pubs.acs.org/NanoLett
“White Graphenes”: Boron Nitride Nanoribbons via Boron Nitride Nanotube Unwrapping Haibo Zeng,*,† Chunyi Zhi,† Zhuhua Zhang,‡ Xianlong Wei,† Xuebin Wang,† Wanlin Guo,*,‡ Yoshio Bando,† and Dmitri Golberg*,† †
International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, and ‡ Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China ABSTRACT Inspired by rich physics and functionalities of graphenes, scientists have taken an intensive interest in two-dimensional (2D) crystals of h-BN (analogue of graphite, so-called “white” graphite). Recent calculations have predicted the exciting potentials of BN nanoribbons in spintronics due to tunable magnetic and electrical properties; however no experimental evidence has been provided since fabrication of such ribbons remains a challenge. Here, we show that few- and single-layered BN nanoribbons, mostly terminated with zigzag edges, can be produced under unwrapping multiwalled BN nanotubes through plasma etching. The interesting stepwise unwrapping and intermediate states were observed and analyzed. Opposed to insulating primal tubes, the nanoribbons become semiconducting due to doping-like conducting edge states and vacancy defects, as revealed by structural analyses and ab initio simulations. This study paves the way for BN nanoribbon production and usage as functional semiconductors with a wide range of applications in optoelectronics and spintronics. KEYWORDS “White graphene”, boron nitride (BN), nanoribbon, nanosheet, two-dimensional crystal
T
he excellent performance of graphene1,2 and amazing edge states of its ribbons3-5 have aroused general curiosity about two-dimensional (2D) crystals of other materials. Hexagonal boron nitride (h-BN), a structural analogue of graphite, has attracted special interests due to its superb thermal and chemical stabilities and intrinsic electrical insulation.6-15 In addition to many traditional propertiesdriven applications, for example, in highly thermal-conductive and insulating composite materials,9 some novel physical properties can be emerged in atomically thin BN 2D crystals due to the small dimensions and special edge structures. For example, it was reported that magnetism could be induced by the replacement of B or N with Be, B, C, N, O, Al, and Si, or with vacancy defects.16-18 On the other hand, atomically thin BN nanoribbons (NRs) were predicted to have narrowed band gap and improved conductivity tuned by a transverse electric field or edge structure.17,19-23 These recent calculations have envisaged magnetic and conductive BNNRs, and hence some exciting potential applications in optoelectronics and spintronics. However, no experimental evidence has been provided since fabrication of such atomically thin ribbons still remains a grand challenge in the BN nanomaterials field.
Compared with graphite, h-BN is much more oxidationresistant (stable up to 900 °C in air) and intercalationresistant due to its weaker electropositivity.24 These make h-BN hard to be an electron acceptor and entail large difficulty to exfoliate h-BN into atomically thin 2D structures using conventional routes.25 Recently, fabrication of BN nanosheets has been attempted by mechanical exfoliation, liquid phase sonication, reacting boric acid with urea, and chemical vapor deposition (CVD),7-14 but the large scale fabrication of BN sheets with a large percentage of single or few layer products is still a challenge, and there have been no reports yet on BNNRs. The experimental difficulties and intrinsic insulating character of BN notably depress research enthusiasm with respect to h-BN nanoderivatives.15 Recently, unzipping carbon nanotubes through catalytic cutting or plasma etching has been explored to fabricate high-quality graphene NRs, but such procedure has solely been limited to carbon materials until now.26-30 Here, we show that few- and single-layered BNNRs, socalled “white graphenes”, mostly terminated with zigzag edges, can be produced by unwrapping multiwalled BN nanotubes through delicate plasma etching. The interesting stepwise unwrapping was demonstrated through careful observation on the intermediate states. The improved conductivity was highlighted according to the observed edge structures and ab initio simulations. This study should pave the way for BN nanoribbon production and utilization as functional semiconductors for a wide range of optoelectronic and spintronic devices.
* To whom correspondence should be addressed. E-mail: (H.Z.) ZENG.Haibo@ nims.go.jp,
[email protected]; (W.G)
[email protected]; (D.G.) GOLBERG.Dmitri@ nims.go.jp. Received for review: 09/14/2010 Published on Web: 10/28/2010 © 2010 American Chemical Society
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cut into ribbonlike structures (shown with a slight tilt) are displayed in Figure 1c. The slight inclination of NRs is in an accord with the bottom stripes of primal tubes. We then used atomic force microscopy (AFM) to characterize the overall transition process and the typical intermediate products as shown in Figure 2. During AFM-runs, the NRs were easily distinguished from the NTs because of their obviously smaller visible heights. Three marginally etched tubes (80 W; 5 s) lie on the substrate in Figure 2a. According to the corresponding height profile (Figure 2c), their diameters are similar to the primal NTs, whereas the heights become notably smaller (h < d). This clearly demonstrates that the top part of many walls has been cut away, resulting in the opened tube state. With the heavier etching (100 W; 15 s), more and more top wall domains become opened and further restructure through unraveling, leading to the lying down side walls, as shown in Figure 2b. Additional etching on the lying walls results in their complete exfoliation, while the residual trunks become NRs. The corresponding height profile in Figure 2d reveals that the inner tube width is similar to the primal NTs diameters, while the structure basement width including the lying walls becomes much larger and approaches the starting tube circumference (L ∼ 2π(d/2)). Figure 2e clearly illustrates these two typical intermediate states and their transitions with the ongoing plasma etching-induced unwrapping. The ribbons’ widths can be as narrow as ∼15 nm and their lengths are ranged from several hundred nanometers to several micrometers (inset of Figure 1c and Supporting Information, Figure S3). Figure 3a presents a typical AFM image of a single-layered BN NR with a height of 0.5 nm as documented in Figure 3b. The measured heights of the thinnest ribbons are in the range of 0.5 to 0.9 nm, and most of them are of 0.8 nm, as shown in Figure 3c. These BNNRs can be assigned to a single-layered BN. The intralayer spacing of h-BN is almost equal to that of graphitic carbon, thus such assignment is in accord with single-, bi-, and trilayer graphene verifications proven previously (Supporting Information, Figure S4).26 It is obvious that with the intensification of plasma etching, the fewer layer BN nanoribbons are formed, as shown in Figure 3d. While utilizing Ar plasma etching under selected conditions, 80 W and 100 s, the present method converted ∼25% of multiwalled nanotubes into single-, double-, and triple-layer NRs. The conversion ratio is relatively high, compared with standard graphenes produced through plasma etching of carbon NTs.4 Excessive etching results in porous/perforated single- and few-layered BNNRs (Supporting Information, Figure S5). The BNNRs were characterized by high-resolution transmission electron microscopy (HRTEM), as shown in Figure 4. Typical HRTEM images of few-, tri-, bi-, and single-layered BN NR are depicted in Figure 4 panels a-d, respectively. The interlayer spacing is measured as ∼3.4 Å at folded edges, corresponding to the (002) planes of h-BN. The enlarged HRTEM image of the surface of a few-layered NR in Figure
FIGURE 1. Unwrapping multiwalled BN nanotubes. (a) Schematic of the unwrapping processes induced by plasma etching, the stepwise opening/unzipping, removing and exfoliating of tube walls to form “white graphene” nanoribbons is sketched. (b,c) Typical TEM images of products before and after etching, demonstrating the morphology transition. The inset in b reveals the open and nearly circular end of the initial tube, and the inset in c shows four formed ribbons; the scale bars are 20 nm.
Figure 1a illustrates the designed approach to fabricate BNNRs. In this process, the raw materials (multiwalled BN nanotubes)15 were first deposited on a Si substrate and spincoated with a PMMA film. Then, the PMMA film was peeled off and turned over to form a film containing BN nanotubes whose side and bottom parts had been protected by PMMA. After that, the composite PMMA-BN nanotube film was subjected to Ar plasma etching with the following parameters: pressure P ) 0.1 Pa, flow rate ) 2 cm3/min, plasma power Pplasma ) 100 W, bias power Pbias ) 25 W, and etching time ) 100 s (Supporting Information Figure S1). After etching, PMMA was removed by acetone vapor to release the formed BNNRs, which were further calcined at 600 °C for 6 h to remove the PMMA residue and oxidize possible carbon-containing contaminants. As the side and bottom walls of the tubes were initially embedded in the PMMA film, only the top part of the tube shell segments was exclusively etched and gradually removed by the plasma, resulting in nanotube opening/unzipping through cutting top walls. Further thinning of the residual bottom strips and exfoliation of side walls eventually led to the formation of layered BNNRs. Figure 1b,c presents the typical transmission electron microscopy (TEM) images of a primal nanotube and formed “white graphenes” NRs. These clearly display the structural transition under nanotube-unwrapping/unzipping. The starting BN nanotubes have an average outer diameter ∼100 nm and a length up to several micrometers (Supporting Information Figure S2). The inset in Figure 1b reveals the intact (nearly circular) cross-section of tubes. Those that have been © 2010 American Chemical Society
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FIGURE 2. Typical intermediate states. (a,b) AFM images of partially unwrapped tubes after 80 W 5 s and 100 W 15 s plasma etchings. (c,d) Height profiles corresponding to (a,b). (e) Cartoons indicating the top wall domains cutting and their gradual unraveling.
field can narrow the bandgap19,22 and induce possible metallic/semiconducting/half-metallic transitions.20 Very recently, Chen et al. have demonstrated that a semiconductor/ half-metal/metal transition may occur in the partially hydrogenated zigzag BNNRs.23 Experimentally, Jin et al. and Alem et al. documented that the N-terminated zigzag edge structures had been dominated in the electron beam irradiationformed BN edges.31,32 For the present BNNRs, atomically resolved images of edges were obtained by a low-voltage (120 kV) aberration-corrected HRTEM, as depicted in Figure 4g. According to the orientation of the honeycomb-like patterns in a phase image, the B-N hexagon placement in line with the [100] direction can be distinguished along the NR edges, as illustrated by drawing in Figure 4g. The spacing
4e clearly shows the expected honeycomb BN lattice. The distance between the bright dots is ∼2.5 Å, in line with the (100) lattice constant of h-BN (Supporting Information, Figure S6). This distance also equals the separations between each two nearest B atoms or N atoms within a hexagonal BN layer.12 Electron energy loss spectroscopy (EELS) in Figure 4f shows only characteristic B- and N- K-edges with a B/N atomic ratio equal to ∼1. No impurities are found. The core edges exhibit sharp π* and σ* peaks, that is, the characteristics of a sp2-hybridized BN. Theoretically, it has been predicted that all hydrogenpassivated and bare armchair BNNRs are wide bandgap insulators; however, bare zigzag BNNRs are electrically conductive.20,21 Furthermore, the applied transverse electric © 2010 American Chemical Society
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FIGURE 3. Layer number adjustments. (a) AFM image of a single-layered BN nanoribbon. (b) The corresponding height profile; the inset presents the cartoon of a single-layered BN nanoribbon. (c) Statistics of thickness distribution of ribbons with a thickness less than 1 nm. (d) Layer number statistics of products after etching at 80 W over different times.
between the protuberant dots on the edges is ∼0.25 nm, in accord with (100) BN lattice constant, and the intensity profile ratio of the neighboring dots is ∼1:0.8 (Supporting Information Figure S7). These characters imply that that the edge has a N-terminated zigzag type structure,22 as schematically illustrated in Figure 4h. During plasma etching, many defects (with a dominant triangle-like morphology) are produced on the surface of few-layered BNNRs (Figure 4i and Supporting Information, Figure S8). The knock-on thresholds of B and N atoms in BN were calculated to be 74 and 84 keV33 (or 79.5 and 118.6 keV in the latest report).34 It has been documented that electron beam irradiations under a voltage intermediate between these two values, or over 84 keV and up to 120 kV, can dominantly lead to the Nterminated zigzag edges and produce surface vacancies.31,32 Therefore, the zigzag edges and surface vacancies in the present BNNRs may be produced through the similar knockon effects under the plasma process. The electrical transport of the present BNNRs was then directly studied using a “Nanofactory Instruments” scanning tunneling microscopy (STM)-TEM holder integrated in the HRTEM.35,36 As illustrated in Figure 5a, an NR was first attached onto a counter electrode; then the STM tip was pushed toward the contact with the sample. The representative I-V curves are displayed in Figure 5b, these show the © 2010 American Chemical Society
characteristics of a semiconductor. For the initial BN tubes, the resistance is more than 10 GΩ and any current can hardly be detected by the same measurement system. Previously, only a 30 pA current could be obtained under a 1 V voltage from a BNNT field-effect transistor (FET).37 In a sharp contrast, here, the measured currents through BNNRs are 1.5-2 µA at 9 V, as shown in Figure 5b, thus being over 4 orders of magnitude higher than those in the “mother” BN nanotubes under the same voltage. Moreover, as demonstrated in Figure 5c, the current can reach as high as 15 µA before the BNNR is broken due to Joule overheating when a high voltage of ∼20 V is applied. Results for multiple samples show the similar trends, as verified in the Supporting Information, Figures S9-S11. The measured metal electrode/ semiconductor/metal tip system can be regarded as a tandem circuit including an Au/BN Schottky junction, a resistor, and another BN/W Schottky junction. The semiconductor parameters can be retrieved from the I-V curves (Supporting Information, Figure S12).38 The logarithmic plots of current I as a function of voltage V give approximately straight lines in a high voltage region, as depicted in Figure 5d. Thus, the resistivity F, hole concentration n, and carrier mobility µ can be obtained by fitting the acquired data. The resistance of BNNRs is reduced to ∼3.2 from ∼104 MΩ peculiar to the starting BN tubes.35 The corresponding NR 5052
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FIGURE 4. Microstructure characterizations. (a-d) HRTEM images of typical few-, tri-, bi-, and single-layer BN nanoribbons. (e) HRTEM image of the surface of a few-layer nanoribbon, that clearly displays the honeycomb B-N lattice. (f) EEL spectrum of BN ribbons. (g) Atomic imaging of edges of a two-layer ribbon, the B-N hexagon arrangement and zigzag edge structure are pictured. (h) Structural model of the corresponding N-terminated zigzag BN edge. (i) HRTEM image of the ribbon surface, showing the triangular-shaped vacancies. The scale bars are 1 nm.
electrical conductivity is ∼104 S/m. The hole concentrations of BNNRs are in the order of 1017, whereas the carrier mobilities are as high as 58.8 cm2/V s, which is the highest value for BN nanostructures.35,37 Although the details of electric transport need more systematic investigations, it is believed that the conductivity has been greatly improved under the tube-ribbon structural transition. The first-principles calculations were then performed to further elucidate the conductivity origin in the produced BNNRs (calculation details are provided in the Supporting Information, Figure S13). We first examined the edge effect by calculating a perfectly shaped bare BNNR. The electronic structure of a bare zigzag BNNR with a width of 3.8 nm is shown in Figure 6a. There are three bands crossing the Fermi level. The bands II and III are from the σ-dangling bond states at the edge N and B atoms, respectively, while the band I is originated from the edge N π states. The © 2010 American Chemical Society
dispersion character indicates that the three bands actually serve as shallow acceptor levels, featuring the whole BNNR as a p-type doped semiconductor. The partially occupied states due to the zigzag shaped edges will greatly enhance the conductivity of the unwrapped BNNRs, although a gap remains between the bands III and IV at the Brillouin zone boundary. Further calculations showed that the shallow acceptor-like states could even exist in bare BNNRs with irregular edges as long as the ribbon contains a part of zigzag shaped edges (Supporting Information, Figure S13) but could not be found in bare armchair edged BNNRs. Next, we introduce a B vacancy in a 3.35 nm width zigzag BNNR to examine the defect effect. The corresponding band structure shows that the number of shallow acceptor-like levels is increased from the case of perfect BNNR (Figure 6b), thereby further enhancing the conductivity of the defective BNNR. In this case, the corresponding charge densities for electronic 5053
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FIGURE 5. Transformation of electrical transport. (a) TEM image of a ribbon connected to the counter gold electrode (upper black part) and tungsten STM tip (bottom black part), the insets present the experimental configuration, the scale bar is 20 nm. (b) Typical I-V curves of several BN nanoribbons. (c) I-V curve of a BN nanoribbon under an increased voltage until its structural degradation induced by Joule overheating. (d) Typical lnI vs V plot of a BN nanoribbon.
So the improved electrical conductivity in the unwrapped BNNRs mainly comes from the bare zigzag edges and the vacancy defects, which behave as a doping source to introduce a number of shallow acceptor-like levels in the ribbons. This is accordant with the present microstructure characterizations and other theoretical works.19-23 Similarly to the obvious effects of dopant concentration on the electronic structure of carbon nanotubes,39 the population of zigzag edges and vacancy defects have also an important role for BN white graphenes. As revealed by the calculations presented in Supporting Information, Figure S14, the electronic states around the Fermi level remarkably increase with increasing B vacancies concentration. Moreover, the energy dispersions of these defect states around the Fermi level are also increased owing to the enhanced coupling between the defect states. Therefore, the electronic conductivity will be better at a higher concentration of B vacancies. In summary, atomically thin “white graphene” BNNRs, structural analogues of graphene NRs, were fabricated through unwrapping multiwalled BN nanotubes under delicate Ar plasma etching. The typical intermediate states of the etched nanotubes were observed to confirm the unwrapping mechanism. The produced BNNRs mainly displayed N-terminated zigzag edges and vacancy defects and were proven to be semiconductors. The high currents of ∼2 µA under a 9 V voltage and ∼15 µA under a 18 V voltage, high
FIGURE 6. Calculated electronic structures. (a) Band structures and density of states of a 3.8 nm width zigzag BN NR. (b) Band structures and density of states of a 3.35 nm width zigzag BN NR with a B vacancy. Below each figure is the isosurface plot (0.2 e/Å3) of the charge density corresponding to the bands crossing the Fermi level, together with the optimized atomic geometries.
states around the Fermi level are not only distributed along the edges, but also extended from the doubly coordinated N atoms around the defect site (bottom panel in Figure 6b). © 2010 American Chemical Society
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conductance of 104 S/m and carrier mobility of 58.8 cm2/V s were measured in such 2D-BN crystals without a gate bias. First-principles calculations verified that the edge states and dangling bond states located in the zigzag edges and experimentally observed surface vacancies had been the driving sources for the distinct carrier transport in the fabricated ribbons. The present approach makes the BNNRs easily accessible for addressing a wide range of fundamental issues and applications in electronic and spintronic devices.
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Acknowledgment. The authors are grateful to Dr. Chuanhong Jin for his help with low-voltage TEM imaging and related discussions and to Drs. Masanori Mitome, Akihiko Nukui, and Isamu Yamada for the experimental support. This work was supported by the Japan Society for Promotion of Science (JSPS) in the form of a fellowship tenable at the National Institute for Materials Science (NIMS), Tsukuba, Japan (H.B.Z). Y.B., C.Y.Z., X.L.W., and D.G. are indebted to the International Center for Materials Nanoarchitectonics (MANA) of the National Institute for Materials Science (NIMS) for a financial support. W.G. and Z.Z. are supported by 973 program and NSF of China. Supporting Information Available. (1) Fabrication of PMMA-BN film; (2) pristine BN nanotubes; (3) TEM and SEM of typical nanoribbons; (4) AFM of typical BN nanoribbons; (5) AFM of porous nanoribbons after excessive etching; (6) BN lattice and composition examination; (7) edge structure of BN nanoribbons; (8) surface vacancies of BN nanoribbons; (9) on-off I-V measurements; (10) EELS before and after I-V measurements; (11) to-and-fro scanning I-V measurements; (12) measurement and analysis of electrical transport; (13) models and calculations of electrical transport of BN white graphenes; (14) impact of B vacancies concentration. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9)
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