Dynamics of molecular and polymeric interfaces probed with atomic beam scattering and scanning probe imaging.

The scattering of atomic and molecular beams from well-characterized surfaces is a useful method for studying the dynamics of gas-surface interactions, providing precise information on the energy and momentum exchange which occur in such encounters. We apply this technique to new systems including disordered films of macromolecules, complex interfaces of macromolecular systems, and hybrid organic-semiconductor interfaces. Time-lapse atomic force microscopy studies of diblock copolymer structural evolution and fluctuations complement the scattering data to give a more complete understanding of dynamical processes in these complex disordered films. Our new scattering findings quantitatively characterize changes in interfacial dynamics including confinement in thin films of poly(methyl methacrylate) and changes in the physical properties of poly(ethylene terephthalate) films as they transform from the glassy to their semicrystalline phase. Further measurements on a hybrid organic-semiconductor interface, methyl-terminated silicon (111), reveal that the surface thermal motion and gas-surface energy accommodation are dominated by local molecular vibrations while the interfacial lattice dynamics remain accessible through helium scattering. High temperature atomic force microscopy allows direct, real-time visualization of structural reorganization and defect migration in poly(styrene)-block-poly(methyl methacrylate) films, revealing details of film reorganization and thermal annealing. Moreover, we employed lithographically created channels to guide the alignment of polymer microdomains. This, in turn, allows direct observation of the mechanisms for diffusion and annihilation of dislocation and disclination defects. In summary, this paper elaborates on the power of combining atom scattering and scanning probe microscopy to interrogate the vibrational dynamics, energy accommodation, energy flow, and structural reorganization in complex interfaces.

Dynamics of the sputtering of water from ice films by collisions with energetic xenon atoms.

The flow of energy from the impact site of a heavy, translationally energetic xenon atom on an ice surface leads to several non-equilibrium events. The central focus of this paper is on the collision-induced desorption (sputtering) of water molecules into the gas-phase from the ice surface. Sputtering is strongly activated with respect to xenon translational energy, and a threshold for desorption was observed. To best understand these results, we discuss our findings in the context of other sputtering studies of molecular solids. The sputtering yield is quite small; differential measurements of the energy of xenon scattered from ice surfaces show that the ice efficiently accommodates the collisional energy. These results are important as they quantitatively elucidate the dynamics of such sputtering events, with implications for energetic non-equilibrium processes at interfaces.

Determination of the sticking coefficient and scattering dynamics of water on ice using molecular beam techniques.

The sticking coefficient for D(2)O impinging on crystalline D(2)O ice was determined for incident translational energies between 0.3 and 0.7 eV and for H(2)O on crystalline H(2)O ice at 0.3 eV. These experiments were done using directed molecular beams, allowing for precise control of the incident angle and energy. Experiments were also performed to measure the intensity and energy of the scattered molecules as a function of scattering angle. These results show that the sticking coefficient was near unity, slightly increasing with decreasing incident energy. However, even at the lowest incident energy, some D(2)O did not stick and was scattered from the ice surface. We observe under these conditions that the sticking probability asymptotically approaches but does not reach unity for water sticking on water ice. We also present evidence that the scattered fraction is consistent with a binary collision; the molecules are scattered promptly. These results are especially relevant for condensation processes occurring under nonequilibrium conditions, such as those found in astrophysical systems.

Comparative surface dynamics of amorphous and semicrystalline polymer films.

The surface dynamics of amorphous and semicrystalline polymer films have been measured using helium atom scattering. Time-of-flight data were collected to resolve the elastic and inelastic scattering components in the diffuse scattering of neutral helium atoms from the surface of a thin poly(ethylene terephthalate) film. Debye-Waller attenuation was observed for both the amorphous and semicrystalline phases of the polymer by recording the decay of elastically scattered helium atoms with increasing surface temperature. Thermal attenuation measurements in the specular scattering geometry yielded perpendicular mean-square displacements of 2.7•10(-4) Å(2) K(-1) and 3.1•10(-4) Å(2) K(-1) for the amorphous and semicrystalline surfaces, respectively. The semicrystalline surface was consistently ∼15% softer than the amorphous across a variety of perpendicular momentum transfers. The Debye-Waller factors were also measured at off-specular angles to characterize the parallel mean-square displacements, which were found to increase by an order of magnitude over the perpendicular mean-square displacements for both surfaces. In contrast to the perpendicular motion, the semicrystalline state was ∼25% stiffer than the amorphous phase in the surface plane. These results were uniquely accessed through low-energy neutral helium atom scattering due to the highly surface-sensitive and nonperturbative nature of these interactions. The goal of tailoring the chemical and physical properties of complex advanced materials requires an improved understanding of interfacial dynamics, information that is obtainable through atomic beam scattering methods.

Helium atom diffraction measurements of the surface structure and vibrational dynamics of CH(3)-Si(111) and CD(3)-Si(111) surfaces.

The surface structure and vibrational dynamics of CH(3)-Si(111) and CD(3)-Si(111) surfaces were measured using helium atom scattering. The elastic diffraction patterns exhibited a lattice constant of 3.82 Å, in accordance with the spacing of the silicon underlayer. The excellent quality of the observed diffraction patterns, along with minimal diffuse background, indicated a high degree of long-range ordering and a low defect density for this interface. The vibrational dynamics were investigated by measurement of the Debye-Waller attenuation of the elastic diffraction peaks as the surface temperature was increased. The angular dependence of the specular (θ(i)=θ(f)) decay revealed perpendicular mean-square displacements of 1.0×10(-5) Å(2) K(-1) for the CH(3)-Si(111) surface and 1.2×10(-5) Å(2) K(-1) for the CD(3)-Si(111) surface, and a He-surface attractive well depth of ∼7 meV. The effective surface Debye temperatures were calculated to be 983 K for the CH(3)-Si(111) surface and 824 K for the CD(3)-Si(111) surface. These relatively large Debye temperatures suggest that collisional energy accommodation at the surface occurs primarily through the Si-C local molecular modes. The parallel mean-square displacements were 7.1×10(-4) and 7.2×10(-4) Å(2) K(-1) for the CH(3)-Si(111) and CD(3)-Si(111) surfaces, respectively. The observed increase in thermal motion is consistent with the interaction between the helium atoms and Si-CH(3) bending modes. These experiments have thus yielded detailed information on the dynamical properties of these robust and technologically interesting semiconductor interfaces.

Polymer surface and thin film vibrational dynamics of poly(methyl methacrylate), polybutadiene, and polystyrene.

Inelastic helium atom scattering has been used to investigate the vibrational dynamics at the polymer vacuum interface of poly(methyl methacrylate), polystyrene, and polybutadiene thin films on SiO(x)Si(100). Experiments were performed for a large range of surface temperatures below and above the glass transition of these three polymers. The broad multiphonon feature that arises in the inelastic scattering spectra at surface temperatures between 175 and 500 K is indicative of the excitation of a continuum of surface vibrational modes. Similarities exist in the line shapes of the scattering spectra, indicating that helium atoms scatter from groups of similar mass on the surface of these polymer thin films. The line shapes obtained were further analyzed using a semiclassical scattering model. This study has shown that quite different polymer thin films can have similar interfacial dynamics at the topmost molecular layer.

Effects of film thickness and molecular weight on the interfacial dynamics of atactic poly(methyl methacrylate).

We have investigated the surface vibrational dynamics at the polymer-vacuum interface of atactic poly(methyl methacrylate) thin films supported on SiO(x)/Si(100) as a function of film thickness and molecular weight. The highly surface-sensitive and nondestructive technique of inelastic helium atom scattering probes the vibrational dynamics at the true polymer-vacuum interface due to the lack of helium atom penetration into the film. For higher molecular weight samples (350 kg/mol), scattering spectra differ between films of thicknesses of fractions of the bulk radius of gyration compared with thicker films due to substrate interactions. A difference in the line shape of the scattering spectra is also present for lower molecular weight samples (60 kg/mol) compared with the thick, higher molecular weight samples. The differences in scattering spectra indicate a reduction of interfacial surface vibrational motion for both the thin, high molecular weight, and low molecular weight films as compared to thick, high molecular weight films. Our experiments demonstrate that dynamics at the interface of polymer thin films are unchanging until the films are thin enough for substrate interactions to influence vibrations at the true interface.

Unfolding of alanine-based peptides using electron spin resonance distance measurements.

We describe a scheme for tagging an alanine-based peptide with a Cu(II) and a nitroxide to measure unfolding transitions. The enhancement in longitudinal relaxation rate of the nitroxide due to the presence of Cu(II) was measured at physiological temperatures by pulsed electron spin resonance (ESR). The change in relaxation rate provided the average interspin distance between the Cu(II) and the nitroxide. Control experiments on a proline-based peptide verify the robustness of the method. The change in interspin distances with temperature for the alanine-based peptide is in accord with the change in helicity measured by circular dichroism. The data provide an opportunity to examine the unfolding process in polyalanine peptides. The distance in the folded state is in concordance with molecular dynamics. However, the ESR experiment measures an average distance of 17 A in the unfolded state, whereas molecular dynamics indicates a distance of 42 A if the unfolded geometry was a polyproline type II helix. Therefore, ESR demonstrates that the unfolded state of this alanine-based peptide is not an ideal extended polyproline type II helix.