Steven B. Fairchild

Hysteresis during field emission from chemical vapor deposition synthesized carbon nanotube fibers

Hysteresis in the field emission (FE) data of a chemical vapor synthesized carbon nanotube fiber cathode is analyzed in the regime where self-heating effects are negligible. In both the forward and reverse applied field sweeps, various FE modes of operation are identified: including Fowler-Nordheim (FN) tunneling and space-charge limited emission from the fiber tip and FN emission from the fiber sidewall. Hysteresis in the FE data is linked to the difference in the field enhancement factors in the different FE modes of operation in the forward and reverse sweeps and related to changes in the fiber morphology.

Physical properties of lanthanum monosulfide thin films grown on (100) silicon substrates

Thin films of lanthanum monosulfide (LaS) have been deposited on Si (100) substrates by pulsed laser deposition. The films are golden yellow in appearance with a mirrorlike surface morphology and a sheet resistance around 0.1Ω∕◻, as measured using a four-probe measurement technique. The thin films are characterized by atomic force microscopy (AFM), x-ray diffraction (XRD) analysis, high resolution transmission electron microscopy (HRTEM), ellipsometry, and Raman spectroscopy. The root-mean-square variation of (1μm thick) film surface roughness measured over a 1μm2 area by AFM was found to be 1.74nm. XRD analysis of fairly thick films (micrometer size) reveals the growth of the cubic rocksalt structure with a lattice constant of 5.863(7)A, which is close to the bulk LaS value. HRTEM images reveal that the films are comprised of nanocrystals separated by regions of amorphous material. Two beam bright field TEM images show that there is a strain contrast in the Si substrate right under the interface with the...

Patchwork field emission properties of lanthanum monosulfide thin films

The field emission (FE) properties of lanthanum monosulfide (LaS) films, deposited on Si and InP substrates by pulsed laser deposition, have been thoroughly analyzed via the scanning anode field emission microscopy technique (SAFEM, Fig. 1) at different surface locations and at different temperatures. For one location, the full set of measured I-V characteristics (total measured current versus applied voltage) for different values of Z, the distance between the cathode surface and the probe ball, were then analyzed in order to extract the current density J versus actual applied field F (J-F data), within the approximation that the LaS surface is a plane. A characteristic J-F variation is shown in Fig. 2. The work function of the LaS thin film has been extracted from the slope of the plot ln(J/F/sup 2/) vs 1/F, by using the conventional Fowler-Nordheim relation, leading to a value of /spl sim/0.65 eV which is in agreement with the onset of the electric field needed to observe an emission current density of 1 mA/cm/sup 2/ at an applied electric field across the vacuum gap around 230 V//spl mu/m.

Multiscale model of heat dissipation mechanisms during field emission from carbon nanotube fibers

A multiscale model of field emission (FE) from carbon nanotube fibers (CNFs) is developed, which takes into account Joule heating within the fiber and radiative cooling and the Nottingham effect at the tip of the individual carbon nanotubes (CNTs) in the array located at the fiber tip. The model predicts the fraction of CNTs being destroyed as a function of the applied external electric field and reproduces many experimental features observed in some recently investigated CNFs, such as order of magnitude of the emission current (mA range), low turn on electric field (fraction of V/μm), deviation from pure Fowler-Nordheim behavior at large applied electric field, hysteresis of the FE characteristics, and a spatial variation of the temperature along the CNF axis with a maximum close to its tip of a few hundred  °C.

Low work function CsI coatings for enhanced field emission properties

Thin films of cesium iodide (CsI) were deposited by pulsed laser deposition and by thermal evaporation onto Si substrates and were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and Kelvin probe measurements. The thermally evaporated films were found to be stoichiometric whereas the pulsed laser deposited films showed the presence of a Cs/CsI mixture. The latter is supported by UPS measurements whose Fermi edge indicates the presence of a metallic component (elemental Cs). The presence of a Cs/CsI mixture is also supported by the Kelvin probe work function values found to be in the range of 2.6–2.8 eV, a value in excess of the 2.1 eV reported for elemental Cs. This paper addresses the physical mechanisms responsible for the presence of the elemental Cs in the films and its ramification to their field emission properties.

Approach to multifunctional device platform with epitaxial graphene on transition metal oxide

Heterostructures consisting of two-dimensional materials have shown new physical phenomena, novel electronic and optical properties, and new device concepts not observed in bulk material systems or purely three dimensional heterostructures. These new effects originated mostly from the van der Waals interaction between the different layers. Here we report that a new optical and electronic device platform can be provided by heterostructures of 2D graphene with a metal oxide (TiO2). Our novel direct synthesis of graphene/TiO2 heterostructure is achieved by C60 deposition on transition Ti metal surface using a molecular beam epitaxy approach and O2 intercalation method, which is compatible with wafer scale growth of heterostructures. As-grown heterostructures exhibit inherent photosensitivity in the visible light spectrum with high photo responsivity. The photo sensitivity is 25 times higher than that of reported graphene photo detectors. The improved responsivity is attributed to optical transitions between O 2p orbitals in the valence band of TiO2 and C 2p orbitals in the conduction band of graphene enabled by Coulomb interactions at the interface. In addition, this heterostructure provides a platform for realization of bottom gated graphene field effect devices with graphene and TiO2 playing the roles of channel and gate dielectric layers, respectively.

Pulsed-Laser Deposited Transition-Metal Carbides for Field-Emission Cathode Coatings

Thin films of transition-metal carbides ZrC, HfC, and TiC were deposited by pulsed-laser deposition under vacuum. The surface chemistry of the films was characterized with ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy in situ. X-ray diffraction was used to characterize the film structure. TiC was shown to be nearly stoichiometric and polycrystalline. The TiC was applied to a vertically aligned carbon nanotube sample and characterized by field emission. Field-emission results showed enhanced current and current density at a film thickness, 5 nm, not previously reported in the literature. Emission from TiC films was also shown to be less affected by adsorbates during field emission. Pulsed-laser deposition of TiC offers a distinct advantage over other techniques in that high-quality films can be obtained under ultrahigh vacuum conditions without the use of a reactive background gas or excessively high annealing temperatures. The application of TiC by pulsed-laser deposition as a cathode coating shows potential for integration into a fabrication process.

Review Article: Rare-earth monosulfides as durable and efficient cold cathodesa)

In their rocksalt structure, rare-earth monosulfides offer a more stable alternative to alkali metals to attain low or negative electron affinity when deposited on various III-V and II-VI semiconductor surfaces. In this article, the authors first describe the successful deposition of lanthanum monosulfide via pulsed laser deposition on Si and MgO substrates. These thin films have been characterized by x-ray diffraction, atomic force microscopy, high resolution transmission electron microscopy, ellipsometry, Raman spectroscopy, ultraviolet photoelectron spectroscopy, and Kelvin probe measurements. For both LaS/Si and LaS/MgO thin films, the effective work function of the submicron thick thin films was determined to be about 1 eV from field emission measurements using the scanning anode field emission microscopy technique. The physical reasons for these highly desirable low work function properties were explained using a patchwork field emission model of the emitting surface. In this model, nanocrystals of ...

Characterization and field emission properties of lanthanum monosulfide nanoprotrusion arrays obtained by pulsed laser deposition on self-assembled nanoporous alumina templates

Three distinct types of nanostructures—nanodomes, nanodots, and nanowires—have been simultaneously self-assembled by pulsed laser deposition of lanthanum monosulfide on anodic alumina films containing hexagonal arrays of pores about 50nm wide and 500nm deep. The nanostructures have been characterized by x-ray diffraction, atomic force microscopy (AFM), and field emission scanning electron microscopy (FE-SEM). Nanodomes preferentially grow on the boundary separating regions (grains) of the alumina template that have near perfect pore ordering, and their density is ∼109∕cm2. The diameter of a nanodome at the base is about 100nm and their aspect ratio (height/diameter at the base) is between 1 and 3. Additionally, nanodots nucleate on top of the alumina walls that separate adjacent pores. They have a diameter of ∼50nm, a density equal to the pore density (1010∕cm2), and an aspect ratio less than 1. Finally, cross sectional FE-SEM images of the templates indicate that LaS nanowires grow inside the pores with ...

1.5: Development of field emission cathodes, electron gun and a slow wave structure for a terahertz traveling wave tube

High power terahertz (THz) sources and amplifiers hold the potential to greatly improve remote sensing and high bandwidth communication. To enable these applications, a Traveling Wave Tube (TWT) operating at 0.22 THz and a multi-cathode Field Emission (FE) electron gun are developed and characterized using a Particle-in-Cell simulation. Three candidate high current density cathode materials, Halfnium Carbide (HfC), carbon fibers, and Carbon Nanotubes (CNTs) were tested, characterized and their emission properties compared and used to verify simulations. A current of 3.0 mAmps for a single 100 micron diameter single walled nanotube rope was experimentally achieved and used as the basis of the FE gun design. Simulations of the FE gun and THz TWT were coupled and the effects of multiple and single tip FE gun beam characteristics on the TWT gain, bandwidth, and efficiencies are examined for several beam optic configurations.