Investigation of ablation efficiency during the pulsed laser ablation of a zinc metal target in a distilled water environment

Mohammad Rasoul Khodaverdi; Elnaz Irani

doi:10.1364/OSAC.438834

1. Introduction

Laser Ablation Synthesis in Solution (LASiS) is a common method for novel nanoparticles (NPs) generation from various solvents. LASiS is a fast, safe and green technique capable of controlling the structure of NPs which has unique applications in the nano science, photonic, biology and medical field. The irradiation of a metal submerged in a liquid solution by a laser beam condenses a plasma plume that produces NPs. It is a reliable top-down method that provides an alternative solution to conventional chemical reduction of metals to synthesis metal-based NPs [1]. In recent years, the LASiS induced nanomaterials have gained increasing interest for its versatility, low cost and ease of execution.

In the process of NP formation in LASiS, the parameters of the synthesis are divided into two categories: material parameters (bulk target, solvent and solutes, system temperature and pressure) and laser parameters (wavelength, time duration, energy, repetition rate, number of laser pulses and the spot area on the target) [2]. In some cases, the phase, size, and shape of nanostructures can be readily controlled by tuning laser parameters and assisting factors and therefore, can be achieved to NPs with new properties [3]. Since a nanosecond is much longer than the phonon–electron relaxation time, the interaction of nanosecond laser with the material is essentially a photo-thermal process. In LASiS method, a very complex physical and chemical coupling reaction takes place in a short time. The process starts with the absorption of the laser pulse by the target, then a plasma plume containing the ablated material expands into the surrounding liquid, complemented by the emission of a shockwave. During the expansion, the plasma plume cools down and releases energy to the liquid environment. This mechanism generates a cavitation bubble and then collapses by emission of another shockwave. This is the last physical process related to laser ablation of a target in liquid environment [2].

Since LASiS can produce different types of NPs from metallic to polymeric materials without toxic, pyrophoric chemical precursors, the number of scientific reports on LASiS of functional materials for specific nanotechnology applications is continually growing. Dittrich et al. compared the productivity and ablation efficiency of laser ablation of gold in water and in air with change in laser parameters [4]. Brause et al. investigated Characterization of laser-ablated and chemically reduced silver colloids in aqueous solution [5]. Elsied et al. investigated nanosecond laser ablation of Al and W targets at different ambient conditions [6]. Yan et al. simulated the nanosecond laser ablation of titanium target with considering plasma shield [7]. Also there have been several reports focused on the synthesis of pure zinc oxide NPs generated by laser ablation over the past decade due to the unique effects of their direct wide band-gap and large exciton binding energy. Guillen et al. investigated the effects of temperature and energy on the structure and morphology of ZnO, Zn(OH)₂ NPs resulting from laser ablation of zinc metal in different water temperatures with a wavelength of 532 nm [8]. Gavrilenko et al. compared the properties of zinc oxide NPs synthesized in both air and water environments by laser ablation of Zn target and using wavelength of 1064 nm [9]. Chen et al. synthesized ZnO NPs by pulsed laser ablation of zinc powders in distilled water. They used 532 nm wavelength, 5 Hz repetition rate, 10 ns duration and 50, 80, 110, and 140 mJ energies to produce ZnO NPs and then compared and investigated the optical properties of the NPs in each sample [10]. Despite some explained experimental reports, the variety of experimental conditions reported in the literature and inconsistencies in the reported results makes it difficult to visualize a comprehensive picture of the process. However, the simulation models cannot always accurately explain some experimental observations when some approximations are used to reduce the complexity of the calculations and should be improved by extensive theoretical models.

This work mainly focuses on the characterization of ZnO NPs to have a quantitative picture of nanosecond laser ablation of zinc target in distilled water medium. The morphology and properties of generated NPs are explored using both experimental and simulation approaches. The effect of plasma shielding on the ablation efficiency is also investigated. Although several reports on the synthesis ZnO NPs by laser ablation have been studied by many researchers, a proper and comprehensive experimental and exclusively extensive simulation model have been least studied. Because of the direct band gap in the ultraviolet range (3.37 eV) and a large exciton binding energy (59 meV) of ZnO, we have motivated to explore the interesting issue for finding the optimal experimental parameters of laser ablation and controlling the properties of ZnO NPs. These NPs have a wide applications in the field of photonics, sensing, lasing, energy production, light emitting diodes, solar cells, sensors, and field emitters [11]. To provide deep insight in this aim and overcome the limitations of the ultrafast measuring tools, the interaction of the laser with the metal target must be managed. Therefore, by finite element modeling simulating the process of laser interaction with zinc metal, we have tried to obtain useful information about the temperature distribution created at the depth and surface of the zinc target, ablation rate of the zinc target, the effect of plasma shielding on the target and the optimal amount of energy applied to the zinc target surface. The good agreement between our results and experimental results confirms that plasma shielding plays a relevant role in the ablation efficiency of zinc metal in distilled water environment.

2. Experimental section

Figures 1(a) and (b) show the schematic experimental setup of the laser ablation of zinc metal target in distilled water solution and the physical phenomena of this process. Briefly, a pulsed Nd:YAG laser with 1064 nm wavelength, 3 Hz repetition rate and 9 ns time duration was focused on to the target by using a doublet lens with focal length 5 cm. For nanosecond laser ablation there is no photochemical ablation during this time duration and only photothermal ablation occurs. So, we have simulated this process by the heat transfer equation. Also, since we wanted the thermodynamic conditions to be the same for each laser pulse, we set the repetition rate to 3 Hz; so that there is enough time for the target to cool and the NPs disperse. The beam spot diameter after passing through the lens on the target surface was equal to 2 mm. A beaker was used to hold the target and distilled water. Also, a rotating plate was used to establish the same conditions in ablation of the target surface. In this study we used a zinc metal plate as target with a purity of 99.9% and dimensions of 2 × 2×0.1 cm³. The target was cleaned with ethanol and distilled water before irradiation then placed at the bottom of a beaker with radius of 1.5 cm at the depth of 1 cm filled with 7 ml of distilled water (see Fig. 1(a)).

Fig. 1. (a) Schematic of the experimental setup of the laser ablation of zinc metal target in distilled water solution and (b) the physical phenomena of laser ablation.

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In this process, when a laser beam with high intensity, reaches the Zn target surface, some of the laser pulse energy is absorbed by the target, and after receiving the energy, this energy is converted into heat, causing the target surface to melt. When the temperature rises, the temperature of the material rapidly exceeds the boiling point, and some of the material is vaporized. At this stage, vaporization is the main removal mechanism because the energy density is relatively low. The vapor molecules will form a very thin Knudsen layer on the surface of the target, and the velocity and pressure will regain the equilibrium distribution. Then, the high-temperature vaporized material leaves Knudsen layer and rises into the liquid environment [7]. Then, a portion of the energy of laser beam is absorbed by the vapor above the target surface. This action increases the pressure and temperature of the vaporized material on the target surface. As the vapor temperature rises, it ionizes the surface and forms an environment of plasma on the target surface. When time duration pulse is of the order of 10⁻¹¹ s or more, the plasma plume itself can absorb energy from the laser pulse. This effect is called ‘‘plasma shielding’’ because it reduces the amount of laser energy directly delivered to the target surface, while increasing the lifetime and the temperature of the plasma plume. The plasma plume usually disappears after 10⁻⁸−10⁻⁷ s. The energy released by the plasma plume to the surrounding liquid induces the rise of a cavitation bubble. During its expansion, bubble’s temperature decreases and internal pressure drops to a value lower than in the surrounding liquid. At this stage, the bubble collapses emitting a shockwave [2]. The physical phenomena of LASiS are shown in Fig. 1(b).

We try to explore the effective parameters on the morphology of generated NPs, productivity rate and ablation efficiency. The effective parameters of the synthesis are divided into material features and laser parameters such as energy, wavelength, number of laser pulses, time duration, repetition rate, the spot area on the metal target. Laser ablation induces physical modification on the target, due to the fragmentation from metal plate into NPs. Plasma shielding effect plays a main role in the analysis of the laser ablation process on the Zn target which is the main aim of this research. For this purpose, the effect of laser energy on the ablation efficiency is investigated using the experimental procedure described above for four different laser energies of 100, 200, 400 and 600 mJ. After the laser ablation process, a colloidal solution containing the synthesized NPs was formed inside the beaker. After about 20 minutes and in room temperature, the effects of agglomeration appeared and caused the NPs to settle. Then was formed a white sedimentation spontaneously, which is shown in Fig. S1, Supplement 1. The samples of NPs synthesized in distilled water medium with energies of 600, 400, 200 and 100 mJ, marked by A, B, C and D, respectively.

The immediate evidence for proper fabricating the ZnO NPs and exploring the features of the ablated material comes from the analyzing the UV-Vis absorption spectroscopy, X-Ray diffraction spectroscopy (XRD) and scanning electron microscope (SEM) for each of the samples. X-ray diffraction (XRD) patterns were recorded by a Philips Expert instrument with Cu kα radiation. UV-Vis spectra was collected by a Perkin-Elmer Lambda 25 spectrophotometer. Hitachi S-4160 scanning electron microscope (SEM) (Tokyo, Japan) was used for characterization of size and morphology of NPs.

According to Fig. S1, it seems that the amount of NPs synthesized in the samples is D < A ≤ C < B. In other words, the rate of laser ablation with energy of 400 mJ is almost higher than other synthesized samples and also the NPs synthesized with energy of 100 mJ, show the lowest rate. But this view alone is not reliable; because in the following it becomes clear that due to the special size and morphology that exists in NPs, during sedimentation, it creates empty space between NPs and more analysis is needed on the results. UV-Vis absorption spectroscopy helps to make a more accurate analysis which is shown in Fig. 2. Before analyzing each sample, the samples were placed in an ultrasonic device for 2 minutes to prevent agglomeration. According to obtained results in Fig. 2, the amount of NP production in the samples is D < A <B ≤ C. The explanation for this fact is that the strong shielding effect is formed for sample A, which is synthesized with high energy of 600 mJ. This effect reduces the ablation efficiency and the production of NPs due to the reduction of the absorption wave obtained at the special size and morphology that exist in sample A, during sedimentation.

Fig. 2. The UV-Vis absorption spectroscopy diagram of samples synthesized by laser ablation of zinc metal with laser parameters of 1064 nm wavelength, 9 ns time duration, 3 Hz repetition rate, 1000 laser pulses and different energies of 100, 200, 400 and 600mJ.

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According to Fig. 2, the absorption spectra have sharp peaks centered at 340, 334, 333 and 345 nm with a long tail towards longer wavelength for samples A, B, C and D, respectively which is similar to the range of absorption wavelength of ZnO NPs experimentally reported by valid Refs. [12,13]. The size distributions of NPs in samples B and C are smaller than the size distributions of samples A and D. Based on the peak intensity of the absorption wavelength and XRD patterns of each sample (see Fig. 3), the concentration of NPs in samples B and C is much higher than samples A and D. In other words, the production of NPs in samples B and C is higher than the other two samples and sample D shows the lowest production compared to other samples. As a result, it can be said that low energy applied has reduced the production of NPs and also the high energy applied has caused part of the energy to be absorbed by the plasma shield and leads to decrease in production and increase in the size distribution of NPs. For characterization of structure of the prepared zinc oxide NPs, the band gap values are obtained from the Tauc plot method using the data obtained from the UV-Vis absorption spectrum diagram of each sample (see Fig. S2, Supplement 1). The energy gaps of samples A, B, C and D are obtained 3.34, 3.36, 3.35, and 3.22 eV, respectively.

Fig. 3. The XRD spectra of samples synthesized by laser ablation of zinc metal with laser parameters of 1064 nm wavelength, 9 ns time duration, 3 Hz repetition rate, 1000 laser pulses and different energies of 100, 200, 400 and 600 mJ.

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The crystalline nature of the prepared ZnO NPs is determined by XRD analysis and Fig. 3 represents the XRD patterns of synthesized NPs by LASiS.

The patterns show diffraction peaks at peak position 2θ = 31.79°, 34.41°, 36.24°, 47.50° and 56.57° which are indexed to (100), (002), (101), (102), (110) lattice planes for crystalline hexagonal wurtzite structure of ZnO NPs [9]. However, two wide peaks at 2θ = 33.13° and 28.27° are not indexed for the ZnO wurtzite structure and they are considered to stem from the ZnOOH NPs as pointed out by Singh et al. [14,15]. They claimed that unstable ZnO₂ formed via reaction of Zn NPs with O₂ may react further with hydroxide ion to produce ZnOOH.

{\rm{Zn(cluster)}} + {{\rm{O}}_2} \to {\rm{Zn}}{{\rm{O}}_2}({{\rm{unstable}}} )

{\rm{Zn}}{{\rm{O}}_2}({{\rm{unstable}}} )+ {\rm{O}}{{\rm{H}}^ - } \to {\rm{ZnOOH}} + {{\rm{O}}^ - }

{\rm{Zn}}({{\rm{cluster}}} )+ {{\rm{O}}^ - } \to {\rm{ZnO}}

{\rm{Zn(cluster)}} + 2{{\rm{H}}_2}{\rm{O}} \to {\rm{Zn}}{({\rm{OH}})_2} + {{\rm{H}}_2}

{\rm{Zn}}{({\rm{OH}})_2} \to {\rm{ZnO}} + {{\rm{H}}_2}{\rm{O}}

This diffraction pattern shows that the ZnO NPs produced with 600 mJ energy are amorphously formed and the NPs are crystalline in the other three samples. Also, by comparing the peak intensity of each sample, it was found that the production of ZnO crystalline NPs in sample B is more than the other two samples and sample D had the lowest amount of crystalline NPs among the other two crystalline samples. The crystallite size of ZnO NPs was estimated between 15 nm and 20 nm for sample B, 7 nm and 29 nm for sample C and 5 nm and 27 nm for sample D, using Scherer equation.

Figure 4 shows the SEM images of the nanostructures synthesized by laser ablation of zinc metal with different energies. According to SEM images, it is found that the nanostructures synthesized have two types of spherical and nanosheets morphology. Spherical NPs are composed of agglomerated ZnO hexagonal NPs. In some reports, this nanosheets structures is called honeycomb structure [14].

Fig. 4. SEM photographs of samples synthesized by laser ablation of zinc metal with laser parameters of 1064 nm wavelength, 9 ns time duration, 3 Hz repetition rate, 1000 laser pulses and different energies of A=600 mJ, B=400 mJ, C=200 mJ and D=100 mJ.

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Figure 4 shows that the NPs of sample A, synthesized with 600 mJ energy, have only nanosheets structure and no traces of hexagonal crystalline NPs are observed. In three samples B, C, D, in addition to the nanosheets structure, the structure of hexagonal crystalline NPs with spherical agglomerated effects can be observed. The average size of these crystalline NPs in all three samples is measured in the range of 10–35 nm using image processing software. However, due to the agglomeration of NPs, this amount increases and shows up to about 135 nm. It is noticed that if the drying time of nanoparticles is reduced, the probability of nanoparticles agglomeration will be decreased. By comparing the results of SEM images and XRD analysis results, it is found that ZnO nanosheets are amorphous.

3. Numerical model and theoretical calculations

Earlier it was also shown that the reason behind the reduced ablation efficiency in ZnO synthesized with 600 mJ energy is due to plasma shielding effect. The dynamics of heat generation in the laser ablation, phase changes, plasma formation, and absorption are further numerically investigated using finite difference method. Moreover, the effect of plasma shielding on laser energy due to inverse Bremsstrahlung (IB) absorption, is carefully investigated. Based on the results, a better understanding of the ablation depth rate can be obtained after nanosecond ablation coupled with the plasma shielding phenomenon. The simulation results are calculated for interacting zinc metal with Nd:YAG laser pulse with 1064 nm wavelength, pulse duration of 9 ns and 2, 4, 8 and 12 J/cm² Fluences (These values are considered in proportion to the values of energy used in the experimental section). The profile of laser pulse is shown in Fig. S3(a), Supplement 1. In this calculation, the cell size with 11 mm×10 mm in Z and r (axial symmetry cylindrical coordinate radius section) directions are used and a numerical grid in the two-dimensional real space is defined as Δr = 0.05 µm and Δt = 0.01 ns. The target is located at Z = 0, and the laser irradiation axis is normal to it. The geometric model applied in the simulation is shown in Fig. S3(b), Supplement 1. The interface of Liquid/target is solved using the moving mesh method. For modelling the dynamics of laser ablation in distilled water environment, a two-fluid heat transfer model is used. The heat diffusion process is governed by the heat conduction equation [7].

(1)$$\rho {C_p}\left( {\frac{{\partial T}}{{\partial t}} + \nabla \cdot ({{u_{ev}}T} )} \right) - \nabla \cdot (k\nabla T) = 0,$$

where k is the material thermal conductivity, $\rho $ is the material density, ${C_p}$ refers to the specific heat, and ${u_{ev}}$ is the velocity of surface recession that during the evaporation stage can be calculated as [16,17]:

(2)$${u_{ev}} = 0.32{\left( {\frac{m}{{{k_{\rm{B}}}{T_{\rm{S}}}}}} \right)^{1/2}}\frac{{{p_{\rm{s}}}}}{\rho },$$

where ${T_{\rm{s}}}$ and ${p_{\rm{s}}}$ are the target surface temperature and the corresponding saturated vapor pressure, respectively, m is the atomic mass of the target, and ${k_{\rm{B}}}$ is the Boltzmann constant. The correction coefficient 0.32 in Eq. (2) account for the influence of the Knudsen layer on the plasma parameters [17]. The temperature, density and the other parameters of the material will have a jump across the Knudsen layer. The jump boundary conditions of the Knudsen layer are given by Eqs. (3) and (4) [16].

(3)$$\frac{{{\rho _{\rm{V}}}}}{{{\rho _{\rm{s}}}}} = \sqrt {\frac{{{T_{\rm{V}}}}}{{{T_{\rm{s}}}}}} \left[ {({m_0^2 + 0.5} )\exp ({m_0^2} ){\mathop{\rm erf}\nolimits} ({{m_0}} )- \frac{{{m_0}}}{{\sqrt \pi }}} \right] + 0.5\frac{{{T_{\rm{s}}}}}{{{T_{\rm{V}}}}}\left[ {1 - \sqrt \pi {m_0}\exp ({m_0^2} ){\mathop{\rm erf}\nolimits} ({{m_0}} )} \right]$$

(4)$$\frac{{{T_{\rm{V}}}}}{{{T_{\rm{s}}}}} = \left[ {\sqrt {1 + \pi {{\left( {\frac{{{\gamma_{\rm{v}}} - 1}}{{{\gamma_{\rm{v}}} + 1}}\frac{{{m_0}}}{2}} \right)}^2}} - \sqrt \pi \frac{{{\gamma_{\rm{v}}} - 1}}{{{\gamma_{\rm{v}}} + 1}}\frac{{{m_0}}}{2}} \right],$$

where ${\rho _{\rm{V}}}$ and ${T_{\rm{V}}}$ are the density and temperature of the vapor on the liquid side of Knudsen layer, respectively, while ${\rho _{\rm{s}}}$ and ${T_{\rm{s}}}$ are the density and temperature of the target surface, respectively. In addition, ${\gamma}$ is the specific heat ratio. The vapor plume is considered as an ideal single particle gas column with the specific heat ratio of 5/3. Additionally, ${m_0} = \sqrt {\frac{{{\gamma _{\rm{v}}}}}{2}} M$, ${\rm{erf}}({{m_0}} )$ is the error function, and M is the Mach number of the vapors at the edge of Knudsen layer [16]. Since the value of Mach number is about 1, we assume that its value to be constant and equal to 1 and calculate the other parameters based on it. So, we obtain the following estimations of temperature ${T_{\rm{v}}}$, pressure ${p_{\rm{v}}}$ and density ${\rho _{\rm{v}}}$ [18,19]:

(5)$${T_{\rm{v}}} \approx 0.67{T_{\rm{s}}},{p_{\rm{v}}} \approx 0.21{p_{\rm{s}}},{\rho _{\rm{v}}} \approx 0.31{\rho _{\rm{s}}}$$

The saturation vapor pressure is obtained by Clausius-Clapeyron equation [19]:

(6)$${p_s} = {p_0}\exp \left[ {\frac{{m\Delta {H_{\rm{V}}}({{T_{\rm{b}}}} )}}{{{k_{\rm{B}}}}}\left( {\frac{1}{{{T_{\rm{b}}}}} - \frac{1}{{{T_s}}}} \right)} \right],$$

where ${p_0} = {p_{{\rm{\;H}}2{\rm{O}}}} + 1\;{\rm{atm}}$ and ${\rm{\Delta }}{H_{\rm{v}}}$ is the vaporization enthalpy at the normal boiling point ${T_{\rm{b}}}$.When the target surface temperature exceeds ${T_{\rm{b}}}$, according to Eq. (2) evaporation rate increases significantly, and the laser beam will be partially absorbed by plasma before it reaches the target. Additionally, it consists of different species, such as electrons, ions, and neutral particles. Due to this, the absorption of the laser by plasma occurs through three different mechanisms, namely the inverse Bremsstrahlung (IB), photoionization (PI) and Mie scattering absorption. The absorption of laser radiation by free electrons in the plasma is described by the IB mechanism, while the absorption by excited ions and neutral atoms is described by the PI mechanism. Furthermore, the absorption by vapor clusters inside the plasma is described by Mie absorption. However, PI and Mie scattering could be neglected due to its very small contribution to the absorption of laser. Due to this reason, the absorption coefficient of laser-induced plasma is expressed by Eq. (7) [7,16,18].

(7)$${\alpha _{{\rm{IB}}e - i}} = {\left( {\frac{{2\pi }}{{3m_e^3{k_{\rm{B}}}}}} \right)^{1/2}}{\left( {\frac{{e_0^2}}{{4\pi {\varepsilon_0}}}} \right)^3}\frac{{4{n_e}Z_i^2{n_i}{\lambda ^3}}}{{3h{c^4}{T^{1/2}}}}\left( {1 - \exp \left( { - \frac{{hc/\lambda }}{{{k_{\rm{B}}}T}}} \right)} \right),$$

where ${m_e}$, ${n_e}$ and ${e_0}$ are the mass, number density and electric charge of electron, respectively. ${n_i}$ and ${z_i}$ are the number density and the average ionized degree of ions. Furthermore, $\;\lambda $ is the wavelength of laser, ${k_{\rm{B}}}$ is the Boltzmann constant, h is the Planck constant and ${\varepsilon _0}$ is the vacuum permittivity. The density of free electrons in the vapors can be calculated using the Saha–Eggert equations [18].

(8)$$\frac{{{x_e}{x_{i1}}}}{{{x_0}}} = \frac{1}{{{n_{{\rm{vap }}}}}}{\left( {\frac{{2\pi {m_e}{k_{\rm{B}}}T}}{{{h^2}}}} \right)^{3/2}}\exp \left( { - \frac{{{\rm{I}}{{\rm{P}}_1}}}{{{k_{\rm{B}}}T}}} \right)$$

(9)$$\frac{{{x_e}{x_{i2}}}}{{{x_{i1}}}} = \frac{1}{{{n_{{\rm{vap }}}}}}{\left( {\frac{{2\pi {m_e}{k_{\rm{B}}}T}}{{{h^2}}}} \right)^{3/2}}\exp \left( { - \frac{{{\rm{I}}{{\rm{P}}_2}}}{{{k_{\rm{B}}}T}}} \right)$$

Here, ${n_{{\rm{vap}}}}$ represents the total vapor number density (${n_{{\rm{vap}}}} = \rho /m$), ${x_e}$, ${x_{i1}}$, ${x_{i2}}$ and ${x_0}$ stand for the fraction of electrons, singly charged (Zn⁺) and doubly charged (Zn²⁺) ions and neutral Zn atoms, respectively, which are defined as ${x_e} = {n_e}/{n_{{\rm{vap}}}}$, etc. Further ${\rm{I}}{{\rm{P}}_1}$ and ${\rm{I}}{{\rm{P}}_2}$ are the ionization energy of the ions, h is the Planck constant, and the other symbols have been explained above. Two Saha–Eggert equations are combined with two other equations, i.e., for the conservation of matter, and the conservation of charge, respectively [20]:

(10)$${x_0} + {x_{i1}} + {x_{i2}} = 1$$

(11)$${x_{i1}} + 2{x_{i2}} = {x_e}$$

In this study, only the first degree of ionization is considered and used ${n_i} = {n_e}$ as a reasonable assumption [21]. Assuming that only singly charged (Zn⁺) ions are formed, and combining the Saha–Eggert equation with the equations for conservation of matter (${x_0} + {x_{i1}} = 1$) and conservation of charge (${x_{i1}} = {x_e}$), yields three equations for three unknowns.

The distribution laser energy in time and space before entering the liquid medium has been calculated by Eq. (12) (see Fig. S4). Due to the fact that laser energy obeys the Gaussian distribution in the time and space domains, the heat conduction equation’s boundary conditions are given by Eq. (12) [7,16].

(12)$$\begin{aligned} {\left. {k\frac{{\partial T}}{{\partial z}}} \right|_{z = 0}} &= \frac{{4(1 - R){I_0}}}{{{t_p}\sqrt {\pi /\ln 2} }}\exp \left[ { - \frac{{2{r^2}}}{{r_0^2}} - 2\sqrt {2\ln (2)} {{\left( {\frac{t}{{{t_{\rm{p}}}}} - \frac{3}{2}} \right)}^2} - \int_z^{ + \infty } \alpha (r,z)dz} \right]\\ &- {u_{ev}}\rho {L_{ev}} - h({T - {T_{en}}} )- \varepsilon \cdot \sigma ({{T^4} - T_{en}^4} ), \end{aligned}$$

where ${I_0}$ is the laser fluence, R is the reflection coefficient, ${r_0}$ is the spot radius, ${t_{\rm{p}}}$ is the full width at half maximum (FWHM) of the laser pulse, $\;$ and $\alpha ({r.z} )$ is the absorption coefficient of laser-induced plasma, water and target. ${u_{ev}}$ is the vaporization rate of target, ${L_{ev}}$ is the latent heat of vaporization, $\;\varepsilon $ is the emissivity of the surface, $\;\sigma $ is the Stefan-Boltzmann constant, h is the surface conductance and taken from Refs. [19], and ${T_{en}}$ is the environment temperature. Since most of the target region in the bottom is unaffected by laser, the heat loss of vaporization and radiation is assumed to occur only on the top surface of the target and could be neglected on the other surface. Also, the target reflection and absorption coefficients can be calculated by Eqs. (13) and (14) [6].

(13)$$R = \{{{{(n - 1)}^2} + {k^2}} \}/\{{{{(n + 1)}^2} + {k^2}} \}$$

(14)$$\alpha = 4\pi k/\lambda $$

The thermophysical parameters of the target and environment are shown in Tables 1 and 2, respectively.

The effect of the plasma shield on the distribution intensity at various fluences is determined using Eq. (12). The results are shown in Fig. 5 for 2,4,8 and 12 J/cm² laser fluences. At high fluences, the plasma shield created on the surface of the zinc target attenuates the incoming laser intensity and also reduces the ablation efficiency. According to the model results, the plasma shielding effect is significant for 8 and 12 J/cm² laser fluences.

Fig. 5. Comparison of incoming laser intensity before and after the attenuation of plasma shield at different laser fluences of 2, 4, 8 and 12 J/cm².

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Table 1. Thermal and optical properties of zinc metal

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Table 2. Thermal and optical properties of distilled water

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Figure 6 shows the temperature distribution diagram generated on the depth and surface of zinc metal for different laser fluences when it reaches its maximum temperature by a single laser pulse and 1064 nm wavelength.

Fig. 6. Temperature distributions in R–Z plane at the moment where the maximum temperature is reached with different laser fluences; (a) 2 J/cm², (b) 4 J/cm², (c) 8 J/cm², and (d) 12 J/cm².

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Figure 7 shows the time evolution of the central point temperature of the target surface for different laser fluences by a single laser pulse and 1064 nm wavelength. Based on this figure, the ablation thresholds for laser fluences of 2, 4, 8 and 12 J/cm² are shown at 7.1, 5.8, 4.5 and 3.8 ns, respectively. As shown, the maximum temperatures created on the surface of zinc metal for laser fluences of 2, 4, 8 and 12 J/cm² are equal to 4430, 9010, 12900, and 13400 °K, respectively. The maximum temperatures are calculated at 15.3, 15.2, 13.9 and 13.7 ns, respectively.

Fig. 7. Time evolution of the central point temperature of target surface at different laser fluences of 2, 4, 8 and 12 J/cm².

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The radius and depth of ablation can be obtained by calculating the velocity of surface recession during the evaporation of zinc metal and using the spatial range of the part of the material whose temperature has increased to a temperature higher than the boiling point of the material. Figure 8 shows ablation depth diagram of the zinc metal created by different fluences.

Fig. 8. The effect of IB plasma shield on the laser ablation depth of zinc metal for single shot and different laser fluences.

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As shown in Fig. 8, the erosion threshold for zinc metal sample occurs at 0.2 J/cm² fluence. As a result, at higher fluences (F> 12 J/cm²) the significant effect on increasing the ablation rate is not observed and a saturation state is achieved. This apparent saturation state can be related to the effect of plasma shield formation based on IB absorption. The model predictions are in good agreement with the obtained experimental results. According to the Eqs. (8) and (12), as the temperature increases, the density of free electrons in the vapors increases and, as a result, the plasma absorption coefficient increases and prevents the laser beam from reaching the target surface. It should be noticed that for simplicity we assume that the effects of the Mie adsorption coefficient and the photoionization absorption coefficient are negligible while the effects of these two processes can be effective for higher fluences above 15 J/cm². This can even lead to a slight reduction in the ablation efficiency observed in the obtained experimental results.

4. Conclusions

We have investigated the laser ablation of zinc metal target in distilled water environment for synthesis of ZnO NPs via experimental and numerical simulation approaches. Herein, we have analyzed the morphology, size distribution and productivity rate of the nanostructures obtained from laser ablation. Two forms of hexagonal crystal and amorphous nanosheets have been fabricated by interaction of zinc metal with a Nd:YAG laser with 1064 nm wavelength, 9 ns time duration, 1000 shot number and pulse energies in the range of 100–600 mJ. Precise measurements of the ablation efficiency for various laser pulse energies allow for determination of plasma shielding role on the reduced ablation efficiency for 600 mJ laser energy. In order to overcome the limitations of the experimental investigation technique due to lack of ultrafast measurement tools for exploring the dynamics of laser ablation, the theoretical thermal model of nanosecond pulsed laser ablation is developed. The evolution of temperature distributions, phase changes, plasma formation, and the efficiency of plasma shield are numerically investigated using two-fluid heat transfer model and finite difference method. Fairly good agreement between the simulation results and experiment measurements has been achieved. This model allows a better understanding of the physical process of laser ablation and plasma shielding and consequently the optimal parameters are achieved to overcome the limitations of the experimental investigation technique.

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.

Supplemental document

See Supplement 1 for supporting content.

References

1. A. M. Ealias and M. P. Saravanakumar, “A review on the classification, characterisation, synthesis of nanoparticles and their application,” in IOP Conference Series: Materials Science and Engineering (IOP Publishing, 2017), Vol. 263, p. 32019.

2. V. Amendola and M. Meneghetti, “What controls the composition and the structure of nanomaterials generated by laser ablation in liquid solution?” Phys. Chem. Chem. Phys. 15(9), 3027–3046 (2013). [CrossRef]

3. J. Xiao, P. Liu, C. X. Wang, and G. W. Yang, “External field-assisted laser ablation in liquid: An efficient strategy for nanocrystal synthesis and nanostructure assembly,” Prog. Mater. Sci. 87, 140–220 (2017). [CrossRef]

4. S. Dittrich, R. Streubel, C. McDonnell, H. P. Huber, S. Barcikowski, and B. Gökce, “Comparison of the productivity and ablation efficiency of different laser classes for laser ablation of gold in water and air,” Appl. Phys. A 125(6), 432 (2019). [CrossRef]

5. R. Brause, H. Möltgen, and K. Kleinermanns, “Characterization of laser-ablated and chemically reduced silver colloids in aqueous solution by UV/VIS spectroscopy and STM/SEM microscopy,” Appl. Phys. B Lasers Opt. 75(6-7), 711–716 (2002). [CrossRef]

6. S. Sinha, “Nanosecond laser ablation of Thoria fuel pellets for microstructural study,” J. Nucl. Mater. 396(2-3), 257–263 (2010). [CrossRef]

7. Z. Yan, X. Mei, W. Wang, A. Pan, Q. Lin, and C. Huang, “Numerical simulation on nanosecond laser ablation of titanium considering plasma shield and evaporation-affected surface thermocapillary convection,” Opt. Commun. 453, 124384 (2019). [CrossRef]

8. G. G. Guillén, M. I. M. Palma, B. Krishnan, D. Avellaneda, G. A. Castillo, T. K. D. Roy, and S. Shaji, “Structure and morphologies of ZnO nanoparticles synthesized by pulsed laser ablation in liquid: Effects of temperature and energy fluence,” Mater. Chem. Phys. 162, 561–570 (2015). [CrossRef]

9. E. A. Gavrilenko, D. A. Goncharova, I. N. Lapin, A. L. Nemoykina, V. A. Svetlichnyi, A. A. Aljulaih, N. Mintcheva, and S. A. Kulinich, “Comparative study of physicochemical and antibacterial properties of zno nanoparticles prepared by laser ablation of zn target in water and air,” Materials 12(1), 186 (2019). [CrossRef]

10. W. Chen, C. Yao, J. Gan, K. Jiang, Z. Hu, J. Lin, N. Xu, J. Sun, and J. Wu, “ZnO colloids and ZnO nanoparticles synthesized by pulsed laser ablation of zinc powders in water,” Mater. Sci. Semicond. Process. 109, 104918 (2020). [CrossRef]

11. B. Seipel, A. Nadarajah, B. Wutzke, and R. Könenkamp, “Electrodeposition of ZnO nanorods in the presence of metal ions,” Mater. Lett. 63(9-10), 736–738 (2009). [CrossRef]

12. R. Zamiri, A. Zakaria, H. A. Ahangar, M. Darroudi, A. K. Zak, and G. P. C. Drummen, “Aqueous starch as a stabilizer in zinc oxide nanoparticle synthesis via laser ablation,” J. Alloys Compd. 516, 41–48 (2012). [CrossRef]

13. H. Zeng, W. Cai, Y. Li, J. Hu, and P. Liu, “Composition/structural evolution and optical properties of ZnO/Zn nanoparticles by laser ablation in liquid media,” J. Phys. Chem. B 109(39), 18260–18266 (2005). [CrossRef]

14. Q. L. Ma, R. Xiong, B. G. Zhai, and Y. M. Huang, “Water assisted conversion of ZnO from metallic zinc particles,” in Key Engineering Materials (Trans Tech Publ, 2013), Vol. 538, pp. 38–41.

15. S. C. Singh and R. Gopal, “Synthesis of colloidal zinc oxide nanoparticles by pulsed laser ablation in aqueous media,” Phys. E Low-Dimensional Syst. Nanostructures 40(4), 724–730 (2008). [CrossRef]

16. D. Marla, U. V. Bhandarkar, and S. S. Joshi, “A model of laser ablation with temperature-dependent material properties, vaporization, phase explosion and plasma shielding,” Appl. Phys. A 116(1), 273–285 (2014). [CrossRef]

17. M. Stafe, C. Negutu, and I. M. Popescu, “Theoretical determination of the ablation rate of metals in multiple-nanosecond laser pulses irradiation regime,” Appl. Surf. Sci. 253(15), 6353–6358 (2007). [CrossRef]

18. R. Rozman, I. Grabec, and E. Govekar, “Influence of absorption mechanisms on laser-induced plasma plume,” Appl. Surf. Sci. 254(11), 3295–3305 (2008). [CrossRef]

19. D. Bäuerle, Laser Processing and Chemistry, 4th ed. (Springer Science & Business Media, 2011).

20. A. Bogaerts, Z. Chen, R. Gijbels, and A. Vertes, “Laser ablation for analytical sampling: What can we learn from modeling?” Spectrochim. Acta - Part B At. Spectrosc. 58(11), 1867–1893 (2003). [CrossRef]

21. S. Ghalamdaran, P. Parvin, M. Javad Torkamany, and J. Sabbagh Zadeh, “Two-dimensional simulation of laser ablation with 235 nanosecond pulses,” J. Laser Appl. 26(1), 012009 (2014). [CrossRef]

22. ThermTest Inc.Materials Database.Thermal Properties, “Materials Database - Thermal Properties - Thermtest Inc.,” https://thermtest.com/materials-database.

23. W. M. Haynes, CRC Handbook of Chemistry and Physics, 95th ed. (CRC press, 2016).

24. W. A. Deskin, Inorganic Chemistry: Principles of Structure and Reactivity (Huheey, James E.), 4th ed. (HarperCollins College Publishers, 1973), Vol. 50.

25. M. Querry, Optical Constants of Minerals and Other Materials from the Millimeter to the Ultraviolet (Chemical Research, Development and Engineering Center, US Army Armament …, 1987).

26. G. M. Hale and M. R. Querry, “Optical Constants of Water in the 200-nm to 200-µm Wavelength Region,” Appl. Opt. 12(3), 555 (1973). [CrossRef]

Parameters of zinc metal	Values	Ref.
Mass density, ρs (solid) (kg/m3)	7140	[22]
Mass density, ρl (liquid) (kg/m3)	6800	[22]
Melting point, Tm (K)	692.68	[23]
Boiling point, Tb (K)	1180	[23]
Specific heat capacity, Cp s (solid) (J/ (K Kg))	389	[22]
Specific heat capacity, Cp l (liquid) (J/ (K Kg))	502	[22]
Thermal conductivity, Ks (solid) (W/ (m K))	111.71	[22]
Thermal conductivity, Kl (liquid) (W/ (m K))	59.413	[22]
Latent heat of evaporation, ΔHV (kJ/mol)	115	[23]
First ionization potential (KJ/mol)	906.4	[24]
Molar mass, m (gr/mol)	65.38	[23]
Complex refractive index (wavelength = 1064 nm)	4.599 + 3.87i	[25]
Emissivity	0.6	[19]
Initial temperature T0 (K)	300

Parameters of distilled water	Values	Ref.
Mass density, ρ (kg/m3)	1000	[22]
Specific heat capacity, Cp (liquid) (J/ (K Kg))	4184	[22]
Thermal conductivity, K (W/ (m K))	0.603	[22]
Complex refractive index (wavelength = 1064 nm)	1.326 + 0.00000513i	[26]
environment temperature Ten (K)	300

Parameters of zinc metal	Values	Ref.
Mass density, ρs (solid) (kg/m3)	7140	[22]
Mass density, ρl (liquid) (kg/m3)	6800	[22]
Melting point, Tm (K)	692.68	[23]
Boiling point, Tb (K)	1180	[23]
Specific heat capacity, Cp s (solid) (J/ (K Kg))	389	[22]
Specific heat capacity, Cp l (liquid) (J/ (K Kg))	502	[22]
Thermal conductivity, Ks (solid) (W/ (m K))	111.71	[22]
Thermal conductivity, Kl (liquid) (W/ (m K))	59.413	[22]
Latent heat of evaporation, ΔHV (kJ/mol)	115	[23]
First ionization potential (KJ/mol)	906.4	[24]
Molar mass, m (gr/mol)	65.38	[23]
Complex refractive index (wavelength = 1064 nm)	4.599 + 3.87i	[25]
Emissivity	0.6	[19]
Initial temperature T0 (K)	300

Parameters of distilled water	Values	Ref.
Mass density, ρ (kg/m3)	1000	[22]
Specific heat capacity, Cp (liquid) (J/ (K Kg))	4184	[22]
Thermal conductivity, K (W/ (m K))	0.603	[22]
Complex refractive index (wavelength = 1064 nm)	1.326 + 0.00000513i	[26]
environment temperature Ten (K)	300

Investigation of ablation efficiency during the pulsed laser ablation of a zinc metal target in a distilled water environment

Abstract

1. Introduction

2. Experimental section

3. Numerical model and theoretical calculations

4. Conclusions

Disclosures

Data availability

Supplemental document

References

Supplementary Material (1)

Data availability

Cited By

Figures (8)

Tables (2)

Equations (19)

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