Band filling and correlation effects in alkali metal doped carbon nanotubes

15 July 2002

Physics Letters A 299 (2002) 601–606 www.elsevier.com/locate/pla

Band filling and correlation effects in alkali metal doped carbon nanotubes Jaewu Choi a,∗ , Iran Amildo Samayoa b , Seong-Chu Lim c , Chulsu Jo c , Young Chul Choi d , Young Hee Lee c , P.A. Dowben e a Department of Electrical and Computer Engineering, Wayne State University, 5050 Anthony Wayne Dr. #3100, Detroit, MI 48202, USA b Center for Advanced Microstructures & Devices, Louisiana State University, Baton Rouge, LA 70806, USA c Department of Physics, Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, South Korea d Materials Technology Laboratory, Corporate R&D Center, Samsung SDI, Suwon 442-391, South Korea e Department of Physics and Astronomy, Center for Material Research and Analysis, Behlen Laboratory of Physics,

University of Nebraska, Lincoln, NE 68588-0111, USA Received 11 January 2002; accepted 29 April 2002 Communicated by L.J. Sham

Abstract We have investigated modification of the electronic structure of the vertically aligned multi-walled carbon nanotubes (MWCNTs) as a function of sodium doping. The changes in band structure can be largely associated with shifts of the Fermi level position relative to the multi-walled carbon nanotube band structure. The changes in the apparent density of states in the vicinity of the Fermi level suggest effects associated with the repulsive on-site Coulomb interaction (the correlation energy) although the intrinsic poor screening in the quasi-one-dimensional carbon nanotubes, particularly near the tube termination, cannot be neglected entirely. The results are compared with ‘unaligned’ single-walled and multi-walled carbon nanotube films (or mats).  2002 Elsevier Science B.V. All rights reserved. PACS: 71.20.Tx; 36.20.Kd; 73.22.-f; 79.60.Dp

The influence of correlation energy and electron– phonon coupling, as well as the search for the Peierls transition, has motivated a number of photoemission studies of one-dimensional and quasi-one-dimensional conductors including the alkali metal tungsten and molybdenum bronzes (A0.9 Mo6 O17 ) [1], tetracyanoquinodimetahne (TTF-TCNQ) [2], and the Bechgaard salts [3] among other examples [4]. For single-walled

* Corresponding author.

E-mail address: [email protected] (J. Choi).

carbon nanotubes (CNTs), band structure calculations [5–7] suggest that alkali metal doping generally leads to a transition from semi-metal character to more metallic character and to modest band filling. This is to say that the Fermi level is positioned at higher energies relative to graphite like band structure of the carbon nanotubes and, qualitatively, the band structure of the carbon nanotube is left largely unperturbed. Conductance measurements [8,9] on single-walled carbon nanotubes suggest that correlation energies, even weak correlation energies, may exert some influence on the band structure and density of states near the

0375-9601/02/$ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 6 0 1 ( 0 2 ) 0 0 6 2 4 - 2

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Fig. 1. Scanning electron microscopic images of the vertically aligned multi-walled carbon nanotubes (a), mechanically deformed (bent) multi-walled carbon nanotubes (b). A magnified view of nanotubes doped with sodium but after annealing to 900 ◦ C show damage to ensemble multi-walled carbon nanotube ends (c) and in transmission electron microscopy, both the damage and the sodium decoration can be discerned (d).

Fermi level [9,10]. Generally, conductance measurements on single-walled carbon nanotubes, both for individual single-walled carbon nanotubes and ropes, suggest that, as the conductance increases monotonically with doping of lithium [11,12], potassium [13, 14] (but not cesium [14]), the alkali metals add electrons to the system and the band filling arguments dominate, except perhaps at lower temperatures, typically well below 50 K [12,14,15]. Nonetheless, with aligned multi-walled carbon nanotubes, an opening of a pseudogap at EF is expected to occur due to weak intertube interaction in metallic multi-walled nanotubes [16–19]. In this Letter, we show that the changes in electronic structure with the alkali metal doping of carbon nanotubes is dominated by effects attributable to band filling, except in the vicinity of the Fermi level. The vertically aligned multi-walled carbon nanotube films (mats) with tube diameters ranging from 30

to 50 nm width and ∼15 µm long were used for this study [20], as shown in Fig. 1(a). The distance between the carbon nanotubes is about 100 nm, providing a filling factor of nanotube mats about 20% from the top view, as measured from the high resolution transmission electron microscopy images. The vertically aligned multi-walled carbon nanotubes were grown on Ni-coated Si substrates using the plasma enhanced chemical vapor deposition, as described elsewhere [21]. Single-walled carbon nanotubes powders were prepared using catalytic arc discharge at low pressure, with a reduction in the transition metal content by acid treatment to less than 1 wt%. The thickness of the aligned carbon nanotube films was about 7 µm. Alignment of the multi-walled carbon nanotubes was established by mapping the band dispersion along the axis of the nanotubes in normal emission angle-

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Fig. 2. Electronic band dispersion as a function of k-parallel momentum to the tube axis for (a) the first four low binding energy features (A, B, C and D) near the Fermi level, and (b) the next five higher binding energy features (E, F, G, H and I).

resolved photoemission, as shown in Fig. 2 [20], by changing the photon energy, and is similar to previous intramolecular band structure studies of vertically aligned long chain molecules [22]. The Brillouin zone center and edge critical points are well defined over several Brillouin zones experimentally [20], in spite of a very small Brillouin zone and only modest dispersion. The angle-resolved photoemission studies were conducted using monochromatic synchrotron radiation dispersed by a 3 m toroidal grating monochromator at Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University. The combined resolution of the photoemission spectrometer and beamline is about 150 meV. The aligned samples were annealed for several stages up to 700 ◦ C in

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vacuum for the photoemission measurements to remove gas adsorbates and were then doped with sodium using SAES sodium getters. Sodium evaporation rate was approximately 5 × 1013 Na atoms/min/cm2 . Sodium was deposited on the ends of our aligned multi-walled carbon nanotubes. Although persistent high coverages of Na films under ultra high vacuum (UHV) conditions is unlikely, we have established that capillary action resulted in the uptake of sodium within our aligned multi-walled carbon nanotubes, following sodium deposition. Intercalation of alkali metals into multi-walled carbon nanotubes is fairly well established [23]. Following annealing treatments of the aligned multi-walled carbon nanotubes at temperature greater than 600 ◦ C, the Na shallow core and valence states increased as a result of de-intercalation. This de-intercalation appears similar to that reported elsewhere [23]. The rapid de-intercalation process (rapid annealing of the Na-doped multi-walled carbon nanotubes to 900 ◦ C) opens the end of the aligned multi-walled carbon nanotubes as seen in Fig. 1(c). The top ends of the tubes become substantially damaged, as is evident in the bigger end diameters and the coalescence of tube ends with next neighbor nanotube ends. High-resolution transmission electron microscope image of an individual multi-walled carbon nanotubes (Fig. 1(d)), separated from the sample of Fig. 1(c), shows the tip is opened in a fashion akin to a rupture and scattered sodium clusters appear attached to the sides of the damaged nanotube. This clearly indicates that the sodium has been intercalated in between carbon layers as mere sodium ‘dressing’ of the aligned multi-walled carbon nanotubes would not result in such damage during de-intercalation of the sodium as is observed. To study the change of the electronic valence structure of the vertically aligned multi-walled carbon nanotubes, the photoemission spectra were taken at normal emission and 45◦ light incidence at room temperature as a function of sodium evaporation time, as shown in Fig. 3(b). The photoemission spectra, as in other studies of carbon nanotubes [24–26], resembles graphite and dominated by features at about 3, 5 and 8 eV binding energy. With the aligned multiwalled carbon nanotubes, a number of discrete bands (as seen in the blue curves in Fig. 3(b)) can be resolved which were not observed in previous efforts [24–26]. The sodium induced feature at 6 eV bind-

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Fig. 3. Photoemission spectra from (a) single-walled carbon nanotube films, (b) multi-walled carbon nanotubes mats or films. We compare vertically aligned (blue), and intentionally bent (and therefore unaligned) carbon nanotubes mats (red). The photoemission spectra are taken at normal emission as a function of the sodium doping using 50 eV photons. Thus the geometry has emission perpendicular to the single-walled carbon nanotube axis and parallel with the multi-walled carbon nanotube axis.

ing energy increases with increasing sodium exposure, while peak intensities of 3 and 8 eV features monotonically decrease and broaden. In our aligned multi-walled carbon nanotubes, the binding energies of all the multi-walled CNT photoemission features increase with increasing alkali metal doping until Na coverages corresponding to 11 minute Na-evaporation time (0.1 Na/C ratio). With increasing Na doping beyond this point (0.1 Na/C ratio), the binding energy of the valence bands of our aligned multi-walled carbon nanotubes decreases. This is summarized by the binding energy shift of the first highly degenerated carbon nanotube π -states with ∼3 eV binding energy in Fig. 4(c). The density of states near the Fermi level of the aligned multi-walled carbon nanotubes is relatively low compared to a transition metal and relatively constant with increasing sodium concentration. As seen in Fig. 4(a) (blue squares), the density of states near the Fermi level, derived from the integrated

Fig. 4. Changes in electronic structure are plotted as a function sodium doping for both multi-walled and single-walled carbon nanotubes. The density of states near Fermi level for multi-walled carbon nanotubes (both vertically aligned (blue squares), and bent (red circles)) are plotted in (a) while the density of states, both experimental (black circles) are compared with our theoretical expectations (black squares), is shown for single-walled carbon nanotubes in (b). The binding energy shift of the degenerate π -states, at about ∼3 eV binding energy, have been plotted for both vertically aligned (blue squares), and bent (red circles) multi-walled carbon nanotubes in (c), and single-walled carbon nanotubes (black circles) are compared with our theoretical expectations of EF − EFo (red triangles) in (d). The relative charge transfer (red triangles) from the alkali metal to the single-walled CNT, based on band calculations, is also shown in (b). The work function change for the vertically aligned multi-walled carbon nanotube mats (e), and binding energy shift of the sodium 2p states consistent with increased metallization of the single-walled nanotubes (f) are also shown.

counts/intensity from −0.5 to 0.5 eV, changes little with increasing sodium coverage. The binding energy shift of the aligned multiwalled carbon nanotube valence bands to higher binding energies and then to lower binding energies is consistent, at least partly, with the expectations of sodium doping on the basis of the calculated charge trans-

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fer (the red triangles) and the calculated density of states at the Fermi level (the black squares) shown in Fig. 4(b) and the shift of the Fermi level relative to the band structure (the red triangles showing EF − EFo in Fig. 4(d)). The absence of a substantial increase in the density of states near the Fermi energy is not, however, consistent with the increase in conductivity and density of states at EF expected from band filling arguments. In first principles calculations for single-walled carbon nanotubes, the molecular orbital binding energies (based on the Fermi level shift: EF − EFo ) shows a minimum (the red triangle symbols in Fig. 4(d) [27]), which can be interpreted as an electron donation (band filling) to the carbon nanotube at the initial stages of the doping, followed by a diminution of charge donation due to Coulomb repulsion as seen in Fig. 4(b) [27]. The general increase in the valence band binding energies has been observed, with initial alkali metal doping, has been observed with Cs intercalation and doping of single-walled carbon nanotube bundles [25]. We have reproduced this binding energy shift with the decoration (not intercalation) of Na on unaligned single-walled carbon nanotubes, as seen in Fig. 3(a) and summarized in Fig. 4(d). There is clear qualitative agreement with theory. These shifts in valence band binding energies, with alkali metal doping, are also in qualitative agreement with our results for the valence bands of our aligned multi-walled carbon nanotubes. From our model calculations, increased sodium doping is also expected to lead to an increase in the density of states, as shown in Fig. 4(b). We do not observe this with our aligned multi-walled carbon nanotubes. We also do not observe this substantial increase in the density of states with the unaligned single-walled carbon nanotubes (Fig. 4(b)), nor does this appear to be the case with Cs intercalation of single-walled carbon nanotube bundles [25]. The work function does decrease with alkali metal doping initially (Fig. 4(e)) as expected, and similar decreases in work function were observed with Cs intercalation of single-walled carbon nanotube bundles [25]. The increase of the work function at high alkali metal doping levels is, however, difficult to explain in a simple electron donation scheme. Suppression of the density of states near the Fermi level with alkali metal doping suggests the formation of a pseudogap and/or electron correlation effects such as those leading to

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the formation of Hubbard bands with the alkali metal doping of molecular systems [28]. Pseudogap formation is predicted to occur in aligned (and undoped) nanotubes [16–19]. To eliminate any possibility that the alignment of the multi-walled carbon nanotubes is responsible for persistent pseudogap formation with alkali metal doping, we mechanically deformed the multi-walled carbon nanotubes in order to introduce misalignment, as seen in Fig. 1(b). Characteristic of unaligned carbon nanotubes, the photoemission features are now much broader than with aligned multi-walled carbon nanotubes and the resolution of many of the discrete photoemission bands is lost in the mechanically deformed multi-walled carbon nanotubes, as seen in Fig. 3(b). With alkali metal doping of the mechanically deformed multi-walled carbon nanotubes, the binding energy increases and decreases in a fashion similar to that observed with the aligned multi-walled carbon nanotubes. The density of states near the Fermi level shows that the gap/pseudogap persists for most alkali metal coverages, as summarized in Fig. 4(a). Only at the highest alkali metal coverages/doping levels is an increase observed in the density of states at the Fermi level and this increase is not monotonic. In fact, at least two distinct metallic states are observed at different Na concentrations. In this respect, the unaligned multi-walled carbon nanotubes resemble alkali metal doped C60 more than an increasingly metallized 1-d conductor. This means that the persistent gap or pseudogap at the Fermi level, with alkali metal doping, is not a consequence of weak intertube interactions, though this may play a role in enhancing metallization at the highest metal doping level explored in the unaligned multi-walled carbon nanotubes. A strong influence of the multi-walled carbon nanotube termination structure on the electron correlation effects observed here cannot be excluded. In the photoemission geometry of the results presented here, the surface of the film are the ‘ends’ of the multi-walled carbon nanotubes and we acknowledge that photoemission is surface sensitive. Extensive damage of the multi-walled carbon nanotube ends, of the type in Fig. 1(c) and (d), does substantially increase the density of states at the Fermi level in photoemission. Unlike other alkali metals, Na doping/intercalation of C60 (which could be imitated by the dome-like nanotube end structure) has generally higher resistance than the

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observed value with other alkali metals and does not form the A3 C60 phase with metallic conductivity but rather exhibits activated nonmetallic conductivity of an insulator akin to a Mott–Hubbard nonmetal [29]. The photoemission results strongly suggest that intramolecular (intratube) electron correlation effects play a role with initial alkali metal doping, though the band shifts of the carbon nanotubes photoemission features are consistent with electron donation and band filling of the multi-walled carbon nanotube band structure. The onset of electron correlation effects would tend to offset decreases in the work function due to alkali metal doping, as is observed. The energy gaps and corresponding low density of states near the Fermi level recently observed in single-walled carbon nanotubes [9], previously thought to be metallic, is consistent with the photoemission measurements near the Fermi level reported here, in particular those data for the single-walled carbon nanotubes. Band shifts relative to the Fermi level, due to alkali metal doping seem to occur as expected, thus there is a promise of forming p–n junctions by doping, as suggested elsewhere for single-walled carbon nanotubes [7]. The results presented here, nonetheless, suggest that anomalous transport effects must be expected occur at low bias voltage or at high alkali metal concentrations in doped multi-walled carbon nanotubes.

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Acknowledgements

[22]

Authors thank to the help of Josef Hormes and acknowledge helpful discussions with David Tománek. This project was undertaken with support from NSF (CCR-0196553), CAMD/LSU, MOST through the NRL and New Frontier program, the Office of Naval Research, the Nebraska Research Initiative in Nanostructured Materials and BK21 program in Korea.

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