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(38 x 38)

"Additional commensurate structures have been observed, depending on surface conditions.

"Additional commensurate structures have been observed, depending on surface conditions.

for the C60 overlayer on Cu (111), two of which are displaced from each other by one third of the Cu-Cu distance in the (Oil) direction, consistent with a hexagon of C60 located over the hollow site of the Cu (111) substrate, as shown in Fig. 17.18, where the locations of the three surrounding pentagons are indicated [17.86],

Interestingly, the Au (111) surface with a0 = 2.89 A shows a minority phase (2^3 x 2v/3)R30° commensurate structure for the epitaxial C60 monolayer, but C60 on Au (111) shows a majority phase with a large unit cell relative to the substrate (38x38), although having poor lattice matching to the substrate. The unit cell for this majority phase contains (11x11) C60 molecules. A second epitaxial phase [(7/2) x (\/3/2)]R13.90 for C60 adsorption on Ag (111) has also been reported [17.122]; this phase can coexist with the (2V3xV3)R30° structure, depending on the substrate temperature and the film growth conditions. The C60 molecule is more likely to adsorb to specific sites on Ag (111) than on Au (111), indicating a somewhat stronger binding of C60 to the Ag (111) surface. This stronger binding is also confirmed by measurement of the reorientation times of C60 molecules on Ag (111) and Au (111) surfaces [17.88], but thermal measurements indicate that the binding of C60 to Ag (111) is only slightly greater than to Au (111). The structures listed in Table 17.3 for C60 adsorbed on various substrates are clearly only a partial list, since other structures are expected to occur under different growth conditions.

Fig. 17.18. Proposed adsorption model of the Cft0 overlayer on Cu (111), showing a (4 x 4) superlattice and threefold symmetry. The intermolecular CU]~CM distance is 10.2 A, equal to four times the Cu-Cu nearest-neighbor distance (see Table 17.3). Three pentagonal rings located in the upper portion of the CM molecules are shaded for better viewing of the rotational freedom of each C60 molecule. Four distinct domains are shown in (/), (/'), (II), and (II'), where each C^o is adsorbed over a different hollow site in the (4 x 4) superlattice [17.86].

In the Au (111) surface, steps tend to run parallel to the [lTO] direction and perpendicular to the reconstruction lines. The orientation of the steps determines the orientation of the C60 overlayer for the Au (111) substrate and to a lesser degree for Ag (111). In the growth process on noble metal surfaces, the first layer of the growth tends to be completed before C60 molecules are added to the second layer [17.88]. The molecules in the second layer tend to occupy the threefold hollow sites of the first layer. The second layer molecules show little affinity for steps, in contrast with the first layer. The second layer C60 molecules spin rapidly (109/s), while the first layer molecules are tightly bound. By the time the third layer forms, there is little explicit memory of the substrate, as discussed below. Well-ordered C60 overlayers of thickness >2 monolayers can be grown on all three Cu surfaces, but only the Cu (111) substrate yields single-domain epitaxial growth. For ordered C60 growth, elevated substrate temperatures (~300°C) are necessary [17.51], The surface energy for C60 on Cu (111) is much lower than that for Cu (100) and Cu (110).

When a gold overlayer was deposited on a single monolayer of C60 on Au (111), clustering on top of the C60 molecules was observed by STM [17.123]. However, when an Au overlayer was deposited on two layers of C60, then the Au diffused under the second monolayer to form 2D clusters, resulting in a granular structure for the second C60 layer.

Interesting charge transfer studies were also carried out using the EELS technique (see §17.2) for C50 on Au (110) surfaces [17.55], By monitoring the EELS peaks in the 1-5 eV range, in the carbon Is core excitation spectra, and using high-resolution EELS to measure the vibrational spectra, a charge transfer of 1.3 ± 0.7e from the Au (110) surface to the C60 was obtained. By depositing a monolayer of K on the Au (110) surface before introducing the C60, the charge transfer was increased to 2.3 ± 0.7e (almost comparable to that observed in K3C60), and when several K layers were deposited on Au (110) before introducing the C60, a value of 5.3 ± 0.7 electrons of charge transfer per C60 molecule was reported (almost comparable to that observed for KgQo) [17.55]. A C60 monolayer on Au (001) is commensurate with the gold substrate, but because of the lattice mismatch, a uniaxial stress along the (110) direction develops. This stress causes the charge density around the C60 molecule to deform, and a deformed charge density is observed in scanning tunneling spectroscopy (STS) data [17.89]. These experiments point toward a way to control charge transfer to adsorbed fullerenes by control of the substrates (with different work functions) prior to the introduction of the fullerenes. These observations also point to the importance of using STM to characterize substrates before and after the deposition of the fullerenes to determine charge transfer.

One unique feature of the field ion scanning tunneling microscopy study of C60 on Cu (111) is the observation of a unique threefold symmetry pattern [17.86], which agrees well with the calculated charge density around the molecule (see §7.1.2). After all the step sites are occupied, two-dimensional C60 islands start to grow with a close-packed arrangement from the step edges toward the upper terraces, eventually forming a monolayer, before the second layer starts to grow; this growth pattern seems to be characteristic for C60 on (111) noble metal surfaces. STM determination of the height of the C60 monolayer on Cu (111) ranges from 4.2 to 5.0 A at a bias voltage between -3 V and +3 V. The monolayer step height is significantly smaller than the height of the second layer (8.2 A), reflecting the difference in the local density of states between the Cu (111) surface and the C60 molecule. Higher-magnification images reveal a threefold symmetry (see Figs. 17.19 and 7.3) for a single C60 molecule on a Cu (111) surface, indicative of an excess of surface states near the surrounding three pentagonal rings and a deficiency of states [indicated by the doughnut shape pattern of Fig. 17.19(a)] over the central hexagon of this cluster. The patterns of Fig. 17.19 suggest that a hexagonal ring of the C60 molecule lies face down on the Cu (111) surface, flanked by three pentagons forming a triangular pattern about this hexagon. If a single carbon atom or the carbon double bond of a C60 molecule were to make contact with the surface [as occurs for an Si (100)(2 x 1) surface], twofold rather than threefold symmetry would be seen [17.119],

17.9.2. C60 on Transition Metal Surfaces

Thus far, only a few studies have been done on the interaction between C60 and transition metal surfaces. Two surfaces which have been studied are W (100) [17.53] and Rh (111) [17.54], The available data on transition metal

Fig. 17.19. Magnified (22 x 22 A) STM images of C60 molecules uniformly adsorbed on the Cu (111) surface at bias voltages of (a) -2.0 V, (b) -0.10 V, and (c) 2.0 V. STM images reveal a strong dependence of the images on the bias voltage, but all three images exhibit the threefold symmetry of the adsorbed layer [17.86],

Fig. 17.19. Magnified (22 x 22 A) STM images of C60 molecules uniformly adsorbed on the Cu (111) surface at bias voltages of (a) -2.0 V, (b) -0.10 V, and (c) 2.0 V. STM images reveal a strong dependence of the images on the bias voltage, but all three images exhibit the threefold symmetry of the adsorbed layer [17.86], surfaces are not yet sufficient for forming general conclusions, although relatively strong binding seems to occur between C60 and transition metal substrates, as is the case for the noble metal surfaces, and no tendency for exohedral intercalation of transition metals into crystalline phase C^ interstices is observed.

Photoemission studies of C^ on W (100) surfaces provide evidence for electron charge donation from the metal surface to C60 to form C60 anions, as for the case of C60 monolayers on noble metal surfaces. Annealing a multilayer C60 film at a temperature as low as 100°C appears to result in sublimation of the C60 overlayers, leading to monolayer C60 coverage on the transition metal surface, consistent with the stronger bonding of Qq to W (100) as compared with C60-C60 bonding. Consistent with the behavior of C60 on noble metal surfaces, it is concluded that the Fermi energy is pinned to the fullerene LUMO level for C60 on the W (100) surface [17.53].

Studies of the interaction between C60 and the Rh (111) surface have been carried out using several complementary measurement techniques, including Auger electron spectroscopy, high-resolution electron energy loss spectroscopy, work function measurements, and ultraviolet photoelectron spectroscopy [17.54]. For Rh (111), the C60-Rh interaction is stronger than the C60-C6() binding, as determined from temperature-programmed desorption studies, although no peaks associated with the C^-Rh interaction could be identified in the ultraviolet photoelectron spectroscopy (UPS) spectra [17.54]. As shown by Auger electron spectroscopy studies, the growth mechanism favors layer-by-layer growth rather than island growth, at least up through 3 ML. However, some spectral features in the optical spectra show upshifts in energy toward EF. These energy upshifts have been interpreted to imply that electrons are transferred from the C60 to the Rh substrate, in contrast to the sign of the charge transfer for W and the noble metals. This sign of the charge transfer is further supported by the blue shift of the vibrational modes of C60, as observed by HREELS [17.54], and decreases in the work function of the Rh (111) surface by 0.35 eV, upon C60 adsorption, although no fundamental explanation has been given for this sign of charge transfer. From the HREELS study, a positive induced dipole moment of 2.7 x 10~3 Cm was deduced.

17.9.3. C60 on Si Substrates

Because of the high density of dangling bonds on Si (111) 7x7 [17.124] and Si (100) 2x1 surfaces [17.125], a considerable amount of charge transfer can be expected for C60 on Si (111) and Si (100) surfaces, and such effects have indeed been observed [17.71,119,126]. STM results for a C60 monolayer on an Si (100) 2x2 surface heated to 330°C show strong bonding between C60

and the Si substrate and show that the preferred adsorption site for the C60 is the trough site surrounded by four Si dimers or eight Si atoms, as shown in Fig. 17.20, without any preference for step edges or defect sites. (Other studies of Qq on Si (111) 7x7 [17.71,90,98] show no preferred absorption sites.) The STM scans show that two different structures form for C60 on Si (100) 2x1, namely centered c(4 x 3) and c(4 x 4) ordered overlayer structures, as shown in Fig. 17.20. For the c(4 x 4) overlayer, the nearest-neighbor C60-C60 distance is 10.90 A (in-plane density of 8.42 x 1013/cm2), where the C6(l-C60 distance is somewhat larger than the 10 A separation in the fee bulk phase [17.127], In contrast, the c(4x3) structure (see Fig. 17.20) has a distance of 11.5 A between A-A molecules and 9.6 A between A-B' molecules, leading to a planar molecular density of 1.13 x 1014/cm2. We note in Fig. 17.20 that the A site has four dangling bonds nearby, while the B' site has eight dangling bonds. In comparison, the C60 density on an fee (111) surface of crystalline C60 is 1.15 x 1014/cm2. The strong bonding of C60 to the Si (100) 2x1 surface leads to strained overlayer structures. These strains are relieved by islandic growth on multiple C60 layers in the Stranski-Krastanov growth regime with C60-C60 distances of 10.4 A. In this islandic C60 growth (see Fig. 17.21), the dominant structures are fee (111) surfaces, which appear clearly after three ML of C60 have grown. The first and second ML of the island image, as obtained by STM, are somewhat

Fig. 17.20. Schematic diagram of the adsorption position of the first layer of Cw on the Si (100) 2x1 surface: (a) this arrangement represents the c(4 x 4) configuration, while (b) represents the c(4 x 3) configuration, and (c) is the side view of (a) and (b) at position A. Small open circles are the bulk Si atoms, hatched and solid circles are buckled Si surface dimers, with the solid circles representing the upper and the hatched circles representing the lower Si atoms in the dimers. The large open circles are C60 molecules [17.126],

snA :nA

Fig. 17.21. (a) STM image of a five-layer C^ island showing the close-packed fee (111) surface, (b) An enlarged STM image of the top layer of the surface shown in (a), reveals uniform brightness of individual molecules (tip voltage and current being Vs = —3.5 V, I, = 20 pA) [17.126], snA :nA

Fig. 17.21. (a) STM image of a five-layer C^ island showing the close-packed fee (111) surface, (b) An enlarged STM image of the top layer of the surface shown in (a), reveals uniform brightness of individual molecules (tip voltage and current being Vs = —3.5 V, I, = 20 pA) [17.126], disordered. Typical defect structures are vacancies and screw dislocations [17.126].

The local electron density of states as measured by STM shows internal structures consisting of four stripes running normal to the dimer rows [17.126], in agreement with calculations of the density of states at the HOMO level [17.128] for the c(4 x 3) structure. These calculations further show that the Fermi level is shifted toward the LUMO band because of charge transfer from the Si surface. However, overall the strong coupling between C60 and the Si (100) 2x1 substrate does not result in major changes in the overall electronic structure, but predominantly affects the Fermi level, similar to observations for K doping of C60. The literature also cites various unsuccessful attempts to grow ordered films of C60 on Si (100) 2 x 1 [17.129] and on Si (111) 7x7 surfaces [17.90,98,119].

Single-crystal domains of C60 on clean Si (111) 7 x 7 surfaces have in fact been achieved [17.99] by careful growth of C60 on heated (~200°C) substrates. We show in Fig. 17.22 schematic diagrams of the two crystalline orientations of C60 on Si (111) 7x7, each making an angle of ±11° between the C60 [110] axis and the Si [211] axis. These two phases become stabilized after a sufficient number of C60 layers have been deposited on a clean Si (111) substrate [17.99], In the ordered phases that are shown in Fig. 17.22, the C60 molecules favor locations at either the corner hole site or bridge sites of the Si (111) 7 x 7 structure as shown in Fig. 17.22(c). Also shown in the figure are the related preferred C60 sites on the same Si (111) 7x7 surface for submonolayer coverage.

CeoMIO]

CeoMIO]

corner hole site

/ \ preferred sites for submonolayer coverage

Fig. 17.22. Top: (a) and (b) show the proposed double domain interfacial structure for CM crystals grown on Si (111) 7x7. Bottom: (c) shows the two basic configurations for the proposed interfacial structure beneath Cm crystals and also shows the preferred bonding sites for CM on Si (111) 7 x 7 at submonolayer coverage [17.99].

corner hole site

/ \ preferred sites for submonolayer coverage

Fig. 17.22. Top: (a) and (b) show the proposed double domain interfacial structure for CM crystals grown on Si (111) 7x7. Bottom: (c) shows the two basic configurations for the proposed interfacial structure beneath Cm crystals and also shows the preferred bonding sites for CM on Si (111) 7 x 7 at submonolayer coverage [17.99].

When potassium is evaporated on a CM) film grown on an Si (100) 2x1 surface, no change is found in the STM image. However, the STS spectra for the K-doped C60 film show an appreciable density of states near the Fermi level, consistent with the XPS measurements of alkali metal-doped Cm [17.91,92],

Auger electron spectroscopy studies (§17.3) show that the number of binding sites for C60 is about twice as high on an Si (111) 7x7 surface, as compared to an Si (100) 2 x 1 surface. The C(272 eV)/Si(90 eV) Auger peak height ratio increases in the 620-1070 K temperature range, indicative of the opening of the C60 cage; this cage opening occurs at a lower temperature for Si (111) 7x7 than for Si (100) 2x1, indicative of the greater activity of the Si (111) 7 x 7 surface [17.71], The opening of the C60 cage has been successfully imaged by STM for C60 on Si (111) 7x7 [17.71],

For a GaAs (110) substrate, STM studies show that C60 adsorption occurs initially at surface steps and because of the large number of inequivalent structures formed at 300 K, a complex surface structure is observed. Multilayer growth shows close-packed layer sequencing. At 450 K a uniform first layer forms, followed by thermodynamically favored growth in the (111) direction [17.91]. Smooth, well-ordered surfaces were preserved upon potassium doping of C60 to K3C60 [17.91], The surfaces for the K3C60-doped sample appeared to be conducting based on I-V measurements, and the surface ordering was found to be consistent with potassium in tetrahedral and octahedral sites. The absence of potassium-related STM features also provides evidence for complete charge transfer in K3C60. Additional potassium incorporation eventually led to a nonmetallic overlayer, with a great deal of disorder, consistent with a crystalline transformation from an fee to a bcc structure and with a high density of grain boundaries and defects [17.91],

17.9.5. Higher-Mass Fullerenes on Various Substrates

Soon after the discovery of C60 in the solid phase, STM experiments on fullerenes on gold surfaces were undertaken, showing images with dimensions comparable to those of fullerenes [17.130]. Although no distinction could be made between the images of C60 and C70 in this early work, an image was reported with a diameter of ~1.5 nm (possibly associated with a spherical giant fullerene C„c corresponding to nc ~ 180-240 carbon atoms). Subsequently, STM studies on deposits characterized by mass spectra, which had a high content of higher-mass fullerenes, showed large spherical objects in the 10-20 Á range, suggestive of C„c with 60 < nc < 330 carbon atoms [17.131].

Experimental STM results show that good-quality C70 thin films can be grown on an Si (100) 2x1 surface at a substrate temperature of 120°C. The behavior of C70 on the Si (100) 2x1 surface is similar to that of C60, showing strong bonding of the first monolayer and weaker bonding of subsequent C70 layers. The first strongly bonded layer is stable up to 1000°C, but reacts at higher temperatures to form SiC (100) 3x2 islands, which are desorbed for T > 1300°C, leaving a rough substrate surface behind [17.132]. Band calculations show that the band dispersion curves for C70 on Si (100) 2 x 1 are similar to those of bulk crystalline C70, at least much more so than for the corresponding case of C60 on Si (100) 2x1 [17.128]. One early report of C70 on GaAs (110) shows that C70 favors growth initiation near a substrate step. No intramolecular features could be resolved, indicating that the molecules are rotating near room temperature [17.133], The characteristics of the growth of C70 and other higher-mass fullerenes on GaAs (110) are summarized in Table 17.4.

Several studies of C84 on semiconductor substrates have also been reported [17.126,134], Cg4 can easily be distinguished from C60 by STM because of its 10% larger size. The binding of C84 to an Si (100) 2x1 substrate is at the trough sites just as for C60 (see Fig. 17.20), with a C84-C84 distance of ~14 A at low coverage and with less local ordering than for C60 because of the larger misfit between the size of C84 and an Si (100) 2x1 unit cell [17.126]. At room temperature, no ordered structure for C84 on Si (100) 2x1 has yet been reported. However, in the 100-150°C range, an ordered monolayer can be observed with a C84-C84 nearest-neighbor distance of 12.1 A and a lattice constant of 17.1 A [17.126]. Unlike the case of C60, individual C84 molecules appear with different brightness on an Si (100) 2x1 surface, with height differences of ~ 1.2 A between the lowest and highest imaged C84 molecules, indicative of the presence of more than one C84 isomer. In fact, there are 24 different isomers of C84 satisfying the isolated pentagon rule (see §3.2) [17.134], with each isomer expected to bind differently to Si (100). A preliminary identification has been made of the observed internal structure of the C84 isomer #22 (using the notation of Ref. [17.134]) with D2 symmetry, which was identified as the dominant isomer present in the STM images [17.126],

By analyzing the geometrical relation between two C84 domains on an Si (100) surface, it is concluded that the island growth of C84 on Si (100) 2x 1 is

Table 17.4

Characteristics of (4 x 2) island structure of C„ fullerenes on GaAs (110) substrates [17.133],

Table 17.4

Characteristics of (4 x 2) island structure of C„ fullerenes on GaAs (110) substrates [17.133],

"c

Monolayer height (Â)

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