Elsevier

Optical Materials

Volume 35, Issue 2, December 2012, Pages 268-273
Optical Materials

Cathodoluminescence and Raman analysis of the finite-size effects in mer-Alq3 structure

https://doi.org/10.1016/j.optmat.2012.08.017Get rights and content

Abstract

As synthesized mer-Alq3 powder, obtained by wet chemical synthesis, has been subjected to different new characterization techniques such as Energy Dispersive X-ray, Cathodoluminescence and Scanning Electron Microscopy in order to identify their structural and photophysical properties. Other common techniques like X-ray diffraction, Raman and Fourier Transform Infrared spectroscopy have been involved in order to prove the formation of mer-Alq3 compound. The vibrational spectroscopic properties were interpreted by using the output files from Gaussian software with B3YLP functionals and 6-311 (d) basic sets. The growth kinetic of mer-Alq3 nanocrystals in the α-phase has been evaluated by using the finite-size effects based on the Raman position peaks and their linewidths, which confirms the semiconductor character of the mer-Alq3 compound. The XRD results suggest that the isothermal annealing increases the mer-Alq3 nanocrystals, which modify the regular shape of typical spectral parameters from Raman peaks.

Highlights

► Growth kinetic of mer-Alq3 has been evaluated by using finite-size effect of nanocrystallinity. ► Cathodoluminescence image was compared with the secondary electron image. ► The energy dispersive X-ray spectrum confirms the formation of mer-Alq3 structure from the atomic ratios.

Introduction

Tris 8-hydroxyquinoline aluminum (Alq3) is one of the most known materials used as the emissive layer (EML) in organic light emitting devices (OLEDs) due to its long lived phosphorescence. The interest in organometallic materials for use in organic light-emitting diodes (OLEDs) began with Tang and Van Slyke in the first report about efficient green electroluminescence from Alq3 [1]. Alq3 is used as electron-transport layer and/or electron-injecting layer in multilayer devices and as effective host material for various dyes [2].

Alq3 is an octahedral coordinated chelate complex of the type M(N^O)3, where M is a trivalent metal and N and O are the nitrogen and oxygen atoms in the bidentate quinolinolate ligand. Due to the bidentate nature of the ligand, the Alq3 complex can be obtained in two isomeric forms (facial and meridional), depending on the orientations of the ligands [3].

Beside of these two isomers, the thermal annealing processes induces different crystalline structures called α, β, γ and even δ phase [4]. These phases can be determined from Differential Scanning Calorimetry (DSC) in which the first crystallographic change appears at 295 °C up to the melting point, around 420 °C [5].

Because the Alq3 powder can be used also as the electron transport layer, it is important to know the properties of this electroluminescent material under direct electron excitation.

The aim of this paper is a deeper understanding of the phosphorescence mechanism of mer-Alq3 powder, together with the structural changes which appears during thermal annealing, slightly below the first phase changing temperature (295 °C). Several experimental techniques were used in order to investigate these properties like Raman, cathodoluminescence (CL), Fourier Transform Infrared (FT-IR) and photoluminescence (PL) for the photophysical properties and X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) for structural characterization. The spectral changes are interpreted in terms of nanocrystallinity-induced finite-size effects associated with the slow growth kinetic. X-ray diffraction experiments were used to determine nanocrystals sizes which were coupled with the Raman line-shape results. The experimental results were compared with the chemical computation results, obtained by using Gaussian software, especially for the Infrared and Raman spectra.

Section snippets

Synthesis of mer-Alq3 compound

Organic molecule tris (8-hydroxyquinoline) aluminum (Alq3) was synthesized by mixing 8-hidroxyquinoline and Al(OH)3 at 95 °C. In a typical synthesis, 0.39 g (0.005 mol) of Al(OH)3 was gradually dropped into 80 ml distilled water which contains 1.45 g (0.01 mol) of 8-hidroxyquinoline. The ratio between 8-hidroxyquinoline and Al(OH)3 was chosen as 2:1, which in the fast reaction conditions (up to 24 h), results in the mer-Alq3 formation. When this ratio is 3:1, the synthesis leads to the formation of

X-ray diffraction

The XRD measurements have been performed on a BRUKER D8 ADVANCE type X-ray diffractometer, in focusing geometry, with a vertical theta–theta goniometer and horizontal sample carrier. BRUKER D8 ADVANCE is equipped with copper target X-ray tube with CuKα1 radiation (λCuKα1 = 1.5406 Å) and nickel Kβ filter. LynxEye one-dimensional detector ensures a collection rate with two orders of magnitudes higher than that of conventional point detectors and very good angular resolutions. The working parameters

Density functional theory (DFT)

Molecular geometry of mer-Alq3 was optimized using Density Functional Theory (DFT) [8] with aid of Gaussian 03W software. We have used the B3LYP hybrid exchange correlation functional of Becke‘s three-parameter theory and 6-31g basic sets, including geometry connectivity. After initial geometry optimization, the next steps of calculation were dedicated to Raman and infrared computational spectra.

XRD analysis

In order to investigate the structural characteristics of as synthesized mer-Alq3 powder, the X-ray diffraction was the first step in the crystalline structure identification. The XRD peaks were refined by using one crystal sleuth application of the Bartelmehs and Downs version 3.0 (RUFF) in order to identify the cell parameters of the crystalline phase [9].

X-ray diffraction of the yellowish powder obtained by wet synthesis (after washing and drying) shows all characteristics of the triclinic

Conclusions

The XRD, Raman, FTIR and photoluminescence spectra prove the formation of stable mer-Alq3. The Raman and FT-IR spectra were interpreted by using Gaussian DFT software with B3YLP functionals and 6-311 (d) basic sets.

Structural analysis shows the formation of partial crystalline structure of mer-Alq3, which is fluorescent under high energy electron beam. The cathodoluminescence spectrum shows similar features with the photoluminescence spectrum but at the long wavelengths, exhibits a small

Acknowledgements

The work has been funded by the Nucleus Program Contract number PN-45 and Sectoral Operational Programme Human Resources Development 2007–2013 of the Romanian Ministry of Labour, Family and Social Protection through the Financial Agreement POSDRU/107/1.5/S/76903.

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