Low-Temperature Thermal Conductivity of Submilimeter Organic Molecular Crystals

Abstract

We disclose a method of quantitatively characterizing crystalline defects by measuring low-temperature thermal conductivity in relatively clean organic molecular crystals. A new thermodevice is developed to measure low-temperature thermal conductivity of organic single crystals that often grow only minute.The technique of Micro Electro Mechanical Systems (MEMS) is employed, so that samples with the length from a few tens to several hundreds of μm can be mounted.Thermal conductivity of rubrene single crystals is measured down to 0.5 K in order to estimate the density of crystalline defects quantitatively from their phonon mean-free paths.The temperature profile of the rubrene crystals exhibit a pronounced peak at ~ 10 K in the thermal conductivity due to very long mean-free paths of their phonons, which indicates extremely low-level defect density in the region of 10¹⁵-10¹⁶ cm^-3 depending on different growth methods.The crystals grown from the gas phase tend to have less defects than those grown from solution.