Qun Shen

Synthesis and characterization of PbSe quantum dots in phosphate glass

The controlled synthesis of PbSe nanocrystal quantum dots with narrow size distributions was achieved through phase decomposition of the PbSe solid solution in a phosphate glass host. Structural characterization by electron microscopy and x-ray diffraction shows that the dots have mean diameters between 2 and 15 nm. The exciton Bohr radius aB=46 nm in PbSe, so these quantum dots provide unusual and perhaps unique access to the regime of strong quantum confinement. The optical absorption spectra are compared to the predictions of a theoretical treatment of the electronic structure. The theory agrees well with experiment for dots larger than ∼7 nm, but for smaller dots there is some deviation from the theoretical predictions.

Diffractive imaging of nonperiodic materials with future coherent X-ray sources

Coherent diffractive imaging using a coherent X-ray source promises to be a useful microscopic method for imaging noncrystalline objects at high spatial resolution. In this article a simple method to estimate the coherently scattered signal as a function of resolution is presented, and it is shown that the required X-ray flux or dose scales as the inverse third power of resolution for a specimen of constant volume and density. A simulated case study using the proposed energy-recovery linac source is also presented, which confirms the estimated flux requirement.

Energy recovery linacs as synchrotron radiation sources (invited)

Practically all synchrotron x-ray sources to data are based on the use of storage rings to produce the high current electron (or positron) beams needed for synchrotron radiation (SR). The ultimate limitations on the quality of the electron beam, which are directly reflected in many of the most important characteristics of the SR beams, arise from the physics of equilibrium processes fundamental to the operation of storage rings. It is possible to produce electron beams with superior characteristics for SR via photoinjected electron sources and high-energy linacs; however, the energy consumption of such machines is prohibitive. This limitation can be overcome by the use of an energy recovery linac (ERL), which involves configuring the electron-beam path to use the same superconducting linac as a decelerator of the electron beam after SR production, thereby recovering the beam energy for acceleration of new electrons. ERLs have the potential to produce SR beams with brilliance, coherence, time structure, and source size and shape which are superior to even the best third-generation storage ring sources, while maintaining flexible machine operation and competitive costs. Here, we describe a project to produce a hard x-ray ERL SR source at Cornell University, with emphasis on the characteristics, promise, and challenges of such an ERL machine.

Performance of a hard x-ray undulator at CHESS (invited)

A 3.3‐cm period Nd‐Fe‐B hybrid undulator has been designed and successfully operated in the Cornell Electron Storage Ring (CESR). This 2‐m‐long, 123‐pole insertion device is a prototype of one of the undulators planned for the Advanced Photon Source. In dedicated operation, the undulator produced the expected brightness at 5.437 GeV with the fundamental x‐ray energy ranging from 4.3 to 7.9 keV corresponding to a change in gap from 1.5 to 2.8 cm.

New energy recovery linac source of synchrotron X-rays

Introduction Cornell University and Jefferson Laboratory physicists have been studying the properties of a new type of synchrotron radiation machine, called an Energy Recovery Linac (ERL), based on a superconducting linac configured for energy recovery with a return ring. A high energy, high current ERL could produce electron beams of order 10 microns in diameter. These could be used as an ultra-high brilliance x-ray source with many desirable characteristics, including: transversely coherent, diffraction-limited hard x-ray beams, very short (~100 fs) frequent (1 – 2 GHz) pulses, no limits on beam lifetime, and very flexible modes of operation. This combination of characteristics opens up new possibilities and could significantly advance the state of the art in x-ray research.

Triplet-phase measurements using reference-beam X-ray diffraction

Reference-beam diffraction (RBD) is a recently developed phase-sensitive X-ray diffraction technique that incorporates the principle of multiple-beam diffraction into the standard oscillating-crystal data-collection method [Shen (1998). Phys. Rev. Lett. 80, 3268-3271]. Using this technique, a large number of multiple-beam interference profiles can be recorded simultaneously on an area detector, from which a large number of triplet phases of Bragg reflections can be determined in a crystallography experiment. In this article, both the theoretical developments and the experimental procedures of the RBD technique are described in detail. Approximate theoretical approaches for RBD are outlined and simple analytical expressions are obtained that provide the basis for an automated data-analysis procedure that can be used to extract triplet phases from a large number of measured reference-beam diffraction profiles. Experimental examples are given for a variety of crystals including GaAs, tetragonal lysozyme and AlPdMn quasicrystal, using both image plates and a charge-coupled device (CCD) as the area detector. Possible uses of the measured phases for crystal structure determination are discussed as well as future prospects of the RBD technique.

Anomalous difference signal in protein crystals

A scattering-angle-dependent formula is obtained for estimation of anomalous diffraction signals in Friedel pairs in protein crystals. In addition to the angle dependences of atomic form factor and temperature factor, a correlation form factor is introduced for the first time to take into account anomalous-scattering clusters such as disulfide bonds. Good agreements with experimental observations suggest that such correlated anomalous difference signals may be visible from measurements before a complete structural solution is obtained.

Enantiomorph determination using inverse reference-beam diffraction images

It is shown that enantiomorph structures of a noncentrosymmetric crystal can be determined, in the absence of anomalous diffraction signals, by measuring two series of reference-beam oscillation diffraction patterns related by an inverse-beam geometry. The corresponding intensities of the Friedel pairs recorded on the two sets of images exhibit the characteristic three-beam interference effects that provide the unambiguous phase information. The experimental arrangement and the data-analysis procedure are demonstrated through an experimental example on tetragonal lysozyme.

Characterization of large‐area arrays of nanoscale Si tips fabricated using thermal oxidation and wet etching of Si pillars

Two‐dimensional periodic arrays containing 108 Si pillars with heights of 600–700 nm, widths of 100 nm, and repeat spacings of 300 nm have been fabricated using electron beam lithography on Si(001) substrates. These pillars have subsequently undergone wet oxidation at 800 °C and etching in hydrofluoric acid to produce an array of sharp tips with a height of ∼4000 A. The x‐ray diffraction from this array appears to be dominated by scattering from the bases of the tips. Correlated variations in tip shape, observed with scanning electron microscopy, produce a modulated diffuse background in the diffracted x‐ray intensity. These observations demonstrate the feasibility of using high‐resolution x‐ray diffraction for studying defects in large‐area arrays of periodic structures.

A complete characterization of x‐ray polarization state by combination of single and multiple Bragg reflections

We present a simple method for complete determination of the x‐ray polarization state, using just one Bragg reflection from a single‐crystal analyzer. For the linear polarization components P1 and P2, we show that the usual method of using a 90° Bragg reflection can be extended to using any Bragg reflection with 2θ≠90°. For circular component P3, we use the intensity modulation profile in an azimuthal rotation caused by the phase‐sensitive interference around a multiple‐beam Bragg reflection. The combination of the two measurements allows a straightforward complete determination of x‐ray polarization, including an unpolarized component, in a broad applicable energy range.