Metastability effects on the photoluminescence of ZnO nano-micro structures grown at low temperature and influence of the precursors on their morphology and structure

Abstract

Nanocrystalline ZnO films were grown on silicon substrate by hydrothermal synthesis at 125 °C, using diethanolamine as additive. A powder containing ZnO spheres, with diameters between 100 to 200 nm and formed by aggregation of ZnO nanoparticles, was also obtained as a secondary reaction product. The samples were studied by scanning electron microscopy, X-ray diffraction and photoluminescence (PL) spectroscopy. The effects of the [diethanolamine]/[Zn 2+ ] molar ratio on morphological, structural and optical properties were studied, as well as the effect of laser illumination (λ=325 nm) and annealing treatment on photoluminescence properties. The film samples exhibited a compact columnar structure, with thickness between 180 to 210 nm, which were not strongly affected by the diethanolamine concentration. The X-ray diffraction patterns from the films evidenced preferred orientation along the c-axis of the ZnO wurzite structure; while the nanospheres did not show any preferential crystalline direction. The PL spectra from the films showed large initial UV emission and a weak defect band centered in the yellow. A PL evolution while the samples were UV illuminated was observed. The relaxation of metastable phases (formed during the low temperature growth) involving the creation of point defects, is suggested. The predominance of the

Structural, optical and vibrational properties of ZnO:M (M=Al3+ and Sr2+) nano and micropowders grown by hydrothermal synthesis

“…This morphology seems to be a consequence of particle agglomeration in the solution during hydrothermal growth. Indeed, this result is in agreement with previous reports where the hydrothermal synthesis of ZnO using DEA was observed to lead to powders composed of spheres with similar characteristics [41,44]. This particular morphology is consequence of the addition of DEA in the hydrothermal synthesis, which causes, in a first stage, the formation of ZnO nanoparticles and, in a final stage, their agglomeration [41].…”

“…Among these techniques, the hydrothermal synthesis appears as an excellent and versatile option since, in addition to the good composition control, it is low cost, environmentally friendly, easily scalable and operates at relatively low temperatures [39,40]. Futhermore, the hydrothermal synthesis of ZnO nano and microstructured powders is a one-step and high yield process that offers good control of morphology [41][42][43]. As it can be seen, all samples are composed of particles with a spherical morphology, independently of the metal constituent (Zn 2+ , Al 3+ or Sr 2+ ).…”

“…nucleation growth rate), which would primarily affect the crystallite size. The DEA is involved in multiples stages in the hydrothermal synthesis of ZnO, including (i) chelating the Zn 2+ (M n+ ) cations by the formation of DEA -Zn -O -Zn -DEA chains (forming a stable colloidal phase), (ii) providing a growth medium at basic pH, (iii) controlling the morphology by the coordination to specific crystal faces and (iv) promoting nanoparticle agglomeration[41]. Although the microscopic chelation mechanism is still poorly understood, it is clear that addition of Al 3+ modifies the DEA -Zn -O -Zn -DEA chain formation process and interferes with nanoparticle aggregation, inhibiting the formation of large spheres.…”

Effects of methanol on morphology and photoluminescence in solvothermal grown ZnO powders and ZnO on Si

“…ZnO seeds were deposited on (100) Si substrates following the procedure previously reported [5]. Samples were synthesized by solvothermal synthesis in 25 mL polytetrafluoroethylene (PTFE) vessels within a stainless-steel autoclave, using zinc acetate dihydrate (CAS 5970-45-6, Alfa Aesar, >98%), HMTA (CAS 100-97-0, PanReac, > 99%) and three different solvent systems: water, water/methanol (1:1 ratio) and methanol (CAS 67-56-1, Cicarelli, 99.8%).…”

“…However, the role of key growth parameters on the morphology has remained elusive. For instance, using ethanolamine family reagents as alkaline additives, spherical ZnO particles were reported [5]. In contrast, HMTA usually leads to ZnO nanorods [3].…”