selective cleavage of functional groups in the ...

Surface Review and Letters, Vol. 9, No. 1 (2002) 305–311 c World Scientific Publishing Company

SELECTIVE CLEAVAGE OF FUNCTIONAL GROUPS IN THE FUNCTIONALIZED ORGANIC MONOLAYERS BY SYNCHROTRON SOFT X-RAYS TAI-HEE KANG,∗‡ KI-JEONG KIM,∗ CHAN-CUK HWANG,∗ KYU-WOOK IHM,∗ HYUN-JOON SHIN,∗† MIN-KYU LEE,∗ BONGSOO KIM∗† and CHONG-YUN PARK‡ ∗ Beamline Research Division, Pohang Accelerator Laboratory, Pohang Institute of Science and Technology, Pohang, 790-784, Korea † Department of Physics, Pohang Institute of Science and Technology, Pohang, 790-784, Korea ‡ Department of Physics and BK21, Sung Kyun Kwan University, Suwon, 440-746, Korea YOUNG-HYE LA, JOONG HO MOON, HYUN JU KIM and JOON WON PARK Center for Integrated Molecular Systems, Department of Chemistry, Division of Molecular and Life Sciences, Pohang Institute of Science and Technology, 790-784, Korea Aminosilylated surface was treated with halide-substituted aromatic aldehydes, and the resulting molecular layers were examined with synchrotron X-ray photoelectron spectroscopy at the 2B1 SGM and 4B1 microscopy beamline in the Pohang Accelerator Laboratory. It was observed that the halide group of the film diminished upon the irradiation, but the other bands were constant in terms of the intensity and the shape. This observation indicates that the functional groups of the organic monolayers are cleaved selectively by soft X-rays. The cleavage rate was measured as a function of photon energy and normalized with photon flux. The cleavage is first-order to the concentration of the functional group. Its rate constant is sensitive to the molecular structure of the organic monolayers and the kind of substituents on the aromatic ring.

1. Introduction

interest, because the final efficiency or property of the materials strongly depends on the chemical and physical properties of the SAM.4 X-ray photoelectron spectroscopy (XPS) has been considered one of the most powerful techniques that are very sensitive to organic monolayer surface.5 It gives valuable information on the electronic structure of atoms, molecules and solids. The electronic structure determines many of their fundamental properties, such as chemical, mechanical, optical, electrical and magnetic properties. Furthermore, the tunable synchrotron photon energy can have extra

The self-assembled monolayer (SAM) with a terminal functionality has been extensively studied because of the ease of introducing new functional groups in defined ways through chemical or physical reaction.1 Various types of the SAM has been used not only as a protecting layer,2 such as antifouling, anticorrosion and lubrication layer, but also as a building block3 for functional materials such as catalyst, sensor, electrical material, and supporter for biological interests. Comprehensive understanding of the features of the SAM has been an issue of

PACS: 33.60.Cv, 33.60.Fy

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306 T.-H. Kang et al.

merit for providing the photon of a particular energy band that excites a specific atom selectively. Although XPS has been widely used in many fields, considering the analysis of thin films, the bond cleavage or damage to organic materials has been observed by soft X-ray irradiation while taking the photoemission spectra.6–8 Sagiv et al.8 surveyed the damage or degradation phenomena of various substances including polymers and thin layers. We previously reported that C–N bond cleavage in the nitrobenzaldimine monolayer is indeed selective.9 In this report, the cleavage of the halide functional groups from functionalized benzaldimine monolayers by soft X-rays is presented. Functional groups include various halides F-, Cl-, Br- and I-. Moreover, rate constants and cross-sections of the cleavage are measured upon the incident photon energy to unveil its mechanism. In addition, analysis on the valence band structure of the irradiated sample is presented.

2. Experimental The polished prime Si(100) wafers (dopant — phosphorous; resistivity — 1.5–2.1 Ω cm) were obtained from MEMC Electronic Materials, Inc., and cut into pieces of 30 mm × 10 mm. For the preparation of imine-formed monolayer, the aminosilylated substrate was immersed into anhydrous ethanol solution (20 mL) containing a corresponding benzaldehyde (approx. 10 mg), under nitrogen atmosphere. The benzaldehyde derivatives were condensed with the amine of the monolayer prepared with aminopropyldiethoxymethyl-silane (APDES).10,11 After the condensation, the substrates were sonicated in ethanol, methanol, acetone and dichloromethane, sequentially. For each sonication step, 2 min was allowed. Finally, the substrates were dried under vacuum. The formed benzaldimine substrates were transferred into an ultrahigh vacuum chamber of the end station of the beamline. The photoemission spectroscopy experiments were performed at the 2B1 spherical grating monochromator (SGM) beamline11 and 4B1 photoemission/ microscopy beamline13 in the Pohang Accelerator Laboratory (PAL). The photon energy was selected to give the most-enhanced photoemission intensity and/or the best-resolved surface-related structure in the spectra. In the case of bromobenzaldimine

monolayer, for example, 500 eV and 250 eV energies were selected for the largest photoionization crosssection of the N 1s orbit and Br 3d orbit respectively. The photocurrent was measured directly from the IRD AXUV-100 photodiode.

3. Results and Discussion 3.1.

Investigation of formation of the halide-substituted benzaldimine monolayers

The benzaldimine monolayers were prepared by treating the aminosilylated silicon wafer with halide-substituted benzaldehydes. Figure 1 shows the procedure of formation of halide-substituted benzaldimine monolayers. The aldehydes include 4fluorobenzaldehyde, 4-chlorobenzaldehyde, 4-bromobenzaldehyde and 4-iodobenzaldehyde. The absolute density of the imines on the surface was determined by the same method applied to 4nitrobenzaldehyde,10 and the pertinent extinction coefficients were used for the calculation. For example, the 4-fluorobenzaldimine-formed substrate (1×3 cm) was dipped into water of the known volume for the hydrolysis, and the concentration of the generated aldehyde was measured by a UV-visible spectrophotometer. The absolute density of the imine was obtained from the amount of the aldehyde and the known surface area of the substrate. Invariably absolute density of approximately 3 amines/nm2 was observed for each sample.

Fig. 1. The benzaldimine monolayers were prepared by treating the aminoasilylated silicon wafer with halidesubstituted benzaldehydes.

Selective Cleavage of Functional Groups in the Functionalized Organic Monolayers 307

arising from the core level of corresponding halides [such as F(1s), Cl(2p), Br(3d), and I(3d)] and other components [such as C(1s), N(1s), O(1s), Si(2s), and Si(2P)].

3.2.

Fig. 2. Survey spectra of the 4-fluorobenzaldimine (a), 4-chlorobenzaldimine (b), 4-bromobenzaldimine (c) and 4-iodobenzaldimine (d) obtained at 820 eV photon energy.

The successful transformation of the reaction was confirmed by the existence of peaks from XPS survey spectra. The spectra (Fig. 2) of the benzaldimine monolayers, obtained at 820 eV, showed the bands

Cleavage of the halide-substituted benzaldimine monolayers by photons

The analysis of the spectra obtained along the irradiation times showed that the carbon-halide bond of halogen-substituted benzaldimine monolayers was cleaved as our previous observation9 in which the intensity for the N(1s) peak of the nitrobenzaldimine monolayer was decreased as the irradiation. Photon energies were selected for the largest photoionization cross-section, which were 820, 350, 250 and 780 eV for F(1s), Cl(2p), Br(3d) and I(3d), respectively. The change of representative F(1s), Cl(2p), Br(3d) and I(3d) peaks is shown in Figs. 3(a)–3(d). A spectrum obtained immediately after the light shutter was open is composed of filled squares (). The second (), third (•), fourth (◦), fifth (N) and sixth (4) data were recorded at time intervals of 10 min under continuous soft X-ray irradiation at each photon energy. The insets in Fig. 3 show that the rate of diminution is first-order to the integrated peak area of the halide group. The logarithm of the relative peak area for the halogen group decreases linearly with respect to the irradiation time. Peaks coming from the C(1s), N(1s) and O(1s) levels were also examined as function of the irradiation time (Fig. 4). These spectra were obtained from the 4-bromobenzaldimine monolayer and the other halide-substituted benzaldimine showed the same constancy. The characteristic features and the intensity of the peaks were invariant under soft X-ray irradiation at 500 [for C(1s) and N(1s)] and 820 [for O(1s)] eV within the error bound. Therefore, it is clear that the carbon-halide bond is cleaved selectively by soft X-ray irradiation, leaving the phenyl ring intact on the surface. To examine the effect of the incident photon energy on the carbon-halide bond cleavage, we measured the rate constants of the bromide-substituted benzaldimine monolayer at various energies (150– 800 eV). To increase the accuracy of the measurement, we chose the 3, 5-dibromo-salicylaldimine monolayer, which exhibits a stronger Br (3d) band


Fig. 3. The change of representative F(1s), Cl(2p), Br(3d) and I(3d) peaks as the irradiation continues; the photon energies were 820, 350, 250 and 780 eV, respectively. A spectrum obtained immediately after the light shutter was open is composed of filled squares (). The second (), third (•), fourth (◦), fifth (N) and sixth (4) data were recorded at time intervals of 10 min under continuous soft X-ray irradiation at each photon energy. The insets show that the rate of diminution is first-order to the integrated relative peak area of the halide group.


Fig. 4. C(1s), N(1s) and O(1s) levels of the 4-bromobenzalidimine monolayer were also examined as a function of the irradiation time. The other halide-substituted benzaldimine showed the same constancy. The characteristic features and the intensity of peaks were invariant under soft X-ray irradiation at 500 [for C(1s) and N(1s)) and 820 (for O(1s)] eV within the error bound.

than the 4-bromobenzaldimine monolayer. The normalized rate constants were 1.83 × 10−4 s−1 (at 150 eV), 1.24 × 10−4 s−1 (at 250 eV) and 0.83 × 10−4 s−1 (at 820 eV). For the normalization, photon flux for 500 eV (6.6 × 1011 photons/cm2 s)

was selected for the reference. The rate constant decreased slightly with increasing photon energy, and the examined photon energy range was enough to cover the possible photoexcitation and ionization from various Br energy levels, including


Br(3s)(256 eV), Br(3p)(182, 189 eV) and Br(3d)(70, 71 eV) levels. This observation supports that the bond cleavage does not occur via direct excitation or ionization from any of these levels. If direct photoninduced cleavage occurred, the cleavage yield would exhibit a substantial increase at or above the corresponding excitation energy. Currently, many of the indirect photoinduced dissociation processes are thought to occur via the electron-stimulated desorption (EDS) mechanism.14 Rieley et al. reported that a carbon-chloride bond is cleaved by irradiation on ω-chloroalkanethiol monolayers.15 Recently, Khan et al. observed that physisorbed alkyl bromides on GaAs could be cleaved by photoinduced electrons, resulting in the bromide anion and an alkyl radical.16 In these cases, bond cleavages are proposed to be caused by electron attachment to occupy the σ ∗ orbital. A similar mechanism may be operative in the present case.

3.3.

Fig. 5. The valence band spectra of (a) intact 4-bromobenzaldimine monolayer, (b) 4-bromobenzaldimine monolayer irradiated at a photon energy of 30 eV for 30 min and (c) aminosilane monolayer prepared from APDES. Inset: Br(3d) region of (a) intact and (b) irradiated layer (at 30 eV for 30 min) scanned at 250 eV. The bromide intensity diminished to over 94% of the initial intensity during the irradiation.

Valence band study on 4-bromobenzaldimine monolayer

We further investigated the cleavage process by using a low energy photon (30 eV). The photon energy 30 eV is the lowest obtainable energy with a suitable flux at the 2B1 SGM beamline. Figure 5 shows the valence band spectra of the 4-bromobenzaldimine monolayer. The filled squares () constitute a spectrum obtained right after a light shutter was open. The open squares () represent a spectrum obtained after the irradiation of the 30 eV for 30 min. Although the flux (3.2 × 1011 photons/cm2 s) of 30 eV photon energy was smaller than 500 eV (6.6 × 1011 photons/cm2 s), it was high enough to cleave the C–Br bond significantly in 30 min. It was estimated based on the rate constant that over 94% of the bromide was cleaved during the irradiation time. While the Br(3d) band fully disappeared after the irradiation [inset (b)], the valence band structure was invariant. This result indicates the phenyl moiety of the benzaldimine monolayer is not damaged during the irradiation. When the spectra from benzaldimine monolayers [(a) and (b)] are compared to the aminosilane one [(c)], it is clear the phenyl moiety is intact. The relative weak intensity near 3–6 eV for aminosilane layer (c) as compared with the benzaldimine ones [(a) and (b)] is due to the absence of the phenyl moiety in the aminosilane layer. From the experimental data of the valence

band, we can demonstrate that: (1) the cleavage rate is independent of the incident photon energy; (2) the C–Br bond is cleaved selectively, leaving the phenyl ring intact on the surface.

4. Conclusions The synchrotron soft X-ray induced cleavage of halide groups on the organic monolayers was examined by monitoring the peak intensity as the irradiation continued. The constant cleavage efficiency over the photon energy ranging from 150 to 820 eV, even 30 eV in case of 4-bromobenzaldimine, was observed. The cleavage is selective and its rate is independent of the incident photon energy. The valence band structure stays unchanged upon the irradiation. It reconfirms that the carbon-halide bonds were cleaved selectively. Overall, the induced electrons of low energy cause the selective cleavage of the carbon-halide bonds on the benzaldimine monolayer.

Acknowledgments This work at PLS is supported by the Ministry of Science and Technology and POSCO. This experiment is also supported by the Korean Science


and Engineering Foundation through the AtomicScale Surface Science Research Center (ASSRC). Student fellowships of Brain Korea 21 are gratefully acknowledged, and also the work is supported by KOSEF through CIMS.

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