γ-Al2O3 nanoparticles synthesised by pulsed laser ablation in liquids: a plasma analysis.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 963

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c-Al2O3 nanoparticles synthesised by pulsed laser ablation in liquids: a plasma analysis Julien Lam,abc David Amans,*abc Frederic Chaput,abd Mouhamed Diouf,abc Gilles Ledoux,abc Nicolas Mary,ae Karine Masenelli-Varlot,ae Vincent Motto-Rosabc and Christophe Dujardinabc Pulsed laser ablation has proved its reliability for the synthesis of nano-particles and nano-structured materials, including metastable phases and complex stoichiometries. The possible nucleation of the nanoparticles in the gas phase and their growth has been little investigated, due to the difficulty of following the gas composition as well as the thermodynamic parameters. We show that such information can be obtained from the optically active plasma during its short lifetime, only a few microseconds for each laser pulse, as a result of a quick quenching due to the liquid environment. For this purpose, we follow the laser ablation of an a-Al2O3 target (corindon) in water, which leads to the

Received 4th September 2013, Accepted 1st November 2013 DOI: 10.1039/c3cp53748j

synthesis of nanoparticles of g-Al2O3. The AlO blue-green emission and the AlI 2P0–2S doublet emission provide the electron density, the density ratio between the Al atoms and AlO molecules, and the rotational and vibrational temperatures of the AlO molecules. These diagnostic considerations are discussed in the framework of theoretical studies from the literature (density functional theory). We have found that starting from a hot atomized gas, the nucleation cannot occur in the first microseconds. We also raise the question of

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the influence of water on the control of the stoichiometry.

1 Introduction Pulsed laser ablation in liquids (PLAL) of a solid target is a powerful bottom-up nanoparticle synthesis method with many specific advantages. The solution of as-produced nanoparticles has an extremely high colloidal stability since the process naturally creates charged particles.2,3 Complexing molecules can be added during or after synthesis to increase the stability even more. Moreover, the use of organic agents during production allows controlling the size distribution of the produced particles,4 and is sometimes necessary to prevent their hydroxidation.5 The method is (i) versatile, because a large variety of materials can be synthesized,6–12 and (ii) green, because no toxic reducing agents or solvents are necessary. The method allows synthesizing complex stoichiometry13–15 and dense phases, such as diamond,7,16–21 cubic BN22,23 and C3N4.24–26 Sesquioxides, silicates, tantalates and oxysulfides have also been successfully synthesized.9,27 But, as has been argued in the case of yttrium aluminium garnet (YAG),27 or LiCoO2,15 the stoichiometry of the target is not

a

PRES-Universite´ de Lyon, F-69361 Lyon, France Universite´ Lyon 1, F-69622 Villeurbanne, France c UMR5306 CNRS, Institut Lumie`re Matie`re, France. E-mail: [email protected]; Fax: +33 (0)4 72 43 11 30; Tel: +33 (0)4 72 44 83 37 d UMR5182 CNRS, Laboratoire de Chimie, ENS Lyon, F-69364 Lyon, France e UMR5510 CNRS, MATEIS, INSA-Lyon, F-69621 Villeurbanne, France b

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always preserved. As an illustration, we have observed in the case of a YAG target ablation a competition between different phases and stoichiometries, YAG, Al2O3 and yttrium aluminium perovskite. It is thus challenging to understand the main processes and parameters that define the stoichiometry, the crystallographic phase, and the size distribution of the products. Fig. 1, which will be discussed in greater detail later, gives a summary scheme of the various steps involved during PLAL at different time scales (see plasma imaging in the literature1,28–31), neglecting the fragmentation and the post-irradiation. If the nucleation has been discussed in some specific cases,32,33 the potential nucleation and growth of the nanoparticles in the liquid confined gas phase, in the time scale from a few microseconds to a few hundreds of microseconds after each laser pulse, has barely been investigated. For this purpose, the possibility of following the gas composition as well as the thermodynamic parameters is a key issue. But plasma spectroscopy in a liquid environment is still a challenge. A few teams have succeeded in performing such measurements,28 including notable contributions from Sakka et al.,1,34–38 and De Giacomo et al.39–41 As far as we know, only Kumar et al.30,42,43 and more recently De Giacomo et al.41 have reported such measurements in the context of nanoparticle synthesis by PLAL. We propose to demonstrate here that nucleation can occur only after cooling of the gas, i.e. in the range of tens of microseconds after the laser pulse. For a nanosecond laser pulse with a fluence of about 10 J cm2, the plasma generated

Phys. Chem. Chem. Phys., 2014, 16, 963--973 | 963

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Fig. 1 Main stages occurring during the relaxation of the system for each laser pulse. The electron density in the inset comes from Fig. 5. The inset pictures come from Sakka et al.1 They show the shadowgraphy of the laser induced bubble.

by the front of the laser pulse interacts with the end of the laser pulse. The ablated matter is atomized, or only contains small molecules. If the nucleation arises in a bubble confined by the liquid, the nucleation is driven by the gas temperature and the density fluctuation. Starting from a hot atomized gas, the growth of large molecules and then small clusters is possible only when the temperature decreases sufficiently, after a few microseconds in our case. We then focused our efforts on obtaining this information from the optically active plasma as a function of time during the first ten microseconds. The thermodynamical parameters measured, and the species observed, are discussed in the framework of the available theoretical calculations in the literature. This study has been performed on alumina (Al2O3). It exists as several phases,44 the corundum phase a being particularly interesting. The pressure dependence of the R fluorescence lines in ruby (a-Al2O3:Cr3+) is commonly used as a continuous pressure sensor45 in nanofluidic systems. Al2O3:C also shows photostimulable properties, and is used as an optically stimulated luminescence (OSL) dosimeter46 in radiation protection applications or in environmental dosimetry. However, if a-Al2O3 is the thermodynamically stable phase of crystalline aluminum oxide, McHale et al. have shown that g-Al2O3 has a lower surface energy and becomes energetically stable at a smaller size.47 According to the theoretical calculation of the enthalpy, g-Al2O3 becomes the thermodynamically stable polymorph for particle sizes smaller than 10 nm. Gram scale synthesis of a-Al2O3 nanoparticles (NP), with an average size larger than 30 nm, has been reported previously.48 But no evidence of a-Al2O3 NP with sizes smaller than 10 nm has been reported. Kumar et al.42 also reported PLAL synthesis of spherical Al2O3 nanoparticles with an average size of 23 nm. But no crystallographic phase was mentioned. In general, the PLAL of Al target in water mainly leads to g-Al2O349–52 or hydroxide.52–54 In order to favour the synthesizis of Al2O3 in the right stoichiometry with respect to the hydroxide phase, the ablation of Al2O3 target has been preferred. However, Al2O3 is transparent in the visible and IR

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wavelength ranges. We thus used a UV laser source to ensure appropriate absorption. In the first part, we characterize the synthesized products and we show the plasma spectroscopy performed during the synthesis. In the second part, we extract the temporal evolution of the physical parameters from the spectroscopy: the electron density, the density ratio between the Al atoms and the AlO molecules, and the rotational and vibrational temperatures of the AlO molecules. The discussion is divided into three parts. The extraction of the physical parameters from the spectroscopy is discussed in the framework of local thermodynamic equilibrium. The temporal behavior of our plasma is compared to that reported in the PLAL literature. As a result, a growth process scenario is proposed.

2 Experimental details The corundum Al2O3 target was synthesized using a Czochralski process. The ablation was performed on an unpolished flat surface. The targets were placed at the bottom of a 50 mL beaker and covered by 50 mL of pure deionized water. The third harmonic of a YAG laser (l = 355 nm, Dt = 5 ns, repetition rate = 10 Hz) was focused on the surface of the target. The energy per pulse was set to 35 mJ per pulse. The crater diameter was measured using optical microscopy. An average value of 485 mm was observed for 10 successive pulses at the same site on the target. The irradiation time was 20 min in order to (i) keep the solvent clear and to ensure the stability of the thermodynamic conditions, and (ii) limit any post-irradiation that could lead to fragmentation. The upper 30 mL of the solutions was collected after a settling time of one hour. For each synthesis, a droplet of the colloidal solution was poured onto 300-mesh copper grids covered with a holey carbon film (S147-3 from Agar scientific). The solvent was then eliminated by natural evaporation. A dry powder composed of the synthesised products was obtained


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through freeze-drying of the solution. X-ray diffraction was performed on the powder. Transmission Electron Microscopy (TEM) experiments were carried out on a JEOL 2010F microscope operating at 200 KV. High resolution images were acquired using a Gatan Orius 200 camera. Electronic diffraction patterns were analysed with the Digital Micrograph software from Gatan. Energy Dispersive X-Ray Spectroscopy (EDS) characterisations were simultaneously performed using an SDD Xmax80 detector from Oxford Instruments. The interreticular distances were measured on the fast Fourier transforms of the pictures. X-ray powder diffraction patterns were recorded at room temperature on a Bruker D8 Advance diffractometer equipped with a sealed Cu X-ray tube and a linear LynxEye detector. The plasma spectroscopy was performed during the synthesis. The light from the plasma plume was collected using a single lens, using a 2f/2f setup, and imaged onto the circular entrance of a fiber bundle. The circular to rectangular fiber bundle from LOT Oriel (LLB552-UV-0,22) is composed of UV silica fibers of core diameter 100 mm and numerical aperture 0.22. The UV silica lens of 2 inch diameter and 75 mm focal length collects the light in a 0.16 numerical aperture cone, which fits the fiber numerical aperture. The entrance of the fiber bundle acts as a spatial filter. According to the 2f–2f setup, the size of the confocal volume observed corresponds to the circular bundle diameter, 800 mm. Due to the strong spatial confinement of the plume by the liquid, we expected to collect the light from the whole plume. Moreover, the confocal mode ensures the suppression of the optical noise from the environment. The rectangular end of the fiber is imaged onto the entrance slit of a monochromator, coupled with an iStar intensified CCD (iCCD) from Andor technology. An optical system is added to match the monochromator f-number. The laser and the iCCD were triggered by a pulse generator DG645 from Stanford Research Systems. The laser was double triggered to ensure an overall timing resolution (jitter) of 5 ns. For all spectra, the delay time shown corresponds to the start time of the time gate of signal integration. Two different monochromators have been used, one to favour the high spectral resolution and the other, a wide spectral range. All spectral resolutions were formulated in full width at half maximum (FWHM), and measured using a calibrated spectral lamp. The high resolution was reached using a monochromator Ramanor U1000, from Jobin Yvon, with 1 m focal length and a 1800 lines per mm grating. It had been modified to accept the iCCD. We were able to achieve a spectral resolution of 0.048 nm (FWHM), which corresponds to seven pixels on the iCCD. The wide spectral range was obtained using a monochromator Shamrock 303, from Andor Technology, with a 300 lines per mm grating. The spectral resolution measured was 0.72 nm (FWHM). Calibration of the wavelengths was systematically performed for all sets of measurements. All spectra have been corrected from the overall optical response of the system using a calibrated blackbody source. The optical response was measured for every experimental configuration and every parameter used. When possible, the continuum background emission has been numerically removed. The procedures are described in Section 3.2.


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3 Results 3.1

Synthesis products

Fig. 2 shows the representative high-resolution picture of g-Al2O3 nanoparticles obtained by synthesis in pure deionized water. The interreticular distances and the angles fit the ones reported in the ICDD file 04-007-2479. Considering the small sizes observed, mostly less than 10 nm, the production of the g phase in pure water is consistent with the expected results from the theoretical calculation of McHale et al.47 X-ray diffraction has been performed. The diffraction pattern only possesses a large amorphous band (not shown). This is in agreement with the TEM pictures of the products that show deformations and crystal twinning in the nanoparticles.

3.2

Plasma emission

Fig. 3 shows typical spectra measured on the Al2O3 target immersed in pure deionized water. The features are attributed to the neutral AlI doublet at 394.4006 nm (2P01/2–2S1/2) and 396.1520 nm (2P03/2–2S1/2), and to the AlO rovibronic emissions between the B2S+and X2S+states. The light emission from the

Fig. 2 Representative high-resolution picture of g-Al2O3 nanoparticles obtained from synthesis in pure deionized water. Picture (a) corresponds to a particle of 23 nm diameter, diffracting in the zone axis [112]: d440 % = 0.143 nm, d220 % = 0.280 nm, d31% 1% = d131 % = 0.241 nm, and d111% = 0.456 nm. The inset corresponds to the fast Fourier transform of the boxed area. Pictures (b) and (c) correspond to a particle of 5.5 nm diameter, diffracting in the zone axis [001]: d440 = 0.143 nm, d040 = d400 = 0.202 nm.


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to the vibrational quantum number of the upper state B2S+ (respectively of the down states X2S+). Fig. 3b shows high resolution spectra of the AlI doublet. The grey lines correspond to the tabulated wavelengths of both transitions 2P01/2–2S1/2 (394.4006 nm) and 2P03/2–2S1/2 (396.1520 nm). The shift and the broadening of both lines allow deduction of the plasma electronic density Ne. The background emission, including the electron-ion recombination and bremsstrahlung, has been numerically removed in Fig. 3a and b. The background of the curve in Fig. 3a is evaluated using the spectral ranges where no spectral lines (atomic and molecular) are identified. A polynomial fit is then performed and leads to a soft varying background. The spectral range of the curve in the Fig. 3b is narrow: 7 nm. Here, the constant background is removed. We use the signal intensity averaged over a narrow range located around 392 nm. Fig. 3c shows the molecular AlO blue-green emission in the spectral range of the vibrational progression Dn = 0. The background emission has not been removed. The spectra for the three delays are normalized to 1. From these spectra, one can deduce the rotational and vibrational temperatures using the procedure described below. 3.3

Fig. 3 (a) Plasma emission measured at 800 ns delay time with a time gate of 200 ns. The spectral resolution is 0.72 nm (FWHM). The background emission, including the electron-ion recombination and bremsstrahlung, has been numerically removed. Dn = n 00 n 0 corresponds to the vibrational progressions, where n 0 (respectively n 00 ) corresponds to the vibrational quantum number of the upper state B2S+ (respectively, of the down states X2S+). The grey boxes indexed (b) and (c) show the spectral ranges displayed in the homonymous panels. (b) High resolution spectra of the AlI doublet displayed for five delays. The time gate is 100 ns for each spectrum. The spectral resolution is 0.053 nm (FWHM). The grey lines correspond to the tabulated wavelengths of both transitions 2P01/2–2S1/2 (394.4006 nm) and 2P03/2–2S1/2 (396.1520 nm). The background emission, including the electron-ion recombination and bremsstrahlung, has been numerically removed. The spectra are scaled between 0 and 1, and display shifted to improve the readability. (c) Molecular AlO rovibronic emission in the spectral range of the vibrational progression Dn = 0. The three arrows show the band heads of the vibrational transitions 0–0, 1–1 and 2–2. The background emission is not removed. The spectra for the three delays are normalized to 1. The time gate is 100 ns for each spectrum. The spectral resolution is 0.061 nm (FWHM).

aluminium atoms produced by a pulsed laser irradiation on an aluminium metal–water interface,30,35 or produced using Laser Induced Breakdown Spectroscopy in AlCl3 solution,39 has been previously observed. But, as far as we know, the AlO blue-green rovibronic emission has never been observed in plasma confined in water. Fig. 3a shows a typical spectrum recorded over a wide spectral range. The molecular AlO bands are identified as Dn = 2, Dn = 1, Dn = 0 and Dn = 1 vibrational progressions, assuming Dn = n 00 n 0 , where n 0 (respectively n 00 ) corresponds


Temperatures of the AlO molecules

Software has been developed to simulate the AlO blue-green (B2S+ X2S+) emission. The line wavenumbers are calculated from vibrational and rotational molecular constants,60,61 and the corresponding Fortrat diagram is controlled using the measured data from the literature.61,63 The relative intensities of each transitions are derived from the vibrational and rota¨ln London factors,64 and absolute band tional temperatures, Ho 62 strengths. In the measured spectra, the main contribution to the spectral linewidth is due to the monochromator spectral resolution. Therefore, the calculated spectra correspond to the convolution of the band system with a Gaussian function. The experimental full width at half maximum (FWHM) was measured using atomic lines from a calibration lamp in the spectral range of interest. Due to the experimental spectral resolution, each line observed in the measured spectra corresponds to the sum of several transitions between steady states. The rotational temperature Trot can be deduced from the intensity ratios between the lines in the spectral range from 486 nm to 486.7 nm where the lines correspond to 0–0 transitions only, without any contribution from the 1–1 and upper transitions. Moreover, several lines are well separated, allowing a suitable measurement of the ratios. One can then calculate the abacus of Trot with respect to these intensity ratios, at each experimental spectral resolution. In the second step, Tvib comes from the fit of the whole spectrum. However, due to self-absorption, the 0–0 band head cannot be reproduced with accuracy. Fig. 3c shows that the first stages are dominated by the emission due to the electron-ion recombination and bremsstrahlung. After 2 ms, the continuous background emission becomes negligible, and the described method can be applied. The spectra recorded with a time gate of 100 ns between 2 ms and 2.5 ms do not evolve significantly. From these spectra, we deduced a rotational temperature of 3400 500 K and a


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Fig. 4 The black curve corresponds to a measured AlO rovibronic spectrum recorded 2.2 ms after the laser pulse. The spectral resolution is 0.061 nm (FWHM). The grey curve corresponds to a simulated spectrum assuming a rotational temperature of 3400 K and a vibrational temperature of 6000 K.

vibrational temperature of 6000 1000 K assuming measurement uncertainties and fit errors. Fig. 4 shows a typical spectrum recorded 2.2 ms after the laser pulse, and its fit. These temperatures are lower than the temperatures usually reported for ablation performed in ambient air with comparable laser parameters. But the temperatures obtained for laser ablation in water are deemed to be lower. Indeed, our results are in good agreement with the temperature reported for laser ablation in liquids,30,38,65 and more specifically with rotational37 and vibrational36 temperatures in C2 molecules. 3.4

The electron impact shift d and the electron impact width o, displayed in Table 1, correspond to a temperature of the electron gas of 5000 K. These parameters are independent of the electron density, but change with the electron temperature. The value of 5000 K is consistent with the measured temperatures. The electron densities deduced from each parameter are displayed in Fig. 5a. The error bars come from the uncertainties in the electron temperature as well as in the parameters d and o,58,66 and of the fit. Fig. 5b shows the ratio between the electron densities deduced from the Stark width of 2P01/2–2S1/2 2 0 2 oa (Nob e ) and the Stark width of P3/2– S1/2 (Ne ). Both transitions have the same electron impact width parameter, which means that the ratio r must be equal to one. But, in the early stages, the ratio is far from one and finally tends to one. This behavior is indubitably the signature of self-absorption. The selfabsorption implies an enlargement of the lines which leads to an overestimation of the full widths at half maximum, and to the corresponding electron densities. The LS splitting of the 2P0 terms is very much lower than kTe. The ratio between the two spin degeneracy terms is equal to two. Therefore, the density na of aluminum in the 2P03/2 level is twice the density nb of aluminum in the 2P01/2 level. Assuming the same Einstein coefficient B12 for both lines, the self-absorption from 2P03/2 should be twice the self-absorption from 2P01/2. This explains the ob higher values of Noa e in comparison with Ne . This is the reason why we are more confident in the electron density obtained from the Stark shift.67 The electron density deduced from the Stark shift of 2P03/2–2S1/2 is then fitted with a phenomenological exponential function: t

Nesa ¼ Aet ;

Electron density

The electron density can be obtained via Stark effects from the temporal behavior of the AlI 2P0–2S doublet (see Fig. 3b). The Stark shift Dl and the Stark broadening s (FWHM) of both lines are deduced using two Voigt functions. The electron density Ne [cm3] is then calculated using the linear dependence Dl = dNe

(1)

s = 2oNe.

(2)

(3)

with A = 4.32 1018 cm3 and t = 265 ns. It should be stressed that the reciprocal function,34 usually used as a solution of the rate equation for the recombination process,68 does not fit our data. In the configuration of high spectral resolution, we do not observe the 2P0–2S transitions after 800 ns (compared to the low resolution configuration). But, from the fit, we expect the electron density to drop below 1016 cm3 after only 1600 ns.

Table 1 Parameters used for AlI. a, b, and 2 correspond, respectively, to the levels 2P03/2, 2P01/2 and 2S1/2. 3 and 4 correspond to the terms 4P and 2D, without specification of the J values. The upper levels above 4 eV are not shown. Considering our thermal energy kTe, the upper levels are not implied in the partition function

Symbol

Definition

A2a A2b l2a l2b o d B12 E1b/g1b E1a/g1a E2/g2 E3/g3 E4/g4

Radiative rate for 2P03/2–2S1/2 transition Radiative rate for 2P01/2–2S1/2 transition Wavelength in air of 2P03/2–2S1/2 transition Wavelength in air of 2P01/2–2S1/2 transition Stark width parameter (see eqn (2)) Stark shift parameter (see eqn (1)) Einstein’s B factor Energy level/degeneracy of 3s23p configuration, term 2P01/2 Energy level/degeneracy of 3s23p configuration, term 2P03/2 Energy level/degeneracy of 3s24s configuration, term 2S1/2 Energy level/degeneracy of 3s3p2 configuration, term 4P Energy level/degeneracy of 3s23d configuration, term 2D


Value [unit] 7

Ref. 1

9.8 10 [s ] 4.93 107 [s1] 396.1520 nm 394.4006 nm 1.34 1019 [nm cm3] 1.64 1019 [nm cm3] 1.82 1020 [J1 m3 Hz s1] 0 [eV]/2 0.014 [eV]/4 3.14 [eV]/2 3.60 [eV]/12 4.02 [eV]/10

55, 55, 55, 55, 58 58 55 55, 55, 55, 55, 55,

56 56 56 56

57 57 57 57 57


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Assuming a canonical ensemble, the density of atomic aluminium at level p is gp NAl Ep Np ¼ ; (5) exp ZAl kTe

Fig. 5 (a) Electron densities deduced fromB the Stark width of 2P03/2–2S1/2 2 0 2 ob 2 0 2 (Noa e ), & the Stark width of P1/2– S1/2 (Ne ),E the Stark shift of P3/2– S1/2 sa 2 0 2 sb (Ne ), and ’ the Stark shift of P1/2– S1/2 (Ne ). The black line corresponds to ob oa an exponential fit of Nsa e . (b) Ratio Ne /Ne .

3.5

Species density

The last piece of information concerns the composition of the gas after 2 ms, when the plasma turns into a neutral gas, according to the electron density measurements. In Fig. 3a, the horizontal brackets indexed by IAl and IAlO correspond, respectively, to the wavelength ranges used to calculate the integrated intensities emitted by the atomic Al and the molecular AlO. Recording simultaneously both emissions insures a better reliability in the comparison of the integrated intensities. The ratio IAlO/IAl is shown in Fig. 6. The emission intensity recorded by the iCCD is proportional to the number of photons and furthermore to the densities NAlO/NAl. NAlO IAlO ¼f : NAl IAl

(4)

From the thermodynamic parameters, one can estimate the proportionality factor f.

Fig. 6 The circles correspond to the ratio IAlO/IAl defined in Fig. 3a. The black line is a linear fit with an adjusted R2 of 97.2%. The inset is a zoom on the short times.


where Te is the electronic temperature, NAl is the total density of the aluminium, ZAl is the partition function, Ep is the energy of level p, and gp is the degeneracy of level p. The number of photons corresponding to the transition from level p to level q emitted per second, in a volume dV, is then given by Ipq = ApqNpdV, where Apq is the radiative rate. For the 2P0–2S doublet, it is then (A2a + A2b)N2dV (see Table 1). In order to simplify the partition functions, kTe is assumed to be lower than 1 eV. This limit is consistent with the vibrational temperature Tvib = 6000 K (0.517 eV) of the AlO molecules if we keep in mind that the vibrational processes quickly tend to an equilibrium with the electronic processes. However, further considerations about the local thermodynamic equilibrium (LTE) will be discussed in the next section. The partition function ZAl is approximated as follows (see Table 1): X Ep ZAl ¼ gp exp (6) g1a þ g1b ¼ 6; kTe p with E1a;1b exp 1; kTe

Ep2 exp 1: kTe

(7)

The number of photons in the 2P0–2S doublet emission, emitted per second in a volume dV, is then g2 E2 IAl ¼ NAl ðA2a þ A2b Þ dV: (8) exp ZAl kTe The AlO partition function can be approximated by summation over the two first electronic states, with kTe lower than the electronic energy of the third level B2S+(see Table 2): X ZðlÞ; (9) ZAlO ¼ l

= Z(1) + Z(2),

(10)

Ee;1 ¼ Zs;v;r;n ð1Þ exp kTe Ee;2 ; þ Zs;v;r;n ð2Þ exp kTe

(11)

with Zs,v,r,n(l) the partition function of the electronic state l, including the electronic spin, the vibration of the molecule, the rotation of the molecule, and the nuclear spins. The electronic states 1, 2 and 3 refer, respectively, to the levels X2S+, A2Pi, and B2S+. AlO is a heteronuclear molecule, no condition on the parity of the wave function is imposed, which means that the emission intensities do not depend on the nuclear spin. One can neglect the nuclear spin in the equations. The average value of the electronic spin-doubling constants g61 in the first three electronic levels is 102 cm1. The spin-doubling hcKg is negligible in comparison with the energy of the rigid rotator hcBeK(K + 1) with K as the rotational quantum number, and Be


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Table 2 Parameters used for the calculation of the total intensity emitted by molecular AlO in the blue-green emission. The indices 1, 2 and 3 refer, 0 respectively, to the levels X2S+, A2Pi, and B2S+. If Ann 00 is the Einstein coefficient of the transition from a vibrational level n 0 of B2S+ to a vibrational level n 00 * + P n0 of X2S+, A is the average value: An 00


n 00

n0

Symbol

Definition

Value [unit]

Te1/Ee,1 Te2/Ee,2 Te3/Ee,3 oe1/Eo,1 oe2/Eo,2 oe3/Eo,3 Be1/EB,1 Be2/EB,2 Be3/EB,3 A

Electronic energy Electronic energy Electronic energy Harmonic oscillator energy Harmonic oscillator energy Harmonic oscillator energy Rotational constant Rotational constant Rotational constant Average Einstein coefficient

0 [cm1]/0 [eV] 5406.11 [cm1]/6.70 101 20 689.06 [cm1]/2.57 [eV] 979.524 [cm1]/1.21 101 729.762 [cm1]/9.05 102 870.369 [cm1]/1.08 101 0.64165 [cm1]/7.96 105 0.53717 [cm1]/6.66 105 0.60897 [cm1]/7.55 105 8.61 0.14 106 [s1]

as the rotational constant (see Table 2). Moreover, hcKg { kTrot, the electronic spin is then treated as a degeneracy of 2 for the three electronic levels, leading to Zs,v,r,n(l) = 2 Zvib(l)Zrot(l),

(12)

with Zvib ðlÞ ¼

X

zvib ðl; nÞ ¼

X

n

Zrot ðlÞ ¼

n

X K

¼

X K

Eo;l ðn þ 0:5Þ ; exp kTvib

Ref.

where n 0 and K 0 (respectively n 00 and K 00 ) correspond to the vibrational and rotational quantum numbers of the upper state 0 ¨ln B2S+ (respectively of the lower states X2S+). SKK00 are the Ho London factors normalized by X 0 SKK00 ¼ 1: (18)

(13)

0

According to the Einstein coefficients Ann 00 reported in Herbert 0

EB;l KðK þ 1Þ : ð2K þ 1Þ exp kTrot

(14)

et al.,62 at n 0 constant, the summation over n 00 of the Ann00 leads to an average value of 8.61 106 s1 with a standard deviation of 0.14 106 s1 (for n 0 and n 00 in [0,10]): * + 10 X n0 0 An 00 ¼ 8:61 0:14 106 s1 : (19) A hA n in 0 ¼ n 00 ¼0

Assuming a vibrational temperature of 6000 K, the population of the vibrational states n can be calculated from the harmonic oscillator energies oe of the three first levels, which drops quickly when n is greater than 7. According to Table 2, the energy Eo,l(n + 0.5) is greater than the expected kTvib when n is greater than 7. For such values, the anharmonicity of the vibration and the corrections to the rigid rotator model can be neglected, leading to rotational constant Be and oscillator energy oe independent of the vibrational quantum number n (see the data in Saksena et al.61). The vibrational partition function of a state l is then 1

Eo;l 2 sinh kTvib

:

(15)

One can also assume a rigid rotator model with kTrot c EB,l. Thus, the rotational partition function of a state l is Zrot ðlÞ ¼

[eV] [eV] [eV] [eV] [eV] [eV]

K 00

zrot ðl; KÞ

Zvib ðlÞ ¼

59 59 60 61 59 61 61 59 61 62

[eV]

kTrot : EB;l

(16)

The number of photons in the blue-green emission, emitted per second in a volume dV, is then 2 Ee;3 IAlO ¼ NAlO dV exp ZAlO kTe (17) X 0 0 Ann 00 SKK00 zvib ð3; n 0 Þzrot ð3; K 0 Þ; n 0 ;n 00 ;K 0 ;K 00


n0

Finally, the AlO emission is 2 Zrot ð3ÞZvib ð3Þ Ee;3 dV; exp IAlO ¼ NAlO A ZAlO kTe

(20)

with Zvib and Zrot following eqn (15) and (16). In conclusion, f can be calculated from eqn (4), (8) and (20): A2a þ A2b g2 ZAlO E2 Ee;3 ; (21) exp f ¼ A 2 Zrot ð3ÞZvib ð3ÞZAl kTe f depends slightly on the vibrational and rotational temperatures because the rotational constants Be and the oscillator energies oe are roughly constant for the first three electronic states of the AlO molecules. For values of Te between Trot and Tvib, the factor f for delays larger than 2 ms is between 0.79 and 2.3, close to unity. From Fig. 6, we can conclude that the aluminium atoms are mainly oxidized 2 ms after the laser pulse. Note that Fig. 6 does not give the density ratio because (i) the temperatures are unknown before 2 ms and (ii) the temperatures change over time. However, Fig. 6 gives the general trend. Indeed, the factor f slowly increases with the temperature, from 0.58 at 3000 K to 5.1 at 12 000 K. This trend induces an underestimation of the NAlO/ NAl density ratio when the temperature is underestimated. The temperature decreases from t = 0 to 2 ms. The increase with time of the NAlO/NAl density ratio is certainly slower than the 112 ms1 observed in the Fig. 6 for IAlO/IAl.


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4 Discussion


4.1

Local thermodynamic equilibrium

To obtain f (eqn (21)), we have assumed that the electronic temperatures experienced by the Al atoms and the AlO molecules are the same. This assumption raises the question of the local thermodynamic equilibrium (LTE). Drawin69 obtains a criterion of the electron density for an optically thin plasma where the excited levels are populated due to electronic collisions. This criterion is known to be necessary but not sufficient to insure a LTE, since the transient nature of the plasma must also be considered. The electron density must obey the relation gmax DE 3 kTelec 0:5 DE Ne 6:5 1016 F ; (22) 1 kTelec gmin E1H E1H where Ne is the electron density in [cm3], DE = Emax Emin is the largest possible energy gap between two adjacent levels Emax and Emin, gmax (resp. gmin) is the degeneracy of the level Emax (resp. Emin), EH 1 = 13.59 eV, and Telec is the electron gas temperature. The coefficient F1 contains the quantum mechanical correction of the classical result from Griem.70 The numerical values of the function can be found in Drawin.69 In our system Al/AlO, the largest gap corresponds to the transition in the aluminium atoms between the degenerate ground level E1a,b and the first excited level E2. Assuming kTelec lower than 1 eV (in compliance with kTvib = 0.517 eV), gmax = g2 = 2, and gmin = g1a + g1b = 6, DE = 3.14 eV, we find F1 = 3.67 leading to Ne Z 2.65 1014 cm3. According to the fit of our data (eqn (3)), the criterion is fulfilled. But this value appears very low compared to the generally accepted value of 1016 cm3 for kTelec = 1 eV. Indeed, eqn (22) has been derived and checked to ensure a complete LTE in the case of hydrogen and hydrogen-like ions, and when Ne is larger than the density of the heavy species. Our current situation for a time delay larger than 2 ms does not exactly square with these assumptions. The LTE criterion in eqn (22) is derived to ensure that the population densities follow the Saha–Eggert equation for all electronic levels. The condition to ensure the validity of f should be less restrictive, because we only have to ensure that the population of level Ee,3 of the AlO molecules is driven by the same process as the level E2 of the aluminium atoms. In addition, a comparison of the characteristic times is also necessary in the case of the transient nature of the plasma, composed of atoms and molecules. Considering a Maxwellian velocity distribution for the heavy species (Al, AlO. . .), the collision frequency between the heavy species which compose the gas is defined by t1 % where Ng is the gas col = Ng s u, density, s an average collisional cross-section for atoms and 8kTg is the average velocity with mAB diatomic molecules, u ¼ pmAB the reduced mass for a collision between two species A and B, and Tg is the kinetic gas temperature. The rotation of the diatomic molecules being due to collisions, we can assume Tg = Trot, leading to u% E 1.5 103 m s1. With a typical crosssection of s = 1015 cm2 and Ng Z 1018 cm3, the mean time tc between two collisions is less than 7 ns, which leads to a free


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path of 10 mm. The mean time between two collisions is smaller than the time evolution of the thermodynamic parameters: Trot ðt þ tc Þ Trot ðtÞ 1 Trot ðtÞ

(23)

for t > 2 ms. Moreover, the free path is smaller than the expected bubble size, and so is smaller than the variation length of the thermodynamic parameters. A local kinetic equilibrium of the heavy species is then reasonable, and in good agreement with Tg = Trot. The assumption of a common electronic temperature leading to f seems reasonable, especially since the energy difference E2 Ee,3 = 0.57 eV is close to the measured values of kTrot and kTvib. 4.2

Comparison with the PLAL literature

In the PLAL bottom-up approach, the nucleation and the growth processes may have different physical or chemical origins at different stages. In the nanosecond regime, the laser pulse creates a hot and dense plasma confined by the liquid. The shadowgraphy of the laser-induced bubble has been extensively reported in the literature.1,71,72 An overview of five different stages is shown in Fig. 1, and described as follows: (i) the laser-target and the laser-plasma interactions, (ii) the relaxation of the plasma with a characteristic lifetime correlated with the electron density drop, (iii) the lifetime of the liquid-confined bubble, (iv) the bubble collapse, and (v) the aging in the liquid phase. The laser irradiance on the target drives the ablation regime, between thermal ablation and Coulomb explosion or explosive boiling.73 Due to inverse bremsstrahlung absorption, the plasma heating is increased with infrared wavelengths and nanosecond pulses compared to ultraviolet wavelengths and picosecond or femtosecond pulses. As a consequence, the results reported by teams using different irradiance values, wavelengths, and pulse durations, are difficult to compare. We have measured the optical emission of the plasma due to electron-ion recombination, bremsstrahlung, atomic emission, and rovibronic emission. We have shown that the emission disappears after times between 1 ms and 10 ms, and this result is in complete agreement with the plasma imaging performed by several teams.1,28–31 The quenching of the emission results from the quick cooling of the plasma and the drop of the electron density.43 After microseconds, the plasma plume leads to a bubble. The bubble expands, may oscillate over a few periods, and then collapses after a period of from hundreds of microseconds to one millisecond.1,8,74 The collapse stage remains unclear in the PLAL process. It can lead to high pressures and temperatures as is observed in cavitation bubble collapses.33,75 But unlike cavitation experiments, the amount and the nature of the dissolved gas in the solvent are not controlled, and the results obtained in the studies on cavitation bubbles may not be effective. The collapse may drive chemical reactions or phase transitions.33,76 After the collapse, matter is expelled into the liquid where several processes can compete with each other: aggregation, growth, and chemical reactions with the solvent or added chemical agents.


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The nature of the matter that appears in the bubble has not been clearly identified, and may contain small molecules, small clusters larger than the nucleus size, or bigger nanoparticles. Rayleigh scattering has been used to probe the growth dynamics of titanium oxide nanoparticles in the first 250 ms,43,77 but those measurements do not provide a size distribution. Recently, a structural view of the particle formation inside a cavitation bubble has been obtained by Plech et al.78 using X-ray scattering as an ultrafast probe of the structure formation. This experiment is certainly a key to understanding the growth kinetics. But the method does not allow observation of particles with sizes smaller than a few nanometers. The key issue of when the nucleation begins is left open. Nucleation times shorter than 1 ns have been reported for diamond nucleation43 and titanium oxide nucleation.79 In the following section, we will show that a nucleation in the first microseconds is not possible under our ablation conditions. 4.3

Considerations of chemical reactions and stoichiometry

After 2 ms, the bubble is mainly composed of a hot gas, essentially neutral, at a kinetic temperature which corresponds to the rotational temperature of 3400 K. The AlO molecule density is several orders of magnitude larger than the density of the Al atoms. These results can then be compared with the theoretical study of Patzer et al.80 They have applied density functional theory (DFT) to AlxOy (x,y = 1–4). Taking into account all chemical reactions between the species in an Al–O system, a gas phase equilibrium is reported. For temperatures larger than 3000 K, and a ratio Al/O about unity, they demonstrate that the most stable species are AlO and Al2O. These theoretical results agree with our measurements at 2 ms after the laser pulse. Moreover, the energetically most stable species tend to follow x = y when both x and y grow, and the gas composition is entirely governed by the aluminum oxides (AlO)x. The abundance of large x increases with decreasing temperature from 3000 K to 1000 K. We can thus state that starting from a hot atomized gas, nucleation cannot occur in the first microseconds. In the framework of their study, i.e. x,y o 4, the smallest cluster of AlxOy grows with a stoichiometry different from that of Al2O3. Nevertheless, the calculation performed by Patzer et al. must be treated with great care for two reasons. First, our solvent H2O is not included in their calculations. Second, the calculations correspond to an equilibrium state while our plasma is certainly a transient system. The gas also certainly contains H, OH and H2O species even if the optical signatures of the H and OH species are not observed because their excitation energies are much larger than the available thermal energy kT (whatever the process). A dissociation of the water can occur in the very first stage of the plasma. The water molecules provide oxygen for chemical reactions in the gas phase. As an example, the enthalpy of reaction of the chemical reaction Al2O + H2O - Al2O2 + H2 is negative.81 We still need to look at the issue of the stoichiometry of a large cluster. In many bottom-up physical methods of the synthesis of oxides, an excess of oxygen atoms is often necessary to ensure the stoichiometry. Plasma enhanced chemical vapor


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deposition (PECVD) uses an oxygen plasma to grow layers of oxides.82,83 Pulsed laser deposition (PLD) of Al2O3 has been performed in an oxygen gas atmosphere to enhance the stoichiometry of the layer.84 In low energy cluster beam deposition under vacuum (LECBD), it is necessary to inject O2 during the cluster growth to improve the stoichiometry of the ZnO nanoparticles.85 But compared to PLD, PECVD or LECBD methods, the PLAL method does not allow an accurate control of the chemical composition of the liquid-confined bubble generated by each laser pulse, because of the uncontrolled exchanges between the bubble and the liquid phase. In particular, regarding the stoichiometry, the synthesis of oxides raises the question of the water in the liquid environment. In PLAL using metallic targets, there have been observed metastable suboxides, such as titanium monoxide,86 iron monoxide87 and Al2O.42 The oxygen atoms clearly come from the water molecules because the amount of dilute oxygen is not sufficient. In a hot environment filled with water, hydroxylation of the clusters followed by a deprotonation due to the high temperature may lead to the Al2O3 stoichiometry. Bulky a-Al2O3 is commonly obtained from the annealing of hydroxides: 800 K if one anneals a diaspore and 1300 K if one anneals boehmite or bayerite. Kumar et al.43 have proposed the same chemical reactions to explain the synthesis of TiO2 particles using laser ablation of titanium metal in de-ionized water. In addition to the gas phase thermodynamic control, we raise the question of the influence of water on the control of the stoichiometry.

5 Conclusion In the framework of the synthesis of g-Al2O3 by PLAL in a water environment, we report the spectroscopy results of the Al and AlO species. From the electron density and the rotational and vibrational temperatures of the AlO molecules, we show that after 2 ms the plasma turns into a hot neutral gas of 3400 K. The quick oxidation of the Al atoms leading to AlO molecules is in good agreement with the DFT calculations of Patzer et al. The same theoretical study shows that only small molecules are stable when the temperature is higher than 3000 K. We can thus state that nucleation cannot take place before a few microseconds. If the process leading to g-Al2O3 nanoparticles is not yet clear, the water environment seems to favour the Al2O3 stoichiometry, but the question is still open. As a further experimental development, it would be of great interest to perform laser-induced fluorescence (LIF) measurements to obtain a more complete time-resolved composition analysis of the gas phase species. In a complementary way, we are developing DFT calculations on small AlxOy clusters and then pseudopotential calculations for larger AlxOy clusters.

Acknowledgements We would like to thank the Centre Lyonnais des Microscopies (CLYM) for access to the JEOL 2010F microscope. We would


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´de ´ration de Recherche Andre ´ Marie also like to thank the Fe `re for financial support. Ampe


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