J. Phys. Chem. B 2007, 111, 11929-11934
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Switchable Semiconductive Property of the Polyhydroxylated Metallofullerene Jun Tang,†,§ Gengmei Xing,† Yuliang Zhao,*,†,| Long Jing,† Hui Yuan,† Feng Zhao,† Xueyun Gao,† Haijie Qian,‡ Run Su,‡ Kurash Ibrahim,‡ Weiguo Chu,⊥,| Lina Zhang,⊥ and Katsumi Tanigaki§ Lab for Bio-EnVironmental Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China; Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, The Chinese Academy of Sciences, Beijing 100049, China; Tsinghua-Foxconn Nanotechnology Research Center, Beijing 100084, China; National Center for Nanosciences and Technology of China, Beijing 100080, China; and Department of Physics, Graduate School of Science, Tohoku UniVersity, Aoba Aramaki Aoba-ku, Sendai, Miyagi 980-8578, Japan ReceiVed: June 21, 2007; In Final Form: August 1, 2007
The temperature-sensitive property of polyhydroxylated metallofullerene film of Gd@C82(OH)x with special hydroxyl number was studied using synchrotron radiation ultraviolet photoelectron spectroscopy (UPS) and TEM techniques. From room temperature (RT) to 4 °C the photoelectron onset energy of the spectra of Gd@C82(OH)12 shifted from 1.9 to 0.2 eV, indicating that Gd@C82(OH)12 automatically shifted from insulator at RT to semiconductor at 4 °C. However, this could not be observed for Gd@C82(OH)20. TEM experiments show that the variation of conductivity can be ascribed to formation of a microcrystal under low temperature. The dipole moment induced unique intermolecular interactions and self-assembled microcrystalline structures for Gd@C82(OH)12. This may cause reconstruction of the upper valence band formed by π-like electrons as well as the density of states (DOS) around the Fermi level (EF) and reconstruct the deeper valence band formed by σ-like electrons, eventually resulting in a shift to a semiconducting nature. These findings revealed a novel nature for polyhydroxylated Gd@C82(OH)x materials: Their insulating properties can be controllably tuned into semiconducting ones as a function of temperature.
Introduction Endohedral metallofullerenes, as well as their special structural and electronic properties,1-6 have attracted much research interest due to their promising applications in the fields of nanomaterial and biomedical sciences.7-10 The properties of fullerenes, such as electronic and magnetic properties, etc., are mainly controlled by the electron distribution around the EF. Therefore, investigation of the upper valence band around the EF level is indeed necessary and significant. For fullerenes/ metallofullerenes the upper valence band from EF to 4.5 eV is due to π-like (pseudo-π-like) electrons.11-13 UPS is known to be a very sensitive tool to probe the difference of DOS near the Fermi edge by which the electronic properties of valence band around 0-10 eV of fullerenes or metllofullerenes have widely been investigated.11-19 However, most of the practical applications are their derivatives generated by chemical coatings/ modifications of the fullerene or metllofullerene, not fullerenes or metllofullerenes themselves. There are thousands of fullerene derivatives reported so far, but almost no study was designed to investigate how their valence-band electronic properties change with the added groups on the fullerene cage surface. This is probably due to the difficulty of preparing pure samples of the fullerene derivatives, for example, Gd@C82(OH)x, an * To whom correspondence should be addressed. Phone/Fax: +86-108823-3191. Email:
[email protected]. † Lab for Bio-Environmental Effects of Nanomaterials and Nanosafety, Chinese Academy of Sciences. ‡ Beijing Synchrotron Radiation Facility, Chinese Academy of Sciences. ⊥ Tsinghua-Foxconn Nanotechnology Research Center. | National Center for Nanosciences and Technology of China. § Tohoku University.
important metallofullerene derivative showing many promising biomedicine applications.9,10,20-23 By investigating the electronic properties of the metal atoms encaged in the metallofullerenol Gd@C82(OH)x via X-ray photoelectron spectroscopy we reported that the energy levels of the encaged Gd atom can be tuned by polyhydroxylation of the outer surface of the fullerene cage.24 When the OH groups in Gd@C82(OH)x reach a certain number, the emission properties of the valence-band electrons show periodical changes depending on how many hydroxyl groups were added to the outer surface of the fullerene cages.25 In this article, we studied the upper valence-band electronic properties of Gd@C82(OH)x by changing x (0, 12, 20) and temperature using UPS and TEM techniques. The results show that the upper valence-band electronic properties around EF can be greatly altered by temperature, and a dramatic transformation from insulating into semiconducting properties of the Gd@C82(OH)x film is induced. This dramatic transformation was only observed at a special given OH number. In this paper, a possible process for this novel and important phenomenon is proposed with a detailed study of microcrystal formation and the structural properties of Gd@C82(OH)x. Experimental Section The metallofullerenes were synthesized using the arc discharge method and have been presented in our previous publications.10,22,24,25,32,37 Briefly, separation and isolation of Gd@C82 were performed using high-performance liquid chromatography (HPLC, LC908-C60, Japan Analytical Industry Co.) coupling with 5PBB and then Buckyprep columns (Nacalai Co. Japan, 20 × 250 mm). The synthesis method for water-soluble
10.1021/jp074863r CCC: $37.00 © 2007 American Chemical Society Published on Web 09/25/2007
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Figure 1. UPS of Gd@C82(OH)x (x ) 0, 12, 20) samples prepared at RT (a1-c1) and 4 °C (a2-c2). The middle column shows the schematic drawings for hydroxyl addition and structural information of metallofullerenols.
Gd-fullerenols was the alkaline reaction at RT, and details are available in our previous publications.24,33 Sephadex G-15 column chromatography with an eluent of neutralized water was used to separate various Gd@C82(OH)x with different hydroxyl numbers. XPS measurements combined with elemental analysis were used to determine the number of hydroxyl groups.24 The Gd@C82 and Gd@C82(OH)x samples were deposited onto the high-purity platinum substrates to obtain thin films for UPS measurements. The UPS spectra were measured using a synchrotron radiation light source at the photoelectron station of the Beijing Synchrotron Radiation Facility, the Chinese Academy of Sciences. Before measurement the samples were placed into the sample chamber of the ultrahigh vacuum chamber with a background pressure of ∼8 × 10-10 Torr long enough to remove the possibly absorbed air or solvent molecules. The experimental resolution was estimated to be 100 meV, and the UPS scan of Au substrate was also carried out for calibration. UPS spectra of the valence band of samples were acquired with an incident photon energy of 32 eV after the samples were stored at RT and 4 °C for 1 week. High-resolution TEM imaging was performed using a Tecnai F20 (FEI com) transmission electron microscope operated at 200 kV. Gd@C82(OH)12 of 0.2 mg/mL aqueous solution was dropped on a grid to form a thin film. Before the TEM experiment the grids were kept at RT and 4 °C for 1 week. Precautions were taken to minimize the heating and irradiation damage on the samples. It is worth noting that the possible presence of water remaining in the sample could influence the measured data. To this end, we did some pretreatment of samples. First, before measurement He ion beam scans on the sample surface were performed to remove any contamination of the sample. Second, to remove the potential influence of impurity (oxygen, water, etc.) absorbed by fullerene cage, samples were degassed by placing them into a vacuumizing sample chamber of 10-8 Torr vacuum for 24 h before measurements. Third, synchrotron radiation X-ray photoelectron spectroscopy was carried out in an ultrahigh vacuum chamber with a background pressure of ∼10-10 Torr during measurement. Using these careful pretreatments the adsorbed water can be removed.
Results and Discussion The UPS spectra of Gd@C82(OH)x (x ) 0, 12, 20) prepared at RT are shown in Figure 1(a1-c1). Gd@C82 was used as the reference because the metallofullerene was known to form a crystalline structure easily when its solution was dried on the substrate.1,13 The shape of the spectrum and the photoelectron onset energy (0.3 eV below EF) of Gd@C82 are similar to the reported results,13 supporting the reliability of the present observations. Compared with the reported data of fullerenes/ metallofullerenes17,26,27 whose valence-band structures were resolved in the region from 1 to 6 eV, only a big hump was observed in the present spectra. Due to the presence of multiple he hydroxyls on the cage surface, different isomers of Gd@C82(OH)x can lead to a relatively lower resolution. Nevertheless, the photoelectron onset energy of Gd@C82 (0.3 eV below EF) agrees well with the earlier result.13 The relatively lower UPS resolution of polyhydroxylated metallofullerene actually will not affect the discussion below. When hydroxyls were introduced onto the Gd@C82 cage surface the ultraviolet photoelectron spectra of Gd@C82(OH)x were greatly changed, although it retains a similar spectrum shape to that of Gd@C82. In the case of x ) 12, the photoelectron onset energy of Gd@C82(OH)12 shifted to 1.9 eV, while when x ) 20, the photoelectron onset energy of Gd@C82(OH)20 became 3.5 eV. When the number of OH changes from 0, to 12, to 20, the photoelectron onset energy of Gd@C82(OH)x shifts systematically from 0.3, to 1.9, to 3.5 eV, respectively. The higher onset energy means a wider band gap between the HOMO and LUMO levels. Supposing EF (set to zero) is located in the middle between the valence band and conduction band, the band gap Eg for Gd@C82(OH)12 and Gd@C82(OH)20 can be estimated to be about 3.8 and 7.0 eV, respectively. More interestingly, when the Gd@C82(OH)x film (samples) was kept at 4 °C for 1 week, the energy level around EF was largely changed (Figure 1(a2-c2)). In particular, the photoelectron onset energy of Gd@C82(OH)12 shifted from 1.9 to 0.2 eV, which is even smaller than that of metallofullerenes
[email protected] The intensity of DOS around EF also increased after low-temperature treatment. This indicates that its insulating properties can be controllably tuned into semiconducting properties as a function of temperature. However, this is not
Switchable Semiconductive Property of Gd@C82(OH)x Film
Figure 2. Temperature-induced variation of photoelectron onset energy of Gd@C82(OH)x (x ) 0, 12, 20).
the case for Gd@C82(OH)20. The photoelectron onset energy of Gd@C82(OH)20 is 3.3 eV, even after the 4 °C treatment, and this is only a slight change of ∼0.2 eV from RT. Figure 2 displays the comparison of the photoelectron onset energy of Gd@C82(OH)x under different temperatures. The photoelectron onset energy shows a nearly linear variation versus the number of OH groups at RT, but this relationship was largely influenced under lower temperature. This revealed that both the OH number and temperature become the key factors which dominate the valence-band properties around EF. It is interesting that the upper valence band of Gd@C82(OH)12 is controllable as a function of temperature. To understand this result we performed further high-resolution TEM studies of Gd@C82(OH)12 and Gd@C82(OH)20. The TEM results of Gd@C82(OH)12 films prepared at RT and 4 °C are displayed in Figure 3. Gd@C82(OH)12 did not show any crystallinity at RT. The inset of Figure 3a shows selected area electron diffraction patterns (SAED) with a hollow ring pattern, which can indicate a noncrystal structure of Gd@C82(OH)12 at RT. After the film was stored at low temperature, 4 °C, Gd@C82(OH)12 became a self-assembled cluster (Figure 3b). Apparently, higher resolution TEM images revealed different phases of the Gd@C82(OH)12 clusters (Figure 3c). We marked these typical phases as I, II, III, and IV, and magnifications of these selected phases are displayed as the insets in Figure 3c. Figure 3c-I shows an outof-order phase, while Figure 3c-II and 3c-III show crystalline structure with clear molecular arrays. More interestingly, Figure 3c-IV displays a perfect molecular pattern that clearly indicates a molecular distortion of Gd@C82(OH)12. It revealed not only a clear microcrystalline structure but also more intensive information for the polycrystalline structure. The distance between the cages is 0.76 nm, as measured by the electrondiffraction pattern (Figure 3d). The diameter of undistorted C82 is nearly 1.1 nm.1 Taking into account the highly distorted structure of the fullerene cage induced by addition of multihydroxyls,28 the observed distance between the cages in Figure 3c is reasonably size consistent. Previous studies of the electronic interactions among encaged Gd atoms, the carbon cage, and the outer OH groups of Gdmetallofullerenols revealed that the encaged metal atom usually has a strong interaction with the carbon cage.7,24,25 The Gd atom partly donates its valence electrons to the cage, resulting in not only the electron density overlap between endohedral metal and the nearest carbon atoms but also appearance of dipole moment.14,29 The existence of an electric dipole moment in metallofullerenes can lead endohedral fullerene to a crystalline structure.1 When metallofullerene Gd@C82 was polyhydroxylated, the hydroxyl groups were asymmetrically added on the C82 cage
J. Phys. Chem. B, Vol. 111, No. 41, 2007 11931 because of the localized distribution of the HOMO frontier orbitals.30,31 The asymmetrical distribution of hydroxyls on the cage surface also leads to a molecular dipole moment that can directly affect formation of crystalline structures. The electronic interactions between the encaged Gd atom and C82 cage as well as those between C82 and OH groups are known to vary with OH number of Gd@C82(OH)x.24 Figure 4a shows a possible schematic structure of a self-assembled pattern of Gd@C82(OH)12 observed in Figure 3. In Gd@C82(OH)12 the localized OH groups would flatten the fullerene cage, leading to an arenelike structure. In Gd@C82(OH)12 the hydroxyl groups were asymmetrically distributed on the Gd@C82 cage due to the localized HOMO distribution.30 On one side of the cage the OH groups serve as the electron donor due to the conjugation effect of the sp2 hybridization with the “aryl” on the cage. The hydroxyl addition is similar to the fluorine addition of fullerene cages, where fluorination resulted in a similar arene-like structure.32 This electronic donation from the OH groups to the C82 cage leads to the following important nature for the Gd@C82(OH)12 molecule: The first is a direct back-donation of electrons from carbon to Gd through the sandwich-type electronic interactions,24 which locally enhances the electronic interactions among the route of Gd-C-OH, resulting in a stronger molecular dipole moment. The second is the more abundant π electrons for a single molecule, which increase the intensity of DOS around EF when they form a crystalline structure as observed in Figure 3b and 3c. On the opposite side of the cage the localized charge distribution leads the OH groups to serve as an electronic acceptor. As illustrated in Figure 4a, neighboring Gd@C82(OH)12 molecules may be self-assembled via an intermolecular hydrogen bond along the dipole moment direction as long as the molecular rotation (which is considered to be weak because of hydroxyl addition on the cage surface and intermolecular interactions between different GdC82(OH)12 molecules) of GdC82(OH)12 themselves can be suppressed by lowering the temperature. When the number of OH groups on the fullerene surface is not too large (e.g., 12 hydroxyls) the cage still holds its conjugation system. When more OH groups (e.g., 20 hydroxyls) were added onto the cage the conjugation system was known to be locally broken by this addition process.25 This will weaken the electronic interactions between the metal atom and the cage as well as that between the cage and hydroxyls and hence decrease the intensity of the π valence band around EF. On the other hand, this structure shift will also lead to a weaker electric dipole moment among Gd@C82(OH)20 than that of Gd@C82(OH)12. Hence, it becomes difficult to generate a strong enough intermolecular interaction between neighboring Gd@C82(OH)20 by means of a hydrogen bond; they could not form a regular molecular array as illustrated in Figure 4b. This inference is supported by the present experimental observations. In Figure 5 TEM results for Gd@C82(OH)20 at RT and 4 °C clearly indicate that no crystalline structures were formed. These are also consistent with the results of UPS observations of Gd@C82(OH)20 (Figure 1(c1) and 1(c2)). The intermolecular force of Gd@C82(OH)12 molecules is negligible due to the strong thermal motion at RT. However, the thermal motion is largely suppressed at lower temperature, and therefore, the dipole interaction overcomes the thermal motion of Gd@C82(OH)12 and can induce a molecular selfassembly process, i.e., gradual formation of the microcrystalline structure. Because of the enhanced dipolar moment of Gd@C82(OH)12, its microcrystalline structure should be much more closely packed than that of Gd@C82. Formation of the regular
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Figure 3. TEM image of Gd@C82(OH)12 film. (a) TEM image of Gd@C82(OH)12 sample prepared at RT. (Inset) Microdiffraction patterns of the microfilm. (b) TEM micrograph of Gd@C82(OH)12 sample stored at 4 °C for 1 week. (c) TEM image of the same microfilm with higher magnification of a selected area. (Insets) Magnified images of the I, II, III, and IV areas. (d) Corresponding electron diffraction pattern suggesting formation of microcrystalline structure in Gd@C82(OH)12 microfilm.
Figure 4. Schematic drawings for formation of crystalline structure of Gd@C82(OH)12 (a) and Gd@C82(OH)20 (b) due to intermolecular interactions. The Gd@C82(OH)12 molecules self-assemble along the dipole moment direction. The combined pattern of Gd@C82(OH)12 molecules is similar to the molecular arrays shown in the inset image. Gd@C82(OH)20 molecules can only form out-of-order cluster due to the lower dipole moment which weakened the intermolecular interactions.
and closely packed geometric structure of the microcrystal consequently alters the valence band. First, the upper valence band formed by π-like electrons was reconstructed, which would enhance the intensity of DOS around EF. Second, the deeper valence band of σ-like electrons ranging from 4.5 to ∼10.0 eV was reconstructed, which could cause a splitting of the σ-valence band (Figure 1b). In the case of Gd@C82(OH)20, dispersion of hydroxyl groups leads to a depolarization of the cage. The intermolecular force becomes weak, and hence it is difficult to overcome the thermal motion of the cage. The small intermolecular force was demonstrated by the fact that the valence band of Gd@C82(OH)20 was less split (Figure 1(c2)) as compared to that of Gd@C82(OH)12 (Figure 1(b2)). Hence, it is reasonable to speculate that the small intermolecular force of Gd@C82-
(OH)20 cannot induce molecular self-assembling to form a microcrystalline structure like that of Gd@C82(OH)12,. This is consistent with the UPS results (Figure 1) and supported by TEM results (Figures 3 and 5). It should be noted that when the hydroxyl number in Gd@C82(OH)x is largely changed, their structural, chemical, and physical properties are largely altered.24,25,33 Accordingly, it may give only averaged information when one studies the properties of a mixed sample of Gd@C82(OH)x. It was difficult to isolate a Gd@C82(OH)x that contains only a specific OH number using the previous separation techniques. In the processes of addition chemistry, because of the multiple π bonds on the cage surface, many hydroxyls are simultaneously added onto the cage surface. This generates Gd@C82(OH)x containing different OH num-
Switchable Semiconductive Property of Gd@C82(OH)x Film
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Figure 5. TEM image of Gd@C82(OH)20 prepared at RT (a) and after being stored at 4 °C for 1 week (b). (Insets) Microdiffraction patterns of the microfilm.
bers.23,34-36 In our experiments we placed a lot of effort on the separation processes to obtain Gd@C82(OH)x having a fixed OH number as narrow as possible.10,21,22,24,25,33,37 First, we used highperformance liquid chromatographic techniques with various types of columns and mobile phases, but it was difficult to isolate the anticipated species from the synthesized mixtures of Gd@C82(OH)x. The effective way we ultimately found for this preparation was use of Sephadex G-15 chromatography with a long enough column and neutralized water as the eluent. It still cannot yield Gd@C82(OH)x having only a fixed OH number. Finally, we were only able to obtain the final Gd@C82(OH)x product with a distribution of OH number varying with (2. Although with the current separation techniques a fixed number of the hydroxyls could not be accurately obtained, an OH number ranging within x ( 2 is the narrowest one we could achieve for Gd@C82(OH)x so far. As Gd@C82(OH)12 and Gd@C82(OH)20 exhibited an evident difference in XAS, TEM, and XPS results, these demonstrated that (i) the separating degree of the Gd@C82(OH)x mixture into Gd@C82(OH)12 and Gd@C82(OH)20 is adequate and (ii) this OH range of (2 should not have a significant influence on the present measurement and discussions. This was certified by elemental analysis as well as X-ray photoemission spectroscopy.22,24,25,33 To the best of our knowledge, this is the narrowest x range that has been so far obtained for Gd@C82(OH)x. Nevertheless, it is still not a pure molecule for Gd@C82(OH)12. This would be the reason that we could not obtain a monolayer film or single crystal for Gd@C82(OH)12 (Figure 4). Gd@C82(OH)10 and Gd@C82(OH)14 mixed with Gd@C82(OH)12 make it difficult to generate a complete microcrystal. Conclusion We found that Gd@C82(OH)12 showed an insulator nature with a larger onset energy of 1.9 eV at RT. From RT to 4 °C the onset energy shifted from 1.9 to 0.2 eV, which is even smaller than that of the semiconductive Gd@C82. The band gap of Gd@C82(OH)12 can be controllably shifted from the insulated to semiconductive state as temperature changes, but this nature could not be observed in Gd@C82(OH)20. The stronger electric dipole moment induced unique intermolecular interactions and self-assembled microcrystalline structures, which may cause reconstruction of the upper valence band formed by π-like electrons and hence the deeper valence band formed by σ-like electrons, while in Gd@C82(OH)20 the dispersion of hydroxyl groups led to a depolarization of the cage. The weaker electric dipole moment caused the weaker intermolecular force, and then
it was difficult to overcome the thermal motion of the cage and we could not induce the molecular self-assembling to form microcrystalline structures. This is consistent with the fact that the upper valence-band structure and electronic properties of Gd@C82(OH)20 were less sensitive to temperature. The present findings revealed novel functions for polyhydroxylated metallofullerenes: Their insulating or semiconducting nature can be controlled and regulated by temperature. Acknowledgment. The authors acknowledge support from MOST (2006CB705601 and 2007CB935604), the National Natural Science Foundation of China (10525524, 20571076, 10675141), and the Chinese Academy of Sciences (Applications of Nanotechnology in Some Important Fields, KJCX-NM, 2007). References and Notes (1) Beyers, R.; Kiang, C.-H.; Johnson, R. D.; Salem, J. R.; Vries, M. S. d.; Yannoni, C. S.; Bethune, D. S.; Dorn, H. C.; Burbank, P.; Harich, K.; Stevenson, S. Nature 1994, 370, 196. (2) Kodama, T.; Ozawa, N.; Miyake, Y.; Sakaguchi, K.; Nishikawa, H.; Ikemoto, I.; Kikuchi, K.; Achiba, Y. J. Am. Chem. Soc. 2002, 124, 1452. (3) Masashi, S.; Kana, S.; Ryuichiro, M.; Masafumi, A. Phys. ReV. B 2003, 68, 235414. (4) Tsuchiya, T.; Wakahara, T.; Maeda, Y.; Akasaka, T.; Waelchli, M.; Kato, T.; Okubo, H.; Mizorogi, N.; Kobayashi, K.; Nagase, S. Angew. Chem. 2005, 117, 3346. (5) Nishibori, E.; Iwata, K.; Sakata, M.; Takata, M.; Tanaka, H.; Kato, H.; Shinohara, H. Phys. ReV. B 2004, 69, 113412. (6) Iiduka, Y.; Wakahara, T.; Nakahodo, T.; Tsuchiya, T.; Sakuraba, A.; Maeda, Y.; Akasaka, T.; Yoza, K.; Horn, E.; Kato, T.; Liu, M. T. H.; Mizorogi, N.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 2005, 127, 12500. (7) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843. (8) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abrun˜a, H. D.; McEuen, P. L.; Ralph, D. C. Nature 2002, 417, 722. (9) Bolskar, R. D.; Benedetto, A. F.; Husebo, L. O.; Price, R. E.; Jackson, E. F.; Wallace, S.; Wilson, L. J.; Alford, J. M. J. Am. Chem. Soc. 2003, 125, 5471. (10) Chen, C.; Xing, G.; Wang, J.; Zhao, Y.; Li, B.; Tang, J.; Jia, G.; Wang, T.; Sun, J.; Xing, L.; Yuan, H.; Gao, Y.; Meng, H.; Chen, Z.; Zhao, F.; Chai, Z.; Fang, X. Nano Lett. 2005, 5, 2050. (11) Hino, S.; Umishita, K.; Iwasaki, K.; Miyazaki, T.; Kikuchi, K.; Achiba, Y. Phys. ReV. B 1996, 53, 7496. (12) Iwasaki, K.; Umishita, K.; Hino, S.; Miyamae, T.; Kikuchi, K.; Achiba, Y. Phys. ReV. B 1999, 60, 5044. (13) Hino, S.; Umishita, K.; Iwasaki, K.; Miyazaki, T.; Miyamae, T.; Kikuchi, K.; Achiba, Y. Chem. Phys. Lett. 1997, 281, 115. (14) Hino, S.; Takahashi, H.; Iwasaki, K.; Matsumoto, K.; Miyazaki, T.; Hasegawa, S.; Kikuchi, K.; Achiba, Y. Phys. ReV. Lett. 1993, 71, 4261. (15) Hino, S.; Umishita, K.; Iwasaki, K.; Miyamae, T.; Inakuma, M.; Shinohara, H. Chem. Phys. Lett. 1999, 300, 145.
11934 J. Phys. Chem. B, Vol. 111, No. 41, 2007 (16) Hino, S.; Wanita, N.; Iwasaki, K.; Yoshimura, D.; Ozawa, N.; Kodama, T.; Sakaguchi, K.; Nishikawa, H.; Ikemoto, I.; Kikuchi, K. Synth. Met. 2005, 152, 357. (17) Hino, S.; Wanita, N.; Iwasaki, K.; Yoshimura, D.; Ozawa, N.; Kodama, T.; Sakaguchi, K.; Nishikawa, H.; Ikemoto, I.; Kikuchi, K. Chem. Phys. Lett. 2005, 402, 217. (18) Hino, S.; Wanita, N.; Kato, M.; Iwasaki, K.; Yoshimura, D.; Inoue, T.; Okazaki, T.; Shinohara, H. J. Electron Spectrosc. 2005, 239, 144-147. (19) Hino, S.; Wanita, N.; Iwasaki, K.; Yoshimura, D.; Akachi, T.; Inoue, T.; Ito, Y.; Sugai, T.; Shinohara, H. Phys. ReV. B 2005, 72, 195424. (20) Sitharaman, B.; Bolskar, R. D.; Rusakova, I.; Wilson, L. J. Nano Lett. 2004, 4, 2373. (21) Wang, J. X.; Chen, C. Y.; Li, B.; Yu, H. W.; Zhao, Y. L.; Sun, J.; Li, Y. F.; Xing, G. M.; Yuan, H.; Tang, J.; Chen, Z.; Meng, H.; Gao, Y. X.; Ye, C.; Chai, Z.; Zhu, C. F.; Ma, B. C.; Fang, X. H.; Wan, L. J. Biochem. Pharmacol. 2006, 71, 872. (22) Qu, L.; Cao, W.; Xing, G.; Zhang, J.; Yuan, H.; Tang, J.; Cheng, Y.; Zhang, B.; Zhao, Y.; Lei, H. J. Alloys Compd. 2006, 400, 408-412 (23) Mikawa, M.; Kato, H.; Okumura, M.; Narazaki, M.; Kanazawa, Y.; Miwa, N.; Shinohara, H. Bioconjugate Chem. 2001, 12, 510. (24) Tang, J.; Xing, G.; Yuan, H.; Cao, W.; Jing, L.; Gao, X.; Qu, L.; Cheng, Y.; Ye, C.; Zhao, Y.; Chai, Z.; Ibrahim, K.; Qian, H.; Su, R. J. Phys. Chem. B 2005, 109, 8779. (25) Tang, J.; Xing, G.; Zhao, Y.; Jing, L.; Gao, X.; Cheng, Y.; Yuan, H.; Zhao, F.; Chen, Z.; Meng, H.; Zhang, H.; Qian, H.; Su, R.; Ibrahim, K. AdV. Mater. 2006, 18, 1458.
Tang et al. (26) Iwasaki, K.; Umishita, K.; Hino, S.; Miyamae, T.; Kikuchi, K.; Achiba, Y. Phys. ReV. B 1999, 60, 5044. (27) Iwasaki, K.; Wanita, N.; Hino, S.; Yoshimura, D.; Okazaki, T.; Shinohara, H. Chem. Phys. Lett. 2004, 398, 389. (28) Avent, A. G.; Clare, B. W.; Hitchcock, P. B.; Kepert, D. L.; Taylor, R. Chem. Commun. 2002, 2370. (29) Poirier, M. D.; Knupfer, M.; Weaver, J. H.; Andreoni, W.; Laasonen, K.; Parrinello, M.; Bethune, D. S.; Kikuchi, K.; Achiba, Y. Phys. ReV. B 1994, 49, 17403. (30) Senapati, L.; Schrier, J.; Whaley, K. B. Nano Lett. 2004, 4, 2073. (31) Ros, D. T.; Prato, M. Chem. Commun. 1999, 663. (32) Neretin, I. S.; Lyssenko, K. A.; Antipin, M. Y.; Slovokhotov, Y. L.; Boltalina, O. V.; Troshin, P. A.; Lukonin, A. Y.; Sidorov, L. N.; Taylor, R. Angew. Chem., Int. Ed. 2000, 39, 3273. (33) Xing, G.; Zhang, J.; Zhao, Y.; Tang, J.; Zhang, B.; Gao, X.; Yuan, H.; Qu, L.; Cao, W.; Chai, Z.; Ibrahim, K.; Su, R. J. Phys. Chem. B 2004, 108, 11473. (34) Chiang, L. Y.; Upasani, R. B.; Swirczewski, J. W. J. Am. Chem. Soc. 1992, 114, 10154. (35) Chiang, L. Y.; Upasani, R. B.; Swirczewski, J. W.; Soled, S. J. Am. Chem. Soc. 1993, 115, 5453. (36) Chiang, L. Y.; Wang, L. Y.; Tseng, S. M.; Wu, J. S.; Hsieh, K. H. Synth. Met. 1995, 70, 1477. (37) Tang, J.; Xing, G.; Zhao, F.; Yuan, H.; Zhao, Y. J. Nanosci. Nanotechnol. 2007, 7, 1085.
