C60-Propylamine Adduct Monolayers at the Gas/Water Interface: A Brewster Angle Microscopy and X-Ray Scattering Study

Brewster angle microscopy (BAM), x-ray specular reflectivity and grazing-incidence x-ray diffraction (GID) studies of C60-propylamine adduct monolayers at the gas/water interface as a function of molecular area are reported. At large molecular areas (A> ∼ 150 Å2/molecule), BAM images reveal macroscopic heterogeneity in the film, consisting of the coexistence between regions covered with uniform solidlike monolayer and bare water surface. After compression to a limiting molecular area of 150 Å2/molecule, the film is observed to be homogeneous, with the uniform monolayer covering the entire available surface. Both the x-ray reflectivity results and the GID patterns are consistent with the formation of a uniform monolayer at A ∼ 150 Å2/molecule, while the little dependence that the GID patterns have on the molecular area for A> ∼ 150 Å2/molecule is consistent with the heterogeneity in the film. Upon further compression to higher densities (A< ∼ 120 Å2/molecule), the x-ray reflectivity results suggest the formation of a partial layer either at the molecule/gas interface or at the molecule/water interface. In this high density regime, the shift in the observed GID pattern with molecular area is much smaller than would be expected if the film were to remain a homogeneous monolayer, also consistent with the formation of an inhomogeneous partial layer. The analysis of the broad GID pattern observed from a uniform monolayer in terms of a model 2D radial distribution function, implies a short range positional correlation, extending to only a few molecular distances. The average nearest neighbor distance (d ∼ 13 Å), extracted from the GID analysis, is consistent with the limiting molecular area (A ∼ 150 Å2/molecule) assuming local hexagonal packing. These results together with the sharp facets observed in the BAM images demonstrate that the monolayer when uniform is a two-dimensional amorphous solid.Engineering and Applied Science

Monolayer/Bilayer Transition in Langmuir Films of Derivatized Gold Nanoparticles at the Gas/Water Interface: An X-Ray Scattering Study

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.Engineering and Applied Science

Toward large-area sub-arcsecond x-ray telescopes II

In order to advance significantly scientific objectives, future x-ray astronomy missions will likely call for x-ray telescopes with large aperture areas (≈3 m[superscript 2]) and fine angular resolution (≈1[superscript 2 ]). Achieving such performance is programmatically and technologically challenging due to the mass and envelope constraints of space-borne telescopes and to the need for densely nested grazing-incidence optics. Such an x-ray telescope will require precision fabrication, alignment, mounting, and assembly of large areas (≈600 m2) of lightweight (≈2 kg/m[superscript 2] areal density) high-quality mirrors, at an acceptable cost (≈1 M$/m[superscript 2] of mirror surface area). This paper reviews relevant programmatic and technological issues, as well as possible approaches for addressing these issues-including direct fabrication of monocrystalline silicon mirrors, active (in-space adjustable) figure correction of replicated mirrors, static post-fabrication correction using ion implantation, differential erosion or deposition, and coating-stress manipulation of thin substrates

Internal segregation and side chain ordering in hairy-rod polypeptide monolayers at the gas/water interface: An x-ray scattering study

We report studies of the structure and packing of Langmuirmonolayers (LMs) of polypeptide poly(γ-4-(n-hexadecyloxy)benzyl α,L-glutamate) (C16–O–PBLG) on the surface of water. The molecule is a “hairy rod” and consists of side attachments of hexadecyloxy chains (–O–C16) to the rigid rod-like core made up of α-helical poly(γ-benzyl L-glutamate) (PBLG). Measurements include surface pressure (Π) versus area/monomer (A) isotherms, x-ray specular reflectivity (XR), and grazing incidence diffraction(GID). In contrast to the LM of bare PBLG on water, which undergoes a monolayer/bilayer transition with increasing Π, monolayers of C16–O–PBLG remain stable up to the highest densities. On the basis of XR and GID results, the structure of the C16–O–PBLG monolayer is characterized by the following main features. First, hydrophobicity causes the –O–C16 chains to segregate towards the film/gas interface and away from water and the PBLG cores, which sit parallel to and near the water/film interface. Since the attachment position of some of the side chains is at the core/water interface, the segregation forces these chains into the space between neighboring core rods. Compression associated with increasing Π thickens the film but the internally segregated structure is maintained for all Π (i.e., >∼30 dyne/cm). Second, the C16–O–PBLG rods form domains in which the rods are aligned parallel to each other and to the interface. The correlation length for the interhelix positional order of the rods is short and typically comparable to or less than the length of the rods. With increasing Π the spacing d between nearest-neighbor rods decreases linearly with A at high Π, indicating a direct correspondence between the macroscopic compressibility and the microscopic interhelix compressibility. Third, as Π increases past ∼5 dyne/cm, the local packing of tethered –O–C16 chains displays the same herringbone (HB) order that is common for high-density bulk and monolayer phases of alkyl chains. Various features of the observed GID peaks also imply that the HB order of –O–C16 chains is oriented with respect to the helical axes of aligned PBLG cores. We propose that the HB order is established initially by one-dimensionally confined chains between aligned rods at low Π and grows laterally with compression

High-precision figure correction of x-ray telescope optics using ion implantation

ABSTRACT Achieving both high resolution and large collection area in the next generation of x-ray telescopes requires highly accurate shaping of thin mirrors, which is not achievable with current technology. Ion implantation offers a promising method of modifying the shape of mirrors by imparting internal stresses in a substrate, which are a function of the ion species and dose. This technique has the potential for highly deterministic substrate shape correction using a rapid, low cost process. Wafers of silicon and glass (D-263 and BK-7) have been implanted with Si+ ions at 150 keV, and the changes in shape have been measured using a Shack-Hartmann metrology system. We show that a uniform dose over the surface repeatably changes the spherical curvature of the substrates, and we show correction of spherical curvature in wafers. Modeling based on experiments with spherical curvature correction shows that ion implantation could be used to eliminate higher-order shape errors, such as astigmatism and coma, by using a spatially-varying implant dose. We will report on progress in modelling and experimental tests to eliminate higher-order shape errors. In addition, the results of experiments to determine the thermal and temporal stability of implanted substrates will be reported

Structure of Poly(γ\gamma-Benzyl-L-Glutamate) Monolayers at the Gas–Water Interface: A Brewster Angle Microscopy and X-Ray Scattering Study

This paper reports Brewster angle microscopy (BAM), x-ray specular reflectivity (XR), grazing incidence diffraction (GID) and off-specular diffuse scattering (XOSDS) measurements of Langmuir monolayers formed on water by both mono- and polydisperse samples of $\alpha$-helical poly($\gamma$-benzyl L-glutamate) (PBLG) as a function of area/monomer A. The microscopic behavior does not exhibit any discernible effects due to differing dispersity. At low surface densities (A > $\sim$ 21  Å$^2$/monomer, surface pressure $\Pi$ = 0), BAM images reveal partial surface coverage by solidlike monolayer islands. GID measurements show an interhelix peak corresponding to a local parallel alignment of rodlike PBLG molecules, indicating their tendency to aggregate laterally without external pressure. Compression to A < 21  Å$^2$/monomer first leads to full and uniform surface coverage by the monolayer, followed by a steep rise in $\Pi$ that is accompanied by a decrease in the interhelix distance. Further compression results in a plateau of constant $\Pi$ in the $\Pi$-A isotherm ( $\sim$ 11.5 < A < $\sim$ 18.5  Å$^2$/monomer, $\Pi$ $\sim$ 9 dyn/cm), which has previously been attributed to a first-order monolayer–bilayer transition. The interfacial electron density profiles determined by the XR measurements on both sides of the coexistence plateau provide direct evidence for this transition. On the basis of x-ray scattering results, the film on the high-density side of the plateau is shown to consist of a newly formed incomplete and incommensurate second layer that sits on top of and has lower average density than a homogeneous first layer. GID measurements indicate that the second layer can be characterized by larger interhelix d-spacing than the first layer, while XOSDS measurements on the bilayer suggest that the second layer is microscopically inhomogeneous. For both mono- and bilayers, the analysis of observed GID peak widths indicates that the extent of lateral positional correlations between parallel PBLG rods ranges from a few to no more than $\sim$15 interhelix distances, implying short-range order.Engineering and Applied Science