RESUMEN
The physisorption of N2 molecules has long been a model system of molecular adsorption. We present a low-energy electron diffraction (LEED) study of the adsorption structures and thermodynamics of monolayer N2 on Pb(1 1 1). The results indicate that the monolayer structure has a triangular incommensurate center-of-mass lattice, and that the N2-substrate interaction is weaker than that observed on other metal surfaces. The N2 monolayer undergoes a phase transition between an orientationally ordered phase (low-temperature) and an orientationally disordered phase at a temperature of 20 K. Potential energy and quasiharmonic calculations indicate that the weak N2-Pb(1 1 1) interaction is the main contributing factor for the difference in orientational order of incommensurate N2 monolayers on Pb(1 1 1) and other similar metal surfaces.
RESUMEN
The quantitative structure determination of adsorbed species on quasicrystal surfaces has so far appeared to present insurmountable problems. The normal incidence standing x-ray wave field technique offers a simple solution, without extensive data sets or large computations. Its application to quasicrystals raises several conceptual difficulties that are related to the phase problem in x-ray diffraction. We demonstrate their solution for the case of Si atoms adsorbed on the decagonal Co-rich modification of the Al-Co-Ni quasicrystal to determine the local structure, comprising 6-atom clusters in particular hollow sites.
RESUMEN
The geometry of adsorbed C(60) influences its collective properties. We report the first dynamical low-energy electron diffraction study to determine the geometry of a C(60) monolayer, Ag(111)-(2 square root of 3 x 2 square root of 3) 30 degrees -C(60), and related density functional theory calculations. The stable monolayer has C(60) molecules in vacancies that result from the displacement of surface atoms. C(60) bonds with hexagons down, with their mirror planes parallel to that of the substrate. The results indicate that vacancy structures are the rule rather than the exception for C(60) monolayers on close-packed metal surfaces.
RESUMEN
Large-scale copy number variants (CNVs) have recently been recognized to play a role in human genome variation and disease. Approaches for analysis of CNVs in small samples such as microdissected tissues can be confounded by limited amounts of material. To facilitate analyses of such samples, whole genome amplification (WGA) techniques were developed. In this study, we explored the impact of Phi29 multiple-strand displacement amplification on detection of CNVs using oligonucleotide arrays. We extracted DNA from fresh frozen lymph node samples and used this for amplification and analysis on the Affymetrix Mapping 500k SNP array platform. We demonstrated that the WGA procedure introduces hundreds of potentially confounding CNV artifacts that can obscure detection of bona fide variants. Our analysis indicates that many artifacts are reproducible, and may correlate with proximity to chromosome ends and GC content. Pair-wise comparison of amplified products considerably reduced the number of apparent artifacts and partially restored the ability to detect real CNVs. Our results suggest WGA material may be appropriate for copy number analysis when amplified samples are compared to similarly amplified samples and that only the CNVs with the greatest significance values detected by such comparisons are likely to be representative of the unamplified samples.
