Science and Application of Nanotubes
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D. Tomanek, R.J. Enbody (Eds.)
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Science and Application of Nanotubes
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Kluwer Academic Publishers
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9780306470981
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Naturwissenschaft
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This volume is a result of the Nanotube '99 Workshop, held in late July 1999. It is the most up-to-date compendium representing the state of the art in the rapidly developing field of nanotubes. An important focus is on the relative merits of the various techniques used to synthesize nanotubes of carbon and other materials. Another important issue addressed is the effect of synthesis conditions on the multitude of morphologies that have by now been observed, and the characterization of the structures. Important advances are reported in the understanding of mechanical and chemical properties of nanotubes, including their potential use for hydrogen storage and nano-manipulators in micro-electromechanical systems (MEMS). The unusual electronic properties of nanotubes, which seem to attract most attention in the field, are represented accordingly, alongside the first likely commercial application of nanotubes in high-efficiency flat-panel displays.
PHYSICS OF THE METAL-CARBON NANOTUBE INTERFACES: CHARGE TRANSFERS, FERMI-LEVEL"PINNING" AND APPLICATION TO THE SCANNING TUNNELING SPECTROSCOPY
(p. 121-122)
Yongqiang Xue1 and Supriyo Datta
2
1 School of Electrical and Computer Engineering, Purdue University,
West Lafayette, IN 47907, USA
Email: yxue@ecn.purdue.edu
2 School of Electrical and Computer Engineering, Purdue University,
West Lafayette, IN 47907, USA
Email: datta@ecn.purdue.edu
INTRODUCTION
After its discovery in 1991,1 carbon nanotube has rapidly emerged as the most promis-- ing candidate for molecular electronics due to its quasi-one dimensional structure and the unique characterization of its electronic structure in terms of two simple geometric indices.2 Besides its huge technological potential, carbon nanotube also serves as the artificial laboratory in which one-dimensional transport can be investigated,3 sim-- ilar to the semiconductor quantum wire.4 However, unlike its semiconductor cousin where transport is mostly ballistic, the study of transport in carbon nanotube has been distressed by the difficulty of making low resistance contact to the measuring electrodes. The high resistances reported in various two- and three-terminal measurements5 have led Tersoff6 (and also independently by one of the authors7) to suggest that wavevector conservation at the metal-carbon nanotube contact may play an important role in explaining the high contact resistance.8 The complexity and importance of the metal-carbon nanotube interface makes it an immediate challenge to both theorists and experimentalists.
The single most important property of the metal-carbon nanotube interface (and in general, of any interface involving metal) is the position of the energy bands (or energy levels) of the nanotube relative to the Fermi-level of the metal which manifests itself in the electronic transport property of the interface. Depending on the contact geometry, transport can occur in the direction parallel to the nanotube axis, in the case of the nanotube field-effect-transistor (FET),5,9 or perpendicular to it, in the case of the STS measurement.10,11 In the STS measurement, the Fermi-level is found to have shifted to the valence band edge of the semiconducting nanotube.10 Such observed Fermi-level"pinning" has been used to explain the operation of the nanotube FETs with highresistance contacts,5 where the measured two-terminal resistance for metallic nanotube is Recently low resistance contacts with two-terminal resistance as low as have been obtained.9 However, low temperature transport measurements using these low resistance contacts show that the Fermi-level is located between the valence and the conductance band of the semiconducting nanotube, instead of being"pinned" at the valence band edge. This conflict raises the important question of whether the Fermi-level positioning may depend on the contact geometry and the interface coupling. In this paper we present a theory of the scanning tunneling spectroscopy of a singlewall carbon nanotube (SWNT) supported on the Au(111) substrate.
The central idea is that the work function difference between the gold substrate and the nanotube leads to charge transfers across the interface, which induce a local electrostatic potential perturbation on the nanotube side. This atomic-scale interfacial potential perturbation shifts the energy level of the nanotube relative to the gold Fermi-level, and gives rise to the observed Fermi-level shift in the STS current-voltage characteristics. However, for transport in the direction parallel to the nanotube axis, as in the case of nanotube transistors, such local potential perturbation at the interface is not important in determining the Fermi-level position if the coupling between the metal and the nanotube is strong (i. e. , low resistance contact). In this case, the metal-induced gap states (MIGS) model provides a good starting point for determining the Fermi-level position. Based on this model, we expect that any discrepancy between the metal Fermi-level and the nanotube"charge-neutrality level" should be rapidly screened out by the metal-induced gap states in the nanotube side,12–14 leading to the"pinning" of the Fermi-level. Another important feature in our theory is that we have taken into account the localized 5d orbitals of the platinum tip in our treatment of the STS which can have significant effects on the interpretation of the STS data.15 Our discussion is restricted to the low temperature regime, in correspondence with the experimental works.
METHODS
A convenient way of characterizing the band lineup problem at any interface is to find a reference level, the role of which is to put all materials forming the interface on a common absolute energy scale.13,14 If the position of the reference level depends only on the bulk property, then the relative position of the energy bands at the interface can be determined trivially by merely lining up the reference levels. This is the elegant idea of"charge-neutrality level",12 which has been applied with impressive success by Tersoff13,14 to various metal-semiconductor junctions and semiconductor heterojunctions. For metal, the reference level is the Fermi-level while for semiconductor, it is the so called"charge-neutrality level" which can be taken as the energy where the gap states cross over from valence- to conduction-band character.
This approach greatly simplifies the band lineup problem and gives quantitatively accurate prediction of the Schottky barrier height when applied to the metalsemiconductor interface.13 The success of this model relies on the fact that there exists a continuum of gap states around at the semiconductor side of the metalsemiconductor interface due to the tails of the metal wavefunction decaying into the semiconductor, which can have significant amplitude a few atomic layers away from the interface.12 Any deviation from the local charge neutrality condition in the interface region will be screened out rapidly by these metal-induced gap states (MIGS). In this way, the local charge and potential perturbations right at the interface are not important in determining the barrier height observed in the transport characteristics since the range of this local perturbation is only a few atomic layers, and the charge carriers can easily tunnel through this region.
SERIES PREFACE
6
PREFACE
8
CONTENTS
11
FILLING CARBON NANOTUBES USING AN ARC DISCHARGE
15
ABSTRACT
15
INTRODUCTION
15
EXPERIMENTAL PROCEDURES
17
STRUCTURAL AND CHEMICAL ANALYSIS
17
ANALYSIS OF THE ROLE OF SULFUR
24
DISCUSSION: GROWTH MECHANISM
24
CONCLUSION
29
SIMULATION OF STM IMAGES AND STS SPECTRA OF CARBON NANOTUBES
31
INTRODUCTION
31
STM THEORY
32
GRAPHITE
34
PERFECT NANOTUBES
36
STM images
38
MULTI-WALL NANOTUBES AND ROPES
41
DEFECTS IN NANOTUBES
42
CONCLUSIONS
45
ACKNOWLEDGMENT
45
REFERENCES
45
APPLICATIONS RESEARCH ON VAPOR-GROWN CARBON FIBERS
48
VAPOR-GROWN CARBON FIBERS
48
INFILTRATION
51
COMPARATIVE FIBER PROPERTIES
54
COMPOSITE THEORY
54
SURFACE TREATMENTS
57
PROPERTIES IMPROVEMENTS
61
ELECTRICAL CONDUCTIVITY
62
ACKNOWLEDGMENTS
63
REFERENCES
63
THE GROWTH OF CARBON AND BORON NITRIDE NANOTUBES: A QUANTUM MOLECULAR DYNAMICS STUDY
65
INTRODUCTION
65
1. MICROSCOPIC GROWTH MECHANISMS FOR CARBON NANOTUBES
66
2. FRUSTRATION EFFECTS AND GROWTH MECHANISMS FOR BORONNITRIDE NANOTUBES
70
CONCLUSION
75
ACKNOWLEDGEMENTS
76
REFERENCES
76
NANOSCOPIC HYBRID MATERIALS: THE SYNTHESIS, STRUCTURE AND PROPERTIES OF PEAPODS, CATS AND KIN
78
INTRODUCTION
78
EXPERIMENTAL METHOD
78
EXPERIMENTAL RESULTS AND DISCUSSION
79
SUMMARY
85
ACKNOWLEDGEMENTS.
86
REFERENCES
86
LINEAR AUGMENTED CYLINDRICAL WAVE METHOD FOR NANOTUBES: BAND STRUCTURE OF
88
INTRODUCTION
88
FORMALISM
88
APPLICATIONS
92
ACKNOWLEDGEMENT
93
REFERENCES
93
COMPARATIVE STUDY OF A COILED CARBON NANOTUBE BY ATOMIC FORCE MICROSCOPY AND SCANNI
