A molecular dynamics study on vibration spectra of a-SiO2 surface

The charge equilibration (QEq) method was used in the molecular dynamics study of the vibration spectra on the a-SiO₂ surface. The vibrational density of states (VDOS) of a silica glass surface and partial contribution of Si and O atoms were compared with those obtained from the fixed-charge (FQ) model. The VDOS in each of the samples has two groups of vibrational bands, i.e., wide-lower-frequency band (WB) between 0-25 THz and narrow-higher-frequency band (NB) between 25 and 40 THz, which is similar to the bulk VDOS obtained experimentally. The QEq surface exhibits a peak and two shoulders while two peaks are found for the FQ surface. At the surface, an excess peak (at 1-2 THz) was observed by substracting the Debye VDOS from the calculated VDOS, which is regarded as the experimentally observed the boson peak (BP). The position of the BP is shorter than that in the case of the bulk. In the FQ surface, the VDOS consists of narrower bands, while in the QEq sample, wider bands exist due to charge variation.     

Twenty-five years of international collaboration in reference materials via ISO/REMCO

This paper looks back on a quarter century of history of the Council Committee on Reference Materials of the International Organization for Standardization – REMCO. It begins however with the period before its formation in 1976 to describe how the Committee came into being as a response to a growing need by the analytical community for the number and variety of reference materials (RMs) as well as a need for the assurance of the quality of RMs and ends with a view of REMCO activities in the near future. This is not intended as a detailed history but instead to describe the evolution of REMCO by identifying major activities and accomplishments of REMCO. Received: 21 January 2002 Accepted: 22 January 2002     

Development of standards for reliable surface analyses by ISO technical committee 201 on surface chemical analysis

The need for reliable surface analyses together with quality‐management requirements for analytical laboratories led the International Organization for Standardization (ISO) to form its Technical Committee (TC) 201 on Surface Chemical Analysis in 1991. This article describes the organization of TC 201, the strategies that have been found useful for identifying and assessing possible projects for new international standards, and the 57 international standards and other documents prepared to date by TC 201. Standards have now been developed for Auger‐electron spectroscopy, glow‐discharge spectroscopy, various types of scanning probe microscopy, secondary‐ion mass spectrometry, sputter‐depth profiling, total‐reflection X‐ray fluorescence spectroscopy, X‐ray photoelectron spectroscopy, and X‐ray reflectometry. In addition, standards have been developed with definitions of terms used in surface chemical analysis; the handling, preparation of specimens for surface analysis; information and data‐transfer formats; and methods for determining the lateral resolution of beam‐based methods of surface analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Total error and uncertainty: Friends or foes?

Guidelines ISO 17025 and ISO 15189 aim to improve the quality-assurance scheme of laboratories. Reliable analytical results are of central importance due to the critical decisions that are taken with them. ISO 17025 and ISO 15189 therefore require that analytical methods be validated and that laboratories can routinely provide the measurement uncertainty of the results of measurements. To evaluate the fitness of purpose of analytical methods, total error is increasingly applied to assess the reliability of results generated by analytical methods. However, the ISO requirement to estimate measurement uncertainty seems opposed to the concept of total error, leading to delays in laboratories implementing ISO 17025 and ISO 15189 and confusion for the analysts. This article therefore aims to clarify the divergences between total error and measurement uncertainty, but also to discuss their main similarities and emphasize their implementation.     

Defect levels in the band gap of amorphous selenium

Energy locations in the band gap have been determined for the thermally accessible levels of the negative-U defect centers in amorphous selenium. Both the temperature dependence of the steady-state photocurrents, and an analysis of emission currents in the post-transit regime of a time-of-flight transient photoconductivity experiment on the same samples, agree on the presence of defect levels at (0.42 ± 0.04) eV above the valence band mobility edge and (0.53 ± 0.06) eV below the conduction band. Both measured current levels and the resolved energy positions of the defects are subject to the Poole-Frenkel effect.     

Statistical analysis of findings in external and internal audits and assessments in a testing laboratory

A statistical analysis of findings in assessments by an accreditation body, external audits and internal audits in an accredited testing laboratory for gamma-ray spectrometry is presented. The frequencies of observations reported in different areas were examined and analysed. The differences are interpreted in terms of the varying importance associated with the different areas as perceived by auditors and assessors.     

Magnetic Properties of Iron Clusters in Silver

Time differential perturbed γ-γ angular correlation technique was used to measure the magnetic hyperfine field (MHF) at Tb sites in the intermetallic compound Tb₃In₅ using the ¹⁴⁰La → ¹⁴⁰Ce nuclear probe. The measurements were carried out in the temperature range of 8 to 295 K. Two different temperature dependent magnetic frequencies were observed below 30 K, which were assigned as ¹⁴⁰Ce substituting the two inequivalent Tb sites in the orthorhombic structure of Tb₃In₅. The temperature dependence of MHF also shows a possible deviation from an expected Brillouin-like behavior for temperatures below 18 K. A Néel transition at 27 K was observed from magnetization measurements in the samples. The magnetization as a function of the applied magnetic field was measured at two temperatures, 5 and 40 K, and the results show antiferromagnetic and a typical paramagnetic behavior, respectively. In both cases it was not observed saturation under high magnetic field.     

Potassium surface enrichment in mixed alkali glass irradiated with electrons

Sodium-potassium-silicate glass was irradiated with electrons of energy of 1600 eV. The changes in the surface composition were analyzed by means of the angular-resolved X-ray induced photoelectron spectroscopy (ARXPS). Low electron dose irradiation enriched the uppermost glass surface with alkali ions, considerably more with potassium than with sodium ions.     

Structure and properties of Ge2.5PSx glasses

Ge_2.5PS_x glasses were studied with a combination of Raman spectroscopy, nuclear magnetic resonance, and neutron diffraction. From these experiments the distribution of bonding configurations was determined, and used to explain the compositional dependence of the index of refraction and the glass transition temperature. On reducing the sulfur content of these glasses below the stoichiometric amount, the sulfur deficit is accommodated by the progressive loss of the non-bridging sulfur of SPS_3/2 groups, followed by the conversion of the resultant PS_3/2 groups into species such as P₄S₃ characterized by P-P bonding. The presence of metal-metal bonds involving germanium, found in samples with the lowest sulfur content, was found to be the most important structural feature in determining the optical response.