Person: Pershan, Peter
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Pershan
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Peter
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Pershan, Peter
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Publication Crystalline monolayer surface of liquid Au–Cu–Si–Ag–Pd: Metallic glass former(AIP Publishing, 2011) Mechler, S.; Yahel, E.; Pershan, Peter; Meron, M.; Lin, B.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.Publication Review of the Highlights of X-Ray Studies of Liquid Metal Surfaces(American Institute of Physics, 2014) Pershan, PeterX-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.Publication X-ray scattering: Liquid metal/vapor interfaces(Springer Science + Business Media, 2011) Pershan, PeterWe 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.Publication Photoconductivity in High Purity, Semiconducting CdF2(Elsevier, 1972) Feldman, Bernard J.; Pershan, PeterThe 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.Publication Self-Assembly of Organic Films on a Liquid Metal(Nature Publishing Group, 1996) Magnussen, Olaf M.; Ocko, Benjamin M.; Deutsch, Moshe; Regan, Michael; Pershan, Peter; Abernathy, Douglas; Grubel, Gerhard; Legrand, Jean-FrancoisThe structure and phase behaviour of organic thin films result from the subtle interplay of intermolecular Van der Waals interactions, which promote self-assembly and long-ranged order, and the more complex interactions between the end groups of the organic chains and the substrate. The structure of molecular films of amphiphiles has been extensively studied on subphases of dielectric liquids, notably water (Langmuir mono-layers) and on solid surfaces (self-assembled monolayers, SAMs. Here we report structural studies, by synchrotron X-ray scattering, of an intermediate case: densely packed alka-nethiol films on the surface of liquid mercury. While, like SAMs, these films form strong chemical bonds to the subphase, this subphase is smooth and unstructured, as in the case of Langmuir monolayers. But unlike either of these, our films have no in-plane long-range order. We suggest that the strong interaction of the thiol group with the underlying disordered liquid dominates here over the order-promoting interactions of the alkyl chains.Publication Complete Wetting of a Rough Surface: An X-Ray Study(American Physical Society, 1991) Tidswell, I. M.; Rabedeau, T. A.; Pershan, Peter; Kosowsky, S. D.The evolution of the surface structure of a wetting film on a rough surface as a function of the film thickness has been studied by x-ray specular reflection and surface diffusion scattering. For thin films (\(\lesssim\)60 Å) the liquid surface is characterized by static undulations induced by the roughness of the substrate; however, with increasing film thickness the structure is dominated by thermally induced capillary waves. The data are quantitatively described by a model with exclusively van der Waals liquid-substrate interactions.Publication X-Ray Reflectivity of a Polymer Monolayer at the Water/Vapor Interface(American Chemical Society, 1991) Schlossman, M. L.; Schwartz, D. K.; Kawamoto, E. H.; Kellogg, G. J.; Pershan, Peter; Kim, M. W.; Chung, T. C.X-ray specular reflectivity from a monolayer of partially hydroxylated 1,2-polybutadiene (50% butyl alcohol random substitution) at the water/vapor interface was measured from below the critical wavevector for water, 0.0217 Å\(^{-1}\), to \(Q_z\) \(\cong\) 0.6 Å\(^{-1}\) (where \(Q_z\) is the transferred momentum normal to the interface). The sample was prepared on a Langmuir trough, and measurements were made at five different surface densities in the high-pressure, or saturated, region of the isotherm. The measured reflectivity is interpreted to obtain a profile of the average electron density p(z) as a function of distance z along the surface normal. The profile has a local maximum that is approximately 10% larger than the electron density in bulk crystalline 1,2-polybutadiene. At the highest pressures the width of the liquid/vapor interface is \(\sim\) 10% larger than the value calculated from thermal capillary waves using in situ measurements of the surface tension; however, for smaller pressures the measured and calculated values agree.Publication X-Ray Specular Reflection Studies of Silicon Coated by Organic Monolayers (Alkylsiloxanes)(American Physical Society, 1990) Tidswell, I. M.; Ocko, B. M.; Pershan, Peter; Wasserman, S. R.; Whitesides, G. M.; Axe, J. D.X-ray specular reflectivity has been used to characterize the structure of silicon–silicon-oxide surfaces coated with chemisorbed hydrocarbon monolayer films (alkylsiloxanes). Using synchrotron radiation the reflectivity was followed over 9 orders of magnitude, from grazing incidence to an incident angle of φ≊6.5°, or q=(4π/λ)sin(φ)=0.8 Å-1, allowing a spatial resolution of features approximately π/0.8≊4.0 Å along the surface normal. Analysis was performed by fitting the data to reflectivities calculated from models of the surface electron density and by calculating Patterson functions directly from the data. Although the measured reflectivities could be equally well described by different sets of model parameters, the electron densities calculated from these different parameters were remarkably alike. Inspection of the electron densities allowed identification of a layer of SiO2 (≊17-Å thick), a layer of head-group region where the alkylsiloxane adsorbs to the SiO2, and the hydrocarbon layer. Fitting the data also required that the various interfaces have different widths. The fact that the same local hydrocarbon density of 0.85 g/cm3 was observed for both fully formed and partially formed monolayers with alkane chains of varying length excluded a model in which the partially formed monolayer was made up of separated islands of well-formed monolayers. Measurements before and after chemical reaction of a monolayer in which the alkyl chain was terminated by an olefinic group demonstrated the ability to use x-ray reflectivity to characterize chemical changes. The effects of radiation damage on these types of measurements are described.Publication Incommensurate Smectic Order at the Free Surface in the Nematic Phase of \(4-N-Heptylphenyl-4'-(4"-Nitrobenzoyloxy)Benzoate (DB7NO_2)\)(American Physical Society, 1987) Ocko, B. M.; Pershan, Peter; Safinya, C. R.; Chiang, L. Y.We report x-ray reflectivity measurements on the free surface of \(4-n-heptylphenyl-4'-(4"-nitrobenzoyloxy)benzoate (DB7NO_2)\) at the nematic to smectic-A phase transition, \(T_{NA}=99.9^{\circ}C\). The free surface in the nematic phase exhibits smecticlike ordering at two q vectors, one which is commensurate with the smectic-A monolayer q vector \(q_2\). The other q vector is incommensurate corresponding to ordering at \(\sim 0.59q_2\). The commensurate peak constructively interferes with the air-liquid interface while the incommensurate peak destructively interferes. These results are compared with bulk-phase x-ray scattering measurements.Publication X-Ray Studies of Transitions between Nematic, \(Smectic-A_1, -A_2, and -A_d\) Phases(American Physical Society, 1986) Chan, K. K.; Pershan, Peter; Sorensen, L. B.; Hardouin, F.We report high-resolution x-ray scattering measurements of the critical fluctuations in mixtures of hexylphenyl cyanobenzoyloxy benzoate \((DB_6)\) and terephtal-bis-butylaniline (TBBA). The phase sequence exhibited on cooling pure \(DB_6\) (or mixtures with a low concentration of TBBA), is nematic (N) to \(smectic-A_d (A_d)\) to \(smectic-A_2 (A_2)\). Mixtures with \(\gtrsim 12\) molar percent (mol %) of TBBA have a \(smectic-A_1 (A_1)\) phase between the nematic and \(smectic-A_2\) phases. For each of the second-order transitions the critical-temperature dependence of the susceptibility and correlation lengths are fit to power laws of the form \(t^x\) where \(t=(T-T_c)/T_c\). For the \(N-A_d\) transition in pure \(DB_6\) the susceptibility exponent \(\beta=1.29 \pm 0.05\) and the parallel and perpendicular correlation-length exponents are \(\nu_{\parallel} 0.67 \pm 0.03\) and \(\nu_{\perp} =0.52 \pm 0.03\), respectively. Close to the multicritical point (12 mol% TBBA) where the second-order \(N-A_1\) line meets the first-order portion of the \(A_1-A_2\) line, the \(N-A_1\) exponents are \(\beta =1.09 \pm 0.05, \mu_{\parallel} 0.57 \pm 0.03, and \nu_{\perp} =0.43 \pm 0.03\). The correlation length anisotropy \((\nu_{\parallel} \neq \nu_{\perp})\) persists along the entire \(N-A_1\) line, with the observed exponents decreasing as the concentration approaches the multicritical point. The \(A_1-A_2\) line has both first-order and second-order regions. All the measured exponents \((\beta, \nu_{\parallel}, \nu_{\perp})\) were invariant along the second-order portion of the \(A_1-A_2\) line and the correlation-length exponents were isotropic \((\nu_{\parallel} = \nu_{\perp})\). The measured exponents were \(\beta = 1.46 \pm 0.05\), and \(\nu_{\parallel} = \nu_1 = 0.74 \pm 0.03\). These numbers also held close to the tricritical point where the \(A_1-A_2\) transition became first order.