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Bi2O3 nanowire growth from high-density Bi nanowires grown at a low temperature using aluminum–bismuth co-deposited films
Sensors and Actuators B 156 (2011) 709–714
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Bi2 O3 nanowire growth from high-density Bi nanowires grown at a low temperature using aluminum–bismuth co-deposited films Yeon-Woong Park a , Hyun-June Jung a , Soon-Gil Yoon a,b,∗ a b
School of Nano Science and Technology, Chungnam National University, Daeduk Science Town, 305-764 Daejeon, Republic of Korea Graduate School of Analytical Science and Technology (GRAST), Chungnam National University, Daeduk Science Town, 305-764 Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 September 2010 Received in revised form 10 February 2011 Accepted 14 February 2011 Available online 21 February 2011 Keywords: Bi2 O3 nanowires Bi nanowires Bi–Al co-sputtered films Low temperature Gas sensing DC sputtering
a b s t r a c t Single crystalline Bi2 O3 nanowires were prepared by annealing in oxygen ambient using pure Bi nanowires grown at a low vacuum (∼10−6 Torr) with Bi–Al co-sputtered films. The ability to grow Bi nanowires using Bi–Al co-sputtered films can be attributed to the suppression of the oxidation of the bismuth by the preferred oxidation of aluminum in co-sputtered ambient (∼mTorr). The Bi nanowires from the Bi–Al co-sputtered films could be grown even at the low temperature of 230 ◦ C in low-vacuum ambient. The Bi2 O3 nanowires prepared from the Bi nanowires showed a single crystalline structure with (1 1 1), (1¯ 2 2), (1 2 0), and (0 1 2) planes. The current–voltage (I–V) relationship of the Bi2 O3 nanowire revealed that the Bi2 O3 nanowire exhibited a semiconducting property with a resistivity of 14.6 -cm. Variations in resistance of the Bi2 O3 single nanowire as a function of time at 350 ◦ C showed reproducible response and recovery time characteristics for each concentration of NO. The electric resistance of the Bi2 O3 single nanowire was sensitive to NO gas even at 10 ppm. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The search for new growth methods for one-dimensional structures with a nanometer diameter, such as nanowires, continues to be of central importance in nanoscience and nanotechnology, since these represent a unique system for exploring dimension dependent nanoscale properties and are expected to play a crucial role in the future of electronic, optoelectronic and electromechanical devices [1–3]. Considering the importance of metal oxides in catalysis, electrochemistry, optics, functional ceramics, and sensors, fabrication in one-dimensional nanostructured morphology appears to be a particularly attractive goal and has been pursued by a number of laboratories. To date, many types of metal oxide nanowires have been successfully synthesized. Bismuth oxide (Bi2 O3 ) is fascinating to scientists owing to its unique structure and physical properties such as a large energy band gap, a high refractive index, a dielectric permittivity and a high oxygen conductivity, as well as remarkable photoconductivity and photoluminescence [4–6]. Despite the scientific and technological attractiveness of Bi2 O3 , the synthesis of a 1D nanostructure using Bi2 O3 has not received great attention. Bismuth oxide nanowires
∗ Corresponding author at: Graduate School of Analytical Science and Technology (GRAST), Chungnam National University, Daeduk Science Town, 305-764 Daejeon, Republic of Korea. Tel.: +82 42 821 6638, fax: +82 42 822 3206. E-mail addresses: [email protected] (Y.-W. Park), np [email protected] (H.-J. Jung), [email protected] (S.-G. Yoon). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.02.023
have been prepared using various techniques such as metalorganic chemical vapor deposition (MOCVD) [7], chemical methods [8], and an oxidative metal vapor transport deposition technique [9]. These successful techniques for the synthesis of Bi2 O3 nanowires are based mainly on high-temperature, solid-state chemistry reaction routes, which are energy intensive and problematic for integration with silicon-based microelectronics. Herein, high-quality single-crystalline Bi nanowires were prepared using a novel stress-induced method at low temperature on a silicon substrate with subsequent anneal at low temperature in oxygen ambient. Single-crystalline Bi nanowires have already been grown using Bi thin films in an ultrahigh vacuum (UHV, ∼10−7 Torr) [10]. For the convenient growth of single-crystalline Bi nanowires from Bi films at a low vacuum (below 10−6 Torr), high-density was achieved using Bi–Al co-sputtered films, and the growth mechanism is addressed in the present study. The gas-sensing properties of NO using a Bi2 O3 single nanowire also are presented. 2. Experimental For Bi–Al films on SiO2 /Si substrates, 290-nm-thick Bi–Al cosputtered films were prepared for 10 min at dc powers of Bi (10 W) and Al (20 W), and a working pressure of 3 mTorr in argon ambient. Deposition rates of the Bi and Al films were approximately 14.3 and 2.7 nm/min, respectively. The film thickness for the determination of the deposition rate was measured using the SEM cross-sectional image of the films grown at a constant deposition
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Fig. 1. SEM images of the Bi films annealed for 10 h at different annealing temperatures: (a) unannealed, (b) 230 ◦ C, (c) 250 ◦ C, and (d) 270 ◦ C using 290-nm-thick Bi thin films grown by dc sputtering. Here, the annealing of the Bi films was performed at 3 × 10−6 Torr.
time. The Bi–Al co-sputtered films prepared under these deposition conditions exhibited a composition of Bi:Al = 1:1. The composition of the co-sputtered films was determined using an electron-probe micro-analyzer (EPMA). The bismuth nanowires were prepared by annealing at 270 ◦ C for 10 h in 3 × 10−6 Torr using Bi–Al cosputtered films. The Bi2 O3 nanowires were prepared by annealing at 300 ◦ C for 2 h in O2 ambient using Bi nanowires. The crystallinity and microstructure of co-sputtered Bi–Al films were characterized using X-ray diffraction (XRD) excited by Cu K␣ radiation and scanning electron microscopy (SEM), respectively. The identification of the oxidation state of the co-sputtered films was performed by
X-ray photoelectron spectroscopy (XPS). The crystal structure of the single-crystalline Bi and Bi2 O3 nanowires was examined using high-resolution transmission electron microscopy (HRTEM). The current–voltage (I–V) characteristics of the Bi2 O3 nanowires were investigated by fabricating two terminal devices with Pt contacts (50 m separation) deposited by a focused-ion-beam (FIB, Helios Nano Lab 600). To measure the sensing properties, Pt electrodes 100 m wide and 3 mm long were patterned onto the SiO2 /Si substrates and a single nanowire was connected to the Pt electrodes. The sensor resistance of the Bi2 O3 single nanowire was measured using a programmable electrometer (Keithley 2400) controlled by
Fig. 2. XPS spectra of the (a) Bi 4f in the Bi films and (b) Al 2p in the Al films, (c) Bi 4f and (d) Al 2p in the Bi–Al co-sputtered films. Each spectra in (a)–(d) were obtained before and after etching treatment for 30 s.
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Fig. 3. XRD patterns of (a) Bi, Al, and Bi–Al co-sputtered films (BA films) and (b) of narrow regions from 35◦ to 40◦ in each of the films.
a personal computer using the Labview program for various concentrations of NO gas at 350 ◦ C.
3. Results and discussion Fig. 1 shows the SEM images of the samples annealed for 10 h at different temperatures using 290-nm-thick Bi thin films. Here, the annealing of the Bi films was performed at 3 × 10−6 Torr. Bismuth thin film deposition and the annealing of the films were performed in situ without breaking the vacuum. As shown in Fig. 1(a), asdeposited Bi films showed irregular grain shapes and the only growth of the Bi grains was observed as the annealing temperature was increased (as shown in Fig. 1(b)–(d)) from 230 to 270 ◦ C, while Bi nanowires were not observed in any of the films. Because bismuth is not a noble metal, oxidation of the bismuth films is possible during deposition at a vacuum ambient of ∼mTorr by dc sputtering. XPS analysis to identify the oxidation of as-grown bismuth films was performed. Fig. 2(a) shows the Bi 4f spectra of the bismuth films before and after etching the surface for 30 s. As shown in Fig. 2(a), after etching of the surface, binding energies indicating the bismuth oxide phase were observed at 158.6 and 163.9 eV. This
result suggests that the bismuth was oxidized in the interior as well as at the surface. The highest peak of the bismuth oxide observed before etching on the surface was attributed to the oxidation of the surface during the handling of the sample. Therefore, in order to conveniently grow Bi nanowire from bismuth films, the oxidation of bismuth should be effectively suppressed, which should allow achievement of the co-deposition of a metal with an oxidization ability that is higher than bismuth. This is supported by a comparison of the Gibbs free energy for the formation of bismuth oxide and aluminum oxide, as follows [11]: 2Al + 3/2O2 = Al2 O3
Go = −1584.0 kJ(at27 ◦ C)
2Bi + 3/2O2 = Bi2 O3
Go = −496.6 kJ(at27 ◦ C)
Based on thermodynamic expectations, because the formation of an aluminum oxide is more stable than that of bismuth oxide, the oxidation of the bismuth was expected to be suppressed in the case of the Bi–Al co-deposited films. This result was clearly confirmed by the XPS Al 2p spectrum of the Al film itself, and by the Bi 4f and Al 2p spectra in the Bi–Al cosputtered films, as shown in Fig. 2(b)–(d), respectively. As shown
Fig. 4. SEM surface images (image tilted by 10◦ ) of the samples annealed in a 3 × 10−6 Torr vacuum for 10 h at (a) unannealed, (b) 230 ◦ C, (c) 250 ◦ C, and (d) 270 ◦ C using Bi–Al co-sputtered films.
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Fig. 5. (a) STEM image and the STEM mapping images of the (b) Bi, (c) Al, and (d) O elements in Bi nanowire grown at 270 ◦ C. The inset of (a) indicates the EDS spectrum showing the composition of the Bi nanowire.
in Fig. 2(b), as-grown Al films before the surface etching showed a higher concentration of aluminum oxide than that of aluminum metal because of the severe surface oxidation. After surface etching for 30 s, Al bulk films showed an equal concentration of Al metal and aluminum oxide. However, from the Al 2p spectra in the co-sputtered films shown in Fig. 2(d), the films after etching showed a higher concentration of aluminum oxide than of aluminum metal. This result suggests that the aluminum in the cosputtered films exists mostly as aluminum oxide. With this result, the preferential formation of the aluminum oxide in the Bi–Al
co-sputtered films was expected to suppress the oxidation of the bismuth. Although the above results are possible from both a kinetic as well as thermodynamic perspective, the growth of the Bi nanowires is impossible if the Bi–Al co-sputtered films form a Bi–Al alloy. From the Bi–Al phase diagram [12], because the solubility limit of aluminum into the bismuth is very narrow, the formation of the Bi–Al alloy is difficult at a low temperature. In order to ascertain whether or not the Bi–Al co-sputtered films formed the alloy, the XRD patterns of Bi, Al, and Bi–Al co-sputtered films (BA films) are shown in
Fig. 6. (a) STEM image and the STEM mapping images of (b) Bi, (c) Al, and (d) O element in the Bi2 O3 nanowire prepared by annealing at 300 ◦ C for 2 h in an oxygen ambient using Bi nanowire grown at 270 ◦ C. The inset of (a) indicates the EDS spectrum showing the composition of the Bi2 O3 nanowire.
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Fig. 7. (a) The process of cutting the nanowire using a focused ion beam (FIB) technique; (b) the TEM image of a cross-section of the Bi2 O3 nanowire; (c) the HRTEM image of the Bi2 O3 nanowire; and, (d) the selected area electron diffraction pattern of the nanowire.
Fig. 3. Because the peaks observed at around 38◦ for various films showed a clear differences (see Fig. 3(a)), they are shown in detail in Fig. 3(b). The Bi (1 0 4) (2 = 37.82◦ ) and Al (1 1 1) (2 = 38.02◦ ) peaks can be clearly observed in the XRD-narrow range in Fig. 3(b). The residual stress initiated by the mixture of the Bi and the Al in the films shifted to lower angles in the mixture films, compared with the peak positions of the Bi and Al films. From the XRD results, the mixture of the Bi and the Al, not Bi–Al alloys in the films, plays an important role in the growth of Bi nanowires. The SEM images (image tilted by 10◦ ) of the samples annealed for 10 h in a vacuum that was 3 × 10−6 Torr at various temperatures using Bi–Al co-sputtered films are shown in Fig. 4. As shown in Fig. 4(a), as-deposited Bi–Al films had small grains with regular shapes, compared with the as-deposited Bi film itself (see Fig. 1(a)). Bi nanowires grew even at a temperature as low as 230 ◦ C (Fig. 4(b)). At annealing temperatures of 250 (Fig. 4(c)) and 270 ◦ C (Fig. 4(d)), a number of nanowires had formed with an average diameter of
300 nm and an average length of 10 m. The average diameter of the grown nanowires was consistent with the mean grain size of the films. This result suggests that the diameter of the nanowire can be controlled by the grain size of the co-sputtered films. The insets in Fig. 4(c) and (d) show tilted images, which indicate clear nanowires. Fig. 5 shows the scanning transmission electron microscopy (STEM) elemental mapping images of a Bi single nanowire grown at 270 ◦ C. Fig. 5(a) shows an image of the Bi nanowire, and the inset indicates the EDS (EDS attached at STEM) spectrum showing the composition of the nanowire. From the EDS spectrum, the nanowire includes the Bi element alone, indicating neither Al nor O elements. Here, the carbon and copper peaks come from the carbon deposited on the copper grid in order to measure the composition of the nanowire. The STEM elemental mapping image clearly shows that the bismuth is still uniformly distributed along the length and breadth of the nanowire, as shown in Fig. 5(b). On the
Fig. 8. (a) The relationship between current and voltage (I–V) of a Bi2 O3 single nanowire. The inset shows the SEM image of the pad-pattern for the measurement of the electrical properties of the nanowire. (b) Schematic diagram of the sensor device. (c) Variations in resistance of the Bi2 O3 single nanowire measured at 350 ◦ C as a function of time for various concentrations of NO gas.
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other hand, as shown in Fig. 5(c) and (d), Al and O are widely spread along the length and breadth of the nanowire, indicating no existence of either element in the nanowire. The STEM elemental mapping images of Bi2 O3 nanowire prepared by an annealing of the Bi nanowire at 300 ◦ C for 2 h in oxygen ambient are shown in Fig. 6. Fig. 6(a) shows the image of the Bi2 O3 nanowire and the inset shows the EDS spectrum, which indicates the existence of the Bi and O elements. The Bi (Fig. 6(b)) and O (Fig. 6(d)) are still uniformly distributed along the nanowire, while the aluminum (Fig. 6(c)) is widely spread, indicating no Al element in the Bi2 O3 nanowire. In order to investigate the crystal structure of the Bi2 O3 nanowire using TEM, a high-resolution TEM (HRTEM) image and a microdiffraction pattern was obtained by cutting (see Fig. 7(a)) the nanowire using a focused ion beam (FIB) technique. Fig. 7(b) shows the TEM image of the cross-section area of a Bi2 O3 nanowire and a high-resolution TEM (HRTEM) image of the nanowire (see Fig. 7(c)), which shows its well-crystallized structure. From the selected area of the box indicated in Fig. 7(c), the lattice distance measured from the (1 2 0) plane was about 3.26 A˚ of Bi2 O3 . A microdiffraction pattern is shown in Fig. 7(d), and the (1 1 1), (1¯ 2 2), (1 2 0), and (0 1 2) planes exhibiting the Bi2 O3 single phase are clearly identifiable. As a result, nanowires prepared by annealing at a low temperature using Bi nanowire were identified as a Bi2 O3 single phase, which also confirms the single crystalline nature of the nanowires. The electrical properties of the Bi2 O3 nanowires were investigated by fabricating two terminal devices using the Pt contacts (7 m separation) deposited by the focused-ion-beam (FIB). The inset of Fig. 8(a) shows the SEM image of the pad-pattern for the measurement of the electrical properties of the nanowire. The red box in the inset of (a) shows a high-magnification SEM image in order to clearly show the connection of the nanowire to a platinum electrode. The large rectangular form connected to the Pt electrode is also the Pt pad used for the measurement of electrical properties. Here, the length of the Bi2 O3 nanowire used to measure the electrical properties was approximately 8.8 m. Fig. 8(a) shows the relationship between the current and voltage of the nanowires. The current of the nanowires linearly increased with increasing voltage. Resistivity of the Bi2 O3 nanowire from the I–V curve was approximately 14.6 -cm, indicating an important semiconducting property [13]. A major application of Bi2 O3 is likely to be the sensing (detection) of important molecules either for environmental protection or human health purposes. Earlier, it was used for smoke detection [14]. Here, the gas used to measure the sensing properties of a single nanowire was a mixture of NO (1000 ppm) and N2 . The sensing performance of the Bi2 O3 single nanowire was investigated using various concentrations of NO. Fig. 8(b) is a schematic diagram of the device for the measurement of the sensor properties with a single Bi2 O3 nanowire connected to both sides of the Pt pattern. Variations in the resistance of the nanowire as a function of time were measured at 350 ◦ C. As shown in Fig. 8(c), a Bi2 O3 single nanowire operated well as a sensor for NO gas. A longer response and recovery time for each concentration of NO was attributed to a decrease in the sensitivity of a single nanowire with a large diameter (about 300 nm). However, the electric resistance of the Bi2 O3 single nanowire was sensitive to NO gas even at 10 ppm. This result suggests that it is still possible for a Bi2 O3 single nanowire with a single crystalline structure to detect NO gas in a small concentration. 4. Conclusions Single crystalline Bi2 O3 nanowires were prepared by annealing in an oxygen ambient using pure Bi nanowires grown at low vacuum (∼10−6 Torr) with Bi–Al co-sputtered films. The possibility of Bi nanowire growth using Bi–Al co-sputtered films was attributed
to the suppression of the oxidation of the bismuth through the preferred oxidation of aluminum in a co-sputtered ambient (∼mTorr). The Bi nanowires from the Bi–Al co-sputtered films could be grown at a temperature as low as 230 ◦ C and in a low-vacuum ambient. The Bi2 O3 nanowires prepared from the Bi nanowires showed a single crystalline structure with (1 1 1), (1¯ 2 2), and (0 1 2) planes. The current–voltage (I–V) relationship of the Bi2 O3 nanowire revealed that the Bi2 O3 nanowire exhibited semiconducting properties with a resistivity of 14.6 -cm. Variations in the resistance of the Bi2 O3 single nanowire as a function of time at 350 ◦ C showed a longer response and recovery time for each concentration of NO. However, the electric resistance of the Bi2 O3 single nanowire was sensitive to NO gas even at 10 ppm. Acknowledgements This work was funded by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MOST) (R01-2007-000-21017-0), and by a KOSEF grant funded by the Korea Government (MEST) (no. 2009-0079164). This work is also the outcome of a Manpower Development Program for Energy & Resources supported by the Ministry of Knowledge and Economy (MKE), and was supported by the Brain Korea 21 Project in 2006. References [1] W. Lu, C.M. Lieber, Semiconductor nanowires, J. Phys. D: Appl. Phys. 39 (2006) R387–R406. [2] Y.N. Xia, P.D. Yang, Y.G. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, One-dimensional nanostructures: synthesis, characterization, and applications, Adv. Mater. 15 (2003) 353–389. [3] Y. Cui, C.M. Lieber, Functional nanoscale electronic devices assembled using silicon nanowire building blocks, Science 291 (2001) 851–853. [4] V. Fruth, M. Popa, D. Berger, R. Ramer, M. Gartner, A. Ciulei, M. Zaharescu, Deposition and characterisation of bismuth oxide thin films, J. Eur. Ceram. Soc. 25 (2005) 2171–2174. [5] H.T. Fan, X.M. Teng, S.S. Pan, C. Ye, G.H. Li, L.D. Zhang, Optical properties of ␦-Bi2 O3 thin films grown by reactive sputtering, Appl. Phys. Lett. 87 (2005) 231916–231918. [6] R.L. Thayer, C.A. Randall, S. Trolier-McKinstry, Medium permittivity bismuth zinc niobate thin film capacitors, J. Appl. Phys. 94 (2003) 1941–1947. [7] H.W. Kim, J.H. Myung, S.H. Shim, One-dimensional structures of Bi2 O3 synthesized via metalorganic chemical vapor deposition process, Solid State Commun. 137 (2006) 196–198. [8] T.P. Gujar, V.R. Shinde, C.D. Lokhande, S.-H. Han, Fibrous nanorod network of bismuth oxide by chemical method, Mater. Sci. Eng. B 133 (2006) 177–180. [9] Y. Qiu, D. Liu, J. Yang, S. Yang, Controlled synthesis of bismuth oxide nanowires by an oxidative metal vapor transport deposition technique, Adv. Mater. 8 (2006) 2604–2608. [10] W.Y. Shim, J.H. Ham, K.I. Lee, W.Y. Jeung, M. Johnson, W.Y. Lee, On-film formation of Bi nanowires with extraordinary electron mobility, Nano Lett. 9 (2009) 18–22. [11] G. Humpston, D.M. Jacobson, Principles of Soldering, The Materials Information Society, 2004, p. 105. [12] J.R. Davis, Aluminum and Aluminum Alloys, ASM International Handbook Committee, J.R. Davis & Associates, 1993, p. 32. [13] A. Cabot, A. Marsal, J. Arbiol, J.R. Morante, Bi2 O3 as a selective sensing material for NO detection, Sens. Actuators B 99 (2004) 74–89. [14] A.Z. Adamyan, Z.N. Adamian, V.M. Aroutiounian, Smoke sensor with overcoming of humidity cross-sensitivity, Sens. Actuators B 93 (2003) 416–421.
Biographies Yeon Woong Park is now in a master’s course in materials science and engineering, Chungnam Nat. Univ., Korea. His research interest is the preparation of nanowires, including CuO and Bi2 O3 , for gas sensors. Hyun June Jung is now in a Ph.D. course in materials science and engineering, Chungnam Nat. Univ., Korea. His research interest is the preparation of nano-floating gate memory devices and a high dielectric constant films. Soon Gil Yoon received his Ph.D. from the Korea Advanced Institute of Science and Technology, Korea in 1988. He is a professor in the School of Nanotechnology in the Department of Materials Science and Engineering at Chungnam Nat. Univ., Korea. His current research interests are ferroelectrics, DRAM, transparent and dielectric materials, solar cells, nanowires as gas sensors, PRAM, spintronics, etc.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Bi2 O3 nanowire growth from high-density Bi nanowires grown at a low temperature using aluminum–bismuth co-deposited films Yeon-Woong Park a , Hyun-June Jung a , Soon-Gil Yoon a,b,∗ a b
School of Nano Science and Technology, Chungnam National University, Daeduk Science Town, 305-764 Daejeon, Republic of Korea Graduate School of Analytical Science and Technology (GRAST), Chungnam National University, Daeduk Science Town, 305-764 Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 September 2010 Received in revised form 10 February 2011 Accepted 14 February 2011 Available online 21 February 2011 Keywords: Bi2 O3 nanowires Bi nanowires Bi–Al co-sputtered films Low temperature Gas sensing DC sputtering
a b s t r a c t Single crystalline Bi2 O3 nanowires were prepared by annealing in oxygen ambient using pure Bi nanowires grown at a low vacuum (∼10−6 Torr) with Bi–Al co-sputtered films. The ability to grow Bi nanowires using Bi–Al co-sputtered films can be attributed to the suppression of the oxidation of the bismuth by the preferred oxidation of aluminum in co-sputtered ambient (∼mTorr). The Bi nanowires from the Bi–Al co-sputtered films could be grown even at the low temperature of 230 ◦ C in low-vacuum ambient. The Bi2 O3 nanowires prepared from the Bi nanowires showed a single crystalline structure with (1 1 1), (1¯ 2 2), (1 2 0), and (0 1 2) planes. The current–voltage (I–V) relationship of the Bi2 O3 nanowire revealed that the Bi2 O3 nanowire exhibited a semiconducting property with a resistivity of 14.6 -cm. Variations in resistance of the Bi2 O3 single nanowire as a function of time at 350 ◦ C showed reproducible response and recovery time characteristics for each concentration of NO. The electric resistance of the Bi2 O3 single nanowire was sensitive to NO gas even at 10 ppm. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The search for new growth methods for one-dimensional structures with a nanometer diameter, such as nanowires, continues to be of central importance in nanoscience and nanotechnology, since these represent a unique system for exploring dimension dependent nanoscale properties and are expected to play a crucial role in the future of electronic, optoelectronic and electromechanical devices [1–3]. Considering the importance of metal oxides in catalysis, electrochemistry, optics, functional ceramics, and sensors, fabrication in one-dimensional nanostructured morphology appears to be a particularly attractive goal and has been pursued by a number of laboratories. To date, many types of metal oxide nanowires have been successfully synthesized. Bismuth oxide (Bi2 O3 ) is fascinating to scientists owing to its unique structure and physical properties such as a large energy band gap, a high refractive index, a dielectric permittivity and a high oxygen conductivity, as well as remarkable photoconductivity and photoluminescence [4–6]. Despite the scientific and technological attractiveness of Bi2 O3 , the synthesis of a 1D nanostructure using Bi2 O3 has not received great attention. Bismuth oxide nanowires
∗ Corresponding author at: Graduate School of Analytical Science and Technology (GRAST), Chungnam National University, Daeduk Science Town, 305-764 Daejeon, Republic of Korea. Tel.: +82 42 821 6638, fax: +82 42 822 3206. E-mail addresses: [email protected] (Y.-W. Park), np [email protected] (H.-J. Jung), [email protected] (S.-G. Yoon). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.02.023
have been prepared using various techniques such as metalorganic chemical vapor deposition (MOCVD) [7], chemical methods [8], and an oxidative metal vapor transport deposition technique [9]. These successful techniques for the synthesis of Bi2 O3 nanowires are based mainly on high-temperature, solid-state chemistry reaction routes, which are energy intensive and problematic for integration with silicon-based microelectronics. Herein, high-quality single-crystalline Bi nanowires were prepared using a novel stress-induced method at low temperature on a silicon substrate with subsequent anneal at low temperature in oxygen ambient. Single-crystalline Bi nanowires have already been grown using Bi thin films in an ultrahigh vacuum (UHV, ∼10−7 Torr) [10]. For the convenient growth of single-crystalline Bi nanowires from Bi films at a low vacuum (below 10−6 Torr), high-density was achieved using Bi–Al co-sputtered films, and the growth mechanism is addressed in the present study. The gas-sensing properties of NO using a Bi2 O3 single nanowire also are presented. 2. Experimental For Bi–Al films on SiO2 /Si substrates, 290-nm-thick Bi–Al cosputtered films were prepared for 10 min at dc powers of Bi (10 W) and Al (20 W), and a working pressure of 3 mTorr in argon ambient. Deposition rates of the Bi and Al films were approximately 14.3 and 2.7 nm/min, respectively. The film thickness for the determination of the deposition rate was measured using the SEM cross-sectional image of the films grown at a constant deposition
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Fig. 1. SEM images of the Bi films annealed for 10 h at different annealing temperatures: (a) unannealed, (b) 230 ◦ C, (c) 250 ◦ C, and (d) 270 ◦ C using 290-nm-thick Bi thin films grown by dc sputtering. Here, the annealing of the Bi films was performed at 3 × 10−6 Torr.
time. The Bi–Al co-sputtered films prepared under these deposition conditions exhibited a composition of Bi:Al = 1:1. The composition of the co-sputtered films was determined using an electron-probe micro-analyzer (EPMA). The bismuth nanowires were prepared by annealing at 270 ◦ C for 10 h in 3 × 10−6 Torr using Bi–Al cosputtered films. The Bi2 O3 nanowires were prepared by annealing at 300 ◦ C for 2 h in O2 ambient using Bi nanowires. The crystallinity and microstructure of co-sputtered Bi–Al films were characterized using X-ray diffraction (XRD) excited by Cu K␣ radiation and scanning electron microscopy (SEM), respectively. The identification of the oxidation state of the co-sputtered films was performed by
X-ray photoelectron spectroscopy (XPS). The crystal structure of the single-crystalline Bi and Bi2 O3 nanowires was examined using high-resolution transmission electron microscopy (HRTEM). The current–voltage (I–V) characteristics of the Bi2 O3 nanowires were investigated by fabricating two terminal devices with Pt contacts (50 m separation) deposited by a focused-ion-beam (FIB, Helios Nano Lab 600). To measure the sensing properties, Pt electrodes 100 m wide and 3 mm long were patterned onto the SiO2 /Si substrates and a single nanowire was connected to the Pt electrodes. The sensor resistance of the Bi2 O3 single nanowire was measured using a programmable electrometer (Keithley 2400) controlled by
Fig. 2. XPS spectra of the (a) Bi 4f in the Bi films and (b) Al 2p in the Al films, (c) Bi 4f and (d) Al 2p in the Bi–Al co-sputtered films. Each spectra in (a)–(d) were obtained before and after etching treatment for 30 s.
Y.-W. Park et al. / Sensors and Actuators B 156 (2011) 709–714
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Fig. 3. XRD patterns of (a) Bi, Al, and Bi–Al co-sputtered films (BA films) and (b) of narrow regions from 35◦ to 40◦ in each of the films.
a personal computer using the Labview program for various concentrations of NO gas at 350 ◦ C.
3. Results and discussion Fig. 1 shows the SEM images of the samples annealed for 10 h at different temperatures using 290-nm-thick Bi thin films. Here, the annealing of the Bi films was performed at 3 × 10−6 Torr. Bismuth thin film deposition and the annealing of the films were performed in situ without breaking the vacuum. As shown in Fig. 1(a), asdeposited Bi films showed irregular grain shapes and the only growth of the Bi grains was observed as the annealing temperature was increased (as shown in Fig. 1(b)–(d)) from 230 to 270 ◦ C, while Bi nanowires were not observed in any of the films. Because bismuth is not a noble metal, oxidation of the bismuth films is possible during deposition at a vacuum ambient of ∼mTorr by dc sputtering. XPS analysis to identify the oxidation of as-grown bismuth films was performed. Fig. 2(a) shows the Bi 4f spectra of the bismuth films before and after etching the surface for 30 s. As shown in Fig. 2(a), after etching of the surface, binding energies indicating the bismuth oxide phase were observed at 158.6 and 163.9 eV. This
result suggests that the bismuth was oxidized in the interior as well as at the surface. The highest peak of the bismuth oxide observed before etching on the surface was attributed to the oxidation of the surface during the handling of the sample. Therefore, in order to conveniently grow Bi nanowire from bismuth films, the oxidation of bismuth should be effectively suppressed, which should allow achievement of the co-deposition of a metal with an oxidization ability that is higher than bismuth. This is supported by a comparison of the Gibbs free energy for the formation of bismuth oxide and aluminum oxide, as follows [11]: 2Al + 3/2O2 = Al2 O3
Go = −1584.0 kJ(at27 ◦ C)
2Bi + 3/2O2 = Bi2 O3
Go = −496.6 kJ(at27 ◦ C)
Based on thermodynamic expectations, because the formation of an aluminum oxide is more stable than that of bismuth oxide, the oxidation of the bismuth was expected to be suppressed in the case of the Bi–Al co-deposited films. This result was clearly confirmed by the XPS Al 2p spectrum of the Al film itself, and by the Bi 4f and Al 2p spectra in the Bi–Al cosputtered films, as shown in Fig. 2(b)–(d), respectively. As shown
Fig. 4. SEM surface images (image tilted by 10◦ ) of the samples annealed in a 3 × 10−6 Torr vacuum for 10 h at (a) unannealed, (b) 230 ◦ C, (c) 250 ◦ C, and (d) 270 ◦ C using Bi–Al co-sputtered films.
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Fig. 5. (a) STEM image and the STEM mapping images of the (b) Bi, (c) Al, and (d) O elements in Bi nanowire grown at 270 ◦ C. The inset of (a) indicates the EDS spectrum showing the composition of the Bi nanowire.
in Fig. 2(b), as-grown Al films before the surface etching showed a higher concentration of aluminum oxide than that of aluminum metal because of the severe surface oxidation. After surface etching for 30 s, Al bulk films showed an equal concentration of Al metal and aluminum oxide. However, from the Al 2p spectra in the co-sputtered films shown in Fig. 2(d), the films after etching showed a higher concentration of aluminum oxide than of aluminum metal. This result suggests that the aluminum in the cosputtered films exists mostly as aluminum oxide. With this result, the preferential formation of the aluminum oxide in the Bi–Al
co-sputtered films was expected to suppress the oxidation of the bismuth. Although the above results are possible from both a kinetic as well as thermodynamic perspective, the growth of the Bi nanowires is impossible if the Bi–Al co-sputtered films form a Bi–Al alloy. From the Bi–Al phase diagram [12], because the solubility limit of aluminum into the bismuth is very narrow, the formation of the Bi–Al alloy is difficult at a low temperature. In order to ascertain whether or not the Bi–Al co-sputtered films formed the alloy, the XRD patterns of Bi, Al, and Bi–Al co-sputtered films (BA films) are shown in
Fig. 6. (a) STEM image and the STEM mapping images of (b) Bi, (c) Al, and (d) O element in the Bi2 O3 nanowire prepared by annealing at 300 ◦ C for 2 h in an oxygen ambient using Bi nanowire grown at 270 ◦ C. The inset of (a) indicates the EDS spectrum showing the composition of the Bi2 O3 nanowire.
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Fig. 7. (a) The process of cutting the nanowire using a focused ion beam (FIB) technique; (b) the TEM image of a cross-section of the Bi2 O3 nanowire; (c) the HRTEM image of the Bi2 O3 nanowire; and, (d) the selected area electron diffraction pattern of the nanowire.
Fig. 3. Because the peaks observed at around 38◦ for various films showed a clear differences (see Fig. 3(a)), they are shown in detail in Fig. 3(b). The Bi (1 0 4) (2 = 37.82◦ ) and Al (1 1 1) (2 = 38.02◦ ) peaks can be clearly observed in the XRD-narrow range in Fig. 3(b). The residual stress initiated by the mixture of the Bi and the Al in the films shifted to lower angles in the mixture films, compared with the peak positions of the Bi and Al films. From the XRD results, the mixture of the Bi and the Al, not Bi–Al alloys in the films, plays an important role in the growth of Bi nanowires. The SEM images (image tilted by 10◦ ) of the samples annealed for 10 h in a vacuum that was 3 × 10−6 Torr at various temperatures using Bi–Al co-sputtered films are shown in Fig. 4. As shown in Fig. 4(a), as-deposited Bi–Al films had small grains with regular shapes, compared with the as-deposited Bi film itself (see Fig. 1(a)). Bi nanowires grew even at a temperature as low as 230 ◦ C (Fig. 4(b)). At annealing temperatures of 250 (Fig. 4(c)) and 270 ◦ C (Fig. 4(d)), a number of nanowires had formed with an average diameter of
300 nm and an average length of 10 m. The average diameter of the grown nanowires was consistent with the mean grain size of the films. This result suggests that the diameter of the nanowire can be controlled by the grain size of the co-sputtered films. The insets in Fig. 4(c) and (d) show tilted images, which indicate clear nanowires. Fig. 5 shows the scanning transmission electron microscopy (STEM) elemental mapping images of a Bi single nanowire grown at 270 ◦ C. Fig. 5(a) shows an image of the Bi nanowire, and the inset indicates the EDS (EDS attached at STEM) spectrum showing the composition of the nanowire. From the EDS spectrum, the nanowire includes the Bi element alone, indicating neither Al nor O elements. Here, the carbon and copper peaks come from the carbon deposited on the copper grid in order to measure the composition of the nanowire. The STEM elemental mapping image clearly shows that the bismuth is still uniformly distributed along the length and breadth of the nanowire, as shown in Fig. 5(b). On the
Fig. 8. (a) The relationship between current and voltage (I–V) of a Bi2 O3 single nanowire. The inset shows the SEM image of the pad-pattern for the measurement of the electrical properties of the nanowire. (b) Schematic diagram of the sensor device. (c) Variations in resistance of the Bi2 O3 single nanowire measured at 350 ◦ C as a function of time for various concentrations of NO gas.
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other hand, as shown in Fig. 5(c) and (d), Al and O are widely spread along the length and breadth of the nanowire, indicating no existence of either element in the nanowire. The STEM elemental mapping images of Bi2 O3 nanowire prepared by an annealing of the Bi nanowire at 300 ◦ C for 2 h in oxygen ambient are shown in Fig. 6. Fig. 6(a) shows the image of the Bi2 O3 nanowire and the inset shows the EDS spectrum, which indicates the existence of the Bi and O elements. The Bi (Fig. 6(b)) and O (Fig. 6(d)) are still uniformly distributed along the nanowire, while the aluminum (Fig. 6(c)) is widely spread, indicating no Al element in the Bi2 O3 nanowire. In order to investigate the crystal structure of the Bi2 O3 nanowire using TEM, a high-resolution TEM (HRTEM) image and a microdiffraction pattern was obtained by cutting (see Fig. 7(a)) the nanowire using a focused ion beam (FIB) technique. Fig. 7(b) shows the TEM image of the cross-section area of a Bi2 O3 nanowire and a high-resolution TEM (HRTEM) image of the nanowire (see Fig. 7(c)), which shows its well-crystallized structure. From the selected area of the box indicated in Fig. 7(c), the lattice distance measured from the (1 2 0) plane was about 3.26 A˚ of Bi2 O3 . A microdiffraction pattern is shown in Fig. 7(d), and the (1 1 1), (1¯ 2 2), (1 2 0), and (0 1 2) planes exhibiting the Bi2 O3 single phase are clearly identifiable. As a result, nanowires prepared by annealing at a low temperature using Bi nanowire were identified as a Bi2 O3 single phase, which also confirms the single crystalline nature of the nanowires. The electrical properties of the Bi2 O3 nanowires were investigated by fabricating two terminal devices using the Pt contacts (7 m separation) deposited by the focused-ion-beam (FIB). The inset of Fig. 8(a) shows the SEM image of the pad-pattern for the measurement of the electrical properties of the nanowire. The red box in the inset of (a) shows a high-magnification SEM image in order to clearly show the connection of the nanowire to a platinum electrode. The large rectangular form connected to the Pt electrode is also the Pt pad used for the measurement of electrical properties. Here, the length of the Bi2 O3 nanowire used to measure the electrical properties was approximately 8.8 m. Fig. 8(a) shows the relationship between the current and voltage of the nanowires. The current of the nanowires linearly increased with increasing voltage. Resistivity of the Bi2 O3 nanowire from the I–V curve was approximately 14.6 -cm, indicating an important semiconducting property [13]. A major application of Bi2 O3 is likely to be the sensing (detection) of important molecules either for environmental protection or human health purposes. Earlier, it was used for smoke detection [14]. Here, the gas used to measure the sensing properties of a single nanowire was a mixture of NO (1000 ppm) and N2 . The sensing performance of the Bi2 O3 single nanowire was investigated using various concentrations of NO. Fig. 8(b) is a schematic diagram of the device for the measurement of the sensor properties with a single Bi2 O3 nanowire connected to both sides of the Pt pattern. Variations in the resistance of the nanowire as a function of time were measured at 350 ◦ C. As shown in Fig. 8(c), a Bi2 O3 single nanowire operated well as a sensor for NO gas. A longer response and recovery time for each concentration of NO was attributed to a decrease in the sensitivity of a single nanowire with a large diameter (about 300 nm). However, the electric resistance of the Bi2 O3 single nanowire was sensitive to NO gas even at 10 ppm. This result suggests that it is still possible for a Bi2 O3 single nanowire with a single crystalline structure to detect NO gas in a small concentration. 4. Conclusions Single crystalline Bi2 O3 nanowires were prepared by annealing in an oxygen ambient using pure Bi nanowires grown at low vacuum (∼10−6 Torr) with Bi–Al co-sputtered films. The possibility of Bi nanowire growth using Bi–Al co-sputtered films was attributed
to the suppression of the oxidation of the bismuth through the preferred oxidation of aluminum in a co-sputtered ambient (∼mTorr). The Bi nanowires from the Bi–Al co-sputtered films could be grown at a temperature as low as 230 ◦ C and in a low-vacuum ambient. The Bi2 O3 nanowires prepared from the Bi nanowires showed a single crystalline structure with (1 1 1), (1¯ 2 2), and (0 1 2) planes. The current–voltage (I–V) relationship of the Bi2 O3 nanowire revealed that the Bi2 O3 nanowire exhibited semiconducting properties with a resistivity of 14.6 -cm. Variations in the resistance of the Bi2 O3 single nanowire as a function of time at 350 ◦ C showed a longer response and recovery time for each concentration of NO. However, the electric resistance of the Bi2 O3 single nanowire was sensitive to NO gas even at 10 ppm. Acknowledgements This work was funded by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MOST) (R01-2007-000-21017-0), and by a KOSEF grant funded by the Korea Government (MEST) (no. 2009-0079164). This work is also the outcome of a Manpower Development Program for Energy & Resources supported by the Ministry of Knowledge and Economy (MKE), and was supported by the Brain Korea 21 Project in 2006. References [1] W. Lu, C.M. Lieber, Semiconductor nanowires, J. Phys. D: Appl. Phys. 39 (2006) R387–R406. [2] Y.N. Xia, P.D. Yang, Y.G. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, One-dimensional nanostructures: synthesis, characterization, and applications, Adv. Mater. 15 (2003) 353–389. [3] Y. Cui, C.M. Lieber, Functional nanoscale electronic devices assembled using silicon nanowire building blocks, Science 291 (2001) 851–853. [4] V. Fruth, M. Popa, D. Berger, R. Ramer, M. Gartner, A. Ciulei, M. Zaharescu, Deposition and characterisation of bismuth oxide thin films, J. Eur. Ceram. Soc. 25 (2005) 2171–2174. [5] H.T. Fan, X.M. Teng, S.S. Pan, C. Ye, G.H. Li, L.D. Zhang, Optical properties of ␦-Bi2 O3 thin films grown by reactive sputtering, Appl. Phys. Lett. 87 (2005) 231916–231918. [6] R.L. Thayer, C.A. Randall, S. Trolier-McKinstry, Medium permittivity bismuth zinc niobate thin film capacitors, J. Appl. Phys. 94 (2003) 1941–1947. [7] H.W. Kim, J.H. Myung, S.H. Shim, One-dimensional structures of Bi2 O3 synthesized via metalorganic chemical vapor deposition process, Solid State Commun. 137 (2006) 196–198. [8] T.P. Gujar, V.R. Shinde, C.D. Lokhande, S.-H. Han, Fibrous nanorod network of bismuth oxide by chemical method, Mater. Sci. Eng. B 133 (2006) 177–180. [9] Y. Qiu, D. Liu, J. Yang, S. Yang, Controlled synthesis of bismuth oxide nanowires by an oxidative metal vapor transport deposition technique, Adv. Mater. 8 (2006) 2604–2608. [10] W.Y. Shim, J.H. Ham, K.I. Lee, W.Y. Jeung, M. Johnson, W.Y. Lee, On-film formation of Bi nanowires with extraordinary electron mobility, Nano Lett. 9 (2009) 18–22. [11] G. Humpston, D.M. Jacobson, Principles of Soldering, The Materials Information Society, 2004, p. 105. [12] J.R. Davis, Aluminum and Aluminum Alloys, ASM International Handbook Committee, J.R. Davis & Associates, 1993, p. 32. [13] A. Cabot, A. Marsal, J. Arbiol, J.R. Morante, Bi2 O3 as a selective sensing material for NO detection, Sens. Actuators B 99 (2004) 74–89. [14] A.Z. Adamyan, Z.N. Adamian, V.M. Aroutiounian, Smoke sensor with overcoming of humidity cross-sensitivity, Sens. Actuators B 93 (2003) 416–421.
Biographies Yeon Woong Park is now in a master’s course in materials science and engineering, Chungnam Nat. Univ., Korea. His research interest is the preparation of nanowires, including CuO and Bi2 O3 , for gas sensors. Hyun June Jung is now in a Ph.D. course in materials science and engineering, Chungnam Nat. Univ., Korea. His research interest is the preparation of nano-floating gate memory devices and a high dielectric constant films. Soon Gil Yoon received his Ph.D. from the Korea Advanced Institute of Science and Technology, Korea in 1988. He is a professor in the School of Nanotechnology in the Department of Materials Science and Engineering at Chungnam Nat. Univ., Korea. His current research interests are ferroelectrics, DRAM, transparent and dielectric materials, solar cells, nanowires as gas sensors, PRAM, spintronics, etc.