Li2SnO3 branched nano- and microstructures with intense and broadband white-light emission

Exploiting the synergy between microstructure, morphology and dimensions by suitable nanomaterial engineering, can effectively upgrade the physical properties and material performances. Li2SnO3 elongated nano- and microstructures in form of belts, wires, rods and branched structures have been fabricated by a vapor-solid method at temperatures ranging from 700 to 900 °C using metallic Sn and Li2CO3 as precursors. The achievement of these new morphologies can face challenging applications for Li2SnO3, not only in the field of energy storage, but also as building blocks in optoelectronic devices. The micro- and nanostructures grown at 700 and 800 °C correspond to monoclinic Li2SnO3, while at 900 °C complex Li2SnO3/SnO2 core-shell microstructures are grown, as confirmed by X-ray diffraction and Raman spectroscopy. Transmission electron microscopy reveals structural disorder related to stacking faults in some of the branched structures, which is associated with the presence of the low-temperature phase of Li2SnO3. The luminescent response of these structures is dominated by intense emissions at 2, 2.5 and 3 eV, almost completely covering the whole range of the visible light spectrum. As a result, white-light emission is obtained without the need of phosphors or complex quantum well heterostructures. Enhanced functionality in applications such as in light-emitting devices could be exploited based on the high luminescence intensity observed in some of the analysed Li2SnO3 structures.

X'Pert Pro diffractometer using Cu Kα radiation. The samples studied in this work will be named, hereinafter, as S-700, S-800 and S-900, indicating the corresponding growth temperature. XRD measurements have been carried out in normal incidence for the as-grown samples, which present nano-and/or microstructures on the pellet surface.
As a result, information from the nano-and microstructures, as well as from the surface of the pellets, is extracted from the XRD signal.
Analogous luminescent behaviors are obtained by photoluminescence acquired at room temperature, showing similar emissions as those already described for CL. PL spectra have been acquired in a confocal microscope, which allows to analyze isolated structures detached from the pellets with micrometric resolution. Figure 8 shows normalized PL spectra acquired, at 300 K using a UV laser ( = 325 nm) as excitation source, in one tree-like and one brush-like structure from sample S-800.
Li + has several allowed intra-ionic emissions, as reported by different authors. Specifically, it presents an intraionic emission between the 2 S-2 P0 levels centered at 713.5 nm (1.74 eV). [35] In other systems, such as Li doped TiO2, this emission has been observed slightly shifted at 724 nm. [36] In Li doped Ga2O3, I. López et al. [37] observed the presence of a narrow and intense emission around 717 nm (1.73 eV), which was associated with the intra-ionic emission of Li + ions between the 2 S-2 P0 levels again. Even when this energy could be shifted from one matrix to another, [38] in our case a possible relation of the weak emission at 1.5 eV observed in the PL spectrum in Figure 8 with the intra-ionic transitions of Li + ion could be discarded.
It should be also noticed that CL and PL measurements confirm high luminescence from the samples under study, even at room temperature.
Among the analyzed samples, S-900 shows a remarkable high luminescence, so that a bright and intense spot can be observed during the luminescence measurements, as shown in Figure 9a and 9b. This spot presents a white color for which the broad luminescence spectra, ranging from the near infrared to the ultraviolet, is responsible. The two traditional approaches to obtain white light from a light emitting diode (LED) are combining LEDs from the three primary colors (red, green and blue) [42] or the use of a phosphor material, which in combination with a blue or UV-LED usually of GaN or InGaN, broaden the final emission [43,44].