- Email: [email protected]
Atomically thin boron nitride nanodisks
Materials Letters 106 (2013) 409–412
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Atomically thin boron nitride nanodisks Lu Hua Li a,n, Ling Li a,b, Xiujuan J. Dai a, Ying Chen a a b
Institute for Frontier Materials, Deakin University, Geelong Waurn Ponds Campus, Victoria 3216, Australia MEMS Center, Harbin Institute of Technology, Harbin 150001, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 January 2013 Accepted 18 May 2013 Available online 28 May 2013
Boron nitride (BN) nanomaterials are of great scientific and technological interest. Here we report formation of BN nanodisks which are only 1–2 nm thick and have a relative uniform diameter of 30– 40 nm. The nanodisks are produced on surfaces of BN nanosheets by argon plasma treatment at room temperature. The nanodisks show a polycrystalline structure and some levels of oxidation. It is found that the oxidation (patched B–O bond) may serve as masks during the plasma etching and lead to the formation of the nanodisks. & 2013 Elsevier B.V. All rights reserved.
Keywords: Boron nitride (BN) nanodisks Nanosheets Plasma NEXAFS
1. Introduction
2. Experimental
Hexagonal boron nitride (hBN) has an analogous structure to graphite and therefore can also form various nanostructures of great interest, including zero-dimensional (0D) Buckyballs [1,2] and nanoparticles [3,4], 1D nanotubes [5,6], nanofiber [7] and nanoribbons [8–10] as well as 2D nanosheets [11]. BN nanomaterials have many properties different from their carbon counterparts. BN are electrical insulating and suitable for optoelectronics. BN nanostructures are more thermal stable in air and therefore more preferable for high temperature applications. Similar to the cases of carbon nanomaterials, physical and chemical properties of BN nanomaterials vary greatly depending on dimensionalities. For example, BN monolayers have a different band structure to 3D hBN crystals [12] and BN nanoribbons could be metallic and magnetic [13]. Many efforts have been devoted to the preparation of 0D BN structures. High-temperature or laser-assisted chemical reactions can produce spherical BN particles [14,15]. Recently, spray pyrolysis, solid-state metathesis and chemical vapour deposition methods have been demonstrated to synthesize monodisperse BN nanoparticles [3,16,17], and the solid-state method is able to well control the particle size. Here, we report the formation of atomically thin partially oxidized BN nanodisks which are only 1–2 nm in thickness and 30–40 nm in diameter. The nanodisks are produced on hBN crystal surfaces after Ar plasma treatment.
Commercial hBN particles with an average size of ∼45 mm (Momentive, PT110) were used as starting material. X-ray diffraction (XRD, Philips 3020) confirmed that the particles contain only hexagonal phase and energy dispersive x-ray spectroscopy (EDX) showed a chemical purity ∼99.9%. The hBN particles were first deposited on a 300 nm oxide covered silicon wafer (SiO2/Si) by the mechanical cleavage method which has been used for graphene production [11] as well as crystal alignment [18,19]. That is, the hBN particles were repeatedly exfoliated between Scotch tape before transferred to the SiO2/Si substrate. The exfoliated hBN crystals on the substrate were heated in air at 450 1C for 2 h to remove tape residues. Low-pressure plasma treatment was carried out in a home-built system which has an inductively coupled radio frequency (RF, 13.56 MHz) plasma reactor, a cylindrical glass reaction chamber enclosed in a Faraday cage, and an antenna around the chamber to transfer RF power into the reaction area. Plasma treatments were conducted in continuous mode with a power of 200 W for 10 min at room-temperature. Before the plasma treatment, the reaction chamber was repeatedly vacuumed and cleaned by Ar gas (purity 99.992%) to remove air. During the plasma treatment, the pressure was kept constant at 5.0 10−2 mbar. The sample surface morphology was characterized by a Cypher atomic force microscopy (AFM) using NCSTR (spring constant 7.4 N/m, NanoWorld) Si cantilever in tapping mode in air. The transmission electron microscopy (TEM) studies used a JEOL-2010 machine. The x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) measurements were performed in ultrahigh vacuum chamber (∼10−10 mbar) at the undulator soft x-ray spectroscopy beamline of the Australian Synchrotron, Victoria, Australia. This beamline is
n
Corresponding author. Tel.: +61 3 52272589. E-mail addresses: [email protected], [email protected] (L.H. Li). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.090
410
L.H. Li et al. / Materials Letters 106 (2013) 409–412
equipped with a hemispherical electron analyzer (SPECS Phoibos 150) and a microchannel plate detector. The photon energy for XPS measurement was 700 eV and the raw XPS data were corrected using the binding energy position of C–C at 284.6 eV. The raw NEXAFS data were normalized to the photoelectron current of the photon beam, measured on an Au grid.
3. Results and discussion After the hBN particles transferred to SiO2/Si by the Scotch tape method, they are scattered and mostly aligned with the (002) plane parallel to the surface of the substrate (see Supporting Information, Figure S1) [18,19]. After the Ar plasma treatment, nanodisks are found on the surface of the hBN particles (Fig. 1a–c). Few layer BN sheets of small thickness can completely turn to nanodisks (Fig. 1d–e). The nanodisks are located where the hBN particles or nanosheets are, which indicates that the nanodisks are formed directly from hBN rather than plasma vaporization and redeposition. Actually, the room-temperature plasma used is not energetically strong enough to evaporate hBN. The nanodisks are 30–40 nm in diameter and 1–2 nm in thickness. The nanodisks can be separated from BN particles or sheets by ultrasonication in liquid (e.g. water or ethanol) and collected for TEM investigation [20]. The TEM studies confirm the size of the nanodisks (Fig. 2a). At high magnification the nanodisks have a low contrast due to their small thicknesses (Fig. 2b). Lattice fringes representing (002) spacing in hBN can sometimes be resolved from the nanodisks (inset of Fig. 2b), revealing a polycrystalline nature. The nanodisks only cover surfaces of the hBN particles, so surface sensitive techniques including XPS and NEXAFS are used to analyze their chemical composition. The XPS spectrum (Fig. 3a) shows that besides B and N, there are also Si, O and C signals (the C signal is probably from surface contamination). Due to the presence of O signal from the silicon oxide substrate, it is not straightforward to determine the O level in the material from the XPS. NEXAFS is a more powerful technique that can precisely probe chemical environments around B atoms. Fig. 3b shows the NEXAFS spectra of partial electron yield (PEY) in B K-edge region
from the sample before and after Ar plasma treatment. Before the plasma, the hBN shows a typical NEXAFS spectrum: a sharp πn resonance at 192.0 eV and two weak satellite peaks centered at 192.6 and 193.2 eV. The sharp πn resonance represents transitions of core level electrons to the unoccupied antibonding πn orbitals in a B–3N hexagonal ring structure. The asymmetric split of this resonance is possibly due to coupling of core excitons to lattice vibrations [19]. The two satellite peaks representing one and two O atoms substituting N atoms in a hexagonal ring (i.e. B–2N–O and B–N–2O) are often observed from hBN particles [21], BN thin films [22], nanotubes [23,24] and nanosheets [25]. These indicate that the hBN particles before plasma mainly have a standard hBN structure with a small amount of point defects in the forms of one and two nitrogen vacancies decorated by O atoms. After the Ar plasma treatment, the πn resonance at 192.0 eV remains, but the two satellite peaks become more pronounced and a new peak at 194.2 eV corresponding to the scenario of three O atoms replacing all N atoms in a hexagonal ring (B–3O or B2O3 bond) appears [21]. Because PEY signals are surface sensitive (from the 1 to 2 nm top surface of a sample), they can be deemed as signals from the nanodisks. These changes clearly show that the nanodisks still mainly have a hBN structure (consistent with the TEM results), but are partially oxidized. The oxidation may come from gas impurity (including air residue in the reaction chamber), silicon oxide substrate as well as exposure to air (it is found that point defects in hBN can be healed via oxidation after exposed to air) [21]. Surface morphology change of the hBN particles after different periods of plasma treatment has been tested for better understanding of the mechanism of nanodisk formation. Before the plasma treatment, the hBN surface is atomically flat with a roughness (Ra) of 104.7 pm (Fig. 4a). After 5 min plasma treatment under the same condition, the surface becomes corrugated, reflected by the almost tripled roughness value of 296.7 pm (Fig. 4b). The increased roughness which has been observed from plasma treatments on both graphite and BN before [26,27] can be attributed to B–N bond breaking as well as chemisorption of O atoms over B–N bond, which elongates the B–N bond length and uplifts the base plane of hBN [28]. The O chemisorption on the surface of hBN is energetically more favorable to stay close to each
Fig. 1. Typical AFM images of the surfaces of two hBN particles after Ar plasma treatment for 10 min.
L.H. Li et al. / Materials Letters 106 (2013) 409–412
411
Fig. 2. TEM images of the BN nanodisks on carbon supporting film (a). At higher magnifications the nanodisks become almost transparent due to their small thickness and a nanodisk (circled) is shown in (b). The inset shows the (002) lattice fringes of hBN.
Fig. 3. (a) XPS spectrum of the hBN particles after the Ar plasma treatment; (b) NEXAFS PEY signals of the hBN particles before and after the plasma treatment.
Fig. 4. AFM images of the surface of hBN particles: (a) before plasma treatment; (b) after Ar plasma treatment for 5 min and (c) for 7.5 min.
other rather than dispersed homogeneously [28]. As a result, small oxidized patches become to form on hBN surface. During this process, O atoms also diffuse into hBN and replace N atoms. The B–O bond is stronger than the B–N bond because O is more electronegative than N: the binding energy of B–N is smaller than 8 eV [29]; while that of B–O is larger than 9 eV [30]. This means that the B–N bond is more vulnerable under the plasma bombardment. With the increase of plasma treating time to 7.5 min, B–O covered nanodisks start to form via the etching of the B–N bond surrounding B–O patches (Fig. 4c). In other words, the patches of B–O bond serve as masks on top of B–N bond to allow nanodisks of relative uniform sizes to form. So oxidation plays an important role during nanodisk formation. The O, we believe, is mainly from the silicon oxide substrate due to plasma induced decomposition. In a control experiment in which hBN particles are placed on a pure Si wafer instead of oxide covered wafer, we find that no nanodisk is produced and etched holes are found on the hBN particle surfaces (see Supporting information, Figure S2). However, if a small amount of air is purposely left in the reaction chamber while pure
Si wafers are used, nanodisks can be produced again. These experiments justify the proposed mechanism that O helps the formation of the nanodisks. However, if pure O2 plasma is used, no nanodisk is formed (see Supporting information, Figure S3). This suggests that a suitable amount of O is important to the formation of the nanodisks.
4. Conclusions In summary, atomically thin partially oxidized BN nanodisks of 1–2 nm in thickness and 30–40 nm in diameter have been produced from hBN particles by Ar plasma treatment. New properties and applications are normally associated with the decrease of dimensions. Similarly, these 0D BN nanodisks could possess different optical, electrical, magnetic and tribological properties than other BN nanostructures. It should also be mentioned that few layer BN nanosheets up to centimeter sizes have been successfully grown on different substrates [31,32]. If these BN
412
L.H. Li et al. / Materials Letters 106 (2013) 409–412
nanosheets are used for the plasma treatment, it is possible to produce surfaces of large areas fully covered by the BN nanodisks.
Acknowledgments We thank the assistance from Mr. Zhiqiang Chen during plasma treatments and Dr. Bruce Cowie during XPS and NEXAFS measurements. This research was partly undertaken on the soft x-ray beamline at the Australian Synchrotron, Victoria, Australia.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.05.090.
References [1] Stephan O, Bando Y, Loiseau A, Willaime F, Shramchenko N, Tamiya T, et al. Appl Phys A 1998;67:107–11. [2] Golberg D, Bando Y, Stephan O, Kurashima K. Appl Phys Lett 1998;73:2441–3. [3] Lian G, Zhang X, Zhu LL, Tan M, Cui DL, Wang QL. J Mater Chem 2010;20:3736–42. [4] Bernard S, Salles V, Li JP, Brioude A, Bechelany M, Demirci UB, et al. J Mater Chem 2011;21:8694–9. [5] Chopra NG, Luyken RJ, Cherrey K, Crespi VH, Cohen ML, Louie SG, et al. Science 1995;269:966–7. [6] Li LH, Chen Y, Glushenkov AM. Nanotechnology 2010;21:105601. [7] Salles V, Bernard S, Brioude A, Cornu D, Miele P. Nanoscale 2010;2:215–7. [8] Zeng HB, Zhi CY, Zhang ZH, Wei XL, Wang XB, Guo WL, et al. Nano Lett 2010;10:5049–55.
[9] Erickson KJ, Gibb AL, Sinitskii A, Rousseas M, Alem N, Tour JM, et al. Nano Lett 2011;11:3221–6. [10] Bechelany M, Brioude A, Stadelmann P, Bernard S, Cornu D, Miele P. J Phys Chem C 2008;112:18325–30. [11] Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV, et al. Proc Natl Acad Sci 2005;102:10451–3. [12] Ribeiro RM. Peres NMR. Phys Rev B 2011;83:235312. [13] Barone V, Peralta JE. Nano Lett 2008;8:2210–4. [14] Lindquist DA, Kodas TT, Smith DM, Xiu XM, Hietala SL, Paine RT. J Am Ceram Soc 1991;74:3126–8. [15] Baraton MI, Boulanger L, Cauchetier M, Lorenzelli V, Luce M, Merle T, et al. J Eur Ceram Soc 1994;13:371–8. [16] Tang CC, Bando Y, Huang Y, Zhi CY, Golberg D. Adv Funct Mater 2008;18:3653–61. [17] Salles V, Bernard S, Li JP, Brioude A, Chehaidi S, Foucaud S, et al. Chem Mater 2009;21:2920–9. [18] Li LH, Chen Y, Cheng BM, Lin MY, Chou SL, Peng YC. Appl Phys Lett 2012;100:261108. [19] Li LH, Petravic M, Cowie BCC, Xing T, Peter R, Chen Y, et al. Appl Phys Lett 2012;101:191604. [20] Lin Y, Williams TV, Xu TB, Cao W, Elsayed-Ali HE, Connell JW. J Phys Chem C 2011;115:2679–85. [21] Caretti I, Jimenez I. J Appl Phys 2011;110:023511. [22] Niibe M, Miyamoto K, Mitamura T, Mochiji K. J Vac Sci Technol A 2010;28:1157–60. [23] Petravic M, Peter R, Kavre I, Li LH, Chen Y, Fan LJ, et al. Phys Chem Chem Phys 2010;12:15349–53. [24] Li L, Li LH, Chen Y, Dai X, Xing T, Petravic M, et al. Nanoscale Res Lett 2012;7:417. [25] Li LH, Chen Y, Behan G, Zhang H, Petravic M, Glushenkov AM. J Mater Chem 2011;21:11862–6. [26] Zhang J, Cui Q, Li X, He Z, Li W, Ma Y, et al. Chem Phys Lett 2004;399:451–5. [27] Paredes JI, Martinez-Alonso A, Tascon JMD. J Mater Chem 2000;10:1585–91. [28] Zhao Y, Wu XJ, Yang JL, Zeng XC. Phys Chem Chem Phys 2012;14:5545–50. [29] Ooi N, Rairkar A, Lindsley L, Adams JB. J Phys Condens Matter 2006;18:97–115. [30] Wenthold PG, Kim JB, Jonas KL, Lineberger WC. J Phys Chem A 1997;101:4472–4. [31] Shi YM, Hamsen C, Jia XT, Kim KK, Reina A, Hofmann M, et al. Nano Lett 2010;10:4134–9. [32] Song L, Ci LJ, Lu H, Sorokin PB, Jin CH, Ni J, et al. Nano Lett 2010;10:3209–15.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Atomically thin boron nitride nanodisks Lu Hua Li a,n, Ling Li a,b, Xiujuan J. Dai a, Ying Chen a a b
Institute for Frontier Materials, Deakin University, Geelong Waurn Ponds Campus, Victoria 3216, Australia MEMS Center, Harbin Institute of Technology, Harbin 150001, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 January 2013 Accepted 18 May 2013 Available online 28 May 2013
Boron nitride (BN) nanomaterials are of great scientific and technological interest. Here we report formation of BN nanodisks which are only 1–2 nm thick and have a relative uniform diameter of 30– 40 nm. The nanodisks are produced on surfaces of BN nanosheets by argon plasma treatment at room temperature. The nanodisks show a polycrystalline structure and some levels of oxidation. It is found that the oxidation (patched B–O bond) may serve as masks during the plasma etching and lead to the formation of the nanodisks. & 2013 Elsevier B.V. All rights reserved.
Keywords: Boron nitride (BN) nanodisks Nanosheets Plasma NEXAFS
1. Introduction
2. Experimental
Hexagonal boron nitride (hBN) has an analogous structure to graphite and therefore can also form various nanostructures of great interest, including zero-dimensional (0D) Buckyballs [1,2] and nanoparticles [3,4], 1D nanotubes [5,6], nanofiber [7] and nanoribbons [8–10] as well as 2D nanosheets [11]. BN nanomaterials have many properties different from their carbon counterparts. BN are electrical insulating and suitable for optoelectronics. BN nanostructures are more thermal stable in air and therefore more preferable for high temperature applications. Similar to the cases of carbon nanomaterials, physical and chemical properties of BN nanomaterials vary greatly depending on dimensionalities. For example, BN monolayers have a different band structure to 3D hBN crystals [12] and BN nanoribbons could be metallic and magnetic [13]. Many efforts have been devoted to the preparation of 0D BN structures. High-temperature or laser-assisted chemical reactions can produce spherical BN particles [14,15]. Recently, spray pyrolysis, solid-state metathesis and chemical vapour deposition methods have been demonstrated to synthesize monodisperse BN nanoparticles [3,16,17], and the solid-state method is able to well control the particle size. Here, we report the formation of atomically thin partially oxidized BN nanodisks which are only 1–2 nm in thickness and 30–40 nm in diameter. The nanodisks are produced on hBN crystal surfaces after Ar plasma treatment.
Commercial hBN particles with an average size of ∼45 mm (Momentive, PT110) were used as starting material. X-ray diffraction (XRD, Philips 3020) confirmed that the particles contain only hexagonal phase and energy dispersive x-ray spectroscopy (EDX) showed a chemical purity ∼99.9%. The hBN particles were first deposited on a 300 nm oxide covered silicon wafer (SiO2/Si) by the mechanical cleavage method which has been used for graphene production [11] as well as crystal alignment [18,19]. That is, the hBN particles were repeatedly exfoliated between Scotch tape before transferred to the SiO2/Si substrate. The exfoliated hBN crystals on the substrate were heated in air at 450 1C for 2 h to remove tape residues. Low-pressure plasma treatment was carried out in a home-built system which has an inductively coupled radio frequency (RF, 13.56 MHz) plasma reactor, a cylindrical glass reaction chamber enclosed in a Faraday cage, and an antenna around the chamber to transfer RF power into the reaction area. Plasma treatments were conducted in continuous mode with a power of 200 W for 10 min at room-temperature. Before the plasma treatment, the reaction chamber was repeatedly vacuumed and cleaned by Ar gas (purity 99.992%) to remove air. During the plasma treatment, the pressure was kept constant at 5.0 10−2 mbar. The sample surface morphology was characterized by a Cypher atomic force microscopy (AFM) using NCSTR (spring constant 7.4 N/m, NanoWorld) Si cantilever in tapping mode in air. The transmission electron microscopy (TEM) studies used a JEOL-2010 machine. The x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) measurements were performed in ultrahigh vacuum chamber (∼10−10 mbar) at the undulator soft x-ray spectroscopy beamline of the Australian Synchrotron, Victoria, Australia. This beamline is
n
Corresponding author. Tel.: +61 3 52272589. E-mail addresses: [email protected], [email protected] (L.H. Li). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.090
410
L.H. Li et al. / Materials Letters 106 (2013) 409–412
equipped with a hemispherical electron analyzer (SPECS Phoibos 150) and a microchannel plate detector. The photon energy for XPS measurement was 700 eV and the raw XPS data were corrected using the binding energy position of C–C at 284.6 eV. The raw NEXAFS data were normalized to the photoelectron current of the photon beam, measured on an Au grid.
3. Results and discussion After the hBN particles transferred to SiO2/Si by the Scotch tape method, they are scattered and mostly aligned with the (002) plane parallel to the surface of the substrate (see Supporting Information, Figure S1) [18,19]. After the Ar plasma treatment, nanodisks are found on the surface of the hBN particles (Fig. 1a–c). Few layer BN sheets of small thickness can completely turn to nanodisks (Fig. 1d–e). The nanodisks are located where the hBN particles or nanosheets are, which indicates that the nanodisks are formed directly from hBN rather than plasma vaporization and redeposition. Actually, the room-temperature plasma used is not energetically strong enough to evaporate hBN. The nanodisks are 30–40 nm in diameter and 1–2 nm in thickness. The nanodisks can be separated from BN particles or sheets by ultrasonication in liquid (e.g. water or ethanol) and collected for TEM investigation [20]. The TEM studies confirm the size of the nanodisks (Fig. 2a). At high magnification the nanodisks have a low contrast due to their small thicknesses (Fig. 2b). Lattice fringes representing (002) spacing in hBN can sometimes be resolved from the nanodisks (inset of Fig. 2b), revealing a polycrystalline nature. The nanodisks only cover surfaces of the hBN particles, so surface sensitive techniques including XPS and NEXAFS are used to analyze their chemical composition. The XPS spectrum (Fig. 3a) shows that besides B and N, there are also Si, O and C signals (the C signal is probably from surface contamination). Due to the presence of O signal from the silicon oxide substrate, it is not straightforward to determine the O level in the material from the XPS. NEXAFS is a more powerful technique that can precisely probe chemical environments around B atoms. Fig. 3b shows the NEXAFS spectra of partial electron yield (PEY) in B K-edge region
from the sample before and after Ar plasma treatment. Before the plasma, the hBN shows a typical NEXAFS spectrum: a sharp πn resonance at 192.0 eV and two weak satellite peaks centered at 192.6 and 193.2 eV. The sharp πn resonance represents transitions of core level electrons to the unoccupied antibonding πn orbitals in a B–3N hexagonal ring structure. The asymmetric split of this resonance is possibly due to coupling of core excitons to lattice vibrations [19]. The two satellite peaks representing one and two O atoms substituting N atoms in a hexagonal ring (i.e. B–2N–O and B–N–2O) are often observed from hBN particles [21], BN thin films [22], nanotubes [23,24] and nanosheets [25]. These indicate that the hBN particles before plasma mainly have a standard hBN structure with a small amount of point defects in the forms of one and two nitrogen vacancies decorated by O atoms. After the Ar plasma treatment, the πn resonance at 192.0 eV remains, but the two satellite peaks become more pronounced and a new peak at 194.2 eV corresponding to the scenario of three O atoms replacing all N atoms in a hexagonal ring (B–3O or B2O3 bond) appears [21]. Because PEY signals are surface sensitive (from the 1 to 2 nm top surface of a sample), they can be deemed as signals from the nanodisks. These changes clearly show that the nanodisks still mainly have a hBN structure (consistent with the TEM results), but are partially oxidized. The oxidation may come from gas impurity (including air residue in the reaction chamber), silicon oxide substrate as well as exposure to air (it is found that point defects in hBN can be healed via oxidation after exposed to air) [21]. Surface morphology change of the hBN particles after different periods of plasma treatment has been tested for better understanding of the mechanism of nanodisk formation. Before the plasma treatment, the hBN surface is atomically flat with a roughness (Ra) of 104.7 pm (Fig. 4a). After 5 min plasma treatment under the same condition, the surface becomes corrugated, reflected by the almost tripled roughness value of 296.7 pm (Fig. 4b). The increased roughness which has been observed from plasma treatments on both graphite and BN before [26,27] can be attributed to B–N bond breaking as well as chemisorption of O atoms over B–N bond, which elongates the B–N bond length and uplifts the base plane of hBN [28]. The O chemisorption on the surface of hBN is energetically more favorable to stay close to each
Fig. 1. Typical AFM images of the surfaces of two hBN particles after Ar plasma treatment for 10 min.
L.H. Li et al. / Materials Letters 106 (2013) 409–412
411
Fig. 2. TEM images of the BN nanodisks on carbon supporting film (a). At higher magnifications the nanodisks become almost transparent due to their small thickness and a nanodisk (circled) is shown in (b). The inset shows the (002) lattice fringes of hBN.
Fig. 3. (a) XPS spectrum of the hBN particles after the Ar plasma treatment; (b) NEXAFS PEY signals of the hBN particles before and after the plasma treatment.
Fig. 4. AFM images of the surface of hBN particles: (a) before plasma treatment; (b) after Ar plasma treatment for 5 min and (c) for 7.5 min.
other rather than dispersed homogeneously [28]. As a result, small oxidized patches become to form on hBN surface. During this process, O atoms also diffuse into hBN and replace N atoms. The B–O bond is stronger than the B–N bond because O is more electronegative than N: the binding energy of B–N is smaller than 8 eV [29]; while that of B–O is larger than 9 eV [30]. This means that the B–N bond is more vulnerable under the plasma bombardment. With the increase of plasma treating time to 7.5 min, B–O covered nanodisks start to form via the etching of the B–N bond surrounding B–O patches (Fig. 4c). In other words, the patches of B–O bond serve as masks on top of B–N bond to allow nanodisks of relative uniform sizes to form. So oxidation plays an important role during nanodisk formation. The O, we believe, is mainly from the silicon oxide substrate due to plasma induced decomposition. In a control experiment in which hBN particles are placed on a pure Si wafer instead of oxide covered wafer, we find that no nanodisk is produced and etched holes are found on the hBN particle surfaces (see Supporting information, Figure S2). However, if a small amount of air is purposely left in the reaction chamber while pure
Si wafers are used, nanodisks can be produced again. These experiments justify the proposed mechanism that O helps the formation of the nanodisks. However, if pure O2 plasma is used, no nanodisk is formed (see Supporting information, Figure S3). This suggests that a suitable amount of O is important to the formation of the nanodisks.
4. Conclusions In summary, atomically thin partially oxidized BN nanodisks of 1–2 nm in thickness and 30–40 nm in diameter have been produced from hBN particles by Ar plasma treatment. New properties and applications are normally associated with the decrease of dimensions. Similarly, these 0D BN nanodisks could possess different optical, electrical, magnetic and tribological properties than other BN nanostructures. It should also be mentioned that few layer BN nanosheets up to centimeter sizes have been successfully grown on different substrates [31,32]. If these BN
412
L.H. Li et al. / Materials Letters 106 (2013) 409–412
nanosheets are used for the plasma treatment, it is possible to produce surfaces of large areas fully covered by the BN nanodisks.
Acknowledgments We thank the assistance from Mr. Zhiqiang Chen during plasma treatments and Dr. Bruce Cowie during XPS and NEXAFS measurements. This research was partly undertaken on the soft x-ray beamline at the Australian Synchrotron, Victoria, Australia.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.05.090.
References [1] Stephan O, Bando Y, Loiseau A, Willaime F, Shramchenko N, Tamiya T, et al. Appl Phys A 1998;67:107–11. [2] Golberg D, Bando Y, Stephan O, Kurashima K. Appl Phys Lett 1998;73:2441–3. [3] Lian G, Zhang X, Zhu LL, Tan M, Cui DL, Wang QL. J Mater Chem 2010;20:3736–42. [4] Bernard S, Salles V, Li JP, Brioude A, Bechelany M, Demirci UB, et al. J Mater Chem 2011;21:8694–9. [5] Chopra NG, Luyken RJ, Cherrey K, Crespi VH, Cohen ML, Louie SG, et al. Science 1995;269:966–7. [6] Li LH, Chen Y, Glushenkov AM. Nanotechnology 2010;21:105601. [7] Salles V, Bernard S, Brioude A, Cornu D, Miele P. Nanoscale 2010;2:215–7. [8] Zeng HB, Zhi CY, Zhang ZH, Wei XL, Wang XB, Guo WL, et al. Nano Lett 2010;10:5049–55.
[9] Erickson KJ, Gibb AL, Sinitskii A, Rousseas M, Alem N, Tour JM, et al. Nano Lett 2011;11:3221–6. [10] Bechelany M, Brioude A, Stadelmann P, Bernard S, Cornu D, Miele P. J Phys Chem C 2008;112:18325–30. [11] Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV, et al. Proc Natl Acad Sci 2005;102:10451–3. [12] Ribeiro RM. Peres NMR. Phys Rev B 2011;83:235312. [13] Barone V, Peralta JE. Nano Lett 2008;8:2210–4. [14] Lindquist DA, Kodas TT, Smith DM, Xiu XM, Hietala SL, Paine RT. J Am Ceram Soc 1991;74:3126–8. [15] Baraton MI, Boulanger L, Cauchetier M, Lorenzelli V, Luce M, Merle T, et al. J Eur Ceram Soc 1994;13:371–8. [16] Tang CC, Bando Y, Huang Y, Zhi CY, Golberg D. Adv Funct Mater 2008;18:3653–61. [17] Salles V, Bernard S, Li JP, Brioude A, Chehaidi S, Foucaud S, et al. Chem Mater 2009;21:2920–9. [18] Li LH, Chen Y, Cheng BM, Lin MY, Chou SL, Peng YC. Appl Phys Lett 2012;100:261108. [19] Li LH, Petravic M, Cowie BCC, Xing T, Peter R, Chen Y, et al. Appl Phys Lett 2012;101:191604. [20] Lin Y, Williams TV, Xu TB, Cao W, Elsayed-Ali HE, Connell JW. J Phys Chem C 2011;115:2679–85. [21] Caretti I, Jimenez I. J Appl Phys 2011;110:023511. [22] Niibe M, Miyamoto K, Mitamura T, Mochiji K. J Vac Sci Technol A 2010;28:1157–60. [23] Petravic M, Peter R, Kavre I, Li LH, Chen Y, Fan LJ, et al. Phys Chem Chem Phys 2010;12:15349–53. [24] Li L, Li LH, Chen Y, Dai X, Xing T, Petravic M, et al. Nanoscale Res Lett 2012;7:417. [25] Li LH, Chen Y, Behan G, Zhang H, Petravic M, Glushenkov AM. J Mater Chem 2011;21:11862–6. [26] Zhang J, Cui Q, Li X, He Z, Li W, Ma Y, et al. Chem Phys Lett 2004;399:451–5. [27] Paredes JI, Martinez-Alonso A, Tascon JMD. J Mater Chem 2000;10:1585–91. [28] Zhao Y, Wu XJ, Yang JL, Zeng XC. Phys Chem Chem Phys 2012;14:5545–50. [29] Ooi N, Rairkar A, Lindsley L, Adams JB. J Phys Condens Matter 2006;18:97–115. [30] Wenthold PG, Kim JB, Jonas KL, Lineberger WC. J Phys Chem A 1997;101:4472–4. [31] Shi YM, Hamsen C, Jia XT, Kim KK, Reina A, Hofmann M, et al. Nano Lett 2010;10:4134–9. [32] Song L, Ci LJ, Lu H, Sorokin PB, Jin CH, Ni J, et al. Nano Lett 2010;10:3209–15.