- Email: [email protected]
Nanostructured unsaturated carbon from laser-photo-polymerization of diacetylene
Carbon 42 (2004) 2521–2526 www.elsevier.com/locate/carbon
Nanostructured unsaturated carbon from laser-photo-polymerization of diacetylene J. Pola
a,*
, A. Ouchi
b,*
, Z. Bastl c, K. Vacek a, J. Boh acek d, H. Orita
b
a
c
Laser Chemistry Group, Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 16502 Prague, Czech Republic b National Institute of Advanced Industrial Science and Technology, AIST, Tsukuba, Ibaraki 305-8565, Japan J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 18223 Prague 8, Czech Republic d z, Czech Republic Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 25068 Re Received 20 February 2004; accepted 17 May 2004 Available online 19 June 2004
Abstract KrF laser irradiation of gaseous diacetylene induces photo-polymerization of diacetylene and results in chemical vapor deposition of nanostructured hydrogenated carbon whose structure is assessed from IR, Raman, photoelectron and Auger spectra as highly unsaturated C/H polymer containing carbon atoms mostly in sp2 state. The EPR spectrum of the deposit is strongly affected by the presence of oxygen. The results demonstrate the first example of laser chemical vapor deposition of unsaturated carbon phase from polyalkynes. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Vapor grown carbon; B. Laser irradiation; C. Photoelectron spectroscopy, Raman spectroscopy; D. Chemical structure
1. Introduction It has been recently recognized that high intense laser irradiation of suitable hydrocarbons affords chemical vapor deposition of hydrogenated carbon films. Both photolytic and thermolytic laser action has been demonstrated: the amorphous hydrogenated carbon films were deposited from ArF laser photolysis of acetylene [1–3], ArF and XeCl laser photolysis [4] and IR laser thermolysis [5] of 3-butyn-2-one, and from Arþ cw laser [6], XeCl laser [7] and ArF laser [8] photolysis of methylene iodide. The degree of unsaturation (hybridization) of carbon and the hydrogen concentration in the films deposited in these processes is controlled by the gas-phase chemistry and the structure of the edduct. Thus, the ArF laser photolysis of acetylene yields H-rich and graphite-like [1,2] or highly C (sp3 )-based [3] films, while Arþ , KrF
* Corresponding authors. Tel.: +420-2-2039-0308; fax: +420-2-20920661 (J. Pola); tel.: +81-29-861-4550; fax: +81-29-861-4421 (A. Ouchi). E-mail addresses: [email protected] (J. Pola), ouchi.akihiko@aist. go.jp (A. Ouchi).
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.05.021
and ArF laser photolysis of CH2 I2 gives rise to H-rich and fairly unsaturated C films [7–9]. Carbonaceous materials with some degree of unsaturation were for decades judged as easily photochemically produced from unsaturated hydrocarbons (e.g. [9]). However, recent recognition of laser-photolysis of alkynes leading to rather saturated polymeric frameworks revealed [10–14] that hydrocarbons with isolated CBC bond are not suitable precursors for laser chemical vapor deposition of unsaturated carbonaceous materials. The possibility of the use of alkadiynes for photolytic conversion to unsaturated carbonaceous materials has not been yet attempted. Of these highly unsaturated hydrocarbons, diacetylene appears most suited for studies due to its relative stability and good absorption in the UV region [15]. Its UV photochemistry over 220– 245 nm is known as controlled by triplet metastable C4 H2 * state [15] and by its reactions with C4 H2 molecules to produce C6 H2 , C8 H2 and C8 H3 (primary products) and C10 H3 and C12 H3 (secondary products) [16,17]. Conversely to the well established photolytic mechanism, there have been yet no studies on the deposited diacetylene photoproduct; it can be merely judged that
2522
J. Pola et al. / Carbon 42 (2004) 2521–2526
the structure is contributed by a sequence of alternating single and triple carbon bonds, cumulene (@C@C@) units, or by cyclic structures possessing some sp3 carbons. This paper is a continuation of our studies on laserinduced formation of carbonaceous solid materials [4,5,13,18] and its objective is to determine the structure and properties of the carbonaceous material laserdeposited from diacetylene and to reveal main path through which this carbonaceous material is formed.
2. Experimental The KrF laser photolytic experiments were carried out on gaseous samples of diacetylene (100 Torr) in nitrogen (total pressure 760 Torr) using an LPX 210i laser (KrF radiation) operating at 248 nm with a pulse energy of 370 mJ incident on 2.5 cm2 (fluence 148 mJ/ cm2 ) and a repetition frequency of 10 Hz. The fluence used allowed an efficient chemical vapor deposition of dark carbonaceous solid material, which was not possible at lower fluences (20–60 mJ/cm2 ). The diacetylene– N2 samples were irradiated with unfocused laser beam in a reactor (140 ml in volume) which was equipped with a sleeve with a rubber septum and a PTFE valve connecting it to a standard vacuum manifold and consisted of two orthogonally positioned Pyrex tubes, one fitted with two quartz plates and the other furnished with two KBr windows. The progress of the photolysis was monitored by periodically removing the reactor and placing it in the cell compartment of an FTIR (a Shimadzu FTIR 4000) spectrometer. The measurement was carried out several minutes after the irradiation had been ceased, which allowed the dark fog descend completely to the reactor bottom. The depletion of diacetylene was determined by using absorption bands at 2020 and 845 cm1 . The progress of the photolysis was also followed by gaschromatography (a Gasukuro Kogyo 370 chromatograph, a 2 m SUS Unipak S column, programmed temperature 30–150 °C). The chromatograph was equipped with flame-ionization detector and connected with a Shimadzu CR 5A Chromatopac data processor. Sampling was made by a gas-tight syringe (Dynatech Precision Sampling). The analysis of the irradiated samples was performed by FTIR spectroscopy (ethyne at 730 cm1 ) and by GC– MS method (a Shimadzu QP 5050 mass spectrometer (60 m capillary column Neutrabond-1, programmed temperature 30–200 °C). The photochemical quantum yield of diacetylene depletion was determined on the basis of knowledge of the amount of diacetylene depleted and laser energy absorbed. The incident pulse energy and pulse energy
absorbed in the gaseous diacetylene was measured by a joule meter coupled with a 10 MHz storage oscilloscope. The reactor accommodated tin, silicon and KBr substrates which, covered with the deposited material, were transferred for measurements of physical properties of the deposits by FTIR, Raman, EPR, X-ray photoelectron and Auger spectroscopy and by electron microscopy. The X-ray C 1s, O 1s photoelectron and C KLL Auger electron spectra of the deposit (as prepared and after mild sputtering with argon ions; E ¼ 4 keV, I ¼ 15 lA, 5 min) were measured using an ESCA 310 (Scienta) electron spectrometer with a base pressure better than 10-7 Pa and using AlKa radiation (1486.6 eV) for electron excitation. The overlapping features of the C 1s spectra were resolved into individual components using the lines of Gaussian–Lorentzian shape and a modified non-linear least-square procedure [19]. The surface composition of the deposited film was determined by correcting the spectral intensities for subshell photoionization cross-sections [20]. The measurements of Raman spectra were carried out at room temperature with LabRam Infinity spectrometer (Jobin Yvon, resolution less than 5 cm1 ) equipped with a He–Ne laser (632.8 nm, 20 mW), a microscope sample stage and CCD detector. FTIR spectra of the films were acquired on a Jasco FTIR spectrometer (model 610). Field emission scanning electron microscopy measurements were performed on a dual stage DS-720 Topcon electron microscope and TEM analysis was conducted on a Philips 201 transmission electron microscope at 80 kV. EPR spectra of the deposit (1.47 mg) were measured at room temperature in vacuum and in air by a cw EPR spectrometer working in X band (9.3 GHz and 1 mW) with 100 kHz magnetic field modulation (0.115 mT). The spectrometer was equipped with a digital frequency counter and an NMR magnetometer used for g-factor calculation. Quantitative estimation of spin concentration is based on TEMPOL and Mn2þ (internal standard). Diacetylene was prepared according to the earlier reported procedure [21].
3. Results and discussion 3.1. Photolytic features The KrF laser irradiation of gaseous diacetylene in nitrogen results in the depletion of diacetylene and the formation of a dark fog that fills all the space of the reactor and slowly deposits on the reactor bottom. The fog deposition on the quartz window through which the laser pulses enter the reactor leads to a continuous
J. Pola et al. / Carbon 42 (2004) 2521–2526
2523
Diacetylene, Torr
100
80
60
40
20 0
500
1000
1500
Number of pulses
Fig. 2. SEM image of the deposit (bar ¼ 1 lm).
Fig. 1. Photolytic depletion of diacetylene.
decay of the laser power within the reactor, because the formed film absorbs at 248 nm. This phenomenon makes the photolysis progress higher than 60% difficult to achieve even when the irradiation is directed through both reactor windows. This is illustrated by the dependence of the depletion of diacetylene on the number of pulses (Fig. 1). Acetylene, benzene and triacetylene are the only detectable volatile products. Within 25–56% photolysis progress the amount of acetylene amounts to less than 3% of the depleted diacetylene. Benzene and triacetylene (mass spectrum––m/z, (relative intensity): 74 (100), 73 (35), 72 (5), 60 (5), 37 (27), 36 (9)) are only trace products. The initial depletion rate of diacetylene (0.17 Torr per pulse) corresponds to a photochemical quantum yield 1.8, which is in agreement with that (2.0 0.5) observed for the low pressure photolysis throughout the region of 255–144 nm [15]). We note that the loss of diacetylene in the absence of the laser radiation is very slow; only 5% of diacetylene is depleted from the gas phase after 12 h. The above facts imply that photo-polymerization of diacetylene is a major process responsible for chemical vapor deposition of the solid films. 3.2. Properties of the deposited films The deposited films show poor adhesion to glass, KBr and tin and can be easily removed from their surface as a black powder. The films are insoluble in organic solvents (methanol, chloroform, toluene, hexane and tetrahydrofuran). The SEM image of the solid film (Fig. 2) shows spongy structures composed of irregular chain-like agglomerates that are discovered in the TEM image (Fig. 3) as 50 nm thick interconnecting chains.
Fig. 3. TEM image of the deposit.
XPS analysis of the deposit shows that the stoichiometry before ion sputtering is C1:00 O0:25 and that after sputtering is C1:00 O0:03 . This is indicative of oxidation of a few superficial layers. The XP spectra of C 1s electrons (Fig. 4) reveal that 11% of carbon is bonded to OH functionalities and 7% of carbon is incorporated in COOH group. The energy difference between the most positive and most negative excursions in the C KLL first derivative Auger spectra (Fig. 5) being a measure of the population of C atoms in sp3 and sp2 states [22–24] reveals a dominating occurrence of the sp2 hybridized carbon. It appears that the minimum for the as received deposit coincides with that of graphite (and not of diamond). The ion sputtering shifts the minimum towards lower kinetic energies, which corresponds to a small contribution of sp3 hybridized carbon. This results from the decreased p-electron conjugation due to the defects produced by sputtering and is commonly observed upon ion sputtering of graphite [25].
2524
J. Pola et al. / Carbon 42 (2004) 2521–2526
The visible Raman spectrum (Fig. 6), being very mostly due to the sp2 phase, shows two features around 1587 and 1350 cm1 that respectively assign to the G and D bands. The former reflects bond stretches of all
pairs of sp2 atoms in both rings and chains and the latter corresponds to breathing modes of rings (e.g. Refs. [26,27]). The observed bands being of almost equal intensity and a broad G band (the full width at half maximum 80 cm1 ) positioned at 1588 cm1 resemble those of amorphous hydrogenated carbon with H content estimated [27] as 10 at%. Such low H content in the deposit is in keeping with the earlier established photolytic mechanism of diacetylene through dehydrogenative photo-polymerization [15–17]. The observed Raman spectra of the diacetylene photoproduct differ from those of a-C:H phases laser-deposited from CH2 I2 [6–8] and 3-butyn-2-one [5] that have their G band more intense than the D band and possess consequently less ring structures. We remark that similarly equally intense G band (1613 cm1 ) and D (1558 cm1 ) bands were observed for carbon films prepared by the ArF laser photolysis of acetylene and ascribed to individual graphite layers with equiseparation [2]. The IR spectrum of the deposit (Fig. 7) typically consists of absorption bands covering the region of H–C (2894–3290 cm1 ), CBC (2000–2200 cm1 ) and C@C stretches (ca. 1600–1700 cm1 ) and the broader region of CAH deformation modes (700–1600 cm1 ) that can be contributed by m(C@O) and m(CAO) vibrations appearing at 1500–1700 cm1 and 1000–1300 cm1 [28– 30]. In the first region, the most intense band at 3290 cm1 assigns to m(H–Csp ), that of less intensity at 3043 cm1 corresponds to m(H–Csp2 ) and the still less intense bands at 2942 and 2925 cm1 relate to m(H–Csp3 ). In the second region, the week bands at 2100 and 2200 cm1 are respectively due to isolated and conjugated CBC bonds. In the third region, the band at 1678 cm1 is attributed to isolated C@C and C@O bonds, whereas more intense band at 1583 cm1 relates to conjugated C@C bonds. In the fourth region, there appear very intense bands at 1180, 1236 and 1297 cm1 and medium bands at 1041, 964 and 742 cm1 . These are due to d(@CAH) modes and relate to structures of aromatic hydrocarbons. We note that the band at 1041 cm1 can be also due to CAO stretching vibration.
Fig. 6. Raman spectrum of the deposited film.
Fig. 7. FTIR spectrum of the film deposited on KBr plate.
Fig. 4. Fitted XPS C 1s core level spectra of the deposit.
Fig. 5. C KLL X-ray excited derivative Auger spectra of the diamond (1), graphite (2), deposit as such (3) and deposit after ion sputtering (4).
J. Pola et al. / Carbon 42 (2004) 2521–2526
The EPR spectrum of the deposit (Fig. 8) shows a single line with g-factor equal to 2.0037 and linewidth of 0.298 mT and it corresponds to total concentration of 4.98 1018 spins per gram. Useful information about the nature of amorphous carbon can be gained from several EPR parameters as g-factor, concentration of unpaired spins, spectral line-shape and width, microwave saturation, thermal behaviour and effect of atmospheric oxygen on them. For the interpretation of paramagnetic centers in amorphous carbon [31] both a carbon with sp3 hybridization and that with sp2 hybridization are commonly used. Both models differ slightly in g-factor (2.0028 and 2.0182 respectively) and their resolution is impossible due the linewidth values in X-band EPR. Moreover, the graphite-like g-factor depends on dimensions of paramagnetic particles and it decreases to the 2.005, if the grain size is about 7.5 nm [32]. The relationship between Dg ¼ g 2:0023 and the apparent crystalline diameter for sp2 hybridized carbon [32] together with the experimentally determined g ¼ 2:0037 and Dg ¼ 0:0014 values allow us to consider that the irregular chain-like agglomerates (Fig. 3) are composed of crystallites smaller than of 7.5 nm. The EPR spectral line of the carbonaceous deposit is singlet and the ratio of Gauss/Lorentz form and the line width are therefore the only parameters which could be evaluated. The ratio value (0.14) obtained by simulation and the line width value indicate the interaction among the spins. Microwave saturation was observed at power level over 10 mW and for this reason all measurements were carried out at 1 mW. In the temperature interval 155–148 °C the sample fulfills the Curie law for paramagnetic particles. The EPR spectrum is dramatically affected by presence of atmospheric oxygen (Fig. 8). The presence of oxygen changes not only the shape of the EPR spectrum by line broadening, but also diminishes the value of double integral that is proportional to the number of paramagnetic species. These changes are reversible and pronounced and suggest that the carbon deposit can be used as an oxygen sensor. We note that similar EPR
2525
spectral changes are typical [33] for some charcoals possessing carbon-centered radicals [34] and that these charcoals were identified as potential sensors for in vivo EPR oximetry [33]. 3.3. Structure and mechanism of photo-polymer formation The above spectral data on the solid deposited material show that the KrF laser irradiation of diacetylene yields a photo-polymer whose structure is dominated by sp2 -hybridized (AC@CA) carbon atoms that are arranged in both rings and chains and that contains some contributions of CAC and CBC bonds. The IR spectrum of the deposit and relative absorbance of the m (H–Csp ), m(H–Csp2 ) m(H–Csp3 ) bands (Fig. 7) reveal that the H atoms attached to carbon atoms prefer C(sp) sites, but are also bonded to C(sp2 ) and C(sp3 ) atoms. These facts allow us to draw conclusion about reactions of polyalkynes C6 H2 , C8 H2 , C8 H3 species [15–17] that were determined as primary products of UV laser irradiation of diacetylene and assumed as arising through combination and dehydrogenative steps. These species undergo further polymerization that takes place at the CBC bonds and that leads to a polymer containing a multitude of conjugated C@CAC@CA bonds, which is only little prone to undergo further polymerization reactions. H
H
H H
+
X H
X
X H
H
X=H, polyalkyne
We remark that this 1,2-polymerization at the triple bond differs from the known [35,36] thermal and photochemical solid state polymerization of polyacetylenes with HO-functional groups, which proceeds via 1,4addition at the conjugative triple bonds to form a polymer with three cumulated double bonds.
4. Conclusion
Fig. 8. EPR spectrum of the deposit.
KrF laser irradiation of diacetylene results in effective depletion of diacetylene (photochemical quantum yield 1.8) and affords chemical vapor deposition of dark nanostructured hydrogenated carbon coating that is
2526
J. Pola et al. / Carbon 42 (2004) 2521–2526
easily removed from the surface of reactor and resembles soot. The material structure is dominated by sp2 -hybridized carbon framework, contains small portions of spand sp3 -carbons and has more H atoms attached to the C(sp) than to the C(sp2 ) and C(sp3 ) sites. The material formation is explained in terms of efficient polymerization at the CBC bonds. The results reveal that the suitability of KrF laser photolysis of diacetylene (and perhaps of higher polyalkynes) to yield unsaturated carbon phases and give the first example of the laser photolytic chemical vapor deposition of these materials from polyalkynes and R– CBCH compounds (R6¼H). Acknowledgements This work was supported by the Ministry of Education, Sport and Youth of the Czech republic (grant no. ME 611). The authors thank Ms. T. Watanabe for SEM measurements. References [1] Kitahama K, Hirata K, Nakamatsu H, Kawai S, Fujimoro N, Imai T, et al. Appl Phys Lett 1986;49:634–5. [2] Kitahama K. Appl Phys Lett 1988;53:1812–4. [3] Dischler B, Bayer E. J Appl Phys 1990;68:1237–41. [4] Pola J, Ouchi A, Saito K, Ishikawa K, Koga Y. Chem Phys Lett 1996;262:279–83. [5] Drınek V, Urbanova M, Bastl Z, Gregora I, Vorlıcek V, Subrt J, et al. Appl Phys A 1998;66:503–9. [6] Lindstam M, Boman M, Piglmayer K. Appl Surf Sci 1999;138– 139:413–7. [7] Lindstam M, Boman M, Piglmayer K. Thin Solid Films 2001; 394:115–24. [8] Stenberg G, Piglmayer K, Boman M, Carlsson J-O. Appl Surf Sci 1997;109–110:549–53.
[9] Srinivasan R. In: Advances in photochemistry, 4. New York: Wiley; 1966. p. 113–42. [10] Pola J, Morita H. Tetrahedron Lett 1997;38:7809–12. [11] Pola J, Urbanova M, Bastl Z, Morita H. Macromol Rapid Commun 2000;21:178–81. [12] Pola J, Urbanova M, Bastl Z, Subrt J, Sakuragi M, Ouchi A, et al. Polymer 2001;42:1311–8. [13] Pola J, Bastl Z, Ouchi A. Surf Coat Technol 2002;157:55–8. [14] Pola J, Bastl Z, Ouchi A, Subrt J, Morita H. Surf Coat Technol 2002;149:129–34. [15] Glicker S, Okabe H. J Phys Chem 1987;91:437–40. [16] Bandy RE, Lakshminarayan C, Frost RK, Zwier TS. J Chem Phys 1993;98:5362–74. [17] Bandy RE, Lakshminarayan C, Frost RK, Zwier TS. Science 1992;258:1630–3. [18] Pola J, Urbanova M, Bastl Z, Plzak Z, Subrt J, Vorlıcek V, et al. Carbon 1997;35:605–11. [19] Hughes AE, Sexton BA. J Electron Spectrosc Relat Phenom 1988;46:31–42. [20] Scofield JH. J Electron Spectrosc Relat Phenom 1976;8:129–37. [21] Armitage JB, Jones ERH, Whiting MC. J Chem Soc 1951:44–8. [22] Lascovich LC, Giorgi R, Scaglione S. Appl Surf Sci 1991;47:17– 21. [23] Galuska AA, Maden HH, Allred RE. Appl Surf Sci 1988;32:253– 72. [24] Jackson ST, Nuzzo RG. Appl Surf Sci 1995;90:195–203. [25] Bastl Z. Coll Czech Chem Commun 1994;60:383–92. [26] Ferrari AC, Robertson J. Phys Rev B 2000;61:14095–107. [27] Schwan J, Ulrich S, Batori V, Ehrhardt H. J Appl Phys 1996;80:440–7, and references therein. [28] Rositani F, Antonucci PL, Minotuli M, Giordano N. Carbon 1987;25:325–32. [29] Bellamy LJ. In: The infrared spectra of complex molecules. London: Chapman and Hall; 1975. [30] Dolphin D, Wick A. Tabulation of Infrared Spectral Data. New York: Wiley; 1977. [31] Bardeleben J, Cantin JL, Zeinert A, Racine B, Zellama K, Hai PN. Appl Phys Lett 2001;78:2843–5. [32] Arnold GM. Carbon 1967;5:33–42. [33] Jordan BV, Baudelet C, Gallez B. Magn Res Mater Phys Biol Med (MAGMA) 1998;7:121–9. [34] Petrakis L, Grandy DW. Anal Chem 1978;50:303–8. [35] Wegner G. Makromol Chem 1971;145:85–94. [36] Wegner G. Z Naturforsch B 1969;24:824–32.
Nanostructured unsaturated carbon from laser-photo-polymerization of diacetylene J. Pola
a,*
, A. Ouchi
b,*
, Z. Bastl c, K. Vacek a, J. Boh acek d, H. Orita
b
a
c
Laser Chemistry Group, Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 16502 Prague, Czech Republic b National Institute of Advanced Industrial Science and Technology, AIST, Tsukuba, Ibaraki 305-8565, Japan J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 18223 Prague 8, Czech Republic d z, Czech Republic Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 25068 Re Received 20 February 2004; accepted 17 May 2004 Available online 19 June 2004
Abstract KrF laser irradiation of gaseous diacetylene induces photo-polymerization of diacetylene and results in chemical vapor deposition of nanostructured hydrogenated carbon whose structure is assessed from IR, Raman, photoelectron and Auger spectra as highly unsaturated C/H polymer containing carbon atoms mostly in sp2 state. The EPR spectrum of the deposit is strongly affected by the presence of oxygen. The results demonstrate the first example of laser chemical vapor deposition of unsaturated carbon phase from polyalkynes. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Vapor grown carbon; B. Laser irradiation; C. Photoelectron spectroscopy, Raman spectroscopy; D. Chemical structure
1. Introduction It has been recently recognized that high intense laser irradiation of suitable hydrocarbons affords chemical vapor deposition of hydrogenated carbon films. Both photolytic and thermolytic laser action has been demonstrated: the amorphous hydrogenated carbon films were deposited from ArF laser photolysis of acetylene [1–3], ArF and XeCl laser photolysis [4] and IR laser thermolysis [5] of 3-butyn-2-one, and from Arþ cw laser [6], XeCl laser [7] and ArF laser [8] photolysis of methylene iodide. The degree of unsaturation (hybridization) of carbon and the hydrogen concentration in the films deposited in these processes is controlled by the gas-phase chemistry and the structure of the edduct. Thus, the ArF laser photolysis of acetylene yields H-rich and graphite-like [1,2] or highly C (sp3 )-based [3] films, while Arþ , KrF
* Corresponding authors. Tel.: +420-2-2039-0308; fax: +420-2-20920661 (J. Pola); tel.: +81-29-861-4550; fax: +81-29-861-4421 (A. Ouchi). E-mail addresses: [email protected] (J. Pola), ouchi.akihiko@aist. go.jp (A. Ouchi).
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.05.021
and ArF laser photolysis of CH2 I2 gives rise to H-rich and fairly unsaturated C films [7–9]. Carbonaceous materials with some degree of unsaturation were for decades judged as easily photochemically produced from unsaturated hydrocarbons (e.g. [9]). However, recent recognition of laser-photolysis of alkynes leading to rather saturated polymeric frameworks revealed [10–14] that hydrocarbons with isolated CBC bond are not suitable precursors for laser chemical vapor deposition of unsaturated carbonaceous materials. The possibility of the use of alkadiynes for photolytic conversion to unsaturated carbonaceous materials has not been yet attempted. Of these highly unsaturated hydrocarbons, diacetylene appears most suited for studies due to its relative stability and good absorption in the UV region [15]. Its UV photochemistry over 220– 245 nm is known as controlled by triplet metastable C4 H2 * state [15] and by its reactions with C4 H2 molecules to produce C6 H2 , C8 H2 and C8 H3 (primary products) and C10 H3 and C12 H3 (secondary products) [16,17]. Conversely to the well established photolytic mechanism, there have been yet no studies on the deposited diacetylene photoproduct; it can be merely judged that
2522
J. Pola et al. / Carbon 42 (2004) 2521–2526
the structure is contributed by a sequence of alternating single and triple carbon bonds, cumulene (@C@C@) units, or by cyclic structures possessing some sp3 carbons. This paper is a continuation of our studies on laserinduced formation of carbonaceous solid materials [4,5,13,18] and its objective is to determine the structure and properties of the carbonaceous material laserdeposited from diacetylene and to reveal main path through which this carbonaceous material is formed.
2. Experimental The KrF laser photolytic experiments were carried out on gaseous samples of diacetylene (100 Torr) in nitrogen (total pressure 760 Torr) using an LPX 210i laser (KrF radiation) operating at 248 nm with a pulse energy of 370 mJ incident on 2.5 cm2 (fluence 148 mJ/ cm2 ) and a repetition frequency of 10 Hz. The fluence used allowed an efficient chemical vapor deposition of dark carbonaceous solid material, which was not possible at lower fluences (20–60 mJ/cm2 ). The diacetylene– N2 samples were irradiated with unfocused laser beam in a reactor (140 ml in volume) which was equipped with a sleeve with a rubber septum and a PTFE valve connecting it to a standard vacuum manifold and consisted of two orthogonally positioned Pyrex tubes, one fitted with two quartz plates and the other furnished with two KBr windows. The progress of the photolysis was monitored by periodically removing the reactor and placing it in the cell compartment of an FTIR (a Shimadzu FTIR 4000) spectrometer. The measurement was carried out several minutes after the irradiation had been ceased, which allowed the dark fog descend completely to the reactor bottom. The depletion of diacetylene was determined by using absorption bands at 2020 and 845 cm1 . The progress of the photolysis was also followed by gaschromatography (a Gasukuro Kogyo 370 chromatograph, a 2 m SUS Unipak S column, programmed temperature 30–150 °C). The chromatograph was equipped with flame-ionization detector and connected with a Shimadzu CR 5A Chromatopac data processor. Sampling was made by a gas-tight syringe (Dynatech Precision Sampling). The analysis of the irradiated samples was performed by FTIR spectroscopy (ethyne at 730 cm1 ) and by GC– MS method (a Shimadzu QP 5050 mass spectrometer (60 m capillary column Neutrabond-1, programmed temperature 30–200 °C). The photochemical quantum yield of diacetylene depletion was determined on the basis of knowledge of the amount of diacetylene depleted and laser energy absorbed. The incident pulse energy and pulse energy
absorbed in the gaseous diacetylene was measured by a joule meter coupled with a 10 MHz storage oscilloscope. The reactor accommodated tin, silicon and KBr substrates which, covered with the deposited material, were transferred for measurements of physical properties of the deposits by FTIR, Raman, EPR, X-ray photoelectron and Auger spectroscopy and by electron microscopy. The X-ray C 1s, O 1s photoelectron and C KLL Auger electron spectra of the deposit (as prepared and after mild sputtering with argon ions; E ¼ 4 keV, I ¼ 15 lA, 5 min) were measured using an ESCA 310 (Scienta) electron spectrometer with a base pressure better than 10-7 Pa and using AlKa radiation (1486.6 eV) for electron excitation. The overlapping features of the C 1s spectra were resolved into individual components using the lines of Gaussian–Lorentzian shape and a modified non-linear least-square procedure [19]. The surface composition of the deposited film was determined by correcting the spectral intensities for subshell photoionization cross-sections [20]. The measurements of Raman spectra were carried out at room temperature with LabRam Infinity spectrometer (Jobin Yvon, resolution less than 5 cm1 ) equipped with a He–Ne laser (632.8 nm, 20 mW), a microscope sample stage and CCD detector. FTIR spectra of the films were acquired on a Jasco FTIR spectrometer (model 610). Field emission scanning electron microscopy measurements were performed on a dual stage DS-720 Topcon electron microscope and TEM analysis was conducted on a Philips 201 transmission electron microscope at 80 kV. EPR spectra of the deposit (1.47 mg) were measured at room temperature in vacuum and in air by a cw EPR spectrometer working in X band (9.3 GHz and 1 mW) with 100 kHz magnetic field modulation (0.115 mT). The spectrometer was equipped with a digital frequency counter and an NMR magnetometer used for g-factor calculation. Quantitative estimation of spin concentration is based on TEMPOL and Mn2þ (internal standard). Diacetylene was prepared according to the earlier reported procedure [21].
3. Results and discussion 3.1. Photolytic features The KrF laser irradiation of gaseous diacetylene in nitrogen results in the depletion of diacetylene and the formation of a dark fog that fills all the space of the reactor and slowly deposits on the reactor bottom. The fog deposition on the quartz window through which the laser pulses enter the reactor leads to a continuous
J. Pola et al. / Carbon 42 (2004) 2521–2526
2523
Diacetylene, Torr
100
80
60
40
20 0
500
1000
1500
Number of pulses
Fig. 2. SEM image of the deposit (bar ¼ 1 lm).
Fig. 1. Photolytic depletion of diacetylene.
decay of the laser power within the reactor, because the formed film absorbs at 248 nm. This phenomenon makes the photolysis progress higher than 60% difficult to achieve even when the irradiation is directed through both reactor windows. This is illustrated by the dependence of the depletion of diacetylene on the number of pulses (Fig. 1). Acetylene, benzene and triacetylene are the only detectable volatile products. Within 25–56% photolysis progress the amount of acetylene amounts to less than 3% of the depleted diacetylene. Benzene and triacetylene (mass spectrum––m/z, (relative intensity): 74 (100), 73 (35), 72 (5), 60 (5), 37 (27), 36 (9)) are only trace products. The initial depletion rate of diacetylene (0.17 Torr per pulse) corresponds to a photochemical quantum yield 1.8, which is in agreement with that (2.0 0.5) observed for the low pressure photolysis throughout the region of 255–144 nm [15]). We note that the loss of diacetylene in the absence of the laser radiation is very slow; only 5% of diacetylene is depleted from the gas phase after 12 h. The above facts imply that photo-polymerization of diacetylene is a major process responsible for chemical vapor deposition of the solid films. 3.2. Properties of the deposited films The deposited films show poor adhesion to glass, KBr and tin and can be easily removed from their surface as a black powder. The films are insoluble in organic solvents (methanol, chloroform, toluene, hexane and tetrahydrofuran). The SEM image of the solid film (Fig. 2) shows spongy structures composed of irregular chain-like agglomerates that are discovered in the TEM image (Fig. 3) as 50 nm thick interconnecting chains.
Fig. 3. TEM image of the deposit.
XPS analysis of the deposit shows that the stoichiometry before ion sputtering is C1:00 O0:25 and that after sputtering is C1:00 O0:03 . This is indicative of oxidation of a few superficial layers. The XP spectra of C 1s electrons (Fig. 4) reveal that 11% of carbon is bonded to OH functionalities and 7% of carbon is incorporated in COOH group. The energy difference between the most positive and most negative excursions in the C KLL first derivative Auger spectra (Fig. 5) being a measure of the population of C atoms in sp3 and sp2 states [22–24] reveals a dominating occurrence of the sp2 hybridized carbon. It appears that the minimum for the as received deposit coincides with that of graphite (and not of diamond). The ion sputtering shifts the minimum towards lower kinetic energies, which corresponds to a small contribution of sp3 hybridized carbon. This results from the decreased p-electron conjugation due to the defects produced by sputtering and is commonly observed upon ion sputtering of graphite [25].
2524
J. Pola et al. / Carbon 42 (2004) 2521–2526
The visible Raman spectrum (Fig. 6), being very mostly due to the sp2 phase, shows two features around 1587 and 1350 cm1 that respectively assign to the G and D bands. The former reflects bond stretches of all
pairs of sp2 atoms in both rings and chains and the latter corresponds to breathing modes of rings (e.g. Refs. [26,27]). The observed bands being of almost equal intensity and a broad G band (the full width at half maximum 80 cm1 ) positioned at 1588 cm1 resemble those of amorphous hydrogenated carbon with H content estimated [27] as 10 at%. Such low H content in the deposit is in keeping with the earlier established photolytic mechanism of diacetylene through dehydrogenative photo-polymerization [15–17]. The observed Raman spectra of the diacetylene photoproduct differ from those of a-C:H phases laser-deposited from CH2 I2 [6–8] and 3-butyn-2-one [5] that have their G band more intense than the D band and possess consequently less ring structures. We remark that similarly equally intense G band (1613 cm1 ) and D (1558 cm1 ) bands were observed for carbon films prepared by the ArF laser photolysis of acetylene and ascribed to individual graphite layers with equiseparation [2]. The IR spectrum of the deposit (Fig. 7) typically consists of absorption bands covering the region of H–C (2894–3290 cm1 ), CBC (2000–2200 cm1 ) and C@C stretches (ca. 1600–1700 cm1 ) and the broader region of CAH deformation modes (700–1600 cm1 ) that can be contributed by m(C@O) and m(CAO) vibrations appearing at 1500–1700 cm1 and 1000–1300 cm1 [28– 30]. In the first region, the most intense band at 3290 cm1 assigns to m(H–Csp ), that of less intensity at 3043 cm1 corresponds to m(H–Csp2 ) and the still less intense bands at 2942 and 2925 cm1 relate to m(H–Csp3 ). In the second region, the week bands at 2100 and 2200 cm1 are respectively due to isolated and conjugated CBC bonds. In the third region, the band at 1678 cm1 is attributed to isolated C@C and C@O bonds, whereas more intense band at 1583 cm1 relates to conjugated C@C bonds. In the fourth region, there appear very intense bands at 1180, 1236 and 1297 cm1 and medium bands at 1041, 964 and 742 cm1 . These are due to d(@CAH) modes and relate to structures of aromatic hydrocarbons. We note that the band at 1041 cm1 can be also due to CAO stretching vibration.
Fig. 6. Raman spectrum of the deposited film.
Fig. 7. FTIR spectrum of the film deposited on KBr plate.
Fig. 4. Fitted XPS C 1s core level spectra of the deposit.
Fig. 5. C KLL X-ray excited derivative Auger spectra of the diamond (1), graphite (2), deposit as such (3) and deposit after ion sputtering (4).
J. Pola et al. / Carbon 42 (2004) 2521–2526
The EPR spectrum of the deposit (Fig. 8) shows a single line with g-factor equal to 2.0037 and linewidth of 0.298 mT and it corresponds to total concentration of 4.98 1018 spins per gram. Useful information about the nature of amorphous carbon can be gained from several EPR parameters as g-factor, concentration of unpaired spins, spectral line-shape and width, microwave saturation, thermal behaviour and effect of atmospheric oxygen on them. For the interpretation of paramagnetic centers in amorphous carbon [31] both a carbon with sp3 hybridization and that with sp2 hybridization are commonly used. Both models differ slightly in g-factor (2.0028 and 2.0182 respectively) and their resolution is impossible due the linewidth values in X-band EPR. Moreover, the graphite-like g-factor depends on dimensions of paramagnetic particles and it decreases to the 2.005, if the grain size is about 7.5 nm [32]. The relationship between Dg ¼ g 2:0023 and the apparent crystalline diameter for sp2 hybridized carbon [32] together with the experimentally determined g ¼ 2:0037 and Dg ¼ 0:0014 values allow us to consider that the irregular chain-like agglomerates (Fig. 3) are composed of crystallites smaller than of 7.5 nm. The EPR spectral line of the carbonaceous deposit is singlet and the ratio of Gauss/Lorentz form and the line width are therefore the only parameters which could be evaluated. The ratio value (0.14) obtained by simulation and the line width value indicate the interaction among the spins. Microwave saturation was observed at power level over 10 mW and for this reason all measurements were carried out at 1 mW. In the temperature interval 155–148 °C the sample fulfills the Curie law for paramagnetic particles. The EPR spectrum is dramatically affected by presence of atmospheric oxygen (Fig. 8). The presence of oxygen changes not only the shape of the EPR spectrum by line broadening, but also diminishes the value of double integral that is proportional to the number of paramagnetic species. These changes are reversible and pronounced and suggest that the carbon deposit can be used as an oxygen sensor. We note that similar EPR
2525
spectral changes are typical [33] for some charcoals possessing carbon-centered radicals [34] and that these charcoals were identified as potential sensors for in vivo EPR oximetry [33]. 3.3. Structure and mechanism of photo-polymer formation The above spectral data on the solid deposited material show that the KrF laser irradiation of diacetylene yields a photo-polymer whose structure is dominated by sp2 -hybridized (AC@CA) carbon atoms that are arranged in both rings and chains and that contains some contributions of CAC and CBC bonds. The IR spectrum of the deposit and relative absorbance of the m (H–Csp ), m(H–Csp2 ) m(H–Csp3 ) bands (Fig. 7) reveal that the H atoms attached to carbon atoms prefer C(sp) sites, but are also bonded to C(sp2 ) and C(sp3 ) atoms. These facts allow us to draw conclusion about reactions of polyalkynes C6 H2 , C8 H2 , C8 H3 species [15–17] that were determined as primary products of UV laser irradiation of diacetylene and assumed as arising through combination and dehydrogenative steps. These species undergo further polymerization that takes place at the CBC bonds and that leads to a polymer containing a multitude of conjugated C@CAC@CA bonds, which is only little prone to undergo further polymerization reactions. H
H
H H
+
X H
X
X H
H
X=H, polyalkyne
We remark that this 1,2-polymerization at the triple bond differs from the known [35,36] thermal and photochemical solid state polymerization of polyacetylenes with HO-functional groups, which proceeds via 1,4addition at the conjugative triple bonds to form a polymer with three cumulated double bonds.
4. Conclusion
Fig. 8. EPR spectrum of the deposit.
KrF laser irradiation of diacetylene results in effective depletion of diacetylene (photochemical quantum yield 1.8) and affords chemical vapor deposition of dark nanostructured hydrogenated carbon coating that is
2526
J. Pola et al. / Carbon 42 (2004) 2521–2526
easily removed from the surface of reactor and resembles soot. The material structure is dominated by sp2 -hybridized carbon framework, contains small portions of spand sp3 -carbons and has more H atoms attached to the C(sp) than to the C(sp2 ) and C(sp3 ) sites. The material formation is explained in terms of efficient polymerization at the CBC bonds. The results reveal that the suitability of KrF laser photolysis of diacetylene (and perhaps of higher polyalkynes) to yield unsaturated carbon phases and give the first example of the laser photolytic chemical vapor deposition of these materials from polyalkynes and R– CBCH compounds (R6¼H). Acknowledgements This work was supported by the Ministry of Education, Sport and Youth of the Czech republic (grant no. ME 611). The authors thank Ms. T. Watanabe for SEM measurements. References [1] Kitahama K, Hirata K, Nakamatsu H, Kawai S, Fujimoro N, Imai T, et al. Appl Phys Lett 1986;49:634–5. [2] Kitahama K. Appl Phys Lett 1988;53:1812–4. [3] Dischler B, Bayer E. J Appl Phys 1990;68:1237–41. [4] Pola J, Ouchi A, Saito K, Ishikawa K, Koga Y. Chem Phys Lett 1996;262:279–83. [5] Drınek V, Urbanova M, Bastl Z, Gregora I, Vorlıcek V, Subrt J, et al. Appl Phys A 1998;66:503–9. [6] Lindstam M, Boman M, Piglmayer K. Appl Surf Sci 1999;138– 139:413–7. [7] Lindstam M, Boman M, Piglmayer K. Thin Solid Films 2001; 394:115–24. [8] Stenberg G, Piglmayer K, Boman M, Carlsson J-O. Appl Surf Sci 1997;109–110:549–53.
[9] Srinivasan R. In: Advances in photochemistry, 4. New York: Wiley; 1966. p. 113–42. [10] Pola J, Morita H. Tetrahedron Lett 1997;38:7809–12. [11] Pola J, Urbanova M, Bastl Z, Morita H. Macromol Rapid Commun 2000;21:178–81. [12] Pola J, Urbanova M, Bastl Z, Subrt J, Sakuragi M, Ouchi A, et al. Polymer 2001;42:1311–8. [13] Pola J, Bastl Z, Ouchi A. Surf Coat Technol 2002;157:55–8. [14] Pola J, Bastl Z, Ouchi A, Subrt J, Morita H. Surf Coat Technol 2002;149:129–34. [15] Glicker S, Okabe H. J Phys Chem 1987;91:437–40. [16] Bandy RE, Lakshminarayan C, Frost RK, Zwier TS. J Chem Phys 1993;98:5362–74. [17] Bandy RE, Lakshminarayan C, Frost RK, Zwier TS. Science 1992;258:1630–3. [18] Pola J, Urbanova M, Bastl Z, Plzak Z, Subrt J, Vorlıcek V, et al. Carbon 1997;35:605–11. [19] Hughes AE, Sexton BA. J Electron Spectrosc Relat Phenom 1988;46:31–42. [20] Scofield JH. J Electron Spectrosc Relat Phenom 1976;8:129–37. [21] Armitage JB, Jones ERH, Whiting MC. J Chem Soc 1951:44–8. [22] Lascovich LC, Giorgi R, Scaglione S. Appl Surf Sci 1991;47:17– 21. [23] Galuska AA, Maden HH, Allred RE. Appl Surf Sci 1988;32:253– 72. [24] Jackson ST, Nuzzo RG. Appl Surf Sci 1995;90:195–203. [25] Bastl Z. Coll Czech Chem Commun 1994;60:383–92. [26] Ferrari AC, Robertson J. Phys Rev B 2000;61:14095–107. [27] Schwan J, Ulrich S, Batori V, Ehrhardt H. J Appl Phys 1996;80:440–7, and references therein. [28] Rositani F, Antonucci PL, Minotuli M, Giordano N. Carbon 1987;25:325–32. [29] Bellamy LJ. In: The infrared spectra of complex molecules. London: Chapman and Hall; 1975. [30] Dolphin D, Wick A. Tabulation of Infrared Spectral Data. New York: Wiley; 1977. [31] Bardeleben J, Cantin JL, Zeinert A, Racine B, Zellama K, Hai PN. Appl Phys Lett 2001;78:2843–5. [32] Arnold GM. Carbon 1967;5:33–42. [33] Jordan BV, Baudelet C, Gallez B. Magn Res Mater Phys Biol Med (MAGMA) 1998;7:121–9. [34] Petrakis L, Grandy DW. Anal Chem 1978;50:303–8. [35] Wegner G. Makromol Chem 1971;145:85–94. [36] Wegner G. Z Naturforsch B 1969;24:824–32.