**Advances in Resistive Switching Memories Based on Graphene Oxide**

Fei Zhuge, Bing Fu and Hongtao Cao

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51260

## **1. Introduction**

[56] Volk C, Fringes S, Terres B, Dauber J, Engels S, Trellenkamp S, and Stampfer C,.Nano

[57] Wang L. J, Cao G, Li H. O, Tu T, Zhou C, Hao X. J, Guo G. C, and Guo G. P, Chi‐

[58] Wang L. J, Cao G, Tu T, Li H. O, Zhou C, Hao X. J, Su Z, Guo G. C, Jiang H. W, and

[59] Wang L. J, Guo G. P, Wei D, Cao G, Tu T, Xiao M, Guo G. C, and Chang A.M, Appl.

[60] Wang L. J, Li H. O, Tu T, Cao G, Zhou C, Hao X. J, Su Z, Xiao M, Guo G. C, Chang

Lett. 11, 3581 (2011).

184 New Progress on Graphene Research

nese .Physics. Letters. 28, 067301 (2011b).

Guo G. P, Appl. Phys. Lett. 97, 262113 (2010).

A.M, and Guo G. P, Appl. Phys. Lett. 100, 022106(2012).

[61] Yang S, Wang X, and Das Sarma S, Phys. Rev. B 83, 161301(R) (2011).

[63] Ziegler R, Bruder C, and Schoeller H, Phys. Rev. B 62, 1961 (2000).

[62] Zhang H, Guo G. P, Tu T, and Guo G. C, Phys. Rev. A 76, 012335 (2007).

Phys. Lett. 99, 112117 (2011a).

Memory devices are a prerequisite for today's information technology. In general, two dif‐ ferent segments can be distinguished. Random access type memories are based on semicon‐ ductor technology. These can be divided into static random access memories (SRAM) and dynamic random access memories (DRAM). In the following, only DRAM will be consid‐ ered, because it is the main RAM technology for standalone memory products. Mass storage devices are traditionally based on magnetic- and optical storage. But also here semiconduc‐ tor memories are gaining market share. The importance of semiconductor memories is con‐ sequently increasing (Mikolajick et al., 2009). Though SRAM and DRAM are very fast, both of them are volatile, which is a huge disadvantage, costing energy and additional periphery circuitry. Si-based Flash memory devices represent the most prominent nonvolatile data memory (NVM) because of their high density and low fabrication costs. However, Flash suf‐ fers from low endurance, low write speed, and high voltages required for the write opera‐ tions. In addition, further scaling, i.e., a continuation in increasing the density of Flash is expected to run into physical limits in the near future. Ferroelectric random access memory (FeRAM) and magnetoresistive random access memory (MRAM) cover niche markets for special applications. One reason among several others is that FeRAM as well as convention‐ al MRAM exhibit technological and inherent problems in the scalability, i.e., in achieving the same density as Flash today. In this circumstance, a renewed nonvolatile memory concept called resistance-switching random access memory (RRAM), which is based on resistance change modulated by electrical stimulus, has recently inspired scientific and commercial in‐ terests due to its high operation speed, high scalability, and multibit storage potential (Beck et al., 2000; Lu & Lieber, 2007; Dong et al., 2008). The reading of resistance states is nondes‐ tructive, and the memory devices can be operated without transistors in every cell (Lee et

© 2013 Zhuge et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Zhuge et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

al., 2007; Waser & Aono, 2007), thus making a cross-bar structure feasible. A large variety of solid-state materials have been found to show these resistive switching characteristics, in‐ cluding solid electrolytes such as GeSe and Ag2S (Waser & Aono, 2007), perovskites such as SrZrO3 (Beck et al., 2000), Pr0.7Ca0.3MnO3 (Liu et al., 2000; Odagawa et al., 2004; Liao et al., 2009), and BiFeO3 (Yang et al., 2009; Yin et al., 2010), binary transition metal oxides such as NiO (Seo et al., 2004; Kim et al., 2006; Son & Shin, 2008), TiO2 (Kim et al., 2007; Jeong et al., 2009; Kwon et al., 2010), ZrO2 (Wu et al., 2007; Guan et al., 2008; Liu et al., 2009), and ZnO (Chang et al., 2008; Kim et al., 2009; Yang et al., 2009), organic materials (Stewart et al., 2004), amorphous silicon (a-Si) (Jo and Lu, 2008; Jo et al., 2009), and amorphous carbon (a-C) (Sinit‐ skii & Tour, 2009; Zhuge et al., 2010) (Zhuge et al., 2011).

ratio >100, a retention time >105

the "voltage–time dilemma".

**2. Resistive switching and RRAM**

s, and switching threshold voltages <1 V. In section 4, the

Advances in Resistive Switching Memories Based on Graphene Oxide

http://dx.doi.org/10.5772/51260

187

resistive switching mechanisms of conjugated-polymer-functionalized GO thin films and memory properties of corresponding RRAM cells are described. In this case, the resistive switching is ascribed to electron/hole transfer between graphene sheets and polymer mole‐ cules. The RRAM cells exhibit excellent memory performances, such as large ON/OFF ratio, good endurance, and high switching speed. In the last section, it is proposed that the realiza‐ tion of bidirectional or reversible electron transfer in graphene-based hybrid systems is ex‐ pected to overcome the "voltage–time dilemma" (i.e., one could not realize high write/erase speed and long retention time simultaneously) in pure electronic mechanism-based RRAM cells (Schroeder et al., 2010). Pure electronic mechanisms in RRAM cells postulate the trap‐ ping and detrapping of electron in immobile traps as the reason for the resistance changes, also known as Simmons & Verderber model (Simmons & Verderber, 1967). While in gra‐ phene-based hybrid systems, the electron transfer occurs between graphene sheets and functional molecules covalently or non-covalently bonded to graphene, which may avoid

A resistive switching memory cell in an RRAM is generally composed of an insulating or resistive material sandwiched between two electron-conductive electrodes to form metal-in‐ sulator-metal (MIM) structure. By applying an appropriate voltage, the MIM cell can be switched between a high-resistance state (HRS or OFF-state) and a low-resistance state (LRS or ON-state). Switching from OFF-state to ON-state is called the SET process, while switch‐ ing from ON-state to OFF-state is called the RESET process. These two states can represent the logic values 1 and 0, respectively. Depending on voltage polarity, the resistive switching behavior of an RRAM device is classified as unipolar and bipolar. For unipolar switching, resistive switching is induced by a voltage of the same polarity but a different magnitude, as shown in Fig. 1(a) (Waser & Aono, 2007). For bipolar switching, one polarity is used to switch from HRS to LRS, and the opposite polarity is used to switch back into HRS, as shown in Fig. 1(b) (Waser & Aono, 2007). A current compliance (CC) is usually needed dur‐ ing the SET process to prevent the device from a permanent breakdown. (Li et al., 2011)

In the simplest approach, the pure memory element can be used as a basic memory cell, result‐ ing in a configuration where parallel bitlines are crossed by perpendicular wordlines with the switching material placed between wordline and bitline at every cross-point. This configura‐ tion is called a cross-point cell. Since this architecture will lead to a large parasitic current flow‐ ing through nonselected memory cells, the cross-point array has a very slow read access. A selection element can be added to improve the situation. A series connection of a diode in ev‐ ery cross-point allows to reverse bias all nonselected cells. This can be arranged in a similar compact manner as the basic cross-point cell. Finally a transistor device (ideally a MOS Tran‐ sistor) can be added which makes the selection of a cell very easy and therefore gives the best random access time, but comes at the price of increased area consumption. Figure 2 illustrates the different cell type possibilities. For random access type memories, a transistor type archi‐

In last decades, carbon-based materials have been studied intensively as a potential candi‐ date to overcome the scientific and technological limitations of traditional semiconductor devices (Rueckes et al., 2000; Novoselov et al., 2004; Avouris et al., 2007). It is worthy men‐ tioning that most of the work on carbon-based electronic devices has been focused on fieldeffect transistors (Wang et al., 2008; Burghard et al., 2009). Thus, it would be of great interest if nonvolatile memory can also be realized in carbon so that logic and memory devices can be integrated on a same carbon-based platform. Graphene oxide (GO) with an ultrathin thickness (~1 nm) is attractive due to its unique physical-chemical properties. A GO layer can be considered as a graphene sheet with epoxide, hydroxyl, and/or carboxyl groups at‐ tached to both sides. GO can be readily obtained through oxidizing graphite in mixtures of strong oxidants, followed by an exfoliation process. Due to its water solubility, GO can be transferred onto any substrates uniformly using simple methods such as drop-casting, spin coating, Langmuir-Blodgett (LB) deposition and vacuum filtration. The as-deposited GO thin films can be further processed into functional devices using standard lithography proc‐ esses without degrading the film properties (Eda et al., 2008; Cote et al., 2009). Furthermore, the band structure and electronic properties of GO can be modulated by changing the quan‐ tity of chemical functionalities attached to the surface. Therefore, GO is potentially useful for microelectronics production. Considering that although a large variety of solid-state materi‐ als have been found to show resistive switching characteristics, none of them can fully meet the requirements of RRAM applications, exploration of new storage media is still a key project for the development of RRAM (Zhuge et al., 2011). This review focuses on GO-based RRAM cells, highlighting their advantages as the next generation memories. Section 2 de‐ scribes the basic concepts of resistive switching and resistance-switching random access memory and physical storage mechanisms. In section 3, the resistive switching mechanisms of GO thin films and memory properties of GO-based RRAM cells are presented. Detailed current–voltage measurements show that in metal/GO/metal sandwiches, the resistive switching originates from the formation and rupture of conducting filaments. An analysis of the temperature dependence of the ON-state resistance reveals that the filaments are com‐ posed of metal atoms due to the diffusion of the top electrodes under a bias voltage. More‐ over, the resistive switching is found to occur within confined regions of the metal filaments. The resistive switching effect is also observed in GO/metal structures by conduct‐ ing atomic force microscopy. It is attributed to the redox reactions between GO and adsor‐ bed water induced by external voltage biases. The GO-based RRAM cells show an ON/OFF ratio >100, a retention time >105 s, and switching threshold voltages <1 V. In section 4, the resistive switching mechanisms of conjugated-polymer-functionalized GO thin films and memory properties of corresponding RRAM cells are described. In this case, the resistive switching is ascribed to electron/hole transfer between graphene sheets and polymer mole‐ cules. The RRAM cells exhibit excellent memory performances, such as large ON/OFF ratio, good endurance, and high switching speed. In the last section, it is proposed that the realiza‐ tion of bidirectional or reversible electron transfer in graphene-based hybrid systems is ex‐ pected to overcome the "voltage–time dilemma" (i.e., one could not realize high write/erase speed and long retention time simultaneously) in pure electronic mechanism-based RRAM cells (Schroeder et al., 2010). Pure electronic mechanisms in RRAM cells postulate the trap‐ ping and detrapping of electron in immobile traps as the reason for the resistance changes, also known as Simmons & Verderber model (Simmons & Verderber, 1967). While in gra‐ phene-based hybrid systems, the electron transfer occurs between graphene sheets and functional molecules covalently or non-covalently bonded to graphene, which may avoid the "voltage–time dilemma".

#### **2. Resistive switching and RRAM**

al., 2007; Waser & Aono, 2007), thus making a cross-bar structure feasible. A large variety of solid-state materials have been found to show these resistive switching characteristics, in‐ cluding solid electrolytes such as GeSe and Ag2S (Waser & Aono, 2007), perovskites such as SrZrO3 (Beck et al., 2000), Pr0.7Ca0.3MnO3 (Liu et al., 2000; Odagawa et al., 2004; Liao et al., 2009), and BiFeO3 (Yang et al., 2009; Yin et al., 2010), binary transition metal oxides such as NiO (Seo et al., 2004; Kim et al., 2006; Son & Shin, 2008), TiO2 (Kim et al., 2007; Jeong et al., 2009; Kwon et al., 2010), ZrO2 (Wu et al., 2007; Guan et al., 2008; Liu et al., 2009), and ZnO (Chang et al., 2008; Kim et al., 2009; Yang et al., 2009), organic materials (Stewart et al., 2004), amorphous silicon (a-Si) (Jo and Lu, 2008; Jo et al., 2009), and amorphous carbon (a-C) (Sinit‐

In last decades, carbon-based materials have been studied intensively as a potential candi‐ date to overcome the scientific and technological limitations of traditional semiconductor devices (Rueckes et al., 2000; Novoselov et al., 2004; Avouris et al., 2007). It is worthy men‐ tioning that most of the work on carbon-based electronic devices has been focused on fieldeffect transistors (Wang et al., 2008; Burghard et al., 2009). Thus, it would be of great interest if nonvolatile memory can also be realized in carbon so that logic and memory devices can be integrated on a same carbon-based platform. Graphene oxide (GO) with an ultrathin thickness (~1 nm) is attractive due to its unique physical-chemical properties. A GO layer can be considered as a graphene sheet with epoxide, hydroxyl, and/or carboxyl groups at‐ tached to both sides. GO can be readily obtained through oxidizing graphite in mixtures of strong oxidants, followed by an exfoliation process. Due to its water solubility, GO can be transferred onto any substrates uniformly using simple methods such as drop-casting, spin coating, Langmuir-Blodgett (LB) deposition and vacuum filtration. The as-deposited GO thin films can be further processed into functional devices using standard lithography proc‐ esses without degrading the film properties (Eda et al., 2008; Cote et al., 2009). Furthermore, the band structure and electronic properties of GO can be modulated by changing the quan‐ tity of chemical functionalities attached to the surface. Therefore, GO is potentially useful for microelectronics production. Considering that although a large variety of solid-state materi‐ als have been found to show resistive switching characteristics, none of them can fully meet the requirements of RRAM applications, exploration of new storage media is still a key project for the development of RRAM (Zhuge et al., 2011). This review focuses on GO-based RRAM cells, highlighting their advantages as the next generation memories. Section 2 de‐ scribes the basic concepts of resistive switching and resistance-switching random access memory and physical storage mechanisms. In section 3, the resistive switching mechanisms of GO thin films and memory properties of GO-based RRAM cells are presented. Detailed current–voltage measurements show that in metal/GO/metal sandwiches, the resistive switching originates from the formation and rupture of conducting filaments. An analysis of the temperature dependence of the ON-state resistance reveals that the filaments are com‐ posed of metal atoms due to the diffusion of the top electrodes under a bias voltage. More‐ over, the resistive switching is found to occur within confined regions of the metal filaments. The resistive switching effect is also observed in GO/metal structures by conduct‐ ing atomic force microscopy. It is attributed to the redox reactions between GO and adsor‐ bed water induced by external voltage biases. The GO-based RRAM cells show an ON/OFF

skii & Tour, 2009; Zhuge et al., 2010) (Zhuge et al., 2011).

186 New Progress on Graphene Research

A resistive switching memory cell in an RRAM is generally composed of an insulating or resistive material sandwiched between two electron-conductive electrodes to form metal-in‐ sulator-metal (MIM) structure. By applying an appropriate voltage, the MIM cell can be switched between a high-resistance state (HRS or OFF-state) and a low-resistance state (LRS or ON-state). Switching from OFF-state to ON-state is called the SET process, while switch‐ ing from ON-state to OFF-state is called the RESET process. These two states can represent the logic values 1 and 0, respectively. Depending on voltage polarity, the resistive switching behavior of an RRAM device is classified as unipolar and bipolar. For unipolar switching, resistive switching is induced by a voltage of the same polarity but a different magnitude, as shown in Fig. 1(a) (Waser & Aono, 2007). For bipolar switching, one polarity is used to switch from HRS to LRS, and the opposite polarity is used to switch back into HRS, as shown in Fig. 1(b) (Waser & Aono, 2007). A current compliance (CC) is usually needed dur‐ ing the SET process to prevent the device from a permanent breakdown. (Li et al., 2011)

In the simplest approach, the pure memory element can be used as a basic memory cell, result‐ ing in a configuration where parallel bitlines are crossed by perpendicular wordlines with the switching material placed between wordline and bitline at every cross-point. This configura‐ tion is called a cross-point cell. Since this architecture will lead to a large parasitic current flow‐ ing through nonselected memory cells, the cross-point array has a very slow read access. A selection element can be added to improve the situation. A series connection of a diode in ev‐ ery cross-point allows to reverse bias all nonselected cells. This can be arranged in a similar compact manner as the basic cross-point cell. Finally a transistor device (ideally a MOS Tran‐ sistor) can be added which makes the selection of a cell very easy and therefore gives the best random access time, but comes at the price of increased area consumption. Figure 2 illustrates the different cell type possibilities. For random access type memories, a transistor type archi‐ tecture is preferred while the cross-point architecture and the diode architecture open the path toward stacking memory layers on top of each other and therefore are ideally suited for mass storage devices (Mikolajick et al., 2009; Pinnow & Mikolajick, 2004).

**2.1. Write operation**

**2.2. Read operation**

**2.3. Resistance ratio**

**2.4. Endurance**

**2.5. Retention**

read voltage pulses.

Write voltages should be in the range of a few hundred mV to be compatible with scaled CMOS to few V (to give an advantage over Flash which suffers from high programming vol‐ tages). The length of write voltage pulses is desired to be <100 ns in order to compete with DRAM specifications and to outperform Flash which has a programming speed of some 10

Advances in Resistive Switching Memories Based on Graphene Oxide

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189

Read voltages need to be significantly smaller than write voltages in order to prevent a change of the resistance during the read operation. Because of constraints by circuit design, read volt‐ age cannot be less than approximately one tenth of write voltage. An additional requirement originates from the minimum read current. In the ON-state, read current should not be less than approximately 1mA to allow for a fast detection of the state by reasonably small sense am‐

Although an ON/OFF (ROFF/RON) ratio of only 1.2 to 1.3 can be utilized by dedicated circuit design as shown in MRAM, ON/OFF ratios >10 are required to allow for small and highly efficient sense amplifiers and, hence, RRAM devices which are cost competitive with Flash.

ing on the type. RRAM should provide at least the same endurance, preferably a better one.

A data retention time of >10 years is required for universal NVM. This retention time must be kept at thermal stress up to 85 ºC and small electrical stress such as a constant stream of

Despite a bursting body of experimental data that is rapidly becoming available, the precise mechanism behind the physical effect of resistive switching remains elusive. A few qualita‐ tive models have been proposed emphasizing different aspects: electric-field-induced defect migration (Baikalov et al., 2003; Nian et al., 2007), phase separation (Tulina et al., 2001), tun‐ neling across interfacial domains (Rozenberg et al., 2004), control of the Schottky barrier's height (Jeong et al., 2009), etc., as shown in Fig. 3. A general consensus has emerged on the empirical relevance of three key features: (i) a highly spatially inhomogeneous conduction in the low resistive state, (ii) the existence of a significant number of defects, and (iii) a pre‐ eminent role played by the interfaces, namely, the regions of the oxide that are near each of

the metallic electrodes which often form Schottky barriers. (Rozenberg et al., 2010)

and 107

, depend‐

plifiers. The read time must be in the order of write time or preferably shorter.

Contemporary Flash shows a maximum number of write cycles between 103

ms, or even <10 ns to approach high-performance SRAM.

**Figure 1.** (a) Unipolar switching. The SET voltage is always higher than the voltage at which RESET takes place, and the RESET current is always higher than the CC during SET operation. (b) Bipolar switching. The SET operation takes place on one polarity of the voltage or current, and the RESET operation requires the opposite polarity. (Waser & Aono, 2007)

**Figure 2.** Three different cell architectures for RRAM cells: (a) cross-point cell, (b) diode cell, and (c) transistor cell to‐ gether with their respective area consumption in F2. F denotes the minimum feature size of the fabrication technolo‐ gy. (Mikolajick et al., 2009)

Based on the circuit requirements of high-density NVM today such as Flash and taking pre‐ dictions about technology scaling of the next 15 years into account, one can collect a number of requirements for RRAM cells (Waser et al., 2009):

#### **2.1. Write operation**

tecture is preferred while the cross-point architecture and the diode architecture open the path toward stacking memory layers on top of each other and therefore are ideally suited for mass

**Figure 1.** (a) Unipolar switching. The SET voltage is always higher than the voltage at which RESET takes place, and the RESET current is always higher than the CC during SET operation. (b) Bipolar switching. The SET operation takes place on one polarity of the voltage or current, and the RESET operation requires the opposite polarity. (Waser & Aono, 2007)

**Figure 2.** Three different cell architectures for RRAM cells: (a) cross-point cell, (b) diode cell, and (c) transistor cell to‐ gether with their respective area consumption in F2. F denotes the minimum feature size of the fabrication technolo‐

Based on the circuit requirements of high-density NVM today such as Flash and taking pre‐ dictions about technology scaling of the next 15 years into account, one can collect a number

gy. (Mikolajick et al., 2009)

of requirements for RRAM cells (Waser et al., 2009):

storage devices (Mikolajick et al., 2009; Pinnow & Mikolajick, 2004).

188 New Progress on Graphene Research

Write voltages should be in the range of a few hundred mV to be compatible with scaled CMOS to few V (to give an advantage over Flash which suffers from high programming vol‐ tages). The length of write voltage pulses is desired to be <100 ns in order to compete with DRAM specifications and to outperform Flash which has a programming speed of some 10 ms, or even <10 ns to approach high-performance SRAM.

#### **2.2. Read operation**

Read voltages need to be significantly smaller than write voltages in order to prevent a change of the resistance during the read operation. Because of constraints by circuit design, read volt‐ age cannot be less than approximately one tenth of write voltage. An additional requirement originates from the minimum read current. In the ON-state, read current should not be less than approximately 1mA to allow for a fast detection of the state by reasonably small sense am‐ plifiers. The read time must be in the order of write time or preferably shorter.

#### **2.3. Resistance ratio**

Although an ON/OFF (ROFF/RON) ratio of only 1.2 to 1.3 can be utilized by dedicated circuit design as shown in MRAM, ON/OFF ratios >10 are required to allow for small and highly efficient sense amplifiers and, hence, RRAM devices which are cost competitive with Flash.

#### **2.4. Endurance**

Contemporary Flash shows a maximum number of write cycles between 103 and 107 , depend‐ ing on the type. RRAM should provide at least the same endurance, preferably a better one.

#### **2.5. Retention**

A data retention time of >10 years is required for universal NVM. This retention time must be kept at thermal stress up to 85 ºC and small electrical stress such as a constant stream of read voltage pulses.

Despite a bursting body of experimental data that is rapidly becoming available, the precise mechanism behind the physical effect of resistive switching remains elusive. A few qualita‐ tive models have been proposed emphasizing different aspects: electric-field-induced defect migration (Baikalov et al., 2003; Nian et al., 2007), phase separation (Tulina et al., 2001), tun‐ neling across interfacial domains (Rozenberg et al., 2004), control of the Schottky barrier's height (Jeong et al., 2009), etc., as shown in Fig. 3. A general consensus has emerged on the empirical relevance of three key features: (i) a highly spatially inhomogeneous conduction in the low resistive state, (ii) the existence of a significant number of defects, and (iii) a pre‐ eminent role played by the interfaces, namely, the regions of the oxide that are near each of the metallic electrodes which often form Schottky barriers. (Rozenberg et al., 2010)

and Au)/GO/Pt devices (Zhuge et al., 2011). They considered that the moisture in air affects the ON/OFF ratio of metal/GO/Pt memory cells severely. Furthermore, Jeong et al. present‐ ed a GO based memory that can be easily fabricated using a room temperature spin-casting method on flexible substrates and has reliable memory performance in terms of retention and endurance, as shown in Fig. 5 (Jeong et al., 2010). Therefore, the GO memory is an excel‐ lent candidate to be a memory device for future flexible electronics (Hong et al., 2010).

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191

**Figure 5.** (a) A schematic illustration of a GO based flexible crossbar memory device. (b) Typical I–V curve of a Al/GO/Al/PES device plotted on a semilogarithmic scale. The arrows indicate the voltage sweep direction. The left in‐ set is a real photo image of a device. (c) Continuous bending effect of a Al/GO/Al/PES device. The insets show photo‐ graphs of repeated two bending states. (d) The resistance ratio between the HRS and LRS as a function of the bending radius (*R*). The inset is a photograph of an I-V measurement being performed under a flexed condition. (e) Retention test of Al/GO/Al/PES device read at –0.5 V. (f) Endurance performance of an Al/GO/Al/PES device measured during

As to the mechanism of the resistive switching effect in GO thin films, Zhuge et al. pointed out that in metal/GO/Pt sandwiches, the resistive switching originates from the formation and rupture of conducting filaments, as schematically shown in Fig. 6 (Zhuge et al., 2011).

100 sweep cycles. (Jeong et al., 2010)

**Figure 3.** Reported several resistive switching mechanisms. (a) Filamentary model (Yang et al., 2009). (b) Domain mod‐ el (Rozenberg et al., 2004). (c) Electrical field induced oxygen vacancy migration model (Szot et al., 2006). (d) Schottky barrier modulation model (Sawa et al., 2004).

#### **3. Resistive switching and memory properties in GO-based RRAM cells**

He et al. firstly reported reliable and reproducible resistive switching behaviors in GO thin films prepared by the vacuum filtration method on 2009 (He et al., 2009). The Cu/GO/Pt structure showed an ON/OFF ratio of about 20, aretention time of more than 104 s, and switching threshold voltages of less than 1 V, as shown in Fig. 4.

**Figure 4.** (a) A schematic configuration of the Cu/GO/Pt sandwiched structure. (b) I–V characteristics of the Cu/GO/Pt structure. The arrows indicate the sweep direction. The inset shows the I–V characteristics in semilogar‐ ithmic scale. (He et al., 2009)

It indicates that GO is potentially useful for future nonvolatile memory applications. At a later time, Zhuge et al. achieved larger ON/OFF ratios of more than 100 in metal (Cu, Ag, Ti, and Au)/GO/Pt devices (Zhuge et al., 2011). They considered that the moisture in air affects the ON/OFF ratio of metal/GO/Pt memory cells severely. Furthermore, Jeong et al. present‐ ed a GO based memory that can be easily fabricated using a room temperature spin-casting method on flexible substrates and has reliable memory performance in terms of retention and endurance, as shown in Fig. 5 (Jeong et al., 2010). Therefore, the GO memory is an excel‐ lent candidate to be a memory device for future flexible electronics (Hong et al., 2010).

**Figure 3.** Reported several resistive switching mechanisms. (a) Filamentary model (Yang et al., 2009). (b) Domain mod‐ el (Rozenberg et al., 2004). (c) Electrical field induced oxygen vacancy migration model (Szot et al., 2006). (d) Schottky

**3. Resistive switching and memory properties in GO-based RRAM cells**

He et al. firstly reported reliable and reproducible resistive switching behaviors in GO thin films prepared by the vacuum filtration method on 2009 (He et al., 2009). The Cu/GO/Pt structure showed an ON/OFF ratio of about 20, aretention time of more than 104

**Figure 4.** (a) A schematic configuration of the Cu/GO/Pt sandwiched structure. (b) I–V characteristics of the Cu/GO/Pt structure. The arrows indicate the sweep direction. The inset shows the I–V characteristics in semilogar‐

It indicates that GO is potentially useful for future nonvolatile memory applications. At a later time, Zhuge et al. achieved larger ON/OFF ratios of more than 100 in metal (Cu, Ag, Ti,

switching threshold voltages of less than 1 V, as shown in Fig. 4.

s, and

barrier modulation model (Sawa et al., 2004).

190 New Progress on Graphene Research

ithmic scale. (He et al., 2009)

**Figure 5.** (a) A schematic illustration of a GO based flexible crossbar memory device. (b) Typical I–V curve of a Al/GO/Al/PES device plotted on a semilogarithmic scale. The arrows indicate the voltage sweep direction. The left in‐ set is a real photo image of a device. (c) Continuous bending effect of a Al/GO/Al/PES device. The insets show photo‐ graphs of repeated two bending states. (d) The resistance ratio between the HRS and LRS as a function of the bending radius (*R*). The inset is a photograph of an I-V measurement being performed under a flexed condition. (e) Retention test of Al/GO/Al/PES device read at –0.5 V. (f) Endurance performance of an Al/GO/Al/PES device measured during 100 sweep cycles. (Jeong et al., 2010)

As to the mechanism of the resistive switching effect in GO thin films, Zhuge et al. pointed out that in metal/GO/Pt sandwiches, the resistive switching originates from the formation and rupture of conducting filaments, as schematically shown in Fig. 6 (Zhuge et al., 2011).

their own concentration gradient and the applied electric field, disconnecting the metal fila‐ ment (Tsuruoka et al., 2010). Zhuge et al. also observed the resistive switching effect in GO/Pt structures by conducting atomic force microscopy (CAFM), as shown in Fig. 7 (Zhuge et al., 2011). It is attributed to the redox reactions between GO and adsorbed water induced by external voltage biases. While for Al/GO/Al memory cells, Jeong et al. attributed the bi‐ polar resistive switching behavior to rupture and formation of conducting filaments at the top amorphous interface layer formed between the GO film and the top Al metal electrode,

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**Figure 8.** Schematic of the proposed bipolar resistive switching model for Al electrode/GO/Al electrode crossbar memory device. (a) The pristine device is in the OFF-state due to the (relatively) thick insulating top interface layer formed by a redox reaction between vapor deposited Al and the GO thin film. (b) The ON-state is induced by the for‐ mation of local filaments in the top interface layer due to oxygen ion diffusion back into the GO thin film by an exter‐

Furthermore, Hong et al. pointed out that for Al/GO/metal memory devices, the resistive switching operation is governed by dual mechanism of oxygen migration and Al diffusion (Hong et al., 2011). The Al diffusion into the graphene oxide is the main factor to determine the switching endurance property which limits the long term lifetime of the device. The elec‐ trode dependence on graphene oxide RRAM operation has been analyzed as well and is at‐ tributed to the difference in surface roughness of graphene oxide for the different bottom electrodes, as shown in Fig. 9 (Hong et al., 2011). Interestingly, Panin et al. observed both diode-like (rectifying) and resistor-like (nonrectifying) resistive switching behaviors in an Al/GO/Al planar structure, as shown in Fig. 10 (Panin et al., 2011). Electrical characterization of the Al/GO interface using the induced current identifies a potential barrier near the inter‐ face and its spatial modulation, caused by local changes of resistance at a bias voltage,

as shown in Fig. 8 (Jeong et al, 2010).

nal negative bias on the top electrode. (Jeong et al., 2010)

**Figure 6.** A schematic diagram for the mechanism of the resistive switching in metal/GO/Pt memory cells. (Zhuge et al., 2011)

**Figure 7.** (a), (b) and (c) AFM images of virgin GO films, GO films in LRS, and GO films in HRS. The light-colored ribbons represent folded regions. (d), (e) and (f) the corresponding CAFM images under a read voltage of 1 V. (Zhuge et al., 2011)

An analysis of the temperature dependence of the ON-state resistance reveals that the fila‐ ments are composed of metal atoms due to the diffusion of the top electrodes under a bias voltage. Tsuruoka et al. pointed out that the formation of a metal filament is due to inhomo‐ geneous nucleation and subsequent growth of metal, based on the migration of metal ions in the oxide matrix (Tsuruoka et al., 2010). Recently, they reported that the ionization of metal at the anode interfaces is likely to be attributed to chemical oxidation via residual water in the oxide layer, and metal ions migrate along grain boundaries in the oxide layer, where a hydrogen-bond network might be formed by moisture absorption (Tsuruoka et al., 2012). Moreover, the switching occurs within confined regions of the metal filaments. The RESET process is considered to consist of the Joule-heating-assisted oxidation of metal atoms at the thinnest part of the metal filament followed by diffusion and drift of the metal ions under their own concentration gradient and the applied electric field, disconnecting the metal fila‐ ment (Tsuruoka et al., 2010). Zhuge et al. also observed the resistive switching effect in GO/Pt structures by conducting atomic force microscopy (CAFM), as shown in Fig. 7 (Zhuge et al., 2011). It is attributed to the redox reactions between GO and adsorbed water induced by external voltage biases. While for Al/GO/Al memory cells, Jeong et al. attributed the bi‐ polar resistive switching behavior to rupture and formation of conducting filaments at the top amorphous interface layer formed between the GO film and the top Al metal electrode, as shown in Fig. 8 (Jeong et al, 2010).

**Figure 6.** A schematic diagram for the mechanism of the resistive switching in metal/GO/Pt memory cells.

**Figure 7.** (a), (b) and (c) AFM images of virgin GO films, GO films in LRS, and GO films in HRS. The light-colored ribbons represent folded regions. (d), (e) and (f) the corresponding CAFM images under a read voltage of 1 V.

An analysis of the temperature dependence of the ON-state resistance reveals that the fila‐ ments are composed of metal atoms due to the diffusion of the top electrodes under a bias voltage. Tsuruoka et al. pointed out that the formation of a metal filament is due to inhomo‐ geneous nucleation and subsequent growth of metal, based on the migration of metal ions in the oxide matrix (Tsuruoka et al., 2010). Recently, they reported that the ionization of metal at the anode interfaces is likely to be attributed to chemical oxidation via residual water in the oxide layer, and metal ions migrate along grain boundaries in the oxide layer, where a hydrogen-bond network might be formed by moisture absorption (Tsuruoka et al., 2012). Moreover, the switching occurs within confined regions of the metal filaments. The RESET process is considered to consist of the Joule-heating-assisted oxidation of metal atoms at the thinnest part of the metal filament followed by diffusion and drift of the metal ions under

(Zhuge et al., 2011)

192 New Progress on Graphene Research

(Zhuge et al., 2011)

**Figure 8.** Schematic of the proposed bipolar resistive switching model for Al electrode/GO/Al electrode crossbar memory device. (a) The pristine device is in the OFF-state due to the (relatively) thick insulating top interface layer formed by a redox reaction between vapor deposited Al and the GO thin film. (b) The ON-state is induced by the for‐ mation of local filaments in the top interface layer due to oxygen ion diffusion back into the GO thin film by an exter‐ nal negative bias on the top electrode. (Jeong et al., 2010)

Furthermore, Hong et al. pointed out that for Al/GO/metal memory devices, the resistive switching operation is governed by dual mechanism of oxygen migration and Al diffusion (Hong et al., 2011). The Al diffusion into the graphene oxide is the main factor to determine the switching endurance property which limits the long term lifetime of the device. The elec‐ trode dependence on graphene oxide RRAM operation has been analyzed as well and is at‐ tributed to the difference in surface roughness of graphene oxide for the different bottom electrodes, as shown in Fig. 9 (Hong et al., 2011). Interestingly, Panin et al. observed both diode-like (rectifying) and resistor-like (nonrectifying) resistive switching behaviors in an Al/GO/Al planar structure, as shown in Fig. 10 (Panin et al., 2011). Electrical characterization of the Al/GO interface using the induced current identifies a potential barrier near the inter‐ face and its spatial modulation, caused by local changes of resistance at a bias voltage, which correlated well with the resistive switching of the whole structure. Recently, Wang et al. found that the speed of the SET and RESET operations of the Al/GO/ITO resistive memo‐ ries is significant asymmetric (Wang et al., 2012). The RESET speed is in the order of 100 ns under a –5 V voltage while the SET speed is three orders of magnitude slower (100 μs) un‐ der a 5 V bias. The behavior of resistive switching speed difference is elucidated by voltage modulated oxygen diffusion barrier change, as shown in Fig. 11 (Wang et al., 2012).

**Figure 11.** Pulse behavior of the Al/GO/ITO/PET memory cell, HRS and LRS is read at 0.3 V. (a) the SET and RESET oper‐ ations of the devices with different pulsing width at ±5. The HRS and LRS of the devices are measured at 0.3V and the SET operation is found to be three orders of magnitude slower than the RESET operation; (b) a schematic of oxygen

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**4. Resistive switching and memory properties in GO-polymer hybrid**

**Figure 12.** Plausible switching mechanism of GO-PVK. RGO stands for reduced graphene oxide. (Liu et al., 2009)

Zhuang et al. synthesized a novel conjugated-polymer-modified graphene oxide (TPAPAM-GO), which was successfully used to fabricate a TPAPAM-GO-based RRAM device (Zhuang et al., 2010). The device exhibits a typical bistable electrical switching and nonvolatile rewrit‐

Liu et al. prepared a solution-processable and electroactive complex of poly(*N*-vinylcarba‐ zole)-derivatized graphene oxide (GO-PVK) via amidation of end-functionalized PVK, from reversible addition fragmentation chain transfer polymerization, with tolylene-2,5 diisocyanate-functionalized graphene oxide (Liu et al., 2009). The Al/GO-PVK/ITO device exhibits bistable electrical conductivity switching and nonvolatile rewritable memory ef‐ fects. The resistive switching is attributed to electron transfer between GO and PVK, as

hopping barrier change model. (Wang et al., 2012)

shown in Fig. 12 (Liu et al., 2009).

**RRAM cells**

**Figure 9.** The contact angles of graphene oxide solution on four different surfaces of ITO, TaN, Su, and Pt. UV treat‐ ment is done to promote adhesion. (Hong et al., 2011)

**Figure 10.** I–V curves of Al/GO/Al structures pre-formed at different forming voltages. (Panin et al., 2011)

which correlated well with the resistive switching of the whole structure. Recently, Wang et al. found that the speed of the SET and RESET operations of the Al/GO/ITO resistive memo‐ ries is significant asymmetric (Wang et al., 2012). The RESET speed is in the order of 100 ns under a –5 V voltage while the SET speed is three orders of magnitude slower (100 μs) un‐ der a 5 V bias. The behavior of resistive switching speed difference is elucidated by voltage

**Figure 9.** The contact angles of graphene oxide solution on four different surfaces of ITO, TaN, Su, and Pt. UV treat‐

**Figure 10.** I–V curves of Al/GO/Al structures pre-formed at different forming voltages. (Panin et al., 2011)

ment is done to promote adhesion. (Hong et al., 2011)

194 New Progress on Graphene Research

modulated oxygen diffusion barrier change, as shown in Fig. 11 (Wang et al., 2012).

**Figure 11.** Pulse behavior of the Al/GO/ITO/PET memory cell, HRS and LRS is read at 0.3 V. (a) the SET and RESET oper‐ ations of the devices with different pulsing width at ±5. The HRS and LRS of the devices are measured at 0.3V and the SET operation is found to be three orders of magnitude slower than the RESET operation; (b) a schematic of oxygen hopping barrier change model. (Wang et al., 2012)

## **4. Resistive switching and memory properties in GO-polymer hybrid RRAM cells**

Liu et al. prepared a solution-processable and electroactive complex of poly(*N*-vinylcarba‐ zole)-derivatized graphene oxide (GO-PVK) via amidation of end-functionalized PVK, from reversible addition fragmentation chain transfer polymerization, with tolylene-2,5 diisocyanate-functionalized graphene oxide (Liu et al., 2009). The Al/GO-PVK/ITO device exhibits bistable electrical conductivity switching and nonvolatile rewritable memory ef‐ fects. The resistive switching is attributed to electron transfer between GO and PVK, as shown in Fig. 12 (Liu et al., 2009).

**Figure 12.** Plausible switching mechanism of GO-PVK. RGO stands for reduced graphene oxide. (Liu et al., 2009)

Zhuang et al. synthesized a novel conjugated-polymer-modified graphene oxide (TPAPAM-GO), which was successfully used to fabricate a TPAPAM-GO-based RRAM device (Zhuang et al., 2010). The device exhibits a typical bistable electrical switching and nonvolatile rewrit‐ able memory effect, with a SET voltage of about–1V and an ON/OFF ratio of more than 103 , as shown in Fig. 13 (Zhuang et al., 2010).

tractive candidate for applications in next-generation high-density nonvolatile flash memo‐ ries (Wu et al., 2011). Yu et al. reported bistable resistive switching characteristics for writeonce-read-many-times (WORM) memory devices using a supramolecular hybrid route to hydrogen-bonded block copolymers (BCP) and GO as charge storage materials (Yu et al., 2012). The ITO/7 wt% GO composite/Al device exhibits a one-time programmable effect

shown in Fig. 15 (Yu et al., 2012). The switching phenomena were attributed to the charge trapping environment operating across the BCP/GO interface and from the GO intrinsic de‐ fect. Controlling the physical interaction of BCP and functional GO sheets can generate a well-dispersed charge storage composite device for future flexible information technology.

**Figure 14.** (a) FE-SEM image of the cross sectional view for the GO-PI film. (b) Schematic of Ag/PI/GO:PI/PI/ITO mem‐

Recently, Hu et al. prepared a novel RRAM device based on reduced GO noncovalently functionalized by thionine (Hu et al., 2012). The device shows nonvolatile resistive

ns, long retention time of >105 s, and good endurance. The resistive switching in such memory device is attributed to electron transfer reaction between reduced graphene ox‐

Noting that besides GO, graphene can also be used for resistive switching memory devices. Standley et al. developed a nonvolatile resistive memory element based on graphene break junctions which demonstrates thousands of writing cycles and long retention times (Standley et al., 2008). They proposed a model for device operation based on the formation and breaking of carbon atomic chains that bridges the junctions, as shown in Fig. 16 (Standley et al., 2008). Recently, He et al. reported a planar graphene/SiO2 nanogap structure for multilevel resistive switching (He et al., 2012). Such two-terminal devices exhibited excellent memory characteris‐

ing speed down to 500 ns. At least five conduction states with reliability and reproducibility were demonstrated in these memory devices, as shown in Fig. 17 (He et al., 2012). The mecha‐ nism of the resistive switching was attributed to a reversible thermal-assisted reduction and

oxidation process that occurred at the breakdown region of the SiO2 substrate.

cycles, long retention time more than 105

switching behaviors with an ON/OFF ratio of more than 104

s and a 108 read pulse of –1.0 V, as

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, fast switching speed of <5

s, and fast switch‐

at –1.0 V, a retention of 104

with an ON/OFF ratio of 105

ory device. (Wu et al., 2011)

ide sheets and thionine molecules.

tics with good endurance up to 104

**Figure 13.** J–V characteristics and stability tested in either ON- or OFF-state under stimulus by read pulses of a 0.16 mm2 ITO/TPAPAM-GO/Al device. Inset: schematic diagram of the single-layer memory devices. (Zhuang et al., 2010)

Both the ON- and OFF-state are stable under a constant voltage stress and survive up to 108 read cycles at a read voltage of –1 V. As to the switching mechanism, they deduced that at the switching threshold voltage, electrons transit from the hole transporting (electron donat‐ ing) polymer TPAPAM (highest occupied molecular orbital, HOMO) into the graphene monoatom layer (lowest unoccupied molecular orbital, LUMO) via intramolecular chargetransfer (CT) interaction (Ling et al., 2008). The transferred electrons can delocalize effective‐ ly in the giant p-conjugation system, and reduce graphene oxide to graphene (Elias et al., 2009; Robinson et al., 2008). Upon electrochemical reduction of the functionalized graphene oxide, electrons can propagate with less scattering, giving rise to a substantially enhanced room temperature conductivity (~102 S m–1) of the composite material (Robinson et al., 2008; Stankovich et al., 2006). Along with the increase of CT interaction, dual-channel chargetransport pathways will form via interplane hopping in graphene films and switch the ITO/ TPAPAM-GO/Al device from the OFF-state to the ON-state (Ling et al., 2008). The applica‐ tion of a reverse positive bias to the device can, however, extract electrons from the reduced graphene nanosheet, returning it to the initial less-conductive form and programming the device back to the OFF-state (Zhuang et al., 2010). Wu et al. fabricated GO-polyimide (PI) hybrid RRAM cells, as shown in Fig. 14 (Wu et al., 2011). The functionalization of GO sheets with PI enables the layer-by-layer fabrication of a GO-PI hybrid resistive-switch device and leads to high reproducibility of the memory effect. The current-voltage curves for the as-fab‐ ricated device exhibit multilevel resistive-switch properties under various reset voltages. The capacitance-voltage characteristics for a capacitor based on GO-PI nanocomposite indi‐ cate that the electrical switching may originate from the charge trapping in GO sheets. The high device-to-device uniformity and unique memory properties of the device make it an at‐ tractive candidate for applications in next-generation high-density nonvolatile flash memo‐ ries (Wu et al., 2011). Yu et al. reported bistable resistive switching characteristics for writeonce-read-many-times (WORM) memory devices using a supramolecular hybrid route to hydrogen-bonded block copolymers (BCP) and GO as charge storage materials (Yu et al., 2012). The ITO/7 wt% GO composite/Al device exhibits a one-time programmable effect with an ON/OFF ratio of 105 at –1.0 V, a retention of 104 s and a 108 read pulse of –1.0 V, as shown in Fig. 15 (Yu et al., 2012). The switching phenomena were attributed to the charge trapping environment operating across the BCP/GO interface and from the GO intrinsic de‐ fect. Controlling the physical interaction of BCP and functional GO sheets can generate a well-dispersed charge storage composite device for future flexible information technology.

able memory effect, with a SET voltage of about–1V and an ON/OFF ratio of more than 103

**Figure 13.** J–V characteristics and stability tested in either ON- or OFF-state under stimulus by read pulses of a 0.16 mm2 ITO/TPAPAM-GO/Al device. Inset: schematic diagram of the single-layer memory devices. (Zhuang et al., 2010)

Both the ON- and OFF-state are stable under a constant voltage stress and survive up to 108 read cycles at a read voltage of –1 V. As to the switching mechanism, they deduced that at the switching threshold voltage, electrons transit from the hole transporting (electron donat‐ ing) polymer TPAPAM (highest occupied molecular orbital, HOMO) into the graphene monoatom layer (lowest unoccupied molecular orbital, LUMO) via intramolecular chargetransfer (CT) interaction (Ling et al., 2008). The transferred electrons can delocalize effective‐ ly in the giant p-conjugation system, and reduce graphene oxide to graphene (Elias et al., 2009; Robinson et al., 2008). Upon electrochemical reduction of the functionalized graphene oxide, electrons can propagate with less scattering, giving rise to a substantially enhanced

Stankovich et al., 2006). Along with the increase of CT interaction, dual-channel chargetransport pathways will form via interplane hopping in graphene films and switch the ITO/ TPAPAM-GO/Al device from the OFF-state to the ON-state (Ling et al., 2008). The applica‐ tion of a reverse positive bias to the device can, however, extract electrons from the reduced graphene nanosheet, returning it to the initial less-conductive form and programming the device back to the OFF-state (Zhuang et al., 2010). Wu et al. fabricated GO-polyimide (PI) hybrid RRAM cells, as shown in Fig. 14 (Wu et al., 2011). The functionalization of GO sheets with PI enables the layer-by-layer fabrication of a GO-PI hybrid resistive-switch device and leads to high reproducibility of the memory effect. The current-voltage curves for the as-fab‐ ricated device exhibit multilevel resistive-switch properties under various reset voltages. The capacitance-voltage characteristics for a capacitor based on GO-PI nanocomposite indi‐ cate that the electrical switching may originate from the charge trapping in GO sheets. The high device-to-device uniformity and unique memory properties of the device make it an at‐

S m–1) of the composite material (Robinson et al., 2008;

as shown in Fig. 13 (Zhuang et al., 2010).

196 New Progress on Graphene Research

room temperature conductivity (~102

,

**Figure 14.** (a) FE-SEM image of the cross sectional view for the GO-PI film. (b) Schematic of Ag/PI/GO:PI/PI/ITO mem‐ ory device. (Wu et al., 2011)

Recently, Hu et al. prepared a novel RRAM device based on reduced GO noncovalently functionalized by thionine (Hu et al., 2012). The device shows nonvolatile resistive switching behaviors with an ON/OFF ratio of more than 104 , fast switching speed of <5 ns, long retention time of >105 s, and good endurance. The resistive switching in such memory device is attributed to electron transfer reaction between reduced graphene ox‐ ide sheets and thionine molecules.

Noting that besides GO, graphene can also be used for resistive switching memory devices. Standley et al. developed a nonvolatile resistive memory element based on graphene break junctions which demonstrates thousands of writing cycles and long retention times (Standley et al., 2008). They proposed a model for device operation based on the formation and breaking of carbon atomic chains that bridges the junctions, as shown in Fig. 16 (Standley et al., 2008). Recently, He et al. reported a planar graphene/SiO2 nanogap structure for multilevel resistive switching (He et al., 2012). Such two-terminal devices exhibited excellent memory characteris‐ tics with good endurance up to 104 cycles, long retention time more than 105 s, and fast switch‐ ing speed down to 500 ns. At least five conduction states with reliability and reproducibility were demonstrated in these memory devices, as shown in Fig. 17 (He et al., 2012). The mecha‐ nism of the resistive switching was attributed to a reversible thermal-assisted reduction and oxidation process that occurred at the breakdown region of the SiO2 substrate.

**Figure 15.** (a) I–V characteristics of 7 wt% GO composite device. The inset shows the switching behavior in different memory cells. (b) Retention time test. (c) Stimulus effect of read pulses. (Yu et al., 2012)

**Figure 17.** Multilevel resistive switching properties of graphene/SiO2 nanogap structures. (a) Typical I–V characteristics of a device with a width of 1 μm, length of 0.4 μm, and thickness of 2.3 nm. The vertical line cut at 1 V indicates five resistance states. By sweeping the reset voltage from 0 to 5 V, the OFF1 state (red) was established. The subsequent reset voltages sweep up to higher voltage of 7 V (purple), 9 V (orange), and even higher to 11 V (olive) from 0 V, and lower conduction states of OFF2, OFF3, and OFF4 were achieved subsequently. (b) Top: series of bias pulses with dif‐ ferent magnitudes of 3, 5, 7, 9, and 11 V, corresponding to the sweep voltages in (a) with three reading pulses of 1 V after each programming pulse was applied. Bottom: resistance changes corresponding to the each voltage pulse in the top panel. (c) Cycled switching of the device under various reset voltages. (d) Retention time of more than 104 s for

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Resistive random access memory based on the resistive switching effect induced by electri‐ cal stimulus has inspired scientific and commercial interests due to its high operation speed, high scalability, and multibit storage potential. The reading of resistance states is nondes‐ tructive, and the memory devices can be operated without transistors in every cell, thus making a cross-bar structure feasible. Although a large variety of solid-state materials have been found to exhibit the resistive switching effect, GO is a very promising material for RRAM applications since due to an ultrathin thickness (~1 nm) and its unique physical– chemical properties. Both GO and GO-polymer hybrid exhibit good memory performances, such as high ON/OFF ratio and long retention time. The resistive switching of GO is always related to defect migration, such as metal ions and oxygen vacancies, whereas the switching of GO-polymer hybrid is considered to be attributed to charge transfer reaction between GO sheets and polymer molecules. Since both high switching speed and good retention could be

each conduction state tested by a continuous 1 V pulse. (He et al., 2012)

**5. Summary and prospect**

**Figure 16.** (a) SEM image of the device before (left panel) and after breakdown (right panel). The arrows indicate the edges of the nanoscale gap. (b) Proposed schematic atomic configurations in the ON and OFF states. (Stand‐ ley et al., 2008)

**Figure 17.** Multilevel resistive switching properties of graphene/SiO2 nanogap structures. (a) Typical I–V characteristics of a device with a width of 1 μm, length of 0.4 μm, and thickness of 2.3 nm. The vertical line cut at 1 V indicates five resistance states. By sweeping the reset voltage from 0 to 5 V, the OFF1 state (red) was established. The subsequent reset voltages sweep up to higher voltage of 7 V (purple), 9 V (orange), and even higher to 11 V (olive) from 0 V, and lower conduction states of OFF2, OFF3, and OFF4 were achieved subsequently. (b) Top: series of bias pulses with dif‐ ferent magnitudes of 3, 5, 7, 9, and 11 V, corresponding to the sweep voltages in (a) with three reading pulses of 1 V after each programming pulse was applied. Bottom: resistance changes corresponding to the each voltage pulse in the top panel. (c) Cycled switching of the device under various reset voltages. (d) Retention time of more than 104 s for each conduction state tested by a continuous 1 V pulse. (He et al., 2012)

#### **5. Summary and prospect**

**Figure 15.** (a) I–V characteristics of 7 wt% GO composite device. The inset shows the switching behavior in different

**Figure 16.** (a) SEM image of the device before (left panel) and after breakdown (right panel). The arrows indicate the edges of the nanoscale gap. (b) Proposed schematic atomic configurations in the ON and OFF states. (Stand‐

memory cells. (b) Retention time test. (c) Stimulus effect of read pulses. (Yu et al., 2012)

ley et al., 2008)

198 New Progress on Graphene Research

Resistive random access memory based on the resistive switching effect induced by electri‐ cal stimulus has inspired scientific and commercial interests due to its high operation speed, high scalability, and multibit storage potential. The reading of resistance states is nondes‐ tructive, and the memory devices can be operated without transistors in every cell, thus making a cross-bar structure feasible. Although a large variety of solid-state materials have been found to exhibit the resistive switching effect, GO is a very promising material for RRAM applications since due to an ultrathin thickness (~1 nm) and its unique physical– chemical properties. Both GO and GO-polymer hybrid exhibit good memory performances, such as high ON/OFF ratio and long retention time. The resistive switching of GO is always related to defect migration, such as metal ions and oxygen vacancies, whereas the switching of GO-polymer hybrid is considered to be attributed to charge transfer reaction between GO sheets and polymer molecules. Since both high switching speed and good retention could be simultaneously achieved in GO-polymer hybrid RRAM device, such memory device is ex‐ pected to overcome the "voltage–time dilemma" (i.e., one could not realize high write/erase speed and long retention time simultaneously in pure electronic mechanism-based RRAM cells). Pure electronic mechanisms in RRAM cells postulate the trapping and detrapping of electron in immobile traps as the reason for the resistance changes, also known as Simmons & Verderber model. While in GO-polymer hybrid systems, the electron transfer occurs be‐ tween graphene sheets and functional molecules covalently or non-covalently bonded to graphene, which may avoid the "voltage–time dilemma".

[4] Burghard, M., Klauk, H., & Kern, K. (2009). Carbon-Based Field-Effect Transistors for

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[5] Chang, W. Y., Lai, Y. C., Wu, T. B., Wang, S. F., Chen, F., & Tsai, M. J. (2008). Unipo‐ lar resistive switching characteristics of ZnO thin films for nonvolatile memory ap‐

[6] Cote, L. J., Kim, F., & Huang, J. X. (2009). Langmuir−Blodgett Assembly of Graphite Oxide Single Layers. *Journal of the American Chemical Society*, 131(3), 1043-1049,

[7] Dong, Y., Yu, M. G., Mc Alpine, C., Lu, W., & Lieber, C. M. (2008). Si/α-Si core/shell nanowires as nonvolatile crossbar switches. *Nano Letters*, 8(2), 386-391, 1530-6984. [8] Eda, G., Fanchini, G., & Chhowalla, M. (2008). Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. *Nature Nanotechnolo‐*

[9] Elias, D. C., Nair, R. R., Mohiuddin, T. M. G., Morozov, S. R., Blake, P., Halsall, M. P., & Ferrari, A. C. (2009). Control of graphene's properties by reversible hydrogenation:

[10] Guan, W. H., Long, S. B., Jia, R., & Liu, M. (2007). Nonvolatile resistive switching memory utilizing gold nanocrystals embedded in zirconium oxide. *Applied Physics*

[11] Guan, W. H., Liu, M., Long, S. B., Liu, Q., & Wang, W. (2008). On the resistive switch‐ ing mechanisms of Cu/ZrO2:Cu/Pt. *Applied Physics Letters*, 93(22), 223506, 0003-6951. [12] He, C. L., Zhuge, F., Zhou, X. F., Li, M., Zhou, G. C., Liu, Y. W., Wang, J. Z., Chen, B., Su, W. J., Liu, Z. P., Wu, Y. H., Cui, P., & Li, R. W. (2009). Nonvolatile resistive switching in graphene oxide thin films. *Applied Physics Letters*, 95(23), 232101,

[13] He, C. L., Shi, Z. W., Zhang, L. C., Yang, W., Yang, R., Shi, D. X., & Zhang, G. Y. (2012). Multilevel resistive switching in planar graphene/SiO2 nanogap structures.

[14] Hong, S. K., Kim, J. E., Kim, S. O., Choi, S. Y., & Cho, B. J. (2010). Flexible resistive switching memory device based on graphene oxide. *IEEE Electron Device Letters*,

[15] Hong, S. K., Kim, J. E., Kim, S. O., & Cho, B. J. (2011). Analysis on switching mecha‐ nism of graphene oxide resistive memory device. *Journal of Applied Physics*, 110(4),

[16] Hu, B. L., Quhe, R. G., Chen, C., Zhuge, F., Zhu, X. J., Peng, S. S., Chen, X. X., Pan, L., Wu, Y. Z., Zheng, W. G., Yan, Q., Lu, J., & Li, R. W. (). *Electrically controlled electron transfer and resistance switching in graphene oxide noncovolently functionalized with dye*,

Nanoelectronics. *Advanced Materials*, 21(25-26), 2586-2600, 0935-9648.

plications. *Applied Physics Letters*, 92(2), 22110, 0003-6951.

Evidence for graphane. *Science*, 323(5914), 610-613, 0036-8075.

0002-7863.

0003-6951.

*gy*, 3(5), 270-274, 1748-3387.

*Letters*, 91(6), 062111, 0003-6951.

*ACS Nano*, 6(5), 4214-4221, 1936-0851.

31(9), 1005-1007, 0741-3106.

044506, 0021-4922.

unpublished.

However, to meet the requirements of future memory applications, GO-based resistance memories should overcome several hurdles. Firstly, the size and chemical composition of GO sheets must be controllable, for example, the type, number and distribution of oxygen functional groups attached to both sides of graphene sheets. Secondly, the resistive switch‐ ing mechanism of GO is still not clear. For metal/GO/metal sandwiches, although the forma‐ tion/rupture of metal filaments is considered to be responsible for the resistive switching, the filament growth and inhibition kinetics remains ambiguous. As to the switching of GOpolymer hybrid, no direct evidences have been provided to support the charge transfer hy‐ pothesis so far. Thirdly, it is a real challenge to improve the thermal stability of GO and GOpolymer hybrid since memory devices may work at elevated temperature.

## **Author details**

Fei Zhuge1,2\*, Bing Fu1 and Hongtao Cao1\*

\*Address all correspondence to: zhugefei@nimte.ac.cn and h\_cao@nimte.ac.cn

1 Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, People's Republic of China

2 State Key Laboratory of Silicon Materials, Zhejiang University, People's Republic of China

#### **References**


[4] Burghard, M., Klauk, H., & Kern, K. (2009). Carbon-Based Field-Effect Transistors for Nanoelectronics. *Advanced Materials*, 21(25-26), 2586-2600, 0935-9648.

simultaneously achieved in GO-polymer hybrid RRAM device, such memory device is ex‐ pected to overcome the "voltage–time dilemma" (i.e., one could not realize high write/erase speed and long retention time simultaneously in pure electronic mechanism-based RRAM cells). Pure electronic mechanisms in RRAM cells postulate the trapping and detrapping of electron in immobile traps as the reason for the resistance changes, also known as Simmons & Verderber model. While in GO-polymer hybrid systems, the electron transfer occurs be‐ tween graphene sheets and functional molecules covalently or non-covalently bonded to

However, to meet the requirements of future memory applications, GO-based resistance memories should overcome several hurdles. Firstly, the size and chemical composition of GO sheets must be controllable, for example, the type, number and distribution of oxygen functional groups attached to both sides of graphene sheets. Secondly, the resistive switch‐ ing mechanism of GO is still not clear. For metal/GO/metal sandwiches, although the forma‐ tion/rupture of metal filaments is considered to be responsible for the resistive switching, the filament growth and inhibition kinetics remains ambiguous. As to the switching of GOpolymer hybrid, no direct evidences have been provided to support the charge transfer hy‐ pothesis so far. Thirdly, it is a real challenge to improve the thermal stability of GO and GO-

polymer hybrid since memory devices may work at elevated temperature.

\*Address all correspondence to: zhugefei@nimte.ac.cn and h\_cao@nimte.ac.cn

1 Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences,

2 State Key Laboratory of Silicon Materials, Zhejiang University, People's Republic of China

[1] Avouris, P., Chen, Z. H., & Perebeinos, V. (2007). Carbon-based electronics. *Nature*

[2] Baikalov, A., Wang, Y. Q., Shen, B., Lorenz, B., Tsui, S., Sun, Y. Y., Xue, Y. Y., & Chu, C. W. (2003). Field-driven hysteretic and reversible resistive switch at the Ag-

[3] Beck, A., Bednorz, J. G., Gerber, Ch., Rosse, C. L., & Widmer, D. (2000). Reproducible switching effect in thin oxide films for memory applications. *Applied Physics Letters*,

Pr0.7Ca0.3MnO3 interface. *Applied Physics Letters*, 83(5), 957-959, 0003-6951.

and Hongtao Cao1\*

graphene, which may avoid the "voltage–time dilemma".

**Author details**

**References**

Fei Zhuge1,2\*, Bing Fu1

200 New Progress on Graphene Research

People's Republic of China

*Nanotechnology*, 2(10), 1748-3387.

77(1), 139-141, 0003-6951.


[17] Jeong, D. S., Schroeder, H., & Waser, R. (2009). Abnormal bipolar-like resistance change behavior induced by symmetric electroforming in Pt/TiO2/Pt resistive switch‐ ing cells. *Nanotechnology*, 20(37), 375201, 0957-4484.

[30] Ling, Q. D., Liaw, D. J., Zhu, C. X., Chan, D. S. H., Kang, E. T., & Neoh, K. G. (2008). Polymer electronic memories: Materials, devices and mechanisms. *Progress in Poly‐*

Advances in Resistive Switching Memories Based on Graphene Oxide

http://dx.doi.org/10.5772/51260

203

[31] Liu, S. Q., Wu, N. J., & Ignatiev, A. (2000). Electric-pulse-induced reversible resist‐ ance change effect in magnetoresistive films. *Applied Physics Letters*, 76(19), 2749-2751,

[32] Liu, Q., Long, S. B., Wang, W., Zuo, Q. Y., Zhang, S., Chen, J. N., & Liu, M. (2009). Improvement of resistive switching properties in ZrO2-based ReRAM with implant‐

[33] Liu, G., Zhuang, X. D., Chen, Y., Zhang, B., Zhu, J. H., Zhu, C. X., Neoh, K. G., & Kang, E. T. (2009). Bistable electrical switching and electronic memory effect in a sol‐ ution-processable graphene oxide-donor polymer complex. *Applied Physics Letters*,

[34] Lu, W., & Lieber, C. M. (2007). Nanoelectronics from the bottom up. *Nature Materials*,

[35] Mikolajick, T., Salinga, M., Kund, M., & Kever, T. (2009). Nonvolatile memory con‐ cepts based on resistive switching in inorganic materials. *Advanced Engineering Mate‐*

[36] Nian, Y. B., Strozier, J., Wu, N. J., Chen, X., & Ignatiev, A. (2007). Evidence for an oxygen diffusion model for the electric pulse induced resistance change effect in

[37] Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., Grigorieva, I. V., & Firsov, A. A. (2004). Electric field effect in atomically thin carbon

[38] Odagawa, A., Sato, H., Inoue, I. H., Akoh, H., Kawasaki, M., & Tokura, Y. (2004). Co‐ lossal electroresistance of a Pr0.7Ca0.3MnO3 thin film at room temperature. *Physical Re‐*

[39] Panin, G. N., Kapitanova, O. O., Lee, S. W., Baranov, A. N., & Kang, T. W. (2011). Re‐ sistive switching in Al/graphene oxide/Al structure. *Japanese Journal of Applied Phys‐*

[40] Pinnow, C. U., & Mikolajick, T. (2004). Material aspects in emerging nonvolatile memories. *Journal of the Electrochemical Society*, 151(6), K13-K19, 0013-4651.

[41] Robinson, J. T., Perkins, F. K., Snow, E. S., Wei, Z., & Sheehan, P. E. (2008). Reduced graphene oxide molecular sensors. *Nano Letters*, 8(10), 3137-3140, 1476-1122.

[42] Rozenberg, M. J., Inoue, I. H., & Sanchez, M. J. (2004). Nonvolatile memory with mul‐ tilevel switching: A basic model. *Physical Review Letters*, 92(17), 178302, 0031-9007.

transition-metal oxides. *Physical Review Letters*, 98(14), 146403, 0031-9007.

ed Ti ions. *IEEE Electron Device Letters*, 30(12), 1335-1337, 0741-3106.

*mer Sciecne*, 33(10), 917-978, 0079-6700.

0003-6951.

95(25), 253301, 0003-6951.

6(11), 841-850, 1476-1122.

*rials*, 11(4), 235-240, 1438-1656.

films. *Science*, 306(5696), 666-669, 0036-8075.

*view B*, 70(22), 224403, 1098-0121.

*ics*, 50(7), 070110, 0021-4922.


[30] Ling, Q. D., Liaw, D. J., Zhu, C. X., Chan, D. S. H., Kang, E. T., & Neoh, K. G. (2008). Polymer electronic memories: Materials, devices and mechanisms. *Progress in Poly‐ mer Sciecne*, 33(10), 917-978, 0079-6700.

[17] Jeong, D. S., Schroeder, H., & Waser, R. (2009). Abnormal bipolar-like resistance change behavior induced by symmetric electroforming in Pt/TiO2/Pt resistive switch‐

[18] Jeong, D. S., Schroeder, H., & Waser, R. (2009). Mechanism for bipolar switching in a Pt/TiO2/Pt resistive switching cell. *Physical Review B*, 79(19), 195317, 1098-0121. [19] Jeong, H. Y., Kim, J. Y., Kim, J. W., Hwang, J. O., Kim, J. E., Lee, J. Y., Yoon, T. H., Cho, B. J., Kim, S. O., Ruoff, R. S., & Choi, S. Y. (2010). Graphene oxide thin films for flexible nonvolatile memory applications. *Nano Letters*, 10(11), 4381-4386, 1476-1122.

[20] Jo, S. H., & Lu, W. (2008). CMOS Compatible Nanoscale Nonvolatile Resistance

[21] Jo, S. H., Kim, K. H., & Lu, W. (2009). Programmable Resistance Switching in Nano‐

[22] Jo, S. H., Kim, K. H., & Lu, W. (2009). High-Density Crossbar Arrays Based on a Si

[23] Kim, D. C., Lee, M. J., Ahn, S. E., Seo, S., Park, J. C., Yoo, I. K., Baek, I. G., Kim, H. J., Yim, E. K., Lee, J. E., Park, S. O., Kim, H. S., In, Chung. U., Moon, J. T., & Ryu, B. I. (2006). Improvement of resistive memory switching in NiO using IrO2 . *Applied Phys‐*

[24] Kim, K. M., Choi, B. J., Shin, Y. C., Choi, S., & Hwang, C. S. (2007). Anode-interface localized filamentary mechanism in resistive switching of TiO2 thin films. *Applied*

[25] Kim, S., Moon, H., Gupta, D., Yoo, S., & Choi, Y. K. (2009). Resistive Switching Char‐ acteristics of Sol-Gel Zinc Oxide Films for Flexible Memory Applications. *IEEE Trans‐*

[26] Kwon, D. H., Kim, K. M., Jang, J. H., Jeon, J. M., Lee, M. H., Kim, G. H., Li, X. S., Park, G. S., Lee, B., Han, S., Kim, M., & Hwang, C. S. (2010). Atomic structure of conduct‐ ing nanofilaments in TiO2 resistive switching memory. *Nature Nanotechnology*, 5(2),

[27] Lee, M. J., Park, Y., Suh, D. S., Lee, E. H., Seo, S., Kim, D. C., Jung, R., Kang, B. S., Ahn, S. E., Lee, C. B., Seo, D. H., Cha, Y. K., Yoo, I. K., Kim, J. S., & Park, B. H. (2007). Two Series Oxide Resistors Applicable to High Speed and High Density Nonvolatile

[28] Li, Y. T., Long, S. B., Liu, Q., Lv, H. B., Liu, S., & Liu, M. (2011). An overview of resis‐ tive random access memory devices. *Chinese Science Bulletin*, 56(28-29), 3072-3078,

[29] Liao, Z. L., Wang, Z. Z., Meng, Y., Liu, Z. Y., Gao, P., Gang, J. L., Zhao, H. W., Liang, X. J., Bai, X. D., & Chen, D. M. (2009). Categorization of resistive switching of metal-

Pr0.7Ca0.3MnO3-metal devices. *Applied Physics Letters*, 94(25), 253503, 0003-6951.

ing cells. *Nanotechnology*, 20(37), 375201, 0957-4484.

202 New Progress on Graphene Research

Switching Memory. *Nano Letters*, 8(2), 392-397, 1476-1122.

Memristive System. *Nano Letters*, 9(2), 870-874, 1476-1122.

*ics Letters*, 88(23), 232106, 0003-6951.

*Physics Letters*, 91(1), 012907, 0003-6951.

148-153, 1748-3387.

1001-6538.

*actions on Electron Devices*, 56(4), 696-699, 0018-9383.

Memory. *Advanced Materials*, 19(22), 3919-3923, 0935-9648.

scale Two-Terminal Devices. *Nano Letters*, 9(1), 496-500, 1476-1122.


[43] Rozenberg, M. J., Sanchez, M. J., Weht, R., Acha, C., Gomez-Marlasca, F., & Levy, P. (2010). Mechanism for bipolar resistive switching in transition-metal oxides. *Physical Review B*, 81(11), 115101, 1098-0121.

[56] Tsuruoka, T., Terabe, K., Hasegawa, T., Valov, I., Waser, R., & Aono, M. (2012). Ef‐ fects of moisture on the switching characteristics of oxide-based, gapless-type atomic

Advances in Resistive Switching Memories Based on Graphene Oxide

http://dx.doi.org/10.5772/51260

205

[57] Tulina, N. A., Zver'kov, S. A., Mukovskii, Y. M., & Shulyatev, D. A. (2001). Current switching of resistive states in normal-metal manganite single-crystal point contacts.

[58] Wang, L. H., Yang, W., Sun, Q. Q., Zhou, P., Lu, H. L., Ding, S. J., & Zhang, D. W. (2012). The mechanism of the asymmetric SET and RESET speed of graphene oxide based flexible resistive switching memories. *Applied Physics Letters*, 100(6), 063509,

[59] Wang, X. R., Ouyang, Y. J., Li, X. L., Wang, H. L., Guo, J., & Dai, H. J. (2008). Room-Temperature All-Semiconducting Sub-10-nm Graphene Nanoribbon Field-Effect

[60] Waser, R., & Aono, M. (2007). Nanoionics-based resistive switching memories. *Na‐*

[61] Waser, R., Dittmann, R., Staikov, G., & Szot, K. (2009). Redox-based resistive switch‐ ing memories-Nanoionic mechanisms, prospects, and challenges. *Advanced Materials*,

[62] Wu, X., Zhou, P., Li, J., Chen, L. Y., Lv, H. B., Lin, Y. Y., & Tang, T. A. (2007). Repro‐ ducible unipolar resistance switching in stoichiometric ZrO2 films. *Applied Physics*

[63] Wu, C. X., Li, F. S., Zhang, Y. A., Guo, T. L., & Chen, T. (2011). Highly reproducible memory effect of organic multilevel resistive-switch device utilizing graphene oxide sheets/polyimide hybrid nanocomposite. *Applied Physics Letters*, 99(4), 042108,

[64] Yang, C. H., Seidel, J., Kim, S. Y., Rossen, P. B., Yu, P., Gajek, M., Chu, Y. H., Martin, L. W., Holcomb, M. B., He, Q., Maksymovych, P., Balke, N., Kalinin, S. V., Baddorf, A. P., Basu, S. R., et al. (2009). Electric modulation of conduction in multiferroic Ca-

[65] Yang, Y. C., Pan, F., Liu, Q., Liu, M., & Zeng, F. (2009). Fully Room-Temperature-Fab‐ ricated Nonvolatile Resistive Memory for Ultrafast and High-Density Memory Ap‐

[66] Yin, K. B., Li, M., Liu, Y. W., He, C. L., Zhuge, F., Chen, B., Lu, W., Pan, X. Q., & Li, R. W. (2010). Resistance switching in polycrystalline BiFeO3 thin films. *Applied Physics*

[67] Yu, A. D., Liu, C. L., & Chen, W. C. (2012). Supramolecular block copolymers: gra‐ phene oxide composites for memory device applications. *Chemical Communications*,

doped BiFeO3 films. *Nature Materials*, 8(6), 485-493, 1476-1122.

plication. *Nano Letters*, 9(4), 1636-1643, 1476-1122.

switches. *Advanced Functional Materials*, 22(1), 70-77, 1616-301X.

Transistors. *Physical Review Letters*, 100(20), 206803, 0031-9007.

*Europhysics Letters*, 56(6), 836-841, 1286-4854.

*ture Materials*, 6(11), 833-840, 1476-1122.

21(25-26), 2632-2663, 1476-1122.

*Letters*, 90(18), 183507, 0003-6951.

*Letters*, 97(4), 042101, 0003-6951.

48(3), 383-385, 1359-7345.

0003-6951.

0003-6951.


[56] Tsuruoka, T., Terabe, K., Hasegawa, T., Valov, I., Waser, R., & Aono, M. (2012). Ef‐ fects of moisture on the switching characteristics of oxide-based, gapless-type atomic switches. *Advanced Functional Materials*, 22(1), 70-77, 1616-301X.

[43] Rozenberg, M. J., Sanchez, M. J., Weht, R., Acha, C., Gomez-Marlasca, F., & Levy, P. (2010). Mechanism for bipolar resistive switching in transition-metal oxides. *Physical*

[44] Rueckes, T., Kim, K., Joselevich, E., Tseng, G. Y., Cheung, C. L., & Lieber, C. M. (2000). Carbon nanotube-based nonvolatile random access memory for molecular

[45] Sawa, A., Fujii, T., Kawasaki, M., & Tokura, Y. (2004). Hysteretic current-voltage characteristics and resistance switching at a rectifying Ti/Pr0.7Ca0.3MnO3 interface. *Ap‐*

[46] Schroeder, H., Zhirnov, V. V., Cavin, R. K., & Waser, R. (2010). Voltage-time dilemma of pure electronic mechanisms in resistive switching memory cells. *Journal of Applied*

[47] Seo, S., Lee, M. J., Seo, D. H., Jeoung, E. J., Suh, D. S., Joung, Y. S., Yoo, I. K., Hwang, I. R., Kim, S. H., Byun, I. S., Kim, J. S., Choi, J. S., & Park, B. H. (2004). Reproducible resistance switching in polycrystalline NiO films. *Applied Physics Letters*, 85(23),

[48] Simmons, J. G., & Verderber, R. R. (1967). New conduction and reversible memory phenomena in thin insulating films. *Proceedings of the Royal Society of London Series A-*

[49] Sinitskii, A., & Tour, J. M. (2009). Lithographic graphitic memories. *ACS Nano*, 3(9),

[50] Son, J. Y., & Shin, Y. H. (2008). Direct observation of conducting filaments on resis‐ tive switching of NiO thin films. *Applied Physics Letters*, 92(22), 222106, 0003-6951.

[51] Standlety, B., Bao, W. Z., Zhang, H., Bruck, J., Lau, C. N., & Bockrath, M. (2008). Gra‐ phene-based atomic-scale switches. *Nano Letters*, 8(10), 3345-3349, 1476-1122.

[52] Stankovich, S., Dikin, D. A., Dommett, G. H. B., Kohlhaas, K. M., Zimney, E. J., Stach, E. A., Piner, R. D., Nguyen, S. T., & Ruoff, R. S. (2006). Graphene-based composite

[53] Stewart, D. R., Ohlberg, D. A. A., Beck, P. A., Chen, Y., Williams, R. S., Jeppesen, J. O., Nielsen, K. A., & Stoddart, J. F. (2004). Molecule-Independent Electrical Switching

[54] Szot, K., Speier, W., Bihlmayer, G., & Waser, R. (2006). Switching the electrical resist‐ ance of individual dislocations in single-crystalline SrTiO3 . *Nature Materials*, 5(4),

[55] Tsuruoka, T., Terabe, K., Hasegawa, T., & Aono, M. (2010). Forming and switching mechanisms of a cation-migration-based oxide resistive memory. *Nanotechnology*,

in Pt/Organic Monolayer/Ti Devices. *Nano Letters*, 4(1), 133-136, 1476-1122.

*Mathematical and Physical Sciences*, 301(1464), 77, 1364-5021.

materials. *Nature*, 442(7100), 282-286, 0028-0836.

*Review B*, 81(11), 115101, 1098-0121.

204 New Progress on Graphene Research

*Physics*, 107(5), 054517, 0021-4922.

5655-5657, 0003-6951.

2760-2766, 1936-0851.

312-320, 1476-1122.

21(42), 425205, 0957-4484.

computing. *Science*, 289(5476), 94-97, 0036-8075.

*plied Physics Letters*, 85(18), 4073-4075, 0003-6951.


[68] Zhuang, X. D., Chen, Y., Liu, G., Li, P. P., Zhu, C. X., Kang, E. T., Neoh, K. G., Zhang, B., Zhu, J. H., & Li, Y. X. (2010). Conjugated-polymer-functionalized graphene oxide: Synthesis and nonvolatile rewritable memory effect. *Advanced Materials*, 22(15), 1731-1735, 1476-1122.

**Chapter 8**

**Surface Functionalization of Graphene with Polymers**

Graphene, a single-atom-thick sheet of hexagonally arrayed sp2 bonded carbon atoms, has been under the spotlight owning to its intriguing and unparalleled physical properties [1]. Because of its novel properties, such as exceptional thermal conductivity, [2] high Young's modulus, [3] and high electrical conductivity,[4] graphene has been highlighted in fabricat‐ ing various micro-electrical devices, batteries, supercapacitors, and composites [5, 7]. Espe‐ cially, integration of graphene and its derivations into polymer has been highlighted, from the point views of both the spectacular improvement in mechanical, electrical properties, and the low cost of graphite [8, 9]. Control of the size, shape and surface chemistry of the reinforcement materials is essential in the development of materials that can be used to pro‐ duce devices, sensors and actuators based on the modulation of functional properties. The maximum improvements in final properties can be achieved when graphene is homogene‐ ously dispersed in the matrix and the external load is efficiently transferred through strong filler/polymer interfacial interactions, extensively reported in the case of other nanofillers. However, the large surface area of graphene and strong van der Waals force among them result in severe aggregation in the composites matrix. Furthermore, the carbon atoms on the graphene are chemically stable because of the aromatic nature of the bond. As a result, the reinforcing graphene are inert and can interact with the surrounding matrix mainly through van der Waals interactions, unable to provide an efficient load transfer across the graphene/ matrix interface. To obtain satisfied performance of the final graphene/polymer composites, the issues of the strong interfacial adhesion between graphene–matrix and well dispersion

To date, the mixing of graphene and functionalized graphene with polymers covers the most of the published studies, and the direct modification of graphene with polymers is a

> © 2013 Zheng et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2013 Zheng et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

distribution, and reproduction in any medium, provided the original work is properly cited.

**for Enhanced Properties**

http://dx.doi.org/10.5772/50490

of graphene should be addressed.

**1. Introduction**

Wenge Zheng, Bin Shen and Wentao Zhai

Additional information is available at the end of the chapter

