## 1 Introduction  
Although ZnO has been extensively studied for many years, recently, it has become more relevant in the frame of different applications of high technological interest. As it is well known, ZnO is a wide bandgap II-VI semiconductor (3.37 eV) with a native concentration of donor defects that makes it conductive; hence it is one of the most studied Transparent Conductive Oxides (TCO). In addition to the wide bandgap, the large exciton binding energy (60 meV), the chemical stability and some properties associated with the wurtzite structure (lack of a symmetrical center) make this material a perfect candidate for a variety of applications in the field of optoelectronics, sensing, photocatalysis or piezoelectricity [1; 2; 3; 4]. In the nano- and microscale, an enhancement of the electroconductivity or a high specific area must be added, widening the scope for new applications [5; 6; 7; 8]. On the other hand, when we move to the micro- and nanoscale, the morphology of the structures plays a key role, and many of the properties of the material, for instance, luminescent properties, are strongly dependent on morphology [8; 9; 10]. Behind the morphology-dependent properties are the crystal structure and the surface and surface defect structure derived from it. The most stable crystal structure for ZnO is hexagonal wurtzite that may be described as alternating Zn\({}^{2+}\) and O\({}^{2-}\) planes stacked along the \(z\)-axisin tetrahedral coordination. The highest surface energy corresponds to the (0001) basal planes; then, the most common growth morphology is that of rods along the c-axis, which minimizes the surface of the basal planes. However, different morphologies can be obtained by several synthesis procedures, giving rise to different surface configurations and hence different properties. As an example, in Figure 1, a scheme of the faces that may appear in the most common morphologies is drawn [4, 11, 12, 13, 14]. The role of dopants in improving the properties of semiconductor materials is well known; in fact, it is the primary route to modify and control them, either by the direct supply of carriers or by the modification of the defect structure necessary to accommodate the impurities. For instance, in our focus material, ZnO, the conductivity increases up to 10 orders of magnitude due to the presence of impurities and native defects. An important drawback, however, is the difficulty of obtaining p-type ZnO due to self-compensation mechanisms [15, 16, 17, 18, 19]. Among the different dopants used, rare-earth (RE) elements occupy a prominent position, and the number of studies of RE-doped ZnO-based systems with good luminescent properties has increased considerably [8, 20, 21, 22, 23]. Although one major interest is in the telecom field, in particular for Er doping, the technological interest in RE doping goes further beyond this field [21]. Rare-earth or lanthanides elements are those found between La and Yb in the periodic table. They are characterized by an electronic structure of the form [Xe]4H6s2, with a partially filled 4f shell shielded by the 5s25p6 electrons. In lanthanide ions, three basic types of optical transitions may occur: 4f-4f; 5d-4f and charge transfer between the host and the dopant. The 4f-5d transitions are allowed and have short lifetimes, in the order of nanoseconds; 5d orbitals are strongly affected by the crystal field and polarizability of the host material, which allows easy tuning in the visible region by changing some characteristics of the host [22]. The f orbitals are shielded by the 5s and 5p and they are very insensitive to the environment, i.e., the crystal field, providing very sharp transitions from UV to infrared. Contrary to the 4f-5d, the 4f-4f transitions are parity forbidden, hence with much longer lifetimes, in the order of microseconds; however, in proper hosts, the influence of the ligand field or non-centrosymmetric interactions may lead to a relaxation of the parity rule, giving rise to sharp emission lines [24, 25].