Crystal growth of organic compound from the melt
Hong, Irvine Huamin
Date of Issue2014
School of Materials Science and Engineering
Crystal Growth Group
Demands on organic scintillator devices for fast neutron detection have been rising in the field of nuclear energy development for industrial usage, biology, chemistry, geology, medicine, and atmospheric science as well as security checks. Organic semiconductors are in principle cheaper, light weight, portable and highly sensitive to large fast neutron fluxes, offering attractive potential as effective radiation detectors. Features present in organic semiconductors which enable fast neutrons detections include the existence of aromatic rings for efficient scintillation; large amount of hydrogen presence for interaction with neutrons; low atomic number constituents, such as carbon or hydrogen, to avoid unnecessary interaction with gamma radiation; a delayed emission to better exhibit energy resolution. Relentless efforts been dedicated to uncover the potential of these scintillation devices (light yield) based on organic single crystals as well as to study their intrinsic charge transport properties so as to harness their application in organic electronics. Crystal defects and foreign materials do affect the optical as well as electronic properties of organic crystals. Regardless of a high volume of investigations, detailed comprehension of the crystal optical and charge transport behavior and performance remain an unresolved issue for organic based gadgets such as transistors as well as scintillation detectors due to the influence of defects/impurities present in the host. Findings have shown that small concentration of impurities and defects can substantially alter (trapping mechanism) optical/transport characteristics of organic semiconductors. The defects can be caused by the presence of chemical contaminants during the crystal development process and after the synthesis owning to the thermal deformation and photo chemical reactions. Since vital to the determination of optical/charge transport mechanism in organic semiconductors is the capability to develop pure, orderly structured molecular crystals for evaluation of photo-luminescence/intrinsic carrier mobility; ultra purification of solid organic crystals consisting of benzene aromatic rings has been a topic of considerable discussion among researchers. Pure organic crystallized materials are therefore the best option to provide efficient scintillation with high light yield and desired carrier mobility in the field of organic transistors (OFETs). Organic single crystals have been commonly attained from the solution, the vapor and the melt phase. Melt growth is more promising than solution growth in that the process takes up less time and free from the issues involving solvent inclusions. Meanwhile, growth of organic crystals from the vapor phase usually results in slow growth of very thin crystals because of the multi-nucleation involved. Therefore, when large single crystals of increased perfection for commercial application are required, these must be grown from the melt. In this study, single crystals of selenium (proof of concept), anthracene and trans-stilbene up to few centimeters were developed from the melt based on a self established, inexpensive and versatile zone refining apparatus. Platelets cut from these crystals by applying disc cutter (selenium ingot) and thread saw (anthracene together with stilbene ingot) were evaluated for purity and perfection by Powder X-ray Diffraction (XRD), Energy Dispersive X-Ray analysis (EDX), Laser Desorption Ionization-Time of Flight Mass Spectrometry (LDI-ToF MS), Fourier-Transform Infrared Spectroscopy (FT-IR), Atomic Force Microscopy (AFM) and fluorescence study. Based on the purity analysis, impurity such as carbon within selenium has been effectively segregated. Meanwhile, the outcomes of characterization studies support the purity of organic crystals grown from melt in comparison with solution technique and Bridgman method. In addition, chemical defects (impurities) within commercial anthracene powder such as carbazole and 9, 9’Bianthryl have been removed significantly as indicated by LDI-ToF MS analysis. As for trans-stilbene, contaminants such as 1-methylphenanthrene & 2-Phenylindene had been identified. Upon the refinement, both the impurities been minimized within the mid zone of the ingot. The light yield generated within these VZM organic crystals (anthracene and trans-stilbene) upon reduction of the impurities have been greatly improved.