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A novel method for formation of single crystalline tungsten nanotip

A novel method for formation of single crystalline tungsten nanotip A point electron source is desired to improve performance of high brightness electron beam instruments. It is valu- able 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 polycrystal- line 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 signifi- cant 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. Background 1 3 3 2 2 2 More than 100  years, thermal electrons emitted from e F 8π(2m) φ J = exp − (1) heated filament have been widely utilized for any X-ray 8πhφ 3heF tube including medical use. Although X-ray tubes with where e, m, and h are elementary charge, electron mass, thermal electron source would become rather huge, and Planck’s constant, respectively. If an emission current ones with cold field emission electron source could be I [A] is taken to AJ (A: emission area), and the electric constructed tiny, because of operating at room tempera- field F is supposed to be βV (β: field enhancement factor, ture, and what’s more utilized to extended application to V: supplied voltage), we can numerically reformulate (1) diagnosis or therapy like a fiber scope in various medical as follows; fields. Field emission is a phenomenon of emitting electrons −6 2 7 at room temperature by quantum mechanical tunneling 2 I 1.54 × 10 Aβ 6.83 × 10 φ ln = ln − effect, when we supply high electric field (more than 2 V φ βV 10  V/m) on a metal surface, which is illustrated in Fig. 1. (2) Current density of the field emission J [A/cm ] was for- This formula represents linear relation between ln (I/V ) mulated by Fowler and Nordheim [1], by using of an elec- and 1/V. We call this Fowler and Nordheim plot, or tric field F [V/cm] and a work function φ[eV] of the metal shortly F-N plot, if ordinate and abscissa are chosen as ln as follows; (I/V ) and 1/V, respectively. Therefore, we call field emis - sion is occurred when the F-N plot shows linear relation. −1 The field enhancement factor β [cm ] and the emis- *Correspondence: s-hayasi@kyoto-msc.jp sion area A [cm ] are calculated from the slope ζ and Department 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/), 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. Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 Page 2 of 6 V(x) Tungsten Needle Δφ (Distance d) Phosphor Tungsten Filament -e /4x E -eFx Current meter Viewing port Δx -7 Pump(<10 Pa) Fig. 2 Apparatus of experimental equipment for field emission V(x)=E -eFx-e /4x Fig. 1 Principle of field emission The mirror potential (–e /4x) is remove surface contamination from the sample W nee- overlapped by supplying high electric field potential (–eFx ) on a metal, which is finally responsible for reducing the surface potential dle. The ordinary polycrystalline W needle has a sharp barrier. Therefore, electrons in the metal can penetrate the surface apex manufactured by electrochemical etching in 1  N by quantum mechanical tunneling effect, because of decreasing the KOH solution, which is shown in upper right of Fig.  2. thickness (Δx) of surface potential barrier 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 ordinate intercept b of the F-N plot line by use of formula observe FEM (Field Emission Microscope) image from (2) as follows; emitted electrons, out of chamber through an optical fiber plate attaching on the phosphor screen. This illus - 6.83 × 10 × φ φexp(b) β =− , A = (3) trates as a viewing port in Fig. 2. By supplying high volt- −6 2 ζ 1.54 × 10 × β age to the anode plate, emission current from the W needle can be measured by a fast current meter, by which These formulae show that the field enhancement factor we can continuously gather split current (repetition time β is in inverse proportion to the slope ζ, and the emis- is 0.2 s) through a personal computer. sion area A is in exponential proportion to the ordinate intercept b. If we know the value of the work function φ Simulation of electric field of the metal, we can estimate the values of β and A from We calculated an electric field by using a software code of the formulae (3), through experimentally obtained F-N electromagnetic field, ELFIN (ELF Corp.) [2]. The ELFIN plot line. code can calculate any electric field at a sharp apex with high precision by using of an original analytical integral Experimental equipment method, not an ordinary finite element method. Figure  2 shows apparatus of experimental equipment Figure  3 shows a sample of calculated electric field for field emission. The main chamber can be evacuated −8 distribution, where a sharp earthed W needle is placed up to 10  Pa by TMP (Turbo Molecular Pump) and RP perpendicular to an anode plate supplied by high volt- (Rotary Pump), not so as to be interrupted by residual age. The electric field at the apex of W needle was cal - gases in the chamber, when electrons are emitted from culated as function of the distance d between the the metal surface. In case of degrading the vacuum by cathode and anode by the ELFIN code, which is shown increasing emission current, liquid nitrogen was supplied in Fig.  4. We ascertained the calculated electric field as in the server of the chamber so as to keep the vacuum of −8 high as 10   V/m, where the diameter and apex curva- 10  Pa order. ture radius of the W needle is assumed to be 0.3 mm and A sample of sharp tungsten (W) needle spot-welding 100  nm, respectively and the supplying anode voltage on a half circled W filament is mounted on normal to an to be 1000  V. This variation curve shows that calculated anode plate, which is illustrated in upper left of Fig.  2. electric field at the sharp W needle apex is extremely This shape is suitable for resistance heating in order to Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 Page 3 of 6 mentioned above. Each curve is corresponding to varia- Anode tion of the distance d between the cathode W and anode (plate) plate. This represents that the threshold voltage to start field emission is increased, as the distance d is increased. It is readily recognized from decreasing the supplied electric field as increasing the distance d. Figure 6 shows variation of the F-N plot versus the dis- tance d, which is derived from the experimental results Cathode in Fig.  5 by use of above formula (2). These F-N plots (needle) are likely to have linear relation for any distance d. This Fig. 3 Calculated electric field distribution between the cathode and shows that field emission is occurred at room tempera - anode ture by supplying high voltage in case of the sharp ordi- nary polycrystalline W needle. Table 1 shows variation of −1 2 β [cm ], r [cm], and A [cm ] versus the distance d calcu- lated from the F-N plots through the formulae (3), where the curvature radius r is assumed to be 1/(5β) [3]. These values of β and r are reasonable on account of the sharp ordinary polycrystalline W needle. The field enhance - ment factor β is increased, as the distance d is decreased. Therefore, the curvature radius r is decreased, as the distance d is decreased. This means that the field emis - sion electrons tend to emit only from the most top of the sharp W needle apex, as the distance d is decreased. This tendency corresponds to the calculated electric field, which shows extreme increase as decreasing the distance Fig. 4 Variation of calculated electric field at the W needle apex with the distance d between cathode and anode -26 increased below 3 mm of the distance d, although gradu- d [mm] -28 ally decreased over 3 mm. -30 Experimental results 8 -32 Field emission from sharp polycrystalline W needle -34 13 Figure  5 shows voltage-current characteristics obtained -36 from field emission from a sharp ordinary polycrystalline -38 W needle manufactured by electrochemical etching as -40 00.2 0.40.6 0.81 1.2 10 /V [1/V] Fig. 6 Variation of F-N plots from the sharp ordinary polycrystalline W needle with the distance d between the cathode and anode d [mm] 15 8 −1 2 Table 1 Variations of β [cm ], r [cm], and A [cm ] with dis- tance d between  the cathode and  anode calculated from the F-N plots in Fig. 6 −1 2 d [mm] β [cm ] r [cm] A [cm ] 3 7.11E+04 2.81E−06 1.83E−13 8 2.53E+04 7.92E−06 2.17E−10 13 1.89E+04 1.06E−05 4.02E−10 V [V] 18 1.78E+04 1.12E−05 6.48E−11 Fig. 5 Voltage-current characteristics emitted from the sharp ordi- nary polycrystalline W needle 23 1.47E+04 1.36E−05 6.04E−11 I [μA] ln(I/V ) Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 Page 4 of 6 d. On the contrary, the emission area A is fluctuated on These also show that field emission is occurred at room the distance d. The reason is not sure at the present time. temperature in case of the W needle coated with any thickness of h-BN thin film. Furthermore, we found that Field emission from sharp W needle coated with h‑BN thin an extremely sharp and straight W apex was grown up in film the W needle coated with thickness of 500 nm h-BN thin Hexagonal boron nitride (h-BN) is well known to have film. On the contrary, curved W apexes were grown up layered structure binding with van der Waals force like in the W needle coated with thickness of 200 or 300 nm graphite as shown in Fig.  7. Nevertheless, h-BN is an h-BN thin film. The reason of growing up curved W insulator unlike graphite. We had tried to improve field needle apexes coated with thickness of 200 or 300  nm emission characteristics by coating with h-BN thin film h-BN thin film is not clear, but the thinner h-BN thin on various materials [4–6]. We coated the h-BN thin film film would be influenced by supplying high electric field. on the W needle at a substrate temperature of 350 °C, by These SEM (Scanning Electron Microscope) images of an ion plating method with Ar and N gases in electron the W needle apexes with h-BN thin films are shown in bombardment on B source. inserts of Fig. 8. We carried out similar field emission experiments by using the W needle coated with h-BN thin film. Figure  8 FEM image of W needle coated with h‑BN thin film shows the results of experimentally obtained F-N plots, Figure  9 shows FEM image emitted from the W needle which are also likely to have linear relation for three coated with 500  nm h-BN thin film, in which the h-BN thicknesses (200, 300 and 500  nm) of h-BN thin film. thin film is blown up in some way. The FEM image shows formation of crystallographic facets of single crystalline W apex. TEM image of W needle coated with h‑BN thin film Figure  10 shows TEM (Transmission Electron Micro- scope) image emitted from the W needle coated with 500  nm h-BN thin film. The black and light gray areas of the TEM image would correspond to W and h-BN, respectively. The curvature radius of the topmost tung - sten 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 crys- talline 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 film and creating the single crystalline W apex is surely caused by supply- Fig. 7 Model of atomic structure of h-BN (hexagonal born nitride) ing high electric filed on the W needle. Especially, h-BN ab c -28 -28 -28 -30 -30 -30 d [mm] d [mm] d [mm] 1.75 -32 -32 1.5 -32 2 2 2.5 -34 2 -34 -34 5.5 5.5 3 -36 -36 -36 -38 -38 -38 00.5 1 00.2 0.40.6 0.81 00.5 1 10 /V [1/V] 3 3 10 /V [1/V] 10 /V [1/V] h-BN:200nm h-BN:300nm h-BN:500nm Fig. 8 Variation of F-N plots with thickness of h-BN thin film as well as the distance d between the cathode and anode. Inserts show SEM images of the W needle apexes. a h-BN:200nm, b h-BN:300nm, c h-BN:500nm ln (I/V ) ln(I/V ) ln(I/V ) Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 Page 5 of 6 Fig. 9 FEM image of the W needle coated with 500 nm h-BN thin film after carrying out field emission experiment Fig. 11 TEM image from the apex of Fig. 10, shows clearly atomic structure of the single crystalline W nanotip 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. Conclusion We will make a summary of this article as follows; (1) We confirmed field emission at room temperature by supplying high voltage from sharp ordinary poly- crystalline W needle, which was manufactured by electrochemical etching. (2) We compared calculated electric field with the field Fig. 10 TEM image of the W needle coated with 500 nm h-BN thin enhancement factor β and the emission area A, film after carrying out field emission experiment which were obtained from F-N plots through field emission experiment, in order to elucidate precisely the mechanism of field emission. We confirmed that thin film must be easily blown away from the substrate W the field emission electrons emitted only from the needle apex because of the layered structure. most top of the sharp W needle. One proposed process to create the single crystalline (3) We could create the W nanotip with an extremely W nanotip is as follows. First, electro migration in ordi sharp apex, under carrying out field emission exper - nary polycrystalline W needle was occurred by supply- iment for ordinary polycrystalline W needle coated ing high electric field. Secondly, a small migrated chip in with h-BN thin film. And we ascertained the W the W needle was penetrated into the h-BN thin film by needle coated with thickness of 500  nm h-BN thin supplied high electric force. Thirdly, sudden large current film to grow up an extremely sharp no-curved apex was flown from the W needle chip, and the chip was par and to show formation of single crystalline W from tially heated by field emission current. Finally, the h-BN the FEM and TEM images. The emission current thin film was blown up by the extreme large current and from the W nanotip is measured to exceed 0.1 mA. Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 Page 6 of 6 Competing interests (4) We proposed one mechanical process to create the The authors declare that they have no competing interests. single crystalline W nanotip by supplying high elec- tric field. In near future, we will confirm the forma - Received: 21 January 2016 Accepted: 29 May 2016 tion mechanism of single crystalline W, by observ- ing diffraction patterns of X-ray or electron beam. The created W nanotip would be applied to realize high brightness electron point source which makes References it possible to improve any electron beam instru- 1. Folwer RH, Nordheim L (1928) Electron emission in intense electric fields. ments, including tiny X-ray tubes for medical or Proc R Soc London A 119:173–181 2. ELF Corp [in Japanese]. http://elf.co.jp/index.php?FrontPage. Accessed 7 industrial use, as well as a cantilever of scanning June 2016 probe microscope. 3. Gomer R (1961) Filed emission and field ionization. Harvard University Press, Cambridge Authors’ contributions 4. Sugino T, Kimura C, Yamamoto T (2002) Electron field emission from SH conceived of the study, and participated in its design and coordination and boron-nitride nanofilms. Appl Phys Lett 80:3602–3604 helped to draft the manuscript. MO, ST and HN carried out the experiments, 5. Sugino T, Yamamoto T, Kimura C, Murakami H, Hirakawa M (2002) Field and summarized the data. All authors read and approved the final manuscript. emission characteristics of carbon nanofiber improved by deposition of boron nitride nanocrystalline film. Appl Phys Lett 80:3808–3810 6. Morihisa Y, Kimura C, Yukawa M, Aoki H, Kobayashi T, Hayashi S, Akita S, Acknowledgements Nakayama Y, Sugino T (2008) Improved field emission characteristics of We appreciate Mr. Yano of ELF Corp. for supplying the electromagnetic field individual carbon nanotube coated with boron nitride nanofilm. J Vac Sci simulation code ELFIN, in order to calculate the electric field at an extremely Technol B26(2):872–875 sharp apex of the W needle. This work was supported by JSPS KAKENHI Grant Number 26670302. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Micro and Nano Systems Letters Springer Journals

A novel method for formation of single crystalline tungsten nanotip

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Publisher
Springer Journals
Copyright
Copyright © 2016 by The Author(s)
Subject
Engineering; Circuits and Systems; Electrical Engineering; Mechanical Engineering; Nanotechnology; Applied and Technical Physics
eISSN
2213-9621
DOI
10.1186/s40486-016-0029-3
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See Article on Publisher Site

Abstract

A point electron source is desired to improve performance of high brightness electron beam instruments. It is valu- able 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 polycrystal- line 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 signifi- cant 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. Background 1 3 3 2 2 2 More than 100  years, thermal electrons emitted from e F 8π(2m) φ J = exp − (1) heated filament have been widely utilized for any X-ray 8πhφ 3heF tube including medical use. Although X-ray tubes with where e, m, and h are elementary charge, electron mass, thermal electron source would become rather huge, and Planck’s constant, respectively. If an emission current ones with cold field emission electron source could be I [A] is taken to AJ (A: emission area), and the electric constructed tiny, because of operating at room tempera- field F is supposed to be βV (β: field enhancement factor, ture, and what’s more utilized to extended application to V: supplied voltage), we can numerically reformulate (1) diagnosis or therapy like a fiber scope in various medical as follows; fields. Field emission is a phenomenon of emitting electrons −6 2 7 at room temperature by quantum mechanical tunneling 2 I 1.54 × 10 Aβ 6.83 × 10 φ ln = ln − effect, when we supply high electric field (more than 2 V φ βV 10  V/m) on a metal surface, which is illustrated in Fig. 1. (2) Current density of the field emission J [A/cm ] was for- This formula represents linear relation between ln (I/V ) mulated by Fowler and Nordheim [1], by using of an elec- and 1/V. We call this Fowler and Nordheim plot, or tric field F [V/cm] and a work function φ[eV] of the metal shortly F-N plot, if ordinate and abscissa are chosen as ln as follows; (I/V ) and 1/V, respectively. Therefore, we call field emis - sion is occurred when the F-N plot shows linear relation. −1 The field enhancement factor β [cm ] and the emis- *Correspondence: s-hayasi@kyoto-msc.jp sion area A [cm ] are calculated from the slope ζ and Department 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/), 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. Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 Page 2 of 6 V(x) Tungsten Needle Δφ (Distance d) Phosphor Tungsten Filament -e /4x E -eFx Current meter Viewing port Δx -7 Pump(<10 Pa) Fig. 2 Apparatus of experimental equipment for field emission V(x)=E -eFx-e /4x Fig. 1 Principle of field emission The mirror potential (–e /4x) is remove surface contamination from the sample W nee- overlapped by supplying high electric field potential (–eFx ) on a metal, which is finally responsible for reducing the surface potential dle. The ordinary polycrystalline W needle has a sharp barrier. Therefore, electrons in the metal can penetrate the surface apex manufactured by electrochemical etching in 1  N by quantum mechanical tunneling effect, because of decreasing the KOH solution, which is shown in upper right of Fig.  2. thickness (Δx) of surface potential barrier 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 ordinate intercept b of the F-N plot line by use of formula observe FEM (Field Emission Microscope) image from (2) as follows; emitted electrons, out of chamber through an optical fiber plate attaching on the phosphor screen. This illus - 6.83 × 10 × φ φexp(b) β =− , A = (3) trates as a viewing port in Fig. 2. By supplying high volt- −6 2 ζ 1.54 × 10 × β age to the anode plate, emission current from the W needle can be measured by a fast current meter, by which These formulae show that the field enhancement factor we can continuously gather split current (repetition time β is in inverse proportion to the slope ζ, and the emis- is 0.2 s) through a personal computer. sion area A is in exponential proportion to the ordinate intercept b. If we know the value of the work function φ Simulation of electric field of the metal, we can estimate the values of β and A from We calculated an electric field by using a software code of the formulae (3), through experimentally obtained F-N electromagnetic field, ELFIN (ELF Corp.) [2]. The ELFIN plot line. code can calculate any electric field at a sharp apex with high precision by using of an original analytical integral Experimental equipment method, not an ordinary finite element method. Figure  2 shows apparatus of experimental equipment Figure  3 shows a sample of calculated electric field for field emission. The main chamber can be evacuated −8 distribution, where a sharp earthed W needle is placed up to 10  Pa by TMP (Turbo Molecular Pump) and RP perpendicular to an anode plate supplied by high volt- (Rotary Pump), not so as to be interrupted by residual age. The electric field at the apex of W needle was cal - gases in the chamber, when electrons are emitted from culated as function of the distance d between the the metal surface. In case of degrading the vacuum by cathode and anode by the ELFIN code, which is shown increasing emission current, liquid nitrogen was supplied in Fig.  4. We ascertained the calculated electric field as in the server of the chamber so as to keep the vacuum of −8 high as 10   V/m, where the diameter and apex curva- 10  Pa order. ture radius of the W needle is assumed to be 0.3 mm and A sample of sharp tungsten (W) needle spot-welding 100  nm, respectively and the supplying anode voltage on a half circled W filament is mounted on normal to an to be 1000  V. This variation curve shows that calculated anode plate, which is illustrated in upper left of Fig.  2. electric field at the sharp W needle apex is extremely This shape is suitable for resistance heating in order to Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 Page 3 of 6 mentioned above. Each curve is corresponding to varia- Anode tion of the distance d between the cathode W and anode (plate) plate. This represents that the threshold voltage to start field emission is increased, as the distance d is increased. It is readily recognized from decreasing the supplied electric field as increasing the distance d. Figure 6 shows variation of the F-N plot versus the dis- tance d, which is derived from the experimental results Cathode in Fig.  5 by use of above formula (2). These F-N plots (needle) are likely to have linear relation for any distance d. This Fig. 3 Calculated electric field distribution between the cathode and shows that field emission is occurred at room tempera - anode ture by supplying high voltage in case of the sharp ordi- nary polycrystalline W needle. Table 1 shows variation of −1 2 β [cm ], r [cm], and A [cm ] versus the distance d calcu- lated from the F-N plots through the formulae (3), where the curvature radius r is assumed to be 1/(5β) [3]. These values of β and r are reasonable on account of the sharp ordinary polycrystalline W needle. The field enhance - ment factor β is increased, as the distance d is decreased. Therefore, the curvature radius r is decreased, as the distance d is decreased. This means that the field emis - sion electrons tend to emit only from the most top of the sharp W needle apex, as the distance d is decreased. This tendency corresponds to the calculated electric field, which shows extreme increase as decreasing the distance Fig. 4 Variation of calculated electric field at the W needle apex with the distance d between cathode and anode -26 increased below 3 mm of the distance d, although gradu- d [mm] -28 ally decreased over 3 mm. -30 Experimental results 8 -32 Field emission from sharp polycrystalline W needle -34 13 Figure  5 shows voltage-current characteristics obtained -36 from field emission from a sharp ordinary polycrystalline -38 W needle manufactured by electrochemical etching as -40 00.2 0.40.6 0.81 1.2 10 /V [1/V] Fig. 6 Variation of F-N plots from the sharp ordinary polycrystalline W needle with the distance d between the cathode and anode d [mm] 15 8 −1 2 Table 1 Variations of β [cm ], r [cm], and A [cm ] with dis- tance d between  the cathode and  anode calculated from the F-N plots in Fig. 6 −1 2 d [mm] β [cm ] r [cm] A [cm ] 3 7.11E+04 2.81E−06 1.83E−13 8 2.53E+04 7.92E−06 2.17E−10 13 1.89E+04 1.06E−05 4.02E−10 V [V] 18 1.78E+04 1.12E−05 6.48E−11 Fig. 5 Voltage-current characteristics emitted from the sharp ordi- nary polycrystalline W needle 23 1.47E+04 1.36E−05 6.04E−11 I [μA] ln(I/V ) Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 Page 4 of 6 d. On the contrary, the emission area A is fluctuated on These also show that field emission is occurred at room the distance d. The reason is not sure at the present time. temperature in case of the W needle coated with any thickness of h-BN thin film. Furthermore, we found that Field emission from sharp W needle coated with h‑BN thin an extremely sharp and straight W apex was grown up in film the W needle coated with thickness of 500 nm h-BN thin Hexagonal boron nitride (h-BN) is well known to have film. On the contrary, curved W apexes were grown up layered structure binding with van der Waals force like in the W needle coated with thickness of 200 or 300 nm graphite as shown in Fig.  7. Nevertheless, h-BN is an h-BN thin film. The reason of growing up curved W insulator unlike graphite. We had tried to improve field needle apexes coated with thickness of 200 or 300  nm emission characteristics by coating with h-BN thin film h-BN thin film is not clear, but the thinner h-BN thin on various materials [4–6]. We coated the h-BN thin film film would be influenced by supplying high electric field. on the W needle at a substrate temperature of 350 °C, by These SEM (Scanning Electron Microscope) images of an ion plating method with Ar and N gases in electron the W needle apexes with h-BN thin films are shown in bombardment on B source. inserts of Fig. 8. We carried out similar field emission experiments by using the W needle coated with h-BN thin film. Figure  8 FEM image of W needle coated with h‑BN thin film shows the results of experimentally obtained F-N plots, Figure  9 shows FEM image emitted from the W needle which are also likely to have linear relation for three coated with 500  nm h-BN thin film, in which the h-BN thicknesses (200, 300 and 500  nm) of h-BN thin film. thin film is blown up in some way. The FEM image shows formation of crystallographic facets of single crystalline W apex. TEM image of W needle coated with h‑BN thin film Figure  10 shows TEM (Transmission Electron Micro- scope) image emitted from the W needle coated with 500  nm h-BN thin film. The black and light gray areas of the TEM image would correspond to W and h-BN, respectively. The curvature radius of the topmost tung - sten 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 crys- talline 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 film and creating the single crystalline W apex is surely caused by supply- Fig. 7 Model of atomic structure of h-BN (hexagonal born nitride) ing high electric filed on the W needle. Especially, h-BN ab c -28 -28 -28 -30 -30 -30 d [mm] d [mm] d [mm] 1.75 -32 -32 1.5 -32 2 2 2.5 -34 2 -34 -34 5.5 5.5 3 -36 -36 -36 -38 -38 -38 00.5 1 00.2 0.40.6 0.81 00.5 1 10 /V [1/V] 3 3 10 /V [1/V] 10 /V [1/V] h-BN:200nm h-BN:300nm h-BN:500nm Fig. 8 Variation of F-N plots with thickness of h-BN thin film as well as the distance d between the cathode and anode. Inserts show SEM images of the W needle apexes. a h-BN:200nm, b h-BN:300nm, c h-BN:500nm ln (I/V ) ln(I/V ) ln(I/V ) Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 Page 5 of 6 Fig. 9 FEM image of the W needle coated with 500 nm h-BN thin film after carrying out field emission experiment Fig. 11 TEM image from the apex of Fig. 10, shows clearly atomic structure of the single crystalline W nanotip 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. Conclusion We will make a summary of this article as follows; (1) We confirmed field emission at room temperature by supplying high voltage from sharp ordinary poly- crystalline W needle, which was manufactured by electrochemical etching. (2) We compared calculated electric field with the field Fig. 10 TEM image of the W needle coated with 500 nm h-BN thin enhancement factor β and the emission area A, film after carrying out field emission experiment which were obtained from F-N plots through field emission experiment, in order to elucidate precisely the mechanism of field emission. We confirmed that thin film must be easily blown away from the substrate W the field emission electrons emitted only from the needle apex because of the layered structure. most top of the sharp W needle. One proposed process to create the single crystalline (3) We could create the W nanotip with an extremely W nanotip is as follows. First, electro migration in ordi sharp apex, under carrying out field emission exper - nary polycrystalline W needle was occurred by supply- iment for ordinary polycrystalline W needle coated ing high electric field. Secondly, a small migrated chip in with h-BN thin film. And we ascertained the W the W needle was penetrated into the h-BN thin film by needle coated with thickness of 500  nm h-BN thin supplied high electric force. Thirdly, sudden large current film to grow up an extremely sharp no-curved apex was flown from the W needle chip, and the chip was par and to show formation of single crystalline W from tially heated by field emission current. Finally, the h-BN the FEM and TEM images. The emission current thin film was blown up by the extreme large current and from the W nanotip is measured to exceed 0.1 mA. Hayashi et al. Micro and Nano Syst Lett (2016) 4:3 Page 6 of 6 Competing interests (4) We proposed one mechanical process to create the The authors declare that they have no competing interests. single crystalline W nanotip by supplying high elec- tric field. In near future, we will confirm the forma - Received: 21 January 2016 Accepted: 29 May 2016 tion mechanism of single crystalline W, by observ- ing diffraction patterns of X-ray or electron beam. The created W nanotip would be applied to realize high brightness electron point source which makes References it possible to improve any electron beam instru- 1. Folwer RH, Nordheim L (1928) Electron emission in intense electric fields. ments, including tiny X-ray tubes for medical or Proc R Soc London A 119:173–181 2. ELF Corp [in Japanese]. http://elf.co.jp/index.php?FrontPage. Accessed 7 industrial use, as well as a cantilever of scanning June 2016 probe microscope. 3. Gomer R (1961) Filed emission and field ionization. Harvard University Press, Cambridge Authors’ contributions 4. Sugino T, Kimura C, Yamamoto T (2002) Electron field emission from SH conceived of the study, and participated in its design and coordination and boron-nitride nanofilms. Appl Phys Lett 80:3602–3604 helped to draft the manuscript. MO, ST and HN carried out the experiments, 5. Sugino T, Yamamoto T, Kimura C, Murakami H, Hirakawa M (2002) Field and summarized the data. All authors read and approved the final manuscript. emission characteristics of carbon nanofiber improved by deposition of boron nitride nanocrystalline film. Appl Phys Lett 80:3808–3810 6. Morihisa Y, Kimura C, Yukawa M, Aoki H, Kobayashi T, Hayashi S, Akita S, Acknowledgements Nakayama Y, Sugino T (2008) Improved field emission characteristics of We appreciate Mr. Yano of ELF Corp. for supplying the electromagnetic field individual carbon nanotube coated with boron nitride nanofilm. J Vac Sci simulation code ELFIN, in order to calculate the electric field at an extremely Technol B26(2):872–875 sharp apex of the W needle. This work was supported by JSPS KAKENHI Grant Number 26670302.

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Micro and Nano Systems LettersSpringer Journals

Published: Jun 8, 2016

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