**3.3. Self-organization of memristors based on graphene/graphene oxide**

The photocatalytic oxidation of graphene coated with a layer of 10–15 nm ZnO nanoparticles under ultraviolet (UV) irradiation conditions led to the formation of self-organized G/GO memristors with very high density (1012 cm−2) [16, 17]. **Figure 7** shows the scheme of photocatalytic oxidation of graphene with ZnO nanoparticles. A 2–3-layer graphene coated with

**Figure 5.** Scanning electron microscope (SEM)-remote induced current (REBIC) images of the Al/GO/Al structure with the modulation of the built-in potential barrier near the negatively biased Al electrode at different bias (V<sup>b</sup> ) and forming (Vf ) voltages. (a) Vb = 0; Vf = 0 (SE mode); (b) V<sup>b</sup> = 0; Vf = 0 (REBIC mode); (c) V<sup>b</sup> = 0; Vf = 5 V (REBIC mode); (d) V<sup>b</sup> = 0; Vf = 5 V (SE mode); (e) V<sup>b</sup> = 0. 2 V; Vf = 7 V (REBIC mode); (f) V<sup>b</sup> = 0. 5 V; Vf = 7 V (REBIC mode, same area as in (e)). A scale mark of 100 μm (e) and (f), 50 μm in (a)–(d). The images in (a)–(d) were obtained by sequentially switching signals of secondary electrons (SE) and remote induced current (REBIC) during scanning of the electron beam [12].

interface, which provides a hole flux (3.3 eV) to graphene. As a result, graphene is decorated

**Figure 8.** Schematic electronic diagram of the G/ZnO interface under UV irradiation. Electron-hole pairs generated in ZnO (3.3 eV) under UV irradiation (reaction 1) are separated in a built-in electric field at the G/ZnO interface, providing

**Figure 7.** Scheme of photocatalytic oxidation of graphene coated with ZnO nanoparticles under UV light to form G/GO

Controlling the distribution of ZnO nanoparticles on graphene with a well reproducible size (10–15 nm) makes it possible to create highly scalable nanoheterojunctions of G/GO for ultrahigh-density memory (up to 1012 cm−2 or 1 Tb on a chip for the vertical geometry of crossing

Memristors with a floating photogate are electrically read with or without optical excitation. The I-V curve of the graphene sample before oxidation demonstrates linear behavior and high conductivity of graphene (**Figure 10(a)**, black curve). The photocatalytic process leads to a decrease in current through the sample by two orders of magnitude and a nonlinear behavior indicating the formation of a bandgap in the oxidized graphene (**Figure 10(a)**, red curve).

The rise in the temperature of moist air reduces the oxidation time of graphene. The G/GO heterostructures obtained by photocatalytic oxidation by blowing moist air at room temperature for 30 min and at 80°C for 5 min demonstrate a nonlinear behavior with a GO band width of about 3 eV, which reduces the conductivity of oxidized graphene by two orders of magnitude compared to graphene. The formed G/GO nanostructures demonstrated good

**3.4. Memristors based on graphene with a floating gate of ultrahigh density**

− and H2

O2

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73

(reactions 3–5) processes

with highly reactive hydroxyl radicals (· OH) through O2

substrate [17].

electrodes, **Figure 9**).

a flux of holes to graphene [16].

heterostructures on a Si/SiO2

of photodecomposition of water molecules from moist air.

**Figure 6.** sp3 (left) and sp2 (right) of the carbon configuration.

particles was irradiated in a moist air stream at room temperature or above (80°C) using a quartz UV lamp with a light flux of 0.03 J min−1 × cm2 . Light with a wavelength exceeding 365 nm was filtered. The time of ultraviolet irradiation ranged from 5 to 90 min. After ultraviolet treatment, the ZnO nanoparticles were dissolved in dilute 0.1 M HCl, the graphene substrate was washed with deionized water and dried in nitrogen.

ZnO nanoparticles play a key role in the process of photooxidation of graphene. **Figure 8** shows the electronic diagram of graphene/ZnO interface under UV irradiation. The bending of the bands upward in the ZnO nanoparticles is caused by a lower electron work function in ZnO (3.6 eV) compared to graphene (4.5 eV). Electron–hole pairs generated in ZnO (3.3 eV) under UV irradiation (reaction 1) are separated in a built-in electric field at the graphene/ZnO

**Figure 7.** Scheme of photocatalytic oxidation of graphene coated with ZnO nanoparticles under UV light to form G/GO heterostructures on a Si/SiO2 substrate [17].

**Figure 8.** Schematic electronic diagram of the G/ZnO interface under UV irradiation. Electron-hole pairs generated in ZnO (3.3 eV) under UV irradiation (reaction 1) are separated in a built-in electric field at the G/ZnO interface, providing a flux of holes to graphene [16].

interface, which provides a hole flux (3.3 eV) to graphene. As a result, graphene is decorated with highly reactive hydroxyl radicals (· OH) through O2 − and H2 O2 (reactions 3–5) processes of photodecomposition of water molecules from moist air.

#### **3.4. Memristors based on graphene with a floating gate of ultrahigh density**

particles was irradiated in a moist air stream at room temperature or above (80°C) using a

**Figure 5.** Scanning electron microscope (SEM)-remote induced current (REBIC) images of the Al/GO/Al structure with

) voltages. (a) Vb = 0; Vf = 0 (SE mode); (b) V<sup>b</sup> = 0; Vf = 0 (REBIC mode); (c) V<sup>b</sup> = 0; Vf = 5 V (REBIC mode); (d) V<sup>b</sup> = 0; Vf = 5 V (SE mode); (e) V<sup>b</sup> = 0. 2 V; Vf = 7 V (REBIC mode); (f) V<sup>b</sup> = 0. 5 V; Vf = 7 V (REBIC mode, same area as in (e)). A scale mark of 100 μm (e) and (f), 50 μm in (a)–(d). The images in (a)–(d) were obtained by sequentially switching signals

the modulation of the built-in potential barrier near the negatively biased Al electrode at different bias (V<sup>b</sup>

of secondary electrons (SE) and remote induced current (REBIC) during scanning of the electron beam [12].

365 nm was filtered. The time of ultraviolet irradiation ranged from 5 to 90 min. After ultraviolet treatment, the ZnO nanoparticles were dissolved in dilute 0.1 M HCl, the graphene

ZnO nanoparticles play a key role in the process of photooxidation of graphene. **Figure 8** shows the electronic diagram of graphene/ZnO interface under UV irradiation. The bending of the bands upward in the ZnO nanoparticles is caused by a lower electron work function in ZnO (3.6 eV) compared to graphene (4.5 eV). Electron–hole pairs generated in ZnO (3.3 eV) under UV irradiation (reaction 1) are separated in a built-in electric field at the graphene/ZnO

. Light with a wavelength exceeding

) and forming

quartz UV lamp with a light flux of 0.03 J min−1 × cm2

72 Advances in Memristor Neural Networks – Modeling and Applications

(Vf

**Figure 6.** sp3

(left) and sp2

substrate was washed with deionized water and dried in nitrogen.

(right) of the carbon configuration.

Controlling the distribution of ZnO nanoparticles on graphene with a well reproducible size (10–15 nm) makes it possible to create highly scalable nanoheterojunctions of G/GO for ultrahigh-density memory (up to 1012 cm−2 or 1 Tb on a chip for the vertical geometry of crossing electrodes, **Figure 9**).

Memristors with a floating photogate are electrically read with or without optical excitation. The I-V curve of the graphene sample before oxidation demonstrates linear behavior and high conductivity of graphene (**Figure 10(a)**, black curve). The photocatalytic process leads to a decrease in current through the sample by two orders of magnitude and a nonlinear behavior indicating the formation of a bandgap in the oxidized graphene (**Figure 10(a)**, red curve).

The rise in the temperature of moist air reduces the oxidation time of graphene. The G/GO heterostructures obtained by photocatalytic oxidation by blowing moist air at room temperature for 30 min and at 80°C for 5 min demonstrate a nonlinear behavior with a GO band width of about 3 eV, which reduces the conductivity of oxidized graphene by two orders of magnitude compared to graphene. The formed G/GO nanostructures demonstrated good

The vertical structure of the G/GO/ZnO nanorods (NR) allows selective excitation with UV light of 380 nm. Resistive switching in heterostructures of G/GO/ZnO NR was observed at

The structure of resistive memory based on graphene and ZnO NR is promising for memris-

Electron beam annealing GO stimulates a radical mechanism for the reduction of GO due to the formation of hot electrons. These electrons destroy the weak C-O and C-H bonds

**Figure 11.** Scheme of arrays of G/GO/ZnO NR photomemristors in vertical geometry (left) and a SEM image of the

**Figure 12.** I-V characteristics of the vertical structure G/GO/ZnO in a semilogarithmic scale (a) without forming and (b)

structure (lower right) with their current-voltage characteristics (upper right) [16].

after the forming process [16].

after the forming process at 1 V

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and H·

, which

75

voltages <1 V with the ratio of high/low resistance of 103

tive devices with high density and low power consumption.

**3.5. Graphene/graphene oxide memristors formed by an electron beam**

(in comparison with strong C-C bonds) and form highly reactive radicals O·

(**Figure 12(b)**).

**Figure 9.** Scheme of arrays of G/GO photomemristors in vertical geometry obtained by photocatalytic oxidation of graphene with ZnO nanoparticles [17].

**Figure 10.** (а) I-V characteristics of the 2–3 layer G/ZnO structure before (black) and after (red) photocatalytic oxidation in moist air for 30 min at room temperature. Insert-scheme for measuring the structure with lateral gold electrodes. (b) I-V characteristics for the G/GO nanostructure preliminarily polarized (+5 V, 15 min) with white light (black) and in the dark (red). (c) Resistive states of the G/GO photomemristor, which are switched by a voltage of −3.8/3.3 V (Reset/Set) in the dark and −3.5/4 V (Set/Reset) under white light pulses (d) and read at 2.5 V [17].

photosensitivity to white light and photoresistive switching. The photocurrent increased approximately six times at a bias voltage greater than 3 V. This indicates that the electron– hole pairs generated by light are effectively separated in the biased G/GO heterojunctions. **Figure 10(b)** shows the I-V characteristics of the preformed G/GO nanostructure (+5 V, 15 min) when sweep voltage of −4 to 4 V under white light (black) and in the dark (red). Well reproducible bipolar hysteresis indicates a resistive switching of the structure with an on/off ratio of about 10 for 4 different resistive states HRSD, LRSD, LRSL and HRSL in the dark and light with switching voltages of −3.8/3.3 V (Reset/Set) and −3.5/4 V (Set/Reset), respectively (**Figure 10(c)** and **(d)**). To form vertical memristive structures, ZnO nanorods (NR) grown on graphene can also be used instead of ZnO nanoparticles (**Figure 11**) [16].

The vertical structure of the G/GO/ZnO nanorods (NR) allows selective excitation with UV light of 380 nm. Resistive switching in heterostructures of G/GO/ZnO NR was observed at voltages <1 V with the ratio of high/low resistance of 103 after the forming process at 1 V (**Figure 12(b)**).

The structure of resistive memory based on graphene and ZnO NR is promising for memristive devices with high density and low power consumption.

#### **3.5. Graphene/graphene oxide memristors formed by an electron beam**

Electron beam annealing GO stimulates a radical mechanism for the reduction of GO due to the formation of hot electrons. These electrons destroy the weak C-O and C-H bonds (in comparison with strong C-C bonds) and form highly reactive radicals O· and H· , which

**Figure 11.** Scheme of arrays of G/GO/ZnO NR photomemristors in vertical geometry (left) and a SEM image of the structure (lower right) with their current-voltage characteristics (upper right) [16].

photosensitivity to white light and photoresistive switching. The photocurrent increased approximately six times at a bias voltage greater than 3 V. This indicates that the electron– hole pairs generated by light are effectively separated in the biased G/GO heterojunctions. **Figure 10(b)** shows the I-V characteristics of the preformed G/GO nanostructure (+5 V, 15 min) when sweep voltage of −4 to 4 V under white light (black) and in the dark (red). Well reproducible bipolar hysteresis indicates a resistive switching of the structure with an on/off ratio of about 10 for 4 different resistive states HRSD, LRSD, LRSL and HRSL in the dark and light with switching voltages of −3.8/3.3 V (Reset/Set) and −3.5/4 V (Set/Reset), respectively (**Figure 10(c)** and **(d)**). To form vertical memristive structures, ZnO nanorods (NR) grown on

**Figure 10.** (а) I-V characteristics of the 2–3 layer G/ZnO structure before (black) and after (red) photocatalytic oxidation in moist air for 30 min at room temperature. Insert-scheme for measuring the structure with lateral gold electrodes. (b) I-V characteristics for the G/GO nanostructure preliminarily polarized (+5 V, 15 min) with white light (black) and in the dark (red). (c) Resistive states of the G/GO photomemristor, which are switched by a voltage of −3.8/3.3 V (Reset/Set) in

**Figure 9.** Scheme of arrays of G/GO photomemristors in vertical geometry obtained by photocatalytic oxidation of

graphene with ZnO nanoparticles [17].

74 Advances in Memristor Neural Networks – Modeling and Applications

graphene can also be used instead of ZnO nanoparticles (**Figure 11**) [16].

the dark and −3.5/4 V (Set/Reset) under white light pulses (d) and read at 2.5 V [17].

**Figure 12.** I-V characteristics of the vertical structure G/GO/ZnO in a semilogarithmic scale (a) without forming and (b) after the forming process [16].

recombine in H2 O, H2 , O2 , and the uncompensated charge in GO is used to restore the sp2 carbon bond. It should be noted that the electron beam annealing process excites the electronic subsystem selectively, and the energy of the generated hot electrons can be resonantly absorbed by the functional groups of graphene oxide. To remove oxygen groups, several eV are required, which is comparable to the energy between orbitals. Primary beam electrons are high-energy and can participate in annealing only through the process of energy absorption by graphene oxide to form hot electrons with an energy close to the GO bandgap (E<sup>g</sup> ). Electron-stimulated annealing of GO can occur due to the generation of a high concentration of charge carriers in this material (E<sup>g</sup> = 1–6 eV) (an electron beam with an electron energy of 3–10 keV creates 10<sup>3</sup> electron-hole pairs per incident electron). The process of electron-stimulated annealing by an electron beam is more effective than laser annealing, in which one photon produces only one electron-hole pair, and therefore the thermal effects in laser annealing make the main contribution. Electron beam annealing allows the direct formation of rGO/GO memristive nanostructures with controlled reduction without the use of a mask. **Figure 13** shows a SEM image of a GO film with a superimposed stripe pattern (green) for electron-beam exposure (a) and a rGO/GO/rGO structure obtained by direct "writing" by an electron beam with a dose of 150 mA × s/cm2 (b, c). The change in image contrast in the secondary electron emission (SEE) of graphene oxide after electron beam processing (b, c) indicates a change in composition and its electronic properties.

(**Figure 14(a)**). The forming process at 20 V led to an increase in the conductivity of the structure by several orders of magnitude and to a pronounced nonlinearity. A bipolar hysteresis was observed that indicated a resistive switching of the structure from the high-resistive resistive state (HRS) ((1.2 ± 0.1) × 1011 Ω) to the low-resistive resistive state (LRS) ((6.7 ± 0.4) × 108 Ohm) (~2 orders of magnitude) at a low switching voltage of 0.8–0.9 V (**Figure 14(b)**). The electron beam annealed structures showed good reproducibility with a small spread of

**Figure 14.** I-V characteristics of the Pt/GO/Pt structure after electron irradiation before (a) and after (b) the forming

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Memory with the ability to store more than one bit per cell, that is, having multilevel memory states, is very attractive, since it offers a simple and economical way to increased memory capacity (e.g., modern CMOS NAND-Flash usually stores 2 or 3 bits per cell). Combining this capability with tiered storage with extremely high scalability is especially effective for implementing memory with ultrahigh storage volumes. Access to four very well-separated and stable memory states in nanoscale GO cells by monitoring the duration and amplitude of the write pulse was recently demonstrated at IBM [19]. Excitation pulses with amplitudes from 2 to 6 V and duration from 20 to 80 ns were used to determine the conditions for successful recording and erasing of multilevel memory states in Pt/GO/Ti/Pt and monitoring of the

The cells were completely switched from the RESET state, which can be considered as state 00 to memory states 01, 10 and 11 using pulses of −2.5 V/60 ns, −3.5 V/60 ns and − 4.5 V/60 ns respectively (**Figure 15(a)**). Erasing of cells from 01, 10 and 11 states back to state 00 was successfully achieved for pulses +3 V/60 ns, +4 V/60 ns and +5 V/60 ns, respectively (**Figure 15(b)**). Separation of intermediate resistance levels is very good (see **Figure 15(a)**), which allows a reliable reading process. Intermediate levels showed excellent reliability (**Figure 15(c)**) and were stable over time (**Figure 15(d)**), both on rigid and flexible substrates. The reversible resistive switching observed in these devices was due to the migration of oxygen, which led to a

**3.6. Multilevel ultrafast nonvolatile memory based on graphene oxide**

switching voltages (0.05–0.1 V).

process.

change in the conductivity.

resulting cell resistance, see **Figure 15(a)** and **(b)**.

The electron beam annealing of GO allows for more efficient formation of a resistive switching structure. The lateral structure of rGO/GO/rGO obtained by electron beam irradiation with a dose of 200 mA × s/cm2 exhibited soft resistive switching without the forming process. The curve of the I-V structure, after irradiation, was nonlinear with a small hysteresis

**Figure 13.** SEM images of a GO film on a SiO<sup>2</sup> /Si substrate with Pt electrodes (white) and superimposed stripe pattern (green) for electron beam writing (a) and rGO/GO/rGO structure after irradiation with an electron beam (b, c). The narrow bands of the brighter SEE contrast are regions of the reduced rGO after irradiation.

recombine in H2

bandgap (E<sup>g</sup>

properties.

with a dose of 200 mA × s/cm2

**Figure 13.** SEM images of a GO film on a SiO<sup>2</sup>

sp2

O, H2

an electron energy of 3–10 keV creates 10<sup>3</sup>

, O2

76 Advances in Memristor Neural Networks – Modeling and Applications

, and the uncompensated charge in GO is used to restore the

electron-hole pairs per incident electron). The

exhibited soft resistive switching without the forming pro-

/Si substrate with Pt electrodes (white) and superimposed stripe pattern

(b, c).

 carbon bond. It should be noted that the electron beam annealing process excites the electronic subsystem selectively, and the energy of the generated hot electrons can be resonantly absorbed by the functional groups of graphene oxide. To remove oxygen groups, several eV are required, which is comparable to the energy between orbitals. Primary beam electrons are high-energy and can participate in annealing only through the process of energy absorption by graphene oxide to form hot electrons with an energy close to the GO

high concentration of charge carriers in this material (E<sup>g</sup> = 1–6 eV) (an electron beam with

process of electron-stimulated annealing by an electron beam is more effective than laser annealing, in which one photon produces only one electron-hole pair, and therefore the thermal effects in laser annealing make the main contribution. Electron beam annealing allows the direct formation of rGO/GO memristive nanostructures with controlled reduction without the use of a mask. **Figure 13** shows a SEM image of a GO film with a superimposed stripe pattern (green) for electron-beam exposure (a) and a rGO/GO/rGO structure

obtained by direct "writing" by an electron beam with a dose of 150 mA × s/cm2

The change in image contrast in the secondary electron emission (SEE) of graphene oxide after electron beam processing (b, c) indicates a change in composition and its electronic

The electron beam annealing of GO allows for more efficient formation of a resistive switching structure. The lateral structure of rGO/GO/rGO obtained by electron beam irradiation

cess. The curve of the I-V structure, after irradiation, was nonlinear with a small hysteresis

(green) for electron beam writing (a) and rGO/GO/rGO structure after irradiation with an electron beam (b, c). The

narrow bands of the brighter SEE contrast are regions of the reduced rGO after irradiation.

). Electron-stimulated annealing of GO can occur due to the generation of a

**Figure 14.** I-V characteristics of the Pt/GO/Pt structure after electron irradiation before (a) and after (b) the forming process.

(**Figure 14(a)**). The forming process at 20 V led to an increase in the conductivity of the structure by several orders of magnitude and to a pronounced nonlinearity. A bipolar hysteresis was observed that indicated a resistive switching of the structure from the high-resistive resistive state (HRS) ((1.2 ± 0.1) × 1011 Ω) to the low-resistive resistive state (LRS) ((6.7 ± 0.4) × 108 Ohm) (~2 orders of magnitude) at a low switching voltage of 0.8–0.9 V (**Figure 14(b)**). The electron beam annealed structures showed good reproducibility with a small spread of switching voltages (0.05–0.1 V).

#### **3.6. Multilevel ultrafast nonvolatile memory based on graphene oxide**

Memory with the ability to store more than one bit per cell, that is, having multilevel memory states, is very attractive, since it offers a simple and economical way to increased memory capacity (e.g., modern CMOS NAND-Flash usually stores 2 or 3 bits per cell). Combining this capability with tiered storage with extremely high scalability is especially effective for implementing memory with ultrahigh storage volumes. Access to four very well-separated and stable memory states in nanoscale GO cells by monitoring the duration and amplitude of the write pulse was recently demonstrated at IBM [19]. Excitation pulses with amplitudes from 2 to 6 V and duration from 20 to 80 ns were used to determine the conditions for successful recording and erasing of multilevel memory states in Pt/GO/Ti/Pt and monitoring of the resulting cell resistance, see **Figure 15(a)** and **(b)**.

The cells were completely switched from the RESET state, which can be considered as state 00 to memory states 01, 10 and 11 using pulses of −2.5 V/60 ns, −3.5 V/60 ns and − 4.5 V/60 ns respectively (**Figure 15(a)**). Erasing of cells from 01, 10 and 11 states back to state 00 was successfully achieved for pulses +3 V/60 ns, +4 V/60 ns and +5 V/60 ns, respectively (**Figure 15(b)**). Separation of intermediate resistance levels is very good (see **Figure 15(a)**), which allows a reliable reading process. Intermediate levels showed excellent reliability (**Figure 15(c)**) and were stable over time (**Figure 15(d)**), both on rigid and flexible substrates. The reversible resistive switching observed in these devices was due to the migration of oxygen, which led to a change in the conductivity.

writing the ON state, **Figure 16(b)**). When the applied voltage changes from 0 to positive voltage (4.2 V), the device returns to HRSL6 (RESET operation to clear the state ON to OFF). The memristive behavior of the device in darkness and in light is well reproduced up to 1000 cycles (**Figure 16(c)** and **(d)**) and demonstrates the possibility of obtaining in the device a multilevel resistive switching and its control by means of an electric field in the dark and

is a faster process than ion transport, and the frequency of optical access is much higher than

nanospheres

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/Au. I-V characteristics in the dark or under

It should be noted that resistive switching controlled by the polarization of MoS2

**Figure 16.** Resistive switching of the nanospheric photomemristor Au/MoS2

the device when excited by white light after several cycles [20].

white light (spectral maxima at 2.7 eV and 1.8 eV; device diagram on the inset in **Figure 16(a)** with light excitation). The arrows on the curves indicate the direction of the voltage sweep; (a) I-V curves after 3 V voltage polarization. The device smoothly switches from HRSL3 to LRSL3 under light and from HRSD3 to LRSD3 in the dark with an on/off ratio of about 2 and 4 at 1.2 V and 0.7 V, respectively; (b) I-V curves after a 6 V voltage polarization. The device shows abrupt changeover of resistance when excited by light, from HRSL6 to LRSL6 at −9.2 V with an on/off ratio of about 10 and a smooth transition from HRSD6 to LRSD6 without light excitation with a switching factor on/off about 3 at 0.7 V. (c) Memristive characteristics of the device without excitation by light after several cycles. (d) Memristive characteristics of

when excited by light.

electrical addressing.

**Figure 15.** (a) Record and (b) erase multilevel states in a 75 nm GO memory cell (8 nm-thick GO layer) by controlling the amplitude and pulse width. (c) reliability and (d) storage of states of a multilevel, nanoscale graphene oxide cell [19].

#### **4. Memristor with floating MoS<sup>2</sup> photogate**

A memristor with a floating MoS<sup>2</sup> photogate polarized in an electric field under different lighting conditions demonstrates a multilevel switching [20]. **Figure 16** shows the current– voltage curves (I-V) of the Au/MoS2 /Au structure (an inset in **Figure 16(a)**) after polarization at 3 and 6 V. The nonlinear characteristics of a device with hysteresis indicate a memristive behavior. Furthermore, the memristor demonstrates a high photoresponse when illuminated with white light. When the device is polarized at 3 V, a smooth switching from HRSL3 to LRSL3 is observed under light illumination and from HRSD3 to LRSD3 in the dark with a ratio of on/off currents of about 2 and 4 at 1.2 V and 0.7 V, respectively (**Figure 16(a)**). At a higher voltage (6 V), the device shows a sharp switching when excited by white light, from HRSL6 to LRSL6 at −2.9 V with an on/off ratio of about 10 and a smooth switching from HRSD6 to LRSD6 in the dark with an on/off ratio of about 3 at 0.7 V (the SET process of writing the ON state, **Figure 16(b)**). When the applied voltage changes from 0 to positive voltage (4.2 V), the device returns to HRSL6 (RESET operation to clear the state ON to OFF). The memristive behavior of the device in darkness and in light is well reproduced up to 1000 cycles (**Figure 16(c)** and **(d)**) and demonstrates the possibility of obtaining in the device a multilevel resistive switching and its control by means of an electric field in the dark and when excited by light.

It should be noted that resistive switching controlled by the polarization of MoS2 nanospheres is a faster process than ion transport, and the frequency of optical access is much higher than electrical addressing.

**4. Memristor with floating MoS<sup>2</sup>**

78 Advances in Memristor Neural Networks – Modeling and Applications

A memristor with a floating MoS<sup>2</sup>

voltage curves (I-V) of the Au/MoS2

 **photogate**

**Figure 15.** (a) Record and (b) erase multilevel states in a 75 nm GO memory cell (8 nm-thick GO layer) by controlling the amplitude and pulse width. (c) reliability and (d) storage of states of a multilevel, nanoscale graphene oxide cell [19].

lighting conditions demonstrates a multilevel switching [20]. **Figure 16** shows the current–

at 3 and 6 V. The nonlinear characteristics of a device with hysteresis indicate a memristive behavior. Furthermore, the memristor demonstrates a high photoresponse when illuminated with white light. When the device is polarized at 3 V, a smooth switching from HRSL3 to LRSL3 is observed under light illumination and from HRSD3 to LRSD3 in the dark with a ratio of on/off currents of about 2 and 4 at 1.2 V and 0.7 V, respectively (**Figure 16(a)**). At a higher voltage (6 V), the device shows a sharp switching when excited by white light, from HRSL6 to LRSL6 at −2.9 V with an on/off ratio of about 10 and a smooth switching from HRSD6 to LRSD6 in the dark with an on/off ratio of about 3 at 0.7 V (the SET process of

photogate polarized in an electric field under different

/Au structure (an inset in **Figure 16(a)**) after polarization

**Figure 16.** Resistive switching of the nanospheric photomemristor Au/MoS2 /Au. I-V characteristics in the dark or under white light (spectral maxima at 2.7 eV and 1.8 eV; device diagram on the inset in **Figure 16(a)** with light excitation). The arrows on the curves indicate the direction of the voltage sweep; (a) I-V curves after 3 V voltage polarization. The device smoothly switches from HRSL3 to LRSL3 under light and from HRSD3 to LRSD3 in the dark with an on/off ratio of about 2 and 4 at 1.2 V and 0.7 V, respectively; (b) I-V curves after a 6 V voltage polarization. The device shows abrupt changeover of resistance when excited by light, from HRSL6 to LRSL6 at −9.2 V with an on/off ratio of about 10 and a smooth transition from HRSD6 to LRSD6 without light excitation with a switching factor on/off about 3 at 0.7 V. (c) Memristive characteristics of the device without excitation by light after several cycles. (d) Memristive characteristics of the device when excited by white light after several cycles [20].

#### **4.1. 8-Level memristor system with an MoS2 floating photogate**

The diagram of the operation of the 8-level memristor system with the MoS<sup>2</sup> floating photodetector is shown in **Figure 17**, where the resistance states formed after the SET/RESET operation of the MoS2 memristor polarized at voltages of 3 V and 6 V in the dark or when excited by light are shown. A memristor polarized at 3 V in darkness or in white light demonstrates four states that are read at a voltage of 0.7 V (HRSD3 and LRSD3) and 1.2 V (HRSL3 and LRSL3) in the dark or in white light (**Figure 17(a)**).

LRSL6) in darkness or in light (**Figure 17(a)**). These states are controlled electrically and optically, which is confirmed by the iterative operation of the memristor under various conditions of writing and reading (**Figure 17(c)** and **(d)**) Polarization of nanospheres in a photomemristor using an electric field and light pulses creates multilevel states. An analysis of the conductivity in these states of resistance shows that the polarization of nanospheres when excited by light leads to the formation of conductive paths. Reducing the gap between the electrodes can greatly minimize the operating voltage of the device. Modulation of the barrier height at the boundaries of the nanospheres in an external electric field by light due to repolarization is a highly efficient process for high-speed signal processing. The memristor polarized at 3 V and 6 V has different states that can be electrically read at optical excitation in the form of four high-resistance states and four low-resistance states. The optical and electrical polarization of the memristor provides several nonlinear dynamic processes that allow us to build a system

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A photonic chip containing 70 photon synapses was demonstrated in 2017 by a team from the universities of Oxford, Münster and Exeter [21]. The recording, erasure and reading of information in this case are carried out completely by optical methods (**Figure 18**). The photon synapse consists of a cone-shaped waveguide (dark blue) with discrete islands of phase-change

**Figure 18.** Photon synapse on a crystal. (A) The structure of the neuron and the synapse. Insert: Illustration of the synapse junction. (B) Scheme of the integrated photon synapse resembling the function of a neural synapse. The synapse is based on a cone-shaped waveguide (dark blue) with discrete PCM islands from the top, optically connecting presynaptic (preneural) and postsynaptic (postneural) signals. The red open circle is a circulator with port 2 and port 3, connecting the synapse and postneuron; weighing pulses are fed through port 1 to the synapse. (C) An optical microscope image of a device with an active region (red rectangle) as a photon synapse. The optical input and output of the device is carried out through apodized diffraction couplers (white rectangles). Box: A typical photonic chip containing 70 photon synapses is smaller than a coin. (D) Scanning electron microscope image of the photon synapse active region corresponding to

(GST) strips (1 × 3 μm, yellow) at the tip of the waveguide (blue). Insert: An

with a neuromorphic architecture, similar to a neural network.

**5. Photonic chip with photon synapse**

the red rectangle in (C) with six Ge2

Sb2 Te5

increased conical waveguide structure, marked with a white dotted frame [21].

Polarization of the memristor at 6 V in darkness or under light leads to the formation of four more states that are read at a voltage of 0.7 V (HRSD6 and LRSD6) and 4 V (HRSL6 and

**Figure 17.** The operation of the MoS2 photomemristor, polarized at different voltages in the dark or when excited by light. (1) high and low resistive states obtained using SET/RESET operations at −3 V/3 V and −6 V/+6 V in the dark (HRSD3, LRSD3 and HRSD6, LRSD6) and under white light (HRSL3, LRSL3, LRSL6 and HRSL6). (2) reading diagram under impulse voltage. Resistive states are read at 0.7 V (HRSD3, LRSD3, HRSD6 and LRSD6), 1.2 V (HRSL3 and LRSL3) and 4 V (LRSL6 and HRSL6) in dark (D) or white light (L). (3) excitation scheme by pulses of white light. SET/RESET and the READ operation is controlled by switching off the light pulses (black) (HRSD3, LRSD3, HRSD6 and LRSD6) and turned on (blue) (HRSL3, LRSL3, HRSL6 and LRSL6). A 3 V polarized memristor demonstrates four states that are read as HRSD3, LRSD3, HRSL3 and LRSL3, while a memristor polarized at 6 V demonstrates the other four states: HRSD6, LRSD6, HRSL6 and LRSL6, which can be read in the dark or in the light [20].

LRSL6) in darkness or in light (**Figure 17(a)**). These states are controlled electrically and optically, which is confirmed by the iterative operation of the memristor under various conditions of writing and reading (**Figure 17(c)** and **(d)**) Polarization of nanospheres in a photomemristor using an electric field and light pulses creates multilevel states. An analysis of the conductivity in these states of resistance shows that the polarization of nanospheres when excited by light leads to the formation of conductive paths. Reducing the gap between the electrodes can greatly minimize the operating voltage of the device. Modulation of the barrier height at the boundaries of the nanospheres in an external electric field by light due to repolarization is a highly efficient process for high-speed signal processing. The memristor polarized at 3 V and 6 V has different states that can be electrically read at optical excitation in the form of four high-resistance states and four low-resistance states. The optical and electrical polarization of the memristor provides several nonlinear dynamic processes that allow us to build a system with a neuromorphic architecture, similar to a neural network.
