Spectroscopy and microscopy of transition metal chalcogenide nanomaterials

This thesis details the physical and electronic structure of several technologically important transition metal chalcogenides (TMCs) using a combination of transmission electron microscopy (TEM) and surface science experimental techniques. The materials studied include CuxTe and CdTe, which find application in high efficiency, low weight photovoltaic devices. CuxTe alloys are frequently used as an electrical back contact in high efficiency CdTe photovoltaics. Here, we examine the alloying of Te on the Cu(111), polycrystalline Cu and Cu(643) surfaces. Chapter 3 of this thesis shows that the alloying of Te and Cu(111) is facile at room temperature, contrary to previous reports. Two distinct surface phases exist, depending on Te surface concentration. Below a coverage of 0.33 monolayers (MLs) of Te a surface substitutional alloy (SSA) is found to exist, where a Te adatom substituted for a surface Cu atom. For Te coverages greater than 0.66 ML, an unusual Cu3Te2 alloy continually grows on the surface, stabilised by a good lattice match to the Cu(111) substrate. The surface alloying of the Cu-Te system displays an intriguing dependence on the surface termination of the Cu substrate. Of the three Cu substrates studied here, Cu(111), Cu(643)R and polycrystalline Cu, a 1 ML film of Te gave ordered alloy structures with stoichiometries of Cu3Te2, CuTe and Cu2Te, respectively. In chapter 4, the study of thin film photovoltaics is extended to the deposition of CdTe onto Cu and CuxTe substrates. CdTe is observed to grow three dimensionally on Cu(111), Cu3Te2 and Cu2Te. Cu+ diffusion, crucial for photovoltaic performance, is detected for CdTe thicknesses greater than 2 ML and is assigned, predominately, to Cu2Te crystallites forming within the CdTe layer, with a minor amount of Cu residing in interstitial sites in the host CdTe structure. Chapter 5 describes the alloying of Te with a intrinsically chiral surface, Cu(643)R, the first study of its kind. The results of this study reveal that step mediated alloying occurs between Cu and Te with significant faceting of the surface. Two ordered CuTe alloy phases were observed for sub-monolayer Te coverages. The low coverage alloy exists for Te coverages between 0.18 ML and 0.45 ML and has a chiral unit cell. The high coverage alloy exists for Te coverages between 0.45 ML and 1.5 ML and has an achiral unit cell. The atomic positions of these surface alloys are tentatively interpreted from the scanning tunnelling microscopy (STM) images. In contrast to the thin film experiments in chapters 3-5, chapter 6 describes a study of TaS3 nanoribbons. These studies reveal that the nanoribbons have a distinct core-shell type structure. Characterisation with surface science techniques shows that the shell is nonstoichiometric and amorphous while TEM shows a crystalline core to the material. Interestingly, the TaS3 are observed to be unstable when interfaced on a Au substrate, with the shell persistently losing S to the substrate, which have potential implications in device integration

Nonlocality of Kohn-Sham exchange-correlation fields in dielectrics

The theory of the macroscopic field appearing in the Kohn-Sham exchange-correlation potential for dielectric materials, as introduced by Gonze, Ghosez and Godby, is reexamined. It is shown that this Kohn-Sham field cannot be determined from a knowledge of the local state of the material (local crystal potential, electric field, and polarization) alone. Instead, it has an intrinsically nonlocal dependence on the global electrostatic configuration. For example, it vanishes in simple transverse configurations of a polarized dielectric, but not in longitudinal ones.Comment: 4 pages, two-column style with 2 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/index.html#dv_gg

Effects of long-term soluble vs. insoluble dietary fiber intake on high-fat diet-induced obesity in C57BL/6J mice

Although most of the proposed beneficial effects of fiber consumption have been attributed to viscous and gel-forming properties of soluble fiber, it is mainly insoluble cereal fiber and whole grains that are strongly associated with reduced diabetes risk in prospective cohort studies, indicating that other unknown mechanisms are likely to be involved. We performed a long-term study investigating potential protective effects of adding soluble guar fiber (10% w/w) vs. insoluble cereal fiber (10% w/w) to an isoenergetic and macronutrient matched high-fat diet in obesity-prone C57BL/6J mice. After 45 weeks, mice fed soluble vs. insoluble fiber showed both significantly increased body weight (41.8±3.0 vs. 33.6±1.5 g, P=.03) and elevated markers of insulin resistance. In mice fed soluble fiber, energy loss via the feces was significantly lower and colonic fermentation with production of short chain fatty acids (SCFA) was markedly increased. Gene expression analysis in white adipose tissue showed significantly increased levels of the fatty acid target G-protein coupled receptor-40 in soluble fiber-fed mice. Liver gene expression in the insoluble fiber group showed a pattern consistent with increased fatty acid oxidation. The present results show that soluble vs insoluble dietary fiber added to a high-fat, Western-style diet differently affected body weight and estimates of insulin sensitivity in obesity-prone mice. Soluble fiber intake with increased SCFA production significantly contributed to digested energy, thereby potentially outweighing the well known short-term beneficial effects of soluble fiber consumption

The blocks of the partition algebra in positive characteristic

In this paper we describe the blocks of the partition algebra over a field of positive characteristic

Polarization and Strong Infra-Red Activity in Compressed Solid Hydrogen

Under a pressure of ~150 GPa solid molecular hydrogen undergoes a phase transition accompanied by a dramatic rise in infra-red absorption in the vibron frequency range. We use the Berry's phase approach to calculate the electric polarization in several candidate structures finding large, anisotropic dynamic charges and strongly IR-active vibron modes. The polarization is shown to be greatly affected by the overlap between the molecules in the crystal, so that the commonly used Clausius-Mossotti description in terms of polarizable, non-overlapping molecular charge densities is inadequate already at low pressures and even more so for the compressed solid.Comment: To appear in Phys. Rev. Let

Density-Polarization Functional Theory of the response of a periodic insulating solid to an electric field.

The response of an infinite, periodic, insulating, solid to an infinitesimally small electric field is investigated in the framework of Density Functional Theory. We find that the applied perturbing potential is not a unique functional of the periodic density change~: it depends also on the change in the macroscopic {\em polarization}. Moreover, the dependence of the exchange-correlation energy on polarization induces an exchange-correlation electric field. These effects are exhibited for a model semiconductor. We also show that the scissor-operator technique is an approximate way of bypassing this polarization dependence.Comment: 11 pages, 1 Fig

How Exposures to Biologics Influence the Induction and Incidence of Asthma

A number of environmental factors can affect the development and severity of allergy and asthma; however, it can be argued that the most significant inhaled agents that modulate the development of these conditions are biologics. Sensitization to environmental allergens is an important risk factor for the development of asthma. Innate immune responses are often mediated by receptors on mononuclear cells whose primary ligands arise from microorganisms. Many pathogens, especially viruses, target epithelial cells and affect the host immune response to those pathogens. The acquired immune response to an allergen is influenced by the nature of the innate immune system. Products of innate immune responses to microbes promote T(H)1-acquired responses. In the absence of T(H)1 responses, T(H)2 responses can dominate. Central to T(H)1/T(H)2 balance is the composition of contaminants that derive from microbes. In this review we examine the biology of the response to allergens, viruses, and bacterial products in the context of the development of allergy and asthma

Cloud condensation nuclei in pristine tropical rainforest air of Amazonia: size-resolved measurements and modeling of atmospheric aerosol composition and CCN activity

Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) are key elements of the hydrological cycle and climate. We have measured and characterized CCN at water vapor supersaturations in the range of <i>S</i>=0.10–0.82% in pristine tropical rainforest air during the AMAZE-08 campaign in central Amazonia. <br><br> The effective hygroscopicity parameters describing the influence of chemical composition on the CCN activity of aerosol particles varied in the range of &kappa;&asymp;0.1–0.4 (0.16&plusmn;0.06 arithmetic mean and standard deviation). The overall median value of &kappa;&asymp;0.15 was by a factor of two lower than the values typically observed for continental aerosols in other regions of the world. Aitken mode particles were less hygroscopic than accumulation mode particles (&kappa;&asymp;0.1 at <i>D</i>&asymp;50 nm; &kappa;&asymp;0.2 at <i>D</i>&asymp;200 nm), which is in agreement with earlier hygroscopicity tandem differential mobility analyzer (H-TDMA) studies. <br><br> The CCN measurement results are consistent with aerosol mass spectrometry (AMS) data, showing that the organic mass fraction (<i>f</i>_org) was on average as high as ~90% in the Aitken mode (<i>D</i>&le;100 nm) and decreased with increasing particle diameter in the accumulation mode (~80% at <i>D</i>&asymp;200 nm). The κ values exhibited a negative linear correlation with <i>f</i>_org (<i>R</i>²=0.81), and extrapolation yielded the following effective hygroscopicity parameters for organic and inorganic particle components: &kappa;_org&asymp;0.1 which can be regarded as the effective hygroscopicity of biogenic secondary organic aerosol (SOA) and &kappa;_inorg&asymp;0.6 which is characteristic for ammonium sulfate and related salts. Both the size dependence and the temporal variability of effective particle hygroscopicity could be parameterized as a function of AMS-based organic and inorganic mass fractions (&kappa;_p=&kappa;_org&times;<i>f</i>_org +&kappa;_inorg&times;<i>f</i>_inorg). The CCN number concentrations predicted with &kappa;_p were in fair agreement with the measurement results (~20% average deviation). The median CCN number concentrations at <i>S</i>=0.1–0.82% ranged from <i>N</i>_CCN,0.10&asymp;35 cm^&minus;3 to <i>N</i>_CCN,0.82&asymp;160 cm^&minus;3, the median concentration of aerosol particles larger than 30 nm was <i>N</i>_CN,30&asymp;200 cm^&minus;3, and the corresponding integral CCN efficiencies were in the range of <i>N</i>_CCN,0.10/<i>N</i>_CN,30&asymp;0.1 to <i>N</i>_CCN,0.82/<i>N</i>_CN,30&asymp;0.8. <br><br> Although the number concentrations and hygroscopicity parameters were much lower in pristine rainforest air, the integral CCN efficiencies observed were similar to those in highly polluted megacity air. Moreover, model calculations of <i>N</i>_CCN,<i>S</i> assuming an approximate global average value of &kappa;&asymp;0.3 for continental aerosols led to systematic overpredictions, but the average deviations exceeded ~50% only at low water vapor supersaturation (0.1%) and low particle number concentrations (&le;100 cm^&minus;3). Model calculations assuming a constant aerosol size distribution led to higher average deviations at all investigated levels of supersaturation: ~60% for the campaign average distribution and ~1600% for a generic remote continental size distribution. These findings confirm earlier studies suggesting that aerosol particle number and size are the major predictors for the variability of the CCN concentration in continental boundary layer air, followed by particle composition and hygroscopicity as relatively minor modulators. <br><br> Depending on the required and applicable level of detail, the information and parameterizations presented in this paper should enable efficient description of the CCN properties of pristine tropical rainforest aerosols of Amazonia in detailed process models as well as in large-scale atmospheric and climate models

Polarized QPOs from the INTEGRAL polar IGRJ14536-5522 (=Swift J1453.4-5524)

We report optical spectroscopy and high speed photometry and polarimetry of the INTEGRAL source IGRJ14536-5522 (=Swift J1453.4-5524). The photometry, polarimetry and spectroscopy are modulated on an orbital period of 3.1564(1) hours. Orbital circularly polarized modulations are seen from 0 to -18 per cent, unambiguously identifying IGRJ14536-5522 as a polar. Some of the high speed photometric data show modulations that are consistent with quasi-periodic oscillations (QPOs) on the order of 5-6 minutes. Furthermore, for the first time, we detect the (5-6) minute QPOs in the circular polarimetry. We discuss the possible origins of these QPOs. We also include details of HIPPO, a new high-speed photo-polarimeter used for some of our observations.Comment: Accepted for publication by MNRAS. The paper contains 7 figures and 1 tabl

Warp propagation in astrophysical discs

Astrophysical discs are often warped, that is, their orbital planes change with radius. This occurs whenever there is a non-axisymmetric force acting on the disc, for example the Lense-Thirring precession induced by a misaligned spinning black hole, or the gravitational pull of a misaligned companion. Such misalignments appear to be generic in astrophysics. The wide range of systems that can harbour warped discs - protostars, X-ray binaries, tidal disruption events, quasars and others - allows for a rich variety in the disc's response. Here we review the basic physics of warped discs and its implications.Comment: To be published in Astrophysical Black Holes by Haardt et al., Lecture Notes in Physics, Springer 2015. 19 pages, 2 figure