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Electric-Field Control of Giant On/Off Current Ratio in Graphene Through Reversible Hydrogenation

Graphene-based electronic devices have attracted tremendous attention owing to its single-atomic-layer thickness and high electron mobility for potential applications in the next-generation electronics. The main obstacle for widespread implementation of graphene transistors is the poor on/off current ratio arising from the absence of essential bandgap in the Dirac-like dispersion. A promising approach for opening an energy gap in graphene is by the hydrogenation process that transforms graphene into highly insulating graphane with sp3 bonding. Here we demonstrate an electric-field control of conductor-insulator transition in microscale graphene through highly reversible hydrogenation reaction in a liquid electrolyte. Most remarkably, the sheet resistance of fully hydrogenated graphene shows a lower limit of 200 Gohm/sq, leading to a giant on/off current ratio larger than 10^8 at room temperature. Similar behaviors are also demonstrated in bilayer graphene, while the trilayer one remains highly conductive after the hydrogenation. In-situ Raman spectroscopy reveals lattice deformation due to the change of C-C bond from sp2 to sp3 hybridization, which is supported by first-principles calculations. Our finding lays out a new roadmap for the graphene-based electronics, and the electric-field-controlled hydrogenation method may also be generalized to other two-dimensional materials.


graphene-based logic transistors.
The hydrogenation of graphene requires rather powerful reagents to initiate the chemical reactions owing to its weak hydrogen affinity and chemical inertness 4 . Therefore, all hydrogenation approaches require the activation of either hydrogen or the graphene lattice to make the reaction feasible 5 . A number of precursive routes to hydrogenated graphene have been demonstrated since its initial synthesis in 2009 (Ref. 6). The main approaches fall into three categories: (1) atomic/plasma hydrogen reaction 6-14 , (2) chemical reduction [15][16][17][18] , and (3) electrochemical hydrogenation [19][20][21] .
In our experimental setup (Fig. 1a), the graphene devices are immersed in a hydrogen-ion electrolyte (HIE). A thin Pt foil close to graphene is used as the gate electrode (see detailed structure in Fig. S1). When a positive VG is applied between the gate and source electrode of graphene, the electric field drives hydrogen ions (H + ) to accumulate on top of the graphene surface. Thus, the resistance of graphene is strongly modulated due to the formation of an electric double layer (EDL) at the interface between graphene and HIE. With increasing VG to above the hydrogenation potential, graphene lattice becomes highly activated and the chemical bonding between H + ions and C atoms starts to form, as illustrated in Fig. 1b. which reveals a characteristic V-shaped profile [29][30][31] for -1 V < VG < 1.5 V arising from the linear dispersion of relativistic Dirac fermions intrinsic to graphene 1,4 . Surprisingly, when VG is increased to 1.7 V, IDS drops dramatically to nearly zero and remains pinch-off up to VG = 4 V (Fig. S2). Therefore, the hydrogenated MLG transforms into a widebandgap insulator analogous to graphane 2,3 . When VG is swept back from 2.2 V, IDS stays in the "off" state till 0.2 V and then starts to increase gradually. It nearly recovers the original "on" state at -1 V, where the MLG is dehydrogenated and the sp 2 bond hybridization is restored. The subsequent two cycles of gate sweeping between -1 V to 2.2 V generate nearly identical IDS traces. Such unprecedented consistency further demonstrates the highly reversible hydrogenation and dehydrogenation processes without the degradation of graphene lattice.
To accurately determine the high resistance value of hydrogenated MLG, we carefully restrain the leakage current (IG) between the gate and graphene electrodes. By linearly fitting the IDS + IG as a function of VDS as shown in Figs. S3 and S4, we can derive that the sheet resistance of hydrogenated MLG has a lower limit of 200 G/sq at room temperature (see more discussion in the Supplementary Information). When the cell is cooled down to 0 °C to further suppress IG, the sheet resistance can reach up to 300 G/sq (Fig. S5), approaching the measurement limit of our instrument. Such high resistance offers an ideal "off" state for graphene-based logic devices. In the "on" state at 1.5 V and -1 V, the MLG is highly conductive with sheet resistance merely ~ 410 /sq. Therefore, the on/off ratio can reach a record high value of 4.910 8 at room temperature, which is seven orders of magnitude higher than that ever reported on microscale MLG FETs (less than 20) 32 .
The changes of graphene lattice induced by hydrogenation are corroborated by Raman spectroscopy, as shown in Fig. 1d. The main features in the Raman spectra of MLG are the G and 2D peaks at around 1580 and 2670 cm −1 , resulting from the inplane vibrational (E2g) mode and two-phonon inter-valley triple resonance scattering, respectively 33,34 . The single and sharp 2D peak at charge neutral point (VG ~ 0.3 V) are unambiguous fingerprints of MLG 33,34 . As more electrons are injected at VG ~ 1.5 V, the G peak becomes sharper and shifts to 1590 cm −1 , consistent with previous reports 29,35 .
With the further increase of VG to 2.0 V, the hydrogenation reaction results in three distinctive features in Raman spectra: (1) the appearance of sharp D and D′ peaks at 1340 and 1600 cm -1 , (2) the significant broadening of the 2D peak, and (3) the emergence of combined (D + D′) and overtone (2D′) modes around 2930 and 3190 cm −1 .
The D peak is undetectable in disorder-free MLG because its activation requires a defect via an intervalley double-resonance Raman process 33,34 . Therefore, the intensity of D peak relative to G peak can serve as a measure for the defect density in graphene.
Here, the sharp D peak in hydrogenated graphene reflects the breaking of translational symmetry of C-C sp 2 bonds and the formation of massive C-H sp 3 bonds 6 , behaving like a large amount of defects in graphene lattice. Besides, the observation of D′, D + D′, and 2D′ peaks further proves the presence of defect-assisted activation during the resonance Raman processes. It is worth noticing that the intensity of all Raman peaks are greatly increased compared to that without hydrogenation, which is probably caused by the surface-enhanced Raman scattering 36 induced by the enhanced electromagnetic field between neighboring isolated and randomly-sized nanoscale pristine graphene domains embedded in the hydrogenated area 18 , as further supported by the broad fluorescence background in the Raman spectra (Fig. 1d).The restoration of graphene lattice during the dehydrogenation reaction for VG from 2 V to -1 V is also confirmed by the Raman spectroscopy (Fig. 1d). When VG is lowered from 2 V, all the hydrogenation-induced Raman peaks (D, D′, D+D′, and 2D′) gradually decrease in intensity and completely disappear at around -0.5 V, leaving only the G and 2D peaks characteristic of graphene. These observations clearly demonstrate that the hydrogenated graphene can be fully recovered to its original lattice by electric-field control, which is further proved by the highly reversible and reproducible Raman spectra with multicycle sweeps (Fig. S6). Moreover, the agreement between Raman spectra and transport measurements indicates that the insulating graphene is indeed caused by the hydrogenation-induced transformation from sp 2 to sp 3 hybridization.
To confirm that the current pinch-off is due to hydrogenation rather than bad connection between drain/source electrodes and graphene flake, we performed the control experiments in a three-segment device on a single graphene ribbon with shared drain/source electrodes as shown in Fig. 2a-c. Only the middle segment is exposed to HIE, while the left and right ones are covered by a layer of 300-nm poly(methyl methacrylate) (PMMA) to obstruct the hydrogenation reaction. We first ground the shared electrode 2 and record the IDS of both the left and middle segments simultaneously. The middle segment reproduces the pinch-off state at VG ~ 2 V (Fig.   2b), while the IDS of the left segment remains "on" at the same VG (Fig. 2a). This proves that the shared electrode 2 has a good electrical connection with the graphene ribbon even when the middle segment is fully hydrogenated. We then performed similar measurements in the middle and right segments. Likewise, the results in Fig. 2b and 2c completely rule out the possibility that electrode 3 is detached from the graphene ribbon during hydrogenation. It is worth mentioning that IDS of the left and right segments still The gate-controlled switching capability is another key factor for the application of graphene FETs. As shown in Fig. 2d, when VG is quickly switched between -0.5 V and 2.4 V, the IDS should exhibit sharp transitions between on and off states, which is explicitly demonstrated by the alternating VDS across graphene channel in the voltage divider circuit (Fig. S8). After one million switching cycles (Fig. 2e), the graphene FET remains readily functional without any decay of the divided voltage, indicating the outstanding reversibility and stability of the hydrogenation and dehydrogenation in MLG. All these findings provide a long-sought solution to obtain a giant on/off current ratio and reliable switching capability in graphene FETs, which would significantly advance the applications of graphene logic devices. Figure 2f and g present the switching speeds of the MLG device under the control of switching VG between -1 V and 2.6 V. We find that the rise time (10 %  90 %) of VDS is 0.4 ms and the turn-off delay time is around 2.2 ms, which strongly depends on the turn-off gate voltage and the distance between gate electrode and graphene channel.
Thus, this delay time can be attributed to the migration time of H + ions onto graphene surface driven by the electric field. In Fig. 2g, the fall time to the "on" state is hard to evaluated due to the presence of undesired signal when switching VG from 2.6 V to -1 V, which probably arises from the large parasitic capacitance in our electrochemical FET. Here we hypothesize the trace of switch curve (blue dashed line) by following the tendency of VDS. Thus, the fall time can be estimated to be around 0.13 ms, much lower than the rise time (0.4 ms). And the delay time is around 1.1 ms. This can be interpreted that the strip off of H atoms from graphene should be faster than the migration and accumulation to the graphene surface. Compared with the previous studies on electrochemical graphene FETs 24,25 , the performance of our MLG device is much more superior in terms of degradation, response time, and on/off ratio. The key improvement in our devices is utilizing the HIE with dissociative H + ions other than that from the hydrolysis of residual water in the moister air 24 or organic dielectrics 25 . Moreover, we only applied low gate voltages (between -1 and 2.6 V) to avoid the electrolysis of electrolyte and the irreversible reaction with anions (e.g. TFSIor OH -), which may induce strong physical stress and break the C-C bonds in the graphene channel.
Besides monolayer graphene, the electric-field-driven reversible hydrogenation in multilayer graphene has also been studied. As shown in Fig. 3a, the IDS of bilayer graphene (BLG) presents a characteristic V-shaped feature 31 similar to MLG before the occurrence of hydrogenation (VG < 1.5 V). A new feature here is that BLG presents a layer-by-layer hydrogenation behavior, as reflected by the two-step falloff of IDS for VG > 1.6 V. Fully hydrogenated BLG is also highly insulating with the sheet resistance larger than 50 G/sq (Fig. S9). As VG is swept back to -1 V, IDS of hydrogenated BLG progressively recovers its original conducting state. Two subsequent cycles of reversible hydrogenation display reproducible IDS traces (Fig. 3a), with a slight difference in the onset voltages of the second jump at VG > 1.7 V, which presumably corresponds to the hydrogenation of the bottom layer. The Raman spectra in Fig. 3b further corroborate the electric-field control of reversible hydrogenation in BLG. Near the charge neutral point (VG ~ 0 V), both the significant broadening and blue-shift of the 2D peak (to 2700 cm -1 ) reveal the fingerprint of BLG. When the top graphene layer is hydrogenated at VG = 1.7 V, the Raman spectrum exhibits similar changes as that in MLG and remains nearly unchanged during the hydrogenation of bottom layer at VG = 2.1 V, which indicates consistent hydrogenation mechanism and structure both for the MLG and BLG. As the dehydrogenation is induced by sweeping VG to -1 V, the Raman spectrum progressively restores its original characteristics of pristine BLG.
For the trilayer graphene (TLG), the IDS traces in Fig. 3c are nearly identical to that of MLG and BLG before the hydrogenation (-1 V < VG < 1. To understand the microscopic mechanism of the electrochemical reaction between hydrogen and graphene, we performed density functional theory (DFT) calculations. Before the formation of H-C bond, there is an energy barrier of ~ 0.2 eV corresponding to the H atom at ~1.8 Å above the C atom, consistent with previous reports 3, 37 . When the electric field is applied on top of graphene, a linear background is superposed to the potential energy of the graphene and H atom system. Accordingly, the energy barrier gradually flattens with increasing electric field to ~ 2 V/nm, which is readily achieved in our experiments at VG = 2 V by the strongly localized electric field of the ~1 nm EDL on graphene surface 30,31 . Moreover, as illustrated in Fig. 4b, the energy barrier is lowered by electron doping to an experimentally accessible level of ~ 1  10 14 cm -1 (Ref. 29,31). This finding indicates that the hydrogenation can be greatly facilitated by the strong electric field and high electron doping, in good agreement with our observation that MLG is highly conductive at VG ~ 1.5 V, which is right before the reaction. For the dehydrogenation process, it is unexpected that MLG gradually becomes conductive with VG < 0.2 V because the energy barrier of H desorption is as high as 1.0 eV. We attribute such discrepancy to the metastable hydrogenated lattice susceptible to Eley-Rideal reactions between H + ions in HIE and chemically bonded H atoms 37,38 , or the desorption of recombined H atom pairs on graphene surface 39,40 .
Among all the structures of fully hydrogenated graphene, the most stable one is supposed to have the chair conformation with the hydrogen atoms alternating on both sides of the plane 2 . Although only the top surface of MLG is exposed to HIE in our devices, we believe that hydrogen atoms are chemically bonded to carbon atoms on both sides based on the following reasons. Firstly, the most stable composition for single-side hydrogenation is C4H as demonstrated both experimentally and theoretically 10,[41][42][43] . However, the binding energy of C4H (1.98 eV/H atom) 41 is much smaller than that of double-side hydrogenated chair conformer (6.56 eV/H atom) 2 .
Additionally, in the general case with two H atoms bonded to the nearest C atoms, H positions on opposite sides of the graphene sheet are much more favorable than those on the same side 3 . Secondly, the single-side hydrogenated graphene can only be obtained when the hydrogen is completely restricted to one side 41 . Given the high diffusivity of H + ions under electric field [26][27][28] , they can easily penetrate through the top surface 44,45 or intercalate into the interface between graphene and SiO2 substrate through the exposed edges 46 . Therefore, H atoms prefer to bond both sides of graphene, which is further facilitated by the progressive reduction of formation energies with the increase of H concentrations 3,47 . Thirdly, due to the frustrated hydrogenation domains arising from the random bonding of H atoms to A or B sites of graphene lattice 48 , the single-side hydrogenated C4H with strong disorders is still conductive 10 with sheet resistance much lower than our results. Finally, compared with the plasma-induced single-side hydrogenation 6-10 , our Raman spectra are more analogous to that from the chemical reduction method, which has been used to produce high H coverage over 50% (Ref. [15][16][17]. Based on the above analyses, we employ the chair conformer as the structure of fully hydrogenated MLG and calculate the density of states (DOS). As shown in Fig.   4c, the hydrogenation process converts gapless MLG into an insulator with bandgap up to 3.5 eV. BLG with only the top layer hydrogenated is equivalent to a graphane/graphene (B-A) heterostructure, which is still conductive. Upon complete hydrogenation, the highly insulating BLG possesses a large bandgap corresponding to bilayer graphane (Fig. 4d). For the case of TLG, the applied electric field is strongly localized on the top one/two layers, thus at least one bottom graphene layer cannot be fully hydrogenated and serves as a gapless conducting channel as displayed in Fig. 4e with the graphane/BLG (B-A-A) and bilayer-graphane/MLG (B-B-A) configurations.
The electronic structure calculations thus are highly consistent with the transport behavior of all the devices with different numbers of graphene layers.
The electric-field-induced reversible hydrogenation provides a powerful way to create and annihilate a sizable bandgap in the microscale graphene. The giant roomtemperature on/off current ratio and stable switching behavior pave the road for practical applications of graphene-based electronics. These findings shed new lights on the construction of an all-graphene-based integrated circuit with MLG working as the essential FET and TLG as the interconnection line 49 . Compared with the chemical method 3,50 , the electric-field control of reversible adsorption of H atoms also represents a feasible approach for hydrogen storage in atomic-thin graphene membranes.
According to the previous studies of H + ion control in metal oxides [26][27][28] , we believe that our electric-field-induced hydrogenation can be used as a universal method for reversibly tuning the atomic bonding and band structures of various 2D crystals, which opens up new opportunities in searching for emergent materials with novel functionalities.

Sample preparation
Graphene crystals were mechanically exfoliated on silicon wafers with 285 nm SiO2.
The thickness was identified by the atomic force microscope and Raman spectroscopy.
After that, standard electron-beam lithography and thermal evaporation were used to

Electric transport measurements
A direct voltage VDS = 5 mV was applied between the drain and source electrodes by Keithley 2400 SourceMeter, and IDS was recorded simultaneously. Another 2400 SourceMeter was used to apply VG between the Pt electrode and graphene source electrode and measure IG in the meanwhile. The change of VG was performed only at room temperature (~ 25 °C) with the changing rate at 2 mV/s. When measuring the resistance of hydrogenated graphene, VG was kept unchanged while VDS was swept back and forth with the rate at 5 mV/s. All the sheet resistance values were obtained from the two-terminal resistance by considering the size effect.

Raman spectroscopy measurements
Raman spectroscopy was measured by HORIBA LabRAM HR Evolution Spectrometer with 532 nm laser excitation and 1800 l/mm grating. All the Raman spectra have subtracted the background signals contributed by HIE. During Raman measurements, two Keithley 2400 SourceMeters worked together to control VG and monitor IDS respectively. When VG reached the target voltage, the Raman signals were acquired until IDS became nearly stable.

Theoretical calculations
First-principles calculations were performed in the framework of density functional theory using the Vienna ab initio Simulation Package 52 . The energy cutoff of the planewave basis was fixed at 500eV. The projector augmented wave (PAW) method and the Perdew-Burke-Ernzerhof type generalized gradient approximation (GGA) 53 were adopted to model the electron-ion interactions and the exchange correlations between electrons, respectively. The crystal structure relaxations were performed with the force criteria of 0.01 eV/Å. The van der Waals interaction between neighboring layers was considered by the DFT-D3 methods 54 . A vacuum layer of 20 Å was added in the direction perpendicular to 2D planes to avoid interactions between adjacent periodic images. The Monkhorst-Pack k-point meshes of 40×40×1, 9×9×1 were adopted for pristine lattices and supercells of 3×3, respectively.

Data availability:
All raw and derived data used to support the findings of this work are available from the authors on request.