SYNTHESIS, ATOMIC STRUCTURES AND ELECTRONIC STATES OF BORON NITRIDE NANOCAGE CLUSTERS AND NANOTUBES

Takeo Oku*, Ichihito Narita and Atsushi Nishiwaki

Institute of Scientific and Industrial Research, Osaka University Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan

*e-mail: [email protected]

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Abstract

Boron nitride (BN) nanocage clusters (BnNn: n = 12~60), endohedral BN clusters Y@BnNn and BN nanotubes were synthesized by an arc-melting method, and detected by mass spectrometry and high-resolution electron microscopy. The BN clusters consisted of 4-, 6-, 8- and 10-membered BN rings satisfying the isolated tetragonal rule, which was optimized by molecular orbital calculations. Total energy calculation showed that some elements stabilize and expand the B36N36 structure. Bandgap energies of the B36N36 clusters were found to be reduced by introducing a metal atom inside the cluster, which indicates controllability of the energy gap. Chiralities of BN nanotubes with zigzag- and armchair-type structures were directly determined from high-resolution images, and structure models were proposed. Total energies of BN nanotubes with a zigzag-type structure were lower than those of armchair-type structure, and these results agreed well with the experimental data of disordered tube structure. BN nanotubes encapsulating BN clusters and a yttrium nanowire were also found. The present work indicates that the new BN nanocage fullerene materials with various atomic structures and properties can be produced, and a guideline for designing the BN fullerene materials is summarized.

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INTRODUCTION

Since the discovery of C60 fullerene1 and carbon nanotubes2, various carbon-based nanocage structures, such as fullerene clusters, nanotubes, nanocapsules, nanopolyhedra, cones, cubes and onions, have been studied because of great potential for using materials with low dimensions in an isolated environment.1-3 Boron nitride (BN) nanostructured materials with a bandgap energy of ~6 eV and non-magnetism are also expected to show various properties.4 Recently, many studies have been reported on BN nanomaterials such as nanotubes,5-22 bundled tubes,23 nanocorns,24,25 nanocapsules,4,26-30 nanoparticles,31,32 clusters3,4,33-36 and BN metalllofullerenes,4,37,38 which are expected to be useful as electronic devices,39 high heat-resistance semiconductors, insulator lubricants, nanowires40-42 and gas storage materials.4,43-45 Theoretical calculations on BN nanotubes,46-51 cluster-included BN nanotubes,52 BN clusters53-62 and BN cluster solids63-65 had also been carried out for prediction of the properties. By controlling the size, layer numbers, helicity, compositions and included clusters, these cluster-included BN/C nanocage structures with bandgap energy of 0-6 eV and non-magnetism are expected to show various electronic, optical and magnetic properties such as Coulomb blockade, photoluminescence and superparamagnetism, as shown in Fig. 1(a). Possible applications of the BN/C fullerene materials are also shown in Fig. 1(b). The purpose of the present work is twofold. The first is to prepare the new BN nanocage clusters and nanotubes. In the present work, an arc-melting method was selected for the formation of the BN nanomaterials. An yttrium element was selected as catalysis for the BN nanomaterial formation. The second purpose is to understand the atomic structures of these BN nanocage materials from high-resolution electron microscopy (HREM), which is a powerful method for direct structure analysis in atomic scale.66-68 In order to confirm the atomic structures and to investigate stabilities and electronic states, total energy calculations by molecular mechanics and molecular orbital calculations were carried out. These studies will give us a guideline for designing and synthesis of the BN fullerene materials, which are expected for the future nanoscale devices. 3

EXPERIMENTAL PROCEDURES

The YB6 powder (4.0 g, 99.6%, Kojundo Chemical Lab. Co., Ltd) was used for the formation of BN clusters.69 BN nanotubes were also synthesized from YB6 with Ni powder.70 The YB6 powder (4.0 g, 99.6%, Kojundo Chemical Lab. Co., Ltd) and Ni powder (0.8g, 99.9%, Kojundo Chemical Lab. Co., Ltd) with the atomic ratio of 1:1 were set on a copper mold in an electric-arc furnace, which was evacuated down to 1×10-3 Pa. After introducing a mixed gas of Ar (0.025 MPa) and N2 (0.025 MPa), arc-melting was applied to the samples at an accelerating voltage of 200 V and an arc current of 125 A for 10 s. Arc-melting was performed with a vacuum arcmelting furnace (NEV-AD03, Nisshin Engineering Co., Ltd), and gray to white powder were obtained around the copper mold. An AXIMA-CFR (Shimadzu/Kratos, Manchester, UK) instrument was used to obtain laser desorption time-of-flight (LD-TOF) mass spectra.71 The operating conditions were as fallows: nitrogen laser (337 nm); linear mode; accelerating voltage at 20 kV; detection of positive ions. The sample powder (50 mg) was suspended in pyridine (C5H5N, 200 µl) with ultrasonination. The aliquot (1-2 µl) was spotted on the sample plate and dried at room temperature. The mass spectra were corrected by using C60 and C70 clusters.72 Samples for HREM observation were prepared by dispersing the materials on holey carbon grids. HREM observation was performed with 300 kV electron microscope (JEM-3000F) having a point-to-point resolution of 0.17 nm. To compare observed images with calculated ones, HREM images were calculated by the multi-slice method using the MacTempas software (Total Resolution, CA, USA). The parameters used in the calculations are as follows: accelerating voltage = 300 kV, radius of the objective aperture = 5.9 nm-1, spherical aberration Cs = 0.6 mm, spread of focus ∆ = 8 nm, semi-angle of divergence α = 0.55 mrad, under defocus values ∆f = -10 ~ 90 nm, unit cell (one cluster) = 2×2×2 nm, and crystal thickness (unit-cell thickness, 7 slices) t = 2 nm. space group P1, and assumed temperature factors73,74 0.02 nm2 (B and N) and 0.14 nm2 (Y). 4

Theoretical calculations for structural stability of the clusters were carried out by molecular mechanics calculations (MM2) and semi-empirical molecular orbital calculations (Hamiltonian： Parameterized Model Revision 5: PM5) using CS Chem3D Ultra (CambridgeSoft, MA, USA) and WinMOPAC Professional (Fujitsu Corp., Chiba. Japan). The energy levels and density of states were also calculated by the first principles calculation with discrete variational (DV)-Xα method.

RESULTS AND DISCUSSION

BN clusters

Figure 2(a) shows a mass spectrum for BN clusters in pyridine solution, and demonstrates the existence of (BN)n (n = 12~60) clusters. In addition, (BN)n (n = ~80) clusters were detected in the present work. Further, endohedral BN clusters [Yx@(BN)n] with yttrium atoms encapsulated inside the BN clusters were detected. A mass spectrum for pyridine solution, which was used for matrix in LD-TOF mass spectrometry, is also shown for reference in Fig. 2(b). The mass spectrum peaks have a somewhat broad distribution because of the two isotopes of

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B and

11

B. For localized

structures of BN clusters, it should be noted that the isotopic ratio of boron atoms might be different from the natural averaged ratio because of minimization of the clusters’ structural energy. Mass spectrum peaks for (BN)n clusters are observed in the range of m/z=600~1600, and the distribution peak is around n~40, which supports previously reported B36N36 clusters with high symmetry.4,38,58 Although fullerenes satisfy the isolated pentagon rule, the present BN cage clusters satisfy the isolated tetragonal rule, as proposed for structural models of (BN)n (n = 12, 24, 28, 36, 48 and 60) in Fig. 3. All BN clusters have tetragonal BN rings isolated by hexagonal BN rings. In addition to the tetragonal and hexagonal rings, octagonal and decagonal BN rings63 were introduced for B24N24 and B60N60 clusters in the present work.

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In order to confirm the structure of the BN clusters, HREM observations were carried out. Hollow BN clusters were often observed on and inside the BN nanotubes, and the BN fullerene clusters had a single BN sheet, as shown in Fig. 4(a). Sizes of the BN clusters were in the range of 0.7-1.0 nm, which corresponds to the size of B36N36 clusters.4,38,58 The B36N36 cluster consists of six 4-membered rings and thirty-two 6-membered rings, as shown in Fig. 4(d). In addition to the empty BN cage clusters, BN clusters with doping atom inside were also observed in HREM images. A typical HREM image of the BN clusters is shown in Fig. 4(b) and 4(c), and a ring contrast indicates B36N36 cluster. A dark contrast is observed inside the BN clusters, which indicates the existence of an yttrium atom inside the B36N36 clusters. During arc-melting of YB6 powder, yttrium atoms could be introduced inside the B36N36 clusters. Structure models for Y@B36N36 metallofullerene are proposed as shown in Fig. 4(e) and 4(f). The structures are optimized by molecular mechanics calculations (MM2), and the yttrium atom is closed to the center of the B36N36 cluster. Dark contrast is also observed outside of the BN cluster, which would be due to the quantum noise of electrons. However, inside the BN cluster, the contrast is strongly dependent on the diffraction conditions.4,38,66 Therefore, the dark contrast inside the BN clusters is believed to be due to the existence of an yttrium atom. Based on the projected structure models of Figs. 4(d-f), image calculations on the B36N36 and Y@B36N36 clusters were carried out to investigate the cluster structures. Figures 5 are HREM images of the B36N36 and Y@B36N36 clusters calculated along the [100] direction of the projected unit cell as a function of defocus values. The experimental images of Fig. 4(a-c) agree with calculated HREM images (Fig. 5a-c), especially at defocus value of -40 nm, which is nearly Scherzer defocus (∆fs = -41.2 nm). This indicates the yttrium atom is included inside the BN cluster. The dark contrast corresponding to the yttrium atom inside the B36N36 cluster is smeared a little compared to the simulated image, which would be due to the vibration of the yttrium atom by electron beam irradiation. In previous works, formation of BN endohedral fullerene clusters including lanthanum and iron atoms had been reported.4 In the present work, the endohedral BN 6

metallofullerenes of Y@BnNn (n = 36, 37 and 48) were produced and characterized from HREM and LD-TOF mass spectrometry. Various lanthanide elements could be introduced into the BN clusters, as reported in the carbon-based metallofullerenes.75 Densities of states (DOS) for B12N12, B24N24, B48N48 and B60N60 clusters are calculated and shown in Fig. 6(a-d), respectively. Strong effects of B-2p and N-2p orbitals on the densities of states are observed. The structural optimization and electronic structures of the BN clusters performed by molecular orbital calculations (PM5) are summarized in Table 1. The B36N36 cluster shows the largest energy gap of 5.367 eV between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and the B24N24 and B60N60 clusters with octagonal and decagonal BN rings shows the smallest energy gap of 4.9 eV. The B36N36 and B48N48 clusters show the lowest total energies per atom as listed in Table 1, which agreed with the results of LD-TOF mass spectroscopy in the range of m/z = 600-1600. Figure 7 is heats of formation (total energies) per atom of B36N36 clusters with doping element M (M@B36N36) clusters presented in periodic table. A total energy of B36N36 per atom was calculated to be -19.68 kcal/atom. Comparing this value to other energies in Fig. 7, energies of K@B36N36 and Ga@B36N36 show lower energies than that of B36N36 clusters. This indicates that potassium and gallium elements introduced inside the BN cluster stabilize the B36N36 structure, and the B36N36 clusters were found to be expanded by an atomic doping. Other many elements show to have higher energies than that of B36N36 clusters. As indicate by asterisks, when molybdenum and barium elements were introduced, the structures of B36N36 were highly distorted or broken. Electronic structures of B36N36, Y@B36N36, Fe@B36N36 and K@B36N36 clusters were investigated as shown in Fig. 8. DOS of endohedral M@B36N36 clusters show effect of doping elements in BN clusters. Energy gaps of B36N36, Y@B36N36, Fe@B36N36 and K@B36N36 clusters were calculated to be 5.367, 0.114, 0.107 and 0.522 eV, respectively. Reduction of bandgap energies for Y@B36N36, Fe@B36N36 and K@B36N36 are due to the Y4d, Fe3d and K4s orbitals. The present results indicate that the energy gap Eg of the B36N36 can be controlled by introducing an 7

atom inside the B36N36 cluster, and that some atoms such as potassium or gallium in the B36N36 cluster might stabilize the BN clusters by doping. The BN cluster is a molecule with polarity because of a positive charge at boron atom positions and a negative charge at nitrogen atom positions; so an electrophilic or nucleophilic reagent would work as a solution for BN clusters. Since C60 fullerene clusters have no polarity and are soluble in nonpolar solvents such as toluene76 and benzene, they have difficulty in solving in a polar solvent. In the present work, pyridine (C5H5N) did work well for the extraction of BN clusters because of an electrophilic reaction; pyrrole (C4H4NH) would also work as a nucleophilic reagent. In order to investigate these BN nanocage clusters further, separation technique using high performance liquid chromatography should also be developed.

BN nanotubes

A low magnification image of BN nanotubes produced from YB6/Ni powder is shown in Fig. 9(a). The length and width of BN nanotubes is ~5 µm and 3-50 nm, respectively. A HREM image of a B36N36 cluster inside a BN nanotube is shown in Fig. 9(b). The BN nanotube has a multi-walled structure, and a diameter of the most inner tube is 1.75 nm. An atomic structure model of the center of Fig. 9(b) is shown in Fig. 9(c). Diameter and chirality of the BN nanotube are 1.747 nm and (22,0), respectively. This kind of peapod-type self-organized structure would be useful for the future nanoscale devices.21 Another HREM image of BN nanotubes with a bundled structure is shown in Fig. 9(d), and an atomic structure model observed from three different directions is shown in Fig. 9(e). There are some spaces among the BN nanotubes, and the space would be useful for gas storage such as hydrogen. In the present work, yttrium worked as a good catalytic element to produce BN nanotubes. Catalytic metals for formation of BN nanotubes, nanocapsules and nanocages, which were confirmed by experiments on arc-method, are summarized in Fig. 10(a) as periodic table. It has 8

been reported that Zr, Hf, Ta, W, Nb and La can be good catalytic metals for synthesis of BN nanotubes.5-22 On the other hand, other metals could not form BN nanotubes, although BN nanocapsules or nanocages were formed. The relationship between catalytic metals and structures of BN fullerene materials would be summarized by formation enthalpy with nitrogen and boron. About some metals, formation enthalpies with boron (HforB) and nitrogen (HforN) were theoretically calculated.77 Difference of formation enthalpy (HforN - HforB) is also shown in Fig. 10(b). From our previous results,4,70 BN nanotubes and BN nanocapsules (or nanocages) were formed when HforN HforB was negative and positive, respectively. From Fig. 10(b), BN nanotubes are formed when rare earth metals are used as catalytic metals, such as Y, Zr, Nb, La, Hf, Ta, Sc, Ti and V, would work as good catalytic elements, which agreed well with the experimental results. In the case of YB6/Ni powder, the yttrium and nickel worked as good catalytic elements to produce bundled BN nanotubes. From the results of only YB6 powder, yttrium atoms would work as core element to produce BN nanotubes, and Ni atoms would have a role for combination of each BN nanotube. Therefore, existence of nickel atoms would have an effect on formation of bundled BN nanotubes, and the nickel atoms might exist among BN nanotubes. Further studies will be needed for the role of nickel atoms in bundled BN nanotubes. Figure 11(a) is a HREM image of a quadruple-walled BN nanotube. In the present work, all HREM images were taken close to the Scherzer defocus (∆fs = -41.2 nm) in order to investigate atomic structures in detail. HREM observations and electron diffraction analysis on BN nanotubes have been reported,78,79 and direct observations of nanotube chirality were tried in the present work. An enlarged HREM image is shown in Fig. 11(b), which indicates lattice fringes in the BN nanotubes. A filtered Fourier transform of Fig. 11(b) showed that this nanotube had a zigzag-type structure as shown in Fig. 11(c). A HREM image with clear contrast was processed after Fourier noise filtering as shown in Fig. 11(d). The intervals of the bright and dark dots are 0.14 nm, which corresponds to the structure of hexagonal BN rings, as shown in Fig. 11(e). Layer intervals of each

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tube are 0.35 nm, as shown in Fig. 11(f). Diameters of each nanotube are 2.8 nm, 3.5 nm, 4.2 nm and 4.9 nm from the inside to outside. Another HREM image of BN nanotube produced from YB6 powder is shown in Fig. 12(a). Width of the multi-walled BN nanotube is 8.5 nm. The BN nanotube consists of 9 layers and has asymmetry layer-arrangements. Layer distances are in the range of 0.34 ~ 0.51 nm, which is larger than that of {002} of ordinary hexagonal BN (0.34 nm). Diameters of the first and second internal nanotubes are 1.7 nm and 2.6 nm, respectively. Hexagonal net planes of BN nanotube are observed in an enlarged image of Fig. 12(b). Figure 12(c) is a filtered Fourier transform of Fig. 12(b), which indicates 002 and 100 reflections of BN structure. Inverse Fourier transform of Fig. 12(c) is shown in Fig. 12(d), which indicates lattice fringes of hexagonal networks clearly. A hexagonal BN ring is shown in Fig. 12(d), and the BN has an armchair-type structure. Atomic structure models were proposed from observed diameters of BN nanotubes, which were based on layer intervals of 0.34-0.35 nm. The chirality of (n, m) is derived from the next equation. dt =

3 a B − N n 2 + nm + m 2

π

(1)

The dt means a diameter of BN nanotube with nm scale, and the aB-N corresponds to the nearest distance of boron and nitrogen atoms. For the BN nanotubes, the value of aB-N is 0.144 nm. When a BN nanotube has a zigzag structure, the value of m is zero. Figure 13(a) shows a proposed structure model of the quadruple-walled BN nanotube. Chiralities of each zigzag BN nanotube are (35, 0), (44, 0), (53, 0), and (62, 0) from the inside to outside. These chiralities were derived from the equation (1). The arrangement of boron and nitrogen atoms was reversed at each layer, as boron atoms exist just above the nitrogen atoms with keeping the layer intervals of 0.35 nm. Calculated images of the proposed model as a function of defocus values are shown in Fig. 13(b). Contrast of hexagonal rings was clearly imaged at the

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defocus values in the range of -40 to -50 nm, and these simulated images agree well with the observed HREM image of Fig. 11(d). A proposed structure model of double-walled BN nanotube corresponding to Fig. 12 is shown in Fig. 13(c). Chiralities of the BN nanotube are (13, 13) and (19, 19) for the first and second layers, respectively. Layer intervals of lattice fringes of {002} planes are accorded with observed ones in Fig. 12(a). Based on the projected structure model, image calculations were carried out for various defocus values, as shown in Fig. 13(d), and a HREM image calculated at -40 nm agrees well with the experimental data of Fig. 12(d). When the zigzag-type BN nanotubes are taken by HREM, the BN atom pairs at sides of the nanotubes are sometimes imaged as dots, as observed in Fig. 11(f). Taking such dot contrast would be difficult for armchair-type BN nanotubes because of high density distribution of atoms along nanotube axis, and HREM image contrast would show straight lines at the sides of nanotubes. Multi-helix BN nanotubes would also show unclear dot-contrast at the side of the nanotube, which indicates that the contrast of side edges of BN nanotubes would also give us useful information on the chirality. If clear dot-contrast is observed at the sides of BN nanotubes, possibility of zigzagtype BN nanotube is high. Image contrast also could be changed by rotation of nanotubes, and the further study on the rotation of BN nanotubes was presented.80 Structural stabilities of BN nanotubes were investigated and summarized as listed in Table 2. Atomic structure models of B66N66, B65N65, B990N990, B992N992 and B36N36@B990N990 nanotubes were used for the molecular mechanics calculations (MM2), as shown in Fig. 14. Since an effect of nanotube edges should be considered, nanotubes with short and long lengths were investigated. For Table 2, total energies of zigzag-type structures show lower values than those of armchair-type structures, which indicates the zigzag-type is more stable compared to the armchair-type structures. This also agrees with distorted nanotube structures in experimental data of Fig. 12. A similar experimental result was also reported,81 and the present calculations also confirmed the stability of the zigzag-type BN nanotubes. Encapsulation of a BN cluster in a BN nanotube showed reduction 11

of total energy as shown in Table 2, which indicates that the encapsulation of the BN cluster in BN nanotube would stabilize the BN cluster. Another HREM image of a BN nanotube synthesized from YB6 powder is shown in Fig. 15(a), which was taken nearly at Scherzer defocus. Number of BN {002} layers is 12, and lattice fringes are observed in the BN nanotube. A filtered Fourier transform of Fig. 15(a) is shown in Fig. 15(b). Spots of BN 002 are observed as bright spots. In addition, reflections corresponding to the yttrium structure are observed and indexed with the incident electron beam along the [101] direction. Figure 15(c) is an inverse Fourier transform of Fig. 15(b), and BN{002} layers are clearly observed in the image. An enlarged image of Fig. 15(c) is shown in Fig. 15(d), which indicates lattice fringes at the center of the BN nanotube. Lattice parameters of yttrium with a hexagonal structure, as determined by X-ray diffraction analysis,82 were a = 0.36474 nm and c = 0.57306 nm, which agrees well with the present lattice fringes. Dark contrast corresponds to yttrium atom pairs, as indicated in Fig. 15(d). Based on the observations, an atomic structure model of yttrium along [101] was constructed as shown in Fig. 15(e), which indicates the yttrium atom pairs. Figure 15(f) is a calculated diffraction pattern of Fig. 15(e), and agree well with the observed Fourier transform of Fig. 15(b). Since YB6 powders formed BN nanotubes in the present work, boron atoms were consumed preferentially. As a result, yttrium element would be remained in the BN nanotube as a nanowire. These BN nanotubes with metal nanowires would be interesting nanomaterials for nanocables.

CONCLUSIONS

BnNn (n = 12~60) nanocage clusters and endohedral Y@BnNn were synthesized by an arcmelting method from YB6 powder in N2/Ar mixture gas, and detected by LD-TOF mass spectrometry and HREM. The BN clusters consisted of tetragonal, hexagonal, octagonal and decagonal BN rings satisfying the isolated tetragonal rule, which was optimized by molecular orbital calculations. Atomic and electronic structures of endohedral M@B36N36 clusters with doping 12

elements were also studied by molecular orbital calculations. Total energy calculation showed that potassium and gallium stabilize the B36N36 structure. Bandgap energies of the B36N36 clusters were found to be reduced by introducing a metal atom inside the cluster, which indicates controllability of the energy gap. BN nanotubes with zigzag- and armchair-type structures were synthesized and investigated by HREM, image simulation and total energy calculation. Hexagonal networks of BN nanotubes were directly observed by HREM in atomic scale, and chiralities of the BN nanotubes were directly determined from HREM images. Atomic structure models for quadruple- and double-walled nanotubes were proposed, and simulated images based on these models agreed well with experimental HREM images. Molecular mechanics calculations showed good stability of a zigzagtype structure compared to the armchair-type structure, which agreed well with the experimental data of disordered armchair-type BN nanotubes. BN nanotubes encapsulating a B36N36 cluster and a yttrium nanowire were also produced and confirmed by HREM and diffraction calculation. These new BN clusters, metallofullerenes and nanotubes can be expected to have applications in a wide variety of future nanodevices in combination with carbon-based fullerenes and nanotubes.

ACKNOWLEDGEMENTS The authors would like to acknowledge Mr. M. Gonda for mass spectrometry measurements.

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18

Table 1. Calculated values for BnNn (n = 12-60) clusters.

B12N12

B24N24

B28N28

B36N36

B48N48

B60N60

-298.3

-832.4

-1162.6

-1597.8

-2312.3

-2311.1

-12.4

-17.3

-20.8

-22.2

-24.1

-19.3

Tetragonal BN rings

6

12

6

6

6

30

Hexagonal BN rings

8

8

24

32

52

20

Octagonal BN rings

0

6

0

0

0

0

Decagonal BN rings

0

0

0

0

0

12

BN10-6 (nm)

─

─

─

─

─

0.1399

BN10-4 (nm)

─

─

─

─

─

0.1492

BN8-6 (nm)

─

0.1425

─

─

─

─

BN8-4 (nm)

─

0.1503

─

─

─

─

BN6-4 (nm)

0.1527

0.1529

0.1512

0.1509

0.1511

0.1532

BN6-6 (nm)

0.1462

─

0.1493

0.1487

0.1472

─

dmax (nm)

0.488

0.683

0.780

0.743

1.063

1.102

dmin (nm)

0.488

0.682

0.647

0.882

0.862

1.102

εHOMO (eV)

-3.123

-3.127

-2.993

-3.051

-2.983

-2.966

εLUMO (eV)

1.957

1.818

2.230

2.316

2.221

1.944

Energy gap Eg (eV)

5.080

4.945

5.222

5.367

5.205

4.910

Heat of formation (kcal/mol) Heat of formation per atom (kcal)

─ : Data were not available.

19

Table 2. Calculated values for BN nanotubes.

B66N66 Structure type Chirality Total energy (kcal/mol) Total energy per atom (kcal)

B65N65

B990N990

B992N992

B36N36

B36N36@B990N990

zigzag armchair

zigzag

armchair

cluster

zigzag

(22, 0)

(13, 13)

(22, 0)

(13, 13)

─

(22, 0)

7.87

10.11

67.68

2949.17

215.21

260.08

0.0596

0.0778

0.0342

1.4865

2.9878

0.1267

20

(a)

Atom

Cluster < 10 nm

Doping, Intercalation Quantum size effect Self-organization

Solid clusters Superconductivety Ferromagnetism Onion Multi-shell Elec. St

Fullerene Metallofullerene Superatom

h-BN Graphite-FL Eg = 0~1.7eV Eg = ~5eV Non-magnetism Chemical Inertness High-T stability Lubricity

Nanotube Eg – chirality, diameter SET, FEE, Gas storage Nanodiode, Catalysis

Nanocapsule Cluster protection Superparamagnetism Luminescence Coulomb blockade SS Lubricant, NBB Radioactive elements

(b) Medicines

C60

Nanotube B

Nano-transportation

Nanocoil (Permanent current)

Oxide layer Gate Drain

Nanocapsule Nanospring Source Nanotube

Nanomachine Armchair Junction Zigzag

H2 H2

Nanodiode C nanotube Eg=0∼1.7eV

Display device

C60 superconductor

BN nanotube Eg=∼5eV

STM tip/ nanotube

H2 > 6 wt.% Hydrogen storage

Fig. 1. (a) Structures and properties of BN/C fullerene materials. (b) Future of BN/C fullerene materials.

21

(a)

×10 250

×500 500

×1000

600

(BN)24

Intensity (A. U.)

BN clusters

(BN)37 Y@(BN) 36

Y@(BN)37 (BN)46 Y2@(BN)44 (BN) Y@(BN)48

(BN)36 (BN)34

48

B12

(BN)54 (BN)56 Y@(BN)54 (BN)60

(BN)12 (BN)28

0

200

(b)

400 ×10

800 m/z

×500 500

1000

1200

1400

1600

×1000

600

Pyridine (Background)

Intensity (A. U.)

250

600

0

200

400

600

800 m/z

1000

1200

1400

1600

Fig. 2. LD-TOF mass spectrum for (a) BN clusters and (b) pyridine (background).

22

(b) (a)

N N

B

(c)

B

(d) N B N

B

(f) (e)

N

B

N

B

Fig. 3. Atomic structure models of (a) B12N12, (b) B24N24, (c) B28N28, (d) B36N36, (e) B48N48 and (f) B60N60 clusters viewed along hexagonal BN rings. Tetragonal BN rings are indicated by star marks.

23

a

(BN)36

(d)

1nm

b

(BN)36

(e)

Y

Y

B

N

1nm

c

(BN)36

(f)

Y N

Y

B

1nm

Fig. 4. HREM images of (a) B36N36 and (b),(c) Y@B36N36 clusters. Structure models of (d) B36N36 and (e),(f) Y@B36N36 clusters corresponding to (a) and (b),(c), respectively. The structure model (e) is viewed perpendicular to (f).

24

a

Δf = -10nm

-20nm

-30nm

-40nm

-50nm

-60nm

-70nm

-80nm

-90nm

b

Δf = -10nm

-20nm

-30nm

-40nm

-50nm

-60nm

-70nm

-80nm

-90nm

c

Δf = -10nm

-20nm

-30nm

-40nm

-50nm

-60nm

-70nm

-80nm

-90nm

Fig. 5. Calculated HREM images of (a) B36N36 and (b),(c) Y@B36N36 clusters as a function of defocus values along the same directions of Fig. 4d-f, respectively.

25

(a) 10

(b) 10 Total

Total

0 N-2p -5

-10

B-2s N-2p -5

N-2s

B12N12

-15

Density of States

(d) 10

(c) 10 Total

B-2p

5 Energy (eV)

Energy (eV)

B24N24

-15

Density of States

Total

0 N-2p -5 B-2s

Total

B-2p

Total 0 N-2p

B-2s -5

-10

-10 -15

Total

0

-10

B-2s N-2s

5

Total

B-2p 5 Energy (eV)

Energy (eV)

5

B-2p

B48N48

N-2s

-15

N-2s

B60N60

Density of States

Density of States

Fig. 6. Densities of states for (a) B12N12, (b) B24N24, (c) B48N48 and (d) B60N60 clusters.

26

1 2 3 4

1/I 2/II H -18.68 Li Be -18.75 16.38 Na Mg -19.41 -19.67 K Ca -20.27 -11.61 Rb Sr

3

4

5

Cs Fr

Ba * Ra

7

Sc

Ti

V

Cr

Mn

Y

Zr

Nb

La-Lu

Hf

Ta

Mo * W

Ac-Lr

Rf

Db

Sg

5 6

6

8

9

Tc

Fe -16.71 Ru

Co Rh

Re

Os

Ir

Bh

Hs

Mt

10

11

12

13/III

14/IV

15/V

16/VI

17/VII 18/VIII He

B C N O F -17.47 -16.42 -17.79 Al Si P S Cl -18.49 -17.53 -17.81 -18.74 -19.00 Ni Cu Zn Ga Ge As Se Br -17.93 -17.81 -19.74 -18.97 -18.48 -19.00 Pd Ag Cd In Sn Sb Te I -18.84 -17.78 -17.40 Pt Au Hg Tl Pb Bi Po At -15.17 -17.84 -18.04 Unn Unu Unb

Ne Ar Kr Xe Rn

7

Fig. 7. Heat of formation (kcal/mol) of endohedral M@B36N36 clusters with doping element M. Total Energy of B36N36 per atom is -19.68 kcal/atom. Asterisks indicate B36N36 structure was highly distorted or broken.

27

(a) 10 B-2p

Total

5

Total

Energy (eV)

Energy (eV)

5

(b) 10

0 N-2p -5

B-2s2p 0

Total -5 N-2s2p

-10

-10 B36N36

N-2s

Y@B36N36

-15 Density of States

Density of States

(c) 10

(d) 10

5

5

0

Fe-3d4s4p

-5

N-2s2p

Energy (eV)

Total Energy (eV)

Y-4d5s5d

B-2s

-15

Total

Total

K-4s4p 0

Total Total

-5 N-2s2p

B-2s2p -10 -15

-10 Fe@B36N36

B-2s2p K@B36N36

-15

Density of States

Density of States

Fig. 8. Densities of states for (a) B36N36, (b) Y@B36N36, (c) Fe@B36N36 and (d) K@B36N36 clusters.

28

a

d

3nm

10nm

(e)

b

z x

z

2nm

y

y

(c)

z x

Fig. 9. (a) Low magnification image of BN nanotubes. (b) HREM image of B36N36 cluster in BN nanotube. (c) Structure model of the center of (b). (d) HREM image of bundled BN nanotubes. BN clusters are indicated by arrows. (f) Structure models for bundled BN nanotubes.

29

(a) 1/I H

2/II

Li

Be

Na

Mg × Ca

3

4

5

6

7

8

9

10

11

12

13/III 14/IV 15/V 16/VI 17/VII 18/VIII He

1 2 3 K

Sc

Ti 〓●

4 Rb 5 Cs 6 Fr

Sr

Y 〓 Ba La-Lu 〓 Ra Ac-Lr

Zr 〓 Hf 〓 Rf

V ● Nb 〓● Ta 〓 Db

Cr

Mn Tc

Fe ●〓 Ru

Co ● Rh

Mo W 〓 Sg

Re

Os

Ir

Bh

Hs

Mt

Ni ● Pd ● Pt

Cu ● Ag

Zn Cd

B × Al ● Ga ○ In

C

N

O

F

Ne

Si × Ge ● Sn

P

S

Cl

Ar

As

Se

Br

Kr

Sb

Te

I

Xe

Tl

Pb

Bi

Po

At

Rn

Au Hg ● Unn Unu Unb

7

Formation enthalpy (HforN-HforB(kJ))

(b)

150 Ir Pt

100 Fe 50

Cr

Pd

Co Ni

Nanocage Nanocapsule

Mo

Mn

0 V -50

Nb

Ta

Ti -100 Sc -150

Nanotube

Hf

Zr La

Y

Elements

Fig. 10. (a) Catalysis metals for BN fullerene nanomaterials confirmed by experiments on arcmethod (=, BN nanotube; ● , BN nanocapsule; ○ , BN nanocage; × , non BN fullerene nanomaterials). (b) Difference formation enthalpy (HforN - HforB) of nitrides and borides.

30

a

b

0.5nm

2nm

c

d 002 010

100

000

110

0.5nm

e

f 0.14 nm

N B 0.35 nm

N B

Fig. 11. (a) HREM image of zigzag-type BN nanotube. (b) Enlarged HREM image of (a). (c) Filtered Fourier transform of (b). (d) Inverse Fourier transform of (c). (e), (f) Enlarged images of (d).

31

a

b

0.5nm

2nm

c

d 002 010

000 100 0.14 nm

110

N B

Fig. 12. (a) HREM image of armchair-type BN nanotube. (b) Enlarged HREM image of (a). (c) Filtered Fourier transform of (b). (d) Inverse Fourier transform of (c).

32

(b)

B

Δf = -10nm

-20nm

-30nm

-40nm

-50nm

-60nm

-70nm

-80nm

-90nm

3.49 nm (44,0) 4.21 nm (53,0) 4.92 nm (62,0)

2.78 nm (35,0)

(a)

N

(d) (c) 0.48 nm

-20nm

-30nm

-40nm

-50nm

-60nm

-70nm

-80nm

-90nm

2.61 nm

Δf = -10nm

0.34 nm

Fig. 13. (a) Proposed structure model of quadruple-walled BN nanotube. Chiralities of zigzag BN nanotubes are (35,0), (44,0), (53,0) and (62,0) from inside to outside. (b) Calculated images of the proposed model (a) as a function of defocus values. (c) Proposed structure model of double-walled BN nanotube. Chiral vectors of nanotube are (13, 13) and (19, 19) for the first and second layers, respectively. (d) Calculated images of the proposed model (c). 33

(a)

(b)

B

N

N B

1.8 nm

1.8 nm

(c)

(d)

(e)

Fig. 14. Atomic structure models of (a) B66N66, (b) B65N65, (c) B990N990 (d) B992N992 and (e) B36N36@B990N990 nanotubes. (a,c,e) and (b,d) are zigzag- and armchair-type structures, respectively.

34

a

b 011-Y

102-Y

000 002-BN

2nm

c

d

Y Y

0.3nm 1nm

e

f 011-Y Y 102-Y 000

Fig. 15. (a) HREM image of BN nanotube synthesized from YB6 powder. (b) Filtered Fourier transform of (a). (c) Inverse Fourier transform of (b). (d) Enlarged image of (c). (e) Atomic structure model of yttrium along [101]. (f) Calculated diffraction pattern of (e).

35