MoS2 memristor with photoresistive switching

A MoS2 nanosphere memristor with lateral gold electrodes was found to show photoresistive switching. The new device can be controlled by the polarization of nanospheres, which causes resistance switching in an electric field in the dark or under white light illumination. The polarization charge allows to change the switching voltage of the photomemristor, providing its multi-level operation. The device, polarized at a voltage 6 V, switches abruptly from a high resistance state (HRSL6) to a low resistance state (LRSL6) with the On/Off resistance ratio of about 10 under white light and smooth in the dark. Analysis of device conductivity in different resistive states indicates that its resistive state could be changed by the modulation of the charge in an electric field in the dark or under light, resulting in the formation/disruption of filaments with high conductivity. A MoS2 photomemristor has great potential as a multifunctional device designed by using cost-effective fabrication techniques.

Molybdenum disulfide (MoS 2 ), as a semiconducting analogue of graphene, with an indirect band gap of ~1.2 eV for the bulk material and with a direct gap of ~1.8 eV for the monolayer 24 has tremendous potential for both electronic and optical applications. MoS 2 is composed of stacked planes of covalently bonded S and Mo atoms with spacing of ~6.5 Å, which are weakly bound to each other by van der Waals forces. MoS 2 in the form of flat flakes or nanospheres can be obtained by using the hydrothermal process and are uniformly distributed on a substrate using drop casting, vacuum filtration, Langmuir-Blodgett deposition, or spin coating processes 7,[25][26][27][28][29][30] . The resistive switching in MoS 2 structures in both vertical and lateral geometry has been reported [18][19][20] . In the case of 2 H MoS 2 nanospheres consisting of monolayers, the polarization phenomenon can control the switching resistance 27 . Furthermore, nanosphere lattice, as reported 31 can affect the electronic band gap, while the optical spectrum of nanoparticles can be changed in an electric field [32][33] or under light [34][35] . Photodetectors constructed, for example, using ZnO spheres as building blocks demonstrate a high and fast photoresponse 36 .
In this article, we report a MoS 2 nanosphere photomemristor obtained from the solution using a simple hydrothermal method. The memristor, through the charge polarization of nanospheres, can be switched by an electric field in the dark or under illumination with white light. Modulation of charge in an electric field by photons controlled switching of the photomemristor, providing multi-level resistance operation with great potential for advanced multi-functional non-volatile memories.

Results and Discussion
The MoS 2 nanospheres synthesized using the hydrothermal method (see Methods) were 550~750 nm in diameter ( Fig. 1(a)) with well-defined XRD spectrum ( Fig. 1(b) [37][38][39][40] . The (002) peak indicates that MoS 2 grows with well-stacked layers along the (002) direction. Figure 1(c) shows a typical HRTEM image of the nanospheres. MoS 2 interlayer distances of 0.62 and 0.22 nm are indexed (002) and (103) planes of 2H-MoS 2 , which agree well with the results of XRD measurements. Raman spectra of MoS 2 nanospheres are shown in Fig. 1(d). The peaks at 375.5 and 401.3 cm −1 are characteristic for as-grown sample, but they are at 383.5 and 406.2 cm −1 for the sample annealed in the H 2 / N 2 atmosphere. They correspond to E 1 2 g and A 1 g vibrations of MoS 2 atoms in plane and out of plane 38,39,41 . The shift of Raman peaks indicates an increase in van der Waals interaction between adjacent layers after annealing 32 . The energy difference Δ E between E 1 2 g and A 1 g modes corresponds to ~24 and ~22.7 cm −1 for 3-4 layers of MoS 2 as-grown and annealed, respectively. This is in good agreement with the HRTEM measurements ( Fig. 1(c)).
X-ray photoelectron spectra of MoS 2 nanospheres are shown in Fig. 2. For as-grown nanospheres, Mo 3d 5/2 peak comprises of four peaks centered at 229.32, 230.0, 231.01, and 232.65 eV, which correspond to MoS 2 , Mo 2 S 5 , MoS 3 (Mo(S 2 ) 2− S 2− )), and MoO 3 , respectively (Table 1). S 2p 3/2 peak is divided into two peaks centered at 161.44 and 162.16 eV, which correspond to MoS 2 and intermediate product of Mo 2 S 5 and MoS 3 , possibly due to incomplete reduction during the hydrothermal process [42][43][44][45][46] . After annealing, Mo 3d 5/2 peak is divided into two peaks centered at 229.49 and 233.19 eV, which correspond to S-Mo-S and O-Mo-O, respectively. The S 2p 3/2 peak shows only one peak at 161.19 eV (Table 1). It indicates that the Mo 4+ state is predominant in annealed samples, and the intermediate phases of Mo 2 S 5 and MoS 3 are reduced in a stream H 2 /N 2 and transferred to MoS 2 phase. However, small quantities of MoO 3 is present in the grown and annealed samples, possibly due to oxidation of MoS 2 during the hydrothermal process. Finding of the S 2 s peak at 226.61 eV and atomic S/Mo ratio of about 2 for the annealed sample (Table 1) indicates that the MoS 2 nanospheres are close to the stoichiometric composition. Figure 3 shows the current-voltage (I-V) curves of the Au/MoS 2 nanospheres/Au structure after sweeping the poling voltage (3 and 6 V) at a constant rate (0.05 V s −1 ). A schematic image of the device is shown in Inset of Fig. 3(a), and Supplementary Information, Fig. S5. Nonlinear characteristics of the device with a marked  Supplementary Information, Fig. S1). The fitted peaks at 375.5 and 401.3 cm −1 are characteristic for as-grown sample, but 383.5 and 406.2 cm −1 for the sample annealed in the H 2 /N 2 atmosphere. The energy difference Δ E between E 1 2 g and A 1 g modes corresponds to ~24 and ~22.7 cm −1 for 3-4 layers of MoS 2 as-grown and annealed, respectively.
Scientific RepoRts | 6:31224 | DOI: 10.1038/srep31224 hysteresis indicates the memristive behaviour. Furthermore, the memristive structure showed a high photoresponse to white light (spectral maxima at 2.7 and 1.8 eV), as the increase in current 1500% and 2000% at a voltage of 3 and 6 V, respectively. When the device is polarized at 3 V smooth switching occurs from HRS L3 to LRS L3 under a white light, and from HRS D3 to LRS D3 in the dark with a ratio of On/Off resistances about 2 and 4 at 1.2 V and 0.7 V, respectively ( Fig. 3(a)). At higher poling voltage (6 V), the device shows a sharp resistive switching under white light from HRS L6 to LRS L6 at − 2.9 V with a ratio of On/Off resistances about 10 and a smooth switch from HRS D6 to LRS D6 in the dark with a ratio of On/Off resistances about 3 at 0.7 V (Fig. 3(b)). The SET operation from OFF to ON state represents a "writing" process. When the applied voltage runs from 0 to the positive voltage (4.2 V), the structure reverts to the HRS L6 . The operation RESET from the ON to the OFF position acts as a function of erasing. Memristive behavior of the device in the dark and under white light is well reproduced in the process of iteration to 1000 cycles with excellent stability of the states (Fig. 3(c,d) and Supplementary Information,  Fig. S3). It indicates that the multi-level resistive switching of the memristor nanostructure can be obtained and controlled by an electric field in the dark and under white light.

Peak and indensity
Peak position (eV) (at.%) described by the SCLC model, which is dominant in the carrier transport process of the OFF state and originates from the charge trapping and detrapping by MoS 2 nanosphere interfaces. Defects at MoS 2 interfaces serve as charge traps, assisting the nanosphere polarization. When negative voltage is applied to the structure, the injected electrons are captured by the defects and fill the traps. Resistance behavior changes from trap-unfilled (S1) to trap-filled (S2) which describes the SCLC contribution to charge emission. In the LRS/ON state, when the voltage is less than 1 V, the plots of Ln(I) vs. V 1/2 from 0 to 0.2 V (denoted as T1) and 0.25 to 0.95 V (denoted as T2) are well fitted by using the thermionic emission model to straight lines with a slope of 5.11 and 9.98 for T1 and T2, respectively (Fig. 4b). When the voltage exceeds 1 V, the I-V curve obeys the ohmic behavior with a slope of 4.53, demonstrating that good conductive channels are established in the ON state (Fig. 4(c)). Formation of the conduction channels along the boundaries of polarized nanospheres, which are interrupted by a reverse bias could be the driving force behind the resistance switching. An electric field can control the polarization of the nanospheres and the shift of the polarization domain in the dark 27 and under the light through the additional charge generating by photons with energies that exceed the band gap of MoS 2 (1.5-1.9 eV). Electrons and holes are separated at nanosphere interfaces, affecting the collective polarization of the nanospheres and the interface barrier height. Figure 5 shows the schematic band diagram of the memristor structure. The band alignments (Fig. 5(a)) are based on reported work functions of n-type MoS 2 (Φ n-MoS2 = 4.6 eV), and Au (Φ Au = 5.1 eV) 47 . Two Schottky barriers (Δ Φ SB = Φ Au − Φ n-MoS2 = 0.5 eV) and Shottky-like double barriers of nanosphere interfaces can separate photo-generated electron-hole pairs which produce a photocurrent. Poling the structure (Fig. 5 (b,c)) forms the interface barriers and polarization P of nanospheres (brown arrows). White light exposure to the nanospheres decreases the interface barrier hight (Fig. 5 (d,f)). The nanospheres are likely to play a significant role in the memristor behavior. An external electric field charges the nanospheres and polarizes them, forming the percolation conduction channels along the interfaces. The charge injection by light contributes to nanosphere polarization over a large area between the lateral electrodes and effect on switching characteristics. The injection of electrons into nanospheres through tunneling process and photogeneration in a high electric field decreases the barrier height at the interface and forms a conductive channel. The memristor is switched at − 2.9 V under white light from HRS L6 to LRS L6 (SET operation) (Fig. 5(f)). A reversely applied electric field leads to opposite charge injection and recovery of the interface barrier height, hindering electron tunneling. The conductive channel is destroyed at 4.2 V (Fig. 5(f)), and the memristor is switched to HRS L6 (RESET). SET operation in the dark results in a higher resistance state due to a higher barrier at the interface. Electron tunneling through the barrier at − 6 V forms LRS D6 (Fig. 5(e)). At an applied voltage of 6 V the memristor sets a high barrier at the interface of oppositely polarized nanospheres and switches back to HRS D6 (Fig. 5e). It should be noted that a high density of point defects may affect the behavior of the memristive device, especially on the nanoscale. Resistive switching in the semiconductor 2 H MoS 2 phase with grain boundaries in the presence of a high density of sulfur vacancies was reported 19 . However, MoS 2 nanospheres used in our experiments were close to the stoichiometric composition ( Table 1). The ion drift at a low concentration of point defects in the semi-insulating nanospheres hardly could control the resistive switching on the observed micrometer scale. But the nanospheres can be charged and polarized by an external electric field, as previously reported 27,48 , changing their resistance because of the motion of the collective polarization domain. It is worth noting that a T ferroelectric phase, recently discovered in MoS 2 20,49 can also cause non-linearity of current-voltage characteristics and the resistive switching. Distortion of the 1 T MoS 2 lattice under an external electric field can cause resistance to change. However, the 1 T phase can be easily converted into a 2 H phase at moderate annealing temperatures (above 200 °C) 50 . In addition, the T phase is highly conductive and thus has a low photosensitivity. The charge modulation by photons allows control of the switching voltage of the 2 H MoS 2 photomemristor, providing the multilevel resistive switching operation. The multilevel switching of resistance of the polarized device poled at different voltages under white light or in the dark was observed in our experiment (Fig. 3). The natural application for resistive switching is nonvolatile resistive random access memory, however their dynamical nonlinear behavior also suggests that it could be used to develop alternative logic architectures. Figure 6 demonstrates the resistance states formed after the SET/RESET operations of the MoS 2 memristor polarized at voltages of 3 V and 6 V in the dark or under light excitation. The memristor polarized at 3 V in the dark or under white light demonstrates the four states, which can be read at a voltage of 0.7 V (HRS D3 and LRS D3 ) and 1.2 V  (HRS L3 and LRS L3 ) in the dark or under light ( Fig. 6(a)). The memristor polarization at 6 V in the dark or light led to the formation of the other four states, which are read at a voltage of 0.7 V (HRS D6 and LRS D6 ) and 4 V (HRS L6 and LRS L6 ) in the dark or light (Fig. 6(a)). All of these states are well controlled electrically at room temperature with the applied voltage and light excitation as it is confirmed by the iterative operation of the memristor under different conditions of writing and reading ( Fig. 3(с,d) and Supplementary Information, Fig. S3). Polarization of nanospheres in the photomemristor using pulses of electric field and light creates multi-level resistance states. Analysis of conductivity in the resistance states indicates that charging and nanosphere polarization modulated by light could generate and interrupt conductive filaments to switch resistance. Reducing the gap between the electrodes in a planar geometry or vertical, can greatly minimize the operating voltage of the device. Modulation of the barrier height at the interfaces of nanospheres in an external electric field by light appears to be highly promising for high-speed optoelectronics, since the polarization process is much faster than the ion drift 36 .

As-grown Annealed
In conclusion, we have demonstrated a MoS 2 nanosphere-based photomemristor. The resistance of the device can be controlled by the polarization of nanospheres and switched by an electric field in the dark and under white light. The charge modulation by photons allows to control the switching characteristics of the photomemristor, providing the multi-level resistance switching operation. The memristor polarized at different voltages demonstrates several resistance states readable under white light and in the dark. We believe that the photomemristor has great potential as a new multifunctional device made by simple and inexpensive methods.

Methods
Fabrication of the Au/MoS 2 /Au structure. The MoS 2 nanospheres were grown by using the hydrothermal technique. Sodium molybdate (Na 2 MoO 4 ·2H 2 O, 0.03 g) and thioacetamide (CH 3 CSNH 2 , 0.06 g) were dissolved in 20 mL distilled water. After stirring for 1 hour, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 220 °C for 24 hours. After cooling down to 22 °C. the black precipitates of MoS 2 were first retrieved from the solution, collected, and washed with acetone, methanol, distilled water, and absolute ethanol for five times, and dried in vacuum oven at 80 °C for 6 hours. Then, the dispersion of MoS 2 nanospheres mixed with isopropyl alcohol (IPA) and was spin-coated on a Si substrate with 275-nm-thick SiO 2 and baked at 200 °C for 15 minutes twice, followed by a rapid thermal annealing at 800 °C for 5 minutes in a stream of Characterizations. The crystal structure of the MoS 2 sample was analyzed by X-ray diffraction techniques (XRD, Bruker D8 Advance) with a Cu Kα radiation (λ = 0.154 nm), high resolution transmission electron microscopy (HRTEM, JEOL JEM 2010), and a room temperature micro-Raman spectrometry (LabRAM HR800, He-Ne laser, 488 nm). The morphology and chemical composition of MoS 2 were examined by a field emission scanning electron microscopy (FESEM, S4800) and X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Mg Kα excitation at 1253.6 eV, 150 W). The current-voltage (I-V) characteristics of fabricated Au/MoS 2 /Au structures were investigated at room temperature in ambient air in the dark and under white light excitation (spectral maxima at 2.7 and 1.8 eV) using a Keithley 4200 SCS semiconductor parameter analyzer with LabVIEW software in voltage sweeping mode.