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Growth and Characterization of Mn Doped SnO2 Nanowires, Nanobelts and Microplates Manuel Herrera Zaldivar, David Maestre, Ana Cremades, and Javier Piqueras J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4007894 • Publication Date (Web): 05 Apr 2013 Downloaded from http://pubs.acs.org on April 8, 2013

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Growth and characterization of Mn doped SnO2 nanowires, nanobelts and microplates Manuel Herrera#, David Maestre, Ana Cremades* and Javier Piqueras. Departamento de Física de Materiales, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Madrid 28040, Spain.

AUTHOR E-MAIL: [email protected]

#

: Permanent address: Centro de Nanociencias y Nanotecnología, UNAM. 22800-Ensenada, Baja California, México. *:Corresponding author. Tel.: +34 913944521. E-mail address: [email protected] (A. Cremades).


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ABSTRACT: Undoped and Mn doped SnO2 nanowires, nanobelts and microplates have been grown by a thermal evaporation method that enables to control the morphology and the Mn content in the structures. The structural and morphological characterization was carried out by scanning and transmission electron microscopy (SEM and TEM) and electron backscattered diffraction (EBSD). A crystallographic model has been proposed to describe the Mn:SnO2 microplates. X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) demonstrated the incorporation of Mn into the SnO2 lattice in concentrations up to 1.6 at % depending on the thermal treatment employed for the growth of the structures. Variations in the luminescence of the doped nanostructures as a function of the Mn content have been studied. A correlation between facets of the SnO2:Mn microplates, identified by EBSD, with higher Mn content, and the increase of the luminescence emissions associated to oxygen vacancies related defects was demonstrated.


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1. INTRODUCTION Tin oxide is a wide band gap semiconductor (Eg = 3.6 eV) with potential technological applications as gas sensors, solar cells, optoelectronic devices, or catalyst.1-4 In the past years SnO2 has received special attention for spintronic applications because it exhibits ferromagnetism at room temperature not only when doped with magnetic impurities, such as manganese5-7 , but also due to the presence of some other impurities such as N.8 The origin of this ferromagnetism is still under discussion, and several mechanisms are proposed. Many efforts have been devoted to optimize the synthesis of SnO2:Mn and to achieve control of its point defect generation. As an example, several authors reported that point defects such as O and Sn vacancies in SnO2 contribute significantly to the generation of ferromagnetism.9-11 Further studies are required to gain new insight in the relation between the crystallographic planes forming SnO2 micro and nanostructures and their characteristic structure of defects in order to achieve deeper understanding of the doping process, as the incorporation of dopants could be favored in certain faces. Studies on the luminescence of SnO2 as a function of the Mn dopant concentration will help to understand the doping process and the generation of point defects. The optical properties of SnO2:Mn as thin films and nanoparticles have been often investigated, but there are few reports on elongated nano- and microstructures,12-15 as those presented in this work. This works reports on the fabrication and characterization of Mn doped SnO2 elongated nano- and microstructures, such as nanowires, nanobelts and microplates. Structural, compositional and optical characterization have been carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy


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dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), cathodoluminescence (CL) in SEM, and X-ray photoelectron spectroscopy (XPS). 2. EXPERIMENTAL Undoped and Mn doped SnO2 elongated nano- and microstructures were fabricated by a catalyst-free evaporation-deposition method, using SnO2

ceramic substrates prepared by

compacting SnO2 powder and then sintering at 700 °C for 3 hours in air. The synthesis was performed in a horizontal furnace equipped with an alumina sealed tube operated at temperatures up to 1400 °C, and low vacuum conditions (up to 1 x 10-2 mbar) under argon flow. Mixtures of commercial SnO2 (Alfa Aesar 99.99 %) and MnCO3 (Alfa Aesar 99.9 %) powders with 5 or 20 wt % of MnCO3 were used as precursors. Also a reference sample was prepared with pure SnO2 as precursor. The powder mixtures were compacted and then annealed under argon flow at 1200 ºC or 1400 ºC during 10 hours (Table 1). Deposition on the substrates placed at a colder region in the furnace leads to the growth of elongated micro- and nanostructures. The substrates were placed on the downstream side of the furnace onto a linear manipulator equipped with an N-type thermocouple, which permitted the control on the growth temperature by placing the substrates at different positions along the tubular furnace. The deposition temperatures are shown in the Table 1. The crystalline structure of the samples was analyzed by XRD with a Siemens D500 Philips diffractometer working at 45 kV and 40 mA, using the Cu Kα radiation. The size and morphology of the samples were studied with a FEI Inspect S SEM. For the analysis of the chemical composition, a Bruker AXS XFlash 4010 X-ray microanalysis system attached to a Leica 440 SEM was used. A Bruker e-Flash electron backscattered diffractometer (EBSD) was


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used for structural analysis of single microstructures in SEM, which were dispersed onto Si(100) to assure a correct surface orientation. The microscopic structure of the samples was further investigated using a Philips CM30 TEM operated at 300 kV. CL measurements were performed at temperatures between 90 and 300 K in the visible range with a Hamamatsu R928 photoncounting. Spatially resolved X-ray photoemission spectroscopy was performed at the ESCA microscopy beamline of the Elettra-Synchrotron in Trieste, Italy. Photoemission spectra were measured with a 640 eV photon energy while keeping a 0.2 eV energy resolution.

3. RESULTS AND DISCUSSION XRD results showed that all the grown samples correspond to the SnO2 rutile structure. No traces of initial MnCO3, manganese oxides, or ternary compounds were detected in the patterns, as shown in Figure 1. Figure 2 shows SEM images of the undoped SnO2 sample (sample 1) with a high concentration of SnO2 nanowires and nanobelts. The nanowires show widths ranging from 100 to 300 nm and lengths of up to tens of microns. Some of the nanowires exhibit changes along their growth direction resulting in elbow-like and zigzag structures (Figure 2). Different authors have explained the zigzag structures, forming angles of 68º and 112º, by an alternate growth between the [101] and [10-1] directions as they are equivalent in the crystal structure of tetragonal rutile SnO2 and, therefore, slight fluctuations of the growth kinetics conditions can cause the variation in the preferential growth.16-17 The presence of Mn induced changes in the growth conditions and the morphology of the resulting SnO2 structures. In particular, Mn doping decreases the minimum temperature required


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to grow SnO2 nanowires. Figure 3a shows the Mn doped SnO2 nanowires corresponding to the sample 2 obtained at 935 ºC (Table 1), with widths ranging from 50 to 200 nm and with lengths of some microns. The averaged Mn content measured by EDS in these structures is around 0.8 at. %, In this case, no zigzag nanowires were observed as a difference with the undoped nanowires, which reveals the influence of Mn on the final morphology of the nanowires. Higher growth temperatures increase the size of the Mn doped structures. Sample 3, grown at 1090 ºC, shows a high concentration of elongated SnO2 microplates, as shown in Figure 3b. These plates with lengths of tens of microns, present thickness below 1µm and widths between 2 and 4 µm. Higher Mn concentrations of up to 1.6 at.% on average, as detected by EDS, were incorporated into these microplates as compared with the nanowires, which can be explained by the higher growth temperature of the plates. Increasing the Mn content in the initial mixture to 20 wt.% of MnCO3 (sample 4) substantially lowers the temperature needed to grow the nanowires down to 820 ºC. This lower temperature leads to nanowires and nanobelts with Mn content below 1 at.% in spite of the relative large amount of MnCO3 used (Figure 3c). However, in that case the final morphology is highly influenced by the precursor used and a significant amount of secondary growth such as branch-like nanowires (Figure 3d) is observed. These structures may grow following a self-catalyst process18-19 originated by the formation of liquid metallic nanoparticles on nanowire-tails, as explained below for some SnO2 wires. Some of the thinnest wires and belts of samples 1, 2 and 4 were studied by TEM in order to assess their crystallinity. Figure 4a shows a TEM image of one undoped SnO2 nanowire of sample 1, which shows a variable growth direction forming angles of around 140º. SAED pattern (inset in Figure 4a), under the [010] zone axis, indicates that this nanowire grows along the and directions in SnO2 rutile type structure. Secondary growth is seldom obtained


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for these undoped nanowires. In one of the wires presenting secondary growth a rounded Sn particle (encircled in Figure 4b), is observed at the tip of the secondary wire, which is an indication of a self-catalytic vapor-liquid-solid process. Figure 4c shows a typical TEM image of the nanostructures grown in the sample 4, revealing formation of straight and undulated nanowires and nanobelts. Most of the belts show faceted sidewalls, which could be induced by the presence of Mn, in analogy to the formation of rippled Si nanowires due to the presence of Au.20 The diffraction contrast observed in these belts could be mainly related to stacking faults occurred during the growth and bend contours. Figure 4c also shows some straight nanobelts with widths below 250 nm and thickness lower than 50 nm. The SAED pattern under the [001] zone axis shown in Figure 4d indicates that this nanobelt extends along the [100] growth direction. The structures that are not thin enough for TEM studies were analyzed by EBSD. Figures 5a and 5b show a microplate of the sample 3 and a typical EBSD pattern acquired at its main face. The EBSD pattern corresponds to the (-101) plane of the rutile unit cell as shown in Figure 5c, which is parallel with the XY plane of the detector. The polar figure of the {-101} planes was obtained during the measurement presenting the (10-1) plane at its center (Figure 5d). The planes assigned for the microplate faces are shown in Figure 5a, and agree with the planes reported by Dai et al. for SnO2 nanoribbons.21 These authors suggested that the (10-1) plane, which is the closest packing plane for the Sn cations in the rutile structure, form the nanoribbons wide surface, while the (010) surface is parallel to the closest packed plane for the O anions. Therefore these planes with lower surface energies tend to form the microplate side surfaces, as described in the schemes shown in Figure 6. According to the crystallographic study of these samples, the Mn doped SnO2 microplates grow along the [101] direction and are formed by a main surface


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corresponding to the (-101) plane, bounded by the (010) and (121) planes as the lateral faces, as shown in the schematic models included in Figure 6. The theoretical angle between (121) and (010) planes corresponds to 138º, while (121) and (-101) planes form angles of 75º in the SnO2 rutile structure which correspond with those observed in the SEM images of the microplates. The triangular tip of the microplates exhibits an angle around 80º which agrees with that formed between (121) and (1-21) planes in the SnO2 rutile structure. A lateral view of one microplate is shown in Figure 6c, where it can be clearly appreciated that the main surface, which corresponds to the (-101) plane, is not orthogonal to the (121) lateral face. A schematic model of the atomic arrangement of the (-101) and (010) crystal faces is also included in Figures 6b and 6d respectively. In addition, more complex microplates are also observed, as that shown in Figure 6e, with beveled-like tips composed by the (1±21) and (1±41) planes, a scheme of which is shown in Figure 6f. These structures could correspond to a later stage of growth from the initial smaller microplates with triangular shape tips. CL measurements show differences in the luminescence signal from undoped and Mn doped SnO2 samples. The deconvolutions of the CL spectra from undoped SnO2 nanostructures (sample 1) acquired at 100 K present two main emissions centered at 2.58 eV and 1.94 eV and a weaker one at 2.25 eV, as shown in Figure 7a. The blue emission (2.58 eV) has been attributed to shallow levels in the SnO2 electronic structure involving surface defects.22,23 The orange emission (1.94 eV), which dominates the spectrum, has been frequently observed in SnO2, and in general it has been associated with oxygen vacancies related defects.24-26 Some authors associated the green emission at 2.25 eV with radiative transitions involving oxygen vacancies related states and intrinsic surface states.27 Luminescence from samples with low Mn content (samples 2 and 4) has been also analyzed at 100 K (Figure 7a). CL spectra are dominated by the


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1.94 eV emission, with a contribution at 2.25 eV more evident in sample 4. The blue emission centered at 2.58 eV is not observed in the spectra from samples 2 and 4, with a low Mn content below 1 % at. These results suggest that Mn doping in low concentrations promotes the formation of oxygen vacancies in the nanowires, which results in the increase of the intensity of the orange (1.94 eV) and green (2.25 eV) emissions and the relative decrease of the 2.58 eV band. In the CL spectra of the microplates of sample 3 with higher Mn content, of about 1.6 at. %, the 2.58 eV emission is the most intense (Figure 7b). This suggests that low Mn concentrations in SnO2 favors the orange emission (1.94 eV) but for higher amounts of Mn the blue emission (2.58 eV) dominates the CL spectra. Similar results have been also reported in Mn doped ZnO systems,28 where low amounts of Mn favors the green emission from ZnO, while Mn concentrations higher than 2 at. % causes its gradual decrease. Neither excitonic emissions nor band edge transitions have been observed,29 due to the dipole-forbidden nature of the SnO2 energy band structure.Local CL spectra acquired at the faces of the microplates of sample 3 show dependence of the relative intensity of the CL bands on the surface probed. The intensity ratio Iblue /Iorange at the peak positions has values of about 4.7, 2.7 and 2.9 at the points marked as A, B and C respectively in the microplate imaged in the insets of Figure 7b. This shows that the relative intensity of the orange emission (1.94 eV) is higher in the regions marked as B and C, which correspond to the (010) and (121) faces of the microplates, as measured by EBSD. We have previously reported that the blue emission (2.58 eV) intensity is sensitive to specific rutile crystal faces, and it does not depend on the orange band.24 The decrease of the intensity ratio Iblue/Iorange, found on the faces (010) and (121) of the SnO2 microplates (B and C in Figure 7b), indicates a higher concentration of oxygen vacancies related defects in those regions.


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Figure 8 shows the SEM and CL images of the microplates observed in sample 3 (Figure 3b). An enhanced CL signal can be appreciated in the central part along the axis of the microplates, marked with arrows in the CL image (Figure 8b). This could be related to a complex distribution of defects along the cross section, as that observed in Sn doped In2O3 or ZnO nanorods grown by a method similar to that used in this work.30,31 The surface chemical composition of single SnO2:Mn microplates (sample 3) was studied by XPS. Only peaks corresponding to tin, oxygen, manganese and adventitious carbon are observed. The XPS spectra analyzed in this work were calibrated using the C (1s) peak (284.6 eV) as a reference. Figure 9 shows XPS spectra acquired at different faces of a microplate (sample 3) marked in the insets as A, B, and C. According to the EBSD results, these lateral faces correspond to (-101), (010), and (121) planes, respectively. At point A, the Sn 3d5/2 signal presents a binding energy (BE) of 488.4 eV (Figure 9a). At the lateral edge of the microplate (point B), the Sn 3d5/2 signal shows a broad peak composed by a main component centered at 488.4 eV and a weak band at 487.4 eV (Figure 9b). This energy difference of about 1 eV between the two components agrees with that reported in the NIST database for the chemical states Sn4+ and Sn2+. At the tip of the microplate (point C), the component of 487.4 eV assigned to Sn2+dominates the Sn3d5/2 signal (Figure 9c). At regions B and C, the Mn 3p core level shows high intensity of a peak centered at 48.9 eV, which is hardly observed when probing point A (Figure 9d). XPS spectra in Figure 9d were normalized in order to avoid the influence of the relative orientation of the probed surfaces with respect to the detector. These results indicate a non uniform distribution of Mn in the microplates, which tends to accumulate in the (010) and (121) faces (points B and C). The incorporation of dopants could modify the surface energy of certain faces involving a preferential incorporation of Mn in some specific surfaces, as observed


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in this work. XPS spectra are consistent with the presence of Mn3+ substituting Sn4+ in the SnO2 lattice. In this process, oxygen vacancies are formed in order to maintain charge neutrality, which agrees with the CL results. It has been reported that the thermal decomposition of MnCO3 at 600 ºC leads to the formation of Mn2O3 crystallites,32 whereas at temperatures of 1000ºC crystallites with a Mn3+ /Mn2+ ratio equal to 2 are obtained,33 due to the oxidation and reducing reaction of Mn ions. Therefore it can be assumed that at the temperatures used to grow the samples of this work most of the manganese in the gas phase is in the Mn3+ charge state. In ref. (34) the incorporation of Mn into the indium oxide lattice has been suggested to depend on the heat of adsorption release upon trapping of the dopant ions on the nanocrystal surface. In this process, the dopant would undergo ligand substitution and eventually a change in coordination can be generated. The dopant incorporation depends on how similar are the crystalline structures of the host (SnO2) and of the stable compound of the dopant with the host lattice anion. The stable compound of Mn3+ with oxygen is Mn2O3, which presents a cubic bixbyite crystalline structure. As the incorporation of Mn in the samples is low, we suggest that the limitation on the dopant incorporation is due to dissimilarities between the coordination of the cations in the SnO2 rutile and the Mn2O3 cubic structures, which supports the inhomogeneous incorporation of Mn depending on the surface orientation observed for the microplates. Moreover, the presence of Sn2+ at the tip and lateral faces of the microplates indicates a surface local coordination of Sn comparable to that of SnO, also reported by other authors,35 which can be explained by the high concentration of oxygen vacancies in those regions promoted by the presence of Mn. Usually the charge imbalance between the host cations (Sn4+) and the dopant results in the formation of Sn2+ and oxygen vacancies related defects. The XPS, EDS and CL results also show a higher concentration of Mn and oxygen vacancies in the (010) and (121) faces (points B and C) of the


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microplates where the presence of Sn2+ has been also detected. This effect is favored in surfaces with orientations (121) and (010) rather than (-101), being the latter parallel to the closest packing plane for Sn cations in the rutile structure which could hinder Mn incorporation.

4. CONCLUSIONS Manganese doped SnO2 micro- and nanostructures have been grown on ceramic SnO2 substrates by a thermal evaporation method using a mixture of SnO2 and MnCO3 as precursor. Nanowires, nanobelts and microplates have been obtained as a function of the temperature of growth and the initial amount of Mn. The presence of Mn decreases the temperature required for the growth of these nanostructures. It has been observed that the final amount of Mn in the as-grown structures is favored by increasing the temperature of growth, whereas higher Mn content in the precursor does not influence the amount of Mn doping. Undoped SnO2 nanowires occasionally show alternate growth along the and directions, which is not observed in Mn doped SnO2 nanowires and nanobelts. The belts tend to grow along the direction. EBSD results show that Mn doped SnO2 microplates are formed by (-101), (121) and (010) planes. Mn doping induces changes in the CL signal in different ways depending on whether the amount of Mn is low or high. SnO2 nanostructures doped with low Mn concentrations, below 1 at. %, show CL signal dominated by the emission at 1.94 eV associated with oxygen vacancies, similar to that corresponding to undoped SnO2. The doped microplates, which contain about 1.6 at. % of Mn, present a CL signal dominated by the emission centered at 2.58 eV. Combined EBDS – CL XPS study enabled to associate the observed CL signal with a defective local stoichiometry at


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different crystalline faces of the doped SnO2 microplates. An increase of the luminescence emissions associated to oxygen vacancies related defects has been observed in areas with higher Mn content. XPS results depict an inhomogeneous distribution of the Mn which tends to accumulate at the (010) and (121) faces of the SnO2 microplates, inducing local charge state changes for Sn atoms.

Acknowledgements This work was supported by MICINN (Projects MAT- 2009- 07882, MAT-2012-31959 and CSD-2009-00013). MHZ thanks for the financial support from PASPA-UNAM and Conacyt 102519 project. We thank M. Amatti and L. Gregoratti for their help during the XPS measurements.

References. (1) Yin X, M.; Li, C. C.; Zhang, M.; Hao Q. Y.; Liu, S.; Li, Q. H.; Chen, L. B.; Wang, T. H. Nanotechnol. 2009, 20, 455503. (2) Sambhaji, S. B.; Gauri, A. T.; Arif, V. S.; Oh-Shim, J.; Myung-Mo, S.; Rajaram, S. M.; Anil, V. G.; Sung-Hwan, H. Materials Letts. 2012, 79, 29. (3) Cannella, G.; Principato, F.; Foti, M.; Di Marco, S.; Grasso, A.; Lombardo, S., J. Appl. Phys. 2011, 110, 024502. (4) Shu-Guo, Z.; Shuang-Feng, Y.; Yu-Dan, W.; Sheng-Lian, L.; Chak-Tong, A. Materials Letts. 2012, 79, 29.


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(5) Fitzgerald, C. B.; Venkatesan, M.; Dorneles, L. S.; Gunning, R.; Stamenov, P.; Coey, J. M. D.; Stampe, P. A.; Kennedy, R. J.; Moreira, E. C.; Sias, U. S. Phys Rev B 2006, 74, 115307. (6) Espinosa, A.; García-Hernández, M.; Menéndez, N.; Prieto, C.; De-Andrés, A. Phys Rev B 2010, 81, 064419. (7) Prellier, W.; Fouchet, A.; Mercey, B. J. Phys.: Condens. Matter 2003, 15, R1583. (8) Long, R.; English, N.J. Phys. Lett. A 2009,374, 319. (9) Wang, C.; Wu, Q.; Ge, H. L.; Shang, T.; Jiang, J. Z. Nanotechnol. 2012, 23, 075704. (10) Nomura, K.; Okabayashi, J.; Okamura, K.; Yamada, Y., J. Appl. Phys. 2011, 110, 083901. (11) Zhang, L.; Ge, S.; Zuo, Y.; Zhang, B.; Xi, L. J. Phys. Chem. C 2010, 114, 7541. (12) Chil-Hyoung, L.; Bo-Ae, N.; Won-Kook, C.; Jeon-Kook, L.; Doo-Jin, C; Young-Jei, O. Mater. Letts. 2011, 65, 722. (13) Xiao, Y.; Ge, S.; Xi, L.; Zuo, Y.; Zhou, X.; Zhang, B.; Zhang, L.; Li, C.; Han, X.; Wen, Z. Appl. Surface Sci 2008, 254, 7459. (14) Chang, J.; Mironov, V. L.; Bribkov, B. A.; Fraerman, A. A.; Gusev, S. A.; Vdovichev, S. N. J. Appl. Phys. 2006, 100, 104304. (15) Chi, J.; Ge, H.; Wang, J.; Zuo, Y.; Zhang, L. J. Appl. Phys. 2011, 110, 83907. (16) Huang, L.; Pu, L.; Shi, Y.; Zhang, R.; Gu, B.; Du, Y. Appl Phys Lett 2005, 87, 163124. (17) Wu, J.; Yu, K.; Li, L.; Xu, J.; Shang, D. ; Xu, Y. ; Zhu Z. J. Phys. D: Appl. Phys. 2008, 41,185302. (18) Chen, Y. X.; Campbell, L. J.; Zhou, W. L., J. Cryst. Growth 2004, 270, 505. (19) Jiang, X.; Zhang, L.; Wang, T.; Wan, Q. J. Appl. Phys. 2009, 106, 104316. (20) Ross, F. Rep. Prog. Phys. 2010, 73, 114501. (21) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. Solid State Comm. 2001, 118, 351-354.


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(22) Luo, S.; Fan, J.; Liu, W.; Zhang, M.; Song, Z.; Lin, C.; Wu, X.; Chu, P. K. Nanotechnol. 2006, 17, 1695. (23) Kim, S.; Lim, T.; Ju, S. Nanotechnol. 2011, 22, 305704. (24) Maestre, D.; Cremades, A.; Piqueras, J. J. Appl. Phys. 2004, 95, 3027. (25) Luo, S.; Chu, P. K.; Liu, W.; Zhang, M.; Lin, C. Appl. Phys. Letts. 2006, 88, 183112. (26) Chen, H. T.; Xiong, S. J.; Wu, X. L.; Zhu, J.; Shen, J. C. Nano Letts. 2009, 9, 1927. (27) Zhou, X. T.; Heigl, F.; Murphy, M.W.; Sham, T.K. Apppl. Phys. Lett. 2006, 89, 213109. (28) Shuang, D.; Zhu, X.X.; Wang, J.B.; Zhong, Z.L., Huang; Hr, G.J.C. Appl. Surf. Sci. 2011, 257, 6085-6088. (29) Pan, S.S.; Tian, Y.H.; Luo, Y.Y.; Zhang, Y.X.; Wang, S.; Li, G.H. Appl. Phys. Lett. 2010, 97, 221105. (30) Maestre, D.; Häussler, D.; Cremades, A.; Piqueras, J. J. Phys. Chem. C 2011, 115, 18083. (31) Ortega, Y.; Dieker, Ch.; Jäger, W.; Piqueras, J.; Fernández, P. Nanotechnol. 2010, 21, 225604. (32) Li-Xia, Y.; Ying-Jie, Z.; Hua, T.; Wei-Wei, W. Ultrasonics Sonochemistry 2007, 14, 259. (33) Gillot, B.; El-Guendouzi, M.; Laarj, M. Mater. Chem. and Phys. 2001, 70, 54. (34) Maestre, D.; Martínez de Velasco, I.; Cremades, A.; Amati, M.; Piqueras, J. J. Phys. Chem. C 2010, 114, 11748. (35) Togo, A.; Oba, F.; Tanaka, I.; Tatsumi, K. Phys. Rev. B 2006, 74, 195128.


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Sample

Growth Temperature (°C)

MnCO3 nominal (wt. %)

Mn (at. %) ±0.1%

1

1040

-

-

2

935

5

0.8

3

1090

5

1.6

4

820

20

0.8

TABLES:


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Table 1. Growth temperatures and proportions of MnCO3 used to obtain undoped and Mn doped SnO2 samples. The last column shows the averaged atomic concentrations of Mn in the as grown structures measured by EDS.

FIGURES:


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Figure 1. XRD spectra acquired from the sample 3 revealing a structure type rutile. The diffracted peaks labeled with (*) correspond to residual Ag-paint used in SEM measurements.


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Figure 2. (a) Typical SEM image of the undoped sample 1 with (b) elbow-like nanowires and nanobelts.


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Figure 3. Typical SEM images of the (a) sample 2 with straight nanowires, (b) sample 3 with microplates, and (c) sample 4 with nanobelts and (d) branched nanowires. The inset in (d) shows some nanoparticles adhered on a SnO2 nanowire-tail.


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Figure 4. (a) TEM images of single SnO2 nanowire grown along the [101] and [100] directions. (b) Lateral growth self-catalyzed by metallic Sn (encircled) observed in a SnO2 nanowire. (c) TEM images of the nanostructures from sample 4 with straight and undulated nanobelts. (d) TEM image of a nanobelt grown along the [100] direction and their corresponding SAED pattern under the [001] axis.


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Figure 5. (a) SEM image of a microplate of the sample 3 with its corresponding rutile-planes. (b) Typical EBSD pattern acquired from its main surface. (c) Orientation of the unit cell that corresponds with the EBSD pattern. (d) Polar figure generated for this unit cell orientation.


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Figure 6. (a) SEM image of a SnO2:Mn microplate with a (b) scheme showing the planes which form the structure grown along the [101] direction. A lateral view of a microplate with the corresponding schematic model are shown in (c, d). (e-f) SEM image and scheme of a thicker microplate which tip is formed by (1±21) and (1±41) planes. Insets in (b) and (d) show the schematic atomic arrangement of the (-101) and (010) planes, respectively.


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Figure 7. (a) CL spectra acquired at 100 K from the samples 1, 2 and 4 with a strong emission of 1.94 eV. (b) Local CL spectra acquired at the (-101), (010) and (121) faces (points A, B and C) of a microplate of the sample 3.


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Figure 8. (a) Typical SEM and (b) CL images of the Mn doped SnO2 microplates of the sample 3. The arrows in (b) indicate the high CL intensity in their central region along the growth direction.


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Figure 9. XPS spectra showing (a-c) Sn (3d) XPS core leves and (d) normalized Mn (3p) core levels acquired at the (-101), (010) and (121) faces (points A, B and C) of a microplate of the sample 3.


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Growth and Characterization of Mn Doped SnO2 ... - ACS Publications

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