Sample preparation: 1 x 1 cm 2 Si samples were cut from a n-type (100) Si wafer. Si substrates were cleaned with 2-propanol and acetone in an ultrasonic bath and subsequently dried using a stream of ultrapure N2. A 30 μm thick free-standing PLA film was supplied by Corapack ® . PLA was dissolved in chloroform and spin coated on the1 x 1 cm 2 Si samples forming ~15 nm thick polymer films. PMMA (Mn = 15 kg mol -1 , PDI = 1.09) was acquired from Polymer Source. Inc. and dissolved in toluene. Then ~15 nm thick PMMA films were prepared by spin coating the PMMA solution on the 1 x 1 cm 2 Si samples. Before infiltration, the PMMA films were annealed at 200 °C for 300 s on a hot plate to remove residual toluene. A simplified scheme depicting the final structure of the samples is shown in Figure .