High Moisture Sensitivity of the Elements Based on Carbon Nanotubes Array

doi:10.4236/msa.2013.45A002

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Materials Sciences and Applications, 2013, 4, 8-10

http://dx.doi.org/10.4236/msa.2013.45A002 Published Online May 2013 (http://www.scirp.org/journal/msa)

High Moisture Sensitivity of the Elements Based on

Carbon Nanotubes Array

Sergei Bulyarsky1, Vyacheslav Galperin2, Levan Ichkitidze3*, Michael Ermakov1, Alexander Pavlov4,

Yuri Shaman2

1Ulyanovsk State University, Ulyanovsk, Russia; 2Scientific Manufacturing Complex “Technological Centre”, Moscow, Russia;

3National Research University of Electronic Technology (MIET), Moscow, Russia; 4Institute of Nanotechnology of Microelectronics

RAS, Moscow, Russia.

Email: *[email protected]

Received January 29th, 2013; revised March 27th, 2013; accepted April 14th, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

The results of the research and development of the moisture-sensitive elements based on the carbon nanotubes (CNT)

array are presented. It was shown that CNT arrays that were grown by low-temperature plasma enhanced chemical va-

por deposition (PECVD) method on the planar Si structures exhibit extremely high moisture sensitivity. The structure

resistance ratio in dry and moisture conditions exceed 400. Such relatively high change in resistances is conditioned by

the pattern of change of the charge carrier’s conductivity between certain nanotubes in the bundle when water mole-

cules adsorption occurs.

Keywords: Carbon Nanotubes; Moisture-Sensitive Elements; Conductivity; Molecules Adsorption

CNT possess a number of unique properties including high

sorption capacity of the various nature molecules [1].

Water absorption of the carbon structures has a number

of features because of high interactions between the wa-

ter molecules and relatively weak interaction between the

water and carbon [2]. The calculation shows that in the

pores formed by adjacent nanotubes in the bundle there

are the chains of water molecules near the nanotubes sur-

face [2] which in turn should lead to the changes of the

CNT electronic structure [3]. Such features are fully mani-

fested in CNT arrays, creating a high sensitivity to moisture.

This article presents the results of the fabrication and

study the moisture-sensitive elements based on CNT ar-

rays. The vertical arrangement of the nanotubes in the

array that was grown on the planar Si structure combined

with a current flow with perpendicular direction to the

CNT axis allowed us to obtain high resistance ratio of the

structure in the dry and moisture conditions which is

more then 400 times.

CNT arrays synthesis was performed by PECVD

method at 500˚C for 20 min. FeNiCo20 film 5 nm thick

deposited on the Ti buffer layer 50 nm thick was used as

the growth catalyst. Such film was prepared on the planar

Si structure surface.

Current conducting electrodes were made from poly-

silicon. Before the CNT array synthesis the catalyst was

subjected to oxygen annealing at a substrate temperature

280˚С for 5 minutes. CNT array that was obtained in this

process is shown in Figure 1.

Figure 1. CNT array of the moisture-sensitive element.

*Corresponding author.

High Moisture Sensitivity of the Elements Based on Carbon Nanotubes Array 9

The average nanotubes high in the array was 2.5 µm.

Catalyst sublayer looked like meander. The contact to

CNT array was done by a catalyst sublayer and by side

contacts from polysilicon tightly adjacent to array by

means of features of the substrate relief. Before the

measurements unoxidized catalyst residues were burned

out by short current impulse. After the last operation

current magnitude dramatically dropped, and the voltage-

current characteristics became nonlinear instead of ohmic

law.

Before the measurements, samples were dried in dry

oxygen at 200˚С for at least 3 hours. Then, in the dark at

room temperature the voltage-current characteristic was

measured. First measurement after above-mentioned dry-

ing mode showed high resistance samples with non-lin-

ear voltage-current characteristic. Before the second meas-

urements the samples were placed into tinted closed ves-

sel directly above the water surface. With identical volt-

age at room temperature current increase 105 times. After

the several cycling (drying-measurements in dry atmos-

phere-measurements in moisture atmosphere) it was ob-

tained stable, repeatable voltage-current characteristic

shown in Figure 2.

Continuous cyclic changes in the experiment condi-

tions stabilized the current through the sample in dry and

moist atmospheres. Such ratio decreased to 400 times

still showing high sensitivity to moisture.

Such form of the voltage-current characteristic is well

described by the formulas for tunnel recombination ob-

tained in works [4,5]. In consistent with the results of

these works, at specified voltage the transfer channel,

Figure 2. Voltage-current characteristic of the moisture-

sensitive element that was stabilized by the cyclic change of

the experiment conditions. 1, current value in dry atmos-

phere; 2, current value in moist atmosphere.

associated with tunnel or hopping transfer from a single

localized state, saturates. Such limitation associated with

a value of the density of states, between which there is an

electron transition. Low density limits the current value.

In that cases when electron transfer from tube to tube

is described by Mott hopping conductivity, probability of

the tunneling transfer will be taken as the value that de-

pends just on the overlap integral [4,6]:







 

exp 2

LR RL

wE wE wEra







, (1)

where



—frequency of attempts to overcame the poten-

tial barrier, equal to frequency of the characteristic pho-

non;



2a



mE—localization length [4], —over-

age jump distance equal to overage distance between the

traps, which in turn defined by their concentration:



.

In addition the current value is related to the concen-

tration of the localized states (N) by the formula [4,5]:



exp

kTdU

UU aN















, (2)

where r—is the current density at forward bias voltage

; k—contact potential difference; S—(p-n-) junc-

tion area;

U U





dU —width of the space-charge region,

k—Boltzmann constant; T—temperature;



—the

number of attempts per time unit of electron to overcome

the potential barrier, which separates adjacent localized

states.

The current ratio of the sensitive element in dry and

moist atmosphere is associated with the concentration

change of the localized states and with the localization

length of the electron on the nanotube:

 

13 13

22 21

exp ,

jN aN aN



 



 

 

(3)

We can make the following assumptions about the

causes of the significant increase in the conductivity of

the tubes in a moist environment. Firstly, adsorption of

water molecules is accompanied by a change in density

of localized states [1] which leads to the increase in the

current ratio in accordance with the Formula (3). Sec-

ondly, water molecules on the nanotube surface create

the attractive interaction which in turn reduces the dis-

tance between nanotubes in the bundle. On this account

the length of electron localization а reduced, which is

equal to overage distance that could be overcome by the

electron when it transfers between adjacent nanotubes.

This also leads to the increase in the current ratio.

Thus, in this study we have presented a new techno-

logy of growing CNT arrays, which essential point are

two factors: the use of the solid catalyst film FeNiCo20,

instead of gaseous ferrocene, which is usually used [1],

and the oxidation of the catalyst before the synthesis of

High Moisture Sensitivity of the Elements Based on Carbon Nanotubes Array

CNT array. It’s experimentally shown that the movement

of the electrons across the beam which is accompanied

by their transition from one tube to another creates the

conditions for significant change in the conductivity in

the presence of water molecules adsorption. Such change

can be explained by changes in the density of electronic

states accompanying adsorption, and by the change in the

distance between nanotubes in the bundle which impacts

on the probability of electron transition between adjacent

nanotubes.

Acknowledgements

We thank professors E. V. Blagov and S. V. Selishchev

for their support of this research.

This work is partially funded by Ministry of Education

of the Russian Federation (state contract No. 16.426.

11.0043 and 16.740.11.0765), and by Russian Fund Ba-

sic Research (state contract No. 10678p/19537).

REFERENCES

[1] S. Bulyarsky, “Carbon Nanotubes: Technology, Property

Control, Application,” Ulyanovsk State University (Rus-

sia), Ulyanovsk, 2011, 478 pages.

http://artlib.osu.ru/web/books/content_all/332.pdf

[2] A. Striolo, A. A. Chialvo, K. E. Gubbins and P. T. Cum-

mings, “Water in Carbon Nanotubes: Adsorption Isotherms

and Thermodynamic Properties from Molecular Simula-

tion,” Journal of Chemical Physics, Vol. 122, No. 23,

2005, Article ID: 234712. doi:10.1063/1.1924697

[3] O. B. Tomilin and U. U. Muryumin, “Carbon Nanotubes

Adsorption on the Graphene Surface and Their Energy

Spectrum,” Solid State Physics (Russia), Vol. 48, No. 3,

2006, pp. 563-571.

[4] S. V. Bulyarsky, N. S. Grushko and A. Zhukov, “Calcu-

lating the Probability of Optical Transitions in Strong

Electric Fields,” Journal of Experimental and Theoretical

Physics (Russia), Vol. 118, No. 11, 2000, pp. 1092-1098.

[5] S. V. Bulyarsky, Y. V. Rud, L. N. Vostrezova, A. S. Ka-

garmanov and O. A. Trifonov, “Tunneling Recombina-

tion in Semiconductor Structures with Nanodisorder,”

Physics and Technology of Semiconductors (Russia), Vol.

43, No. 4, 2009, pp. 460-466.

[6] N. Mott and A. Davis, “Electronic Processes in Non-

Crystalline Materials,” Clarendon Press, Oxford, 1979,

590 pages.