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Influence of Aromatic Environments on the Physical Properties of β-Carotene Kazuhiro Yanagi,*,†,# Yasumitsu Miyata,‡ Zheng Liu,§ Kazu Suenaga,§ Susumu Okada,|,# and Hiromichi Kataura⊥,# Department of Physics, Tokyo Metropolitan UniVersity, Hachiouji 192-0397, Japan, Department of Chemistry, Nagoya UniVersity, Nagoya 464-8602, Japan, Research Center for AdVanced Carbon Materials, National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba 305-8562, Japan, Institute of Physics, Center for Computational Science, UniVersity of Tsukuba, Tennodai, Tsukuba 305-8571, Japan, Nanotechnology Research Institute, AIST, Tsukuba 305-8565, Japan, and CREST, Japan Science Technology Agency, Saitama 332-0012, Japan ReceiVed: NoVember 5, 2009; ReVised Manuscript ReceiVed: December 29, 2009
The influence of the surrounding aromatic environment on the properties of β-carotene (Car) was investigated for a simple system comprising single-walled carbon nanotubes (SWCNTs) encapsulating Car molecules. Both metallic- and semiconducting-type SWCNTs encapsulating Car were prepared, and their physical properties were investigated using optical measurements and first-principles calculations. The optical absorption peaks of encapsulated Car in metallic and semiconducting SWCNTs were slightly different, which is thought to be caused by the difference in polarizability of the two types of SWCNTs. The Raman frequency of the CdC stretching mode of Car in the metallic SWCNTs was 3 cm-1 down-shifted from that in the semiconducting SWCNTs. This down-shift could not be explained by the difference of dielectric environments of the metallic and the semiconducting SWCNTs. One possible origin for the shift is a difference in the amount of charge on the encapsulated Car, which was supported by theoretical calculations. From the results of this study, it can be concluded that the electronic structure of the nanotube walls influences the properties of encapsulated molecules. Introduction Single-walled carbon nanotubes (SWCNTs) are graphitic tubes with diameters of approximately 1 nm. SWCNTs exhibit metallic or semiconducting behavior depending on their chiralities,1 and their properties have been intensively studied because of their potential applications such as electronic devices and conductive films.2 Since the discovery of fullerene encapsulation by Smith et al.,3 it is known that it is possible to encapsulate various kinds of organic molecules inside the hollow nanospace.4 Encapsulated molecules can influence the physical properties of nanotubes, for example molecular encapsulations affect the band gaps5 and carrier concentrations of nanotubes.6 Therefore, such encapsulation is one approach to controlling the physical properties of SWCNTs.7 In order to develop a method to tune the properties of SWCNTs, it is very important to correctly understand how the encapsulated molecules influence the physical properties of the surrounding nanotubes or vice versa. It is noteworthy that the samples used in previous studies of molecule-SWCNT complexes were usually mixtures of metallic and semiconducting nanotubes, because SWCNTs are always produced in a mixed chirality state. Thus, it has been very difficult to experimentally determine how the electronic type of the nanotubes influences the physical properties of encapsulated molecules. Theoretically, it is expected that such electronic differences will affect the physical properties of * To whom correspondence should be addressed. Tel: +81-42-677-2494. Fax: +81-42-677-2483. E-mail:
[email protected]. † Tokyo Metropolitan University. ‡ Nagoya University. § Research Center for Advanced Carbon Materials, National AIST. | University of Tsukuba. ⊥ Nanotechnology Research Institute, AIST. # CREST.
molecule-SWCNT complexes.8,9 Recently, it has become possible to prepare high-purity metallic or semiconducting SWCNTs.10,11 Therefore, in this study the effect of the electronic type was investigated by preparing metallic or semiconducting SWCNT-encapsulated molecules. In a previous study, we have reported the encapsulation of a carotenoid molecule, β-carotene (Car), inside the nanotubes.12–15 Carotenoids are one-dimensional π-conjugated molecules, and they are important natural pigments found both in photosynthetic organisms and in animals.16,17 In photosynthetic pigment-protein complexes, carotenoids exhibit important functions such as lightharvesting, photoprotection, and structure-stabilization.18 In protein matrixes, carotenoids are surrounded by various types of amino-acid residues and other pigments (as an example, a carotenoid in a reaction center complex is shown in Figure 1).19 Interactions between carotenoids and these surrounding components strongly influence the physical properties of carotenoids. 20,21 For example, polarizability interactions (sometimes called as dispersive interactions) between the carotenoids and surrounding components induce peak shifts in the optical absorption spectra of carotenoids.22 The excited-state properties of carotenoids in vivo are modified (or controlled) by various pigment-pigment and pigment-protein interactions (See review papers by Cogdell20 and Koyama23). In order to elucidate the physical mechanisms governing the functions of carotenoids in biological complexes, fundamental knowledge concerning the influence of the surrounding environment on the properties of carotenoids is of great importance. For that purpose, a number of studies have been performed, and the physical properties of carotenoids have been intensively investigated by systematically changing environmental parameters such as the amount of
10.1021/jp910568k 2010 American Chemical Society Published on Web 01/22/2010
Physical Properties of β-Carotene
Figure 1. Schematic illustrations of a biological complex (a reaction center complex of photosynthetic bacteria, which was drawn with the VMD program based on 1K6L of the protein data bank. Carotenoid and bacteriochlorophylls (Bchl) are drawn in red and green, respectively. Other pigments and amino-acid residues are drawn in white) and a SWCNT (white) encapsulating Car (red), Car@SWCNT. Carotenoids are surrounded by aromatic components in both complexes.
applied pressure,24–26 the types of solvents,22,27–31 and the types of amino-acid residues.32,33 Inside SWCNTs, Car is surrounded by graphitic nanotube walls, which do not have the freedom of movement (such as rotation or bending) that amino acid residues and molecules possess. Thus, inside the nanotubes, we can ignore the influences of such dynamic behavior of the surrounding components on the properties of Car. X-ray diffraction patterns and the lightpolarization dependence of optical absorption spectra have indicated that Car is located at an off-center position inside the nanotubes and aligned along the nanotube-axis.13 Thus Car is assumed to be statically located at van der Waals contacts with the nanotube-walls through π-π interactions between the polyene chain and the nanotube-walls; such a situation is in fact theoretically predicted for the case of polyacetylene.34 Although conformation changes of retinal molecules have been observed during high-resolution transmission electron microscopy (HRTEM) observations, these changes are caused by the electron-beam irradiation,35 and in the absence of such strong perturbations conformation changes will not occur. SWCNTs exhibit different electronic structures depending on their chiralities,1 and this can therefore be used to examine the relationship between the electronic structure of the aromatic environment and the properties of Car located at van der Waals contacts. This in turn may provide insights into the properties of carotenoids surrounded by aromatic components in biological complexes. Therefore, in this study, we tried to elucidate the influence of the electronic structure of the graphitic walls on the properties of carotenoids. Although it is still difficult to prepare SWCNTs with a single chirality, preparation of metallic or semiconducting SWCNTs with a high degree of purity is now possible (for a good review, see Hersam11).10,11,36–38 In this study, Car molecules encapsulated by both metallic and semiconducting SWCNTs were prepared, and the effects on the physical properties of Car was investigated by optical measurements and theoretical calculations. Experimental Section Metallic or semiconducting SWCNT-encapsulated Car complexes (referred to as Car-peapods) were prepared as follows. First, SWCNTs with average diameters of 1.4 nm were produced by a laser-vaporization method. Car was encapsulated inside the SWCNTs in a manner similar to that reported previously.12 Metallic or semiconducting peapods were separated from the
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2525 mixture using density gradient ultracentrifugation (DGU), as first reported for the separation of metal-semiconductor nanotubes by Arnold et al.10 The experimental setup and procedures used are similar to these reported in ref 36. Metallic peapods were obtained in a manner similar to that given in ref 36. However, semiconducting peapods were obtained using the following sample conditions: the surfactants and gradient medium used in the centrifuge tube sample-layer were sodium dodecyl sulfate 2.4%, sodium cholate 0.6%, sodium deoxycholate 0.33%, and iodixanol 40%. For optical measurements, thin films were prepared in a manner similar to that described in ref 36 in order to remove the surfactants and gradient medium from the samples. Metallic and semiconducting SWCNTs without Car were obtained by annealing the metallic and semiconducting Car-peapods at 400 °C for 2 h under high vacuum. The relationship between the annealing processes and the degradation of the encapsulated Car is shown in the Supporting Information. Optical absorption spectra were recorded using a UV-vis-NIR spectrophotometer (Shimadzu, SolidSpec-3700DUV). Raman spectra were measured using a triple monochromator system equipped with a charge-coupled-device detector (Photon Design Co., PDPT3-640S), and an Ar+ laser (Spectra Physics, 2016) operated at 488 nm (2.54 eV). Only spectral shifts larger than 1 cm-1 were considered because of the resolution limit, 0.6 cm-1, of Raman measurements in our experimental setup. All measurements were performed at room temperature. There have been no reports on the preparation of metallic or semiconducting peapods by a density-gradient method, and it was therefore necessary to first determine whether this was possible. For this purpose, C60 was chosen as the encapsulated molecule since it can be easily identified in TEM observations. Therefore, metallic/semiconducting SWCNT-encapsulated C60 (C60-peapods) were also prepared using the same method (see the Supporting Information). C60 was encapsulated in the vapor phase, and metal-semiconductor separations were then performed in a manner similar to that described above. Absorption spectra of the metallic and semiconducting C60 peapods are shown in the Supporting Information. The purity of the metallic and semiconducting peapods was estimated to be more than 95% from the optical absorption spectra. The HRTEM images clearly indicated the presence of the C60 inside these SWCNTs, which confirmed that metallic and semiconducting C60-peapods were successfully prepared. Therefore, it was concluded that this method could also produce metallic or semiconducting Carpeapods. HRTEM measurements on the Car-peapods were carried out at 120 kV using a JEM-2010F equipped with a CEOS postspecimen Cs corrector; the Cs value was set at 5 to 10 µm. The specimens were kept at ambient temperature during observation. A Gatan 894 CCD camera was used for digital recording of the HRTEM images. Different observation conditions were used for the C60- and the Car-peapods. The HRTEM images of the C60-peapod specimens were recorded at the Scherzer defocus under a beam density of about 80 000 electrons/(nm2 second) or 1.27 C/cm2. The HRTEM images of Car-peapods were recorded with an additional underfocus to enhance the contrast of the encapsulated Car molecules with respect to the graphene network of the SWCNTs; in this way the contrast of the SWCNTs could be minimized by fabricating a desirable contrast transfer function. In addition, the beam density was set to about 15 000 electrons/(nm2 second) or 0.24 C/cm2 to minimize electron beam damage to the Car. HRTEM image simulations of Car-peapods were performed using standard multislice procedures on an atomic model of Car that
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Figure 2. Absorption spectra of (a) metallic and (b) semiconducting Car-peapod (Car@SWCNTMetal and Car@SWCNTSemi) thin films, shown as red lines. In both panels, the empty metallic and semiconducting SWCNTs (SWCNTMetal and SWCNTSemi) are also shown as dashed lines. The differential spectra of samples with and without Car are indicated by dotted lines (enlarged by a factor of 3). Carotene bands (filled yellow regions) are clearly identified from the differential spectra. Arrows in the spectra indicate excitation positions in the Raman measurements. Mii and Sii indicate the i-th optical transitions of the metallic and the semiconducting SWCNTs, respectively. The centrifuge tubes used to obtain high purity (c) metallic and (d) semiconducting Car-peapods are also shown. Dotted circles indicate the regions in which high-purity metallic and semiconducting Car-peapod samples were obtained.
was geometrically optimized using density functional (DFT) theory. As shown in the Supporting Information, the simulation results exactly matched the experimental observations, strongly suggesting that the observed molecules were in fact Car. Although further confirmation of the identity of the observed molecules should be a subject for future study, at this moment the HRTEM results are consistent with the presence of Car inside the nanotubes. Total-energy electronic-structure calculations based on DFT were performed on the Car-peapods. The local density approximation was adopted to treat the exchange-correlation energy of the interacting electrons. To describe the electronion interaction, we used norm-conserving pseudopotentials generated by the Troullier-Martins scheme with separable forms. When constructing the pseudopotentials, we adopted 1.5 bohrs for the core radii of both the C 2s and 2p states. The valence wave functions were expanded in terms of the plane-wave basis set with a cutoff energy of 36 Ry. We carried out the calculations for (10,10) and (17,0) nanotubes containing Car molecules to clarify how the electronic structure of the SWCNT affects the physical properties of the resultant peapods. We adopted a supercell model in which a peapod was placed with its nanotube wall separated 6 Å from the wall of an adjacent peapod. We imposed a commensurability condition between the onedimensional periodicity of the atomic arrangement in the nanotube and that of an array of Car. Consequently, the lattice parameters along the tube axis are 34.384 and 34.031 Å for armchair and zigzag nanotubes, respectively, which give sufficient spacing between adjacent Car molecules. Results and Discussion Absorption Spectra. Figure 2 shows absorption spectra of the metallic and semiconducting Car-peapods. The spectra clearly indicate that both metallic and semiconducting SWCNT enriched samples were successfully prepared. The purity of both types of samples was estimated to be more than 95% from their
Yanagi et al.
Figure 3. Raman spectra of (a) metallic (Car@SWCNTMetal) and (b) semiconducting (Car@SWCNTSemi) Car-peapods at 488-nm excitation. The dotted red line indicates the Raman spectra for empty metallic (SWCNTMetal) and semiconducting SWCNTs (SWCNTSemi) around G-band. The intensity of the Raman G-band for empty SWCNTs was normalized to that of the peapods. Peak frequencies of Raman modes, V1, V2, V3, and V4, for encapsulated Car are indicated.
absorption spectra, using the intensity ratio of the absorption bands due to the first optical transition of the metallic (M11) or the semiconducting (S11) SWCNTs.39,40 Absorption spectra of empty metallic and semiconducting SWCNTs without Car are also shown in Figure 2. The empty SWCNTs were obtained by the annealing processes described in the Experimental section. An absorption band (Carotene band) due to encapsulated Car was easily identified from the difference in the spectra between SWCNTs with and without Car. On the basis of previous results,12,13 the weight percent of encapsulated Car was estimated to be approximately 1.5% from the absorbance ratio of the Carotene band and the SWCNT bands. The peak positions of the Carotene bands in the metallic and the semiconducting SWCNTs were estimated to be 2.56 and 2.57 eV, respectively. Here we derived the values from the results of a fitting analysis using Gaussian functions. It is well-known that the peak positions of optical absorption bands are influenced by the dielectric environment through dispersive interactions.22,26–29,31 Therefore, the 10 meV difference of the peak positions is likely to be caused by the difference of the polarizability of the metallic and the semiconducting SWCNTs.41 Vibrational structures, which are clearly observed for Car in nonpolar solvents,17 were not observed in the absorption lineshapes of the Carotene-band, indicating the presence of inhomogeneous distributions of some factors in the Car-peapods. The following two factors are thought to be possible origins for the structureless absorption band: (1) inhomogeneity of Car configurations inside the nanotubes and (2) distribution in the chirality of the nanotubes. These two factors will be discussed in more detail in regard to the Raman measurement results. Raman Spectra. Raman spectra of the metallic and semiconducting Car-peapods are shown in Figure 3. Four Raman peaks due to Car, V1 (CdC stretching), V2 (C-C stretching), V3 (C-H in-plane deformation), and V4 (C-H out of plane wagging),42,43 were easily identified in both types of peapods. In addition, the G-band of SWCNTs was also clearly seen. The radial breathing modes (RBMs) of these peapods are shown in the Supporting Information, and separation of the metallic and semiconducting peapods was confirmed also from the intensities of the RBMs. As shown in previous studies, the anomalously strong V4 mode signal indicates that the Car has a twisted form.12,44 Such a conformation is not stable outside the nanotubes, and is only observed when the Car is constrained by solid
Physical Properties of β-Carotene matrices such as proteins44 or carbon nanotubes.12,15 Thus, the strong signals of the V4 mode indicate that these Raman signals originated from encapsulated Car. The relative intensities of the four Raman modes of Car in the metallic SWCNTs were similar to those in the semiconducting SWCNTs, which indicates similar Car structures in both types of SWCNT.43 However, the intensity ratio of the Raman signals of Car to the G-band of SWCNTs was remarkably different between the metallic and semiconducting peapods. For the metallic Car-peapods, the intensity of the Car V1 mode was significantly larger than that of the SWCNT G-band. However, for the semiconducting Car-peapods, the intensity of the Car stretching mode was smaller than that of the G-band. The initial SWCNTs used to prepare the metallic Car-peapods were the same as those for the semiconducting Car-peapods, with no significant difference in either the average diameters or the filling ratio of Car. Therefore, it is assumed that these characteristics were not the cause of the observed differences in Raman intensity. According to the absorption spectra shown in Figure 2, the semiconducting SWCNTs have strong extinction coefficients in the spectral region of the excitation light, but the metallic SWCNTs do not. Therefore, the incident light could selectively excite the Car through the surrounding SWCNT walls in the metallic SWCNTs. In contrast, for the semiconducting SWCNTs, the excitation light was absorbed by the surrounding semiconducting SWCNTs, and hence the Car was not efficiently photoexcited. Physical Background of the Absorption Line Shape of Car in SWCNTs (Inhomogeneity in Car-Peapods). In the optical absorption spectra of Car-peapods, we suggested two possible reasons for the lack of vibrational structures in the Carotene band: (a) inhomogeneity of the Car structure (b) chiral distribution of SWCNTs. We will first discuss the details of Car configurations from the Raman spectra of Car-peapods with reference to a review paper by Koyama and Fujii.45 The intensity of C10-C11 stretching around 1140 cm-1 of Car in SWCNTs was quite small (it was very difficult to identify the peak), indicating that Car in SWCNTs does not take 7-cis, 9-cis, or 13-cis configurations. Moreover, the Raman peak around 1245 cm-1, which is observed in 15-cis isomers, was not observed in the encapsulated Car; thus, Car is not 15-cis. The relative intensity of the C6-C7 stretching around 1170 cm-1 to the C14-C15 stretching around 1160 cm-1 (ν2 mode) of Car in SWCNTs is similar to that of all-trans, but not to 11-cis. Therefore, the Raman spectra indicate that most of the Car in SWCNTs has an all-trans configuration. Thus, it is difficult to form a conclusion concerning the inhomogeneity of Car structures. However, as mentioned above, the intensity of the ν4 Raman mode is large, indicating that the Car has a twisted form preserving an all-trans configuration. Therefore, differences in the degree of twisting might be one of the origins of the inhomogeneity. A more probable origin for the inhomogeneity is a distribution in the chirality of the metallic or semiconducting nanotubes. Although the two types of nanotubes were separated, variations in chirality still exist within each type.46 As pointed out in this study, the electronic structures of the nanotube walls will influence the properties of Car. Therefore, it is very likely that the chirality distribution was responsible for the inhomogeneity in the interactions between Car and SWCNTs. ν1 Frequency of Car in Metallic and Semiconducting SWCNTs. The peak positions of the V2, V3, and V4 modes of Car in the metallic SWCNTs were the same as those in semiconducting SWCNTs. However, the peak of the V1 mode
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Figure 4. Peak positions of the V1 modes for Car encapsulated in metallic (solid line, Car@SWCNTMetal) and semiconducting (dotted line, Car@SWCNTSemi) SWCNTs. The line shapes were obtained from the difference between the Raman spectra of the peapods and the empty SWCNTs in order to remove the contribution from the G-band of SWCNTs on the peaks of the V1 modes (see Figure 3).
in the metallic SWCNTs was notably downshifted 3 cm-1 from that in the semiconducting SWCNTs (see Figures 3 and 4). The half-widths of the V1 modes of the metallic and the semiconducting SWCNTs were rather large at 13 and 14 cm-1, respectively. Moreover, the line shape of the V1 mode of Car in the metallic SWCNTs was slightly asymmetric. Here we discuss the origin of these features in more detail. Shift of the Peak Position of the ν1 Frequency. It is known that the peak position of the V1 mode can be affected by pressure, conformational changes, difference of polarizability, and doping.24,26,28,30,43,47 As previously mentioned, the relative intensities of Raman modes of Car in the metallic SWCNTs are similar to those in the semiconducting SWCNTs, indicating that the packing conditions and the Car configurations in the metallic and semiconducting SWCNTs were similar. Therefore, the observed shift cannot be due to pressure or configuration differences. The presence of defects on the surface of the SWCNTs may influence the structure of the encapsulated Car,48 and thereby change the Raman peak positions. Although such structural changes should cause a shift of all the Raman modes, the difference was only observed for the V1 mode as shown in Figure 3. This implies that SWCNT defects cannot be the main cause for the shift in the Raman peak positions. Polarizability interactions (dispersive interactions) influence the energy of the optical excitation from the ground 11Ag-to the 11Bu+ state of carotenoids (the Carotene band is caused by this transition).22 It is known that the ν1 frequency of the ground state of β-Carotene, ν1(11Ag-), is influenced by the excitation energy through vibronic coupling processes.26,28,30 According to the vibronic coupling model, the ν1(11Ag-) frequency is approximately expressed as30
ν1(11Ag) ≈ Ω -
V201 E1(21Ag)
-
V202 E2(11B+ u)
(1)
Here Ω is the diabatic ν1 frequency. V01 and V02 are vibronic coupling integrals between the ground state and the first excited singlet state (optically forbidden 21Ag- state), and between the ground state and the second excited singlet state (optically allowed 11Bu+ state), respectively. E1(21Ag-) and E2(11Bu+) are the energies of the 11Ag--21Ag- and 11Ag--11Bu+ transitions. It should be noted that Car has a C2h symmetrical structure, and
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-1 1 + Figure 5. Relationship between ν1(11Ag ) (cm ) and E2(1 Bu ) (eV) of Car. This graph was depicted using the data reported by Noguchi et al.30 eq 2 was derived from the least-squares fitting of the data.
thus V02 is expected to be very small from vibronic coupling theory.30 The E1(21Ag-) energy is not strongly influenced by the surrounding polarizability.28 Thus, it is reported that polarizability interactions influence the E2(11Bu+) energy, but do not 30 strongly affect the ν1(11Ag ) frequency. (In contrast, it is noted, 1 1 the ν1 mode of the 2 Ag state ν1(2 Ag ) is strongly influenced by the surrounding polarizability).28,30 From the data reported by Noguchi et al.,30 we depict the relationship between the ν1(11Ag-) and the E2(11Bu+) of Car in Figure 5. From eq 1 and the reported data, the ν1(11Ag-) frequency (cm-1) can be expressed as a function of E2(11Bu+) (eV) as
ν1(11Ag ) ) 1585.2 -
172.63 E2(11B+ u)
(2)
We can now evaluate the influence of polarizability interactions on the ν1 frequency of Car in SWCNTs by using eq 2. The E2(11Bu+) energy can be determined from the optical absorption spectra of carotenoids. In the case of Car-peapods, the E2(11Bu+) energy of Car in the metallic SWCNT was estimated to be 10 meV smaller than that in the semiconducting SWCNT. This difference of E2(11B+ u ) energy will cause a downshift in the ν1 frequency of Car in metallic SWCNTs relative to that in semiconducting SWCNTs. However, according to eq 2, the down shift caused by a difference of 10 meV is only 0.26 cm-1. Thus, polarizability interactions through vibronic coupling can not be the origin of the observed 3 cm-1 shift.
It is known that the V1 mode is quite sensitive to the amount of charge on the Car.47 For example, Harada et al. reported that when Car is lightly doped with iodine the frequency of the V1 mode is slightly decreased.47 Therefore, if the amount of charge on the Car was different between the metallic and semiconducting SWCNTs, the observed difference of the peak position of V1 mode could be explained by this mechanism. To evaluate whether such a charge difference can occur or not, we performed the following theoretical calculations. The electronic energy bands of the Car-peapods are shown in Figure 6. In the case of the metallic (10,10) SWCNT, the highest occupied (HO) state of Car crosses two linear dispersion bands of the SWCNT just above the crossing point [Figure 6a]. In this case, the electrons are transferred from the HO state of Car to the π states of the SWCNT. Therefore, our first-principles calculations clearly indentify the mechanism of charge transfer from Car to a metallic SWCNT. The amount of transferred charge is calculated to be 0.481 e. Wave function analysis suggests that the number of electrons in the π band of Car decreases under encapsulation (see the Supporting Information), which would result in softening of the stretching mode of Car. Thus, softening of the V1 mode would occur in metallic SWCNTs. In contrast, for the semiconducting Car-peapods, the HO band of Car emerges in the fundamental gap of the semiconducting SWCNT [Figure 6, panels b and d]. Thus, charge transfer does not take place. The expected amount of charge transferred from the Car to the metallic SWCNTs (0.481 e) corresponds to the case for lightly doped Car in ref 47. Therefore, the charge-transfer mechanism can be one of the plausible origins for the observed 3 cm-1 frequency shift of the V1 mode in the metallic SWCNTs. Finally, we comment on the physical mechanisms behind the slightly asymmetrical line shapes of the ν1 frequency mode of Car in the metallic SWCNTs. As mentioned previously, there are chirality variations within the metallic and semiconducting SWCNTs. Sato et al. revealed that armchair and chiral SWCNTs with large chiral angles (>20°) are dominant in metallic SWCNTs, whereas such a noticeable preference for a particular chirality was not observed for semiconducting SWCNTs.46 The chirality distribution of the surrounding nanotube walls will cause inhomogeneity in the interactions between Car and SWCNTs, and we believe that this could lead to the slightly asymmetrical line shape of the V1 frequency mode of Car in the metallic SWCNTs.
Figure 6. Electronic energy bands [and density of states (DOS)] for (a) [(c)] armchair (10,10) metallic and (b) [(d)] zigzag (17,0) semiconducting SWCNTs containing Car. Contributions from the highest (HO) and the second highest (HO-1) occupied states of Car are indicated as arrows. Red dots [red lines in DOS] denote the HO state of Car. Other contributions in the panels are from SWCNTs. Energies are measured from the Fermi level energy and the top of the valence band for the armchair (10,10) and zigzag (17,0) SWCNTs, respectively.
Physical Properties of β-Carotene Conclusion The influence of a graphitic component on the physical properties of carotenoids located at van der Waals contacts was clarified. Metallic and semiconductor SWCNTs with encapsulated Car were prepared and their optical properties were investigated. There was a small difference of 10 meV between the peak energies of the optical absorption band of Car in the two types of SWCNTs. This is likely to be caused by the difference in the dielectric environment of the metallic and semiconducting SWCNTs through dispersive interactions. We observed a clear down-shift of the V1 Raman frequency of Car in the metallic SWCNTs relative to that in the semiconducting SWCNTs. This down-shift could not be explained by a vibronic coupling model. Our theoretical calculations revealed that the electronic structures of the SWCNT walls strongly influence the physical properties of Car. In the metallic SWCNTs, electron transfer from the Car to the nanotube walls can take place, but this is not the case in the semiconducting SWCNTs. We proposed that a difference in the amount of charge transferred to the Car between the metallic and semiconducting SWCNTs is the most plausible origin for the shift of the V1 Raman mode. These results provide experimental evidence that the electronic band structures of aromatic nanotube walls do affect the physical properties of encapsulated π-conjugated molecules. These results will contribute to the fundamental understanding of the influences of the surrounding environment on carotenoids, which is of great importance to elucidate the functions of carotenoids in biological systems. A simple artificial complex like a carbon nanotube-encapsulated molecule is a useful tool for investigating the interactions of materials located at a van der Waals contact. SWCNTs thus provide us with fascinating cages for encapsulation of various molecules. The encapsulated molecules can transfer excited light energy to the cages,13,49 and thus these molecular-SWCNT complexes have the potential to exhibit the functionality of photosynthetic systems. It is hoped that our studies of the physical properties of Car-SWCNT complexes, which somewhat mimic photosynthetic pigment-protein complexes, will contribute to the future development of artificial photosynthetic systems. Acknowledgment. Support from the Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency (JST) is gratefully acknowledged. This work was supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas (No. 21108523, “π-Space”) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Industrial Technology Research Grant Program in 2007 from New Energy and Industrial Technology Development Organization (NEDO) of Japan. Supporting Information Available: RBMs of Car-peapods, the wave function analysis of the electrons in the π band of Car, the relationship between the annealing processes and the degradation of encapsulated Car, the separation results on C60peapods, and the observed HRTEM images of Car-peapods. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Appl. Phys. Lett. 1992, 60, 2204–2206.
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