Transport properties of Mo6S3I6 nanowire networks

Article abstract of DOI:10.1063/1.2166475

We present measurements of resistivity on Mo6 S3 I6 nanowire networks under different conditions. The room-temperature values of conductivity of as-grown nanowire networks are on the order of 300K =0.04 Sm and show linear current-voltage characteristics, indicating that-in agreement with band structure calculations-there is a finite density of states at the Fermi level. The conductivity of pristine samples is thermally activated, closely following three-dimensional variable range hopping (VRH) behavior of the form = 0 exp- (T0 T)Β, where Β=14. Removal of interstitial iodine from the network by annealing in a vacuum gives rise to a cross over to one-dimensional VRH with Β=12 and a concurrent increase in room-temperature conductivity. The introduction of water vapor leads to a decrease in conductivity and reveals that the resistivity of the network is sensitive to interstitial water molecules.

Full text of DOI:10.1063/1.2166475

Transport properties of Mo 6 S 3 I 6 nanowire networks
Boštjan Berčič, Urša Pirnat, Primož Kusar, Damjan Dvorsek, Dragan Mihailovic, Damjan Vengust, and Boštjan
Podobnik
Citation: Applied Physics Letters 88, 173103 (2006); doi: 10.1063/1.2166475
View online: http://dx.doi.org/10.1063/1.2166475
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/17?ver=pdfcov
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APPLIED PHYSICS LETTERS 88, 173103 ͑2006͒
Transport properties of Mo S I nanowire networks
6
3 6
a͒
Boštjan Berčič, Urša Pirnat, Primož Kusar, Damjan Dvorsek, and Dragan Mihailovic
Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
Damjan Vengust
Mo6 d.o.o. Teslova 30, 1000 Ljubljana, Slovenia
Boštjan Podobnik
LPKF d.o.o. Planina 3, 4000 Kranj, Slovenia
͑
Received 14 June 2005; accepted 16 December 2005; published online 26 April 2006͒
We present measurements of resistivity on Mo S I nanowire networks under different conditions.
6
3 6
The room-temperature values of conductivity of as-grown nanowire networks are on the order of
␴300K=0.04 S/m and show linear current-voltage characteristics, indicating that—in agreement
with band structure calculations—there is a finite density of states at the Fermi level. The
conductivity of pristine samples is thermally activated, closely following three-dimensional variable
␤
range hopping ͑VRH͒ behavior of the form ␴=␴ exp−͑T /T͒ , where ␤=1/4. Removal of
0
0
interstitial iodine from the network by annealing in a vacuum gives rise to a cross over to
one-dimensional VRH with ␤=1/2 and a concurrent increase in room-temperature conductivity.
The introduction of water vapor leads to a decrease in conductivity and reveals that the resistivity
of the network is sensitive to interstitial water molecules. © 2006 American Institute of Physics.
͓
DOI: 10.1063/1.2166475͔
New quasi-one-dimensional materials such of different
types, have important applications in electronics. Metallic
nanowires or nanotubes, due to their high aspect ratios, are
of great interest as additives to organic polymers enabling
the manufacture of transparent conducting composites for
example. They might also be very useful for nanoscale hori-
zontal interconnections and vias in device circuits and for the
construction of various sensors. Recently, electronic devices
have been proposed based on carbon nanotube ͑CNT͒ net-
works, which do not rely on positioning and orientation of
resistivity, the influence of removal of interstitial iodine by
high-temperature annealing and the effect of humidity on the
resistivity of the networks.
Mo6S3I6 was synthesized directly from the elements in a
3
quartz tube as described previously. The material is dis-
persed in water using a ultrasonic bath, filtered through a
quantitative membrane filter and dried. Annealing was per-
formed at 500, 700, or 900 °C for 12 h in a sealed quartz
ampoule evacuated to 10− bar. Upon annealing at 700 °C
and above a deposit on the inside of the quartz tube was
observed. The deposit was dissolved in the ethanol and ana-
lyzed with ultraviolet-visible spectrometer. The obtained
spectrum showed two characteristic iodine peaks at 292 nm
and at 360 nm, which confirmed that iodine was released
from the samples upon annealing. The scanning electron mi-
croscope ͑SEM͒ image of the material after annealing is
shown in Fig. 1͑a͒. X-ray diffraction ͑XRD͒ was also per-
formed to check if the structure had changed, or if the addi-
tional impurity phases, particularly Mo S I , which is metal-
7
1
individual nanotubes. CNTs exist in both semiconducting
and metallic form, but presently they cannot be easily sepa-
rated to a high degree of purity and are not very soluble.
Introducing surfactants increases their solubility, but signifi-
cantly degrades internanotube contact, resulting in inferior
2
electrical conductivity characteristics of the composite. As a
promising alternative, Mo6S9−xIx nanowires have recently
been found to be easily synthesizable in bulk quantities, and
thus could potentially replace carbon nanotubes in numerous
6
6 2
lic, might be present ͓Fig. 1͑d͔͒. Since neither a change of
the structure nor presence of impurity phases were observed,
we believe that released iodine was coming from interstitial
sites in between nanowires or bundles. The material was then
pressed into pellets which results in a network of intercon-
nected nanowire bundles with typically 75±5% of theoreti-
cal density. Four contacts were made using silver paste ͑SPI
No. 4999͒ and measurements were made with a Keithley 238
current source and Keithley 2000 voltmeter in an Oxford
instruments cryostat.
The current-voltage ͑I-V͒ characteristics of as-grown
and annealed samples are shown in Fig. 1͑c͒. ␴300K varies by
less than 20% in different pellets, indicating that a high de-
gree of reproducibility is achieved in sample preparation.
applications
overcoming
their
abovementioned
3
shortcomings. Moreover, they were found to exhibit very
good dispersion characteristics in a variety of solvents, in-
cluding water. Mo S I with x=6 is predicted to be semi-
metallic on the basis of local density approximation band
structure
4
6
9−x x
5
calculations.
Recent
measurements
of
Mo6S I /polymethyl-methacrylate composites have given
9−x x
remarkably low percolation thresholds and relatively high
conductivities at low loadings, suggesting very competitive
use in a variety of applications. A remarkable feature of the
6
material are the very weak interwire forces, which suggests
that
transport
properties
should
be
highly
5
–7
one-dimensional. In this letter, we concentrate on macro-
scopic transport measurements of Mo S I nanowire bundle
networks, reporting on the temperature dependence of the
6
3 6
␴300K
is found to change upon annealing, from approxi-
mately ␴300K=0.04 S/m in unannealed to ␴300K=0.2 S/m
upon annealing at 900 °C. In all cases, the I-V characteristic
was found to be linear over a large range of electric field E.
a͒Electronic mail: dragan.mihailovic@ijs.si
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003-6951/2006/88͑17͒/173103/3/$23.00 88, 173103-1 © 2006 American Institute of Physics
4.88.136.149 On: Fri, 12 Dec 2014 13:36:27
0
8

173103-2
Berčič et al.
Appl. Phys. Lett. 88, 173103 ͑2006͒
FIG. 3. The resistance change as a function of time in pressed pellets of
Mo S I for pristine samples ͑open circles͒ and after one wetting/drying
6
3 6
cycle ͑filled squares͒. The annealed samples have a dramatically reduced
humidity dependence ͑solid and dashed line͒.
FIG. 1. ͑a͒ Scanning electron micrograph ͑SEM͒ of the Mo S I . ͑b͒ The
6
3
6
connected to a saturated water vapor bath at 30 °C, and the
resistivity was monitored as a function of time. The results,
shown in Fig. 3, show that the pristine ͑unannealed͒ samples
display a rapid change of resistance when exposed to mois-
ture within the first minute, after which time the resistance
saturates at a value which is approximately 10% higher than
in a dry atmosphere. One might expect that the presence of
water inside the sample should have a shorting effect, which
would result in a decrease in the resistivity. Instead, the re-
sistivity increases, suggesting that water has an effect in re-
ducing the interbundle conductivity, similar to the effect of
iodine. Remarkably, the effect of moisture under identical
conditions on the annealed sample is much smaller, being
less than 1% indicating that interstitial material ͑removed
upon annealing͒ has an important role in determining the
overall network resistance and suggests that pre-existing io-
dine in pristine samples increases the sensitivity to water. To
see if the moisture effect is reversible, we have subsequently
dried the exposed sample in an oven at 50 °C for 10 h. The
resistivity change upon exposure was then remeasured. From
the results, shown in Fig. 3, it appears that the change of
moisture-induced resistivity ⌬R/R is very similar before and
after cycling, although a small increase in absolute value is
observed after the first cycle which is attributed to inevitable
irreversible changes of interbundle contacts resulting from
temperature/moisture cycling.
room temperature conductivity ␴300 as a function of annealing temperature
T . ͑c͒ Current-voltage characteristic of bulk Mo S I for pristine ͑line͒ and
A
6 3 6
annealed samples at 500, 700, and 900 °C. No discernible difference is
observed after annealing. ͑d͒ An XRD diffraction pattern of the measured
sample shows no trace of impurities and no detectable change of structure,
only a slight change in peak intensities after annealing.
The dependence of ␴300K on T is shown in Fig. 1͑b͒.
A
In Fig. 2, we show the temperature dependence of the
resistivity of the as-grown and annealed material. We clearly
observe thermally activated conductivity behavior from 50 K
to room temperature. ͑Below 50 K, the resistance becomes
too high to measure reliably͒. Figure 2͑b͒ shows that the
behavior very closely follows an exponential law of the form
␤
␴
=␴ exp-͑T /T͒ , where ␤=1/4 is characteristic of three-
0
0
dimensional Mott variable range hopping ͑VRH͒ ͑Ref. 9͒ for
the pristine sample, and remarkably, ␤=1/2 for the annealed
samples ͓Fig. 2͑d͔͒. The latter exponent is characteristic of
one-dimensional VRH. Repeated measurements on different
samples reveal identical temperature behavior, consistently
showing a cross over from ␤=1/4 to ␤=1/2 upon annealing
at 700 °C and above.
Since the pellets contain some empty space, it might be
expected that water vapor, or other gases may permeate into
the network voids, which may cause a change in the inter-
wire and interbundle contact resistivity. As an example, to
test the pressed pellet sensitivity to water molecules, we have
performed measurements of the resistivity under saturated
water vapor conditions. The sample was placed in a chamber
When trying to understand the origin of the observed
behavior, both interwire and interbundle transport need to be
considered. Band structure calculations on Mo S I suggest
6
3 6
that the material is best described as a highly anisotropic
5
semimetal. Narrow bands cross the Fermi energy EF for
wave vectors along the nanowire ͑i.e., crystal c͒ axes, while
no band crossings exist perpendicular to the nanowire axis
͑
along a or b͒. This implies that coherent electron transport is
not possible perpendicular to the nanowire axis. Along the c
axis ͑i.e., along the nanowire͒, coherent transport may also
be strongly subject to localization due to the highly one-
dimensional character of these materials. Thus, the electron
transport properties of extended networks are expected to be
dominated by interbundle and interwire hopping. The obser-
vation of VRH temperature dependence with a three-
dimensional exponent ␤=1/4 is thus not surprising, and
similar behavior has also been observed previously in CNT
8
networks. It is somewhat surprising that the exponent
changes upon high-temperature annealing to ␤=1/2. Which
9
could, according to theory, be attributed to a change of di-
FIG. 2. Resistivity as a function of temperature of bulk Mo S I : ͑a͒ For
6
3 6
mensionality from three to one dimensions. The change of
dimensionality could be understood in terms of the removal
of atoms at contact points in between nanowires, which play
pristine samples, and ͑c͒ for annealed samples ͑T =700 °C and 900 °C͒.
A
͑
b͒, ͑d͒ By using proper scaling, we show that VRH-like behavior is obeyed
over a wide range of temperatures for both, but with different exponents:
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␤
=1/4 for pristine samples, and ␤=1/2 for annealed samples. a role of conducting ͑tunnelling͒ bridges between nanowires.
8
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173103-3
Berčič et al.
Appl. Phys. Lett. 88, 173103 ͑2006͒
However, the fact that at the same time ␴ increases is not
the very different chemical nature of the Mo6S9−xIx nano-
wires offer complementary detection capabilities to CNT
networks.
0
straightforward to understand and must be caused by another
mechanism. Impurities, such as Mo S I crystals, can in
6
6 2
principle give rise to increased conductivity, but XRD spec-
tra do not show the presence of additional impurity phases
after high-temperature annealing. The fact that the change in
resistivity upon annealing ͑Fig. 1͒ is linear with annealing
temperature, excludes the possibility of a structural phase
transition, which is confirmed by the XRD data ͑Fig. 1͒.
Removal of interstitial iodine atoms has two additional
effects: ͑a͒ It reduces the number of scattering centers, thus
The authors wish to acknowledge support from EU
Nanotemp RTN and DESYGN-IT project for this work.
1
E. S. Snow, J. P. Novak, P. M. Campbell and D. Park, Appl. Phys. Lett.
8
2
2, 2145 ͑2003͒; K. Brdley, J.-C. P. Gabriel, and G. Gruener, Nano Lett.
, 1353 ͑2003͒.
2
3
J. N. Coleman, A. Fleming, S. Maier, S. O’Flaherty, A. Minett, M. S.
Ferreira, S. Hutzler, and W. J. Blau, J. Phys. Chem. B 108, 3446 ͑2004͒.
D. Vrbanic, M. Remskar, A. Jesih, A. Mrzel, P. Umek, M. Ponikvar, B.
Jancar, A. Meden, B. Novosel, S. Pejovnik, P. Venturini, J. C. Coleman,
and D. Mihailovic, Nanotechnology 15, 635 ͑2004͒.
increasing the mean-free path ᐉ , and ͑b͒ effectively causes
m
doping with electrons. Assuming that iodine is bound in pris-
tine samples by a partial charge transfer, then I removal in
4
V. Nicolosi, D. Vrbanic, A. Mrzel, J. McCauley, S. O’Flaherty, C.
McGuiness, G. Compagnini, D. Mihailovic, W. J. Blau, and J. N.
Coleman, J. Phys. Chem. B 109, 7124 ͑2005͒.
the form of I results in a net charge transfer of electrons to
2
the nanowires, increasing the carrier density, and conse-
quently the conductivity. The greater sensitivity to humidity
for the pristine samples implies that iodine acts as a sensi-
tizer for humidity detection.
5
A. Meden, A. Kodre, J. Padežnik Gomilšek, I. Arčon, I. Vilfan, D.
Vrbanic, A. Mrzel, and D. Mihailovic, Nanotechnology 16, 1578 ͑2005͒.
R. Murphy, V. Nicolosi, Y. Hernandez, D. McCarthy, D. Rickard, D.
Verbanic, A. Mrzel, D. Mihailovic, W. J. Blau, and J. N. Coleman, AIP
Conf. Proc. 786, 382 ͑2005͒.
A. Kis, D. Mihailovic, M. Remskar, A. Mrzel, A. Jesih, I. Piwonski, A. J.
Kulik, W. Benoit, and L. Forro, Adv. Mater. ͑Weinheim, Ger.͒ 15, 733
6
To conclude, it is clear from this work that the overall
transport properties of Mo S I networks are governed by
7
6
3 6
interwire and interbundle hopping, and as a result the con-
ductivity of networks appears to strongly dependent on the
presence of interstitial atoms or molecules, such as iodine or
water. This suggests that a variety of MoSI-based network
molecular sensors could be developed, which would, due to
8͑
2003͒.
J. M. Benoit, B. Corraze, and O. Chauvet, Phys. Rev. B 65, 241405
9͑
2002͒.
N. F. Mott and E. A. Davies, Electronic Processes in Noncrystalline Ma-
terials ͑Clarendon, Oxford, 1979͒.
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