Optical emission spectroscopic studies on laser ablated TiO2 plasma

Applied Surface Science 255 (2009) 8730–8737

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Optical emission spectroscopic studies on laser ablated TiO2 plasma Gaurav Shukla *, Alika Khare Laser & Photonics Lab, Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 20 March 2009 Received in revised form 28 May 2009 Accepted 29 May 2009 Available online 10 June 2009

Optical emission spectroscopic investigations of the plasma produced during Nd:YAG laser ablation of sintered TiO2 targets, in oxygen and argon gas environments are reported. The spatial variations of electron temperature (Te) and electron number density (Ne) are studied. The effect of oxygen/argon pressure on electron temperature (Te) and electron number density (Ne) is presented. The kinematics of the emitted particles and expansion of plume edge are discussed. Spatio-temporal variations of various species in TiO2 plasma were recorded and corresponding velocities were calculated. The effect of oxygen pressure on intensity of neutral/ion species and their corresponding velocities is also reported. ß 2009 Elsevier B.V. All rights reserved.

PACS: 32.30.Jc 32.70.Jz Keywords: Optical emission spectroscopy Laser produced plasma Stark broadening Titanium oxide Pulsed laser ablation

1. Introduction TiO2 is a n-type, wide-band semiconductor, with many industrial and research applications, including its use as a photocatalyst, as excellent anti-reflecting optical coating, in environmental sanitation, in detoxification of waste water, for fabrication of electrodes for solar energy conversion, etc. [1–5]. The dependence of its properties, including the electrical ones, on its nanostructure has motivated the use of nanometer-sized TiO2 particles for ceramics, in optical devices, sensors, self-cleaning coatings, etc. [6,7]. In contrast to extensive research relating to TiO2 thin films produced by pulsed laser deposition (PLD), focused on the study of their properties [8–14], there appears to have little effort directed towards the characterization of the ablation plume from which such films are produced using optical emission spectroscopy (OES) [15–17]. Pulsed laser deposition (PLD) has emerged as a potential technique for the fabrication of thin films and nanostructures of various materials because of the ability to control the size and shape of nano-deposits by varying the laser parameters, the gas atmosphere, target-substrate distance and substrate temperature [18]. This technique is highly suitable for depositing oxide thin films at a relative high deposition rate and low cost. However, the

* Corresponding author. Tel.: +91 361 2582748/+91 9954604503. E-mail address: [email protected] (G. Shukla). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.05.162

interaction of laser light with the solid targets is a complicated process and is not yet understood. It consists of different stages: the laser ablation of the target; plasma generation; laser interaction with the plasma; plasma expansion and collision with a substrate. Laser-produced plasma is transient in nature with characteristic parameters that evolve quickly and are highly dependent on irradiation conditions such as incident laser intensity and pulse duration, laser wavelength, irradiation spot size, ambient gas composition, and ambient pressure. The stages of the plasma generation, laser interaction with plasma and plasma expansion play a very important role in the thin film growth process. In fact, during temporal evolution of the laser induced plasma (LIP), excitation and ionization of the evaporated material occurs, therefore, it is important to define its thermodynamic parameters, such as electron number density Ne, neutral number density N, temperatures of electrons Te, atoms Ta and ions Ti. In order to optimize and control the thin film growth process it is necessary to study the dependence of these parameters of the LIP on the deposition conditions and to develop suitable diagnostics. For measuring and controlling the parameters of the LIP the method of OES is used extensively [19–23]. It is based on the study of the spectral distribution of line intensity and broadening in emission spectra. In this paper we report on optical emission spectroscopic studies of TiO2 plasma plume accompanying pulsed laser ablation. Atomic and molecular species are identified through the characteristic emissions in recorded spectra. The electron temperature

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Fig. 1. Experimental setup.

Te and number density Ne are determined from relative atomic and ionic line intensity measurements using Boltzmann and Stark broadening measurement techniques [24,25]. The properties of the created plume were investigated by acquiring spectra at different positions from target surface and at various ambient pressures. The velocity of emitted species is determined by recording spatiotemporal variations at different position from target surface.

excited species within expanding plasma, the output of Photo Multiplier Tube (PMT) was fed into Tektronix 2000 Digital storage oscilloscope (DSO) interfaced with computer.

2. Experimental setup

The plasma emission was recorded at several distances normal and parallel to target surface at different background gas pressures. At distance (<2 mm) close to the target surface an inverse continuum emission was observed. The emission spectrum is attributed to both the elastic collisions of electrons with the ions and atoms (free–free emission) and recombination of electrons with the ions (free–bound emission). Fig. 2 shows the typical optical emission spectrum of TiO2 at 0.1 mbar oxygen ambient and Fig. 3 shows optical emission spectrum of TiO2 at 0.1 mbar argon ambient, at a distance of 1 mm away and parallel to target surface at laser intensity of 7  1010 W/cm2. Various marked transitions of neutral titanium (Ti I), neutral oxygen (O I), neutral argon (Ar I) and single ionized titanium (Ti II) were identified using standard table [26]. Some of the lines have not been marked for the sake of clarity in presentation, however most of the unmarked lines correspond to either Ti (I) or Ti (II) transitions. Fig. 4 shows the emission bands 3 þ þ 3 of excited oxygen molecules (L Su  b Sg , Schumann–Runge system) typically degraded to a longer wavelength. In particular the (0.15) vibrational transition headed at 351.7 nm dominates [27]. It should be noted that the intensity ratio between measurements shown in Figs. 2 and 4 is about 1:100. Because of transitory nature of laser-produced plasmas, the atomic and ionic population present in the plume rapidly evolved with time and position. The line intensity of the observed transitions increases with ambient pressure. Also intensities of Ti I and Ti II lines are higher in the presence of oxygen than that in argon.

The experimental setup is shown in Fig. 1. A Q-switched Nd:YAG (HYL-101 Quanta systems) laser with pulse width of 8 ns at full width half maximum, laser energy 240 mJ/pulse maximum at a repetition rate of 10 Hz, operating in the second harmonic mode (532 nm) was used for creating titanium oxide (TiO2) plasma in the presence of oxygen and argon gases. The laser beam was focused using a lens onto the titanium oxide (99.99% pure, sintered at 1200 8C for 18 h) target to a spot size of 130 mm. The vacuum chamber was evacuated to a base pressure of 106 Torr and was flushed with argon gas many times before introducing the gas in a controlled manner. The gas (O2/Ar) pressure in the chamber and the laser irradiance incident on the target were varied from 0.001 to 10 mbar and from 3.0  1010 to 8  1010 W/cm2, respectively. The target was continuously translated with an external motor so that each laser pulse falls on a fresh target surface. Plasma was imaged onto the slit of the monochromator (SPEX 750 M, Jobin Yvon) with a 25 cm focal length lens so as to have one-to-one correspondence with the plasma and its image on the slit of the monochromator. The plasma was imaged at various distances from TiO2 target surface in its direction of propagation perpendicular to the target surface. The output from the monochromator was detected with a photomultiplier tube IP28, Hamamatsu and results were processed by a computer interfaced with monochromator. To measure spatio-temporal variation and expansion velocity of

3. Results and discussion 3.1. Plasma emission

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3

þ

3

þ

Fig. 4. (0.15) Vibrational transition of O2 Schumann–Runge system (L Su  b Sg ) resolved with 1200 l/mm grating during TiO2 laser ablation.

Fig. 2. Emission spectra of laser ablated TiO2 plasma in 0.1 mbar oxygen ambient at a distance of 1 mm from the target surface.

where Imn, gm, Amn, lmn and Emn are the line intensity, statistical weight, transition probability, transition wavelength and excited level energy, respectively; T and k are the electron temperature and Boltzmann constant, respectively; N(T) is the number density of neutrals, and U(T) is the partition function. For the transition upper state is labeled as m and lower state by n. We have estimated the excitation temperature by using five lines of neutral titanium at 399.8, 461.7, 462.3, 464.5 and 484.0 nm. The spectroscopic details of the transition lines are listed in Table 1. 3.3. Electron number density The plasma density can be estimated from the broadened profile of the spectral lines. The main contributions to the broadened line profile are the Stark effect, Doppler broadening, and instrumental width. The Doppler broadening can be estimated from the relation [29,30],

Dld ¼ 2l

Fig. 3. Emission spectra of laser ablated TiO2 plasma in 0.1 mbar argon ambient at a distance of 1 mm from the target surface.

3.2. Electron temperature The characterization of the laser-induced plasma through the determination of their main parameters, i.e., electron temperature (Te) and electron number density (Ne), is important to understand the dynamics of the process. The electron temperature is a crucial parameter and it is estimated by the Boltzmann plot method [28],   NðTÞ lmn Imn Em ¼ ; exp UðTÞ g m Amn kT

(1)

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2kT ln 2 mc2

(2)

Here l(m) is the wavelength, k (J K1) is the Boltzmann constant, T (K) is the absolute temperature, m (kg) is the atomic mass and c (m s1) is the velocity of light. At a temperature of 10,000 K, the Doppler width is estimated 0.005 nm for the transition at 374.1 nm. The instrumental width of the Spectra Max 750 M spectrometer system is determined as 0.01 nm using spectra of two mercury lines at 578.88 and 576.75 nm. Thus, the dominating contribution to the line width is attributed to the Stark effect. The Stark broadening of various spectral lines of Ti (I) at different O2 pressures is shown in Fig. 5. The full width at half maximum of a Stark broadened line is expressed as [31,32], 

Dl ¼ 2v

Ne

10

16



    Ne 1=4 Ne 1=3 ð1  1:2ND Þv þ 3:5A ; 16 16 10 10

(3)

Table 1 Spectroscopic parameters of neutral Ti transition lines used to calculate electron temperature (Te).

Ti Ti Ti Ti Ti

I I I I I

Wavelength (A˚)

Aki (s1)

Ei (cm1)

Ek (cm1)

Configurations

Term

Ji–Jk

gi–gk

3,998.64 4,617.27 4,623.09 4,645.19 4,840.87

4.08e + 07 8.51e + 07 5.74e + 07 8.57e + 07 4.08e + 07

386.874 14,105.68 14,028.47 13,981.75 7,255.369

25,388.334 35,751.51 35,652.95 35,503.40 27,907.026

3d2 3d3 3d3 3d3 3d2

a a a a a

3

4–4 3–4 2–3 1–0 2–2

9–9 7–9 5–7 3–1 5–5

4s2–3d2 (3F)4s4p(1P8) (4P)4s–3d3 (4P)4p (4P)4s–3d3 (4P)4p (4P)4s–3d3 (4P)4p 4s2–3d2(1D)4s4p(1P8)

F–y 3F8 5 P–w 5D8 5 P–w 5D8 5 P–w 5D8 1 D–y 1D8

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Fig. 6. Variation of electron temperature and electron number density with distance (square: O2 ambient; circle: Ar ambient).

form, 

Dl ¼ 2v

Ne



(4)

16

10

The Stark broadened emission line profile is generally Lorentzian and the spectral lines obtained in the present study shown in Fig. 5 fit very well with the typical Lorentzian profile. The calculations for the electron temperature have been carried out under the assumption that the plasma is at the local thermodynamic equilibrium (LTE); i.e., equilibrium exists in a small region of space, although it may differ from region to region. One of the conditions that shows that the plasma is at LTE is McWhirter’s criterion [33], 3

Ne ðcm3 Þ  1:6  1012 T 1=2 ðDEÞ

(5)

Here, T (K) is the electron temperature and DE (eV) is the difference between the energy states. At the highest evaluated temperature of 12,200 K (close to the target surface), Eq. (5) yields Ne 5.8  1015 cm3, whereas the experimentally determined value is 1.24  1017 cm3 for the TiO2 plasma at 0.1 mbar O2 pressure close to the target surface, which indicates that the plasma can be considered at LTE. The transition line used to determine the electron number density is an isolated line at 374.1 nm {3d2 4s2– 3d2(1D) 4s4p(3P8)}. We have obtained the same values of electron density using Stark broadening measurements for the transition line at 453.3 nm. 3.4. Spatial variation of electron temperature and electron number density

Fig. 5. Stark broadened profiles of Ti (I) spectral lines with O2 pressure (a) Ti I (374.1 nm), (b) Ti I (453.3 nm), and (c) O I (777.1 nm).

Here, v(nm) is the electron impact parameter, which is a weak function of temperature, A (nm) is the ion broadening parameter, Ne (cm3) is the electron number density, and ND is the number of particles in the Debye sphere. The contribution due to the ion broadening is much smaller than the broadening due to electron impact; therefore, it can be neglected and, thus, only the first term on the right is sufficient to extract the number density. Hence Eq. (3) reduces to a well known simple

In the experiments on spatial behavior of the plasma generated by the second harmonic of the Nd:YAG laser, the laser irradiance is adjusted to 7  1010 W cm2 and the sample is placed in the environment of 0.1 mbar of argon and oxygen pressure. The spectra were recorded at different axial distances from the target surface, from 0 to 4.0 mm, and the electron temperature and electron number densities are estimated at each axial point. The variation of electron temperature and electron number density with distance in O2 and Ar ambient is shown in Fig. 6. It is observed that the electron temperature decreases from 12,200 to 7800 K for 4 mm distance variation from the target surface. The number densities close to the surface of the target are estimated as 12  1016 cm3 decrease to 1.2  1016 cm3 as the spatial distance is varied from 0 mm to 4 mm away from the target in both O2 and Ar ambient. As the plasma expands, the temperature rapidly drops. However, the variation is small at a

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Fig. 7. Variation of electron temperature and electron number density with pressure (square: O2 ambient; circle: Ar ambient).

later time because energy is regained in the recombination of the ions. Similarly, at the initial stage, the electron number density rapidly decreases, and at latter distances, the variation is small. It is observed that the electron number density achieves a maximum value near the surface of the target and decreases as a function of distance, which is due to the fact that when the laser photons strike a solid surface, particles are emitted during the laser pulse. The free electrons initially grow by multiphoton ionization. However, with the creation of these electrons, the cascade ionization starts and dominates the production of free electrons. When a high electron concentration approaches a critical electron density, the growth of the density is slowed down by the onset of the electron-ion recombination process. On the trailing edge of the laser pulse, the electron recombination cannot be compensated by the production of free electrons because of the decrease in the laser irradiance. It appears that the temperature and density approach similar values at large distances irrespective of the initial condition of the plasma. 3.5. Effect of background pressure The ablation dynamics of the plume in ambient gas is quite different from its expansion in vacuum. The plume expansion dynamics is determined by the interaction of the plasma species with the background gas, which results in a change in the properties of the plasma emission. An increase in the pressure of ambient buffer gas causes an increase in the emission intensity of

Fig. 8. Variation of electron temperature and electron number density with laser irradiance in O2 and Ar ambient (square: O2 ambient; circle: Ar ambient).

the transition lines. Since the cooling due to collisions takes place when the plume expands in an ambient gas, the efficiency of cooling and intensity of different plasma species, strongly depends on the ambient gas parameters like elastic collisions, electron heating due to colllisional de-excitation of metastable ions and the recombination effects. The elastic collision term depends among various other factors on mass of the ambient gas while recombination effects are much more likely to occur due to presence of low energy electron in plume. In the presence of oxygen gas the emission signal is enhanced than in the presence of argon gas because the cooling rate due to plasma expansion is higher in a light gas than in a heavy gas. This result in increase of three body recombination rate hence we observe increase in excited neutral and ionic species and their line intensities. The electron temperature and electron number density of the TiO2 plasma at different pressures and gas environments is estimated using Eqs. (1) and (4) respectively, which is shown in Fig. 7. In Ar and O2 ambient environments the temperature varies as (7800–10,500) K and (7800  11,400) K respectively, whereas the electron number density varies as (10.7  1016–1.4  1016) and (11.6  1016–1.2  1016) cm3 respectively with pressure. The estimated value of the temperature and the electron number density is higher in the oxygen environment as compared to the argon environment. The increase in temperature and the electron number density of the expanding plume in background gas is mainly due to three processes: (i) excitations due to collisions with background atoms, (ii) electron impact excitation and (iii) the shock waves are created in the ambient gas by the rapidly expanding plasma of the ablated matter and these shock waves ionizes the gas. 3.6. Effect of laser irradiance We have determined the electron temperature and electron number density for different values of the laser irradiance. The laser irradiance is varied from 3  1010 to 8  1010 W cm2 and the sample is placed in an environment of 0.5 mbar of oxygen or argon pressures. Fig. 8 shows the variation of the electron temperature for the laser produced plasma with respect to the laser irradiance at a distance of 0.5 mm from the target surface. The electron temperatures increase from 4400 to 10,600 K for oxygen and 4000 to 10,200 for argon as the laser irradiance is varied from 3  1010 to 8  1010 W cm2. Similarly, Fig. 8 shows the variation in the electron number density as a function of laser irradiance.

Fig. 9. Variation of neutral O (I) intensity with distance for different O2 pressures.

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Fig. 10. Variation of neutral Ti (I) intensity as a function of distance for different O2 pressures.

3.7. Intensity variation with pressure The emission intensity of O I line (777.14 nm) as a function of distance from the target surface is shown in the Fig. 9. This gives a measure of the amount of excited species within the plume, arriving at a given point. The intensity of O I emission, generated during the ablation at various pressures of ambient gas, showed steady increase up to 12 mm distance from the target and then a monotonic decrease with further increase in distance. This is due to the increase in the excitation of oxygen atoms by the collision with the particles in the plume in the early stage of plume expansion. At the later stage the collision probability decreases due to the increase in the mean free path and hence the observed decrease in intensity. However, it should be noted that the increase in oxygen pressure increases the emission intensities. In the presence of argon gas, oxygen line intensities were found to be very small and insensitive to distance variations. The variation of neutral Ti (I) intensity and single ionized Ti (II) intensity as a function of distance for different O2 pressures is shown in Figs. 10 and 11 respectively. Increase in oxygen pressure enhanced the emission line intensities of the neutral Ti and neutral oxygen while the emission line intensities of Ti ions decreased. This could be attributed to the fact that increase in pressure will decrease the mean free path of particles in the plume, thereby increasing the probability of collision of Ti ions with electrons in the plume. This will enhance the rate of electron-ion recombination, leading to the increase in the number of excited neutral titanium atoms.

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Fig. 11. Variation of single ionized Ti (II) intensity as a function of distance for different O2 pressures.

while velocities of neutral species seem to be vary slowly with O2 pressure. Fig. 13(a and b) shows typical spatio-temporal variations of Ti I [399.8 nm, 3d24s2–3d2(3F)4s4p(1P8)] and O I [777.1 nm, 2s22p3(4S8)3s–2s22p3(4S8)3p] transition lines at 0.1 mbar of O2 pressure respectively. While Fig. 13(c and d) shows spatiotemporal variations of Ti II [430.0 nm, 3d3–3d2(3F)4p] transition lines at 10 and 0.1 mbar O2 pressure respectively. The time evolution of the spectral emission profiles shown in Fig. 13 clearly reveals that the species ejected during laser ablation of TiO2 exhibit multiple peaks for the TOF (time of flight) distribution. This type of multiple peak distribution in laser produced plasma has been reported previously [34–36]. This behavior can be attributed to formation of Knudsen layer with stopped and/or backward moving species developed due to collisions between the ejected species in the high pressure region of the initial expansion [37]. The ions located at the front of the plasma acquire the largest energy during hydrodynamic acceleration and the interaction time for recombination is very much reduced. As observed in the spatio-temporal studies, these ions propagate with very high expansion velocities (>8  104 m/s).

3.8. Spatio-temporal variation and measurement of neutral and ion velocity The velocity measurements and spatiotemporal variations of Ti I, O I and Ti II species were done by feeding PMT output to DSO. The expansion velocities of kinetic peaks are strongly affected by the background gas pressure. With increasing background pressure, due to enhanced collisions between excited target species and ambient gas species, expansion velocity of various excited species tend to decrease. Thus, the ambient gas acts as a retarding media, slowing the plume development and bringing the ion energy closer to the energy of excited neutral atomic species in the plasma. This is very much evident from Fig. 12. Fig. 12 shows Ti I, O I neutral and Ti II ion velocity variation with O2 pressure. It can be seen that Ti II ion velocity decreases exponentially with increase in O2 pressure

Fig. 12. Ti I, O I neutral and Ti II ion velocity variation with O2 pressure.

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Fig. 13. Spatio-temporal variations of (a) Ti I (399.8 nm), (b) O I (777.1 nm) at 0.1 mbar O2 pressure, (c) Ti II (430.0 nm) at 10 mbar O2 pressure, and (d) Ti II (430.0 nm) at 0.1 mbar O2 pressure.

These ions interpenetrate into background gas without much attenuation in the kinetic energy. The ions located in the inner plume layers are accelerated much less due to hydrodynamic expansion and remain in a denser state subjected to strong recombination. This phenomenon finally results into the plume sharpening and splitting with time. Plume splitting behavior suggests that higher kinetic energy particles are emitted closer to the target surface as also supported by our electron temperature measurements. 3.9. Implications for PLD of TiO2 thin films The steady behavior of electron density and electron temperature above 4 mm distance from the target, at 5.5  1010 to 7  1010 W/cm2 laser irradiance and in the range of 5–50 mbar O2 ambient pressure suggest that high quality stoichiometric TiO2 thin films can be deposited under these optimum conditions. Also maximum intensities of Ti (I) and O (I) around 10 mm from target surface indicates single phase film deposition can be expected when substrate is placed beyond this distance. Recent reports show that stochiometric and crystalline thin films of TiO2 can be deposited using PLD in O2 ambient by heating and keeping the substrate at a few centimeters away from the target [38–40].

species such as TiO, TiO2 could be identified except O2 band at 351.7 nm. The electron temperature (Te) has been determined from the Boltzmann plot method and electron number density (Ne) is estimated from the Stark broadened profile of the spectral lines. The electron temperature and electron number density show similar spatial behavior. Both decrease with distance from the target surface and become more or less steady after 4 mm distance from the target. The electron temperature and electron number density found to be decrease exponentially with increase in O2 or Ar pressure. The estimated value of the temperature and the electron number density is higher in the oxygen environment as compared to the argon environment. The variation of electron temperature and electron number density with laser irradiance is reported. The electron temperature and electron number density increases with laser irradiance. Intensity variation of neutral and ion species within laser produced TiO2 plasma with O2 pressure and distance from target surface is presented. Increase in oxygen pressure enhanced the emission line intensities of the neutral Ti and neutral oxygen while the emission line intensities of Ti ions decreased. The velocity measurement of neutral and ion species has been done using spatiotemporal variations of transition lines of respective species. Ti II ion velocity decreases exponentially with increase in O2 pressure while velocities of neutral species seem to be vary slowly with O2 pressure.

4. Conclusion References In conclusion, the titanium oxide plasma has been generated during pulsed laser deposition process by the second harmonic of a Nd:YAG laser. Emission spectra were mainly due to neutral and ionized Ti and neutral O species. No spectra attributed to molecular

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