Field-induced Electron Emission Research Articles

Field induced electron emission from graphene nanostructures

Electric fields play a crucial role in modulating the electronic properties of nanoscale materials. Electron emission, induced by an electric field, is a representative phenomenon. Experimental and theoretical aspects of such electron emission from graphene are briefly reviewed. The emission occurs at the edge of graphene flakes, not at the surface, because the edge highly concentrates the electric field. Emission currents are sensitive to the edge shapes and edge functionalization. This review provides guiding principles for designing high-efficiency field-emission devices by using graphene nanostructures.

A stable LaB6 nanoneedle field-emission electron source for atomic resolution imaging with a transmission electron microscope

Field-induced electron emission offers the best performance point electron source for electron microscopes. Herewith we report a new cold field-emission point electron source utilizing a nanoneedle made of lanthanum hexaboride (LaB6) and its implementation in a spherical aberration-corrected transmission electron microscope (TEM). A sub-ångstrom resolution of 0.96 Å has been obtained with the TEM operated at an acceleration voltage of 60 kV and a smaller energy spread was also observed compared with the contemporary W(310) electron source. In particular, the LaB6 nanoneedle cold field-emission electron source required no thermal flashing for continuous operation of extended hours (>100 hours) while exhibiting and sustaining a high stability in emission current (<1%). We attribute the excellent performance of the LaB6 nanoneedle electron source to (i) a low work function of LaB6; (ii) excellent alignment of the nanoneedle emitter by the electron and ion dual-beam processing with nanoscale precision and (iii) robustness of the nanometric structure that suppressed mechanical vibrations of the LaB6 emitter even in a high electric field. This new LaB6 nanoneedle cold field-emission electron source enables stable high-resolution imaging in TEM and it will also benefit cryogenic electron microscopy, scanning electron microscopy, electron beam lithography, and other electron beam technologies.

Low-Field Electron Emission Capability of Thin Films on Flat Silicon Substrates: Experiments with Mo and General Model for Refractory Metals and Carbon.

Herein, we describe a study of the phenomenon of field-induced electron emission from thin films deposited on flat Si substrates. Films of Mo with an effective thickness of 6–10 nm showed room-temperature low-field emissivity; a 100 nA current was extracted at macroscopic field magnitudes as low as 1.4–3.7 V/μm. This result was achieved after formation treatment of the samples by combined action of elevated temperatures (100–600 °C) and the electric field. Morphology of the films was assessed by AFM, SEM, and STM/STS methods before and after the emission tests. The images showed that forming treatment and emission experiments resulted in the appearance of numerous defects at the initially continuous and smooth films; in some regions, the Mo layer was found to consist of separate nanosized islets. Film structure reconstruction (dewetting) was apparently induced by emission-related factors, such as local heating and/or ion irradiation. These results were compared with our previous data obtained in experiments with carbon islet films of similar average thickness deposited onto identical substrates. On this basis, we suggest a novel model of emission mechanism that might be common for thin films of carbon and refractory metals. The model combines elements of the well-known patch field, multiple barriers, and thermoelectric models of low-macroscopic-field electron emission from electrically nanostructured heterogeneous materials.

Gas pressure measurement using micro-corona-discharging effect in surface acoustic wave resonators

This paper reports a gas pressure sensor using surface acoustic wave resonator (SAWR) based on both the field-induced electron emission and the micro-corona-discharging (MCD) phenomena. With the micron-scale finger-gap of electrodes covered with SiO2 passivation layer, the enhanced field-effect emission (FEE) at electrodes result in the micro-corona-discharging and thus the change in the dynamic resistance of SAWR. A high Q-factor SAWR was fabricated on ST-cut quartz substrate using microelectronics processing technology, and the gas-pressure-dependent return-loss was tested, and the MCD theory and the equivalent circuit model of SAWR were used to analyze the test results. Under argon pressure range from 0 to 601 kPa, the dynamic resistance of the SAWR varies from 24.569 to 41.589 Ω at 25 °C. The resistance rate reaches 69.3%, which is far higher than that of traditional piezoresistive pressure sensors. At the same time, the oxygen pressure was also tested by the SAWR as a comparison. The SAW-based gas pressure sensors have the potential for measurement of the super-high pressure.

Research on effects of space charge field in relativistic backward wave oscillator

The breakdown problem in the relativistic backward wave oscillator (RBWO) limits the microwave pulse energy and should be solved. The slow wave structure (SWS) is the most key component of the RBWO. The breakdown of SWS may be triggered by field-induced electron emission and electron bombardment on the wall. This paper demonstrates that the space charge field of the relativistic electron beam (REB) can enhance the electric field forbidding field-induced electron emission and decrease the electric field igniting the RF breakdown, and therefore, the RF breakdown can be significantly suppressed. However, the enhanced electric field will enlarge the radial displacements of electrons, especially the high-energy electrons. When the RBWO operates with a low guiding magnetic field or a long-pulse REB, this situation will worsen and some electrons may possess large enough radial displacements and bombard on the SWS surface with the help of radial oscillation of REB and radial expansion of cathode plasmas. This may aggravate the breakdown of SWS.

Nanodiamond vacuum field emission microtriode

Vacuum field emission (VFE) microtriodes utilizing nanodiamond emitters, integrated with a self-aligned silicon gate and an anode and fabricated by the mold-transfer patterning technique on a silicon-on-insulator (SOI) substrate, have been developed. The nanodiamond VFE microtriodes were fabricated by an integrated circuit-compatible microfabrication process in conjunction with chemical vapor deposition of nanodiamond into the inverted-pyramidal molds micropatterned on the SOI substrate, which provides precision controlled emitter-gate alignment and spacing. The devices exhibited triode characteristics showing anode field induced electron emission with gate controlled emission current modulation at low operating voltages, agreeing with its electron emission transport model. A high current density of 150 mA/cm2 is achievable from the device with the anode-emitter spacing of 4 μm at low operating voltages of Va = 48.5 V and Vg = 5 V. The ac characteristics of the microtriodes for signal amplification were experimentally evaluated, and the results conformed to the proposed small signal equivalent circuit model. The triode small signal parameters were found to be dependent on the device geometry, which could be tailored to meet various applications by designing the physical structures of the device with the desired parameters. These results demonstrate the potential use of the nanodiamond VFE microtriodes for vacuum microelectronic applications.

Operating modes of field emission assisted microplasmas in the microwave regime

The operating modes of microwave microplasma devices integrated with field emitting cathodes are studied using one-dimensional particle-in-cell with Monte Carlo collision (PIC-MCC) simulations. The PIC-MCC simulations predict operation in two modes—an α-mode characterized by a positive differential resistance with negligible influence of boundary processes and a γ-mode with significant field-induced electron emission. PIC-MCC results are presented for two representative 0.5 GHz argon microplasmas operating in the α and γ modes. The field emission-induced transition to γ-mode modifies the electron number density profiles in the sheath apart from leading to a higher contribution of conduction current in the sheath. The interpretation of electrical characteristics using time history of voltage and current demonstrates that the microplasma device impedance decreases as a result of the thinner sheath. It is also shown that the presence of field emitting cathodes leads to lower power requirements (about 64% of the case presented without field emission) to achieve a given plasma density.

Modeling of the effect of temperature and field-induced electron emission from the cathode with a thin insulating film on the Townsend discharge ignition voltage in argon–mercury mixture

A model of the low-current (Townsend) discharge in argon–mercury mixture in the presence of a thin insulating oxide film on the cathode is developed. It takes into account the cathode ion-electron emission and the electron emission from the cathode metal substrate under the strong electric field generated in the film by the ion surface charge. An influence of the mixture temperature and the oxide film on the discharge ignition voltage is estimated and it is shown that formation of a thin insulating film on the cathode surface can facilitate the discharge ignition in mercury-containing gas discharge devices at low temperatures.

Field-induced electron emission from nanoporous carbon of various types

The influence of fabrication technology on field electron emission properties of nanoporous carbon (NPC) has been investigated. Samples of NPC derived from different carbides via chlorination at different temperatures demonstrated similar low-field emission ability with the threshold electric field strength of 2–3 V/μm. This property correlated with the presence of nanopores with the characteristic size of 0.5–1.2 nm determining high values of specific surface area (more than 800 m2/g) of the material. In most cases, voltage–current characteristics of emission were approximately linear in Fowler–Nordheim (FN) coordinates (excluding the low-current part near the emission threshold), but the plot slope angles were in notable disagreement with the known material morphology and electronic properties, and this could not be explained within the frames of FN emission theory. We suggest that the actual emission mechanism for NPC involves hot electrons generated at internal boundaries, and that emission centers may be associated with relatively large (20–100 nm) onion-like particles observed in many microscopy images.

Field‐Induced Electron Emission from Nanoporous Carbons

Influence of fabrication technology on field electron emission properties of nanoporous carbon (NPC) was investigated. Samples of NPC derived from different carbides via chlorination at different temperatures demonstrated similar low‐field emission ability with threshold electric field 2‐3 V/μm. This property correlated with presence of nanopores with characteristic size 0.5–1.2 nm, determining high values of specific surface area (&gt;800 m2/g) of the material. In most cases, current characteristics of emission were approximately linear in Fowler‐Nordheim coordinates (excluding a low‐current part near the emission threshold), but the plots’ slope angles were in notable disagreement with the known material morphology and electronic properties, unexplainable within the frames of the classical emission theory. We suggest that the actual emission mechanism for NPC involves generation of hot electrons at internal boundaries and that emission centers may be associated with relatively large (20–100 nm) onion‐like particles observed in many microscopic images. Such particles can serve two functions: to provide additional “internal” enhancement of the electric field and to inhibit relaxation of hot charge carriers due to the “phonon bottleneck” effect.

A rapid high power microwave switch based on multipactor electrons flow

A plasma waveguide switch based on multipactor effect for switching low/high power microwave is proposed. No external triggering and no actuator are needed for the switch. Through controlling the electron flow formed on the waveguide sidewall based on field-induced electron emission and secondary electron multipactor, the high power capacity, low loss, and rapid activation time are realized in the microwave switch simultaneously. The power loss is about −20 dB, turn-on time is 5 ns, and the power capacity of hundreds of megawatt can be achieved. The switch has been used for backward wave oscillator injection locking.

Field-Induced Electron Emission from Graphitic Nano-Island Films at Silicon Substrates

Numerous carbonic nano-structured materials were reported to demonstrate electron emission in moderate electric field (1 kV/mm or less), even if their surface morphology shows no edges or tips capable of strong local field enhancement. In many such cases, the actual mechanism of emission facilitation remains unclear, partially due to the complex structure of the investigated materials usually representing a three-dimensional system of domains with different properties. In the presented work, we investigated a simpler, quasi-2d system, presumably providing better possibilities for unambiguous description of the emission mechanism. Emitters were produced by chemical deposition of small amounts of graphitic carbon at silicon substrates. The films had nano-island structure and demonstrated good emission properties with threshold electric field as low as 550 V/mm.

Potential barrier decrease at cathode boundary under high-voltage gas-discharge conditions

The influence of atoms, molecules, or layers adsorbed on the cathode surface on the initiation of high-voltage gas discharge is considered. It is shown that the presence of island films of an adsorbate leads to an increase in the field-induced electron emission current that initiates breakdown of the gas-filled interelectrode gap. The formation of a continuous adsorbed film on the cathode accounts for the subsequent weak dependence of the emission characteristics in high-voltage (in particular, picosecond pulsed) discharge on the gas phase composition.

On Possible Structure of Field-Induced Electron Emission Centers of Nanoporous Carbon

Analysis of emission characteristics of a nanostructured porous carbon material measured in pulsed electric field demonstrated that their hysteresis complies with two-barrier emission model in the additional assumption that electron transport through the internal barrier is controlled by electric charging of the surface. Quantitative parameters of emission centers were derived from emission I-V plots. These parameters comply with the model of emission center as a single onion-like particle at the emitter surface connected with the rest of emitter via a tunnel junction.

Interpretation of Dynamic and DC Field-Emission Characteristics of Nanocarbons in Terms of Two-Stage Emission Model

Experimental data on hysteresis of field-induced electron emission from nanocarbon materials in pulsed field are compared with results of simulations performed with simplified dynamic model of two-barrier emission mechanism. Transformation of I-V plots with variation of emitter properties and field pulse parameters was found to agree with theoretical predictions for special emission regimes, which allows us to associate parameters of I-V plots with quantitative parameters of emission centers.

“Fine Structure” of Emission I-V Characteristics of Nanodispersed Films

I-V plots of field-induced electron emission from nanostructured materials often deviate from predictions of Fouler-Nordheim theory. For samples of nanoporouse carbon, we observed reproducible “fine” structure of emission I-V plots with intervals of saturation and even negative current derivative. Such features find explanation in two-barrier emission model, proposed earlier to explain phenomena of low-field emission and pulsed-field hysteresis.

PREPARATION AND CHARACTERIZATION OF ORGANIC FERROELECTRIC CATHODES

ABSTRACT The phenomenon of electron emission from ferroelectric cathodes has attracted much interest and today is a subject of investigation all over the world. In this paper, organic PVDF (polyvinylidene fluoride) thin films were prepared by ultrasonic atomization method, and pulse excited electric field induced electron emission from the poled organic PVDF ferroelectric cathode has been investigated. The results indicated that the reason of electron emission was the compensated charges on the surface of organic ferroelectric cathode unbalancing caused by fast reversal of the polarization. Repeatable emissions were obtained from the organic PVDF ferroelectric cathode, which could also be modulated at high electric field. It is anticipated that an electron source, based on emission from PVDF ferroelectric cathode, will comprehensively trigger a novel type of display.

ELECTRON EMISSION OUT OF POLY(VINYLIDENE FLUORIDE) THIN FILMS

ABSTRACT Pulse electric field induced electron emission from the ferroelectric polymer has been investigated. Electron emission of unpoled and poled poly(vinylidene fluoride) thin films prepared by spin coating was measured under increasing driving field with fixed pulse-exited frequency. The results showed that the emission current from poled polymer thin film is attributed to its polarization, including the polarization reversal in the crystalline phase or amorphous phase, and the emission of excess charges in the Maxwell–Wagner interface. And besides, it is found that the charges frozen and injected in the Maxwell–Wagner interface under an appropriate driving pulse field played an important role of electron emission for unpoled thin film sample.

Electron emission from conduction band of diamond with negative electron affinity

Experimental evidence explaining the extremely low-threshold electron emission from diamond reported in 1996 has been obtained for the first time. Direct observation using combined ultraviolet photoelectron spectroscopy/field emission spectroscopy (UPS/FES) proved that the origin of field-induced electron emission from heavily nitrogen (N)-doped chemical vapour deposited (CVD) diamond was at conduction band minimum (CBM) utilising negative electron affinity (NEA). The significance of the result is that not only does it prove the utilisation of NEA as the dominant factor for the extremely low-threshold electron emission from heavily N-doped CVD diamond, but also strongly implies that such low-threshold emission is possible from other types of diamond, and even other materials having NEA surface. The low-threshold voltage, along with the stable intensity and remarkably narrow energy width, suggests that this type of electron emission can be applied to develop a next generation vacuum nano-electronic devices with long lifetime and high energy resolution.

Interpretation of the field enhancement factor for electron emission from carbon nanotubes

The local electric fields in the vicinity of the tips of metallic nanotubes are calculated. The variation in the field enhancement factor as a function of internanotube separation, anode-substrate separation, and height and radius of the nanotube is reported. Calculations show that the field induced electron emission current, based on the Fowler–Nordheim theory, is reduced when the intertube separation is less than twice the height of the nanotube. The location of the anode plane is shown to be important when the anode-substrate separation is less than three times the height of the nanotube. The results also predict that the macroscopic threshold field for electron emission should decrease as the anode-substrate separation D decreases. For separations greater than three times the height of the nanotube, the threshold field becomes constant and independent of anode-cathode geometry. Conversely, the manner in which applied electric field is defined is crucial if the results are be understood better. Experimental field emission measurements made on an isolated carbon nanotube confirms the need for a new interpretation of the electric field around stand alone point emitters.