K.L. Jensen

K.L. Jensen
United States Naval Research Laboratory | NRL · Materials Science and Technology Division

Doctor of Philosophy

About

273
Publications
17,110
Reads
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4,404
Citations
Additional affiliations
October 1987 - present
United States Naval Research Laboratory
Position
  • Research Physicist

Publications

Publications (273)
Article
A recent algorithm developed by Tripathi et al. [J. Appl. Phys. 128, 025017 (2020); Erratum, J. Appl. Phys. 131, 169901 (2022)] is modified to study the effects of thermal contact resistance on the field emission (FE) properties of a carbon nanotube (CNT). The model takes into account the temperature dependence of the CNT electrical and thermal con...
Article
Full-text available
Electron sources exploiting field emission generally have sharp geometries in the form of cones and wires. Often, they operate under elevated temperatures. A sharply curved emitter affects the emission barrier past which the electrons must be emitted via thermal-field processes, as does as space charge in metal-insulator-metal (MIM) and metal-oxide...
Preprint
Full-text available
Experimental observations of thermionic electron emission demonstrate a smooth transition between TL and FSCL regions of the emitted-current-density-versus-temperature (J-T) (Miram) curve and the emitted-current-density-versus-voltage (J-V) curve. Knowledge of the temperature and shape of the TL-FSCL transition is important in evaluating the thermi...
Article
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An electron wave packet tunneling through a barrier has a transmission (or "group delay") time τg that, for a rectangular barrier, is commonly held to become independent of the barrier width L as the width increases (the McColl-Hartman effect). In the present study, it is shown that: first, the McColl-Hartman effect for a rectangular barrier is dep...
Article
We advocate the use of lookup tables in the development of extremely fast and accurate multiscale models based on the coupling of a quantum-mechanical wave impedance approach and finite-element simulations to determine the local emission current density (LECD) from a metallic emitter of arbitrary shape. The lookup tables are prepopulated with numer...
Article
Full-text available
Photocathodes emit electrons when illuminated, a process utilized across many technologies. Cutting-edge applications require a set of operating conditions that are not met with current photocathode materials. Meanwhile, halide perovskites have been studied extensively and have shown a lot of promise for a wide variety of optoelectronic application...
Article
Full-text available
A numerical solution to the time evolution equation of the Wigner distribution function (WDF) with an accuracy necessary to simulate the passage of a wave packet past a barrier is developed, where quantum effects require high accuracy and fine discretization. A wave packet incident on a barrier, a portion of which tunnels through, demonstrates beha...
Article
Full-text available
Carbon fibers passing current are subject to resistive heating. Where failure occurs is related to its local temperature. The failure temperature and its location is estimated. The temperature variation is calculated using analytical models for electrical and thermal conductivities based on the temperature dependent electron-phonon relaxation time....
Chapter
Traditionally electron sources are characterized as thermal, field, and photoemission cathodes (a fourth, secondary, is not considered here), each governed by a canonical emission equation (Richardson—Laue—Dushman, Fowler—Nordheim, and Fowler—DuBridge, respectively) for current density. Modern electron sources operate such that more than one regime...
Article
An efficient algorithm is described to calculate the spatial dependence of the temperature distribution along a carbon nanotube (CNT) during field emission (FE). The algorithm considers the effects of Joule heating in the CNT and radiative losses from the CNT sidewall and tip. The CNT emission current density and the rate of heat exchange per unit...
Article
Full-text available
The complexity of photocathode designs and detector materials, and the need to model their performance for short pulse durations, the response to high-frequency photons, the presence of coatings and/or thinness of the absorptive layer, necessitates modifications to three-step and moments models of photoemission that are used in simulation codes. In...
Article
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Tunneling barriers are an essential component of electron sources, sensors, detectors, and vacuum nanoelectronics and a pivotal factor in their performance, but the barriers themselves routinely depart from the analytic models used to model their behavior. A new formalism is developed to analytically and accurately model emission through and over b...
Article
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A methodology for implementing the recently developed reformulated General Thermal Field (rGFT) equation describing simultaneous thermal and field emission contributions to electron emission is developed, with modifications directed to enhancing speed and accuracy of computation as demanded by emitter characterization and electron beam simulations....
Article
Quantum Efficiency Enhancement Phototube with accelerator technology relevant antimonide photocathodes (K2CsSb) deposited on atomically thin two‐dimensional (2D) crystal layers such as graphene. Quantum efficiency enhancement occurs in a reflection mode, when a 2D crystal is placed in between the photocathodes and optically reflective substrates. M...
Article
We report quantum efficiency (QE) enhancements in accelerator technology relevant antimonide photocathodes (K2CsSb) by interfacing them with atomically thin 2D crystal layers. The enhancement occurs in a reflection mode, when a 2D crystal is placed in between the photocathodes and optically reflective substrates. Specifically, the peak QE at 405 nm...
Preprint
Full-text available
We report quantum efficiency (QE) enhancements in accelerator technology relevant antimonide photocathodes (K2CsSb) by interfacing them with atomically thin two-dimensional (2D) crystal layers. The enhancement occurs in a reflection mode, when a 2D crystal is placed in between the photocathodes and optically reflective substrates. Specifically, the...
Article
Full-text available
The analytic Wigner function for a single well with infinite walls is extended to the configuration where half of the well is raised and the weighting of the wave functions is in accordance with a thermal Fermi-Dirac distribution. This requires a method for determining the energy eigenstates particularly near the height of the higher half-well. The...
Article
We report the first operation of field-emission cathodes produced from electrically conductive polymers using 3-D printing. These cathodes consisted of a single high-aspect-ratio emitter printed using a commercially available fused filament fabrication (FFF) printer. Emitters were printed from one of three commercially available polylactic acid (PL...
Article
Full-text available
Theoretical models of thermal, field, and thermal-field emission of electrons generally require the evaluation of the Gamow, or tunneling, factor. A rapid means to do so is developed using shape factor methods for general potentials, but in particular, for the image charge (or Schottkky-Nordheim) barrier from which the Fowler-Nordheim (field emissi...
Article
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A methodology of modeling non-planar surfaces, in which the micro-scale features of the emission sites can be orders of magnitude smaller than the meso-scale features defining the active emission area, has been developed and applied to both ordered arrays of identical emitters and random variations characteristic of a roughened surface. The methodo...
Article
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Schottky's Conjecture posits that the geometric field enhancement produced by a hybrid shape formed from a small perturbation on a larger base is the product of the individual field enhancement factors of the base and perturbation in isolation. This is a powerful concept with practical applications to understanding field emitter design and operatio...
Article
Full-text available
Schottky's Conjecture (SC) posits that when a compound conductive structure is formed by placing a protrusion on top of an underlying base, the total field enhancement factor is the product of the field enhancement factors that would be produced by the base and protrusion in isolation. This is a powerful concept, which, in principle, allows separat...
Article
Full-text available
The Wigner function is assembled from analytic wave functions for a one-dimensional closed system (well with infinite barriers). A sudden change in the boundary potentials allows for the investigation of time-dependent effects in an analytically solvable model. A trajectory model is developed to account for tunneling when the barrier is finite. The...
Article
The development of photoemission electron sources to specifically address the competing and increasingly stringent requirements of advanced light sources such as x-ray free-electron lasers (XFELs) motivates a comprehensive material-centric approach that integrates predictive computational physics models, advanced nanosynthesis methods, and sophisti...
Article
Accelerator technology‐relevant photocathodes are now successfully deposited on free‐standing atomically thin substrates of graphene. This advances an ultimate goal of enhancing the lifetime of photocathodes without sacrificing the high quantum efficiency by using an atomically thin protection layer. More details can be found in the article number...
Article
We report successful deposition of high quantum efficiency (QE) bialkali antimonide K2CsSb photocathodes on graphene films. The results pave a pathway towards an ultimate goal of encapsulating technologically-relevant photocathodes for accelerator technology with an atomically-thin protecting layer to enhance lifetime while minimizing QE losses. A...
Article
Full-text available
Electron emission from coated surfaces as a result of thermal, field, and photoemission effects is often described theoretically using models dependent on the Kemble approximation for the transmission probability D(k). The validity of the approximation for the simple potential profiles (rectangular, triangular, parabolic) is examined, and generaliz...
Article
Full-text available
Recent experimental measurements of a bulk material covered with a small number of graphene layers reported by Yamaguchi et al. [NPJ 2D Mater. Appl. 1, 12 (2017)] (on bialkali) and Liu et al. [Appl. Phys. Lett. 110, 041607 (2017)] (on copper) and the needs of emission models in beam optics codes have lead to substantial changes in a Moments model o...
Article
A compact introduction to the history and the canonical equations of electron emission is given for thermal emission (Richardson), field emission (Fowler-Nordheim), photoemission (Fowler-DuBridge), and secondary emission (Baroody), as well as the space-charge-limited flow (Child-Langmuir law). A general equation is derived and related to the canoni...
Article
A theoretical model of delayed emission following photoexcitation from metals and semiconductors is given. Its numerical implementation is designed for beam optics codes used to model photocathodes in rf photoinjectors. The model extends the Moments approach for predicting photocurrent and mean transverse energy as moments of an emitted electron di...
Article
Full-text available
We have successfully deposited Fe4N thin films with (111) out-of-plane orientation on thermally oxidized Si substrates using a facing-target-sputtering system. A Ta/Ru composite buffer layer was adopted to improve the (111) orientation of the Fe4N thin films. The N2 partial pressure and substrate temperature during sputtering were optimized to prom...
Article
Quantum tunneling by field emission from nanoscale features or sharp field emission structures for which the anode-cathode gap is nanometers in scale (“nano diodes”) experience strong deviations from the planar image charge lowered tunneling barrier used in the Murphy and Good formulation of the Fowler-Nordheim equation. These deviations alter the...
Article
Predictive models of field emission remain elusive, in part, due to the sensitivity of this process to emitter surface details at length scales ranging from macroscopic to atomic. Moving towards more fully predictive models requires that we develop techniques to disentangle contributions of features on the largest length scales, which can be easily...
Conference Paper
Research and development on high efficiency, robust, long-life photocathodes is in progress for accelerator, light source, and other commercial applications. The research is investigating detailed physics of photoemission and developing a computational capability to predict performance. Reservoir technology will significantly increase lifetime and...
Article
Full-text available
The hexagonal structure of graphene gives rise to the property of gas impermeability, motivating its investigation for a new application: protection of semiconductor photocathodes in electron accelerators. These materials are extremely susceptible to degradation in efficiency through multiple mechanisms related to contamination from the local imper...
Article
Analytic image charge approximations exist for planar and spherical metal surfaces but approximations for more complex geometries, such as the conical and wirelike structures characteristic of field emitters, are lacking. Such models are the basis for the evaluation of Schottky lowering factors in equations for current density. The development of a...
Article
Future advanced light sources and x-ray free electron lasers require fast response from the photocathode to enable short electron pulse durations as well as pulse shaping, and so the ability to model delays in emission is needed for beam optics codes. The development of a time-dependent emission model accounting for delayed photoemission due to tra...
Article
Photocathodes can benefit from a thin protection layer and attain long-term stability. Graphene is potentially a good candidate for such application. We report direct growth of single-layer graphene on single crystal Cu(110) photocathodes using chemical vapor deposition and the effective protection of copper photocathodes with graphene against degr...
Article
Full-text available
In cold field and thermo-field emission, positive ions or adsorbates very close to the cathode surface can enhance emission current by both resonant and non-resonant processes. In this paper, resonant tunneling behavior is investigated by solving the one-dimensional Schrödinger equation in the presence of an ion, and the enhancement due to resonant...
Article
Full-text available
This paper reports an experiment that studies the emission characteristics of small number of field emitters. The experiment consists of nine carbon fibers in a square configuration. Experimental results show that the emission characteristics depend strongly on the separation between each emitter, providing evidence of the electric field screening...
Conference Paper
Building on recent efforts to characterize carbon nanotube fibers (CNFs) and electron emission suitable for compact, high power, high frequency, vacuum electronic devices, this paper describes a proposed exhaustive approach towards optimizing CNF field emission (FE) properties. It outlines how a platform geared towards meaningful comparisons betwee...
Article
In arrays of ungated field emitters, nearby emitters introduce a screening effect, reducing each emitter's field enhancement factor below its value in isolation. At the edge of the array, however, the strength of this screening effect is reduced because the number of nearby emitters is reduced, leading to a variation in the emitted current wit...
Article
Full-text available
A Point Charge Model of conical field emitters, in which the emitter is defined by an equipotential surface of judiciously placed charges over a planar conductor, is used to confirm Schottky’s conjecture that field enhancement factors are multiplicative for a small protrusion placed on top of a larger base structure. Importantly, it is shown that S...
Article
In arrays of ungated field emitters, the field enhancement factor of each emitter decreases as the distance between the emitters decreases, an effect known as screening. At the edge of these arrays, emitters experience reduced screening, leading to higher field enhancement factors than emitters at the array center, causing nonuniform emission acros...
Article
Secondary electron transmission measurements are used to examine the electron diffusion and thermalization processes in B-doped nanocrystalline diamond membranes of thickness 2.3 μm and 650 nm. Specifically, transmitted energy spectra are measured following impact by an electron beam that penetrates deeper in the membrane as the beam energy E o inc...
Article
Full-text available
Field emitter arrays are high-brightness electron sources with important current and future applications in vacuum electronics, particle accelerators, and directed energy. The current produced by individual emitters in these arrays is controlled by the total electric field on their surfaces, which depends on the background field, the space charge f...
Article
Although Fowler and Nordheim developed the basics of field emission nearly one century ago with their introduction of the Fowler-Nordheim equation (FNE), the topic continues to attract research interest particularly with the development of new materials that have been proposed as field emitters. The first order analysis of experiments typically rel...