Stoichiometry of laser ablated brass nanoparticles in water and air D. N. Patel,1,* Pramod K. Pandey,2 and Raj K. Thareja1 1 2
Department of Physics, Indian Institute of Technology, Kanpur, India
WCI-Center for Quantum-Beam Based Radiation Research, Korea Atomic Energy Research Institute, South Korea *Corresponding author: [email protected]
Received 2 April 2013; revised 22 September 2013; accepted 23 September 2013; posted 7 October 2013 (Doc. ID 188111); published 29 October 2013
We report on the stoichiometric analysis of laser ablated brass plasma nanoparticles (NPs) in water and ambient air. Morphological study of the deposited NPs in water showed smaller spherical NPs compared to micrometer sized spherical particles in air. The smaller particles were Zn enriched and the concentration decreased with increases in size. Photoluminescence of particles at 380 nm corresponding to ZnO showed higher concentrations of Zn with smaller sized deposited NPs, whereas the micrometer sized particles showed multiple peaks at 415 and 440 nm, which implied that there was an abundance of the Cu fraction in the NPs. Plasma plume parameters, electron temperature, electron density, and evolution of the plasma plume were studied using optical emission spectroscopy and 2-dimensional imaging of the plume. The mass ablation rate in water was observed to be greater than that in air. Higher electron density and temperature of the plasmoid in water was attributed to confinement of the plasma plume near the target surface in water. © 2013 Optical Society of America OCIS codes: (140.3440) Laser-induced breakdown; (300.2140) Emission; (300.6500) Spectroscopy, time-resolved; (350.5030) Phase; (350.5400) Plasmas. http://dx.doi.org/10.1364/AO.52.007592
In recent years, tremendous research efforts have been put forth to generate and characterize nanoparticles (NPs). Nanoparticulate materials and composites are being used in biosensing, medical diagnostics, therapeutic in vivo applications, and several opto-electronic applications because of their size-specific and unique properties [1–3]. Several techniques such as aqueous chemical synthesis, hot thermolysis/colloidal synthesis, photochemical synthesis, and sol gel methods  have been proposed to synthesize NPs. Synthesis of NPs using chemical routes has an inherent drawback of contributions from reducing reactions, i.e., the liquid always contains other ions and reaction products that are difficult to separate from the liquid. Laser 1559-128X/13/317592-10$15.00/0 © 2013 Optical Society of America 7592
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ablation is an alternative method for the generation of nanoclusters/nanostructures and the nanostructuring of surfaces . This technique is based on ablation of a solid target using intense laser radiation, which leads to the ejection of target constituents and the formation of nanoclusters. NPs/nanoclusters can be deposited on a suitably placed substrate at some distance from the target to get nanostructured thin-films [6–8]. Laser ablation of a solid target in a liquid medium is a simple way to generate NPs. The ejected NPs remain in the liquid where they form a NP colloidal solution . Laser ablation of a solid target in water results in debris-free deposition of NPs/microstructures that lack any surface active substances and counter ions. Nanostructuring of an irradiated target is due to the close proximity of a dense medium to the molten layer of the target, which acts as source for instabilities in the melt and viscous flows of the vapors of the surrounding liquid . However, laser
ablation of solids in liquid environments for NP generation of binary alloys such as brass (Cu–Zn), bronze (Cu–Sn), and other alloys is somewhat problematic owing to differences in melting temperatures and chemical properties of the constituents . The NPs formed during laser ablation have the same composition as that of the solid irradiated target; however, a problem arises when the alloy targets are irradiated. For example, in the case of metallic alloys (e.g., Al alloys, bronze, and brass), various constituents of the alloys have different thermal conductivities, and hence, the stoichiometry of the ablated material may in turn affect the formed NPs because of condensation from the ablated plume. The stoichiometry of the ablated plume approaches the sample compositions for higher laser fluence . Henglein and Cotton [12–14] reported pulsed laser ablation of pure metal targets in various solvents. Ablation with femtosecond laser pulses involves direct solid vapor transitions and NPs are formed during fragmentation of a highly pressurized fluid undergoing rapid quenching during expansion . Laser ablated inductively coupled plasma mass spectroscopy (LA-ICP-MS) has been used for elemental analysis of solid materials; however, one needs matrix matched standards for calibration . Mao et al.  looked at stoichiometry issues of brass during laser ablation with inductively coupled plasma atomic emission spectroscopy and concluded that ultraviolet (UV) short laser pulses (picoseconds) with high power density are better suited for chemical analysis and stoichiometric ablation. Pulsed laser deposition (PLD) of alloys using multicomponent targets has shown a noncongruent transfer of target composition primarily due to differential scattering in the plume itself, though the average composition of the particulates in the film has the stoichiometry of the bulk target . The ablation of brass samples using high power femtosecond and UV lasers has been shown to reduce fractionation during ablation [19,20]. The study of laser ablation of solids, especially, metals in liquid environments, has mainly focused on the preparation of NPs via laser ablation [20–25] and laser-induced modifications of the size and shape of the NPs [26–31]. We performed a comparative study of laser ablation using 1064 nm alloys (brass) in air and a liquid environment to optimize the parameters needed for NP generation of the required stoichiometry, size, and shape. Depending on physical properties of the target, such as thermal conductivity, diffusivity, and reflectivity, and the laser parameters, such as pulse width, shape, and wavelength, an intense local heating of the surface will result in a sharp rise of the surface temperature of the material, which leads to material vaporization and the formation of plasma . The spatial period of the structures formed during laser ablation of a solid target immersed in water depends on numerous experimental parameters including the laser fluence, the nature of the surrounding liquid, and the laser spot size [9,33].
In the present work, we report on the laser ablation of brass in water and ambient air and conduct a stoichiometric analysis of the corresponding deposited NPs. An attempt was made to correlate laser ablated brass plasma parameters like electron density and electron temperature with the stoichiometry of the deposited NPs. The photoluminescence (PL) spectra were used to reveal the optical properties of the deposited NPs in different ambient environments. This paper is organized as follows. Section 2 gives the details of the experimental setup. The results and discussion are presented in Section 3 and the conclusions are presented in Section 4. 2. Experimental Setup
Figure 1 shows a schematic diagram of the experimental setup used. A Q-switched Nd-YAG laser (DCR-4G, Spectra Physics) was used for ablation of solid brass targets. The laser had a maximum energy of 600 mJ∕pulse, a fundamental wavelength of 1064 nm, and a pulse width of 8 ns at a repetition rate of 10 Hz. The laser intensities used in this study were 1, 2.9, 5.7, 8.6, 11.4, and 14.3 GW∕cm2 . To produce the plasma in ambient water/air, the laser beam was focused onto the target (brass) surface using a 25 cm focal length lens. The optical emission from the plasma plume was recorded by imaging the plume onto an array of fibers placed perpendicular to the direction of plasma expansion and coupled with a spectrograph (Shamrock SR 303i, ANDOR Technology, USA) with a gated intensified charged-coupled device (ICCD, DH-720, ANDOR Technology, USA) using a quartz lens so as to have a 1∶1 image of the plume. The data collecting system (i.e., spectrograph equipped with ICCD) was monitored by a computer using ANDOR i-Star software. In order to record plasma plume images, the collecting lens and optical fiber array were replaced with an externally triggered ICCD interfaced with a computer. The structural and morphological properties of the material deposited on the target itself and NPs collected in the liquid were investigated by use of a scanning electron microscope (SEM) (SUPRA 40 VP Zeiss). The stoichiometry of the particles was
Fig. 1. Schematic of the experimental setup. 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
analyzed using energy-dispersive x-ray (EDX) analysis. The optical properties of NPs were studied using PL with an excitation wavelength of λex 355.0 nm (Fluorolog (R)-3 Jobin Yvon Horiba) and Raman measurements with an excitation wavelength of 515 nm (Witec). 3. Results and Discussion
In order to understand the stoichiometry of the NPs, it was imperative to have a comparative study of the laser ablation results for brass in water and air. Therefore, we did an extensive investigation of the mass ablation rate of the target material in ambient water and air, plume dynamics, and spectroscopic and optical measurements. A.
Laser ablation of solid targets in liquid environments is an intriguing technique for producing micro/NPs, and it is imperative to have an in depth understanding of the ablation mechanism, i.e., how the material is librated from the targets. It has been observed that ablation of a target is mitigated by the strong absorption of the laser during the breakdown of plasma in environments composed of water . The intense laser pulse irradiating the target raises the surface temperature of the target and the heat generated at the target surface is confined within the thermal diffusion length Lth 2Dth τL 1∕2 , where Dth is the thermal diffusion constant (Dth K∕ρc, here,ρ is the density of the material and K is the thermal conductivity) and τL is the pulsed width of the laser. Depending on the laser irradiance, the surface of the target can be melted and liquid can successively undergo normal heating processes, superheating, and explosive phase changes. The surface temperature of the target is given by [35,36] 1 − RI Tz; t T 0 2 ρC
r t z ierfc p ; Dth 2 Dth t
∼105 K, plasma is initiated at the water and brass target boundary where it equilibrates quickly with the surrounding water. During this process, some of the water vaporizes or dissociates, which generates an assembly of bubbles at the center of the ablation area . This leads to smooth region surrounding the crater dig as shown in Fig. 2(a). Figure 2 shows the SEM images of the crater dig formed by laser ablation of brass in water and ambient air along with the corresponding depth profiles. The images were taken after an exposure of 400 shots of the laser on the sample. The crater diameter in air was larger compared to that in water. The radiation incident on the surface of the water induces a radiation pressure force (∼2 × 10−2 N) that is much greater than the surface tension force of water (∼2 × 10−4 N), which results in a strong surface lens effect . The water layer in contact with the beam becomes curved and acts as a converging lens that causes smaller focused diameters in water. The fluence was maintained constant by taking into account an absorption coefficient of water of ∼0.1 cm−1 for 1064 nm during the experiment. We observed a larger ablation depth of ∼235 μm with a clean crater profile in water compared to ∼23 μm of ablation depth in air for 400 shots of laser pulses as shown in Figs. 2(a) and 2(c). The ablation rate was estimated by assuming a conical shape of the crater with radius r and depth h. The ablated mass was calculated by multiplying the density of the material by the ejected volume, 1∕3πr2 h. The ablation rates in water and air were ∼1.16 × 10−6 g∕s and ∼0.61× 10−6 g∕s, respectively. In order to understand the larger crater depth as well as the large ablation rate in water, we looked at the confinement of the plasma in two ambient environments using imaging and spectroscopy of the ablated plumes. It has been reported that the rate of temperature rise at the irradiated surface is much greater than that necessary
where T 0 is the temperature at time t 0, R is the reflectivity of the material, I is the laser intensity, Dth is the thermal diffusivity, ρ is the density of the material, C is the heat capacity of the material, “ierfc” is the inverse of the complementary error function, “z” is the coordinate parallel to the laser beam, and t is the time. Equation (1) can be used to get the surface (z 0) temperature of the target: T0; t T 0
21 − RI p tDth ∕π : K
Using values of various parameters in Eq. (2), including an 8 ns laser pulse width, R 0.8 reflectivity of brass , 109.9 W∕mK thermal conductivity , and 3.752 × 10−5 m2 ∕s thermal diffusivity , we obtained a surface temperature of brass equivalent to ∼105 K. Once the surface temperature rises to 7594
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Fig. 2. SEM images of the brass crater at 40 mJ pulse energy and 400 laser shots in (a) water and (b) air and the corresponding depth profiles in (c) water and (d) air.
Fig. 3. (a) Mass ablation rate as a function of energy and (b) ablated depth with number of shots at 40 mJ energy in water.
for a phase explosion to occur ∼109 K∕s . Rapid temperature increase during ablation produces cavitation bubbles in the water. The shock wave generated by bubble implosion and liquid jet formation during bubble collapse generates a very high pressure impact on the target . In order to calculate the plasma pressure in the ambient liquid, we used the Hugoniot relation : P kbar 0.1
α 2α 3
1 2 ZI 0 ;
linearly. This peculiar behavior may be due to less ablated mass dissolved in water at lower energies that does not affect the transmissivity of water for the laser radiation to reach the target surface. At laser energies greater than 20 mJ, the suspended particles in the water act as scattering centers; also, part of the energy goes towards dissociating the water [45,46]. The presence of ablated material, which is largely suspended nano/microparticles, and the dissociation of water inhibit radiation from reaching the target. Since the volume of water probed by the laser beam is the same for each pulse and the amount of energy required to dissociate the water is greater than 20 mJ (∼2.5 GW∕cm2 ), the energy deposited on the target decreases because of the dissociation of water. Furthermore, increases in irradiation cause ionization of the water resulting in UV emissions that are reabsorbed by the target, thereby enhancing the ablation rate. B. Plume Imaging
where α is the correction factor (0.2 for water and 1.0 for air confinement), Z g∕cm2 s is the shock impedance of the target-ambient system, and I 0 GW∕cm2 is the incident laser intensity. The shock impedance for the surroundings and the target were: water Zw 0.165 × 106 g∕cm2 s , air ZAir 40 g∕cm2 s, and brass ZBrass 3.67 × 106 g∕cm2 s. The estimated shock pressures in water and air were 3.14 GPa and 96 MPa, respectively. At high pressures and temperatures, the molten layer becomes less viscous, which results in intense shock wave generation that hammers the solid target resulting further ablation. Figure 3(a) shows the ablation rate of brass as a function of the incident laser energy. The ablation rate measurement was based on the average results of multiple pulses. Figure 3(b) shows an increase of depth with the number of laser shots. It follows from Fig. 3(a) that the mass ablation rate increases linearly attaining a maximum value at 20 mJ of laser energy, beyond which it starts decreasing to reach a minimum at 40 mJ, and then, again increases
In order to investigate the confinement of the plasma plume in liquid and air near the target surface, we performed plume imaging in water/air and plotted the plume front position verses time (R-t plot) (Fig. 4). The plasma plume experienced higher background pressure in water as compared to that in ambient air. Because of the higher background pressure, the plasma plume dragged slowly into the background surroundings. The plasma plume expansion in water and air followed the drag model. It was clear from Fig. 4 that plume confinement was more pronounced in water than in air. The plasma plume expanded up to 1.1 mm in water compared to 1.3 mm from the target surface in ambient air. It follows from the R-t plot in Fig. 4 that the plume in water stops near the target surface and the high pressure (3.14 GPa) and temperature results in a high ablation rate in water as discussed above. C.
Optical Emission Spectroscopy
To gain more insight into the nature and constituents of expanding plasma plume characteristics induced by laser ablation of brass using a 1064 nm Nd-YAG laser, we performed time resolved optical emission spectroscopic (OES) studies of the expanding plasma
Fig. 4. Brass plasma plume images in water and ambient air and the corresponding R-t plots. 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
plume in water and air. The spectra were taken at a 1 mm distance from the target surface. Figures 5 and 6 show the time evolution emission spectra of the Cu I and Zn I lines, respectively, at different laser energies viz 10, 20, 30, and 40 mJ in water. Figure 5 shows the variation of the Cu I (4p3 P3∕2 − 4s 2 D5∕2 , 4d 2 D3∕2 − 4p 2 P1∕2 , 4d 2 D5∕2 − 4p 2 P3∕2 ) line intensity with incident laser energy. The intensity of emitted lines increased with input energy attaining a maximum at 20 mJ. Then, the intensity decreased because at higher laser energies the transitions may be absorbed during water breakdown. Similar behavior was also seen for Zn I transition with various laser energies (Fig. 6).
Figure 7 shows emission of Cu I (510.5, 515.3, and 521.8 nm) atomic lines (transitions 4p3 P3∕2 − 4s 2 D5∕2 , 4d 2 D3∕2 − 4p2 P1∕2 , 4d 2 D5∕2 − 4p 2 P3∕2 ) and Zn I (468.01, 472.2, and 481.1 nm) lines (transitions 4s5s 3 S1 − 4s4p 3 P0 , 4s5s 3 S1 − 4s4p 3 P1 , 4s5s 3 S1 − 4s4p 3 P2 ) in water and air, respectively. The line intensities of Cu I and Zn I diminish because of absorption by the medium (water). However, it was observed that in water, the ambient Cu I line at 510.5 nm dominated over the other Cu I lines (515.3 and 521.8 nm); whereas in air, there was a dominance of Cu I lines at 515.3 and 521.8 nm over 510.5 nm. It has been reported that the lifetime of Cu (515.3 nm) decreases gradually with increases in
Fig. 5. Time evolution of the Cu I line at different energies. The lines shown in the spectrum are 4p 2 P3∕2 − 3d9 4s2 2 D5∕2 at 510.5 nm, 4d 2 D3∕2 − 4p 2 P1∕2 at 515.3 nm, and 4d 2 D5∕2 − 4p 2 P3∕2 at 521.8 nm.
Fig. 6. Time evolution of the Cu I and Zn I lines at different laser energies. The lines shown in the spectrum are Cu I at 465.1 nm (4s5s 4 D7∕2 − 4s 4p 4 F 9∕2 ), Zn I at 468.0 nm (4s5s 3 S1 − 4s 4p 3 P0 ), 472.2 nm (4s5s 3 S1 − 4s 4p 3 P1 ), and 481.1 nm (4s5s 3 S1 − 4s 4p 3 P2 ). 7596
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Fig. 7. Time evolution of the Cu I lines at 510.5 nm, 515.3 nm, and 521.8 nm in (a) water and (b) air and Zn I lines at 472.2 nm and 481.1 nm in (c) water and (d) air.
the ambient pressure . The reduction in lifetime is likely due to increased collisions induced by rapid cooling of the plasma at higher pressures . In our case, in water the pressure during plasma expansion was very high compared to air. Therefore, at higher pressure due to the enhanced collisional events between Cu–Cu and Zn–Cu atoms, the energized Cu atoms lose their energy through nonradiative transition and populate the 2 P energy level (3.82 eV) with an effective decay route of the 2 P state through the 2 D state. The corresponding decay time was rather long (0.5 μs) because the transition 4p3 P3∕2 − 4s 2 D5∕2 is weakly allowed . Hence, a relatively stronger line at 510.5 nm was found in water (Figs. 7(a) and 7(b)). Whereas, if we observe the Zn I line transitions (Figs. 7(c) and 7(d)) in water, the line intensity was almost negligible in comparison to the ambient air. This may be due to zinc being more reactive than copper; hence, Zn atoms react with CuO to form ZnO rather than emitting radiation. The oxidation state of Cu2 is more stable than Cu . Thus, in water there is more of an abundance of CuO than cuprous oxide. Zn reacts with CuO to form ZnO by releasing the Cu atoms, which may be the reason for the feeble intensity of Zn I lines in comparison to Cu I lines in water. The temperature and density of the plasma was estimated using the ratio of the intensity of spectral lines and Stark broadened spectral lines, respectively . Assuming the plasma is in local thermodynamic equilibrium (LTE), the electron temperature can be determined using a Boltzmann plot  ln
I ki λ gk Aki
1 hcne ; Ej ln 4πUT e kB T e
where I ki is the intensity corresponding to the wavelength λ having degeneracy of the upper state gk and
transition probability Aki. Ej is the atomic energy of the upper state, kB is the Boltzmann constant, and T e is the electron temperature. h is Planck’s constant, c is the velocity of light, ne is the electron density, and UT e is the partition function. The electron temperature can be calculated easily from the slope of the plot of lnI ki λ∕gk Aki versus Ej of Eq. (4). The various parameters used in Eq. 4 were taken from the literature . We used Cu I transitions 4s5s 4 D7∕2 − 4s4p4 F 9∕2 at 465.11 nm, 4p 2 P3∕2 − 3d9 4s2 2 D5∕2 at 510.55 nm, 4d 2 D3∕2 − 4p 2 P1∕2 at 515.32 nm, 4d 2 D5∕2 − 4p 2 P3∕2 at 521.82 nm, and 4p 2 P3∕2 − 3d9 4s 2 D3∕2 at 570.02 nm for calculating the electron temperature. The estimated temperatures using Cu I lines were higher (1.12 eV) in air than in water (0.66 eV) after 500 ns with respect to the ablating pulse. The electron temperature calculated using the relative line intensities yielded a low electron temperature of water because of the very feeble line intensity in water. The rapid cooling of the plasma plume in water may also act to decrease the electron temperature. Plasma electron densities corresponding to Cu I 4p 2 P3∕2 − 3d9 4s2 2 D5∕2 at the 510.5 nm transition were calculated using their Stark broadened profile . The density was estimated at times where discrete lines were observed; hence, the plasma was optically thin (Figs. 5–7). Moreover, the observed lines did not show saturation/dip at the transition center frequency, which is characteristic of self absorption. Since Stark broadening is weakly dependent on the temperature, the full width at halfmaximum (FWHM) Δλ1∕2 of the Stark broadened line is defined as 
ne nm 2W ; 1016
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Fig. 8. Temporal evolution of electron density in (a) water and (b) air. In both cases, electron density followed an exponential decay with a mean lifetime of 99 12 ns in water and 86 8 ns in air.
where W is the electron impact parameter and ne is the electron density in cm−3 . The electron impact parameter W is a weak function of temperature and its value is specified in the literature . Figure 8 shows the time evolution of electron density at 1 mm from the target surface calculated using the Stark broadened Cu I (λ 510.5 nm) line. Figures 8(a) and 8(b) show the temporal evolution of the electron density in water and air, respectively. The time evolution of electron density followed exponential decay in both water and ambient air. The mean lifetime (99 12 ns) in water was longer compared to air (86 8 ns). The electron density was found to be three times higher in water than in ambient air. The higher electron density in water can be attributed to the combined effect of plasma and the contribution from the background . The higher electron density in water may be due to the mixing of laserinduced plasma and plasma-induced plasma. Morphological and optical properties of the deposited NPs onto the target were examined using SEM and PL spectra. Figure 9 shows SEM images of the NPs deposited back onto the target surface during
laser ablation in water and air and their corresponding particle size distributions on the target. The particle size was measured using commercially available Image J software and the particle distributions were fitted with the Gaussian profile. We observed that the deposited particles in water were of an average particle size of ∼55 nm, which was larger than the value of ∼2.5 μm in air. As explained, the molten layer undergoes phase explosion because of the very high pressures and temperatures, which creates an intense wave that hammers the solid target resulting in further plasma-induced ablation. The vaporized water in the form of a bubble assembly and the bubble motion contributed to the removal of ablated particles, which were redeposited on the target surface. In order to study the stoichiometry, we performed an EDX analysis on the deposited NPs in water and air. Figure 10 shows variations in the Zn/Cu ratio for deposited NPs with different particle diameters. It was found that for smaller sized particles, the Zn/ Cu ratio was roughly the same in both the ambient conditions, but for large sized particles, the Cu percentage increased with particle size in air. The large size particles were absent in water. It was not possible to measure the Zn/Cu ratio for the smaller sized particles deposited in water because of experimental limitations. From Fig. 10, it can be inferred that as the NP size decreases, the Zn/Cu ratio increases . Assuming that the pulse hits the target at time t 0, the absorption of the laser pulse by the bulk solid target occurs from t 0 to τp (pulse width). The laser light penetrates into the target material and forms a molten pool of depth Dth τL 0.5 , where Dth is the thermal diffusivity and τL is the laser pulse width. For nanosecond laser pulses, thermal processes
Fig. 9. SEM images of NPs of brass deposited back onto the target surface in (a) water and (b) ambient air and particle size distributions in (c) water and (d) air. 7598
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Fig. 10. Zn/Cu ratio versus particle diameter in (a) water and (b) air.
including melting, boiling, vaporization, and thermionic emission occur simultaneously with photoionization. Furthermore, increases in temperature due to increases in the irradiance results in evaporation, and eventually, the formation of plasma when the energy deposited approximately equals the latent −1∕2 heat of sublimation  Ls Iτp1∕2 ρ−1 Dth , where ρ is the density of the solid target and I is the laser intensity on the target. Laser heating causes elements of higher volatility to vaporize quickly, which leaves melted residue that contains low volatility elements . Consequently, the particles formed by melt ejection or from vapor condensation would have a different chemical composition. For nanosecond pulsed laser ablation, most of the mass removed (∼95%) is from melt ejection . It has been shown that smaller particles are formed by the nucleation and condensation of vapors, while bigger particles may be ejected from the melted liquid pool because of surface instability or recoil pressure from the expanding vapor plume . The liquid droplet is ejected from the melted pool when the acceleration of the thermal expansion of liquid exceeds the force holding the liquid to the surface . The stoichiometry of the particle changes because of the deposition of the smaller condensate particles on the ejected droplets. It follows from the vaporization rate p J P M∕2πkB T, where P is the vapor pressure of the element, M is the mass of the atom, kB is the Boltzmann constant, and T is the temperature, that the higher vaporization rate of Zn compared to Cu increases the mole fraction of Cu in the melted liquid layer forming the larger Cu enriched particles. However, in case of the water, nanosized particles were observed that were Zn enriched. These NPs were formed by the condensation of the ablated species. Since the quenching time of the plasma in water is shorter than in air, the early vaporization of the Zn condenses to form the NPs, which were mostly Zn enriched. Figure 11 shows the PL spectra of a fresh (nonexposed) brass target and that of deposited NPs on the target in water and ambient air. An excitation wavelength λex 355 nm(Eex 3.5 eV) was used to excite the samples. The observed PL spectra for the fresh sample showed a peak at λ 380 nm (3.26 eV) corresponding to PL peak of ZnO . Since Zn has more reactive properties than copper with oxygen, on the surface of the fresh brass target there may be dominating ZnO formations rather than
CuO, and hence, the observed ZnO PL spectra peaked at 380 nm (see inset of Fig. 11). The NPs deposited in water though were smaller in size (∼55 nm) with a large fraction of Zn (Fig. 11(a)) that formed ZnO after reacting with the oxygen present in water. This resulted in enhancements of the PL peak intensity at 380 nm. The PL spectra of the NPs deposited on the target in air showed three peaks: one at 380 nm corresponding to ZnO and two other peaks at 415 and 440 nm possibly due to the abundance of the Cu fraction in the NPs due to the enlarged size of the NPs. The micrometer sized particles were deposited back onto the target surface by ablating the brass (Cu–Zn alloy). The ejected liquid droplets during ablation could be from melted material formed when the acceleration of liquid thermal expansion exceeds the force holding the liquid to the surface . The condensation of small particles on the ejected droplets gives the change in composition of the particles. The EDX analysis of the deposited particles (Fig. 10) showed that the larger particles (i.e., micrometer sized) were enriched in Cu. The appearance of two peaks (415 and 440 nm) in the PL spectra can be attributed to the dominance of Cu in the micrometer sized particles. At the high excitation energy (3.5 eV), the two peaks appeared because of the radiative recombination of electrons near the Fermi level and photo-excited holes in the first and second d bands . Since the abundance of copper is higher in large particles, the particles behave more like metal rather than alloy. We have already reported on the PL spectra of a Cu target, with deposited nanoclusters showing two peaks at λ 435 nm (E 2.85 eV) and λ 410 nm (E 3.03 eV) and increased emission intensity compared to that of the fresh target . The shift in the PL peak of 5 nm may be due to presence of Zn. In order to explain the presence of the two PL peaks, we assumed that the initial layer of the deposited particles was spherical in shape (Fig. 9(b)), and in the presence of an external electric field, the deposited spherical structures get distorted to spheroids . Accordingly, the surface polarizability tensor of a spheroid splits into two parts corresponding to parallel and perpendicular directions in relation to the major axis of the spheroid. This, in turn, gives rise to two surface plasmon resonances: one arising due to the induced dipole parallel to the
Fig. 11. PL spectra of NPs in (a) water and (b) air. Inset shows the PL of the fresh brass target. 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
Fig. 12. (a) Nanostructures of ZnO formed in water. The inset shows a zoomed image of the area in the red colored circle. (b) Raman spectrum of the nanostructures. The inset shows the new window of the circled part.
major axis that occurs at low energy and one corresponding to the perpendicular component that occurs at high energy. In order to confirm the ZnO formation in water, we performed micro-Raman measurements on the deposited nanostructures. Figure 12 shows the deposited nanostructures in water and the corresponding Raman spectrum. The inset of Fig. 12(a) shows a zoomed-in image of the nanostructures. The Raman peaks at 283, 436, and 564 cm−1 [60,61] resemble the ZnO nanostructures redeposited on the target in water. The Raman peak at 383 cm−1 corresponds to the A1 (TO) mode, 436 cm−1 corresponds to E2 (high), and 564 cm−1 corresponds to the A1 (LO) phonon mode. The group theory prediction of the Raman active zone-center optical phonons are A1 2E2 E1 . The phonons of A1 and E1 symmetry are polar phonon, which are both Raman and infrared (IR) active exhibiting different frequencies for longitudinal optical (LO) and transverse optical (TO) phonons. The E2 symmetry, related with nonpolar phonon modes, has two frequencies: E2 (low) is associated with the Zn sublattice and E2 (high) is associated specifically with the motion of oxygen atoms [60,62]. 4. Conclusion
The comparative study of laser-induced brass craters via plasma imaging and spectroscopy was done in water and air. The EDX and PL of deposited NPs were correlated with the spectroscopy and imaging of brass plasma characteristics. The high shock wave intensity induced in water caused more depth and a new phase developed at the surface of the target that prevented redeposition, and hence, clean depth in the brass sample. The imaging investigation of plasma in water and air revealed that the confinement of the plasma in water caused an enhancement in electron density. The spectroscopic investigations in water and air concluded that the Cu I line at 510.5 nm was more intense in water than the other lines; however, in ambient air, the Cu line at 510.5 nm was weaker than the other lines. The distribution of NPs in water was narrower than in air and the deposited particle size was smaller in water than in air. The EDX analysis showed that there were higher concentrations of Zn in smaller particles in water as well as in air. The PL of the 7600
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