing the sample for more than 10 h at 950°, heating the sample to 1500° for 2 minutes while keeping the chamber pressure below 2.0 x 10^-9 torr(18) , followed by several minutes at 1200° and finally cooling down over a period of 30 minutes to room temperature. An optical pyrometer was employed to measure the sample temperature. C60 (>99.8%) powder was loaded in a Knudson cell with a BN crucible and outgassed at 570° for more than 24 hours. Submonolayer C60 molecules were deposited on the sample surface via sublimation at 550°, typically for 30 seconds. The coverage was estimated directly from STM images, where one monolayer corresponds to 8.4 x 10^13 molecules per square centimeter(15) . During the C60 deposition, the chamber pressure was maintained below 2.0 x 10^-10 torr. STM measurements were performed in-situ using a modified McAllister STM, equipped with control electronics by Digital Instruments. All images presented in this paper were taken in the constant-current mode using tunneling currents between 0.5 nA and 0.9 nA, and a sample bias between -1.6 V and -3.0 V.
Figure 1 shows an STM image of a Si(100) surface covered with a 0.02 monolayer (ML) of C60 molecules. The deposition was made in two steps. A 0.01 ML of C60 molecules was first deposited on the silicon surface at room temperature, and then annealed to 900° for several seconds. After the sample cooled back to room temperature, the second deposition of a 0.01 ML of C60 was made. This procedure provides both the pre- and post-annealed C60 molecules on the same Si(100) surface. In addition to the two orthogonally oriented domains of Si(100)-2x1 dimer row structures, which are separated by a monatomic step, the image also shows the C60 adsorbates which appear as white spherical protrusions on the surface. Figure 2 is a high resolution STM image showing several isolated C60 adsorbates on the Si(100) surface where individual Si dimers are clearly resolved. By inspecting the adsorbates' bonding positions and measuring their apparent sizes and heights, we found that there are three types of C60 adsorbates with different characteristics, which are labeled in Fig. 1 and Fig. 2 as A, B, and C, respectively. Among these adsorbates, type A molecules have the largest apparent size and height. They are located in the troughs between dimer rows, and are readily moved by the scanning tip (17) . One molecule near the center of Fig. 1 (pointed out by the arrow) shows the tip-induced motion during the scan. In contrast, type C molecules have the smallest apparent size and height. Both type B and type C molecules are adsorbed on top of the dimer rows instead of in-between the dimer rows. It is of interest to note that there is always a defect-like feature on the substrate adjacent to every type C molecule. Also noted is that neither type B nor type C molecules in the scanning area are moved by the scanning probe under a variety of scanning conditions, indicating a much stronger adsorbate-substrate interaction compared to that of type A molecules. According to a previous study (17) , type B and type C molecules can be identified as post-annealed C60 adsorbates and type A molecules as pre-annealed C60 molecules.
Chen and Sarid (17) suggested a dipole-induced dipole interaction between the Si(100) surface and C60 molecules deposited at room temperature. Thermal annealing promotes the C60-Si(100) bonding nature from physisorption to chemisorption. This change in bonding nature is characterized by changes in the bonding locations and changes in the local charge densities of the adsorbates. In this study, we observed both the pre- and post-annealed C60 molecules on the same sample surface proving unambiguously that such an irreversible transition did indeed occur during the thermal annealing process. Also, the high resolution of the STM image (see Fig. 3a) allows us to make a detailed inspection of the symmetry of the adsorption-induced dimer buckling under the type A molecules. As illustrated in Fig. 3a, the C60 adsorption causes buckling of dimers which form c(4 x 2) reconstruction in the vicinity of the adsorbates(17). It is found that the alternatively buckled dimers of the two dimer rows below two type A molecules have a 180° phase shift right below the adsorbate. In other words, for the four dimers under each molecule, two belonging to the same row are buckled in the same direction as shown schematically in Fig. 3b. This result is consistent with the dipole-induced dipole model proposed in previous work (17) . The changes in the occupied local density of states of the post-annealed molecules can be explained in terms of the change in bonding properties caused by the thermal annealing. The thermal annealing drives the C60-Si(100) bonding to a lower energy state where some carbon atoms in the C60 molecules form covalent bonds with the underlying silicon atoms. Because of the formation of these covalent bonds, some electrons, which originally belonged to the highest occupied molecular orbital (HOMO) of a C60 molecule, are now localized at the newly formed covalent bonds so that the charge density of the HOMO decreases, giving rise to the smaller apparent size and height of the type B and C molecules. The height difference between type B (4.7 Å ) and type C (3.3 Å ) molecules can be understood by considering the formation of covalent bonds of type C C60 with the second layer Si atoms at surface defects, such as missing dimer defects. Here, C60 molecules covalently bond to the dangling bonds of the exposed Si atoms on the second layer. The defect-like features adjacent to each type C molecule are believed to be the silicon atom rearrangement caused by the strong bonding between the molecules and the substrate. A detailed analysis of these thermally induced bonding properties will be discussed elsewhere.
The co-adsorption of pre- and post-annealed C60 on Si(100)-2x1 surfaces reveals that the bonding nature of C60 molecules adsorbed at room temperature is different from that observed after annealing to 900°. The pre-annealed adsorption is governed by the dipole-induced dipole physisorption interaction between C60 and the Si(100)-2x1 surface. In contrast, post-annealed C60 molecules chemisorb on top of dimer rows and missing dimer defects, and involve bond breaking and the formation of covalent bonds.
The authors would like to thank D.R. Huffman and L.D. Lamb for providing the C60 samples. This research is supported by the National Science Foundation, the Air Force Office of Scientific Research, and the Ballistic Missile Defense Initiative.
