1 Electronic Transport in Ropes of Single Wall Carbon Nanotubes Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der Rheinisch-Westfälischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation vorgelegt von Diplom-Physiker Heiko Stahl aus Böblingen. Berichter: Universitätsprofessor Dr. B. Lengeler Universitätsprofessor Dr. H. Lüth Tag der mündlichen Prüfung: 18. August 2000
3 Electronic Transport in Ropes of Single Wall Carbon Nanotubes Dissertation approved by the Faculty for Mathematics, Informatics and Natural Sciences at the Aachen University of Technology in order to obtain the academic degree of a Doctor rerum naturalium presented by Diplom-Physiker Heiko Stahl from Böblingen. Expert witnesses: Universitätsprofessor Dr. B. Lengeler Universitätsprofessor Dr. H. Lüth Date of oral examination: August 18, 2000
5 Contents 1 Quo Vadis 7 2 Basics Wrapping Up How Do Electrons Feel In Here? One Way Only T he Nanotube Family First Steps How To Make Contact When Current Flows Inside T he T ube To Better Roads Connections Holes In T he Road It s Getting Tough T he Passing Lane How T o Change T he Lane Moving On Beyond T emperature What Makes T he Difference It s Not Over 97 A Do It Yourself 99 B What And Where 109 5
6 6 CONTENTS
7 Chapter 1 Quo Vadis Since their discovery in 1991 , carbon nanotubes have attracted a great deal of attention... A sentence like this opens most of the literature on carbon nanotubes. It sounds promising and at the same time decent enough. But it does not reveal the excitement the nanotubes caused, not only among scientists, but also in the general public. They contain such a wealth of most interesting characteristics that one does not know where to begin. Obviously, the carbon nanotubes are made from carbon, the most versatile element in the world. It forms brittle structures like graphite and durable material like diamond, not mentioned its innumerable compounds in chemistry and biology. The newer descendants of carbon, the buckminster fullerenes, occupied research in the 80 s and enriched the community with ideas of cages for atoms and frictionless ball bearings. And they supplied the solid state physics with an instructive example for a solid at the frontier between molecule and crystal. The worthy successors are the long, hollow molecules of the carbon nanotubes. Their exceptional strength and their remarkable electronic properties do indeed attract a great deal of attention. Besides the basic interest of physics in these one dimensional systems, a vast number of applications has been proposed, ranging from ultrasmall probes in scanning probe microscopy  over artificial muscles  to hydrogen storage , from field emitters for flat panel displays  to nanoelectronics [6, 7]. As is often observed, the basic research trails the application since the question why it is working is felt less important as long as it is working. Most of the applications relying on the geometry of the nanotubes (SPM, hydrogen storage, field emission) have produced impressive results. But the picture is very different in case of the electronic applications. Though this field stimulated the most fantastic ideas about the use of carbon nanotubes, it is the field in which a detailed knowledge about the intrinsic properties and the interaction of the tubes is essential for any kind of progress. This work will expand the understanding of the electronic properties of the nanotubes in order to give impulses for application as well as basic research. Chapter 2 will give a bottom-to-top review of the basic knowledge about 7
8 8 CHAPTER 1. QUO VADIS nanotubes as it emerged from research so far. It will cover the geometric and especially the electronic characteristics of the single nanotubes and will address the challenges encountered when dealing with complex arrangements of nanotubes. Supplied with this necessary background, chapter 3 will turn to the experimental characterization of such arrangements of tubes, called nanotube ropes. The findings will yield information on current transport in nanotube ropes and, even more important, they will show that the most important task in research in nanotube electronics is the study of the electronic interaction of the tubes. Exactly this point will be the topic of chapter 4. The measurements will supply valuable information on the coupling mechanism between and coherent transport in carbon nanotubes. Chapter 5 will move on to deepen the insight in the mechanisms having influence on current transport.
9 Chapter 2 Basics First of all, it has to be clarified, what a carbon nanotube is, how it is constructed from carbon atoms, and what it looks like. The beauty and aesthetics of these hollow molecules goes along with a high rigidity and strength. But the main interest of this work is in the electronic characteristics of nanotubes and their interaction with one another. Thus, once a picture of the geometry of the nanotubes is drawn, the second section will move on to their electronic behavior. The band structure of the tubes will be studied thoroughly and conclusions will be drawn about current transport in a single tube. The fundamental implications of the reduced dimensionality of these systems will be addressed in the third section, before ensembles of tubes can be studied. These ensembles will be the focus of the last chapter, regarding both, their geometry as well as their electronics, before the main part of this investigation can begin. 2.1 Wrapping Up The geometry of the nanotubes is that of a hollow cylinder. The carbon atoms, which the tubes consist of, form the surface of the cylinder and are arranged in a honeycomb crystalline lattice. This lattice is the same as found in the atomic planes of graphite. Graphite consists of planes of atoms stacked onto one another. This description of a crystal is commonly used when dealing with diffraction experiments and is not an appropriate view for most properties of the crystal, but in case of graphite this picture already captures most of the physics. While strong covalent bonds exist between atoms inside a plane, there is only van der Waals interaction between the planes, making graphite a structurally very anisotropic material. Its brittle consistence is the macroscopic evidence for the weak inter-plane interaction. Inside one plane, the carbon atoms are hybridized in the sp 2 configuration. Thus, they share three equal bonds with other carbon atoms in the plane and the remaining electron populates a π orbital perpendicular to this plane. This electron is delocalized, similar to electrons delocalized in rings of benzene. One such atomic plane is called graphene and figure 2.1 shows its crystal structure. Graphite is obtained, when many of these 9
10 10 CHAPTER 2. BASICS Figure 2.1: Honeycomb lattice of graphene, a single atomic plane of carbon atoms, and the crystal structure of graphite, consisting of several stacked graphene planes. The nearest-neighbor distance d is 1.42 Å, a 1 and a 2 are the basis vectors of the underlying Bravais lattice with a = a 1 = a 2 = 3 d =2.46Å. atomic planes are stacked onto one another. The honeycomb lattice of the graphene planes is a triangular Bravais lattice with a two-atom basis. The nanotubes are formed by rolling up such an atomic sheet to a cylinder. The most comprehensive way to picture a nanotube is to make one by oneself. At the end of this work there are some sheets printed with a honeycomb lattice and the reader is welcome to remove them and build his or her own nanotubes. To preserve the periodicity of the lattice, the roll-up may not be completely arbitrary. One has to match the atomic positions at the two edges of the sheet. This is fulfilled only if an atomic site is rolled onto another equivalent site of the lattice. Since the lattice is a Bravais lattice with a basis not all atomic positions are equivalent. (This can be seen e.g. in figure 2.1 where half of the atoms have a bond going strictly to the right, the other half has a bond going to the left.) The triangular Bravais lattice connects all equivalent positions by the lattice vectors R = n 1 a 1 + n 2 a 2. Let us consider the vector C = n a 1 + m a 2, as shown in figure 2.2. If the sites connected by C are positioned onto one another when rolling up the cylinder, the lattice matches and a nanotube is constructed successfully. The vector C now runs around the circumference of the tube, which is hence called a (n,m) tube. (One of the do-it-yourself sheets results in a (10,10) tube, another in a (13,6) tube.) The circumference is simply C and the diameter of a(n,m)tubeis D = C /π = a/π n 2 + n m + m 2 (2.1) (a = 2.46 Å) . The vector P =ñ a 1 + m a 2 marks the new periodicity along the tube axis, with ñ =(n +2m)/lcd and m =( 2n m)/lcd and lcd being the largest common divisor of (n +2m) and( 2n m). For tubes build in this way, the term Single Wall carbon NanoTube or simply SWNTestab-
11 2.1. WRAPPING UP 11 Figure 2.2: Graphene sheet with a lattice vector C =5 a 1 +2 a 2.When rolling up the sheet to a (5,2) nanotube, the atomic sites connected by C have to be matched. The shaded area will then be the cylinder surface and C will run around the circumference. The vector P =3 a 1 4 a 2 is perpendicular to C and marks the periodicity of the nanotube along its axis. lished. Some examples of tubes with different roll-up vector are depicted in figure 2.3. Now, every choice of n and m results in a nanotube with somewhat different appearance, but not all types of tubes are really unique. E.g. the (13,6) and (6,13) tubes are just mirror images. Moreover, the threefold symmetry of the lattice further degenerates the choices for n and m, making for example the tubes (10,10), (-10,-10), (10,-20), (-10,20), (20,-10), and (-20,10) indistinguishable. Figure 2.4 depicts the choices for (n,m) which are sufficient to reproduce all tubes possible. Some combinations result in tubes of special symmetry. The (n,0) tubes are called zig-zag tubes since their circumference runs along a zig-zag chain of bonds as shown in figure 2.3. The bonds following the circumference of the (n,n) tubes form an armchair (an example for the imagination of physicists), as can be seen best at one of the do-it-yourself models. The remaining tubes are called helical or chiral. These symmetries will become very important in the next section, when talking about the electronic structure of the tubes. To get an impression what the size of the nanotubes is, let s put some numbers. A typical tube of the (10,10) type (an armchair tube) has a diameter
12 12 CHAPTER 2. BASICS Figure 2.3: Several examples for tubes with different roll-up vectors . The diameters of the tubes, according to formula 2.1, is 0.68 nm, 0.7 nm, and 1.04 nm, respectively. Note the zig-zag like chain of bonds running around the circumference of the (9,0) tube and the armchair like chain in the case of the (5,5) tube. of 1.35 nm. Of course, such a tube is not visible by bare eye, and a bundle of 100 of them is necessary to be spotted with the best optical microscope. Using scanning tunneling microscopy [10, 11, 12] the crystalline structure of the tubes was verified. Despite their small diameter, their length can be micrometers , which makes nanotubes the (geometrically) most anisotropic molecules in the world. E.g. the do-it-yourself model (appendix A) of the (10,10) tube has a diameter of 4.5 cm, so in a consistent scale it should extend over 50 m from end to end. Concerning the ends of the nanotube, these are the points where the symmetry, or better the periodicity, of these macromolecules is broken. Sometimes the tubes are found to terminate in an open end, quite like rolled-up paper sheets serving as the simplest models for nanotubes, but usually they are closed by hemispherical end caps . Figure 2.5 shows the end of a nanotube with a cap being one half of a spherical fullerene (a bucky ball ) of the appropriate size
13 2.1. WRAPPING UP 13 Figure 2.4: Any nanotube possible can be constructed by connecting the origin (large gray circle) with one of the marked lattice sites by a vector C, which serves as roll-up vector for the tube. The use of any unmarked lattice site results (due to the symmetry) in a nanotube which can also be constructed by using one of the marked sites. (cf. figure 2.3). The end caps, which are curved in two dimensions, are formed by inserting pentagons in the usual hexagonal structure of the graphene sheet. Until now, the only restriction posed on constructing nanotubes has been the conservation of the lattice periodicity of the graphene honeycomb lattice. But the roll-up process itself already changes the graphene structure since the atomic sheet is no longer planar. It is clear that there has to be a limit for the curvature of the sheet the bonds between the carbon atoms are able to follow. And indeed the smallest tube found to date has a diameter of 0.5 nm, which corresponds to a (6,0) tube . On the other hand, the largest tubes with diameters of 20 nm or more are expected to collapse due to interactions with other tubes or a substrate (figure 2.6). But between these extremes all possible structures are found when producing nanotubes. Of course, the tubes are not produced by rolling up atomic sheets of carbon atoms, but they grow under certain conditions in a self-assembly process (for a review, see ). The material for the growth is usually supplied by an arc discharge between graphite electrodes or by evaporizing material from a block of graphite with a laser beam. Chemical vapor deposition of carbon is another process in development. The carbon material deposits on an anode
14 14 CHAPTER 2. BASICS Figure 2.5: Model of the end of a (10,10) tube . Like in fullerenes, the hemispherical end cap is formed by inserting pentagons into the usual array of hexagons. or on the walls of the process chamber and the nanotubes grow starting at a carbon pentagon or part of an end cap, which are discussed to act as nucleation centers. In most processes, some metallic catalysts like Ni or Co are added to the graphite to help producing large (i.e. long) nanotubes rather than the short, more or less spherical fullerenes. According to the growth conditions, the produced nanotube material, the soot, contains various forms of nanotubes together with fullerenes, graphite and amorphous carbon. The most prominent process to produce large quantities of single-wall carbon nanotubes is a laser ablation technique developed at Rice University . It produces nanotubes with diameters close to 1.35 nm, the diameter of the (10,10) tube. Though first investigation pointed to most of the tubes being indeed of the (10,10) type , newer results showed that no special chirality (a single (n,m) type) is preferred . To date, no technique is known to produce a specific type of nanotube, though it would clearly be very valuable for research and future applications. Once the nanotubes are separated from the by-products of the process, the investigation can begin. It turned out, that the tubes are very rigid and stable structures. When placed on a surface and manipulated with a tool like an atomic force microscope, they can sometimes be moved, but it proved very difficult to cut them into parts . They showed very high bending stiffness with a Young modulus of several TPa [19, 20]. Their sound velocity, which is a measure for the elasticity modulus and hence for their rigidity, is around m/s  (compare this to Y 0.2 TPa and c = m/s for steel).the nanotubes were observed to follow the surface corrugation of the substrate. In
15 2.2. HOW DO ELECTRONS FEEL IN HERE? 15 Figure 2.6: Cross section of several single wall nanotubes (SWNTs) with different diameters lying on a substrate. While the van der Waals interaction tries to increase the contact area between tube and surface, the strain build up when deforming the tube tries to maintain the circular shape. The resulting equilibrium depends on the rigidity of the tube. The larger tubes can be deformed more easily and are thus flattened by interaction with the surface . view of their rigidity, this is only possible if the adhesion forces are rather strong. The adhesion was found to be around 0.8 ev/å. This is strong enough to overcome the deformation energy needed to flatten the tube (figure 2.6) or to bend it along surface features, and it explains, why it is often difficult to move tubes over the surface. In conclusion, carbon nanotubes are found to be small hollow cylinders with the atoms arranged in a graphene crystal structure on the cylinder surface. The tubes can be characterized by a roll-up vector (n,m) which determines their diameter and the alignment of the lattice with respect to the tube axis. The tubes are very long compared to their diameter (several micrometers compared to several nanometers), stable and rigid, while at the same time flexible enough to be bend along obstacles or to be pushed over the surface of a substrate without being broken apart. Thus, they are often cited as ideal candidates for tayloring nanoelectronic devices. For this purpose, their electronic characteristics have to be studied, which will be the topic of the next section. 2.2 How Do Electrons Feel In Here? The relation between nanotubes and graphene regarding the geometry was already pointed out in the last section. This relation holds for the electrical properties, too. In graphene, three of the four valence electrons of carbon are hybridized in sp 2 orbitals forming covalent σ bonds to neighboring atoms, and the one remaining electron populates a p orbital. The half filled p orbital forms a valence bond with one bonding π and one antibonding π orbital with the neighboring atoms. The bonding π orbital is filled, while the antibonding π orbital is empty. Since all atoms contribute an electron in a p orbital, the electrons in the π orbital are delocalized over the entire atomic plane. This
16 16 CHAPTER 2. BASICS Figure 2.7: Graphene lattice with basis vectors a 1 and a 2 (left hand side) and the corresponding reciprocal lattice with basis vectors b 1 and b 2 (right hand side). The hexagons in the reciprocal lattice are constructed according to the method of Wigner and Seitz, the shaded one being the first Brillouin zone. situation is very similar to the delocalized electrons in conjugate double bonds, e.g. in benzene. But in contrast to benzene, where the electrons are restricted to the limited geometry of the benzene ring, in graphene the electrons can move in the plane with momentum. This leads to the band structure of graphene. The band structure is usually presented in the first Brillouin zone of the reciprocal lattice which is depicted in figure 2.7 for graphene. The Bravais lattice of graphene is a triangular lattice with lattice constant a = 2.46Å. The reciprocal lattice is thus triangular, too, with a lattice constant b =4π/ 3a and the first Brillouin zone has the shape of a hexagon. Figure 2.8 shows the band structure along several directions in the Brillouin zone. The lower lying valence bands formed by the σ bonds (as well as the antibonding σ bands) and the bonding and antibonding π and π bands can be seen, the π and π bands being emphasized. As was stated above, the bonding π orbital is filled and the antibonding π orbital is empty, thus the Fermi level is located between these bands. The π and π bands touch at the K points, that is in the corners of the Brillouin zone, and the Fermi level is fixed by this. Since the bands only touch the Fermi level (and one another) at single points of the Brillouin zone, the density of states vanishes at the Fermi level and graphene is a zero gap semiconductor. (In graphite, the weak interaction between the graphene planes results in a small overlap of the π and π bands, hence a small but finite density of states at the Fermi level is populated with electrons and holes, which makes graphite a semimetal [22, 23].) Since electronic transport is dominated by the electronic states near the Fermi level, the K points are the most interesting ones in the Brillouin zone. Figure 2.9 shows the electronic bands near the Fermi level, i.e. parts of the π and π bands. The bands touch at the corners of the Brillouin zone and form
17 2.2. HOW DO ELECTRONS FEEL IN HERE? 17 Figure 2.8: Band structure of graphene along several lines in the Brillouin zone . The σ and σ bands are derived from the sp 2 hybrid orbitals forming the in-plane bonds. The π and π bands (emphasized) result from the delocalized p orbitals. Since every carbon atom supplies one electron in its p orbital, the bonding π band is completely filled, while the antibonding π band is completely empty. These bands touch each other at the K point, that is in the corners of the Brillouin zone, and thus the Fermi level is pinned there, making graphene a zero gap semiconductor. cones above and below the Fermi level. They can be described by E(k) =E F ± h v F k k F where k F is the momentum at a K point and v F = v(k) = h 1 de/dk 10 6 m/s. This description holds for a range of several hundred mev around the Fermi level . The states at the K points can be visualized very easily. In figure 2.10 the six momenta belonging to the K points are retransferred into the real lattice. They correspond to a motion along the zig-zag directions in the graphene lattice. This is reasonable, as the zig-zag directions offer the most
18 18 CHAPTER 2. BASICS Figure 2.9: Three dimensional plot of the graphene band structure near E F. The cones represent the π and π bands touching in the corners of the Brillouin zone. Figure 2.10: The momenta of the K points in the reciprocal lattice (right hand side) correspond to a motion along the zig-zag directions in the graphene sheet (left hand side). direct path for current transport if transport is imagined as moving an electron from bond to bond. Now, what are the similarities and differences between the electronic structure of graphene and the one of nanotubes? Geometrically, a nanotube is constructed from graphene by rolling up the sheet of atoms to a cylinder, matching the lattice at the ends of the sheet. So, the (n,m) nanotube is periodic along
19 2.2. HOW DO ELECTRONS FEEL IN HERE? 19 Figure 2.11: A graphene sheet as to be rolled up to form a (3,3) tube. In the real lattice (left hand side), the vectors P and C mark the periodicity of the tube along and perpendicular to its axis. The tube surface is shaded. In the reciprocal lattice (right hand side), the dashed lines cutting through the Brillouin zone mark the momenta which are allowed states in the nanotube according to quantization condition 2.2. its circumference with C = n a 1 + m a 2. This is a first class example for periodic boundary conditions. All electronic states of the tube have to match this periodicity around the circumference and thus their momentum k has to fulfill k C =2π i, (2.2) with i as an integer . This is the quantization condition for the momentum parallel to C, around the circumference, while the momentum along the tube axis (i.e. parallel to P ) is not affected. In figure 2.11 the situation is visualized. The dashed lines mark the momenta which satisfy the quantization condition and are thus allowed states in this special nanotube. The only degree of freedom for the electronic momentum left is along the tube axis. The momentum around the circumference is quantized. Thus, each of the bands of graphene (σ, σ, π and π ) splits into a number of one dimensional subbands (labeled by i), depicted in figure The subbands are called one dimensional, although the belonging wave functions extend over the cylinder circumference and most of the subbands (i 0) even possess angular momentum. This is in analogy to the electronic wave functions of atoms, where the states are called zero dimensional while they have an extent in three directions. One puzzling fact about figure 2.12 has to be addressed, that is the unusual presentation of the band scheme. The bands in this diagram extend over different ranges along the momentum axis. That is, since the quantization of the graphene band structure in the first Brillouin zone was done solely according to the periodic boundary condition along the circumference. As soon as the quantization lines reach the boundary of the first Brillouin zone of the graphene plane, the subbands end in this figure. But now, one has a new, one dimensional crystal with a new periodicity along the tube axis. The periodicity along
20 20 CHAPTER 2. BASICS Figure 2.12: When the grahene sheet is rolled up to a nanotube, every band of the graphene band structure (figure 2.8) splits into several subbands labeled by the subband index i, according to quantization condition 2.2. The momentum k is directed along the nanotube axis. The emphasized lines in the figure are the subbands of the π and π bands of graphene.
21 2.2. HOW DO ELECTRONS FEEL IN HERE? 21 Figure 2.13: Band structure of a (3,3) nanotube. This is derived by splitting the band structure of graphene (figure 2.8) into subbands according to quantization condition 2.2 (figure 2.12), and subsequently transferring all subbands into the first Brillouin zone of the nanotube ( π/a < k < π/a). Again, the π and π derived subbands are emphasized. Two of these subbands meet at the Fermi level.
22 22 CHAPTER 2. BASICS the axis is P, thus all subbands have to be transferred into the first Brillouin zone of the tube between π/ P <k<π/ P. This is done in figure 2.13 for the (3,3) tube, the example considered so far. As in all the (n,n) tubes, the period along the tube axis is P = a, and these parts of the subbands which reach beyond the new Brillouin zone boundaries of ±π/a are retransferred. Now, what can be learned from the band structure of the nanotubes when they are derived in this way? In graphene, the states at the corners of the Brillouin zone were the most interesting ones regarding electronic transport. At these K points the π and π bands meet at the Fermi level. Are these states allowed in a (n,m) nanotube, that is, do they fulfill the quantization condition? The band structure of the (3,3) tube depicted in figure 2.13 supplies an example, where the K points are included in the quantization condition (for i =0andi = ±3) and thus the appropriate subbands derived from the π and π bands meet at the Fermi level. In contrast to graphene, where the bands meet at the boundary of the Brillouin zone (more precisely, at the corners), the meeting points are now inside the Brillouin zone of the tube at ±2/3 π/a. As was discussed in the last paragraph, this happens due to the new periodicity of the tube. When evaluating the condition at the K points, one finds that if n m =3 i! (i integer), the K points are allowed states in a (n,m) tube . That means, if n-m is not an integer multiple of 3, there are no states at the Fermi level allowed and the filled π and empty π subbands are separated by a finite energy gap. Thus, these tubes are semiconductors, e.g. a (13,6) tube has a gap of about 600 mev. On the other hand, when n-m is an integer multiple of 3, the K points are allowed states and at least one pair of the π and π subbands meets at the Fermi level. Again, it is a good approximation to describe the bands by a linear dispersion around E F  E(k) =E F ± h v F (k k F ), but now the states are one dimensional (note that k and k F are now scalars). A linear dispersion relation in one dimension leads to a finite density of states and hence the tubes with n m = 3i are metallic. Sorting all possible tubes into these categories, one third of the tubes is found to be metallic, i.e. fulfill n m =3i, while two thirds of the tubes are semiconducting. Figure 2.14 shows how the quantization condition results in semiconducting and metallic tubes when applied to the graphene band structure. If the quantization lines include the K points, the tube is metallic (i.e. there are allowed states at the Fermi level), while if the quantization misses these points, an energy gap between the subbands results. As can be seen, the closer the quantization is to the K points, the smaller the energy gap will be. When the diameter of the tube is increased, the interval between quantization lines for different subbands i shrinks and the lines draw closer to the K points, so the gap decreases. Using the quantization condition 2.2, one can compute how close (or how far) the K points are missed by the subbands. For a metallic tube, this distance is zero, at least for one subband-k point combination. For
23 2.2. HOW DO ELECTRONS FEEL IN HERE? 23 Figure 2.14: Three dimensional plot of the graphene band structure near E F, together with the quantization condition marked by dashed lines in the Brillouin zone. The resulting subbands in the π and π bands (cones) are marked by the dotted lines. The upper figure shows a situation, where the K points are included in one of the quantization lines (i=0 and i=±3), hence the belonging subbands are touching (or crossing) at the Fermi level and the tube is metallic. In the lower figure, the quantization lines miss the K points and a finite energy gap between the subbands results.
24 24 CHAPTER 2. BASICS Figure 2.15: Construction of a (2,4) tube from a graphene sheet. The quantization lines in the reciprocal lattice are spaced by k =2π/ C and miss the K points by at least δk =1/3 k. the semiconducting tubes the minimum distance between a subband and a K point turns out to be δk = 1 3 2π C = 1 3 k, where k is the separation of the quantization lines in the reciprocal space (cf. figure 2.15). Due to the linear dispersion relation around the K points, the energy gap for the semiconducting tubes turns out to be E gap =2 hv F δk = 3 hv 2 2π F C = 4 hv F 3 D. (2.3) The gap energy scales inversely with the diameter D of the tube. Figure 2.16 visualizes the dependence of the gap energy on the tube diameter. These are the main features in the band structure of nanotubes, summarized in the statement that, depending on the roll-up vector (n,m), the tubes can be metallic (n m = 3i) or semiconducting (n m 3i). But there are two add-ons to be made. The first add-on concerns the value and the multiplicity of the Fermi points in the metallic nanotubes. It seems to be, there should be six Fermi points according to the six K points of the graphene Brillouin zone, but this is not the case. Each K point belongs to three Brillouin zones so only 6 1/3 =2 K points belong to the first Brillouin zone. This becomes clear, if another unit cell is chosen for the first Brillouin zone as in figure Unambiguously, this unit cell contains only two K points and the quantization results in two Fermi points for the nanotube. Figure 2.18 shows a band structure calculation for a (10,10) tube, in which the subbands resulting from the quantization of the graphene band structure, as well as the Fermi points at ±2/3 π/a can be seen.
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