During the last twenty years a large number of organic-inorganic hybrid compounds based on metal-halide units have been prepared and studied, in our institute and in collaboration with others. Compounds of the type (CH3C6H4CH2NH3)2MX4 (M=Pb, Sn; X=I, Br, Cl) behave as two-dimensional (2D) semiconducting systems. Compounds of the type (CH3NH3)n-1 (CH3C6H4CH2NH3)2MnX3n+1 (n=2: bilayer; n=3: trilayer; n?3:multilayer) behave as quasi-two-dimensional (q-2D) semiconducting sytems. Compounds of the type (CH3NH3)MX3 or (CH3NH3)n-1(CH3C6H4CH2NH3)2MnX3n+1 with n>3 behave as three-dimensional (3D) semiconducting systems. Investigations of the optical properties on single crystals of (CH3NH3)n-1 (CH3C6H4CH2NH3)2MnX3n+1 showed that the excitonic optical absorption (OA) and photoluminescence (PL) bands are shifted to longer wavelengths with increasing the number of layers (i.e., n). For large values of n the excitonic band occurs close to that of the corresponding 3D material (CH3NH3)MX3. In the most cases, single crystals or polycrystalline powders, contaminated by several different species (n=1,2,3.). For example, single crystals or polycrystalline pallets of the 2D systems (CH3C6H4CH2NH3)2 PbBr4 obtained from solutions or from melts exhibit a sharp excitonic PL band at ca 408 nm, and crystals of the 3D system (CH3NH3)PbBr3 exhibit an excitonic PL band at ca 530 nm. Q-2D systems exhibit exhitonic bands at intermediate position. Also, it has been observed that, after intensive grinding of crystals of the q-2D and 3D compounds, considerable shifts of the PL (and OA) bands and enhancement of the PL bands take place. For example, in the cases of (CH3NH3)n-1(CH3C6H4CH2NH3)2PbnBr3n+1, (q-2D) and (CH3NH3)PbBr3 (3D) the PL bands occur at ca 520nm after grinding the crystals. The green photoluminescence of the grinding samples is observed with naked eye. Similar results were obtained by spreading solutions of compounds on quartz plates.
By changing M and halide, it was observed that the strong low frequency PL bands cover a wide spectral region, e.i. from ca 400 to ca 700nm. A possible explanation of the enhancement of the low frequency PL band is based on the Foerster (or fluorescence ) resonant energy transfer.
PL (a,b) and OA (c) spectra of (CH3NH3)n-1(CH3C6H4CH2NH3)2PbnBr3n+1 obtained from melts, before (a) and after (b,c) grinding the sample; excitation 350 nm.
Three and low dimensional inorganic semiconductors, G. C. Papavassiliou, Progr. Solid State Chem. 25 125 (1999).
Organic - inorganic Hybrid Compounds Based on Lead-halide Units: Preparation from Melts and Grinding Effects, G. C. Papavassiliou, G. A. Mousdis, G. C. Anyfantis, Z. Naturforsch. 65b, 516 (2010).
2. Organic Conducting and Superconducting Systems
Until recently it was believed that electrical conductivity and metallic behavior in general was a property exclusive of inorganic materials and, especially, of metals. However, during the second half of the 20 th century a number of organic conducting materials were synthesized. Most of them are based on tetrathiafulvalenes (TTF's) and metal 1,2 dithiolenes (M 1,2-DTs). There are at least four types of organic conducting systems:
crystalline radical or charge transfer salts,
single component systems
C 60 and nanotubes.
A large variety of organic conductors can be synthesized through appropriate chemical manipulations. The tuning of properties guides the synthesis, and leads to hundreds of new conducting organic materials including some superconducting ones. The study of these materials revealed many new physical phenomena such as Peierls transition, spin density transition, negative magnetoresistance, de Hass-van Alphen oscillations, quantum Hall effect, etc., and contributed to a better understanding of solid state physics.
The Development of Organic Conductors
The first "organic metal" discovered was TTF-TCNQ in 1973. Since then a vast number of organic conductors has been prepared, most of them based on radical salts of the TTF molecule. Over 50 organic superconductors [e.g. (MDT-TTF) 2AuI 2] have been discovered in the past 15 years.
In our effort to design and prepare new Organic Conductors and Superconductors , we combine the expertise of several disciplines in chemistry, physics, and materials science.
The experimental capabilities in our laboratory, for the synthesis of organic, inorganic, and organometallic molecular components, crystal growth, and characterization, are interfaced with the capabilities of collaborating groups.
The principal activities of our research team concern the design and synthesis of the precursors of organic conductors. The precursors are π -donor organic molecules that can be readily oxidized to stable cation-radicals or reduced to anion-radicals. In combination with appropriate counterions, crystalline cation-radical or anion-radical salts of the organic molecules are synthesized by a highly purifying process called electrocrystallization.
Our efforts focus mainly on the synthesis and investigation of new organic quasi-2-dimensional metals the so called τ-phase organic conductors. Full structural characterization of the crystals by X-ray diffraction at ambient temperature is being conducted in collaboration with X-RAY Crystallography Laboratory of IMS-NCSR "Demokritos". Knowledge of crystal structure allows the calculation of their band electronic structures, in a collaboration with L. Ducasse group ( C.R.C.M University Bordeaux France). Four-probe electrical conductivity measurements in a collaboration with K. Murata group (Graduate School of Science,Osaka City University, Japan), reveal the nature of the normal state-to-superconducting, Peierls, spin-Peierls, charge-density wave, or spin-density wave phase transitions that frequently occur in these materials. The magnetoelectric properties at low temperatures, measured by the K. Murata group and for very high magnetic fields measured by the J. S. Brooks group (NHMFL, University of Florida USA), reveal interesting phenomena such as negative magnetoresistance and Shubnikov de Haas (SdH) oscillations. In collaboration with the Lyubovskaya group (Institute of Problems of Chemical Physics RAS, ChernogoloVka, Russia) we studied new materials based on asymmetrical donors. The combined approach to synthetic metal development described above is essential to the development of new and improved organic conductors. This research strategy has been quite successful, and many new conducting salts including two superconducting ones have been discovered in recent years. Recently, a number of single component symmetrical and unsymmetrical metal 1,2-dithiolenes and their structural, optical and electrical properties were reported.
Structure and conductivity of unsymmetrical p-donor ethylenedithio-dithiadiselenafulvalene iodomercurate, (EDT-DTDSF)4Hg3I8E.I. Zhilyaeva, A.Yu. Kovalevskyi, S.A. Torunova, G.A. Mousdis, R.B. Lyubovskii, G.C. Papavassiliou, P. Coppens, R.N. LyubovskayaSynth. Metals 150, (3), 245-250 (2005).
G. C. Papavassiliou, in: J. Yamada and T. Sugimoto (Eds.), TTF Chemistry, Kodansha and Springer, 2003, Chap.2.
Optical properties of the conducting salt τ -(P-S,S-DMEDT-TTF) 2(AuBr2) (AuBr2)y (y » 0.75) A. Lapinski, A. Graja, G. C. Papavassiliou, G. A. Mousdis. Synthetic Metals 139 405 (2003).
New ambient pressure organic superconductor with T c = 8.1 K based on unsymmetrical donor ethylenedithiotetrathiafulvalene: b -(EDT-TTF)4Hg 2.83I E. Zhilyaeva, O. Kazheva, S. Torunova, R. Lyubovskaya, O. Dyachenko, G. Mousdis, G. Papavassiliou, J. Perenboom, S. Pesotskii, R. Lyubovskii Synth. Metals 140 151 (2004).
Magnetization, thermoelectric, and pressure studies of the magnetic field-induced metal to insulator transition in tau phase organic conductors. D. Graf, E. S. Choi, J. S. Brooks, N. Harrison, K. Murata, T. Konoike, G.C.Papavassiliou and G. A. Mousdis Physical Review B71 (4) 045117 (2005)
Precursors of Organic Conductors and Superconductors
The majority of the crystalline organic conductors are based on π -donor molecules that are similar to TTF and can be easily and reversible oxidized to +1 or +2
Replacement of S with Se and addition of other groups gave new π -donor molecules symmetrical or unsymmetrical. The most important are:
G. C. Papavassiliou, in: J. Yamada and T. Sugimoto (Eds.), TTF Chemistry, Kodansha and Springer, 2003, Chap. 2.
B. Barszcz, A. Graja, G. Soras, N. Psaroudakis, G. A. Mousdis. J. Phys. Chem. Solids. 68 , 1364 (2007)
Electrocrystallization is an excellent technique to prepare high-quality conductive crystals. The process takes place by passing a current through a pair of platinum electrodes in an electrolyte solution containing electron donor molecules. Redox reactions occur near the electrode, and the radical cations generated are trapped by the anions from the electrolyte to form black and lustrous crystals on the anode. The procedure is a clean and self-purifying one. It is applicable to aqueous, organic, or molten salt solutions.
τ -Phase Organic Conductors
The majority of organic metals are crystallized in different phases called α , β , θ , λ , κ , τ etc. One of the most important phases is the τ -phase. Organic conductors of τ -phase have been obtained by electrocrystallization of some unsymmetrical π -donor molecules, such as EDO-DMEDT-TTF and P-DMEDT-TTF, with linear anions such as AuI 2, I 3, CuBr 2. They are crystallized in the tetragonal system, with the formula τ -(D) 2 (X) 1 (X) y . Their electronic band structure predicts a star - like Fermi surface.
The organic conductors or more precisely the organic-inorganic hybrid conductors of τ -phase are composed of mixed organic cation-anion layers alternating with anion layers, and the resulting crystals are metallic in directions parallel to the layers, i.e. in the ab plane. They remain metallic with decreasing temperature and become semiconducting or insulating at low temperatures.
Due to their structure their conductivity is isotropic at the directions a, b and highly anisotropic at the direction c, R c>>1000 Rab.
They show negative Hall coefficient, indicating negative carriers (electrons), and negative magnetoresistance at low fields. At high fields they exhibit giant magnetoresistance oscillations, quantum Hall effect and chiral surface states, characteristic of the quasi-2-dimensional systems.
"Magnetization, thermoelectric, and pressure studies of the magnetic field-induced metal to insulator transition in tau phase organic conductors." D. Graf, E. S. Choi, J. S. Brooks, N. Harrison, K. Murata, T. Konoikr, G.C.Papavassiliou, G. A. Mousdis. Physical Review B 71 (4) 045117 (2005)
" Structural and Physical Properties of τ-(EDO-S,S-DMEDTTTF) 2 (AuBr2)1+y and τ-(P-S,S-DMEDT-TTF)2 (AuB 2 )1+y ." G.C.Papavassiliou, G. A. Mousdis, G.C. Anyfantis, K.Murata, L.Li, H.Yoshino, H. Tajima, T. Konoike, J.S.Brooks, D. Graf and E.S. Choi. Z. Naturforsch. 59a 952 (2004).
"New donor molecules and their τ -p hase conducting salts" G. C. Papavassiliou, G. A. Mousdis, A. Terzis, C. P. Raptopoulou K. Murata. T. Konoike, H. Yoshino, A. Graja, and A. Lapinski. Synth . Metals 135-136 651 (2003)
" Low Temperature Electric Nature of τ Phase Contuctors", T. Konoike, A. Oda, K. Iwashita, T. Yamamoto, H. Tajima, H. Yoshino, K. Ueda, T. Sugimoto, K. Hiraki, T. Takahashi, T. Sasaki, Y. Nishio, K. Kajita, G. C. Papavassiliou, G. A. Mousdis and Keizo Murata. Synth. Metals 120 801 (2001).
" Preparation, Structure and Physical Properties of Some New Organic Conductors of τ -Phase " G. C. Papavassiliou, G. A. Mousdis, A. Terzis, C. Raptopoulou, K. Murata, T. Konoike and Y. Yoshino. Synth. Metals 120 743 (2001).
''Low dimensional organic conductors as thermoelectric materials'', H. Yoshino, G. C. Papavassiliou and K. Murata, J. Therm. Anal. Cal. 92 457 (2008).
A New Superconductor
The synthesis of the donor molecule EDT-TTF was done in 1988 by our group. In collaboration with the Russian Groub (R.N. Lyubovskayas) the preparation and study of superconducting (EDT-TTF)4[Hg3I8]0.973 radical cation salt were done in 2006. All of the of the crystals have a layered structure, in which organic and inorganic layers are alternate along the c-axis. It has been proved for the (EDT-TTF)4[Hg3I8]1-x salts that when x 10 then there is a metal superconductor transition at ambient or under low pressure.
''Low temperature measurements of the electrical conductivities of some change transfer salts with the assymetric. (MDT-TTF) 2Au 2 a new superconductor'' G. C. Papavassiliou, G. A. Mousdis, J. S. Zambounis, A. Terzis, A. Hountas, B. Hilti, G. W. Mayer and J. Pfeiffer. Synt. Metals 27, B379-B383 (1988).
''Anion Chain Structure Controlled Behavior of Phase Transition in Quasi-Two-Dimensional Organic Metal (EDT-TTF)4[Hg3I8]1-x'' E. I. Zhilyaeva, A. Y. Kovalevsky, R. B. Lyubovskii, S. A. Torunova, G. A. Mousdis, G. C. Papavassiliou, and R. N. Lyubovskaya. Crystal Growth & Design, 7 (12), 2768-2773 (2007)T
''New ambient pressure organic superconductor with Tc = 8.1 K based on unsymmetrical donor ethylenedithiotetrathiafulvalene: b-(EDT-TTF)4Hg2.83I8.'', E. Zhilyaeva, O. Kazheva, S. Torunova, R. Lyubovskaya, O. Dyachenko, G. Mousdis, G. Papavassiliou, J. Perenboom, S. Pesotskii, R. Lyubovskii. Synth. Metals 140 151-154 (2004).
Single component metal 1,2-dithiolene complexes (M 1,2-DTs)
We have prepared and studied some single component M 1,2-DTmolecules (similar to TTF's) of the following formulas:
where R'=H, CH3, Ph, 2R'= -CH2CH2CH2- etc; 2R = -CH2CH2CH2-, -CH2CH2-, C=X; X=S,O.
Also we prepared similar molecules by replacing some S by Se.
Most of them show third order non-linear optical properties. According to the Cyclic Voltametry measurements the LUMO and HOMO energy states are about 4.5 and 5.4eV respectively, showing that these complexes are stable in air.
These molecules can be used as materials for electronic devices, because the Fermi level of Au (that is the most common electrode) is 4.8eV, and is in between the LUMO and HOMO levels of M 1,2DT complexes .
We fabricated an ambipolar Field -Effect transistor based on Ni(dpedt)(dmit) that showed very good characteristics, which remain unchanged after exposure of the device to ambient atmosphere for 3 months, at least.
''Transient nonlinear optical response of novel neutral unsymmetrical nickel dithiolene complexes'',P. Aloukos, S. Couris, J.B. Koutselas, G.C. Anyfantis and G.C. Papavassiliou,Chem. Phys. Lett. 428, 109 (2006).
''Some New Nickel Dichalcogenolene Complexes as Single Component Semiconductors'', G.C. Papavassiliou, G.C. Anyfantis, B.R. Steele, A. Terzis, C.P. Raptopoulou, G. Tatakis, G. Chaidogiannos, N. Glezos, Y.F. Weng, H. Yoshino, and K. Murata, Z. Naturforsch. 62b, 679 (2007).
"Air-stable ambipolar organic transistors" T.D. Anthopoulos, G. C. Anyfantis, G.C. Papavassiliou and D.M. deLeeuw, Appl. Phys. Lett. 90 122105 (2007)
''Some unsymmetrical metal 1,2 - dithiolenes based on Pd, Pt and Au'', G.C. Papavassiliou, G. C. Anyfantis, A. Terzis, V. Psycharis, P. Kyritsis and P. Paraskeyopoulou.Z. Naturforsch 63b, 1377 (2008).