Silver nanowires grown in the pores of a silica gel

Abstract

Silver nanowires of diameter ;40 nm and length ;0.3 mm have been grown by electrodeposition
within the pores of silica gels which were heat treated in the temperature range 523 to 823 K and,
subsequently, soaked in a silver nitrate solution. A staircase current–voltage characteristic was
observed in the direction of electrodeposition after nanowires were disrupted by the application of
a dc voltage pulse. Such gels containing interrupted nanowires of silver showed a dielectric constant
value ;104 both in directions parallel and perpendicular to that of electrodeposition.

Full text

Silver nanowires grown in the pores of a silica gel
S. Bhattacharyya, S. K. Saha, and D. Chakravorty
Citation: Appl. Phys. Lett. 77, 3770 (2000); doi: 10.1063/1.1327278
View online: http://dx.doi.org/10.1063/1.1327278
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Published by the American Institute of Physics.
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Silver nanowires grown in the pores of a silica gel
S. Bhattacharyya and S. K. Saha
Department of Physics, Bidhannagar College, Salt Lake City, Calcutta 700 064, India
D. Chakravortya)
Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India
共Received 6 July 2000; accepted for publication 20 September 2000兲
Silver nanowires of diameter ⬃40 nm and length ⬃0.3 mm have been grown by electrodeposition
within the pores of silica gels which were heat treated in the temperature range 523 to 823 K and,
subsequently, soaked in a silver nitrate solution. A staircase current–voltage characteristic was
observed in the direction of electrodeposition after nanowires were disrupted by the application of
a dc voltage pulse. Such gels containing interrupted nanowires of silver showed a dielectric constant
value ⬃104both in directions parallel and perpendicular to that of electrodeposition. © 2000
American Institute of Physics. 关S0003-6951共00兲02047-7兴
Investigation of nanostructured materials has assumed
great importance in recent years because of new properties
and physics associated with such systems.1,2 Preparation and
study of quantum dots3,4 and quantum wires5,6 have been
reported extensively. Quantum wires of semiconductors7and
metallic alloys8have been found to exhibit interesting elec-
trical and magnetic properties. In nanodevice fabrication
where quantum dots figure extensively, metallic nanowires
will be needed for bringing about interelement electrical con-
nection. In recent years, templates involving pores in anod-
ized Al2O3films have been used for fabricating metallic
nanowires.9We have explored the possibility of using the
nanopores within a silica gel to grow nanowires of silver by
an electrodeposition process. The pore diameters could be
conveniently changed by subjecting the gel to different heat
treatments thereby changing the diameter of the metallic
nanowires subsequently grown. A staircase voltage–current
characteristic and high dielectric constant were also found
after bringing about a disruption of the nanowire by a volt-
age pulse. The details are reported in this letter.
For preparing the silica gel, a solution was prepared by
mixing 4 ml tetraethylorthosilicate 共TEOS兲, 40 ml of ethyl
alcohol and 2 ml of distilled water. The solution was stirred
for 6 h and then allowed to gel for a period of 3 days in a
petri dish. The gelled samples were given heat treatments at
temperatures ranging from 523 to 823 K for a duration of 6
h. This was done to change the pore diameter within the gel.
Heat treated sample pieces of an approximate area of 0.6 cm2
and a thickness of 0.3 mm were then dipped into a solution
containing1gofsilver nitrate in 20 ml of distilled water for
16 h. The samples were taken out of the solution and the
surfaces washed with distilled water. This step brought about
the filling up of the pores by AgNO3. Silver paste 共supplied
by Acheson Colloiden B. V., Holland兲electrodes were ap-
plied on two opposite faces of the treated sample. A dc volt-
age of 8 V was applied across the specimen for half an hour.
The current through the sample increased from a few micro-
amperes to a few milliamperes which indicated the formation
of metallic nanowires from one electrode to the other.
The microstructure of the samples was studied by using
a JEM 200 CX transmission electron microscope. Figure 1共a兲
shows a typical micrograph obtained from a sample heat
treated at 723 K for 6 h, soaked in AgNO3and then subjected
to electrodeposition. Figure 1共b兲is the electron diffraction
pattern corresponding to Fig. 1共a兲. The interplanar spacings
dhkl calculated from the diffraction ring diameters are com-
pared with standard ASTM data for silver in Table I. These
results confirm that the nanowires shown in Fig. 1共a兲indeed
consist of metallic silver. The diameter of the wires is seen to
be ⬃40 nm.
In Fig. 2, the resistance variation as a function of tem-
perature is shown for three specimens which were heat
a兲Electronic mail: m1sdc@mahendra.iacs.res.in; also at Jawaharlal Nehru
Center for Advanced Scientific Research, Bangalore 560 064, India.
FIG. 1. 共a兲Transmission electron micrograph for silica gel heat treated at
723 K/6 h soaked in AgNO3and subjected to electrodeposition. 共b兲Electron
diffraction pattern for 共a兲.
APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 23 4 DECEMBER 2000
37700003-6951/2000/77(23)/3770/3/$17.00 © 2000 American Institute of Physics
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treated at 523, 723 and 823 K, respectively before elec-
trodeposition. We refer to these as specimen numbers 1, 2,
and 3, respectively, in subsequent discussion. It is evident
that the resistance level increases as the heat treatment is
enhanced. This result is consistent with the fact that increas-
ing heat treatment decreases the pore diameter10,11 which in
turn gives rise to nanowires of decreasing diameter. Figure 3
shows the transmission electron micrograph for a sample
heat treated at 823 K for 6 h, soaked in AgNO3and then
subjected to electrodeposition. The diameter of the silver
wire is seen to be ⬃20 nm. This result substantiates the
conclusion that increasing heat treatment brings about a de-
crease in the pore diameter of the silica gel. The latter causes
an increasing trend in the resistance value.
It has been possible to form some double-junction
systems9in the present nanowire composites. For this, a volt-
age pulse of 0.6 V of duration 5 s was applied across the
specimen containing silver nanowires. Such a pulse brought
about the disruption of the metal filament of nanometer di-
mensions. This optimum pulse was determined by applying
voltage pulses of 5 s duration of gradually increasing ampli-
tudes and measuring the resultant resistance of the sample
after application of the voltage pulse. It was found that with
0.6 V pulse of 5 s duration, the resistance of the sample
showed a sharp increase by several orders of magnitude in-
dicating a disruption in the metal filament. This is believed to
be caused by heating of some of the neck regions which
causes a selective melting of narrow zones in the filament.
Such disruption has been discussed earlier in connection
with the memory switching effect in glass–metal
composites.12–14 Figure 4 gives the voltage–current charac-
teristics of specimen No. 3 obtained at a temperature of 80
K. The measuring voltage was applied along the direction of
the electrodeposition voltage. The points represent the ex-
perimental data and the solid line is drawn to guide the eye.
The results obtained are reversible. It is evident that a stair-
case behavior, which resembles the characteristics of single-
electron tunneling is exhibited.9This is typical of all the
specimens. From the theory of single electron tunneling into
ultrasmall capacitance particles,15 the voltage gap between
successive current increments is given by e/Cwhere eis the
electronic charge and Cthe capacitance of the nanoparticle
of silver. The gap measured here is seen to be 0.2 V. Using
the aforementioned expression, we calculate a value of C
⫽8.0⫻10⫺19 F. The offset in the I–Vcurve is found to be
0.1 V which is half the above gap. This is consistent with the
FIG. 2. Variation of resistance as a function of temperature for silver
nanowire composites with different heat treated silica gels. 共䉱兲Specimen
No. 1, 共䊏兲specimen No. 2, 共䊉兲specimen No. 3.
FIG. 3. Transmission electron micrograph for silica gel heat treated at 823
K for 6 h soaked in AgNO3and subjected to electrodeposition.
FIG. 4. Current–voltage characteristic for specimen No. 3 with interrupted
silver nanowire induced by a dc voltage pulse.
TABLE I. Comparison of interplanar spacings dhkl obtained from electron
diffraction pattern with standard ASTM data. Specimen: silica gel heat
treated at 723 K/6 h, soaked in silver nitrate and subjected to electrodepo-
sition.
Observed
共nm兲
ASTM for
silver
共nm兲
0.233 0.235 9
0.204 0.204 4
0.145 0.144 5
0.125 0.123 1
0.104 0.102 15
3771Appl. Phys. Lett., Vol. 77, No. 23, 4 December 2000 Bhattacharyya, Saha, and Chakravorty
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fact that a coulomb gap has to exist for the charging of the
capacitor by a single electron. Evidently, the disruption of
the nanowire by a voltage pulse causes formation of silver
nanoparticles separated by barrier layers comprised of silica.
Using an expression for Cas15
C⫽␧r共1⫹r/2t兲,共1兲
where, ␧is the dielectric permittivity for silica, ris the radius
of silver particle, and tis the separation between the latter
and the remaining part of the nanowire which acts as the
electrode here. Substituting the above value of C, we calcu-
late a value of r⫽11.3nm. Considering the nanowire diam-
eter of ⬃40 nm, this looks reasonable. Evidently, the voltage
pulse brings about a break up of the nanowires into smaller
particles at certain sites.
In Fig. 5, we show the variation of dielectric constant as
a function of frequency for specimen No. 2 at room tempera-
ture. The measurements were carried out with electrodes
having a configuration either parallel or perpendicular to the
direction of the electrodeposition voltage. It can be seen that
the dielectric constant is quite high being of the order of 104
and there is no dispersion in the high frequency range. The
dissipation factor in the high frequency range ⬃0.06. Such
behavior is typical of all the specimens studied here. The
dielectric constant of the silica gel is ⬃4.0. The large value
of dielectric constant is ascribed to the presence of nanowires
of silver which are disjointed from each other and is quan-
tum mechanical in origin.16 This type of behavior was ob-
served earlier in other systems containing nanosized metal
filaments of a smaller length.17 It is to be noted here that
some metal nanowires are formed at an angle to the elec-
trodeposition direction because of the random orientation of
the pores within the silica gel. The high dielectric constant in
a direction perpendicular to the electrodeposition direction
therefore arose due to the contribution of such metal fila-
ments.
In summary, silver nanowires of a diameter ⬃40 nm and
a length ⬃0.3 mm have been grown within a silica gel heat
treated at temperatures in the range 523–823 K. A staircase
voltage–current characteristic was observed in specimens
where the metal nanowires were disrupted by the application
of a voltage pulse. A large value of dielectric constant, viz.
⬃104was measured in such pulse-treated specimens.
This research has been supported by the Department of
Science and Technology, Government of India, New Delhi.
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FIG. 5. Variation of dielectric constant as a function of frequency in speci-
men No. 2 with silver nanowires interrupted by application of a dc voltage
pulse.
3772 Appl. Phys. Lett., Vol. 77, No. 23, 4 December 2000 Bhattacharyya, Saha, and Chakravorty
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