| Citation: |
Shuangyuan FENG, Shin KAJITA, Masayuki TOKITANI, Daisuke NAGATA, Noriyasu OHNO. Rapid growth of nanostructure on tungsten thin film by exposure to helium plasma[J]. Plasma Science and Technology, 2023, 25(4): 042001. DOI: 10.1088/2058-6272/ac9f2d
|
A fiberform nanostructure was synthesized by exposing high-density helium plasma to a 100 nm thick tungsten thin film in the linear plasma device NAGDIS-II. After helium plasma exposure, the cross-section of samples was observed by a scanning electron microscope, transmission electron microscope, and focused ion beam scanning electron microscope. It is shown that the thickness of the nanostructured layer increases significantly for only a short irradiation time. The optical absorptivity remains high, even though it is exposed to helium plasma for a short time. The usage of the thin film can shorten the processing time for nanostructure growth, which will be beneficial for commercial production.
Tungsten (W) is primarily used as a plasma-facing material in fusion reactors [1–4]. When exposed to tokamak-based helium plasma in plasma devices at a laboratory scale, the surface of tungsten will grow fiberform nanostructures (FNs), often referred to as 'fuzz, ' under certain conditions [5–11]. The FNs are composed of fibers with diameters of 10–50 nm. There have been many studies on FNs researching detailed mechanisms [6, 10, 12–14], conditions of growth [10, 15], characteristics, and so on. It has been found that favorable formation conditions are surface temperature in the range of 1000–2000 K [15] and incident ion energy greater than 20 eV [10]. Owing to the formation of FN on the material surface, various properties are largely changed in electronics, mechanics, magnetics, optics, etc [16–20]. Although the property changes caused by the formation of FNs have a detrimental effect on fusion reactor performance and operation [21, 22], they have potential applications in many fields and devices, such as solar absorbers [18, 19, 23], photocatalysis [24–26], gas sensors [27], and so on. Petty et al first used a magnetron sputtering device to create a tungsten nanostructure [11]. Although the usage of widely available magnetron sputtering expanded the potential application of FNs to various fields, the formation of FNs took a long time, typically more than six hours. The requirement for the formation of FNs is high helium ion fluence, namely, it requires high density and/or a long time. Rapid preparation of FNs is beneficial to better promote the fiberform nanostructure of effective utilization in many industrial fields. Recently, non-uniform isolated nanostructures were formed when He plasma exposure was performed on W thin-film samples in which the film thickness was 100 nm or less [28]. The results suggested that the growth property of FNs on thin films can be quite different from sheet samples, especially when the film thickness is 100 nm or less. In the present article, 100 nm thick tungsten thin films were exposed in a linear plasma device NAGDIS-II [28, 29]. This study compared the growth rates of the FNs on sheet and thin-film samples. We will discuss the difference in the growth process of FNs on the thin-film sample exposed to He plasma from that on the sheet sample. We will also show the optical absorptivity of nanostructured thin-film samples under low helium fluence and briefly discuss the formation mechanism of the FNs on the thin-film sample.
The substrate used in this experiment was 10 × 10 mm2 quartz glass (Labo-USQ, 1 mm thickness). 100 nm thick tungsten thin films were deposited on substrates by a radio frequency (RF) magnetron sputtering apparatus. After deposition, the linear plasma irradiation device NAGDIS-II was used to obtain fiberform nanostructure in the experiment. High-density helium plasma could be formed by a direct current arc discharge using a LaB6 cathode. A single probe was used to measure the electron density (~1018–1019 m−3) and temperature (~5 eV). A radiation pyrometer was applied to measure the surface temperature after starting the plasma irradiation. Negative bias was applied to the tungsten thin-film samples. The negative bias of all samples was −90 V. In this study, −90 V was chosen to obtain high incident ion energy and easier generation of fiberform nanostructures. The irradiation conditions of different samples are summarized in table 1. The samples were numbered from W0 to W5, as shown in table 1. In this study, the He fluence was altered from 6.6 × 1024 (W0) to 1.2 × 1026 (W5) m−2 at the surface temperature of 1090–1173 K. To evaluate the effects of helium ion fluence on the thickness of the nanostructured layer, the tungsten thin-film samples were irradiated for various helium ion fluences. The surface and cross-sectional morphology observations were processed by a scanning electron microscope (SEM), a focused ion beam scanning electron microscope (FIB-SEM, nanoDUE'T NB5000, Hitachi High-Technologies Corp.), and a transmission electron microscope (TEM). The optical reflectivity was carried out using a UV-Vis spectrophotometer (UV-2600). The transmissivity was measured using a UV-VIS-NIR spectrophotometer (UV-3600i Plus).
| No. | Ts (K) | Fluence (m−2) |
| W0 | 1108 | 6.6 × 1024 |
| W1 | 1149 | 8.7 × 1024 |
| W2 | 1169 | 1.0 × 1025 |
| W3 | 1173 | 1.3 × 1025 |
| W4 | 1167 | 5.2 × 1025 |
| W5 | 1090 | 1.2 × 1026 |
DownLoad:
CSV
Figures 1(a) and (b) show the surface and cross-sectional micrographs of the W1 sample, respectively, which was exposed to the helium plasma with a helium fluence of 8.7 × 1024 m−2. The island-shaped nanostructures were formed on the surface, and the thickness of the layer was approximately 550 nm. In figure 1(b), the island structure cannot be clearly identified because the sample was not cut to a thin film, using a focused ion beam (FIB). Figures 2(a)–(c) show TEM micrographs of the W2 sample. The TEM sample was prepared with FIB milling after a carbon coating. The sample was prepared in the form of a thin film, so the individual island-shaped nanostructures can be clearly seen. Some self-assembling processes should exist behind the formation of such island structures; the mechanism has yet to be understood. One of the possible causes of the island-shaped nanostructure is that due to the difference in the thermal expansion coefficient, the nanostructure cannot follow the thermal expansion of the SiO2 substrate.
The part without nanostructures (the red circle part) in figure 2(a) corresponds to the middle of the island-shaped nanostructures in figure 1(a). It can be seen from figures 2(b) and (c) that the thin film became fibers almost totally without remaining in the bulk layer, and it seems that fibers also grow downward. This is probably because of the swelling process due to the growth of He bubbles, as was discussed in [30]. Because the thickness of W thin film is just 100 nm, the implanted He atoms probably cannot easily diffuse to a deeper region due to the barrier between the SiO2 substrates, as no He bubbles can be identified on the SiO2 substrate beneath the fuzz layer. Due to limited diffusion in the depth direction, the He density in the top layer can be higher than those in sheet sample cases. In addition, in the region where W atoms were removed and He ions could reach the SiO2 substrate, nanocones were identified on the SiO2 surface, similar to Si nanocones formed by He ion irradiation [31]. Because a darker droplet can be found on the tip of the nanocone, which is marked with a red circle in figure 2(c), the formation mechanism is the same as that on the Si substrate, on which sputtering played a major role for the nanocone formation.
Figure 3 shows the wavelength dependence of optical absorptivity, derived from the specular reflection spectrum and transmission spectrum, for W0. The helium fluence is 6.6 × 1024 m−2. The red and blue lines with markers are the wavelength dependence of the absorptivity of the non-irradiated W and irradiated W sheet sample with a helium fluence of 4.6 × 1024 m−2, respectively. The absorptivity reaches 90% in the visible region, even though the irradiation time of W0 is very short (180 s). The helium fluence of nanostructured thin-film samples is much lower, as compared to that of nanostructured sheet samples, in order to achieve the same optical absorptivity. The absorptivity of the nanostructured sheet sample under a similar helium fluence is about 60% [19].
Figure 4 plots the thickness of the nanostructured layer grown on thin film and sheet samples as a function of the helium fluence. The helium fluence of W2 is 1.0 × 1025 m−2, which is slightly lower than that of W3 (1.3 × 1025 m−2). As the helium fluence increased slightly, the nanostructured layer became thicker by comparing W3 (~819 nm) to W2 (~668 nm). The nanostructured layer thickness of W3 was approximately 1.2 times that of W2. As compared to the increase in the helium fluence, the thickness of the nanostructured layer did not change significantly from W3 (~819 nm) and W4 (~1000 nm), though the fluence increased more than four times. The helium fluence of W4 and W5 are 5.2 × 1025 and 1.2 × 1026 m−2, respectively. The thickness of the nanostructured layer did not increase with the helium fluence, comparing W4 and W5 (~1000 nm). The results indicate that as the helium fluence increases, the nanostructured layer thickness increases and finally approaches a saturated value (~1 μm). As can be seen from figure 4, the thickness of the nanostructured layer on thin-film samples is thicker than that on the sheet samples when the helium fluence is low, say lower than ~5 × 1024 m−2. Petty and his colleagues collected data from various devices and concluded that He fluence is a sole key factor to determining the fuzzy layer thickness. In other words, the fuzzy thickness can be determined by the He fluence when other competing processes such as annealing [33] and sputtering [34] do not work. That is, the growth process is determined by some diffusion-limited process, though the exact process has yet to be identified. Numerous studies have investigated the growth model of nanostructures under high fluence, and it follows a square root of fluence (Φ1/2) fit [6]. Petty investigated the growth model under low fluence and found it also followed a Φ1/2 fit. It is reported by Baldwin et al that there was an incubation time necessary for fuzz growth [35]. It is more convenient to discuss using fluence rather than time so as to be able to compare all devices [36]. Researchers have been able to use incubation fluence for experiments with low fluxes and long exposure times. However, in the case of large flux and an extremely short irradiation time, such a time is not enough for fuzz growth. This implies that some inherent incubation time is necessary for fuzz to grow. Regarding the results of figure 4, it seems that the present thin-film samples in this study under low fluence have a short incubation time. Although an enhanced growth process has recently been identified with auxiliary W deposition [37], the enhanced growth identified in this work is an interesting process because it violates the diffusion-limited process, especially when the He fluence is low (< 1025 m−2). One of the key factors that is not consistent with the square root of fluence, as Petty stated, is likely to be the increase in the density of He atoms on the top layer because of the limited diffusion toward the deeper region. One possible reason is the thermal deformation of the SiO2 substrate that formed the island structure. As can be seen from figure 2(a), the deformation of the SiO2 substrate has already occurred and there is little tungsten thin film in the concave part of the substrate. The convex part of the substrate was far more likely to be entered than the concave part. The island structure can be formed more easily. In the future, it will be of interest to investigate the amount of He atoms on the thin layer by using thermal desorption spectroscopy. One of the disadvantages of this work is the non-uniformity of the fuzzy layer. It is of importance to investigate this non-uniformity in lower fluence.
In conclusion, in this study, tungsten thin-film samples were exposed to helium plasma and it was found that as the helium fluence increased, the nanostructured thickness grew rapidly. The optical absorptivity of W0 at low helium fluence is very high, up to about 90%, which will provide the foundation for its optical application. As mentioned earlier, the preparation time of existing preparation methods for generating fiberform nanostructures is very long [11]. Irradiation with thin-film samples brings many advantages, such as a short preparation time and high optical absorptivity, which may make the application scope for obvious expansion. This synthesizes the merits of high optical absorptivity and short formation time, and offers a new effective option for optical application.
| [1] |
Ueda Y et al 2014 Fusion Eng. Des. 89 901 doi: 10.1016/j.fusengdes.2014.02.078
|
| [2] |
Neu R et al 2016 Fusion Eng. Des. 109–111 1046 doi: 10.1016/j.fusengdes.2016.01.027
|
| [3] |
Wurster S et al 2013 J. Nucl. Mater. 442 S181 doi: 10.1016/j.jnucmat.2013.02.074
|
| [4] |
Dasgupta D et al 2019 Nucl. Fusion
59 086057 doi: 10.1088/1741-4326/ab22cb
|
| [5] |
Takamura S et al 2006 Plasma Fusion Res. 1 051 doi: 10.1585/pfr.1.051
|
| [6] |
Baldwin M J and Doerner R P 2008 Nucl. Fusion
48 035001 doi: 10.1088/0029-5515/48/3/035001
|
| [7] |
Wang K et al 2017 Sci. Rep. 7 42315 doi: 10.1038/srep42315
|
| [8] |
Yang Q et al 2015 Sci. Rep. 5 10959 doi: 10.1038/srep10959
|
| [9] |
Wright G M et al 2013 J. Nucl. Mater
438 S84 doi: 10.1016/j.jnucmat.2013.01.013
|
| [10] |
Kajita S et al 2009 Nucl. Fusion
49 095005 doi: 10.1088/0029-5515/49/9/095005
|
| [11] |
Petty T J and Bradley J W 2014 J. Nucl. Mater. 453 320 doi: 10.1016/j.jnucmat.2014.07.023
|
| [12] |
Krasheninnikov S I et al 2011 Phys. Scr. T145 014040 doi: 10.1088/0031-8949/2011/T145/014040
|
| [13] |
Martynenko Y V and Nagel M Y 2012 Plasma Phys. Rep. 38 996 doi: 10.1134/S1063780X12110074
|
| [14] |
Lasa A, Tähtinen S K and Nordlund K 2014 Europhy. Lett. 105 25002 doi: 10.1209/0295-5075/105/25002
|
| [15] |
Sakaguchi W et al 2010 Plasma Fusion Res. 5 S1023 doi: 10.1585/pfr.5.S1023
|
| [16] |
Nishijima D et al 2011 J. Nucl. Mater
415 S96 doi: 10.1016/j.jnucmat.2010.12.017
|
| [17] |
Patino M, Raitses Y and Wirz R 2016 Appl. Phys. Lett. 109 201602
|
| [18] |
Kajita S et al 2010 Appl. Phys. Express
3 085204 doi: 10.1143/APEX.3.085204
|
| [19] |
Kajita S et al 2011 Jpn. J. Appl. Phys. 50 08JG01 doi: 10.7567/JJAP.50.08JG01
|
| [20] |
Kajita S et al 2016 Results Phys. 6 877 doi: 10.1016/j.rinp.2016.10.025
|
| [21] |
Wirth B D et al 2015 J. Nucl. Mater. 463 30 doi: 10.1016/j.jnucmat.2014.11.072
|
| [22] |
Kajita S et al 2014 Nucl. Fusion
54 033005 doi: 10.1088/0029-5515/54/3/033005
|
| [23] |
Tripathi J K, Novakowski T J and Hassanein A 2015 Appl. Surf. Sci. 353 1070 doi: 10.1016/j.apsusc.2015.06.173
|
| [24] |
Kajita S et al 2013 J. Appl. Phys. 113 134301 doi: 10.1063/1.4798597
|
| [25] |
de Respinis M et al 2013 ACS Appl. Mater. Interfaces
5 7621 doi: 10.1021/am401936q
|
| [26] |
Feng S Y et al 2022 Appl. Surf. Sci. 580 151979 doi: 10.1016/j.apsusc.2021.151979
|
| [27] |
Ibano K et al 2018 Jpn. J. Appl. Phys. 57 040316 doi: 10.7567/JJAP.57.040316
|
| [28] |
Feng S Y et al 2020 Mater. Res. Express
7 075007 doi: 10.1088/2053-1591/aba394
|
| [29] |
Ohno N et al 2001 Nucl. Fusion
41 1055 doi: 10.1088/0029-5515/41/8/309
|
| [30] |
Kajita S et al 2015 New J. Phys. 17 043038 doi: 10.1088/1367-2630/17/4/043038
|
| [31] |
Takamura S et al 2019 Appl. Surf. Sci. 487 755 doi: 10.1016/j.apsusc.2019.05.034
|
| [32] |
Kajita S et al 2011 J. Nucl. Mater. 418 152 doi: 10.1016/j.jnucmat.2011.06.026
|
| [33] |
De Temmerman G Doerner R P Pitts R A 2019 Nucl. Mater. Energy
19 255 doi: 10.1016/j.nme.2019.01.034
|
| [34] |
Noiri Y, Kajita S and Ohno N 2015 J. Nucl. Mater. 463 285 doi: 10.1016/j.jnucmat.2015.01.036
|
| [35] |
Baldwin M J et al 2009 J. Nucl. Mater. 390–391 886 doi: 10.1016/j.jnucmat.2009.01.247
|
| [36] |
Petty T J et al 2015 Nucl. Fusion
55 093033 doi: 10.1088/0029-5515/55/9/093033
|
| [37] |
Kajita S et al 2018 Sci. Rep. 8 56 doi: 10.1038/s41598-017-18476-7
|
| [1] | Ling LI (李玲), Hailin GUO (郭海林), Junqin ZONG (宗俊勤), Jingbo CHEN (陈静波), Yi WANG (汪毅), Jianjian LI (李建建), Dandan LI (李丹丹), Hanliang SHAO (邵汉良), Jianxiu LIU (刘建秀). Influence of low-vacuum helium cold plasma pre-treatment on the rooting and root growth of zoysiagrass (Zoysia Willd.) stolon cuttings[J]. Plasma Science and Technology, 2019, 21(5): 55504-055504. DOI: 10.1088/2058-6272/aaf368 |
| [2] | N C ROY, M M HASAN, A H KABIR, M A REZA, M R TALUKDER, A N CHOWDHURY. Atmospheric pressure gliding arc discharge plasma treatments for improving germination, growth and yield of wheat[J]. Plasma Science and Technology, 2018, 20(11): 115501. DOI: 10.1088/2058-6272/aac647 |
| [3] | Ling LI (李玲), Jiangang LI (李建刚), Hanliang SHAO (邵汉良), Yuanhua DONG (董元华). Effects of low-vacuum helium cold plasma treatment on seed germination, plant growth and yield of oilseed rape[J]. Plasma Science and Technology, 2018, 20(9): 95502-095502. DOI: 10.1088/2058-6272/aac3d0 |
| [4] | Jiafeng JIANG (蒋佳峰), Jiangang LI (李建刚), Yuanhua DONG (董元华). Effect of cold plasma treatment on seedling growth and nutrient absorption of tomato[J]. Plasma Science and Technology, 2018, 20(4): 44007-044007. DOI: 10.1088/2058-6272/aaa0bf |
| [5] | Jinkui FENG (冯金奎), Decheng WANG (王德成), Changyong SHAO (邵长勇), Lili ZHANG (张丽丽), Xin TANG (唐欣). Effects of cold plasma treatment on alfalfa seed growth under simulated drought stress[J]. Plasma Science and Technology, 2018, 20(3): 35505-035505. DOI: 10.1088/2058-6272/aa9b27 |
| [6] | Xiaochun MA (马小春), Xiaogang CAO (曹小岗), Lei HAN (韩磊), Zhiyan ZHANG (张志艳), Jianjun WEI (韦建军), Fujun GOU (芶富均). Characterization of high flux magnetized helium plasma in SCU-PSI linear device[J]. Plasma Science and Technology, 2018, 20(2): 25104-025104. DOI: 10.1088/2058-6272/aa936e |
| [7] | Nasrin SAFARI, Alireza IRANBAKHSH, Zahra ORAGHI ARDEBILI. Non-thermal plasma modified growth and differentiation process of Capsicum annuum PP805 Godiva in in vitro conditions[J]. Plasma Science and Technology, 2017, 19(5): 55501-055501. DOI: 10.1088/2058-6272/aa57ef |
| [8] | TONG Jiayun(童家赟), HE Rui(何瑞), ZHANG Xiaoli(张晓丽), ZHAN Ruoting(詹若挺), CHEN Weiwen(陈蔚文), YANG Size(杨思泽). Effects of Atmospheric Pressure Air Plasma Pretreatment on the Seed Germination and Early Growth of Andrographis paniculata[J]. Plasma Science and Technology, 2014, 16(3): 260-266. DOI: 10.1088/1009-0630/16/3/16 |
| [9] | JIANG Jiafeng(蒋佳峰), HE Xin(何昕), LI Ling(李玲), LI Jiangang(李建刚), SHAO Hanliang(邵汉良), XU Qilai(徐启来), YE Renhong(叶仁宏), DONG Yuanhua(董元华). Effect of Cold Plasma Treatment on Seed Germination and Growth of Wheat[J]. Plasma Science and Technology, 2014, 16(1): 54-58. DOI: 10.1088/1009-0630/16/1/12 |
| [10] | Takanori MIYAMOTO, Shuichi TAKAMURA, Hiroaki KURISHITA. Recovery of Tungsten Surface with Fiber-Form Nanostructure by Plasmas Exposures[J]. Plasma Science and Technology, 2013, 15(2): 161-165. DOI: 10.1088/1009-0630/15/2/17 |
| 1. | Farahani, F.A., Depla, D. Phase composition of sputter deposited tungsten thin films. Surface and Coatings Technology, 2024. DOI:10.1016/j.surfcoat.2024.131447 |
| 2. | Shi, Q., Kajita, S., Ohno, N. Tungsten fuzz growth at low temperatures (<900 K) on a surface with nanocones. Nuclear Materials and Energy, 2024. DOI:10.1016/j.nme.2024.101668 |
| 3. | Kajita, S., Yagi, T., Ohno, N. Thermal Conductivity Measurement of Fuzzy W Using a Thermoreflectance Method. IEEE Transactions on Plasma Science, 2024, 52(9): 4365-4370. DOI:10.1109/TPS.2023.3348465 |
| 4. | Feng, S., Kajita, S., Yasuhara, R. et al. Formation of corrugated nano-fuzz tungsten thin film on silicon via helium plasma irradiation. Japanese Journal of Applied Physics, 2024, 63(1): 010904. DOI:10.35848/1347-4065/ad12ef |
| 5. | Feng, S., Natsume, H., Kajita, S. et al. Fabrication of tungsten-based optical diffuser using fiberform nanostructure via efficient plasma route. Optics Express, 2023, 31(16): 25438-25445. DOI:10.1364/OE.493993 |
| 6. | Zhou, J., Lu, F., Cong, J. et al. Effect of plasma jet on electrochemical properties of silk fibroin hydrogel doped with PEDOT:PSS. Energy Science and Engineering, 2023, 11(7): 2521-2534. DOI:10.1002/ese3.1471 |
| 7. | Kajita, S., Yagi, T., Ohno, N. Thermal conductivity measurement of fuzzy W using a thermoreflectance method. Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV, 2023. DOI:10.23919/ISDEIV55268.2023.10200254 |
Figures(4) / Tables(1)
Supported by: Beijing Renhe Information Technology Co., Ltd. 
| No. | Ts (K) | Fluence (m−2) |
| W0 | 1108 | 6.6 × 1024 |
| W1 | 1149 | 8.7 × 1024 |
| W2 | 1169 | 1.0 × 1025 |
| W3 | 1173 | 1.3 × 1025 |
| W4 | 1167 | 5.2 × 1025 |
| W5 | 1090 | 1.2 × 1026 |
DownLoad:
CSV
