Solution‐Based Phototransformation of C60 Nanorods: Towards Improved Electronic Devices

A modified liquid–liquid interface precipitation synthesis of C₆₀ nanorods, effects and opportunities following an in situ photochemical transformation in the liquid state, and an electronic characterization using a field‐effect transistor (FET) geometry are reported. The nanorods feature a high aspect ratio of ≈10³ and a notably small average diameter of 172 nm. Interestingly, it is found that a decreased nanorod diameter appears to correlate with distinctly improved electronic properties, and an average electron mobility of 0.30 cm² V^?1 s^?1, as measured in a FET geometry, is reported for as‐grown nanorods, with the peak value being an impressive 1.0 cm² V^?1 s^?1. A photoexposure using green laser light (λ = 532 nm) is demonstrated to result in the formation of a polymer‐C₆₀ shell encapsulating a monomer‐C₆₀ bulk; such photo‐transformed nanorods exhibit an electron mobility of 4.7 × 10^?3 cm² V^?1 s^?1. It is notable that the utilized FET geometry only probes the polymer‐C₆₀ nanorod surface shell, and that the monomer‐C₆₀ bulk is anticipated to exhibit a higher mobility. Importantly, photoexposed nanorods can be conveniently processed as a stabile dispersion in common hydrophobic solvents, and this finding is attributed to the insoluble character of the polymer‐C₆₀ shell.     

X-ray photoelectron spectroscopy and near-edge X-ray-absorption fine structure of C60 polymer films

We report core-level and valence-band X-ray photoelectron spectroscopy (XPS) and carbon [ _]K near-edge X-ray-absorptionfine structure spectroscopy (NEXAFS) results of plasma-polymerized C₆₀. In comparison with evaporated C₆₀ the C 1s peak is broader and asymmetric for the C₆₀ polymer and its shake-up satellites diminished. Furthermore, the features of the valence-band as well as the features of the π^* antibonding orbitals of the C₆₀ polymer are broader and reduced in intensity. Changes in the electronic structure are attributed to the polymerization of C₆₀, the post-plasma functionalization of the surface by oxygen after exposure to atmosphere, and the occurrence of amorphous carbon. Received: 28 May 1999 / Accepted: 31 August 1999 / Published online: 8 March 2000     

High dynamic range optical limiting in fullerene doped porous glasses

A diffusion-controlled technique of obtaining gradient in molecular distribution of C₆₀ molecules in porous vycor glass is presented. Here toluene solution of C₆₀ is allowed to diffuse only in one-dimension. The molecular distribution in the doped glass is measured by transmission method. By comparing optical limiting performance in the graded and uniformly doped glasses, we show that there is about 10-fold increase in the dynamic range in graded device. Received: 10 July 2001 / Accepted: 11 January 2002 / Published online: 3 May 2002 RID="*" ID="*"Corresponding author. E-mail: mukesh@cat.ernet.in     

Optical properties of fullerene and non-fullerene peapods

Single-wall carbon nanotubes (SWNTs) encapsulating fullerenes, so-called fullerene peapods, were synthesized in high yield by using diameter-selected nanotubes as pods. Transmission electron microscopy revealed high-density fullerene chains inside the nanotubes. X-ray-diffraction measurements indicate 85% filling for C₆₀ and 72% filling for C₇₀ molecules as a total yield. Interestingly, C₆₀ peas do not show any thermal expansion while C₇₀ peas show normal behavior. Room-temperature Raman spectra show one-dimensional photopolymerization of C₆₀ inside nanotubes by blue-laser irradiation, suggesting molecular rotation inside them. In C₇₀ peapods, no photopolymerization was observed but the relative Raman intensity of each peak is different from the C₇₀ 3D crystal. This is probably caused by mixing of two different crystal structures in C₇₀ peas. Furthermore, we synthesized Zn-diphenylporphyrin peapods. Optical absorption and Raman spectra suggest that the encapsulated molecules are deformed by interaction with the SWNT. Received: 12 November 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Effect of a buffer layer, composition and annealing on the performance of a bulk heterojunction polythiophene/fullerene composite photovoltaic device

Post-annealing of P3HT/C₆₀-based plastic solar cells, composite ratio and, incorporation of a buffer layers at the ITO–polymer/fullerene composite interface and the polymer/fullerene composite–Al interface have shown significant improvement in the performance of the photovoltaic device, both in terms of open-circuit voltage and short-circuit current. The annealing temperature, time duration and P3HT/C₆₀ composite ratios have been optimized for the best performance of typical ITO/PEDOT:PSS/P3HT:C₆₀/LiF/Al solar cell. The C₆₀ content is very crucial because it improves the photocurrent in the beginning but after a certain value it leads to decrease in absorption, thereby decreasing the photocurrent.     

On interface dipole layers between C60 and Ag or Au

C₆₀ layers on polycrystalline Ag and Au are studied by photoelectron spectroscopy. At these metal/C₆₀ interfaces an electron transfer occurs from the metal to the lowest unoccupied orbital of C₆₀. We found in the case of the polycrystalline Ag/C₆₀ interface a dipolar layer with its associated electric field in the direction corresponding to the charge transfer, so pointing from the substrate to the adsorbent. Yet, at the Au/C₆₀ interface we observed an overall electric field pointing from C₆₀ towards the metal. We discuss our observations in terms of charge transfer, screening and hybridization effects and propose the occurrence of a hybridization mechanism similar to back-bonding at the Au/C₆₀ interface. We show that the alignment of energy levels at the metal/C₆₀ interface cannot simply be deduced using the metal workfunction and the frontier orbitals of C₆₀, including screening effects, since hybridization effects may strongly alter the interfacial energy level structure. Our experimental findings on the polycrystalline metal/C₆₀ interfaces indicate an at-most weak dependence of the Fermi level of the C₆₀ overlayer on the workfunction of the polycrystalline metal substrate. These interfaces are found in donor–acceptor-based organic photovoltaic devices and our results may help to understand the electrical characteristics of these devices. Received: 26 September 2001 / Accepted: 15 January 2002 / Published online: 3 June 2002     

Photoluminescence in fullerene-doped porous glasses

We report enhanced and broad-band photoluminescence (PL) behavior of C₆₀ doped in porous silica glasses. By carrying out photoluminescence and photoluminescence-excitation spectroscopy of C₆₀ doped in surface-unpassivated and -passivated porous glasses, we show that the interaction of C₆₀ with the silica surface is quite strong and that the modified C₆₀ molecules contribute only in the orange-red region (∼1.9 eV) of the observed PL spectrum. The PL intensity in the blue-green region (∼2.3 eV) observed in doped glasses is found to originate in the porous glass itself. Received: 13 July 2000 / Revised version: 24 January 2001 / Published online: 9 May 2001     

Properties of C60 thin film transistor based on polystyrene

This paper reports that the n-type organic thin-film transistors have been fabricated by using C60 as the active layer and polystyrene as the dielectric.The properties of insulator and the growth characteristic of C60 film were carefully investigated.By choosing different source/drain electrodes,a device with good performance can be obtained.The highest electron field effect mobility about 1.15 cm 2 /(V·s) could reach when Barium was introduced as electrodes.Moreover,the C60 transistor shows a negligible 'hysteresis effect' contributed to the hydroxyl-free of insulator.The result suggests that polymer dielectrics are promising in applications among n-type organic transistors.     

Chemical adsorption of C60 on diamond (100)(2×1) surfaces

Molecular-dynamics simulations (MDSs) and ab initiocalculations are used to investigate the adsorption behavior of C₆₀ molecules on a clean dimer-reconstructed (100)(2×1) diamond surface. C₆₀ molecules have some probability to be adsorbed on the diamond surface at low incident energy (6∼45 eV). Electron-density contours show strong chemical interaction between C₆₀ molecules and the substrate surface. The adsorption property depends strongly on the incident energy and the impacting point. An incident energy of 18 eV may be an appropriate energy to grow a sub-monolayer or monolayer C₆₀ film on a clean C(100)(2×1) surface at room temperature. Received: 5 July 2000 / Accepted: 17 October 2000 / Published online: 28 February 2001     

Vibrational dynamics of fullerene molecules adsorbed on metal surfaces studied with synchrotron infrared radiation

Infrared (IR) spectroscopy of chemisorbed C₆₀ on Ag (111), Au (110) and Cu (100) reveals that a non-IR-active mode becomes active upon adsorption, and that its frequency shifts proportionally with the charge transferred from the metal to the molecule by about 5 cm^-1 per electron. The temperature dependence of the frequency and the width of this IR feature have also been followed for C₆₀/Cu (100) and were found to agree well with a weak anharmonic coupling (dephasing) to a low-frequency mode, which we suggest to be the frustrated translational mode of the adsorbed molecules. Additionally, the adsorption is accompanied by a broadband reflectance change, which is interpreted as due to the scattering of conduction electrons of the metal surface by the adsorbate. The reflectance change allows determination of the friction coefficient of the C₆₀ molecules, which results in rather small values (∼2×10⁹ s^-1 for Ag and Au, and ∼1.6×10⁹ s^-1for Cu), consistent with a marked metallic character of the adsorbed molecules. Pre-dosing of alkali atoms onto the metal substrates drastically changes the IR spectra recorded during subsequent C₆₀ deposition: anti-absorption bands, as well as an increase of the broadband reflectance, occur and are interpreted as due to strong electron–phonon coupling with induced surface states. Received: 6 June 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

Structural and electronic properties of pyrrolidine-functionalized [60]fullerenes

We have investigated energetic, geometric, electronic, and field emission properties of three recently synthesized fulleropyrrolidines based on the density functional theory method B3LYP/6-31G(d). Fulleropyrrolidines show higher conductivity, and solubility in water, and smaller work function in comparison with the pristine C₆₀ fullerene. The functionalization of C₆₀ with different pyrrolidines containing 3NH₂, NO, or NO₂ groups transforms it to an n-type semiconductor. The functionalization can also dramatically enhance the electrophilicity of the C₆₀ about 23–37%. Moreover, it should be mentioned that the work function is mainly influenced by the pyrrolidine containing 3NO₂ group whereas the conductivity is largely affected by the one containing 3NH₂ functionality.     

Ambipolar organic field-effect transistors on unconventional substrates

In this paper we report on the realization of flexible all-organic ambipolar field-effect transistors (FETs) realized on unconventional substrates, such as plastic films and textile yarns. A double layer pentacene-C₆₀ heterojunction was used as the semiconductor layer. The contacts were made with poly(ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of soft lithography microcontact printing (μCP). Very interestingly growing C₆₀ on a predeposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the film so it obtains n-type conduction despite the very high electron injection barrier at the interface between PEDOT:PSS and C₆₀. As a result, it was possible to obtain all-organic ambipolar FETs and to optimize their electrical properties by tuning the thicknesses of the two employed active layers. Moreover, it will be shown that modifying the triple interface between dielectric/semiconductor/electrodes is a crucial point for optimizing and balancing injection and transport of both kinds of charge carriers. In particular, we demonstrate that using a middle contact configuration in which source and drain electrodes are sandwiched between pentacene and C₆₀ layers allows significantly improving the electrical performance in planar ambipolar devices. These findings are very important because they pave the way for the realization of low-cost, fully flexible and stretchable organic complementary circuits for smart wearable and textile electronics applications.     

Electronic properties of the organic semiconductor hetero-interface CuPc/C60

We present a study of the electronic properties of the interface between the well-established molecular organic semiconductor copper phthalocyanine (CuPc) and the fullerite C₆₀ using photoelectron spectroscopy and the Kelvin-probe (KP) method. Upon deposition of CuPc on C₆₀, we found interfacial shifts of the vacuum level indicating the formation of a dipole layer, while band bending is found to be negligible. The interface dipole of 0.5 eV measured with KP is close to the difference between the work functions of bulk CuPc and C₆₀. No evidence for a chemical interaction at the interface is concluded from the absence of additional features in the core-level spectra at the earliest stages of deposition. The energy-level alignment diagram at the CuPc/C₆₀ interface is derived.     

Properties of C60 polymerized under high pressure and temperature

60 polymers. Pure and mixed phase polymeric samples were synthesized by simultaneously subjecting microcrystalline C₆₀ powder or pellets to various pressures () and temperatures (). The optical spectra of the orthorhombic, tetragonal, and rhombohedral C₆₀ polymer phases are observed to be quite distinct and rich. These spectra exhibit numerous lines and an overall downshift in frequency relative to C₆₀ is observed, consistent with a loss of double bonds from the fullerene cage. The LDMS spectra of a sample synthesized at under hydrostatic conditions and , exhibited a succession of clear peaks at mass numbers corresponding to , similar to the LDMS data on the C₆₀ photopolymer. This is taken as further evidence for interfullerene bonds in these high-pressure polymers. The XRD pattern of this sample indicates the presence of a strong texture in the sample. Received: 14 November 1996/Accepted: 8 January 1997     

Photorefractive properties of Cr-doped Sr0.61Ba0.39Nb2O6 related to crystal purity and doping concentration

The purity and the concentrations of the constituents Sr, Ba, Nb and Cr were determined in single crystals of chromium-doped Sr_0.61Ba_0.39Nb₂O₆ (SBN) by instrumental neutron activation analysis (INAA) and X-ray fluorescence analysis (XRF). Experiments with different Cr concentrations C_Cr reveal constant Sr and Ba concentrations as well as a decreasing Nb concentration with an increasing C_Cr. Therefore, Cr is incorporated at Nb lattice sites. The distribution coefficient of Cr is 1.2 between 100 and about 7000 ppm Cr and 1 above. From measurements of the holographic two-beam coupling gain Γ for different C_Cr, we deduced response times for the buildup of holographic gratings. The saturation value of Γ depends in a nonlinear manner on C_Cr and reaches its maximum at about 6000 ppm Cr. The inverse photorefractive response time of the grating erasure depends linearly on the erasure intensity for all doping concentrations. Thus a one-center model can be assumed for the charge transport in SBN:Cr. Received: 8 December 2000 / Revised version: 23 January 2001 / Published online: 21 March 2001     

Formation of metallofullerenes by laser ablation of externally doped fullerenes C60Mx (M=Sm, Pt and Ni)

Photofragmentation of metal fullerides C₆₀M_x (M=Sm, Pt and Ni) has been studied by excimer laser ablation–TOF mass spectrometry. Metallofullerenes of the type C_nM (n<60) have been observed in both the positive and negative ionic modes, with C₅₉M being the most prominent species. It is supposed that the metal atom is incorporated into the network of the fullerene cage to replace one carbon atom of the cage, forming substitutional metallofullerene. The occurrence of the C₅₉M, C₅₈M, C₅₇M clusters in the mass spectra is confirmed by the coincidence of the intensity distribution of the mass peaks with the isotopic abundance pattern calculated from the natural abundance of isotopes of C and M. Odd-numbered high-carbon clusters are observed in our laser ablation study of all the metal fullerides in the negative ion channel. The evolution of the mass spectra of these samples with laser irradiation shots indicates that the transformation process from an externally doped fullerene to the substitutionally doped fullerene involves the loss of metal carbide, MC. The structures of metallofullerenes C_2n+1M and C_2nM with even and odd total numbers of atoms respectively are discussed. Formation mechanisms with the participation of odd-numbered all-carbon fullerene clusters as intermediates are supposed. Received: 18 June 2001 / Accepted: 28 September 2001 / Published online: 2 May 2002     

Reverse saturable absorption of a novel rhodamine-B cation fulleride salt at 532 nm

A novel rhodamine-B cation fulleride salt was synthesized by metathetical reaction. Its tetrahydrofuran solution was prepared and the fluence-dependent transmission was measured with 10-ns 532-nm laser pulses in a collimated optical setup. With increases in the input fluence, its transmittance decreased significantly, which indicated reverse saturable absorption behavior. For comparison, the input fluence-dependent transmittance of fullerene C₆₀ and rhodamine-B solution were also measured. The nonlinear optical limiting effect of the fulleride salt in tetrahydrofuran is slightly lower than that of C₆₀ in toluene and the explanations are provided. Received: 25 August 2000 / Accepted: 6 November 2000 / Published online: 3 April 2001     

Structures and energetics of carbon bridged C 60 clusters

The structures and energetics of carbon bridged C₆₀ clusters (C ₆₀ ) _n C_m have been studied by simulated annealing technique within the tight-binding molecular-dynamics. The “sp² addition” ball-and-chain dimers exhibit odd-even alternations over the number of chain atoms, with the dimers containing even chain atoms more stable against dissociation than their immediate neighbors containing odd chain atoms. In addition to the usual “sp² addition” dimers, a pentagon-linked C₁₂₁ isomer and a hexagon-linked C₁₂₂ isomer are also found to be stable. Based on our tight-binding calculations, trimers and larger clusters can be simply regarded as being made up of independent or weakly interacting dimers, if the C-C₆₀ joints on a single cage are not too close to each other. Large C₆₀ clusters connected by chains each containing only one or two carbon atoms have similar stability to that of constituent dimers, indicating the possibility to form stable C₆₀-carbon polymers. Received 17 January 2001 and Received in final form 26 February 2001     

Charge‐transfer‐induced doping at an electron donor (α‐sexithiophene)/acceptor (C60) interface and mobility improvement

We studied various aspects relating to surface charge‐transfer‐induced doping at an organic/organic interface using in situ electrical measurements with a field‐effect transistor (FET) during the formation of the electron donor/acceptor interface. Adsorption of the electron‐accepting molecules (C₆₀) on top of the electron donating molecules (α‐6T) led to an increase in the FET hole mobility in an α‐6T film. Under illumination, the FET hole mobility in the α‐6T film with C₆₀ deposition was significantly increased in comparison with that in the dark due to exciton dissociation at the C₆₀/α‐6T interface, resulting in a large threshold voltage shift. The origin of the mobility increase is explained by the multiple trapping and release (MTR) model in which the mobility is determined by the carrier density. Various phenomena relevant to charge transfer and charge transport at the organic/organic interface are reported and their origins are discussed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In situ high-resolution valence photoelectron spectra of a peanut-shaped C60 polymer

To elucidate the origins of the metallic properties of the peanut-shaped C₆₀ polymer, we examined the valence photoelectron spectra of the polymer using in situ high-resolution photoelectron spectroscopy and found that the density-of-states (DOS) of the polymer clearly comes across the Fermi edge in a manner similar to that of Au film used as a reference, indicating that the peanut-shaped C₆₀ polymer exhibits metallic properties.