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Abstract
Herein, we report on the one-step formation of a novel microstructure on the surface of crystalline ZnO in ambient air excited by a single femtosecond laser beam (central wavelength 400 nm, pulse duration 35fs), which has photon energy close to the bandgap of ZnO. A two-dimensional surface structure with a controlled period of ∼2-6 μm is observed, with its orientation independent on the status of laser polarization (linear, circular, or elliptical polarization). We find that the orientation of this two-dimensional structure is defined by the direction of the crystal a and c axes. This structural period of ∼2-6 micrometers and the independence of its orientation on the laser polarization are in sharp contrast with the traditional laser induced periodic surface structure (LIPSS). In the meantime, surface cracks with a feature size of ∼30 nm are observed at the bottom of the valley of the two-dimensional structure and theoretical results show there exists strong electric field enhancement on the cracks under 400 nm femtosecond laser irradiation. In view of these unusual features, we attribute the formation of this two-dimensional structure to the mechanical cracking of the ZnO crystal along its (11-20) and (0001) planes induced by the multiple-cyclic heating due to linear absorption of the femtosecond pulses.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Micro/nano-scale surface structuring of dielectrics induced by ultrafast lasers has attracted enormous interest in the last decades, due to the exceptional structural [1] and electrical properties [2] of the dielectrics surface after structuring. Most of the studies have been focused on laser-induced periodic surface structure (LIPSS) by femtosecond laser pulses, which is simpler and more efficient in comparison to laser ablation and melting with nanosecond pulses. The LIPSSs generally present sub-micronic spatial-scales spikes and ripple with the landscape details. As to the periodicity of the surface ripple, two types of LIPSSs have been mostly reported. One is called as the high spatial frequency LIPSS(HSFL) in which the spatial period is considerably smaller than the incident laser wavelength (<λ/2) and another one is the low spatial frequency LIPSS (LSFL) with the period close to the incident wavelength [3–7]. In further, the physical mechanisms for LSFL and HSFL have been attributed to the interference of the incident laser with the surface plasmon polaritons [8,9], self-organization [10], nanoplasmonic excitations [11], or second-harmonic generation [12]. One attractive property of LIPSS-based structures is its high controllability because it commonly depends on the material and laser irradiation conditions including the laser wavelength, laser pulse number, angles of incidence, and the polarization state of the laser electric field [13,14,15]. Moreover, the sophisticated micro/nano-scale structures have proven great potentials in various applications such as surface self-cleaning [16], reduction of friction [17], structural coloring [18].
It is noteworthy that these novel micro/nano-scale surface structures, in condition that they are fabricated on some functional materials, have shown unexpected performance. For example, zinc oxide crystal (ZnO), a compound semiconductor with direct band gap of 3.34 eV [19] and a large exciton-binding energy of 60 meV [20], is regarded as one of the representative materials due to their unique property in light emission operating in UV spectral region. For this, many groups have carried out researches on laser induced ZnO micro/nano structuring. Depending on the laser parameters, nanorods, nanoparticles and nanobelts [21–23], or periodic surface structures [24–26] on the ZnO crystal have been observed, with the latter showing applications in many fields such as nanolaser [27–29], photoluminescence [30–32], solar cells [33] or chemical sensors [34]. Most of the studies mentioned above on ZnO are concentrated upon the one-dimensional surface structures. In the meantime, two-dimensional surface structuring of ZnO with more powerful and extensive functions has also been proposed, which can further improve the light absorption rate [35], ultraviolet luminescence [36,37] or result in more advanced surface functions [38,39]. However, the two-dimensional array construction can usually be achieved at present by femtosecond laser two-beams or three-beams interference [35,40], or a two-step irradiation method by the single femtosecond pulse or temporally shaped pulses with adjusting of the pulse delay [41–45]. These techniques are much intricate and it has not been demonstrated that a controllable two-dimensional structure on ZnO can be formed with a single beam irradiation method up to the best of our knowledge.
In this study, we demonstrate the one-step formation of a novel two-dimensional periodic surface structure on the crystalline ZnO excited by a single femtosecond beam. We irradiate the (10-10) surface of crystalline ZnO with the lasers of 400 nm wavelength, which has the photon energy close to the bandgap of ZnO. The formation threshold of the two-dimensional structure, the evolution of its period and depth, and the influence of the crystal orientation and scanning direction are investigated in details. In particular, the orientation of this two-dimensional periodic structure is found to be determined by the internal axis of the crystal, regardless of the incident laser polarization. More interestingly, surface nanocracks with the width below 30 nm are observed at the bottom of the two-dimensional periodic structure. The generation and evolution of nanocracks is experimentally obtained by changing the pulse number irradiated on the ZnO surface. Furthermore, FDTD method is adopted to simulate the electric field distribution on the nanocracks. Based on the simulation and experimental results, we propose one interpretation on the formation of this novel two-dimensional periodic surface structure.
2. Experimental details
In the experiment, a commercial femtosecond laser system (Legend DUO, 40 fs, 1 kHz, 800 nm), which delivers laser pulses with maximum energy of 12 mJ is employed. The schematic experiment setup is presented in Fig. 1(a). We use a BBO crystal to generate the second harmonic of the fundamental pulses. The conversion efficiency of the 400 nm pulses can be optimized by rotating the azimuthal angle of BBO crystal for phase-matching. A pair of dichroic mirrors, which reflect 400 nm pulses and transit 800 nm pulses, are used to separate the beams of 800 nm and 400 nm lasers. To completely filter out the 800 nm laser, we also use a glass filter (BG 40) with thickness of 2 mm to absorb the residual 800 nm pulses. The horizontally polarized component of the 400 nm pulses is selected by a Glan polarizer. A quarter wave-plate is placed in the light path which can change the polarization of 400 nm pulses from linearly polarized state to circularly, as well as elliptically one. The second harmonic laser beam is finally focused by a parabolic reflective Ag mirror with a focal length 100 mm. The diameter of the focus is measured to be 50 μm, which is defined by the 1/e2 of the peak intensity of the Gaussian beam. The monocrystalline ZnO sample from hefei crystal technical material Co., Ltd. with the dimension 10 mm×10 mm×0.5 mm is used in our experiment, with the lattice structure of ZnO shown in Fig. 1(b). ZnO in the wurtzite phase is a non-centrosymmetric hexagonal crystal structure composed of alternate layers of positive (Zn) and negative(O), which is presented by four coordinates. The three a-axes at the bottom are in the (0001) plane and the angles between them are 120 degrees. The c axis is perpendicular to the a-axes plane. The sample is cut along the m-plane. The ZnO crystal is mounted on a motorized translation stage, and its movement in the horizontal plane along the x direction is controlled electronically. The front surface of the ZnO crystal is placed around the focus of the parabolic mirror, and its (10-10) plane is perpendicular to the laser incident direction. To improve the fabrication efficiency, we place the sample about 1.6 mm in front of the focal point and the laser beam diameter on the sample surface measured by a Charge Couple Detector (CCD) beam profiler is ∼126 μm. In our experiment, the moving velocity of the sample is set to 1.4 mm/s. The details of the surface structures were measured using white light interference (Bruker), a field emission-scanning electron microscopy (FE-SEM, ZEISS, Germany) and an atomic force microscope (AFM from Bruker), respectively.
Fig. 1. (a) Schematic experimental setup for single femtosecond laser beam irradiation. (b) Unit cell illustrating crystalline orientations of ZnO. Fs: femtosecond; QWP: quarter-wave plate; Pol: polarizer; DM: dichroic mirror, PRM: parabolic reflective mirror.
3. Results and discussion
In order to examine the topography of the surface structures, the optical microscope and the white light interferometer are used. The observed structures are presented in Fig. 2. Figure 2(a) shows the optical pattern of structures with the laser fluence of 1.32 J/cm2. It can be seen that a two-dimensional periodic structure exists in the middle of the irradiated area. The depth information of the whole scanning area is measured by the white light interferometer, as shown in Fig. 2(b). The overall cross-section ablated by the femtosecond laser pulses is a semicircular crater, and the whole width of the crater structure is approximately 120 μm. Figure 2 (c) shows that the depth of the crater structure is increased from 2.4 μm to 4.4 μm for the laser fluence ranging from 1.1 to 2.4 J/cm2. It is worth noting that the crater depth presents a linear dependence on the laser fluence and it is most likely due to the linear absorption at 400 nm wavelength which is close to the bandgap of the ZnO sample. The details of the two-dimensional periodic columnar dot matrix induced by femtosecond laser with different fluence are shown in Fig. 3. The linearly polarized laser pulses with vertical polarization direction are used in the experiments. The polarization direction of laser pulse is perpendicular to c-axis and the laser scanning direction. When the incident laser fluence is 1.08 J/cm2, the LIPSS structures with periodicity about 300 nm are observed in the direction perpendicular to laser polarization (encircled by the green lines), which is in accord with the traditional high-frequency LIPSS, whereas some little microgrooves also appear along the polarization direction on the irradiated area in Fig. 3(a). With the increase of the laser fluence, the microgrooves in the polarization direction become distinct and show certainly regular periodization. At the same time, some unexpected microgrooves in the c-axis direction appear in the irradiated area as shown in Fig. 3(b). When the laser fluence is 1.32 J/cm2, it can be clearly seen that the microgrooves have become a two-dimensional columnar dot matrix and the distribution in the c-axis and polarization direction exhibit periodicity, as shown in Fig. 3(c). The spatial period is approximately 4 μm in the c-axis direction and 2 μm in the polarization direction. When the pulse energy fluence is further increased, the columnar dot matrix distribution becomes more prominent and regular as presented in Fig. 3(d-f). From Fig. 3(d) to Fig. 3(f), it can be obtained that the spatial period in the horizontal direction slightly enlarges with increasing of the incident laser fluence, while that in the vertical direction keeps almost constant. The depth features of the two-dimensional periodic structure are also measured by an AFM working in contact mode and presented in Fig. 4, which correspond to the laser fluence in Fig. 3. From the section-cross profile of the structures, it is clearly shown that the regular periodicity of two-dimensional columnar dot matrix is observed and the crater depth displays an increasing tendency under different laser fluence irradiated.
Fig. 2. Optical morphologies (a) and white-light interferometer topography (b) of femtosecond laser-induced periodic structures on ZnO surface with laser fluence of 1.32 J/cm2 (400 nm, 35fs, 1 kHz and 1.4 mm/s) (c) the relationship between the crater depth and laser fluence, the inset is the cross-section of (b). The error bar is set ±5%. Scale bar: 10µm.
Fig. 3. SEM morphologies of the periodic surface structures as a function of the laser fluence (400 nm, 35fs, 1 kHz and 1.4 mm/s). (a) 1.08 J/cm2 (b) 1.32 J/cm2 (c) 1.56 J/cm2 (d) 1.80 J/cm2 (e) 2.05 J/cm2 (f) 2.29 J/cm2. The two areas encircled by the green and yellow lines correspond to the typical LIPSS and two-dimensional periodic structures, respectively. P: polarization direction of the linear polarized femtosecond pulse, C: crystal c-axis direction, S: scanning direction. Scale bar: 3 µm.
Fig. 4. AFM topography of the inner periodic surface structures in the three-dimensional space corresponding to Fig. 3 (a-f). The section depths of the surface structures are curves are depicted in the curves below the AFM results
Figure 5 shows the dependence of period in the a-axis and c-axis directions and the ablation depth as a function of laser fluence. In Fig. 5(a), it can be seen that the ablation depth with error bar ±5% is approximately proportional to the laser fluence, which indicates that the two-dimensional periodic structure is related to strong linear absorption of the 400 nm femtosecond laser. In Fig. 5(b), we find that there exist two kinds of diverse tendencies of the period variation. In the c-axis direction, perpendicular to the direction of polarization, the period variety is approximately linear. For the determination of the period, each measurement is repeated several times and we presented the averaged value to reduce the experimental errors within the range from ∼1.9 μm to ∼3 μm. For the a-axis direction, parallel to the direction of polarization and perpendicular to c-axis, it displays a decreasing trend as the laser fluence is increased.
Fig. 5. Height (a) and period (b) of the two-dimension periodic structures as a function of laser fluence. The error bar is set ±5%.
It is well-known that laser polarization is an important factor to the laser induced micro/nanostructure formation, especially for the LIPSS. In Fig. 6, the effect of laser polarization on the two-dimensional structure is presented. Herein, we rotated the quarter wave plate to transform the polarization state of the linearly polarized 400 nm beam to vertical, circularly and elliptically polarized beam as indicated in the Fig. 6. The dotted and line arrows refer to the scanning direction and c-axis direction, respectively. It is noticed that such two-dimensional columnar dot matrix structure can be obtained with any polarization states, including linearly, circularly, or elliptically polarized laser pulses. More interestingly, the orientation of the two-dimensional periodic structure always remains unchanged no matter how the laser polarization states change. One axis of the two-dimensional periodic structure is along the c-axis and another one is along the a-axis. These results demonstrate that the orientation of the microscale two-dimensional periodic structure is only determined by the crystal orientation of ZnO itself and not by the polarization of the laser, which is in sharp contrast with the widely observed LIPSS where the polarization of the pump laser plays a deterministic role [9,14,15]. Additionally, we also analyse the influence of the laser scanning direction with respect to the c-axis angle. The results with the laser fluence of 1.08 J/cm2 is presented in Fig. 7. From Fig. 7(a) to Fig. 7(f), the angles between the laser scanning direction and c-axis are changed by rotating the ZnO crystal in 30-degree interval so that they can be set to 0, 30, 60, 90, 120, 150-degrees. Experimentally, the two-dimensional dot matrix structure direction also rotates with the rotation of the crystal and its direction is always along the c-axis direction of the ZnO crystal. It shows that the orientation of the microscale two-dimensional periodic structure depends on the c-axis orientation, regardless the laser scanning direction and the laser polarization state.
Fig. 6. SEM morphologies of the periodic surface structures as a function of the laser polarization (F = 1.92 J/cm2, laser parameters: 400 nm, 35fs, 1 kHz and 1.4 mm/s). (a) Circular polarization (CP); (b) Elliptical polarization (EP); (c) Vertical linear polarization (VP) and (d) Horizontal linear polarization (HP); C: crystal c-axis, S: scanning direction. Scale bar: 3 µm.
Fig. 7. SEM Morphologies of micro scale periodic surface structure as a function of the angle between the laser scanning direction and the crystal c-axis (laser parameters: 400 nm, 35fs, 1 kHz and 1.4 mm/s). (a) 0°, (b) 30°, (c) 60°, (d) 90°, (e) 120°, (f) 150°. C: crystal c-axis, S: scanning direction. Scale bar: 3 µm.
How should we understand the formation of the two-dimensional periodic micro-structure on the ZnO irradiated by a single beam of 400 nm femtosecond laser? We noticed that there exist some subtle cracks with a width of about 30 nm in the center of the microgroove structure along the c-axis as shown in Fig. 8. This nanocracks are also observed under the irradiation with the femtosecond KrF laser (248 nm, 450 fs) [46]. According to the first-principles and ab initio density functional calculations of the wurtzite ZnO [47,48], the surface energies for the planes (11-20) and (0001) are lower than other planes containing the c- and a- axes direction. When a single laser pulse irradiates on the ZnO surface, the internal thermal stress is generated due to ultrafast heating on the femtosecond time scale associated with the laser pulses and subsequent heat dissipation. One therefore can expect that the (11-20) and (0001) planes are prone to experience cleavage in presence of sufficient internal stress [46,49]. The formation of nanocracks can be expected due to the accumulation of internal thermal stress reinforced by multiple pulse radiation. In view that the orientation of the two-dimensional periodic structure only depends on the c-axis orientation, we speculate that this type of nanocracks can be the origin of the two-dimensional periodic structure. To verify this assumption, we experimentally observed the generation and evolution of nanocracks by changing the pulse number irradiated on the identical location of the sample (without translation of the sample). In the above experiments, about sixty pulses irradiate on the same location of the sample. In these experiments, the nanocracks are observed in the c-axis before the appearance of the two-dimensional periodic structure under thirteen pulses irradiation. Surprisingly, another nanocrack with smaller width of ∼10 nm is also found in the direction of the a-axis, as shown in Fig. 9(a). As the number of pulses increases to seventeen, a group of crisscross microgrooves begin to appear and their orientations are along the a and c axes as shown in Fig. 9(b), which are identified to be the predecessor of the periodic columnar dot matrix structure.
Fig. 8. SEM Morphologies of thin crack with a width of about 10∼30 nm in the center of micro scale periodic surface structure (laser parameters: 400 nm, 35fs, 1 kHz and 1.4 mm/s). The energy fluence of the laser are (a)1.08 J/cm2 and (b) 1.56 J/cm2. Scale bar in (a) and (b): 3 µm. Scale bar in (a) and (b): 3 µm.
Fig. 9. (a) and (b), SEM Morphologies of experimentally observed nanocracks on the ZnO surface along the c-axis and a-axis induced by thirteen pulses (a) and seventeen pulses (b) (laser parameters: 400 nm, 35fs, 1 kHz and 1.4 mm/s). The energy fluence of the laser is 1.16 J/cm2. (c) and (d), Calculated electric field intensity of the incident 400 nm laser pulse on the surface structure in (a) and (b). The red color area corresponds to enhancement of the strength of the laser field. Scale bar: 500 nm.
In terms of the formation of the two-dimensional periodic structures, the appearance of nanocracks can be regarded as pre-formed structure defects, which can lead to in site strong absorption on incident laser energy. Thus, we used the finite-difference time-domain (FDTD) method to simulate the electric field distribution and analyze the absorption of incident Gaussian laser light field irradiated upon the surface structures. In the FDTD simulation, the laser beam with wavelength of 400 nm is used as the light source. The structure model is derived from the nanocracks in Fig. 9(a) and (b). We measure the width and depth of the nanocracks by SEM and AFM, then set the identical area with the measured parameters of the nanocracks in the model. We take the periodic boundary condition in the x and y plane.
In Fig. 9(c) and (d), the results corresponding to Fig. 9(a) and (b) are presented. In Fig. 9(c), a prominent electric field enhancement effect (red color is much stronger than blue) is observed in the nanocrack position, which signifies that the nanocracks can give rise to strong linear absorption of the 400 nm femtosecond pulse and damages are easier to produce around these nanocracks. This is also verified by the experimental result in Fig. 9(b). In Fig. 9(d), the electric field distribution around the microgroove presents more significant enhancement. This indicates that more efficient ablation occurs along the microgroove, then two-dimensional columnar dot matrix structure can be ultimately formed as the pulse number accumulation. We therefore conclude that the nanocracks in the c and a-axes directions are formed due to the efficient ultrafast heating of the surface and the resulting nanocracking of crystal boundary in the (0001) and (11-20) planes, since the photon energy of 400 nm pulse is close to the bandgap of the ZnO and strong linear absorption takes place.
4. Conclusion
In summary, a two-dimensional periodic columnar dot matrix structure is formed on crystalline ZnO surface irradiated by a single femtosecond beam at 400 nm. Different from the commonly observed LIPSS or nano-gratings formed on the dielectric surface, this two-dimensional structure is formed regardless of the laser polarization state and the direction of laser scanning. This structure presents laser intensity-dependent period which varies from 2 μm to 6 μm. Moreover, by rotation of the crystal orientation of ZnO crystal, we discover that the orientation of the two-dimensional structure is determined by the c-axis and a-axis. In the meantime, nanocracks with feature size about 30 nm are found at the centre of valleys of the two-dimensional periodic structure and their initial formation can be traced back on the surface before the appearance of the two-dimensional periodic structure. Theoretical results obtained by FDTD method show there exists strong electric field enhancement on the nanocracks under 400 nm femtosecond laser irradiation. We therefore attribute these nanocracks as the origin of this two-dimensional periodic structure, which is formed to enhance ablation along the internal crystalline surfaces due to the efficient heating and resulting in nanocracking of the crystal surfaces since the photon energy of 400 nm femtosecond pulse is close to the bandgap of the ZnO crystal.
Funding
Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-07-E00007); Shanghai Sailing Program (18YF1426300); National Natural Science Foundation of China (12034013, 61905263).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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