**3. Direct growth of graphene on flexible organic substrates at low temperature**

To avoid the problems arising in the graphene transferring process, two growth approaches have been suggested for the direct formation of graphene on flexible insulating substrates without additional transfer processes: (i) catalytic growth with the help of an external metal catalyst [51], and (ii) non-catalytic direct growth of graphene on a dielectric substrate without a metal catalyst [57–59].

The direct growth of graphene is a process on flexible substrates (PI, PDMS and Willow glass, mica) and rigid substrates (glass, AlN, GaN, sapphire, quartz, Si, textured Si, SiO<sup>2</sup> , SiC, fused silica, MgO, h-BN, MnO<sup>2</sup> , TiO<sup>2</sup> , and HfO<sup>2</sup> ) without transfer processes [6, 52–63, 74, 76–100], compared with conventional indirect growth processes on metal substrates (Cu, Ni, Ge, etc.) which need additional transfer processes onto arbitrary substrates [1–5, 8–51]. Using this method, we can avoid the complicated transfer process, which induces the defects, residues, and tears that degrade the performance of graphene-based devices. Various approaches for direct growth of graphene were classified into three major types: (i) catalyst-free and polymer-free, (ii) based on both catalyst and polymer, and (iii) based on metal catalyst (**Figure 3**).

**Figure 3.** Generally methods for direct-growth of graphene onto target insulating substrate.

Compared with the conventional catalytic growth approach, the direct growth of graphene on dielectric substrate without any external catalysts has various advantages, such as low process cost, and the shorter experimental processes. However, the drawback of this method is that without using catalysts, the chemical reactions for the excitation of the kinetic energy of the graphene growth process is not sufficient to obtain high quality direct-grown graphene for commercialization, compared with the catalytic direct growth described in previous sections.

Direct growth of graphene on flexible organic substrates has a huge potential in applications related to flexible and stretchable electronics, such as e-skin and health monitoring on the human body [101–103]. However, the limited thermal stability of organic substrates, which can be easily melted, deformed or damaged at high temperatures (>300°C), leads to a serious limitation in direct growth of graphene on the flexible substrates, because the quality of graphene grown at low temperatures (<400°C) is much lower than that of the one at high temperatures (~1000°C). Owing to these constraints, graphene growth on flexible substrates is only recently being studied [77, 78]. To reduce the process temperature, most of the studied growth methods involve the catalytic conversion of organic precursors to graphitic layers on the flexible organic substrates with the help of catalytic metal layers.

and patternable process to synthesize graphene-dielectric bi-layer (GDB) films on solutionprocessed polydimethylsiloxane (PDMS) under a Ni capping layer (**Figure 4g**) [78]. Seo et al. deposited the Ni film as the catalyst and encapsulation layer on a PDMS layer that was a few micrometer thick; this layer enabled direct growth of GDB between the substrate and Ni layer. PDMS (4 μm)/Ni (400 nm) films on the substrate were thermally annealed under vacuum, forming a PDMS/MLG/Ni/MLG structure. At the interface of the PDMS layer and the Ni film, the carbon atoms in the PDMS surface diffused into the Ni layer under high temperature, and carbon atoms were released to form MLG on both sides of the Ni layer during cooling. With this method the GDB structure was fabricated simultaneously and directly on the substrate, by thermal conversion of the PDMS without using additional graphene transfer and patterning process or formation of an expensive dielectric layer, which makes the device fabrication

**Figure 4.** Diagram of direct-growth of graphene onto flexible substrates: (a–f) PI, and (g) PDMS. (a–f) Reproduced with permission from [77], copyright 2012, IOP publishing. (g) Diagram of direct-growth of bilayer graphene on PDMS

Direct Growth of Graphene on Flexible Substrates toward Flexible Electronics: A Promising…

http://dx.doi.org/10.5772/intechopen.73171

77

substrate based on Ni catalyst (reproduced with permission from [78], copyright 2017, IOP Publishing).

In 2015, Sun et al. revealed a growth method of graphene-graphitic carbon (G-GC) at

growth time in 1 h) using PECVD system as depicted in **Figure 5** [108]. The advantage of direct PECVD process is that graphene films could be formed on flexible substrate, e.g.

gas, pressure 100 W, and

the growth conditions (low-temperature range 400–600°C, CH<sup>4</sup>

process much easier.

In 2012, Kim et al. reported a low-temperature (300°C) growth of graphene-graphitic carbon (G-GC) films on Cu layer deposited on polyimide (PI) substrate using inductively coupled plasma-enhanced CVD (ICP-CVD), and a direct transfer of the G-GC films onto a underlying flexible PI substrate using wet etching of Cu layer (**Figure 4a–f**) [77]. The optical and electrical characteristics of G-GC are affected by the varying growth temperature, plasma power and growth time. More recently, in 2016, Seo et al. revealed a simple, inexpensive, scalable Direct Growth of Graphene on Flexible Substrates toward Flexible Electronics: A Promising… http://dx.doi.org/10.5772/intechopen.73171 77

**Figure 4.** Diagram of direct-growth of graphene onto flexible substrates: (a–f) PI, and (g) PDMS. (a–f) Reproduced with permission from [77], copyright 2012, IOP publishing. (g) Diagram of direct-growth of bilayer graphene on PDMS substrate based on Ni catalyst (reproduced with permission from [78], copyright 2017, IOP Publishing).

Compared with the conventional catalytic growth approach, the direct growth of graphene on dielectric substrate without any external catalysts has various advantages, such as low process cost, and the shorter experimental processes. However, the drawback of this method is that without using catalysts, the chemical reactions for the excitation of the kinetic energy of the graphene growth process is not sufficient to obtain high quality direct-grown graphene for commercialization, compared with the catalytic direct growth described in previ-

**Figure 3.** Generally methods for direct-growth of graphene onto target insulating substrate.

Direct growth of graphene on flexible organic substrates has a huge potential in applications related to flexible and stretchable electronics, such as e-skin and health monitoring on the human body [101–103]. However, the limited thermal stability of organic substrates, which can be easily melted, deformed or damaged at high temperatures (>300°C), leads to a serious limitation in direct growth of graphene on the flexible substrates, because the quality of graphene grown at low temperatures (<400°C) is much lower than that of the one at high temperatures (~1000°C). Owing to these constraints, graphene growth on flexible substrates is only recently being studied [77, 78]. To reduce the process temperature, most of the studied growth methods involve the catalytic conversion of organic precursors to graphitic layers on the flexible organic substrates with the help of catalytic metal

lower

In 2012, Kim et al. reported a low-temperature (300°C) growth of graphene-graphitic carbon (G-GC) films on Cu layer deposited on polyimide (PI) substrate using inductively coupled plasma-enhanced CVD (ICP-CVD), and a direct transfer of the G-GC films onto a underlying flexible PI substrate using wet etching of Cu layer (**Figure 4a–f**) [77]. The optical and electrical characteristics of G-GC are affected by the varying growth temperature, plasma power and growth time. More recently, in 2016, Seo et al. revealed a simple, inexpensive, scalable

ous sections.

76 Flexible Electronics

layers.

and patternable process to synthesize graphene-dielectric bi-layer (GDB) films on solutionprocessed polydimethylsiloxane (PDMS) under a Ni capping layer (**Figure 4g**) [78]. Seo et al. deposited the Ni film as the catalyst and encapsulation layer on a PDMS layer that was a few micrometer thick; this layer enabled direct growth of GDB between the substrate and Ni layer. PDMS (4 μm)/Ni (400 nm) films on the substrate were thermally annealed under vacuum, forming a PDMS/MLG/Ni/MLG structure. At the interface of the PDMS layer and the Ni film, the carbon atoms in the PDMS surface diffused into the Ni layer under high temperature, and carbon atoms were released to form MLG on both sides of the Ni layer during cooling. With this method the GDB structure was fabricated simultaneously and directly on the substrate, by thermal conversion of the PDMS without using additional graphene transfer and patterning process or formation of an expensive dielectric layer, which makes the device fabrication process much easier.

In 2015, Sun et al. revealed a growth method of graphene-graphitic carbon (G-GC) at the growth conditions (low-temperature range 400–600°C, CH<sup>4</sup> gas, pressure 100 W, and growth time in 1 h) using PECVD system as depicted in **Figure 5** [108]. The advantage of direct PECVD process is that graphene films could be formed on flexible substrate, e.g.

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

on flexible PDMS-based FET device (**Figure 6**) will be a promising potential in future flexible

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

for LED

**direct-grown graphene**

Flexible CVD Ni Graphene pattern High flexibility pattern [98]

Ni FET Electron mobility

promising

Direct Growth of Graphene on Flexible Substrates toward Flexible Electronics: A Promising…

PI Flexible ICP-CVD Cu Strain sensor Transmittance (%T)

**Results Ref.**

http://dx.doi.org/10.5772/intechopen.73171

)

)

/Vs On/off ratio (1.1 × 10<sup>4</sup>

High transparent and

[77]

79

[78]

[100]

(77% at 550 nm) Sheet resistance (R<sup>s</sup>

(80 KΩ/sq)

μ<sup>e</sup> = 0.01 cm<sup>2</sup>

flexibility

**Substrate Property Method Catalyst Applications of** 

and thermal annealing, CVD

Mica Flexible PECVD Transparent circuit

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.

electronics [78].

Flexible Willow

Glass

PDMS Flexible Spin coat

Note: "NA" means "not applicable".

**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 multifunctional electrodes in solar cell, smart window, and transparent electronic.
