Direct Growth of Graphene Films on 3D Grating Structural Quartz

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Direct Growth of Graphene Films on 3D Grating Structural Quartz Substrates for High-performance Pressure-Sensitive Sensor Xuefen Song, Tai Sun, Jun Yang, Leyong Yu, Dacheng Wei, Liang Fang, Bin Lu, Chunlei Du, and Dapeng Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04526 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 9, 2016

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ACS Applied Materials & Interfaces

Direct Growth of Graphene Films on 3D Grating Structural Quartz Substrates for High-performance Pressure-Sensitive Sensor Xuefen Song,a,b Tai Sun,a Jun Yang,a Leyong Yu,a Dacheng Wei,c Liang Fang,b Bin Lu,a Chunlei Dua and Dapeng Weia* a

Chongqing Key Laboratory of Multi-scale Manufacturing Technology, Chongqing Institute of

Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China b

State Key Laboratory of Mechanical Transmission, College of Physics, Chongqing University,

Chongqing, 400044, PR China c

State Key Laboratory of Molecular Engineering of Polymers & Department of Macromolecular

Science, Fudan University, Shanghai 200433, China

KEYWORDS: Conformal graphene, Grating-structured quartz substrates, Direct CVD growth, Pressure-Sensitive Sensor, Wind pressure. ABSTRACT: The conformal graphene films have directly been synthesized on the surface of grating micro-structured quartz substrates by a simple chemical vapor deposition (CVD) process. The wonderful conformality and relatively high quality of the as-prepared graphene on threedimensional substrate have been verified by scanning electron microscopy (SEM) and Raman

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spectra. This conformal graphene films possess excellent electrical and optical properties with the sheet resistance of < 2000sq-1 and the transmittance of > 80% (at 550nm), which can be attached with a plat graphene film on PDMS substrate, and then could work as a pressuresensitive sensors. This device possesses the high pressure sensitivity of -6.524kPa-1in a lowpressure range of 0-200 Pa. Meanwhile, This pressure-sensitive sensor exhibits super reliability (≥5000 cycles) and ultrafast response time (≤4ms). Owing to these features, this pressuresensitive sensors based on the 3D conformal graphene is adequately introduced to test wind pressure, expressing higher accuracy and lower background noise level than a market anemometer. 1. INTRODUCTION Graphene, a two-dimensional honeycomb lattice of carbon atoms,1 possesses remarkable mechanical2-3, electrical and optical properties4, and was considered as an ideal candidate of the next-generation transparent conductive films5. For the practical applications, three dimensional (3D)superficial structures exist in a lot of functional devices, such as black silicon solar cells,6 cambered micro-optics7 and three dimensional Micro-electromechanical systems (MEMS) sensors8 etc. It is necessary that graphene film need be conformally covered on the 3D structural surface. For traditional approach, graphene film was grown on the catalytic metal wafer, and then was transferred to the dielectric substrates by poly(methyl methacrylate) PMMA media.9 However, this traditional transfer method does not only bring the risk of damage and


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contamination, but was also difficult to conformally and tightly adhere 2D graphene film onto the 3D-structural surface. 10

The direct growth technique of graphene on dielectric substrates might provide one of the optional ways to solve the above-mentioned problems. Until now, graphene films have been directly produced by chemical vapor deposition (CVD) method or Plasma enhanced chemical vapor deposition (PECVD) method on non-catalytic planar substrates, such as Si,11SiO2,12 Ge,13 SiN,14 sapphire,15 and quartz16, and display the poly-crystalline structure.

11-12

However, in these

reports,11 the growth surfaces were planar, so these experimental results could not fully reflect and deduce the growth of graphene on 3D micro-structured dielectric surface. Different from the planar growth, the perturbation of gas flow on the 3D structured surface might bring the nonuniform nucleation and growth. Hence, a more systematic and comprehensive investigation is necessary for understanding the crystallinity, uniformity and conformality of the graphene films on the whole surface of 3D substrate. Moreover, the transparent conductive property of 3Dstructured graphene film need also be carefully investigated.

Herein, we demonstrated that the direct CVD method is a simple and effective approach to produce conformal graphene films on 3D grating structural quartz substrate. The wonderful conformality and relatively high quality of graphene film on 3D micro-structured quartz wafer have been verified by Scanning Electron Microscopy (SEM) and Raman spectra. Furthermore, this conformal graphene film possesses the relatively high conductive ability and


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transparency(sheet resistance 80%, 550nm), and can be directly used as an electrode in pressure-sensitive sensors, which provide higher sensitivity in low-pressure regimes than previous reports.17 And the pressure-sensitive sensor based on the 3D conformal structured graphene exhibits super reliability (≥5000 cycles) and ultrafast response time (≤4ms). Simultaneously, the pressure-sensitive sensor could be used to test wind pressure, and exhibits higher accuracy than a market anemometer.

2. EXPERIMENTAL SECTION 2.1. Preparation of the 3Dgrating structural quartz substrates: The micro-scale patterning on quartz substrates were prepared by photo-lithography systems, and were etched by ion beam etching (IBE) system to obtain the 3D grating structural quartz substrates. 2.2. Synthesis of graphene: Graphene films were produced using a chemical vapor deposition (CVD) system. The clean substrates were placed into the center of quartz tube, and were heated at 950°C. Under atmospheric pressure, mixture gases of methane (CH4), hydrogen (H2) and argon (Ar) were introduced into the chamber. The graphene growth maintained for 60-120 minutes, and then the CH4 gas was turned off. Finally, the samples were cooled to room temperature under Ar gas flow. 2.3. Preparation of the pressure-sensitive sensor: According to our previous work,3 the preparation of polydimethylsiloxane/graphene (PDMS/Gr) film includes two major steps. Firstly,


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graphene films were deposited via CVD process on copper wafer (Cu). The graphene film was then transferred onto the flexible polymer PDMS to obtain a composite structure of PDMS/Gr by PDMS coating, solidification, and Cu etching. The pressure-sensitive sensor could be assembled by attaching PDMS/Gr film on to the upside of Gr/GQ. 2.4. Measurements and Characterization: The surface morphology of the conformal graphene was directly characterized by ﬁeld-emission scanning electron microscopy (SEM, JSM-7800F). Raman spectra (Renishaw inVia ManualWiRE3.4, with 532 nm laser) were used to characterize the crystallinity and uniformity of graphene. An UV–visble–NIR spectrophotometer (Hitachi U4100) was used to measure the optical properties of the Gr/GQ. The sheet resistance of Gr/GQ and the pressure-sensitive sensor were measured by electrochemical workstation system. The details of experiments are shown in methods of the supplementary materials. 3. RESULTS AND DISCUSSION

The preparation and test process of conformal graphene on 3D grating micro-structured quartz (GQ) is shown in Figure 1(a). First of all, GQ was fabricated by photo-lithography and ion beam etching method, and was employed as substrate for the deposition of graphene films. Then, graphene films were conformally deposited on the whole surface including the top, bottom and side of 3D grating microstructures, by CVD method. After etching the edge of graphene using oxygen plasma, silver paste was brushed on the both ends of conformal graphene on GQ (Gr/GQ) as electrodes for electrical conductivity test. In general, graphene films could be prepared on the


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Cu or Ni,

1-3,18

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and these metal substrates could work as catalysts for nucleation and growth of

graphene. Differently, the dielectric substrate were believed to possess no catalytic activity.

20-22

The catalysis-free growth of graphene film needs longer CVD process, owing to slow nucleation and growth processes, which was observed by Wei et. al.19 Figure 2b exhibits the cross-sectional schematic diagram of Gr/GQ, indicating that the graphene film conformally cover the periodic grating surface. Figure 1c is the photograph of Gr/GQ with the Ag electrodes on the both ends. The grating area of 11cm2 on the quartz wafer (size: 22cm2) was conformally covered by graphene film. The Gr/GQ is transparent, and displays interference fringes, indicating the excellent uniformity and regularity of the surface of Gr/GQ patterns.

Figure 1.(a)Schematic illustration of the conformal graphene preparation and test: i. Directly depositing graphene film on the surface of grating quartz substrate; ii. Etching the graphene boundary by oxygen plasma; iii. Printing electrodes and testing the sheet resistance of conformal graphene by external circuit. (b) The cross section view of Gr/GQ. (c) Photograph of the Gr/GQ


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with Ag electrodes.

The electrical and optical properties of square conformal graphene films were respectively measured by ultraviolet-visible spectrophotometer and electrochemical workstation system. The current-voltage (I–V) characteristic curves in Figure 2a show excellent linearity and increasing slope with the growth time, indicating the decreasing sheet resistance of conformal graphene film. In the visible range, the transmittances of all the samples are higher than 80% at 550nm, as shown

in

Figure

2b.

Furthermore,

Figure

2c

reveals

the

dependence

of

electrical and optical properties on the growth time. With the increasing growth time, the layer number of graphene would increase, leading to the decrease of transmittance and the rise of conductive ability. In our experiments, the minimum sheet resistance is ~1200 sq-1 (with the transmittance of ~80%), which is almost same as the sheet resistance measured on transferred graphene from metal nickel (Ni)2 and clearly smaller than the sheet resistance measured on directly deposited graphene films on SiO212, 23, with the same transmittance of ~80%. Figure.2d shows that the graphene films deposited on Cu,24 the few-layer graphene directly grown on SiO2,12 the pristine SWNTs

25

and the calculated value of graphene

26

possess the

similar tendency of transmittance-Sheet resistance curves. Remarkably, viewed against the upper trend curve of few-layer graphene directly grown on planar SiO2 (FL Gr/SiO2),12 the Gr/GQ in our work has lower

sheet resistance with the same transmittance, indicating the better

conductivity and crystallinity of graphene film in our work. However the transparent conductive


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performance of Gr/GQ is still not comparable to that of ITO27, but the current results could fulfill the requirements for transparent electrode in some applications, such as solar cells12 and pressure-sensors (this work). The conductivity of conformal graphene still remains a challenge since the desired sheet resistance in theory 26 is as low as a few tens of ohmssq-1 or even lower.

Figure 2.(a) I-V and (b) transmittance curves of conformal graphene films.(c) Sheet resistance and transmittance (at 550 nm) of conformal graphene as a function of the growth time. (d) Comparison of sheet resistance and transmittance with our work and other references. The dashed arrows indicate the expected sheet resistances at lower transmittance. The red trend curve: few-layer graphene directly grown on SiO2 (ref.12); yellow-green and green trend curves:


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graphene grown on Ni and Cu, respectively (ref.2, ref.24); blue trend curve: single-walled carbon nanotubes (SWNTs) (ref.25); blue-green trend curve: ITO (ref.26); purplish red trend curve: calculated values (ref.27); and the black curve is our work.

The detailed morphology features of Gr/GQ were characterized by Scanning electron microscopy (SEM).Figure 3a shows the as-prepared quartz grating structure with the period of 4 µm, and the inset of Figure 3a reveals conformal graphene film on the surface of grating (more details of SEM images are shown in Figure S1). Raman spectroscopy is a simple and effective tool to directly characterize the nature of graphene, including the defects,28-30 sizes of crystal grains,31 atomic layer number,32-33 and so on. The quality of conformal graphene at the top, bottom and side (as shown in the inset of Figure 3a) was investigated by micro-Raman spectra in Figure 3b. The similar Raman peak shapes and positions were observed from different spots (the top, bottom and side) of Gr/GQ, which reveals that the 3D surface of quartz grating was fully covered by graphene film.31-32 In Raman spectra, weak D peaks manifest less defects and relatively high-quality graphene.28-29 The specific intensity ratio of G peak (at ~1580cm-1)to 2D peak (at ~2700cm-1)(IG/I2D) is ~1/2,which signifies that monolayer graphene on micro-structured quartz.34 With the increase of growth time, the IG/I2D would increase up to about 1.3, as shown in Figure S2. In this case, graphene film was thicker, and become few-layer graphene.

The Gr/GQ could works as an electrode to fabricate transparent high-performance pressure-sensitive sensors. In detail, the period of grating quartz was 4 m, and a flat graphene


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film on polydimethylsiloxane(Gr/PDMS) was covered onto Gr/GQ by face to face as another electrode. So, the device structure was named as PDMS/Gr/Gr/GQ. In order to investigate the sensitivity of this sensor, some small glass plates with the same size (8 mg) was placed on the top of sensors to exert tiny pressure.8 The working mechanism of this pressure-sensitive sensor is illustrated in Figure 4a. A constant voltage of 1 V was applied across both the electrodes, and the equivalent circuit diagram of conformal graphene pressure-sensor is shown in Figure 4b. When the flexible sensor was pressed by external force, the contact area between the grating patterns and the top assembled PDMS/Gr, including the tops, corners, edges or bottoms of gratings, would be enlarged, which was equivalent to the increase of parallel conducting paths. Hence, this process would cause the increase in current intensity, owing to the decrease in contact resistance. Once the pressure is released, the deformation of elastic PDMS would be recovered, and the contact resistance would return back to initial value. The typical current-voltage (I–V) characteristic curves are shown in Figure 4c. The excellent linearity testify stable resistance and perfect contact between Gr/PDMS and Gr/GQ, meanwhile increasing slope with pressure indicates that the decreasing resistance, owing to larger contact areas.


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Figure 3. (a)the SEM image of the Gr/GQ. The inset shows the cross-section of Gr/GQ. (b) Raman spectra of the conformal graphene films on the top, side and bottom of micro-grating patterns.

The variation ratios of electronic resistance is defined as ΔR/R0=(R-R0)/R0,35-36where R0is initial resistance, and R denotes the resistance with applied pressure. As shown in Figure 4d, ΔR/R0of the PDMS/Gr/Gr/GQ sensor is plotted as a function of the applied pressure. The sensitivity of pressure sensor is defined as S=δ(ΔR/R0)/δP,35-36 where P is the applied pressure. So, the S can be evaluated by the slope of the trace in Figure 4d. Clearly, the pressure sensitivity(S) is -0.0012 kPa-1 for plat graphene pressure sensor, which is assembled by two components: flat Gr/PDMS and graphene film on flat quartz (Gr/FQ). While the pressure sensitivity of PDMS/Gr/Gr/GQ sensor is -1.5kPa-1, almost 1000 times larger than that of PDMS/Gr/Gr/FQ sensor. As shown in Figure 4d, the response of PDMS/Gr/Gr/GQ sensor is divided into two segments: (1) in the pressure range of 1kPa, the conformal graphene shows a sensitivity of S2=-


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0.0005kPa-1.The dramatically discriminative sensitivities in two pressure ranges were related to the 3Dmicro-structure of gratings. In the first range (0–1000Pa), a rapid increase in contact area between the elastic PDMS/Gr and conformal Gr/GQ caused the resistance to drop sharply, when pressure was loaded. In the second range (>1000Pa), deformation of the PDMS/Gr would tend to saturation, so that any additional pressure can only induce a small change in contact area. Furthermore, the magnified pressure response curve of the PDMS/Gr/Gr/GQ sensor in Figure 4e shows a pressure sensitivity of -6.524kPa-1 (0-200Pa), indicating that this sensor would possess the higher sensitivity for smaller pressure loading.

Figure 4.(a) Schematic illustration of working mechanism. The contact area between the grating patterns and the top assembled Gr/PDMS electrode is enlarged as the applied pressure increases. (b) The equivalent circuit diagram of conformal graphene pressure-sensor. Each grating is equivalent to a variable resistor (Rn).(c) The typical I–V curves of the PDMS/Gr/Gr/GQ sensor


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with different applied pressure.(d) Pressure-response curves for the flat Gr/Q and conformal Gr/GQ films, respectively. The PDMS/Gr/Gr/GQ sensor with grating structures exhibit much higher pressure sensitivity than the PDMS/Gr/Gr/Q sensor. (e)The magnified pressure-response curves for PDMS/Gr/Gr/GQ sensor on 0-900 Pa.

For practical applications, the fast response rate, high repeatability and consistency are important to pressure sensors. In order to investigate its practicability, the current responses to a series of pressure loading and unloading cycles (5, 10, 200, 500, and 1000 Pa) were plotted in Figure 5a. For the same pressure loadings, the response signals could maintain the same current level. Once the pressure forces were unloaded, the current signals could return to original level, revealing high repeatability and consistency of the PDMS/Gr/Gr/GQ sensor. As shown in Figure 5b, this sensor could ultrafast respond the pressure signals, with a short response time of only 4 milliseconds (ms) for the loading or unloading of 5 Pa. Moreover, in order to investigate the reliability and cycling stability, the 1000 Pa pressure loading and releasing were repeated for 5000 cycles. As shown in Figure 5c, a consistent current response can be maintained for thousands of cycles, serving as another evidence for the practical utility of the PDMS/Gr/Gr/GQ sensor. Here, we also confirm that the graphene films on the both surfaces of GQ and PDMS would not be cracked during the compression of PDMS elastomer, as shown in Figure S3 and S4.

Furthermore, we directly tested the response of the PDMS/Gr/Gr/GQ sensor to the different wind speed or pressure. This pressure sensor could work as an anemometer,


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demonstrating a successful application case. In general, a gas flowing will generate wind pressure, so the wind pressure is related to wind speed. Wind force scale could be defined by distinct range of wind speed or pressure, as shown in Table 1. For the PDMS/Gr/Gr/GQ sensor, the response current could precisely reflect the pressure force exerted on the surface of elastic PDMS/Gr film. Figure 6a shows current response curve as a function of wind pressure. A standard formula(y=8.3610-13exp[x/58.61]+6.3710-7exp[x/506.82]-5.410-7; y: response current, x: wind pressure) was calculated from their fitting curve.Hence, the wind force scale could be determined, according to the ranges of response current, which are marked in Figure 6a. In detail, a standard wind scale chart in Table 1 provides more information, including wind pressure, wind speed, wind force scale, wind name and response current. It is clear that this wind pressure-sensitive sensor could satisfy all of the routine wind force tests, from 1 to 12 level.

Figure5.(a) The current change of the sensor at different pressure values of 5Pa, 10Pa, 200Pa, 500 Pa and 1000Pa. The inset shows a corresponding photograph of PDMS/Gr/Gr/GQ sensor. (b) The magnified current change of the sensor, when loading and unloading the pressure of 5Pa.The inset shows response time of this pressure-sensitive sensor.(c)Stability of the sensor is demonstrated by repeatedly applying and releasing a pressure of 1000 Pa for 5000 cycles.


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In order to investigate the practicability of the anemometer based on structural graphene, a series of winds, including different wind force scales of 2, 4, 6, 8, 10, and 12 levels, were vertically blown on the surface of elastic PDMS/Gr film. As shown in Figure 4b, a real-time current response curve of the PDMS/Gr/Gr/GQ sensor reveals high sensitivity, high consistency and fast response without obvious hysteresis. Meanwhile, the current signals could return to original level if the winds stopped, exhibiting prefect recoverability of anemometer. Moreover, another series of winds, with the pressure of ~250 Pa, speed of 20-22 m/s, and different blowing durations, including 4, 10, 20, 40, 120 s, were supplied onto the assembled sensor in turn. Figure 4c shows the immediate response to a continuous wind condition, highlighting the high response performance of the PDMS/Gr/Gr/GQ anemometer.

In addition, we further verified the potential high performance of PDMS/Gr/Gr/GQ pressure sensor by comparing with a market anemometer (DT 618) on the same wind conditions. In Figure 6d, the red curves represent the variation of the wind force, and the black curves on the upside and down side represent the real-time response signals from PDMS/Gr/Gr/GQ sensor and market anemometer, respectively. For the PDMS/Gr/Gr/GQ sensor, the response current curve has the same tendency of wind force curve. Meanwhile, according to the response current range in Figure 6a and Table 1, the wind force scales could be evaluated and are consistent withactual wind force. For example, at 140 s, the actual wind force was 4, and the response current of our sensor was about 0.135 µA, which corresponded to the reference range of moderate breeze


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(0.1212-0.148µA). For the market anemometer, the obvious noise existed in real-time response signal, while the anemometer could detect the variation of the wind force. Particularly, the market anemometer displayed lower accuracy and larger noise, for the low wind speed (wind force scale of 1-2). Hence, the PDMS/Gr/Gr/GQ sensor could provide real-time and accurate response signal, which was enough to analyze the wind force. On the whole, the performance of the PDMS/Gr/Gr/GQ sensor was higher than the market anemometer (DT 618), especially at low background noise level.

Figure 6.Wind pressure response based on the PDMS/Gr/Gr/GQ sensor. (a) Current response curve as a function of wind pressure, which defines the wind force scales of 0-12 levels. The inset shows a magnified curve (0-8 wind force scales).(b) The current change of the PDMS/Gr/Gr/GQ sensor under different wind force scales of 2, 4, 6, 8, 10 and 12. (c) The


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current response curve of the PDMS/Gr/Gr/GQ sensor to different supplied wind duration time, with the wind pressure of ~250Pa, wind force scale of 8 and wind speed of 20-22 m/s. (d) The comparison of the wind response between the PDMS/Gr/Gr/GQ sensor and the market anemometer.

Table 1.Standard wind scale chart Wind Wind name Wind speed force scale Km/h m/s 0 1 2 3 4 5 6 7 8 9 10 11 12 >12

Calm light air light breeze Gentle breeze Moderate breeze Fresh breeze Strong breeze Moderate gale Fresh gale Strong gale Whole gale Storm Hurricane Super-strong hurricane

Reference pressure(Pa)

(Our work) Reference current

117

8.0-10.7 10.8-13.8 13.9-17.1 17.2-20.7 20.8-24.4 24.5-28.4 28.5-32.6 >32.6

40-71.6 10.8-13.8 120.8-182.8 184.9-267.8 270.4-372.1 375.2-504.1 507.7-664.2 664.2-851 >851

0.1493-0.1937 0.1955-0.2656 0.2685-0.3737 0.3747-0.5406 0.5461-0.7879 0.7960-1.1994 1.1994-1.8919 1.8919-4.5654 >4.5654

4. CONCLUSION

In summary, we have demonstrated a simple and effective method to directly produce conformal graphene films on the surface of micro-structured quartz substrates using CVD system. The wonderful conformality, uniformity, and relatively high-quality of Gr/GQ have been verified by SEM and Raman spectra. Compared with previous reports, the conformal graphene


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film possesses comparable electro-conductibility and transmittance. The high-quality conformal graphene and its 3D micro grating structures directly provide ultrahigh sensitivity and ultrafast response rate for the application of pressure-sensitive sensor. Moreover, the conformal graphene sensors own high repeatability, consistency, reliability, and practicability, and are suitable to the application in detecting wind speed. The PDMS/Gr/Gr/GQ pressure-sensitive sensor expresses higher accuracy and lower background noise level than a market anemometer, manifesting the high practicability.

ASSOCIATED CONTENT

Supporting Information.

The 3D image of grating quartz and Raman spectra of conformal graphene in this work, and the sectional schematic of conformal graphene are offered. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected];

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was supported by NSFC (No.11404329, No. 61504148, No.61306079, No.51402291), National High-tech R&D Program of China (863 Program, No. 2015AA034801), Natural Science Foundation Project of CQ CSTC (CSTC2014jcyjjq50004), and the CAS Western Light Program.

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Direct Growth of Graphene Films on 3D Grating Structural Quartz

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