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Abstract
A method for direct growth of graphene nanowalls (GNWs) on an insulating substrate by plasma enhanced chemical vapor deposition (PECVD) is reported. The effects of growth temperature, plasma power, carbon source concentration, gas ratio and growth time on the quality of GNWs are systematically studied. The Raman spectrum shows that the obtained GNWs have a relatively high quality with a D to G peak ratio (ID/IG) of 0.42. Based on the optimization of the quality of GNWs, a field-effect transistor (FET) photodetector is prepared for the first time, and its photo-response mechanism is analyzed. The responsivity of the photodetector is 160 mA/W at 792 nm and 55 mA/W at 1550 nm. The results reveal that the GNWs are promising for high performance photodetectors.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Graphene nanowalls (GNWs) are few-layer graphene corrugated nanosheets standing vertically on a substrate. Due to the special morphology of exposed sharp edges, non-stacked state and huge surface-to-volume ratio [1], they have realized various applications, such as field emission [2,3], energy applications [4,5], biosensor [6,7]. Recently, GNWs have attracted increasingly interests for photo detection [8–13]. Shen et al. reported a typical Schottky heterojunction photodetector based on GNWs-Si structure, which achieved ultralow current noise and high specific detectivity [11]. It benefits from the fact that compared with the monolayer structure, the nanowalls structure has the characteristics of improving electron transmission efficiency and reducing reflection loss, which have also been found in the nanowalls structures of GaN, ZnO and Ag other than graphene [14–16]. However, the performance of this type of device in broadband detection is poor because of the limitation of Si absorption bandwidth [12].
In addition, the poor quality of GNWs leads to the reduction of mobility, which is also the key to damage the performance of photodetectors. Currently, radio frequency (RF) -plasma enhanced chemical vapor deposition (PECVD) is a widely used method for synthesizing GNWs [8,17–19]. With the aid of RF plasma source, the carbon precursor is decomposed into free radicals and deposited vertically on the substrate to grow GNWs [20,21]. But this kind of growth system has complex structure and high power consumption [22]. Besides, the low electron mean energy of RF plasma reduces collision probability of between electrons and molecules (atoms), resulting in a low decomposition rate of the precursor and weak ion activation [23]. Consequently, the preparation of high quality GNWs and their application in high performance photodetectors have potential research value.
In this paper, we propose a method for direct growth of high quality GNWs on dielectric surface by low frequency (LF) -PECVD. Raman spectroscopy was utilized to study the effects of growth techniques including temperature, plasma power, CH4 concentration, gas ratio and growth time on the quality of GNWs. The morphology of GNWs with different height was characterized by scanning electron microscope (SEM). Furthermore, for the first time, we used these optimized GNWs to fabricate a field-effect transistor (FET) photodetector, and demonstrated the photoresponse mechanism of GNWs different from monolayer graphene (MLG). The finding of response delay presented by this device was also analyzed. The strong absorption of light by GNWs enables the device to have excellent responsivity in the infrared band. The application of high quality GNWs in FETs provides an opportunity for photo detection with both broadband and high responsivity.
2. Materials and methods
2.1 Growth of GNWs
The growth mechanism of GNWs is shown in Fig. 1(a). In the first step, the gas molecules (CH4, H2 and Ar) in the chamber are dissociated by LF plasma. Then the carbon radicals reach the substrate surface and form a base graphite layer in the direction parallel to the substrate. Note that there are many cracks in this layer, which are mainly caused by internal stress [20]. With the continuous decomposition and deposition, the density of the cracks increases and the crack edges curl upward. The last step is that the electric field perpendicular to the substrate plane induced by the plasma sheath promotes carbon atoms preferentially bond to the crack edges, and the graphene nanosheets begin to grow vertically [21].
Fig. 1. (a) Deposition mechanism of GNWs: i Gas molecules are decomposed. ii Graphite base with cracks is formed. iii The density of the cracks increases and the crack edges curl upward. iv Graphene nanosheets grow vertically. (b) Schematic of the PECVD system used for GNWs deposition. (c) Photograph of the plasma sheath.
The GNWs in this work are grown by the vertical cold wall LF (15KHz) -PECVD (Black Magic, Aixtron) shown in Fig. 1(b). The chamber of the system is mainly composed of two graphite plates. The upper graphite plate acts as a heater. The lower graphite plate is used as one of the electrodes of the plasma source and forms the positive/negative electrodes of the LF electric field with the chamber wall. The substrate is placed on the graphite electrode instead of the heater. This is because the plasma sheath, a key factor in the formation of GNWs [21], is mainly concentrated on the surface of the lower graphite plate [Fig. 1(c)]. Before growth, the dielectric substrates (including SiO2, quartz and sapphire) were cleaned with acetone, ethanol and deionized water, and the chamber was flushed with Ar and then heated to the temperature required for growth in a H2 atmosphere (320 sccm, 10 mbar) at the rate of 300 °C/min. The total flow rate of the mixture gas (CH4, H2 and Ar) during the growth stage was 320 sccm. We adjusted the parameters such as temperature, plasma power, CH4 concentration, gas ratio and growth time to analyze their influences on the quality of GNWs.
2.2 Fabrication of photodetectors
The GNWs photodetector adopts a top-gated FET structure, and its preparation process is shown in Figs. 2(a)–2(e). The GNWs were synthesized on the quartz substrate (5 mm × 5 mm, ∼95% transmittance) using a mixture gas of CH4, H2 and Ar (with the ratio of 1:3:60) at 930 °C, with plasma power of 30 W, for 40 min. The obtained GNWs were 25 nm high and were patterned into a 100 µm × 15 µm band by photolithography and oxygen-plasma etching. Subsequently, Ti/Au (15 nm/200 nm) were sputtered as the source/drain electrodes. HfO2 with a thickness of 40 nm was deposited as the gate dielectric layer by Atomic layer deposition (ALD). Finally, another Ti/Au (15 nm/200 nm) was sputtered as the gate electrode. The advantage of this structure is that the dielectric layer acts as a package for the GNWs. In order to prevent the incident light from being blocked by the gate electrode, quartz with high transparency is used as a substrate and light is incident from the back of the photodetector, as shown in Fig. 2(f). The MLG device was prepared using commercial graphene grown on Cu foil by chemical vapor deposition (CVD). The graphene was transferred to quartz substrate, and the subsequent preparation steps were the same as those of GNWs photodetector.
Fig. 2. (a)-(e) Schematic diagram of the preparation process of the GNWs photoconductive photodetector with top gate FET. (f) Schematic sketch of the GNWs photodetector and light source incident mode of photodetector: back incident.
2.3 Characterization of materials and devices
Raman investigations were performed on a Horiba LabRAM HR Evolution system using a 532 nm laser operating at 50 mW, × 50 objective lens with about 1 µm diameter spot size, and 600 lines per mm grating with about 0.65 cm−1 spectral resolution. SEM images were obtained using a Zeiss Merlin scanning electron microscope operating at 15 kV. The transfer characteristics and the photocurrent of the photodetectors were measured with a Keysight B1500A semiconductor device analyzer. The transmittance spectra of GNWs and MLG were measured with a Hitachi U-4100 spectrophotometer.
3. Results and discussion
Figures 3(a)–3(e) exhibits the Raman spectra for GNWs grown on the variation in temperature, plasma power and CH4 concentration, gas ratio and growth time, respectively. The Raman spectra of all samples contain three typical peaks of graphene, which are D at ∼1350 cm−1, G at ∼1580 cm−1 and 2D at ∼2700 cm−1. The G peak originates from the E2g phonon at the Brillouin zone center and corresponds to the graphitized structure, while the D and 2D peaks are attributed to double-resonant Raman scattering [23–25]. The intensity ratio of the D to G peak (ID/IG) represents the degree of disorder and defects in graphene, that is, the lower the D peak relative to the G peak, the less defects the sample has. The intensity ratio of the 2D to G peak (I2D/IG) indicates the quality and number of layers of graphene. Moreover, the I2D/IG ratio is very sensitive to the environment [26].
Fig. 3. Raman spectra of GNWs grown at various (a) temperatures, (b) plasma powers, (c) CH4 concentrations, (d) ratios of CH4 to H2, (e) growth times.
As can be seen in Fig. 3(a), the D peak of GNWs is the weakest and the 2D peak is the strongest when the temperature is 930 °C (limit temperature of the equipment used). This indicates that higher temperature is helpful to improve the quality of GNWs. It has been proved that high temperature plays an important role in increasing the activity of precursor and crystallinity of the sp2 phase [27]. Figure 3(b) shows that 30W is the best plasma power for growth. Plasma power actually represents the magnitude of plasma strength, which can make the decomposition efficiency of carbon source in reactor much higher than that in thermal CVD without plasma [28]. However, for this growth procedure, when the plasma power is too high, the reactive ions and active groups near the substrate are saturated and partially recombine before being deposited on the substrate. When the plasma power is too low, the carbon source cannot be fully decomposed. The preferred CH4 concentration value is illustrated in Fig. 3(c). This means that a lower CH4 concentration can reduce the defect density and the number of graphene layers in the nanowalls. The reason is that the low CH4 concentration can effectively suppress the nucleation density, which in turn increases the domain size of graphene [29,30]. It is observed from Fig. 3(d) that the sample grown at CH4/H2 = 1/3 [although its Raman spectrum seems to be close to the sample with CH4/H2 = 1/1, its ID/IG value is smaller, as shown in Fig. 4(d)] is desirable. Here, the main function of H2 with a moderate composition is to etch amorphous graphite phase and promote the catalytic decomposition of hydrocarbon gases to improve the quality of graphene [31]. As shown in Fig. 3(e), the D peak of samples grown for no less than 40 minutes is significantly weaker. It implies that at least 40 minutes of growth time is required to obtain GNWs with fewer defects. In addition, the G and the 2D peaks show a slight asymmetry, which is a common phenomenon in GNWs materials. The reason is the measurement environment and the double resonance process caused by defects [32,33].
Fig. 4. Intensity ratios of Raman peaks corresponding to sample in Fig. 3: ID/IG (Orange Lines) and I2D/IG (violet lines).
The values and trends of Raman peak intensity ratios (ID/IG and I2D/IG) of GNWs grown under different conditions [corresponding to Fig. (3)] are shown in Fig. 4. The ID/IG values of all the samples ranged from 0.4 to 1.2, while the I2D/IG values are around 0.2-0.5. It suggest that the samples have a crystalline, but defective graphene nanosheets structure with few layers. Figures 4(a), 4(c) and 4(e) show that ID/IG value decreases with the increase of temperature, the decrease of CH4 concentration and the extension of growth time, and no extreme point appears. However, with the increase of plasma power and CH4/H2 ratio, ID/IG value first decreases and then increases [Figs. 4(b) and 4(d)]. In the process of regulating these five parameters, the trend of I2D/IG is always opposite to ID/IG, which is also observed by Qi et al. [18]. In the above, almost all the main parameters of PECVD are investigated, and the optimal solution for growing GNWs is obtained. The ID/IG value of optimized GNWs is 0.42, and the I2D/IG value is 0.52. As shown in Table 1, compared with the previous literature, the ID/IG value of our GNWs is the smallest, which indicates that our sample has fewer defects and better quality.
Table 1. Comparison of the growth parameters and quality of the GNWs in this work and other reports.
In order to investigate the morphology evolution of GNWs, we observed the samples with different growth time by SEM. It should be noted that since the growth time also affects the quality of the sample, the quality factor does not need to be taken into account. In detail, these samples were synthesized at 750 °C, under 8 mbar, with mixture gas of CH4, H2 and Ar (with the ratio of 1:1:14), with plasma power of 80 W, for 0.5–24 min. Figures 5(a)–5(e) reflect the appearance evolution of GNWs as the height (growth time) increases. When the growth time is only 0.5 min, the nanosheets have a very small size. The reason is that nucleation has just begun in the vertical direction [corresponding to iii in Fig. 1(a)]. Along with a longer growth time, the height and size of the free-standing graphene nanosheets are increased. At 24 min, these nanosheets extend and converge into larger network structures [21]. The Raman spectrum of the sample shown in Fig. 5(a) is the pink curve in Fig. 3(a), which is roughly the same as that of the other four samples with longer growth time. It is confirmed that there is no obvious effect to reduce the defects of GNWs by extending the growth time at relatively low temperature. Figure 5(f) is a side view SEM image of the sample in Fig. 5(d), which shows the typical sidewall morphology of GNWs. It can be seen that GNWs are composed of numerous vertical nanosheet groups. This is the result of graphene deposition in the vertical direction driven by the plasma sheath electric field.
Fig. 5. Top view SEM images of the GNWs with height (growth time) of (a) 30 nm (0.5 min), (b) 125 nm (2 min), (c) 450 nm (8 min), (d) 650 nm (12 min), (e) 1100 nm (24 min). (f) typical sidewall morphology of GNWs (corresponding to (d)).
The GNWs with optimized growth parameters [the SEM topography is shown in Fig. 6(a), and the Raman spectrum is the red line in Fig. 3(e)] was used to prepare the FET photodetector. The transfer characteristic curves of the GNWs photodetector under different source-drain voltages (VSD) are shown in Fig. 6(b). Both the minimum conductivities appear at the gate voltage (VG) of +10 V. It is found that the slopes of the two curves are relatively gentle in the range of VG from –10 V to +10 V. We suggest that this is because the GNWs have a certain height, and the regulation of VG on the conductivity is limited to the part of the GNWs closer to the gate dielectric surface. This part shrinks as the VG decreases. When the VG is small, most of the GNWs are no longer affected by the VG, and the source-drain current (ISD) changes slightly. Figure 6(c) shows the photocurrent Iph (Iph = Ion – Ioff, Ion and Ioff are the ISD of device with and without the illumination of the incident light) of the GNWs photodetector at different VG. The two curves have the same trend, and the height of the black curve (VSD = +100 mV) is about twice the height of the red curve (VSD = +50 mV), which corresponds to their VSD values. However, these trends are significantly different from those of the traditional two-dimensional graphene. According to the research results of Freitag et al., for graphene, when the Fermi level is at the position of Dirac point, the photovoltaic effect of graphene is the strongest, and the device has the largest positive Iph. When the Fermi level is far away from the Dirac point, the photovoltaic effect is gradually weakened and the Iph tends to zero [37]. However, for GNWs, we found the Iph tends to a nonzero value with the increase of the absolute VG. We think that the difference is also because of the height of the GNWs. The Fermi level of the part of the GNWs far from the gate dielectric surface will remain unchanged with the VG, and the Iph of this part will remain unchanged. Therefore, the Iph will tend to a fixed positive value when the VG is far away from the Dirac point.
Fig. 6. (a) SEM image of GNWs used to prepare photodetector. (b) Transfer characteristic curves of GNWs FET at VSD of + 50 mV and + 100 mV. (c) Photocurrent (Iph) of the GNWs photodetector at various gate voltages.
Figures 7(a) and 7(b) shows the light response of the GNWs photodetector and MLG photodetector under the illumination of 792 nm laser and 1550 nm laser. We have evaluated the performance of the photodetectors based on the responsivity (R). The responsivity is considered as the benchmark for photoelectric conversion ability of a photodetector,and it can be expressed in following equation [14]:
where Po is the power density, which is 2.5 W/cm2 and 3 W/cm2 respectively at the 792 nm and 1550 nm laser sources, and APD (100 µm×15 µm) is the area of the photosensitive region of the device. Illuminated by 792 nm laser, the responsivity of the GNWs photodetector (160 mA/W) is about 12 times than that of the MLG photodetector (13 mA/W). Under the illumination of 1550 nm laser, the responsivity of the GNWs photodetector (55 mA/W) is about 18 times than the MLG photodetector (3 mA/W). Table 2 indicates that the response of our device is better than conventional pure graphene (monolayer or multilayer) detectors and GNWs detectors even with a high light power. We believe that this is due to the high light absorption of GNWs. As shown in Fig. 7(c), the transmittance of MLG is 94–97% in the wavelength range of 0.3 µm to 2.1 µm, while the transmittance of the GNWs is only 38–62%. GNWs have numerous inner walls created by free-standing graphene nanosheets. Multiple reflections of light between the inner walls increase the light capture rate. GNWs have the combined benefits of large surface-to-volume ratio and excellent optical properties, which makes them promising candidates for photo detection that are superior to other carbon-based materials.Fig. 7. The time-dependent Iph measurements of the GNWs photodetector and MLG photodetector working at their respective Dirac point voltage under the illumination of (a) 792 nm laser and (b) 1550 nm laser, VSD = 100 mV. (c) Transmittance of the GNWs and MLG.
Table 2. Comparison of the responsivity of our GNWs photodetector with those in the literature.
It can be seen from Figs. 7(a) and 7(b) that, as the GNWs photodetector illuminated with a laser source, source-drain current (ISD) rises rapidly (lasting for about 0.75s, 792 nm)after that rises slowly with time. Similarly, the ISD falls rapidly (lasting for about 0.4s, 792 nm) after the laser source is turned off, and then falls slowly . Obviously, the response delay of this device cannot be ignored. It can be attributed to the capture and decapture of photo-generated carriers by the trapping centers [41]. In fact, the response delay is usually found in nanowalls structure photodetectors of graphene and other materials [14–16]. Due to the huge surface-to-volume ratio and too many exposed edges, the density of trapping centers in the nanowalls structure is quite high, which prolongs the lifetime of photo-generated carriers and slows down the speed of collecting photo-generated carriers. Consequently, the response speed of the device based on the nanowalls structure is slow. In addition, Zhang et al. proved this from another angle. They introduced trapping centers into high-quality MLG by etching, and then found that the response time was significantly longer [2]. However, some GNWs-Si Schottky detectors have a faster response speed [8,11]. This is because Si dominates the detection when the light wavelength is within the absorption spectrum range of Si (less than 1100 nm). Once the wavelength exceeds 1100 nm, as mentioned in Ref. [12], GNWs will dominate the detection and the phenomenon of slow response will appear again.
4. Conclusions
We have achieved the growth of high quality GNWs by the vertical cold wall LF-PECVD system. The Raman intensity ratio of the best sample is 0.42 (ID/IG) and 0.52 (I2D/IG). Higher temperature, lower CH4 concentration, appropriate plasma power, right ratio of CH4 to H2 and longer growth time have positive effects on improving the quality of this material. Because of the high light absorption, the GNWs photodetector based on FET has higher responsivity than the MLG device with the same structure and size. Moreover, for the first time, the GNWs were found to have a difference from graphene in the variation of photocurrent with gate voltage and a delay of photoresponse. In the follow-up work, it is the key to effectively separate the photo-generated carriers and increase the photoconductive gain by designing the electric field reasonably.
Funding
National Key Research and Development Program of China (2018YFA0209000); National Natural Science Foundation of China (11674016, 61874145, 62074011, 61604007, 61774175); Natural Science Foundation of Beijing Municipality (4182012); Beijing Municipal Science and Technology Commission (Z161100002116032); Beijing Nova Program (Z201100006820096).
Disclosures
The authors declare no conflicts of interest.
References
1. Z. Bo, Y. Yang, J. Chen, K. Yu, J. Yan, and K. Cen, “Plasma-enhanced chemical vapor deposition synthesis of vertically oriented graphene nanosheets,” Nanoscale 5(12), 5180–5204 (2013). [CrossRef]
2. S. Wang, J. Wang, P. Miraldo, M. Zhu, R. Outlaw, K. Hou, X. Zhao, B. C. Holloway, D. Manos, T. Tyler, O. Shenderova, M. Ray, J. Dalton, and G. McGuire, “High field emission reproducibility and stability of carbon nanosheets and nanosheet-based backgated triode emission devices,” Appl. Phys. Lett. 89(18), 183103 (2006). [CrossRef]
3. L. Jiang, T. Yang, F. Liu, J. Dong, Z. Yao, C. Shen, S. Deng, N. Xu, Y. Liu, and H. Gao, “Controlled synthesis of large-scale, uniform, vertically standing graphene for high-performance field Emitters,” Adv. Mater. 25(2), 250–255 (2013). [CrossRef]
4. J. L. Qi, X. Wang, J. H. Lin, F. Zhang, J. C. Feng, and W. D. Fei, “A high-performance supercapacitor of vertically-oriented few-layered graphene with high-density defects,” Nanoscale 7(8), 3675–3682 (2015). [CrossRef]
5. J. Liu, W. Sun, D. Wei, X. Song, T. Jiao, S. He, W. Zhang, and C. Du, “Direct growth of graphene nanowalls on the crystalline silicon for solar cells,” Appl. Phys. Lett. 106(4), 043904 (2015). [CrossRef]
6. D. Seo, A. Rider, S. Kumar, L. Randeniya, and K. Ostrikov, “Vertical graphene gas- and bio-sensors via catalyst-free, reactive plasma reforming of natural honey,” Carbon 60, 221–228 (2013). [CrossRef]
7. Q. Chen, T. Sun, X. Song, Q. Ran, C. Yu, J. Yang, H. Feng, L. Yu, and D. Wei, “Flexible electrochemical biosensors based on graphene nanowalls for the real-time measurement of lactate,” Nanotechnology 28(31), 315501 (2017). [CrossRef]
8. Q. Zhou, X. Liu, E. Zhang, S. Luo, J. Shen, Y. Wang, and D. Wei, “The controlled growth of graphene nanowalls on Si for Schottky photodetector,” AIP Adv. 7(12), 125317 (2017). [CrossRef]
9. H. Zhang, K. Zhao, S. Cui, J. Yang, D. Zhou, L. Tang, J. Shen, S. Feng, W. Zhang, and Y. Fu, “Anomalous temperature coefficient of resistance in graphene nanowalls/polymer films and applications in infrared photodetectors,” Nanophotonics 7(5), 883–892 (2018). [CrossRef]
10. X. Liu, Q. Zhou, S. Luo, H. Du, Z. Cao, X. Peng, W. Feng, J. Shen, and D. Wei, “Infrared photodetector based on the photothermionic effect of graphene-nanowall/silicon heterojunction,” ACS Appl. Mater. Interfaces 11(19), 17663–17669 (2019). [CrossRef]
11. J. Shen, X. Liu, X. Song, X. Li, J. Wang, Q. Zhou, S. Luo, W. Feng, X. Wei, S. Lu, S. Feng, C. Du, Y. Wang, H. Shi, and D. Wei, “High-performance Schottky heterojunction photodetector with directly grown graphene nanowalls as electrodes,” Nanoscale 9(18), 6020–6025 (2017). [CrossRef]
12. L. Li, Y. Dong, W. Guo, F. Qian, F. Xiong, Y. Fu, Z. Du, C. Xu, and J. Sun, “High-responsivity photodetectors made of graphene nanowalls grown on Si,” Appl. Phys. Lett. 115(8), 081101 (2019). [CrossRef]
13. H. Wang and Y. Fu, “Graphene-Nanowalls/Silicon hybrid heterojunction photodetectors,” Carbon 162, 181–186 (2020). [CrossRef]
14. C. Ramesh, P. Tyagi, B. Bhattacharyya, S. Husale, K. K. Maurya, M. S. Kumar, and S. S. Kushvaha, “Laser molecular beam epitaxy growth of porous GaN nanocolumn and nanowall network on sapphire (0001) for high responsivity ultraviolet photodetectors,” J. Alloys Compd. 770, 572–581 (2019). [CrossRef]
15. J. Agrawal, T. Dixit, P. A. Iyamperumal, and V. Singh, “Electron Depleted ZnO Nanowalls-Based Broadband Photodetector,” IEEE Photonics Technol. Lett. 31(20), 1639–1642 (2019). [CrossRef]
16. W. Zhao, N. Du, C. Xiao, H. Wu, H. Zhang, and D. Yang, “Large-scale synthesis of Ag-Si core-shell nanowall arrays as high-performance anode materials of Li-ion batteries,” J. Mater. Chem. A 2(34), 13949–13954 (2014). [CrossRef]
17. K. Shiji, M. Hiramatsu, A. Enomoto, M. Nakamura, H. Amano, and M. Hori, “Vertical growth of carbon nanowalls using rf plasma-enhanced chemical vapor deposition,” Diamond Relat. Mater. 14(3-7), 831–834 (2005). [CrossRef]
18. J. L. Qi, F. Zhang, X. Wang, L. X. Zhang, J. Cao, and J. C. Feng, “Effect of catalyst film thickness on the structures of vertically-oriented few-layer graphene grown by PECVD,” RSC Adv. 4(84), 44434–44441 (2014). [CrossRef]
19. S. Vizireanu, S. D. Stoica, C. Luculescu, L. C. Nistor, B. Mitu, and G. Dinescu, “Plasma techniques for nanostructured carbon materials synthesis. A case study: carbon nanowall growth by low pressure expanding RF plasma,” Plasma Sources Sci. Technol. 19(3), 034016 (2010). [CrossRef]
20. A. Malesevic, R. Vitchev, K. Schouteden, A. Volodin, L. Zhang, G. V. Tendeloo, A. Vanhulsel, and C. V. Haesendonck, “Synthesis of few-layer graphene via microwave plasma-enhanced chemical vapour deposition,” Nanotechnology 19(30), 305604 (2008). [CrossRef]
21. M. Zhu, J. Wang, B. C. Holloway, R. A. Outlaw, X. Zhao, K. Hou, V. Shutthanandan, and D. M. Manos, “A mechanism for carbon nanosheet formation,” Carbon 45(11), 2229–2234 (2007). [CrossRef]
22. H. T. Kim, M. J. Kim, and S. H. Sohn, “Characterization of TiN thin films grown by low-frequency (60 Hz) plasma enhanced chemical vapor deposition,” J. Phys. Chem. Solids 73(7), 931–935 (2012). [CrossRef]
23. H. T. Kim, D. K. Park, and W. S. Choi, “Measurements of plasma parameters in low-frequency (60 Hz) hydrogen discharge,” J. Korean Phys. Soc. 42, S916–S919 (2003).
24. E. M. Ferreira, M. O. Moutinho, F. Stavale, M. Lucchese, R. B. Capaz, C. Achete, and A. Jorio, “Evolution of the Raman spectra from single-, few-, and many-layer graphene with increasing disorder,” Phys. Rev. B 82(12), 125429 (2010). [CrossRef]
25. C. Yang, H. Bi, D. Wan, F. Huang, X. Xie, and M. Jiang, “Direct PECVD growth of vertically erected graphene walls on dielectric substrates as excellent multifunctional electrodes,” J. Mater. Chem. A 1(3), 770–775 (2013). [CrossRef]
26. K. Hu, Z. Xue, Y. Liu, H. Long, B. Peng, H. Yan, Z. Di, X. Wang, L. Lin, and W. Zhang, “Tension-Induced Raman Enhancement of Graphene Membranes in the Stretched State,” Small 15(2), 1804337 (2019). [CrossRef]
27. K. Teii, S. Shimada, M. Nakashima, and A. T. H. Chuang, “Synthesis and electrical characterization of n-type carbon nanowalls,” J. Appl. Phys. 106(8), 084303 (2009). [CrossRef]
28. D. B. Hash and M. Meyyappan, “Model based comparison of thermal and plasma chemical vapor deposition of carbon nanotubes,” J. Appl. Phys. 93(1), 750–752 (2003). [CrossRef]
29. Y. Hao, M. S. Bharathi, L. Wang, L. Y. Liu, H. Chen, S. Nie, X. Wang, H. Chou, C. Tan, B. Fallahazad, and H. Ramanarayan, “The Role of Surface Oxygen in the Growth of Large Single-Crystal Graphene on Copper,” Science 342(6159), 720–723 (2013). [CrossRef]
30. Y. Okigawa, R. Kato, T. Yamada, M. Ishihara, and M. Hasegawa, “Electrical properties and domain sizes of graphene fifilms synthesized by microwave plasma treatment under a low carbon concentration,” Carbon 82, 60–66 (2015). [CrossRef]
31. X. Zhang, L. Wang, J. Xin, B. I. Yakobson, and F. Ding, “Role of Hydrogen in Graphene Chemical Vapor Deposition Growth on a Copper Surface,” J. Am. Chem. Soc. 136(8), 3040–3047 (2014). [CrossRef]
32. S. Kurita, A. Yoshimura, H. Kawamoto, T. Uchida, K. Kojima, M. Tachibana, P. Molina-Morales, and H. Nakai, “Raman spectra of carbon nanowalls grown by plasma-enhanced chemical vapor deposition,” J. Appl. Phys. 97(10), 104320 (2005). [CrossRef]
33. E. H. Hasdeo, A. R. T. Nugraha, M. S. Dresselhaus, and R. Saito, “Breit-Wigner-Fano line shapes in Raman spectra of graphene,” Phys. Rev. B 90(24), 245140 (2014). [CrossRef]
34. B. Liu, I. S. Chiu, and C. S. Lai, “Improvements on thermal stability of graphene and top gate graphene transistors by Ar annealing,” Vacuum 137, 8–13 (2017). [CrossRef]
35. N. Soin, S. S. Roy, C. O’Kane, J. A. D. McLaughlin, T. H. Lim, and C. J. D. Hetherington, “Exploring the fundamental effects of deposition time on the microstructure of graphene nanoflakes by Raman scattering and X-ray diffraction,” CrystEngComm 13(1), 312–318 (2011). [CrossRef]
36. J. Sun, Y. Chen, X. Cai, B. Ma, Z. Chen, M. K. Priydarshi, K. Chen, T. Gao, X. Song, Q. Ji, X. Guo, D. Zou, Y. Zhang, and Z. Liu, “Direct low-temperature synthesis of graphene on various glasses by plasma-enhanced chemical vapor deposition for versatile, cost-effective electrodes,” Nano Res. 8(11), 3496–3504 (2015). [CrossRef]
37. M. Freitag, T. Low, F. Xia, and P. Avouris, “Photoconductivity of biased graphene,” Nat. Photonics 7(1), 53–59 (2013). [CrossRef]
38. T. Mueller, F. Xia, and P. Avouris, “Graphene photodetectors for high-speed optical communications,” Nat. Photonics 4(5), 297–301 (2010). [CrossRef]
39. M. C. Lemme, F. H. L. Koppens, A. L. Falk, M. S. Rudner, H. Park, L. S. Levitov, and C. M. Marcus, “Gate-activated photoresponse in a graphene p-n junction,” Nano Lett. 11(10), 4134–4137 (2011). [CrossRef]
40. M. Freitag, T. Low, and P. Avouris, “Increased esponsivity of suspended graphene photodetectors,” Nano Lett. 13(4), 1644–1648 (2013). [CrossRef]
41. L. Liu, C. Yang, A. Patane, Z. Yu, F. Yan, K. Wang, H. Liu, J. Li, and L. Zhao, “High-detectivity ultraviolet photodetectors based on laterally mesoporous GaN,” Nanoscale 9(24), 8142–8148 (2017). [CrossRef]
