Competition between electron doping and short-range scattering in hydrogenated bilayer graphene on hexagonal boron nitride

Abstract

We studied the electron doping of bilayer graphene (BLG) on hexagonal boron nitride (h-BN) by dissociative H₂ adsorption. Charge transfer phenomena were investigated by the gate voltage (V_g)-dependent electrical conductivity σ(V_g) and Raman spectroscopy with respect to the H₂ exposure. The shift of the charge neutrality point toward the negative V_g region was observed and the charge scattering mechanism was found with the variation of σ(V_g). The charge transfer at the interface as well as the lattice distortion of BLG due to hydrogenation were verified by Raman spectroscopy. From these results, we concluded that the electron doping and short-range scattering in the BLG exposed to high H₂ pressure (11 bar) are intrinsic features, which were achieved using a van der Waals interface consisting of BLG and h-BN.

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1. Introduction

In contrast to the linear dispersion of single-layer graphene (SLG), bilayer graphene (BLG), which consists of two graphene sheets with Bernal stacking, has a parabolic electronic band structure.^1,2 Given that the electronic band gap of BLG can be opened by applying a perpendicular electric field which breaks the inversion symmetry, extensive research has been carried out via dual-gated configurations^3,4 and asymmetric chemical doping.^5–8 With band gap engineering, the realization of n-type BLG transistors is essential for graphene-based electronics such as complementary logic devices.⁶ Various methods have been applied to achieve n-type graphene.^6–9 Hydrogenation is one of the approaches which can be used to functionalize graphene^10–15 and reportedly provides an n-type dopant in graphene layers.^12–14 Using the electron doping properties due to dissociative H₂ adsorption,¹⁴ a p–n junction has also been realized in SLG devices.¹⁶ In contrast, few investigations of hydrogenated BLG have been done in spite of its considerable applicability.

It is known that adsorption on a graphene-based system can be related in an extrinsic environment and through intrinsic chemical interaction. Thus, as adsorption with I₂ (ref. 17 and 18) and O₂ (ref. 19 and 20) is carried out, the interface between graphene and the underlying SiO₂ is a crucial for the variation of the electrical properties. Chu et al. noted that intercalated halogen molecules screen charge impurities which are embedded in SiO₂.¹⁸ Silvestre et al. also reported that interfacial O₂ decreases the scattering due to defects and corrugations in the substrate and increases the average distance between charge carriers and substrate impurities.²⁰ Meanwhile, hexagonal boron nitride (h-BN) has attracted attention as a building block for two-dimensional devices because h-BN substrates play an important role in eliminating the substrate effect, thus enabling intrinsic electronic properties.^21–29 Furthermore, the interface between graphene and h-BN, i.e., the van der Waals interface, is tight enough to provide a seal against adsorbate intercalation.^29,30 Therefore, the BLG/h-BN device structure has become a platform on which to access intrinsic chemical-reacted phenomena.

Here, we report dissociative H₂ adsorption on BLG supported by an h-BN substrate. As with SLG, electron doping was observed in BLG/h-BN, which was probed by the gate voltage-dependent electrical conductivity σ(V_g) and Raman spectroscopy. The variation of the mobility (μ) and the minimum conductivity (σ_min) upon H₂ exposure provide information about the scattering mechanism. The Raman spectroscopy results show that the compressive lattice strain and the charge transfer both contribute significantly to the electronic transport properties of hydrogen adsorbed BLG. We expect that the hydrogenation of BLG is a key step in the realization of n-type transistors for graphene-based electronics.

2. Experimental

The BLG/h-BN device was fabricated by a conventional exfoliation and transfer process.²¹ High-quality h-BN (HQ-graphene) was exfoliated on a SiO₂ (285 nm)/Si substrate. A thickness of 10 nm was measured by an atomic force microscope (AFM). The BLG was exfoliated from HOPG (SPI supplies) and transferred by means of micro-alignment. The BLG was then identified by Raman spectroscopy with a 532 nm laser (LabRam 300, JY-Horiba). A standard electron beam lithography process and metal evaporation (Cr/Au = 5/50 nm) were also used. A pristine device was obtained by annealing at 380 K under a high vacuum (∼5 × 10⁻⁶ Torr) for 10 hours using home-made high temperature and high pressure chamber. The σ(V_g) with a three-terminal configuration was measured using 4200-SCS Keithley semiconductor analyzer under H₂ exposure at a pressure of 11 bar (99.9999%) at 350 K. The DC bias voltage (V_ds) was fixed to 1 mV and the electrical current (I_ds) was measured as a function of back gate voltage (V_g). With fixed H₂ pressure and temperature, the σ(V_g) measurements were performed as exposed time.

3. Results and discussion

Fig. 1 shows σ(V_g) of BLG/h-BN as a function of the H₂ exposure time. The black curve corresponding to σ(V_g) of the pristine BLG/h-BN shifted to the negative V_g region and was finally saturated to the yellow curve in the Fig. 1. The electron doping due to H₂ adsorption was consistent with a previous experiment on BLG on SiO₂ substrates.¹⁴ The inset indicates optical (top) and AFM (bottom) images of the BLG/h-BN device, respectively.

Fig. 1 The conductivity (σ) versus gate voltage (V_g) curves of BLG/h-BN as H₂ exposure time. Insets show optical (top) and AFM (bottom) image of BLG/h-BN device. P and H denote pristine and H₂ treated case, respectively.

The electronic transport properties were investigated by the temporal evolution of the charge neutrality point (V_CNP) and the electron/hole mobility (μ_e and μ_h, respectively) (Fig. 2). Fig. 2a shows the variation of V_CNP (black squares) due to H₂ exposure and the fitting curve (red line). The negative shift of V_CNP is direct evidence of electron doping in BLG/h-BN, which can be understood as a charge transfer due to dissociative H₂ adsorption.¹⁴ The adsorption is saturated to a certain degree because the number of pre-existing interaction sites determines the saturation coverage.¹² The temporal dependence of V_CNP can be described by a first-order Langmuir-type adsorption model, −ΔV_CNP = −ΔV^Sat_CNP(1 − e^−kt), where −ΔV^Sat_CNP and k are the magnitude of the saturated V_CNP and the kinetic coefficient, respectively. This type of first-order adsorption has been widely observed in SLG and BLG regardless of the adsorbate type used, such as H₂, O₂, or NH₃.^12,20,31 We obtained values of 18 V and 0.05 h⁻¹ for each fitting parameter and a value of 0.994 for the adj. R².

Fig. 2 Temporal evolution of (a) the charge neutrality point, V_CNP, (b) electron/hole mobility, μ_e and μ_h, and (c) −ΔV_CNP versus 1/μ. The black and red lines are guide for eye. (d) The minimum conductivity, σ_min versus −ΔV_CNP.

We observed reductions of μ_e and μ_h, of which the carrier densities correspond to V_CNP ±10 V, respectively with H₂ adsorption as shown in Fig. 2b. The field effect mobility was determined via , where c_g is the back gate capacitance per unit area. It was found that O₂ adsorption on BLG/SiO₂ enhances the mobility asymmetry between μ_e and μ_h. Silvestre et al.²⁰ reported that interfacial O₂ between BLG and SiO₂ contributes to an increase of μ_e and μ_h due to the screening of embedded impurities in the substrate, while the μ_e decreases due to short-range scattering, as anticipated by the resonant states above CNP through density functional theory calculations.²⁰ Likewise, we reported mobility asymmetry (increased μ_e and decreased μ_h) in SLG and BLG on SiO₂ substrates during H₂ adsorption.¹⁴ Considering that the BLG device in this study was fabricated on a h-BN substrate, which is known to mitigate the effects of embedded impurities in SiO₂ substrate, the extrinsic factor for increasing mobility can be excluded. Therefore, the mobility reduction is in good agreement with earlier research on hydrogenated single-layer graphene (SLG).¹²

The impurity density, n_imp is proportional to −ΔV_CNP, , where e is the charge element.²⁰ It has been theoretically reported that the short-range scattering in BLG yields mobility which is inversely proportional to n_imp.³² In Fig. 2c, for electron and hole corresponding to V_CNP ±10 V, has a linear relationship with −ΔV_CNP, that is . It should be noted that the behavior in BLG is not a sufficient but a necessary condition to confirm short-range scattering, as the same behavior is expected during long-range scattering.⁹ However, chemisorbed H atoms on graphene are generally known to be short-range scatterers.^12,31,32 Furthermore, the results of a Raman study, which will be discussed below, show definite C–H bonding which promotes short-range scattering.³² Therefore, the condition of in BLG due to H₂ dissociative adsorption reveals that the hydrogenation results in the short-range scattering in BLG as well as SLG.^12,33

The minimum conductivity in SLG, σ_min depending on n_imp, shows various behaviors depending on the type of adsorbate. K, Ti, and Pt doping induce a reduced σ_min, while σ_min remains constant after Au and Fe doping.^34–36 Chu et al. noted that various σ_min behaviors can be explained by the competition between the mobility and induced carrier density, n₀ (i.e., σ_min ∼ μn₀).¹⁸ This type of competitive behavior was also observed during H₂ adsorption, during which σ_min decreases¹² or increases.³⁷ Fig. 2d shows a nearly constant σ_min with H₂ adsorption, which can be understood by balancing μ and n₀.

Fig. 3a shows the Raman spectra acquired before (black) and after (red) H₂ exposure, showing typical G and 2D peaks for BLG as well as h-BN (1366 cm⁻¹) peak. After H₂ exposure, the D + G mode (2935 cm⁻¹) in Fig. 3a, the D (1350 cm⁻¹) and D′ (1620 cm⁻¹) peaks in Fig. 3b were developed. These D and D′ peaks indicate successful hydrogenation on BLG/h-BN.^10–15,33 The D + G mode is also evidence of hydrogenation, which results in C–H bonds.^11,14,15 The typical Raman spectra in BLG also provide significant information related to the atomic structure and charge transfer properties. Similar to SLG, the peak position and full width half maximum (FWHM) of the Raman spectra in BLG are sensitive to the charge carrier density³⁸ and the lattice strain.^39,40 The G peak position (ω_G) and the FWHM (Γ_G), for pristine BLG/h-BN are 1582.7 cm⁻¹ and 13.8 cm⁻¹, respectively, as shown by the black curve in Fig. 3c. The values of ω_G and Γ_G indicate that the Fermi energy of BLG/h-BN is initially close to the CNP.³⁸ The Raman G peak results are in good agreement with σ(V_g), as shown in Fig. 1, where V_CNP is close to zero for the pristine case. The G peak after the H₂ treatment was blue-shifted and broadened; that is, ω_G and Γ_G are 1584.2 cm⁻¹ and 21.1 cm⁻¹, respectively, as denoted by the blue curves in Fig. 3c. In contrast, the 2D peak position (ω_2D) is red-shifted, as shown in Fig. 3d. The upper plot in Fig. 3d shows the 2D peak of pristine BLG, which consists of multi-components, 2D_1A, 2D_1B, 2D_2A and 2D_2B. We observed that all components became red-shifted after H₂ adsorption, as shown in the lower plot of Fig. 3d. Specifically, the 2D_1A and 2D_2A peak positions changed from 2689 cm⁻¹ and 2708 cm⁻¹ to 2679 cm⁻¹ and 2698 cm⁻¹, respectively. In a previous study, these two components were at 2690 cm⁻¹ and 2710 cm⁻¹ at the CNP.³⁸ Upon electron doping, these positions are both monotonically red-shifted. These G and 2D peak shifts collectively indicate electron doping, consistent with the results of σ(V_g). On the one hand, the FWHM of the G peak increased from 13.8 cm⁻¹ to 21.1 cm⁻¹ after H₂ exposure. In the unstrained case, the FWHM of BLG is limited below ∼14 cm⁻¹ at a wide charge carrier density range, |E_F| < 0.2 eV.³⁸ On the other hand, the G peak broadens and is finally separated by the G⁺ and G⁻ modes under tensile or compressive strain.⁴⁰ Furthermore, we observed that dissociative H₂ adsorption results in compressive strain in SLG.¹⁴ Thus, the increase in the FWHM can be explained by the introduction of compressive lattice strain. Finally, we note that the Raman 2D peak maintained a multi-component structure after the H₂ treatment, which means that decoupling due to intercalation between the graphene layers was not detected during H₂ adsorption on BLG.

Fig. 3 (a–d) The black/red curves correspond to the Raman spectra acquired before/after H₂ exposure, respectively. (a) Full spectra, enlarged regions of (b) h-BN, D and D′ peaks, (c) G peak, and (d) 2D peak. All blue curves in (c and d) are single Lorentzians formulated to fit a multi-component spectrum.

4. Conclusions

In summary, we investigated the intrinsic electron doping and short-range scattering phenomena in BLG upon dissociative H₂ adsorption. The results were achieved using the van der Waals interface with the BLG/h-BN structure. We observed (i) a negative shift of V_CNP, (ii) saturating behavior of electron doping, (iii) inverse n_imp-dependent mobility, and (iv) σ_min with competition between the mobility and the induced carrier density. The results are consistent with the Raman spectra which show hydrogenation of the carbon structure. These fundamental studies are expected to contribute to carbon electronics.

Acknowledgements

This work was supported by the Swedish-Korean Basic Research Cooperative Program (No. 2014R1A2A1A12067266) of the NRF, Korea. M.L. and D.H.J. acknowledge the support of the Pioneer Research Center Program through the NRF of Korea, funded by the Ministry of Science, ICT & Future Planning (No. NRF-2011-0027888). B.H.K. acknowledges the support from the National Research Foundation of Korea (No. NRF-2014R1A1A1002467).

Notes and references

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