www.elsevier.com/locate/tsf

Thin Solid Films 474

Stability improvement of porous silicon surface structures by grafting

polydimethylsiloxane polymer monolayers

Bing Xia, Shou-Jun Xiao*, Jing Wang, Dong-Jie Guo

State Key Laboratory of Coordination Chemistry, College of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China

Received 5 July 2004; received in revised form 20 August 2004; accepted 26 August 2004

Abstract

Organic silicone, polydimethylsiloxane (PDMS), was covalently grafted onto the porous silicon (PS) surface to improve its stability to

oxidation. Transmission Fourier-transform infrared (FTIR), photoluminescence (PL) and interferometric reflectance spectra have been

recorded to evaluate the surface modification and the passivation effect. After modification, the porous nanostructure retained, the stability

against oxidation was significantly improved and the PL still kept.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Porous silicon; Polydimethylsiloxane; Nanostructure; Photoluminescence

1. Introduction

Much attention has been attracted in the field of porous

silicon (PS) for its diverse applications in bio- and

chemical sensing [1–3]. Unique properties for theses

applications include the significantly increased surface

interaction area, simplicity and repeatability of fabrication,

and compatibility with the well-established silicon micro-

fabrication technology. But the major barrier preventing

commercial applications of PS is the instability of its

native interface with a metastable Si–Hx termination. The

metastable hydrosilicon can undergo spontaneous oxida-

tion in ambient atmosphere and results in the degradation

of surface structures. Therefore, surface passivation is

crucial for the technological success of this material. Many

organic molecules such as alkenes or alkynes had been

covalently grafted onto the PS surface by wet chemical

approaches to passivate its surface [4,5]. Recently, polymer

films, instead of hydrocarbon monomer monolayers, on PS

are of special interest because they provide a strong

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.08.121

* Corresponding author. Tel.: +86 25 83595706; fax: +86 25

83314502.

E-mail address: sjxiao@nju.edu.cn (S.-J. Xiao).

armature to protect the surface nanostructure. To our

knowledge, the well-known and successful product of

hydrosilylation is polydimethylsiloxane (PDMS) by the

reaction of hydrosilicone with vinyl silicone. With this in

mind, we grafted the vinyl silicone on porous silicon

surfaces. The grafted PDMS was characterized with the

transmission Fourier-transform infrared spectroscopy

(FTIR) and the contact angle goniometer. The retention

of the surface nanostructure is characterized by an

interferometric reflectance spectrometer. The stability and

photoluminescence (PL) of PS were tested under stringent

conditions to present the advantage of using PDMS for

preventing PS from oxidation and degradation.

2. Experimental

Silicon wafers were purchased from Huajing Micro-

electronics (China). Sylgard 184 Silicone Elastomer Base

(B) was purchased from Dow Corning (USA). Single side

polished (100) oriented p-type silicon wafers (7.3–9.0 V

cm resistivity) were cleaned in 3:1 (v/v) concentrated

H2SO4/30% H2O2 for 30 min at room temperature and

then rinsed copiously with deionized water (resistance N18

MV cm). The cleaned wafers were immersed in 40%

(2005) 306–309

Fig. 1. Transmission FTIR spectra for PS (a) freshly hydride-terminated

samples and (b) grafted with PDMS.

B. Xia et al. / Thin Solid Films 474 (2005) 306–309 307

aqueous HF solution for 1 min at room temperature to

remove the native oxide. The hydrogen-terminated surfaces

were electrochemically etched in a 1:1 (v/v) pure ethanol

and 40% aqueous HF for 30 min at a current density of 6

mA/cm2. After etching, the samples were rinsed with pure

ethanol and dried under a stream of dry nitrogen prior to

use. The freshly prepared PS surface (1.54 cm2) was

covered with 20 Al a vinyl-PDMS (Sylgard184 B) and

incubated in an oven at 80 8C for 2 h. The excess of

unreacted and physisorbed reagent was rinsed three times

with toluene at room temperature and dried under a stream

of dry N2.

Transmission FTIR spectra were recorded using a Bruker

IFS66/S spectrometer at 0.25 cm�1 resolution. Typically, 128

interferograms were acquired per spectrum. The samples

were mounted in a purged sample chamber. Background

spectra (except those for alkaline solution measurements)

were obtained using a flat untreated Si(100) wafer.

Contact angle measurements of the hydride-terminated

and PDMS-grafted surfaces with deionized water were

carried in air by the sessile-drop method with a contact

angle goniometry (Rame-Hart, USA). Readings were taken

after the angles were observed to be stable with time.

Reported data are an average of the readings taken at three

different spots for each sample.

Interferometric reflectance spectra of PS were recorded by

using an HR2000CG-UV-MR spectrometer (Ocean Optics,

USA) fitted with a bifurcated fiber optic probe. A Xe light

source was focused onto the center of a porous silicon

surface with a spot size of approximately 1–2 mm. Spectra

were recorded with a charge-coupled-device detector in the

wavelength range 400–1000 nm. The illumination of the

surface as well as the detection of the reflected light was

performed along an axis coincident with the surface normal.

The optical thickness of the porous silicon was determined

by Eq. (1) [6].

nd ¼ Dkk1k22 k1 � k2ð Þ ð1Þ

where n is the average refractive index of the porous silicon

film, d the film thickness, k1 and k2 the wavelengths of twoadjacent minima or maxima, respectively, and for two

adjacent minima or maxima the difference in order (Dk) is

1.0.

All of the PL spectra were measured at room temperature

in air by a SLM 4800 DSCF/AB2 fluorescence photo-

spectrometer (SLM, USA) using 450-W Xe laser, excited by

a 366-nm line.

A Si(100) wafer and a modified PS sample were put into

the boiling 0.01 M NaOH aqueous solution. The FTIR

spectra of the modified PS samples were recorded after 0,

5,10, 20, 40, 60 and 120 min treatment in the base solution

and thorough washing with water. Background spectra were

obtained by using a flat Si(100) wafer by the same treatment

with the base solution.

3. Results

The grafting of PDMS on the PS surface is confirmed by

the transmission FTIR. The spectrum of the freshly prepared

PS (Fig. 1) is similar to that reported by Buriak [4]. The

spectrum of a freshly hydride-terminated PS (Fig. 1a)

exhibits a typical tripartite band for Si–Hx (x=1–3) stretch-

ing modes (2087 cm�1 for tSi–H, 2114 cm�1 for tSi–H2and

2138 cm�1 for tSi–H3). The Si–Hx bending modes are

observed at 916, 669 and 630 cm�1. Fig. 1b is the spectrum

of PDMS-grafted PS. In the spectra, vibrations of CH3 in

PDMS are characterized by the aliphatic tC–H stretching

modes at 2961 cm�1 and deformation modes at 1440 cm�1.

The absorption peaks at 1260 cm�1 is attributed to the

stretching bands of Si–O. The unit Si–O constructs the

backbone of PDMS. Therefore, functionalization of porous

silicon with PDMS is successful from the FTIR measure-

ment. All bands of Si–Hx from PS after modification are still

observable but exhibit a significant decrease. The average

conversion efficiency E is related to the change in the

integrated intensity A of the Si–Hx region (2000–2200

cm�1) after modification:

E ¼ A0 � A1ð Þ=A0

where A0 and A1 are the integrated peak areas of the freshly

etched and the modified samples in the Si–Hx region (2000–

2200 cm�1), respectively. According to this equation the

value E is 13.4%, which corresponds well to the reported

average value 15% for organic monomer molecules [7].

Another evidence for the PDMS attachment is the

contact angle measurement. The measurement revealed that

the PS surfaces had an advancing contact angle of 1208,while the value for PDMS-grafted PS surfaces was found

to be 1078 [8].

The well-established method to analyze the surface

nanostructure of PS is the interferometric reflectance

spectra. The freshly etched and the modified PS samples

show well-resolved Frabry-Perot fringes in their reflection

spectra (Fig. 2). The spectra confirm the retaining of the

Fig. 2. Interferometric reflectance spectra for (a) a freshly etched hydride-

terminated PS and (b) a PS sample grafted with PDMS.

Fig. 4. PL spectra of PDMS-grafted PS samples treated with 0.01 M NaOH

at room temperature for (a) 0, (b) 5, (c) 30, (d) 60 and (e) 90 min.

B. Xia et al. / Thin Solid Films 474 (2005) 306–309308

nanostructure of PS after grafting PDMS [9]. The freshly

etched PS sample typically displays an optical thickness (the

value from Eq. (1)) of 5131.94 nm, while the value for the

PDMS-grafted PS sample is 5255.9 nm. PDMS-grafted PS

samples have a slightly higher optical thickness because the

PDMS monolayer increases the value of d in Eq. (1).

In Fig. 3, the PL emission spectra of the freshly etched

and the PDMS-modified PS samples were collected with an

excitation wavelength of 366 nm at room temperature. The

hydride-terminated PS samples (Fig. 3a) exhibit a blue-red

PL emission between 400 and 800 nm. The maximum PL

emission is at 646 nm, and other two peaks at 488 and 797

nm, respectively. Fig. 3b shows the PL spectra of the PDMS

modified samples. It can be seen that the functionalization

does not change the maximum PL peak position at 646 nm,

but results in a reduction (75%) of the PL intensity. A

typical stringent PL test for PS is the treatment with an

alkaline solution. It is reported that the PL disappears in

seconds to minutes in this case for the freshly etched PS [4].

We placed our PDMS-modified samples into a 0.01 M

NaOH aqueous solution at room temperature for 0, 5, 30, 60

Fig. 3. PL spectra for (a) a freshly etched hydride-terminated sample and (b)

a PS sample grafted with PDMS.

and 90 min. Fig. 4 shows the evolution of PL for the base

treatment. With increasing the time, the maximum PL peak

at 646 nm due to the silicon hydride nanostructure gradually

shifts to 577 nm with a slightly decreased intensity. The

peak 488 nm due to the silicon oxide [10] becomes much

stronger and reaches its maximum at 60 min, then it begins

to decrease because of the degradation of the surface

nanostructures. The peak 797 nm due to the surface

dangling bonds disappears after 5 min.

To test the chemical resistance of the derivatized PS, we

select a stringent environment: exposure to a boiling

alkaline solution. The evolutional FTIR spectra were

recorded to witness the stability. Fig. 5 exhibits the spectra

of PDMS-modified PS samples after exposure to a 0.01 M

boiling NaOH aqueous solution for 0, 5, 10, 20, 40, 60 and

120 min, respectively. It had been reported that under such a

stringent condition (an alkaline solution at pH=12), the

tSi–Hxcentered around 2100 cm�1 disappears completely

within tens of seconds [4]. However in our situation, the

tSi–Hxis observable even after 40-min exposure to the base.

The strong methyl bands exist even after 120-min exposure.

Fig. 5. Transmission FTIR spectra of PDMS-grafted PS samples treated

with a boiling 0.01 M NaOH solution for (a) 0, (b) 5, (c) 10, (d) 20, (e) 40,

(f) 60 and (g) 120 min.

B. Xia et al. / Thin Solid Films 474 (2005) 306–309 309

No obvious stripping or degradation of PS structures was

observed during the first 40-min exposure.

4. Discussion

PL phenomena and Frabry-Perot fringes in the reflection

spectra are unique characters of PS. After modification, these

optical properties are still retained. PS is very sensitive to the

base solution and its surface nanostructures will be destroyed

very quickly. As a result, its PL will disappear after tens of

seconds or minutes if PS is inserted into a base solution.

While in our case, the PDMS-modified PS retains the PL

even after 90-min treatment of a NaOH solution at room

temperature. The blue-red peak at 644 nm shifts to 588 nm

indicates the existence and some change of surface Si–Hx

nanostructures. The Si–Hx structures are proved by FTIR

measurements (the spectra are ignored because of the

similarity with those in Fig. 5). The peak 488 nm becomes

much stronger due to oxidation of partial Si–Hx nano-

structures. The co-existence of both Si–Hx and Si–O

nanostructures contributes to the yellowish PL after the base

treatment. These results show that PDMS monolayers

provide a good protection and some characteristic improve-

ment for PL of PS.

The native Si–Hx terminated porous silicon surfaces do

not provide adequate stability for most potential applications.

Therefore, stabilization of the surface nanostructures of PS

through introduction of a tunable monolayer is exclusively

necessary and would be a revolution for technological

applications of PS. Not only limited for this, it is also

beneficial for fundamental studies such as the quantum

confinement effect of some embedded nanocrystallities in the

future. Though the chemical reaction does not consume all of

the non-oxidized Si–Hx (because of the steric hindrance at the

surface), the density of the covered organic species on the

surface is high enough to protect the remained Si–Hx bonds

against the oxidation. We used a stringent condition of a

boiling base solution to test its chemical resistance of the

PDMS-modified PS. The Si–Hx bond is observable after 40-

min treatment and the PDMS monolayer observable after

120-min treatment. This indicates that PDMS monolayer has

a strong chemical resistance to the base treatment at both

room and boiling temperatures. We also measured the PL of

the samples after the boiling treatment. Their PL disappears

after 10 min because the surface nanostructures are destroyed

(spectra not shown because of the similarity with Fig. 4).

From the above discussions, we conclude that the PL is

related not only to the surface components but also to the

surface nanostructures. The surface nanostructure retains at

room temperature but is vulnerable at the boiling temperature.

This is the reason to explain the difference of the PL observed

at both room and boiling temperatures. Such a long

endurance of PL and chemical resistance is rarely reported.

5. Conclusion

The results of FTIR and contact angle measurements

confirm the presence of PDMS after modification. The

passivation effect of the PDMS monolayer on PS was

investigated by FTIR and PL. The measurements show that

the PDMS monolayer provides a strong armature to PS

under a variety of stringent conditions such as in the base

solution. This passivation technique might have a bright

outlook for practical applications in the future.

Acknowledgment

The authors would like to thank Mr. Qing-Wei Hang,

Mr. Hong-Qi Shi, Mr. De-Jun Liu and the Center of

Analytical Instruments of Nanjing University for their

helps in FTIR and PL measurements. We are grateful to

the financial support of SRF for ROCS, SEM, the High

Technology (Industry) Project BG2003028 of Jiangsu

Province and the Nanjing University Talent Development

Foundation.

References

[1] V.S. Lin, K. Motesharei, K.S. Dancil, M.J. Sailor, M.R. Ghadiri,

Science 278 (1997) 840.

[2] J. Gao, T. Gao, M.J. Sailor, Appl. Phys. Lett. 77 (2000) 901.

[3] S. Letant, M.J. Sailor, Adv. Mater. 12 (2000) 355.

[4] J.M. Buriak, Chem. Rev. 105 (2002) 1271.

[5] D.D.M. Wayner, R.A. Wolkow, J. Chem. Soc., Perkin Trans. 2

(2002) 23.

[6] G. Kraus, G. Gauglitz, Fresenius. J. Anal. Chem. 344 (1992)

153.

[7] J.M. Buriak, M.J. Stewart, T.W. Geders, M.J. Allen, H.C. Choi, J.

Smith, M.D. Raftery, L.T. Canham, J. Am. Chem. Soc. 121 (1999)

11491.

[8] S.J. Clarson, J.A. Semlyen, Siloxane Polymers, Prentice Hall, Engle-

wood Cliffs, NJ, 1993.

[9] A. Janshoff, K.-P.S. Dancil, C. Steinem, D.P. Greiner, V.S.-Y. Lin, C.

Gurtner, K. Motesharei, M.J. Sailor, M.R. Ghadiri, J. Am. Chem. Soc.

120 (1998) 12108.

[10] J. Chun, A. Boccarsly, T. Cottrell, J. Benziger, J. Lee, J. Am. Chem.

Soc. 115 (1993) 3024.