Nanostructured hybrid films containing nanophosphor: Fabrication and electronic spectral properties

Abstract

The intensive research of the optical properties of rare-earth ions is due to the high quantum efficiency of their emission, very narrow bands, and excellent fluorescence monochromaticity. The photoluminescence data presented here show that the nanophosphor remains a green emitter in Layer-by-Layer (LbL) films leading to potential application in optical devices or biological labeling. The LbL technique, an established method for thin film fabrication with molecular architecture control, is used in the manufacture of a hybrid film containing the cationic polyelectrolyte poly (allylamine hydrochloride) (PAH) and Y₂O₃: Er, Yb nanophosphor. The spectroscopic properties of this luminescent nanomaterial are extracted from the spectral data of the powder, cast film and LbL films. The growth of the LbL film was monitored by absorbance versus concentration plots in ultraviolet–visible (UV–Vis) absorption spectroscopy. The presence of both PAH and nanophosphor in the LbL film was confirmed by Fourier transform infrared (FTIR) absorption spectroscopy. The FTIR data also ruled out the existence of chemical interactions between the PAH and nanophosphor layers, which means that secondary interactions (like Van der Waals forces) might be the driving forces for LbL film growth. The morphology and the spatial distribution of the LbL film components along the film surface were probed with micrometer spatial resolution combining optical microscopy and Raman mapping. In addition, the observation of intense electronic emission lines from doping ions showed micro-spectroscopy as a highly sensitive analytical technique for characterization of films containing nanophosphors.

Introduction

Rare-earth based inorganic luminescent nanoparticles (NPs), or the so called nanophosphors, are inorganic host lattices doped with activator ions containing rare-earth ions in their crystalline system that have the potential to open a vast field of optical applications. Rare-earth doped NPs are highly photostable, exhibit long luminescence lifetimes and narrow emission bands and their emission color can be adjusted by the choice of the host material and dopants. They commonly ensure high chemical stability and bright luminescent intensity, which is dependent on the concentration of doping ions and on the host material. For rare-earth dopant trivalent ions as Er³⁺ and Yb³⁺, hosts such as REPO₄ (RE = Y, La, Gd, Lu), GdVO₄, Y₂O₃, SnO₂, Gd₂O₃ etc. are of considerable interest due to their high chemical durability and thermal stability [1], [2], [3]. In addition, Parchur et al. [4] reported the preparation of Eu³⁺ doped CaMoO₄ nanoparticles with attractive proprieties such as high melting point (1445–1480 °C), high refractive index (1.98), photo electron yield of 9% besides chemically resistant and non-hygroscopic [5]. On the other hand, Luwang et al. [6], [7] and Phaomei et al. [3] reported the synthesis of nanoparticles based on hosts such as REPO₄ (RE = Y and La) type. These hosts have crystal structure changed from tetragonal to monoclinic as the ionic size of RE³⁺ increases. However, if molecules of water are associated in a unit cell during preparation, a hexagonal phase is formed. This phase is stable even at higher temperatures. Cheng et al. [8] and Ansari et al. [9] reported the properties of the REVO₄ (RE = Y and La) hosts such as broad absorption and emission bands, low solubility in water, blinking proprieties and moderate quantum yields.

All nanophosphor features mentioned before and their performance depend directly on the preparation method. Parchur et al. [4] reported the advantages of using urea hydrolysis in ethylene glycol to obtain CaMoO₄:Eu³⁺ nanoparticles. The addition of urea improved the co-precipitation at low temperature and crystallinity, and the use of ethylene glycol prevented the agglomeration of the nanoparticles, although the as-prepared samples had to be annealed at higher temperatures (500–900 °C) in order to provide CaMoO₄ crystalline phase. The same nanophosphor was prepared using sol–gel synthesis by Wu et al. [10] and compared with solid state reaction. Sol–gel method provided precursors that were calcinated under 800 °C and yield phosphors with smaller grain size and enhanced luminescence when compared with solid state synthesis. Finally, Y₂O₃: Er, Yb nanophosphor was also investigated by Guo et al. [11] using a simple solvothermal method followed by a calcination at 900 °C for 4 h that generated monodisperse microspheres with diameter of 1.5–3 μm and nanoparticles with size of about 50 nm. However, in this present work, it was chosen the Pechini’s method to prepare Y₂O₃: Er, Yb nanophosphor. This methodology proves to be as advantageous as the methods previously cited due the possibility of preparing complex compositions, obtaining good homogeneity through mixing at the molecular level in solution and stoichiometric control. Moreover the method leads to powder particles that are more adequate for preparation of analytes with nanometric dimensions [1]. The thermal decomposition at high temperature leads the increased size of the nanoparticles and the crystallinity [4] removing crystal defects and reducing non-radiative processes [10]. In this last case, as the particle/grain size increases, the relative surface area is reduced, decreasing the number of OH⁻ and ${\text{CO}}_3^{2-}$ groups that can attach to the surface and affect the luminescence features [12].

Yttrium oxide host (Y₂O₃) has been reported in the literature due its attractive features for luminescent applications. When compared with the other hosts previously cited, it shows properties that make it an excellent host lattice for rare-earth ions doping and a potential host for integrated optics [1], [2]. Broad transparency range (0.2–8 μm), high refractive index (>1.9), large band gap (5.8 eV), low phonon energy, low lattice mismatch with Si (a_Y2O3 = 1.060 nm, a_Si × 2 = 1.086 nm) are also important properties that belong to this rare earth oxide and turn it interesting to be applied and to support our choice to use it in this work [13].

Concerning suitable optical features, the use of both Er³⁺ and Yb³⁺ activators for doping inorganic host lattices, an important characteristic for the produced nanophosphor is the conversion of different types of energy excitation sources into light throughout Stokes or anti-Stokes processes [14]. Considering the Stoke process, the activator center (Er³⁺) can be directly excited by ultraviolet (UV) source, emitting in the visible region [15]. However, if the activator (Er³⁺) is excited under near infrared excitation (NIR) directly or indirectly via the sensitizer (Yb³⁺) and emits in the ultraviolet–visible (UV–Vis) region, the anti-Stokes process takes place, which is also known as upconversion process [16], [17], [18]. In the case of Y₂O₃: Er³⁺, Yb³⁺ nanophosphor the upconversion process has already been extensively studied using infrared excitation due its applicability in upconverting phosphor technology (UPT), which involves the detection of analytes via immunoassay [1], [19], [20]. Downconversion process involving the Y₂O₃: Er³⁺, Yb³⁺ nanophosphor has also been extensively studied due its applications as light-emitting devices and photonic-based telecommunication [1], [16]. Because most of these applications involve devices based on thin films [21], [22], [23] as signal transducer, the optimization conditions to grow LbL films containing the nanophosphor is an important step considering potential applications. There are reports on the Layer-by-Layer (LbL) technique used to fabricate thin film structures containing rare-earth doped nanoparticles [24], [25]. Originally this technique was introduced by Iler et al. [26] for colloidal particles with opposite charges. Decher and co-workers [27], [28], [29], [30] extended this technique to deposition of multilayer alternating adsorption of cationic and anionic polyelectrolytes on charged surfaces. The method consists in the immersion of a solid substrate in an aqueous solution containing the material to be adsorbed followed by rinsing to remove the excess, drying, and then other immersion in a solution containing the second material of opposite charge, forming the first bilayer. Commonly, LbL thin films are formed by cationic and anionic molecular bilayers adsorbed alternately. The use of materials with opposite charges in aqueous solutions permits bilayer growth through electrostatic interactions [31], [32], [33]. Multilayer LbL films containing inorganic nanoparticles [34], [35], ceramics [36], [37] and even biological macromolecules such as proteins [38], [39] and DNA [40], [41] have been produced based on this type of interaction.

The fabrication of LbL films containing rare earth ions has also been explored. Liu et al. [34] reported the immobilization of LaPO₄ nanoparticles by LbL technique taking advantage of chemical interactions between lanthanide nitrate and phosphate supporting layer. Zhou et al. [42] reported the fabrication of (DNA/Eu)_n LbL film based on electrostatic interaction between DNA and Eu³⁺. Cui et al. [43] also demonstrated the possibility of immobilizing the positive Eu(DBM)₃ nanoparticles onto anionic supporting layers of poly(sodium 4-styrene sulfonate) (PSS) through electrostatic interactions. Here, we present unique results describing the immobilization of nanophosphors in supporting layers of polyelectrolytes using the LbL technique. In the case of Y₂O₃:Er³⁺, Yb³⁺ nanophosphor, the use of this technique is very important for the deposition of alternating layers on a solid substrate, considering that the material is partially soluble in water. Therefore, the use of techniques like LB (Langmuir–Blodgett) generally applied in fabrication of thin films containing rare-earth ions is not viable. Moreover, LbL technique also leads to a good quality of the films in terms of thickness control and surface morphology. In this work the viability of the LbL technique to fabricate multilayer thin films containing the cationic polyelectrolyte PAH (polyallylamine hydrochloride) alternated with the nanophosphor Y₂O₃: Er, Yb (2%, 1%) is demonstrated, allowing manipulation of their optical properties for practical applications. The LbL films of PAH/YOErYb, YOErYb refers to Y₂O₃: Er, Yb (2%, 1%), were compared to the nanophosphor bulk form, i.e. powder form, as well as to the cast film containing only the nanophosphor. The molecular architecture of the LbL films was characterized using different techniques. The film growth was monitored by UV–Vis absorption spectroscopy and the morphology at micrometer scale was investigated via micro-Raman technique, which combines morphological and chemical information by coupling an optical microscope and a Raman spectrograph. Moreover, Raman scattering and FTIR absorption spectroscopes were used to confirm the presence of the components in the film and possible interactions between them. Photoluminescence spectroscopy (PL) was performed by using UV excitation source to provide optical, structural and local activator environment information about the nanophosphor optical features from both LbL and cast films.