Irregular Modulation of Density-of-States of Nano-Peapods

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Irregular Modulation of Density-of-States of Nano-Peapods Encapsulating Gd@C82 Metallofullerenes Kazunori Ohashi,† Naoki Imazu,† Ryo Kitaura,† and Hisanori Shinohara*,† †

Department of Chemistry and Institute for Advanced Research, Nagoya University, Nagoya 464-8602, Japan ABSTRACT: The modifications of density-of-states (DOS) of single-wall carbon nanotubes (SWCNTs) induced by encapsulation of Gd@C82 metallofullerenes have been studied by a combination of scanning tunneling microscopy/spectroscopy (STM/STS) and DFT. We demonstrate that internal orientation of Gd@C82 molecules inside SWCNTs can strongly affect the electronic structure of SWCNTs. The present findings suggest that not only encapsulated molecules but also their molecular orientations can be a tunable parameter with which we can modulate local electronic structures of SWCNTs.

1. INTRODUCTION Single-wall carbon nanotubes (SWCNTs) have been attracting considerable interest for their potential use for miniaturizing electronics beyond the micro electromechanical scale currently used because they have characteristic nanoscale structures and exhibit unique electronic properties.1,2 Particularly, their bandgap can be varied from 0 to 2 eV depending on the geometrical structure of SWCNTs, and their electrical conductivity can show metallic or semiconducting behavior according to their chirality. Moreover, controlling the electronic structure of SWCNTs is a crucial issue to apply SWCNTs to nanoelectronics devices such as field effect transistors (FETs),3 sensors,4 and so forth. One of the most effective ways to control the electronic structure of SWCNTs is to insert fullerenes, endohedral metallofullerenes, and other molecules or atoms into their central hollow space.5-8 In fact, encapsulation of C60 can change photoluminescence properties of SWCNTs depending on the nanotube diameters.9 Also, we have reported that FET characteristics of SWCNTs encapsulating Gd@C82 molecules (the so-called peapods) show an ambipolar behavior which is completely different from empty SWCNTs or SWCNTs encapsulating C60.10 Recent theoretical study by Kim and co-workers has shown that Gd nanowires inside SWCNTs can also modulate electronic states of SWCNTs as well as Gd metallofullerenes.7 To understand the origin of the modulation of electronic properties of SWCNTs induced by the encapsulations, detailed studies on the interaction between SWCNTs and encapsulated fullerenes are of great importance. In contrast to C60, which can be simply regarded as a spherical ball, Gd@C82 has a lower molecular symmetry. Moreover, Gd@C82 has a large dipole moment because Gd ion is known to be located at an off-centered position inside C82 cage due to the coulombic attraction exerted between Gd3þ and C823-.11 The local electronic structure of the Gd@C82-peapods may well depend on the orientation of Gd@C82 molecules inside the SWCNTs. r 2011 American Chemical Society

Controlling the orientation and position of metallofullerenes in SWCNT is, therefore, potentially a key parameter to properly modulate the electronic properties of SWCNTs. Scanning tunneling microscopy and spectroscopy (STM/STS) have known to be key techniques to obtain information on the electronic structure of peapods as they allow for measurement of electronic local density-of-states (DOS) near Fermi level with atomic resolution. In fact, an STM study on C60-peapods has shown that the DOS of the nanotube are oscillating along the nanotube axis reflecting the 1D array of C60.12 Furthermore, our previous STM/STS studies by Kuk and co-workers on Gd@C82peapods have demonstrated that the spatial modulation of the nanotube electronic bandgap is observed and that the bandgap of 0.5 eV is narrowed down to 0.1 eV where Gd@C82 fullerenes are inserted tightly into SWCNT of small diameters.13,14 Theoretical study by Ihm and co-workers has shown that the orbital hybridization of the orbital of metallofullerenes with the nanotube states explains the peak in the STS experiments.15 This technique for fabricating an array of quantum dots could be used for nanoelectronics and nano-optoelectronics. Here, we report electronic DOS modulations of SWCNTs induced by encapsulation of Gd@C82 molecules depending on the orientation of Gd@C82 in terms of a combination of STM/ STS and DFT. In the STM observation, we have demonstrated that the position of Gd@C82 can be determined but the orientations of Gd@C82 relative to nanotube axes are not identified. By the use of site-dependent detailed STS measurements, we have shown that the relative positions of valence band peaks and conduction band peaks appeared in STS spectra are varying irregularly depending on the position of SWCNTs. This variation of the local DOS indicates that Gd@C82 molecules are situated in a mutually random orientation inside SWCNTs irrespective of the Received: December 29, 2010 Published: February 23, 2011 3968

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dipole-dipole interaction exerted among the fullerene molecules.

2. EXPERIMENTAL SECTION SWCNTs produced by arc-discharge were purchased from MEIJO NANO CARBON Co., Ltd. (ARC-SO). The average diameter and the diameter distribution of the SWCNTs were estimated to be 1.4 and 0.1 nm respectively from high-resolution transmission electron microscopy (HRTEM) observations. Gd@C82 metallofullerenes were synthesized by dc arc-discharge and were purified by using multistage high-performance liquid chromatography (HPLC) with toluene as eluent. The purity of Gd@C82 metallofullerene was checked by both positive and negative laser desorption time-of-flight mass spectrometry as well as by HPLC analyses. Encapsulation of Gd@C82 into SWCNTs has been performed via the gas-phase reaction method,5 and a high filling ratio (>90%) of Gd@C82 was confirmed by HRTEM observations.16 The metallofullerene peapods were dispersed in 1,2-dichloroethane by sonication for more than 1 h. After sonication, three drops of the dispersion solution were deposited by spin coating on an Au(111) substrate (UNISOKU), which was annealed up to 800 K in air before deposition. The samples were then introduced into UHV and degassed at 600 K before STM measurements. The STM/STS measurements were carried out with an Omicron VT-STM in the constant-current mode operated at room temperature. Scanning tips used in the present study were electrochemically etched from W wires. To visualize the STM images obtained we used a program of WSxM.17 Theoretical calculations for structural and electronic properties of Gd@C82-peapods were also performed. All calculations were carried out based on DFT using Vienna ab initio simulation package (VASP).18,19 Here, generalized gradient approximation (GGA) was used, and a plane-wave basis set was employed with a cutoff energy of 250 eV. This cutoff energy has been reported to provide reliable results for the present purpose.20 The interaction between the ionic cores and valence electrons were implemented through the projector augmented wave (PAW) method.21 We have employed a special pseudopotential for Gd supplied with VASP, in which f-electrons are kept frozen in the core. The atomic positions are relaxed with residual forces smaller than 0.03 eV/Å by using the conjugate gradient method incorporating one irreducible k point with the coordinate (0, 0, 1/4)(2π/T), where T is the translational period. For the band-structure calculation, 10 irreducible k points [(0, 0, n/20)(2π/T), n = 0, 1,...,10] were used. Gaussian smearing with the parameter σ = 0.02 eV was applied to broaden the one-electron eigenenergies. The metallofullerenes were repeatedly placed along the direction of the nanotube axis with around 1 nm intermolecular distance to be commensurate with the periodicity of the nanotube. 3. RESULTS AND DISCUSSION A. STM Image of Gd@C82-Peapods. Parts a and b of Figure 1 show typical STM images of a Gd@C82-peapod taken with two different bias voltages, 1.0 and 0.5 V, and with a set point current of 100 pA at exactly the same position. In the STM image of part a of Figure 1, a typical lattice image originating from the atomic lattice of SWCNTs is observed. However, when we applied a different bias voltage of 0.5 V (part b of Figure 1), we have found additional periodic modulations of DOS (indicated by white arrows). This kind of additional modulation of DOS has never

Figure 1. (a) STM image of Gd@C82-peapod taken at a bias voltage of 1.0 V. (b) STM image of the same area as (a) at a different bias voltage of 0.5 V. Spatial modulations of DOS are indicated by arrows. (c) The illustration of a (20,2)-carbon nanotube encapsulating Gd@C82 metallofullerenes for the most possible candidate of the structure of the STM images.

been observed so far in pristine SWCNTs at any bias voltages. Moreover, the periodicity of this additional modulation is approximately 1.2 nm, which is consistent with the intermolecular distance of each adjacent [email protected] We therefore conclude that the additional periodic modulation of DOS results from encapsulated Gd@C82 inside SWCNTs. From STM images, the chiral indices of SWCNTs can be estimated.22,23 The chiral angle of this peapod is measured to be 4-6°. The diameter is also roughly estimated to be 1.5 nm based on the apparent height and width of the peapod. Although several candidates of chiral index can be considered, we simply assume that the periodic modulations resulting from Gd@C82 are commensurate to the translation vector of the outside SWCNT for computational limitations. Judging from these, the most probable candidate of the SWCNT possesses a chirality of (20,2) and has a diameter of 1.67 nm, a chiral angle of 4.72°, and a translation vector length of 1.52 nm. The structural model of (Gd@C82)@(20,2) SWCNT is shown in part c of Figure 1. STM images are not simply images of sample surface geometry but can be significantly affected by the DOS distribution over the sample surface. To interpret the STM images obtained in detail, we have performed DFT calculations followed by the simulations for the STM images based on Tersoff-Hamann theory.24 For STM simulations, we used partial (band decomposed) charge densities calculated by using VASP. After the geometry optimization, we found that Gd@C82 encapsulated is not located at the center of (20,2)-nanotube but along the sidewall of the nanotube. The calculated distance between the wall of the nanotube and the nearest carbon atom of Gd@C82 is 0.32 nm, which is close to the known interlayer distance of graphite (0.334 nm). We also found that (20,2)-nanotube keeps its original (perfect) cylindrical shape even after the Gd@C82 encapsulation; where the differences between major and minor axes of the peapod are within 0.03 nm, suggesting that only a little mechanical deformation induced by the encapsulation of Gd@C82. Part a of Figure 2 shows band structure of (20,2)-nanotube obtained with DFT calculations. Two energy bands near Fermi energy cross each other, which is consistent with metallic nature of (20,2)-nanotube. The (20,2)-nanotube encapsulating Gd@C82 is shown in part b of Figure 2. In the case of the peapod, many bands 3969

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The Journal of Physical Chemistry C with virtually no dispersion appear, which originate from the Gd@C82 molecules encapsulated. As clearly seen, the band structure of the (20,2)-nanotube is significantly modified due to the encapsulation. In particular, the bandgap opens up to about 0.5 eV, and the valence band top is located near the energy level where the Gd@C82 band is appearing. This implies that there are substantial orbital hybridizations between Gd@C82 and (20,2)-nanotube. Such modulated electronic structure can affect the STM images. Figure 3 shows simulated STM images of the (20,2)-nanotube encapsulating Gd@C82 at two different bias voltages and with two different Gd@C82 orientations together with the corresponding structural model. In the simulated images, periodic peaks are observed at the site of Gd@C82 molecules inserted. On the basis of the close similarity between the DFT and experimental images, we thus conclude that bright peaks in the STM images of the peapod correspond to the Gd@C82 positions. It should be noted that the relative heights of the periodic peaks are bias-dependent. In particular, in the DFT image at 0.5 V (part b of Figure 3), the hexagonal lattice image due to the SWCNT’s sidewall is severely modified and the bright periodical peaks are most clearly observable among the three images. These features are consistent with the experimental STM images in parts a and b of Figure 1; the periodic peaks originating from Gd@C82 are disappeared at a relatively high voltage of 1.0 V.

Figure 2. Band structures obtained with the DFT calculations. (a) (20,2)-nanotube, (b) (20,2)-nanotube encapsulating Gd@C82. The Fermi level is set to zero.

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One might concern the difference between the experimental STM images and theoretical ones; where, in the theoretical images, periodic modulations appear only on the top of the peapod although these modulations appear uniformly perpendicular to the nanotube axis. This is mainly because of the fact that the experimental tunneling current tends to flow to the direction perpendicular to the nanotube axis because tunneling occurs between the shortest paths and this effect has not been considered in the STM simulations. The calculated STM images are somewhat different (parts a, b, d, and e of Figure 3) and may depend on the mutual orientation of Gd@C82 with respect to the SWCNT wall. The periodic modulations in parts d and e of Figure 3 originating from Gd@C82 are less obvious than those of parts a and b of Figure 3. However, these differences are so small that one cannot distinguish these two orientations only by comparing STM images and the corresponding simulations. B. STS Spectra of Gd@C82-Peapods. To further investigate the modification of electronic states induced by Gd@C82 encapsulation, we performed site-dependent STS measurement on Gd@C82-peapod. Part a of Figure 4 shows an intensity plot from STS spectra of a SWCNT before the encapsulation taken along the SWCNT axis. We have applied 512  512 pixel resolution for STM imaging of a 30  30 nm2 region, and STS spectra were taken after every 5 pixels. The normalized differential conductance as shown in Figure 4 corresponds to the local DOS at each position.25 As shown in part a of Figure 4, peaks in surface DOS are clearly observed at 0.2, -0.6, and -1.0 V, which originates from the van Hove singularity of the SWCNT. Positions of these peaks do not change along the SWCNT axis. The STS spectra of Gd@C82-peapods are shown in part b of Figure 4. Periodic peaks are clearly observable in both occupied and unoccupied states along the tube axis. The periodicity is approximately 1 nm, which is consistent with typical intermolecular distance of Gd@C82 inside the peapods. Detailed analyses of the STS spectra reveal that the periodicity observed in STS spectra is not perfectly regular. Part c of Figure 4 shows the peak positions of the spectra in part b of Figure 4. The lines in red and black show the peak positions in valence band (at -0.3 V) and in

Figure 3. STM image by DFT calculations of (20,2)-nanotube encapsulating Gd@C82 molecules at different energy levels of (a),(d) 1.0 V, and (b),(e) 0.5 V. (c),(f) Optimized structure based on DFT of a (20,2)-nanotube encapsulating Gd@C82. These two have different orientation of the Gd@C82 metallofullerenes. (a) and (b) is based on the structural model (c): C2 axis of Gd@C82 is perpendicular to the tube axis. Gd ion is near side of the theoretical STM tip; and (d) and (e) is based on the structural model (f): C2 axis of Gd@C82 is perpendicular to the tube axis. 3970

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Figure 4. Intensity plots from STS spectra taken along a (a) SWCNT and (b) Gd@C82-peapod axis; (c) peak positions of the STS spectra of Gd@C82-peapods. The intensity plot is the same as (b), in an energy range of -0.5 to 0.5 V. Lines in red show the peak positions in valence band (at -0.3 V) and the Lines in black show the peak positions in conduction band (at 0.2 V). The normalized differential conductance ((dI/dV)/(I/V)) is plotted as color scale.

conduction band (at 0.2 V), respectively. As clearly seen, the relative positions of both valence and conduction band peaks change depending on the position along the peapod axis. Moreover, one peak in conduction band and one peak in valence band are located in every 1.1 nm length. (See the scale mark at the interval of 1.1 nm shown at the bottom of part c of Figure 4.) These results suggest the presence of a 1.1 nm interval array of Gd@C82, and the random orientations of Gd@C82 inside SWCNTs may affect the electronic structure of the peapods. To obtain information on the effect of the Gd@C82 orientation on the STS spectra, we performed the simulation of STS spectra based on DFT. Figure 5 shows simulated STS spectra for (20,2)-nanotube encapsulating Gd@C82 molecules. Parts a and b of Figure 5 exhibit different orientation of Gd@C82 molecule. The structural model of parts a and b of Figure 5 are the same as those of STM image simulation in parts c and f of Figure 3, respectively. In both cases, Gd ion is located on the C2 axis of Gd@C82 as previously reported.11 In part a of Figure 5, periodic peaks are observed similar to the experimental STS spectra. The most salient peaks are observed at 0.5 V. It should be noted that these peaks are located between the Gd@C82 molecules, not on the top of the molecules. This suggests that these peaks are mainly originating from the outer SWCNT. Because STS measures the DOS on the surface, the contributions of outer SWCNTs to STS spectra should be bigger than that of inner Gd@C82. The relative peak heights and positions are different when Gd@C82 molecules inside SWCNT have a different orientation. One of the most important differences between parts a and b of Figure 5 is the peak positions on the tube axis. In part b of Figure 5, new peaks appeared at 0.1 V that are located on the top of the Gd ions while the most of the peaks still remain between the Gd@C82 molecules as part a of Figure 5. The STS peak positions derive from Gd@C82 encapsulation are, therefore, sensitive to the molecular orientations of Gd@C82. The results described above strongly suggest that Gd@C82 metallofullerenes are not rotating inside SWCNTs during the

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Figure 5. Simulated STS intensity plot of (20,2)-nanotube encapsulating Gd@C82 with two different Gd@C82 orientations. Structural models are shown on the top of the figure. The arrows on the structural model show the theoretical STM tip direction. The structural models of a and b are the same as those of c and f, respectively.

STS measurements. In fact, previous studies suggest that the fullerene cages of Gd@C82 are not rotating even at room temperature because of strong interactions exerted between the wall of SWCNT and [email protected],26 Locally modified electronic structure of SWCNTs can significantly affect the electronic properties of SWCNTs such as conductivity and optical properties. Here, we demonstrate that not only the kinds of SWCNT and materials to be encapsulated but also the orientation of the materials is an important key factor to control the electronic states of peapods.

4. CONCLUSIONS In summary, we have observed electronic modulations of SWCNTs induced by encapsulation of Gd@C82 molecules by using STM/STS. Comparison between STM images and DFT calculations has allowed us to determine the position of Gd@C82 molecules inside the SWCNTs. The STS results reveal that both conduction and valence bands of SWCNTs have periodic peaks originating from a 1D array of Gd@C82. We have found that the relative positions of valence band peaks and conduction band peaks vary depending on the position of SWCNTs. This variation can be explained as due to random orientation of Gd@C82 inside SWCNTs. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work has been supported by the Grant-in-Aids for Specific Area Research (No. 19084008) on Carbon Nanotube NanoElectronics and for Scientific Research A (No. 19205003) of MEXT, Japan, and partly by the Global COE Program in Chemistry, Nagoya University. ’ REFERENCES (1) Iijima, S. Nature 1991, 354, 56. 3971

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