fullerene 
 c60 is a cage molecule formed by 20 carbon 
 hexagons and 12 pentagons . the curvature of the molecule originates 
 from pentagons , which make fullerenes more reactive compared to planar 
 graphene , where the carbon atoms are bonded entirely in hexagons . 
 for example , c60 easily reacts with molecular hydrogen 
 at elevated conditions forming c60hx ( also called fulleranes ) with  x  
 approaching the maximal possible value of 60 , while hydrogenation of graphene is more difficult 
 and typically performed using atomic hydrogen . hydrogenation of fullerenes has been extensively studied over the 
 past two decades as a route to possible high density hydrogen storage 
 materials . recently published studies 
 alkali metal doped c60 hydrogenation have revived the interest 
 in this application of fulleranes . it is 
 known that prolonged exposure of c60 to molecular 
 hydrogen at 400450 c results first in formation of fulleranes 
 c60hx ( x up 
 to 52 ) , followed by cage fragmentation ( c59hx , c58hx , etc . ) 
 and collapse of cage structure with formation of a complex mixture 
 of hydrocarbon molecules . large fullerene fragments , for example , polycyclic aromatic hydrocarbons 
 ( pahs ) , have been identified as some of the final cage fragmentation 
 products . formation of pahs is expected under the conditions of hydrogenation 
 reaction since any dangling bond that appears in the process of fullerene 
 cage fragmentation will be immediately terminated by hydrogen . the hydrogenation of the c60 cage is only partly reversible , 
 especially for strongly hydrogenated samples . annealing of c60hx in inert 
 gas or vacuum results in c  h bond breaking , hydrogen release , 
 and at least partial recovering of pristine c60 . according 
 to thermogravimetric analysis ( tga ) experiments , the release of hydrogen 
 occurs at around 480550 c . therefore , it can be expected that hydrogen reaction 
 with c60 in this temperature region will proceed into direct 
 breakup of the fullerene cage rather than formation of fulleranes . recently , we reported that increasing the temperature for the hydrogenation 
 reaction above the limits of c  h bond stability in fulleranes 
 induces rapid collapse of the c60 cage structure with formation 
 of unidentified carbon / hydrogen products . the main aim of this previously 
 published study was to observe hydrogen - driven collapse of c60 inside carbon nanotubes . unexpectedly , 
 we also found several single layered graphene sheets among  super - hydrogenated peapods which could form only as a result of hydrogen reaction with 
 carbon materials available in studied sample ( fullerenes and carbon 
 nanotubes ) . this intriguing observation motivated more detailed study 
 of hydrogen - driven unzipping and collapse of c60 at around 
 550 c . here , we report detailed study of hydrogen - driven 
 unzipping of 
 the c60 cage at elevated conditions ( 500550 c , 
 50 bar h2 pressure ) performed on both bulk powder and thin 
 film samples . the reaction of c60 with hydrogen at higher 
 temperatures leads to direct etching of the c60 cage . the 
 fullerene molecule collapse induced by hydrogen is found to result 
 in the formation of  nano - graphene annealing c60 thin films 
 in hydrogen at elevated conditions results in the formation of graphene 
 islands with spectral characteristics similar to previously observed 
 coronene oligomer samples . on the basis of our 
 results , we suggest a possible mechanism of the cage opening , fragmentation , 
 and the formation of nanographenes . fullerene c60 powder ( > 99.9% , mer corporation , 
 tucson , 
 arizona , usa ) was placed into an alumina container , sealed in a stainless 
 steel reactor , and subjected to annealing in hydrogen at a pressure 
 of about 50 bar and temperatures of 500550 c for 624 
 h. the weight of samples decreased as a result of the reaction of 
 c60 with hydrogen due to evaporation of light hydrocarbons . maximal weight loss of about 50% was observed for powder samples annealed 
 under hydrogen at 550 c for 24 h. mass spectrometry was performed 
 with a matrix - assisted laser desorption ionization time - of - flight 
 ( maldi - tof ) mass spectrometer ( voyager de - str , applied biosystems , 
 carlsbad , california , usa ) . nuclear magnetic resonance ( nmr ) spectra 
 of the soluble fractions were recorded on a varian unity ( h at 399.97 mhz , c at 100.58 mhz ) or a varian inova 
 ( h at 499.93 mhz , c at 125.71 mhz ) spectrometer . the chemical shift regions are reported using the residual solvent 
 signal as an indirect reference to tms ( h : chcl3 : 7.26 , toluene - d8 : 2.03 ( ch3 ) , c : cdcl3 : 77.0 ppm ) . chemical shift simulations 
 were performed using mestrenova v6.0.2 software ( mestrelab research 
 s.l . x - ray photoelectron spectra ( xps ) were recorded with a 
 kratos axis ultra electron spectrometer equipped with a delay line 
 detector . a monochromated al kr source operated at 150 w , a hybrid 
 lens system with a magnetic lens , providing an analysis area of 0.3 
 mm  0.7 mm , and a charge neutralizer were used for the measurements . raman spectra were recorded with 514 nm laser excitation using a renishaw 
 invia raman spectrometer . ft - ir spectra were collected using a bruker 
 ifs 66 v / s using transmission mode on finely ground kbr diluted samples . the high - resolution tem images were collected on a jeol jem-2200fs 
 with a corrector of spherical aberrations ( cs  0.005 mm ) . 
 to minimize the destructive influence of electron irradiation , all 
 samples were analyzed at 80 kv acceleration voltage and the minimal 
 possible electron illumination time . the c - afm measurements 
 ( veeco multimode v , veeco , usa / nanonis , 
 specs , switzerland ) were carried out at room temperature in an ambient 
 pressure environmental controlled chamber of 6.5l in volume flushed 
 with a flow of 99.99% of argon ( scientific argon , messer , slovenia ) 
 at a constant flow rate of 0.2 l / min . during measurement of iv curves , the doped diamond tip ( nanosensors dt - nchr , nanoworld ag , switzerland ) 
 was brought into contact at a constant force of 0.2 nn . resistances 
 were estimated by performing current  voltage curve measurements 
 at different points on the c60 transformed nanoislands . the islands resistances were estimated based on differential profile 
 measurement over islands and gold substrate . the c - afm tip cleanliness 
 and contact reproducibility were checked on au substrate before and 
 after collecting data on islands to ensure good contact and linearity 
 of the iv curves . the atomic force microscopy ( afm ) imaging was carried 
 out by conventional tapping mode , while electrical measurements were 
 performed under constant force in afm contact mode . prior to contact , 
 a slow approach soft landing was performed . this approach allows the 
 tip to avoid jumping into contact , which has a high risk of damaging 
 soft materials such as c60 nanoislands . iv measurements were performed by ramping sample bias from 
 1 v to + 1 v , while monitoring the current . the upper limit 
 of the current was set to the a range by connecting a limiting 
 resistor of 1 m  in series with sample to protect it and the 
 tip from electron migration . the scanning tunneling microscopy 
 ( stm ) topographic images were 
 collected by recording the tip height at a constant sample bias . typical 
 imaging parameters are 2 v and 0.2 na for bias voltage and tunneling 
 current respectively . it was anticipated from our previous 
 experience that hydrogen - driven c60 collapse results in 
 complex mixtures of products that are difficult to analyze . the light 
 hydrocarbons formed as a result of hydrogen treatment of c60 would evaporate from the samples leaving only relatively large hydrocarbons 
 with melting points higher than the temperature of annealing . the 
 residual material is particularly difficult for analysis as it is 
 composed mostly of hydrocarbon molecules which are too large to be 
 soluble in common organic solvents . in line with expectations , the powder collected after 18 h annealing of c60 at 550 
 c had a weight loss of about 50% due to sublimation of reaction 
 products . the samples obtained after hydrogen annealing at 500 and 
 550 c were mostly not soluble . therefore , we performed characterization 
 of as - prepared powder and then extracted the soluble fraction for 
 separate analysis ( see next section ) . raman spectra recorded 
 from powders obtained by hydrogen annealing 
 at 500 and 550 c are shown in figure 1 . it should be noted that fulleranes c60hx obtained in our previous studies using lower hydrogenation 
 temperatures typically exhibited very strong luminescence background , 
 and their raman spectra could not be recorded with 514 , 633 , 780 nm 
 lasers . figure 1 shows that some luminescence 
 background is still present in the spectra of 500 c sample and 
 completely disappeared in 550 c sample indicating the absence 
 of hydrogenated fullerenes in these samples . figure 1 shows two broad features in the spectral region typical for 
 g and d bands of amorphous carbon . however , unlike true amorphous 
 carbon , the spectra exhibit several other peaks , while the g- and 
 d - bandlike features clearly consist of several components . this type 
 of spectrum is typical for larger pahs molecules ; see for example 
 spectra of c78 and c114 in ref ( 23 ) . analysis of raman spectra 
 from various progressively larger pahs was reported for example in 
 a study by castiglioni et al . which demonstrated 
 how spectral features of  nano - graphenes ( see also 
 ref ( 25 ) ) are transformed 
 into g- and d - bands when the molecule size is increased . raman spectra 
 recorded from carbon deposits obtained by hydrogen 
 annealing of c60 at 500 and 550 c . more detailed information about composition of 
 samples can be extracted 
 from fourier transform infrared ( ftir ) spectra . figure 2 shows spectra recorded from powders hydrogenated at 450 c 
 ( mostly hydrogenated fragmented fullerenes and some pahs ) and samples hydrogenated at 500 and 550 c . strong broadening of all spectral features and appearance of new ones 
 are obvious for the samples hydrogen annealed at higher temperatures . 
 especially clear is the difference in the region of c  h vibrations 
 ( 27003200 cm ) . in figure 2 , the arrow points at the major peak at 3089 cm with a smaller component at 3083 cm , see table 1 . the peaks are 
 in the spectral region typical for c  h of pah molecules and 
 are assigned to large fragments of the c60 cage formed 
 as a result of cage unzipping . ftir spectra recorded from ( a ) powder 
 samples obtained by hydrogen 
 annealing of c60 at 500 and 550 c ; ( b ) powder samples 
 obtained by hydrogen annealing at 500 c with duration of treatment 
 6 and 24 h. the spectral region 28003000 
 cm also 
 shows a very complex combination of c  h peaks with several 
 components distinguished ( see table 2s and deconvolution plots in supporting information ) ; these peaks are distinctly 
 different compared to peaks found in this region for highly hydrogenated 
 c60hx . as noted above , formation 
 of c60hx as a major product 
 is not expected at 500550 c as these temperatures are 
 above the stability limit of c therefore , 
 most of the peaks observed in the ftir spectra should be assigned 
 to various ( noncage ) hydrocarbon molecules . the collapse of 
 the c60 cage structure is relatively 
 rapid as confirmed by the ftir spectra recorded from powder samples 
 obtained by hydrogen annealing at 500 c for 6 and 24 h , shown 
 in figure 2b . the spectra are almost identical , 
 which demonstrates that reaction was mostly completed after 6 h. raman 
 and ftir spectra prove that the samples obtained by hydrogen annealing 
 do not consist anymore of fullerenes or their hydrogenated modifications . 
 the spectra point to formation of larger hydrocarbon molecules . however , the samples are even more complex as revealed by high 
 resolution tem images ; see figure 3 . the analysis 
 of tem data shows that the sample prepared at 550 c is mostly 
 composed of few - layered ( 15 layers ) defect graphene sheets . the fft shows two circles of spots with d - spacings 
 2.27 and 1.26  , which correspond to graphene lattice while the 
 distance between the layers is around 3.5  . the inset of figure 3 shows a zoomed part of the sheet which exhibit 
 some traces of periodicity , probably from the parent c60 lattice , as the size of spots which originate from corrugation of 
 the 2d surface geometry corresponds approximately to the diameter 
 of fullerene molecules . the graphene sheets can be formed as a result 
 of hydrogen - driven c60 unzipping either by direct fusion 
 of unzipped molecules or by reaction between c60 fragments 
 formed as a result of cage collapse . the clues to understanding of 
 the mechanism of hydrogen - driven c60 unzipping and fragmentation 
 can be obtained by analysis of the soluble part of hydrogen - annealed 
 samples ; see next section . high resolution tem recorded from the sample 
 obtained by hydrogen 
 annealing of c60 at 550 c : ( a ) image showing defect 
 few - layered graphene sheets , ( b ) fft image , ( c ) zoomed area of graphene 
 sheet . solutions extracted from samples prepared by 
 hydrogen annealing 
 in visible and uv light . from the left to the right : dmf ( 500 and 
 550 c samples ) , toluene ( 500 and 550 c ) , acetone ( 500 
 and 550 c ) , and ethanol ( 500 c sample ) . the solubility of samples produced from c60 by hydrogen annealing at 500 and 550 c were tested for several 
 common solvents , see figure 4 . the samples 
 were subjected to sonication in toluene , acetone , dimethylformamide 
 ( dmf ) , and ethanol for a period of about 14 h. the best solubility 
 was observed for dmf and toluene ; no solubility was observed for ethanol . it is clear also that the sample prepared at lower temperature ( 500 
 c ) contains a larger fraction of soluble products . strong luminescence 
 is typical for many pah molecules ; for example , perylene has very 
 bright luminescence , whereas fullerane samples ( c60hx with average composition x  36 ) do not show visible luminescence in uv light when dissolved 
 in the same solvents . this further supports the assumption that fulleranes 
 are not present in the samples of the present study . the solutions 
 were used for mass spectrometric characterization and nmr analysis . 
 using extraction with dmf and evaporation of solvent , we were able 
 also to prepare several milligrams of powder which was used to record 
 ftir spectra , figure 5 . comparing these spectra 
 with spectra of parent powder allows one to distinguish contributions 
 from soluble and nonsoluble fractions . ftir spectra recorded 
 from dmf extracts prepared using c60 powder annealed in 
 hydrogen at 500 c with variation of treatment 
 duration . the reference spectrum from mostly c60hx sample prepared at 450 c the spectra shown in figure 5 are 
 clearly 
 distinct from spectra of c60hx , once again proving that no hydrogenated c60 is present 
 in these samples . this difference is particularly clear in the region 
 of c  h vibrations , where the strongly hydrogenated c60 sample ( synthesized by reaction with h2 gas at 400 c ) 
 showed four major peaks at 2826 , 2858 , 2909 , and 2946 cm . the main peaks found in dmf extracts of the sample hydrogen annealed 
 at 500 c for 18 h are found at 2858 , 2898 , 2925 , and 2955 cm ; see all peak positions found for these samples in 
 table 1s , supporting information . figure 5 also shows that prolonging the duration of hydrogen 
 annealing does not result in significant changes in composition of 
 products : already after 6 h of treatment all fullerenes are transformed 
 into hydrocarbons . the relative intensity of c  h vibrations 
 in dmf extracts 
 is significantly lower compared to the spectrum obtained for fulleranes . indeed , the fullerane sample shown in figure 5 has at least two - thirds of carbon atoms hydrogenated ( the 
 maximal possible c / h ratio is 1:1 for c60h60 fullerane ) , while for planar hydrocarbon molecules only carbon atoms 
 situated on the edges of molecules are hydrogenated . therefore , for 
 larger planar pah compounds , weaker c  h peaks can be expected 
 than for smaller pahs . it is also very interesting to compare 
 the spectra of the dmf - extracted 
 powders with those of the parent powder samples of figure 2 . the main difference is the absence of 3039 cm and 3083 cm peaks in the spectra 
 of the extracted fraction . these peaks can be confidently assigned 
 then to c  h vibrations of insoluble larger pahs which are basically 
 fragments of graphene sheets (  nanographenes  ) terminated 
 by hydrogen atoms . this suggestion is also compatible with numerous 
 peaks observed 
 in the 7001000 cm region of ftir spectra . considering the complex nature of samples composed of at least several 
 main products , precise evaluation of the molecular composition using 
 ftir is not possible . however , one can note that several larger pahs 
 consisting of hexagons and pentagons ( with geometry that corresponds 
 to c60 cage fragments ) are known to exhibit peaks in the 
 this spectral region . for example , the ftir spectrum of benzo[k]fluoranthene 
 has major peaks at 746 cm , 823 cm , and 883 cm , and the related 
 benzo[j]fluoranthene has peaks at 739 cm , 769 
 cm , and 815 cm assigned to 
 out - of - plane ch bending vibrations typical for molecules containing 
 aromatic or condensed aromatic rings . spectra recorded from the hydrogen 
 collapsed c60 samples showed peaks at similar positions , 
 747757 cm , 820823 cm , and 866873 cm ( table 2s , supporting information ) . the mass spectrum recorded from the sample obtained 
 by hydrogen annealing at 500 c shows a very complex mixture 
 of products with masses below 600 amu , figure 6 . the molecules lighter than 250 amu are almost absent in 
 the spectrum , which is the expected result : light hydrocarbons with 
 low melting points evaporate from the powder samples in the process 
 of hydrogen annealing . remarkably , this spectrum shows no peaks from 
 pristine c60 ( 720 amu ) or hydrogenated fragmented fullerenes 
 which are typically found in the samples hydrogenated at lower temperatures 
 ( 400450 c ) after prolonged reaction durations . maldi - ms 
 spectrum of dmf extract from sample hydrogen annealed 
 at 500 c with hypothetical assignment of some major peaks . the spectrum shown in figure 6 exhibits 
 a very distinct pattern with strongest peaks in the following trend : 
 peaks with c2 ( 24 amu ) difference and with c2h2 difference ( 26 amu ) . these peaks are interpreted as 
 chain products of c60 cage fragmentation with a consequently 
 smaller size of fragments . it should be emphasized that the peaks 
 observed in this spectrum represent true composition of products present 
 in our sample and can not be assigned to fragmentation of larger molecules 
 induced in the process of ms recording . this conclusion is supported 
 by a separate experiment where we attempted to separate reaction products 
 obtained by hydrogen annealing of c60 using sublimation 
 with a temperature gradient inside of vacuum sealed glass tube . as 
 a result of this experiment , we observed differently colored areas 
 in the reaction tube with orange products accumulated in the region 
 of higher temperatures , yellowish products in the colder part , and 
 some microcrystals precipitated in the coldest part of the tube . the 
 microcrystals were unambiguously identified using raman spectroscopy 
 as pyrene , while yellow and orange fractions of the samples exhibited 
 luminescence background , which do not allow recording of raman spectra . maldi - ms from orange and yellow sublimation products indeed showed 
 that they consist of heavier and lighter molecular species ( see figure 
 2s in supporting information ) with major 
 ms peaks corresponding to the overall composition of parent sample 
 which was similar to the spectrum shown in figure 6 . this experiment proves that major peaks observed in the 
 mass spectra of our samples can be considered mostly as true fragments 
 of the c60 cage , not a result of fragmentation induced 
 in the process of ms recording . it is impossible to identify all peaks 
 observed in figure 6 with certain molecules , 
 but analysis of trends allows one to make reasonable suggestions about 
 possible fragmentation path of the c60 cage in the hydrogen 
 driven collapse process . it is logical to suggest that most 
 abundant products of c60 fragmentation and collapse will 
 consist only of pentagons and hexagons ; 
 these molecules should also be molecular fragments of the fullerene 
 cage terminated by hydrogen atoms . below we propose a possible model 
 of c60 fragmentation which is speculated to explain the 
 sequence of peaks found in the ms spectra ( see figure 7 ) . the model suggests direct unzipping of the c60 molecule with formation of mostly flat molecules . the cuplike molecules 
 ( e.g. , corrannulene ) are less likely to appear as major products due 
 to their lower stability caused by strain . the molecule 2 represents flat hydrocarbon consisting 
 of 60 carbon atoms which can be obtained from c60 by consequent 
 breaking of some c  c bonds on pentagon  hexagon edges . 
 one of the possible intermediate products which illustrate a pathway 
 of unzipping is shown in figure 7 as molecule 1 . most of the peaks observed in the mass spectra of our samples 
 now can be explained if we suggest that they are formed by the breakup 
 of molecule 2 on smaller fragments which consist of pentagons and 
 hexagons ; one example of such fragmentation is shown in figure 7 and results in formation of molecules 3 and 4 . figure 6 shows possible 
 assignment of peaks found in our spectra using various fragments of 
 molecule 2 produced by cage unzipping of c60 . of course , 
 more than one fragmentation pathway is possible in the process of 
 hydrogen driven fragmentation of c60 . detailed analysis 
 of the hydrocarbon species present in solvent 
 extracted samples ( e.g. , using separation by hplc ) was so far not 
 successful due to the very complex sample compositions and , for chromatography 
 purposes , poor solubility . however , some of evidence supporting the 
 suggested fragmentation model was obtained using nmr analysis of soluble 
 reaction products from two reaction temperatures ; the parent powder 
 of batch 1 was produced at 500 c and that of batch 2 at 550 
 c . the h nmr analysis indicates the presence 
 of aromatic 
 and polyaromatic structure elements as well as branched and nonbranched 
 aliphatic hydrocarbons , some of which may be attached to aromatic 
 units ( figure 8 , lowest trace ) . in both batches , the relative proportion of vinylic / alkynylic hydrogens in 
 conjugated alkene  alkene / alkyne or arene - alkene / alkyne structural 
 elements ( 57 ppm ) is lower than the relative proportion of 
 hydrogens in arene ( 68 ppm ) and aliphatic ( 04 ppm ) 
 structural elements . the relative proportion of aromatic structures 
 is lower in batch 2 than in batch 1 , and the observed chemical shifts 
 are quite consistent with what should be expected from a mixture of 
 the compounds detected in the maldi - ms analyses ( simulated , middle 
 trace of figure 8) . the broad signals in the 
 aliphatic region indicate the presence of compounds even further reduced . assuming hydrogenation at the more reactive positions of the set of 
 compounds used for the generation of the middle trace , the chemical 
 shift simulation of the mixture gives a spectrum even closer to the 
 experimental ones , considering only the chemical shifts as the simulations 
 were carried out for unlikely mixtures with one entity of each compound 
 and toluene . the further reduced compounds , having sp centers , 
 should be more soluble in used solvents than the more planar sp pah degradation products , and hence the relative proportion 
 detected by h nmr is not reflecting the composition of 
 the entire sample . moreover , it is not unlikely that certain unsaturated 
 degradation products undergo oligomerization or rearrangement processes , 
 leading to even more complex mixtures . such functional groups may form 
 by oxidation of terminal allylic or certain benzylic positions in 
 air . both oligomerization and partial aerobic oxidations are likely 
 during the sonication - assisted extraction and subsequent solvent evaportations , 
 and would explain the ( poor ) solubility and macroscopic  sticky  
 appearance of the materials . lower trace , left panel : h nmr in 
 toluene - d8 of toluene - soluble fraction 
 of  batch 2  , 
 produced at 550 c . middle trace : simulated chemical shifts for 
 the pah structures suggested from the maldi - ms data . upper trace : 
 simulated chemical shifts for partly reduced pahs ( depicted in panel 
 right ) . simulated signals in red are for toluene ( extraction solvent 
 for  batch 2  and likely the most soluble arene component 
 of the sample ) . the presence of aliphatic 
 structural elements is evident also from 
 the c nmr spectrum of batch 2 ( not shown ) , with four 
 signals in the region 1035 ppm . arene signals from the sample , 
 except for the signal originating from the very small quantity of 
 unreacted c60 ( hardly distinguishable on tlc ) are expected 
 to be buried in the solvent signal . summarizing the nmr data , aromatic 
 and polyaromatic structural elements are present in studied samples , 
 and the sample annealed in hydrogen at higher temperature shows larger 
 proportion of these elements as expected in fragmentation pathway 
 shown in figures 6 and 7 . in conclusion of this section , the soluble fraction of hydrogen - collapsed 
 c60 samples represents smaller part of the as - produced 
 powders but allows the proposal of a pathway for the fragmentation 
 of fullerene into pah entities with carbon skeletons that correspond 
 to fragments of the cage structure . more detailed analysis of the 
 mixture of molecules is complicated not only by complex combinations 
 of such fragments in the studied samples but also by reaction of these 
 fragments with each other . the larger part of our samples consist 
 of nonsoluble molecules and few - layered graphene sheets revealed in 
 our samples , which points to reactions that include fusion of the 
 planar c60 fragments into nanographenes of progressively 
 larger size . the few - layered graphene 
 sheets found in powder samples of hydrogen - collapsed c60 indicated a hypothetical possibility to synthesize single - layered 
 graphene using c60 as a precursor . we suggested that using 
 sufficiently thin film of c60 for hydrogen annealing reaction 
 may result in surface - assisted formation of single - layered graphene 
 instead of few - layered sheets randomly formed in the powder . to test 
 the hypothesis , c60 films of different thickness were deposited 
 on gold - plated mica substrates and subjected to hydrogen annealing 
 at 550 c at the conditions similar to those used for powders . a significant part of the material was expected to evaporate from 
 the initial films in the form of gaseous hydrocarbons , similar to 
 experiments with bulk powders described above . analysis of the films 
 after hydrogen treatment revealed that at least part of carbon material 
 formed because hydrogen - driven fullerene fragmentation had preserved 
 as thin film on the gold surface . the raman spectra recorded 
 from the films revealed a significant difference compared to the spectra 
 collected form bulk powders , and the difference is especially strong 
 for the thinnest films ; see figure 9 . the film 
 with an initial thickness of 30 nm showed raman spectra similar to 
 those of bulk powder samples : only two broad but rather asymmetric 
 features which correspond to multicomponent d- and g - bands of larger 
 pahs or defected graphitic carbon . the peaks become less broad in 
 the spectra recorded from film with an initial thickness of 10 nm , 
 with some clearly distinct peaks that indicate a larger fraction of 
 nanographenes and hydrocarbon molecules . raman spectra recorded 
 from c60 films of various initial 
 thicknesses subjected to hydrogen annealing at 550 c . the thinnest c60 films 
 ( initial thickness of 3 nm ) consisted 
 of only 45 monolayers of c60 molecules prior the 
 reaction . raman spectra recorded from this film after hydrogen annealing 
 showed strikingly different features with a number of rather sharp 
 peaks . it should be noted that many accumulations had to be used to 
 collect the spectra due to small thickness of resulting films and 
 weak signals . the spectra exhibit three distinct and rather sharp 
 peaks instead of a broad g - band feature , whereas d - band region exhibits 
 several peaks spread over a broader spectral range . the sharp peaks 
 found in the spectra are indicative of a more homogeneous reaction 
 product formed in the thinnest films . according to the data obtained 
 for bulk powders , it is expected also that at least one - half of the 
 material is evaporated from the films in the form of gaseous hydrocarbons . the raman spectra recorded from thinnest hydrogen annealed films 
 show striking similarity to the previously reported raman spectra 
 of coronene annealing products ( coronene oligomers ) . the spectrum 
 recorded from bulk toluene - insoluble sample produced by heat treatment 
 of coronene is shown as a reference in figure 9 . the detailed molecular structure of 
 coronene oligomers is not completely clear , but likely they consist 
 of linear or possibly branched coronene chains of various length . therefore , we repeated 
 the same set of experiments , again using three c60 films 
 with different thicknesses to verify reproducibility of the thickness - dependent 
 effect in the hydrogen annealing treatment . the second set of 
 experiments showed the same trends : very broad 
 spectral features were observed for films with higher thickness and 
 many sharp peaks for thinnest films . the peak positions found for 
 thinnest films almost exactly coincide with the spectra of coronene 
 oligomers . coronene and dicoronylene 
 ( coronene dimer ) would sublimate away from our samples at the temperature 
 of reaction , and only larger molecules could survive annealing and 
 preserve on the substrate . therefore , the raman peaks are likely to 
 originate from larger nanographenes formed by fusion of coronene - like 
 pah molecules formed on the substrate at 550 c . it is interesting 
 that coronene is not a fragment of the c60 cage structure 
 as it consists of six hexagons fused into a ring . therefore , the coronene 
 oligomers can not be formed only as a result of c60 unzipping 
 and most likely formed in surface - assisted reactions between fullerene 
 fragments . this suggestion is confirmed by observation of spectral 
 features from coronene oligomers only in the thinnest of hydrogen - annealed 
 c60 films . the collapse of c60 molecules 
 in thin films subjected 
 to hydrogen annealing at 550 c was confirmed also using xps . the spectrum of a pristine 13 nm thick film was typical for 
 c60 spectrum with the c1s peak at 284.5 ev . the film was 
 not continuous as evident from detection of 29.9 atom % of gold ( from 
 material of substrate ) . the proportion of gold increased after hydrogen 
 annealing of the film to 55.5 atom % . that is in good correlation 
 with experiments on powder samples described above where about 50% 
 mass loss was observed as a result of the hydrogen treatment . the 
 new nanographene carbon phase is characterized by a shift of the main 
 c1s peak to 284.3 ev and new weaker peaks found at 285.7 ev ( due to 
 c  h ) and 287.1 ev , which is possibly a result of partial sample 
 oxidation after exposure to air ( see supporting 
 information ) . the characterization of nanographene islands 
 was performed also 
 using afm and stm , and the conductivity of individual islands was 
 measured . the characterization 
 of hydrogen - annealed films was performed using afm and stm , and conductivity 
 of individual islands was measured . figure 10 shows afm and stm topographic images of islands formed by hydrogen 
 annealing of thinner c60 film deposited on the gold surface . the average size of islands is 50 nm and height is about 
 10 nm . we selected several islands to perform local electrical conductivity 
 measurements at 15 spots on each islands , figure 11 . an ohmic behavior at low bias was observed with no saturation 
 at higher voltages indicating a metallic behavior that is completely 
 different from the pristine parent c60 material which is 
 known to be a semiconductor . topographic afm images in tapping mode : ( a ) 
 nanoislands formed 
 by hydrogen collapse of c60 thin film ( b ) individual nanoislands 
 before exfoliation ( c ) after exfoliation ; ( d ) stm image of island 
 on the au(111 ) surface . scanning parameter are 0.5 na and 0.6 v for 
 tunneling current and sample bias respectively . local voltage - current graph measured on nanoisland ( shown in the 
 inset ) . the curves exhibit linear behavior , indicating the metallic 
 character for the nanoislands . to protect the island and the afm tip 
 from atom migration , the maximum current was set to 0.1 microamp using 
 a series resistor of 10 m. moreover , the island can be exfoliated using an afm tip at 
 a relatively 
 low lateral force of 100 n as shown in figure 10b , c , indicating weak vertical bonding and evidence of multilayered 
 structures . in this experiment , the tip is moved laterally about 50 
 nm away from the edge of a designated nanoisland , and then the feedback 
 loop is interrupted and the tip is brought 510 nm above the 
 gold surface . as scanning is resumed ( without feedback control ) , the 
 tip suffers a torque due to shear stress as it reaches the island 
 about 510 nm below its surface . this torque causes the afm 
 tip to exert a horizontal force as it scans through the island . bond 
 rupturing occurs when this force is larger than bonding forces and 
 finally exfoliating the nanoisland . we find also that the island formed 
 by exfoliation maintained its metallic characters similar to its parent 
 island . the change in electrical character of c60 materials 
 after the hydrogenation process has prompted us to conduct stm measurements 
 as shown in figure 10d . unlike the case of 
 pure c60 islands of similar height , which could not be 
 imaged with stm , the imaging of transformed materials can be established 
 at low bias of 0.2 v and tunneling current of 0.1 na . the average 
 size of the islands is about 30 nm with a height of 15 nm . therefore , 
 it can be concluded that thin c60 films exposed 
 to hydrogen treatment at 550 c have been transformed into few - layered 
 nanographene islands . the afm / stm results are also in agreement with 
 raman spectra recorded from thin films which are typical for large 
 hydrogen - terminated nanographene samples . the experiments performed here could serve as a proof of concept 
 for the possibility to prepare graphene films by hydrogen annealing 
 of thin fullerene film . simple calculation shows that the number of 
 carbon atoms in a monolayer c60 film is sufficient only 
 for the formation of double - layered graphene film . considering that 
 some carbons are evaporated in the process of hydrogen annealing in 
 form of light gaseous hydrocarbons , it is reasonable to suggest that 
 the synthesis of single - layered graphene might be possible in the 
 future using a c60 precursor and hydrogen annealing . the 
 advantage of such a synthesis method would be the low temperatures 
 required for c60 film deposition , the variety of possible 
 substrates available , and the homogeneous hydrogen termination of 
 graphene edges .
annealing of c60 in hydrogen 
 at temperatures above the 
 stability limit of c 
 h bonds in c60hx ( 500550 c ) is found to result in direct 
 collapse of the cage structure , evaporation of light hydrocarbons , 
 and formation of solid mixture composed of larger hydrocarbons and 
 few - layered graphene sheets . 
 only a minor part of this mixture is 
 soluble ; this was analyzed using matrix - assisted laser desorption / ionization 
 ms , fourier transform infrared ( ftir ) , and nuclear magnetic resonance 
 spectroscopy and found to be a rather complex mixture of hydrocarbon 
 molecules composed of at least tens of different compounds . 
 the sequence 
 of most abundant peaks observed in ms , which corresponds to c2h2 mass difference , suggests a stepwise breakup 
 of the fullerene cage into progressively smaller molecular fragments 
 edge - terminated by hydrogen . a simple model of hydrogen - driven c60 unzipping 
 is proposed to explain the observed sequence of 
 fragmentation products . the insoluble part of the product mixture 
 consists of large planar polycyclic aromatic hydrocarbons , as evidenced 
 by ftir and raman spectroscopy , and some larger sheets composed of 
 few - layered graphene , as observed by transmission electron microscopy . 
 hydrogen annealing of c60 thin films showed a thickness - dependent 
 results with reaction products significantly different for the thinnest 
 films compared to bulk powders . 
 hydrogen annealing of c60 films with the thickness below 10 nm was found to result in formation 
 of nanosized islands with raman spectra very similar to the spectra 
 of coronene oligomers and conductivity typical for graphene .