I.N. Rivera1*, D.J. Ramos², G. C. Vásquez2, and D. Maestre2 

1 Inter American University of Puerto Rico, 2 Departamento de Física de Materiales, Facultad de CC. Físicas, Universidad Complutense de Madrid, 28040, Madrid, Spain

*irivera2151@interbayamon.edu

Since nano- and microstructured materials exhibit distinct properties from those of bulk materials due to their peculiar structure characteristics and size effect, different approaches have been explored for their synthesis. Combinations of p- and n-type oxides or introduction of dopants have been considered as promising strategies to adequate the characteristics and applicability of nano- and microstructured materials to the required performances. Among these materials, the well-known n-type oxide SnO2 is a wide-bandgap semiconductor (Eg  3.6 eV) that displays high electrical conductivity, and good optical transparency in the visible range, which are suitable for multiple applications [1]. In the other hand, NiO stands out as one of the few p-type semiconducting oxides, which still remains underexplored, with a wide-bandgap around 3.6-4.3 eV that exhibits interesting electrical, optical, and magnetic properties [2, 3]. In that manner, we report the synthesis and characterization of elongated micro- and nanostructures based on SnO2 and Ni doped SnO2, p-n heterostructures based on NiO-SnO2, and NiO structures grown by a vapor-solid method using diverse precursors, including SnO2, metallic Sn and Ni, and C in controlled ratio. Different thermal treatments were carried out at temperatures between 900-1400 C under a controlled N2 flow. All the samples were characterized by SEM (Scanning Electron Microscopy), EDS (Energy Dispersive x-ray Spectroscopy), Photoluminescence (PL), and micro-Raman Spectroscopy using different excitation sources.  

The results established that treatments at high temperature (1400 C) on Sn:Ni:C precursor promotes the growth of elongated structures based on SnO2 with large dimensions at the microscale, combined with NiO-rich regions, as compared with treatments performed at lower temperature (900 C) . Also, high-yield growth of nanowires is obtained by using metallic Sn as precursor and temperatures of 900 C. Raman spectroscopy measurements confirm the presence of rutile SnO2 in most of the samples, as well as cubic NiO in some cases. PL signal from SnO2 shows two main contributions in the visible range related to oxygen deficiency which slightly vary among the samples, as a function of the thermal treatments and the employed precursor. Moreover, both p-n heterostructures based on NiO-SnO2, as well Sn doped NiO and Ni doped SnO2 were obtained by a vapor-solid method, which confirms the versatility and efficiency of the synthesis route. The physical mechanisms involved in the growth of these oxide-based nano- and microstructures are discussed. Further research will lead to the achievement of controlled morphology, dimensions and compositions, in the search of improved applications based on these oxides.

References

[1] A. Vázquez-López, D. Maestre, J. Ramírez-Castellanos et al. J. Phys. Chem. C, 124, 18490-18501, (2020)

[2] M. Taeño, D. Maestre and A. Cremades. Materials Letters, 271, 127801, (2020). https://doi.org/10.1016/j.matlet.2020.127801. Taeño, J. Bartolomé, L. Gregoratti et. al.. Crystal Growth & Design, 20 (6), 4082-4091 (2020).

[4] I. Gontia, M. Baibarac, and I. Baltog.. Physica status solidi (b), 248 (6), 1494-1498 (2011).