A research on MoTe2-based memristor and switching stability improvement

Electronic synapse based on two-dimensional material are equivalent to synapses because of their excellent properties, which is very pivotal for constructing neuromorphic computing to break through the traditional Von Neumann architecture. In the study, a vertical double-ended memristor is prepared by using the dispersion of molybdenum disulfide nanosheets. Memristors based on MoTe2 structure show stable bipolar nonvolatile resistive behavior. Based on this, we introduce carbon dots layer into the original device structure, and improve the device performance by using carbon atoms to form new conductive filaments driven by electric field. This research provides a reliable solution for the next generation of neuromorphic computing.


Introduction
In the era of electronic information technology, it illustrated that data processing is particularly important.Given the information transmission bottleneck caused by storage and computing parallel units in von Neumann computing systems, the processing of massive data is not satisfactory [1][2][3].Similar to the biological neurons, a data storage and transmission behavior can be completed through synapses, which are fast, small in size, and low power.Learn from this behavior, building devices that simulate synaptic functions has shown great appeal.As an emerging electronic device, the sandwich structure based dual ended memory based on MIM exhibits a biological synaptic transmission principle and historical dependence [4][5][6].
Compared with other materials, two-dimensional transition metal disulfide compounds (TMDCs) shows the unique optical and optical properties, and other excellent chemical and physical properties, layered structure between atomic layers, which causing widespread research interest [7][8][9].As one kind of rising TMDCs, MoTe2 exhibits high carrier mobility and anisotropic crystal structure.Compared with the most widely studied memristor based on MoS2, the development of memristor centered on MoTe2 is rare.In a word, the exploit of MoTe2 based-memristor deserves more attention.
As a new class of carbon-based materials, carbon quantum dots (CQDs) have become the most potential competitor to common quantum dots due to thee excellent physical, chemical, and electronic properties [10][11][12][13].Excitingly, as an emerging zero dimensional sample with a size less than 10 nm, CQD combines traditional quantum dots' optical properties with carbon nanomaterials' electrical properties, thus demonstrating important research and development value in many fields.CQD is usually mainly composed of sp 2 carbon, with the same lattice spacing as graphite.It is worth noting that unlike graphene quantum dots (GQDs), GQDs show more defects.The edge' carbon atoms defect form weak bonds with adjacent carbon atoms.This means that carbon atoms are more likely to undergo directional migration driven by an electric field, forming conductive filaments.Compared with active metal ions, the weaker diffusion of carbon improves the random formation behavior of conductive filaments.This provides a solution that fully utilizes the drawbacks of CQD.
In this work, a Ag/MoTe2/ITO based memristor and Pt/MoTe2/C/ITO structure were exploited.The device exhibits stable and reliable resistive switching behavior.In addition, we exploit the defects on the surface of carbon dots to form carbon conductive filaments driven by electric fields to improve the performance of the devices.This provides a new plan for developing new computing device.

Experimental section 2.1 Preparation of MoTenanosheets
We use an electrochemical ablation process to prepare MoTe2 nanosheets.Put 20 mg of solid MoTe2 into 40 ml of NMP (N-methylpyrrolidone), heat and stir for 10 hours.The sample was then centrifuged twice (5000 rpm for 10 min each).The supernatant was collected and centrifuged twice (12000 rpm for 5 min each).Collect the supernatant containing the manganese dioxide nanosheets again.

Preparation of device
We use pin coating and thermal evaporation to prepare Ag/MoTe2/ITO memristors.We dropped the MoTe2 nanosheet dispersion on the ITO surface and spin-coated it (1200 rpm for 25 s).The devices were then dried on a drying bench and repeated twice.Finally, we used a thermal evaporation process to deposit Ag top electrodes on MoTe2 nanosheet films through a mask with a diameter of 200 m.We fabricated Pt/MoTe2/C/ITO memristors by spin coating and magnetron sputtering.The CQDs were spin-coated onto the ITO surface using a spin-coating process.The first spin coating was performed at 500 rpm for 10 s, and the second spin coating was performed at 2000 rpm for 30 s.Then, drop the MoTe2 nanosheet dispersion onto the cqd surface and spin-coat the first spin-coating at 500 rpm for 5 s and the second spin-coating at 1500 pm for 15 s, and repeat this behavior twice.Finally, a platinum top electrode is deposited on the molybdenum dioxide film through a magnetron sputtering mask.

The electrical performance and conductive mechanism of device
In this work, the bottom electrode is ITO, the top electrode is Ag, the resistance switching (RS) layer is MoTe2 film.The diagram of electrical performance measurement is shown in Fig. 2 a. Fig. 2b shows the classic current-voltage (I-V) characteristic curve of 50 cycles for a memristor.We next discuss the conduction mechanism of Ag/MoTe2/ITO memristors, the I-V characteristic is redrawn and fitted in the double logarithmic coordinate mode, as shown in HRS and LRS in Figs.2c and 2d.In HRS (Fig. 2c), the fitting curve can be composed of three parts: Ohm's Law, Child's Law, and trap-free regions.This suggests that the dominant conduction mechanism of HRS can be attributed to the space charge limited current (SCLC) mechanism.In LRS (Fig. 2d), the mechanism follows Ohm's law, and when the Ag conductive filament breaks, device will perform the reverse process [17].

The physical mechanism of the device
For Ag/MoTe2/ITO memristor, the mechanism is mainly attributed to the formation and rupturing of Ag conductive filaments.When we apply a forward voltage to the device, the Ag around the top electrode will be dissolved into Ag + through oxidation reaction, and in the process of migration to the bottom electrode under the effect of electric field, Ag conductive filaments will be formed through reduction reaction and electron combination.With the gradual formation and accumulation of Ag conductive filaments, the memristor will switch from HRS to LRS (Fig. 3a).On the contrary, when we apply a negative voltage to the device, the fracture process of the Ag conductive filament is opposite to the above process, and the device will switch back in HRS (Fig. 3b).

3.5
The electrical performance and conductive mechanism of device Measure the I-V curve of the device in DC mode, as shown in Fig. 5a.The results indicate that the device can achieve stable I-V switching behavior for 100 cycles at very small programmable voltages (-1V to 1V).The formation and fracture behavior of carbon conductive wires under the RS mechanism were discussed.Temperature dependence of HRS and LRS of measuring devices (Vread=0.1 V).It indicates that HRS decreases with increasing temperature (Fig. 5b).In stark contrast, LRS increases with increasing temperature (Fig. 5c), and metal behavior can be observed in the temperature dependence of LRS.Based on the above, the fitting of the LRS part of the I-V curve on the logarithmic coordinate axis is completely linear, with a slope of 1.00, indicating that the LRS belongs to Ohmic contact behavior (Fig. 5d).Therefore, it can be inferred that LRS is attributed to the formation of carbon conductive filaments (CFs) [21,22].

Conclusion
In summary, We fabricated MoTe2 nanosheets as the RS layer of memristors.The Ag/MoTe2/ITObased memristors exhibited excellent RS curve.Based on this, we introduced carbon dots to further improve the stability of resistive switching at a smaller operating voltage, which is attributed to the orientation of carbon atoms to form a strong conductive filament driven by an electric field (Pt/MoTe2/C/ITO memristors), robust carbon conductive filaments allow the device to exhibit excellent stability over more duty cycles.This provides a new plan for developing new computing device.

Figure 1 .
Figure 1.(a) The SEM image of MoTe2 nanosheets.(b) The SEM image of MoTe2 film.(c) The XRD of MoTe2.(d) The Raman spectrum of MoTe2.

Figure 2 .
Figure 2. (a) Diagram of electrical measurement.(b) The I-V curve of the device.(c) The conduction mechanism of memristor in HRS.(d) The conduction mechanism of memristor in LRS.

Figure 3 . 4
Figure 3. (a-b) Diagram of the physical mechanism of the memristor.

Figure 5 .
Figure 5. Classical I-V curves of device.(b, c) Dependence of device HRS and LRS current on temperature (I-T).(d) The fitting curve of LRS (Ln(I) versus Ln(V)).

Figure 6 .
Figure 6.(a-b) Diagram of the physical mechanism of the memristor.