1,720,970 research outputs found
Off-axis multilayer zone plate with 16 nm × 28 nm focus for high-resolution X-ray beam induced current imaging
No full textUsing multilayer zone plates (MZPs) as two-dimensional optics, focal spot sizes of less than 10 nm can be achieved, as we show here with a focus of 8.4 nm × 9.6 nm, but the need for order-sorting apertures prohibits practical working distances. To overcome this issue, here an off-axis illumination of a circular MZP is introduced to trade off between working distance and focal spot size. By this, the working distance between order-sorting aperture and sample can be more than doubled. Exploiting a 2D focus of 16 nm × 28 nm, real-space 2D mapping of local electric fields and charge carrier recombination using X-ray beam induced current in a single InP nanowire is demonstrated. Simulations show that a dedicated off-axis MZP can reach sub-10 nm focusing combined with reasonable working distances and low background, which could be used for in operando imaging of composition, carrier collection and strain in nanostructured devices
Time-Resolved Photoluminescence Studies of InGaP Nanowires for Improving the Internal Quantum Efficiency
No full textSemiconductor Nanowires are promising building blocks for advanced optoelectronic devices since their small diameter give rise to quantization effects. The small diameter also makes them susceptible to non-radiative recombination due to surface states. A consequence of non-radiative surface recombination is a reduction of a total recombination lifetime. This is in turn limits an internal quantum efficiency (IQE) of optoelectronic devices because the IQE is defined as a ratio between the radiative recombination lifetime and the total recombination lifetime. It is therefore important to produce the nanowires with as long total lifetime as possible in order to achieve a significant IQE. This thesis aims to improve the IQE of the nanowires by using the surface passivating layer with a larger bandgap material on ternary alloy (InGaP) nanowires. The larger bandgap of the passivating layer will allocate the charge carriers into the center nanowire which, therefore, decrease the chance of the recombination at the surface. The passivating layer covers the side facet of the nanowires as a shell. The nanowires with this shell layer are called core-shell nanowires. Photoluminescence (PL) spectroscopy was used to evaluate the material composition and the bandgap of the core nanowire as well as the shell layer. Various compositions of shell materials give different band offsets between the core and the shell. Time-resolved photoluminescence (TRPL) was performed on the nanowires with and without the shell to measure the recombination lifetime. Finally, the energy structure of the non-radiative recombination center was studied by a temperature dependent PL (TDPL) and a power dependent TRPL. For the plain nanowires, the result shows that the non-radiative recombination is indeed related to surface states as expected. The total lifetime of the thinner wires is shorter than the thicker wires. Therefore, the IQE will degrade for the devices based on the thin nanowires. The TDPL reveals that the intensity is two orders of magnitude lower at room temperature (300 K) compared with the lowest measured temperature (4 K). The decreasing intensity is a result of two quenching processes. As the temperature increases, charge carriers gain higher thermal energies. The first quenching process occurs as the charge carriers escape the potential confinement with their higher thermal energies. Another process corresponds to a non-radiative recombination center with an activation energy of 17 meV. Power dependent TRPL at low (5 K) and high (100 K) temperature confirmed that the non-radiative recombination center is thermally activated.The performance of the computer processor has improved drastically over the past decade through an increasing number of transistors employed on a single processor chip. A simple approach for having a larger amount of transistors on a single circuit is to scale down the device. The smallest feature of a transistor is for example only a few nanometers (nm=0.000000001m) large which is about ten thousand times smaller than human’s hair. However, electrical and optical properties of miniaturized devices are impacted by quantum effects, such as energy quantization, and tunneling. These effects can either limit or improve the performance of the devices. A new nanostructure, Semiconductor Nanowire, is introduced as a building block of advanced electrical devices. It is a semiconductor material with a cylindrical shape, in which the diameter can be on a scale of nanometers and the length is about a few micrometers. The configuration of the nanowire provides a large surface comparing to its volume. With its large surface, it is therefore suitable for sensing applications, especially as an optoelectronic device. The purpose of the optoelectronic devices is to be either a light detector or a light source. For the nanowire as the light detector, the absorbed light excites electrons from a stationary state to move freely within the nanowire. Those free electrons make the device electrically conductive. For the light source, the electrons can be excited by an external energy; such as electricity or light. The excited electrons would then relax back to their stationary state whereupon release the excess energy in the form of light. The efficiency of those devices is determined by how well the energy is transferred between the free electrons and the light. An ideal condition is that every electron should emit the light as it relaxes for the light source. In contrast for the light detector, all the absorbed light should give rise to the free electrons. However, defects in the material and the surfaces of the devices can create “dark” pathways for the electrons to relax without the light emission. This makes the device less efficient. This is problematic since the nanowires are so thin that electrons are always close to the surface. There is a method called surface passivation which most surface of the wire is covered by another semiconductor material. With the proper passivation, the free electrons will move away from the surface. Consequently, the “dark” pathways can be avoided. In this study, there are several measurements performed on the nanowires with and without surface passivation in order to compare the device performance. The performance of the nanowires is expected to be improved when employing the surface passivation. Also, the study is to better understand the “dark” pathways involving the surface condition of the nanowires
Nanowire devices for X-ray detection
Full text linkHigh spatial resolution X-ray microscopy has become a dedicated tool to study nanocrystals and nanostructure devices in recent years. In general, the spatial resolution of X-ray microscopy depends on the spot size of the X-ray beam and the pixel size of X-ray detectors. High-resolution X-ray detection ideally requires a minimal active region with a sufficient thickness for the X-ray absorption, which leads to nanowire-shaped structures. This thesis made use of semiconductor nanowires to create a single-pixel X-ray detector at nanoscale resolution. The basic interaction between X-rays and nanowire devices can best be investigated by choosing a sample geometry where the nanowire is oriented in-plane with the substrate and orthogonal to the beam. X-ray beam induced current (XBIC), which is the physical process used in X-ray detectors, was used as the primary method to investigate the electrical response from nanowire devices. Different aspects of the XBIC process were investigated in two nanowire materials, InP and InGaP, with two types of doping profiles, n-i-n and p-i-n.The spectrally resolved XBIC measurements shed light on the underlying XBIC signal generation process in nanowire devices, showing that the XBIC signal originates at the atomic level with photoelectric absorption. Then, the X-ray flux variation revealed that the n+-i-n+ doped InGaP nanowire devices were affected by charge trapping leading to photogating and photodoping effects. In contrast, both kinds of p-i-n doped nanowire devices illustrated a linear response as function of the X-ray photon flux, which makes this doping profile more suitable for X-ray detectors. The XBIC measurements of this thesis could reveal the spatially resolved charge collection efficiency (CCE) or internal quantum efficiency (IQE) of the nanowire device. This result emphasizes the key ability of XBIC to be used in the development of nanowire solar cells. Furthermore, calculations based on the finite element method (FEM) was used to get a better understanding of the XBIC results.Although the in-plane nanowire devices can be used for understanding of the XBIC process at the nanoscale, they are not ideal for X-ray detection. The spatial resolution is still limited by the length of the active region, and the diameter of the nanowire limits the absorbing length. A novel fabrication process was therefore developed for a single vertical nanowire device where standing as-grown nanowires were turned into single pixel devices. With this configuration, the incident X-rays can be absorbed along the nanowire axis instead of the diameter. The nanowire used for this device is a p-i-n doped InP nanowire with a diameter of 60 nm as pixel size. Unlike the horizontal NW devices, the flux variation XBIC measurement reveals a sub-linear behaviour.The vertical nanowire device was used to make a high-resolution 3D image of a 90 nm nanofocused X-ray beam by scanning the device in different planes along the beam. The measurements reveal details of the intensity distribution that agree well with calculations based on ptychography. Instead of the nanowire diameter, the spatial detection was limited to about 100 nm due to the stability of the measurement system and X-ray absorption in the top contact. In the future, the device design with as-grown nanowires could scale up into multi-pixel array detectors operating much like conventional X-ray detectors
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
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
Optical demonstration of crystallography and reciprocal space using laser diffraction from Au microdisc arrays
No full textCrystallography is an invaluable tool in materials science, solid state physics and protein science. Understanding crystallography requires grasping the powerful but abstract concept of reciprocal space. Here a simple but insightful experiment using a laser pointer and Au microdisc arrays to explore and illustrate Bragg diffraction and reciprocal space is demonstrated. The Au microdisc arrays were manufactured using standard semiconductor fabrication techniques. The flexibility of the array design allows the demonstration of basic concepts such as lattice and atomic form factor, but also more advanced ones such as quasicrystal and shape function
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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