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Секция 11.Физика
Section 11. Physics Секция 11. Физика
Gadzhieva Nushaba Nubarak Institute of Radiation Problems, Azerbaijan National Academy of Sciences
E-mail: Sevinc.m@rambler.ru
An IR-spectroscopy study of radiation-stimulated adsorption of n-hexane on the silicon surface
Abstract: The peculiarities of radiation-stimulated adsorption of n-hexane on the silicon surface under the action ofy-quanta at room temperature have been studied by infrared reflection-absorption spectroscopy. It has been shown that adsorption of n-hexane on the silicon surface occurs by the molecular and dissociative mechanisms. It has been found that at absorbed dose Dy~10-35 kGy observes activated dissociative chemisorption which accompanied by the formation of silicon alkyls and surface silicon hydrides. A possible mechanism of this process has been discussed.
Keywords: silicon, n-hexane, the radiation-stimulated adsorption of n-hexane, y- radiation, the infrared reflec-
tion-absorption spectroscopy (IRRAS).
Introduction
Research into the adsorption and radiation-chemical transformation ofhydrocarbons in metal — hydrocarbon and metal — semiconductor heterogeneous systems is of particular interest for radiation heterogeneous catalysis and atomic hydrogen power engineering and of fundamental importance for solving a number of environmental problems. The use of various metals with an activated surface accelerates the radiation-chemical decomposition of paraffins and increases the efficiency of hydrogen production by the environmentally safe radiolysis of hydrocarbons. Some features ofthe interaction and activation of paraffins on metal surfaces have been discussed in [1-4]. However, to date, the occurrence ofradiative processes in a metal-hydrocarbon heterogeneous system has been insufficiently studied [4-9]. In addition, literature data on the adsorption and chemical transformation of paraffins on the surface of semiconductors stimulated by y- radiation are hardly available [10].
This work presents the results of IR-spectroscopic studies of regularities of radiation-stimulated adsorption of n-hexane on the silicon surface exposed to y-rays at room temperature.
Experimental
Polished single-crystal silicon wafers with sizes of 20x10x2 mm exhibiting high reflectivity R of 0.95 ±
0.05 in the infrared band at wavelength \ of 15-2.2 pm were used [9, 10].
In order to avoid impurity contamination, the samples were exposed to solvents (ethanol, acetone) and predried at room temperature in an argon atmosphere. To conduct surface dehydroxylation and complete cleaning from organic impurities, the samples were placed in quartz cells and subjected to an additional heat treatment at 673 K in a vacuum of P = 10 6 Pa for 6 h. The adsorbate was unsaturated vapors of reagent-grade n-hexane; impurity gases were removed from the vapors by repeated freezing in a trap with liquid nitrogen and subsequent evacuation. The adsorption of n-hexane was studied as described in [8].
The samples were irradiated using an isotopic source of 60Co у rays at a dose rate of dDY/dt= 1.03 Gy s-1 Dosimetry was performed with chemical dosimeters; ferrosul-phate and methane methods [12]. The absorbed dose in the systems under study was carried out by comparing the electron densities. In this case, absorbed dose was Dy = 5-50 kGy.
The IR reflection spectra for linearly polarized radiation incident on the sample at an angle close to the glancing angle (ф= 80°) were measured on FT-IR spectrometer (Varian 640IR) in a frequency range of 4000-400 cm-1 at room temperature using a special optical device (Shimadzu, Japan). To this end, a quartz cell with a CaF2 window was designed and constructed for recording the spectra of adsorbed n-hexane and monitoring the changes resulting from its decomposition under y-radiation [10]. When overlapping bands we performed
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the decomposition of the contour into the individual components by method [13]. The optical density of the absorption bands was determined by the formula
D=-Ig (R/R,), (1)
where R and R0 are the reflection coefficients ofpure silicon and silicon with adsorbate (n-hexane) respectively [7]. According to formula (1) we calculated the optical density D, D0 of adsorption bands of C-H and Si-H stretching vibrations (D0-optical density in the initial sample, D-in treated samples) and we determined their ratio D/D0.
Results and discussion
The IR-reflection spectra of n-hexane adsorbed on the surface of the dehydroxylated silicon in the stretching vibration C-H at room temperature are shown in Figure 1 (curve l). It is obvious that the adsorption of
n-hexane on a silicon surface is accompanied by a series of absorption bands (a. b.); narrow at vmax= 2950, and 2920 cm-1, weak-at 2870 and 2830 cm-1 and an intense broad at 2650 cm-1- The narrow bands are close to the position of the bands featured for v (C-H) in the spectra of n-C6 H in the gas phase [14] which allows us to assign them to a physically adsorbed n-hexane. Asymmetric broad band with a maximum at 2650 cm-1 attributed to the variation of one of the CH-bonds in a molecule of n-C6 H perturbed silicon surface centers. Unusually low frequency vibrations, as well as a relatively high intensity and width demonstrate the strong perturbation of molecules of n-hexane at adsorption. The complex to which these a. b. refer is unstable and collapses when degassing at room temperature. This allows the band to include n-hexane adsorbed in the molecular form.
Fig.1. IR- absorption spectra of n-hexane, adsorbed on the silicon surface; before (1) and after Y-irradiated (2) at D=20 ^y, T=300K, р=20Pа.
The occurrence of the three forms ofadsorption is also confirmed bands in the region of the C-H bending vibrations peaking at vmax=1590, 1540,1520,1470,1400 and1 370 cm-1 [10].The formation of molecular complexes is proved theoretically within ab initio quantum chemical calculations of the potential energy profile of dissociative energy of methane on the surface of Ni and experimentally found in the study of its adsorption on metal surfaces (Fe, Ni, Pt) [2,7]. The formation of a molecular complex spectrally has been also proved by us in n-hexane adsorption on the surface of aluminum [7]. When comparing the spectra of this complex with n-hexane adsorption on the surfaces of aluminum and silicon showed that in the case
of silicon, a. b. complex is much narrower (half-width decreases from v1/2= 110 to 85 cm-1, the difference is the halfwidth Av1/2=25 cm-1), its maximum shifts to Av=30 cm-1 (from v = 2680 to 2650 cm-1) and it is less asymmetric in the low frequency region. The observed effects are apparently related by the difference in the extent and density of surface states of silicon defects.
The weak absorption bands (a. b.) with maxima at 2870 and 2830 cm-1 show insignificant dissociative adsorption of n-hexane at the surface Si, that is associated with even lower than for the H-bonded complexes, concentrations of such forms, in particular at low temperature interaction.
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In the study of the adsorption of n-hexane on an y-pre-irradiated silicon samples we found out that starting from certain values ofthe radiation dose (D^=10kGy) at room temperature, there is a strong dissociative chemisorption. It results from the interaction of n-hexane with surface centers formed under the action of y-quanta silicon. This is evidenced by the increase and redistribution of intensities a. b. at 2870 and 2830 cm-1; as well as the appearance in the IR spectrum of the new a. b. with a peak at 2780 cm-1 (Fig. 1, curve 2). The observable absorption bands appear to relate to the stretching vibrations of CH-bonds fragments CH3, C2H5, etc., associated with silicon (silicon alkyls).
The process of dissociative adsorption also confirmed the appearance in the IR spectrum (Fig. 2, curve 1) in the region of valence (v«2200-1900 cm-1) and deformation (v«900-600 cm-1) vibrations Si-H
bands with frequencies 2100, 2000 and 640 cm-1 which are Si-H bonds [15, 16]. With increasing the doses of y-irradiation up to 40 kGy of the intensity of these a. b. are redistributed: the intensity of the bands with maxima at 2000 and 640 cm-1 decrease, and vice versa, the intensity of the band 2100 cm-1 increase and a new strong band with a maximum at 895 cm-1 appears. It points to the accumulation of hydrogen in the form of its hydride (Fig. 2, curve 2,3). A further increase in value up to 50 kGy is accompanied by the formation of a stable silicon hydride SiH2 at room temperature (a. b. with maxima at 2100 and 895 cm-1) [16].The formation of the surface silicon hydrides is confirmed by the appearance of the absorption bands in the overtone region of the Si-H bond (v«4300-3800 cm-1) with maxima at 4200 and 4000 cm-1, the intensity of which depends on the absorbed dose of gamma — irradiation redistributed.
Figure2. IR- absorption spectra of the surface silicon hydrides, resulting in the adsorption of n-hexane on Si at Dy=10 (1), 15 (2) and 30 kGy.
In addition, we recorded in the Raman spectra, after the adsorption of n-hexane on the silicon surface plates (Raman spectra were recorded on Perkin-Elmer LR-3 spectrometer, Ar-laser, \xc =514,5 nm) along peaks due to lattice vibrations of the silicon doublet observed in the stretching vibration of Si-H bond (v«2000-2100 cm-1) with maxima at 2040 and 2100 cm-1 [17]. The ratio of intensities of these peaks depending on the dose of gamma — irradiation increases in favor of formation of silicon hydride SiH2 It should be noted that the problem of radiation-induced hydrogenation of silicon is of particular interest, and will be a subject of separate studies. In order to identify the spectro-kinetic regularities of the radiation-stimulated adsorption of n-hexane on the surface of silicon the adsorption of kinetic curves has
been studied, i. e. the dependences of the changes of the related optical density of absorbance bands of molecular and dissociative adsorbed forms of n-hexane on the absorbed y-radiation dose which are shown in Figure 3a.
It can be seen that the kinetic feature of radiation-induced chemisorption consists of a certain initial induction period when D £10kGy (associated with healing of biographical defects), the linear region at 10£ D^£35kGy (caused by the generation of adsorption-active sites and adsorption on these centers of additional molecules of n-C6 H ), as well as stationary saturation region at D^£35kGy (Fig.3a, curve 1). Apparently, under the action y-radiation is generated in new silicon surface active states, the density of which increases by the rise of y-radiation dose in silicon, and increases their probabil-
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ity of interacting with the adsorbed molecules of n-hex-ane, it causes them to dissociate. At the same time, the kinetic curve of the molecular adsorption of n-hexane is characterized by two areas: a formation is observed in
325kGy the destruction of molecular H-complex occurs (Fig. 3a, curve 2). The presence of activated dissociative chemisorption in 10 £ D^£ 35kGy is also confirmed by the progress of a kinetic curve obtained for surface sili-
Figure 3.The dependences of the related optical density of absorbance bands of dissociative (1) and molecular (2) absorbed forms of n-hexane (a) and the optical density of absorption bands of the surface silicon hydrides (1,2) (b) on the absorbed Y-radiation dose: vmax = 2870 (1a), 2650cm-1 (2a) and vmax=2100 (1b), 2000cm-1 (2b).
Thus, an activated dissociative chemisorption is detected in the absorbed dose range of 10 £ D^£ 35kGy which explains both an increase of the number of centers of activated adsorption by the surface-excited states of silicon and a decomposition of H-bonded complexes in the result of the transfer of excitation energy to the molecules of n-hexane. Activated adsorption of n-hexane was also observed on the surface of nickel and aluminum [2, 7].
Data of IR- spectroscopic analysis suggest that, under the action of y-radiation the radiation-stimulated adsorption of n-hexane on the silicon surface occurs. The mechanism of this process can be represented as follows: under the action of y-radiation active states S * (ions, localized charges, etc.) are formed on the Si surface and it emits secondary electron radiation (esec) [18]:
Its interaction with n-hexane leads to excitation of n-C,H.. molecules:
6 14
H - C6H14 esec yH - C6H-U (ads.),
S*
where n- C6H14* (ads) -is the excited adsorbed state of the n-hexane molecules.
The excitation of n-hexane molecules on the surface-active states S* occurs through complexation followed by the transfer of the excitation energy of the complex to the n-C6H14 molecules. After that, the excited molecules of n-hexane undergo decomposition by the homogeneous mechanism [19, 20], and is accompanied by the formation of intermediate active relaxing particles (H, CH, C2 Hs).
The intermediate active decomposition particles can interact with surface active states to form silicon hydrides and alkyls:
=Si+2H^Si-H2,
=Si+C H2 2 ■» Si -CH, ,
n 2n+2 n 2n+1
Conclusions
Our experimental data have shown that reflection-absorption spectroscopy can be used to study the radiative processes in the silicon-n-hexane system at room temperature. Based on the analysis of the FT- IR spectra of n-hexane adsorbed on the surface of the y-irradiated silicon samples we made a conclusion about a complex formation with a perturbation of C-H bond (adsorption molecular form) and the possibility of occurrence
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of dissociative adsorption accompanied by the formation of surface silicon alkyls and silicon hydrides. There has been detected an activated dissociative adsorption at absorbed dose 10 £ D^£ 35kGy due to both an in-
crease in the number of surface-active centers formed by the action of y-rays in silicon and the destruction of the molecular complexes that are a transitional state in adsorption processes.
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