In Situ Silanization Reaction on the Surface of Freshly Prepared

Hong-Liang Li,* Ai-Ping Fu,† Dong-Sheng Xu, Guo-Lin Guo, Lin-Lin Gui, and. You-Qi Tang. State Key Laboratory for Structural Chemistry of Unstable an...
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In Situ Silanization Reaction on the Surface of Freshly Prepared Porous Silicon Hong-Liang Li,* Ai-Ping Fu,† Dong-Sheng Xu, Guo-Lin Guo, Lin-Lin Gui, and You-Qi Tang State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Physical Chemistry, Peking University, Beijing 100871, People’s Republic of China Received November 6, 2001. In Final Form: February 1, 2002 An in situ silanization reaction on the surface of freshly prepared porous silicon (PS) has been achieved at room temperature with a trace of amine as the catalyst. Infrared absorption spectra (FTIR) and X-ray photoelectron spectroscopy measurements show that the silanes are successfully attached to the surface of the PS. The effect of the silanization on the stability and the photoluminescence properties of the PS layer has been studied. A mechanism for the silanization reaction based on the alkalinity of the organic amine has been proposed. A new approach for the modification of the surfaces of PS layers by in situ silanization has been established.

* To whom correspondence should be addressed. E-mail: lhlfap@ hotmail.com. Fax: 86-10-62751725. † Permanent address: Department of Chemistry, Qufu Normal University, Qufu 273165, P. R. China.

organic molecules under various conditions has been reported.15-18 Traditionally, the most common method used for surface derivatization of the crystalline silicon is the silanization reaction.19,20 This method requires a pretreatment to form a Si-OH intermediate on the surface of c-Si by chemical oxidization.20,21 However, the nanometer c-Si particles have a much higher reactivity than that of the bulk silicon crystal and are very sensitive to oxygen in air and other chemical oxidants.10,22,23 To preserve the semiconductor characters of PS, heavy oxidation should be avoided. Hence, to obtain the Si-OH groups on the surface of PS using the conventional pretreatment is not very appropriate. Although a direct silanization method had been studied, the formation of the Si-OH intermediate is not very feasible.24,25 Therefore, a flexible method to obtain the Si-OH intermediate that avoids the heavy oxidation is very desirable. Our experimental results here show that an in situ silanization reaction occurs on the surface of the freshly prepared PS when a trace of amine, such as n-propylamine or n-butylamine, is used as a catalyst. Subsequently, the indirect substitution of the H- ion on the freshly prepared PS surface by silane such as (C2H5O)3Si(CH2)3SH has been achieved in an inert atmosphere. The influence of the in situ modification on the chemical stability and the PL properties of the PS has been studied.

(1) Waltenburg, H. N.; Yates, J. T. Chem. Rev. 1995, 95, 1589. (2) Sailor, M. J.; Lee, E. J. Adv. Mater. 1997, 9, 783. (3) (a) Sheen, C. W.; Shi, J. X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514. (b) Namyong, Y. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 2297. (4) Buriak, J. M. Adv. Mater. 1999, 11, 265. (5) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (6) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (7) Hamilton, B. Semcond. Sci. Technol. 1995, 10, 1187. (8) Hirschman, K. D.; Tsybeskov, L.; Duttagupta, S. P.; Fauchet, P. M. Nature 1996, 384, 338. (9) Lin, V. S.; Moteshare, K.; Dancil, K. S.; Sailor, M. J.; Ghadiri, M. R. Science 1997, 278, 840. (10) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. J. Am. Chem. Soc. 1999, 121, 7925. (11) Stewart, M. D.; Buriak, J. M. Adv. Mater. 2000, 12, 859. (12) Widberg, E.; Amberger, E. In Hydrides of the Elements of Main Group I-IV; Elsevier: Amsterdam, 1971; pp 462-621. (13) Tischler, M. A.; Collins, R. T.; Stathis, J. H.; Stang, J. C. Appl. Phys. Lett. 1992, 60, 639. (14) Bsiesy, A. In Properties of porous silicon; Canham, L. T., Ed.; INSPEC: London, 1997; pp 283-289.

(15) Buriak, J. M. Chem. Commun. 1999, 1051. (16) Boukherroub, R.; Morin, S.; Wayner, D. D. M.; Bensebaa, F.; Sproule, G. I.; Baribeau, J.-M.; Lockwood, D. J. Chem. Mater. 2001, 13, 2002. (17) Bateman, J. E.; Eagling, R. D.; Worrall, D. R.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. 1998, 37, 2683. (18) Okubo, T.; Tsuchiya, H.; Sadakata, M.; Yasuda, T.; Tanaka, K. Appl. Surf. Sci. 2001, 171, 252. (19) Sato, T.; Hoko, D. G.; Ahmed, H. J. J. Vac. Sci. Technol., B 1997, 15, 45. (20) Wang, J.; Zhu, T.; Song, J.; Liu, Z. F. Thin Solid Films 1998, 327, 591. (21) Haller, I. J. J. Am. Chem. Soc. 1978, 100, 8050. (22) Wolkin, M. V.; Jorne, J.; Fauchet, P. M.; Allan, G.; Delerue, C. Phys. Rev. Lett. 1999, 82, 197. (23) Bateman, J. E.; Eagling, R. D.; Horrocks, B. R.; Houlton, A.; Worrall, D. R. Chem. Commun. 1997, 23, 2275. (24) Anderson, R. C.; Muller, R. S.; Tobias, C. W. J. Electrochem. Soc. 1993, 140, 1393. (25) Dubin, V. M.; Vieillard, C.; Ozanam, F.; Chazalviel, J. N. Phys. Status Solidi B 1995, 190, 47.

1. Introduction Silicon surface chemistry is of fundamental importance because of the ubiquitous role of silicon in modern technology and yet is just beginning to be investigated.1-4 Control of Si surface chemistry is crucial to allow access to technologically interesting thin films for fabrication of new electronic devices.2,5 Porous silicon (PS) is a wellknown versatile inorganic material. It can be easily synthesized directly from single-crystal silicon wafers and has been found to exhibit photoluminescence (PL).6 During the last 10 years, PS has attracted considerable attention because of its potential use in the development of siliconbased optoelectronic devices7,8 or microsensors.9-11 However, the instability of the PL due to the variability of the surface composition of the nanometer crystal silicon (cSi) particles hinders the applications of this novel material.12-14 Therefore, the modification of the PS surface appears highly attractive, since it might provide a more stable or more selective surface for device applications or for testing fundamental models aimed at explaining the mechanism involved in the luminescence of PS. Recently, a direct derivatization of the PS surface with different

10.1021/la015664g CCC: $22.00 © 2002 American Chemical Society Published on Web 03/20/2002

Silanization Reaction on Porous Silicon

Figure 1. The FTIR spectra: A, the freshly prepared PS sample; B, the sample modified by the in situ silanization reaction; C, the sample immersed in a toluene solution of silane under N2; D, the sample in water vapor at room temperature; E, the sample immersed in toluene in a dilute atmosphere of amine; F, the samples immersed in toluene in the dilute atmosphere of amine with a trace of water.

The mechanism of the in situ silanization reaction will also be discussed briefly in this paper. 2. Experimental Section The in situ silanization reaction was carried out in a standard Schlenk flask. (C2H5O)3Si(CH2)3SH was used as received (Aldrich). Toluene, n-propylamine, and n-butylamine were dried and then distilled under N2 prior to use. The Fourier transform infrared (FTIR) spectra were collected on a Nicolet Magna 550 operating in transmission mode. The X-ray photoelectron spectra were accumulated on a VG ESCALAB 5 multitechnique electron spectrometer system with an Al KR standard X-ray source, and the binding energies were calibrated by C1s. Static PL spectra were measured at room temperature by a Hitachi FL-4500 fluorescence photospectrometer with a 350 nm wavelength filter, excited by a 345 nm line. Porous silicon films were prepared by anodically etching p-type Si (100) (6-9 Ω cm) in an ethanol/hydrofluoric acid (30%) mixture (2:3 v/v). A current density of 22 mA/cm2 was applied for 12 min, and a porosity of about 70% was obtained. After etching, the samples were washed with ethanol and dried thoroughly in a stream of high-purity N2 (99.999%) before the treatments. In a typical in situ reaction, two uncovered weighing bottles were put into a flask, and the freshly prepared PS layers were placed in one weighing bottle. Then, 5 mL of a solution of 10% (C2H5O)3Si(CH2)3SH in toluene was injected into the bottle containing the PS layers, and 2 mL of a 1% amine solution in toluene was injected into the empty bottle. Pure water vapor at room temperature was then instilled into the flask in a stream of N2 for about 20 min. After 14 h, the liquid was removed by syringe and the PS samples were rinsed three times by toluene. The samples were then removed from the flask and dried under a stream of high-purity N2. In control experiments, PS layers were treated in the toluene solution of silane under N2 for 20 h, in pure water vapor for 50 h, in pure toluene under a N2 and dilute amine atmosphere for 20 h, and in pure toluene under a dilute amine atmosphere with a trace of water for 6 h, respectively. A 0.1 M aqueous NaOH solution was used to test the effect of the in situ modification on the composition and the PL properties of the PS.

3. Results and Discussion 3.1. FTIR and X-ray Photoelectron Spectoscopy (XPS). Curve A in Figure 1 is a typical FTIR spectrum of the freshly prepared PS, which displays the characteristic of surface Si-Hx (x ) 1-3) stretching modes at 2080-2200 cm-1 and the δ(Si-H2) scissors mode at 908 cm-1.26 The Si-O-Si mode around 1100 cm-1 is very weak. (26) Rao, A. V.; Ozanam, F.; Chazalviel, J. N. J. Electrochem. Soc. 1991, 138, 153.

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Curve B in Figure 1 is the FTIR spectrum of the sample derived from the in situ silanization reaction. Compared with that of the freshly prepared PS, the stretching mode of Si-Hx disappeared and a C-Hx stretching mode at 2800-3000 cm-1 27 appeared. The broad peak at 1100 cm-1 is the characteristic mode of Si-O-Si.28 These FTIR properties are typical of the surface composition of the sample covered with silane. Although in theory we could get some information about the covalent anchoring of the silanes on the PS surface by analyzing the character of the Si-O stretching band around 1100 cm-1, the disordered structure of the PS layer makes this difficult. As some spectra show, this band is very noisy, and a detailed analysis of this band is problematical. The spectrum of the control sample immersed in a toluene solution of silane (10% v/v) under N2 is the same as that of the freshly etched PS. The intense Si-Hx stretching modes and the weak Si-O-Si signal were preserved after the treatment, and no C-Hx signal was detected in the FTIR spectrum (curve C). It follows that the Si-Hx groups on the surface of freshly prepared PS do not react with the silane molecules in the absence of amine and water. Compared with that of the freshly prepared sample, no obvious change appeared in the FTIR spectrum of the control sample placed in water vapor for about 50 h. This indicates that the hydrolysis of the Si-Hx groups on the surface of PS is very difficult in a pure water atmosphere (curve D). Curve E in Figure 1 shows the FTIR spectrum of the control sample immersed in toluene under N2 with dilute amine for 20 h. The Si-Hx stretching modes at around 2100 cm-1 are almost as intense as those of the freshly prepared PS. Although the Si-O-Si peaks around 1100 cm-1 are enhanced, the C-Hx characteristic modes are absent from the FTIR spectrum. These control experiments reveal that no detectable reactions take place between the pure amine or pure water and the Si-Hx groups covering the surface of freshly prepared PS. Curve F in Figure 1 is the FTIR spectrum of the PS immersed in toluene under N2 and dilute amine in which a trace of water vapor at room temperature was instilled into the flask in a stream of N2. From the curve, we can see that most of the Si-Hx stretching modes around 2100 cm-1 have disappeared. An obvious OH stretching mode around 3500 cm-1 and a bending mode at 1630 cm-1 appear.29 It also shows an intense, broad peak of the characteristic stretching mode of Si-O-Si around 1100 cm-1. The FTIR properties of this control sample reveal that hydrolysis of the Si-Hx groups covering the surface of the PS takes place when a trace of amine and water are present together. Comparing the FTIR spectrum of this control sample (curve F) with that of the sample treated by the in situ silanization reaction (curve B), we see obvious differences between their FTIR properties: the former shows a more intense Si(O-H)x (x ) 1-3) stretching mode around 3500 cm-1 and a bending mode at 1630 cm-1 as compared to the latter, while the latter shows more intense C-Hx stretching modes around 2900 cm-1 as compared to the former. The appearance of the characteristic IR absorption of Si-O-Si around 1100 cm-1 and the obvious characteristic IR stretching mode of C-Hx around 28003000 cm-1 28 indicates that silane groups have bonded to the surface of the PS layer after the in situ silanization reaction. Although this control sample shows an observable C-Hx stretching mode at 2800-3000 cm-1, it is (27) Bateman, J. E.; Eagling, R. D.; Worrall, D. R.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. Engl. 1998, 37, 2683. (28) Pai, P. G.; Chao, S. S.; Takigi, Y.; Lucovsky, G. J. Vac. Sci. Technol., A 1985, 4, 689. (29) Yan, J.; Shih, S.; Jung, K. H.; Kwong, D. L.; Kovar, M.; White, J. M.; Gnade, B. E.; Magel, L. Appl. Phys. Lett. 1994, 64, 1374.

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Figure 3. XPS surveys in the Si2p and Si2s regions before (A) and after (B) the exposure to an amine/water atmosphere.

Figure 2. (a) The full XPS survey and (b) the high-resolution spectra at the S2p region: A, the sample modified by the in situ method; B, the sample immersed in toluene in an atmosphere of dilute amine; C, the sample immersed in toluene in an atmosphere of dilute amine with a trace of water.

weaker than that in curve B. We believe that the obvious C-Hx stretching modes in curve B correspond to the alkyl chain of the silane molecules bonded to the surface of the PS, while the very weak C-Hx modes in curve F originate from adsorbed organic amine. The absence of the observable Si-N stretching modes30 rules out a bonding reaction between the PS and the amine. The variation in the intensity of the Si(OH)x vibration modes indicates that some Si(OH)x intermediates have been depleted by the silane molecules during the in situ silanization reaction. The modified samples, as well as the control sample, show a weak peak at about 2250 cm-1. This peak can be attributed to the characteristic mode of (O)Si-H, which may derive from partial oxidization of the unsubstituted Si-Hx species. Figure 2a shows the full XPS surveys of the sample modified by the in situ silanization reaction (curve A), the control sample immersed in toluene under a N2 and dilute amine atmosphere (curve B), and the control sample immersed in toluene under a N2 and dilute amine atmosphere with a trace of water (curve C). Figure 2b shows the S2p high-resolution XPS surveys of the above samples. From Figure 2a, we see that the C1s peak at 285 eV in curve A is much more intense than that in curve B and curve C, while the Si2s peak at 154 eV, the Si2p peak at 103 eV, and the O1s peak at 533 eV31,32 in curve A are weaker than those in curve B and curve C. These XPS properties indicate that the surface of the samples modified by the in situ silanization is covered with some carbon compounds. The O1s peak at 533 eV is much more intense in curve C than that in curve A and curve B, in accordance with the FTIR results showing that the surface of the samples treated in dilute amine (30) (a) Lucovsk, G.; Richard, P. D.; Tsu, D. V.; Lin, S. Y.; Markunas, R. J. J. Vac. Sci. Technol. 1986, A4, 681. (b) Tsu, D. V.; Lucovsk, G.; Mantini, M. J. Phys. Rev. B 1986, 33, 7069. (31) Bansal, A.; Li, X. L.; Lauermann, I.; Lewis, N. S. J. Am. Chem. Soc. 1996, 118, 7225. (32) Kovtyukhova, N. I.; Buzaneva, E. V.; Waraksa, C. C.; Martin, B. R.; Mallouk, T. E. Chem. Mater. 2000, 12, 383.

Figure 4. The FTIR spectra of the modified PS layer before (A) and after (B) immersion in a 0.1 M NaOH aqueous solution for 30 min.

with a trace of water is covered by Si(OH)x groups. From Figure 2b, we see that the S2p peak of the SH groups, centered at 164 eV,33 is very prominent in the XPS spectrum of the sample modified by in situ silanization, while the control samples do not show such a characteristic peak. We believe that this S2p signal originates from the SH groups of the attached Si(CH2)3SH moieties on the surface of the modified PS. The XPS results further confirm the attachment of the silane groups on the surface of the PS, which is consistent with the FTIR result that the C-Hx stretching mode corresponds to the alkyl chain of the silane molecules bonded to the PS surface. It has been known that the Si2p and Si2s peaks will have a high energy shift with the increase of the O content in the SiOx (0 e x e 2) compositions.32 Figure 3 gives the XPS surveys in the Si2p and Si2s regions of the PS before (A) and after (B) exposure to an amine/water atmosphere. From the spectra, we can see that both the Si2p and the Si2s peak positions do not change after the exposure, which means that no heavy oxidization took place afterward. 3.2. Stability Test. The freshly prepared PS layer can be dissolved in an aqueous solution of base,7 which is applied to measure the thickness of the PS layer,6 and the PL will be quenched after immersion in base solution even for the oxidized PS.34,35 Here, the stability of the modified layers and their PL properties were studied after corroding the samples in an aqueous NaOH solution. Figure 4 shows the FTIR spectra of the modified sample before (curve A) and after (curve B) immersion in 0.1 M NaOH for about 30 min. From the figure, we can see that even after immersion in 0.1 M NaOH for 30 min, the FTIR spectrum of the modified sample shows no change in the C-Hx stretching modes around 2900 cm-1. This reveals that the (33) Zhang, Q.; Huang, H. Z.; He, H. X.; Chen, H. F.; Shao, H. B.; Liu, Z. F. Surf. Sci. 1999, 440, 142. (34) Chun, J.; Boccarsly, A.; Cottrell, T.; Benziger, J.; Lee, J. J. Am. Chem. Soc. 1993, 115, 3024. (35) Fukuda, Y.; Zhou, W.; Furuya, K.; Suzuki, H. J. Electrochem. Soc. 1999, 146, 2697.

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Based on our experimental results, a reasonable mechanism for the amine-catalyzed in situ silanization on the surface of the PS can be proposed. It is known that the hydrolysis of the SiH4 molecule is difficult in pure water but occurs very readily in base.38 Numerous studies have shown that the properties of the Si-Hx species covering the surface of silicon are very similar to that of the SiH4 molecule.39-42 The amine here provides an alkaline environment for the hydrolysis of the Si-Hx species, which can be expressed in the following equations:

H2O + C3H7NH2 f OH- + C3H7NH2H+

(1)

Si-H + OH- + C3H7NH2H+ f SiOH + H2v + C3H7NH2 (2)

Figure 5. The PL spectra before (curve A) and after (curve B) immersion in 0.1 M NaOH solution: (a) the modified PS layer after 30 min; (b) the control sample after 30 s.

silanes bonded to the PS surface are very stable, and the silane layer can protect the PS layer from corrosion by base solution. Figure 5a is the PL spectra of the modified PS before and after immersion in a 0.1 M NaOH solution for 30 min. The PL was only slightly quenched by a 0.1 M NaOH solution after about 30 min. On the contrary, the PL spectrum of the control sample treated only in toluene solution under an amine atmosphere with a trace of water was quenched very rapidly after immersion. Comparing Figure 5a with Figure 5b, we can see that the PL stability is enhanced after in situ modification. This result further demonstrates the role of the bonded silane to the stability of the modified PS.35 3.3. A Proposed Mechanism. A number of researchers have studied the influence of amine on the PL properties of the PS layer.34,36,37 However, they did not explain in detail the chemical reaction that takes place between the amine and the PS. V. M. Dubin et al.25 thought that the presence of a trace of water in the reaction system could make the hydrolysis of the Si-Hx groups covering the surface of PS possible in several hours, and then a silanization reaction could take place through the Si(OH)x intermediates derived from the hydrolysis reaction. Our results here show that the hydrolysis of the Si-Hx species covering the surface of PS is negligible over a period of tens of hours in the toluene solution of silane, and even in saturated water vapor at room temperature. However, it can be promoted by a trace of amine. Therefore, we propose that the function of the amine is to catalyze the hydrolysis of the Si-Hx groups covering the surface of the PS. Considering the amidocyanogen unit in the silane that V. M. Dubin et al. used, it is reasonable that the silanization could take place in their experiment. (36) Coffer, J. L.; Lilley, S. C.; Martin, R. A.; Filessesler, L. A. J. Appl. Phys. 1993, 74, 2094. (37) Henderson, R. R. C.; Krawice, B. S.; Coffer, J. L. J. Phys. Chem. 1995, 99, 8851.

In the first step, the amine effects the disassociation of the water molecules, leading to the formation of the OHion (eq 1). The latter then displaces H- ions2 from the Si-Hx groups, releasing H2 and forming the Si(OH)x groups. The subsequent silanization reaction is a condensation reaction between the intermediate Si(OH)x groups and the silanes that are present. Thus, a complete in situ silanization reaction is achieved on the PS surface. This proposed mechanism of the in situ silanization is confirmed by the FTIR spectra. As shown in Figure 1, the sample immersed in pure toluene in an atmosphere of dilute amine and a trace of water shows more intense Si(O-H)x modes at 3500 and 1630 cm-1 (Curve F), and this agrees with a Si(OH)x surface coating of the hydrolyzed PS. In contrast, the Si(O-H)x characteristic absorption modes in the FTIR spectrum of the samples treated by in situ silanization are much weaker than those in the former because some of the Si-OH groups had been depleted by in situ reaction with the silane molecules present (Curve B in Figure 1). 4. Conclusion An in situ silanization reaction on the surface of freshly prepared PS has been achieved in an inert atmosphere at room temperature in the presence of a trace of water and amine. The stability of the composition of the PS layer and its PL are obviously enhanced after in situ silanization. It has been demonstrated that the amine provides the alkalinity for the hydrolysis of the Si-Hx groups on the PS surface, forming the intermediate Si(OH)x groups which subsequently undergo the silanization. This in situ silanization does not require separate pretreatment as in the traditional method, and we expect that it will be useful for derivatization of the PS surface. This constitutes a new approach to the modification of silicon materials that are oxidizer sensitive. Further development of this in situ surface modification may make PS useful as a high surface area material for integrated sensors or optoelectronic devices. (38) Bailar, J. C.; Emeleus, H. J.; Trotman-Dickenson, A. F. In Comprehensive Inorganic Chemistry; Pergamon Press: Elmsford, NY, 1973; Vol. 1, pp 1367-1368. (39) Asao, N.; Sudo, T.; Yamamoto, Y. J. Org. Chem. 1996, 61, 7654. (40) Zazzera, L. A.; Evans, J. F.; Deruelle, M.; Tirrell, M.; Kessel, C. R.; Mckeown, P. J. Electrochem. Soc. 1997, 144, 2184. (41) Buriak, J. M.; Allen, M. J. J. Am. Chem. Soc. 1998, 120, 1339. (42) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; Van der Mass, J. H.; De Jeu, W. H.; Zuihof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759.

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Acknowledgment. This work was supported by the Chinese National Natural Science Foundation and the Major State Research Development Program (Grant No. 2000077500). The authors thank Dr. Shaul Wallach and

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Mrs. Louise Braverman, Department of Chemistry, BarIlan University, Israel, for editorial assistance. LA015664G