pubs.acs.org/JPCL
Rate-Limiting Processes Determining the Switching Time
in a Ag2S Atomic Switch
Alpana Nayak,*,† Takuro Tamura,‡ Tohru Tsuruoka,† Kazuya Terabe,† Sumio Hosaka,‡
Tsuyoshi Hasegawa,† and Masakazu Aono†
†
WPI Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1Namiki, Tsukuba, Ibaraki 305-0044,
Japan and ‡Department of Production Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu 376-8515, Japan
ABSTRACT The switching time of a Ag2S atomic switch, in which formation and
annihilation of a Ag atomic bridge is controlled by a solid-electrochemical reaction
in a nanogap between two electrodes, is investigated as a function of bias voltage
and temperature. Increasing the bias voltage decreases the switching time exponentially, with a greater exponent for the lower range of bias than that for the
higher range. Furthermore, the switching time shortens exponentially with raising
temperature, following the Arrhenius relation with activation energy values of
0.58 and 1.32 eV for lower and higher bias ranges, respectively. These results
indicate that there are two main processes which govern the rate of switching, first,
the electrochemical reduction Agþ þ e-fAg and, second, the diffusion of Agþ ions.
This investigation advances the fundamental understanding of the switching
mechanism of the atomic switch, which is essential for its successful device
application.
SECTION Electron Transport, Optical and Electronic Devices, Hard Matter
N
room temperature, enable the development of a novel
programmable logic device5 that can achieve all functions
with a single chip.
Because the operating mechanism of the atomic switch is
very different from that of conventional semiconductor devices, a fundamental understanding of the switching mechanism is necessary prior to its use in commercial devices.
Though several intriguing results, both experimental3,6-8 and
theoretical,9,10 have already been obtained concerning this
switch, its actual working mechanism has not been well
clarified yet. Accordingly, we measured the switching time
of a Ag2S atomic switch with a nanogap as a function of bias
voltage and temperature. We determined, for the first time,
the activation energy values for switching, which are of prime
importance in understanding the mechanism. Our results
suggest that, in addition to the electrochemical reaction, the
diffusion of Agþ ions plays a significant role in determining
the rate of switching.
In this study, a Ag2S atomic switch is realized across a gap
of 1 nm between a Ag2S electrode and a Pt tip of a scanning
tunneling microscope (STM) as a counter electrode. Figure 1
shows a scanning electron microscope (SEM) image of the
Ag2S electrode, which was used as the solid-electrolyte electrode, with crystals ranging from submicrometer to micrometer size. A schematic representation of the Ag2S atomic
switch and the corresponding electric potential distribution
anoionics-based resistive switching devices have
been attracting much attention in recent years to
overcome the physical and economical limitations of
current semiconductor technology.1 A lot of research has been
aimed at finding a reliable switching mechanism that can
permit ever smaller and more powerful electronics.2 Recently,
we have developed a conceptually new nanodevice called an
atomic switch, in which formation and annihilation of a metal
atomic bridge across a nanogap between a solid-electrolyte
electrode and a counter metal electrode is controlled by a
solid-electrochemical reaction.3 The switching operation can
be achieved by only changing the polarity of the bias voltage
applied to either electrode. For instance, applying a positive
bias voltage to the solid-electrolyte electrode, the metal ions in
the electrode reduce to metal atoms, forming a conductive
atomic bridge between the electrodes. This decreases the
resistance between the two electrodes to a certain ON resistance, which means that the switch is turned ON. When the
polarity of the applied voltage is reversed, the metal atoms in
the conductive atomic bridge are oxidized and are incorporated back into the solid-electrolyte electrode. This annihilates
the conductive bridge between the two electrodes, turning the
switch OFF. Similar controlled formation and annihilation of
an atomic bridge has also been achieved in an ionic conductive material sandwiched between two electrodes using
the solid-electrochemical reaction.4 The ease of operation and
simple structure of the atomic switch make it suitable for
configuring logic gates3 and memory devices.4 In addition, its
unique features, namely, low ON resistance, scalability down
to nanometer size, low power consumption, and operation at
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Received Date: December 8, 2009
Accepted Date: January 6, 2010
Published on Web Date: January 11, 2010
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DOI: 10.1021/jz900375a |J. Phys. Chem. Lett. 2010, 1, 604–608
pubs.acs.org/JPCL
Figure 1. SEM image of the Ag2S electrode showing crystals of
submicrometer to micrometer size. The Ag protrusions, which
grew during scanning, are indicated by arrows.
Figure 3. Switching time tsw as a function of bias voltage Vsw
measured at room temperature (∼25 °C). The mean value of tsw
for each Vsw is shown by open circle (O) symbol. The exponential
fits of the data, considering the mean values of tsw, are shown by
the solid lines. The extrapolations of the exponential fits are
shown by the dashed lines.
dashed lines in Figure 3. Interestingly, the exponent β is about
2 times greater for the lower bias range than that for the
higher range.
It should be noted that the resistance of the Ag2S electrode
(RAg2S) is on the order of 100 Ω;, therefore, most of the voltage
drop occurs in the vacuum gap of much higher resistance
(1 MΩ-12.9 kΩ) between the Ag2S electrode and the STM tip
(Figure 2). Hence, the bias voltage VAg2S effectively applied to
the Ag2S electrode is much smaller than Vsw and is given by
It RAg2S, where It is the tunneling current flowing in the
atomic switch during the switch-ON process. Consequently, it
is useful to examine tsw as a function of VAg2S. We find that the
tsw decreases exponentially as well with VAg2S, consistent with
our earlier report on the growth rate of Ag protrusion.12 This
suggests that the rate of the solid-electrochemical reaction
increases exponentially with increasing bias and that the
charge-transfer coefficient1 is proportional to the exponent
β. Furthermore, a smaller value of β for higher Vsw indicates
that the operating mechanism might be accompanied by
some additional activation barrier at higher Vsw. More insight
into the switching mechanism can be obtained from the
temperature dependence of tsw.
Upon increasing the temperature of the Ag2S electrode, the
tsw decreases exponentially following the Arrhenius relation
tsw µ exp-Ea/kBT, where Ea is the activation energy for switching. The temperature (T) dependence of tsw for Vsw of 0.15
and 0.25 V are shown in Figure 4a and b, respectively. The
Ea values extracted from the slope of the Arrhenius plots
(cf. insets of Figure 4a and b) are 0.51 and 1.26 eV, respectively. Similarly, we have extracted the Ea values from the
Arrhenius plots of tsw versus T data for various Vsw. Figure 5
shows the Ea values for Vsw in the range of 0.1-0.275 V.
Interestingly, the Ea values at the lower bias range are smaller
than those at the higher bias range, complementing our
results from the voltage dependence measurements. The
mean values of Ea are found to be 0.58 and 1.32 eV for lower
and higher bias ranges, respectively. We infer from these
results that, at the higher Vsw, the supply of Agþ ions might
become slower than the rate of electrochemical reduction,
leading to a higher value of Ea.10
Figure 2. Schematic representation of the Ag2S atomic switch and
the corresponding electric potential distribution induced by applying a bias voltage Vsw for switching. The bias voltage effectively
applied to the Ag2S electrode is shown by VAg2S.
induced by applying a bias voltage (Vsw) for switching are
shown in Figure 2. Most of the potential drop occurs in the
vacuum gap, and the potential drop in the Ag2S electrode
(VAg2S) occurs mainly at the surface and the interface. We
define switching time (tsw) as the time taken for the resistance
between the Ag2S electrode and the STM tip to decrease from
an initial OFF resistance to 12.9 kΩ after applying a Vsw. Here,
12.9 kΩ is considered as the ON resistance corresponding to a
single atomic contact.11
Figure 3 shows the tsw as a function of Vsw measured at
room temperature (∼25 °C) for an initial OFF resistance of
1 MΩ. Switching becomes exponentially faster with increasing Vsw.3,8 The variation in the values of tsw at a particular Vsw is
acceptable, taking into account that the measurements were
done at different positions on the Ag2S surface. As can be seen
in Figure 1, Ag precipitation depends on the local conditions of
surface structure and ionic distribution. However, considering
the mean value of tsw for each Vsw, we find that the data fits
well with a simple exponential function tsw µ exp-βVsw.
Further, this exponential relation appears to consist of two
components with different values of exponent β, indicating
that more than one process participates in controlling the rate
of switching. The exponential fits to the data are shown by the
solid lines, and the extrapolations of the fits are shown by the
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DOI: 10.1021/jz900375a |J. Phys. Chem. Lett. 2010, 1, 604–608
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Figure 6. Schematic illustration of the operating mechanism in a
Ag2S atomic switch. The solid line represents the rate of precipitation of Ag atoms, and the dashed line represents the Agþ ionic
density at the surface of the Ag2S electrode due to diffusion. At
lower bias voltages (Vsw), the electrochemical reduction for the
precipitation of Ag atoms is the rate-limiting process, whereas the
diffusion of Agþ ions toward the surface of Ag2S becomes the ratelimiting process at higher Vsw. The crossover between these two
rate-limiting processes occurs at around a Vsw referred as the
critical point. Accordingly, the activation energy (Ea) for switching
is greater at higher Vsw than that at lower Vsw.
toward the Pt tip leading to the formation of an atomic bridge.
We believe, on the basis of our results, that more than one
process participates in controlling the overall kinetics of the
atomic switch and that the final rate-limiting step depends on
the applied bias voltage for switching. Since the electric field
applied during switching is very high (∼106 V/cm), the precipitated Ag atoms are most likely to grow toward the Pt tip,
leading to the formation of an atomic bridge, and hence, step
(iii) does not seem to be a rate-limiting process. This renders
steps (i) and (ii) as the probable but competitive rate-limiting
steps determining the kinetics of the switch-ON process.
At lower Vsw, the time taken for switching is on the order of
milliseconds or greater, providing enough time for the diffusion of sufficient Agþ ions to the Ag2S surface for the electrochemical reduction.13,14 In other words, the rate of diffusion
of Agþ ions is faster than the rate of electrochemical reduction. Therefore, in this range of Vsw, the rate of electrochemical
reduction for the precipitation of Ag atoms is the most
probable rate-limiting step. On the other hand, at higher
Vsw, the rate of precipitation of Ag atoms becomes faster
due to the increased It. However, a greater value of Ea and a
smaller value of β indicates that the availability of Agþ ions at
the Ag2S surface might not be sufficient to accompany the fast
precipitation of Ag atoms. This is because there is not enough
time for the diffusion of sufficient Agþ ions toward the surface
of the Ag2S electrode. Therefore, at higher Vsw, the rate of
diffusion of Agþ ions in Ag2S toward its surface is the most
probable rate-limiting step.
On the basis of our results and the above discussion, we
have schematically illustrated, in Figure 6, the mechanism for
the switch-ON process of the Ag2S atomic switch for lower and
higher Vsw. The solid line represents the rate of precipitation of
Ag atoms with increasing Vsw, and the dashed line represents
Figure 4. Temperature (T) dependence of the switching time (tsw)
for bias voltages (Vsw) of (a) 0.15 and (b) 0.25 V. The error bars
correspond to the standard deviation of the measured tsw. The
solid lines correspond to the exponential fits to the data. The
Arrhenius plots and the extracted activation energy (Ea) values are
shown in the insets of the respective figures.
Figure 5. The activation energy (Ea) values extracted from the
Arrhenius plots of switching time versus temperature data for bias
voltages (Vsw) in the range of 0.1-0.275 V.
The switch-ON process of the atomic switch at a sufficient
Vsw includes the following steps: (i) a charge-transfer process
that involves reduction of Agþ ions at the Ag2S surface leading
to the precipitation of Ag atoms and the counter reaction
representing the oxidation and dissolution of Agþ ions into the
Ag2S electrolyte (Figure 2), (ii) diffusion of Agþ ions across the
Ag2S electrolyte under the action of an applied electric field,
and (iii) preferential growth of the precipitated Ag atoms
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the Agþ ionic density at the Ag2S surface due to diffusion. At
lower Vsw, the electrochemical reduction for the precipitation
of Ag atoms is the rate-limiting process, whereas the diffusion
of Agþ ions toward the surface of Ag2S becomes the ratelimiting process at higher Vsw. The crossover between these
two rate-limiting processes occurs at around a Vsw of 0.2 V,
referred as the critical point in Figure 6. Further investigation
to clarify the existence of such a critical point is in progress.
Moreover, there are possibilities of other rate-determining
processes; for instance, the electron migration process to trap
defects on the Ag2S surface for the formation of a Ag cluster
could also play a significant role. Investigating such processes
would require a precise knowledge of the distribution of
defects on the surface of the Ag2S electrode together with
an atomistic study based on first-principles calculations. Thus,
in order to exploit the potential of atomic switch to the limits, a
considerable research effort is still needed with respect to a
deeper understanding of the mechanism that governs the rate
of switching.
The Ag2S electrode used in this study was prepared by
sulfurizing a Ag plate (8 5 0.2 mm3) at 150 °C for 2 h with
sulfur vapor. Under this sulfurization condition, the Ag2S
electrode is expected to exhibit the acanthite R-phase characterized by a monoclinic unit cell with semiconducting
properties and is stable at room temperature.15-17 The
temperature dependence measurements were performed
by an indirect resistive heating of the Ag2S electrode. During
all of the measurements, the STM was operated under high
vacuum conditions (∼1 10-4 Pa).
We measured the switching time by the following
procedure: (i) The Pt tip was fixed with a tunneling resistance of 1 MΩ (V = -20 mV, I = 20 nA) as the initial
OFF resistance. Consequently, a gap of about 1 nm was
maintained between the Pt tip and the Ag2S electrode.
(ii) The feedback system of the STM was disabled, and a
positive Vsw was applied to the Ag2S electrode. (iii) The current
flowing between the tip and the Ag2S surface was recorded.
Since the resistance of the Ag2S electrode was less than 100 Ω,
the ON resistance was calculated from the current flowing
between the tip and the Ag2S electrode. The Ag protrusion
formed on the surface of Ag2S during each switch-ON process
was completely eliminated by applying a negative Vsw
for a few minutes to the Ag2S electrode before the next
measurement.
In conclusion, the effects of bias voltage and temperature
on the switching time of a Ag2S atomic switch were investigated. Switching becomes exponentially faster with increasing bias voltage. However, the exponential relation exhibits a
greater exponent for the lower bias range than that for the
higher range, suggesting an additional activation barrier for
switching at higher bias voltages. Raising the temperature
shortens the switching time exponentially, following the Arrhenius relation. The activation energy values are determined
to be 0.58 and 1.32 eV for the lower and higher bias ranges,
respectively. This is the first experimental determination of
the activation energy for switching in an atomic switch and is
of prime importance in understanding the switching behavior.
Our results suggest that the electrochemical reduction for the
precipitation of Ag atoms is the rate-limiting process at lower
r 2010 American Chemical Society
bias voltages, whereas the diffusion of Agþ ions toward the
surface of Ag2S becomes the rate-limiting process at higher
bias voltages. These findings provide physical insight into the
operating mechanism of the atomic switch.
AUTHOR INFORMATION
Corresponding Author:
*To whom correspondence should be addressed. E-mail: NAYAK.
[email protected].
ACKNOWLEDGMENT Part of this work was conducted under the
Key-Technology Research Project, “Atomic Switch Programmed
Device”, supported by the MEXT and the Strategic JapaneseGerman Cooperative Program supported by JST.
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