Controlled Synthesis of Atomically Layered Hexagonal Boron Nitride

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Nov 29, 2016 - Abstract: Hexagonal boron nitrite (h-BN) is an attractive material for many applications including electronics as a complement to graphene, ...
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Controlled Synthesis of Atomically Layered Hexagonal Boron Nitride via Chemical Vapor Deposition Juanjuan Liu 1 , R. Govindan Kutty 2 and Zheng Liu 2, * 1 2

*

School of Geography and Environmental Sciences, Guizhou Normal University, Guizhou 550001, China; [email protected] Center for Programmable Materials, School of Material Science and Engineering, Nanyang Technological University, Singapore 637798, Singapore; [email protected] Correspondence: [email protected]; Tel.: +65-6513-7352

Academic Editor: Derek J. McPhee Received: 17 August 2016; Accepted: 24 November 2016; Published: 29 November 2016

Abstract: Hexagonal boron nitrite (h-BN) is an attractive material for many applications including electronics as a complement to graphene, anti-oxidation coatings, light emitters, etc. However, the synthesis of high-quality h-BN is still a great challenge. In this work, via controlled chemical vapor deposition, we demonstrate the synthesis of h-BN films with a controlled thickness down to atomic layers. The quality of as-grown h-BN is confirmed by complementary characterizations including high-resolution transition electron microscopy, atomic force microscopy, Raman spectroscopy and X-ray photo-electron spectroscopy. This work will pave the way for production of large-scale and high-quality h-BN and its applications as well. Keywords: chemical vapor deposition; hexagonal boron nitride; synthesis PACS: 61.72.jd; 61.72.uj; 68.37.Og

1. Introduction Hexagonal boron nitride (h-BN), as a typical large-bandgap 2D crystal, has been attracting increasing attention due to its unique properties such as high thermal conductivity and excellent electrical insulation, etc. [1–7]. Due to its atomically smooth surface and close in-plane lattice match to graphene, h-BN has been considered an important layered material complementary to graphene [8–12]. Previous work reported that h-BN can help to improve the mobility of graphene as a substrate [5], fabricate high current cut-off devices for radio frequency applications [12], engineer a bandgap in stacked graphene/h-BN (G/h-BN) [10,13–16], and enhance the ON/OFF ratio of the graphene field effect transistor (GFET) in the vertical G/h-BN heterostructure [11]. These progresses, together with the high thermal conductivity, thermal stability and large bandgap, make the h-BN 2D crystal a promising component in both fundamental uses and applications such as optics, electronics, etc. Unlike the mature technology of the production of highly-oriented pyrolytic graphite (HOPG), the synthesis of bulk h-BN crystals such as highly-oriented pyrolytic boron nitride (HOPBN) [17] is still a great challenge, although a few works have reported on the synthesis of h-BN crystals using high-temperature and high-pressure methods [18–20]. There is an urgent need to develop an alternative approach to prepare large-scale and high-quality h-BN for further applications for next-generation electronics. Previous efforts have demonstrated that atomically layered h-BN can grow on copper foil, using the chemical vapor deposition (CVD) method, with ammonia borane as a precursor at a temperature from 900 ◦ C to 1000 ◦ C [2,21] Some further works, with optimized growing conditions,

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have advanced the thicknesses of h-BN to a single layer [22,23]. In most of the recent works, h-BN films films are grown on copper quality of such CVD h-BN cannotbebecompared comparedwith withthe the exfoliated exfoliated are grown on copper foil. foil. TheThe quality of such CVD h-BN cannot samples, and and tends tends to to aa relatively relatively low low performance. performance. samples,

2. Results 2. Results wewe demonstrated that, with as a substrate at a high temperature °C), In this thispaper, paper, demonstrated that,Ni with Ni as a and substrate and at a high(1000~1200 temperature ◦ high-quality h-BN can be prepared. The thicknesses of as-grown h-BN can be well controlled, from (1000~1200 C), high-quality h-BN can be prepared. The thicknesses of as-grown h-BN can be well single-layered, few-layered, tooreven bulk, as the images show in Figure 1. show in Figure 1. controlled, fromorsingle-layered, few-layered, to optical even bulk, as the optical images

Figure 1. 1. Optical of h-BN h-BN on on Ni Ni substrates Figure Optical images images (a–d) (a–d) and and illustrations illustrations (e–h) (e–h) of of controllable controllable growth growth of substrates 2 . Scale bar in in (e–h) is for bulk (a,e), several layers (b,f), few layers (c,g) and a single layer (d,h) on SiO for bulk (a,e); several layers (b,f); few layers (c,g) and a single layer (d,h) on SiO2 . Scale bar (e–h) 2 mm. (i) SEM image of few-layered h-BN on Ni foil (left, scale bar 1 mm) and optical image of few-layered is 2 mm; (i) SEM image of few-layered h-BN on Ni foil (left, scale bar 1 mm) and optical image of (scale µm). (j) Bulk h-BN domains on Ni foils. Scale bar: 100 µm. h-BN film onh-BN SiO2film few-layered onbar SiO100 2 (scale bar 100 µm); (j) Bulk h-BN domains on Ni foils. Scale bar: 100 µm.

Figure 11 shows shows optical optical images images and and illustrations illustrations of of as-transferred as-transferred h-BN h-BN films. films. Figure 1a–d are are Figure Figure 1a–d illustrations of bulk, six-layer, double-layer and single-layer h-BN, respectively. Figure 1e–h are illustrations of bulk, six-layer, double-layer and single-layer h-BN, respectively. Figure 1e–h are corresponding optical h-BN films on SiO 2 substrates (thickness: 285 nm). Their corresponding opticalimages imagesofofas-transferred as-transferred h-BN films on SiO 2 substrates (thickness: 285 nm). layer numbers from left to right are >10, five to six, two to three, and to two Their layer numbers from left to right are >10, five to six, two to one three, and layers, one torespectively. two layers, The thick h-BN on SiO 2 is green while thinner h-BN turns to light purple. We noticed that, for severalrespectively. The thick h-BN on SiO2 is green while thinner h-BN turns to light purple. We noticed that, layer h-BN (>3 nm), the thickness and coverage not as good few-layered h-BN on for several-layer h-BN (>3 nm), theuniformity thickness uniformity andiscoverage is notasasfor good as for few-layered Ni (Supplementary Materials Figure S1). As it can be seen in Figure 1e, there are tiny holes in film. h-BN on Ni (Supplementary Materials Figure S1). As it can be seen in Figure 1e, there are tinythe holes in The h-BN films in Figure 1e–h were prepared at a growing temperature of 1000 °C (Figure 1i). If the ◦ the film. The h-BN films in Figure 1e–h were prepared at a growing temperature of 1000 C (Figure 1i). growing temperature is high (>1100 °C) and the duration is long (>1 h), high crystalline h-BN stars If the growing temperature is high (>1100 ◦ C) and the duration is long (>1 h), high crystalline h-BN can be found (Figure 1j). stars can be found (Figure 1j). The thicknesses of CVD h-BN can be well controlled from a single layer to bulk with optimized The thicknesses of CVD h-BN can be well controlled from a single layer to bulk with optimized growing conditions. We should note that the temperature is critical to control the thickness of h-BN. A growing conditions. We should note that the temperature is critical to control the thickness of h-BN. high temperature up to 1100 °C is required in order to synthesize thick h-BN film. The melting point of A high temperature up to 1100 ◦ C is required in order to synthesize thick h-BN film. The melting point Cu foil is around 1050 °C while Ni foils resist higher temperatures up to 1400 °C. So, the substrate (Ni foil) of Cu foil is around 1050 ◦ C while Ni foils resist higher temperatures up to 1400 ◦ C. So, the substrate is important to produce h-BN films with a controlled thickness. The growing conditions are listed in (Ni foil) is important to produce h-BN films with a controlled thickness. The growing conditions Table 1. Monolayer h-BN can be synthesized at under ~1000 °C by heating a precursor as low as 40 °C. are listed in Table 1. Monolayer h-BN can be synthesized at under ~1000 ◦ C by heating a precursor The growing time is 1 h. A longer growing time results in a thicker h-BN film. As expected, a higher as low as 40 ◦ C. The growing time is 1 h. A longer growing time results in a thicker h-BN film. precursor temperature and growing temperature will enhance the growing rate of h-BN. Figure 1i is a As expected, a higher precursor temperature and growing temperature will enhance the growing rate close-up area of few-layered h-BN film. A high furnace temperature (1200 °C) and precursor temperature of h-BN. Figure 1i is a close-up area of few-layered h-BN film. A high furnace temperature (1200 ◦ C) will result in very thick h-BN up to tens of nanometers, as shown in Figure 1j. The h-BN flakes can be observed in the optical images. Each flake has a few legs and the angles between them are multiples of 30°. We also realized that the domain orientation may play an important role in the formation of

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and precursor temperature will result in very thick h-BN up to tens of nanometers, as shown in Figure 1j. The h-BN flakes can be observed in the optical images. Each flake has a few legs and the angles between them are multiples of 30◦ . We also realized that the domain orientation may play an important role in the formation of h-BN, especially at high temperatures. At low temperatures, Molecules 2016, 21, x 3 of 8 the ammonia borane precursors are not fully condensed into h-BN, and therefore it results in relatively nanometers, as shown inh-BN Figure films 1j. The h-BN flakes can be in the optical images. Each flake low-quality but high-coverage (Figure S1). Atobserved high temperatures, highly crystalline h-BN a few legs and the angles between them are multiples of 30°. We also realized that the domain can be easilyhas obtained but is also highly selective to the lattice orientation of the domains of the orientation may play an important role in the formation of h-BN, especially at high temperatures. At substrates. Therefore, we can observe thick h-BN flakes various thicknesses at Ni foils at high low temperatures, the only ammonia borane precursors are not fully of condensed into h-BN, and therefore results in S2). relatively but high-coverage h-BN filmsreports (Figure S1). At high temperatures, temperaturesit(Figure Thislow-quality is also confirmed by previous [24]. highly crystalline h-BN can be easily obtained but is also highly selective to the lattice orientation of the domains of the substrates. Therefore, we can only observe thick h-BN flakes of various thicknesses Table 1. Specific growth conditions of h-BN films in various layers. at Ni foils at high temperatures (Figure S2). This is also confirmed by previous reports [32]. Table 1. Specific conditions layers. Thickness of h-BN 1–2 growth L 2–3 L of h-BN films 5–6 in L variousFew Layers

of h-BN 1000 1–2◦ C L GrowthThickness temperature Growthtime temperature 1000 Growth 1 h °C Growth time Precursor temperature 40 ◦1Ch Precursor temperature

40 °C

Bulk

Commented [M2]: References should order.

2–3 L ◦ C 5–61000 L ◦ CFew Layers 1000 1000 ◦ C Bulk>1100 ◦ C 1000 1000 °C 1 h >1100 °C 0.5 h 1.5°Ch 1000 °C1 h 1.5 h◦ 1h ◦ 1h 40 C 50 C 70 ◦ C 0.5 h 70 ◦ C 40 °C 50 °C 70 °C 70 °C

Figure 2. TEM images, diffractions and EELS of single-layered and few-layered h-BN films. (a–e) TEM

Figure 2. TEM images, diffractions and(2 EELS of (3single-layered and few-layered films. (a–e) TEM images of single(1 L), doubleL), threeL), four- (4 L) and seven-layered (7 L)h-BN h-BN films, respectively. Scale bar: 2 nm; TEM image of area of(4single-layered film and its FFT pattern images of single(1 L), double(2(f)L), three(3random L), fourL) and seven-layered (7 L) h-BN films, 10 nm; (g) HRTEM image of single-layered h-BN film and its FFT patterns from the respectively. (inset). Scale Scale bar: bar: 2 nm; (f) TEM image of random area of single-layered film and its FFT pattern white-dashed region. Scale bar: 5 nm; (h) TEM image of few-layered h-BN and its FFT patterns. Scale (inset). Scalebar: bar: 10 nm; (g) image single-layered h-BN andof its FFT patterns from 2 nm; (i) A typical HRTEM diffraction pattern of of few-layered h-BN in a large area;film (j) EELS few-layered h-BN film. the white-dashed region. Scale bar: 5 nm; (h) TEM image of few-layered h-BN and its FFT patterns. Scale bar: 2 nm; (i) A typical diffraction pattern of few-layered h-BN in a large area; (j) EELS of Figure 2 shows the TEM images, diffraction patterns and elemental analysis of single-layered few-layered h-BN film.h-BN films. Figure 2a–e show the edges of single-layered (1 L), double-layered (2 L), and few-layered three-layered (3 L), four-layered (4 L) and seven-layered (7 L) h-BN films, respectively. The scale bar is 2 nm. The thickness of an individual h-BN layer is estimated to be ~0.35 nm. Figure 2f shows the Figure 2TEM shows the TEM images, diffraction patterns and elemental analysis of single-layered image of single-layered h-BN film. The scale bar is 10 nm. The inset is the corresponding fast few-layered h-BN films. Figure 2a–e show the edges of single-layered (1 L),2gdouble-layered (2 L), Fourier transform (FFT) pattern, showing individual six-fold symmetry spots. Figure is a high-

and three-layered (3 L), four-layered (4 L) and seven-layered (7 L) h-BN films, respectively. The scale bar is 2 nm. The thickness of an individual h-BN layer is estimated to be ~0.35 nm. Figure 2f shows the TEM image of single-layered h-BN film. The scale bar is 10 nm. The inset is the corresponding fast Fourier transform (FFT) pattern, showing individual six-fold symmetry spots. Figure 2g is a high-resolution TEM (HRTEM) image of single-layered h-BN film. The scale bar is 5 nm. The inset shows the corresponding FFT patterns. Figure 2h shows the morphology of few-layered h-BN and its FFT pattern

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(inset). Figure 2i is a typical diffraction pattern of a random area of few-layered h-BN film. The electron energy loss spectrum (EELS) is shown in Figure 2j. Both the boron edge (~188 eV) and nitrogen edge 2016, 21, 1636 4 of 8 (~410 eV)Molecules are marked by black arrows. spectrum (EELS) is shown in Figure 2j. Both the boron edge (~188 eV) and nitrogen edge (~398 eV)

3. Discussion are marked by black arrows.

The thicknesses and morphologies of h-BN films are further confirmed by AFM in Figure 3. 3. Discussion Figure 3a–c show the height topography of atomically layered h-BN on SiO2 . Their sizes are The thicknesses and morphologies of h-BN films are further confirmed by AFM in Figure 3. 10 µm × 10 µm, 30 µm × 30 µm and 1.5 µm × 1.5 µm, and the typical thicknesses are 0.7, 1.3 and Figure 3a–c show the height topography of atomically layered h-BN on SiO2. Their sizes are 10 µm × 2.0 nm (corresponding toµm 1~2and L,1.5 3~4 L h-BN), considering effect. 10 µm, 30 µm × 30 µmL,× 5~6 1.5 µm, and the respectively, typical thicknesses are 0.7, 1.3the andsubstrate 2.0 nm The roughness of the to thin be as respectively, low as ~300 pm (thethe dashed region Figure 3c), (corresponding 1~2layer L, 3~4h-BN L, 5~6can L h-BN), considering substrate effect.in The roughness the thinof layer can bepm) as low as ~300 pm (the region in Figure comparable comparable to theofvalue SiOh-BN [12]. Figure 3ddashed shows thick h-BN 3c), domains on a Ni foil. 2 (~250 to the value of SiO 2 (~250 pm) [12]. Figure 3d shows thick h-BN domains on a Ni foil. Figure 3e, f are Figure 3e,f are the height topography and corresponding current sensing image of the h-BN thick film. the height topography and corresponding current sensing image of the h-BN thick film. The thickness The thickness is ~20 nm (more than 50 layers). Due to the insulating behavior of h-BN, the h-BN films is ~20 nm (more than 50 layers). Due to the insulating behavior of h-BN, the h-BN films can be easily can be easily found from thesensing current sensing image. found from the current image.

AFM morphologies of as-transferred h-BN on SiO2 with thicknesses of 0.78 nm (a), 1.3 nm Figure 3.Figure AFM 3.morphologies of as-transferred h-BN on SiO2 with thicknesses of 0.78 nm (a); 1.3 nm (b) (b) and 2.5 nm (c), respectively, corresponding to one- to two-layered, three- to four-layered and sixand 2.5 nm (c), respectively, corresponding to one- to two-layered, three- to four-layered and six- to to seven-layered h-BN. Inset of each image shows the height profile of the cross-sections. The sizes of seven-layered h-BN. image shows the of the image cross-sections. Figure (a–c) are Inset 10 µm of × 10each µm, 30 µm × 30 µm= and 1.5height µm × 1.5profile µm. (d) Optical of h-BN bulk.The sizes Topography and × corresponding AFM image h-BN foil. (d) Size:Optical 20 µm × image of of Figure(e,f) (a–c) are 10 µm 10 µm, 30 current µm ×sensing 30 µm= and 1.5 ofµm × on 1.5Niµm; 20 µm. ~20 nm. h-BN bulk; (e,f)Thickness: Topography and corresponding current sensing AFM image of h-BN on Ni foil. Size: 20 µm × 20 µm. Thickness: ~20 nm.

Raman spectroscopy is a powerful technique to determine the thickness and quality of as-grown h-BN films. Figure 4a shows typical Raman spectra taken from one-layer, two-layer, three-layers and bulkspectroscopy h-BN film with ais514.5 nm laser excitation which performed at athickness power of 20and mW.quality The Raman Raman a powerful technique to was determine the of as-grown vibrating mode of h-BN (E2g) locates at around 1368 cm−1 [25,26]. From bulk to single-layer h-BN films, h-BN films. Figure 4a shows typical Raman spectra taken from one-layer, two-layer, three-layers and the E2g peak shifts from 1367 to 1371 cm−1, along with a dramatic drop of the Raman intensity (Figure 4a,b). bulk h-BN film with a 514.5 nm laser excitation which was performed at a power of 20 mW. The Raman For most single-layered h-BN on SiO2, these results agree with a previous report on the Raman −1 vibratingsignature mode of h-BN (E2gexfoliated ) locatesh-BN at around bulk to(HWHM) single-layer h-BN of mechanical films [7].1368 Also,cm the half[25,26]. width at From half-maximum of − 1 2g 2g peaks is analyzed to evaluate the1371 quality of h-BN filmswith (Figure For most h-BNof samples grown intensity films, theEE peak shifts from 1367 to cm , along a 4c). dramatic drop the Raman on Ni foil, HWHM are ~11 cm−1, comparable the value of mechanical exfoliated h-BN (10~12 cm−1). (Figure 4a,b). Forthe most single-layered h-BN ontoSiO 2 , these results agree with a previous report on As a comparison, the HWHM of E2g peak from h-BN on a Cu substrate is ~16 cm−1. All these Raman data the Raman signature of mechanical exfoliated h-BN films [7]. Also, the half width at half-maximum demonstrate that the high-quality and atomically layered h-BN films can be grown on Ni foil.

(HWHM) of E2g peaks is analyzed to evaluate the quality of h-BN films (Figure 4c). For most h-BN samples grown on Ni foil, the HWHM are ~11 cm−1 , comparable to the value of mechanical exfoliated h-BN (10~12 cm−1 ). As a comparison, the HWHM of E2g peak from h-BN on a Cu substrate is ~16 cm−1 .

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All these Raman data demonstrate that the high-quality and atomically layered h-BN films can be grown on Ni foil. Molecules 2016, 21, 1636 5 of 8 Molecules 2016, 21, 1636

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4. Raman signature of h-BN films. (a) Raman spectra of one-layer, two-layer and three-layer Figure 4.Figure Raman signature of h-BN films. (a) Raman spectra of one-layer, two-layer and three-layer h-BN films and bulk h-BN; (b) Peak position (E2g) of h-BN film versus thickness; (c) HWHM of E2g h-BN films and bulk h-BN; (b) Peak position (E2g ) of h-BN film versus thickness; (c) HWHM of E2g peak4.versus thickness. Figure Raman signature of h-BN films. (a) Raman spectra of one-layer, two-layer and three-layer peak versus thickness.

h-BN films and bulk h-BN; (b) Peak position (E2g) of h-BN film versus thickness; (c) HWHM of E2g The XPS spectra of h-BN films with various thicknesses are represented in Figure 5a,b. It was peak versus thickness.

reported that the B 1s with core level of hexagonal boron are nitride locates at ~190 eV and the N It was Thepreviously XPS spectra of h-BN films various thicknesses represented in Figure 5a,b. 1s core level locates at 398 eV. For the single-layered and few-layered h-BN film that was grown, the XPS spectra of h-BN filmslevel with of various thicknesses represented 5a,b. It was previouslyThe reported that the B 1s core hexagonal boronare nitride locatesinatFigure ~190 eV and the N 1s peak from B located from 190.3 to 190.7 eV, and the peak from N locate from 379.9 to 398.2 eV, consistent previously reported thatFor thethe B 1ssingle-layered core level of hexagonal boron nitride locates at ~190 eV and the N peak core level locates at 398 eV. and few-layered h-BN film that was grown, the with the reported values. Furthermore, h-BN powder is used for comparison. The B and N peak 1s core level locates at 398 eV. For the single-layered and few-layered h-BN film that was grown, the from B located 190.3and to 397.9 190.7eV, eV,close andtothe N locate from 398.2 eV, consistent positionsfrom are ~190.3 the peak value from from CVD growth. The379.9 atomictoconcentration of from B located from 190.3 to 190.7 eV, and the peak from N locate from 379.9 to 398.2 eV, consistent with peak thethe reported Furthermore, h-BN powder is stoichiometry used for comparison. B and h-BN thatvalues. was grown is 50% ± 4%, showing an excellent ratio of B andThe N. The slightN peak with the reported values. Furthermore, h-BN powder is usedAlso, for comparison. Theconditions, B and N the peak redundancy may come from nitricto residual during transfer. underThe the same positions are ~190.3 and 397.9 eV,theclose the value from CVD growth. atomic concentration of positions are ~190.3 and 397.9 eV, close to the value from CVD growth. The atomic concentration intensity of h-BN becomes while the h-BN is less than five layers. ratio of B and N. Theofslight the h-BN that was grown is 50%weak ± 4%, showing anfilm excellent stoichiometry the h-BN that was grown is 50% ± 4%, showing an excellent stoichiometry ratio of B and N. The slight redundancy maymay come from residualduring during transfer. theconditions, same conditions, redundancy come fromthe thenitric nitric residual transfer. Also,Also, underunder the same the the intensity becomesweak weakwhile while h-BN is less five layers. intensityof of h-BN h-BN becomes thethe h-BN filmfilm is less than than five layers.

Figure 5. XPS signature of h-BN films. (a,b) XPS spectra of B and N 1s core level of samples in various thicknesses.

Figure 5. XPS signature of h-BN films. (a,b) XPS spectra of B and N 1s core level of samples in various

Figure 5. XPS signature of h-BN films. (a,b) XPS spectra of B and N 1s core level of samples in thicknesses. various thicknesses.

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To find out how the thickness depends on the thicknesses of h-BN, we further collected the To find out how the thickness depends on the thicknesses of h-BN, we further collected the UV-visible absorption spectrum of the atomically layered h-BN film (Figure 6a). The h-BN samples UV-visible absorption spectrum of the atomically layered h-BN film (Figure 6a). The h-BN samples (~1 cm × 1 cm) were first transferred to quartz substrates. Before measurements, a blank quartz (~1 cm × 1 cm) were first transferred to quartz substrates. Before measurements, a blank quartz plate was plate was used to collect the background which was subtracted in the following measurements used to collect the background which was subtracted in the following measurements of as-transferred of as-transferred h-BN films. The bandgap of h-BN films, Eg , was derived from Tauc’s equation h-BN films. The bandgap of h-BN films, Eg, was derived from Tauc’s equation ω2ε = (hω − Eg)2, where 2 2 ω εε =is(hω − Eg ) ,absorbance where ε isand the ω optical absorbance and ω is the angular frequency of the incident the optical is the angular frequency of the incident radiation (ω = 2π/λ, and λ radiation (ω = 2π/λ, and λ is the wavelength) [27–29] Theg optical bandgap Efrom g = hc/λ g can be calculated is the wavelength) [27–29] The optical bandgap Eg = hc/λ can be calculated the intersection point 1/2 from intersection point of ε estimation /λ with the x-axis.the Such estimation requires ε1/2 /λline versus ofthe ε1/2/λ with the x-axis. Such requires plot of ε1/2/λ versus 1/λthe to plot be a of straight in 1/λthe to corresponding be a straight line in the corresponding spectral range, which is clearly shown in Figure spectral range, which is clearly shown in Figure 6b. The optical bandgap for thick6b. Theh-BN optical bandgap fornm), thickclose h-BN eV (220 nm), closereport to the on previous experimental is ~5.6 eV (220 to is the~5.6 previous experimental h-BN bulk [27–29] andreport the on h-BN bulk [27–29] [30,31].the For the few-layered theconsistent optical bandgap calculations [30,31]. and For the the calculations few-layered h-BN, optical bandgap is h-BN, ~5.8 eV, with ouris ~5.8previous eV, consistent with our previous study on the few-layered h-BN grown on Cu substrates [32]. study on the few-layered h-BN grown on Cu substrates [32]. The maximum optical bandgap found in single-layered h-BNiswith a value of ~6.0 eV, as shown in Figure 5b, which excellently meetsin Theismaximum optical bandgap found in single-layered h-BN with a value of ~6.0 eV, as shown the theoretically predicatedmeets valuethe [15]. Figure 5b, which excellently theoretically predicated value [15].

Figure 6. Ultraviolet-visible adsorptionspectra spectraof ofh-BN h-BN films films of of various various thicknesses Figure 6. Ultraviolet-visible adsorption thicknessestaken takenatatroom room 1/2/λ 1/2 temperature. (a) UV adsorption spectra of 1 L, 5 L and thick (>10 L) h-BN films (ε versus 1/λ);1/λ); (b) temperature. (a) UV adsorption spectra of 1 L, 5 L and thick (>10 L) h-BN films (ε /λ versus Calculated optical bandgap for each h-BN film. (b) Calculated optical bandgap for each h-BN film.

4. Materials and Methods 4. Materials and Methods The growth of h-BN was carried out in a quartz tube furnace. A nickel foil with a thickness of The growth of h-BN was carried out in a quartz tube furnace. A nickel foil with a thickness of 25 µm (Purity: 99.9%, American Elements, Inc., 1093 Broxton Ave. Suite 2000, Los Angeles, CA, USA) 25 µm Elements, Inc.,The 1093 Broxton Ave.pump Suitedown 2000,to Los Angeles, USA) was(Purity: put in the99.9%, centerAmerican of the quartz tube furnace. quartz tube was 60 mT duringCA, growth. wasThe puttemperature in the centerwas of the quartz tube furnace. The quartz tube was pump down to 60 mT during raised to 1000 °C in 50 min under the protection of 300 mT Ar/H2 gas flow ◦ C in 50 min under the protection of 300 mT Ar/H growth. The temperature raised toand 1000 (15 vol % H2 balanced bywas 85 vol % Ar), subsequently ammonia borane (NH3-BH3), as precursors, 2 gaswas flowsublimated (15 vol %atH~40–80 balanced by 85 vol % Ar), and subsequently ammonia borane 2 3 -BH °C as the source of B and N. The typical growth time is about (NH 1–2 h. The3 ), ◦ C as the source of B and N. The typical growth time is as precursors, was sublimated at ~40–80 temperature of ammonia borane and the growing time are two main factors to control the thickness about 1–2 h. The temperature offurnace ammonia borane the growing timeslowly. are two main factors of h-BN film. After growth, the cooled downand to room temperature Therefore, the Nito control thickness of h-BN film. After growth, the (PMMA) furnace cooled downh-BN to room temperature slowly. foils the were coated with poly(methyl methacrylate) to transfer to other substrates for further characterizations. resolution transmission electron microscopy JEOL-2100) and Therefore, the Ni foils were High coated with poly(methyl methacrylate) (PMMA)(HRTEM, to transfer h-BN to other elemental (Gatan GIF) were employed for selected area electron diffraction (SAED), (HRTEM, electron substrates formapping further characterizations. High resolution transmission electron microscopy energy loss (EELS) measurements and atomic image. (AFM, JEOL-2100) andspectroscopy elemental mapping (Gatan GIF) were employed for Atomic selectedforce area microscopy electron diffraction Agilent PicoScan 5500 and Veeco Digital Instrument Nanoscope IIIA was used to obtain thicknesses (SAED), electron energy loss spectroscopy (EELS) measurements and atomic image. Atomic force and topographical variations of the 5500 samples. spectroscopy (XPS)IIIA was was performed microscopy (AFM, Agilent PicoScan and X-ray Veecophotoelectron Digital Instrument Nanoscope used to

obtain thicknesses and topographical variations of the samples. X-ray photoelectron spectroscopy

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(XPS) was performed using monochromatic aluminum KR X-rays. Raman spectroscopy (Renishaw inVia, was performed at 514.5 nm laser excitation. 5. Conclusions In this work, we show that by controlling the growing temperature and reaction time, we can produce highly crystalline h-BN film from star-like shapes down to a few layers. The high-quality as-grown h-BN is confirmed by our characterization. The h-BN peaks can be in the Raman spectra and HRTEM shows the clean surface of the h-BN film, as well as the hexagonal lattice. XPS confirms a 1:1 ratio of the B and N elements in the sample. Also, the absorption spectra indicate the bandgap evolution of h-BN from 5.8 eV (bulk) to 6.0 eV (a few layers). Our work will shed light on the large-scale production of high-quality ultra-thin h-BN films for their applications including heat dissipation and anti-oxidation coating. Supplementary Materials: Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/21/ 12/1636/s1. Acknowledgments: This research is supported by the Singapore National Research Foundation through the NRF RF Award No. NRF-RF2013-08, and the Singapore Ministry of Education through the Academic Research Fund under Projects MOE RG10/14 Tier 1. Author Contributions: All authors contributed extensively to this work. J.L., R.G.K. and Z.L. conceived the research. J.L. and R.G.K. synthesized and characterized the samples. Z.L. performed the TEM measurements. J.L., R.G.K. and Z.L. analyzed data, discussed and interpreted detailed results, and wrote the manuscript with input from all authors. Conflicts of Interest: The authors declare no conflict of interest.

References 1.

2.

3. 4.

5.

6. 7.

8. 9.

10.

Shi, Y.; Hamsen, C.; Jia, X.; Kim, K.; Reina, A.; Hofmann, M.; Hsu, A.; Zhang, K.; Li, H.; Juang, Z. Synthesis of Few-Layer Hexagonal Boron Nitride Thin Film by Chemical Vapor Deposition. Nano Lett. 2010, 10, 4134–4139. [CrossRef] [PubMed] Song, L.; Ci, L.; Lu, H.; Sorokin, P.; Jin, C.; Ni, J.; Kvashnin, A.; Kvashnin, D.; Lou, J.; Yakobson, B. Large Scale Growth and Characterization of Atomic Hexagonal Boron Nitride Layers. Nano Lett. 2010, 10, 3209–3215. [CrossRef] [PubMed] Ci, L.; Song, L.; Jin, C.; Jariwala, D.; Wu, D.; Li, Y.; Srivastava, A.; Wang, Z.; Storr, K.; Balicas, L. Atomic layers of hybridized boron nitride and graphene domains. Nat. Mater. 2010, 9, 430–435. [CrossRef] [PubMed] Xue, J.; Sanchez-Yamagishi, J.; Bulmash, D.; Jacquod, P.; Deshpande, A.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.; LeRoy, B.J. Scanning tunnelling microscopy and spectroscopy of ultra-flat graphene on hexagonal boron nitride. Nat. Mater. 2011, 10, 282–285. [CrossRef] [PubMed] Dean, C.R.; Young, A.F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K.L.; et al. Boron nitride substrates for high-quality graphene electronics. Nat. Nano 2010, 5, 722–726. [CrossRef] [PubMed] Corso, M.; Auwärter, W.; Muntwiler, M.; Tamai, A.; Greber, T.; Osterwalder, J. Boron Nitride Nanomesh. Science 2004, 303, 217–220. [CrossRef] [PubMed] Gorbachev, R.V.; Riaz, I.; Nair, R.R.; Jalil, R.; Britnell, L.; Belle, B.D.; Hill, E.W.; Novoselov, K.S.; Watanabe, K.; Taniguchi, T. Hunting for Monolayer Boron Nitride: Optical and Raman Signatures. Small 2011, 7, 465–468. [CrossRef] [PubMed] José Eduardo, P.; Renato Borges, P.; Adalberto, F. Bilayer graphene on h-BN substrate: Investigating the breakdown voltage and tuning the bandgap by electric field. J. Phys. Condens. Matter 2012, 24, 075301. Decker, R.G.; Wang, Y.; Brar, V.W.; Regan, W.; Tsai, H.-Z.; Wu, Q.; Gannett, W.; Zettl, A.; Crommie, M.F. Local Electronic Properties of Graphene on a BN Substrate via Scanning Tunneling Microscopy. Nano Lett. 2011, 11, 2291–2295. [CrossRef] [PubMed] Quhe, R.; Zheng, J.; Luo, G.; Liu, Q.; Qin, R.; Zhou, J.; Yu, D.; Nagase, S.; Mei, W.; Gao, Z. Tunable and sizable band gap of single-layer graphene sandwiched between hexagonal boron nitride. NPG Asia Mater. 2012, 4, e6. [CrossRef]

Molecules 2016, 21, 1636

11.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

8 of 8

Britnell, L.; Gorbachev, R.V.; Jalil, R.; Belle, B.D.; Schedin, F.; Mishchenko, A.; Georgiou, T.; Katsnelson, M.I.; Eaves, L.; Morozov, S.V. Field-Effect Tunneling Transistor Based on Vertical Graphene Heterostructures. Science 2012, 335, 947–950. [CrossRef] [PubMed] Han, W.; Taychatanapat, T.; Hsu, A.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.; Palacios, T. BN/Graphene/BN Transistors for RF Applications. Electron. Device Lett. IEEE 2011, 32, 1209–1211. Bokdam, M.; Khomyakov, P.A.; Brocks, G.; Zhong, Z.; Kelly, P.J. Electrostatic Doping of Graphene through Ultrathin Hexagonal Boron Nitride Films. Nano Lett. 2011, 11, 4631–4635. [CrossRef] [PubMed] Fan, Y.; Zhao, M.; Wang, Z.; Zhang, X.; Zhang, H. Tunable electronic structures of graphene/boron nitride heterobilayers. Appl. Phys. Lett. 2011, 98, 083103. [CrossRef] Kharche, N.; Nayak, S.K. Quasiparticle Band Gap Engineering of Graphene and Graphone on Hexagonal Boron Nitride Substrate. Nano Lett. 2011, 11, 5274–5278. [CrossRef] [PubMed] Ramasubramaniam, A.; Naveh, D.; Towe, E. Tunable Band Gaps in Bilayer Graphene-BN Heterostructures. Nano Lett. 2011, 11, 1070–1075. [CrossRef] [PubMed] Pacile, D.; Meyer, J.C.; Girit, C.O.; Zettl, A. The two-dimensional phase of boron nitride: Few-atomic-layer sheets and suspended membranes. Appl. Phys. Lett. 2008, 92, 133107. [CrossRef] Paine, R.T.; Narula, C.K. Synthetic routes to boron nitride. Chem. Rev. 1990, 90, 73–91. [CrossRef] Taniguchi, T.; Watanabe, K. Synthesis of high-purity boron nitride single crystals under high pressure by using Ba–BN solvent. J. Cryst. Growth 2007, 303, 525–529. [CrossRef] Watanabe, K.; Taniguchi, T.; Kanda, H. Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal. Nat. Mater. 2004, 3, 404–409. [CrossRef] [PubMed] Liu, Z.; Song, L.; Zhao, S.; Huang, J.; Ma, L.; Zhang, J.; Lou, J.; Ajayan, P.M. Direct Growth of Graphene/Hexagonal Boron Nitride Stacked Layers. Nano Lett. 2011, 11, 2032–2037. [CrossRef] [PubMed] Kim, K.K.; Hsu, A.; Jia, X.; Kim, S.; Shi, Y.; Hofmann, M.; Nezich, D.; Joaquin, F.; Nieva, R.; Dresselhaus, M. Synthesis of Monolayer Hexagonal Boron Nitride on Cu Foil Using Chemical Vapor Deposition. Nano Lett. 2011, 12, 161–166. [CrossRef] [PubMed] Wang, B.; Bocquet, M.-L. Monolayer Graphene and h-BN on Metal Substrates as Versatile Templates for Metallic Nanoclusters. J. Phys. Chem. Lett. 2011, 2, 2341–2345. [CrossRef] Lee, Y.; Liu, K.K.; Lu, A.; Wu, C.; Lin, C.; Zhang, W.; Su, W.; Hsu, C.; Lin, T.; Wei, K. Growth selectivity of hexagonal-boron nitride layers on Ni with various crystal orientations. RSC Adv. 2012, 2, 111–115. [CrossRef] Geick, R.; Perry, C.H.; Rupprecht, G. Normal Modes in Hexagonal Boron Nitride. Phys. Rev. 1966, 146, 543. [CrossRef] Babich, I.L. Raman spectrum of hexagonal boron nitride. Theor. Exp. Chem. 1974, 8, 594–595. [CrossRef] Kubota, Y.; Watanabe, K.; Tsuda, O.; Taniguchi, T. Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure. Science 2007, 317, 932–934. [CrossRef] [PubMed] Watanabe, K.; Taniguchi, T. Hexagonal Boron Nitride as a New Ultraviolet Luminescent Material and Its Application. Int. J. Appl. Ceram. Technol. 2011, 8, 977–989. [CrossRef] Li, L.H.; Chen, Y.; Lin, M.-Y.; Glushenkov, A.M.; Cheng, B.-M.; Yu, J. Single deep ultraviolet light emission from boron nitride nanotube film. Appl. Phys. Lett. 2010, 97, 141104. [CrossRef] Blase, X.; Rubio, A.; Louie, S.G.; Cohen, M.L. Quasiparticle band structure of bulk hexagonal boron nitride and related systems. Phys. Rev. B 1995, 51, 6868–6875. [CrossRef] Hoffman, D.M.; Doll, G.L.; Eklund, P.C. Optical properties of pyrolytic boron nitride in the energy range 0.05–10 eV. Phys. Rev. B 1984, 30, 6051–6056. [CrossRef] Lee, K.H.; Shin, H.-J.; Lee, J.; Lee, I.-Y.; Kim, G.-H.; Choi, J.-Y.; Kim, S.-W. Large-Scale Synthesis of High-Quality Hexagonal Boron Nitride Nanosheets for Large-Area Graphene Electronics. Nano Lett. 2012, 12, 714–718. [CrossRef] [PubMed]

Sample Availability: Samples of the h-BN films are available from the authors. © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).