Single bath electrodeposition of samarium oxide/zinc oxide nanostructured films with intense, broad luminescence

Abstract

Electrochemical deposition from a solution containing zinc and samarium ions, leads to a samarium oxide/zinc oxide sandwich-like structure with an intense, visible, broad luminescence peak centered at 550 nm. The successive deposition of the two materials is related to the bath composition and overpotential, taking place for values higher than a certain threshold. The zinc oxide film, first one to be deposited, presents typical hexagonal prism morphology while samaria coating films present a porous, nanowall like structure. The photoluminescence emission is at least 10 times more intense than in the case of typical electrodeposited ZnO films of similar thickness and does not appear in Sm₂O₃ films electrodeposited from solutions containing only Sm ions. Samples prepared in different conditions were characterized from the point of view of composition, structure, morphology and optical properties. The characteristics of the emission spectra of the films make them interesting for solid state lightning applications.

Introduction

Electrochemical deposition of semiconductors represents an interesting approach for the preparation of different electronic or optoelectronic devices [1], [2], [3], [4], [5]. Even if not as widely employed as other deposition methods, the technique shows the promises of low infrastructure cost and high scalability, these two characteristics making it appealing for cheap, high yield production such is the case of solar photovoltaics or solid state lighting. One other important aspect of electrodeposition when compared with other techniques is related to the fact that it works on nonplanar substrates.

Zinc oxide is a direct band-gap semiconductor extremely interesting for optoelectronic applications. Its band gap of approximately 3.3 eV is well suited for ultraviolet light emitting devices such as LED's or lasers [6], [7], [8]. Thus, due to its direct band structure, ZnO shows a strong excitonic emission peak centered at about 385 nm with an intensity depending strongly on material quality. Besides the narrow excitonic peak the emission spectra of ZnO contains several defect related broad emission bands. These bands are related to specific point defects, either intrinsic or extrinsic [9], [10], [11], [12]. Electrochemical deposition of zinc oxide was intensively studied during the last decade, the main goal being related to the fabrication of high quality material on different substrates [13], [14], [15]. Extremely interesting is the fact that by appropriately tuning the deposition conditions one succeeds in tailoring the properties of the film such as morphology or optical properties. Thus, by changing the deposition conditions, one can obtain various structures such as hexagonal rods, hexagonal prisms, hexagonal platelets, pyramids or hollow prisms [16], [17], [18], [19]. Also by varying the deposition parameters one can produce films with intense UV emission or with intense defect emission [20]. Doping can also be achieved for a wide range of impurities, such as, for example, dopants with magnetic or optic properties.

Electrochemical deposition of rare earth metals or rare earth compounds became increasingly important during the last years due to the special high interest in the field of magnetic films (e.g. deposition of CoSm alloys) but also in protective coatings (e.g. deposition of ceria or samaria thin films) [21], [22], [23], [24]. Of major interest is considered the deposition of rare earth metal doped zinc oxide, since its potential in light emitting optoelectronic devices.

In the present paper we report our results regarding the preparation of a zinc oxide/samarium oxide multilayer by a process of electrochemical deposition from aqueous solutions. The deposition process leads initially to a hexagonal prisms ZnO layer followed by a nanostructured/nanowall layer of samarium oxide. Besides the interesting process which leads to this complex architecture, the deposit possesses a highly intense, broad photoluminescence emission, centered at about 550 nm. This makes the material appealing for applications in the field of solid state lighting.