Decoupling of surface diffusion and relaxation SEE COMMENTARY dynamics of molecular glasses Yue Zhanga and Zahra Fakhraaia,1 aDepartment of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323 Edited by Pablo G. Debenedetti, Princeton University, Princeton, NJ, and approved March 13, 2017 (received for review January 25, 2017) Tobacco mosaic virus is used as a probe to measure surface mics that are more similar to those of the free surface. These diffusion of ultrathin films of N,N0-Bis(3-methylphenyl)-N,N0- studies can help investigate whether the decoupling between sur- diphenylbenzidine (TPD) (12 nm < h < 53 nm, where h is the film face diffusion and bulk relaxation dynamics in aged and ultra- thickness) at various temperatures below the glass transition tem- stable molecular glass films is due to large differences between the perature, Tg, of all films. As the film thickness is decreased, Tg values [at least six orders of magnitude at Tg for ordinary glass decreases rapidly and the average film dynamics are enhanced and growing with lowering fictive temperature (11)] or whether by 6–14 orders of magnitude. We show that the surface diffu- it has a more fundamental origin. In addition, the typical thick- sion is invariant of the film thickness decrease and the result- nesses of molecular glasses used in applications such as coatings ing enhanced overall mobility. The values of the surface diffu- and organic electronics are less than 100 nm, and as such it is sion coefficient and its temperature dependence are invariant of imperative to directly characterize the surface mobility on these film thickness and are the same as the corresponding bulk val- nanosized glasses and to understand its role and effect in enhanc- ues (h = 400 nm). For the thinnest films (h < 20 nm), the effective ing the dynamics in ultrathin films used in these applications. activation energy for rearrangement (temperature dependence of Here we apply our recently developed tobacco mosaic virus relaxation times) becomes smaller than the activation energy for (TMV)-probe method (4, 11) to measure the surface diffusion surface diffusion. These results suggest that the fast surface dif- of ultrathin TPD glass films supported on silicon substrates. The fusion is decoupled from film relaxation dynamics and is a solely important advantage of the TMV-probe method is that it gen- free surface property. erates a mild perturbation on the free surface and requires no additional modification of the sample’s surface. As such, this is a fast surface diffusion j relaxation times j molecular glass j ultrathin films robust method that can be easily extended to study the surface of ultrathin molecular glass films. In this method, the surface mobil- ity can be evaluated by monitoring the temporal evolution of the he diffusion coefficients on the surfaces of molecular glasses surface response to TMV’s perturbation. In this study, the TMV- Tare reported to be orders of magnitude faster than the probe method is applied on the surfaces of ultrathin molecular bulk diffusion (1–6), with weaker temperature dependences and glass films, ranging from 12 nm to 53 nm, to measure their sur- stronger dependences on the molecular size or intermolecular face diffusion. The surface diffusion coefficients on these ultra- interactions (5–7). The measured fast surface diffusion on molec- thin films are found to be constant within the experimental error ular glass systems has been hypothesized to affect the frequently at four measuring temperatures below the bulk or thin film Tgs. observed fast surface crystallization (8, 9) and the formation of The results show that the surface diffusion has no dependence ultrastable glasses by physical vapor deposition (10). on the film thickness, whereas the Tg is reduced by as much as We recently investigated the surface diffusion of liquid- 20 K in 12-nm films and the average dynamics in these films are N N0 N N0 quenched bulk , -Bis(3-methylphenyl)- , -diphenylben- enhanced by 6–14 orders of magnitude. Furthermore, the acti- zidine (TPD) glasses (4) as well as aged and vapor-deposited vation energy of the thinnest films (h < 20 nm, where h is the ultrastable TPD glasses (11), spanning a range of 35 K in fic- tive temperatures and 13–20 orders of magnitude variations in structural relaxation times of the glass. We found that when held Significance below bulk glass transition temperature, Tg, the surface diffusion coefficient remains fast (about 6 orders of magnitude faster than Diffusion on the surfaces of molecular glasses is observed to the corresponding bulk diffusion coefficient of the ordinary glass be greatly enhanced compared with the bulk diffusion with at Tg), has a lower activation energy than bulk, and is invariant lower activation energies. However, the physical nature of the of the bulk dynamics on aged and ultrastable glass films. The lack fast surface diffusion and its relation to the glassy dynamics of correlation between the surface diffusion and bulk dynamics remain unclear. Relaxation dynamics of these glasses can be of aged or ultrastable glasses motivates further exploration of the enhanced by up to 14 orders of magnitude by reducing their nature of the observed fast surface diffusion on glassy surfaces. film thickness below 50 nm. By investigating the relationship Nanosized polymer films have been shown to have properties between surface diffusion and relaxation times of ultrathin that deviate strongly from the corresponding bulk properties. In films, we find that the fast surface diffusion remains invari- particular, their glass transition temperatures are reduced com- ant of the films’ relaxation dynamics even when the activation pared with their corresponding bulk values (12–17). The reduced energy of the film becomes lower than the activation energy Tg has been linked to enhanced relaxation dynamics near the for the surface diffusion, indicating a complete decoupling of PHYSICS free surface for both supported (12, 18, 19) and freestanding the relaxation dynamics and surface diffusion. (20, 21) films and the resulting enhancement of the overall relax- ation dynamics (15, 22, 23) of ultrathin films. We have recently Author contributions: Y.Z. and Z.F. designed research; Y.Z. performed research; Y.Z. and demonstrated that the viscosity and relaxation dynamics in ultra- Z.F. analyzed data; and Y.Z. and Z.F. wrote the paper. thin films of molecular glass TPD are similarly enhanced by 6–14 The authors declare no conflict of interest. orders of magnitude, depending on the measuring temperature This article is a PNAS Direct Submission. (24), but a direct comparison with fast surface diffusion to our See Commentary on page 4854. knowledge has not been made in molecular glass systems. 1To whom correspondence should be addressed. Email: [email protected]. By investigating surface diffusion coefficients on ultrathin TPD This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. films, surface diffusion can be measured in films with overall dyna- 1073/pnas.1701400114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1701400114 PNAS j May 9, 2017 j vol. 114 j no. 19 j 4915–4919 Downloaded by guest on September 26, 2021 film thickness) is lower than that of the surface diffusion on the A same films. These results suggest that the fast surface diffusion is fully decoupled from the overall film relaxation dynamics, down to film thicknesses where the film dynamics become compara- ble with the surface diffusion enhancement. Once the film is no longer glassy, the virus embeds into the film and surface diffusion can no longer be measured. Results and Discussion Tg Reduction and Enhanced Overall Dynamics in Ultrathin Films. The model system studied here is the organic molecular glass TPD (Tg = 330 K, molecular structure shown in Fig. 1B). Fig. 1A shows the measured Tg values as a function of film thick- ness. (Sample preparation, temperature ramping, and ellipsom- etry details can be found in Figs. S1–S4.) Fig. 1A, Inset shows representative normalized thickness profiles for four different films during the cooling ramps. The glass transition temperature B is defined as the intercept of the supercooled liquid line with the glassy line. As the temperature is decreased, ultrathin films maintain equilibrium at lower temperatures compared with the 120-nm film and show broader Tg transitions. As a result, the Tg is reduced as the film thickness is decreased, with an onset of devi- ation from bulk Tg around 50 nm. This observation is consistent with previous measurements of Tg reductions in ultrathin films of molecular (24–26) and polymeric glasses (12, 13, 15, 22, 23) and shows a similar range of thickness over which the Tgs of the ultrathin films are affected by the enhanced surface dynamics. We recently measured the effective viscosity and average relaxation times of ultrathin TPD films at various temperatures below bulk Tg (24). Using cooling-rate–dependent Tg (CR- Tg) measurements, the film’s average relaxation time at Tg can be estimated based on the inverse of cooling rates. Isothermal dewetting measurements were also performed in these studies to Fig. 1. (A) Glass transition temperatures, Tg, vs. film thickness for TPD films characterize the effective viscosity within films of various thick- supported on Si substrates measured using ellipsometry at a cooling rate of 10 K/min. A, Inset shows representative normalized thickness vs. temper- nesses and were related to relaxation times. Within the error ature plots for various film thicknesses. The black dashed lines show the of the experiments, both measures of dynamics (isothermal and slopes for thermal expansion coefficients of a bulk film in the supercooled temperature ramps) produced similar average relaxation times in liquid and glassy regimes. The black and green arrows show the values of Tg ultrathin films. Using these data from ref.