Graphene-based Flexible Devices Research Articles

Engineering the flexibility and elastic modulus of graphene by heterojunctions

Graphene shows both superior flexibility and excellent mechanical strength. The fabricated graphene samples usually contain various defects like grain boundaries, which can either enhance or weaken the mechanical strength of graphene. So, exploring the effects of grain boundaries on the flexibility of graphene is useful in designing graphene-based flexible devices. Employing the first-principles calculation, flexibilities of graphene heterojunctions were studied, aiming to tailor the flexibility of graphene by heterojunctions. Here, by connecting armchair (AC) and zigzag (ZZ) graphene through grain boundaries, graphene heterojunctions with tunable AC to ZZ ratio were constructed. It was found that bending moduli, as well as Young’s moduli, of graphene heterojunctions are lower than the pristine graphene and can be further tailored by the AC to ZZ ratio, making graphene heterojunctions more flexible than graphene. Particularly, changing the AC to ZZ ratio can even alter the relative flexibility of graphene heterojunctions in different directions. Therefore, graphene heterojunction provides an approach to engineer the flexibility of graphene, which is helpful in understanding the mechanical properties of two-dimensional materials and designing the flexible devices.

Laser-Induced Graphene-based Flexible Substrate with Photothermal Conversion and Photoresponse Performance on Polyimide Film.

Graphene-based flexible electronic devices are widely used in photoelectric components and photodetectors. However, it remains a huge challenge to fabricate graphene-based flexible devices efficiently and economically. Compared with the flexible electronic devices made by combining the flexible film with metal and semiconductor materials, the graphene-based flexible substrate (GFS) can be efficiently and conveniently induced by laser direct writing on the flexible film. In this paper, the GFS with a resistance of as low as 15 Ω was successfully induced by CO2 laser on a polyimide (PI) film in one step, and the GFS surface covered with carbon nanoparticles (GFSC) with a resistance of 25 Ω was further induced by femtosecond (fs) laser reprocessing. Benefiting from the laser-induced porous graphene structure, the absorptivity of GFS is up to 90% in the wavelength range of 200-2000 nm. The formation of carbon nanoparticles on the GFSC surface further improves the absorptivity to 97.5% in a wide spectral range. Under white light irradiation of 1 sun, the surface temperature of GFS reaches 65.7 °C and that of GFSC is up to 70.8 °C within 2 min. Under the irradiation of a light-emitting diode (LED) with a central wavelength of 365 nm, the highest photoresponsivity of GFS and GFSC was 8.8 and 1.3 mA/W, respectively. The response time and recovery time of GFS are 8 and 7.3 s, and those of GFSC are 8.3 and 6.7 s, respectively. Importantly, GFSC has a more stable photoresponse performance due to the better electron capture and transfer capability of carbon nanoparticles. It is believed that GFS and GFSC have great application potential in flexible photodetectors and sensors.

In Situ Growth of Graphene on Polyimide for High-Responsivity Flexible PbS-Graphene Photodetectors.

Graphene is an ideal material for flexible optoelectronic devices due to its excellent electrical and optical properties. However, the extremely high growth temperature of graphene has greatly limited the direct fabrication of graphene-based devices on flexible substrates. Here, we have realized in situ growth of graphene on a flexible polyimide substrate. Based on the multi-temperature-zone chemical vapor deposition cooperated with bonding a Cu-foil catalyst onto the substrate, the growth temperature of graphene was controlled at only 300 °C, enabling the structural stability of polyimide during growth. Thus, large-area high-quality monolayer graphene film was successfully in situ grown on polyimide. Furthermore, a PbS-graphene flexible photodetector was fabricated using the graphene. The responsivity of the device reached 105 A/W with 792 nm laser illumination. The in-situ growth ensures good contact between graphene and substrate; therefore, the device performance can remain stable after multiple bending. Our results provide a highly reliable and mass-producible path for graphene-based flexible devices.

The split-up of G band and 2D band in temperature-dependent Raman spectra of suspended graphene

Dedicated Raman spectroscopy investigations in the temperature range from 78 K to 298 K were performed for the graphene suspended above round holes on SiO2/Si substrate (the suspended graphene, SUG), in comparison with the surrounding graphene adhered to solid SiO2/Si substrate (the substrate supported graphene, SSG). The results confirmed that one-layer SUG (1L-SUG) was subjected to strains originated from the surrounding 1L-SSG due to the different area and shape around the hole along different directions. In further, the split-up of G band and 2D band were observed at low temperature because of the enhanced anisotropic strains in 1L-SUG due to the mismatch of thermal expansion coefficient (TEC) between the 1L-SSG and SiO2/Si substrates. The temperature-dependent (T-dependent) frequency shifts for the G band and 2D band in 1L-SUG and 1L-SSG were consistent with previous Raman investigations. Therefore, the micro-hole boundary effect in suspended graphene play a significant role in determination of the properties for graphene materials and graphene-based flexible devices.

Domain-boundary independency of Raman spectra for strained graphene at strong interfaces

Strains in graphene play a significant role in graphene-based flexible devices, but many aspects of the domain boundary effects in strained graphene remain unclear, such as the evolution of Raman spectra. Here we present a systematic investigation on the domain boundary effects on the Raman spectra of strained graphene, using a designed strong interface formed by formvar resins. We achieve in single-crystal graphene by far the largest strain up to 2%, significantly improved from the previous highest value of 1.3%, as well as the redshift and splitting for its G and 2D peaks. More importantly, the Raman spectra of strained bi-crystal graphene with a domain boundary show that the spectral evolution follows the same trend as the single crystal, and this trend was also confirmed by the result in polycrystalline graphene, demonstrating that the straining effect on the G and 2D peaks of graphene at strong interfaces is actually independent of its domain boundaries, different from the previous report of graphene deposited on weak interfaces. We attribute it to the efficient interfacial stress transfer at the formvar interfaces, and believe they can provide new insights into the understanding of graphene mechanical behaviors and valuable guidance for graphene-based flexible electronics.

Graphene film development on flexible substrate using a new technique: temperature dependency of gauge factor for graphene-based strain sensors

Purpose The purpose of this paper is to develop a new simple technique to synthesize graphene film on a flexible polyethylene terephthalate (PET) substrate and applied as a strain sensor. Design/methodology/approach Graphene film was synthesized using laser treatment of graphene oxide (GO) film deposited on PET substrate. A universal laser system was used to simultaneously reduce and pattern the GO film into laser reduced graphene oxide (LRGO) film. Findings The laser treatment synthesizes a multilayer graphene film with overlapped flakes, which shows structure integrity, mechanical flexibility and electrical conductivity of 1,330 S/m. The developed LRGO/PET film was used to fabricate a high sensitivity strain sensor. The sensitivity and temperature dependency of its gauge factor (GF) was examined at applied strains up to 0.25 per cent and operating temperatures up to 80°C. The fabricated sensor shows stable GF of approximately 78 up to 60°C with standard error of the mean not exceeding approximately ± 0.2. Originality/value The proposed method offers a new simple and productive technique of fabricating large-scale graphene-based flexible devices at a low cost.

Substrate wettability requirement for the direct transfer of graphene

The direct transfer method, wherein graphene is transferred from its growth metal to a soft substrate, is widely used to fabricate various devices, and the interfacial bonding condition between the substrate and the graphene is vital for transfer success. In this letter, we present a theoretical model to derive the wettability requirements of the soft substrate to sustain the direct transfer of graphene, and verify the theoretical analysis with experiments. We find that the surface energy components of the substrate have a crucial effect upon the graphene transfer, and that substrates possessing a strong polar surface energy are not suitable for transfer. The theoretical model predicts the critical water contact angle of the soft substrate for graphene transfer to be about 50°, and the experiments measure it to be about 60°. These results provide guidelines for choosing proper substrates to transfer graphene during the fabrication of graphene-based flexible devices.

Surface treatment of polyimide substrates for the transfer and multitransfer of graphene films

This work explores the use of polyimide substrates for the development of flexible graphene-based devices. Single layer graphene was synthesized by chemical vapour deposition and transferred to untreated and treated polyimide substrates. Of the four different surface treatments, the tetrafluoromethane/oxygen plasma and, especially, hydrochloric acid bath treatments showed the best performance, as they reduced the resistivity of the graphene samples by as much as 46%. At the same time, resistivity was also reduced by as much as 96% due to the increase in the graphene coverage area as a result of repeating the transfer process up to four times. The combination of HCl treatment and transfer iteration led to the samples acquiring a sheet resistance of 800Ω/sq. To the best of our knowledge, this is the lowest value ever reported in the literature for graphene samples on conventional flexible polymeric substrates and, in particular, in polyimide, the most widely used material for flexible applications. This result represents a major contribution to the development of graphene-based flexible devices.

Fabrication of graphene-based flexible devices utilizing a soft lithographic patterning method

There has been considerable interest in soft lithographic patterning processing of large scale graphene sheets due to the low cost and simplicity of the patterning process along with the exceptional electrical or physical properties of graphene. These properties include an extremely high carrier mobility and excellent mechanical strength. Recently, a study has reported that single layer graphene grown via chemical vapor deposition (CVD) was patterned and transferred to a target surface by controlling the surface energy of the polydimethylsiloxane (PDMS) stamp. However, applications are limited because of the challenge of CVD-graphene functionalization for devices such as chemical or bio-sensors. In addition, graphene-based layers patterned with a micron scale width on the surface of biocompatible silk fibroin thin films, which are not suitable for conventional CMOS processes such as the patterning or etching of substrates, have yet to be reported. Herein, we developed a soft lithographic patterning process via surface energy modification for advanced graphene-based flexible devices such as transistors or chemical sensors. Using this approach, the surface of a relief-patterned elastomeric stamp was functionalized with hydrophilic dimethylsulfoxide molecules to enhance the surface energy of the stamp and to remove the graphene-based layer from the initial substrate and transfer it to a target surface. As a proof of concept using this soft lithographic patterning technique, we demonstrated a simple and efficient chemical sensor consisting of reduced graphene oxide and a metallic nanoparticle composite. A flexible graphene-based device on a biocompatible silk fibroin substrate, which is attachable to an arbitrary target surface, was also successfully fabricated. Briefly, a soft lithographic patterning process via surface energy modification was developed for advanced graphene-based flexible devices such as transistors or chemical sensors and attachable devices on a biocompatible silk fibroin substrate. Significantly, this soft lithographic patterning technique enables us to demonstrate a simple and efficient chemical sensor based on reduced graphene oxide (rGO), a metallic nanoparticle composite, and an attachable graphene-based device on a silk fibroin thin film.

Electrical and lattice vibrational behaviors of graphene devices on flexible substrate under small mechanical strain

We present systematic experimental study on the electrical response and two-phonon Raman scattering mode of graphene under small uniaxial strain. The graphene, which was initially grown by chemical vapor deposition, was transferred to a transparent-flexible-polyethylene-terephthalate substrate. It was found that the electrical resistance increases as the stain is increased after a slight decrease in very small strain regimes of <0.20%. This is due to a relaxation of intrinsic ripples created during the transfer of the graphene to the polyethylene-terephthalate substrate. The gauge factor in the linear response regime was found to be about 22. Also, the 2D Raman bands of the strained graphene showed a distinct red-shift of −37 cm−1 per 1% strain for the 2D+ mode and −46 cm−1 per 1% strain for the 2D− mode. Finally, we determined the Gruneisen parameters of γ2Δ+ ∼ 2.05 and γ2Δ−∼ 2.55 for the phonons in free-standing graphene without a substrate. Our results provide electro-mechanical parameters for graphene-based flexible devices and show the potential of graphene for measuring strain in future flexible electronics.