Session E3: Poster Session III

Hall Effect Calibration of Silicon Doped GaAs Grown by Molecular Beam Epitaxy (MBE

Calibration of equipment is paramount for any system to ensure quality and control over the properties of a material. A molecular beam epitaxy (MBE) system needs careful calibration to ensure accuracy in the properties of the semiconductor materials it makes. Specifically, we need to precisely control our semiconductor's carrier type (electron or hole), carrier density, and carrier mobility. Hall measurement is one of several experimental techniques used to study a material's electronic properties, and in this study, we utilized the Van der Pauw method to take those measurements. We investigated several thin gallium arsenide (GaAs) films doped with different concentrations of silicon, and epitaxially grown on GaAs (001) substrates. The samples were all tested at room temperature with a 0.545 T magnet, which revealed a linear Arrhenius relationship between the electron concentration and the temperature of the silicon source used to dope the sample. From this plot, a confident determination of specific electron concentration in future GaAs samples can be prescribed and grown by MBE.   

Functionalization of Graphene Oxide by Atomic Layer Deposition using Titanium Oxide for Improving Electrocatalysis

Oxygen reduction reactions (ORR) is a lethargic process that lowers the efficiency of renewable energy devices such as fuel cells and metal air batteries. Though ORR can be effectively catalyzed by platinum, and other known catalyst, there is still a persistent search for a catalyst material that is inexpensive and more effective. Recent studies have shown that the titanium oxide (TiO$_{\mathrm{x}})_{\mathrm{\thinspace }}$can have catalytic activity towards ORR. This study investigates the catalytic activities of titanium oxide incorporated onto graphene oxygen (GO) by atomic layer deposition (ALD). The catalytic activity was systematically measured by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) using a rotating disk electrode (RDE). Evidence shows that TiO$_{\mathrm{x}}$ bonded on the surface of GO is catalytic active. Comparing the CV before and after ALD of TiO$_{\mathrm{x}}$ the catalytic activity increased. Temperature effects were measured by electrochemical techniques and the trends show that at higher temperature there is an increase in catalytic activity, which is due to the simultaneous reduction and addition of TiO$_{\mathrm{x}}$ during ALD.   

Computational generation of void-rich hydrogenated amorphous silicon

We present computational generation of void-rich hydrogenated amorphous silicon (a-Si:H) for use in studying the electronic properties for tandem solar cells. We are specifically interested in void regions in structures of varying densities which are likely to constitute defects limiting carrier mobilities and may be implicated in the light-induced degradation of the Staebler-Wronski effect. We generate structures using the Wooten-Winer-Weaire classical-potential Monte Carlo method. We create hydrogenated structures of varying densities and study the response of the bond angle deviation ($\Delta \theta )$ and average bond length and compare them to non-hydrogenated a-Si structures. We utilize comparisons of the total energy and $\Delta \theta $ to identify and avoid crystalline structures as well as a class of artifactual structures of distinctively high energy. We attempt to characterize the size and position of voids using Voronoi polyhedra and persistent homology.   

Exploring Defects in Topological Insulators Using X-ray Diffraction and Atomic Force Microscopy

Topological insulators such as the Bi- and Sb- chalcogenides are of great interest to because they have unusual metallic surface states that allow the spin of electrons moving through them to be controlled and used for spintronic devices. Much progress has been made in the epitaxial growth of thin films of these materials. However, the common defects present need to be better characterized so that higher quality materials can be synthesized. Our aim was to better understand the structural defects present within the various topological insulators synthesized via molecular beam epitaxy. The methods used were primarily high-resolution x-ray diffraction and atomic force microscopy. These two techniques were used in conjunction in order to better understand both the surface defects and the defects present within the crystalline structure. The quality of the thin films is known to depend on the type of substrate the films are deposited upon. We report detailed characterization of Bi$_{\mathrm{2}}$Se$_{\mathrm{3\thinspace }}$grown on sapphire substrates and compare our results with those reported for growth on other substrates such as InP and SrTiO$_{\mathrm{3}}$.   

Assessment of Thermodynamic Properties of Carbon Nanotubes(CNTs) and Hydrated C60 Fullerenes(C60HyFn) as Potential Agents to Suppress Reactive Oxygen Species

Oxidation of the neural tissues in human brain causes neurodegenerative disease; however, information on the sub-cellular localization of oxidative molecules is not provided in detail thus far. It is highly desirable to visualize the activities of Reactive Oxygen Species (ROS) in living cells on a microscopic level for the proper mechanism of the role of peroxidation. Multiple pathways through oxidative stress can produce cell injury; and thus, the oxidative reactions in biomembranes are particularly important. It can result in the impairment of lipid–protein interaction and modification and fragmentation of membrane proteins as well, thereby leading to the cell injury and aging. A free-radical chain reaction capable of propagating in space is the major oxidative reaction in biomembranes. In this paper, the functionalized Carbon Nanotubes(CNTs) and hydrated C60 Fullerenes(C60HyFn) molecules were thermodynamically studied to determine whether the molecules stabilize or destabilize the molecules. The Auto Optimize Tool in the computational software was used for each Carbon Nanotubes(CNTs) and hydrated C60 Fullerenes(C60HyFn) derivatives modeled in this project to determine its optimization energy. The Universal Force Field (UFF) option was selected for all the molecules modeled.   

Experimental measurement of the temperature dependence of high purity single-walled carbon nanotube networks.$\backslash $pard

h $pardabstract-$\backslash $expnd0We measure the temperature dependence of the conductance of high purity thin film semiconducting single-walled carbon nanotube (s-SWNT) networks to determine how charge transport occurs in these networks. We use a cryostat to cool devices from 300K to 21K. The conductance is measured in 1K increments using custom LabVIEW programs, both as the device is cooled, and as it equilibrates back to room temperature. A variety of effects are observed which hinder our ability to clearly interpret the functional dependence of the conductance. These include hysteresis in the conductance versus temperature curves for warming and cooling, possibly due to doping. In addition we find that the accuracy of the temperature is significantly impacted by the proximity of the temperature sensor to the device. Improvements are presented here which allow us to now measure the temperature dependence accurately./abstract-$\backslash $pard$\   

A Study on the Soccer Ball Mechanics and Dynamics Using Physical and Computational Simulations

Soccer ball mechanics and dynamics involve aerodynamics, the study of forces and the resulting motion of objects as they fly through the air. The way a soccer ball curves through the air is determined by the values of factors such as speed of the kick, direction, angle, and even weather conditions. Some of the factors contributing to weather conditions include pressure, temperature, humidity, and altitude of the environment in which the soccer ball is being kicked. Other elements such as distance, vertical angle, horizontal angle are to be calculated when analyzing the aerodynamics of a soccer ball. In this paper, we studied and analyzed for the equations involving various factors that can be applied to soccer ball mechanics through a specially designed computer application, deciphering possible bending and spinning of the soccer ball. In this research, with mechanics and a program, we studied how a soccer player bends the ball, creating the Magnus effect by changing the values of the factors that affect the lift and drag aerodynamic forces on the ball. Also, we considered whether the condition is a laminar flow or turbulent flow which can be identified by the Reynold’s number.   

Memory loss in a particle swelling model

Liquid suspensions of particles that are sheared back and forth repetitively will self-organize and ``adapt'' to that shearing so long as its amplitude is below some critical value. This organization creates a memory in the system that can later be ``read out'' by observing how many particles are perturbed by a given shear. We use a model that allows us to efficiently study this behavior. In place of shear, particles swell to a given amplitude, and overlapping particles are repelled. This process repeats while the system is monitored. By training the system on progressively larger memories, the system ``forgets'' memories at smaller amplitudes. We present a study of this ``forgetting'' as the system approaches the critical swelling amplitude.   

Thermal Reflow Method for the Fabrication of PDMS Molds to investigate of Microtubule Tracking

With the growing interest in studying protein or macro-molecule mobility, the creation of modified surfaces has been an increasingly important component of active matter studies. Modified surfaces can be developed using a simple single patterned structure with the use of photo-lithography. We present a thermal reflow method for the creation of curved dome-like structures from fabricated cylinders developed using a mylar photomask. The initial cylinders are formed from a single exposure onto a positive tone photoresist (AZ P4620) spun onto a 4 inch Silicon wafer. After development, the wafer is placed on a hotplate at 150\textdegree C for 2 minutes to induce photoresist reflow. Result show 8(\textpm 1)\textmu m thick domes with diameters less than 300\textmu m. Larger diameters were not able to reflow. Once cooled, a PDMS mold is placed over the structures to produce the bowls to be used as a surface for Microtubule gliding. Bowl depths were recorded as 8(\textpm 1)\textmu m, true to size of their dome counterparts. In terms of reliability, the dome structures can be used multiple times for repeated use without loss of quality. We demonstrate microtubule gliding on these novel substrates as a proof of concept.   

Physical and Thermodynamic Analysis of Nano-scaled Aryl Azo Molecules Causing Dermal Disease

Recently, computational biomedical simulation technology is perceived as a means of new approach to an alternative method for future solution of research on chemical agents causing dermal disease. As the use of artificial colorant molecules increases, scientist have tried to study those complexes as they are believed to be able to affect human dermal cells. In this paper, we have tested selectivity and stereochemical aspects for several types of aryl azo compounds and their derivatives as a biological agent causing dermal disease. For this purpose, we used chemical computer programs to model, optimize, and compare the resulting molecular enthalpy of the aryl azo clusters. The theoretical structure of each feasible nano-scaled compounds has been studied in this project. Based on the predicted stability of each molecule, it can be predicted which compound can be used more efficiently to assess the thermodynamic stability. Optimization configuration energy was collected in order to compare each chemical compound's stability. Calculations show some compounds converge easily to lower in enthalpy, which makes them suitable to use as biochemical compound in the disease treatment.   

A microbeam scanning method to determine the x-ray attenuation of the soft tissue in an L-shell x-ray fluorescence lead detection in a soft tissue and bone phantom assembly

Lead (Pb) is a well-known toxic element residing in the human bone for many years. Hence, \textit{in vivo} measurement of bone Pb concentration is a good metric of long-term human Pb exposure. The L-shell x-ray fluorescence (LXRF) is a non-invasive quantitative method suitable for large population bone Pb surveys since it can use portable x-ray tubes and detectors. In past studies the x-ray attenuation of the soft tissue (XAST) overlying the bone was initially measured using ultrasound soft tissue thickness (STT) measurements and generic elemental composition of the soft tissue to calculate its linear attenuation coefficient ($\mu )$. An in-depth analysis revealed the procedure is inaccurate in its simplifying assumptions. A cylindrical plaster-of-Paris Pb-doped (75 $\mu $g/g) bone phantom (BP) and a 3-mm thick cylindrical-shell polyoxymethylene (POM) soft tissue phantom were used to test a new scanning microbeam method for the XAST measurement. The method measured the STT by positioning the microbeam in 0.1 mm steps perpendicular to the BP and the $\mu $ of POM in the direction parallel to BP. The measurements were compared to the $\mu_{\mathrm{\thinspace }}$of POM calculated value and generated an accurate Pb concentration (\textless 5{\%}) using a bare BP calibration line data.   

Reversibility in a Contact Line

When a liquid spreads over a solid surface, it forms an irregular border between the liquid and the air called a contact line. If this contact line is repeatedly driven back and forth through small displacements, its shape may eventually self-organize so that the motion becomes reversible. We have designed and built an apparatus to study this reversibility. A syringe pump injects and withdraws water repeatedly with a set volume amplitude in a quasi-2D acrylic cell. A camera is used to measure how much the contact line changes in each cycle. Our preliminary data shows evidence of self-organization in the system, indicated by a transient period of irreversibility followed by a steady state. When the amplitude is changed, we observe another irreversible transient, suggesting that the system may form a specific memory of driving. We discuss methods of experimentation and the results of further experiments to test for memory.   

Spectroscopic Modeling of X-Ray Spectra for Xe High Energy Density Plasmas

The study of plasma is a rich field of interest with far reaching applications, e.g. understanding astrophysical phenomena, controlled fusion, and effects of radiation physics. Moreover, the use of X-ray spectroscopy is a very important tool in the study of high energy density (HED) plasmas. Particularly, M-shell radiation from Xe HED plasmas is of special theoretical interest due to the large number of ionic transitions and their overlap compared to K-shell and L-shell radiation. M-shell radiation is more complex to study and thus presents a challenge and needs an experimental benchmark. The goal of this research is to isolate M-shell Xe spectral features and identify specific peaks to learn about the properties of the Xe HED plasma created in interaction of femtosecond laser pulse with gas puff plasma using the Titan laser at Lawrence Livermore National Laboratory. Theoretical spectra are produced using the Spectroscopic Collisional-Radiative Atomic Model (SCRAM). Electron temperature and density dependences to analyze the M-shell transitions are presented for the spectral range of 9-15 {\AA}, as well as the theoretical modeling of the experimental M-shell Xe data from the Titan laser.   

Experimental study of the conductive properties of high purity single-walled carbon nanotube networks

The conductance of single-walled carbon nanotube (SWNT) networks near percolation is highly sensitive to changes in the tube composition, density, and size of the network. Networks of primarily semiconducting SWNTs show promise in bio-sensing applications due to their electrical sensitivity to the environment. Balancing the tube composition between semiconducting and metallic tubes is essential for these applications. In this study, we measure thin film SWNT network devices of high semiconducting purity to determine the impact that network size, tube density, and composition has on the overall conductance. This is done by electrostatically gating networks to determine their conductance. Our results indicate that when we are close to the percolation threshold for the metallic tubes the device properties vary significantly.