Anomalous Lattice Dynamics of EuSi_{2} Nanoislands: Role of Interfaces Unveiled

We report a systematic lattice dynamics study of EuSi_{2} films and nanoislands by in situ nuclear inelastic scattering on ^{151}Eu and ab initio theory. The Eu-partial phonon density of states of the nanoislands exhibits anomalous excess of phonon states at low and high energies, not present in the bulk and at the EuSi_{2}(001) surface. We demonstrate that atomic vibrations along the island-substrate interface give rise to phonon states both at low and high energies, while atomic vibrations across the island-island interface result in localized high-energy phonon modes.

X-ray Bragg magnifier microscope as a linear shift invariant imaging system: image formation and phase retrieval

We present the theoretical description of the image formation with the in-line germanium Bragg Magnifier Microscope (BMM) and the first successful phase retrieval of X-ray holograms recorded with this imaging system. The conditions under which the BMM acts as a linear shift invariant system are theoretically explained and supported by the experiment. Such an approach simplifies the mathematical treatment of the image formation and reconstruction as complicated propagation of the wavefront onto inclined planes can be avoided. Quantitative phase retrieval is demonstrated using a test sample and a proof of concept phase imaging of a spider leg is also presented.

Potential use of V-channel Ge(220) monochromators in X-ray metrology and imaging

While channel-cut crystals, in which the diffracting surfaces in an asymmetric cut are kept parallel, can provide beam collimation and spectral beam shaping, they can in addition provide beam compression or expansion if the cut is V-shaped. The compression/expansion ratio depends in this case on the total asymmetry factor. If the Ge(220) diffraction planes and a total asymmetry factor in excess of 10 are used, the rocking curves of two diffractors will have a sufficient overlap only if the second diffractor is tuned slightly with respect to the first one. This study compares and analyses several ways of overcoming this mismatch, which is due to refraction, when the Cu Kα₁ beam is compressed 21-fold in a V21 monochromator. A more than sixfold intensity increase was obtained if the matching was improved either by a compositional variation or by a thermal deformation. This provided an intensity gain compared with the use of a simple slit in a symmetrical channel-cut monochromator. The first attempt to overcome the mismatch by introducing different types of X-ray prisms for the required beam deflection is described as well. The performance of the V-shaped monochromators is demonstrated in two applications. A narrow collimated monochromatic beam obtained in the beam compressing mode was used for high-resolution grazing-incidence small-angle X-ray scattering measurements of a silicon sample with corrupted surface. In addition, a two-dimensional Bragg magnifier, based on two crossed V15 channel monochromators in beam expansion mode and tuned by means of unequal asymmetries, was successfully applied to high-resolution imaging of test structures in combination with a Medipix detector.

Excitation Dependence of Steady-State Photoluminescence in CdSe Nanocrystal Films

The excitonic and deep-level photoluminescence (PL) in CdSe nanocrystal (NC) films (wurtzite type) was studied under continuous-wave excitation as a function of excitation power, temperature, and time of photoaging. It was shown that the intensity-power dependencies are identical for excitonic and deep-level emissions in a wide temperature range. At low temperatures (80-100 K), both emissions were saturated at the laser power used, which generates more than one exciton per nanocrystal. A transition point from the linear to the saturated region was dependent on the temperature, size, and quality of the NCs. A clear inverse dependency between the intensities of excitonic and deep-level emissions was revealed at 80 K over the entire sample area. At room-temperature, the quantum yield dropped significantly and a higher laser power was needed to reach PL saturation. An increase in temperature led to worsening of the reverse dependence between excitonic and deep-level emissions, and at room-temperature, they became uncorrelated. These results can be explained by Auger recombination and also by an increase of nonradiative recombination in the surface states with increasing temperature.