Arrangement Of Atoms In Crystals Research Articles

Mitigating Surface Defects in Tin-Based Perovskite Films with α-Tocopherol for Enhanced Photovoltaic Performance.

Surface defects in tin-based perovskite films disrupt the periodic arrangement of atoms in crystals, making surface atoms more susceptible to interactions with water and oxygen molecules in the surrounding environment. The diffusion of oxygen ions into the perovskite interior leads to the formation of severe bulk defects, which compromises the performance of tin-based perovskite solar cells (PSCs). As a result, surface defects are recognized as the primary source of degradation and require special attention. In this study, α-Tocopherol (also known as vitamin E) into tin-based perovskite films is introduced. Experimental results show that because of its larger volume, α-Tocopherol does not enter the perovskite lattice. Instead, it forms van der Waals and hydrogen bond interactions with the formamidine ion (FA+) and the [SnI6]4- octahedron at the perovskite terminals. Through α-Tocopherol passivation, both surface and interior oxidation of the perovskite are significantly suppressed as α-Tocopherol firmly embeds itself on the perovskite surface. Density functional theory analysis confirms the inhibition of I─Sn antisite defects (ISn) and Sn interstitial defects (Sni), which possess deep trap states within the bandgap. Ultimately, it is demonstrated that α-Tocopherol enhances the power conversion efficiency (PCE) from 9.19% to 13.14% and prolongs the lifetime of tin-based PSCs to over 50 days.

Energy dependence of interference phenomena in the forward-scattering regime of photoelectron diffraction

X-ray photoelectron diffraction is a powerful spectroscopic technique in the direct legacy of C.S. Fadley that combines high sensitivity to the arrangement of atoms in crystals and element specificity providing unique fingerprints of selected atomic sites in matter. When used with kinetic energies between 500 eV and 1500 eV, its interpretation and description is based on the fact that the atomic scattering factors are strongly forward-peaked in such a way that low-angles scattering and backscattering events are respectively dominant and almost irrelevant in the photoemission process. In this paper we aim to demonstrate with the help of multiple-scattering simulations that energy scans of high-energy (500–1500 eV) forward-scattering photoelectron diffraction can provide valuable structural and chemical information about thin epitaxial films or stacking of two-dimensional materials.

X-ray diffraction and the development of the mineral crystal chemistry

After the epochal experience of Friedrich and Knipping suggested and interpreted by Laue, the scene rapidly moved to England where the young Bragg correctly explained the results of the Munich discovery and showed how to use X-rays to determine the arrangement of atoms in crystals and, in few years, actually derived the structures of several inorganic compounds. Their comparison permitted Bragg to assert that crystals are composed of “inelastic spheres in contact” and that “it is possible to assign to the sphere representing an atom… a constant diameter.” The development of these concepts by various authors (Wasastjerna, Goldschmidt, …) led to the well-known Table of ionic radii by Pauling, who—at the same time—proposed the rules for the stability of ionic compounds, based on the concept of coordination polyhedra as building elementary modules of inorganic compounds. The stability rules of Pauling, as well as the conception, formulated by Bragg, of the structures of complex oxides as characterized by ‘close packing’ of oxygen anions, were the main guiding lines in the ‘trial and error’ procedures to determine the atomic arrangements in minerals and in inorganic compounds in general. The increasing number of known structures raised the need for proper classification schemes, largely based on elementary structural units (as the silicon tetrahedra in silicates) and the various ways of their connection. At the same time the wide basis of structural data and the increased precision and accuracy of those data stimulated the revision of the old compilations of ionic radii as well as a re-formulation of the Pauling’s rule of valence-bond balance. Examples are presented of the application of the crystal chemical concepts to some important mineralogical, geochemical and geophysical problems. An explosive growth of structural knowledge in the last decades of the twentieth century was stimulated by the new techniques of data collection, the high speed of computing and the advent of direct methods of structure solution, which resulted not only in an enormous increase in the number of known structures, but also in unraveling very complex arrangements. This increasing structural knowledge accompanied and stimulated the development of a new way to look at the arrangements of inorganic compounds, based on their modular aspects. The concept of assembling different, geometrically compatible modules, mainly structural layers, to build up complex structures has been very productive; in fact it not only consented a deep knowledge of the structural relationships inside wide families of natural and synthetic compounds, but also favored the solution of complex structural problems and permitted careful previsions of new possible structural arrangements. Few lines of future developments of the crystal chemical study of minerals are briefly presented and discussed: ‘synergy’ between mineralogists and material chemists; computational simulations of structural arrangements; new experimental approach through automated electron diffraction tomography.

Complete minigaps for effective-mass carriers in three-dimensional semiconductor superlattices

This paper examines the possibility of obtaining complete gaps in the density of states of effective-mass carriers in a semiconductor, at the conduction or valence band fundamental edge. The dispersion relations are defined for waves associated with the effective-mass electrons or holes in a direct-gap material periodically modulated on a length scale significantly larger than the base crystal interatomic spacing. The specific case of cubic interacting quantum dots of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ in a GaAs matrix is considered. A striking similarity between the effective-mass spectral density and the atomic gallium arsenide host electronic structure is found for an effective-mass superstructure derived from the zinc-blende crystal atomic arrangement. The charge carrier density has also been examined in order to shed some light on this strong similarity with atomic GaAs.

Use of dynamical diffraction effects on x-ray fluorescence to determine the polarity of GaP single crystals

The dynamical theory of x-ray diffraction, which predicts a highly structured internal field intensity during symmetric Bragg reflection from large single crystals, was applied to the \ifmmode\pm\else\textpm\fi{} (111) and \ifmmode\pm\else\textpm\fi{} (222) Bragg reflections from noncentrosymmetric GaP crystals. It was shown that theory predicts an internal field intensity with maximal and minimal surfaces. At the low-angle side of the region of total reflection, the surface of maximal field intensity is positioned to sense the least amount of charge density. The field intensity moves into the crystal during the scan of the total reflection range until it senses the maximum amount of charge density at the high-angle limit of total reflection. During scans of the (111) and (\ifmmode\bar\else\textasciimacron\fi{} 1 \ifmmode\bar\else\textasciimacron\fi{} 1 \ifmmode\bar\else\textasciimacron\fi{} 1) reflections, the field intensity moves across the atomic planes in different sequences so that the polarity of the crystal may be determined by monitoring the x-ray fluorescence from the phosphorus or gallium atoms. This polarity determination does not reguire an anomalously scattered component in the diffracted beam as have all previous determinations. Experimental phosphorus fluorescence profiles were obtained during diffraction of $\mathrm{Cu} K\ensuremath{\alpha}$ radiation from the \ifmmode\pm\else\textpm\fi{} (111) and \ifmmode\pm\else\textpm\fi{} (222) reflections. The long-wavelength phosphorus $K\ensuremath{\alpha}$ radiation provided distinct advantages in detecting the internal field motion, and the \ifmmode\pm\else\textpm\fi{} (222) reflection profiles provided internal checks on the \ifmmode\pm\else\textpm\fi{} (111) results. The experimental profiles agreed with theoretical predictions and determined the GaP crystal polarity. The polarity was also determined by the traditional anomalous dispersion method using $\mathrm{Mo} K\ensuremath{\alpha}$ radiation. The two methods gave consistent results providing experimental confirmation of the predictions of dynamical and kinematic theories of x-ray diffraction. The fluorescence technique also provides a means for determining the arrangement of atoms in crystals in a new and direct way.

Modern Physics and the Discovery of X-Rays

Among the most striking practical developments resulting from the discoveries of modern physics are radio and the atomic bomb. Both are fruits of the discovery of x-rays. In our effort to understand the nature of the world around us, prominent recent advances include knowledge of the arrangement of atoms in crystals and molecules, recognition of the several elemental particles of which atoms are built, the electron and the nucleus, the proton and the neutron, the positron, the mesotrons, and so on, and something of the way in which these particles combine to form atoms. We have learned in the theory of relativity the laws of motion of stars and atoms moving at very high speeds and more precise laws of gravitation. In the quantum theory we have greatly improved our understanding of the nature of light and x-rays and have learned how to describe the motions of atoms and the parts of atoms. All of these new findings stem from Röntgen's discovery of x-rays fifty years ago, and in their development x-rays themselves have been used as a vitally important tool. We might point out, also, how chemistry, geology, biology, and philosophy have been enriched by Röntgen's discovery. We could show how the electronic tools have stimulated the growth of industry, how the electron tube has made possible not only the radio but also the long-distance telephone and greatly improved telegraphic communication. We could describe the use of x-radiation and radium in the diagnosis and treatment of diseases. All of these have come from the discovery and use of x-rays. They are, however, part of a larger story that we cannot here take time to tell. Two years before the discovery of x-rays, in his statement of the purpose for which the new Ryerson Physical Laboratory of the University of Chicago was built, Professor A. A. Michelson noted that the fundamental principles of physics had been well established. The future of physics research, he explained, lay in making more precise measurements of the known physical constants. It was for such precision measurements that the new building was designed. This attitude toward physics was common to the leading thinkers of the period, who from the time of Galileo, through Newton, Faraday, Maxwell, and Helmholtz had developed an elegantly organized description of how events in the physical world happen. Ours was a determined world, precisely predictable according to laws that were clearly understood. X-Rays As typical of the scientific work of the period, Wilhelm Conrad Röntgen was then engaged in a careful study of the densities of various crystals. It seems that the immediate occasion for turning his interest to new fields was a publication by Lenard of an experiment with cathode rays striking a thin window from which rays (which came to be called “Lenard rays”) were observed to emerge into the surrounding air.

Prof. W. L. Bragg, O.B.E., F.R.S.

PROF. W. L. BRAGG, who has been appointed director of the National Physical Laboratory, has held the Langworthy chair of physics in the University of Manchester since 1919. Beginning his research work at Cambridge in 1912, about the time when v. Laue had announced his discovery of the diffraction of X-rays by crystals, Prof. Bragg was attracted to this field of work. One of his early papers explained, in terms of the ‘reflection’ principle, the nature of the patterns observed in the Laue photographs. Along with his father, Sir William Bragg, he developed the X-ray spectrometer. The determination of the structures of the simpler crystals followed, until in 1915, their work was recognized by the joint award of the Nobel Prize for Physics. After his appointment at Manchester, Prof. Bragg quickly established a school of workers engaged in investigations of the solid state of matter. His own particular interest has been in the orderly arrangement of atoms in crystals and in relations between atomic arrangement and the properties of a crystalline substance. The field is a very wide one, and is of interest, not only to the physicist, but also to the chemist who is seeking to understand the nature of the forces which bind atoms together and to the industrial research worker who wants information about the structure of his working materials.

The Arrangement of Atoms in Crystals

IN NATURE for January 6, p. 609, was published a note in which some figures given in my paper on “The Arrangement of Atoms in Crystals” (Phil. Mag., vol. xl., August, 1920) were contrasted with similar figures given by Wyckoff (Amer. Journ. Sci., [iv], vol. 1., pp. 317–60, November, 1920). These figures were estimates of the distances between atoms of metal, carbon, and oxygen in the crystals calcite, CaCO3, rhodochrosite, MnCO3, and siderite, FeCO3. In this note it is stated that our data differ considerably, “the deviations rising to 0.6 A. m the distance from carbon to metal.”

The arrangement of atoms in crystals : W. Lawrence Bragg. ( Phil. Maq., August, 1920.)

The arrangement of atoms in crystals : W. Lawrence Bragg. ( Phil. Maq., August, 1920.)