The presence and stability of sp hybridized atoms in free carbon nanoparticles was investigated by NEXAFS spectroscopy. The experiments show that a predominant fraction of carbon atoms is found in linear sp-chains and that conversion into sp(2) structures proceeds already at low temperature and in the gas phase

Amati, Matteo

Bongiorno, Gero

Coreno, Marcello

Devetta, Michele

Evangelista, Fabrizio

Lenardi, Cristina

Mazza, Tommaso

Milani, Paolo

Piseri, Paolo

Ravagnan, Luca

Rudolf, Petra

Chemical Communications

The presence and stability of sp hybridized atoms in free carbon nanoparticles was investigated by NEXAFS spectroscopy. The experiments show that a predominant fraction of carbon atoms is found in linear sp-chains and that conversion into sp(2) structures proceeds already at low temperature and in the gas phase.</p

University of Groningen

   University of Groningensp hybridization in free carbon nanoparticles-presence and stability observed by near edge X-ray absorption fine structure spectroscopyRavagnan, Luca; Mazza, Tommaso; Bongiorno, Gero; Devetta, Michele; Amati, Matteo;Milani, Paolo; Piseri, Paolo; Coreno, Marcello; Lenardi, Cristina; Evangelista, FabrizioPublished in:Chemical CommunicationsDOI:10.1039/c0cc03778hIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ravagnan, L., Mazza, T., Bongiorno, G., Devetta, M., Amati, M., Milani, P., ... Rudolf, P. (2011). sphybridization in free carbon nanoparticles-presence and stability observed by near edge X-ray absorptionfine structure spectroscopy. Chemical Communications, 47(10), 2952-2954.https://doi.org/10.1039/c0cc03778hCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Electronic Supplementary Information Section  Here we provide a concise description of the experimental setup used for time resolved NEXAFS measurements on free clusters. A more extensive description can be found in P. Piseri et. al, J. El. Spectr. Rel. Phen., 2008, 166-167, 28 (ref. [22] in the paper).   Experimental setup To characterize clusters in the gas phase by X-ray absorption we developed a dedicated apparatus that was directly interfaced to the Gas Phase beamline of the Elettra synchrotron radiation facility in Trieste (Italy). The apparatus, consists of three differentially evacuated UHV chambers. The first chamber hosts the free jet expansion from the PMCS. The cluster source is equipped with an aerodynamic lens system focusing the clusters into a highly collimated molecular beam [K. Wegner et al., J. Phys. D: Appl. Phys., 2006, 39, R439]. Since the conductance of the focusing nozzle is low, at every working cycle of the pulsed source a time in the order of 100 ms is required for emptying completely the cluster source cavity. During the time the clusters spend in the cavity they can undergo cluster-cluster interactions leading to their further aggregation and rearrangement: thus the cluster size and structure evolve with increasing residence time in the source. As the clusters exit from the source cavity and get into the supersonic beam, their further evolution is quenched owing to the collision-less environment of the supersonic beam and to the further cooling occurring in the free jet expansion.   The central part of the cluster beam enters the second chamber of the apparatus through a skimmer. Here the ionic component of the cluster beam is deflected by an electrostatic field, while neutral clusters reach the interaction chamber through a small diaphragm, where they cross the soft x-ray photon beam coming from the GasPhase beamline. Differential pumping of the three chambers and the high collimation of the cluster beam ensure a pressure in the interaction chamber in the range of 10-9 mbar during source operation. Photo-emitted electrons from the clusters are collected by a channel electron multiplier connected to counting/timing electronics.    The detector signal is fed to a time to digital converter (TDC) triggered by the PMCS discharge, so that electron counts can be resolved by detection time within each cluster source pulse.   Time resolved measurements To catch the full timing information relative to each event, without reduction in counting efficiency, a large dynamic range TDC system with multiple channels and multi-hit capability has been set up.  The system is based on a TDC chip from Acam GmbH (8 channels, resolution better than 100ps, <5 ns dead time) and implemented using the ATMD-GPX PCI board test system from the same manufacturer (see http://www.acam.de/fileadmin/Download/pdf/English/DB AMGPX e.pdf for details). The board is installed in a dedicated computer running on Linux OS and managed by an ad hoc developed server application that can be controlled via the PC’s Ethernet interface in a client/server architecture. The TDC is set in a self-retriggering mode and coupled to a software counter indexing retriggering cycles; the server application stores the timing information relative to any possibly occurred stop event. This system allows high-resolution acquisition of a very large number of stop hits, in an indefinite time range after a single start. System testing proved its effectiveness for average counting rates up to 300 KHz, well above the regime encountered in present experiment.  The signals from the channel electron multiplier is fed to the one ot the TDC stop channels after discrimination (Ortec® CF8000, pulse rise time 1 ns, pulse width 10 ns). As previously said, the TDC start is triggered by a logic signal synchronous to the cluster source pulsed-valve opening. Thanks to this every count is recorded together with the delay time lapsed between the starting of the cluster source and the detection of the electron. This delay time correspons, once subtracted the short delay between the pulsed-valve opening and the source disccharge (~300 μs) and the small time of flight of the cluster from the source to the interaction region (~1 ms), to the residence time of the cluster inside of the source. A second identical TDC system is synchronously triggered and used to measure complementary information with pulse-to-pulse resolution; these are namely: the ion count signals from the beam diagnostics detectors in the third chamber; photon and cluster beam fluxes, after Supplementary Material (ESI) for Chemical CommunicationsThis journal is (c) The Royal Society of Chemistry 2011conversion of the photodiode and microbalance signals with volt-to-frequency converters (VFC); Ar ion counts from  a gas-cell in the diagnostics chamber of the GasPhase beamline, for absolute photon flux determination. Separate files are recorded containing the full information relative to 30 s accumulation time; each file corresponds to one row in a log file where information read from the beamline control system is stored (storage ring parameters, photon energy, undulator’s gaps, etc.).   Time resolved NEXAFS spectra are obtained by a posteriori analysis of the data recorded in the mass storage device of the PC. For each photon energy used, the electron counts collected by the TDC stop channel are selected within the desired intervals of residence time (in the present case 11 delay time intervals were chosen: 7 ms long, ranging from 15 to 82 ms). The resulting electron yields are then normalized to the measured x-ray photon flux and cluster flux.   Evaluation of the fraction of NEXAFS signal related to sp-bonds (Xsp)  In order to study the evolution of the sp-bonds contents in the carbon NP as a function of delay time, we calculated, for each time resolved NEXAFS spectra in Figure 1B (see the paper), the fraction of NEXAFS signal related to sp-bonds: Xsp.  Xsp is defined as       where A[π*C≡C] and A[π*C=C] are integral intensities from the spectra in Fig. 1B in two 0.6 eV wide energy windows centred at 284.7 eV and 285.9 eV, respectively (shaded regions in Fig. 1B). Thanks to this definition Xsp is the percentage of the integrated intensity of the NEXAFS spectra in the energy region relative to the π* resonance for sp hybridization, in respect to the integrated intensity in the regions relative to both sp2 and sp hybridizations.    In order to estimate the influence of the choice of the width of the energy windows used for evaluating A[π*C≡C] and A[π*C=C] on the obtained value of Xsp, we calculated Xsp also when using widths ranging from 0.1 to 1 eV. The small variations in the obtained values of Xsp account for the ordinate error bars in Figure 1C. Although the used definition of Xsp is only qualitative, it avoids the high arbitrariness involved in spectra fitting, especially concerning the choice of the fitting curve for the post edge region.  NEXAFS measurements on cluster assembled films  In order o measure the NEXAFS spectra of thin films obtained by depositing the carbon nanoparticles, a copper substrate holder can be inserted by a vacuum translator at the crossing point between the cluster and the photon beam. The substrate holder temperature can be varied during the deposition in the range 270 - 350 K. Furthermore, the substrate holder is connected to ground through a pico-amperometer, so that NEXAFS measurements on clusters assembled films can be easily performed also during cluster deposition and thus at conditions where the sample is continuously refreshed.  [ ][ ] [ ]CCCCCCsp AAAX≡=≡+⋅= ***100 πππSupplementary Material (ESI) for Chemical CommunicationsThis journal is (c) The Royal Society of Chemistry 2011

sp hybridization in free carbon nanoparticles-presence and stability observed by near edge X-ray absorption fine structure spectroscopy

NARCIS 

   University of Groningensp hybridization in free carbon nanoparticles-presence and stability observed by near edge X-ray absorption fine structure spectroscopyRavagnan, Luca; Mazza, Tommaso; Bongiorno, Gero; Devetta, Michele; Amati, Matteo;Milani, Paolo; Piseri, Paolo; Coreno, Marcello; Lenardi, Cristina; Evangelista, FabrizioPublished in:Chemical CommunicationsDOI:10.1039/c0cc03778hIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ravagnan, L., Mazza, T., Bongiorno, G., Devetta, M., Amati, M., Milani, P., Piseri, P., Coreno, M., Lenardi,C., Evangelista, F., & Rudolf, P. (2011). sp hybridization in free carbon nanoparticles-presence and stabilityobserved by near edge X-ray absorption fine structure spectroscopy. Chemical Communications, 47(10),2952-2954. https://doi.org/10.1039/c0cc03778hCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.2952 Chem. Commun., 2011, 47, 2952–2954 This journal is c The Royal Society of Chemistry 2011Cite this: Chem. Commun., 2011, 47, 2952–2954sp hybridization in free carbon nanoparticles—presence and stabilityobserved by near edge X-ray absorption fine structure spectroscopywzLuca Ravagnan,a Tommaso Mazza,a Gero Bongiorno,a Michele Devetta,a Matteo Amati,aPaolo Milani,a Paolo Piseri,*a Marcello Coreno,b Cristina Lenardi,a Fabrizio Evangelistacand Petra RudolfcReceived 10th September 2010, Accepted 16th December 2010DOI: 10.1039/c0cc03778hThe presence and stability of sp hybridized atoms in free carbonnanoparticles was investigated by NEXAFS spectroscopy. Theexperiments show that a predominant fraction of carbon atomsis found in linear sp-chains and that conversion into sp2 struc-tures proceeds already at low temperature and in the gas phase.Since their discovery, fullerenes have become the paradigmof carbon nanoparticles (NPs). These highly symmetric, even-numbered carbon aggregates with mixed sp2 and sp3 hybridiza-tion were first identified experimentally by mass spectrometryof free carbon clusters formed in a plasma,1 and predictedtheoretically to be the energetically most stable structures forclusters with more than 20 carbon atoms.2High yield production of fullerenes from a carbon plasma isobtained only in cluster sources where the aggregate condensationoccurs at temperatures above 1000 K.1,3 At lower temperaturesthe fullerene yield is low and ill-defined carbon NPs that build upthe so-called ‘carbon soot’ are favoured.3 Fullerene productiontherefore represents the end point of the carbon NPs aggregationprocess, the ‘fullerene road’, which nevertheless is not necessarilyreached. Several theoretical models of the fullerene formationsteps predict the existence of intermediate highly disordered pre-fullerenic NPs (PFNPs).4,5 These simulations indicate that PFNPscan consist not only of sp2 and sp3 hybridized carbons but alsohave a significant amount of sp hybridization and be remarkablystable.4,5 The development of the experimental production andcharacterization of PFNPs represents a very challenging criticalstep on the route to complex carbon structures.6The presence of sp hybridization in the solid state carbonhas been considered for a long time as very unlikely if notimpossible.7 Although free carbon NPs with less than 20 atomshave sp-based structures as most stable configuration (sp carbonchains, spCCs),2 they react exothermically to form sp2 structureswhen deposited8 due to the dangling bonds at the extremitiesof the isolated spCCs.Despite the long lasting scepticism about stable spCCs in thesolid phase, the feasibility of producing the sp-rich solid carbonwas demonstrated a few years ago by some of us by depositing,at room temperature (RT) and in high vacuum, pure carbon NPsproduced by a Pulsed Microplasma Cluster Source (PMCS).9,10In this source carbon atoms aggregate at a remarkably lowertemperature than in standard fullerene sources.4 The massdistribution of the produced NPs ranges between 102 and103 atoms,11 and thus fullerenes should be the most stableconfiguration;2 however, in situRaman spectra of deposited filmsof these NPs clearly identified the presence of a relevant fractionof sp bonds.9 This sp component decays very slowly in highvacuum at RT, while it is rapidly depleted when the film isexposed to oxygen or moderately heated.11 If the substrate iskept at 150 K during deposition, a remarkable increase in sphybridized atoms is observed.12 The NPs produced by the PMCSmust therefore be PFNPs with high spCCs content and the spchains metastable. In PFNPs the sp chains are embedded in adisordered sp2 structure and their reactive extremities are termi-nated by bonding to sp2 (or sp3) carbon atoms4,5 which producesa stabilization of the spCCs,13 comparable to that achieved byterminating them with bulky endgroups15 or with graphene.14While the spectroscopic study of free PFNPs is essential fora better understanding of the intermediate steps of carbon NPformation, to date sp chains have been characterized only inthe solid state and not as free NPs prior to deposition.Here we present the first gas phase characterization performedby near edge X-ray absorption fine structure spectroscopy(NEXAFS) of the bonding structure of sp-rich PFNPs producedby a PMCS.In NEXAFS a core electron is excited to an unoccupiedmolecular orbital and the resulting core hole filled throughfluorescent decay or an Auger process.16 For carbon the latteris preponderant and by collecting the emitted electrons oneobtains a peak in the spectrum each time the exciting photonenergymatches the difference between the core level and an emptys* or p* orbital. NEXAFS is thus capable of identifying thechemical bonds present and, via a building block approach,quantify their abundance.16 In particular, in the C K-edgeNEXAFS spectra of C2H2 and C2H4, the simplest organicaCIMAINA, Università degli Studi di Milano, V, ia Celoria 16,I-20133 Milano, Italy. E-mail: paolo.piseri@fisica.unimi.itbCNR-IMIP, Montelibretti (Rome) and Gas Phase BeamlineELETTRA, Area Science Park, I-34149 Trieste, ItalycZernike Institute for Advanced Materials, University of Groningen,Nijenborgh 4, 9747AG Groningen, The Netherlandsw This article is part of the ‘Carbon Nanostructures’ web-theme issuefor ChemComm.z Electronic supplementary information (ESI) available: detailedexperimental section. See DOI: 10.1039/c0cc03778hChemComm Dynamic Article Linkswww.rsc.org/chemcomm COMMUNICATIONDownloaded by University of Groningen on 17 March 2011Published on 17 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CC03778HThis journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 2952–2954 2953molecules with sp and sp2 bonds, resonances occur at 285.9 eV forexcitation into p*CRC and at 284.7 eV for p*CQC.16,17 Similarly,distinct resonances (at B286 eV for sp and B284.5 eV for sp2)have been identified in more complex sp–sp2 systems.18 For purecarbon systems only recently few authors claimed the identifica-tion of distinct resonances for sp and sp2 hybridizations.19,20Although this might be surprising given the numerous NEXAFSstudies in the past, the reason becomes clear when consideringthat experiments were usually performed ex situ, thus exposingthe samples to air before characterization. Since the sp species aredestroyed by oxygen,9 their identification by NEXAFS waspossible only recently on NP films produced in situ in a UHVcharacterization facility,19 or transported in vacuum to it.20To characterize NPs in the gas phase by NEXAFS wedeveloped a dedicated apparatus, consisting of three differentiallypumped UHV chambers,21 and directly interfaced with the GasPhase Beamline of the Elettra synchrotron in Trieste, Italy.22 Thefirst chamber hosts the PMCS where the NPs are produced via theaggregation of carbon atoms ablated from a graphite rod throughHe ion sputtering by a plasma plume.10 After aggregation, themixture of NPs and He expands through a focusing nozzle in thevacuum chamber, forming a highly collimated molecular beam.10Since the nozzle’s conductance is low,B100 ms are required aftereach of the source’s working cycles for emptying completely thesource cavity. During that time, the NPs in the cavity can interact,aggregate and structurally rearrange at a temperature as low asB100 K.4 As a result the NP size and structure evolve withresidence time in the source. After exiting from the cavity, theirfurther evolution is quenched by the collisionless and cool environ-ment of the molecular beam.10 The central part of the NP beamenters the second chamber through a skimmer. Here the ioniccomponent of the beam is deflected by an electrostatic field, whileneutral NPs reach the interaction chamber through a smalldiaphragm, where they cross the soft X-ray beam coming fromthe beamline. Differential pumping of the three chambers andthe high collimation of the NP beam ensure a pressure in the10ÿ9mbar range in the interaction chamber. The emitted electronsare collected by a channeltron connected to counting/timingelectronics;21 the detector signal is fed into a time-to-digitalconverter triggered by the PMCS discharge, so that electroncounts can be resolved as a function of detection time within eachsource pulse. Time resolved NEXAFS spectra are obtained bybinning the data and by normalizing the electron yield to theX-ray flux and to the NP flux.21Fig. 1A shows the electron yield measured at differentdelay times after the electrical discharge producing carbonvaporization in the PMCS (shading is used to highlight thetime windows applied for time resolved data shown in Fig. 1Band C). The delay time is the residence time of the NPs in thecavity plus a small interval due to the time of flight from thesource to the interaction region (B1 ms, with a little spread fordifferent parts of the NP beam pulse).Fig. 1B presents the NEXAFS spectra for each of the elevendelay time intervals marked in Fig. 1A. A total electron yieldNEXAFS spectrum of the film obtained by depositing the NPsat 300 K on a copper substrate is also shown for comparison.A clear evolution can be observed: the spectra of the first NPsexiting from the source are sharply peaked at B285.7 eV, anenergy consistent with the acetylene-type p*CRC resonanceat 285.9 eV (marked by a dotted line in the figure).17 Thusthe first NPs exiting the source are characterized by apredominantly sp bond structure. To the best of our knowledgethis is the first NEXAFS spectrum of a pure carbon systemdisplaying a sharp p* peak indicative of sp hybridization.Fig. 1 (A) Electron yield measured for free carbon NPs as a function of the delay time between the PMCS discharge and the electron detection(i.e. the NP residence time in the source). (B) NEXAFS spectra of the free NPs acquired in the 11 delay time windows shaded in panel (A),compared to the in situ NEXAFS spectrum of the NP assembled film deposited at RT (dep. 300 K). The acetylene and ethylene p* resonances areindicated as dotted lines.16,17 (C) Evolution of Xsp (see definition in text) with the delay time; the value of Xsp for the deposited film is indicated as adashed line.Downloaded by University of Groningen on 17 March 2011Published on 17 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CC03778H2954 Chem. Commun., 2011, 47, 2952–2954 This journal is c The Royal Society of Chemistry 2011As the delay time increases (going from spectrum 1 to 11 inFig. 1B), the peak at 285.7 eV progressively decreases, stillremaining the predominant contribution to the p* near edgepart of the spectrum, while a shoulder at B284.7 eV(i.e. p*CQC) grows. Finally, in the spectrum of the deposited film,the acetylene-type p*CRC peak is not any longer the predominantfeature and the p*CQC peak shows a similar intensity.Fig. 1C shows the evolution of the fraction of NEXAFS signalrelated to sp-bonds as a function of delay time (i.e. NP residencetime inside the source). This fraction (Xsp) is calculated asdetailed in the ESIz. Although this definition of Xsp is onlyqualitative, it avoids the high arbitrariness involved in spectrafitting, especially concerning the choice of the fitting curve for thepost edge region.20,23 As can be clearly seen in Fig. 1C, theamount of sp bonds present in the NPs decreases monotonouslywith increasing residence time, remaining nevertheless muchhigher than the amount of sp detected on the NP assembled filmdeposited at RT (dashed line in Fig. 1C). This indicates thatduring all phases of the NP formation in the PMCS, theaggregates are composed of a major fraction of spCCs, in goodagreement with the theoretical prediction for the low temperatureaggregation process of carbon.4,5 The chains are metastable andevolve progressively towards sp2 structures during residence timein the source; nevertheless a relevant fraction of the spCCs is ableto survive even after NP deposition at RT, confirming that theend-capping of the chains by bonding to sp2 groups afford aremarkable stability to carbon atomic wire structures.13To further follow the decay process of spCCs, we deposited theNP also on the copper substrate kept at 350 K. As shown inref. 11 and 12 the fraction of spCCs that survive the depositionprocess, is strongly affected by the substrate temperature. Fig. 2shows the NEXAFS spectra, normalized to the post edge region,from the film deposited at room temperature and at 350K, as wellas the difference between these two spectra. Substrate heatingcauses a substantial change in both the p* and the s* regions ofthe spectrum: as evident from the difference (see the fit in theinset), the change in the p* region is related to the decay ofthe p*CRC band at 285.9 eV and the corresponding increase ofthe p*CQC band at 285 eV. This further supports the metastabilityof the spCCs, and their decay in sp2 carbon when annealed.In conclusion, we demonstrated the sp hybridizationof free carbon NPs by NEXAFS and followed the evolutionof their structure on a millisecond time scale under lowtemperature conditions in an inert atmosphere. Our resultsshow that free carbon NPs can have a complex structure withvarying relative amounts of sp and sp2 bonds. This has veryimportant consequences for a wide variety of fields wherecarbon NPs play a role such as astrochemistry and environ-mental sciences. Indeed carbon NPs rich in spCCs are theintermediate steps in the ‘fullerene road’ and could be basicconstituents of both the interstellar medium and of combus-tion exhausts.Measurements were supported by the ‘‘Access to ResearchInfrastructure’’ action of the EU Human Potential Program.Notes and references1 R. E. Smalley, Acc. Chem. Res., 1992, 25, 98.2 R. O. Jones, J. Chem. Phys., 1999, 110, 5189.3 T. Wakabayashi, D. Kasuya, H. Shiromaru, S. Suzuki, K. Kikuchiand Y. Achiba, Z. Phys. D, At., Mol. Clusters, 1997, 40, 414.4 Y. Yamaguchi, L. Colombo, P. Piseri, L. Ravagnan and P. Milani,Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 134119.5 G. Zheng, Z. Wang, S. Irle and K. Morokuma, J. Nanosci.Nanotechnol., 2007, 7, 1662.6 G. Benedek, H. Vahedi Tafreshi, E. Barborini, P. Piseri, P. Milani,C. Ducati and J. Robertson, Diamond Relat. Mater., 2003, 12, 768.7 R. B. Heinmann, in Carbyne and Carbynoid Structures, ed.R. B. Heimann, S. E. Evsyukov and L. Kavan, Kluwer AcademicPress, Dordrecht, 1999, pp. 7–15.8 T. Wakabayashi, A.-L. Ong, D. Strelnikov and W. Krätschmer,J. Phys. Chem. B, 2004, 108, 3686.9 L. Ravagnan, F. Siviero, C. Lenardi, P. Piseri, E. Barborini,P. Milani, C. S. Casari, A. Li Bassi and C. E. Bottani, Phys.Rev. Lett., 2002, 89, 285506.10 K. Wegner, P. Piseri, H. Vahedi Tafreshi and P. Milani, J. Phys. D:Appl. Phys., 2006, 39, R439.11 C. S. Casari, A. Li Bassi, L. Ravagnan, F. Siviero, C. Lenardi,P. Piseri, G. Bongiorno, C. E. Bottani and P. Milani, Phys. Rev. B:Condens. Matter Mater. Phys., 2004, 69, 75422.12 L. Ravagnan, P. Piseri, M. Bruzzi, S. Miglio, G. Bongiorno,A. Baserga, C. S. Casari, A. Li Bassi, C. Lenardi,Y. Yamaguchi, T. Wakabayashi, C. E. Bottani and P. Milani,Phys. Rev. Lett., 2007, 98, 216103.13 L. Ravagnan, N. Manini, E. Cinquanta, G. Onida, D. Sangalli,C. Motta, M. Devetta, A. Bordoni, P. Piseri and P. Milani, Phys.Rev. Lett., 2009, 102, 245502.14 C. Jin, H. Lan, L. Peng, K. Suenaga and S. Iijima, Phys. Rev. Lett.,2009, 102, 205501.15 W. A. Chalifoux and R. R. Tykwinski, C. R. Chim., 2009, 12, 341;S. Kim, Angew. Chem., Int. Ed., 2009, 48, 7740.16 J. Stöhr, NEXAFS spectroscopy, Springer, New York, 1992.17 A. P. Hitchcock and D. C. Mancini, J. Electron Spectrosc. Relat.Phenom., 1977, 10, 749.18 F. Matsui, H. W. Yeom, I. Matsuda and T. Ohta, Phys. Rev. B:Condens. Matter, 2000, 62, 5036; A. Fujimori, M. Ishitsuka,H. Nakahara, E. Ito, M. Hara, K. Kanai, Y. Ouchi and K. Seki,J. Phys. Chem. B, 2004, 108, 13153.19 L. Ravagnan, G. Bongiorno, D. Bandiera, E. Salis, P. Piseri,P. Milani, C. Lenardi, M. Coreno, M. de Simone andK. C. Prince, Carbon, 2006, 44, 1518.20 J. Diaz, O. R. Monteiro and Z. Hussain, Phys. Rev. B: Condens.Matter Mater. Phys., 2007, 76, 094201.21 P. Piseri, T. Mazza, G. Bongiorno, M. Devetta, M. Coreno andP. Milani, J. Electron Spectrosc. Relat. Phenom., 2008, 166–167, 28.22 K. C. Prince, R. R. Blyth, R. Delaunay, M. Zitnik, J. Krempasky,J. Slezak, R. Camilloni, L. Avaldi, M. Coreno, G. Stefani,C. Furlani, M. de Simone and S. Stranges, J. Synchrotron Radiat.,1998, 5, 565.23 I. Jiménez, R. Gago and J. M. Albella, Diamond Relat. Mater.,2003, 12, 110.Fig. 2 Total electron yield NEXAFS spectra (normalized to the postedge region as indicated by the arrow) of NP assembled filmsdeposited at 300 K (RT) and at 350 K. The difference spectrum isindicated in black, and its Gaussian fit is shown in the inset.Downloaded by University of Groningen on 17 March 2011Published on 17 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CC03778H

English

The presence and stability of sp hybridized atoms in free carbon nanoparticles was investigated by NEXAFS spectroscopy. The experiments show that a predominant fraction of carbon atoms is found in linear sp-chains and that conversion into sp2 structures proceeds already at low temperature and in the gas phase

L. Ravagnan

T. Mazza

G. A. Bongiorno

M. Devetta

M. Amati

P. Milani

P. G. C. Piseri

C. Lenardi

M. Coreno

F. Evangelista

P. Rudolf

AIR Universita degli studi di Milano

sp hybridization in free carbon nanoparticles—presence and stability observed by near edge X-ray absorption fine structure spectroscopy

University of Groningensp hybridization in free carbon nanoparticles-presence and stability observed by near edge X-ray absorption fine structure spectroscopyRavagnan, Luca; Mazza, Tommaso; Bongiorno, Gero; Devetta, Michele; Amati, Matteo;Milani, Paolo; Piseri, Paolo; Coreno, Marcello; Lenardi, Cristina; Evangelista, FabrizioPublished in:Chemical CommunicationsDOI:10.1039/c0cc03778hIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ravagnan, L., Mazza, T., Bongiorno, G., Devetta, M., Amati, M., Milani, P., Piseri, P., Coreno, M., Lenardi,C., Evangelista, F., & Rudolf, P. (2011). sp hybridization in free carbon nanoparticles-presence and stabilityobserved by near edge X-ray absorption fine structure spectroscopy. Chemical Communications, 47(10),2952-2954. https://doi.org/10.1039/c0cc03778hCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 30-12-20252952 Chem. Commun., 2011, 47, 2952–2954 This journal is c The Royal Society of Chemistry 2011Cite this: Chem. Commun., 2011, 47, 2952–2954sp hybridization in free carbon nanoparticles—presence and stabilityobserved by near edge X-ray absorption fine structure spectroscopywzLuca Ravagnan,a Tommaso Mazza,a Gero Bongiorno,a Michele Devetta,a Matteo Amati,aPaolo Milani,a Paolo Piseri,*a Marcello Coreno,b Cristina Lenardi,a Fabrizio Evangelistacand Petra RudolfcReceived 10th September 2010, Accepted 16th December 2010DOI: 10.1039/c0cc03778hThe presence and stability of sp hybridized atoms in free carbonnanoparticles was investigated by NEXAFS spectroscopy. Theexperiments show that a predominant fraction of carbon atomsis found in linear sp-chains and that conversion into sp2 struc-tures proceeds already at low temperature and in the gas phase.Since their discovery, fullerenes have become the paradigmof carbon nanoparticles (NPs). These highly symmetric, even-numbered carbon aggregates with mixed sp2 and sp3 hybridiza-tion were first identified experimentally by mass spectrometryof free carbon clusters formed in a plasma,1 and predictedtheoretically to be the energetically most stable structures forclusters with more than 20 carbon atoms.2High yield production of fullerenes from a carbon plasma isobtained only in cluster sources where the aggregate condensationoccurs at temperatures above 1000 K.1,3 At lower temperaturesthe fullerene yield is low and ill-defined carbon NPs that build upthe so-called ‘carbon soot’ are favoured.3 Fullerene productiontherefore represents the end point of the carbon NPs aggregationprocess, the ‘fullerene road’, which nevertheless is not necessarilyreached. Several theoretical models of the fullerene formationsteps predict the existence of intermediate highly disordered pre-fullerenic NPs (PFNPs).4,5 These simulations indicate that PFNPscan consist not only of sp2 and sp3 hybridized carbons but alsohave a significant amount of sp hybridization and be remarkablystable.4,5 The development of the experimental production andcharacterization of PFNPs represents a very challenging criticalstep on the route to complex carbon structures.6The presence of sp hybridization in the solid state carbonhas been considered for a long time as very unlikely if notimpossible.7 Although free carbon NPs with less than 20 atomshave sp-based structures as most stable configuration (sp carbonchains, spCCs),2 they react exothermically to form sp2 structureswhen deposited8 due to the dangling bonds at the extremitiesof the isolated spCCs.Despite the long lasting scepticism about stable spCCs in thesolid phase, the feasibility of producing the sp-rich solid carbonwas demonstrated a few years ago by some of us by depositing,at room temperature (RT) and in high vacuum, pure carbon NPsproduced by a Pulsed Microplasma Cluster Source (PMCS).9,10In this source carbon atoms aggregate at a remarkably lowertemperature than in standard fullerene sources.4 The massdistribution of the produced NPs ranges between 102 and103 atoms,11 and thus fullerenes should be the most stableconfiguration;2 however, in situRaman spectra of deposited filmsof these NPs clearly identified the presence of a relevant fractionof sp bonds.9 This sp component decays very slowly in highvacuum at RT, while it is rapidly depleted when the film isexposed to oxygen or moderately heated.11 If the substrate iskept at 150 K during deposition, a remarkable increase in sphybridized atoms is observed.12 The NPs produced by the PMCSmust therefore be PFNPs with high spCCs content and the spchains metastable. In PFNPs the sp chains are embedded in adisordered sp2 structure and their reactive extremities are termi-nated by bonding to sp2 (or sp3) carbon atoms4,5 which producesa stabilization of the spCCs,13 comparable to that achieved byterminating them with bulky endgroups15 or with graphene.14While the spectroscopic study of free PFNPs is essential fora better understanding of the intermediate steps of carbon NPformation, to date sp chains have been characterized only inthe solid state and not as free NPs prior to deposition.Here we present the first gas phase characterization performedby near edge X-ray absorption fine structure spectroscopy(NEXAFS) of the bonding structure of sp-rich PFNPs producedby a PMCS.In NEXAFS a core electron is excited to an unoccupiedmolecular orbital and the resulting core hole filled throughfluorescent decay or an Auger process.16 For carbon the latteris preponderant and by collecting the emitted electrons oneobtains a peak in the spectrum each time the exciting photonenergymatches the difference between the core level and an emptys* or p* orbital. NEXAFS is thus capable of identifying thechemical bonds present and, via a building block approach,quantify their abundance.16 In particular, in the C K-edgeNEXAFS spectra of C2H2 and C2H4, the simplest organicaCIMAINA, Università degli Studi di Milano, V, ia Celoria 16,I-20133 Milano, Italy. E-mail: paolo.piseri@fisica.unimi.itbCNR-IMIP, Montelibretti (Rome) and Gas Phase BeamlineELETTRA, Area Science Park, I-34149 Trieste, ItalycZernike Institute for Advanced Materials, University of Groningen,Nijenborgh 4, 9747AG Groningen, The Netherlandsw This article is part of the ‘Carbon Nanostructures’ web-theme issuefor ChemComm.z Electronic supplementary information (ESI) available: detailedexperimental section. See DOI: 10.1039/c0cc03778hChemComm Dynamic Article Linkswww.rsc.org/chemcomm COMMUNICATIONDownloaded by University of Groningen on 17 March 2011Published on 17 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CC03778HThis journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 2952–2954 2953molecules with sp and sp2 bonds, resonances occur at 285.9 eV forexcitation into p*CRC and at 284.7 eV for p*CQC.16,17 Similarly,distinct resonances (at B286 eV for sp and B284.5 eV for sp2)have been identified in more complex sp–sp2 systems.18 For purecarbon systems only recently few authors claimed the identifica-tion of distinct resonances for sp and sp2 hybridizations.19,20Although this might be surprising given the numerous NEXAFSstudies in the past, the reason becomes clear when consideringthat experiments were usually performed ex situ, thus exposingthe samples to air before characterization. Since the sp species aredestroyed by oxygen,9 their identification by NEXAFS waspossible only recently on NP films produced in situ in a UHVcharacterization facility,19 or transported in vacuum to it.20To characterize NPs in the gas phase by NEXAFS wedeveloped a dedicated apparatus, consisting of three differentiallypumped UHV chambers,21 and directly interfaced with the GasPhase Beamline of the Elettra synchrotron in Trieste, Italy.22 Thefirst chamber hosts the PMCS where the NPs are produced via theaggregation of carbon atoms ablated from a graphite rod throughHe ion sputtering by a plasma plume.10 After aggregation, themixture of NPs and He expands through a focusing nozzle in thevacuum chamber, forming a highly collimated molecular beam.10Since the nozzle’s conductance is low,B100 ms are required aftereach of the source’s working cycles for emptying completely thesource cavity. During that time, the NPs in the cavity can interact,aggregate and structurally rearrange at a temperature as low asB100 K.4 As a result the NP size and structure evolve withresidence time in the source. After exiting from the cavity, theirfurther evolution is quenched by the collisionless and cool environ-ment of the molecular beam.10 The central part of the NP beamenters the second chamber through a skimmer. Here the ioniccomponent of the beam is deflected by an electrostatic field, whileneutral NPs reach the interaction chamber through a smalldiaphragm, where they cross the soft X-ray beam coming fromthe beamline. Differential pumping of the three chambers andthe high collimation of the NP beam ensure a pressure in the10ÿ9mbar range in the interaction chamber. The emitted electronsare collected by a channeltron connected to counting/timingelectronics;21 the detector signal is fed into a time-to-digitalconverter triggered by the PMCS discharge, so that electroncounts can be resolved as a function of detection time within eachsource pulse. Time resolved NEXAFS spectra are obtained bybinning the data and by normalizing the electron yield to theX-ray flux and to the NP flux.21Fig. 1A shows the electron yield measured at differentdelay times after the electrical discharge producing carbonvaporization in the PMCS (shading is used to highlight thetime windows applied for time resolved data shown in Fig. 1Band C). The delay time is the residence time of the NPs in thecavity plus a small interval due to the time of flight from thesource to the interaction region (B1 ms, with a little spread fordifferent parts of the NP beam pulse).Fig. 1B presents the NEXAFS spectra for each of the elevendelay time intervals marked in Fig. 1A. A total electron yieldNEXAFS spectrum of the film obtained by depositing the NPsat 300 K on a copper substrate is also shown for comparison.A clear evolution can be observed: the spectra of the first NPsexiting from the source are sharply peaked at B285.7 eV, anenergy consistent with the acetylene-type p*CRC resonanceat 285.9 eV (marked by a dotted line in the figure).17 Thusthe first NPs exiting the source are characterized by apredominantly sp bond structure. To the best of our knowledgethis is the first NEXAFS spectrum of a pure carbon systemdisplaying a sharp p* peak indicative of sp hybridization.Fig. 1 (A) Electron yield measured for free carbon NPs as a function of the delay time between the PMCS discharge and the electron detection(i.e. the NP residence time in the source). (B) NEXAFS spectra of the free NPs acquired in the 11 delay time windows shaded in panel (A),compared to the in situ NEXAFS spectrum of the NP assembled film deposited at RT (dep. 300 K). The acetylene and ethylene p* resonances areindicated as dotted lines.16,17 (C) Evolution of Xsp (see definition in text) with the delay time; the value of Xsp for the deposited film is indicated as adashed line.Downloaded by University of Groningen on 17 March 2011Published on 17 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CC03778H2954 Chem. Commun., 2011, 47, 2952–2954 This journal is c The Royal Society of Chemistry 2011As the delay time increases (going from spectrum 1 to 11 inFig. 1B), the peak at 285.7 eV progressively decreases, stillremaining the predominant contribution to the p* near edgepart of the spectrum, while a shoulder at B284.7 eV(i.e. p*CQC) grows. Finally, in the spectrum of the deposited film,the acetylene-type p*CRC peak is not any longer the predominantfeature and the p*CQC peak shows a similar intensity.Fig. 1C shows the evolution of the fraction of NEXAFS signalrelated to sp-bonds as a function of delay time (i.e. NP residencetime inside the source). This fraction (Xsp) is calculated asdetailed in the ESIz. Although this definition of Xsp is onlyqualitative, it avoids the high arbitrariness involved in spectrafitting, especially concerning the choice of the fitting curve for thepost edge region.20,23 As can be clearly seen in Fig. 1C, theamount of sp bonds present in the NPs decreases monotonouslywith increasing residence time, remaining nevertheless muchhigher than the amount of sp detected on the NP assembled filmdeposited at RT (dashed line in Fig. 1C). This indicates thatduring all phases of the NP formation in the PMCS, theaggregates are composed of a major fraction of spCCs, in goodagreement with the theoretical prediction for the low temperatureaggregation process of carbon.4,5 The chains are metastable andevolve progressively towards sp2 structures during residence timein the source; nevertheless a relevant fraction of the spCCs is ableto survive even after NP deposition at RT, confirming that theend-capping of the chains by bonding to sp2 groups afford aremarkable stability to carbon atomic wire structures.13To further follow the decay process of spCCs, we deposited theNP also on the copper substrate kept at 350 K. As shown inref. 11 and 12 the fraction of spCCs that survive the depositionprocess, is strongly affected by the substrate temperature. Fig. 2shows the NEXAFS spectra, normalized to the post edge region,from the film deposited at room temperature and at 350K, as wellas the difference between these two spectra. Substrate heatingcauses a substantial change in both the p* and the s* regions ofthe spectrum: as evident from the difference (see the fit in theinset), the change in the p* region is related to the decay ofthe p*CRC band at 285.9 eV and the corresponding increase ofthe p*CQC band at 285 eV. This further supports the metastabilityof the spCCs, and their decay in sp2 carbon when annealed.In conclusion, we demonstrated the sp hybridizationof free carbon NPs by NEXAFS and followed the evolutionof their structure on a millisecond time scale under lowtemperature conditions in an inert atmosphere. Our resultsshow that free carbon NPs can have a complex structure withvarying relative amounts of sp and sp2 bonds. This has veryimportant consequences for a wide variety of fields wherecarbon NPs play a role such as astrochemistry and environ-mental sciences. Indeed carbon NPs rich in spCCs are theintermediate steps in the ‘fullerene road’ and could be basicconstituents of both the interstellar medium and of combus-tion exhausts.Measurements were supported by the ‘‘Access to ResearchInfrastructure’’ action of the EU Human Potential Program.Notes and references1 R. E. Smalley, Acc. Chem. Res., 1992, 25, 98.2 R. O. Jones, J. Chem. Phys., 1999, 110, 5189.3 T. Wakabayashi, D. Kasuya, H. Shiromaru, S. Suzuki, K. Kikuchiand Y. Achiba, Z. Phys. D, At., Mol. Clusters, 1997, 40, 414.4 Y. Yamaguchi, L. Colombo, P. Piseri, L. Ravagnan and P. Milani,Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 134119.5 G. Zheng, Z. Wang, S. Irle and K. Morokuma, J. Nanosci.Nanotechnol., 2007, 7, 1662.6 G. Benedek, H. Vahedi Tafreshi, E. Barborini, P. Piseri, P. Milani,C. Ducati and J. Robertson, Diamond Relat. Mater., 2003, 12, 768.7 R. B. Heinmann, in Carbyne and Carbynoid Structures, ed.R. B. Heimann, S. E. Evsyukov and L. Kavan, Kluwer AcademicPress, Dordrecht, 1999, pp. 7–15.8 T. Wakabayashi, A.-L. Ong, D. Strelnikov and W. Krätschmer,J. Phys. Chem. B, 2004, 108, 3686.9 L. Ravagnan, F. Siviero, C. Lenardi, P. Piseri, E. Barborini,P. Milani, C. S. Casari, A. Li Bassi and C. E. Bottani, Phys.Rev. Lett., 2002, 89, 285506.10 K. Wegner, P. Piseri, H. Vahedi Tafreshi and P. Milani, J. Phys. D:Appl. Phys., 2006, 39, R439.11 C. S. Casari, A. Li Bassi, L. Ravagnan, F. Siviero, C. Lenardi,P. Piseri, G. Bongiorno, C. E. Bottani and P. Milani, Phys. Rev. B:Condens. Matter Mater. Phys., 2004, 69, 75422.12 L. Ravagnan, P. Piseri, M. Bruzzi, S. Miglio, G. Bongiorno,A. Baserga, C. S. Casari, A. Li Bassi, C. Lenardi,Y. Yamaguchi, T. Wakabayashi, C. E. Bottani and P. Milani,Phys. Rev. Lett., 2007, 98, 216103.13 L. Ravagnan, N. Manini, E. Cinquanta, G. Onida, D. Sangalli,C. Motta, M. Devetta, A. Bordoni, P. Piseri and P. Milani, Phys.Rev. Lett., 2009, 102, 245502.14 C. Jin, H. Lan, L. Peng, K. Suenaga and S. Iijima, Phys. Rev. Lett.,2009, 102, 205501.15 W. A. Chalifoux and R. R. Tykwinski, C. R. Chim., 2009, 12, 341;S. Kim, Angew. Chem., Int. Ed., 2009, 48, 7740.16 J. Stöhr, NEXAFS spectroscopy, Springer, New York, 1992.17 A. P. Hitchcock and D. C. Mancini, J. Electron Spectrosc. Relat.Phenom., 1977, 10, 749.18 F. Matsui, H. W. Yeom, I. Matsuda and T. Ohta, Phys. Rev. B:Condens. Matter, 2000, 62, 5036; A. Fujimori, M. Ishitsuka,H. Nakahara, E. Ito, M. Hara, K. Kanai, Y. Ouchi and K. Seki,J. Phys. Chem. B, 2004, 108, 13153.19 L. Ravagnan, G. Bongiorno, D. Bandiera, E. Salis, P. Piseri,P. Milani, C. Lenardi, M. Coreno, M. de Simone andK. C. Prince, Carbon, 2006, 44, 1518.20 J. Diaz, O. R. Monteiro and Z. Hussain, Phys. Rev. B: Condens.Matter Mater. Phys., 2007, 76, 094201.21 P. Piseri, T. Mazza, G. Bongiorno, M. Devetta, M. Coreno andP. Milani, J. Electron Spectrosc. Relat. Phenom., 2008, 166–167, 28.22 K. C. Prince, R. R. Blyth, R. Delaunay, M. Zitnik, J. Krempasky,J. Slezak, R. Camilloni, L. Avaldi, M. Coreno, G. Stefani,C. Furlani, M. de Simone and S. Stranges, J. Synchrotron Radiat.,1998, 5, 565.23 I. Jiménez, R. Gago and J. M. Albella, Diamond Relat. Mater.,2003, 12, 110.Fig. 2 Total electron yield NEXAFS spectra (normalized to the postedge region as indicated by the arrow) of NP assembled filmsdeposited at 300 K (RT) and at 350 K. The difference spectrum isindicated in black, and its Gaussian fit is shown in the inset.Downloaded by University of Groningen on 17 March 2011Published on 17 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CC03778H

University of Groningen Research Database

  
 University of Groningen
sp hybridization in free carbon nanoparticles-presence and stability observed by near edge X-
ray absorption fine structure spectroscopy
Ravagnan, Luca; Mazza, Tommaso; Bongiorno, Gero; Devetta, Michele; Amati, Matteo;
Milani, Paolo; Piseri, Paolo; Coreno, Marcello; Lenardi, Cristina; Evangelista, Fabrizio
Published in:
Chemical Communications
DOI:
10.1039/c0cc03778h
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2011
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Ravagnan, L., Mazza, T., Bongiorno, G., Devetta, M., Amati, M., Milani, P., ... Rudolf, P. (2011). sp
hybridization in free carbon nanoparticles-presence and stability observed by near edge X-ray absorption
fine structure spectroscopy. Chemical Communications, 47(10), 2952-2954. DOI: 10.1039/c0cc03778h
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
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Download date: 10-02-2018
Electronic Supplementary Information Section 
 
Here we provide a concise description of the experimental setup used for time resolved 
NEXAFS measurements on free clusters. A more extensive description can be found in P. 
Piseri et. al, J. El. Spectr. Rel. Phen., 2008, 166-167, 28 (ref. [22] in the paper).  
 
Experimental setup 
To characterize clusters in the gas phase by X-ray absorption we developed a dedicated 
apparatus that was directly interfaced to the Gas Phase beamline of the Elettra synchrotron 
radiation facility in Trieste (Italy). The apparatus, consists of three differentially evacuated UHV 
chambers. The first chamber hosts the free jet expansion from the PMCS. The cluster source 
is equipped with an aerodynamic lens system focusing the clusters into a highly collimated 
molecular beam [K. Wegner et al., J. Phys. D: Appl. Phys., 2006, 39, R439]. Since the 
conductance of the focusing nozzle is low, at every working cycle of the pulsed source a time 
in the order of 100 ms is required for emptying completely the cluster source cavity. During the 
time the clusters spend in the cavity they can undergo cluster-cluster interactions leading to 
their further aggregation and rearrangement: thus the cluster size and structure evolve with 
increasing residence time in the source. As the clusters exit from the source cavity and get into 
the supersonic beam, their further evolution is quenched owing to the collision-less 
environment of the supersonic beam and to the further cooling occurring in the free jet 
expansion. 
  The central part of the cluster beam enters the second chamber of the apparatus 
through a skimmer. Here the ionic component of the cluster beam is deflected by an 
electrostatic field, while neutral clusters reach the interaction chamber through a small 
diaphragm, where they cross the soft x-ray photon beam coming from the GasPhase beamline. 
Differential pumping of the three chambers and the high collimation of the cluster beam ensure 
a pressure in the interaction chamber in the range of 10-9 mbar during source operation. Photo-
emitted electrons from the clusters are collected by a channel electron multiplier connected to 
counting/timing electronics.  
  The detector signal is fed to a time to digital converter (TDC) triggered by the PMCS 
discharge, so that electron counts can be resolved by detection time within each cluster source 
pulse.  
 
Time resolved measurements 
To catch the full timing information relative to each event, without reduction in counting 
efficiency, a large dynamic range TDC system with multiple channels and multi-hit capability 
has been set up.  
The system is based on a TDC chip from Acam GmbH (8 channels, resolution better than 
100ps, <5 ns dead time) and implemented using the ATMD-GPX PCI board test system from the 
same manufacturer (see http://www.acam.de/fileadmin/Download/pdf/English/DB AMGPX e.pdf for 
details). The board is installed in a dedicated computer running on Linux OS and managed by an 
ad hoc developed server application that can be controlled via the PC’s Ethernet interface in a 
client/server architecture. The TDC is set in a self-retriggering mode and coupled to a software 
counter indexing retriggering cycles; the server application stores the timing information relative to 
any possibly occurred stop event. This system allows high-resolution acquisition of a very large 
number of stop hits, in an indefinite time range after a single start. System testing proved its 
effectiveness for average counting rates up to 300 KHz, well above the regime encountered in 
present experiment.  
The signals from the channel electron multiplier is fed to the one ot the TDC stop channels 
after discrimination (Ortec® CF8000, pulse rise time 1 ns, pulse width 10 ns). As previously said, 
the TDC start is triggered by a logic signal synchronous to the cluster source pulsed-valve opening. 
Thanks to this every count is recorded together with the delay time lapsed between the starting of 
the cluster source and the detection of the electron. This delay time correspons, once subtracted 
the short delay between the pulsed-valve opening and the source disccharge (~300 μs) and the 
small time of flight of the cluster from the source to the interaction region (~1 ms), to the residence 
time of the cluster inside of the source. 
A second identical TDC system is synchronously triggered and used to measure 
complementary information with pulse-to-pulse resolution; these are namely: the ion count signals 
from the beam diagnostics detectors in the third chamber; photon and cluster beam fluxes, after 
Supplementary Material (ESI) for Chemical Communications
This journal is (c) The Royal Society of Chemistry 2011
conversion of the photodiode and microbalance signals with volt-to-frequency converters (VFC); Ar 
ion counts from  a gas-cell in the diagnostics chamber of the GasPhase beamline, for absolute 
photon flux determination. Separate files are recorded containing the full information relative to 30 
s accumulation time; each file corresponds to one row in a log file where information read from the 
beamline control system is stored (storage ring parameters, photon energy, undulator’s gaps, etc.). 
  Time resolved NEXAFS spectra are obtained by a posteriori analysis of the data 
recorded in the mass storage device of the PC. For each photon energy used, the electron 
counts collected by the TDC stop channel are selected within the desired intervals of residence 
time (in the present case 11 delay time intervals were chosen: 7 ms long, ranging from 15 to 82 
ms). The resulting electron yields are then normalized to the measured x-ray photon flux and 
cluster flux.  
 
Evaluation of the fraction of NEXAFS signal related to sp-bonds (Xsp)  
In order to study the evolution of the sp-bonds contents in the carbon NP as a function of delay 
time, we calculated, for each time resolved NEXAFS spectra in Figure 1B (see the paper), the 
fraction of NEXAFS signal related to sp-bonds: Xsp.  
Xsp is defined as  
 
 
 
 
 
where A[π*C≡C] and A[π*C=C] are integral intensities from the spectra in Fig. 1B in two 0.6 eV 
wide energy windows centred at 284.7 eV and 285.9 eV, respectively (shaded regions in Fig. 
1B). Thanks to this definition Xsp is the percentage of the integrated intensity of the NEXAFS 
spectra in the energy region relative to the π* resonance for sp hybridization, in respect to the 
integrated intensity in the regions relative to both sp2 and sp hybridizations.  
  In order to estimate the influence of the choice of the width of the energy windows used 
for evaluating A[π*C≡C] and A[π*C=C] on the obtained value of Xsp, we calculated Xsp also when 
using widths ranging from 0.1 to 1 eV. The small variations in the obtained values of Xsp 
account for the ordinate error bars in Figure 1C. Although the used definition of Xsp is only 
qualitative, it avoids the high arbitrariness involved in spectra fitting, especially concerning the 
choice of the fitting curve for the post edge region. 
 
NEXAFS measurements on cluster assembled films  
In order o measure the NEXAFS spectra of thin films obtained by depositing the carbon 
nanoparticles, a copper substrate holder can be inserted by a vacuum translator at the 
crossing point between the cluster and the photon beam. The substrate holder temperature can 
be varied during the deposition in the range 270 - 350 K. Furthermore, the substrate holder is 
connected to ground through a pico-amperometer, so that NEXAFS measurements on clusters 
assembled films can be easily performed also during cluster deposition and thus at conditions 
where the sample is continuously refreshed. 
 
[ ]
[ ] [ ]CCCC
CC
sp AA
AX
≡=
≡
+⋅= **
*100 ππ
π
Supplementary Material (ESI) for Chemical Communications
This journal is (c) The Royal Society of Chemistry 2011


ARTS repository - University of Groningen

University of Groningensp hybridization in free carbon nanoparticles-presence and stability observed by near edge X-ray absorption fine structure spectroscopyRavagnan, Luca; Mazza, Tommaso; Bongiorno, Gero; Devetta, Michele; Amati, Matteo;Milani, Paolo; Piseri, Paolo; Coreno, Marcello; Lenardi, Cristina; Evangelista, FabrizioPublished in:Chemical CommunicationsDOI:10.1039/c0cc03778hIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ravagnan, L., Mazza, T., Bongiorno, G., Devetta, M., Amati, M., Milani, P., Piseri, P., Coreno, M., Lenardi,C., Evangelista, F., & Rudolf, P. (2011). sp hybridization in free carbon nanoparticles-presence and stabilityobserved by near edge X-ray absorption fine structure spectroscopy. Chemical Communications, 47(10),2952-2954. https://doi.org/10.1039/c0cc03778hCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 24-12-20252952 Chem. Commun., 2011, 47, 2952–2954 This journal is c The Royal Society of Chemistry 2011Cite this: Chem. Commun., 2011, 47, 2952–2954sp hybridization in free carbon nanoparticles—presence and stabilityobserved by near edge X-ray absorption fine structure spectroscopywzLuca Ravagnan,a Tommaso Mazza,a Gero Bongiorno,a Michele Devetta,a Matteo Amati,aPaolo Milani,a Paolo Piseri,*a Marcello Coreno,b Cristina Lenardi,a Fabrizio Evangelistacand Petra RudolfcReceived 10th September 2010, Accepted 16th December 2010DOI: 10.1039/c0cc03778hThe presence and stability of sp hybridized atoms in free carbonnanoparticles was investigated by NEXAFS spectroscopy. Theexperiments show that a predominant fraction of carbon atomsis found in linear sp-chains and that conversion into sp2 struc-tures proceeds already at low temperature and in the gas phase.Since their discovery, fullerenes have become the paradigmof carbon nanoparticles (NPs). These highly symmetric, even-numbered carbon aggregates with mixed sp2 and sp3 hybridiza-tion were first identified experimentally by mass spectrometryof free carbon clusters formed in a plasma,1 and predictedtheoretically to be the energetically most stable structures forclusters with more than 20 carbon atoms.2High yield production of fullerenes from a carbon plasma isobtained only in cluster sources where the aggregate condensationoccurs at temperatures above 1000 K.1,3 At lower temperaturesthe fullerene yield is low and ill-defined carbon NPs that build upthe so-called ‘carbon soot’ are favoured.3 Fullerene productiontherefore represents the end point of the carbon NPs aggregationprocess, the ‘fullerene road’, which nevertheless is not necessarilyreached. Several theoretical models of the fullerene formationsteps predict the existence of intermediate highly disordered pre-fullerenic NPs (PFNPs).4,5 These simulations indicate that PFNPscan consist not only of sp2 and sp3 hybridized carbons but alsohave a significant amount of sp hybridization and be remarkablystable.4,5 The development of the experimental production andcharacterization of PFNPs represents a very challenging criticalstep on the route to complex carbon structures.6The presence of sp hybridization in the solid state carbonhas been considered for a long time as very unlikely if notimpossible.7 Although free carbon NPs with less than 20 atomshave sp-based structures as most stable configuration (sp carbonchains, spCCs),2 they react exothermically to form sp2 structureswhen deposited8 due to the dangling bonds at the extremitiesof the isolated spCCs.Despite the long lasting scepticism about stable spCCs in thesolid phase, the feasibility of producing the sp-rich solid carbonwas demonstrated a few years ago by some of us by depositing,at room temperature (RT) and in high vacuum, pure carbon NPsproduced by a Pulsed Microplasma Cluster Source (PMCS).9,10In this source carbon atoms aggregate at a remarkably lowertemperature than in standard fullerene sources.4 The massdistribution of the produced NPs ranges between 102 and103 atoms,11 and thus fullerenes should be the most stableconfiguration;2 however, in situRaman spectra of deposited filmsof these NPs clearly identified the presence of a relevant fractionof sp bonds.9 This sp component decays very slowly in highvacuum at RT, while it is rapidly depleted when the film isexposed to oxygen or moderately heated.11 If the substrate iskept at 150 K during deposition, a remarkable increase in sphybridized atoms is observed.12 The NPs produced by the PMCSmust therefore be PFNPs with high spCCs content and the spchains metastable. In PFNPs the sp chains are embedded in adisordered sp2 structure and their reactive extremities are termi-nated by bonding to sp2 (or sp3) carbon atoms4,5 which producesa stabilization of the spCCs,13 comparable to that achieved byterminating them with bulky endgroups15 or with graphene.14While the spectroscopic study of free PFNPs is essential fora better understanding of the intermediate steps of carbon NPformation, to date sp chains have been characterized only inthe solid state and not as free NPs prior to deposition.Here we present the first gas phase characterization performedby near edge X-ray absorption fine structure spectroscopy(NEXAFS) of the bonding structure of sp-rich PFNPs producedby a PMCS.In NEXAFS a core electron is excited to an unoccupiedmolecular orbital and the resulting core hole filled throughfluorescent decay or an Auger process.16 For carbon the latteris preponderant and by collecting the emitted electrons oneobtains a peak in the spectrum each time the exciting photonenergymatches the difference between the core level and an emptys* or p* orbital. NEXAFS is thus capable of identifying thechemical bonds present and, via a building block approach,quantify their abundance.16 In particular, in the C K-edgeNEXAFS spectra of C2H2 and C2H4, the simplest organicaCIMAINA, Università degli Studi di Milano, V, ia Celoria 16,I-20133 Milano, Italy. E-mail: paolo.piseri@fisica.unimi.itbCNR-IMIP, Montelibretti (Rome) and Gas Phase BeamlineELETTRA, Area Science Park, I-34149 Trieste, ItalycZernike Institute for Advanced Materials, University of Groningen,Nijenborgh 4, 9747AG Groningen, The Netherlandsw This article is part of the ‘Carbon Nanostructures’ web-theme issuefor ChemComm.z Electronic supplementary information (ESI) available: detailedexperimental section. See DOI: 10.1039/c0cc03778hChemComm Dynamic Article Linkswww.rsc.org/chemcomm COMMUNICATIONDownloaded by University of Groningen on 17 March 2011Published on 17 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CC03778HThis journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 2952–2954 2953molecules with sp and sp2 bonds, resonances occur at 285.9 eV forexcitation into p*CRC and at 284.7 eV for p*CQC.16,17 Similarly,distinct resonances (at B286 eV for sp and B284.5 eV for sp2)have been identified in more complex sp–sp2 systems.18 For purecarbon systems only recently few authors claimed the identifica-tion of distinct resonances for sp and sp2 hybridizations.19,20Although this might be surprising given the numerous NEXAFSstudies in the past, the reason becomes clear when consideringthat experiments were usually performed ex situ, thus exposingthe samples to air before characterization. Since the sp species aredestroyed by oxygen,9 their identification by NEXAFS waspossible only recently on NP films produced in situ in a UHVcharacterization facility,19 or transported in vacuum to it.20To characterize NPs in the gas phase by NEXAFS wedeveloped a dedicated apparatus, consisting of three differentiallypumped UHV chambers,21 and directly interfaced with the GasPhase Beamline of the Elettra synchrotron in Trieste, Italy.22 Thefirst chamber hosts the PMCS where the NPs are produced via theaggregation of carbon atoms ablated from a graphite rod throughHe ion sputtering by a plasma plume.10 After aggregation, themixture of NPs and He expands through a focusing nozzle in thevacuum chamber, forming a highly collimated molecular beam.10Since the nozzle’s conductance is low,B100 ms are required aftereach of the source’s working cycles for emptying completely thesource cavity. During that time, the NPs in the cavity can interact,aggregate and structurally rearrange at a temperature as low asB100 K.4 As a result the NP size and structure evolve withresidence time in the source. After exiting from the cavity, theirfurther evolution is quenched by the collisionless and cool environ-ment of the molecular beam.10 The central part of the NP beamenters the second chamber through a skimmer. Here the ioniccomponent of the beam is deflected by an electrostatic field, whileneutral NPs reach the interaction chamber through a smalldiaphragm, where they cross the soft X-ray beam coming fromthe beamline. Differential pumping of the three chambers andthe high collimation of the NP beam ensure a pressure in the10ÿ9mbar range in the interaction chamber. The emitted electronsare collected by a channeltron connected to counting/timingelectronics;21 the detector signal is fed into a time-to-digitalconverter triggered by the PMCS discharge, so that electroncounts can be resolved as a function of detection time within eachsource pulse. Time resolved NEXAFS spectra are obtained bybinning the data and by normalizing the electron yield to theX-ray flux and to the NP flux.21Fig. 1A shows the electron yield measured at differentdelay times after the electrical discharge producing carbonvaporization in the PMCS (shading is used to highlight thetime windows applied for time resolved data shown in Fig. 1Band C). The delay time is the residence time of the NPs in thecavity plus a small interval due to the time of flight from thesource to the interaction region (B1 ms, with a little spread fordifferent parts of the NP beam pulse).Fig. 1B presents the NEXAFS spectra for each of the elevendelay time intervals marked in Fig. 1A. A total electron yieldNEXAFS spectrum of the film obtained by depositing the NPsat 300 K on a copper substrate is also shown for comparison.A clear evolution can be observed: the spectra of the first NPsexiting from the source are sharply peaked at B285.7 eV, anenergy consistent with the acetylene-type p*CRC resonanceat 285.9 eV (marked by a dotted line in the figure).17 Thusthe first NPs exiting the source are characterized by apredominantly sp bond structure. To the best of our knowledgethis is the first NEXAFS spectrum of a pure carbon systemdisplaying a sharp p* peak indicative of sp hybridization.Fig. 1 (A) Electron yield measured for free carbon NPs as a function of the delay time between the PMCS discharge and the electron detection(i.e. the NP residence time in the source). (B) NEXAFS spectra of the free NPs acquired in the 11 delay time windows shaded in panel (A),compared to the in situ NEXAFS spectrum of the NP assembled film deposited at RT (dep. 300 K). The acetylene and ethylene p* resonances areindicated as dotted lines.16,17 (C) Evolution of Xsp (see definition in text) with the delay time; the value of Xsp for the deposited film is indicated as adashed line.Downloaded by University of Groningen on 17 March 2011Published on 17 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CC03778H2954 Chem. Commun., 2011, 47, 2952–2954 This journal is c The Royal Society of Chemistry 2011As the delay time increases (going from spectrum 1 to 11 inFig. 1B), the peak at 285.7 eV progressively decreases, stillremaining the predominant contribution to the p* near edgepart of the spectrum, while a shoulder at B284.7 eV(i.e. p*CQC) grows. Finally, in the spectrum of the deposited film,the acetylene-type p*CRC peak is not any longer the predominantfeature and the p*CQC peak shows a similar intensity.Fig. 1C shows the evolution of the fraction of NEXAFS signalrelated to sp-bonds as a function of delay time (i.e. NP residencetime inside the source). This fraction (Xsp) is calculated asdetailed in the ESIz. Although this definition of Xsp is onlyqualitative, it avoids the high arbitrariness involved in spectrafitting, especially concerning the choice of the fitting curve for thepost edge region.20,23 As can be clearly seen in Fig. 1C, theamount of sp bonds present in the NPs decreases monotonouslywith increasing residence time, remaining nevertheless muchhigher than the amount of sp detected on the NP assembled filmdeposited at RT (dashed line in Fig. 1C). This indicates thatduring all phases of the NP formation in the PMCS, theaggregates are composed of a major fraction of spCCs, in goodagreement with the theoretical prediction for the low temperatureaggregation process of carbon.4,5 The chains are metastable andevolve progressively towards sp2 structures during residence timein the source; nevertheless a relevant fraction of the spCCs is ableto survive even after NP deposition at RT, confirming that theend-capping of the chains by bonding to sp2 groups afford aremarkable stability to carbon atomic wire structures.13To further follow the decay process of spCCs, we deposited theNP also on the copper substrate kept at 350 K. As shown inref. 11 and 12 the fraction of spCCs that survive the depositionprocess, is strongly affected by the substrate temperature. Fig. 2shows the NEXAFS spectra, normalized to the post edge region,from the film deposited at room temperature and at 350K, as wellas the difference between these two spectra. Substrate heatingcauses a substantial change in both the p* and the s* regions ofthe spectrum: as evident from the difference (see the fit in theinset), the change in the p* region is related to the decay ofthe p*CRC band at 285.9 eV and the corresponding increase ofthe p*CQC band at 285 eV. This further supports the metastabilityof the spCCs, and their decay in sp2 carbon when annealed.In conclusion, we demonstrated the sp hybridizationof free carbon NPs by NEXAFS and followed the evolutionof their structure on a millisecond time scale under lowtemperature conditions in an inert atmosphere. Our resultsshow that free carbon NPs can have a complex structure withvarying relative amounts of sp and sp2 bonds. This has veryimportant consequences for a wide variety of fields wherecarbon NPs play a role such as astrochemistry and environ-mental sciences. Indeed carbon NPs rich in spCCs are theintermediate steps in the ‘fullerene road’ and could be basicconstituents of both the interstellar medium and of combus-tion exhausts.Measurements were supported by the ‘‘Access to ResearchInfrastructure’’ action of the EU Human Potential Program.Notes and references1 R. E. Smalley, Acc. Chem. Res., 1992, 25, 98.2 R. O. Jones, J. Chem. Phys., 1999, 110, 5189.3 T. Wakabayashi, D. Kasuya, H. Shiromaru, S. Suzuki, K. Kikuchiand Y. Achiba, Z. Phys. D, At., Mol. Clusters, 1997, 40, 414.4 Y. Yamaguchi, L. Colombo, P. Piseri, L. Ravagnan and P. Milani,Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 134119.5 G. Zheng, Z. Wang, S. Irle and K. Morokuma, J. Nanosci.Nanotechnol., 2007, 7, 1662.6 G. Benedek, H. Vahedi Tafreshi, E. Barborini, P. Piseri, P. Milani,C. Ducati and J. Robertson, Diamond Relat. Mater., 2003, 12, 768.7 R. B. Heinmann, in Carbyne and Carbynoid Structures, ed.R. B. Heimann, S. E. Evsyukov and L. Kavan, Kluwer AcademicPress, Dordrecht, 1999, pp. 7–15.8 T. Wakabayashi, A.-L. Ong, D. Strelnikov and W. Krätschmer,J. Phys. Chem. B, 2004, 108, 3686.9 L. Ravagnan, F. Siviero, C. Lenardi, P. Piseri, E. Barborini,P. Milani, C. S. Casari, A. Li Bassi and C. E. Bottani, Phys.Rev. Lett., 2002, 89, 285506.10 K. Wegner, P. Piseri, H. Vahedi Tafreshi and P. Milani, J. Phys. D:Appl. Phys., 2006, 39, R439.11 C. S. Casari, A. Li Bassi, L. Ravagnan, F. Siviero, C. Lenardi,P. Piseri, G. Bongiorno, C. E. Bottani and P. Milani, Phys. Rev. B:Condens. Matter Mater. Phys., 2004, 69, 75422.12 L. Ravagnan, P. Piseri, M. Bruzzi, S. Miglio, G. Bongiorno,A. Baserga, C. S. Casari, A. Li Bassi, C. Lenardi,Y. Yamaguchi, T. Wakabayashi, C. E. Bottani and P. Milani,Phys. Rev. Lett., 2007, 98, 216103.13 L. Ravagnan, N. Manini, E. Cinquanta, G. Onida, D. Sangalli,C. Motta, M. Devetta, A. Bordoni, P. Piseri and P. Milani, Phys.Rev. Lett., 2009, 102, 245502.14 C. Jin, H. Lan, L. Peng, K. Suenaga and S. Iijima, Phys. Rev. Lett.,2009, 102, 205501.15 W. A. Chalifoux and R. R. Tykwinski, C. R. Chim., 2009, 12, 341;S. Kim, Angew. Chem., Int. Ed., 2009, 48, 7740.16 J. Stöhr, NEXAFS spectroscopy, Springer, New York, 1992.17 A. P. Hitchcock and D. C. Mancini, J. Electron Spectrosc. Relat.Phenom., 1977, 10, 749.18 F. Matsui, H. W. Yeom, I. Matsuda and T. Ohta, Phys. Rev. B:Condens. Matter, 2000, 62, 5036; A. Fujimori, M. Ishitsuka,H. Nakahara, E. Ito, M. Hara, K. Kanai, Y. Ouchi and K. Seki,J. Phys. Chem. B, 2004, 108, 13153.19 L. Ravagnan, G. Bongiorno, D. Bandiera, E. Salis, P. Piseri,P. Milani, C. Lenardi, M. Coreno, M. de Simone andK. C. Prince, Carbon, 2006, 44, 1518.20 J. Diaz, O. R. Monteiro and Z. Hussain, Phys. Rev. B: Condens.Matter Mater. Phys., 2007, 76, 094201.21 P. Piseri, T. Mazza, G. Bongiorno, M. Devetta, M. Coreno andP. Milani, J. Electron Spectrosc. Relat. Phenom., 2008, 166–167, 28.22 K. C. Prince, R. R. Blyth, R. Delaunay, M. Zitnik, J. Krempasky,J. Slezak, R. Camilloni, L. Avaldi, M. Coreno, G. Stefani,C. Furlani, M. de Simone and S. Stranges, J. Synchrotron Radiat.,1998, 5, 565.23 I. Jiménez, R. Gago and J. M. Albella, Diamond Relat. Mater.,2003, 12, 110.Fig. 2 Total electron yield NEXAFS spectra (normalized to the postedge region as indicated by the arrow) of NP assembled filmsdeposited at 300 K (RT) and at 350 K. The difference spectrum isindicated in black, and its Gaussian fit is shown in the inset.Downloaded by University of Groningen on 17 March 2011Published on 17 January 2011 on http://pubs.rsc.org | doi:10.1039/C0CC03778H

sp hybridization in free carbon nanoparticles-presence and stability observed by near edge X-ray absorption fine structure spectroscopy

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