Growth Of Low-dimensional Nanostructures Research Articles

(Invited) Eu- Oxide and Oxyhydroxide Nanofilms for Luminescent Photonic Components

Optically-doped rare-earth (RE) nanostructures find application in a diverse range of fields, including lighting (e.g. LEDs), displays, quantum memories, and biosensors such as luminescent nano-thermometers [1-5]. This is possible thanks to the ability of RE ions to provide robust and reliable light emission when incorporated in a solid. Since their performance depends critically on the composition and structure of the host material in order to successfully implement these RE-doped solids in technological applications it is desirable to achieve the controlled growth of low dimensional nanostructures that show efficient emission. This will enable on-chip component fabrication and their integration in photonic devices. The starting point of such technological development is the preparation of light emitting nanometer-thick films.In this contribution, we present an alternative approach for the development of europium-based nanofilms, departing from conventional doping methods that rely on host matrices, typically oxides and oxinitrides of diverse elements, to encapsulate the Eu2+ or Eu3+ emitter ions [4-6]. We highlight the efficacy of employing direct Eu compounds, maximizing the number of active emitters in the nanofilms and resulting in the observation of intense emission from these nanostructures. Our search started with original studies on Eu oxides (EuOx 1/3<x<1), and demonstrated a broad tuning of their emission by controlling their stoichiometry [1]. In this context, we present the progress achieved through the successful fabrication of europium oxyhydroxide (EuOOH) nanofilms [7]. It is noteworthy that the conventional method for producing hydroxides and oxyhydroxides involves chemical hydrothermal procedures with multiple steps, including dehydration, ultimately yielding powders and micro-crystals unsuitable for integration. In contrast, our methodology employs a clean physical evaporation technique under vacuum conditions, yielding thin films ready for integration. Crystalline monoclinic EuOOH nanofilms were successfully grown on silicon substrates by using pulsed laser deposition (PLD) and a subsequent annealing at a low temperature (250ºC). We will provide evidence of the physical mechanisms governing OH incorporation and discuss the influence of oxygen (O) and hydroxyl (OH) content on the resulting red emission of the Eu3+ ions related to the 5D0→2FJ transition, and on the optical properties of the films through the visible and infrared spectrum.[1] A. Mariscal-Jiménez et al, J. Phys. Chem. C 2020, 124, 15434.[2] P. Gomez-Rodriguez, et al, Nanophotonics 2021, 16, 3995-4007.[3] T. Zhong and P. Goldner, Nanophotonics 2019, 8, 2015.[4] D. Jaque and F. Vetrone, Nanoscale 2012, 4, 4301.[5] I. Camps, A. Mariscal-Jiménez, and R. Serna, Appl. Surf. Sci. 2023, 613, 1.[6] C. W. Bond et al. Opt. Mater. 2021, 122, 111796.[7] A. Caño, B. Galiana, G. B. Perea, J. Gonzalo and R. Serna, Appl. Surf. Sci. 2023, 640, 158236.

Influence of oxygen-related surface adsorbates on the growth of low dimensional nanostructures and enhanced luminescence due to superoxide charge-transfer states in ZnO for application in optoelectronic devices

Kinetically controlled growth of low dimensional zinc oxide (ZnO) nanostructures like nanorods, nanoflowers etc. have been demonstrated with enhanced photoluminescent emission by tuning of the hydroxide ion concentration in the precursor irrespective of the synthesis techniques. Change in concentration of the hydroxide ions in the precursor during the synthesis of ZnO nanostructured samples has been found to tune the aspect ratio of the nanostructures, viz. nanorods grown by electrodeposition, and morphology of nanoflowers to nanogranules synthesized by direct precipitation method. The photoluminescent emission from these samples has been found to increase synchronously with the increase in the concentration of hydroxide ions in the precursor. Observed enhancement in photoluminescence has been proposed to be due to the formation of superoxide (O2−) charge transfer (CT) states CTO2− due to the adsorption of molecular oxygen directly at grain boundaries or induced by hydroxide ions adsorbed over the surface of the nanostructures. The results are in contradiction to the established role of defect states behind photoluminescent emission from ZnO nanostructures. In this work, mathematical modelling has been used to establish the dominant role that superoxide CT states can play in enhancing the efficiency of ZnO based optoelectronic devices such as light-emitting diodes and photodiodes.