Functionalized polysiloxane thin films deposited by matrix-assisted pulsed

laser evaporation for advanced chemical sensor applications

E.J. Houser a, D.B. Chrisey a, M. Bercu b, N.D. Scarisoreanu c, A. Purice c,D. Colceag c, C. Constantinescu c, A. Moldovan c, M. Dinescu c,*

a US Naval Research Laboratory, Washington, DC 20375-5345, USAb University of Bucharest, Faculty of Physics, Magurele, Bucharest, Romania

c National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-16,

Magurele, 077125 Bucharest, Romania

Received 3 May 2005; accepted 15 July 2005

Available online 5 December 2005

Abstract

High-quality thin films of fragile chemoselective polymers with precise and accurate thickness, density and chemical integrity are required for

advanced chemical sensor applications. While these attributes are difficult to achieve by conventional methods, we have successfully demonstrated

the matrix-assisted pulsed laser evaporation (MAPLE) deposition of thin films of especially synthesized fluoro-alcohol substituted carbo-

polysiloxane polymer coatings. The quadrupled output of a Nd:YAG laser (265 nm) served as the laser source and depositions were done in a

background pressure of N2. Using various solvents appropriate to solvate this polymer (e.g. tetrahydrofuran, acetone and chloroform) and varying

the laser fluence, we optimized the deposition of high-quality thin films on 1 cm2 double-polished silicon substrates. The best solvent used as

matrix was proved to be acetone. Under these conditions, the important functional groups were reproduced and observed by Fourier Transform

Infrared Spectroscopy (FTIR) as compared to the drop cast films and the surface roughness was analysed using Atomic Force Microscopy (AFM)

and found to be much smoother than conventional wet deposition techniques.

Crown Copyright # 2005 Published by Elsevier B.V. All rights reserved.

Keywords: MAPLE; Polymer; Pulsed laser

www.elsevier.com/locate/apsusc

Applied Surface Science 252 (2006) 4871–4876

1. Introduction

In the past few years, matrix-assisted pulsed laser evaporation

has been of growing interest as a method of deposition for

polymeric and other organic films [1]. The polymeric and organic

films can be used in medicine, electronics, optics and as chemical

sensors [1–3]. Thin polymer films can be used as chemoselective

coatings [4] for SAW devices or as drug delivery coatings [1].

Functionalized polysiloxanes can possess the desirable physical

properties for chemical sensing applications. They show

environmental stability and good thermal stability.

Pulsed laser ablation is commonly used for the deposition of

various thin films, most of them being inorganic materials [1,5].

PLD cannot be used for all kinds of polymers, biopolymers and

* Corresponding author.

0169-4332/$ – see front matter. Crown Copyright # 2005 Published by Elsevier

doi:10.1016/j.apsusc.2005.07.159

proteins because of the high laser fluences used that can create

photochemical decompositions. Even at low fluences, some

polymers are extremely photosensitive [6].

Recently, matrix-assisted pulsed laser evaporation (MAPLE)

has been used instead of PLD for polymers deposition. In

MAPLE, the target consists of the material (usually 0.1–2 wt%)

dissolved in a solvent [7]. The target is cooled down to a solid

state and it is evaporated using a laser emitting in UV. The

material is collected on a nearby substrate parallel to the target

and the solvent is eliminated through the vacuum system.

Compared to PLD, MAPLE is a gentle deposition technique for

polymers and organic films [6]; the laser energy is absorbed only

by the solvent, and therefore, the bond dissociation and chemical

modification can be avoided [2].

In this paper, we report the synthesis of fluoro-alcohol

substituted carbo-polysiloxane using MAPLE. We used three

different solvents: acetone, tetrahydrofuran and chloroform and

the obtained films were studied and compared.

B.V. All rights reserved.

E.J. Houser et al. / Applied Surface Science 252 (2006) 4871–48764872

Fig. 1. AFM image on a 20 mm � 20 mm area of a sample deposited from a chloroform and polymer target; RMS roughness = 5.6 A; vertical scale 5.1 nm.

2. Experimental

2.1. Method

A Nd:YAG laser working at a repetition rate of 10 Hz,

265 nm wavelength (5–7 ns) and an incident fluence in the

range of 0.03–0.18 J cm�2 was used for MAPLE. As substrates

we used 1 cm2 double-polished silicon placed at a distance of

5 cm from the target. The substrates were held at room

temperature during the deposition. In most of the cases, the

number of pulses was 20,000. In order to have uniform

evaporation the laser beam was translated onto the target, while

Fig. 2. Images of a sample deposited from a tetrahydrofuran and polymer target at 0.1

image of a 20 mm � 20 mm area, topography with slope shading, vertical scale 214

microscope on an approximately 1 mm wide area of the film.

the target was rotated with a motion feedthrough driven by a

motor. For controlling the temperature, two thermocouples

were placed in two different spots of the target holder. The

depositions took place in a nitrogen background pressure in the

range of 50 to 1000 Pa. During some depositions the pressure

varied probably due to non-concordance between the evapo-

rated solvent and the pumping flux.

2.2. Target preparation

We used different types of targets, depending on the solvent.

The solvents were tetrahydrofuran, acetone and chloroform.

7 J cm�2 laser fluence and nitrogen pressure of approximately 100 Pa: (a) AFM

nm; (b) phase image of the same area; (c) image obtained with the AFM’s video

E.J. Houser et al. / Applied Surface Science 252 (2006) 4871–4876 4873

Fig. 3. AFM on a 5 mm � 5 mm area of a drop, on a sample deposited from a tetrahydrofuran and polymer target; RMS roughness = 5.8 A; vertical scale 5.5 nm.

We prepared solutions of 4% polymer and 96% solvent. The

target holder, made of copper, was chilled with liquid nitrogen.

During deposition, the target temperature had to be smaller than

the freezing temperature of the solvent, so that the target

remained solid. The freezing temperature for tetrahydrofuran is

�108.4 8C, for acetone�94.9 8C and for chloroform�63.5 8C.

2.3. Analysis

The films surface aspect and roughness was analysed using

Atomic Force Microscopy (AFM). The chemical composition

was investigated by Fourier Transform Infrared Spectroscopy

(FTIR) technique. The sensitivity has been increased by the

accumulation of 30–60 spectra in the range of 400–4000 cm�1.

The incident beam was normal to the back-side of the sample,

being collected as a transmitted flux. The measurements were

performed at room temperature in the presence of a He–Ne

Fig. 4. (a) AFM image of 40 mm � 40 mm on a sample deposited from an acetone an

scale 93 nm and (b) image obtained with the AFM’s video microscope; the width

laser beam (60 mW) used by FTIR technique. The intensities of

the absorption bands shown in the figures are not relevant

because the width of the resulted samples were different.

3. Results and discussion

As seen in the AFM images, some of the samples reveal a

continuous, extremely smooth surface, covering the entire

exposed area of the substrate. The roughness (RMS deviation)

of the area shown in Fig. 1 (20 mm � 20 mm area of a sample

deposited from a chloroform and polymer target at a laser

fluence of 0.136 J cm�2 and a nitrogen background pressure of

100 Pa) is 5.6 A approximately, which is extremely low for

such type of materials.

On some other samples, only a fraction of the substrate’s

surface has been covered by the deposited polymer, as can be

clearly seen by the contrast between the different areas in the

d polymer target at a fluence of 0.05 J cm�2; RMS roughness = 9.8 nm; vertical

of the cantilever is 40 mm.

E.J. Houser et al. / Applied Surface Science 252 (2006) 4871–48764874

Fig. 5. FTIR spectra of the resulted polymers evaporated from targets prepared

by using acetone, THF and chloroform as solvents.Fig. 7. The components of the IR absorption band related to C–H bond

stretching vibration for the initial polymer.

phase image of Fig. 2 (for a sample deposited from a

tetrahydrofuran and polymer target as a result of 30,000 pulses

at 0.17 J cm�2 laser fluence and a nitrogen pressure of

approximately 100 Pa). The deposited polymer formed drops

with diameters up to 50 mm (Fig. 2(c)).

The AFM image of a 5 mm � 5 mm area on top of the

biggest drop presented in Fig. 2 shows a very smooth surface,

with a roughness of 5.8 A (Fig. 3).

Samples obtained when acetone was used as a solvent also

cover the entire exposed area of the substrates forming very

smooth films (Fig. 4(a)). Fig. 4(b) shows an approximately

1 mm wide area of one of the films.

FTIR analysis was used in order to identify the best

deposition conditions for the polymer thin films.

Fig. 6. CHn bond stretching vibrations of polymeric films deposited by MAPLE

technique at laser fluences in the range of 0.03–0.136 J cm�2. Acetone was used

as solvent for target preparation. The sample number and the amplification

factor for each curve is mentioned on the left side.

A first conclusion concerns the general comparison between

the use of the three solvents: tetrahydrofuran, acetone and

chloroform. In Fig. 5 are presented the spectra of three layers

obtained by using the three different solvents, in the same

experimental conditions identified to be the best as composition

reproducibility. It can be easily seen that tetrahydrofuran is not

the best choice as solvent, as many of the important infrared

active bands are not reproduced. After an accurate parametric

study, it resulted that the layers prepared by samples using

acetone as solvent fulfill the requirements for further

applications, as they are better reproducing the initial

composition. Thus, the further results presented in this paper

refer only to the layers prepared using acetone as solvent. For a

better investigation of the chemical composition of the

deposited layers, the spectra were divided in several parts.

Thus, in Fig. 6, there are presented the FTIR spectra in the range

of 2800–3100 cm�1 for samples deposited at different fluences

from 0.03 to 0.136 J cm�2. Each spectrum was amplified and

shifted along the vertical axis to reveal the changes of the IR

spectra resulted by increasing the laser pulse energy.

Fig. 8. The fluence dependence of the ratio between the integrated intensity of

the components extracted at 2868 and 2885 cm�1 and the absorption band at

2925 cm�1.

E.J. Houser et al. / Applied Surface Science 252 (2006) 4871–4876 4875

Fig. 9. FTIR spectra of polymeric films deposited by laser evaporation from targets prepared with acetone in the range of: (a) 1550–1760 cm�1 and (b) 1000–

1080 cm�1. The sample number and the amplification factor for each curve is mentioned on the left side.

We applied an extraction procedure of Gaussian components

and we obtained the absorption bands centered at the

wavelengths: 2864, 2885, 2970, 2925 and 3032 cm�1. For

the starting polymer, Fig. 7 shows a good match between the

experimental spectrum and the sum of the components. The

same extraction procedure has been applied to all the spectra.

The ratio between the dominant components of CHn

stretching vibration modes (2864, 2885 cm�1) and the most

intense absorption band at 2925 cm�1 (Fig. 6) has been

determined in relation with the laser beam fluence, as shown in

Fig. 8. The other stretching vibration modes (2970 and

3032 cm�1) were not discussed because of their relative

weakness. The two resulting dependences of I/I2925 on the

fluence at 2864 and 2885 cm�1, respectively, have opposite

Fig. 10. The fluence dependence of the ratio between the integrated intensity of

the absorption bands mentioned and the total absorption of CHn bonds in

between 2800 and 3070 cm�1.

behaviors. The first one is increasing to saturation and the

second one shows an almost exponential decay. The horizontal

lines correspond to the values of I/I2925 for the initial polymer

used for target preparation. The fluences which determine the

same ratio I/I2925 for the starting material and for the deposited

film are around 0.06 J cm�2. By increasing the fluence from

0.03 to 0.06 J cm�2, the structure of the films becomes more

similar to the structure of the polymer (we are referring for the

moment only to the CHn stretching vibration modes). By

further increasing the fluence, the degradation increases too.

In Fig. 9(a and b) are presented the FTIR spectra in the

ranges of 1550–1760 and 1000–1070 cm�1, respectively.

We have considered the ratio between the integral intensities

of the absorption bands, found at 1716, 1612, 1033 cm�1 and

Fig. 11. The fluence dependence of the absorption bands integral intensity at

1033, 1612, 1716 and 2922 cm�1.

E.J. Houser et al. / Applied Surface Science 252 (2006) 4871–48764876

the total integral absorbance of CHn groups in the range of

2800–3100 cm�1. The results are shown in Fig. 10, where

horizontal lines indicate the values of the absorption bands for

the initial polymer. Laser fluences between 0.03 and

0.08 J cm�2 determine almost the same ratios for the films

and for the starting material at 1612 and 1716 cm�1. These

absorption bands were assigned to C C and C O stretching

bond vibrations, respectively, by comparison with other reports

and spectra libraries.

Si–O bond stretching vibration can be found in the range of

970–1095 cm�1. It seems to correspond to the absorption band

at 1033 cm�1 for all the samples. Fig. 10 shows that a fluence of

about 0.136 J cm�2 is needed for having the same ratio for the

film and starting material, but this fluence is too high for the

other groups, such as CHn, C O and C C.

The rate of polymer deposition versus laser fluence has been

determined relative to the chemical bonds population men-

tioned above by using the corresponding integral intensities of

the absorption bands at: 1033, 1612, 1716, 2922 cm�1. They

were divided at the corresponding number of laser pulses used

for each sample fabrication (Fig. 11). Two regimes of the

deposition rate for all populations were identified. The first one

is related to a non-linear increasing of the deposited material up

to 0.08 J cm�2 followed by saturation at higher laser beam

fluence. It is interesting to mention that the highest resemblance

between the chemical structure of the deposited film and the

starting polymer was obtained in the first regime, namely below

0.08 J cm�2, related to specific bond population, such as CHn,

C O, C C.

4. Conclusions

MAPLE can be used successfully to grow thin films of

fluoro-alcohol substituted carbo-polysiloxane. The paper

presents an approach based on FTIR technique on the

characterization of MAPLE deposited polysiloxane onto

silicon. The spectra indicate that acetone solvents give the

best results relative to the chemical structure of the starting

polymer.

The structure of CHn bonds populations of the deposited

polymer was found to be the same as for the starting material at

fluences between 0.05–0.08 J cm�2 for samples prepared from

targets containing acetone as solvent. Smooth films can be

obtained from the same targets for an established window of

experimental conditions. Even at low laser fluences, the films

cover large areas of the substrates. On the other hand, it seems

that samples deposited from THF and polymer targets exhibit a

tendency to form drops at high laser fluences.

References

[1] D.M. Bubb, P.K. Wu, J.S. Horwitz, J.H. Callahan, M. Galicia, A. Vertes,

R.A. McGill, E.J. Houser, B.R. Ringeisen, D.B. Chrisey, J. Appl. Phys. 91

(4) (2002) 2055.

[2] P.K. Wu, B.R. Ringeisen, D.B. Krizman, C.G. Frondoza, M. Brooks, D.M.

Bubb, R.C.Y. Auyeung, A. Pique, B. Spargo, R.A. McGill, D.B. Chrisey,

Rev. Sci. Instrum. 74 (2003) 2546.

[3] A. Gutierrez-Llorente, R. Perez-Casero, B. Pajot, J. Roussel, R.M. Defour-

neau, D. Defourneau, J.L. Fave, E. Millon, J. Perriere, Appl. Phys. A 77

(2003) 785–788.

[4] R.A. McGill, V.K. Nguyen, C. Russell, R.E. Shaffer, D. DiLella, J.L.

Stepnowski, T.E. Mlsna, D.L. Venezky, D. Dominguez, Sens. Actuators B

Chem. 65 (2000) 10.

[5] D.M. Bubb, R.A. McGill, J.S. Horwitz, J.M. Fitz-Gerald, E.J. Houser, R.M.

Stroud, P.W. Wu, B.R. Ringeisen, A. Pique, D.B. Chrisey, J. Appl. Phys. 89

(2001) 5739.

[6] D.B. Chrisey, A. Pique, R.A. McGill, J.S. Horwitz, B.R. Ringeisen, D.M.

Bubb, P.K. Wu, Chem. Rev. 103 (2) (2003) 553.

[7] P.K. Wu, B.R. Ringeisen, J. Callahan, M. Brooks, D.M. Bubb, H.D. Wu, A.

Pique, B. Spargo, R.A. McGill, D.B. Chrisey, Thin Solid Films 398–399

(2001) 607–614.