Formation of Multiscale Protrusions on Commercially Pure Titanium and Stainless Steel by Two-Stage Sputter Etching Using Different Cathode Disks

Abstract

Argon ion sputter etching was applied to two types of commercially pure titanium (JIS: TP340 and 550) and a martensitic stainless steel (JIS: SUS420J2) specimens placed on a SUS304 stainless steel cathode disk, and a second sputter etching stage was carried out by placing these specimens on a tungsten or tantalum cathode disk. The first sputter etching formed cone-shaped protrusions with base diameters of about 3 µm on the titanium specimens and 20–30 µm on the SUS420J2 specimen, and the second sputter etching yielded fine ridge-shaped protrusions less than 1 µm thick on the cone-shaped protrusions. In addition, nanometer-scale fine stick-shaped protrusions were formed along the tops of the ridge-shaped protrusions. Thus, surfaces with multiscale protrusions were obtained by two-stage sputter etching using different cathode disks. The specimens with ridge-shaped protrusions ensured high absorptance of more than 96% of visible light.

1. Introduction

Fine protrusions of various shapes, sizes and distributions can be formed on the surfaces of metals and alloys by sputter etching using argon or xenon plasma.^1–3) The protrusion formation mechanisms depend on the sputter etching conditions and the kinds of alloys used.⁴⁾ In the present research we consider two mechanisms. (1) Very fine cone-, rod-, tube- or ridge-shaped protrusions are formed when a specimen is sputtered by placing it on or near a cathode disk with a lower sputter rate than that of the specimen. The particles (atoms including ions) sputtering out of the disk arrive at the specimen surface and gather to form clusters. They act as masks or seeds^5,6) for the formation of protrusions. One example of such a sputtering combination is a copper specimen and a molybdenum disk.⁵⁾ This mechanism is called the artificial seed mechanism.⁴⁾ (2) Cone-shaped protrusions with vertex angles of about 40° and base diameters of 20–50 µm are formed by the sputter etching of austenitic or martensitic stainless steels placed on a stainless steel disk.^7–9) Very fine cone-shaped protrusions smaller than 1 µm are also formed by the sputter etching of tool steels containing strong carbide formation elements, such as W, V and Mo.^10,11) In stainless and tool steels, fine carbides precipitated during sputter etching become the origins of high densities of protrusions,^7–11) i.e., not only a high temperature and high vacancy density but also a high temperature and vacancy density gradient existing near the surface promote the rapid precipitation of carbides. These carbides grow perpendicular to the surface as pillars, and cone-shaped protrusions grow stably around the pillars. Also, on a commercially pure titanium,⁴⁾ cone-shaped protrusions with base diameters of about 3 µm form around the precipitated titanium dioxide pillars. In these cases, the sputter rate of a cathode disk should be close to or higher than that of the specimen so that the particles sputtering out of the disk do not stick to the specimen surface as seeds. Because the origins of these protrusions are the precipitates near the surface of the specimen, this mechanism is called the precipitates seed mechanism.⁴⁾

These two seed mechanisms raise the possibility of obtaining multiscale protrusions by using two-stage sputter etching, i.e., the formation of considerably large cone-shaped protrusions as a first stage using the precipitates seed mechanism followed by the formation of very fine protrusions on the cone-shaped protrusions as a second stage using the artificial seed mechanism.

One application of such multiscale protrusions is as a catalyst or support for a catalytic material. Because the multiscale protrusions have wider surface areas and more corners or edges compared with protrusions that have smooth surfaces, they will increase the activity and efficiency of chemical reactions. For example, fine ridge-shaped protrusions smaller than 1 µm absorb more than 95% of visible light within wide incident angles from 0 to 70°.¹¹⁾ In addition, the protrusions also absorb about 90% of near-infrared rays with wavelengths shorter than 3 µm.¹²⁾ Moreover, the surface oxidation of the protrusion promotes the absorptance of infrared rays.¹²⁾ Therefore, if oxide layers or oxide particles having high photocatalytic ability are formed or deposited on multiscale protrusions, the photocatalytic reaction can be promoted by high absorptance of solar light as well as a large reaction area. This may be an effective method of solving environmental and energy problems.^13,14)

Another possible application of multiscale protrusions is for the formation of bioactive surfaces. It is well known that titanium and titanium alloys have high tissue compatibility.^15,16) Various surface modifications¹⁷⁾ are carried out to promote tissue compatibility, e.g., the formation of protrusions¹⁸⁾ by sputter etching, the addition of meshes¹⁷⁾ and the addition of roughness,¹⁹⁾ as well as the formation of an oxide layer²⁰⁾ or the formation of titanium oxide nanotubes²¹⁾ on the surfaces of implants. On the other hand, it has already been reported that sharp protrusions decrease the survival rate of bacteria²²⁾ and that titanium oxide nanopatterns promote this effect.²³⁾

To realize these potential modifications, in the present research we applied two-stage sputter etching to form multiscale protrusions on pure titanium and stainless steel specimens. In addition, we measured the absorptance of visible light to confirm that the multiscale protrusions formed on the specimens exhibit the high absorptance that has been reported on fine protrusions of tool steel,¹¹⁾ e.g., more than 95%.

2. Experimental Procedures

The materials used for the experiments were two types of commercially pure titanium (JIS: TP340 and TP550; ASTM: Grade 2 and Grade 4) and martensitic stainless steel (JIS: SUS420J2; AISI: Type 420). The chemical compositions of these materials are shown in Table 1. Both of the titaniums were supplied as 1-mm-thick sheets, and they were cut into square specimens at 20 mm per side. The specimens were not polished because the surfaces were smooth as received. SUS420J2 steel was supplied as a rod with a 20 mm diameter. It was cut into 5-mm-thick specimens, and the surface of each specimen was polished using emery paper up to a grit of #1000.

Table 1 Chemical compositions of specimens (mass%).

Figure 1 schematically shows two stages of sputter etching applied to a titanium specimen to obtain multiscale protrusions. Stage 1 sputter etching (Fig. 1(a)) was carried out as follows: the specimen was placed on a 2-mm-thick SUS304 stainless steel disk set on a copper cathode of an RF magnetron sputtering apparatus (SP300-M; Sanvac Co.). To obtain the highest sputter rate, the specimen was placed on the disk corresponding to the circle range between a pair of magnets. After the vacuum pressure fell below 6 × 10⁻³ Pa, argon gas (purity: 99.999%) was introduced and maintained at a pressure of 0.67 Pa. Then the specimens were sputter etched with a sputter power of 250 W for 3.6–10.8 ks. The purpose of this process is to form cone-shaped protrusions on the specimens.^4,9) After the SUS304 stainless steel disk was changed to a tungsten or tantalum disk, stage 2 sputter etching (Fig. 1(b)) was carried out at 250 W for 0.9, 1.8 and 3.6 ks to form fine protrusions on the original cone-shaped protrusions. In the present research, the disk placed on the copper cathode is called the cathode disk, and it is called either the seed disk or simply the disk depending on the protrusion formation mechanism. Also, for the SUS420J2 steel specimen, the same two-stage sputter etching was carried out using the SUS304 steel disk and the tungsten seed disk, respectively.

Fig. 1

Schematic illustration of two-stage sputter etching to form multiscale protrusions. Stage 1: Argon ion sputter etching of a titanium specimen to form cone-shaped protrusions by placing the specimen on a SUS304 disk (a). Stage 2: Sputter etching to form fine protrusions by placing the specimen on a tungsten or tantalum seed disk (b).

The surface morphologies of the specimens after each sputter etching were observed by a scanning electron microscope (SEM; JEOL, JSM-6510A), and element analysis was carried out by using an energy-dispersive X-ray spectrometry (EDX) method. The relationship between the wavelength and reflectance of visible light was measured by a spectrophotometer (NF333; Nippon Denshoku Co.). Reflectance was calibrated by using two standard samples: one with a reflectance of 87.8% (white standard) and one with a reflectance of 0% (black). This apparatus is a simple system that uses a light incident angle of 2° and a detection angle of 45°; the measured reflectance is a little larger than the absolute reflectance, e.g., the reflectance of a tool steel (JIS: SKH51; AISI: M2) specimen with fine protrusions measured by this apparatus is 3.8%, whereas the absolute reflectance measured using a geminated ellipsoid mirror is 0.25%.¹¹⁾

3. Results and Discussion

3.1 SEM images of multiscale protrusions formed by two-stage sputter etching of TP340 specimen using SUS304 steel and tungsten cathode disks

The present authors⁴⁾ have already reported that the sputter etching of a commercially pure titanium specimen of TP340 (hereinafter the TP340 specimen) placed on a SUS304 stainless steel cathode disk (SUS304 disk) or an oxygen-free copper disk (copper disk) forms cone-shaped protrusions with base diameters of 2–5 µm. Figure 2 shows such protrusions formed by sputter etching of the TP340 specimen at 250 W for 7.2 ks on the SUS304 disk. Although more than half of the grains are covered with cone-shaped protrusions, there remain many grains on which no or very few protrusions form due to the crystal orientation dependency of protrusion formation.⁴⁾ As is schematically shown in Fig. 1(a), during the stage 1 sputter etching process, all of the constituent elements of the SUS304 disk and titanium specimens, including Fe, Cr, Ni, and Ti atoms, are sputtered, and a small portion of these atoms that collide with argon particles (atoms and ions) rebound and arrive on the specimen surface. As we also reported,⁴⁾ the shapes and sizes of cone-shaped protrusions formed on the TP340 specimen are almost the same regardless of whether the SUS304 disk or the copper disk is used, but EDX analyses show no detectable Fe, Cr, Ni, or Cu elements on these specimens. This means that the constituent elements of the SUS304 and copper disks were unable to become seeds for the protrusions because the sputter rates of these atoms are smaller than that of the Ti atom, i.e., these atoms are sputtered away even if they can stick to the specimen surface. Thus, SUS304 and copper disks cannot serve as seed disks for titanium specimens. On the other hand, no cone-shaped protrusions were formed by sputter etching of a high-purity (99.9%) titanium containing only 0.031% oxygen placed on the copper disk.⁴⁾ This means also that the Ti atoms that are sputtered from the TP340 specimen and then return to the specimen surface are not the origin of the cone-shaped protrusions. Considering these results, the present authors have concluded that the origin of the cone-shaped protrusion formed by sputter etching of the TP340 specimen containing 0.11% oxygen placed on the SUS304 and copper disks is titanium dioxide⁴⁾ precipitated near the surface of the specimen corresponding to the precipitates seed mechanism, as explained above.

Fig. 2

SEM images of protrusions formed by sputter etching of a TP340 titanium specimen placed on a SUS304 steel disk at 250 W for 7.2 ks. Image (a) is a top view, and (b) and (c) are 45° inclined side views.

Figure 3 shows SEM images of the TP340 specimen sputter etched at 250 W for 7.2 ks on the tungsten disk. All of the surface is densely covered with ridge-shaped protrusions with deep grooves, and the ridges are less than 500 nm wide. The tops of the ridges are not flat but feature nanometer-sized stick-shaped protrusions. Similar dense ridges are observed on the specimen sputter etched at 250 W for 3.6 ks, although the ridges are a little narrower. According to the EDX area analysis of a 90 µm × 60 µm region of the specimen shown in Fig. 3, a tungsten amount of 3.87% is detected. This means that the protrusion formation mechanism in this case corresponds to the artificial seed mechanism,^1,4,5) i.e., as shown in Fig. 1(b), a small portion of sputtered tungsten atoms arrive and stick to the surface of the specimen. They gather to form clusters by surface diffusion, and the clusters become seeds or masks for the formation of the ridge-shaped protrusions. Thus, the tungsten disk is the seed disk for the TP340 specimen.

Fig. 3

SEM images of ridge-shaped protrusions formed by sputter etching of a TP340 titanium specimen placed on a tungsten disk at 250 W for 7.2 ks. Image (a) is a top view and (b) is a 45° inclined side view.

It is difficult to determine the shape and distribution of the tungsten clusters from EDX analysis, but if the tungsten atoms gather as sphere-shaped clusters, cone-shaped protrusions rather than ridge-shaped protrusions would be formed beneath the clusters. The formation of ridge-shaped protrusions suggests that the tungsten particles have migrated to form line-shaped clusters as stable shapes in the midst of three simultaneous actions: argon ion sputtering of titanium substrate, the sputtering of tungsten clusters themselves, and the adhesion of tungsten atoms to the surface. According to Fig. 3(a), the shape and direction of the ridge line differ from grain to grain, which means that the sticking of tungsten particles depends on the crystal orientation. Once the sputter etching of the titanium matrix around the line-shaped clusters progresses, newly arriving tungsten particles stick only to the tops of the ridges. The formation of stick-shaped very fine protrusions on the ridge suggests that the sticking of tungsten particles is not homogeneous and that the ridge itself is sputtered irregularly. Further sputter etching eventually forms deep grooves among the ridge-shaped protrusions.

Although there is a possibility that the cone-shaped protrusions in Fig. 2 are also formed during the sputter etching of the specimen on the tungsten seed disk, no such protrusion is observed in Fig. 3. This suggests that the ridge forms so rapidly that the growth of cone-shaped protrusions should be prevented even if the precipitation of titanium dioxide pillars occurs.

Figure 4 shows SEM images of the TP340 specimen sputter etched at 250 W for 7.2 ks on the SUS304 disk followed by additional sputter etching at 250 W for 3.6 ks with the specimen on the tungsten seed disk. By this two-stage sputter etching, the very fine ridge-shaped protrusions shown in Fig. 3 form not only on the cone-shaped protrusions (e.g., region A in Fig. 4(b)) but also on the grains on which cone-shaped protrusions do not form at the first stage (the region B). The EDX area analyses of Fig. 4(b) show that the tungsten contents in these areas are almost the same. Thus, after the second stage of sputter etching, triple-scale protrusions coexist: cone-shaped protrusions and flat surfaces around the cones on a micrometer scale, ridge-shaped protrusions on the cones and on the regions around the cones on a submicrometer scale, and fine stick-shaped protrusions along the tops of the ridges on a nanometer scale. A comparison of Figs. 2 and 4 shows that the sharpness and height of a cone-shaped protrusion are decreased by the second-stage sputter etching. This means that the tungsten particles landing on cone-shaped protrusions do not remain permanently but are also sputtered away from the surface of the cone.

Fig. 4

SEM images of protrusions formed by sputter etching of a TP340 titanium specimen placed on a SUS304 steel disk at 250 W for 7.2 ks followed by sputter etching of the specimen on a tungsten disk at 250 W for 3.6 ks. Images (a) and (b) are top views, and (c) and (d) are 45° inclined side views.

3.2 SEM images of multiscale protrusions formed by two-stage sputter etching of the TP550 specimen using SUS304 steel and tantalum cathode disks

Figure 5 shows SEM images of the TP550 specimen sputter etched at 250 W for 3.6 ks on the SUS304 disk. Despite the shorter sputter etching time of 3.6 ks, the cone-shaped protrusions are denser but smaller than those of the TP340 specimen sputter etched for 7.2 ks (Fig. 2). This is because the higher oxygen content of the TP550 specimen relative to the TP340 specimen (0.32% vs. 0.11%) promotes the precipitation of a larger number of titanium dioxides, leading to the formation of cone-shaped protrusions. However, it was difficult to form dense protrusions covering all of the surface even when the sputter etching time was prolonged to 10.8 ks.

Fig. 5

SEM images of protrusions formed by sputter etching of a TP550 titanium specimen placed on a SUS304 steel disk at 250 W for 3.6 ks. All the images are 45° inclined side views.

Figure 6 shows an SEM image after the sputter etching of the TP550 specimen at 250 W for 0.9 ks on the tantalum disk. Very fine and deep holes with inside diameters smaller than 500 nm are densely formed. This suggests that the tantalum particles arriving on the surface aggregate into ring-shaped clusters rather than into line-shaped clusters and that they become the seeds or masks to form holes. The surfaces of ridges around the holes are rather flat, and there seem to be no distinct stick-shaped protrusions on the ridge-shaped protrusions as observed for the TP340 specimen sputter etched on the tungsten seed disk (Fig. 3).

Fig. 6

SEM images of protrusions formed by sputter etching of a TP550 titanium specimen placed on a tantalum disk at 250 W for 0.9 ks. The image is a 45° inclined side view.

Figure 7 shows SEM images of the surface of the TP550 specimen sputter etched at 250 W for 7.2 ks on the SUS304 disk followed by sputter etching of the specimen on the tantalum seed disk at 250 W for 1.8 ks (Fig. 7(a) and (b)) and 3.6 ks (Fig. 7(c) and (d)), respectively. Many holes are formed both on and around the cone-shaped protrusions; i.e., double-scale protrusions are formed. The cone is shorter and its top rounder than those formed by the sputter etching of the TP340 specimen on the tungsten seed disk at the same sputter etching time of 3.6 ks (compare Fig. 7(c) and (d) to Fig. 4). Similar double-scale protrusions have already been formed at a shorter sputter etching time of 0.9 ks.

Fig. 7

SEM images of protrusions formed by sputter etching of a TP550 titanium specimen placed on a SUS304 steel disk at 250 W for 3.6 ks followed by sputter etching of the specimen on a tantalum disk at 250 W for 1.8 (a, b) or 3.6 ks (c, d). All images are 45° inclined side views.

EDX area analyses showed that the tantalum proportions on the specimens are 1.25% for the sputter etching time of 0.9 ks, 3.38% for 1.8 ks, and 3.29% for 3.6 ks. These values are similar to those of the TP340 specimen sputter etched on the tungsten disk, i.e., 0.65–1.51% for 0.9 ks, 2.72% (average value) for 3.6 ks (Fig. 4) and 3.87% for 7.2 ks (Fig. 3). Although the amounts of adhered tantalum and tungsten are almost the same, the resultant flatness and surface morphology of the cone-shaped protrusions are different.

3.3 SEM images of multiscale protrusions formed by two-stage sputter etching of the SUS420J2 specimen using SUS304 steel and tungsten cathode disks

Figure 8 shows an SEM image of the SUS420J2 specimen sputter etched at 250 W for 10.8 ks on the SUS304 disk.⁹⁾ Cone-shaped protrusions are formed, similar to the cases of the TP340 and TP550 specimens. However, the protrusions are more than ten times larger than those of the TP340 and TP550 specimens and there are fine protrusions around the cones. Although the protrusions are not very dense, their distribution is rather homogeneous and is independent of crystal grains. The cone-shaped protrusions originate from the precipitation of chromium carbides,⁹⁾ which corresponds to the precipitation of titanium dioxides in the TP340 specimen.⁴⁾

Fig. 8

SEM images of protrusions formed by sputter etching of a SUS420J2 steel specimen placed on a SUS304 steel disk at 250 W for 10.8 ks. The image is a 45° inclined side view.

Figure 9 shows SEM images of the SUS420J2 specimen sputter etched at 250 W for 10.8 ks on the SUS304 disk followed by the sputter etching at 250 W for 1.8 ks on the tungsten disk. Similar to the case with the TP340 specimen, fine ridge-shaped protrusions with stick-shaped protrusions form densely all over the surface. Because EDX analysis detected a 3.51% proportion of tungsten in the area shown in Fig. 9(a), the tungsten clusters are the seeds of the ridge-shaped protrusions in this case as well. However, unlike the case with the TP340 and TP550 specimens, in this specimen the cone almost maintains its original height and sharpness. The ridges and sticks are more complex in shape and they are smaller than those observed on the TP340 specimen (Fig. 4).

Fig. 9

SEM images of protrusions formed by sputter etching of a SUS420J2 steel specimen placed on a SUS304 steel disk at 250 W for 10.8 ks followed by sputter etching of the specimen on a tungsten disk at 250 W for 1.8 ks. Images (a) and (b) are top views, and images (c) and (d) are 45° inclined side views.

3.4 Visible light absorptance of multiscale protrusions

One of the important functions of fine protrusions is their high absorptance of visible and infrared light.^11,12) The multiscale protrusions formed on the TP340 and SUS420J2 specimens cover the wavelengths of ultraviolet light, visible light, and near-infrared rays. Figure 10 shows the relationships between wavelength and absorptance of visible light obtained for the TP340 specimens with various protrusions, where absorptance α was calculated from measured reflectance R (α = 100-R). Specimen D with multiscale protrusions (Fig. 4) and specimen C with only ridge-shaped protrusions (similar to those shown in Fig. 3, but with a sputter etching time of 3.6 ks) each reveal an absorptance of more than 97%, much larger than those of as-received specimen A and as-sputter-etched specimen B (Fig. 2). The reason for the high absorptance of specimens C and D with fine ridge-shaped protrusions is the moth-eye effect, i.e., the light that has entered the space among protrusions repeats the reflection on the protrusion wall and is absorbed without getting outside. The 97% absorptance will be almost an upper limit value considering that the absolute absorptance is larger than that measured with the spectrophotometer used in the present research.¹¹⁾

Fig. 10

Relationships between wavelength and visible light absorptance of a TP340 titanium specimen. A: The as-received specimen without protrusions. B: A specimen sputter etched at 250 W for 7.2 ks placed on a SUS304 steel disk. C: A specimen sputter etched at 250 W for 3.6 ks placed on a tungsten (W) disk. D: A specimen sputter etched on a SUS304 steel disk followed by sputter etching of the specimen placed on a W disk at 250 W for 3.6 ks. E: Specimen D oxidized at 773 K for 1.8 ks in air.

Although the absorptance of ultraviolet (UV) and near-infrared light was not measured on the present protrusions, the absorptance of these lights will still be large. For example, the absorptance of infrared rays of the sputter-etched SUS316 stainless steel and the SKH2 tool steel with ridge-shaped protrusions is more than 90% within the wavelength of 3 µm.¹²⁾ The fact that the absorptance of the multiscale protrusions at 400 nm is about 98% (Fig. 10), combined with the fact that the minimum width of a ridge is near 300 nm (Fig. 4), means that the multiscale protrusions will absorb a large amount of UV-A light (wavelength 400–315 nm), which is the main part of the UV light contained in solar light. Specimen E was prepared by heating specimen D in air at 773 K for 1.8 ks to form titanium oxide on the surface. The absorptance of this specimen is a little larger than those of specimens C and D, especially at a longer wavelength. The reason for this is the additional absorptance of titanium or tungsten oxide due to electron transition, as has also been observed for the oxidized protrusions of tough pitch copper and phosphor bronze.¹²⁾

Figure 11 shows the relationships between the wavelength and absorptance of visible light obtained for the SUS420J2 specimens. Also in this case, the visible light absorptance of specimen D with multiscale protrusions (Fig. 9), or that of specimen C with only ridge-shaped protrusions, is more than 96%, much larger than that of the as-sputter-etched specimen B (Fig. 8).

Fig. 11

Relationships between wavelength and visible light absorptance of a SUS420J2 steel specimen. B: A specimen sputter etched at 250 W for 10.8 ks placed on a SUS304 steel disk. C: A specimen sputter etched at 250 W for 1.8 ks placed on a tungsten (W) disk. D: A specimen sputter etched on a SUS304 steel disk followed by sputter etching at 250 W for 1.8 ks placed on a W disk.

Although the light absorptance of the surface with multiscale protrusions formed on the TP340 and SUS420J2 specimens is almost the same as those of the surfaces with only ridge-shaped protrusions, the total surface area and the numbers of reaction sites of the former are much larger than those of the latter. The combination of high visible light absorptance and a large number of reaction sites of multiscale protrusions can be applied to promote a photo-catalytic reaction by oxidizing the titanium specimens, although it is essential that the titanium oxide layer itself has high photo-catalytic ability. The multiscale protrusions of SUS420J2 steel can be used as catalyst supports for the deposition of high chemical- or photo-catalytic thin film or nanoparticles.

4. Conclusions

Argon ion sputter etching was applied to two types of commercially pure titanium specimens (JIS: TP340 and 550) and to a martensitic stainless steel (JIS: SUS420J2) specimen placed on a SUS304 stainless steel cathode disk, and an additional sputter etching was carried out by placing these specimens on a tungsten or tantalum cathode disk. The results obtained are as follows.

1. (1)    The first sputter etching process formed cone-shaped protrusions with base diameters of about 3 µm on the TP340 and TP550 titanium specimens and of 10–30 µm on the SUS420J2 specimen. The second sputter etching of the TP340 and SUS420J2 specimens on the tungsten disk formed fine ridge-shaped protrusions less than 500 nm thick on the cone-shaped protrusions. In addition, nanometer-sized fine stick-shaped protrusions formed along the tops of the ridge-shaped protrusions. The second sputter etching of the TP550 specimen on the tantalum disk formed fine holes with inside diameters smaller than 500 nm. Thus, multiscale protrusions were obtained by two-stage sputter etching using different disks.
2. (2)    The formation of cone-shaped protrusions in the first sputter etching stage was due to the precipitates seed mechanism, i.e., the origins of the protrusions were the precipitated titanium dioxides in the TP340 and TP550 specimens and the precipitated chromium carbides in the SUS420J2 specimen. Ridge-shaped protrusions or fine holes formed on the cone-shaped protrusions due to an artificial seed mechanism, i.e., the tungsten or tantalum particles coming from the disk gathered to form line-shaped or ring-shaped clusters on the specimen surface and acted as seeds or masks.
3. (3)    The TP340 and SUS420J2 specimens with multiscale protrusions exhibited a high visible-light absorptance of more than 96%.

Acknowledgments

This work was supported by JSPS KAKENHI Grant Number JP 16K06780. We greatly appreciate the guidance provided by Dr. A. Terayama and Mr. N. Fuyama of Hiroshima Prefectural Technology Research Center regarding the use of SEM and of the EDX apparatus. We also express our gratitude to Mr. Y. Yoshikawa and Mr. Y. Mikami of Hiroshima Kokusai Gakuin University for their assistance in preparing the samples.

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