Keng S. Liang

A small/wide-angle X-ray scattering instrument for structural characterization of air-liquid interfaces, thin films and bulk specimens

At the National Synchrotron Radiation Research Center, a small/wide-angle X-ray scattering (SAXS/WAXS) instrument has been installed at the BL23A beamline with a superconducting wiggler insertion device. This beamline is equipped with double Si(111) crystal and double Mo/B4C multilayer monochromators, and an Si-based plane mirror that can selectively deflect the beam downwards for grazing-incidence SAXS (GISAXS) studies of air–liquid or liquid–liquid interfaces. The SAXS/WAXS instrument, situated in an experimental hutch, comprises collimation, sample and post-sample stages. Pinholes and slits have been incorporated into the beam collimation system spanning a distance of ∼5 m. The sample stage can accommodate various sample geometries for air–liquid interfaces, thin films, and solution and solid samples. The post-sample section consists of a 1 m WAXS section with two linear gas detectors, a vacuum bellows (1–4 m), a two-beamstop system and the SAXS detector system, all situated on a motorized optical bench for motion in six degrees of freedom. In particular, the vacuum bellows of a large inner diameter (260 mm) provides continuous changes of the sample-to-detector distance under vacuum. Synchronized SAXS and WAXS measurements are realized via a data-acquisition protocol that can integrate the two linear gas detectors for WAXS and the area detector for SAXS (gas type or Mar165 CCD); the protocol also incorporates sample changing and temperature control for programmable data collection. The performance of the instrument is illustrated via several different measurements, including (1) simultaneous SAXS/WAXS and differential scanning calorimetry for polymer crystallization, (2) structural evolution with a large ordering spacing of ∼250 nm in a supramolecular complex, (3) SAXS for polymer blends under in situ drawing, (4) SAXS and anomalous SAXS for unilamellar lipid vesicles and metalloprotein solutions, (5) anomalous GISAXS for oriented membranes of Br-labeled lipids embedded with peptides, and (6) GISAXS for silicate films formed in situ at the air–water interface.

An instrument for time-resolved and anomalous simultaneous small- and wide-angle X-ray scattering (SWAXS) at NSRRC

A SWAXS (small- and wide-angle X-ray scattering) instrument was recently installed at the wiggler beamline BL17B3 of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The instrument, which is designed for studies of static and dynamic nanostructures and correlations between the nano (or meso) structure (SAXS) and crystalline structure (WAXS), provides a flux of 1010–1011 photon s−1 at the sample at energies between 5 and 14 keV. With a SAXS area detector and a WAXS linear detector connected to two data acquisition systems operated in master–slave mode, the instrument allows one to perform time-resolved as well as anomalous scattering measurements. Data reduction algorithms have been developed for rapid processing of the large SWAXS data sets collected during time-resolved measurements. The performance of the instrument is illustrated by examples taken from different classes of ongoing projects: (i) time-resolved SAXS/WAXS/differential scanning calorimetry (DSC) with a time resolution of 10 s on a semicrystalline poly(hexamethylene terephthalate) sample, (ii) anomalous SAXS/WAXS measurements on a nanoparticulate PtRu catalyst, and (iii) grazing-incidence SAXS of a monolayer of oriented semiconductor quantum wires, and humidity-controlled ordering of Alamethicin peptides embedded in an oriented lipid membrane.

Electron diffractive imaging of nano-objects using a guided method with a dynamic support

We present a phase recovery technique that utilizes a guiding method with a dynamic support to reconstruct the shape and exit wave of a single MgO nanoparticle in a coherent electron diffraction experiment. The proposed method provides an optimal solution deduced from the electron diffraction pattern alone. The recovered shape has spatial resolution 3.1 nm. The complex exit wave encodes the projected atomic structure of the nanocrystal with resolution about 0.15 nm, and agrees with a multislice simulation. The possibility of imaging nanosized objects at diffraction-limited resolution using a field emission electron microscope is thus demonstrated.

Size dependence of tetrahedral bond lengths in CdSe nanocrystals

The structural characteristics of organically passivated CdSe nanocrystals (NCs) were investigated with x-ray diffraction and extended x-ray absorption fine structure. As the NC size decreases, the axial bond length R(1) for an atomic tetrahedron extends but the equatorial bond length R(2) contracts, with a similar tendency of distortion for the lattice parameters of the wurtzite structure. The authors suggest that the observed hexagonal distortion is attributed to the surface stress of the NCs related to the organic passivation effect and the relaxation of atomic positions at the stacking fault interface.

Electron coherent diffraction tomography of a nanocrystal

Coherent diffractive imaging (CDI) with electron or x-ray sources is a promising technique for investigating the structure of nanoparticles down to the atomic scale. In electron CDI, a two-dimensional reconstruction is demonstrated using highly coherent illumination from a field-emission gun as a source of electrons. In a three-dimensional (3D) electron CDI, we experimentally determine the morphology of a single MgO nanocrystal using the Bragg diffraction geometry. An iterative algorithm is applied to invert the 3D diffraction pattern about a (200) reflection of the nanoparticle measured at an angular range of 1.8°. The results reveal a 3D image of the sample at ∼8 nm resolution, and agree with a simulation. Our work demonstrates an alternative approach to obtain the 3D structure of nanocrystals with an electron microscope.

Grazing Incidence Small Angle X-Ray Scattering Study on Low Dielectric Thin Films

Highly porous silica films with pore size in the nanometer scale are being extensively studied as potential candidates for interlevel dielectrics. Because these dielectric materials appear in the form of thin films with a thickness of only several thousand Angstroms, conventional techniques are difficult to be readily applied to study their structure and porosity. We employed small angle scattering in the grazing incidence geometry in this study. Using high resolution xray beamline with synchrotron radiation source, we demonstrate that the small angle x-ray scatteirng (SAXS) data of the porous films can be obtained. The structure of sol-gel derived silica - xerogel films on silicon substrate studied by specular reflectivity and grazing incidence small angle x-ray scattering (GISAXS) will be presented.

The Structure and Composition of Semiconductor Quantum Materials

The size, shape, strain distribution, compositional profile, and spatial distribution are the critical factors determining the electronic levels and thus the physical properties of semiconductor nanostructures. For MBE-grown quantum dots, the interplay between lattice mismatch, surface segregation, interface diffusion and various kinetic effects makes their formation mechanism very complicated. In fact, the structure and the formation mechanism of these self-assembled quantum dots are still not well understood. In this work, we applied grazing incidence X-ray scattering methods including reciprocal space map and small angle X-ray scattering to study the strain field, shape and spatial distribution of InGaAs quantum dots. In particular, we focus on the application of grazing incidence resonant X-ray scattering technique to determine the compositional distribution within the quantum dots and its correlation with strain field. With the data obtained, we profiled the distribution of elastic energy within the dots.