Successful cold field emission from single-tip emitters has up to now only been possible in UHV conditions with sub-micron tip radii, typically in the 0. The experiments reported here were carried out on a few-layer graphene-coated Ni wire cathode design, and demonstrate it to have an ultralow work function 1. This significantly lower electric field strength requirement makes it possible to have field emission from larger diameter tips in the micron range , and also reduces the kinetic energies of back-bombarding positive ions.
This leads to better current stability and less damage to the cathode tip, and also makes it feasible to operate the gun at less stringent vacuum conditions. Based upon experimentally measured angular intensity results, the Graphene-Ni single-tip cathode is predicted to have a higher reduced brightness than comparable conventional cold field emission electron sources, and there is no obvious need for regular thermal flashing of the cathode. These advantages come from its ultralow work function and relatively low applied electric field strength at the tip apex.
A two-step process Fig. Detail of this growth process is described in the Methods section of this paper. It is worth mentioning that Ni is not only conducive for the growth of uniform layers of graphene over large areas with high crystallinity 22 , but it also significantly reduces the work function of graphene 23 , This lowering of work function is discussed in detail in later parts of this paper.
Typical scanning electron microscope SEM images of the graphene-coated Ni tips of four different tip radii as depicted in Fig. Due to the ultrathin nature of the few-layer graphene, the grain boundaries of the underlying polycrystalline Ni, formed by high temperature annealing during the CVD process, are visible in Fig. Wrinkles are observed only on the lateral surface of Ni where graphene was grown, which was possibly induced by thermal stress around step edges and defect lines SEM characterization of the Graphene-Ni field emitter.
To confirm that the Ni cathode wire is covered with graphene after the CVD process, energy-dispersive X-ray spectroscopy EDS was employed to examine the chemical composition of the grown film. Presence of a carbon peak after the CVD coating process clearly suggests the presence of carbon-based material. Raman spectroscopy was then utilized to determine the type of carbon that is present, e. The position and shape of the G peak confirms the formation of sp 2 phase carbon and provides further evidence of the presence of graphene In addition to the relatively high G peak, the D peak which shows the presence of sp 3 carbon atoms or defects is close to the background level, indicating insignificant defects in the as-grown graphene layers and the film exhibits high graphitic crystallinity Characterization of the grown graphene.
A graphene flake was extracted from the surface of the graphene-coated emitter. Selected area electron diffraction SAED and high-resolution transmission electron microscopy HRTEM were used to further investigate the crystallinity, lattice structure and the thickness of the graphene flake. Different regions of the graphene flake were examined and the results consistently indicated the high crystallinity of the deposited graphene film Supplementary Fig. A high-resolution TEM image is shown in Fig. The lattice fringes along the in-plane direction of the graphene layer can be clearly observed and its crystallinity is confirmed.
The interlayer spacing was estimated to be 3. An experimental cold field emission electron gun setup Supplementary Fig. The emitter tip was positioned about 0. A negative voltage V c was applied to the cathode while the rest of the setup was grounded and the current leaving the anode plate, referred to here as total current I t , and the current reaching the Faraday Cup, referred to here as the sample current I s , were recorded.
It can be observed that the field emission current increases exponentially as the cathode becomes more negative, following a typical Fowler—Nordheim F-N curve The F-N formula is given as:. The total current I t reaches a maximum value of 4. Field electron emission characteristics. One of the stand out features of the Graphene-Ni point cathodes reported here is their ultralow work function values. This significant finding was extracted from the experimental results shown in Fig.
The distance d was measured accurately in a SEM, and SEM imaging also provided an estimate of the tip radius, which was approximated to be spherical in shape. The work function estimates from the F-N plots using this method were calculated to be 5.
The 5. Further confirmation of the accuracy comes from calculating the work function for the graphene-coated Ni tip by taking the ratio of the two F-N plots shown in Fig. The work function value for the graphene-coated Ni tip using this second method was calculated to be 1. The cathode surface area S p which contributes to the current collected by the Faraday Cup, was also estimated using simple direct ray tracing simulations Supplementary Fig. There are two mechanisms that help explain the significant lowering of the work function: i n-type doping of graphene due to chemisorption on Ni, which reduces the work function to the order of 0.
The combination of these two effects is most likely responsible for the dramatic reduction of work function value, as measured here, a value of 1. B r values for graphene-coated point cathodes versus their tip radii are presented in Fig.
The estimated B r value of 2. It is worth noting that B r values from even larger size tips are still relatively high and comparable to the B r value obtained from the state-of-art tungsten field emitters. Electron optical characterization of the Graphene-Ni field emitter. The uncertainty in the x -axis of a , b is from the determination of tip size. The stability of the electron beam is of major concern for focused electron beam applications. Conventional cold field emission electron sources are prone to instability due to the dynamics of residual gas adsorption and ion back bombardment.
One of the most important advantages of Graphene-Ni point cathodes over the conventional metal cold field emitters is the chemical inertness of its carbon surface 36 , which is less likely to adsorb residual gas molecules and obviously much more stable. In addition, a lower turn-on electric field is desirable for cold field emitters since it will reduce the kinetic energies up to which the back-bombarding gas-ions are accelerated to when they collide with the cathode surface.
21.3 Radioactive Decay
The local electric field strength was estimated by using the enhancement factor derived from simulation Supplementary Fig. For comparison, the electric field strength required to obtain the same angular current density from widely used tungsten cold field emitters 5 , 6 and Schottky thermal emitters 34 , as reported previously, are plotted on the same graph. It is clear that there is around an order of magnitude reduction in the local electric field strength requirement for the graphene-coated pointed cathodes as compared to the field strength required for tungsten cold field emitters, typically in the range of 0.
These findings help to explain why the graphene-coated point cathode is able to provide stable field emission for micron diameter cathode tips and operate in much less stringent vacuum conditions. The degree of current instability and damage to the cathode tip not only depends on the chemical inertness of the carbon surface and the kinetic energies of the back-bombardment ions, but also the size of the cathode tip.
Since for the same emission angle, a larger radius tip has a greater area of emission, the relatively large-diameter graphene-coated cathode tips in the micrometer range , as reported in this paper, are therefore expected to have an order of magnitude lower RMS noise ratio values as compared to conventional tungsten cold field emitters. Stepwise changes in emission currents were observed and such fluctuations may be attributed to events such as adsorption, desorption or flip-flop of adsorbate molecules at the tip of the emitter In the case of the graphene-coated cathode, there was no obvious need for flashing of the cathode tip.
Repeated field emission tests of the graphene-coated point cathode were carried out in a HV chamber, confirming that the graphene-coated point cathode has highly repeatable field emission characteristics see Supplementary Fig.
Radioactive Decay – Chemistry
The spectrum was obtained by the FFT of a probe current 1. Supplementary Fig. From the plots, the intrinsic full-width at half-maximum FWHM energy spreads are calculated to be 0. The intrinsic TED of electron emission is only one contributor to the energy spread, and another contribution comes from longitudinal Coulomb interactions also known as Boersch effect. These preliminary simple analytical considerations point towards new opportunities for obtaining smaller energy spreads with the Graphene-Ni cathode, which comes from its ability to produce stable field emission from relatively large cathode tip radii.
Schottky emitter used for calibration purposes. The FWHM of 0.
Energy spread and its dependence on emission current and reduced brightness. The W T. The uncertainty in the y -axis is from the determination of tip size and current instability. The FWHM values increase with increasing total emission current. The lowest measured value is 0. Plotted in Fig.
The overall energy spread values are lower than or comparable with state-of-the-art conventional cold field emission sources. As predicted by our analytical calculations, the small energy spreads expected for the Graphene-Ni cathode will be approximately off-set by the Boersch effect for the small tip sizes, and the total energy spread is comparable to conventional tungsten cold field emitters.
In this paper, these preliminary experimental results demonstrate that by using a few-layer graphene-coated Ni wire point cathode, it is possible to obtain stable cold field emission for electron microscopy and lithography applications in HV conditions and, additionally, use relatively large point cathode tip diameters in the micron range. The feasibility of using such large size tips and relatively poor vacuum conditions comes from their experimentally measured ultralow work function value of 1. The estimated reduced brightness of these new type of cold field emission sources, as well as their measured energy spread, are comparable to conventional single crystal tungsten cathode cold field emission sources.
These results establish the promising prospect of using them as high brightness high-resolution electron sources for electron microscopy and lithography applications, similar in performance to conventional single crystal tungsten cathode cold field emission sources, while at the same time having better emission stability and less stringent vacuum requirements. Electron gun structures that can accommodate the Graphene-Ni electron point source need to be developed in which the source is accurately aligned with optical axis and accelerated directly after emission to avoid Coulomb interactions; this will be the subject for future studies.
A typical electrochemical etching process was used for preparing a sharp Ni tip having a radius of a few hundred nanometers The sharp Ni tip serves as a template and catalyst for the growth of graphene. In this work, the deposition of a few-layer graphene is achieved by using the CVD method with solid carbon source PMMA poly methyl methacrylate as feedstock, since this method avoids the use of high temperatures which may change the morphology of the sharpened tip.
As shown in the schematic, the Ni tip was placed in a ceramic holder positioned at the center of the tube furnace. An Al 2 O 3 boat loaded with solid PMMA was placed at the inlet side of the quartz tube, just outside of the heating zone. A schematic of the experimental electron gun setup that was used to analyze the performance of the cathode is shown in Supplementary Fig. The emitter tip was positioned 0.
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A cylindrical sector energy analyzer Focus Electronics CSA was installed to measure the electron energy distribution. The data that support the findings of this study are available from the corresponding authors upon request. Crewe, A. Electron gun using a field emission source.
De Jonge, N. High brightness electron beam from a multi-walled carbon nanotube. Nature , — Zhang, H. An ultrabright and monochromatic electron point source made of a LaB6 nanowire. Houdellier, F. Ultramicroscopy , — Swanson, L. A review of the cold-field electron cathode. Imaging Electron Phys. Schwind, G. Comparison of parameters for Schottky and cold field emission sources.
B Microelectron. Nanometer Struct. New carbon cone nanotip for use in a highly coherent cold field emission electron microscope. Carbon 50 , — Yeong, K. Field-emission properties of ultrathin 5 nm tungsten nanowire. Jin, C. These housings have multiplicity of ports for electrical and water feed-throughs, laser beam windows, vacuum pump connections, and instrumentation diagnostics and monitoring with opening and closure provisions to allow refurbishment of internal components.
Especially designed or prepared components or devices for collecting uranium product material or uranium tails material following illumination with laser light.
In one example of molecular laser isotope separation, the product collectors serve to collect enriched uranium pentafluoride UF 5 solid material. Components of these compressors that come into contact with process gas are made of, or protected by, materials resistant to UF 6 corrosion. Especially designed or prepared systems for fluorinating UF 5 solid to UF 6 gas.
These systems are designed to fluorinate the collected UF 5 powder to UF 6 for subsequent collection in product containers or for transfer as feed for additional enrichment. In both approaches, equipment is used for storage and transfer of fluorine or other suitable fluorinating agents and for collection and transfer of UF 6.
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Especially designed or prepared mass spectrometers capable of taking on-line samples from UF 6 gas streams and having all of the following characteristics:. Especially designed or prepared process systems or equipment for enrichment plants made of or protected by materials resistant to corrosion by UF 6 , including:. Especially designed or prepared process systems for separating UF 6 from carrier gas. The laser system typically contains both optical and electronic components for the management of the laser beam or beams and the transmission to the isotope separation chamber.
The laser system for atomic vapor based methods usually consists of tunable dye lasers pumped by another type of laser e. The laser system for molecular based methods may consist of CO 2 lasers or excimer lasers and a multi-pass optical cell. Lasers or laser systems for both methods require spectrum frequency stabilization for operation over extended periods of time.