Thermal Decomposition of the Fullerene Precursor C60H21F9

Article pubs.acs.org/JPCC

Thermal Decomposition of the Fullerene Precursor C60H21F9 Deposited on Graphite Seyithan Ulas,† Jürgen Weippert,† Konstantin Amsharov,‡,§ Martin Jansen,§ Monica Loredana Pop,†,∥ Mircea V. Diudea,∥ Dmitry Strelnikov,† Artur Böttcher,*,† and Manfred M. Kappes*,†,⊥ †

Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany University Erlangen-Nürnberg, Institut für Organische Chemie, Henkestr. 42, 91054 Erlangen, Germany § Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany ∥ Faculty of Chemistry and Chemical Engineering, University Babes-Bolyai Cluj-Napoca, Arany Janos Str. 11, 400084, Cluj, Romania ⊥ Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡

S Supporting Information *

ABSTRACT: Specially fluorinated polycyclic aromatic hydrocarbons (F-PAHs) are of interest as precursors for transition metal catalyzed CVD growth of chiral-index pure singlewalled carbon nanotubes as well as for the rational synthesis of fullerenes. Laser desorption/ ionization of a prototypical F-PAH has recently been shown to lead to C60 via a sequence of regioselective intramolecular cyclodehydrofluorination steps: C60H21F9 → C60H20F8 + HF → C60H19F7 + HF ... → C60 (Kabdulov et al. Chem.−Eur. J. 2013, 19, 17262). We have studied the thermal stability of solid C60H21F9 films on graphite under UHV conditions toward exploring the extent to which such intramolecular dehydrofluorination can also occur on a hot chemically inert surface and to what extent intermolecular interactions influence such transformation processes. C60H21F9 films were probed in situ by ultraviolet photoionization, X-ray ionization, Raman spectroscopy, and thermal desorption mass spectrometry, as well as by ex situ atomic force microscopy. Heating multilayer films results first in C60H21F9 emission from the bulk (peaked at ∼630 K) followed at higher temperatures by desorption from the interface region (in the range 750−850 K). Sublimation from the interface region is also associated with some on-surface cyclo-dehydrofluorination as indicated by C60H21−nF9−n, n = 1, 2, 3 emission. C60 was not observed in the desorbed material suggesting that complete cage closure cannot be achieved on HOPG. Furthermore, C60H21F9 deposits cannot be fully removed from HOPG. Instead, competing on-surface polycondensation of reactive intermediates yields a fluorinated carbon phase, which remains stable up to at least ∼1000 K. To complement these studies we have also used mass selective ion beam soft-landing to probe the desorption properties of monodispersed films consisting of mass-selected C60H21−nF9−n fragments, n = 1, 2. suitable X-PAH precursors, flash vacuum pyrolysis still yields only small amounts of fullerenes.10 MNCs have also been generated by heating deposits of planar PAHs on catalytically active transition metal surfaces.11 At submonolayer coverages, the associated fragmentation reactions modify the shape of the individual, well-separated adsorbate molecules, from planar to dome-shaped (“inverted Bucky-bowls”). The transformation typically occurs at substantially lower temperatures than necessary for cyclodehydrogenation by flash vacuum pyrolysis in gas phase, as seen, for example, for C60H30 on Pt(111) .12 Along similar lines, planar C84H42 molecules deposited onto Pt(111) have been efficiently converted into dome-shaped objects via heating to ∼753 K.13,14 The structural selectivity of surface-catalyzed thermal con-

1. INTRODUCTION There is much present interest in large polycyclic aromatic hydrocarbons (PAHs) and in their halogenated derivatives (XPAHs)1,2 as building blocks for molecular electronics and also as precursors for the selective synthesis of various molecular nanocarbon forms (MNCs), such as graphene nanoribbons,3,4 carbon nanotubes,5 and fullerenes.6−10 In all such selective syntheses, the MNCs form by sequential fragmentation of the PAH or X-PAH building blocks. For example, laser irradiation of the planar PAH C60H30 causes cyclodehydrogenation and results in traces of C60.1 Increased C60 yields (up to 1%) have been observed upon flash vacuum pyrolysis of the X-PAH C60H27Cl3.8 This suggested that specific isomers of larger fullerenes might be targeted by pyrolysis of appropriately designed planar X-PAHs, allowing for a series of regioselective intramolecular cyclo-dehyrohalogenation steps (leading first to strain-induced curvature and eventually to cage closure).9 Unfortunately, in spite of considerable advances in synthesizing © 2015 American Chemical Society

Received: January 20, 2015 Revised: March 15, 2015 Published: March 16, 2015 7308

DOI: 10.1021/acs.jpcc.5b00588 J. Phys. Chem. C 2015, 119, 7308−7318

Article

The Journal of Physical Chemistry C

Figure 1. Schematic molecular structures of C60H21F9 and C60H30, as used in this study (carbon atoms are black, hydrogens white, and fluorines green). Both are known precursors for the (low yield) formation of C60(Ih) by flash vacuum pyrolysis.

version has also been studied for C60H30 on TiO2(110),15 confirming that cyclodehydrogenation rather than (internal) Stone−Wales rearrangement is responsible for “inverted Buckybowl” formation. In a related recent development, it was shown that thermal transformation of the planar PAH C96H54 deposited at low coverages onto a catalytically active platinum substrate yields dome-shaped objects, which are themselves able to initiate chiral index selective growth of single-walled carbon nanotubes (SWCNTs) under chemical vapor deposition (CVD) conditions.5 This observation holds promise for unravelling the CVD growth mechanism of SWCNTs at a molecular level. As a prerequisite, it is necessary to better understand surfacecatalyzed planar → dome transformations themselves. Research on the conversion of PAHs or X-PAHs to MNCs has so far focused mainly on the activation of isolated molecules (in gas phase or isolated on a surface) in order to elucidate the elementary intramolecular transformations and fragmentation steps taking place.9,10 In this contribution, we instead explore how X-PAH···X-PAH interactions modify this fragmentation scenario. For this we have studied the thermal decomposition of solid X-PAH films of various thicknesses ranging from submonolayer to multilayer. These were grown under ultrahigh vacuum conditions on inert graphite substrates. C60H21F9 has been chosen as the most suitable prototypical X-PAH for study, as its (unimolecular) fragmentation cascade is comparatively well understood.16 Furthermore, due to recent synthetic progress it is available in sufficient amounts for an extensive surface science study. Thermal decomposition (and associated molecular fragmentation) of C60H21F9 films has been studied by analyzing the mass spectra of all species sublimating from the films during heating up to >900 K as well as by characterizing the properties of the solids surviving the thermal treatment (by UPS, XPS, Raman, TDS, AFM). Heating thin C60H21F9 films on graphite leads to significant desorption of the parent molecules, C60H21F9, as well as to the evolution of molecular fragments, C60H21−nF9−n (n = 1−3), resulting from up to three sequential HF-elimination steps. The activation barriers for surface-mediated HF elimination are considerably higher than that found for sublimation of parent

molecules. Further insight into the influence of the X-PAH··· HOPG interaction on C60H21F9 fragmentation has been obtained by recording thermal desorption from submonolayers deposited mass-selectively by means of the low-energy cluster beam deposition technique (C60H21−nF9−n (n = 0−2), LECBD17). Sublimation from thick, nominally van der Waals-stabilized films has been found to be accompanied by pronounced transformation of the material surviving the heating procedure. The electronic properties of the residue (valence and core states) reveal that sublimation triggers covalent interlinking of the building blocks as well as considerable modifications of the substrate.

2. EXPERIMENTAL SECTION Solid C60H21F9 for deposition studies was synthesized and purified as previously described.9 For the near-planar reference substance C60H30 (also used in this study), we followed the preparation method described in ref 16. The schematic molecular structures of both substances are shown in Figure 1. C60H21F9 films (as well as C60H30 reference films) were prepared under ultrahigh vacuum conditions (base pressure < 5 × 10−10 mbar) by deposition onto freshly cleaved graphite, HOPG (SPI Incorporated, SPI-II quality). Prior to film growth, the HOPG sample was flashed several times up to 1100 K to remove −OH and −C−H terminations of step edges.18 This procedure yields clean substrates as confirmed by AFM-based topography imaging as well as photoelectron spectroscopy (UPS and XPS). Two deposition methods were used to prepare C60H21F9 films, depending on desired coverage. Substrates were exposed to either: (1) a thermal flux of neutral C60H21F9 molecules sublimed from a hot Knudsen cell (T = 660 K) or (2) a flux of low energy mass-selected C60H21F9+ ions (or alternatively massselected C60H21−nF9−n+ fragments) generated by electronimpact ionization of neutrals sublimating from a Knudsen cell.19 To preclude (further) fragmentation ionic species were deposited at typical kinetic energies of ∼6 eV, similar to the procedure used in our previous studies of non-IPR fullerene materials.20 Ion beam soft-landing generated an elliptically 7309


Article


Figure 2. (T,M) maps recorded by temperature resolved mass spectrometry during heating C60H21F9 (=CHF(0)) thin films deposited on HOPG (0.09, 0.23, and ∼1 ML). Desorption features observed near ∼800 K correspond to evolution of both intact CHF(0) and its fragments CHF(n). The corresponding mass spectral fragmentation patterns are contributed to by two different processes: (1) electron-impact ionization induced heating (following desorption) and (2) on-surface fragmentation (before desorption), see text. Beyond a critical coverage, TD spectra begin to exhibit a second well-separated set of peaks (at ∼670 K) reflecting the onset of multilayer formation. As in all other sublimation experiments a heating rate of 5 K/s was applied.

shaped deposit of roughly 3 × 4 mm2. Neutral deposition was carried out under conditions leading to uniform coverage of the whole substrate surface 7 × 7 mm2. Film thicknesses were estimated on the basis of a series of C60H21F9 desorption mass spectra recorded after depositing stepwise increasing amounts of C60H21F9 molecules, as determined by integrating the incident ion current (see also section 3). C60H30 reference films were generated analogously. Substrates were held at room temperature throughout all depositions. Thermal desorption of as-prepared C60H21F9 films was studied by temperature resolved mass spectrometry (quadrupole mass spectrometer, Extrel, 70 eV electron impact ionization, typical mass resolution m/Δm ∼ 300). For this, the flux of desorbed species R was monitored over the mass range ΔM = 690−1000 amu while linearly increasing the sample temperature at a constant heating rate (T = 300 K + βt, β = 5 K/s). Generally, we rapidly and iteratively acquired mass spectra (24 scans per minute) during heating. These Tdependent mass scans were converted to the corresponding (T,M)-diagrams or sublimation maps, as previously described.21 Ideally, sublimation maps allow to directly identify desorbed species, their sublimation temperatures as well as the corresponding desorption activation energies. After correcting for ionization induced fragmentation, the temperature dependence of the ion signal on a given mass channel M*+ can be related to the desorption flux of the neutral precursor species M* (=standard thermal desorption spectrum of M*). Sample temperature during TD scans was measured by means of a Ktype thermocouple spot-welded to the back side of the sample holder. The temperature in the central part of the sample at its front side was also measured by means of a calibrated radiation pyrometer (Keller PZ20AF).

The valence-band electronic structure of C60H21F9-derived films was studied by ultraviolet photoelectron spectroscopy, UPS (21.2 eV), using a hemispherical electron energy analyzer (ESI 125, Omicron, energy resolution of ∼0.1 eV) and a HeIdischarge lamp (Omicron). He lamp and electron analyzer were aligned so as to detect the surface normal component of the photoemission. The UP spectra reflect the density of states in the valence region, VB-DOS, from which the work function, ϕ, and surface ionization potential, IP, of the film can be determined (all referenced to the well-defined Fermi level of HOPG). C1s and F1s band XP spectra were taken with a twin anode X-ray source (Mg Kα line 1253.6 eV, DAR 400 Omicron) using the same hemispherical energy analyzer as for UPS (at pass energy of 20 eV). The vibrational properties of thick C60H21F9-derived films were analyzed in situ by Raman spectroscopy (RXN1 Analyzer, Kaiser Optical Systems, excitation wavelength λ = 785 nm, laser power usually 20 mW, acquisition time 60 s). Spectra were compared to those calculated for isolated C60H21F9 molecules by DFT using the TURBOMOLE program package (TURBOMOLE V6.0 2009, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989− 2007, TURBOMOLE GmbH, since 2007; available from http://www.turbomole.com).22 Geometry optimization and vibrational analysis was performed at the RI-DFT BP86/defSV(P) level of theory without symmetry restrictions. Frequency-dependent Raman intensities were calculated for 785 nm excitation (which corresponds to the laser wavelength in our Raman spectrometer). The surface topography of as-prepared C60H21F9 films was measured ex situ by means of atomic force microscopy, AFM (AFM, Veeco Instruments CP-II), in noncontact mode. A 5 μm 7310


Article


Figure 3. Panel I: (T,M) map for a thick CHF(0) film (∼225 ML) on HOPG. Band A corresponds to sublimation from the bulk. B reflects desorption from the interface region. The periodic stripes to the left of the parent ion signals reflect fragmentation by sequential loss of n HF molecules. For thick films this occurs primarily as a result of vibrational excitation associated with electron impact ionization (i.e., after desorption of mainly intact CHF(0) molecules), as indicated by the nearly constant desorption temperatures corresponding to parent and fragment signals. Panel II: Mass spectra highlighting bulk and interface sublimation, black and gray lines, respectively, as obtained by projecting the sublimation map shown in panel I onto the mass-axis. Panel III: Thermal desorption spectrum of CHF(0), obtained by projecting the parent ion intensity from the sublimation map (T,M) onto its temperature axis.

desorption temperature of ∼670 K, indicating multilayer formation. We use the onset of the latter feature (>1 ML) to internally calibrate surface coverage. Assuming dense packing of planar C60H21F9 on HOPG, 1 ML corresponds to a lateral density of ∼3 × 1013 molecules/cm2. 3.1.1. Multilayer Films. Figure 3 shows a typical (T,M) thermal desorption map (panel I) obtained by heating a thick film of ∼225 ML C60H21F9. Now, the two sets of desorption features can be more clearly distinguished: Band A comprises six equidistantly spaced mass peaks at the same temperature T* ∼ 646 K. The most intense of these corresponds to the parent ion C60H21F9+. Additionally, a series of fragment ions spaced by ∼20 amu is observed (panel II) corresponding to sequential loss of up to five HF molecules. (Note: So as to ensure adequate signal intensities, mass spectral resolution was set to m/Δm ∼ 300 throughout. This allowed to assign the dominant fragmentation channel to (sequential) HF loss but did not allow us to exclude other minor decay processes, such as H or H2 loss). The constant desorption temperature observed for all fragment ions indicates that they are predominantly generated by electron impact heating in gas phase, that is, after desorption. Band B observed above 700 K consists of six parallel, vertically elongated features. Each of these exhibits

scanner and NCS15 or NCS18 cantilevers with well-defined spring constants were typically used.

3. RESULTS AND DISCUSSION 3.1. Thermal Desorption Mass Spectrometry. Figure 2 contains three typical (T,M) thermal desorption maps for increasing amounts of C60H21F9 soft landed onto HOPG in the thin film regime (0.09 → ∼1 ML). The sublimation maps show >5 vertical stripes centered in temperature near ∼800 K equidistantly spaced on the mass axis. To first order, these features correspond to the desorption of C60 H 21−n F 9−n (abbreviated as CHF(n) in the following), which we detect by ionization. (Note: The heating of the films is also accompanied by the emission of HF molecules which for experimental reasons could not be monitored simultaneously with C60H21F9 desorption). In addition to parent ion signals (CHF(n) + e → CHF(n)+ + e), which are roughly proportional to the CHF(n) fluxes evolved from the surface, the detected ion counts in a given CHF(n)+ mass channel are also contributed to (sometimes dominantly, see below) by ionization induced fragmentation (CHF(n − 1) + e → CHF(n − 1)+* + e → CHF(n)+ + HF). Beyond a critical coverage, TD spectra exhibit two well separated sets of peaks. Apart from the features at 800 K, a second set of peaks is observed at a significantly lower 7311


Article

The Journal of Physical Chemistry C three distinguishable components (at common temperatures of T*1 = 770 K, T*2 = 802 K, and T*3 = 835 K (see panel III)). Again, the common desorption temperatures indicate that band B mostly arises from ionization induced fragmentation of desorbed CHF(0), with on-surface fragmentation negligible by comparison. While the mass spectra in both regions A and B look very similar (CHF(n)+ intensities scale as I(n) = I(n − 1) exp(−0.58 ± 0.03)), the integral ion signal in the hightemperature region is considerably weaker. Rationalization of the CHF(n)+, signals (n = 1−5) seen in bands A and B of Figure 3, as due to electron impact induced fragmentation after desorption is additionally supported by measurements on deposits generated by soft landing of preformed mass-selected CHF(n) fragment ions, Figure 4. Corresponding (T,M) maps show analogous fragmentation cascades, which set in at the masses deposited. Gas-phase dehydrofluorination, as induced by electron impact heating, is consistent with the known thermal fragmentation behavior of 1- and 2- fluorobenzo[c]phenanthrenes, as seen in flash vacuum pyrolysis (homo-HF elimination).23 This can in turn be rationalized by the generally low activation barriers for 1,5HF elimination to yield five-membered ring closure, as indicated by DFT calculations for isolated molecules (3.48 eV23). Note that our (T,M) maps show no evidence of a C60+ fragment, that is, nine-step sequential HF elimination cascade followed by (or interleaved with) multiple dehydrogenation steps could not be induced by 70 eV electron impact ionization under our conditions. This is in apparent contradiction with laser desorption ionization mass spectrometry probes of C60H21F9, where C60− can be observed as a product (in gas phase).16 However, the excitation regimes and reaction times before detection differ widely in the two experiments. The (T,M) map shown in Figure 3 can be projected onto the temperature axis to obtain the thermal desorption spectrum of CHF(0), panel III. The two well separated desorption bands, A and B, resemble the desorption profile of thick alkane films on single crystal graphite.24 In analogy, we assign the CHF(0)-A band to sublimation from the bulk (i.e., the film component not affected by the surface). The higher temperature band CHF(0)B appears upon removal of the bulk material. We therefore attribute it to desorption from the film component affected by the interface (=interface region). Bulk sublimation (CHF(0)-A) proceeds according to zeroorder kinetics: R = dN/dt = ν exp(−E(0)/kT), that is, the desorption rate R does not depend on the film thickness. Correspondingly, the preexponential factor, ν, as well as the activation barrier E(0) can be directly determined by fitting the leading edge in the TD measurement, resulting in values of E(0) = 2.44 ± 0.06 eV and ν = 7.3 × 1025±1 Hz, which compare well to the numbers for other smaller planar PAHs (e.g., ovalene, E = 2.2 eV and ν = 5 × 1021±3.25 Interestingly, in terms of binding energy per carbon atom, the E(0) value derived for bulk CHF(0) (0.04 eV/C) is only slightly lower than the exfoliation energy of a graphene layer from HOPG (0.043 eV/ C).26 This suggests that the cohesion of solid C60H21F9 is dominated by van der Waals-stabilized stacking of the constituent (near) planar molecules. Apparently, −C−F and −C−H terminating groups do not significantly perturb the molecular planarity. Note that the activation barrier for intact sublimation (E(0) = 2.44 eV) is considerably lower than HFabstraction energy estimated for an analogous fluorinated PAH on the basis of calculations.

Figure 4. (T,M) maps taken from monodispersed CHF(n), n = 0, 1, and 2, films grown by soft-landing of mass-selected CHF(0)+, CHF(1)+, and CHF(2)+ (Ekin ∼ 6 eV), respectively. The related amounts deposited corresponded to 2.5, 1.5, and C−F bonds should appear at ∼290 ± 0.2 eV, whereas a semi-ionic >C···F−δ bond is expected in the range 287−288 eV.34−36 Thus, the β0(287.7 eV) feature is again consistent with surface-mediated charging (and localization of excess negative on the most electronegative adsorbate atom). This inference is also supported by the structure of the F1s band (Figure 8, right panel). Both Gaussian components, a0(687.5 eV) and the much 7315


Article


Figure 8. Dependence of the C1s and F1s core states of a 225 ML thick CHF(0) film upon thermal treatment, as determined by XPS (1253.6 eV): (a) after preparation (300 K), (b) after heating to 715 K, and (c) after heating to 1000 K (panels I, II, and III, respectively). For assignment, see text. Widths and relative intensities of all Gaussian components used to fit the spectra are tabulated in Supporting Information (Table 1S).

weak C1s spectral component β* (i.e., reflecting surviving >C··· F−δ bonds), we assign component a*, as due to F anions intercalated in residual CHF stacks, in analogy to fluorinated graphite (which shows comparable XPS features for C/F ratios higher than 2037). Note that the inference of CHF stacks interconnected by ···F−δ··· species (via the image-potential (Ebind ∼ δ2e2/4z38)) helps to explain the higher thermal stability of the interface layer as observed in (T,M) maps. In contrast, the weak component δ* may be due to formation of more covalent −C−F bonds (∼290.6 eV39). Further heating from 715 → 1000 K is accompanied by emission of CHF(n), n = 0−3, from the interface region as indicated by the (T,M) maps shown in the previous section (band B). The fragments result from surface-mediated dehydrofluorination, followed by desorption. However, part of the initial deposit is transformed into a nondesorbable material with a characteristic XPS signature (III). The C1s band is now well fit by superposing five Gaussian components: α•(284.4 eV), β•(286.3 eV), γ•(288.3 eV), δ•(290.9 eV), and λ•

more intense component b0(688.3 eV), are clearly located below the region typical for covalent −C−F bonds (689.6 eV31,37). Component a0 (687.5 eV) reflects fluorine atoms with a somewhat higher localized charge density than is the case for b0 (688.3 eV) species. Heating CHF(0) films up to T* = 715 K (removal of the bulk) considerably modifies their C1s and F1s XPS bands. The resulting C1s profiles (II) can be fit by superimposing four Gaussian peaks: α*(284.4 eV), β*(286.2 eV), γ*(288.3 eV), and δ*(290.9 eV). Whereas the α*(284.4 eV) component originates from the previously buried HOPG substrate, the components β*, γ*, and δ* reflect contributions from carbon atoms associated with strongly modified −C−F bonds. We assign the heating-induced shift, β0 → β*, to increasing polarization of remaining >C···F−δ bonds (δ → 1) due to charge transfer. Correspondingly, the F1s band is constituted by three resolved peaks: a*(684.4 eV), b*(687.5 eV), and a weak component c*(690 eV). Both a* and b* also reflect increased charging. Whereas peak b* can be correlated with the 7316


Article


of charged intermediates along their fragmentation cascade after ionization, fully in the gas phase. In particular, ion mobility measurements would be useful in this regard.42 C60H21F9 and C60H30 are two of the largest molecules for which TDS measurements have so far been reported. We find the C60H21F9 desorption activation energy to be comparable to the activation energy for on-surface fragmentation to form desorbable CHF(n). The large amounts of nondesorbable residue observed after heating, indicate that thermally activated polycondensation processes are also kinetically competitive in this system. Comparison to the materials formed by thermal decomposition of soft-landed CHF(n) fragments may help to understand the mechanism of such polycondensation processes. We note in closing that selective electron-impact ionization induced fragmentation of F-PAHs in gas phase in combination with ion beam soft-landing could also be applied to decorating surfaces with preformed dome-like species for subsequent studies of their surface reactivity, for example, in the context of carbon nanotube CVD growth.

centered at 283.8 eV (panel III in Figure 8). The F1s band now comprises only a weak feature a•(687.3 eV). The C1s-(γ•, δ•) and F1s-(a•) components provide evidence that strong covalent C−F bonds survive the heating procedure applied.40 The intense C1s-(α•,β•) components cover the region, which has previously been assigned to sp3-hybridized carbon atoms. When combining the two findings it appears that −C−C−F/sp3 species are created in the nondesorbable material. In contrast a*(684.4 eV) and b*(687.5 eV) components as assigned to ionic species have been entirely removed. Apparently, only a minority ionic fluorine species survives heating. Interestingly, terminating fluorine atoms can be removed from fluorinated graphite by heating to a comparable temperature (dissociation onset < 1000 K41), thus, supporting this assignment. Fluorine atom elimination would allow additional covalent C−C interlinking. This is consistent with a C1s component β• at 286.3 eV, as seen for sp3-hybridized carbons in literature.32,38

4. SUMMARY The thermal stability of C60H21F9 (=CHF(0)) films grown on HOPG under UHV conditions has been investigated over the temperature range 300−1000K and for film thicknesses ranging from 0.09−225 ML. Heating at a rate of 5 K/s induces onsurface reactions and is also associated with significant sublimation of intact CHF(0). For multilayer films, this proceeds in two separate temperature windows: (1) emission in the interval 580−650 K (activation barrier ∼ 2.44 eV) is assigned to sublimation of the bulk material unaffected by the substrate and (2) evolution of CHF(0) in the range 750−850 K (activation barrier > 3.9 eV) is attributed to desorption from the interface region. The latter is also associated with the evolution of some CHF(n), n = 1−3, formed at the interface either by unimolecular dehydrofluorination or by reactions of individual molecules with the surrounding carbon network. Desorption of CHF(1) and CHF(2) occurs at higher temperatures than CHF(0) desorption. The (additional) activation energy necessary for their evolution is coverage dependent. By contrast, heating a C60H30/HOPG sample does not lead to measurable dehydrogenation confirming that HF elimination in CHF(0) films is much more facile than H2 elimination. Heating CHF(0) films to 1000 K on HOPG converts a significant fraction of the deposit (dependent on the initial coverage) to a nondesorbable covalently stabilized carbon network material containing F atoms in a bonding environment significantly different from that of the as-prepared CHF(0) van der Waals solid. Based on our electronic spectroscopy results, we speculate that much of the thermally induced on-surface/ interface region chemistry is facilitated by charging of terminal electronegative F atoms in CHF(0) molecules via electron transfer from HOPG. In contrast to laser desorption studies,16 we observed no evolution of C60 during heating of C60H21F9 films on HOPG, neither from the C60H21F9 bulk nor from the interface region. We attribute this discrepancy to the widely different energy fluxes and reaction times pertaining in thermal versus laser desorption/ionization setups. Furthermore, steric constraints as well as competing intermolecular condensation can be expected to inhibit complete cage closure at interfaces in contrast to analogous transformation cascades in gas-phase. An analogous (steric) argument can be invoked to explain dome formation in surface-catalyzed PAH transformations. To sort these issues out, it will be informative to study the structural development

■

ASSOCIATED CONTENT

S Supporting Information *

(1) A representative (T,M) sublimation map of a thick C60H30 film recorded under conditions similar to those of the C60H21F9 measurement shown in Figure 3; (2) Raman spectra calculated for an isolated C60H21F9 molecule in comparison to Raman spectra recorded for C60H21F9 powder; (3) Representative AFM images of a HOPG surface taken before and after the deposition of a ∼2 ML-thick C60H21F9 film. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge support by the DFG Center for Functional Nanostructures (CFN − Subproject C4.6). M.L.P. acknowledges fellowship support from The Sectoral Operational Programme Human Resources Development (Contract POSDRU 6/1.5/S/3 − “Doctoral studies: through science towards society”). K.A. acknowledges support by Heisenberg fellowship of the DFG for synthesis of C60H21F9 and C60H30.

■

REFERENCES

(1) Boorum, M. M.; Vasilev, Y. V.; Drewello, T.; Scott, L. T. Groundwork for a Rational Synthesis of C60: Cyclodehydrogenation of a C60H30 Polyarene. Science 2001, 294, 830−831. (2) Feng, X.; Pisula, W.; Müllen, K. Large Polycyclic Aromatic Hydrocarbons: Synthesis and Discotic Organization. Pure Appl. Chem. 2009, 81, 2203−2224. (3) Feng, X.; Wu, J.; Ai, M.; Pisula, W.; Zhi, L.; Rabe, J. P.; Müllen, K. Triangle-Shaped Polycyclic Aromatic Hydrocarbons. Angew. Chem., Int. Ed. 2007, 46, 3033−3036. (4) Chen, L.; Hernandez, Y.; Feng, X.; Mü llen, K. From Nanographene and Graphene Nanoribbons to Graphene Sheets: Chemical Synthesis. Angew. Chem., Int. Ed. 2012, 51, 7640−7654. (5) Sanchez-Valencia, J. R.; Dienel, T.; Gröning, O.; Shorubalko, I.; Mueller, A.; Jansen, M.; Amsharov, K.; Ruffieux, P.; Fasel, R. Controlled Synthesis of Single-Chirality Carbon Nanotubes. Nature 2014, 512, 61−64.

7317


Article

The Journal of Physical Chemistry C (6) Kadish, K. M.; Ruoff, R. S. Fullerenes: Chemistry, Physics, and Technology; Wiley-VCH: Weinheim, 2000. (7) Richter, H.; Labrocca, A. J.; Grieco, W. J.; Taghizadeh, K.; Lafleur, A. L.; Howard, J. B. Generation of Higher Fullerenes in Flames. J. Phys. Chem. B 1997, 101, 1556−1560. (8) Scott, L. T.; Boorum, M. M.; McMahon, B. J.; Hagen, S.; Mack, J.; Blank, J.; Wegner, H.; de Meijere, A. A Rational Chemical Synthesis of C60. Science 2002, 295, 1500−1503. (9) Kabdulov, M. A.; Amsharov, K. Yu.; Jansen, M. A Step Toward Direct Fullerene Synthesis: C60 Fullerene Precursor With Fluorine in Key Positions. Tetrahedron 2010, 66, 8587−8593. (10) Amsharov, K. Y.; Jansen, M. Synthesis of a Higher Fullerene Precursor - an “Unrolled” C84 Fullerene. Chem. Commun. 2009, 19, 2691−2693. (11) Rim, K. T.; Siaj, M.; Xiao, S.; Myers, M.; Carpentier, V. D.; Liu, L.; Su, C.; Steigerwald, M. L.; Hybertsen, M. S.; McBreen, C.; et al. Forming Aromatic Hemispheres on Transition-Metal Surfaces. Angew. Chem. 2007, 119, 8037−8041. (12) Otero, G.; Biddau, G.; Sánchez-Sánchez, C.; Caillard, R.; López, M.-F.; Rogero, C.; Palomares, F. J.; Cabello, N.; Basanta, M. A.; Ortega, J. J. A.; et al. Fullerenes From Aromatic Precursors by SurfaceCatalysed Cyclodehydrogenation. Nature 2008, 454, 865−868. (13) Amsharov, K.; Abdurakhmanova, N.; Stepanow, S.; Rauschenbach, S.; Jansen, M.; Kern, K. Towards the Isomer-Specific Synthesis of Higher Fullerenes and Buckybowls by the SurfaceCatalyzed Cyclodehydrogenation of Aromatic Precursors. Angew. Chem., Int. Ed. 2010, 49, 9392−9396. (14) Huang, C.-P.; Su, C.-C.; Ho, M.-S. Intramolecular Structures of C60 and C84 Molecules on Si(111)-7 × 7 Surfaces by Scanning Tunneling Microscopy. Appl. Surf. Sci. 2008, 254, 7712−7717. (15) Sánchez-Sánchez, C.; Martínez, J. I.; Lanzilotto, V.; Biddau, G.; Gómez-Lor, B.; Pérez, R.; Floreano, L.; López, M. F.; Martín-Gago, J. Á . Chemistry and Temperature-Assisted Dehydrogenation of C60H30 Molecules on TiO2(110) Surfaces. Nanoscale 2013, 5, 11058−11065. (16) Kabdulov, M. A.; Jansen, M.; Amsharov, K. Y. Bottom-Up C60 Fullerene Construction from a Fluorinated C60H21F9 Precursor by Laser-Induced Tandem Cyclization. Chem.Eur. J. 2013, 19, 17262− 17266. (17) Löffler, D.; Ulas, S.; Jester, S.; Weis, P.; Böttcher, A.; Kappes, M. M. Properties of Non-IPR Fullerene Films Versus Size of the Building Blocks. Phys. Chem. Chem. Phys. 2010, 27, 10671−10684. (18) Böttcher, A.; Löffler, D.; Bajales, N.; Ulas, S.; Machatschek, R.; Malik, S.; Brenner, P.; Kappes, M. M. Nanostructured Arrays of Stacked Graphene Sheets. Nanotechnology 2012, 23, 415302−415315. (19) Böttcher, A.; Weis, P.; Jester, S.-S.; Löffler, D.; Bihlmeier, A.; Klopper, W.; Kappes, M. M. C58 on HOPG: Soft-Landing Adsorption and Thermal Desorption. Phys. Chem. Chem. Phys. 2004, 6, 5213− 5217. (20) Löffler, D.; Bajales, N.; Cudaj, M.; Weis, P.; Lebedkin, S.; Bihlmeier, A.; Tew, D. P.; Klopper, W.; Böttcher, A.; Kappes, M. M. Non-IPR C60 Solids. J. Chem. Phys. 2009, 130, 164705−164711. (21) Löffler, D.; Weis, P.; Böttcher, A.; Kappes, M. M. Thermal Stability, Phase Segregation, and Sublimation of Cesium Fulleride Thin Films. Phys. Rev. B 2008, 77, 155405−155416. (22) Furche, F.; Ahlrichs, R.; Hättig, C.; Klopper, W.; Sierka, M.; Weigend, F. Turbomole. WIREs Comput. Mol. Sci. 2014, 4, 91−96. (23) Amsharov, K. Yu.; Kabdulov, M. A.; Jansen, M. HomoElimination of HF- An Efficient Approach for Intramolecular Aryl− Aryl Coupling. Chem.Eur. J. 2010, 16, 5868−5871. (24) Paserba, K. R.; Gellman, A. J. Kinetics and Energetics of Oligomer Desorption from Surfaces. Phys. Rev. Lett. 2001, 86, 4338− 4341. (25) Zacharia, R.; Ulbricht, H.; Hertel, T. Interlayer Cohesive Energy of Graphite From Thermal Desorption of Polyaromatic Hydrocarbons. Phys. Rev. B 2004, 69, 155406−155413. (26) Girifalco, L. A.; Lad, R. A. Energy of Cohesion, Compressibility, and the Potential Energy Functions of the Graphite System. J. Chem. Phys. 1956, 25, 693−697.

(27) Redhead, P. H. Thermal Desorption of Gases. Vacuum 1962, 12, 203−211. (28) van de Craats, A. M.; Warman, J. M.; Fechtenkötter, A.; Brand, J. D.; Harbison, M. A.; Müllen, K. Record Charge Carrier Mobility in a Room-Temperature Discotic Liquid-Crystalline Derivative of Hexabenzocoronene. Adv. Mater. 1999, 11, 1469−1472. (29) Aiharaa, J.; Fujiwaraa, K.; Harada, A.; Ichikawab, H.; Fukushimaa, K.; Hirota, F.; Ishidad, T. The C-H Bond Dissociation Energies of Polycyclic Aromatic Hydrocarbons. J. Mol. Struct. 1996, 366, 219−226. (30) Kundu, R.; Mishra, P.; Sekhar, B. R.; Maniraj, M.; Barman, S. R. Electronic Structure of Single Crystal and Highly Oriented Pyrolytic Graphite from ARPES and KRIPES. Physica B 2012, 407, 827−832. (31) Kera, S.; Yabuuchi, Y.; Yamane, H.; Setoyama, H.; Okudaira, K. K.; Kahn, A.; Ueno, N. Impact of an Interface Dipole Layer on Molecular Level Alignment at an Organic-Conductor Interface Studied by Ultraviolet Photoemission Spectroscopy. Phys. Rev. B 2004, 70, 085304−085310. (32) Ulas, S.; Bundschuh, S.; Jester, S.-S.; Eberl, C.; Kraft, O.; Hölscher, H.; Böttcher, A.; Kappes, M. M. Mechanical Properties of C58 Materials and Their Dependence on Thermal Treatment. Carbon 2014, 68, 125−137. (33) Mérel, P.; Tabbal, M.; Chaker, M.; Moisa, S.; Margot, J. Direct Evaluation of the sp3 Content in Diamond-Like-Carbon Films by XPS. Appl. Surf. Sci. 1998, 136, 105−110. (34) Nansé, G.; Papirer, E.; Fioux, P.; Moguet, F.; Tressaud, A. Fluorinated Carbon Blacks: Influence of the Morphology of the Starting Material on the Fluorination Mechanism. Carbon 1997, 35, 175−194. (35) Lee, Y.-S. Syntheses and Properties of Fluorinated Carbon Materials. J. Fluorine Chem. 2007, 128, 392−403. (36) Piraux, L.; Bayot, V.; Issi, J. P.; Dresselhaus, M. S.; Endo, M.; Nakajima, T. Electrical and Thermal Properties of FluorineIntercalated Graphite Fibers. Phys. Rev. B 1990, 41, 4961−4969. (37) Palchan, I.; Crespin, M.; Estrade-Schwarckopf, H.; Rousseau, B. Graphite Fluorides: An XPS Study of a New Type of C-F bonding. Chem. Phys. Lett. 1989, 157, 321−327. (38) Lang, N. D.; Kohn, W. Theory of Metal Surfaces: Induced Surface Charge and Image Potential. Phys. Rev. B 1973, 7, 3541−3550. (39) Tressaudt, A.; Moguet, F.; Flandroiss, S.; Chambon, M.; Guimon, C.; Nanset, G.; Papireri, E.; Gupta, V.; Bahl, O. P. On the Nature of C-F bonds in Various Fluorinated Carbon Materials: XPS and TEM Investigations. J. Phys. Chem. Solids 1996, 57, 745−751. (40) Dillon, E. P.; Crouse, C. A.; Barron, A. E. Synthesis, Characterization, and Carbon Dioxide Adsorption of Covalently Attached Polyethyleneimine Functionalized Single-Wall Carbon Nanotubes. ACS Nano 2008, 2, 156−164. (41) Cadman, P.; Scott, J. D.; Thomas, J. M. The Fluorination of the Surfaces of Elemental Carbon I: X-ray Photoelectron Spectroscopic Studies of Fluorinated Graphite. Carbon 1977, 15, 75−86. (42) Weis, P.; Gilb, S.; Gerhardt, P.; Kappes, M. M. A Time-of-Flight, Drift Cell, Quadrupole Apparatus for Ion Mobility Measurements. Int. J. Mass Spectrom. 2002, 216, 59−73.

7318


Thermal Decomposition of the Fullerene Precursor C60H21F9

Recommend Documents