The descriptions of SnO2 nanowires growth procedures are getting more and more frequent in the current literature. However, studies on the growth mechanisms are still lacking. In particular, no investigation has been reported on the growth process when the growth mechanisms are not based, as in the case of whiskers, on vapour-liquid-solid (VLS) transitions. In this paper, a new procedure is reported by the authors for growing SnO2 nanowires, based on the presence of liquid-tin droplets on the substrate. The Sn vapour pressure developed by these droplets, which find themselves very close to the growing tip of the wire, gives rise to a sufficiently high supersaturation to enable the fast growth rate usually observed. The principal features and results of this new procedure, as well as possible growth mechanisms, are also discussed

Calestani, D.

Paorici, C.

Zanotti, L.

Zha, M.

English

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DOI 10.1393/ncc/i2005-10012-xIL NUOVO CIMENTO Vol. 27 C, N. 5 Settembre-Ottobre 2004Study of quasi-1D SnO2 nanowiresD. Calestani(1), M. Zha(1), L. Zanotti(1) and C. Paorici(2)(1) IMEM, CNR - Parma, Italy(2) Dipartimento di Fisica, Universita` di Parma - Parma, Italy(ricevuto il 20 Gennaio 2005)Summary. — The descriptions of SnO2 nanowires growth procedures are gettingmore and more frequent in the current literature. However, studies on the growthmechanisms are still lacking. In particular, no investigation has been reported onthe growth process when the growth mechanisms are not based, as in the case ofwhiskers, on vapour-liquid-solid (VLS) transitions. In this paper, a new procedureis reported by the authors for growing SnO2 nanowires, based on the presence ofliquid-tin droplets on the substrate. The Sn vapour pressure developed by thesedroplets, which find themselves very close to the growing tip of the wire, gives riseto a sufficiently high supersaturation to enable the fast growth rate usually observed.The principal features and results of this new procedure, as well as possible growthmechanisms, are also discussed.PACS 81.10.Aj – Theory and models of crystal growth; physics of crystal growth,crystal morphology and orientation.PACS 81.10.Bk – Growth from vapor.PACS 81.07.-b – Nanoscale materials and structures: fabrication and characteri-zation.1. – IntroductionNew interesting results on the capability to synthesize semiconducting oxides likeSnO2, ZnO, In2O3 and others, in quasi-1D form (in literature these structures are oftencalled “nanowires”, “nanobelts”, “nanoribbons”, etc. . . . ) by simple vapour transporttechniques have recently been reported (e.g., see the review by Wang in ref. [1]). Theyhave stirred a great interest as to the potential applications in optics and microelectronicsas well as the physico-chemical properties of their nanostructures.A particular attention has been focused on the possible extension of these nanomate-rials to the gas-sensor field. In fact, several semiconducting oxides have been successfullyemployed as “sensing” materials during the last years, mainly in the thick- and thin-filmform. But a single-crystalline quasi-1D nanostructure (i.e. the nanowires) may sub-stantially increase the “sensing” properties of metal-oxide (MOX) sensors, thanks to theincreased surface/bulk ratio and the assumed larger time-stability. Preliminary tests ofc© Societa` Italiana di Fisica 539540 D. CALESTANI, M. ZHA, L. ZANOTTI and C. PAORICIFig. 1. – Metallic Sn micro-droplets condensed on the substrate during the first step of thegrowth. Their presence is fundamental for the nanowire growth.SnO2 nanowires-based sensor devices developed by this group confirmed a large increaseof sensitivity, in comparison with the one of a traditional SnO2 sensor [2].Further investigation on the crystal growth mechanisms is needed if a good controlof the nanowires growth process has to be achieved. In particular it is important toextend the technique to a reliable and large-scale growth on the desired substrates, orto different semiconducting oxides. Unfortunately this information is not available andonly uncompleted and contradictory data can be retrieved from literature. Object ofthis paper is to report on a novel vapour-phase technique suitable for growing SnO2nanowires. Details on the growth procedure, as well as possible growth mechanisms, arealso discussed especially in view of a future optimisation of the technique.2. – SnO2 nanowires growth and characterizationTin oxide nanowires were grown in a home-made furnace, inside a quartz tube inwhich it is possible to have vacuum and flow different gasses. SnO powders (the sourcematerial) were located next to the substrates (Al2O3, Si, Si/SiO2, . . . ) within an aluminaboat, in the central part of the tube. The growth-system is then heated to 800–900 ◦Cin an Ar flow. At high temperature SnO decomposes into liquid Sn and SnO2 (e.g., seeref. [3]) and Sn vapour can be carried on the substrates, where it condenses in micron-sizedroplets (fig. 1). The reaction with oxygen then promotes the crystallization of the SnO2nanowires. The mechanism of this crystal-growth will be discussed later in this paper.The obtained samples have been then characterized by X-ray diffraction analysis,Scanning Electron Microscope (SEM) imaging, Transmission Electron Microscopy (TEM)investigations, Photoluminescence (PL) and Cathodoluminescence (CL) spectra measure-ments [4].These measurements revealed that the nanowires are characterized by a rutile-likeSnO2 single-crystal structure. They generally show a “belt-like” morphology, with fewtens of nanometers in thickness and hundreds of micrometers in length (fig. 2).3. – DiscussionIn spite of the presence of Sn droplets in the first step of the process, no evidenceof a liquid phase could be found on the growing tip at the end of the growth. ThisSTUDY OF QUASI-1D SNO2 NANOWIRES 541Fig. 2. – SEM images of the obtained samples of SnO2 nanowires grown on alumina substrate, atdifferent magnification. The nanowires are homogeneously distributed on the substrate. Theirmorphology is mainly “belt-like”. The thinnest side is few tens of nanometers wide, while thelength can reach hundreds of micrometers.observation should exclude any whiskers-like VLS growth mechanism, though some otherauthors claimed to have evidence of this mechanism in their growth process [5-7]. Ourobservation is however in agreement with what reported in some recent papers [8-11],where VLS mechanisms were excluded.An alternative explanation may be given by considering that the growth rate of thenanowires is rather high, as they can reach 50–500 µm in length in a 30–60 minutes run.The nanowires often grow in a slightly bended or twisted shape, usually elongated forhundreds of micrometres in directions which are generally parallel to the substrate andclose to it (usually the distance is smaller than 50 µm).Now, simple considerations show that such high growth rates cannot be justified bythe vapour transport of Sn (the limiting element in the reaction with oxygen) from sourceto substrate. Both diffusion and convection through the vapour phase are in fact notable to provide a sufficient amount of Sn to justify the crystallization flux necessary toexplain the observed growth rate. In the case of diffusion of Sn through the carrier gas,542 D. CALESTANI, M. ZHA, L. ZANOTTI and C. PAORICIthe flux can be estimated byJdiff = −(D [ pSn(1) − pSn(2)])/RTd ,(1)in which D is the binary diffusion coefficient of Sn in Ar, R is the gas constant, T theaverage temperature in the temperature profile between source and substrate a distantd far apart, pSn(1) and pSn(2) are the equilibrium partial pressures of Sn at source andsubstrate temperature, respectively. When D = 10−4 m2s−1, T = 1150 K, (pSn(1) −pSn(2)) = 10−4 mbar, d = 0.1 m, one gets Jdiff ≈ 10−9 mol m−2 s−1.As the convective flux (Jdrift), a simple estimation leads toJdrift = (vdrift pSn)/(PVm)(2)in which vdrift is the drift velocity, Vm the molar volume of Ar (the carrier gas), pSn theaverage partial pressure of Sn and P the total pressure in the system. When Vm = 3.33 m3mol−1, pSn = 0.5 × 10−4 mbar, and vdrift = 0.05 m s−1, one obtains Jdrift ≈ 10−8 molm−2 s−1.One should notice that these values are estimated for the typical conditions of ourgrowth system in which ∆T = 100 K (temperature drop between source and substratewhen d = 0.1 m) and a flow of inert gas of about 100 cm3 min−1 at 300 mbar.The obtained values may be compared with the Sn flux that is required to assure themass balance (Jmb) at the observed growth rate (at least 0.1 mm/h): Jmb ≈ 10−6 molcm−2 s−1. It is easy to note that the value is much larger than the ones obtained ineqs. (1) and (2), meaning that the Sn transport from the source cannot steadily sustainthe nanowires growth.As previously mentioned, the nanowires usually grow near to the substrate surface,where the liquid-Sn droplets are located. In the experimental conditions that have beenhere reported, a particle mean free path λ ≈ 10 µm can be evaluated. So, λ is comparablewith the distance between the Sn drop and the nanowire growing tip: the Sn vapourtransport in this range is much larger, because the mean square velocity (u) of a Snparticle is obviously larger than the average long-range transport velocity mediated bycollisions. In this short range a new Sn flux (Jsr) can be evaluated byJsr = 1/4 u n ≈ 10−6 molm−2 s−1 ,(3)where u (≈ 4 m s−1) is the mean square velocity and n (≈ 5 × 10−7 mol m−3) is themolar density derived in the ideal-gas approximation. This value is comparable withthe required growth molar flux (Jmb). So, if Sn evaporation kinetics from the dropletsis high enough to support such transport, the liquid phase on the substrate has to beconsidered the effective feeding material for the nanowires growth and the mechanismmay be considered as a “short-range vapour-solid” growth.The above considerations underline the important role of the liquid-Sn droplets forthe growth of nanowires. In fact, as experimentally evidenced, when the temperaturegradient inside the furnace and the inert-gas flow are modified to prevent the condensationof the liquid phase on the substrate, leaving Sn vapours only coming from the source, thereaction with oxygen only gives rise to the formation of SnO2 nano-crystalline powders.Another problem, which has deserved little attention so far, regards which are thegrowth mechanisms of these quasi-nanocrystalline structures and how they assemble ina supersaturated vapour phase.STUDY OF QUASI-1D SNO2 NANOWIRES 543Difficulties arise when attempting to explain the strong difference (usually amountingto more than three orders of magnitudes) between the fast-growth orientation parallel tothe long size of the “wire” crystal and the other low-growth orientations.As fast-growth rates are typical of atomically rough interfaces between the growingcrystal and its nutrient phase (even at very small supersaturation) [12, 13], one has topostulate the existence of this type of interfaces in a crystal-vapour system such asours. The difficulty is here that the atomistic models up to now developed [13-16] wouldonly predict, for these systems, low roughness levels, i.e. atomically smooth interfaces,typically characterized by slow growth kinetics.As often reported (e.g., see [17]), a way out is here relying on VLS growth mechanisms.The presence of a tiny liquid layer on the growing tip of the filamentary crystal wouldact, from the atomistic viewpoint, as an almost ideally rough interface, thus favouringa very fast growth. Unfortunately, no experimental evidence exists in our case, of theformation of such liquid phase.A further way for explaining the observed fast-grow rates might very likely be in re-lation with possible modifications of the strength field at the growing interface due tothe rearrangements of the unsaturated bonds. As pointed out after revisiting the earlieratomistic models [18], surface reconstructions should favour an increase in surface rough-ness in the presence of catalyst impurities and/or marked off-stoichiometry in compoundcrystals.Research work is presently under way, along these lines, in our laboratory.4. – ConclusionsA new technique for growing SnO2 nanowires has been developed and studied indetails. It is shown that the formation of liquid-tin droplets appears to be crucial foran efficient growth process, since the Sn liquid phase acts as a feeding material forthe growth. The short distance between the Sn droplets and the growing tip of thenanowire enables a fast vapour Sn transport, which allows to achieve the needed highsupersaturation and to keep it for the entire growth process.By optimisation of this procedure it was possible to extend the nanowires depositionarea from a few square millimetres to about some square centimetres, by improving inthe same time the deposition homogeneity and quality.Finally, since a further optimisation of the growth technique requires a deeper under-standing of the growth mechanisms, possible approaches to the atomistic growth modelshave been briefly discussed, especially in view of future research work along these lines.∗ ∗ ∗The authors thank Prof. G. Sberveglieri, for his collaboration in the preparationof a high-performance reliable gas-sensor device, and they would like to acknowledge thefinancial support of the Italian Ministry for University and Research (MIUR), throughthe PRIN 2004 Project “Growth and properties of semiconducting-oxide based quasione-dimensional nanocrystals”.REFERENCES[1] Wang Z. L., Materials Today, 7 (2004) 26.[2] Comini E., Faglia G., Sberveglieri G., Calestani D., Zanotti L. and Zha M., inpress on Sensors and Actuators B.544 D. CALESTANI, M. ZHA, L. ZANOTTI and C. PAORICI[3] Aurbach D., Nimberger A., Markovsky B., Levi E., Sominski E. and GedankenA., Chem. Mater., 14 (2002) 4155.[4] Calestani D., Lazzarini L., Salviati G. and Zha M., in press on Crystal Res. Technol.[5] Dang H. Y., Wang J. and Fan S. S., Nanotechnology, 14 (2003) 738.[6] Chen Y., Cui X., Zhang K., Pan D., Zhang S., Wang B. and Hou J. G., Chem. Phys.Lett., 369 (2003) 16.[7] Jian J. K., Chen X. L., Wang W. J., Dai L. and Xu Y. P., Appl. Phys. A, 76 (2003)291.[8] Pan Z. W., Dai Z. R. and Wang Z. L., Science, 291 (2001) 1947.[9] Dai L., Chen X. L., Jian J. K., He M., Zhou T. and Hu B. Q., Appl. Phys. A, 75(2002) 687.[10] Hu J. Q., Ma X. L., Shang N. G., Xie Z. Y., Wong N. B., Lee C. S. and Lee S. T.,J. Phys. Chem., 106 (2002) 3823.[11] Zhang J., Jiang F. and Zhang L., J. Phys. D, 36 (2003) L21.[12] Rosenberger F., in Interfacial Aspects of Phase Transformations, Crystal GrowthKinetics, edited by Mutaftshief B. (Reidel, Rotterdam) 1982, pp. 315-464.[13] Mutaftschief B., in The Atomistic Nature of Crystal Growth (Springer, Berlin) 2001,chapt. 16.[14] Burton W. K., Cabrera N. and Frank F. C., Philos. Trans. R. Soc. London, Ser. A,243 (1951) 299.[15] Bennema P. and Gilmer G. H., in Crystal Growth: an Introduction, Kinetics of CrystalGrowth, edited by Hartman P. (North Holland, Amsterdam) 1973, pp. 263-327.[16] Jackson K. A., J. Crystal Growth, 5 (1969) 13.[17] Givargizov E. I., in Current Topic Mat. Sci., Growth of Whiskers by the VLS mechanism,edited by Kaldis E., Vol. 1 (North Holland, Amsterdam) 1978.[18] Chen G. S., Ming N. B. and Rosenberger F., J. Chem. Phys., 84 (1986) 2365.

Study of quasi-1D SnO2 nanowires

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Study of quasi-1D SnO2 nanowires

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