Morphological and Elastic Transition of Polystyrene Adsorbed Layers on Silicon Oxide.

Herein we present a study on the formation of irreversibly adsorbed layer of polystyrene molecules on silicon oxide surfaces. Various scanning probe microscopy techniques have been employed to study both the morphology and the mechanical properties of these self-assembled thin polymeric layers. More in detail, standard contact mode, force versus distance spectroscopy and ultrasonic force microscopy have been employed to obtain spatially-resolved maps and, thus, observe the physisorption of polystyrene on native silicon oxide substrate in function of time. Thick films, spin coated from a toluene solution, have been annealed at a temperature above the glass transition for increasing time intervals, and finally thoroughly rinsed in toluene. We have found that isolated islands of adsorbed chains are already present after an annealing time of half an hour. Prolonged annealing determines a progressive increase of the covered areas, whereas the formation of a complete flat layer requires 24 h. The pattern observed is in line with expected evolution of an unstable system, corresponding to the phenomenon of spinodal dewetting. Adhesion measurements show that the films present a reduced snap-off and the formation of a meniscus between tip and surface for annealing time up to 8 h. On the other hand, elastic measurements allow us to observe a progressive increase of the elastic modulus, with a complete transition for annealing time above 20 h. This is indication that a dense packing of the polystyrene molecules occurs, in line with the predictions of current models on the kinetics of irreversible adsorption. LAY DESCRIPTION: Herein we present a study on the formation of irreversibly adsorbed layer of polystyrene molecules on silicon oxide surfaces. Various scanning probe microscopy techniques have been employed to study both the morphology and the mechanical properties of these self-assembled thin polymeric layers. Thick polystyrene films, spin coated from a toluene solution, have been thermally annealed at a temperature above the glass transition for increasing time intervals, and finally thoroughly rinsed in toluene. We have found that isolated islands of adsorbed chains are already present after an annealing time of half an hour. Prolonged annealing determines a progressive increase of the covered areas, whereas the formation of a complete flat layer requires twenty-four hours. The adsorption pattern observed is in line with expected evolution of an unstable system, corresponding to the phenomenon of spinodal dewetting. Adhesion and elastic measurements have allowed us to observe a progressive increase of the packing density of the polystyrene molecules, in agreement with the predictions of current models on the kinetics of irreversible adsorption.

Scanning probe nanoimprint lithography.

The present paper reports on a novel lithographic approach at the nanoscale level, which is based on scanning probe microscopy (SPM) and nanoimprint lithography (NIL). The experimental set-up consists of an atomic force microscope (AFM) operated via software specifically developed for the purpose. In particular, this software allows one to apply a predefined external load for a given lapse of time while monitoring in real-time the relative distance between the tip and the sample as well as the normal and lateral force during the embossing process. Additionally, we have employed AFM tips sculptured by means of focused ion beam in order to create indenting tools of the desired shape. Anti-sticking layers can also be used to functionalize the tips if one needs to investigate the effects of different treatments on the indentation and de-molding processes. The lithographic capabilities of this set-up are demonstrated on a polystyrene NIL-patterned sample, where imprinted features have been obtained upon using different normal load values for increasing time intervals, and on a thermoplastic polymer film, where the imprint process has been monitored in real-time.

Differential near-field scanning optical microscopy with THz quantum cascade laser sources.

We have realized a differential Near-field Scanning Optical Microscope (NSOM) working with subwavelength resolution in the THz spectral region. The system employs a quantum cascade laser emitting at lambda approximately 105 microm as source, and the method, differently from conventional NSOM, involves diffracting apertures with size comparable to the wavelength. This concept ensures a higher signal-to-noise level at the expense of an additional computational step. In the implementation here reported lambda/10 resolution has been achieved; present limiting factors are investigated through finite difference time domain simulations.

The optical visibility of graphene: interference colors of ultrathin graphite on SiO(2).

Monatomic layers of graphite are emerging as building blocks for novel optoelectronic devices. Experimental studies on a single graphite layer (graphene) are today possible since very thin graphite can be identified on a dielectric substrate using a normal optical microscope. We investigate the mechanism behind the strong visibility of graphite, and we discuss the importance of substrates and of the microscope objective used for the imaging.

Resonant transport in Nb /GaAs /AlGaAs heterostructures: realization of the de Gennes-Saint-James model.

Resonant transport is demonstrated in a hybrid superconductor-semiconductor heterostructure junction grown by molecular beam epitaxy on GaAs. This heterostructure realizes the model system introduced by de Gennes and Saint-James in 1963 [P. G. de Gennes and D. Saint-James, Phys. Lett. 4, 151 (1963)]. At low temperatures a single marked resonance peak is shown superimposed to the characteristic Andreev-dominated subgap conductance. The observed magnetotransport properties are successfully analyzed within the random matrix theory of quantum transport, and ballistic effects are included by directly solving the Bogoliubov-de Gennes equations.