Nonwetting and Optical Properties of BN Nanosheet Films

Surface Innovations http://dx.doi.org/10.1680/si.12.00007 Research Article Received 13/09/2012 Accepted 18/10/2012 Nonwetting and optical properties of BN Published online 30/10/2012 nanosheet films Keywords: nanostructures/superhydrophobicity/thin film/ vapor deposition Pakdel, Bando, Shtansky and Golberg ice | science ICE Publishing: All rights reserved

Nonwetting and optical properties of BN nanosheet films

1 Amir Pakdel PhD* 3 Dmitry Shtansky PhD International Center for Materials Nanoarchitectonics (MANA), National University of Science and Technology (MISIS), Moscow, National Institute for Materials Science (NIMS), Tsukuba, Japan Russia 2 Yoshio Bando PhD 4 Dmitri Golberg PhD* International Center for Materials Nanoarchitectonics (MANA), International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan National Institute for Materials Science (NIMS), Tsukuba, Japan

1 2 3 4

This study begins with a brief discussion on the high-temperature chemical vapor deposition synthesis of transparent boron nitride nanosheet films on silicon/silicon dioxide substrates. The compact nanosheets grew perpendicular to the substrate surface, and the majority of them had thicknesses of less than 5 nm. Ultraviolet-visible spectroscopy measurements demonstrated a wide optical band gap of ~5·6 eV of nanosheets, and cathodoluminescence spectroscopy showed their strong luminescence emission in the ultraviolet region. The nanor- ough surface morphology of the films induced nonwetting and self-cleaning features with water-contact angles reaching ~153°. Such transparent superhydrophobic films can be utilized for the preparation of nonwetting ultraviolet light-emitting surfaces for optoelectronics applications, antifouling surfaces on marine vessels or oil–water separation equipments.

1. Introduction binary structures, two-dimensional zinc oxide pore arrays, and car- The control of the surface wettability is a research topic of funda- bon nanofiber and nanosphere arrays.4 The usage of polymeric and mental interest and is essential in a variety of applications, such as organic superhydrophobic surfaces is limited by their short lifetime marine vehicles, stain resistant materials and clothing, and fluid due to mechanical erosion and heat degradation. Strong acids, bases power system components.1 The wettability of a surface can be and UV irradiation from the sunlight accelerate instability and deg- measured by the equilibrium contact angle (CA) of a water droplet radation of these surfaces.5 Therefore, an important breakthrough on it. If the water droplet CA is larger than 150°, the surface is supe- is to fabricate superhydrophobic surfaces from inorganic materials rhydrophobic. When a superhydrophobic surface with a small CA with high chemical and thermal stability. Such durable surfaces can hysteresis is tilted, water droplets can move spontaneously on that also exhibit stable optical and electrical properties. surface.2 It has been documented that the water repellency of a solid surface mainly depends on two factors: its chemical composition Boron nitride (BN) low-dimensional materials are among the most and functionality, as well as its micro/nano morphological features.3 promising inorganic nanosystems explored so far. BN is a chemical However, a lot of questions still remain in this field, and further compound, consisting of equal numbers of boron (B) and nitrogen research is necessary to fully realize the potentials. On the basis (N) atoms, which is not found in nature and is therefore produced of the regarded two factors, many superhydrophobic surfaces have synthetically.6 Hexagonal BN (h-BN) is an analogue of graphite in been fabricated, for instance, organosilane films, mixed inorgan- which alternating B and N atoms substitute for carbon (C) atoms in a ic–organic coatings, gold cluster films, silicon pyramid/nanowire honeycomb network with sp2 bonding. Within each layer of h-BN, B

*Corresponding author e-mail addresses: [email protected]; [email protected]

1 Surface Innovations Nonwetting and optical properties of BN nanosheet films Pakdel, Bando, Shtansky and Golberg

and N atoms are bound by covalent bonds, whereas the layers are held coating by a microsyringe. A high-resolution Keyence VH-5000 together by van der Waals forces.7 Unlike the popular graphene, mon- optical instrument equipped with a WinROOF V5·03 analysis soft- olayer BN sheets have rarely been observed8,9 due to the peculiar B−N ware was used for measuring the water CA on the films. stacking characteristics. The hexagons of neighboring planes in h-BN are superposed, that is, B and N atoms are in succession along the 3. Results and discussion c-axis, while in graphite, they are shifted by half a hexagon. Moreover, due to the difference in electronegativity of B and N, the B−N bonds 3.1 Growth and structure are partially ionic, in contrast with the purely covalent C−C bonds in Figure 1a and 1b shows typical SEM images of a BN film con- graphitic structures. This can lead to the so-called “lip−lip” interac- sisting of partially aligned nanosheets along the vertical directions between neighboring layers in BN nanosheets, that is, chemical tion. The nanosheets are uniformly distributed over a large area bonds form bridges or “spot-welds” between the atoms of adjacent and display a compact and curly morphology. Compared with the layers. Therefore, formation of multilayers stabilizes the structure.10 BN nanosheets synthesized at lower temperatures,15 the present ones show a branching feature, that is, subnanosheets grow on the BN structures exhibit unique features such as superb thermal surface of the main nanosheets producing a peculiar three-dimen- conductivity, excellent mechanical and chemical stability, and sional nanostructure. The suggested mechanism is illustrated in a stable wide band gap.11,12 After the successful realization of Figure 1c. During the heterogeneous nucleation and quick growth superhydrophobic coatings based on insulating and chemically of preferential crystal planes on the substrate at a high temperature inert BN nanotubes,13,14 the present authors developed a chemical (1300°C), abundant growth vapor (trapped in the combustion boat) vapor deposition (CVD) method to prepare BN nanosheet coatings caused additional growth steps on the pre-existing nanosheets, with controllable water repellency.15 To further investigate the mer- which resulted in the outgrowth of new crystal planes. This repeti- its of such coatings, in this manuscript, the authors describe the tive branching led to the formation of a hierarchical BN nanos- high-temperature CVD formation of nonwetting h-BN films that tructure on the substrate. Another possibility is the intergrowth consist of nanosheets assembled in a perpendicular-to-the-substrate of several nanosheets in different directions, as shown in Figure fashion and their superhydrophobic and optical properties. 1d. In this case, on the initial heterogeneous nucleation of the BN nanosheets on the substrate, continuous supply of growth species could lead to their growth in various directions along the energeti- 2. Experimental cally favorable axis until their collision. As a result, flexible per- The CVD growth of the crystalline BN nanosheets was performed pendicular-to-the-substrate BN nanosheets intermeshed with each in a horizontal tube furnace, as described elsewhere.15 In brief, the other and formed well-aligned and highly dense BN networks. precursor powders were mechanically mixed and positioned in an alumina combustion boat covered with a Si/SiO substrate. The boat 2 A typical TEM image of the nanosheets is illustrated in Figure 2a. was then set into an alumina test tube inside vacuum chamber. The This indicates the compact BN network with very thin nanosheets chamber was evacuated to 1 Torr, and then ammonia gas flow was ∼ that are almost transparent to the electron beam. In addition, intrin- introduced at the rate of 0·4 mL/min. The precursors were heated sic bending and scrolling of the nanosheets can be noticed in Figure to 1300°C, held for 30 min and then cooled to the room tempera- 2a, similar to previously reported BN nanosheets prepared by other ture. The morphology of the films was studied by a field-emission methods.16,17 Typical HRTEM images of the nanosheets in Figures scanning electron microscope (FE-SEM; Hitachi S4800, Japan). 2b and 2c reveal that they are less than 5 nm in thickness. Figure Chemical composition and structural features of the nanosheets 2b depicts highly ordered lattice fringes denoting a well-crystallized were investigated by an X-ray photoelectron spectrometer (XPS; product. The average spacing between adjacent fringes in Figure 2c is Thermo Scientific Theta Probe, USA) and a high-resolution field- ~0·33 nm, which indicates the formation of layered (002) BN planes. emission transmission electron microscope (HRTEM; JEOL JEM- 2100F, Japan) equipped with an electron energy loss spectrometer (EELS, Gatan, USA). Fourier transform infrared (FTIR) spectros- 3.2 Characterization copy (Nicolet 4700, USA), Raman spectroscopy (LabRam HR-800, To establish the elemental composition and structural features of Japan), ultraviolet-visible (UV-Vis) spectroscopy (Jasco V-570, the nanosheets, EELS measurements were carried out, as depicted Japan) and cathodoluminescence (CL) spectroscopy (Gemini elec- in Figure 3a. The EEL spectrum shows two distinct absorption tron gun; Omicron, inside FE-SEM; Hitachi S4600, Japan) were features for B and N K-shell ionization edges at 188 and 401 eV, used to investigate the optical properties of the nanosheets at room respectively. The authors have observed similar peak positions in temperature. The topographical images of the films were obtained BN nanosheets prepared at lower temperatures (1000−1200°C).15 by a JEOL JSPM-5200 scanning probe microscope in the tapping Quantification analysis of the EEL spectrum gives a B/N atomic atomic force microscopy (AFM) mode at ambient conditions. The ratio of ~1·0. The sharp peaks on the left side of B-K and N-K CA measurement was carried out by a sessile drop method using a edges correspond to 1s→π* antibonding orbits, and the peaks deionized water droplet of about 10-μL volume positioned on the on the right side of the absorption edges correspond to 1s→σ*

2 Surface Innovations Nonwetting and optical properties of BN nanosheet films Pakdel, Bando, Shtansky and Golberg

(a) (b)

1 µm 200 nm

(c)

(d)

Figure 1. (a) Typical SEM image of the BN nanosheet films. (b) Higher magnification SEM image showing branching of the nanosheets. (c and d) Suggested growth mechanisms for the vertically aligned BN nanosheets. BN, boron nitride; SEM, scanning electron microscope. antibonding orbits. This type of EELS edge structure is typical of characteristic peak at 1365 cm–1 is attributed to the B–N high-fre- 2 the sp -hybrizied layered h-BN. quency vibrational mode (E2g) within h-BN layers, analogous to the G peak in graphene.21 The previous measurements show Raman XPS analysis identified the chemical composition and bonding shifts of 1363−1365 cm−1 for BN nanosheets synthesized at lower states in the synthesized films. A typical XPS spectrum of the temperatures15 and 1366 cm−1 for BN nanotubes.22 The reported nanosheets is presented in Figure 3b. The photoelectron peaks Raman shift for different BN structures is in the range of 1366–1374 observed at ~191 and ~399 eV are related to B and N, respectively. cm–1.9,23 Considering the multilayer nature of the nanosheets, it can A very weak peak related to O can also be seen at around 533 eV, be expected that they show the same features as bulk h-BN materi- possibly due to a slight oxidation of the nanosheets. The sharp B als (Raman shift at 1366 cm–1). The observed 1 cm–1 red shift can 1s and N 1s peaks in Figure 3c and 3d are attributed to the B−N be attributed to the possible local temperature increase caused by bonding in pure BN7,18, and the π plasmon loss peak located at ~9 the laser9 and generation of stress in nanosheets due to folding and eV away from the center of the B 1s and N 1s peaks in Figure 3b is interactions with the substrate.7 Figure 3e also proves the absence of characteristic of the hexagonal phase BN.19,20 carbon G band trace at ~1600 cm–1, indicating the high purity of the present BN nanosheets. The full width at half maximum (FWHM) Raman spectroscopy was used to further characterize the film. of the Raman peaks can be used to evaluate the crystallinity of BN Figure 3e shows a typical Raman spectrum of the nanosheets. The nanomaterials. The FWHM of present BN nanosheets was 16 cm−1,

3 Surface Innovations Nonwetting and optical properties of BN nanosheet films Pakdel, Bando, Shtansky and Golberg

(a) (b) (c)

0·33nm

50 nm 10 nm 2nm

Figure 2. (a) Low-magnification TEM image of the BN nanosheets. (b and c) Typical HRTEM images of the nanosheets indicating their good crystallinity. BN, boron nitride; HRTEM, high-resolution field-emission transmission electron microscope; TEM, transmission electron microscope.

smaller than the BNNSs prepared at lower temperatures (19−37 of the surface layer (associated with θe) and the roughness of the cm−1),15 which indicates the good crystallinity of the product. wetted area (portion of the film surface in contact with the liquid

droplet, fs). Therefore, water CA higher than 150° (superhydro- Figure 3f is a typical FTIR spectrum of the film, showing an absorp- phobicity) can be achieved on any surface (even an intrinsically tion peak at ~815 cm–1 and one broad absorption band in the range hydrophilic material such as BN) provided that it is roughened of ~1350–1580 cm–1 with its bottom at 1377 cm–1, which can be enough. Moreover, airborne adsorbates on the BN films could be attributed to the A2u (B–N–B bending vibration mode parallel to the an additional factor to enhance their hydrophobic properties, as has 5 c-axis) and E1u (B–N stretching vibration mode perpendicular to the been recently reported for BN nanotube films. c-axis) modes of h-BN, respectively.16 The peak at ~1084 cm–1 is related to the Si/SiO2 substrate, as shown at the onset of Figure 3f. The practical interest in superhydrophobic films is closely related The FTIR results are in a good agreement with previously reported to the dynamic properties of droplets on them. The first interest- ones for BN nanocrystals,16,24 which confirm the good crystallinity ing property is a very low degree of sticking to the surface. The of the present BN nanosheets. authors performed an experiment by pushing a suspended water droplet from a syringe on the film and moving it on the sample. The suspended droplet did not attach to the surface and was easily 3.3 Nonwetting properties sliding over the film indicating that the adhesion between the film Nonwetting properties of the BN films were then measured. A and water was considerably weaker than that between the syringe peculiar surface morphology can generate superhydrophobicity in needle and the water drop. nanostructured films.16,25 Figure 4a displays the round shape of a deionized water droplet on the BN film. The measured CA is 153 ± Self-cleaning behavior is also a very important feature of super- 1·8° indicating the superhydrophobic feature of the coating. Figure hydrophobic films. The authors observed that on the present BN 4b is a typical AFM topography image of the BN nanosheet films. films most of the dust was picked up under water droplet rolling, Quantitative AFM measurements showed that the maximum height and no residue was left behind. It means that the adhesion between difference on the film surface, and its average roughness were 188 water droplets and dust particles is larger than that between the and ~26 nm, respectively. This implies that the strong nonwetting surface and the dust. This results from the conjunction of a very tendency could be ascribed to the nanoscale surface roughness of large CA (which reduces the solid/liquid surface area) and a very the BN films. Cassie-Baxter model considers a liquid droplet sit- small hysteresis. A liquid flowing on a solid is conceived not to ting on a rough surface as partly on the nanostructure and partly on slip at the interface between the phases, but a microscopic slip may air and describes the CA of the droplet as,26 exist if the solid behaves in a hydrophobic manner.27 This effect can become more notable if the hydrophobic solid is textured and air is cos=θ− 1+f 1 + cos θ 1. CB se() trapped in the textures (Cassie-Baxter model), as is the case of the present films. The self-cleaning characteristic makes the regarded where fs is the fraction of solid–liquid contact and θe is the CA films suitable for rendering a surface antimicrobial. The ability of in Young’s mode (for an ideally smooth surface). Obviously, the water to easily slide on such films opens the door for the reduction main factors affecting the CA value are the chemical composition of energy required to pump fluids in pipe networks. The extremely

4 Surface Innovations Nonwetting and optical properties of BN nanosheet films Pakdel, Bando, Shtansky and Golberg

(a) (b) N 1s

B-K edge

B 1s N-K edge Intensity (Arbitrary units) Intensity (Arbitrary units)

200 300 400 500 0 200 400 600 800 1000 1200 Energy loss (eV) Binding energy (eV)

186 189 192 195 392 396 490 404 900 1200 1500 1800 2100 2400 2700 Binding energy (eV) Binding energy (eV) Raman shift (cm−1) (f) Si/SiO2 substrate

1500 1000 500

N-K edge ransmittance (Arbitrary units) T

2400 2000 1600 1200 800 Figure 3. (a) Typical EEL spectrum from a BN nanosheet. (b) WXPSavenumber (cm−1) survey spectrum of the BN film. (c and d) N 1s and B 1s core-level XPS spectra, respectively. (e and f) Raman and FTIR spectra of the BN film. BN, boron nitride; EEL, electron energy loss; FTIR, Fourier transform infrared spectroscopy; XPS, X-ray photoelectron spectrometer. low density makes them a good candidate for applications in can bounce back, after experiencing an almost elastic collision. This avionics as a moisture barrier or an ice-resistant material. is also a reason for these films to remain dry even after coming in contact with some liquid. The rebound becomes possible due to small In another set of experiments, the authors noticed that water droplets energy dissipation as the drop impacts the solid, that is, because of the falling on the present superhydrophobic films with a decent velocity high CA, viscous dissipation close to the moving contact line (which

5 Surface Innovations Nonwetting and optical properties of BN nanosheet films Pakdel, Bando, Shtansky and Golberg

usually is the primary cause of viscous loss) becomes nearly negli- gap of ~5·7 eV (according to Tauc’s calculation method),29 gible. A drop impacting a superhydrophobic film deforms; however, larger than those of the previously reported BN nanoribbons and due to the very large CA it can store its kinetic energy under surface nanosheets.24,30 However, theoretical studies of band structures deformation and thus bounces back. The drop therefore behaves as a of a single-layer h-BN predict a 6-eV band gap.31 The measured spring, whose stiffness displays the surface tension of the liquid.28 smaller gap in the present multilayer BN nanosheets could be attributed to an increase in the electronic band dispersion because of the layer–layer interactions.31 A dip is observed at ~230−300 3.4 Optical properties nm, which could be due to defect transitions. The measurements The optical absorption properties reveal the electronic state of a on various BN nanostructures indicate that some strong lumi- material and can be used to verify the band gap of semiconduc- nescence emissions at ~300−330 nm can be noticed in them due tors. UV-Vis absorption spectroscopy was utilized to investigate to excitation at 240−290 nm. Multiple absorption peaks in this the optical energy gap (E ) of the synthesized films. Figure 5a g region may also originate from absorption centers associated with shows the UV-Vis absorption spectrum of the BN film. Since the both sp2- and sp3-bonded structures, since the peaks may be the Si/SiO substrate is opaque to UV and visible light, the UV-Vis 2 reflection of the phonon–electron coupling associated with the measurements were performed in the reflection mode. Reflectance infrared vibration modes.32 (R) was then converted to absorbance (A) automatically by the machine’s software considering zero transmittance (T) through The optical properties of the nanosheets were further investigated the opaque substrate (A + R + T = 100%). A sharp absorption by CL spectroscopy. Figure 5b depicts the CL spectrum of a BN peak is observed at ~212 nm, corresponding to an optical band film at room temperature, indicating two luminescence bands at ~334 and ~377 nm. This strong ultraviolet CL emission can (a) (b) be attributed to the deep-level emissions associated with defect- related centers (B or N vacancy-type defect-trapped states).30 Although theoretical calculations and experiments on bulk BN and BN nanotubes have demonstrated that strong Frenkel type (nm) 1·88 excitonic transitions at ~211–234 nm take place at temperatures 4·4 ≤100 K,33–35 this luminescence emission did not appear in the present BN films at room temperature.30 In fact, the BN nanosheets 0 displayed strong luminescence emission in the ultraviolet range, 0·0 m) (µ although the near band edge CL emission was not recognized in (µm) their spectrum.

4·1 0·0

Figure 4. (a) Typical optical photograph of a water droplet on a 4. Conclusion BN film. (b) Atomic force micrograph showing the film surface In summary, transparent h-BN films were synthesized at 1300°C topography. BN, boron nitride. via a CVD method on Si/SiO2 substrates. The films were composed of compactly aligned nanosheets protruding out of the substrate

(a) (b) Intensity (Arbitrary units) Absorbance (Arbitrary units)

200 400 600 800 0200 400600 800 1000 Wavelength Wavelength

Figure 5. (a) Ultraviolet-visible absorption spectrum of a BN film. (b) Cathodoluminescence spectrum from the hexagonal BN nanosheets. BN, boron nitride.

6 Surface Innovations Nonwetting and optical properties of BN nanosheet films Pakdel, Bando, Shtansky and Golberg

surface. The majority of the nanosheets were less than 5-nm thick. 9. Arenal, R.; Ferrari, A. C.; Reich, S.; Wirtz, L.; Mevellec, J. Y.; Their chemical composition and structural features were studied Lefrant, S.; Rubio, A.; Loiseau, A. Raman spectroscopy of by EELS, XPS, FTIR, Raman spectroscopy, CL spectroscopy and single-wall boron nitride nanotubes. Nano Letters 2006, 6, UV-Vis spectroscopy. The nanosheets displayed a wide band gap 1812–1816. and strong CL emission in the ultraviolet region at room tempera- 10. Blase, X.; De Vita, A.; Charlier, J. C.; Car, R. Frustration effects ture, as well as excellent nonwetting behavior due to their rough and microscopic growth mechanisms for BN nanotubes. morphology and nanoscale features. No sticking between water Physical Review Letters 1998, 80, 1666–1669. droplets and the films was observed, and the droplet-film collision 11. Chen, Y.; Chen, H.; Liu, Y. Cathodoluminescence of boron was almost elastic. The present films are envisaged to be valuable nitride nanotubes doped by ytterbium. Journal of Alloys and for diverse applications such as self-cleaning, nonfogging displays Compounds 2010, 504, S353–S355. and protection from acid rain corrosion. 12. Zeng, H. B.; Zhi, C. Y.; Zhang, Z. H.; Wei, X. L.; Wang, X. B.; Guo, W. L.; Bando, Y.; Golberg, D. “White graphenes”: boron nitride nanoribbons via boron nitride nanotube unwrapping. Acknowledgments Nano Letters 2010, 10, 5049–5055. This work was supported by the WPI Center for Materials 13. Lee, C. H.; Drelich, J.; Yap, Y. K. Superhydrophobicity of Nanoarchitectonics (MANA) of the National Institute for Materials boron nitride nanotubes grown on silicon substrates. Langmuir Science (NIMS; Tsukuba, Japan). Amir Pakdel is grateful to Prof. 2009, 25, 4853–4860. Tomonobu Nakayama, Prof. Takashi Sekiguchi, Dr. Chunyi Zhi and 14. Li, L. H.; Chen, Y. Superhydrophobic properties of nonaligned Mr. Xuebin Wang for useful discussions and also to Dr. Yoshihiro boron nitride nanotube films.Langmuir 2010, 26(7), Nemoto, Dr. Shinichi Hara and Dr. Kentaro Watanabe for their 5135–5140. technical support. 15. Pakdel, A.; Zhi, C. Y.; Bando, Y.; Nakayama, T.; Golberg, D. Boron nitride nanosheet coatings with controllable water References repellency. ACS Nano 2011, 5, 6507–6515. 1. Steele, A.; Bayer, I.; Moran, S.; Cannon, A.; King, W. P.; 16. Yu, J.; Qin, L.; Hao, Y. F.; Kuang, S.; Bai, X. D.; Chong, Y. Loth, E. Conformal ZnO nanocomposite coatings on micro- M.; Zhang, W. J.; Wang, E. Vertically aligned boron nitride patterned surfaces for superhydrophobicity. Thin Solid Films nanosheets: chemical vapor synthesis, ultraviolet light 2010, 518, 5426–5431. emission, and superhydrophobicity. ACS Nano 2010, 4, 2. McCarthy, T. J.; Oner, D. Ultrahydrophobic surfaces: effects of 414–422. topography length scales on wettability. Langmuir 2000, 16, 17. Zhi, C. Y.; Bando, Y.; Tang, C. C.; Kuwahara, H.; Golberg, 7777–7782. D. Large-scale fabrication of boron nitride nanosheets and 3. Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. Super water- their utilization in polymeric composites with improved and oil-repellent surfaces resulting from fractal structure. thermal and mechanical properties. Advanced Materials 2009, Journal of Colloid and Interface Science 1998, 208, 287–294. 21, 2889–2893. 4. Wang, B.; Li, G. X.; Liu, Y.; Song, X. M.; Li, E.; Yan, 18. Park, K. S.; Lee, D. Y.; Kim, K. J.; Moon, D. W. Observation H. Preparation of transparent BN films with superhydrophobic of a hexagonal BN surface layer on the cubic BN film grown surface. Applied Surface Science 2008, 254, 5299–5303. by dual ion beam sputter deposition. Applied Physics Letters 5. Boinovich, L. B.; Emelyanenko, A. M.; Pashinin, A. S.; Lee, C. 1997, 70, 315–317. H.; Drelich, J.; Yap, Y. K. Origins of thermodynamically stable 19. Pakdel, A.; Wang, X. B.; Zhi, C. Y.; Bando, Y.; Watanabe, K.; superhydrophobicity of boron nitride nanotubes coatings. Sekiguchi, T.; Nakayama, T.; Golberg, D. Facile synthesis Langmuir 2012, 28, 1206–1216. of vertically aligned hexagonal boron nitride nanosheets 6. Pakdel, A.; Zhi, C. Y.; Bando, Y.; Golberg, D. Low- hybridized with graphitic domains. Journal of Materials dimensional boron nitride nanomaterials. Materials Today Chemistry 2012, 22, 4818–4824. 2012, 15, 256–265. 20. Jiang, L. D.; Fitzgerald, A. G.; Rose, M. J.; Lousa, A.; Gimeno, 7. Song, L.; Ci, L.; Lu, H.; Sorokin, P. B.; Jin, C.; Ni, J.; Kvashnin, S. Characterisation of cubic boron nitride films at different A. G.; Kvashnin, D. G.; Lou, J.; Yakobson, B. I.; Ajayan, stages of deposition. Applied Surface Science 2000, 167, P. M. Large-scale growth and characterization of atomic 89–93. hexagonal boron nitride layers. Nano Letters 2010, 10, 21. Walukiewicz, W.; Wu, J.; Han, W. Q.; Ager, J. W.; Shan, W.; 3209–3215. Haller, E. E.; Zettl, A. Raman spectroscopy and time-resolved

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