nanostructures on copper by femtosecond laser

Accepted Manuscript One-step fabrication of multifunctional fusiform hierarchical micro/nanostructures on copper by femtosecond laser

Kaiwen Ding, Cong Wang, Yu Zheng, Zheng Xie, Zhi Luo, Shu Man, Biwei Wu, Ji'an Duan PII: DOI: Reference:

S0257-8972(19)30364-0 https://doi.org/10.1016/j.surfcoat.2019.04.005 SCT 24500

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

24 January 2019 21 March 2019 1 April 2019

Please cite this article as: K. Ding, C. Wang, Y. Zheng, et al., One-step fabrication of multifunctional fusiform hierarchical micro/nanostructures on copper by femtosecond laser, Surface & Coatings Technology, https://doi.org/10.1016/j.surfcoat.2019.04.005

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ACCEPTED MANUSCRIPT

One-step fabrication of multifunctional fusiform hierarchical micro/nanostructures on copper by femtosecond laser Kaiwen Dinga, Cong Wanga,*, Yu Zhenga,*, Zheng Xieb,

State Key Laboratory of High Performance and Complex Manufacturing, College of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China b College of Mechanical Engineering, Hunan Industry Polytechnic, Changsha 410083, China *Corresponding author: [email protected] and [email protected]

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Zhi Luoa, Shu Mana, Biwei Wua and Ji’an Duana

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Abstract

In this paper, one-step fabrication of fusiform hierarchical micro/nanostructures on copper surface by femtosecond laser is proposed. According to the preferential valley ablation, Marangoni

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effect based on melt hydrodynamics and the coupling of the incident light and surface plasmon polaritons, the formation mechanism of the fusiform hierarchical micro/nanostructures is explained.

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And the effect of laser parameters including the defocusing distance, laser power, and scanning speed on the surface morphology is investigated. Also, the quantitative analysis of the fusiform tapers is

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carried out. It is demonstrated that the three-dimensional size of the fusiform hierarchical micro/nanostructures could be controlled by adjusting laser parameters. In addition, the potential applications of the fusiform hierarchical micro/nanostructures in microliter droplet directional

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transportation and light absorption are explored. Keywords: femtosecond laser, hierarchical micro/nanostructures, anisotropic wettability, light

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absorption

1. Introduction

After millions of years of evolution, survivals of the fittest in natural selection have their preponderant characteristics, such as iridescent structural colors [1-3], compound eyes array [4], anisotropic sliding wetting surface [5] and so on. These special characteristics of the creatures are attributed to the existence of the hierarchical micro/nanostructures on their surfaces. Inspired by the numerous natural creatures and their preponderant characteristics, hierarchical micro/nanostructures have attracted considerable attentions in recent years. 1

ACCEPTED MANUSCRIPT Various methods have been applied for the fabrication of hierarchical micro/nanostructures, such as electrodeposition [6-8], selective deposition with mask [9], chemical bath deposition [10], hydrothermal method [11-13], scanning probe lithography and wet chemical etching [14]. For example, Hang et al. used electrodeposition to produce hierarchical micro/nanostructures on nickel surface to get superhydrophobic surface [6]. Chen and his colleagues made a colorful solar selective absorber by employing selective deposition with mask [9]. Velayi et al. proposed a chemical bath

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deposition method to generate superhydrophobic zinc oxide film on stainless steel mesh, by which annealing temperature dependent reversible wettability could be obtained [10]. Li and his co-workers presented a hydrothermal method to manufacture hierarchically porous micro/nanostructures on

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copper foil, which presented enhanced antireflection and hydrophobicity [12]. However, the

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strategies mentioned above are multi-step, complicated, costly or hazardous to the environment. In addition, most of the micro/nanostructures are produced by bottom-up methods, by which the

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multi-scale structures are hard to be achieved.

Recently, this study has been extended by employing femtosecond (fs) lasers. Due to the extremely high irradiance and ultrashort pulse duration, fs laser has been proved to be a promising

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top-down method for one-step processing [15-18]. For instance, Zhu and his colleagues found elliptic and conical spikes on fs laser irradiated silicon surface [16]. Kietzig and his co-workers observed the microscale tapers on metal surfaces by laser processing [17]. Fraggelakis et al. obtained

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micro-spikes with coral-like roughness by texturing metal surface with high repetition fs laser [18].

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However, the relationship between micro/nanostructures’ size and fs laser processing parameters is still unknown. In addition, less research has been carried out to explore or expand the practical applications of the hierarchical micro/nanostructures.

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In this study, we propose a facile and controllable method for the fabrication of fusiform hierarchical micro/nanostructures on copper substrate by femtosecond laser. The evolution of the

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fusiform hierarchical micro/nanostructures is explained by preferential valley ablation, Marangoni effect based on melt hydrodynamics and the coupling of the incident light and surface plasmon polaritons. And the effect of laser parameters such as the defocusing distance, laser power, and scanning speed on the three-dimensional micro/nanostructures’ size is investigated. Also, the quantitative analysis of the fusiform tapers is carried out. In addition, the potential applications of the fusiform hierarchical micro/nanostructures in microliter droplet directional transportation and light absorption are explored.

2. Materials and methods The material used in this study was commercially available pure copper plate (30 mm × 30 mm 2

ACCEPTED MANUSCRIPT × 0.8 mm) with initial roughness of Ra ≤ 0.2 μm. The proportion of impurities (Bi, Sb, As, Fe, Pb, S) in the copper was less than 0.10 %. Before and after fs laser processing, the samples were washed in an ultrasonic cleaner with anhydrous ethanol and deionized water for 30 min respectively. The as-prepared samples were irradiated with a linearly polarized femtosecond laser (Pharos from Light Conversion, Lithuania) with a wavelength of 1030 nm, repetition rate of 75 kHz, and pulse duration of 215 fs. The output laser power was varied from 2 W to 6 W. The laser beam was

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focused by a biconvex lens with a focal length of 50 mm perpendicularly onto the surface of copper substrate, which was mounted on a computer-controlled XYZ three-axis translation stage (Suruga Seiki, Japan). The scanning speed was varied from 0.1 mm/s to 1 mm/s. The focused diameter after

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the lens of the Gaussian-profile laser beam at 1/e2 of its maximum intensity was approximately 12.6

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μm. The defocusing distance was set from 2 mm to 5 mm. The laser fabrication strategy was line scanning. The interval of adjacent laser scanning lines was set to 10 μm, which meant consequent

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overlap of laser spots to eliminate the influence of scanning lines. The experimental parameters considered were the pulse peak fluence, the pulses per spot (PPS, the pulse overlapping on one scanning line only considered) and the total amount of pulses irradiated per spot (PPStot, the pulse

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overlapping and the scanning overlapping both considered).

After fs laser treatment, the morphology of the samples was characterized by a MIRA3 LMU scanning electron microscope (SEM, TESCAN, Czech). The size of the microscale tapers was

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obtained by a SEM images analysis software (Nano Measurer System, 1.2.5). The height of the

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microscale tapers was measured by a laser scanning confocal microscope (LCSM, Carl Zeiss, Germany). The wettability of the samples was evaluated by measuring the apparent water contact angle (WCA). Before the characterization of the wettability, the fabricated surface had been exposed

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to air for a month in a storage tank with constant ambient temperature 22 ºC and humidity 20 %. Then, the WCA was measured in air at room temperature by an optical contact angle meter with 8 μL

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water droplets (Optical Tensiometer, Biolin Scientific, Finland). Optical wavelength-dependent reflectivity measurement in the visible region for the fs laser structured surfaces was carried out with a Lambda 950 spectrophotometer (PerkinElmer, USA) incorporated with an integrating sphere.

3. Results and discussion Fig. 1 shows the SEM images of fs laser fabricated copper surface. The defocusing distance, laser power, and scanning speed of the fs laser were firstly set to 3 mm, 5 W and 1 mm/s, respectively. And the corresponding peak fluence, PPS and PPStot were 0.17 J/cm2, 23 and 736, respectively. It is observed that the copper surface is regularly covered with hierarchical micro/nanostructures. According to Fig. 1(a), a good deal of stereoscopic gullies and cones exist on 3

ACCEPTED MANUSCRIPT the fs laser irradiated surface, which monolithically present fusiform. As displayed in the amplified SEM image in Fig. 1(b), a single taper is so much like a dried carya cathayensis shrouded by wrinkle, which is fs laser induced periodic surface structures (LIPSSs) [19-21]. According to the previous studies, the orientation of the LIPSSs is determined by the laser polarization, scanning direction and speed [22, 23]. In our study, the laser polarization and scanning direction were along the same direction. It is noted that the orientation of the LIPSSs is perpendicular to the polarization of the fs

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laser. However, the LIPSSs are located spatially upon the microscale taper from the top down, the orientation of which is distorted with a curved shape according to Fig. 1(b). In addition, a great many cracks are formed over every ridge of LIPSSs, as presented in Fig. 1(c). According to the gray values

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order

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formation

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further demonstrates the existence of the LIPSSs and cracks.

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depicted in Fig. 1(d), which come out of the cross profile from a single taper (the orange line), it mechanism

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hierarchical

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micro/nanostructures, a series of experiments were carried out. Fig. 2 shows the fs laser defocusing distance dependence of the fabricated surface’s morphology. The laser power and laser scanning speed were fixed to 5 W and 1 mm/s, respectively. It is found that the fusiform hierarchical

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micro/nanostructures were generated only when the defocusing distance was close to 3 mm (0.17 J/cm2, 23 PPS, 736 PPStot), as displayed in Fig. 2(b). Due to the strong ablation of 2 mm defocused fs laser (0.39 J/cm2, 16 PPS, 328 PPStot) in Fig. 2(a), the micro/nanostructures obtained were coral-like.

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The microscale ridges were tangled, and the detailed structures on the ridges were fragmentized.

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Instead of structures with a dozen or dozens of microns, when the defocusing distance reached to 4 mm (0.10 J/cm2, 31 PPS, 1308 PPStot), the tapers became small and flat in Fig. 2(c). As the defocusing distance continually increased to 5 mm (0.06 J/cm2, 39 PPS, 2043 PPStot) in Fig. 2(d),

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conventionally complanate LIPSSs became dominant rather than bumpy tapers. According to previous studies, the LIPSSs is mainly derived from the interference between incident laser and

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surface plasmons [24-26]. Especially, the formation of LIPSSs needs the appropriate laser fluence within specific value [27]. It is implied that the defocusing distance between the focal point and copper surface is one of the important parameters for the generation of the LIPSSs hierarchically on the fusiform hierarchical micro/nanostructures, since the defocus brings greater nonuniformity on energy distribution. The nonlinear energy deposition due to the change of defocusing distance plays an important role in tilting the micro/nanostructures on copper in our experiments. Also, the effect of laser power on germination of the three-dimensional fusiform micro/nanostructures was investigated, as distributed in Fig. 3. For the sake of observation, SEM images in Fig. 3 were tilted. The laser defocusing distance and scanning speed were set to 3 mm and 1 mm/s, respectively. For the case of low laser power of 2 W (0.07 J/cm2, 23 PPS, 736 PPStot) in Fig. 4

ACCEPTED MANUSCRIPT 3(a), the complanate LIPSSs were formed on the fs laser treated copper surface, which only led to small scale roughness. As the laser power was raised to 3 W (0.10 J/cm2, 23 PPS, 736 PPStot) in Fig. 3(b), the tiny bumps appeared. Then, the microscale tapers were formed for the case of 4 W (0.14 J/cm2, 23 PPS, 736 PPStot) in Fig. 3(c). Finally, as the laser power was increased to 6 W (0.21 J/cm2, 23 PPS, 736 PPStot) in Fig. 3(d), the strong ablation emerged, which resulted in further growing of the bumps. It is conducted that the surface becomes bumpier and bumpier with the increase of the

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laser power. The formation mechanism of fusiform microscale tapers has been discussed in previous studies [28-33]. Considering the above surface growth (ASG) and below surface growth (BSG) mechanisms

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characterized by Craig A. Zuhlke and his co-workers, a significant difference to distinguish them is

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the relative height between structured and original surface. As shown in Figs. 4(a) and 4(b), the tapers formed on copper are both lower than the original surface for the cases of 1 mm/s (0.17 J/cm2,

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23 PPS, 736 PPStot) and 0.1 mm/s (0.17 J/cm2, 235 PPS, 7362 PPStot). Within the dashed yellow box in Fig. 4(b), the bottom of the tapers tends to be vertical rather than fastigiate as a cone. And the gullies penetrate to the substrate slightly, which is derived from preferential valley ablation (PVA) as

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depicted in Fig. 4(c). High fluence in the center of the laser leads to initial bumpers on the surface. As the subsequent pulses irradiated on the surface, a part of the photons would be scattered to the valleys [29-31]. Additionally, since the more drastic fluctuation on surface profile, the irradiated area

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on valleys is smaller than that in peaks. Consequently, the fluence on the valleys would be larger than

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others, which leads to more ablation in the valleys. Then, the grandiose gullies and tapers were formed. Considering that the height of the tapers has discrepancy, there are some tapers lying in the lower position during their germination process. According to the mechanism of PVA, the low-lying

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tapers would be ablated in the end by the light reflected from the cant of adjacent tapers. As for the fusiform shape of the microscale tapers, it could be explained as follows. On the one

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hand, the reflectance of laser polarization is generally lower than that of other polarizations at off normal incidence [28]. Thus, a larger amount of material is ablated from the surfaces facing the laser beam with laser polarization, which leads to the initial difference on the surface. On the other hand, the ablation of preferential valley is more fiercely among tapers with higher slope. Therefore, the ablation difference from polarization could be further amplified. A compensatory element for the formation of the grandiose gullies and tapers could be explained by the Marangoni effect based on melt hydrodynamics [27]. The area with high temperature has a lower surface tension compared to the area with low temperature, so there would be a difference of surface tension. Then, the area with higher surface tension is going to pull the liquid from there with lower surface tension. It means that the surface tension gradient produces a 5

ACCEPTED MANUSCRIPT flow from the low surface tension area to the high surface tension area. In brief, the material on the surface would melt during and after fs laser irradiation. After that, as illustrated in Fig. 4(d), the convection generated by the Marangoni shear leads to liquid flow, which compensates the process of ablation to the valleys and then makes the tapers avoid being fully ablated. In order to obtain controllable fusiform hierarchical micro/nanostructures, more detailed experiments and quantitative analysis were carried out. The upper panel of Fig. 5 depicts the SEM

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images of the fusiform tapers structures generated under different scanning speed. The defocusing distance and laser power were set to 3 mm and 5 W, respectively. It is obviously found that the size of regularly covered microstructures is quite different in micron scale, which indicates that the

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structures could be changed by adjusting laser scanning speed. During the strong fs laser-material

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interactions in the case of low scanning speed, a plenty of splashes were generated as depicted in Fig. 4(d), which would deposit on the featured structures. This would enhance the formation of scattered

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nano particles, which could be observed in Fig. 5(a) for the case of low scanning speed. Nevertheless, faster scanning speed weakens the fs laser-material interactions, which only leads to cracks on the LIPSSs without plentiful nano particles, as shown in Fig. 1(c). In addition, based on the statistical

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calculation of the corresponding SEM images, the typical size distribution histograms of the fusiform tapers are expressed in the under panel of Fig. 5. In the statistical calculation, the size here was all obtained from the short axis direction. For a relatively low scanning speed, large size microscale

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tapers are obtained. Also, the dispersion in the size distribution of the large size tapers is generally

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broad according to Fig. 5(a). For the case of 0.3 mm/s (0.17 J/cm2, 78 PPS, 2454 PPStot), the size distribution has a mean size of 16.93 μm and a standard deviation of 4.47 μm. The tapers are still large and dispersed in the case of 0.5 mm/s (0.17 J/cm2, 47 PPS, 1472 PPStot). The corresponding

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mean size is 12.93 μm and the standard deviation is 3.33 μm, as exhibited in Fig. 5(b). As the scanning speed is 0.7 mm/s (0.17 J/cm2, 34 PPS, 1052 PPStot) in Fig. 5(c), a narrow size distribution

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of tapers is obtained. The corresponding mean size is 11.00 μm and the standard deviation is 2.75 μm. As the scanning speed further increases to 0.9 (0.17 J/cm2, 26 PPS, 818 PPStot) mm/s, more small size tapers are formed with a narrower distribution. The smaller tapers are obtained with a mean size of 9.73 μm and a standard deviation of 2.19 μm in Fig. 5(d). In conclusion, the mean size and the dispersion in the size distribution decrease with the increasing of laser scanning speed. It is demonstrated that the size of the tapers could be controlled by adjusting laser scanning speed. Fig. 6 describes the quantitative analysis of the fusiform tapers as a function of the scanning speed. It is revealed that the size, height, taper ratio, and density of the fusiform tapers are different with the change of laser scanning speed. Due to the fusiform geometrical shape of the tapers, the diameters along the short and long axis directions were both measured. As distributed in Fig. 6(a), 6

ACCEPTED MANUSCRIPT the diameters of the short axis direction gradually decrease with the increase of the laser scanning speed. However, this trend is not apparent for the diameters of the long axis direction with the change of the scanning speed. Considering that the PVA effect is weakened on cants with smaller slope, the long axis direction gets less impact from the PVA. Therefore, the change at the long axis direction was not apparent compared with that along the long axis direction when the scanning speed was faster than 0.5 mm/s. Meanwhile, according to Fig. 6(b), the height of the tapers has the same

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trend with the diameters of the short axis direction. In this study, the taper ratio is defined as the mean size divided by mean height. On account of the simultaneous changing tendency with the size and height at short direction, the corresponding taper ratio basically remains unchanged. Thus, the

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corresponding taper ratio along the long axis direction increases with the increase of the scanning

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speed. Generally, due to the small slope of the short axis direction, the PVA effect would be stronger along the short axis direction. Thus, the taper ratio is always larger along the long axis direction than

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that along the short axis direction. As for the density of the fusiform tapers in Fig. 6(d), it increases as the scanning speed raises, which further indicates the ablation of the low-lying tapers. In addition, the potential application of the copper surface with fusiform hierarchical

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micro/nanostructures has been explored. Due to the anisotropic period in two orthogonal axes, the WCAs along two different directions have been tested. The upper panel of Fig. 7 presents the optical photographs of an 8 μL water droplet on the laser structured surface in two orthogonal viewing

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angles. Fig. 7(b) analyzes the evolution of water contact angle in two orthogonal orientations. It is

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found that the WCAs along short axis were always larger than that along long axis. Due to the fusiform geometrical shape of the tapers, larger period with wider interspace along the long axis leads to deeper embedment of water droplet inside the tapers. On the solid-liquid contact surface,

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there is a mixed state combined the Cassie state with the Wenzel state simultaneously [34]. For the case with larger period, the portion of Cassie state reduces and Wenzel state increases. Therefore, the

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WCA along short axis is larger than that along long axis. In our study, it is assumed that the anisotropic wettability here is due to the anisotropic size of the microscale tapers. It is worth noting that the laser scanning lines here were all along the short axis direction, which rules out the possibility that the laser scanning direction results in the anisotropic WCAs. Fig. 7(c) reveals the covariation between anisotropic wettability and the microscale tapers, which further identifies the assumption. The surface with anisotropic wettability along different axis has potential applications in microliter droplet directional transportation [35]. Another

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application

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surface

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micro/nanostructures in light absorption is explored. As presented in Fig. 8(a), the absorption of three characteristic surfaces as a function of wavelength in visual spectrum were measured. The 7

ACCEPTED MANUSCRIPT defocusing distance, laser power and scanning speed for the fabrication of surface with fusiform hierarchical micro/nanostructures were 3 mm, 5 W and 0.6 mm/s, respectively, while these were 3 mm, 10 W, and 200 mm/s for the structured surface only with LIPSSs. It is obvious that the absorption of the LIPSSs structured surface is higher than that of original surface. And the absorption of the surface with fusiform hierarchical micro/nanostructures is highest. The result about light absorption of these three different surfaces could be explained by the coupling of the incident light

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and surface plasmon polaritons (SPPs) [36], as shown in Fig. 8(b-d). The coupling of the incident light and SPPs occurs on the surface with LIPSSs, which results in the efficient absorption of the incident light. The reason why the surface with fusiform hierarchical micro/nanostructures performs

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comparably best in light absorption could be explained as follows. On the one hand, the interspace

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between the adjacent tapers plays an important role in geometrical light trapping. On the other hand, every reflection among the two tapers leads to a loss of the incident light wave. When the tapers have

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LIPSSs inside, the coupling between the incident light and SPPs increased, which further enhances the light absorption. The surface with fusiform hierarchical micro/nanostructures could be useful for stealth, enhancement of thermal-electrical conversion efficiency, extraction of light from LEDs

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[37-39], and so on.

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4. Conclusion

In summary, we propose a facile and controllable method for the fabrication of fusiform

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hierarchical micro/nanostructures on copper substrate by femtosecond laser. According to the Marangoni effect based on preferential valley ablation, melt hydrodynamics and the coupling of the incident light and surface plasmon polaritons, the formation mechanism of the fusiform hierarchical

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micro/nanostructures is explained. Besides, the relationship between micro/nanostructures’ three-dimensional size and defocusing distance, laser power, and scanning speed is studied. Also, the

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quantitative analysis of the fusiform tapers is carried out. It is demonstrated that the fusiform hierarchical micro/nanostructures’ size could be controlled by adjusting laser parameters. Moreover, the fs laser structured surface exhibits good performance in anisotropic wettability and light absorption, which could be used in microliter droplet directional transportation, stealth, extraction of light from LEDs, and so on.

Acknowledgement This research is supported by the National Key R&D Program of China (Grant No. 2017YFB1104300). 8

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[19] J. E. Sipe, J. F. Young, J. S. Preston, H. M. Driel, Laser-induced periodic surface structure. I. Theory, Phys. Rev. B 27 (1983) 1141-1154. [20] J. F. Young, J. S. Preston, H. M. Driel, J. E. Sipe, Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass, Phys. Rev. B 27 (1983) 1155-1172. [21] M. Ardron, N. Weston, D. Hand, A practical technique for the generation of highly uniform LIPSS, Appl. Surf. Sci. 313 (2014) 123-131. [22] S. Gräf, F. A. Müller, Polarization-dependent generation of fs-laser induced periodic surface structures, Appl. Surf. Sci. 331 (2015) 150-155. [23] P. Liu, L. Jiang, J. Hu, S. Zhang, Y. Lu, Self-organizing microstructures orientation control in femtosecond laser patterning on silicon surface, Opt. Express 22 (2014) 16669-16675. [24] J. Wang, C. Guo, Formation of extraordinarily uniform periodic structures on metals induced by femtosecond laser pulses, J. Appl. Phys. 100 (2006) 023511. [25] M. Huang, F. Zhao, Y. Cheng, N. Xu, Z. Xu, Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident laser, ACS Nano 3 (2009) 4062-4070. [26] F. Garrelie, J. P. Colombier, F. Pigeon, S. Tonchev, N. Faure, M. Bounhalli, S. Reynaud, O. Parriaux, Evidence of surface plasmon resonance in ultrafast laser-induced ripples, Opt. Express 19 (2011) 9035-9043. [27] G. D. Tsibidis, C. Fotakis, E. Stratakis, From ripples to spikes: a hydro-dynamical physical mechanism to interpret femtosecond laser induced self-assembled structures, Phys. Rev. B 92 (2015) 041405. [28] T. Y. Hwang, C. Guo, Polarization and angular effects of femtosecond laser-induced conical microstructures on Ni, J. Appl. Phys. 111 (2012) 083518. [29] C. A. Zuhlke, T. P. Anderson, D. R. Alexander, Formation of multiscale surface structures on nickel via above surface growth and below surface growth mechanisms using femtosecond laser pulses, Opt. Express 21 (2013) 8460-8473. [30] E. Peng, A. Tsubaki, C. A. Zuhlke, M. Wang, R. Bell, M. J. Lucis, T. P. Anderson, D. R. Alexander, G. gogos, J. E. Shield, Experimental explanation of the formation mechanism of surface mound-structures by femtosecond laser on polycrystalline Ni60Nb40, Appl. Phys. Lett. 108 (2016) 031602. [31] E. Peng, A. Tsubaki, C. A. Zuhlke, M. Wang, R. Bell, M. J. Lucis, T. P. Anderson, D. R. Alexander, G. Gogos, J. E. Shield, Micro/nanostructures formation by femtosecond laser surface processing on amorphous and polycrystalline Ni60Nb40, Appl. Surf. Sci. 396 (2017) 1170-1176. [32] K. M. T. Ahmmed, C. Grambow and A. Kietzig, Fabrication of micro/nano structures on metals by femtosecond laser micromachining, Micromachines 5 (2014) 1219-1253. [33] J. Long, Z. Cao, C. Lin, C. Zhou, Z. He, X. Xi, Formation mechanism of hierarchical Micro- and nanostructures on copper induced by low-cost nanosecond lasers, Appl. Surf. Sci. 464 (2019) 412-421. [34] Y. Song, C. Wang, X. Dong, K. Yin, F. Zhang, Z. Xie, D. Chu, J. Duan, Controllable superhydrophobic aluminum surfaces with tunable adhesion fabricated by femtosecond laser, Opt. Laser Technol. 102 (2018) 25-31. [35] N. A. Malvadkar, M. J. Hancock, K. Sekeroglu, W. J. Dressick, M. C. Demirel, An engineered anisotropic nanofilm with unidirectional wetting properties, Nat. Mater. 9 (2010) 1023-1028. [36] Z. Ou, M. Huang, F. Zhao, The fluence threshold of femtosecond laser blackening of metals: the effect of laser-induced ripples, Opt. Laser Technol. 79 (2016) 79-87. [37] S. J. An, J. H. Chae, G. Yi, G. H. Park, Enhanced light output of GaN-based light-emitting diodes with ZnO nanorod arrays, Appl. Phys. Lett. 92 (2008) 121108.

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[38] G. Li, J. Li, C. Zhang, Y. Hu, X. Li, J. Chu, W. Huang, D. Wu, Large-area one-step assembly of three-dimensional porous metal micro/nanocages by ethanol-assisted femtosecond laser irradiation for enhanced antireflection and hydrophobicity, ACS Appl. Mater. Inter. 7 (2015) 383-390. [39] P. Fan, H. Wu, M. Zhong, H. Zhang, B. Bai, G. Jin, Large-scale cauliflower-shaped hierarchical copper nanostructures for efficient photothermal conversion, Nanoscale 8 (2016) 14617-14624.

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ACCEPTED MANUSCRIPT FIGURE CAPTIONS FIG.1. SEM images of the fusiform tapers with hierarchical LIPSSs. The zoomed-in subordination is marked by the red dashed lines. The green dashed lines are along the orientation of the LIPSSs. (d) The corresponding gray values of the cross profile along the orange line. The E

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mm, (b) 3 mm, (c) 4 mm, and (d) 5 mm respectively.

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FIG.3. The evolution of the fusiform tapers produced by different laser power: (a) 2 W, (b) 3 W, (c)

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FIG.4. Cross-sectional SEM images at boundary with scanning speed (a) 1 mm/s and (b) 0.1 mm/s. Schematic diagrams about (c) preferential valley ablation and (d) low-lying taper’s ablation

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FIG.5. Quantitative analysis of the size distribution of fusiform tapers formed by different scanning speeds: (a) 0.3, (b) 0.5, (c) 0.7 and (d) 0.9 mm/s, respectively. Herein, d represents

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FIG.6. The fusiform tapers’ size, height, taper ratio, and density as a function of the laser scanning speed.

FIG.7. (a) Optical photographs of an 8 μL water droplet on the laser structured surface in two orthogonal viewing angles. (b) Evolution of the corresponding WCAs. (c) Difference of contact angle in two orthogonal axes and the size of laser structured taper as a function of laser scanning speed. FIG.8. (a) The light absorption of the untreated copper substrate, surface with LIPSSs and surface 12

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Fig. 1 SEM images of the fusiform tapers with hierarchical LIPSSs. The zoomed-in subordination is marked by the red dashed lines. The green dashed lines are along the orientation of the LIPSSs. (d) The corresponding gray values of the cross profile along the orange line. The E stands for the laser polarization direction, while S is the laser scanning direction.

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Fig. 2 The surface’s morphology fabricated by fs laser with different defocusing distance: (a) 2 mm, (b) 3 mm, (c) 4 mm, and (d) 5 mm respectively.

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Fig. 3 The evolution of the fusiform tapers produced by different laser power: (a) 2 W, (b) 3 W, (c) 4 W, and (d) 6 W, respectively.

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Fig. 4 Cross-sectional SEM images at boundary with scanning speed (a) 1 mm/s and (b) 0.1 mm/s. Schematic diagrams about (c) preferential valley ablation and (d) low-lying taper’s ablation process.

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Fig. 5 Quantitative analysis of the size distribution of fusiform tapers formed by different scanning speeds: (a) 0.3, (b) 0.5, (c) 0.7 and (d) 0.9 mm/s, respectively. Herein, d represents mean size, and σ is standard deviation.

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Fig. 6 The fusiform tapers’ size, height, taper ratio, and density as a function of the laser scanning speed.

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Fig. 7 (a) Optical photographs of an 8 μL water droplet on the laser structured surface in two orthogonal viewing angles. (b) Evolution of the corresponding WCAs. (c) Difference of contact angle in two orthogonal axes and the size of laser structured taper as a function of laser scanning speed.

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Fig. 8 (a) The light absorption of the untreated copper substrate, surface with LIPSSs and surface with the fusiform hierarchical micro/nanostructures. (b)-(d) Schematics for light absorption of three different surfaces.

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1. One step fabrication of fusiform hierarchical micro/nanostructures on copper surface by femtosecond laser is proposed. 2. The formation mechanism of the fusiform hierarchical micro/nanostructures is explained by preferential valley ablation, Marangoni effect and surface plasmons model. 3. Effect of laser parameters on the three-dimensional micro/nanostructures' size is investigated. 4. The potential applications of the fusiform hierarchical micro/nanostructures in microliter droplet directional transportation and light absorption are explored.

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