Metallic phase in the metal-intercalated higher fullerene Rb8.8(7)C84

A new material of higher fullerene, RbxC84, was synthesized by intercalating Rb metal into C-84 crystals. The RbxC(84) crystals showed a simple cubic (sc) structure with lattice constant, a, of 16.82 (2) angstrom at 6.5 K, and 16.87 (2) angstrom at 295 K. The Rietveld refinements were achieved with the space group, Pa (3) over bar, based on a model that the C-2 axis of D2d-C84 aligned along [111]. The sample composition was determined to be Rb-8.8(7) C-84. The ESR spectrum at 303 K was composed of a broad peak with peak-to-peak linewidth Delta H-pp of 220 G, and a narrow peak with Delta H-pp of 24 G. Temperature dependence of the broad peak clearly showed a metallic behavior. The metallic behavior was discussed based on a theoretical calculation. This finding of new metallic phase in a higher fullerene is the first step for a development of new types of fullerene materials with novel physical properties such as superconductivity.</p

Photophysics of pentacene-doped picene thin films

Here were report a study of picene nano-cristalline thin films doped with pentacene molecules. The thin films were grown by supersonic molecular beam deposition with a doping concentration that ranges between less than one molecules of pentacene every 104 picene molecules up to about one molecule of pentacene every 102 of picene. Morphology and opto-electronic properties of the films were studied as a function of the concentration of dopants. The optical response of the picene films, characterized by absorption, steady-state and time-resolved photoluminescence measurements, changes dramatically after the doping with pentacene. An efficient energy transfer from the picene host matrix to the pentacene guest molecules was observed giving rise to an intense photoluminescence coming out from pentacene. This efficient mechanism opens the possibility to exploit applications where the excitonic states of the guest component, pentacene, are of major interest such as MASER. The observed mechanism could also serve as prototypical system for the study of the photophysics of host guest systems based on different phenacenes and acenes.Comment: 15 pages, 6 figure

Accessing surface Brillouin zone and band structure of picene single crystals

We have experimentally revealed the band structure and the surface Brillouin zone of insulating picene single crystals (SCs), the mother organic system for a recently discovered aromatic superconductor, with ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction with laser for photoconduction. A hole effective mass of 2.24 m_0 and the hole mobility mu_h >= 9.0 cm^2/Vs (298 K) were deduced in Gamma-Y direction. We have further shown that some picene SCs did not show charging during UPS even without the laser, which indicates that pristine UPS works for high-quality organic SCs.Comment: 5 pages, 4 figures, Phys. Rev. Lett. 108, 226401 (2012

Evaluation of Effective Field-Effect Mobility in Thin-Film and Single-Crystal Transistors for Revisiting Various Phenacene-Type Molecules

The magnitude of the field-effect mobility mu of organic thin-film and single-crystal field-effect transistors (FETs) has been over-estimated in certain recent studies. These reports set alarm bells ringing in the research field of organic electronics. Herein, we report a precise evaluation of the mu values using the effective field-effect mobility, mu(eff), a new indicator that is recently designed to prevent the FET performance of thin-film and single-crystal FETs based on various phenacene molecules from being overestimated. The transfer curves of a range of FETs based on phenacene are carefully categorized on the basis of a previous report. The exact evaluation of the value of mu(eff) depends on the exact classification of each transfer curve. The transfer curves of all our phenacene FETs could be successfully classified based on the method indicated in the aforementioned report, which made it possible to evaluate the exact value of mu(eff) for each FET. The FET performance based on the values of mu(eff) obtained in this study is discussed in detail. In particular, the mu(eff) values of single-crystal FETs are almost consistent with the mu values that were reported previously, but the mu(eff) values of thin-film FETs were much lower than those previously reported for mu, owing to a high absolute threshold voltage, vertical bar V-th vertical bar. The increase in the field-effect mobility as a function of the number of benzene rings, which was previously demonstrated based on the mu values of single-crystal FETs with phenacene molecules, is well reproduced from the mu(eff) values. The FET performance is discussed based on the newly evaluated mu(eff) values, and the future prospects of using phenacene molecules in FET devices are demonstrated

Scanning tunneling microscopy/spectroscopy studies of two isomers of Ce@C82 on Si(111)-(7×7&#65289;surfaces

Scanning tunneling microscopy images for two isomers of Ce@C-82 were observed on Si(111)-(7x7) at 295 K. The Ce@C-82 molecules in the first layer were bound to the Si surfaces, and the motions were frozen even at 295 K. The multilayer of the Ce@C-82 isomer I (Ce@C-82-I) produced a close-packed structure in the surface layer by annealing the Si substrate at 473 K. The distance between the nearest-neighboring molecules was 1.15(4) nm whose value was consistent with that, 1.12 nm, estimated from x-ray diffraction of the Ce@C-82-I crystals. This implies that the close-packed structure is dominated by van der Waals forces, as in crystals of Ce@C-82-I. The internal structure of Ce@C-82-I was observed in the first layer due to a freeze of molecular motion caused by strong interactions between the molecule and the Si adatoms in the surface. Scanning tunneling spectroscopy revealed that the energy gaps for Ce@C-82-I and -II in the first layer opened to gap energies, E-g of 0.7 and 1.0 eV, respectively. This fact suggests that these molecules are semiconductors with smaller value of E-g than those for C-60 and C-70.</p

Facile synthesis of picenes incorporating imide moieties at both edges of the molecule and their application to n-channel field-effect transistors

Picene derivatives incorporating imide moieties along the long-axis direction of the picene core (Cn-PicDIs) were conveniently synthesized through a four-step synthesis. Photochemical cyclization of dinaphthylethenes was used as the key step for constructing the picene skeleton. Field-effect transistor (FET) devices of Cn-PicDIs were fabricated by using ZrO2 as a gate substrate and their FET characteristics were investigated. The FET devices showed normally-off n-channel operation; the averaged electron mobility (μ) was evaluated to be 2(1) × 10−4, 1.0(6) × 10−1 and 1.4(3) × 10−2 cm2 V−1 s−1 for C4-PicDI, C8-PicDI and C12-PicDI, respectively. The maximum μ value as high as 2.0 × 10−1 cm2 V−1 s−1 was observed for C8-PicDI. The electronic spectra of Cn-PicDIs in solution showed the same profiles irrespective of the alkyl chain lengths. In contrast, in thin films, the UV absorption and photoelectron yield spectroscopy (PYS) indicated that the lowest unoccupied molecular orbital (LUMO) level of Cn-PicDIs gradually lowered upon the elongation of the alkyl chains, suggesting that the alkyl chains modify intermolecular interactions between the Cn-PicDI molecules in thin films. The present results provide a new strategy for constructing a high performance n-channel organic semiconductor material by utilizing the electronic features of phenacenes

Low-voltage organic thin-film transistors based on [n]phenacenes

Low-voltage p-channel organic thin-film transistors based on [n]phenacene (n = 5, 6 or 7) were fabricated on glass and on flexible poly(ethylene 2,6-naphthalate) (PEN) substrates. For the first time, these phenacenes were combined with two ultrathin gate dielectrics based on aluminium oxide and a monolayer of octadecyl-phosphonic acid in three different transistor structures. Regardless of the substrate and the transistor structure, the field-effect mobility is found to increase with increasing length of the conjugated [n]phenacene core, leading to the best performance for [7]phenacene. The largest average field-effect mobility we have obtained is 0.27 cm2/V·s for transistors on glass and 0.092 cm2/V·s for transistors on flexible PEN

Transistor properties of exfoliated single crystals of 2H-Mo(Se1-x Te-x) 2 ( 0 <= x <= 1)

Field-effect transistors (FETs) were fabricated using exfoliated single crystals of Mo(Se1-x Te-x)(2) with an x range of 0 to 1, and the transistor properties fully investigated at 295 K in four-terminal measurement mode. The chemical composition and crystal structure of exfoliated single crystals were identified by energy-dispersive x-ray spectroscopy (EDX), single-crystal x-ray diffraction, and Raman scattering, suggesting the 2H - structure in all Mo(Se1-x Te-x)(2). The lattice constants of a and c increase monotonically with increasing x, indicating the substitution of Se by Te. When x 0.4. In contrast, the polarity of a thick single-crystal Mo(Se1-x Te-x)(2) FET did not change despite an increase in x. The change of polarity in a thin single-crystal FET was well explained by the variation of electronic structure. The absence of such change in the thick single-crystal FET can be reasonably interpreted based on the large bulk conduction due to naturally accumulated electrons. The mu value in the thin single-crystal FET showed a parabolic variation, with a minimum mu at around x = 0.4, which probably originates from the disorder of the single crystal caused by the partial replacement of Se by Te, i.e., a disorder that may be due to ionic size difference of Se and Te

Preparation of new superconductors by metal doping of two-dimensional layered materials using ethylenediamine

We have studied new superconductors prepared by metal doping of two-dimensional (2D) layered materials, FeSe and FeSe0.5Te0.5, using ethylenediamine (EDA). The superconducting transition temperatures (T(c)s) of metal-doped FeSe and metal-doped FeSe0.5Te0.5, i.e., (EDA)(y)MxFeSe and (EDA)(y)MxFeSe0.5Te0.5 (M: Li, Na, and K), were 31-45 K and 19-25 K, respectively. The stoichiometry of each sample was clarified by energy dispersive x-ray (EDX) spectroscopy, and the x-ray powder diffraction pattern indicated a large expansion of lattice constant c, indicating the cointercalation of metal atoms and EDA. The pressure dependence of superconductivity in (EDA)(y)NaxFeSe0.5Te0.5 has been investigated at a pressure of 0-0.8GPa, showing negative pressure dependence in the same manner as (NH3)(y)NaxFeSe0.5Te0.5. The T-c-c phase diagrams of MxFeSe and MxFeSe0.5Te0.5 were drawn afresh from the T-c and c of (EDA)(y)MxFeSe and (EDA)(y)MxFeSe0.5Te0.5, showing that the T-c increases with increasing c but that extreme expansion of c reverses the T-c trend