The epitaxial growth of thin metal films studied by helium atom scattering

Available from British Library Document Supply Centre-DSC:D064070 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Developments in the analysis of helium atom scattering data: the sequential filtering Lomb periodogram technique

A generalized methodology to analyse lattice-rod scans from helium atom scattering (HAS) has been developed, allowing in situ measurement of interlayer spacing during thin film growth. A Lomb periodogram technique is used to analyse the interference fringes observed in a lattice-rod scan of a stepped crystal surface. The algorithm is ideally suited to the small, unevenly sampled HAS data sets typically collected and has the advantage that the significance of peaks in the resulting frequency distribution can be calculated. We show how the method can be used to assess the significance of each interlayer spacing quantitatively. Using the growth of cobalt on copper(111) as an example, the statistical significance of the surface step height distribution is calculated directly. This paper represents a considerable improvement over existing fast Fourier transform-based methodologies

Multiple scattering in scanning helium microscopy

Using atom beams to image the surface of samples in real space is an emerging technique that delivers unique contrast from delicate samples. Here, we explore the contrast that arises from multiple scattering of helium atoms, a specific process that plays an important role in forming topographic contrast in scanning helium microscopy (SHeM) images. A test sample consisting of a series of trenches of varying depths was prepared by ion beam milling. SHeM images of shallow trenches (depth/width 1) exhibited an enhanced intensity. The scattered helium signal was modeled analytically and simulated numerically using Monte Carlo ray tracing. Both approaches gave excellent agreement with the experimental data and confirmed that the enhancement was due to localization of scattered helium atoms due to multiple scattering. The results were used to interpret SHeM images of a bio-technologically relevant sample with a deep porous structure, highlighting the relevance of multiple scattering in SHeM image interpretation

Nano-pathways: Bridging the divide between water-processable nanoparticulate and bulk heterojunction organic photovoltaics

Here we report the application of a conjugated copolymer based on thiophene and quinoxaline units, namely poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1), to nanoparticle organic photovoltaics (NP-OPVs). TQ1 exhibits more desirable material properties for NP-OPV fabrication and operation, particularly a high glass transition temperature (Tg) and amorphous nature, compared to the commonly applied semicrystalline polymer poly(3-hexylthiophene) (P3HT). This study reports the optimisation of TQ1:PC71BM (phenyl C71 butyric acid methyl ester) NP-OPV device performance by the application of mild thermal annealing treatments in the range of the Tg (sub-Tg and post-Tg), both in the active layer drying stage and post-cathode deposition annealing stage of device fabrication, and an in-depth study of the effect of these treatments on nanoparticle film morphology. In addition, we report a type of morphological evolution in nanoparticle films for OPV active layers that has not previously been observed, that of PC71BM nano-pathway formation between dispersed PC71BM-rich nanoparticle cores, which have the benefit of making the bulk film more conducive to charge percolation and extraction

Probing surfaces with thermal He atoms: scattering and microscopy with a soft touch

Helium atom scattering (HAS) is a well established technique, particularly suited for the investigation of insulating and/or fragile materials and light adsorbates including hydrogen. In contrast to other beam techniques based on Xrays or electrons, low energy (typically less than 100 meV) He atoms are scattered by the tail of the electron density distribution which spill out from a surface, therefore HAS is strictly a nonpenetrating technique without any sample damage. HAS has been used to investigate structural properties of crystalline surfaces, including precise determination of atomic step heights, for monitoring thin film growth, to study surface transitions such as surface melting and roughening and for determining the presence and properties of adsorbates. Energy resolved HAS can provide information about surface vibrations (phonons) in the meV range and surface diffusion. This chapter provides a brief introduction to HAS with an outlook on a new, promising surface science technique: Neutral Helium Microscopy