Direct Chemical-Vapor-Deposition-Fabricated, Large-Scale Graphene

Direct Chemical-Vapor-Deposition-Fabricated, Large-Scale Graphene Glass with High Carrier Mobility and Uniformity for Touch Panel Applications Jingyu Sun,† Zhaolong Chen,† Long Yuan,§ Yubin Chen,† Jing Ning,∥ Shuwei Liu,⊥ Donglin Ma,† Xiuju Song,† Manish K. Priydarshi,† Alicja Bachmatiuk,#,∇ Mark H. Rümmeli,#,∇,○ Tianbao Ma,⊥ Linjie Zhi,∥ Libai Huang,§ Yanfeng Zhang,*,†,‡ and Zhongfan Liu*,† †

Center for Nanochemistry (CNC), Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering and ‡Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China § Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States ∥ National Center for Nanoscience and Technology, Beiyitiao No. 11, Zhongguancun, Beijing 100190, P. R. China ⊥ State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, P. R. China # Center of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej 34, Zabrze 41-819, Poland ∇ Institute for Complex Materials, IFW Dresden, P.O. Box 270116, Dresden 01171, Germany ○ College of Physics, Optoelectronics, and Energy and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, P. R. China S Supporting Information *

ABSTRACT: In this work, we report the transfer-free measurement of carrier dynamics and transport of direct chemical vapor deposition (CVD) grown graphene on glass with the aid of ultrafast transient absorption microscopy (TAM) and demonstrate the use of such graphene glass for high-performance touch panel applications. The 4.5 in.-sized graphene glass was produced by an optimized CVD procedure, which can readily serve as transparent conducting electrode (TCE) without further treatment. The graphene glass exhibited an intriguing optical transmittance and electrical conductance concurrently, presenting a sheet resistance of 370−510 Ω·sq−1 at a transmittance of 82%, much improved from our previous achievements. Moreover, direct measurement of graphene carrier dynamics and transport by TAM revealed the similar biexponential decay behavior to that of CVD graphene grown on Cu, along with a carrier mobility as high as 4820 cm2·V−1·s−1. Such large-area, highly uniform, transparent conducting graphene glass was assembled to integrate resistive touch panels that demonstrated a high device performance. Briefly, this work aims to present the great feasibility of good quality graphene glass toward scalable and practical TCE applications. KEYWORDS: graphene glass, direct CVD, touch panel, transparent conducting electrode, carrier transport

T

possesses excellent features with respect to both transmittance (90%) and conductance (10 Ω·sq−1);9 however, it has faced limitations such as high cost ($20−90 per m2),10 indium scarcity, and chemical instability. For this reason, various

he growing demand for favorable performance and inexpensive material candidates as transparent conducting electrodes (TCEs) has been fueled by the rapidly expanding markets of touch panel displays, lightemitting diodes, and smart windows.1−8 Over the past decades, the dominant TCE material used in commercialized devices is indium-doped tin oxide (ITO), which can normally be obtained by sputtering deposition under vacuum conditions. ITO © 2016 American Chemical Society

Received: September 8, 2016 Accepted: November 22, 2016 Published: November 22, 2016 11136

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Figure 1. Tailored preparation and detailed characterization of graphene glass by a direct CVD route. (a) Photograph of a 6 cm × 4 cm graphene glass sample featuring a clear view of the Weiming Lake and Boya Tower of Peking University. (b) Raman spectra of directly grown graphene on glass possessing single layer (red), few layer (blue), and of bare glass substrate (black). (c) OM image of a graphene film transferred onto a 300 nm SiO2/Si substrate. Scale bar: 10 μm. (d) AC-HRTEM image revealing the good quality of the graphene. Scale bar: 1 nm. (e) C 1s XPS spectrum of as-grown graphene indicative of its high-quality. Inset: Atomically resolved STM image of graphene honeycomb lattice. Scale bar: 0.5 nm. (f) Sheet resistance information and UV−vis transmittance spectra of as-produced graphene glass. Note that the asmeasured transmittance data correspond to the double-side graphene transmittance of graphene glass.

materials, including metal nanowires,7,11−13 metallic nanoparticle inks,14,15 metal oxide films,8 carbon nanotubes, and graphene,1,5,16−19 have recently been explored as alternatives. Among them, graphene has garnered substantial attention due to its two-dimensional structure, chemical stability, wide availability, and more significantly, its excellent combination of electrical conductance and optical transmittance in the visible range. Large-area graphene films have been realized by chemical vapor deposition (CVD) route on Cu foils, readily showing 30 Ω·sq−1 sheet resistance (Rs) with 90% transmittance (T).5,20 However, the complicated transfer procedure during graphenebased TCE fabrication may lead to various process complexity, severe film quality degradation, as well as high production costs. In contrast, solution-processed, printed graphene coatings based on liquid-exfoliated graphene nanoplatelets or reduced graphene oxides (rGO) otherwise hold promise for potentially low costs,21−24 but would normally generate much larger sheet resistance than that required for TCEs. Recent years have witnessed a growing interest in the employment of direct CVD routes for transfer-free graphene syntheses on a plethora of dielectric and insulating substrates.25,26 Recently, our group has reported the tailored production of graphene glass via a series of direct CVD approaches,27−30 where the resulting graphene glass materials demonstrated advanced surface properties (e.g., conductivity, hydrophobicity, and biocompatibility) as distinguished from or complementary to those of traditional glass. Nevertheless, the limited sample dimensions (so far up to several cm in length) as well as the limited graphene quality (3.5 kΩ·sq−1 at a T of ∼83%)27 of as-produced graphene glass narrow its range for practical TCE applications, e.g., touch panels. In this work, we report the controllable synthesis of largearea, uniform, and high-quality graphene glass by our bespoke direct CVD route by scaling-up the furnace dimensions as well as using optimized metal-assisted methods. Of particular note,

the as-produced graphene glass can readily serve as TCEs without any graphene transfer procedure, exhibiting rather high transparency and conductivity (i.e., Rs = 370−510 Ω·sq−1 at T = 82%) as well as uniform transmittance in the visible region. In addition to the traditional analyses, the high quality of the asgrown graphene on glass was reflected by the direct measurement of graphene carrier transport employing transient absorption microscopy, showing a rather high graphene mobility value of 4820 cm2·V−1·s−1. Based on these traits, we further demonstrated that our large-area (4.5 in. in diagonal length) graphene glass-based TCEs can be assembled as resistive touch panels, which exhibited excellent device performance (fluent and stable writing input without functional decay and device linearity comparable to ITO-based touch panels). Such results represent the direct measurement of directly grown graphene carrier dynamics and transport employing transient absorption microscopy as well as detailed demonstration by employing our CVD-derived graphene glass in integrated high-performance touch screen devices. This makes the graphene glass very promising toward scalable applications as next-generation TCEs.

RESULTS AND DISCUSSION The direct CVD approach offers concurrent benefits in both fabrication efficiency and product uniformity of graphene glass. By utilizing CH4-atmospheric pressure CVD at a reaction temperature of 1050 °C, graphene/quartz glass (6 × 4 cm2) samples with good optical transparency (Figure 1a) were obtained in a batch of 6 pieces with a transmittance of 97.1% at 550 nm, as demonstrated by ultraviolet−visible (UV−vis) transmittance spectroscopy measurement (Supporting Information Figure S1). Figure 1b shows the representative Raman spectra of as-produced graphene on glass, revealing that the graphene thickness can be effectively controlled during the synthesis. Note that small optimization of the synthetic route, 11137

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Figure 2. (a) Comparison of the sheet resistance and transmittance of the graphene films obtained in this work and other available data in the literature. (b) T and Rs data for various graphene-based TCEs plotted according to eq 1, mainly including exfoliated graphite (ref 22), rGO (ref 32), directly grown graphene (refs 34−37), and graphene glass in this work, where the slope of the curves is equal to (FoM)−1. (c) Schematic of the construction of a graphene glass-based LED panel. The as-grown graphene is patterned as electrodes and button-type commercial LEDs are attached. (d) Photograph of a 2 cm × 1 cm LED panel on glass demonstrating the large-scale electrically conductive uniformity of the graphene glass TCE (The background “SGG” denotes the acronym for “super graphene glass”). (e) Contour map of surface temperature on a 2 cm × 1 cm graphene glass heating device under an input voltage of 30 V. (f) Frost removal time as a function of input voltage with graphene glass operating as a transparent defroster. Inset: Photographs depicting the defrost performance of device before (upper) and after (lower) heating at 12 V for 3 min.

e.g., employing remote catalysis with Cu vapors31 and/or using optimized growth conditions (higher growth temperature with low carbon flow rate and prolonged growth duration) could result in the quality improvement on the directly grown graphene as compared with our previous studies. For instance, such a route is essentially different from that of our recent work by employing ‘molten-bed CVD route’ for graphene fabrication on normal low softening point glass,29,30 via the same synthetic temperature range and carbon feedstock type. Notably, the current route takes advantage of Cu vapor to aid the decomposition of methane for growing better quality graphene on high-temperature-resistant, solid glass as well as bypasses the use of sample molding and tedious cooling steps usually used for the molten-bed CVD synthesis route. Hence, our current method could be more beneficial and straightforward for practical applications of resultant graphene glass materials. The uniformity of synthesized few-layer graphene at a macroscopic level was evaluated by optical microscopy (OM) imaging of a transferred film (Figure 1c), showing a relatively uniform color contrast. Atomic force microscopy (AFM) image in Figure S2

depicts the morphology of the as-derived CVD graphene on a quartz glass substrate. Moreover, the excellent large-scale uniformity of as-grown graphene was further witnessed by the water condensation experiments on both graphene glass and normal glass, displaying uniform dropwise condensation (hydrophobic) on graphene glass surfaces as compared to film-wise condensation (hydrophilic) on normal glass surfaces (Figure S3).27 Furthermore, atomic-scale characterizations by employing aberration-corrected, high-resolution TEM (ACHRTEM) (Figure 1d) and scanning tunneling microscopy (STM) (Figure 1e inset) clearly show the crystal lattice of obtained graphene, indeed suggesting the rather high crystal quality of graphene on glass. As presented in Figures S4 and 1e, a full-range X-ray photoelectron spectroscopy (XPS) spectrum acquired after the CVD reaction suggests a direct CVD process taken place, while the C 1s spectrum reveals the characteristic signals of high-quality graphene, respectively. It is well-known that the performance of TCE is mainly governed by two key parameters: the sheet resistance and the visible transmittance. Figure 1f displays the transparencies and 11138

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Figure 3. (a) Schematics of TAM setup for hot carrier diffusion measurements in direct CVD grown graphene on glass (OPO: optical parametric oscillator; AOM: acoustic optical modulator). (b) Transient carrier dynamics of the direct CVD fabricated graphene on glass at a photogenerated carrier density of 1.32 × 1013 cm−2 with a pump wavelength of 790 nm and probe wavelength of 620 nm. The red line shows the fitting results with biexponential decay convoluted with a Gaussian response function. (c) Carrier distribution at different time delays, which are fitted with a Gaussian function (red lines). (d) Diffusion coefficient obtained by lineally fitting the variance of Gaussian profiles at different time delays.

The attractive TCE properties (low Rs values at a reasonably high T) of graphene glass facilitate the design of transparent circuits directly on glass. This was reflected by the full operation of a graphene glass-based transparent light-emitting diode (LED) panel (Figure 2c,d), where button-type LEDs were attached on the patterned graphene glass surface with the aid of Ag paste. Furthermore, the optical transparency and electrical conductivity of graphene film directly grown on glass enable Joule heating and hence a surface temperature rise, making our graphene glass suitable for transparent heating applications. A contour map of the surface temperature with a 30 V input (Figure 2e) shows a uniform temperature distribution (49.5 ± 2.1 °C) excluding the electrode areas, also substantiating the high conductivity uniformity of graphene glass samples. To demonstrate the feasibility of graphene glass as transparent heaters, a defroster device with an optical transmittance of 80% and an active working area of 10 × 6 cm2 was constructed. Such a device affords an advanced defrosting performance with a defrost completion time of 180 s with a 12 V input, which is comparable to that of metal-wire-based defrosting windows found in modern vehicles and superior to that of bare glass substrates under identical conditions (Figure 2f and inset). To carry out transfer-free measurement of the carrier mobility for graphene quality evaluation, the carrier dynamics and transport of such direct CVD grown graphene were characterized by ultrafast transient absorption microscopy (TAM; schematic illustration in Figure 3a). The carrier dynamics exhibit biexponential decay behavior, similar to the carrier dynamics in CVD graphene grown on Cu,38 implying comparable carrier scattering pathways. The decay constant of fast component is about 0.5 ps, which accounts for 95% of the total signal (Figure 3b). The fast decay component does not

sheet resistances for the graphene samples directly grown on quartz glass, reflecting our capacity to tailor the optical and electrical properties of graphene glass. Herein, the transparency was measured by the UV−vis transmittance spectroscopy, and the sheet resistance was measured by using the four-probe method. Specifically, the graphene glass sample possessing a sheet resistance of 2.1 kΩ·sq−1 can be reproducibly obtained with a transmittance of 91%. More conductive samples with Rs ∼ 200 Ω·sq−1 can also be realized with an acceptable transmittance of 70% (Figure 1f). The produced graphene glass with tailored graphene coating thickness, simultaneously harnessing optical transparency and electrical conductivity, merits its potential use in TCE applications. Figure 2a compares the Rs of our work with those of other studies as a function of T at 550 nm. Apparently, direct CVD-derived graphene glass attains a Rs of 370−510 Ω· sq−1 with a T ∼ 82%, which is superior to those of rGO,32,33 liquid-exfoliated graphite,22 as well as typical directly grown graphene films,34−37 and are comparable to those of metalcatalyzed CVD graphene2 and certain metallic nanoparticle inks.15 To further quantify the comparison, the figures of merit (FoM, defined as the ratio of DC to optical conductivity, σDC/ σOp) is extracted based on the following expression, reflecting a relationship between T and Rs for a transparent conducting thin film: 2 ⎛ Z σOp ⎞ T = ⎜1 + 0 · ⎟ 2R s σDC ⎠ ⎝

(1)

It is evident that high values of FoM give rise to desirable performances of TCEs.17 As presented in Figure 2b, graphene glass in this work has the highest value of FoM. 11139

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Figure 4. Large-scale, high-quality graphene glass for touch panel applications. (a) Schematic illustration of a graphene glass-based touch panel structure. (b) UV−vis transmittance spectrum in the wavelength range of 350−800 nm of as-produced graphene glass used as touch panel electrode showing a transmittance of 82.7% at a wavelength of 550 nm. Inset: Photograph of a four-wire resistive touch panel using graphene glass as top and bottom electrodes. (c) Linearity test of the graphene glass-based touch panel showing the detected and simulated voltage of the touch screen along its Y-axis (graphene glass responsivity). Inset: Sheet resistance mapping on a 5 × 5 cm2 area of the directly grown graphene film grown on glass. (d) Linearity test result of our touch panel device. The black dots indicate the detected points while the intersections of lines indicate the practical touch points. (e, f) Demonstrations of operating a 4.5 in., four-wire resistive touch panel device connected to a computer (e) and a smart phone (f) with control software by recording “PKU” (computer) and “graphene glass” (smart phone). The word “PKU” is the acronym for “Peking University”. For a movie of its operation see Supporting Information Videos S1, S2, and S3.

CVD graphene was deduced to be 4820 cm2·V−1·s−1, which is superior to those of directly grown films on insulators (277 cm2·V−1·s−1 on sapphire, 472−531 cm2·V−1·s−1 on SiO2, 1510 cm2·V−1·s−1 on Si3N4)26 and comparable with those of transferred films grown on metals (3700 cm2·V−1·s−1 from Ni, 4050 cm2·V−1·s−1 from Cu),2,20 suggesting a rather high quality of our directly grown graphene. Specifically, the TCE properties of our high-quality graphene glass are rigorously tested in touch panel applications. These applications were reported to rely upon low-cost, high-quality, and large-scale graphene films.5,43 In our 4.5 in., four-wire resistive touch screen device (schematics showing in Figure 4a), two pieces of large-area, uniform graphene glass TCEs with a T of ∼83% (Figure 4b) were combined to construct a resistive touch screen (Figure 4b inset; see Methods for touch screen fabrication details). The excellent film uniformity is further evidenced by the Rs mapping result (Figure 4c inset, displaying a uniform Rs value of ∼400−600 Ω·sq−1). By connecting a commercial USB-interface touch screen controller, our device works without any functional decay for both PC (Figure 4e) and smart phone interfaces (Figure 4f). Moreover, linearity tests employing touch panel devices with ITO-PET serving as the top electrode strongly suggest that our device is of high performance, as evidenced by a linearity of 1.3% on SGGresponsible y axis, which is very close to that of typical ITObased materials (1.5% as an industrial criterion) and rGO-based touch screens (1.2−1.4%)43 (Figure 4c,d). Furthermore, the performance stability of the touch panels was evaluated after two-week exposure in ambient environment, still demonstrating fluent writing input of “PKU CNC” without any noticeable decay (Figure S7). Altogether, such results represent the detailed demonstration for the use of directly grown graphene

show obvious power dependence behavior and is assigned to the relaxation of excited hot carriers coupled with an optical phonon of graphene.38 The slow decay component is determined to be ∼3.0 ps (Figure S6) at a carrier density of 4.41 × 1012 cm−2 and increases to ∼3.6 ps at 1.32 × 1013 cm−2. The slow-down of the slow decay component at higher carrier density is assigned to a hot phonon effect.38 Hot carrier transport is directly measured using transient absorption microscopy, which has been successfully employed to study exciton and charge transport in organic semiconductors and perovskite thin films.39,40 The normalized carrier distribution profiles at different time delays fitted by Gaussian functions are shown in Figure 3c. The carrier diffusion coefficient of CVD graphene is related to the time-dependent variance of the Gaussian profiles and can be written as σt , x 2 = σ0, x 2 + 2Dt

(2)

where σt,x and σt,0 are the variance at time delay of zero and t, respectively. More details about the eq 2 can be found in Supporting Information. As shown in Figure 3d, the variance with different time delay is fitted well with a linear function indicative of diffusive transport. We calculated the diffusion coefficient D of directly grown graphene to be 1700 cm2/s. We employ Einstein relation to convert D to carrier mobility μ by

μ=

eD kBT

(3)

where e, kB, and T is elementary charge, Boltzmann constant, and carrier temperature, respectively.41 Here the temperature of carriers in CVD graphene is determined to be 4100 K from the photogenerated carrier density (1.32 × 1013 cm−2) in the diffusion measurement.42 The charge carrier mobility in direct 11140

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Figure 5. (a, b) Lateral force versus displacement curve of directly grown graphene on quartz glass and transferred graphene onto quartz glass, respectively. The hatched portions represent the area under the curve, i.e., the energy required to detach graphene from the glass substrate. The vertical dashed line indicates the point to start tearing the graphene. (c) Comparison plot of both curves (a and b) showing marked increase in the lateral force of directly grown graphene as compared to transferred graphene. Note that several tests were carried out per sample under the same condition to show good reproducibility of data. (d) Zoomed-in Raman spectra taken both on directly grown graphene on quartz glass (lower plot) and transferred graphene on quartz glass produced from Cu foils (upper plot).

the adhesion energy of directly grown graphene and transferred graphene (average value) is 17.74 J·m−2 and 0.49 J·m−2, respectively. These data show a 36-fold increase in the adhesion energy of directly grown graphene as compared to the transferred one, suggesting strong adhesion between graphene and glass for our produced samples. This strong interface interaction is particularly beneficial to touch screen applications. It is furthermore worth-noting that, the electrical conductivity σ of our directly grown graphene on glass is lower than that of transferred graphene from Cu foils (see Figure 2a). According to the Drude formula (σ = neμ), σgraphene depends on the carrier mobility μ and carrier density n. With regard to μ, our TAM measurements have shown a comparable value of directly grown graphene with that of transferred graphene grown from Cu. Hence we believe that the decrease in σ in directly grown graphene is mostly attributed to its lower carrier density, i.e., a lower doping level45 in comparison with transferred graphene. To testify this, we compare the Raman spectra of directly grown and transferred graphene on quartz glass, since it is well-known that the G and 2D peak positions are sensitive to the changes in the carrier density.45,46 The representative Raman spectra in Figure 5d display marked changes in peak positions: the G (2D) peak for the directly grown film (green solid lines) starts at 1585 cm−1 (2690 cm−1) and upshifts to 1595 ± 2 cm−1 (2697 ± 1 cm−1) for the transferred film (green dashed lines). These upshifts of G and 2D peaks could be attributed to hole doping rather than strain effect, owing to the fact that the ratios of the upshifting change of G and 2D peaks are not consistent with a known value (1:3).47,48 Namely, no doping process originating from Cu

on glass in integrating high-performance touch panel devices. Compared to other touch screen materials (ITO, Ag nanowires, rGO, Ni ink), the potential use of graphene glass in touch panel and related TCE applications is greatly encouraging, owing to its low-cost, robustness, availability, facile scalability, and corrosion-resistance. From a touch panel application point of view, it is meaningful herein to measure the adhesion force/energy between directly grown graphene and the underneath glass substrate, since strong adhesion is essential for device fabrication. In this work, the adhesion force/energy of the graphene film on glass was measured using a mechanically nanoscratch method44 with the aid of an AFM system (Cypher Asylum Research). The tip radius for the experiment was ∼7 nm, and a normal load of 19.6 μN was applied. A 350 nm long scratch was made with a velocity of 100 nm·s−1, starting on the substrate (∼150 nm away from the graphene edge), going through the graphene film, and tearing it along the scratch length. For an accurate estimation, the lateral force curve was subtracted by the value of the lateral force on the bare substrate (without graphene). The integral of the area under the lateral force versus displacement curve was calculated to estimate the adhesion energy of the graphene. Figure 5a,b displays the lateral force versus displacement curves of nanoscratch measurements for two representative samples of directly grown graphene and transferred graphene on quartz glass, respectively (with Figure S8 displaying friction images of directly grown graphene prior to and after the scratch test). Figure 5c shows a comparison plot between such two curves, clearly illustrating higher lateral force needed to tear the directly grown graphene. Our calculation further indicates that 11141

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70% of the pulse energy was fed into the optical parametric oscillator (Coherent Mira OPO) to generate probe light at 620 nm, while the remainder 30% was served as the pump beam. The pump beam was modulated at 1 MHz using an acoustic optical modulator (model R21080-1DM, Gooch&Housego). A 40× NA = 0.60 objective (CFI Apo TIRF, Nikon Inc.) was used to focus the laser beams onto the sample, and the transmission light was then collected by an optical condenser and detected by an avalanche photodiode (Hamamatsu C5331-04). The change in the probe transmission (ΔT) induced by the pump was detected by a lock-in amplifier (HF2LI, Zurich Instrument). For transient dynamics scan, pump beam and probe beam were overlapped spatially, and a mechanical translation stage (Thorlabs, LTS300) was used to delay the probe with respect to the pump. A two-dimensional galvo mirror (Thorlabs GVS012) was used to scan the probe beam relative to the pump beam in space to obtain the carrier diffusion. Touch Panel Device Fabrication. Ag ink was printed on both pieces of graphene glass as x- and y-axes electrodes, respectively. UVcured adhesive was then printed on the bottom glass as a space dot layer. Flexible printed circuit was connected to the leading-out terminals of the four Ag electrodes. Finally, the bottom and top conductive layers were accurately aligned and sealed together. To fabricate ITO-graphene glass-based touch panel device, ITO was used as the top conductive layer instead of graphene glass.

etchants as well as from the transfer procedures could result in a relatively lower electrical conductivity of the directly grown graphene on glass.

CONCLUSIONS In summary, our data show that the direct CVD synthesis of high-quality graphene on glass enables the as-produced graphene glass to become promising for a next-generation TCE material. The graphene glass TCE exhibits highperformance (i.e., Rs = 370−510 Ω·sq−1 at T = 82%) and uniform transmittance in the visible region. The transfer-free measurement of graphene carrier transport employing transient absorption microscopy shows a high graphene mobility value of 4820 cm2·V−1·s−1. Further application demonstration on touch panel devices integrated by using such graphene glass TCE material is representatively presented. This work demonstrates the great feasibility of using graphene glass material for scalable and practical TCE applications. METHODS Large-Scale Production of High-Quality Graphene Glass. The graphene glass was fabricated according to the modified CVD recipes reported in our previous work. Briefly, in a typical CVD process, quartz or sapphire glass was thoroughly cleaned with deionized water, acetone, and isopropyl alcohol before loaded into a horizontal quartz tube (3 in. inner diameter) placed inside a threezone high-temperature furnace. For each growth, a surrounding Cu foil aiming to supply the floating Cu catalyst was selectively placed along the tube wall at the upper stream, where the distance between the Cu strip and glass substrate was approximately 4 cm. The CVD system was flushed with 500 sccm Ar to remove air prior to ramping the temperature. The furnace was then heated to the desired growth temperature (normally 1050−1100 °C) and stabilized for about 10 min. Optimized growth conditions for growing uniform and highquality graphene on glass were concerned with a gas mixture of 500 sccm Ar, 50 sccm H2, and 13.5 sccm CH4, a growth temperature of 1100 °C, and growth duration at 6 h. The thickness and uniformity of graphene samples can be controlled by varying the growth time and the flow rate of gases or by altering the growth temperature. Characterizations. To systematically characterize the as-obtained graphene glasses, we employed optical microscopy (Olympus DX51), SEM (Hitachi S-4800; operating at 1 kV), Raman spectroscopy (Horiba, LabRAM HR-800; 514 nm laser excitation), UV−vis spectroscopy (PerkinElmer Lambda 950 spectrophotometer), AFM (Vecco Nanoscope IIIa; working at a tapping mode), XPS (Kratos Analytical Axis-Ultra spectrometer using a monochromatic Al Kα Xray source), and TEM (FET Tecnai F20; operating at 200 kV). A Cu Quantifoil TEM grid was used for TEM characterization, onto which graphene film was transferred with the aid of polymethyl methacrylate coating. The atomically resolved TEM investigations were performed on a third-order aberration corrected (objective lens) FEI Titan3 30080 operating with an acceleration voltage of 80 kV. The sheet resistances of the films were measured using a four-probe resistance measuring meter (Guangzhou 4-probe Tech Co. Ltd., RTS-4) based on four-point probes method after gentle doping the as-grown graphene glass samples with the aid of chloroauric acid (HAuCl4). Four metal probes were aligned in a line at intervals of 1 mm. The outer pair probe was current-carrying electrode, and the inner pair probe was voltage-sensing electrode. For the temperature measurements of graphene-based heaters, a K-type thermocouple with a digital thermometer (HH11B, Omega) was employed. The temperature was kept constant during the measurements. Transient Absorption Microscopy Study. Charge carrier diffusion in direct CVD grown graphene was measured by a homebuilt TAM system shown in Figure 3a. Briefly, a Ti:sapphire oscillator (Coherent Mira 900) pumped by a Verdi diode laser (Verdi V18) was used as the light source (output at 790 nm, 80 MHz repetition rate).

ASSOCIATED CONTENT S Supporting Information *

and videos. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acsnano.6b06066. Graphene glass characterization, data analysis, and supporting figures (PDF) Video 1: Operation of SGG-based resistive touch screen device connected to a PC with control software by recording “SGG” (AVI) Video 2: Operation of SGG-based resistive touch screen device connected to a smart phone with control software by recording “PKU CNC” (AVI) Video 3: Operation of SGG-based resistive touch screen device connected to a smart phone with control software by recording “graphene” (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: zfl[email protected]. *E-mail: [email protected]. ORCID

Zhongfan Liu: 0000-0003-0065-7988 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology of China (grants 2016YFA0200103, 2013CB932603, 2012CB933404, 2013CB934600), the National Natural Science Foundation of China (grants 51432002, 51290272, 51121091, 51222201, 11222434), the Ministry of Education (20120001130010), and the Beijing Municipal Science and Technology Planning Project (Z151100003315013). A.B. and M.H.R. acknowledge the Sino-German Center for Research Promotion (grant GZ 871). L.H. and L.Y. acknowledge the support from the National Science Foundation of United States under the award number 1433490. 11142

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ACS Nano

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DOI: 10.1021/acsnano.6b06066 ACS Nano 2016, 10, 11136−11144