External electric field assisted laser-induced plasma and bubble dynamics for optimizing Mn₂O₃ nanoparticles as UV emitters

Abstract

To gain a deeper understanding of how the size of the nanoparticles synthesized by Electric Field-Assisted Laser Ablation in Liquids (EFLAL) changes with the applied electric field, we use Laser-Induced Breakdown Spectroscopy (LIBS) and a Beam Deflection Set-up (BDS). These tools help us examine how the electric field strength affects the plasma parameters and the bubble dynamics. The findings show that the electron density and plasma temperature are perturbed as the electric field strength can alter the interaction region. The strength of the electric field can cause the bubble size to increase, which also elevates its pressure and temperature. These alterations at the breakdown region lead to changes in the size, properties, and production of the Mn₂O₃ nanoparticles. Thus, the interplay of laser plasma and fluid-assisted bubble phenomena in the presence of an electric field is probed for Mn₂O₃ nanoparticles thereby optimizing as UV emitters.

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1 Introduction

The utilization of external fields in laser ablation in liquids (LAL) has gained prominence in recent decades. Techniques such as Electric Field-assisted Laser Ablation in Liquid (EFLAL), Electrochemistry-assisted Laser Ablation in Liquid (ECLAL), and Magnetic-Field-assisted Laser Ablation in Liquid (MFLAL) play a crucial role.¹ LAL is a versatile method that involves ablating materials in a liquid medium by directing an intense laser beam onto the material surface. This process vaporizes the surface, creating a partially ionized laser-induced plasma containing neutral atoms, ions, and electrons that expands at a supersonic speed against the liquid medium. Upon irradiation, the plasma is characterized by ionization, multiphoton ionization, the inverse Bremsstrahlung process, and simultaneously quanta absorption by the atoms to produce an ion–electron pair. By studying the plasma, valuable information about plasma temperature, electron density, and elemental composition can be extracted.²

At the interface between the hot plasma and water, a vapor layer—referred to as a bubble—forms. This bubble oscillates within the liquid until it dissipates its energy over several hundred microseconds, ultimately collapsing asymmetrically.³ The high kinetic energy of the plasma, coupled with the dynamics of the bubble, significantly influences the shape, size, morphology, and crystallinity of the nanoparticles synthesized through LAL.^4,5 Phase explosion plays a crucial role in generating nano-sized particles during LAL. This process involves homogeneous nucleation of the ablated materials from the irradiated targeted surface, which becomes overheated beyond the thermodynamic stability of the liquid phase threshold, leading to its swift breakdown into a combination of vapor and liquid droplets.⁶ Studies indicate that these particles are ejected both from the expanding plasma and the oscillating bubble.^4,7 Notably, most particles are expelled during the bubble's collapse.⁸ In our recent research, we also observed underwater laser-induced microbubbles known as persistent bubbles. These microbubbles serve as carriers for the laser-produced nanoparticles that are deposited onto substrates.⁹

Many studies have already been reported on the influence of the electric field on the size, shape, and crystallinity of the nanoparticles produced using LAL.^10–12 However, only a few studies have explored the impact of these external fields on plasma kinetics and bubble dynamics.^13–15 For this, a manganese target is subjected to ablation in water leading to the formation of Mn₂O₃ nanoparticles as reported in our previous studies.^16,17 Mn₂O₃ nanoparticles have been synthesized because of their appealing characteristics for optoelectronic applications, particularly in light-emitting devices, owing to their notable photoluminescence emission in the ultraviolet range of the electromagnetic spectrum.^18–20 The goal of this study is to focus on the influence of the electric field strength on the plasma parameters using laser-induced breakdown spectroscopy (LIBS).²¹ The electron density and plasma temperature are determined, and adjustments have been made to counter the self-absorption effect, to showcase the precision of the measurements. A Beam Deflection Set-up (BDS) is also employed to investigate the behavior of a bubble subjected to an external electric field.¹⁶ Additionally, the impact of the external electric field translating on the manganese oxide nanoparticles, synthesized using EFLAL, is scrutinized. The objective is to gain a more profound understanding of how the electric field at the breakdown region can affect the plasma and assisted cavitation bubbles. This will provide more insight into optimizing the material processing of synthesized (Mn₂O₃) nanoparticles for specific applications and the subsequent deposition of the energy-harvesting Mn₂O₃ nanoparticles on the electrodes.

2 Experimental set-up

Manganese (99.5% purity, Sigma-Aldrich) serves as the target material for ablation in distilled deionized water. The process of laser ablation involves focusing a Nd:YAG laser (Continuum Minilite ML II, wavelength = 532 nm, pulse duration = 6 ns, repetition rate = 10 Hz, and energy = 25 mJ) through a plano-convex lens (focal length = 10 cm). This setup generates a spot size of 229 μm and a laser fluence of 61 J cm⁻² onto the manganese target, as depicted in Fig. 1. For the Electric Field-assisted Laser Ablation in Liquids (EFLAL) experiment, a set-up comprising two parallel copper plates positioned 0.8 cm apart is employed. These plates are linked to the terminals of a direct current (D.C.) power supply using connecting wires, generating electric field strengths of 125 V m⁻¹, 1875 V m⁻¹, and 3750 V m⁻¹. The electrodes are securely mounted on holders attached to the bottom of the liquid cell, with the manganese target positioned equidistantly between both electrodes. To investigate the atomic and ionic emission lines, and the parameters of laser-induced plasma, a LIBS set-up is employed. The emission spectrum of the plasma is captured using an optical fiber (Ocean Insight, QP600-1-UV-VIS, diameter = 600 μm and length = 1 m). This fibre channels the emitted light to a calibrated spectrometer (Ocean Insight, HDX-UV-VIS Miniature spectrometer, integrated time = 100 ms, slit width = 10 μm, spectral resolution = 0.73 nm, and spectral range = 200–800 nm). Simultaneously, a collimating lens is positioned at the other end of the optical fiber. To filter out the 532 nm radiation of the incident laser, a notch filter (Thorlabs, NF533-17) is placed in front of the collimating lens. Both the collimating lens and the notch filter are oriented parallel to the plasma. The emission spectrum data are then transferred to a computer for subsequent analysis, where the identified emission peaks are cross-referenced with the NIST-LIBS database.²²

Fig. 1 Laser-induced breakdown spectroscopy (LIBS) in the presence of an external electric field.

To investigate the nanoparticles produced by EFLAL, following 15 minutes of ablation, the nano-colloidal solution is centrifuged and then analyzed using a Horiba Scientific Fluoromax-4 and PerkinElmer UV-Vis spectrometer Lambda 35 to examine the optical characteristics of the nano-colloidal solution produced at room temperature. Energy Dispersive X-ray spectroscopy (EDX) analysis is carried out using Oxford instruments (X-Max 65 T). The centrifuged nano-colloidal solutions, synthesized by EFLAL, are also drop-cast onto a carbon-coated nickel grid for transmission electron microscopy (TEM) analysis using a High-Resolution Transmission Electron Microscope (HR-TEM), (JOEL JEM-2100); the EDX is attached to the HR-TEM instrument.

3 Results

3.1 Laser-induced breakdown spectroscopy analysis

LIBS is a rapid, partially non-destructive, and straightforward technique used for elemental analysis and investigating the dynamics of the laser-induced plasma. Within the plasma, excited atoms and ions undergo de-excitation by emitting radiation, which LIBS captures as the emission spectrum. This spectrum is influenced by factors such as the elemental concentration in the plasma, the properties of the excitation source, and the surrounding environment.²³ From the spectrum obtained using the experimental set-up depicted in Fig. 1, we can determine the electron density and plasma temperature.

The plasma electron density N_e is derived from the hydrogen peak, specifically the H_α line at 656.6 nm wavelength. The linear Stark broadening significantly affects the full-width half maximum (FWHM) of the H_α line. To address this, a pseudo-Voigt profile as shown in Fig. 2 at (a) 125 V m⁻¹, (b) 1875 V m⁻¹, and (c) 3750 V m⁻¹ electric field strength is employed to fit the H_α line and to determine the Stark broadening Δλ_Stark induced by the perturbers. The relationship is given by²⁴

where Δλ_measured represents the FWHM of the H_α line obtained from the fittings [as shown in Fig. 2 for electric fields of (a) 125 V m⁻¹, (b) 1875 V m⁻¹, and (c) 3750 V m⁻¹], and Δλ_instrumental accounts for instrumental error. Subsequently, N_e is calculated using the methodology described in ref. 25.Here ω_s is the Stark broadening parameter of the H_α line at 656.6 nm wavelength. The values of Δλ_instrumental and ω_s are 0.73 nm and 0.549 nm respectively.²⁵

Fig. 2 The H_α line (normalized intensity), located at a wavelength of 656.6 nm, is fitted with a pseudo-Voigt profile. These measurements are performed at three different electric field strengths when a pulsed laser energy of 25 mJ is applied: (a) 125 V m⁻¹, (b) 1875 V m⁻¹, and (c) 3750 V m⁻¹.

The Stark broadening and plasma electron density, obtained using eqn (1) and (2), exhibit variation with the applied electric field, as summarized in Table 2 which is similar to other reported studies.^26,27 At 125 V m⁻¹, the electron density is 4.6 × 10¹⁷ cm⁻³. As the electric field increases to 1875 V m⁻¹, the electron density rises to 5.5 × 10¹⁷ cm⁻³. However, at 3750 V m⁻¹, the electron density decreases to 4.8 × 10¹⁷ cm⁻³.

Determining the plasma temperature involves analyzing the intensities of the Mn lines in the emission spectra as shown in Fig. 3 at different electric field strengths of (a) 125, (b) 1875, and (c) 3750 V m⁻¹. Fig. 3 also illustrates the emission spectral line of O I at 777.4 nm obtained on ablating at an electric field strength of (d) 125 V m⁻¹, (e) 1875 V m⁻¹, and (f) 3750 V m⁻¹.

Fig. 3 The LIBS spectra display Mn I spectral lines, and the insets include a Boltzmann plot. These spectra were captured under three varying electric field strengths: (a) 125 V m⁻¹, (b) 1875 V m⁻¹, and (c) 3750 V m⁻¹. The spectral emission lines of O I are for electric field strengths of: (d) 125 V m⁻¹, (e) 1875 V m⁻¹, and (f) 3750 V m⁻¹.

In plasma emission, re-absorption occurs—where the atoms and ions reabsorb emitted radiation. This re-absorption can distort the actual intensity and shape of the Mn lines. Therefore, correcting this self-absorption effect in the plasma emission spectra is essential. The ratio of the observed line intensity (in counts per second) at its maximum I(λ₀) to the corrected intensity I₀(λ₀) obtained by linear extrapolation at the actual elemental concentration of the linear part of the curve of growth is known as the self-absorption coefficient SA,²³

N_e(line) is the electron density of the Mn I line at wavelength λ₀ that is determined after calculating the FWHM of Stark broadening using the equation^2,28where w_s represents the Stark parameter of the corresponding Mn I lines.²⁹N_e(H_α) is the electron density determined using eqn (2). The SA coefficient varies from 0 to 1. At SA = 0, it is a completely absorbed line; at SA = 1, it is perfectly optically thin.

Once the intensity is determined with the self-absorption (SA) coefficient correction denoted by I₀(λ₀), the plasma temperature using a Boltzmann plot is calculated.² This approach assumes that the laser-induced plasma is in local thermodynamic equilibrium. (LTE) proposed by McWhirter should satisfy the criteria , where ΔE (eV) is the highest energy difference between the upper (E_k) and lower (E_i) energy states. The slope of the Boltzmann plot, given by ln[λ_oI₀(λ₀)/A_kig_ihc] versus E_k yields −1/k_BT, where k_B is the Boltzmann constant. This value provides the plasma temperature T_e utilizing the I₀(λ₀) values from the Mn I spectral lines. In this context, λ_o, A_ki, g_i, E_k, h, and c are the wavelengths corresponding to the optical transitions of Mn I, the transition probability, the statistical weight of the upper state, the upper state energy level, Planck's constant, and the speed of light respectively.²¹ The relevant parameters for determining the plasma temperature are detailed in Table 1.

Table 1 The parameters of Mn I spectral lines used for determining the plasma temperature

Element	λ (nm)	E i (eV)	E k (eV)	A ki g i (10^8 s^−1)
Mn I	403.3	0.00	3.08	1.40
Mn I	408.3	2.16	5.19	2.20
Mn I	423.9	2.94	5.86	0.78
Mn I	446.1	3.08	5.85	7.00
Mn I	476.4	2.89	5.49	7.83
Mn I	478.4	2.30	4.89	3.21
Mn I	482.2	2.32	4.89	3.99

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The plasma temperature values obtained from the slope of the Boltzmann plot, as shown in the insets in Fig. 3, are shown in Table 2, and are similar to other reported studies.^26,27,30 The plasma temperature increases from 4300 K to 7900 K on increasing the electric field from 125 V m⁻¹ to 1875 V m⁻¹. However, with a further increase in the electric field to 3750 V m⁻¹, the plasma temperature decreases to 6400 K.

Table 2 Variation of plasma electron density and temperature with the applied electric field

External applied electric field (V m^−1)	ΔλStark(nm)	N e (cm^−3)	T e (K)
125	3.14 ± 0.10	4.6 × 10^17	4300
1875	3.49 ± 0.09	5.5 × 10^17	7900
3750	3.19 ± 0.10	4.8 × 10^17	6400

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It is reported by Long et al. from the ns-resolved shadowgraph that the onset of cavitation bubbles starts in <30 ns.³¹ Tian et al. observed the continuum emission from the plasma persisting beyond 40 ns. Thus, it is highly likely that plasma constituents trapped inside cavitation bubbles get perturbed under the external electric field.³² Recent reports have shown that near the bubble wall, the localized peak electric field is ∼5300 V m⁻¹ for an applied electric field of 3750 V m⁻¹.¹⁶ This is substantially stronger to perturb the plasma than the localized electric field for lower applied electric field strengths (1875 V m⁻¹ and 125 V m⁻¹). Thus, a localized electric field during plasma formation facilitates the separation of charges, lowering the plasma density and temperature at higher applied electric fields.

3.2 Laser-induced bubble studies

The breakdown region is characterized by the plasma formation succeeded by the development of a bubble on several hundred-microsecond timescales, which expands, attains a maximum size, and subsequently contracts.² This bubble subsequently oscillates until it attains equilibrium with the surrounding liquid. To investigate the impact of an externally applied electric field on a laser-induced bubble, we employed a technique known as the beam deflection set-up as described in our previous work.¹⁶ In this current study, we varied the electric field strength from 0 V m⁻¹ to 3750 V m⁻¹ and conducted experiments at different laser energies (8 mJ, 15 mJ, 22 mJ, 24 mJ, and 25 mJ). By analyzing the beam deflection signal (ref. 16), the key parameters such as the maximum bubble radius, bubble energy, bubble pressure, and bubble temperature are determined. The bubble maximum radius R_m, as shown in Fig. 4(a), is determined using the equation^16,33Here t_c, P_∞, P_v, and ρ are the Rayleigh collapsed time, hydrostatic pressure, vapor pressure of the water, and density of water. Fig. 4(a) shows how the maximum bubble radius varies with the applied field at different laser energies. The maximum radius of the bubble formed on ablating manganese in distilled water rises with the external electric field strength at all five laser energies.

Fig. 4 (a) Bubble maximum radius and (b) bubble energy at various electric field strengths and laser energies.

The potential energy of the bubble is proportional to the cube of the maximum bubble radius as given by¹⁶

It also increases with the strength of the electric field at all the laser energies as shown in Fig. 4(b). The bubble energy rises considerably when the electric field strength increases.

The time-dependent pressure P(t)_B and temperature T(t)_B of the bubble, as it oscillates in water to dissipate its energy against the surrounding medium, are determined using Hardcore van der Waals equations^16,33

where R(t) is the time-dependent radius, d = R_m/9.174 is the estimated actual radius, and σ, δ_w, and T_∞ are the surface tension of water, the specific heat ratio of water, and the standard room temperature respectively.

Time-dependent pressure and temperature are obtained using eqn (7) and (8). Fig. 5 shows the variation of the peak (a) pressure P_p and (b) temperature T_p recorded when the bubble collapses (∼260 μs) on ablating the target at various laser energies in the presence of an external electric field. The pressure rises with the increase in applied electric field strength. The temperature follows a similar trend, increasing with the electric field strength and laser energy.

Fig. 5 (a) Pressure and (b) temperature at various electric field strengths and laser energies.

This finding supports the LIBS analysis, where the electron density and plasma temperature rise when the electric field strength increases from 125 V m⁻¹ to 1875 V m⁻¹. At an external electric field strength of 3750 V m⁻¹, the fluid dynamical processes continue with a similar trend, but the localized electric field starts perturbing the plasma lowering its temperature and density. The fluid dynamic-assisted bubble solely depends on the radius of the oscillating bubble. Therefore, the additional energy provided by the electric field (Fig. 4) is transferred to the bubble, causing an increase in its size and subsequently, its pressure and temperature (Fig. 5). When the bubble collapses, its pressure peaks. This pressure is transferred to the liquid, causing an increase in the liquid pressure, which significantly affects the growth of nanoparticles. The pressure of the liquid also increases with the increase in the electric field strength. As previously mentioned, the oscillation of the bubble sways the movement of the microbubbles in the liquid medium. These microbubbles eventually collide with the surface of the substrate, which is placed parallel to the expanding plasma and the laser beam, leading to the deposition of the synthesized nanoparticles on the substrate. This deposition increases with the electric field strength because the gradient in the electric field due to the non-uniform electric field beyond the bubble wall propels the microbubbles toward the electrodes.⁹

3.3 Nanoparticle characterization

The optical studies reveal two broad humps in the UV-Vis spectra at 230 nm and 280 nm, revealing an electronic π → π* transition, as shown in Fig. 6(a), that occurs when an electron transitions from a filled to a vacant band within the cubic bixbyite α-Mn₂O₃ nanoparticles synthesized by EFLAL.³⁴ On increasing the applied field, the hump at 280 nm merges with the tail of the broad hump at 230 nm. An increase in the concentration of α-Mn₂O₃ nanoparticles is evident by changes in the absorbance spectra. These changes reflect a higher concentration of the sample prepared under the influence of an external electric field. This phenomenon aligns with the findings of H. Mozaffari et al., who proposed that the presence of an electric field enhances the ablation rate of the targeted surface, increasing the concentration of the nano-colloidal solution.³⁵

Fig. 6 (a) UV-Vis and (b) photoluminescence spectra of the nanoparticles formed by EFLAL at 25 mJ laser energy.

The photoluminescence (PL) emission spectrum of the prepared samples in the presence of an electric field exhibits four distinct peaks, as observed in Fig. 6(b). The 395 nm peak is a deep-level ultraviolet emission that arises from the radiative annihilation of excitons within the α-Mn₂O₃ nanoparticles. The blue-green peaks at 434, 470, and 545 nm correspond to the radiative recombination of a photogenerated hole with an electron occupying a vacant oxygen site. They signify the presence of defect states within the synthesized α-Mn₂O₃ particles.²⁰ There is a red shift when the electric field strength is 125 V m⁻¹, and the PL spectra shift by 10 nm towards longer wavelengths. Specifically, the peaks now occur at 405, 444, 480, and 555 nm. In addition, as the applied field strength increases, the intensity of the defect state at 470 nm reduces, while the annihilation peak at 395 nm becomes more pronounced, indicating a better crystallinity in the sample prepared with higher electric field strength. This behavior also underscores the fact that α-Mn₂O₃ nanoparticles exhibit excellent UV-emitting properties.²⁰

The elemental analysis of Mn₂O₃ nanoparticles formed during the ablation of a manganese target in distilled water under the influence of an external electric field using the Energy Dispersive X-ray (EDX) characterization technique reveals an intriguing trend. It shows that as the applied electric field strength increases (as depicted in Fig. 7 for (a) 125 V m⁻¹ and (b) 1875 V m⁻¹), the manganese content also rises. This observation supports the UV-Vis spectra results, which indicate that the presence of an external electric field enhances the ablation rate of manganese, resulting in a higher yield of the prepared sample.³⁵Fig. 7(c) shows that the proportion of Mn decreases at an electric field strength of 3750 V m⁻¹ contrary to the UV-Vis measurements. This is attributed to self-absorption by the primary X-rays during EDX measurements. To mitigate this, the sample thickness should be minimized.³⁶ The sample dropcast onto the carbon-coated nickel grid is thicker due to a more significant number of synthesized nanoparticles, leading to increased self-absorption and, consequently, a reduction in the weight percentage of Mn.

Fig. 7 Energy dispersive X-ray (EDX) spectra of Mn₂O₃ nanoparticles formed by EFLAL at (a) 125 V m⁻¹, (b) 1875 V m⁻¹, and (c) 3750 V m⁻¹ when the laser energy of 25 mJ is applied.

The transmission electron microscopy (TEM) image reveals the creation of manganese oxide nanoparticles at the nanoscale using EFLAL at (a) 125 and (b) 1875 V m⁻¹ as shown in Fig. 8. The scale bar in the image corresponds to 100 nanometres. Analysis of the transmission electron microscopy (TEM) images reveals a clear trend that as the electric field strength increases, the size of the nanoparticles decreases. This observation is further supported by the experimental and theoretical particle size distribution data from our previous study, which demonstrates that the presence of an electric field (at 3750 V m⁻¹) leads to a drop in the nanoparticle size.¹⁶

Fig. 8 TEM images of the nanoparticle formed on ablating the manganese target in distilled water using a laser energy of 25 mJ at an electric field strength of (a) 125 and (b) 1875 V m⁻¹. HR-TEM and SAED diffraction ring patterns of the samples are shown in the insets.

Additionally, the high-resolution TEM (HR-TEM) image, with a scale bar of 2 nm, along with the selected area electron diffraction (SAED) pattern shown in the insets, offers valuable insights into the interplanar spacing and diffraction rings of the cubic bixbyite α-Mn₂O₃ nanoparticles. Specifically, the interplanar spacings of 0.26 nm, 0.21 nm, 0.15 nm, and 0.11 nm correspond to the diffraction ring patterns of the (222), (204), (622), and (662) crystallographic planes within the cubic bixbyite α-Mn₂O₃ nanoparticles.^16,37

4 Conclusion

LIBS research shows that the electron density and plasma temperature are perturbed when the electric field strength reaches 3750 V m⁻¹. This leads to a decline in electron density and plasma temperature which is attributed to a reduction in the number of particles within the interaction region, as the charged particles are disturbed by the stronger electric field. To ensure higher precision, plasma temperature measurements are adjusted for self-absorption. The radius, pressure, and temperature of fluid-assisted, laser-induced bubbles show upward trends in response to changes in laser energy and electric field strength. Examination of nanoparticles created using EFLAL indicates that their size decreases with an increasing electric field, while nanoparticle production is boosted due to a rise in the ablation rate caused by the electric field. Furthermore, the defect states of laser-produced nanoparticles are substantially reduced exhibiting a pronounced annihilation peak at higher applied electric field strengths of 3750 V m⁻¹. This signifies the requirement of optimization in external electric fields for plasma-meditated synthesis of Mn₂O₃ nanoparticles as powerful UV emitters.

Data availability

The data will be made available by the authors upon request through email.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors sincerely acknowledge DST-SERB, India (Project File No. ECR/2017/001705) for financial support and SAIF NEHU, Shillong, India for performing the TEM analysis.

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