## 3 Results and Discussion  
Let us first describe the morphology of the structures grown from the sample without lithium used as reference. A very dense shell of structures grew onto the pellet that simultaneously acted as a source and substrate (Figure 2a). As shown in Figure 2b, the grown structures were mainly in the form of wires or rods with a hexagonal cross-section with diameters of a few hundred nanometers and lengths of several microns. In some cases, bundles of comb-like structures (Figure 2c) also appeared. Due to the flux distribution at the edges of the pellet, the structures were more regularly arranged, demonstrating columnar growth, with columns larger than the rods observed in the central part of the sample (Figure 2d). Beyond the differences in morphology, the influence of lithium doping was clear even for the lowest Li content. When Li was present, the growth occurred mainly on the alumina boat, and only for the lowest Li content did a few structures grow onto the pellet. On the other hand, the density of grown structures was much lower in samples Eu5_Li1 and Eu5_Li10, and only for the intermediate Li content (Eu5_Li5) was a density of structures comparable to that of the samples without Li (Eu5) observed. As mentioned, in the Eu5-Li1 sample, a few structures in the form of hexagonal rods grew onto the pellet. They were smaller than those observed in the sample Eu5, with diameters between 2 and 6 \(\upmu\)m and 10-20 \(\upmu\)m in length, and did not show any preferred  
Figure 2: SEM images of the different structures grown on the samples with no Li included: (**a**) Compacted shell of structures, (**b**) hexagonal wires and rods, (**c**) bundles of comb-like structures and (**d**) columnar growth observed in the rims of the pellets. orientation (Figure 3a). The structures grown onto the alumina boat or at the rims of the pellets were mainly ribbons or sword-like, around 15 \(\upmu\)m wide, several hundred microns long and a few microns thick (Figure 3b,c). The Eu5_Li5 sample showed the highest density of structures among all the co-doped samples. The structures, mainly in the form of ribbons, grew onto the alumina boat. As shown in Figure 4, they have very large aspect ratios, with lateral dimensions in the range of several tens of microns and several millimetres long. With a closer look, let us propose a growth model in which the comb-like structures constitute the first step. The formation of comb-like structures is related to different activities shown by the polar faces of ZnO. The preferential growth would take place on the Zn terminated face, and then initially, the prongs would develop from it (Figure 4b). Finally, the space between prongs would progressively fill, forming the ribbons (Figure 4c). Finally, in the Eu5_Li10 samples, the growth was much scarcer and occurred on the alumina boat. The structures were less regular, ribbons, swords and wires, sometimes interlinked, and with dimensions of a few tens of microns (width) and hundreds of microns in length, as shown in Figure 5a. In some cases, small plates, very well oriented with respect to the crystallographic directions, are observed in Figure 5b. As drawn in part c of this figure, this suggests a growth modelsuch as that described in Figure 1 (middle). Figure 3: SEM images of the structures obtained on the sample with 1 wt% Li. (**a**) Hexagonal rods grown on the pellet, (**b,c**) ribbons and sword-like structures grown both on the rims of the pellet and on the alumina boat. To further investigate the crystalline orientations of the structures, Electron Backscattered Diffraction (EBSD) was performed in one of the combs, which showed secondary growth in one of its lateral faces (Figure 6). Part a of the figure shows the SEM image of the selected structure, which was isolated and deposited on a silicon wafer; the orientation of the detector is indicated by the X, Y and Z axes drawn in the lower right corner. Figure 6b show the orientation map on the Z-axis.