Surface diffusion of a glassy discotic organic semiconductor and the surface mobility gradient of molecular glasses

Surface diffusion has been measured in the glass of an organic semiconductor, MTDATA, using the method of surface grating decay. The decay rate was measured as a function of temperature and grating wavelength, and the results indicate that the decay mechanism is viscous flow at high temperatures and surface diffusion at low temperatures. Surface diffusion in MTDATA is enhanced by 4 orders of magnitude relative to bulk diffusion when compared at the glass transition temperature T-g. The result on MTDATA has been analyzed along with the results on other molecular glasses without extensive hydrogen bonds. In total, these systems cover a wide range of molecular geometries from rod-like to quasi-spherical to discotic and their surface diffusion coefficients vary by 9 orders of magnitude. We find that the variation is well explained by the existence of a steep surface mobility gradient and the anchoring of surface molecules at different depths. Quantitative analysis of these results supports a recently proposed double-exponential form for the mobility gradient: logD(T, z) = logD(v)(T) + [logD(0) - logD(v)(T)]exp(-z/xi), where D(T, z) is the depth-dependent diffusion coefficient, D-v(T) is the bulk diffusion coefficient, D-0 approximate to 10(-8) m(2)/s, and xi approximate to 1.5 nm. Assuming representative bulk diffusion coefficients for these fragile glass formers, the model reproduces the presently known surface diffusion rates within 0.6 decade. Our result provides a general way to predict the surface diffusion rates in molecular glasses. Published under an exclusive license by AIP Publishing.

Automated summary

Surface diffusion in organic semiconductor glass has been studied and compared with other molecular glasses. The presence of surface mobility gradient and anchoring of surface molecules at different depths are found to significantly influence the surface diffusion behavior. A double-exponential model is proposed to explain the variation in surface diffusion rates.