T. Siegrist

A soluble and air-stable organic semiconductor with high electron mobility

Electronic devices based on organic semiconductors offer an attractive alternative to conventional inorganic devices due to potentially lower costs, simpler packaging and compatibility with flexible substrates. As is the case for silicon-based microelectronics, the use of complementary logic elements—requiring n- and p-type semiconductors whose majority charge carriers are electrons and holes, respectively—is expected to be crucial to achieving low-power, high-speed performance. Similarly, the electron-segregating domains of photovoltaic assemblies require both n- and p-type semiconductors. Stable organic p-type semiconductors are known, but practically useful n-type semiconductor materials have proved difficult to develop, reflecting the unfavourable electrochemical properties of known, electron-demanding polymers. Although high electron mobilities have been obtained for organic materials, these values are usually obtained for single crystals at low temperatures, whereas practically useful field-effect transistors (FETs) will have to be made of polycrystalline films that remain functional at room temperature. A few organic n-type semiconductors that can be used in FETs are known, but these suffer from low electron mobility, poor stability in air and/or demanding processing conditions. Here we report a crystallographically engineered naphthalenetetracarboxylic diimide derivative that allows us to fabricate solution-cast n-channel FETs with promising performance at ambient conditions. By integrating our n-channel FETs with solution-deposited p-channel FETs, we are able to produce a complementary inverter circuit whose active layers are deposited entirely from the liquid phase. We expect that other complementary circuit designs can be realized by this approach as well.

New phases of C60 synthesized at high pressure

The fullerene C60 can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C60 on reheating to 300�C at ambient pressure. For synthesis temperatures between 300� and 400�C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced with a lattice parameter ao = 13.6 angstroms. When treated at 500� to 800�C at the same pressure, C60 transforms into a rhombohedral structure with hexagonal lattice parameters of ao = 9.22 angstroms and co = 24.6 angstroms. The intermolecular distance is small enough that a chemical bond can form, in accord with the reduced solubility of the pressure-induced phases. Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C60 molecules are linked.

Superconductivity near 70 K in a new family of layered copper oxides

A new family of high-temperature superconductors is described, with the general formula Pb2Sr2ACu3O8+δ. Although they have the planes of CuO5 square pyramids characteristic of the other copper-oxide superconductors, the new compounds belong to a distinct structural series, with wide scope for elemental substitution. Their unusual electronic configuration also gives new insight into the role of charge distribution among the structural building blocks in controlling superconductivity.

Disorder-induced localization in crystalline phase-change materials

Localization of charge carriers in crystalline solids has been the subject of numerous investigations over more than half a century. Materials that show a metal-insulator transition without a structural change are therefore of interest. Mechanisms leading to metal-insulator transition include electron correlation (Mott transition) or disorder (Anderson localization), but a clear distinction is difficult. Here we report on a metal-insulator transition on increasing annealing temperature for a group of crystalline phase-change materials, where the metal-insulator transition is due to strong disorder usually associated only with amorphous solids. With pronounced disorder but weak electron correlation, these phase-change materials form an unparalleled quantum state of matter. Their universal electronic behaviour seems to be at the origin of the remarkable reproducibility of the resistance switching that is crucial to their applications in non-volatile-memory devices. Controlling the degree of disorder in crystalline phase-change materials might enable multilevel resistance states in upcoming storage devices.

Physical vapor growth of centimeter-sized crystals of α-hexathiophene

Abstract The processes in physical vapor transport: vaporization, transport and crystal growth, the regimes for transport: molecular flow path limited, diffusion-limited, convection-limited and forced-convection-limited are analyzed and the results are used to guide a systematic investigation of physical vapor transport and crystal growth of α-hexathiophene (α6T), a promising thin-film transistor organic material. Successful growth occurred when the gas pressure was such that the regime was convective and when deliberate inert-gas flow (forced convection) improved volatilization. Plate-like growth morphology and thickness differences between the high-temperature and low-temperature polymorphs is explained on the basis of differing atomic structure. Conditions for the reproducible growth of crystals of up to 1 cm in size are reported. We feel that the analyses and procedures reported here can be used to grow crystals of other organic materials.

Crystal Structure and the Paraelectric-to-Ferroelectric Phase Transition of Nanoscale BaTiO3

We have investigated the paraelectric-to-ferroelectric phase transition of various sizes of nanocrystalline barium titanate (BaTiO3) by using temperature-dependent Raman spectroscopy and powder X-ray diffraction (XRD). Synchrotron X-ray scattering has been used to elucidate the room temperature structures of particles of different sizes by using both Rietveld refinement and pair distribution function (PDF) analysis. We observe the ferroelectric tetragonal phase even for the smallest particles at 26 nm. By using temperature-dependent Raman spectroscopy and XRD, we find that the phase transition is diffuse in temperature for the smaller particles, in contrast to the sharp transition that is found for the bulk sample. However, the actual transition temperature is almost unchanged. Rietveld and PDF analyses suggest increased distortions with decreasing particle size, albeit in conjunction with a tendency to a cubic average structure. These results suggest that although structural distortions are robust to changes in particle size, what is affected is the coherency of the distortions, which is decreased in the smaller particles.

Zinc-indium-oxide: A high conductivity transparent conducting oxide

We report the fabrication and characterization of zinc‐indium‐oxide films with similar electrical conductivity and better transparency in both the visible and infrared compared with indium–tin–oxide, a widely used transparent conductor in many technological applications. Dramatically superior transmission properties in the 1–1.5 μm range in particular make zinc–indium–oxide attractive for use in infrared devices, where transparent electrodes are required. Resisitivities as low as 400 μΩ cm result from doping with small quantities of Sn; Al, Ga, and Ge are also effective dopants. Deposition on glass and quartz substrates as amorphous films by pulsed laser deposition and dc reactive sputtering is described.

Studies of oxygen-deficient Ba2YCu3O7−δ and superconductivity Bi(Pb)SrCaCuO

Abstract Ambient temperature measurements of the crystallographic cell parameters for oxygen deficient Ba 2 YCu 3 O 7−δ prepared by gettered annealing indicate the presence of microscopic differences in the oxygen configuration at fixed δ dependent on annealing temperature. The loss of superconductivity with increasing oxygen deficiency is shown to be due to a step-like increase in length of the plane copper-apical oxygen bond, and not to the orthorhombic to tetragonal transition. The crystal structure of the 84K superconductor Bi 2 Sr 2 CaCu 2 O 8+δ is described, as is the stabilization of 110K superconductivity via partial Pb/Bi substitution with long time annealing.

A new layered cuprate structure-type, (A1−xA′x)14Cu24O41

Abstract Phases crystallizing in a new structure-type with general formula (A1−xA′x)14Cu24O41 (A = alkaline earth metal, A′ = trivalent metal) and symmetry Cccm with an extended stoichiometry range have been found in studies of the SrYCuO, BaSrBiCuO and CaLaCuO systems. Single crystal X-ray studies on several crystals grown from different alkaline earth/metal oxide-cuprate melts reveal a common orthorhombic F-centered subcell of a =11.3 A , b =13.0 A and c =3.9 A . Superstructure is observed in crystals, leading to a 7-fold increase of the c-axis and a change in symmetry to space group Cccm. As in Ba2YCu3O7, the Cu atoms are found as CuO planes and linear CuO chains. Due to shear in the planes, half of the CuO squares share edges, producing CuCu zigzag chains, similar to the planes observed in CaCu2O3. In the linear CuO chains, the CuO squares share edges as well, leading to a short CuCu contact of 2.75A. Experiments on ceramic samples indicate that the oxygen content is fixed and that the samples are semiconducting.

Single-crystal field-effect transistors based on copper phthalocyanine

Copper phthalocyanine (Cu–Pc) single crystals were grown by physical vapor transport and field-effect transistors (FETs) on the surface of these crystals were prepared. These FETs function as p-channel accumulation-mode devices. Charge carrier mobilities of up to 1cm2∕Vs combined with a low field-effect threshold were obtained. These remarkable FET characteristics, along with the highly stable chemical nature of Cu–Pc, make it an attractive candidate for device applications.