Femtosecond laser induced periodic surface structures for the enhancement of field emission properties of tungsten

Direct femtosecond laser ablation enables the maskless fabrication of nanoand micro-scale structures on variety of materials. A typical example is the formation of femtosecond Laser Induced Periodic Surface Structures (fs-LIPSSs), which can lead to strong modification in electrical, optical, wetting, and field emission properties of materials. Here, we study the field emission properties of fs-LIPSSs. We created fs-LIPSSs and fs-LIPSSs covered with nano and micro-scale structures at different laser fluences on Tungsten (W) and showed that these structures offer significant enhancement in electron field emission properties. We provide a phenomenological model to explain the enhancement of electron emission parameters. The enhancement in the field emission properties of laser irradiated W is explained based on the convergence of electric field lines at the ridges of the fs-LIPSSs and fsLIPSSs covered with nanoscale structures, which in turn, enhances the local field intensity and the electron emission parameters. The direct fabrication of 1D subwavelength structures is an important step towards the creation of lowcost cathodes for various potential applications. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Subwavelength periodic surface structures were shown to modify the electrical, optical, mechanical and wetting properties of materials [1][2][3][4]. Ultrafast laser surface-processing can successfully produce different types of subwavelength structures such as cones, holes and fs-LIPSSs [1, 2,5,6]. Among these structures, fs-LIPSSs have been extensively studied as they offer a facile way of achieving a well-organized and highly precise patterns [7][8][9]. The underlying mechanism of fs-LIPSSs is not fully understood and widely discussed in the literature [7][8][9], however, on metals, it is commonly believed to be a result of interference between the incident light and the scattered surface waves, namely, surface plasmons polaritons (SPPs) [1,10]. Briefly, the irradiation of a smooth surface create surface roughness with multiple femtosecond laser pulses and excite SPPs that interferes with the incident radiation resulting in a spatial distribution of the field intensity across the material surface which leads to selective, periodic ablation [1,11]. Fs-LIPSSs have been employed for wide range of applications such as structural colorization, perfect light absorption, anti-reflective surfaces and improved wetting properties [1, [12][13][14].
On the other hand, electron sources with high stability and enhanced efficiency are desirable for various applications such as flat panel displays and field emission electron microscopes [15,16]. Periodic arrays of microtips are commonly used as large area field emission (FE) cathode in integrated circuits, flat-panel displays and microwave power amplifiers [15,17]. The development of such cathodes operating at low voltages with high current density is significantly important and reported by many research groups [18][19][20][21]. Large area regular arrays of FE tips were fabricated using electron beam lithography, electrodeposition, hydrothermal or electrochemical techniques [18][19][20]22]. Although, the emission from such high aspect-ratio cathodes is highly directional [23][24][25], these methods are complex and expensive. On the other hand, laser induced surface structuring as a result of direct laser ablation is a potential alternative for the fabrication of large area FE cathodes. This method exhibits simplicity, robustness and ease of fabrication. Recently, enhancement in the FE properties have been reported for the nanosecond and femtosecond laser irradiated structures [26][27][28][29][30][31]. In addition, we reported the surface-plasmon-enhanced photoelectron emission from nanostructure-covered periodic grooves on metals [32]. Nevertheless, the FE enhancement from fs-LIPPSs formed on metallic surfaces has not been reported previously.
Here, we study the enhancement of field emission properties from the fs-LIPSSs and fs-LIPSSs covered with nano-and micro-scale structures. All type of Fs-LIPSSs structures show a significantly higher enhancement in the electron emission parameters. We show that fs-LIPSSs enhanced FE properties depend on laser fluence F. We provide a phenomenological model to explain the enhancement of electron emission parameters. Our model employs the convergence of electric field at the ridges of the fs-LIPSSs, which in turn, enhance the electron emission parameters.

Experimental setup
A diagram of the experimental setup is illustrated in Fig. 1, where a Ti: sapphire femtosecond laser amplifier (Femtopower Compact Pro, Femtolasers produktions GmbH, Austria) was employed as an irradiation source to deliver horizontally polarized pulse trains at the repetition rate of 1 kHz, with central wavelength of λ=800 nm and a pulse duration of τ=30 fs.
The maximum pulse energy delivered by the laser system is 1 mJ, which was attenuated with the help of neutral density filters. A half wave-plate (λ/2) was employed to change the light polarization from horizontal to vertical direction. An electromechanical shutter was used to select a single shot for controlling the number of shots. The laser was focused by a lens of focal length 10 cm and incident at normal incidence. A bulk circular disk of W (obtained from Alfa Aesar company with 99.95% purity) of diameter 20 mm and thickness 5 mm was used as a target material due to its high electronic emissivity, superior thermal and chemical stability and excellent mechanical strength [33,34]. The sample was mounted at a computerized XYZ precision stage and translated at a fixed speed of 1 mm/s for all experiments, which give 99.2 percent overlap between pulses. In order to avoid overlap of scanned lines, the step size or interspacing between two lines is kept constant of 100 µm. Accordingly, the scanned lines are formed throughout the surface. The laser fluence was varied by increasing the power. After the laser microstructuring process, the surface morphology was analyzed by Scanning Electron Microscopy (SEM) and electron emission performance was investigated through field emission measurements. The experimental setup for field emission measurements is shown elsewhere [31].

Results and Discussions
To determine the effect of surface structuring for the enhancement of FE properties, we first investigate the formation of fs-LIPSSs at various laser fluences. The overall scanned pattern of the laser irradiated region is shown in the Fig. 2(a). The optimal results suitable for FE enhancement are presented in Fig. 2(b-e) and were obtained using laser fluences of (b) 0.09 J/cm , (c) 0.18 J/cm , (d) 0.90 J/cm , and (e) 1.81 J/cm . As shown in Fig. 2(b), a well-defined fs-LIPSSs structures observed with periodicity of Λ~ (634 ± 0.48) nm. Accordingly, low-spatialfrequency LIPSSs (LSFLs) are created with spatial orientation perpendicular to the incident laser polarization (vertically polarized) as reported by others [35,36]. Closer inspection of Fig 2(b) reveal that such surface structures have a slanted distribution of ridges and grooves, i.e., elongated grooves and ridges in the range of 10's of microns. The observed slanted behavior of fs-LIPSSs is due to random scattering that acts as a seed points for fs-LIPSSs [7]. We note here the decrease in fs-LIPSSs period to (590 ± 0.88) by increasing the fluence to 0.18 J/cm (Fig. 2c). In addition, the ridge width is reduced, and grooves become more pronounced. By further increasing the fluence to 0.90 J/cm , the scanned line region in Fig. (2d) shows enhanced nanoscale particulates at the grooves and ridges of fs-LIPSSs which make them diffusive. The measured fs-LIPSSs groove periodicity of (602 ± 0.88) nm is observed across the ablated region, which is considerably smaller than the period obtained with the lowest fluence of (0.09 J/ cm ). We also note that at the fluence of ( = 0.9 J/cm ), extensive nanoscale structures are formed that are randomly distributed (Fig. 2d). For the highest fluence ( = 1.81 J/cm ), nano-and micro-scale structures are formed and the overall LSFLs morphology is distorted due to increased melting and re-solidification (Fig. 2e). Craters formation can be seen along with microscale structures. Figure 2(f) shows two-dimensional Fast Fourier transform (2D-FFT) of the fs-LIPSS obtained at the fluence of 0.09 J/cm which reflects its uniformity. We note that the fs-LIPSSs periodicity for fluences ( = 0.09, 0.18 and 0.9 J/cm ), were obtained using the 2D-FFT analysis. The formation of fs-LIPSSs has been extensively investigated by us and others [11,37,38]. Briefly, the irradiation of a smooth surface with multiple fs laser pulses excite surface waves SPPs that interferes with the incident radiation resulting in a spatial distribution of the field intensity across the material surface which leads to selective, periodic ablation. The formed grooves, thus, have a wave-vector G such that G = -, where = 2π|λ | and = 2π|λ | are the incident laser beam and the SPP wave-vectors, respectively and λ and λ are the incident laser wavelength and SPP wavelength, respectively. For an incident angle θ (measured from the normal), G = sin θ-, since only the wavevector component parallel to the metaldielectric interface excites SPPs. The grooves' period is given by Λ= 2π | | ⁄ = 2π |( − )| . Consequently Λ= λ |( θ − )| , and finally, we obtain the period Λ of near subwavelength fs-LIPSSs as below [38]; For normal incidence (when (θ = 0°), as in our case, Λ=λ . Thus, the period of fs-LIPSSs is λ . For a metal its complex permittivity is given by ε = ε + iε , where, ε and ε are the real and imaginary parts of the metal permittivity. The excitation of a surface wave at a metal-dielectric interface, thus, requires a phase mismatch with the dielectric, i.e., the guided mode is to the right of the light-line, λ is highly sensitive to the metal and dielectric permittivity [38], λ = λ / , where ε is the dielectric permittivity and ε is the real part of the metal's complex permittivity [38,39]. In other words, at fluence higher than damage threshold, the fs-LIPSSs formed on the material will be equal to λ ,which is always smaller than the incident wavelength as we observed. Although W is not a metal at the excitation wavelength of 800nm (ε (800 ) = 5.22), the excitation of SPP is still possible due to the high intensity associated with fs laser pulses which promotes enough electrons beyond the W bandgap such that it transiently behaves as a metal, i.e., ε < -1 [39]. Furthermore, the reduced Λ for fluences = (0.18 and 0.9 J/cm , compared to 0.09 J/cm ), is due to increase in the effective dielectric permittivity as surface roughness formed on the W surface, which leads to the reduction in λ [37,40].

Field Emission analysis
The electron emission performance such as turn-on field, field enhancement factor and current density of fs-LIPSSs and fs-LIPSSs covered micro and nanoscale structures with respect to untreated target are analyzed using the Fowler-Nordheim (FN) equation based on the FE theory proposed for metallic electron emitters [41]. It relates the emitted J to the applied field (E) at the metallic emitter surface as follows; where a and b are constants whose values are listed as 1.4 × 10 and −6.83 × 10 , respectively. Since E = , where, "E, V and d" are the applied electric field, voltage and inter-electrode spacing, respectively. In our experiments, we varied the voltage ranging from 0.1 kV to 2.5 kV and kept the inter-electrode spacing to d= 200 µm. The current density is generally defined as, J= , where, A is area of electrode, that is 15 mm 2 , and is a work function. As we have irradiated the samples in ambient air and presence of oxides peak is evident from the XRD analysis, hence, we have used work function of tungsten oxide (5.7 eV) for the field emission analysis. An important factor in field emission is the relationship between the applied and the local electric field where electron tunneling occurs [31,41,42]. For this tunneling, the field enhancement factor ( ) is defined by; = , where is the local field enhancement and is the applied electric field, plays a key role.
Equation (2), can be written in terms of I and V and inserting the values of constants "a" and "b", we obtain; The value of can be extracted from the linear slope (m) of the plot between ln (I/V 2 ) versus 1/V as follows; = −6.83 × 10 (4) Figures 3 (a-d) illustrate J versus E plots along with the corresponding FN plots in insets for the laser irradiated W samples at different fluences of (a) 0.09 J/cm (b) 0.18 J/cm , (c) 0.90 J/cm and (d) 1.81 J/cm . The current density essentially remains zero until turn-on field, which in our case, we defined as the applied electric field require to attain a current density of ~10 μA/cm 2 and the current exponentially increases afterwards. From Fig. 3(a-d), the linear nature of the FN plots with a single negative slope and minimum deviation from linearity ensures the field emission as the origin of the measured current. The maximum applied voltage and hence the field is limited to pre-  Table   Table 1. Obtai Fluenc he fluence of Enhancement ct ratio such as ficantly localiz anar diode con induced field n of the appli of ridges (prot that the electric cal mechanism inv a of fs-LIPSSs stru m field at the ridges reasing the flu as compared to hibits a decrea nsity to a valu efore saturatio 1. / a oscale structure The applied e ld [41]. On the he absence of s tructures enab particular, fs-L he order of ~ 1 gure 4. nm is (390 ± ximum n the value supports our proposed model for the enhancement of field emission properties due to the convergence of field lines at the ridges. Lesser the ridge width corresponds to more area occupied by grooves, as we quantitively shown above, hence less will be the value of . The significant increase in the current density with a decreased suggests either an increase in the emission area or decrease in the work function [45]. This can also be explained on the basis of reduced screening effect as it is reported that emission current increases with the increase of spacing (in our case the groove width) between the emitter up to a certain value [46]. For the sample irradiated at 0.90 J/cm , a low turnon field of 7 V/μm, and a high beta factor of value 1917 is achieved along with a maximum current density of 217 μ A / cm 2 , at 16 V / μm. This is attributed to the nanostructures covered fs-LIPSSs morphology. Such nanostructures have been shown for the enhancement of field emission due to increased surface area [47]. For W irradiated with maximum fluence, a significant increase in the value of =5950 is observed as depicted in Fig. 3(d). However, the turn-on field slightly increased to 8 V /μm whereas, maximum current density decreases to 200 μA/cm 2 .
Quantitatively, for the highest fluence of = 1.81 J/cm , the density of nanoscale structures (Fig. 3 (f)) is lower than what we obtained for the fluence of = 0.90 J/cm (Fig. 3(e)). Accordingly, the increase in the size of the nanostructures that cover the fs-LIPSSs correspond to an increased value of . The increase in the turn-on and decrease of the maximum current density for the highest fluence is explained based on screening effect [46].  (1 1 0) and W (2 0 0) orientations and are attributed to the JCPD reference pattern (00-004-0806). The peak at an angle of 43.849 o is representing W 2 N (2 0 0) phase and is attributed to the JCPD reference pattern (00-025-1257). The remaining small peak at 50.886 o corresponds to the WO 3 (3 2 2) phase and is indexed on the basis of JCPD reference pattern (00-020-1323). However, the W target irradiated at high fluence shows more pronounced peak shifting and decrease in intensities as compared to W target irradiated at low fluence. The reduction in both W (110) and W (200) is due to the loss of crystallinity, due to the fs laser surface ablation.

Conclusions
We have successfully employed femtosecond laser irradiation to create periodic surface structures on the bulk tungsten surface. It is surprisingly found that these fs-LIPSSs structures show tremendous enhancement in electron field emission parameters. We show that fs-LIPSSs enhanced FE properties depends on laser fluence F. We provide a plausible explanation for the formation of fs-LIPSSs and enhancement of field emission from these bare fs-LIPSSs and fs-LIPSSs covered with nano-and micro-scale structures. The direct fabrication of structured cathode opens an important step for future investigations for low-cost cathodes for various potential applications.

Funding
This work is financially supported by the Bill & Melinda Gates Foundation (OPP1119542). Mahreen Akram acknowledges Higher Education Commission of Pakistan, for financial support to travel Vienna through IRSIP program.