Charles B. Parker

Ferroelectricity in thin films: The dielectric response of fiber-textured (BaxSr1−x)Ti1+yO3+z thin films grown by chemical vapor deposition

We have investigated the dielectric response of a series of {100} fiber-textured (BaxSr1−x)Ti1+yO3+z samples deposited by liquid-source metalorganic chemical vapor deposition onto Pt/SiO2/Si, as a function of the two most commonly varied microstructural parameters: film thickness and Ti nonstoichiometry y. We find that the overall behavior of these samples is adequately described by mean-field, Landau–Ginzburg–Devonshire theory as for bulk ferroelectrics. However, we quantify the impact of three separable factors for these films that greatly alter the dielectric susceptibility as a function of temperature, compared to that found for bulk ceramic samples at the same Ba/Sr ratio of 70/30: (i) Ti nonstoichiometry; (ii) the apparent interface effect; and (iii) the plane equibiaxial stress state resulting from thermal expansion mismatch strains. When these factors are properly taken into consideration, we show that these fine grained thin films behave in a manner entirely consistent with expectations based on ...

Domain patterns in epitaxial rhombohedral ferroelectric films. I. Geometry and experiments

Possible domain patterns are developed for (001) oriented (pseudocubic indexing) epitaxial rhombohedral perovskite ferroelectric (FR) films. We assume that the films are grown above their Curie temperature (TC) in a cubic paraelectric (PC) state. The rhombohedral distortion consists of a “stretch” along one of the four 〈111〉 crystallographic directions of the cubic perovskite unit cell. Domain pattern formation is concurrent with the PC→FR transformation on cooling from the growth temperature. The domain patterns form to minimize elastic energy in the film, at the energetic expense of both forming domain boundaries and developing local stresses in the substrate. Eight possible domains may form, half of which are related by inversion, thus leading to four mechanically distinct variants. The possible domain walls are determined by mechanical and charge compatibility and follow closely from the analysis of Fousek and Janovec [J. Appl. Phys. 40, 135 (1969)]. Domain patterns may develop with either {100} or {1...

Temperature and thickness dependent permittivity of (Ba,Sr)TiO3 thin films

The temperature and thickness dependence of permittivity of (Ba,Sr)TiO3 has been investigated. The films were deposited by liquid-source metalorganic chemical vapor deposition onto Pt/SiO2/Si, with thicknesses ranging from 15 to 580 nm. The dielectric response was measured from 100 to 520 K. As film thickness decreased, the maximum dielectric constant decreased, the temperature at which the maximum dielectric constant occurred decreased, and the peak in the dielectric constant became more diffuse. A model incorporating a thickness independent interior and a nonferroelectric surface cannot account for these thickness dependencies. To appropriately model these observations a physical model containing thickness and temperature dependent interior and surface components is necessary.

On-chip electron-impact ion source using carbon nanotube field emitters

A lateral on-chip electron-impact ion source utilizing a carbon nanotube field emission electron source was fabricated and characterized. The device consists of a cathode with aligned carbon nanotubes, a control grid, and an ion collector electrode. The electron-impact ionization of He, Ar, and Xe was studied as a function of field emission current and pressure. The ion current was linear with respect to gas pressure from 10−4to10−1Torr. The device can operate as a vacuum ion gauge with a sensitivity of approximately 1Torr−1. Ion currents in excess of 1μA were generated.

Graphenated carbon nanotubes for enhanced electrochemical double layer capacitor performance

This letter reports on nucleation and growth of graphene foliates protruding from the sidewalls of aligned carbon nanotubes (CNTs) and their impact on the electrochemical double-layer capacitance. Arrays of CNTs were grown for different time intervals, resulting in an increasing density of graphene foliates with deposition time. The samples were characterized using electrochemical impedance spectroscopy, scanning electron microscopy, and transmission electron microscopy. Both low and high frequency capacitance increased with increasing foliate density. A microstructural classification is proposed to explain the role of graphene edges, three-dimensional organization, and other features of hybrid carbon systems on their electrochemical properties.

Resistance degradation behavior of Ba0.7Sr0.3TiO3 thin films compared to mechanisms found in titanate ceramics and single crystals

Abstract The resistance degradation behavior of Ba0.7Sr0.3TiO3 (BST) and SrTiO3 (ST) thin films is compared to the degradation behavior of titanate single crystals and ceramics with respect to the dependence on parameters such as temperature, thickness of the sample, applied voltage, acceptor dopant concentration and electrode material. Different model considerations to explain the resistance degradation in titanate thin films are discussed.

Order of Magnitude Signal Gain in Magnetic Sector Mass Spectrometry Via Aperture Coding

AbstractMiniaturizing instruments for spectroscopic applications requires the designer to confront a tradeoff between instrument resolution and instrument throughput [and associated signal-to-background-ratio (SBR)]. This work demonstrates a solution to this tradeoff in sector mass spectrometry by the first application of one-dimensional (1D) spatially coded apertures, similar to those previously demonstrated in optics. This was accomplished by replacing the input slit of a simple 90° magnetic sector mass spectrometer with a specifically designed coded aperture, deriving the corresponding forward mathematical model and spectral reconstruction algorithm, and then utilizing the resulting system to measure and reconstruct the mass spectra of argon, acetone, and ethanol. We expect the application of coded apertures to sector instrument designs will lead to miniature mass spectrometers that maintain the high performance of larger instruments, enabling field detection of trace chemicals and point-of-use mass spectrometry. Graphical Abstractᅟ

Perspectives on the Growth of High Edge Density Carbon Nanostructures: Transitions from Vertically Oriented Graphene Nanosheets to Graphenated Carbon Nanotubes

Insights into the growth of high edge density carbon nanostructures were achieved by a systematic parametric study of plasma-enhanced chemical vapor deposition (PECVD). Such structures are important for electrode performance in a variety of applications such as supercapacitors, neural stimulation, and electrocatalysis. A morphological trend was observed as a function of temperature whereby graphenated carbon nanotubes (g-CNTs) emerged as an intermediate structure between carbon nanotubes (CNTs) at lower temperatures and vertically oriented carbon nanosheets (CNS), composed of few-layered graphene, at higher temperatures. This is the first time that three distinct morphologies and dimensionalities of carbon nanostructures (i.e., 1D CNTs, 2D CNSs, and 3D g-CNTs) have been synthesized in the same reaction chamber by varying only a single parameter (temperature). A design of experiments (DOE) approach was utilized to understand the range of growth permitted in a microwave PECVD reactor, with a focus on identifying graphenated carbon nanotube growth within the process space. Factors studied in the experimental design included temperature, gas ratio, catalyst thickness, pretreatment time, and deposition time. This procedure facilitates predicting and modeling high edge density carbon nanostructure characteristics under a complete range of growth conditions that yields various morphologies of nanoscale carbon. Aside from the morphological trends influenced by temperature, a relationship between deposition temperature and specific capacitance emerged from the DOE study. Transmission electron microscopy was also used to understand the morphology and microstructure of the various high edge density structures. From these results, a new graphene foliate formation mechanism is proposed for synthesis of g-CNTs in a single deposition process.

High voltage microelectromechanical systems platform for fully integrated, on-chip, vacuum electronic devices

We demonstrate a fully integrated, on-chip, vacuum microtriode capable of handling voltages up to 800V. The ability to operate at such high voltages is achieved by the addition of a 10μm thick silicon dioxide layer to the device. The device is fabricated using microelectromechanical systems fabrication principles and utilizes carbon nanotubes as field emitters. A dc amplification factor of 600 was obtained. This is the highest value reported for carbon nanotube-enabled microtriode devices. The high voltage capability of these microscale devices will enable their use in a wider variety of applications.

Two-Dimensional Aperture Coding for Magnetic Sector Mass Spectrometry

AbstractIn mass spectrometer design, there has been a historic belief that there exists a fundamental trade-off between instrument size, throughput, and resolution. When miniaturizing a traditional system, performance loss in either resolution or throughput would be expected. However, in optical spectroscopy, both one-dimensional (1D) and two-dimensional (2D) aperture coding have been used for many years to break a similar trade-off. To provide a viable path to miniaturization for harsh environment field applications, we are investigating similar concepts in sector mass spectrometry. Recently, we demonstrated the viability of 1D aperture coding and here we provide a first investigation of 2D coding. In coded optical spectroscopy, 2D coding is preferred because of increased measurement diversity for improved conditioning and robustness of the result. To investigate its viability in mass spectrometry, analytes of argon, acetone, and ethanol were detected using a custom 90-degree magnetic sector mass spectrometer incorporating 2D coded apertures. We developed a mathematical forward model and reconstruction algorithm to successfully reconstruct the mass spectra from the 2D spatially coded ion positions. This 2D coding enabled a 3.5× throughput increase with minimal decrease in resolution. Several challenges were overcome in the mass spectrometer design to enable this coding, including the need for large uniform ion flux, a wide gap magnetic sector that maintains field uniformity, and a high resolution 2D detection system for ion imaging. Furthermore, micro-fabricated 2D coded apertures incorporating support structures were developed to provide a viable design that allowed ion transmission through the open elements of the code. Graphical Abstractᅟ