Robust sandwiched fluorinated graphene for highly reliable flexible electronics

Applied Surface Science 499 (2020) 143839

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full length article

Robust sandwiched fluorinated graphene for highly reliable flexible electronics

T

Mamina Sahooa,1, Jer-Chyi Wanga,b,f,h,1, Yuta Nishinac, Zhiwei Liud, Jong-Shing Bowe, ⁎ Chao-Sung Laia,b,g,i, a

Department of Electronic Engineering, Chang Gung University, No. 259, Wenhua 1st Road, Guishan District, Taoyuan City, Taiwan Biosensor Group, Biomedical Engineering Research Center Chang Gung University, No. 259, Wenhua 1st Road, Guishan District, Taoyuan City, Taiwan c Research Core for Interdisciplinary Sciences, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan d State Key Laboratory of Electronic Thin Films and Integrate Devices, University of Electronic Science and Technology of China, Chengdu 610054, China e Integrated Service Technology, No.10-1, Lixing 1st Rd., East Dist., Hsinchu City 30078, Taiwan f Department of Neurosurgery, Chang Gung Memorial Hospital, Guishan District, Taoyuan 33305, Taiwan g Department of Nephrology, Chang Gung Memorial Hospital, Guishan District, Taoyuan 33305, Taiwan h Department of Electronic Engineering, Ming Chi University of Technology, Taishan District, New Taipei City 24301, Taiwan i Department of Materials Engineering, Ming Chi University of Technology, Taishan District, New Taipei City 24301, Taiwan b

A R T I C LE I N FO

A B S T R A C T

Keywords: CVD graphene Sandwiched fluorinated graphene Flexible and transparent Field effect transistor Bending Strain

The high sensitivity of graphene to the surface condition of the gate dielectric layer and its poor van der Waals adhesion with a flexible substrate result in interfacial sliding and fracturing of graphene at low strains, making the successful utilization of pristine graphene (PG) in flexible electronics challenging. Here, we report a facile method for the fabrication of flexible graphene field effect transistors (F-GFETs) using sandwiched fluorinated graphene (FG). The “FG-PG-FG” sandwich structure shows a high optical transparency (> 94%) with an average carrier mobility above 340 cm2/V·s, higher than that obtained when GO and Ion gel were used as gate dielectric materials on F-GFETs and a relatively low gate leakage current of ~160 pA. Furthermore, we observed a high mechanical stability, retaining > 88% of the original current output against bending deformation of up to 6 mm and > 77% after 200 bending cycles by applying a tensile strain of 1.56%, compared to the control sample. This improved performance is attributed to the fact that the sandwiched FG provides a good dielectric environment by tuning the C/F ratio, which tightly fixes the PG under strain. These findings provide a new route for the future development of graphene-based flexible electronics.

1. Introduction To meet the growing demand for flexible electronics, researchers are focusing more on the fabrication of user-friendly flexible electronic devices, such as displays, E-paper, and wearable and artificial skin devices [1–3]. These flexible devices must be fabricated at a low cost and in such a manner to provide the same high electrical performance as devices on a rigid substrate. The successful fabrication of flexible field effect transistors (FETs) requires proper understanding of the key materials (active layer and dielectric layer) and the fabrication process. In the early stage of development, conventional organic polymers [4] and amorphous silicon [5] were the building blocks for flexible electronics due to their intrinsic flexibility. However, these traditional

materials have lower carrier mobility and poor chemical stability, which limits their use in many practical applications. In this regard, graphene has been proposed as an ideal material for high-performance flexible FETs due to its superb electronic properties, high optical transparency (> 90%) and excellent mechanical flexibility, with a fracture strain of approximately 25% and a Young's modulus of approximately 1 TPa, which is sufficient for flexible devices [6–8]. However, the lower on/off ratio of graphene due to its zero-bandgap nature is a serious hurdle for its application in flexible digital logic gates. On the other hand, a larger on/off ratio is not important for radio-frequency (RF) applications [9]. During the last few years, several efforts have been made to utilize the full potential of graphene in flexible RF transistors [10,11]. Several groups have demonstrated

⁎

Corresponding author at: Biosensor Group, Biomedical Engineering Research Center Chang Gung University, No. 259, Wenhua 1st Road, Guishan District, Taoyuan City, Taiwan. E-mail address: [email protected] (C.-S. Lai). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apsusc.2019.143839 Received 13 June 2019; Received in revised form 16 August 2019; Accepted 30 August 2019 Available online 07 September 2019 0169-4332/ © 2019 Published by Elsevier B.V.

Applied Surface Science 499 (2020) 143839

M. Sahoo, et al.

graphene-based RF devices on flexible substrates [12,13]. Despite many demonstrations of graphene transistors on flexible substrates, the performance of these devices is far inferior to that of devices on rigid substrates. Moreover, the performance of flexible graphene FETs (FGFETs) strongly depends on the gate dielectrics and bottom substrate because graphene is highly sensitive to the surface condition of the dielectric layer and the underlying substrate [14]. Usually, for F-GFETs, the CVD graphene is transferred from its growth substrate to the desired flexible polymer substrate, which actually degrades the interfacial adhesion between the graphene and the substrate due to weaker van der Waals forces. This process causes interfacial sliding, even at low strains of ~0.3%, producing an interfacial scattering charge and thus hindering the progress of graphene for use in future flexible electronic devices [15]. To prevent charge scattering from the top and bottom sides of graphene, various dielectric layers have been developed for graphene encapsulation. Several high-k dielectrics, such as Al2O3, HfO2 and SiO2, have been used for GFETs [16–18]. However, the high-temperature processing, low bendability and high interfacial trapping charge with the graphene surface are not preferable for F-GFETs [19]. To overcome the interfacial trapping charge and low bendability of gate dielectrics, Nicolas Petron et al. reported a uniform h-BN [20] for use as an encapsulating layer for F-GFETs. Although the F-GFETs have shown promising results, the synthesis of high-quality and large-area h-BN is challenging, thus limiting the utilization of h-BN in many practical applications. One type of graphene based insulator known as Graphene oxide (GO) have been demonstrated as novel gate dielectric materials in many applications with excellent performance, such as energy storage, solar cells and GFETs [21–24]. However, the decrease in resistivity with increased temperature (even under current flow) limits the application of these materials in flexible electronics. Therefore, to resolve the above limitations of F-GFETs, we require a highly flexible dielectric layer that can be integrated as the gate dielectric and substrate buffer layers. In an effort to overcome the above challenges and limitations, fluorinated graphene (FG) is proposed as an ideal candidate for the gate dielectric and substrate buffer layers in F-GFETs. FG, which is one of the thinnest 2D insulators, has received increased attention due to its excellent mechanical flexibility characteristics, such as the maintenance of pristine graphene (PG), super hydrophobicity, and good chemical and thermal stability [25]. Importantly, the electronic properties of FG can be tuned by modulating the carbon-to-fluorine ratio (C/F), which changes the bandgap within the range of 0 to 3.4 eV [26]. Several recent studies have discussed the interaction between FG and graphene [27,28]. In particular, Ho et al. reported the fabrication of top-gate GFETs on a rigid substrate using FG as the gate dielectric [29], and the devices showed high electrical breakdown (~10 MV cm−1) with a low leakage current. Additionally, the decoupling properties of a PG/FG heterostructure on a rigid SiO2/Si substrate exhibited a tenfold mobility enhancement compared with graphene devices on a SiO2/Si substrate [30]. These unique properties of FG make it a promising candidate for use in F-GFETs. Note that the development of flexible electronics depends not only on new materials but also on the combination of novel and traditional materials and fabrication methodologies. In the present study, by taking advantage of PG and FG, we propose a promising nondestructive method for the fabrication of top-gate GFETs on a polyethylene terephthalate (PET) substrate with sandwiched FG (where FG was used as both the gate dielectric and the substrate buffer layers). The “FG-PG-FG” sandwich structure shows a high optical transparency (> 94%) due to the formation of a large band gap (> 3.1 eV) after precise control of the C/F ratio. The low number of dangling bonds at the FG/PG heterointerface results in high carrier mobility ~340 cm2/V·s (higher than that of Ion gel and GO) and meanwhile a relatively low gate leakage current (160 pA). We attribute this high mobility to the low scattering effects due to the sandwich structure. Furthermore, we observed a high mechanical stability, retaining > 88% of the original current output against bending deformation of up to 6 mm and > 77% after 200 bending cycles by

applying a tensile strain of 1.56%, whereas the F-GFETs without FG retained only 59% of their starting output current. Most importantly, this work demonstrated that the FG-PG-FG sandwich structure introduces many opportunities for the future development of flexible and low-cost graphene-based RF electronics applications. 2. Experimental section 2.1. Graphene growth Monolayer graphene was grown over a copper foil (Alfa Aesar, No. 13382, thickness 25 μm, purity 99.8%) using the low-pressure chemical vapor deposition (LPCVD) method [29]. The copper foil was placed on a quartz plate and then loaded into a quartz-glass tube (TF55030, Lindberg/Blue/M). Then, the chamber temperature was increased from room temperature to 1000 °C at a pressure 5 mTorr. During the growth step, gas composed of CH4/H2 = 1 sccm/100 sccm was introduced into the chamber for 20 min at a pressure of 450 mTorr. After growth of the graphene, the system was cooled to room temperature at a rate of ~5 °C/s. The produced PG was transferred to the PET substrate via the conventional wet transfer method (Fig. S1a). 2.2. Graphene fluorination Graphene fluorination was performed using the plasma-enhanced chemical vapor deposition (PECVD) method. The chamber was evacuated to ~5 mTorr by increasing the temperature to 200 °C, because at a high temperature of ~260 °C, FG becomes unstable and starts to decompose [31]. Afterward, CF4 gas was introduced to the chamber with a controlled gas flow and an exposure time of 30 min. In this work, graphene fluorination (1L-FG and 3L-FG) was performed over copper foil as shown in Fig. S1b and c to avoid PET substrate deformation and shrinkage, as the PET substrate exhibits a low glass transition temperature (~76 °C) and a crystallizing temperature of 120 °C [32]. Trilayer FG (3L-FG) was obtained by transferring PG (layer by layer) onto copper foil to form 3L-PG. Then, CF4 plasma treatment was performed from the top side of the 3 L-PG/copper foils to form 3L-FG (Fig. S1c). 2.3. F-GFETs with sandwiched FG device fabrication Fig. 1 shows the schematic illustration of the fabrication process of F-GFETs using sandwiched FG on PET substrates. The device fabrication process begins by cleaning the PET substrate with deionized water (DI water), acetone and isopropyl alcohol. One layer of FG (1L-FG) was transferred to the PET substrate (~188 μm) as the substrate buffer layer, followed by transfer of the PG as the active layer, as shown in Fig. 1a and b. Thereafter, the source and drain electrodes (50 nm Ni) were formed using photolithography, followed by the thermal evaporation and lift-off process shown in Fig. 1c. Prior to formation of the channel layer, a 1L-FG, as the gate dielectric layer, was transferred over the source and drain, as shown in Fig. 1d, and then the channel and gate dielectric (1L-FG) pattern were formed together using photolithography, followed by O2 plasma etching to form the channel and top-gate dielectric layer, as shown in Fig. 1e. Finally, the top-gate electrode (160 nm Al) was formed using photolithography, followed by the thermal evaporation and lift-off process shown in Fig. 1f. 2.4. Device characterization For confirmation of the CVD graphene lattice and graphene after CF4 plasma treatment, Raman spectral analysis was performed. The Raman spectra were collected using an NT-MDT confocal Raman microscope system with a laser beam at a wavelength of 473 nm focused on a spot with a size of ~0.5 μm. Prior to the measurement, calibration was performed by setting the Si peak at 520 cm−1. The chemical compositions of PG and FG were analyzed by an X-ray photoelectron 2

Applied Surface Science 499 (2020) 143839

M. Sahoo, et al.

Fig. 1. Schematic illustration of the flexible top gate graphene field effect transistor with sandwich fluorinated graphene, including the key steps involved in the fabrication process; (a) a monolayer fluorinated graphene (1L-FG) as substrate buffer layer is transferred onto the PET substrate. (b) CVD graphene is transferred over the 1L-FG. (c) Formation of source and drain contact using photolithography followed by thermal evaporation. (d) 1L-FG transferred over the source and drain electrode as gate dielectric. (e) Both the channel and gate dielectric patterned using an O2 plasma reactive ion etching process (80 sccm and 120 W). (f) Top-gate electrode is patterned using photolithography followed by thermal evaporation.

were observed at ~1355 and ~2705 cm−1, respectively, after CF4 plasma treatment for 3L-FG. This observation is likely the result of the introduction of lattice disorder via fluorine chemisorption on the surface and the reduction of sp2 hybridized CeC bonds [34]. In contrast, in the spectrum of 3L-PG (Fig. S2), the G peak at ~1589 cm−1 became broader than that of 3L-FG, and formation of the shoulder D′ peak at ~1618 cm−1 reflected intravalley Raman scattering [35].There is one new peak D + D′ peak observed at 2950 cm−1 is the combination of D and D′ peaks. For 1L-FG, almost all peaks were unidentified, reflecting the complete destruction of π electron conjugation due to the doublesided functionalization of the graphene sheet. This destruction causes the surface of 1L-FG to become highly insulated, resulting in a higher ID/IG ratio [36]. The decrease in the ID/IG ratio for 3L-FG compared with that of 1L-FG possibly indicates that the top graphene layers of 3LFG are highly insulated (first and second top layers) with more sp3 bonds after the CF4 plasma treatment via fluorine chemisorption on the surface, whereas the bottom layer is partially insulated with some sp2 hybridized CeC bonds on the graphene plane. The Raman spectra of PG, 1L-FG and 3L-FG over PET were shown in Fig. S3. The PG/PET shows a low intensified 2D peak at 2691 cm−1 due to the poor optical contrast between the graphene and PET. Unfortunately, the G and D peak of PG/PET strongly overlapped with the PET peaks. However, for 1L-FG/PET the 2D peak is not clearly identified as shown in 1L-FG/Cu indicating the1L-FG to become highly insulated after CF4 plasma treatment. Whereas, for 3L-FG/PET a weak 2D peak was observed at 2704 cm−1 while the G and D peak of 3 L-FG/PET partially overlapped with the PET peaks. In addition, the optical transmittances of PG, 1L-FG and 3L-FG on a PET substrate were measured to be ~98%, ~96.5% and 94% at 550 nm, respectively, as shown in the right ordinate of Fig. 2c. The optical

spectrometer (XPS) equipped with a Mg Kα X-ray source for sample excitation. In addition, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed to examine the sandwiched FG device structure. The surface hydrophobicity of PG and FG over PET were measured according to the water contact angle using a deionized water droplet. The optical transparencies of PG and FG over PET were measured via UV–vis spectroscopy. Atomic force microscopy (AFM) was performed to examine the surface morphology of the PG, 1L-FG and 3L-FG. Finally, all electrical measurements were performed using an Agilent B1500A semiconductor device parameter analyzer at room temperature. 3. Results and discussion Before analyzing the electrical characteristics of F-GFETs with sandwiched FG, we first investigated the physical and electrical characteristics of GFETs with FG as the gate dielectrics. A schematic of the F-GFETs with FG as the dielectrics is illustrated in Fig. 2a. The fabrication process of F-GFETs with FG as the dielectrics is the same as that of the F-GFETs with sandwiched FG (as mentioned in the device fabrication section) but without the substrate buffer layer. Fig. 2b and c shows the Raman spectral analysis of PG, 1L-FG and 3L-FG over copper foil. The Raman spectrum of PG exhibits three peaks at 1350, 1585 and 2700 cm−1, which are attributed to the D, G and 2D peaks, respectively. The high I2D/IG (~2.13) and low ID/IG values indicate that the fabricated PG is single-layer graphene of high quality with negligible defects and is suitable for use in the further transfer process to the desired PET substrate. Further, the domain size of (La) of graphene was estimated using Cancado's formula is 5.45 μm indicates high quality single layer graphene [33]. However, a high-intensity D peak and a weak 2D peak 3

Applied Surface Science 499 (2020) 143839

M. Sahoo, et al.

Fig. 2. (a) Schematic illustration of top gate GFET with fluorinated graphene as gate dielectric over PET substrate. (b) The evolution of Raman spectra of PG, 1L-FG and 3L-FG after 30 min of CF4 plasma treatment over copper foil. (c) Peak intensities ratio ID/IG and optical transmittance of PG, 1L-FG and 3L-FG. Inset: image of PG and 1L-FG film over PET substrate. XPS analysis result of (d) PG (e) 1L-FG (f) 3L-FG where the C1s core level and several carbon–fluorine components are labeled. The inset shows the fluorine peak (F 1s) at 688.5 eV.

and carbon atoms of graphene after the CF4 plasma treatment, as shown in Fig. 2d–f. The main peak due to the CeC bond located at 284.4 eV is attributed to sp2 hybridized carbon, indicating that the C atoms were arranged in a honeycomb lattice structure in the PG. Additionally, two weak deconvoluted peaks at 286.1 eV and 288.6 eV were also identified. These peaks were attributed to the CeO and C]O groups, respectively [38,39]. These peaks possibly arose from the remaining

transmittance of the FG graphene layers was high due to the decrease in the optical absorption coefficient after the CF4 plasma treatment [34]. However, below 400 nm, the optical transmittance started to decrease due to loss of the optical absorption and reflection of FG [37] (Fig. S4). The inset of Fig. 2c shows optical images of PG/PET and 1L-FG/PET. XPS analyses of PG, 1L-FG and 3L-FG on a SiO2/P+ Si substrate were conducted to understand the chemical interaction between the fluorine 4

Applied Surface Science 499 (2020) 143839

M. Sahoo, et al.

formed Al2Ox as the gate dielectrics [45]), w/1L-FG and w/3L-FG samples. The linear output characteristics with a high drain current indicate a lower ohmic contact resistance between the metal contact (Ni) and the graphene channel [46]. Moreover, a higher drain current was observed for the w/3L-FG sample than for the w/1L-FG and w/oFG samples because when w/3L-FG is used as the dielectric layer, the top layers of FG are highly insulated, whereas the bottom layer is not fully insulated but is partially semiconducting (as discussed above in the Raman analysis). This bottom layer of 3L-FG combined with the active graphene channel might have been responsible for the enhancement of the drain current. Fig. 3b illustrates the transfer characteristics (IDS−VGS) of the w/o-FG, w/1L-FG and w/3L-FG samples at a fixed bias of 0.5 V. The transfer curve shows the ambipolar behavior of GFETs, regardless of the different layers of FG used as the gate dielectrics. The Dirac points for w/o-FG, w/1L-FG and w/3L-FG were 0.2 V, 0.6 V and 1.5 V, respectively, indicating that the graphene channel is of P-type doping. The greater shift of the Dirac point towards positive values for w/1L-FG and w/3L-FG was caused by the fluorinedoped graphene above the graphene channel. However, the w/o-FG sample exhibited p-type characteristics because of the unintentional doping of adsorbates (e.g., water, oxygen, or organic residue) during fabrication [47]. The field-effect carrier mobility was estimated from the IDS-VGS characteristics according to the gate voltage-dependent transconductance using the following Eq. (1) [48]:

residue of PMMA after graphene transfer or resulted from unintentional native oxidation [40]. Some different peaks were generated for 1L-FG and 3L-FG after the CF4 plasma treatment, which were attributed to CeC (284.4 eV), CeCF (286.3 eV) [29], CF (288.7 eV) and CF2 (290.7 eV) according to previous XPS studies [41,42]. The inset figures show the F1s binding energy regions of the 1L-FG and 3L-FG spectra observed at ~688.5 eV [43], supporting the formation of covalent CeF bonds due to the chemical absorption of fluorine over the carbon atoms [36]. The XPS area ratio of C/F was 1.1 after 30 min of CF4 plasma treatment, which indicates double-sided FG [30]. Following the XPS analysis, we performed water contact angle measurements of PG, 1L-FG and 3L-FG on a PET substrate to demonstrate the hydrophobic properties of FG (Fig. S5). The measured water contact angle values of PG, 1L-FG and 3L-FG were 59°, 70° and 78°, respectively. This result indicates that the presence of oxygen-containing functional groups in PG makes the surface hydrophilic, whereas the fluorine-containing functional groups in FG make the surfaces of 1LFG and 3L-FG hydrophobic [44]. The qualitative difference between 1LFG and 3L-FG is expected to be due to nonideal effects such as the difference in surface roughness between 1L-FG (2.90 nm) and 3L-FG (4.66 nm), which is shown in Fig. S6. The increase in the surface roughness of 3L-FG was caused by unavoidable formation of wrinkle and PMMA residue during the layer-by-layer graphene transfer process before the CF4 plasma treatment. Because of its highly hydrophobic nature, FG is a candidate material for use in future hydrophobic devices. Fig. 3 exhibits the DC electrical characterization of top-gate F-GFETs with FG as the gate dielectric material on a PET substrate at room temperature under ambient conditions. Fig. 3a illustrates the typical drain current (ID) vs. drain voltage (VD) curves of the w/o-FG (naturally

(

)

μ = L W COX VD (dI/dV)

(1)

where L and W are the graphene channel length (20 μm) and width (50 μm) under top gate modulation, the dI/dV is derived from the linear region of the transportation figure, COX is the gate capacitance of w/o-

Fig. 3. (a) Output characteristics (Id vs. Vd) of w/o-FG, w/1L-FG and w/3L-FG samples after 30 min of CF4 plasma. (b) Transfer characteristics (Id vs. Vg) of w/o-FG, w/1L-FG and w/3L-FG samples at a fixed value of drain to source voltage, Vds of 0.5 V. (c) Gate leakage current of w/o-FG (naturally formed Al2OX as gate dielectric), w/1L-FG and w/3L-FG samples, revealing the insulating properties of fluorinated graphene. 5

Applied Surface Science 499 (2020) 143839

M. Sahoo, et al.

decrease in the carrier mobility (μ) was observed compared with the carrier mobility under flat conditions before bending (μ0). The changes in the carrier mobility Δμ/μ0 (where Δμ is μ-μ0) for w/o-FG, w/1L-FG and w/3L-FG with decreasing bending radii were 52.46%, 31.10% and 37.72%, respectively. The changes in the ION (ΔI/I0) of w/o-FG, w/1LFG and w/3L-FG with decreasing bending radii were 23%, 16% and 18%, respectively. The decrease in carrier mobility and ION of w/o-FG, w/1L-FG and w/3L-FG in comparison with that of flat conditions further indicates the shifts of electronic band structure [54] and increase in channel resistance (Fig. S8) of graphene under tensile strain which may not be completely recovered again at flat condition after bending. Furthermore, endurance testing of the w/o-FG, w/1L-FG and w/3L-FG samples was performed to reveal the mechanical stability of the devices. Fig. 4d and e show the carrier mobility and ION of the w/o-FG, w/ 1L-FG and w/3L-FG samples as a function of bending cycles at a bending radius of 6 mm (1.56% tensile strain). The carrier mobility variations of w/o-FG, w/1L-FG and w/3L-FG were found to be 58, 42 and 50.7%, respectively, with respect to their initial values. The changes in ION of w/o-FG, w/1L-FG and w/3L-FG after 200 bending cycles were found to be 46, 37 and 39%, respectively, relative to their initial values. The large difference in the carrier mobility and ION of the flexible cycle after several bending cycles provides further evidence of interlayer sliding between the PG and the PET substrate, and this sliding limits the electrical performance by making the GFET mechanically unstable. Moreover, the preexisting strain in the lattice during the preparation and transfer of graphene causes a gradual increase in the deformation when external strain is applied [59]. Hence, the suppression of the strain transfer discrepancy between PG and the PET substrate is an essential prerequisite for future flexible graphene applications. By comparing the electrical and mechanical values of the w/o-FG, w/1L-FG and w/3L-FG samples, we conclude that the w/1L-FG sample exhibits the smallest variations in both carrier mobility and ION as a function of the bending radius and bending cycles. However, the performance of the F-GFET with FG as the gate dielectrics is far different from the electrical performance of the original F-GFET, which has caused graphene to be called a magical material among other 2D materials [60]. We anticipate that applying a highly flexible buffer layer over PET will be an effective method to further enhance the carrier mobility and mechanical stability of F-GFETs, and such enhancement is desired for many practical applications. Finally, following the above experimental demonstration of F-GFET with FG as the gate dielectrics as a proof of concept, F-GFETs using sandwiched FG (FG as both the gate dielectric and substrate buffer layers) were investigated. Fig. 5a shows the Raman analysis of PG over the FG/PET substrate. Although the optical contrast between the graphene and PET is poor compared with that against a SiO2/Si substrate, a low-intensity 2D peak was observed at approximately 2691 cm−1 for PG/PET. Unfortunately, the G peak of PG/PET strongly overlapped with the PET peaks. In contrast, PG/FG/PET showed a highly intensified 2D peak and a partially overlapped G peak at approximately 2687 and 1583 cm−1, respectively. However, the downshift of the 2D peak indicates a p-type doping of the graphene via the bottom FG, which acts as a buffer layer between the PG and the PET substrate. In addition, the Raman spectrum of sandwiched FG over PET (FG/PG/FG/PET) demonstrated a highly intensified 2D peak (Fig. S9a). The sandwiched FG over PET showed a high optical transparency of ~94%, which can fulfill the requirement for application in flexible display devices (inset of Fig. S9a). As the G and D peaks of PG/PET were not clearly visible, in this work, we focused on the 2D peak for further Raman analysis. Fig. 5b shows the HRTEM image of 1L-FG/PET, reflecting an amorphous-like structure. This result is due to the breakage of the hexagonal structure and basal plane of graphene via the CF4 plasma treatment, which thereby causes the surface to be insulated [61]. This result supports the Raman spectral analysis in Fig. 2b. Furthermore, the cross-sectional TEM shows the graphene layer located between the sandwich FG layers over PET (Fig. S9b).

FG, w/1L-FG and w/3L-FG G-FETs. The measurement of the gate capacitance as a function of the applied gate voltage (−3 to 3 V) is shown in Fig. S7a. The measured gate capacitance of w/o-FG, w/1L-FG, and w/3L-FG G-FETs were 290, 215,182 nF/cm2 respectively. The effective oxide thickness of naturally formed Al2OX in between graphene and Al for w/o-FG sample is ~6 nm calculated from the EDX analysis as shown in Fig. S7b. According to Chun-Chieh Lu et.al [45] the oxide thickness of naturally formed Al2Ox ranges from 5 to 12 nm which supports our result. The extracted carrier mobilities were 152, 170.4 and 160 cm2/ V.s for w/o-FG, w/1L-FG and w/3L-FG samples respectively. The higher carrier mobility of the FG samples indicates the suppression of longrange coulomb scattering due to the low number of dangling bonds and the smooth and stable interface between the FG/PG heterostructure [49].However, the lower on/off ratio of w/o-FG (1.3), w/1L-FG (1.3) and w/3L-FG (1.33) samples were due to the absence of intrinsic bandgap (Eg) of graphene. There are several studies reported the GFETs on flexible substrate showing low on/off ratio (summarized and discussed in Table S1). Fig. 3c shows the gate leakage current (Ig) of the w/o-FG (naturally formed Al2Ox as the gate dielectrics), w/1L-FG and w/3L-FG samples as a function of VGS with a drain bias of VDS 0.5 V. The low gate leakage currents of the FG samples (~600 pA) prove the excellent insulating properties of the dielectric layer for GFETs on a PET substrate. This observation of a low gate leakage current for FG samples used as gate dielectrics is in good agreement with the observation of Ho et al. [29] on a Si substrate. Furthermore, to investigate the mechanical stability of F-GFETs with FG as the dielectrics, the carrier mobility as a function of the bending radius (r) and bending cycles is shown in Fig. 4. A schematic illustration of the device bending measurement setup is shown in Fig. 4a. The sample devices (w/o-FG, w/1L-FG, and w/3L-FG) were wrapped around a set of rods with different bending radii of 10, 8, and 6 mm. The substrates were bent along the channel such that the applied tensile strain was perpendicular to the channel direction. Fig. 4b and c show the carrier mobility and ION of w/o-FG, w/1L-FG and w/3L-FG as a function of the bending radius (here, the symbol ∞ indicates a flat substrate) and the data points are the average value taken from a total of ~10 devices measurement. The mechanical strain induced on the substrate by decreasing the bending radius was calculated using the following equation (Eq. (2)) [50]:

Tensile Strain =

TPET + TFG + TAl2Ox ∗ 100 2R

(2)

where TFG is the thickness of the FG dielectric layer (~0.5 nm) [29], TPET is the thickness of the PET substrate (~188 μm) and TAl2Ox is the thickness of naturally formed Al2Ox (~6 nm). For the different bending radii of 10, 8, and 6 mm, the tensile strains applied on the substrate were 0.94, 1.17, and 1.56%, respectively. The figure indicates no significant impact from strain (1.17%) on the carrier mobility and ION of the w/o-FG and FG samples up to a bending radius of 8 mm. At a lower bending radius, we observed a gradual deterioration in both the carrier mobility and ION under applied tensile strain. This phenomenon was attributed to (i) the change in the intermolecular distance of the graphene caused by bending due to the applied strain [51] (ii) interlayer sliding between the graphene and PET caused by the strain discrepancy effect. Because graphene starts to slide beyond the applied strain of 1.2% [52] as a result of the poor van der Waals attraction with PET and the high difference between the mechanical flexibilities of PG and PET, fracturing of the graphene at low strains [53] and the formation of an interfacial charge occur, thereby increasing the channel resistance and (iii) the change in work function and gate capacitance of graphene as a result of tensile strain affect the coupling of gate to electron and holes [54,55], thereby decreasing the carrier mobility. Similar results in other flexible GFETs with metal electrode have also been reported [56–58]. Furthermore, bending the substrate below 6 mm caused device failure via the dislocation or breakage of the metal electrodes. Once the bending samples were released and measured under flat conditions, a 6

Applied Surface Science 499 (2020) 143839

M. Sahoo, et al.

Fig. 4. (a) Schematic illustration of bending measurement setup at different bending radius. (i) Device measurement at (i) flat condition (ii) bending radius of 10 mm (iii) 8 mm (iv) 6 mm. Inset shows the photograph of measurement setup. Change in (b) carrier mobility (c) ION of w/o-FG, w/1L-FG and w/3L-FG samples as a function of bending radius. The symbol ∞ represents the flat condition. Change in (d) carrier mobility (e) ION of w/o-FG, w/1L-FG and w/3L-FG samples as a function of bending cycles (Strain = 1.56%).

heavy p-type doping via the top and bottom FG layers. The F-GFET with sandwiched FG exhibits an enhancement of the on-state drain current and a carrier mobility of ~340.5 cm2/V·s compared with F-GFETs with 1L-FG as gate dielectrics. This result is attributed to the suppression of interfacial scattering by the FG buffer layer between the graphene and PET due to the similar lattice structures between PG and FG. However, this value is lower than that of GFETs with FG over a rigid substrate

Fig. 5c shows that the linear output characteristics (Id vs. Vd) of a sandwiched FG sample at different gate voltages (Vg = 0 V, ± 1 V and ± 2 V) follows the ohmic law (I ∝ V). Fig. 5d shows the transfer characteristics (Id vs. Vg) of a sandwiched FG sample under an applied gate voltage ranging from −1.5 V to +3.5 V at a fixed Vd = 0.5 V. The value of VDirac of the sandwiched FG is ~2.4 V, which is more positive than that of w/1L-FG (~0.7 V), as shown in Fig. 3b. This result indicates 7

Applied Surface Science 499 (2020) 143839

M. Sahoo, et al.

Fig. 5. Physical and electrical measurements of sandwich FG sample (a) Raman spectra of PG/PET and PG/FG/PET substrate. (b) High-resolution transmission electron microscopy image for 1L-FG, Scale bar, 2 nm. (c) Id vs. Vd of 1L-FG/PG/1L-FG device at variable vg (−2 to 2 V). (d) Id vs. Vg of 1L-FG/PG/1L-FG. Raman spectra of devices under different bending radius (e) PG/PET (Inset shows the 2D peak) (f) PG/FG/PET (inset shows the 2D peak). (g) Change in Mobility (h) change in ION of PG/PET and PG/FG/PET as a function of bending radius between bending radii of ∞ to 1.6 mm in tensile mode. (i) Change in Mobility. (j) Change in ION of PG/PET and PG/FG/PET as a function of bending cycles. Inset of (i) shows the photograph of F-GFETs with sandwich FG on the PET substrate. (k) Schematic evolution of proposed strain transfer mechanism through PG/PET and PG/FG/PET. The inset of PG/PET sample shows the generation of sliding charge due to interfacial sliding between PG and PET.

substrate also impact on the degradation of carrier mobilities compared to the SiO2 substrate [64,65]. Moreover, a low gate leakage current (~160 pA) was observed, with the leakage current being almost fivefold smaller than the drain current, as shown in Fig. 5d. Note that FG shows excellent dielectric properties, even when functioning as a substrate buffer layer. To investigate the mechanical flexibility and stability of GFETs with

[29] but higher than the highest reported mobility of F-GFETs when GO [24] and Ion gel [56] are used as the gate dielectric materials (summarized in Table S2). The decrease in carrier mobility is due to (i) the unavoidable wrinkles formation on the graphene and the unwanted PMMA residue on the graphene surface during the graphene transferred to the flexible substrate act as an internal and external scattering of charge carriers [62,63]. (ii) The remote phonon scattering of polymeric 8

Applied Surface Science 499 (2020) 143839

M. Sahoo, et al.

a tensile strain of 1.56%. There was no significant enhancement in resistance (Fig. S11) and degradation in the carrier mobility and ION under different applied tensile strains, suggesting the suppression of the interfacial sliding of PG and PET substrate. The formation of the sp3 bond in FG [25] makes the FG mechanically flexible, as is the case with PG, which helps reduce the strain discrepancy between PG and FG by transferring the complete strain applied from the PET substrate to FG. Therefore, the F-GFETs using sandwiched FG show high electrical and mechanical flexibility compared with the F-GFETs with FG as the dielectrics, as summarized in Table 1. Fig. 5k shows the schematic of the strain transfer mechanisms of FG/PG/PET and FG/PG/FG/PET under bending. In the case of FG/PG/ PET under bending, the strain applied on the substrate is larger than the strain applied on the PG (ΔεSub > ΔεPG) [69]. The strain discrepancy debonded the graphene from the PET substrate, and then the graphene started to slide, resulting in some interfacial sliding charge between PG and PET. This sliding charge affected the bottom surface of the graphene, resulting in a decrease in the carrier mobility and in the current observed in the abovementioned electromechanical results. In contrast, in the “FG-PG-FG” sandwich structure, both of the FG layers fixed the active PG (channel layer) as the skeleton of the sandwich structure because of the formation of sp3 bonding in the FG layer, resulting in high mechanical flexibility similar to that of the PG layer and the ability to transfer the complete strain from the FG layer to the PG layer (ΔεSub > ~ΔεFG ~ ΔεPG), thereby reducing the strain discrepancy and causing suppression of the interracial sliding charge. We hereby concluded that the active graphene layer will not fail until the top and bottom FG layers become ineffective. Therefore, the FGFETs with sandwiched FG showed improved electrical and mechanical performance. More detailed work is required to understand the complete strain transfer mechanism, but such work is beyond the scope of this report. A brief comparison of the mechanical flexibility of GFETs with that of different dielectric layers under strain and current work is shown in Table 2. We compared the carrier mobility degradation ratio w.r.t. the applied strain using Eq. (2),

Table 1 Summary of the electrical and mechanical performance of flexible w/o-FG, w/ 1L-FG, w/3L-FG and sandwich FG (FG/PG/FG) samples. Device

w/o-FG w/1L-FG w/3L-FG FG/PG/FG

Ion/Ioff

1.26 1.33 1.33 1.37

Mobility (cm2/V·s)

152 170.4 160 340.57

Ig (A) (−3 to 3 V)

2.7n 400p 200p 160p

Bending radius sequence

Bending cycles (R = 6 mm)

Δμ (%)

ΔION (%)

Δμ (%)

ΔION (%)

52.46 31.1 37.72 25

23 16 18 12

58.69 42 50.72 36.19

46 37 39 23

sandwiched FG, physical and electrical analyses were performed under bending by applying repeated strain. Raman analysis is an ideal method to investigate the effect of strain on the graphene lattice [66]. Fig. 5e and f show the Raman analysis of PG/PET and PG/FG/PET under a tensile strain of 0.58 and 0.94%, respectively, at different bending radius values of 16 and 10 mm. In both cases, the D and G peaks were strongly overlapped by the PET peaks; however, an intensified 2D peak was observed for PG/PET and PG/FG/PET at 2690.5 and 2688.8 cm−1, respectively. It can be clearly observed that by applying a tensile strain from 0% to 0.94% on PET under bending, the 2D peak position of PG/ PET starts to shift from the initial position of ~2690.5 cm−1 to 2679.6 cm−1, whereas for PG/FG/PET, the 2D peak shifts from 2688.8 cm−1 to 2684.5 cm−1 (as shown in the inset of Fig. 5e and f). The shift of the 2D band indicates elongation and weakening of the CeC bond [67]. The larger shift of the 2D peak of PG/PET may indicate that the complete amount of strain from the PET substrate could not be transferred to the graphene due to weak van der Waals adhesion. In contrast, the smaller shift in the 2D peak under strain for our PG/FG/ PET sample was caused by the suppression of the FG interfacial sliding. Because of the excellent mechanical flexibility and interface coupling of the PG/FG heterostructure [68], the electromechanical durability was evaluated by applying a tensile strain to F-GFETs with sandwiched FG. The bending radius setup was the same as that described above. Fig. 5g and h exhibits the carrier mobility and ON current (ION) of F-GFETs with sandwiched FG as a function of the bending radius of up to 6 mm achieved by applying tensile strain. The observed degradation percentages of the carrier mobility and ION were 25% 12% of their initial values within the range of 0%–1.5% tensile strain, which were smaller than the previously obtained changes in the carrier mobility and ION of the w/1L-FG sample (31% and 16%). Furthermore, small degradations of the carrier mobility (36%) and ION (23%) of 1LFG/PG/1L-FG/PET were observed after 200 bending cycles by inducing

μM = μ/μ 0∗Strain

(2)

where μ and μ0 are the carrier mobilities of the device after bending and before bending, respectively. By using Eq. (2), the previously reported F-GFETs [45] show low mechanical stability (~0.23) under an applied strain of 0.62%, whereas the F-GFETs using sandwiched FG show high mechanical stability (~1.02) under an applied tensile strain of 1.56%. These results further demonstrate the benefits of utilizing FG in FGFETs.

Table 2 Comparison of electrical and mechanical performance of flexible graphene field effect transistors with different gate dielectric. Structure

Dielectric

Operating voltage (V)

Mobility (cm2/V·s)

Bending radius (mm)

Tensile strain (%)

Mechanically dependent mobility w.r.t tensile straina

Δμ (%)b

References

Natural Al2O3/graphene/ PET Ion gel/graphene/PET Graphene/PLZT/F-Mica Organic DMSO/graphene/ PET Yox/graphene/PI Yox/graphene/PENc ALD Al2O3/SiO2/ graphene/PI FG/graphene/FG/PET

Natural Al2O3

−3 to 2.5

280

8

0.62

0.24

NA

[45]

Ion gel PLZT Organic DMSO

−3 to 2 −2 to 2 −4 to 4

203 54.8 154.6

6 6 NA

NA NA NA

NA NA NA

NA NA NA

[56] [57] [58]

Yox Yox ALD Al2O3/ SiO2 FG

−4 to 3 −0.8 to 0.8 −5.0 to 5.0

102 13,540 295

12.5 7.8 NA

NA 8 NA

NA 0.8 NA

NA NA NA

[70] [71] [72]

−1.5 to 3.5

340

6

1.56

1.02

25%

This work

Mobility degradation w.r.t tensile strain = μ (Mobility after bending) / μ0 (Mobility before bending) ∗ Strain. Change in mobility Δμ = μ0 (After bending Flat) − μ (Before bending Flat) / μ (Before bending Flat). c Although the Yox/graphene/PEN structure shows the high mobility due to the less substrate roughness of PEN and better thermal stability then PET reduces the thermal deformation and stress of graphene but still it shows a maximum bending radius up to 7.8 mm beyond that led to failure of the devices. a

b

9

Applied Surface Science 499 (2020) 143839

M. Sahoo, et al.

4. Conclusion

[16] M. Friedemann, M. Woszczyna, A. Müller, S. Wundrack, T. Dziomba, T. Weimann, F.J. Ahlers, Versatile sputtering technology for Al2O3 gate insulators on graphene, Sci. Technol. Adv. Mater. 13 (2012) 025007. [17] L. Liao, J. Bai, R. Cheng, Y.C. Lin, S. Jiang, Y. Huang, X. Duan, Top-gated graphene nanoribbon transistors with ultrathin high-k dielectrics, Nano Lett. 10 (2010) 1917–1921. [18] M.C. Lemme, T.J. Echtermeyer, M. Baus, H. Kurz, A graphene field-effect device, IEEE Electr. Device Lett. 28 (2007) 282–284. [19] S.-L. Li, H. Miyazaki, A. Kumatani, A. Kanda, K. Tsukagoshi, Low operating bias and matched input-output characteristics in graphene logic inverters, Nano Lett. 10 (2010) 2357–2362. [20] N. Petrone, T. Chari, I. Meric, L. Wang, K.L. Shepard, J. Hone, Flexible graphene field-effect transistors encapsulated in hexagonal boron nitride, ACS Nano 9 (2015) 8953–8959. [21] B. Martin-Garcia, Y. Bi, M. Prato, D. Spirito, R. Krahne, G. Konstantatos, I. Moreels, Reduction of moisture sensitivity of PbS quantum dot solar cells by incorporation of reduced graphene oxide, Sol Energ Mat Sol C 183 (2018) 1–7. [22] Z. Liu, Z. Chen, F. Yu, Microencapsulated phase change material modified by graphene oxide with different degrees of oxidation for solar energy storage, Sol Energ Mat Sol C 174 (2018) 453–459. [23] X. Maa, Y. Liu, H. Liu, L. Zhang, B. Xu, F. Xiao, Fabrication of novel slurry containing graphene oxide-modified microencapsulated phase change material for direct absorption solar collector, Sol Energ Mat Sol C 188 (2018) 73–80. [24] S.K. Lee, H.Y. Jang, S. Jang, E. Choi, B.H. Hong, J. Lee, S. Park, J.H. Ahn, All graphene-based thin film transistors on flexible plastic substrates, Nano Lett. 12 (2012) 3472–3476. [25] R.R. Nair, W.C. Ren, R. Jalil, I. Riaz, V.G. Kravets, L. Britnell, P. Blake, F. Schedin, A.S. Mayorov, S.J. Yuan, M.I. Katsnelson, H.M. Cheng, W. Strupinski, L.G. Bulusheva, A.V. Okotrub, I.V. Grigorieva, A.N. Grigorenko, K.S. Novoselov, A.K. Geim, Fluorographene: a two-dimensional counterpart of Teflon, Small 6 (2010) 2877–2884. [26] K.J. Jeon, Z. Lee, E. Pollak, L. Moreschini, A. Bostwick, C.M. Park, R. Mendelsberg, V. Radmilovic, R. Kostecki, T.J. Richardson, E. Rotenberg, Fluorographene: a wide bandgap semiconductor with ultraviolet luminescence, ACS Nano 5 (2011) 1042–1046. [27] I.-V. Antonova, I.-A. Kotin, I.-I. Kurkina, A.-I. Ivanov, E.-A. Yakimchuk, N.A. Nebogatikova, V.-I. Vdovin, A.-K. Gutakovskii, R.-A. Soots, Graphene/fluorinated graphene systems for a wide spectrum of electronics application, J. Mater. Sci. Eng. 6 (2017) 1000379. [28] Y. Li, F. Li, Z. Chen, Graphane/fluorographene bilayer: considerable C−H···F−C hydrogen bonding and effective band structure engineering, J. Am. Chem. Soc. 134 (2012) 11269–11275. [29] K.I. Ho, C.H. Huang, J.H. Liao, W. Zhang, L.J. Li, C.S. Lai, C.Y. Su, Fluorinated graphene as high performance dielectric materials and the applications for graphene nanoelectronics, Sci. Rep. 4 (2014) 5893. [30] K.I. Ho, M. Boutchich, C.Y. Su, R. Moreddu, E.S. Marianathan, L. Montes, C.S. Lai, A self-aligned high-mobility graphene transistor: decoupling the channel with fluorographene to reduce scattering, Adv. Mater. 27 (2015) 6519. [31] J.A. Conesa, R. Font, Polytetrafluoroethylene decomposition in air and nitrogen, Polym. Eng. Sci. 41 (2001) 2137–2147. [32] W.S. Wong, A. Salleo, Flexible Electronics: Materials and Applications, Springer, New York, 2009. [33] L. Cancado, K. Takei, T. Enoki, M. Endo, Y.A. Kim, H. Mizusaki, A. Jorio, L.N. Coelho, R. Magalhaes-Paniago, M.A. Pimenta, General equation for the determination of the crystallite size La of nanographite by Raman spectroscopy, Appl. Phys. Lett. 88 (2006) 163106. [34] K.I. Ho, J.H. Liao, C.H. Huang, C.L. Hsu, W. Zhang, A.Y. Lu, L.J. Li, C.S. Lai, C.Y. Su, One-step formation of a single atomic-layer transistor by the selective fluorination of a graphene film, Small 10 (2013) 989–997. [35] H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, Evaluation of solution-processed reduced graphene oxide films as transparent conductors, ACS Nano 2 (2008) 463–470. [36] J.T. Robinson, J.S. Burgess, C.E. Junkermeier, S.C. Badescu, T.L. Reinecke, F.K. Perkins, M.K. Zalalutdniov, J.W. Baldwin, J.C. Culbertson, P.E. Sheehan, E.S. Snow, Properties of fluorinated graphene films, Nano Lett. 10 (2010) 3001–3005. [37] Z. Lu, L. Ma, J. Tan, H. Wang, X. Ding, Transparent multi-layer graphene/polyethylene terephthalate structures with excellent microwave absorption and electromagnetic interference shielding performance, Nanoscale 8 (2016) 16684–16693. [38] B. Liu, C.M. Yang, Z. Liu, C.S. Lai, N-doped graphene with low intrinsic defect densities via a solid source doping technique, Nanomaterials 7 (10) (2017) 302. [39] P. Bazylewski, D.W. Boukhvalov, A.I. Kukharenko, E.Z. Kurmaev, A. Hunt, A. Moewes, Y.H. Lee, S.O. Cholakhd, G.S. Chang, The characterization of Co-nanoparticles supported on graphene, RSC Adv. 5 (2015) 75600. [40] J. Chan, A. Venugopal, A. Pirkle, S. McDonnell, D. Hinojos, C.W. Magnuson, R.S. Ruoff, L. Colombo, R.M. Wallace, E.M. Vogel, Reducing extrinsic performance limiting factors in graphene grown by chemical vapor deposition, ACS Nano 6 (2012) 3224–3229. [41] Z. Wang, J. Wang, Z. Li, P. Gong, X. Liu, L. Zhang, J. Ren, H. Wang, S. Yang, Synthesis of fluorinated graphene with tunable degree of fluorination, Carbon 50 (2012) 5403–5410. [42] V. Urbanova, F. Karlicky, A. Matej, F. Sembera, Z. Janousek, J.A. Perman, V. Ranc, K. Cepe, J. Michl, M. Otyepka, R. Zboril, Fluorinated graphenes as advanced biosensors effect of fluorine coverage on electron transfer properties and adsorption of biomolecules, Nanoscale 8 (2016) 12134. [43] Xu Wang, Weimiao Wang, Yang Liu, Mengmeng Ren, Huining Xiao, Xiangyang Liu,

In summary, we investigated the gate-dielectric and substrate buffer strength of Fluorinated graphene on a flexible graphene field effect transistor (F-GFET) using sandwiched structure (FG-PG-FG).This FGPG-FG sandwich structure exhibited a twofold enhancement of the carrier mobility (~340.57 cm2/V·s) with a relatively low gate leakage current of 160 pA compared with those of F-GFETs without FG due to the reduction of scattering mechanism. In addition, the structure shows excellent mechanical stability, with > 88% and 77% of the original output current against bending deformation of up to 6 mm and after 200 bending cycles. This mechanism was likely associated with reduction of interfacial sliding and the high mechanical flexibility of the FG-PG-FG sandwich structure, which creates a suitable insulating environment for the PG and tightly fixes the PG within the sandwich structure under different bending conditions. The combination of high electrical performance and mechanical stability with a low-cost fabrication method makes sandwiched FG a promising candidate for use in the development of future flexible RF electronic devices. Acknowledgments This research was supported by the Ministry of Science and Technology, Taiwan (107-2218-E-182-006, 107-2911-I-182-502, NCRPD2HP011) and Chang Gung Memorial Hospital (CMRPD2G0102). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.143839. References [1] P. Mach, S.J. Rodriguez, R. Nortrup, P. Wiltzius, J.A. Rogers, Monolithically integrated, flexible display of polymer-dispersed liquid crystal driven by rubberstamped organic thin-film transistors, Appl. Phys. Lett. 78 (2001) 3592–3594. [2] J.A. Rogers, Z. Bao, K. Baldwin, A. Dodabalapur, B. Crone, V.R. Raju, V. Kuck, H. Katz, K. Amundson, J. Ewing, P. Drzaic, Paper-like electronic displays: large-area rubberstamped plastic sheets of electronics and microencapsulated electrophoretic inks, PANS 98 (2001) 4835–4840. [3] Kenry, J.C. Yeo, C.T. Lim, Emerging flexible and wearable physical sensing platforms for healthcare and biomedical applications, Microsyst. Nanoeng. 2 (2016) 16043. [4] U. Zschieschang, F. Ante, T. Yamamoto, K. Takimiya, H. Kuwabara, M. Ikeda, T. Sekitani, T. Someya, K. Kern, H. Klauk, Flexible low-voltage organic transistors and circuits based on a high-mobility organic semiconductor with good air stability, Adv. Mater. 22 (2010) 982–985. [5] L. Han, K. Song, P. Mandlik, S. Wagner, Ultra-flexible amorphous silicon transistors made with a resilient insulator, Appl. Phys. Lett. 96 (2010) 042111. [6] X. Du, I. Skachko, A. Barker, E.Y. Andrei, Approaching ballistic transport in suspended graphene, Nat. Nanotech 3 (2008) 491–495. [7] R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, N.M. Peres, A.K. Geim, Fine structure constant defines visual transparency of graphene, Science 320 (2008) 1308. [8] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. [9] Y. Wu, Y.M. Lin, A.A. Bol, K.A. Jenkins, F. Xia, D.B. Farmer, Y. Zhu, P. Avouris, High-frequency, scaled graphene transistors on diamond-like carbon, NATURE 472 (2011) 74–78. [10] H. Wang, D. Nezich, J. Kong, T. Palacios, Graphene frequency multipliers, IEEE Electron Device Lett 30 (5) (2009) 547–549. [11] C. Dimitrakopoulos, Y.M. Lin, A. Grill, D.B. Farmer, M. Freitag, Y. Sun, S.J. Han, Z. Chen, K.A. Jenkins, Y.J. Zhu, Wafer-scale epitaxial graphene growth on the Siface of hexagonal SiC (0001) for high frequency transistors, Vac. Sci. Technol. B. 28 (5) (2010) 985–992. [12] O.M. Nayfeh, Graphene transistors on mechanically flexible polyimide incorporating atomic-layer-deposited gate dielectric, IEEE Electron Device Lett 32 (10) (2011) 1349–1351. [13] C. Sire, F. Ardiaca, S. Lepilliet, J.W.T. Seo, M.C. Hersam, G. Dambrine, H. Happy, V. Derycke, Flexible gigahertz transistors derived from solution-based single-layer graphene, Nano Lett. 12 (2012) 1184–1188. [14] D.J. Cole, P.K. Ang, K.P. Loh, Ion adsorption at the graphene/electrolyte interface, J. Phys. Chem. Lett. 2 (2011) 1799–1803. [15] T. Jiang, R. Huang, Y. Zhu, Interfacial sliding and buckling of monolayer graphene on a stretchable substrate, Adv. Funct. Mater. 24 (2014) 396–402.

10

Applied Surface Science 499 (2020) 143839

M. Sahoo, et al.

[44]

[45]

[46] [47] [48]

[49]

[50]

[51] [52]

[53]

[54] [55]

[56]

[57]

[58]

Controllable defluorination of fluorinated graphene and weakening of C–F bonding under the action of nucleophilic dipolar solvent, Phys. Chem. Chem. Phys. 18 (2016) 3285–3293. G. Hong, Y. Han, T.M. Schutzius, Y. Wang, Y. Pan, M. Hu, J. Jie, C.S. Sharma, U. Müller, D. Poulikakos, On the mechanism of hydrophilicity of graphene, Nano Lett. 16 (2016) 4447. C.-C. Lu, Y.-C. Lin, C.-H. Yeh, J.-C. Huang, P.-W. Chiu, High mobility flexible graphene field-effect transistors with self-healing gate dielectrics, ACS Nano 6 (2012) 4469–4474. W.S. Leong, H. Gong, J.T.L. Thong, Low-contact-resistance graphene devices with nickel-etched-graphene contacts, ACS Nano 8 (2014) 994–1001. K. Alexandrou, N. Petrone, J. Hone, I. Kymissis, Encapsulated graphene field-effect transistors for air stable operation, Appl. Phys. Lett. 106 (2015) 113104. K.K. Liu, W. Zhang, Y.H. Lee, Y.C. Lin, M.T. Chang, C.Y. Su, C.S. Chang, H. Li, Y. Shi, H. Zhang, C.S. Lai, L.J. Li, Growth of large-area and highly crystalline MoS2 thin layers on insulating substrates, Nano Lett. 12 (2012) 1538–1544. Z. Ao, Q. Jiang, S. Li, H. Liu, F.M. Peeters, S. Li, G. Wang, Enhancement of the stability of fluorine atoms on defective graphene and at graphene/fluorographene interface, ACS Appl. Mater. Interfaces 7 (2015) 19659–19665. X. Zhao, B. Zhang, Q. Tang, X. Ding, S. Wang, Y. Zhou, Y. Tong, Y. Liu, Conformal transistor arrays based on solution-processed organic crystals, Sci. Rep. 7 (2017) 15367. R.J. Young, L. Gong, I.A. Kinloch, I. Riaz, R. Jalil, K.S. Novoselov, Strain mapping in a graphene monolayer nanocomposite, ACS Nano 5 (2011) 3079–3084. C. Xu, T. Xue, J. Guo, Q. Qin, S. Wu, H. Song, H. Xie, An experimental investigation on the mechanical properties of the interface between large-sized graphene and a flexible substrate, J. Appl. Phys. 117 (2015) 164301. J.H. Lee, D.W. Jang, S.G. Hong, B.C. Park, J.H. Kim, H.J. Jung, S.B. Lee, Fracture mechanism and electromechanical behavior of chemical vapor deposited graphene on flexible substrate under tension, Carbon 118 (2017) 475–485. S.M. Choi, S.H. Jhi, Y.-W. Son, Effects of strain on electronic properties of graphene, Phys. Rev. B 81 (2010) 81407. G.S. Kliros, Analytical modeling of uniaxial strain effects on the performance of double-gate graphene nano-ribbon field-effect transistors, Nanoscale Res. Lett. 9 (2014) 65. B.J. Kim, H. Jang, S.K. Lee, B.H. Hong, J.H. Ahn, J.H. Cho, High-performance flexible graphene field effect transistors with ion gel gate dielectrics, Nano Lett. 10 (2010) 3464–3466. G. Hu, J. Wu, C. Ma, Z. Liang, W. Liu, M. Liu, J.Z. Wuc, C.L. Jiabd, Controlling the Dirac point voltage of graphene by mechanically bending the ferroelectric gate of a graphene field effect transistor, Mater. Horiz. 6 (2019) 302–310. K.A. Kam, B.I.C. Tengan, C.K. Hayashi, R.C. Ordonez, D.G. Garmire, Polar organic

[59] [60] [61]

[62]

[63]

[64] [65]

[66]

[67] [68]

[69]

[70]

[71]

[72]

11

gate dielectrics for graphene field-effect transistor-based sensor technology, Sensors 18 (2018) 2774. M.A. Bissett, M. Tsuji, H. Ago, Strain engineering the properties of graphene and other two-dimensional crystals, Phys. Chem. 16 (2014) 11124. B. Zhang, Y. Wang, G. Zhai, Biomedical applications of the graphene-based materials, Mater. Sci. Eng. C. 61 (2016) 953–964. L. Zhan, S. Yang, Y. Wang, Y. Wang, L. Ling, X. Feng, Fabrication of fully fluorinated graphene nanosheets towards high-performance lithium storage, Adv. Mater. Interfaces 1 (2014) 1300149. N.J.G. Couto, D. Costanzo, S. Engels, D.K. Ki, K. Watanabe, T. Taniguchi, C. Stampfer, F. Guinea, A.F. Morpurgo, Random strain fluctuations as dominant disorder source for high-quality on-substrate graphene devices, Phys. Rev. X 4 (2014) 041019. C. Neumann, S. Reichardt, P. Venezuela, M. Drogeler, L. Banszerus, M. Schmitz, K. Watanabe, T. Taniguchi, F. Mauri, B. Beschoten, S.V. Rotkin, C. Stampfer, Raman spectroscopy as probe of nanometre-scale strain variations in graphene, Nat. Commun. 6 (2015) 8429. S. Fratini, F. Guinea, Substrate-limited electron dynamics in graphene, Phys. Rev. B 77 (2008) 195415. F. Giannazzo, S. Sonde, R.L. Nigro, E. Rimini, V. Raineri, Mapping the density of scattering centers limiting the electron mean free path in graphene, Nano Lett. 11 (2011) 4612–4618. J. Zabel, R.R. Nair, A. Ott, T. Georgiou, A.K. Geim, K.S. Novoselov, C. Casiraghi, Raman spectroscopy of graphene and bilayer under biaxial strain: bubbles and balloons, Nano Lett. 12 (2012) 617–621. M. Chhikara, L. Gaponenko, P. Paruch, A.B. Kuzmenko, Effect of uniaxial strain on the optical Drude scattering in graphene, 2D Mater. 4 (2017) 025081. M. Zhong, D. Xu, X. Yu, K. Huang, X. Liu, Y. Qu, Y. Xu, D. Yang, Interface coupling in graphene/fluorographene heterostructure for high-performance graphene/silicon solar cells, Nano Energy 28 (2016) 12–18. H. Kumar, L. Dong, V.B. Shenoy, Limits of coherency and strain transfer in flexible 2D van der Waals heterostructures: formation of strain solitons and interlayer debonding, Sci. Rep. 5 (2016) 21516. C. Sire, F. Ardiaca, S. Lepilliet, J.W. Seo, M.C. Hersam, G. Dambrine, H. Happy, V. Derycke, Flexible gigahertz transistors derived from solution-based single-layer graphene, Nano Lett. 12 (2012) 1184–1188. Y. Liang, X. Liang, Z. Zhang, W. Li, X. Huo, L. Peng, High mobility flexible graphene field-effect transistors and ambipolar radio-frequency circuits, Nanoscale 7 (2015) 10954. O.M. Nayfeh, Graphene transistors on mechanically flexible polyimide incorporating atomic-layer-deposited gate dielectric, IEEE Electron Device Lett 32 (10) (2011) 1349–1351.