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Sensors and Actuators B 121 (2007) 208–213 Synthesis and characterization of semiconducting nanowires for gas sensing G. Sberveglieri ∗ , C. Baratto, E. Comini, G. Faglia, M. Ferroni, A. Ponzoni, A. Vomiero SENSOR Lab of CNR-INFM and Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali, Brescia University, via Valotti 9, 25133 Brescia, Italy Available online 27 October 2006 Abstract Quasi one-dimensional nanostructures of semiconducting metal oxides are promising for the development of nano-devices. Tin, indium, and zinc oxides were produced in form of single-crystalline nanowires through condensation from vapor phase. Such a growth occurs in controlled thermo- dynamical condition and size reduction effects on the electrical and optical response to gases have been exploited. Preparation, microstructural, and electrical characterization of nanowires are presented and the peculiarities of these innovative structures are highlighted. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanowires; SnO 2 ;In 2 O 3 ; ZnO; Ozone 1. Introduction A new generation of nanostructures has been recently pro- duced and has attracted the interest of a wide research com- munity [1]. These fascinating quasi one-dimensional nanostruc- tures, namely nanowires, nanorods, and nanobelts, exhibit a single-crystalline arrangement and feature unusual electrical and optical properties, which arise from size reduction or quantum confinement as crystal size is comparable to the wavelength of the electronic wave-function [2]. Presently, the synthesis of nanowires of semiconduct- ing metal oxides (MOX) is based on thermal decomposi- tion of precursor powders followed by vapor–solid (VS) or vapor–liquid–solid (VLS) growth [3]. Such a growth in con- trolled thermodynamic condition appears highly promising for nanostructure fabrication, due to its simplicity and low cost with respect to the technology of silicon processing and to other top- down approaches. A potential application of nanowires is gas sensing, which MOX are widely employed for. Tin-, indium-, and zinc-oxide nanowires may constitute the building blocks for a novel class of nano-devices. Some authors of the present work demonstrated first the gas sensing properties of SnO 2 nanowires [4]. Indeed, ∗ Corresponding author. E-mail address: sbervegl@sensor.ing.unibs.it (G. Sberveglieri). nanowires may overcome some typical limitations of sensing layers based on polycrystalline nanostructures. In general, the fabrication of polycrystalline sensing layers is directed to con- trol diffusion phenomena, which greatly influence the structural and electrical properties and contribute either positively or nega- tively to the long-term stability. Most of thetechniquesemployed for conventional synthesis (sol–gel, condensation from liquid or gas phase, chemical or physical vapor deposition) require thermal treatment [5–8]. Calcination, firing, or annealing defini- tively stabilizes stoichiometry, crystalline phase, and determines the other non-equilibrium characteristics such as porosity, inter- faces, and defects. Unfortunately, thermal treatment promotes grain coarsening and causes degradation of the functionality by suppressing the surface-to-volume ratio [9]. Differently, newly developed quasi one-dimensional nanos- tructures envisage long durability owing to their exceptionally high degree of crystallinity [10]. The transverse dimension of nanowires may result even smaller than the Debye length asso- ciated to the surface space-charge region and in such condition the detection efficiency of gas molecules adsorbedatsurface may reach very high value [11]. This extraordinary sensing potential has been recently demonstrated for operation in liquid envi- ronment or at room temperature [12–14]. Among the possible applications in the field of bio-nanotechnology, sensitive DNA and protein detection are presently under investigation [15]. This paper summarizes the preparation and characterization of tin, zinc and indium oxide nanowires. The electrical and 0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.09.049 G. Sberveglieri et al. / Sensors and Actuators B 121 (2007) 208–213 209 Table 1 Basic operating parameters for nanowire growth from vapor condensation Nanowires Precursor Decomposition temperature ( ◦ C) Substrate temperature ( ◦ C) Duration (min) Pressure (mbar) Substrate Catalyst Sn oxide SnO 2 1370 330 30 100 Poly-Al 2 O 3 Absent SnO 2 1370 470 30 100 Poly-Al 2 O 3 Pt Zn oxide Zn 600 600 30 1000 Poly-Al 2 O 3 Absent Zn 600 600 30 1000 Silicon Absent In oxide In 2 O 3 1500 1100 60 100 Poly-Al 2 O 3 Absent optical properties of nanowires were investigated with particular regard to gas sensing behavior. 2. Experimental The growth of MOX nanowires from vapor phase is based on the evaporation–condensation technique [10]. The oxide pre- cursor powder is placed at the center of an alumina tube and then temperature is raised above the limit of decomposition for the oxide (from 600 ◦ C for zinc oxide to 1500 ◦ C for indium oxide) [16]. A controlled flow of inert gas (usually argon) is maintained during decomposition and the overall pressure mea- sures hundreds of mbar. The temperature gradient downstream the gas flow promotes condensation of cations on clean alumina substrates and allows interaction with the residual oxygen. The peculiar thermodynamic conditions promote growth of nano- sized one-dimensional structures instead of equi-axed grains. Fig. 1 shows the nucleation of indium oxide nanowires as achieved by the evaporation–condensation process. The SEM image shows the crystal habit for the nanowires: the section appears to be squared and the apex of the wires is tapered. In Fig. 1. Nucleation of indium oxide nanowires over polycrystalline alumina. general, no epitaxial relationship between the orientation of the wire and the alumina grains has been observed. Control over the direction of growth as well as pattering of the substrates may be achieved by assisting the growth mechanism through dispersion of catalysts [17]. By varying the operating conditions, nanostructures can be produced with different length and shapes [18]. During tem- perature transients, the argon flow is reversed in order to pre- vent uncontrolled condensation. Table 1 summarizes the basic operating parameters for production of SnO 2 , ZnO, and In 2 O 3 nanowires. Despite the deposition technique is relatively simple, cleanness of the alumina tubes and purity of the atmosphere are the key factor for the reproducibility of deposition. 3. Microstructural characterization Scanning and transmission electron microscopy (SEM and TEM) have been carried out in order to determine the degree of homogeneity and crystalline arrangement. High-resolution TEM imaging is useful for investigation of the termination of the nanowire lateral sides and apex. Electron diffraction (ED) and analysis of zero-order and higher-order Laue-zones allows precise determination of unit cell and space group. Incoherent imaging techniques such as STEM with the High- Angle-Annular-Dark-Field detector (STEM-HAADF) were used for the investigation of the shape of the nanowires and impurities and local variations in the composition (Z-contrast). 3.1. SnO 2 nanowires The nanowires prepared featured a very high aspect ratio as the length exceeds several microns and the width is smaller than 100 nm. As shown in Fig. 2a, the length and width of the nanowire measure 25.3 m and about 50 nm, respectively. The length and flexibility allows nano-manipulation for removal and positioning over Si-based substrates for functional characteriza- tion (see Fig. 2b). High-resolution TEM and electron diffraction showed that the wire is single crystalline, with atomically sharp termination of lateral sides. Measured Bragg reflections and the whole symmetry of the ED pattern (see Fig. 2c) agree with the cassiterite tetragonal SnO 2 phase (P42/mnm - SG 136). The direction of the electron beam is parallel to the [0 1 0] zone-axis of the reciprocal lattice and the nanowire grows along to the [1 0 0] direction. 210 G. Sberveglieri et al. / Sensors and Actuators B 121 (2007) 208–213 Fig. 2. Characteristics of SnO 2 nanowires: (a) low-magnification TEM image of a very long SnO 2 nanowire, (b) removal of nanowires from the alumina substrate through manipulators for structural and electrical characterization, (c) ED pattern of nanowire, (d) STEM-HAADF image of a nanowire, and (e) linescan of the HAADF signal (solid line) and numerical fit of the shape of the nanowire (dashed line). As both composition and phase can be considered uniform for the crystalline SnO 2 nanowire; STEM-HAADF directly visu- alizes variations in the projected thickness. Fig. 2d shows a STEM-HAADF image of a SnO 2 nanowire, about 45 nm in width: the contrast of the wire is not constant along its section, indicating a variation of thickness. The thickness fitted from HADDF profile measures 48 ± 0.2 nm (see Fig. 2e), based on the approximation of a circular section of the wire. A width- to-thickness ratio very close to 1 may thus be considered. The asymmetry of the line profile with respect to the circular fit curve indicates that the shape of the wire section is more likely to be a regular polyhedron, as it is expected for a wire with crystalline habit and crystal facets as lateral sides. 3.2. In 2 O 3 nanowires Synthesis of indium oxide nanowires is difficult because of the high temperature required for decomposition of the pre- cursor oxide [18]. In addition, In 2 O 3 usually crystallizes in a highly symmetric cubic structure, and the thermodynamic con- ditions required for producing anisotropic growth are critical to achieve. The SEM image, presented in Fig. 3a, shows two nanowires of indium oxide. The wires are capable to bend because of their very small transverse dimension. TEM analysis (see Fig. 3b and c) highlighted that the nanowires are single crystalline. ED determined that the crystalline phase for the nanowire is Ia−3 body-centered cubic In 2 O 3 and that the growth direction is par- allel to the [1 0 0] direction. 3.3. ZnO nanowires ZnO nanowires may be produced at relatively low decom- position temperature (see Table 1); the size and shape of the obtained nanowires is however sensitive to the condensation condition. Fig. 4 shows that ZnO nanowires smaller than 10 nm in width can be produced. The capability to control the lateral dimension of the nanowires will allow the systematic investiga- tion of size reduction effects on the electrical and gas sensing behavior of ZnO nanowires. Fig. 3. Characteristics of In 3 O 2 nanowires: (a) SEM image of In 3 O 2 nanowires, (b) TEM image of nanowire 70 nm in width, and (c) ED pattern from the nanowire. G. Sberveglieri et al. / Sensors and Actuators B 121 (2007) 208–213 211 Fig. 4. Characteristics of ZnO nanowires: (a–c) variation of the size for the ZnO nanowires for different growth conditions, (d) TEM image of ZnO crystalline nanowire, (e) high-resolution TEM image of the hexagonal nanowire lattice, and (f) digital diffractogram and sketch of the indexed Bragg reflections. TEM observation confirms the regular crystalline arrange- ment for the nanowires. No evidences of extended crystal defects governing the growth have been recorded. The high-resolution TEM image and the corresponding digital diffractogram indicate that the lattice symmetry is hexagonal and that the longitudinal direction of growth is parallel to the c-axis of the crystal unit cell. 4. Electrical characterization For the electrical characterization, the electrical Pt contacts were deposited by sputtering, while a Pt heating meander is realized on the opposite side of the substrate. Gas sensing characterization was carried out by volt- amperometric technique; the sensors were biased by 1 V and the electrical current was measured by a picoammeter. The refer- ence atmosphere of synthetic air was maintained at the constant condition of 0.3 l/min flow, 20 ◦ C temperature, and 50% relative humidity. Nanowires were tested towards ozone generated by a UV lamp discharge. Its concentration was measured at the chamber outlet by a detector based on the wet chemical Brewer–Milford principle. Fig. 5 shows that SnO 2 nanowire and In 2 O 3 nanowires exhibit good response towards ozone together with an appreciable capa- bility to distinguish among different ozone concentrations. The complete recovery of the baseline value after ozone injection indicates that no poisoning effects occurred, as is sometimes encountered for conventional MOX-based sensors in sensing of oxidizing species [19]. ZnO nanowires exhibit low response to ozone. By observ- ing the dynamic of response, three processes with different time constant can be observed: a quick decrease in conductance occurred after the ozone injection and is followed by a conduc- tance increase; finally a very slow process prevented the sensor response from reaching a steady-state value even after 1 h from ozone injection. Despite this phenomenon, the response keeps reversible. The high response of the nanowires can be attributed to their small lateral dimension. Indeed, when the lateral dimensions of the nanowire are sufficiently reduced, then the nanowire can be completely depleted and the response to gases increases [20]. Fig. 6 shows the ozone sensing capability of SnO 2 and In 2 O 3 nanowires as a function of the operating temperature. ZnO nanowires are not reported because of their slow response. The highest response is obtained for an operating temperature of 400 ◦ C for both the samples. 5. Optical characterization Photoluminescence (PL) spectroscopy was performed over a wide temperature and wavelength range for the purpose of inves- tigating the effect of adsorbed gases on the optical properties of zinc oxide nanowires. As visible in Fig. 7, the PL in the visible and ultra-violet region of the light spectrum is quenched by 12 ppm of NO 2 . The Fig. 5. Variation of current as function of ozone concentration for (a) SnO 2 nanowires operated at 400 ◦ C, (b) ZnO nanowires operated at 350 ◦ C, and (c) In 2 O 3 nanowires operated at 400 ◦ C. The reference atmosphere is synthetic air at 20 ◦ C and 50% relative humidity. 212 G. Sberveglieri et al. / Sensors and Actuators B 121 (2007) 208–213 Fig. 6. Response of the nanowires of SnO 2 (solid line), and In 3 O 2 (dashed line) as a function of the operating temperature. Ozone concentration is 280 ppb. Fig. 7. Spectrum of photoluminescence at room temperature for ZnO nanowires in dry air (open squares), 20 min after NO 2 introduction (open triangles) and 20 min after dry air restoration (open circles). effect is fast (time scale order of seconds) and fully reversible. The amplitude of quenching achieves its maximum at room temperature and the influence of humidity and other reducing gases is negligible. This feature could be interesting for applica- tion of nanowires as a selective optical sensor working at room temperature. 6. Concluding remarks Nanowires of semiconducting MOX can be effectively pro- duced through evaporation–condensation process. Control over the size of the nanowires is achieved by proper modification of the operating conditions. Nanowires of SnO 2 ,In 2 O 3 and ZnO have been produced in their stable and common crystalline phase. 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Williams, NanoLetters 4 (2004) 245–247. [14] Y. Cui, C.M. Lieber, Science 291 (2001) 851–853. [15] D. Zhang, C. Li, X. Liu, S. Han, T. Tang, C. Zhou, Proceedings of IEEE NANO, San Francisco, 2003, p. 8. [16] C. Li, D. Zhang, X. Liu, S. Han, T. Tang, J. Han, C. Zhou, Appl. Phys. Lett. 82 (2003) 1613. [17] K.C. Kam, F.L. Deepak, A.K. Cheetham, C.N.R. Rao, Chem. Phys. Lett. 397 (2004) 329. [18] X.Y. Kong, Y. Ding, R.S. Yang, Z.L. Wang, Science 303 (2004) 1348–1351. [19] S.R. Utembe, G.M. Hansford, M.G. Sanderson, R.A. Freshwater, K.F.E. Pratt, D.E. Williams, R.A. Cox, R.L. Jones, Sens. Actuators B 114 (2006) 507–512. [20] S. Bianchi, E. Comini, M. Ferroni, G. Faglia, A. Vomiero, G. Sberveglieri, Sens. Actuators B 118 (2006) 204–207. Biographies G. Sberveglieri was born on 17 July 1947 and received his degree in physics cum laude from the University of Parma (Italy), where he started in 1971 his research activities on the preparation of semiconducting thin film solar cells. He is now the director of the CNR, INFM Sensor Laboratory (http://sensor.ing.unibs.it ) at Brescia University where more than 20 researchers are working. In 1988 he established the Gas Sensor Lab, mainly devoted to the preparation and char- acterization of thin film chemical sensors based on nanostructured metal oxide semiconductors and, since the mid 1990s, to the area of electronic noses. In 1994, he was appointed full professor in physics. He is referee of many inter- national journals and associate editor of IEEE Sensor Journal and has acted as chairman in several Conferences on Materials Science and on Sensors. He G. Sberveglieri et al. / Sensors and Actuators B 121 (2007) 208–213 213 has been the general chairman of IMCS11th (11th International Meeting on Chemical Sensors) and he is the chair of the Steering Committee of the IMCS series Conference. During 30 years of scientific activity he published more than 250 papers in international journals; he presented more than 250 oral commu- nications to international congresses (12 plenary talks and 45 invited talks). He also is an evaluator of European Union, in the area of nanoscience and nanomaterials, and the coordinator of the EU Project NANOS4 (nanostructured solid-state gas sensors with superior performance) and several Italian projects on gas sensors. C. Baratto was born in Brescia in 1972 and has received the degree in applied physics at the University of Parma in 1997. In 1998 she started her collaboration with the Sensor Lab and in 2002 she received the PhD degree. Now she works as a researcher at the Sensor Lab. Research topics are study and development of innovative gas sensors (metal oxide thin films and nanobelts, porous silicon, car- bon nanotubes). Main activities are thin film deposition by magnetron sputtering, electrical characterization of gas sensor, optical characterization of gas sensor (photoluminescence, reflectance and surface photovoltage measurements). E. Cominiwas born on 21 November 1972and shereceived her degree inphysics at Pisa University in 1996. She is presently working on chemical sensors. She received her PhD in material science at the University of Brescia. She is now an assistant professor at the University of Brescia. G. Faglia received an MS degree from the Polytechnic of Milan in 1991 with a thesis on gas sensors. In 1992, he has been appointed as a researcher by the Thin Film Lab at the University of Brescia. He is involved in the study of the interactions between gases and semiconductor surfaces and in gas sensors electrical characterization. In 1996, he has received the PhD degree by discussing a thesis on semiconductor gas sensors. In 2000, he has been appointed associate professor in experimental physics at University of Brescia. During his career Guido Faglia has published more than 80 articles on International Journals with referee. M. Ferroni received his PhD degree in physics at the University of Ferrara in 1998, and became researcher at the University of Brescia in 2004. His mean research activity concerns the characterization of nanostructured metal oxides by means of transmission and scanning electron microscopy. Presently, Matteo Ferroni is in charge of the high-resolution scanning electron microscopy facility at the CNR-INFM SENSOR laboratory in Brescia. A. Ponzoni was born in 1976. He received the degree in physics from the Uni- versity of Parma in 2000. In 2006, he received the PhD degree in material engineering from the University of Brescia with a thesis on nanostructured metal oxides for gas sensing applications. His main activity regards synthesis and elec- trical characterization of metal oxides for gas sensing applications. Presently, he is researcher at the CNR-INFM Sensor Lab, Brescia. A. Vomiero received his degree in physics at the University of Padova in 1999, and his PhD in electronic engineering at the University of Trento in 2003. His main activities deal with the synthesis of thin films and nanostructured materials by the means of PVD techniques and the application of low energy nuclear techniques to materials science. Presently, he is researcher at the CNR-INFM SENSOR Lab, Brescia. . Sensors and Actuators B 121 (2007) 208–213 Synthesis and characterization of semiconducting nanowires for gas sensing G. Sberveglieri ∗ ,. metal oxides for gas sensing applications. His main activity regards synthesis and elec- trical characterization of metal oxides for gas sensing applications.