Deposition of nanostructured Cr2O3 on amorphous substrates under laser irradiation of the solid–liquid interface

Applied Surface Science 138–139 Ž1999. 449–454

Deposition of nanostructured Cr2 O 3 on amorphous substrates under laser irradiation of the solid–liquid interface S.I. Dolgaev, N.A. Kirichenko, G.A. Shafeev

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General Physics Institute, Russian Academy of Sciences, 38, VaÕiloÕ street, Moscow, 117942, Russian Federation

Abstract The deposition of epitaxial films of Cr2 O 3 , Fe 2 O 3 , and MnO 2 under laser irradiation of the interface sapphire-absorbing liquid has been reported recently. In similar experimental conditions, laser irradiation of the amorphous solid–liquid interface results in deposition of a polycrystalline film. In the present paper, the deposition of Cr2 O 3 on a glass substrate induced by radiation of a Cu vapor laser is studied. Irradiation of the interface glass–aqueous solution of CrO 3 at fluence of 2–5 Jrcm2 at l s 510.6 nm results in the deposition of Cr2 O 3 which consists of oriented nanoclusters with size of 8–20 nm. The subsequent chemical etching of the glass results in a free-standing film of Cr2 O 3 with lateral dimensions of several mm2 and 30–50 mm thick. The mathematical model of the deposition process is considered based on the semi-analytical solution of the non-stationary heat diffusion equation for a gaussian profile of laser beam. It is shown that during a ns laser pulse the maximum of the temperature shifts from the interface towards the absorbing liquid. The results of calculations are qualitatively consistent with experimental data on the dependence of the thickness of Cr2 O 3 deposit on the heat diffusion coefficient of a solid substrate. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Cr2 O 3 deposit; Amorphous substrates; Laser irradiation; Solid–liquid interface

1. Introduction Laser irradiation of the interface transparent solid-absorbing liquid is characterised by a strong coupling of various phenomena which take place in both media in the vicinity of interface. The irradiation of the interface through the substrate transparent at laser wavelength results in high temperature gradients in the solid and its fast ablation w1x. For instance, the ablation of sapphire induced by a Cu vapor laser at its interface with absorbing liquids can )

Corresponding author. Fax: q7-095-135-0376; E-mail: [email protected]

be as high as 0.2 mmrs which is limited by the depth of heat diffusion length during the laser pulse. The absorbing liquid is completely replenished to the forthcoming laser pulse owing to strong convective flows induced in the liquid phase. During the laser pulse the liquid within the radiation absorption depth is superheated which results in its decomposition. In some cases the products of this decomposition are not soluble in the liquid, so a dense suspension is formed in the vicinity of the interface. Further behaviour of this suspension depends on experimental parameters, such as laser pulse duration, its intensity, absorption coefficient, etc. For instance, if the surface of sapphire is ablated, the condensation of the

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 4 3 9 - 5

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S.I. DolgaeÕ et al.r Applied Surface Science 138–139 (1999) 449–454

oxide suspension leads to the formation of epitaxial oxide layer on its surface, as it has been demonstrated recently for Cr2 O 3 , Fe 2 O 3 , and MnO 2 w2x. The crystallographic orientation of the deposited film is determined both by orientation of the substrate and crystallographic structure of the particles of suspension. One might expect that the structure of the film deposited on amorphous substrate will depend mostly on the structure of the oxide clusters and, hence, will stem more light on the dynamics of clusters formation. This paper deals with deposition of Cr2 O 3 on a glass substrate upon exposure of its interface with highly absorbing aqueous solution of CrO 3 . The average size of clusters that make up the deposited film is about 10 nm. 2. Experimental Deposition of Cr2 O 3 film was initiated with the help of a copper vapor laser with unstable resonator, wavelength l s 510 nm, pulse duration of 10 ns FWHM, repetition rate of 8 kHz. The radiation was focused with the help of an objective with N.A.s 0.3

onto the interface glassrliquid through the glass substrate under a typical laser fluence of 2–5 Jrcm2 . The aqueous solution of CrO 3 Ž6 molerl. was used as the absorbing liquid with measured weak signal absorption coefficient of 500 cmy1 at the laser wavelength. The weak signal absorption coefficient was measured using a calibrated glass cell and photodiode at average intensity of laser beam of several mWrcm2 that causes no decomposition of the solution. The liquid in a Teflon cell was covered by a horizontal glass substrate transparent at the laser wavelength. The cell was mounted on a computerdriven X–Y stage for the displacement of the cell under the laser beam with an accuracy of a 2 mm and scanning velocities ranging from 30 to 3000 mmrs. The morphology of the deposited film was studied with help of a scanning electron microscope ŽSEM.. The chemical composition of the film was identified by X-ray diffraction and by Raman spectroscopy. Arq laser at 488 nm was used as the excitation source, spectral resolution was 2 cmy1 and spatial resolution was 2 mm. The structure of the film was

Fig. 1. Raman spectra of Cr2 O 3 layer deposited on glass substrate from aqueous solution of CrO 3 . Thickness of the layer is about 30 mm. The spectrum is taken from the side of the film which was in contact with the solution.

S.I. DolgaeÕ et al.r Applied Surface Science 138–139 (1999) 449–454

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studied using the transmission electron microscopy ŽTEM..

3. Results Scan-irradiation of the interface glass–aqueous solution of CrO 3 at 1–2 mmrs and at laser fluence of 2–5 Jrcm2 results in deposition of a dense film with little or no damage to the glass substrate. At higher laser fluences, the glass surface is covered by cracks, and the partial detachment of the deposited film occurs. The decrease of the scanning velocity also leads to the damage of glass substrate. X-ray analysis of the deposited film shows the peaks of polycrystalline Cr2 O 3 , according to JCPDS reference data w2x. The peaks are broadened which indicates the small size of the regions of coherent scattering of X-radiation. The Raman spectrum of the film shows strong peaks at 306, 350, 550 and 730 cmy1 Žsee Fig. 1.. The latter peak is broadened compared to reference data. The deposited film is stable to HF that allows detaching it from the glass substrate. In typical experimental conditions its thickness is comprised between 10 and 30 mm, in contrast to Cr2 O 3 film deposited on sapphire in the same experimental conditions, where the film thickness is 2–3 mm w2x. Fig. 2 shows the morphology of the deposited film from the side of glass substrate Ža. and from the side of liquid phase Žb.. From the glass side, the film exhibits a cell-like structure with average period of the structure of order of the size of the laser spot. This structure is likely due to the deposition of Cr2 O 3 film on the cracks in the glass surface produced by its inhomogeneous heating from the liquid. So the surface in Fig. 2, a is a Cr2 O 3 replica of the glass surface. The inset in Fig. 2, a shows the enlarged view of the cells’ wall made of particles of about 1 mm. Also, one can see that the wall consists of rounded particles of 3–4 mm in size. The outer side of the deposited film is more smooth, though a certain relief still can be seen despite to large thickness of the deposited film ŽFig. 2b.. The TEM analysis of the deposited film shows that it consists of large aggregates of about 100 nm is size which in turn is composed of smaller particles of 8 = 20 nm. These smaller Cr2 O 3 particles have

Fig. 2. SEM view of Cr2 O 3 film detached from the glass substrate in HF solution. Glass side Ža., solution side Žb.. Scale bar denotes 30 mm. Inset in Ža. shows the enlarged view of the film with spherical particles. Scale bar denotes 2 mm.

the hexagonal structure. Thus, the hierarchy of dimensions observed in the deposited Cr2 O 3 film is as follows: 8–20, 100, 1000, 4000 nm. Since Cr2 O 3 is transparent at the laser wavelength and has higher refractive index than surrounding liquid, the laser radiation may be captured in the film. So, a fractal structure of the deposited film might be expected.

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S.I. DolgaeÕ et al.r Applied Surface Science 138–139 (1999) 449–454

4. Model of the temperature distribution In the following model the solid substrate Ž z - 0. is considered as semi-infinite and transparent, while the gaussian laser beam is absorbed in the semi-infinite liquid Ž z ) 0. with absorption coefficient a . Laser radiation is switched on at t s 0 and has constant intensity I0 during the pulse duration t .

1 E T1 a1 E t 1 E T2 a2 E t

1 E s

r Er 1 E

s

r Er

E T1

ž / ž / r

r

I Ž r , z . s I0 Ž t . eyr

Er

E T2 Er

2

q

E 2 T1

q

E 2 T2

r r 02 y a z

e

E z2 Ez ;

2

;

z-0

a q k2

I Ž r , z ,t . ;

I0 Ž t . s

½

I0 , 0,

tFt t)t

Fig. 3. The calculated distribution of dimensionless T Ž z,t . after the end of the laser pulse. Time is normalized to t c s Ž r 0 . 2ra, where a is the heat diffusivity coefficient and r 0 is the laser beam radius. t s 0 Ž1., 10y2 Ž2., 2 = 10y2 Ž3., 3 = 10y2 Ž4., 4 = 10y2 Ž5.. The temperature is normalized to T0 s a aI0 tr2 k 2 . Interface sapphire–water Ža., glass–water Žb..

S.I. DolgaeÕ et al.r Applied Surface Science 138–139 (1999) 449–454

z s 0,

°T s T ET ¢k E z s k

~

1

2

aIsy

E T2

1

1

t s 0,

453

2

Ez

T1 s T2 s 0

EI Ez

The final solution may be significantly simplified assuming that the heat diffusivities of both media are equal: a1 s a 2 s a. The results of numerical calculations for the interface sapphire–water and glass water are shown in Fig. 3. The characteristic time of the model is given by t c s Ž r 0 . 2ra s 10y5 s in the experimental conditions of the present work, and the time is normalized to this value. The temperature is normalized to T0 s a aI0 tr2 k 2 . Within the present model, the only difference between both solids lies in the ratio of thermal conductivities: k 1rk 2 s 50 for sapphire and 5 for water. One can see, however, that the temperature distribution for both substrates is different: the temperature of the glass substrate is higher than that of sapphire. In both cases, the maximum of the temperature is situated in the absorbing liquid and shifts towards the bulk of the liquid with time. The glass substrate has higher temperature than sapphire one, while the maximal temperature in the liquid is independent on the substrate. The calculated temperature distribution allows suggesting the following sequence of the deposition process. The decomposition of CrO 3 molecules takes place in the region of high temperature, where the liquid is in the superheated state. In this region the size of Cr2 O 3 nuclei is small, since the high temperature does not favour their agglomeration. During the diffusion to the interface to low temperature region, these nuclei commence to agglomerate, since the critical radius of the nuclei in this region is higher. The time of condensation is limited by fast cooling of the whole irradiated region, so only some clusters may reach the interface while the others remain in the bulk of liquid phase. These suggestions are illustrated in Fig. 4. The diffusion of clusters near the interface proceeds much slower than in the high temperature region, so the higher temperature of the glass substrate favours the deposition of a thicker film compared to sapphire.

Fig. 4. Sketch of the process of cluster condensation at the interface transparent solid-absorbing liquid exposed to laser radiation through the substrate. Temperature distribution Ža., relative concentration of clusters Žb.. The lines b.p. and c.p. in Ža. indicate the boiling point and critical point for water, respectively. The size of clusters increases upon diffusion to the cold substrate.

The existence of relatively cold layer of liquid near the interface causes a fast condensation of clusters. On the other hand, larger clusters have lower mobility w3x, so big clusters may not reach the interface at all. The size of deposited clusters depends on a number of experimental parameters, such as laser pulse duration, nature of substrate, liquid, etc. Unlike Cr2 O 3 , which is transparent at the laser wavelength, the metal clusters are highly absorbing, and in this case the thickness of the deposited film decreases with the increase of laser beam intensity w3,4x. Thus, the irradiation of the interface glass-absorbing liquid ŽCrO 3 . with laser pulses of ns duration results in deposition of polycrystalline Cr2 O 3 film. The film consists of oriented clusters with size of several nanometers. The model of the temperature distribution predicts the existence of relatively cold layer of liquid adjacent to the interface. The temperature of the transparent substrate depends on its thermal conductivity, as well as the thickness of deposited oxide film.

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S.I. DolgaeÕ et al.r Applied Surface Science 138–139 (1999) 449–454

Acknowledgements

References

The authors are indebted to I.I. Vlasov for the Raman measurements, to S.V. Lavrischev for SEM pictures, and to V. Babina for TEM characterization of deposited films. P. Hoffmann and B.S. Luk’yanchuk are thanked for valuable discussions of the results.

w1x S.I. Dolgaev, A.A. Lyalin, A.V. Simakin, G.A. Shafeev, Quantum Electron. 26 Ž1. Ž1996. 65. w2x S.I. Dolgaev, V.V. Voronov, G.A. Shafeev, Appl. Phys. A 66 Ž1998. 87–92. w3x G.A. Shafeev, Thin Solid Films 218 Ž1992. 187. w4x K. Bali, T. Szorenyi, M.R. Brook, G.A. Shafeev, Appl. Surf. ¨ Sci. 69 Ž1993. 75.