Electron Affinity Of C60 Research Articles

(Invited) Photoexcited Electron Transfer from Semiconducting Carbon Nanotubes to Non-Fullerene Acceptors

We have previously demonstrated efficient electron transfer from photoexcited small diameter (< 1 nm) semiconducting carbon nanotubes to C60 fullerene electron acceptors in planar, bilayer heterojunction diode devices, with internal quantum efficiency of 90%. However, the out-of-plane exciton diffusion length in planar nanotube thin films is poor (~ 5 nm), limiting the optical density of nanotube films that can be implemented in these devices, which in turn has limited external quantum efficiency to 50% at the nanotube bandgap and to << 50% off-resonance. Polymer solar cells also face a similar limitation (poor exciton diffusion length) and overcome this problem by blending the polymer and acceptor together into a bulk heterojunction. These bulk heterojunctions are typically formed by co-casting the polymer with a solubilized fullerene derivative, phenyl-C61 (or 71) -butyric acid methyl ester (PBCM) to create the blend. However, this strategy has not yet worked well to create nanotube-based bulk heterojunction devices that are more efficient that bilayered devices. One problem is that the electron affinity of PCBM is shallower than the electron affinity of C60, which detrimentally impacts the nanotube to acceptor charge transfer efficiency. To address this shortcoming, new electron acceptors are needed. We have surveyed several new non-fullerene electron acceptors, including both small molecule and polymers, as potential C60 and PCBM replacements. In this talk, the results of this survey will be presented. Moreover, the use of the most promising new non-fullerene acceptors in bulk heterojunctions with semiconducting nanotubes will be discussed.

Electronic structure of Ba(OH)2 interface in inverted organic photovoltaics: Improved electron transport by charged state of C60

Lowering the work function (WF) of the cathode in inverted organic photovoltaics (OPVs) is critically important to obtaining a high power conversion efficiency (PCE). The insertion of functional interlayers between the cathode and acceptor is a widely employed strategy to lower the WF. Among these functional materials, Ba(OH)2 is known to be an efficient solution-processable cathode buffer layer that improves the electron transport in organic optoelectronic devices. Despite several reports of device performance enhancement with the use of a Ba(OH)2 layer, its interfacial energetics is yet to be clearly understood. In this study, we investigated the electronic structure of Ba(OH)2 interfaces and the improvement in the PCE of inverted small molecule OPVs with the use of a Ba(OH)2 layer. On implementing the optimum thickness of the Ba(OH)2 layer, the PCE of the OPVs was significantly enhanced from 1.29% to 3.41%, and the S-shaped kink in the current-density–voltage curve was eliminated. To elucidate the underlying mechanism of this phenomenon, we explored the interfacial electronic structures of C60/indium tin oxide (ITO) and C60/Ba(OH)2/ITO using in situ photoelectron spectroscopy. The spin-coated Ba(OH)2 was physisorbed onto the ITO, which significantly reduced its WF. Owing to the Ba(OH)2, the reduced WF of the ITO is lower than the electron affinity of C60. Thus, a charge transfer is induced from Ba(OH)2/ITO to C60, and the charged states of C60 are observed within the monolayer. These charged states result in significant band bending in the C60 layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level (EF) of the Ba(OH)2/ITO. As a result, the energy offset between the C60 LUMO level and the cathode EF is substantially reduced from 0.45 eV to 0.15 eV with the use of the Ba(OH)2 layer. This is the origin of the enhanced device performance of OPVs.

A computational DFT study of the physicochemical properties of anticancer drug gemcitabine on the surface of Al-doped C60 and C70 fullerenes

Targeted drug delivery systems are very suitable for medical usages. This investigation studies the interaction of anti-cancer drug gemcitabine with C60 and C70 fullerenes by the DFT method at a B3LYP/cc-pvdz quantum chemical level. In addition, the investigation is carried out on the drug delivery capability of gemcitabine. To achieve suitable and optimized adsorption of the drug on the surface of fullerenes (C60 and C70), an carbon atom is replaced by an aluminum atom in fullerene structures. Then, the geometric parameters, HOMO and LUMO energies, and the energy gap are reported and compared, and also the density of state (DOS) plots for Al-doped fullerenes and their complexes with the drug are traced. The obtained results show that the orbital energy levels decrease significantly by the gemcitabine bonding to C60 and C70 fullerenes. Other specifications such as chemical potential, chemical hardness, and electrophilicity also vary significantly in these structures. The interpretation of these variations may depend on the electron affinity of C60 and C70 fullerenes.

Study of the Physicochemical Properties of Anti-Cancer Drug Gemcitabine on the Surface of Al Doped C60 and C70 Fullerenes: A DFT Computation

Targeted drug delivery systems are very suitable for medical usages. This investigation studies the interaction of anti-cancer drug gemcitabine with C60 and C70 fullerenes by the DFT method at a B3LYP/cc-pvdz quantum chemical level. In addition, the investigation is carried out on the drug delivery capability of gemcitabine. To achieve suitable and optimized adsorption of the drug on the surface of fullerenes (C60 and C70), an carbon atom is replaced by an aluminum atom in fullerene structures. Then, the geometric parameters, HOMO and LUMO energies, and the energy gap are reported and compared, and also the density of state (DOS) plots for Al-doped fullerenes and their complexes with the drug are traced. The obtained results show that the orbital energy levels decrease significantly by the gemcitabine bonding to C60and C70 fullerenes. Other specifications such as chemical potential, chemical hardness, and electrophilicity also vary significantly in these structures. The interpretation of these variations may depend on the electron affinity of C60 and C70 fullerenes.

Probing the interaction between the encapsulated water molecule and the fullerene cages in H2O@C60- and H2O@C59N.

We report a high-resolution photoelectron imaging study of cryogenically-cooled H2O@C60- and H2O@C59N- endohedral fullerene anions. The electron affinity (EA) of H2O@C60 is measured to be 2.6923 ± 0.0008 eV, which is 0.0088 eV higher than the EA of C60, while the EA of H2O@C59N is measured to be 3.0058 eV ± 0.0007 eV, which is 0.0092 eV lower than the EA of C59N. The opposite shifts are found to be due to the different electrostatic interactions between the encapsulated water molecule and the fullerene cages in the two systems. There is a net coulombic attraction between the guest and host in H2O@C60-, but a repulsive interaction in H2O@C59N-. We have also observed low-frequency features in the photoelectron spectra tentatively attributed to the hindered rotational excitations of the encapsulated H2O molecule, providing further insights into the guest-host interactions in H2O@C60- and H2O@C59N-.

Orientation-dependent ionization potential of CuPc and energy level alignment at C60/CuPc interface

The electronic structure evolution of interfaces of fullerene (C60) with copper phthalocyanine (CuPc) on highly oriented pyrolytic graphite (HOPG) and on native silicon oxide has been investigated with ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The LUMO edge of C60 was found to be pinned at the interface with CuPc on SiO2. A substantial difference in the electron affinity of CuPc on the two substrates was observed as the orientation of CuPc is lying flat on HOPG and standing up on SiO2. The ionization potential and electron affinity of C60 were not affected by the orientation of CuPc due to the spherical symmetry of C60 molecules. We observed band bending in C60 on the standing-up orientation of CuPc molecules, while the energy levels of C60 on the flat-lying orientation of CuPc molecules were observed to be flat. The observation points to a dependence of photoexcited charge transfer on the relative molecular orientation at the interface.

Convergence Acceleration of Parallel Monte Carlo Second-Order Many-Body Perturbation Calculations Using Redundant Walkers.

A Monte Carlo (MC) integration of the second-order many-body perturbation (MP2) corrections to energies and self-energies eliminates the usual computational bottleneck of the MP2 algorithm (i.e., the basis transformation of two-electron integrals), thereby achieving near-linear size dependence of its operation cost, a negligible core and disk memory cost, and a naturally parallel computational kernel. In this method, the correlation correction expressions are recast into high-dimensional integrals, which are approximated as the sums of integrands evaluated at coordinates of four electron random walkers guided by a Metropolis algorithm for importance sampling. The gravest drawback of this method, however, is the inevitable statistical uncertainties in its results, which decay slowly as the inverse square-root of the number of MC steps. We propose an algorithm that can increase the number of MC sampling points in each MC step by many orders of magnitude by having 2m electron walkers (m > 2) and then using m(m - 1)/2 permutations of their coordinates in evaluating the integrands. Hence, this algorithm brings an O(m(2))-fold increase in the number of MC sampling points at a mere O(m) additional cost of propagating redundant walkers, which is a net O(m)-fold enhancement in sampling efficiency. We have demonstrated a large performance increase in the Monte Carlo MP2 calculations for the correlation energies of benzene and benzene dimer as well as for the correlation corrections to the energy, ionization potential, and electron affinity of C60. The calculation for C60 has been performed with a parallel implementation of this method running on up to 400 processors of a supercomputer, yielding an accurate prediction of its large electron affinity, which makes its derivative useful as an electron acceptor in bulk heterojunction organic solar cells.

Pinning of fullerene lowest unoccupied molecular orbital edge at the interface with standing up copper phthalocyanine

The electronic structure evolution of interfaces of fullerene (C60) with copper phthalocyanine (CuPc) on highly oriented pyrolitic graphite (HOPG) and on native silicon oxide has been investigated with ultra-violet photoemission spectroscopy and inverse photoemission spectroscopy. The lowest unoccupied molecular orbital edge of C60 was found to be pinned at the interface with CuPc on SiO2. A substantial difference in the electron affinity of CuPc on the two substrates was observed as the orientation of CuPc is lying flat on HOPG and standing up on SiO2. The ionization potential and electron affinity of C60 were not affected by the orientation of CuPc due to the spherical symmetry of C60 molecules. We observed band bending in C60 on the standing-up orientation of CuPc molecules, while the energy levels of C60 on the flat lying orientation of CuPc molecules were observed to be flat.

Vibrational cooling in a cold ion trap: Vibrationally resolved photoelectron spectroscopy of cold C60− anions

We demonstrate vibrational cooling of anions via collisions with a background gas in an ion trap attached to a cryogenically controlled cold head (10-400 K). Photoelectron spectra of vibrationally cold C60(-) anions, produced by electrospray ionization and cooled in the cold ion trap, have been obtained. Relative to spectra taken at room temperature, vibrational hot bands are completely eliminated, yielding well-resolved vibrational structures and a more accurate electron affinity for neutral C60. The electron affinity of C60 is measured to be 2.683+/-0.008 eV. The cold spectra reveal complicated vibrational structures for the transition to the C60 ground state due to the Jahn-Teller effect in the ground state of C60(-). Vibrational excitations in the two A(g) modes and eight H(g) modes are observed, providing ideal data to assess the vibronic couplings in C60(-).

Electron affinities and C60 anion clusters of certain bowl-shaped polycyclic aromatic hydrocarbons

The kinetic method is applied to the determination of the electron affinities (EAs) of the bowl-shaped polycyclic aromatic hydrocarbons (PAHs) dibenzo[a,g]corannulene (C28H14) 1 and diindenochrysene (C26H12) 2 using electron attachment desorption chemical ionization mass spectrometry (DCI-MS) and triple-quadrupole tandem mass spectrometry. The estimated EAs of 1 and 2 are 1.00±0.20 and 1.16±0.20 eV respectively. In an analogous experiment, loosely bound dimeric radical anions comprising C60 with 1 and C60 with 2 were generated in the gas phase by electron attachment desorption chemical ionization. The collision-induced dissociation of each of these cluster ions yields both the monomeric radical anions as products and yields calculated electron affinities of C60 of 1.20±0.20 eV (C60/1 dimer) and 1.24±0.20 eV (C60/2 dimer). These values are significantly different from the known electron affinity of C60 (2.65 eV) and support previous studies on electron-bound dimers comprising C6O and coronene [G. Chen, R. G. Cooks, E. Corpuz and L. T. Scott, J. Am. Soc. Mass Spectrom. 7, 619 (1996)] in showing that the dissociation of electron-bound dimers comprising C60 and reference PAHs is controlled by the electron affinity of a portion of the C60 surface, i.e. that a ‘local’ electron affinity of C60 is measured. The ‘local’ electron affinity value corresponds approximately to the electron affinity of the reference PAHs, C60 exhibiting an affinity for the ‘extra’ electron in the cluster which approximately matches that of its PAH partner. © 1997 by John Wiley & Sons, Ltd.

Enthalpies of formation of buckminsterfullerene (C60) and of the parent ions C60 +, C60 2+, C60 3+ and C60 ?

The standard enthalpy of formation of buckminsterfullerene (C60) in the crystalline state has been determined as 2278.1 ± 14.4 kJ mol–1 using a microcombustion calorimeter. Statistical mechanical calculations of the heat capacity of gaseous C60 as a function of the temperature and a critical survey of the literature data for the enthalpy of sublimation, the heat capacity in the crystalline state, the first, second and third ionization energies and the electron affinity of C60, enabled the enthalpy of formation of gaseous C60 and the enthalpies of formation of the ions C60+, C602+, C603+ and C60– to be derived.