short-term plasticity and long-term potentiation … plasticity and long-term potentiation mimicked...

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Supplementary Information

Short-term plasticity and long-term potentiation

mimicked in single inorganic synapses

Takeo Ohno, Tsuyoshi Hasegawa, Tohru Tsuruoka, Kazuya Terabe, James K.

Gimzewski and Masakazu Aono

1. Current-voltage (I–V) characteristics of the Ag2S atomic switch

Figure S1 shows typical I–V characteristics of the Ag2S atomic switch when

bias voltage is swept to a value large enough to cause bipolar switching. The I–V

characteristics are similar to those observed in the operation of conventional resistance

random access memories (ReRAMs), although the operating mechanisms are different.

Namely, the switch was turned on at around +80 mV when the bias voltage was swept

to +100 mV, and it was turned off at around –30 mV. The on-state is achieved by the

formation of a metallic atom bridge.

After formation of the atomic bridge, widening/thinning of its diameter takes

place depending on the polarity of the applied bias voltage. The manner of

widening/thinning shows memristive behaviour, as described in our previous paperS1.

The stability of the atomic bridge is the origin of the long-term potentiation (LTP)

demonstrated in this paper.

On the other hand, as described in this paper, the Ag2S atomic switch shows a

spontaneous decay in conductance (or current) before the metallic atom bridge

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formation without any bias application. This decay phenomenon, which is a

characteristic unique to atomic switches, enables the operation of the synaptic

behaviours reported in this paper.

Figure S1. I–V characteristics of the Ag2S atomic switch under the ON/OFF

switching operation. The bias voltage was swept as indicated by the arrows.

2. Retention time of a robust atomic bridge in the LTP state

Figure S2 shows the change in the conductance of an inorganic synapse versus

the measurement time, when a metallic atom bridge is formed by input pulses. Once an

atomic bridge is formed, the inorganic synapse shows a longer retention time. For

instance, the atomic bridge formed by a voltage pulse (V = 100 mV, W = 0.5 s) had

existed over 400 s, as shown in Figure S2. Since the range of the measured conductance

(about 2 G0, where G0 indicates the quantized conductance unit) corresponds to the

contact size of a few atoms, the result supports the contention that the point contact

formation is the origin of the robustness of an inorganic synapse in the LTP state.

Although the conductance changes due to the atomic rearrangement soon after

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the bridge formation, especially for a thinner bridge showing a conductance of about 1

G0, it mostly occurs in a few seconds. Therefore, we decided to judge the robustness of

the atomic bridge after 20 s has passed from its formation.

In Figure S2, we believe that the sudden increase and the decay in the

conductance at around 400 s was due to an accidental mechanical caused by the

scanning tunnelling microscope (STM) system. This is because our STM has a

capability to maintain a relative position between a tip and sample over 600 s, as

discussed in the following.

Figure S2. Change in the conductance of an inorganic synapse after a robust

atomic bridge was formed. The atomic bridge had existed over 400 s. Input pulse

was applied at 0 s, followed by a bias of 10 mV to measure the conductance of

the inorganic synapse.

Usually, a STM system has thermal/mechanical drift that changes the relative

position between a tip and sample. Since the thermal/mechanical drift may modify the

formed atomic bridge, we confirmed the stability of STM we used in this study. Figure

S3 shows the change in the tunnelling current when the feedback circuit of STM was





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switched off. The constant current suggests that the STM maintained a constant

nanogap without any feedback over 600 s, which is longer than the measured retention

time and much longer than the LTP formation judgement time of 20 s.

Figure S3. Change in the tunnelling current during the feedback off condition of

STM system.

Although LTP retention time over 400 s was confirmed as shown in Figure S2,

we measured a potential retention time using a crossbar-type inorganic synapseS2, in

which the two electrodes can be fixed completely for a longer period of time such as

months. Ideally, the inorganic synapse made by STM and the crossbar-type inorganic

synapse should show similar characteristics. The reason why we employed a STM to

make an inorganic synapse in this study is that a STM can make the initial condition

such as a nanogap size same among the measurements, whereas the crossbar-type

inorganic synapse still has a variation of the initial condition due to the fabrication

process at the present stageS2.

The potential retention time was much longer, as shown in Figure S4. The

atomic bridge formed by a voltage pulse (V = 130 mV, W = 0.5 s) had existed for 2 ×





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104 s, until stopping the measurement. This result further supports our contention that

the point contact formation is the origin of the robustness of an inorganic synapse in the

LTP state.

Figure S4. Change in the conductance of a crossbar-type inorganic synapse

after a robust atomic bridge was formed. Although a small change in the

conductance can be seen, the atomic bridge had existed for a longer time. 5 mV

was applied to measure the conductance of the inorganic synapse.

3. Threshold value distribution among inorganic synapses

Device-to-device reproducibility is discussed in relation to the distribution of

the threshold values achieving unstable potentiation. Each inorganic synapse operating

in the short-term plasticity (STP) state shows a certain distribution of the threshold

value of the number of input pulses. For instance, the threshold value is four in the

measurement shown in Figure 1b.

Figure S5 shows the distribution of the number of input pulses required for

unstable potentiation. Both inorganic synapses #1 and #2 show similar distributions,





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which secures the device-to-device reproducibility of the switching threshold for use as

inorganic synapses in certain future circuits.

As described in the method section of the main paper, the device structure of

the inorganic synapses was made using a scanning tunnelling microscope. Therefore,

the distribution shown in Figure S5 was measured at two different positions of the Ag2S

substrate surface.

Figure S5. The number of input pulses required for unstable potentiation. The

conditions of the input pulses were V = 80 mV, W = 0.5 s and T = 10 s. #1 and #2

indicate different inorganic synapses.

4. The effect of electronic phenomena in Ag2S

We considered the purely electronic phenomena, such as the trapping and

detrapping of electrons in Ag2S, which might cause the decay in the conductance of the

STP state after a pulse input. It is expected that the defect levels in the Ag2S film,

1 2 3 7 8 9 10 4 5 6





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originating mainly from the non-stoichiometric composition of the Ag2S, trap electrons

by a pulse input. Detrapping of the electrons could cause the change in the conductance

after the pulse input. Since the time-scale of the trapping and detrapping phenomena

could be similar to the conductance decay in the STP state in this experiment, we

performed the following experiment to confirm the purely electronic effect.

The change in current was measured using a platinum tip in contact with Ag2S

film, in order to observe the effect of the phenomena occurring in the Ag2S film. Figure

S6 shows the change in current when the input pulse was applied. As can be seen, the

current levels before and after the pulse input are almost the same. The decay after the

pulse input, such as seen in Figures 1b and 1c, was not observed in this experiment. We

believe that this is because the low level of resistivity of the Ag2S film we used makes

the effect of trapping and detrapping invisible. From this experiment, it is expected that

the spontaneous decay in the conductance discussed in this paper is caused by a

phenomenon occurring in a nanogap due to atomic/ionic movement; as discussed in the

main text.

Figure S6. Change in current when a platinum tip is intentionally in contact with

the Ag2S film. The condition of the input pulse was identical to Figure 1b.





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5. Supplementary movie

Movie demonstrating image memorizing into an inorganic synapse array.

As shown in the supplementary movie, we carried out the storing into a 7 × 7

inorganic synapse array of two images; the letters ‘1’ and ‘2’, with intervals of T = 2 s

and 20 s. The movie shows the input image (left-hand side) and the stored image

(right-hand side). The darker color in the stored image indicates the higher conductance

value of the inorganic synapse. The conductance which corresponds to sensory memory

(SM) is not actually displayed in the movie because its conductance (a few µS) is much

smaller than the full scale of conductance (100 µS), as shown in Figure 4a. The movie

replay time is ten times faster than the actual time.

References

S1. Hasegawa, T. et al. Memristive operations demonstrated by gap-type atomic

switches. Appl. Phys. A 102, 811–815 (2011).

S2. Terabe, K., Hasegawa, T., Nakayama, T. & Aono, M. Quantized conductance

atomic switch. Nature 433, 47–50 (2005).





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