Person:

Pershan, Peter

The microscopic structure of Langmuir films of derivatized gold nanoparticles has been studied as a function of area/particle on the water surface. The molecules (AuSHDA) consist of gold particles of mean core diameter D ∼ 22 Å that have been stabilized by attachment of carboxylic acid terminated alkylthiols, HS–(CH2)15–COOH. Compression of the film results in a broad plateau of finite pressure in the surface pressure versus area/particle isotherm that is consistent with a first-order monolayer/bilayer transition. X-ray specular reflectivity (XR) and grazing incidence diffraction show that when first spread at large area/particle, AuSHDA particles aggregate two dimensionally to form hexagonally packed monolayer domains at a nearest-neighbor distance of a = 34 Å. The lateral positional correlations associated with the two-dimensional (2D) hexagonal order are of short range and extend over only a few interparticle distances; this appears to be a result of the polydispersity in particle size. Subsequent compression of the film increases the surface coverage by the monolayer but has little effect on the interparticle distance in the close-packed domains. The XR and off-specular diffuse scattering (XOSDS) results near the onset of the monolayer/bilayer coexistence plateau are consistent with complete surface coverage by a laterally homogeneous monolayer of AuSHDA particles. On the high-density side of the plateau, the electron-density profile extracted from XR clearly shows the formation of a bilayer in which the newly formed second layer on top is slightly less dense than the first layer. In contrast to the case of the homogeneous monolayer, the XOSDS intensities observed from the bilayer are higher than the prediction based on the capillary wave model and the assumption of homogeneity, indicating the presence of lateral density inhomogeneities in the bilayer. According to the results of Bragg rod measurements, the 2D hexagonal order in the two layers of the bilayer are only partially correlated.

The photoconductivity spectra of high purity, semiconducting CdF2, containing low concentrations of rare-earth donors, are interpreted in terms of a shallow trap (1260 \pm 100cm^{-1} ) below the bottom of the conduction band and an excited bound state (1030cm^{-1} ) above the ground state.

X-ray studies of the interface between liquid metals and their coexisting vapor are reviewed. After a brief discussion of the few elemental liquid metals for which the surface Debye-Waller effect is sufficiently weak to allow measurement, this paper will go on to discuss the various types of surface phenomena that have been observed for liquid metal alloys. These include surface adsorption, surface freezing, surface aggregation of nm size atomic clusters, and surface chemistry that leads to new 3D crystalline phases.

We will review the principal x-ray scattering measurements that have been carried out on the free surface of liquid metals over the past two decades. For metals such as K, Ga, In Sn, Bi etc the surface induces well-defined layering with atomic spacing ‘d’ that penetrates into the bulk a distance of the order of the bulk liquid correlation length. As a consequence the angular dependence of the surface structure factor observed by x-ray reflectivity displays a broad peak at wavevector transfer ∼ 2π/d with a half width that is comparable to the width of the bulk liquid structure factor. Quantitative measurement of this surface structure factor requires correction for a singular Debye-Waller like effect arising from thermally excited capillary waves. For liquid metal alloys the layering is accompanied by chemical segregation (i.e. Gibbs absorption) that can be characterized from the energy dependence of the reflectivity. Particularly interesting are the temperature dependence and elasticity of the two-dimensional surface frozen phases that form on the surface of the Au82Si18 liquid eutectic. Surface freezing, although not observed near the eutectic points of alloys such as Au-Ge, Pd-Ge and Pd-Si, has been observed at the free surface of the glass forming alloy Au49Ag5.5Pd2.3Cu26.9Si16.3.

Brillouin-scattering results are reported for monodomain samples of (\beta)-methyl butyl p [(p-methoxy benzylidine)amino] cinnamate in both the smectic-A and -B phases. For both phases theoretical predictions are confirmed concerning the number and polarizations of the propagating hypersonic waves. We report quantitative measurements of various thermodynamic derivatives, including the coupling between density and smectic-layer spacing. We discuss preliminary evidence concerning one model of the smectic-B phase.

Raman-scattering techniques have been used to obtain a new quantitative measure of orientational statistics of individual molecules in a nematic liquid crystal. A marked discrepancy is observed between these measurements and the predictions of existing theories of nematic ordering.

The six fundamental bands of (CH_3I) and (CD_3I), both as pure liquids and in solution with (CS_2), have been studied in order to obtain information about the molecular rotational and vibrational dynamics. Emphasis was placed on the features of the spectra that can be interpreted independent of a particular model. An examination for Boltzmann asymmetry in the (A_1) and E bands indicates that the former, being symmetric, are amenable to classical description, while the latter, being asymmetric, definitely require quantum mechanical interpretations. With respect to the validity of the assumption that rotation‐vibration coupling can be ignored, we give evidence of substantial coupling effects in a nondegenerate (A_1) mode ((\nu1)) as well as in the doubly degenerate E modes. We emphasize that, in attempting to obtain dynamical information, several bands of any given liquid must be studied and compared in order to decide which can be used with confidence in a detailed analysis. The consistency of the rotational diffusion model for describing the tumbling motion of the methyl iodide molecules has been confirmed.

Distortions of the plane texture of smectic-A and cholesteric liquid crystals are shown to combine with the uniaxial birefringence of the plane texture to produce a strong characteristic light-scattering effect. We have observed static deformations, arising from surface defects, as well as dynamic deformations arising from thermal or mechanical stress. An immediate application of this effect is to confirm theoretical speculations on the elasticity of smectic-A phases.

It is demonstrated by means of x-ray synchrotron reflectivity and diffraction that the surface of the liquid phase of the bulk metallic glass forming alloy Au_49 Cu_26.9 Si_16.3 Ag_5.5 Pd_2.3 consists of a two-dimensional crystalline monolayer phase for temperatures of up to about 50 K above the eutectic temperature. The present alloy as well as glass forming Au_82 Si_18 and Au–Si–Ge alloys containing small amounts of Ge are the only metallic liquids to exhibit surface freezing well above the melting temperature. This suggests that the phenomena of surface freezing in metallic liquids and glass forming ability are related and probably governed by similar physical properties.

A brief description of the essential features of amphiphillic molecules will be presented. This will be followed by a discussion of specific physical phenomena and their relation to a number of contemporary research areas. Foremost amongst these is the possible application of amphiphillic monolayers to the physics of two-dimensional systems. Specific experimental and theoretical examples will largely be drawn from studies of synthetic phosphatidylcholine (e.g., lecithin) type lipids. Recent experimental results from our laboratory will be mentioned.