Heterogeneous growth of BCN nanotubes by laser ablation

21 November 1997

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 279 (1997) 264-269

Heterogeneous growth of B - C - N nanotubes by laser ablation Y. Zhang ", H. Gu b, K. Suenaga c, S. Iijirna " a Fundamental Research Laboratories, NEC Corporation, 34 M~vukigaoka. Tsukuba, lbaraki 305, Japan b JSTlnternational Research Project on Ceramics Superplasticity, 2-4-1, Mutsuno, Atsuta-ku, Nagoya 456, Japan c Laboratoire de Physique des Solides, URA 002, Universit~ de Paris-Sud, Bat. 510, 91405 Orsay, France

Received 21 August 1997

Abstract Multi-wall nanotubes have been synthesized by metal catalyzed laser ablation of a BN and carbon composite target in a tube furnace heated to 1000°C or 1200°C and flowing with nitrogen gas. The products are characterized by scanning transmission electron microscopy and electron energy-loss spectroscopy. No single-wall nanotube has been found in the products. Thinner nanotubes with a few layers are usually composed of pure carbon. B - C - N layers exist at the outer surface of thicker nanotubes. The layer number and chemical composition vary along the tube axis. The growth mechanism of the heterogeneous nanotubes is discussed. © 1997 Elsevier Science B.V.

1. Introduction The physical and geometrical perfection of carbon nanotubular structures [1,2] have surprised scientists and stimulated extensive investigations on fabrication and characterization as well as theoretical calculations of this novel type of material (see for example, Ref. [3]). At the same time, the scope has been extended from carbon to other materials, such as h-BN and the B - C - N ternary system, which possess an energetically preferable graphitic structure. Theoretical calculations [4-7] indicated that the h-BN, BC2N, BC 3 tubular structure may be stable since the energy increase for tubule formation is less than that of a carbon nanotube. The electronic properties of these tubules and their heterojunctions [8] are mainly determined by their chemical composition rather than tubule geometry. These theoretical results suggest a considerable potential in applications of B - C - N nanotubes in nanosized electronic and photonic devices [8] if their production can be controlled. Exper-

imental synthesis of BN [9-12] and B - C - N [ 13-16] nanotubes has been realized by electric arc-discharge and pyrolysis methods. However, no relationship between composition variation along tubule axis and growth mechanism has been established, which is important for precise control of the B - C - N stoichiometry within an individual nanotube. In this Letter, we report the synthesis of B - C - N nanotubes by a new method in which a pressed pellet of carbon, BN and metal mixture are ablated by a high-power pulsed laser in flowing nitrogen gas. The heterogeneously grown nanotubes were characterized by scanning transmission electron microscopy. The axial and radial composition variations of the tubes are attributed to a two-step nucleation and growth process.

2. Experimental The laser ablation apparatus is similar to that used for synthesizing high-yield single-wall carbon nan-

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Y. Zhang et al. / Chemical Physics Letters 279 (1997) 264-269 -

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Fig. 1. Typical morphologiesof laser ablated tubular structures. (a) A thin and clean pure carbon multi-wallnanotube.(b) A non-uniformly thickened B-C-N nanotube with a small inner diameter. Boron concentrationis higher in the thicker part (solid arrow) than that in the thinner part (hollow arrow). (c) A graduallythickenedB-C-N nanotubewith a larger innerdiameter. BN are mainly doped in segments I1 and III.

otubes [17,18]. In this experiment, we used the secondary harmonic beam (532 nm) from a pulsed Nd:YAG laser (Quanta-Ray GCR-200). The pulse repetition rate was 10 Hz and pulse width 6 - 7 ns. The beam was focused on the target surface to get an energy density of about 3 J / c m z per shot. The target, a pressed pellet made of mixed powder of BN, carbon, nickel and cobalt with an atomic ratio of 100:100:0.6:0.6, was placed at the center of an electric tube furnace heated to 1000°C or 1200°C. Nitrogen gas was introduced with a constant flowing rate of 300 sccm. The system was evacuated to keep a pressure of about 500 Torr. The soot deposited on the down-stream side of the furnace was collected, sonicated in ethanol and then put on a micro-grid for microscopy. Scanning transmission electron microscopes (STEM VG-HB601 and VG-HB501, 100 kV) equipped with parallel energy loss spectrometers (Gatan PEELS 666) were employed to characterize the tubular materials obtained. The spatial resolution was about 0.2 nm for imaging and 0.5 nm for electron energy loss spectroscopy (EELS). The acquisition time of an individual EELS spectrum was set as short as possible in order to avoid electron irradiation damage. Hydrogenic cross-sections were used to quantify the B, C, and N K-edge spectra [19]

and to give the chemical composition of nanotubes with an associated error of about 10%.

3. Results and discussion Similar tubular materials can be formed at both temperatures of 1000°C and 1200°C. In the products, --~

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Fig. 2. A series of EELS spectra from the four segments in Fig. lc. B, C, and NK-edges rise at an energy loss ot 188, 284, and 399 eV, respectively. The spectra were normalizedby carbon (r peaks.

Y. Zhang et al. / Chemical Physics Letters 279 (1997) 264 269

266

we found a lot of micrometer-sized h-BN crystalline particles in addition to the usual soot which contained multi-wall nanotubes, nanoparticles, and amorphous B - C - N phase. No single-wall nanotube was found in the products although the experimental conditions were the same as those for single-wall

carbon nanotube synthesis [17,18], except for the addition of BN into the target. Typical tubular morphologies observed in the soot are shown in Fig. 1. The relatively thin tubes with a few layers are usually composed of pure carbon (Fig. l a). They usually have an inner diameter of 2-3 nm

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Fig. 3. High-resolution STEM images showing detail structure of the nanotube in Fig. lc. (a) The joint point between segments I (upper part) and II (lower part). (b) The joint point between segments II (upper part) and III (lower part). (c) A part in segment III. (d) The blunt end of segment IV and an attached nanoparticle. The area in the black square contains only boron carbide without metal while the area in the white square contains metal.

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K Zhang et al./Chemical Physies Letters 279 (1997) 264-269

and outer diameter of 4 - 1 0 nm (the one in Fig. la has an outer diameter of about 4.5 nm). Their morphology and chemical composition are relatively homogeneous. Few nanoparticles and amorphous materials are attached to their walls. The B - C - N nanotubes, on the contrary, usually have a larger outer diameter, non-uniform morphology, heterogeneous chemical composition, and relatively 'dirty" surfaces. These thick nanotubes can be sorted into two types according to their inner diameter (Fig. l b,c). The tube morphology and axial distribution of chemical composition are also dependent on the inner diameter. Fig. lb shows a non-uniformly thickened multi-wall nanotube with an inner diameter similar to that in Fig. l a, i.e. 2 - 3 nm. The outer diameter varies from about 9 to 20 nm. Boron concentration is higher in the thicker than that in the thinner part. A quantitative elemental analysis from EELS data gives the atomic ratios of B / C and B / N to be 0.14 and 1.3, respectively, at a thick part (indicated by a solid arrow in Fig. lb). At a thinner part (indicated by a hollow arrow in Fig. lb), the ratios are 0.10 and 1.1, respectively. This result implies that boron tends to dope into the outer layers in the thick part. The deviation of the B / N ratio from unity indicates that a small number of boron atoms doped into a graphite layer are bonded only by carbon atoms. Fig. lc shows another type of B - C - N nanotube with a larger inner diameter, in the range of 5 - 7 nm. The thickening happens gradually along the tube axis, and the outer diameter changes from 13 nm at one end to 29 nm at the other. The tube can be divided into four segments as indicated in Fig. lc, according to the thickness and chemical composition. The chemical composition is homogeneous within each segment. A series of core excitation EELS spectra from the four segments are shown in Fig. 2. The well-defined near-edge structure (ELNES) of carbon K-edges indicates that the nanotube preserves a graphitic structure throughout the four segments. The thinnest part, segment I, contains nearly pure carbon. Segments I1 and III have similar chemical composition ( B / C ~ 0.12, B / N ~ 1). The sharp "rr peak in boron K-edge ELNES shows that boron atoms are doped into the graphite lattice with sp 2 hybridization. The doping of nitrogen is expected in the same way although the ELNES of N K-edges are

tOO weak to be distinguished in the spectra owing to the short acquisition time. The thickest part, segment IV, contains an unexpectedly low concentration of boron ( B / C ~ 0.04: nitrogen is under the detectable limit). This gives an example of axially heterogeneous growth independent of outer diameter. Detail features of this heterogeneous nanotube are shown by high-resolution STEM images (Fig. 3). Nanoparticles and cage shells were found attached at the joint points of these segments (Fig. 3a,b). The particles are composed of boron carbide (Fig. 3d, black square), cobalt and nickel (Fig. 3d, white square). The composition of cage shells is similar to that of nearest tube walls. Abrupt change of chemical composition happens at the joint point between segments I and II (Fig. 3a), while the inner layers can be seen extending continuously through the junction. This also implies that B - C - N layers are wrapped on the carbon nanotube. There is a small variation in inner diameter in segments lI and Ill (Fig. 3a,b,c). The layer number also changes within each segment except tbr segment I. Fig. 3d shows the blunt end of segment IV which also observed in arc-discharged compound tubules [ 14]. EELS line scan technique [20] has been used to study the radial distribution of boron atoms. Fig. 4 shows the carbon and boron profiles scanned perpendicularly across an B - C - N nanotube. The profiles were normalized to hydrogenic cross-sections and

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268

Y. Zhang et al. / Chemical Physics Letters 279 (1997) 264-269 "' ..... ~

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Fig. 5. Schematic growth models of B-C-N nanotubes corresponding to (a) Fig. lb and (b) Fig. lc. The solid and dotted lines represent carbon and B-C-N layers, respectively.

zero-loss intensity, and hence represent the distribution of atomic concentration [19,20]. Computer simulations of these profiles suggest the following model best fits the experimental result of this particular nanotube: a tubule with inner diameter ~ 5 nm and outer diameter ~ 14 nm; six inner layers are pure carbon and seven outer layers are BC7N. The result confirmed that B - C - N layers do indeed tend to exist at a tube surface. This is consistent with the results of arc-discharge nanotubes [ 13,15], although a recent study has reported a C / B N / C sandwich structure for tubules with much higher B / C ratio [21]. In order to explain the heterogeneous structure shown above, we propose a two-step nucleation and growth mechanism. The first step is the nucleation and growth of a multi-wall carbon nanotube. The second step is partial coating of B - C - N layers. The first step determines the inner diameter and basic morphology. The nucleation of B - C - N in the second step may originate from a defect or attached particle on the carbon nanotube surface. The growth of the B - C - N layers can also be terminated by a similar defect or particle. Fig. 5 schematically demonstrates the formation of heterogeneous nanotubes in Fig. lb,c. A key question for this mechanism is why doping of B and N does not happen in the first step. One possible answer is that the first step growth happens at the center of the plasma region where doped B and N easily diffuse out of the graphite lattice and form h-BN crystallite. This diffusion process can be stopped by high-pressure ambient gas when the nanotube moves to the edge of the plasma in the second growth step. This explanation is supported by the facts that a lot of h-BN crystallites were observed in

the present experiment and that high-pressure inert gas is needed for doping BN in arc-discharged graphite nanotubes [14]. This scheme may also be used to explain the absence of single-wall nanotubes since the doping and diffusion of B and N atoms can destroy the perfection of a single-wall structure and introduce a nucleation site for another layer. They can also serve as the bridging atoms in lip-lip interaction [22,23] to stimulate the growth of multilayers.

4. C o n c l u s i o n

The laser ablation method has been shown to be able to synthesize heterogeneous B - C - N nanotubes. A growth mechanism has been proposed based on the detailed characterization of the axial and radial distribution of chemical composition as well as the morphological variation. Under this mechanism, the doping of BN in a graphite lattice is controlled by the matter transportation velocity in the plasma and ambient gas pressure. Thus, precise control of the experimental conditions may eventually lead to a controllable fabrication of a heterojunction structure similar to that between segments III and IV in Fig. lc.

Acknowledgements

The project is partially supported by the Special Coordination Funds of the Science and Technology Agency of the Japanese Government.

K Zhang et al./Chemical Physics Letters 279 (1997) 264-269

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