Organic/inorganic-doped aromatic derivative crystals: Growth and properties
Introduction
In the last decades, organic molecular crystals have been considered potential substitutes for inorganic crystals in nonlinear optical applications because they have high values for the nonlinear coefficients, large birefringence values, high damage thresholds in laser beams, and a large transparency domain [1], [2]. In the field of nonlinear optical (NLO) phenomena, there is interest in second harmonic generation (SHG) for medical applications and two-photon absorption fluorescence emission (TPF) for potential applications in frequency up conversion lasing, optical power limiting, 3D fluorescence imaging, 3D optical data storage, 3D lithographic micro-fabrication, and photodynamic therapy. Recently, special attention was paid to the study of the synthesis, crystal growth, and characterization of bi-component organic systems for NLO applications [3], [4].
Aromatic derivatives are a group of organic compounds that could be interesting for optical nonlinear applications because of the delocalized cloud of π electrons. Meta-dinitrobenzene and benzil are characterized by a large optical band gap and high nonlinear coefficients. These two compounds show different particularities of the chemical structure at the molecular level, but both show important optical nonlinear phenomena.
Meta-dinitrobenzene (C6H4N2O4, m-DNB) is a negative biaxial crystal that belongs to the Pbn21 space group and the point symmetry group mm2. It crystallizes in the orthorhombic system at room temperature and shows good transparency in the range of 0.4–2.5 μm. The crystal has a pyramidal shape with four molecules in the unit cell characterized by the following dimensions: a=13.20 Å, b=13.97 Å, and c=3.80 Å [5]. Diphenyl-α,β-diketone also known as benzil (C6H5COCOC6H5, Bz), is an uniaxial crystal that belongs to the trigonal space group P3121 and the point symmetry group . Good optical quality crystals with a large transparency domain from UV through near IR can be grown. In the benzil unit cell, three molecules are helically disposed and closed packed around the 31 axis, and the hexagonal unit cell dimensions are a=8.42 Å and b=13.75 Å [6]. There are few studies about the growth from the melt in vertical configuration of m-DNB [3], [7], [8] and benzil [9], [10], [11], [12] crystals but the problem of the growth interface stability was not investigated.
The aim of this paper is to determine the influence of crystal growth conditions on dopant incorporation and the effect of dopant incorporation on the optical properties of pure m-DNB and Bz crystals. We studied two different aspects: (1) the effect of different iodine concentrations (c=1 wt% and 2 wt%) on two different matrices, namely m-DNB and Bz; (2) the behavior of the same aromatic derivative, m-DNB, as a matrix and a dopant. The systems composed by an aromatic derivative matrix and organic/inorganic dopant have been studied from the point of view of the growth interface stability criterion to analyze dopant incorporation and its effect on the quality of the crystal. We have evaluated the stability limits and the experimental conditions such as the temperature gradient ΔT at the growth interface, and the interface displacement velocity v of the ampoule in the furnace for initiating instabilities. These instabilities could be associated with structural defects and compositional non-homogeneities in the doped crystal of m-DNB. The influence of crystal quality, determined by the particularities of dopant incorporation, on the optical band gap and intensity of the SHG and TPF emissions has been presented for the first time, comparing pure and doped m-DNB and Bz crystals.
Section snippets
Experimental
The starting materials provided by Aldrich Inc. (purity 98 wt%) were purified by zone refining in a multiple passages process (between 15 and 25). The m-DNB and Bz crystals were grown by the Bridgman–Stockbarger method. The growth from melt was chosen because both compounds are stable and do not decompose at the melting point. The high purity compounds were introduced into the growth chamber, which was evacuated and sealed under vacuum because the organic compounds show a high affinity for the
Crystal growth and interface stability
One of the most difficult problems related to the growth of doped organic molecular crystals is their homogeneity, which is related to dopant incorporation in the crystal and is mainly determined by the stability of the growth interface. In order to understand the growth interface stability, we investigated the growth of m-DNB in the selected Bridgman–Stockbarger configuration and evaluated the conditions that favor the generation of the morphological instability associated with defects in the
Relationship between optical properties of crystals and growth conditions
The incorporation of the dopant, controlled by ΔT and v, affects the properties of the crystals. Although the system is situated in the stable growth region under the experimental conditions mentioned above, the deviation from the plane growth interface induces a certain disorder in the crystalline material associated with the generation of defects. The defects affect the optical transitions and generate variations in the optical band gap and strong bulk absorption, “masking” the intrinsic
Conclusions
Pure and doped m-DNB and Bz crystals have been grown in the Bridgman–Stockbarger configuration in the following experimental conditions: ΔT=25 °C and v=1.7 mm/h for pure crystals and ΔT=30 °C and v=2.0 mm/h for doped crystals. Applying the interface stability criterion, we have deduced for m-DNB-based systems limits of the growth interface stability compatible with those previously obtained for Bz-based systems. Therefore, it is possible to grow m-DNB and BZ crystals in the same experimental
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