Epitaxial Growth f SnO2 Nanorods n R-Cut Sapphire Substrate

S. M. Loya-Mancilla, J. T. Holguín-Momaca, W. Antúnez-Flores, F. Espinosa-Magaña, J. A. Matutes-Aquino, S. F. Olive-Méndez.

Centro de Investigación en Materiales Avanzados, S.C. (CIMAV), NanoTech, Miguel de Cervantes No. 120, C.P. 31109, Chihuahua, Chih., México.

SnO2 nanostructures have attracted much attention for its potential sensitivity for gas sensing, photoluminescence emission, transparency and the capability to modify charge carriers by modifying oxygen vacancies [1].

SnO2 nanorods (NRs) were grown using vapor transport technique and the vapor-liquid-solid (VLS) process in a tubular vacuum furnace at 1100 °C for 10 min. Substrates were coatedwith a ≈5 nm Au layer that produces gold nanodroplets (GND), once the temperature exceeds 750 °C, which act as a catalytic element for located NRs growth. Sn powder was used asmetal precursor. A mixture of Ar/O2 (10:1) was employed to transport the material from an alumina boat to the substrate surface; the flux was fixed to 150 sccm with a partial pressure of 1.5×10-1 Torr. Scanning electron microscopy (SEM), using a FESEM JSMF-7401F, wasperformed to obtain the morphology of such NRs. The distance between the alumina boat and the substrates varies from 1 to 5 cm. Fig. 1a depicts the obtained nanowires (NWs); there are some SnO2 agglomerates, whose formation is due to the excess of flux of Sn-O. Neat NWswere obtained for a separation of 2 cm and no agglomerates were observed (Fig. 1b). All the NW growth process takes place at the GND, since a minor quantity of atoms arrives to the sample. Atoms have enough time to form a saturated solution in the GND and the excess of the solute precipitates on the growth front of the NW. NRs were obtained when separation was equal or higher than 5cm (Fig. 1c). One can suggest that as a very small amount of material arrives to the sample, ad-atoms have enough time to diffuse over the surface of the wall side of the NR until they find energetically convenient positions.

The VLS process favors the deposition of material over the GND.! However, we have observed that material deposits on bare sapphire substrates for experiments longer than 60 min. This fact suggests that ad-atoms also deposit on the wall side of NRs, and only a small amount of material arrives directly to the GND. Actually, as the NRs have lower surface area (i.e. lower surface energy) NRs system seems to be more stable than the NWs system. In the insert of Fig. 1c shows the shape of the edge of the NRs, these facets were identified as {-11-1}, the formation energy for this pyramidal termination equal to -0.87 Jm-2 implies that (20-1) plane will be more stable than observed facets.[2] However, at the end of the NR growth a change on the composition leads to the pyramidal shape. Fig. 1d shows a cross section SEMmicrograph of the NRs; also there is an angle of 15° with the normal to the plane and relative to [12-1] direction of the substrate; this value differs from an angle of 22° normally found on SnO2 NWs/r-cut sapphire [3]. A high-resolution micrograph obtained by means of transmission electron microscopy (TEM) using a FETEM-JEM-2200FS+Cs, shows atomic resolution of a NR near the wall side. Fig. 2b is the fast Fourier transform (FFT) of the HR-TEM micrograph of the body of a NR, indexation of the dots as well as cross section micrographs obtained by SEM (Fig. 2c) were used to identify that the growth direction is

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Microsc. Microanal. 18 (Suppl 2), 2012© Microscopy Society of America 2012

along [101]SnO2 and to find the epitaxial relationship between SnO2(403)//α-Al2O3(1-12) and[010]SnO2//[100]Al2O3. HR-TEM and FFT of the interface NR/substrate are in progress to confirm the obtained epitaxial relationship, which differs from the one of SnO2 NWs/r-cutsapphire.[4] A Raman spectrum was obtained (Fig. 1h), which excitation was induced using aHe-Ne laser line with 632.8 nm wavelength. There are three classical Raman scattering peaks at 477, 636, 779 cm-1 related to the typical bulk feature of the rutile phase SnO2 [5], so the tetragonal structure is still in NR. Some broad bands are observed in Eg peak, whose intensity is increased compared to the B2g, suggesting the presence of a small amount of a secondary phase such as SnO.

References:[1] H. Wang et al. Journal of Magnetism and Magnetic Materials 321. (2009). 3114-3119.[2] J. Oviedo and M. J. Gillan. Surface Science. 463 (2000) 93-101.[3] W-S Kim. Advanced Materials Processing Lab. 2011.[4] S. M. Loya-Mancilla and S. F. Olive-Méndez to be published.[5] B. Cheng et al. Materials Chemistry and Physics 129 (2011) 713-717.

Fig. 1 SEM Micrographs of different morphologies were obtained with different distances between Sn andsubstrates; a) 1cm, b) 2cm, c) 5cm (inset image shows the morphology of the obtained tip in the NRs); d) Cross-section cut of NRs in a r-sapphire substrate from where epitaxial relationship was obtained, aided with FFT from HR-TEMmicrograph.

Fig. 2 a) NR micrograph obtained by means of TEM of a single NR; b) HR-TEM micrograph of the atomic arrangement of the rutile tetragonal structure of SnO2; c) FFT of the HR-TEMmicrograph of the NR, showing the indexation of the representative dots used to find epitaxial relationship; d) NR Raman spectrum obtained by means of a MicroRaman Horiba Spectrometer, comparison with a reported NW Raman spectrum obtained from bulk SnO2.

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