Temporal coherence envelope function of field emission in electron microscopy

Imaging in electron microscopy is adversely affected by partial electron spatial and temporal coherence. Temporal coherence has been treated theoretically in the past using the method pioneered fifty years ago by Hanßen and Trepte, who assumed a Gaussian energy distribution. However, state-of-the-art instruments employ field emission (FE) sources that emit electrons with a non-Gaussian energy distribution. We have updated the treatment of temporal coherence to describe the effects of an arbitrary energy distribution on image formation. The updated approach is implemented in Fourier optics simulations to explore the effect of FE on image formation in conventional, non-aberration-corrected (NAC) and aberration-corrected (AC) low energy electron microscopy. It is found that the resolution that can be achieved for the FE distribution is only slightly degraded compared to a Gaussian distribution with the same energy spread. FE also produces a focus offset. These two effects are weaker for AC than for NAC microscopy. These and other insights may be relevant to the selection of the aperture size that optimizes resolution and to analyses that make use of focal image series. The approach developed here is also applicable to transmission electron microscopy.

Properties of blade-like field emitters

Blade-Like Field Emitters (BFE), as defined here, are emitters expanded in one direction, forming a sharp emitting edge instead of a sharp tip. These structures have four main advantages compared to their needle counterparts, i.e., they are mechanically firmer, are better electrical and thermal conductors, and provide a larger emission area. We focus on the optimization of the last of these. We evaluate the emission properties of three types of BFEs, which we short-named hSoC-blade, HCP-blade and Elli-blade. Each is built from the expansion of a hemisphere-on-a-cone (hSoC), hemisphere-on-a-cylindrical-post (HCP) and an ellipsoidal (Elli) emitter, respectively. The characteristics of the field enhancement factor, the local electrostatic field distribution on each blades' edges and their notional area (An) of emission as a function of the expansion length are described. Finally, we point out how to improve the edge of the HCP-blade to obtain the optimal profile, which yield the largest An.

The effect of size distribution and degradation of carbon nanotubes on the Fowler-Nordheim plot behavior

The behavior of the Folwer-Nordheim (FN) plot is investigated for large-area emitters arrays composed of perfectly aligned single-walled carbon nanotubes (CNTs). The field emission currents are calculated based on the standard FN model using the Murphy-Good equation. The aspect ratio of the CNTs is varied such that the height of the CNT has predefined uniform or Gaussian distributions. Continuous degradation of the high-aspect-ratio CNTs is assumed to take place at predefined values of the scaled barrier field. The effects, on the FN plot behavior, of the size distribution and the scenario of the degradation of the emitter are determined and discussed. These factors are found to produce the two behaviors of the experimental FN plots, i.e. the upward and the downward bending behaviors. This work shows the possibility to expand the application of the standard FN model for the investigation of the field emission characteristics from nanoscaled emitters such as CNTs.

Numerical simulations of field emission characteristics of open CNT

Numerical simulations on field emission properties of an open single-walled carbon nanotube with radius r = 1 nm have been carried out. Using finite element method, we have calculated the local electric field F_loc and field enhancement factor γ distribution over the surface of the open CNT. From these calculations, we plot theoretical current-voltage characteristics and assess effective field characteristics for CNT with wall thickness w = 0.2 nm and height h = {3, 11, 101, 1001} nm. It was revealed that the maximum emission current did not change when applied field at the top of the CNT corresponds to the scaled barrier field f = 0.45. Obtained values of effective emission area are in good agreement with surface area, which has the highest field enhancement factor. In addition, we have plotted a "map" of maximum field enhancement factor γ_a for open CNTs with various wall thickness - w ϵ [0.1; 0.9] nm and heights h ϵ [11; 1001] nm. It was shown that maximum field enhancement factor is a nonlinear function of h/r and h/w ratios.

Fabrication and Physical Properties of Single-Crystalline Βeta-FeSi2 Nanowires

In this study, self-catalyzed β-FeSi₂ nanowires, having been wanted but seldom achieved in a furnace, were synthesized via chemical vapor deposition method where the fabrication of β-FeSi₂ nanowires occurred on Si (100) substrates through the decomposition of the single-source precursor of anhydrous FeCl₃ powders at 750-950 °C. We carefully varied temperatures, duration time, and the flow rates of carrier gases to control and investigate the growth of the nanowires. The morphology of the β-FeSi₂ nanowires was observed with scanning electron microscopy (SEM), while the structure of them was analyzed with X-ray diffraction (XRD) and transmission electron microscopy (TEM). The growth mechanism has been proposed and the physical properties of the iron disilicide nanowires were measured as well. In terms of the magnetization of β-FeSi₂, nanowires were found to be different from bulk and thin film; additionally, longer β-FeSi₂ nanowires possessed better magnetic properties, showing the room-temperature ferromagnetic behavior. Field emission measurements demonstrate that β-FeSi₂ nanowires can be applied in field emitters.

Development of analytical ultrafast transmission electron microscopy based on laser-driven Schottky field emission

A new design scheme for ultrafast transmission electron microscopy (UTEM) has been developed based on a Schottky-type field emission gun (FEG) at the Institute of Physics, Chinese Academy of Sciences (IOP CAS). In this UTEM setup, electron pulse emission is achieved by integrating a laser port between the electron gun and the column and the resulting microscope can operate in either continuous or pulsed mode. In pulsed mode, the optimized electron beam properties are an energy width of ~0.65 eV, micrometer-scale coherence lengths and sub-picosecond pulse durations. The potential applications of this UTEM, which include electron diffraction, high-resolution imaging, electron energy loss spectroscopy, and photon-induced near-field electron microscopy, are demonstrated using ultrafast electron pulses. Furthermore, we use a nanosecond laser (~10 ns) to show that the laser-driven FEG can support high-quality TEM imaging and electron holography when using a stroboscopic configuration. Our results also indicate that FEG-based ultrafast electron sources may enable high-performance analytical UTEM.

Conduction mechanisms and voltage drop during field electron emission from diamond needles

We report results of experimental investigation of field electron emission from diamond nanoemitters. The measurements were performed with single crystal diamond needles fixed at tungsten tips. The voltage drop along diamond needles during emission was revealed and measured using electron energy spectroscopy. The observed linear dependence of the voltage drop in diamond on voltage applied to the tungsten tip is explained in the frame of a simple macroscopic electrical model combining Poole-Frenkel conduction along the diamond tip and Fowler-Nordheim tunneling at the diamond-vacuum junction. Experimental evidences of electron emission sensitivity to laser illumination are discussed for possible modification of diamond emitter characteristics and voltage drop.

Development of a high brightness ultrafast Transmission Electron Microscope based on a laser-driven cold field emission source

We report on the development of an ultrafast Transmission Electron Microscope based on a cold field emission source which can operate in either DC or ultrafast mode. Electron emission from a tungsten nanotip is triggered by femtosecond laser pulses which are tightly focused by optical components integrated inside a cold field emission source close to the cathode. The properties of the electron probe (brightness, angular current density, stability) are quantitatively determined. The measured brightness is the largest reported so far for UTEMs. Examples of imaging, diffraction and spectroscopy using ultrashort electron pulses are given. Finally, the potential of this instrument is illustrated by performing electron holography in the off-axis configuration using ultrashort electron pulses.

Flexible Field Emitter for X-ray Generation by Implanting CNTs into Nickel Foil

This paper reports on a flexible Ni micro wire with CNTs embedded into its surface. By using micromachining technology, for the first time, we could implant nanoscale materials into micro-scale metal substrate at room temperature. Thanks to the effective direct contact and the strong interactions between CNTs and the substrate, field emission current of 1.11 mA (current density of 22.2 mA/cm(2)) could be achieved from the micro wire. Moreover, the wire shows excellent mechanical properties for large amplitude bending, which is beneficial for geometric designing. To check the practical application of the wire, a simplified X-ray imaging system was set up by modifying a conventional tube. The gray shade that appears on the sensitive film after being exposed to the radiation confirms the X-ray generation.

Flexible Field Emitter for X-ray Generation by Implanting CNTs into Nickel Foil

This paper reports a novel implanting micromachining technology. By using this method, for the first time, we could implant nano-scale materials into milli-scale metal substrates at room temperature. Ni-based flexible carbon nanotube (CNT) field emitters were fabricated by the novel micromachining method. By embedding CNT roots into Ni foil using polymer matrix as transfer media, effective direct contact between Ni and CNTs was achieved. As a result, our novel emitter shows relatively good field emission properties such as low turn-on field and good stability. Moreover, the emitter was highly flexible with preservation of the field emission properties. The excellent field emission characteristics attributed to the direct contact and the strong interactions between CNTs and the substrate. To check the practical application of the novel emitter, a simple X-ray imaging system was set up by modifying a traditional tube. The gray shadow that appears on the sensitive film after being exposed to the radiation confirms the successful generation of X-ray.

Wide-range Vacuum Measurements from MWNT Field Emitters Grown Directly on Stainless Steel Substrates

The field emission properties and the vacuum measurement application are investigated from the multi-walled carbon nanotubes (MWNTs) grown directly on catalytic stainless steel substrates. The MWNT emitters present excellent emission properties after the acid treatment of the substrate. The MWNT gauge is able to work down to the extreme-high vacuum (XHV) range with linear measurement performance in wide range from 10(-11) to 10(-6) Torr. A modulating grid is attempted with improved gauge sensitivity. The extension of the lower pressure limit is attributed largely to low outgassing effect due to direct growth of MWNTs and justified design of the electron source.

High Current Emission from Patterned Aligned Carbon Nanotubes Fabricated by Plasma-Enhanced Chemical Vapor Deposition

Vertically, carbon nanotube (CNT) arrays were successfully fabricated on hexagon patterned Si substrates through radio frequency plasma-enhanced chemical vapor deposition using gas mixtures of acetylene (C2H2) and hydrogen (H2) with Fe/Al2O3 catalysts. The CNTs were found to be graphitized with multi-walled structures. Different H2/C2H2 gas flow rate ratio was used to investigate the effect on CNT growth, and the field emission properties were optimized. The CNT emitters exhibited excellent field emission performance (the turn-on and threshold fields were 2.1 and 2.4 V/μm, respectively). The largest emission current could reach 70 mA/cm(2). The emission current was stable, and no obvious deterioration was observed during the long-term stability test of 50 h. The results were relevant for practical applications based on CNTs.

Chemical ionization mass spectrometry using carbon nanotube field emission electron sources

A novel chemical ionization (CI) source has been developed based on a carbon nanotube (CNT) field emission electron source. The CNT-based electron source was evaluated and compared with a standard filament thermionic electron source in a commercial explosives trace detection desktop mass spectrometer. This work demonstrates the first reported use of a CNT-based ion source capable of collecting CI mass spectra. Both positive and negative modes were investigated. Spectra were collected for a standard mass spectrometer calibration compound, perfluorotributylamine (PFTBA), as well as trace explosives including trinitrotoluene (TNT), Research Department explosive (RDX), and pentaerythritol tetranitrate (PETN). The electrical characteristics, lifetime at operating pressure, and power requirements of the CNT-based electron source are reported. The CNT field emission electron sources demonstrated an average lifetime of 320 h when operated in constant emission mode under elevated CI pressures. The ability of the CNT field emission source to cycle on and off can provide enhanced lifetime and reduced power consumption without sacrificing performance and detection capabilities. Graphical Abstract ᅟ.

Effect of the laser pulse width on the field evaporation behavior of metals and oxides

The laser assisted field-evaporation of metals and oxides strongly depends on the illumination conditions. Here we study the effect of laser pulse width using two different laser systems, with a pulse duration of a few tens of femtoseconds and a few tens of picoseconds, respectively. Adjusting the laser wavelength by nonlinear optical conversion systems, we study the evaporation behavior of FeCu and MgO samples. We prove that the laser pulse width does not affect the evaporation behavior, in the range of duration explored. These results are explained taking into account the absorption behavior of nanometric samples and their thermal properties.

Single-crystalline chromium silicide nanowires and their physical properties

In this work, chromium disilicide nanowires were synthesized by chemical vapor deposition (CVD) processes on Si (100) substrates with hydrous chromium chloride (CrCl3 · 6H2O) as precursors. Processing parameters, including the temperature of Si (100) substrates and precursors, the gas flow rate, the heating time, and the different flow gas of reactions were varied and studied; additionally, the physical properties of the chromium disilicide nanowires were measured. It was found that single-crystal CrSi2 nanowires with a unique morphology were grown at 700°C, while single-crystal Cr5Si3 nanowires were grown at 750°C in reducing gas atmosphere. The crystal structure and growth direction were identified, and the growth mechanism was proposed as well. This study with magnetism, photoluminescence, and field emission measurements demonstrates that CrSi2 nanowires are attractive choices for future applications in magnetic storage, photovoltaic, and field emitters.

Synthesis and field emission studies of tower-like GaN nanowires

Tower-like GaN nanowires were successfully fabricated on Au-coated Si substrates by chemical vapor deposition. The tower-like nanowire consisted of a nanowire at the center and microcrystal layers stacked one by one around the nanowire. The tower-like nanowires grew along the [0001] direction, and the exposed surfaces of the microcrystal layers are [Formula: see text] and [Formula: see text] facets. The growth mechanism of the tower-like GaN nanowires was proposed. The field emission property of tower-like GaN nanowires was tested. Due to the sharp tips, nearly vertical alignment and rough surfaces caused by the microcrystal layers, the tower-like GaN nanowires show excellent performance in field emission with a turn-on field of 2.44 V/μm which is lower than those of other GaN one-dimensional (1D) nanomaterials.

PACS

81.15.Gh; 68.37.Lp; 68.37.Vj.

Patterned growth of carbon nanotubes over vertically aligned silicon nanowire bundles for achieving uniform field emission

A fabrication strategy is proposed to enable precise coverage of as-grown carbon nanotube (CNT) mats atop vertically aligned silicon nanowire (VA-SiNW) bundles in order to realize a uniform bundle array of CNT-SiNW heterojunctions over a large sample area. No obvious electrical degradation of as-fabricated SiNWs is observed according to the measured current-voltage characteristic of a two-terminal single-nanowire device. Bundle arrangement of CNT-SiNW heterojunctions is optimized to relax the electrostatic screening effect and to maximize the field enhancement factor. As a result, superior field emission performance and relatively stable emission current over 12 h is obtained. A bright and uniform fluorescent radiation is observed from CNT-SiNW-based field emitters regardless of its bundle periodicity, verifying the existence of high-density and efficient field emitters on the proposed CNT-SiNW bundle arrays.

Tunable field emission characteristics of ZnO nanowires coated with varied thickness of lanthanum boride thin films

Lanthanum boride (LaB(x)) thin films with various thicknesses were deposited on ZnO nanowire arrays by electron beam evaporation. Field emission characteristics of ZnO nanowires show close dependence on LaB(x) coating thickness. The turn-on field increases with increasing LaB(x) coating thickness from 10 nm to 50 nm. The observed phenomena were explained by a model that the tunneling at ZnO/LaB(x) interface dominates the emission process.

WE-C-217BCD-03: Restricted Data Set Reconstruction Based on Respiration Quality to Improve Prospectively Gated in Vivo Micro-CT of Mice

PURPOSE

Micro-CT is commonly employed for lung imaging of mice; prospective gating allows for in-vivo imaging of free-breathing subjects. While this technique is successfully executed for healthy animals, results are less consistent for some disease models whose symptoms include irregular or unstable respiration. The purpose of this work is to repair the quality of high-blur images that arise from respiration instability using a retrospective method of motion reduction which identifies the individual x-ray projection images contributing most to the motion blur. Reconstructions were performed after the exclusion of these projections (the so-called restricted set).

METHODS

Sixteen mice were imaged using field emission cone beam micro-CT and prospective gating with a bellows-type respiration sensor. The scanner was operated in step-and-shoot mode; 400 projection images were acquired per scan. An algorithm was developed to analyze the respiration trace file and segment the individual breath corresponding to each projection image. We tested three different criteria to define a bad breath shape (correlation, mean breath height, or mode breath height), and restricted data set reconstructions were performed using each of these criteria to exclude projections corresponding to bad breaths. Each restricted set was compared against the full unrestricted data set image; the slope perpendicular to the diaphragm was used as a quantitative assessment of motion blur.

RESULTS

All image sets saw a reduction in motion blur with at least one restriction technique. In 22 of 27 images, improvement was measured regardless of the removal criterion. Five percent total projection removal is optimal; a more aggressive correction increases the likelihood of under-sampling artifacts.

CONCLUSIONS

Removing a subset of bad projections from otherwise complete image sets measurably decreases motion blur in respiratory-gated imaging. An approach based on breath height generally provides the best results. The technique is applicable to a variety of imaging modalities.

On the morphology, structure and field emission properties of silver-tetracyanoquinodimethane nanostructures

Silver-tetracyanoquinodimethane(Ag-TCNQ) nanostructured arrays with different morphologies were grown by an organic vapor-transport reaction under different conditions. The field emission properties of nanostructured arrays were studied systematically. Their morphology and crystal structure were characterized by SEM and XRD, respectively. It was found that the field emission properties were strongly dependent on the reaction temperature and the initial Ag film thickness. The lowest turn-on field with 10-nm-thick silver film is about 2.0 V/μm, comparable to that of carbon nanotubes. The film crystal structure and the morphology are contributed to the final emission performance.

Field Emission of ITO-Coated Vertically Aligned Nanowire Array

An indium tin oxide (ITO)-coated vertically aligned nanowire array is fabricated, and the field emission characteristics of the nanowire array are investigated. An array of vertically aligned nanowires is considered an ideal structure for a field emitter because of its parallel orientation to the applied electric field. In this letter, a vertically aligned nanowire array is fabricated by modified conventional UV lithography and coated with 0.1-μm-thick ITO. The turn-on electric field intensity is about 2.0 V/μm, and the field enhancement factor, β, is approximately 3,078 when the gap for field emission is 0.6 μm, as measured with a nanomanipulator in a scanning electron microscope.

Synthesis and enhanced field-emission of thin-walled, open-ended, and well-aligned N-doped carbon nanotubes

Thin-walled, open-ended, and well-aligned N-doped carbon nanotubes (CNTs) on the quartz slides were synthesized by using acetonitrile as carbon sources. As-obtained products possess large thin-walled index (TWI, defined as the ratio of inner diameter and wall thickness of a CNT). The effect of temperature on the growth of CNTs using acetonitrile as the carbon source was also investigated. It is found that the diameter, the TWI of CNTs increase and the Fe encapsulation in CNTs decreases as the growth temperature rises in the range of 780-860°C. When the growth temperature is kept at 860°C, CNTs with TWI = 6.2 can be obtained. It was found that the filed-emission properties became better as CNT growth temperatures increased from 780 to 860°C. The lowest turn-on and threshold field was 0.27 and 0.49 V/μm, respectively. And the best field-enhancement factors reached 1.09 × 105, which is significantly improved about an order of magnitude compared with previous reports. In this study, about 30 × 50 mm2 free-standing film of thin-walled open-ended well-aligned N-doped carbon nanotubes was also prepared. The free-standing film can be transferred easily to other substrates, which would promote their applications in different fields.

Stable Field Emitters for a Miniature X-ray Tube Using Carbon Nanotube Drop Drying on a Flat Metal Tip

Stable carbon nanotube (CNT) field emitters for a vacuum-sealed miniature X-ray tube have been fabricated. The field emitters with a uniform CNT coating are prepared by a simple drop drying of a CNT mixture solution that is composed of chemically modified multi-walled CNTs, silver nanoparticles, and isopropyl alcohol on flat tungsten tips. A highly thermal- and electrical-conductive silver layer strongly attaches CNTs to the tungsten tips. Consequently, the field emitters exhibit good electron emission stability: continuous electron emission of around 100 μA at 2.3 V/μm has stably lasted over 40 h even at non-high vacuum ambient (~10-3 Pa).