C60 Surface Research Articles

Multiple Additions of Palladium to C60

Density functional theory calculations on exohedral metallofullerenes Pd n C60 show that the palladium–fullerene bond energy remains essentially constant for n=1−6. A novel Pd2(η2−C60) structure with the two metal atoms bridging over a six‐membered ring has been identified as the most stable arrangement of two palladium atoms on the surface of C60, although entropy considerations suggest that both isolated atoms and weakly bonded metal aggregates may exist in equilibrium. Both metal atoms benefit from η2 coordination at (6–6′) junctions as well as some metal–metal interaction. Binding of Pd atoms to the fullerene is preferred over palladium dimerization.

Structural and optical properties of highly hydroxylated fullerenes: Stability of molecular domains on the C60 surface

The excitation spectra and the structural properties of highly hydroxylated C(60)(OH)(x) fullerenes (so-called fullerenols) are analyzed by comparing optical absorption experiments on dilute fullerenol-water solutions with semiempirical and density functional theory electronic structure calculations. The optical spectrum of fullerenol molecules with 24-28 OH attached to the carbon surface is characterized by the existence of broad bands with reduced intensities near the ultraviolet region (below approximately 500 nm) together with a complete absence of optical transitions in the visible part of the spectra, contrasting with the intense absorption observed in C(60) solutions. Our theoretical calculations of the absorption spectra, performed within the framework of the semiempirical Zerner intermediate neglect of diatomic differential overlap method [Reviews in Computational Chemistry II, edited by K. B. Lipkowitz and D. B. Boyd (VCH, Weinheim, 1991), Chap. 8, pp. 313-316] for various gas-phase-like C(60)(OH)(26) isomers, reveal that the excitation spectra of fullerenol molecules strongly depend on the degree of surface functionalization, the precise distribution of the OH groups on the carbon structure, and the presence of impurities in the samples. Interestingly, we have surprisingly found that low energy atomic configurations are obtained when the OH groups segregate on the C(60) surface forming molecular domains of different sizes. This patchy behavior for the hydroxyl molecules on the carbon surface leads in general to the formation of fullerene compounds with closed electronic shells, large highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps, and existence of an excitation spectrum that accounts for the main qualitative features observed in the experimental data.

Clustering of Ti on a C60 Surface and Its Effect on Hydrogen Storage

Recent efforts in finding materials suitable for storing hydrogen with large gravimetric density have focused attention on carbon-based nanostructures. Unfortunately, pure carbon nanotubes and fullerenes are unsuitable as hydrogen storage materials because of the weak bonding of the hydrogen molecules to the carbon frame. It has been shown very recently that coating of carbon nanostructures with isolated transition metal atoms such as Sc and Ti can increase the binding energy of hydrogen and lead to high storage capacity (up to 8 wt % hydrogen, which is 1.6 times the U.S. Department of Energy target set for 2005). This prediction has led to a great deal of excitement in the fuel cell community [see The Fuel Cell Review, http://fcr.iop.org/articles/features/2/7/4]. However, this prediction depends on the assumption that the metal atoms coated on the fullerene surface will remain isolated. Using first-principles calculations based on density functional theory, we show that Ti atoms would prefer to cluster on the C60 surface, which can significantly alter the nature of hydrogen bonding, thus affecting not only the amount of stored hydrogen but also their thermodynamics and kinetics.

Gold cluster formation on a fullerene surface

The growth of Au clusters on a fullerene thin film was investigated by in situ photoelectron spectroscopy in the ultraviolet (UPS) and x-ray (XPS) regime. Due to its highly corrugated surface fullerene films provide a wide range of bonding sites which could be exploited as molecular templates and serve to create a cluster superstructure. To gain insight into the fullerene-Au interaction two types of experiments were performed: (i) the deposition of Au on a fullerene surface, and (ii) the deposition of fullerenes on a Au surface. In both experiments an island growth mode is observed. The deposition of submonolayer amounts of C60 onto a gold film showed that the main interaction of the two species is due to chemisorption of the first C60 monolayer. In addition a constant band bending in the fullerene film is detected, but the UPS valence-band spectra show that there is no charge transfer from the Au to the C60 lowest unoccupied molecular orbital. In the reverse experiment, the cluster growth of Au on the corrugated C60 surface, the analysis of the Au core level does not reveal a specific bonding or nucleation site for Au atoms and clusters. This is in contrast to observations with Si clusters, which prefer to reside in the troughs between the fullerene molecules. The Au clusters grow continually from a size of about 55 atoms for the early stages of growth up to 150 atoms for the deposition of a nominal coverage of 1.5 nm. These data are derived from an analysis of the d-band splitting and the Au 4f core-level shift due to delayed photohole relaxation. The thermal stability of the Au-clusters-covered fullerene film was investigated by annealing in situ up to temperatures of 650 degrees C. For temperatures up to 450 degrees C a continuous growth of the clusters is detected, which is accompanied by a slight drop in Au concentration in the range of XPS for annealing temperatures higher than 350 degrees C. This may be due to a ripening of the clusters. The presence of Au apparently delays fullerene sublimation. The film shows a very good thermal stability and even after annealing at 650 degrees C there is still a fullerene film detectable in the photoelectron spectroscopy spectra.

A New Route to an Endohedral Fullerene by Way of σ-Framework Transformations

Abstract The molecular surgical approach has been shown to be applicable to preparation of an endohedral fullerene. Thus, an orifice large enough to insert an atom or molecule has been constructed on the surface of fullerene C60. The orifice has been enlarged to a 13-membered ring by insertion of a sulfur atom on the rim, and 100% encapsulation of molecular hydrogen has been attained. Then, the size reduction of the orifice, without the loss of incorporated hydrogen, and complete closure of the orifice have been achieved to afford a new endohedral fullerene, H2@C60.

Tether-Directed Multiple Functionalization of Fullerene[60

Chemical functionalization of fullerenes is the first step to investigate their intriguing properties and applications. The versatile functionalization methods have enabled fullerenes to be extensively studied in many fields, and they are found to have uses in medicines, electronics and optoelectronics. The mono-functionalization of C60 is straightforward, however its multiple functionalization is a challenging job for organic chemists. The stepwise multiple additions to C60 yield a mixture of geometric isomers with low yield and low selectivity for respective isomers, and tedious chromatographic separations are required. A general solution to the multiple addition problem is to find a generic linker which can direct the second addition to expected positions on C60 surface, then the third, and so on. Many research groups have developed tether-directed synthetic methodologies to selectively synthesize C60 multiple adducts. The design and synthesis of C60 isomeric multiple adducts using tether-directed approach will be reviewed in this article.

Effect of Indium-Tin Oxide Surface Micromodification and Improvement of Long-Wavelength Sensitivity on Photovoltaic Properties of Photovoltaic Cell with Conducting Polymer/C60 Interpenetrating Heterostructure

Photovoltaic cells, with a conducting polymer/C60 interpenetrating heterostructure fabricated by spin-coating a conducting polymer onto a C60 thin film, have been investigated and demonstrated a high efficiency as solar cells based on organic materials. The photovoltaic properties of the photovoltaic cell with a structure of indium-tin oxide (ITO)/C60/poly(3-hexylthiophene)/Au have been improved by the micromodification of the ITO surface and codeposition of metal-free phthalocyanine and C60.

Spectroscopic Studies on Inclusion Properties of Fullerenocalix[4]arene Conjugates with Metal Ions

Metal-chelating properties-in the ground and excited states-of fullerenocalix[4]arenes containing two malonamide substituents at the upper rim and four alkyl ester chains at the lower rim have been studied by means of steady-state absorption, fluorescence spectroscopy, and time-resolved transient absorption spectra. In particular, the influence that Ag+ enforces on the fullerene electronic spectra is due to direct interactions between Ag+ and the surface of C60. The effects stemming from Na+, Mg2+, and Ba2+, on the other hand, are indirect and are introduced through chelating the metal ions to the calix[4]arene moiety. They strongly depend on the molecular structure of the fullerenocalix[4]arenes. No spectroscopic evidence was obtained for any influence caused by Mn2+, although the malonamide groups provide good chelating ability even for this transition metal ion.

Coordination chemistry of buckybowls: from corannulene to a hemifullerene

This work highlights the progress made in coordination chemistry of transition-metal centers to open geodesic polyaromatic hydrocarbons that map onto the surface of C60, the family of compounds known as buckybowls or 'fullerene fragments'. In particular, an overview of our recent gas-phase coordination studies of several bowl-shaped polyarenes toward the dinuclear metal complex, [Rh2(O2CCF3)4], is given. Selected buckybowls include corannulene (C20H10) and two of its derivatives, namely dibenzo[a,g]corannulene (C28H14) and 1,3,5,7,9-penta-tert-butylcorannulene (C40H50), as well as a hemifullerene (C30H12). This study has resulted in the first X-ray structural characterization of buckybowl coordination complexes and has revealed eta2-rim coordination preferences of open geodesic polyarenes in rhodium(ii) binding reactions.

Hollow porous carbon nanospheres with large surface area and stability, assembled from oxidized fullerenes

Hollow porous carbon nanospheres with large surface area and stability were self-assembled using gentle oxidization of fullerenes. The C60 framework, functional groups, composition, structure, surface area and stability of the nanospheres were investigated using scanning electron microscopy, X-ray photoelectron spectroscopy, FT-IR spectroscopy, solid state 13C NMR spectroscopy, high resolution transmission electron microscopy, adsorption and thermogravimetric analysis. The narrow distribution of the nanospheres was centred on 8 nm, the BET surface area was 222.63 m2 g−1, and the residue at 1000 °C was 75%. Thus, a novel nanomaterial with the highest surface area and highest stability in the family of fullerenes has been prepared.

Fullerenols revisited as stable radical anions.

The first exhaustive purification and characterization of the much-studied "fullerenols", prepared by reaction of C(60) in toluene with an oxygenated, aqueous NaOH solution using tetrabutylammonium hydroxide as a phase transfer catalyst, has been performed. The resulting fullerenol is not simply polyhydroxylated C(60) but rather is a structurally and electronically complex C(60) radical anion with a molecular formula of Na(+)(n)[C(60)O(x)(OH)(y)](n)(-) (where n = 2-3, x = 7-9, and y = 12-15) for three different, but identical, preparations. Surprisingly, Na(+)-fullerenol is paramagnetic, exhibiting mu(B) values in aqueous solution of 1.9-2.1 B.M. at 0.5 T and 300 K and R(1) proton relaxivities of 0.55-0.77 mM(-1)s(-1) at 20 MHz and 40 degrees C, values both slightly higher than those expected for a pure S = 1/2 spin system. ESR studies (ESE-FS and 2D nutation) of frozen aqueous solutions at 1.5 and 5.0 K establish that Na(+)-fullerenol is mainly S = 1/2 with a minor, but significant, component of S = 1. Thus, this is the first report to characterize these widely studied, water-soluble fullerenols as stable radical anions. The stability of the S = 1/2 Na(+)-fullerenol radical is likely due to a highly derivatized C(60) surface that protects a cyclopentadienyl radical center on the fullerene.

Interaction of Fe+ with the C60 surface: A study about the feasibility of endohedral doping

The feasibility of endohedral iron doping of C60 is studied using a mass selected ion beam to create an Fe+ beam with variable energy (60–380 eV, 1.2×10−3 C) which is directed at a C60 thin film. The surface is characterized by x-ray photoelectron spectroscopy, and oxidation of the samples is used to discriminate between Fe@C60 and carbides. The fullerene cages are damaged during the irradiation, and participate in direct Fe–C bond formation. The reaction with oxygen indicates the absence of sizable amount of Fe@C60. Comparison with reference samples composed of iron and damaged fullerene layers (by Ar+ irradiation) supports this interpretation. The experiment serves as a model study for the investigation of different metal-fullerene combinations.

Physisorption of molecular oxygen on C60 thin films.

The interaction of oxygen molecules with a fullerene surface has been studied using high resolution electron energy loss spectroscopy and temperature programmed desorption. Vibrational excitation of the adsorbed oxygen is observed at 190 meV, an energy value comparable with that for molecular oxygen in the gas phase. We take this to indicate physisorption of molecular oxygen on the C(60) surface. Thermal desorption results also show that the bonding of oxygen molecules to the C(60) overlayer is comparable to that on a graphite surface. A detailed study of the energy dependence of the vibrational excitation reveals an inelastic electron resonance scattering process. The angular dependence of the resonant vibrational excitation exhibits features distinctively different from those for molecular oxygen physisorbed on the related graphite surface, at a comparable coverage. One possible reason is that the corrugated surface potential, due to the curvature of the C(60) molecules, promotes the preferential ordering of the physisorbed oxygen molecules perpendicular to the surface plane of the C(60) overlayer.

Preparation and Photoelectrochemical Properties of Fullerene Modified ITO Electrodes Using Self-Assembled Monolayer

Fullerene (C60) immobilized indium-tin-oxide (ITO) electrodes were prepared by a technique of self-assembled monolayers (SAMs) using five compounds of alkyldiamine (NH2(CH2)nNH2; n = 4, 6, 8, 10 and 12). These C60/diamine-SAMs modified ITO (C60/n/ITO; n = 4, 6, 8, 10 and 12) electrodes were examined in terms of properties of electrochemical and photoelectrochemical responses. The C60 surface coverage of C60/n/ITOs estimated was ca. 3×10−10 to 4×10−10 mol cm−2. The stable anodic photocurrent appeared immediately after the irradiation of the C60/n/ITO electrode in 0.1 M Na2SO4 containing 50 mM ascorbic acid (AA) as an electron sacrificer with λ = 400−600 nm, light power of 4.0−6.5 mW cm−2 and electrode potential of −0.15 V to +0.15 V vs. Ag/AgCl (saturated KCl). The photocurrents generated by C60/n/ITOs increased with the number of the methylene unit (n) except for n = 12. These results have shown that increasing the length of the methylene contributes to only immobilization of C60, because of highly ordered structure of diamine-SAMs.

Scratching the surface of buckminsterfullerene: the barriers for Stone-Wales transformation through symmetric and asymmetric transition states.

General-gradient approximation (PBE) and hybrid Hartree-Fock density functional theories (B3LYP) in conjunction with basis sets of up to polarized triple-zeta quality have been applied to study the Stone-Wales transformation of buckminsterfullerene (BF) to yield a C(60) isomer of C(2)(v) symmetry with two adjacent pentagons (#1809). In agreement with earlier investigations, two different transition states and reaction pathways could be identified for the rearrangement from BF to C(60)-C(2)(v) on the C(60) potential energy surface (PES). One has C(2) molecular point group symmetry with the two migrating carbon atoms remaining close to the fullerene surface. The other one has a high-energy carbene-like (sp(3)) structure where a single carbon atom is significantly moved away from the C(60) surface. The carbene intermediate and the second transition state along the stepwise reaction path characterized previously at lower levels of theory do not exist as stationary points with the density functionals utilized here. The classical barriers of both mechanisms are essentially identical, 6.9 eV using PBE and 7.3 eV with B3LYP.

Scanning probe microscopy and computer simulations: Complementary techniques for nanostructured materials and thin films

AbstractIt is demonstrated how scanning force microscopy in combination with recent simulation techniques and computer power can be used in a complementary way in the analysis and explanation of thin film growth and nanoindentation on crystal surfaces. Simulation techniques such as Monte Carlo models and molecular dynamic models are applied for thin film growth and nanoindentation. The formation of the first molecular layers in the growth of C60 films on silicon can be modelled by classical molecular dynamics to show the different bonding behaviour between C60 molecules, which results in hexagonal and cubic structures with stacking faults within the C60 film. Ab initio calculations of C60 bonding sites to the silicon surface classify bond types by the length and the energy of the C‐Si bonds. Strong binding energies between the C60 molecule and the Si atoms are obtained to fix the C60 molecule at a definite position. Different growth modes such as island and layer growth of C60 films can be studied by Monte Carlo simulations, including diffusion of atoms within the cluster and cluster diffusion over the surface itself. A brief experimental view of the nanoindentation process is given in combination with molecular dynamics simulations for the penetration of the tip into a C60 surface. The crystal symmetry is reflected during the plastic deformation in the pile‐up structure around the indentation hole in relation to the orientation of the indenter and the crystal surface. As an example of these investigations results are given for nanoindentation into the Fe{110} single crystal surface.

The photochemical reaction of 1,2-digermacyclobutane with C60: possible example of a closed [6,5]-bridged fullerene derivative of germacyclopropane

The photolysis of 1,2-digermacyclobutane derivatives ( 1) with bulky bistrimethylsilylmethyl (disyl) groups in a toluene solution of C60 provided the germylene ( 2) and germacyclopropane ( 3) adducts of C60, respectively. The structures of the two products were characterized using 1H- and 13C-NMR spectra. The signals showed a dramatic change in line shape with temperature. The non-equivalence of the two disyl groups at high temperature, as well as the presence of quaternary aliphatic carbon, suggested that the germylene adduct ( 2) may be a closed [6,5] adduct: a possible first example of a closed [6,5]-bridged fullerene adduct. PM3 and ab initio calculations also support an energetically preferred closed [6,5] germacyclopropane adduct over an open [6,5] germa-annulene. The non-equivalence of the two disyl groups at low temperatures, as well as the presence of two methylene groups, indicated a germacyclopropane adduct ( 3), having a germacyclopentane structure that includes a closed [6,6] bond on the C60 surface. To take into account the regio-selective formation of the [6,5] adducts, a fragmentation mechanism of the digermacyclobutane on the C60 surface is proposed.

Derivatization of fullerene dimer C120 by the Bingel reaction and a 3He NMR study of 3He@C120 monoadducts.

Cyclopropanation with diethyl bromomalonate and base (the Bingel reaction) was conducted on fullerene dimer C120 to give a mixture of "monoadducts" (45% yield) and "bisadducts" (< or =37% yield), while 18% of the C120 remained unchanged. The "monoadducts" were separated into five positional isomers, i.e., e(face), e(edge), trans-4, trans-3, and trans-2, by preparative HPLC. Assignments were made based on 1H (and 13C) NMR and confirmed by theoretical calculations of the addends' 1H NMR chemical shifts. The relative yields of these isomers were in fair agreement with those observed for the Bingel bisaddition of C60. The Bingel reaction was also carried out on the dimer C120 encapsulating 3He in one of the C60 cages. Each positional isomer of the "monoadduct" exhibited a pair of 3He NMR signals corresponding to an isomer with functionalization on the 3He-containing cage and the other isomer with functionalization on the empty cage. Using the 3He NMR spectroscopy, a pair of signals for the trans-1 isomer, which eluded detection by 1H NMR, were observed, in addition to pairs of signals for e(face), e(edge), trans-4, trans-3, and trans-2 isomers. The 3He NMR signals for isomers with functionalization on the 3He-containing cage were spread out over a 1.82-ppm range reflecting the direct effects of the addition pattern on the C60 surface. In contrast, the isomers with functionalization on the empty cage exhibited 3He NMR signals that appeared over a 0.14-ppm range, which was shown to be primarily due to changes in the diamagnetism of the functionalized cage based on theoretical calculations of 3He NMR chemical shifts for the model system in which the C60 cage encapsulating 3He was removed.

Synchrotron radiation photoemission spectrum study on K3C60 film

K3C60, single crystal film was prepared on the cleaved (111) surface of C60, single crystal. Synchrotron radiation angle-resolved photoemission spectra were measured at normal emission with sample temperature at × 150K. Up to four subpeaks of LUMO-derived band were observed. These sub-peaks exhibit distinct energy dispersions which resemble in general the theoretical ones calculated for K3C60 at low temperature with the so-called one-dimensional disordered structure. But there is large deviation of experimental sub-band intervals from the theoretical values. This result is meaningful for the studies of the physical properties of alkali-doped C60 solids, e.g. the mechanism for superconductivity.

EPR of a fullerene-molecule-derived paramagnetic center as a mesoscopic conducting object

Discussion of theg-factor value of fullerene is based on the model of itinerant electrons restricted to the surface of the fullerene molecule C60. The Ag shift, i.e., the difference between the experimentalg-factor and theg-factor of free electron Δg = g − 2.0023 for C60−1 is negative as for a very small metallic conducting particle.g-factor value is proportional to the interaction between itinerant electrons in the conduction band, thus the Δg is negative for C60−1 and C60−3 having less than half filled conduction band, while Δg is positive for C60+ where the conduction band is almost filled.