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Physica E 27 (2005) 319–324

www.elsevier.com/locate/physe

Crystalline boron oxide nanowires on silicon substrate

Qing Yanga, Jian Shab, Lei Wanga, Yu Zoua, Junjie Niua,Can Cuia, Deren Yanga,�

aState Key Laboratory of Silicon Materials, Zhejiang University, 20 Yu Gu Road, Hangzhou 310027, People’s Republic of ChinabDepartment of Physics, Zhejiang University, 20 Yu Gu Road, Hangzhou 310027, People’s Republic of China

Received 26 November 2004; accepted 20 December 2004

Abstract

Crystalline boron oxide nanowires have been synthesized on silicon substrates by chemical vapor deposition (CVD)

process without the use of catalysts or templates. It is pointed out that the boron oxide nanowires are cubic and single

crystalline, and the diameter of the nanowires is in the range of 20–80 nm: Some of the nanowires branched, and thediameters of the branches and stems of the branched boron oxide nanowires are in the range of 20–80 and 100–200nm;respectively. The crystallinity, morphology, and structure features of the as-prepared boron oxide nanowires were

investigated by field emission scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and

selected area electron diffraction. Furthermore, Raman spectrum and Fourier transform infrared spectroscopy of the

nanowires were also investigated.

r 2005 Elsevier B.V. All rights reserved.

PACS: 81.05.Je; 61.46.+w; 81.15.Gh

Keywords: B2O3; Nanowires; CVD

1. Introduction

The trivalent elemental boron, characterized byits short covalent radius, sp2 hybrid valenceorbital, three-center deficiency bonds, high meltingpoint of 2300 �C, hardness that is similar to that ofdiamond, etc. has been intensively focused on [1].

e front matter r 2005 Elsevier B.V. All rights reserve

yse.2004.12.015

ng author. Tel.: +860571 87951667;

7952322.

ss: mseyang@zju.edu.cn (D. Yang).

The curious discovery of new superconductingmaterial MgB2 above 40K has accelerated boronand boron-related compounds’ research [2–4].Boron oxide is one of the additives widely usedto modify textural and acid-based properties ofmetal oxides such as Al2O3; TiO2; and MgO [5],and metal borate is an excellent antiwear andreducing friction additive [6]. One-dimensional(1D) materials, having the potential to go farbeyond the limits of top-down manufacturing, areimportant for future nanodevices and fundamental

d.

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research [7,8]. Till now, boron and boride com-pound nanostructures such as amorphous [9,10]and crystalline boron nanowires [11–13], crystal-line boron nanobelts [14], MgB2 nanowires [15,16],metal borate nanowires and nanotubes [17,18],and amorphous boron oxide nanowires with adiameter of 40–100 nm on Mg surface [19] havebeen synthesized. To achieve the functionality ofthese quantum wires as building blocks in poten-tial nanoscale electronic and mechanical devices,several objectives are important. One objective isto fabricate nanowires on silicon substrates andintegrate them to useful devices. Forming nano-wires by chemical vapor deposition (CVD) isespecially attractive because of the compatibilityof CVD with conventional semiconductor devicefabrication and also the ease of scaling fromresearch to production-size systems.Silicon nanotubes [20] and nanowires [21], silica

nanowires [22], boron nanowires [12], and MgB2nanowires [16] etc. have been synthesized by ourgroup using CVD as the fabrication method andnano-channel Al2O3 or silicon as the substrates.Here, we report the preparation of crystallineboron oxide nanowires on silicon substrates usinga simple chemical vapor deposition (CVD) methodwhich is different from the amorphous boronoxide nanowires synthesized through infraredirradiation reported by Ma et al. [19], the as-synthesized nanowires are single crystalline. Forconsideration of the compatibility with the inte-grated circuits, single crystal silicon substrateswere used in our experiments. The optical proper-ties of these nanowires have also been character-ized by Raman scattering spectroscopy andFourier transform infrared spectroscopy.

2. Experimental details

The experimental apparatus consists of ahorizontal tube furnace (75 cm in length), areacting chamber made of quartz tube(f 5:5� 150 cm), a rotary pump system, and agas supply and control system. The ultimatevacuum for this configuration is � 70Pa: Severalsilicon substrates were placed downstream one byone from the center of the furnace to the end of the

furnace, for collecting growth products. Thefurnace chamber was first pumped down to100Pa and heated. As the temperature reached1000 �C, the gas mixture of argon, hydrogen anddiborane with a flow ratio of 50:20:1 was allowedinto the chamber. The pressure and temperature inthe chamber were kept at 2000Pa and 1000 �C,respectively. After 5 h deposition the chamber wascooled to room temperature with the furnace.Boron oxide nanowires were collected on thedownstream place of 30 cm from the center ofthe furnace at the temperature of about 400 �C.Field emission scanning electron microscopy(FESEM, Sirion), energy-dispersive X-ray spectro-scopy (EDX) attached to the FESEM, transmis-sion electron microscopy (TEM, JEM200CX,200 kV), selected-area electron diffraction(SAED), and X-ray diffraction (XRD, D/max-rA, with CuKa radiation) were carried out for themorphological and structural observation. Ramanscattering spectroscopy (Nicolet, Almega) andFourier transform infrared (FTIR, Bruker, IFS66v/S) were performed for the vibrational investi-gation. All the measurements were carried out atroom temperature.

3. Results and discussion

Fig. 1 shows the XRD pattern of the as-synthesized nanowires. All the three diffractionpeaks can be perfectly indexed to the cubiccrystalline B2O3 ða ¼ 10:05 (AÞ; and the diffractiondata are in agreement with JCPDS card of 06-0297, not only in the peaks’ positions, but also intheir relative intensity. The two peaks in thespectrum can be readily indexed as (3 1 0) and(4 2 0) crystal planes of the cubic structure B2O3;respectively.The morphology of the B2O3 nanowires was

observed via SEM and TEM, as shown in Figs. 2and 3. Fig. 2 displays the SEM image of the B2O3nanowires. A large quantity of curved and verylong nanowires could be seen. The TEM image ofthe nanowires is depicted in Fig. 3(a); it can beseen that the diameter of the nanowires is inthe range of 20–80 nm: SAED pattern of thenanowires shows that the nanowires are single

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Fig. 2. SEM image of the boron oxide nanowires on silicon

substrates.

Fig. 3. TEM images of the B2O3 nanowires. (a) TEM image of

curved long boron oxide nanowires. (b) A branched nanowire.

10 20 30 40 500

150

300

450

600

420

310

CP

S

2θ2θ

B2O3

Fig. 1. XRD patterns of the boron oxide nanowires synthesized

by CVD process. All the three diffraction peaks can be perfectly

indexed to the cubic crystalline B2O3 ða ¼ 10:05 (AÞ; while thediffraction data are in agreement with JCPDS card of 06-0297.

Q. Yang et al. / Physica E 27 (2005) 319–324 321

crystalline B2O3 with the lattice constant of a ¼

10:05 (A; consistent with the XRD result shown inFig. 1. Some branched boron oxide nanowirescould also be found in the TEM characterization(Fig. 3(b)), which shows that the nanowiresnucleate and grow on the sidewall of the backbonenanowire and form multiple nanojunctions. Thediameters of the branches and stems are in therange of 20–80 and 100–200 nm; respectively. TheSAED pattern taken from the branched nanowire

shown in the inset of Fig. 3(b) illustrates that thebranched nanowire is also a single crystal cubiclattice structure of boron oxide with the latticeconstant of a ¼ 10:05 (A: The ripple-like contrastobserved in the TEM images is due to the strainresulting from the bending of the nanowires [23].Fig. 4 shows the Raman spectra of the as-

synthesized B2O3 nanowires (curve a) and bulkB2O3 (curve b). There are two peaks in thespectrum of the B2O3 nanowires. The dominantline at 878 cm�1 is generally attributed to abreathing mode of the three oxygen atoms withina boroxol ring [24]. Its broadened width andasymmetric shape compared with the Ramanspectrum of bulk boron oxide could be due tothe strain of the defects existing on the nanowires.The relatively weaker line at 500 cm�1 in theRaman spectrum of the boron oxide nanowires isattributed to the Si–B stretching mode [25].

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600 800 10000

1500

3000

4500

600050

0

875

878

(b)

(a)

Ram

an In

tens

ity

Raman Shift (cm-1)

Fig. 4. Raman spectra of the boron oxide nanowires (a)

compared with that of bulk boron oxide (b).

1000 2000 3000 4000

1.8

2.4

3.0

3.6

3207

.4

2360

.725

08.9

1456

.1

1195

.810

83.9

Abs

orba

nce

Wavenumbers (cm-1)

2260

.4

Fig. 5. FTIR spectrum of the boron oxide nanowires.

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Compared with the Si–B stretching mode in bulkstructure at 515 cm�1; the downshift of it might becaused by the tensile deformation between the Sisubstrate and the boron oxide nanowires. Theexistence of the Si–B bond shows that the as-synthesized boron oxide nanowires are chemicallyconnected with silicon substrates.Fig. 5 shows the FTIR spectrum of the B2O3

nanowires subtracting the spectrum of the siliconsubstrate. For eliminating the influence of thesilicon substrate, a same silicon plate without theboron oxide nanowires was measured by means ofFTIR at the same measurement conditions. It iswell known that the band at around 3207:4 cm�1 isrelated to O–H stretching bond with the hydrogenbeing involved in a hydrogen bridge bond [26]. Thesmall peak appears at 2260:4 cm�1; it can beattributed to O–Si–H bond [27]. The three bandsat 1083.9, 1195.3, and 1456:1 cm�1 can be attrib-uted to B–O–B stretching mode, B–O–H d bond,and B–O terminal bond, respectively [28]. Theother two bands at 2360.7 and 2508:9 cm�1 areattributed to B–H terminal bonds [29]. Theexistences of B–H, B–O–H, and O–H bonds showthat the boron oxide nanowires are hydrogenatedbecause of the gas of diborane and the hydrogenused for synthesis of the nanowires.Vapor–liquid–solid (VLS) mechanism [30], so-

lution–liquid–solid (SLS) mechanism [31], vapor-phase (VS) mechanism [32], and oxide-assistedgrowth mechanism [33] are the most successful

mechanisms for explaining the growth of 1Dnanostructures. In VLS and SLS mechanisms,the 1D growth of nanowires is mainly induced byliquid droplets at the tip of nanowires. It seemsthat the VLS and SLS model are not suited for ourcase because no metal catalysts were used in ourexperiments. In addition, no metal or alloyparticles can be found at the tips of the boronoxide nanowires. We suggest that our use of theCVD method to grow crystalline boron oxidenanowires may involve a mechanism complex ofoxide-assisted and VS mechanism. The growthprocess might be as follows: first, diboranedecomposed and formed boron clusters thatcondensed on the substrates placed in the centerof the furnace to form boron particles. Then theparticles were oxidized by the residual oxygen andimpure reaction gases in the quartz tube to formB2O3 particles. The chemical reactions might be

B2H6 ! 2Bþ 3H2, (1)

4Bþ 3O2 ! 2B2O3. (2)

Boron oxide has a low melting point (� 450 �C).When heated to 1000 �C; the boron oxide ismolten. The molten boron oxide produced B2O3vapor, and B2O3 vapor was transported by thecarrier gas to the low-temperature region andbecame super cold droplets. When the temperatureis lower than the melting point of B2O3; the supercold droplets will condense on the substrates

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placed at the low-temperature zone as the nuclei ofthe nanostructures and boron oxide nanowiresformed finally. Some nanowires could nucleate onthe defects of other nanowires and formedbranched nanowires. It should be noted thatboron oxide nanowires could not form whenboron oxide powders were put in the center ofthe furnace and the furnace was heated to 1000 �Cunder the flow of a gas mixture of Ar and H2: Thereason might be that the volatilization of bulkB2O3 is too fast at a high temperature and toomuch B2O3 vapor formed at the same time.Therefore, just boron oxide particles, but noboron oxide nanowires, could form. However, inthe CVD system, the quantity of diborane could becontrolled, and the process of forming B2O3 vaporis slow. B2O3 nanowires could form throughvapor–solid mechanism. Additionally, when thetemperature was not high enough (lower than950 �C) and the vacuum degree was high, boronnanowires formed instead of boron oxide nano-wires because of the low oxidization of boronclusters [12].

4. Conclusions

In summary, crystalline boron oxide nanowireshave been synthesized on silicon substrates by theCVD process without the use of catalysts ortemplates. The diameter of the nanowires is inthe range of 20–80 nm: Some of the nanowires arebranched. Raman spectrum and FTIR spectro-scopy have been checked on the nanowires. Thebroadened width and asymmetric shape of peaksin the Raman spectrum could be due to the strainof the defects existing on the nanowires. The FTIRspectrum of the B2O3 nanowires shows that thenanowires are hydrogenated. The growth mechan-ism of the B2O3 nanowires may involve amechanism complex of oxide-assisted and VSmechanism.

Acknowledgements

This work is financially supported by theNational Natural Science Foundation of China

(Project nos 50272057 and 60225010), and the keyproject of the Education Department of China.And the authors would thank Mr. Youwen Wangfor his great help in TEM and SEM measurement.

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