Low-temperature thermal conductivity of bulk and film-like rubrene single crystals

Thermal conductivity of rubrene single crystals is measured for both bulk and film-like crystals 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 10${}^{15}$–10${}^{16}$ cm${}^{-3}$ depending on different growth methods. The crystals grown from the gas phase tend to have less defects than those grown from solution. The method is applied even for micrometer-thick crystals used for field-effect transistors developed for a new membrane device for thermal-conductivity measurement of film-like samples.

Figure 1

(a) A schematic diagram of the steady-state thermal conductivity measurement for bulk organic crystals. (b) A newly developed homemade MEMS device for thermal conductivity measurement of submillimeter organic crystals. Samples are to be mounted on the area described by dotted lines so that the working part of the heater and two thermometers are located on the samples. The whole device is kept under vacuum less than ${10}^{-5}$ Torr during the measurement.

Figure 2

Thermal conductivity of four bulk rubrene crystals grown by different techniques. Sample A is grown by PVT while B and C are from aniline and $p$-xylene solutions, respectively. Sample D is a film-like rubrene crystal grown by PVT. The inset is a magnified view to show the low-temperature peak of the phonon thermal conductivity.

Figure 3

(a) Thermal conductivity of rubrene crystals sample A, B, and D as a function of $T^2$ at low temperatures. (b) Specific heat of sample A as a function of $T^3$ at low temperatures.