2.6 Synthesis of BNNS
2.6.1 Top-Down Approach
This method is to exert certain forces (shearing force for example) to overcome the van der Waals forces that hold BN layers together. This method is useful for large- scale production of BNNSs, but it generally produces BNNSs with a large distribution of layer numbers, various lateral sizes, and contamination.
Synthesis of BNNS
Top-Down Approach Bottom-Up Approach
Micromechanical Cleavage
Chemical Vapour Deposition Sonication Assisted
Exfoliation
Physical Vapour Deposition Electron Beam
Irradiation
23 2.6.1.1 Micromechanical Cleavage
Scotch tape has been used to peel off layered materials (graphite, BN, MoS2, NbSe2, and Bi2Sr2CaCu2Ox) down to mono- and few-layers [163]. The effective procedure to extracts the 2D crystallites was implemented, where a fresh surface of a layered crystal was rubbed against another surface, which left variety of flakes attached to it (the rubbing process is similar to drawing by a chalk on a chalkboard). This technique can lead to BNNS with large lateral size and relatively free of contamination (aside from the tape adhesive which can be removed by a thermal anneal). BNNS obtained in this way can be easily stacked on top of another 2D material.
For example, flexible field effect transistors (FETs) prepared by stacking exfoliated BNNS and graphene, have shown exceptional room temperature carrier mobility [164, 165, 166] and flexibility [166]. Instead of the direct peeling applied in the scotch-tape method, shearing forces were employed in the ball milling process to exfoliate BNNS from bulk h-BN powders [167, 168]. Direct contact of the h-BN powders with the grinding balls can be avoided by carrying out the process in a solvent medium, which improves the efficiency of the process and the quality of the nanosheets [169]. Ball milling without using a solvent generates a large numbers of defects and impurities in the nanosheets [170-172].
2.6.1.2 Sonication-Assisted Exfoliation
Sonication in common organic solvents was reported to effectively overcome the van der Waals forces between the layers in highly oriented layered materials. An extensive study of sonication-assisted exfoliation was conducted by the Coleman et al. [173] where they exfoliated h-BN, MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2,
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NiTe2, and Bi2Te3 in a wide range of solvents, and the best 20 solvents for h-BN, MoS2, and WS2 exfoliation were identified [173].
The amount of nanosheet exfoliation and retention in the solvents was found to be dependent on the solvents‘ surface tension value, and better exfoliation was seen for 40 mJ/m2. Several reports were published afterward on exfoliating BNNS by intercalating ions and compounds inside the layers [174-177]. Molten hydroxides of NaOH and KOH were used to exfoliate h-BN powders down to mono- and few-layers of BNNS by reaction, curling, cutting, and peeling mechanisms [178].
Interestingly, even water was found to be suitable for obtaining mono- and few-layer BNNS by sonication-assisted hydrolysis of boron nitride bulk powders [179]. Besides conventional solvents, BNNS were exfoliated in 1, 2 dichloroethane solution of poly (m-phenyl-enevinylene)-co-(2, 5-dictoxy-p-phenylenevinylene) [180] (1.2 mg polymer in 10 ml solution) by ultra-sonication. Although BNNSs are very inert towards acid and basic attacks, functionalisation of the BNNSs has been attempted to fully utilise its properties in polymer composites. Liu et al. [181] used block copolymers [(P(S-b- MMA)), where PS is polystyrene, PMMA is methylmethacrylate and b represents the double block polymer], to interact with the exfoliated BNNS [181]. The different solubility of polystyrene and PMMA in the block copolymer broadened the dispersion range of the nanosheets even where the Hansen solubility parameters were mismatched. For example, BNNSs can be dispersed in acetone and toluene using the block copolymers, although acetone and toluene are considered unsuitable for BNNSs dispersion. In another work, BNNS were ball milled at high energy to incorporate mechanical defects. These defects were utilised to effectively exfoliate and functionalise the nanosheets by gold nanoparticles [182]. Amine functionalised
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exfoliated stable BNNS dispersion in organic solvents and water was achieved using octadecylamine (ODA) for the functionalised molecules [183].
Functionalisation is achieved due to the strong Lewis acid-base interaction between electron deficient boron and electron rich amine molecules. In a recent work by Sainsbury et al. [184], oxygen radical functionalisation of BNNS was also reported [184]. Firstly, the exfoliated BNNS in N-methyl-2-pyrrolidone (NMP) were tertbutoxy functionalised using di-tert-butylperoxide reagent in a high-pressure autoclave at 120
°C for 12 hours. In the second stage, tertbutoxy functionalised BNNS was reacted with piranha solution (H2SO4: H2O2, 3:1) to make hydroxyl functionalised BNNS.
Nanocomposites prepared by the addition of 0.1% hydroxyl functionalised BNNS to polyvinyl alcohol (PVA) showed very high elastic modulus, strength, elongation, and toughness values compared to bare PVA and non-functionalised BNNS incorporated in PVA. All the properties were enhanced due to effective load transfer in functionalised nanosheets by cross-linking of hydroxide ions with PVA and nanosheets [79].
2.6.1.3 Electron Beam Irradiation
According to Jin et al. [185], the h-BN specimens with reduced number of layers are prepared and thinned to a single layer in situ using electron beam irradiation at 120 kV inside the TEM. The single layered h-BN are prepared within a desired region in a well-controlled manner [186]. During the electron beam thinning process, many lattice defects such as vacancies are subsequently introduced, mainly due to the knock-on effect of the incident electron beam [185]. In addition, it has been reported through the studies [185] that, by means of the exit-wave (EW) reconstruction of through-focus image series, individual boron and nitrogen atoms have been
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experimentally distinguished. Defects in boron nitride mono-layer created by irradiation damage, such as the dominated boron mono-vacancies and the large vacancies with nitrogen atom terminated zigzag edge, have also been atomically resolved [185].