2 Symmetry. 2.1 Structure of carbon nanotubes

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Chad Phillips
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1 2 Symmetry Carbon nanotubes are hollow cylinders of graphite sheets. They can be viewed as single molecules, regarding their small size ( nm in diameter and µm length), or as quasi-one dimensional crystals with translational periodicity along the tube axis. There are infinitely many ways to roll a sheet into a cylinder, resulting in different diameters and microscopic structures of the tubes. These are defined by the chiral index that gives the angle of the hexagon helix around the tube axis. Some properties of carbon nanotubes like their elasticity [48] can be explained within a macroscopic model of an homogeneous cylinder, whereas others depend crucially on the microscopic structure of the tubes. The latter include, for instance, the electronic band structure, in particular, their metallic or semiconducting nature (Chap. 3). The fairly complex microscopic structure with tens to hundreds of atoms in the crystal unit cell can be described in a very general way with the help of the nanotube symmetry. This greatly simplifies understanding and calculating physical properties like optical absorption, phonon eigenvectors, and electron phonon coupling. In this chapter we first describe the geometric structure of carbon nanotubes and the construction of their Brillouin zone in relation to that of graphite (Sect. 2.1). The symmetry properties of single-walled tubes are presented in Sect We explain how to obtain the entire tube of a given chirality from one single carbon atom by applying the symmetry operations. Furthermore, we give an introduction to the theory of line groups of carbon nanotubes [46], and explain the quantum numbers, irreducible representations, and their notation. 2.1 Structure of carbon nanotubes A tube made of a single graphite layer rolled up into a hollow cylinder is called a single-walled nanotube (SWNT); a tube comprising several, concentrically arranged cylinders is referred to as a multiwall tube (MWNT). Single-walled nanotubes, as typically investigated in the work presented here, are produced by laser ablation, high-pressure CO conversion (HiPCO), or the arc-discharge technique, and have a Gaussian distribution of diameters d with mean diameters d nm [32, 33, 49, 5]. The chiral angles [Eq. (2.1)] are evenly distributed [51].

2 2.1 Structure of carbon nanotubes 8 Single-walled tubes form hexagonal-packed bundles during the growth process. The wallto-wall distance between two tubes is in the same range as the interlayer distance in graphite ( 3.41 Å). Multiwall nanotubes have similar lengths to single-walled tubes, but much larger diameters. Their inner and outer diameters are around 5 and 1 nm, respectively, corresponding to 3 coaxial tubes. Confinement effects are expected to be less dominant than in singlewalled tubes, because of the larger circumference. Many properties of multiwall tubes are similar to those of graphite. Because the microscopic structure of carbon nanotubes is derived from that of graphene 1, the tubes are usually labeled in terms of the graphene lattice vectors. Figure 2.1 shows the graphene honeycomb lattice. The unit cell is spanned by the two vectors a 1 and a 2 and contains two carbon atoms at the positions 1 3 (a 1 + a 2 ) and 2 3 (a 1 + a 2 ), where the basis vectors of length a 1 = a 2 = a = 2.461Å form an angle of 6. In carbon nanotubes, the graphene sheet is rolled up in such a way that a graphene lattice vector c = n 1 a 1 + n 2 a 2 becomes the circumference of the tube. This circumferential vector c, which is usually denoted by the pair of integers (n 1,n 2 ), is called the chiral vector and uniquely defines a particular tube. We will see below that the electronic band structure or the spatial symmetry group of nanotubes vary dramatically with the chiral vector, even for tubes with similar diameter and direction of the chiral vector. For example, the (1,1) tube contains 4 atoms in the unit cell and is metallic; the close-by (1,9) tube with 184 atoms in the unit cell is a semiconducting tube. In Fig. 2.1, the chiral vector c = 8a 1 + 4a 2 of an (8,4) tube is shown. The circles indicate the four points on the chiral vector that are lattice vectors of graphene; the first and the last circle coincide if the sheet is rolled up. The number of lattice points on the chiral vector is given by the greatest common divisor n of (n 1,n 2 ), since c = n(n 1 /n a 1 + n 2 /n a 2 ) = n c is a multiple of another graphene lattice vector c. The direction of the chiral vector is measured by the chiral angle θ, which is defined as the angle between a 1 and c. The chiral angle θ can be calculated from cosθ = a 1 c a 1 c = n 1 + n 2 /2 n n 1 n 2 + n 2 2. (2.1) For each tube with θ between and 3 an equivalent tube with θ between 3 and 6 exists, but the helix of graphene lattice points around the tube changes from right-handed to left-handed. Because of the six-fold rotational symmetry of graphene, to any other chiral vector an equivalent one exists with θ 6. We will hence restrict ourselves to the case n 1 n 2 (or θ 3 ). Tubes of the type (n,) (θ = ) are called zig-zag tubes, because they exhibit a zig-zag pattern along the circumference, see Fig (n, n) tubes are called armchair 1 Graphene is a single, two-dimensional layer of graphite.

3 2.1 Structure of carbon nanotubes 9 Figure 2.1: Graphene honeycomb lattice with the lattice vectors a 1 and a 2. The chiral vector c = 8a 1 + 4a 2 of the (8,4) tube is shown with the 4 graphenelattice points indicated by circles; the first and the last coincide if the sheet is rolled up. Perpendicular to c is the tube axis z, the minimum translational period is given by the vector a = 4a 1 + 5a 2. The vectors c and a form a rectangle, which is the unit cell of the tube, if it is rolled along c into a cylinder. The zigzag and armchair patterns along the chiral vector of zig-zag and armchair tubes, respectively, are highlighted. ( 4,5), a 2 a 1 (,4) armchair zig-zag circumference (8,4) (8,) tubes; their chiral angle is θ = 3. Both, zig-zag and armchair tubes are achiral tubes, in contrast to the general chiral tubes. The geometry of the graphene lattice and the chiral vector of the tube determine its structural parameters like diameter, unit cell, and its number of carbon atoms, as well as size and shape of the Brillouin zone. The diameter of the tube is simply given by the length of the chiral vector: d = c π = a π n n 1 n 2 + n 2 2 = a N, (2.2) π with N = n n 1 n 2 + n 2 2. The smallest graphene lattice vector a perpendicular to c defines the translational period a along the tube axis. For example, for the (8,4) tube in Fig. 2.1 the smallest lattice vector along the tube axis is a = 4a 1 + 5a 2. In general, the translational period a is determined from the chiral indices (n 1,n 2 ) by a = 2n 2 + n 1 nr a 1 + 2n 1 + n 2 nr a 2, (2.3) and a = a = 3(n n 1 n 2 + n 2 2 ) a, nr (2.4) where R = 3 if (n 1 n 2 )/3n is integer and R = 1 otherwise. Thus, the nanotube unit cell is formed by a cylindrical surface with height a and diameter d. For achiral tubes, Eqs. (2.2) and (2.4) can be simplified to a Z = 3 a c Z = na (zig-zag) (2.5) a A = a c A = 3 na (armchair).

4 2.1 Structure of carbon nanotubes 1 Figure 2.2: Structure of the (17,), the (1,1) and the (12,8) tube. The unit cells of the tubes are highlighted; the translational period a is indicated. a a a (17,) (1,1) (12,8) Figure 2.3: Brillouin zone of graphene with the high-symmetry points Γ, K, and M. The reciprocal lattice vectors k 1, k 2 in Cartesian coordinates are k 1 = (,1)4π/ and k 2 = (.5 3,.5)4π/. k 1 2 K e y e x 2 4 M 2 /a M k 2 For chiral tubes, a and c have to be calculated from Eqs. (2.2) and (2.4). Tubes with the same chiral angle θ, i.e., with the same ratio n 1 /n 2, possess the same lattice vector a. In Fig. 2.2 the structures of (17,), (1,1), and (12,8) tubes are shown, where the unit cell is highlighted and the translational period a is indicated. Note that a varies strongly with the chirality of the tube; chiral tubes often have very long unit cells. The number of carbon atoms in the unit cell, n c, can be calculated from the area S t = a c of the cylinder surface and the area S g of the hexagonal graphene unit cell. The ratio of these two gives the number q of graphene hexagons in the nanotube unit cell q = S t /S g = 2(n2 1 + n 1 n 2 + n 2 2 ). (2.6) nr Since the graphene unit cell contains two carbon atoms, there are n c = 2q = 4(n2 1 + n 1 n 2 + n 2 2 ) nr (2.7) carbon atoms in the unit cell of the nanotube. In achiral tubes, q = 2n. The structural parameters given above are summarized in Table 2.1. After having determined the unit cell of carbon nanotubes, we now construct their Brillouin zone. For comparison, we show in Fig. 2.3 the hexagonal Brillouin zone of graphene with the

5 2.1 Structure of carbon nanotubes 11 Table 2.1: Structural parameters of armchair (A), zig-zag (Z) and chiral (C) nanotubes. The symbols are explained in the text. Tube N q = n c /2 A (n,n) 3n 2 2n Z (n,) n 2 2n C (n 1,n 2 ) n n 1 n 2 + n 2 2 2N/(nR) A Diameter d Translational period a Chiral angle θ 3na /π a 3 Z na /π C N a /π 3N a /(nr) arccos(n 1 + n 2 /2)/ N high-symmetry points Γ, K, and M and the distances between these points. In the direction of the tube axis, which we define as the z-axis, the reciprocal lattice vector k z corresponds to the translational period a; its length is k z = 2π/a. (2.8) As the tube is regarded as infinitely long, the wave vector k z is continuous; the first Brillouin zone in the z-direction is the interval ( π/a, π/a]. The bracket types ( ) and [ ] indicate open and closed intervals, respectively. Along the circumference c of the tube, any wave vector k is quantized according to the boundary condition m λ = c = π d k,m = 2π λ = 2π c m = 2 m, (2.9) d where m is an integer taking the values q/2+1,...,,1,...,q/2. A wave with wave vector k,m = 2 d m has 2m nodes around the circumference. The maximum k,m (minimum wavelength) follows from the number of atoms (2q) in the unit cell: a projection of the carbon atoms on the circumference of the tube leads to equidistant pairs of carbon atoms; then at least 4 atoms are neccessary for defining a wavelength, i.e., m q/2. The first Brillouin zone then consists of q lines parallel to the z-axis separated by k = 2/d and k ( π/a, π/a]. The quantized wave vector k and the reciprocal lattice vector k z are found from the conditions k c =2π k a = (2.1) k z c = k z a =2π.

6 2.1 Structure of carbon nanotubes 12 k k 2/a k 1 2 k 1 k2 2 /a /a k z 2 /a k 2 k z Figure 2.4: Brillouin zone of a (7,7) armchair and a (13,) zig-zag tube (thick lines). The background is a contour plot of the electronic band structure of graphene (white indicates the maximum energy). The graphene Brillouin zone in the right panel is rotated by 3. The Brillouin zone consists of 2n (i.e., 14 and 26, respectively) lines parallel to k z, where k z is the reciprocal lattice vector along the tube axis. Each line is indexed by m [ n,n], where m = corresponds to the line through the graphene Γ point (k = ). Note that the Brillouin-zone boundary π/a is given by π/a for armchair and π/ for zig-zag tubes. It can be seen from the symmetry of the graphite hexagonal Brillouin zone that lines with index m and m are equivalent. The same holds for k and k with the same index m. This yields k = 2n 1 + n 2 qnr k 1 + 2n 2 + n 1 qnr k 2 (2.11) k z = n 2 q k 1 + n 1 q k 2. (2.12) In Fig. 2.4 the Brillouin zones of a (7,7) armchair and a (13,) zig-zag tube are shown for m [ n,n] in relation to the graphene Brillouin zone. The line through the graphene Γ point has the index m =. The position of the lines with m = and m = n is the same for all zig-zag and all armchair tubes, respectively, independent of their diameter. With increasing diameter the number of lines increases while their distance decreases. For chiral tubes, see Fig To a first approximation, the properties of carbon nanotubes are related to those of graphite by taking from graphene the lines that correspond to the nanotube Brillouin zone, according to Eqs. (2.11) and (2.12). For example, the electronic band structure of a particular nanotube is found by cutting the two-dimensional band structure of graphene (see background of Fig. 2.4) into q lines of length 2π/a and distance 2/d parallel to the direction of the tube axis. This approach is called zone folding and is commonly used in nanotube research. Since the zone-

2.2 Symmetry of single-walled carbon nanotubes 13 Figure 2.5: Scanning tunneling microscopy images of an isolated semiconducting (a) and metallic (b) singlewalled carbon nanotube on a gold substrate.

Based on the diameter and chiral angle determined from the STM image, the tube in (a) was assigned to a (14, 3) tube and in (b) to a (12,3) tube.

7 2.2 Symmetry of single-walled carbon nanotubes 13 Figure 2.5: Scanning tunneling microscopy images of an isolated semiconducting (a) and metallic (b) singlewalled carbon nanotube on a gold substrate. The solid arrows are in the direction of the tube axis; the dashed line indicates the zig-zag direction. Based on the diameter and chiral angle determined from the STM image, the tube in (a) was assigned to a (14, 3) tube and in (b) to a (12,3) tube. The semiconducting and metallic behavior of the tubes, respectively, was confirmed by tunneling spectroscopy at specific sites. From Ref. [17]. folding procedure, however, neglects any effects of the cylinder geometry and curvature of the tube walls, results obtained by zone folding have to be used with great care. Experimentally, the atomic structure of carbon nanotubes can be investigated either by direct imaging techniques, such as transmission electron microscopy [2,52] and scanning probe microscopy [17,19,53 58], or by electron diffraction [59 62], i.e., imaging in reciprocal space. Scannig tunneling microscopy (STM) offers measurements with atomic resolution, see Fig From both, STM and electron diffraction, the chiral angle and tube diameter can be determined, and hence the chiral vector (n 1,n 2 ), in principle, can be found experimentally. Interpretation of the images is, however, delicate and often requires computations of the images and crosschecking with other experimental results. 2.2 Symmetry of single-walled carbon nanotubes The symmetry of carbon nanotubes is described by the so-called line groups, which were introduced by Damnjanović et al. [28 3, 63, 64]. Line groups are the full space groups of one-dimensional systems including translations in addition to the point-group symmetries like rotations or reflections. Therefore, they provide a complete set of quantum numbers. Damnjanović et al. [46,47,65] showed that every nanotube with a particular chirality (n 1,n 2 ) belongs to a different line group. Only armchair and zig-zag tubes with the same n belong to the same symmetry group. Moreover, starting from a single carbon atom, the successive application of all symmetry operations of the group constructs the whole tube. Because the relation between the carbon atoms and the symmetry operations is one-to-one, the nanotubes can be viewed as

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