A novel method for formation of single crystalline tungsten nanotip

Abstract

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.

Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 DOI 10.1186/s40486-016-0029-3

A novel method forformation ofsingle crystalline tungsten nanotip

Shigeki Hayashi*http://orcid.org/0000-0001-5958-079X

Web End = , Masashi Ono, Shinya Tomonaga and Haruka Nakanishi

http://orcid.org/0000-0001-5958-079X

Web End = Background

More than 100 years, thermal electrons emitted from heated lament 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 eld emission electron source could be constructed tiny, because of operating at room temperature, and whats more utilized to extended application to diagnosis or therapy like a ber scope in various medical elds.

Field emission is a phenomenon of emitting electrons at room temperature by quantum mechanical tunneling eect, when we supply high electric eld (more than 109V/m) on a metal surface, which is illustrated in Fig.1. Current density of the eld emission J [A/cm2] was formulated by Fowler and Nordheim [1], by using of an electric eld F [V/cm] and a work function [eV] of the metal as follows;

where e, m, and h are elementary charge, electron mass, and Plancks constant, respectively. If an emission current I [A] is taken to AJ (A: emission area), and the electric eld F is supposed to be V (: eld enhancement factor, V: supplied voltage), we can numerically reformulate (1) as follows;

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 eld emission is occurred when the F-N plot shows linear relation.

The eld enhancement factor [cm1] and the emission area A [cm2] are calculated from the slope and

J =

[notdef]

e3F2 8~h

exp

8~(2m)12 32 3heF

(1)

l[notdef]

I

V 2

= l[notdef]

3

2

1.54 1062

6.83 107

~V

(2)

*Correspondence: [email protected] of Radiological Technology, Faculty of Medical Science, Kyoto College of Medical Science, 1-3 Imakita, Oyama-higashi, Sonobe, Nantan, Kyoto 622-0041, Japan

2016 The Author(s). This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

Web End =http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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ordinate intercept b of the F-N plot line by use of formula (2) as follows;

These formulae show that the eld enhancement factor is in inverse proportion to the slope , and the emission area A is in exponential proportion to the ordinate intercept b. If we know the value of the work function ~ of the metal, we can estimate the values of and A from the formulae (3), through experimentally obtained F-N plot line.

Experimental equipment

Figure 2 shows apparatus of experimental equipment for eld emission. The main chamber can be evacuated up to 108Pa by TMP (Turbo Molecular Pump) and RP (Rotary Pump), not so as to be interrupted by residual gases in the chamber, when electrons are emitted from the metal surface. In case of degrading the vacuum by increasing emission current, liquid nitrogen was supplied in the server of the chamber so as to keep the vacuum of 108Pa order.

A sample of sharp tungsten (W) needle spot-welding on a half circled W lament 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

N .

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 ber 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 0.2s) through a personal computer.

Simulation ofelectric eld

We calculated an electric eld by using a software code of electromagnetic eld, ELFIN (ELF Corp.) [2]. The ELFIN code can calculate any electric eld at a sharp apex with high precision by using of an original analytical integral method, not an ordinary nite element method.

Figure 3 shows a sample of calculated electric eld distribution, where a sharp earthed W needle is placed perpendicular to an anode plate supplied by high voltage. The electric eld at the apex of W needle was calculated as function of the distance d between the cathode and anode by the ELFIN code, which is shown in Fig. 4. We ascertained the calculated electric eld as high as 109 V/m, where the diameter and apex curvature radius of the W needle is assumed to be 0.3mm and 100 nm, respectively and the supplying anode voltage to be 1000V. This variation curve shows that calculated electric eld at the sharp W needle apex is extremely

~ = 6.83 104

32

, A

= exp(b)1.54 106 ~2

(3)

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mentioned above. Each curve is corresponding to variation of the distance d between the cathode W and anode plate. This represents that the threshold voltage to start eld emission is increased, as the distance d is increased. It is readily recognized from decreasing the supplied electric eld as increasing the distance d.

Figure6 shows variation of the F-N plot versus the distance d, which is derived from the experimental results in Fig. 5 by use of above formula (2). These F-N plots are likely to have linear relation for any distance d. This shows that eld emission is occurred at room temperature by supplying high voltage in case of the sharp ordinary polycrystalline W needle. Table1 shows variation of [cm1], r [cm], and A [cm2] versus the distance d calcu-

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d. On the contrary, the emission area A is uctuated on the distance d. The reason is not sure at the present time.

Field emission fromsharp W needle coated withhBN thin lm

Hexagonal boron nitride (h-BN) is well known to have layered structure binding with van der Waals force like graphite as shown in Fig. 7. Nevertheless, h-BN is an insulator unlike graphite. We had tried to improve eld emission characteristics by coating with h-BN thin lm on various materials [46]. We coated the h-BN thin lm on the W needle at a substrate temperature of 350C, by an ion plating method with Ar and N2 gases in electron bombardment on B source.

We carried out similar eld emission experiments by using the W needle coated with h-BN thin lm. Figure8 shows the results of experimentally obtained F-N plots, which are also likely to have linear relation for three thicknesses (200, 300 and 500 nm) of h-BN thin lm.

These also show that eld emission is occurred at room temperature in case of the W needle coated with any thickness of h-BN thin lm. Furthermore, we found that an extremely sharp and straight W apex was grown up in the W needle coated with thickness of 500nm h-BN thin lm. On the contrary, curved W apexes were grown up in the W needle coated with thickness of 200 or 300nm h-BN thin lm. The reason of growing up curved W needle apexes coated with thickness of 200 or 300 nm h-BN thin lm is not clear, but the thinner h-BN thin lm would be inuenced by supplying high electric eld. These SEM (Scanning Electron Microscope) images of the W needle apexes with h-BN thin lms are shown in inserts of Fig.8.

FEM image ofW needle coated withhBN thin lm

Figure9 shows FEM image emitted from the W needle coated with 500nm h-BN thin lm, in which the h-BN thin lm is blown up in some way. The FEM image shows formation of crystallographic facets of single crystalline W apex.

TEM image ofW needle coated withhBN thin lm

Figure 10 shows TEM (Transmission Electron Microscope) image emitted from the W needle coated with 500 nm h-BN thin lm. The black and light gray areas of the TEM image would correspond to W and h-BN, respectively. The curvature radius of the topmost tungsten is a few times 10 nm. Furthermore, the topmost TEM picture of Fig.10, as shown Fig.11, indicates clearly atomic image. We plan to carry out future experiments to determine precisely the orientation of the single crystalline W. In any case, the extreme sharp apex seems to grow on the top of ordinary polycrystalline W needle. The formation mechanism is not well known, but the reason why blowing up the h-BN thin lm and creating the single crystalline W apex is surely caused by supplying high electric led on the W needle. Especially, h-BN

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thin lm must be easily blown away from the substrate W needle apex because of the layered structure.

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 eld. Secondly, a small migrated chip in the W needle was penetrated into the h-BN thin lm by supplied high electric force. Thirdly, sudden large current was own from the W needle chip, and the chip was partially heated by eld emission current. Finally, the h-BN thin lm was blown up by the extreme large current and

nm

, which were obtained from F-N plots through eld emission experiment, in order to elucidate precisely the mechanism of eld emission. We conrmed that the eld emission electrons emitted only from the most top of the sharp W needle.(3) We could create the W nanotip with an extremely sharp apex, under carrying out eld emission experiment for ordinary polycrystalline W needle coated with h-BN thin lm. And we ascertained the W needle coated with thickness of 500nm h-BN thin lm 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 0.1mA.

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(4) We proposed one mechanical process to create the single crystalline W nanotip by supplying high electric eld. In near future, we will conrm the formation mechanism of single crystalline W, by observing diraction 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.

Authors contributions

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 nal manuscript.

Acknowledgements

We appreciate Mr. Yano of ELF Corp. for supplying the electromagnetic eld simulation code ELFIN, in order to calculate the electric eld at an extremely sharp apex of the W needle.

This work was supported by JSPS KAKENHI Grant Number 26670302.

Competing interests

The authors declare that they have no competing interests.

Received: 21 January 2016 Accepted: 29 May 2016

References

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2. ELF Corp [in Japanese]. http://elf.co.jp/index.php?FrontPage

Web End =http://elf.co.jp/index.php?FrontPage . Accessed 7 June 2016

3. Gomer R (1961) Filed emission and eld ionization. Harvard University Press, Cambridge

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5. Sugino T, Yamamoto T, Kimura C, Murakami H, Hirakawa M (2002) Field emission characteristics of carbon nanober improved by deposition of boron nitride nanocrystalline lm. Appl Phys Lett 80:38083810

6. Morihisa Y, Kimura C, Yukawa M, Aoki H, Kobayashi T, Hayashi S, Akita S, Nakayama Y, Sugino T (2008) Improved eld emission characteristics of individual carbon nanotube coated with boron nitride nanolm. J Vac Sci Technol B26(2):872875