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N-type graphene induced by dissociative H2 adsorption at room temperature

Studies of the interaction between hydrogen and graphene have been increasingly required due to the indispensable modulation of the electronic structure of graphene for device applications and the possibility of using graphene as a hydrogen storage material. Here, we report on the behaviour of molecular hydrogen on graphene using the gate voltage-dependent resistance of single-, bi-, and multi-layer graphene sheets as a function of H2 gas pressure up to 24 bar from 300 K to 345 K. Upon H2 exposure, the charge neutrality point shifts toward the negative gate voltage region, indicating n-type doping, and distinct Raman signature changes, increases in the interlayer distance of multi-layer graphene, and a decrease in the d-spacing occur, as determined by TEM. These results demonstrate the occurrence of dissociative H2 adsorption due to the existence of vacancy defects on graphene.

Studies of the interaction between hydrogen and graphene have been increasingly required due to the indispensable modulation of the electronic structure of graphene for device applications and the possibility of using graphene as a hydrogen storage material. Here, we report on the behaviour of molecular hydrogen on graphene using the gate voltage-dependent resistance of single-, bi-, and multi-layer graphene sheets as a function of H 2 gas pressure up to 24 bar from 300 K to 345 K. Upon H 2 exposure, the charge neutrality point shifts toward the negative gate voltage region, indicating n-type doping, and distinct Raman signature changes, increases in the interlayer distance of multi-layer graphene, and a decrease in the d-spacing occur, as determined by TEM. These results demonstrate the occurrence of dissociative H 2 adsorption due to the existence of vacancy defects on graphene. G raphene has been regarded as the successor of silicon in the electronics industry due to its remarkably high carrier mobility, transparency, anomalous quantum Hall effect [1][2][3][4] , and exceptionally high Young's modulus 5 . However, the electronic structure of graphene, which features gap opening and n-and p-type properties, must be controlled to achieve this demand. One of the methods used to control the properties of graphene is hydrogenation. Hydrogenated graphene, known as graphane, is obtained from hydrogen plasma and shows a hole-doping effect, a reversible insulating state, and the disappearance of the quantum Hall effect [6][7][8][9][10] . Along with the hydrogenation of graphene, considerable attention has been paid to the adsorption behaviour of molecular hydrogen on graphitic materials, as well as the interaction between atomic hydrogen and graphene, because multilayered graphene has been proposed as a good candidate for hydrogen storage [11][12][13][14][15] . For physisorption, the attraction between H 2 and graphene occurs via overlapping of the s molecular orbital of H 2 with the electrons in the p state of graphene, with no charge transfer 16 . However, the dissociative adsorption of hydrogen molecules on a graphite surface with vacancies 17 and at the armchair edge of graphite 18 has been theoretically suggested. Herein, we report on the dissociative adsorption of hydrogen with gate-voltage (V G )-dependent current variations of single-, bi-, and multi-layer graphene (SLG, BLG, and MLG, respectively) as a function of H 2 gas pressure from vacuum (approximately 10 26 Torr) to 24 bar at temperatures T $ 300 K. As the H 2 gas pressure increases, the charge neutrality point (CNP) shifts toward the negative V G region (n-doping). This charge neutrality point does not return to the origin, even after degassing is performed (vacuum and heating at 393 K). Long exposure times and high temperatures in a H 2 atmosphere enhance the n-doping effect. H 2 exposure leads to the development of defect-related D (approximately 1350 cm 21 ) and D' (approximately 1620 cm 21 ) peaks and a peak corresponding to the C-H bond (approximately 2932 cm 21 ), a blueshift in the G peak, and a change in stacking order deleted by the variation in the 2D peak, as observed in Raman spectra. An increase in the interlayer distance of MLG was observed by atomic force microscopy (AFM) and X-ray diffraction (XRD). Transmission electron microscopy (TEM) showed in-plane compression after H 2 exposure. We also found that the extent of CNP variation is related to the number of graphene layers present. These results show that dissociative H 2 adsorption is mainly caused by the vacancy defects of graphene, thus providing a better understanding of the interaction between the H 2 molecule and graphene and offering evidence of the possibility of accessing more potential degrees of freedom for electronic applications using graphene.

Results
The V G -dependent resistance (R) of SLG (Fig. 1a), BLG (Fig. 1b), and MLG (Fig. 1c) as a function of H 2 pressure up to 24 bar was measured. We observed a shift in the CNP toward the negative V G region in a H 2 atmosphere. This shift indicates n-type behaviour, which becomes prominent as the layer number increases. Upon exposure to H 2 (1 bar), the graphene sheets seemed to be doped by electrons. By further exposing the graphene sheets to H 2 molecules, a high H 2 pressure enhances this effect. This effect was also observed in the V Gdependent current-voltage (I-V) characteristics ( Supplementary Fig.  S2). Upon H 2 exposure, (1) the maximum resistance (R M ) increases in BLG and MLG but the R M of SLG decreases, and (2) far from V G at the CNP (V CNP ), R in the V G , V CNP region increases while R in the V G . V CNP region decreases. The main carriers are holes and electrons at V G , V CNP and V G . V CNP , respectively. Hence, the n-doping effect induced by H 2 exposure produces result (2) mentioned above. For result (1), the increase in R M in BLG and MLG can be understood with respect to the increase in the impurity carrier density 19 but not for the reduction of R M in SLG. Hence, we tentatively suggest that the reduction in R M results from competition between the residual carrier density and the impurity carrier density, i.e., H 2 exposure contributes more to the increase in the residual carrier density than the impurity density in SLG.
Desorption occurred as the H 2 pressure was reduced, as shown in the right panels of Figure 1, and only a slight shift in V CNP to the negative V G region was detected, which will be discussed later. More interestingly, even after applying a high vacuum (approximately 10 26 Torr) at 393 K, the CNP did not return to the original value obtained before H 2 exposure. This result suggests dissociative H 2 adsorption of graphene. The variation in CNP as the H 2 pressure was increased and decreased is clearly shown in Supplementary Fig. S3.
With regard to graphene, the attached hydrogen atoms can be considered impurities that exert the n-doping effect on graphene. Raman spectroscopy can provide information regarding this type of defect. The D peak near 1350 cm 21 is a breathing mode of A 1g symmetry, which is ascribed to the presence of disorder. The G peak near 1580 cm 21 is the in-plane stretching mode of sp 2 carbon 20 . The G band is blueshifted by the variation in charge density 21 . Development of the D peak and a blueshift in the G peak were observed after H 2 exposure (Fig. 2a). Before H 2 exposure, the G bands were located at 1586 cm 21 , 1582 cm 21 , and 1583 cm 21 for SLG, BLG, and MLG, respectively. After exposing the graphene to a H 2 pressure of 24 bar, these peaks were shifted to 1591 cm 21 , 1584 cm 21 , and 1588 cm 21 , respectively, and a D peak developed near 1350 cm 21 (for BLG, the D peak can be more clearly observed in Supplementary Fig. S4). The 2D band (approximately 2700 cm 21 ) for SLG and BLG showed little change, but this was not the case for MLG. In the Raman spectra of graphene, the 2D band was very sensitive to the stacking order of the graphene layers. As a result, turbostratic graphite, which lacks a stacking order, can be considered 2-dimensional (2-D) graphite 22 . The 2D band of MLG consisted of two sub-bands at 2686.52 6 0.60 cm 21 and 2720.91 6 0.16 cm 21 (Fig. 2b). These peaks are ascribed to contributions from 2-D graphite and the highly oriented 3-D structure of graphite, respectively 23 . Before H 2 exposure, the peak (P 1 ) corresponding to 2-D graphite was smaller than the peak (P 2 ) originating from the 3-D graphitic structure. After H 2 exposure, P 1 increases, but P 2 decreases. This is also observed in graphene grown by CVD without electrodes (see Supplementary Fig. S4). Note that the development of the peak at approximately 2930 cm 21 was observed in both MLG obtained by mechanical exfoliation and that produced by CVD growth (green arrows in Figs. 2b and 2c). This peak corresponds to the sp 3 C-H stretching mode 24 . The C-H bending mode, which occurs at approximately 1330 cm 21 , overlapped with the D peak 24,25 . To confirm the development of these peaks, we measured the H 2 pressure-dependent Raman spectra of CVD graphene (Fig. 2c). Upon reaching a H 2 pressure of 10 bar, the D and D' (approximately 1620 cm 21 ) peaks increased and the peak for the C-H stretching mode at 2930 cm 21 developed. As the H 2 pressure was increased to 20 bar, the D and C-H peaks were enhanced. After evacuation at 393 K for 3 hr, the intensities of the two peaks were weakened, but the peaks were not identical to those obtained before H 2 exposure. This indicates that a small degree of C-H bonding remains even after evacuation, which is similar to the result observed for the V G -dependent R after the release process. The inset of Figure 2c shows the change in the I D /I G ratio as a function of the H 2 pressure. I D /I G increased with the increase in H 2 pressure and then decreased after evacuation. The results obtained from Raman study reveals that the attached H atoms dissociated by graphene produce an incommensurate structure such as turbostratic graphite.
By AFM, we observed an increase in the thickness of MLG from 3.2 6 0.52 nm to 7.2 6 0.82 nm after H 2 exposure ( Supplementary  Fig. S5). An increase in the interlayer distance was also observed in the graphene prepared by CVD methods. Before H 2 exposure, very small XRD peaks near 2h 5 15.8u and 26.5u (3.36 Å ) were observed. The small peaks are attributed to the few-layer graphene grown by CVD. Upon H 2 exposure, even at 1 bar of H 2 pressure, the peak near 2h 5 15.8u (5.61 Å ) became larger and shifted to a lower angle: 5.99 Å at 10 bar and 6.16 Å at 20 bar, indicating that the interlayer distance of few-layer graphene increased (Supplementary Fig. S4).
The impurity carrier density is proportional to the DV CNP induced by H 2 exposure 26 . To investigate the effect of the impurity carrier density for these three types of graphene, we obtained values for DV CNP /A (Fig. 2d). A is the area of the three types of graphene between two electrodes. Increasing the number of graphene layers leads to an increase in DV CNP . At 24 bar, DV CNP /A5 -0.05189 V/ mm 2 for SLG, -0.3065 V/mm 2 for BLG, and 21.6216 V/mm 2 for MLG. If we assume that the same number of dissociated hydrogen atoms is attached to each graphene layer, the specific DV CNP /A relationship for each of the three graphenes can be determined. Considering the DV CNP /A of SLG (20.05189 mm 22 ), the DV G /A due to the interspace of BLG becomes 20.2546 V/mm 2 , which shows that hydrogen atoms are attached to not only the edge but also the central region of the graphene. The MLG used in the measurement has seven layers, indicating that it has six BLG interspaces and one layer of SLG (AFM images in Supplementary Fig. S5). Therefore, a variation of 21.5796 V/mm 2 can be estimated by the variation in V CNP for SLG and the interspace of BLG. This value is similar to that obtained for the variation in the CNP in MLG (1.6216 V/mm 2 , the small discrepancy between the two values may be due to the error in the height obtained by AFM). This similarity indicates that the shift in CNP toward the n-type carrier regime is due to the graphene itself and not other effects such as dielectric screening caused by hydrogen molecules between the graphene and the SiO 2 substrate 27 . An increase in interlayer distance at high H 2 pressure 13 is theoretically expected, and an enhancement in the peak near 2h 5 15u in the XRD patterns of CVD graphene (see Supplementary Fig. S4) shows that the structure of hydrogen molecule-intercalated MLG can be interpreted as second-stage structures such as graphite bromine lamellar compounds 28 . However, because electrical charge cannot be transferred from hydrogen molecules to graphene 16 , the physisorption of H 2 molecules can be attributed to the increase in the interlayer distance, but this is not the reason for the n-type doping of graphene.
Strong evidence for chemisorption was obtained from TEM images of single-layer graphene grown by CVD methods (Figs. 2e and 2f). Structural modulation was indicated by the electron-diffraction (ED) patterns for 30 different regions of a graphene sample obtained before and after H 2 exposure. The d-spacings of pristine graphene were distributed over distances ranging from 2.197 to 2.256 Å . In the case of H 2 -exposed graphene, the d-spacing was reduced (2.181-2.232 Å ), indicating that in-plane compression occurs due to the chemical bonding of hydrogen 6 .
Thus far, we have investigated the properties of graphene upon H 2 exposure at 300 K. The results indicate that the dissociation barrier of H 2 is reduced on graphene. Accordingly, the V G -dependent R of MLG at constant H 2 pressure (10 bar) was measured with increasing temperature up to 340 K (Fig. 3). Upon exposing the graphene to 10 bar of H 2 pressure, the CNP shifted from 7.40 V to 6.80 V and then remained constant up to T 5 310 K. Beyond T 5 315 K, the CNP started to shift again (6.65 V at 315 K and 6.30 V at 320 K) and was slightly enhanced at 325 K (5.80 V at 325 K). At T . 330 K, the CNP changes significantly from 5.30 V to 1.20 V at 340 K (Fig. 3a). The dissociation of H 2 was much more favourable at high temperature. This finding indicates that the probability of overcoming the dissociation barrier increases, even for a small thermal energy. During cooling, we observed an interesting feature. Figure 3b shows the variation in CNP as a function of time at 330 K. Although T decreased from 340 K to 330 K, the CNP continuously shifted to 24.1 V (70 min elapsed after the measurement at 340 K). With increasing time, the CNP decreased to 27.9 V at a constant temperature T 5 330 K. This shift is natural because the thermal energy at a temperature of 330 K exceeds the dissociation barrier, as shown in Figure 3a. Further shifts in V G during the release process ( Fig. 1 and Supplementary Fig. S3) can be understood in the same way. The CNP continued to shift up to 320 K, but it remained constant beyond T 5 310 K (Fig. 3c). The CNPs at 295 K also remained unchanged during evacuation (dotted box in Figs. 3c and 3d). However, heat treatment at 393 K in vacuum for 3 hr caused the CNP to return to approximately its original value (6.8 V). The variation in CNP with decreasing temperature is summarised in Figure 3e.
The same experiment with a different MLG sample was performed for a long exposure time (t) and up to a slightly higher temperature (345 K). Before H 2 exposure, measurements of the V G -dependent R as a function of T were carried out. The CNP was observed to remain constant up to 345 K (see Supplementary  Fig. S6). Upon H 2 exposure, an n-doping effect was also observed. However, the shift in CNP and the decrease in hole mobility became significant compared with the results described in Figure 3, as is clearly shown in Figure 4a. Although DV CNP /Dt increased as the temperature increased, DV CNP /Dt seemed to saturate at each temperature at even longer times, which implies that both sufficient time and high temperature are necessary to enhance the n-doping effect. As a result, the hole mobility strongly decreased at 345 K after 750 min (Fig. 4b). This behaviour was maintained at 295 K. However, although the ambipolarity is recovered, the current level does not return to its original value in air after heating in a vacuum due to the re-adsorption of admolecules such as O 2 and H 2 O in air. If a desorption process (heating in vacuum) is not performed, the CNP moves toward the positive V G region and is then saturated, i.e., the n-type behaviour is maintained even in air ( Supplementary Fig. 7).

Discussion
The dissociative H 2 adsorption barrier on graphite is approximately 3.3 eV 29 . Although this barrier appears prohibitive, dissociation can occur in particular cases. If a hydrogen molecule approaches a vacancy defect on a graphene surface, the molecule becomes largely polarised and finally dissociates 17 . At the edge of graphite, the energy barrier is reduced, causing dissociative adsorption 18,30 . Other possible factors for dissociative H 2 adsorption, including electric fields 31 and mechanical stress 32 , have been suggested. In the present study, the C-H bonding of CVD graphene was clearly detected compared with that of mechanically exfoliated graphene. The results show that vacancy defects promote dissociative adsorption because graphene grown by CVD methods has more structural defects than mechanically exfoliated graphene 33 .
H molecules dissociated on graphene can break C5C double bonds in the structure of graphene. Consequently, two unpaired electrons are produced. An unpaired electron participates in the formation of a C-H bond, and the other is delocalised [34][35][36][37] . This delocalised electron induces an n-type doping effect on graphene. In addition, we can rule out other possibilities for n-doping on graphene in a H 2 atmosphere, including the presence of a Ti adhesion layer ( Supplementary Figs. S8 and S9).
Collectively, the exposure of graphene to H 2 molecules causes an increase in the interlayer distance of graphene, and molecular hydrogen is partially dissociated due to the existence of vacancy defects on graphene, as theoretically proposed. The dissociated H atoms are expected to be chemisorbed at the edge 30 and in the vacancies of the graphene 17,38 , resulting in the n-type doping of graphene due to delocalised unpaired electrons. Upon exposing the graphene to 10 bar of H 2 at 295 K, the CNP shifts toward the negative V G region. As the temperature increases up to 310 K, the CNP remains nearly constant. In fact, it shifts but cannot be defined because of the resolution (a V G sweep interval of 50 mV). Beyond 315 K, the CNP shifts again, and the variation in CNP increases with temperature up to 340 K. (b) During the cooling process, we set the temperature to 330 K. As the exposure time increases, the CNP continuously shifts along the same direction and R M increases slightly. The insets of (a) and (b) clearly show the variation in the CNP. (c) The shift is maintained down to 320 K but not below 310 K. (d) At 295 K, although the H 2 pressure is released, no shift in the CNP is observed. (e) The variation in the CNP during the release process is depicted. and t. Upon exposure to 10 bar of H 2 pressure at 300 K, the CNP changes; it then changes slightly at 320 K and t 5 300 min after exposure. Beyond 320 K/ 300 min, the slope of DV CNP /Dt increases. (b) Beyond 340 K/540 min, the hole mobility decreases significantly and finally reaches zero. After heating to 393 K for 3 hr, the ambipolarity is restored. Methods Graphene samples were extracted from highly oriented pyrolytic graphite (HOPG) by mechanical exfoliation. The graphene samples were deposited on 300 nm SiO 2 / highly p-doped Si wafers. The samples were selected by the colour contrast observed in optical microscopy images and characterised by micro-Raman spectroscopy (LabRam 300 with a 532 nm laser line of less than 1 mW, JY-Horiba) and AFM (SPA-400, Seiko instrument). A TEM investigation (HRTEM and diffraction pattern) of single-layer graphene was performed using an FEI Titan cube G2 60-300 installed at UNIST, equipped with an image aberration corrector. The microscope was operated at an accelerating voltage of 80 kV to reduce beam damage. Electrodes were fabricated by standard electron beam lithography (VEGA MM5150 with 30 keV filament, Tescan), the evaporation of Ti/Au (5/50 nm) and Cr/Au (10/40 nm) (ZZS550-2/D, Maestech), and lift-off procedures.
The devices were electrically characterised by a back gate sweep with a drain-source bias of V ds 5 1.0 or 2.0 mV using a semiconductor characterisation system (4200-SCS, Keithley) in a high-pressure chamber, which was completely made of stainless steel. The procedure is as follows. First, the devices were loaded into the chamber, which was evacuated at 393 K until the pressure reached approximately 10 26 Torr. Then, the graphene surfaces were exposed to 99.9999 % H 2 gas at pressures ranging from 1.0 to 25 bar.