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Theoretical Study on Carrier Mobility of Hydrogenated Graphene/Hexagonal BoronNitride Heterobilayer
Nanoscale Research Letters volume 13, Article number: 376 (2018)
Abstract
Hydrogenated graphene (HG)/hexagonal boron nitride (hBN) heterobilayer is an ideal structure for the highperformance field effect transistor. In this paper, the carrier mobilities of HG/hBN heterobilayer are investigated based on the firstprinciples calculations by considering the influence of stacking pattern between HG and hBN, hydrogen coverage and hydrogenation pattern. With the same hydrogenation pattern, the electron mobility monotonously decreases when the hydrogen coverage increases. With the same hydrogen coverage, different hydrogenation patterns lead to significant changes of mobility. For 25% and 6.25% HGs, the μ_{e} (ΓK) of 25% pattern I is 8985.85 cm^{2}/(V s) and of 6.25% pattern I is 23,470.98 cm^{2}/(V s), which are much higher than other patterns. Meanwhile, the hBN substrate affects the hole mobilities significantly, but it has limit influences on the electron mobilities. The hole mobilities of stacking patterns I and II are close to that of HG monolayer, but much lower than that of stacking patterns III and IV.
Introduction
Hydrogenated graphene (HG) [1, 2] is one of the most promising graphenebased materials. It has aroused widespread attention due to its extensive applications, such as hydrogen storage [3], ferromagnetism [4], fluorescence [5], and thermal rectification [6]. In contrast to metallic graphene, HG is predicted to be the semiconductor with a tunable band gap [7, 8]. Thus, it can be used as the channel material of fieldeffect transistor (FET) [9]. Excellent FETs should have an ultrahigh carrier mobility of the channel material. As is well known, the traditional SiO_{2} substrate has a significant negative effect on FET performance [10]. Recently, the studies show that monolayer hexagonal boron nitride (hBN) [11, 12] is a promising candidate for the substrate of graphenebased FET. Monolayer hBN and HG are latticematched structures, indicating a better contact performance. Hence, HG/hBN heterobilayer is an ideal structure of the FET’s channel. Unfortunately, there are only a few related studies about the electronic properties of HG/hBN heterobilayer structure. The carrier mobility performance of HG/hBN heterobilayer is still an open question.
Most of the current studies on HG are devoted to engineering the desired electronic properties via hydrogenation [13,14,15,16,17,18]. Gao et al. [13] studied the hydrogen coverage and configuration dependence of the band gap of HG. Sahin et al. [14] compared the effect of adatompatterned (hydrogenation) and holepatterned (removal of carbon atom) graphene nanomeshes on band structure. Shkrebtii et al. [15] investigated the band structure of HG, where the structure of HG is limited in C_{16}H_{n} system (n = 0,2,8,16). Song et al. [16] calculated the band gap of HGs with different hexagon vacancies. Bruzzone et al. [17] calculated the mobilities of HG with different hydrogen coverage (100%, 75%, 25%) by abinitio simulations and found 25% HG got the highest mobility. There are also some studies about applying the hydrogenation in hBN. Chen et al. [19] utilized the hydrogenation to realize semiconductor to metal transition in hBN. Liang et al. [20] studied the interactions between 100% HG and 100% hydrogenated hBN. It shows that the electron mobility of HG/hydrogenated hBN is only 50 cm^{2}/(V s) which is far away from that of graphene.
In a word, the current studies on the carrier mobility of HG/hBN heterobilayer are still not enough. The main factors which affect the carrier mobility of HG/hBN heterobilayer, namely hydrogen coverage, hydrogenation pattern and the stacking pattern between HG and hBN, should be clarified. In this paper, the carrier mobilities of HG/hBN heterobilayer structures were investigated based on the firstprinciples calculations. Firstly, the effect of the hBN substrate on the mobilities of HG was investigated. Secondly, the electronic properties of HG with different hydrogen coverage were compared. Finally, different hydrogenation patterns were applied in 25% and 6.25% HG to reveal the influence of hydrogenation pattern.
Methods
All calculations were implemented in Atomistix ToolKit (ATK) [21] based on the density functional theory (DFT). The exchange correlation is the generalized gradient approximation (GGA) with the PerdewBurkeErnzerhof (PBE) functional. Van der Waals (vdW) correction adopted Grimme DFTD2 method [22] for the heterobilayer structures. The cell length in z direction (perpendicular to the HG plane) is 20 Å, in order to eliminate the effect of its periodic images. The kpoint sampling is 33 × 33 × 1 MonkhorstPack grid.
Deformation potential approximation (DPA) method [23] is used to investigate the carrier mobility; the expression of the carrier mobility of 2D material [24, 25] is:
where e is the electron charge, ћ is reduced Planck constant, k_{B} is Boltzmann constant, T is the temperature (it is set to be 300 K in the cases), and C_{2D} is the elastic modulus of the propagation direction. E_{1} is the deformation potential constant defined by E_{1} = ΔV/(Δl/l_{0}). ΔV is the energy change under proper cell compression and dilatation. The change of the conduction band minimum (CBM) is used for electrons and the valence band maximum (VBM) for holes. l_{0} is the lattice length in the transport direction and Δl is its deformation (Δl/l_{0} is set to be − 0.01, − 0.005, 0, 0.005, 0.01). m^{*} is the effective mass in the transport direction, calculated by:
where k is the wave vector and E is the energy. m_{d} is the equivalent densityofstate mass defined as m_{d} = (m_{x}m_{y})^{0.5}. Deformation potential constant and effective mass can be deduced from band structures, while the elastic modulus is extracted from phonon dispersion relations. It should be emphasized that the DPA method may overestimate the mobilities of arsenene, antimonene [26], and silicene [27] because it does not consider the effect of flexural acoustic (ZA) phonons. Shuai et al. [28, 29] discussed the applicability of DPA and found that it can estimate the electronic properties of graphene and graphyne well. The ZA phonons play a minor role in electronphonon interactions for twodimensional carbon materials. The electronic mobility of graphene [28] at room temperature is estimated to be 3.4 × 10^{5} cm^{2}/(V s) by DPA method and 3.2 × 10^{5} cm^{2}/(V s) [28] by considering all the electronphonon interactions. As for HG, we will reanalyze the effect of ZA phonons in the next part.
Results and Discussion
Firstly, different stacking patterns between hBN and HG were investigated, where the HG is 100% hydrogenated. It should be emphasized that the interaction between HG and hBN is vdW force, which is far weaker than covalent bond. Hence, it is unnecessary to analyze the other HG/hBN heterobilayers. There are four possible stacking patterns for the heterobilayer, as seen in Fig. 1a–d, where “a” is lattice parameter and “d” is interlayer distance. The interlayer distance is defined as the distance between the geometrical centers of HG layer and hBN layer, as marked in Fig. 1a. In patterns I and II, the two skeletons are in AA stacking, while in patterns III and IV are in AB stacking. The structures were geometry optimized by the LBFGS optimizer method firstly. The convergence criteria for force tolerance are less than 0.001 eV/Å. After geometry optimization, the unit cell parameter is 2.52 Å for all the stacking patterns, while the interlayer distance depends on the stacking pattern. The interlayer distance of pattern I is the lowest, and pattern III is the highest. The vdW corrections of the four patterns are − 651.69 meV, − 658.14 meV, − 658.22 meV, and − 651.54 meV, respectively. Obviously, the tendency of vdW interaction coincides with that of the interlayer distance.
Band structure is one of the most important electronic properties. The corresponding band structures of stacking patterns I–IV are shown in Fig. 2. The two bold lines in each figure represent the bands including CBM (up) and VBM (down), respectively. Γ (0,0,0), M (0,0.5,0), K (0.333,0.333,0) are the symmetry points in the Brillouin zone. The main band structure information, including direct band gap (DBG), indirect band gap (IBG), CBM, and VBM positions, should be noticed. Generally, the four patterns have similar band structures. For patterns I–IV, the CBM and VBM are at point K and Γ, respectively. Patterns I and IV have similar DBG (4.35 eV) and IBG (3.25 eV), while the DBG and IBG of patterns II and III are about 4.22 eV and 2.98 eV. By comparing their interlayer distance, it can be concluded that the stronger interlayer interaction leads to the wider band gap. It should be emphasized that the band structure of single layer hBN is also calculated with PBE. The band gap of hBN is 4.65 eV which agrees well with the value reported in [30]. Overall, the method is suitable for hBN.
Secondly, the influences of hydrogen coverage and hydrogenation patterns are considered, whereas the influence of hydrogenation origins from the changing of covalent bonds, which is much stronger than vdW force. Hence, only HG monolayer is investigated in this part. The considered structures are shown in Fig. 3, where “” and “”denote the carbon atoms bonded with hydrogen atom at different sides. For the sake of the stability of the whole structure, hydrogen atoms are evenly distributed on each side. For 100% HG, it only has one stable pattern. Twentyfive percent HG composed by 8C and 2H has three different patterns. For 6.25% HG, it has 32C and 2H in the primitive cell. Only two patterns of 6.25% HG are considered. As shown in Fig. 3b, c, two hydrogenated carbon atoms are adjacent to each other in pattern I and away from each other in pattern II. It should be noticed that 6.25% pattern I, 25% pattern I and 100% HG are the same type (two hydrogenated carbon atoms are adjacent). In Fig. 3, E_{f} is the formation energy per atom
where E_{total} is the total energy of HG, E_{graphene} refers to the energy of pristine graphene, E_{H} is the energy per atom of the H_{2} molecule, and n_{H} is the number of the adsorbed hydrogen atoms. E_{f} is used to check the stability of the structure, and the negative E_{f} suggests thermodynamics stability. The results in Fig. 3 imply that all the listed HGs are stable. η denotes the percentage rise of the lattice parameter of HG in contrast to graphene (the minimum unit cell length of graphene is 2.47 Å). On the whole, the lattice enhancement decreases with the decreasing hydrogen coverage. For 6.25% HG, η is almost negligible. Besides the hydrogen coverage, hydrogenation pattern also influences the lattice. For 25% HG, pattern I is enlarged least among the three patterns, mainly because the hydrogenated carbon atoms are adjacent. Δ is the buckling parameter, which is defined as the standard deviation of the outofplane displacements of the carbon atoms. Generally, the buckling parameter increases with the increased hydrogen coverage.
The band structures of the above HGs are shown in Fig. 4. The band gap of 100% HG is about 4.14 eV, in good agreement with the previous literature [16, 31]. For 25% HG, the band gap is strongly affected by the hydrogenation pattern. Pattern II has an IBG of 3.0 eV, while the IBG of pattern III is 0 eV. The IBG from zero to nonzero indicates a transition from metallic to semiconductor. In addition, pattern II has different DBG and IBG, suggesting that its CBM and VBM are at different points. For 6.25% HG, the VBM and CBM are at the same points for both of the two patterns, which of pattern I is (0.153, 0.423, 0) and pattern II is (0.24, 0.24, 0). The band gap of two 6.25% HGs are 0 eV and 0.49 eV, both of which reduced significantly in contrast to that of 100% HG. Generally, both hydrogen coverage and hydrogenation patterns are effective methods to modulate band gap.
Table 1 presents the estimated values of elastic modulus C_{2D}, effective mass m^{*} and deformation potential constant E_{1}. C_{2D} and m^{*} are directiondependent parameters. Among all the directions, ΓM and ΓK are the most concerned. Hence, C_{2D} (ΓM/ΓK) and m* (ΓM/ΓK) are listed in Table 1. C_{2D} = ρv_{g}^{2}, where ρ is the density and v_{g} denotes group velocity of acoustic phonon. Because hydrogenation has few effects on group velocity, C_{2D} of different HGs are similar with each other. The HG v_{g} is about 23 km/s in ΓK direction and 19.4 km/s in ΓM, so C_{2D} (ΓK) is much higher than C_{2D} (ΓM). The deformation potential constant has no regular tendency with the different patterns. Generally, the vdW interaction between HG and hBN increases the deformation potential constant.
Effective mass is more complicated, since it depends on carrier and direction. There are three points that should be noted on effective mass. First, the electron effective mass of 100% HG and 100%HG/hBN heterobilayer are isotropic, i.e., m*(ΓM) = m*(ΓK). The heterobilayer structure leads to a slight drop of electron effective mass compared with 100% HG monolayer. The stacking pattern has slight influence on the electron effective mass (all of the four stacking patterns are about 0.90). Second, under the same hydrogenation pattern (i.e., 100%, 25% pattern I and 6.25% pattern I), the electron m*(ΓK) decreases with the decreased hydrogen coverage. It is shown that the limit is 0.024 (the effective mass of graphene) as the hydrogen coverage reduces to zero. Third, under the same hydrogen coverage, effective mass is also affected by hydrogenation pattern. For 25% HG, the electron effective mass of pattern I is much lower than the other two. In a word, the effective mass is more likely to be affected by hydrogenation but not the elastic modulus and deformation potential constant.
In Table 2, the electron and hole mobilities are computed based on the above parameters. Because the effective mass is more likely to be affected, the tendency of mobility is similar with that of effective mass. Generally speaking, hydrogenation dramatically reduces the mobility of graphene. The theoretical mobility of graphene (3.2 × 10^{5} cm^{2}/(V s)[28]) is several orders of magnitude higher than that of HG. In addition, HGs have asymmetric (μ_{e} ≠ μ_{h}) and anisotropic (μ(ΓM) ≠ μ (ΓK)) mobilities. There are three details that should be noticed. First, under the same hydrogenation pattern, the electron mobility monotonously decreases with the increasing hydrogen coverage. But, if under different hydrogenation pattern, the conclusion is not always established. For example, the mobilities of 25% pattern II are lower than that of 100% HG. Second, for 25% and 6.25% HGs, pattern I has a higher μ_{e} compared to the other patterns. The μ_{e} (ΓK) of 25% pattern I is 8985.85 cm^{2}/(V s) and of 6.25% pattern I is 23,470.98 cm^{2}/(V s), much higher than black phosphorene [24] and MoS_{2} [32]. Third, the hBN substrate affects the hole mobilities significantly, while it has little effect on the electron mobilities. It indicates the hole mobilities of stacking patterns I and II are close to that of HG monolayer, but much lower than that of stacking patterns III and IV. Hence, different stacking patterns have significant effects on hole mobilities but little effects on electron mobilities.
Moreover, the mobility of 100% HG was recalculated by considering all the electronphonon interactions, namely longitude acoustic (LA), transverse acoustic (TA) and ZA phonons. The results show that the electron mobility is 105 cm^{2}/(V s) in ΓK direction. Figure 5 gives the electronphonon interaction matrix elements g of LA, TA and ZA phonons. It shows that the LA phonons dominate in electronphonon interactions. On the whole, LA phonons have larger interaction strength with electrons compared with the TA and ZA phonons. Although the mobility value is slightly lower than that calculated by DPA method, the difference of two methods in HG is much less than that in arsenene, antimonene, and silicene. Generally, the DPA method is feasible in our study.
Conclusions
In summary, the carrier mobilities of HG/hBN heterobilayer were investigated based on the firstprinciples calculations in this paper. The influence on mobilities is discussed in terms of the stacking patterns of HG/hBN heterobilayer, hydrogen coverage, and hydrogenation pattern. The elastic modulus C_{2D}, effective mass m^{*}, and deformation potential constant E_{1} are calculated to analyze the mobilities. The deformation potential constant has no regular tendency with the different patterns. The elastic modulus and the effective mass in HGs are directiondependent. The results show that ΓK direction has a higher elastic modulus. The effective mass is more likely to be affected by different hydrogenations and stacking patterns. Under the same hydrogenation pattern, the electron mobility monotonously decreases with the increasing hydrogen coverage. Under the same hydrogen coverage, different patterns lead to a significant change of mobilities. For 25% and 6.25% HGs, the μ_{e} (ΓK) of 25% pattern I is 8985.85 cm^{2}/(V s) and of the μ_{e} (ΓK) 6.25% pattern I is 23,470.98 cm^{2}/(V s); both are much higher than the other patterns. As for the influence of hBN substrate, different stacking patterns affect the hole mobilities significantly, but hardly affect the electron mobilities. The hole mobilities of stacking patterns I and II are close to that of HG monolayer, but much lower than that of stacking patterns III and IV. Overall, HG/hBN heterobilayer has a considerable carrier mobility and band gap under a specific hydrogenation pattern, which has promising application prospects in electronics and photonics.
Abbreviations
 ATK:

Atomistix ToolKit
 CBM:

Conduction band minimum
 DBG:

Direct band gap
 DFT:

Density functional theory
 DPA:

Deformation potential approximation
 FET:

Fieldeffect transistor
 GGA:

Generalized gradient approximation
 hBN:

Hexagonal boron nitride
 HG:

Hydrogenated graphene
 IBG:

Indirect band gap
 PBE:

PerdewBurkeErnzerhof
 VBM:

Valence band maximum
 vdW:

van der Waals
References
 1.
Elias DC, Nair RR, Mohiuddin TMG et al (2008) Control of graphene’s properties by reversible hydrogenation. Science 323(5914):610–613.
 2.
Zhou C, Chen S, Lou J et al (2014) Graphene’s cousin: the present and future of graphane. Nanoscale Res Lett 9(1):26.
 3.
Hussain T, Sarkar AD, Ahuja R (2014) Functionalization of hydrogenated graphene by polylithiated species for efficient hydrogen storage. Int J Hydrogen Energ 39(6):2560–2566.
 4.
Zhou J, Wang Q, Sun Q, Chen XS, Kawazoe Y, Jena P (2009) Ferromagnetism in semihydrogenated graphene sheet. Nano Lett 9(11):3867–3870.
 5.
Schäfer RA, Englert JM, Wehrfritz P et al (2013) On the way to graphanepronounced fluorescence of polyhydrogenated graphene. Angew Chem Int Ed 52(2):754–757.
 6.
Rajabpour A, Allaei SMV, Kowsary F (2011) Interface thermal resistance and thermal rectification in hybrid graphenegraphane nanoribbons: a nonequilibrium molecular dynamics study. Appl Phys Lett 99(5):666–670.
 7.
He H, Pan B, Argyraki A et al (2014) Advances in wide bandgap SiC for optoelectronics. Eur Phys J B 87(3):58–87.
 8.
Son J, Lee S, Kim SJ et al (2016) Hydrogenated monolayer graphene with reversible and tunable wide band gap and its fieldeffect transistor. Nat Commun 7:13261.
 9.
Fiori G, Lebègue S, Betti A et al (2010) Simulation of hydrogenated graphene fieldeffect transistors through a multiscale approach. Phys Rev B 82(15):1462–1465.
 10.
Bartolomeo AD, Giubileo F, Iemmo L et al (2016) Sidegate leakage and field emission in allgraphene field effect transistors on SiO_{2}/Si substrate. Appl Phys Lett 109:023510.
 11.
Dean CR, Young AF, Meric I et al (2010) Boron nitride substrates for highquality graphene electronics. Nat Nanotechnol 5(10):722–726.
 12.
Xue J, SanchezYamagishi J, Bulmash D et al (2011) STM spectroscopy of ultraflat graphene on hexagonal boron nitride. Nat Mater 10:282–285.
 13.
Gao H, Wang L, Zhao J, Ding F, Lu J (2011) Band gap tuning of hydrogenated graphene: H coverage and configuration dependence. J Phys Chem C 115(8):3236–3242.
 14.
Sahin H, Ciraci S (2011) Structural, mechanical, and electronic properties of defectpatterned graphene nanomeshes from first principles. Phys Rev B 84(3):035452.
 15.
Shkrebtii AI, Heritage E, McNelles P et al (2012) Graphene and graphane functionalization using hydrogen and nitrogen: electronic, optical and vibrational signatures. Phys Status Solidi 9(6):1378–1383.
 16.
Song EH, Ali G, Yoo SH, Jiang Q, Cho SO (2014) Tuning electronic and magnetic properties of partially hydrogenated graphene by biaxial tensile strain: a computational study. Nanoscale Res Lett 9(1):491–491.
 17.
Bruzzone S, Fiori G (2011) Abinitio simulations of deformation potentials and electron mobility in chemically modified graphene and twodimensional hexagonal boronnitride. Appl Phys Lett 99:22108.
 18.
Samarakoon DK, Wang XQ (2010) Tunable band gap in hydrogenated bilayer graphene. ACS Nano 4(7):4126–4130.
 19.
Chen W, Li Y, Yu G et al (2010) Hydrogenation: a simple approach to realize semiconductor−halfmetal−metal transition in boron nitride nanoribbons. J Am Chem Soc 132(5):1699–1705.
 20.
Liang Q, Jiang J, Meng R et al (2016) Tuning the electronic properties and work functions of graphane/fully hydrogenated hBN heterobilayers via heteronuclear dihydrogen bonding and electric field control. Phys Chem Chem Phys 18(24):16386–16395.
 21.
Atomistix Toolkit version 2018.06, Synopsys QuantumWise A/S. (www.quantumwise.com).
 22.
Grimme S (2006) Semiempirical GGAtype density functional constructed with a longrange dispersion correction. J Comput Chem 27(15):1787–1799.
 23.
Shockley W, Bardeen J (1950) Energy bands and mobilities in monatomic semiconductors. Phys Rev 77(3):407–408.
 24.
Qiao J, Kong X, Hu ZX, Yang F, Ji W (2014) Highmobility transport anisotropy and linear dichroism in fewlayer black phosphorus. Nat Commun 5:4475.
 25.
Gao S, Xiang H, Xu et al. Theoretical study of carrier mobility in twodimensional tetragonal carbon allotrope from porous graphene. Chinese Phys Lett 2016; 33(8):083101.
 26.
Wang Y, Huang P, Ye M et al (2017) Manybody effect, carrier mobility, and device performance of hexagonal arsenene and antimonene. Chem Mater 29(5):2191–2201.
 27.
Gunst T, Markussen T, Stokbro K, Brandbyge M (2016) Firstprinciples method for electronphonon coupling and electron mobility: applications to twodimensional materials. Phys Rev B 93:035414.
 28.
Xi JY, Nakamura Y, Zhao TQ, Wang D, Shuai ZG (2018) Theoretical studies on the deformation potential, electronphonon coupling, and carrier transports of layered systems. Acta Phys Chim Sin 34(9):961–976.
 29.
Xi JY, Long MQ, Tang L, Wang D, Shuai ZG (2012) Firstprinciples prediction of charge mobility in carbon and organic nanomaterials. Nanoscale 4:4348–4369.
 30.
Topsakal M, Aktürk E, Ciraci S (2009) Firstprinciples study of twoand onedimensional honeycomb structures of boron nitride. Phys Rev B 79:115442.
 31.
Li Y, Li F, Chen Z (2012) Graphane/fluorographene bilayer: considerable CH···FC hydrogen bonding and effective band structure engineering. J Am Chem Soc 134(27):11269–11275.
 32.
Xiao J, Long M, Li X, Xu H, Huang H, Gao Y (2014) Theoretical prediction of electronic structure and carrier mobility in singlewalled MoS_{2} nanotubes. Sci Rep 4:4327.
Acknowledgements
This research was supported by the National Key Research and Development Program of China (Grant No. 2016YFC0801200, 2016YFC0801300 and 2017YFC0803703), the National Natural Science Foundation of China (Grant No. 61773233, 61675111 and 61575103), and the China Postdoctoral Science Foundation funded project (Grant No. 2017M620781). The authors deeply appreciate the supports.
Funding
This study was funded by (1) the National Key Research and Development Program of China (Grant No. 2016YFC0801200, 2016YFC0801300 and 2017YFC0803703), (2) the National Natural Science Foundation of China (Grant No.61773233, 61675111 and 61575103), and (3) the China Postdoctoral Science Foundation funded project (Grant No. 2017M620781).
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ZY carried out the ATK simulations and drafted the manuscript. HG assisted in the data analysis and the manuscript writing and revision. XZ supported and supervised the whole research. All the authors discussed the results and approved the final manuscript.
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Ye, Z., Geng, H. & Zheng, X. Theoretical Study on Carrier Mobility of Hydrogenated Graphene/Hexagonal BoronNitride Heterobilayer. Nanoscale Res Lett 13, 376 (2018). https://doi.org/10.1186/s1167101827802
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Keywords
 Hydrogenated graphene
 Hexagonal boron nitride
 Effective mass
 Carrier mobility