Materials Science and Engineering B 299 (2024) 116941 Available online 11 October 2023 0921-5107/© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). Morphological, structural and luminescent characterization of Nd-doped ZnO nano- and microstructures grown by vapor-solid method P. Jara a, R. Fernández-Jiménez b, A. Ferreiro a, A. Urbieta b,*, M.E. Rabanal a, P. Fernández b a University Carlos III of Madrid and IAAB, Dept. of Materials Science and Engineering and Chemical Engineering, Avda. Universidad 30, 28911 Leganés, Madrid, Spain b Depto. de Física de Materiales, Facultad de Físicas, Univ. Complutense, 28040 Madrid, Spain A R T I C L E I N F O Keywords: Zinc Oxide Neodymium Synthesis Vapor-solid Energy Transitions A B S T R A C T This work focuses on synthesizing neodymium-doped ZnO nano- and microstructures using the vapor–solid method. The efficiency of Nd incorporation into the ZnO lattice has been analyzed. The characterization was carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), cathodoluminescence (CL), μ-photoluminescence (μ-PL) and Raman Spectroscopy (RS). The formation of the solid solution corresponding to a hexagonal wurtzite phase of ZnO was observed for the samples doped with 1% and 5% wt. Additional phases related to neodymium oxides were also detected in the samples doped with 10% wt. The incorporation of Nd in the Zn lattice has been monitored by means of EDS, CL and XRD. The main changes observed are the increase of the lattice parameter as the Nd content increases, the changes in the morphology of micro- and nanostructures, and the influence on the luminescent properties with higher intensity of Nd intraionic lines. 1. Introduction ZnO (group II-VI compound) has attracted much attention due to its broad and direct bandgap (~3.3 eV) and n-type conductivity, associated with its lack of stoichiometry that results mainly in the presence of ox ygen vacancies. It also has a high exciton binding energy (60 meV) at room temperature, ease of synthesis, biocompatibility, excellent elec tronic properties, high electrochemical stability, acoustic characteris tics, high infrared reflectivity and transparency in the visible region and antibacterial properties [1–5]. Zinc oxide has been extensively studied for decades; however, in recent years, it has been the object of new in terest as it is one of the most versatile and technologically essential semiconductor materials, suitable for many different applications as solar cells [6,7], photodetectors [8], sensors [9–13] or photocatalysis [14–19], among others. Numerous investigations have focused on doped ZnO since doping processes open new applications and improve the best-known charac teristics [20–22]. Among the dopants, rare earth (RE) elements have been the object of extensive study due to their unique optical properties and promising applications in many technological fields, such as opto electronic devices, flat panel displays, and biological labels [23–26]. In most of these applications, efficient energy transfer from the host to RE3+ is required, and the use of a wide band gap semiconductor as a host material, such as ZnO, favors the sensitization of RE3+ ions to produce an efficient luminescence emission [24,25]. The photocatalytic activity of ZnO could be improved, by doping with rare earth ions, limiting the recombination of electron-hole pairs, and improving light absorption [18,19,27]. However, not only the doping processes are important to tailor the materials, but many properties can also be modified playing with the particle size, morphology and crystallinity of the materials. Recently, the study of ZnO nanostructures has become one of the pillars of nanoscience and nanotechnology. Due to the inherent anisotropic crystallographic growth and its high sensitivity to growth conditions, ZnO can be grown in many different ways at the nanoscale [28]. ZnO nano- and microstructures are attractive, as morphology, size, and chemical and physical properties are suitable for assembling nanoscale devices. ZnO nanostructures with morphologies such as: nanowires, nanobelts, nanoneedles, whiskers, and nanotubes, nanorings, nanobeads [4,29] have unique properties and offer the possibility of being used as building blocks for electronic and photonic devices, including light- emitting diodes (LEDs), flat and plasma displays, fluorescent lamps, electrodes in dye-sensitized solar cells and potentially for hydrogen * Corresponding author. E-mail addresses: pjaravargas@gmail.com (P. Jara), rubefe03@ucm.es (R. Fernández-Jiménez), adferrei@ing.uc3m.es (A. Ferreiro), anaur@ucm.es (A. Urbieta), eugenia@ing.uc3m.es (M.E. Rabanal), arana@ucm.es (P. Fernández). Contents lists available at ScienceDirect Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb https://doi.org/10.1016/j.mseb.2023.116941 Received 17 July 2023; Received in revised form 6 October 2023; Accepted 7 October 2023 mailto:pjaravargas@gmail.com mailto:rubefe03@ucm.es mailto:adferrei@ing.uc3m.es mailto:anaur@ucm.es mailto:eugenia@ing.uc3m.es mailto:arana@ucm.es www.sciencedirect.com/science/journal/09215107 https://www.elsevier.com/locate/mseb https://doi.org/10.1016/j.mseb.2023.116941 https://doi.org/10.1016/j.mseb.2023.116941 https://doi.org/10.1016/j.mseb.2023.116941 http://crossmark.crossref.org/dialog/?doi=10.1016/j.mseb.2023.116941&domain=pdf http://creativecommons.org/licenses/by-nc-nd/4.0/ http://creativecommons.org/licenses/by-nc-nd/4.0/ Materials Science & Engineering B 299 (2024) 116941 2 storage [1,4,14,20,30]. Since the properties of ZnO structures depend on the size and morphology of the nano- and microparticles, it is essential to develop easy methods to control the nucleation and growth steps of nano structures. In the literature, various mechanisms have been used to synthesize semiconductor nanostructures composed of ZnO, using techniques such as vapor–liquid–solid (VLS) and vapor–solid (VS). 1D semiconductor nanostructures have also been obtained by laser ablation catalytic growth, oxide-assisted growth, template-induced growth, so lution-liquid–solid growth in organic solvents, and metal–organic chemical vapor deposition (MOCVD) [29], physicochemical methods such as sol–gel, chemical vapor deposition (CVD), hydrothermal, elec trodeposition, precipitation [18,31,32]. Rare earth elements, in particular those from lanthanum family are characterized by an electronic structure [Xe]4fn6s2, with 4f shell partially filled. The electrons on shells above it (5s25p6) shield strongly the electrons in the 4f shell, giving rise to very well defined transitions, barely influenced by the host. The main transition processes take place between 4f and 4f and 5d-4f shells or as a result of charge transfer be tween the host and the dopant. The 4f-5d transitions are allowed and have short lifetimes (in the order of ns), while the 4f-4f transitions are forbidden by the quantum selection rules, which give much longer lifetimes (in the order of μs). This fact may be overcome by choosing the proper host. For instance, in a non-centrosymmetric material as ZnO, become important different interactions that may lead to a relaxation of the parity rule. In these cases, 4f-4f emission lines that otherwise would not appear [33,34] are visible. In this work, we will focus on the growth of Nd doped ZnO structures using the vapor–solid method and study the influence of the concentration of neodymium in the precursor on the energy levels and luminescence transitions of the obtained nano- and microstructures. Although the RE doping in ZnO has been extensively studied, the use of Nd is not so frequent in the literature. On the other hand, the efficiency of the Nd intraionic lines in the visible region constitute a good chance to improve the absorption of visible light of ZnO, of great importance in fields as photocatalysis [35,36]. 2. Experimental method The samples have been synthesized from a powder mixture of zinc sulphide (ZnS 99.99%, No. CAS 1314-–98-3 from Acros Organics (Aldrich Chemical Company)) and neodymium oxide (Nd2O3 99,99% No.CAS 1313-97-9) from Aldrich Chemical Company. An analytical balance (Adventurer Ohaus) has been used to perform an accurate measure of the different weight proportions of the precursors. Homog enization of the mixtures has been carried out by a milling process in a high-energy ball mill (Retsch S100) with 20 mm agate balls during 5 h at 180 rpm. The nominal concentration of dopant has been 1, 5 and 10 wt %. Once milled, the powders were compacted (Millennium 30 T Mega KP-30ª) to form a pellet (2 mm thickness, 7 mm diameter), under a pressure of 2.5 T for 1 min Nano- and microstructures of Nd-doped ZnO have been synthesized by Vapor -Solid method in a horizontal tubular furnace (CHESA) under a continuous N2 flux. During the thermal treatment ZnS is oxidized to ZnO and the Nd is incorporated into the Zn lattice. According to previous experiments [37], the temperature for thermal treatments has been set up at 900◦, and the treatment duration was 10 h. During the whole treatment (including heating and cooling ramps), the nitrogen flux was kept at 1.5 l/min. Scanning Electron Microscopy (SEM) has been carried out through a FEI Inspect S microscope operating at 15 kV. X-ray diffraction experi ments have been performed in a Philips X’Pert, (40 kV, 40 mA) at the Cu Kα line (1,540598 Å). Measurement range (2θ) was 20–80◦ with a scan step size of 0.02◦/s. The composition of the samples was investigated by X-Ray microanalysis (EDX) in an SEM Leica 440 Stereoscan (operated at 20 kV, 5nA) equipped with a Bruker AXS system and an XFlash Detector 4010. The software package “Espirit” has been used for data analysis. Cathodoluminescence (CL) measurements have been done in a Hitachi S2500 SEM (operated at 20 kV). All the measurements were carried out at room temperature in the range 200–960 nm. Spectra have been recorded by a CCD camera (Hamamatsu PMA-11) coupled to the mi croscope. µ-Photoluminescence (PL) and Raman spectroscopy (RS) measurements have been performed in a confocal microscope (Horiba Jobin Yvon LabRAM HR800) equipped with a He-Cd laser (λ = 325 nm) for PL measurements and a He-Ne laser (λ = 632.8 nm) for RS experiments. 3. Results and discussion Both, pressed samples and VS-grown structures have been carefully characterized by X-Ray diffraction. Diffraction patterns obtained from the precursors fit well with the data in the corresponding standard files. In the case of ZnS, the diffraction peaks correspond to those of the sphalerite structure (JCPDS 01-077-2100). The diffraction pattern for Nd2O3 is consistent with the reference card of hexagonal Nd(OH)3 (JCPDS 01–70-215), reflecting the hygroscopic nature of Nd2O3. As ex pected, the mixtures of the two precursors do not show additional phases. After the thermal treatments, nano- and microstructures of different morphologies are obtained. The diffraction patterns show the peak characteristics of ZnO with a wurtzite structure (JCPDS No. 00-75- 1526). All the peaks are very well-defined and intense, indicating that the structures have a high crystalline quality (Fig. 1a). From the zoom of Fig. 1b, a shift toward lower diffraction angles is appreciated. The corresponding interplanar distances can be obtained from the positions of the different peaks. The data in Table 1 show an increase in the interplanar distances as the Nd nominal content increases, corre sponding to the shift to lower angles observed. From the data in Table 1, the increase in the interplanar distances as the nominal content in neodymium increases is rather apparent; how ever, we can also calculate the lattice parameters and the unit cell vol ume (Table 2) by means of the following equations 1 d2 hkl = 4 3 ( h2 + hk + k2 a2 ) + l2 c2 V = ca2cos30 Where h,k, l are the standard Miller indexes, a and c the lattice pa rameters, dhkl the interplanar distance for the hkl plane and V is the cell volume. This behavior could be attributed to the incorporation of neodymium ions into the Zn host lattice since the ionic radius of Nd+3 is larger than that of Zn+2 (rNd+3 = 0.98Å; rZn+2 = 0.74Å, according to ref. [38]). The data in Table 2 show that the main lattice distortion is taking place in the basal planes (a axis) while along the c axis the lattice parameter only shows a very slight increase (0.5%) for the heaviest doped sample. On the other hand, no additional phases are observed from diffrac tion patterns for the samples ZN1 and ZN5. In contrast, in the sample ZN10, the diffraction pattern (Fig. 1c) shows diffraction peaks corre sponding to different neodymium oxide phases, in particular Nd2O3 (JCPDS 01-74-1147), Nd2O3 (JCPDS 00-24-0779) and Nd2O2S (JCPDS 00-27-0321). This could indicate that in the samples ZN1 and ZN5, Nd has been successfully incorporated into the Zn lattice [39], while, in the sample ZN10, with the highest Nd nominal content, the solubility limit has been reached, and consequently, the precipitation of secondary oxide phases has occurred [26]. Diffraction patterns of the of ZN1, ZN5 and ZN10 have been analyzed to determine the average crystallite size (ACS), however the differences observed in the broadening of the main three peaks (Fig. 1b) in the different samples are very small. The low magnitude of these variations makes difficult to establish a sound tendency of the crystallite size, since they are within the error interval (Fig. 1d). In fact, previous research report on similar variations of the FWHM of the peaks which is P. Jara et al. Materials Science & Engineering B 299 (2024) 116941 3 attributed to differences in the average particle size distribution [40]. It is important to emphasize that in other investigations, as the concentration of the dopant element (lanthanides) increases, the average crystallite size (ACS) decreases due to the stress induced in the host lattice by the doping process [35,41]. N.K Divya et al. observed a decrease in the ACS of Nd-doped ZnO nanostructures and also reported the change in 2θ due to the lattice strain [42]. Although the values of the ACS of ZnO: Nd were not conclusive enough, according to the previous literature, a decrease in ACS with increasing Nd content would be expected. Raman spectroscopy measurements have also been done to assess the structural quality of the structures. Fig. 2 shows the Raman spectra at room temperature (RT) of the ZnO: Nd samples. The three main active Raman modes, E1, E2, and A1, and the corresponding longitudinal (LO) and transverse (TO) components are distinguished [43]. As labelled in Fig. 2, the peaks at 96 cm− 1 and 434 cm− 1 correspond to the Elow 2 and Ehigh 2 modes, the high intensity and sharpness of these bands indicate the good crystallinity of the wurtzite phase of the samples. All prominent Raman bands usually described for undoped ZnO are also observed in Nd-doped ZnO samples, but as the Nd content increases, the Raman peaks become broader. These observations reveal that the local sym metry in the ZnO: Nd samples is slightly different from that of the Fig. 1. a) Diffractogram of the structures grown from samples ZN1, ZN5, ZN10, compared to ZnO (JCPDS 75-1526); b) zoom in the three first peaks; c) Diffraction pattern from the structures grown on sample ZN10 marking also the peaks related to neodymium oxide phases (JCPDS 01-74-1147, JCPDS 00-24-0779, JCPDS 00-27- 0321; d) Estimation of the mean crystallite size. Table 1 Interplanar distances (in Å) estimated from the position of the peaks for the different doped samples. Sample Miller indexes (hkl) (100) (002) (101) (102) (110) (103) (200) (112) (201) ZN1 2.8109 2.5999 2.4730 1.9096 1.6237 1.4759 1.4057 1.3777 1.3570 ZN5 2.81815 2.6038 2.4779 1.9117 1.6259 1.4768 1,4068 1.3787 1.3591 ZN10 2,8270 2.6131 2.4852 1.9148 1.6281 1.4793 1.4091 1.3805 1.3603 Table 2 Lattice parameters and unit cell volume obtained from the data in Table 1. Sample Lattice parameters Unit cell Vol. (Å3) c/a a (Å) c (Å) ZN1 3.246 ± 0.001 5.204 ± 0.002 47.485 ± 0.055 1.603 ± 0.001 ZN5 3.252 ± 0.001 5.209 ± 0.001 47.706 ± 0.044 1.602 ± 0.001 ZN10 3.255 ± 0.001 5.215 ± 0.002 47.849 ± 0.055 1.602 ± 0.001 ZnO (JCPDS75- 1526) 3.220 5.200 46.691 1.615 P. Jara et al. Materials Science & Engineering B 299 (2024) 116941 4 undoped ZnO, the higher disorder degree associated with the increasing amount of Nd incorporated is responsible for the broadening of the Raman peaks. As shown in Fig. 3, the pellet is fully covered with nanostructures. This is true for all the Nd contents, although the distribution of the morphologies obtained varies among the samples with different nominal dopant concentrations, as can be observed in Fig. 4a to f. In the samples from the ZN1 series, two main morphologies are observed, as shown in Fig. 4a and b. In the center of the pellet, the structures are rod-shaped forming bundles that apparently grow from a single nucleation site; we will name them hedgehog structures (Fig. 4a). The single rods have a diameter of about 0.5 μm and lengths of around 5 μm. As we approach the rims of the pellet, the structures appear in the form of needles, much longer, tens of microns in length, and thinner, around 100 nm in diameter (Fig. 4b). The samples from the ZN5 series also show a high density of struc tures. Similar to the samples with the lowest Nd nominal content, the structures observed at the center of the pellet are hedgehog-like, although the length of the individual rods is higher than in ZN1, around 15–20 μm (Fig. 4c). As mentioned for the ZN1 samples, as we approach the rim of the pellet the morphology changes to long tapered needles, ending with an inverted hexagonal pyramid (Fig. 4d). In a closer look at these structures, two different morphologies are appreci ated at the end of the structures, that could correspond to different growth stages. At a first stage, a star-like with hexagonal symmetry would form, the space between the blades would progressively fill with redeposited material, and finally the inverted pyramid would appear [44]. This model is sketched in Fig. 4g. Examples of tapered structures have been observed in ZnO with different dopants [24] [and references there in]. The samples with the highest dopant content show similar mor phologies to those already described; nevertheless, the average size of the structures is considerably higher, giving rise to highly compacted bundles of hexagonal rods (Fig. 4e-f), with a final diameter of around 10 μm. In some cases, the rods grow so close to each other that they coa lesce, as shown in Fig. 4e-f. As in series ZN1 and ZN5, as we approach the pellet’s border, the structures’ morphology changes to individual long hexagonal rods (Fig. 4f, bottom) similar to those found in previous studies [20]. The evolution of the morphologies obtained as the dopant content increases is sketched in the diagram from Fig. 4g. The same phenomenon discussed above has been observed in pre vious studies, as in the case of Ga or Eu doped systems [45–47]. As well, in the case of Nd, as the nominal dopant concentration increases, fewer but thicker nanostructures are obtained, with a crystalline habit of hexagonal prisms. It is well known that these prisms’ lateral surfaces correspond to the structure’s nonpolar lattice planes, while the bases of the prims correspond to the polar (0001) faces. This kind of rod-shaped morphology directly results from the significant difference between the surface energy of the nonpolar and polar reticular planes [34][and ref erences therein]. In previous works, it has been found that the different positions of the source / substrate with respect to the gas inlet give rise to important changes in morphology. For localized regions closer to the gas inlet, structures with high aspect ratios can be observed, while in areas opposite to the flow inlet, longer but thicker structures are obtained (aspect ratios are much lower) [48,49]. On the other hand, it has been previously reported the influence of dopants on the preferential growth direction. This influence is mainly due to two factors: changes in the slip systems due to stress associated to the doping process, and changes of surface energy associated to the uneven incorporation of dopant on the different faces, this is the case for instance with Eu as a result of the lattice mismatches introduced by doping. This causes the stability of the surface to decrease and leads to changes in the direction of structural growth [50] and could also be related to changes in the preferential growth directions associated with the transport flow. In previous in vestigations of the vapor–solid growth method, the presence of a variety of morphologies has been reported. In most cases, the morphology de pends on the parameters (flow) and the amount of dopant [36,37,39]. The morphologies of spheres composed of cones with hexagonal tips, observed in ZN10, could also be related to the nucleation mechanism in the growth process. As mentioned earlier in the X-ray analysis section, the ionic radius of RE3+ is greater than Zn2+, which leads to a limited nucleation rate and, therefore a lower growth rate of ZnO [31]. It has been observed in previous studies that the decreases in the size of RE- doped ZnO structures were induced by the formation of RE-O-Zn, pre venting crystal growth. It has been observed that the agglomeration of smaller structures can give rise to larger structures. This phenomenon could contribute to the behavior observed in our nanorods, they could be agglomerating and as a result, structures with greater dimensions are formed [29,31]. The catalyst-free growth mechanism of ZnO nanorods has been thoroughly investigated, but it has been previously observed that the main reason for anisotropic growth is the anisotropic surface energy in ZnO, which is dependent on the crystalline faces of wurtzite. Further more, high-velocity laminar gas flow under certain growth conditions can induce turbulent flow between nanostructures, resulting in Fig. 2. Raman spectra corresponding to the three series of samples. Fig. 3. Overview of the structures grown on the pellet. The image corresponds to a sample from series ZN5. P. Jara et al. Materials Science & Engineering B 299 (2024) 116941 5 adsorption of fresh reactive gases only at the nanorod tips. Since there are more surface exposed at the nanorod tips, the growth rate of nanorod is higher at the nanorod tips than at the side walls [51]. The incorporation of neodymium ions into the wurtzite structure has been assessed by X-ray microanalysis (EDX). Fig. 5 shows the compari son among the spectra obtained from the different samples. The result of a semi-quantitative analysis is shown in Table 3. From the data in this table, it is apparent the increase in oxygen content as the dopant proportion in the precursor mixture is increased. This behavior was expected since the precursor for ZnO is ZnS, and hence the main oxygen source is the neodymium oxide. Photoluminescence spectra also reveal this oxygen deficiency. Fig. 6 compares the photoluminescence spectra obtained by excitation with He-Cd laser (325 nm). The spectra consist of two main bands: near band edge (NBE) in the UV range and the green band related to different kind of defects, including oxygen vacancies. The relative intensity of the green band with respect to the NBE decreases as the Nd content in creases, and consequently an apparent shift towards higher energy is also observed. The luminescence properties of the samples have been more extensively studied by means of cathodoluminescence. Fig. 7(a) shows the CL spectra of the grown nanostructures of ZnO doped with different Nd content. All the CL spectra have shown a dominant deep-level emission (DLE) band centered around 508 nm (~2.4 eV), known as the green band. A lower intensity peak has also been observed in the UV range and this emission has been observed to vary slightly between samples, from ~390 nm (3.18 eV) to 382 nm (3.24 eV). The spectra of the nanostructures also show small peaks in the range of 529–950 nm that are better defined as the amount of dopant (Nd3+) in the samples increases. The emission associated with the green band has been attributed to photogenerated holes/electrons belonging to surface defects, which generate radiative recombination [28,31]. In some investigations, they have observed that those photogenerated holes / electrons which lead to the wide visible emission band around 500 nm, could correspond to the single ionized VO in ZnO and can result from the recombination of a photogenerated hole with this defect. They have also observed that the stronger the emission intensity of the visible emission band, the higher the content of ionized VO is [28]. However, other authors have identified additional factors that could influence emissions peaks in the green Fig. 4. SEM images from the morphologies obtained: a) ZN1 structures from the center of the pellet and b) from the rim; c) ZN2 center of the pellet and d) rim; e) ZN3 center of the pellet and f) rim; g) sketch of the evolution of the needle-like structures to hexagonal- shaped rods. P. Jara et al. Materials Science & Engineering B 299 (2024) 116941 6 zone. They have observed that they could also be related to interstitial O / Zn states, individual oxygen and zinc vacancies (VO and VZn) [52,53] or doubly ionized vacancies (Vo++ / Vo+) [26]. It has also been revealed that these emissions may be associated with impurities or elemental contamination [31]. Liu et al [54] have observed similar bands in the visible range on ZnO / Zn (S, O) nanostructures, extending from 400 to 650 nm and concluded that it is due to the presence of numerous energy levels associated with defects, such as those already mentioned. In the nanostructures of the sample ZN1, small shoulders are appreciated at approximately 530, 549 and 607 nm. These features appear as defined components in the samples from series ZN5 and ZN10 samples, and are present in all the morphologies studied, then it could be related to the presence of Nd3+ ions. This effect has been observed in other investigations when doping with REs and has been linked to transitions between discrete states of the RE ions [31]. Fig. 7(b) shows the different peaks related to the Nd-related transi tions. Although all samples show these emissions, in the case of ZN1 and ZN5 they are much weaker; hence we have chosen samples from series ZN10 to identify the different bands. Fig. 7b shows a spectrum with the different peaks labeled according to the diagram level in Fig. 7c. The emissions observed in the analyzed nanostructures in the ZN10 sample at 549 nm (1.31 eV), 746 nm (1.67 eV), 638–624 nm (1.95–1.99 eV), 608–578 nm (2.04–2.15 eV), 557–531 nm (2.23–2.34 eV), 5011–502 nm (2.43–2.47 eV), 476 nm (2.61 eV) and 405–392 nm (3.06–3.17 eV) are linked with the transitions of 4 F3/2, 4 F7/2+ 4 S3/2, 2 H11/2, 4 G5/2, 4 G7/2, 4 G9/2 + 2 K13/2, 2 G9/2, 4 G11/2 + 2 K15/2, respectively, up to the ground state (ground state)4 I9/2 [25,55–57]. On the other hand, regarding the near band edge emission, in all series of samples, it seems to depend on the morphology of the struc tures. Two main factors would be behind these differences: surface states and changes in the apparent band gap due to either band renormaliza tion (BN) or Burstein-Moss (BM) effect. These two effects may be simultaneously present, then either an increase (if BM is dominant) or a decrease (if the dominant effect is BN) in the apparent band gap would be observed. In fact, in previous works a tendency change, i.e, a change in the dominant mechanism, has been observed as a function of dopants content. In the samples studied in this work, this change in the dominant mechanism is also observed. The NBE band (Fig. 7a) shifts first to higher energies (from samples ZN1 to ZN5) and then to lower energies (from ZN5 to ZN10). This behavior has been widely reported in semi conductors [7]. In this work, we have also studied the guiding of light in the struc tures of the samples. For this purpose, a µ-photoluminescence study has been carried out in individual structures of each sample. As the same results are observed for the three series of samples, we are going to give Fig. 5. Comparison of typical EDS spectra obtained from nanostructures with different dopant content. Table 3 Data from EDS semi-quantitative analysis (Atomic %) for the samples with different nominal dopant content. % atomic percentage Quantitative Analysis Zn O Nd 10% Nd 48.39 ± 2.23 49.65 ± 2.29 0.55 ± 0.08 5% Nd 57.65 ± 2.48 40,90 ± 1.80 0.49 ± 0.07 1% Nd 63.27 ± 2.58 34,14 ± 1.55 0.38 ± 0,07 Fig. 6. Comparison of the photoluminescence spectra obtained from the different series of samples. P. Jara et al. Materials Science & Engineering B 299 (2024) 116941 7 an example, in this case for a structure from series ZN1. As can be seen in Fig. 8a, photoluminescence is excited at the impact point of the laser and is guided along the structure axis and exits at the edges. In part b) of the figure, the two spectra recorded at both excitation and exit point are shown. They both have a peak in the UV region; however, the short wavelength component is not visible in the spectrum from the exit point (red line). Also, small modulations appear superimposed to the main luminescence bands, this allows us to appreciate that the structures can act as resonant cavities. This effect is due to morphology, that is, the structures act as a resonant cavity if interference occurs. Due to the high refractive index of ZnO (n □ 2.4 in the spectral region under study), a total internal reflection is induced within the faces creating a Fig. 7. a) CL spectra obtained from the different morphologies in series ZN1 (black line) series ZN5 (red line) and series ZN10 (blue line); b) Intraionic Transitions observed in nanostructures of ZnO:Nd samples. The spectrum corresponds to a sample of series ZN10. c) Scheme of intraionic transitions associated to Nd+3 ion. Fig. 8. a) µ-PL image of an hexagonal rod grown in a sample from ZN1 series, b) PL spectra recorded at both excitation (black line) and exit point (red line). P. Jara et al. Materials Science & Engineering B 299 (2024) 116941 8 confinement of the light. The existence of modulations has been observed before in different nanostructures and microstructures of non- doped ZnO [58] and also in microrods of ZnO doped with several ele ments like Tb [39] or Li [59]. In all these cases, the modulations have been attributed to Whispering Gallery (WG), Quasi-Whispering Gallery (Q-WGM) or Fabry-Perot (FP) according to the geometry of the resonant cavity. In our case, the geometry of the cavity is hexagonal; so WG modes are expected. 4. Conclusions Nd-doped ZnO nano- and micro-structures have been synthesized by the vapor–solid technique. The samples present different morphologies depending on dopant content and growth site. The synthesized struc tures present a wurtzite-type structure with hexagonal symmetry and high crystallinity. A lattice expansion associated with the substitutional incorporation of Nd ions in the Zn lattice positions is observed. In the ZN10 samples, secondary phases (Nd2O3, Nd2O2S) are identified in addition to the wurtzite phase. A trend of decreasing ACS with increasing nominal Nd ion content has been observed. Morphological changes are observed with increasing nominal Nd concentration: hedgehog morphologies, nanowires, tapered nanowires with hexagonal tips, hexagonal prisms, and nanocones with irregular and hexagonal tips. By semi-quantitative analysis, the incorporation of Nd into the ZnO lattice and it increase with the nominal composition of the initial mixtures have been assessed. Both PL and CL spectra have been performed. All the CL spectra of the nanostructures show a deep level emission band (~2.4 eV) known as the green band and a lower intensity peak in the UV range identified as the band edge of ZnO and the emission. The spectra also show several emissions corresponding to intraionic transitions of Nd, which are more intense and better resolved for structures grown in the samples with higher Nd nominal content. The behavior of the microstructures as waveguides and optical resonators have been also established. CRediT authorship contribution statement P. Jara: Validation, Formal analysis, Investigation, Visualization, Writing – original draft. R. Fernández-Jiménez: Validation, Formal analysis, Investigation, Visualization, Writing – original draft. A. Fer reiro: Validation, Formal analysis, Investigation, Visualization. A. Urbieta: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing – review & editing, Visualization, Supervision. M. E. Rabanal: Conceptualization, Methodology, Validation, Formal analysis, Resources, Writing – review & editing, Visualization, Super vision, Project administration, Funding acquisition. P. Fernández: Conceptualization, Methodology, Validation, Formal analysis, Re sources, Writing – original draft, Writing – review & editing, Visuali zation, Supervision, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgements This work has been financially supported by Comunidad de Madrid, Spain (S2018/NMT-4411) and Ministry of Economia, Industría y Com petividad (MAT2016-80875-C3-3-R). The authors are also grateful to the Complutense University of Madrid and Banco Santander for support via the project UCM-Santander 2019 (PR87/19-22613). References [1] A. Kolodziejczak-Radzimska, T. Jesionowski, Zinc oxide- from synthesis to application: a review, Materials 7 (2014) 2833–2881. [2] Ü. Özgür, YaI. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doǧan, V. Avrutin, S. J. Cho, H. Morkoç, A comprehensive review of ZnO materials and devices, Appl. Phys. Rev. 98 (2005), 041301-103. [3] A. Sirekhatim, S. Mahmud, A. Seeni, Review on zinc oxide nanoparticles: antibacterial activity and toxicity mechanism, Nano-micro Letters 7 (2015) 219–242. [4] A.B. Djurǐsić, Y.H. Leung, Optical properties of ZnO nanostructures, Small 2 (2006) 944–961. [5] Z. Fan, J.G. Lu, Zinc oxide nanostructures: synthesis and properties, J. Nanosci. Nanotechnol. 5 (2005) 1561–1573. [6] S. Salam, M. Islam, A. Akram, Sol-gel synthesis of intrinsic and aluminum-doped Zn thin films as as transparent conducting oxides for thin film solar cells, Thin Solid Films 529 (2013) 242–247. [7] A. Saboor, S.M. Shah, H. Hussain, Band gap tuning and applications of ZnO nanorods in hybrid solar cell: Ag-doped versus Nd-doped ZnO nanorods, Mater. Sci. Semicond. Process. 93 (2019) 215–225. [8] W. Zhang, D. Jiang, M. Zhao, Y. Duan, X. Zhou, X. Yang, C.h. Shan, J. Qin, S. Gao, Q. Liang, J. Hou, Piezo-phototronic effect for enhanced sensitivity and response range of ZnO thin film flexible UV photodetectors, J. Appl. Phys. 125 (2019), 024502. [9] Mat. Sci.&Eng. B 176 (2011) 1409–1421. [10] V. Bahti, M. Hojamberdiev, M. Kumar, Enhanced sensing performance of ZnO nanostructures-based gas sensors: a review, Energy Rep. 6 (2020) 46–62. [11] M. Suchea, s. Christoulakis, K. Moschovis, N. Katsarakis and G. Kiriakidis,, ZnO transparent thin films for gas sensor applications, Thin Solid Films 515 (2006) 551–554. [12] P. Bharathi, M. Krishna, V. Mohan, S. Shalini, M. Harish, J. Navaneethan, M. G. Archana, P. Kumar, S. Dhivya, M.S. Ponnusamy, Y. Hayakawa, Growth and influence of Gd doping on ZnO nanostructures for enhanced optical, structural properties and gas sensing applications, Appl. Surf. Sci. 499 (2020), 143857, https://doi.org/10.1016/j.apsusc.2019.143857. [13] D.J. Ramos-Ramos, B. Sotillo, A. Urbieta, P. Fernández, Fabrication and characterization of ZnO:CuO electronic composites for their application in sensing processes, IEEE Sens. J. 21 (2021) 2573–2580. [14] C. Ong, L. Ng, A. Mohammad, A review of ZnO nanoparticles as solar photocatalysts: synthesis, mechanisms and applications, Renew. Sustain. Energy Rev. 81 (2018) 536–551. [15] N. Güy, M. Özacar, The influence of noble meals on photocatalytic activity of ZnO for Congo red degradation, Int. J. Hydrogen Energy 41 (2016) 20100–20112. [16] K. V. Chandekar M. Shkir, A. Khan, B.M. Al-Shehri, M. S. Hamdy, S. AlFaify, M.A. El-Toni, A. Aldalbahi, A. A. Ansari, H. Ghaithan, A facile one-pot flash combustion synthesis of La@ZnO nanoparticles and their characterizations for optoelectronic and photocatalysis applications, J. Photochem. Photobiol. A Chem. 395 (2020) 112465 10.1016/j.jphotochem.2020. [17] J. Akhtar, M.B. Tahir, M. Sagir, H.S. Bamufleh, Improved photocatalytic performance of Gd and Nd co-doped ZnO nanorods for the degradation of methylene blue, Ceram. Int. 46 (2020) 11955–11961, https://doi.org/10.1016/j. ceramint.2020.01.234. [18] M. Rodríguez-Peña, G. Flores-Carrasco, A. Urbieta, M.E. Rabanal, P. Fernández, Growth and characterisation of ZnO micro/nanostructures doped with cerium for photocatalytic degradation applications, J. Alloy. Compd. 820 (2020), 153146, https://doi.org/10.1016/j.jallcom.2019.153146. [19] O. Batza, A. Urbieta, S. Trasobares, J. Piqueras, P. Fernández, M. Addou, J. J. Calvino, A.B. Hungría, In-depth structural and optical analysis os Ce-modified ZnO nanopowders with enhanced photocatalytic activity prepared by microwave- assisted hydrothermal method, Catalysts 10 (2020) 551. [20] J.D. Bryan, d.R. Gamelin,, Doped semiconductor nanocrystals: synthesis, characterization, physical properties and applications, Prog. Inorg. Chem. 54 (2005) 47–126. [21] C.W. Zou, M. Li, H.J. Wang, M.L. Yin, C.S. Liu, L.P. Guo, D.J. Fu, T.W. Kang, Ferroelectricity in Li-implanted ZnO thin films, Nucl. Instrum. Methods Phys. Res. B 267 (2009) 1067–1071. [22] J. Zhang, K. Tse, M. Wong, Y. Zhang, J. Zhu, A brief review of codoping, Front. Phys. 11 (2016) 117405–117421. [23] D. Daksh, Y.K. Agrawal, Rare-Earth doped Zinc oxide nanostructures: a review, Rev. Nanosci. Nanotechnol. 5 (2016) 1–27. [24] S. Shukla, D.K. Sharma, A review on rare earth (Ce and Er)-doped zinc oxide nanostructures, Mater. Today:. Proc. 34 (2021) 793–801, https://doi.org/10.1016/ j.matpr.2020.05.264. [25] Y. Liu, W. Luo, R. Li, X. Chen, Optical properties of Nd 3+ ion-doped ZnO nanocrystals, J. Nanosci. Nanotechnol. 10 (2010) 1871–1876, https://doi.org/ 10.1166/jnn.2010.2140. [26] E.H.H. Hasabeldaim, O.M. Ntwaeaborwa, R.E. Kroon, E. Coetsee, H.C. Swart, Photoluminescence and cathodoluminescence of spin coated ZnO films with different concentration of Eu3+ ions, Vacuum 169 (2019), 108889, https://doi. org/10.1016/j.vacuum.2019.108889. [27] L.T.T. Nguyen, L.T.H. Nguyen, A.T.T. Duong, B.D. Nguyen, N.Q. Hai, V.H. Chu, T. D. Nguyen, L.G. Bach, Preparation, characterization and photocatalytic activity of P. Jara et al. http://refhub.elsevier.com/S0921-5107(23)00683-9/h0005 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0005 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0010 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0010 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0010 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0015 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0015 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0015 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0020 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0020 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0025 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0025 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0030 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0030 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0030 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0035 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0035 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0035 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0040 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0040 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0040 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0040 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0045 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0050 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0050 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0055 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0055 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0055 https://doi.org/10.1016/j.apsusc.2019.143857 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0065 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0065 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0065 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0070 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0070 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0070 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0075 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0075 https://doi.org/10.1016/j.ceramint.2020.01.234 https://doi.org/10.1016/j.ceramint.2020.01.234 https://doi.org/10.1016/j.jallcom.2019.153146 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0095 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0095 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0095 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0095 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0100 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0100 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0100 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0105 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0105 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0105 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0110 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0110 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0115 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0115 https://doi.org/10.1016/j.matpr.2020.05.264 https://doi.org/10.1016/j.matpr.2020.05.264 https://doi.org/10.1166/jnn.2010.2140 https://doi.org/10.1166/jnn.2010.2140 https://doi.org/10.1016/j.vacuum.2019.108889 https://doi.org/10.1016/j.vacuum.2019.108889 Materials Science & Engineering B 299 (2024) 116941 9 La-doped zinc oxide nanoparticles, Materials 12 (2019) 1195, https://doi.org/ 10.3390/ma12081195. [28] Y.J. Lu, Z.F. Shi, C.X. Shan, D.Z. Shen, ZnO Nanostructures and Lasers, in: Nanoscale Semiconductor Lasers, Elsevier, 2019, pp. 75–108. [29] G.C. Yi, Semiconductor Nanostructures for Optoelectronic Devices: Processing, Characterization and Applications, (Springer), 2012. [30] Z.L. Wang, Novel nanostructures of ZnO for nanoscale photonics, optoelectronics, piezoelectricity, and sensing, Appl. Phys. A Mater. Sci. Process. 88 (2007) 7–15, https://doi.org/10.1007/S00339-007-3942-8. [31] V. Kumar, O.M. Ntwaeaborwa, T. Soga, V. Dutta, H.C. Swart, Rare Earth doped Znic Oxide nanophosphor powder: a future material for solid state lighting and solar cells, ACS Photonics 4 (2017) 2613–2637. [32] P.P. Pal, J. Manam, Enhanced luminescence of ZnO:RE3+ (RE=Eu, Tb) nanorods by Li+ doping and calculations of kinetic parameters, J. Lum. 145 (2014) 340–350. [33] S.V. Eliseeva, J.-C.-G. Bünzli, D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, Chem. Soc. Rev. Crit. Rev. 39 (1) (2010) 1–380, https://doi.org/10.1039/ b905604c. [34] A.K. Singh, S.K. Singh, S.B. Rai, Role of Li + ion in the luminescence enhancement of lanthanide ions: favorable modifications in host matrices, RSC Adv. 4 (51) (2014) 27039–27061, https://doi.org/10.1039/C4RA01055H. [35] M. Samadi, M. Zirak, A. Naseri, E. Khorashadizade, A.Z. Moshfegh, Recent progress on doped ZnO nanostructures for visible-ligth photocatalysis, Thin Solid Films 605 (2016) 2–19. [36] A. Ferreiro, G. Flores-Carrasco, M. Quevedo-Lopez, A. Urbieta, P. Fernández, M. E. Rabanal, Effect of lithium codoping on the structural, morphological and photocatalytic properties of Nd-doped ZnO, Ceram. Int. 49 (2023) 33513–33524, https://doi.org/10.1016/j.ceramint.2023.07.248. [37] R. Ariza, M. Dael, B. Sotillo, A. Urbieta, J. Solis, P. Fernández, Vapor-solid growth ZnO:ZrO2 micro and nanocomposites, J. Alloys Compd. 877 (2021), https://doi. org/10.1016/j.jallcom.2021.160219. [38] L. Honglin, L. Yingbo, L. Jinzhu, Y. Ke, Experimental and first-principles studies of structural and optical properties of rare earth (RE = La, Er, Nd) doped ZnO, J. Alloy. Compd. 617 (2014) 102–107, https://doi.org/10.1016/j. jallcom.2014.08.019. [39] G.N. Dar, A. Umar, S.A. Zaidi, A.A. Ibrahim, M. Abaker, S. Baskoutas, M.S. Al- Assiri, Ce-doped ZnO nanorods for the detection of hazardous chemical, Sensors Actuators, B Chem. 173 (2012) (2012) 72–78, https://doi.org/10.1016/j. snb.2012.06.001. [40] P. Kaur, S.K. Rahul, D. Kriti, K.A. Arora, D.P. Singh, Correlation between lattice deformations and optical properties of Ni doped ZnO at dilute concentration, Mater. Today:. Proc. 26 (2020) 3436–3441, https://doi.org/10.1016/j. matpr.2019.12.001. [41] A. Paulson, N.A. Muhammed Sabeer, P.P. Pradyumnan, A synergetic approach of band gap engineering and reduced lattice thermal conductivity for the enhanced thermoelectric property in Dy ion doped ZnO, J. Alloys Compd. 786 (2019) 581–587, https://doi.org/10.1016/j.jallcom.2019.01.336. [42] N.K. Divya, P.P. Pradyumnan, Enhancement of photocatalytic activity in Nd doped ZnO with an increase in dielectric constant, J. Mater. Sci. Mater. Electron. 28 (2017) 2147–2156, https://doi.org/10.1007/s10854-016-5779-4. [43] R. Cuscó, E. Alarcón-Lladó, J. Ibáñez, L. Artús, J. Jiménez, B. Wang, M.J. Callahan, Temperature dependence of Raman scattering in ZnO, Phys. Rev. B 75 (2007), 165202. [44] J. Grym, P. Fernández, J. Piqueras, Growth and spatially resolved luminescence of low dimensional structures in sintered ZnO, Nanotechnology 16 (2005) 931–935. [45] G. Pineda-Hernández, A. Escobedo-Morales, U. Pal, E. Chigo-Anota, Morphology evolution of hydrothermally grown ZnO nanostructures on gallium doping and their defect structures, Mater. Chem. Phys. 135 (2012) 810–817, https://doi.org/ 10.1016/j.matchemphys.2012.05.062. [46] Y.K. Ryu, P. Fernández, J. Piqueras, Growth and characterization of Er-doped ZnO elongated nanostructures, Phys. Status Solidi Appl. Mater. Sci. 208 (2011) 868–873, https://doi.org/10.1002/pssa.201026632. [47] Y. Ortega, P. Fernández, J. Piqueras, Growth and cathodoluminescence of Eu doped ZnO nanoneedles and branched nanoneedle structures, J. Nanosci. Nanotechnol.. 10 (2010) 502–507, https://doi.org/10.1166/jnn.2010.1586. [48] I. Iwantono, S.K. Md Saad, R. Yuda, M.Y. Abd Rahman, A.A. Umar, Structural and properties transformation in ZnO hexagonal nanorod by ruthenium doping and its effect on DSSCs power conversion efficiency, Superlattices Microstruct. 123 (2018) 119–128, https://doi.org/10.1016/j.spmi.2018.05.041. [49] A. Urbieta, R. Del Campo, R. Pérez, P. Fernández, J. Piqueras, Luminescence and waveguiding behavior in Tb doped ZnO micro and nanostructures, J. Alloy. Compd. 610 (2014) 416–421, https://doi.org/10.1016/j.jallcom.2014.05.007. [50] F. Pavón, A. Urbieta, P. Fernández, Characterization, luminescence and optical resonant modes of Eu-Li co-doped ZnO nano- and microstructures, Appl. Sci. 12 (2022) 6948. [51] G.C. Yi, C. Wang, W.I. Park, ZnO nanorods: synthesis, characterization and applications, Semicond. Sci. Technol. 20 (2005) S22, https://doi.org/10.1088/ 0268-1242/20/4/003. [52] T. Thangeeswari, G. Parthipan, S. Shanmugan, Raju, Synthesize of gadolinium- doped ZnO nano particles for energy applications by enhance its optoelectronic properties, Mater. Today:. Proc. 34 (2020) 448–452, https://doi.org/10.1016/j. matpr.2020.02.662. [53] E.H.H. Hasabeldaim, O.M. Ntwaeaborwa, R.E. Kroon, E. Coetsee, H.C. Swart, Cathodoluminescence degradation study of the green luminescence of ZnO nanorods, Appl. Surf. Sci. 484 (2019) 105–111, https://doi.org/10.1016/j. apsusc.2019.04.113. [54] B.Y. Liu, X.G. Bi, C.M. Xiong, L.Y. Shang, Composition-gradient ZnO/Zn(S, O) heterostructure nanorod arrays and their cathodoluminescence, Mater. Sci. Semicond. Process. 91 (2019) 362–366. [55] A.A.S. Da Gama, G.F. De Sá, P. Porcher, P. Caro, Energy levels of Nd3+ in LiYF4, J. Chem. Phys. 75 (1981) 2583–2587. [56] J.D. Zuegel, W. Seka, Upconversion and reduced 4F3/2 upper-state lifetime in intensely pumped Nd : YLF, Appl. Opt. 38 (1999) 2714–2723. [57] A. Barrera-Villatoro, C. Boronat, T. Rivera-Montalvo, V. Correcher, J. Garcia- Guinea, J. Zarate-Medina, Cathodoluminescence response of natural and synthetic lanthanide-rich phosphates (Ln3+: Ce, Nd), Radiat. Phys. Chem. 141 (2017) 271–275. [58] J. Liu, S. Lee, Y.H. Ahn, J.-Y. Park, K.H. Koh, K.H. Park, Identification of dispersion-dependent hexagonal cavity modes of an individual ZnO nanonail, Appl. Phys. Lett. 92 (2008), 263102. [59] R. Ariza, B. Sotillo, F. Pavón, A. Urbieta, P. Fernández, Evolution of whispering gallery modes in Li-doped ZnO hexagonal micro- and nanostructures, Appl. Sci. 10 (2021), 160319. P. Jara et al. https://doi.org/10.3390/ma12081195 https://doi.org/10.3390/ma12081195 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0140 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0140 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0145 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0145 https://doi.org/10.1007/S00339-007-3942-8 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0155 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0155 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0155 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0160 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0160 https://doi.org/10.1039/b905604c https://doi.org/10.1039/b905604c https://doi.org/10.1039/C4RA01055H http://refhub.elsevier.com/S0921-5107(23)00683-9/h0175 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0175 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0175 https://doi.org/10.1016/j.ceramint.2023.07.248 https://doi.org/10.1016/j.jallcom.2021.160219 https://doi.org/10.1016/j.jallcom.2021.160219 https://doi.org/10.1016/j.jallcom.2014.08.019 https://doi.org/10.1016/j.jallcom.2014.08.019 https://doi.org/10.1016/j.snb.2012.06.001 https://doi.org/10.1016/j.snb.2012.06.001 https://doi.org/10.1016/j.matpr.2019.12.001 https://doi.org/10.1016/j.matpr.2019.12.001 https://doi.org/10.1016/j.jallcom.2019.01.336 https://doi.org/10.1007/s10854-016-5779-4 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0215 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0215 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0215 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0220 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0220 https://doi.org/10.1016/j.matchemphys.2012.05.062 https://doi.org/10.1016/j.matchemphys.2012.05.062 https://doi.org/10.1002/pssa.201026632 https://doi.org/10.1166/jnn.2010.1586 https://doi.org/10.1016/j.spmi.2018.05.041 https://doi.org/10.1016/j.jallcom.2014.05.007 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0250 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0250 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0250 https://doi.org/10.1088/0268-1242/20/4/003 https://doi.org/10.1088/0268-1242/20/4/003 https://doi.org/10.1016/j.matpr.2020.02.662 https://doi.org/10.1016/j.matpr.2020.02.662 https://doi.org/10.1016/j.apsusc.2019.04.113 https://doi.org/10.1016/j.apsusc.2019.04.113 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0270 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0270 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0270 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0275 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0275 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0280 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0280 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0285 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0285 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0285 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0285 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0290 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0290 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0290 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0295 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0295 http://refhub.elsevier.com/S0921-5107(23)00683-9/h0295 Morphological, structural and luminescent characterization of Nd-doped ZnO nano- and microstructures grown by vapor-solid m ... 1 Introduction 2 Experimental method 3 Results and discussion 4 Conclusions CRediT authorship contribution statement Declaration of Competing Interest Data availability Acknowledgements References