Materials Sciences and Applications, 2013, 4, 8-10 Published Online May 2013 (
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: *
Received January 29th, 2013; revised March 27th, 2013; accepted April 14th, 2013
Copyright © 2013 Sergei Bulyarsky et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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.
Copyright © 2013 SciRes. MSA
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
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
wE wE wEra
, (1)
—frequency of attempts to overcame the poten-
tial barrier, equal to frequency of the characteristic pho-
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]:
, (2)
where r—is the current density at forward bias voltage
; k—contact potential difference; S—(p-n-) junc-
tion area;
dU —width of the space-charge region,
k—Boltzmann constant; T—temperature;
number of attempts per time unit of electron to overcome
the potential barrier, which separates adjacent localized
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
 
 
 
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
Copyright © 2013 SciRes. MSA
High Moisture Sensitivity of the Elements Based on Carbon Nanotubes Array
Copyright © 2013 SciRes. MSA
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
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).
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