Source: https://pubs.rsc.org/en/content/articlehtml/2019/na/c8na00181b
Timestamp: 2019-04-25 12:01:45+00:00

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In the race to find novel transparent conductors for next-generation optoelectronic devices, graphene is supposed to be one of the leading candidates, as it has the potential to satisfy all future requirements. However, the use of graphene as a truly transparent conductor remains a great challenge because its lowest sheet resistance demonstrated so far exceeds that of the commercially available indium tin oxide. The possible cause of low conductivity lies in its intrinsic growth process, which requires further exploration. In this work, I have approached this problem by controlling graphene nucleation during the chemical vapor deposition process as well as by adopting three distinct procedures, including bis(trifluoromethanesulfonyl)amide doping, post annealing, and flattening of graphene films. Additionally, van der Waals stacked graphene layers have been prepared to reduce the sheet resistance effectively. I have demonstrated an efficient and flexible transparent conductor with the extremely low sheet resistance of 40 Ω sq−1, high transparency (Tr ∼90%), and high mechanical flexibility, making it suitable for electrode materials in future optoelectronic devices.
Significant efforts have been made so far to reduce the Rs of graphene film by chemical doping,17–19 substitutional doping,20–22 or stacking of graphene films7,23,24 in parallel while maintaining a high Tr to improve its potential to be harnessed in optoelectronic applications. Although chemical doping has been shown to effectively reduce the Rs of graphene down to 150 Ω sq−1 at Tr = 87%,12 125 Ω sq−1 at Tr = 97.4%,13 and 80 Ω sq−1 at Tr = 80% for eight-layer stacked graphene,7 the adsorption of moisture, other atoms, and molecules onto graphene after chemical treatment leads to the increase of Rs within few days.25,26 Similarly, substitutional doping induces disorder in the graphene, thereby reducing its mobility as a result of scattering of Dirac electrons.27,28 Additionally, most doping processes decrease the optical transparency of graphene film by changing its band structure or by forming optically reflective nanoparticles at the surface,29,30 making it unsuitable for optoelectronic applications. Recently, a new growth technique has been introduced by Han et al.31 which involves cyclic injection of carbon precursor (CH4) during chemical vapor deposition (CVD) to suppress the multilayer patches generally formed using continuous CH4 flow, thereby improving both the optical and electrical performance of graphene. Similarly, other approaches, such as trifluoromethanesulfonimide (TFSA) doping,19,32–34 post annealing,35,36 and flattening of graphene films37 have been carried out to reduce the Rs of graphene. However, no systematic study exists so far that combines all the aforementioned approaches, which is highly desired to achieve superior graphene film suitable for the transparent conducting electrode.
In the present work, I have prepared a high-quality graphene film using pulsed-grown technique and investigated its optical and electrical performance using the combined effects of TFSA doping, stacking, post annealing and flattening. The prepared graphene film shows Rs of a few hundred Ω sq−1, on/off ratio of ∼13, along with a high I2D/IG ratio (>1.5). TFSA doped four layer stacked graphene films show a low Rs of 40 Ω sq−1 and transmittance of 90%, which are almost comparable to those of ITO. The combination of TFSA doping, post annealing and flattening of graphene film is found to reduce the Rs by 80%, the highest value reported so far. Finally, the mechanical flexibility of this highly conducting and transparent film has been demonstrated, making it suitable for application as a flexible transparent conducting electrode.
Growth of monolayer graphene on Cu substrate using CVD has been a well-established technique since 2009,38–40 where the nucleation and growth of graphene usually occurs by exposure of the Cu surface to a hydrocarbon gas under atmospheric pressure, low pressure or ultra-high vacuum (UHV) conditions. However, the grain boundaries in such graphene films act as scattering centers for electron waves, limiting electron mobility.41,42 To minimize the grain boundary effect, I have modified the graphene growth process by supplying CH4 in short pulses31 instead of the conventional continuous CH4 flow during the CVD growth of graphene on Cu, as illustrated in Fig. 1. The details of the synthesis process are provided in Fig. S1 (see ESI†). In the case of continuous CH4 flow used in literature so far, the rate of nucleation is very high, which leads to high density of graphene nucleation seeds over the Cu surface.43–47 These nucleation seeds interfere each other when growth persists, and hence, the graphene domains do not grow in the preferred orientation, and the average graphene domain size shrinks. On the other hand, in the pulsed-grown method, graphene domains are seen to coalesce in a preferred direction to form larger islands of ∼100 μm size (shown by arrows in Fig. S2 of ESI†). This provides evidence that the nucleation rate is expected to be slow in the pulsed-grown method, which leads to low density of graphene nucleation seeds. It can be noted that the preliminary annealing of the Cu substrate leads to a reconstruction of atomic distribution over the surface,48 which causes directional growth of graphene domains, but not coalescence. As the Cu substrates used in this work were not pre-annealed, the directional growth due to substrate effect can be ruled out.
Fig. 1 (a) Schematic of the designed CVD system with introduction of periodic CH4 pulses into the reaction zone. (b) OM image of pulse-grown graphene film transferred onto marked 300 nm SiO2/Si substrate. The graphene film is clearly distinguishable from bare SiO2 with the color contrast. (c) Camera image of 3.4-inch graphene film transferred onto 125 μm thick PET substrate. (d) Raman spectrum of transferred graphene film on SiO2 substrate. Both G and 2D band peaks are very symmetric with full width at half maximum (FWHM) of 15 and 30 cm−1, respectively. The I2D/IG intensity ratio is >1.5, which confirms that the prepared graphene is monolayer. A slight D band intensity is present in the Raman spectrum, which probably arises from the etching process. The inset shows the R vs. Vg plot for graphene-based FET in back-gate configuration.
Several methods have been perused in order to reduce the Rs of graphene, including doping with TFSA, post annealing, flattening and stacking of graphene films. The schematics of the above processes are mentioned in Fig. S5 (see ESI†), and the overall processes are summarized below.
Fig. 2 Effect of TFSA doping on the Raman spectra of pulsed-grown graphene. (a) Optimized position of TFSA molecules absorbed on the surface of monolayer graphene. The constituent atoms of TFSA, such as N, S, O, C and F are marked in blue, yellow, red, grey and green, respectively. (b) Band structure calculation of TFSA-doped graphene using DFT. The band gap is almost zero; i.e., the gap between conduction band minimum and valence band maximum at the high symmetry K point of the Brillouin zone and the Fermi level shifts below VBM due to p-doping. (c) Raman spectra of CVD graphene before and after TFSA doping. The G and 2D peaks of both the graphene films are extrapolated and plotted in separate panels for comparison. Compared to pristine graphene, doped graphene shows a large blue shift for both G peak (ΔG = 6 cm−1) and 2D peak (Δ2D = 33 cm−1), which confirms that there is charge transfer from graphene to TFSA.
In the pursuit of transparent conducting applications, the optical and electrical properties of 1 × 1 cm2 pristine and doped graphene films prepared by pulsed-grown method were analyzed, as shown in Fig. 3. The optical transmittance of the graphene films in the visible range was measured using an UV-VIS-NIR spectrophotometer (UV-3600, Shimadzu). For this measurement, graphene films were transferred on a cover glass as a single layer and as multiple stacked layers, and a bare cover glass was used as a reference. Multiple-layered “graphene stacks” were created using the transfer process sequentially. Fig. 3a depicts the wavelength dependence of the optical transparency of one- to four-layer-stacked pristine graphene films. The transmittance, Tr (%), of stacked graphene films at λ = 550 nm extracted from (a) was found to be 97.3, 94.6, 92.9 and 90 for one, two, three and four layers, respectively (Fig. 3b). Because each graphene layer has an opacity of 2.3%,8 the transmittance decreases by ∼2.3% for each added layer, indicating that the film behaves as a set of individual graphene layers.8,59 A slight variation in the transmittance values as per layer numbers predicted from theoretical calculation may be due to impurities present in the interface of the artificially built weak van der Waals stacked layers.60 Similar behavior has also been observed by Karsy et al.7 for stacked graphene layers. The evolution of graphene layers is also detectable from the change in color contrast observed from the optical images, as shown in the inset of Fig. 3b, which is minimum for one layer and maximum for four layers. The optical transmittance of TFSA-doped graphene also varies in a similar fashion to pristine graphene. For comparison, the optical transmittance of pristine and doped monolayer graphene is shown in Fig. 3c. It is observed that the Tr values of both films at λ = 550 nm are almost same, and hence, TFSA doping has no effect on the Tr of the graphene film. Fig. 3d shows the Rs of pristine and doped graphene films measured using van der Pauw method. The details of Rs measurement of graphene films are shown in Fig. S6 (see ESI†). In both the cases, Rs decreases with the increase in number of graphene layers. The Rs of monolayer graphene film is 288 Ω sq−1 with Tr of 97.3%, whereas that of four-layer graphene is 86 Ω sq−1, while still maintaining a relatively high Tr of about 90%. Similarly, the Rs of doped graphene varies from 134 Ω sq−1 to a lowest value of 40 Ω sq−1 with the increase in number of graphene layers from one to four, while maintaining the same Tr values as those of pure graphene layers. In principle, the Rs of multilayer film, Rsn, is related to the Rs of one layer film, Rs1, by the relation Rsn = Rs1/n, where n is the number of graphene layers if all layers are acting independently of each other. However, the average Rs of our films decreases slightly by stacking four graphene layers. The discrepancy might be explained by considering that the cracks in one film are bridged by its neighboring films, thus increasing the conductivity.61 As stated earlier, when an electron is transferred from the graphene to TFSA, graphene becomes p-doped according to eqn (2). This results in a shift of the Fermi level (Fig. 3b), which increases the carrier concentration and thus the conductivity of the graphene layers. With the increase in layer numbers, there is further increment in the carrier concentration and density of states (DOS) at the Dirac point, which results in higher conductivity. This effect was first observed with single-walled carbon nanotubes (SWCNTs), where the doping of the SWCNTs with TFSA led to an increase in the conductivity of SWCNT films.52 Since SWCNTs and graphene have essentially the same chemical structure, it is worth noting that TFSA doping yields lower sheet resistance.
Fig. 3 Evaluation of pulsed-grown graphene films as transparent conductor. (a) Transmittance spectra of one- to four-layer stacked graphene films in the visible region of the energy spectrum. (b) Tr (%) as a function of the number of graphene layers extracted from (a) at λ = 550 nm. Inset shows photographs of stacked graphene layers. (c) Comparison of transmittance spectra between one-layer pristine graphene and TFSA-doped graphene. (d) Rsvs. Tr (%) of pristine and TFSA-doped graphene films for one to four stacked layers.
Fig. 4 (a) Comparison of Rs of the pulsed-grown graphene films with those reported in the literature as a function of Tr (%) at λ = 550 nm. An extremely low Rs of 40 Ω sq−1 was observed for TFSA-doped four-layer stacked graphene films at a Tr of 90%, which is almost comparable with that of ITO.62,63 The Rs values of pristine (black squares) and TFSA-doped graphene films (red circles) of the present work are very low compared to those of wet transferred graphene (blue-green triangles)61 and HNO3 doped graphene (purple triangles).60 However, these values are slightly higher than the theoretical calculated values.64 For the monolayer case, the Rs of the present graphene films is also low compared to CHCl3-doped graphene (olive-green triangle)24 and annealed graphene (chocolate-brown pentagon),65 but slightly higher than KI-doped graphene (blue diamond).66 (b) Reduction in Rs (ΔRs) of graphene films by TFSA doping (T), post annealing (A), flattening (F) and some combination of these processes. The maximum ΔRs is 68% for TFSA doping compared to post annealing (66%) and flattening (22%). The combination of all three effects (T + A + F) shows maximum ΔRs of 80%, whereas other combinations (A + T), (F + T) and (A + F) show ΔRs of 79, 70 and 64%, respectively.
Besides TFSA doping (T), I carried out post annealing (A) and flattening (F) of graphene films to reduce the Rs, as shown in Fig. 4b. The maximum reduction in the Rs (ΔRs) was observed for T (68%) compared to A (66%) and F (22%). Interestingly, the combination of all three effects (T + A + F) shows a maximum ΔRs of 80%, whereas other combinations (A + T), (F + T) and (A + F) show ΔRs of 79%, 70% and 64%, respectively. The above methods also have been applied to graphene prepared by continuous CH4 flow in literature, including TFSA doping,19,33,67 annealing,35,36 and stacking,7,23,24 as shown in ESI Table S1.† The reduction is Rs value is 30–75%, 7–30% and 50–58% for TFSA doping, annealing and stacking, respectively, which are comparatively less with reference to pulsed-grown graphene. During post annealing, the residual PMMA present on the surface of graphene decomposes at a temperature > 160 °C.49 It is expected that the decomposition products of this molecule form cross links with the graphene surface by forming dangling bonds, and the surface absorbs O2 or water vapor during cooling to room temperature, becoming heavily p-doped, which leads to increased conductance. Flattening of graphene films was carried out using PMMA, in which the PMMA/graphene stack was prepared and heated at elevated temperatures (∼150–160 °C). Heating of PMMA resulted in softening and expanding PMMA, allowing it to relax flat upon the substrate and thereby minimizing any wrinkling of the graphene, in turn restricting mechanical and electrical degradation, as reported by Goniszewski et al.37 However, the effect of flattening was found to be very insignificant compared to TFSA doping and post annealing (Fig. 4b).
Fig. 5 Analysis of electromechanical properties of graphene films. (a) Photograph showing highly transparent graphene film transferred onto flexible PET substrate. (b) Reduction in Rs of graphene film after TFSA doping and its environmental stability test. The Rs remains preserved up to 18 days, revealing superior environmental stability. (c) The Rsvs. bending angle of pristine graphene measured on PET substrate under different bending conditions. The arrows show the direction of bending and recovery. Inset shows a photograph of the mechanical flexibility test setup. (d) The Rsvs. bending cycles for single-layer graphene/PET substrate. The Rs remains unperturbed up to 100 bending cycles. Inset shows the relative change in Rsversus strain due to bending. At a maximum tensile strain of 1.77%, the Rs of the graphene film increased by 10–11% only.
where t is the thickness of the PET substrate. Fig. 4c inset shows the relative change in Rsversus strain due to bending. At a maximum tensile strain of 1.77%, the Rs of the graphene film increased by 10–11%, which is much smaller than the 321% observed for ITO at 2.14% tensile strain.74 Such a minor change in the Rs implies that the prepared graphene films are much more robust against bending and have advantages over ITO films in terms of superior mechanical flexibility.
Graphene sheets used in the current study were grown by atmospheric pressure chemical vapor deposition (APCVD) on polycrystalline Cu foils, using methane (99.99%) as carbon source. Prior to growth, the Cu foils were cleaned with acetone and isopropyl alcohol (IPA) by sonication, followed by etching in acetic acid for 30 min to remove surface oxides. Then, the Cu foils were mounted in the CVD chamber, and graphene growth process was configured in several steps in the presence of steady flow of hydrogen and argon gas in the proportion of 10–20 : 300 sccm. The schematic of the growth process is shown in Fig. S1 (see ESI†). In the first step, the furnace was ramped up to 1050 °C over 40 min. In the second step, pre-annealing was carried out by holding the temperature at 1050 °C for 30 min. By this step, the oxide on the copper foil surface was being removed and the internal stress was released by recrystallization to make the surface of copper foil smoother. Then, methane with flow rate of <1 sccm was fed into the reaction chamber for 50 min (15–25 cycles) in step 3, during which graphene growth occurred. In each cycle, CH4 pulse was given for 30 s and then released for 1 min. This process was carried out by means of a control device, which controlled the import flow rate of the CH4 gas by controlling the opening and closing of the gas valve. That is to say, the CH4 supply during this reaction step was changed continuously (first increased and then decreased in one cycle), which is different from the constant gas supply process in prior literature. Finally, the Cu foils were moved to the cooling zone, which is equipped with a cooling system, in step 4.
Graphene layers on Cu foils were coated with 300 nm thick PMMA, followed by etching in a 16.5% aqueous hydrochloric acid solution. The PMMA film, along with the attached graphene, was detached from the metal substrate after a whole night. The PMMA/graphene film was then rinsed with DI water, transferred to a silicon or target substrate, and rinsed with warm acetone to remove the PMMA. Photographs of the graphene transfer process are shown in Fig. S4 (see ESI†).
TFSA-doped graphene was prepared by spinning a few droplets of TFSA over graphene film at 3000 rpm for 30 s. Post annealing was carried out in two steps, first at 250 °C in air and then at 250 °C in the presence of mixed H2 (200 sccm) and Ar (400 sccm) atmosphere for 2 h, similar to Lin et al.49 Flattening of graphene film was performed by spin coating of PMMA at 3000 rpm for 30 s, followed by heating at elevated temperatures (∼150–160 °C) and, finally, removing PMMA using warm acetone. Stacked graphene films were prepared by sequential transfer of one graphene layer over another, up to four layers. No adhesive was used at the interfaces, and the interlayer bonding is only due to van der Waals force.
Band structure calculation was performed by placing the TFSA molecule on the top of single-layer graphene. The pseudopotential density functional SIESTA package was used for band structure calculations,75,76 with the local density approximation (LDA) representing the exchange correlation potential77,78 and with an energy mesh cutoff of 150 Ry. Optimization of initial experimental structures was performed using analytical energy gradient with respect to atomic coordinates and unit cell parameters. The structures were relaxed until the force on each atom was less than 0.01 eV Å−1. The Brillouin zone was sampled with a grid of 5 × 5 × 1 k-points within the Monkhorst–Pack scheme.79 Band structures were calculated along the high symmetry points using the path Г–M–K–Г.
In summary, high-quality graphene was prepared by supplying methane pulses instead of the conventional continuous methane flow used in CVD. The lowest sheet resistance of a few hundred Ω sq−1, the on/off ratio of ∼13 for the graphene-based FET, and the high I2D/IG ratio of >1.5 demonstrate the quantitative signatures of graphene quality. Several processes, including TFSA doping, post annealing, flattening and stacking of graphene films, were carried out in order to reduce the sheet resistance, making them suitable for application as transparent conducting electrode. An extremely low sheet resistance of 40 Ω sq−1 for TFSA-doped four-layer stacked graphene films at a transmittance of 90% was reported, which is almost comparable with that of ITO. TFSA doping was found to be very effective for increasing the conductivity of graphene film. Interestingly, the combination of TFSA doping, post annealing and flattening of graphene films was found to reduce the sheet resistance by 80%, which is the highest value reported so far. Besides, the superior mechanical flexibility of this highly conducting and transparent film was also demonstrated, making it suitable as a transparent conducting electrode. This work provides a new pathway to obtain a suitable transparent conducting electrode for applications in various optoelectronic devices such as solar cells, touch screens, and transparent displays in the future.
The author would like to acknowledge the financial support from the Department of Science and Technology, India, with sanction order No. SERB/F/5379/2017-2018 under Ramanujan Fellowship. The author also acknowledges Chao-Hui Yeh of National Tsing Hua University, Taiwan, for graphene transfer as well as useful discussion.
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