Field Emission Patterns Research Articles

Self-joule heating assisted field emission following the Child–Langmuir law

In this study, stable and long-term field emission properties that completely follow the Child–Langmuir law were successfully observed. A tungsten tip covered with a liquid gallium metal was used. The current characteristics showed three phases. The electron emission first began below half of the threshold voltage for the emission from a bare W chip, and the current increased by 20 μA. Then, the field emission pattern showed multiple disordered blinking spots, which originated Ga Taylor cones and the emission current value reached several mAs. Then, emission current began to follow the Child–Langmuir law, and a clear field emission pattern from {011}-oriented tungsten was observed. Electrons emitted from the submicrometer sized area that is heated by itself with joule heating of current densities greater than 107 A/cm2.

Oxygen Adsorption, Subsurface Oxygen Layer Formation and Reaction with Hydrogen on Surfaces of a Pt\u2013Rh Alloy Nanocrystal

The oxygen adsorption and its catalytic reaction with hydrogen on Pt–Rh single crystals were studied at the nanoscale by Field Emission Microscopy (FEM) and Field Ion Microscopy (FIM) techniques at 700 K. Both FEM and FIM use samples prepared as sharp tips, apexes of which mimic a single nanoparticle of catalyst considering their similar size and morphology. Oxygen adsorption on Pt-17.4 at.%Rh samples leads to the formation of subsurface oxygen, which is manifested in the field emission (FE) patterns: for O2 exposure of ~3 Langmuir (L), {113} planes appear bright in the emission pattern, while for higher oxygen doses, i.e. 84 L, the bright regions correspond to the high index planes between the {012} and {011} planes. Formation of subsurface oxygen is probably accompanied by a surface reconstruction of the nanocrystal. The subsurface oxygen can be effectively reacted off by subsequent exposure of the sample to hydrogen gas at 700 K. The hydrogenation reaction was observed as a sudden, eruptive change of the brightness seen on the FE pattern. This reaction resulted in the recovery of the initial field emission pattern characteristic of a clean tip, with {012} facets being the most visible. It was shown that the oxygen accumulation-reduction process is completely reversible. The obtained results indicate that the presence of subsurface species must be considered in the description of reactive processes on Pt–Rh catalysts.

Stable Heating Above 900 K in the Field Emission of ZnO Nanowires: Mechanism for Achieving High Current in Large Scale Field Emitter Arrays

AbstractThe thermal runaway of a quasi‐1D (Q1D) field emitter is an important cause of vacuum breakdown, which limits the field emission current density in field emitter arrays (FEAs). Comprehensive knowledge on the self‐heating process of zinc oxide (ZnO) nanowires is important for obtaining a high breakdown field for activating more emitters. This work investigates the self‐heating model of individual ZnO nanowire by considering the thermal field emission current distribution along the nanowire. Theoretical calculations suggest that the thermal field emission distribution along the nanowire can be reflected on the profile of the field emission pattern, which offers a method for determining its temperature. The field emission ring pattern is experimentally observed in the ZnO nanowire, whose intensity profile indicates that the nanowire can be steadily heated above 900 K. Considering the high breakdown field of high‐thermal stability ZnO nanowires, the field emission current density of ≈1.4 mA cm−2 can be achieved using these nanowires in a 3.5‐inch FEA. These findings provide an alternative method for measuring the Q1D field emitter temperature and a guideline for improving the field emission current from large‐scale Q1D FEAs.

Fine Fabrication and Optical Waveguide Characteristics of Hexagonal tris(8-hydroxyquinoline)aluminum(Ⅲ) (Alq3) Crystal

Herein, we reported on the precise growth and optical waveguide characteristics of hexagonal tris(8-hydroxyquinoline)aluminum(Ⅲ) (Alq3) micro-crystals (MCs). The hexagonal Alq3 MCs were prepared using surfactant-assisted assembly growth with the help of cetyltrimethylammoniumbromide (CTAB), in which the crystallization occurred as a result of molecular assembly and packing. Also, we adjusted the molar ratio of Alq3 and CTAB for the control degree of crystallization. The formation and structure of Alq3 MCs were investigated using field-emission scanning electron microscopy and X-ray diffraction pattern experiments, respectively. The solid-state laser confocal microscope-photoluminescence spectra and charge-coupled device images for the Alq3 MCs were measured to study the luminescence efficiency and colors, respectively. The optical waveguide performance of the hexagonal Alq3 MCs was measured for each side direction. According to our results, crystalline Alq3 micro-crystals are promising materials for application to the development of optical communication devices.

Quantum Dots Luminescence Collection Enhancement and Nanoscopy by Dielectric Microspheres

AbstractIn recent years, dielectric microspheres have been used in conjunction with optical microscopes to beat the diffraction limit and to obtain superresolution imaging. The use of microspheres on quantum dots (QDs) is investigated, for the first time, to enhance the light coupling efficiency. The enhancement of the QD luminescence collection in terms of extraction and directionality is demonstrated, as well as the enhancement of spatial resolution. In particular, it is found that a dielectric microsphere, placed on top of an epitaxial QD, increases the collected radiant energy by about a factor of 42, when a low numerical aperture objective is used. Moreover, if two or more QDs are present below the microsphere, the modification of the far field emission pattern allows selective collection of the luminescence from a single QD by simply changing the collection angle. Dielectric microspheres present a simple and efficient tool to improve the QD spectroscopy, and potentially QD‐based devices.

Field emission patterns showing symmetry of electronic states in graphene edges

Intriguing field emission microscopy (FEM) images reflecting subnanometer‐sized structures of emitting sites and/or electronic orbitals have been observed from graphene edges. Graphene emitters with free edges (i.e. open edges) show a striped pattern (dubbed as a ‘lip pattern’) consisting of an array of streaked spots (or oval‐shaped spots); the direction of striation is perpendicular to the graphene sheet, and each stripe is divided into two wings by a central dark band running parallel to the graphene sheet. The dark band may be due to the distractive interference of electrons emitted from π orbitals with a phase difference of π on either side of the graphene. From the magnification calibration using FEM images of an aluminum cluster with atomic resolution, the spacing of the streaked spots is found to be close to the distance between adjacent carbon atoms aligned along the zigzag and armchair edges. These observations suggest that the lip pattern reflects the symmetry of π states strongly delocalized at edge atoms. Copyright © 2016 John Wiley & Sons, Ltd.

Phonon-assisted lasing in ZnO microwires at room temperature

We report on room temperature phonon-assisted whispering gallery mode (WGM) lasing in ZnO microwires. For WGM laser action on the basis of the low gain phonon scattering process high quality resonators with sharp corners and smooth facets are prerequisite. Above the excitation threshold power PTh of typically 100 kW/cm2, the recombination of free excitons under emission of two longitudinal optical phonons provides sufficient gain to overcome all losses in the microresonator and to result in laser oscillation. This threshold behavior is accompanied by a distinct change of the far and near field emission patterns, revealing the WGM related nature of the lasing modes. The spectral evolution as well as the characteristic behavior of the integrated photoluminescence intensity versus the excitation power unambiguously proves laser operation. Polarization-resolved measurements show that the laser emission is linear polarized perpendicular to the microwire axis (TE).

Engineering the mode parity of the ground state in photonic crystal molecules

We propose a way to engineer the design of photonic molecules, realized by coupling two photonic crystal cavities, that allows an accurate control of the parity of their ground states. The spatial distribution of the fundamental mode of photonic molecules can be tuned from a bonding to an antibonding character by a local and continuous modification of the dielectric environment in between the two coupled cavities. In the systems that we investigate the transition could be experimentally accomplished by post-fabrication methods in either a reversible or an irreversible way. We notably find that the mode parity exchange is tightly related to a dramatic variation of the far field emission pattern, leading to the possibility to exploit these systems and techniques for future applications in optoelectronics.

Fabrication of a single-atom electron source by noble-metal surface diffusion

For application as single-atom emitters, the authors have developed a new method for preparing atomic-scale pyramids with three {211}-facet sides (nanopyramids). In the new method, palladium covers the “backward” area of the tungsten tip, approximately 1 mm from the sharpened end, rather than the end itself. The palladium was deposited via surface diffusion promoted by elevating the temperature. Field ion microscopy exhibited typical signs indicating that, with the added annealing, identical nanopyramids grew spontaneously. The field emission characteristics of these nanopyramids were investigated and compared with those produced by the two established preparation methods. The authors found that field emission patterns for single-atom tips were narrow circles at low extractor voltages and three-pronged stars, indicating the presence of three ridges of the nanopyramid, at high voltages. The patterns are the same for tips prepared by whichever method. As for field emission stability, clear differences were also not seen across the preparation methods, except for a minute difference in fluctuation ratios obtained from time-dependent current data.

Optical parametric oscillaton in 1D semiconductor microcavities

AbstractWe present an experimental investigation of parametric scattering processes and optical parametric oscillations in wire shaped, one‐dimensional semiconductor microcavities. Far field emission patterns and corresponding band dispersion are studied by polarization resolved measurements and power dependence measurements under resonant and non‐resonant excitation. The multiplicity of the photonic bands allows for an efficient engineering of interbranch parametric scattering processes. We demonstrate the onset of optical parametric oscillation of perfectly balanced twin beams, degenerate in energy and split in momentum space.

Ring-shaped field emission patterns from carbon nanotube films

Highly symmetric ring-shaped field emission patterns were observed from broad-area flat cathodes prepared by growing a film of vertically aligned carbon nanotubes (CNTs) on TiN coated Si substrates. The images were obtained utilizing a luminescent screen of a specially designed triode cell composed of parallel electrodes. The emission rings sporadically appeared during voltage scans in which the emission patterns and cathode currents were recorded. The fine structure and stability of the rings suggests that their formation is due to an emission state of an individual CNT. The observed patterns are consistent with models that predict the formation of emission rings produced by the inhomogeneous electron emission from CNTs. The macroscopic value of the electric field when the rings were observed was between 0.7 and 2.5 V/μm, and the emission current corresponding to individual rings was estimated to be in the range of 2–4 μA. Numerical simulation of electron trajectories for sidewall emission from similar shaped metallic structures is in qualitative and quantitative agreement with the experimentally observed ring-shaped field emission patterns. The results also appear consistent with a recent model [Marchand M, Journet C, Adessi C, Purcell ST. Phys Rev B 2009;80:245425] based on thermal-field emission due to Joule heating.

Design and investigation of surface addressable photonic crystal cavity confined band edge modes for quantum photonic devices

We propose to use a localized Γ-point slow Bloch mode in a 2D-Photonic Crystal (PC) membrane to realize an efficient surface emitting source. This device can be used as a quantum photonic device, e.g. a single photon source. The physical mechanisms to increase the Q/V factor and to improve the directivity of the PC microcavity rely on a fine tuning of the geometry in the three directions of space. The PC lateral mirrors are first engineered in order to optimize photons confinement. Then, the effect of a Bragg mirror below the 2DPC membrane is investigated in terms of out-of-plane leakages and far field emission pattern. This photonic heterostructure allows for a strong lateral confinement of photons, with a modal volume of a few (λ/n)3 and a Purcell factor up to 80, as calculated by two different numerical methods. We finally discuss the efficiency of the single photon source for different collection set-up.

Field-emission properties of patterned ZnO nanowires on 2.5D MEMS substrate

We fabricated a nanowire-based field-emission display (FED) device on a 2.5D substrate using a photolithography, lift-off, thermal-evaporation, and plasma-etching process. We first fabricated a 3×3 array of microholes (diameter = 400 μm and depth = 50 μm) on a Si substrate and fabricated ZnO nanowires inside the microholes by using a thermal CVD process. The field-emission pattern image of the 3×3 array of microholes was clearly apparent. The threshold emission field was ca. 5.6 V/μm and we obtained considerable brightness when the applied voltage was 1900 V (i.e. 6.3 V/μm). Because the fabrication processes used in this study are standard semiconductor fabrication routes, the study suggests the feasibility of mass producing a nanowire-based FED device.

STEM observation of tungsten tips sharpened by field-assisted oxygen etching

Tungsten tips oriented toward the <111> direction were fabricated by field-assisted O 2 etching, and observed by means of scanning transmission electron microscopy (STEM) and field ion microscopy (FIM). The radii of curvature of the tip apexes were sharpened from 16–20 nm to less than 2 nm. The O 2 etching is considered to start from the O 2 imaging region depending on the field distribution around the tip apex and shank. We estimated the effect of field distribution derived from a shank shape and applied bias voltage. The results showed that the tip with a cylindrical shank before the O 2 etching became sharper than tips having an initial paraboloidal shape, with respective cone angles (defined in Fig. 3) of 58° and 80°. The field emission (FE) patterns of these etched tips became a single spot derived from the W(111) plane, and their opening angles (defined by the full width at half maximum) were 14.4° and 7.8°, respectively.

Laser-induced field emission from a tungsten tip: Optical control of emission sites and the emission process

Field-emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses have been investigated. Strongly asymmetric field-emission intensity distributions are observed depending on three parameters: (1) the polarization of the light, (2) the azimuthal and (3) the polar orientation of the tip apex relative to the laser incidence direction. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity of a few tens of nanometers. Simulations of local fields on the tip apex and of electron emission patterns based on photo-excited nonequilibrium electron distributions explain our observations quantitatively. Electron emission processes are found to depend on laser power and tip voltage. At relatively low laser power and high tip voltage, field-emission after two-photon photo-excitation is the dominant process. At relatively low laser power and low tip voltage, photoemission processes are dominant. As the laser power increases, photoemission from the tip shank becomes noticeable.

Mode manipulation in system of coupled microcavities with whispering gallery modes

In recent years the studies of electromagnetic modes in solid spherical microcavities have been of great interest both for their potential applications and fundamental optical properties. A system of coherently coupled microcavities may be called a “photonic molecule” and can be employed in the tight-binding device in order to manipulate photons in micrometer length scale. In this work we demonstrate the possibility of mode manipulation in systems of symmetric photonic molecules formed by placing several high-Q micro-spheres in contact. We observe photonic nanojets that reflect the symmetry of the photonic molecule, with 3 jets located at 120 degrees with respect to each other for the triangular photonic molecule. A benzene molecule-like structure consisting of a 7-microspheres cyclic photonic molecule shows a field emission pattern similar to the spatial distribution of the orbitals of the benzene molecule. We also present some results showing the coexistence of whispering gallery modes and photonic nanojets in the same structure.

Ring patterns in high-current field emission from carbon nanotubes

In this paper we explain the origin of the ring structure that sometimes forms a sharp border surrounding field-emission electron microscopy patterns from carbon nanotubes (CNTs) at high current. The rings turn out to be due to the self-focusing of thermal-field electrons emitted from the near-cap shank of a CNT that reaches high temperature through Joule heating. To prove this we have simulated the electrostatic fields, the dependence of electron emission on field, temperature, and position on the CNT, and the emitted electron trajectories for different CNT/electrode geometries. The rings are formed when the manifold of emitted electron trajectories folds over onto itself due to self-focusing by the back-support plane. Sufficient thermal electron currents can only be emitted because CNTs can maintain a continuous high-temperature self-heating state, a state for now unique to carbon nanotubes. We do not need to evoke space charge effects and a program based on Green's functions is to calculate the field emission current for all values of field and temperature. The original observation of this phenomenon goes back to at least the pulsed high current experiments on W emitters in the 1950s [W. P. Dyke, J. K. Trolan, E. E. Martin, and J. P. Barbour, Phys. Rev. 91, 1043 (1953); W. W. Dolan, W. P. Dyke, and J. K. Trolan, Phys. Rev. 91, 1054 (1953)] and thus this work resolves a long-standing riddle in field emission.

Optical Control of Field-Emission Sites by Femtosecond Laser Pulses

We have investigated field-emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses. Strongly asymmetric modulations of the field-emission intensity distributions are observed depending on the polarization of the light and the laser incidence direction relative to the azimuthal orientation of tip apex. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity. Simulations of local fields on the tip apex and of electron emission patterns based on photoexcited nonequilibrium electron distributions explain our observations quantitatively.

Electronic and Field Emission Properties of Carbon Nanocones: A Density Functional Theory Investigation

Using density functional theory calculations, we investigate the electronic structures and field emission properties of carbon nanocones (CNCs). We find that the cohesive and formation energies for various types of CNCs are dependent on the cone angles, while the work function, local density of states, redistribution of the charge, and field emission pattern are sensitive to the morphologies of CNCs that are governed by the position of pentagonal rings in the cone apex. Most importantly, the nanocone with three pentagons in the cone apex exhibits the best field emission property.

Work function of W(100) field emitter modified with lutetium oxide and measured with photoemission electron microscope

The work function of the W(100) surface has been successfully reduced to 2.6eV by heating with a thin layer of lutetium oxide, as measured by use of the Fowler-Nordheim plot. Lu2O3 fine powder is attached on shank of a sharp W(100) needle and heated at 10−7Pa. Field emission pattern has a fourfold symmetry with a bright center spot. The lowered work function of Lu2O3∕W(100) surface was also measured by using photoemission electron microscope. The measured value of work function is 2.56eV. This resultant value of optical work function is in fairly good agreement with that from the Fowler-Nordheim plot.