Session C46: SPS Undergraduate III

X-ray Magnetic Circular Dichroism Study of La$_{\mathrm{(1-x)}}$Sr$_{\mathrm{x}}$MnO$_3$ Thin Films

The perovskite manganite La$_{\mathrm{(1-x)}}$Sr$_{\mathrm{x}}$MnO$_3$ (LSMO) has attracted great attention recently due to its fundamental physics and potential applications in spintronics and data storage. In this work, we report a temperature-dependent x-ray magnetic circular dichroism (XMCD) study of epitaxial LSMO thin films deposited on orthorhombic NdGaO$_3$ (NGO) substrates grown by the molecular beam epitaxy (MBE) method. Small angle x-ray reflectivity and atomic force microscopy (AFM) results confirmed good epitaxial quality. XMCD measurements were performed at beamline 4-ID-C of the Advanced Photon Source at Argonne National Laboratory. XMCD spectra were taken in a 0.5 tesla field at temperatures ranging from 5 K to 180 K after the 0.5 tesla field cool. The total electron yield absorption spectra showed the oxide state characteristics of Mn, and the shapes of the Mn and O dichroism spectra change with temperature.   

Synthesis of Ag$_{2}$O Films using RF Magnetron Sputtering

Silver oxide (Ag$_{2}$O) thin films were successfully grown using reactive RF magnetron sputtering onto SiO$_{2}$ and Al$_{2}$O$_{3}$ substrates at room temperature. Synthesis of these films was achieved in a gaseous mixture of oxygen and argon which was 40{\%} oxygen. X-Ray diffraction tests yielded numerous peak intensities at angles correlating directly to Ag$_{2}$O. Deposition rates were shown to be a significantly greater on Al$_{2}$O$_{3}$ in comparison to SiO$_{2}$. Understanding this difference is a point of future investigations. ASTM D3359 adhesion tests as well as four terminal conductivity tests were also performed on the films and will be reported.   

Properties of Ti$_{8}$C$_{5}$ thin films created at different temperatures using magnetron sputtering

We were able to create thin films of Ti$_{8}$C$_{5}$ on c-axis oriented single crystal Al$_{2}$O$_{3}$ using both co-deposition magnetron sputtering and reactive magnetron sputtering. While TiC is generally used as a precursor film when making ``on-chip'' super capacitors, Ti$_{8}$C$_{5}$ is of similar composition and may have some advantages when making super capacitors. The Ti$_{8}$C$_{5}$ is more porous and demonstrates slightly different properties than TiC. Film deposition was optimized using elemental composition data obtained by WDXRF and characterized using XRD. It was found that composition and phase of Ti$_{8}$C$_{5}$ greatly depended on the temperatures at which the samples were grown. We outline the different parameters at which Ti$_{8}$C$_{5}$ grows best by outlining features of the Ti-C phase diagram.   

Field Directed Ordering in Magnetic Nanocrystal Structures

Iron oxide nanocrystals (NCs) have been the focus of intense research owing to the observation of tunable magnetic properties which could lead to advances in many fields including magnetic storage devices and medicine. We have been targeting the use of iron oxide NCs as magnetoresistance (MR) based sensors using ordered NC arrays. In this work, we will present our efforts toward using external magnetic fields to induce intraparticle ordering in iron oxide NC drop cast films. We use x-ray diffraction to analyze effects of the external fields on the NC array structure, while using SQUID magnetometry to probe the effects of NC interactions on the magnetic properties of iron oxide NCs ranging from 5 - 20 nm in diameter. MR measurements suggest large changes in the MR ratio can be achieved using the directed ordering approach for NC arrays. Our work could provide new avenues towards the fabrication of new magnetic devices.   

Photocatalysis of Thin Films of TiO$_{2}$ on Al$_{2}$O$_{3}$ Substrates

Titanium dioxide (TiO$_{2})$ has grown to be one of the most promising photocatalysts in recent years because of extensive applications in renewable and clean energy. The rise in demand for these new energies has driven an increase in research on metal oxides and their properties. Our interest in growing the rutile structure of TiO$_{2}$ stems from its lower excitation energy (3.0 eV) when compared to anatase (3.2 eV), which indicates it has better activity in the visible portion of the spectrum. It has been shown that sapphire (Al$_{2}$O$_{3})$ substrates are conducive to epitaxial rutile growth. In this study, we measured the photocatalytic activity of thin films of TiO$_{2}$ on r-Al$_{2}$O$_{3}$ (1 -1 0 2) substrates. We used PLD and MBE to grow the films, which were characterized using XPS and AFM. Photoactivity was measured via the decomposition of methyl orange on the film's surface using a UV/VIS spectrophotometer. The decomposition of this organic compound is driven by oxidation-reduction reactions on the surface of the TiO$_{2}$ film. From this, we calculated the charge carrier diffusion length and compared it to that of anatase.   

Synthesis and Characterization of Ni-NiO Nanocomposites for Optoelectronic Applications

LEDs and solar cells are becoming increasingly ubiquitous in modern society as they offer low energy consumption in a world where energy concerns are becoming increasingly prominent. Nonetheless, these devices have to overcome several shortfalls before they will be able to effectively replace traditional devices. In particular, these devices are fabricated using diodes, which depend on p-n junctions. While n-type oxide semiconductors are relatively plentiful, p-types are harder to produce. This research attempts to create a p-type oxide semiconductor with long lifespan and low resistivity. Using pulse laser deposition, NiO thin films with Ni nanoparticles were fabricated on quartz and Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ substrates. Detailed structures of the thin films were studied by X-Ray diffraction, scanning electron microscopy, and Raman spectroscopy techniques. Physical parameters such as magnetic moment of nickel, carrier concentration, and bandgap have been estimated using ultra violet-visible spectroscopy, photoluminescence, Hall effect, and magnetization data. Detailed results will be discussed in the presentation. This work is supported by NSF (Award Number DMR-0907037).   

Experimental and analytical study of ionic self-assembly of silica and titania nanoparticles

Using the ionically self-assembled monolayers (ISAM) technique we investigate the time dependence of the surface coverage of thin films that consist of alternating layers of silica or titania nanoparticles deposited on polymer substrates. We conduct experiments in order to investigate the significant observable factors that affected the quality of the coatings including the dipping time, pH, and the molarity of the silica, titania, and PDDA solutions. Using SEM micrographs, we analyzed the surface coverage and compared it to analytical results obtained using a cooperative sequential adsorption model.   

Nanoscale Thermal Analysis of Organic Solar Cells

Our research uses atomic force microscopy (AFM) and a Nanoscale Thermal Analysis (NanoTA) system from Anasys Instruments to correlate the morphology of local structures with the thermal material properties of organic solar cells. The NanoTA system uses AFM probes that can be heated up to 350$^{\circ}$C over a 50 nm region to quantify the melting transition temperatures of nanoscale regions. We show results for two materials: D$_{\mathrm{i}}$PSQ[OH]$_{2}$ and PCBM, for both pure and systematically blended thin-films. We have characterized the morphology and melting points of the blend films with increasing anneal time, and differences in melting points of blended as compared to pure samples.   

Characterization of organic solar cell morphology

The morphology of organic solar cell bulk heterojunctions were characterized using atomic force microscopy (AFM). The RMS roughness of solar cells composed of 1,3-bis[4-(N,N-diisopentylamino)-2,6-dihydroxyphenyl]squaraine [DiPSQ(OH)$_2$] and phenyl[C$_{61}$]-butyric acid methyl ester [PCBM] through spin casting were measured. Solar cells of various blend concentrations, anneal times, and cooling methods were characterized. Through RMS roughness analysis we can study the crystallization process in solar cell fabrication. Morphology will be related to device characterization.   

Computational study of a class of cooperative sequential adsorption models on Cayley trees and two- dimensional lattices

We present a Monte Carlo simulation study of a class of cooperative sequential adsorption models with constant and variable attachment rates and their possible applications for ionic self-assembly of thin films, drug encapsulation of nanoparticles and susceptible-infected-recovered epidemic models. We do a comparison study of these models on a Cayley tree and a two - dimensional lattice and discuss the cases for which four-branch Cayley trees are good approximations for two -dimensional lattices.   

Characterization of Ion Movement in Light-Emitting Electrochemical Cells via ToF-SIMS

An emerging advantage of organic semiconductors is their ability to conduct ions in applications such as light-emitting electrochemical cells (LECs), photovoltaic devices, and electrochromic devices. This ability of organic materials to conduct both ionic and electronic currents in the solid state sets these materials apart from their inorganic counterparts. However the fundamental electrochemical processes are not well characterized. Evidence suggests that the profiles of ions and electrochemical doping in the polymer film during operation significantly impact the performance and stability of the device. Here, we present our findings from direct profiling of ion distributions in LECs following application of voltage, via time-of-flight secondary ion mass spectrometry. Ion distributions were characterized with regard to film thickness, salt concentration, applied voltage, and relaxation over time. Results provide insight into correlation between ion profiles and device performance and potential approaches to tuning electrochemical doping processes in LECs.   

Asymmetric laser sideband generation with a tapered semiconductor amplifier

We have constructed a free-space, frequency-shifted feedback amplifier using a tapered semiconductor gain element. The general layout of the system is similar to that described in Littler, et al., \textit{Opt. Comm.} \textbf{88}, 523 (1992). Traveling-wave feedback is demonstrated with the $m=-1$ order of several different acousto-optic modulators driven at variable frequency. Asymmetric sideband production is observed in the rf spectrum of a fast photodiode and in the transmission of a scanning Fabry-Perot interferometer. The number of asymmetric modes is controlled with the AOM rf drive power and the seed laser optical power.   

Synchronization in a network of phase-coupled oscillators: the role of learning and time delay

We investigate numerically the interplay of network ``learning'' and finite signal speed in one and two-dimensional arrays of coupled Kuramoto oscillators. The finite signal speed is introduced into the dynamical system via a time-delay in the coupling. The network structures we examine include various one and two-dimensional arrays with both long and short-range connectivity; the structure of these arrays is imposed via a time delay and a connection matrix. The learning is governed by the Hebbian learning rule which allows the coupling strengths between pairs of oscillators to vary dynamically. It corresponds to a neurological type of learning in which the synapses between neural oscillators increase in strength when they fire action potentials together. We explore the coherent spatio-temporal patterns that can emerge as a function of model parameters such as learning rate and signal speed.