One-Dimensional Nanostructures: CNT (1)

Dr. Chunlei (Peggy) WangMechanical and Materials Engineering

Florida International University

CNTs

• Carbon nanotubes (CNT) were discovered by Iijima as elongated fullerenes in 1991.

• CNTs posses unique electronic and extraordinary mechanical properties.

• They can be metallic or semiconducting. • The high strength, elastic modulus, and

other mechanical properties hold promise for high-strength composites for structural applications. The potential of CNTs in a wide range of applications includes nanoelectronics, sensors, hydrogen storage …

W. Z. Zhu et al., Mater. Chem. Phys., 82 (2003) 638–647

S. Lebedkin et al., Carbon, 40 (2002) 417-423

Introduction

Carbon Nanotubes (CNT) History

.In 1960 scientist Roger Bacon of Union Carbideproduced structures of graphitic basal layers (e.g.graphene sheets) rolled into scrolls, supporting hisdiscovery with both diffraction and microscopy data.

.Nearly two decades later, Peter Wiles, JonAbrahamson, and Brian Rhoades of the University ofCanterbury reported finding hollow fibers on theanode of a carbon arc-discharge apparatus. Thesefibers consisted of concentric layers of wrappedgraphene, spaced by essentially the usual graphiticinterlayer separation (c.a. 0.34 nm).

. Actual discovery was not until 1991 by electronmicroscopist Sumio Iijima at NEC Japan.

.Carbon nanotubes belong to the fullerene family.

• A single-wall carbon nanotube (SWCNT) - a rolled-up tubular shell of

graphene sheet which is made of benzene-type hexagonal rings of carbon

atoms. The ends are capped by half-dome-shape half-fullerene molecules.

• The nature curvature in the side walls is due to the rolling of the sheet into

the tubular structure, whereas the curvature in the end caps is due to the

presence of topological (pentagonal rings) defects in the other wise

hexagonal structure of the underlying lattice. The role of a pentagonal ring

defect is to give a positive curvature to the surface, which helps in closing

the tube at the two ends.

Y. Zhang et al., Phil. Mag. Lett, 79 (1999) 473- 479S. Lebedkin et al., Carbon, 40 (2002) 417-423

Robert F. Curl, Jr., Sir Harold W. Kroto, and Richard Smalley Winners of the 1996 Nobel Prize in Chemistry12 pentagons

20 hexagons

SWCNT

Carbon bonding

If carbon forms bonds with only three other atoms (sp2 hybridization), the remaining valence electron forms a double-bond, also known as a π bond. Π bonds are delocalized. That is, they can be shared over an entire molecule like the valence electrons of metal can be shared by all atoms.. Delocalized π bonds provide interesting electrical and optical properties.. Fullerenes and nanotubes have each carbon bonded to three nearest neighborcarbons and thus have pi bonds throughout their structure.

SWCNT

M. Meyyappan and D. Srivastava, Handbook of Nanoscience, Engineering and Technology

armchair

zigzag

armchair

zigzag

chiral

SWCNT

SWCNT

� The (n,m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite graphene sheet that describes how to "roll up" the graphene sheet to make the nanotube. T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space.

� Armchair nanotubes: \_/¯\_/ shape is perpendicular to the tube axis and have a symmetry along the axis with a short unit cell (0.25 nm) that can be repeated to make the entire section of a long nanotube.

� Zigzag nanotubes: \ /\ / shape is perpendicular to the axis as well as the short unit cell (0.43 nm) along the axis.

� All the remaining nanotubes are known as chiral or helical nanotubes and have a longer unit cell sizes along the tube axis.

SWCNT

SWCNT & MWCNT

• A multi-wall carbon nanotube (MWCNT) is a rolled-up stack of graphene

sheets into concentric SWCNTs, with the ends again either capped by half-

fullerenes or kept open.

W. Z. Zhu et al., Mater. Chem. Phys., 82 (2003) 638–647

W. Z. Zhu et al., Mater.c Chem. Phys., 82 (2003) 638–647

P. Collins et al., Science 292, 706-709 (2001) 6.

MWCNT

• The arc process involves striking a DC discharge in an inert gas (such as argon or helium) between a set of graphite electrodes.

• The electric arc vaporizes a hollow graphite anode packed with a mixture of a transition metal (such as Fe, Co, or Ni) and graphite powder.

• The inert gas flow is maintained at 50-600 Torr. Nominal conditions involve 2000 to 3000 oC, 100 amps, and 20 volts. This produces SWCNTs in a mixture of MWCNTs and soot.

• The gas pressure, flow rate, and metal concentration can be varied to change the yield of nanotubes; but these parameters do not seem to change the diameter distribution. Typical diameter distribution of SWCNTs by this process appears to be 0.7 to 2 nm.

Graphite electrode arc chamber1, motor for anode position control to maintain a

constant inter-electrode distance, a similar arrangement is used at the cathode;

2, quartz optical access ports for plasma

spectroscopy;

3, anode;

4, cathode; 5, electrode holding platform within vacuum

chamber;

6, glass dumbbell.

K. Saidane et al., J. Phys. D: Appl. Phys. 37 (2004) 232–239

Synthesis – Arc Process

• A target consisting of graphite mixed with small amount of transition metal particles as catalysts is placed at the end of a quartz tube enclosed in a furnace.

• The target is exposed to an argon ion laser beam that vaporizes graphite and nucleates carbon nanotubes in the shockwave just in front of the target. Argon flow through the reactor heated to about 1200 oC by the furnace carries the vapor and nucleated nanotubes, which continue to grow. The nanotubes are deposited on the cooler walls of the quartz tube downstream from the furnace. This produces a high percentage of SWCNTs (~70%) with the rest being catalyst particles and soot.

A. A. Puretzky et al., Appl. Phys. A, 70 (2000) 153–160.

Synthesis – Laser Ablation

• A thermal CVD reactor is simple and inexpensive to construct consisting of a quartz tube enclosed in a furnace.

• The substrate material may be Si, mica, quartz, or alumina. The setup needs a few mass flow controllers to meter the gases and a pressure transducer to measure the pressure. The growth may be carried out at atmospheric pressure or slightly reduced pressures using a hydrocarbon or CO feedstock.

• The growth temperature is in the range of 700 to 900 oC. A high kinetic energy (and thus a high temperature, >900 oC) and a limited, low supply of carbon are necessary to form SWCNTs. CO and CH4 are the two gases that have been reported to give SWCNTs.

J. W. Seo et al., New Journal of Physics 5 (2003) 120.1–120.22.

Synthesis – CVD (1)

• MWCNTs are grown using CO, CH, and other higher hydrocarbons at lower temperatures of 600 to 750 oC.

• CNT growth requires a transition metal catalyst. The type of catalyst, particle size, and the catalyst preparation techniques dictate the yield and quality of CNTs.

• Plasma-grown nanotubes appear to be more vertically oriented than that possibly by thermal CVD. This is attributed to the electric field in the plasma normal to the substrate. Because the plasma is very efficient in tearing apart the precursors and creating radicals. Plasma-based CVD always results in MWCNTs and filaments.

• The plasma reactor consists of a high vacuum chamber to hold the substrate, mass flow controllers, a mechanical (roughing) pump and, if necessary, a turbopump (plasma reactors almost always run at reduced pressure, 0.1 to 50 Torr), pressure gauges, and discharge source.

H. Sato et al., J. Vac. Sci. Technol. B 21 (2003) 2564-2568

S. G. Wang et al., Diamond and Related Materials

12 (2003) 2175–2177

Synthesis – CVD (2)

• First, 0.5 g (0.09 mmol) of Pluronic P-123 triblock

copolymer is dissolved in 15 cc of a 2:1 mixture of ethanol

and methanol.

• Next, SiCl4 (0.85 cc, 7.5 mmol) is slowly added using a

syringe into the triblock coploymer/alcohol and stirred for

30 minutes at room temperature. Stock solutions of AlCl

•6H2O, CoCl2 • 6H2O, and Fe(NO3)3 • 6H2O are prepared

at the concentration of the structure-directing agent (SDA)

and inorganic salts.

• The catalyst solutions are filtered through 0.45 um

polytetrafluoroethylene (PTFE) membranes before being

applied to the substrate. The substrate with the catalyst

formation is loaded onto a furnace and heated at 700 oC

for 4 hours in air or render the catalyst active by the

decomposition of the inorganic salts and removal of the

SDA.

• A nanotube tower with millions of multi-walled tubes

supporting each other by van der Waals force is seen. If

the catalyst solution forms a ring during annealing, then a

hollow tower results. M. Meyyappan and D. Srivastava, Handbook of Nanoscience, Engineering and Technology

Synthesis – Catalyst Preparation (1)

• A typical solution-based technique for catalyst preparation involves several steps lasting hours.

• Physical process such as sputtering and e-beam deposition not only can deal with very small patterns but are also quick and simple in practice.

• Delzeit et al. reported catalyst preparation using ion beam sputtering wherein an underlayer of Al (~ 10 nm) is deposited first, followed by 1 nm of Fe active catalyst layer.

• Methane feedstock at 900 oC was used to produce SWCNTs. The same catalyst formation at 750 oC with ethylene as the source gas yields MWCNT towers.

M. Meyyappan and D. Srivastava, Handbook of Nanoscience, Engineering and Technology

Synthesis – Catalyst Preparation (2)

• One of the most successful approaches to obtain oriented arrays of nanotubes uses a nanochannel alumina template for catalyst patterning.

• First, alumina is anodized on a substrate such as Si or quartz, which provides ordered, vertical pores. Anodizing conditions are varied to tailor the pore diameter, height, and spacing between pores.

• This is followed by electrochemical deposition of a cobalt catalyst at the bottom of the pores.

• The catalyst is activated by reduction at 600 oCfor 4 hours.

• The figure shows an ordered array of MWCNTs(mean diameter 47 nm) grown by CVD from 10% acetylene in nitrogen. The use of a template provides uniformity and vertically oriented nanotubes.

M. J. Kim et al., Thin Solid Films 435 (2003) 312–317

M. Meyyappan and D. Srivastava, Handbook of Nanoscience, Engineering and Technology

Synthesis – Catalyst Preparation (3)

• Instead of supported catalysts, nanotubes have also been produced using catalysts in the gas phase. This approach is designed for production of large quantities of nanotubes in a continuous process.

• The earliest report of such a process involved pyrolysis of a mixture containing benzene and a metallocene (such as ferrocene, cobaltocene, or nickelocene).

• In the absence of metallocene, only nanospheres of carbon were seen, but a small amount of ferrocene yields large quantities of nanotubes.

• The growth system uses a two-stage furnace wherein a carrier gas picks up the metallocene vapor at around 200 oC in the first stage, and the decomposition of the metallocene as well as pyrolysis of the hydrocarbon and catalytic reactions occur in the second stage at elevated temperatures (>900 oC).

• Acetylene with ferrocene or iron pentacarbonyl at 1100 oC has been shown to yield SWCNTs in a continuous process in the same two-stage system.

Synthesis – Continuous, High-Throughput Processes

1. Please read a paper about how to grow Y or T junction carbon nanotubes.

Please describe the synthesis procedure and the form mechanisms of Y

or T junctions

HW