w.elsevier.com/locate/tsf

Thin Solid Films 496 (

Controlled modification of octadecyltrichlorosilane self-assembled

monolayer by CO2 plasma

Nicolas Delorme a,b,c, Jean-Francois Bardeau a,c,*, Alain Bulou a,c, Fabienne Poncin-Epaillard b,c,*

a Laboratoire de Physique de l’Etat Condense (UMR CNRS 6087), Universite du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, Franceb Polymeres Colloıdes et Interfaces (UMR CNRS 6120), Universite du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France

c Institut de recherche en ingenierie moleculaire et materiaux fonctionnels, (CNRS FR 2575), Universite du Maine, Avenue Olivier Messiaen,

72085 Le Mans Cedex 9, France

Received 11 April 2005; received in revised form 31 August 2005; accepted 15 September 2005

Available online 27 October 2005

Abstract

CO2-plasma is used to introduce functional groups on the uppermost surface of an alkoxysilane self-assembled monolayer (SAM). The

structural and chemical modifications of the material surface were monitored by X-ray reflectometry, atomic force microscopy, X-ray

photoelectrons spectroscopy and water contact angle measurements. Optimization of the plasma parameters is performed in order to achieve a

maximum functionalization and to prevent degradation of the SAM. Finally, the ability of grafting organic compounds onto the plasma modified

SAMS was demonstrated by the formation of an alkoxysilane bilayer.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Plasma processing; Octadecyltrichlorosilane; Atomic force microscopy; X-ray photoelectron spectroscopy

1. Introduction

Self-assembled monolayers (SAMs) have attracted increas-

ing interests over the past 20 years due to their ability to form

spontaneously highly organized organic structures on mica and

silicon substrates. SAMs can be easily formed by adsorption

from solution onto a solid surface with molecules having an

appropriate terminal anchoring group with specific chemical

affinity (physic-sorption, chemi-sorption or covalent bond) to

the substrate surface. A long linear hydrocarbon chain favors

upright monolayer ordering on the surface and a terminal

functional group defines the surface functionality [1]. The

quality of the SAMs monolayer (chains orientation, packing. . .)is directly controlled by two main parameters: the reaction

temperature [2–5], and the water traces in the reaction solvent

[2,6]. Alkylsiloxane SAMs have emerged as one of the most

0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2005.09.105

* Corresponding authors. Jean-Francois Bardeau is to be contacted at

Laboratoire de Physique de l’Etat Condense (UMR CNRS 6087), Universite

du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.

E-mail addresses: jean-francois.bardeau@univ-lemans.fr

(J.-F. Bardeau), fabienne.poncin-epaillard@univ-lemans.fr

(F. Poncin-Epaillard).

widely studied SAMs [7] due to their convenient attachment to

hydroxyled and/or oxided surfaces and to their significant

applications in many important fields such as microfluidic [8,9],

protein adsorption [10], chromatography [11], and molecular

electronic [12].

The surface modification of alkoxyloxane SAMs was little

studied [13–16]. Reported experiments mainly concern SAMs

subjected to UV-light [14,16,17], electrons [15,18–20], or X-

ray exposures [21,22]. In most of the cases such an exposure

results in either partial or total damage. One of the perspectives

of modification of alkylsiloxane SAMs is the plasma proces-

sing. Whereas this method is widely applied in microelectron-

ics or chemical modification of polymeric surfaces, there are

only few examples for plasma treatment on SAMs [13,23–25].

The major reason is probably the large complexity of the

physical and chemical processes involving during the interac-

tions of plasma with these systems. Both ions, electrons, free

radicals and UV-light generated by the plasma, affect the SAM

at the same time, involving degradation and functionalization

in most cases [13,23–25].

In this study, octadecyltrichlorosilane (OTS) was selected

as a model of highly packed organic monolayer and because

of its widespread use in the literature [7,20,26–29]. The

2006) 612 – 618

ww

N. Delorme et al. / Thin Solid Films 496 (2006) 612–618 613

thickness and the density of the monolayer were controlled by

X-ray reflectivity measurements whereas the surface homo-

geneity was studied by atomic force microscopy (AFM)

[21,30–32].

CO2-plasma surface modification of the OTS monolayer

was characterized by X-ray photoelectron spectroscopy (XPS)

and contact angle measurements whereas structural modifica-

tions were detected by X-ray reflectivity (XR) and AFM

measurements. The systematic evaluation of the importance of

these two methods allowed us, mainly by controlling the

plasma parameters, to favor the functionalization of the

monolayer surface with regards to the degradation.

2. Experimental details

2.1. Materials

Octadecyltrichlorosilane (95%) was purchased from Aldrich

and was used without further purification. Sulfuric acid and

hydrogen peroxide were analytical grade and obtained from

Aldrich. Single side polished Si {100} (Siltronix) wafers were

used as substrates. Wafers were irradiated by a UV-ozone

cleaner (Jelight Company Inc.).

2.2. Substrate preparation

Before silanization, silicon wafers were ultrasonically

cleaned in methanol for 5 min and treated by a freshly

prepared Piranha solution (70% H2SO4, 30% H2O2) at 100 -Cfor 5 min in order to eliminate all the contaminants. Afterward,

wafers were rinsed with a copious amount of ultrapure water

and dried under a high-purity nitrogen stream. The surface

silanol groups were formed after placing substrates into the

chamber of a UV-ozone cleaner for 10 min. Samples were used

immediately to avoid further contamination. After this cleaning

procedure XPS measurements do not show any signal

corresponding to carbon element indicating the absence of

organic contaminants onto the surface. Furthermore, a total

wetting of the silicon surface with a drop of ultrapure water

was observed demonstrating the presence of silanol groups

onto the surface [29].

2.3. Silanization

OTS silanization was performed by immersing a cleaned

silicon wafer into a solution of OTS (5 I10�3 M) in bicyclo-

hexyl (cyclohexylcyclohexane) for 2 min. Then samples were

ultrasonically rinsed in toluene. This process was run two times

more in order to achieve the formation of a dense OTS

monolayer.

2.4. Contact angles measurements

Contact angles measurements were performed using a

Rame-Hart Inc. goniometer. Droplets of ultrapure water

(Millipore) with a resistivity higher than 18 MV/cm (2 AL)were used to measure the wettability of the surface. The

reported results are the averaged values of measurements

performed with five drops on each sample surface.

2.5. X-ray reflectivity

X-ray reflectivity measurements were performed using a

Philips X’Pert MPD with a parallel beam optics configura-

tion. X-ray was produced by a Cu-Ka-radiation (k=1.54 A)

at 40 keV and 30 mA. The experimental data are shown after

the theoretical Fresnel normalization [33,34] for favoring the

observation of the well defined Kiessig fringes. A quantitative

analysis of the X-ray reflectivity curves has been carried out

using the matrix method [33], rather than the Born

approximation [35]. The electron density profiles of the

samples are modelized by successive layers of uniform

electron density. Each layer is parameterized by an electron

density, a thickness and a smoothing/roughness parameter. In

the fitting procedure, these parameters can be either kept

fixed or varied.

2.6. CO2-plasma treatment

A tubular quartz sample chamber with dimension 76�500

mm (diameter� length) was used. Power (P) was supplied by

a microwave generator (433 MHz). A primary pump and a

diffusion pump were used to attempt a 10�4 Pa ultimate

pressure in the chamber. The plasma was turned on 5 min after

gas introduction. In the following, d denotes the distance

between the bottom of the plasma source and the sample, t the

treatment time, and F the gas flow.

2.7. X-ray photoelectron spectroscopy analysis

XPS spectra were acquired with an ESCA LHS 12

instrument (Leybold) at the Laboratoire de Physique des

Couches Minces (Institut des Materiaux de Nantes). The

photo-emission was excited by a monochromatic Mg Ka beam

at 1253.6 eV. Emission was analyzed at a take-off angle of 90-relative to the horizontal, yielding a sampling depth around 10

nm [36] due to the mean free path of the electrons. Calibration

was made on the C1s peak of C–C bonds at 285.0 eV. Curve

fitting was performed using Peak Fit 4.0 software (Jandel

Scientific). The Full Width at Half Maximum was narrower

than 1.5. Quantitative analysis of the surface composition was

estimated from the integrated peak areas normalized by the

relative sensitivity factors (RSF), the electronmeanfree path,

and the apparatus transmission function provided by the

manufacturer. The spectra were fitted assuming 80% Gaussian

and 20% Lorentzian lineshapes [37]. The surface composition

was expressed in atom percent, and the typical uncertainty was

5% of the measured values for the peaks above 10 at.% the

spectra processing.

2.8. Atomic force microscopy

AFM measurements were accomplished in air with a

Scientec Picoplus system. Silicon tips (k =42 N/m—Nano-

0.0 0.1 0.2 0.3 0.4 0.510-10

10-9

10-8

10-7

10-6

Measured Calculated

R/R

F [a

rb. u

nits

]

Qz [A-1]

Fig. 1. Fresnel normalized X-ray reflectivity results from an OTS-SAM onto

silicon (square) together with the calculated curve (full line).

832 pm

0 nm 50

0

50

100

150

100 150

0 pm

Fig. 2. AFM image of the OTS monolayer on silicon wafer in air (tapping

mode—image size=180�180 nm).

90

95

100

105

110

115

ngle

[deg

rees

]

N. Delorme et al. / Thin Solid Films 496 (2006) 612–618614

world-Switzerland) were used for intermittent contact mode.

The surface roughness Rq was evaluated by the root mean

square (RMS) which can be expressed as:

Rq ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

MN~

M�1

k¼0

~N�1

l¼0

z xk ; ylð Þ � l½ �2s

where M�N is the image size and l is the mean height.

The reported Rq values were calculated using WS�M 4.0

software developed by Nanotec Electronica S.L. To make any

comparison meaning between the Rq values, the AFM images

were measured on the different samples with the same scan

size, pixel resolution, tip-sample force, and AFM tip.

3. Results and discussion

3.1. Formation of a compact OTS monolayer

Quality of the OTS monolayer was preliminary demonstrat-

ed by the high water contact angle (110 T2-) [7], and by the

analysis of X-ray reflectivity curves. In Fig. 1, we show the

Fresnel normalized XR curve of silicon wafer functionalized

by OTS-SAM and the best reflectivity fit as the solid line

which describes the two well-defined Kiessig fringes. The

fringes spacing of 0.240 A�1 which is inversely proportional to

the total film thickness and the Qz value of the first minimum at

0.125 A�1 (denoted Qzmin) are good indicators of the quality of

the SAM yielding to an approximate total film thickness of

2.5(T0.1) nm [34]. The full analysis of the XR curve gives a

fully description of both orientation and density packing of the

OTS monolayer. All the parameters found in this analysis and

Table 1

Parameters obtained from the fit to the experimental X-ray reflectivity curve of

OTS film grafted on the silicon substrate

q [e� A�3] Roughness [A] Thickness [A]

Substrate 0.710(*) 0.7 –

Oxide layer 0.670(*) 0.2 12.9

Transition layer 0.601 3.1 3.5

OTS layer 0.316(*) 2.8 23.0

(*) Parameters marked with an asterisk are maintained fixed during the fit.

0 25 50 75 100 125 150 175 200 22560

65

70

75

80

85

Wat

er c

onta

ct a

CO2-plasma treatment duration [s]

Fig. 3. Evolution of the water contact angle with the CO2-plasma treatmen

duration.

summarized in Table 1 were obtained from a four-component

model: a silicon substrate, a silicon oxide layer, a transition

layer and the OTS monolayer. The silicon substrate was

supposed to have an infinite thickness and an electron density

of 0.710 e� A�1 [34]. The electron density of silicon oxide

was fixed to 0.670 e� A�1 [21]. The introduction of a thin

transition layer is necessary to take into account a complex

interface between the silicon oxide layer and the OTS film

[21,33,34,38]. The structure of OTS monolayers which has

been investigated by FTIR-ATR spectroscopy, X-ray diffrac-

tion and X-ray reflectivity evidenced that OTS monolayers

were composed of close packed, generally vertical chains all-

trans extended [3]. As the total film thickness calculated from

the Qzmin is in agreement with a highly dense OTS monolayer

with alkyl chains in all-trans conformation [33,39], we have

performed constrained fits in which the length and the electron

density of the hydrocarbon chain was fixed to it expected

nominal values i.e. 0.316 e� A�1. The parameters which are

allowed to vary during the fit are therefore the thickness of the

OTS layer, the thickness of the silicon oxide layer, the

roughness of each interface and the transition oxide layer

parameters. The whole parameters are summarized in Table 1.

The fit yields roughness below 0.4 nm indicating the

presence of atomic-level smooth interfaces [33]. The thickness

of the silicon oxide layer was found to be 1.29 nm which was

in good agreement with the value proposed by Brzoska et al.

t

282 283 284 285 286 287 288 289 290

0

5000

10000

15000

20000

25000

285.0 eVC-C

Inte

nsity

[arb

. uni

ts]

Binding energy [eV]

(A)

282 283 284 285 286 287 288 289 290

0

2000

4000

6000

8000

10000

12000

14000

16000

288.7 eVCOO

287.4 eVC=O

286.1 eVC-O

285.0 eVC-C

Inte

nsity

[arb

. uni

ts]

Binding energy [eV]

(B)

Fig. 4. C1s-high resolution XPS spectra of (a) non treated OTS-film and (b)

CO2-plasma treated substrate (t =120 s).

0.0 0.1 0.2 0.3 0.4 0.5 0.6

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

20 s

90 s

210 s

120 s

60 s

0 s

30 s

R/R

F [a

rb. u

nits

]

Qz [A-1]

45 s

Fig. 5. Evolution of the Fresnel normalized X-ray reflectivity curves of OTS-

SAM deposited on silicon wafer with the CO2-plasma treatment duration.

N. Delorme et al. / Thin Solid Films 496 (2006) 612–618 615

for the typical thickness of the native silicon oxide layer on

silicon substrate (1.0–1.5 nm) [3,40]. The parameters obtained

for the transition layer (i.e. electron density and thickness) are

attributed to a complex interface between the silicon oxide

layer and the alkyl chains of the OTS monolayer [33].

Therefore, the fitting analysis of the X-ray reflectivity curve

provides a reliable measure of the thickness of the layers, the

electron density and interface roughness consistent with a

highly ordered OTS-SAM with fully extended hydrocarbon in

all-trans conformation [21,38,41].

The coverage of the monolayer surface was studied by AFM

(Fig. 2). RMS roughness determined from the AFM image

(180�180 nm) is found to be 0.15 nm indicating that, at this

scale, surface is smooth. The lack of default in a wider scale

(1�1 Am) confirms the high degree of homogeneity of the

OTS-SAM. Thus, this observation is consistent with the

Table 2

Total atomic percentage and C1s atomic percentage of the different carbon moieties

film

Plasma treatment time [s] Total atomic percentage [%]

Si [%] C [%] O [%] C

0 20 60 20 3.

20 19 59 22 2.

45 21 54 25 2.

60 22 51 27 1.

120 22 52 26 2.

210 28 46 26 1.

structural studies which clearly indicated the formation of a

dense and perpendicularly oriented OTS chains monolayer.

3.2. CO2-plasma surface modification of the OTS monolayer

Plasma containing oxygen species is a well-known tech-

nique to quantitatively introduce polar reactive groups onto

organic surfaces [42–44]. However, in order to chemically

modify the surface of the OTS monolayer, CO2-plasma was

preferred to O2 plasma because the degradation processes are

less efficient [13,45]. Four parameters can be monitored with

the plasma reactor used in this study: P the power, F the gas

flow, t the treatment duration and d the distance between the

bottom of the excitator and the sample. Parameters P and F

were chosen to be the most effective to introduce preferentially

hydroxyl (C–OH) and carboxyl groups (–COOH) onto

organic surface [42,45,46]. The distance ‘‘d’’ was kept to be

maximal (¨70 cm). Such positioning was expected to expose

OTS monolayer predominantly to radicals that are responsible

of the functionalization [47] and to avoid interaction with

electrons and ions responsible of the degradation that do not

exist in the post-discharge.

The effect of the plasma treatment was directly controlled

by contact angle measurement which is known to be sensitive

to the modification of the uppermost surface. The effect of the

plasma treatment onto the OTS monolayer was evaluated by

following the evolution of the water contact angle as a function

of the plasma treatment duration (Fig. 3). A decrease of the

water contact angle is observed since the first seconds of the

determined by XPS analysis versus the CO2-plasma treatment time on the OTS

Atomic percentage of C [%]

/O C–C [%] C–O [%] C_O [%] COO [%]

0 100 0 0 0

7 85 10 3 2

2 81 12 5 2

9 70 19 7 4

0 68 20 8 4

8 71 18 8 3

0 pm

978 pm

0 nm 50 100 150

0

50

100

150

0 pm

918 pm

0 nm 50 100 150

0

50

100

150

tplasma= 0 s

RRMS= 0.23 nm RRMS= 0.28 nm RRMS= 0.49 nm

tplasma= 120 s tplasma= 210 s

0 nm 50 100 150

0

50

100

150

832 pm

0 pm

Fig. 6. Evolution of AFM topography images and RMS roughness of monolayers as function of CO2-plasma treatment time (tapping mode—image

size=400�400 nm).

10-9

10-8

10-7

10-6

10-5

OTS+ plasma (t= 120 s)+ OTS

OTS+ plasma (t= 120 s)

R/R

F [a

rb. u

nits

]

OTS

N. Delorme et al. / Thin Solid Films 496 (2006) 612–618616

plasma treatment until treatment duration of 150 s, and then a

plateau is observed. Such variation can be assigned to the

progressive introduction of polar functions onto the monolayer

surface [13] leading, for the plasma duration of 150 s, to a

surface saturation with polar functions.

To check these results, surface chemical composition was

analyzed by X-ray photoelectron spectroscopy (XPS). Fig. 4

shows a comparison between two XPS spectra depicting the

differences of the surface chemical compositions before and

after a CO2-plasma treatment (t =120 s). After decomposition

of the XPS signal of the plasma treated monolayer, the

appearance of three components at 286.1, 287.4, and 288.7

eV can be observed corresponding to C–O, C_O, and COO

groups, respectively. Thus, XPS analysis confirms that CO2-

plasma introduces reactive polar groups onto the surface of the

OTS film. XPS analysis as a function of the plasma treatment

duration also indicates a progressive functionalization as

confirmed by water contact angle measurements. Indeed, Table

2 shows a progressive increase of the polar groups propor-

tions (C–O, C_O, and COO) with the treatment duration

until 210 s. For longer plasma treatments, there is a decrease

of the proportions of polar functions. This can be attributed to

the degradation of the monolayer surface, presumably caused

by fragmentation of chains containing polar groups. Since

after the plasma treatment the samples are transferred through

the laboratory atmosphere, a reaction between the plasma

modified OTS surface and the laboratory atmosphere cannot

be avoided. However, the clear dependency of the plasma

treatment time with the proportion of introduced groups

demonstrates that the effect of the reaction with the laboratory

atmosphere is not predominant for the surface modification.

Table 3

Evolution of the water contact angle at each step of the formation of the OTS

bilayer

Sample Water contact angle [-]

OTS monolayer 110 (T2)CO2-plasma treated (120 s) 72 (T2)

Second OTS layer 103 (T2)

The effect of the plasma treatment on the SAM has been

accurately controlled by X-ray reflectivity measurements. In

Fig. 5, it can be observed that for a treatment time below 120 s,

the position and the height of the first minimum are quite

similar. Such results indicate that under these conditions,

electron density and thickness of the plasma treated mono-

layers remain the same as the non treated one. This result

clearly indicates that until the treatment duration of 120 s, the

degradation of the monolayer may be negligible.

On the contrary, for plasma treatment times over 210 s

alterations of both height and position of the first minimum of

the reflectivity curve are observed indicating a decrease of the

monolayer electron density. The shift of the first minimum

towards low Qz wave vector transfers and the decrease of the

contrast are likely due to the decrease of the initial OTS

monolayer density and the polymerization of removed molec-

ular chains on the top-most surface. Consistently, the decrease of

the densities of polar groups observed by XPS measurements

was interpreted as the result of the degradation of the monolayer.

Surface structure of the plasma treated monolayer was

monitored using AFM. Topography images (Fig. 6) indicate

that the plasma treatment of 120 s does not significantly modify

0.0 0.1 0.2 0.3 0.4 0.510-11

10-10

Qz [A-1]

Fig. 7. The Fresnel normalized X-ray reflectivity curves of (a) the initial OTS

monolayer, (b) after CO2-plasma treatment (t =120s) and (c) after the grafting

of a second OTS layer.

N. Delorme et al. / Thin Solid Films 496 (2006) 612–618 617

the surface structure of the monolayer. Furthermore RMS

rugosities were comparable between plasma treated monolayer

(RRMS=0.28 nm) and the initial OTS monolayer (RRMS=0.23

nm); this support the previous conclusion that under these

conditions, the monolayer is not degraded by the plasma

treatment. On the contrary, AFM image of the 210 s plasma

treated monolayer shows morphological modification of the

surface with the appearance of hills and holes. This is

quantitatively evidenced by the large increase of the RMS

roughness (0.49 nm) which clearly proves a degradation of the

monolayer.

3.3. Grafting onto the CO2-plasma modified OTS monolayer

In order to prove the ability of grafting organic compounds

onto the plasma functionalized SAM, the grafting of a second

layer of OTS molecules was performed. Indeed, the presence of

C–OH and COOH groups onto the modified surface should

allow the grafting of alkoxysilane molecules [18,33]. The

evolution of the surface energy was studied by measuring the

water contact angle (Table 3).

The formation of a second layer of OTS molecules was

confirmed by X-ray reflectivity (Fig. 7) since the position of

the first minimum was shifted from 0.128 A�1 for the plasma

treated OTS monolayer to 0.65 A�1 after the deposition of the

second OTS layer. This shift correspond to an increase of the

thickness from 2.45 to 4.83 nm [21] confirming the presence of

a second OTS monolayer. Quantitative analysis of the X-ray

reflectivity curves and contact angle measurements have shown

that the second OTS layer is less organized than the first

monolayer. This is probably due to the random positioning of

the reactive groups introduced by the plasma treatment.

4. Conclusion

In this study, self-assembled OTS on silicon was used as a

platform to demonstrate the possibility to functionalize the

topmost surface of SAM deposited on silicon by CO2 plasma

treatment. In one hand, a combination of X-ray photoelectron

spectroscopy and contact angle measurements allowed to

monitor the surface modification of the OTS monolayer as a

function of the plasma parameters. On the other hand, the

control of the monolayer structure (density and thickness) by

a quantitative analysis of the X-ray reflectivity curves and by

the study of the surface topography using atomic force

microscopy have clearly shown a quantitative functionaliza-

tion of the topmost OTS monolayer without nanoscale

degradation of the highly organized organic structures.

Finally, the grafting of a second OTS layer onto the plasma

modified SAM has shown that this method should be suitable

for the grafting of a large number of organic compounds onto

SAMs.

Acknowledgment

This work was supported by the French PIR ‘‘Microfluidi-

que et Microsystemes fluidiques 2003’’ under projects

‘‘MI2F03-44’’. The authors thank E. Lepleux and L. Pacheco

for providing the AFM images.

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