Excellent electrical conductivity of the exfoliated and fluorinated hexagonal boron nitride nanosheets
© Xue et al.; licensee Springer. 2013
Received: 19 October 2012
Accepted: 12 November 2012
Published: 24 January 2013
The insulator characteristic of hexagonal boron nitride limits its applications in microelectronics. In this paper, the fluorinated hexagonal boron nitride nanosheets were prepared by doping fluorine into the boron nitride nanosheets exfoliated from the bulk boron nitride in isopropanol via a facile chemical solution method with fluoboric acid; interestingly, these boron nitride nanosheets demonstrate a typical semiconductor characteristic which were studied on a new scanning tunneling microscope-transmission electron microscope holder. Since this property changes from an insulator to a semiconductor of the boron nitride, these nanosheets will be able to extend their applications in designing and fabricating electronic nanodevices.
KeywordsBoron nitride Exfoliation Fluorination Electrical conductivity
Innovative and constructive doping into nanomaterials has attracted considerable attention, because a specific dopant could bring a revolutionary change on the materials’ properties and applications, such as in the fields of energy storage[1, 2], photovoltaics[3, 4], and biosensor. Graphene exfoliated from graphite is a good example, which is doped by some elements (e.g., N[6, 7] and B[6, 8]) has been explored many fascinating properties and applications. The hexagonal boron nitride nanosheets (h-BNNSs) are a structural analogue of graphene, so-called ‘white-graphene’, in which B and N atoms alternatively substitute for C atoms. However, in contrast to the comprehensive researches on graphene[6, 11–13], especially the breakthrough in semiconductor devices[14, 15], the study on h-BNNSs, including their exfoliation, properties (by doping or functionalizing), and applications, is in its infancy. This may attribute to the ‘lip-lip’ ionic characteristic of the bonding between neighboring boron nitride (BN) layers, which is stronger than the weak Van der Waals force between graphene layers and the wide band gap of h-BNNS (approximately 4–6 eV), making it as an insulator. If the two aforesaid challenging problems are solved, h-BNNS will exhibit more novel properties and applications in nanoelectronics and nanophotonics. Of particular interest is that minishing the band gap of h-BNNS by doping into some featured elements could lead an amazing change from an insulator to a semiconductor.
Doping preferentially takes place at the more vulnerable sites, so it will be much easier to perform doping experiment with fewer-layered h-BNNSs. Though several methods have been presented to prepare few-layered or mono-layered h-BNNSs[17, 18], the rigorous conditions restrict these methods to be widely conducted. Recently, Golberg and Coleman et al. have put forward a facile route to few-layered or mono-layered h-BNNSs by sonicating the bulk BN in a common liquid solvent. Speaking of doping, several methods have been reported such as placing peculiar dopant into well-defined regions of h-BN nanotubes (h-BNNTs). Wei et al. used the electron-beam-induced strategy and Wang et al. applied the noncovalent functionalization method to dope carbon (C) into the h-BNNTs, which demonstrated the electrical conductivity increased with the C content. In comparison with C, doping of fluorine (F) may be a new pathway to regulate the electrical properties of h-BN. Since F is a highly electronegative element and has excessive valence electrons compared to B and N, doping F into some nanomaterials should reliably yield a p-type semiconductor at low coverages and even a conductor at high coverages[23, 24]. Some theoretical calculations have predicted the possible functions of doping F into h-BNNTs and h-BNNSs[24–26]. Only Tang et al. reported the electrical conductivity of h-BNNTs which were fluorine-functionalized during the nanotubes’ growth. Doping F into h-BNNSs and examining their corresponding electrical properties have not been realized experimentally. Therefore, it is of crucial importance to develop a facile method for doping F into h-BNNSs and explore its electrical properties.
Herein, we doped F into few- and mono-layered h-BNNSs and first pursued their electrical properties with the scanning tunneling microscope-transmission electron microscope (STM-TEM) holder. The few-layered h-BNNSs were exfoliated from the bulk BN using a modified chemical solution route in isopropanol (IPA) at 50°C and with ultrasonicating, and subsequently fluorinated with a solution of fluoboric acid (HBF4). The fluorinated h-BNNSs exhibit a significant characteristic of a semiconductor, with a current up to 15.854 μA.
All chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used without further purification.
Exfoliation of bulk BN to few-layered or mono-layered h-BNNSs
In a typical exfoliation process, the bulk boron nitride (BN) powders (0.25 g) were dispersed in a solvent of IPA contained in a 100-mL round-bottomed flask, and then as-formed solution was heated at 50°C for 24 h under magnetic stirring. Subsequently, the solution was subjected to further ultrasonication for 20 h in a low power sonic bath. Then the resulted solution in the flask was stood for 2 days, and the supernatant solution was removed to the centrifugal tube followed by centrifugation at 14,000 rpm for 10 min. Afterwards, the precipitate was washed with acetone several times to remove the IPA absolutely and dried at 60°C overnight. Finally, a milk-white solution of few-layered and mono-layered h-BN nanosheets (h-BNNSs) were obtained.
Fluorination of h-BNNSs
In a representative fluorination experiment, as-prepared h-BN nanosheets (0.25 g) and HBF4 (50 mL) were mixed in a 100-mL round-bottomed flask. Then the mixture was heated at 50°C for 8 h under magnetic stirring. After this treatment, the mixture was cooled to room temperature naturally. Finally, the fluorinated products were removed to the centrifugal tube, washed with deionized water several times, and dried at 60°C for several hours.
The morphologies and structures of the exfoliated and fluorinated products were characterized by a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800, Tokyo, Japan) equipped with an X-ray energy-dispersive spectrometer (EDS), a transmission electron microscope (TEM, JEOL, JEM-2010F, JEOL Ltd., Tokyo, Japan), an atomic force microscope (AFM, NanoScope IV Veeco Instruments Inc., Plainview, NY, USA), and a D/max-2550 PC powder X-ray diffractometer (XRD, Rigaku Co., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) spectra were conducted on an Axis Ultra DLD X-ray photoelectron spectroscopy (Kratos Co., Manchester, UK). Fourier transform infrared (FTIR) spectroscopy investigations were performed on an IR Rrestige-21 FTIR spectrometer (Shimadzu Co., Kyoto, Japan).
Results and discussion
In order to further identify doping F into the h-BNNSs, we analyzed the chemical composition of the products by XPS (Figure3c) and EDS (Figure S5 in Additional file1). Figure3c shows the XPS spectra of the exfoliated (I) and further fluorinated (II) products, respectively. The results reveal that B, C, N, O and F elements exist in the fluorinated products, in which the binding energy of B 1s, C 1s, N 1s, O 1s, and F 1s is corresponding to 197.6, 288.4, 401.7, 530.0, and 686.6 eV, respectively. The existence of C and O elements commonly seen could attribute to the carbon contamination and water adsorbing from the atmosphere. Comparatively, the curve I only show an existence of the B, C, N and O elements. It suggests the F element appearing in the fluorinated products is the key factor contributing to the excellent electrical conductivity of the h-BNNSs. If the F only attaches to the surface of BNNSs, it will be too unstable to exist under the beam irradiation in the electron microscope[23, 24], resulting in electrical conductivity that will not be significantly improved. So, we deduce that the excellent electrical conductivity of the fluorinated BN nanosheets alternatively confirms the F was doped into the few-layered h-BNNSs successfully.
In summary, an excellent electrical conductivity of the exfoliated and fluorinated h-BNNSs, i.e., transferring from the insulator to the semiconductor, has been reported. A facile chemical route was developed to exfoliate the bulk BN into few- and mono-layered h-BNNSs, then a simple chemical solution route successfully fluorinated the BNNSs. Importantly, the fluorinated BNNSs possesses the excellent electrical property with a current up to 15.854 μA, showing a typical semiconductor characteristic, which will open a new opportunity in designing and fabricating electronic nanodevices.
This work was financially supported by the National Natural Science Foundation of China (grant no. 21171035), the Science and Technology Commission of Shanghai-based ‘Innovation Action Plan’ Project (grant no. 10JC1400100), Ph.D. Programs Foundation of Ministry of Education of China (grant no. 20110075110008), Key Grant Project of Chinese Ministry of Education (grant no. 313015), Shanghai Rising-Star Program (grant no. 11QA1400100), Fundamental Research Funds for the Central Universities, the Shanghai Leading Academic Discipline Project (grant no. B603), and the Program of Introducing Talents of Discipline to Universities (grant no. 111-2-04).
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