Undoped C60 Research Articles

Adsorption mechanism, structural and electronic properties: 4-Phenylpyridine & undoped or doped (B or Si) C60

Interaction mechanisms of fullerene derivatives and various types of molecules have been paid increasing attention. In this research, interaction mechanism, structural and electronic properties, natural bond orbital (NBO) analyses of 4-Phenylpyridine (4-phpy) interacted undoped or doped (B or Si) C60 fullerenes were examined based on quantum mechanical calculations. Bond order and binding energy results suggest a possible chemisorption between nitrogen atom of 4-phpy and boron or silicon atoms of doped fullerene cages and the examined B- or Si-doped complexes yielded with higher binding energies in water media calculations. Results also show that unlike C60…4-phpy indicating a very weak binding energy, B or Si doped C60 fullerenes can be used to carry 4-phpy molecule for various purposes including drug delivery systems.

Calorimetric measurements on Li4C60 and Na4C60.

We show specific heat data for Na4C60 and Li4C60 in the range 0.4-350 K for samples characterized by Raman spectroscopy and X-ray diffraction. At high temperatures, the two different polymer structures have very similar specific heats both in absolute values and in general trend. The specific heat data are compared with data for undoped polymeric and pristine C60. At high temperatures, a difference in specific heat between the intercalated and undoped C60 polymers of 100 J K(-1) mol(-1) is observed, in agreement with the Dulong-Petit law. At low temperatures, the specific heat data for Li4C60 and Na4C60 are modified by the stiffening of vibrational and librational molecular motion induced by the polymer bonds. The covalent twin bonds in Li4C60 affect these motions to a somewhat higher degree than the single intermolecular bonds in Na4C60. Below 1 K, the specific heats of both materials become linear in temperature, as expected from the effective dimensionality of the structure. The contribution to the total specific heat from the inserted metal ions can be well described by Einstein functions with TE = 386 K for Li4C60 and TE = 120 K for Na4C60, but for both materials we also observe a Schottky-type contribution corresponding to a first approximation to a two-level system with ΔE = 9.3 meV for Li4C60 and 3.1 meV for Na4C60, probably associated with jumps between closely spaced energy levels inside "octahedral-type" ionic sites. Static magnetic fields up to 9 T had very small effects on the specific heat below 10 K.

Optimized inverted polymer solar cells incorporating Cs2CO3-doped C60 as electron transport layer

An efficient charge transfer between co-sublimed cesium carbonate (Cs2CO3) and fullerene C60 provides an n-type material exhibiting an electrical conductivity above 1 S/cm. This type of doped layers can be used in organic optoelectronic devices to reduce ohmic losses at organic-electrode interfaces. We report here an analysis of inverted polymer-based solar cells incorporating Cs2CO3 doped C60 as electron transport layer (ETL). The optimization of both dopant concentration and thickness resulted in a maximum efficiency of 3.79% compared to 3% for similar devices using undoped C60 as ETL and 2.13% for devices without any ETL.

Acridine orange base as a dopant for n doping of C60 thin films

We present a study on n doping of C60 thin films by acridine orange base [3,6-bis(dimethylamino)acridine(AOB)] combining conductivity, field effect, and Seebeck measurements. An increase of more than six orders of magnitude in conductivity is observed for a doping ratio of 6mol%, accompanied by a decrease in the activation energy from 0.64to0.15eV compared to the undoped C60. We observe a clear doping effect immediately after sample preparation, but also a further activation by annealing or illumination. The field effect and Seebeck measurements confirm n-type conduction of C60 thin films and show that deep donor states are formed in AOB-doped C60 thin films. A field effect mobility of 0.2cm2∕Vs is achieved for a doping level of 1.8mol%. Near Infrared (NIR) and Fourier transform infrared (FTIR) spectra demonstrate electron transfer from the dopant to the matrix: For C60 doped with AOB, C60− is present in NIR absorption and FTIR spectra. On the other hand, a peak corresponding to acridine orange [3,6-bis(dimethylamino)acridinium chloride (AOBH+)] is also observed in the FTIR spectrum of C60:AOB, where AOBH+ corresponds to AOB with one additional proton attached. Electrochemical data of AOB and AOBH+ in acetontrile suggest that the AOB radical cation is not stable, but is rapidly transformed into a compound with similar properties to AOBH+. Conductivities of C60 thin films doped with bis(ethylenedithio)-tetrathiafulvalene were also investigated to confirm that the doping effect of AOB in C60 does not result from a simple electron transfer from AOB to C60.

Universality of the intermediate phase and high-temperature superconductivity in C 60

With the discovery of Tc s over 100K, continuously biased hole-doped semiconductive C60+y has joined the family of high-temperature superconductors (HTSC) previously dominated by the ceramic cuprates. The phase diagrams of these two quite different materials are remarkably similar, despite the absence from undoped C60+y of magnetic order. This supports percolative models of HTSC based on strong electron-phonon interactions. These models explain observed fine structure in the C60+y phase diagrams, and furthermore suggest novel paths to still higher Tc s.

Electronic structure of K-doped C60 monolayer films adsorbed on Si([formula omitted])-(2×1) and Si([formula omitted])-(7×7) surfaces

We have measured the valence spectra of K-doped C60 monolayer films adsorbed on Si(001)-(2×1) and Si(111)-(7×7) surfaces. No density of states is observed at the Fermi level for the undoped C60 monolayer films on both Si surfaces. After a small K doping, a density of states is clearly observed at the Fermi level on a Si(001)-(2×1) surface. On the other hand, no density of states is observed at the Fermi level at any amount of K doping on a Si(111)-(7×7) surface. The difference observed on both Si surfaces indicates that the symmetry of C60 molecules is almost the same as that in solid phase on a Si(001)-(2×1) surface, and is broken due to a stronger C60-substrate and/or C60–C60 interaction on the Si(111)-(7×7) one.

Band-Like Charge Transport in C60 Single Crystals

Charge transport properties of vapor grown C60 single crystals are investigated in the temperature range from 5–300 K by field-effect transistor and space charge limited current measurements in vacuum. Room temperature mobilities of 2.1 and 1.8 cm2/Vs are observed for holes and electrons, respectively. Moreover, the mobilities increase with decreasing temperature following a power law behavior (∝T—1.6). Values as high as 60 (electrons) and 95 cm2/Vs (holes) are achieved at low temperatures. The discontinuity around 255 K is ascribed to the first-order phase transition of C60. The pronounced electric field dependence can be explained by acoustic phonon scattering. Hence, we conclude, that band-like charge transport in delocalized states rather than hopping motion can be assumed as intrinsic charge transport mechanism in undoped C60.

First principles calculations of Si doped fullerenes: Structural and electronic localization properties in C59Si and C58Si2

Si-doped heterofullerenes C59Si and C58Si2, obtained from C60 by replacing one and two C atoms with Si atoms, are investigated via first principles calculations. Static geometry optimizations show that structural deformations occur in the vicinity of the dopant atoms and give rise to Si–C bonds significantly larger than the ordinary C–C bonds of the fullerene cage. In the case of C58Si2, the lowest energy isomer has two Si atoms located at distances corresponding to third nearest neighbors. The electronic structure of these heterofullerenes, although globally close to that of C60, is characterized by a strong localization of both the HOMO’s and the LUMO’s on the Si sites. Charge transfer occurs from the dopant atoms to the nearest neighbor C atoms, contributing to the formation of polar Si–C bonds. A detailed analysis of the charge localization, based on the electron localization function and maximally localized Wannier function approaches, reveals that the bonding of Si in the fullerene cage consists of two single and one weak double bond, thus preserving the conjugation pattern of the undoped C60. Beside the charge localization along the bonds, we observe a peculiar region of charge localization outside the cage above each Si atom. These features are discussed in comparison with the corresponding patterns exhibited by the C60 system.

Experimental access to doped fullerene polymers

compound could be obtained by slow cooling the high temperature fcc phase and by quenching with subsequent annealing. The various phases after quenching and during annealing were studied in detail. No evidence for a direct doping of undoped C60 to the polymeric AC60 phase was found. Due to the local character of the doping process the formation of A3C60 clusters is observed. The same results were obtained from doping experiments performed with undoped polymeric structures like phototransformed and pressure polymerized C60.

Charge migration in pulse-irradiated undoped C60 powder studied with the Time-Resolved Microwave conductivity technique

The conductivity of C60 buckminsterfullerene was studied using the pulse-radiolysis time resolved microwave conductivity (PR-TRMC) technique. Conductivity transients with disperse decay kinetics and a first half-life of 2 ns were obtained in the temperature range from 210 to 450 K. The charge carrier mobility at high temperatures (T>300 K) is thermally activated (E a=46 meV). A minimum for the sum of the charge carrier mobilities of holes and electrons was found to be 1×10 −5 m 2/Vs.

ELECTRICAL CHARACTERIZATION OF SOLID C60/Si HETEROJUNCTIONS——RECTIFYING PROPERTIES, ENERGY-BAND MODELS, AND BIAS-TEMPERATURE EFFECT

This work investigates electrical properties of contacts between undoped solid C60 and n-or p-type Si. Current-voltage measurements show that the two contacts result in strong rectification with opposite conduction directions, indicating two different kinds of barriers for carrier transportation at the interface of the two contacts. Current-temperature measurements show that the current is an exponential function of reciprocal temperature, from which we estimate the effective barrier height to be 0.30 and 0.48 eV for C60/n-Si and C60/p-Si, respectively. Within energy-band models we interpret successfully the above experimental results. Based on the energy-band model and experimental data we derived the electron affinity of solid C60 film to be 3.92 eV and its energy gap to be below 1.72 eV. Highfrequency capacitance-voltage measurements show that biastemperature treatments have considerable effect on the C-V characteristics of the heterojunction. Assuming the existence of mobile negative charges in the solid C60 film we explain the effect successfully and estimate the density of the negative charges to be 3.1×1012cm-2. By use of the C-V results we determine the dielectric constant of the solid C60 film to be 3.7±0.1 in the temperature range of 300-370 K.

Band structure studies on crystalline C60, Ca3C60, and Ca5C60

AbstractThe three‐dimensional EHMO crystal orbital calculations for crystalline C60, Ca3C60, and Ca5C60 are reported. The ground states of both undoped solid C60 and partially doped Ca3C60 are found to be insulating with an indirect energy gap of 1.2 and 0.5 eV, respectively. In contrast, Ca5C60 forms a metallic conducting phase with a set of three half‐filled bands crossing the Fermi level, which is found to be located close to a peak of the density of state. The character of crystal orbitals near the Fermi level for both Ca3C60 and Ca5C60 is completely carbonlike. In both cases, the Ca atoms are almost fully ionized and C60 molecules form a stable negative charge state with six to 10 additional electrons. © 1995 John Wiley & Sons, Inc.

Raman spectroscopy of undoped and doped single crystal C60

Summary form only given. We present high resolution Raman spectra of undoped single crystals at low temperatures using laser exitations in the red and green spectral range. A splitting of the H/sub g/ modes in A/sub g/, E/sub g/ and F/sub g/ modes was found. A maximum of 8 splitted components could be observed. The doping of the single crystals with potassium was performed at temperatures slightly above room temperature and the doping process was studied by using in-situ Raman spectroscopy. Up to the penetration depth of the laser homogeneous single phases of K/sub 1/ C/sub 60/ and K/sub 3/C/sub 60/ could be obtained. The doped phases were identified by the position of the A/sub g/ pinch mode, which is very sensitive relative to the charge transfer. At cooling the high temperature phase K/sub 1/C/sub 60/ was observed to undergo a sharp transition into C/sub 60/ and K/sub 3/C/sub 60/ between 430 K and 410 K. In addition temperature dependent Raman measurements were performed on single phase K/sub 3/C/sub 60/. At liquid nitrogen temperature an unexpected strong splitting and selective line broadening of the H/sub g/ modes was observed.

Alkali Rb-doped C60: The movement of the valence band (abstract)

Using synchrotron radiation light source, we have measured the photoemission spectra of undoped C60 and alkali Rb-doped C60 to reveal their electron structure. We observed a complex movement of the top of the valence band through the process of alkali Rb doping. When Rb is doped into C60, at first the spectrum moves to the high binding energy side for about 0.5 eV. As doping continues, the spectrum moves to the low binding energy side again. The spectrum moves back toward the original site by a small amount until the stoichiometry reaches about 1. This phenomenon has not been reported in other alkali doped C60 systems. We suggest the backward movement is due to the instability of the RbC60, Rb3C60, and other phases.

Quantum chemical calculations on the buckminsterfullerene (C60) system

Abstract We performed ab initio SCF LCAO calculations for a single C60 molecule and for interacting undoped C60 systems with different lattice constants (corresponding to the undoped case and to the K3C60 and Rb3C60 crystals, respectively). We also used different orbitals for different spin computations on single K3C60 and Rb3C60 molecules. To obtain convergence of the large number of basis functions in both programs (which belong to the Gaussian 90 package), we applied the direct SCF option. The analysis of the σ- and π-type MO splittings showed that in all cases the σ splittings are at least one order of magnitude smaller than the π splittings in the region of the Fermi level. This raises the possibility of explaining the likely electronic contribution to the superconductivity of doped C60 systems by using the so-called “two band” model. In this model the polarization of the localized electrons in filled narrow bands gives rise to an effective attraction between the delocalized electrons in the broad metallic band which facilitates Cooper pair formation. A simple mathematical derivation has shown that within the framework of the ‘two-band’ model, the decrease in level splittings with the increase in lattice constants (a0) (which we calculated for two C60 molecules) approximately accounts for the linear increase in the super-conducting transition temperature (Tc) with a0 or its linear decrease with pressure. These results are in quite good agreement with the experimental findings.

Femtosecond optical dynamics of C60 and M3C60

The initial, ultrafast scattering of optically excited carriers in M3C60 for M=K, Rb is investigated by femtosecond pump–probe measurements. An initial fast response (τ≊0.2 ps) of similar magnitude for both Rb3C60 and K3C60 is observed, while τ for undoped C60 is more than an order of magnitude longer. These results are discussed in terms of the molecular character of C60 and the bandlike character of M3C60.

Intercalation of sodium heteroclusters into the C60 lattice

INTERCALATION of sodium into C60 has been shown1 to yield a range of compounds Nax C60 (2<x<6). Unlike the other alkali-metal-doped compounds, there is no evidence of superconductivity within this range of doping. Furthermore, Na6C60 retains the face-centred-cubic (f.c.c.) structure of undoped C60 whereas the analogous x = 6 phases with K, Rb or Cs are body-centred cubic (b.c.c.)2. Here we report the preparation of sodium-doped C60 with x up to about 10; the structure remains f.c.c. throughout. Rietveld refinement of X-ray diffraction data yields a structure with ideal composition Na11C60, in which the octahedral interstitial site contains a nine-atom body-centred cluster of sodium atoms while the tetrahedral sites are singly occupied. Clusters of four to nine sodium atoms in the octahedral site provide sufficient 'chemical pressure' to prevent the low-temperature lattice distortion that occurs when this site is singly occupied, as in Na3C60 (ref. 1). This distortion is thought to suppress superconductivity in the latter compound. Thus we suggest that new sodium-doped superconduct-ing phases may be found in the range 4< x< 11.