Facile fabrication of boron nitride nanosheets–amorphous carbon hybrid film for optoelectronic applications

Abstract

A novel boron nitride nanosheets (BNNSs)–amorphous carbon (a-C) hybrid film has been deposited successfully on silicon substrates by simultaneous electrochemical deposition, and showed a good integrity of this B–C–N composite film by the interfacial bonding. This synthesis can potentially provide the facile control of the B–C–N composite film for the potential optoelectronic devices.

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Two dimensional boron–carbon–nitrogen (B–C–N) ternary materials show a series of electronic properties from insulation to semi-metallic in terms of nanosheets, nanoribbons and films, and they show profound potential in optoelectronic devices, luminescent devices, transistors, and micro-electrical-mechanical system (MEMS).^1–3 Techniques for the synthesis of a B–C–N ternary material are mostly chemical vapour deposition (CVD) and physical vapour deposition (PVD). As a result, a spectrum of polymorphic structures in such B–C–N systems are often obtained, which are associated with the homophilicity (C–C) or the heterophilicity (C–B, C–N and BN),⁴ making the precise control of their chemical stoichiometry and geometry a challenging problem. Essentially, the band-gap of the B–C–N system closely depends not only on the stoichiometry but also on the particular atomic arrangements,⁵ and it is thereby a challenging opportunity to obtain the appropriate structure and best performance of a well-mixed B–C–N system, which can be easily tuned by the complex variants during a facile film deposition.

Amorphous carbon (a-C), a wide-band gap and thin-film semiconductor, has been intensively studied for its application as the active layer in photodetectors, electroluminescent devices and field emission displays.⁶ In particular, by choosing different types and sizes of the doped component, one can obtain a nanocomposite carbon material with desired optoelectronic properties. Recent investigations on the structural similarity of graphene–h-BN hetero-junctions motivate the alloying of amorphous carbon and h-BN to achieve a novel ternary B–C–N hybrid film,^7–9 e.g. nanoscale h-BN sheets doped into the a-C matrix of the promising optoelectronic layer.¹⁰ However, the precise intermixing of BNNSs in a-C matrix and the interface integrity between them in such ternary material is far from understood.

Herein, we report the fabrication of B–C–N ternary films by a facile electrochemical deposition, which allows us to easily control the precise stoichiometry of this type of B–C–N system and their electronic structure of the optoelectronic devices.

A schematic of the two-step synthesis of B–C–N ternary films is shown in Fig. 1. After ultrasonication, the obtained h-BNNSs in water (∼0.1 mg ml⁻¹) show a milky white colour. This homogenous dispersion is stable, and only a small amount of precipitation can be observed over a few days, as shown in Fig. 1b. The corresponding characterizations of h-BNNSs can be found in the ESI.† Two strong peaks at 1380 cm⁻¹ and 810 cm⁻¹ of the Fourier transform infrared spectroscopy (FTIR) are the in-plane stretching of the B–N rings and out-of-plane bending vibration of the B–N–B rings of sp²-hybridized, respectively, which are typical characteristics of h-BN.¹¹ Furthermore, a peak of ∼3414 cm⁻¹ is attributed to the stretching signal of O–H, indicating a number of hydroxyl groups on the BNNS's surface.¹² The obtained h-BNNSs have an average grain size of around 100 nm, which is determined by transmission electron microscopy (TEM). The selected area electron diffraction (SAED) pattern corresponds to the (100) plane and the (002) plane of h-BN.¹² The electrolysis of the BNNSs–methanol electrolyte is performed under the electric field of 2400 V cm⁻¹ and at 60 ± 5 °C. Finally a uniformly grey film with a thickness of about 1000 nm is obtained on the silicon substrate, as shown in Fig. 1d.

Fig. 1 Schematic of the two-step preparation of BNNSs–a-C hybrid film by sonication and electrolysis: commercially boron nitride powder (a), h-BNNSs in water after sonication (b), setup of electrolysis (c), and BNNSs–a-C hybrid film (d).

In Fig. 2, h-BNNSs are homogeneously dispersed in a-C matrix with a grain size of about 5 nm, and the adjacent nanocrystalline BNNSs are around 2 nm apart. Note that there is no clear crystalline boundary among the mixing hetero-structure interfaces. In other words, carbon atoms probably interact with B or N atoms of BNNSs under intensive electric field during the co-deposition process. The SAED pattern shows the asymmetric matrix spots with a d-spacing of 0.25 nm, which agrees well with h-BN. Moreover, three distinct absorption peaks can be identified at ∼188, ∼284, and ∼401 eV in the electron energy loss spectroscopy (EELS), which belongs to the K-shell ionization edges of B, C, and N, respectively.¹³ These values well support the sp²-hybridized bonding among B, C and N atoms, according to their sharp π*, forming the novel B–C–N structure of this hybrid film. In this study BNNSs–a-C composite film is easily obtained by the simple electrolysis of h-BNNSs–methanol dispersion as expected. However, the grain size of h-BNNS in a-C matrix is smaller than the obtained h-BNNS in water, and it can be explained that the smaller is the grain size of h-BNNS in the methanol electrolyte, the easier is the incorporation of h-BNNS into a-C matrix during the electrodepositing process.

Fig. 2 Microstructure of the h-BNNSs–a-C film: HRTEM image (a–c), the corresponding SAED pattern (c, inset), and EELS spectrum of the film (d).

Quantitative XPS analysis shows the atomic concentration of B and N in a-C matrix is around 1.8 at%. In Fig. 3b, the B1s spectrum can be well fitted with a dominant peak at 189.6 eV, attributed to B=N bonding in h-BNNSs, and one peak positioned at 191.0 eV assigned to the B–O bonding state.¹⁴ Because the electro-negativity of the C atom is lower than that of the N atom, the charge displacement from B to C is lower than that from B to N. The peak at ∼188.8 eV is possibly from the bonding configurations of B and C.¹⁵ From Fig. 3a, the sp² B–N bonding in h-BN contributes to the peak at ∼397.5 eV, and the other one at ∼399.4 eV is in association with the C–N/C=N bonding.^14,15 The B1s and N1s spectra both signify the graphitic B–N bonding of h-BNNSs. A further illustration of carbon bonding is shown by the fitted C1s spectrum in Fig. 3c. A shoulder peak at ∼283.8 eV is attributed to B–C bonds, which is consistent with BC_x compounds that show a peak at 283.0 eV for B₄C.¹⁶ The binding energy of ∼284.5 eV can be assigned to C–C bonds, whereas the peak at ∼285.4 eV is a combined contribution of C=C bonds and C=N bonds because of the highly electronegative nitrogen atoms.¹⁷ For the peak at ∼288.8 eV, it is believed to be related to sp³ C–O bonding because some oxidation of the surface often occurs. It indicates that the interfacial bonding along the heterostructural boundary of the hybrid film happens. Furthermore, an increasing value of the sp²-C/sp³-C ratio from 0.8 to 1.0 identifies more graphitic phase in the hybrid film as compared to pure a-C film. As shown in Fig. 3d and e, two fitted peaks at ∼1361 and ∼1580 cm⁻¹, an increasing intensity of D peak, and a simultaneous G peak narrowing can be found as well as an increasing ratio of I_D/I_G from 0.38 to 2.96, which confirms the amorphous nature of the film.¹⁰ Moreover, more graphitic carbon domains appear in the hybrid film as compared with pure a-C film.

Fig. 3 XPS analysis of h-BNNSs–a-C film, the curve-fitted N1s spectrum (a), B1s spectrum (b), C1s spectrum (c). The curve-fitted Raman spectra of a-C (d) and h-BNNSs–a-C (e) films.

During electrolysis, the methanol electrolyte is broken down by the high electric field between electrodes, generating CH₃⁺ ions, which will be absorbed on the negative electrode.¹⁸ Thus, there will be a bonding probability of methyl ions with B and N atoms in h-BN sheets during the simultaneous depositing process. A periodic DFT calculation using DMol³ code is further carried out to calculate the adsorption energy of the CH₃⁺ ions perpendicular to the h-BN structure: on the top of boron and nitrogen atoms, as shown in Fig. 4. After the optimization, the shorter distance between the adsorbed CH₃⁺ radical and the plane of h-BN sheet are obtained: 1.734 Å for CH₃⁺–B (h-BN) and 1.559 Å for CH₃⁺–N (h-BN). The calculated results shows that the absorption energy of CH₃⁺ group to the B atom (CH₃⁺–B (h-BN)) is around −1442.36 kJ mol⁻¹, while the absorption energy of CH₃⁺ group to the N atom (CH₃⁺–N (h-BN)) is about −950.30 kJ mol⁻¹, indicating the chemisorption of CH₃⁺. Concerning the polarity, a charge transfer from methyl (cationic) to h-BN sheet takes place by the characteristic bending of the sheet, as shown in Fig. 4, inducing an increment in the polarity of the sheets. Furthermore, in the case of the dipolar moment, the values of the CH₃⁺–BN structure go from 4.3 × 10⁻³ to 8.3 × 10⁻³ Debye obtained from the N, whereas 4.3 × 10⁻³ to 9.5 × 10⁻³ Debye from the B, which is in agreement with the investigation reported.¹⁹ These calculated results well supports the fact that CH₃⁺ can be chemically absorbed on h-BNNSs, probably forming B–C and C=N bonding.

Fig. 4 The obtained configuration of CH₃⁺ absorbed on h-BN sheet: perpendicular to the B atom (a), and perpendicular to the N atom (b).

As for the possibility of the co-deposition of h-BNNSs and a-C, it is undoubted that CH₃⁺ ions are necessarily moved towards the silicon substrate of the cathode.¹⁷ Moreover, h-BNNSs will be polarized under the effect of the intensive electric field of 2400 V cm⁻¹, and a charged density increases toward the longitudinal edges of h-BNNSs.²⁰ Furthermore, it will be absorbed on the cathode surface, achieving the entrapment of h-BNNSs into the a-C matrix. As references say, h-BNNSs aligned themselves parallel to the electric field to minimize the electrostatic energy and to overcome the free energy of the system, which will play a template of the growth of graphitic carbon in a-C matrix.^21,22 XPS and Raman analysis in this study well reveal the more graphitic carbon present in the hybrid film.

Especially, during the co-deposition process the interfacial interactions of the cationic CH₃⁺ of methanol chemically bonded onto h-BNNSs greatly contributes to the good integrity of BNNSs–a-C hybrid film.²³ Moreover, the filler of h-BNNSs in the nanoscale often has a wrinkled sheet topology obtained from the exfoliation process, which will improve the mechanical interlocking of BNNSs with the 3D cross-linked a-C matrix.²³ In this study, the carbon atoms will bond with B, N atoms of h-BNNSs, forming C=N and/or B–C bonds along the BNNSs–a-C interface.²³ Note that considerable amounts of –OH groups are chemically anchored onto B atoms of h-BNNNs during the exfoliation process in water medium,²⁴ thus making the h-BNNS's surface more hydrophilic. These abovementioned aspects greatly ensure the co-deposition of h-BNNNs and CH³⁺ species from the stable dispersion, and finally forms a homogeneous distribution of h-BNNs in a-C matrix, as shown in Fig. 2.

The integrity of the obtained B–C–N hybrid film can be identified by the improvement in the mechanical property of BNNSs–a-C hybrid films using a nanoindentation test. The increase in hardness from 2.35 GPa to 5.76 GPa and Young's modulus from 44.35 GPa to 105.90 GPa is achieved by the incorporation of h-BNNSs into a-C matrix, this improvement in mechanical property closely depends on a good integrity of the nanocrystalline/amorphous hybrid film.^25,26

This remarkable heterostructure further allows us to readily modulate the electronic structure of BNNSs–a-C hybrid film and its optoelectronic property mainly by tuning the sheet or tube's concentration in the organic electrolyte. Moreover, some experiments have shown that the band gap of B–C–N thin films depends on the relative C and BN fractions; however, there has been a controversy concerning the structural properties of B–C–N thin films: some investigations indicate the partial segregation between C and BN,²⁷ whereas others propose the models in which C, B, and N atoms are mixed.²⁸ For instance, a considerable decrease of the band gap of BN nanomaterial can be tuned by 10% C doping.^29,30

In conclusion, we synthesized a novel h-BNNSs–a-C hybrid film by a facile electrochemical deposition, which gives a new approach to the facile fabrication of B–C–N ternary system. The interfacial bonding between carbon and h-BNNSs contributes to the high integrity of this hybrid film. This graphite-like h-BNNSs–a-C hybrid film shows promising applications in optoelectronic devices.

Acknowledgements

Authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (no. 51102247 and 51105352).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13268h

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