Influence of SiO₂ or h-BN substrate on the room-temperature electronic transport in chemically derived single layer graphene

Abstract

The substrate effect on the electronic transport of graphene with a density of defects of about 0.5% (^0.5%G) is studied. Devices composed of monolayer ^0.5%G, partially deposited on SiO₂ and h-BN were used for transport measurements. We find that the ^0.5%G on h-BN exhibits ambipolar transfer behaviours under ambient conditions, in comparison to unipolar p-type characters on SiO₂ for the same flake. While intrinsic defects in graphene cause scattering, the use of h-BN as a substrate reduces p-doping.

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Wet-chemically prepared graphene from graphite can be stabilized in solution by covalently bound oxo-groups using established oxidation protocols.^1–3 In general, the materials obtained are termed graphene oxide (GO). However, the chemical structure varies and the carbon lattice may even be amorphous due to the evolution of CO₂ during synthesis.⁴ Thus, in this study we use oxo-functionalized graphene (oxo-G), a type of GO with a more defined structure, as proven in our previous work.³ The oxygen-containing groups on the graphene basal plane and rims of flakes and holes make GO a p-type semiconductor with a typical resistance of 10¹⁰–10¹³ Ω sq^−1 5,6 and a band gap of about 2.2 eV.^7,8 The reductive defunctionalization of GO leads to a certain type of graphene (G), often named reduced GO (r-GO).^4,9 Removal of oxo-groups from the surface can be achieved by chemical reduction,^9,10 electrochemical methods,^11,12 electron beam treatment¹³ and was observed in situ by transmission electron microscopy.¹³ Thermal processing of GO instead leads to a disproportionation reaction forming carbon with additional vacancy defects and CO₂.¹⁴ In general, the reduction of GO turns r-GO from a semi-conductive material to a semi-metal. Mobility values were determined in field effect transistor (FET) devices.^15,16 Generally, the quality of graphene strongly depends on the integrity of the hexagonal carbon lattice. Thus, mobility values of 10⁻³ and up to 10³ cm² V⁻¹ s⁻¹ were reported,^3,17,18 with the resistance fluctuating between 10³ and 10⁶ Ω sq⁻¹.^19–21 We reported on the highest mobility values of chemically reduced oxo-G (with about 0.02% of lattice defects) of 1000 cm² V⁻¹ s⁻¹,³ determined by Hall-bar measurements at 1.6 K.

Hexagonal boron nitride (h-BN) has been proved to be an excellent substrate for matching graphene-based materials owing to its atomic flatness, chemical inertness and electronic insulation due to a bandgap of ∼5.5 eV.²² Up to now, most studies with graphene deposited on h-BN were restricted to measurements with virtually defect-free graphene.²³ To the best of the authors knowledge, no studies reported transport measurements based on single layers of GO or oxo-G on h-BN substrates. No studies are reported with graphene derived from GO or oxo-G on single-layer level. Recently, we found that chemical reactions can be selectively conducted close to the rims of defects.²⁴ However, before functionalized devices can be studied, the lack of knowledge on the ambient environment device performances of graphene with defects and the influence of substrates must be addressed. Therefore, we fabricated the devices composed of ^0.5%G, partially deposited on SiO₂ (SiO₂/^0.5%G) and h-BN (h-BN/^0.5%G) (Fig. 1). Areas of the same flake on both materials are used to ensure reliable measurements and to prove that the results stem from the influence of the substrate rather than from the difference between devices. Thereby, the ^0.5%G exhibits an I_D/I_G ratio of about 3–4, corresponding to 0.5% of defects, according to the model introduced by Lucchese and Cançado.^25–28 Our results demonstrate that the h-BN layer is responsible for a downshift of the Dirac point and a more narrow hysteresis, resulting in ambipolar transfer behaviours in h-BN/^0.5%G.

Fig. 1 (a) Optical image of the fabricated h-BN/^0.5%G heterostructure on SiO₂. (b) The h-BN/^0.5%G heterostructure device. Electrodes 1 and 2 define the SiO₂/^0.5%G FET device. Electrodes 1 and 3 define the ^0.5%G on overlapped SiO₂/h-BN hetero-substrate device. Electrodes 3 and 4 define the h-BN/^0.5%G FET device. Distance between the electrodes 1–2 and 3–4 is 1.5 μm and 3 μm, respectively. (c) 3D illustration of the h-BN/^0.5%G transistor device.

Results and discussion

To gain structural information of ^0.5%G, flakes of ^0.5%G were deposited on HOPG surface and examined by scanning tunnelling microscopy (STM) under an ultra-high vacuum (10⁻¹⁰ mbar). The average height of a single ^0.5%G flake was determined as 0.9 nm (Fig. 2a). At higher resolution, two different morphologies are detected in the ^0.5%G flake, as depicted in Fig. 2b. The atomically resolved structure is assigned for the dark region while the resolution fades away in the bright region. The diffraction spots marked in dashed red indicate the hexagonal lattice of graphene in the dark regions, shown in Fig. 2c. The bright regions are caused by the oxygen-containing groups attached to the carbon lattice, which breaks the lattice periodicity of graphene and subsequently lead to no apparent diffraction feature in the reciprocal space (Fig. 2d).

Fig. 2 (a) Overview STM topographic image of the ^0.5%G on highly oriented pyrolytic graphite (HOPG) substrate (200 nm × 200 nm; I_t = 0.6 nA, V_s = −0.5 V). The inset is the height profile of the ^0.5%G flake. (b) STM topographic image of the ^0.5%G (12 nm × 12 nm; I_t = 0.4 nA, V_s = −0.3 V). (c) and (d) Fast Fourier transform (FFT) images of the dark region and bright region in (b), respectively. (e) and (f) I (V) spectrum (averaged over >100 single spectra) and corresponding dI/dV curves recorded at the dark area (red curve), bright area (blue curve) and whole area (black curve), respectively.

Atomic scale electronic properties of ^0.5%G were explored using scanning tunnelling spectroscopy (STS). Fig. 2e displays the I (V) spectrum of the ^0.5%G surface. Compared to the tunnelling current at the dark region, there exists an apparent suppression of current at the bright region owing to a lower conductivity in the distorted graphene lattices. For the averaged I (V) spectra of the whole area, the metallic behaviour of the ^0.5%G flake is found. This phenomenon is also confirmed by the differential conductivity (dI/dV) curves in Fig. 2f. The Dirac point is determined from the minimum value in dI/dV curves. The Dirac point in dark region is located at 0.0 V, suggesting low impurity-related doping level. In contrast, the bright regions exhibit a positive shift of the Dirac point of about 50 mV, likely due to the presence of oxygen groups. For the entire scanned areas, the ^0.5%G flake exhibits a p-type electronic doping feature with the average Dirac point at about 20 mV.

For the fabrication of the heterostructure of h-BN/^0.5%G or SiO₂/^0.5%G, flakes of oxo-G were first deposited on SiO₂ substrate by Langmuir–Blodgett technique,²⁹ as shown in Fig. 3a. Then ^0.5%G flakes were prepared by reduction using vapor of HI/TFA (in inset of Fig. 3b).³⁰ The h-BN flakes used in this study were exfoliated from h-BN single crystals.³¹ Next, the heterostructures of h-BN/^0.5%G or SiO₂/^0.5%G were prepared by a dry transfer technique.³²

Fig. 3 (a) Schematic illustration of Langmuir–Blodgett assembly of oxo-G single layers. (b) Optical image of collected ^0.5%G flakes on a silicon wafer with 300 nm SiO₂, obtained after hydroiodic acid (HI) and trifluoroacetic acid (TFA) reduction as shown in the inset.

Fig. 4a shows an AFM image of a h-BN/^0.5%G heterostructure, which consists of SiO₂ substrate with multilayer h-BN flake and a monolayer ^0.5%G flake (∼25 × 10 μm²) partially covering the h-BN. The AFM image in Fig. 4b, obtained within the marked area in Fig. 4a, revealed that the transfer process induced wrinkles and folds in ^0.5%G. The height profile of the single ^0.5%G flake on SiO₂ is shown in Fig. 4c (compare Fig. S1†) and depicts a thickness of about 2 nm. This height is much thicker than 0.9 nm measured by STM for similar monolayer ^0.5%G on HOPG. A major plausible reason is that e.g. water molecules are inevitably adsorbed on the hydrophilic SiO₂ surface (treated by O₂ plasma) leading to an approximately nanometer-thick hydrogen-bonded water layer and cleaved oxo-groups captured between SiO₂ and ^0.5%G.³³ In contrast, although small amounts of polymer residues are likely trapped between h-BN and ^0.5%G, the measured thickness of the same ^0.5%G flake on h-BN is ∼1 nm as shown in Fig. 4c, which is almost the same result as the thickness determined by STM. The ^0.5%G flake on ∼6 nm thick h-BN (Fig. 4c) possesses a lower roughness (∼0.5 nm) than on SiO₂ (∼1 nm). Therefore, h-BN, as a passivation layer, can not only negate the influence of trapped water on graphene, but also improves accuracy in the AFM thickness measurements of monolayer 2D flakes.

Fig. 4 (a) AFM image of a h-BN/^0.5%G heterostructure on a Si/SiO₂ substrate. (b) AFM image obtained from the area within the white square in (a). (c) Height profiles of ^0.5%G on SiO₂, ^0.5%G on h-BN and h-BN layer, which are corresponding to the black lines in (b).

Average Raman spectra of the ^0.5%G supported by SiO₂ and h-BN, respectively, are shown in Fig. 5a. The primary peaks are the D peak near 1340 cm⁻¹, the G peak near 1555–1557 cm⁻¹ and the 2D peak near 2667 cm⁻¹. The D peak of ^0.5%G on each interface is mainly activated by defects in the carbon skeletons. The G and 2D peaks closely relate to the quality of graphene. The almost unchanged positions of the three peaks indicate that wrinkles and residual polymers induced during the transfer processes do not produce obvious doping effect on the single layer ^0.5%G. We use scatter plots of I_D/I_G versus Γ_2D to further confirm the quality of the ^0.5%G in Fig. 5b. For the ^0.5%G on h-BN, the I_D/I_G ratio is about 3.3, within the standard deviation of the I_D/I_G ratio of 3.1 determined on SiO₂. Based on the model introduced by Lucchese and Cançado et al.,^25,26 the density of lattice defects is related to 0.5% for the devices on h-BN and SiO₂. This density of defects relates to the average distance between defects of around 3 nm. The related defect density (n_D) is 4.0 × 10¹² cm⁻² on h-BN and SiO₂, respectively, calculated from the equation n_D (cm⁻²) = 10¹⁴/(πL²_D).²⁵ The Γ_2D of the Raman 2D band is sensitive to the presence of defects. For the monolayer ^0.5%G on h-BN, only a slightly smaller Γ_2D of ∼70 cm⁻¹ is observed than on SiO₂ (∼72 cm⁻¹). The same monolayer ^0.5%G, partially deposited on SiO₂ and h-BN, presents almost the same Γ_2D. Therefore, the quality of the investigated flake is the same on SiO₂ and h-BN, respectively.

Fig. 5 Statistical Raman microscopy measured with each pixel corresponding to an area of ∼0.7 × 0.7 μm² at 532 nm laser excitation wavelength. The laser power is below 1 mW to avoid heating induced by laser. (a) Average Raman spectra of ^0.5%G on SiO₂ and h-BN. (b) I_D/I_G ratio vs. Γ_2D.

Reference experiments to determine the contact resistance were conducted using four-probe measurements. The surface resistance is determined to roughly 21 kOhm in four-probe configuration and 23.5 kOhm in two-probe configuration (Fig. S2†). Thus, further investigations were conducted in two-probe configuration under ambient conditions. For our transport measurements, we prepared one device with monolayer ^0.5%G on SiO₂ substrate (Fig. S3†), two devices with the same monolayer ^0.5%G flake that are in part on SiO₂ and on h-BN (Fig. 1 and S4†) and one device with monolayer ^0.5%G on h-BN substrate (Fig. S5†). The patterning of the electrodes was achieved by standard electron beam lithography processing and subsequent deposition of 5 nm Cr/70 nm Au by thermal evaporation. The electrical performance of the ^0.5%G flake on h-BN and SiO₂, respectively, is summarised in Table 1. The resistance of ^0.5%G on h-BN and SiO₂ measured at V_bg = 0 V ranges widely, from 5.0 kΩ to 34.4 kΩ. But the resistances are significant lower compared to >10⁶ Ω reported for similar devices.¹⁷

Table 1 Summary of electrical performances for the ^0.5%G on h-BN, overlapped SiO₂-h-BN hetero-substrate and SiO₂

	Resistance/kΩ	Mobility/cm^2 V^−1 s^−1	Dirac point voltage/V	Channel length/μm
a Reference device of ^0.5%G on SiO2, see Fig. S3 (channel: 1–2).b Reference device of ^0.5%G on SiO2, see Fig. S3 (channel: 2–3).c Reference device of ^0.5%G on SiO2, see Fig. S4 (channel: 1–2).d Reference device of ^0.5%G on h-BN, see Fig. S4 (channel: 3–4).e Reference device of ^0.5%G on h-BN, see Fig. S5 (channel: 1–2).
h-BN/^0.5%G	34.4	5.6	∼20	3
SiO2/^0.5%G	15.6	11.6	>43	1.5
SiO2/^0.5%G ^a	5.0	14.2	>50	3
SiO2/^0.5%G ^b	5.7	14.5	>50	3
SiO2/^0.5%G ^c	6.8	7.4	>60	2
h-BN/^0.5%G ^d	18.7	5.3	∼37	2
h-BN/^0.5%G ^e	32.5	8.5	∼22	1.5

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Transfer curves (I_ds–V_ds) of ^0.5%G on h-BN is shown in Fig. 6a. The Dirac points are located at around +20 V. The hysteresis effect of the ^0.5%G on h-BN is observed in ambient environment for sweeping continuously from −50 to 50 V in forward direction and then back to −50 V (backward direction). From the red dashed lines presented in Fig. 6a, a room-temperature hole mobility (μ_h) of 5.6 cm² V⁻¹ s⁻¹ is extracted using the equation μ = (L/W) × (1/(C_oxV_ds)) × (dI_ds/dV_bg),³⁴ where C_ox = 1.15 × 10⁻⁸ F cm⁻². As the output curves (I_ds–V_ds) exhibit ohmic behaviour (Fig. 6b) we conclude that there is no Schottky contact between ^0.5%G and metal electrodes. For the ^0.5%G deposited on the overlapped SiO₂-h-BN hetero-substrate (transport measurements performed between electrodes 2 and 4, shown in Fig. 1c), we observe only p-type character of the I_ds–V_ds curves with the Dirac point shifted to about +30 V (Fig. 6c).

Fig. 6 (a), (c) and (e) Transfer characteristics under ambient conditions for ^0.5%G on h-BN, overlapped SiO₂/hBN hetero-substrate and SiO₂ with V_ds = 50 mV. The gate voltage is swept continuously from −50 to 50 V and back to −50 V. (b), (d) and (f) Related I_ds–V_ds curves acquired for V_bg values from −40 V to 40 V in steps of 10 V.

In contrast to ^0.5%G on h-BN and overlapped SiO₂-h-BN hetero-structure, the ^0.5%G on SiO₂ exhibits unipolar p-type character (Fig. 6e). The point of the minimum conductivity in the I_ds–V_bg curve is not observed and the Dirac point moves to higher positive voltage (>43 V). Obviously, electrical transport of the ^0.5%G on SiO₂ is completely governed by holes with hole mobility μ_h estimated to about 11.6 cm² V⁻¹ s⁻¹. In addition, the I_ds–V_bg curves exhibit an increase of hysteresis in SiO₂/^0.5%G device with a shift of V_bg (ΔV_bg ≈ 7.3 V) between the forward and reverse sweeps, compared to the h-BN/^0.5%G device with ΔV_bg ≈ 2.6 V. Substrate change from h-BN to SiO₂ induces trapped holes with density higher than 1.6 × 10¹² cm⁻² using Δn_t = ΔV_{Dirac point} (C_ox/q),² where q is the elementary charge, ΔV_{Dirac point} > 43–20 = 23 V. In general, a high density of charge traps can cause hysteresis and lead to reduced mobility of graphene samples.³⁵ However, as summarized in Table 1, mobility values on SiO₂ are higher and the resistance is lower than on h-BN. The main reason for that contradictory finding is that for ^0.5%G defects are the dominant scatterers reducing the carrier mobility. This is consistent with Raman results of Fig. 5b. As further reference experiments we conducted transport measurements of defective graphene, here ^0.8%G on SiO₂. As shown in Fig. S6,† due to the higher density of defects the hole mobility values are 0.6 cm² V⁻¹ s⁻¹ in ambient and 0.9 cm² V⁻¹ s⁻¹ in vacuum. However, the Dirac point shifts only from 60 V in ambient to 30 V in vacuum. Those results are in agreement with the STS measurements, which indicate p-doping of ^0.5%G in vacuum. It could however be expected that oxo-groups with −I and −M effects,^2,3 decorating the rims of vacancy defects, may be responsible for trapping hole carriers. However, the experimental results, such as transport and AFM measurements, give evidence that p-doping is strongly induced by the SiO₂ substrate and cleaved oxo-species, such as water or organosulfate, which are trapped between SiO₂ and ^0.5%G. Therefore, based on the AFM height determination on SiO₂, the knowledge about the chemical structure and the reduction mechanism of oxo-G to ^0.5%G we propose that molecules, such as water or hydrogensulfate stemming from oxo-G (Fig. 7a) are trapped between the SiO₂ substrate surface and ^0.5%G (Fig. 7b). In comparison, h-BN is affected by the local polarity of h-BN/^0.5%G. As a result, spurious dopant molecules may get squeezed out (Fig. 7c), as is also supported by the measured height and roughness results determined by AFM.

Fig. 7 Proposed model of trapped species upon cleavage of oxo-groups upon reduction and influence of substrate. (a) Chemical sketch of the structure of oxo-G with the graphene lattice decorated by hydroxyl-, epoxy- and organosulfate groups. (b) ^0.5%G prepared by chemical reduction of oxo-G; covalently bound oxo-groups are cleaved and at least partially trapped between ^0.5%G and the SiO₂ substrate. (c) ^0.5%G on h-BN; cleaved oxo-groups may not be trapped between h-BN and ^0.5%G because they are squeezed out.

Conclusions

^0.5%G is a p-doped material and defects determine the scattering of charge carriers. Using h-BN as substrate leads to less trapped molecules, which are responsible for p-doping. In this regard, most likely hydrogen-bonded water and other cleaved oxo-species are captured between SiO₂ and ^0.5%G causing p-doping, as a consequence of chemical reduction of oxo-G. The ambipolar behaviour with V_{Dirac point} of +20 V was therefore observed for the h-BN/^0.5%G structure while unipolar p-type response was shown for the same ^0.5%G flake on SiO₂. Transfer characteristics show a reduction of hysteresis in the h-BN/^0.5%G. The mobility of the SiO₂/^0.5%G is determined to 7.4–14.5 cm² V⁻¹ s⁻¹ and for h-BN/^0.5%G to 5.6–8.5 cm² V⁻¹ s⁻¹ at ambient conditions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is supported by the China Scholarship Council (CSC), the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), project number 392444269 and 249559513, and ERC Grant No. 639739.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra09197a

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