CARBON 49 (2011) 625– 630

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Polyyne synthesis and amorphous carbon nano-particle formation by femtosecond irradiation of

M.J. Wesolowski *, S. Kuzmin, B. Moores, B. Wales, R. Karimi, A.A. Zaidi, Z. Leonenko, J.H. Sanderson, W.W. Duley

Department of Physics and Astronomy, University of Waterloo, 200 University Ave. W., Waterloo, Ontario, Canada N2L 3G1

ARTICLE INFO ABSTRACT

Article history: A range of carbon nanoparticles together with polyyne chains are formed when liquid ben- Received 23 September 2010 zene is irradiated with 120 fs pulses of intensity 1 · 1014 Wcm2. The morphology and Accepted 8 October 2010 composition of these nanoparticles are reported. Atomic force microscopy (AFM) shows Available online 14 October 2010 irradiation result in the formation of spherical, ellipsoidal and toroidal nanoparticles with sizes between 1 and 50 nm. Surface enhanced Raman (SERS) indicates that the composition of these particles is primarily amorphous carbon reinforced by polyyne chains with 8–14 carbon atoms. Time of flight mass spectroscopy (TOF-MS) has been used to determine the precursor created in the irradiation of benzene and shows the importance of ring-opening reactions in particle formation. A simple chemical mechanism is proposed to explain the appearance of polyyne chains in irradiated benzene. Crown Copyright 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction degrees of success to synthesize polyyne molecules [5].We have recently shown that certain polyyne molecules can be Polyynes are 1-D carbon chain with sp bond hybridization and formed by the irradiation of pure liquid hydrocarbons, such C2nH2. The synthesis of these molecules as , hexane or octane, with high intensity femtosec- has garnered much attention because of their unique proper- ond laser pulses [6–8]. This method is of interest because it ties and potential applications in nanotechnology. Polyynes is simple and yields a high concentration of polyynes. As re- are molecular conductors [1] with high mechanical strength ported previously [8], the primary step in the growth of poly-

[2] and are therefore excellent candidates for use as wires in yne chains in irradiated solutions involves C2 and C3 insertion molecular electronics. Such applications require the develop- reactions in smaller chains. These, and other small molecules ment of generic synthesis methodologies whereby molecules and molecular ions, are generated in the dissociation of the having specific lengths can be generated in a reproducible hydrocarbon precursor by multi-photon excitation. The disso- manner. ciation of the precursor is then a critical step in the The first stable polyynes were synthesized using chemical formation of polyynes via fs-laser irradiation. This process is techniques such as oxidative coupling [3]. These syntheses expected to be more difficult in ring compounds because of are complex and can also be quite dangerous in certain cases the inherent stability of the ring structure. Previous studies + [4]. While preparative chemical techniques are still in current [9–11] have shown that C6H6 is the dominant ion produced use, other simpler techniques have recently been shown to be in the fs-dissociation of benzene, although other molecules, effective. Submerged electric arc, D.C. magnetron sputtering, indicative of ring opening have also been detected in mass combustion flame, and laser ablation of graphite and carbo- spectra. Given these constraints, it is of interest to see if poly- naceous material in solution, have all been used with varying yne molecules analogous to those seen in the fs-irradiation of

* Corresponding author: Fax: +1 519 888 4567x33620. E-mail address: [email protected] (M.J. Wesolowski). 0008-6223/$ - see front matter Crown Copyright 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.10.008 626 CARBON 49 (2011) 625– 630

other hydrocarbons can also be generated in the irradiation of tion of gaseous benzene. The dual stage TOF system is of Wi- liquid benzene. ley-McLaren type with a drift region of 0.92 m. The same Ti:Sapphire laser used for irradiation of liquid benzene was 2. Experimental utilized as a dissociation source [8].

High performance liquid chromatography (HPLC) grade ben- 3. Results and discussion zene (P99.9%) was irradiated with 120 fs pulses from a 300 lJ regeneratively amplified Ti:Sapphire tabletop laser with When the laser beam was focused into liquid benzene an in- a 1 kHz repetition rate and wavelength of 800 nm. Liquid ben- tense region of filamentation, accompanied by the emission zene was placed in a rectangular polished quartz cuvette of a white light continuum, was immediately observed. Fila- (3.5 ml capacity, 1 cm path length) and irradiated for 1.5 h. mentation is common when ultra-short laser pulses pass The incident beam was focused with a 4 cm focal length lens through transparent nonlinear dispersive media and has the resulting in a peak intensity of 1014 Wcm2 at the focus. After effect of capping the focal intensity (1013–1014 Wcm2) and irradiation particles were examined using atomic force increasing the interaction volume. The emission of a white microscopy (AFM) and surface enhanced Raman spectroscopy light continuum is also typical under these conditions [12]. (SERS). After approximately 20 min of irradiation the liquid began A50ll droplet of solution was placed on a standard glass to develop a light yellow/brown color. This was followed by microscope slide and dried in air. The topography of the the appearance of precipitates after further irradiation resulting film was studied with a Nanowizard II AFM (JPK (Fig. 1a). Instruments AG, Berlin, Germany) in intermittent air contact A similar result was recently reported by Nakamura et al. mode using Nanoworld NCH tips with 42 Nm1 spring con- [13] when liquid benzene was irradiated with 780 nm, 100 fs stants. SERS measurements were carried out using a Reni- laser pulses at 9 · 1018 Wcm2. They found that the liquid shaw micro-Raman Spectrometer with an objective contained a mono-disperse solution of amorphous carbon magnification of 50· and an excitation wavelength of nanoparticles roughly 10 nm diameter. The typical topogra- 488 nm (0.5 mW). Silver nanoparticles were added to the irra- phy of precipitates produced in the current experiment is diated benzene solution to facilitate the SERS effect. The shown in Figs. 1b and 2. Numerous nanoparticles of varying spectral resolution of this system was better than ±1 cm1. size are shown in the 2 lm · 2 lm atomic force micrograph Time of flight mass spectrometry (TOF-MS) was used to study (Fig. 1b). The diameter of particles cannot be accurately deter- the atomic and molecular species produced from fs-irradia- mined due to probe broadening however it is common prac-

Fig. 1 – Precipitates form in benzene (a) when it is irradiated by fs-laser pulses and are composed of agglomerated poly- dispersed nanoparticles (b) as seen by AFM. The inset shows the cross-sectional height profile of the white line in the 2 lmby 2 lm micrograph. CARBON 49 (2011) 625– 630 627

Fig. 2 – Atomic force micrograph of the smallest nanoparticles created by the irradiation of benzene with fs-laser pulses. The inset is a magnified image of the toroidal particle contained by the white square.

tice to assign particles a given geometry and then use a height profile to attain approximate estimate of their size [14]. A rep- resentative cross-sectional height profile (Fig. 1b) indicates that the smallest particles in our samples are 1nm in height while the largest structures, which appear to be cluster assembled from smaller particles, are about 25 nm in height. A higher resolution (1 lm · 1 lm) image of the smallest parti- cles was taken to better characterize their shape and size. Three distinct types of nanoparticles are visible (Fig. 2); spher- ical particles with heights ranging between 1 and 3 nm, pro- late spheroids with similar heights, and toroidal particles that occasionally encircle a small spherical particle with heights of 1 nm (Fig. 2 inset). The spherical and prolate parti- cles appear similar to those previously observed in fs irradi- ated benzene [13] and are composed of amorphous carbon. The toroidal particles on the other hand exhibit a novel qua- si-molecular morphology. These may consist of fullerenes [15] or cyclic polyynes [16] however it is also possible that these Fig. 3 – Surface enhanced Raman spectra of pure benzene structures are assembled from smaller amorphous carbon (blue), irradiated deuterated benzene (cyan), and irradiated nanoparticles. benzene (green). Vibrational modes in the fingerprint In order to determine the molecular structure of these par- regions of sp (polyynes) and sp2 (amorphous) carbon are ticles and of the overall irradiated benzene solution we use clearly present in the irradiated benzene solution(For surface enhanced Raman spectroscopy (SERS). Fig. 3 shows interpretation of the references to colour in this figure the SERS spectra for both pure and irradiated liquid benzene. legend, the reader is referred to the web version of this The spectrum of pure sample shows all of the characteristic article.). benzene peaks with the two strongest modes being the a ring stretching mode (991.6 cm1) and the b C–H stretching mode (3060.7 cm 1). The irradiated sample has spectral features Lorentzian functions and are summarized in Table 1. The dis- associated with benzene, amorphous carbon and polyynes. order induced D (1363 cm1) and combination G (1560 cm1, The energies of features present in the SERS spectrum of irra- 1601 cm1) bands clearly indicate the presence of amorphous diated benzene were determined by fitting with Gaussian or carbon in this material. The additional peak in this region at 628 CARBON 49 (2011) 625– 630

Table 1 – SERS peak positions for irradiated benzene. Peak position (cm1) Assignment

991.6 C6H6 (a) 1175.7 C6H6 1288.6 C2H2 (sp) 1363.7 D (sp2) 1560 G (sp2) 1601 G (sp2) 1605.9 C6H6 (sp) 1974.3 C14H2 (sp) 2075.1 C8H2 (sp) 2135 C„CorC„O (sp) 2948.2 C6H6 3060.7 C6H6 (b) 3165.4 C6H6 3186.2 C6H6

1289 cm1, has been previously associated with or poly-acetylenic compounds [17,18]. The fingerprint region for linear chain polyynes occurs between 1800 and 2200 cm1, and within this region there are three large spectral features in the spectrum of irradiated benzene occurring at 1974, 2075 and 2135 cm1. The first two peaks can be attributed to

C14H2 and C8H2 respectively [19], while the third peak at 2135 cm1 is most probably the carbyne anti-symmetric stretching mode [20]. It is also possible that this feature may originate from the C„O bond of [21] as the irradiation was carried out in air. The vibration modes of medium sized monocyclic polyyne isomers also occur in this region [22,23], and suggest that cyclic polyynes could be present in the irradiated sample. These data indicate that the nanostructures present in the residue extracted from irra- Fig. 4 – Normalized time of flight mass spectrum resulting diated benzene are primarily amorphous carbon but that they from the irradiation of gaseous benzene (bottom) and also contain a variety of polyynes. These polyyne molecules deuterated benzene (top) by a 120 fs pulse at 800 nm and are the result of laser-induced dissociation of benzene to- peak intensity of 1014 W/cm2. gether with subsequent radical reactions in solution. The dissociation of benzene in the gaseous phase by fs- pulsed laser irradiation has been studied extensively [9–11]. not detect polyynes or amorphous carbon in the SERS spec-

Pulses from a Ti:Sapphire laser operating at 800 nm produce trum of irradiated liquid C6D6 (Fig. 3). The clamping intensity little ionization in benzene at intensities in the 1013 Wcm2 of a fs-laser filament depends inversely on the nonlinear range. For both 50 fs [9] and 200 fs [10] pulse durations the refractive index, n2, and is given as I ¼ qðIÞ=2n2qc, where, þ parent ion C6H6 was the predominant species in TOF spectra qðIÞ, is the photo-generated electron density and, qðcÞ, is the at this intensity. At higher intensities (up to 2.5 · 1014 Wcm2) critical plasma density [9]. The ratio of intensities within each liquid is ID 0:4 indicating that the maximum intensity within the benzene ring was found to dissociate into numerous IB þ molecular fragments, CnHn where n is from 1 to 6. At intensi- the C6D6 is less than half of that of the C6H6 which accounts ties between 1016 and 1017 Wcm2 it was found that multiply for the lack of fragmentation within the liquid benzene-d6. charged carbon ions dominated in time of flight mass spectra Laser-induced multi-photon ionization is the source of frag- [11]. The relative role of ionization and fragmentation in irra- mentation of the benzene ring and is the process that domi- diated benzene is therefore a strong function of intensity, so nates in the generation of the reactive fragments that that the products produced as the result of irradiation will initiate a complex in irradiated liquid benzene. be dependent on the particular laser system used. Fig. 4 The products of this chemistry include an amorphous carbon shows the TOF spectrum resulting from the irradiation of precipitate as observed in AFM images and detected in the pure gaseous benzene with the present Ti:Sapphire laser. As SERS spectrum. reported by Castillejo [9] the predominant carbon species While polyynes have been detected in solution after irradi- þ are seen to be the parent ion C6H6 , as well as fragments such ation of alkane and acetone liquids, [6,8] no evidence has pre- þ þ + 2+ as C3Hn ,CHn ,C, and C produced from the dissociation of viously been found for the formation of amorphous carbon. In the benzene ring. The same distribution of dissociation prod- a model proposed recently [8] to explain the formation of ucts is also seen in the TOF spectrum of C6D6 obtained under polyynes in the fs irradiation of liquid alkanes, long chain + identical radiation conditions (Fig. 4), but significantly we do polyynes are the result of the insertion of C, C ,C2, and other CARBON 49 (2011) 625– 630 629

primary dissociation products into small carbon chains. The REFERENCES present study shows that the formation of amorphous carbon is initiated by the reaction of primary dissociation products with C6H6 molecules in solution. A variety of substituted ring [1] Akagi K, Nishiguchi M, Shirakawa H. One-dimensional compounds would result from these reactions leading even- conjugated carbyne – synthesis and properties. Synth Met tually to amorphous carbon. Polyynes appear to be the sp- 1987;17:557–62. [2] Itzhaki L, Altus E, Basch H, Hoz S. Harder than diamond: bonded component of this structure, and it seems likely that determining the cross-sectional area and young’s modulus of the compounds seen in the SERS spectrum are not separate mechanical rods. Angew Chem 2005;117:7598–601. molecular species, but take the form of chains attached to [3] Eastmond R, Johnson TR, Walton DRM. Silylation as a aromatic rings. These chains probably act as bridges between protective method for terminal in oxidative clusters of sp2-bonded rings. We note that SERS spectra do couplings: a general synthesis of the parent polyynes 1 @ not show a feature at 3330 cm that would be typical of ter- H(C C)nH(n = 4–10, 12). Tetrahedron 1972;28:4601–16. minal CH groups attached to free polyyne molecules. There is [4] Baughman RH. Dangerously seeking linear carbon. Science 2006;312:1009–10. also no evidence for spectral features at 2946 and 2924 cm1 [5] Cataldo F. Polyynes Synthesis, Properties, and associated with CH3 and CH2 groups suggesting that the Applications. CRC Press Taylor and Francis Group; 2006. amorphous carbon generated by fs-irradiation of liquid ben- [6] Hu A, Sanderson JH, Zaidi AA, Zhang T, Zhou Y, Duley WW. 2 3 zene has a (sp, sp )-bonded structure rather than the (sp , Direct synthesis of polyyne molecules in acetone by sp2)-bonded structure that is characteristically observed in dissociation using femtosecond laser irradiation. Carbon amorphous carbons [24]. A similar composition has also been 2008;46:1823–5. observed in the fs-irradiation of graphite surfaces [25]. The [7] Sato Y, Kodama T, Shiromaru H, Sanderson JH, Fujino T, Wada Y, et al. Synthesis of polyyne molecules from hexane by absence of amorphous carbon and polyynes in solution after irradiation of intense femtosecond laser pulses. Carbon fs irradiation of C6D6 is consistent with this reaction scheme, 2010;48:1673–6. since the laser intensity in liquid C6D6 is insufficient to pro- [8] Zaidi AA, Hu A, Wesolowski MJ, Fu X, Sanderson JH, Zhou Y, duce fragmentation. et al. Time of flight mass spectrometry of polyyne formation in the irradiation of liquid alkanes with femtosecond laser pulses. Carbon 2010;48:2517–20. 4. Conclusion [9] Castillejo M, Couris S, Koudoumas E, Martı´n M. Ionization and fragmentation of aromatic and single-bonded hydrocarbons with 50 fs laser pulses at 800 nm. Chem Phys We have shown that the irradiation of pure liquid benzene by Lett 1999;308:373–80. fs-laser pulses leads to the formation of nanoparticles with [10] Talebpour A, Bandrauk AD, Chin SL. Fragmentation of three distinct morphologies. Spherical, prolate and toroidal benzene in an intense Ti:Sapphire laser pulse. Laser Phys nanoparticles ranging in height from 1 to 25 nm were ob- 2000;10(1):210–5. served by AFM. The composition of these nanoparticles was [11] Shimizu S, Kou J, Kawato S, Shimizu K, Sakabe S, Nakashima determined by SERS to be amorphous carbon, however not N. Coulomb explosion of benzene irradiated by an intense of the characteristic the (sp3,sp2)-bonded structure. Linear femtosecond laser pulse. Chem Phys Lett 2000;307:609–14. [12] Couairon A, Mysyrowicz A. Femtosecond filamentation in or cyclic polyynes (C ,C ), appear to act as bridges between 8 14 transparent media. Phys Rep 2007;441:47–189. 2 clusters of sp -bonded aromatic rings resulting in a (sp, [13] Nakamura T, Mochidzuki Y, Sato S. Conference on Lasers and 2 sp )-bonded amorphous carbon material. The dissociation Electro-Optics/Quantum Electronics and Laser Science of gaseous benzene by fs-laser pulses was shown by TOF- Conference and Photonic Applications Systems MS to produce a number of predominant carbon species Technologies, OSA Technical Digest (CD), Optical Society of þ America; 2007. JThD89. including the parent ion C6H6 , as well as fragments such as [14] Wong SS, Woolley AT, Odom TW, Huang J-L, Kim P, Vezenov C Hþ,CHþ,C+, and C2+. In liquid benzene laser-induced ioni- 3 n n DV, et al. Single-walled probes for high- zation is the source of fragmentation. The formation of (sp, resolution nanostructure imaging. Appl Phys Lett 2 sp )-bonded amorphous carbon is initiated by the reaction 1998;73:3465–7. of primary dissociation products with C6H6 molecules in solu- [15] Jasti R, Bhattacharjee J, Neaton JB, Bertozzi CR. Synthesis, tion. A variety of substituted ring compounds result from characterization, and theory of [9]-, [12]-, and [18] these reactions leading eventually to the observed amor- cycloparaphenylene: carbon nanohoop structures. J Am phous carbon. This process was not observed in deuterated Chem Soc 2008;130:17646–7. [16] Diederich F, Rubin Y, Knobler CB, Whentren RL, Schriver KE, benzene since the laser intensity in liquid C D is insufficient 6 6 Houk KN, et al. All-carbon molecules: evidence for the to produce fragmentation due to its higher nonlinear refrac- generation of cyclo[18]carbon from a stable organic tive index. precursor. Science 1989;245:1088–90. [17] Baker C, Turner DW. High resolution molecular photoelectron spectroscopy III. and aza-acetylenest. Proc Roy Acknowledgments Soc A 1968;308:19–37. [18] Pfeiffer R, Kuzmany H, Knoll P, Bokova S, Salk N, Gu¨ nther B. Evidence for trans- in nano-crystalline The authors acknowledge financial support from the Natural diamond films. Diam Relat Mater 2003;12:268–71. Sciences and Engineering Research Council of Canada [19] Tabata H, Fujii M, Hayashi S, Dio T, Wakabayashi T. Raman (NSERC), the Canada Foundation for Innovation (CFI) and and surface-enhanced Raman scattering of a series of size- the Ontario Research Fund (ORF). separated polyynes. Carbon 2006;44:3168–76. 630 CARBON 49 (2011) 625– 630

[20] Cataldo F. From dicopper acetylide to carbyne. Polym Int frequencies of size-selected C16,C18, and C20 clusters. J 1999;48:15–22. Chem Phys 1998;109(22):9652–5. [21] Wood TH, Klein MV, Zwemer DA. Enhanced Raman scattering [24] Ferrari AC, Robertson J. Interpretation of Raman spectra of from adsorbates on metal films in ultra high vacuum. Surf Sci disordered and amorphous carbon. Phys Rev B 1981;107:625–35. 2000;61(20):14095–107. [22] Hutter J, Lu¨ thi HP, Diederich F. Structures and vibrational [25] Hu A, Rybachuk M, Lu Q-B, Duley WW. Direct synthesis of sp- frequencies of the carbon molecules C2–C18 calculated by bonded carbon chains on graphite surface by femtosecond density functional theory. J Am Chem Soc 1994;116:750–6. laser irradiation. Appl Phys Lett 2007;91:131906-3. [23] Ott AK, Rechtsteiner GA, Felix C, Hampe O, Jarrold MF, Van Duyne RP, et al. Raman spectra and calculated vibrational