could not be analysed in pure polar solvents since fulle-

renes are almost completely insoluble in polar and H-

bonding solvents like acetone, tetrahydrofurane, aceto-

nitrile, the only exception being NMP in spite of its very

high dielectric constant (e = 32.2). This unusual solubil-

ity of fullerenes in such a polar solvent appears to be dueto fullerene-specific interactions involved in fullerenes

solubilization in NMP which are also responsible for

the observed spectroscopic changes. Aggregation in pure

NMP is probably due to the difference of polarity be-

tween fullerene molecules and NMP which forces the

molecules of fullerenes to aggregate in order to minimize

the contact with the solvent by means of weak p–p inter-

actions between fullerenes. In other words solute-soluteinteractions are favoured with respect to solute-solvent

interactions, resulting in the fullerenes aggregation. A

much larger number of interactions occur in solvent

mixtures like toluene/NMP that specifically are: NMP-

toluene, fullerene-fullerene, NMP-fullerene and tolu-

ene-fullerene interactions [5,6]. The larger dimensions

of the aggregates in toluene-NMP mixed solvents have

to be due to the predominance of NMP-toluene interac-tions which favours the fullerenes-fullerenes interaction.

This is the first time, on our knowledge, that fullerene

aggregation has been studied by DLS and TRFPA

methods measuring the size of aggregates in NMP and

toluene/NMP mixtures solvents. The use of NMP as dis-

persant/solvent for carbon-rich materials like fullerenes,

soot, etc., is promising for their structural analysis pro-

vided that the interactions of these materials with NMP

are recognised.

References

[1] Frackowiak E, Beguin F. Interaction between electroconducting

polymers and C60. J Phys Chem Solids 1996;57(6–8):983–9.

[2] Yevlampieva NP, Biryulin YuF, Melenevskaja EYu, Zgonnik VN,

Rjumtsev EI. Aggregation of fullerene C60 in N-methylpyrrolidi-

none. Colloids Surf A 2002;209:167–71.

[3] Brandelik DM, McLean DG, Brandt MC, Sutherland RL, Frock

LR, Crane RL. Complex formations in C60 solutions. Mat Res Soc

Symp Proc 1995;374:299–304.

[4] Apicella B, Ciajolo A, Barbella R, Tregrossi A, Morgan TJ, Herod

AA, et al. Size exclusion chromatography of particulate produced

in fuel-rich combustion of different fuels energy fuels

2003;17(3):565–70.

[5] Nath S, Pal H, Sapre A. Effect of solvent polarity on the

aggregation of C60. Chem Phys Lett 2000;327:143–8.

[6] Nath S, Pal H, Sapre A. Effect of solvent polarity on the

aggregation of fullerenes: a comparison between C60 and C70.

Chem Phys Lett 2002;360:422–8.

[7] Liu S, Lu Y, Kappes MM, Ibers JA. The structure of the C60

molecule: X-Ray crystal structure determination of a twin at 110K.

Science 1991;254:408–10.

Room-temperature negative differential conductancein carbon nanotubes

Jingqi Li *, Qing Zhang

Microelectronics Center, School of Electrical and Electronic Engineering, Nanyang Technological University, S1-B2c-20, Singapore 639798, Singapore

Received 27 August 2004; accepted 13 October 2004

Available online 25 November 2004

Keywords: A. Carbon nanotubes; D. Electrical (electronic) properties

Single-wall carbon nanotubes (SWNTs) have at-tracted much attention due to their unique electronic,

thermal and mechanical properties. The one-dimen-

sional (1D) structure with nanometer diameter and long

length (of the order of micrometer) makes it possible to

realize on-tube electronic devices, especially quantum

devices. Nanoscale intratube junctions and devices on

SWNTs have been prepared by chemical doping [1–3]

and mechanical bending techniques [4,5]. The fabricateddevices exhibit unique characteristics, such as single elec-

tron transistor (SET) characteristics [2,4,5], low temper-

ature negative differential conductance (NDC) [1] and

intratube rectifying characteristics [3], etc. In this paper,

we report a significant NDC phenomenon of a bundle of

SWNTs at room temperature. This room temperature

SWNT-NDC could be quite useful for practical applica-

tions of fast switching, oscillator and amplifying devices,etc. Possible mechanisms for the NDC phenomenon are

discussed.

* Corresponding author. Tel.: +65 679 045 57; fax: +65 679 333 18.

E-mail address: p144703764@ntu.edu.sg (J. Li).

0008-6223/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2004.10.019

Letters to the Editor / Carbon 43 (2005) 651–673 667

The samples used here are back-gated carbon nano-

tube (CNT) based field effect transistors (CNTFETs)

[6–8]. P-type silicon with 500nm thick thermally grown

silicon dioxide layer is used as the back gate. Source

and drain electrodes are made of 20nm thick Ti fol-

lowed by 40nm thick Au prepared by standard UVlithography and lift-off processes. The gap between the

source and drain electrode is 2lm.

Commercial SWNTs with a high purity of �95% are

used in the experiment. The diameter of the SWNTs is

around 1.4nm. SWNTs are bridged across the source

and drain electrodes using an AC dielectrophoresis

method [8]. From AFM images, the diameter of the

bridging SWNT bundle is about 20nm. The drain cur-rent as a function of the drain-source voltage (Id–Vds

curve) is measured using a HP-4156B semiconductor

analyzer at ambient atmosphere. Fig. 1(a) shows Id verse

Vds as a function of the gate voltage Vg from �16 to

16V. One can see that when Vg > 0V, Vg has little influ-

ence on the Id–Vds curves. We suggest that Id (for

Vg > 0V) is dominated by those metallic SWNTs in

the bundle. To burn off the metallic SWNTs using elec-trical burning method [8,9], a gate voltage of +6V is ap-

plied to switch off and protect the semiconducting

SWNTs while Vds is scanned from zero to a positive

value, Vpds which is increased gradually from a lower

value, say, 5V, upto a higher one until some m-SWNTs

are destroyed. Two apparent current drops at

Vpds = 10.2V (Curve 2 in Fig. 1(b)) and 12.2V (Curve 4),

respectively, suggest the breakdown of some metallic

SWNTs.

After the above tube-burning process, the resultantId–Vds curves show significant NDC characteristics at

room temperature. It can be seen from Fig. 2(a) that

the current peak and valley occur at Vds = 9.2V and

9.8V, respectively, under Vg = 6V. The peak-to-valley

current ratio is �2. The NDC characteristics are repeat-

able at a given Vg as shown in Fig. 2(a) (Curves 2 and 3).

Fig. 2(b) shows the NDC two days after measuring the

Id–Vds curves in Fig. 2(a). At Vg = 6V, the peak voltageof the NDC is exactly at the same voltage value

although the peak current changes slightly. More inter-

estingly, the peak voltage gradually decreases from 9.2V

to 7.8V with Vg decreasing from 6V to �6V.

It is important to note that the NDC phenomenon

was not observed at the first round of the tube-burning

process even if Vds was higher than 9.2V (the peak volt-

age of the NDC), see Curves 1 and 2 in Fig. 1(b). There-fore, the bundle structure and imperfect contacts

between the tube bundle and electrodes should not be

responsible for the NDC. We suggest that it is caused

by a quantum well in the semiconducting CNT as the

appearance of the NDC is consistent with the hole reso-

nant tunneling through a quantum well [10]. At Vds = 0,

holes in the well are confined to discrete energy levels,

say En, in thermal equilibrium condition, as shown inFig. 3(a). As Vds is increased, an accumulation region

forms near the barrier by the drain side and a depletion

region forms near the barrier by the source side. How-

ever, very few holes can tunnel through the energy bar-

rier until Vds reaches a value (for example 9.2V) at

-6

-3

0

3

6

-1.5 -0.5 0.5 1.5Vds (V)

Id(x

10-7

A) (a) Vg = -16V~16V

Step: 4V

0

2

4

6

0 8 12Vds (V)

Id(x

10-5

A) (b)

1

234

54

Fig. 1. (a) Id as a function of Vds measured for Vg = � 16–16V in steps

of 4V. (b) Id as a function of Vds during the tube burning process.

Curve 1 is the one before burning. Curves 2 and 3 are sweepings from 0

to 10.2V and 11V, respectively. Curve 4 is sweeping from 0 to 12.2V

with a large conductance decrease at 11.8V. Curve 5 is the last

sweeping.

0

3

6

9

12

0 3 6 9 12 15Vds (V)

Vds (V)

I d (x

10-6

A)

A

B

C123

(a)

4

6

8

10

12

6 8 10 12

I d (x

10-6

A)

(b) Vg (V)

63

0-3

-6

Fig. 2. (a) Id as a function of Vds at Vg = 6V. Curves 1–3 were

measured under the same conditions. For clarity, Curves 2 and 3 for

the repeated measures are shifted up by 2.5 · 10�6A and 4 · 10�6A,

respectively. (b) Id as a function of Vds at Vg = 6, 3, 0, �3 and �6V.

668 Letters to the Editor / Carbon 43 (2005) 651–673

which the Fermi level of the drain side is aligned with En

in the well (Fig. 3(b)). When this happens, holes could

tunnel through the right barrier into the well at a much

high tunneling rate and subsequently through the left

barrier into unoccupied states in the valence band of

the source side. As Vds is increased further, the va-lence-band edge on the right is below En (Fig. 3(c)), so

that the tunneling rate of holes is significantly reduced.

As a result, the current begins to decrease with increas-

ing the voltage, resulting in an apparent NDC

phenomenon.

The energy levels in the well can be approximately

expressed as

En ��h2

2m�npl

� �2

; ð1Þ

where n is the quantum number labeling the energy lev-

els, l is the width of the well, and m* is the effective massof holes. When the energy difference between the levels,

E2 � E1, is larger than the thermal energy kBT =

0.026eV, i.e.,

3� �h2

2m�pl

� �2

P 0:026� 1:6� 10�19; ð2Þ

the NDC phenomenon could be observed at room tem-

perature. Using m* = 9.31 · 10�32 [11], we have

l � 8nm. The obtained well width is consistent with

the dimension of Leonard and Tersoff’s simulated car-

bon nanotube devices which show a strong room tem-

perature NDC [12].The shift of the peak voltage as a function of Vg can

be attributed to the variation of the conductance of the

SWNT bundle and the shift of the En with respect to the

Fermi level in the quantum well. Vds should drop on

the contacts between the source and drain electrodes

and SWNTs (Vc), the quantum well (Vp) and the

SWNTs beyond the quantum well (Vs), i.e., Vds = Vc +

Vp + Vs. Because of a large contact resistance [7] and

large SWNT (channel) resistance, Vp could be quite

small. Since the channel resistance decreases with

decreasing gate voltage from 6V to �6V for a p-type

CNTFET, more voltage will drop on the quantum well

at a given Vds. Therefore, Vds for triggering the resonant

tunneling decreases with the gate voltage changing from6 to �6V, resulting in the down shift of the peak volt-

age, as observed in Fig. 2(b). The peak current could

be constrained by the tunneling process so that it may

not be significantly influenced by the variation of the

channel resistance or Vg. On the other hand, the up-shift

of En with respect to the Fermi level induced by decreas-

ing Vg [13] would also lead to a decrease in the peak

voltage.Taking the burning process into account, we suggest

two possibilities for the formation of the quantum well.

Local depletion of adsorbed oxygen on the s-SWNT

walls could be one of the reasons. The significant drops

of Id in Fig. 1(b) indicate burning-off of the m-SWNTs

in the bundle [9,14]. We suggest that the tube-burning

mainly occurs in a small area far from the source and

drain so that the rest parts of the broken m-SWNTs stillkeep contact with the metal electrodes, as observed by

Collions et al. [9,14]. At the burning sites, oxygen that

initially adsorbed on s-SWNTs could react with carbon

atoms during the tube burning process and some rem-

nants [15] of the burned m-SWNTs might leave behind

on the surface of s-SWNTs. The remnants prevent oxy-

gen in air from being adsorbed again. Theoretic study

[16,17] has suggested that the chemisorption of oxygencould drastically decrease the energy band gap from

0.99 to 0.38eV for a (8, 4) SWNT. A reasonable specu-

lation is that the band gap of a SWNT could be strongly

associated with the amount of adsorbed oxygen on the s-

SWNT wall. Therefore, the s-SWNTs covered with the

remnants could form local energy barriers for hole

transport. In addition, variation of the diameter of the

SWNT bundle could be another reason. According toReich et al. [18], interaction between SWNTs in the bun-

dle could lead to a decrease in the energy gap of the

semiconducting SWNTs. At the tube-burning sites, the

tube number should be small due to the burning-off of

the m-SWNTs (the local bundle diameter should be

thinned, as observed by Collions et al. [9]) and the en-

ergy gap at the sites could be larger than the rest of

the bundle, resulting in energy barriers for a quantumwell.

In summary, room-temperature negative differential

conductance phenomenon has been observed in a bun-

dle of SWNTs in a CNTFET. The peak-to-valley cur-

rent ratio is about 2. This phenomenon is attributed to

a quantum well between two adjacent energy barriers

formed on semiconducting SWNTs due to the burning

off metallic SWNTs in the bundle. The conductance var-iation of the SWNTs and the shift of the energy level in

the quantum well are suggested to be responsible for the

EF (a)

(b)

(c)

E1

E1

E1

S D

S

S

D

D

Fig. 3. (a)–(c) Bias voltage dependent band diagrams for the points A,

B, and C on the Id–Vds curve in Fig. 2(a). Only the valence band is

shown here. The dashed lines are the Fermi level EF. S and D represent

the source and drain, respectively.

Letters to the Editor / Carbon 43 (2005) 651–673 669

peak voltage shifts from 9.2V to 7.8V with decreasing

the gate voltage from 6V to �6V.

References

[1] Zhou C, Kong J, Yenilmez E, Dai H. Modulated chemical doping

of individual carbon nanotubes. Science 2000;290:1552–5.

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carbon nanotubes by organic Amine compounds. J Phys Chem

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[8] Li J, Zhang Q, Yang D, Tian J. Fabrication of carbon nanotube

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tubes and nanotube circuits using electrical breakdown. Science

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holes in AIAs-GaAs-AIAs heterostructures. Appl Phys Lett

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A. Is the intrinsic thermopower of carbon nanotubes positive?

Phys Rev Lett 2001;85(20):4361–4.

[12] Leonard F, Tersoff J. Multiple functionality in nanotube transis-

tors. Phys Rev Lett 2002;88(25):258302-1–258302-4.

[13] Luryi S, Capasso F. Resonant tunneling of two-dimensional

electrons through a quantum wire: A negative transconductance

device. Appl Phys Lett 1985;47(12):1347–9.

[14] Collions PG, Hersam M, Arnold M, Martel R, Avouris Ph.

Current saturation and electrical breakdown in carbon nanotubes.

Phys Rev Lett 2001;86(14):3128–31.

[15] Yao N, Lordi V, Ma SXC, Dujardin E, Krishnan A, Treacy

MMJ, Ebbesen TW. Structure and oxidation patterns of carbon

nanotubes. J Mater Res 1998;13(9):2432–7.

[16] Barone V, Heyd J, Scuseria GE. Effect of oxygen chemisorp-

tion on the energy band gap of a chiral semiconducting

single-walled carbon nanotube. Chem Phys Lett 2004;389:

289–92.

[17] Chan S, Chen G, Gong XG, Liu Z. Oxidation of carbon

nanotubes by singlet O2. Phys Rev Lett 2003;90:086403-1–

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[18] Reich S, Thomsen C, Ordejon P. Electronic band structure of

isolated and bundled carbon nanotubes. Phys Rev B

2002;65:155411-1–155411-11.

Sonication assisted deposition of Cu2O nanoparticles onmultiwall carbon nanotubes with polyol process

Ying Yu a,b, Li-Li Ma a, Wen-Ya Huang a, Fei-Peng Du a, Jimmy C. Yu c,Jia-Guo Yu c, Jian-Bo Wang d, Po-Keung Wong b,*

a College of Physical Science and Technology, Central China Normal University, Wuhan 430079, Chinab Department of Biology, The Chinese University of Hong Kong, Shatin, NT, Hong Kong, SAR China

c Department of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong Kong, SAR Chinad Center for Electron Microscopy, Wuhan University, Wuhan 430072, China

Received 23 July 2004; accepted 18 October 2004

Available online 25 November 2004

Keywords: A. Carbon nanotubes; B. Chemical treatment; C. Electron microscopy; D. Microstructure

Due to the special structure, the extraordinary

mechanical and unique electronic properties and the po-

tential applications, carbon nanotubes (CNTs) have

attracted considerable attention since they were discov-

ered. To generate new functionalities, it is necessary to

attach other nanostructures to the surface of CNTs.

The combination of carbon nanotubes and nanocrys-

tals are useful in field emission displays, nanoelectronic

devices, novel catalysts and polymer or ceramic rein-

forcement [1]. The coating of CNTs with metals, inor-

ganic compounds and polymers has been reported

0008-6223/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2004.10.031

*Corresponding author. Tel.: +852 2609 6783; fax: +852 2603 5767.

E-mail address: pkwong@cuhk.edu.hk (P.-K. Wong).

670 Letters to the Editor / Carbon 43 (2005) 651–673