**4. Application of direct-grown graphene on flexible electronics**

A wide range of functional devices (transistors, solar cells, sensors, resistors, diffusion barriers, heat-resistant devices, photocatalytic plates and energy-saving smart windows) of graphene directly grown on various dielectric substrates using different growth methods, catalysts and device performances to date have been introduced, as briefly classified in **Table 1**.

#### **4.1. Transistors (FETs)**

The transfer-free growth of graphene will provide a new way to fabricate the FET-based electronic applications of graphene simply and inexpensively, while avoiding the transfer process. In general, FETs based on transfer-free direct graphene growth on various substrates (PDMS, sapphire, quartz, SiO<sup>2</sup> and h-BN), have been investigated thoroughly in previous studies [1, 6, 52, 59, 60, 62, 78, 82, 86, 88, 90, 92, 95, 97]. In particular, direct-grown graphene Direct Growth of Graphene on Flexible Substrates toward Flexible Electronics: A Promising… http://dx.doi.org/10.5772/intechopen.73171 79


**Table 1.** A brief classification of direct-grown graphene on various flexible substrates and their applications to date.

on flexible PDMS-based FET device (**Figure 6**) will be a promising potential in future flexible electronics [78].

#### **4.2. Strain sensor**

mica substrate. The uniform and high-quality graphene films directly integrated with lowcost used flexible mica glass will unlock a promising perspective in fabrication of multi-

**Figure 5.** (a) Diagram of direct-growth of graphene onto flexible mica glass substrate: (b) PECVD system utilized in this graphene growth with a single-zone electrical chamber (left) and RF plasma source (right). (a, b) reproduced with

A wide range of functional devices (transistors, solar cells, sensors, resistors, diffusion barriers, heat-resistant devices, photocatalytic plates and energy-saving smart windows) of graphene directly grown on various dielectric substrates using different growth methods, catalysts and

The transfer-free growth of graphene will provide a new way to fabricate the FET-based electronic applications of graphene simply and inexpensively, while avoiding the transfer process. In general, FETs based on transfer-free direct graphene growth on various substrates

studies [1, 6, 52, 59, 60, 62, 78, 82, 86, 88, 90, 92, 95, 97]. In particular, direct-grown graphene

and h-BN), have been investigated thoroughly in previous

functional electrodes in solar cell, smart window, and transparent electronic.

permission from [100], copyright 2015, Springer and Tsinghua University Press.

**4. Application of direct-grown graphene on flexible electronics**

device performances to date have been introduced, as briefly classified in **Table 1**.

**4.1. Transistors (FETs)**

78 Flexible Electronics

(PDMS, sapphire, quartz, SiO<sup>2</sup>

Graphene can be used as 3D structured electrodes in multifunctional devices, such as pressure sensors [59], black silicon solar cells [66], cambered micro-optics [67], and MEMS sensors [68]. In general, graphene grown on catalytic metal substrate is almost impossible to be

**Figure 6.** (a) Schematic of flexible PDMS-based FET device. (b) I-V curves of this FET device (black: Forward bias, blue: Reverse bias). Inset: Optical image of patterned source/drain graphene electrodes on a-PDMS. Scale bar: 1 mm. (c) Output characteristics of this FET device (channel length: 100 μm). Inset: Output characteristics at low voltage. (a-c) Reproduced with permission from [88], copyright 2017, IOP Publishing.

conformally transferred onto the 3D structural surface without mechanical damages [104]. Therefore, the direct growth of graphene on the 3D structured device surface can be a potential way to solve the limitations and problems above.

In 2012, by using CVD growth at low-temperature (300°C) for graphene films on a dielectric PI flexible substrate, Kim et al. successfully fabricated a graphene-based strain sensor on a PI substrate, and demonstrated the resistance modulation at different strains [77]. The resistance of graphene films showed a gradually increasing tensile strain of ~0.8% for 340 s (**Figure 7a**). Particularly, it linearly increased in the range of 31.64–31.69 MΩ with the applied strains of 0.1–0.8% (**Figure 7b**).

#### **4.3. Strain pattern**

To demonstrate the potential for applications of Ni-catalyzed direct-grown graphene on Willow flexible glass, Marchena et al. successfully produced ribbons and a square pattern of graphene (**Figure 8**) [98]. In particular, growing graphene directly on an ultrathin flexible Willow glass has huge potential in future flexible electronics.

> Recently, Sun et al. fabricated and designed a transparent circuit based on PECVD directgrown graphene on flexible mica glass sheet (2 × 8 cm) by utilizing photolithography, as shown in **Figure 9a** [100]. The results showed the patterned graphene electrode on a flexible mica glass can lighten up a green light-emitting diode (LED) indicator (**Figure 9b**). They also

> **Figure 9.** (a) Photograph of transparent circuit based on PECVD direct-grown graphene/flexible mica glass. The inset shows OM image of this device. (b) Photograph of patterned PECVD graphene on white glass showing the transparent conductivity to lighten up a green LED device. (c) Resistance with various bending values of direct-grown graphene film on mica glass. The inset shows the bending test. Reproduced with permission from [100], copyright 2015, Springer and

Tsinghua University Press.

**Figure 8.** Sequences of direct-grown graphene pattern fabrication on willow flexible glass using Ni catalyst by UV

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81

lithography. Reproduced with permission from [98], copyright 2016, OSA Publishing.

#### **4.4. Transparent circuit for green LED device**

The directly grown graphene/glass sample, for the use in a range of transparent conductive applications (like transparent circuit) in industry, requires uniformity, low-cost, flexibility and good quality graphene on transparent flexible substrates (Corning Willow glass, or mica). It is currently in high demand to explore the novel functions of circuits on flexible glass, which may have application potentials in optoelectronics, gas/moisture/bio sensors etc. For instance, the resistances of the pattern graphene within circuit devices, would show a noticeable change without the harmful wet transfer graphene process applied directly on devices. In addition, regarding the potential for integration into flexible electronic devices, mechanical durability of the directly grown graphene is an important factor. To the best of our knowledge, such properties have not been studied so far.

**Figure 7.** (a) Resistance changes of flexible strain sensor assisted direct-grown graphene on PI flexible substrate at various treatment time. The inset is strain-applied sensor. (b) Resistance changes of this device under applied strains. (a, b) reproduced with permission from [77], copyright 2012, IOP Publishing.

Direct Growth of Graphene on Flexible Substrates toward Flexible Electronics: A Promising… http://dx.doi.org/10.5772/intechopen.73171 81

conformally transferred onto the 3D structural surface without mechanical damages [104]. Therefore, the direct growth of graphene on the 3D structured device surface can be a poten-

In 2012, by using CVD growth at low-temperature (300°C) for graphene films on a dielectric PI flexible substrate, Kim et al. successfully fabricated a graphene-based strain sensor on a PI substrate, and demonstrated the resistance modulation at different strains [77]. The resistance of graphene films showed a gradually increasing tensile strain of ~0.8% for 340 s (**Figure 7a**). Particularly, it linearly increased in the range of 31.64–31.69 MΩ with the applied

To demonstrate the potential for applications of Ni-catalyzed direct-grown graphene on Willow flexible glass, Marchena et al. successfully produced ribbons and a square pattern of graphene (**Figure 8**) [98]. In particular, growing graphene directly on an ultrathin flexible

The directly grown graphene/glass sample, for the use in a range of transparent conductive applications (like transparent circuit) in industry, requires uniformity, low-cost, flexibility and good quality graphene on transparent flexible substrates (Corning Willow glass, or mica). It is currently in high demand to explore the novel functions of circuits on flexible glass, which may have application potentials in optoelectronics, gas/moisture/bio sensors etc. For instance, the resistances of the pattern graphene within circuit devices, would show a noticeable change without the harmful wet transfer graphene process applied directly on devices. In addition, regarding the potential for integration into flexible electronic devices, mechanical durability of the directly grown graphene is an important factor. To the best of our knowl-

**Figure 7.** (a) Resistance changes of flexible strain sensor assisted direct-grown graphene on PI flexible substrate at various treatment time. The inset is strain-applied sensor. (b) Resistance changes of this device under applied strains. (a,

tial way to solve the limitations and problems above.

Willow glass has huge potential in future flexible electronics.

**4.4. Transparent circuit for green LED device**

edge, such properties have not been studied so far.

b) reproduced with permission from [77], copyright 2012, IOP Publishing.

strains of 0.1–0.8% (**Figure 7b**).

**4.3. Strain pattern**

80 Flexible Electronics

**Figure 8.** Sequences of direct-grown graphene pattern fabrication on willow flexible glass using Ni catalyst by UV lithography. Reproduced with permission from [98], copyright 2016, OSA Publishing.

Recently, Sun et al. fabricated and designed a transparent circuit based on PECVD directgrown graphene on flexible mica glass sheet (2 × 8 cm) by utilizing photolithography, as shown in **Figure 9a** [100]. The results showed the patterned graphene electrode on a flexible mica glass can lighten up a green light-emitting diode (LED) indicator (**Figure 9b**). They also

**Figure 9.** (a) Photograph of transparent circuit based on PECVD direct-grown graphene/flexible mica glass. The inset shows OM image of this device. (b) Photograph of patterned PECVD graphene on white glass showing the transparent conductivity to lighten up a green LED device. (c) Resistance with various bending values of direct-grown graphene film on mica glass. The inset shows the bending test. Reproduced with permission from [100], copyright 2015, Springer and Tsinghua University Press.

synthesized graphene directly on flexible mica glass and measured the change of the resistance through bending tests, with a bending variation of ~45%, and the full recovery after bending indicates good mechanical stability and flexibility of the graphene electrode (**Figure 9c**).

possible strategies are proposed: (i) to explore a more efficient way to reduce the reaction barrier for graphene direct growth; (ii) to obtain epitaxial, direct-grown well-aligned graphene domains and seamlessly stitch them together into a complete single-crystal film. One potential way to realize strategy (i) is probably to introduce a gas phase catalyst enhancing the catalytic conversion of carbon precursors to graphene layer. In fact, strategy (ii) has been implemented for conventional indirect-grown graphene on catalytic single-crystal substrates such as Ge (110) [5], or Cu (111) [109, 110]. However, the single-crystal graphene layer has limited size due to the limited singlecrystal substrate size, and often the quality is less satisfactory owing to the imperfect alignment of individual graphene islands. Thus, to achieve a large-area, high-quality direct-grown graphene film, preparation of the large-area flexible substrate for releasing larger graphene nucleation seeds and an improvement in graphene alignment are critical issues in future works.

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In addition, direct-growth of graphene on flexible substrate assisted by metal powder precursors (solid, diluted solution) contained inside a sub-chamber (as high temperature region) for a direct evaporation process into an innovative and re-designed-CVD main-chamber (containing flexible substrate as low temperature region) in order to allow graphene formation on

dielectric substrates is an attractive topic and currently under investigation.

This work is supported by the research and development grant, South Korea.

SKKU Advanced Institute of Nano Technology, Sungkyunkwan University, Suwon,

[1] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, Ahn JH, Kim P, Choi JY, Hong BH. Large-scale pattern growth of graphene films for stretchable. Nature. 2009;**457**:706-

**Acknowledgements**

**Conflict of interest**

**Author details**

Viet Phuong Pham

South Korea

**References**

There are no conflicts of interest to declare.

710. DOI: 10.1038/nature07719

Address all correspondence to: pvphuong85@ibs.re.kr
