**5. Applications of graphene nanosheets in devices**

## **5.1. Hierarchical graphene structure for lithium ion batteries**

The hierarchical three-dimensional (3-D) structure for lithium ion battery (LIB) anodes has great potential for high electrochemical performances such as high-power densities and enhanced Coulomb's rates (C-rates). The efficient monolithic structure has a large specific surface area with numerous active sites, stable multistacking with short diffusion length, and high percolation threshold with high electrical conductivity as shown in **Figure 10(a)**. Unlike the porous-like graphene structuring on anode described in previous studies, monolithic graphene is similar to densely branched pine trees as shown in **Figure 10(b)**. The structure has high mechanical strength and flexibility, as well as high adhesion stability on the current collector. **Figure 10(c)** shows the growth mechanism for the structure by freeze-drying with water-soluble polymer [23]. The monolithic graphene anodes induced ultrafast charge/discharge rates with outstanding cycling stability with high capacitance as seen in **Figure 10(d)** and **(e)**. The fabrication method is simple and straightforward and suitable for high performance LIB anodes.

groups in GO is possible for the application of IR detectors. (Bae et al. [25]) Here, GO is synthesized using thermal chemical vapor deposition (TCVD) and its electrical and optical properties are characterized using low-temperature measurements and Raman spectroscopy. The electrical conductivity of GO-based devices under IR irradiation was subsequently measured. The electrical characterization process can be described as follows. Single layered graphene (SLG) is synthesized on a copper substrate using the CVD method. Following its synthesis,

is deposited on a silicon substrate patterned using oxygen plasma etching. Furthermore, the metal electrode is deposited on the patterned substrate using Au as shown in **Figure 11(b)**. To produce GO, we performed chemical treatments using immersion in an aqueous acid solution as shown in **Figure 11(c)**. Subsequently, GO is cleaned with water to remove the residual acid and dried in vacuum as shown in **Figure 11(d)**. It should be noted that, in order to control the oxygen functional groups with certain resistance, it is reduced by annealing at optimum temperature as shown in **Figure 11(e)**. The optical microscopy image of the GO device is shown in **Figure 11(f)**. The dotted area indicates the fabricated GO channel between the metal electrodes. Raman spectra illustrate the chemical doping effect by charge transfer between graphene and oxygen molecules. The spectra revealed significant changes in intensity ratio that can be described by the I(D)/I(G) and I(2D)/I(G) ratios. Moreover, the G band at 1590 cm−1 can be shifted due to the oxidation treatment. Four different samples: pristine CVD graphene, acid treated graphene, and annealed graphene after acid treatment are shown in **Figure 12(a)**. Initially, the I(D)/I(G) ratio increased after acid treatment compared to that of pristine graphene. This is caused by oxygen moieties generating intervalley scattering. After mild reduction by thermal treatment, the ratio decreases due to the partial elimination of oxygen functional groups. Thus, Raman spectra illustrate the doping effect of the oxygen functional groups. Moreover, charge transfer behavior presents as a p-type characteristic. The I(2D)/I(G) ratio is significantly decreased and the G band shows a red shift. Unlike graphene, GO has characteristic IR absorption peaks at

**Figure 11.** Fabrication schematics of the GO device: (a) transfer of CVD-grown graphene onto the Si/SiO<sup>2</sup>

(b) patterning of graphene and deposition of electrodes, (c) acid treatment, (d) rinsing and drying, (e) reduction of GO,

substrate as shown in **Figure 11(a)**. Subsequently, SLG

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substrate,

the graphene is transferred onto SiO2

and (f) optical image of GO device [25].

### **5.2. Fabrication of graphene oxide-based mid-IR detectors**

Graphene oxide can be defined as chemically functionalized graphene containing oxygen. Moreover, GO has a large bandgap, which implies an insulating behavior. The bandgap can be decreased with decreasing the oxidation level of GO. Thus, this is a promising method for bandgap tuning that can be applicable to various optical and electronic devices. The spectrum ranges of GO-based photodetectors are limited within the visible and near-infrared (IR) wavelengths because they are based on the photovoltaic effect. Therefore, a new mechanism for mid-IR detection using GO sensors is required. Significantly, the control of oxygen functional

**Figure 10.** (a) FE-SEM image for monolithic RGO structure (inset: Vertically aligned RGO structure on current collector), (b) high magnification view of monolithic RGO, (c) growth mechanism for the monolithic structure during the freezedrying process, (d) voltage profiles for monolithic-MGO (M-MGO) and 2-D RGO, (e) charge/discharge capacities for five cycles as the capacity rates increase from 0.2 to 100 C with respect to natural graphite and M-RGO, and (f) capacity retention for M-RGO (red line) and natural graphite (black line) [24].

groups in GO is possible for the application of IR detectors. (Bae et al. [25]) Here, GO is synthesized using thermal chemical vapor deposition (TCVD) and its electrical and optical properties are characterized using low-temperature measurements and Raman spectroscopy. The electrical conductivity of GO-based devices under IR irradiation was subsequently measured. The electrical characterization process can be described as follows. Single layered graphene (SLG) is synthesized on a copper substrate using the CVD method. Following its synthesis, the graphene is transferred onto SiO2 substrate as shown in **Figure 11(a)**. Subsequently, SLG is deposited on a silicon substrate patterned using oxygen plasma etching. Furthermore, the metal electrode is deposited on the patterned substrate using Au as shown in **Figure 11(b)**. To produce GO, we performed chemical treatments using immersion in an aqueous acid solution as shown in **Figure 11(c)**. Subsequently, GO is cleaned with water to remove the residual acid and dried in vacuum as shown in **Figure 11(d)**. It should be noted that, in order to control the oxygen functional groups with certain resistance, it is reduced by annealing at optimum temperature as shown in **Figure 11(e)**. The optical microscopy image of the GO device is shown in **Figure 11(f)**. The dotted area indicates the fabricated GO channel between the metal electrodes.

**5. Applications of graphene nanosheets in devices**

138 Graphene Oxide - Applications and Opportunities

**5.1. Hierarchical graphene structure for lithium ion batteries**

**5.2. Fabrication of graphene oxide-based mid-IR detectors**

retention for M-RGO (red line) and natural graphite (black line) [24].

The hierarchical three-dimensional (3-D) structure for lithium ion battery (LIB) anodes has great potential for high electrochemical performances such as high-power densities and enhanced Coulomb's rates (C-rates). The efficient monolithic structure has a large specific surface area with numerous active sites, stable multistacking with short diffusion length, and high percolation threshold with high electrical conductivity as shown in **Figure 10(a)**. Unlike the porous-like graphene structuring on anode described in previous studies, monolithic graphene is similar to densely branched pine trees as shown in **Figure 10(b)**. The structure has high mechanical strength and flexibility, as well as high adhesion stability on the current collector. **Figure 10(c)** shows the growth mechanism for the structure by freeze-drying with water-soluble polymer [23]. The monolithic graphene anodes induced ultrafast charge/discharge rates with outstanding cycling stability with high capacitance as seen in **Figure 10(d)** and **(e)**. The fabrication

method is simple and straightforward and suitable for high performance LIB anodes.

Graphene oxide can be defined as chemically functionalized graphene containing oxygen. Moreover, GO has a large bandgap, which implies an insulating behavior. The bandgap can be decreased with decreasing the oxidation level of GO. Thus, this is a promising method for bandgap tuning that can be applicable to various optical and electronic devices. The spectrum ranges of GO-based photodetectors are limited within the visible and near-infrared (IR) wavelengths because they are based on the photovoltaic effect. Therefore, a new mechanism for mid-IR detection using GO sensors is required. Significantly, the control of oxygen functional

**Figure 10.** (a) FE-SEM image for monolithic RGO structure (inset: Vertically aligned RGO structure on current collector), (b) high magnification view of monolithic RGO, (c) growth mechanism for the monolithic structure during the freezedrying process, (d) voltage profiles for monolithic-MGO (M-MGO) and 2-D RGO, (e) charge/discharge capacities for five cycles as the capacity rates increase from 0.2 to 100 C with respect to natural graphite and M-RGO, and (f) capacity Raman spectra illustrate the chemical doping effect by charge transfer between graphene and oxygen molecules. The spectra revealed significant changes in intensity ratio that can be described by the I(D)/I(G) and I(2D)/I(G) ratios. Moreover, the G band at 1590 cm−1 can be shifted due to the oxidation treatment. Four different samples: pristine CVD graphene, acid treated graphene, and annealed graphene after acid treatment are shown in **Figure 12(a)**. Initially, the I(D)/I(G) ratio increased after acid treatment compared to that of pristine graphene. This is caused by oxygen moieties generating intervalley scattering. After mild reduction by thermal treatment, the ratio decreases due to the partial elimination of oxygen functional groups. Thus, Raman spectra illustrate the doping effect of the oxygen functional groups. Moreover, charge transfer behavior presents as a p-type characteristic. The I(2D)/I(G) ratio is significantly decreased and the G band shows a red shift. Unlike graphene, GO has characteristic IR absorption peaks at

**Figure 11.** Fabrication schematics of the GO device: (a) transfer of CVD-grown graphene onto the Si/SiO<sup>2</sup> substrate, (b) patterning of graphene and deposition of electrodes, (c) acid treatment, (d) rinsing and drying, (e) reduction of GO, and (f) optical image of GO device [25].

is the conductance and T is the temperature. Consequently, under IR irradiation, NNH is more dominant than variable range hoping (VRH) in the electron transport of GO, which is usually observed at a higher temperature range in a disordered system, jumping electrons to other defect sites due to thermal activation. Therefore, the different nature of the electron transport induced by IR irradiation contributes to the different temperature dependence exhibited by *G*. The activation energy *W*, extracted from the slope of the plot in **Figure 13(c)** is approximately 164 meV. The energy is easily obtained by the incident IR. The specific NNH transport phenomena is confirmed by plotting ln(*W*) as a function of ln(*T*) as shown in **Figure 13(d)**. The temperature exponent is −0.97, indicating thermal transport in GO under IR irradiation. The plot exhibits a negative slope between 250 and 350 K. These results demonstrate that the conducting mechanism can be attributed to VRH. The shift from VRH to NNH transport occurs

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Controlling the structure of graphene emitters such as the aspect ratio, density, and alignment, which are of practical importance for applications in field emission devices, is not readily achievable. Jeong et al. introduced a simple method for fabricating tubular structured graphene arrays with controlled tube lengths and alignment as shown in **Figure 14**. They used the filtration of RGO suspensions with a polycarbonate membrane. The interactions between hydrophobic graphene and the pore walls, but not the top surfaces, of a polycarbonate membrane were tuned, and the filtration rate was varied to control the length and alignment of the graphene arrays. They observed that the lengths of the graphene arrays increased with increasing filtration rate, but a maximum field emission efficiency was reached for intermediate filtration rates due to field screening for array lengths longer than an optimum value. The turn-on field and field enhancement factor for an optimum length of graphene arrays were

Another useful approach for fabricating graphene field emitters is a thermal welding-peeling method as shown in **Figure 15**. The graphene film was formed on a polytetrafluoroethylene membrane by filtering the dispersed RGO solutions. The CNT/polyethersulfone (PES)

**Figure 14.** (a) Schematic and (b) SEM image of RGO emitters fabricated by filtration-transfer method. (c) J-E curves of

due to the increase in the temperature of GO due to the incident IR irradiation.

**5.3. Graphene emitters**

1.89 V/μm and 4624, respectively.

RGO emitters as a function of filtration rate [27].

**Figure 12.** (a) Raman spectra of graphene, GO, and RGO and (b) IR absorption spectra of graphene and GO [25].

750–2250 cm−1, as shown in **Figure 12(b)**. The Fourier transform infrared (FTIR) spectra of GO reveal oxygen functional groups from 960 to 1860 cm−1, without graphene [26].

The optical properties of GO show in the mid-IR range at 7–14 μm. Depending on the functionality of GO by control of mild reduction, it reveals larger resistance changes as temperature. When the IR source is turned on, the resistance of the GO immediately drops from 31 to 7.4 MΩ, as shown in **Figure 13(a)**. This is a promising approach for obtaining sensitive mid-IR sensors by controlling GO functionalities. To describe the response of GO as a function of IR irradiation, the curve is fitted using nearest neighbor hopping (NNH) model as shown in **Figure 13(b)** and **(c)**. **Figure 13(c)** reveals a linear fit of ln(*G*)-*T−1* in the 200–350 K temperature range, where G

**Figure 13.** Electrical transport properties of GO under IR irradiation: (a) change in GO resistance, (b) GO resistancetemperature plot between 50 and 350 K, (c) temperature dependence of GO electrical conductance plotted for the NNH mechanism between 260 and 350 K, (d) ln(*W*) vs. ln(*T*) plot for GO [25].

is the conductance and T is the temperature. Consequently, under IR irradiation, NNH is more dominant than variable range hoping (VRH) in the electron transport of GO, which is usually observed at a higher temperature range in a disordered system, jumping electrons to other defect sites due to thermal activation. Therefore, the different nature of the electron transport induced by IR irradiation contributes to the different temperature dependence exhibited by *G*. The activation energy *W*, extracted from the slope of the plot in **Figure 13(c)** is approximately 164 meV. The energy is easily obtained by the incident IR. The specific NNH transport phenomena is confirmed by plotting ln(*W*) as a function of ln(*T*) as shown in **Figure 13(d)**. The temperature exponent is −0.97, indicating thermal transport in GO under IR irradiation. The plot exhibits a negative slope between 250 and 350 K. These results demonstrate that the conducting mechanism can be attributed to VRH. The shift from VRH to NNH transport occurs due to the increase in the temperature of GO due to the incident IR irradiation.

## **5.3. Graphene emitters**

750–2250 cm−1, as shown in **Figure 12(b)**. The Fourier transform infrared (FTIR) spectra of GO

**Figure 12.** (a) Raman spectra of graphene, GO, and RGO and (b) IR absorption spectra of graphene and GO [25].

The optical properties of GO show in the mid-IR range at 7–14 μm. Depending on the functionality of GO by control of mild reduction, it reveals larger resistance changes as temperature. When the IR source is turned on, the resistance of the GO immediately drops from 31 to 7.4 MΩ, as shown in **Figure 13(a)**. This is a promising approach for obtaining sensitive mid-IR sensors by controlling GO functionalities. To describe the response of GO as a function of IR irradiation, the curve is fitted using nearest neighbor hopping (NNH) model as shown in **Figure 13(b)** and **(c)**. **Figure 13(c)** reveals a linear fit of ln(*G*)-*T−1* in the 200–350 K temperature range, where G

**Figure 13.** Electrical transport properties of GO under IR irradiation: (a) change in GO resistance, (b) GO resistancetemperature plot between 50 and 350 K, (c) temperature dependence of GO electrical conductance plotted for the NNH

mechanism between 260 and 350 K, (d) ln(*W*) vs. ln(*T*) plot for GO [25].

reveal oxygen functional groups from 960 to 1860 cm−1, without graphene [26].

140 Graphene Oxide - Applications and Opportunities

Controlling the structure of graphene emitters such as the aspect ratio, density, and alignment, which are of practical importance for applications in field emission devices, is not readily achievable. Jeong et al. introduced a simple method for fabricating tubular structured graphene arrays with controlled tube lengths and alignment as shown in **Figure 14**. They used the filtration of RGO suspensions with a polycarbonate membrane. The interactions between hydrophobic graphene and the pore walls, but not the top surfaces, of a polycarbonate membrane were tuned, and the filtration rate was varied to control the length and alignment of the graphene arrays. They observed that the lengths of the graphene arrays increased with increasing filtration rate, but a maximum field emission efficiency was reached for intermediate filtration rates due to field screening for array lengths longer than an optimum value. The turn-on field and field enhancement factor for an optimum length of graphene arrays were 1.89 V/μm and 4624, respectively.

Another useful approach for fabricating graphene field emitters is a thermal welding-peeling method as shown in **Figure 15**. The graphene film was formed on a polytetrafluoroethylene membrane by filtering the dispersed RGO solutions. The CNT/polyethersulfone (PES)

**Figure 14.** (a) Schematic and (b) SEM image of RGO emitters fabricated by filtration-transfer method. (c) J-E curves of RGO emitters as a function of filtration rate [27].

**Figure 15.** (a) Schematic diagram of RGO emitters fabricated using a thermal welding-peeling technique. SEM image of (b) RGO sheets and (c) fabricated RGO emitters [28].

substrate used as cathode was then coated with an adhesive polycarbonate layer to prepare an upper graphene thin film under pressed thermal treatment of the sample. Vertically aligned graphene emitters were finally constructed by peeling off the polytetrafluoroethylene membrane from the welded sample. The average height and interspace of graphene emitters were 1.2 and 0.8 μm, respectively, which could be controlled by experimental conditions such as the size of the GO sheets and peeling force of the membrane. A large field enhancement factor for the RGO emitters was achieved by optimum vertical alignment of GO nanosheets, resulting in a high emission current density and a low turn-on field.

Freeze-drying of highly concentrated water-based RGO/polymer paste is one of the fabricating methods for 3-D graphene emitters with random micropores. Kim et al. first reported 3-D monolithic graphene structures as shown in **Figure 17**. They used highly concentrated water-based RGO paste prepared using monovalent cation-π interactions. After bar coating the paste on substrate, freezing was performed by immersing the sample in a liquid nitrogen bath. Low temperature freezing using liquid nitrogen resulted in the rapid formation of ice nuclei, hence the growth of relatively small ice crystals. After the sublimation of the ice crystals by vacuum drying, monolithic 3-D graphene structures with cylindrical pores could be obtained. Although the randomly distributed pores of the 3-D graphene structure were

**Figure 17.** (a) Photo image of a water-based RGO paste. Inset shows the SEM image of the RGO sheets. The scale bar is 5 μm. (b) Fabrication method of the monolithic 3-D graphene structure using bar coating of the RGO paste, followed by subsequent freeze-drying. (c) a 3-D structured graphene fabricated on a plastic substrate. (d) Cross-sectional and (e) top view SEM images of a 3-D graphene structure. The scale bars are 300 μm. (f) J–E and (g) F-N plots of the 3-D graphene

**Figure 16.** (a) Schematic diagram of the graphene arrays obtained using the 'breath figure" method, and pattern structures obtained at (b) and (d) low and (c) and (e) high solution viscosities, respectively. (f) J-E and (g) F-N plots of

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graphene arrays fabricated using various GO concentrations [29].

emitters as a function of graphene concentrations [30].

The "breath figure" technique is a simple and versatile self-assembly method for fabricating porous nanomaterial patterns with high regularity. Although highly ordered 3-D graphene assemblies with high porosity have been fabricated, their use for field emitters was not readily achieved because the flat or smooth surface structures of the graphene assemblies did not emit electrons. Vertically aligned graphene ordered structures for an efficient field emitter was first fabricated using the "breath figure" method as shown in **Figure 16**. Octadecylamine (ODA)-functionalized GO solution dispersed in toluene was uniformly coated onto a substrate by spin coating. The ODA-functionalized GO was self-assembled under high humidity conditions and presented high periodicity due to the surface energy difference between the water droplet and toluene. Moreover, the ODA-functionalized GO nanosheets tended to encapsulate water droplets and precipitate at the water–solution interface, thereby preventing coalescence of the water droplets. The patterned ODA-functionalized GO array structure was obtained after the complete evaporation of the toluene and water droplets. The structure of GO array, the size, shape, and homogeneity, was dependent on the viscosity of the ODAfunctionalized GO solution, large hexagonally structured GO patterns were fabricated with solutions having a low viscosity. The vertically aligned tip structures were formed at the interfaces between pores. However, with highly viscous solutions, small spherical patterns generated without tip formation. The graphene array prepared using a 2.0 g/L GO solution displayed the lowest turn-on field of 2.04 V/μm of all the arrays prepared using various GO concentrations.

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**Figure 16.** (a) Schematic diagram of the graphene arrays obtained using the 'breath figure" method, and pattern structures obtained at (b) and (d) low and (c) and (e) high solution viscosities, respectively. (f) J-E and (g) F-N plots of graphene arrays fabricated using various GO concentrations [29].

substrate used as cathode was then coated with an adhesive polycarbonate layer to prepare an upper graphene thin film under pressed thermal treatment of the sample. Vertically aligned graphene emitters were finally constructed by peeling off the polytetrafluoroethylene membrane from the welded sample. The average height and interspace of graphene emitters were 1.2 and 0.8 μm, respectively, which could be controlled by experimental conditions such as the size of the GO sheets and peeling force of the membrane. A large field enhancement factor for the RGO emitters was achieved by optimum vertical alignment of GO nanosheets, result-

**Figure 15.** (a) Schematic diagram of RGO emitters fabricated using a thermal welding-peeling technique. SEM image of

The "breath figure" technique is a simple and versatile self-assembly method for fabricating porous nanomaterial patterns with high regularity. Although highly ordered 3-D graphene assemblies with high porosity have been fabricated, their use for field emitters was not readily achieved because the flat or smooth surface structures of the graphene assemblies did not emit electrons. Vertically aligned graphene ordered structures for an efficient field emitter was first fabricated using the "breath figure" method as shown in **Figure 16**. Octadecylamine (ODA)-functionalized GO solution dispersed in toluene was uniformly coated onto a substrate by spin coating. The ODA-functionalized GO was self-assembled under high humidity conditions and presented high periodicity due to the surface energy difference between the water droplet and toluene. Moreover, the ODA-functionalized GO nanosheets tended to encapsulate water droplets and precipitate at the water–solution interface, thereby preventing coalescence of the water droplets. The patterned ODA-functionalized GO array structure was obtained after the complete evaporation of the toluene and water droplets. The structure of GO array, the size, shape, and homogeneity, was dependent on the viscosity of the ODAfunctionalized GO solution, large hexagonally structured GO patterns were fabricated with solutions having a low viscosity. The vertically aligned tip structures were formed at the interfaces between pores. However, with highly viscous solutions, small spherical patterns generated without tip formation. The graphene array prepared using a 2.0 g/L GO solution displayed the lowest turn-on field of 2.04 V/μm of all the arrays prepared using various GO

ing in a high emission current density and a low turn-on field.

(b) RGO sheets and (c) fabricated RGO emitters [28].

142 Graphene Oxide - Applications and Opportunities

concentrations.

Freeze-drying of highly concentrated water-based RGO/polymer paste is one of the fabricating methods for 3-D graphene emitters with random micropores. Kim et al. first reported 3-D monolithic graphene structures as shown in **Figure 17**. They used highly concentrated water-based RGO paste prepared using monovalent cation-π interactions. After bar coating the paste on substrate, freezing was performed by immersing the sample in a liquid nitrogen bath. Low temperature freezing using liquid nitrogen resulted in the rapid formation of ice nuclei, hence the growth of relatively small ice crystals. After the sublimation of the ice crystals by vacuum drying, monolithic 3-D graphene structures with cylindrical pores could be obtained. Although the randomly distributed pores of the 3-D graphene structure were

**Figure 17.** (a) Photo image of a water-based RGO paste. Inset shows the SEM image of the RGO sheets. The scale bar is 5 μm. (b) Fabrication method of the monolithic 3-D graphene structure using bar coating of the RGO paste, followed by subsequent freeze-drying. (c) a 3-D structured graphene fabricated on a plastic substrate. (d) Cross-sectional and (e) top view SEM images of a 3-D graphene structure. The scale bars are 300 μm. (f) J–E and (g) F-N plots of the 3-D graphene emitters as a function of graphene concentrations [30].

several tens of micrometers in size, the pore walls had numerous sharp edges. The size, shape, and homogeneity of these pores could be adjusted by choosing different freezing temperatures, solution concentrations, and solvents.

getters, inner walls, and phosphors can destruct electron emitters, resulting in a critical reduction of the emitter characteristics. To enhance the current stability, Jeong et al. introduced the ZnO sol coating as a protective layer for the RGO emitters as shown in **Figure 19** [27]. Zinc oxide is an n-type semiconductor with a wide band gap and a low resistivity in the order of 10−2 to 10−3 Ω cm. The ZnO layer on the graphene surface was realized by hydrogen bonding between the amine groups of the ZnO sol and carboxyl groups of RGO and subsequent thermal treatment. A life time test showed stable emission for the ZnO-coated graphene emitters, which might be due to the ZnO protection of the emission site from reactive ion bombardment. Since the development of graphene-based thin film fabrication techniques on polymeric substrates, research into graphene-based flexible electrodes for display application has advanced. In addition to the high electrical and mechanical flexibility of graphene-based thin films, the interface between an electrode and an emitter material should be strong and provide ohmic contact to achieve highly flexible field emitters. Thus, SWNT-coated polymer substrates have been used as electrodes [31]. Moreover, RGO emitters were fabricated on SWNT-coated PET substrates. A

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**Figure 19.** (a) Illustration of formation of a ZnO protective layer on the RGO surface. (b) SEM images and (c) EDAXS plot of the RGO emitters modified with a ZnO layer. Scale bars in (b) are 5 μm. (d) Current stability of the RGO array emitters

**Figure 20.** (a) J–E and (b) F–N plots of the RGO emitters as a function of the bending angle. The insets show schematic diagrams of the flexible field emission setup and an emission pattern at a 30° bending angle, respectively [26].

[27].

wit and without a ZnO layer in vacuum and after exposure to O2

The tunneling barrier at the interface between a material surface and vacuum can be modulated by varying the physical properties of an emitter material. In this context, the field-emission characteristics are critically dependent on the work function of an emitter material. Low work functions decrease the barrier height causing the enhancement of electron tunneling for a given applied electric field, which results in high field-emission characteristics. Chemical doping can be a useful approach for modulating the work function of graphene because the intrinsic Fermi level of graphene can be readily shifted, due to charge transfer between the dopant and graphene. Jeong et al. reported the work-function engineering of graphene field emitters using chemical doping. Gold chloride as a p-type dopant and aluminum chloride powder as an n-type dopant were dissolved in distilled water and mixed with a GO solution. After centrifugation, the solution was filtered and the doped graphene films were dried. The schematic diagram in **Figure 18** shows the charge transfer from graphene to the gold and aluminum ions and the corresponding band diagrams. The charge transfer upon chemical doping was confirmed using various techniques such as Raman, X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). Due to decreasing the work-function of graphene, the Al-doped graphene emitters showed lower turn-on field than those of undoped and Au-doped graphene emitters. A similar study was conducted by mixing dopant solution with graphene paste.

Long-term emission stability of electron emitters is needed for thigh quality field emission devices. The ion bombardment of residual gas species which are degassed from cathodes,

**Figure 18.** (a) Schematic diagram showing the electron transfer from graphene to the Au ion and from Al to graphene and corresponding band diagrams under application of a certain external voltage. (b) Raman, (c) XPS, and (d) UPS spectra for the undoped, Au-doped, and Al-doped graphene emitters [28].

getters, inner walls, and phosphors can destruct electron emitters, resulting in a critical reduction of the emitter characteristics. To enhance the current stability, Jeong et al. introduced the ZnO sol coating as a protective layer for the RGO emitters as shown in **Figure 19** [27]. Zinc oxide is an n-type semiconductor with a wide band gap and a low resistivity in the order of 10−2 to 10−3 Ω cm. The ZnO layer on the graphene surface was realized by hydrogen bonding between the amine groups of the ZnO sol and carboxyl groups of RGO and subsequent thermal treatment. A life time test showed stable emission for the ZnO-coated graphene emitters, which might be due to the ZnO protection of the emission site from reactive ion bombardment.

Since the development of graphene-based thin film fabrication techniques on polymeric substrates, research into graphene-based flexible electrodes for display application has advanced. In addition to the high electrical and mechanical flexibility of graphene-based thin films, the interface between an electrode and an emitter material should be strong and provide ohmic contact to achieve highly flexible field emitters. Thus, SWNT-coated polymer substrates have been used as electrodes [31]. Moreover, RGO emitters were fabricated on SWNT-coated PET substrates. A

**Figure 19.** (a) Illustration of formation of a ZnO protective layer on the RGO surface. (b) SEM images and (c) EDAXS plot of the RGO emitters modified with a ZnO layer. Scale bars in (b) are 5 μm. (d) Current stability of the RGO array emitters wit and without a ZnO layer in vacuum and after exposure to O2 [27].

**Figure 20.** (a) J–E and (b) F–N plots of the RGO emitters as a function of the bending angle. The insets show schematic diagrams of the flexible field emission setup and an emission pattern at a 30° bending angle, respectively [26].

**Figure 18.** (a) Schematic diagram showing the electron transfer from graphene to the Au ion and from Al to graphene and corresponding band diagrams under application of a certain external voltage. (b) Raman, (c) XPS, and (d) UPS

several tens of micrometers in size, the pore walls had numerous sharp edges. The size, shape, and homogeneity of these pores could be adjusted by choosing different freezing tempera-

The tunneling barrier at the interface between a material surface and vacuum can be modulated by varying the physical properties of an emitter material. In this context, the field-emission characteristics are critically dependent on the work function of an emitter material. Low work functions decrease the barrier height causing the enhancement of electron tunneling for a given applied electric field, which results in high field-emission characteristics. Chemical doping can be a useful approach for modulating the work function of graphene because the intrinsic Fermi level of graphene can be readily shifted, due to charge transfer between the dopant and graphene. Jeong et al. reported the work-function engineering of graphene field emitters using chemical doping. Gold chloride as a p-type dopant and aluminum chloride powder as an n-type dopant were dissolved in distilled water and mixed with a GO solution. After centrifugation, the solution was filtered and the doped graphene films were dried. The schematic diagram in **Figure 18** shows the charge transfer from graphene to the gold and aluminum ions and the corresponding band diagrams. The charge transfer upon chemical doping was confirmed using various techniques such as Raman, X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). Due to decreasing the work-function of graphene, the Al-doped graphene emitters showed lower turn-on field than those of undoped and Au-doped graphene emitters. A

similar study was conducted by mixing dopant solution with graphene paste.

Long-term emission stability of electron emitters is needed for thigh quality field emission devices. The ion bombardment of residual gas species which are degassed from cathodes,

tures, solution concentrations, and solvents.

144 Graphene Oxide - Applications and Opportunities

spectra for the undoped, Au-doped, and Al-doped graphene emitters [28].

PET film was used as the spacer and a white phosphor-coated SWNT layer on a PET substrate was used as the anode as shown in **Figure 20**. Strong π-π carbon bonds allowed the RGO arrays to form strong mechanical contact with the SWNT network. The emission current density of the RGO emitters did not decrease much with increasing the bending angle. This stable emission is likely due to the strong adhesion of the 3-D RGO emitters to the SWNT-coated PET substrate.

**References**

10.1063/1.3028339

10.1126/science.1216744

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