Science and Application of Nanotubes
1st Edition
Published on 17. December 2005
XIII, 398 pages
978-0-306-47098-1 (ISBN)
Description
This series of books, which is published at the rate of about one per year, addresses fundamental problems in materials science. The contents cover a broad range of topics from small clusters of atoms to engineering materials and involve chemistry, physics, materials science, and engineering, with length scales ranging from Ångstroms up to millimeters. The emphasis is on basic science rather than on applications. Each book focuses on a single area of current interest and brings together leading experts to give an up-to-date discussion of their work and the work of others. Each article contains enough references that the interested reader can access the relevant literature. Thanks are given to the Center for Fundamental Materials Research at Michigan State University for supporting this series. M. F. Thorpe, Series Editor E-mail: thorpe@pa. msu. edu East Lansing, Michigan V PREFACE It is hard to believe that not quite ten years ago, namely in 1991, nanotubes of carbon were discovered by Sumio Iijima in deposits on the electrodes of the same carbon arc apparatus that was used to produce fullerenes such as the "buckyball". Nanotubes of carbon or other materials, consisting ofhollow cylinders that are only a few nanometers in diameter, yet up to millimeters long, are amazing structures that self-assemble under extreme conditions. Their quasi-one-dimensional character and virtual absence of atomic defects give rise to a plethora of unusual phenomena.
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D. Tománek | R.J. Enbody
Science and Application of Nanotubes
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03/2000
Kluwer Academic / Plenum Publishers
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Content
Morphology, Characterization, and Formation of Nanotubes.- Filling Carbon Nanotubes Using an ARC Discharge.- Simulation of STM Images and STS Spectra of Carbon Nanotubes.- Applications Research on Vapor-Grown Carbon Fibers.- The Growth of Carbon and Boron Nitride Nanotubes: A Quantum Molecular Dynamics Study.- Nanoscopic Hybrid Materials: The Synthesis, Structure and Properties of Peapods, Cats and Kin.- Linear Augmented Cylindrical Wave Method for Nanotubes: Band Structure of [Cu@C20]?.- Comparative Study of a Coiled Carbon Nanotube by Atomic Force Microscopy and Scanning Electron Microscopy.- Investigation of the Deformation of Carbon Nanotube Composites Through the Use of Raman Spectroscopy.- Electronic States, Conductance and Localization in Carbon Nanotubes with Defects.- Physics of the Metal-Carbon Nanotube Interfaces: Charge Transfers, Fermi-Level "Pinning" and Application to the Scanning Tunneling Spectroscopy.- Single Particle Transtort Through Carbon Nanotube Wires: Effect of Defects and Polyhedral Cap.- Carbon Nanotubes from Oxide Solid Solution: A Way to Composite Powders, Composite Materials and Isolated Nanotubes.- Impulse Heating an Intercalated Compound Using a 27.12 MHz Atmospheric Inductively Coupled Argon Plasma to Produce Nanotubular Structures.- The Synthesis of Single-Walled Carbon Nanotubes by CVD Catalyzed with Mesoporous MCM-41 Powder.- Mechanical and Chemical Properties of Nanotubes.- Mechanical Properties and Electronic Transport in Carbon Nanotubes.- Electrochemical Storage of Hydrogen in Carbon Single Wall Nanotubes.- Direct Measurement of Binding Energy Via Adsorption of Methane on SWNT.- Electronic Properties of Nanotubes.- Electrical Properties of Carbon Nanotubes: Spectroscopy Localization and Electrical Breakdown.- Field Emissionof Carbon Nanotubes from Various Tip Structures.- First and Second-Order Resonant Raman Spectra of Single-Walled Carbon Nanotubes.- On the ? - ? Overlap Energy in Carbon Nanotubes.- Electronic and Mechanical Properties of Carbon Nanotubes.- Low Energy Theory for STM Imaging of Carbon Nanotubes.- Quantum Transport in Inhomogeneous Multi-Wall Nanotubes.- Conductivity Measurements of Catalytically Synthesized Carbon Nanotubes.- Applications of Nanotubes.- Fabrication of Full-Color Carbon-Nanotubes Field-Emission Displays: Large Area, High Brightness, and Hig Stability.- Free Space Construction with Carbon Nanotubes.
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 Datta2
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.
Yongqiang Xue1 and Supriyo Datta2
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.
