1,720,996 research outputs found
Molecular Simulations of Nanoscale Transformations in Ionic Semiconductor Nanocrystals
No full textThe aim of the study described in this thesis is to obtain a profound understanding of transformations in NCs at the atomic level, by performing molecular simulations for such transformations, and by comparing the simulation results with available experimental high resolution transmission electron microscopy (HRTEM) data to validate the simulations and to reveal underlying physical mechanisms. These transformations include structural and morphological transitions and cation exchange processes in ionic nanocrystals (II-VI and IV-VI semiconductors). The main simulation method used is classical Molecular Dynamics (MD) simulation. First principles density functional theory (DFT) calculations were used to develop empirical force fields that are able to accurately reproduce phase transitions. Using these newly developed force fields, large scaled classical MD simulations were carried out and linked to HRTEM experiments. The partially charged rigid ion model (PCRIM) was chosen for the force fields. This PCRIM approach has a simple functional form with a few number of parameters and has a clear physical meaning for ionic crystals. To simulate cation exchange in colloidal NC systems at the NC/solution interface, we used a combination of all-atom force fields and a coarse-grained model. In Chapter 2, an ab-initio based force field for ZnO is developed within the framework of the PCRIM approach. The values of the partial charges were determined by Bader charge analysis of DFT calculations on various ZnO phases. Beside Coulombic interactions, only short-ranged pairwise interatomic interactions were included. An initial guess of the parameters of the short-ranged pair potentials were first obtained by the lattice inversion method. The parameters were further adjusted by an ab-initio potential surface fitting procedure. The new ZnO force field has a very simple functional form is able to accurately reproduce several important physical properties of ZnO materials. These physical properties include the lattice parameters and phase stability of several ZnO polymorphs, as well as the elastic constants, bulk moduli, phonon dispersion, and melting points of wurtzite ZnO. The transition pressure of the wurtzite-to-rocksalt transition calculated with the force field equals 12.3 GPa, in agreement with experimental measurements and DFT calculations. A wurtzite-to-honeycomb phase transition is predicted at an uniaxial pressure of 8.8 GPa. We found a rational and effective way to derive force fields with simple functional forms for accurate simulations of phase transitions in ionic crystals. In Chapter 3, we developed a transferable force field for CdS-CdSe-PbSPbSe solid systems. The selection of the force field and the fitting procedure are similar to that of the ZnO force field in Chapter 2. The challenges when developing this force field were to maintain the transferability of this force field for four materials (CdS, CdSe, PbS, and PbSe) and to describe their mixed phases. This was solved by assuming that different cations/anions have the same values of the partial charges, and that shortranged interatomic interactions between two cations/anions are the same in different materials. For the mixed phases, DFT calculations of the mixed phases were included in both the training and validation sets. This work is the first step for further simulation studies of these II-VI and IV-VI semiconductor NCs and heteronanocrystals (HNCs). In Chapter 4, a thermally induced morphological and structural transition of CdSe NCs was investigated using MD simulations. In MD simulations, a CdSe nanosphere with the ZB structure transforms into a tetrapodlike morphology at 800 K. In a CdSe tetrapod, four WZ legs attach to the {111} facets of a tetrahedral ZB core. This transformation is achieved by a layer-by-layer slip of the ZB-{111} bilayer. Simulations show that the slips are mediated by the formation of Cd vacancies on the surface of the NCs to overcome the potentially large energy barriers associated with slip. The morphology of the annealed NCs is found to be temperature and size-dependent. An octapod-like morphology is found in NCs with a relatively large NC size and in a certain range of the heating temperature. Surprisingly, nanoscale transformations of CdSe NCs have been directly observed in HRTEM in situ heating experiments. Our findings provide a simple method to modify the morphology of ionic NCs and can potentially be used in the synthesis of branched NCs. The cation exchange process of PbSe/CdSe HNCs has been investigated by HRTEM in situ heating experiments in combination with MD simulations and DFT calculations in Chapter 5. In the HRTEM experiments, we bserved that Cd atoms in PbSe/CdSe nanodumbbells (CdSe rods with one or two PbSe tip(s)) are replaced by Pb atoms. The exchange rate depends on the heating temperature and the amount of Pb atoms present in the system. Sometimes, fully converted PbSe nanodumbells can be observed. MD simulations were performed to investigate the mechanism of this cation exchange process. It was found that the the CdSe domains near the PbSe/CdSe interfaces have significant structural disorder. These findings are in line with the experimental observation that the exchange process proceeds in a layer-by-layer fashion along the WZ-direction. We concluded that cation exchange in PbSe/CdSe HNCs is mediated by the local structural disorder which enables the formation of vacancies and accelerated the motion of cations. In Chapter 6, a coarse-grained psuedoligand model was introduced to simulate cation exchange in PbS colloidal NCs taking into account the cation-solvent interactions. Modelling colloidal NC systems including interactions with the solvent has long been a challenge due to the large system size and long time scales. Here, we incorporated the effects of ligands and solvents into negatively charged large spherical coarse-grained psuedoligands. MD simulations combining coarse-grained and all-atom models can successfully reproduce the cation exchange process in PbS colloidal NCs. Simulations show that the exchange rate and system equilibrium can be controlled by the temperature and by changing ligands. The exchange process is directly related to vacancy formation and the high mobility of Cd ions at the PbS/CdS interface. Our simulations also predict that high-pressure conditions will be beneficial for achieving fast exchange at elevated temperatures. Our coarse-grained model can be easily extended to other systems for the computational investigation of transformations in nanostructures.Process and EnergyMechanical, Maritime and Materials Engineerin
Nanosystems in Ceramic Oxides Created by Means of Ion Implantation
No full textThe material properties of nanometer-sized clusters are dependent on the cluster size. Changing the cluster dimensions induces structural phase transformations, metal-insulator transitions, non-linear optical properties and widening of the band gap of semiconductors. In this work, nanoclusters are created by ion implantation followed by thermal annealing. The ceramic oxides MgO and Al2O3 are used as embedding materials because of their stability and optical transparency. All clusters were created by means of ion implantation (typical dose 1016 ions cm-2, energy range 0.031.0 MeV) and subsequent thermal annealing (temperature 9001300K). Many experimental techniques were applied to investigate the defect evolution and the properties of the nanoclusters, amongst others optical absorption spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and positron annihilation spectroscopy. The formation, growth and dissociation of He and Kr bubbles in MgO and Al2O3 was investigated. It is found that the Kr nanoclusters in MgO are solid and have the fcc crystal structure. The lattice parameter of the Kr in individual clusters was found to vary as a function of the cluster size. Metallic Li, Zn, Ag and Au nanoclusters in MgO are also studied. XRD and TEM investigations reveal that Ag and Au nanoclusters are in a cube-on-cube orientation relationship with the rocksalt MgO. Large Li clusters (> 40nm) have the bcc crystal structure, while small Li clusters (<20nm) adopt the unusual fcc crystal structure. The MgO:Li system exhibits very strong positron confinement. The positron wavefunction is three-dimensionally spatially confined in the nanoclusters, so that 95% of the positrons annihilate in Li nanoclusters even though the atomic concentration of Li is only 1.3at.%. Semiconductor CdSe nanoclusters are created by sequential Cd and Se ion implantation. TEM analysis reveals that CdSe nanoclusters smaller than 5nm have the unusual rocksalt crystal structure, while nanoclusters larger than 10nm have the sphalerite or wurtzite structure. It can be concluded that nanoclusters are easily created in ceramic oxides by ion implantation and exhibit fascinating size-dependent material properties.Applied Science
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Chemistry and interactions of silica based particles studied by liquid cell electron microscopy
No full textLiquid Phase Electron Microscopy (LPEM) is a rapidly improving technique
that utilizes the powerful capabilities of the electron microscope in order to
image and investigate liquid-based processes at the nanoscale. However, the
presence of the liquid and particles dispersed in the liquid and the high energy
electron beam can greatly affect the ongoing processes inside the liquid cell
thus the results need to be carefully interpreted. In this thesis, we utilized and developed the LPEM technique to reliably study the chemistry and interactions of silica based colloidal particles which eventually paves the way to study the self-assembly of the colloidal nanoparticles using LPEM technique with a negligible effect of the electron beam
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Structural, electronic, and magnetic properties of iron carbide Fe7C3 phases from first-principles theory
No full textThe iron carbide Fe7C3 exhibits two types of basic crystal structures, an orthorhombic (o-) form and a hexagonal (h-) one. First-principles calculations have been performed for the basic Fe7C3 forms and for the related ?-Fe3C cementite phase. Accurate total-energy calculations show that the stability of Fe7C3 is comparable to that of ?-Fe3C. The o-Fe7C3 phase is more stable than the hexagonal one, in contrast to recent atomistic simulations. Furthermore, the calculations also show a rather low energy for a carbon vacancy in the o structure, which implies possible C deficiency in the lattice. Both Fe7C3 phases are ferromagnetic metals. Electronic band-structure calculations show that all Fe atoms exhibit high-spin states with the majority of their 3d states being almost fully occupied. From an analysis of the structural and energetic properties, the formation of the o phase in steel treatment processes and of h form in carburization of ferrite is discussed.Kavli Institute of NanoscienceApplied Science
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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