Popov V.N., Lambin P. (eds.) Carbon Nanotubes

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*To whom correspondence should be addressed. Marie-Faith Fiawoo; e-mail: fiawoo@onera.fr

V.N. Popov and P. Lambin (eds.), Carbon Nanotubes, 47–48.

TEM SAMPLE PREPARATION FOR STUDYING THE INTERFACE

CNTS-CATALYST-SUBSTRATE

MARIE-FAITH FIAWOO, ANNICK LOISEAU

LEM UMR 104 CNRS/ONERA, 92322 Châtillon, France

ANNE-MARIE BONNOT, ANTONIO IAIA

LEPES-CNRS UPR11, 38042 Grenoble, France

VINCENT BOUCHIAT

CRTBT-CNRS UPR5001, 38042 Grenoble, France

JANY THIBAULT

TECSEN CNRS UMR 6122, Univ. Paul Cézanne, 13397

Marseille, France

Abstract. We study by Transmission Electron Microscopy (TEM) in-situ self-

assembled carbon nanotubes (CNTs) grown by the Hot Filament Chemical

Vapor Deposition (HFCVD) process on silicon substrates used for electronic

devices. We present the different methods we have developed for extracting

CNTs and studying their interface with the substrate as they are observable by

TEM.

Keywords: TEM preparation technique, nanotubes, SACT, cleavage, ion milling

For studying specimens by TEM and performing analysis (EFTEM, EELS), the

sample needs to be thin (20 to 30 nm), transparent to electrons, smaller than 3

nm diameter to fit in the TEM holder and keep its intrinsic features. We want to

study the interface CNTs-catalyst-substrate in localized CNTs synthesis (Marty

et al., 2002) aiming to understand the CNTs growth by Chemical Vapor

Deposition (CVD) process like previously done for high temperature synthesis

(Loiseau et al., 2004). Here we present different preparation methods which we

tried in order to reach the required thickness necessary for the study of the

interface of nanotubes with a single crystal silicon substrate.

Four types of preparation have been tested: scratching the sample, ion

milling, Si

membrane system, and small angle cleavage technique (SACT).

Some of them enable plane view, cross-section observations or both. Scratching

the sample with a tweezer or a diamond scribe to deposit on a TEM grid seldom

enables observations of the interface because of the thickness of the piece

scratched. Ion milling technique involves a mechanical polishing before

thinning the sample by pulling out atoms of the sample with an ion beam of

argon Ar+ (Fig. 1, left). A disadvantage of this technique is the introduction of

artifacts. The Si

membrane with a rectangular hole has CNTs grown on the

membrane as well as across the hollow. Unfortunately, the hole's edge in not

thin enough to see the interface. SACT consists in cleaving the sample along

the (120) and (110) planes, which form an angle of 18.43° (Fig. 1, right). This

small angle of the apex of the sample allows its transparency to electrons

(Walck et al., 1997). The disadvantage of this technique is the small area

transparent to electrons, which is not adapted for a sample having a few CNTs.

Figure 1. Left: Ion milling preparation stages; Right: Different stages of SACT.

For the time being, the ion milling technique is the only one, which enables

the systematical observation and analysis of the interface. Further

investigations, such as focused ion beam cutting and other membrane systems,

are presently being carried out.

References

Marty, L., Bouchiat, V., Bonnot, A.M., Chaumont, M., Fournier, T., Decossas, S., and Roche, S.

2002, Batch processing of nanometer-scale electrical circuitry based on in-situ grown single-

walled carbon nanotubes, Microelectronic Engineering 61-62:485-489.

Loiseau, A., Gavillet, J., Ducastelle, F., Thibault, J., Stéphan, O., Bernier, P., and Thair, S., 2004,

Nucleation and growth of SWNT: TEM studies of the role of the catalyst, C. R. Physique

4:975-991.

Walck, S.D., and McCaffrey, J.P, 1997, The small angle cleavage technique applied to coatings

and thin films, Thin Solid Films 308-309:399-405.

*To whom correspondence should be addressed. Roman Caudillo; email: rcaudillo@mail.utexas.edu

V.N. Popov and P. Lambin (eds.), Carbon Nanotubes, 49–50.

A METHOD TO SYNTHESIZE AND TAILOR CARBON NANOTUBES

BY ELECTRON IRRADIATION IN THE TEM

R. CAUDILLO,* M. JOSÉ-YACAMAN

Materials Science and Engineering and Texas Materials Institute,

University of Texas at Austin, 1 University Station C2201, Austin,

Texas 78712, USA

H. E. TROIANI

Centro Atómico Bariloche and Instituto Balseiro, CNEA and

UNC, 8400 Bariloche, Rio Negro, Argentina

M. A. L. MARQUES, A. RUBIO

Donostia International Physics Center, Paseo Manuel Lardizábal

4, 20018 San Sebastián, Spain

Abstract. Carbon nanotubes (CNTs) can be synthesized from amorphous

carbon thin films by electron irradiation in the transmission electron microscope

(TEM). We propose that irradiation effects impart a tensile stress and introduce

defects to a CNT thus formed and show that the CNTs can be made to fracture

by a brittle or ductile mechanism. We also report a healing of the carbon film

that is observed after some critical amount of irradiation.

TEM is normally only used to characterize CNTs synthesized by standard

techniques, such as arc discharge, laser ablation, or chemical vapor deposition

(CVD); however, Troiani et al.

1,2

have shown that it is also possible to use

electron irradiation in a TEM to construct single-walled carbon nanotubes

(SWCNT) from an amorphous carbon film. In this method, two through-holes

are created in an amorphous carbon film using electron-beam irradiation in a

TEM. A thin graphitic fiber forms in between the two holes and is narrowed

until forming a SWCNT. Here we show that by controlling the current density

on a CNT formed by this technique, it is possible to cause fracture and to

observe the fracture mechanism in the TEM. We observe that the CNT fractures

either by a brittle or ductile fracture mechanism, depending on the amount of

current density on the sample.

We propose two mechanisms working to achieve the narrowing of the

bridge: (1) deformation in response to an axial stress resulting from a

contraction of the film due to radiation-induced graphitization and (2) creation

of defects in the bridge due to irradiation, resulting in vacancy clustering that

leads to brittle fracture or atomic rearrangements that lead to plastic fracture.

Figure 1 shows CNT formation at the neck of a bridge and the formation of a

linear chain of carbon atoms during the ductile deformation of the CNT.

Figure 1. (a) TEM image showing the formation of a CNT at the neck of a bridge, (b) TEM image

showing the formation of a linear chain of carbon atoms (signaled by arrow) in a CNT neck that

has undergone ductile deformation.

We also observe a surprising phenomenon during prolonged irradiation of

carbon nanotubes and carbon nanofibers formed by electron irradiation in the

TEM, in which a sudden diffusion of carbon atoms to the bridge structure and

periphery of the holes results in a reconstruction of the carbon film, which in

some cases results in a complete closing of the holes.

These results indicate that CNTs, as well as other carbon nanostructures, can

be synthesized and tailored by electron irradiation, and suggests the possibility

of larger scale, carbon nanostructure design by e-beam lithography and

complementary techniques.

References

1. H. E. Troiani, M. Miki-Yoshida, G. A. Camacho-Bragado, M. A. L. Marques, A. Rubio, J. A.

Ascencio, and M. Jose-Yacaman, Direct observation of the mechanical properties of single-

walled carbon nanotubes and their junctions at the atomic level, Nano Letters 3(6), 751-755

(2003).

2. M. A. L. Marques, H. E. Troiani, M. Miki-Yoshida, M. Jose-Yacaman, and A. Rubio, On the

breaking of carbon nanotubes under tension, Nano Letters 4(5), 811-815 (2004).

3. R. Caudillo, H. E. Troiani, M. Miki-Yoshida, M. A. L. Marques, A. Rubio, and M. Jose-

Yacaman, A viable way to tailor carbon nanomaterials by irradiation-induced

transformations, Radiation Physics and Chemistry (In Press) (2005).

V.N. Popov and P. Lambin (eds.), Carbon Nanotubes, 51–52.

SCANNING TUNNELING MICROSCOPY STUDIES OF NANOTUBE-

LIKE STRUCTURES ON THE HOPG SURFACE

I. N. KHOLMANOV,

1,2 1 1

1 1,3

INFM and Dipartimento di Matematica e Fisica, Università

Cattolica del Sacro Cuore, Via dei Musei 41, 25121 Brescia, Italy

Thermophysics Department, Academy of Sciences, Katartal str.,

28, 700135 Tashkent, Uzbekistan

Laboratorio Nazionale TASC-INFM, Strada Statale 14, Km

163.5, Basovizza, I-34012 Trieste, Italy

Abstract. We have studied the scanning tunneling microscopy tip interaction

with the naturally formed nanotube-like (NTL) structures on highly oriented

pyrolytic graphite (HOPG) surface. Shape variations of the NT-like structures,

caused by the modulation of scanning parameters, were observed and analyzed.

Keywords: STM; highly oriented pyrolytic graphite; nanotube-like structures

Scanning tunneling microscopy (STM) and spectroscopy (STS) are widely

employed to obtain information on local electronic properties, surface

geometry, and nanostructure-surface interactions.

We report on STM studies of the NTL structures formed on HOPG surface.

STM (OMICRON ultra-high vacuum (UHV) system) measurements have been

carried out in UHV at room temperature, in constant-current mode using

tungsten tips. HOPG specimen has been prepared by cleaving (adhesive tape) in

air, and annealed in UHV chamber at 900 °C.

Naturally-formed NTL structures located on steps were formed by the

spontaneous folding of the HOPG graphene sheets, or by STM tip

manipulation.

The NTL, presented in Fig. 1, is located at a HOPG step edge,

with an apparent width of 1.77 nm. The height difference of the graphene layers

between the right and left side of the NTL structure is 0.86 nm, quite close to

M. CASELLA, M. SANCROTTI

M. FANETTI, L. GAVIOLI,

the height of two graphene sheets. The image has been acquired with two

different tunneling currents, corresponding to region A (0.1 nA) and B (0.2 nA),

respectively. Region A corresponds to the typical appearance of the NTL

structure with continuity of the distorted graphite lattice on the curved structure.

Figure 1. STM image (8u8 nm2) of NTL structure on the step edge of the HOPG graphene sheet.

Scanning parameters: gap voltage +0.253 V and current 0.1 nA (region A) and 0.2 nA (region B).

Variation of tunneling parameters leads to dramatic changes of the NTL

apparent shape. In region B, the NTL presents a large modification of the NTL

curvature that becomes a large depression in the graphite structure. The

maximum depth of the depression, with respect to the unmodified structure, is

0.4 nm. Such variation is not observed when the tip is moving from right side to

left side of Fig.1, i.e., the backward direction. Hence the shape change behavior

is asymmetric. The NTL recovers its original shape by simply restoring a lower

current, without presenting any hysteresis in the apparent height variation.

These observations suggest that the deformations may actually be induced

by the modification of the tip-substrate interaction strength, and that the NTL

structure reacts in an elastic way to the STM tip. Although these features may

support deformation-based mechanism of the shape-changes, further detailed

analysis is required to elucidate the obtained experimental results. The results

suggest a possible development of STM as a tool to investigate the mechanical

properties of nanosystems.

References

1. T. W. Odom, J. H. Hafner, Ch. M. Lieber, in: Carbon Nanotube: Synthesis, Structures,

Properties and Applications, edited by M. S. Dresselhaus, G. Dresselhaus and Ph. Avouris

(Springer-Verlag, Berlin, 2001) pp. 177-215.

2. H.-V. Roy, C. Kallinger, B. Marsen, and K. Sattler, J. Appl. Phys. 83, 4695-4699 (1998).

*To whom correspondence should be addressed. Antal Koós; e-mail: koos@mfa.kfki.hu

V.N. Popov and P. Lambin (eds.), Carbon Nanotubes, 53–54.

INFLUENCE OF CATALYST AND CARBON SOURCE ON THE

SYNTHESIS OF CARBON NANOTUBES IN A SEMI-CONTINUOUS

INJECTION CHEMICAL VAPOR DEPOSITION METHOD

Z. E. HORVÁTH, A. A. KOÓS,* Z. VÉRTESY, L. TAPASZTÓ,

Z. OSVÁTH, P. NEMES INCZE, L. P. BIRÓ

Research Institute for Technical Physics and Materials Science,

P.O. Box 49, H-1525, Budapest, Hungary

K. KERTÉSZ, Z. SÁRKÖZI, AL. DARABONT

Babes-Bolyai University, Faculty of Physics, Kogãlniceanu 1,

RO-3400, Cluj-Napoca, Romania

Abstract. The injection chemical vapor deposition (CVD) method allows the

semi-continuous production of pure multi-wall carbon nanotubes (MWCNTs).

In order to find the most efficient catalyst material and carbon source, we

investigated the quality and quantity of carbon nanotubes, when different

metallocenes (ferrocene, cobaltocene and nickelocene) and hydrocarbons

(benzene, toluene, xylene, cyclohexane, cyclohexanone, n-hexane, n-heptane,

n-octane and n-pentane) are injected into the reaction furnace. The obtained

samples were analyzed by Transmission Electron Microscopy (TEM). The

highest yield and the best quality were obtained when a mixture of ferrocene-

nickelocene was used as catalyst and xylene as carbon source. The weight of

purified carbon nanotubes was higher than 50% of the weight of catalyst

material for xylene and reached the 10% for all investigated carbon sources.

Keywords: carbon nanotubes; chemical vapour deposition; electron microscopy

The major advantage of the injection CVD method is the in-situ and continuous

generation of catalytic particles throughout the entire growth cycle. Therefore,

the injection CVD method does not need a catalyst synthesis step and can be

scaled up for continuous or semi-continuous production. We investigated the

quality and quantity of carbon nanotubes when different metallocenes and

hydrocarbons were injected into the reaction furnace.

We used the following reaction parameters: 6 g metallocene in 100 ml

hydrocarbon catalyst concentration (except for the cases when the solubility of

the metallocene in the hydrocarbon was too low), 1 ml/min solution flow-rate,

875 °C furnace temperature and 500 l/h Ar (carrier gas) flow-rate. The samples

were purified using a 45% HNO

aqueous solution.

Figure 1. Left: comparison of the weight of purified carbon nanotubes produced with different

catalysts and carbon sources (black) and the weight of purified carbon nanotubes normalized to

the weight of the catalyst material (grey). Right: TEM image of MWCNTs prepared using xylene

as a carbon source.

Pure MWCNTs were grown. The MWCNTs were produced with maximum

yield when ferrocene – nickelocene catalyst mixture was used and xylene was

found to be the most efficient carbon source. For xylene, the weight of purified

carbon nanotubes normalized to the weight of catalyst material was higher than

50%.

Work supported by the Sapientia Research Programs Institute in the

framework of Fellowships 822/2001 K/513/2003.03.25, and by the MTA-MEH

Grant "Nanogas" and by OTKA through Grant nr. T043685 and M 041689.

*To whom correspondence should be addressed. Alexander Malesevic, VITO Materialen, Boeretang 200,

2400 Mol; email: alexander.malesevic@vito.be

V.N. Popov and P. Lambin (eds.), Carbon Nanotubes, 55–56.

PECVD GROWTH OF CARBON NANOTUBES

ALEXANDER MALESEVIC,* A. VANHULSEL

Flemish Institute for Technological Research, Boeretang 200,

2400 Mol, Belgium

C. VAN HAESENDONCK

Solid State Physics, KULeuven, Celestijnenlaan 200 D, 3001

Heverlee, Belgium

Abstract. We report on the growth of carbon nanotubes by plasma-enhanced

chemical vapor deposition (PECVD). We used acetylene and ammonia gas

mixtures with a process pressure of 1 Torr for the growth of carbon nanotubes

on nickel layers. The metal catalyst, just a few nanometers thick, was patterned

on oxidized silicon substrates by electron beam lithography. Our aim was to

develop a cheap mass production method to grow high quality, aligned carbon

nanotubes. We used different plasma setups and scanned individual growth

parameters in order to achieve that goal. In this work, we compare the different

results obtained by direct current (DC) plasma deposition.

Keywords: carbon nanotubes; plasma-enhanced chemical vapor deposition; DC plasma

deposition; mass production

Whereas most groups concentrate on only one particular type of plasma for the

growth of carbon nanotubes, we have built a unique setup which combines DC,

radio-frequency (RF) and inductively-coupled (ICP) plasma sources. A scheme

of the setup is drawn in Fig. 1, left. In the case of a DC or RF bias, we apply a

negative voltage to a resistively heated electrode and ignite plasma between the

electrode and the grounded reactor walls. A dense RF-ICP source can be used

in addition to or in combination with the DC or RF substrate biasing.

In order to compare the results of different types of plasma, we started to

grow carbon nanotubes with the DC plasma enhanced chemical vapor

deposition (PECVD) technique.

Figure 1. Left: Schematic view of a PECVD setup. Right: SEM image of carbon nanotubes

obtained with an acetylene to ammonia ratio of 25/100 sccm.

The samples consist of a 5 nm thick nickel layer that is deposited by a

conventional molecular beam epitaxy (MBE) technique on a silicon (100)

substrate with a silicon oxide buffer layer of 200 nm. The samples were placed

on the electrode and the reactor is pumped down to a base pressure of 10

-6

mBarr. The electrode was resistively heated up to 700°C in a continuous flow

of 100 sccm ammonia at 1 mBarr. The samples were annealed at 700°C during

10 minutes and afterwards pretreated in ammonia plasma of 600 V during 10

minutes. The growth was initiated by allowing the carbon containing gas,

diluted with ammonia, in the reactor for 20 minutes.

As carbonaceous gas, we used acetylene, methane, propane and butadiene in

combination with ammonia. We varied the ratio of the carbonaceous gas to

ammonia from 15/100 to 33/100 sccm. The combination acetylene with

ammonia was each time successful for the growth of carbon nanotubes,

independent of the gas ratio. A typical result is presented in Fig. 1, right.

Among the other gas mixtures, only the combination of methane with ammonia

yielded carbon nanotubes.