3.1 Raman Spectroscopy
Figure 1a. shows the conveyor CVD system process. The substrate was first loaded into the left cartridge and placed on the copper metal foil; the foil acted as a conveyor belt and transported the cartridge to the center of the furnace, where the substrate underwent heat annealing and growth. It was then transported to the right cartridge. Simultaneously, during transport of the substrate from the loading chamber to the growth area, the temperature increased rapidly from room temperature. As reaction with a carbon source during transport can cause the deposition of amorphous carbon, synthesis gas was not supplied, and only hydrogen was supplied for reduction. The furnace was operated at various target temperatures—850, 900, 950, and 1000 ℃—for a 1 min hydrogen treatment to optimize the heat treatment temperature. The heat treatment time was varied to find the minimum temperature for rapid synthesis (Figure S2.).
Figure 1c. shows the Raman spectra of pristine and nitrogen-doped graphene synthesized on copper at 900 and 1000°C, respectively, for 60 s. For the pristine graphene, the G peak appeared at ∼1588 cm–1, and the 2D peak at ∼2698 cm–1. Meanwhile, for the nitrogen-doped graphene, the G peak appeared at ∼1595 cm–1, and the 2D peak at ∼2724 cm–1; both the G-peak and 2D peak exhibited blue shifts of approximately 7 cm–1 and 6–7 cm–1, respectively.
To determine the maximum graphene area achievable through conveyor-type CVD synthesis, graphene samples were synthesized at various positions along the furnace; the temperature was measured at each position, and the synthesized graphene samples were analyzed using Raman spectroscopy (Fig. 2.). Using a 45-cm furnace as the reference, we confirmed the synthesis of high-quality graphene in an area measuring 5 cm × 15 cm, approximately equal to the size of a smartphone screen.
Figure 3. shows the Raman spectra of pristine and nitrogen-doped graphene synthesized using butane and pyridine as precursors, respectively, and transferred onto SiO2/Si substrates. Figure 3a. shows the Raman spectra for pristine graphene synthesized using butane at varying temperatures of 850, 900, 950, and 1000 ℃ for 60 s, while Fig. 3c. shows the Raman spectra for pristine graphene synthesized at 900 ℃ for varying growth times of 30, 45, 60, 90 s. Figure 3b. and d. show the corresponding data for nitrogen-doped graphene synthesized under the same conditions using pyridine as the precursor. Notably, the high-speed synthesis conditions afforded both pristine and doped graphene in just 60 s at a growth temperature of 900 ℃, and were applicable even when using a Si substrate with a thin copper film deposited on top (300-nm-Cu/SiO2/Si). This demonstrated the suitability of these conditions for device fabrication processes (Figure S1.).
As shown in Fig. 3a., pristine graphene samples synthesized at temperatures above 900 ℃ exhibited almost identical characteristics to that synthesized at 900°C, indicating the high quality of the synthesized monolayer of pristine graphene. For N-doped graphene synthesized under the same conditions, the D/G ratio decreased and the 2D/G ratio increased with increasing temperature (Fig. 3c.). While there is no clear relationship between synthesis time and Raman data for pristine graphene at 900 ℃ and N-doped graphene at 1000 ℃ (Fig. 2c, d.), examining the peak ratio based on synthesis time for pristine graphene reveals that a low D/G ratio and high 2D/G ratio were achieved only at a synthesis time of 60 s, indicating the synthesis of high-quality and uniform graphene with small error bars (Figure S2.). Considering cost and energy efficiency, as well as the potential for mass production and industrial competitiveness, the most suitable conditions for the synthesis of pristine graphene are 900 ℃ and 60 s. For nitrogen-doped graphene, the conditions at 1000 ℃ resulted in the highest 2D peak. However, the optimal conditions must be adjusted based on the required qualities and application purpose; this can be achieved by combining XPS and electrical property evaluation, considering factors such as the nitrogen content and charge neutrality point (Dirac point).
Based on the Raman data, additional analyses were conducted on the graphene samples synthesized at 900, 950, and 1000 ℃ for 60 s. For pristine graphene, the analysis was focused on the sample synthesized at the lowest temperature, 900 ℃, as the results did not differ significantly with varying temperatures. In other words, when the growth temperature was sufficiently high and the characteristics were saturated, there was minimal variation in the samples with temperature.
In Fig. 4., the G peaks and 2D peaks for pristine and doped graphene synthesized at 900, 950, and 1000 ℃ were compared using Raman data measured at 30 spots for each sample. For pristine graphene, the G peak and 2D peak were located at approximately 1583 and 2684 cm–1, respectively, independent of growth temperature. Meanwhile, for doped graphene, at growth temperatures of 900, 950, 1000 ℃, the G peak was located at approximately 1595, 1594, and 1589 cm–1, respectively, and the 2D peak at approximately 2704, 2694, and 2688 cm–1, respectively. The purple dots represent the reference values for exfoliated freestanding graphene unaffected by strain or doping (G peak: 1581.6 ± 0.2 cm–1, 2D peak: 2676.9 ± 0.7 cm–1).23 The gray lines indicate the Raman changes due to strain, with a slope of 2.2 ± 0.2 (Pos(2D)/Pos(G)).23 For graphene synthesized using butane, three distinct graphene clusters were formed near the reference points, indicating uniform pristine graphene.
According to previous research, hole doping causes Raman spectra to exhibit a linear shift toward higher frequencies at a slope of 0.70 ± 0.05 (Pos(2D)/Pos(G)) and blue shifts toward higher frequencies.[27] However, studies on the effect of electron doping on the Raman spectra are relatively limited, though they have been reported as nonlinear shifts.[28, 29] Additionally, a previous study utilized freestanding graphene in top-gate transistors, making it challenging to directly compare and explain our Raman results for nitrogen-doped graphene, in which the dopant replaced carbon. In case of N-doped graphene using pyridine, both the G and 2D peaks exhibited a blue shift and nonlinear changes with decreasing synthesis temperature.
3.2 X-Ray Photoelectron Spectroscopy
Figure 5. shows the XPS data obtained for the elemental analysis of each graphene sample. Figure 5a. shows the XPS spectrum of pristine graphene synthesized at 900 ℃, while Fig. 5b–d. show the XPS spectra of nitrogen-doped graphene synthesized at 1000, 950, and 900 ℃, respectively. The C1s spectra for both graphene types include a dominant peak at 284.7 eV corresponding to the C–C bond, a small oxide bond that can occur during the process at 286.3 eV corresponding to the C–O bond, and peaks at 287.5 and 289.0 eV corresponding to the O–C = O and C = O bonds, respectively.[13–15, 20, 30–32] For nitrogen-doped graphene, there was an additional clear peak corresponding to the C–N bond at 285.7 eV, and all C1s peaks exhibited overall broadening. The N1s spectra can comprise three main peaks corresponding to three different bonds: the pyridinic N bond (398.6 eV) and pyrrolic N bond (400.3 eV) represent the bonding of one and two nitrogen atoms, respectively, with carbon through π bonds, and the graphitic N bond at 401.5 eV corresponds to the substitution of carbon with nitrogen.[13, 14] Although all three bonds involve nitrogen and carbon, pyridinic N and pyrrolic N contribute to p-type doping, whereas graphitic N contributes to n-type doping. [33, 34] These peaks are not present for pristine graphene, indicating the absence of nitrogen. Meanwhile, all doped graphene samples were confirmed to contain nitrogen owing to the presence of all three peaks, with the nitrogen content being similar independent of the synthesis temperature; however, as the synthesis temperature decreased from 1000 to 900 ℃, the ratio of graphitic-N increased.
3.3 Electrical Properties
Figure 6a. shows the transfer curve of pristine graphene with respect to the growth temperature. Pristine graphene synthesized at three different temperatures exhibited Dirac points ranging from 0 to + 1 V, and the values of the source-drain current with respect to the gate voltage were similar. As observed from the Raman data, the electrical properties of pristine graphene were nearly identical when synthesized at growth temperatures above 900 ℃. This is consistent with Fig. 5 (a), in which the absence of C–N and N1s is logically presented. The electron mobility was calculated to be 1430–1498 cm2 V–1 s–1 at a charge concentration of n = + 1e12, and the hole mobility was calculated to be 1468–1572 cm2 V–1 s–1 at n=–1e12.
In the case of N-doped graphene, the Dirac point voltage was consistently below 0 V, indicating n-type behavior. For graphene samples synthesized at 900, 950, and 1000°C, the Dirac point voltages were − 117, − 82, and − 51 V, respectively, electron mobilities at n = + 1e12 were calculated as 253, 317, and 402 cm2 V–1 s–1, respectively, and hole mobilities at n=–1e12 were calculated as 255, 326, and 404 cm2 V–1 s–1, respectively. As the growth temperature decreased, the relative abundance of graphitic N increased, leading to a larger absolute value of the Dirac point voltage in the negative direction. However, overall, the graphitic structure decreased, and the increased scattering sites resulted in a reduction in mobility (Figure S4.).
3.4 Gas Sensing Application
Figure 7. shows the potential applicability of gas sensors using pristine graphene synthesized at 900 ℃ and N-doped graphene synthesized at 1000 ℃. The two types of graphene were transferred onto SiO2 substrates to create simple FET devices, which were then applied as gas-sensor channels for NO2 and NH3 at room temperature and atmospheric pressure. Typically, CVD-grown graphene, the dominant carriers are holes, is a p-type channel; NO2 absorbed onto the graphene surface withdraws free electrons such that the resistance increases, while NH3 donates free electrons to the graphene surface, thus the resistance decreases. Meanwhile, nitrogen-doped graphene, the dominant carriers are electrons, is an n-type channel that exhibits the opposite electrical response characteristics. This leads to opposite changes in the reaction characteristics.
The graphene-based gas sensors were exposed to NO2 and NH3 gases for 9 min each, and the resistance changes were measured repeatedly. Pristine graphene exhibited a polarity characteristic similar to that of a previous study,[35] showing adsorption and desorption based on the gas type. In contrast, the N-doped graphene exhibited reversed polarity. Figure 8a. shows the second cycle of the graphs in Fig. 7., grouped according to graphene and gas types. Pristine and N-doped graphene exhibited reactivities of − 25% and 79%, respectively, when exposed to NO2 for 9 min, and reactivities of 18% and − 12%, respectively, when exposed to NH3 for 9 min. Figure 8b. shows the reactivity of the graphene samples after 2 min of exposure to each gas; nitrogen-doped graphene exhibited higher sensitivity and faster response compared to pristine graphene for both gases.