Substrate Temperature to Control Moduli and Water Uptake in Thin Films of Vapor Deposited N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD)

Ultrastable glasses are generated by vapor deposition on substrates heated near the glass transition temperature (T_g), but it is unclear if the remarkable properties of such glasses are present in ultrathin (<100 nm) films. Here, we demonstrate that the moduli of 50 nm thick N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD) film can be increased from 1.5 to 2.5 GPa by simply increasing the temperature of the substrate during deposition with a maximum in modulus found at T/T_g = 0.94. This maximum in modulus is the same modulus obtained for very thin (<15 nm) NPD films deposited at 295 K (T/T_g = 0.80). However, the modulus of films deposited at this lower temperature abruptly decreases to approximately 1.5 GPa for thicker films; the modulus from deposition at T/T_g = 0.94 is thickness independent. In addition to the thin film modulus, the substrate temperature significantly impacts the water uptake in NPD films. From QCM, the volume fraction of water at equilibrium with nearly saturated water vapor decreases from nearly 4% to less than 1% as the substrate temperature increases from T/T_g = 0.82 to T/T_g = 0.93. The substrate temperature provides a simple route to control mechanical properties and water uptake into vapor-deposited NPD, and these concepts are likely extendable to other organic electronic materials; the increased moduli and decreased water uptake could enable improved performance and lifetime of small molecule glasses for a variety of organic electronic applications.