A novel method for formation of single crystalline tungsten nanotip
© The Author(s) 2016
Received: 21 January 2016
Accepted: 29 May 2016
Published: 8 June 2016
A point electron source is desired to improve performance of high brightness electron beam instruments. It is valuable to create nano-sized tungsten (W) tip from sharp ordinary polycrystalline W needle. The sharp W needle, which is manufactured by electrochemical etching, has been practically utilized as a cold field emission electron source. A novel method for formation of single crystalline W nanotip on the top of h-BN coated conventional polycrystalline tungsten, by supplying high voltage, has been found. The W nanotip with an apex radius as small as a few times 10 nm would be grown on the top of the polycrystalline W needle. Field emission characteristics of obtained W nanotip are measured, and the field emission microscopic (FEM) and transmission emission microscopic (TEM) images are observed. The emission current from the W nanotip is measured to exceed 0.1 mA. The FEM image shows significant electron emission from the crystallographic facets of the W single crystal. From these results, the present method for formation of the single crystalline W nanotip would be expected as a key technology to realize a point electron source with a nano-sized apex which makes it possible to improve the performance of high brightness electron beam instruments, especially tiny X-ray tubes for medical use, as well as a cantilever of scanning probe microscope.
More than 100 years, thermal electrons emitted from heated filament have been widely utilized for any X-ray tube including medical use. Although X-ray tubes with thermal electron source would become rather huge, ones with cold field emission electron source could be constructed tiny, because of operating at room temperature, and what’s more utilized to extended application to diagnosis or therapy like a fiber scope in various medical fields.
This formula represents linear relation between ln (I/V 2) and 1/V. We call this Fowler and Nordheim plot, or shortly F-N plot, if ordinate and abscissa are chosen as ln (I/V 2) and 1/V, respectively. Therefore, we call field emission is occurred when the F-N plot shows linear relation.
These formulae show that the field enhancement factor β is in inverse proportion to the slope ζ, and the emission area A is in exponential proportion to the ordinate intercept b. If we know the value of the work function \(\phi\) of the metal, we can estimate the values of β and A from the formulae (3), through experimentally obtained F-N plot line.
A sample of sharp tungsten (W) needle spot-welding on a half circled W filament is mounted on normal to an anode plate, which is illustrated in upper left of Fig. 2. This shape is suitable for resistance heating in order to remove surface contamination from the sample W needle. The ordinary polycrystalline W needle has a sharp apex manufactured by electrochemical etching in 1 N KOH solution, which is shown in upper right of Fig. 2. The W needle can be linearly moved from outside of the chamber, and therefore we can easily vary the distance between the W needle and anode plate. The anode plate has also a phosphor screen, by which we can directly observe FEM (Field Emission Microscope) image from emitted electrons, out of chamber through an optical fiber plate attaching on the phosphor screen. This illustrates as a viewing port in Fig. 2. By supplying high voltage to the anode plate, emission current from the W needle can be measured by a fast current meter, by which we can continuously gather split current (repetition time is 0.2 s) through a personal computer.
Simulation of electric field
We calculated an electric field by using a software code of electromagnetic field, ELFIN (ELF Corp.) . The ELFIN code can calculate any electric field at a sharp apex with high precision by using of an original analytical integral method, not an ordinary finite element method.
Field emission from sharp polycrystalline W needle
Variations of β [cm−1], r [cm], and A [cm2] with distance d between the cathode and anode calculated from the F-N plots in Fig. 6
Field emission from sharp W needle coated with h-BN thin film
FEM image of W needle coated with h-BN thin film
TEM image of W needle coated with h-BN thin film
One proposed process to create the single crystalline W nanotip is as follows. First, electro migration in ordinary polycrystalline W needle was occurred by supplying high electric field. Secondly, a small migrated chip in the W needle was penetrated into the h-BN thin film by supplied high electric force. Thirdly, sudden large current was flown from the W needle chip, and the chip was partially heated by field emission current. Finally, the h-BN thin film was blown up by the extreme large current and the chip would grow a single crystalline W nanotip under the high electric field.
At last, we express that we cannot find such FEM images of single crystalline W with thickness of 200 or 300 nm h-BN thin film, which could not be explained by now. It would be related to formation of the curved apexes of the W needle with thickness of 200 or 300 nm h-BN thin film.
We confirmed field emission at room temperature by supplying high voltage from sharp ordinary polycrystalline W needle, which was manufactured by electrochemical etching.
We compared calculated electric field with the field enhancement factor β and the emission area A, which were obtained from F-N plots through field emission experiment, in order to elucidate precisely the mechanism of field emission. We confirmed that the field emission electrons emitted only from the most top of the sharp W needle.
We could create the W nanotip with an extremely sharp apex, under carrying out field emission experiment for ordinary polycrystalline W needle coated with h-BN thin film. And we ascertained the W needle coated with thickness of 500 nm h-BN thin film to grow up an extremely sharp no-curved apex and to show formation of single crystalline W from the FEM and TEM images. The emission current from the W nanotip is measured to exceed 0.1 mA.
We proposed one mechanical process to create the single crystalline W nanotip by supplying high electric field. In near future, we will confirm the formation mechanism of single crystalline W, by observing diffraction patterns of X-ray or electron beam. The created W nanotip would be applied to realize high brightness electron point source which makes it possible to improve any electron beam instruments, including tiny X-ray tubes for medical or industrial use, as well as a cantilever of scanning probe microscope.
SH conceived of the study, and participated in its design and coordination and helped to draft the manuscript. MO, ST and HN carried out the experiments, and summarized the data. All authors read and approved the final manuscript.
We appreciate Mr. Yano of ELF Corp. for supplying the electromagnetic field simulation code ELFIN, in order to calculate the electric field at an extremely sharp apex of the W needle.
This work was supported by JSPS KAKENHI Grant Number 26670302.
The authors declare that they have no competing interests.
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