Zhengqing Huang

A synchrotron x-ray liquid surface spectrometer

The design of a synchrotron x-ray liquid surface spectrometer at beamline X19C at the National Synchrotron Light Source is described. This spectrometer is capable of performing the full range of x-ray surface scattering techniques. A few examples of measurements made using this spectrometer are presented, including studies of organic monolayers on the surface of water and of the structure of strongly fluctuating oil–microemulsion interfaces. The measurements discussed illustrate the accuracy, resolution, and capabilities of the spectrometer.

X-ray scattering from monolayers of F(CF2)10(CH2)2OH at the water–(hexane solution) and water–vapor interfaces

Synchrotron x-ray reflectivity is used to study the structure of a monolayer of F(CF2)10(CH2)2OH self-assembled at the liquid–liquid interface from a solution in hexane placed in contact with water. It is demonstrated that this monolayer is in a high density (solid) phase below a transition temperature. This is in contrast to the conventional expectation that soluble surfactants form disordered monolayers at the liquid–liquid interface. Above the transition temperature the monolayer desorbs into the hexane solution, leaving behind an interface with a very low density of surfactants. Hysteresis in the formation of the monolayer occurs when the temperature is scanned through the transition temperature. The success of these measurements relied upon the development of a novel technique to flatten the liquid–liquid interface to the extent required for x-ray reflectivity. The measurements of F(CF2)10(CH2)2OH at the liquid–liquid interface are compared to x-ray surface diffraction measurements of monolayers of t...

Surface segregation and layering in the liquid–vapor interface of a dilute bismuth:gallium alloy

The density distribution normal to the liquid–vapor interface of a bismuth–gallium mixture (0.18% Bi and 99.82% Ga) has been studied by measurement of the x‐ray reflectivity as a function of angle of incidence. The data obtained confirm the predicted stratification of the density distribution along the normal to the interface and the segregation of the excess surface concentration of Bi into a monolayer atop the bulk mixture.

X-ray studies of polymer/gold nanocomposites

We demonstrate that x-ray reflection standing wave fluorescence spectroscopy is a highly sensitive technique for the determination of the distribution of metal nanoparticles inside polymer thin films. We investigate both the depth profile and the in-plane spatial correlation of gold nanoparticles in an asymmetric polystyrene-b-poly(2-vinylpyridine) block copolymer film. These copolymer films self-assemble into alternating, nanometer-sized domains that are, upon annealing, selectively decorated by thermally evaporated metal nanoparticles to form metal/polymer composites.

Structural studies of semifluorinated hydrocarbon monolayers at the air/water interface

We report the results of experimental studies of the equation of state and structure of water-supported monolayers of the semifluorinated hydrocarbons F(CF2)8(CH2)12H, F(CF2)10(CH2)11H and F(CF2)12(CH2)18H at temperatures from 4 to 30 °C. We find that monolayers of these molecules are metastable to stable, and that the stability improves as the length of the fluorinated block increases. Grazing-incidence X-ray diffraction measurements of the F(CF2)12(CH2)18H monolayer show that it is capable of forming ordered structures defined by hexagonal close packing of the fluorinated blocks of the adsorbed molecules. This monolayer undergoes slow relaxation, which is accompanied by increasing order and increasing macroscopic inhomogeneity. X-Ray reflectivity studies of the F(CF2)12(CH2)18H monolayer were carried out on a flat area of the surface; an analysis of the reflectivity data reveals a hydrocarbon-down, fluorocarbon-up orientation for the F(CF2)12(CH2)18H molecule.

STRUCTURE OF THE LIQUID-VAPOR INTERFACE OF A SN:GA ALLOY

The atomic distributions along the normal to, and in the plane of, the liquid–vapor interface of a dilute (9:91 at. %) Sn:Ga alloy have been studied by x-ray specular reflectivity and grazing incidence x-ray diffraction at 57 °C, 41 °C, 26 °C, and 12 °C. Surface tensions, calculated from sample shape measurements, have also been determined at the same temperatures. The results of these experiments show that the atomic distribution in the liquid–vapor interface of the Sn:Ga alloy studied has both similarities and differences with the atomic distribution in the liquid–vapor interface of a dilute Bi:Ga alloy. In both alloys the atomic distribution along the normal to the liquid–vapor interface is stratified, and the composition of the outermost layer is, sensibly, 100% of the constituent with lower (pure liquid) surface tension, respectively, Sn and Bi. In both alloys the in-plane structure of the outermost layer is that of a liquid. In the homovalent Bi:Ga alloy the excess Bi segregates into a monolayer whi...

In‐plane structure of the liquid–vapor interfaces of dilute bismuth:gallium alloys: X‐ray‐scattering studies

The in‐plane structure functions in the liquid–vapor interfaces of two bismuth–gallium alloys (0.18 at. % Bi and 0.07 at. % Bi) have been studied by grazing incidence x‐ray diffraction. The higher concentration alloy exhibits, at 25, 43, and 77 °C, segregation of Bi into nearly complete monolayers atop the bulk alloy; at each temperature the Bi monolayer has the structure of a two‐dimensional supercooled liquid. The intensity distribution of the diffuse x‐ray scattering at qz=1.0 and 1.5 A−1 is used to calculate the surface tension of the alloy at 77 °C; the value obtained is 570±30 dyn/cm. This value, which is very different from the value obtained by extrapolating the surface tension of liquid Bi to 77 °C, is related to the structure of the liquid–vapor interface. We have compared experimental grazing incidence x‐ray diffraction studies on the liquid–vapor interface of the 0.07% alloy with integral‐equation theoretical calculations on a two‐dimensional hard‐disk fluid mixture. The results suggest that t...

Urease and Hexadecylamine-Urease Films at the Air-Water Interface: An X-Ray Reflection and Grazing Incidence X-Ray Diffraction Study

We report the results of surface x-ray scattering measurements performed on urease and hexadecylamine-urease films at the air-aqueous solution interface. It is demonstrated that although hexadecylamine does not form a stable monolayer on the pure aqueous surface, it does self-assemble into a stable, well-organized structure when spread on top of a urease film at the air-water interface. It is also likely that protein and hexadecylamine domains coexist at the interface.

COHERENT X-RAY SCATTERING FROM AN OPTICAL GRATING

X-ray speckles due to scattering of partially (transverse) coherent x rays from an optical reflection grating are observed. The speckles indicate the presence of surface inhomogeneities of the grating that are otherwise undetectable with either visible laser light or transversely incoherent x-ray scattering. Qualitative analysis of the speckle patterns provide information on the surface morphology of the grating. The underlying order due to the periodicity of the grating enhances the detection of the speckles.