Field Emitter Tips Research Articles

Oscillations and Bistability in the Catalytic Formation of Water on Rhodium in High Electric Fields

A comprehensive theory for the adsorption of H2 and O2 on a nanometric rhodium field emitter tip is developed to describe the equilibrium properties, the adsorption−desorption kinetics, as well as its observed nonlinear reaction behavior and oscillatory states. The basis is a kinetic mean-field model for hydrogen, oxygen, and subsurface oxygen which takes into account the anisotropy of the tip’s surface. The resulting model reproduces the correct anisotropy, period and form of the oscillations, as well as the bistability diagram for a varying temperature, hydrogen pressure, and external electric field as observed in a field ion microscope.

Polarization Control of Electron Tunneling into Ferroelectric Surfaces

We demonstrate a highly reproducible control of local electron transport through a ferroelectric oxide via its spontaneous polarization. Electrons are injected from the tip of an atomic force microscope into a thin film of lead-zirconate titanate, Pb(Zr0.2Ti0.8)O3, in the regime of electron tunneling assisted by a high electric field (Fowler-Nordheim tunneling). The tunneling current exhibits a pronounced hysteresis with abrupt switching events that coincide, within experimental resolution, with the local switching of ferroelectric polarization. The large spontaneous polarization of the PZT film results in up to 500-fold amplification of the tunneling current upon ferroelectric switching. The magnitude of the effect is subject to electrostatic control via ferroelectric switching, suggesting possible applications in ultrahigh-density data storage and spintronics.

Field electron emission from bunchy flake-like nano-carbon films

This paper reports that bunchy flake-like nano-graphite crystallite films (BNGCFs) were deposited on Si substrates by using the microwave chemical vapour deposition technique. Furthermore the BNGCFs were characterized by x-ray diffraction spectra, scanning electron microscopy, Raman spectra and field emission (FE) I–V measurements, and a lowest turn-on field of 1.5 V/μm, and a high average emission current density of 30 mA/cm2 at a macroscopic electric field of 8.0 V/μm were obtained. The J–E data did not follow the original Fowler–Nordheim (F–N) relation since they were not well represented in the F–N plot by a straight line. A model considering the F–N mechanism, and the statistic effects of FE tip structures has been applied successfully to explain all the FE data observed for E < 8.8 V/μm.

Nanometric chemical clocks.

Field ion microscopy combined with video techniques and chemical probing reveals the existence of catalytic oscillatory patterns at the nanoscale. This is the case when a rhodium nanosized crystal--conditioned as a field emitter tip--is exposed to hydrogen and oxygen. Here, we show that these nonequilibrium oscillatory patterns find their origin in the different catalytic properties of all of the nanofacets that are simultaneously exposed at the tip's surface. These results suggest that the underlying surface anisotropy, rather than a standard reaction-diffusion mechanism, plays a major role in determining the self-organizational behavior of multifaceted nanostructured surfaces. Surprisingly, this nanoreactor, composed of the tip crystal and a constant molecular flow of reactants, is large enough for the emergence of regular oscillations from the molecular fluctuations.

Ionization of helium gas with a tungsten tip

The ionization of a helium atom was investigated as a function of gas pressure, with the use of a tungsten tip. The tungsten tip, to which the external voltage was applied, was used to generate a constant electron current. The ionization current of helium gas was measured as a function of gas pressure. Effective ionization occurred in the pressure range of 0.5–20 torr when the distance between the field emission tip and the collector was 1 cm. The ionization current was linearly proportional to the voltage that was applied to the tungsten tip.

Electron Emission Properties of Single Field Emitter Covered with Diamond-Like Carbon (DLC) Film Prepared by Electro-Deposition in Methanol

A film formed by electro-deposition in methanol was successfully grown on single tungsten field emission tip. Carbon clusters containing hydrogen were detected from the coated tip by femto-second laser atom-probe. It was found that electrons which were needed to capture one carbon atom into the film were an order of 5. Emission current vs. voltage characteristic from the protrusion at the apex of the covered tip obeyed Fowler-Nordheim relationship, and gave work function about 4.1 eV. Good short-term stability in emission current was observed.

Field ionization of helium in a supersonic beam: Kinetic energy of neutral atoms and probability of their field ionization

High detection efficiency combined with spatial resolution on a nm-scale makes the field ionization process a promising candidate for spatially resolved neutral particles detection. The effective cross-sectional area σ eff can serve as a measure for the effectiveness of such a field ion detector. In the present contribution, we combine quantum-mechanical calculations of the field-modified electron density distribution near the tungsten tip surface and of the resulting local field distributions, performed using the functional integration method, with a classical treatment of the atom trajectories approaching the tip in order to calculate the σ eff values for ionization of free He atoms over an apex of a tungsten field emitter tip. The calculated values are compared with experimental data for supersonic He atomic beams at two different temperatures 95 and 298 K.

Enhanced field ionization of water adsorbed on a carbon monoxide-covered, platinum field emitter tip

The influence of carbon monoxide, adsorbed on a platinum field emitter tip, on field ionization of adsorbed water was examined. Ramped field desorption (RFD) measurements of water ionization were performed at 108K for water layer thicknesses up to 80ML on a clean or CO-saturated tip surface. In RFD the applied field is ramped linearly in time until water ionization is detected, giving the onset field of ionization. Water ionization yields hydrated hydroxide ions and protons; the hydroxide ions remain within the water layer on the tip, while the hydrated protons are emitted into vacuum. At a low water coverage of 1.5ML, the CO adlayer substantially reduced the onset field of ionization (that is, facilitated ionization) of water by 40%, from a value of 0.43V/Å for water on clean Pt to 0.26V/Å for water on CO-covered Pt. The extent of the reduction gradually diminished with thicker coverages of water and was absent at coverages of 20ML or greater. The characteristic decay length of the field enhancement was 4.7±1ML. The results were analyzed with the charge exchange model of ionization kinetics and changes in dipole moments of water adsorbed without and with CO. The analysis reveals that a change in water structure (dipole moment) caused by CO is an important contributor in field enhancement and that the dipole moment for hydrated hydroxide ion in an ice-like layer must be greater than that for bulk ice-like water. The significance of these results with respect to electrochemical oxidation of CO is discussed.

Fullerene-coated field emitters activated by a potassium ion flux in high electric fields

Activation of fullerenes covering tip field emitters by a potassium ion flux is studied. New data for the effect of the coating thickness, emitter temperature during coating application, time of ion processing, and ion energy on the activation efficiency are derived. These data are used for optimization of the activation process. The result is a twofold decrease in the voltage necessary to obtain fixed emission currents. The performance of activated emitters in strong electric fields is investigated, and conditions under which the emission current is high are established. It is shown that emitters with a diameter of the top of 0.3 μm can stably operate at emission currents of up to 100 μm A.

High-field versus high-pressure: Weakly adsorbed CO species on Pt(1 1 1)

Adsorption of carbon monoxide on a nanosized apex facet of a [1 1 1]-oriented Pt field emitter tip was studied under high electrostatic field conditions (15–20 V/nm) by field ion appearance energy spectroscopy (FIAES). Weakly bound highly mobile CO species have been detected and their possible role in surface reactions such as CO oxidation on Pt was elucidated. The FIAES results are compared with those of vibrational sum frequency generation (SFG) spectroscopy of CO adsorption on a macroscopic Pt(1 1 1) single crystal, in a gas pressure range from UHV to atmospheric. SFG and FIAES are complementary in that SFG mainly provides information on strongly bound CO molecules, whereas FIAES nearly exclusively detects weakly bound species. Both high-field and high-pressure studies indicate the presence of a compressed CO adlayer including weakly bound species. The high-field mimics the high gas pressure, leading to similar high adsorbate coverage, which is required for the formation of weakly bound highly mobile CO species, which may be the active entities in CO oxidation.

Field emission characteristics of nano-sheet carbon films deposited by quartz-tube microwave plasma chemical vapour deposition

Nano-sheet carbon films are prepared on Si wafers by means of quartz-tube microwave plasma chemical vapour deposition (MPCVD) in a gas mixture of hydrogen and methane. The structure of the fabricated films is investigated by using field emission scanning electron microscope (FESEM) and Raman spectroscopy. These nano-carbon films are possessed of good field emission (FE) characteristics with a low threshold field of 2.6 V/μm and a high current density of 12.6 mA/cm2 at an electric field of 9V/μm. As the FE currents tend to be saturated in a high E region, no simple Fowler–Nordheim (F-N) model is applicable. A modified F-N model considering statistic effects of FE tip structures and a space-charge-limited-current (SCLC) effect is applied successfully to explaining the FE data observed at low and high electric fields, respectively.

Investigation of an electron string ion source with field emission cathode

The string mode of operation for an electron beam ion source uses axially oscillating electrons in order to reduce power consumption, also simplifying the construction by omitting the collector with cooling requirements and has been called electron string ion source (ESIS). We have started a project (supported by INTAS and GSI) to use Schottky field emitting cathode tips for generating the electron string. The emission from these specially conditioned tips is higher by orders of magnitude than the focused Brillouin current density at magnetic fields of some Tesla and electron energies of some keV. This may avoid the observed instabilities in the transition from axially oscillating electrons to the string state of the electron plasma, opening a much wider field of possible operating parameters for an ESIS. Besides the presentation of the basic features, we emphasize in this paper a method to avoid damaging of the field emission tip by backstreaming ions.

Field emission properties of nano-structured carbon films with carbon needles deposited on Si using high-power-density microwave-plasma CVD method

Nano-structured carbon films (NCFs) were fabricated on Si wafer chips with hydrogen–methane gas mixture by creating high-power-density microwave (MW) plasma in a quartz tube with a low-output MW power source. Carbon sheets with many nano-protuberances were complicatedly twisted each other in the center region of the NCF while carbon needles were additionally formed in the fringe region of the NCF. These NCFs yielded field emission (FE) characteristics with the FE current density of 68 mA/cm 2 at a macroscopic electric field, E, of 10 V/μm. Since the FE currents tended to be saturated even in a medium E region, no simple Fowler–Nordheim (F–N) model was applicable. A model considering the F–N mechanism, statistic effects of FE tip structures and a space-charge-limited-current effect has been applied successfully to explain all the FE data observed for E < 10 V/μm. It turned out that E-dependent parameters such as the effective total emission area and the effective field enhancement factor should be employed to obtain sufficient quantitative agreements between the model and the experiments.

Field Emission from W Tips Sharpened by Field-Assisted Nitrogen and Oxygen Etching

Field emission properties were studied for tips sharpened using field-assisted nitrogen and oxygen etching. Sharp single crystal tungsten -oriented tips were fabricated and evaluated by Fowler-Nordheim plots and field-ion microscopy observations. The results demonstrated emissions at lower bias voltages due to the sharpening of the tip apex using the etching. The field emission properties and tip shapes after nitrogen etching were compared with those obtained after oxygen etching. [DOI: 10.1380/ejssnt.2008.152]

Modification of the field enhancement factor for a field emitter with a surrounding electrode stabilized using a field effect transistor

One of the useful methods for stabilizing field emission current in field emitter arrays (FEAs) is to control them using field effect transistors (FETs) connected in series. However, one cannot use conventional FETs, because the electron extraction voltage (typically hundreds of volts) of FEAs is much higher than the permitted source-drain voltage of FETs. The authors conducted a detailed investigation of the mechanism for current stabilization of the field emission current while controlling the field emitter tip wtih a FET. The electron source consists of the FET controlled tip and surrounding electrode, which is kept at a constant potential. In this paper the authors discuss how the field enhancement factor (β) is modified by the potential distribution, which is determined by the potentials of the tip and the surrounding electrode, Making use of this effect, the authors could control a FEA that has a high electron extraction voltage with a FET with a lower source-drain voltage.

Extension of lifetime of silicon field emitter arrays in oxygen ambient by carbon negative ion implantation

Extension of lifetime of silicon field emitter arrays (Si-FEAs) in oxygen ambient was achieved by implanting carbon negative ions into the emitter surface. The authors examined three samples, each of which has 1024 field emission tips and implanted with 5keV carbon negative ions to the doses of 1×1016, 3×1016, and 1×1017ionscm−2. The lifetime, the time when the emission current becomes half the maximum value after starting operation, was extended in accordance with the implantation dose. The lifetime of the sample which was dosed with 1×1017ionscm−2 was 50h, which was about 17 times as long as that of as-fabricated Si-FEA.

Field desorption ion source development for neutron generators

A new approach to deuterium ion sources for deuterium-tritium neutron generators is being developed. The source is based upon the field desorption of deuterium from the surfaces of metal tips. Field desorption studies of microfabricated field emitter tip arrays have been conducted for the first time. Maximum fields of 3 V/Å have been applied to the array tip surfaces to date, although achieving fields of 2 V/Å to 2.5 V/Å is more typical. Both the desorption of atomic deuterium ions and the gas-phase field ionization of molecular deuterium have been observed at fields of roughly 2 and 2–3 V/Å, respectively, at room temperature. The desorption of common surface adsorbates such as hydrogen, carbon, water, and carbon monoxide is observed at fields exceeding ∼1 V/Å. In vacuo heating of the arrays to temperatures of the order of 800 °C can be effective in removing many of the surface contaminants observed.

Neutron production from feedback controlled thermal cycling of a pyroelectric crystal

The LLNL Crystal Driven Neutron Source is operational and has produced record ion currents of approximately 10 nA and neutron output of 1.9(+/-0.3)x10(5) per thermal cycle using a crystal heating rate of 0.2 degrees C/s from 10 to 110 degrees C. A 3 cm diameter by 1 cm thick LiTaO(3) crystal with a socket secured field emitter tip is thermally cycled with feedback control for ionization and acceleration of deuterons onto a deuterated target to produce D-D fusion neutrons. The entire crystal and temperature system is mounted on a bellows which allows movement of the crystal along the beam axis and is completely contained on a single small vacuum flange. The modular crystal assembly permitted experimental flexibility. Operationally, flashover breakdowns along the side of the crystal and poor emitter tip characteristics can limit the neutron source. The experimental neutron results extend earlier published work by increasing the ion current and pulse length significantly to achieve a factor-of-two higher neutron output per thermal cycle. These findings are reviewed along with details of the instrument.

Field emission characteristics of lanthanum hexaboride coated silicon field emitters

Lanthanum hexaboride(LaB6) films have been deposited on silicon tip field emitters by electron-beam evaporation. The field emission characteristics are studied in a diode test cell in a vacuum system. The experimental results show that the field emission stability of the LaB6-coated Si-tip FEA can be improved and the emission current is significantly enhanced to 75 µA, in contrast to pure Si-tip FEA of 500 nA and Mo-coated Si-tip FEA of 300 nA with 1500 V applied to the anode. Furthermore, even at low vacuum (>1.5 × 10−5 Torr) it still exhibits good emission properties and strong ability to withstand ion bombardment. This LaB6-coated Si-tip FEA is an excellent electronic candidate, in particular a fit for travelling-wave tubes, klystrons and other large vacuum apparatus.

Mechanism of surface-tension reduction by electric-field application: Shape changes in single-crystal field emitters under thermal-field treatment

The shape change of the ⟨100⟩-oriented tungsten single-crystal field emitter tip under thermal-field (TF) treatment was systematically studied with increasing remolding field (i.e., reversely polarized field opposite to field-emission polarity) at a constant temperature. The initially hemispherical emitter tip builds up to polyhedral shapes with increasing remolding voltage. These shapes are formed by certain low-index crystal planes. On further increase of the voltage, an overremolding state is reached in which the emitter takes a rugged surface structure. A thermodynamic model is proposed to explain the buildup mechanism utilizing the reduction of the surface tension under the influence of a strong electric field. The theory assumes the emitter-extractor system to comprise an electric capacitor and includes the electrostatic-field energy stored around the emitter into the system under consideration. It is shown that the electrostatic-field energy term in the Gibbs free energy could be interpreted as an effective reduction of the surface tension. The initially moderate anisotropy of the intrinsic surface tension is increasingly enhanced by the electrostatic-field energy through the reduction mechanism. The facets on the emitter surface consequently develop in accordance with the Wulff construction of the equilibrium crystal shape (ECS). By application of an electric field, the surface free-energy anisotropy can be made large enough to induce complete faceting of the crystal even at an elevated temperature where the crystal shape can immediately follow the ECS.