1,721,059 research outputs found
Utilizing electron microscopy and spectroscopy methods to understand water structure and water doping
Get PDFWater is the second most common element in the universe and the most studied material on earth. Most of the studies concerning water are from the fields of chemistry and biology. Hence, the structure of water molecules and short range order and interactions are well characterized and understood. However, the collective arrangement of water molecules and the long range order are still missing. Understanding of this long range order in water is needed, as it is the key to many water activities. To fill this gap, this study utilizes a new direct method for characterization of water in the vapor phase. Water samples from different water types were characterized using electron energy loss spectroscopy (EELS) within a transmission electron microscope (TEM). Prior to characterizing water vapor, the measurement method for in-situ gas analysis was developed using pure gases. Water samples were also characterized using more conventional techniques, including: using cryogenic scanning electron microscopy (Cryo-SEM) in the solid state, after rapid freezing; and using high resolution TEM (HRTEM) and scanning TEM (STEM) after drying. Many other characterization techniques were evaluated but most of them were found to be not suitable, mainly due to detection limits. EELS characterization showed that samples from different water types have different electronic configurations, and they all have structures that are large enough in order to scatter electrons. From cryo SEM characterization it was found that water has nanoparticles inside with a size range of 10-100 nm, and these particles are ~500 nm apart. HRTEM/STEM characterization showed that particles from different water types have different shapes. The presence of particles provide surfaces to support water structures and the difference between the particles can explain the different properties of different water types Using tools and methods that are conventional in materials science for characterization of bulk materials and long range orders, resulted in characterization of water clusters that are significantly larger than what was known until now. By this we contribute a new point of view for water structure, which together with new insights on water properties can potentially advance later use of water as an active ingredient
Investigating carbon nanotube growth with the transmission electron microscope
No full textVertically aligned single walled carbon nanotube carpets or arrays are of great technological importance, with various potential applications inspired by their unique properties. Nonetheless, these structures are still not widely utilized, largely because of a lack of understanding of the termination mechanism whereby carpet growth suddenly stops after growing to a certain height. In order to solve this perplexing problem, we systematically perform a series of experiments, including controlled thermal annealing, carpet growth, and in-situ monitoring of carpet growth. These experiments are closely correlated with both ex-situ plan-view and cross section imaging in the transmission electron microscope (TEM), and real time observation of individual carbon nanotube growth and termination in-situ to the TEM. With these various approaches, we show that catalyst particles that have existing nanotubes can undergo dynamic evolution in size and that carbon nanotube growth can cease due to complete loss of the catalyst particle. Also, we develop a solid correlation between termination of carpet growth and the dynamic evolution of catalyst particles during thermal annealing or growth processes. The dynamic evolution of catalyst particles is governed by Ostwald ripening and sub-surface diffusion. Based on this correlation, we can explain four growth termination related phenomena: (i) how can a small amount of H2O dramatically enhance the lifetime of catalyst? (ii) how can only 30 seconds of a rapid reduction process using atomic hydrogen significantly enhance the lifetime of catalyst? (iii) why is the growth temperature is so sensitive to the carpet growth termination? (iv) why is the support material so important in carpet growth? In addition, based on the understanding of the termination mechanism obtained through this work, we demonstrate one possible way to improve the carpet growth
An influence of crystal structure and interface formation on the electrical properties of nanoparticle printed thin films
No full textThe creation of inexpensive electronic devices through the direct printing of organic semiconductors is of interest for the clear advantages associated with device cost reduction through mass production using cheap substrates. However, the low inherent carrier mobility of organics places fundamental limitations on both the performance and types of devices that can be created with this approach. As a substitution for organics, printing of inorganic nanoparticles followed by subsequent sintering to form conductive films has been proposed. However, the high required sintering temperatures have restricted their usage. Here, core-shell nanoparticles are proposed to reduce the sintering temperature of inorganic nanoparticles, with three semiconductor (or metal)-metal core-shell nanoparticle systems investigated; Ge-Ag, SnO2-Ag, and Cu-Ag. These were chosen because (1) silver exhibits a high surface self-diffusion, (2) silver has a low solubility in the core materials, and (3) silver dewets on the core materials. Due to the difficulty of synthesizing Ge-Ag and SnO 2-Ag core-shell nanoparticle systems, alternate approaches were taken to analyze the thermal behavior of these systems. The thermal behavior of pure Ge nanoparticles was investigated, as first approach to determine the feasibility of using simple inorganic particles in printed electronics. Based on in situ transmission electron microscopy (TEM) annealing experiments mass transport temperature was at ≈ 200 °C. Additionally, it was found that different synthesis routes and sintering conditions lead to differences in the crystal structure of the Ge nanoparticles. As a proof of principle experiment to predict the sintering behavior of SnO2-Ag core-shell nanoparticles, the annealing behavior of a SnO2/Ag/SnO2 trilayer was examined. Ag layer pinch off and the formation of both Ag rods and islands was observed. This implies that a potential SnO2-Ag core-shell nanoparticle system would exhibit sintering at lower temperatures than clean SnO2 nanoparticle system. Finally, from the investigation of the sintering behavior of Ag-Cu core-shell nanoparticles, it was determined conclusively that the Ag shell leads to accelerated sintering of the copper, thereby validating the general principle. Based on the results, the use of core-shell nanoparticles is a viable route to fabricate thin films using inorganic nanoparticles in printed electronics through the use of lower sintering temperatures
In situ transmission electron microscopy studies of initial stages of vapor-liquid-solid growth of silicon nanowire
No full textA critical component in the successful application of Si nanowire in electronic and optical devices is a fundamental understanding of the initial stages of vapor-liquid-solid (VLS) process via which most of semiconducting nanowires are grown. In this dissertation, in-situ transmission electron microscopy is used to investigate each of the stages of the VLS process in detail, in order to understand the kinetics of each stage and to assess how they impact overall nanowire growth process. Utilizing the unique capabilities of a specially constructed ultra-high vacuume transmission electron microscopye (UHV-TEM) equipped with in-situ ultra-high vacuum chemical vapor deposition (UHV-CVD) growth facilities, detailed measurements and analysis of the nucleation and growth process in nanoscale systems are presented. Observations of individual events, as opposed to data from ensembles, allow direct probing of classical nucleation theories in detail. The system described here is directly related to nanowire formation: nucleation of Si from a Au-Si eutectic liquid is examined at increasing supersaturation, as in the VLS process, in contrast to nucleation on cooling as in most other nucleation studies. A simple theoretical model is presented which provides an accurate description of the measurements of nucleation at the nanoscale. No size effects on nucleation are observed, even in systems below 12 nm. The result that the phase diagram is insensitive to size down to this value is in stark contrast to recent reports for AuGe, which lacked the quantitative rigor of this study, and suggests that it may be relatively simple to design processes to create reproducible structures at these size scales in technologically relevant applications. Moreover, this model allows determination of the supersaturation required to initiate nucleation. This supersaturation exhibits a strong temperature dependence, and - surprisingly - a higher supersaturation is required at higher nucleation temperatures. The examination of the three stages of the vapor-liquid-solid process, prior to the axial growth of Si nanowires - the dissolution of the solid Au catalyst in the AuSi eutectic liquid, Si saturation in the liquid AuSi, and Si nucleation was also carried out using the in situ transmission electron microscope (TEM). Our quantitative analyses show that the incubation time for Si nucleation linearly increases with liquid droplet radius. This linear dependence is explained by taking into account the invariant characteristics of the following two factors in time; the shape of liquid AuSi droplet, and the sticking coefficient of disilane on the AuSi. Cross sectional TEM imaging shows the shape of the liquid drop remains constant with time. From dark field imaging and measurement of the dissolution rate of solid Au, we find that the solid to liquid transformation occurs from the surface inwards and the solid volume linearly decreases with time. Based upon these results, we suggest that the rate of Si addition to the droplet is constant over time – a simple conclusion to estimate the supersaturation at the moment of Si nucleation. Finally, quantitative in situ measurements of coarsening/decay of individual AuSi droplets with simple models accounting for kinetic behaviors of the droplets were implemented. From the measurements of droplet volumes as a function of time in combination with both surface diffusion - and attachment - models of droplet decay, it is found that attachment/detachment of Au adatoms on the Si (100) surface is the rate-limiting step. Both coarsening and decay kinetics agree well with a simple kinetic equation, and the “reservoir effect” reflected in it would result in the apparent size dependence of decay rate. These results have important implications for acquiring the capability of tailoring the structural properties of Si nanowires (i.e. diameter, distribution and density). Throughout this dissertation, emphases are placed on the advantages of quantitative measurements of all the early stages of VLS growth of Si nanowires. These kinetic analyses yield insight into such fundamental process as crystal growth, phase transformation and nucleation, as well as into technologically important aspects of designing nano-scale devices
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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